Stage-specific association of apolipoprotein A-I and E in developing mouse retina

To characterize the intercellular lipid transport systems in differentiating retina.

Developing mouse retinas were evaluated for the expression of apolipoproteins (apoE, apoA-I) by Western blot analysis and reverse transcription-polymerase chain reaction (RT-PCR). They were compared with changes in the lipid content and association of retinal proteins, such as postsynaptic density protein 95, glial fibrillary acidic protein, and cellular retinaldehyde-binding protein. Intraretinal distribution of apolipoproteins and their receptors was examined by immunofluorescence and in situ hybridization of prenatal and postnatal retinal sections. In vitro culture of dissociated cells was also examined.

Although apoE is known to be present in the mature retina, the neonatal retina remarkably expressed apoA-I mRNA and protein. This protein was present until postnatal day (P)3, and its putative receptor, scavenger receptor class B-I, was present until P5 to P7. This state subsequently exhibited a dramatic switchover to an apoE-rich one, in parallel with the stratification. Whereas apoE was synthesized at low levels until P7, apoE mRNA was clearly concentrated in Müller glia cells, which extended long apoE-bound processes to the plexuses and contours of photoreceptor cells. These acceptor cells expressed LDL receptor-related protein 1 as a putative receptor. ApoE genes were not transcribed in ganglion cells, though they were associated with a high level of the protein throughout the development. ApoE protein in ganglion cells initially appeared to be synthesized by astrocytes but later were observed to be supplied from an extraretinal space.

The present results document several new aspects of apoA-I and apoE in the developing retina. The switchover of the lipoprotein systems runs a parallel course with the differentiation.