Role of the store‐operated calcium entry proteins Stim1 and Orai1 in muscarinic cholinergic receptor‐stimulated calcium oscillations in human embryonic kidney cells

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Abstract
We have investigated the nature of the Ca2+ entry supporting [Ca2+]i oscillations in human embryonic kidney (HEK293) cells by examining the roles of recently described store-operated Ca2+ entry proteins, Stim1 and Orai1. Knockdown of Stim1 by RNA interference (RNAi) reduced the frequency of [Ca2+]i oscillations in response to a low concentration of methacholine to the level seen in the absence of external Ca2+. However, knockdown of Stim1 did not block oscillations in canomical transient receptor potential 3 channel (TRPC3)-expressing cells and did not affect Ca2+ entry in response to arachidonic acid. The effects of knockdown of Stim1 could be reversed by inhibiting Ca2+ extrusion with a high concentration of Gd3+, or by rescuing the knockdown by overexpression of Stim1. Similarly, knockdown of Orai1 abrogated [Ca2+]i oscillations, and this was reversed by use of high concentrations of Gd3+; however, knockdown of Orai1 did not affect arachidonic acid-activated entry. RNAi targeting 34 members of the transient receptor potential (TRP) channel superfamily did not reveal a role for any of these channel proteins in store-operated Ca2+ entry in HEK293 cells. These findings indicate that the Ca2+ entry supporting [Ca2+]i oscillations in HEK293 cells depends upon the Ca2+ sensor, Stim1, and calcium release-activated Ca2+ channel protein, Orai1, and provide further support for our conclusion that it is the store-operated mechanism that plays the major role in this pathway.