Thermosensitive TRPV4 channels mediate temperature-dependent microglia movement

Transient Receptor Potential Channels: Multiple Modulators of Peripheral Neuropathic Pain in Several Rodent Models

Neuropathic pain, a prevalent chronic condition in clinical settings, has attracted widespread societal attention. This condition is characterized by a persistent pain state accompanied by affective and cognitive disruptions, significantly impacting patients' quality of life. However, current clinical therapies fall short of addressing its complexity. Thus, exploring the underlying molecular mechanism of neuropathic pain and identifying potential targets for intervention is highly warranted. The transient receptor potential (TRP) receptors, a class of widely distributed channel proteins, in the nervous system, play a crucial role in sensory signaling, cellular calcium regulation, and developmental influences. TRP ion channels are also responsible for various sensory responses including heat, cold, pain, and stress. This review highlights recent advances in understanding TRPs in various rodent models of neuropathic pain, aiming to uncover potential therapeutic targets for clinical management.

ER and SOCE Ca2+ signals are not required for directed cell migration in human microglia

The central nervous system (CNS) is constantly surveilled by microglia, highly motile and dynamic cells deputed to act as the first line of immune defense in the brain and spinal cord. Alterations in the homeostasis of the CNS are detected by microglia that respond by migrating toward the affected area. Understanding the mechanisms controlling directed cell migration of microglia is crucial to dissect their responses to neuroinflammation and injury. We used a combination of pharmacological and genetic approaches to explore the involvement of calcium (Ca2+) signaling in the directed migration of induced pluripotent stem cell (iPSC)-derived microglia challenged with a purinergic stimulus. This approach mimics cues originating from injury of the CNS. Unexpectedly, simultaneous imaging of microglia migration and intracellular Ca2+ changes revealed that this phenomenon does not require Ca2+ signals generated from the endoplasmic reticulum (ER) and store-operated Ca2+ entry (SOCE) pathways. Instead, we find evidence that human microglial chemotaxis to purinergic signals is mediated by cyclic AMP in a Ca2+-independent manner. These results challenge prevailing notions, with important implications in neurological conditions characterized by perturbation in Ca2+ homeostasis.

TRP Channels in Stroke

Ischemic stroke is a devastating disease that affects millions of patients worldwide. Unfortunately, there are no effective medications for mitigating brain injury after ischemic stroke. TRP channels are evolutionally ancient biosensors that detect external stimuli as well as tissue or cellular injury. To date, many members of the TRP superfamily have been reported to contribute to ischemic brain injury, including the TRPC subfamily (1, 3, 4, 5, 6, 7), TRPV subfamily (1, 2, 3, 4) and TRPM subfamily (2, 4, 7). These TRP channels share structural similarities but have distinct channel functions and properties. Their activation during ischemic stroke can be beneficial, detrimental, or even both. In this review, we focus on discussing the interesting features of stroke-related TRP channels and summarizing the underlying cellular and molecular mechanisms responsible for their involvement in ischemic brain injury.

pH/Temperature Dual-Responsive Protein-Polymer Conjugates for Potential Therapeutic Hypothermia in Ischemic Stroke

Thrombolytic therapy for ischemic stroke still has several limitations, such as a narrow therapeutic time window and adverse effects. Therapeutic hypothermia is a neuroprotective strategy for stroke. In this study, we developed pH/temperature dual-responsive protein-polymer conjugates (PEG-uPA-PEG-PPG-PEG) by modifying a urokinase-type plasminogen activator (uPA) with polyethylene glycol (PEG) and poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEG-PPG-PEG, a thermosensitive polymer) via pH-sensitive imine bonds and disulfide bonds, respectively. At 37 °C and pH 7.4 (normothermia and physiological pH), PEG-uPA-PEG-PPG-PEG exhibits antiprotease hydrolysis and masked bioactivity of uPA due to the protective effect of the polymer segments wrapped around the protein surface. However, at 33 °C and pH 6.0 (hypothermia and pH at the thrombotic site), uPA loses the protective effect and recovers its bioactivity due to PEG dissociation and PEG-PPG-PEG stretching. The masked bioactivity of uPA at normothermia and physiological pH could reduce the risk of acute hemorrhage complication, and the recovery of protein activity at acidic pH and 33 °C is of great significance for thrombolytic therapy at mild hypothermia. Thus, PEG-uPA-PEG-PPG-PEG provides promising potential for therapeutic hypothermia in ischemic stroke.

Modulation of TRPM8 alters the phagocytic activity of microglia and induces changes in sub-cellular organelle functions

In this work, we investigated the presence and function of TRPM8, a non-selective and cold-sensitive Ca2+-permeable ion channel in the primary microglia cell as well as in microglia cell line BV2. We demonstrate that primary microglia as well as BV2 express TRPM8 endogenously. Both pharmacological activation or inhibition of TRPM8 causes enhanced uptake of bacterial particles at early time points of infection. In BV2, TRPM8 activation and/or LPS-signaling alters its surface expression and cytosolic ROS production. TRPM8 modulation in the absence and presence of LPS causes differential regulation of cytosolic pH and lysosomal pH. Notably, TRPM8 modulation also alters the correlation between lysosomal pH and cytosolic pH depending on TRPM8 modulation and the presence or absence of LPS. Collectively our data suggest that TRPM8 is involved in the regulation of subcellular organelle, i.e. mitochondrial and lysosomal functions. Data also suggest that primarily TRPM8 activation, but often deviation from endogenous TRPM8 function is linked with better innate immune function mediated by microglial cells. We suggest that TRPM8-mediated regulations of sub-cellular organelle functions are more context-dependent manner. Such understanding is relevant in the context of microglial cell functions and innate immunity.

Functional material-mediated wireless physical stimulation for neuro-modulation and regeneration

Nerve injuries and neurological diseases remain intractable clinical challenges. Despite the advantages of stem cell therapy in treating neurological disorders, uncontrollable cell fates and loss of cell function in vivo are still challenging. Recently, increasing attention has been given to the roles of external physical signals, such as electricity and ultrasound, in regulating stem cell fate as well as activating or inhibiting neuronal activity, which provides new insights for the treatment of neurological disorders. However, direct physical stimulations in vivo are short in accuracy and safety. Functional materials that can absorb energy from a specific physical field exerted in a wireless way and then release another localized physical signal hold great advantages in mediating noninvasive or minimally invasive accurate indirect physical stimulations to promote the therapeutic effect on neurological disorders. In this review, the mechanism by which various physical signals regulate stem cell fate and neuronal activity is summarized. Based on these concepts, the approaches of using functional materials to mediate indirect wireless physical stimulation for neuro-modulation and regeneration are systematically reviewed. We expect that this review will contribute to developing wireless platforms for neural stimulation as an assistance for the treatment of neurological diseases and injuries.

Movers and shakers: Microglial dynamics and modulation of neural networks

Microglia are multifaceted cells that act as immune sentinels, with important roles in pathological events, but also as integral contributors to the normal development and function of neural circuits. In the last decade, our understanding of the contributions these cells make to synaptic health and dysfunction has expanded at a dizzying pace. Here we review the known mechanisms that govern the dynamics of microglia allowing these motile cells to interact with synapses, and recruit microglia to specific sites on neurons. We then review the molecular signals that may underlie the function of microglia in synaptic remodeling. The emerging picture from the literature suggests that microglia are highly sensitive cells, reacting to neuronal signals with dynamic and specific actions tuned to the need of specific synapses and networks.

Astrocytic TRPV4 Channels and Their Role in Brain Ischemia

Transient receptor potential cation channels subfamily V member 4 (TRPV4) are non-selective cation channels expressed in different cell types of the central nervous system. These channels can be activated by diverse physical and chemical stimuli, including heat and mechanical stress. In astrocytes, they are involved in the modulation of neuronal excitability, control of blood flow, and brain edema formation. All these processes are significantly impaired in cerebral ischemia due to insufficient blood supply to the tissue, resulting in energy depletion, ionic disbalance, and excitotoxicity. The polymodal cation channel TRPV4, which mediates Ca²⁺ influx into the cell because of activation by various stimuli, is one of the potential therapeutic targets in the treatment of cerebral ischemia. However, its expression and function vary significantly between brain cell types, and therefore, the effect of its modulation in healthy tissue and pathology needs to be carefully studied and evaluated. In this review, we provide a summary of available information on TRPV4 channels and their expression in healthy and injured neural cells, with a particular focus on their role in ischemic brain injury.

Nanoparticles-mediated ion channels manipulation: From their membrane interactions to bioapplications

Ion channels are transmembrane proteins ubiquitously expressed in all cells that control various ions (e.g. Na⁺, K⁺, Ca²⁺ and Cl^- etc) crossing cellular plasma membrane, which play critical roles in physiological processes including regulating signal transduction, cell proliferation as well as excitatory cell excitation and conduction. Abnormal ion channel function is usually associated with dysfunctions and many diseases, such as neurodegenerative disorders, ophthalmic diseases, pulmonary diseases and even cancers. The precise regulation of ion channels not only helps to decipher physiological and pathological processes, but also is expected to become cutting-edge means for disease treatment. Recently, nanoparticles-mediated ion channel manipulation emerges as a highly promising way to meet the increasing requirements with respect to their simple, efficient, precise, spatiotemporally controllable and non-invasive regulation in biomedicine and other research frontiers. Thanks the advantages of their unique properties, nanoparticles can not only directly block the pore sites or kinetics of ion channels through their tiny size effect, and perturb active voltage-gated ion channel by their charged surface, but they can also act as antennas to conduct or enhance external physical stimuli to achieve spatiotemporal, precise and efficient regulation of various ion channel activities (e.g. light-, mechanical-, and temperature-gated ion channels etc). So far, nanoparticles-mediated ion channel regulation has shown potential prospects in many biomedical fields at the interfaces of neuro- and cardiovascular modulation, physiological function regeneration and tumor therapy et al. Towards such important fields, in this typical review, we specifically outline the latest studies of different types of ion channels and their activities relevant to the diseases. In addition, the different types of stimulation responsive nanoparticles, their interaction modes and targeting strategies towards the plasma membrane ion channels will be systematically summarized. More importantly, the ion channel regulatory methods mediated by functional nanoparticles and their bioapplications associated with physiological modulation and therapeutic development will be discussed. Last but not least, current challenges and future perspectives in this field will be covered as well.

Menstrual blood-derived endometrial stem cells inhibit neuroinflammation by regulating microglia through the TLR4/MyD88/NLRP3/Casp1 pathway

Neuroinflammation is a common response in various neurological disorders. Mesenchymal stem cell-based treatment has become a promising therapy for neuroinflammation-associated diseases. However, the effects of mesenchymal stem cells are controversial, and the underlying mechanism is incompletely understood. In the present study, menstrual blood-derived endometrial stem cells were intravenously transplanted into a mouse model of neuroinflammation established by peripheral injection of lipopolysaccharide. Microglial cells challenged with lipopolysaccharide were cultured with conditioned medium from endometrial stem cells. The levels of cytokines were detected by enzyme-linked immunosorbent assay. Cell proliferation and death were detected by Cell Counting Kit 8 and flow cytometry, respectively. The expression levels of Toll-like receptor 4 (TLR4), myeloid differentiation primary response gene 88 (MyD88), NLR family pyrin domain containing 3 (NLRP3) and caspase 1 (Casp1) were evaluated by western blotting. The results showed that intravenous transplantation of endometrial stem cells downregulated proinflammatory factors and upregulated anti-inflammatory factors in the brain of mice with neuroinflammation. Conditioned medium suppressed the inflammatory reaction and hyperactivation of microglial cells and protected microglial cells from cell death induced by lipopolysaccharide in vitro. The expression of TLR4, MyD88, NLRP3 and Casp1 in the brain of mice with neuroinflammation and in lipopolysaccharide-stimulated microglial cells was downregulated by endometrial stem cells and conditioned medium, respectively. These data suggested that menstrual blood-derived endometrial stem cells may suppress neuroinflammatory reactions partially by regulating microglia through the TLR4/MyD88/NLRP3/Casp1 signalling pathway. Our findings may be very useful for the development of an alternative stem cell-based therapy for neuroinflammation-associated disorders.

A TRPV4-dependent neuroimmune axis in the spinal cord promotes neuropathic pain

Microglia, resident macrophages of the CNS, are essential to brain development, homeostasis, and disease. Microglial activation and proliferation are hallmarks of many CNS diseases, including neuropathic pain. However, molecular mechanisms that govern the spinal neuroimmune axis in the setting of neuropathic pain remain incompletely understood. Here, we show that genetic ablation or pharmacological blockade of transient receptor potential vanilloid type 4 (TRPV4) markedly attenuated neuropathic pain-like behaviors in a mouse model of spared nerve injury. Mechanistically, microglia-expressed TRPV4 mediated microglial activation and proliferation and promoted functional and structural plasticity of excitatory spinal neurons through release of lipocalin-2. Our results suggest that microglial TRPV4 channels reside at the center of the neuroimmune axis in the spinal cord, which transforms peripheral nerve injury into central sensitization and neuropathic pain, thereby identifying TRPV4 as a potential new target for the treatment of chronic pain.

Purinoreceptors and ectonucleotidases control ATP-induced calcium waveforms and calcium-dependent responses in microglia: Roles of P2 receptors and CD39 in ATP-stimulated microglia

Adenosine triphosphate (ATP) and its metabolites drive microglia migration and cytokine production by activating P2X- and P2Y- class purinergic receptors. Purinergic receptor activation gives rise to diverse intracellular calcium (Ca2+ signals, or waveforms, that differ in amplitude, duration, and frequency. Whether and how these characteristics of diverse waveforms influence microglia function is not well-established. We developed a computational model trained with data from published primary murine microglia studies. We simulate how purinoreceptors influence Ca2+ signaling and migration, as well as, how purinoreceptor expression modifies these processes. Our simulation confirmed that P2 receptors encode the amplitude and duration of the ATP-induced Ca2+ waveforms. Our simulations also implicate CD39, an ectonucleotidase that rapidly degrades ATP, as a regulator of purinergic receptor-induced Ca2+ responses. Namely, it was necessary to account for CD39 metabolism of ATP to align the model's predicted purinoreceptor responses with published experimental data. In addition, our modeling results indicate that small Ca2+ transients accompany migration, while large and sustained transients are needed for cytokine responses. Lastly, as a proof-of-principal, we predict Ca2+ transients and cell membrane displacements in a BV2 microglia cell line using published P2 receptor mRNA data to illustrate how our computer model may be extrapolated to other microglia subtypes. These findings provide important insights into how differences in purinergic receptor expression influence microglial responses to ATP.

Retinal TRP channels: Cell-type-specific regulators of retinal homeostasis and multimodal integration

Transient receptor potential (TRP) channels are a widely expressed family of 28 evolutionarily conserved cationic ion channels that operate as primary detectors of chemical and physical stimuli and secondary effectors of metabotropic and ionotropic receptors. In vertebrates, the channels are grouped into six related families: TRPC, TRPV, TRPM, TRPA, TRPML, and TRPP. As sensory transducers, TRP channels are ubiquitously expressed across the body and the CNS, mediating critical functions in mechanosensation, nociception, chemosensing, thermosensing, and phototransduction. This article surveys current knowledge about the expression and function of the TRP family in vertebrate retinas, which, while dedicated to transduction and transmission of visual information, are highly susceptible to non-visual stimuli. Every retinal cell expresses multiple TRP subunits, with recent evidence establishing their critical roles in paradigmatic aspects of vertebrate vision that include TRPM1-dependent transduction of ON bipolar signaling, TRPC6/7-mediated ganglion cell phototransduction, TRP/TRPL phototransduction in Drosophila and TRPV4-dependent osmoregulation, mechanotransduction, and regulation of inner and outer blood-retina barriers. TRP channels tune light-dependent and independent functions of retinal circuits by modulating the intracellular concentration of the 2nd messenger calcium, with emerging evidence implicating specific subunits in the pathogenesis of debilitating diseases such as glaucoma, ocular trauma, diabetic retinopathy, and ischemia. Elucidation of TRP channel involvement in retinal biology will yield rewards in terms of fundamental understanding of vertebrate vision and therapeutic targeting to treat diseases caused by channel dysfunction or over-activation.

The role and mechanism of NLRP3 inflammasome-mediated astrocyte activation in dehydrocorydaline against CUMS-induced depression

Background: Depression is a common and potentially life-threatening mental illness, and currently, there is a lack of effective treatment. It has been reported that dehydrocorydaline (DHC) can inhibit monoamine transporter uptake in depressed CUMS mice, but more possible mechanisms of action remain to be further studied. Methods: C57BL/6 mice were exposed to chronic unpredictable mild stress (CUMS) for five consecutive weeks. The mice were administrated with dehydrocorydaline or fluoxetine (FLU) for four consecutive weeks. Behavioral tests including sucrose preference test (SPT), tail suspension test (TST), and forced swimming test (FST) were applied. In parallel, hematoxylin and eosin (H&E) staining and Nissl staining were used to explore the effect of DHC on pathological changes in the hippocampus. The concentrations of depression-related factors (5-HT and DA) and inflammatory factors (TNF-α, IL-6, and IL-1β) in the hippocampus and serum were assessed by ELISA assay. NLRP3 inflammasome pathway-related proteins (NLRP3, IL-18, IL-1 IL-1α, and caspase-1) were detected by western blot. The activation of microglia and astrocytes was subjected to immunofluorescent staining. Additionally, microglia were treated with DHC (100 mg/L) for 24 h following incubation with 100 ng/ml LPS for 12 h. ov-NC or ov-NLRP3 plasmid was transfected into microglia 6 h before LPS induction for exploring the effect of NLRP3 overexpression on DHC-inhibited microglia activation. Then, conditioned media of microglia were collected from each group, followed by intervention of astrocytes for 24 h to explore the effect of NLRP3 overexpression of microglia on astrocyte activation. Results: In vivo administration of DHC was found to ameliorate depressive-like behaviors and attenuate neuron damage of CUMS mice. DHC increased neurotransmitter concentration, reduced the proinflammatory factor levels, attenuated NLRP3 inflammasome activation, and decreased A1 astrocyte and microglia activation in the hippocampus of CUMS mice. Furthermore, in vivo results showed that activated microglia induced activation of A1 astrocytes but not A2 astrocytes. Conclusion: Taken together, we provided evidence that DHC exhibited antidepressive effects on CUMS mice possibly via NLRP3 inflammasome-mediated astrocyte activation.

Ischemic stroke: From pathological mechanisms to neuroprotective strategies

Ischemic stroke (IS) has complex pathological mechanisms, and is extremely difficult to treat. At present, the treatment of IS is mainly based on intravenous thrombolysis and mechanical thrombectomy, but they are limited by a strict time window. In addition, after intravenous thrombolysis or mechanical thrombectomy, damaged neurons often fail to make ideal improvements due to microcirculation disorders. Therefore, finding suitable pathways and targets from the pathological mechanism is crucial for the development of neuroprotective agents against IS. With the hope of making contributions to the development of IS treatments, this review will introduce (1) how related targets are found in pathological mechanisms such as inflammation, excitotoxicity, oxidative stress, and complement system activation; and (2) the current status and challenges in drug development.

Acute inhibition of transient receptor potential vanilloid-type 4 cation channel halts cytoskeletal dynamism in microglia

Microglia, the resident macrophages of the central nervous system, are highly motile cells that support brain development, provision neuronal signaling, and protect brain cells against damage. Proper microglial functioning requires constant cell movement and morphological changes. Interestingly, the transient receptor potential vanilloid 4 (TRPV4) channel, a calcium-permeable channel, is involved in hypoosmotic morphological changes of retinal microglia and regulates temperature-dependent movement of microglial cells both in vitro and in vivo. Despite the broad functions of TRPV4 and the recent findings stating a role for TRPV4 in microglial movement, little is known about how TRPV4 modulates cytoskeletal remodeling to promote changes of microglial motility. Here we show that acute inhibition of TRPV4, but not its constitutive absence in the Trpv4 KO cells, affects the morphology and motility of microglia in vitro. Using high-end confocal imaging techniques, we show a decrease in actin-rich filopodia and tubulin dynamics upon acute inhibition of TRPV4 in vitro. Furthermore, using acute brain slices we demonstrate that Trpv4 knockout microglia display lower ramification complexity, slower process extension speed and consequently smaller surveyed area. We conclude that TRPV4 inhibition triggers a shift in cytoskeleton remodeling of microglia influencing their migration and morphology.

TRPV4: A trigger of pathological RhoA activation in neurological disease

Transient receptor potential vanilloid 4 (TRPV4), a member of the TRP superfamily, is a broadly expressed, cell surface-localized cation channel that is activated by a variety of environmental stimuli. Importantly, TRPV4 has been increasingly implicated in the regulation of cellular morphology. Here we propose that TRPV4 and the cytoskeletal remodeling small GTPase RhoA together constitute an environmentally sensitive signaling complex that contributes to pathological cell cytoskeletal alterations during neurological injury and disease. Supporting this hypothesis is our recent work demonstrating direct physical and bidirectional functional interactions of TRPV4 with RhoA, which can lead to activation of RhoA and reorganization of the actin cytoskeleton. Furthermore, a confluence of evidence implicates TRPV4 and/or RhoA in pathological responses triggered by a range of acute neurological insults ranging from stroke to traumatic injury. While initiated by a variety of insults, TRPV4-RhoA signaling may represent a common pathway that disrupts axonal regeneration and blood-brain barrier integrity. These insights also suggest that TRPV4 inhibition may represent a safe, feasible, and precise therapeutic strategy for limiting pathological TRPV4-RhoA activation in a range of neurological diseases.

Both heat-sensitive TRPV4 and cold-sensitive TRPM8 ion channels regulate microglial activity

Microglia, the brain-resident macrophages, perform a myriad of functions directed towards development of neural circuits, and their maintenance. A plethora of ion channels aid in microglial activities that are critical for overall brain functioning. Notably, different functions of microglial cells are sensitive to minute temperature changes, as well as mechanical forces. Therefore, among all the players involved in the regulation of microglial functions, thermosensitive TRP ion channels are potentially important. In this study, we report the endogenous and functional presence of a heat-sensitive ion channel TRPV4 and a cold-sensitive ion channel TRPM8 in primary rat microglia and microglial cell line, N9. We demonstrate that pharmacological modulations of both these channels affect intracellular Ca²⁺-levels, cellular morphology, migration, and motility. Thus, TRPV4 and TRPM8 act as potential regulators of microglial activities. These findings may have broad implications in understanding neuro-glia interactions in neurodevelopmental and neurodegenerative pathologies with overall bio-medical applications.

Mechanosensing and the Hippo Pathway in Microglia: A Potential Link to Alzheimer's Disease Pathogenesis?

The activation of microglia, the inflammatory cells of the central nervous system (CNS), has been linked to the pathogenesis of Alzheimer’s disease and other neurodegenerative diseases. How microglia sense the changing brain environment, in order to respond appropriately, is still being elucidated. Microglia are able to sense and respond to the mechanical properties of their microenvironment, and the physical and molecular pathways underlying this mechanosensing/mechanotransduction in microglia have recently been investigated. The Hippo pathway functions through mechanosensing and subsequent protein kinase cascades, and is critical for neuronal development and many other cellular processes. In this review, we examine evidence for the potential involvement of Hippo pathway components specifically in microglia in the pathogenesis of Alzheimer’s disease. We suggest that the Hippo pathway is worth investigating as a mechanosensing pathway in microglia, and could be one potential therapeutic target pathway for preventing microglial-induced neurodegeneration in AD.