The Ca2+ export pump PMCA clears near-membrane Ca2+ to facilitate store-operated Ca2+ entry and NFAT activation

STIM Proteins: The Gas and Brake of Calcium Entry in Neurons

Stromal interaction molecules (STIM)s are Ca2+ sensors in internal Ca2+ stores of the endoplasmic reticulum. They activate the store-operated Ca2+ channels, which are the main source of Ca2+ entry in non-excitable cells. Moreover, STIM proteins interact with other Ca2+ channel subunits and active transporters, making STIMs an important intermediate molecule in orchestrating a wide variety of Ca2+ influxes into excitable cells. Nevertheless, little is known about the role of STIM proteins in brain functioning. Being involved in many signaling pathways, STIMs replenish internal Ca2+ stores in neurons and mediate synaptic transmission and neuronal excitability. Ca2+ dyshomeostasis is a signature of many pathological conditions of the brain, including neurodegenerative diseases, injuries, stroke, and epilepsy. STIMs play a role in these disturbances not only by supporting abnormal store-operated Ca2+ entry but also by regulating Ca2+ influx through other channels. Here, we review the present knowledge of STIMs in neurons and their involvement in brain pathology.

Ca2+ puffs underlie adhesion-triggered Ca2+ microdomains in T cells

Ca2+ signalling is pivotal in T cell activation, an essential process in adaptive immune responses. Key to this activation are Ca2+ microdomains, which are transient increases in cytosolic Ca2+ concentration occurring within narrow regions between the endoplasmic reticulum (ER) and the plasma membrane (PM), lasting a few tens of milliseconds. Adhesion Dependent Ca2+ Microdomains (ADCM) rely on store-operated Ca2+ entry (SOCE) via the ORAI/STIM system. The nanometric scale at which these microdomains form poses challenges for direct experimental observation. Following the previous work of Gil et al. [1], which introduced a three-dimensional model of the ER-PM junction, this study combines a detailed description of the Ca2+ fluxes at the junction with stochastic dynamics of a cluster of D-myo-inositol 1,4,5 trisphosphate receptors (IP3R) located in the ER surrounding the junction. Because the consideration of Ca2+ release through the IP3R calls for the simulation of a portion of the cytoplasm considerably larger than the junction, our study also investigates the spatial distribution of PMCAs, revealing their likely localization outside the ER-PM junction. Simulations indicate that Ca2+ puffs implying the opening of 2-6 IP3Rs create ADCMs by provoking local depletions of ER Ca2+ stimulating Ca2+ entry through the ORAI1 channels. Such conditions allow the reproduction of the amplitude, duration and spatial extent of the observed ADCMs. By integrating advanced computational techniques with insights from experimental studies, our approach provides valuable information on the mechanisms governing early Ca2+ signalling in T cell activation, paving the way for a deeper understanding of immune responses.

Trim21-mediated CCT2 ubiquitination suppresses malignant progression and promotes CD4+T cell activation in breast cancer

Breast cancer remains a significant global health challenge, and its mechanisms of progression and metastasis are still not fully understood. In this study, analysis of TCGA and GEO datasets revealed a significant increase in CCT2 expression in breast cancer tissues, which was associated with poor prognosis in breast cancer patients. Functional analysis revealed that CCT2 promoted breast cancer growth and metastasis through activation of the JAK2/STAT3 signaling pathway. Additionally, the E3 ubiquitin ligase Trim21 facilitated CCT2 ubiquitination and degradation, significantly reversing the protumor effects of CCT2. Most interestingly, we discovered that exosomal CCT2 derived from breast cancer cells suppressed the activation and proinflammatory cytokine secretion of CD4+ T cell. Mechanistically, exosomal CCT2 constrained Ca2+-NFAT1 signaling, thereby reducing CD40L expression on CD4+ T cell. These findings highlight CCT2 upregulation as a potential driver of breast cancer progression and immune evasion. Our study provides new insights into the molecular mechanisms underlying breast cancer progression, suggesting that CCT2 is a promising therapeutic target and prognostic predictor for breast cancer.

Mammalian cysteine-rich secretory proteins interact with plasma membrane Ca2+ exporter PMCA4b

BACKGROUND

Mammalian cysteine-rich secretory proteins (CRISPs) are predominantly expressed in the male reproductive tract. Knockout mice lacking two or more CRISPs show defects in sperm transport, sperm-egg interaction and Ca2+ homeostasis. CRISPs play redundant and specific roles via their binding partners. To understand this, a comprehensive analysis of CRISP interactome needs to be undertaken.

OBJECTIVES

This study aimed to analyse CRISP4 binding partners on the plasma membrane of rat caudal spermatozoa.

MATERIALS AND METHODS

Total proteins from rat caudal spermatozoa were subjected to immunoprecipitation using anti-CRISP4 antibody followed by liquid chromatography-mass spectrophotometry analysis. Plasma membrane localised proteins were shortlisted, and a key target was validated by co-immunoprecipitation and co-localisation. Co-transfection followed by co-immunoprecipitation was carried out for studying the interaction of full-length as well as deletion mutants of CRISPs with human plasma membrane calcium ATPase, isoform b (hPMCA4b). Calcium assays were performed using Fura-2-AM. The cholesterol binding ability of different CRISPs was evaluated in silico.

RESULTS

The membrane-specific interactome of rat CRISP4 (rCRISP4) from caudal spermatozoa revealed PMCA4b as a novel binding partner, and their interaction was validated in rat spermatozoa. Human CRISP1 (hCRISP1) and hCRISP3 also interacted with PMCA4b via the N-terminal domain. Interestingly, hCRISP1 and rCRISP4 delayed PMCA4b-mediated calcium extrusion but hCRISP3 did not. In silico analysis demonstrated that hCRISP1 and rCRISP4 have higher binding affinity towards cholesterol than hCRISP3. The secretion profile of different CRISPs also showed that the ratio of secreted to cell-associated proteins was highest for hCRISP3.

CONCLUSION

Our study identifies PMCA4b as a target of multiple mammalian CRISPs and unravels a new role of CRISPs in regulating calcium homeostasis. Differences in the interaction of different CRISPs with cholesterol may regulate their enrichment in the lipid rafts and redistribution in the membrane post-capacitation, thereby affecting their interaction with PMCA4b.

Plasma membrane Ca2+ pump isoform 4 function in cell migration and cancer metastasis

The Ca2+ ion is a universal second messenger involved in many vital physiological functions including cell migration and development. To fulfil these tasks the cytosolic Ca2+ concentration is tightly controlled, and this involves an intricate functional balance between a variety of channels and pumps of the Ca2+ signalling machinery. Among these proteins, plasma membrane Ca2+ ATPases (PMCAs) represent the major high-affinity Ca2+ extrusion systems in the cell membrane that are effective in maintaining free Ca2+ concentration at exceedingly low cytosolic levels, which is essential for normal cell function. An imbalance in Ca2+ signalling can have pathogenic consequences including cancer and metastasis. Recent studies have highlighted the role of PMCAs in cancer progression and have shown that a particular variant, PMCA4b, is downregulated in certain cancer types, causing delayed attenuation of the Ca2+ signal. It has also been shown that loss of PMCA4b leads to increased migration and metastasis of melanoma and gastric cancer cells. In contrast, an increased PMCA4 expression has been reported in pancreatic ductal adenocarcinoma that coincided with increased cell migration and shorter patient survival, suggesting distinct roles of PMCA4b in various tumour types and/or different stages of tumour development. The recently discovered interaction of PMCAs with basigin, an extracellular matrix metalloproteinase inducer, may provide further insights into our understanding of the specific roles of PMCA4b in tumour progression and cancer metastasis.

Multifaceted control of T cell differentiation by STIM1

In T cells, stromal interaction molecule (STIM) and Orai are dispensable for conventional T cell development, but critical for activation and differentiation. This review focuses on novel STIM-dependent mechanisms for control of Ca2+ signals during T cell activation and its impact on mitochondrial function and transcriptional activation for control of T cell differentiation and function. We highlight areas that require further work including the roles of plasma membrane Ca2+ ATPase (PMCA) and partner of STIM1 (POST) in controlling Orai function. A major knowledge gap also exists regarding the independence of T cell development from STIM and Orai, despite compelling evidence that it requires Ca2+ signals. Resolving these and other outstanding questions ensures that the field will remain active for many years to come.

Physiology of intracellular calcium buffering

Calcium signaling underlies much of physiology. Almost all the Ca2+ in the cytoplasm is bound to buffers, with typically only ∼1% being freely ionized at resting levels in most cells. Physiological Ca2+ buffers include small molecules and proteins, and experimentally Ca2+ indicators will also buffer calcium. The chemistry of interactions between Ca2+ and buffers determines the extent and speed of Ca2+ binding. The physiological effects of Ca2+ buffers are determined by the kinetics with which they bind Ca2+ and their mobility within the cell. The degree of buffering depends on factors such as the affinity for Ca2+, the Ca2+ concentration, and whether Ca2+ ions bind cooperatively. Buffering affects both the amplitude and time course of cytoplasmic Ca2+ signals as well as changes of Ca2+ concentration in organelles. It can also facilitate Ca2+ diffusion inside the cell. Ca2+ buffering affects synaptic transmission, muscle contraction, Ca2+ transport across epithelia, and the killing of bacteria. Saturation of buffers leads to synaptic facilitation and tetanic contraction in skeletal muscle and may play a role in inotropy in the heart. This review focuses on the link between buffer chemistry and function and how Ca2+ buffering affects normal physiology and the consequences of changes in disease. As well as summarizing what is known, we point out the many areas where further work is required.

Trogocytosis of cancer-associated fibroblasts promotes pancreatic cancer growth and immune suppression via phospholipid scramblase anoctamin 6 (ANO6)

In pancreatic ductal adenocarcinoma (PDAC), the fibroblastic stroma constitutes most of the tumor mass and is remarkably devoid of functional blood vessels. This raises an unresolved question of how PDAC cells obtain essential metabolites and water-insoluble lipids. We have found a critical role for cancer-associated fibroblasts (CAFs) in obtaining and transferring lipids from blood-borne particles to PDAC cells via trogocytosis of CAF plasma membranes. We have also determined that CAF-expressed phospholipid scramblase anoctamin 6 (ANO6) is an essential CAF trogocytosis regulator required to promote PDAC cell survival. During trogocytosis, cancer cells and CAFs form synapse-like plasma membranes contacts that induce cytosolic calcium influx in CAFs via Orai channels. This influx activates ANO6 and results in phosphatidylserine exposure on CAF plasma membrane initiating trogocytosis and transfer of membrane lipids, including cholesterol, to PDAC cells. Importantly, ANO6-dependent trogocytosis also supports the immunosuppressive function of pancreatic CAFs towards cytotoxic T cells by promoting transfer of excessive amounts of cholesterol. Further, blockade of ANO6 antagonizes tumor growth via disruption of delivery of exogenous cholesterol to cancer cells and reverses immune suppression suggesting a potential new strategy for PDAC therapy.

HIV-1 gp120 protein promotes HAND through the calcineurin pathway activation

For over two decades, highly active antiretroviral therapy (HAART) was able to help prolong the life expectancy of people living with HIV-1 (PLWH) and eliminate the virus to an undetectable level. However, an increased prevalence of HIV- associated neurocognitive disorders (HAND) was observed. These symptoms range from neuronal dysfunction to cell death. Among the markers of neuronal deregulation, we cite the alteration of synaptic plasticity and neuronal communications. Clinically, these dysfunctions led to neurocognitive disorders such as learning alteration and loss of spatial memory, which promote premature brain aging even in HAART-treated patients. In support of these observations, we showed that the gp120 protein deregulates miR-499-5p and its downstream target, the calcineurin (CaN) protein. The gp120 protein also promotes the accumulation of calcium (Ca2+) and reactive oxygen species (ROS) inside the neurons leading to the activation of CaN and the inhibition of miR-499-5p. gp120 protein also caused mitochondrial fragmentation and changes in shape and size. The use of mimic miR-499 restored mitochondrial functions, appearance, and size. These results demonstrated the additional effect of the gp120 protein on neurons through the miR-499-5p/calcineurin pathway.

Ca2+ Homeostasis by Plasma Membrane Ca2+ ATPase (PMCA) 1 Is Essential for the Development of DP Thymocytes

The strength of Ca²⁺ signaling is a hallmark of T cell activation, yet the role of Ca²⁺ homeostasis in developing T cells before expressing a mature T cell receptor is poorly understood. We aimed to unveil specific functions of the two plasma membrane Ca²⁺ ATPases expressed in T cells, PMCA1 and PMCA4. On a transcriptional and protein level we found that PMCA4 was expressed at low levels in CD4^-CD8^- double negative (DN) thymocytes and was even downregulated in subsequent stages while PMCA1 was present throughout development and upregulated in CD4⁺CD8⁺ double positive (DP) thymocytes. Mice with a targeted deletion of Pmca1 in DN3 thymocytes had an almost complete block of DP thymocyte development with an accumulation of DN4 thymocytes but severely reduced numbers of CD8⁺ immature single positive (ISP) thymocytes. The DN4 thymocytes of these mice showed strongly elevated basal cytosolic Ca²⁺ levels and a pre-mature CD5 expression, but in contrast to the DP thymocytes they were only mildly prone to apoptosis. Surprisingly, mice with a germline deletion of Pmca4 did not show any signs of altered progression through the developmental thymocyte stages, nor altered Ca²⁺ homeostasis throughout this process. PMCA1 is, therefore, non-redundant in keeping cellular Ca²⁺ levels low in the early thymocyte development required for the DN to DP transition.

PMCA Ca2+ clearance in dental enamel cells depends on the magnitude of cytosolic Ca2

Enamel formation (amelogenesis) is a two-step process whereby crystals partially grow during the secretory stage followed by a significant growth expansion during the maturation stage concurrent with an increase in vectorial Ca2+ transport. This requires tight regulation of cytosolic Ca2+ (c Ca2+ ) concentration in the enamel forming ameloblasts by controlling Ca2+ influx (entry) and Ca2+ extrusion (clearance). Gene and protein expression studies suggest that the plasma membrane Ca2+ -ATPases (PMCA1-4) are likely involved in c Ca2+ extrusion in ameloblasts, yet no functional analysis of these pumps has been reported nor whether their activity changes across amelogenesis. PMCAs have high Ca2+ affinity and low Ca2+ clearance which may be a limiting factor in their contribution to enamel formation as maturation stage ameloblasts handle high Ca2+ loads. We analyzed PMCA function in rat secretory and maturation ameloblasts by blocking or potentiating these pumps. Low/moderate elevations in c Ca2+ measured using the Ca2+ probe Fura-2-AM show that secretory ameloblasts clear Ca2+ faster than maturation stage cells through PMCAs. This process was completely inhibited by an external alkaline (pH 9.0) solution or was significantly delayed by the PMCA blockers vanadate and caloxin 1b1. Eliciting higher c Ca2+ transients via the activation of the ORAI1 Ca2+ channel showed that the PMCAs of maturation ameloblasts were more efficient. Inhibiting PMCAs decreased the rate of Ca2+ influx via ORAI1 but potentiation with forskolin had no effect. Our findings suggest that PMCAs are functional Ca2+ pumps during amelogenesis regulating c Ca2+ upon low and/or moderate Ca2+ stimulus in secretory stage, thus participating in amelogenesis.

Suppression of Ca2+ signaling enhances melanoma progression

The role of store-operated Ca2+ entry (SOCE) in melanoma metastasis is highly controversial. To address this, we here examined UV-dependent metastasis, revealing a critical role for SOCE suppression in melanoma progression. UV-induced cholesterol biosynthesis was critical for UV-induced SOCE suppression and subsequent metastasis, although SOCE suppression alone was both necessary and sufficient for metastasis to occur. Further, SOCE suppression was responsible for UV-dependent differences in gene expression associated with both increased invasion and reduced glucose metabolism. Functional analyses further established that increased glucose uptake leads to a metabolic shift towards biosynthetic pathways critical for melanoma metastasis. Finally, examination of fresh surgically isolated human melanoma explants revealed cholesterol biosynthesis-dependent reduced SOCE. Invasiveness could be reversed with either cholesterol biosynthesis inhibitors or pharmacological SOCE potentiation. Collectively, we provide evidence that, contrary to current thinking, Ca2+ signals can block invasive behavior, and suppression of these signals promotes invasion and metastasis.

Plasma Membrane Ca2+ ATPase Activity Enables Sustained Store-operated Ca2+ Entry in the Absence of a Bulk Cytosolic Ca2+ Rise

In many cell types, the rise in cytosolic Ca2+ due to opening of Ca2+ release-activated Ca2+ (CRAC) channels drives a plethora of responses, including secretion, motility, energy production, and gene expression. The amplitude and time course of the cytosolic Ca2+ rise is shaped by the rates of Ca2+ entry into and removal from the cytosol. However, an extended bulk Ca2+ rise is toxic to cells. Here, we show that the plasma membrane Ca2+ ATPase (PMCA) pump plays a major role in preventing a prolonged cytosolic Ca2+ signal following CRAC channel activation. Ca2+ entry through CRAC channels leads to a sustained sub-plasmalemmal Ca2+ rise but bulk Ca2+ is kept low by the activity of PMCA4b. Despite the low cytosolic Ca2+, membrane permeability to Ca2+ is still elevated and Ca2+ continues to enter through CRAC channels. Ca2+-dependent NFAT activation, driven by Ca2+ nanodomains near the open channels, is maintained despite the return of bulk Ca2+ to near pre-stimulation levels. Our data reveal a central role for PMCA4b in determining the pattern of a functional Ca2+ signal and in sharpening local Ca2+ gradients near CRAC channels, whilst protecting cells from a toxic Ca2+ overload.

Low expression and Hypermethylation of ATP2B1 in Intrahepatic Cholangiocarcinoma Correlated With Cold Tumor Microenvironment

Background

The efficacy of current therapeutic schedule is limited owing to fibroproliferative tumor microenvironment (TME) of cholangiocarcinoma, compelling a search for new therapeutic targets.

Methods

Gene expression profiles and methylation profiles were obtained from UCSC Xena. Consensus clustering was performed on the transcriptome data of cholangiocarcinoma to determine the different immune subtypes. The differentially expressed genes (DEGs) between hot tumor and cold tumors were identified. ESTIMATE was used to assess immune score, and the cases were separated into relatively superior and inferior immune score groups. Single-sample gene set enrichment analysis was applied to assess 28 immune cells in the cholangiocarcinoma microenvironment. Unsupervised consensus was applied for methylation profiling to distribute the high and low methylation groups. The correlation between DNA methylation and mRNA expression was investigated, and the relationship between the ATP2B1 gene and the immune microenvironment was explored. Finally, 77 cases of intrahepatic cholangiocarcinoma (ICC) were collected for verification.

Results

Seven subtypes were related to patient outcomes (P=0.005). The proportions of CD8⁺ T cells in the “hot” immune type was significantly greater than that in the “cold” immune type (P<0.05). Next, DEGs and DNA methylation-governed genes were intersected, and ATP2B1 was identified as a prognosis factor in ICC (P=0.035). ATP2B1 expression was positively correlated with immune scores (P=0.005, r=0.458), the levels of infiltrating CD8⁺ T cells (P=0.004, r=0.47), and CD4⁺ T cells (P=0.027, r=0.37). Immunohistochemistry confirmed that the amounts of CD8⁺ and CD4⁺ T cells were significantly higher in ICC tissue samples than in tissues with ATP2B1 overexpression (P<0.05).

Conclusions

ATP2B1 overexpression can activate immune signals and prompt cold tumor response.

Impaired regulation of PMCA activity by defective CFTR expression promotes epithelial cell damage in alcoholic pancreatitis and hepatitis

Alcoholic pancreatitis and hepatitis are frequent, potentially lethal diseases with limited treatment options. Our previous study reported that the expression of CFTR Cl- channel is impaired by ethanol in pancreatic ductal cells leading to more severe alcohol-induced pancreatitis. In addition to determining epithelial ion secretion, CFTR has multiple interactions with other proteins, which may influence intracellular Ca2+ signaling. Thus, we aimed to investigate the impact of ethanol-mediated CFTR damage on intracellular Ca2+ homeostasis in pancreatic ductal epithelial cells and cholangiocytes. Human and mouse pancreas and liver samples and organoids were used to study ion secretion, intracellular signaling, protein expression and interaction. The effect of PMCA4 inhibition was analyzed in a mouse model of alcohol-induced pancreatitis. The decreased CFTR expression impaired PMCA function and resulted in sustained intracellular Ca2+ elevation in ethanol-treated and mouse and human pancreatic organoids. Liver samples derived from alcoholic hepatitis patients and ethanol-treated mouse liver organoids showed decreased CFTR expression and function, and impaired PMCA4 activity. PMCA4 co-localizes and physically interacts with CFTR on the apical membrane of polarized epithelial cells, where CFTR-dependent calmodulin recruitment determines PMCA4 activity. The sustained intracellular Ca2+ elevation in the absence of CFTR inhibited mitochondrial function and was accompanied with increased apoptosis in pancreatic epithelial cells and PMCA4 inhibition increased the severity of alcohol-induced AP in mice. Our results suggest that improving Ca2+ extrusion in epithelial cells may be a potential novel therapeutic approach to protect the exocrine pancreatic function in alcoholic pancreatitis and prevent the development of cholestasis in alcoholic hepatitis.

The roles of transmembrane family proteins in the regulation of store-operated Ca2+ entry

Store-operated Ca2+ entry (SOCE) is a major pathway for calcium signaling, which regulates almost every biological process, involving cell proliferation, differentiation, movement and death. Stromal interaction molecule (STIM) and ORAI calcium release-activated calcium modulator (ORAI) are the two major proteins involved in SOCE. With the deepening of studies, more and more proteins are found to be able to regulate SOCE, among which the transmembrane (TMEM) family proteins are worth paying more attention. In addition, the ORAI proteins belong to the TMEM family themselves. As the name suggests, TMEM family is a type of proteins that spans biological membranes including plasma membrane and membrane of organelles. TMEM proteins are in a large family with more than 300 proteins that have been already identified, while the functional knowledge about the proteins is preliminary. In this review, we mainly summarized the TMEM proteins that are involved in SOCE, to better describe a picture of the interaction between STIM and ORAI proteins during SOCE and its downstream signaling pathways, as well as to provide an idea for the study of the TMEM family proteins.

Structure, Function and Regulation of the Plasma Membrane Calcium Pump in Health and Disease

In this review, I summarize the present knowledge of the structural and functional properties of the mammalian plasma membrane calcium pump (PMCA). It is outlined how the cellular expression of the different spliced isoforms of the four genes are regulated under normal and pathological conditions.

The interplay between mitochondria and store-operated Ca2+ entry: Emerging insights into cardiac diseases

Store‐operated Ca²⁺ entry (SOCE) machinery, including Orai channels, TRPCs, and STIM1, is key to cellular calcium homeostasis. The following characteristics of mitochondria are involved in the physiological and pathological regulation of cells: mitochondria mediate calcium uptake through calcium uniporters; mitochondria are regulated by mitochondrial dynamic related proteins (OPA1, MFN1/2, and DRP1) and form mitochondrial networks through continuous fission and fusion; mitochondria supply NADH to the electron transport chain through the Krebs cycle to produce ATP; under stress, mitochondria will produce excessive reactive oxygen species to regulate mitochondria‐endoplasmic reticulum interactions and the related signalling pathways. Both SOCE and mitochondria play critical roles in mediating cardiac hypertrophy, diabetic cardiomyopathy, and cardiac ischaemia‐reperfusion injury. All the mitochondrial characteristics mentioned above are determinants of SOCE activity, and vice versa. Ca²⁺ signalling dictates the reciprocal regulation between mitochondria and SOCE under the specific pathological conditions of cardiomyocytes. The coupling of mitochondria and SOCE is essential for various pathophysiological processes in the heart. Herein, we review the research focussing on the reciprocal regulation between mitochondria and SOCE and provide potential interplay patterns in cardiac diseases.

Plasma Membrane and Organellar Targets of STIM1 for Intracellular Calcium Handling in Health and Neurodegenerative Diseases

Located at the level of the endoplasmic reticulum (ER) membrane, stromal interacting molecule 1 (STIM1) undergoes a complex conformational rearrangement after depletion of ER luminal Ca²⁺. Then, STIM1 translocates into discrete ER-plasma membrane (PM) junctions where it directly interacts with and activates plasma membrane Orai1 channels to refill ER with Ca²⁺. Furthermore, Ca²⁺ entry due to Orai1/STIM1 interaction may induce canonical transient receptor potential channel 1 (TRPC1) translocation to the plasma membrane, where it is activated by STIM1. All these events give rise to store-operated calcium entry (SOCE). Besides the main pathway underlying SOCE, which mainly involves Orai1 and TRPC1 activation, STIM1 modulates many other plasma membrane proteins in order to potentiate the influxof Ca²⁺. Furthermore, it is now clear that STIM1 may inhibit Ca²⁺ currents mediated by L-type Ca²⁺ channels. Interestingly, STIM1 also interacts with some intracellular channels and transporters, including nuclear and lysosomal ionic proteins, thus orchestrating organellar Ca²⁺ homeostasis. STIM1 and its partners/effectors are significantly modulated in diverse acute and chronic neurodegenerative conditions. This highlights the importance of further disclosing their cellular functions as they might represent promising molecular targets for neuroprotection.

Plasma Membrane Calcium ATPase Regulates Stoichiometry of CD4+ T-Cell Compartments

Immune responses involve mobilization of T cells within naïve and memory compartments. Tightly regulated Ca²⁺ levels are essential for balanced immune outcomes. How Ca²⁺ contributes to regulating compartment stoichiometry is unknown. Here, we show that plasma membrane Ca²⁺ ATPase 4 (PMCA4) is differentially expressed in human CD4⁺ T compartments yielding distinct store operated Ca²⁺ entry (SOCE) profiles. Modulation of PMCA4 yielded a more prominent increase of SOCE in memory than in naïve CD4⁺ T cell. Interestingly, downregulation of PMCA4 reduced the effector compartment fraction and led to accumulation of cells in the naïve compartment. In silico analysis and chromatin immunoprecipitation point towards Ying Yang 1 (YY1) as a transcription factor regulating PMCA4 expression. Analyses of PMCA and YY1 expression patterns following activation and of PMCA promoter activity following downregulation of YY1 highlight repressive role of YY1 on PMCA expression. Our findings show that PMCA4 adapts Ca²⁺ levels to cellular requirements during effector and quiescent phases and thereby represent a potential target to intervene with the outcome of the immune response.

The Plasma Membrane Ca2+ Pump PMCA4b Regulates Melanoma Cell Migration through Remodeling of the Actin Cytoskeleton

Simple Summary

Earlier we demonstrated that the plasma membrane Ca²⁺ pump PMCA4b inhibits migration and metastatic activity of BRAF mutant melanoma cells, however, the exact mechanism has not been fully understood. Here we demonstrate that PMCA4b acted through actin cytoskeleton remodeling in generating a low migratory melanoma cell phenotype resulting in increased cell–cell connections, lamellipodia and stress fiber formation. Both proper trafficking and calcium transporting activity of the pump were essential to complete these tasks indicating that controlling Ca²⁺ concentration levels at specific plasma membrane locations such as the cell front played a role. Our findings suggest that PMCA4b downregulation is likely one of the mechanisms that leads to the perturbed cancer cell cytoskeleton organization resulting in enhanced melanoma cell migration and metastasis.

Abstract

We demonstrated that the plasma membrane Ca²⁺ ATPase PMCA4b inhibits migration and metastatic activity of BRAF mutant melanoma cells. Actin dynamics are essential for cells to move, invade and metastasize, therefore, we hypothesized that PMCA4b affected cell migration through remodeling of the actin cytoskeleton. We found that expression of PMCA4b in A375 BRAF mutant melanoma cells induced a profound change in cell shape, cell culture morphology, and displayed a polarized migratory character. Along with these changes the cells became more rounded with increased cell–cell connections, lamellipodia and stress fiber formation. Silencing PMCA4b in MCF-7 breast cancer cells had a similar effect, resulting in a dramatic loss of stress fibers. In addition, the PMCA4b expressing A375 cells maintained front-to-rear Ca²⁺ concentration gradient with the actin severing protein cofilin localizing to the lamellipodia, and preserved the integrity of the actin cytoskeleton from a destructive Ca²⁺ overload. We showed that both PMCA4b activity and trafficking were essential for the observed morphology and motility changes. In conclusion, our data suggest that PMCA4b plays a critical role in adopting front-to-rear polarity in a normally spindle-shaped cell type through F-actin rearrangement resulting in a less aggressive melanoma cell phenotype.

Mechanisms and functions of calcium microdomains produced by ORAI channels, d-myo-inositol 1,4,5-trisphosphate receptors, or ryanodine receptors

With the discovery of local Ca²⁺ signals in the 1990s the concept of 'elementary Ca²⁺ signals' and 'fundamental Ca²⁺ signals' was developed. While 'elementary Ca²⁺signals' relate to optical signals gained by activity of small clusters of Ca²⁺channels, 'fundamental signals' describe such optical signals that arise from opening of single Ca²⁺channels. In this review, we discuss (i) concepts of local Ca²⁺ signals and Ca²⁺ microdomains, (ii) molecular mechanisms underlying Ca²⁺ microdomains, (iii) functions of Ca²⁺ microdomains, and (iv) mathematical modelling of Ca²⁺ microdomains. We focus on Ca²⁺ microdomains produced by ORAI channels, D-myo-inositol 1,4,5-trisphosphate receptors, or ryanodine receptors. In summary, research on local Ca²⁺ signals in different cell models aims to better understand how cells use the Ca²⁺ toolkit to produce Ca²⁺ microdomains as relevant signals for specific cellular responses, but also how local Ca²⁺ signals as building blocks merge into global Ca²⁺ signaling.

Proteins Interacting with STIM1 and Store-Operated Ca2+ Entry

The endoplasmic reticulum (ER) Ca²⁺ sensor stromal interaction molecule 1 (STIM1) interacts with ORAI Ca²⁺ channels at the plasma membrane to regulate immune and muscle cell function. The conformational changes underlying STIM1 activation, translocation, and ORAI1 trapping and gating, are stringently regulated by post-translational modifications and accessory proteins. Here, we review the recent progress in the identification and characterization of ER and cytosolic proteins interacting with STIM1 to control its activation and deactivation during store-operated Ca²⁺ entry (SOCE).

Plasma membrane Ca2+ ATPase 1 (PMCA1) but not PMCA4 is critical for B-cell development and Ca2+ homeostasis in mice

The amplitude and duration of Ca²⁺ signaling is crucial for B-cell development and self-tolerance; however, the mechanisms for terminating Ca²⁺ signals in B cells have not been determined. In lymphocytes, plasma membrane Ca²⁺ ATPase (PMCA) isoforms 1 and 4 (PMCA1 and PMCA4, aka ATP2B1 and ATP2B4) are the main candidates for expelling Ca²⁺ from the cell through the plasma membrane. We report here that Pmca4 (Atp2b4) KO mice had normal B-cell development, while mice with a conditional KO of Pmca1 (Atp2b1) had greatly reduced numbers of B cells, particularly splenic follicular B cells, marginal zone B cells, and peritoneal B-1a cells. Mouse and naïve human B cells showed only PMCA1 expression and no PMCA4 by western blot, in contrast to T cells, which did express PMCA4. Calcium handling was normal in Pmca4^-/- B cells, but Pmca1 KO B cells had elevated basal levels of Ca²⁺ , elevated levels in ER stores, and reduced Ca²⁺ clearance. These findings show that the PMCA1 isoform alone is required to ensure normal B-cell Ca²⁺ signaling and development, which may have implications for therapeutic targeting of PMCAs and Ca²⁺ in B cells.

Comprehensive Analysis of the Expression, Relationship to Immune Infiltration and Prognosis of TIM-1 in Cancer

TIM-1 is a critical gene that regulates T-helper cell development. However, little research has revealed the distribution, prognosis, and immune infiltration of TIM-1 in cancers. TCGA, GEO, Oncomine, TIMER, Kaplan-Meier, PrognoScan, GEPIA, TISIDB, and HPA databases were used to analyze TIM-1 in cancers. High TIM-1 expression was observed in bladder, cholangio, head and neck, colorectal, gastric, kidney, liver, lung adenocarcinoma, skin, uterine corpus endometrial, and pancreatic cancers compared to the normal tissues, and immunofluorescence shows that TIM-1 is mainly localized in vesicles. Simultaneously, high TIM-1 expression was closely related with poorer overall survival in gastric, lung adenocarcinoma, and poorer disease-specific survival in gastric cancer in the TCGA cohort, and was validated in the GEO cohort. Moreover, high expression of TIM-1, correlated with clinical relevance of gastric cancer and lung adenocarcinoma, was associated with tumor-infiltrating lymphocytes in lung adenocarcinoma and gastric cancer. Finally, immunohistochemistry showed TIM-1 expression was higher in lung adenocarcinoma and gastric cancer compared to the normal tissues. In summary, we applied integrated bioinformatics approaches to suggest that TIM-1 can be used as a prognostic biomarker in gastric and lung adenocarcinoma, which might provide a novel direction to explore the pathogenesis of gastric and lung adenocarcinoma.

Extracellular Vesicles Derived from Human Umbilical Cord Mesenchymal Stem Cells Protect Liver Ischemia/Reperfusion Injury by Reducing CD154 Expression on CD4+ T Cells via CCT2

As a cause of postoperative complications and early hepatic failure after liver transplantation, liver ischemia/reperfusion injury (IRI) still has no effective treatment during clinical administration. Although the therapeutic potential of mesenchymal stem cells (MSCs) for liver IRI has been previously shown, the underlying mechanisms are not completely clear. It is accepted that MSC-derived extracellular vesicles (MSC-EVs) are newly uncovered messengers for intercellular communication. Herein, it is reported that umbilical cord-derived MSCs (UC-MSCs) improve liver IRI in mice through their secreted EVs. It is also visualized that UC-MSC-EVs mainly concentrate in liver after 6 h of reperfusion. Furthermore, UC-MSC-EVs are found to significantly modulate the membranous expression of CD154 of intrahepatic CD4+ T cells, which is an initiation of inflammatory response in liver and can aggravate liver IRI. Mechanistically, protein mass spectrum analysis is performed and it is revealed that Chaperonin containing TCP1 subunit 2 (CCT2) enriches in UC-MSC-EVs, which regulates the calcium channels to affect Ca2+ influx and suppress CD154 synthesis in CD4+ T cells. In conclusion, these results highlight the therapeutic potential of UC-MSC-EVs in attenuating liver IRI. This finding suggests that CCT2 from UC-MSC-EVs can modulate CD154 expression of intrahepatic CD4+ T cells during liver IRI through the Ca2+-calcineurin-NFAT1 signaling pathway.

Suppression of Ca2+ signals by EGR4 controls Th1 differentiation and anti-cancer immunity in vivo

While the zinc finger transcription factors EGR1, EGR2, and EGR3 are recognized as critical for T-cell function, the role of EGR4 remains unstudied. Here, we show that EGR4 is rapidly upregulated upon TCR engagement, serving as a critical "brake" on T-cell activation. Hence, TCR engagement of EGR4^-/- T cells leads to enhanced Ca²⁺ responses, driving sustained NFAT activation and hyperproliferation. This causes profound increases in IFNγ production under resting and diverse polarizing conditions that could be reversed by pharmacological attenuation of Ca²⁺ entry. Finally, an in vivo melanoma lung colonization assay reveals enhanced anti-tumor immunity in EGR4^-/- mice, attributable to Th1 bias, Treg loss, and increased CTL generation in the tumor microenvironment. Overall, these observations reveal for the first time that EGR4 is a key regulator of T-cell differentiation and function.