Balancing calcium signals through TRPC5 and TRPC6 in podocytes.
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.Molecular pharmacology 08/2014; 86(5). DOI:10.1124/mol.114.093229 · 4.12 Impact Factor
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ABSTRACT: Background: Proteinuria is a cardinal sign of chronic kidney disease, which is a major healthcare problem that affects millions of people worldwide. Recent advances in molecular genetics and cell biology have revealed the podocyte as the primary functional regulator of the tri-layered glomerular filter. Since podocyte foot processes (FP) and their interposed slit diaphragms (SD) form the final barrier to protein loss, podocyte injury causes proteinuric kidney disease. Summary: A fundamental mechanism of proteinuric glomerular diseases is podocyte FP effacement and the loss of podocyte SD integrity, both of which involve the active rearrangement of the podocyte actin cytoskeleton. Initially, these early changes are reversible, but later can progress to cell detachment and death. Based on the importance of the actin cytoskeleton for podocyte development and the maintenance of the glomerular filter, podocyte research is heavily focused on studying actin’s molecular make-up and regulation. In this review we provide a comprehensive summary of the about 100 actin-associated proteins that have been described in podocytes to date, and we point out that so far only about one quarter of them have been shown to be functionally relevant for podocyte function in rodents or humans. Since actinmediated cell plasticity is a key feature of normal podocyte function, and alterations in actin dynamics appear to be a major driver in changing podocyte morphology and glomerular permeability, we discuss the current work on proteins and mechanisms that regulate actin polymerization and stress fiber contraction in podocyte FP in greater detail. Key Message: Without a doubt, the actin cytoskeleton is the key component of podocytes and proper glomerular filtration. Over the past 20 years many actin-associated proteins and actin-regulating mechanisms have been identified in podocytes. However, since most of these proteins are widely expressed and regulate actin in different cell types, it remains unclear if the podocyte actin cytoskeleton can be specifically targeted, and if and how actin-associated proteins can serve as novel drug targets in proteinuric kidney disease.Contributions to nephrology 07/2014; 2014(183):22-53. DOI:10.1159/000359923 · 1.53 Impact Factor
Nephrology Dialysis Transplantation 06/2014; DOI:10.1093/ndt/gfu214 · 3.49 Impact Factor