Mouse models of accelerated aging in musculoskeletal research for assessing frailty, sarcopenia, and osteoporosis – A review

Musculoskeletal aging encompasses the decline in bone and muscle function, leading to conditions such as frailty, osteoporosis, and sarcopenia. Unraveling the underlying molecular mechanisms and developing effective treatments are crucial for improving the quality of life for those affected. In this context, accelerated aging models offer valuable insights into these conditions by displaying the hallmarks of human aging. Herein, this review focuses on relevant mouse models of musculoskeletal aging with particular emphasis on frailty, osteoporosis, and sarcopenia. Among the discussed models, PolgA mice in particular exhibit hallmarks of musculo-skeletal aging, presenting early-onset frailty, as well as reduced bone and muscle mass that closely resemble human musculoskeletal aging. Ultimately, findings from these models hold promise for advancing interventions targeted at age-related musculoskeletal disorders, effectively addressing the challenges posed by musculoskeletal aging and associated conditions in humans.


Introduction
Musculoskeletal aging denotes the age-associated alterations that manifest within the bones, muscles, tendons, ligaments, and joints (Roberts et al., 2016).This natural progression often gives rise to conditions such as frailty, osteoporosis, and sarcopenia, all of which contribute to reduced physical capabilities and heightened susceptibility to stressors, ultimately increasing the likelihood of adverse health outcomes among older individuals.
The collective state of vulnerability and reduced resilience is commonly referred to as frailty, encompassing the interconnected challenges posed by musculoskeletal aging (Milte and Crotty, 2014).Osteoporosis, on the one hand, a widespread ailment in older adults, is marked by decreased bone density and structural deterioration, significantly increasing the risk of fractures, especially in weight-bearing bones (Demontiero, Vidal, and Duque, 2012).On the other hand, sarcopenia refers to the progressive loss of muscle mass and strength and further contributes to functional decline, reduced mobility, and an increased risk of falls and fractures (Walston, 2012).
There is a closely intertwined relationship between frailty, agerelated osteoporosis, and sarcopenia which collectively contribute to the physical decline observed in older individuals.Sarcopenia and osteoporosis exhibit many parallels with frailty and can significantly contribute to their development.As people age, the decline in bone and muscle mass and strength can diminish the capacity for routine daily activities of individuals resulting in an increased risk of falls and prolonged periods of bed rest (Fedarko, 2011;Fulop et al., 2010).Decreased muscle function and strength frequently co-occur with the deterioration of bone microarchitecture in elderly populations (Greco, Pietschmann, and Migliaccio, 2019).Osteosarcopenia, therefore, is a condition that describes the simultaneous presence of osteoporosis and sarcopenia and it is associated with an increased risk of fractures, frailty, reduced mobility, chronic diseases hospitalization, and death (Hirschfeld, Kinsella, and Duque, 2017).Osteosarcopenic individuals had lower grip strength, lower T-scores, and worse balance compared to those with sarcopenia or osteoporosis alone indicating that the combination of osteoporosis and sarcopenia may predict adverse outcomes beyond the risk posed by each condition individually (Sepulveda-Loyola et al., 2020).
Moreover, frailty contributes to decreased physical function while sarcopenia and osteoporosis, when considered separately, may not always be directly associated with physical function in older adults.Physical inactivity, characterized by a lack of regular exercise and sedentary behavior, is a common precursor to frailty (Del Pozo-Cruz et al., 2017).Sedentary lifestyles, coupled with bed rest or immobilization, accelerate muscle atrophy, leading to muscle weakness and reduced functional capacity, both defining features of frailty.In addition, sedentary behavior has been linked to elevated levels of oxidative stress and inflammation (Carter, Hartman, Holder, Thijssen, and Hopkins, 2017).Therefore, physical exercise serves as an effective intervention to enhance various aspects of musculoskeletal health, including anti-oxidant effects, increased muscle strength, and improved functional capacity in the elderly.
Gaining a deeper understanding of the underlying mechanisms and discovering effective strategies to mitigate these musculoskeletal conditions is crucial for promoting healthy aging and improving the quality of life for older adults.In this regard, mouse models have emerged as valuable tools for investigating musculoskeletal aging, owing to their genetic similarity to humans and the ability to manipulate their genomes easily (Jilka, 2013).In particular, accelerated aging mouse models have received significant attention in recent years through genetically induced alterations that accelerate the aging process, thereby displaying hallmarks of musculoskeletal aging and associated conditions seen in humans (Hasty and Vijg, 2004).On that note, this review focuses on musculoskeletal aging (Table 1) with particular emphasis on four specific mouse models of accelerated aging that exhibit

Table 1
Comparison of musculoskeletal aging conditions in humans and mice with emphasis on evaluation methods and criteria.

Mouse
Frailty FP: evaluating grip strength, walking speed, physical activity, and endurance ▪ not frail: 0 criteria present ▪ pre-frail: 1-2 criteria present ▪ frail ≥ 3 criteria present (Liu et al., 2014) Valencia score: evaluating weight loss, running time and speed, grip strength and motor coordination ▪ not frail: 0 criteria present ▪ pre-frail: 1-2 criteria present ▪ frail ≥ 3 criteria present (Gomez-Cabrera et al., 2017) FI: evaluating cumulative health deficiency score of 31 parameters ▪ 0: no deficiency ▪ 0.5: mild deficiency ▪ 1: severe deficiency (Whitehead et al., 2014) Osteoporosis BMD: ▪ DXA (Jilka, 2013) Bone hallmarks of frailty, osteoporosis, and sarcopenia with their validated assessment methods (Table 2).Our selection of these four mouse models is not intended to replicate the entirety of human aging, nor do they possess precise mutations specific to musculoskeletal tissues.Instead, our aim is to glean valuable insights into specific aspects of aging, such as DNA repair mechanisms or Lamin A deficiency, which have notable effects on musculoskeletal aging.Thereby, this review offers a comprehensive yet manageable overview of accelerated aging mouse models that encompass a range of musculoskeletal aging-related research, with their validated experimental evaluation methods.

Frailty
Frailty is characterized by the loss of physiological function and reduced strength and endurance, leading to increased susceptibility to adverse health outcomes in aged humans (Fedarko, 2011).It involves a decline of function in various physiological systems, leading to reduced resilience, diminished functional capacity, and increased risk of disability, morbidity, and mortality.The underlying causes of frailty are not fully understood, but possible mechanisms include increased oxidative stress, DNA damage, chronic inflammation (inflammaging), cellular senescence, and mitochondrial dysfunction (Fedarko, 2011;Fulop et al., 2010).Oxidative stress arises from an imbalance between the production of reactive oxygen species (ROS) and the body's ability to counteract their harmful effects (El Assar, Angulo, and Rodriguez-Manas, 2020).Over time, chronic oxidative stress can result in changes in the redox signaling at the skeletal muscle level.
Mitochondrial dysfunction in muscle cells can lead to an increased generation of ROS, which in turn triggers a self-perpetuating cycle of deterioration.In aging muscle, oxidative stress increases the deterioration of type II fibers, and increased ROS production stimulates protein degradation through the activation of muscle proteases and decreases the proteasomal breakdown of damaged proteins contributing to the progression of frailty (Baumann, Kwak, Liu, and Thompson, 2016).Inflammaging, on the other hand, is a state of chronic inflammation marked by persistent elevation in proinflammatory cytokines and chemokines, notably Interleukin-6 (IL-6), and tumor necrosis factor-α (TNF-α) all of which have the direct capacity to increase muscle loss and decrease muscle regeneration, consequently amplifying the risk of frailty (Baylis et al., 2013;Franceschi et al., 2000;Fulop et al., 2010).Recently, cellular senescence, the gradual deterioration of cells over time, has emerged as a potential contributor to frailty.A prominent feature of cellular senescence is the chronic release of proinflammatory cytokines and chemokines (von Zglinicki, Wan, and Miwa, 2021).One pathway leading to senescence involves the activation of p53-p21 and/or p16INK4a genes, which inhibits the activation of CDK4/6 kinases and subsequently, the pRB protein, resulting in cell-cycle arrest (d'Adda di Fagagna, F, 2008).When p53, a tumor suppressor gene is activated, it initiates responses such as DNA repair, cell cycle arrest, and programmed cell death.Additionally p21, a protein induced by p53 functions as a cyclin-dependent kinase (CDK) inhibitor preventing CDK activity and leading to cell cycle arrest in the G1 phase.Another protein, p16INK4a, encoded by the CDKN2A gene, inhibits CDK4 and CDK6, which are critical for cell cycle progression.Finally, p16INK4a blocks phosphorylation of the retinoblastoma protein (pRB), maintaining it in an unphosphorylated state that cell cycle progression, thereby  (Takeda et al., 1997) (Takeda, 1999) Lamin A deficiency (Bergo et al., 2002) DNA damage repair impairement (Weeda et al., 1997) mitochondrial dysfunction (Trifunovic et al., 2004) (Kujoth, Leeuwenburgh, and Prolla, 2006 Parameters: Frailty Index (FI), Bone Mineral Density (BMD), Intravertebral Disk Degeneration (Henson et al., 2015).*Only the PolgA mice are subjected to clinical mouse frailty scoring indicated as FI.The frailty-associated features exhibited in this table for the other mouse models are based on the mouse monitoring.
D. Yılmaz et al. contributing to cellular senescence and the prevention of replication of damaged or aging cells (d'Adda di Fagagna, F, 2008).Several aspects of senescence contribute to frailty development including persistent DNA damage response (DDR), senescence-associated secretory phenotype (SASP), senescence-associated mitochondrial dysfunction (SAMD), disruptions in metabolic and nutrient signaling and epigenetic reprogramming (von Zglinicki et al., 2021).Various evaluation methods have been developed to capture the multidimensional nature of frailty.One widely used approach is the frailty phenotype (FP) proposed by Fried et al.This approach considers people frail if they exhibit at least three out of the following five characteristics: exhaustion, low physical activity, weakness, slow walking speed, and unintended weight loss (Fried et al., 2001).Another method is the frailty index (FI), developed by Rockwood et al., which calculates the ratio of the number of health deficiencies present in an individual divided by the total number of deficiencies evaluated (Rockwood and Mitnitski, 2011;Rockwood et al., 2005).Rockwood's FI method evaluates overall health status, comorbidities, and functional impairments.It has been shown to be a valuable predictor of adverse health outcomes and rapid physical and cognitive deterioration in older adults.A higher FI score indicates greater susceptibility to functional decline, and increased risk of falls, hospitalizations, and mortality.It serves as an indicator of the overall vulnerability and frailty of an individual.Building upon the FI, the Clinical Frailty Scale (CFS) was developed to provide a simplified and standardized measure of frailty correlated with the FI method (Rockwood et al., 2005).The CFS categorizes individuals into one of nine categories ranging from "very fit" (level 1) to "terminally ill" (level 9), with each level representing a different degree of frailty.A score of 5 or above indicates that a patient is frail.The CFS offers a quick and practical way to estimate the level of frailty of an individual, providing a common language for clinicians, researchers, and healthcare professionals to communicate and make informed decisions about care and interventions.The FI captures a more detailed and nuanced evaluation of health deficits, while the CFS provides a simplified scale to estimate the overall level of frailty.In contrast, the FP method focuses on physical features rather than clinical symptoms and signs of diseases.Overall, both approaches demonstrate that frailty increases with age and is associated with morbidity and mortality in old humans and identify individuals who are at higher risk of adverse outcomes.
In preclinical research, efforts have been made to develop mouse models that mimic human frailty (Baumann, Kwak, and Thompson, 2020;Liu, Graber, Ferguson-Stegall, and Thompson, 2014;Parks et al., 2012).Liu et al. assessed the frailty of 27-28 months-old wild-type C57BL/6 male mice applying the human FP approach, evaluating grip strength (inverted grip test), walking speed (rotarod test), physical activity (voluntary wheel running), and endurance (grip strength+rotarod test) (Liu et al., 2014).Similar to human criteria, mice were classified as non-frail, mildly or pre-frail, and frail based on the number of characteristics present.Valencia score is another method of assessing frailty in mice based on the human FP (Gomez-Cabrera et al., 2017).The same criteria were used to define frailty in mice, with weight loss being the only exception: Instead of the lowest 20% of the mice displaying weight loss, a > 5% body weight was used as a cut-off value for being frail (Gomez-Cabrera et al., 2017).Additionally, Whitehead et al. developed a simplified mouse frailty assessment based on the clinical human FI (Whitehead et al., 2014).The integument (skin), vestibulocochlear/auditory system (hearing and balance), ocular/nasal systems, digestive/urogenital system, respiratory systems, body weight, body surface temperature, and signs of discomfort are 31 parameters assessed by this method (Whitehead et al., 2014).A FI score was calculated for each parameter by assigning a value of 0 to indicate no deficiency, 0.5 to represent a mild deficiency, and 1 to indicate severe deficiency.To determine body weight and surface temperature changes, young adult mice were used as references.FI scoring allows for the quantification of the level of impairment or deficiency in each parameter, providing a comprehensive assessment of frailty.Thus, this approach showed a progressive increase in FI score on aging C57BL/6 mice (between months and 28 months) (Whitehead et al., 2014).Other groups have used this clinical mouse FI method by modifying the parameters, such as generating a 27-item FI (Sukoff Rizzo et al., 2018), which has shown promise in assessing frailty in aging mice considering factors other than FP (e.g., physical activity).Overall, the two mouse frailty assessment approaches could contribute to developing effective interventions for frailty.
In addition to the utilization of naturally aging mice in the FP or FI approach, transgenic mouse models have been employed as well to delve into the multifaceted mechanism of frailty.For instance, Interleukin (IL)− 10 homozygous knockout mice (IL-10 tm/tm ), originally developed for the study of inflammatory bowel disease, have also proven valuable in the investigation of frailty.Watson et al. demonstrated the IL-10 tm/tm mice exhibited a more rapid decline in muscle strength accompanied by increased pro-inflammatory cytokine IL-6 and serum levels compared to the C57BL/6 mice at 50 weeks (Walston et al., 2008).Furthermore, there were notable alterations in the skeletal muscle gene expression levels particularly related to apoptosis and mitochondrial function in the IL-10 knockout mice, aligning with the changes observed in frail humans (Walston et al., 2008).However, these mice did not display all aspects of the frailty phenotype.Specifically, there were no significant differences between the C57BL/6 and IL-10 knockout mice in terms of weight, activity levels, or mortality rates up to 18 months of age despite the average life span being reduced to 21 months which was approximately 10% less compared to the C57BL/6 mice.
Another proposed model for frailty is Cu/Zn superoxide dismutase knockout mouse (Sod1 -/-) (Deepa et al., 2017).These mice exhibited increased levels of oxidative stress in many tissues due to the lack of antioxidant (Cu/Zn superoxide dismutase) enzyme.Moreover, Sod1 -/- mice exhibit a notable reduction in average lifespan, approximately 30%, in comparison to their wild-type (WT) counterparts (Kim et al., 2013).This premature mortality in Sod1 knockout mice is attributed to the manifestation of accelerated aging characteristics, including hearing and weight loss, muscle weakness, depletion of muscle mass, and increased exhaustion, emphasizing that oxidative stress contributes to the development of frailty (Keithley et al., 2005;Muller et al., 2006;Zhang et al., 2013).Moreover, a caloric restriction diet in these mice has been shown to prevent the onset of frailty similarly observed in C57BL/6 mice (Zhang et al., 2013).Both IL-10 and Sod1 knockout mice are well-established models for investigating the mechanisms of frailty, however, it is worth noting that neither IL-10 tm/tm nor Sod1 -/-mice have been formally quantified for frailty, unlike the naturally aged mice discussed earlier.
Furthermore, a recent addition to the repertoire of mouse models for frailty is the Glucose 6-phosphate Dehydrogenase (G6PD) overexpression model.G6PD is an essential enzyme involved in the pentose phosphate pathway (PPP) and plays a central role in cellular antioxidant defense by generating nicotinamide adenine dinucleotide phosphate (NADPH), which is crucial for neutralizing ROS (Fernandez-Marcos and Nobrega-Pereira, 2016).Increased G6PD activity leads to higher levels of NADPH and reduced damage from ROS.This in turn is associated with extended lifespan and suggested to have a protective effect against age-related damage and delays the onset of frailty in mice overexpressing G6PD (G6PD-Tg) (Arc-Chagnaud et al., 2021).Valencia score-based frailty assessment on these mice aged between 18 and months was performed, evaluating body weight, motor coordination, maximal grip strength, running time, and running speed.In comparison to their WT counterparts, G6PD-Tg mice were less frail.This difference was particularly pronounced in aged mice, with only 13% of G6PD-Tg mice identified as frail, whereas nearly 50% of WT mice showed signs of frailty (Arc-Chagnaud et al., 2021).Notably, G6PD-Tg mice exhibited a reduction in the age-dependent accumulation of oxidized lipids in their skeletal muscles, and a decrease in intramuscular adipose tissue levels was observed in these mice, as indicated by reduced levels of fatty acid infiltrations (FABP4) (Arc-Chagnaud et al., 2021).These findings suggest that overexpression of G6PD protects against frailty, and the damage associated with oxidative stress plays a role in the onset of frailty.
Furthermore, physical activity has been shown to delay the onset of frailty in mice as well.For instance, voluntary wheel running reversed frailty, improved muscle mass, and increased strength in 28-30-monthold male C57BL/6 mice, even with just 4 weeks of training (Graber, Ferguson-Stegall, Liu, and Thompson, 2015).Another noteworthy example is the use of high-intensity interval training (HIIT) performed for 10 min, three times a week, over 8-16 weeks, which significantly improved physical performance and reduced frailty in aging male mice (Seldeen et al., 2018).Similarly, 8 weeks of HIIT attenuated frailty in females (Seldeen et al., 2019).Hence, these studies collectively demonstrated that exercise can mitigate the effects of aging and help prevent frailty in mice regardless of sex, mirroring the observations in humans.Overall, these mouse models serve as important tools for investigating the mechanisms underlying frailty, indicating the significant roles of inflammation and oxidative stress, and evaluating the efficacy of interventions aimed at preventing this condition.

Osteoporosis
Age-related bone loss is a prevalent condition characterized by the gradual decline in bone mass and deterioration of bone tissue over time (Raisz and Rodan, 2003).It is a natural part of the aging process and can contribute to the development of osteoporosis.Osteoporosis, conversely, is characterized by low bone mass and deterioration of bone tissue, resulting in increased bone fragility and a higher risk of fractures (Akkawi and Zmerly, 2018).
While age-related bone loss is a broader term that encompasses the natural decline in bone health with aging, osteoporosis refers to a more severe disorder where bone loss reaches a point where it significantly increases the risk of fractures, thereby impacting the quality of life of an individual.In the context of this review, the focus is specifically on agerelated bone loss associated with osteoporosis, excluding specific aspects such as fracture risk assessment or postmenopausal osteoporosis.This allows for a more targeted examination of the gradual decline in bone mass and its implications without delving into the more severe manifestations of osteoporosis.
The peak bone mass of the skeleton is typically achieved during young adulthood, followed by a gradual decline in bone mass as individuals age (Raisz and Seeman, 2001).While postmenopausal women are more susceptible to osteoporosis due to a decline in estrogen levels, age-related bone loss remains a concern for both sexes.Evaluating age-related bone loss in humans involves various assessment methods to measure bone density, strength, and overall bone health.Dual-energy X-ray absorptiometry (DXA) is considered the gold standard for assessing bone mineral density (BMD) and is widely used in clinical practice to help diagnose osteoporosis by comparing the BMD to reference ranges (Ammann and Rizzoli, 2003).A T-score of ≤ − 2.5 determined by DXA indicates osteoporosis (Sheu and Diamond, 2016).Additionally, quantitative computed tomography (QCT) and high-resolution peripheral quantitative computed tomography (HR-pQCT) provide further insights into the bone microarchitecture (Sheu and Diamond, 2016).These advanced imaging techniques revealed that cortical and trabecular thinning, increased cortical porosity, and trabecular bone loss are key factors contributing to the decline in bone function and strength observed in older individuals (Engelke et al., 2008).
Moreover, age-related bone loss in humans is associated with an increased number of marrow adipocytes, which correlates with a decline in BMD.This is attributed to a shift in the osteogenic differentiation where mesenchymal stem cells (MSCs) favor adipocytes over osteoblasts during aging (Paccou, Penel, Chauveau, Cortet, and Hardouin, 2019)- (Fazeli et al., 2013).Therefore, dysfunctional osteoblasts with reduced bone formation, rather than increased resorption, are suggested to be the primary mechanism underlying osteoporosis (Almeida, 2012).
One example of a pathway that promotes osteogenic differentiation is Wnt signaling.With age, the level of Wnt signaling declines, resulting in reduced bone formation (Kim et al., 2013).Another factor contributing to age-related decreases in bone formation is the increased levels of sclerostin, a Wnt antagonist secreted by osteocytes (Kim et al., 2013).Sclerostin inhibits the Wnt signaling pathway, which plays a crucial role in promoting osteogenic differentiation and bone formation.With age, the levels of sclerostin tend to rise and result in reduced bone formation.Additionally, osteocytes produce RANKL, a key mediator of osteoclastogenesis (Cheng, Chen, and Chen, 2022).The increased presence of RANKL in aging individuals leads to an imbalance between bone formation and resorption, ultimately contributing to increased bone resorption and the development of osteoporosis.Thus, osteocytes play a direct role in bone formation events of osteoblasts with aging, thereby promoting osteoporosis.Importantly, as age progresses, the osteocyte lacunocanalicular network (OLCN) undergoes deterioration characterized by reduced lacunar density, increased apoptosis, and the accumulation of microcracks (Busse et al., 2010).These changes in the OLCN negatively impact the communication and signaling between osteocytes and other bone cells, impairing their ability to regulate bone remodeling effectively.Consequently, the compromised functionality of osteocytes hinders the normal bone formation process carried out by osteoblasts, leading to a net loss of bone density and strength.Thereby, the deteriorating OLCN and the associated dysregulation of osteocyte function contribute to the promotion of osteoporosis.Together, the intrinsic mechanisms of age-related osteoporosis in humans involve altered cellular signaling, increased bone marrow adiposity, and the role of osteocytes in bone formation events.
When comparing age-related bone loss between humans and mice, it is important to consider the similarities and differences.The pattern of bone development in mice, in contrast to humans, exhibits distinct characteristics that continue beyond sexual maturity.Peak bone mass in most mouse strains is typically achieved between 4 to 6 months of age, a considerably earlier timeframe than in humans (Jilka, 2013).Furthermore, while humans undergo a cessation of longitudinal bone growth with the onset of puberty, mice continue to experience longitudinal growth at a diminished rate after reaching sexual maturity, which typically occurs at 6-8 weeks of age (Jilka, 2013).The process of longitudinal bone growth occurs at the growth plate located at the epiphyseal side in mice.Here, new cartilage is generated, while at the metaphyseal side, the previously formed cartilage is replaced by new bone.This balance between cartilage formation and bone replacement allows bones to lengthen without altering the width of the growth plate.In contrast, in humans, the deposition of cartilage ceases at puberty, leading to the fusion of the metaphysis with the epiphysis and the disappearance of the growth plate (Jilka, 2013;Parfitt, 2002).Despite longitudinal growth significantly slowing at puberty in mice, it does not come to a complete end and the growth plates do not entirely fuse and disappear.Instead, growth continues at a gradual pace, albeit at varying rates among different mouse strains, with some showing no significant changes in bone length during aging (Jilka, 2013).For instance, femoral length remains relatively stable between 6 and 12 months of age in C57BL/6 mice despite the persistence of open growth plates (Glatt, Canalis, Stadmeyer, and Bouxsein, 2007).Moreover, humans typically experience gradual age-related bone loss, while mice often exhibit more rapid bone loss due to their shorter lifespan (Jilka, 2013).Human osteoporosis is influenced by various factors, including hormonal changes, genetics, lifestyle, and nutrition, while mouse models often focus on specific aspects, such as hormonal deficiencies or genetic modifications.Despite these differences, mouse models have proven valuable in understanding the pathophysiology of age-related bone loss.
Various mechanisms such as caloric restriction, genetic alterations, diet-induced, and unloading approaches have been used to study agerelated osteoporosis in mice.The C57BL/6 mouse, which is one of the most commonly used naturally aging models, exhibits a decline in BMD that affects both cancellous and cortical bone, reflecting changes observed in humans (Jilka, 2013).In C57BL/6 mice, BMD starts to decline between 16 to 25 months before they reach their median age (Jilka, 2013).Similar age-associated bone loss has also been reported in other mouse strains.For instance, the outbred strains, such as CW1 mice, show a decline in peak bone mass at 12, 15, and 18 months of age, while inbred strains, such as C3H/HeJ and BALB/cByj mice, experience age-associated bone loss between 7 to28 months of age (Beamer, Donahue, Rosen, and Baylink, 1996).In addition to the decline in BMD, C57BL/6 mice exhibit a bone phenotype that closely resembles osteoporotic humans, characterized by reduced bone mass and compromised quality.Specifically, with advanced aging, there is a notable impact on trabecular and cortical bones, as shown by the significant decrease in trabecular bone volume fraction observed between 6 to 24 months of age (Ferguson, Ayers, Bateman, and Simske, 2003).Moreover, the osteogenic potential of bone marrow stromal cells (BMSCs) was reported to decrease with age in mice.In C57BL/6 mice, the number and differentiation capacity of these stem cells were found to be reduced by 18 months (Zhang et al., 2008).Similarly, age-related bone loss led to a decrease in the proliferation capacity of stem cells in BALB/c mice at 24 months (Bergman et al., 1996).Furthermore, RANKL expression increases with age in C57BL/6 mice, suggesting a similar mechanism of bone resorption as observed in humans (Cao, Venton, Sakata, and Halloran, 2003).Age-related bone loss affects OLCN in mice as well.In C57BL/6 mice, there is a significant decline in osteocyte density and dendriticity accompanied by reduced canaliculi and lacunae, indicating degeneration in the OLCN with aging (Tiede- Lewis et al., 2017).These findings highlight the occurrence of age-related bone loss in various mouse strains due to the decreased osteogenic potential of BMSCs, reduced proliferation capacity of stem cells, increased RANKL expression, and alterations in the OLCN, which collectively mirror the process seen in humans.
Furthermore, disuse osteoporosis is a form of osteoporosis resulting from prolonged immobility or reduced physical activity (Watanabe et al., 2004).Inactivity disrupts the normal mechanical loading of bones during activities such as walking or weight-bearing exercises (Sakata et al., 1999).In mice, tail suspension or hind limb immobilization models are used to simulate disuse osteoporosis.Tail suspension model, in which the mice are suspended by their tails, effectively unloading their hind limbs leading to reduced mechanical stress on the bones, ultimately resulting in bone loss and decreased bone density over time (Sakata et al., 1999).For instance, in C57BL/6 mice, a decrease in bone formation coupled with an increase in bone resorption upon tail suspension experiments (Amblard et al., 2003).
In addition, genetically modified mice exhibiting multiple agerelated characteristics have been utilized to investigate the specific roles of cellular pathways or genes in age-related osteoporosis.One prominent example is Ku80-deficient mice.Ku80 is a critical protein involved in DNA repair, and its deficiency leads to accelerated aging phenotype and accumulation of DNA damage over time (Vogel, Lim, Karsenty, Finegold, and Hasty, 1999).Other examples include the models targeting genes involved in telomere maintenance (Wrn/Terc), mitochondrial DNA mutations (PolgA), cell cycle arrest (TRp53), oxidative stress response (Klotho) (Chang et al., 2004;Kawaguchi et al., 1999;Trifunovic et al., 2004;Tyner et al., 2002).
Consequently, these models provide an understanding of the intricate molecular and cellular mechanisms that underlie age-related bone loss and they offer insights into the relationship between DNA repair, cellular senescence, and bone health, shedding light on how genetic factors influence age-related osteoporosis development.

Sarcopenia
Sarcopenia, loss of skeletal muscle mass and function, is one of the health concerns in aging humans.It leads to reduced physical performance, impaired mobility, and increased risk of falls (Frontera, Zayas, and Rodriguez, 2012).It can contribute to the development of frailty, as weakened muscles can make individuals more susceptible to physical limitations and disability.At around age 40, sarcopenia becomes apparent, and by age 80, 30-50% of the muscle mass and function are reported to be lost (Walston, 2012).Gait speed, grip strength, and lean muscle mass are determinant factors in predicting age-related locomotor activity (Shefer et al., 2006).Reduced gait speed is considered an indicator of slowness and is often used to assess physical performance in individuals.Weak grip strength, on the other hand, serves as a reliable measure of muscle weakness and can provide valuable insights into overall muscle function (Wu et al., 2020).In addition to these assessments, DXA scans are commonly utilized to determine low lean muscle mass, which is a key characteristic of sarcopenia (Messina et al., 2018).Furthermore, imaging techniques such as CT, MRI, and ultrasound can be employed to identify specific alterations in muscle size and function in patients with sarcopenia, providing a more comprehensive evaluation of the condition (Cruz-Jentoft et al., 2019).In addition to these direct evaluation methods, various functional and clinical indicators are used to identify sarcopenia and assess its impact on the health of an individual.The Short Physical Performance Battery (SPPB), for instance, combines gait speed, chair stands, and balance tests to provide an overall assessment of physical performance (Puthoff, 2008).The scoring system for the SPPB ranges from 0 (indicating the worst performance) to 12 (indicating the best performance).Numerous studies have demonstrated the predictive validity of the SPPB, showing that individuals with lower scores are at a higher risk for adverse outcomes such as mortality, nursing home admission, and disability.In addition, the European Working Group on Sarcopenia in Older People has established diagnostic criteria for sarcopenia that include the measurement of muscle mass, muscle strength, and physical performance (Cruz-Jentoft et al., 2010).In accordance with this diagnostic criteria, the identification of probable sarcopenia is based on Criterion 1, which stipulates the presence of low muscle strength.To confirm the diagnosis, additional documentation of Criterion 2 is required, which involves low muscle quantity or quality.If all three criteria, including Criterion 3, which relates to low physical performance, are met, the diagnosis of sarcopenia is considered severe.These criteria provide a comprehensive framework for assessing and categorizing sarcopenia based on different aspects of muscle function and performance (Cruz-Jentoft et al., 2019).
Sarcopenia can be attributed to various underlying factors, including mitochondrial dysfunction, oxidative stress, fat infiltration, and decreased satellite cells, leading to reduced muscle regeneration capacity (Bellanti, Lo Buglio, and Vendemiale, 2021).Consequently, sarcopenia is associated with a decline in type II muscle fibers, myofiber number, and cross-sectional muscle area (Narici and Maffulli, 2010;Shefer et al., 2006).Additionally, various signaling pathways are implicated in age-associated changes to the regulation of muscle protein synthesis and regeneration.The phosphatidylinositol 3-kinase (PI3K)/Akt pathway is known to influence protein synthesis and contribute to muscle loss in sarcopenia (Ziaaldini, Marzetti, Picca, and Murlasits, 2017).Similarly, the transforming growth factor beta (TGFβ) family governs muscle regeneration, while activation of the mammalian target of rapamycin (mTOR) pathway promotes muscle hypertrophy (Rommel et al., 2001).The age-related decline in Notch signaling affects satellite cell self-renewal capacity, further compromising muscle regeneration (Bi et al., 2016).Furthermore, the shift from canonical to non-canonical Wnt signaling hampers satellite cell function and impairs muscle regeneration (Florian et al., 2013).Collectively, these signaling pathways contribute to the development and progression of sarcopenia, underscoring the need to comprehensively understand and target these factors for potential interventions and treatments that can mitigate muscle loss in older individuals.
Mouse models of sarcopenia often involve genetically modified mice, age-induced muscle wasting, or disuse-induced muscle atrophy models (Rydell-Tormanen and Johnson, 2019).Similar to humans, mice are evaluated for muscle strength and physical performance using grip strength and functional performance tests.Additionally, evaluating D. Yılmaz et al. muscle mass, muscle fiber type distribution, and muscle fiber cross-sectional area are among the methods used to study sarcopenia in mice (Xie et al., 2021).For instance, at 18 months of age, grip strength and endurance were considerably lower compared to 10 weeks old C57BL/6 mice (Kim and Hwang, 2020).Furthermore, hindlimb muscle weight was reduced in 25-month-old C57BL/6 mice compared to 10-month-old mice, accompanied by a significant decline in grip strength and maximum muscle strength (van Dijk et al., 2017).These findings further support the notion that age-related muscle loss develops in C57BL/6 mice as they age.
In addition to naturally aging mice, genetically altered mouse models have played a crucial role in understanding the underlying pathways and mechanisms involved in sarcopenia.As previously mentioned, IL-10 and Sod1 knockout mice have been widely employed in research focused on sarcopenia, primarily because they demonstrate both muscle weakness and a reduction in muscle mass.IGF-1, PI3K, and Akt1 knockout and TNF-α overexpression mice, have been particularly valuable in studying the pathways involved in the development of sarcopenia (Baek et al., 2014).By targeting specific genes and signaling pathways, these models provide insights into the molecular mechanisms underlying age-related muscle loss.
Similar to age-related osteoporosis, dietary induction, caloric restriction, and immobilization are the approaches used to investigate sarcopenia in mice.For instance, muscle atrophy and weakness are prominent outcomes when mice undergo hindlimb unloading, a procedure in which the hind limbs of mice are immobilized (Morey-Holton and Globus, 2002).This mirrors the wasting of muscle seen in sarcopenic patients since hindlimb unloading can disrupt cellular processes by inducing oxidative imbalance, impairing mitochondrial function, interfering with intercellular communication, and affecting protein synthesis and degradation (Mankhong et al., 2020).These methods in mice enable the investigation of muscle atrophy, weakness, and other age-related changes associated with sarcopenia which provides a better understanding of the pathophysiology of this condition.
Table 1 summarizes the measurements that define the musculoskeletal aging conditions with the cut-off values in humans and mice.

Accelerated aging mouse models with musculoskeletal aging phenotypes
Accelerated aging mouse models play a crucial role in elucidating the fundamental molecular pathways and mechanisms associated with musculoskeletal aging and associated conditions.These mouse models exhibit accelerating aging due to specifically targeted single gene mutations leading to multiple aging pathologies (Hasty and Vijg, 2004).They offer distinct advantages over naturally aging mice requiring longer lifespans to exhibit age-related phenotypes, making it costly and challenging to study the progression of musculoskeletal aging over time.In contrast, accelerated aging mouse models enable the study of these age-related changes within a relatively short timeframe (Gurkar and Niedernhofer, 2015).These models exhibit accelerated manifestations of age-related pathologies, including bone and muscle loss, frailty, and other hallmarks of musculoskeletal aging, such as cellular senescence that are similarly observed in humans.
Interventions involving senolytic compounds or the genetic elimination of senescent cells have demonstrated the potential to prolong the period of healthy living and diminish frailty in accelerated aging mice.One example is Bubr1 H/H , an accelerated aging mouse model that exhibits a reduced lifespan and various age-related characteristics, such as infertility, lordokyphosis, sarcopenia, cataracts, and fat loss (Baker et al., 2004).However, the inactivation of p16INK4a, a key marker for senescence, in these mice resulted in an increased lifespan and decreased effects of age-related pathologies in skeletal muscle and fat tissues (Baker et al., 2008;Baker et al., 2011).Remarkably, similar findings were observed in WT mice, albeit with a rather significant delay.(Baker et al., 2016;Yousefzadeh et al., 2019).Ercc1 − /Δ accelerated aging mice provide another example.When these mice were treated with an HSP90 inhibitor or a combination of senolytics like dasatinib and quercetin, they demonstrated reduced symptoms associated with aging and frailty, including poor coat conditions, kyphosis, reduced grip strength, tremors, and poor overall body condition (Fuhrmann-Stroissnigg et al., 2017).Another example of this phenomenon is the T cells with impaired mitochondria, resulting from a lack of mitochondrial transcription factor A (TFAM), which is thought to expedite the onset of cellular senescence.In Tfam knock-out mice (Tfam fl/fl Cd4 Cre ), the mitochondria within T cells were destroyed, triggering a significant release of inflammatory cytokines and TNF-α which in turn led to the development of senescence, neuromuscular dysfunction, and vascular dysfunction resulting in premature aging (Desdin-Mico et al., 2020).At the age of months, these mice exhibited impaired mitochondrial function and a shift in T cell metabolism toward glycolysis accompanied by an increased production of type 1 inflammatory cytokines, particularly TNF-α.Consequently, 2-month-old Tfam knock-out mice displayed the characteristics of mitochondrial dysfunction of 22-month-old WT mice.Notably, besides this proinflammatory phenotype, Tfam fl/fl Cd4 Cre mice at 2 months exhibited an immunocompromised state comparable to that observed in WT mice at 22 months (Desdin-Mico et al., 2020).Around months of age, these mice demonstrated several aging-related characteristics that are also observed in humans including weight loss, decreased mobility, spinal deformities, cardiovascular impairments, skeletal muscle wasting, and a 50% reduction in their life span (Desdin-Mico et al., 2020).The unique characteristic of the Tfam fl/fl Cd4 Cre mouse model is its ability to manifest mitochondrial dysfunction specifically in a single cell type within the immune system, namely T cells unlike other accelerated aging mice associated with mitochondrial dysfunction.However, what makes this model particularly intriguing is its capacity to induce harmful systemic consequences that affect numerous other tissues and organs throughout the body, which is primarily attributed to the release of proinflammatory cytokines and TNFα, and includes the onset of senescence (Desdin-Mico et al., 2020).
Overall, cellular senescence is indeed a contributing factor to frailty, and it has detrimental effects on the musculoskeletal system through various mechanisms.However, for the purpose of this review, we will not delve deeply into these mechanisms.Instead, our focus will be on four specific accelerated aging mouse models that exhibit hallmarks of musculoskeletal aging conditions along with their validated assessment methods as mentioned previously (Table 2).

Senescence accelerated mouse
Senescence accelerated mouse (SAM) is broadly utilized for studying age-related musculoskeletal changes.This model developed by selective inbreeding of the AKR/J mice and, based on their senescent phenotype, categorized into two groups: SAMP (senescence-accelerated prone) mice demonstrate faster aging with a shorter life span, whereas SAMRs (senescence resistant) mice display moderately less accelerated aging and have a typical lifespan (Takeda, 1999;Takeda, Hosokawa, and Higuchi, 1997;Takeda et al., 1981).
Among the SAMP strains, SAMP6 is one of the most extensively characterized models used in research on senile osteoporosis (Matsushita et al., 1986;Silva, Brodt, and Ettner, 2002).SAMP6 mice display lower BMD as assessed with DXA compared to the controls (Kasai et al., 2004).Additionally, micro-CT evaluation of the caudal vertebra revealed trabecular bone loss in these mice (Jilka, Weinstein, Takahashi, Parfitt, and Manolagas, 1996).Similar findings have indicated bone loss with aging in the lumbar vertebra and tibia of 3-4 months SAMP6 mice (Chen, Zhou, Emura, and Shoumura, 2009;Jilka et al., 1996).This indicates that SAMP6 mice exhibit osteoporotic features, such as reduced bone density and mass which is consistent with the development of osteoporosis during aging.Furthermore, advanced aging in SAMP6 mice is suggested to be associated with increased bone resorption due to increased maturation of osteoclasts, increased adipogenesis, and reduced osteoblastogenesis (Kajkenova et al., 1997).In addition, the secretion of the frizzled-related protein (Srfp4), a Wnt pathway antagonist, is increased in SAMP6 mice, contributing to reduced osteoblastogenesis (Nakanishi et al., 2006).Notably, at four months of age, SAMP6 mice demonstrate little or no difference in mechanosensitivity relative to their controls when subjected to in vivo tibia loading (Silva and Brodt, 2008), suggesting that SAMP6 mice do not display significant alterations in their bone's ability to sense and respond to mechanical forces while they exhibit other age-related phenotypes, such as osteoporosis.
Moreover, age-associated musculoskeletal changes in these mice are assessed through physical (open field-home cage activity) and motor activity tests (grip strength, hanging, and rotation rod) (Niimi and Takahashi, 2014).It has been observed that adult SAMP6 (4-6 months old) mice have higher activity compared to old (8-12 months old) and the age-matched controls (Niimi and Takahashi, 2014).However, both adult and old SAMP6 mice display impaired intrinsic motor coordination as evidenced by decreased latency to fall (rotarod test) compared to control animals (Niimi and Takahashi, 2014).These findings suggest an age-related decline in neuromuscular function in SAMP6 mice.
In contrast to SAMP6, SAMP8 mice are commonly used in sarcopenia research due to their rapid muscle aging phenotypes compared to normal aging mice and SAMR controls.The aging characteristics of SAMP8 mice have been extensively studied, revealing several agerelated alterations in multiple aspects.
Firstly, SAMP8 mice exhibit various physical changes associated with aging, such as a reduction in the body condition score, increased kyphosis (excessive curvature of the spine), and loss of fur color (Liu et al., 2020).Additionally, these mice experience a decrease in body weight as they age (Liu et al., 2020).These observations indicate that SAMP8 mice manifest visible signs of accelerated aging compared to their control counterparts.When it comes to muscle function, SAMP8 mice demonstrate a decline in motor skills and muscle performance as they age.This has been assessed through various tests, including the treadmill-exhaustion test, grip strength measurements, and voluntary wheel running tests (Liu et al., 2020).In each of these assessments, SAMP8 mice perform poorly compared to the controls, indicating an age-related decline in motor abilities and early signs of aging.
Sarcopenic muscle characteristics of SAMP8 mice further support the accelerated muscle aging phenotype.These mice exhibit reduced mass in gastrocnemius and extensor digitorum longus (EDL), decreased crosssectional area and decrease in soleus muscle fiber size at approximately 10-12 months (Liu et al., 2020).Additionally, SAMP8 mice display lower grip strength, decreased muscle contraction time, increased fatigue rate, and a reduction in type II muscle fibers in the soleus and gastrocnemius muscles (Liu et al., 2020).These findings collectively indicate the development of muscle atrophy and deterioration in SAMP8 mice as they age.
At the cellular level, SAMP8 mice show decreased mTOR and Akt signaling, which leads to the upregulation of genes associated with muscle protein degradation and mitochondrial dysfunctionality (Liu et al., 2020).This suggests that molecular mechanisms underlying muscle aging, such as increased protein degradation and impaired mitochondrial function, contribute to the accelerated muscle aging phenotype observed in SAMP8 mice.
It is worth noting that SAMP6 and SAMP8 mouse models have also been extensively utilized in neurodegenerative disease research.These strains exhibit a decline in cognitive skills with aging, suggesting that progressive neurodegenerative conditions may contribute to the observed aging-related muscle and bone phenotypes.Collectively, SAMP6 mice serve as a useful model for studying age-related musculoskeletal changes, including osteoporosis and motor coordination impairments and SAMP8 mice serve as valuable models for studying sarcopenia due to their accelerated muscle aging phenotypes compared to normal aging mice.

Lamin A deficiency
Zmpste24 knockout mice (Zmpste24 -/-) are widely used as a model to study the effects of impaired Lamin A processing on musculoskeletal aging.Zmpste24 is a zinc metalloproteinase responsible for processing Lamin A, which plays a crucial role in maintaining the integrity of the nuclear envelope and ensuring proper cell proliferation and genome stability (Bergo et al., 2002).By knocking out Zmpste24 in mice, the normal processing of Lamin A is disrupted, mimicking the defective processing observed in Hutchison-Gilford progeria syndrome (HPGS).The characteristics of this disease involve accelerated aging, including alopecia and cardiovascular complications (Denecke et al., 2006).Mice deficient with Zmpste24 exhibited an accelerated aging phenotype, displaying gradual weight and hair loss, as well as kyphosis (abnormal curvature of the spine), alopecia, and shortened life span of approximately 20 weeks overall frailty-like characteristics (Bergo et al., 2002;Wang et al., 2022).Additionally, Zmpste24 -/-mice exhibit various impairments in bone structure and function, as analyzed by micro-CT.At 3 months of age, these mice demonstrate significantly lower bone volume and density and thinner trabeculae (Bergo et al., 2002;Rivas, Li, Akter, Henderson, and Duque, 2009).These findings indicate a substantial loss of bone in Zmpste24 -/-mice.Histomorphometry analysis further reveals a notable decrease in the numbers of osteoblasts and osteocytes, suggesting that the accumulation of prelamin A, resulting from Zmpste24 deficiency, contributes to bone loss (Rivas et al., 2009).Furthermore, the reduced osteoblastogenesis observed in Zmpste24 -/-mice is associated with decreased expression of key transcription factors, including Runx2, osteocalcin (OCN), osteopontin (OPN), and bone sialoprotein (BSP) (Rivas et al., 2009).This shift towards reduced osteoblastogenesis is concurrent with an increase in adipogenesis, suggesting a preference for adipocyte formation over osteoblast differentiation in this mouse model.These musculoskeletal changes observed in Zmpste24 -/-mice closely resemble the characteristics of age-related osteoporosis seen in humans.
Muscle-related phenotypes are also observed in Zmpste24 -/-mice.One notable finding is the reduction in grip strength and whole body tension in Zmpste24 -/-mice, indicating a decline in locomotor activity (Bergo et al., 2002;Greising et al., 2012).These mice also exhibit decreased ankle mobility and increased passive torque, suggesting the presence of stiff joints (Greising et al., 2012).These musculoskeletal impairments contribute to reduced physical activity and compromised movement in Zmpste24 -/-mice.In terms of specific muscle groups, Zmpste24 -/-mice exhibit dystrophic quadriceps, characterized by the increased presence of abnormal small round muscle fibers (Pendas et al., 2002).The contractile capacity of the posterior leg muscles (soleus, plantaris, and gastrocnemius) remains unaffected in these mice (Greising et al., 2012).However, the anterior leg muscles (extensor hallucis longus, tibialis anterior, extensor digitorum longus) demonstrate significant weakness compared to the controls.
The size and distribution of soleus muscle fibers also show distinct characteristics in Zmpste24 -/-mice.Although the mean cross-sectional area of soleus muscle fibers is similar between Zmpste24 knockout and control mice, the fiber size distribution is significantly broader in Zmpste24 -/-mice.Additionally, the percentage of myosin heavy chain type I, IIa, and IIx fibers in the soleus muscles is similar between the two groups, but Zmpste24 -/-mice have a higher number of myonuclei (Greising et al., 2012;Song et al., 2013).These findings suggest altered muscle composition and potential compensatory mechanisms in response to Zmpste24 deficiency.In particular, muscle-derived mesenchymal stem cells (MSCs) in Zmpste24 -/-mice exhibit reduced proliferation and myogenic differentiation, which contributes to impaired muscle regeneration.Moreover, the absence of Zmpste24 leads to age-related bone loss and abnormal fat infiltration within the bone marrow without affecting the subcutaneous fat layer (Heizer et al., 2020).This aberrant fat accumulation within the bone marrow is associated with reduced differentiation of MSCs toward the adipogenic D. Yılmaz et al. lineage.Furthermore, downstream signaling pathways involved in stem cell differentiation and function, such as Notch and Wnt, are impaired in Zmpste24 -/-mice (Song et al., 2013).These signaling pathways play essential roles in regulating the balance between different cell lineages and maintaining tissue homeostasis.
In summary, Zmpste24 -/-mice exhibit various muscle-related phenotypes, including decreased grip strength, mobility impairments, muscle weakness in specific muscle groups, altered muscle fiber characteristics, impaired muscle regeneration, and abnormal fat infiltration within the bone marrow.These findings highlight the complex interactions between Zmpste24 deficiency, muscle function, stem cell dynamics, and age-related musculoskeletal changes.

DNA damage repair mechanism dysfunction
Ercc1 mutant mouse (Ercc1 -/Δ ) is another valuable model for studying musculoskeletal aging.This model exhibits a deficiency in the ERCC1-XPF endonuclease protein complex, which plays a crucial role in DNA repair mechanisms (Weeda et al., 1997).In humans, mutations or deletions of Errc1 or XPF gene are associated with accelerated aging, leading to a range of clinical diseases and developmental problems (Eriksson et al., 2003).
Notably, Ercc1 knockout mice (Ercc1 -/-) display significantly accelerated aging, with a remarkably short lifespan of only 3 weeks (McWhir et al., 1993).These mice show musculoskeletal aging phenotypes and other health abnormalities, including liver abnormalities and neuronal degeneration underscoring the critical role of Ercc1 in DNA repair and overall organismal health.On the other hand, Ercc1 -/Δ mutant mice, which retain 10% of XPF-Ercc1 expression, have normal development until adulthood but subsequently experience accelerated aging with a reduced life span (Dolle et al., 2011;Weeda et al., 1997).
One of the prominent musculoskeletal changes observed in Ercc1 -/Δ mice is a decline in bone density and structure exhibiting age-related osteoporosis characteristics (Chen et al., 2013).Micro-CT analysis of Ercc1 -/Δ mice reveals age-related changes in the bone microarchitecture, including reduced trabecular thickness and number and increased vertebral porosity in the lumbar vertebra (Chen et al., 2013).Particularly these mice develop age-related intervertebral disc degeneration (Henson et al., 2015) associated with an osteoporotic bone phenotype characterized by loss of disk height and degenerative changes within the disc (Vo et al., 2010).In 20-week-old Ercc1 -/Δ mice, a significantly lower height of the lumbar vertebral disc was observed.This reduction is equivalent to the vertebral height loss observed in mice naturally aged to 2 years (Vo et al., 2010).Thus, the osteoporotic phenotype in mutant Ercc1 mice occurs at an earlier age than in normal aging 2-year-old mice.
Additionally, elevated expression of tumor necrosis factor α (TNFα), and receptor activator of NF-Ϗβ, suggests an early onset of osteoporosis in Ercc1-deficient mice (Flores et al., 2017;Kim et al., 2020).This is attributed to increased senescence in bone marrow stem cells, leading to reduced osteoblast functionality and increased osteoclastogenesis in these mice.
In terms of muscle-related phenotypes, Ercc1 -/Δ mice exhibit reduced grip strength, poor body condition, gait impairment, kyphosis, and tremors, indicating muscle dysfunction and wasting (Alyodawi et al., 2019;de Waard et al., 2010).Correspondingly, the decline in locomotor activity, assessed through the grip strength, rotarod, and open field test, supports the reduced muscle strength and function in these mice compared to controls (de Waard et al., 2010).Moreover, at 16 weeks, Ercc1 -/Δ mice demonstrate a sarcopenic muscle phenotype characterized by smaller muscle sizes and decreased mass in muscles like the EDL and soleus (Alyodawi et al., 2019).This muscle loss is attributed to age-associated muscle mass decline.In addition, Ercc1 -/Δ mice exhibit a higher proportion of dying muscle fibers due to apoptosis and necrosis compared to aged wild-type mice (Alyodawi et al., 2019).
Furthermore, Akt pathway, known for its involvement in regulating muscle hypertrophy, is upregulated in mutant Ercc1 mice (Alyodawi et al., 2019).This finding suggests that the activation of protein synthesis pathways counteracts the observed atrophic muscle phenotype in these mice.Consequently, it is plausible that stimulating the protein synthesis pathways could potentially serve as a compensatory mechanism to compensate the severe muscle wasting observed in this mouse model.
Beyond these findings of the musculoskeletal phenotype of mutant Ercc1 mice, several studies have investigated interventions aimed at extending the health span and improving symptoms in mutant Ercc1 mice.These interventions include stem cell transplantation from young wild-type mice, dietary restrictions, and inhibition of the NF-Ϗβ pathway (Lavasani et al., 2012).These approaches aim to mitigate the effects of accelerated aging and promote healthier aging in Ercc1-deficient mice.
Overall, the Ercc1-XPF complex is crucial for the DNA repair process, and deficiencies in Ercc1 or XPF genes can lead to accelerated aging and various clinical conditions.Mice with Ercc1 gene mutations or deletions exhibit accelerated aging phenotypes with notable effects on the musculoskeletal system, including bone loss and muscle dysfunction.

Mitochondrial dysfunction
PolgA D257A/D257A mice (referred to as PolgA), also known as mutator mice, carry a mutated form of the proofreading domain found exclusively in the mitochondrial DNA (mtDNA) polymerase (Trifunovic et al., 2004).This mutation leads to a substantial increase (3-5 times higher) in the accumulation of point mutations within mtDNA compared to normal mice (Dobson et al., 2020).Consequently, this accelerated mutation rate gives rise to compromised mitochondrial function, affecting various cellular processes and resulting in accelerated aging.
PolgA mice display several hallmarks of aging, including kyphosis, alopecia, hearing impairments, and gradual weight loss.These phenotypes are evident throughout their relatively short lifespan of approximately 48 weeks (Kujoth, Leeuwenburgh, and Prolla, 2006;Trifunovic et al., 2004).The age-related decline in PolgA mice has been assessed through the clinical mouse FI method.Notably, at 40 weeks of age, these mice demonstrate a higher prevalence of aging-associated characteristics, including graying fur, ruffled fur, distended abdomen, kyphosis, etc., when compared to the control group.This observation indicates an increased FI in PolgA mice, reflecting that they experience accelerated frailty progression relative to their aged match counterparts, even at a relatively young age (Scheuren, D'Hulst et al., 2020).Additionally, the aging phenotypes observed in PolgA mice are primarily attributed to increased mitotic cell apoptosis rather than elevated oxidative stress, reduced cellular proliferation, or cellular senescence.This suggests that the accumulation of mitochondrial DNA mutations, resulting from the PolgA mutation, drives the accelerated aging phenotype in these mice rather than increased oxidative stress induced by reactive oxygen species (ROS), causing damaged mitochondria and leading to accelerated aging (Geurts et al., 2020).
Moreover, PolgA mice demonstrate a pronounced propensity for the development of osteoporosis.These mice display significant alterations in bone density and structure, reflecting the characteristic features of osteoporosis.Specifically, at 40 weeks of age, they display reduced BMD in femurs, which closely resembles the BMD decline observed in aging humans (Trifunovic et al., 2004).Micro-CT analysis further confirms age-related impairments in bone microarchitecture evident by the decrease in bone volume and quality together with a reduction in multiple bone morphometric parameters in comparison to the age-matched controls in caudal vertebrae and tibia (Geurts et al., 2020;Scheuren, D'Hulst et al., 2020;Scheuren, Kuhn, and Muller, 2020).Similar findings of bone loss were observed in the lumbar vertebrae and femurs of PolgA mice at 4-11 months (Dobson et al., 2020).In addition, PolgA mice were found to be more susceptible to the development of knee osteoarthritis by showing increased numbers of hypertrophic chondrocytes, specifically in the articular calcified cartilage.Importantly, Geurts et al. reported the prevalence of low-grade cartilage degeneration primarily characterized by proteoglycan alterations in this mouse model.The study also demonstrated a significant correlation between trabecular thickness and osteocyte apoptosis, along with an elevation in osteoclast numbers (Geurts et al., 2020).These findings suggest that mitochondrial DNA mutations impact chondrocyte function and subchondral bone dynamics.
Furthermore, Polg A mice with advanced aging show a lack of mechanosensitivity upon in vivo mechanical loading of the caudal vertebra, indicating impaired bone response to mechanical stimuli.This is reminiscent of the reduced bone turnover observed in aging humans but without achieving peak bone mass (Scheuren et al., 2020).This impaired bone response and loss in PolgA mice was accompanied by a significant decrease in bone formation rate, reduced densities of osteoblasts, along with increased density of osteoclasts (Dobson et al., 2020).In this regard, in vitro experiments have revealed significant impairments in the formation of mineralized matrix as well as an increased ability to resorb bone through osteoclast activity in these mice (Dobson et al., 2020).These findings suggest that the PolgA mutation significantly affects bone health, leading to impaired bone formation and increased resorption that contribute to the accelerated rate of bone loss observed in these mice.
Regarding muscle characteristics, PolgA mice demonstrate deficiencies in motor coordination, locomotor activity, and muscle functionality, all of which contribute to their musculoskeletal deterioration phenotype.For instance, rotarod and voluntary wheel running tests showed decreased performance in PolgA mice, including reduced running capacity, decreased distance covered, and diminished overall physical activity compared to wild-type mice (Ross et al., 2019).Decline in the grip strength in PolgA mice with aging supported the findings in compromised muscle strength and function, mirroring the characteristics of sarcopenia (Scheuren, D'Hulst et al., 2020).Moreover, muscle fiber atrophy and reduced muscle mass were reported in PolgA mice at 40-46 weeks (Scheuren et al., 2020).Such pathological alterations further contribute to the impairment of muscle function and the development of sarcopenia in PolgA mice.
Another important feature of PolgA mice is that they exhibit dysfunctional stem cells starting from embryogenesis.The self-renewal capacity of these cells is reduced in vitro, while in vivo there is a decrease in quiescent stem cell populations (Ahlqvist et al., 2012).Treatment with N-acetyl-L-cysteine improved abnormalities, suggesting that mtDNA mutagenesis-induced ROS/redox changes modulate stem cell function.Additionally, PolgA mice showed an enlarged small intestine with apoptosis in the stem cell zones (Fox, Magness, Kujoth, Prolla, and Maeda, 2012).Stem cell-derived organoids from these mice exhibited impaired development highlighting the early effects of mtDNA mutagenesis on stem cell function.
On that note, voluntary wheel running has been shown to improve the visible aging phenotypes mentioned earlier in PolgA mice, as well as behavioral parameters like locomotion and rearing (Ross et al., 2019).The study also investigated the effects of exercise on mtDNA mutation load and mtDNA copy number.It was found that voluntary running resulted in a reduction of mtDNA mutation load in actively running PolgA mice compared to sedentary or exercised wild-type mice.However, mtDNA copy number did not increase with exercise in PolgA mice, possibly due to the presence of defective stem cell populations in these mice (Ahlqvist et al., 2012;Fox, Magness, Kujoth, Prolla, and Maeda, 2012).Furthermore, exercise improved the dysregulated proteins observed in the gastrocnemius muscle and striatum of mtDNA mutator mice (Ross et al., 2019).These findings suggest that exercise in aging humans may contribute to a decrease in mtDNA mutation load and the normalization of skeletal muscle protein levels based on the findings in PolgA mice.Overall, these results are consistent with those observed in human sarcopenia patients, suggesting the PolgA mouse model is a promising model for studying musculoskeletal aging.

Discussion
Musculoskeletal aging is a complex process that involves the deterioration of the skeletal system and muscles, leading to conditions such as frailty, sarcopenia, and osteoporosis.Mouse models that exhibit accelerated aging have become invaluable tools for studying hallmarks of musculoskeletal aging and associated conditions in a shorter time frame than observed in natural aging.With this regard, numerous models of accelerated aging have been developed to aid in the development of interventions for slowing down aging and age-related skeletal and muscle pathologies.From the array of well-established and extensively studied accelerated aging models such as BubR1 H/H mice, Terc1 and Sod1 knock-out mice, we have carefully selected models that specifically target key pathways involved in musculoskeletal aging, including DNA damage repair, mitochondrial dysfunction, and lamin A deficiency exhibiting prominent musculoskeletal aging phenotypes similar to humans.Hence, in this comprehensive review, we have focused on discussing the significance and findings of SAMP6, SAMP8, Zmpste24, Ercc1, and PolgA mice, shedding light on their invaluable contributions to the understanding of musculoskeletal aging and associated conditions.
Firstly, SAMP6 and SAMP8 mice exhibit certain characteristics resembling musculoskeletal aging in humans.However, these models possess a polygenic background and exhibit some deviations from the complete spectrum of human musculoskeletal aging.For instance, the lack of mechanosensitivity in SAMP6 mice and slower muscle atrophy observed in SAMP8 mice limits their representation of the full complexity of human musculoskeletal aging.
While Zmpste24 -/-model, which provides insights into accelerated aging and nuclear envelope defects, shows a rapid aging phenotype and decreased bone turnover around 30 weeks, its applicability to studying sarcopenia and other musculoskeletal aging aspects may be limited due to exhibiting selective muscle weakness.
Additionally, Ercc1 -/Δ model offers the opportunity to investigate the impact of DNA repair deficiency on aging, including its effect on the musculoskeletal system.This model displays aging-related bone and muscle phenotypes as early as 20 weeks; however, its specific emphasis on DNA repair may limit its direct applicability to musculoskeletal aging research.Furthermore, the impact of Ercc1 deficiency on fertility underscores a crucial factor to consider when utilizing this model to study musculoskeletal aging and its related conditions.
Finally, among the discussed models, PolgA mice appear to best represent the hallmarks of musculoskeletal aging and associated conditions similar to humans.They display early onset frailty, robust reductions in muscle and bone mass, and degeneration of bone cells.However, it is important to acknowledge certain limitations of the PolgA mouse model.These include the need for strategic breeding to prevent mitochondrial accumulation in females and the observed decrease in fertility in both male and female mice by the age of 20 weeks.Additionally, further research, particularly in the areas of transcriptomics and metabolomics, is necessary to fully establish the suitability of PolgA mice as a comprehensive model for investigating musculoskeletal aging and associated conditions.
Future research should focus on further characterizing accelerated aging mouse models and conducting detailed analyses to gain a comprehensive understanding of the molecular changes associated with musculoskeletal aging research.Moreover, the translation of findings from these mouse models to human clinical applications remains a crucial area of investigation.

Conclusion
In conclusion, mouse models of accelerated aging have proven to be valuable tools in studying musculoskeletal aging and associated conditions (frailty, osteoporosis, and sarcopenia), Therefore, in this review, we discussed four specific accelerated aging mouse models that, while D. Yılmaz et al. not perfect replicas of human aging, offer valuable insight into the underlying mechanism of aging in musculoskeletal research due to their specifically targeted key pathways involved in aging, including DNA damage repair, mitochondrial dysfunction, and Lamin A deficiency.Despite not having mutations specific to musculoskeletal tissues, these models exhibit musculoskeletal aging phenotypes associated with frailty, osteoporosis, and sarcopenia, akin to those observed in aged WT mice and humans regardless of their young age.These phenotypes have been characterized using valid assessment methods at those respective ages including Ercc1 -/Δ and Zmpste24 -/-mice, having shorter lifespan compared to PolgA and SAMP strains.These models have exhibited their well-established relevance to the hallmarks of aging, widespread use in musculoskeletal research, and the confirmation of their age-related phenotype through validated assessment methods.In particular, the PolgA mouse model provided important insights due to exhibiting hallmarks of musculoskeletal aging.
While we concentrated on Lamin A deficiency, DNA damage, and mitochondrial dysfunction as key aging hallmarks, we acknowledged the existence of other hallmarks of aging, such as cellular senescence, inflammation, stem cell exhaustion, and epigenetic alterations, which may be influenced by different mouse models, such as p53 knockout or p16INK4a overexpression mice (Donehower et al., 1992;Serrano, Lin, McCurrach, Beach, and Lowe, 1997).However, not all of these models exhibit sufficient characteristics related to frailty, age-related osteoporosis, and sarcopenia, which were our primary criteria for model selection in this review.Our aim was to provide a comprehensive yet manageable overview of accelerated aging mouse models, covering a spectrum of musculoskeletal aging-related research with validated experimental assessment methods.

Table 2
Comparing the hallmarks of aging to musculoskeletal conditions observed in accelerated aging mouse models.