Date Published: January 18, 2017
Publisher: Public Library of Science
Author(s): Alberto Serrano-Pozo, Manuel A. Sánchez-García, Antonio Heras-Garvín, Rosana March-Díaz, Victoria Navarro, Marisa Vizuete, José López-Barneo, Javier Vitorica, Alberto Pascual, Madepalli K. Lakshmana.
Recent epidemiological evidence has linked hypoxia with the development of Alzheimer disease (AD). A number of in vitro and in vivo studies have reported that hypoxia can induce amyloid-β peptide accumulation through various molecular mechanisms including the up-regulation of the amyloid-β precursor protein, the β-secretase Bace1, or the γγ-secretase complex components, as well as the down-regulation of Aβ-degrading enzymes.
To investigate the effects of acute and chronic sustained hypoxia in Aβ generation in vivo.
2–3 month-old C57/Bl6J wild-type mice were exposed to either normoxia (21% O2) or hypoxia (9% O2) for either 4 to 72 h (acute) or 21–30 days (chronic sustained) in a hermetic chamber. Brain mRNA levels of Aβ-related genes were measured by quantitative real-time PCR, whereas levels of Bace1 protein, full length AβPP, and its C-terminal fragments (C99/C88 ratio) were measured by Western blot. In addition, 8 and 14-month-old APP/PS1 transgenic mice were subjected to 9% O2 for 21 days and levels of Aβ40, Aβ42, full length AβPP, and soluble AβPPα (sAβPPα) were measured by ELISA or WB.
Hypoxia (either acute or chronic sustained) did not impact the transcription of any of the Aβ-related genes in young wild-type mice. A significant reduction of Bace1 protein level was noted with acute hypoxia for 16 h but did not correlate with an increased level of full length AβPP or a decreased C99/C83 ratio. Chronic sustained hypoxia did not significantly alter the levels of Bace1, full length AβPP or the C99/C83 ratio. Last, chronic sustained hypoxia did not significantly change the levels of Aβ40, Aβ42, full length AβPP, or sAβPPα in either young or aged APP/PS1 mice.
Our results argue against a hypoxia-induced shift of AβPP proteolysis from the non-amyloidogenic to the amyloidogenic pathways. We discuss the possible methodological caveats of previous in vivo studies.
Alzheimer disease (AD) is the most common neurodegenerative disease and the most prevalent dementia. AD is defined neuropathologically by the presence of amyloid plaques and neurofibrillary tangles (NFTs) in sufficient number and extension within the cortex. While NFTs are intraneuronal somatodendritic aggregates of the hyperphosphorylated and misfolded microtubule-associated protein tau, amyloid plaques are extracellular deposits of amyloid-β (Aβ) peptide, which is released by the neurons to the interstitial space . Aβ is a normal by-product of the transmembrane amyloid-β precursor protein (AβPP) after its sequential cleavage by the transmembrane aspartyl proteases β- and γ-secretases. Specifically, the cleavage of AβPP by the β-site AβPP cleaving enzyme 1 (Bace1) produces two fragments: soluble AβPPβ (sAβPPβ) and a 99 amino acid C-terminal fragment (βCTF or C99). The latter is next cleaved by γ-secretase to produce Aβ peptides of different lengths from 37 to 43 amino acids depending on the cleaving site. The γ-secretase is, in fact, a complex of four proteins: presenilin 1 or 2—which contains the catalytic proteolytic site—, Aph1 (with one of three isoforms A, B, or C), nicastrin, and Pen-2.
We present several lines of evidence arguing against a hypoxia-induced shift of AβPP proteolysis from the non-amyloidogenic to the amyloidogenic pathways in both wild-type and AD mice. First, exposure of young wild-type mice to either AH or CSH did not significantly change the transcription of any of the Aβ-related genes. Second, Bace1 protein level was not only not increased by hypoxia, but decreased by both AH for 16 h and, to a lesser extent, CSH for 30 days, in young wild-type mice. Nonetheless, AβPP processing—as indicated by WBs for full length AβPP and C99/83 ratio—was not altered by either hypoxia protocol in these young wild-type mice. Last, Aβ40 and Aβ42, full length AβPP, and sAβPPα levels were not significantly changed by exposure to CSH in either young (low Aβ burden) or aged (high Aβ burden) APP/PS1 mice.
In summary, our results argue against a hypoxia-induced shift of AβPP processing leading to an increased Aβ generation. The possibility remains that reoxygenation after hypoxia (rather than hypoxia itself) is associated with an enhanced amyloidogenic processing of AβPP. Also, the potential indirect effects of hypoxia on Aβ metabolism, for example mediated through Vegf [41,42], were not investigated here. More studies are needed to elucidate the molecular mechanism(s) that underlie(s) the epidemiological evidence linking hypoxia and cerebrovascular ischemic disease with Alzheimer disease.