A minimally lipidated form of cell-derived apolipoprotein E exhibits isoform-specific stimulation of neurite outgrowth in the absence of exogenous lipids or lipoproteins

Within the central nervous system,;apolipoprotein E (apoE) synthesis Is increased in response to nerve injury, a finding that may reflect a role for apoE in neuronal remodeling.,, Recent studies show that apoE3 promotes and apoE4 inhibits neurite outgrowth in cultured neuronal cells, interestingly, these isoform-specific effects are observed only when apoE is presented to cells in the presence of an exogenous lipid source such as rabbit beta-very low density lipoprotein (beta-VLDL, making, it difficult to discern the biologically active form of apoE or to understand the role of the lipid source. In the present study we tested whether a cell-derived lipidated form of apoE can alter neurite outgrowth in the? absence of beta-VLDL by constructing Neuro-2a cell lines expressing high levels of apoE. Our results showed that endogenous apoE3 stimulated neurite outgrowth, whereas the endogenous apoE4 isoform was neutral, Furthermore, beta-VLDL antagonized the stimulatory effects of the endogenous apoE3, Characterization of the secreted apoE3 indicated that the neurite outgrowth-stimulating activity could be recovered,from culture medium with an anti-apoE immunoaffinity column and was present in a poorly lipidated particle with a density between 1.19 and 1.26 g/ml. These results indicated that the biological activity of apoE3 in stimulating neurite outgrowth was inherent in the cell-derived apoE particle and was not dependent on either (a) an interaction of apoE3 with an artificial lipid source or (b) independent actions of apoE3 and beta-VLDL.