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

Abstract

Astrocytes are actively involved in the physiological and pathophysiological functions of the brain. Ca²⁺ release from the endoplasmic reticulum (ER) is considered to be crucial for the regulation of astrocytic functions. Inositol 1,4,5-trisphosphate receptor type 2 (IP₃R2)-knockout (KO) mice are reportedly devoid of astrocytic Ca²⁺ signaling, and thus widely used to explore the significance of astrocytic Ca²⁺ signaling. However, functional deficits in IP₃R2-KO mice have been found in some reports, but not in others. To address this controversy, we re-evaluated the assumption that Ca²⁺ release from the ER is abolished in IP₃R2-KO astrocytes. We expressed the ER luminal Ca²⁺ indicator G-CEPIA1er in astrocytes to directly visualize 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.