Dynll1-PI31 Interaction Enhances Proteolysis via the Proteasome, Representing a Novel Therapeutic Target for INF2-Related FSGS

Abstract

Background: The p.Arg218Gln (R218Q) mutation in the inverted formin 2 (INF2) gene causes podocytopathy prone to focal segmental glomerulosclerosis (FSGS). This mutation disrupts the ability of INF2 to sequester DYNLL1, thus promoting dynein-mediated mistrafficking of the slit diaphragm protein, nephrin, to proteolytic pathways. Bortezomib, a proteasome inhibitor (PI), stabilizes nephrin in R218Q knockin (KI) podocytes, suggesting a role for the ubiquitin proteasome system (UPS) in dynein-driven pathogenesis. However, the link between dynein and the UPS is unknown. This study tested the hypothesis that INF2 R218Q promotes proteasome-mediated degradation of nephrin via an increased interaction between Dynll1 and Proteasomal Inhibitor of 31kD (PI31), a Dynll1 adaptor that potentially couples the UPS with dynein cargoes.

Methods: The essential role of PI31 in UPS-mediated degradation of nephrin, a known dynein cargo, was studied in cultured R218Q KI mouse podocytes by applying genetic or chemical interventions to inhibit the activity of PI31 or of the proteasome. The protective effect of bortezomib in dynein-driven podocytopathy and FSGS was tested in R218Q KI mice challenged with puromycin aminonucleoside (PA), a murine model of FSGS.

Results: The R218Q mutation in Inf2 disrupted sequestration of Dynll1 by Inf2, allowing Dynll1 to be captured by PI31 and promoting dynein-mediated transport of nephrin to the proteasome. Each of the following manipulations was sufficient to restore nephrin proteostasis in R218Q KI podocytes: knocking down Dynll1 or PI31, inactivating dynein, or inhibiting the activity of the proteasome. In R218Q KI mice challenged with PA, dynein-mediated mistrafficking and depletion of nephrin were correlated with increased Dynll1-PI31 interaction; the resulting podocytopathy and FSGS were ameliorated by bortezomib.

Conclusions: The Dynll1-PI31 interaction facilitates dynein-driven trafficking of nephrin to the proteasome and proteasome-mediated degradation of nephrin in INF2-R218Q-mediated podocytopathy. This mechanism offers new therapeutic strategies for INF2-related FSGS by using pharmacologically available proteasome inhibitors.