Human chromosome 10q21-22 harbors USH1F in a region of conserved synteny to mouse chromosome 10. This region of mouse chromosome 10 contains Pcdh15, encoding a protocadherin gene that is mutated in Ames waltzer and causes deafness and vestibular dysfunction. Two mutations of protocadherin 15 (PCDH15) have been found in two families segregating Usher syndrome type 1F. A Northern blot probed with the PCDH15 cytoplasmic domain showed expression in the retina, consistent with its pathogenetic role in the retinitis pigmentosa associated with USH1F (Ahmed, 2001).
The molecular basis for Usher syndrome type 1F (USH1F) has been determined in two families segregating for this type of syndromic deafness. By fluorescence in situ hybridization, the human homolog of the mouse protocadherin Pcdh15 has been placed in the linkage interval defined by the USH1F locus. The genomic structure of this novel protocadherin was determined, and a single-base deletion was found in exon 10 in one USH1F family and a nonsense mutation in exon 2 in the second. Consistent with the phenotypes observed in these families, expression of PCDH15 was demonstrated in the retina and cochlea by RT-PCR and immunohistochemistry. This report shows that protocadherins are essential for maintenance of normal retinal and cochlear function (Alagramam, 2001b).
Recessive splice site and nonsense mutations of PCDH15, encoding protocadherin 15, are known to cause deafness and retinitis pigmentosa in Usher syndrome type 1F (USH1F). Non-syndromic recessive hearing loss (DFNB23) is caused by missense mutations of PCDH15. This suggests a genotype-phenotype correlation in which hypomorphic alleles cause non-syndromic hearing loss, while more severe mutations of this gene result in USH1F. Protocadherin 15 has been localized to inner ear hair cell stereocilia and to retinal photoreceptors by immunocytochemistry. These results further strengthen the importance of protocadherin 15 in the morphogenesis and cohesion of stereocilia bundles and retinal photoreceptor cell maintenance or function (Ahmed, 2003).
Protocadherin 15 (Pcdh15) is associated with the Ames waltzer mutation in the mouse. By situ hybridization Pcdh15 is found to be expressed in the sensory epithelium in the developing inner ear, in Rathke's pouch, and broadly throughout the brain with the highest level of expression being detected at embryonic day 16 (E16). Pcdh15 transcripts are also found in the developing eye, dorsal root ganglion, and the dorsal aspect of the neural tube, floor plate and ependymal cells adjacent to the neural canal. Additionally, expression is also detected in the developing glomeruli of the kidney, surface of the tongue, vibrissae, bronchi of the lung, and in the epithelium of the olfactory apparatus, gut and lung (Murcia, 2001).
Natural killer cells are well known to play an important role in immune defense against tumor development and viral infections. To further characterize new functionally relevant structures in these cells, A series of monoclonal antibodies were studied that were raised against the NK cell line YT. One of these antibodies previously described as AY19, recognizes a 85 kD surface glycoprotein. This study reports the identification of a new secreted isoform of protocadherin 15, PCDH15C, which represents a potential associated protein for p85. Importantly, whereas protocadherins are absent from the surface of normal hematopoietic cells, this study shows that PCDH15 is expressed in cytotoxic tumor-derived T- and NK-cell lines as well as in biopsies of nasal NK/T-cell lymphomas (Rouget-Quermalet, 2006).
The neuroepithelia of the inner ear contain hair cells that function as mechanoreceptors to transduce sound and motion signals. Mutations affecting these neuroepithelia cause deafness and vestibular dysfuction in humans. Ames waltzer (av) is a recessive mutation found in mice that causes deafness and a balance disorder associated with the degeneration of inner ear neuroepithelia. As reported here, the gene that harbours the av mutation encodes a novel protocadherin. Cochlear hair cells in the av mutants show abnormal stereocilia by 10 days after birth (P10). This is the first evidence for the requirement of a protocadherin for normal function of the mammalian inner ear (Alagramam, 2001a).
The deaf-circling Ames waltzer (av) mouse harbors a mutation in the protocadherin 15 (Pcdh15) gene and is a model for inner ear defects associated with Usher syndrome type 1F. Earlier studies showed altered cochlear hair cell morphology in young av mice. In contrast, no structural abnormality consistent with significant vestibular dysfunction in young av mice was observed. Light and scanning electron microscopic studies showed that vestibular hair cells from presumptive null alleles Pcdh15(av-Tg) and Pcdh15(av-3J) are morphologically similar to vestibular sensory cells from control littermates, suggesting that the observed phenotype in these alleles might be a result of a central, rather than peripheral, defect. In the present study, a combination of physiologic and anatomic methods was used to more thoroughly investigate the source of vestibular dysfunction in Ames waltzer mice. Analysis of vestibular evoked potentials and angular vestibulo-ocular reflexes revealed a lack of physiologic response to linear and angular acceleratory stimuli in Pcdh15 mutant mice. Optokinetic reflex function was diminished but still present in the mutant animals, suggesting that the defect is primarily peripheral in nature. These findings indicate that the mutation in Pcdh15 results in either a functional abnormality in the vestibular receptor organs or that the defects are limited to the vestibular nerve. AM1-43 dye uptake has been shown to correlate with normal transduction function in hair cells. Dye uptake was found to be dramatically reduced in Pcdh15 mutants compared to control littermates, suggesting that the mutation affects hair cell function, although structural abnormalities consistent with significant vestibular dysfunction are not apparent by light and scanning electron microscopy in the vestibular neuroepithelia of young animals (Alagramam, 2005).
Mutations in genes coding for cadherin 23 and protocadherin 15 cause deafness in both mice and humans. Evidence is provided that mutations at these two cadherin loci can interact to cause hearing loss in digenic heterozygotes of both species. Using a classical genetic approach, mice were generated that were heterozygous for both Cdh23 and Pcdh15 mutations on a uniform C57BL/6J background. Significant levels of hearing loss were detected in these mice when compared to age-matched single heterozygous animals or normal controls. Cytoarchitectural defects in the cochlea of digenic heterozygotes, including degeneration of the stereocilia and a base-apex loss of hair cells and spiral ganglion cells, were consistent with the observed age-related hearing loss of these mice beginning with the high frequencies. In humans, evidence has been obtained for a digenic inheritance of a USH1 phenotype in three unrelated families with mutations in CDH23 and PCDH15. Altogether, these data indicate that CDH23 and PCDH15 play an essential long-term role in maintaining the normal organization of the stereocilia bundle (Zheng, 2005).
In the sensory receptors of both the eye and the ear, specialized apical structures have evolved to detect environmental stimuli such as light and sound. Despite the morphological divergence of these specialized structures and differing transduction mechanisms, the receptors appear to rely in part on a shared group of genes for function. For example, mutations in Usher (USH) genes cause a syndrome of visual and acoustic-vestibular deficits in humans. Several of the affected genes have been identified, including the USH1F gene, which encodes protocadherin 15 (PCDH15). Pcdh15 mutant mice also have both auditory and vestibular defects, although visual defects are not evident. Zebrafish have two closely related pcdh15 genes that are required for receptor-cell function and morphology in the eye or ear. Mutations in pcdh15a cause deafness and vestibular dysfunction, presumably because hair bundles of inner-ear receptors are splayed. Vision, however, is not affected in pcdh15a mutants. By contrast, reduction of pcdh15b activity using antisense morpholino oligonucleotides causes a visual defect. Optokinetic and electroretinogram responses are reduced in pcdh15b morpholino-injected larvae. In electron micrographs, morphant photoreceptor outer segments are improperly arranged, positioned perpendicular to the retinal pigment epithelium and are clumped together. These results suggest that both cadherins act within their respective transduction organelles: Pcdh15a is necessary for integrity of the stereociliary bundle, whereas Pcdh15b is required for alignment and interdigitation of photoreceptor outer segments with the pigment epithelium. It is concluded that after a duplication of pcdh15, one gene retains an essential function in the ear and the other in the eye (Seiler, 2005).
Defects in myosin VIIa, harmonin (a PDZ domain protein), cadherin 23, protocadherin 15 and sans (a putative scaffolding protein), underlie five forms of Usher syndrome type I (USH1). Mouse mutants for all these proteins exhibit disorganization of their hair bundle, which is the mechanotransduction receptive structure of the inner ear sensory cells, the cochlear and vestibular hair cells. Harmonin interacts with cadherin 23 and myosin VIIa. This study addresses the extent of interactions between the five known USH1 proteins. The previously suggested sans-harmonin interaction has beem established; sans also binds to myosin VIIa. Sans can form homomeric structures and harmonin b can interact with all harmonin isoforms. Harmonin also binds to protocadherin 15. Molecular characterization of these interactions indicates that through its binding to four of the five USH1 proteins, the first PDZ domain (PDZ1) of harmonin plays a central role in this network. Sans localizes in the apical region of cochlear and vestibular hair cell bodies underneath the cuticular plate. In contrast to the other four known USH1 proteins, no sans labeling is detected within the stereocilia. It is proposed that via its binding to myosin VIIa and/or harmonin, sans controls the hair bundle cohesion and proper development by regulating the traffic of USH1 proteins en route to the stereocilia (Adato, 2005).
The human Usher syndrome (USH) is the most common form of deaf-blindness. Usher type I (USH1), the most severe form, is characterized by profound congenital deafness, constant vestibular dysfunction and prepubertal onset of retinitis pigmentosa. Five corresponding genes of the seven USH1 genes have been cloned over the years. Recent studies indicated that three USH1 proteins, namely myosin VIIa (USH1B), SANS (USH1G), and cadherin 23 (USH1D) interact with the USH1C gene product harmonin. In these protein-protein complexes harmonin acts as the scaffold protein binding these USH1 molecules via its PDZ domains. The aim of the present study was to analyze whether or not the fifth identified USH1 protein protocadherin 15 (Pcdh15) also binds to harmonin and where these putative protein complexes might be localized in mammalian rod and cone photoreceptor cells. In vitro binding assays (GST pull-down, yeast two-hybrid assay) were applied. Antibodies against bacterial expressed USH1 proteins were generated. Affinity purified antibodies were used in immunoblot analyses of brain fractions and isolated retinas, in immunofluorescence studies, and in immunoelectron microscopic studies of rodent retinas. Pcdh15 (USH1F) has been shown to interact with harmonin PDZ2. Immunocytochemistry revealed that Pcdh15 is expressed in photoreceptor cells of the mammalian retina, where it is colocalized with harmonin, myosin VIIa, and cadherin 23 at the synaptic terminal. Colocalization of Pcdh15 with harmonin was found at the base of the photoreceptor outer segment, where newly synthesized disk membranes are present. These data indicate that harmonin-Pcdh15 interactions probably play a role in disk morphogenesis. Furthermore, evidence is provided that a complex composed of all USH1 molecules may assemble at the photoreceptor synapse. This USH protein complex can contribute to the cortical cytoskeletal matrices of the pre- and post-synaptic regions, which are thought to play a fundamental role in the structural and functional organization of the synaptic junction. Defects in any of the USH1-complex partners may result in photoreceptor dysfunction causing retinitis pigmentosa, the clinical phenotype in the retina of USH1 patients (Reiners, 2005).
Hair cells of the mammalian inner ear are the mechanoreceptors that convert sound-induced vibrations into electrical signals. The molecular mechanisms that regulate the development and function of the mechanically sensitive organelle of hair cells, the hair bundle, are poorly defined. Two gene products that have been associated with deafness and hair bundle defects, protocadherin 15 (PCDH15) and myosin VIIa (MYO7A), can be linked into a common pathway. PCDH15 binds to MYO7A and both proteins are expressed in an overlapping pattern in hair bundles. PCDH15 localization is perturbed in MYO7A-deficient mice, whereas MYO7A localization is perturbed in PCDH15-deficient mice. Like MYO7A, PCDH15 is critical for the development of hair bundles in cochlear and vestibular hair cells, controlling hair bundle morphogenesis and polarity. Cochlear and vestibular hair cells from PCDH15-deficient mice also show defects in mechanotransduction. Together, these findings suggest that PCDH15 and MYO7A cooperate to regulate the development and function of the mechanically sensitive hair bundle (Senften, 2006).
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