Vascular diseases are always related to the cognitive impairment (Duncombe et al., 2017; Tang et al., 2021; Ungvari et al., 2021). More and more evidence show that vascular risk factors can lead to neurodegeneration, cognitive impairment and dementia (Duncombe et al., 2017). Certain areas of the brain responsible for the memory, cognition, and behavior are particularly vulnerable to insufficient blood supply (Thammisetty et al., 2021). Chronic cerebral hypoperfusion (CCH) often occurs due to diseases that affect the cerebral circulatory system, such as hypertension, diabetes, atherosclerosis and smoking, and is one of the prime factors leading to the development of vascular cognitive impairment and dementia (VCID) in the elderly (Thammisetty et al., 2021). CCH is also considered as a preclinical condition of mild cognitive impairment and precursor of dementia (Xu et al., 2020). In addition, CCH is a common and important cause of cognitive impairment associated with many neurodegenerative diseases, such as Alzheimer’s disease (AD) and Parkinson’s disease, and it plays an indispensable role in the development of these diseases (Tang et al., 2017; Xie et al., 2018; Feng et al., 2021). Recent studies conducted by the American AD Research Center revealed that up to 80% of the sporadic and late onset AD have some form of brain vascular pathology (Duncombe et al., 2017; Xu et al., 2020). The inflammation, oxidative stress, and amyloid-β (Aβ) accumulation commonly existed in AD pathology have been reported to be associated with CCH. In addition, CCH was found to appear in the “early” stage of AD. Before the obvious symptoms of AD appear, arterial spin-labeled magnetic resonance imaging has detected changes in cerebral blood flow, indicating that CCH may be a potential biomarker of AD (Duncombe et al., 2017; Xu et al., 2020). In general, it has been demonstrated that there is considerable overlap between the characteristics of VCID and AD. CCH seems to be the common underlying pathophysiological mechanisms, and it is the main contributor to cognitive decline and degenerative development of dementia (Duncombe et al., 2017; Xu et al., 2020).

It is well known that neuroinflammation is considered to be the important pathogenic mechanism of CCH-induced cognitive impairment (Du et al., 2020). Considerable evidence shows that CCH can promote the occurrence and progression of cognitive impairment and dementia through neuroinflammation (Kim et al., 2017). Astrocytes are the most abundant cells and play multiple roles in the central nervous system (CNS). It has been confirmed that astrocytes participate in the immune response of the CNS, and are potential sentinels in the brain parenchyma together with microglia (Leardini-Tristao et al., 2020). In addition, astrocytes are not only involved in the regulation of neurotransmission, neurodevelopment, cerebral blood flow, metabolism and neurogenesis, their coverage integrity but also related to the brain homeostasis and cognition (Leardini-Tristao et al., 2020; Miyamoto et al., 2020). Recent studies have confirmed that astrocytes have two phenotypes, the neurotoxic A1 and neuroprotective A2 phenotype (Liddelow et al., 2017). However, astrocytes activated after CCH mainly transform to A1 phenotype, which lose their normal function and aggravate the neuroinflammation progression (Miyamoto et al., 2020; Jiang et al., 2021). On the one hand, the loss of normal A1 astrocytes function and the interruption of their interaction with blood vessels may lead to homeostasis destruction of neurovascular units and cognitive impairment (Leardini-Tristao et al., 2020; Presa et al., 2020). On the other hand, it has been shown that the neuroinflammatory processes aggravated by the activated neurotoxic astrocytes are essential for the initiation and progression of cognitive disorders (Kim et al., 2017). Therefore, modulating the polarization of activated astrocytes from neurotoxicity to neuroprotective phenotype may be one of the potential therapeutic targets for CCH-induced cognitive impairment.

There is evidence that primary cilia play important roles in regulating inflammation, and the disturbance of primary cilia may lead to gliosis and neuroinflammation (Singh et al., 2019). Primary cilia are essential for the development and maintenance of neural homeostasis in the mammalian brain (Sipos et al., 2018). They act as the “sensing antenna” of cells and are the central hub for receiving and transducing extracellular signals to elaborate biological responses to a wide range of developmental and physiological processes. Primary cilia always play key roles in cell cycle control, cell proliferation, migration, and polarity (Alvarez-Satta et al., 2019; Alhassen et al., 2021). Additionally, large numbers of convincing studies have shown that there are strong correlations between primary cilia and cognition (Hu et al., 2017; Alhassen et al., 2021). Defects in primary cilia can lead to age-related cognitive decline, and primary cilia may be potential targets for AD and/or other dementia (Alvarez-Satta et al., 2019). Most mammalian cells have primary cilia, including astrocytes (Sipos et al., 2018). In response to various brain injuries, such as traumatic brain injury, ischemia, and neurodegenerative diseases, astrocytes proliferation is coordinated with the ciliogenesis and dismantling of astrocytes cilia (Sterpka and Chen, 2018). A study has reported that primary cilia dysfunction caused by congenital deletion of BBSome protein 8 can induce A1-like astrocyte activation, neuroinflammation, and changes in postsynaptic density (Singh et al., 2019). Primary cilia are an important component of the sonic hedgehog (Shh) signaling pathway, and the activation of the latter can promote astrocyte polarization toward to A2 phenotype (Payan-Gomez et al., 2018). These studies indicate that primary cilia are involved in the regulation of the A1/A2 phenotypic polarization of astrocytes. At present, little is known about the physiological and pathological functions of astrocytic primary cilia (Sterpka et al., 2020). Further exploration of the role of primary cilia in reactive astrocytes will not only help to better understand the regulatory mechanism of astrocytes polarization, but also to develop potential therapeutics for cognitive disorders.

Over the years, there have been many studies on the benefits of physical exercise. A lot of scientific evidences show that there is a positive correlation between active sports lifestyle and health benefits (De la Rosa et al., 2020; Marques-Aleixo et al., 2021; Padilha et al., 2021). Physical exercise is an effective non-toxic strategy to prevent and treat many chronic diseases. Its positive effects in reducing the risk of all-cause death and extending life span have been widely documented (Scheffer and Latini, 2020). More and more evidences showed that physical exercise, including aerobic and resistance exercise training, can alleviate age-related cognitive impairment (Brach et al., 2021; Chow et al., 2021; Pronk, 2021). Physical exercise can promote different physiological phenomena including neurogenesis, synaptogenesis, angiogenesis, and neurotrophic factor stimulation by inducing a series of cellular and molecular processes, thereby causing changes in the brain at the cellular, molecular and anatomic level, and eventually enhancing learning, memory and brain plasticity (De la Rosa et al., 2020). Additionally, the beneficial effects of physical exercise on health outcomes also involve the regulation of the immune system. Regular physical exercise can prevent viral and bacterial infections and enhance the immune response to vaccines and pathogens. This makes physical exercise a valuable tool for preventing infectious diseases such as COVID-19 (Scheffer and Latini, 2020). Moreover, studies have shown that physical inactivity favors promoting the polarization of immune cells toward a pro-inflammatory phenotype. In contrast, regular moderate-intensity physical exercise can direct the immune response to an anti-inflammatory state, which is considered to be the main molecular mechanism required for the improvement of health outcomes (Scheffer and Latini, 2020). Our previous study also found that physical exercise can regulate the polarization of astrocytes from pro-inflammatory A1 to neuroprotective A2 phenotype, and improve the cognitive impairment of rats with CCH (Jiang et al., 2021).

In this study, we will investigate the effects of physical exercise on primary ciliogenesis and astrocyte polarization in rats with CCH. In addition, we will further investigate the role of MAPKs signaling cascades, including ERK, JNK, and P38 signaling pathways in this process.

Physical exercise has beneficial effects on both peripheral tissues and CNS, and is a kind of safe and effective intervention to improve cognitive function (Maugeri et al., 2021). In human and animal models, the multiple benefits of regular physical exercise have been fully demonstrated. Whether aerobic exercise, anaerobic exercise or resistance exercise, are all considered to be beneficial to physical and mental health (Xu et al., 2021). More and more evidence showed that physical exercise can help maintain optimal cerebrovascular function, thereby preventing or slowing down the occurrence and development of cognitive impairment (Bliss et al., 2021). In this study, we found that wheel running exercise for 2 months can improve the spatial memory and non-spatial memory of CCH rats (Figures 1B, 2A–E). This is consistent with our previous study, which found that physical exercise alleviated the cognitive impairment of 2VO rats, and the effects tended to be more pronounced as the duration of exercise increased (Jiang et al., 2021). Similarly, some studies have shown that long-term physical exercise has positive effects through delaying the onset of physiological memory loss. However, it is worth mentioning that the late-onset exercise intervention has also shown positive impacts in delaying brain aging (De la Rosa et al., 2020). At present, the effects of pharmacological treatment for patients with cognitive impairment and dementia are limited, which not only brings a huge economic burden to society, but also seriously affects the quality of life of families facing such conditions. Since physical exercise has significant beneficial effects on the cognitive function of different diseases, it has aroused a lot of research interests as a kind of safe and effective non-drug treatment (Chan et al., 2021; Consorti et al., 2021). Physical exercise cannot only be used as a therapeutic protocol for neurological diseases, but also help to unlock the potential molecular targets of pharmacological methods (Consorti et al., 2021). Recently, exercise mimetics have been proposed as a class of therapeutics that can specifically mimic or enhance the therapeutic effects of physical exercise. Clarifying the mechanism mediating the beneficial impacts of exercise at the molecular, cellular and system levels is essential for the development of novel exercise mimetics (Gubert and Hannan, 2021). However, the underlying mechanism by which exercise may improve cognition remains unclear.

Recently, more and more studies have shown that the changes in astrocytes may be a key mechanism accounting for the improvement of cognitive and executive functions related to exercise (Li et al., 2021; Maugeri et al., 2021). Astrocytes are the main homeostasis and defense elements of the brain, helping to maintain cognitive reserve through a variety of mechanisms (Verkhratsky et al., 2021). They can communicate with neurons and other astrocytes in a bidirectional manner by releasing transmitters, increasing synaptic activity and strength, and play an active role in storing and processing information in the brain (Maugeri et al., 2021). It has been shown that the multiple effects of physical exercise on astrocytes include the number of new astrocytes increase, the basal level of catecholamines maintenance, the glutamate uptake enhance, the trophic factors release, and the better astrocytic coverage of cerebral blood vessels (Maugeri et al., 2021). In this study, we found that the astrocytes were activated in the brain of rats 2 months after 2VO, and they were mainly polarized toward the neurotoxic A1 phenotype. While physical exercise can switch the polarization of astrocytes from neurotoxic A1 to neuroprotective A2 phenotype (Figures 3A–C, 4A–C). Our previous study also reported that physical exercise cannot only modulate the transition of microglia from M1 to M2 phenotype, but also regulate the phenotypic polarization of astrocytes (Jiang et al., 2021). Consistently, similar studies have revealed that the low intensity motor balance and coordinated exercise can suppress the phenotypic polarization of M1 microglia and A1 astrocytes in the hippocampus of AD mice, contributing to the inhibition of Aβ accumulation and counteraction of behavioral and cognitive decline (Nakanishi et al., 2021). Recent studies have also emphasized the importance of physical activity as an intervention to prevent development of a severe form of COVID-19, as exercise can suppress excessive immune responses and trigger anti-inflammatory functions to promote psychological health (Maugeri et al., 2021). Interestingly, studies have shown that astrocytes may be the perspective targets of exercise mimetics (Lalo and Pankratov, 2021). In general, astrocytes are the central components for physical exercise-induced cognitive improvement in many neurodegenerative and neurovascular diseases (Li et al., 2021). The modulation of astrocytes polarization from neurotoxic A1 to neuroprotective A2 phenotypes may be an important mechanism involved.

As mentioned before, studies have indicated that primary cilia are involved in the regulation of the A1/A2 phenotypic polarization of astrocytes. It is well known that primary cilia can regulate astrocyte proliferation, polarization and tissue regeneration (Sterpka et al., 2020). Since primary cilia originate from the maternal centrioles, astrocytes proliferation must be coordinated with the dismantling and ciliogenesis of astrocytic cilia (Sterpka and Chen, 2018). Studying astrocytic primary cilia will provide useful clues for intervention in reactive astrocytes to combat various neuropathologies such as brain injury and ischemia (Sterpka and Chen, 2018). Primary cilia can be used as sensors for chemical and mechanical cues from cellular environment and play critical roles in transducing environmental stimuli and concentrating signal molecules to regulate cellular properties (Sun et al., 2021; Tereshko et al., 2021). However, current research about the effects of physical exercise on primary cilia is still limited. In this study, we found that the average length of primary cilia in the corpus callosum varied between 2.62 ± 0.33 μm in the sham group. Compared with the sham group, the length of astrocytic primary cilia in rats with CCH was shortened. In addition, the incidence of cilia also showed a decreasing trend, but there was no significant difference. However, physical exercise can significantly increase the expression and length of astrocytic primary cilia in the brain of rats 2 months after 2VO (Figures 5A–C). The length of primary cilia is closely related to their functions. Studies have found that primary cilia in hypocretin/orexin cells of obese mice were shorter, resulting in losing their ability to sense the local environment in the lateral/perifonical hypothalamus (Tan et al., 2020). Similarly, the shortened Arl13b-positive astrocytic cilia can be observed in the hippocampus of mice with spontaneous seizures (Sterpka et al., 2020). Whereas the ciliary length elongation has been proposed to fine-tune signaling activity of organelles in response to changes in the extracellular environment. The longer the cilia, the more functional they are and the greater capacity for signaling transduction in their internal receptor system (Brodsky et al., 2017; Sterpka et al., 2020). Therefore, astrocytic primary cilia are shortened after CCH, which may impair the ability of astrocytes to maintain normal function and perceive local damage environment, leading astrocytes polarization primarily toward the neurotoxic A1 phenotype. In contrast, physical exercise can rescue this negative impact and modulate astrocytes polarization to neuroprotective A2 phenotype by promoting primary cilia expression and elongation.

In order to clarify the possible mechanisms involved in the regulation of primary ciliogenesis and astrocyte polarization by physical exercise, we further observed the expression of mitogen-activated protein kinases (MAPKs) cascades. The MAPKs cascades mainly include JNK, P38, and ERK signaling pathways. They are involved in the regulation of cell proliferation, differentiation, migration, apoptosis and senescence, and are core components in signaling networks in various mammalian cells (Asih et al., 2020). Additionally, MAPKs are also the main effectors of inflammatory and mechanical stress (He et al., 2016). In this study, we found that compared with the sham group, the ERK and JNK signaling pathways in the brain of rats with CCH were activated, and physical exercise significantly reduced the expression of phosphorylated ERK and JNK, but without significant effects on the expression of the phosphorylated P38 (Figures 6A–D). These results are consistent with our previous study. We previously revealed that physical exercise can reduce ERK and JNK phosphorylation in the brain of rats 28 days after 2VO and promote microglia polarization from neurotoxic M1 to neuroprotective M2 phenotype, and finally promote remyelination and cognition improvement (Jiang et al., 2021). These findings suggest that the ERK and JNK signaling pathways may be the important effectors of exercise-induced anti neuroinflammatory responses. The inhibition of MAPKs activation, such as ERK, JNK, or P38, cannot only reduce inflammatory factors released from astrocytes and prevent astrocytes forming neurotoxic activity, but also increase neurotrophic factors release, maintain stable expression of glutamine synthetase from astrocytes, and promote neuroprotective effects of astrocytes (Efremova et al., 2017; Li et al., 2017; Liu et al., 2017; Wang et al., 2018, 2021; Zhou et al., 2019). Studies have also shown that mechanical signals sensed by primary cilia were transduced into biochemical signals involving pathways including ERK, JNK, and P38 (He et al., 2016). In some ciliopathy and cystic disease, MAPKs are important effector pathways closely associated with the process of ciliogenesis (Ma et al., 2013). Therefore, physical exercise may reduce the activation of ERK and JNK signaling pathways through regulating astrocytic primary cilia, and promote the activated astrocytes polarization from neurotoxic A1 to A2 phenotype to exert neuroprotective effects.

In summary, we have provided new evidence for clarifying the mechanism of physical exercise induced cognitive function recovery in CCH. Physical exercise can promote reactive astrocytes to exert neuroprotective roles by enhancing primary cilia length and ciliogenesis, and finally improve the cognitive function of rats with CCH. Moreover, the JNK and ERK signaling pathways are at least partly involved in this regulatory process (Figure 7).

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding authors.

The animal study was reviewed and approved by Institutional Animal Care and Use Committee of Wuhan University Center for Animal Experiment.

TJ and WJL conceived and designed the experiments. WYC, JYL, and BRW performed research and data acquisition. JBL and WYC analyzed and interpreted data. TJ and WX wrote the manuscript. WJL, WX, and JBL reviewed and edited the manuscript. All authors read the submitted version of this article and approved it for publication.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.