ABSTRACT
Glomerular visceral epithelial cells, also known as podocytes, are highly specialized epithelial cells that cover the outer layer of the glomerular basement membrane. Podocytes consist of cell bodies, major processes and foot processes (FP) of neighbouring cells, with the filtration slits bridged by the slit membrane between them. The function of podocytes is largely based on their specialized cell architecture and functions such as stabilization of glomerular capillaries and participation in the barrier function of the glomerular filter. Therefore, they form the final barrier to protein loss, which explains why podocyte injury is typically associated with marked proteinuria. Under pathological conditions, podocytes exhibit various changes. Among these changes, FP effacement represents the most characteristic change in cell shape of podocytes. FP effacement is dependent on disruption of the actin cytoskeletal network in the podocytes, The mechanisms of organization and re-organization of actin in the FP of podocytes are discussed in this review.
Subject(s)
Actin Cytoskeleton/physiology , Podocytes/pathology , Podocytes/physiology , Proteinuria/pathology , Proteinuria/physiopathology , Animals , HumansABSTRACT
Podocytes of the renal glomerulus are unique cells with a complex cellular organization consisting of a cell body, major processes and foot processes. Podocyte foot processes form a characteristic interdigitating pattern with foot processes of neighboring podocytes, leaving in between the filtration slits that are bridged by the glomerular slit diaphragm. The highly dynamic foot processes contain an actin-based contractile apparatus comparable to that of smooth muscle cells or pericytes. Mutations affecting several podocyte proteins lead to rearrangement of the actin cytoskeleton, disruption of the filtration barrier and subsequent renal disease. The fact that the dynamic regulation of the podocyte cytoskeleton is vital to kidney function has led to podocytes emerging as an excellent model system for studying actin cytoskeleton dynamics in a physiological context.
Subject(s)
Actins/metabolism , Cytoskeleton/metabolism , Podocytes , Adaptor Proteins, Signal Transducing/metabolism , Animals , Basement Membrane/metabolism , Basement Membrane/ultrastructure , Cadherins/metabolism , Calcium/metabolism , Cytoskeletal Proteins/metabolism , Humans , Membrane Proteins/genetics , Membrane Proteins/metabolism , Microfilament Proteins/metabolism , Models, Anatomic , Oncogene Proteins/metabolism , Podocytes/metabolism , Podocytes/ultrastructure , Signal Transduction/physiology , TRPC Cation Channels/metabolism , TRPC6 Cation Channel , Wiskott-Aldrich Syndrome Protein/metabolism , rho GTP-Binding Proteins/metabolismABSTRACT
The actin-based foot processes of kidney podocytes and the interposed slit diaphragm 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. Synaptopodin is an actin-associated protein essential for the integrity of the podocyte actin cytoskeleton because synaptopodin-deficient mice display impaired recovery from protamine sulfate-induced foot process effacement and lipopolysaccharide-induced nephrotic syndrome. Moreover, bigenic heterozygosity for synaptopodin and CD2AP is sufficient to induce spontaneous proteinuria and focal segmental glomerulosclerosis-like glomerular damage in mice. Mechanistically, synaptopodin induces stress fibers by blocking the proteasomal degradation of RhoA. Here we show that synaptopodin directly binds to IRSp53 and suppresses Cdc42:IRSp53:Mena-initiated filopodia formation by blocking the binding of Cdc42 and Mena to IRSp53. The Mena inhibitor FP(4)-Mito suppresses aberrant filopodia formation in synaptopodin knockdown podocytes, and when delivered into mice protects against lipopolysaccharide-induced proteinuria. The identification of synaptopodin as an inhibitor of Cdc42:IRSp53:Mena signaling defines a novel antiproteinuric signaling pathway and offers new targets for the development of antiproteinuric therapeutic modalities.
Subject(s)
Cytoskeletal Proteins/metabolism , Microfilament Proteins/metabolism , Nerve Tissue Proteins/metabolism , Podocytes/metabolism , Proteinuria/metabolism , cdc42 GTP-Binding Protein/metabolism , Animals , COS Cells , Cell Line , Cells, Cultured , Chlorocebus aethiops , Cytoskeletal Proteins/genetics , DNA-Binding Proteins/genetics , DNA-Binding Proteins/metabolism , Female , Humans , Male , Mice , Mice, Knockout , Microfilament Proteins/genetics , Microfilament Proteins/physiology , Multiprotein Complexes/physiology , Nephrotic Syndrome/genetics , Nephrotic Syndrome/metabolism , Nephrotic Syndrome/pathology , Nerve Tissue Proteins/genetics , Podocytes/cytology , Protein Binding , Proteinuria/genetics , Pseudopodia/physiology , Rats , Rats, Sprague-Dawley , Signal Transduction/physiology , Transfection/methods , cdc42 GTP-Binding Protein/geneticsABSTRACT
The Rho family of small GTPases (RhoA, Rac1 and Cdc42) controls signal-transduction pathways that influence many aspects of cell behaviour, including cytoskeletal dynamics. At the leading edge, Rac1 and Cdc42 promote cell motility through the formation of lamellipodia and filopodia, respectively. On the contrary, RhoA promotes the formation of contractile actin-myosin-containing stress fibres in the cell body and at the rear. Here, we identify synaptopodin, an actin-associated protein, as a novel regulator of RhoA signalling and cell migration in kidney podocytes. We show that synaptopodin induces stress fibres by competitive blocking of Smurf1-mediated ubiquitination of RhoA, thereby preventing the targeting of RhoA for proteasomal degradation. Gene silencing of synaptopodin in kidney podocytes causes the loss of stress fibres and the formation of aberrant non-polarized filopodia and impairment of cell migration. Together, these data show that synaptopodin is essential for the integrity of the podocyte actin cytoskeleton and for the regulation of podocyte cell migration.
