Endocytotic Pathways in Neurons

Summary

The amyloid precursor protein is a cell surface protein of neurons that undergoes rapid endocytosis and degradation in lysosomes. In diseased brains the proteolysis of the protein in the lysosomes is aberrant, giving rise to novel proteolytic products, including the ß-amyloid fragment. We have examined the processes of endocytosis and protein degradation in neurons and the neuroendocrine PC12 cell line. We find evidence for two different classes of endosome that internalize different surface proteins. One of these classes of endosomes is enriched in cell bodies, where it carries out housekeeping functions. The other is in axonal processes, where its functions appear to include synaptic vesicle biogenesis. We have followed the biogenesis of synaptic vesicles in PC12 cells using a synaptic vesicle protein, VAMP or synaptobrevin, that is epitope tagged on its lumenal domain. No neural-specific protein is required for the endocytosis of VAMP since it is endocytosed rapidly in transfected CHO cells. Mutational analysis of VAMP showed that the signal for endocytosis was constrained to a small sequence which showed no homologies with other known endocytotic signals. Domains required for sorting to synaptic vesicles were distributed throughout the length of the molecule. The sequence required for endocytosis, which was contained within a region predicted to be a coiled coil, was also required for targeting to synaptic vesicles.

Internalization of synaptic vesicle proteins into the nerve terminal can also be studied using the neuromuscular junction of Drosophila larvae. The shibire locus controls endocytosis. Mutations in this locus arrest the synaptic vesicle life cycle at the internalization step, leading to loss of synaptic vesicle content and paralysis. We show that the shibire arrest precedes a calcium-independent step.

To study what targets proteins from the endosome to the lysosome, we studied a surface protein that is rapidly degraded in cells, P-selectin. The rapid degradation in lysosomes of this protein could be attributed to an exon that codes for a 10 amino acid region of the cytoplasmic tail. The amyloid precursor, which is also rapidly degraded, may have an equivalent lysosomal targeting domain.