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J Am Soc Nephrol. 2020 Jan 10. pii: ASN.2019050443. doi: 10.1681/ASN.2019050443. [Epub ahead of print]

FSGS-Causing INF2 Mutation Impairs Cleaved INF2 N-Fragment Functions in Podocytes.

Author information

1
Division of Nephrology, Department of Medicine, and.
2
Department of Pathology, Beth Israel Deaconess Medical center, Harvard Medical School, Boston, Massachusetts.
3
Department of Biochemistry, Geisel School of Medicine, Dartmouth College, Hanover, New Hampshire; and.
4
Broad Institute of Harvard and Massachusetts Institute of Technology, Cambridge, Massachusetts.
5
Division of Nephrology, Department of Medicine, and mpollak@bidmc.harvard.edu.

Abstract

BACKGROUND:

Mutations in the gene encoding inverted formin-2 (INF2), a member of the formin family of actin regulatory proteins, are among the most common causes of autosomal dominant FSGS. INF2 is regulated by interaction between its N-terminal diaphanous inhibitory domain (DID) and its C-terminal diaphanous autoregulatory domain (DAD). INF2 also modulates activity of other formins, such as the mDIA subfamily, and promotes stable microtubule assembly. Why the disease-causing mutations are restricted to the N terminus and how they cause human disease has been unclear.

METHODS:

We examined INF2 isoforms present in podocytes and evaluated INF2 cleavage as an explanation for immunoblot findings. We evaluated the expression of INF2 N- and C-terminal fragments in human kidney disease conditions. We also investigated the localization and functions of the DID-containing N-terminal fragment in podocytes and assessed whether the FSGS-associated R218Q mutation impairs INF2 cleavage or the function of the N-fragment.

RESULTS:

The INF2-CAAX isoform is the predominant isoform in podocytes. INF2 is proteolytically cleaved, a process mediated by cathepsin proteases, liberating the N-terminal DID to function independently. Although the N-terminal region normally localizes to podocyte foot processes, it does not do so in the presence of FSGS-associated INF2 mutations. The C-terminal fragment localizes to the cell body irrespective of INF2 mutations. In podocytes, the N-fragment localizes to the plasma membrane, binds mDIA1, and promotes cell spreading in a cleavage-dependent way. The disease-associated R218Q mutation impairs these N-fragment functions but not INF2 cleavage.

CONCLUSIONS:

INF2 is cleaved into an N-terminal DID-containing fragment and a C-terminal DAD-containing fragment. Cleavage allows the N-terminal fragment to function independently and helps explain the clustering of FSGS-associated mutations.

KEYWORDS:

Actin; Cathepsins; FSGS; INF2; cytoskeleton; podocyte

PMID:
31924668
DOI:
10.1681/ASN.2019050443

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