New application of beta-galactosidase complementation to monitor tau self-association.

Neurofibrillary tangles composed of hyperphosphorylated and aberrantly cleaved microtubule-associated protein tau are a major neuropathological hallmark of Alzheimer's disease. Recent studies suggest that the predominant neurotoxic effect of pathologically processed tau is mediated by intermediate tau multimers rather than the mature tau tangles, thus underscoring the importance of studying tau self-association preceding tangle formation. However, experimental approaches for such studies are limited. Here, we describe a modification of the beta-galactosidase (beta-gal) complementation assay, which provides a simple, sensitive and quantitative system to monitor pre-tangle tau-tau interactions in a cell model. Full-length tau (T4) and tau truncated at D421 (C3, to mimic caspase-cleaved tau) were fused to one of a pair of weakly complementing beta-gal mutants (Deltaalpha and Deltaomega) and expressed in human embryonic kidney cells. The tau-tau interactions and the subsequent juxtapositioning of Deltaalpha and Deltaomega led to beta-gal complementation and an increase in beta-gal activity which was detected by histochemical staining and quantified by chemiluminescent assays. After cross-linking with disuccinimidyl suberate, tau formed high molecular weight complexes which were detected on denaturing acrylamide gels, further confirming the close proximity among self-associated tau molecules. The self-association of C3 appeared to be less efficient than that of T4. Furthermore, treatment with lithium decreased beta-gal complementation of both T4 and C3 indicating that the interaction of these proteins was attenuated. Overall, this study suggests that beta-gal complementation assay can be a useful tool to monitor tau self-association.