Distribution of transcript and protein isoforms of the synaptic glycoprotein neuroplastin in rat retina

PURPOSE. To examine the expression and localization of the neuroplastins (n p), two synapse-enriched members of the immunoglobulin(Ig) superfamily of c ell-adhesion molecules, in the developing and adult retina and optic nerve. METHODS. Expressions of the two isoforms np55 and np65 and carboxyl-termina l splice variants were investigated by immunocytochemistry. Western blot an alysis, RT-PCR, and in situ hybridization. RESULTS. Immunoreactivity for both neuroplastins was confined to the two sy naptic layers of the retina: the inner (IPL) and outer plexiform layer (OPL ). Significant overlap was found in staining at synaptic structures with sy naptophysin. A large proportion of immunoreactivity for both isoforms, howe ver, was of perisynaptic origin. In situ hybridization studies were suggest ive of a pre- and postsynaptic localization of np65 in the OPL. Transcripts for np55 were already present at birth in the inner retina, but the hybrid ization signals increased during postnatal development. Np65 transcripts an d immunosignals appeared at later developmental ages, concomitant with syna pse formation in the OPL. Several C-terminal neuroplastin cDNA clones harbo r an insert of 12 bp, coding for four amino acids (DDEP) in the intracellul ar domain of neuroplastins. Splice isoforms containing the insert exhibited a developmental expression pattern similar to that of np55; however, both neuroplastins could harbor the C-terminal insert. Neuroplastins: were also detected in optic nerve homogenates. RT-PCR and blockade of axonal transpor t by nerve crush confirmed transcript and protein expression in optic nerve tissue. CONCLUSIONS. Tile findings suggest a role for neuroplastins in cell adhesio n in the plexiform layers during histogenesis, as well as in maintenance of connections between specific cellular structures.