␥-Secretase is known to play a pivotal role in the pathogenesis of Alzheimer disease through production of amyloidogenic A␤42 peptides. Early onset familial Alzheimer disease mutations in presenilin (PS), the catalytic core of ␥-secretase, invariably increase the A␤42:A␤40 ratio. However, the mechanism by which these mutations affect ␥-secretase complex formation and cleavage specificity is poorly understood. We show that our in vitro assay system recapitulates the effect of PS1 mutations on the A␤42:A␤40 ratio observed in cell and animal models. We have developed a series of small molecule affinity probes that allow us to characterize active ␥-secretase complexes. Furthermore we reveal that the equilibrium of PS1-and PS2-containing active complexes is dynamic and altered by overexpression of Pen2 or PS1 mutants and that formation of PS2 complexes is positively correlated with increased A␤42:A␤40 ratios. These data suggest that perturbations to ␥-secretase complex equilibrium can have a profound effect on enzyme activity and that increased PS2 complexes along with mutated PS1 complexes contribute to an increased A␤42:A␤40 ratio.

␥-Secretase processing of the amyloid precursor protein (APP) generates A␤ 40 and A␤ 42 , peptides that constitute the principal components of the ␤-amyloid plaque pathology of Alzheimer's disease (AD). The ␥-secretase activity is executed by a high-molecular-weight complex of which presenilin 1 (PS1) is an essential component. PS1 is a multi-pass membrane protein, and the large hydrophilic loop domain between transmembrane domains 6 and 7 has been shown to interact with various proteins. To determine the physiological function of the loop domain, we created a strain of PS1 knock-in mice in which the exon 10, which encodes most of the hydrophilic loop sequence, was deleted from the endogenous PS1 gene. We report here that the homozygous exon 10-deleted mice are viable but exhibit drastically reduced ␥-secretase cleavage at the A␤ 40 , but not the A␤ 42 , site. Surprisingly, this reduction of A␤ 40 is associated with exacerbated plaque pathology when expressed on APP transgenic background. Thus, the PS1 loop plays a regulatory role in ␥-secretase processing, and decreased A␤ 40 , not increased A␤ 42 is likely the cause for the accelerated plaque deposition in these animals. Our finding supports a protective role of A␤ 40 against amyloid pathology and raises the possibility that impaired ␥-secretase activity could be the basis for AD pathogenesis in general.

Amyloid precursor protein (APP) is implicated in axonal elongation, synaptic plasticity, and axonal transport. However, the role of APP on axonal transport in conjunction with the microtubule associated protein tau continues to be debated. Here we measured in vivo axonal transport in APP knockout mice with Manganese Enhanced MRI (MEMRI) to determine whether APP is necessary for maintaining normal axonal transport. We also tested how overexpression and mutations of tau affect axonal transport in the presence or absence of APP. In vivo axonal transport reduced significantly in the absence of functional APP. Overexpression of human wildtype tau maintained normal axonal transport and resulted in a transient compensation of axonal transport deficits in the absence of APP. Mutant R406Wtau in combination with the absence of APP compounded axonal transport deficits and these deficits persisted with age. These results indicate that APP is necessary for axonal transport, and overexpression of human wildtype tau can compensate for the absence of APP at an early age.

Abstractα-synuclein is a small soluble, cytosolic protein which associates with vesicular membranes. It is a component of intracellular Lewy bodies present in Parkinson's disease and a subset of Alzheimer's disease (AD). In addition, early studies identified a fragment of α-synuclein in the amyloid plaques of AD patients. Hypothesizing that α-synuclein might modify the AD pathogenic process, we crossed the Tg2576 strain of APP transgenic mice onto an α-synuclein knockout background to determine the effects of α-synuclein on Aβ production and plaque deposition. We found that α-synuclein deficiency does not affect the Aβ levels, nor does it alter the age of onset of plaque pathology. To our surprise, however, loss of α-synuclein leads to a significant increase in plaque load in all areas of the forebrain at 18 months of age. This is associated with an increase in another synaptic protein, synaptophysin. We thus conclude that α-synuclein is not involved in seeding of the plaques, but rather suppresses the progression of plaque pathology at advanced stages.

Alzheimer's disease (AD) is a significant and growing health problem in the aging population. Although definitive mechanisms of pathogenesis remain elusive, genetic and histological clues have implicated the proteins presenilin (PS) and tau as key players in AD development. PS mutations lead to familial AD, and although tau is not mutated in AD, tau pathology is a hallmark of the disease. Axonal transport deficits are a common feature of several neurodegenerative disorders and may represent a point of intersection of PS and tau function. To investigate the contribution of wild-type, as opposed to mutant, tau to axonal transport defects in the context of presenilin loss, we used a mouse model postnatally deficient for PS (PS cDKO) and expressing wild-type human tau (WtTau). The resulting PS cDKO;WtTau mice exhibited early tau pathology and axonal transport deficits that preceded development of these phenotypes in WtTau or PS cDKO mice. These deficits were associated with reduced neurotrophin signaling, defective learning and memory and impaired synaptic plasticity. The combination of these effects accelerated neurodegeneration in PS cDKO;WtTau mice. Our results strongly support a convergent role for PS and tau in axonal transport and neuronal survival and function and implicate their misregulation as a contributor to AD pathogenesis.