Faculty Bibliography

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

Much evidence indicates that abnormal processing and extracellular deposition of amyloid-beta peptide (A beta), a proteolytic derivative of the beta-amyloid precursor protein (betaAPP), is central to the pathogenesis of Alzheimer's disease (reviewed in ref. 1). In the PDAPP transgenic mouse model of Alzheimer's disease, immunization with A beta causes a marked reduction in burden of the brain amyloid. Evidence that A beta immunization also reduces cognitive dysfunction in murine models of Alzheimer's disease would support the hypothesis that abnormal A beta processing is essential to the pathogenesis of Alzheimer's disease, and would encourage the development of other strategies directed at the 'amyloid cascade'. Here we show that A beta immunization reduces both deposition of cerebral fibrillar A beta and cognitive dysfunction in the TgCRND8 murine model of Alzheimer's disease without, however, altering total levels of A beta in the brain. This implies that either a approximately 50% reduction in dense-cored A beta plaques is sufficient to affect cognition, or that vaccination may modulate the activity/abundance of a small subpopulation of especially toxic A beta species

The accumulation of neurofilaments required for postnatal radial growth of myelinated axons is controlled regionally along axons by oligodendroglia. Developmentally regulated processes previously suspected of modulating neurofilament number, including heavy neurofilament subunit (NFH) expression, attainment of mature neurofilament subunit stoichiometry, and expansion of interneurofilament spacing cannot be primary determinants of regional accumulation as we show each of these factors precede accumulation by days or weeks. Rather, we find that regional neurofilament accumulation is selectively associated with phosphorylation of a subset of Lys-Ser-Pro (KSP) motifs on heavy neurofilament subunits and medium-size neurofilament subunits (NFMs), rising >50-fold selectively in the expanding portions of optic axons. In mice deleted in NFH, substantial preservation of regional neurofilament accumulation was accompanied by increased levels of the same phosphorylated KSP epitope on NFM. Interruption of oligodendroglial signaling to axons in Shiverer mutant mice, which selectively inhibited this site-specific phosphorylation, reduced regional neurofilament accumulation without affecting other neurofilament properties or aspects of NFH phosphorylation. We conclude that phosphorylation of a specific KSP motif triggered by glia is a key aspect of the regulation of neurofilament number in axons during axonal radial growth

Absence of functional presenilin 1 (PS1) protein leads to loss of gamma-secretase cleavage of the amyloid precursor protein (betaAPP), resulting in a dramatic reduction in amyloid beta peptide (Abeta) production and accumulation of alpha- or beta-secretase-cleaved C-terminal fragments of betaAPP (alpha- or beta-CTFs). The major C-terminal fragment (CTF) in brain was identified as Abeta[11-98], which is consistent with the observation that cultured neurons generate primarily Abeta[11-40]. In PS1-/- murine neurons and fibroblasts expressing the loss-of-function PS1Asp385Ala mutant, CTFs accumulated in the endoplasmic reticulum (ER), Golgi, and lysosomes, but not late endosomes. There were some subtle differences in the subcellular distribution of CTFs in PS1-/- neurons as compared to PS1Asp385Ala mutant fibroblasts. However, there was no obvious redistribution of full-length betaAPP or of markers of other organelles in either mutant. Blockade of ER-to-Golgi trafficking indicated that in PS1-/- neurons (as in normal cells) trafficking of betaAPP to the Golgi compartment is necessary before alpha- and beta-secretase cleavages occur. Thus, while we cannot exclude a specific role for PS1 in trafficking of CTFs, these data argue against a major role in general protein trafficking. These results are more compatible with a role for PS1 either as the actual gamma-secretase catalytic activity or in other functions indirectly related to gamma-secretase catalysis (e.g., an activator of gamma-secretase, a substrate-adaptor for gamma-secretase, or delivery of gamma-secretase to betaAPP-containing compartments)