The Insulin/Insulin-Like Growth Factor Axis Is Critically Important in the Kidney Podocyte Controlling Gene Transcription and Spliceosome Function

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Background: Insulin signalling to the podocyte via the insulin receptor (IR) is crucial for kidney function and insulin-like growth factor 1 (IGF1) signalling through the structurally related insulin-like growth factor 1 receptor (IGF1R) is also known to directly affect the podocyte. Since the IR and IGF1R may act redundantly in some contexts, this study sought to elucidate the role of the insulin/IGF1 axis in podocyte function using mouse and cell culture models deficient in both receptors. Methods: To examine the effects of combined receptor loss in vivo, a transgenic mouse model with conditional inactivation of podocyte IR and IGF1R was generated. In vitro, conditionally immortalised genetic IR/IGF1R dual knockout podocytes were characterised using global proteomic and transcriptomic analysis. Results: Podocyte specific IR/IGF1R knockout mice developed significant albuminuria and a severe renal phenotype with global sclerosis, renal failure and death occurring between 4 and 24 weeks. >90% loss of IR/IGF1R in cultured mouse podocytes was also detrimental resulting in >50% cell death 7 days after gene knockdown. Enrichment analysis of total proteomic data revealed a striking downregulation of gene ontology terms associated with DNA repair, splicing and RNA processing activity in IR/IGF1R knockdown cells. Long-read RNA sequencing was performed to obtain an overview of global splicing differences in the IR/IGF1R knockdown transcriptome and to determine the effect of spliceosome depletion on the transcriptomic profile of these cells. Analysis of this data revealed higher levels of intron retention and an increase in the proportion of transcripts containing premature termination codons in IR/IGF1R knockdown podocytes along with differential expression of fibronectin splice variants. Conclusions: This work underlines the critical importance of podocyte insulin/IGF signalling and reveals a novel role for this extrinsic hormonal signalling axis in the maintenance of genomic integrity and in RNA processing by regulating spliceosome activity in this cell type. Funding: Government Support - Non-U.S.

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