The pathogenesis involved in age-related cognitive impairment is somewhat complex and remains unclear. Though progressive accumulation of misfolded protein, like amyloid-beta (Aβ) within plaques, is considered a critical mechanism in most age-related cognitive decline diseases, the pathogenesis of production and clearance of Aβ remains to be investigated (Bloom, 2014; Upadhaya et al., 2012). Emerging evidence indicated that the “glymphatic” system dependent on the polarization of aquaporin 4 (AQP4) expressed at high density on the astrocytic end feet is a novel and practical pathway for the clearance of neurotoxic molecules and waste metabolites like Aβ (Arbel-Ornath et al., 2013; Harrison et al., 2020; Palazzo et al., 2019). However, it may be markedly disrupted by several factors like systemic inflammation (Mogensen et al., 2021). A growing body of evidence demonstrates that the gut–brain axis (GBA) is involved in regulating the progression of neurodegenerative diseases, including age-related cognitive impairment disease (Cheng et al., 2022; Toledo et al., 2022; Ambrosini et al., 2019). Patients with inflammatory bowel diseases (IBDs) are associated with higher dementia risk compared to age-matched individuals without IBDs (Zhang et al., 2021; Hopkins et al., 2021). Furthermore, IBDs may advance the average onset age of dementia in aged people (Zhang et al., 2021). Peripheral intestinal inflammation may exacerbate the neuropathological process of age-related cognitive impairment by secreting pro-inflammatory cytokines, disrupting the Blood-Brain Barrier (BBB), and disturbing the composition and diversity of intestinal microbiota, etc (El-Hakim et al., 2022). Our earlier research indicated that chronic colitis induced by dextran sodium sulfate (DSS) exacerbated neuroinflammation and cognitive impairment in aged mice in an NLRP3-dependent manner (He et al., 2021). Meanwhile, it is accompanied by depolarization of AQP4 and impaired glymphatic clearance function (He et al., 2021). Glymphatic clearance efficiency decreases during aging (Benveniste et al., 2019). A dynamic imaging study demonstrated a widespread impairment of glymphatic transport in the aged brain of rats compared to the young brain (Li et al., 2022). Therefore, it is of great importance to investigate both the mechanism involved in impaired glymphatic clearance function and effective interventions promoting glymphatic system clearance in the aged brain. Except for the microglia-mediated inflammatory response, the role of astrocytes, essential for the glymphatic system and BBB integrity, in regulating neuroinflammation has attracted great attention (Feng et al., 2023). Various studies demonstrated that the pro-inflammatory phenotype of astrocyte (A1-like astrocyte) could augment the inflammatory response, resulting in the loss of AQP4 polarization (Feng et al., 2023; Iliff et al., 2014). However, the mechanism by which reactive astrocytes impair AQP4 polarization and glymphatic clearance function remains to be further investigated. Moreover, a lack of therapeutic intervention targets the modulation of astrocyte-mediated inflammation and AQP4 & glymphatic system.

Studies have confirmed that physical exercise (PE) alleviates neurological function deficits in age-related cognitive impairment patients and animals (De la Rosa et al., 2020; Valenzuela et al., 2020; Guitar et al., 2018; Yang et al., 2022; Wu et al., 2020). Though the mechanism remains unclear, it is believed that the role of exercise is multi-target, including alleviation of neuronal loss, modulation of neuroinflammation, promotion of misfolded protein clearance, and increased synaptic plasticity, etc (De la Rosa et al., 2020). Voluntary exercise is considered a critical exercise model with high accessibility. Our previous study demonstrated that voluntary wheel exercise promoted glymphatic clearance of Aβ and modulated neuroinflammation in aged mice, accompanied by alleviation of cognition impairment and anxiety-like behavior (He et al., 2017). AQP4 knockout abolished the effects of alleviating Aβ deposition and perivascular AQP4 mislocalization induced by voluntary wheel exercise in APP/PS1 mice (Liu et al., 2022), suggesting the critical role of the AQP4-dependent glymphatic system in mediating exercise's effect for AD. A recent study indicates that high-intensity interval training ameliorates Alzheimer's disease-like pathology by modulating astrocytic phenotype transformation and astrocytic phenotype-associated AQP4 polarization, and thus accelerates clearance of misfolded protein in the brain through the glymphatic system and the kidney (Feng et al., 2023). However, the mechanisms by which voluntary exercise promotes AQP4 polarization, glymphatic clearance of Aβ, as well as modulates phenotype transformation of astrocytes in chronic colitis mice should be further investigated.

A variety of studies indicated that TRPV4, a calcium-permeable non-selective cation channel that functions as an environmental sensor, could form a compound with AQP4 and modulate its subcellular structure distribution via TRPV4/CaM/AQP4 signal pathway (Benfenati et al., 2011; Jo et al., 2015). Mechanosensitive TRPV4 channel promotes calcium ions influx into astrocytes and activates CaM, which interacts with adenylate cyclase, and thus activates protein kinase A (PKA) in a cyclic adenosine phosphate (cAMP)-dependent manner (Kitchen et al., 2020). AQP4 phosphorylation by PKA relocates it to the cell membrane, leading to subcellular relocalization of AQP4 (Kitchen et al., 2020). Moreover, it has been proposed that TRPV4 blockade preserves the BBB integrity (Zhao et al., 2018), suggesting its potential role in peripheral inflammation-induced BBB disruption. Whether exercise promoted AQP4 polarization via modulation of the TRPV4 pathway is unknown.

Hence, the present study investigates whether and how TRPV4 is implicated in hyperreactive calcium activity of astrocytes in aged mice with chronic colitis, plays a critical role in voluntary exercise-promoted AQP4 polarization, glymphatic clearance of Aβ, as well as modulation of astrocytic phenotype transformation. We found that voluntary wheel exercise alleviated Aβ deposition in the brains of aged mice with colitis, which was also accompanied by the protection of neurons, increased synaptic plasticity, modulation of astrocytic phenotype transformation, as well as attenuation of cognitive decline and anxiety-like behavior. Furthermore, voluntary wheel exercise-mediated alleviation of Aβ deposition was partly attributed to the promotion of AQP4 polarization via inhibition of TRPV4-related calcium activity in astrocytes and consequently enhanced efficiency of glymphatic clearance, as voluntary exercise remained unable to promote AQP4 polarization in chronic colitis mice treated with TRPV4 activator, GSK1.