The Emory laboratories of Keqiang Ye and David Weinshenker recently published a paper on ApoE, the most common genetic risk factor for late-onset Alzheimer’s. The findings, published in Acta Neuropathologica, suggest how the risk-conferring form of ApoE (ApoE4) may exacerbate pathology in the locus coeruleus.
The LC, part of the brainstem, is thought to be the first region of the brain where pathological signs predicting future cellular degeneration show up. The LC (“blue spot”) gets its name from its blue color; it regulates attention, arousal, stress responses and cognition. The LC is also the major site for production of the neurotransmitter norepinephrine.
ApoE, which packages and transports cholesterol, was known to modulate the buildup of the toxic protein fragment beta-amyloid, but this proposed mechanism goes through Tau. Tau is the other pesky protein in Alzheimer’s, forming neurofibrillary tangles that are the earliest signs of degeneration in the brain. Tau pathology correlates better with dementia and cognitive impairments than beta-amyloid, which several proposed Alzheimer’s therapeutics act on.
The new paper shows that ApoE4 inhibits the enzyme VMAT2, which packages norepinephrine into vesicles. As a result, free/unpackaged norepinephrine lingers in the cytoplasm, and forms a harmful oxidative byproduct that triggers enzymatic degradation of Tau. Thus, norepinephrine may have a “too hot to handle” role in Alzheimer’s – with respect to the LC — somewhat analogous to dopamine in Parkinson’s, which has also been observed to form harmful byproducts. Dopamine and norepinephrine are similar chemically and both are substrates of VMAT2, so this relationship is not a stretch.
The Emory results make the case for inhibiting the enzyme AEP (asparagine endopeptidase), also known as delta-secretase, as an approach for heading off Alzheimer’s. AEP is the Tau-munching troublemaker, and is activated by the norepinephrine byproduct DOPEGAL
An alternative approach may be to inhibit monoamine oxidase (MAO-A above) enzymes — several old-school antidepressants are available that accomplish this.
Delta-secretase’s name positions it in relation to beta- and gamma-secretase, enzymes for processing APP (amyloid precursor protein) into beta-amyloid, but AEP/delta-secretase has the distinction of having its fingers in both the beta-amyloid and Tau pies.
We have to caution that most of the recent research on delta-secretase has been in mouse models. Ye’s collaborators in China have been testing an inhibitor of delta-secretase in animals but it has not reached human studies yet, he reports. That said, this work has been oriented toward figuring out the web of interactions between known players such as ApoE and Tau, whose importance has been well-established in studies of humans with Alzheimer’s.