Faculty Bibliography

Store-operated Ca2+ entry (SOCE) is the main Ca2+ influx pathway in lymphocytes and is essential for T cell function and adaptive immunity. SOCE is mediated by Ca2+ release-activated Ca2+ (CRAC) channels that are activated by stromal interaction molecule (STIM) 1 and STIM2. SOCE regulates many Ca2+-dependent signaling molecules, including calcineurin, and inhibition of SOCE or calcineurin impairs antigen-dependent T cell proliferation. We here report that SOCE and calcineurin regulate cell cycle entry of quiescent T cells by controlling glycolysis and oxidative phosphorylation. SOCE directs the metabolic reprogramming of naive T cells by regulating the expression of glucose transporters, glycolytic enzymes, and metabolic regulators through the activation of nuclear factor of activated T cells (NFAT) and the PI3K-AKT kinase-mTOR nutrient-sensing pathway. We propose that SOCE controls a critical "metabolic checkpoint" at which T cells assess adequate nutrient supply to support clonal expansion and adaptive immune responses.

Dental enamel is one of the most remarkable examples of matrix-mediated biomineralization. Enamel crystals form de novo in a rich extracellular environment in a stage-dependent manner producing complex microstructural patterns that are visually stunning. This process is orchestrated by specialized epithelial cells known as ameloblasts which themselves undergo striking morphological changes switching function from a secretory role to a cell primarily engaged in ionic transport. Ameloblasts are supported by a host of cell types which combined represent the enamel organ. Fully mineralized enamel is the hardest tissue found in vertebrates owing its properties partly to the unique mixture of ionic species represented and their highly organized assembly in the crystal lattice. Among the main elements found in enamel, Ca2+ is the most abundant ion yet how ameloblasts modulate Ca2+ dynamics remains poorly known. This review describes previously proposed models for passive and active Ca2+ transport, the intracellular Ca2+ buffering systems expressed in ameloblasts and provide an up-dated view of current models concerning Ca2+ influx and extrusion mechanisms, where most of the recent advances have been made. We also advance a new model for Ca2+ transport by the enamel organ

Loss-of-function mutations in stromal interaction molecule 1 (STIM1) impair the activation of Ca2+ release-activated Ca2+ (CRAC) channels and store-operated Ca2+ entry (SOCE), resulting in a disease syndrome called CRAC channelopathy that is characterized by severe dental enamel defects. The cause of these enamel defects has remained unclear given a lack of animal models. We generated Stim1/2K14cre mice to delete STIM1 and its homolog STIM2 in enamel cells. These mice showed impaired SOCE in enamel cells. Enamel in Stim1/2K14cre mice was hypomineralized with decreased Ca content, mechanically weak, and thinner. The morphology of SOCE-deficient ameloblasts was altered, showing loss of the typical ruffled border, resulting in mislocalized mitochondria. Global gene expression analysis of SOCE-deficient ameloblasts revealed strong dysregulation of several pathways. ER stress genes associated with the unfolded protein response were increased in Stim1/2-deficient cells, whereas the expression of components of the glutathione system were decreased. Consistent with increased oxidative stress, we found increased ROS production, decreased mitochondrial function, and abnormal mitochondrial morphology in ameloblasts of Stim1/2K14cre mice. Collectively, these data show that loss of SOCE in enamel cells has substantial detrimental effects on gene expression, cell function, and the mineralization of dental enamel.

Store-operated Ca2+ entry (SOCE) through Ca2+ release-activated Ca2+ (CRAC) channels is critical for lymphocyte function and immune responses. CRAC channels are hexamers of ORAI proteins that form the channel pore, but the contributions of individual ORAI homologues to CRAC channel function are not well understood. Here we show that deletion of Orai1 reduces, whereas deletion of Orai2 increases, SOCE in mouse T cells. These distinct effects are due to the ability of ORAI2 to form heteromeric channels with ORAI1 and to attenuate CRAC channel function. The combined deletion of Orai1 and Orai2 abolishes SOCE and strongly impairs T cell function. In vivo, Orai1/Orai2 double-deficient mice have impaired T cell-dependent antiviral immune responses, and are protected from T cell-mediated autoimmunity and alloimmunity in models of colitis and graft-versus-host disease. Our study demonstrates that ORAI1 and ORAI2 form heteromeric CRAC channels, in which ORAI2 fine-tunes the magnitude of SOCE to modulate immune responses.

Ca2+ signals were reported to control lipid homeostasis, but the Ca2+ channels and pathways involved are largely unknown. Store-operated Ca2+ entry (SOCE) is a ubiquitous Ca2+ influx pathway regulated by stromal interaction molecule 1 (STIM1), STIM2, and the Ca2+ channel ORAI1. We show that SOCE-deficient mice accumulate pathological amounts of lipid droplets in the liver, heart, and skeletal muscle. Cells from patients with loss-of-function mutations in STIM1 or ORAI1 show a similar phenotype, suggesting a cell-intrinsic role for SOCE in the regulation of lipid metabolism. SOCE is crucial to induce mobilization of fatty acids from lipid droplets, lipolysis, and mitochondrial fatty acid oxidation. SOCE regulates cyclic AMP production and the expression of neutral lipases as well as the transcriptional regulators of lipid metabolism, peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC-1alpha), and peroxisome proliferator-activated receptor alpha (PPARalpha). SOCE-deficient cells upregulate lipophagy, which protects them from lipotoxicity. Our data provide evidence for an important role of SOCE in lipid metabolism.

Store-operated Ca²⁺entry (SOCE) is a pathway for increasing intracellular Ca²⁺ levels regulated by stromal interaction molecule 1 (STIM1), STIM2, and the Ca²⁺ channel ORAI1. SOCE-deficient patients suffer from Calcium Release-Activated Calcium (CRAC) channelopathy characterized by immunodeficiency, autoimmunity, myopathy, and anhidrotic ectodermal dysplasia. Several mitochondrial enzymes/complexes depend on Ca²⁺ but the source of Ca²⁺ required for their function are not entirely clear. We recently showed a cell-intrinsic role of SOCE in human mitochondria (Maus M et al. Cell Metab. 2017;25(3):698- 712). MitoView Green showed reduced mitochondrial volume in fibroblasts of patients with ORAI1/STIM1 lossof- function mutations. mtDNA copy numbers and mRNAs expression of selected mitochondrial transcription factors were normal. SDS-PAGE/Western blot analysis showed reduced expression of NADH ubiquinone oxidoreductase subunit-B8, Cytochrome b-c1 complex subunit-2, Cytochrome c oxidase subunit-I, Cytochrome C, Mitochondrial porin and permeability transition pore, etc. Blue native PAGE of isolated mitochondria confirmed reduced expression of CI, CIV and supercomplex CICIII2. SOCE-deficient fibroblasts had reduced mRNA and protein expression of uncoupling protein 2, higher basal mitochondrial membrane potential (MMP) and higher numbers of damaged mitochondria as suggested by increased co-localization of mitochondria and lysosomes and increased MitoKeima reporter activity indicative of lysosomal mitophagy. Oligomycininduced ATP-synthase inhibition revealed decreased electron transport and proton pumping rates measured as MMP hyperpolarization rates and reduced superoxide production assessed by MitoSOX. Maximal O2 consumption rates in SOCE-deficient cells were decreased. Skeletal myocytes had reduced CI and CIV function in 2 out of 3 ORAI1-deficient patients. Gene expression of very long chain acyl-CoA dehydrogenase and long-chain fatty acid transporter carnitine palmitoyltransferase 1B was reduced in patient fibroblasts cultured in either high glucose medium or oleic acid (OA) medium followed by starvation in 2 mM glucose medium. Furthermore, SOCE-deficient fibroblasts were lacking a starvation-induced increase in etomoxir-sensitive mitochondrial respiration in OA medium and showed reduced rates of OA beta-oxidation when cultured in 14C-OA-medium with or without subsequent starvation. Our findings indicate an important new role of SOCE in mitochondrial function

Eccrine sweat glands are essential for sweating and thermoregulation in humans. Loss-of-function mutations in the Ca2+ release-activated Ca2+ (CRAC) channel genes ORAI1 and STIM1 abolish store-operated Ca2+ entry (SOCE), and patients with these CRAC channel mutations suffer from anhidrosis and hyperthermia at high ambient temperatures. Here we have shown that CRAC channel-deficient patients and mice with ectodermal tissue-specific deletion of Orai1 (Orai1K14Cre) or Stim1 and Stim2 (Stim1/2K14Cre) failed to sweat despite normal sweat gland development. SOCE was absent in agonist-stimulated sweat glands from Orai1K14Cre and Stim1/2K14Cre mice and human sweat gland cells lacking ORAI1 or STIM1 expression. In Orai1K14Cre mice, abolishment of SOCE was associated with impaired chloride secretion by primary murine sweat glands. In human sweat gland cells, SOCE mediated by ORAI1 was necessary for agonist-induced chloride secretion and activation of the Ca2+-activated chloride channel (CaCC) anoctamin 1 (ANO1, also known as TMEM16A). By contrast, expression of TMEM16A, the water channel aquaporin 5 (AQP5), and other regulators of sweat gland function was normal in the absence of SOCE. Our findings demonstrate that Ca2+ influx via store-operated CRAC channels is essential for CaCC activation, chloride secretion, and sweat production in humans and mice.

T follicular helper (Tfh) cells promote affinity maturation of B cells in germinal centers (GCs), whereas T follicular regulatory (Tfr) cells limit the GC reaction. Store-operated Ca2+ entry (SOCE) through Ca2+ release-activated Ca2+ (CRAC) channels mediated by STIM and ORAI proteins is a fundamental signaling pathway in T lymphocytes. Conditional deletion of Stim1 and Stim2 genes in T cells abolished SOCE and strongly reduced antibody-mediated immune responses following viral infection caused by impaired differentiation and function of Tfh cells. Conversely, aging Stim1Stim2-deficient mice developed humoral autoimmunity with spontaneous autoantibody production due to abolished Tfr cell differentiation in the presence of residual Tfh cells. Mechanistically, SOCE controlled Tfr and Tfh cell differentiation through NFAT-mediated IRF4, BATF, and Bcl-6 transcription-factor expression. SOCE had a dual role in controlling the GC reaction by regulating both Tfh and Tfr cell differentiation, thus enabling protective B cell responses and preventing humoral autoimmunity.