Daily parathyroid hormone administration enhances bone turnover and preserves bone structure after severe immobilization‐induced bone loss

Abstract Immobilization, as a result of motor‐complete spinal cord injury (SCI), is associated with severe osteoporosis. Whether parathyroid hormone (PTH) administration would reduce bone loss after SCI remains unclear. Thus, female mice underwent sham or surgery to produce complete spinal cord transection. PTH (80 μg/kg) or vehicle was injected subcutaneously (SC) daily starting on the day of surgery and continued for 35 days. Isolated tibias and femurs were examined by microcomputed tomography scanning (micro‐CT) and histology and serum markers of bone turnover were measured. Micro‐CT analysis of tibial metaphysis revealed that the SCI‐vehicle animals exhibited 49% reduction in fractional trabecular bone volume and 18% in trabecular thickness compared to sham‐vehicle controls. SCI‐vehicle animals also had 15% lower femoral cortical thickness and 16% higher cortical porosity than sham‐vehicle counterparts. Interestingly, PTH administration to SCI animals restored 78% of bone volume, increased connectivity to 366%, and lowered structure model index by 10% compared to sham‐vehicle animals. PTH further favorably attenuated femoral cortical bone loss to 5% and prevented the SCI‐associated cortical porosity. Histomorphometry evaluation of femurs of SCI‐vehicle animals demonstrated a marked 49% and 38% decline in osteoblast and osteoclast number, respectively, and 35% reduction in bone formation rate. In contrast, SCI‐PTH animals showed preserved osteoblast and osteoclast numbers and enhanced bone formation rate. Furthermore, SCI‐PTH animals had higher levels of bone formation and resorption markers than either SCI‐ or sham‐vehicle groups. Collectively, these findings suggest that intermittent PTH receptor activation is an effective therapeutic strategy to preserve bone integrity after severe immobilization.


Introduction
Bone is an active organ that undergoes a complex and continuous process of remodeling. This process involves resorption of bone by osteoclasts (multinucleated cells of hematopoietic origin) followed by synthesis of new bone matrix and subsequent mineralization by osteoblasts (cells of mesenchymal origin). This remodeling process is important for maintaining calcium homeostasis and preserving skeletal mass and integrity. Thus, disorders associated with impaired or negatively imbalanced bone remodeling with excessive resorption manifest with bone loss.
Spinal cord injury (SCI) is a major trauma that affects the function of many organ systems, including that of the skeletal system (Kostovski et al. 2010). Bone loss after SCI has been attributed primarily to the loss of muscle innervation/function, disuse muscle atrophy, and the subsequent immobilization. As a consequence of this reduction in mechanical loading, most patients rapidly and dramatically lose bone shortly after injury with the eventual development of osteoporosis (Bauman and Cardozo 2015). As a result, the use of rehabilitation techniques, exercise, and electrical muscle stimulation evolved as measures to restore mechanical stimulation and reduce the bone loss of acute immobilization. Other factors that are believed to further worsen bone loss after acute injury are the dysregulation of sympathetic outflow, impairment of the local circulation, and insufficient anabolic stimulation due to testosterone deficiency in men and suppression of endogenous PTH release (Huang et al. 1997;Maimoun et al. 2006b;Qin et al. 2010;Gaspar et al. 2014). Mechanistic studies suggested that bone loss is attributed to increased osteoclast activity and number, causing an imbalance in bone remodeling by favoring bone resorption over that of bone formation (Demulder et al. 1998;Jiang et al. 2006Jiang et al. , 2007Reiter et al. 2007; Morse et al. 2008Morse et al. , 2011Battaglino et al. 2012;Sabour et al. 2014). This notion has been further supported by the finding that bone resorption markers increase several fold after injury and remain high for months to years thereafter (Uebelhart et al. 1994;Roberts et al. 1998;Nance et al. 1999;Dauty et al. 2000;Maimoun et al. 2002Maimoun et al. , 2005Maimoun et al. , 2006a. These studies promoted the use of antiresorptive agents, such as calcitonin and bisphosphonates. However, the efficacy of this approach in treating bone loss in those who become nonambulatory after SCI remains unclear and controversial (Maimoun et al. 2006a). Furthermore, findings from our investigations strongly suggest that there is no benefit of bisphosphonates, specifically pamidronate or zoledronic acid at the knee in individuals with motorcomplete SCI (Bauman et al. 2014).
An important regulator of bone remodeling is parathyroid hormone (PTH). PTH is a single-chain, 84-amino acid polypeptide hormone secreted by the parathyroid glands in response to hypocalcemia to maintain homeostasis of circulating levels of calcium. The primary function of endogenous PTH is to act directly on the kidney to increase calcium reabsorption and phosphate excretion, on bone to stimulate calcium mobilization, and indirectly on the intestine by the action of vitamin D to enhance calcium absorption.
Repeated transient elevation of PTH achieved by daily (intermittent) administration to animals or humans, increases trabecular bone mass and bone mineral density (BMD), and improves bone biomechanical properties. For these reasons, a regimen of daily PTH administration for a circumscribed period of time has been approved by the FDA for treating primary osteoporosis associated with a high risk of fracture (Neer et al. 2001) and osteoporosis secondary to glucocorticoid use (Saag et al. 2007).
Although many reports have indicated that PTH is beneficial in unloading-associated bone loss (Ma et al. 1995;Halloran et al. 1997;Turner et al. 1998Turner et al. , 2007Ono et al. 2007;Bruel et al. 2013;Sandberg et al. 2014), whether PTH administration could protect against SCI-induced bone loss remains unclear. In this study, we evaluated whether stimulation of bone anabolism using intermittent PTH receptor 1 (PTHR1) activation would preserve bone structure after severe immobilization in SCI animals.

Reagents
Human PTH(1-34) (Teriparatide, Cat. Number H-4835) was purchased from Bachem (Torrance, CA) and was reconstituted in sterile solution of 0.9% NaCl and 10 mmol/L acetic acid. Vehicle (0.9% NaCl/10 mmol/L acetic acid) or PTH (80 lg/kg/day) was injected SC daily (except Sunday) starting on the day of surgery and continued for 35 days. A PTH dose of 40-80 lg/kg/day has been commonly used in studying PTH actions in bone. Additionally, a dose of 80 lg/kg/day in our previous experiments on mice exhibited an optimal bone anabolic response to PTH (Terauchi et al. 2009;Bedi et al. 2012). The great difference between the mouse and human dosage (20 lg/day) is anticipated given that the metabolic rate in mice is almost 10 times higher than humans. Calcein (Cat. Number C0875) was purchased from Sigma-Aldrich (St. Louis, MO) and reconstituted using 2% sodium bicarbonate solution. Calcein (30 mg/kg) was injected to each animal SC twice (10 and 3 days before the end of the experiment). our SCI investigations. First, mice have been widely used as very useful models for studying bone homeostasis in health and disease. Second, the effects of PTH on bone mass have been well demonstrated in normal mice and mouse models of osteoporosis. Therefore, sufficient literature and knowledge are available that can be applied to advance our understanding of bone loss after severe immobilization. Third, since most genetic modifications are generated in mice, many genetically modified mice have become available to further determine the role of different molecules in SCI-induced bone loss. Lastly, mouse genome and physiology are similar to that of humans making the mouse an appropriate model to study human diseases.

Spinal cord injury surgery
Spinal cord transection was performed in the thoracic segment of the spinal cord to result in complete paralysis of the hindlimbs. Animal underwent general anesthesia using controlled isoflurane inhalation from a precision vaporizer or flowmeter.
The surgical site was prepared by wiping three times with alternating 70% ethanol and betadine. A midline incision was made over spinous processes T6-T10 and paravertebral muscles were separated from the vertebrae. The laminae and spinous processes of the T9 vertebra were removed (laminectomy) with fine scissors to expose the underlying spinal cord. The spinal cord was cut with a scalpel or microscissors and a surgical sponge was placed in the gap between the ends of the severed spinal cord. Muscle and connective tissues were closed with a 4-0 to 5-0 absorbable suture, and the skin was closed with monofilament nonabsorbable sutures or surgical clips or staples and tissue glue. Sutures were removed at 7-10 days under anesthesia with inhaled isofluorane. Spinal cord transection results in complete paralysis of the hindlimbs. Mice lose the ability to voluntarily micturate, requiring bladders to be manually expressed three times daily initially, then at least once daily after some automatic emptying of the bladder develops. With complete hindlimb paralysis, mice can still roam freely with their forelimbs and have full access to food and water. At the time of bladder expression, mice were provided with saline or lactated Ringers's solution (1 mL). Mice also received an injection Carprofen analgesic (subcutaneously (SC), 5 mg/kg once 1 h before surgery then once daily for 3 days after surgery and subsequently as needed) and Baytril antibiotic (SC, 2.5-5 mg/kg twice daily on the day of surgery then twice daily for 3 days after surgery in saline solution). At the time of bladder expression, a health check on each mouse was performed for any visible signs of distress such as lack of responsiveness, abnormal stool, porphyria around the eyes, dehydration, weight loss, lack of grooming, lack of mobility, pressure sores, and/or urinary tract infection. If any of these signs were noticed, the mouse was placed back on the warming pad and administered Ringer's solution, Carprofen, and/or other medications, as necessary or recommended by the attending veterinarian. Mice that did not improve after 2 days, were euthanized. After surgery, mice were placed on a warming pad and monitored continuously until fully recovered from anesthesia, then three times daily for 2 weeks, including weekends and then once daily during weekdays until the end of experiment. Animals were weighed before surgery and then twice weekly until the end of experiment. Weight and health status were recorded.
For studies investigating SCI-induced bone loss that results primarily from muscle paralysis and mechanical unloading, spinal transection is an appropriate model. Spinal cord transection in mice provides the most complete denervation and paralysis, thus closely modeling persons with motor-complete SCI, the form of SCI associated with the most severe bone loss. Technically, contusion SCI in mice is more difficult to reproduce and the extent of SCI may vary among animals; this will cause variable degrees of bone loss that would respond differently to treatment, thus complicating the interpretation of the data. The spinal transection approach, therefore, assures similar unloading and bone loss among animals.

Micro-CT measurements of cortical and trabecular bone
Animals were euthanized using isoflurane inhalation (to effect) anesthesia followed by exsanguination and cervical dislocation and femurs and tibias harvested and fixed in 10% neutral buffered formalin for 24 h and then preserved in 70% ethanol. Bones were scanned using a microcomputed tomographic (micro-CT) instrument (VivaCT-40, Scanco Medical AG, Bassersdorf, Switzerland). Standard nomenclature and guidelines for assessment of bone microstructure were followed, as recommended by the American Society for Bone and Mineral Research (Bouxsein et al. 2010). Cross-sectional geometry at the femoral midshaft and trabecular bone volume fraction and microarchitecture in the secondary spongiosa of the proximal tibia were assessed. The bones were scanned at a low resolution, an energy level of 55 kVp, intensity of 145 lA, and a fixed threshold of 220. The VivaCT-40 is calibrated weekly using a phantom provided by Scanco. Scans for the cortical region were measured at the midpoint of each femur, with an isotropic pixel size of 21 lm and slice thickness of 21 lm, and used to calculate the average total cross-sectional area (mm 2 ), bone area (mm 2 ), and marrow area (mm 2 ). For midshaft analysis, the cortical shell was contoured by user-defined threshold and iterated across the 50 slices. Tibial trabecular bone volume fraction and microarchitecture were evaluated in the secondary spongiosa, starting proximately at 0.6 mm distal to the growth plate, and extending distally 1.5 mm. Approximately 230 consecutive slices were made at 10.5 lm interval at the distal end of the growth plate and extending in a proximal direction, and 180 contiguous slices were selected for analysis. The main bone parameters are TV (total volume [mm 3 ]), BV (bone volume [mm 3 ]), BV/TV (the relative volume of calcified tissue in the selected volume of interest (VOI), Tb.Th (the thickness of the trabecular structure), Tb.N (the number of trabeculae), Tb.Sp (trabecular separation; a measurement of the thickness of the spaces between the trabeculae, inversely proportional to the trabecular thickness), Conn. D (connectivity density, 3-D connectivity index; a measure of the degree to which a structure is multiply connected), and TRI-SMI (structure model index; is related to the architecture of the structure and ranges between 0 and 3, SMI toward 0 or lower signifies that the structure is mainly concave plates, whereas a value of three means only cylindrical rods).

Bone histology and histomorphometry
Bone histology and histomorphometry analysis was performed at Yale Core Center for Musculoskeletal Disorders at Yale University School of Medicine (New Haven, CT). The measurements, terminology, and units used were those recommended by the Nomenclature Committee of the American Society for Bone and Mineral Research (Dempster et al. 2013). Femurs were dissected, fixed in 10% neutral buffered formalin for 24 h before transferring to 70% ethanol. Bones were then dehydrated and embedded undecalcified in methyl methacrylate (MMA). Longitudinal sections, 5 lm thick, were cut along the frontal plane from the MMA plastic-embedded blocks using a Leica 2265 microtome (Microm, Richards-Allan Scientific, Kalamazoo, MI, USA) and stained with toluidine blue. For dynamic histomorphometric analyses, each mouse received two SC injections of 30 mg/kg body weight calcein at 10 and 3 days before sacrifice. Static parameters of bone formation and resorption were measured at a standardized site in an area of the distal femoral epiphysis spanning from 75 lm from the growth plate to the endocortical edge of the epiphysis, as described previously (Tawfeek et al. 2010) using an Olympus BX40 microscope interfaced with the Osteomeasure system software and hardware (Osteometrics, Atlanta, GA). Software calculations of various parameters are based on previously defined formulas (Dempster et al. 2013). BV/TV (percentage of bone volume relative to total tissue volume), Tb Th (trabecular thickness), Tb N (trabecular number), OV/ TV (percentage of osteoid volume relative to total tissue volume), OV/BV (percentage of osteoid volume relative to bone volume), BS/BV (percentage of bone surface relative to bone volume), OS/BS (percentage of osteoid surface relative to total bone surface), O Th

Measurement of serum markers of bone turnover
Blood was collected in BD Microtainer TM Capillary Blood Collector and BD Microgard TM Closure (BD cat. #BD 365967) drawn by cardiac puncture which was performed as a terminal procedure on isoflurane-anesthetized animals. Sera were then isolated and frozen at À80°C. Serum markers of bone turnover were measured, as described previously (Gao et al. 2008;Terauchi et al. 2009;Tawfeek et al. 2010) and by following the manufacturer's instructions. Serum collagen type I C-telopeptide or carboxyterminal collagen crosslinks (CTX), a marker of bone resorption, procollagen type 1 intact N-terminal propeptide (P1NP), a specific marker of bone resorption, and tartrate-resistant acid phosphatase (TRAcP-5b, a specific serum marker of osteoclastic activity) were measured using rodent-specific ELISA assay kits from Immunodiagnostic Systems (Scottsdale, AZ).

Statistical analysis
As per experimental protocol, animals were randomized between sham-vehicle, SCI-vehicle, and SCI-PTH groups and the analyses were performed blindly regarding the control and test groups by using unique animal identification codes. The results are presented as the means AE SD. Unpaired Student's t-test was used for comparisons between two groups. For carrying out multiple comparisons between all groups, one-way ANOVA and Tukey's post hoc test were employed. Statistical significance was considered when the P value was less than 0.05 (P < 0.05).

PTH improves trabecular bone structure in SCI animals
To assess the effects of intermittent PTH administration on trabecular bone, sham and SCI animals were injected daily (except Sunday) SC with vehicle or PTH for 35 days. Mice were then sacrificed and the isolated tibias were analyzed by micro-CT. SCI-vehicle animals exhibited a marked decrease in proximal tibial fractional trabecular bone volume (Tb BV/TV) and trabecular thickness (Tb Th) compared to the control sham-vehicle group (Fig. 1A and B). Additionally, two indices that have been implicated in mechanical properties of bone, connectivity density (Conn D), and structure model index (TRI-SMI) were examined after SCI. Although there was a trend for a decrease in Conn D with higher TRI-SMI in the trabecular bone of the SCI-vehicle compared to sham-vehicle animals, these differences did not reach significance ( Fig. 1E and F). Interestingly, PTH treatment protected against SCIinduced trabecular bone loss and preserved bone architecture. SCI animals that received daily PTH injection had significantly higher Tb BV/TV, Tb Th, and Conn D, and lower TRI-SMI, than SCI-vehicle counterparts (Fig. 1A  2017 | Vol. 5 | Iss. 18 | e13446 Page 5 groups ( Fig. 1C and D). Qualitative changes in trabecular bone structure are also demonstrated in micro-CT images of the proximal tibia (Fig. 1G).

PTH attenuates cortical bone loss and reduces porosity in SCI animals
To determine the effects of PTH treatment on cortical bone structure after SCI, sham-and SCI-operated animals were injected daily with vehicle or PTH for 35 days. Midshaft regions of isolated femurs were then analyzed by micro-CT. Micro-CT analysis revealed that SCI-vehicle animals suffered a reduction in cortical bone volume (Ct BV/TV) and thickness (Ct Th), and an elevation in cortical porosity (Ct Porosity) compared to the shamvehicle group ( Fig. 2A-C). In contrast, PTH injection preserved cortical thickness and volume and abolished the increase in porosity observed in femurs of SCI animals ( Fig. 2A-C). Qualitative changes in cortical bone are also demonstrated in micro-CT images of the femur midshaft region (Fig. 2D).

PTH prevents the deleterious effects of SCI on histomophometrical indices of bone formation and resorption
To understand further how PTH protects against SCIinduced bone loss and improves bone structure and microarchitecture, bone histology and histomorphometry were performed on femurs isolated from sham-vehicle, SCI-vehicle, and SCI-PTH animal groups.
To assess the effects of daily PTHR1 activation on osteoblast and osteoclast numbers, static histomorphometry analysis was performed. The results show that two important static indices of bone formation (N.Ob/T.Ar) and bone resorption (N.Oc/T.Ar) that were considerably reduced in SCI-vehicle animals were augmented as a result of PTH treatment (Fig. 3A and B). Furthermore, dynamic bone histomorphometry evaluation demonstrated that SCI animals had pronounced reduction in bone formation rate (BFR) compared to sham-vehicle control (Fig. 3C). This reduction in BFR was completely prevented by PTH treatment of SCI animals and BFR became similar to sham-vehicle controls (Fig. 3C).
Consistently, other indices, namely BV/TV, OV/TV, Tb Sp, Tb N, Ob.S/OS, and N.Ob/Opm were also significantly increased in the SCI-PTH animals compared to SCI-vehicle and/or sham-vehicle controls (Table 1). Other histomorphometrical parameters were not significantly altered (Table 1).

PTH promotes the serum levels of bone turnover markers
To determine the effects of PTH treatment on serum markers of bone turnover after SCI, markers of bone formation and resorption were measured by ELISA. Analysis of sera collected from sham-vehicle and SCI-vehicle mice showed no significant changes in either bone formation (Procollagen I intact N-terminal propeptide, P1NP) or resorption (collagen type 1 cross-linked C-telopeptide, CTX, and osteoclastic activity-specific tartrate-resistant acid phosphatase, TRAcP-5b) markers between sham-vehicle and SCI-vehicle control animals ( Fig. 4A-C). PTH administration, however, caused greater stimulation of bone turnover in SCI animals. PTH caused an elevation of serum P1NP, CTX, and TRAcP-5b to levels that were significantly higher than sham-vehicle and SCI-vehicle groups ( Fig. 4A-C). These results suggest that PTHR1 activation promotes bone turnover in SCI animals.  To determine if serum levels of bone turnover are altered in the early stage of SCI, animals underwent sham and SCI surgery for 14 days. Sera were collected and P1NP and CTX levels were measured by ELISA. The results show that P1NP levels were significantly lower while CTX were higher in SCI than sham control animals after 14 days ( Fig. 4D and E).

Discussion
The high incidence and severity of sublesional osteoporosis after SCI and the absence of effective conservative (rehabilitation) or pharmacological therapeutic regimen has made bone loss in this condition a difficult clinical challenge to address (Maimoun et al. 2006a). There is, therefore, a pressing need for additional and focused research to explore new therapeutic approaches that would reduce fracture risk in patients with SCI.
In this report, using a mouse model, we demonstrate that severe immobilization, as a result of SCI, is associated with marked attenuation of bone turnover, causing severe cortical and trabecular bone loss in the distal femoral and proximal tibial regions. We further show, for the first time, that daily PTH administration protects against SCI-induced bone loss by promoting both bone formation and resorption, thus maintaining bone microarchitecture.
Our static histomorphometry findings demonstrated that both osteoclast and osteoblast cell numbers were evidently reduced in bones of the paralyzed hindlimbs after 35 days. Dynamic histomorphomerty analysis further revealed that bone formation rate was attenuated in immobilized animals. These results support the conclusion that inhibition of bone turnover and uncoupling of bone resorption and formation contributes to bone loss after SCI. These results may explain, at least partly, why patients with osteoporosis after SCI fail to respond to antiresorptive agents that act principally by inhibiting osteoclast formation and activity (Bauman et al. 2014). Of relevance, osteocytic osteolysis and osteocyte-induced bone degradation and demineralization, a concept that was raised decades ago, has recently been reintroduced as a possible mechanism for bone dissolution and osteolysis in many forms of bone loss including immobilization (Baylink et al. 1973;Tazawa et al. 2004;Teti and Zallone 2009;Qing et al. 2012;Wysolmerski 2012Wysolmerski , 2013 Lloyd et al. 2014;Clarke et al. 2015;Nango et al. 2016). Our finding of increased cortical porositiy implies that osteocyte osteolysis could partially account for bone demineralization and loss after SCI. However, further investigation is warranted to verify whether or not osteocytic osteolysis is involved in rapid bone dissolution after SCI. As reported previously (Maimoun et al. 2002(Maimoun et al. , 2005Zehnder et al. 2004;Kostovski et al. 2015), when assessed earlier (14 days), P1NP levels were lower and CTX were higher in SCI than sham control animals. However, at 35 days, despite the clear reduction in the local indices of bone formation and resorption in SCI animals, there was no significant change in the serum levels of bone turnover markers. Collectively, these results suggest that bone loss after SCI is likely initiated by activation of bone resorption before osteoblast lineage and osteoclast cell functions are severely compromised at later stages of SCI; this leads to a status of low bone turnover and subsequent adynamic bone disease. Notably, absence of concomitant decline in the circulating bone makers after 35 days of SCI may be attributed to masking by possible rise in bone turnover subsequent to adaptation to weight bearing on the intact forelimb bones.
With the advancement of medical care, patients with SCI have near-normal longevity, and, with the benefit of technological advances, they are increasingly upright and active. Moreover, powered exoskeletal devices place increased forces on the sublesional long-bones of the lower extremities, which must be strong enough to withstand the added stresses and strains without fracturing. The development of pharmacological approaches that reduce bone loss and retain bone strength would be anticipated to prevent fractures, with associated improved quality of life. With the current uncertainty surrounding the use of antiresorptive agents and other rehabilitation measures to prevent bone loss after SCI, bone anabolic agents emerge as an attractive approach that deserves serious attention. In contrast to the antiresorptive agents that act by preventing further bone loss, bone anabolic agents act primarily on osteoblasts to stimulate bone formation and improve bone mass, structure, and mechanical properties. Thus, assessing the existing food and drug administration (FDA)-approved bone anabolic drugs offers hope for preventing bone loss in patients with SCI.
In this regard, human PTH(1-34) or teriparatide, the amino terminal fragment of PTH, is a proven, effective bone anabolic agent and has been approved by the FDA for treating both postmenopausal (Neer et al. 2001) and glucocorticoid induced (Saag et al. 2007) osteoporosis. In these patients, PTH increases bone mineral density (BMD) at the spine and femur of osteoporotic men and women, and decreases by greater than half the risk of vertebral and nonvertebral fractures. PTH is also relatively safe and is superior to other FDA-approved therapies for osteoporosis (e.g., antiresorptive agents) in improving bone density and quality, and reducing fracture risk.
Surprisingly, there has been relatively insufficient focus on evaluating PTH as a potential therapy for bone loss after SCI in animal models or human studies. There is only one recent, and quite limited, pilot study in man that administered PTH to subjects with SCI. The study had serious limitations and was inconclusive, principally due to the lack of a control group, short duration of treatment (6-12 months of PTH), and small sample size (11 subjects received 6 months of PTH and 7 subjects received 12 months of PTH). Moreover, the subjects were studied 3-21 years after SCI, with a mean duration of injury of approximately 10 years (only 2 subjects were injured less than 5 years-that is, after significant bone loss had already occurred, and the sublesional skeleton was probably in a low turnover state for the majority of subjects studied (Gordon et al. 2013). Regarding the animal studies, rat and mouse models of skeletal unloading have been utilized to assess the effects of PTHR1 activation on disuse osteopenia (Ma et al. 1995;Halloran et al. 1997;Turner et al. 1998Turner et al. , 2007Ono et al. 2007;Bruel et al. 2013;Sandberg et al. 2014). In these studies, hindlimb unloading was induced pharmacologically using botulin toxin A (Botox) (Bruel et al. 2013;Sandberg et al. 2014) or mechanically using tail suspension or limb fixation to the abdomen (Ma et al. 1995;Halloran et al. 1997;Turner et al. 1998Turner et al. , 2007Ono et al. 2007). These investigations have demonstrated that PTH under different experimental conditions namely wide dose range (1-200 lg/kg/day), short-or longterm treatment duration, or daily or continuous administration efficiently counteracts trabecular and cortical bone loss in the unloaded femur and tibia. PTH was further shown to reduce bone loss primarily by enhancing bone formation, but by also reducing osteoclast number in some of these studies.
Accordingly and in light of the urgent demand for effective therapy to reduce bone loss after SCI, we evaluated the efficacy of PTH in reducing bone loss after SCI. Our findings reveal that intermittent PTHR1 activation is a potentially appealing strategy for treatment of bone loss in patients with nonambulatory SCI. Our biochemical and histological analyses strongly demonstrate that PTHR1 activation triggers a strong bone turnover response in immobilized animals. The micro-CT analysis further indicates that the increase in bone turnover positively impacts cortical bone thickness and porosity, enhances trabecular bone volume and architecture, and increases trabecular connectivity and the number of concave plate-over rodlike structures. A decrease in the plate/rod ratio and connectivity are believed to be major contributing factors to fragility fracture in various forms of osteoporosis and with advancing age, while an increase in these parameters induced by treatment with bone anabolic and anti-resorptive agents is beneficial for bone quality and strength (Ding and Hvid 2000;Benito et al. 2003;Borah et al. 2004;Rupprecht et al. 2006;Benhamou 2007;Liu et al. 2013;Altman et al. 2014;Stein et al. 2014;Sutter et al. 2014;Zhou et al. 2014;Chang et al. 2015). These effects of PTH on bone formation and bone structure and architecture are consistent with those reported under other hindlimb unloading conditions (Ma et al. 1995;Halloran et al. 1997;Turner et al. 1998Turner et al. , 2007Ono et al. 2007;Bruel et al. 2013;Sandberg et al. 2014). However, PTH stimulation of both bone formation and resorption has only been observed in SCI further underscoring the differential impact of SCI on osteoblast lineage and osteoclast cell function and their responsiveness to PTH. Lastly, it is important to acknowledge the limitations of the present investigation. First, while the course of PTH treatment started immediately after SCI, further investigations are required to address the efficacy of PTH after chronic SCI. Second, while we anticipate that PTH will be efficacious in male animals, this should be independently verified.
In conclusion, the present study proposes that intermittent PTHR1 activation is an effective anabolic approach that should be seriously evaluated for restoring bone integrity in patients with severe disuse osteoporosis.