Ca2+ signaling in astrocytes from Ip3r2−/− mice in brain slices and during startle responses in vivo

Abstract

Intracellular Ca2+ signaling is considered important for multiple astrocyte functions in neural circuits. However, mice devoid of inositol triphosphate type 2 receptors (IP3R2) reportedly lack all astrocyte Ca2+ signaling, but display no neuronal or neurovascular deficits, implying that astrocyte Ca2+ fluctuations play no role(s) in these functions. An assumption has been that loss of somatic Ca2+ fluctuations also reflects similar loss within astrocyte processes. Here, we tested this assumption and found diverse types of Ca2+ fluctuations within astrocytes, with most occurring within processes rather than in somata. These fluctuations were preserved in IP3R2−/− mice in brain slices and in vivo, occurred in endfeet, were increased by G-protein coupled receptor activation and by startle-induced neuromodulatory responses. Our data reveal novel Ca2+ fluctuations within astrocytes and highlight limitations of studies that used IP3R2−/− mice to evaluate astrocyte contributions to neural circuit function and mouse behavior.

Long-term In Vivo Calcium Imaging of Astrocytes Reveals Distinct Cellular Compartment Responses to Sensory Stimulation

“…1E) and a greater number of signals ( Fig. 1D) than endfeet or somata, comparable to previous reports with other calcium indicators and in other brain regions (Shigetomi et al 2013;Gee et al 2014;Kanemaru et al 2014;Otsu et al 2015;Srinivasan et al 2015;Tang et al 2015). A previous study has also detected three different populations of GCaMP6s peaks in astrocytes: single peaks, multi peaks, and plateaus (Bonder and McCarthy 2014).…”

“…We specifically examined IP3 signaling and found that, similar to a previous study (Srinivasan et al 2015),…”

“…Heterogeneous astrocyte calcium signals have been identified in vivo (Bonder and McCarthy 2014;Srinivasan et al 2015); however, the nature of these signals in response to local synaptic activity remain unclear. We sought to characterize astrocyte calcium signaling in different sub-cellular compartments in terms of long-term stability and temporal synchronicity using chronic in vivo twophoton imaging of GCaMP6s during sensory stimulation and innovative analysis tools.…”

“…Previous studies have shown that inositol-1,4,5-trisphosphate (IP3)-dependent calcium release from the endoplasmic reticulum (ER) is important for astrocyte calcium signaling (Petravicz et al 2008;Nizar et al 2013;Takata et al 2013;Srinivasan et al 2015). We examined the involvement of this pathway in sensory stimulation-evoked astrocyte responses by expressing GCaMP6s in astrocytes from Ip3r2 -/-mice and wild-type (WT) littermate controls (Li et al 2005) and selecting sub-cellular ROIs in the same manner as previous experiments (manual selection for endfeet and somata ROIs and automated detection for processes).…”

Genetic targeting of astrocytes in the gliovascular unit of the adult brain

“…Thus, it is conceivable that knocking-down or knocking-out of the IP 3 receptors specifically in astrocytes may allow us to study astrocyte silencing. To genetically manipulate this signaling, researchers have used the astrocyte-specific IP 3 receptor-2 (ITPR2) gene knockout mice and found that the astrocyte Ca 2+ responses to neuronal activity are somewhat diminished 47 . However, while Ca 2+ responses in the astrocyte somata may be depend on ITPR2 signaling it appears that Ca 2+ flux in distal fine processes appears to be at least partially ITPR2-independent, mediated by extracellular influx or release from mitochondrial stores 47,48 .…”