The endosomal trafficking of apolipoprotein E3 and E4 in cultured human brain neurons and astrocytes

The association of the E4 isoform of apolipoprotein E (apoE) as a genetic r isk factor for late onset Alzheimer's disease (AD) has been well establishe d. Central nervous system (CNS) neurons are specifically affected so that d efining the mechanisms by which two of the major human apoE isoforms act wi thin CNS neurons is important to our understanding of their effect on neuro nal maintenance and function. We have developed a cell culture model using human brain tissue to characterize exogenous apoE transport. We have tracke d the association of apoE3 and E4 with CD63, the GTP-binding protein rab5a and the acidic hydrolase cathepsin D, which localize lysosomes, early endos omes, and late endosomes/lysosomes, respectively. Double immunostaining and confocal laser scanning microscopy revealed by z-series that after 30 min most intraneuronal apoE colocalized with rab5a, whereas no astrocyte apoE/r ab5a colocalization was detected. Conversely, apoE3 and CD63 did not coloca lize in neurons, even after 1 h, but was colocalized in astrocytes. Also, t here was approximately 9% apoE3 colocalization with cathepsin D in neurons, whereas up to 87% of apoE4 vesicles were colocalized. In astrocytes, the p roportion of apoE3 colocalized with cathepsin D was greater than that in ne urons, but still significantly different from that found with apoE4. These immunohistological data demonstrate that, in neurons, apoE can be endocytos ed via a rab5a-regulated vesicle-mediated pathway and that beyond this stag e there may be isoform specific differences in apoE trafficking present in both neurons and astrocytes. (C) 2001 Academic Press.