The enriched expression of aquaporin-2 in the renal inner medulla is mediated by increased chromatin accessibility
Authors: Engel Kristina , Nerger Katrin , Chernyakov Dmitry , Edemir Bayram 
Published: May 2024

Introduction: The osmotic gradient between renal cortex (CTX) and renal inner medulla (IM) is important for the generation of a concentrated urine. In addition, the unique hypertonic environment in the IM induces a specific gene expression pattern. For example the expression of aquaporin-2 (Aqp2) is enriched in the IM compared to CTX. We postulated that these changes must be associated with epigenetic mechanisms that could regulate the spatial expression pattern in CTX and IM. We performed assay for transposase-accessible chromatin (ATAC) sequencing to identify genomic regions with differences in accessibility in CTX and IM. Methods: We used kidneys from control mice and collecting duct principal cell specific nuclear factor of activated t cells (Nfat5)-KO mice and dissected CTX and IM. These samples were subjected for ATAC-seq analysis. Accessible genomic regions were identified with MACS2. We next compared differentially accessible regions between CTX and IM in control mice and between control and Nfat5-KO mice. The results were correlated with corresponding gene expression profiles that have been analyzed in a former study. Results: There are massive differences in the accessibility of genomic regions between CTX and IM. In CTX, more than 100,000 regions are differentially accessible compared to IM, with 48,698 regions having higher accessibility and 55,453 regions having lower accessibility. Genes with higher accessibility in CTX show also higher or even specific gene expression in CTX and vice versa. Since loss of NFAT5 is associated with a massive change in gene expression in kidneys from Nfat5-KO mice, we analyzed if this is affected by differences in chromatin accessibility. Our data shows that not only spatial localization in the kidney affects chromatin accessibility, but also the function of NFAT5. Loss of NFAT5 results in 10,456 regions with higher accessibility and 3,858 regions with closed chromatin in the CTX and 16,452 regions losing accessibility and 9,494 regions with increased accessibility in the IM. The accessibility of the Aqp2 gene is higher in IM compared with CTX. The open chromatin of the Aqp2 promoter region correlates with higher expression of Aqp2 in the IM. This suggests that hypertonicity leads to an improvement in the accessibility of the promoter region of Aqp2. In Nfat5-KO IM the accessibility of this region was decreased, which could explain the reduced Aqp2 expression in Nfat5-KO mice. Conclusions: To date, studying hypertonicity-dependent regulation of chromatin accessibility and associated gene expression in CTX and IM in the kidney were missing. Here we show that the differences in gene expression between CTX and IM might be due to changes in accessibility of genomic regions. We also show that the renal expression of Aqp2 might be a result of an epigenetic mechanism. In this context, NFAT5 was found to be an important regulator of chromatin accessibility and gene expression in the kidney. The authors have nothing to disclose. This project was funded by the Deutsche Forschungsgemeinschaft(ED181/9-3). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.