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

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 (IP₃ R2) 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 IP₃ R2-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 IP₃ R2-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 IP₃ R2-KO astrocytes. This IP₃ R2-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 IP₃ R2-KO astrocytes.