Amyloid β-Mediated Cell Death of Cultured Hippocampal Neurons Reveals Extensive Tau Fragmentation without Increased Full-length Tau Phosphorylation

Abstract

A variety of genetic and biochemical evidence suggests that amyloid ␤ (A␤) oligomers promote downstream errors in Tau action, in turn inducing neuronal dysfunction and cell death in Alzheimer and related dementias. To better understand molecular mechanisms involved in A␤-mediated neuronal cell death, we have treated primary rat hippocampal cultures with A␤ oligomers and examined the resulting cellular changes occurring before and during the induction of cell death with a focus on altered Tau biochemistry. The most rapid neuronal responses upon A␤ administration are activation of caspase 3/7 and calpain proteases. A␤ also appears to reduce Akt and Erk1/2 kinase activities while increasing GSK3␤ and Cdk5 activities. Shortly thereafter, substantial Tau degradation begins, generating relatively stable Tau fragments. Only a very small fraction of fulllength Tau remains intact after 4 h of A␤ treatment. In conflict with expectations based on suggested increases of GSK3␤ and Cdk5 activities, A␤ does not cause any major increases in phosphorylation of full-length Tau as assayed by immunoblotting one-dimensional gels with 11 independent site-and phosphospecific anti-Tau antibodies as well as by immunoblotting twodimensional gels probed with a pan-Tau antibody. There are, however, subtle and transient increases in Tau phosphorylation at 3-4 specific sites before its degradation. Taken together, these data are consistent with the notion that A␤-mediated neuronal cell death involves the loss of full-length Tau and/or the generation of toxic fragments but does not involve or require hyperphosphorylation of full-length Tau.Many neurodegenerative diseases are characterized by the accumulation of aggregated proteins in the brain. For example, the microtubule-associated protein Tau forms aggregates in a variety of neurodegenerative diseases known as tauopathies, including Alzheimer, frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17), progressive supranuclear palsy, corticobasal degeneration, and related dementias (1, 2). Alzheimer disease is distinguished from many other tauopathies by accumulation of a second pathological feature known as amyloid ␤ (A␤) 3 plaques (3, 4). Complementary genetic evidence demonstrates that errors in the action or regulation of either Tau or the amyloid precursor protein (which is proteolytically cleaved to produce A␤) can cause neuronal cell death and dementia in humans (5). Furthermore, experiments in both cultured rodent hippocampal neurons and transgenic mice demonstrate that A␤-mediated neuronal cell death and memory deficits require Tau (6, 7). Taken together, the data suggest an intrinsic relationship between A␤ and Tau dysfunction. Indeed, the widely cited "amyloid cascade hypothesis" proposes that A␤ oligomers induce aberrant effects on Tau, which in turn promotes neurodegeneration and dementia (8 -10).One central feature of Alzheimer and related dementias is that Tau isolated from affected brains is hyperphosphorylated (11,12). This observation led investigators to ...

Biometals and Their Therapeutic Implications in Alzheimer's Disease

“…Treating cell cultures with multiple forms of Aβ, including soluble Aβ1-42 [40], Aβ oligomer [41], and Aβ fibril [42] induces tau hyperphosphorylation. Aβ-induced tau hyperphosphorylation consequently caused microtubule disassembly [41,43], reduction of total soluble tau and an increase in tau fragments [44], missorting of tau into dendritic areas [41], activation of the nuclear transcription factor of activated T cells' apoptotic pathways [45,46], and cell toxicity [41,44]. 2xTg (tau and APP mutant) or 3xTg (tau, APP and PS mutant) mice exhibit enhanced neurofibrillary tangle formation [47][48][49][50], which was attributed to activation of glycogen synthase kinase (GSK)-3β and Cdk5 [49,51], and inhibited proteasome activity [50].…”

Atorvastatin prevents Aβ oligomer-induced neurotoxicity in cultured rat hippocampal neurons by inhibiting Tau cleavage

“…Taken together, the data suggest that there is an intrinsic relationship between Aβ and Tau dysfunction. It has been shown that the deposition of Aβ induced the activation of calpain-1 and caspase-3 proteases [2,5,8] . These proteases cleave Tau proteins at specific sites, generating toxic Tau fragments or enhancing the aggregation properties of Tau protein [2,5,9] .…”

“…It has been shown that the deposition of Aβ induced the activation of calpain-1 and caspase-3 proteases [2,5,8] . These proteases cleave Tau proteins at specific sites, generating toxic Tau fragments or enhancing the aggregation properties of Tau protein [2,5,9] . The activation of these proteases, Tau cleavage, and the extent of neurodegeneration are both time-and dose-dependent [2,5,9] .…”

“…AβOs, which are increasingly considered to act as proximal neurotoxins in AD, interact with glutamate receptors at the dendritic membrane, induce abnormal calcium influx and oxidative stress, block long-term potentiation (LTP), and facilitate long-term depression, ultimately leading to synapse failure [3,4] . It has recently been suggested that the deposition of Aβ might trigger a series of cellular events that lead to posttranslational changes in Tau followed by neurite degeneration [2,5] . Furthermore, experiments in both cultured rodent hippocampal neurons and transgenic mice demonstrate that Tau is intrinsically involved in Aβ-mediated neuronal dysfunction and cell death [6,7] .…”

“…Cognitive deficits in AD are believed to result from progressive synaptic dysfunction and neurodegeneration initiated by soluble amyloid β oligomers (AβOs) and further involving aggregates of hyperphosphorylated Tau [1,2] . AβOs, which are increasingly considered to act as proximal neurotoxins in AD, interact with glutamate receptors at the dendritic membrane, induce abnormal calcium influx and oxidative stress, block long-term potentiation (LTP), and facilitate long-term depression, ultimately leading to synapse failure [3,4] .…”