Apolipoprotein E expression in the cerebellum of normal and hypercholesterolemic rabbits

We have studied the expression of apolipoprotein E (ApoE) mRNA in the cerebella of control and experimental rabbits fed with a cholesterol-rich diet for 8 weeks. Cholesterol-treated rabbits show a dramatic increase in serum cholesterol levels; however, no significant variations in the expression level of cerebellar ApoE mRNA were found in comparison to control rabbits. In addition, no differences were observed between control and hypercholesterolemic rabbits in the in situ hybridization pattern of ApoE mRNA on cerebellar cortex sections. ApoE mRNA was localized in astroglial processes associated with Purkinje cell bodies and dendrites, granule cell clusters, blood vessels and nerve fibers of the white matter. No expression of ApoE mRNA was observed in Purkinje and granule cell neurons. Polarized light examination of cryostat cerebellar sections revealed the absence of cholesterol-rich microglia/macrophage cells induced by the hypercholesterolemia. In this way, neither reactive microglial cells nor perivascular phagocytes were found by ultrastructural analysis in hypercholesterolemic conditions. The pattern of glial fibrillary acidic protein of the astroglial cells of the cerebellar cortex as well as their nuclear size were unchanged following cholesterol treatment, indicating the absence of astroglial activation induced by hypercholesterolemia. Our results suggest that cerebellar ApoE does not contribute to the general cholesterol homeostasis outside of the brain and supports the view that this cerebellar ApoE is involved in paracrine and autocrine functions particularly related with synpase turnover and membrane remodelling of astroglial cells.