Endoplasmic reticulum (ER), or sarcoplasmic reticulum in muscle cells, acts as the largest intracellular Ca2+ store and plays a pivotal role in shaping the spatiotemporal dynamics of Ca2+ sigals to maintain intracellular Ca2+ homeostasis. Reciprocally, calcium is also intimately involved in regulating ER functions, including lipid synthesis, protein synthesis, folding, modifications and translocation. Consequently, the ER employs a series of regulators to control Ca2+ concentration on both sides of the membrane and mediate Ca2+ transfer with the surrounding organelles via membrane contact sites. Deregulated ER Ca2+ homeostasis not only results in ER stress and unfolded protein response (UPR), but also is involved in cardiovascular diseases, neurological diseases, metabolic syndromes and other diseases. Pharmacological modulation and genetic manipulations are often applied to study the ER Ca2+ handling machinery, but these largely irreversible approaches often lack spatial precision and specificity. Optogenetics technology provides a novel approach for modulating ER Ca2+ homeostasis with superior spatiotemporal resolution and high reversibility. Hence, the team proposes to create an innovative optogenetic toolkit, named as Genetically Encoded ER Calcium Actuators (GEECAs), that enable optical interrogation of ER Ca2+ homeostasis, ER-organelle Ca2+ communications and organellar Ca2+-modulated activities in multiple biological systems. In Specific Aim 1, the team will design a set of GEECAs based on ER-localized calcium channels and/or their photoswitchable actuators to photo-tune ER Ca2+ signals with varying kinetic and dynamic properties. In Specific Aim 2, the team seeks to develop an optogenetic platform to control inter-organellar tethering and Ca2+ transfer between ER and its surrounding organelles. The successful execution of this project will provide novel opportunities to achieve noninvasive and precise modulation of ER Ca2+ homeostasis and inter-organellar Ca2+ signaling with high spatiotemporal precision, thereby exerting remote control over cellular physiology, including ER stress, energy metabolism, autophagy and mTOR signaling. From a translational perspective, molecular tools generated from this project will provide novel interventional approaches for human diseases associated with aberrant ER Ca2+ signaling.