Developmental changes in cellular prion protein in primate visual cortex.

Cellular prion protein (PrP(c)) is a cell surface glycoprotein highly expressed in neurons, and a protease-resistant conformer of the protein accumulates in the brain parenchyma in prion diseases. In human prion diseases, visual cortex and visual function can be affected. We examined both the levels and the localization of PrP(c) in developing visual cortex of the common marmoset. Western blot analysis showed that PrP(c) increased from the day of birth through adulthood, and this increase correlated with the progression of synapse formation. Immunohistochemistry showed that PrP(c) was present in fiber tracts of the neonate, and this immunoreactivity was lost with maturation. Within the neuropil, the laminar distribution of PrP(c) changed with age. In the neonate, PrP(c) immunoreactivity was strongest in layer 1, where the earliest synapses form. At the end of the first postnatal week, layer 4C, as identified by its strong cytochrome oxidase activity, was noticeably lighter in terms of PrP(c) immunoreactivity than the adjacent layers. The contrast between the strong immunoreactivity in both supragranular and infragranular layers and weak immunoreactivity in layer 4C increased with age. Layers 2/3 and 5 contained more intense PrP(c) immunoreactivity; these layers receive thalamic input from the koniocellular division of the LGN, and these layers of the LGN also had strong PrP(c) immunoreactivity. Together, these results provide evidence for PrP(c) localization in an identified functional pathway and may shed some light on prion disease pathogenesis.