APP processing and synaptic function.

Alzheimer's disease (AD), the leading cause of dementia worldwide, is characterized by the accumulation of the β-amyloid peptide (Aβ) within the brain along with hyperphosphorylated and cleaved forms of the microtubule-associated protein tau. Genetic, biochemical, and behavioral research suggest that physiologic generation of the neurotoxic Aβ peptide from sequential amyloid precursor protein (APP) proteolysis is the crucial step in the development of AD. APP is a single-pass transmembrane protein expressed at high levels in the brain and metabolized in a rapid and highly complex fashion by a series of sequential proteases, including the intramembranous γ-secretase complex, which also process other key regulatory molecules. Why Aβ accumulates in the brains of elderly individuals is unclear but could relate to changes in APP metabolism or Aβ elimination. Lessons learned from biochemical and genetic studies of APP processing will be crucial to the development of therapeutic targets to treat AD.

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Amyloid Precursor Protein Processing and Alzheimer's Disease

Alzheimer's disease (AD) is characterized by the deposition of senile plaques (SPs) and neurofibrillary tangles (NFTs) in vulnerable brain regions. SPs are composed of aggregated β-amyloid (Aβ) 40/42(43) peptides. Evidence implicates a central role for Aβ in the pathophysiology of AD. Mutations in βAPP and presenilin 1 (PS1) lead to elevated secretion of Aβ, especially the more amyloidogenic Aβ42. Immunohistochemical studies have also emphasized the importance of Aβ42 in initiating plaque pathology. Cell biological studies have demonstrated that Aβ is generated intracellularly. Recently, endogenous Aβ42 staining was demonstrated within cultured neurons by confocal immunofluorescence microscopy and within neurons of PS1 mutant transgenic mice. A central question about the role of Aβ in disease concerns whether extracellular Aβ deposition or intracellular Aβ accumulation initiates the disease process. Here we report that human neurons in AD-vulnerable brain regions specifically accumulate γ-cleaved Aβ42 and suggest that this intraneuronal Aβ42 immunoreactivity appears to precede both NFT and Aβ plaque deposition. This study suggests that intracellular Aβ42 accumulation is an early event in neuronal dysfunction and that preventing intraneuronal Aβ42 aggregation may be an important therapeutic direction for the treatment of AD.

Intraneuronal Aβ42 Accumulation in Human Brain

We studied the effect of microtubule-associated tau protein on trafficking of vesicles and organelles in primary cortical neurons, retinal ganglion cells, and neuroblastoma cells. Tau inhibits kinesin-dependent transport of peroxisomes, neurofilaments, and Golgi-derived vesicles into neurites. Loss of peroxisomes makes cells vulnerable to oxidative stress and leads to degeneration. In particular, tau inhibits transport of amyloid precursor protein (APP) into axons and dendrites, causing its accumulation in the cell body. APP tagged with yellow fluorescent protein and transfected by adenovirus associates with vesicles moving rapidly forward in the axon (∼80%) and slowly back (∼20%). Both movements are strongly inhibited by cotransfection with fluorescently tagged tau (cyan fluorescent protein–tau) as seen by two-color confocal microscopy. The data suggests a linkage between tau and APP trafficking, which may be significant in Alzheimer's disease.

https://rupress.org/jcb/article-pdf/156/6/1051/1302666/jcb15661051.pdf

Tau blocks traffic of organelles, neurofilaments, and APP vesicles in neurons and enhances oxidative stress

Cell-permeable Peptide Antioxidants Targeted to Inner Mitochondrial Membrane inhibit Mitochondrial Swelling, Oxidative Cell Death, and Reperfusion Injury

The excessive generation and accumulation of 40- and 42-aa β-amyloid peptides (Aβ40/Aβ42) in selectively vulnerable brain regions is a major neuropathological feature of Alzheimer’s disease. Aβ, derived by proteolytic cleavage from the β-amyloid precursor protein (βAPP), is normally secreted. However, recent evidence suggests that significant levels of Aβ also may remain inside cells. Here, we have investigated the subcellular compartments within which distinct amyloid species are generated and the compartments from which they are secreted. Three experimental approaches were used: (i) immunofluorescence performed in intact cortical neurons; (ii) sucrose gradient fractionation performed with mouse neuroblastoma cells stably expressing wild-type βAPP695 (N2a695); and (iii) cell-free reconstitution of Aβ generation and trafficking from N2a695 cells. These studies demonstrate that: (i) Aβ40 (Aβ1–40 plus Aβx-40, where x is an NH2-terminal truncation) is generated exclusively within the trans-Golgi Network (TGN) and packaged into post-TGN secretory vesicles; (ii) Aβx-42 is made and retained within the endoplasmic reticulum in an insoluble state; (iii) Aβ42 (Aβ1–42 plus Aβx-42) is made in the TGN and packaged into secretory vesicles; and (iv) the amyloid peptides formed in the TGN consist of two pools (a soluble population extractable with detergents and a detergent-insoluble form). The identification of the organelles in which distinct forms of Aβ are generated and from which they are secreted should facilitate the identification of the proteolytic enzymes responsible for their formation.

https://www.pnas.org/content/pnas/96/2/742.full.pdf

Endoplasmic reticulum and trans-Golgi network generate distinct populations of Alzheimer β-amyloid peptides

Alzheimer's disease (AD) is characterized by the age-related deposition of β-amyloid (Aβ) 40/42 peptide aggregates in vulnerable brain regions. Multiple levels of evidence implicate a central role for Aβ in the pathophysiology of AD. Aβ peptides are generated by the regulated cleavage of an ≈700-aa Aβ precursor protein (βAPP). Full-length βAPP can undergo proteolytic cleavage either within the Aβ domain to generate secreted sβAPPα or at the N- and C-terminal domain(s) of Aβ to generate amyloidogenic Aβ peptides. Several epidemiological studies have reported that estrogen replacement therapy protects against the development of AD in postmenopausal women. We previously reported that treating cultured neurons with 17β-estradiol reduced the secretion of Aβ40/42 peptides, suggesting that estrogen replacement therapy may protect women against the development of AD by regulating βAPP metabolism. Increasing evidence indicates that testosterone, especially bioavailable testosterone, decreases with age in older men and in postmenopausal women. We report here that treatment with testosterone increases the secretion of the nonamyloidogenic APP fragment, sβAPPα, and decreases the secretion of Aβ peptides from N2a cells and rat primary cerebrocortical neurons. These results raise the possibility that testosterone supplementation in elderly men may be protective in the treatment of AD.

https://www.pnas.org/content/pnas/97/3/1202.full.pdf

Bing Hai

Papers

Endoplasmic reticulum and trans-Golgi network generate distinct populations of Alzheimer β-amyloid peptides

Testosterone reduces neuronal secretion of Alzheimer's β-amyloid peptides