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EDITORIAL FOCUS
Department of Pharmacology and Toxicology, Michigan State University, East Lansing, Michigan
CALCIUM ENTRY TRIGGERED BY loss of Ca2+ from endoplasmic reticulum Ca2+ stores in vascular smooth muscle cells was first described by Casteels and Droogmans (6) 25 years ago. Subsequent studies have demonstrated that this is a ubiquitous process occurring in all eukaryotes from yeast to humans (see recent reviews for details: Refs. 4, 21, 31). However, the molecular identity of the ion channels that underlie store-operated Ca2+ (SOC) entry remains one of the significant unanswered questions in this important area of research. Compelling evidence has been presented that members of the canonical transient receptor potential family of channels, particularly TRPC1, are involved in SOC entry in vascular smooth muscle cells (5, 11, 28, 33) and in other cell types (see Ref. 21 for numerous references). However, it is also clear that TRPC1 alone is unlikely to represent the SOC channel (SOCC) expressed in these cells (4, 21). The study by Xu et al. (34) in this issue of the American Journal of Physiology, Heart and Circulatory Physiology, confirms and extends a preliminary report by this same group (35) that TRPC5 is another component of vascular smooth muscle SOCC. These findings are important for at least two reasons. First, the identity of an additional component of SOC entry in native vascular smooth muscle cells is a significant advance in our understanding of this important area of research. While TRP channels long have been implicated as SOCC, the characteristics of channels formed by individual TRP channel isoforms have poorly matched the characteristics of SOCC expressed in native cells. As demonstrated by Xu and colleagues (34, 35), the whole cell currents, ion selectivity, and pharmacology of SOCC in rabbit cerebral arteriolar smooth muscle cells nicely match heterologously expressed TRPC1/TRPC5 heteromultimers, strongly supporting a role for both TRPC isoforms in SOC entry in this system. However, it also should be noted that the hypothesis that TRPC1/TRPC5 heteromers represent at least a component of SOCC in native vascular smooth muscle cells has not been critically tested (see below).
The second important contribution of the study by Xu et al. (34) is methodological. The use of the blocking antibody strategy to study the function of specific TRP channels, an approach championed by this group (5, 33–35), provides what appears to be a general strategy to dissect out the contribution of specific channel proteins involved in ionic currents and functional responses. While used successfully to demonstrate roles for TRPC1 and TRPC5 in SOC entry in vascular smooth muscle cells (5, 33–35), it should be recognized that this approach was first used as a tool to study voltage-gated K+ channels (38) and appears also to be effective in blockade of voltage-gated Na+ channels (35). Thus generation of antibodies directed at extracellular epitopes adjacent to the pore region of ion channels provides a means to study the specific function of a variety of ion channels and, importantly, the role played by distinct channel isoforms. This is a welcomed addition to the pharmacological armamentarium for the study of TRP channels where few selective agents are available. It also provides a means to confirm results derived from gene silencing approaches, which, while theoretically specific, have the potential to produced undesired effects due to alterations/compensations in expression or trafficking of other channel subunits or disruption of signaling complexes.
While the findings of Xu et al. (34) bring us closer to the identity of the molecular composition of vascular smooth muscle SOCC, many very important questions remain. First, as noted above, do TRPC1 and TRPC5 form heteromeric channels that constitute at least part of the structure of SOCC in native vascular smooth muscle cells? While the data presented (34) are consistent with this hypothesis, and studies in other systems support this possibility (26, 27), the hypothesis that TRPC1 and TRPC5 form heteromeric channels in native vascular smooth muscle cells has not been rigorously tested. This remains an important area for future study.
Second, in addition to TRPC1 and TRPC5, what additional subunits are required to reconstitute SOCC in vascular smooth muscle cells? Studies in other systems have implicated additional TRPC family members in SOC entry including TRPC3 (16, 36), TRPC4 (12, 26), and TRPC7 (36). In addition, TRPC1 has been shown to interact with TRPP2 (30), which also forms Ca2+-permeable channels (10). Excitingly, the unrelated proteins stromal interaction molecule 1 (STIM1) and Orai1 recently have been proposed as essential components of SOCC (19, 25) in other systems, and recent studies in platelets indicate that STIM1 interacts with TRPC1 and is involved in SOC entry (17). Given the similarities between Ca2+ handling in platelets and vascular smooth muscle cells, it would not be surprising that STIM1 and perhaps Orai1 also participate in SOC entry in vascular smooth muscle.
Third, how are these channels activated by depletion of Ca2+ stores? In murine aortic smooth muscle cells, it has been suggested that depletion of intracellular Ca2+ stores leads to formation of an as yet unidentified Ca2+ influx factor (CIF) that disinhibits Ca2+-insensitive phospholipase A2 (iPLA2), which, in turn, catalyzes the formation of lysophospholipids that activates SOCC (23). Consistent with this hypothesis and the study by Xu et al. (34), TRPC5 channels stably expressed in HEK-293 cells are activated by lysophospholipids, and lysophospholipids activate a nonselective cation current in murine aortic smooth muscle cells that is similar to that through TRPC1/TRPC5 heteromeric channels (8). On the other hand, considerable evidence has accumulated in other systems implicating inositol-1,4,5-trisphosphate receptors (IP3R) and interactions with TRPC1 in the mechanism underlying activation of SOCC (see Refs. 21 and 31 for numerous references). Future studies will be required to determine which mechanism is involved in native smooth muscle or whether both mechanisms may participate. Furthermore, TRPC1 channels have been demonstrated to interact with a large number of signaling proteins including calmodulin, Gq/11, PLC-
, PLC-
, PKC-
, RhoA, and several types of receptors (see Refs. 2 and 3 for numerous references), and Ca2+ influx through TRPC5 channels expressed in HEK-293 cells is activated by a number of stimuli (37). These data support the idea that SOCC comprised of these channels may be activated by several different mechanisms even in the same cell, consistent with studies in the literature (1).
The final, and perhaps most important, unanswered question is the physiological and pathophysiological role played by SOC entry in the regulation of vascular smooth muscle tone in arterioles and resistance arteries that are involved critically in the regulation of blood flow and blood pressure. While SOC entry has been demonstrated to consistently elicit increases in Ca2+ and contraction of small pulmonary arteries (18, 24), Ca2+ influx through SOCC (in the absence of activation of G protein-coupled receptors) raises intracellular Ca2+ but does not consistently support smooth muscle contraction in systemic arteries (20, 24, 29) or arterioles (9, 34). These latter observations suggest that SOC entry may be functionally compartmentalized and support refilling of endoplasmic reticulum Ca2+ stores or provide a basis for selective activation of Ca2+-dependent cell functions other than contraction, such as gene expression. Pharmacological inhibition of SOC entry in skeletal muscle arterioles with 2-aminoethoxydiphenyl borate (2-APB) suggests a role for SOC entry in pressure-induced smooth muscle tone, myogenic reactivity, and 1-adrenergic agonist-induced vasoconstriction (22). However, 2-APB can block a number of TRPC channels, including TRPC6 (15), which, while likely involved in receptor-mediated vasoconstriction (13) and pressure-induced smooth muscle tone (32), are not a component of SOCC. Thus the role played by SOC entry in the regulation of pressure-induced tone and agonist-induced vasoconstriction remains to be established. However, it is interesting to note that SOC entry appears to be elevated in renal arteriolar vascular smooth muscle cells isolated from hypertensive animals (7), and SOC entry appears to be upregulated in proliferating vascular smooth muscle cells (11, 14, 28), supporting an important role for SOCC in vascular pathophysiology.
Thus, although the findings of Xu et al. (34) importantly move the field forward by demonstrating that TRPC5 is a membrane-spanning component of SOCC in native vascular smooth muscle cells, there remain a large number of unanswered questions that should prove fertile ground for future investigation.
GRANTS
This work was supported by National Heart, Lung, Blood Institute Grant HL-32469.
FOOTNOTES
Address for reprint requests and other correspondence: W. F. Jackson, Dept. of Pharmacology and Toxicology, Michigan State Univ., B420 Life Sciences Bldg., East Lansing, MI 48824 (e-mail: jacks783{at}msu.edu)
REFERENCES
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