Junctophilin-4, a component of the endoplasmic reticulum-plasma membrane junctions, regulates Ca2+ dynamics in T cells

Orai1/STIMs modulators in pulmonary vascular diseases

Calcium (Ca2+) is a secondary messenger that regulates various cellular processes. However, Ca2+ mishandling could lead to pathological conditions. Orai1 is a Ca2+channel contributing to the store-operated calcium entry (SOCE) and plays a critical role in Ca2+ homeostasis in several cell types. Dysregulation of Orai1 contributed to severe combined immune deficiency syndrome, some cancers, pulmonary arterial hypertension (PAH), and other cardiorespiratory diseases. During its activation process, Orai1 is mainly regulated by stromal interacting molecule (STIM) proteins, especially STIM1; however, many other regulatory partners have also been recently described. Increasing knowledge about these regulatory partners provides a better view of the downstream signalling pathways of SOCE and offers an excellent opportunity to decipher Orai1 dysregulation in these diseases. These proteins participate in other cellular functions, making them attractive therapeutic targets. This review mainly focuses on Orai1 regulatory partners in the physiological and pathological conditions of the pulmonary circulation and inflammation.

Behavioral and Transcriptomic Analyses in the Indoxacarb Response of a Non-Target Damselfly Species

Ischnura senegalensis, which widely spreads in paddy fields, has the potential to be used as a natural predator of insect pests. However, the application of insecticides in the field could pose a threat to the survival of I. senegalensis. Among these pesticides, indoxacarb, an oxadiazine insecticide, is renowned for its broad-spectrum efficacy against numerous insect pests. In this study, we examined the toxicity of indoxacarb towards the larvae of I. senegalensis. Behavioral experiments and transcriptome analyses were conducted under indoxacarb treatments. Results revealed that indoxacarb induced abnormal body gestures and significant locomotory impairments, which could ultimately reduce the survival rate of the larvae in their natural habitat. Moreover, transcriptome analyses indicated that genes related to muscle function were significantly affected. Interestingly, at lower concentrations of indoxacarb (0.004 mg/L), the larvae seem to detoxify the indoxacarb with the aid of the cytochrome P450 gene. However, under higher concentrations (0.4 mg/L), the sensory abilities of the larvae were significantly diminished, and they were unable to degrade the toxicity of indoxacarb. Our study underscores the importance of carefully evaluating the impact of insecticides on non-target predatory insects before their widespread application.

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.

Routing of Kv7.1 to endoplasmic reticulum plasma membrane junctions

AIM

The voltage-gated Kv7.1 channel, in association with the regulatory subunit KCNE1, contributes to the IKs current in the heart. However, both proteins travel to the plasma membrane using different routes. While KCNE1 follows a classical Golgi-mediated anterograde pathway, Kv7.1 is located in endoplasmic reticulum-plasma membrane junctions (ER-PMjs), where it associates with KCNE1 before being delivered to the plasma membrane.

METHODS

To characterize the channel routing to these spots we used a wide repertoire of methodologies, such as protein expression analysis (i.e. protein association and biotin labeling), confocal (i.e. immunocytochemistry, FRET, and FRAP), and dSTORM microscopy, transmission electron microscopy, proteomics, and electrophysiology.

RESULTS

We demonstrated that Kv7.1 targeted ER-PMjs regardless of the origin or architecture of these structures. Kv2.1, a neuronal channel that also contributes to a cardiac action potential, and JPHs, involved in cardiac dyads, increased the number of ER-PMjs in nonexcitable cells, driving and increasing the level of Kv7.1 at the cell surface. Both ER-PMj inducers influenced channel function and dynamics, suggesting that different protein structures are formed. Although exhibiting no physical interaction, Kv7.1 resided in more condensed clusters (ring-shaped) with Kv2.1 than with JPH4. Moreover, we found that VAMPs and AMIGO, which are Kv2.1 ancillary proteins also associated with Kv7.1. Specially, VAP B, showed higher interaction with the channel when ER-PMjs were stimulated by Kv2.1.

CONCLUSION

Our results indicated that Kv7.1 may bind to different structures of ER-PMjs that are induced by different mechanisms. This variable architecture can differentially affect the fate of cardiac Kv7.1 channels.

Structure, Function, and Regulation of the Junctophilin Family

In both excitable and nonexcitable cells, diverse physiological processes are linked to different calcium microdomains within nanoscale junctions that form between the plasma membrane and endo-sarcoplasmic reticula. It is now appreciated that the junctophilin protein family is responsible for establishing, maintaining, and modulating the structure and function of these junctions. We review foundational findings from more than two decades of research that have uncovered how junctophilin-organized ultrastructural domains regulate evolutionarily conserved biological processes. We discuss what is known about the junctophilin family of proteins. Our goal is to summarize the current knowledge of junctophilin domain structure, function, and regulation and to highlight emerging avenues of research that help our understanding of the transcriptional, translational, and post-translational regulation of this gene family and its roles in health and during disease.

Stay in touch with the endoplasmic reticulum

The endoplasmic reticulum (ER), which is composed of a continuous network of tubules and sheets, forms the most widely distributed membrane system in eukaryotic cells. As a result, it engages a variety of organelles by establishing membrane contact sites (MCSs). These contacts regulate organelle positioning and remodeling, including fusion and fission, facilitate precise lipid exchange, and couple vital signaling events. Here, we systematically review recent advances and converging themes on ER-involved organellar contact. The molecular basis, cellular influence, and potential physiological functions for ER/nuclear envelope contacts with mitochondria, Golgi, endosomes, lysosomes, lipid droplets, autophagosomes, and plasma membrane are summarized.

Golgi-localized MORN1 promotes lipid droplet abundance and enhances tolerance to multiple stresses in Arabidopsis

Lipid droplet (LD) in vegetative tissues has recently been implicated in environmental responses in plants, but its regulation and its function in stress tolerance are not well understood. Here, we identified a Membrane Occupation and Recognition Nexus 1 (MORN1) gene as a contributor to natural variations of stress tolerance through genome-wide association study in Arabidopsis thaliana. Characterization of its loss-of-function mutant and natural variants revealed that the MORN1 gene is a positive regulator of plant growth, disease resistance, cold tolerance, and heat tolerance. The MORN1 protein is associated with the Golgi and is also partly associated with LD. Protein truncations that disrupt these associations abolished the biological function of the MORN1 protein. Furthermore, the MORN1 gene is a positive regulator of LD abundance, and its role in LD number regulation and stress tolerance is highly linked. Therefore, this study identifies MORN1 as a positive regulator of LD abundance and a contributor to natural variations of stress tolerance. It implicates a potential involvement of Golgi in LD biogenesis and strongly suggests a contribution of LD to diverse processes of plant growth and stress responses.

CRAC and SK Channels: Their Molecular Mechanisms Associated with Cancer Cell Development

Cancer represents a major health burden worldwide. Several molecular targets have been discovered alongside treatments with positive clinical outcomes. However, the reoccurrence of cancer due to therapy resistance remains the primary cause of mortality. Endeavors in pinpointing new markers as molecular targets in cancer therapy are highly desired. The significance of the co-regulation of Ca²⁺-permeating and Ca²⁺-regulated ion channels in cancer cell development, proliferation, and migration make them promising molecular targets in cancer therapy. In particular, the co-regulation of the Orai1 and SK3 channels has been well-studied in breast and colon cancer cells, where it finally leads to an invasion-metastasis cascade. Nevertheless, many questions remain unanswered, such as which key molecular components determine and regulate their interplay. To provide a solid foundation for a better understanding of this ion channel co-regulation in cancer, we first shed light on the physiological role of Ca²⁺ and how this ion is linked to carcinogenesis. Then, we highlight the structure/function relationship of Orai1 and SK3, both individually and in concert, their role in the development of different types of cancer, and aspects that are not yet known in this context.

NMR resonance assignments of the DNA binding domain of mouse Junctophilin-2

Junctophilin-2 (JP2) is a critical structural protein in the heart by stabilizing junctional membrane complexes between the plasma membrane and sarcoplasmic reticula responsible for precise Ca²⁺ regulation. Such complexes are essential for efficient cardiomyocyte contraction and adaptation to altered cardiac workload conditions. Mutations in the JPH2 gene that encodes JP2 are associated with inherited cardiomyopathies and arrhythmias, and disruption of JP2 function is lethal. Interestingly, cardiac stress promotes the proteolytic cleavage of JP2 that triggers the translocation of its N-terminal fragment into the nucleus to repress maladaptive gene transcription. We previously found that the central region of JP2 is responsible for mediating direct DNA binding interactions. Recent structural studies indicate that this region serves as a structural role in the cytosolic form of JP2 by folding into a single continuous α-helix. However, the structural basis of how this DNA-binding domain interacts with DNA is not known. Here, we report the backbone and sidechain assignments of the DNA-binding domain (residues 331-413) of mouse JP2. These assignments reveal that the JP2 DNA binding domain is an intrinsically disordered protein and contains two α-helices located in the C-terminal portion of the protein. Moreover, this protein binds to DNA in a similar manner to that shown previously by electrophoretic mobility shift assays. Therefore, these assignments provide a framework for further structural studies into the interaction of this JP2 domain with DNA for the elucidation of transcriptional regulation of stress-responsive genes as well as its role in the stabilization of junctional membrane complexes.

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.

Structures of the junctophilin/voltage-gated calcium channel interface reveal hot spot for cardiomyopathy mutations

SignificanceIon channels have evolved the ability to communicate with one another, either through protein-protein interactions, or indirectly via intermediate diffusible messenger molecules. In special cases, the channels are part of different membranes. In muscle tissue, the T-tubule membrane is in proximity to the sarcoplasmic reticulum, allowing communication between L-type calcium channels and ryanodine receptors. This process is critical for excitation-contraction coupling and requires auxiliary proteins like junctophilin (JPH). JPHs are targets for disease-associated mutations, most notably hypertrophic cardiomyopathy mutations in the JPH2 isoform. Here we provide high-resolution snapshots of JPH, both alone and in complex with a calcium channel peptide, and show how this interaction is targeted by cardiomyopathy mutations.

The Role of Lipids in CRAC Channel Function

The composition and dynamics of the lipid membrane define the physical properties of the bilayer and consequently affect the function of the incorporated membrane transporters, which also applies for the prominent Ca²⁺ release-activated Ca²⁺ ion channel (CRAC). This channel is activated by receptor-induced Ca²⁺ store depletion of the endoplasmic reticulum (ER) and consists of two transmembrane proteins, STIM1 and Orai1. STIM1 is anchored in the ER membrane and senses changes in the ER luminal Ca²⁺ concentration. Orai1 is the Ca²⁺-selective, pore-forming CRAC channel component located in the plasma membrane (PM). Ca²⁺ store-depletion of the ER triggers activation of STIM1 proteins, which subsequently leads to a conformational change and oligomerization of STIM1 and its coupling to as well as activation of Orai1 channels at the ER-PM contact sites. Although STIM1 and Orai1 are sufficient for CRAC channel activation, their efficient activation and deactivation is fine-tuned by a variety of lipids and lipid- and/or ER-PM junction-dependent accessory proteins. The underlying mechanisms for lipid-mediated CRAC channel modulation as well as the still open questions, are presented in this review.

The Role of Junctophilin Proteins in Cellular Function

Junctophilins (JPHs) comprise a family of structural proteins that connect the plasma membrane to intracellular organelles such as the endo/sarcoplasmic reticulum. Tethering of these membrane structures results in the formation of highly organized subcellular junctions that play important signaling roles in all excitable cell types. There are four JPH isoforms, expressed primarily in muscle and neuronal cell types. Each JPH protein consists of 6 'membrane occupation and recognition nexus' (MORN) motifs, a joining region connecting these to another set of 2 MORN motifs, a putative alpha-helical region, a divergent region exhibiting low homology between JPH isoforms, and a carboxy-terminal transmembrane region anchoring into the ER/SR membrane. JPH isoforms play essential roles in developing and maintaining subcellular membrane junctions. Conversely, inherited mutations in JPH2 cause hypertrophic or dilated cardiomyopathy, while trinucleotide expansions in the JPH3 gene cause Huntington Disease-Like 2. Loss of JPH1 protein levels can cause skeletal myopathy, while loss of cardiac JPH2 levels causes heart failure and atrial fibrillation, among other disease. This review will provide a comprehensive overview of the JPH gene family, phylogeny, and evolutionary analysis of JPH genes and other MORN domain proteins. JPH biogenesis, membrane tethering, and binding partners will be discussed, as well as functional roles of JPH isoforms in excitable cells. Finally, potential roles of JPH isoform deficits in human disease pathogenesis will be reviewed.

The Builders of the Junction: Roles of Junctophilin1 and Junctophilin2 in the Assembly of the Sarcoplasmic Reticulum–Plasma Membrane Junctions in Striated Muscle

Contraction of striated muscle is triggered by a massive release of calcium from the sarcoplasmic reticulum (SR) into the cytoplasm. This intracellular calcium release is initiated by membrane depolarization, which is sensed by voltage-gated calcium channels Ca_V1.1 (in skeletal muscle) and Ca_V1.2 (in cardiac muscle) in the plasma membrane (PM), which in turn activate the calcium-releasing channel ryanodine receptor (RyR) embedded in the SR membrane. This cross-communication between channels in the PM and in the SR happens at specialized regions, the SR-PM junctions, where these two compartments come in close proximity. Junctophilin1 and Junctophilin2 are responsible for the formation and stabilization of SR-PM junctions in striated muscle and actively participate in the recruitment of the two essential players in intracellular calcium release, Ca_V and RyR. This short review focuses on the roles of junctophilins1 and 2 in the formation and organization of SR-PM junctions in skeletal and cardiac muscle and on the functional consequences of the absence or malfunction of these proteins in striated muscle in light of recently published data and recent advancements in protein structure prediction.

Junctophilins: Key Membrane Tethers in Muscles and Neurons

Contacts between the endoplasmic reticulum (ER) and plasma membrane (PM) contain specialized tethering proteins that bind both ER and PM membranes. In excitable cells, ER–PM contacts play an important role in calcium signaling and transferring lipids. Junctophilins are a conserved family of ER–PM tethering proteins. They are predominantly expressed in muscles and neurons and known to simultaneously bind both ER- and PM-localized ion channels. Since their discovery two decades ago, functional studies using junctophilin-deficient animals have provided a deep understanding of their roles in muscles and neurons, including excitation-contraction coupling, store-operated calcium entry (SOCE), and afterhyperpolarization (AHP). In this review, we highlight key findings from mouse, fly, and worm that support evolutionary conservation of junctophilins.

Analysis of merged whole blood transcriptomic datasets to identify circulating molecular biomarkers of feed efficiency in growing pigs

Background

Improving feed efficiency (FE) is an important goal due to its economic and environmental significance for farm animal production. The FE phenotype is complex and based on the measurements of the individual feed consumption and average daily gain during a test period, which is costly and time-consuming. The identification of reliable predictors of FE is a strategy to reduce phenotyping efforts.

Results

Gene expression data of the whole blood from three independent experiments were combined and analyzed by machine learning algorithms to propose molecular biomarkers of FE traits in growing pigs. These datasets included Large White pigs from two lines divergently selected for residual feed intake (RFI), a measure of net FE, and in which individual feed conversion ratio (FCR) and blood microarray data were available. Merging the three datasets allowed considering FCR values (Mean = 2.85; Min = 1.92; Max = 5.00) for a total of n = 148 pigs, with a large range of body weight (15 to 115 kg) and different test period duration (2 to 9 weeks). Random forest (RF) and gradient tree boosting (GTB) were applied on the whole blood transcripts (26,687 annotated molecular probes) to identify the most important variables for binary classification on RFI groups and a quantitative prediction of FCR, respectively. The dataset was split into learning (n = 74) and validation sets (n = 74). With iterative steps for variable selection, about three hundred’s (328 to 391) molecular probes participating in various biological pathways, were identified as important predictors of RFI or FCR. With the GTB algorithm, simpler models were proposed combining 34 expressed unique genes to classify pigs into RFI groups (100% of success), and 25 expressed unique genes to predict FCR values (R² = 0.80, RMSE = 8%). The accuracy performance of RF models was slightly lower in classification and markedly lower in regression.

Conclusion

From small subsets of genes expressed in the whole blood, it is possible to predict the binary class and the individual value of feed efficiency. These predictive models offer good perspectives to identify animals with higher feed efficiency in precision farming applications.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07843-4.

Machine learning compensates fold-change method and highlights oxidative phosphorylation in the brain transcriptome of Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder causing 70% of dementia cases. However, the mechanism of disease development is still elusive. Despite the availability of a wide range of biological data, a comprehensive understanding of AD's mechanism from machine learning (ML) is so far unrealized, majorly due to the lack of needed data density. To harness the AD mechanism's knowledge from the expression profiles of postmortem prefrontal cortex samples of 310 AD and 157 controls, we used seven predictive operators or combinations of RapidMiner Studio operators to establish predictive models from the input matrix and to assign a weight to each attribute. Besides, conventional fold-change methods were also applied as controls. The identified genes were further submitted to enrichment analysis for KEGG pathways. The average accuracy of ML models ranges from 86.30% to 91.22%. The overlap ratio of the identified genes between ML and conventional methods ranges from 19.7% to 21.3%. ML exclusively identified oxidative phosphorylation genes in the AD pathway. Our results highlighted the deficiency of oxidative phosphorylation in AD and suggest that ML should be considered as complementary to the conventional fold-change methods in transcriptome studies.

Junctophilin-4 facilitates inflammatory signalling at plasma membrane-endoplasmic reticulum junctions in sensory neurons

KEY POINTS

Rat somatosensory neurons express a junctional protein, junctophilin-4 (JPH4) JPH4 is necessary for the formation of store operated Ca²⁺ entry (SOCE) complex at the junctions between plasma membrane and endoplasmic reticulum in these neurons. Knockdown of JPH4 impairs endoplasmic reticulum Ca²⁺ store refill and junctional Ca²⁺ signalling in sensory neurons. In vivo knockdown of JPH4 in the dorsal root ganglion (DRG) sensory neurons significantly attenuated experimentally induced inflammatory pain in rats. Junctional nanodomain Ca²⁺ signalling maintained by JPH4 is an important contributor to the inflammatory pain mechanisms.

ABSTRACT

Junctions of endoplasmic reticulum and plasma membrane (ER-PM junctions) form signalling nanodomains in eukaryotic cells. ER-PM junctions are present in peripheral sensory neurons and are important for the fidelity of G protein coupled receptor (GPCR) signalling. Yet little is known about the assembly, maintenance and physiological role of these junctions in somatosensory transduction. Using fluorescence imaging, proximity ligation, super-resolution microscopy, in vitro and in vivo gene knockdown we demonstrate that a member of the junctophilin protein family, junctophilin-4 (JPH4), is necessary for the formation of store operated Ca²⁺ entry (SOCE) complex at the ER-PM junctions in rat somatosensory neurons. Thus we show that JPH4 localises to the ER-PM junctional areas and co-clusters with SOCE proteins STIM1 and Orai1 upon ER Ca²⁺ store depletion. Knockdown of JPH4 impairs SOCE and ER Ca²⁺ store refill in sensory neurons. Furthermore, we demonstrate a key role of the JPH4 and junctional nanodomain Ca²⁺ signalling in the pain-like response induced by the inflammatory mediator bradykinin. Indeed, an in vivo knockdown of JPH4 in the dorsal root ganglion (DRG) sensory neurons significantly shortened the duration of nocifensive behaviour induced by hindpaw injection of bradykinin in rats. Since the ER supplies Ca²⁺ for the excitatory action of multiple inflammatory mediators, we suggest that junctional nanodomain Ca²⁺ signalling maintained by JPH4 is an important contributor to the inflammatory pain mechanisms.

ER-PM Contact Sites - SNARING Actors in Emerging Functions

The compartmentalisation achieved by confining cytoplasm into membrane-enclosed organelles in eukaryotic cells is essential for maintaining vital functions including ATP production, synthetic and degradative pathways. While intracellular organelles are highly specialised in these functions, the restricting membranes also impede exchange of molecules responsible for the synchronised and responsive cellular activities. The initial identification of contact sites between the ER and plasma membrane (PM) provided a potential candidate structure for communication between organelles without mixing by fusion. Over the past decades, research has revealed a far broader picture of the events. Membrane contact sites (MCSs) have been recognized as increasingly important actors in cell differentiation, plasticity and maintenance, and, upon dysfunction, responsible for pathological conditions such as cancer and neurodegenerative diseases. Present in multiple organelles and cell types, MCSs promote transport of lipids and Ca²⁺ homoeostasis, with a range of associated protein families. Interestingly, each MCS displays a unique molecular signature, adapted to organelle functions. This review will explore the literature describing the molecular components and interactions taking place at ER-PM contact sites, their functions, and implications in eukaryotic cells, particularly neurons, with emphasis on lipid transfer proteins and emerging function of SNAREs.

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.

Structures of three MORN repeat proteins and a re-evaluation of the proposed lipid-binding properties of MORN repeats

MORN (Membrane Occupation and Recognition Nexus) repeat proteins have a wide taxonomic distribution, being found in both prokaryotes and eukaryotes. Despite this ubiquity, they remain poorly characterised at both a structural and a functional level compared to other common repeats. In functional terms, they are often assumed to be lipid-binding modules that mediate membrane targeting. We addressed this putative activity by focusing on a protein composed solely of MORN repeats-Trypanosoma brucei MORN1. Surprisingly, no evidence for binding to membranes or lipid vesicles by TbMORN1 could be obtained either in vivo or in vitro. Conversely, TbMORN1 did interact with individual phospholipids. High- and low-resolution structures of the MORN1 protein from Trypanosoma brucei and homologous proteins from the parasites Toxoplasma gondii and Plasmodium falciparum were obtained using a combination of macromolecular crystallography, small-angle X-ray scattering, and electron microscopy. This enabled a first structure-based definition of the MORN repeat itself. Furthermore, all three structures dimerised via their C-termini in an antiparallel configuration. The dimers could form extended or V-shaped quaternary structures depending on the presence of specific interface residues. This work provides a new perspective on MORN repeats, showing that they are protein-protein interaction modules capable of mediating both dimerisation and oligomerisation.

Upregulation of Orai1 and increased calcium entry contribute to angiotensin II-induced human coronary smooth muscle cell proliferation: Running Title: Angiotensin II-induced human coronary smooth muscle cells proliferation

Angiotensin II (Ang II) is an oligopeptide of the renin-angiotensin system, and Ang II-induced vascular smooth muscle cell (VSMC) proliferation is an important pathophysiological process involved in atherosclerosis; however, the underlying mechanism remains unclear. Orai1 and Stim1 are the main components of store-operated Ca²⁺ entry (SOCE), which has an important effect on VSMC proliferation. In the present study, we showed that Ang II-induced human coronary smooth muscle cell (HCSMC) proliferation was associated with increased calcium entry. The expression of Orai1, but not that of Stim1, was significantly upregulated in Ang II-treated HCSMCs. However, knockdown of Orai1 or Stim1 decreased HCSMC proliferation and SOCE activity in Ang II-treated HCSMCs. Orai1 was significantly downregulated in HCSMCs transfected with short interfering RNA (siRNA) against NOX2 or NF-κB. Transfection with siRNA against NOX2 or p65 also decreased Ang II-induced HCSMCs SOCE activation and proliferation. These findings suggested that Ang II upregulated Orai1 via the NF-κB and NOX2 pathways, leading to increased SOCE and HCSMC proliferation. The molecular factors mediating Ang II-induced SOCE upregulation are potential therapeutic targets for the prevention of Ang II-sensitive or Ang II-dependent HCSMC proliferation.

On a Magical Mystery Tour with 8-Bromo-Cyclic ADP-Ribose: From All-or-None Block to Nanojunctions and the Cell-Wide Web

A plethora of cellular functions are controlled by calcium signals, that are greatly coordinated by calcium release from intracellular stores, the principal component of which is the sarco/endooplasmic reticulum (S/ER). In 1997 it was generally accepted that activation of various G protein-coupled receptors facilitated inositol-1,4,5-trisphosphate (IP3) production, activation of IP3 receptors and thus calcium release from S/ER. Adding to this, it was evident that S/ER resident ryanodine receptors (RyRs) could support two opposing cellular functions by delivering either highly localised calcium signals, such as calcium sparks, or by carrying propagating, global calcium waves. Coincidentally, it was reported that RyRs in mammalian cardiac myocytes might be regulated by a novel calcium mobilising messenger, cyclic adenosine diphosphate-ribose (cADPR), that had recently been discovered by HC Lee in sea urchin eggs. A reputedly selective and competitive cADPR antagonist, 8-bromo-cADPR, had been developed and was made available to us. We used 8-bromo-cADPR to further explore our observation that S/ER calcium release via RyRs could mediate two opposing functions, namely pulmonary artery dilation and constriction, in a manner seemingly independent of IP3Rs or calcium influx pathways. Importantly, the work of others had shown that, unlike skeletal and cardiac muscles, smooth muscles might express all three RyR subtypes. If this were the case in our experimental system and cADPR played a role, then 8-bromo-cADPR would surely block one of the opposing RyR-dependent functions identified, or the other, but certainly not both. The latter seemingly implausible scenario was confirmed. How could this be, do cells hold multiple, segregated SR stores that incorporate different RyR subtypes in receipt of spatially segregated signals carried by cADPR? The pharmacological profile of 8-bromo-cADPR action supported not only this, but also indicated that intracellular calcium signals were delivered across intracellular junctions formed by the S/ER. Not just one, at least two. This article retraces the steps along this journey, from the curious pharmacological profile of 8-bromo-cADPR to the discovery of the cell-wide web, a diverse network of cytoplasmic nanocourses demarcated by S/ER nanojunctions, which direct site-specific calcium flux and may thus coordinate the full panoply of cellular processes.

The short isoform of extended synaptotagmin-2 controls Ca2+ dynamics in T cells via interaction with STIM1

Ca²⁺ release-activated Ca²⁺ (CRAC) channels elevate cytoplasmic Ca²⁺ concentration, which is essential for T cell activation, differentiation and effector functions. T cell receptor stimulation induces depletion of the endoplasmic reticulum (ER) Ca²⁺ stores, which is sensed by stromal interaction molecule 1 (STIM1). STIM1 translocates to the ER-plasma membrane (PM) junctions to interact with ORAI1, the pore subunit of the CRAC channels. Here, we show that two members of the extended synaptotagmin (E-Syt) family, E-Syt1, and the short isoform of E-Syt2 (E-Syt2S), contribute to activation of CRAC channels in T cells. Knockdown or deletion of both ESYT1 and ESYT2 reduced store-operated Ca²⁺ entry (SOCE) and ORAI1-STIM1 clustering in Jurkat T cells. Further, depletion of E-Syts in primary T cells decreased Ca²⁺ entry and cytokine production. While the ER-PM junctions were reduced in both HeLa and Jurkat T cells deleted for ESYT1 and ESYT2, SOCE was impaired only in Jurkat T cells, suggesting that the membrane-tethering function of E-Syts is distinct from their role in SOCE. Mechanistically, E-Syt2S, the predominant isoform of E-Syt2 in T cells, recruited STIM1 to the junctions via a direct interaction. This study demonstrates a membrane-tethering-independent role of E-Syts in activation of CRAC channels in T cells.

SEPT7 regulates Ca2+ entry through Orai channels in human neural progenitor cells and neurons

Human neural progenitor cells (hNPCs) are self-renewing cells of neural lineage that can be differentiated into neurons of different subtypes. Here we show that SEPT7, a member of the family of filament-forming GTPases called septins, prevents constitutive Ca²⁺ entry through the store-operated Ca²⁺ entry channel, Orai in hNPCs and in differentiated neurons and is thus required for neuronal calcium homeostasis. Previous work in Drosophila neurons has shown that loss of one copy of the evolutionarily-conserved dSEPT7 gene leads to elevated Ca²⁺ entry via Orai, in the absence of ER-Ca²⁺ store depletion. We have identified an N-terminal polybasic region of SEPT7, known to interact with membrane-localized phospholipids, as essential for spontaneous calcium entry through Orai in hNPCs, whereas the GTPase domain of dSEPT7 is dispensable for this purpose. Re-organisation of Orai1 and the ER-Ca²⁺ sensor STIM1 observed near the plasma membrane in SEPT7 KD hNPCs, supports the idea that Septin7 containing heteromers prevent Ca²⁺ entry through a fraction of STIM-Orai complexes. Possible mechanisms by which SEPT7 reduction leads to opening of Orai channels in the absence of store-depletion are discussed.

Microtubules Stabilization by Mutant Spastin Affects ER Morphology and Ca2+ Handling

The endoplasmic reticulum (ER) extends as a network of interconnected tubules and sheet-like structures in eukaryotic cells. ER tubules dynamically change their morphology and position within the cells in response to physiological stimuli and these network rearrangements depend on the microtubule (MT) cytoskeleton. Store-operated calcium entry (SOCE) relies on the repositioning of ER tubules to form specific ER-plasma membrane junctions. Indeed, the tips of polymerizing MTs are supposed to provide the anchor for ER tubules to move toward the plasma membrane, however the precise role of the cytoskeleton during SOCE has not been conclusively clarified. Here we exploit an in vivo approach involving the manipulation of MT dynamics in Drosophila melanogaster by neuronal expression of a dominant-negative variant of the MT-severing protein spastin to induce MT hyper-stabilization. We show that MT stabilization alters ER morphology, favoring an enrichment in ER sheets at the expense of tubules. Stabilizing MTs has a negative impact on the process of SOCE and results in a reduced ER Ca²⁺ content, affecting the flight ability of the flies. Restoring proper MT organization by administering the MT-destabilizing drug vinblastine, chronically or acutely, rescues ER morphology, SOCE and flight ability, indicating that MT dynamics impairment is responsible for all the phenotypes observed.

Changes in Deoxyribonucleic Acid Methylation Contribute to the Pathophysiology of Multiple Sclerosis

Multiple sclerosis (MS) is an autoimmune disease of the central nervous system characterized by loss of coordination, weakness, dysfunctions in bladder capacity, bowel movement, and cognitive impairment. Thus, the disease leads to a significant socioeconomic burden. In the pathophysiology of the disease, both genetic and environmental risk factors are involved. Gene x environment interaction is modulated by epigenetic mechanisms. Epigenetics refers to a sophisticated system that regulates gene expression with no changes in the DNA sequence. The most studied epigenetic mechanism is the DNA methylation. In this review, we summarize the data available from the current literature by grouping sets of differentially methylated genes in distinct biological categories: the immune system including innate and adaptive response, the DNA damage, and the central nervous system.

Nanoscale coupling of junctophilin-2 and ryanodine receptors regulates vascular smooth muscle cell contractility

Junctophilin proteins maintain close contacts between the endoplasmic/sarcoplasmic reticulum (ER/SR) and the plasma membrane in many types of cells, as typified by junctophilin-2 (JPH2), which is necessary for the formation of the cardiac dyad. Here, we report that JPH2 is the most abundant junctophilin isotype in native smooth muscle cells (SMCs) isolated from cerebral arteries and that acute knockdown diminishes the area of sites of interaction between the SR and plasma membrane. Superresolution microscopy revealed nanometer-scale colocalization of JPH2 clusters with type 2 ryanodine receptor (RyR2) clusters near the cell surface. Knockdown of JPH2 had no effect on the frequency, amplitude, or kinetics of spontaneous Ca²⁺ sparks generated by transient release of Ca²⁺ from the SR through RyR2s, but it did nearly abolish Ca²⁺ spark-activated, large-conductance, Ca²⁺-activated K⁺ (BK) channel currents. We also found that JPH2 knockdown was associated with hypercontractility of intact cerebral arteries. We conclude that JPH2 maintains functional coupling between RyR2s and BK channels and is critically important for cerebral arterial function.

S-Palmitoylation of junctophilin-2 is critical for its role in tethering the sarcoplasmic reticulum to the plasma membrane

Junctophilins (JPH1-JPH4) are expressed in excitable and nonexcitable cells, where they tether endoplasmic/sarcoplasmic reticulum (ER/SR) and plasma membranes (PM). These ER/SR-PM junctions bring Ca-release channels in the ER/SR and Ca as well as Ca-activated K channels in the PM to within 10-25 nm. Such proximity is critical for excitation-contraction coupling in muscles, Ca modulation of excitability in neurons, and Ca homeostasis in nonexcitable cells. JPHs are anchored in the ER/SR through the C-terminal transmembrane domain (TMD). Their N-terminal Membrane-Occupation-Recognition-Nexus (MORN) motifs can bind phospholipids. Whether MORN motifs alone are sufficient to stabilize JPH-PM binding is not clear. We investigate whether S-palmitoylation of cysteine (Cys), a critical mechanism controlling peripheral protein binding to PM, occurs in JPHs. We focus on JPH2 that has four Cys residues: three flanking the MORN motifs and one in the TMD. Using palmitate-alkyne labeling, Cu(I)-catalyzed alkyne-azide cycloaddition reaction with azide-conjugated biotin, immunoblotting, proximity-ligation-amplification, and various imaging techniques, we show that JPH2 is S-palmitoylatable, and palmitoylation is essential for its ER/SR-PM tether function. Palmitoylated JPH2 binds to lipid-raft domains in PM, whereas palmitoylation of TMD-located Cys stabilizes JPH2's anchor in the ER/SR membrane. Binding to lipid-raft domains protects JPH2 from depalmitoylation. Unpalmitoylated JPH2 is largely excluded from lipid rafts and loses the ability to form stable ER/SR-PM junctions. In adult ventricular myocytes, native JPH2 is S-palmitoylatable, and palmitoylated JPH2 forms distinct PM puncta. Sequence alignment reveals that the palmitoylatable Cys residues in JPH2 are conserved in other JPHs, suggesting that palmitoylation may also enhance ER/SR-PM tethering by these proteins.

Molecular determinants of homo- and heteromeric interactions of Junctophilin-1 at triads in adult skeletal muscle fibers

In adult skeletal muscles, 2 junctophilin isoforms (JPH1 and JPH2) tether the sarcoplasmic reticulum (SR) to transverse tubule (T-tubule) membranes, generating stable membrane contact sites known as triads. JPHs are anchored to the membrane of the SR by a C-terminal transmembrane domain (TMD) and bind the T-tubule membrane through their cytosolic N-terminal region, which contains 8 lipid-binding (MORN) motifs. By combining expression of GFP-JPH1 deletion mutants in skeletal muscle fibers with in vitro biochemical experiments, we investigated the molecular determinants of JPH1 recruitment at triads in adult skeletal muscle fibers. We found that MORN motifs bind PI(4,5)P₂ in the sarcolemma, but do not mediate the selective localization of JPH1 at the T-tubule compartment of triads. On the contrary, fusion proteins containing only the TMD of JPH1 were able to localize at the junctional SR compartment of the triad. Bimolecular fluorescence complementation experiments indicated that the TMD of JPH1 can form dimers, suggesting that the observed localization at triads may result from dimerization with the TMDs of resident JPH1. A second domain, capable of mediating homo- and heterodimeric interactions between JPH1 and JPH2 was identified in the cytosolic region. FRAP experiments revealed that removal of either one of these 2 domains in JPH1 decreases the association of the resulting mutant proteins with triads. Altogether, these results suggest that the ability to establish homo- and heterodimeric interactions with resident JPHs may support the recruitment and stability of newly synthesized JPHs at triads in adult skeletal muscle fibers.

IP3 receptors and Ca2+ entry

Inositol 1,4,5-trisphosphate receptors (IP₃R) are the most widely expressed intracellular Ca²⁺ release channels. Their activation by IP₃ and Ca²⁺ allows Ca²⁺ to pass rapidly from the ER lumen to the cytosol. The resulting increase in cytosolic [Ca²⁺] may directly regulate cytosolic effectors or fuel Ca²⁺ uptake by other organelles, while the decrease in ER luminal [Ca²⁺] stimulates store-operated Ca²⁺ entry (SOCE). We are close to understanding the structural basis of both IP₃R activation, and the interactions between the ER Ca²⁺-sensor, STIM, and the plasma membrane Ca²⁺ channel, Orai, that lead to SOCE. IP₃Rs are the usual means through which extracellular stimuli, through ER Ca²⁺ release, stimulate SOCE. Here, we review evidence that the IP₃Rs most likely to respond to IP₃ are optimally placed to allow regulation of SOCE. We also consider evidence that IP₃Rs may regulate SOCE downstream of their ability to deplete ER Ca²⁺ stores. Finally, we review evidence that IP₃Rs in the plasma membrane can also directly mediate Ca²⁺ entry in some cells.

Here, there, and everywhere: The importance of ER membrane contact sites

Our textbook image of organelles has changed. Instead of revealing isolated cellular compartments, the picture now emerging shows organelles as largely interdependent structures that can communicate through membrane contact sites (MCSs). MCSs are sites where opposing organelles are tethered but do not fuse. MCSs provide a hybrid location where the tool kits of two different organelles can work together to perform vital cellular functions, such as lipid and ion transfer, signaling, and organelle division. Here, we focus on MCSs involving the endoplasmic reticulum (ER), an organelle forming an extensive network of cisternae and tubules. We highlight how the dynamic ER network regulates a plethora of cellular processes through MCSs with various organelles and with the plasma membrane.

Ca2+ Entry Through Reverse Mode Na+/Ca2+ Exchanger Contributes to Store Operated Channel-Mediated Neointima Formation After Arterial Injury

BACKGROUND

Na⁺/Ca²⁺ exchange (NCX) reversal-mediated Ca²⁺ entry is a critical pathway for stimulating proliferation in many cell lines. However, the role of reverse-mode NCX1 in neointima formation and atherosclerosis remains unclear. The aims of the present study were to investigate the functional role of NCX1 in the pathogenesis of atherosclerosis and vascular smooth muscle cell (VSMC) proliferation, and to determine the interaction between NCX1 and store depletion in VSMCs.

METHODS

A rat balloon injury model was established to examine the effect of the knockdown of NCX1 on neointima formation after injury. VSMCs were cultured to verify that NCX1 knockdown suppressed serum-induced VSMC proliferation.

RESULTS

The results showed that balloon injury induced neointima formation and upregulated NCX1 expression at 7 and 14 days after injury in rat carotid arteries (1.18- and 1.45-fold, respectively). A lentivirus vector expressing short hairpin (sh)RNA against rat NCX1 dramatically downregulated NCX1, proliferating cell nuclear antigen (PCNA) and Ki-67 expression, and suppressed neointima formation in vivo (62% at 7 days and 70% at 14 days). KB-R7943 (an inhibitor of reverse-mode NCX1) and NCX1 knockdown significantly inhibited serum-induced VSMC proliferation (65% at 72 hours and 41% at 72 hours, respectively), determined according to PCNA and Ki-67 expression and cell counting in vitro, and markedly suppressed store depletion-mediated Ca²⁺ entry and peripheral cytosolic Na⁺ transients in VSMCs.

CONCLUSIONS

Reverse-mode NCX1 is activated by store depletion and is required for proliferative VSMC proliferation and neointima formation after arterial injury.

Molecular physiology and pathophysiology of stromal interaction molecules

Ca²⁺ release from the endoplasmic reticulum is an important component of Ca²⁺ signal transduction that controls numerous physiological processes in eukaryotic cells. Release of Ca²⁺ from the endoplasmic reticulum is coupled to the activation of store-operated Ca²⁺ entry into cells. Store-operated Ca²⁺ entry provides Ca²⁺ for replenishing depleted endoplasmic reticulum Ca²⁺ stores and a Ca²⁺ signal that regulates Ca²⁺-dependent intracellular biochemical events. Central to connecting discharge of endoplasmic reticulum Ca²⁺ stores following G protein-coupled receptor activation with the induction of store-operated Ca²⁺ entry are stromal interaction molecules (STIM1 and STIM2). These highly homologous endoplasmic reticulum transmembrane proteins function as sensors of the Ca²⁺ concentration within the endoplasmic reticulum lumen and activators of Ca²⁺ release-activated Ca²⁺ channels. Emerging evidence indicates that in addition to their role in Ca²⁺ release-activated Ca²⁺ channel gating and store-operated Ca²⁺ entry, STIM1 and STIM2 regulate other cellular signaling events. Recent studies have shown that disruption of STIM expression and function is associated with the pathogenesis of several diseases including autoimmune disorders, cancer, cardiovascular disease, and myopathies. Here, we provide an overview of the latest developments in the molecular physiology and pathophysiology of STIM1 and STIM2. Impact statement Intracellular Ca²⁺ signaling is a fundamentally important regulator of cell physiology. Recent studies have revealed that Ca²⁺-binding stromal interaction molecules (Stim1 and Stim2) expressed in the membrane of the endoplasmic reticulum (ER) are essential components of eukaryote Ca²⁺ signal transduction that control the activity of ion channels and other signaling effectors present in the plasma membrane. This review summarizes the most recent information on the molecular physiology and pathophysiology of stromal interaction molecules. We anticipate that the work presented in our review will provide new insights into molecular interactions that participate in interorganelle signaling crosstalk, cell function, and the pathogenesis of human diseases.

The Drosophila junctophilin gene is functionally equivalent to its four mammalian counterparts and is a modifier of a Huntingtin poly-Q expansion and the Notch pathway

Members of the Junctophilin (JPH) protein family have emerged as key actors in all excitable cells, with crucial implications for human pathophysiology. In mammals, this family consists of four members (JPH1-JPH4) that are differentially expressed throughout excitable cells. The analysis of knockout mice lacking JPH subtypes has demonstrated their essential contribution to physiological functions in skeletal and cardiac muscles and in neurons. Moreover, mutations in the human JPH2 gene are associated with hypertrophic and dilated cardiomyopathies; mutations in JPH3 are responsible for the neurodegenerative Huntington's disease-like-2 (HDL2), whereas JPH1 acts as a genetic modifier in Charcot–Marie–Tooth 2K peripheral neuropathy. Drosophila melanogaster has a single junctophilin (jp) gene, as is the case in all invertebrates, which might retain equivalent functions of the four homologous JPH genes present in mammalian genomes. Therefore, owing to the lack of putatively redundant genes, a jpDrosophila model could provide an excellent platform to model the Junctophilin-related diseases, to discover the ancestral functions of the JPH proteins and to reveal new pathways. By up- and downregulation of Jp in a tissue-specific manner in Drosophila, we show that altering its levels of expression produces a phenotypic spectrum characterized by muscular deficits, dilated cardiomyopathy and neuronal alterations. Importantly, our study has demonstrated that Jp modifies the neuronal degeneration in a Drosophila model of Huntington's disease, and it has allowed us to uncover an unsuspected functional relationship with the Notch pathway. Therefore, this Drosophila model has revealed new aspects of Junctophilin function that can be relevant for the disease mechanisms of their human counterparts.

Summary: This work reveals that the Drosophila Junctophilin protein has similar functions to its mammalian homologues and uncovers new interactions of potential biomedical interest with Huntingtin and Notch signalling.

TRPC6 and TRPC4 Heteromultimerization Mediates Store Depletion-Activated NCX1 Reversal in Proliferative Vascular Smooth Muscle Cells

Store depletion has been shown to induce Ca²⁺ entry by Na+/Ca+ exchange (NCX) 1 reversal in proliferative vascular smooth muscle cells (VSMCs). The study objective was to investigate the role of transient receptor potential canonical (TRPC) channels in store depletion and NCX1 reversal in proliferative VSMCs. In cultured VSMCs, expressing TRPC1, TRPC4, and TRPC6, the removal of extracellular Na⁺ was followed by a significant increase of cytosolic Ca²⁺ concentration that was inhibited by KBR, a selective NCX1 inhibitor. TRPC1 knockdown significantly suppressed store-operated, channel-mediated Ca²⁺ entry, but TRPC4 knockdown and TRPC6 knockdown had no effect. Separate knockdown of TRPC1, TRPC4, or TRPC6 did not have a significant effect on thapsigargin-initiated Na⁺ increase in the peripheral regions with KBR treatment, but knockdown of both TRPC4 and TRPC6 did. Stromal interaction molecule (STIM)1 knockdown significantly reduced TRPC4 and TRPC6 binding. The results demonstrated that TRPC4–TRPC6 heteromultimerization linked Ca²⁺ store depletion and STIM1 accumulation with NCX reversal in proliferative VSMCs.

RASSF4 controls SOCE and ER-PM junctions through regulation of PI(4,5)P2

RAS association domain family 4 (RASSF4) is involved in tumorigenesis. Chen et al. show that RASSF4 regulates store-operated Ca²⁺ entry and ER–PM junctions by affecting PI(4,5)P₂ levels. RASSF4 interacts with and regulates the activity of ARF6, an upstream regulator of PIP5K and PI(4,5)P₂.

RAS association domain family 4 (RASSF4) is involved in tumorigenesis and regulation of the Hippo pathway. In this study, we identify new functional roles of RASSF4. First, we discovered that RASSF4 regulates store-operated Ca²⁺ entry (SOCE), a fundamental Ca²⁺ signaling mechanism, by affecting the translocation of the endoplasmic reticulum (ER) Ca²⁺ sensor stromal interaction molecule 1 (STIM1) to ER–plasma membrane (PM) junctions. It was further revealed that RASSF4 regulates the formation of ER–PM junctions and the ER–PM tethering function of extended synaptotagmins E-Syt2 and E-Syt3. Moreover, steady-state PM phosphatidylinositol 4,5-bisphosphate (PI[4,5]P₂) levels, important for localization of STIM1 and E-Syts at ER–PM junctions, were reduced in RASSF4-knockdown cells. Furthermore, we demonstrated that RASSF4 interacts with and regulates the activity of adenosine diphosphate ribosylation factor 6 (ARF6), a small G protein and upstream regulator of type I phosphatidylinositol phosphate kinases (PIP5Ks) and PM PI(4,5)P₂ levels. Overall, our study suggests that RASSF4 controls SOCE and ER–PM junctions through ARF6-dependent regulation of PM PI(4,5)P₂ levels, pivotal for a variety of physiological processes.

Endoplasmic Reticulum-Plasma Membrane Contact Sites

The endoplasmic reticulum (ER) has a broad localization throughout the cell and forms direct physical contacts with all other classes of membranous organelles, including the plasma membrane (PM). A number of protein tethers that mediate these contacts have been identified, and study of these protein tethers has revealed a multiplicity of roles in cell physiology, including regulation of intracellular Ca²⁺ dynamics and signaling as well as control of lipid traffic and homeostasis. In this review, we discuss the cross talk between the ER and the PM mediated by direct contacts. We review factors that tether the two membranes, their properties, and their dynamics in response to the functional state of the cell. We focus in particular on the role of ER-PM contacts in nonvesicular lipid transport between the two bilayers mediated by lipid transfer proteins.

Immunological Disorders: Regulation of Ca2+ Signaling in T Lymphocytes

Engagement of T cell receptors (TCRs) with cognate antigens triggers cascades of signaling pathways in helper T cells. TCR signaling is essential for the effector function of helper T cells including proliferation, differentiation, and cytokine production. It also modulates effector T cell fate by inducing cell death, anergy (nonresponsiveness), exhaustion, and generation of regulatory T cells. One of the main axes of TCR signaling is the Ca²⁺-calcineurin-nuclear factor of activated T cells (NFAT) signaling pathway. Stimulation of TCRs triggers depletion of intracellular Ca²⁺ store and, in turn, activates store-operated Ca²⁺ entry (SOCE) to raise the intracellular Ca²⁺ concentration. SOCE in T cells is mediated by the Ca²⁺ release-activated Ca²⁺ (CRAC) channels, which have been very well characterized in terms of their electrophysiological properties. Identification of STIM1 as a sensor to detect depletion of the endoplasmic reticulum (ER) Ca²⁺ store and Orai1 as the pore subunit of CRAC channels has dramatically advanced our understanding of the regulatory mechanism of Ca²⁺ signaling in T cells. In this review, we discuss our current understanding of Ca²⁺ signaling in T cells with specific focus on the mechanism of CRAC channel activation and regulation via protein interactions. In addition, we will discuss the role of CRAC channels in effector T cells, based on the analyses of genetically modified animal models.

Regulation of Store-Operated Ca2+ Entry by Septins

The mechanism of store-operated Ca²⁺ entry (SOCE) brings extracellular Ca²⁺ into cells after depletion of intracellular Ca²⁺ stores. Regulation of Ca²⁺ homeostasis by SOCE helps control various intracellular signaling functions in both non-excitable and excitable cells. Whereas essential components of the SOCE pathway are well characterized, molecular mechanisms underlying regulation of this pathway need investigation. A class of proteins recently demonstrated as regulating SOCE is septins. These are filament-forming GTPases that assemble into higher order structures. One of their most studied cellular functions is as a molecular scaffold that creates diffusion barriers in membranes for a variety of cellular processes. Septins regulate SOCE in mammalian non-excitable cells and in Drosophila neurons. However, the molecular mechanism of SOCE-regulation by septins and the contribution of different subgroups of septins to SOCE-regulation remain to be understood. The regulation of SOCE is relevant in multiple cellular contexts as well as in diseases, such as the Severe Combined Immunodeficiency (SCID) syndrome and neurodegenerative syndromes like Alzheimer's, Spino-Cerebellar Ataxias and Parkinson's. Moreover, Drosophila neurons, where loss of SOCE leads to flight deficits, are a possible cellular template for understanding the molecular basis of neuronal deficits associated with loss of either the Inositol-1,4,5-trisphosphate receptor (IP₃R1), a key activator of neuronal SOCE or the Endoplasmic reticulum resident Ca²⁺ sensor STIM1 (Stromal Interaction Molecule) in mouse. This perspective summarizes our current understanding of septins as regulators of SOCE and discusses the implications for mammalian neuronal function.

Junctophilin-2 in the nanoscale organisation and functional signalling of ryanodine receptor clusters in cardiomyocytes

Signalling nanodomains requiring close contact between the plasma membrane and internal compartments, known as 'junctions', are fast communication hubs within excitable cells such as neurones and muscle. Here, we have examined two transgenic murine models probing the role of junctophilin-2, a membrane-tethering protein crucial for the formation and molecular organisation of sub-microscopic junctions in ventricular muscle cells of the heart. Quantitative single-molecule localisation microscopy showed that junctions in animals producing above-normal levels of junctophilin-2 were enlarged, allowing the re-organisation of the primary functional protein within it, the ryanodine receptor (RyR; in this paper, we use RyR to refer to the myocardial isoform RyR2). Although this change was associated with much enlarged RyR clusters that, due to their size, should be more excitable, functionally it caused a mild inhibition in the Ca2+ signalling output of the junctions (Ca2+ sparks). Analysis of the single-molecule densities of both RyR and junctophilin-2 revealed an ∼3-fold increase in the junctophilin-2 to RyR ratio. This molecular rearrangement is compatible with direct inhibition of RyR opening by junctophilin-2 to intrinsically stabilise the Ca2+ signalling properties of the junction and thus the contractile function of the cell.

Live-cell imaging of ER-PM contact architecture by a novel TIRFM approach reveals extension of junctions in response to store-operated Ca2+-entry

Nanometer-spaced appositions between endoplasmic reticulum and plasma membrane (ER-PM junctions) stabilized by membrane-joining protein complexes are critically involved in cellular Ca²⁺-handling and lipid trafficking. ER-PM junctional architecture and plasticity associated with inter-membrane communication are as yet barely understood. Here, we introduce a method to precisely characterize ER-PM junction morphology and dynamics with high temporal resolution and minimal disturbance of junctional intermembrane communication. We show that expression of soluble cytosolic fluorophores in combination with TIRFM enables to delineate ER and PM distance in the range of 10-150 nm. Live-cell imaging of sub-plasmalemmal structures in RBL-2H3 mast cells by this method, designated as fluorescence density mapping (FDM), revealed profound dynamics of ER-PM contact sites in response to store-depletion. We report the existence of a Ca²⁺-dependent process that expands the junctional ER to enlarge its contact surface with the PM, thereby promoting and stabilizing STIM1-Orai1 competent ER-PM junctions.

Piecing Together the Patchwork of Contact Sites

Contact sites are places where two organelles join together to carry out a shared activity requiring nonvesicular communication. A large number of contact sites have been discovered, and almost any two organelles can contact each other. General rules about contacts include constraints on bridging proteins, with only a minority of bridges physically creating contacts by acting as 'tethers'. The downstream effects of contacts include changing the physical behaviour of organelles, and also forming biochemically heterogeneous subdomains. However, some functions typically localized to contact sites, such as lipid transfer, have no absolute requirement to be situated there. Therefore, the key aspect of contacts is the directness of communication, which allows metabolic channelling and collective regulation.