#2305 Mice with a PAX2 missense variant are susceptible to experimental focal segmental glomerulosclerosis (FSGS) and display impaired glomerular repair

Abstract Background and Aims Focal segmental glomerulosclerosis (FSGS) is a histopathologic lesion observed in chronic kidney disease characterized by scarring of the glomerulus due to the injury and irreversible loss of highly specialized glomerular epithelial cells called podocytes. Recent studies suggest that podocytes may be regenerated by another population of glomerular epithelial cells called parietal epithelial cells (PECs), although this remains controversial. PAX2 is a transcription factor protein that regulates glomerular development and its expression persists in adult parietal epithelial cells (PECs), postulated to serve as a reservoir for podocytes in the event of injury and loss. Our team previously found that PAX2 pathogenic missense variants account for 4% of adult FSGS. In our present study, our aim is to determine whether PEC-mediated podocyte regeneration is impaired in mice with a pathogenic Pax2 missense variant (Pax2-MV). Method FSGS was induced in wildtype and Pax2-MV mice using Adriamycin. Urine samples were collected from mice every week for 28 days to measure proteinuria. At 28 days after Adriamycin injury, kidneys were collected for histopathology, transmission electron microscopy, and immunofluorescence staining for PAX2 and podocyte marker WT-1. Glomeruli from Adriamycin-injured mice were also isolated through intra-cardiac perfusion with Dynabeads and subjected to mass-spectrometry for proteomics analysis. Proteins with statistically-significant fold changes in expression levels were further analyzed by Gene Ontology to determine biological and molecular processes involved in pathogenesis. Kidney tissue from an FSGS patient with a pathogenic PAX2 missense variant was also examined by histopathology and PAX2 immunohistochemistry. Results Compared to wildtype, Pax2-MV mice were more susceptible to worsened FSGS after Adriamycin, as demonstrated by worsened glomerular scarring, increased podocyte injury and loss, and increased proteinuria, associated with increased morality in Pax2-MV mice (Fig. 1A-B). Wildtype mice showed fewer PAX2-expressing PECs after injury, accompanied by the occurrence of PAX2/WT-1 co-expressing glomerular tuft cells (Fig. 2A-B). These suggest that in Adriamycin-injured wildtype, PAX2-expressing PECs move to the glomerular tuft, where they trans-differentiate to a podocyte fate. In contrast, Pax2-MV mice showed no changes in numbers of PAX2-expressing PECs after injury, associated with fewer PAX2/WT-1 co-expressing glomerular tuft cells compared to injured wildtype (Fig. 2A-B). These suggest that in Adriamycin-injured Pax2-MV mice, PAX2-expressing PECs are unable to move to the glomerular tuft, resulting in decreased numbers of cells undergoing podocyte trans-differentiation. A subset of ectopic PAX2-expressing glomerular tuft cells after injury was increased in Pax2-MV mice (Fig. 2A), suggesting a pathologic process given the worse outcomes observed in these mice. Consistent with findings from injured Pax2-MV mice, we found that in an FSGS patient with a pathogenic PAX2 missense variant, PAX2-expressing glomerular tuft cells overlap with areas of glomerular scarring, while in a normal human kidney biopsy, PAX2 expression is only observed in PECs but not within glomerular tuft cells (Fig. 2A). Lastly, proteomics Gene Ontology analysis of isolated glomeruli demonstrated biological and molecular processes indicative of maladaptive glomerular repair in injured Pax2-MV mice, including decreased regulation of cell shape, decreased cell-cell junction assembly and decreased actin and cadherin binding compared to wildtype (Fig. 2C). Conclusion Altogether, our findings suggest that PAX2-expressing PECs migrate to the glomerular tuft to assist in podocyte regeneration in wildtype. However, this mechanism is impaired in Pax2-MV mice, leading to worsened podocyte loss and injury and further exacerbation of glomerular disease (Fig. 2C). Our findings provide a further understanding of normal and impaired regeneration mechanisms in FSGS, and our identified pathogenic mechanisms can be further harnessed for future therapeutic development.