ABSTRACT
Thirteen retinoschisis males with genotyped XLRS1 gene mutations were examined by electroretinogram (ERG) techniques to determine photoreceptor involvement and ON-pathway and OFF-pathway sites of dysfunction. Parameters R(max) and logS determined by fitting the mathematical model of the activation phase of phototransduction to the scotopic and photopic a-wave responses, were not significantly different from normal. However, the XLRS photopic a-wave amplitudes were significantly lower than normal across all intensities, consistent with defective signaling in the OFF pathway. Long flash (150 ms) ON-OFF photopic responses showed reduced b-wave amplitude but normal d-wave amplitude, giving a reduced b/d ratio of <1.32 Hz photopic flicker ERG fundamental frequency responses showed reduced amplitude and delayed phase, consistent with abnormal signaling by both the ON- and OFF-pathway components. These results indicate that the XLRS1 protein appears not to affect photoreceptor function directly for most XLRS males, and that ERG signaling abnormalities occur in both the ON- and OFF-pathway components that originate in the proximal retina.
Subject(s)
Photoreceptor Cells, Vertebrate/physiology , Retinal Degeneration/physiopathology , Adolescent , Adult , Analysis of Variance , Case-Control Studies , Child , Color Perception/physiology , Electroretinography/methods , Genetic Linkage , Genotype , Humans , Male , Middle Aged , Models, Neurological , Mutation/genetics , Psychophysics , Retinal Degeneration/genetics , Vision, Ocular/physiology , X ChromosomeABSTRACT
PURPOSE: To correlate retinal functional changes with structural changes in P23H rhodopsin transgenic rats as a model of autosomal dominant retinitis pigmentosa. METHODS: P23H heterozygote (lines 1 and 3) and Sprague-Dawley control rats were studied at 4 to 29 weeks by retinal histology, electroretinogram (ERG), and a-wave transduction modeling. RESULTS: Both line 1 (faster degeneration) and line 3 (slower degeneration) showed progressive rod outer segment (ROS) shortening and outer nuclear layer (ONL) cell loss with age. ERG b-wave maximum amplitude (Vb(max)) decreased with age, but b-wave threshold remained constant within each line despite progressive ONL thinning and ROS shortening. The only exception was in line 1 at 29 weeks, which showed a slight threshold change relative to earlier ages. Va(max) and a-wave threshold changed more rapidly and were more sensitive than the b-wave in reflecting histologic degeneration. Va(max) was linearly proportional to the product of (ROS x ONL) across a two log unit range of data combined from both lines. The photopic b-wave was normal for both lines until the ONL thinned beyond 50%. Phototransduction sensitivity was normal for both lines, and dark-adaptation recovery after bleaching rhodopsin was normal. CONCLUSIONS: The P23H transgenic rat has a slow rod degeneration with initially normal cone function, consistent with clinical findings of P23H patients. However, the normal bleach recovery and the normal phototransduction sensitivity in this rat model are different from human P23H disease. a-Wave measures were more sensitive than the b-wave for tracking changes. b-Wave threshold was inexplicably poor for tracking degeneration. Although line 1 degenerated faster than line 3, the functional-structural correlates were the same. The tight linear relationship between saturated a-wave amplitude and the product of (ROS x ONL) indicates that the density of cGMP-gated channels per unit ROS plasma membrane area remains constant over a wide range of degenerations.
Subject(s)
Retina/physiopathology , Retinitis Pigmentosa/genetics , Retinitis Pigmentosa/pathology , Rhodopsin/genetics , Animals , Animals, Genetically Modified , Dark Adaptation , Electroretinography , Female , Male , Photoreceptor Cells, Vertebrate/physiology , Rats , Rats, Sprague-Dawley , Retina/pathology , Retinitis Pigmentosa/physiopathology , Vision, OcularABSTRACT
Mice carrying a targeted disruption of the rhodopsin gene develop a severe degenerative retinopathy, failing to elaborate rod photoreceptor outer segments (ROS), having no recordable rod electroretinogram (ERG) and losing all of their rod cells over a period of approximately 12 weeks. Murine and human rhodopsins differ in their amino acid sequences. Whether, or to what extent, such variability might influence the ability of human rhodopsin to serve as an adequate structural and functional substitute for the endogenous protein in mouse rod cells bears direct relevance to exploiting the full utility of Rho-/-animals as a model of degenerative retinal disease in man. We crossed Rho-/-mice with mice expressing a wild-type human rhodopsin transgene at levels approximating to those of the endogenous protein. Immunohistological examination of retinal selections from such animals demonstrated ROS of normal number and length and temporal expression of rhodopsin similar to that observed in wild-type animals; that is, immunoreactivity to an anti-rhodopsin antibody became clearly evident by day 3 post-partum. Whereas Rho-/-mice never display a rod ERG response, and even lose cone responses by 12 weeks of age, rescued mice showed 75% normal maximum amplitudes and had ERG b-wave thresholds (based on a 50 microV criterion) within 0.1 log unit of normal wild-type at 20 weeks, and cone amplitudes remained normal at this age. These data demonstrate very substantial structural and functional rescue of the rod photoreceptors of Rho-/-mice and long-term preservation by the human rhodopsin transgene.
