Calcium acts as a second messenger to regulate a myriad of cell functions, ranging from short-term muscle contraction and cell motility to long-term changes in gene expression and metabolism. To study the impact of Ca2+-modulated ‘ON’ and ‘OFF’ reactions in mammalian cells, pharmacological tools and ‘caged’ compounds are commonly used under various experimental conditions. The use of these reagents for precise control of Ca2+ signals, nonetheless, is impeded by lack of reversibility and specificity. The recently developed optogenetic tools, particularly those built upon engineered Ca2+ release-activated Ca2+ (CRAC) channels, provide exciting opportunities to remotely and non-invasively modulate Ca2+ signaling due to their superior spatiotemporal resolution and rapid reversibility. In this review, we briefly summarize the latest advances in the development of optogenetic tools (collectively termed as ‘genetically encoded Ca2+ actuators’, or GECAs) that are tailored for the interrogation of Ca2+ signaling, as well as their applications in remote neuromodulation and optogenetic immunomodulation. Our goal is to provide a general guide to choosing appropriate GECAs for optical control of Ca2+ signaling in cellulo, and in parallel, to stimulate further thoughts on evolving non-opsin-based optogenetics into a fully fledged technology for the study of Ca2+-dependent activities in vivo.
The calcium release-activated calcium (CRAC) channel, composed of ORAI and stromal interaction molecules (STIM), represents a prototypical example of store-operated calcium entry in mammals. The ORAI-STIM signaling occurs at membrane contact sites formed by close appositions between the endoplasmic reticulum (ER) and the plasma membrane. ORAI1 is a four-pass transmembrane protein that forms a highly calcium-selective ion channel in the plasma membrane. STIM1 is an ER-resident, a single-pass transmembrane protein that serves as a calcium sensor within the ER lumen and a potent activator of ORAI1 calcium channels. The intricate interplay between ORAI and STIM controls calcium entry into cells to regulate a myriad of physiological processes. We highlight herein the current knowledge on the structure-function relationship of CRAC channel, with a focus on key structural elements that mediate STIM1 conformational switch and the dynamic coupling between STIM1 and ORAI1. Furthermore, we discuss the physiological roles of STIM-ORAI signaling in various tissues and organs, as well as major pathological conditions arising from loss- or gain-of-function mutations in human ORAI1 and STIM1. © 2017 American Physiological Society. Compr Physiol 8:981-1002, 2018.
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.
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