High-resolution optical analyses of IP3-evoked Ca2+ signals

Ca2+ is a universal intracellular messenger that regulates many cellular responses. Most cells express inositol 1,4,5-trisphosphate receptors (IP3R) that mediate Ca2+ release from the endoplasmic reticulum (ER) when they bind IP3 produced after activation of cell-surface receptors. Vertebrate genomes encode three closely related subtypes of IP3R (IP3R1-3). High-resolution optical analyses have revealed a hierarchy of IP3-evoked Ca2+ signals that are thought to arise from the co-regulation of IP3Rs by IP3 and Ca2+. The smallest events (‘blips’) report the opening of single IP3Rs, Ca2+ ‘puffs’ report the almost simultaneous opening of a few clustered IP3Rs, and as stimulus intensities increase further Ca2+ signals propagate regeneratively as Ca2+ waves. The aim of this study was to establish whether all three IP3R subtypes can generate Ca2+ puffs. I first used a haploid cell line (HAP1 cells) to generate, using CRISPR/Cas9, a line lacking all endogenous IP3Rs. However, for analyses of Ca2+ puffs, I used HEK cells that had been engineered, using CRISPR/Cas9 to disrupt endogenous genes, to express single IP3R subtypes. Local Ca2+ signals evoked by flash-photolysis of caged- IP3 were recorded using Cal520 and total internal reflection fluorescence (TIRF) microscopy in human embryonic kidney (HEK) cells. The Flika algorithm was used, and validated, for automated detection of Ca2+ puffs and to measure their properties. IP3 evoked Ca2+ puffs in wild-type HEK cells and in cells expressing single IP3R subtypes. In wild-type cells, the Ca2+ signals invariably propagated regeneratively to give global increases in cytosolic [Ca2+]. This occurred less frequently in cells expressing single IP3R subtypes, commensurate with their lower overall levels of IP3R expression. The properties of the Ca2+ puffs, including their rise and decay times, durations, the size of the unitary fluorescence steps as channels closed channel during the falling phase, and the estimated number of active IP3Rs in each Ca2+ puff, were broadly similar in each of the four cell lines. The latter observation suggests that despite lower overall levels of IP3R expression (~30%) in cells with single subtypes relative to WT cells, there is a mechanism that ensures formation of similarly sized IP3R clusters. The only significant differences between cell lines were the slower kinetics of the Ca2+ puffs evoked by IP3R2, which may suggest dissociation of IP3 from its receptor contributes to the termination of Ca2+ puffs. My results demonstrate, for the first time, that all three IP3R subtypes can generate Ca2+ puffs. I conclude that Ca2+ puffs are fundamental building blocks of all IP3-evoked Ca2+ signals.Biotechnology and Biological Sciences Research Council (BBSRC

Inherited human group IVA cytosolic phospholipase A(2) deficiency abolishes platelet, endothelial, and leucocyte eicosanoid generation

This research was supported by an Imperial College Junior Research Fellowship (to N.S.K.), Wellcome Trust program grant (0852551Z108/Z to J.A.M. and T.D.W.), British Heart Foundation Ph.D. studentship (FS/10/033/28271 to F.R.), British Heart Foundation project grant (PG/11/39/28890 to D.B.-B.), and by the Intramural Research Program of the U.S. National Institutes of Health, National Institute of Environmental Health Sciences (Z01 ES025034 to D.C.Z.)

All three IP3 receptor subtypes generate Ca2+ puffs, the universal building blocks of IP3-evoked Ca2+ signals.

All three subtypes of inositol 1,4,5-trisphosphate receptor (IP3R) are intracellular Ca2+ channels that are co-regulated by IP3 and Ca2+ This allows IP3Rs to evoke regenerative Ca2+ signals, the smallest of which are Ca2+ puffs that reflect the coordinated opening of a few clustered IP3Rs. We use total internal reflection microscopy (TIRF) microscopy to record Ca2+ signals in HEK cells expressing all three IP3R subtypes or a single native subtype. Ca2+ puffs are less frequent in cells expressing one IP3R subtype, commensurate with them expressing fewer IP3Rs than wild-type cells. However, all three IP3R subtypes generate broadly similar Ca2+ puffs with similar numbers of IP3Rs contributing to each. This suggests that IP3R clusters may be assembled by conserved mechanisms that generate similarly sized clusters across different IP3R expression levels. The Ca2+ puffs evoked by IP3R2 had slower kinetics and more prolonged durations, which may be due to IP3 binding with greater affinity to IP3R2. We conclude that Ca2+ puffs are the building blocks for the Ca2+ signals evoked by all IP3Rs

IP3 Receptors Preferentially Associate with ER-Lysosome Contact Sites and Selectively Deliver Ca2+ to Lysosomes

Summary: Inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs) allow extracellular stimuli to redistribute Ca2+ from the ER to cytosol or other organelles. We show, using small interfering RNA (siRNA) and vacuolar H+-ATPase (V-ATPase) inhibitors, that lysosomes sequester Ca2+ released by all IP3R subtypes, but not Ca2+ entering cells through store-operated Ca2+ entry (SOCE). A low-affinity Ca2+ sensor targeted to lysosomal membranes reports large, local increases in cytosolic [Ca2+] during IP3-evoked Ca2+ release, but not during SOCE. Most lysosomes associate with endoplasmic reticulum (ER) and dwell at regions populated by IP3R clusters, but IP3Rs do not assemble ER-lysosome contacts. Increasing lysosomal pH does not immediately prevent Ca2+ uptake, but it causes lysosomes to slowly redistribute and enlarge, reduces their association with IP3Rs, and disrupts Ca2+ exchange with ER. In a “piston-like” fashion, ER concentrates cytosolic Ca2+ and delivers it, through large-conductance IP3Rs, to a low-affinity lysosomal uptake system. The involvement of IP3Rs allows extracellular stimuli to regulate Ca2+ exchange between the ER and lysosomes. : Ca2+ exchanges between ER and lysosomes regulate cytosolic Ca2+ signals and lysosome behavior. Atakpa et al. show that clusters of IP3 receptors populate ER-lysosome contact sites and facilitate local delivery of Ca2+ from the ER to lysosomes. Keywords: Ca2+, concanamycin A, endoplasmic reticulum, IP3 receptor, genetically encoded Ca2+ sensor, lysosome, membrane contact site, proximity ligation assay, store-operated Ca2+ entr

Cyclic AMP Recruits a Discrete Intracellular Ca2+ Store by Unmasking Hypersensitive IP3 Receptors

Inositol 1,4,5-trisphosphate (IP3) stimulates Ca2+ release from the endoplasmic reticulum (ER), and the response is potentiated by 3′,5′-cyclic AMP (cAMP). We investigated this interaction in HEK293 cells using carbachol and parathyroid hormone (PTH) to stimulate formation of IP3 and cAMP, respectively. PTH alone had no effect on the cytosolic Ca2+ concentration, but it potentiated the Ca2+ signals evoked by carbachol. Surprisingly, however, the intracellular Ca2+ stores that respond to carbachol alone could be both emptied and refilled without affecting the subsequent response to PTH. We provide evidence that PTH unmasks high-affinity IP3 receptors within a discrete Ca2+ store. We conclude that Ca2+ stores within the ER that dynamically exchange Ca2+ with the cytosol maintain a functional independence that allows one store to be released by carbachol and another to be released by carbachol with PTH. Compartmentalization of ER Ca2+ stores adds versatility to IP3-evoked Ca2+ signals

IP3 Receptors Preferentially Associate with ER-Lysosome Contact Sites and Selectively Deliver Ca2+ to Lysosomes.

Inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs) allow extracellular stimuli to redistribute Ca2+ from the ER to cytosol or other organelles. We show, using small interfering RNA (siRNA) and vacuolar H+-ATPase (V-ATPase) inhibitors, that lysosomes sequester Ca2+ released by all IP3R subtypes, but not Ca2+ entering cells through store-operated Ca2+ entry (SOCE). A low-affinity Ca2+ sensor targeted to lysosomal membranes reports large, local increases in cytosolic [Ca2+] during IP3-evoked Ca2+ release, but not during SOCE. Most lysosomes associate with endoplasmic reticulum (ER) and dwell at regions populated by IP3R clusters, but IP3Rs do not assemble ER-lysosome contacts. Increasing lysosomal pH does not immediately prevent Ca2+ uptake, but it causes lysosomes to slowly redistribute and enlarge, reduces their association with IP3Rs, and disrupts Ca2+ exchange with ER. In a "piston-like" fashion, ER concentrates cytosolic Ca2+ and delivers it, through large-conductance IP3Rs, to a low-affinity lysosomal uptake system. The involvement of IP3Rs allows extracellular stimuli to regulate Ca2+ exchange between the ER and lysosomes