Among the products of amyloid precursor protein (APP) processing, the Aβ peptide claims the lion’s share of attention from researchers, but lately, other metabolites are grabbing the spotlight as potential purveyors of pathology. The APP intracellular domain (AICD), a product of γ-secretase cleavage and a putative transcriptional regulator, was recently implicated in neurodegeneration in mice (see ARF related news story on Ghosal et al., 2009). Now, a study from Ralph Nixon and colleagues at the Nathan Kline Institute, Orangeburg, New York, provides evidence that buildup of a different APP fragment, namely the C-terminal product of β-secretase cleavage (βCTF), causes the defects in endocytosis that are seen early in AD and can lead to neurodegeneration. The work suggests that inhibition of β-secretase may be the key to preventing the endosomal aspects of AD pathology.
“With the field questioning whether Aβ is the full answer to AD, another metabolite of APP is of particular interest, especially given its pathogenic properties of disrupting crucial neuronal functions very early in AD,” Nixon told ARF. The work appeared December 28 in the online edition of PNAS.
The study used fibroblasts from patients with Down syndrome (DS), who carry an extra dose of APP and are subject to early onset AD. Like both AD and DS neurons, the fibroblasts show early endosome dysfunction characterized by increased endocytosis, enlargement of endosomes, and an increase in the number of the largest endosomes (Cataldo et al., 2008). Previous work from the lab of William Mobley at Stanford, in collaboration with Nixon’s lab, had shown that the disruption of endocytosis by APP overexpression in a mouse model of DS leads to neurodegeneration, as neurons are unable to support the retrograde transport of trophic factors necessary for their survival (see ARF related news story on Salehi et al., 2006). In the mouse neurons, the disruption of endocytosis depended on APP, but not necessarily on Aβ production.
In the new work, first author Ying Jiang shows that the endosome defects in DS fibroblasts depend on the overexpression of APP and production of the βCTF. RNAi knockdown of either APP or the β-secretase BACE1 restored both normal endosome function (measured by transferrin uptake) and morphology. Conversely, overexpressing APP or the βCTF in normal human fibroblasts induced endosome pathology. Jiang did not see the effect after transfecting a β-cleavage resistant APP mutant (M596V), establishing that β cleavage, and production of the βCTF, was critical to initiating the APP-induced endosome abnormalities.
To show that Aβ was not required, the researchers treated the fibroblasts with γ-secretase inhibitors, which decreased Aβ production while increasing levels of βCTF. In normal fibroblasts, the inhibitors caused an increase in lysosomal size, and in the DS fibroblasts, they worsened lysosomal pathology.
The results suggest that BACE inhibitors may have an advantage over Aβ-targeted therapies, at least where endosome dysfunction is concerned. “I think that the BACE inhibitors are going to cover more bases in having a broader spectrum of effects on potential toxic metabolites of APP. I think our study gives more credibility to that approach for reversing endosome pathology, which we think is very critical to the development of the later pathologies,” Nixon said.
How might βCTF affect endocytosis? In DS neurons, APP and βCTF are both elevated and distributed in early endosomes. “We don’t know very much about what regulates endocytosis, but we strongly suspect that APP is involved,” Nixon said. He cited work from Rachael Neve’s lab showing that APP binds to a protein complex that includes the GTP-binding protein Rab5 (Laifenfeld et al., 2007). Work that Nixon published a year ago with the late Anne Cataldo at McLean Hospital, Belmont, Massachusetts (see Cataldo et al., 2008), showed that Rab5 is markedly upregulated in the DS fibroblasts. “When Anne manipulated Rab5 up or down, she essentially achieved the same circumstances that we achieved by manipulating βCTF,” he said. “Rab5 has become a focal point for us in initiating the downstream effects that lead to endocytosis upregulation and some of the adverse consequences of upregulation. We suspect that the βCTF is also involved as the mediating factor in the APP-endosome interaction that leads to Rab5 activation, or that it is involved in the signaling that is part of the Rab5 mechanism to stimulate endocytosis.”
Neurons appear to be especially dependent on healthy endocytic function, Nixon said, and endosome dysfunction is increasingly related to a range of neurodegenerative diseases. “A lot of genes, not only in AD, but in other neurodegenerative diseases, are starting to hit the endocytic pathway with a frequency that is much greater than expected by chance alone. In ALS, in Charcot-Marie-Tooth, and in some forms of spinal dementia, some genes that cause these conditions are very important functional proteins in either the early endosome or the late endosome. So there is a nexus of neurodegenerative phenomena associated with mutations anywhere along that endocytic pathway, which I think elevates the significance of endosomal dysfunction as a specific pathway to neurodegeneration in a number of diseases,” he said.—Pat McCaffrey