Selective failure of long-term survival of isolated photoreceptors from both homozygous and heterozygous rd (retinal degeneration) mice.

Retinas from homozygous rdle/rdle and heterozygous rdle/++ C57BL/6J mice were dissected and dissociated on postnatal day 2, when they are still essentially indistinguishable. The resulting cell suspensions were seeded on highly adhesive substrata, to which the cells attach as individual units, and grown in vitro for 2 weeks in serum-free, chemically defined media. The behavior of neurons and photoreceptors in vitro was investigated with several techniques; essentially no differences were found between rdle/rdle and rdle/++ cells. Three distinctive cell types could be recognized in cultures of both genotypes towards the end of the first week in vitro: process-free cells, multipolar neurons and rod photoreceptors. There were similarities between rdle/rdle and rdle/++ cultures in the number and morphology of photoreceptor cells, to include the presence of a cilium and a short neurite terminating in a spherule-like body. Moreover, in cultures of both genotypes, only photoreceptors showed opsin immunoreactivity and the antigen recognized by the rod-specific monoclonal antibody RET-P1. Biochemical and autoradiographic studies demonstrated that rdle/rdle and rdle/++ cells also showed similar uptakes of the putative amino acid neurotransmitters glutamate and aspartate (associated with most of the photoreceptors and only some neurons), and gamma-aminobutyric acid (associated with neurons but absent in photoreceptors). Thus, according to several parameters, the properties shown by photoreceptor cells were similar in rdle/rdle and rdle/++ cultures during the first week in vitro. Massive photoreceptor cell death was observed in both genotypes during the second week in vitro, coinciding with the time when photoreceptor degeneration occurs in vivo in rd/rd, but not in rd/+ retinas. Photoreceptor death in culture appeared to be specific, since approx. 80% of the non-photoreceptor neurons survived normally during the period when photoreceptor degeneration took place. Several reports from the literature suggest that the period around postnatal days 8-10 represents a critical stage for rd/rd photoreceptors, since they survive until this time but degenerate thereafter. Genetically normal photoreceptors apparently undergo a comparable crisis during maintenance in primary culture, suggesting the involvement of cell-cell contacts and/or retina-derived environmental signals in the survival or rod visual cells.