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

Abstract

Optogenetic targeting of astrocytes provides a robust experimental model to differentially induce Ca2+ signals in astrocytes in vivo. However, a systematic study quantifying the response of optogenetically modified astrocytes to light is yet to be performed. Here, we propose a novel stochastic model of Ca2+ dynamics in astrocytes that incorporates a light sensitive component—channelrhodopsin 2 (ChR2). Utilizing this model, we investigated the effect of different light stimulation paradigms on cells expressing select variants of ChR2 (wild type, ChETA, and ChRET/TC). Results predict that depending on paradigm specification, astrocytes might undergo drastic changes in their basal Ca2+ level and spiking probability. Furthermore, we performed a global sensitivity analysis to assess the effect of variation in parameters pertinent to the shape of the ChR2 photocurrent on astrocytic Ca2+ dynamics. Results suggest that directing variants towards the first open state of the ChR2 photocycle (o1) enhances spiking activity in astrocytes during optical stimulation. Evaluation of the effect of Ca2+ buffering and coupling coefficient in a network of ChR2-expressing astrocytes demonstrated basal level elevations in the stimulated region and propagation of calcium activity to unstimulated cells. Buffering reduced the diffusion range of Ca2+ within the network, thereby limiting propagation and influencing the activity of astrocytes. Collectively, the framework presented in this study provides valuable information for the selection of light stimulation paradigms that elicit desired astrocytic activity using existing ChR2 constructs, as well as aids in the engineering of future application-oriented optogenetic variants.

Engaging biological oscillators through second messenger pathways permits emergence of a robust gastric slow-wave during peristalsis

“…Previous modeling studies of intercellular Ca 2+ waves in astrocytes [ 35 ] and smooth muscle cells [ 25 ] support this approach of modeling Ca 2+ and IP 3 permeability. The exchange of IP 3 was modeled as proportional to the IP 3 concentration gradient between adjacent cells (see Methods , also [ 83 ]). Although IP 3 can diffuse within the cell cytoplasm and not necessarily through the gap junctions, we assumed that IP 3 only moves to the adjacent cells based on the concentration gradient.…”

“…An exchange of second messengers, namely, Ca 2+ and IP 3 can occur via gap junctions [ 22 , 23 , 25 , 83 ]. This phenomenon has not been considered in previous computational models of ICCs.…”