The human Usher syndrome (USH) is the most frequent cause of combined deaf-blindness. USH is genetically heterogeneous with at least 12 chromosomal loci assigned to three clinical types, USH1-3. Although these USH types exhibit similar phenotypes in human, the corresponding gene products belong to very different protein classes and families. The scaffold protein harmonin (USH1C) was shown to integrate all identified USH1 and USH2 molecules into protein networks. Here, we analyzed a protein network organized in the absence of harmonin by the scaffold proteins SANS (USH1G) and whirlin (USH2D). Immunoelectron microscopic analyses disclosed the colocalization of all network components in the apical inner segment collar and the ciliary apparatus of mammalian photoreceptor cells. In this complex, whirlin and SANS directly interact. Furthermore, SANS provides a linkage to the microtubule transport machinery, whereas whirlin may anchor USH2A isoform b and VLGR1b (very large G-protein coupled receptor 1b) via binding to their cytodomains at specific membrane domains. The long ectodomains of both transmembrane proteins extend into the gap between the adjacent membranes of the connecting cilium and the apical inner segment. Analyses of Vlgr1/del7TM mice revealed the ectodomain of VLGR1b as a component of fibrous links present in this gap. Comparative analyses of mouse and Xenopus photoreceptors demonstrated that this USH protein network is also part of the periciliary ridge complex in Xenopus. Since this structural specialization in amphibian photoreceptor cells defines a specialized membrane domain for docking and fusion of transport vesicles, we suggest a prominent role of the USH proteins in cargo shipment.

contributed equally to this work. Conflict of interest:The authors have declared that no conflict of interest exists. Nonstandard abbreviations used: parathyroid hormone (PTH); 1,25-dihydroxycholecalciferol (1,25-(OH)2D3); transient receptor potential (TRP); TRP cation channel subfamily V, member 5 (TRPV5); TRP cation channel subfamily V, member 6 (TRPV6); last loop of proximal tubules (LPT); hypoxanthine-guanine phosphoribosyl transferase (HPRT); bone volume (BV); total bone marrow volume including trabeculae (TV); trabecular bone volume fraction (BV/TV); trabecular thickness (Tb.Th); trabecular number (Tb.N); connectivity density (CD); structure model index (SMI); cortical volume (Ct.V); endocortical volume (Ec.V); total diaphyseal volume (Dp.V), cortical thickness (Ct.Th); cortical bone volume fraction (Ct.V/Dp.V); moment of inertia (MOI); Tartrate-resistant acid phosphatase (TRAP); number of osteoclasts per bone surface area (N.Oc/BS); surface area of osteoclasts per bone surface area (Oc.S/BS); arginine vasopressin (AVP); Na + -Ca 2+ exchanger (NCX1); vitamin D receptor (VDR). IntroductionCa 2+ is the most abundant cation in the human body and serves a number of important physiological functions, including fertilization, synaptic transmission, muscle contraction, blood clotting, and bone mineralization. The extracellular Ca 2+ concentration is controlled by the kidney, intestine, and bone through the action of the calciotropic hormones, including parathyroid hormone (PTH) and 1,25-dihydroxycholecalciferol (1,25-(OH) 2 D 3 ). In humans, the daily dietary Ca 2+ intake is less than 1,000 mg, of which only 30% is absorbed in the intestinal tract. This percentage is significantly enhanced during growth, pregnancy, and lactation by increased levels of circulating 1,25-(OH) 2 D 3 . Although there is continuous turnover of bone mass, there is no net gain or loss of Ca 2+ from bone in a young and healthy individual. This indicates that healthy adults excrete a maximum of 300 mg Ca 2+ in the urine to balance the intestinal Ca 2+ uptake and that the remaining 98% of the Ca 2+ filtered in the glomeruli is reabsorbed along the nephron. The molecular mechanism responsible for Ca 2+ absorption in the small intestine and the kidney was elusive for a long time.Renal Ca 2+ wasting, hyperabsorption, and reduced bone thickness in mice lacking TRPV5 Ca 2+ ions play a fundamental role in many cellular processes, and the extracellular concentration of Ca 2+ is kept under strict control to allow the proper physiological functions to take place. The kidney, small intestine, and bone determine the Ca 2+ flux to the extracellular Ca 2+ pool in a concerted fashion. Transient receptor potential (TRP) cation channel subfamily V, members 5 and 6 (TRPV5 and TRPV6) have recently been postulated to be the molecular gatekeepers facilitating Ca 2+ influx in these tissues and are members of the TRP family, which mediates diverse biological effects ranging from pain perception to male aggression. Genetic ablation of TRPV5 in the mouse allowed u...

Mutations in the DFNB31 gene encoding the PDZ scaffold protein whirlin are causative for hearing loss in man and mouse. Whirlin is known to be essential for the elongation process of the stereocilia of sensory hair cells in the inner ear, though its complete spatial and temporal expression patterns remained elusive. Here, we demonstrate that, in embryonic development, the gene is not only expressed in the inner ear, but also in the developing brain and the retina. Various isoforms of whirlin are widely and differentially expressed, and we provide evidence that whirlin directly associates with USH2A isoform b and VLGR1b, two proteins that we previously reported to be part of the Usher protein interactome. These proteins co-localize with whirlin at the synaptic regions of both photoreceptor cells and outer hair cells in the cochlea. These findings indicate that whirlin is part of a macromolecular PDZ protein scaffold that functions in the organization of the pre- and/or postsynaptic side of photoreceptor and hair cell synapses. Whirlin might be involved in synaptic adhesion through interaction with USH2A and VLGR1b as well as in synaptic development as suggested by its spatial and temporal expression patterns. In addition, we demonstrate that whirlin, USH2A and Vlgr1b co-localize at the connecting cilium and the outer limiting membrane of photoreceptor cells and in spiral ganglion neurons of the inner ear. Our data show that whirlin is connected to the dynamic Usher protein interactome and indicate that whirlin has a pleiotropic function in both the retina and the inner ear.

contributed equally to this work. Conflict of interest:The authors have declared that no conflict of interest exists. Nonstandard abbreviations used: parathyroid hormone (PTH); 1,25-dihydroxycholecalciferol (1,25-(OH)2D3); transient receptor potential (TRP); TRP cation channel subfamily V, member 5 (TRPV5); TRP cation channel subfamily V, member 6 (TRPV6); last loop of proximal tubules (LPT); hypoxanthine-guanine phosphoribosyl transferase (HPRT); bone volume (BV); total bone marrow volume including trabeculae (TV); trabecular bone volume fraction (BV/TV); trabecular thickness (Tb.Th); trabecular number (Tb.N); connectivity density (CD); structure model index (SMI); cortical volume (Ct.V); endocortical volume (Ec.V); total diaphyseal volume (Dp.V), cortical thickness (Ct.Th); cortical bone volume fraction (Ct.V/Dp.V); moment of inertia (MOI); Tartrate-resistant acid phosphatase (TRAP); number of osteoclasts per bone surface area (N.Oc/BS); surface area of osteoclasts per bone surface area (Oc.S/BS); arginine vasopressin (AVP); Na + -Ca 2+ exchanger (NCX1); vitamin D receptor (VDR). IntroductionCa 2+ is the most abundant cation in the human body and serves a number of important physiological functions, including fertilization, synaptic transmission, muscle contraction, blood clotting, and bone mineralization. The extracellular Ca 2+ concentration is controlled by the kidney, intestine, and bone through the action of the calciotropic hormones, including parathyroid hormone (PTH) and 1,25-dihydroxycholecalciferol (1,25-(OH) 2 D 3 ). In humans, the daily dietary Ca 2+ intake is less than 1,000 mg, of which only 30% is absorbed in the intestinal tract. This percentage is significantly enhanced during growth, pregnancy, and lactation by increased levels of circulating 1,25-(OH) 2 D 3 . Although there is continuous turnover of bone mass, there is no net gain or loss of Ca 2+ from bone in a young and healthy individual. This indicates that healthy adults excrete a maximum of 300 mg Ca 2+ in the urine to balance the intestinal Ca 2+ uptake and that the remaining 98% of the Ca 2+ filtered in the glomeruli is reabsorbed along the nephron. The molecular mechanism responsible for Ca 2+ absorption in the small intestine and the kidney was elusive for a long time.Renal Ca 2+ wasting, hyperabsorption, and reduced bone thickness in mice lacking TRPV5 Ca 2+ ions play a fundamental role in many cellular processes, and the extracellular concentration of Ca 2+ is kept under strict control to allow the proper physiological functions to take place. The kidney, small intestine, and bone determine the Ca 2+ flux to the extracellular Ca 2+ pool in a concerted fashion. Transient receptor potential (TRP) cation channel subfamily V, members 5 and 6 (TRPV5 and TRPV6) have recently been postulated to be the molecular gatekeepers facilitating Ca 2+ influx in these tissues and are members of the TRP family, which mediates diverse biological effects ranging from pain perception to male aggression. Genetic ablation of TRPV5 in the mouse allowed u...

Leber congenital amaurosis (LCA) causes blindness or severe visual impairment at or within a few months of birth. Here we show, using homozygosity mapping, that the LCA5 gene on chromosome 6q14, which encodes the previously unknown ciliary protein lebercilin, is associated with this disease. We detected homozygous nonsense and frameshift mutations in LCA5 in five families affected with LCA. In a sixth family, the LCA5 transcript was completely absent. LCA5 is expressed widely throughout development, although the phenotype in affected individuals is limited to the eye. Lebercilin localizes to the connecting cilia of photoreceptors and to the microtubules, centrioles and primary cilia of cultured mammalian cells. Using tandem affinity purification, we identified 24 proteins that link lebercilin to centrosomal and ciliary functions. Members of this interactome represent candidate genes for LCA and other ciliopathies. Our findings emphasize the emerging role of disrupted ciliary processes in the molecular pathogenesis of LCA.