Expression of amyloid precursor protein (APP) in rat brain and cultured neural cells.

Cell type-specific and developmental patterns of APP expression were investigated in rat brain and cultured neural cells. Nearly all astrocytes were APP-positive, whereas only selected population of neurons appeared to express APP. In these neurons, APP immunoreactivity was preferentially restricted to single processes. mRNAs encoding the major APP isoforms, APP695 and APP770, were co-expressed as 3.4-3.6 kb transcripts both in astrocytes and neurons. In addition, an unusual 2.8 kb mRNA size class was revealed in cultured cerebellar granule neurons by means of the probe recognizing APP770 mRNA. Also, for the first time, APP714 mRNA was detected in rat brain by northern blotting. The steady-state levels of these transcripts were increased from birth up to postnatal day 20, whereas no apparent changes were observed after reaching adulthood. These data hint at the involvement of APP in the major morphogenetic events taking place in rat brain during the first three postnatal weeks.