STIM proteins: integrators of signalling pathways in development, differentiation and disease

Activation mechanisms and structural dynamics of STIM proteins

The family of stromal interaction molecules (STIM) includes two widely expressed single-pass endoplasmic reticulum (ER) transmembrane proteins and additional splice variants that act as precise ER-luminal Ca2+ sensors. STIM proteins mainly function as one of the two essential components of the so-called Ca2+ release-activated Ca2+ (CRAC) channel. The second CRAC channel component is constituted by pore-forming Orai proteins in the plasma membrane. STIM and Orai physically interact with each other to enable CRAC channel opening, which is a critical prerequisite for various downstream signalling pathways such as gene transcription or proliferation. Their activation commonly requires the emptying of the intracellular ER Ca2+ store. Using their Ca2+ sensing capabilities, STIM proteins confer this Ca2+ content-dependent signal to Orai, thereby linking Ca2+ store depletion to CRAC channel opening. Here we review the conformational dynamics occurring along the entire STIM protein upon store depletion, involving the transition from the quiescent, compactly folded structure into an active, extended state, modulation by a variety of accessory components in the cell as well as the impairment of individual steps of the STIM activation cascade associated with disease.

Astragalus membranaceus as a Drug Candidate for Inflammatory Bowel Disease: The Preclinical Evidence

Inflammatory bowel disease (IBD) is a group of chronic inflammatory disorders that include Crohn's disease (CD) and ulcerative colitis (UC). Today, IBD has no successful treatment. As a result, it is of paramount importance to develop novel therapeutic agents for IBD prevention and treatment. Astragalus membranaceus (AMS) is a traditional Chinese medicine found in the AMS root. Modern pharmacological studies indicate that AMS and its constituents exhibit multiple bioactivities, such as anti-inflammatory, anti-oxidant, immune regulatory, anticancer, hypolipidemic, hypoglycemic, hepatoprotective, expectorant, and diuretic effects. AMS and its active constituents, which have been reported to be effective in IBD treatment, are believed to be viable candidate drugs for IBD treatment. These underlying mechanisms are associated with anti-inflammation, anti-oxidation, immunomodulation, intestinal epithelial repair, gut microbiota homeostasis, and improved energy metabolism. In this review, we summarize the efficacy and underlying mechanisms involved in IBD treatment with AMS and its active constituents in preclinical studies.

Familial 4p Interstitial Deletion Provides New Insights and Candidate Genes Underlying This Rare Condition

Chromosome 4p deletions can lead to two distinct phenotypic outcomes: Wolf-–Hirschhorn syndrome (a terminal deletion at 4p16.3) and less frequently reported proximal interstitial deletions (4p11-p16). Proximal 4p interstitial deletions can result in mild to moderate intellectual disability, facial dysmorphisms, and a tall thin body habitus. To date, only 35 cases of proximal 4p interstitial deletions have been reported, and only two of these cases have been familial. The critical region for this syndrome has been narrowed down to 4p15.33-15.2, but the underlying causative genes remain unclear. In this study, we report the case of a 3-year-old female with failure to thrive, developmental and motor delays, and morphological features. The mother also had a 4p15.2-p14 deletion, and the proband was found to have a 13.4-Mb 4p15.2-p14 deletion by chromosome microarray analysis. The deleted region encompasses 16 genes, five of which have a high likelihood of contributing to the phenotype: PPARGC1A, DHX15, RBPJ, STIM2, and PCDH7. These findings suggest that multiple genes are involved in this rare proximal 4p interstitial deletion syndrome. This case highlights the need for healthcare providers to be aware of proximal 4p interstitial deletions and the potential phenotypic manifestations.

MITF is a novel transcriptional regulator of the calcium sensor STIM1: Significance in physiological melanogenesis

Stromal Interaction Molecule1 (STIM1) is an endoplasmic reticulum membrane-localized calcium (Ca²⁺) sensor that plays a critical role in the store-operated Ca²⁺ entry (SOCE) pathway. STIM1 regulates a variety of physiological processes and contributes to a plethora of pathophysiological conditions. Several disease states and enhanced biological phenomena are associated with increased STIM1 levels and activity. However, molecular mechanisms driving STIM1 expression remain largely unappreciated. We recently reported that STIM1 expression augments during pigmentation. Nonetheless, the molecular choreography regulating STIM1 expression in melanocytes is completely unexplored. Here, we characterized the molecular events that regulate STIM1 expression during pigmentation. We demonstrate that physiological melanogenic stimuli α-melanocyte stimulating hormone (αMSH) increases STIM1 mRNA and protein levels. Further, αMSH stimulates STIM1 promoter-driven luciferase activity, thereby suggesting transcriptional upregulation of STIM1. We show that downstream of αMSH, microphthalmia-associated transcription factor (MITF) drives STIM1 expression. By performing knockdown and overexpression studies, we corroborated that MITF regulates STIM1 expression and SOCE. Next, we conducted extensive bioinformatics analysis and identified MITF-binding sites on the STIM1 promoter. We validated significance of the MITF-binding sites in controlling STIM1 expression by performing ChIP and luciferase assays with truncated STIM1 promoters. Moreover, we confirmed MITF's role in regulating STIM1 expression and SOCE in primary human melanocytes. Importantly, analysis of publicly available datasets substantiates a positive correlation between STIM1 and MITF expression in sun-exposed tanned human skin, thereby highlighting physiological relevance of this regulation. Taken together, we have identified a novel physiologically relevant molecular pathway that transcriptionally enhances STIM1 expression.

Deficiency of the Two-Pore Potassium Channel KCNK9 Impairs Intestinal Epithelial Cell Survival and Aggravates Dextran Sodium Sulfate-Induced Colitis

BACKGROUND & AIMS

The 2-pore potassium channel subfamily K member 9 (KCNK9) regulates intracellular calcium concentration and thus modulates cell survival and inflammatory signaling pathways. It also was recognized as a risk allele for inflammatory bowel disease. However, it remains unclear whether KCNK9 modulates inflammatory bowel disease via its impact on immune cell function or whether its influence on calcium homeostasis also is relevant in intestinal epithelial cells.

METHODS

Kcnk9^-/- mice were challenged with 3% dextran sulfate sodium (DSS) to induce experimental acute colitis. Primary cultures of intestinal epithelial cells were generated, and expression of potassium channels as well as cytosolic calcium levels and susceptibility to apoptosis were evaluated. Furthermore, we evaluated whether KCNK9 deficiency was compensated by the closely related 2-pore potassium channel KCNK3 in vivo or in vitro.

RESULTS

Compared with controls, KCNK9 deficiency or its pharmacologic blockade were associated with aggravated DSS-induced colitis compared with wild-type animals. In the absence of KCNK9, intestinal epithelial cells showed increased intracellular calcium levels and were more prone to mitochondrial damage and caspase-9-dependent apoptosis. We found that expression of KCNK3 was increased in Kcnk9^-/- mice but did not prevent apoptosis after DSS exposure. Conversely, increased levels of KCNK9 in Kcnk3^-/- mice were associated with an ameliorated course of DSS-induced colitis.

CONCLUSIONS

KCNK9 enhances mitochondrial stability, reduces apoptosis, und thus supports epithelial cell survival after DSS exposure in vivo and in vitro. Conversely, its increased expression in Kcnk3^-/- resulted in less mitochondrial damage and apoptosis and was associated with beneficial outcomes in DSS-induced colitis.

Crucial Role of Stromal Interaction Molecule-Activated TRPC-ORAI Channels in Vascular Remodeling and Pulmonary Hypertension Induced by Intermittent Hypoxia

Obstructive sleep apnea (OSA), a sleep breathing disorder featured by chronic intermittent hypoxia (CIH), is associate with pulmonary hypertension. Rats exposed to CIH develop lung vascular remodeling and pulmonary hypertension, which paralleled the upregulation of stromal interaction molecule (STIM)-activated TRPC-ORAI Ca2+ channels (STOC) in the lung, suggesting that STOC participate in the pulmonary vascular alterations. Accordingly, to evaluate the role played by STOC in pulmonary hypertension we studied whether the STOC blocker 2-aminoethoxydiphenyl borate (2-APB) may prevent the vascular remodeling and the pulmonary hypertension induced by CIH in a rat model of OSA. We assessed the effects of 2-APB on right ventricular systolic pressure (RVSP), pulmonary vascular remodeling, α-actin and proliferation marker Ki-67 levels in pulmonary arterial smooth muscle cells (PASMC), mRNA levels of STOC subunits, and systemic and pulmonary oxidative stress (TBARS) in male Sprague-Dawley (200 g) rats exposed to CIH (5% O2, 12 times/h for 8h) for 28 days. At 14 days of CIH, osmotic pumps containing 2-APB (10 mg/kg/day) or its vehicle were implanted and rats were kept for 2 more weeks in CIH. Exposure to CIH for 28 days raised RVSP > 35 mm Hg, increased the medial layer thickness and the levels of α-actin and Ki-67 in PASMC, and increased the gene expression of TRPC1, TRPC4, TRPC6 and ORAI1 subunits. Treatment with 2-APB prevented the raise in RVSP and the increment of the medial layer thickness, as well as the increased levels of α-actin and Ki-67 in PASMC, and the increased gene expression of STOC subunits. In addition, 2-APB did not reduced the lung and systemic oxidative stress, suggesting that the effects of 2-APB on vascular remodeling and pulmonary hypertension are independent on the reduction of the oxidative stress. Thus, our results supported that STIM-activated TRPC-ORAI Ca2+ channels contributes to the lung vascular remodeling and pulmonary hypertension induced by CIH.

STIM Proteins: An Ever-Expanding Family

Stromal interaction molecules (STIM) are a distinct class of ubiquitously expressed single-pass transmembrane proteins in the endoplasmic reticulum (ER) membrane. Together with Orai ion channels in the plasma membrane (PM), they form the molecular basis of the calcium release-activated calcium (CRAC) channel. An intracellular signaling pathway known as store-operated calcium entry (SOCE) is critically dependent on the CRAC channel. The SOCE pathway is activated by the ligand-induced depletion of the ER calcium store. STIM proteins, acting as calcium sensors, subsequently sense this depletion and activate Orai ion channels via direct physical interaction to allow the influx of calcium ions for store refilling and downstream signaling processes. This review article is dedicated to the latest advances in the field of STIM proteins. New results of ongoing investigations based on the recently published functional data as well as structural data from nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulations are reported and complemented with a discussion of the latest developments in the research of STIM protein isoforms and their differential functions in regulating SOCE.

Redox Regulation of Ion Channels and Receptors in Pulmonary Hypertension

Significance: Pulmonary hypertension (PH) is characterized by elevated vascular resistance due to vasoconstriction and remodeling of the normally low-pressure pulmonary vasculature. Redox stress contributes to the pathophysiology of this disease by altering the regulation and activity of membrane receptors, K+ channels, and intracellular Ca2+ homeostasis. Recent Advances: Antioxidant therapies have had limited success in treating PH, leading to a growing appreciation that reductive stress, in addition to oxidative stress, plays a role in metabolic and cell signaling dysfunction in pulmonary vascular cells. Reactive oxygen species generation from mitochondria and NADPH oxidases has substantial effects on K+ conductance and membrane potential, and both receptor-operated and store-operated Ca2+ entry. Critical Issues: Some specific redox changes resulting from oxidation, S-nitrosylation, and S-glutathionylation are known to modulate membrane receptor and ion channel activity in PH. However, many sites of regulation that have been elucidated in nonpulmonary cell types have not been tested in the pulmonary vasculature, and context-specific molecular mechanisms are lacking. Future Directions: Here, we review what is known about redox regulation of membrane receptors and ion channels in PH. Further investigation of the mechanisms involved is needed to better understand the etiology of PH and develop better targeted treatment strategies.

AMP-Activated Protein Kinase (AMPK)-Dependent Regulation of Renal Transport

AMP-activated kinase (AMPK) is a serine/threonine kinase that is expressed in most cells and activated by a high cellular AMP/ATP ratio (indicating energy deficiency) or by Ca²⁺. In general, AMPK turns on energy-generating pathways (e.g., glucose uptake, glycolysis, fatty acid oxidation) and stops energy-consuming processes (e.g., lipogenesis, glycogenesis), thereby helping cells survive low energy states. The functional element of the kidney, the nephron, consists of the glomerulus, where the primary urine is filtered, and the proximal tubule, Henle's loop, the distal tubule, and the collecting duct. In the tubular system of the kidney, the composition of primary urine is modified by the reabsorption and secretion of ions and molecules to yield final excreted urine. The underlying membrane transport processes are mainly energy-consuming (active transport) and in some cases passive. Since active transport accounts for a large part of the cell's ATP demands, it is an important target for AMPK. Here, we review the AMPK-dependent regulation of membrane transport along nephron segments and discuss physiological and pathophysiological implications.

To die or not to die SGK1-sensitive ORAI/STIM in cell survival

The pore forming Ca²⁺ release activated Ca²⁺ channel (CRAC) isoforms ORAI1-3 and their regulators STIM1,2 accomplish store operated Ca²⁺ entry (SOCE). Activation of SOCE may lead to cytosolic Ca²⁺ oscillations, which in turn support cell proliferation and cell survival. ORAI/STIM and thus SOCE are upregulated by the serum and glucocorticoid inducible kinase SGK1, a kinase under powerful genomic regulation and activated by phosphorylation via the phosphoinositol-3-phosphate pathway. SGK1 enhances ORAI1 abundance partially by phosphorylation of Nedd4-2, an ubiquitin ligase priming the channel protein for degradation. The SGK1-phosphorylated Nedd4-2 binds to the protein 14-3-3 and is thus unable to ubiquinate ORAI1. SGK1 further increases the ORAI1 and STIM1 protein abundance by activating nuclear factor kappa B (NF-κB), a transcription factor upregulating the expression of STIM1 and ORAI1. SGK1-sensitive upregulation of ORAI/STIM and thus SOCE is triggered by a wide variety of hormones and growth factors, as well as several cell stressors including ischemia, radiation, and cell shrinkage. SGK1 dependent upregulation of ORAI/STIM confers survival of tumor cells and thus impacts on growth and therapy resistance of cancer. On the other hand, SGK1-dependent upregulation of ORAI1 and STIM1 may support survival of neurons and impairment of SGK1-dependent ORAI/STIM activity may foster neurodegeneration. Clearly, further experimental effort is needed to define the mechanisms linking SGK1-dependent upregulation of ORAI1 and STIM1 to cell survival and to define the impact of SGK1-dependent upregulation of ORAI1 and STIM1 on malignancy and neurodegenerative disease.

STIM1 silencing inhibits the migration and invasion of A549 cells

The present study aimed to explore the effects of stromal interaction molecule 1 (STIM1) knockdown on the migration, invasion and metastasis of A549 cells in vitro and in vivo. Western blotting and immunohistochemistry were used to detect protein expression levels. Wound healing and Transwell invasion assays were used to assess the migratory and invasive abilities of A549 cells transfected with STIM1-specific short hairpin (sh)RNA (shSTIM1). In addition, a tail vein metastatic assay was performed. The results demonstrated that the frequency of STIM1 high-expression was significantly increased in metastatic lung cancer tissues (72.2%) compared with in non-metastatic lung cancer tissues (33.0%). STIM1 knockdown inhibited A549 cell migration and invasion in vitro and tumor metastasis in vivo. The protein expression levels of Snail1, Vimentin, matrix metalloproteinase (MMP) 2 and MMP9 were markedly decreased in A549-shSTIM1 compared with in A549 cells transfected with control shRNA (shcon). In addition, the protein expression levels of E-cadherin were markedly increased in A549-shSTIM1 cells compared with in A549-shcon cells. These results suggested that STIM1 knockdown may inhibit the migration and invasion of A549 cells in vitro, and metastasis in vivo.

Higher plasma level of STIM1, OPG are correlated with stent restenosis after PCI

OBJECT

Percutaneous Coronary Intervention (PCI) is one of the most effective treatments for Coronary Heart Disease (CHD), but the high rate of In Stent Restenosis (ISR) has plagued clinicians after PCI. We aim to investigate the correlation of plasma Stromal Interaction Molecular 1 (STIM1) and Osteoprotegerin (OPG) level with stent restenosis after PCI.

METHODS

A total of 100 consecutive patients with Coronary Heart Disease (CHD) received PCI procedure were recruited. Coronary angiography was performed 8 months after their PCI. Then patients were divided into 2 groups: observation group was composed by patients who existing postoperative stenosis after intervention; Control group was composed by patients with no postoperative stenosis. The plasma levels of STIM, OPG in all patients were tested before and after intervention. Pearson correlation and multiple linear regression analysis were performed to analysis the correlation between STIM, OPG level and postoperative stenosis.

RESULTS

35 cases were divided into observation group and other 65 were divided into control group. The plasma levels of STIM, OPG have no statistical difference before their PCI procedure, but we observed higher level of High-sensitivity C-reactive protein (Hs-CRP) existed in observation group. We observed higher level of plasma STIM, OPG in observation group when compared with control group after PCI procedure (P < 0.05). Regression analysis demonstrated that Hs-CRP, STIM1, OPG are independent risk factors for ISR.

CONCLUSION

Elevated levels of plasma STIM1, OPG are independent risk factors for ISR in patients received PCI, which could provide useful information for the restenosis control after PCI.

Downregulation of STIM2 improves neuronal survival after traumatic brain injury by alleviating calcium overload and mitochondrial dysfunction

Although store-operated calcium entry (SOCE) has been implicated in several neurological disorders, the exact mechanism for its role in traumatic brain injury (TBI) has not been elucidated. In this study, we found that TBI upregulated the expression of a calcium sensor protein called stromal interactive molecule 2 (STIM2); however, the levels of its homologue, STIM1, were unaffected. Both STIM1 and STIM2 are crucial components of SOCE, both in vivo and in vitro. Using shRNA, we discovered that downregulation of STIM2, but not STIM1, significantly improved neuronal survival in both an in vitro and in vivo model of TBI, decreasing neuronal apoptosis, and preserving neurological function. This neuroprotection was associated with alleviating TBI-induced calcium overload and preserving mitochondrial function. Additionally, downregulation of STIM2 not only inhibited Ca(2+) release from the endoplasmic reticulum (ER), but also reduced SOCE-mediated Ca(2+) influx, decreased mitochondrial Ca(2+), restored mitochondrial morphology and improved mitochondrial function, including MMP maintenance, ROS production and ATP synthesis. These results indicate that inhibition of STIM2 can protect neurons from TBI by inhibiting calcium overload and preserving mitochondrial function. This suggests that STIM2 might be an effective interventional target for TBI.

Fructus Corni extract-induced neuritogenesis in PC12 cells is associated with the suppression of stromal interaction molecule 1 expression and inhibition of Ca2+ influx

Fructus Corni (Cornus officinalis Sieb. et Zucc.) is commonly prescribed as a traditional Chinese herbal medicine that possesses pharmacological actions against inflammation, diabetic nephropathy, tumors, oxidation and aging. However, its function and mode of action within the nervous system remain largely unclear. In this study, the effects of Fructus Corni extract (FCE) on neuronal differentiation were investigated. It was found that FCE significantly increased the percentage of PC12 cells bearing neurites (P<0.001). Following the generation of neurite outgrowth, FCE treatment decreased the mRNA expression of stromal interaction molecule 1 (STIM1; P<0.05) and suppressed the expression of STIM1 protein (P<0.001). In addition, extracellular calcium (Ca²⁺) influx was inhibited resulting in a reduction in the intracellular Ca²⁺ level, suggesting that the inhibition of Ca²⁺ influx may be involved in the FCE-promoted neurite outgrowth of PC12 cells. These results demonstrate that FCE induces neurite outgrowth in PC12 cells and that this is associated with the suppression of STIM1 expression and the inhibition of Ca²⁺ influx, which may partially explain the FCE-induced neuritogenesis.

Members of the thrombospondin gene family bind stromal interaction molecule 1 and regulate calcium channel activity

The thrombospondins (TSPs) are a family of matricellular proteins that regulate cellular phenotype through interactions with a myriad of other proteins and proteoglycans. We have identified a novel interaction of the members of the TSP gene family with stromal interaction molecule 1 (STIM1). This association is robust since it is preserved in Triton X-100, can be detected with multiple anti-TSP-1 and anti-STIM1 antibodies, and is detected in a wide range of cell types. We have also found that STIM1 co-immunoprecipitates with TSP-4 and cartilage oligomeric matrix protein (COMP), and that a recombinant version of the N-terminal domain of STIM1 binds to the signature domain of TSP-1 and COMP. The association of the TSPs with STIM1 is observed in both the presence and absence of calcium indicating that the calcium-dependent conformation of the signature domain of TSPs is not required for binding. Thus, this interaction could occur in the ER under conditions of normal or low calcium concentration. Furthermore, we observed that the expression of COMP in HEK 293 cells decreases STIM1-mediated calcium release activated calcium (CRAC) channel currents and increases arachidonic acid calcium (ARC) channel currents. These data indicate that the TSPs regulate STIM1 function and participate in the reciprocal regulation of two channels that mediate calcium entry into the cell.

The neglected CRAC proteins: Orai2, Orai3, and STIM2

Plasma-membrane-localized Orai1 ion channel subunits interacting with ER-localized STIM1 molecules comprise the major subunit composition responsible for calcium release-activated calcium channels. STIM1 "translates" the Ca(2+) store content into Orai1 activity, making it a store-operated channel. Surprisingly, in addition to being the physical activator, STIM1 also modulates Orai1 properties, including its inactivation and permeation (see Chapter 1). STIM1 is thus more than a pure Orai1 activator. Within the past 7 years following the discovery of STIM and Orai proteins, the molecular mechanisms of STIM1/Orai1 activity and their functional importance have been studied in great detail. Much less is currently known about the other isoforms STIM2, Orai2, and Orai3. In this chapter, we summarize the current knowledge about STIM2, Orai2, and Orai3 properties and function. Are these homologues mainly modulators of predominantly STIM1/Orai1-mediated complexes or do store-dependent or -independent functions such as regulation of basal Ca(2+) concentration and activation of Orai3-containing complexes by arachidonic acid or by estrogen receptors point toward their "true" physiological function? Is Orai2 the Orai1 of neurons? A major focus of the review is on the functional relevance of STIM2, Orai2, and Orai3, some of which still remains speculative.

Calcium signaling in lacrimal glands

Lacrimal glands provide the important function of lubricating and protecting the ocular surface. Failure of proper lacrimal gland function results in a number of debilitating dry eye diseases. Lacrimal glands secrete lipids, mucins, proteins, salts and water and these secretions are at least partially regulated by neurotransmitter-mediated cell signaling. The predominant signaling mechanism for lacrimal secretion involves activation of phospholipase C, generation of the Ca(2+)-mobilizing messenger, IP3, and release of Ca(2+) stored in the endoplasmic reticulum. The loss of Ca(2+) from the endoplasmic reticulum then triggers a process known as store-operated Ca(2+) entry, involving a Ca(2+) sensor in the endoplasmic reticulum, STIM1, which activates plasma membrane store-operated channels comprised of Orai subunits. Recent studies with deletions of the channel subunit, Orai1, confirm the important role of SOCE in both fluid and protein secretion in lacrimal glands, both in vivo and in vitro.

Role of ASIC1 in the development of chronic hypoxia-induced pulmonary hypertension

Chronic hypoxia (CH) associated with respiratory disease results in elevated pulmonary vascular intracellular Ca(2+) concentration, which elicits enhanced vasoconstriction and promotes vascular arterial remodeling and thus has important implications in the development of pulmonary hypertension (PH). Store-operated Ca(2+) entry (SOCE) contributes to this elevated intracellular Ca(2+) concentration and has also been linked to acute hypoxic pulmonary vasoconstriction (HPV). Since our laboratory has recently demonstrated an important role for acid-sensing ion channel 1 (ASIC1) in mediating SOCE, we hypothesized that ASIC1 contributes to both HPV and the development of CH-induced PH. To test this hypothesis, we examined responses to acute hypoxia in isolated lungs and assessed the effects of CH on indexes of PH, arterial remodeling, and vasoconstrictor reactivity in wild-type (ASIC1(+/+)) and ASIC1 knockout (ASIC1(-/-)) mice. Restoration of ASIC1 expression in pulmonary arterial smooth muscle cells from ASIC1(-/-) mice rescued SOCE, confirming the requirement for ASIC1 in this response. HPV responses were blunted in lungs from ASIC1(-/-) mice. Both SOCE and receptor-mediated Ca(2+) entry, along with agonist-dependent vasoconstrictor responses, were diminished in small pulmonary arteries from control ASIC(-/-) mice compared with ASIC(+/+) mice. The effects of CH to augment receptor-mediated vasoconstrictor and SOCE responses in vessels from ASIC1(+/+) mice were not observed after CH in ASIC1(-/-) mice. In addition, ASIC1(-/-) mice exhibited diminished right ventricular systolic pressure, right ventricular hypertrophy, and arterial remodeling in response to CH compared with ASIC1(+/+) mice. Taken together, these data demonstrate an important role for ASIC1 in both HPV and the development of CH-induced PH.

Prenatal diagnosis and molecular cytogenetic characterization of a de novo proximal interstitial deletion of chromosome 4p (4p15.2→p14)

We present prenatal diagnosis of de novo proximal interstitial deletion of chromosome 4p (4p15.2→p14) and molecular cytogenetic characterization of the deletion using uncultured amniocytes. We review the phenotypic abnormalities of previously reported patients with similar proximal interstitial 4p deletions, and we discuss the functions of the genes of RBPJ, CCKAR, STIM2, PCDH7 and ARAP2 that are deleted within this region.

Role of STIM1- and Orai1-mediated Ca2+ entry in Ca2+-induced epidermal keratinocyte differentiation

The uppermost thin layer on the surface of the skin, called the epidermis, is responsible for the barrier function of the skin. The epidermis has a multilayered structure in which each layer consists of keratinocytes (KCs) of different differentiation status. The integrity of KC differentiation is crucial for the function of skin and its loss causes or is accompanied by skin diseases. Intracellular and extracellular Ca(2+) is known to play important roles in KC differentiation. However, the molecular mechanisms underlying Ca(2+) regulation of KC differentiation are still largely unknown. Store-operated Ca(2+) entry (SOCE) is a major Ca(2+) influx pathway in most non-excitable cells. SOCE is evoked in response to a fall in Ca(2+) concentration in the endoplasmic reticulum. Two proteins have been identified as essential components of SOCE: STIM1, a Ca(2+) sensor in the ER, and Orai1, a subunit of Ca(2+) channels in the plasma membrane. In this study, we analyzed the contribution of SOCE to KC growth and differentiation using RNAi knockdown of STIM1 and Orai1 in the human keratinocyte cell line, HaCaT. KC differentiation was induced by a switch in extracellular Ca(2+) concentration from low (0.03 mM; undifferentiated KCs) to high (1.8 mM; differentiated KCs). This Ca(2+) switch triggers phospholipase-C-mediated intracellular Ca(2+) signals (Ca(2+)-switch-induced Ca(2+) response), which would probably involve the activation of SOCE. Knockdown of either STIM1 or Orai1 strongly suppressed SOCE and almost completely abolished the Ca(2+)-switch-induced Ca(2+) responses, resulting in impaired expression of keratin1, an early KC differentiation marker. Furthermore, loss of either STIM1 or Orai1 suppressed normal growth of HaCaT cells in low Ca(2+) and inhibited the growth arrest in response to a Ca(2+) switch. These results demonstrate that SOCE plays multiple crucial roles in KC differentiation and function.

Knockdown of stromal interaction molecule 1 (STIM1) suppresses store-operated calcium entry, cell proliferation and tumorigenicity in human epidermoid carcinoma A431 cells

Store-operated calcium (Ca(2+)) entry (SOCE) is important for cellular activities such as gene transcription, cell cycle progression and proliferation in most non-excitable cells. Stromal interaction molecule 1 (STIM1), a newly identified Ca(2+)-sensing protein, monitors the depletion of endoplasmic reticulum (ER) Ca(2+) stores and activates store-operated Ca(2+) channels at the plasma membrane to induce SOCE. To investigate the possible roles of STIM1 in tumor growth in relation to SOCE, we established STIM1 knockdown (KD) clones of human epidermoid carcinoma A431 cells by RNA interference. Thapsigargin, an inhibitor of ER Ca(2+)-ATPase, -induced and phospholipase C-coupled receptor agonist-induced SOCEs were reduced in two STIM1 KD clones compared to a negative control clone. Re-expression of a KD-resistant full-length STIM1, but not a Ca(2+) release-activated Ca(2+) channel activation domain (CAD)-deleted STIM1 mutant, in the KD clone restored the amplitude of SOCE, suggesting the specificity of the STIM1 knockdown. The cell growth of the STIM1 KD clones was slower than that of the negative control clone. DNA synthesis assessed by BrdU incorporation, as well as EGF-stimulated EGF receptor activation, decreased in the STIM1 KD clones. Xenograft growth of the STIM1 KD clones was significantly retarded compared with that of the negative control. Cell migration was attenuated in the STIM1 KD clone and the STIM1 silencing effect was reversed by transient re-expression of the full-length STIM1 but not CAD-deletion mutant. These results indicate that STIM1 plays an important role in SOCE, cell-growth and tumorigenicity in human epidermoid carcinoma A431cells, suggesting the potential use of STIM1-targeting agents for treating epidermoid carcinoma.

Store-operated Ca2+ entry is remodelled and controls in vitro angiogenesis in endothelial progenitor cells isolated from tumoral patients

BACKGROUND

Endothelial progenitor cells (EPCs) may be recruited from bone marrow to sustain tumor vascularisation and promote the metastatic switch. Understanding the molecular mechanisms driving EPC proliferation and tubulogenesis could outline novel targets for alternative anti-angiogenic treatments. Store-operated Ca(2+) entry (SOCE), which is activated by a depletion of the intracellular Ca(2+) pool, regulates the growth of human EPCs, where is mediated by the interaction between the endoplasmic reticulum Ca(2+)-sensor, Stim1, and the plasmalemmal Ca(2+) channel, Orai1. As oncogenesis may be associated to the capability of tumor cells to grow independently on Ca(2+) influx, it is important to assess whether SOCE regulates EPC-dependent angiogenesis also in tumor patients.

METHODOLOGY/PRINCIPAL FINDINGS

The present study employed Ca(2+) imaging, recombinant sub-membranal and mitochondrial aequorin, real-time polymerase chain reaction, gene silencing techniques and western blot analysis to investigate the expression and the role of SOCE in EPCs isolated from peripheral blood of patients affected by renal cellular carcinoma (RCC; RCC-EPCs) as compared to control EPCs (N-EPCs). SOCE, activated by either pharmacological (i.e. cyclopiazonic acid) or physiological (i.e. ATP) stimulation, was significantly higher in RCC-EPCs and was selectively sensitive to BTP-2, and to the trivalent cations, La(3+) and Gd(3+). Furthermore, 2-APB enhanced thapsigargin-evoked SOCE at low concentrations, whereas higher doses caused SOCE inhibition. Conversely, the anti-angiogenic drug, carboxyamidotriazole (CAI), blocked both SOCE and the intracellular Ca(2+) release. SOCE was associated to the over-expression of Orai1, Stim1, and transient receptor potential channel 1 (TRPC1) at both mRNA and protein level The intracellular Ca(2+) buffer, BAPTA, BTP-2, and CAI inhibited RCC-EPC proliferation and tubulogenesis. The genetic suppression of Stim1, Orai1, and TRPC1 blocked CPA-evoked SOCE in RCC-EPCs.

CONCLUSIONS

SOCE is remodelled in EPCs from RCC patients and stands out as a novel molecular target to interfere with RCC vascularisation due to its ability to control proliferation and tubulogenesis.

Polyamines regulate intestinal epithelial restitution through TRPC1-mediated Ca²+ signaling by differentially modulating STIM1 and STIM2

Early epithelial restitution occurs as a consequence of intestinal epithelial cell (IEC) migration after wounding, and its defective regulation is implicated in various critical pathological conditions. Polyamines stimulate intestinal epithelial restitution, but their exact mechanism remains unclear. Canonical transient receptor potential-1 (TRPC1)-mediated Ca(2+) signaling is crucial for stimulation of IEC migration after wounding, and induced translocation of stromal interaction molecule 1 (STIM1) to the plasma membrane activates TRPC1-mediated Ca(2+) influx and thus enhanced restitution. Here, we show that polyamines regulate intestinal epithelial restitution through TRPC1-mediated Ca(2+) signaling by altering the ratio of STIM1 to STIM2. Increasing cellular polyamines by ectopic overexpression of the ornithine decarboxylase (ODC) gene stimulated STIM1 but inhibited STIM2 expression, whereas depletion of cellular polyamines by inhibiting ODC activity decreased STIM1 but increased STIM2 levels. Induced STIM1/TRPC1 association by increasing polyamines enhanced Ca(2+) influx and stimulated epithelial restitution, while decreased formation of the STIM1/TRPC1 complex by polyamine depletion decreased Ca(2+) influx and repressed cell migration. Induced STIM1/STIM2 heteromers by polyamine depletion or STIM2 overexpression suppressed STIM1 membrane translocation and inhibited Ca(2+) influx and epithelial restitution. These results indicate that polyamines differentially modulate cellular STIM1 and STIM2 levels in IECs, in turn controlling TRPC1-mediated Ca(2+) signaling and influencing cell migration after wounding.

Regulation of Orai1/STIM1 by the kinases SGK1 and AMPK

STIM and Orai isoforms orchestrate store operated Ca2+ entry (SOCE) and thus cytosolic Ca2+ fluctuations following stimulation by hormones, growth factors and further mediators. Orai1 is a target of Nedd4-2, an ubiquitin ligase preparing several plasma membrane proteins for degradation. Phosphorylation of Nedd4-2 by the serum and glucocorticoid inducible kinase SGK1 leads to the binding of Nedd4-2 to the protein 14-3-3 thus preventing its interaction with Orai1. Nedd4-2 is activated by the energy sensing AMP activated kinase AMPK. Thus, SGK1 disrupts and AMPK fosters degradation of Orai1. New synthesis of both, Orai1 and STIM1, is stimulated by the transcription factor NF-κB (nuclear factor kappa B), which binds to the respective promoter regions of the genes encoding STIM1 and Orai1. SGK1 upregulates and AMPK presumably downregulates NF-κB and thus de novo synthesis of Orai1 and STIM1 proteins. The regulation by SGK1 links SOCE to the signaling of a wide variety of hormones and growth factors, the AMPK dependent regulation of Orai1 and STIM1 may serve to limit inadequate activation of SOCE following energy depletion, which is otherwise expected to activate SOCE by depletion of intracellular Ca2+ stores due to impairment of the ATP consuming sarco/endoplasmatic reticulum Ca2+ ATPase SERCA.

Store-operated channels regulate intracellular calcium in mammalian rods

Exposure to daylight closes cyclic nucleotide-gated (CNG) and voltage-operated Ca(2+) -permeable channels in mammalian rods. The consequent lowering of the cytosolic calcium concentration ([Ca(2+)](i)), if protracted, can contribute to light-induced damage and apoptosis in these cells. We here report that mouse rods are protected against prolonged lowering of [Ca(2+)](i) by store-operated Ca(2+) entry (SOCE). Ca(2+) stores were depleted in Ca(2+)-free saline supplemented with the endoplasmic reticulum (ER) sequestration blocker cyclopiazonic acid. Store depletion elicited [Ca(2+)](i) signals that exceeded baseline [Ca(2+)](i) by 5.9 ± 0.7-fold and were antagonized by an inhibitory cocktail containing 2-APB, SKF 96365 and Gd(3+). Cation influx through SOCE channels was sufficient to elicit a secondary activation of L-type voltage-operated Ca2+ entry. We also found that TRPC1, the type 1 canonical mammalian homologue of the Drosophila photoreceptor TRP channel, is predominantly expressed within the outer nuclear layer of the retina. Rod loss in Pde6b(rdl) (rd1), Chx10/Kip1(-/-rdl) and Elovl4(TG2) dystrophic models was associated with ∼70% reduction in Trpc1 mRNA content whereas Trpc1 mRNA levels in rodless cone-full Nrl(-/-) retinas were decreased by ∼50%. Genetic ablation of TRPC1 channels, however, had no effect on SOCE, the sensitivity of the rod phototransduction cascade or synaptic transmission at rod and cone synapses. Thus, we localized two new mechanisms, SOCE and TRPC1, to mammalian rods and characterized the contribution of SOCE to Ca(2+) homeostasis. By preventing the cytosolic [Ca(2+)](i) from dropping too low under sustained saturating light conditions, these signalling pathways may protect Ca(2+)-dependent mechanisms within the ER and the cytosol without affecting normal rod function.

Chronic hypoxia upregulates pulmonary arterial ASIC1: a novel mechanism of enhanced store-operated Ca2+ entry and receptor-dependent vasoconstriction

Acid-sensing ion channel 1 (ASIC1) is a newly characterized contributor to store-operated Ca(2+) entry (SOCE) in pulmonary vascular smooth muscle (VSM). Since SOCE is implicated in elevated basal VSM intracellular Ca(2+) concentration ([Ca(2+)](i)) and augmented vasoconstriction in chronic hypoxia (CH)-induced pulmonary hypertension, we hypothesized that ASIC1 contributes to these responses. To test this hypothesis, we examined effects of the specific pharmacologic ASIC1a inhibitor, psalmotoxin 1 (PcTX1), on vasoconstrictor and vessel wall [Ca(2+)](i) responses to UTP and KCl (depolarizing stimulus) in fura-2-loaded, pressurized small pulmonary arteries from control and CH (4 wk at 0.5 atm) Wistar rats. PcTX1 had no effect on basal vessel wall [Ca(2+)](i), but attenuated vasoconstriction and increases in vessel wall [Ca(2+)](i) to UTP in arteries from control and CH rats; normalizing responses between groups. In contrast, responses to the depolarizing stimulus, KCl, were unaffected by CH exposure or PcTX1. Upon examining potential Ca(2+) influx mechanisms, we found that PcTX1 prevented augmented SOCE following CH. Exposure to CH resulted in a significant increase in pulmonary arterial ASIC1 protein. This study supports a novel role of ASIC1 in elevated receptor-stimulated vasoconstriction following CH which is likely mediated through increased ASIC1 expression and SOCE.

Endoplasmic reticulum calcium pumps and cancer cell differentiation

The endoplasmic reticulum (ER) is a major intracellular calcium storage pool and a multifunctional organelle that accomplishes several calcium-dependent functions involved in many homeostatic and signaling mechanisms. Calcium is accumulated in the ER by Sarco/Endoplasmic Reticulum Calcium ATPase (SERCA)-type calcium pumps. SERCA activity can determine ER calcium content available for intra-ER functions and for calcium release into the cytosol, and can shape the spatiotemporal characteristics of calcium signals. SERCA function therefore constitutes an important nodal point in the regulation of cellular calcium homeostasis and signaling, and can exert important effects on cell growth, differentiation and survival. In several cell types such as cells of hematopoietic origin, mammary, gastric and colonic epithelium, SERCA2 and SERCA3-type calcium pumps are simultaneously expressed, and SERCA3 expression levels undergo significant changes during cell differentiation, activation or immortalization. In addition, SERCA3 expression is decreased or lost in several tumor types when compared to the corresponding normal tissue. These observations indicate that ER calcium homeostasis is remodeled during cell differentiation, and may present defects due to decreased SERCA3 expression in tumors. Modulation of the state of differentiation of the ER reflected by SERCA3 expression constitutes an interesting new aspect of cell differentiation and tumor biology.

Phosphorylation of Stim1 at serine 575 via netrin-2/Cdo-activated ERK1/2 is critical for the promyogenic function of Stim1

The promyogenic cell surface molecule Cdo is required for activation of extracellular signal-regulated kinase (ERK) and nuclear factor of activated T cells c3 (NFATc3) induced by netrin-2 in myogenic differentiation. However, the molecular mechanism leading to NFATc3 activation is unknown. Stromal interaction molecule 1 (Stim1), an internal calcium sensor of the endoplasmic reticulum store, promotes myogenesis via activation of NFATc3. In this study we investigated the functional interaction between Cdo and Stim1 in myogenic differentiation. Overexpression and depletion of Stim1 enhanced or decreased myotube formation, respectively. Of interest, Stim1 protein levels were decreased in Cdo-deficient perinatal hindlimb muscles or primary myoblasts; this correlates with defective NFATc3 activation in Cdo(-/-) myoblasts upon differentiation. Forced activation of NFATc3 by overexpression of calcineurin restored differentiation of Cdo-depleted C2C12 myoblasts. Furthermore, Cdo and Stim1 formed a complex in 293T cells or in differentiating C2C12 myoblasts. The netrin-2-mediated NFATc3 activation was coincident with robust interactions between Cdo and Stim1 in myoblasts and the ERK-mediated Stim1 phosphorylation at serine 575. The serine 575 phosphorylation was enhanced in C2C12 cells upon differentiation, and the alanine substitution of serine 575 failed to restore differentiation of Stim1-depleted myoblasts. Taken together, the results indicate that cell adhesion signaling triggered by netrin-2/Cdo induces Stim1 phosphorylation at serine 575 by ERK, which promotes myoblast differentiation.

Transcription factor NF-κB regulates expression of pore-forming Ca2+ channel unit, Orai1, and its activator, STIM1, to control Ca2+ entry and affect cellular functions

The serum and glucocorticoid-inducible kinase SGK1 increases the activity of Orai1, the pore forming unit of store-operated Ca(2+) entry, and thus influences Ca(2+)-dependent cellular functions such as migration. SGK1 further regulates transcription factor nuclear factor κB (NF-κB). This study explored whether SGK1 influences transcription of Orai1 and/or STIM1, the Orai1-activating Ca(2+) sensor. Orai1 and STIM1 transcript levels were decreased in mast cells from SGK1 knock-out mice and increased in HEK293 cells transfected with active (S422D)SGK1 but not with inactive (K127N)SGK1 or in (S422D)SGK1-transfected cells treated with the NF-κB inhibitor Wogonin (100 μm). Treatment with the stem cell factor enhanced transcript levels of STIM1 and Orai1 in sgk1(+/+) but not in sgk1(-/-) mast cells and not in sgk1(+/+) cells treated with Wogonin. Orai1 and STIM1 transcript levels were further increased in sgk1(+/+) and sgk1(-/-) mast cells by transfection with active NF-κB subunit p65 as well as in HEK293 cells by transfection with NF-κB subunits p65/p50 or p65/p52. They were decreased by silencing of NF-κB subunits p65, p50, or p52 or by NF-κB inhibitor Wogonin (100 μm). Luciferase assay and chromatin immunoprecipitation defined NF-κB-binding sites in promoter regions accounting for NF-κB sensitive genomic regulation of STIM1 and Orai1. Store-operated Ca(2+) entry was similarly increased by overexpression of p65/p50 or p65/p52 and decreased by treatment with Wogonin. Transfection of HEK293 cells with p65/p50 or p65/p52 further augmented migration. The present observations reveal powerful genomic regulation of Orai1/STIM1 by SGK1-dependent NF-κB signaling.

STIM1-dependent store-operated Ca²⁺ entry is required for pathological cardiac hypertrophy

Alterations in intracellular Ca(2+) homeostasis are an important trigger of pathological cardiac remodeling; however, mechanisms governing context-dependent changes in Ca(2+) influx are poorly understood. Store-operated Ca(2+) entry (SOCE) is a major mechanism regulating Ca(2+) trafficking in numerous cell types, yet its prevalence in adult heart and possible role in physiology and disease are each unknown. The Ca(2+)-binding protein, stromal interaction molecule 1 (STIM1), is a Ca(2+) sensor in the sarcoplasmic reticulum (SR), capable of triggering SOCE by interacting with plasma membrane Ca(2+) channels. We report that SOCE is abundant and robust in neonatal cardiomyocytes; however, SOCE is absent from adult cardiomyocytes. Levels of STIM1 transcript and protein correlate with the amplitude of SOCE, and manipulation of STIM1 protein levels (via shRNA) or activity (via expression of constitutively active or dominant-negative mutants) reveals a critical role for STIM1 in activating SOCE in cardiac myocytes. In neonatal hearts a recently identified STIM1 splice variant (STIM1L) is predominant but diminishes with maturation, only to reemerge with agonist- or afterload-induced cardiac stress. To test for pathophysiological relevance, we evaluated both in vitro and in vivo models of cardiac hypertrophy, finding that STIM1 expression is re-activated by pathological stress to trigger significant SOCE-dependent Ca(2+) influx. STIM1 amplifies agonist-induced hypertrophy via activation of the calcineurin-NFAT pathway. Importantly, inhibition of STIM1 suppresses agonist-triggered hypertrophy, pointing to a requirement for SOCE in this remodeling response. Stress-triggered STIM1 re-expression, and consequent SOCE activation, are critical elements in the upstream, Ca(2+)-dependent control of pathological cardiac hypertrophy.

Calcium Channels in the Development, Maturation, and Function of Spermatozoa

A proper dialogue between spermatozoa and the egg is essential for conception of a new individual in sexually reproducing animals. Ca2+ is crucial in orchestrating this unique event leading to a new life. No wonder that nature has devised different Ca2+-permeable channels and located them at distinct sites in spermatozoa so that they can help fertilize the egg. New tools to study sperm ionic currents, and image intracellular Ca2+ with better spatial and temporal resolution even in swimming spermatozoa, are revealing how sperm ion channels participate in fertilization. This review critically examines the involvement of Ca2+ channels in multiple signaling processes needed for spermatozoa to mature, travel towards the egg, and fertilize it. Remarkably, these tiny specialized cells can express exclusive channels like CatSper for Ca2+ and SLO3 for K+, which are attractive targets for contraception and for the discovery of novel signaling complexes. Learning more about fertilization is a matter of capital importance; societies face growing pressure to counteract rising male infertility rates, provide safe male gamete-based contraceptives, and preserve biodiversity through improved captive breeding and assisted conception initiatives.

A human sterile alpha motif domain polymerizome

The sterile alpha motif (SAM) domain is one of the most common protein modules found in eukaryotic genomes. Many SAM domains have been shown to form helical polymer structures suggesting that SAM modules can be used to create large protein complexes in the cell. Because many polymeric SAM domains form heterogenous and insoluble aggregates that are experimentally intractable when isolated, it is likely that many polymeric SAM domains have gone uncharacterized. We, therefore, developed a method to maintain polymeric SAM domains in a soluble form that allowed rapid screening for potential SAM polymers. SAM domains were expressed as fusions to a super-negatively charged green fluorescent protein (negGFP). The negGFP imparts three useful properties to the SAM domains: (1) the charge helps to maintain solubility; (2) the charge leads to reliable migration toward the cathode on native gels; and (3) the fluorescence emission allows visualization in crude extracts. Using the negGFP-SAM fusions, we screened a large library of human SAM domains for polymerization using a native gel screen. A selected set of hSAM domains were then purified and examined for true polymer formation by electron microscopy. In this manner, we identified a set of new potential SAM polymers: ANKS3, Atherin, BicaudalC1, Caskin1, Caskin2, Kazrin, L3MBTL3, L3MBTL4, LBP, LiprinB1, LiprinB2, SAMD8, SAMD9, and STIM2. While further characterization will be necessary to verify that the SAM domains identified here truly form polymers, our results provide a much stronger working hypothesis for a large number of proteins that was possible from sequence analysis alone.

Endoplasmic reticulum calcium pumps and cancer

Endoplasmic reticulum calcium homeostasis is involved in a multitude of signaling, as well as "house-keeping" functions that control cell growth, differentiation or apoptosis in every human/eukaryotic cell. Calcium is actively accumulated in the endoplasmic reticulum by Sarco/Endoplasmic Reticulum Calcium transport ATPases (SERCA enzymes). SERCA-dependent calcium transport is the only calcium uptake mechanism in this organelle, and therefore the regulation of SERCA function by the cell constitutes a key mechanism to adjust calcium homeostasis in the endoplasmic reticulum depending on the cell type and its state of differentiation. The direct pharmacological modulation of SERCA activity affects cell differentiation and survival. SERCA expression levels can undergo significant changes during cell differentiation or tumorigenesis, leading to modified endoplasmic reticulum calcium storage. In several cell types such as cells of hematopoietic origin or various epithelial cells, two SERCA genes (SERCA2 and SERCA3) are simultaneously expressed. Expression levels of SERCA3, a lower calcium affinity calcium pump are highly variable. In several cell systems SERCA3 expression is selectively induced during differentiation, whereas during tumorigenesis and blastic transformation SERCA3 expression is decreased. These observations point at the existence of a cross-talk, via the regulation of SERCA3 levels, between endoplasmic reticulum calcium homeostasis and the control of cell differentiation, and show that endoplasmic reticulum calcium homeostasis itself can undergo remodeling during differentiation. The investigation of the anomalies of endoplasmic reticulum differentiation in tumor and leukemia cells may be useful for a better understanding of the contribution of calcium signaling to the establishment of malignant phenotypes.

A cytosolic STIM2 preprotein created by signal peptide inefficiency activates ORAI1 in a store-independent manner

Calcium (Ca(2+)) influx through the plasma membrane store-operated Ca(2+) channel ORAI1 is controlled by Ca(2+) sensors of the stromal interaction molecule (STIM) family. STIM1 responds to endoplasmic reticulum (ER) Ca(2+) store depletion by redistributing and activating ORAI1 from regions of the ER juxtaposed to the plasma membrane. Unlike STIM1, STIM2 can regulate ORAI1 in a store-dependent and store-independent manner, but the mechanism by which this is achieved is unknown. Here we find that STIM2 is translated from a highly conserved methionine residue and is directed to the ER by an incredibly long 101-amino acid signal peptide. We find that although the majority of the total STIM2 population resides on the ER membrane, a second population escapes ER targeting to accumulate as a full-length preprotein in the cytosol, signal peptide intact. Unlike STIM2, preSTIM2 localizes to the inner leaflet of the plasma membrane where it interacts with ORAI1 to regulate basal Ca(2+) concentration and Ca(2+)-dependent gene transcription in a store-independent manner. Furthermore, a third protein comprising a fragment of the STIM2 signal peptide is released from the ER membrane into the cytosol where it regulates gene transcription in a Ca(2+)-independent manner. This study establishes a new model for STIM2-mediated regulation of ORAI1 in which two distinct proteins, STIM2 and preSTIM2, control store-dependent and store-independent modes of ORAI1 activation.

Regulation of STIM1 and SOCE by the ubiquitin-proteasome system (UPS)

The ubiquitin proteasome system (UPS) mediates the majority of protein degradation in eukaryotic cells. The UPS has recently emerged as a key degradation pathway involved in synapse development and function. In order to better understand the function of the UPS at synapses we utilized a genetic and proteomic approach to isolate and identify novel candidate UPS substrates from biochemically purified synaptic membrane preparations. Using these methods, we have identified Stromal interacting molecule 1 (STIM1). STIM1 is as an endoplasmic reticulum (ER) calcium sensor that has been shown to regulate store-operated Ca²⁺ entry (SOCE). We have characterized STIM1 in neurons, finding STIM1 is expressed throughout development with stable, high expression in mature neurons. As in non-excitable cells, STIM1 is distributed in a membranous and punctate fashion in hippocampal neurons. In addition, a population of STIM1 was found to exist at synapses. Furthermore, using surface biotinylation and live-cell labeling methods, we detect a subpopulation of STIM1 on the surface of hippocampal neurons. The role of STIM1 as a regulator of SOCE has typically been examined in non-excitable cell types. Therefore, we examined the role of the UPS in STIM1 and SOCE function in HEK293 cells. While we find that STIM1 is ubiquitinated, its stability is not altered by proteasome inhibitors in cells under basal conditions or conditions that activate SOCE. However, we find that surface STIM1 levels and thapsigargin (TG)-induced SOCE are significantly increased in cells treated with proteasome inhibitors. Additionally, we find that the overexpression of POSH (Plenty of SH3′s), an E3 ubiquitin ligase recently shown to be involved in the regulation of Ca²⁺ homeostasis, leads to decreased STIM1 surface levels. Together, these results provide evidence for previously undescribed roles of the UPS in the regulation of STIM1 and SOCE function.