Actin-depolymerizing Factor Cofilin-1 Is Necessary in Maintaining Mature Podocyte Architecture

Abstract

Actin dynamics determines podocyte morphology during development and in response to podocyte injury and might be necessary for maintaining normal podocyte morphology. Because podocyte intercellular junction receptor Nephrin plays a role in regulating actin dynamics, and given the described role of cofilin in actin filament polymerization and severing, we hypothesized that cofilin-1 activity is regulated by Nephrin and is necessary in normal podocyte actin dynamics. Nephrin activation induced cofilin dephosphorylation via intermediaries that include phosphatidylinositol 3-kinase, SSH1, 14-3-3, and LIMK in a cell culture model. This Nephrin-induced cofilin activation required a direct interaction between Nephrin and the p85 subunit of phosphatidylinositol 3-kinase. In a similar fashion, cofilin-1 dephosphorylation was observed in a rat model of podocyte injury at a time when foot process spreading is initially observed. To investigate the necessity of cofilin-1 in the glomerulus, podocyte-specific Cfl1 null mice were generated. Cfl1 null podocytes developed normally. However, these mice developed persistent proteinuria by 3 months of age, although they did not exhibit foot process spreading until 8 months, when the rate of urinary protein excretion became more exaggerated. In a mouse model of podocyte injury, protamine sulfate perfusion of the Cfl1 mutant mouse induced a broadened and flattened foot process morphology that was distinct from that observed following perfusion of control kidneys, and mutant podocytes did not recover normal structure following additional perfusion with heparin sulfate. We conclude that cofilin-1 is necessary for maintenance of normal podocyte architecture and for actin structural changes that occur during induction and recovery from podocyte injury.Glomerular visceral epithelial cells or podocytes play a central role in maintaining the selective filtration barrier of the kidney that prevents the passage of cellular elements and large macromolecules from the blood into the urinary space. Podocytes are unique cells with interdigitating foot-like actin-rich processes that arise from their cell bodies and surround glomerular capillary walls. An ultrafiltrate of serum passes across this specialized intercellular junction, also termed the slit diaphragm, formed at the interface of these interdigitating foot processes.There appears to be a direct relationship between the integrity of the podocyte intercellular junction and the three-dimensional architecture of the podocyte. When injured, podocytes undergo a dramatic change in their morphology termed foot process effacement that appears to result from incompletely understood alterations in cytoskeletal and intercellular junctional architecture. Foot process effacement is a dynamic and reversible process that correlates with the development of proteinuria both in human disease and in experimental models. Recent investigations have demonstrated a functional relationship between molecular components of the foot process intercellular junction and...

Nephrin Tyrosine Phosphorylation Is Required to Stabilize and Restore Podocyte Foot Process Architecture

“…13,54,55 Instead, we find that foot process recovery is impaired in nephrin Y3F/Y3F animals in both injury models, similar to that observed in mice lacking the actin regulators synaptopodin and cofilin. 56,57 Taken together, our data support a model wherein nephrin tyrosine phosphorylation is required to counteract normal podocyte stress and loss of this phosphorylation, as seen in nephrin Y3F/Y3F mice and some human diseases, leads to irreversible injury and chronic disease.…”

Direct Regulation of Nephrin Tyrosine Phosphorylation by Nck Adaptor Proteins

“…Decreased Akt phosphorylation on Ser 473 has been reported during the onset of proteinuria in the rat puromycin aminonucleoside model (9). Furthermore, mice with a podocyte-specific knock-out of cofilin developed renal disease (10), and phosphorylation of cofilin, corresponding with inactivation, was increased in some forms of human glomerular disease including minimal change disease (39). Interestingly, decreased phosphorylation on the Nck binding sites Tyr 1193 and Tyr 1217 has been observed in the same animal models (19,20) and human diseases (20) where decreased p85-based signaling has been reported (9, 39).…”

Focus on ADF/Cofilin: Beyond Actin Cytoskeletal Regulation

“…Activated ADF/cofilin is well distributed in normal podocytes, but inactivated (phosphorylated) ADF/cofilin can be found throughout misfunctional podocyte cells [70]. Of interest, ADF/cofilin can be activated for actin filamental reorganization through receptor tyrosine kinase activation, the so-called NephrinNeph1 receptor complex on the membranes of podocytes [64]. In addition to ADF/cofilin, α-actinin-4 [71], nephrin [72], and podocin [73] are also involved in regulating the dynamics of actin cytoskeleton and are important for the functions of podocytes on renal filtration.…”

“…The role of ADF/cofilin in nephrology has also been defined in last decade. Currently, it is believed that the active form of ADF/cofilin is not only critical for maintaining the morphology of podocytes but also required for the development of podocytes [64]. Podocytes are high-specialized and terminal-differentiated epithelial cell located at the outer aspect of the glomerular basement membrane to prevent the leakage of urine protein, maintenance of glomerular capillary loops integrity, and serve as a barrier to against the intracapillary hydrostatic pressure [65,66].…”