Ординатура / Офтальмология / Английские материалы / Retinal Degenerations biology, diagnostics, and therapeutics_Tombran-Tink, Barnstable_2007

Fig. 3. Structural organization of the known Usher 2 and Usher 3 proteins. The Usher 2A protein, usherin, has four main structural elements. Following a signal peptide sequence (SP), is a domain with homology to the thromospondin family of extracellular matrix proteins (TS), and a laminins (LN) module, which is a globular domain found in many laminins. Like laminins, this domain is followed by 10 consecutive repeats of laminin-epithelial growth factor-like modules (LE). Lastly, it contains three repeats of fibronectin type III (FN) 86. Three different isoforms of the Usher 2C protein, VLGR1, are predicted, with isoforms a and c being abbreviated versions of isoform b. VLGR1b is a G protein coupled receptor (GPCR) with a very large extracellular component, containing 35 CalX-β domains, a LamG/TspN/PTX homology domain (LTP), 7 EAR/EPTP repeats (epilepsy-associated repeat/Epitempin repeat, forming a protein interaction domain), and a GPCR proteolysis site (PS). 7TM, seven transmembrane domains. See references in ref. 37. The Usher 3 protein, clarin1, has four predicted transmembrane regions 38.

observed to undergo a small amount of retinal degeneration, which was not observed in mice that were homozygous mutant for only one of the genes (44).

In protein-binding studies with ear isoforms, two groups found binding of the cytoplasmic region of cadherin23 to the second PDZ domain of harmonin (45,46). One of the groups also reported binding between the first PDZ domain (PDZ1) of harmonin and myosin VIIa (45), whereas the other group reported that the PDZ1 domain of harmonin binds the isoform of cadherin23 that is found in tissues other than the ear, such as kidney, brain, and retina (46). The binding of sans to the harmonin PDZ1 domain has also been reported (34).

Rather controversially, Wolfrum and colleagues have proposed that all the Usher 1 and Usher 2 proteins, including the putative Usher 2B protein, NBC3, form a supramolecular complex in the photoreceptor synapse (47). This suggestion is based on protein-binding data and immunolocalization studies. However, it has been argued that the reported immunolocalization of some of these proteins to the synapse is artefactual (48). Moreover, none of the mouse models for Usher syndrome have been shown to have defective synaptic transmission from the photoreceptor cells. Detailed ERG analyses have been made on mice carrying mutations in Myo7a, Cdh23, and Pcdh15 (orthologs of the Usher 1B, 1D, and 1F genes) (43,49,50), and although the Myo7a and Cdh23 mutants have a reduced a- wave amplitude (indicating a reduced photoreceptor response), the ratio of the b-wave amplitude to a-wave amplitude appeared normal in all three mutants. This ratio is a good indicator of signal transmission from the photoreceptor cells to the second-order neurons; it is reduced in a variety of mice with photoreceptor synaptic defects (51,52).

Although it is likely that some Usher proteins function together, on balance, most evidence points to independent functions of Usher 1 proteins in the retina. There are several subcellular regions in which only one protein is found (Fig. 1). The clearest example of an independent function is the RPE function of myosin VIIa, the most exten- sively-studied Usher protein.

Most of the retinal myosin VIIa is present in the RPE (53), where it performs more than one function. No other Usher protein has been detected in the RPE. Studies on mutant Myo7a mice, including a line that has a null allele, have shown that myosin VIIa is required for the correct localization of the RPE melanosomes (54), by transporting and tethering melanosomes in the apical RPE (55–57). In addition, myosin VIIa functions in the delivery of phagosomes to the basal RPE and, in this role, is important for the efficient degradation of phagosomes (58).

Myosin VIIa is also present in the connecting cilium of photoreceptor cells (59), where it is required for the normal transport of opsin (60). Cadherin23 also appears to be associated with the connecting cilium (46,61) (which may explain the synergistic effect of the combined loss of MYO7A and CDH23 on photoreceptor death [44]). So far, there have been no reports of any other Usher protein associated with this structure, however, protocadherin15 appears to be “nearby,” at the base of the outer segment (83).

Little is known about the Usher 2 and Usher 3 proteins that have been identified. As noted previously, usherin is an extracellular matrix protein. In the retina, it has been described as a component of Bruch’s membrane, which supports the RPE (88). A different group has found it to be localized also in the photoreceptor cells (89).

The Usher 3 protein, clarin1, is unusual among RP proteins in that it is present in the second-order neurons of the retina and, thus, apparently functions in one or more postsynaptic processes (62).

RETINAL PATHOGENESIS

Our understanding of the cellular bases of retinal degeneration in Usher syndrome has been limited by the availability of animal models that mimic Usher syndrome. Curiously, none of the Usher 1 mouse models has been found to undergo retinal degeneration. Such models include different alleles for Myo7a, Ush1c, Cdh23, and Pcdh15. Animal models for Usher 2 and Usher 3 have yet to be published.

The most thoroughly studied model is the shaker1 mouse, which has mutant Myo7a (40), with the 4626SB allele having a null mutation (60,63). As noted here previously, studies of these mice have shown that myosin VIIa is required for normal photoreceptor electrophysiology (49), and several normal cellular functions in the RPE and photoreceptor cells (54,56–58,60), although there is no photoreceptor cell death. Of particular interest to pathogenesis is its role in the turnover of phototransductive membrane. Myosin VIIa is required for both the efficient phagosome delivery to the basal RPE, and its ensuing degradation, as well as the normal flow of opsin along the connecting cilium. Either one or both of these roles results in a retarded turnover of the phototransductive disk membranes (60). Although in the mouse retina, these defects do not result in photoreceptor cell death (at least within the life span of a mouse), this turnover process is clearly critical. Faults at any one of several stages of disk membrane turnover underlie various forms of retinal degeneration, whether it is in the transport of proteins to the outer segment, the formation of new disk membranes (as in Rds mutations) (64–66), or in the phagocytosis of outer segment disks (as in Mertk mutations) (67,68). The retinal degeneration found in patients with Usher 1B might therefore be linked to defects in the turnover of phototransductive disk membrane.

SUMMARY

Most of the genes responsible for Usher syndrome have been identified. Clinical studies of retinal defects have identified minor differences between Usher 1 and Usher 2, but no differences have yet been detected among the different genotypes of a given Usher type. Most of the Usher proteins are present in the photoreceptor cells, although the Usher 1B protein also functions in the RPE cells, the Usher 2A protein is a component of Bruch’s membrane, and the Usher 3 protein is found only in the inner retinal cells. The proteins are more likely to function in cellular organization rather than in processes more directly related to phototransduction.

The previously reported locus for Usher syndrome type 1A (69) has now been shown to be false (90). Hence, there are presently six, not seven, known loci for Usher 1.

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