The role of Rnd3 in kidney morphogenesis and function

Abstract

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is an adult-onset, multi-systemic disorder, which affects ~12.5 million people worldwide. ADPKD is characterised by progressive kidney enlargement caused by continuous growth of cysts, alongside extra-renal phenotypes. ADPKD is caused by mutations in PKD1 and PKD2. The phenotypic variability of ADPKD can be attributed to genic, allelic and gene modifier effects. To understand the involvement of modifier genes in the onset and development of ADPKD phenotypes, a novel mouse model Rnd3tm1b(EUCOMM)Hmgu was generated. Rnd3 is an atypical member of the Rnd subgroup of the Rho family of GTPases and has a role in actin cytoskeleton reorganisation, cell cycle progression, and cell migration. The characterisation of the kidney phenotype revealed the onset of cyst formation in the cortex in 5-month-old adult Rnd3tm1b+/- kidneys. A significant upregulation in Pkd1 and Pkd2 expression was also observed in adult Rnd3tm1b+/- mice. The kidneys of aged 9-month-old Rnd3tm1b+/- mice presented with multiple cysts in the cortex, mimicking moderate-to-severe ADPKD. The expression of Pkd1 and Pkd2 was significantly downregulated in aged Rnd3tm1b+/- mice. RNA-Sequencing performed on 5-month-old Rnd3tm1b+/- kidneys generated 415 statistically significant protein coding differentially expressed genes. Six genes were selected for validation via qRT-PCR which included Smoc2, Col5α3, Slc4a3, Cchcr1, Psmb7, Dnaja4. All validated genes followed an expression pattern that was in accordance with the data generated by RNA-Sequencing. The characterisation of extra-renal phenotypes revealed that hearts of adult and aged Rnd3tm1b+/- mice exhibited a left ventricular hypertrophy phenotype and fibrosis. To dissect the role and function of Rnd3 in the kidney, mouse Inner Medullary Collecting Duct (mIMCD3) cells were utilised. The downregulation of Rnd3 in post-siRNA mediated mIMCD3 cells resulted in the upregulation of Pkd1 and Pkd2 expression. The downregulation of Rnd3 also increased cell proliferation, migration and disassembled the actin cytoskeleton. The findings from this study highlight the importance of Rnd3 as a potential genetic modifier involved in ADPKD by modulating the expression of Pkd1 and Pkd2. It is proposed that there is a critical window where the expression of Pkd1 and Pkd2 is altered, exacerbating the phenotypes between adult and aged Rnd3tm1b+/- mice. Furthermore, the dysregulation of Rnd3 is also proposed to result in the destabilisation of the cAMP-PKA-mTORC-1 signalling cascade, which contributes towards the onset and development of polycystic kidneys, mimicking ADPKD.