Calcium Regulates Podocyte Actin Dynamics Anna Greka, MD, PhD, and Peter Mundel, MD Summary: Ca 2+ -mediated remodeling of the actin cytoskeleton is a dynamic process that regulates cell motility through the modulation of rho guanosine triphosphatase (GTPase) signaling. Kidney podocytes are unique, pericyte-like cells with a complex cellular organization consisting of a cell body, major processes, and foot processes (FPs). The FPs form a characteristic interdigitating pattern with FPs of neighboring podocytes, leaving in between filtration slits that are covered by the slit diaphragm (SD). The actin-based FP and the SD form the final barrier to proteinuria. Mutations affecting several podocyte proteins cause disruption of the filtration barrier and rearrangement of the highly dynamic podocyte actin cytoskeleton. Proteins regulating the plasticity of the podocyte actin cytoskeleton are therefore of critical importance for sustained kidney barrier function. Dynamic regulation of the actin-based contractile apparatus in podocyte FPs is essential for sustained kidney filter function. Thus, the podocyte represents an excellent model system to study calcium signaling and actin dynamics in a physiologic context. Here, we discuss the regulation of podocyte actin dynamics by angiotensin or bradykinin-mediated calcium influx and downstream Rho GTPase signaling pathways and how these pathways are operative in other cells including fibroblasts and cancer cells. Semin Nephrol 32:319-326 © 2012 Elsevier Inc. All rights reserved. Keywords: Cdc42, fibroblasts, synaptopodin, TRPC channels, TRPC5, TRPC6, Rac1, RhoA, tropomyosin C a 2+ crosses the plasma membrane of cells to me- diate a multitude of vital cellular functions such as cell motility, contraction, ion channel gating, vesi- cle secretion, gene transcription, and cell death 1-4 (Fig. 1). Ca 2+ channels on the plasma membrane can control the precise timing and location of each Ca 2+ signal, to secure its specificity for each of these important cellular functions (Fig. 1). 1-4 In all cells, including podocytes, the levels of free cytoplasmic Ca 2+ are extremely low, in the 20 –300 nM range. 1 Podocytes possess homeostasis mechanisms to buf- fer and control the levels of free intracellular Ca 2+ (Fig. 1): the Na + -Ca 2+ exchanger (NCX) and the ATP dependent plasma membrane Ca 2+ pump (PMCA) work to ensure the efflux of excess Ca 2+ ions, the Ca 2+ buffers (the calbindins and parvalbumin) sequester excess Ca 2+ ions and the podo- cyte’s internal Ca 2+ stores (the endoplasmic reticulum (ER) and the mitochondria) also actively capture excess Ca 2+ ions to ensure that the cytoplasmic Ca 2+ levels remain low. 1 CALCIUM AND THE CYTOSKELETON In the late 1800s, Sydney Ringer 6 published seminal reports establishing the relative importance, both qualitative and quantitative, of calcium ions in cardiac conduction. He postulated a “saline” instrumental for life, which is now widely known as “Ringer’s solution.” This was the first glimpse of the importance of Ca 2+ signaling in cellular motility. In muscle cells, Ca 2+ entering the myoplasm is tightly and selectively sequestered by troponin’s Ca 2+ bind- ing sites. 7 Together with calmodulin, these proteins have evolved into powerful detectors of submicromolar changes in cytosolic [Ca 2+ ]. 7 Ca 2+ binding leads to the activation of enzymes, and thus the tiny Ca 2+ signal is amplified to match the potency of entire proteins. For example, activation of the actomyosin adenosine triphosphatase results in shortening of the contractile filament, an essential step in muscle con- traction, and activation of the Ca 2+ -calmodulin– dependent kinase leads to phosphorylation of a plethora of downstream signaling molecules. Indeed, the shape of each cell is de- pendent on the activities of Ca 2+ /calmodulin through con- trol of myosin’s interaction with actin filaments. 4 Vascular smooth muscle cells contract when voltage gated Ca 2+ channels open, Ca 2+ bound calmodulin activates myosin light chain kinase to phosphorylate the myosin head light chain and trigger myosin adenosine triphosphatase activity, leading to contraction. 4-7 In a matter of millisec- onds, homeostatic mechanisms prevail, the cytosolic [Ca 2+ ] decreases, and myosin light chain kinase phosphatases now mediate smooth muscle relaxation. 8-10 The theme that emerges from these conserved pathways is that Ca 2+ is a selective activator of kinases that mediate contraction, and that a decline or change in localized [Ca 2+ ] may fuel phos- phatases to mediate relaxation and/or increased motility. The Ca 2+ -mediated activation of downstream enzymatic activity to promote changes in cell morphology is illustrated further in cardiac hypertrophy models. 11 Increases in cyto- solic [Ca 2+ ] in response to vasoactive hormones such as endothelin and angiotensin II (Ang II) contribute to cardi- Harvard Medical School, Boston, MA. Division of Nephrology, Department of Medicine, Massachusetts General Hospital, Boston, MA. Financial support: Supported in part by an American Society of Nephrology Gottschalk Grant and a National Institutes of Health grant (DK083511 to A.G., and DK57683 and DK062472 to P.M.). Conflict of interest statement: none. Address reprint requests to Anna Greka, MD, PhD, Division of Ne- phrology, Massachusetts General Hospital and Harvard Medical School, 149 13th St, Charlestown, MA 02129. E-mail: greka.anna@ mgh.harvard.edu 0270-9295/ - see front matter © 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.semnephrol.2012.06.003 Seminars in Nephrology, Vol 32, No 4, July 2012, pp 319-326 319