Regulation of Store-Operated Ca²⁺Entry by IP₃Receptors Independent of Their Ability to Release Ca²⁺

Abstract

Loss of ER Ca2+ activates store-operated Ca2+ entry (SOCE) by causing STIM1 to unfurl domains that activate Orai1 channels in the plasma membrane at membrane contact sites (MCS). In human neurons, SOCE evoked by thapsigargin to deplete ER Ca2+ is attenuated by loss of IP3Rs, and restored by expression of IP3Rs even when they cannot release Ca2+, but only if the IP3Rs can bind IP3. In cells expressing poredead IP3Rs, IP3 enhances SOCE evoked by partial store depletion without enhancing Ca2+ release. Proximity ligation assays establish that IP3Rs enhance association of STIM1 with Orai1 in cells with empty stores; this requires an IP3-binding site, but not a pore. Over-expressing STIM1, or extended synaptotagmin-1 to exaggerate MCS, circumvents the need for IP3Rs. Thus, IP3 binding to IP3Rs stimulates SOCE by both mediating ER Ca2+ release and promoting STIM1-Orai1 interactions. Convergent regulation by IP3Rs may tune SOCE to respond selectively to IP3.

Signalling switches maintain intercellular communication in the vascular endothelium

“…Staurosporine‐induced Ca 2+ influx was absent in IP 3 R lacking its cytosolic C‐terminal tail (Khan et al, 2007). IP 3 Rs also facilitate interactions of active STIM1 and Orai to promote store‐operated Ca 2+ entry in a way that is also independent of Ca 2+ release from the store (Chakraborty et al, 2022; Thillaiappan et al, 2019). This additional role for IP 3 Rs is regulated by IP 3 but does not require a functional channel pore (Chakraborty et al, 2022).…”

“…IP 3 Rs also facilitate interactions of active STIM1 and Orai to promote store‐operated Ca 2+ entry in a way that is also independent of Ca 2+ release from the store (Chakraborty et al, 2022; Thillaiappan et al, 2019). This additional role for IP 3 Rs is regulated by IP 3 but does not require a functional channel pore (Chakraborty et al, 2022). These findings reveal links between IP 3 , IP 3 Rs and Ca 2+ signalling that are not mediated by the flux of Ca 2+ across the channel (Chakraborty et al, 2022; Khan et al, 2007).…”

“…This additional role for IP 3 Rs is regulated by IP 3 but does not require a functional channel pore (Chakraborty et al, 2022). These findings reveal links between IP 3 , IP 3 Rs and Ca 2+ signalling that are not mediated by the flux of Ca 2+ across the channel (Chakraborty et al, 2022; Khan et al, 2007). Ion flux‐independent functions of the IP 3 R also include a structural role of receptor in linking the endoplasmic reticulum and mitochondria (Bartok et al, 2019).…”

ER‐Ca2+ stores and the regulation of store‐operated Ca2+ entry in neurons

“…A mechanistic understanding of this regulatory mechanism was obtained by creating knockdowns and knockouts of IP 3 R1 in a human neuroblastoma cell line. Through rescue experiments and proximity ligation assays of STIM1 and Orai1 the authors demonstrate that enhanced STIM–Orai coupling requires IP 3 binding to the IP 3 R, but not an active Ca 2+ channel (Chakraborty et al., 2022). This raises the intriguing possibility that in neurons signals that generate IP 3 are coupled to SOCE through dual regulatory mechanisms of IP 3 ‐binding and ER‐Ca 2+ release, unlike single regulation of SOCE by ER‐Ca 2+ release through the RyR.…”

“…However, these results need to be reviewed carefully, given that complete deletion of all IP 3 R subtypes within a cell is likely to be compensated by mechanisms that change normal cell physiology and Ca 2+ homeostasis. Indeed, knockdown of the IP 3 R1 alone in combination with depletion of basal IP 3 levels significantly reduces SOCE in wild‐type HEK cells, which continue to express IP 3 R2 and IP 3 R3 (Chakraborty et al., 2022). Similar experiments with acute knockdown of single IP 3 R subtypes have so far not been attempted in immune cells to our knowledge.…”