Review: Structure and Activation Mechanisms of CRAC Channels

Ca²⁺ release activated Ca²⁺ (CRAC) channels represent a primary pathway for Ca²⁺ to enter non-excitable cells. The two key players in this process are the stromal interaction molecule (STIM), a Ca²⁺ sensor embedded in the membrane of the endoplasmic reticulum, and Orai, a highly Ca²⁺ selective ion channel located in the plasma membrane. Upon depletion of the internal Ca²⁺ stores, STIM is activated, oligomerizes, couples to and activates Orai. This review provides an overview of novel findings about the CRAC channel activation mechanisms, structure and gating. In addition, it highlights, among diverse STIM and Orai mutants, also the disease-related mutants and their implications.

Photoswitchable Inhibitor of the Calcium Channel TRPV6

Herein we report the first photoswitchable inhibitor of Transient Receptor Potential Vanilloid 6 (TRPV6), a selective calcium channel involved in a number of diseases and in cancer progression. By surveying analogs of a previously reported TRPV6 inhibitor appended with a phenyl-diazo group, we identified a compound switching between a weak TRPV6 inhibitor in its dark, E-diazo stereoisomer (Z/E = 3:97, IC₅₀ ≫ 10 μM) and a potent inhibitor as the Z-diazo stereoisomer accessible reversibly by UV irradiation at λ = 365 nm (Z/E = 3:1, IC₅₀ = 1.7 ± 0.4 μM), thereby allowing precise spatiotemporal control of inhibition. This new tool compound should be useful to deepen our understanding of TRPV6.

A novel STIM1-Orai1 gating interface essential for CRAC channel activation

Calcium signalling through store-operated calcium (SOC) entry is of crucial importance for T-cell activation and the adaptive immune response. This entry occurs via the prototypic Ca2+ release-activated Ca2+ (CRAC) channel. STIM1, a key molecular component of this process, is located in the membrane of the endoplasmic reticulum (ER) and is initially activated upon Ca2+ store depletion. This activation signal is transmitted to the plasma membrane via a direct physical interaction that takes place between STIM1 and the highly Ca2+-selective ion channel Orai1. The activation of STIM1 induces an extended cytosolic conformation. This, in turn, exposes the CAD/SOAR domain and leads to the formation of STIM1 oligomers. In this study, we focused on a small helical segment (STIM1 α3, aa 400-403), which is located within the CAD/SOAR domain. We determined this segment's specific functional role in terms of STIM1 activation and Orai1 gating. The STIM1 α3 domain appears not essential for STIM1 to interact with Orai1. Instead, it represents a key domain that conveys STIM1 interaction into Orai1 channel gating. The results of cysteine crosslinking experiments revealed the close proximity of STIM1 α3 to a region within Orai1, which was located at the cytosolic extension of transmembrane helix 3, forming a STIM1-Orai1 gating interface (SOGI). We suggest that the interplay between STIM1 α3 and Orai1 TM3 allows STIM1 coupling to be transmitted into physiological CRAC channel activation.

Lipid-independent control of endothelial and neuronal TRPC3 channels by light

Lipid-gated TRPC channels are highly expressed in cardiovascular and neuronal tissues. Exerting precise pharmacological control over their activity in native cells is expected to serve as a basis for the development of novel therapies. Here we report on a new photopharmacological tool that enables manipulation of TRPC3 channels by light, in a manner independent of lipid metabolism and with higher temporal precision than lipid photopharmacology. Using the azobenzene photoswitch moiety, we modified GSK1702934A to generate light-controlled TRPC agonists. We obtained one light-sensitive molecule (OptoBI-1) that allows us to exert efficient, light-mediated control over TRPC3 activity and the associated cellular Ca2+ signaling. OptoBI-1 enabled high-precision, temporal control of TRPC3-linked cell functions such as neuronal firing and endothelial Ca2+ transients. With these findings, we introduce a novel photopharmacological strategy to control native TRPC conductances.

Communication between N terminus and loop2 tunes Orai activation

Ca²⁺ release-activated Ca²⁺ (CRAC) channels constitute the major Ca²⁺ entry pathway into the cell. They are fully reconstituted via intermembrane coupling of the Ca²⁺-selective Orai channel and the Ca²⁺-sensing protein STIM1. In addition to the Orai C terminus, the main coupling site for STIM1, the Orai N terminus is indispensable for Orai channel gating. Although the extended transmembrane Orai N-terminal region (Orai1 amino acids 73-91; Orai3 amino acids 48-65) is fully conserved in the Orai1 and Orai3 isoforms, Orai3 tolerates larger N-terminal truncations than Orai1 in retaining store-operated activation. In an attempt to uncover the reason for these isoform-specific structural requirements, we analyzed a series of Orai mutants and chimeras. We discovered that it was not the N termini, but the loop2 regions connecting TM2 and TM3 of Orai1 and Orai3 that featured distinct properties, which explained the different, isoform-specific behavior of Orai N-truncation mutants. Atomic force microscopy studies and MD simulations suggested that the remaining N-terminal portion in the non-functional Orai1 N-truncation mutants formed new, inhibitory interactions with the Orai1-loop2 regions, but not with Orai3-loop2. Such a loop2 swap restored activation of the N-truncation Orai1 mutants. To mimic interactions between the N terminus and loop2 in full-length Orai1 channels, we induced close proximity of the N terminus and loop2 via cysteine cross-linking, which actually caused significant inhibition of STIM1-mediated Orai currents. In aggregate, maintenance of Orai activation required not only the conserved N-terminal region but also permissive communication of the Orai N terminus and loop2 in an isoform-specific manner.

Authentic CRAC channel activity requires STIM1 and the conserved portion of the Orai N terminus

Calcium (Ca²⁺) is an essential second messenger required for diverse signaling processes in immune cells. Ca²⁺ release-activated Ca²⁺ (CRAC) channels represent one main Ca²⁺ entry pathway into the cell. They are fully reconstituted via two proteins, the stromal interaction molecule 1 (STIM1), a Ca²⁺ sensor in the endoplasmic reticulum, and the Ca²⁺ ion channel Orai in the plasma membrane. After Ca²⁺ store depletion, STIM1 and Orai couple to each other, allowing Ca²⁺ influx. CRAC-/STIM1-mediated Orai channel currents display characteristic hallmarks such as high Ca²⁺ selectivity, an increase in current density when switching from a Ca²⁺-containing solution to a divalent-free Na⁺ one, and fast Ca²⁺-dependent inactivation. Here, we discovered several constitutively active Orai1 and Orai3 mutants, containing substitutions in the TM3 and/or TM4 regions, all of which displayed a loss of the typical CRAC channel hallmarks. Restoring authentic CRAC channel activity required both the presence of STIM1 and the conserved Orai N-terminal portion. Similarly, these structural requisites were found in store-operated Orai channels. Key molecular determinants within the Orai N terminus that together with STIM1 maintained the typical CRAC channel hallmarks were distinct from those that controlled store-dependent Orai activation. In conclusion, the conserved portion of the Orai N terminus is essential for STIM1, as it fine-tunes the open Orai channel gating, thereby establishing authentic CRAC channel activity.

Transmembrane helix connectivity in Orai1 controls two gates for calcium-dependent transcription

The channel Orai1 requires Ca2+ store depletion in the endoplasmic reticulum and an interaction with the Ca2+ sensor STIM1 to mediate Ca2+ signaling. Alterations in Orai1-mediated Ca2+ influx have been linked to several pathological conditions including immunodeficiency, tubular myopathy, and cancer. We screened large-scale cancer genomics data sets for dysfunctional Orai1 mutants. Five of the identified Orai1 mutations resulted in constitutively active gating and transcriptional activation. Our analysis showed that certain Orai1 mutations were clustered in the transmembrane 2 helix surrounding the pore, which is a trigger site for Orai1 channel gating. Analysis of the constitutively open Orai1 mutant channels revealed two fundamental gates that enabled Ca2+ influx: Arginine side chains were displaced so they no longer blocked the pore, and a chain of water molecules formed in the hydrophobic pore region. Together, these results enabled us to identify a cluster of Orai1 mutations that trigger Ca2+ permeation associated with gene transcription and provide a gating mechanism for Orai1.

A coiled-coil clamp controls both conformation and clustering of stromal interaction molecule 1 (STIM1)

Store-operated Ca²⁺ entry, essential for the adaptive immunity, is initiated by the endoplasmic reticulum (ER) Ca²⁺ sensor STIM1. Ca²⁺ entry occurs through the plasma membrane resident Ca²⁺ channel Orai1 that directly interacts with the C-terminal STIM1 domain, named SOAR/CAD. Depletion of the ER Ca²⁺ store controls this STIM1/Orai1 interaction via transition to an extended STIM1 C-terminal conformation, exposure of the SOAR/CAD domain, and STIM1/Orai1 co-clustering. Here we developed a novel approach termed FRET-derived Interaction in a Restricted Environment (FIRE) in an attempt to dissect the interplay of coiled-coil (CC) interactions in controlling STIM1 quiescent as well as active conformation and cluster formation. We present evidence of a sequential activation mechanism in the STIM1 cytosolic domains where the interaction between CC1 and CC3 segment regulates both SOAR/CAD exposure and CC3-mediated higher-order oligomerization as well as cluster formation. These dual levels of STIM1 auto-inhibition provide efficient control over the coupling to and activation of Orai1 channels.