Species: Mouse
Genes: TARDBP
Modification: TARDBP: Transgenic
Disease Relevance: Amyotrophic Lateral Sclerosis, Frontotemporal Dementia
Strain Name: B6;C3-Tg(tetO-TARDBP*)4Vle/J
Genetic Background: Transgene injected into fertilized eggs from C57BL/6J x C3HeJ.
Availability: The Jackson Lab: Stock# 014650; Live

Summay

Under physiological conditions TDP-43 is primarily nuclear, but in some people with ALS and FTD the protein relocates to the cytoplasm where it accumulates into hallmark inclusions. To investigate the consequences of TDP-43 mislocalization, this mouse model overexpresses TDP-43 targeted to the cytoplasm through removal of the nuclear localization signal (NLS) from the transgene (Igaz et al., 2011).

This model uses the TET-OFF system to regulate TDP-43 expression, allowing for temporal control over the transgene. The transcriptional transactivator (tTA) is driven by the CAMKIIα promoter, resulting in preferential forebrain expression of tTA, and thus of transgenic TDP-43. An advantage of this model is that human TDP-43 can be suppressed by adding doxycycline (dox) to the diet, bypassing potential confounds of transgene expression during development.

Consistent with the expected pattern for the CAMKIIα promoter, human TDP-43 protein is primarily restricted to forebrain neurons. High levels were seen in the cortex, hippocampus, and olfactory bulb, whereas low to undetectable protein levels were seen elsewhere (e.g., cerebellum, brainstem, and spinal cord). Cortical TDP-43 expression was about eightfold higher than endogenous levels in non-Tg mice. Notably, within one month of dox withdrawal, exogenous TDP-43 expression in the cortex severely downregulated murine TDP-43 expression.

As expected given the absence of a functional NLS, TDP-43 protein was cytoplamic in the majority of neurons in the hippocampus and cortex (~70 to 90 percent). Despite robust levels of protein in the cytoplasm, TDP-43 aggregates were extremely rare, observed in less than 1 percent of cortical neurons. Aggregate levels peaked at about one month after de-repression of the transgene.

Despite a lack of TDP-43 inclusions, these mice develop severe neuronal degeneration shortly after dox withdrawal. Approximately 50 percent of dentate gyrus neurons were gone one month after de-repression. Neurons in the dentate gyrus and deep layers of the neocortex were selectively vulnerable to degeneration, whereas other regions, such as the hippocampal CA1 subfield and olfactory bulb, were resistant, despite similar expression levels.

In hTDP-43ΔNLS mice, extensive gliosis developed in regions affected by neurodegeneration (i.e., the neocortex and dentate gyrus). Gliosis also was noted at all levels of the corticospinal tract, as well as the striatum, cerebral peduncles, and cervical spinal cord. Axons of the corticospinal tract degenerated one month after dox withdrawal. The number of lower motor neurons was unaffected, and there was no evidence of muscle atrophy (Igaz et al., 2011).

The neurodegenerative pathology in these mice is striking; however, it is not dependent on the cytoplasmic mislocalization of TDP-43. Control animals generated in parallel, expressing wild-type TDP-43 with the NLS intact, developed a similar phenotype (Igaz et al., 2011). Furthermore, some of the observed neurodegeneration may be due to expression of tTA itself. tTA-only mice were not assessed for neurodegeneration alongside bigenic hTDP-43ΔNLS mice, and subsequent reports using other TET-OFF mice have found tTA alone can produce smaller forebrains and dentate gyri compared with non-Tg littermates. This difference is reportedly detectable by two months of age and is largely avoided by treating with dox prenatally and for the first six weeks of life (Han et al., 2012; Liu et al., 2015).

These mice exhibit an array of behavioral impairments indicative of motor, cognitive, and social deficits. Initially they were noted to display an abnormal clasping response as early as one week after dox withdrawal (Igaz et al., 2011). A later comprehensive behavioral assessment at one month after dox withdrawal revealed a variety of motor abnormalities, including hyperlocomotion in the open field test, impaired coordination and balance on the Rotarod, and decreased grip strength. They also showed cognitive deficits such as impaired recognition and spatial memory as measured by the novel-object-recognition test and the Y maze. One month after transgene de-repression, hTDP-43ΔNLS mice showed reduced social behavior, which could not be attributed to changes in anxiety or olfaction. Behavioral deficits were not observed in tTA–only littermates (Alfieri et al., 2014).

The TET-OFF system allows the transgene to be suppressed after phenotypes have developed in order to investigate potential reversibility. “Switching off” the transgene for 14 days largely reversed the motor and cognitive phenotypes in young mice (1.5 months). However, the phenotypes were not reversible in older mice (6.5 months), in which extensive neurodegeneration had already occurred. Interestingly, social deficits were not reversible even in young mice (Alfieri et al., 2014).

The data on this page refer to hemizygous mice.

Modification Details

These bigenic mice use the CAMKIIα promoter to drive expression of tetracycline transactivator (tTA) in forebrain neurons. The responder transgene is wild-type human TDP-43 minus the nuclear localization signal (NLS). Human TDP-43 is expressed constitutively unless suppressed by doxycycline.

Phenotype Characterization

COMMENTS / QUESTIONS

1. • Subhojit Roy
     University of California, San Diego
   • Posted: 26 Jan 2011
   • Paper: Dysregulation of the ALS-associated gene TDP-43 leads to neuronal death and degeneration in mice.

   Wonderful, rigorous study, and perhaps a window into the future of neurodegeneration research. Though the thought is slightly disturbing, it is becoming more and more apparent that the aggregates, plaques, and tangles that we neuropathologists love to see (and preach about) may not be as important as we thought.

   View all comments by Subhojit Roy

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References

Paper Citations

1. Igaz LM, Kwong LK, Lee EB, Chen-Plotkin A, Swanson E, Unger T, Malunda J, Xu Y, Winton MJ, Trojanowski JQ, Lee VM. Dysregulation of the ALS-associated gene TDP-43 leads to neuronal death and degeneration in mice. J Clin Invest. 2011 Feb;121(2):726-38. Epub 2011 Jan 4 PubMed.
2. Han HJ, Allen CC, Buchovecky CM, Yetman MJ, Born HA, Marin MA, Rodgers SP, Song BJ, Lu HC, Justice MJ, Probst FJ, Jankowsky JL. Strain background influences neurotoxicity and behavioral abnormalities in mice expressing the tetracycline transactivator. J Neurosci. 2012 Aug 1;32(31):10574-86. PubMed.
3. Liu P, Paulson JB, Forster CL, Shapiro SL, Ashe KH, Zahs KR. Characterization of a Novel Mouse Model of Alzheimer's Disease--Amyloid Pathology and Unique β-Amyloid Oligomer Profile. PLoS One. 2015;10(5):e0126317. Epub 2015 May 6 PubMed.
4. Alfieri JA, Pino NS, Igaz LM. Reversible behavioral phenotypes in a conditional mouse model of TDP-43 proteinopathies. J Neurosci. 2014 Nov 12;34(46):15244-59. PubMed.

External Citations

1. The Jackson Lab: Stock# 014650

Further Reading