The Molecular Diversity of TRPC Channels and Related Proteins

The cytosolic Ca²⁺ serves an intracellular mediator for many extracellular signals. The receptors coupling via heterotrimeric G proteins to different isoforms of phospholipase C (PLC) leads to the breakdown of phosphoinositide to inositol 1,4,5-phosphate (InsP₃) and diacylglycerol (DAG).

After this reaction, a biphasic increase in cytosolic Ca²⁺ concentration occurs. The first stage results from transient InsP₃-mediated release from intracellular stores. The second, more sustained phase, results from store-operated activation of Ca²⁺-permeable membrane channels.

The suggested role of this phase is refilling the intracellular Ca²⁺ stores or prolonging the response^22-24.

Depending on the cell type, the second-phase Ca²⁺ channels may vary from highly selective Ca²⁺ channels to nonselective Ca²⁺-permeable channels³. The best studied store-operated Ca²⁺ current is Ca²⁺-release activated Ca²⁺ channel (CRAC), originally described in mast cells and T lymphocytes^1,2.

The first step for molecular identification of storeoperated Ca²⁺ channels came from the study of Drosophila photoreceptors. In contrast to vertebrates, in insects, light activation of rhodopsin is coupled via a G protein to PLC, which leads to a biphasic Ca²⁺ entry. A mutant called trp (transient receptor potential) was shown to lack the second phase, light-induced current (LIC). It has been shown that trp codes a Ca²⁺-selective channel protein, TRP. Later, another closely related channel, TRPL, has been cloned. It forms a less selective Ca²⁺-permeable channel^{20, 21}.

TRP channels (TRPCs) belong to the superfamily of cation channels with six transmembrane segments (see below). This superfamily includes voltage-gated K⁺, Na⁺, and Ca²⁺ channels, as well as cyclic nucleotide gated (CNG) channels, hyperpolarization-activated cyclic nucleotide gated (HCN) channels, and the most closely related group, polycystins (PKD)^37,38 TRPC contain specific ankyrin domains in the cytosolic N-terminal part and proline-rich motif in the cytosolic C-terminal part.

The search of Caenorhabditis elegans genome database revealed 13 TRPC genes, that can be divided into three families⁴. The first is a family of long TRPC (LTRPC), with a reading frame of ~1600 residues. The second, short TRPC (STRPC), has a reading frame of ~900 residues. The more distant family, OTRPC, named after the homology with osm-9, the first cloned member of this family, also has a ~900 residue reading frame.

The suggested classification, based on C. elegans genome, seems to apply widely⁴. The Drosophila TRP and TRPL channels thus belong to STRPC family.

In mammals, seven members of this family, named TRPC1-7, have been cloned. While this family is more studied then the others, most data for mammalian TRPCs is based on heterologous expression system. There is good agreement that TRPCs are activated downstream of G-protein-coupled receptors, which induce PLC-mediated phosphoinositide breakdown. However, the downstream signaling pathways that finally activate TRPCs remain highly controversial. For nearly all of the functionally expressed TRPCs, there is at leas tone report proposing a store-operated mechanism of activation^5-11. On the other hand, there is growing evidence for the involvement of store-independent pathways in the regulation of TRPC3^12-15, TRPC5¹⁶, TRPC6^15,17, and TRPC7¹⁸. For TRPL, TRPC3, and TRPC6, direct activators have been identified that stimulate the channels in a membrane-confined manner. Polyunsaturated fatty acids were shown to gate Drosophila TRPL¹⁹and DG to activate TRPC3, TRPC6, and TRPC7^15,18.

The tissue localization of TRPC differs greatly among the family members. TRPC1 is expressed ubiquitously, while TRPC3, TRPC4 and TRPC5 are almost exclusively localized in the brain¹¹. In rat, TRPC2 is exclusively expressed in vomeronasal organ. Interestingly, in humans TRPC2 is a pseudogene, possibly in agreement with the lack of vomeronasal organ in higher primates²⁹.

The C. elegans member of OTRPC family, osm-9, is involved in responses to odorants, high osmotic strength, and mechanical stimulation³⁶. Similarly, the mammalian members of this family, VR1³³ and VRL1³⁴, form non-selective Ca²⁺-permeable channels activated by heat and other pain-producing stimuli³⁵. Other members of this family, CaT1 (ECaC)^30,31 and CaT2³⁹, seem to be involved in Ca²⁺ transport in epithelial cells³².

The function of the LTRPC family channels is completely unknown. One of mammalian LTRPC, melastatin, is found in melanocytes and its level is decreased in metastatic cells²⁵. Another one, called TRPC7, but different from TRPC7 belonging to STRPC family¹⁹, has been recently described²⁷. Another one, named MTR1, has been recently cloned²⁶.

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