Microglial activation and polarization in type 2 diabetes-related cognitive impairment: A focused review of pathogenesis

Microglia, as immune cells in the central nervous system, are closely related to cognitive impairment associated with type 2 diabetes (T2D). Preliminary explorations have investigated the relationship between T2D-related cognitive impairment and the activation and polarization of microglia. This review summarizes the potential mechanisms of microglial activation and polarization in the context of T2D. It discusses central inflammatory responses, neuronal apoptosis, amyloid-β deposition, and abnormal phosphorylation of Tau protein mediated by microglial activation and polarization, exploring the connections between microglial activation and polarization and T2D-related cognitive impairment from multiple perspectives. Additionally, this review provides references for future treatment targeting microglia in T2D-related cognitive impairment and for clinical translation.


Introduction
Type 2 diabetes (T2D) is increasingly prevalent worldwide, with epidemiological studies indicating that approximately 537 million adults (aged 20-79) globally had diabetes in 2021, of which T2D accounted for about 90 % of these cases (Saeedi et al., 2019).In recent years, the prevalence of T2D has been steadily rising, with the global prevalence estimated at 9.3 % in 2019, projected to rise to 10.2 % by 2030 and 10.9 % by 2045 (Saeedi et al., 2019).Major risk factors contributing to the increase in T2D include obesity, lack of exercise, unhealthy diet (high sugar and fat), and aging population, which interact in complex ways, making T2D a multifactorial disease (Gregory et al., 2022;Sun et al., 2022).T2D can lead to cognitive impairment, becoming one of its significant chronic complications (Magliano et al., 2021;Srikanth et al., 2020).T2D-related cognitive impairment is closely linked to population aging, with older adult patients with T2D having a 1.5-to 2-fold increased risk of cognitive impairment compared to older adult non-diabetic individuals (Srikanth et al., 2020).Among these, patients with T2D have a 50-100 % increased risk of Alzheimer's disease (AD), a 100-150 % increased risk of vascular dementia (Gudala et al., 2013;Zhang et al., 2017), and a 1.53-fold increased risk of progressing from mild cognitive impairment to dementia (Xue et al., 2019).Key risk factors for T2D-related cognitive impairment mainly include atherosclerosis and related cardiovascular diseases (Katsi et al., 2023).Among patients with T2D, the prevalence and progression of atherosclerosis are higher and faster due to chronic hyperglycemia, dyslipidemia, and hypertension (Wong and Sattar, 2023).Specifically, atherosclerosis of the cerebral and carotid arteries reduces blood supply to the brain, leading to chronic hypoperfusion and ischemic damage, which may impair cognitive function (Sabayan et al., 2023), and patients with T2D Abbreviations: Aβ, amyloid-β; AD, Alzheimer's disease; AGEs, advanced glycation end products; APP, amyloid precursor protein; Arg1, arginase-1; Bax, B-cell lymphoma 2-associated X protein; Bcl-2, B-cell lymphoma 2; CD16, cluster of differentiation 16; CD86, cluster of differentiation 86; CD206, cluster of differentiation 206; CNS, central nervous system; GK, Goto-Kakizaki; Iba1, ionized calcium-binding adapter molecule 1; IDE, insulin-degrading enzyme; IL-1β, interleukin 1β; IL-4, interleukin 4; IL-6, interleukin 6; IL-10, interleukin 10; IL-17A, interleukin 17 A; INOS, inducible nitric oxide synthase; MSCs, mesenchymal stem cells; NFTs, neurofibrillary tangles; PPAR-γ, peroxisome proliferator-activated receptor gamma; P-Tau, phosphorylated Tau; RAGE, receptor for advanced glycation end products; RELM-α, resistin-like molecule-α; SD, Sprague Dawley; STAT1, signal transducer and activator of transcription 1; STAT3, signal transducer and activator of transcription 3; STAT6, signal transducer and activator of transcription 6; STZ, streptozotocin; T2D, type 2 diabetes; TLR, toll-like receptor; TLRs, toll-like receptors; TNFα, tumor necrosis factor α; TREM2, triggering receptor expressed on myeloid cells 2. combined with cardiovascular disease have an increased risk of both ischemic and hemorrhagic strokes (Balasubramanian et al., 2023;Brown et al., 2023).Additionally, hypertension, dyslipidemia, and obesity are common risk factors for both cardiovascular disease and cognitive impairment in patients with T2D, causing T2D and cardiovascular disease to influence and exacerbate each other, creating a vicious cycle (Cai et al., 2023;Filler et al., 2024;Shang et al., 2024;Xie et al., 2024).Therefore, epidemiological evidence strongly supports the association between T2D and an increased risk of cognitive impairment, making effective management of cardiovascular risk factors in patients with T2D crucial for preventing cognitive decline and improving overall quality of life.
Microglia are important immune cells in the central nervous system (CNS), closely and coordinately distributed around neurons.They are crucial for maintaining the stability of the CNS environment and regulating neuronal functions (Cserep et al., 2021).For example, by providing nutrients, clearing metabolic waste, and maintaining ionic balance, microglia ensure the normal functioning of neurons (Stogsdill et al., 2022;Wang et al., 2024).They also participate in and regulate the communication between neurons, helping to maintain the stability and plasticity of neural circuits (Marinelli et al., 2019).Additionally, microglia can phagocytose and degrade abnormal protein aggregates, which reduces toxic effects on nerve cells and is crucial for neuron survival.This function provides a reference for studying cognitive impairment-related diseases (Wakselman et al., 2008).Currently, research on the role and mechanisms of microglia in T2D-related cognitive impairment is still relatively limited and primarily at the preclinical research stage.Revealing the functions and roles of microglia in this context may provide new research directions and therapeutic strategies for preventing and treating T2D-related cognitive impairment.Therefore, this review will focus on previous preclinical research stages, exploring the potential roles and mechanisms of microglia in T2D-related cognitive impairment and providing meaningful theoretical foundation for future in-depth studies.

Clinical manifestations of T2D-related cognitive impairment
T2D-related cognitive impairment is a common condition in T2D patients, characterized by cognitive deficits that surpass the cognitive decline associated with normal aging.These symptoms primarily manifest as cognitive issues, such as declining memory, reduced attention span, and impaired executive functions (Chen et al., 2023;Xue et al., 2019).These symptoms not only affect patients' daily activities but also their overall quality of life.In addition to cognitive symptoms, individuals with T2D-related cognitive impairment may experience emotional fluctuations, behavioural changes, and reduced daily life capabilities (Srikanth et al., 2020).The coexistence of these symptoms makes the clinical manifestations of T2D-related cognitive impairment more complex (McCrimmon et al., 2012).

Hyperglycemia
The chronic hyperglycemic environment has a broad and profound impact on the brain health of patients with T2D, primarily causing severe damage to neurons and synapses.This damage is associated with the accumulation of advanced glycation end products (AGEs) and oxidative stress (Biessels and Despa, 2018).The hyperglycemic environment leads to a large influx of glucose into nerve cells, triggering a series of adverse effects, including the accumulation of AGEs.AGEs are a group of compounds produced under hyperglycemic conditions, which can interact with the receptor for advanced glycation end products (RAGE), leading to various harmful cross-links.For example, the production and accumulation of AGEs can activate microglia under hyperglycemic conditions, stimulate the release of inflammatory factors, and couple through gap junction proteins, especially connexin 43, and connexin 43 which is a downstream effector of the AGEs-RAGE interaction in microglia (Mastrocola et al., 2020;Shaikh et al., 2012).These cascade reactions disrupt neuronal structure and function, impair neural signal transmission, and lead to impaired cognitive function (Uribarri et al., 2015).

Insulin resistance
Epidemiological studies show that insulin resistance, especially in the prediabetes stage, significantly increases the risk of cognitive impairment (Kiliaan et al., 2014;Luchsinger, 2008).Elevated plasma insulin levels, reduced glucose tolerance, and other clinical indicators of insulin resistance are closely related to impaired cognitive function, including observable hippocampus atrophy and memory deficits (Convit et al., 2003).Therefore, insulin resistance, particularly in the hippocampus, maybe a key cause of cognitive impairment (Biessels and Reagan, 2015).Basic research in obese Zucker rats, which exhibit insulin resistance, shows a noticeable decline in learning ability and memory capacity.This decline may be related to the reduced plasma membrane level of glucose transporter 4 in the hippocampus and synaptic plasticity defects under insulin resistance (Kamal et al., 2013;Winocur et al., 2005).Similarly, in animal models such as streptozotocin (STZ)-induced insulin-deficient rat and the db/db mouse with insulin resistance, damage to hippocampal synaptic plasticity and adult neurogenesis are observed, with significant cognitive impairment.This may be due to glucocorticoid-mediated defects in neurogenesis and synaptic plasticity (Stranahan et al., 2008).Furthermore, researchers have used a lentiviral vector carrying an antisense sequence of the insulin receptor to specifically induce insulin resistance in the hippocampal region of Sprague Dawley (SD) rats, where the rats exhibited damage to hippocampal neuroplasticity, with reduced basal phosphorylation levels of glutamate receptor 1 in the hippocampal region, which may contribute to impaired hippocampal-dependent learning functions under insulin-resistant conditions (Grillo et al., 2015).Insulin resistance and the development of T2D mutually influence and accompany each other.Due to this influence, microglia may undergo abnormal activation and polarization, inducing subsequent cognitive impairment (Zhao et al., 2024).

Cerebrovascular pathology
T2D is a chronic metabolic disorder characterized by long-term fluctuations in blood glucose levels, potentially causing damage to the brain's structure, function, and cerebral vasculature.Among these, chronic vascular impairment of the brain plays a pivotal role in diabetic encephalopathy.Hyperglycemic conditions trigger various pathophysiological mechanisms, including inflammatory factors, oxidative stress, and cell apoptosis.These mechanisms lead to endothelial dysfunction and structural alterations in the vascular wall, resulting in vascular pathology that reduces cerebral blood flow.Ultimately, this leads to cerebral hypoperfusion and hypoxia, which can detrimentally impact normal neuronal function and survival (Biessels et al., 2014;Last et al., 2007).Under conditions of cerebral hypoperfusion, damage to the blood-brain barrier can be induced, thereby activating microglia.The activation of microglia then induces subsequent polarization and inflammatory damage, impacting neurons, leading to neuronal loss or apoptosis, and causing cognitive impairment (Gulen et al., 2023;Rajeev et al., 2022;Zhan et al., 2023).

Inflammatory responses
The persistent hyperglycemia and subsequent cerebrovascular pathology in T2D patients often give rise to chronic inflammatory responses within the CNS.These inflammatory reactions can be evaluated and monitored through a variety of biomarkers, with the most common and crucial ones including C-reactive protein(CRP), tumor necrosis factor α (TNF-α), interleukin 1β (IL-1β), and interleukin 6 (IL-6).These pro-inflammatory factors not only display elevated expression in the bloodstream but are also closely linked to neurofunctional impairments.They can disrupt cognitive function by directly or indirectly affecting neuronal activity, synaptic stability, normal neurotransmitter release, and synaptic plasticity (Chung et al., 2015).It is important to note that this CNS inflammation is intricately connected to the activation status of microglia.When exposed to inflammatory stimuli, microglia release a multitude of inflammatory mediators that amplify the inflammatory response, thus perpetuating a vicious cycle (Calsolaro and Edison, 2016;Li and Barres, 2018).

Disruption of brain energy metabolism
Glucose serves as the principal energy substrate for neuronal function.However, in T2D, the brain's insulin resistance and dysregulation of insulin signaling pathways impose a clear constraint on the utilization of glucose, impacting the energy supply to neurons.This can lead to mitochondrial dysfunction and an escalation in oxidative stress, potentially affecting the normal functioning of neurons and causing a significant deficiency in neuronal energy (Sonneville et al., 2015;Stranahan et al., 2008).A cascade of damaging reactions within the brain prompts microglia to undergo rapid morphological changes, migration, and proliferation, initiating a series of immune responses, and the robust activation of microglia may induce neuronal damage, thereby exerting a detrimental effect on cognitive function (Hu et al., 2022).

Deposition of amyloid-β in the brain
Previous relevant research has discovered significantly elevated levels of amyloid-β (Aβ) plaques in the brains of T2D models, accompanied by pronounced cognitive impairments, further reinforcing the connection between T2D and cognitive decline (Meakin et al., 2020).
Studies have shown that in T2D, peripheral hyperinsulinemia results in increased levels of insulin in the brain, which in turn causes insulin resistance in neurons, potentially reducing the key enzyme of glucose metabolism, hexokinase 2, leading to glycolytic disorders in neurons and possibly causing the activation and functional abnormalities of microglia (Chow et al., 2019).These functional abnormalities of microglia inhibit their ability to phagocytose Aβ, resulting in its accumulation in the brain and contributing to cognitive impairment (Leng et al., 2022).Synthesizing information from various aspects, it is evident that T2Drelated cognitive impairment may be linked to multiple factors, including hyperglycemia, insulin resistance, cerebrovascular disease, inflammatory responses, brain energy metabolism disorders, and Aβ deposition in the brain.Hyperglycemia, in particular, plays a crucial role; excessively high blood sugar levels can damage various organs and affect normal brain function.Additionally, microglial activation and functional abnormalities are interwoven with these factors.They interact with each other, forming a complex disease state, leading to cognitive impairment (Fig. 1).

Current treatment strategies for T2D-related cognitive impairment 2.3.1. Glycemic control and lifestyle adjustment
Good glycemic control is essential for preventing and slowing the Y. Tian et al. Neuroscience and Biobehavioral Reviews 165 (2024) 105848 progression of T2D-related cognitive impairment (Biessels et al., 2018).However, for patients with T2D having different health conditions, different glycemic control targets should be set to avoid the risk of hypoglycemia increased by overly stringent glycemic control, as hypoglycemia itself can also have a negative impact on cognitive function (Biessels and Whitmer, 2020).At the same time, in the daily life of patients with T2D, attention should be paid to a healthy diet and regular physical activity to improve glycemic control through non-pharmacological interventions, which may have a positive impact on patients' cognitive function (Koekkoek et al., 2015).In addition, targeted cognitive training programs for patients with T2D may help improve or maintain the cognitive function of patients (Koekkoek et al., 2015).

Pharmacological treatment
Currently, there is no specific drug available for the pharmacological treatment of T2D-related cognitive impairment in clinical practice.However, considering hyperglycemia as the initiating factor, blood sugar control is a fundamental treatment strategy.For instance, metformin, a commonly used hypoglycemic drug, not only lowers blood sugar but may also have a positive impact on cognitive function.Clinical studies have shown that metformin treatment can reduce the risk of cognitive decline in patients with T2D (Zhang et al., 2022a).Nevertheless, studies on drugs such as metformin, glucagon-like peptide-1 receptor agonists, and dipeptidyl peptidase-4 inhibitors are primarily at the basic research level (Hu et al., 2024;Ma et al., 2015;Yan et al., 2019;Yang et al., 2018;Zhang et al., 2021).Therefore, in-depth research is needed to develop specific drugs for treating T2D-related cognitive impairment.

Interdisciplinary collaborative individualized treatment
Given the complexity of T2D-related cognitive impairments, especially the coexistence of multiple diseases in patients with T2D, it requires collaboration among a multidisciplinary team, including physicians, nurses, dietitians, physical therapists, and clinical psychologists, to provide comprehensive care and management: where physicians are primarily responsible for diagnosis, monitoring blood glucose levels, and administering key pharmacological treatments, while also assessing the patient's overall health status, including cardiovascular health, and kidney function; nurses play a key role in patient education, blood glucose monitoring, medication management, and providing daily care; dietitians need to help patients develop personalized meal plans to control blood glucose levels and improve overall health; physical therapists can help patients improve mobility and reduce or prevent other complications caused by T2D; clinical psychologists can provide psychological support for patients, helping them cope with emotional and behavioral issues related to T2D (American Diabetes, 2019).
Furthermore, considering the varying degrees of cognitive impairment, stages of T2D, and patient living conditions, it is crucial to develop a personalized treatment plan.For patients with T2D, determining the degree of cognitive impairment through neuropsychological tests and other tools is necessary, and then providing corresponding pharmacological and non-pharmacological treatments.Additionally, considering the patient's residential environment, social support, and cultural background ensures that the treatment plan is practical and acceptable (Jia et al., 2020).This individualized approach is key to addressing the unique needs and circumstances of each patient with T2D-related cognitive impairment.

Management of cardiovascular risk factors
Managing cardiovascular risk factors is crucial for preventing and treating T2D-related cognitive impairment (van Sloten et al., 2020).Hypertension is an independent risk factor for cognitive decline.Long-term hypertension and hyperlipidemia can lead to atherosclerosis and cerebrovascular damage, affecting the brain's blood supply.Patients with T2D often experience hypertension and hyperlipidemia, increasing their risk of cardiovascular events, stroke, and cognitive impairment (Cai et al., 2023;van Arendonk et al., 2023).Therefore, in the absence of specific drugs for T2D-related cognitive impairment, it is crucial to provide necessary management of cardiovascular risk factors for patients with T2D.Such management includes targeted treatment with antihypertensive and lipid-lowering drugs, a healthy diet, regular exercise, and controlling blood pressure and lipids through smoking cessation and alcohol limitation (Morys et al., 2021).

Activation of microglia in the context of T2D
During T2D, influenced by hyperglycemia, microglial activation can exacerbate cytotoxicity and neuronal damage.As immune cells of the CNS, microglia can participate in maintaining tissue homeostasis and secrete pro-inflammatory or anti-inflammatory factors depending on their activation state.Additionally, chronic activation of microglia can lead to progressive neuronal loss and accelerate the onset of cognitive impairment (Xu et al., 2023).Studies have shown that in T2D models, influenced by fluctuations in blood sugar, microglia undergo stress-induced metabolic reprogramming, transitioning from oxidative phosphorylation to glycolysis, with increased glucose uptake, and increased production of lactate, lipids, succinate, and upregulation of glycolytic enzymes.These metabolic responses result in functional changes in microglia, presenting exacerbated inflammatory responses under high glucose conditions and reduced phagocytic capacity of microglia, thereby exacerbating neurodegenerative changes (Quan et al., 2011;Vinoth Kumar et al., 2016).Galectin-3, a β-galactoside-binding protein in the CNS, plays a key role in the activation of microglia under T2D conditions (Garcia-Revilla et al., 2022).It interacts with key pattern recognition receptors such as toll-like receptor 4 and triggering receptor expressed on myeloid cells 2 (TREM2), affecting the activation state of microglia and, subsequently, the inflammatory response in the CNS (Boza-Serrano et al., 2019;Zhong et al., 2023).Additionally, hyperglycemia can lead to increased levels of reactive oxygen species within cells.In response to oxidative stress, microglia may increase the expression of antioxidant enzymes, such as superoxide dismutase, glutathione peroxidase (Volpe et al., 2018).Despite these protective mechanisms, oxidative stress can activate microglia, prompting them to release pro-inflammatory factors (Simpson and Oliver, 2020).

Polarization of microglia
In the CNS, the functions of microglia are highly dynamic and can adopt different phenotypes based on the microenvironmental signals they receive.This ability to change phenotype, known as polarization, is a key characteristic of microglia, enabling them to adapt to various physiological and pathological conditions (Drager et al., 2022).Microglial polarization is mainly influenced by signaling molecules such as cytokines, chemokines, and pathogen-associated molecular patterns (Li et al., 2023b).The key signaling pathways involved in microglial polarization are diverse, including the signal transducer and activator of transcription 1 (STAT1), signal transducer and activator of transcription 6 (STAT6), peroxisome proliferator-activated receptor gamma (PPAR-γ) pathways, toll-like receptor (TLR) pathways, and interferon-gamma signaling pathways.Polarized microglia can exhibit a range of phenotypes (Bi et al., 2023;Hsu et al., 2023;Li et al., 2023a).

Phenotypes of microglia
Microglial phenotypes refer to the different morphological and functional characteristics that microglia exhibit under various physiological and pathological conditions.Different phenotypes of microglia play important and complex roles in maintaining CNS function and in the development of various neurological diseases.However, they are generally classified into two main phenotypes: M1 and M2 (Ritzel et al., 2023).
M1 phenotype microglia are typically considered pro-inflammatory, playing a key role in immune responses and inflammatory reactions.M1 microglia produce pro-inflammatory cytokines, such as IL-1β and TNF-α, to promote the activation of immune cells and the occurrence of inflammation.Therefore, in cases of infection, injury, and neurodegenerative diseases, activated microglia tend to polarize more towards the M1 phenotype (Colonna and Butovsky, 2017;Ransohoff, 2016).
On the other hand, M2 phenotype microglia are mainly involved in neuroprotection and anti-inflammatory responses.They have antiinflammatory properties and can produce anti-inflammatory cytokines, such as interleukin 4 (IL-4) and interleukin 10 (IL-10).M2 microglia play an important role in reducing inflammation, promoting neural repair, and maintaining neuronal homeostasis.In the later stages of neurodegenerative diseases, the activation of M2 microglia may be crucial in alleviating disease symptoms and reducing neural damage (Cherry et al., 2014;Orihuela et al., 2016).
Additionally, researchers have found that microglia exhibit intermediate and alternative phenotypes that do not fully conform to the M1/ M2 paradigm.For example, M1/M2 mixed phenotypes, where microglia express markers of both M1 and M2 phenotypes, can be observed in cases of brain injury (Choi et al., 2023;Hu et al., 2015).Moreover, there is a type of AD-associated microglia that accumulates in the aging brain, characterized by the upregulation of genes involved in phagocytosis and lipid metabolism, which may serve as cellular targets for treating neurodegenerative diseases (Silvin et al., 2022).
It is worth noting that microglia are not fixed in the M1 or M2 phenotype.They can transform into each other under different environmental stimuli, exhibiting different functional characteristics.This dynamic phenotypic transformation is crucial for adapting to the various needs of the CNS and responding to different pathological states (Fig. 2) (Orihuela et al., 2016).Y. Tian et al. Neuroscience and Biobehavioral Reviews 165 (2024) significantly decreased.Pharmacological intervention reversed the expression of these inflammatory factors and improved cognitive impairment in the mouse model (Sood et al., 2023a).
Reviewing the above four studies (Table 1), Li et al. utilized Goto-Kakizaki (GK) rats to simulate the T2D model (Li et al., 2017), while the other 3 studies induced T2D mouse models through a combination of high-fat diet and STZ intraperitoneal injection (Cui et al., 2021;Sood et al., 2023a;Zhang et al., 2022b).Despite different approaches to model establishment, all studies detected cognitive impairment accompanied by CNS microglial activation and excessive expression of pro-inflammatory factors.Additionally, Sood et al.'s study concurrently investigated anti-inflammatory factors (Sood et al., 2023a), further confirming the close association between T2D-related cognitive impairment and inflammatory responses under microglial activation.Inhibiting the expression of pro-inflammatory factors in the CNS under T2D conditions and upregulating the release of anti-inflammatory factors may represent an ideal strategy to improve T2D-related cognitive impairment.

The impact of the two main phenotypes
The two main phenotypes of microglia are M1 and M2, and the differentiation of these phenotypes represents two important functional characteristics in the neuroimmune system.As discussed, the M1 phenotype of microglia is considered pro-inflammatory.Its main characteristic is to trigger immune responses and inflammation in the face of infection, injury, or other neurological abnormalities.The M1 phenotype can secrete pro-inflammatory cytokines to activate immune cells and promote immune inflammation (Colonna et al., 2017;Ransohoff, 2016).In contrast, M2 phenotype microglia are anti-inflammatory, regulating neuroprotection and anti-inflammatory responses by producing anti-inflammatory cytokines that suppress inflammation, promote neuronal repair, and maintain neuronal homeostasis (Cherry et al., 2014;Orihuela et al., 2016).Therefore, in T2D-related cognitive impairment, the M1 and M2 phenotypes play critical roles.
In the db/db diabetic mouse model, significant cognitive impairment was observed through the Morris water maze test.With the activation of microglia in these model mice, high levels of the M1 phenotype marker cluster of differentiation 86 (CD86) were expressed in the brain, while the M2 phenotype marker cluster of differentiation 206 (CD206) was expressed at low levels (Hui et al., 2023).In the T2D mouse model induced by a high-fat diet combined with STZ, researchers administered pharmacological interventions to the model mice, which inhibited the overactivated microglia and reduced the expression level of Iba1.Additionally, the pharmacological intervention reversed the polarization of microglial phenotypes under T2D conditions, shifting them from the pro-inflammatory M1 type to the anti-inflammatory M2 type.This intervention reduced the expression of the M1 marker inducible nitric oxide synthase (iNOS) while increasing the expression of the M2 markers arginase-1 (Arg1) and CD206, improving the cognitive impairment symptoms in T2D mice (Liu et al., 2022;Sood et al., 2023b).It is noteworthy that another group of researchers conducted similar studies in a high-fat diet-induced C57BL/6 mouse model.The model mice exhibited significant glucose intolerance, and cognitive function tests using the Morris water maze showed a marked decline in cognitive function.To study the role of microglia, researchers used adeno-associated virus-mediated gene transfection technology to overexpress TREM2 in the hippocampus of the mice.This intervention successfully inhibited the activation of microglia in the hippocampus of high-fat diet-fed mice, resulting in a reduction in the number of Iba1-positive cells.Notably, the overexpression of TREM2 promoted the polarization of microglia from the M1 type to the M2 type, reducing the expression of iNOS and increasing the expression of Arg1.During this period, the cognitive function of the model mice significantly improved, as evidenced by a significant decrease in escape latency in the Morris water maze, an increase in the number of platform crossings, and a significant increase in the swimming distance in the target quadrant (Wu et al., 2022).
In the above four studies (Table 2), researchers adopted different modeling approaches to study T2D-related cognitive impairment.A common feature across these studies is the significant activation of microglia in the T2D state.However, the detection of microglial M1 and M2 phenotype markers in these four studies varied, with M1 phenotypes detected by CD86 and iNOS, and M2 phenotypes detected by CD206 and Arg1.Notably, by detecting M1 and M2 phenotypes, we can infer that reversing the polarization of microglial M1 and M2 phenotypes in the brain under T2D conditions, downregulating M1 and upregulating M2, has an ideal therapeutic effect on improving T2D-related cognitive impairment.Additionally, the current mainstream markers for microglial M1 phenotypes include cluster of differentiation 16 (CD16), cluster of differentiation 32, CD86, iNOS, Chemokine (C-C motif) ligand 3, and Chemokine (C-C motif) ligand 5, while mainstream markers for M2 phenotypes include CD206, Arg1, resistin-like molecule-α (RELM-α), Chitinase-3-like protein 3, and Chemokine (C-C motif) ligand 22 (Colonna et al., 2017;Wolf et al., 2017).Therefore, given the incomplete detection of M1 and M2 phenotype markers in the studies above, future research should conduct more comprehensive detection of M1 and M2 phenotype markers in T2D model animals to more precisely identify key factors relevant to the treatment of T2D-related cognitive impairment.

Table 1
The correlation between microglial activation-mediated release of inflammatory cytokines and T2D-related cognitive impairment.

Release of apoptotic factors
In the context of T2D onset, microglia activation is closely related to neuronal apoptosis, which may be a key mechanism inducing cognitive impairment.Previous studies on T2D model mice have found that microglia activation is accompanied by a significant decrease in the expression of the anti-apoptotic factor B-cell lymphoma 2 (Bcl-2) and an increase in the expression of the pro-apoptotic factors B-cell lymphoma 2-associated X protein (Bax) and caspase-3.This results in extensive neuronal apoptosis and significant cognitive decline in model mice (Table 3).Conversely, inhibiting the overactivation of microglia in the brains of T2D models, downregulating pro-apoptotic factors, and upregulating anti-apoptotic factors can reduce neuronal apoptosis and improve T2D-related cognitive impairment (Cui et al., 2021;Hui et al., 2023;Sood et al., 2023aSood et al., , 2023b)).However, different researchers have varying views on the correlation between microglial activation and the release of apoptotic factors in T2D models: Yun et al. proposed that microglial overactivation might induce neuronal apoptosis by activating the signal transducer and activator of transcription 3 (STAT3) pathway, resulting in a significant upregulation of caspase-3 and Bax, along with a substantial decrease in Bcl-2 (Yun et al., 2021).Fang and colleagues' study demonstrates that microglial activation in the brains of diabetic mice leads to polarization towards the M1 phenotype.This polarization results in a significant expression of pro-inflammatory factors, including TNF-α and IL-1β, and induces the overexpression of pro-apoptotic factors, caspase-3 and caspase-9.Consequently, this cascade of events triggers neuronal apoptosis, ultimately leading to cognitive impairment (Fang et al., 2022).
Based on the studies above, we propose a complex interrelated chain involving microglial activation, M1 phenotype polarization, proinflammatory factor release, pro-apoptotic factor release, neuronal apoptosis, and ultimately the occurrence of T2D-related cognitive impairment.As shown in Fig. 3, in the T2D model, influenced by hyperglycemia, microglia in the CNS are typically activated and polarized to the M1 phenotype, releasing pro-inflammatory factors such as IL-1β, interleukin 17 A (IL-17A) and TNF-α.These cytokines play a pivotal role in immune responses, recruiting other immune cells, increasing vascular permeability, and intensifying the inflammatory response.Moreover, pro-inflammatory factors not only contribute to inflammation but also directly or indirectly influence apoptotic factors.For instance, TNF-α can activate Bax, leading to changes in mitochondrial membrane permeability, releasing apoptotic mediators like cytochrome C.This process activates apoptotic factors such as caspase-3, ultimately culminating in neuronal apoptosis (Singh et al., 2024).Understanding this interconnected chain provides a foundation for further research.We currently summarize the relationship between microglia, pro-inflammatory factors, and apoptotic factors in the pathogenesis of T2D-related cognitive impairment as mutually influential and mutually regulatory.

Aβ deposition
The deposition of Aβ plays a key role in the development of cognitive impairment associated with T2D and is a widely accepted mechanistic hypothesis in the academic community (Fan et al., 2024;Zheng et al., 2024).However, in the pathogenesis of T2D cognitive impairment, the interaction between microglial activation and polarization and Aβ has received less attention in the field of basic medicine.Nevertheless, evidence suggests that microglial activation and polarization are closely related to Aβ deposition in the brains of T2D models.For example, studies on GK rats have found that model rats exhibit significant cognitive impairment, accompanied by extensive activation of microglia in the brain, a significant increase in the number of Iba1-positive cells, and a large amount of Aβ deposition.This finding directly indicates that microglial activation and Aβ deposition are closely related in the T2D state and may be key factors inducing T2D cognitive impairment (Quan et al., 2022).Another study in T2D mice also showed extensive microglial activation, brain Aβ deposition, and the occurrence of cognitive impairment in the model mice (Zhang et al., 2023).
Compared to the two studies above (Table 4), the difference lies in the preparation of the models, namely using rat T2D models and mouse T2D models, respectively.However, we may need to focus more on the key commonalities of these two studies, such as the deposition of Aβ in the brains of rats and mice with T2D models under microglial activation, inducing T2D-related cognitive impairment.Additionally, there are similar accompanying manifestations, such as high expression of proinflammatory factors IL-1β and IL-6, overexpression of pro-apoptotic factors caspase-3 and caspase-9, and extensive neuronal apoptosis in the brains of T2D model rats and mice (Quan et al., 2022;Zhang et al., 2023).These accompanying manifestations draw attention to the correlation between Aβ, inflammatory factors, and apoptotic factors under microglial activation and polarization.Currently, there is no direct evidence proving the causal relationship between Aβ deposition and the release of inflammatory and apoptotic factors mediated by microglial activation and polarization in T2D models.Therefore, exploring the relationships between these factors in future research is worth considering.

Abnormal phosphorylation of Tau protein
As a key factor in the pathogenesis of cognitive impairment, phosphorylated Tau (p-Tau) has received increasing attention in research on T2D-related cognitive impairment.Studies on T2D model mice have shown significant microglia activation in their brains, accompanied by a marked increase in p-Tau expression, leading to significant cognitive impairment (Carranza-Naval et al., 2021;Hierro-Bujalance et al., 2020;Infante-Garcia et al., 2016).Notably, these three studies (Table 5) used a model combining amyloid precursor protein (APP) mice with db/db mice, a hybrid model of AD and T2D.This model allows for a more direct observation of microglial activation, accumulation of Aβ, and p-Tau overexpression under conditions of cognitive impairment.However, it has limitations, such as a higher probability of Aβ accumulation and p-Tau overexpression due to AD pathological mechanisms, combined with the confounding effects of various pathological factors under T2D conditions.This complexity makes it difficult for this hybrid model to

Table 3
The correlation between neuronal apoptosis under microglial activation and polarization and T2D-related cognitive impairment.fully determine or independently evaluate whether abnormal phosphorylation of Tau protein and accumulation of Aβ in the brains of model animals are mediated by microglial activation and polarization under T2D conditions.Therefore, future research, should using pure T2D model animals to independently evaluate the correlation between microglial activation and polarization and p-Tau protein, thereby advancing in-depth research on T2D-related cognitive impairment.Abbreviations: Aβ, amyloid-β; APP, amyloid precursor protein; Iba1, ionized calcium binding adapter molecule 1; p-Tau, phosphorylated Tau.

Summary and discussion
Through a comprehensive review of previous studies and extensive consideration, we formally present our team's scientific viewpoint, as shown in Fig. 4. We believe that in the context of T2D, high blood sugar causes stress damage to the CNS.This stress damage activates a large number of microglia, causing them to predominantly convert to the M1 phenotype, which in turn leads to the release of a large number of proinflammatory factors (such as IL-1β and TNF-α).The excessive release of these pro-inflammatory factors further stimulates microglia, forming a vicious cycle.
At the same time, the activation and polarization of microglia and the release of pro-inflammatory factors also lead to the secretion of a large number of pro-apoptotic factors, thereby inducing neuronal apoptosis (Singh et al., 2024).Additionally, accompanying the activation and polarization of microglia, a large amount of Aβ protein is produced in the brain.The excessive accumulation of Aβ and its deposition around neurons ultimately form amyloid plaques, leading to neuronal damage, impairing normal neuronal signal transmission and function, and possibly causing inflammatory responses around neurons   et al., 2023) Table 5 The correlation between microglial activation-mediated abnormal phosphorylation of Tau protein and T2D-related cognitive impairment.and neuronal loss.Additionally, during the disease progression, the abnormal accumulation of Aβ forms amyloid plaques, which may lead to the abnormal phosphorylation of Tau protein, transforming it from a normal microtubule-associated protein to an aggregated, phosphorylated form (Ittner and Gotz, 2011).Due to the abnormal phosphorylation of Tau protein, neurofibrillary tangles (NFTs) form within neurons, which is another significant pathological feature of cognitive impairment.NFTs can disrupt the normal structure and function of neurons, leading to neurodegenerative changes.On the other hand, during the development of T2D, microglia can polarize towards the M2 phenotype under the mediation of related signaling pathways (such as STAT1, STAT6, PPAR-γ, and TLRs), or by inhibiting CNS inflammatory responses, or by reducing oxidative stress damage, thereby secreting anti-inflammatory factors (Bi et al., 2023;Cherry et al., 2014;Hsu et al., 2023;Joshi et al., 2019;Li et al., 2023a).Meanwhile, the release of anti-inflammatory factors, such as IL-4 and IL-10, can also feedback to microglia, inducing the polarization towards the M2 phenotype.This forms a positive cycle, which may exert an ideal protective effect against T2D-related cognitive impairment (Cherry et al., 2014;Joshi et al., 2019).

Preclinical research
Regarding future research, in the preclinical stage, our team has clearly elaborated on the key role of insulin-degrading enzyme (IDE) in the pathogenesis of diabetic cognitive impairment in previous reviews.Specifically, IDE has the important characteristic of degrading Aβ, a feature that may exert protective effects against diabetic cognitive impairment through various pathways (Tian et al., 2023).In light of the perspectives put forward in this review, future basic research could focus on exploring the potential relationships between IDE, microglia, Aβ, and p-tau.Using IDE-related pathways as a reference, researchers could investigate whether microglia are associated with T2D-related cognitive impairment through these pathways, This approach would refine the theoretical understanding of the pathogenesis of T2D-related cognitive impairment, identify potential therapeutic targets and strategies, and provide comprehensive theoretical support for the prevention and treatment of T2D-related cognitive impairment.

Clinical therapy and translation
Considering the importance of microglia in the neuroinflammatory processes associated with T2D-related cognitive impairment, future clinical treatment and translation efforts should focus on targeted therapies for microglia:

Anti-inflammatory drug development
Once activated, microglia can release pro-inflammatory cytokines, leading to neuroinflammation (Qian et al., 2024).Existing anti-inflammatory drugs are not particularly effective against the inflammatory response in the CNS under T2D conditions.Therefore, there is an urgent need to develop more targeted central anti-inflammatory drugs, and targeting microglia to suppress inflammatory responses is a viable therapeutic approach.

Microglial phenotype polarization regulation
The pro-inflammatory M1 phenotype and the anti-inflammatory M2 phenotype of microglia interact during the pathogenesis of T2D-related cognitive impairment.Promoting the polarization of microglia towards the M2 phenotype can enhance neuroprotection.For instance, the activation of the PPAR-γ pathway has been shown to promote M2 polarization (Isali et al., 2021).Additionally, minocycline can promote the M2 polarization and inhibit the M1 polarization of microglia by modulating the STAT1/STAT6 pathways, thereby aiding in neuronal survival and the recovery of neurological function (Lu et al., 2021).Future research can delve into the pathways that promote the polarization of microglia towards the M2 phenotype, develop specific regulatory drugs, target the M2 phenotype polarization of microglia, and promote clinical translation.

Targeting microglial receptors
Microglia express a variety of receptors, such as toll-like receptors (TLRs) and fractalkine signaling pathway receptors, which play important roles in microglia activation (Paolicelli et al., 2014;Rodriguez et al., 2024;Wu et al., 2023).The development of corresponding receptor antagonists can specifically inhibit the over-activation of microglia under stress and injury conditions, reducing the release of inflammatory factors and the occurrence of inflammatory responses, thereby exerting an ideal neuroprotective effect.

Application of AGEs inhibitors
In the T2D state, the accumulation of AGEs can activate microglia, and excessive accumulation of AGEs can lead to adverse outcomes such as oxidative stress and neuronal damage (Zhou et al., 2024).Inhibiting the formation of AGEs or their interaction with the RAGE can reduce microglia over-activation and prevent excessive inflammatory responses in the CNS under hyperglycemic conditions, thereby providing an ideal protective effect against T2D-related cognitive impairment.

Stem cell therapy
Studies have shown that mesenchymal stem cells (MSCs) influence most immune cells through direct contact or by secreting positive microenvironmental factors.For example, MSCs can affect the immune regulatory function by modulating the polarization state of microglia (Yang et al., 2023).This has significant potential for the clinical treatment and translation of T2D-related cognitive impairment.

Gene therapy
Gene therapy is a cutting-edge treatment method that delivers specific genes into a patient's cells using viral or non-viral vectors to regulate or repair abnormal gene expression.This approach holds significant potential in modulating the function of microglia.Viral vectors, such as retroviruses, adenoviruses, adeno-associated viruses, and recombinant adeno-associated viral vectors, are widely studied due to their high transfection efficiency (Lin et al., 2022).These viral vectors can effectively deliver therapeutic genes to the target cells.However, using viral vectors also comes with certain risks, including insertional mutagenesis and immune responses.In contrast, non-viral vectors offer an alternative approach.They have lower immunogenicity, higher safety and stability, and are easy to produce on a large scale.In modulating the function of microglia, gene therapy can deliver genes involved in anti-inflammatory pathways using specific vectors.For instance, gene editing through the clustered regularly interspaced short palindromic repeats/clustered regularly interspaced short palindromic repeats-associated protein 9 can specifically activate or inhibit certain genes in microglia, thereby regulating their immune response (Chang et al., 2023;Yang et al., 2024).Furthermore, microRNAs delivered via non-viral vectors can also be used to regulate the polarization state of microglia, affecting the onset and progression of neuroinflammatory-related diseases (Paunovska et al., 2022).Upon careful consideration, gene therapy offers numerous feasible approaches for treating T2D-related cognitive impairment by targeting microglia in the future.

Conclusion
In this review, we summarized research on the activation and polarization of microglia in models of T2D-related cognitive impairment.We concluded that under the influence of high blood sugar levels, activated and polarized microglia in the CNS are closely associated with inflammatory factors, apoptosis genes, Aβ, and p-Tau proteins.Their abnormal accumulation and interactions lead to neuronal damage, resulting in cognitive decline.Inhibiting the excessive activation of microglia, downregulating M1 polarization, and upregulating M2 polarization may represent an ideal research direction for improving T2Drelated cognitive impairment.

Fig. 3 .
Fig. 3. Release of inflammatory and apoptotic factors mediated by microglial activation and polarization in T2D.Under T2D conditions, influenced by hyperglycemia, microglia in the T2D model brain are activated and primarily polarized to the M1 phenotype, releasing pro-inflammatory factors such as IL-1β, IL-17A, and TNF-α, and activating apoptosis-related genes such as Bax, caspase-3, and caspase-9.Among these, TNF-α can activate Bax, and Bax activation can lead to changes in mitochondrial membrane permeability, releasing Cytochrome C, which in turn activates apoptosis-related factors such as caspase-3, leading to neuronal apoptosis and ultimately resulting in cognitive impairment.Abbreviations: Bax, B-cell lymphoma 2-associated X protein; CNS, central nervous system; IL-1β, interleukin 1β; IL-17A, interleukin 17 A; TNF-α, tumor necrosis factor α.

Fig. 4 .
Fig. 4. Mechanisms of T2D-related cognitive impairment mediated by microglial activation and polarization.In the T2D environment, hyperglycemia causes stress damage to the CNS, activating microglia and polarizing them towards the M1 phenotype.The polarization of the M1 phenotype leads to the release of large amounts of pro-inflammatory and pro-apoptotic factors, with pro-inflammatory factors providing feedback stimulation to microglia, creating a loop.Additionally, microglial activation and polarization are accompanied by a large accumulation of Aβ protein in the brain, eventually forming amyloid plaques.These plaques further lead to abnormal phosphorylation of Tau protein, forming NFTs, and ultimately causing neuronal apoptosis.Under the conditions of pathway mediation, inhibition of inflammatory response, and reduction of oxidative stress damage, microglia can be promoted to polarize towards the M2 phenotype, secreting anti-inflammatory factors such as IL-4 and IL-10.These anti-inflammatory factors can further induce microglial polarization to the M2 type, creating a positive cycle that may alleviate T2D-related cognitive impairment and exert an ideal neuroprotective effect.Abbreviations: Aβ, amyloid-β; Bax, B-cell lymphoma 2-associated X protein; CNS, central nervous system; IL-1β, interleukin 1β; IL-4, interleukin 4; IL-10, interleukin 10; NFTs, neurofibrillary tangles; PPAR-γ, peroxisome proliferator-activated receptor gamma; p-Tau, phosphorylated Tau; STAT1, signal transducer and activator of transcription 1; STAT6, signal transducer and activator of transcription 6; TLRs, toll-like receptors; TNF-α, tumor necrosis factor α.

Table 2
The correlation between microglial M1 and M2 phenotypes and T2D-related cognitive impairment.

Table 4
Correlation between Aβ deposition mediated by microglial activation and T2Drelated cognitive impairment.