Inositol 1,4,5‐trisphosphate receptor type 2‐independent Ca²⁺ release from the endoplasmic reticulum in astrocytes

Abstract

Accumulating evidence indicates that astrocytes are actively involved in the physiological and pathophysiological functions of the brain. Intracellular Ca2+ signaling, especially Ca2+ release from the endoplasmic reticulum (ER), is considered to be crucial for the regulation of astrocytic functions. Mice with genetic deletion of inositol 1,4,5‐trisphosphate receptor type 2 (IP3R2) are reportedly devoid of astrocytic Ca2+ signaling, and thus widely used to explore the roles of Ca2+ signaling in astrocytic functions. While functional deficits in IP3R2‐knockout (KO) mice have been found in some reports, no functional deficit was observed in others. Thus, there remains a controversy regarding the functional significance of astrocytic Ca2+ signaling. To address this controversy, we re‐evaluated the assumption that Ca2+ release from the ER is abolished in IP3R2‐KO astrocytes using a highly sensitive imaging technique. We expressed the ER luminal Ca2+ indicator G‐CEPIA1er in cortical and hippocampal astrocytes to directly visualize spontaneous and stimulus‐induced Ca2+ release from the ER. We found attenuated but significant Ca2+ release in response to application of norepinephrine to IP3R2‐KO astrocytes. This IP3R2‐independent Ca2+ release induced only minimal cytosolic Ca2+ transients but induced robust Ca2+ increases in mitochondria that are frequently in close contact with the ER. These results indicate that ER Ca2+ release is retained and is sufficient to increase the Ca2+ concentration in close proximity to the ER in IP3R2‐KO astrocytes.

Impairments in remote memory caused by the lack of Type 2 IP3 receptors

“…Mice lacking IP 3 R2 have been shown to exhibit a reduction in the amplitude of astrocytic Ca 2+ oscillations, a decrease in their frequency and a complete absence of events with a large spatial spread (Okubo et al, ; Petravicz et al, ; Sherwood et al, ). Such astrocytic impairments were shown to reflect upon neural circuits, namely causing deficits upon cholinergic plasticity in the cerebral cortex (Takata et al, ) and hippocampus (Navarrete et al, ), but whether Ip3r2 −/− mice display alterations upon cognitive processes has only been examined following photothrombosis‐induced ischemia (Li et al, ), leaving such a core question largely outstanding.…”

Visualization of astrocytic intracellular Ca2+ mobilization

“…We then introduce our recent work using intraorganellar Ca 2+ imaging techniques, which may help resolve the controversy (Okubo et al . ).…”

“…These findings clearly indicate that Ca 2+ mobilization from the ER is retained in IP 3 R2‐KO astrocytes, contradicting the previous assumption (Okubo et al . ).…”

“…Comparing the changes in [Ca 2+ ] Cyt and [Ca 2+ ] ER upon agonist stimulation, we estimated the combined effect of the dilution and buffering factors to be about 1/260 in cultured astrocytes (Okubo et al . ). Therefore, the change in the Ca 2+ concentration in the cytosol is only about 0.4% of that within the ER.…”

“…Our results indicate that the NA‐induced [Ca 2+ ] Mito response in astrocytes is mediated by mitochondrial membrane potential‐dependent Ca 2+ uptake, presumably via the MCU (Okubo et al . ). The mitochondrial Na + /Ca 2+ exchanger (NCLX) mediates Ca 2+ efflux to the cytosol (Palty et al .…”