Cation-selective tight junctions in the nephron

• Applicant: Dr. Susanne Milatz
• Subject Area: Anatomy and Physiology
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 455805987

The kidney reabsorbs large quantities of Na+, Ca2+, and Mg2+. A major part of the cation reabsorption is conducted via the paracellular route – the tight junction (TJ). Within the TJ complex the members of the claudin family determine the selectivity depending on solute size and charge. Claudins interact with each other and form complex TJ strand meshworks, sealing the paracellular cleft or forming paracellular channels with particular selectivity. The project applied for aims to unravel the composition and function of cation reabsorbing TJs in the kidney.The proposal’s first part encompasses the functional characterization of different pathogenic variants of the TJ protein claudin-10. Claudin-10 is mainly responsible for the selective paracellular reabsorption of Na+ in the thick ascending limb of Henle’s loop of the nephron. Patients with pathogenic claudin-10 variants suffer from an electrolyte imbalance. Due to the expression of claudin-10 in exocrine glands and skin, patients have a dysfunction of eccrine sweat, salivary and lacrimal glands as well as dermatological symptoms (HELIX syndrome). Since symptoms can differ a lot between patients with particular claudin-10 mutations, the variants will be analysed regarding localization, Na+ selectivity and interaction properties. By this means we aim to shed light on the mechanisms by which different claudin-10 mutants perturb the protein function and cause clinical symptoms. Using this translational approach, we seek to achieve a clear correlation of genotype and phenotype. Moreover, these studies offer the opportunity to elucidate the interplay of claudin structure, strand architecture, paracellular channel function and physiological consequences.The proposal’s second objective is to identify the paracellular pathway for Mg2+ in the proximal nephron, whose composition is completely unknown, in contrast to the route for Na+ und Ca2+. The role of the present claudins for the Mg2+ passage and their interactions will be investigated. Moreover, the role of other candidate proteins possibly modulating the renal ion transport will be analysed. On the one hand, this project aims to close important gaps in understanding of ion reabsorption mechanisms in the kidney. Moreover, we want to open up a new research field by investigating candidate proteins, whose role in the kidney is unknown as yet. The proposed project spans from protein structure to TJ strand composition to permeability function to the individuum. We will use electrophysiological, microscopic and molecular biological methods as well as transgenic mouse models.

• DFG Programme: Research Grants