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.
Highlights
The role of astrocytic calcium signaling in various regulatory mechanisms in the brain is far from being fully understood and is a subject of considerable controversy [see refs [1,2,3,4,5,6]]
Optogenetics is a novel technique involving the targeted delivery of light-sensitive ion channels like Channelrhodopsin-2 (ChR2) to cells
This technique has been expanded to non-neuronal cell types, e.g., astrocytes
Summary
The role of astrocytic calcium signaling in various regulatory mechanisms in the brain is far from being fully understood and is a subject of considerable controversy [see refs [1,2,3,4,5,6]]. Sensory and transcranial direct current stimulation techniques [11,12,13] have been used to elicit Ca2+ changes in astrocytes These methods lack cell specificity due to the concurrent activation of other cell types, including neurons. Optogenetics is an avant-garde, minimally invasive, and reproducible approach [20,21], providing a platform to genetically target specific cell types with high temporal and spatial precision, which can be employed as a tool to exclusively modulate astrocytic Ca2+ signaling in vivo
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