Caspase-3 Causes the Formation ofHuman-Tau-Derived Toxic Fragments During Neuronal Apoptosis

Truncated tau proteins are a hallmark of human Alzheimer’s disease (AD).The cleaved state of tau influences its physiological ability to bind microtubules, to assume ADrelated pathological conformations, to aggregate and assemble into filaments and to induce neuronal death .A transgenic rat model overexpressing truncated human tau has been shown to cause in vivo neurofibrillary tangles, demonstrating that cleaved forms of tau are sufficient to produce AD-like neurofibrillary degeneration by inducing oxidative stress. Previously, we have shown that adenovirus- mediated overexpression of 25-230 human tau fragment evokes a potent neurotoxic effect in primary neuronal cultures by sustained stimulation of NMDA receptor (Amadoro et al.,2006). In order to assess whether the 25-230 fragment is actually produced during apoptosis, we attempted to ascertain its presence using a site-directed, caspase 3- cleaved antibody (Rohn et al., 2002) against amino-terminal consensus cleavage site D25 of tau protein (QGGYTMHQDQ). We provide biochemical evidence that N-tau 25-230 fragments, consistent with the sizes produced by caspase -3 and calpain cleavage of different tau isoforms, are generated in staurosporine-treated differentiated human SY5Y. These findings support the notion that activation of apoptotic mechanism(s) may be directly involved in AD pathogenesis, possibly also via generation of tau fragments, indicating that inhibition of caspase-mediated cleavage of this protein(s) may be protective in vivo.