Synaptopodin Is Dispensable for Normal Podocyte Homeostasis but Is Protective in the Context of Acute Podocyte Injury

Significance Statement Synaptopodin is a podocyte actin-binding protein that regulates the cytoskeleton. Despite the cytoskeleton's critical role in podocytes and its alteration in nephrotic syndrome and FSGS, mutations in the synaptopodin gene SYNPO have not been reported in patients. Studies in Synpo mutant mice suggest that a truncated form of synaptopodin partially compensates for loss of the full-length protein. Experiments using newly generated Synpo mutant mice demonstrated that complete absence of synaptopodin did not cause a phenotype, but it increased susceptibility to Adriamycin nephropathy, an FSGS model. In cultured podocytes from the mice, absence of synaptopodin altered the cytoskeleton. These findings indicate that synaptopodin is dispensable for normal podocyte homeostasis but is protective in injured podocytes, suggesting the cytoskeleton as a target for therapeutic intervention in podocytopathies. Background Synaptopodin (Synpo) is an actin-associated protein in podocytes and dendritic spines. Many functions in regulating the actin cytoskeleton via RhoA and other pathways have been ascribed to Synpo, yet no pathogenic mutations in the SYNPO gene have been discovered in patients. Naturally occurring Synpo isoforms are known (Synpo-short and -long), and a novel truncated version (Synpo-T) is upregulated in podocytes from Synpo mutant mice. Synpo-T maintains some Synpo functions, which may prevent a podocyte phenotype from emerging in unchallenged mutant mice. Methods Novel mouse models were generated to further investigate the functions of Synpo. In one, CRISPR/Cas9 deleted most of the Synpo gene, preventing production of any detectable Synpo protein. Two other mutant strains made truncated versions of the protein. Adriamycin injections were used to challenge the mice, and Synpo functions were investigated in primary cultured podocytes. Results Mice that could not make detectable Synpo ( Synpo −/− ) did not develop any kidney abnormalities up to 12 months of age. However, Synpo −/− mice were more susceptible to Adriamycin nephropathy. In cultured primary podocytes from mutant mice, the absence of Synpo caused loss of stress fibers, increased the number and size of focal adhesions, and impaired cell migration. Furthermore, loss of Synpo led to decreased RhoA activity and increased Rac1 activation. Conclusions In contrast to previous findings, podocytes can function normally in vivo in the absence of any Synpo isoform. Synpo plays a protective role in the context of podocyte injury through its involvement in actin reorganization and focal adhesion dynamics.