TRPC channels and their implication in neurological diseases

Abstract

Calcium is an essential intracellular messenger and serves critical cellular functions in both excitable and non-excitable cells. Most of the physiological functions in these cells are uniquely regulated by changes in cytosolic Ca2+ levels ([Ca2+](i)), which are achieved via various mechanisms. One of these mechanism(s) is activated by the release of Ca2+ from the endoplasmic reticulum (ER), followed by Ca2+ influx across the plasma membrane (PM). Activation of PM Ca2+ channel is essential for not only refilling of the ER Ca2+ stores, but is also critical for maintaining [Ca2+](i) that regulates biological functions, such as neurosecretion, sensation, long term potentiation, synaptic plasticity, gene regulation, as well as cellular growth and differentiation. Alterations in Ca2+ homeostasis have been suggested in the onset/progression of neurological diseases, such as Parkinson's, Alzheimer's, bipolar disorder, and Huntington's. Available data on transient receptor potential conical (TRPC) protein indicate that these proteins initiate Ca2+ entry pathways and are essential in maintaining cytosolic, ER, and mitochondrial Ca2+ levels. A number of biological functions have been assigned to these TRPC proteins. Silencing of TRPC1 and TRPC3 has been shown to inhibit neuronal proliferation and loss of TRPC1 is implicated in neurodegeneration. Thus, TRPC channels not only contribute towards normal physiological processes, but are also implicated in several human pathological conditions. Overall, it is suggested that these channels could be used as potential therapeutic targets for many of these neurological diseases. Thus, in this review we have focused on the functional implication of TRPC channels in neuronal cells along with the elucidation of their role in neurodegeneration.

Fig. (1). Activation and functional regulation of TRPC channels

Binding of an agonist (Ach) to the G protein coupled receptor stimulates G protein, which activates PLCβ, causing the hydrolysis of phosphatidylinositol (4,5) bisphosphate (PIP₂) to inositol (1,4,5)-triphosphate (IP₃) and diacylglycerol (DAG). IP₃ binds to IP₃R, a ligand gated ion channel, which leads to the release of Ca²⁺ from the internal ER stores. Depletion of Ca²⁺ from the internal stores in turn allows STIM1 to aggregate, followed by the activation of the TRPC1, 4 or ORAI Ca²⁺ channels in the plasma membrane, which allows Ca²⁺ to enter the cell. Importantly, these signaling molecules are associated within lipid raft domains, which provide the platform for protein-protein interactions and stimulate activation of TRPC channels.