A better brain? Alternative spliced STIM2 in hominoids arises with synapse formation and creates a gain-of-function variant

Balanced Ca2+ homeostasis is essential for cellular functions. STIM2 mediated Store-Operated Ca2+ Entry (SOCE) regulates cytosolic and ER Ca2+ concentrations, stabilizes dendritic spine formation and drives presynaptic spontaneous transmission and ER stress in neurons. Recently identified alternative spliced variants expand the STIM protein repertoire, uncover unique functions and facilitate our understanding of tissue specific regulation of SOCE. Here, we describe an addition to this repertoire, a unique short STIM2 variant (STIM2.3/STIM2G) present only in old world monkeys and humans with expression in humans starting with the beginning of brainwave activity and upon synapse formation within the cerebral cortex. In contrast to the short STIM1B variant, STIM2.3/STIM2G increases SOCE upon stimulation independently of specific spliced in residues. Basal cluster formation is reduced and analyses of several additional deletion and point mutations delineate the role of functional motifs for Ca2+ entry, NFAT activation and changes in neuronal gene expression. In addition, STIM2.3/STIM2G shows reduced binding and activation of the energy sensor AMPK. In the context of reduced STIM2.3 splicing seen in postmortem brains of patients with Huntingtons disease, our data suggests that STIM2.3/STIM2G is an important regulator of neuronal Ca2+ homeostasis, potentially involved in synapse formation/maintenance and evolutionary expansion of brain complexity.