Ectopic Expression Induces Abnormal Somatodendritic Distribution of Tau in the Mouse Brain.

Tau is a microtubule (MT)-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 mislocalization occurs, we recently developed immunohistochemical tools that can separately detect endogenous mouse and exogenous human tau with high sensitivity, which allows us to visualize not only the pathological but also the pre-aggregated tau in mouse brain tissues of both sexes. Using these antibodies, we found that 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. Endogenous mouse tau and exogenous human tau in human tau knock-in mice exhibited high expression levels during the neonatal period and strong suppression into the adulthood. However, human tau in transgenic mice was expressed continuously and at high levels in adult animals. These results indicated the uncontrolled expression of exogenous tau beyond the developmental period as a cause of mislocalization in the transgenic mice. Superresolution microscopic and biochemical analyses also indicated that the interaction between MTs and exogenous tau was impaired only in the tau-transgenic mice, but not in knock-in mice. Thus, the ectopic expression of tau may be critical for its somatodendritic mislocalization, 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 mislocalization 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 (e.g., human tau in transgenic 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.

Inhibition of microtubule assembly competent tubulin synthesis leads to accumulation of phosphorylated tau in neuronal cell bodies.

Neurofibrillary tangles, a pathological hallmark of Alzheimer's disease (AD), are somatodendritic filamentous inclusions composed of hyperphosphorylated tau. Microtubule loss is also a common feature of affected neurons in AD. However, whether and how the disruptions of microtubules and the microtubule-associated proteins occur in the pathogenesis of AD remain unclear. Recent evidence indicates that reduced expression of tubulin by knocking down a tubulin chaperon can cause tau neurotoxicity. Thus, the disruption of tubulin homeostasis may result in the acquisition of tau pathogenesis and ultimately cause tauopathy. To investigate whether the disruption of tubulin maintenance induces tau abnormalities in mammalian neurons, we developed a miRNA-mediated knockdown system of tubulin-specific chaperon E (Tbce), which is a factor required for the de novo synthesis of tubulin. Tbce knockdown in mouse primary cultured neurons induced an increase in tubulin in the cell body at 14 days in vitro. Accumulated tubulin was not acetylated or incorporated in microtubules, indicating that they were functionally inert. Concomitantly, tau also accumulated in neuronal cell bodies. The mis-localized tau was phosphorylated at Ser202/Thr205 and Ser396/Ser404. These results indicate that Tbce knockdown in mammalian neurons induces not only a reduction in properly folded tubulins, which are microtubule assembly competent, but also an accumulation of phosphorylated tau in the cell body of mammalian neurons. These findings suggest that disruption of the homeostatic mechanism for maintaining tubulin biosynthesis and/or microtubules can cause tau accumulation in the cell body, which is commonly observed in tauopathies.