AAV-compatible optogenetic tools for activating endogenous calcium channels in vivo

Calcium ions (Ca²⁺) play pivotal roles in regulating diverse brain functions, including cognition, emotion, locomotion, and learning and memory. These functions are intricately regulated by a variety of Ca²⁺-dependent cellular processes, encompassing synaptic plasticity, neuro/gliotransmitter release, and gene expression. In our previous work, we developed 'monster OptoSTIM1' (monSTIM1), an improved OptoSTIM1 that selectively activates Ca²⁺-release-activated Ca²⁺ (CRAC) channels in the plasma membrane through blue light, allowing precise control over intracellular Ca²⁺ signaling and specific brain functions. However, the large size of the coding sequence of monSTIM1 poses a limitation for its widespread use, as it exceeds the packaging capacity of adeno-associated virus (AAV). To address this constraint, we have introduced monSTIM1 variants with reduced coding sequence sizes and established AAV-based systems for expressing them in neurons and glial cells in the mouse brain. Upon expression by AAVs, these monSTIM1 variants significantly increased the expression levels of cFos in neurons and astrocytes in the hippocampal CA1 region following non-invasive light illumination. The use of monSTIM1 variants offers a promising avenue for investigating the spatiotemporal roles of Ca²⁺-mediated cellular activities in various brain functions. Furthermore, this toolkit holds potential as a therapeutic strategy for addressing brain disorders associated with aberrant Ca²⁺ signaling.