The authors thank Dr. E. Kominami for antibody to subunit c of mitochondrial ATPase, L. Trakimas and M. Ericsson of the Harvard Medical School Electron Microscope Facility, Dr. David Sulzer for assistance with TEM analysis, Dr. Sylvie Breton for assistance with endocytosis experiments, and Dr. Andre Bernards and James Follen for assistance with the cathepsin D activity assay. This work was supported by NIH/NINDS grant # NS 33648. Dr. S.L. Cotman received fellowship funding from the Batten Disease Support and Research Association (BDSRA).