Caspase-cleaved Tau Expression Induces Mitochondrial Dysfunction in Immortalized Cortical Neurons

Abstract

In Alzheimer disease (AD) mitochondrial abnormalities occur early in the pathogenic process and likely play a significant role in disease progression. Tau is a microtubule-associated protein that is abnormally processed in AD, and a connection between tau pathology and mitochondrial impairment has been proposed. However, few studies have examined the relationship between pathological forms of tau and mitochondrial dysfunction. We recently demonstrated that inducible expression of tau truncated at Asp-421 to mimic caspase cleavage (T4C3) was toxic to immortalized cortical neurons compared with a fulllength tau isoform (T4). In this study we investigated the effects of T4C3 on mitochondrial function. Expression of T4C3 induced mitochondrial fragmentation and elevated oxidative stress levels in comparison with T4-expressing cells. Thapsigargin treatment of T4 or T4C3 cells, which causes an increase in intracellular calcium levels, resulted in a significant decrease in mitochondrial potential and loss of mitochondrial membrane integrity in T4C3 cells when compared with cells expressing T4. The mitochondrial fragmentation and mitochondrial membrane damage were ameliorated in T4C3 cells by pretreatment with cyclosporine A or FK506, implicating the calcium-dependent phosphatase calcineurin in these pathogenic events. Increased calcineurin activity has been reported in AD brain, and thus, inhibition of this phosphatase may provide a therapeutic target for the treatment of AD.Tau is a microtubule-associated protein which in a hyperphosphorylated state forms paired helical filaments; the major component of neurofibrillary tangles (NFTs) 3 (1, 2). These NFTs are one of the primary pathophysiological hallmarks of Alzheimer disease (AD) and were originally suggested to play a major role in facilitating neuronal degeneration (1). However, recent studies now suggest that mature tangles may not be the toxic species (3, 4). For example, in a repressible tau overexpression transgenic mouse model, turning off tau expression attenuated memory impairment and neuronal loss, whereas NFTs continued to accumulate (5). Furthermore, reduction of endogenous wild type tau attenuated behavioral abnormalities in an APP transgenic AD mouse model, in which substantial NFT pathology is absent (6). These and other findings suggest that a form or forms of tau that precede NFT formation may be the toxic species. There is increasing evidence that, in addition to aberrant phosphorylation, caspase cleavage of tau plays a role in the oligomerization and formation of a pathological tau species in AD (7,8). Tau is an in vitro substrate for caspase-3 and is readily cleaved at Asp-421, the caspase-3 cleavage site, located on the carboxyl-terminal end of the protein (7-10). This cleavage event results in a highly fibrillogenic tau isoform which in in vitro studies aggregates more readily and to a greater extent than full-length tau and facilitates aggregate formation of fulllength tau (7,8). Antibodies that specifically recognize Asp-421-truncated tau show ...

Expression of A152T human tau causes age‐dependent neuronal dysfunction and loss in transgenic mice

“…hTau fragments can cause or promote tau aggregation, mitochondrial and lysosomal dysfunction, axonal transport deficits, and increases in NMDA receptor levels 61, 62, 63, 64, 65. Thus, it is interesting that hTau‐A152T (L1) mice, despite their lower levels of tau fragments, had more neuronal dysfunction than hTau‐WT (L32) mice.…”

Tau Accumulation Causes Mitochondrial Distribution Deficits in Neurons in a Mouse Model of Tauopathy and in Human Alzheimer's Disease Brain

“…Tau is a caspase-3 substrate and its cleavage is thought to potentiate oligomerization and enhance tau-induced mitochondrial alterations and other negative consequences to the neuron. 1,43,52 Our data also demonstrate several methodological advances for the field of pathological analyses of mitochondria. First, we confirmed that single section analysis of cytoplasmic mitochondrial area fraction is comparable to volume fractions from whole cell reconstructions.…”

“…The fact that alterations in mitochondrial distribution are evident at early stages, in the 5.5 month rTg4510, supports the notion that transport deficits and accompanying mitochondrial changes in distribution are early events in neurodegenerative cascades. 19,23,24,42,43 Several potential mechanisms for tau overexpression contributing to transport deficits have previously been explored. (1) Studies of tau over-expression in cells and in vitro assays have demonstrated negative consequences to anterograde cellular transport, by directly inhibiting progress of kinesin-1, by forming a "road block" on the microtubule where excessive tau is bound.…”

“…50,51 Furthermore, tau over-expression in vitro has been shown to result in increased mitochondrial fragmentation, alterations in mitochondrial membrane potential, impaired calcium buffering and oxidative stress responses, and diminished respiratory chain function. 19,43,52 For these reasons, we assessed mitochondrial volumes in the 8.5 month-old rTg4510 and nonTg mice to determine whether concomitant disequilibrium in fission and fusion dynamics is evident alongside mitochondrial distribution deficits. We did not find significant differences in mitochondrial size in the neuropil, which is consistent with a proteomic study of P301L mutant tau over-expressing mice.…”

Mitochondrial respiratory chain dysfunction: Implications in neurodegeneration

“…To further complicate the view of mitochondrial dysfunction in AD, the microtubule associated protein tau, which in its phosphorylated state forms the neurofibrillary tangles detected in AD brains, can in turn induce mitochondrial alterations such as decreased expression of complex I and complex V subunits, subsequent decreases in these complexes activities, lower mitochondrial membrane potential, altered calcium buffering, increased ROS production and up-regulation of cellular antioxidants (David et al, 2005;Quintanilla et al, 2009;Rhein et al 2009;Quintanilla et al, 2012). Moreover, tau and Aβ can interact synergistically to impair mitochondrial function.…”