Complex Actions of 2-Aminoethyldiphenyl Borate on Store-operated Calcium Entry

Abstract

Store-operated Ca2؉ entry (SOCE) is likely the most common mode of regulated influx of Ca 2؉ into cells. However, only a limited number of pharmacological agents have been shown to modulate this process. 2-Aminoethyldiphenyl borate (2-APB) is a widely used experimental tool that activates and then inhibits SOCE and the underlying calcium release-activated Ca 2؉ current (I CRAC ). The mechanism by which depleted stores activates SOCE involves complex cellular movements of an endoplasmic reticulum Ca 2؉ sensor, STIM1, which redistributes to puncta near the plasma membrane and, in some manner, activates plasma membrane channels comprising Orai1, -2, and -3 subunits. We show here that 2-APB blocks puncta formation of fluorescently tagged STIM1 in HEK293 cells. Accordingly, 2-APB also inhibited SOCE and I CRAC -like currents in cells coexpressing STIM1 with the CRAC channel subunit, Orai1, with similar potency. However, 2-APB inhibited STIM1 puncta formation less well in cells co-expressing Orai1, indicating that the inhibitory effects of 2-APB are not solely dependent upon STIM1 reversal. Further, 2-APB only partially inhibited SOCE and current in cells co-expressing STIM1 and Orai2 and activated sustained currents in HEK293 cells expressing Orai3 and STIM1. Interestingly, the Orai3-dependent currents activated by 2-APB showed large outward currents at potentials greater than ؉50 mV. Finally, Orai3, and to a lesser extent Orai1, could be directly activated by 2-APB, independently of internal Ca 2؉ stores and STIM1. These data reveal novel and complex actions of 2-APB effects on SOCE that can be attributed to effects on both STIM1 as well as Orai channel subunits.In many cell types, the activation of phospholipase C through G protein-coupled receptors liberates Ca 2ϩ from the lumen of the endoplasmic reticulum (ER current (I CRAC ), first described in mast cells (3) and since recorded in several cell types (4). Until recently, the mechanism by which I CRAC is activated by store depletion, as well as the channels themselves, was unknown. However, the discoveries of both STIM1 (5, 6) and Orai1 (CRACM1) (7-9) have revealed two key molecular components of the I CRAC -signaling pathway.It is now clear that STIM1 functions as the Ca 2ϩ sensor within the ER, whereas members of the family of Orai proteins (including Orai1, -2, and -3) function as pore-forming subunits of CRAC channels in the plasma membrane. When intracellular Ca 2ϩ stores are depleted, STIM1 rearranges from a fibrillar localization that depends on microtubules to discrete punctate structures near the plasma membrane (6, 10 -12). Remarkably, Orai1 channels also rearrange into punctate structures, in response to store depletion, that coincide with those formed by STIM1 (13-15). Thus, highly orchestrated molecular rearrangements underlie I CRAC activation.Overexpression of Orai1 together with STIM1 in HEK293 cells produces unusually large currents with biophysical properties similar to I CRAC (9, 16 -18), suggesting that either these two proteins are sufficien...

STIM protein coupling in the activation of Orai channels

“…In contrast to Orai3, Orai2-CFP expressed in S1/S2 Ϫ/Ϫ cells resulted in no 2-APB activation of Ca 2ϩ entry (Fig. 2I), consistent with previous reports revealing Orai2 is not directly activated by 2-APB (22,25).…”

“…2-APB is a powerful modifier of SOC function (16,(19)(20)(21)(22)(23)(24)(25)(26), and we reveal here that 2-APB induces rapid, profound, and either direct or very close interactions between the STIM and Orai proteins, locking the C terminus of STIM proteins together with Orai1 to facilitate complete functional channel coupling. We show that just the C-terminal coiled-coil domain of STIM proteins is sufficient for Orai1 activation.…”

“…The action of 2-APB on SOCs is complex; recent studies suggest 2-APB may directly activate Orai3 channels (22)(23)(24)(25), an effect thought to be independent of STIM1. We determined that the avian genome is missing the of STIM2 was not addressed.…”

“…Its actions are complex, enhancing SOC activation at 5-10 M, yet blocking it at 50 M (20, 21). Recent evidence emphasizes the remarkable effectiveness and the complexities of 2-APB in modifying STIM protein function and Orai channel gating (22)(23)(24)(25). Although its actions are enigmatic, they indicate 2-APB is a valuable tool in deciphering the STIM-Orai coupling mechanism.…”

Intracrine endothelin signaling evokes IP3-dependent increases in nucleoplasmic Ca2+ in adult cardiac myocytes

“…However, IP3R inhibitors such as 2-APB and the xestospongins have multiple cellular targets, including IP3R, store-operated Ca 2+ entry (SOC), and SERCA [99,100]. In addition, the effect of 2-APB on SOC is dosedependent: 30-50 μM 2-APB is inhibitory whereas lower concentrations of 2-APB stimulate SOC [101]. However, with this said, the 2-APB-sensitive component of the increase in [Ca 2+ ] n evoked by increasing the intracellular ET-1 concentration is likely mediated by IP3R: the identity of the 2-APB-insensitive channel remains to be determined.…”