Self-defense of the kidney: Probing monoallelic expression and functional effects of proteinuria-associated CUBN variants

• Applicant: Professor Dr. Matias Simons
• Subject Area: Human Genetics ; Nephrology
• Term: from 2019 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 426446721

• Final Report Year: 2024

Final Report Abstract

The rate of chronic kidney disease is on the rise worldwide. Many forms of chronic kidney disease are featured by the loss of protein into the urine (proteinuria). When the cause of proteinuria lies within the glomerulus, such as in diabetic kidney disease, then the protein overload in the tubular lumen may lead to damage of the downstream tubular cells. Particularly vulnerable are proximal tubular cells (PTCs), because these cells are specialized in protein reabsorption and have a high metabolic demand. While normal PTC metabolism is fueled by the uptake of fatty acids bound to albumin, an overload of fatty acids can lead to ER and mitochondrial stress. Albumin and other filtered proteins are taken up by the scavenger receptors cubilin and megalin: cubilin is responsible for the uptake of normally filtered albumin in the early PTC segment S1, whereas megalin is less selective and is also expressed in the later PTC segments (S2 and S3). Recently, the glycosphingolipid Gb3 was also shown to facilitate protein reabsorption, but it is not clear in how far it cooperates with the other two receptors. Here, we hypothesized that genetic variants in CUBN are beneficial for PTC homeostasis and resistance against proteinuric kidney disease. This hypothesis is based on our recent findings that 1.) biallelic CUBN variants are very well tolerated by humans despite their proteinuric effects and 2.) that more common CUBN variants are associated with albuminuria and better kidney function. Interesting in this regard is also that in in CUBN-GFP/+ mice CUBN was shown to be expressed in a monoallelic manner, which resulted in cell-to-cell variability of cubilin function when the GFP-labelled loss-of-function allele was in the heterozygous state. We speculate that such cell-to-cell variability may provide the tissue with better resilience under stress conditions and that such a protective mechanism may even be the result of positive selection in the CUBN locus. Therefore, our plan was to analyze the CUBN locus for positive selection signatures and to generate PTCs with heterozygous GFP expression from these mice to test potential heterogenous responses to protein and lipid overload stress. Additionally, we studied in how far inhibition of megalin and Gb3 protects against the same stress. Altogether, our integrative translational approach combines human genetics and experimental studies to explore a new mechanism of proximal tubule homeostasis that may also be applicable to other tissues.

Publications

• Drosophila melanogaster: a simple genetic model of kidney structure, function and disease. Nature Reviews Nephrology, 18(7), 417–434.
  Dow, Julian A. T.; Simons, Matias & Romero, Michael F.
  
• Lysosomal cystine mobilization shapes the response of TORC1 and tissue growth to fasting. Science, 375(6582).
  Jouandin, Patrick; Marelja, Zvonimir; Shih, Yung-Hsin; Parkhitko, Andrey A.; Dambowsky, Miriam; Asara, John M.; Nemazanyy, Ivan; Dibble, Christian C.; Simons, Matias & Perrimon, Norbert
  
• Reducing lipid bilayer stress by monounsaturated fatty acids protects renal proximal tubules in diabetes. eLife, 11.
  Pérez-Martí, Albert; Ramakrishnan, Suresh; Li, Jiayi; Dugourd, Aurelien; Molenaar, Martijn R.; De La Motte, Luigi R.; Grand, Kelli; Mansouri, Anis; Parisot, Mélanie; Lienkamp, Soeren S.; Saez-Rodriguez, Julio & Simons, Matias
  
• ER stress and slit diaphragms: is there a connection?. Kidney International, 103(5), 830-832.
  Hermle, Tobias & Simons, Matias
  
• Lysosomal cystine accumulation activates mTOR signaling in cystinosis: are mTOR inhibitors the cure?. Kidney International, 105(4), 656-658.
  Christer, Salómon & Simons, Matias