Campari2 genomic interrogation of homeostatic calcium activity identifies TIM1 as a negative regulator of T cell function.

Calcium signals regulate crucial cellular functions yet many genes coding for Cahandling proteins remain unknown as their identification relies on low-throughput single-cell approaches. Here we describe a method to measure Ca activity using CaMPARI2, flow cytometry and pooled genome interrogation. CAMPARI2 screen (CaMP-Screen) identified enhancers and inhibitors of homeostatic Ca activity, highlighting a predominant role for store-operated Ca entry (SOCE) and lipid signalling pathways.

STIM1/2 maintain signaling competence at ER-PM contact sites during neutrophil spreading.

Neutrophils are highly motile leukocytes that migrate inside tissues to destroy invading pathogens. Ca2+ signals coordinate leukocytes migration, but whether Ca2+ fluxes mediated by Stim proteins at ER-PM contact sites regulate neutrophil actin-based motility is unclear. Here, we show that myeloid-specific Stim1/2 ablation decreases basal cytosolic Ca2+ levels and prevents adhesion-induced Ca2+ elevations in mouse neutrophils, reducing actin fiber formation and impairing spreading.

S-acylation of Ca transport proteins in cancer.

Alterations in cellular calcium (Ca) signals have been causally associated with the development and progression of human cancers. Cellular Ca signals are generated by channels, pumps, and exchangers that move Ca ions across membranes and are decoded by effector proteins in the cytosol or in organelles. S-acylation, the reversible addition of 16-carbon fatty acids to proteins, modulates the activity of Ca transporters by altering their affinity for lipids, and enzymes mediating this reversible post-translational modification have also been linked to several types of cancers.

S-acylation of Ca2+ transport proteins: molecular basis and functional consequences.

Calcium (Ca2+) regulates a multitude of cellular processes during fertilization and throughout adult life by acting as an intracellular messenger to control effector functions in excitable and non-excitable cells. Changes in intracellular Ca2+ levels are driven by the co-ordinated action of Ca2+ channels, pumps, and exchangers, and the resulting signals are shaped and decoded by Ca2+-binding proteins to drive rapid and long-term cellular processes ranging from neurotransmission and cardiac contraction to gene transcription and cell death. S-acylation, a lipid post-translational modification, is emerging as a critical regulator of several important Ca2+-handling proteins.

The ER stress sensor IRE1 interacts with STIM1 to promote store-operated calcium entry, T cell activation, and muscular differentiation.

Store-operated Ca entry (SOCE) mediated by stromal interacting molecule (STIM)-gated ORAI channels at endoplasmic reticulum (ER) and plasma membrane (PM) contact sites maintains adequate levels of Ca within the ER lumen during Ca signaling. Disruption of ER Ca homeostasis activates the unfolded protein response (UPR) to restore proteostasis. Here, we report that the UPR transducer inositol-requiring enzyme 1 (IRE1) interacts with STIM1, promotes ER-PM contact sites, and enhances SOCE.

Calcium storage in multivesicular endo-lysosome.

It is now established that endo-lysosomes, also referred to as late endosomes, serve as intracellular calcium store, in addition to the endoplasmic reticulum. While abundant calcium-binding proteins provide the latter compartment with its calcium storage capacity, essentially nothing is known about the mechanism responsible for calcium storage in endo-lysosomes. In this paper, we propose that the structural organization of endo-lysosomal membranes drives the calcium storage capacity of the compartment.

The lipid transfer proteins Nir2 and Nir3 sustain phosphoinositide signaling and actin dynamics during phagocytosis.

Changes in membrane phosphoinositides and local Ca2+ elevations at sites of particle capture coordinate the dynamic remodeling of the actin cytoskeleton during phagocytosis. Here, we show that the phosphatidylinositol (PI) transfer proteins PITPNM1 (Nir2) and PITPNM2 (Nir3) maintain phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] homeostasis at phagocytic cups, thereby promoting actin contractility and the sealing of phagosomes. Nir3 and to a lesser extent Nir2 accumulated on endoplasmic reticulum (ER) cisternae juxtaposed to phagocytic cups when expressed in phagocytic COS-7 cells.

Extending the Contacts Breaks the Flow.

In this news and views, we discuss our recent publication where we described how ER-PM membrane contact sites (MCS) are modulated during store operated calcium entry (SOCE). We also examine why enforcing ER-PM MCS by tethering proteins does not not enhance, but rather inhibits SOCE.

SARS-CoV-2 infection alkalinizes the ERGIC and lysosomes through the viroporin activity of the viral envelope protein.

The coronavirus SARS-CoV-2, the agent of the deadly COVID-19 pandemic, is an enveloped virus propagating within the endocytic and secretory organelles of host mammalian cells. Enveloped viruses modify the ionic homeostasis of organelles to render their intra-luminal milieu permissive for viral entry, replication and egress. Here, we show that infection of Vero E6 cells with the delta variant of the SARS-CoV-2 alkalinizes the endoplasmic reticulum (ER)-Golgi intermediate compartment (ERGIC) as well as lysosomes, mimicking the effect of inhibitors of vacuolar proton ATPases.

Discovery and validation of new Hv1 proton channel inhibitors with onco-therapeutic potential.

The voltage-gated hydrogen channel Hv1 encoded in humans by the HVCN1 gene is a highly selective proton channel that allows large fluxes of protons across biological membranes. Hv1 form functional dimers of four transmembrane spanning proteins resembling the voltage sensing domain of potassium channels. Each subunit is highly selective for protons and is controlled by changes in the transmembrane voltage and pH gradient.

TMBIM5 is the Ca /H antiporter of mammalian mitochondria.

Mitochondrial Ca ions are crucial regulators of bioenergetics and cell death pathways. Mitochondrial Ca content and cytosolic Ca homeostasis strictly depend on Ca transporters. In recent decades, the major players responsible for mitochondrial Ca uptake and release have been identified, except the mitochondrial Ca /H exchanger (CHE).

The TAM-associated STIM1 mutation increases ORAI1 channel function due to a reduced STIM1 inactivation break and an absence of microtubule trapping.

Tubular aggregate myopathy (TAM) is a progressive skeletal muscle disease associated with gain-of-function mutations in the ER Ca sensor STIM1 that mediates store-operated Ca entry (SOCE) across the Ca-release-activated (CRAC) Ca channel ORAI1. A frameshift mutation in STIM1 inactivation domain, STIM1, was identified in a TAM patient and reported to decrease SOCE. Using ion imaging and electrophysiology, we show that the STIM1 mutation instead renders STIM1 constitutively active.

Enforced tethering elongates the cortical endoplasmic reticulum and limits store-operated Ca2+ entry.

Recruitment of STIM proteins to cortical endoplasmic reticulum (cER) domains forming membrane contact sites (MCSs) mediate the store-operated Ca2+ entry (SOCE) pathway essential for human immunity. The cER is dynamically regulated by STIM and tethering proteins during SOCE, but the ultrastructural rearrangement and functional consequences of cER remodeling are unknown. Here, we express natural (E-Syt1 and E-Syt2) and artificial (MAPPER-S and MAPPER-L) protein tethers in HEK-293T cells and correlate the changes in cER length and gap distance, as measured by electron microscopy, with ionic fluxes.

The mammalian trafficking chaperone protein UNC93B1 maintains the ER calcium sensor STIM1 in a dimeric state primed for translocation to the ER cortex.

The stromal interaction molecule 1 (STIM1) is an endoplasmic reticulum (ER) Ca sensor that regulates the activity of Orai plasma membrane Ca channels to mediate the store-operated Ca entry pathway essential for immunity. Uncoordinated 93 homolog B1 (UNC93B1) is a multiple membrane-spanning ER protein that acts as a trafficking chaperone by guiding nucleic-acid sensing toll-like receptors to their respective endosomal signaling compartments. We previously showed that UNC93B1 interacts with STIM1 to promote antigen cross-presentation in dendritic cells, but the STIM1 binding site(s) and activation step(s) impacted by this interaction remained unknown.

S-acylation by ZDHHC20 targets ORAI1 channels to lipid rafts for efficient Ca signaling by Jurkat T cell receptors at the immune synapse.

Efficient immune responses require Ca fluxes across ORAI1 channels during engagement of T cell receptors (TCR) at the immune synapse (IS) between T cells and antigen presenting cells. Here, we show that ZDHHC20-mediated S-acylation of the ORAI1 channel at residue Cys143 promotes TCR recruitment and signaling at the IS. Cys143 mutations reduced ORAI1 currents and store-operated Ca entry in HEK-293 cells and nearly abrogated long-lasting Ca elevations, NFATC1 translocation, and IL-2 secretion evoked by TCR engagement in Jurkat T cells.

Control of lysosomal-mediated cell death by the pH-dependent calcium channel RECS1.

Programmed cell death is regulated by the balance between activating and inhibitory signals. Here, we have identified RECS1 (responsive to centrifugal force and shear stress 1) [also known as TMBIM1 (transmembrane BAX inhibitor motif containing 1)] as a proapoptotic member of the TMBIM family. In contrast to other proteins of the TMBIM family, RECS1 expression induces cell death through the canonical mitochondrial apoptosis pathway.

Simultaneous determination of intraluminal lysosomal calcium and pH by dextran-conjugated fluorescent dyes.

The lysosome is the main catabolic organelle in the cell, also serving as a signaling platform. Lysosomes maintain a low intraluminal pH where dozens of hydrolytic enzymes degrade a wide variety of macromolecules. Besides degradation of polymers, the lysosome is involved in various cellular processes, including energy metabolism, plasma membrane repair and antigen presentation.

Proteins Interacting with STIM1 and Store-Operated Ca Entry.

The endoplasmic reticulum (ER) Ca sensor stromal interaction molecule 1 (STIM1) interacts with ORAI Ca 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 Ca entry (SOCE).

Molecular Mechanisms of Calcium Signaling During Phagocytosis.

Calcium (Ca) 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 Ca 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, Ca signals render phagocytic ingestion and phagosome maturation more efficient, and their manipulation make an attractive avenue for therapeutic interventions.

Signaling and functional competency of neutrophils derived from bone-marrow cells expressing the ER-HOXB8 oncoprotein.

Neutrophils play a central role in immunity and inflammation via their intrinsic ability to migrate into inflamed tissue, to phagocytose pathogens, and to kill bacterial and fungi by releasing large quantities of superoxide anions and lytic enzymes. The molecular pathways controlling neutrophil microbicidal functions are still unclear, because neutrophils have a short half-life and are resistant to genetic manipulation. Neutrophil-like cells (NLC) can be generated from myeloid progenitors conditionally immortalized with the ER-HoxB8 oncoprotein, but whether these cells can replace neutrophils in high-throughput functional assays is unclear.

Ultrastructural Characterization of Flashing Mitochondria.

Mitochondria undergo spontaneous transient elevations in matrix pH associated with drops in mitochondrial membrane potential. These require a functional respiratory chain and the profusion protein optic atrophy 1, but their mechanistic basis is unclear. To gain insight on the origin of these dynamic events, we resolved the ultrastructure of flashing mitochondria by correlative light and electron microscopy.

The Bicarbonate Transporter SLC4A7 Plays a Key Role in Macrophage Phagosome Acidification.

Macrophages represent the first line of immune defense against pathogens, and phagosome acidification is a necessary step in pathogen clearance. Here, we identified the bicarbonate transporter SLC4A7, which is strongly induced upon macrophage differentiation, as critical for phagosome acidification. Loss of SLC4A7 reduced acidification of phagocytosed beads or bacteria and impaired the intracellular microbicidal capacity in human macrophage cell lines.

GRAM marks the spot for STIM. Commentary on "GRAM domain proteins specialize functionally distinct ER-PM contact sites in human cells".

GRAM domain proteins were reported as novel ER-PM tethers defining specific membrane contact sites (MCS) subdomains. GRAMD2a pre-marks the sites occupied by STIM1 at MCS and its ablation impairs STIM1 translocation, but not store-operated Ca entry. We discuss these apparently counterintuitive findings in the context of STIM/ORAI signaling at MCS.

The role of STIM proteins in neutrophil functions.

Stromal interaction molecule (STIM) proteins regulate store-operated Ca entry (SOCE) in innate and adaptive immune cells and participate in the Ca signals that control the functions of neutrophils, the first line of host defence against bacterial and fungal infections. Loss-of-function experiments in animal and cellular models indicate that both STIM1 and STIM2 regulate neutrophil functions, but the complexity of the SOCE machinery and the versatility of neutrophils complicate the evaluation of the results. This review aims to summarize the latest progress in the field, with special attention to the details of the experimental designs.