The STIM-Orai Pathway: Light-Operated Ca2+ Entry Through Engineered CRAC Channels

Abstract

Ca signals regulate a plethora of cellular functions that include muscle contraction, heart beating, hormone secretion, lymphocyte activation, gene expression, and metabolism. To study the impact of Ca signals on biological processes, pharmacological tools and caged compounds have been commonly applied to induce fluctuations of intracellular Ca concentrations. These conventional approaches, nonetheless, lack rapid reversibility and high spatiotemporal resolution. To overcome these disadvantages, we and others have devised a series of photoactivatable genetically encoded Ca actuators (GECAs) by installing light sensitivities into a bona fide highly selective Ca channel, the Ca release-activated Ca (CRAC) channel. Store-operated CRAC channel serves as a major route for Ca entry in many cell types. These GECAs enable remote and precise manipulation of Ca signaling in both excitable and non-excitable cells. When combined with nanotechnology, it becomes feasible to wirelessly photo-modulate Ca-dependent activities in vivo. In this chapter, we briefly review most recent advances in engineering CRAC channels to achieve optical control over Ca signaling, outline their design principles and kinetic features, and present exemplary applications of GECAs engineered from CRAC channels.

Optogenetic engineering to probe the molecular choreography of STIM1-mediated cell signaling

“…The prototypical CRAC channel made of ORAI1 and STIM1 serves as a major route for Ca 2+ entry in many cell types. Various optogenetic modules have been engineered into STIM1 to confer light-sensitivity to CRAC channel over the past five years 5,6,[41][42][43][44][45][46][47] . We call these tools as genetically encoded Ca 2+ actuators (GECAs) 6 , as opposed to genetically encoded Ca 2+ indicators (GECIs) that are widely used for monitoring Ca 2+ signals 48 .…”

“…We call these tools as genetically encoded Ca 2+ actuators (GECAs) 6 , as opposed to genetically encoded Ca 2+ indicators (GECIs) that are widely used for monitoring Ca 2+ signals 48 . Two major strategies have been adopted to make GECAs: fusion with CRY2 to mimic Ca 2+ -depletion induced STIM1 oligomerization or replacing CC1 with LOV2 to recapitulate intramolecular autoinhibition with the STIM1 cytoplasmic domain 5,6,[41][42][43][44][45][46][47] . Herein, by using STIM1 as a test case, we have further exploited these optogenetic engineering approaches to study protein activities, interrogate and control cell signaling.…”

Critical parameters maintaining authentic CRAC channel hallmarks

“…Here, light represents an ideal tool to precisely and dynamically control CRAC channel function and downstream signalling processes. Initial attempts have already been taken by several groups to equip STIM1 with light-sensitive modules that allow precise, light-guided activation of CRAC channels (Ishii et al 2015; Kyung et al 2015; Ma et al 2017). Consequently, downstream signalling events can be triggered in a more accurate manner.…”

Isoform-Specific Properties of Orai Homologues in Activation, Downstream Signaling, Physiology and Pathophysiology

“…According to the recent reports, solid evidence that light enables to obtain high spatiotemporal control over the protein of interest is highly emerging. Remarkably, several publications [ 49 , 189 , 190 , 191 ] demonstrated the successful transfer of light sensitivity to STIM proteins and lately also to Orai1 using naturally occurring photo-responsive homo-/heteromerization domains. Although it is still technically challenging to precisely control light-sensitive proteins in native tissue, if successful, it would offer an encouraging therapy option.…”