Activation of Neuronal Caspase-3 by Intracellular Accumulation of Wild-Type Alzheimer Amyloid Precursor Protein

Abstract

Forced overexpression of wild-type Alzheimer amyloid precursor protein (APP) causes postmitotic neurons to degenerate. Caspase-3 (CPP32) is a principal cell death protease involved in neuronal apoptosis during physiological development and under pathological conditions. Here, we investigated whether APP overexpression activates caspase-3 in human postmitotic neurons using adenovirus-mediated gene transfer. When a recombinant adenovirus vector expressing human wild-type APP695 was infected in vitro into neurally differentiated embryonal carcinoma NT2 cells, only postmitotic neurons underwent severe degeneration. Before neurodegeneration, full-length APP- and Abeta-immunoreactive peptides were accumulated in infected neurons, and caspase-3-like protease activity was markedly elevated. Western blot analysis revealed that activated caspase-3 subunits were generated in APP-accumulating neurons. Such neuronal caspase-3 activation was undetectable in NT2 neurons infected with beta-galactosidase-expressing adenovirus. Addition of the caspase-3 inhibitor acetyl-Asp-Glu-Val-Asp-aldehyde to the culture medium significantly reduced the severity of degeneration exhibited by APP-overexpressing neurons. Immunocytochemical analyses revealed that some APP-accumulating neurons contained activated caspase-3 subunits and exhibited the characteristics of apoptosis, such as chromatin condensation and DNA fragmentation. Activation of caspase-3 was also observed in vivo in rat hippocampal neurons infected with the APP-expressing adenovirus. These results suggest that wild-type APP is an intrinsic activator of caspase-3-mediated death machinery in postmitotic neurons.

Cell death induced by a caspase-cleaved transmembrane fragment of the Alzheimer amyloid precursor protein

“…11 The present study has shown that APPDC31 induced caspase-3-independent neuronal death. These findings suggested that APPDC31 induces apoptosis in a manner distinct from that of wild-type APP.…”

“…21,22 Furthermore, tau, a microtubule-associated protein whose abnormal phosphorylation is thought to cause formation of neurofibrillary tangles seen in AD, is cleaved by caspase-3 in vitro, and overexpression of the caspase-cleaved N-terminal fragment of tau induces apoptosis of NT2 stem cells and COS cells. 23,24 These Neuronal specificity of caspase-mediated apoptosis induced by wild-type APP Overexpression of wild-type APP causes caspase-3-mediated apoptosis of postmitotic neurons but not of nonneuronal cells, 11 suggesting that a neuron-specific machinery is involved in APP-induced caspase-3 activation. It has been demonstrated that two adaptor proteins, mammalian Disabled and FE65, which are primarily expressed in neurons, interact with the cytoplasmic tail of APP that contains the adaptor protein-binding motif NPTY.…”

“…The APP mutants in the transgenic mice in vivo may not be accumulated to toxic levels within neurons, whereas adenovirus-mediated overexpression of wild-type APP in vivo and in vitro induces a rapid elevation of APP levels sufficient for neuronal caspase-3 activation. 10,11 Self-association of APP may be responsible for caspase-3 activation It is noteworthy that the APP mutant lacking the Ab(1 ± 20) domain (APPDAb20), like APPDC31, induced no neuronal caspase-3 activation (Figures 2A,B and 6B). These findings suggest that both the Ab domain and the cytoplasmic C31 region are indispensable for caspase-3 activation.…”

“…9 ± 12 In APP-accumulating neurons in vitro, caspase-3 is activated in a cleavagedependent manner, and a caspase-3 inhibitor significantly reduces the severity of degeneration exhibited by APPoverexpressing neurons. 11 These observations suggest that unphysiological levels of wild-type APP induce neuronal apoptosis in a caspase-dependent manner.…”

Proapoptotic Effects of Tau Cleavage Product Generated by Caspase-3

“…Levels of both APP and Tau mRNA are elevated in Down's syndrome (Oyama et al, 1993). It was suggested that the C-terminal fragments of APP, 676 -695, is possibly essential along with Tau to PHF formation (Caputo et al, 1992) and increased level of APP induces activation of caspase (Uetsuki et al, 1999), implying a role of APP in ⌬Tau-induced cell death. Thus far, while a number of putative tauopathies have been identified, the pathological mechanism is not fully understood.…”