No abstract

That neprilysin (NEP) is a major Abeta peptide-degrading enzyme in vivo is shown by higher Abeta peptide levels in the brain of an NEP knockout mouse. In addition, we show that infusion of an NEPinhibitor, but not inhibitors of other peptidases, into the brains of an APP transgenic mouse elevates Abeta levels. We have investigated the use of NEP as a potential therapeutic agent to prevent the accumulation of Abeta peptides in the brain. Lentivirus expressing NEP was initially used to demonstrate the ability of the enzyme to reduce Abeta levels in a model CHO cell line and to make primary hippocampal neurons resistant to Abeta-mediated neurotoxicity. Injection of NEPexpressing lentivirus, but not inactive NEP-expressing lentivirus, GFP-expressing lentivirus, or vehicle, into the hippocampus of 12-20-mo-old hAPP transgenic mice led to an approx 50% reduction in the number of amyloid plaques. These studies provide the impetus for further investigating of the use of NEP in a gene transfer therapy paradigm to prevent the accumulation of Abeta and prevent or delay the onset of Alzheimer's disease.

A novel function for the neural cell adhesion molecule (NCAM) was identified in ephrinA/EphA-mediated repulsion as an important regulatory mechanism for development of GABAergic inhibitory synaptic connections in mouse prefrontal cortex. Deletion of NCAM, EphA3, or ephrinA2/3/5 in null mutant mice increased the numbers and size of perisomatic synapses between GABAergic basket interneurons and pyramidal cells in the developing cingulate cortex (layers II/III). A functional consequence of NCAM loss was increased amplitudes and faster kinetics of miniature inhibitory postsynaptic currents in NCAM null cingulate cortex. NCAM and EphA3 formed a molecular complex and colocalized with the inhibitory presynaptic marker vesicular GABA transporter (VGAT) in perisomatic puncta and neuropil in the cingulate cortex. EphrinA5 treatment promoted axon remodeling of enhanced green fluorescent protein-labeled basket interneurons in cortical slice cultures and induced growth cone collapse in wild-type but not NCAM null mutant neurons. NCAM modified with polysialic acid (PSA) was required to promote ephrinA5-induced axon remodeling of basket interneurons in cortical slices, likely by providing a permissive environment for ephrinA5/EphA3 signaling. These results reveal a new mechanism in which NCAM and ephrinAs/EphA3 coordinate to constrain GABAergic interneuronal arborization and perisomatic innervation, potentially contributing to excitatory/inhibitory balance in prefrontal cortical circuitry.

A number of therapeutic strategies for treating Alzheimer's disease have focused on reducing amyloid burden in the brain. Amongst these approaches, the expression of amyloid β peptide (Aβ)-degrading enzymes in the brain has been shown to be effective, but to date not practical for treating patients. We report here a novel strategy for lowering amyloid burden in the brain by peripherally expressing the Aβ-degrading enzyme neprilysin on leukocytes in the 3×Tg-AD mouse model of Alzheimer's disease. Through transplantation of lentivirus transduced bone marrow cells, the Aβ-degrading protease neprilysin was expressed on the surface of leukocytes. This peripheral neprilysin reduced soluble brain amyloid β peptide levels by ~30% and lowered the accumulation of amyloid β peptides by 50-60% when transplantation was performed at both young and early adult age. In addition, peripheral neprilysin expression reduced amyloid dependent performance deficits as measured by the Morris Water Maze. Unlike other methods designed to lower amyloid β peptide levels in blood, which cause a net increase in peptide, neprilysin expression results in the catabolism of the amyloid β peptide to small innocuous peptide fragments. These findings demonstrate that peripherally expressed neprilysin, and likely other amyloid β peptide degrading enzymes, has the potential for being utilized as a therapeutic approach to prevent and treat Alzheimer's disease and suggest this approach should be further explored.