Faculty Bibliography

Nicastrin is an integral component of the high molecular weight presenilin complexes that control proteolytic processing of the amyloid precursor protein and Notch. We report here that nicastrin is most probably a type 1 transmembrane glycoprotein that is expressed at moderate levels in the brain and in cultured neurons. Immunofluorescence studies demonstrate that nicastrin is localized in the endoplasmic reticulum, Golgi, and a discrete population of vesicles. Glycosidase analyses reveal that endogenous nicastrin undergoes a conventional, trafficking-dependent maturation process. However, when highly expressed in transfected cells, there is a disproportionate accumulation of the endo-beta-N-acetylglucosaminidase H-sensitive, immature form, with no significant increase in the levels of the fully mature species. Immunoprecipitation revealed that presenilin-1 interacts preferentially with mature nicastrin, suggesting that correct trafficking and co-localization of the presenilin complex components are essential for activity. These findings demonstrate that trafficking and post-translational modifications of nicastrin are tightly regulated processes that accompany the assembly of the active presenilin complexes that execute gamma-secretase cleavage. These results also underscore the caveat that simple overexpression of nicastrin in transfected cells may result in the accumulation of large amounts of the immature protein, which is apparently unable to assemble into the active complexes capable of processing amyloid precursor protein and Notch

Robust activation of the neuronal lysosomal system and cellular pathways converging on the lysosome, such as the endocytic and autophagic pathways, are prominent neuropathological features of Alzheimer's disease. Disturbances of the neuronal endocytic pathway, which are one of the earliest known intracellular changes occurring in Alzheimer's disease and Down syndrome, provide insight into how beta-amyloidogenesis might be promoted in sporadic Alzheimer's disease, the most prevalent and least well understood form of the disease. Primary lysosomal system dysfunction in inherited disorders is commonly associated with prominent neurological phenotypes and neurodegeneration. New studies now directly implicate lysosomal cathepsins as proteases capable of initiating, as well as executing, cell death programs. These and other studies support the view that the progressive alterations of lysosomal system function in Alzheimer's disease have broad relevance to the neurodegenerative processes occurring during the disease

The lysosomal system (LS) is markedly activated in vulnerable neuronal populations early in Alzheimer's disease, although lysosomes become less efficient degradative compartments as neurons become more compromised. LS dysfunction, especially altered activity of the lysosomal protease cathepsin D, has been implicated in cell death initiation under various apoptotic conditions in vivo and in vitro. In this study, we observed that cathepsin D content increases nearly 3-fold in the human neocortex during normal aging while lysosomal cysteine protease activities decrease. By contrast, during aging in the mouse, this protease imbalance and other aging-related changes of the LS, such as lipofuscin accumulation, are minimal in these cortical areas. However, when leupeptin (a cysteine protease inhibitor) was infused intraventricularly, an imbalance of cathepsins similar to that in the aging human brain was induced. This was accompanied by changes associated with cell senescence, including ceroid-lipofuscin accumulation and alterations of tau proteolysis. In PSM146L/APPSWE transgenic mice, super-imposition of this aging-related cathepsin imbalance accentuated preexisting LS abnormalities to the level seen in AD brain and also induced neuronal atrophy and neurodegeneration. The minimal degree of 'lysosomal aging' seen in old mice, compared to that in humans, may partly explain the mild neurodegenerative phenotypes in transgenic models of AD pathology. In addition, these studies provide in vivo evidence relating altered lysosomal function to neurodegeneration