Tubule-Specific Compensatory Responses to Cpt1a Deletion in Aged Mice

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Abstract
Key Points: Aging plus high fat diet suppresses expression of genes related to metabolism and fatty acid oxidation in the proximal tubules. Proximal tubules lacking Cpt1a have significant transcriptional increases in Hmgcs2, peroxisomal fatty acid oxidation genes, and omega-oxidation genes. Distal convoluted tubules lacking Cpt1a have fewer changes in metabolic genes, but significantly downregulated expression of differentiation markers. Background: Fatty acid oxidation (FAO) is the preferred energy pathway in the proximal tubule (PT), and carnitine palmitoyltransferase 1A (Cpt1a) is the rate-limiting enzyme of mitochondrial FAO. Cpt1a expression and FAO decrease after renal injury. Our recent work demonstrated that genetic deletion of tubular Cpt1a did not significantly worsen the response to injury or aging and did not completely block FAO, suggesting compensatory metabolic pathways. In addition, Cpt1a was most highly expressed in distal convoluted tubule (DCT), a segment not known for FAO. Therefore, we used single-nuclear RNA sequencing to explore a cell-specific responses to aging with high fat diet (HFD aging), to define compensatory metabolic pathways in PT segments lacking Cpt1a, and to determine the role of Cpt1a in the DCT. Methods: Cpt1a floxed (Cpt1a fl/fl ) and tubule-specific conditional Cpt1a knockout (Cpt1a CKO ) mice were aged for 2 years with HFD. Single-nuclear RNA-sequencing was performed on these HFD-aged mice and young controls. Results: HFD-aged mice had increased fibrosis, inflammation, and more injured PT cells than young mice. Whereas PT segments from HFD-aged mice had significant transcriptional changes in metabolism-related pathways, the DCT had more changes in inflammation-related pathways. Compared with floxed mice, HFD-aged Cpt1a CKO mice had increased lipid deposition and increased inflammation, but no significant differences in fibrosis or renal function. PT segments from HFD-aged Cpt1a CKO mice had significantly upregulated Hmgcs2, a promoter of ketogenesis and FAO, and upregulated genes in peroxisomal FAO and omega-FAO (CYP4A family) pathways. DCT from HFD-aged Cpt1a CKO mice had decreased expression of DCT-specific markers of cell differentiation. Conclusions: The upregulated Hmgcs2, peroxisomal FAO genes, and CYP4A genes may compensate for impaired mitochondrial metabolism of long chain fatty acids in PT cells lacking Cpt1a. Our data suggest that Cpt1a may be important in maintenance of cell differentiation for DCT.

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