Neuroprotective Secreted Amyloid Precursor Protein Acts by Disrupting Amyloid Precursor Protein Dimers

Abstract

The amyloid precursor protein (APP) is implied both in cell growth and differentiation and in neurodegenerative processes in Alzheimer disease. Regulated proteolysis of APP generates biologically active fragments such as the neuroprotective secreted ectodomain sAPP␣ and the neurotoxic ␤-amyloid peptide. Furthermore, it has been suggested that the intact transmembrane APP plays a signaling role, which might be important for both normal synaptic plasticity and neuronal dysfunction in dementia. To understand APP signaling, we tracked single molecules of APP using quantum dots and quantitated APP homodimerization using fluorescence lifetime imaging microscopy for the detection of För-ster resonance energy transfer in living neuroblastoma cells. Using selective labeling with synthetic fluorophores, we show that the dimerization of APP is considerably higher at the plasma membrane than in intracellular membranes. Heparan sulfate significantly contributes to the almost complete dimerization of APP at the plasma membrane. Importantly, this technique for the first time structurally defines the initiation of APP signaling by binding of a relevant physiological extracellular ligand; our results indicate APP as receptor for neuroprotective sAPP␣, as sAPP␣ binding disrupts APP dimers, and this disruption of APP dimers by sAPP␣ is necessary for the protection of neuroblastoma cells against starvation-induced cell death. Only cells expressing reversibly dimerized wild-type, but not covalently dimerized mutant APP are protected by sAPP␣. These findings suggest a potentially beneficial effect of increasing sAPP␣ production or disrupting APP dimers for neuronal survival. The amyloid precursor protein (APP)4 is known both for its important role in the development and plasticity of the nervous system (1-6) and for its involvement in Alzheimer disease (AD) (7,8). Despite intensive research efforts, the initial events that lead to the prevalent sporadic, i.e. non-familial, forms of AD are still unclear. Furthermore, although a higher gene dose of APP (9) or the presence of pathological APP mutations is sufficient to induce familial AD (for review, see Ref. 10), the exact pathological mechanism that is triggered by APP is still under debate.Some fragments of APP, such as the ␤-amyloid peptide (A␤), are thought to contribute to synaptic dysfunction and neurotoxicity (11,12). On the other hand, the ␣-secretasederived extracellular fragment of APP (sAPP␣), which is present at lower levels in AD patients than in controls (13), has been shown to be beneficial for memory function, to possess neuroprotective properties, and to counteract the effects of A␤ (14 -18).Signaling by transmembrane APP may directly contribute to neurodegeneration in AD (19 -24); however, the signal transduction pathway for transmembrane APP remains unknown, although several potential regulatory proteins, glycosaminoglycans, and metal ions are known to bind with high affinity to APP and sAPP␣ (25,26). The most common form of signal transduction for single-pass transm...

Structure and biochemical analysis of the heparin-induced E1 dimer of the amyloid precursor protein

“…It has been reported that dimerization depends on zinc binding (18,30), heparin binding (16), or the simultaneous interaction of several sites within the APP molecule (31) including its transmembrane domain (29) and that it is functionally associated with the concurrent dimerization of the β-site APP cleaving enzyme (28,(31)(32)(33). Recently, Gralle et al corroborated the physiological importance of APP dimerization using single molecule FRET methods (27). However, it is currently unclear by which molecular mechanism the oligomerization of APP is realized.…”

“…22) and several proteins have been described to interact with the ectodomain of APP, therefore representing potential APP ligands (23)(24)(25). Besides, secreted soluble APPectodomain fragments have been shown to interact in a ligandlike manner with membrane-bound APP (26,27).…”

Histone Deacetylase Inhibitor Valproic Acid Inhibits Cancer Cell Proliferation via Down-regulation of the Alzheimer Amyloid Precursor Protein

“…These results clearly indicate that the secreted N-terminal domain of APP plays an essential regulatory role in promoting cellular growth. Several lines of evidence showed that sAPP␣ fulfills criteria of a trophic growth factor (42,45,(57)(58)(59) and that the growth regulating activity of APP is specified to the N-terminal domain of sAPP containing the 5-amino acid sequence RERMS (APP-(328 -332)). Peptides containing this pentapeptide sequence could increase cellular proliferation in fibroblasts and induce FIGURE 6.…”

L-3-n-Butylphthalide Improves Cognitive Impairment and Reduces Amyloid- in a Transgenic Model of Alzheimer's Disease

“…This processing cleaves APP within the sequence of A␤ peptide, releasing a soluble APP fragment (␣APPs) into the extracellular media, thereby precluding the formation of A␤ (Esch et al, 1990;Sisodia et al, 1990). The ␣APPs fragment has been shown to have both neurotrophic (Wallace et al, 1997) and neuroprotective (Mattson et al, 1993;SmithSwintosky et al, 1994;Gralle et al, 2009) activities. Recent studies suggest that ␣APPs might serve as an AD therapeutic target (Etcheberrigaray et al, 2004;Small et al, 2005;Turner et al, 2007).…”