Surf4 modulates STIM1-dependent calcium entry

Disulfidptosis features and prognosis in head and neck squamous cell carcinoma patients: unveiling and validating the prognostic signature across cohorts

BACKGROUND

Head and neck squamous cell carcinoma (HNSCC) is a significant health concern with a variable global incidence and is linked to regional lifestyle factors and HPV infections. Despite treatment advances, patient prognosis remains variable, necessitating an understanding of its molecular mechanisms and the identification of reliable prognostic biomarkers.

METHODS

We analyzed 959 HNSCC samples and employed batch correction to obtain consistent transcriptomic data across cohorts. We examined 79 disulfidptosis-related genes to determine consensus clusters and utilized high-throughput sequencing to identify genetic heterogeneity within tumors. We established a disulfidptosis prognostic signature (DSPS) using least absolute shrinkage and selection operator (LASSO) regression and developed a prognostic nomogram integrating the DSPS with clinical factors. Personalized chemotherapy prediction was performed using the "pRRophetic" R package.

RESULTS

Batch corrections were used to harmonize gene expression data, revealing two distinct disulfidptosis subtypes, C1 and C2, with differential gene expression and survival outcomes. Subtype C1, characterized by increased expression of the MYH family genes ACTB, ACTN2, and FLNC, had a mortality rate of 48.4%, while subtype C2 had a mortality rate of 38.7% (HR = 0.77, 95% CI: 0.633-0.934, P = 0.008). LASSO regression identified 15 genes that composed the DSPS prognostic model, which independently predicted survival (HR = 2.055, 95% CI: 1.420-2.975, P < 0.001). The prognostic nomogram, which included the DSPS, age, and tumor stage, predicted survival with AUC values of 0.686, 0.704, and 0.789 at 3, 5, and 8 years, respectively, indicating strong predictive capability. In the external validation cohort (cohort B), the DSPS successfully identified patients at greater risk, with worse overall survival outcomes in the high-DSPS subgroup (HR = 1.54, 95% CI: 1.17-2.023, P = 0.002) and AUC values of 0.601, 0.644, 0.636, and 0.748 at 3, 5, 8, and 10 years, respectively, confirming the model's robustness.

CONCLUSION

The DSPS provides a robust prognostic tool for HNSCC, underscoring the complexity of this disease and the potential for tailored treatment strategies. This study highlights the importance of molecular signatures in oncology, offering a step toward personalized medicine and improved patient outcomes in HNSCC management.

Surfeit locus protein 4 modulates endoplasmic reticulum function and maintains oocyte quality

Surfeit locus protein 4 is a cargo receptor mediating cargo transport from the endoplasmic reticulum lumen to the Golgi apparatus. Loss of Surf4 gene led to embryonic lethality in mice. However, the role of Surf4 during oocyte development remains unknown. In this study, we generated the mouse model with oocyte-specific knockout of Surf4 gene. We found that adult mice with deletion of Surf4 showed normal folliculogenesis, ovulation and fertility. However, loss of Surf4 slightly impaired oocyte quality, thus led to partial oocyte meiotic arrest and reduced ratio of blastocyst formation. Consistent with this, the distribution of endoplasmic reticulum was disturbed in Surf4-deficient oocytes in mice. These results demonstrated that although Surf4 is dispensable for female mouse fertility, Surf4 modulates endoplasmic reticulum arrangement and participates in regulation of developmental competence of oocytes.

The Cytoplasmic Region of SARAF Reduces Triple-Negative Breast Cancer Metastasis through the Regulation of Store-Operated Calcium Entry

Triple-negative breast cancer has a poor prognosis and is non-responsive to first-line therapies; hence, new therapeutic strategies are needed. Enhanced store-operated Ca2+ entry (SOCE) has been widely described as a contributing factor to tumorigenic behavior in several tumor types, particularly in breast cancer cells. SOCE-associated regulatory factor (SARAF) acts as an inhibitor of the SOCE response and, therefore, can be a potential antitumor factor. Herein, we generated a C-terminal SARAF fragment to evaluate the effect of overexpression of this peptide on the malignancy of triple-negative breast cancer cell lines. Using both in vitro and in vivo approaches, we showed that overexpression of the C-terminal SARAF fragment reduced proliferation, cell migration, and the invasion of murine and human breast cancer cells by decreasing the SOCE response. Our data suggest that regulating the activity of the SOCE response via SARAF activity might constitute the basis for further alternative therapeutic strategies for triple-negative breast cancer.

A Genome-Wide Functional Screen Identifies Enhancer and Protective Genes for Amyloid Beta-Peptide Toxicity

Alzheimer's disease (AD) is known to be caused by amyloid β-peptide (Aβ) misfolded into β-sheets, but this knowledge has not yet led to treatments to prevent AD. To identify novel molecular players in Aβ toxicity, we carried out a genome-wide screen in Saccharomyces cerevisiae, using a library of 5154 gene knock-out strains expressing Aβ_1-42. We identified 81 mammalian orthologue genes that enhance Aβ_1-42 toxicity, while 157 were protective. Next, we performed interactome and text-mining studies to increase the number of genes and to identify the main cellular functions affected by Aβ oligomers (oAβ). We found that the most affected cellular functions were calcium regulation, protein translation and mitochondrial activity. We focused on SURF4, a protein that regulates the store-operated calcium channel (SOCE). An in vitro analysis using human neuroblastoma cells showed that SURF4 silencing induced higher intracellular calcium levels, while its overexpression decreased calcium entry. Furthermore, SURF4 silencing produced a significant reduction in cell death when cells were challenged with oAβ_1-42, whereas SURF4 overexpression induced Aβ_1-42 cytotoxicity. In summary, we identified new enhancer and protective activities for Aβ toxicity and showed that SURF4 contributes to oAβ_1-42 neurotoxicity by decreasing SOCE activity.

STIM Proteins and Regulation of SOCE in ER-PM Junctions

ER-PM junctions are membrane contact sites formed by the endoplasmic reticulum (ER) and plasma membrane (PM) in close apposition together. The formation and stability of these junctions are dependent on constitutive and dynamic enrichment of proteins, which either contribute to junctional stability or modulate the lipid levels of both ER and plasma membranes. The ER-PM junctions have come under much scrutiny recently as they serve as hubs for assembling the Ca²⁺ signaling complexes. This review summarizes: (1) key findings that underlie the abilities of STIM proteins to accumulate in ER-PM junctions; (2) the modulation of Orai/STIM complexes by other components found within the same junction; and (3) how Orai1 channel activation is coordinated and coupled with downstream signaling pathways.

Surf4 facilitates reprogramming by activating the cellular response to endoplasmic reticulum stress

OBJECTIVES

Maternal factors that are enriched in oocytes have attracted great interest as possible key factors in somatic cell reprogramming. We found that surfeit locus protein 4 (Surf4), a maternal factor, can facilitate the generation of induced pluripotent stem cells (iPSCs) previously, but the mechanism remains elusive.

MATERIALS AND METHODS

In this study, we investigated the function and mechanism of Surf4 in somatic cell reprogramming using a secondary reprogramming system. Alkaline phosphatase (AP) staining, qPCR and immunofluorescence (IF) staining of expression of related markers were used to evaluate efficiency of iPSCs derived from mouse embryonic fibroblasts. Embryoid body and teratoma formation assays were performed to evaluate the differentiation ability of the iPSC lines. RNA-seq, qPCR and western blot analysis were applied to validate the downstream targets of Surf4.

RESULTS

Surf4 can significantly facilitate the generation of iPSCs in a proliferation-independent manner. When co-expressed with Oct4, Sox2, Klf4 and c-Myc (OSKM), Surf4 can activate the response to endoplasmic reticulum (ER) stress at the early stage of reprogramming. We further demonstrated that Hspa5, a major ER chaperone, and the active spliced form of Xbp1 (sXbp1), a major mediator of ER stress, can mimic the effects of Surf4 on somatic cell reprogramming. Concordantly, blocking the unfolded protein response compromises the effect of Surf4 on reprogramming.

CONCLUSIONS

Surf4 promotes somatic cell reprogramming by activating the response to ER stress.

Desmin interacts with STIM1 and coordinates Ca2+ signaling in skeletal muscle

Stromal interaction molecule 1 (STIM1), the sarcoplasmic reticulum (SR) transmembrane protein, activates store-operated Ca2+ entry (SOCE) in skeletal muscle and, thereby, coordinates Ca2+ homeostasis, Ca2+-dependent gene expression, and contractility. STIM1 occupies space in the junctional SR membrane of the triads and the longitudinal SR at the Z-line. How STIM1 is organized and is retained in these specific subdomains of the SR is unclear. Here, we identified desmin, the major type III intermediate filament protein in muscle, as a binding partner for STIM1 based on a yeast 2-hybrid screen. Validation of the desmin-STIM1 interaction by immunoprecipitation and immunolocalization confirmed that the CC1-SOAR domains of STIM1 interact with desmin to enhance STIM1 oligomerization yet limit SOCE. Based on our studies of desmin-KO mice, we developed a model wherein desmin connected STIM1 at the Z-line in order to regulate the efficiency of Ca2+ refilling of the SR. Taken together, these studies showed that desmin-STIM1 assembles a cytoskeletal-SR connection that is important for Ca2+ signaling in skeletal muscle.

Nutrient supplementation strategy improves cell concentration and longevity, monoclonal antibody production and lactate metabolism of Chinese hamster ovary cells

A careful selection of culture mediums and feeds has become necessary to maximize yields of recombinant proteins during bioprocesses of mammalian cells. Supplements contain a variety of concentrate nutrients, and their beneficial effects vary according to recombinant cell lines. In this study, the effects of PowerFeed A on growth kinetics, productivity and cellular metabolism were evaluated for two Chinese hamster ovary cell lines producing a monoclonal antibody in a batch culture. Supplemented cultures increased integral viable cell density of CRL-12444 and CRL-12445 cells by 2.4 and 1.6 times through extension of culture time at which viability was above 90% in 72 and 36 h, respectively, and increment of maximal cell concentration in 3.25 × 10⁶ cells/ml (69%) for CRL-12445 cells. Product titer augmented 1.9 and 2.5 times for CRL-12444 and CRL-12445 cells, respectively, without changes in growth rate and specific productivity. Feed supplementation also stimulated full consumption of glucose and free glutamine and reduced 10 times lactate accumulation, while ammonium, sodium and potassium remained at similar concentrations at the end of the culture. About 44% of calcium, mainly provided by feed, was consumed by both cell lines. Maximization of cellular growth, viability and protein titer through feeding encourages extending its use to other cell lines and exploring novel combinations with other basal mediums or feeds. A thorough investigation of its impact on protein quality and the molecular mechanisms behind these effects will allow designing effective feeds and strategies to rationally optimize protein production in the biomanufacturing industry.

ER membrane protein complex 1 interacts with STIM1 and regulates store-operated Ca2+ entry

Store-operated calcium entry (SOCE) is the process by which the emptying of endoplasmic reticulum (ER) Ca2+ stores causes an influx of Ca2+ across the plasma membrane. It is the major Ca2+ influx pathway in non-excitable cells and has a wide array of physiological functions. Upon store depletion, stromal interaction molecule 1 (STIM1), an ER calcium sensor relocates into discrete puncta at the ER-plasma membrane junction region, which results in the coupling of Ca2+ channels to initiate SOCE. However, the mechanism regulating STIM1 activity remains poorly understood. Here, we performed affinity purification of STIM1 and uncovered ER membrane protein complex 1 (EMC1) as a STIM1 binding partner. We showed that this interaction occurred in the ER through the intraluminal region of STIM1. After store depletion, EMC1 does not cluster adjacent to the plasma membrane, which suggests that it is distributed differently from STIM1. EMC1 knockdown with small interfering RNA resulted in a marked decrease in SOCE. Thus, these findings suggest that EMC1 functions as a positive regulator of SOCE.

Compartmentalized Proteomic Profiling Outlines the Crucial Role of the Classical Secretory Pathway during Recombinant Protein Production in Chinese Hamster Ovary Cells

Different cellular processes that contribute to protein production in Chinese hamster ovary (CHO) cells have been previously investigated by proteomics. However, although the classical secretory pathway (CSP) has been well documented as a bottleneck during recombinant protein (RP) production, it has not been well represented in previous proteomic studies. Hence, the significance of this pathway for production of RP was assessed by identifying its own proteins that were associated to changes in RP production, through subcellular fractionation coupled to shot-gun proteomics. Two CHO cell lines producing a monoclonal antibody with different specific productivities were used as cellular models, from which 4952 protein groups were identified, which represent a coverage of 59% of the Chinese hamster proteome. Data are available via ProteomeXchange with identifier PXD021014. By using SAM and ROTS algorithms, 493 proteins were classified as differentially expressed, of which about 80% was proposed as novel targets and one-third were assigned to the CSP. Endoplasmic reticulum (ER) stress, unfolded protein response, calcium homeostasis, vesicle traffic, glycosylation, autophagy, proteasomal activity, protein synthesis and translocation into ER lumen, and secretion of extracellular matrix components were some of the affected processes that occurred in the secretory pathway. Processes from other cellular compartments, such as DNA replication, transcription, cytoskeleton organization, signaling, and metabolism, were also modified. This study gives new insights into the molecular traits of higher producer cells and provides novel targets for development of new sub-lines with improved phenotypes for RP production.

SOCE in the cardiomyocyte: the secret is in the chambers

Store-operated Ca²⁺ entry (SOCE) is an ancient and ubiquitous Ca²⁺ signaling pathway that is present in virtually every cell type. Over the last two decades, many studies have implicated this non-voltage dependent Ca²⁺ entry pathway in cardiac physiology. The relevance of the SOCE pathway in cardiomyocytes is often questioned given the well-established role for excitation contraction coupling. In this review, we consider the evidence that STIM1 and SOCE contribute to Ca²⁺ dynamics in cardiomyocytes. We discuss the relevance of this pathway to cardiac growth in response to developmental and pathologic cues. We also address whether STIM1 contributes to Ca²⁺ store refilling that likely impacts cardiac pacemaking and arrhythmogenesis in cardiomyocytes.

More Than Just Simple Interaction between STIM and Orai Proteins: CRAC Channel Function Enabled by a Network of Interactions with Regulatory Proteins

The calcium-release-activated calcium (CRAC) channel, activated by the release of Ca2+ from the endoplasmic reticulum (ER), is critical for Ca2+ homeostasis and active signal transduction in a plethora of cell types. Spurred by the long-sought decryption of the molecular nature of the CRAC channel, considerable scientific effort has been devoted to gaining insights into functional and structural mechanisms underlying this signalling cascade. Key players in CRAC channel function are the Stromal interaction molecule 1 (STIM1) and Orai1. STIM1 proteins span through the membrane of the ER, are competent in sensing luminal Ca2+ concentration, and in turn, are responsible for relaying the signal of Ca2+ store-depletion to pore-forming Orai1 proteins in the plasma membrane. A direct interaction of STIM1 and Orai1 allows for the re-entry of Ca2+ from the extracellular space. Although much is already known about the structure, function, and interaction of STIM1 and Orai1, there is growing evidence that CRAC under physiological conditions is dependent on additional proteins to function properly. Several auxiliary proteins have been shown to regulate CRAC channel activity by means of direct interactions with STIM1 and/or Orai1, promoting or hindering Ca2+ influx in a mechanistically diverse manner. Various proteins have also been identified to exert a modulatory role on the CRAC signalling cascade although inherently lacking an affinity for both STIM1 and Orai1. Apart from ubiquitously expressed representatives, a subset of such regulatory mechanisms seems to allow for a cell-type-specific control of CRAC channel function, considering the rather restricted expression patterns of the specific proteins. Given the high functional and clinical relevance of both generic and cell-type-specific interacting networks, the following review shall provide a comprehensive summary of regulators of the multilayered CRAC channel signalling cascade. It also includes proteins expressed in a narrow spectrum of cells and tissues that are often disregarded in other reviews of similar topics.

Proteins Interacting with STIM1 and Store-Operated Ca2+ Entry

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

Surfeit 4 Contributes to the Replication of Hepatitis C Virus Using Double-Membrane Vesicles

A number of positive-strand RNA viruses, such as hepatitis C virus (HCV) and poliovirus, use double-membrane vesicles (DMVs) as replication sites. However, the role of cellular proteins in DMV formation during virus replication is poorly understood. HCV NS4B protein induces the formation of a "membranous web" structure that provides a platform for the assembly of viral replication complexes. Our previous screen of NS4B-associated host membrane proteins by dual-affinity purification, liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS), and small interfering RNA (siRNA) methods revealed that the Surfeit 4 (Surf4) gene, which encodes an integral membrane protein, is involved in the replication of the JFH1 subgenomic replicon. Here, we investigated in detail the effect of Surf4 on HCV replication. Surf4 affects HCV replication in a genotype-independent manner, whereas HCV replication does not alter Surf4 expression. The influence of Surf4 on HCV replication indicates that while Surf4 regulates replication, it has no effect on entry, translation, assembly, or release. Analysis of the underlying mechanism showed that Surf4 is recruited into HCV RNA replication complexes by NS4B and is involved in the formation of DMVs and the structural integrity of RNA replication complexes. Surf4 also participates in the replication of poliovirus, which uses DMVs as replication sites, but it has no effect on the replication of dengue virus, which uses invaginated/sphere-type vesicles as replication sites. These findings clearly show that Surf4 is a novel cofactor that is involved in the replication of positive-strand RNA viruses using DMVs as RNA replication sites, which provides valuable clues for DMV formation during positive-strand RNA virus replication.IMPORTANCE Hepatitis C virus (HCV) NS4B protein induces the formation of a membranous web (MW) structure that provides a platform for the assembly of viral replication complexes. The main constituents of the MW are double-membrane vesicles (DMVs). Here, we found that the cellular protein Surf4, which maintains endoplasmic reticulum (ER)-Golgi intermediate compartments and the Golgi compartment, is recruited into HCV RNA replication complexes by NS4B and is involved in the formation of DMVs. Moreover, Surf4 participates in the replication of poliovirus, which uses DMVs as replication sites, but has no effect on the replication of dengue virus, which uses invaginated vesicles as replication sites. These results indicate that the cellular protein Surf4 is involved in the replication of positive-strand RNA viruses that use DMVs as RNA replication sites, providing new insights into DMV formation during virus replication and potential targets for the diagnosis and treatment of positive-strand RNA viruses.

Molecular Mechanisms of Calcium Signaling During Phagocytosis

Calcium (Ca²⁺) is a ubiquitous second messenger involved in the regulation of numerous cellular functions including vesicular trafficking, cytoskeletal rearrangements and gene transcription. Both global as well as localized Ca²⁺ signals occur during phagocytosis, although their functional impact on the phagocytic process has been debated. After nearly 40 years of research, a consensus may now be reached that although not strictly required, Ca²⁺ signals render phagocytic ingestion and phagosome maturation more efficient, and their manipulation make an attractive avenue for therapeutic interventions. In the last decade many efforts have been made to identify the channels and regulators involved in generating and shaping phagocytic Ca²⁺ signals. While molecules involved in store-operated calcium entry (SOCE) of the STIM and ORAI family have taken center stage, members of the canonical, melastatin, mucolipin and vanilloid transient receptor potential (TRP), as well as purinergic P2X receptor families are now recognized to play significant roles. In this chapter, we review the recent literature on research that has linked specific Ca²⁺-permeable channels and regulators to phagocytic function. We highlight the fact that lipid mediators are emerging as important regulators of channel gating and that phagosomal ionic homeostasis and Ca²⁺ release also play essential parts. We predict that improved methodologies for measuring these factors will be critical for future advances in dissecting the intricate biology of this fascinating immune process.

The cargo receptor SURF4 promotes the efficient cellular secretion of PCSK9

PCSK9 is a secreted protein that regulates plasma cholesterol levels and cardiovascular disease risk. Prior studies suggested the presence of an ER cargo receptor that recruits PCSK9 into the secretory pathway, but its identity has remained elusive. Here, we apply a novel approach that combines proximity-dependent biotinylation and proteomics together with genome-scale CRISPR screening to identify SURF4, a homologue of the yeast cargo receptor Erv29p, as a primary mediator of PCSK9 secretion in HEK293T cells. The functional contribution of SURF4 to PCSK9 secretion was confirmed with multiple independent SURF4-targeting sgRNAs, clonal SURF4-deficient cell lines, and functional rescue with SURF4 cDNA. SURF4 was found to localize to the early secretory pathway where it physically interacts with PCSK9. Deletion of SURF4 resulted in ER accumulation and decreased extracellular secretion of PCSK9. These findings support a model in which SURF4 functions as an ER cargo receptor mediating the efficient cellular secretion of PCSK9.

Molecular profiles of oxyphilic and chief cell parathyroid adenoma

CONTEXT

Parathyroid adenomas may be composed of chief cells (conventional or water-clear), oxyphilic cells or a mixture of both cells. The molecular background is rarely studied.

OBJECTIVE

To molecularly characterize parathyroid adenomas of different cell type composition.

DESIGN

Chief and oxyphilic cell adenomas were compared in a cohort of 664 sporadic cases. Extensive analyses of parathyroid tissues were performed in subgroup. Gene expressions of known parathyroid-related genes were quantified by qRT-PCR. Protein expression profiles determined by liquid chromatography - tandem mass spectrometry (LC-MS/MS) were compared between each type of parathyroid adenomas. Selected proteins were analysed by Western blot and immunohistochemistry.

RESULTS

Patients with oxyphilic cell adenoma were found to be older at the time of operation than chief cell adenoma cases but did not differ in gender, serum calcium or tumor weight. The gene expression of CASR, VDR, FGFR1, CYP27B1, CYP24A1, PTHLH, GCM2, NDUFA13, CDKN1B, MEN1 and CNND1 did not differ between the groups. VDR protein levels were weaker in oxyphilic adenomas. The proteomic studies identified a set of novel dysregulated proteins of interest such as nuclear receptor subfamily 2 group C member 2 (TR4), LIM domain only protein 3 (LMO3) and calcium-binding protein B (S100B). LMO3 and S100B showed higher expression in oxyphilic adenoma and may be involve in parathyroid tumorgenesis through the p53 pathway. TR4 showed different subcellular localisation between adenoma and normal rim.

CONCLUSION

Chief and oxyphilic cell parathyroid adenomas have partly overlapping but also distinct molecular profiles. The calmodulin-eEF2K, TR4 and p53 pathways may be involved in the tumor development.

SURF4 has oncogenic potential in NIH3T3 cells

SURF4, which is located in the Surfeit gene cluster, encodes for a conserved integral membrane protein containing multiple putative transmembrane regions. However, the physiological role of SURF4 has not been determined. We found that SURF4 demonstrated aberrant amplification and increased expression in the tumor tissues of several human cancer patients. Overexpression of SURF4 led to increased cell proliferation, migration, and maintenance of anchorage-independent growth. In addition, NIH3T3 cells overexpressing SURF4 induced tumor growth in the mice. Collectively, our findings demonstrate that SURF4 has the potential for inducing cellular transformation and cell migration in vitro and has oncogenic transformation ability in vivo.

ER-plasma membrane junctions: Why and how do we study them?

Endoplasmic reticulum (ER)-plasma membrane (PM) junctions are membrane microdomains important for communication between the ER and the PM. ER-PM junctions were first reported in muscle cells in 1957, but mostly ignored in non-excitable cells due to their scarcity and lack of functional significance. In 2005, the discovery of stromal interaction molecule 1 (STIM1) mediating a universal Ca²⁺ feedback mechanism at ER-PM junctions in mammalian cells led to a resurgence of research interests toward ER-PM junctions. In the past decade, several major advancements have been made in this emerging topic in cell biology, including the generation of tools for labeling ER-PM junctions and the unraveling of mechanisms underlying regulation and functions of ER-PM junctions. This review summarizes early studies, recently developed tools, and current advances in the characterization and understanding of ER-PM junctions. This article is part of a Special Issue entitled: Membrane Contact Sites edited by Christian Ungermann and Benoit Kornmann.

Homeostatic regulation of the PI(4,5)P2-Ca(2+) signaling system at ER-PM junctions

The phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2)-Ca(2+) signaling system is important for cell activation in response to various extracellular stimuli. This signaling system is initiated by receptor-induced hydrolysis of PI(4,5)P2 in the plasma membrane (PM) to generate the soluble second messenger inositol 1,4,5-trisphosphate (IP3). IP3 subsequently triggers the release of Ca(2+) from the endoplasmic reticulum (ER) store to the cytosol to activate Ca(2+)-mediated responses, such as secretion and proliferation. The consumed PM PI(4,5)P2 and ER Ca(2+) must be quickly restored to sustain signaling responses, and to maintain the homeostasis of PI(4,5)P2 and Ca(2+). Since phosphatidylinositol (PI), the precursor lipid for PM PI(4,5)P2, is synthesized in the ER membrane, and a Ca(2+) influx across the PM is required to refill the ER Ca(2+) store, efficient communications between the ER and the PM are critical for the homeostatic regulation of the PI(4,5)P2-Ca(2+) signaling system. This review describes the major findings that established the framework of the PI(4,5)P2-Ca(2+) signaling system, and recent discoveries on feedback control mechanisms at ER-PM junctions that sustain the PI(4,5)P2-Ca(2+) signaling system. Particular emphasis is placed on the characterization of ER-PM junctions where efficient communications between the ER and the PM occur, and the activation mechanisms of proteins that dynamically localize to ER-PM junctions to provide the feedback control during PI(4,5)P2-Ca(2+) signaling, including the ER Ca(2+) sensor STIM1, the extended synaptotagmin E-Syt1, and the PI transfer protein Nir2. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

∆F508 CFTR interactome remodelling promotes rescue of cystic fibrosis

Deletion of phenylalanine 508 of the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) is the major cause of Cystic Fibrosis (CF), one of the most common inherited childhood diseases. The mutated CFTR anion channel is not fully glycosylated and shows minimal activity in bronchial epithelial cells of CF patients. Low temperature or inhibition of histone deacetylases (HDACi) can partially rescue ΔF508 CFTR cellular processing defects and function. A favorable change of ΔF508 CFTR protein-protein interactions was proposed as mechanism of rescue, however CFTR interactome dynamics during temperature-shift and HDACi rescue are unknown. Here, we report the first comprehensive analysis of the wt and ΔF508 CFTR interactome and its dynamics during temperature shift and HDACi. By using a novel deep proteomic analysis method (CoPIT), we identified 638 individual high-confidence CFTR interactors and discovered a mutation-specific interactome, which is extensively remodeled upon rescue. Detailed analysis of the interactome remodeling identified key novel interactors, whose loss promoted enhanced CFTR channel function in primary CF epithelia or which were critical for normal CFTR biogenesis. Our results demonstrate that global remodeling of ΔF508 CFTR interactions is crucial for rescue, and provide comprehensive insight into the molecular disease mechanisms of CF caused by deletion of F508.

Retrograde regulation of STIM1-Orai1 interaction and store-operated Ca2+ entry by calsequestrin

Interaction between the endoplasmic reticulum (ER)-located stromal interaction molecue1 (STIM1) and the plasma membrane-located Ca²⁺ channel subunit, Orai1, underlies store-operated Ca²⁺ entry (SOCE). Calsequestrin1 (CSQ1), a sarcoplasmic reticulum Ca²⁺ buffering protein, inhibits SOCE, but the mechanism of action is unknown. We identified an interaction between CSQ1 and STIM1 in HEK293 cells. An increase in monomeric CSQ1 induced by depleted Ca²⁺ stores, or trifluoperazine (TFP), a blocker of CSQ folding and aggregation, enhanced the CSQ1-STIM1 interaction. In cells with Ca²⁺ stores depleted, TFP further increased CSQ1 monomerization and CSQ1-STIM1 interaction, but reduced the association of STIM1 with Orai1 and SOCE. Over-expression of CSQ1 or a C-terminal (amino acid 388–396) deletion mutant significantly promoted the association of CSQ1 with STIM1, but suppressed both STIM1-Orai1 interaction and SOCE, while over-expression of the C-terminal (amino acid 362–396) deletion mutant had no effect. The physical interaction between low polymeric forms of CSQ1 and STIM1 likely acts by interfering with STIM1 oligimerization and inhibits STIM1-Orai1 interaction, providing a brake to SOCE under physiological conditions. This novel regulatory mechanism for SOCE may also contribute to the pathological Ca²⁺ overload in calsequestrin deficient diseases, such as malignant hyperthermia and ventricular tachycardia.

Role of Calcium Signaling in B Cell Activation and Biology

Increase in intracellular levels of calcium ions (Ca2+) is one of the key triggering signals for the development of B cell response to the antigen. The diverse Ca2+ signals finely controlled by multiple factors participate in the regulation of gene expression, B cell development, and effector functions. B cell receptor (BCR)-initiated Ca2+ mobilization is sourced from two pathways: one is the release of Ca2+ from the intracellular stores, endoplasmic reticulum (ER), and other is the prolonged influx of extracellular Ca2+ induced by depleting the stores via store-operated calcium entry (SOCE) and calcium release-activated calcium (CRAC) channels. The identification of stromal interaction molecule 1(STIM1), the ER Ca2+ sensor, and Orai1, a key subunit of the CRAC channel pore, has now provided the tools to understand the mode of Ca2+ influx regulation and physiological relevance. Herein, we discuss our current understanding of the molecular mechanisms underlying BCR-triggered Ca2+ signaling as well as its contribution to the B cell biological processes and diseases.

The Two-pore channel (TPC) interactome unmasks isoform-specific roles for TPCs in endolysosomal morphology and cell pigmentation

The two-pore channels (TPC1 and TPC2) belong to an ancient family of intracellular ion channels expressed in the endolysosomal system. Little is known about how regulatory inputs converge to modulate TPC activity, and proposed activation mechanisms are controversial. Here, we compiled a proteomic characterization of the human TPC interactome, which revealed that TPCs complex with many proteins involved in Ca(2+) homeostasis, trafficking, and membrane organization. Among these interactors, TPCs were resolved to scaffold Rab GTPases and regulate endomembrane dynamics in an isoform-specific manner. TPC2, but not TPC1, caused a proliferation of endolysosomal structures, dysregulating intracellular trafficking, and cellular pigmentation. These outcomes required both TPC2 and Rab activity, as well as their interactivity, because TPC2 mutants that were inactive, or rerouted away from their endogenous expression locale, or deficient in Rab binding, failed to replicate these outcomes. Nicotinic acid adenine dinucleotide phosphate (NAADP)-evoked Ca(2+) release was also impaired using either a Rab binding-defective TPC2 mutant or a Rab inhibitor. These data suggest a fundamental role for the ancient TPC complex in trafficking that holds relevance for lysosomal proliferative scenarios observed in disease.

Activating mutations in STIM1 and ORAI1 cause overlapping syndromes of tubular myopathy and congenital miosis

Signaling through the store-operated Ca(2+) release-activated Ca(2+) (CRAC) channel regulates critical cellular functions, including gene expression, cell growth and differentiation, and Ca(2+) homeostasis. Loss-of-function mutations in the CRAC channel pore-forming protein ORAI1 or the Ca(2+) sensing protein stromal interaction molecule 1 (STIM1) result in severe immune dysfunction and nonprogressive myopathy. Here, we identify gain-of-function mutations in the cytoplasmic domain of STIM1 (p.R304W) associated with thrombocytopenia, bleeding diathesis, miosis, and tubular myopathy in patients with Stormorken syndrome, and in ORAI1 (p.P245L), associated with a Stormorken-like syndrome of congenital miosis and tubular aggregate myopathy but without hematological abnormalities. Heterologous expression of STIM1 p.R304W results in constitutive activation of the CRAC channel in vitro, and spontaneous bleeding accompanied by reduced numbers of thrombocytes in zebrafish embryos, recapitulating key aspects of Stormorken syndrome. p.P245L in ORAI1 does not make a constitutively active CRAC channel, but suppresses the slow Ca(2+)-dependent inactivation of the CRAC channel, thus also functioning as a gain-of-function mutation. These data expand our understanding of the phenotypic spectrum of dysregulated CRAC channel signaling, advance our knowledge of the molecular function of the CRAC channel, and suggest new therapies aiming at attenuating store-operated Ca(2+) entry in the treatment of patients with Stormorken syndrome and related pathologic conditions.

Calcium signaling in B cells: regulation of cytosolic Ca2+ increase and its sensor molecules, STIM1 and STIM2

Calcium signals are crucial for diverse cellular functions including adhesion, differentiation, proliferation, effector functions and gene expression. After engagement of the B cell receptor, the intracellular calcium ion (Ca(2+)) concentration is increased promoting the activation of various signaling cascades. While elevated Ca(2+) in the cytosol initially comes from the endoplasmic reticulum (ER), a continuous influx of extracellular Ca(2+) is required to maintain the increased level of cytosolic Ca(2+). Store-operated Ca(2+) entry manages this process, which is regulated by an ER calcium sensor, stromal interaction molecule (STIM). STIM proteins sense changes in the levels of Ca(2+) stored within the ER lumen and regulates the Ca(2+)-release activated Ca(2+) channel in the plasma membrane. This review focuses on the signaling pathways leading to Ca(2+) influx and the role of Ca(2+) signals in B cell functions.

STIM1/Orai1-mediated SOCE: current perspectives and potential roles in cardiac function and pathology

Store-operated Ca²⁺ entry (SOCE) is critical for Ca²⁺ signaling in nonexcitable cells; however, its role in the regulation of cardiomyocyte Ca²⁺ homeostasis has only recently been investigated. The increased understanding of the role of stromal interaction molecule 1 (STIM1) in regulating SOCE combined with recent studies demonstrating the presence of STIM1 in cardiomyocytes provides support that this pathway co-exists in the heart with the more widely recognized Ca²⁺ handling pathways associated with excitation-contraction coupling. There is now substantial evidence that STIM1-mediated SOCE plays a key role in mediating cardiomyocyte hypertrophy, both in vitro and in vivo, and there is growing support for the contribution of SOCE to Ca²⁺ overload associated with ischemia/reperfusion injury. Here, we provide an overview of our current understanding of the molecular regulation of SOCE and discuss the evidence supporting the role of STIM1/Orai1-mediated SOCE in regulating cardiomyocyte function.

Orai1-NFAT signalling pathway triggered by T cell receptor stimulation

T cell receptor (TCR) stimulation plays a crucial role in development, homeostasis, proliferation, cell death, cytokine production, and differentiation of T cells. Thus, in depth understanding of TCR signalling is crucial for development of therapy targeting inflammatory diseases, improvement of vaccination efficiency, and cancer therapy utilizing T cell-based strategies. TCR activation turns on various signalling pathways, one of the important one being the Ca(2+)-calcineurin-nuclear factor of activated T cells (NFAT) signalling pathway. Stimulation of TCRs triggers depletion of intracellular Ca(2+) store and in turn, initiates store-operated Ca(2+) entry (SOCE), one of the major mechanisms to raise the intracellular Ca(2+) concentrations in T cells. Ca(2+)-release-activated-Ca(2+) (CRAC) channels are a prototype of store-operated Ca(2+) (SOC) channels in immune cells that are very well characterized. Recent identification of STIM1 as the endoplasmic reticulum (ER) Ca(2+) sensor and Orai1 as the pore subunit has dramatically advanced the understanding of CRAC channels and provides a molecular tool to investigate the physiological outcomes of Ca(2+) signalling during immune responses. In this review, we focus on our current understanding of CRAC channel activation, regulation, and downstream calcineurin-NFAT signaling pathway.

Impairment of TRPC1-STIM1 channel assembly and AQP5 translocation compromise agonist-stimulated fluid secretion in mice lacking caveolin1

Neurotransmitter regulation of salivary fluid secretion is mediated by activation of Ca(2+) influx. The Ca(2+)-permeable transient receptor potential canonical 1 (TRPC1) channel is crucial for fluid secretion. However, the mechanism(s) involved in channel assembly and regulation are not completely understood. We report that Caveolin1 (Cav1) is essential for the assembly of functional TRPC1 channels in salivary glands (SG) in vivo and thus regulates fluid secretion. In Cav1(-/-) mouse SG, agonist-stimulated Ca(2+) entry and fluid secretion are significantly reduced. Microdomain localization of TRPC1 and interaction with its regulatory protein, STIM1, are disrupted in Cav1(-/-) SG acinar cells, whereas Orai1-STIM1 interaction is not affected. Furthermore, localization of aquaporin 5 (AQP5), but not that of inositol (1,4,5)-trisphosphate receptor 3 or Ca(2+)-activated K(+) channel (IK) in the apical region of acinar cell was altered in Cav1(-/-) SG. In addition, agonist-stimulated increase in surface expression of AQP5 required Ca(2+) influx via TRPC1 channels and was inhibited in Cav1(-/-) SG. Importantly, adenovirus-mediated expression of Cav1 in Cav1(-/-) SG restored interaction of STIM1 with TRPC1 and channel activation, apical targeting and regulated trafficking of AQP5, and neurotransmitter stimulated fluid-secretion. Together these findings demonstrate that, by directing cellular localization of TRPC1 and AQP5 channels and by selectively regulating the functional assembly TRPC1-STIM1 channels, Cav1 is a crucial determinant of SG fluid secretion.

Ca(2+) release-activated Ca(2+) (CRAC) current, structure, and function

Store-operated Ca(2+) entry describes the phenomenon that connects a depletion of internal Ca(2+) stores to an activation of plasma membrane-located Ca(2+) selective ion channels. Tremendous progress towards the underlying molecular mechanism came with the discovery of the two respective limiting components, STIM and Orai. STIM1 represents the ER-located Ca(2+) sensor and transmits the signal of store depletion to the plasma membrane. Here it couples to and activates Orai, the highly Ca(2+)-selective pore-forming subunit of Ca(2+) release-activated Ca(2+) channels. In this review, we focus on the molecular steps that these two proteins undergo from store-depletion to their coupling, the activation, and regulation of Ca(2+) currents.