Species: Rat
Genes: MAPT
Mutations: MAPT P301S
Modification: MAPT: Transgenic
Disease Relevance: Frontotemporal Dementia, Alzheimer's Disease
Strain Name: N/A
Genetic Background: Sprague-Dawley
Availability: Contact David Westaway, Institute for Neurodegenerative Diseases, UCSF.

Summary

Tg12099 rats overexpress the 0N4R isoform of human tau with the P301S mutation linked to frontotemporal dementia. The MAPT transgene is driven by the rat Prnp promoter.

Homozygous rats develop tau pathology, including neurofibrillary tangles, and neurodegeneration in corticolimbic structures and die prematurely.

Hemizygous rats do not develop overt tau pathology or neurodegeneration but are competent to propagate tau prions after intracerebral inoculation with recombinant tau fibrils or tau prions derived from the brains of sick Tg12099 homozygotes.

Neuropathology in Tg12099 rats. Left, Bielschowsky-stained neurofibrillary tangles are present in the piriform and entorhinal cortices of Tg12099 rats homozygous for the transgene but are absent from hemizygous animals. Right, neurodegeneration of corticolimbic structures in homozygotes but not hemizygotes; hematoxylin and eosin stain. From Ayers, Lopez et al., 2024; licensed under Creative Commons BY 4.0.

Both males and females were used in the initial characterization of the Tg12099 line, as summarized below (Ayers, Lopez et al., 2024).

Clinical phenotype | Tau expression | Tau pathology | Neurodegeneration | Gliosis | Tg12099 rats on a rat Mapt–null background | Tau propagation |

Clinical phenotype

Homozygotes die prematurely, with a median survival time of 14 months, while hemizygotes live for more than two years. (Animals were examined daily and euthanized if they showed two or more clinical signs—including hindlimb clasping, ataxia, or lack of a righting reflex—that did not resolve or worsened within 24 hours.)

Tau expression

Hemizygous rats carry 21 to 22 copies of the transgene inserted into a single site in rat chromosome 18 (18:20,312,421; rat genome assembly rn6). The insertion did not appear to disrupt any protein-coding genes or cause structural rearrangements.

Levels of tau protein in the brains of homozygous and hemizygous Tg12099 rats, respectively, were approximately 4.5-fold and 1.5-fold higher than in the brains of wild-type Sprague-Dawley rats—as determined by western blotting with the Tau5 antibody, which recognizes both human and rodent tau.

Expression of human tau was stable at least through 18 months of age—as shown by immunoblotting using the Tau13 antibody, selective for human tau.

Human tau was expressed throughout the brain, primarily by neurons.

Tau pathology

Homozygous Tg12099 rats exhibit tau pathology, including neurofibrillary tangles.

Immunoreactivity to AT8, a monoclonal antibody that recognizes tau phosphorylated at serine-202 and threonine-205, is a common marker of tauopathy. AT8 detects “pretangles”  in addition to the mature neurofibrillary tangles and neuropil threads seen with silver stains. AT8-immunoreactive neuronal inclusions first appeared in Tg12099 homozygotes between 6 and 7 months and increased with age. The piriform cortex, entorhinal cortex, and amygdala developed the highest concentrations of AT8-positive inclusions, while hindbrain structures showed little AT8 staining up to 18 months. Only a few AT8-immunoreactive neurons were found in hemizygotes, and these were restricted to the entorhinal cortices of aged animals.

AT8-immunreative bands became detectable on western blots of whole-brain homogenates of homozygotes between 6 and 9 months of age. AT8-immunoreactive tau was found in the sarkosyl-insoluble fraction, containing aggregated tau. AT8 immunoreactivity was not seen in western blots of homogenates from Tg12099 hemizygotes up to 18 months of age.

Monoclonal antibody MC-1 recognizes a tau conformation found in Alzheimer’s disease. MC-1-immunoreactive neuronal inclusions were observed in the cortex and hippocampus of homozygotes, beginning at 6 months and increasing with age. No MC-1 staining was seen in hemizygotes up to 18 months.

Neurofibrillary tangles visualized by Bielschowsky silver staining were present in the piriform and entorhinal cortices of terminal Tg12099 rats homozygous for the transgene. Tangles also stained with ThioflavinS. Tangles were absent in hemizygous rats.

Neurodegeneration

Pronounced neurodegeneration was observed in the forebrains of Tg12099 homozygotes at terminal stages. The piriform cortex of homozygous Tg12099 rats was only about half the size (46 percent reduction) and the amygdala only about one-tenth the size (88 percent reduction) of the corresponding structures in the brains of 18-month-old hemizygotes or wild-type rats. Neuron numbers were reduced by about 28 percent and 87 percent in the piriform cortex and amygdala, respectively, of Tg12099 homozygotes compared with hemizygotes.

Vacuolizations were observed in the cortices and brainstems of homozygotes, but rarely in hemizygotes.

Gliosis

There were signs of astrogliosis in the brains of terminal Tg12099 homozygotes. In the amygdala, there was a significant increase in GFAP immunoreactivity (measured as percent area) compared with aged hemizygotes. GFAP immunoreactivity was also greater in the piriform cortex of homozygotes, although the difference between genotypes did not reach statistical significance in this region.

Tg12099 rats on a rat Mapt–null background

The Tg12099 rats described above expressed endogenous rat tau in addition to human tauP301S. Because rodent tau has been reported to influence the phenotype of transgenic mouse models of tauopathy (see, for example, Ando et al., 2011; Andorfer et al., 2003; Sydow and Mandelkow, 2010; Wegmann et al., 2015), Tg12099 rats were intercrossed with a rat Mapt knockout line established in the same laboratory to isolate the effects of the human tau transgene.

Endogenous rat tau appeared to contribute to the premature death of Tg12099 rats: Homozygous Tg12099 rats on a Mapt-null background lived longer than the parental line expressing rat tau, with a median survival of 580 days versus 440 days.

Transgenic human tau was sufficient to induce tau pathology and neurodegeneration in this rat model. Tg12099; Mapt^-/- rats developed tau pathology—including neurofibrillary tangles—and exhibited similar patterns of brain atrophy as Tg12099 homozygotes expressing rat tau.

Tau propagation

Initial studies with Tg12099 rats focused on the prion-like propagation of tau. (A 2024 review described prion-like behavior as follows: “Prion-like aggregates must be able to 1) escape the cell and tissue region of origin, 2) enter new cells and regions, and 3) induce aggregation via a templated conformational change [Glynn et al., 2024].”)

It was first shown that brain homogenates from Tg12099 homozygotes were capable of seeding tau inclusions in a cellular assay. This assay employs HEK cells that stably express tauP301S conjugated to yellow fluorescent protein (YFP). While diffuse cytoplasmic fluorescence was observed in control cells, application of brain homogenates led to the appearance of dense fluorescent inclusions. The ability to promote these inclusions (“prion infectivity”) was present in brain homogenates from 6-month-old homozygotes, but not from 3-month-old animals, and increased with age. There were also regional differences, with corticolimbic regions showing the highest levels of prion infectivity. No prion infectivity was seen in homogenates from hemizygotes.

As mentioned above, only a few AT8-positive tau inclusions were seen in aged hemizygous Tg12099 rats, and these were restricted to the entorhinal cortex. Nonetheless, hemizygotes are competent to propagate tau prions after intracerebral injection of either fibrils synthesized in vitro from recombinant tau or tau prions derived from the brains of sick Tg12099 homozygotes.

Four months after unilateral injection of tau fibrils into the hippocampus, AT8-immunoreactive inclusions were found in neurons in the entorhinal, frontal, and piriform cortices, hippocampal layer CA3, and the thalamus—both ipsilateral and contralateral to the injection site. Further, brain lysates from the fibril-injected rats could promote tau aggregation in a cellular assay similar to the one described above—with the capacity to promote aggregation depending upon brain region and time after injection of the recombinant fibrils. Despite the generation of tau prions in the brains of fibril-injected hemizygotes, the rats did not develop any observable phenotypes.

Next, tau prions prepared from aged, sick homozygotes were injected bilaterally into the thalamus of 6- to 8-week-old hemizygotes. The recipient animals were killed two, four, six, or eight months post-injection. Histological examination revealed many AT8-immunoreactive neurons in the cortices of the oldest rats. When brain lysates from recipient animals were tested for prion infectivity in the cellular assay, they induced the formation of tau-YFP aggregates in the reporter cells, with infectivity increasing with time post-injection. Here again, despite the presence of tau prions in their brains, recipient Tg12099 hemizygotes did not show any signs of illness.

When intracerebrally injected with brain homogenates from donors with Alzheimer’s disease (AD) or progressive supranuclear palsy (PSP), hemizygous Tg12099 rats did not generate tau prions detectable in the cellular assay. A few AT8-immunoreactive cells were seen in recipient brains that had been injected with PSP donor material, but none were observed in the brains of recipients injected with brain lysates from AD donors.

Phenotype Characterization

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References

Mutations Citations

1. MAPT P301S

AlzAntibodies Citations

1. Tau (AT8); Phospho Tau (Ser 202, Thr 205) 18 May 2022
2. Tau (MC-1) 18 May 2022

Paper Citations

1. Ayers J, Lopez TP, Steele IT, Oehler A, Roman-Albarran R, Cleveland E, Chong A, Carlson GA, Condello C, Prusiner SB. Severe neurodegeneration in brains of transgenic rats producing human tau prions. Acta Neuropathol. 2024 Aug 20;148(1):25. PubMed.
2. Ando K, Leroy K, Héraud C, Yilmaz Z, Authelet M, Suain V, De Decker R, Brion JP. Accelerated human mutant tau aggregation by knocking out murine tau in a transgenic mouse model. Am J Pathol. 2011 Feb;178(2):803-16. PubMed.
3. Andorfer C, Kress Y, Espinoza M, de Silva R, Tucker KL, Barde YA, Duff K, Davies P. Hyperphosphorylation and aggregation of tau in mice expressing normal human tau isoforms. J Neurochem. 2003 Aug;86(3):582-90. PubMed.
4. Sydow A, Mandelkow EM. 'Prion-like' propagation of mouse and human tau aggregates in an inducible mouse model of tauopathy. Neurodegener Dis. 2010;7(1-3):28-31. PubMed.
5. Wegmann S, Maury EA, Kirk MJ, Saqran L, Roe A, DeVos SL, Nicholls S, Fan Z, Takeda S, Cagsal-Getkin O, William CM, Spires-Jones TL, Pitstick R, Carlson GA, Pooler AM, Hyman BT. Removing endogenous tau does not prevent tau propagation yet reduces its neurotoxicity. EMBO J. 2015 Dec 14;34(24):3028-41. Epub 2015 Nov 4 PubMed.
6. Glynn C, Rodriguez JA, Hyman BT. The structural line between prion and "prion-like": Insights from prion protein and tau. Curr Opin Neurobiol. 2024 Jun;86:102857. Epub 2024 Mar 15 PubMed.

Other Citations

1. Mapt knockout

External Citations

1. licensed
2. Creative Commons BY 4.0

Further Reading

No Available Further Reading