Mechanisms of Podocyte Injury Due to Loss of the Nucleoporin NUP93.

KEY POINTS: Loss of NUP93 caused progressive glomerular disease in mice. Nuclear levels of podocyte transcription factors were reduced on loss of NUP93. Loss of NUP93 changed transcriptional programs in podocytes.

BACKGROUND

Genetic variants in six different genes encoding nuclear pore complex (NPC) proteins cause steroid-resistant nephrotic syndrome and FSGS in pediatric patients. All affected proteins are structural components of NPCs, restricted channels across the nuclear envelope that are required for molecular exchange between nucleoplasm and cytoplasm. Although podocyte injury is a unifying feature of these genetic variants, the molecular mechanisms remain poorly understood.

METHODS

To elucidate the pathogenesis of NPC protein-related podocyte injury, a mouse model with conditional knockout of the Nup93 gene was generated and studied. NPC density was measured by stimulated emission depletion microscopy. Transcriptomics, proteomics, and immunofluorescence analysis were used to define molecular pathogenic mechanisms.

RESULTS

Knockout of Nup93 reduced NPC density in podocyte nuclei. While knockout of Nup93 during kidney development led to embryonic lethality, its deletion in mature podocytes ( NPHS2 -Cre) caused progressive glomerular disease with onset around 4 months of age, when a phenotype of minimal change glomerulopathy was observed. At this stage, immunofluorescence staining revealed reduced nuclear abundance of podocyte-specific transcription factors, which was due to cytoplasmic redistribution. Transcriptome and proteome analysis detected altered transcriptional regulation, prominently affecting genes related to cell adhesion and regulation of the actin cytoskeleton. Until 6 months of age, knockout mice developed progressive CKD with FSGS, podocyte loss, and uremia. Before induction of apoptosis, NUP93-deficient podocytes accumulated genomic damage as indicated by activation of the DNA damage response pathway.

CONCLUSIONS

Our data suggest that in early stages of disease, loss of NUP93 impaired podocyte transcription factors and altered transcriptional programs in podocytes, prominently affecting regulators of the actin cytoskeleton. In the late stages, podocytes accumulated genomic damage, leading to apoptosis and progressive glomerulosclerosis.