A novel mouse model that recapitulates the isoform-specific, pathological propagation of tau

Masato Hosokawa (former Chief Researcher of Tokyo Metropolitan Institute of Medical Science (TMIMS), Faculty of Pharmaceutical Science, Fukuoka University), Masami Masuda-Suzukake, Hiroshi Shitara, Aki Shimozawa, Genjiro Suzuki, Hiromi Kondo, Takashi Nonaka, Masato Hasegawa (TMIMS), William Campbell (Telarray Diagnostics) and Tetsuaki Arai (Tsukuba University) reported a novel mouse model of isoform-specific propagation of tau in “Brain”.

Tauopathies, such as Alzheimer's disease (AD), corticobasal degeneration (CBD), and Pick's disease (PiD), are thought to cause dementia by abnormal accumulation of tau protein in neurons and glial cells, resulting in neurodegeneration.  Tau protein is classified into two types: 3R tau, which has three repeats in the microtubule-binding domain, and 4R tau, which has four repeats in the microtubule-binding domain.  In AD, both 3R and 4R tau are accumulated; in CBD, only 4R tau is accumulated; and in PiD, only 3R tau is accumulated.  This indicates that the accumulation of tau isoforms is characteristic of each disease. Furthermore, this pathology has been reported to spread in the brain, a phenomenon known as "propagation". In Alzheimer's disease, there is a correlation between the location and spread of tau accumulation and clinical symptoms.  In order to elucidate the pathological mechanisms of tauopathies and to develop therapeutic agents, it has been necessary to develop mice that can accurately reproduce a specific tau pathology.

In mouse models generated in the past, mouse brains injected with the insoluble fraction of AD patient brains were able to induce the accumulation of 4R tau, but not 3R tau.  Researchers in England also failed to induce the accumulation of 3R tau seen in the insoluble fraction of PiD patient brains.  The reason for this could be the difference in the expression pattern of tau between humans and mice.  In humans, only 3R tau is expressed in the fetal brain, but later both 3R and 4R tau are expressed.  In contrast, wild-type mice express only 3R tau in fetal and juvenile brains, but almost all tau is replaced by 4R tau in the adult, which is a major difference.  Studies reported so far have failed to reproduce the accumulation of isoforms characteristic of the disease because researchers did not take these differences into account.  Therefore, it was necessary to develop tau mice with the same expression patterns as those of humans.  In addition, many experiments reported previously used 4R tau transgenic mice and that was thought to be the reason why researchers could not induce the accumulation of 3R tau.

First, Hosokawa and colleagues created a new mouse that expresses endogenous 6-isoform tau to overcome the inadequancies of previous tau-injected experimental mouse models.  Using CRISPR-Cas9 genome editing technology, they succeeded in creating a novel mouse (Tau 3R/4R mouse) that expresses physiologically normal amounts of both 3R and 4R tau (not an overexpression system) in the adult, just as seen in the adult human brain.

Next, Hosokawa and colleagues extracted detergent-insoluble fractions from autopsy brains of AD, CBD, and PiD patients, and the tau fibrils contained in these fractions were injected into the striatum of Tau 3R/4R mice. Nine months post-injection, they performed immunohistochemical staining using 3R tau-specific and 4R tau-specific antibodies and found that mice injected with AD (3R+4R tauopathy) showed both endogenous 3R and 4R tau accumulation, mice injected with CBD (4R tauopathy) showed only 4R tau accumulation, and PiD (3R tauopathy)-injected mice accumulated only 3R tau.  Thus, they reproduced isoform-specific, seed-dependent amplification of tau pathology (Figure).  Biochemical analysis by immunoblotting also confirmed isoform-specific, seed-dependent accumulation of tau.

In addition, when the brains of mice injected with AD tau fibrils were stained with phosphorylated tau antibody (AT8), the pathology of tau accumulation in the striatum, the site of injection, increased with time, and the propagation of tau accumulation pathology spread to the cerebral cortex, thalamus amygdala and other regions which are directly connected to the striatum.

Finally, when the PiD tau fibril-injected mouse brains were examined in detail, spherical tau accumulation pathologies were observed that were not observed when AD or CBD tau fibrils were injected. These were very similar to the Pick bodies observed in PiD.  This is the first time that Pick-body-like pathology has been reproduced in a tau fibril injection experiment.  However, these inclusions were somewhat smaller than Pick bodies.  The reason for this is that the time that elapsed after injection of the PiD tau fibrils was shorter than the time required for Pick body formation in human PiD.  Moreover, the brains of mice injected with PiD tau fibrils were able to reproduce features of PiD pathology.  Tau accumulation in PiD is characterized by negative Gallyas-Braak staining and the absence of phosphorylation of serine 262 of tau, features which were also reproduced.

Hosokawa and colleagues next examined the process by which mouse tau accumulates after injection with human tau derived from an AD patient.  Injected human AD-derived tau was detected on the ipsilateral side of the brain immediately after the injection, but it became degraded with time and fell below the detection limit at 14 days after injection.  Eight months later, enhanced accumulation of abnormal tau was confirmed.  The abnormal accumulation of tau in these mice was found to consist of endogenous tau, inferring that the injected human tau had crossed the species barrier to convert mouse tau into the abnormal form which then accumulated.

This experiment confirmed the following prion-like properties of tau: (1) abnormal tau has the ability to cause isoform-specific, seed-dependent aggregation, (2) abnormal tau has the ability to propagate, and (3) human tau fibrils injected into the brain can induce the accumulation of endogenous mouse tau across the species barrier. Hosokawa and colleagues believe that this novel mouse model can be used to elucidate the mechanism of tau propagation and to search for drugs that inhibit tau propagation.

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About the Tokyo Metropolitan Institute of Medical Science

The Tokyo Metropolitan Institute of Medical Science (TMIMS) is dedicated to advancing basic and medical research in order to improve human health and quality of life. Founded in 2011 through the consolidation of three medical institutes, TMIMS is funded by the Tokyo metropolitan government and supports basic research in molecular and cellular biology in areas including genome replication, protein degradation, and infectious and neurodegenerative diseases. TMIMS also supports the development of new technologies in areas such as genome editing, control of neural prostheses, and vaccine development, and clinical research in fields such as optimization of nursing care and development of new treatments for psychiatric, neurodegenerative and other diseases. By integrating top-down applied research with bottom-up basic research, a goal of TMIMS is to more efficiently translate basic research results into treatments beneficial for humankind. For more information about TMIMS, see www.igakuken.or.jp/english.