Histochem Cell Biol (2008) 130:55–70 Hereditäry diseases Hereditäry deafness: nonsyndromic recessive deafness DFNB29 The cochlea of the inner ear is divided into two compart- ments of diVerent ionic composition. Hereby, the peri- lymph of the scala vestibuli and scala tympani is high in Na+ but low in K+ concentration, while the endolymph of the scala media resembles the intracellular ionic composi- tion, being high in K+ but low in Na+ (Ferrary and Sterkers 1998). This ion gradient leads to the establishment of an 80–100 mV endocochlear potential, necessary for the depo- larisation of sensory hair cells, enhancing the sensitivity of mechano-activated channels at the top of stereocilia (Hud- speth 1989). Besides, hereditary defects in the gap junction proteins connexin-26 and connexin-43 (Sabag et al. 2005) an associ- ation between tight junctions and hearing loss was estab- lished. The importance of claudin-14 expression in the cochlea was demonstrated by the identiWcation of two diVerent CLDN14 mutations that cause nonsyndromic recessive deafness DFNB29 in two large consanguineous Pakistani families (Wilcox et al. 2001). By in situ hybridiza- tion and immunoXuorescence labeling mouse claudin-14 expression in the sensory epithelium of the organ of Corti was observed. By the generation and characterisation of claudin-14 knockout mice as a model for autosomal-reces- sive deafness DFNB29 it was revealed that deafness occurred due to death of outer hair cells. Searching for the factors that promoted increased death of outer hair cells, morphology of the TJs of claudin-14 knockout mice was evaluated. It was demonstrated that TJ strands between sup- porting cells and between outer hair cells and supporting cells in the organ of Corti are still present and appear mor- phologically normal in freeze fracture replicas. The ana- lysed TJ strands which are composed of multiple claudins were thus concluded to have rather a functional than an ultrastructural defect (Ben-Yosef et al. 2003). Following the reasoning that claudins create charge-selective chan- nels, Ben-Yosef characterised the electrophysiologic prop- erties of claudin 14 by ectopically expressing it in MDCK cells, which revealed a strong discrimination of transfected monolayers against Na+ and K+ ions. Ben-Yosuf and coworkers thus concluded that the absence of claudin 14 from TJs in the organ of Corti might lead to elevated K+ concentration in the space of Nuel, which is normally Wlled with the K+-poor cortilymph and increased death of outer hair cells. Long-term exposure of the basolateral mem- branes of outer hair cells to high K+ concentrations had been shown to be toxic and eventually to lead to cell death before (Zenner 1986). 63 By the identiWcation of two claudin-14 missense muta- tions that were unable to form TJs after ectopic expression of CLDN14 in L mouse Wbroblastes, Wattenhofer et al. (2005) were then able to prove that the basis for the develop- ment of deafness is incorrect recruitment and TJ strand formation in cells expressing mutated CLDN14. Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) Inherited and acquired defects in epithelial membrane transport are responsible for many human diseases. In the kidney, tubular claudins play a role in extracellular or para- cellular permeability. In this context, it has been shown that mutations in claudin-16 (Paracellin-1) impair the function of TJs in the paracellular resorption of Mg2+ and Ca2+ but not of monovalent cations (Simon et al. 1999), probably founding the molecular basis for the development of familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC), an autosomal recessive renal tubular disorder. In FHHNC patients, the impaired tubular reabsorption of Mg2+ and Ca2+ in the thick ascending limb of the loop of Henle with an eventual progression can lead to end-stage renal disease (Lee et al. 2006). Functional tubular reabsorb- tion mechanisms are vitally important, since a normal adult absorbs only 200 mg Ca2+ from the diet each day, but loses 8,000 mg to Wltration across the glomerulus. To achieve normal calcium balance, the majority of the Wltered load must be reabsorbed by the tubule, to approximately 90%, or 7,000 mg/day and this occurs mainly via the paracellular pathway driven by a lumen positive potential across the tubular wall. The structural backbone of the paracellular pathway must provide for the signiWcant substrate selectivity and functional heterogeneity between nephron segments and thus also plays a pathophysiologic role in renal diseases. Clinically, the symptoms and gradual kidney damage caused by FHHNC cannot be alleviated by intravenous or oral supplementation with Mg2+, and kidney trans- plantation is often the only option at later stages of the disease. Recent studies (Simon et al. 1999) have reported that FHHNC is caused by mutations in a gene, which encodes the TJ protein, paracellin-1. Paracellin-1 (PLCN-1) belongs to the claudin family (claudin-16) that allows the paracellu- lar transport of Mg2+ and Ca2+. As a member of the claudin family of TJ proteins claudin-16 exhibits four transmem- brane spans with two extracellular loops and both termini located in the cytoplasm. To date, more than 20 distinct mutations have been identiWed, which aVect either the traYcking of PLCN-1/claudin-16, or its permeability properties (Kausalya et al. 2006). 123