Calcium signalling in T-lymphocytes

Nicotinic Acid Adenine Dinucleotide Phosphate (NAADP)-Mediated Calcium Signaling Is Active in Memory CD4+ T Cells

Nicotinic acid adenine dinucleotide phosphate (NAADP), identified as one of the most potent calcium-mobilizing second messengers, has been studied in different eukaryotic cell types, including lymphocytes. Although aspects of NAADP-mediated calcium release in lymphocytes are still under debate, the organelles pertaining to NAADP-mediated calcium release are often characterized as acidic and related to lysosomes. Although NAADP-mediated calcium release in different subsets of T cells, including naïve, effector and natural regulatory T cells, has been studied, it has not been widely studied in memory CD4+ T cells, which show a different calcium flux profile. Using a pharmacological approach, the effect of Ned-19, an NAADP pathway antagonist, on the involvement of NAADP in TCR activation in murine memory CD4+ T cells and their downstream effector functions, such as proliferation and cytokine production, was studied. According to this study, Ned-19 inhibited TCR-mediated calcium flux and its downstream effector functions in primary memory CD4+ T cells. The study also revealed that both extracellular and intracellular calcium stores, including endoplasmic reticulum and lysosome-like acidic calcium stores, contribute to the TCR-mediated calcium flux in memory CD4+ T cells. NAADP-AM, a cell permeable analogue of NAADP, was shown to release calcium in memory CD4+ T cells and calcium flux was inhibited by Ned-19.

Altered TRPM7-Dependent Calcium Influx in Natural Killer Cells of Myalgic Encephalomyelitis/Chronic Fatigue Syndrome Patients

Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a disabling multisystemic condition. The pathomechanism of ME/CFS remains unestablished; however, impaired natural killer (NK) cell cytotoxicity is a consistent feature of this condition. Calcium (Ca²⁺) is crucial for NK cell effector functions. Growing research recognises Ca²⁺ signalling dysregulation in ME/CFS patients and implicates transient receptor potential ion channel dysfunction. TRPM7 (melastatin) was recently considered in the pathoaetiology of ME/CFS as it participates in several Ca²⁺-dependent processes that are central to NK cell cytotoxicity which may be compromised in ME/CFS. TRPM7-dependent Ca²⁺ influx was assessed in NK cells isolated from n = 9 ME/CFS patients and n = 9 age- and sex-matched healthy controls (HCs) using live cell fluorescent imaging techniques. Slope (p < 0.05) was significantly reduced in ME/CFS patients compared with HCs following TRPM7 activation. Half-time of maximal response (p < 0.05) and amplitude (p < 0.001) were significantly reduced in the HCs compared with the ME/CFS patients following TRPM7 desensitisation. Findings from this investigation suggest that TRPM7-dependent Ca²⁺ influx is reduced with agonism and increased with antagonism in ME/CFS patients relative to the age- and sex-matched HCs. The outcomes reported here potentially reflect TRPM3 dysfunction identified in this condition suggesting that ME/CFS is a TRP ion channelopathy.

Induction of genotoxicity and differential alterations of p53 and inflammatory cytokines expression by acute oral exposure to bulk- or nano-calcium hydroxide particles in mice "Genotoxicity of normal- and nano-calcium hydroxide"

With the high increases in the uses of calcium hydroxide in various applications due its distinctive properties, human exposure has increased to normal- and nano-calcium hydroxide. However, its impact on the DNA integrity, expression of inflammatory cytokines, and induction of oxidative stress has not been clearly studied. Therefore, here we estimate the induction of DNA damage, inflammation, and oxidative stress in mice orally administrated a single dose (100 mg/kg) of normal- or nano-sized calcium hydroxide for 24 hour. Comet, Diphenylamine and laddered DNA fragmentation assays were done to assess DNA damage induction. Acute oral administration of normal- or nano-calcium hydroxide particles disrupted the DNA integrity, caused generation of ROS and also concurrent increases in both the nitric oxide concentration and inducible nitric oxide synthase gene expression in a reverse proportional to the calcium hydroxide particles' size. Increases in the concentration of calcium ions as well as alterations in the expression level of p53 and proinflammatory cytokines were also observed in calcium hydroxide administrated groups. Moreover, administration of normal- or nano-calcium hydroxide particles suspension elevated the level of malondialdehyde and decreased both the glutathione peroxidase activity and the reduced glutathione level, as well as caused tissue injuries (e.g. renal tube degeneration, congested blood vessels, atrophied lymphoid follicles, interstitial inflammatory reaction, and hyalinosis of myocardial muscles). Thus, we conclude that calcium hydroxide acutely orally administrated in its ordinary or nano-particulate form causes DNA damage induction by generating free radicals and altering the expression levels of p53 gene and proinflammatory cytokines.

Immunosuppressive Activity of Artemisia argyi Extract and Isolated Compounds

The need for novel drugs for the treatment of autoimmune diseases is high, since available pharmaceuticals often have substantial side effects and limited efficacy. Natural products are a good starting point in the development of immunosuppressive leads. Since enhanced T cell proliferation is a common feature of autoimmune diseases, we investigated the T cell proliferation inhibitory potential of an extract library of plants used in traditional Chinese medicine. Using a newly established cell-based screening platform, an ethyl acetate extract of Artemisia argyi H.Lév. & Vaniot (Asteraceae, A. argyi) was found to suppress the proliferation of human primary T lymphocytes in vitro in an IL-2-dependent manner. Flow cytometry- and ELISA-based techniques further demonstrated that the A. argyi extract reduced the activation and function of T cells. Transcription factor analysis and flow cytometric calcium influx investigations indicated that the immunomodulatory effect was based on specific modification of T cell signaling in a non-cytotoxic manner which is mediated via the NFAT pathway and a non-sequestrant inhibition of the calcium influx. A series of guaianolide and seco-guaianolide sesquiterpene lactones, as well as a flavonoid, were identified in a previous study as the bioactive compounds in the A. argyi extract. The effects of these bioactive compounds were compared to those of the crude extract. The tested sesquiterpene lactones act via the transcription factor NFAT and NF-κB, thereby exhibiting their immunosuppressive potential, but have an overall effect on T cell biology on a more-downstream level than the crude A. argyi extract.

Flos Magnoliae and its Constituent Linoleic Acid Suppress T Lymphocyte Activation via Store-Operated Calcium Entry

Intracellular calcium signaling is crucial for type 2 helper T cell and mast cell activation, which is essential for allergic inflammation. It is initiated by antigen-mediated receptor stimulation that triggers store-operated calcium entry (SOCE) via ORAI1 calcium channel. Flos Magnoliae (FM) is widely used to treat allergic diseases such as allergic rhinitis and asthma. Although many studies have reported that FM regulates intracellular calcium signaling, research on the exact type of calcium channel modulated by FM is scarce. Therefore, we hypothesized that the anti-allergic effects of FM might result from ORAI1 inhibition in T cells. We investigated whether a 70% ethanolic extract of FM (FMEtOH) and its constituents inhibit ORAI1 channel activity and subsequent T cell activation. We performed conventional whole-cell patch clamp studies in hSTIM1 and hORAI1-overexpressing HEK293T cells (HEKORAI1). Intracellular calcium concentration was determined using Fura-2 dye and cytokine production measurement in Jurkat T lymphocytes. FM_EtOH (0.03 mg/mL) and its fractions, especially hexane fraction (FM_Hex, 0.01 mg/mL), significantly inhibited SOCE and IL-2 cytokine production in Jurkat T lymphocytes. GC/MS analysis showed linoleic acid (LA) as the major component of FM_Hex. FM_Hex at 0.01 mg/mL (equivalent to 10 μM LA) inhibited not only SOCE but also IL-2 production, as well as CD3/CD28 receptor co-stimulation induced calcium signaling in Jurkat T lymphocytes. FM_EtOH and LA suppressed CD4+ T lymphocyte activation, at least in part, by inhibiting ISOCE. Thus, ISOCE inhibition may be a potential strategy to inhibit immune responses in inflammation.

What's Bred in the Bone: Calcium Channels in Lymphocytes

Calcium (Ca²⁺) is an important second messenger in lymphocytes and is essential in regulating various intracellular pathways that control critical cell functions. Ca²⁺ channels are located in the plasma membrane and intracellular membranes, facilitating Ca²⁺ entry into the cytoplasm. Upon Ag receptor stimulation, Ca²⁺ can enter the lymphocyte via the Ca²⁺ release-activated Ca²⁺ channel found in the plasma membrane. The increase of cytosolic Ca²⁺ modulates signaling pathways, resulting in the transcription of target genes implicated in differentiation, activation, proliferation, survival, and apoptosis of lymphocytes. Along with Ca²⁺ release-activated Ca²⁺ channels, several other channels have been found in the membranes of T and B lymphocytes contributing to key cellular events. Among them are the transient receptor potential channels, the P2X receptors, voltage-dependent Ca²⁺ channels, and the inositol 1,4,5-trisphosphate receptor as well as the N-methyl-d-aspartate receptors. In this article, we review the contributions of these channels to mediating Ca²⁺ currents that drive specific lymphocyte functions.

The current status and perspectives regarding the clinical implication of intracellular calcium in breast cancer

Calcium ions (Ca²⁺ ) act as second messengers in intracellular signaling. Ca²⁺ pumps, channels, sensors, and calcium binding proteins, regulate the concentrations of intracellular Ca²⁺ as a key regulator of important cellular processes such as gene expression, proliferation, differentiation, DNA repair, apoptosis, metastasis, and hormone secretion. Intracellular Ca²⁺ also influences the functions of several organelles, that include: the endoplasmic reticulum, mitochondria, the Golgi, and cell membrane both in normal and breast cancer cells. In breast cancer, the disruption of intracellular: Ca²⁺ homeostasis may cause tumor progression by affecting key factors/pathways including phospholipase C (PLC), inositol 1,4,5-trisphosphate (IP3), calmodulin (CaM), nuclear factor of activated T-cells (NFAT), calpain, calmodulin-dependent protein kinase II (CaMKII), mitogen-activated protein kinase (MAPK), epithelial-mesenchymal transition (EMT), vascular endothelial growth factor (VEGF), poly (ADP-Ribose) polymerase-1 (PARP1), estrogen, and estrogen receptor. Because the foregoing molecules play crucial roles in breast cancer, the factors/pathways influencing intracellular Ca²⁺ concentrations are putative targets for cancer treatment, using drugs such as Mephebrindole, Tilapia piscidin 4, Nifetepimine, Paricalcitol, and Prednisolone. We have explored the factors/pathways which are related to breast cancer and Ca²⁺ homeostasis and signaling in this review, and also discussed their potential as biomarkers for breast cancer staging, prognosis, and therapy.

Oral Exposure to Atrazine Induces Oxidative Stress and Calcium Homeostasis Disruption in Spleen of Mice

The widely used herbicide atrazine (ATR) can cause many adverse effects including immunotoxicity, but the underlying mechanisms are not fully understood. The current study investigated the role of oxidative stress and calcium homeostasis in ATR-induced immunotoxicity in mice. ATR at doses of 0, 100, 200, or 400 mg/kg body weight was administered to Balb/c mice daily for 21 days by oral gavage. The studies performed 24 hr after the final exposure showed that ATR could induce the generation of reactive oxygen species in the spleen of the mice, increase the level of advanced oxidation protein product (AOPP) in the host serum, and cause the depletion of reduced glutathione in the serum, each in a dose-related manner. In addition, DNA damage was observed in isolated splenocytes as evidenced by increase in DNA comet tail formation. ATR exposure also caused increases in intracellular Ca²⁺ within splenocytes. Moreover, ATR treatment led to increased expression of genes for some antioxidant enzymes, such as HO-1 and Gpx1, as well as increased expression of NF-κB and Ref-1 proteins in the spleen. In conclusion, it appears that oxidative stress and disruptions in calcium homeostasis might play an important role in the induction of immunotoxicity in mice by ATR.

Low-Voltage-Activated CaV3.1 Calcium Channels Shape T Helper Cell Cytokine Profiles

Activation of T cells is mediated by the engagement of T cell receptors (TCRs) followed by calcium entry via store-operated calcium channels. Here we have shown an additional route for calcium entry into T cells-through the low-voltage-activated T-type CaV3.1 calcium channel. CaV3.1 mediated a substantial current at resting membrane potentials, and its deficiency had no effect on TCR-initiated calcium entry. Mice deficient for CaV3.1 were resistant to the induction of experimental autoimmune encephalomyelitis and had reduced productions of the granulocyte-macrophage colony-stimulating factor (GM-CSF) by central nervous system (CNS)-infiltrating T helper 1 (Th1) and Th17 cells. CaV3.1 deficiency led to decreased secretion of GM-CSF from in vitro polarized Th1 and Th17 cells. Nuclear translocation of the nuclear factor of activated T cell (NFAT) was also reduced in CaV3.1-deficient T cells. These data provide evidence for T-type channels in immune cells and their potential role in shaping the autoimmune response.

Nicotinic Acid Adenine Dinucleotide Phosphate Plays a Critical Role in Naive and Effector Murine T Cells but Not Natural Regulatory T Cells

Nicotinic acid adenine dinucleotide phosphate (NAADP), the most potent Ca(2+) mobilizing second messenger discovered to date, has been implicated in Ca(2+) signaling in some lymphomas and T cell clones. In contrast, the role of NAADP in Ca(2+) signaling or the identity of the Ca(2+) stores targeted by NAADP in conventional naive T cells is less clear. In the current study, we demonstrate the importance of NAADP in the generation of Ca(2+) signals in murine naive T cells. Combining live-cell imaging methods and a pharmacological approach using the NAADP antagonist Ned-19, we addressed the involvement of NAADP in the generation of Ca(2+) signals evoked by TCR stimulation and the role of this signal in downstream physiological end points such as proliferation, cytokine production, and other responses to stimulation. We demonstrated that acidic compartments in addition to the endoplasmic reticulum were the Ca(2+) stores that were sensitive to NAADP in naive T cells. NAADP was shown to evoke functionally relevant Ca(2+) signals in both naive CD4 and naive CD8 T cells. Furthermore, we examined the role of this signal in the activation, proliferation, and secretion of effector cytokines by Th1, Th2, Th17, and CD8 effector T cells. Overall, NAADP exhibited a similar profile in mediating Ca(2+) release in effector T cells as in their counterpart naive T cells and seemed to be equally important for the function of these different subsets of effector T cells. This profile was not observed for natural T regulatory cells.

Increase in intracellular Ca(2+) level by phenylsulfamide fungicides, tolylfluanid and dichlofluanid, in rat thymic lymphocytes

Tolylfluanid, a phenylsulfamide fungicide, is one of the many pesticides that are frequently detected in crops. Therefore, its health risk is a concern. Micromolar concentrations of tolylfluanid induce chromosomal aberrations and micronuclei in mammalian lymphocytes. The findings prompted us to study the cellular actions of tolylfluanid and another frequently detected pesticide, dichlofluanid, at submicromolar and micromolar concentrations. Of the cellular actions of chemicals, the action on cellular Ca(2+) homeostasis is important since Ca(2+) is involved in cell signaling and death. Consequently, in this study, the effects of phenylsulfamide fungicides were examined on rat thymocytes by using fluorescent probes in order to further characterize the cellular actions of phenylsulfamide fungicides. Both phenylsulfamide fungicides exhibited biphasic, early and late, increase in intracellular Ca(2+) levels. The early phase was dependent on intracellular Ca(2+) release and increased membrane Ca(2+) permeability. The late phase was owing to Ca(2+) influx via activation of store-operated Ca(2+) channels and the further increase of membrane ionic permeability. Voltage-dependent Ca(2+) channels were not involved. The increases in intracellular Ca(2+) levels by phenylsulfamide fungicides were observed at drug concentrations of 0.1 μM or more (up to 10 μM). Thus, it is plausible that micromolar concentrations of phenylsulfamide fungicides deregulate intracellular Ca(2+) homeostasis in rat thymocytes. Both phenylsulfamide fungicides at 10 μM promoted the transition from intact living cells to living cells with phosphatidylserine-exposed membranes. This was not the case for phenylsulfamide fungicides at 3 μM. The potency of tolylfluanid was similar to that of dichlofluanid. Although the information on residual concentrations of tolylfluanid and dichlofluanid is very limited, their residual concentrations do not reach micromolar levels. It is unlikely that humans will develop adverse effects on exposure to phenylsulfamide fungicides under present environmental conditions.

Tweeters, Woofers and Horns: The Complex Orchestration of Calcium Currents in T Lymphocytes

Elevation of intracellular calcium ion (Ca²⁺) levels is a vital event that regulates T lymphocyte homeostasis, activation, proliferation, differentiation, and apoptosis. The mechanisms that regulate intracellular Ca²⁺ signaling in lymphocytes involve tightly controlled concinnity of multiple ion channels, membrane receptors, and signaling molecules. T cell receptor (TCR) engagement results in depletion of endoplasmic reticulum (ER) Ca²⁺ stores and subsequent sustained influx of extracellular Ca²⁺ through Ca²⁺ release-activated Ca²⁺ (CRAC) channels in the plasma membrane. This process termed store-operated Ca²⁺ entry (SOCE) involves the ER Ca²⁺ sensing molecule, STIM1, and a pore-forming plasma membrane protein, ORAI1. However, several other important Ca²⁺ channels that are instrumental in T cell function also exist. In this review, we discuss the role of additional Ca²⁺ channel families expressed on the plasma membrane of T cells that likely contribute to Ca²⁺ influx following TCR engagement, which include the TRP channels, the NMDA receptors, the P2X receptors, and the IP₃ receptors, with a focus on the voltage-dependent Ca²⁺ (Ca_V) channels.

Placing ion channels into a signaling network of T cells: from maturing thymocytes to healthy T lymphocytes or leukemic T lymphoblasts

T leukemogenesis is a multistep process, where the genetic errors during T cell maturation cause the healthy progenitor to convert into the leukemic precursor that lost its ability to differentiate but possesses high potential for proliferation, self-renewal, and migration. A new misdirecting "leukemogenic" signaling network appears, composed by three types of participants which are encoded by (1) genes implicated in determined stages of T cell development but deregulated by translocations or mutations, (2) genes which normally do not participate in T cell development but are upregulated, and (3) nondifferentially expressed genes which become highly interconnected with genes expressed differentially. It appears that each of three groups may contain genes coding ion channels. In T cells, ion channels are implicated in regulation of cell cycle progression, differentiation, activation, migration, and cell death. In the present review we are going to reveal a relationship between different genetic defects, which drive the T cell neoplasias, with calcium signaling and ion channels. We suggest that changes in regulation of various ion channels in different types of the T leukemias may provide the intracellular ion microenvironment favorable to maintain self-renewal capacity, arrest differentiation, induce proliferation, and enhance motility.

Calcium channels in Fc receptor signaling

The calcium ion (Ca(2+)) is the main common second messenger involved in signaling transduction subsequent to immunoreceptor activation. Its rapid intracellular elevation induces multiple cellular responses, such as secretion, proliferation, mobility, and gene transcription. Intracellular levels of Ca(2+) need to reach a specific threshold to efficiently transduce the signal to activate transcription factors through the recruitment of Ca(2+)-binding molecules. However, since Ca(2+) cannot be metabolized, its intracellular concentration is tightly regulated to avoid the induction of programmed cell death. This highly controlled regulation of Ca(2+) homeostasis has recently been clarified by the uncovering of new ion channels. The regulation of these channels allows the role of Ca(2+) in Fc receptor transduction pathways to be more precisely defined.

Calcium influx pathways in breast cancer: opportunities for pharmacological intervention

Ca(2+) influx through Ca(2+) permeable ion channels is a key trigger and regulator of a diverse set of cellular events, such as neurotransmitter release and muscle contraction. Ca(2+) influx is also a regulator of processes relevant to cancer, including cellular proliferation and migration. This review focuses on calcium influx in breast cancer cells as well as the potential for pharmacological modulators of specific Ca(2+) influx channels to represent future agents for breast cancer therapy. Altered expression of specific calcium permeable ion channels is present in some breast cancers. In some cases, such changes can be related to breast cancer subtype and even prognosis. In vitro and in vivo models have now helped identify specific Ca(2+) channels that play important roles in the proliferation and invasiveness of breast cancer cells. However, some aspects of our understanding of Ca(2+) influx in breast cancer still require further study. These include identifying the mechanisms responsible for altered expression and the most effective therapeutic strategy to target breast cancer cells through specific Ca(2+) channels. The role of Ca(2+) influx in processes beyond breast cancer cell proliferation and migration should become the focus of studies in the next decade.

Calcium ion influx in microglial cells: physiological and therapeutic significance

Microglial cells, the immunocompetent cells of the central nervous system (CNS), exhibit a resting phenotype under healthy conditions. In response to injury, however, they transform into an activated state, which is a hallmark feature of many CNS diseases. Factors or agents released from the neurons, blood vessels, and/or astrocytes could activate these cells, leading to their functional and structural modifications. Microglial cells are well equipped to sense environmental changes within the brain under both physiological and pathological conditions. Entry of calcium ions (Ca(2+)) plays a critical role in the process of microglial transformation; several channels and receptors have been identified on the surface of microglial cells. These include store-operated channel, Orai1, and its sensor protein, stromal interaction molecule 1 (STIM1), in microglial cells, and their functions are modulated under pathological stimulations. Transient receptor potential (TRP) channels and voltage- and ligand-gated channels (ionotropic and metabotropic receptors) are also responsible for Ca(2+) influx into the microglial cells. An elevation of intracellular Ca(2+) concentration subsequently regulates microglial cell functions by activating a diverse array of Ca(2+)-sensitive signaling cascades. Perturbed Ca(2+) homeostasis contributes to the progression of a number of CNS disorders. Thus, regulation of Ca(2+) entry into microglial cells could be a pharmacological target for several CNS-related pathological conditions. This Review addresses the recent insights into microglial cell Ca(2+) influx mechanisms, their roles in the regulation of functions, and alterations of Ca(2+) entry in specific CNS disorders.

Protein kinase C-dependent activation of CaV1.2 channels selectively controls human TH2-lymphocyte functions

BACKGROUND

In addition to calcium release-activated calcium channel/ORAI calcium channels, the role of voltage-gated calcium (Cav1) channels in T-cell calcium signaling is emerging. Cav1 channels are formed by α1 (CaV1.1 to CaV1.4) and auxiliary subunits. We previously demonstrated that mouse TH2 cells selectively overexpressed CaV1.2 and CaV1.3 channels. Knocking down these channels with Cav1 antisense (AS) oligonucleotides inhibited TH2 functions and experimental asthma.

OBJECTIVE

We investigated the expression profile and role of Cav1 channels in human T-cell subsets, with a focus on TH2 cells.

METHODS

We compared the profile of CaV1 channel subunit expression in T-cell subsets isolated ex vivo from the blood of healthy donors, as well as in vitro-polarized T-cell subsets, and tested the effect of the Cav1 inhibitors nicardipine and Cav1.2AS on their functions.

RESULTS

CaV1.4 expression was detectable in CD4(+) T cells, ex vivo TH1 cells, and TH17 cells, whereas Cav1.2 channels predominated in TH2 cells only. T-cell activation resulted in Cav1.4 downregulation, whereas Cav1.2 expression was selectively maintained in polarized TH2 cells and absent in TH1 or TH9 cells. Nicardipine and CaV1.2AS decreased Ca(2+) and cytokine responses in TH2, but not TH1, cells. Protein kinase C (PKC) α/β inhibition decreased Ca(2+) and cytokine responses, whereas both calcium and cytokine responses induced by PKC activation were inhibited by nicardipine or Cav1.2AS in TH2 cells.

CONCLUSION

This study highlights the selective expression of Cav1.2 channels in human TH2 cells and the role of PKC-dependent Cav1.2 channel activation in TH2 cell function. Blocking PKC or Cav1.2 channel activation in TH2 cells might represent new strategies to treat allergic diseases in human subjects.

GABA protects human islet cells against the deleterious effects of immunosuppressive drugs and exerts immunoinhibitory effects alone

BACKGROUND

We recently found that γ-aminobutyric acid (GABA) protects mouse islet β cells. It prevented autoimmune type 1 diabetes in mice, induced islet β-cell regeneration, and exerted immunoinhibitory effects. However, it is not known whether GABA would be equally active on human islet and immune cells.

METHODS

In vitro culture of human islets and immune cells with or without GABA and immunosuppressive drugs. In vitro analysis of apoptosis, proliferation, nuclear factor (NF)-κB activation, calcium signaling, and insulin secretion.

RESULTS

GABA reduced human islet cell apoptosis in culture, such that the yield of live cells was approximately tripled after 1 week, and it stimulated insulin secretion. It protected against the deleterious effects of rapamycin, tacrolimus, and mycophenolate mofetil. In human immune cells, GABA had inhibitory effects similar to mouse cells, such as suppressed anti-CD3-stimulated T-cell proliferation, in a GABA type A receptor-dependent fashion. The immunosuppressive mechanisms have been unclear, but we found that GABA blocked calcium influx, which is a key activation signal. GABA also suppressed NF-κB activation in both human islet cells and immune cells. We found that it could be combined with rapamycin to increase its suppressive effects.

CONCLUSIONS

GABA improved human islet cell survival and had suppressive effects on human immune cells. It inhibited canonical NF-κB activation in both islet and immune cells. This is important because activation of this pathway is detrimental to islet cells and likely promotes damaging autoimmunity and alloreactivity against transplanted islets. These findings suggest that GABA might find applications in clinical islet transplantation.

Weft, warp, and weave: the intricate tapestry of calcium channels regulating T lymphocyte function

Calcium (Ca(2+)) is a universal second messenger important for T lymphocyte homeostasis, activation, proliferation, differentiation, and apoptosis. The events surrounding Ca(2+) mobilization in lymphocytes are tightly regulated and involve the coordination of diverse ion channels, membrane receptors, and signaling molecules. A mechanism termed store-operated Ca(2+) entry (SOCE), causes depletion of endoplasmic reticulum (ER) Ca(2+) stores following T cell receptor (TCR) engagement and triggers a sustained influx of extracellular Ca(2+) through Ca(2+) release-activated Ca(2+) (CRAC) channels in the plasma membrane. The ER Ca(2+) sensing molecule, stromal interaction molecule 1 (STIM1), and a pore-forming plasma membrane protein, ORAI1, have been identified as important mediators of SOCE. Here, we review the role of several additional families of Ca(2+) channels expressed on the plasma membrane of T cells that likely contribute to Ca(2+) influx following TCR engagement, particularly highlighting an important role for voltage-dependent Ca(2+) channels (CaV) in T lymphocyte biology.

Tespa1 protein is phosphorylated in response to store-operated calcium entry

We previously reported that Tespa1 (thymocyte-expressed, positive selection-associated gene 1) protein expressed in lymphocytes physically interacts with IP3R (Inositol 1,4,5-trisphosphate receptor), a Ca(2+) channel protein spanning endoplasmic reticulum (ER) membrane. However, the biochemical characterization of Tespa1 protein remains unknown. In this study, we have found that Tespa1 protein was posttranslationally modified upon intracellular Ca(2+) increase in thymocytes. Through the analyses using various inhibitors, store-operated Ca(2+) entry (SOCE) was found to be an essential factor for the Tespa1 protein modification induced by T cell receptor (TCR)-stimulation. Remarkably, the Ca(2+)-dependent Tespa1 protein modification was restored by in vitro protein phosphatase treatment, indicating that this modification was due to phosphorylation. Moreover, we examined whether Ca(2+)-dependent phosphorylation of Tespa1 protein would affect the physical association between Tespa1 and IP3R proteins, revealing that physical association of these proteins is maintained regardless of the presence or absence of phosphorylation of Tespa1. In addition, KRAP protein which represents substantial amino acid sequence homology to Tespa1 was also posttranslationally phosphorylated by intracellular Ca(2+) increase in HCT116 human colon cancer cells and HEK293 human embryonic kidney cells, suggesting that common signaling mechanism(s) may contribute to the molecular modification of Tespa1 and KRAP in different cellular processes. All these results suggested a novel molecular modification of Tespa1 and the existence of the regulatory pathway that SOCE affects the Tespa1-IP3R molecular complex.