Neprilysin is a zinc metalloendopeptidase with relatively broad substrate specificity. The enzyme is localized to the plasma membrane of cells where it can function to degrade extracellular peptides. Structural studies show that neprilysin preferentially cleaves peptides on the amino side of hydrophobic amino acids. Neprilysin has been implicated in the catabolism of amyloid β peptides in the brain and as such has received considerable attention, particularly as a therapeutic target for Alzheimers disease. An inverse relationship between neprilysin levels and amyloid β peptide levels and between neprilysin levels and amyloid plaque formation has been observed in human brain. Neprilysin levels decline with aging in the temporal and frontal cortex possibly contributing to higher amyloid β peptide levels. A number of studies have shown that increasing neprilysin levels in the brain leads to a decrease in brain amyloid β peptide levels. Most recently a potential relationship between amyloid β peptide synthesis from the amyloid precursor protein and neprilysin activity has been proposed.

Mid-life obesity and type 2 diabetes mellitus (T2DM) confer a modest, increased risk for Alzheimer’s disease (AD), though the underlying mechanisms are unknown. We have created a novel mouse model that recapitulates features of T2DM and AD by crossing morbidly obese and diabetic db/db mice with APP ΔNL/ΔNL x PS1 P264L/P264L knock-in mice. These mice (db/AD) retain many features of the parental lines (e.g. extreme obesity, diabetes, and parenchymal deposition of β-amyloid (Aβ)). The combination of the two diseases led to additional pathologies-perhaps most striking of which was the presence of severe cerebrovascular pathology, including aneurysms and small strokes. Cortical Aβ deposition was not significantly increased in the diabetic mice, though overall expression of presenilin was elevated. Surprisingly, Aβ was not deposited in the vasculature or removed to the plasma, and there was no stimulation of activity or expression of major Aβ-clearing enzymes (neprilysin, insulin degrading enzyme, or endothelin-converting enzyme). The db/AD mice displayed marked cognitive impairment in the Morris Water Maze, compared to either db/db or APP ΔNL x PS1 P264L mice. We conclude that the diabetes and/or obesity in these mice leads to a destabilization of the vasculature, leading to strokes and that this, in turn, leads to a profound cognitive impairment and that this is unlikely to be directly dependent on Aβ deposition. This model of mixed or vascular dementia provides an exciting new avenue of research into the mechanisms underlying the obesity-related risk for age-related dementia, and will provide a useful tool for the future development of therapeutics.

Neprilysin (NEP) is a zinc metallopeptidase that efficiently degrades the amyloid β (Aβ) peptides believed to be involved in the etiology of Alzheimer disease (AD). The focus of this study was to develop a new and tractable therapeutic approach for treating AD using NEP gene therapy. We have introduced adeno-associated virus (AAV) expressing the mouse NEP gene into the hindlimb muscle of 6-month-old human amyloid precursor protein (hAPP) (3X-Tg-AD) mice, an age which correlates with early stage AD. Overexpression of NEP in muscle decreased brain soluble Aβ peptide levels by ~60% and decreased amyloid deposits by ~50%, with no apparent adverse effects. Expression of NEP on muscle did not affect the levels of a number of other physiological peptides known to be in vitro substrates. These findings demonstrate that peripheral expression of NEP and likely other peptidases represents an alternative to direct administration into brain and illustrates the potential for using NEP expression in muscle for the prevention and treatment of AD.

Choline acetyltransferase (ChAT) synthesizes acetylcholine in neurons and other cell types. Decreases in ChAT activity are associated with a number of disease states, and mutations in ChAT cause congenital neuromuscular disorders. The crystal structure of ChAT reported here shows the enzyme divided into two domains with the active site in a solvent accessible tunnel at the domain interface. A low‐resolution view of the complex with one substrate, coenzyme A, defines its binding site and suggests an additional interaction not found in the related carnitine acetyltransferase. Also, the preference for choline over carnitine as an acetyl acceptor is seen to result from both electrostatic and steric blocks to carnitine binding at the active site. While half of the mutations that cause motor disorders are positioned to affect enzyme activity directly, the remaining changes are surprisingly distant from the active site and must exert indirect effects. The structure indicates how ChAT is regulated by phosphorylation and reveals an unusual pattern of basic surface patches that may mediate membrane association or macromolecular interactions.