Abstract
Tau is a microtubule-associated protein that is localized to the axon. In Alzheimer’s disease, the distribution of tau undergoes a remarkable alteration, leading to the formation of tau inclusions in the somatodendritic compartment. To investigate how this mis-localization occurs, we recently developed immunohistochemical techniques that can separately detect endogenous mouse and exogenous human tau with high sensitivity, which allows us to visualize not only the pathological but pre-aggregated tau in mouse brain tissues of both sex. In tau transgenic mouse brains, exogenous human tau was abundant in dendrites and somata even in the presymptomatic period, whereas the axonal localization of endogenous mouse tau was unaffected. In stark contrast, exogenous tau was properly localized to the axon in human tau knock-in mice. We tracked this difference to the temporal expression patterns of tau. Tau mRNA was continuously expressed in the transgenic mice, whereas endogenous tau and exogenous tau in the knock-in mice exhibited high expression levels during the neonatal period and strong suppression into the adulthood. These results indicated the uncontrolled expression of exogenous tau beyond the developmental period as a cause of mis-localization in the transgenic mice. Super-resolution microscopic and biochemical analyses also indicated that the interaction between microtubules and exogenous tau was indeed impaired in the tau transgenic mice. Thus, the ectopic expression of tau may be critical for its somatodendritic mis-localization, a key step of the tauopathy.
Significance Statement Somatodendritic localization of tau may be an early step leading to the neuronal degeneration in tauopathies. However, the mechanisms of the normal axonal distribution of tau and the mis-localization of pathological tau remain obscure. Our immunohistochemical and biochemical analyses demonstrated that the endogenous mouse tau is transiently expressed in neonatal brains, that exogenous human tau expressed corresponding to such tau expression profile can distribute into the axon, and that the constitutive expression of tau into adulthood (ex. human tau in Tg mice) results in abnormal somatodendritic localization. Thus, the expression profile of tau is tightly associated with the localization of tau, and the ectopic expression of tau in matured neurons may be involved in the pathogenesis of tauopathy.
Acknowledgements
We thank Akihiro Harada and Fumitaka Oyama for provision of the tau-KI mice. This work was supported in part by the Grant-in-Aid for Scientific Research on Innovative Areas “Brain Protein Aging and Dementia Control” (T.M. 26117004), Challenging Exploratory Research (22650074; T.M.), the “Integrated research on neuropsychiatric disorder”, which was carried out under the Strategic Research Program for Brain Sciences (T.M. and Y.I.), Core-to-Core Program A Advanced Research Networks, the Strategic Research Foundation at Private Universities (S1201009), the Mitsubishi Foundation (T.M.), and the JSPS KAKENHI Grant Number 26640030 (A.K.) and 16K07006 (to H.M.).
Footnotes
Conflict of interest: The authors declare no competing financial interests.