Organelle Communication and Maintenance of Podocyte Mitochondrial and Endosomal Homeostasis in Obesity-Related Kidney Diseases

Background: Organelle stress exacerbates podocyte injury, contributing to perturbed lipid metabolism. Although organelles closely interact with one another at membrane contact sites, limited studies have explored their involvement in kidney homeostasis. The endoplasmic reticulum (ER) protein, PDZ domain-containing 8 (Pdzd8), is implicated in multiple organelle tethering processes and cellular lipid homeostasis. In this study, we aimed to elucidate the role of organelle communication in podocyte injury using podocyte-specific Pdzd8-knockout mice. Methods: Podocyte-specific Pdzd8-knockout mice (podocin-Cre: Pdzd8 flox/flox) and littermate wild-type controls (Pdzd8 flox/flox) were generated. The mice were fed either a normal diet or a high-fat diet for 12 weeks. To elucidate the role of Pdzd8 in organelle stress, glomeruli were isolated for a comprehensive proteomic analysis. To corroborate in vivo proteomic phenotypes, cell culture experiments including lipidome analysis were performed using mouse podocyte cells. Results: In vivo, Pdzd8 deletion exacerbated podocyte injury in an accelerated obesity-related kidney disease model. Proteomic analysis of isolated glomeruli revealed that Pdzd8 deletion exacerbated mitochondrial and endosomal dysfunction during podocyte lipotoxicity. Additionally, electron microscopy revealed the accumulation of “fatty abnormal endosomes” in Pdzd8-deficient podocytes during obesity-related kidney diseases. In vitro, Pdzd8 knockdown inhibited mitochondrial and fatty acid oxidation activity in podocytes, in association with a reduction of mitochondria-ER contact sites. In addition, enlarged endosomes were observed in palmitic acids-treated Pdzd8 knockdown podocytes, resembling in vivo “fatty abnormal endosomes.” Lipidomic analysis indicated that glucosylceramide accumulated in Pdzd8-deficient podocytes, owing to accelerated production and decelerated degradation. Conclusion: The organelle-tethering factor, Pdzd8, plays a crucial role in maintaining mitochondrial and endosomal homeostasis during podocyte lipotoxicity. Collectively, our findings highlight the importance of organelle communication at the three-way junction among the ER, mitochondria, and endosomes in preserving podocyte homeostasis. Funding: Government Support – Non-U.S.