TRPC Channels Function Independently Of STIM1 And Orai1

Abstract

Recent studies have defined roles for STIM1 and Orai1 as calcium sensor and calcium channel, respectively, for Ca 2+ -release activated Ca 2+ (CRAC) channels, channels underlying store-operated Ca 2+ entry (SOCE). In addition, these proteins have been suggested to function in signalling and constructing other channels with biophysical properties distinct from the CRAC channels. Using the human kidney cell line, HEK293, we examined the hypothesis that STIM1 can interact with and regulate members of a family of non-selective cation channels (TRPC) which have been suggested to also function in SOCE pathways under certain conditions. Our data reveal no role for either STIM1 or Orai1 in signalling of TRPC channels. Specifically, Ca 2+ entry seen after carbachol treatment in cells transiently expressing TRPC1, TRPC3, TRPC5 or TRPC6 was not enhanced by the co-expression of STIM1. Further, knockdown of STIM1 in cells expressing TRPC5 did not reduce TRPC5 activity, in contrast to one published report. We previously reported in stable TRPC7 cells a Ca 2+ entry which was dependent on TRPC7 and appeared store-operated. However, we show here that this TRPC7-mediated entry was also not dependent on either STIM1 or Orai1, as determined by RNA interference (RNAi) and expression of a constitutively active mutant of STIM1. Further, we determined that this entry was not actually store-operated, but instead TRPC7 activity which appears to be regulated by SERCA. Importantly, endogenous TRPC activity was also not regulated by STIM1. In vascular smooth muscle cells, arginine-vasopressin (AVP) activated non-selective cation currents associated with TRPC6 activity were not affected by RNAi knockdown of STIM1, while SOCE was largely inhibited. Finally, disruption of lipid rafts significantly attenuated TRPC3 activity, while having no effect on STIM1 localization or the development of I CRAC . Also, STIM1 punctae were found to localize in regions distinct from lipid rafts. This suggests that TRPC signalling and STIM1/Orai1 signalling occur in distinct plasma membrane domains . Thus, TRPC channels appear to be activated by mechanisms dependent on phospholipase C which do not involve the Ca 2+ sensor, STIM1.

Store-independent pathways for cytosolic STIM1 clustering in the regulation of store-operated Ca2+ influx

“…The rearrangement of microtubules in the regulation of SOCE has also been demonstrated by the application of colchicine [54][55][56], however, colchicine has no effect on TG-induced STIM1 movement. In addition, disruption of lipid rafts in the PM does not change the STIM1 movement [57].…”

Morphological and functional aspects of STIM1-dependent assembly and disassembly of store-operated calcium entry complexes

“…Store depletion promoted the recruitment of STIM1 to ER structures facing lipid raft domains enriched in the TRPC (canonical transient receptor potential) 1 cation channel, and both SOCE and STIM1 puncta formation were prevented by the disruption of lipid rafts with methyl-β-cyclodextrin or Filipin-III [55]. The involvement of TRPC channels in SOCE is still controversial, with several groups reporting that TRPC channels interact with STIM1 and/or with Orai channels and therefore contribute to SOCE influx [56,57], and others claiming that TRPC channels function independently of STIM and Orai [58]. Membrane lipid rafts may act as a switching platform that convert TRPC channels from receptor-operated channels into storeoperated channels by favouring their interaction with cortical ER structures enriched in STIM1 and facing the lipid rafts [59].…”

Pharmacology of receptor operated calcium entry in human neutrophils

“…Neutrophil motility and migration is dependent on elevations in [Ca 2+ ] i , and in FMLP-stimulated neutrophils, inhibition of MLCK by ML-9 decreases cell motility [14]. In a variety of cultured cell lines and endothelial cells, it has been demonstrated that ML-9 inhibits thapsigargin induced Ca 2+ release and hence SOCE [15][16][17][18]. Recently, using human neutrophils, we demonstrated that ML-9 inhibits Ca 2+ influx stimulated by thapsigargin but not by the agonists FMLP, PAF, or LTB 4 [7].…”