CRACing the Cluster

Abstract

HomeCirculation ResearchVol. 111, No. 1CRACing the Cluster Free AccessEditorialPDF/EPUBAboutView PDFView EPUBSections ToolsAdd to favoritesDownload citationsTrack citationsPermissions ShareShare onFacebookTwitterLinked InMendeleyReddit Jump toFree AccessEditorialPDF/EPUBCRACing the ClusterFunctionally Active Orai1 Channels in the Absence of Clustering With STIM1 Kimberly A. Smith and Jason X.-J. Yuan Kimberly A. SmithKimberly A. Smith From the Department of Medicine, Section of Pulmonary, Critical Care, Sleep, and Allergy; Institute for Personalized Respiratory Medicine, Department of Pharmacology; and Center for Cardiovascular Research, University of Illinois at Chicago. Search for more papers by this author and Jason X.-J. YuanJason X.-J. Yuan From the Department of Medicine, Section of Pulmonary, Critical Care, Sleep, and Allergy; Institute for Personalized Respiratory Medicine, Department of Pharmacology; and Center for Cardiovascular Research, University of Illinois at Chicago. Search for more papers by this author Originally published22 Jun 2012https://doi.org/10.1161/CIRCRESAHA.112.271791Circulation Research. 2012;111:9–11Platelet-derived growth factor (PDGF) signaling is implicated in a wide range of diseases, and PDGF drives the pathological responses in many vascular disorders and fibrotic diseases such as atherosclerosis, restenosis, pulmonary arterial hypertension, and pulmonary fibrosis. In vascular smooth muscle cells (VSMC), PDGF is a potent mitogen which promotes proliferation and migration and contributes to pathological vascular remodeling. Recent studies have demonstrated a role for PDGF in stromal interaction molecule 1 (STIM1)/Orai1-mediated Ca2+ influx in VSMC.1 Regulation of cytosolic Ca2+ concentration ([Ca2+]cyt) is critical for many cellular processes. A rise in [Ca2+]cyt is a major trigger for VSMC proliferation and contraction. One of the major routes of Ca2+ entry into cells is through store-operated Ca2+ (SOC) channels. On depletion of Ca2+ from the stores (sarcoplasmic or endoplasmic reticulum, SR/ER), a Ca2+ deficiency signal is transmitted to the SOC channels in the plasma membrane via STIM1. This causes the SOC to open, allowing Ca2+ to flow into the cytosol, a process referred to as store-operated Ca2+ entry (SOCE). The cytosolic Ca2+ is then sequestered into the stores by the sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA), thus replenishing the stores. Studies have demonstrated a role for Ca2+ release-activated Ca2+ (CRAC) channels in SOCE, with STIM1 and Orai1 as the major components.2Article, see p 66The majority of STIM1 is expressed in the SR/ER membrane, where it senses decreased Ca2+ concentration in the SR/ER when inositol 1,4,5-triphosphate (IP3)-mediated activation of IP3 receptor induces Ca2+ release. STIM1 then undergoes a conformational change allowing STIM1 to multimerize and translocate to discrete puncta near the plasma membrane where it interacts with and activates Orai1, the pore-forming unit of the CRAC channel.3 In this issue, McKeown et al4 investigate the relevance and importance of STIM1/Orai1 redistribution and clustering in PDGF-mediated Ca2+ entry through Orai1 channels. The redistribution and clustering of STIM1 and Orai1 was observed in this study using fluorescently-tagged STIM1 and Orai1. The authors demonstrate that, under normal conditions, Orai1 localizes to intracellular vesicles and has a reasonably uniform distribution in the plasma membrane except in dynamic membrane structures such as membrane ruffles or spiny protrusions, where Orai1 is highly expressed. Additionally, they show that STIM1 can be found in the SR/ER and associated with comet-like structures under nonstimulated conditions. Passive depletion of Ca2+ stores with the SERCA inhibitor thapsigargin (TG) causes significant redistribution and clustering of STIM1 and Orai1 in VSMC, which is clearly demonstrated with the fluorescently-tagged STIM1 and Orai1 used in this study. Interestingly, this study shows that PDGF fails to cause significant STIM1/Orai1 redistribution and that PDGF activates Orai1 channels in the absence of STIM1/Orai1 clustering. The data indicate that PDGF causes significant STIM1/Orai1 redistribution when Orai1 channels are blocked, either by Synta 66 or by 2 loss-of-function mutations (R91W and E106A). Furthermore, the authors show that under conditions of SR/ER stress (such as acidosis), PDFG-mediated Ca2+ influx is associated with redistribution and clustering of STIM1 and Orai1. Based on these data, the authors conclude that 1 PDGF-mediated Ca2+ entry through Orai1 channels is independent of clustering of STIM1 and Orai1, 2 nonclustered Orai1 channels serve to maintain Ca2+ in the stores and prevent store depletion, 3 redistribution and clustering of STIM1 and Orai1 becomes apparent and important when Orai1 channel activity is compromised by Orai1 channel inhibitors or Orai1 mutations, and 4 redistribution and clustering of Orai1 and STIM1 are important for situations where there is SR/ER stress, such as acidosis, and a risk of store depletion.Given that STIM1 is an SR/ER Ca2+ sensor whose response to store depletion is redistribution and clustering, McKeown et al4 suggest that PDGF causes Ca2+ release, but not store depletion, because STIM1 does not cluster in response to PDGF exposure. The authors propose that nonclustering Orai1 channels maintain the stores replete preventing STIM1 clustering. While the exact mechanism by which PDGF activates Orai1 channels without redistribution and clustering is still an unknown, there are several possibilities. It is possible that PDGF binding to the PDGF receptor, a receptor tyrosine kinase, may release a second messenger which activates Orai1 (Figure, A). This indicates that Orai1 channels may also function as a receptor-operated Ca2+ channel (ROC)-like channel following activation by PDGF. Under conditions which cause store depletion, such as acidosis, inhibition of the SERCA pump, or in the absence of extracellular Ca2+, Orai1 channels cluster with STIM1 and participate in classical SOCE. This suggests that the roles of these voltage-independent Ca2+ channels (eg, ROC and SOC) may overlap. Indeed, several studies have shown previously that TRPC channels are important for both receptor- and store-operated Ca2+ entry.5,6 That these channels have overlapping functions underlies the critical importance of regulation of Ca2+ to the cell.Download figureDownload PowerPointFigure. PDGF-mediated activation of Orai1 channels.A, Under normal conditions, PDGF activates Orai1 channels without redistribution or clustering with STIM1 possibly by activating free Orai1 channels, by activating Orai1 channels coupled to plasma membrane localized-STIM1, or by activating constitutively coassembled STIM1/Orai1 channels. B, Under conditions of store depletion or SR/ER stress, PDGF exposure leads to STIM1/Orai1 clustering and classical SOCE. This may involve co-clustering with SERCA in order for more efficient refilling of the Ca2+ stores. PM indicates plasma membrane; SR/ER, sarcoplasmic reticulum/endoplasmic reticulum.Another possibility for the mechanism by which PDGF activates Orai1 in the absence of clustering involves STIM1. McKeown et al4 provide evidence that inhibition of STIM1 with siRNA blocks PDGF-mediated Ca2+ influx suggesting that even in the absence of clustering, activation of Orai1 still requires STIM1. It is possible that nonclustering Orai1 channels interact with STIM1 located on the plasma membrane (Figure, A). STIM1 has been previously shown to be localized in the plasma membrane, where it can interact with TRPC1 and contribute to Ca2+ influx.7 The authors suggest it is possible that a fraction of Orai1 and STIM1 proteins are constitutively coassembled in nonclustered units that do not function until being activated by PDGF (Figure, A). A recent study identified a splice variant of STIM1 in skeletal muscle which forms permanent clusters with Orai1.8 PDGF may activate these Orai1/STIM1 units under normal conditions (eg, 1.5 mmol/L extracellular Ca2+, no SR/ER stress or store depletion) without redistribution or clustering of Orai1 and STIM1 (Figure, A).Another interesting concept proposed by McKeown et al4 is the dual response of Orai1 channels to PDGF. Under normal conditions (eg, 1.5 mmol/L extracellular Ca2+, no SR/ER stress or store depletion), PDGF activates Orai1 channels without clustering. However, PDGF causes STIM1/Orai1 clustering under conditions of store depletion or ER stress. If Orai1 channels are open and allowing Ca2+ entry without clustering in response to PDGF, what advantage does STIM1/Orai1 clustering bring under conditions of ER stress? The authors propose this serves to maximize efficiency of store-filling and minimize store depletion. This suggests that there may be a mechanism by which STIM1/Orai1 clusters allow for direct funneling of Ca2+ to the SERCA pump in order to replenish the Ca2+ stores and prevent an increase in cytosolic Ca2+ (Figure, B). Release of Ca2+ from the stores has been shown to alter the distribution of SERCA and the IP3 receptor.9 STIM1 and Orai1 may also cluster with SERCA in the SR/ER puncta to more efficiently funnel Ca2+ directly into the SR/ER under conditions of store depletion or ER stress (Figure, B).In summary, the data from McKeown et al4 provide evidence of PDGF-mediated activation of Orai1 channels without redistribution or clustering with STIM1. Under normal conditions PDGF activates Orai1 channels in the absence of clustering; however, during times of store depletion or ER stress, STIM1/Orai1 clustering occurs. This dynamic regulation of Orai1 activation may be important for efficiently replenishing the Ca2+ stores and demonstrates the importance of regulation of Ca2+ signaling.Sources of FundingThis work was supported in part by grants from the National Heart, Lung, and Blood Institute of the National Institutes of Health (HL066012 and HL098053).DisclosuresNone.FootnotesThe opinions expressed in this article are not necessarily those of the editors or of the American Heart Association.Correspondence to Jason X.-J. Yuan, MD, PhD, Department of Medicine, University of Illinois at Chicago, COMRB 3131, MC 719, 909 S Wolcott Ave, Chicago, IL 60612. E-mail [email protected]edu