Characterization of Steroid-Resistant Nephrotic Syndrome-Associated Motor Domain Mutations in MYO1E

Background: Myosin1e (Myo1e) is a cytoskeletal protein highly expressed in podocytes and localized to cell-cell junctions/slit diaphragm complexes, which are required for these cells' structural and functional integrity. As podocytes are the final gatekeepers of the filtration barrier, they rely on the actin cytoskeletal organization to maintain their intricate architecture and respond to mechanical stresses from fluid flow and capillary distension. Myo1e binds to actin and may modulate actin cytoskeletal assembly and dynamics. Mutations in the gene encoding Myo1e have been found in patients with steroid-resistant nephrotic syndrome (SRNS) and focal segmental glomerulosclerosis (FSGS). In this project, we have analyzed recently identified MYO1E mutations associated with SRNS/FSGS (Warejko et al. Clin J Am Soc Nephrol 2018, Krendel et al. Pediatr Nephrol 2023) that have not been functionally characterized. Methods: EGFP-tagged Myo1e constructs (mutant or wild-type (WT)) were introduced into the Myo1e-KO podocyte-derived cells via adenovirus-mediated transduction to test the effects of these mutations on Myo1e localization and podocyte junctional properties. Results: Similarly to Myo1e localization in the WT podocytes, WT Myo1e was enriched at cell-cell contact points and colocalized with actin. Unlike WT Myo1e, the mutants showed primarily cytoplasmic subcellular localization. One of the mutants, G562R, was partially localized at the cellular junction, but its enrichment at the cell-cell junctions was significantly lower than that of the WT. Although none of the mutants colocalized with actin at the podocyte-podocyte intersection, their expression as the sole source of Myo1e did not affect the actin enrichment at those sites. Conclusion: WT Myo1e frequently localizes at the cell-cell contacts and may play a role in regulating junctional integrity. Mis-localization of the Myo1e mutants that we have studied indicates that these disease-associated mutations disrupt the ability of Myo1e to perform its functions at the sites of cell-cell contact. Funding: NIDDK Support