Store-independent modulation of Ca2+ entry through Orai by Septin 7

Abstract

Orai channels are required for store-operated Ca2+ entry (SOCE) in multiple cell types. Septins are a class of GTP-binding proteins that function as diffusion barriers in cells. Here we show that Septin 7 acts as a ‘molecular brake’ on activation of Orai channels in Drosophila neurons. Lowering Septin 7 levels results in dOrai-mediated Ca2+ entry and higher cytosolic Ca2+ in resting neurons. This Ca2+ entry is independent of depletion of endoplasmic reticulum Ca2+ stores and Ca2+ release through the inositol-1,4,5-trisphosphate receptor. Importantly, store-independent Ca2+ entry through Orai compensates for reduced SOCE in the Drosophila flight circuit. Moreover, overexpression of Septin 7 reduces both SOCE and flight duration, supporting its role as a negative regulator of Orai channel function in vivo. Septin 7 levels in neurons can, therefore, alter neural circuit function by modulating Orai function and Ca2+ homeostasis.

Mutant IP3 receptors attenuate store-operated Ca2+ entry by destabilizing STIM–Orai interactions in Drosophila neurons

“…S3A,B" type="url"/>) (Agrawal et al, 2010). A similar restoration of SOCE upon overexpression of STIM has been reported in neurons in which IP 3 R expression was reduced by small interfering RNA (siRNA) (Deb et al, 2016). Hence, even though mutant IP 3 Rs do not affect expression of STIM or Orai, the attenuated SOCE can be compensated for by overexpressing STIM and Orai (Fig.…”

Meeting report – shining light on septins

“…First, septins regulate neuronal activity by modulating the influx of Ca 2+ . Gaiti Hasan (NCBS, Bengaluru, India) presented unpublished data showing that septins of the SEPT2 subgroup act as positive regulators of the Orai channel in Drosophila melanogaster neurons, expanding the previously reported role of SEPT7 as a negative regulator of Orai channels (Deb et al, 2016).…”

“…Septins modulate dendritic branching and morphogenesis (Xie et al, 2007), spine formation (Tada et al, 2007) and diffusion of membrane-bound proteins into the spine (Ewers et al, 2014) as well as the excitability of neurons (Deb et al, 2016). Several interesting new results were presented at the workshop that broaden our understanding of how septins regulate neuronal structure and function.…”

The Interplay between Cytoskeleton and Calcium Dynamics

“…Knockdown of SEPT4 gene expression reduced SOCE without afecting ER-Ca ] i locally, an event required for the phosphorylation of Src, PYK2 and FAK kinases, as well as MLC2, which are well-known modulators of invadopodia formation, focal adhesion turnover and actomyosin contractility. entry, suggesting that, in Drosophila neurons, Septin 7 is a negative regulator of dOrai channel function [53]. These authors stated that hetero-hexamers septin ilaments closely associate with the PM and near the ER in resting neurons and that the reduction of SEPT7 results in breaks in the linear septin ilaments present in wild-type cells, leading to the formation of shorter septin ilaments.…”

“…ER-Ca 2+ store depletion reorganizes these ilaments, moving STIM1 to the peripheral ER, promoting the coupling of STIM/dOrai and the activation of Ca 2+ entry through dOrai (SOCE). Shorter septin ilaments due to SEPT7 knockdown leads to STIM1 recruitment to the peripheral ER in resting neurons and the activation of dOrai, resulting in a store-independent activation of dOrai [53].…”