TRPC6 channels and their binding partners in podocytes: role in glomerular filtration and pathophysiology

Abstract

Loss or dysfunction of podocytes is a major cause of glomerular kidney disease. Several genetic forms of glomerular disease are caused by mutations in genes that encode structural elements of the slit diaphragm or the underlying cytoskeleton of podocyte foot processes. The recent discovery that gain-of-function mutations in Ca(2+)-permeable canonical transient receptor potential-6 channels (TRPC6) underlie a subset of familial forms of focal segmental glomerulosclerosis (FSGS) has focused attention on the basic cellular physiology of podocytes. Several recent studies have examined the role of Ca(2+) dynamics in normal podocyte function and their possible contributions to glomerular disease. This review summarizes the properties of TRPC6 and related channels, focusing on their permeation and gating properties, the nature of mutations associated with familial FSGS, and the role of TRPC channels in podocyte cell biology as well as in glomerular pathophysiology. TRPC6 interacts with several proteins in podocytes, including essential slit diaphragm proteins and mechanosensitive large-conductance Ca(2+)-activated K(+) channels. The signaling dynamics controlling ion channel function and localization in podocytes appear to be quite complex.

Fig. 1.

A: domain structure of transient receptor potential-6 (TRPC6) channels. Boxes indicate human mutations known to cause focal segmental glomerular sclerosis (FSGS). All of the known disease-causing mutations are located in the cytoplasmic NH₂ and COOH terminals of TRPC6. B: immunogold localization of TRPC6, TRPC1, and TRPC5 to podocyte foot processes (arrows). The micrograph of TRPC6 expression also shows that it is present along a primary process.

Fig. 2.

Large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels in podocytes. A: topography of the pore-forming subunits of BK_Ca channels (Slo1 proteins) showing Ca²⁺-binding domains in the large cytoplasmic COOH terminal. Also shown is a noncanonical SH3 domain that mediates stretch-sensitive gating by interactions with actin-binding proteins. B: colocalization of Slo1 (red) and synaptopodin (green) in a mouse glomerulus revealed by double-label immunofluorescence and a merged image. Areas of colocalization indicate expression in podocytes. Slo1 also appears to be expressed in the mesangium, where there is no synaptopodin. C: model for coordinated activation of TRPC6 and BK_Ca. Although Ca²⁺ can permeate TRPC6, it also causes pore blockade and with depolarization TRPC6 behaves as a monovalent cation channel. However, as a result of physical interactions and resulting colocalization, Ca²⁺ influx through TRPC6 can cause activation of BK_Ca. This may prevent membrane depolarization, thereby providing a driving force that can sustain Ca²⁺ infux through TRPC6.

Fig. 3.

Models of TRPC6 and BK_Ca function in podocytes. These channels form a complex with each other in the plasma membrane, which ensures that Ca²⁺ influx through TRPC6 can cause coordinated activation of BK_Ca. This serves to prevent membrane depolarization and thereby maintain the Ca²⁺ permeability of TRPC6. In foot processes (left), these channels also interact with nephrin, podocin (P) and with the underlying actin cytoskeleton, which allows them to function as mechanosensors and to allow the resulting Ca²⁺ influx to modulate nephrin signaling and cytoskeletal dynamics acting in part through small GTPases and proteins such as Nck and CD2AP (shown schematically as a black circle). BK_Ca channels are also mechanosensitive through interactions with the cytoskeleton mediated by cortactin (blue circle) or other actin-binding proteins. TRPC6 channels can become active in response to G protein-mediated signaling cascades associated with PLC activation, for example, during angiotensin signaling. The entire slit diaphragm complex may be also regulated by tyrosine kinases such as Fyn. Other TRPC family members are expressed in podocytes, and their functions are unknown. In the cell body (right), they may form heteromeric complexes with TRPC6 and in this way function as Ca²⁺-store operated channels in G protein signaling cascades associated with PLC activation, possibly in complex with other proteins such as STIM1 and the 1,4,5-trisphosphate (IP₃) receptor. Influx of Ca²⁺ in the cell body can regulate gene expression in podocytes. Excessive Ca²⁺ influx through TRPC6 channels as a result of mutations or other pathological events can ultimately lead to loss of podocytes through apoptosis as well as detachment from the glomerular basement membrane (GBM). Foot process effacement and podocyte hypertrophy may represent an adaptive response of the remaining podocytes to maintain coverage of the GBM.