Early N-Terminal Changes and Caspase-6 Cleavage of Tau in Alzheimer's Disease

Abstract

Alzheimer's disease (AD) is a progressive amnestic dementia that involves post-translational hyperphosphorylation, enzymatic cleavage, and conformational alterations of the microtubule-associated protein tau. The truncation state of tau influences many of its pathologic characteristics, including its ability to assume AD-related conformations and to assemble into filaments. Cleavage also appears to be an important marker in AD progression. Although C-terminal truncation of tau at D421 has recently been attributed to the apoptotic enzyme caspase-3, N-terminal processing of the protein remains mostly uncharacterized. Here, we report immunohistochemical staining in a cohort of 35 cases ranging from noncognitively impaired to early AD with a panel of three N-terminal anti-tau antibodies: Tau-12, 5A6, and 9G3-pY18. Of these three, the phosphorylation-independent epitope of 5A6 was the earliest to emerge in the pathological lesions of tau, followed by the appearance of the Tau-12 epitope. The unmasking of the Tau-12 epitope in more mature 5A6-positive tangles was not correlated with tau phosphorylation at tyrosine 18 (9G3-pY18). Still, later in the course of tangle evolution, the extreme N terminus of tau was lost, correlating temporally with the appearance of a C-terminal caspase-truncated epitope lacking residues 422-441. In addition, caspase-6 cleaved the N terminus of tau in vitro, preventing immunoreactivity with both Tau-12 and 5A6. Mass spectrometry confirmed that the in vitro caspase-6 truncation site is D13, a semicanonical and hitherto undescribed caspase cleavage site in tau. Collectively, these results suggest a role for caspase-6 and N-terminal truncation of tau during neurofibrillary tangle evolution and the progression of Alzheimer's disease.

Increase in tau tyrosine phosphorylation correlates with the formation of tau aggregates

“…It is possible that a substantial phosphorylation of Y18 and/or Y29 occurs primarily at later stages of neurofibrillary tangle formation than those observed in the JNPL3 we examined. In this regard, it has been reported that antibodies to tau peptides phosphorylated at Y18 (aa 12-24) recognize fewer tangles in AD brain than those unphosphorylated tau peptides (aa , and that compact fibrillary tangles, but not early diffuse tangles, display phosphorylated Y18 immunoreactivity [8].…”

“…Yen's laboratory [4]], monoclonal Tau12 [aa 9-18; 1:20000; provided by Dr. Lester Binder [8]], and phospho-tau-specific antibodies CP13 (S202 and T205) and PHF-1 (S396 and S404) (both provided by Dr. Peter Davies [6], 1:1000); monoclonal phospho-tyrosine-specific 4G10 antibody (Upstate, NY) (1:200). Secondary antibodies used in Western blot analysis are as follows: peroxidase-conjugated goat anti-rabbit (1:4000) or goat anti-mouse (1:2000) Ig antibodies (Chemicon, Temecula, CA, USA).…”

Caspases and cancer

“…Although caspase-3 seems to be the primary caspase in this case, colocalization with intracellular tau inclusions, in the form of neurophil threads and neurofibrillary tangles, and site-specific processing of tau protein also implicate caspase-6 in this process. 76,77 Moreover, accumulated caspase cleavage-mediated huntingtin fragments represent an early pathological change in the brains of Huntington's disease patients. Using a Huntington's disease mouse model that develops Huntington's-related characteristics with age, it was shown that individuals expressing a mutant huntingtin protein, which is resistant to caspase-6 cleavage, are protected from neuronal dysfunction.…”

Truncated tau from sporadic Alzheimer's disease suffices to drive neurofibrillary degeneration in vivo

“…It has been shown that truncation is closely associated with Alzheimer's disease (AD)-typical conformational changes of the tau protein [5][6][7][8][9][10]. We have hypothesized that truncation could play major role in AD tau pathology [11].…”