Mutations: TARDBP M337V
Modification: TARDBP: Transgenic
Disease Relevance: Amyotrophic Lateral Sclerosis, Frontotemporal Dementia
Strain Name: N/A
Genetic Background: Transgene injected into BL6/SJL oocytes. Founders crossed to C57BL6/J.
These mice, which express mutant human TDP-43 in postnatal neurons, quickly develop several ALS-related phenotypes, including motor impairment, upper motor neuron loss, and premature death (Janssens et al., 2013). The “Mt” in the name of this model stands for “mutant,” and refers to the fact that the transgene contains the M337V mutation associated with familial ALS.
Transgene expression in Mt-TAR6 mice is driven by the Thy-1.2 promoter, resulting in preferential expression in neurons. Because this promoter becomes active about one week after birth, transgene expression does not interfere with critical developmental processes in utero. Once the transgene is on, the disease progresses very rapidly in homozygous mice (Mt-TAR6/6), with a comparatively later onset and milder progression in hemizygotes (Mt-TAR6).
Compared with other transgenic TDP-43 models, these mice express relatively low levels of transgene. Transgene expression in the brains of hemizygous mice is less than endogenous levels of TDP-43, and expression in homozygotes is approximately double endogenous levels. Endogenous TDP-43 is downregulated by expression of transgenic TDP-43.
Consistent with a gene-dosage effect, homozygous mice develop a much more severe phenotype. Starting around postnatal day 12, homozygous Mt-TAR6/6 showed an abnormal clasping reflex when suspended by their tails. Symptoms progressed rapidly, involving a hunched posture, muscle twitches, and reduced mobility by day 15. Complete paralysis and death were close behind, with an average survival of just 17 days. No mice survived past day 20.
In contrast, hemizygous MT-TAR6 mice lived up to 24 months (average survival was 16.4 months). They did not develop an abnormal hindlimb reflex until about one year of age, and Rotarod testing did not show motor impairment until 13 months. In addition, a considerable amount of phenotypic variability was noted in hemizygous mice. A small minority of the mice (about 5 percent) developed a severe motor impairment, involving complete paralysis of the hindlimbs. In this subset, paralysis developed between three and 14 months of age.
Although the Thy-1.2 promoter drives expression in virtually all neurons of the brain and spinal cord, specific neuronal subpopulations appeared to be selectively vulnerable, including cortical layer V motor neurons, spinal anterior horn motor neurons, CA regions of the hippocampus, and thalamic neurons. MT-TAR6/6 mice developed severe neuronal loss in all CA regions of the hippocampus and complete obliteration of CA3 and CA fields, while CA1 and CA2 fields were reduced to a single layer of cells. Astrogliosis and microgliosis occurred in regions affected by neuronal loss, including the motor cortex, spinal cord, and hippocampus.
Cortical motor neurons in homozygous and hemizygous mice developed diffuse cytoplasmic ubiquitin staining. This staining, which was absent in non-Tg mice, was also observed in mice expressing comparable levels of wild-type TDP-43. Ubiquitin immunoreactivity was also present in spinal motor neurons, the hippocampus, pons, and cerebellum, albeit at lower levels.
In addition to diffuse ubiquitin immunoreactivity, some Mt-TAR6/6 mice developed ubiquitin-positive inclusions in layer V cortical neurons, but not in spinal motor neurons. However, inclusions were not universally present in all mice, even at end-stage. Also observed were ubiquitin-negative inclusions, described as “large amorphous eosinophilic inclusions” in the cytoplasm of anterior horn neurons, and to a lesser extent in neurons of the cortical layer V, thalamus, pons, and cerebellum.
Mitochondrial abnormalities were also observed, including deformed cristae and fission deficits.
In this model the Thy-1.2 promoter drives expression of a transgene encoding human TARDBP with the M337V mutation.
When visualized, these models will distributed over a 18 month timeline demarcated at the following intervals: 1mo, 3mo, 6mo, 9mo, 12mo, 15mo, 18mo+.
- NMJ Abnormalities
- Muscle Atrophy
Cortical Neuron Loss
Severe neuronal loss in all CA regions of the hippocampus of homozygous mice. Neuronal loss was also observed in layer V cortical neurons and thalamic neurons.
Lower Motor Neuron Loss
Neuronal loss was observed in the spinal cords of homozygous mice.
Some homozygous mice developed cytoplasmic inclusions in layer V cortical neurons. These were often, but not always, ubiquitin–positive. They were not universally observed, even in end-stage mice.
Elevated astrogliosis and microgliosis compared with non-Tg controls, especially in the motor cortex and spinal cord. Gliosis in the hippocampus was seen at end stage.
Motor impairment developed quickly, by 11 days of age in homozygous mice, starting with an abnormal clasping reflex. They also develop a hunched posture, muscle twitches, and reduced mobility. Paralysis developed within days, leading to death. Hemizygotes do not develop motor symptoms until about one year of age, and impairment varied from mouse to mouse.
Early postnatal growth retardation in homozygous mice. By day 17 their average body weight is about half that of non-Tg controls.
Homozygous mice survived an average of just 17 days. In contrast, hemizygous Mt-TAR6 mice lived up to 24 months (average survival ~16.4 months).
- Janssens J, Wils H, Kleinberger G, Joris G, Cuijt I, Ceuterick-de Groote C, Van Broeckhoven C, Kumar-Singh S. Overexpression of ALS-associated p.M337V human TDP-43 in mice worsens disease features compared to wild-type human TDP-43 mice. Mol Neurobiol. 2013 Aug;48(1):22-35. Epub 2013 Mar 10 PubMed.
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