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The Journal of Neuroscience, September 8, 2004, 24(36):7895-7902; doi:10.1523/JNEUROSCI.1988-04.2004
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Neurobiology of Disease
Early N-Terminal Changes and Caspase-6 Cleavage of Tau in Alzheimer's Disease
Peleg M. Horowitz,1
Kristina R. Patterson,1
Angela L. Guillozet-Bongaarts,1
Matthew R. Reynolds,1
Christopher A. Carroll,4
Susan T. Weintraub,4
David A. Bennett,5
Vincent L. Cryns,1,2
Robert W. Berry,1,3 and
Lester I. Binder1,3
1Department of Cell and Molecular Biology, 2Cell Death Regulation Laboratory, Division of Endocrinology, Metabolism, and Molecular Medicine, Department of Medicine, and 3Cognitive Neurology and Alzheimer's Disease Center, Feinberg School of Medicine, Northwestern University, Chicago, Illinois 60611, 4Department of Biochemistry, University of Texas Health Science Center at San Antonio, San Antonio, Texas 78229, and 5Department of Neurological Sciences and Rush Alzheimer's Disease Center, Rush University Medical Center, Chicago, Illinois 60612
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.
Key words: Alzheimer's disease; tau; neurofibrillary tangle; caspase; truncation; tyrosine phosphorylation
Received May 21, 2004;
revised July 21, 2004;
accepted July 27, 2004.
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