Increased Vascular Smooth Muscle Contractility in TRPC6^−/− Mice

Abstract

Among the TRPC subfamily of TRP (classical transient receptor potential) channels, TRPC3, -6, and -7 are gated by signal transduction pathways that activate C-type phospholipases as well as by direct exposure to diacylglycerols. Since TRPC6 is highly expressed in pulmonary and vascular smooth muscle cells, it represents a likely molecular candidate for receptor-operated cation entry. To define the physiological role of TRPC6, we have developed a TRPC6-deficient mouse model. These mice showed an elevated blood pressure and enhanced agonist-induced contractility of isolated aortic rings as well as cerebral arteries. Smooth muscle cells of TRPC6-deficient mice have higher basal cation entry, increased TRPC-carried cation currents, and more depolarized membrane potentials. This higher basal cation entry, however, was completely abolished by the expression of a TRPC3-specific small interference RNA in primary TRPC6 ؊/؊ smooth muscle cells. Along these lines, the expression of TRPC3 in wild-type cells resulted in increased basal activity, while TRPC6 expression in TRPC6 ؊/؊ smooth muscle cells reduced basal cation influx. These findings imply that constitutively active TRPC3-type channels, which are up-regulated in TRPC6-deficient smooth muscle cells, are not able to functionally replace TRPC6. Thus, TRPC6 has distinct nonredundant roles in the control of vascular smooth muscle tone.The TRP (transient receptor potential) family of ion channels is a growing group of structurally and evolutionarily related cation channels formed of several subfamilies that include the TRPC, TRPM, and TRPV classes of channels (6, 22). TRP-type ion channels are presumed to be homo-or heterotetramers (13,14), each spanning the plasma membrane six times. The founding members of this channel family are the insect TRP and TRPL channels, which are responsible for photoreceptor depolarization in response to light. Mammalian TRPCs (C stands for canonical or classical) (23, 32) are the closest mammalian structural relatives of insect TRPs. Among the TRPC channels, TRPC3, -6, and -7 are 75% identical and gated by signal transduction pathways that activate C-type phospholipases (3, 32) as well as by direct exposure to diacylglycerols (DAG) (15). TRPC3, -6, and -7 interact physically and, upon coexpression, coassemble to form functional channels (14). Expression of TRPC3 and TRPC7 in HEK 293 cells, but not of TRPC6, reveals constitutively active cation channels that are permeable not only to monovalent but also to divalent cations such as Ca 2ϩ , Ba 2ϩ , and Mn 2ϩ (7,23,33). In contrast to members of other TRP families, the functional importance of most members of the TRPC subfamily is still poorly understood. A TRPC channel for which considerable evidence has accumulated for a specific role is TRPC6, which has been proposed to regulate smooth muscle function. The TRPC6 mRNA was originally isolated from mouse brain and was also identified in lung cells (4). By comparative biophysical characterization and gene suppression using antisense oligonucleot...

Imbalance and dysfunction of transient receptor potential channels contribute to the pathogenesis of hypertension

“…Furthermore, some multimerization data show that TRPC3/6 tetramers are formed in a heterologous expression system [42], in brain synaptosomes [43], and in polarized epithelial cells [44]. These findings may be compatible with the hypothesis that, in such heteromultimeric complexes, TRPC6 may suppress the basal activity of TRPC3, leading to a tightly receptor-regulated cation channel complex required for the physiological regu-lation of smooth muscle tone [40]. Recently, the studies indicate a critical role of TRPM4 in myogenic constriction of cerebral arteries.…”

“…In another study, the isolated aortic rings and cerebral arteries of TRPC6-deficient mice showed an enhanced agonist-induced constriction [40]. Likewise, TRPC6-deficient mice showed a higher basal cation entry in VSMC, increased TRPC-carried cation currents, and more depolarized membrane potentials because of constitutively active TRPC3 channels [40]. These data implicate the concept of the formation of TRPC3/6 heteromeric complexes with a tightly receptor-operated TRPC6 subunit suppressing TRPC3 basal activity [41].…”

“…The inhibition of TRPC6 has been found to decrease TRPC6 protein expression and greatly attenuate arterial smooth muscle depolarization and constriction caused by elevated pressure in intact cerebral arteries [39]. In another study, the isolated aortic rings and cerebral arteries of TRPC6-deficient mice showed an enhanced agonist-induced constriction [40]. Likewise, TRPC6-deficient mice showed a higher basal cation entry in VSMC, increased TRPC-carried cation currents, and more depolarized membrane potentials because of constitutively active TRPC3 channels [40].…”

Homer proteins in Ca2+ signaling by excitable and non-excitable cells

“…As indicated above, all Homer isoforms co-localize with mGluRs in the PSD and Homer expression is enriched in dendrites [26][27][28][29]. The localization of the Homer isoforms was further examined in the polarized pancreatic acinar cells and was found to be isoform-specific [30].…”

TRP Channels: Their Function and Potentiality as Drug Targets

“…In contrast, pressure overloadinduced cardiac hypertrophy is suppressed by double deletions of TRPC3/6 genes in C57BL6/J background mice, although single deletion of TRPC3 and TRPC6 genes never suppresses cardiac hypertrophy [ 151 ]. As TRPC3 and TRPC6 form heteromultimer channels and regulate agonist-and mechanical stretch-induced hypertrophic growth of rat neonatal cardiomyocytes [ 35 ] and mice lacking TRPC6 were reported to have mRNA upregulation for TRPC3 [ 152 ], TRPC3 and TRPC6 proteins may compensatively work with each other. TRPC6 is abundantly expressed in cardiac fi broblasts, and fi broblasts lacking the TRPC6 gene were refractory to transdifferentiation [ 149 ].…”