The microtubule-associated protein tau has been implicated in the pathogenesis of Alzheimer's disease (AD) and other neurodegenerative disorders. Reducing tau levels ameliorates AD-related synaptic, network, and behavioral abnormalities in transgenic mice expressing human amyloid precursor protein (hAPP). We used mass spectrometry to characterize the post-translational modification of endogenous tau isolated from wild-type and hAPP mice. We identified seven types of tau modifications at 63 sites in wild-type mice. Wild-type and hAPP mice had similar modifications, supporting the hypothesis that neuronal dysfunction in hAPP mice is enabled by physiological forms of tau. Our findings provide clear evidence for acetylation and ubiquitination of the same lysine residues; some sites were also targeted by lysine methylation. Our findings refute the hypothesis of extensive O-linked N-acetylglucosamine (O-GlcNAc) modification of endogenous tau. The complex post-translational modification of physiological tau suggests that tau is regulated by diverse mechanisms.

Several norindenoisoquinolines substituted with methoxy or methylenedioxy groups have been prepared and their anticancer properties evaluated in cancer cell cultures and in topoisomerase I inhibition assays. 2,3-Dimethoxy-8,9-methylenedioxy-11H-indeno[1,2-c]isoquinoline hydrochloride (14) is a strong topoisomerase I inhibitor and also displays very high cytotoxicity in the NCI cancer cell culture screen (mean graph midpoint of 50 nM). The X-ray crystal structure of norindenoisoquinoline 14 in complex with topoisomerase I and DNA has been solved, providing insight into the structure-activity relationships within this class of new anticancer agents. The number and position of the norindenoisoquinoline substituents have a significant influence on biological activity and demonstrate that substitution on the nitrogen atom is not an absolute requirement for the antitumor effect of the indenoisoquinolines. Removal of the 11-keto group from the lead compound 1 and replacement of the N-alkyllactam with an unsubstituted pyridine ring causes the indenoisoquinoline ring system to flip over in the DNA-enzyme-inhibitor ternary complex. This allows the nitrogen atom to assume the hydrogen bond acceptor role of the 11-keto group, resulting in hydrogen bonding to Arg364.