Studies on the tau lysosomal proteolytic pathway

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Tau is the major component of paired helical filaments (PHF) in Alzheimer's disease (AD) brain. Our results suggested that tau is degraded in lysosomes or their functional equivalents. To confirm the tau degradation pathway in lysosome we studied the binding between tau and lysosome membrane. Tau bound the lysosomes with the dissociation constant or 8 x 10^<-7> M.This observations prompted us to search for a receptor for tau in rat brain lysosomes. Rat brain lysosomal membranes were separated by SDS polyacrylamide gel electrophoresis, transferred to nitrocellulose membrane, and incubated with tau. We found that the proteins with the molecular weight of 130,100, and 76 kDa specifically bound tau. The C-termtinal regions of major lysosomal proteins were expressed as fusion proteins with GST and tested its binding with tau using BIAcore system. We found that both LAMP-1 and LAMP-2 bound tau. On the other hand, we identified glutamate dehydrogenase in the lysosomal fraction as a possible t … More au-binding protein by use of tau affinity chromatography. These results strongly suggest that these proteins are candidate for tau receptors on the lysosomes. Next we overexpressed the tau binding protein in CHO and neuro-2a cells and studied the turnover of tau by pulse chase labelling using the ^<35>S-methionine. We found that the tau levels in the cells are not largely changed for 3 days. In addition, the levels of tau in the cells were unaffected even after overexpressing these receptor proteins. These results suggest the possibility that tau is resistant against proteolysis in the cells and the difficulty in studying on the tau lysosomal proteolytic pathway in the cultured cells.Missense mutations were identified in the family of FTDP-17 patients. This clearly shows that tau and/or its proteolytic products cause the neuronal cell death. To study the effect of the mutations on the tau proteolytic pathway, we have generated knockin mouse lines expressing wild-type or mutant human tau under transcriptional control of the mouse tau promoter. In the brains of knockin mice, the levels of human tau mRNA and protein were similar to those of endogenous mouse tau. We will study the tau lysosomal proteolytic pathway using this mouse model. Less