Balancing calcium signals through TRPC5 and TRPC6 in podocytes.

Division of Nephrology, Massachusetts General Hospital and Harvard Medical School, Charlestown, MA 02129, USA.
Journal of the American Society of Nephrology (Impact Factor: 9.47). 11/2011; 22(11):1969-80. DOI: 10.1681/ASN.2011040370
Source: PubMed

ABSTRACT Calcium (Ca(2+)) ions are important mediators of cellular homeostasis owing to their ability to elicit a dynamic, transient, and tightly regulated range of biochemical responses. More than a decade ago, a nonselective, Ca(2+)-permeable, cationic conductance was identified in podocytes downstream of angiotensin II (Ang II) signaling, but its molecular structure remained elusive. Six years ago, transient receptor potential canonical 6 (TRPC6) mutations were found in families with hereditary FSGS, and TRPC5 and TRPC6 channels are now known as the Ca(2+) influx pathways for this previously described, nonselective, cationic current in podocytes. Ang II activation engages this Ca(2+) influx to modulate the actin cytoskeleton in podocytes. These discoveries dovetail with previously described regulation of actin dynamics by the Ca(2+)-activated phosphatase, calcineurin, and the emergence of Rho GTPases as critical regulators of podocyte function in health and disease. Understanding the interconnected signaling regulated by Ca(2+) currents offers potential new therapeutic targets and highlights the notion that synergistic therapies targeting multiple levels of biochemistry may be useful in treating proteinuric kidney disease.

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    ABSTRACT: TRPC5 is a nonselective, Ca(2+) permeable cation channel which belongs to the large family of transient receptor potential channels. It is predominantly found in the central nervous system with a high expression density in the hippocampus, the amygdala and the frontal cortex. Several studies confirm that TRPC5 channels are implicated in the regulation of neurite length and growth cone morphology. We identified clemizole as a novel inhibitor of TRPC5 channels. Clemizole efficiently blocks TRPC5 currents and Ca(2+) entry in the low micromolar range (IC50 = 1.0 - 1.3 μM), as determined by fluorometric [Ca(2+)]i measurements and patch clamp recordings. Clemizole blocks TRPC5 currents irrespectively of the mode of activation, e.g. stimulation of GPCR, hypoosmotic buffer conditions or by the direct activator riluzole. Electrophysiological whole cell recordings revealed that the block was mostly reversible. Moreover, clemizole was still effective in blocking TRPC5 single channels in excised inside-out membrane patches, hinting to a direct block of TRPC5 by clemizole. Based on fluorometric [Ca(2+)]i measurements, clemizole exhibits a 6-fold selectivity for TRPC5 over TRPC4β (IC50 = 6.4 μM), the closest structural relative of TRPC5 and an almost 10-fold selectivity over TRPC3 (IC50 = 9.1 μM) and TRPC6 (IC50 = 11.3 μM). TRPM3 and M8 as well as TRPV1, V2, V3 and V4 channels were only weakly affected by markedly higher clemizole concentrations. Clemizole was not only effective in blocking heterologously expressed TRPC5 homomers but also TRPC1:TRPC5 heteromers as well as native TRPC5-like currents in the U-87 glioblastoma cell line.
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