α-Actinin 4 Potentiates Nuclear Factor κ-Light-chain-enhancer of Activated B-cell (NF-κB) Activity in Podocytes Independent of Its Cytoplasmic Actin Binding Function

Background: ACTN4 is an actin-binding protein and is associated with kidney diseases. Results: Nuclear ACTN4 interacts with NF-B and is recruited to NF-B-targeted promoters to potentiate its transcription activity. Conclusion: ACTN4 coactivates NF-B activity independently of its cytoplasmic activity in cultured human podocytes. Significance: Nuclear ACTN4 may play an important role in glomerular function. Glomerular podocytes are highly specialized terminally differentiated cells that act as a filtration barrier in the kidney. Mutations in the actin-binding protein, -actinin 4 (ACTN4), are linked to focal segmental glomerulosclerosis (FSGS), a chronic kidney disease characterized by proteinuria. Aberrant activation of NF-B pathway in podocytes is implicated in glomerular diseases including proteinuria. We demonstrate here that stable knockdown of ACTN4 in podocytes significantly reduces TNF-mediated induction of NF-B target genes, including IL-1 and NPHS1, and activation of an NF-B-driven reporter without interfering with p65 nuclear translocation. Overexpression of ACTN4 and an actin binding-defective variant increases the reporter activity. In contrast, an FSGS-linked ACTN4 mutant, K255E, which has increased actin binding activity and is predominantly cytoplasmic, fails to potentiate NF-B activity. Mechanistically, IB blocks the association of ACTN4 and p65 in the cytosol. In response to TNF, both NF-B subunits p65 and p50 translocate to the nucleus, where they bind and recruit ACTN4 to their targeted promoters, IL-1 and IL-8. Taken together, our data identify ACTN4 as a novel coactivator for NF-B transcription factors in podocytes. Importantly, this nuclear function of ACTN4 is independent of its actin binding activity in the cytoplasm.