#5919 PATIENT-SPECIFIC HUMAN INDUCED PLURIPOTENT STEM CELL (HIPSC)-DERIVED PODOCYTES
Authors: Rose Victoria , Sopel Nina , Miedl Denise , Krüger René , Kraus Annalena , Schiffer Mario , Müller-Deile Janina 
Published: June 2023

Abstract

Background and Aims
Podocytes typically represent a distinct morphology with long, interdigitating primary and secondary foot processes wrapping around glomerular capillaries forming the filtration unit of the kidney. In glomerular diseases like genetic focal segmental glomerulosclerosis (FSGS), podocyte alterations are often related to dysfunction of the glomerular filtration barrier and the development of proteinuria. Genetic mutations in podocyte relevant genes, important for foot process or actin cytoskeleton dynamics, are known to induce severe podocyte injury. Analysis of podocyte alterations ex vivo is limited because of their inability to proliferate due to their terminal differentiated state and limited access to primary podocytes. Instead, immortalized podocytes regained proliferation capacity by insertion of a thermosensitive SV40 large T antigen and are useful for many research questions. However, these podocytes usually do not express all podocyte marker and foot process development is often restricted. Since we aim to characterize personalized podocytes from patients suffering from genetic FSGS ex vivo, we generated podocytes from dermal fibroblasts via reprogramming into hiPSCs and subsequent differentiation into hiPSC-derived podocytes keeping the patients’ genetic background.

Method
Dermal fibroblasts were outgrown from a skin punch biopsy from a patient or a healthy donor and subsequently reprogrammed into hiPSCs by electroporation of the common OKSM plasmids and finally differentiated into podocytes. Cells were characterized phenotypically regarding morphology and marker expression using RNA bulk sequencing, western blot, qPCR and immunofluorescence staining. Potential functional alterations of diseased podocytes regarding actin cytoskeleton were assessed by actin polymerization assay.

Results
Patient-specific podocytes could be generated ex vivo by reprogramming dermal fibroblasts into hiPSCs and subsequently differentiating them into mature podocytes representing the patients’ background. During this process, cell morphology changed from spindle-like fibroblasts to small and round hiPSCs growing in colonies and representing a high nuclei-to body ratio as well as high proliferation rate. During differentiation, hiPSCs were induced into intermediate mesoderm, resulting in increased cell size and decreased proliferation rate terminating in 300 nm large star-shaped podocytes developing a distinct network of long primary and secondary foot processes. Patient-derived hiPSC-podocytes with a mutation in the INF2 gene lacked filamentous actin and only develop a limited number of only short foot processes compared to hiPSC-podocytes derived from a healthy control donor. Moreover, expression of actin cytoskeleton associated genes like SYNPO, ACTN4 and CD2AP was decreased while other actin isoforms, cortactin and the GTPase RhoA were increased in diseased patient-specific podocytes. Actin polymerization assay showed that polymerization process itself was not altered in healthy and diseased hiPSC-podocytes.

Conclusion
Analysis of patient-specific podocytes is possible ex vivo via differentiation of hiPSCs, generated from patients’ dermal fibroblasts by episomal reprogramming, into hiPSC-derived podocytes, with the potential to study alterations of disease-specific mutations regarding phenotypical and functional behavior in a personalized manner. The use of hiPSCs bypasses the limitation of restricted podocyte cell number and has the advantage of maintaining the patients’ genetic background at podocyte cell level. This enables future large-scale experiments regarding intercell-cell communication and interaction in glomerular three-dimensional co-cultures or via treatment with therapeutic substances or stress factors.