Unraveling ChR2-driven stochastic Ca2+ dynamics in astrocytes - A call for new interventional paradigms

Control of astrocytes via modulation of Ca2+ oscillations using techniques like optogenetics can prove to be crucial in therapeutic intervention of a variety of neurological disorders. However, a systematic study quantifying the effect of optogenetic stimulation in astrocytes is yet to be performed. Here, we propose a novel stochastic Ca2+dynamics model that incorporates the light sensitive component – Channelrhodopsin2 (ChR2). Utilizing this model, we studied the effect of various light stimulation paradigms on astrocytes for select variants of ChR2 (wild type, ChETA and ChRET/TC) in both an individual and a network of cells. Our results exhibited a consistent pattern of Ca2+ activity among individual cells in response to optogenetic stimulation, i.e., showing a steady rise in the Ca2+ basal level with increase in pulse width, and distinct regions with maximal spiking probability. Furthermore, we performed a global sensitivity analysis to assess the effect of ChR2 parameters and the model Weiner processes on astrocytic Ca2+ dynamics in the presence and absence of light stimulation, respectively. Results indicated that directing variants towards the first open state of the photo-cycle of ChR2 (O1) enhances spiking activity in astrocytes during optical stimulation. Evaluation of the effect of ChR2 transduction efficiency (heterogeneity) on Ca2+ signaling revealed that the optimal stimulation paradigm of a network does not necessarily coincide with that of an individual cell. Simulation for ChETA-incorporated astrocytes suggest that maximal activity of a single cell reduced the spiking probability of the network of astrocytes at higher degrees of transduction efficiency due to elevation of basal Ca2+ beyond physiological levels. Collectively, the framework presented in this study provides valuable information in the selection of paradigms that elicit optimal astrocytic activity using existing ChR2 constructs, as well as aid in the engineering of future optogenetic constructs.