TRANSGENIC MICE CARRYING THE DOMINANT RHODOPSIN MUTATION P347S - EVIDENCE FOR DEFECTIVE VECTORIAL TRANSPORT OF RHODOPSIN TO THE OUTER SEGMENTS

To explore the pathogenic mechanism of dominant mutations affecting th e carboxyl terminus of rhodopsin that cause retinitis pigmentosa, me g enerated five lines of transgenic mice carrying the proline-347 to ser ine (P347S) mutation, The severity of photoreceptor degeneration corre lated with the levels of transgene expression in these lines, Visual f unction as measured by the electroretinogram was approximately normal at an early age when there was little histologic evidence of photorece ptor degeneration, but it deteriorated as photoreceptors degenerated, Immunocytochemical staining showed the mutant rhodopsin predominantly in the outer segments prior to histologically evident degeneration, a finding supported by quantitation of signal intensities in different r egions of the photoreceptor cells by confocal microscopy, A distinct h istopathologic abnormality was the accumulation of submicrometer-sized vesicles extracellularly near the junction between inner and outer se gments, The extracellular vesicles were bound by a single membrane tha t apparently contained rhodopsin as revealed by ultrastructural immuno cytochemical staining with anti-rhodopsin antibodies, The outer segmen ts, although shortened, contained well-packed discs, Proliferation of the endoplasmic reticulum as reported in Drosophila expressing dominan t rhodopsin mutations was not observed, The accumulation of rhodopsin- laden vesicles likely represents aberrant transport of rhodopsin from the inner segments to the nascent disc membranes of the outer segments , It is possible that photoreceptor degeneration occurs because of a f ailure to renew outer segments at a normal rate, thereby leading to a progressive shortening of outer segments, or because of the loss of ce llular contents to the extracellular space, or because of both.