Store-Operated Calcium Entry in Müller Glia Is Controlled by Synergistic Activation of TRPC and Orai Channels

Targeting calciumopathy for neuroprotection: focus on calcium channels Cav1, Orai1 and P2X7

As the uncontrolled entry of calcium ions (Ca2+) through plasmalemmal calcium channels is a cell death trigger, the conjecture is here raised that mitigating such an excess of Ca2+ entry should rescue from death the vulnerable neurons in neurodegenerative diseases (NDDs). However, this supposition has failed in some clinical trials (CTs). Thus, a recent CT tested whether isradipine, a blocker of the Cav1 subtype of voltage-operated calcium channels (VOCCs), exerted a benefit in patients with Parkinson's disease (PD); however, outcomes were negative. This is one more of the hundreds of CTs done under the principle of one-drug-one-target, that have failed in Alzheimer's disease (AD) and other NDDs during the last three decades. As there are myriad calcium channels to let Ca2+ ions gain the cell cytosol, it seems reasonable to predict that blockade of Ca2+ entry through a single channel may not be capable of preventing the Ca2+ flood of cells by the uncontrolled Ca2+ entry. Furthermore, as Ca2+ signaling is involved in the regulation of myriad functions in different cell types, it seems also reasonable to guess that a therapy should be more efficient by targeting different cells with various drugs. Here, we propose to mitigate Ca2+ entry by the simultaneous partial blockade of three quite different subtypes of plasmalemmal calcium channels that is, the Cav1 subtype of VOCCs, the Orai1 store-operated calcium channel (SOCC), and the purinergic P2X7 calcium channel. All three channels are expressed in both microglia and neurons. Thus, by targeting the three channels with a combination of three drug blockers we expect favorable changes in some of the pathogenic features of NDDs, namely (i) to mitigate Ca2+ entry into microglia; (ii) to decrease the Ca2+-dependent microglia activation; (iii) to decrease the sustained neuroinflammation; (iv) to decrease the uncontrolled Ca2+ entry into neurons; (v) to rescue vulnerable neurons from death; and (vi) to delay disease progression. In this review we discuss the arguments underlying our triad hypothesis in the sense that the combination of three repositioned medicines targeting Cav1, Orai1, and P2X7 calcium channels could boost neuroprotection and delay the progression of AD and other NDDs.

Mechanosensitive ion channels in glaucoma pathophysiology

Force sensing is a fundamental ability that allows cells and organisms to interact with their physical environment. The eye is constantly subjected to mechanical forces such as blinking and eye movements. Furthermore, elevated intraocular pressure (IOP) can cause mechanical strain at the optic nerve head, resulting in retinal ganglion cell death (RGC) in glaucoma. How mechanical stimuli are sensed and affect cellular physiology in the eye is unclear. Recent studies have shown that mechanosensitive ion channels are expressed in many ocular tissues relevant to glaucoma and may influence IOP regulation and RGC survival. Furthermore, variants in mechanosensitive ion channel genes may be associated with risk for primary open angle glaucoma. These findings suggest that mechanosensitive channels may be important mechanosensors mediating cellular responses to pressure signals in the eye. In this review, we focus on mechanosensitive ion channels from three major channel families-PIEZO, two-pore potassium and transient receptor potential channels. We review the key properties of these channels, their effects on cell function and physiology, and discuss their possible roles in glaucoma pathophysiology.

Resting trabecular meshwork cells experience constitutive cation influx

A quintessential sentinel of cell health, the membrane potential in nonexcitable cells integrates biochemical and biomechanical inputs, determines the driving force for ionic currents activated by input signals and plays critical functions in cellular differentiation, signaling, and pathology. The identity and properties of ion channels that subserve the resting potential in trabecular meshwork (TM) cells is poorly understood, which impairs our understanding of intraocular pressure regulation in healthy and diseased eyes. Here, we identified a powerful cationic conductance that subserves the TM resting potential. It disappears following Na+ removal or substitution with choline or NMDG+, is insensitive to TTX, verapamil, phenamil methanesulfonate, amiloride and GsMTx4, is substituted by Li+ and Cs+, and inhibited by Gd3+ and Ruthenium Red. Constitutive cation influx is thus not mediated by voltage-operated Na+, Ca2+, epithelial Na+ (ENaC) channels, Piezo channels or Na+/H+ exchange but may involve TRP-like channels. Transcriptional analysis detected expression of many TRP genes, with the transcriptome pool dominated by TRPC1 followed by expression of TRPV1, TRPC3, TRPV4 and TRPC5. Pyr3 and Pico1,4,5 did not affect the standing current whereas SKF96365 promoted rather than suppressed, Na+ influx. SEA-0400 induced a modest hyperpolarization, indicating residual contribution from Na+/Ca2+ exchange. The resting membrane potential in human TM cells is thus maintained by a constitutive monovalent cation leak current with properties not unlike those of TRP channels. This conductance is likely to influence conventional outflow by setting the homeostatic steady-state and by regulating the magnitude of pressure-induced currents in normotensive and hypertensive eyes.

Retinal Connectomics: A Review

The retina is an ideal model for understanding the fundamental rules for how neural networks are constructed. The compact neural networks of the retina perform all of the initial processing of visual information before transmission to higher visual centers in the brain. The field of retinal connectomics uses high-resolution electron microscopy datasets to map the intricate organization of these networks and further our understanding of how these computations are performed by revealing the fundamental topologies and allowable networks behind retinal computations. In this article, we review some of the notable advances that retinal connectomics has provided in our understanding of the specific cells and the organization of their connectivities within the retina, as well as how these are shaped in development and break down in disease. Using these anatomical maps to inform modeling has been, and will continue to be, instrumental in understanding how the retina processes visual signals.

Resting human trabecular meshwork cells experience tonic cation influx

The trabecular meshwork (TM) regulates intraocular pressure (IOP) by converting biochemical and biomechanical stimuli into intracellular signals. Recent electrophysiological studies demonstrated that this process is mediated by pressure sensing ion channels in the TM plasma membrane while the molecular and functional properties of channels that underpin ionic homeostasis in resting cells remain largely unknown. Here, we demonstrate that the TM resting potential is subserved by a powerful cationic conductance that disappears following Na+ removal and substitution with choline or NMDG+. Its insensitivity to TTX, verapamil, phenamil methanesulfonate and amiloride indicates it does not involve voltage-operated Na+, Ca2+ and epithelial Na+ (ENaC) channels or Na+/H+ exchange while a modest hyperpolarization induced by SEA-0440 indicates residual contribution from reversed Na+/Ca2+ exchange. Tonic cationic influx was inhibited by Gd3+ and Ruthenium Red but not GsMTx4, indicating involvement of TRP-like but not Piezo channels. Transcriptional analysis detected expression of most TRP genes, with the canonical transcriptome pool dominated by TRPC1 followed by the expression ofTRPV1, TRPC3 and TRPC5. TRPC3 antagonist Pyr3 and TRPC1,4,5 antagonist Pico1,4,5 did not affect the standing current, whereas the TRPC blocker SKF96365 promoted rather than suppressed, Na+ influx. TM cells thus maintain the resting membrane potential, control Na+ homeostasis, and balance K+ efflux through a novel constitutive monovalent cation leak current with properties not unlike those of TRP channels. Yet to be identified at the molecular level, this novel channel sets the homeostatic steady-state and controls the magnitude of pressure-induced transmembrane signals.

Selective Assembly of TRPC Channels in the Rat Retina during Photoreceptor Degeneration

Transient receptor potential canonical (TRPC) channels are calcium channels with diverse expression profiles and physiological implications in the retina. Neurons and glial cells of rat retinas with photoreceptor degeneration caused by retinitis pigmentosa (RP) exhibit basal calcium levels that are above those detected in healthy retinas. Inner retinal cells are the last to degenerate and are responsible for maintaining the activity of the visual cortex, even after complete loss of photoreceptors. We considered the possibility that TRPC1 and TRPC5 channels might be associated with both the high calcium levels and the delay in inner retinal degeneration. TRPC1 is known to mediate protective effects in neurodegenerative processes while TRPC5 promotes cell death. In order to comprehend the implications of these channels in RP, the co-localization and subsequent physical interaction between TRPC1 and TRPC5 in healthy retina (Sprague-Dawley rats) and degenerating (P23H-1, a model of RP) retina were detected by immunofluorescence and proximity ligation assays. There was an overlapping signal in the innermost retina of all animals where TRPC1 and TRPC5 physically interacted. This interaction increased significantly as photoreceptor loss progressed. Both channels function as TRPC1/5 heteromers in the healthy and damaged retina, with a marked function of TRPC1 in response to retinal degenerative mechanisms. Furthermore, our findings support that TRPC5 channels also function in partnership with STIM1 in Müller and retinal ganglion cells. These results suggest that an increase in TRPC1/5 heteromers may contribute to the slowing of the degeneration of the inner retina during the outer retinal degeneration.

Store-operated calcium entry in the satellite glial cells of rat sympathetic ganglia

Satellite glial cells (SGCs), a major type of glial cell in the autonomic ganglia, closely envelop the cell body and even the synaptic regions of a single neuron with a very narrow gap. This structurally unique organization suggests that autonomic neurons and SGCs may communicate reciprocally. Glial Ca2+ signaling is critical for controlling neural activity. Here, for the first time we identified the machinery of store-operated Ca2+ entry (SOCE) which is critical for cellular Ca2+ homeostasis in rat sympathetic ganglia under normal and pathological states. Quantitative realtime PCR and immunostaining analyses showed that Orai1 and stromal interaction molecules 1 (STIM1) proteins are the primary components of SOCE machinery in the sympathetic ganglia. When the internal Ca2+ stores were depleted in the absence of extracellular Ca2+, the number of plasmalemmal Orai1 puncta was increased in neurons and SGCs, suggesting activation of the Ca2+ entry channels. Intracellular Ca2+ imaging revealed that SOCE was present in SGCs and neurons; however, the magnitude of SOCE was much larger in the SGCs than in the neurons. The SOCE was significantly suppressed by GSK7975A, a selective Orai1 blocker, and Pyr6, a SOCE blocker. Lipopolysaccharide (LPS) upregulated the glial fibrillary acidic protein and Toll-like receptor 4 in the sympathetic ganglia. Importantly, LPS attenuated SOCE via downregulating Orai1 and STIM1 expression. In conclusion, sympathetic SGCs functionally express the SOCE machinery, which is indispensable for intracellular Ca2+ signaling. The SOCE is highly susceptible to inflammation, which may affect sympathetic neuronal activity and thereby autonomic output.

Astroglial S100B Secretion Is Mediated by Ca2+ Mobilization from Endoplasmic Reticulum: A Study Using Forskolin and DMSO as Secretagogues

S100B, a homodimeric Ca2+-binding protein, is produced and secreted by astrocytes, and its extracellular levels have been used as a glial marker in brain damage and neurodegenerative and psychiatric diseases; however, its mechanism of secretion is elusive. We used primary astrocyte cultures and calcium measurements from real-time fluorescence microscopy to investigate the role of intracellular calcium in S100B secretion. In addition, the dimethyl sulfoxide (DMSO) effect on S100B was investigated in vitro and in vivo using Wistar rats. We found that DMSO, a widely used vehicle in biological assays, is a powerful S100B secretagogue, which caused a biphasic response of Ca2+ mobilization. Our data show that astroglial S100B secretion is triggered by the increase in intracellular Ca2+ and indicate that this increase is due to Ca2+ mobilization from the endoplasmic reticulum. Also, blocking plasma membrane Ca2+ channels involved in the Ca2+ replenishment of internal stores decreased S100B secretion. The DMSO-induced S100B secretion was confirmed in vivo and in ex vivo hippocampal slices. Our data support a nonclassic vesicular export of S100B modulated by Ca2+, and the results might contribute to understanding the mechanism underlying the astroglial release of S100B.

Robust expression of the TRPC1 channel associated with photoreceptor loss in the rat retina

Baseline intracellular calcium levels are significantly higher in neuronal and glial cells of rat retinas with retinitis pigmentosa (RP). Although this situation could initiate multiple detrimental pathways that lead to cell death, we considered the possibility of TRPC1 being involved in maintaining calcium homeostasis in the retina by acting as a component of store-operated calcium (SOC) channels with special relevance during photoreceptor degeneration. In this study, we examined by Western blot the expression of TRPC1 in healthy control rat retinas (Sprague-Dawley, SD) and retinas with RP (P23H-1 rats). We also analyzed its specific cellular distribution by immunofluorescence to recognize changes during neurodegeneration and to determine whether its presence is consistent with high basal calcium levels and cellular survival in degenerating retinas. We found that TRPC1 immunostaining was widely distributed across the retina in both rat strains, SD and P23H, and its expression levels significantly increased in the retinas with advanced degeneration compared to the age-control SD rats. In the outer retina, TRPC1 immunoreactivity was distributed in pigment epithelium cells, the photoreceptor inner segments of older animals, and the outer plexiform layer. In the inner retina, TRPC1 labeling was detected in horizontal cells, specific somata of bipolar and amacrine cells, and cellular processes in all the strata of the inner plexiform layer. Somata and processes were also highly immunoreactive in the ganglion cell layer and astrocytes in the nerve fiber layer in all animals. In the P23H rat retinas, the TRPC1 distribution pattern changed according to advancing photoreceptor degeneration and the gliosis reaction, with TRPC1 immunoreactive Müller cells mainly in advanced stages of disease. The cellular TRPC1 immunoreactivity found in this work suggests different mechanisms of activation of these channels depending on the cell type. Furthermore, the results support the idea that photoreceptor loss due to RP is associated with robust TRPC1 protein expression in the rat inner retina and raise the possibility of TRPC1 channels contributing to maintain high basal calcium levels during neurodegeneration and/or maintenance processes of the inner retina.

Inhibition of altered Orai1 channels in Müller cells protects photoreceptors in retinal degeneration

The expressions of ion channels by Müller glial cells (MGCs) may change in response to various retinal pathophysiological conditions. There remains a gap in our understanding of MGCs' responses to photoreceptor degeneration towards finding therapies. The study explores how an inhibition of store-operated Ca2+ entry (SOCE) and its major component, Orai1 channel, in MGCs protects photoreceptors from degeneration. The study revealed increased Orai1 expression in the MGCs of retinal degeneration 10 (rd10) mice. Enhanced expression of oxidative stress markers was confirmed as a crucial pathological mechanism in rd10 retina. Inducing oxidative stress in rat MGCs resulted in increasing SOCE and Ca2+ release-activated Ca2+ (CRAC) currents. SOCE inhibition by 2-Aminoethoxydiphenyl borate (2-APB) protected photoreceptors in degenerated retinas. Finally, molecular simulations proved the structural and dynamical features of 2-APB to the target structure Orai1. Our results provide new insights into the physiology of MGCs regarding retinal degeneration and shed a light on SOCE and Orai1 as new therapeutic targets.

Orai, RyR, and IP3R channels cooperatively regulate calcium signaling in brain mid-capillary pericytes

Pericytes are multifunctional cells of the vasculature that are vital to brain homeostasis, yet many of their fundamental physiological properties, such as Ca²⁺ signaling pathways, remain unexplored. We performed pharmacological and ion substitution experiments to investigate the mechanisms underlying pericyte Ca²⁺ signaling in acute cortical brain slices of PDGFRβ-Cre::GCaMP6f mice. We report that mid-capillary pericyte Ca²⁺ signalling differs from ensheathing type pericytes in that it is largely independent of L- and T-type voltage-gated calcium channels. Instead, Ca²⁺ signals in mid-capillary pericytes were inhibited by multiple Orai channel blockers, which also inhibited Ca²⁺ entry triggered by endoplasmic reticulum (ER) store depletion. An investigation into store release pathways indicated that Ca²⁺ transients in mid-capillary pericytes occur through a combination of IP₃R and RyR activation, and that Orai store-operated calcium entry (SOCE) is required to sustain and amplify intracellular Ca²⁺ increases evoked by the GqGPCR agonist endothelin-1. These results suggest that Ca²⁺ influx via Orai channels reciprocally regulates IP₃R and RyR release pathways in the ER, which together generate spontaneous Ca²⁺ transients and amplify Gq-coupled Ca²⁺ elevations in mid-capillary pericytes. Thus, SOCE is a major regulator of pericyte Ca²⁺ and a target for manipulating their function in health and disease.

The Molecular Heterogeneity of Store-Operated Ca2+ Entry in Vascular Endothelial Cells: The Different roles of Orai1 and TRPC1/TRPC4 Channels in the Transition from Ca2+-Selective to Non-Selective Cation Currents

Store-operated Ca²⁺ entry (SOCE) is activated in response to the inositol-1,4,5-trisphosphate (InsP₃)-dependent depletion of the endoplasmic reticulum (ER) Ca²⁺ store and represents a ubiquitous mode of Ca²⁺ influx. In vascular endothelial cells, SOCE regulates a plethora of functions that maintain cardiovascular homeostasis, such as angiogenesis, vascular tone, vascular permeability, platelet aggregation, and monocyte adhesion. The molecular mechanisms responsible for SOCE activation in vascular endothelial cells have engendered a long-lasting controversy. Traditionally, it has been assumed that the endothelial SOCE is mediated by two distinct ion channel signalplexes, i.e., STIM1/Orai1 and STIM1/Transient Receptor Potential Canonical 1(TRPC1)/TRPC4. However, recent evidence has shown that Orai1 can assemble with TRPC1 and TRPC4 to form a non-selective cation channel with intermediate electrophysiological features. Herein, we aim at bringing order to the distinct mechanisms that mediate endothelial SOCE in the vascular tree from multiple species (e.g., human, mouse, rat, and bovine). We propose that three distinct currents can mediate SOCE in vascular endothelial cells: (1) the Ca²⁺-selective Ca²⁺-release activated Ca²⁺ current (I_CRAC), which is mediated by STIM1 and Orai1; (2) the store-operated non-selective current (I_SOC), which is mediated by STIM1, TRPC1, and TRPC4; and (3) the moderately Ca²⁺-selective, I_CRAC-like current, which is mediated by STIM1, TRPC1, TRPC4, and Orai1.

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.

TRPV4 and TRPC1 channels mediate the response to tensile strain in mouse Müller cells

Müller glia, a pillar of metabolic, volume regulatory and immune/inflammatory signaling in the mammalian retina, are among the earliest responders to mechanical stressors in the eye. Ocular trauma, edema, detachment and glaucoma evoke early inflammatory activation of Müller cells yet the identity of their mechanotransducers and signaling mechanisms downstream remains unknown. Here, we investigate expression of genes that encode putative stretch-activated calcium channels (SACs) in mouse Müller cells and study their responses to dynamical tensile loading in cells loaded with a calcium indicator dye. Transcript levels in purified glia were Trpc1>Piezo1>Trpv2>Trpv4>>Trpv1>Trpa1. Cyclic radial deformation of matrix-coated substrates produced dose-dependent increases in [Ca2+]i that were suppressed by the TRPV4 channel antagonist HC-067047 and by ablation of the Trpv4 gene. Stretch-evoked calcium responses were also reduced by knockdown and pharmacological inhibition of TRPC1 channels whereas the TRPV2 inhibitor tranilast had no effect. These data demonstrate that Müller cells are intrinsically mechanosensitive, with the response to tensile loading mediated through synergistic activation of TRPV4 and TRPC1 channels. Coupling between mechanical stress and Müller Ca2+ homeostasis has treatment implications, since many neuronal injury paradigms in the retina involve calcium dysregulation associated with inflammatory and immune signaling.

Emergent Temporal Signaling in Human Trabecular Meshwork Cells: Role of TRPV4-TRPM4 Interactions

Trabecular meshwork (TM) cells are phagocytic cells that employ mechanotransduction to actively regulate intraocular pressure. Similar to macrophages, they express scavenger receptors and participate in antigen presentation within the immunosuppressive milieu of the anterior eye. Changes in pressure deform and compress the TM, altering their control of aqueous humor outflow but it is not known whether transducer activation shapes temporal signaling. The present study combines electrophysiology, histochemistry and functional imaging with gene silencing and heterologous expression to gain insight into Ca²⁺ signaling downstream from TRPV4 (Transient Receptor Potential Vanilloid 4), a stretch-activated polymodal cation channel. Human TM cells respond to the TRPV4 agonist GSK1016790A with fluctuations in intracellular Ca²⁺ concentration ([Ca²⁺]_i) and an increase in [Na⁺]_i. [Ca²⁺]_i oscillations coincided with monovalent cation current that was suppressed by BAPTA, Ruthenium Red and the TRPM4 (Transient Receptor Potential Melastatin 4) channel inhibitor 9-phenanthrol. TM cells expressed TRPM4 mRNA, protein at the expected 130-150 kDa and showed punctate TRPM4 immunoreactivity at the membrane surface. Genetic silencing of TRPM4 antagonized TRPV4-evoked oscillatory signaling whereas TRPV4 and TRPM4 co-expression in HEK-293 cells reconstituted the oscillations. Membrane potential recordings suggested that TRPM4-dependent oscillations require release of Ca²⁺ from internal stores. 9-phenanthrol did not affect the outflow facility in mouse eyes and eyes from animals lacking TRPM4 had normal intraocular pressure. Collectively, our results show that TRPV4 activity initiates dynamic calcium signaling in TM cells by stimulating TRPM4 channels and intracellular Ca²⁺ release. It is possible that TRPV4-TRPM4 interactions downstream from the tensile and compressive impact of intraocular pressure contribute to homeostatic regulation and pathological remodeling within the conventional outflow pathway.

Orai1 is a crucial downstream partner of group I metabotropic glutamate receptor signaling in dorsal horn neurons

ABSTRACT

Group I metabotropic glutamate receptors (group I mGluRs) have been implicated in several central nervous system diseases including chronic pain. It is known that activation of group I mGluRs results in the production of inositol triphosphate (IP3) and diacylglycerol that leads to activation of extracellular signal-regulated kinases (ERKs) and an increase in neuronal excitability, but how group I mGluRs mediate this process remains unclear. We previously reported that Orai1 is responsible for store-operated calcium entry and plays a key role in central sensitization. However, how Orai1 is activated under physiological conditions is unknown. Here, we tested the hypothesis that group I mGluRs recruit Orai1 as part of its downstream signaling pathway in dorsal horn neurons. We demonstrate that neurotransmitter glutamate induces STIM1 puncta formation, which is not mediated by N-Methyl-D-aspartate (NMDA) or α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. Glutamate-induced Ca2+ entry in the presence of NMDA or AMPA receptor antagonists is eliminated in Orai1-deficient neurons. Dihydroxyphenylglycine (DHPG) (an agonist of group I mGluRs)-induced Ca2+ entry is abolished by Orai1 deficiency, but not affected by knocking down of transient receptor potential cation channel 1 (TRPC1) or TRPC3. Dihydroxyphenylglycine-induced activation of ERKs and modulation of neuronal excitability are abolished in cultured Orai1-deficient neurons. Moreover, DHPG-induced nociceptive behavior is markedly reduced in Orai1-deficient mice. Our findings reveal previously unknown functional coupling between Orai1 and group I mGluRs and shed light on the mechanism underlying group I mGluRs-mediated neuronal plasticity.

Compartmentalized citrullination in Muller glial endfeet during retinal degeneration

Muller glia (MG) play a central role in reactive gliosis, a stress response associated with rare and common retinal degenerative diseases, including age-related macular degeneration (AMD). The posttranslational modification citrullination​ targeting glial fibrillary acidic protein (GFAP) in MG was initially discovered in a panocular chemical injury model. Here, we report in the paradigms of retinal laser injury, a genetic model of spontaneous retinal degeneration (JR5558 mice) and human wet-AMD tissues that MG citrullination is broadly conserved. After laser injury, GFAP polymers that accumulate in reactive MG are citrullinated in MG endfeet and glial cell processes. The enzyme responsible for citrullination, peptidyl arginine deiminase-4 (PAD4), localizes to endfeet and associates with GFAP polymers. Glial cell–specific PAD4 deficiency attenuates retinal hypercitrullination in injured retinas, indicating PAD4 requirement for MG citrullination. In retinas of 1-mo-old JR5558 mice, hypercitrullinated GFAP and PAD4 accumulate in MG endfeet/cell processes in a lesion-specific manner. Finally, we show that human donor maculae from patients with wet-AMD also feature the canonical endfeet localization of hypercitrullinated GFAP. Thus, we propose that endfeet are a “citrullination bunker” that initiates and sustains citrullination in retinal degeneration.

Targeting Ca2+ signaling: A new arsenal against cancer

The drug resistance of cancer cells is a major concern in medical oncology, resulting in the failure of chemotherapy. Ca²⁺ plays a pivotal role in inducing multidrug resistance in cancer cells. Calcium signaling is a critical regulator of many cancer hallmarks, such as angiogenesis, invasiveness, and migration. In this review, we describe the involvement of Ca²⁺ signaling and associated proteins in cancer progression and in the development of multidrug resistance in cancer cells. We also highlight the possibilities and challenges of targeting the Ca²⁺ channels, transporters, and pumps involved in Ca²⁺ signaling in cancer cells through structure-based drug design. This work will open a new therapeutic window to be used against cancer in upcoming years.

Role of Serosal TRPV4-Constituted SOCE Mechanism in Secretagogues-Stimulated Intestinal Epithelial Anion Secretion

As little is known about the role of calcium (Ca²⁺) signaling mediating the small intestinal epithelial anion secretion, we aimed to study its regulatory role in secretagogue-stimulated duodenal anion secretion and the underlying molecular mechanisms. Therefore, intestinal anion secretion from native mouse duodenal epithelia was examined with Ussing chambers to monitor PGE₂-, 5-HT-, and CCh-induced short-circuit currents (I_sc). PGE₂ (10 μM) and 5-HT (10 μM) induced mouse duodenal I_sc, markedly attenuated by serosal Ca²⁺-free solution and selective blockers of store-operated Ca²⁺ channels on the serosal side of the duodenum. Furthermore, PGE₂- and 5-HT-induced duodenal I_sc was also inhibited by ER Ca²⁺ chelator TPEN. However, dantrolene, a selective blocker of ryanodine receptors, inhibited PGE₂-induced duodenal I_sc, while LiCl, an inhibitor of IP₃ production, inhibited 5-HT-induced I_sc. Moreover, duodenal I_sc response to the serosal applications of both PGE₂ and 5-HT was significantly attenuated in transient receptor potential vanilloid 4 (TRPV4) knockout mice. Finally, mucosal application of carbachol (100 μM) also induced duodenal I_sc via selective activation of muscarinic receptors, which was significantly inhibited in serosal Ca²⁺-free solution but neither in mucosal Ca²⁺-free solution nor by nifedipine. Therefore, the serosal TRPV4-constituted SOCE mechanism is likely universal for the most common and important secretagogues-induced and Ca²⁺-dependent intestinal anion secretion. These findings will enhance our knowledge about gastrointestinal (G.I.) epithelial physiology and the associated G.I. diseases, such as diarrhea and constipation.

TRPV4 channels mediate the mechanoresponse in retinal microglia

The physiological and neurological correlates of plummeting brain osmolality during edema, traumatic CNS injury, and severe ischemia are compounded by neuroinflammation. Using multiple approaches, we investigated how retinal microglia respond to challenges mediated by increases in strain, osmotic gradients, and agonists of the stretch-activated cation channel TRPV4. Dissociated and intact microglia were TRPV4-immunoreactive and responded to the selective agonist GSK1016790A and substrate stretch with altered motility and elevations in intracellular calcium ([Ca2+ ]i ). Agonist- and hypotonicity-induced swelling was associated with a nonselective outwardly rectifying cation current, increased [Ca2+ ]i , and retraction of higher-order processes. The antagonist HC067047 reduced the extent of hypotonicity-induced microglial swelling and inhibited the suppressive effects of GSK1016790A and hypotonicity on microglial branching. Microglial TRPV4 signaling required intermediary activation of phospholipase A2 (PLA2), cytochrome P450, and epoxyeicosatrienoic acid production (EETs). The expression pattern of vanilloid thermoTrp genes in retinal microglia was markedly different from retinal neurons, astrocytes, and cortical microglia. These results suggest that TRPV4 represents a primary retinal microglial sensor of osmochallenges under physiological and pathological conditions. Its activation, associated with PLA2, modulates calcium signaling and cell architecture. TRPV4 inhibition might be a useful strategy to suppress microglial overactivation in the swollen and edematous CNS.

Impact of Na+ permeation on collective migration of pulmonary arterial endothelial cells

Collective migration of endothelial cells is important for wound healing and angiogenesis. During such migration, each constituent endothelial cell coordinates its magnitude and direction of migration with its neighbors while retaining intercellular adhesion. Ensuring coordination and cohesion involves a variety of intra- and inter-cellular signaling processes. However, the role of permeation of extracellular Na⁺ in collective cell migration remains unclear. Here, we examined the effect of Na⁺ permeation in collective migration of pulmonary artery endothelial cell (PAEC) monolayers triggered by either a scratch injury or a barrier removal over 24 hours. In the scratch assay, PAEC monolayers migrated in two approximately linear phases. In the first phase, wound closure started with fast speed which then rapidly reduced within 5 hours after scratching. In the second phase, wound closure maintained at slow and stable speed from 6 to 24 hours. In the absence of extracellular Na⁺, the wound closure distance was reduced by >50%. Fewer cells at the leading edge protruded prominent lamellipodia. Beside transient gaps, some sustained interendothelial gaps also formed and progressively increased in size over time, and some fused with adjacent gaps. In the absence of both Na⁺ and scratch injury, PAEC monolayer migrated even more slowly, and interendothelial gaps obviously increased in size towards the end. Pharmacological inhibition of the epithelial Na⁺ channel (ENaC) using amiloride reduced wound closure distance by 30%. Inhibition of both the ENaC and the Na⁺/Ca²⁺ exchanger (NCX) using benzamil further reduced wound closure distance in the second phase and caused accumulation of floating particles in the media. Surprisingly, pharmacological inhibition of the Ca²⁺ release-activated Ca²⁺ (CRAC) channel protein 1 (Orai1) using GSK-7975A, the transient receptor potential channel protein 1 and 4 (TRPC1/4) using Pico145, or both Orai1 and TRPC1/4 using combined GSK-7975A and Pico145 treatment did not affect wound closure distance dramatically. Nevertheless, the combined treatment appeared to cause accumulation of floating particles. Note that GSK-7975A also inhibits small inward Ca²⁺ currents via Orai2 and Orai3 channels, whereas Pico145 also blocks TRPC4, TRPC5, and TRPC1/5 channels. By contrast, gene silence of Orai1 by shRNAs led to a 25% reduction of wound closure in the first 6 hours but had no effect afterwards. However, in the absence of extracellular Na⁺ or cellular injury, Orai1 did not affect PAEC collective migration. Overall, the data reveal that Na⁺ permeation into cells contributes to PAEC monolayer collective migration by increasing lamellipodial formation, reducing accumulation of floating particles, and improving intercellular adhesion.

Calcium signaling in neuroglia

Glial cells exploit calcium (Ca²⁺) signals to perceive the information about the activity of the nervous tissue and the tissue environment to translate this information into an array of homeostatic, signaling and defensive reactions. Astrocytes, the best studied glial cells, use several Ca²⁺ signaling generation pathways that include Ca²⁺ entry through plasma membrane, release from endoplasmic reticulum (ER) and from mitochondria. Activation of metabotropic receptors on the plasma membrane of glial cells is coupled to an enzymatic cascade in which a second messenger, InsP₃ is generated thus activating intracellular Ca²⁺ release channels in the ER endomembrane. Astrocytes also possess store-operated Ca²⁺ entry and express several ligand-gated Ca²⁺ channels. In vivo astrocytes generate heterogeneous Ca²⁺ signals, which are short and frequent in distal processes, but large and relatively rare in soma. In response to neuronal activity intracellular and inter-cellular astrocytic Ca²⁺ waves can be produced. Astrocytic Ca²⁺ signals are involved in secretion, they regulate ion transport across cell membranes, and are contributing to cell morphological plasticity. Therefore, astrocytic Ca²⁺ signals are linked to fundamental functions of the central nervous system ranging from synaptic transmission to behavior. In oligodendrocytes, Ca²⁺ signals are generated by plasmalemmal Ca²⁺ influx, or by release from intracellular stores, or by combination of both. Microglial cells exploit Ca²⁺ permeable ionotropic purinergic receptors and transient receptor potential channels as well as ER Ca²⁺ release. In this contribution, basic morphology of glial cells, glial Ca²⁺ signaling toolkit, intracellular Ca²⁺ signals and Ca²⁺-regulated functions are discussed with focus on astrocytes.

Piezo1 channels mediate trabecular meshwork mechanotransduction and promote aqueous fluid outflow

KEY POINTS

Trabecular meshwork (TM) is a highly mechanosensitive tissue in the eye that regulates intraocular pressure through the control of aqueous humour drainage. Its dysfunction underlies the progression of glaucoma but neither the mechanisms through which TM cells sense pressure nor their role in aqueous humour outflow are understood at the molecular level. We identified the Piezo1 channel as a key TM transducer of tensile stretch, shear flow and pressure. Its activation resulted in intracellular signals that altered organization of the cytoskeleton and cell-extracellular matrix contacts and modulated the trabecular component of aqueous outflow whereas another channel, TRPV4, mediated a delayed mechanoresponse. This study helps elucidate basic mechanotransduction properties that may contribute to intraocular pressure regulation in the vertebrate eye.

ABSTRACT

Chronic elevations in intraocular pressure (IOP) can cause blindness by compromising the function of trabecular meshwork (TM) cells in the anterior eye, but how these cells sense and transduce pressure stimuli is poorly understood. Here, we demonstrate functional expression of two mechanically activated channels in human TM cells. Pressure-induced cell stretch evoked a rapid increase in transmembrane current that was inhibited by antagonists of the mechanogated channel Piezo1, Ruthenium Red and GsMTx4, and attenuated in Piezo1-deficient cells. The majority of TM cells exhibited a delayed stretch-activated current that was mediated independently of Piezo1 by TRPV4 (transient receptor potential cation channel, subfamily V, member 4) channels. Piezo1 functions as the principal TM transducer of physiological levels of shear stress, with both shear and the Piezo1 agonist Yoda1 increasing the number of focal cell-matrix contacts. Analysis of TM-dependent fluid drainage from the anterior eye showed significant inhibition by GsMTx4. Collectively, these results suggest that TM mechanosensitivity utilizes kinetically, regulatory and functionally distinct pressure transducers to inform the cells about force-sensing contexts. Piezo1-dependent control of shear flow sensing, calcium homeostasis, cytoskeletal dynamics and pressure-dependent outflow suggests potential for a novel therapeutic target in treating glaucoma.

Target Molecules of STIM Proteins in the Central Nervous System

Stromal interaction molecules (STIMs), including STIM1 and STIM2, are single-pass transmembrane proteins that are located predominantly in the endoplasmic reticulum (ER). They serve as calcium ion (Ca²⁺) sensors within the ER. In the central nervous system (CNS), they are involved mainly in Orai-mediated store-operated Ca²⁺ entry (SOCE). The key molecular components of the SOCE pathway are well-characterized, but the molecular mechanisms that underlie the regulation of this pathway need further investigation. Numerous intracellular target proteins that are located in the plasma membrane, ER, cytoskeleton, and cytoplasm have been reported to play essential roles in concert with STIMs, such as conformational changes in STIMs, their translocation, the stabilization of their interactions with Orai, and the activation of other channels. The present review focuses on numerous regulators, such as Homer, SOCE-associated regulatory factor (SARAF), septin, synaptopodin, golli proteins, partner of STIM1 (POST), and transcription factors and proteasome inhibitors that regulate STIM-Orai interactions in the CNS. Further we describe novel roles of STIMs in mediating Ca²⁺ influx via other than Orai pathways, including TRPC channels, VGCCs, AMPA and NMDA receptors, and group I metabotropic glutamate receptors. This review also summarizes recent findings on additional molecular targets of STIM proteins including SERCA, IP₃Rs, end-binding proteins (EB), presenilin, and CaMKII. Dysregulation of the SOCE-associated toolkit, including STIMs, contributes to the development of neurodegenerative disorders (e.g., Alzheimer's disease, Parkinson's disease, and Huntington's disease), traumatic brain injury, epilepsy, and stroke. Emerging evidence points to the role of STIM proteins and several of their molecular effectors and regulators in neuronal and glial physiology and pathology, suggesting their potential application for future therapeutic strategies.

Store-Operated Calcium Channels in Physiological and Pathological States of the Nervous System

Store-operated calcium channels (SOCs) are widely expressed in excitatory and non-excitatory cells where they mediate significant store-operated calcium entry (SOCE), an important pathway for calcium signaling throughout the body. While the activity of SOCs has been well studied in non-excitable cells, attention has turned to their role in neurons and glia in recent years. In particular, the role of SOCs in the nervous system has been extensively investigated, with links to their dysregulation found in a wide variety of neurological diseases from Alzheimer’s disease (AD) to pain. In this review, we provide an overview of their molecular components, expression, and physiological role in the nervous system and describe how the dysregulation of those roles could potentially lead to various neurological disorders. Although further studies are still needed to understand how SOCs are activated under physiological conditions and how they are linked to pathological states, growing evidence indicates that SOCs are important players in neurological disorders and could be potential new targets for therapies. While the role of SOCE in the nervous system continues to be multifaceted and controversial, the study of SOCs provides a potentially fruitful avenue into better understanding the nervous system and its pathologies.

Calcium Permeable Channels in Cancer Hallmarks

Cancer, the second cause of death worldwide, is characterized by several common criteria, known as the “cancer hallmarks” such as unrestrained cell proliferation, cell death resistance, angiogenesis, invasion and metastasis. Calcium permeable channels are proteins present in external and internal biological membranes, diffusing Ca²⁺ ions down their electrochemical gradient. Numerous physiological functions are mediated by calcium channels, ranging from intracellular calcium homeostasis to sensory transduction. Consequently, calcium channels play important roles in human physiology and it is not a surprise the increasing number of evidences connecting calcium channels disorders with tumor cells growth, survival and migration. Multiple studies suggest that calcium signals are augmented in various cancer cell types, contributing to cancer hallmarks. This review focuses in the role of calcium permeable channels signaling in cancer with special attention to the mechanisms behind the remodeling of the calcium signals. Transient Receptor Potential (TRP) channels and Store Operated Channels (SOC) are the main extracellular Ca²⁺ source in the plasma membrane of non-excitable cells, while inositol trisphosphate receptors (IP₃R) are the main channels releasing Ca²⁺ from the endoplasmic reticulum (ER). Alterations in the function and/or expression of these calcium channels, as wells as, the calcium buffering by mitochondria affect intracellular calcium homeostasis and signaling, contributing to the transformation of normal cells into their tumor counterparts. Several compounds reported to counteract several cancer hallmarks also modulate the activity and/or the expression of these channels including non-steroidal anti-inflammatory drugs (NSAIDs) like sulindac and aspirin, and inhibitors of polyamine biosynthesis, like difluoromethylornithine (DFMO). The possible role of the calcium permeable channels targeted by these compounds in cancer and their action mechanism will be discussed also in the review.

TRPCs: Influential Mediators in Skeletal Muscle

Ca²⁺ itself or Ca²⁺-dependent signaling pathways play fundamental roles in various cellular processes from cell growth to death. The most representative example can be found in skeletal muscle cells where a well-timed and adequate supply of Ca²⁺ is required for coordinated Ca²⁺-dependent skeletal muscle functions, such as the interactions of contractile proteins during contraction. Intracellular Ca²⁺ movements between the cytosol and sarcoplasmic reticulum (SR) are strictly regulated to maintain the appropriate Ca²⁺ supply in skeletal muscle cells. Added to intracellular Ca²⁺ movements, the contribution of extracellular Ca²⁺ entry to skeletal muscle functions and its significance have been continuously studied since the early 1990s. Here, studies on the roles of channel proteins that mediate extracellular Ca²⁺ entry into skeletal muscle cells using skeletal myoblasts, myotubes, fibers, tissue, or skeletal muscle-originated cell lines are reviewed with special attention to the proposed functions of transient receptor potential canonical proteins (TRPCs) as store-operated Ca²⁺ entry (SOCE) channels under normal conditions and the potential abnormal properties of TRPCs in muscle diseases such as Duchenne muscular dystrophy (DMD).

Pharmacological Preconditioning Using Diazoxide Regulates Store-Operated Ca2 + Channels in Adult Rat Cardiomyocytes

Voltage-dependent Ca²⁺ channels and store-operated Ca²⁺ channels (SOCs) are the major routes of Ca²⁺ entry into mammalian cells. Previously, we reported that pharmacological preconditioning (PPC) leads to a decrease in the amplitude of L-type calcium channel current in the heart. In this study, we examined PPC-associated changes in SOC function. We measured adult cardiomyocyte membrane currents using the whole-cell patch-clamp technique, and we evaluated reactive oxygen species (ROS) production and intracellular Ca²⁺ levels in cardiomyocytes using fluorescent probes. Diazoxide (Dzx) and thapsigargin (Tg) were used to induce PPC and to deplete internal stores of Ca²⁺, respectively. Ca²⁺ store depletion generated inward currents with strong rectification, which were suppressed by the SOC blocker GSK-7975-A. These currents were completely abolished by PPC, an effect that could be countered with 5-hydroxydecanoate (5-HD; a selective mitochondrial ATP-sensitive K⁺ channel blocker), an intracellular mitochondrial energizing solution, or Ni²⁺ [a blocker of sodium-calcium exchanger (NCX)]. Buffering of ROS and intracellular Ca²⁺ also prevented PPC effects on SOC currents. Refilling of intracellular stores was largely suppressed by PPC, as determined by measuring intracellular Ca²⁺ with a fluorescent Ca²⁺ indicator. These results indicate that influx of Ca²⁺ through SOCs is inhibited by their ROS and Ca²⁺-dependent inactivation during PPC and that NCX is a likely source of PPC-inactivating Ca²⁺. We further showed that NCX associates with Orai1. Down-regulation of SOCs by PPC may play a role in cardioprotection following ischemia-reperfusion.

Volume sensing in the transient receptor potential vanilloid 4 ion channel is cell type-specific and mediated by an N-terminal volume-sensing domain

Many retinal diseases are associated with pathological cell swelling, but the underlying etiology remains to be established. A key component of the volume-sensitive machinery, the transient receptor potential vanilloid 4 (TRPV4) ion channel, may represent a sensor and transducer of cell swelling, but the molecular link between the swelling and TRPV4 activation is unresolved. Here, our results from experiments using electrophysiology, cell volumetric measurements, and fluorescence imaging conducted in murine retinal cells and Xenopus oocytes indicated that cell swelling in the physiological range activated TRPV4 in Müller glia and Xenopus oocytes, but required phospholipase A₂ (PLA₂) activity exclusively in Müller cells. Volume-dependent TRPV4 gating was independent of cytoskeletal rearrangements and phosphorylation. Our findings also revealed that TRPV4-mediated transduction of volume changes is dependent by its N terminus, more specifically by its distal-most part. We conclude that the volume sensitivity and function of TRPV4 in situ depend critically on its functional and cell type-specific interactions.

Physiology of Astroglia

Astrocytes are principal cells responsible for maintaining the brain homeostasis. Additionally, these glial cells are also involved in homocellular (astrocyte-astrocyte) and heterocellular (astrocyte-other cell types) signalling and metabolism. These astroglial functions require an expression of the assortment of molecules, be that transporters or pumps, to maintain ion concentration gradients across the plasmalemma and the membrane of the endoplasmic reticulum. Astrocytes sense and balance their neurochemical environment via variety of transmitter receptors and transporters. As they are electrically non-excitable, astrocytes display intracellular calcium and sodium fluctuations, which are not only used for operative signalling but can also affect metabolism. In this chapter we discuss the molecules that achieve ionic gradients and underlie astrocyte signalling.

Enhancement of Cardiac Store Operated Calcium Entry (SOCE) within Novel Intercalated Disk Microdomains in Arrhythmic Disease

Store-operated Ca²⁺ entry (SOCE), a major Ca²⁺ signaling mechanism in non-myocyte cells, has recently emerged as a component of Ca²⁺ signaling in cardiac myocytes. Though it has been reported to play a role in cardiac arrhythmias and to be upregulated in cardiac disease, little is known about the fundamental properties of cardiac SOCE, its structural underpinnings or effector targets. An even greater question is how SOCE interacts with canonical excitation-contraction coupling (ECC). We undertook a multiscale structural and functional investigation of SOCE in cardiac myocytes from healthy mice (wild type; WT) and from a genetic murine model of arrhythmic disease (catecholaminergic ventricular tachycardia; CPVT). Here we provide the first demonstration of local, transient Ca²⁺ entry (LoCE) events, which comprise cardiac SOCE. Although infrequent in WT myocytes, LoCEs occurred with greater frequency and amplitude in CPVT myocytes. CPVT myocytes also evidenced characteristic arrhythmogenic spontaneous Ca²⁺ waves under cholinergic stress, which were effectively prevented by SOCE inhibition. In a surprising finding, we report that both LoCEs and their underlying protein machinery are concentrated at the intercalated disk (ID). Therefore, localization of cardiac SOCE in the ID compartment has important implications for SOCE-mediated signaling, arrhythmogenesis and intercellular mechanical and electrical coupling in health and disease.

Uncovering malathion (an organophosphate insecticide) action on Ca2+ signal transduction and investigating the effects of BAPTA-AM (a cell-permeant Ca2+ chelator) on protective responses in glial cells

Malathion, one of commonly used organophosphate insecticides, has a wide range of toxic actions in different models. However, the effect of this compound on Ca²⁺ homeostasis and its related cytotoxicity in glial cells is elusive. This study examined whether malathion evoked intracellular Ca²⁺ concentration ([Ca²⁺]_i) rises and established the relationship between Ca²⁺ signaling and cytotoxicity in normal human astrocytes, rat astrocytes and human glioblastoma cells. The data show that malathion induced concentration-dependent [Ca²⁺]_i rises in Gibco^® Human Astrocytes (GHA cells), but not in DI TNC1 normal rat astrocytes and DBTRG-05MG human glioblastoma cells. In GHA cells, this Ca²⁺ signal response was reduced by removing extracellular Ca²⁺. In Ca²⁺-free medium, pretreatment with the endoplasmic reticulum Ca²⁺ pump inhibitor thapsigargin abolished malathion-induced [Ca²⁺]_i rises. Conversely, incubation with malathion abolished thapsigargin-induced [Ca²⁺]_i rises. Inhibition of phospholipase C (PLC) with U73122 also blocked malathion-induced [Ca²⁺]_i rises. In Ca²⁺-containing medium, malathion-induced [Ca²⁺]_i rises was inhibited by store-operated Ca²⁺ channel blockers (2-APB, econazole or SKF96365) and the protein kinase C (PKC) inhibitor GF109203X. Malathion (5-25 μM) concentration-dependently caused cytotoxicity in GHA, DI TNC1 and DBTRG-05MG cells. This cytotoxic effect was partially prevented by prechelating cytosolic Ca²⁺ with BAPTA-AM (a selective Ca²⁺ chelator) only in GHA cells. Together, in GHA but not in DI TNC1 and DBTRG-05MG cells, malathion induced [Ca²⁺]_i rises by inducing PLC-dependent Ca²⁺ release from the endoplasmic reticulum and Ca²⁺ entry via PKC-sensitive store-operated Ca²⁺ channels. Furthermore, malathion induced Ca²⁺-associated cytotoxicity, suggesting that Ca²⁺ chelating may have a protective effect on malathion-induced cytotoxicity in normal human astrocytes.

Molecular mechanisms of caffeine-mediated intestinal epithelial ion transports

BACKGROUND AND PURPOSE

As little is known about the effect of caffeine, one of the most widely consumed substances worldwide, on intestinal function, we aimed to study its action on intestinal anion secretion and the underlying molecular mechanisms.

EXPERIMENTAL APPROACH

Anion secretion and channel expression were examined in mouse duodenal epithelium by Ussing chambers and immunocytochemistry. Ca²⁺ imaging was also performed in intestinal epithelial cells (IECs).

KEY RESULTS

Caffeine (10 mM) markedly increased mouse duodenal short-circuit current (I_sc ), which was attenuated by a removal of either Cl^- or HCO₃ ^- , Ca²⁺ -free serosal solutions and selective blockers of store-operated Ca²⁺ channels (SOC/Ca²⁺ release-activated Ca²⁺ channels), and knockdown of Orai1 channels on the serosal side of duodenal tissues. Caffeine induced SOC entry in IEC, which was inhibited by ruthenium red and selective blockers of SOC. Caffeine-stimulated duodenal I_sc was inhibited by the endoplasmic reticulum Ca²⁺ chelator (N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine), selective blockers (ruthenium red and dantrolene) of ryanodine receptors (RyR), and of Ca²⁺ -activated Cl^- channels (niflumic acid and T16A). There was synergism between cAMP and Ca²⁺ signalling, in which cAMP/PKA promoted caffeine/Ca²⁺ -mediated anion secretion. Expression of STIM1 and Orai1 was detected in mouse duodenal mucosa and human IECs. The Orai1 proteins were primarily co-located with the basolateral marker Na⁺ , K⁺ -ATPase.

CONCLUSIONS AND IMPLICATIONS

Caffeine stimulated intestinal anion secretion mainly through the RyR/Orai1/Ca²⁺ signalling pathway. There is synergism between cAMP/PKA and caffeine/Ca²⁺ -mediated anion secretion. Our findings suggest that a caffeine-mediated RyR/Orai1/Ca²⁺ pathway could provide novel potential drug targets to control intestinal anion secretion.

TREK-1 channels regulate pressure sensitivity and calcium signaling in trabecular meshwork cells

The trabecular meshwork (TM) plays a fundamental role in intraocular pressure regulation, but its mechanotransduction pathway is poorly understood. Yarishkin et al. show that the mechanosensing channel TREK-1 regulates TM membrane potential, pressure sensitivity, calcium homeostasis, and impedance.

Mechanotransduction by the trabecular meshwork (TM) is an essential component of intraocular pressure regulation in the vertebrate eye. This process is compromised in glaucoma but is poorly understood. In this study, we identify transient receptor potential vanilloid isoform 4 (TRPV4) and TWIK-related potassium channel-1 (TREK-1) as key molecular determinants of TM membrane potential, pressure sensitivity, calcium homeostasis, and transcellular permeability. We show that resting membrane potential in human TM cells is unaffected by “classical” inhibitors of voltage-activated, calcium-activated, and inwardly rectifying potassium channels but is depolarized by blockers of tandem-pore K⁺ channels. Using gene profiling, we reveal the presence of TREK-1, TASK-1, TWIK-2, and THIK transcripts in TM cells. Pressure stimuli, arachidonic acid, and TREK-1 activators hyperpolarize these cells, effects that are antagonized by quinine, amlodipine, spadin, and short-hairpin RNA–mediated knockdown of TREK-1 but not TASK-1. Activation and inhibition of TREK-1 modulates [Ca²⁺]_TM and lowers the impedance of cell monolayers. Together, these results suggest that tensile homeostasis in the TM may be regulated by balanced, pressure-dependent activation of TRPV4 and TREK-1 mechanotransducers.

Polymodal TRPV1 and TRPV4 Sensors Colocalize but Do Not Functionally Interact in a Subpopulation of Mouse Retinal Ganglion Cells

Retinal ganglion cells (RGCs) are projection neurons that transmit the visual signal from the retina to the brain. Their excitability and survival can be strongly influenced by mechanical stressors, temperature, lipid metabolites, and inflammatory mediators but the transduction mechanisms for these non-synaptic sensory inputs are not well characterized. Here, we investigate the distribution, functional expression, and localization of two polymodal transducers of mechanical, lipid, and inflammatory signals, TRPV1 and TRPV4 cation channels, in mouse RGCs. The most abundant vanilloid mRNA species was Trpv4, followed by Trpv2 and residual expression of Trpv3 and Trpv1. Immunohistochemical and functional analyses showed that TRPV1 and TRPV4 channels are expressed as separate molecular entities, with TRPV1-only (∼10%), TRPV4-only (∼40%), and TRPV1 + TRPV4 (∼10%) expressing RGC subpopulations. The TRPV1 + TRPV4 cohort included SMI-32-immunopositive alpha RGCs, suggesting potential roles for polymodal signal transduction in modulation of fast visual signaling. Arguing against obligatory heteromerization, optical imaging showed that activation and desensitization of TRPV1 and TRPV4 responses evoked by capsaicin and GSK1016790A are independent of each other. Overall, these data predict that RGC subpopulations will be differentially sensitive to mechanical and inflammatory stressors.

High pH-Sensitive Store-Operated Ca2+ Entry Mediated by Ca2+ Release-Activated Ca2+ Channels in Rat Odontoblasts

Odontoblasts play a crucial role in dentin formation and sensory transduction following the application of stimuli to the dentin surface. Various exogenous and endogenous stimuli elicit an increase in the intracellular free calcium concentration ([Ca²⁺]_i) in odontoblasts, which is mediated by Ca²⁺ release from intracellular Ca²⁺ stores and/or Ca²⁺ influx from the extracellular medium. In a previous study, we demonstrated that the depletion of Ca²⁺ stores in odontoblasts activated store-operated Ca²⁺ entry (SOCE), a Ca²⁺ influx pathway. However, the precise biophysical and pharmacological properties of SOCE in odontoblasts have remained unclear. In the present study, we examined the functional expression and pharmacological properties of Ca²⁺ release-activated Ca²⁺ (CRAC) channels that mediate SOCE and evaluated the alkali sensitivity of SOCE in rat odontoblasts. In the absence of extracellular Ca²⁺, treatment with thapsigargin (TG), a sarco/endoplasmic reticulum Ca²⁺-ATPase inhibitor, induced an increase in [Ca²⁺]_i. After [Ca²⁺]_i returned to near-resting levels, the subsequent application of 2.5 mM extracellular Ca²⁺ resulted in an increase in [Ca²⁺]_i which is a typical of SOCE activation. Additionally, application of 2-methylthioadenosine diphosphate trisodium salt (2-MeSADP), a P2Y₁,₁₂,₁₃ receptor agonist, or carbachol (CCh), a muscarinic cholinergic receptor agonist, in the absence of extracellular Ca²⁺, induced a transient increase in [Ca²⁺]_i. The subsequent addition of extracellular Ca²⁺ resulted in significantly higher [Ca²⁺]_i in 2-MeSADP- or CCh-treated odontoblasts than in untreated cells. SOCE, that is activated by addition of extracellular Ca²⁺ in the TG pretreated odontoblasts was then suppressed by Synta66, BTP2, or lanthanum, which are CRAC channel inhibitors. Treatment with an alkaline solution enhanced SOCE, while treatment with HC030031, a TRPA1 channel antagonist, inhibited it. The amplitude of SOCE at pH 9 in the presence of HC030031 was higher than that at pH 7.4 in the absence of HC030031. These findings indicate that CRAC channel-mediated alkali-sensitive SOCE occurs in odontoblasts. SOCE is mediated by P2Y and muscarinic-cholinergic receptors, which are activated by endogenous ligands in odontoblasts.

Mechanism and consequence of abnormal calcium homeostasis in Rett syndrome astrocytes

Astrocytes play an important role in Rett syndrome (RTT) disease progression. Although the non-cell-autonomous effect of RTT astrocytes on neurons was documented, cell-autonomous phenotypes and mechanisms within RTT astrocytes are not well understood. We report that spontaneous calcium activity is abnormal in RTT astrocytes in vitro, in situ, and in vivo. Such abnormal calcium activity is mediated by calcium overload in the endoplasmic reticulum caused by abnormal store operated calcium entry, which is in part dependent on elevated expression of TRPC4. Furthermore, the abnormal calcium activity leads to excessive activation of extrasynaptic NMDA receptors (eNMDARs) on neighboring neurons and increased network excitability in Mecp2 knockout mice. Finally, both the abnormal astrocytic calcium activity and the excessive activation of eNMDARs are caused by Mecp2 deletion in astrocytes in vivo. Our findings provide evidence that abnormal calcium homeostasis is a key cell-autonomous phenotype in RTT astrocytes, and reveal its mechanism and consequence.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Dyslipidemia modulates Müller glial sensing and transduction of ambient information

Unesterified cholesterol controls the fluidity, permeability and electrical properties of eukaryotic cell membranes. Consequently, cholesterol levels in the retina and the brain are tightly regulated whereas depletion or oversupply caused by diet or heredity contribute to neurodegenerative diseases and vision loss. Astroglia play a central role in the biosynthesis, uptake and transport of cholesterol and also drive inflammatory signaling under hypercholesterolemic conditions associated with high-fat diet (diabetes) and neurodegenerative disease. A growing body of evidence shows that unesterified membrane cholesterol modulates the ability of glia to sense and transduce ambient information. Cholesterol-dependence of Müller glia - which function as retinal sentinels for metabolic, mechanical, osmotic and inflammatory signals - is mediated in part by transient receptor potential V4 (TRPV4) channels. Cholesterol supplementation facilitates, whereas depletion suppresses, TRPV4-mediated transduction of temperature and lipid agonists in Müller cells. Acute effects of cholesterol supplementation/depletion on plasma membrane ion channels and calcium homeostasis differ markedly from the effects of chronic dyslipidemia, possibly due to differential modulation of modality-dependent energy barriers associated with the functionality of polymodal channels embedded within lipid rafts. Understanding of cholesterol-dependence of TRP channels is thus providing insight into dyslipidemic pathologies associated with diabetic retinopathy, glaucoma and macular degeneration.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Molecular mechanisms of calcium signaling in the modulation of small intestinal ion transports and bicarbonate secretion

Background and Purpose:

Although Ca²⁺ signaling may stimulate small intestinal ion secretion, little is known about its critical role and the molecular mechanisms of Ca²⁺-mediated biological action.

Key Results

Activation of muscarinic receptors by carbachol(CCh) stimulated mouse duodenal I_sc, which was significantly inhibited in Ca²⁺-free serosal solution and by several selective store-operated Ca²⁺ channels(SOC) blockers added to the serosal side of duodenal tissues. Furthermore, we found that CRAC/Orai channels may represent the molecular candidate of SOC in intestinal epithelium. CCh increased intracellular Ca²⁺ but not cAMP, and Ca²⁺ signaling mediated duodenal Cl^- and HCO₃^- secretion in wild type mice but not in CFTR knockout mice. CCh induced duodenal ion secretion and stimulated PI3K/Akt activity in duodenal epithelium, all of which were inhibited by selective PI3K inhibitors with different structures. CCh-induced Ca²⁺ signaling also stimulated the phosphorylation of CFTR proteins and their trafficking to the plasma membrane of duodenal epithelial cells, which were inhibited again by selective PI3K inhibitors.

Materials and Methods

Functional, biochemical and morphological experiments were performed to examine ion secretion, PI3K/Akt and CFTR activity of mouse duodenal epithelium. Ca²⁺ imaging was performed on HT-29 cells.

Conclusions and Implications

Ca²⁺ signaling plays a critical role in intestinal ion secretion via CRAC/Orai-mediated SOCE mechanism on the serosal side of epithelium. We also demonstrated the molecular mechanisms of Ca²⁺ signaling in CFTR-mediated secretion via novel PI3K/Akt pathway. Our findings suggest new perspectives for drug targets to protect the upper GI tract and control liquid homeostasis in the small intestine.

Calcium influx through TRPV4 channels modulates the adherens contacts between retinal microvascular endothelial cells

KEY POINTS

Endothelial cells employ transient receptor potential isoform 4 (TRPV4) channels to sense ambient mechanical and chemical stimuli. In retinal microvascular endothelial cells, TRPV4 channels regulate calcium homeostasis, cytoskeletal signalling and the organization of adherens junctional contacts. Intracellular calcium increases induced by TRPV4 agonists include a significant contribution from calcium release from internal stores. Activation of TRPV4 channels regulates retinal endothelial barriers in vitro and in vivo. TRPV4 sensing may provide a feedback mechanism between sensing shear flow and eicosanoid modulators, vascular permeability and contractility at the inner retinal endothelial barrier.

ABSTRACT

The identity of microvascular endothelial (MVE) mechanosensors that sense blood flow in response to mechanical and chemical stimuli and regulate vascular permeability in the retina is unknown. Using immunohistochemistry, calcium imaging, electrophysiology, impedance measurements and vascular permeability assays, we show that the transient receptor potential isoform 4 (TRPV4) plays a major role in Ca²⁺ /cation signalling, cytoskeletal remodelling and barrier function in retinal microvasculature in vitro and in vivo. Human retinal MVE cells (HrMVECs) predominantly expressed Trpv1 and Trpv4 transcripts, and TRPV4 was broadly localized to the plasma membrane of cultured cells and intact blood vessels in the inner retina. Treatment with the selective TRPV4 agonist GSK1016790A (GSK101) activated a nonselective cation current, robustly elevated [Ca²⁺ ]_i and reversibly increased the permeability of MVEC monolayers. This was associated with disrupted organization of endothelial F-actin, downregulated expression of occludin and remodelling of adherens contacts consisting of vascular endothelial cadherin (VE-cadherin) and β-catenin. In vivo, GSK101 increased the permeability of retinal blood vessels in wild type but not in TRPV4 knockout mice. Agonist-evoked effects on barrier permeability and cytoskeletal reorganization were antagonized by the selective TRPV4 blocker HC 067047. Human choroidal endothelial cells expressed lower TRPV4 mRNA/protein levels and showed less pronounced agonist-evoked calcium signals compared to MVECs. These findings indicate a major role for TRPV4 in Ca²⁺ homeostasis and barrier function in human retinal capillaries and suggest that TRPV4 may differentially contribute to the inner vs. outer blood-retinal barrier function.

Mouse retinal ganglion cell signalling is dynamically modulated through parallel anterograde activation of cannabinoid and vanilloid pathways

KEY POINTS

Retinal cells use vanilloid transient receptor potential (TRP) channels to integrate light-evoked signals with ambient mechanical, chemical and temperature information. Localization and function of the polymodal non-selective cation channel TRPV1 (transient receptor potential vanilloid isoform 1) remains elusive. TRPV1 is expressed in a subset of mouse retinal ganglion cells (RGCs) with peak expression in the mid-peripheral retina. Endocannabinoids directly activate TRPV1 and inhibit it through cannabinoid type 1 receptors (CB1Rs) and cAMP pathways. Activity-dependent endocannabinoid release may modulate signal gain in RGCs through simultaneous manipulation of calcium and cAMP signals mediated by TRPV1 and CB1R.

ABSTRACT

How retinal ganglion cells (RGCs) process and integrate synaptic, mechanical, swelling stimuli with light inputs is an area of intense debate. The nociceptive cation channel TRPV1 (transient receptor potential vanilloid type 1) modulates RGC Ca2+ signals and excitability yet the proportion of RGCs that express it remains unclear. Furthermore, TRPV1's response to endocannabinoids (eCBs), the putative endogenous retinal activators, is unknown, as is the potential modulation by cannabinoid receptors (CBRs). The density of TRPV1-expressing RGCs in the Ai9:Trpv1 reporter mouse peaked in the mid-peripheral retina. TRPV1 agonists including capsaicin (CAP) and the eCBs anandamide and N-arachidonoyl-dopamine elevated [Ca2+ ]i in 30-40% of wild-type RGCs, with effects suppressed by TRPV1 antagonists capsazepine (CPZ) and BCTC ((4-(3-chloro-2-pyridinyl)-N-[4-(1,1-dimethylethyl)phenyl]-1-piperazinecarboxamide), and lacking in Trpv1-/- cells. The cannabinoid receptor type 1 (CB1R) colocalized with TRPV1:tdTomato expression. Its agonists 2-arachidonoylglycerol (2-AG) and WIN55,122 inhibited CAP-induced [Ca2+ ]i signals in adult, but not early postnatal, RGCs. The suppressive effect of 2-AG on TRPV1 activation was emulated by positive modulators of the protein kinase A (PKA) pathway, inhibited by the CB1R antagonist rimonabant and Gi uncoupler pertussis toxin, and absent in Cnr1-/- RGCs. We conclude that TRPV1 is a modulator of Ca2+ homeostasis in a subset of RGCs that show non-uniform distribution across the mouse retina. Non-retrograde eCB-mediated modulation of RGC signalling involves a dynamic push-pull between direct TRPV1 activation and PKA-dependent regulation of channel inactivation, with potential functions in setting the bandwidth of postsynaptic responses, sensitivity to mechanical/excitotoxic stress and neuroprotection.

Cholesterol regulates polymodal sensory transduction in Müller glia

Over- and underexposure to cholesterol activates glia in neurodegenerative brain and retinal diseases but the molecular targets of cholesterol in glial cells are not known. Here, we report that disruption of unesterified membrane cholesterol content modulates the transduction of chemical, mechanical and temperature stimuli in mouse Müller cells. Activation of TRPV4 (transient receptor potential vanilloid type 4), a nonselective polymodal cation channel was studied following the removal or supplementation of cholesterol using the methyl-beta cyclodextrin (MβCD) delivery vehicle. Cholesterol extraction disrupted lipid rafts and caveolae without affecting TRPV4 trafficking or membrane localization protein. However, MβCD suppressed agonist (GSK1016790A)- and temperature-evoked elevations in [Ca²⁺ ]_i , and suppressed transcellular propagation of Ca²⁺ waves. Lowering the free membrane cholesterol content markedly prolonged the time-course of the glial swelling response, whereas MβCD:cholesterol supplementation enhanced agonist- and temperature-induced Ca²⁺ signals and shortened the swelling response. Taken together, these data show that membrane cholesterol modulates polymodal transduction of agonists, swelling and temperature stimuli in retinal radial glia and suggest that dyslipidemic retinas might be associated with abnormal glial transduction of ambient sensory inputs.

Spatiotemporal control of mitochondrial network dynamics in astroglial cells

Mitochondria are increasingly recognized for playing important roles in regulating the evolving metabolic state of mammalian cells. This is particularly true for nerve cells, as dysregulation of mitochondrial dynamics is invariably associated with a number of neuropathies. Accumulating evidence now reveals that changes in mitochondrial dynamics and structure may play equally important roles also in the cell biology of astroglial cells. Astroglial cells display significant heterogeneity in their morphology and specialized functions across different brain regions, however besides fundamental differences they seem to share a surprisingly complex meshwork of mitochondria, which is highly suggestive of tightly regulated mechanisms that contribute to maintain this unique architecture. Here, we summarize recent work performed in astrocytes in situ indicating that this may indeed be the case, with astrocytic mitochondrial networks shown to experience rapid dynamic changes in response to defined external cues. Although the mechanisms underlying this degree of mitochondrial re-shaping are far from being understood, recent data suggest that they may contribute to demarcate astrocyte territories undergoing key signalling and metabolic functions.

TRPV4 Contributes to Resting Membrane Potential in Retinal Müller Cells: Implications in Cell Volume Regulation

Neural activity alters osmotic gradients favoring cell swelling in retinal Müller cells. This swelling is followed by a regulatory volume decrease (RVD), partially mediated by an efflux of KCl and water. The transient receptor potential channel 4 (TRPV4), a nonselective calcium channel, has been proposed as a candidate for mediating intracellular Ca²⁺ elevation induced by swelling. We previously demonstrated in a human Müller cell line (MIO-M1) that RVD strongly depends on ion channel activation and, consequently, on membrane potential (V_m ). The aim of this study was to investigate if Ca²⁺ influx via TRPV4 contributes to RVD by modifying intracellular Ca²⁺ concentration and/or modulating V_m in MIO-M1 cells. Cell volume, intracellular Ca²⁺ levels, and V_m changes were evaluated using fluorescent probes. Results showed that MIO-M1 cells express functional TRPV4 which determines the resting V_m associated with K⁺ channels. Swelling-induced increases in Ca²⁺ levels was due to both Ca²⁺ release from intracellular stores and Ca²⁺ influx by a pathway alternative to TRPV4. TRPV4 blockage affected swelling-induced biphasic response (depolarization-repolarization), suggesting its participation in modulating V_m changes during RVD. Agonist stimulation of Ca²⁺ influx via TRPV4 activated K⁺ channels hyperpolarizing V_m and accelerating RVD. We propose that TRPV4 forms a signaling complex with Ca²⁺ and/or voltage-dependent K⁺ channels to define resting V_m and V_m changes during RVD. TRPV4 involvement in RVD depends on the type of stimuli and/or degree of channel activation, leading to a maximum RVD response when Ca²⁺ influx overcomes a threshold and activates further signaling pathways in cell volume regulation. J. Cell. Biochem. 118: 2302-2313, 2017. © 2017 Wiley Periodicals, Inc.

Expression of peptidylarginine deiminase 4 in an alkali injury model of retinal gliosis

Citrullination is an important posttranslational modification that occurs during retinal gliosis. We examined the expression of peptidyl arginine deiminases (PADs) to identify the PADs that mediate citrullination in a model of alkali-induced retinal gliosis. Mouse corneas were exposed to 1.0 N NaOH and posterior eye tissue from injured and control uninjured eyes was evaluated for transcript levels of various PADs by reverse-transcription polymerase chain reaction (RT-PCR), and quantitative RT-PCR (qPCR). Retinas were also subjected to immunohistochemistry (IHC) for glial fibrillary acidic protein (GFAP), citrullinated species, PAD2, and PAD4 and tissue levels of GFAP, citrullinated species, and PAD4 were measured by western blots. In other experiments, the PAD4 inhibitor streptonigrin was injected intravitreally into injured eyes ex vivo to test inhibitory activity in an organ culture system. We found that uninjured retina and choroid expressed Pad2 and Pad4 transcripts. Pad4 transcript levels increased by day 7 post-injury (p < 0.05), whereas Pad2 levels did not change significantly (p > 0.05) by qPCR. By IHC, PAD2 was expressed in uninjured eyes along ganglion cell astrocytes, but in injured retina PAD2 was downregulated at 7 days. On the other hand, PAD4 showed increased staining in the retina upon injury revealing a pattern that overlapped with filamentous GFAP staining in Müller glial processes by 7 days. Injury-induced citrullination and soluble GFAP protein levels were reduced by PAD4 inhibition in western blot experiments of organ cultures. Together, our findings for the first time identify PAD4 as a novel injury-inducible druggable target for retinal gliosis.