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Mechanosensitive Piezo1 channel regulation of microglial cell function and implications to neurodegenerative diseases and neuroinflammation

2023; Medknow; Volume: 18; Issue: 11 Linguagem: Inglês

10.4103/1673-5374.371355

1876-7958

Mo Zhang, Philippa Malko, Lin‐Hua Jiang,

Neuroinflammation and Neurodegeneration Mechanisms

Microglial cells are the key immunocompetent cells in the central nervous system (CNS) and play a crucial role in CNS health and disease (Paolicelli et al., 2022). Under the homeostatic conditions, microglial cells assume diverse and dynamic states, depending upon interactions with neighboring cells and structures in local contextual settings, continuously patrol brain parenchyma utilizing their highly mobile fine processes, phagocytize protein aggregates, unwanted synapses and cells to maintain CNS health, and secrete neurotrophic factors to support neuronal function (Colonna and Butovsky, 2017; Paolicelli et al., 2022). In response to damage or infection, microglial cells change to different functional states, in many cases accompanied with distinct morphology characterized by an enlarged cell body with short and thick processes. Mechanistically, such changes in the states of microglial cells, commonly referred to microglial activation, are initiated upon ligation of pattern recognition receptors to damage-associated molecular patterns from host cells, or pathogen-associated molecular patterns from invading pathogens, and enable microglial cells to coordinate an immune response to reinstate CNS homeostasis (Colonna and Butovsky, 2017; Paolicelli et al., 2022). However, incomplete resolution of the immune response leading to chronic inflammation or, more specifically, unwanted production of neurotoxic inflammatory mediators that can damage synapses and neurons and thereby impair CNS function. Such microglial cell-mediated action is an important factor driving the progression of age-related neurodegenerative diseases, including Alzheimer's disease (AD) (Takata et al., 2021). The brain represents one of the softest tissues in the body, but its stiffness can change considerably during development and also as a result of AD progression (Budday et al., 2019; Hu et al., 2023). Brain tissue associated with amyloid plaques, one of the histopathological hallmarks of AD, is substantially stiffer than plaque-free tissue. Microglial cells are equipped with a striking capacity of durotaxis, that is, a preference for migrating towards stiff brain tissue, which enables them to form a physical barrier to restrict neuronal loss (Bollmann et al., 2015). However, it remained elusive how microglial cells sense the mechanical properties of their surrounding microenvironments. Piezo1, a mechanically activated Ca2+-permeable cation channel (insert in Figure 1), has been recognized to serve as part of an evolutionarily conserved intrinsic mechanism that enables cells to detect diverse forms of mechanical forces and modulate cell function accordingly (Wu et al., 2017; Xiao et al., 2020). In this perspective, we highlight recent findings that the Piezo1 channel is a previously unrecognized mechanism that regulates microglial cell function and raises the possibility of targeting the Piezo1 channel to restore the homeostatic states and function of microglial cells as a therapeutic strategy to delay or avert AD progression.Figure 1: Piezo1 channel regulation of microglial cell functions and implications to AD pathogenesis and inflammation.(A) Exposure to stiff amyloid-β (Aβ) fibrils in AD-associated amyloid plaques or Yoda1 activates the Piezo1 channel in microglial cells, which enhances the capacity of migration (and clustering around amyloid plaques, not shown) and phagocytosis of Aβ peptides, thereby attenuating Aβ accumulation and amyloid plaque formation and improving cognitive function. (B) Yoda1-induced Piezo1 channel activation mediates extracellular Ca2+ influx to increase intracellular Ca2+ concentration, which inhibits lipopolysaccharide (LPS)-induced activation of the nuclear factor (NF)-κB signaling pathway that drives the production of neurotoxic proinflammatory cytokines, tumor necrosis factor (TNF)-α and interleukin (IL)-6. See text for more details. Insert: Schematic illustration of activating the Piezo1 channel on cell surface by mechanical force or Yoda1 that mediates Ca2+ influx.Piezo1 channel expression in microglial cells: Several recent studies examined the expression of the Piezo1 channel in microglial cells, using mouse BV2 microglial cells (Liu et al., 2021; Jäntti et al., 2022; Malko et al., 2023), primary mouse microglial cells (Jäntti et al., 2022; Hu et al., 2023; Malko et al., 2023), human SV40 microglial cells and human inducible pluripotent stem cell-derived microglia-like (iMGL) cells (Jäntti et al., 2022). Piezo1 mRNA and/or protein expression was consistently detected in all these cell preparations. As demonstrated in human iMGL cells, membrane pressure induced by puff or membrane stretch induced in hypo-osmotic solution evoked an increase in intracellular Ca2+ concentration (Jäntti et al., 2022). Exposure to Piezo1 channel-specific agonist Yoda1 also raised intracellular Ca2+ concentration in human iMGL cells, BV2 cells and primary mouse microglial cells (Liu et al., 2021; Jäntti et al., 2022; Malko et al., 2023) and, more specifically, Yoda1-induced increases in intracellular Ca2+ in BV2 cells and primary mouse microglial cells were dependent on extracellular Ca2+ influx (Malko et al., 2023). Furthermore, mechanically or chemically induced Ca2+ responses were inhibited by prior treatment with ruthenium red or GsMTx4, which are known to inhibit the mechanosensitive channels including the Piezo1 channel, or small interference RNA (siRNA) to knock down Piezo1 expression (Liu et al., 2019; Jäntti et al., 2022; Malko et al., 2023). Collectively, these studies provide strong evidence to support that the Piezo1 channel is expressed in human and mouse microglial cells as a Ca2+-permeable channel on the cell surface that mediates mechanically or chemically induced Ca2+ signaling. It is perhaps worth mentioning that in both human iMGL cells and primary mouse microglial cells, the Piezo1 protein was detected in the nuclei by immunofluorescent imaging (Jäntti et al., 2022), an interesting observation that however requires further validation and investigation for the functional implication. Amyloid β 42 (Aβ42) fibrils are the key and highly neurotoxic component of amyloid plaques that display greater stiffness than healthy brain tissue. Exposure of primary mouse microglial cells cultured on soft hydrogels to insoluble Aβ42 fibrils (2 µM) or Yoda1 enhanced the amplitude and frequency of Ca2+ transients (Hu et al., 2023). Exposure to Aβ42 fibrils or Yoda1 also induced ionic currents in primary mouse microglial cells, which were abolished by prior treatment with ruthenium red or GsMTx4 (Hu et al., 2023). However, as shown in a separate study, Yoda1-induced Ca2+ response was reduced in human iMGL cells after treatment for 30 minutes or longer with soluble Aβ42 (1 µM) that likely assumes different forms from monomeric to fibrils (Jäntti et al., 2022). The exact reason for such discrepancy remains unclear but the differences in microglial cell preparations and species as well as the different types of Aβ42 peptides used in these studies likely contribute. In addition, noticeable discrepancies exist in these two studies with respect to microglial expression of the Piezo1 channel in the AD mouse model with mutations in five AD genes (5xFAD) and AD patients. One study detected similar Piezo1 mRNA and protein expression in wild-type and 5xFAD mice. However, this study did reveal, based on a detailed analysis of published data, downregulated Piezo1 expression in disease-associated microglial cells in 5xFAD mice and, by contrast, upregulated expression in a cluster of microglial cells in AD patients (Jäntti et al., 2022). In the second study, an increase in Piezo1 protein expression was consistently observed in Iba1-positive microglial cells associated with amyloid plaques in both 5xFAD mice and AD patients (Hu et al., 2023). Further investigations may help clarify the discrepancy in the findings reported by these studies. Piezo1 channel activation mitigates Aβ-induced impairment in migration and phagocytosis of microglial cells and AD-related cognitive dysfunction: Microglial cells play an important role in restricting AD pathology, particularly in the early stages, by migrating towards and clustering around amyloid plaques and forming a physical barrier to inhibit neuronal loss and, in addition, by phagocytosing Aβ to prevent Aβ accumulation. However, as AD progresses, the mobility and particularly the phagocytic capacity of microglial cells are overwhelmed by excessive Aβ and become deficient. As shown in the recent study by Jäntti et al. (2022), exposure of human iMGL cells to Yoda1 accelerated cell migration, enhanced phagocytosis of exogenous soluble Aβ42 in undefined forms and stimulated lysosome function, suggesting that Piezo1 channel activation can facilitate the restoration of migratory and phagocytic functions of microglial cells. The study further showed that daily administration via infusion of Yoda1 into 5-month-old 5xFAD mice over the course of 12 days increased the number of Iba1-positive microglial cells, reduced amyloid plaque formation, and enhanced co-localization of microglial cells with amyloid plaques in both the cortex and hippocampus, brain regions key to cognitive function. As shown in a separate study (Hu et al., 2023), siRNA-mediated knockdown of Piezo1 expression compromised the ability of primary mouse microglial cells to internalize exogenous insoluble Aβ42 fibrils. Furthermore, conditional deletion of microglial Piezo1 expression in 5xFAD mice decreased engulfing of endogenous Aβ by microglial cells, heightened accumulation of both soluble and insoluble Aβ42 peptides and formation of amyloid plaques in the cortex and hippocampus, and led to reduced co-localization of Iba1-positive cells and CD11b/CD45-positive cells with amyloid plaques in the hippocampus (Hu et al., 2023). Piezo1 deficiency in 5xFAD mice also reduced the clustering of microglial cells within amyloid plaques and compacting of amyloid plaques. Conversely, administration of Yoda1 via intraperitoneal injection to 1.5-month-old 5xFAD mice for 3 months attenuated amyloid plaque formation, increased the population of microglial cells in the close vicinity of amyloid plaques, and stimulated engulfing of endogenous Aβ by microglial cells and lysosomal function. Moreover, selective deletion of microglial Piezo1 expression in 5xFAD mice exacerbated Aβ-induced impairment in long-term potentiation and cognitive deficits, revealed by behavioral testing using the Morris water maze and Y-maze, whereas administration of Yoda1 mitigated Aβ-induced cognitive dysfunction (Hu et al., 2023). Overall, these findings from in vitro and in vivo studies provide evidence to suggest that activation of the Piezo1 channel in microglial cells, presumably by stiff amyloid plaques or Aβ42 fibrils, enhances migration of microglial cells towards and clustering around amyloid plaques in addition to phagocytosis of Aβ peptides, thereby controlling Aβ accumulation, amyloid plaque formation and AD progression (Figure 1A). Piezo1 channel activation suppresses the production of proinflammatory cytokines by microglial cells: As introduced above, one physiologically and pathologically important function of microglial cells is to orchestrate an innate immune response, particularly the production of proinflammatory cytokines. Interleukin (IL)-1β, tumor necrosis factor (TNF)-α and IL-6, which represent the three major proinflammatory cytokines. We have recently examined the role of the Piezo1 channel in regulating the production of these proinflammatory cytokines by microglial cells after exposure to lipopolysaccharide (LPS), a widely used pathogen-associated molecular pattern (Malko et al., 2023). In both BV2 cells and primary mouse microglial cells, exposure to LPS induced the production of TNF-α and IL-6, but not IL-1β. Treatment with Yoda1 during exposure to LPS suppressed the production of TNF-α and IL-6. Such inhibition by Yoda1 of LPS-induced production of TNF-α was weakened by siRNA-mediated knockdown of Piezo1 expression, indicating the importance of the Piezo1 channel in mediating such an inhibition. Activation of the nuclear factor-κB proinflammatory signaling pathway was identified to be crucial in driving LPS-induced expression of TNF-α and IL-6. Exposure to Yoda1 inhibited LPS-induced activation of the nuclear factor-κB signaling pathway, and such inhibition was significantly mitigated by siRNA-mediated depletion of Piezo1 expression or treatment with BAPTA-AM to prevent intracellular Ca2+ increase. These findings have led us to propose Piezo1 channel activation as an inhibitory mechanism that dampens the proinflammatory response of microglial cells, particularly the production of proinflammatory cytokines TNF-α and IL-6, by initiating intracellular Ca2+ signaling to inhibit the nuclear factor-κB signaling pathway (Figure 1B). Perspective on the Piezo1 channel as a new therapeutic target for AD and other CNS conditions: It is well recognized that impairment in the migratory and phagocytic capacity of microglial cells and excessive production of proinflammatory cytokines by microglial cells play a crucial role in the pathogenesis of AD and other neurodegenerative diseases. It is evident from the discussion above that recent studies provide evidence to suggest that activation of the Piezo1 channel in microglial cells can restore the homeostatic state of microglial cells and associated cell functions including mobility and phagocytosis as well as the capacity of producing proinflammatory cytokines, and ameliorate AD pathogenesis. Therefore, targeting the Piezo1 channel presents a promising opportunity to assess the widely endorsed idea that re-instatement of the homeostasis of microglial cells is a viable therapeutic strategy for neurodegenerative diseases. There is evidence that exposure to Aβ42, like LPS, stimulates microglial cells to produce neurotoxic proinflammatory cytokines (e.g., Syed Mortadza et al., 2018). As discussed above, the Piezo1 channel has been shown to be activated by Aβ42 fibrils (Hu et al., 2023) but inhibited by soluble Aβ42 in undefined forms (Jäntti et al., 2022) and, currently, it remains unclear how Aβ42 peptides exert such opposite actions on the Piezo1 channel and impact the production of proinflammatory cytokines and AD pathogenesis. In addition, it is unknown whether Piezo1 channel activation regulates the capacity of microglial cells to produce anti-inflammatory mediators, which are crucial in the timely termination of immune responses and promotion of tissue repair. It would also be interesting to examine whether such a Piezo1 channel-mediated mechanism participates in or averts other neurodegenerative diseases and CNS traumatic damage. Clearly, more investigations are warranted to answer these important questions to help gain mechanistic insights into the expression and engagement of the Piezo1 channel in the pathogenesis and development of AD and other CNS pathologies. Nonetheless, the findings from recent studies raise the perspective of targeting the Piezo1 channel in microglial cells as a potential strategy to mitigate or prevent AD. This work was supported by A PhD Studentship from University of Leeds (to PM); Start-Up Fund from Xinxiang Medical University (to LHJ). Open peer reviewer:Miriam Corraliza-Gómez, Universidade de Lisboa Faculdade de Farmacia, Portugal. Additional file:Open peer review report 1.P-Reviewer: Corraliza-Gómez M; C-Editors: Zhao M, Liu WJ, Qiu Y; T-Editor: Jia Y