ENVIRONMENTAL FACTORS IN USHERS SYNDROME

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Usher's syndrome is characterized by combined congenital hearing loss and progressive retinal degeneration. The disease accounts for the majority of cases involving inherited deficits of both hearing and vision. Usher's is unique among the roughly dozen hereditary syndromes involving the ear and eye in that its effects are relatively restricted. These are almost entirely limited to the auditory, visual, and sometimes vestibular systems. The initial loss is a loss of receptor cells--the hair cells of the cochlea and vestibule, and the photoreceptors of the eye. Although Usher's has been subdivided into three phenotypic classes, the genetic forms of this disease number many more. The fact that the multiple types of Usher's have such circumscribed effects suggests that all forms of this disease act upon one or a few structures or processes common to the sensory epithelia of the ear and eye. Since the cascade of events in sensory transduction is quite different in the ear and eye, one common focus may involve some aspect(s) of cellular metabolism. It has long been known that prolonged exposure to intense light causes permanent oxidative damage to the retina (e.g., Noell, 1963). Likewise, hearing losses resulting from noise exposure have been suggested to involve oxidative damage (Siedman et al., 1993). The expression of Usher's may involve inadequate removal or repair of damage associated with reactive oxygen species (ROS) which are generated in the course of normal sensory transduction. ROS-related damage has been implicated as causing or exaCerbating several neurodegenerative diseases (Gotz et al., 1994). Compromised cells are less capable of fending off ROS-related damage to lipids, proteins and DNA, and are less able to compartmentalize the reactions that generate ROS. Presently, almost nothing is known about the role of ROS in inherited degeneration of sensory epithelia. In this revised application, we propose to explore the role of ROS in cochlear and retinal degeneration in two candidate mouse models of Usher's syndrome, tubby and shaker-1. These have received support as Usher's models on the basis of their phenotypic similarity to Usher's and their genetics (Gibson et al., 1995; Well et al, 1995; Heckenlively et al., 1994; Ohlemiller et al., 1995). We will localize and quantify ROS formation in vivo in the organ of Corti and retina of these two models using an oxidation-sensitive fluorescent indicator, 2,7-dichlorofluorescin diacetate, and laser confocal microscopy. The sensory epithelia will be examined at two ages: in young animals prior to the onset of substantial neurodegeneration, and in older animals in which cell loss is ongoing. Moderate noise and light exposure will be used to address whether ROS levels are activity dependent. In addition, tissue content of an important endogenous antioxidant, glutathione, will be determined under similar conditions. Glutathione content is decreased by oxidative injury, so assessment of its concentration may provide evidence for ongoing ROS elevation that is below the detection sensitivity of fluorescent indicators.