Research Models

FUS-R521C

Species: Mouse
Genes: FUS
Mutations: FUS R521C
Modification: FUS: Transgenic
Disease Relevance: Amyotrophic Lateral Sclerosis, Frontotemporal Dementia
Strain Name: B6;SJL-Tg(Prnp-FUS*R521C)3313Ejh/J
Genetic Background: Transgene injected into B6SJL oocytes. Maintained on C57BL/6, therefore subsequent generations have a higher percentage of C57BL/6.
Availability: The Jackson Lab: Stock# 026406; Cryopreserved

Summary

At a young age, this transgenic mouse develops severe motor impairment and other ALS-related phenotypes. Notably, it develops robust neuronal loss in the spinal cord, denervation of neuromuscular junctions, and muscle atrophy. Phenotype development is swift—detectable within weeks of birth—and the mice decline rapidly. Most mice in the original N1F1 generation reached end-stage within three months (Qiu et al., 2014).

At one month of age, the mice express mutant FUS at levels approximately equal to endogenous FUS levels in the brain and spinal cord. The majority of the transgenic FUS-R521C protein in these mice is nuclear. Cytoplasmic protein is occasionally detected; however, less than 10 percent of spinal motor neurons contain cytoplasmic FUS inclusions. Endogenous FUS protein, but not transgenic protein, appears in dendrites as a punctate pattern. Similar to endogenous FUS, FUS-R521C protein is detected in astrocytes and oligodendrocytes, but not in microglia.

The mice develop prominent neuronal loss in the spinal cord. At birth, the number of spinal motor neurons is normal. However, by day 16, the number of ChAT-positive neurons in the anterior horn of the spinal cord is reduced by 20 percent. Degeneration continues, and by end stage, only about half of the neurons remain. The surviving motor neurons have reduced dendritic complexity, synaptic defects, and DNA damage. There is no detectable loss of cortical neurons; however, neurons in the sensorimotor cortex have reduced dendritic complexity and reduced synaptic density.

Despite modest transgene expression, FUS-R521C mice exhibit a variety of motor impairments from a young age, including spastic paraplegia and abnormal hindlimb clasping when lifted by the tail. They also have gait abnormalities, including reduced distance between their hind paws during walking. Performance on the Rotarod is poor.

The majority of mice in the original N1F1 generation reached end stage by postnatal day 100. Mice in subsequent generations (e.g., N2F2, N2F3), which have greater C57BL/6 contribution, live longer; about 40 percent reached end stage by postnatal day 200. Both male and female mice show similar disease phenotypes. Hemizygous males may be sterile, or at least have a much reduced breeding capacity (Eric Huang, personal communication, March 2016).

Data on this page refer to hemizygous mice. The phenotype of homozygous mice have not been reported.

Modification Details

The transgene in this model encodes human FUS with the R521C mutation near the C-terminus. It uses a flag-tagged construct driven by the Syrian hamster prion protein promoter.

Phenotype Characterization

When visualized, these models will distributed over a 18 month timeline demarcated at the following intervals: 1mo, 3mo, 6mo, 9mo, 12mo, 15mo, 18mo+.

Absent

  • Cortical Neuron Loss

No Data

Cortical Neuron Loss

No detectable loss of cortical neurons; however, neurons in the sensorimotor cortex show reduced dendritic complexity and reduced synaptic density.

Lower Motor Neuron Loss

No detectable difference in spinal motor neurons at P0. At P16, about 20% loss of ChAT-positive neurons in the anterior horn of cervical spinal cord. At P30-P60, about 50% loss of anterior horn neurons. Remaining motor neurons show reduced dendritic complexity and synaptic density.

Cytoplasmic Inclusions

Less than 10% of spinal motor neurons have cytoplasmic FUS inclusions.

Gliosis

Prominent increase in microgliosis and astrogliosis in the anterior horn of the spinal cord by end stage.

NMJ Abnormalities

Reduced innervation of neuromuscular junctions in the diaphragm.

Muscle Atrophy

The majority of mice have severe skeletal muscle atrophy in the hindlimb by end stage.

Motor Impairment

Early postnatal motor impairment, including abnormal hindlimb clasping when lifted by the tail, gait abnormalities, and impaired Rotarod performance.

Body Weight

Early postnatal growth is retarded, and the mice experience progressive loss of body weight.

Premature Death

The majority of mice in the N1F1 generation reached end stage and were sacrificed by postnatal day 100. Mice in subsequent generations live longer: about 40% reach end stage by postnatal day 200.

COMMENTS / QUESTIONS

  1. It is always encouraging when another model of ALS is published. The more models we have, the more we can find out and the faster we can get to therapies. This model specifically overexpresses a mutant form of FUS. The onset of disease occurs very early in these mice, which might relate directly to the overall levels of FUS expressed in these animals. Although the mutant form is expressed at a similar level to the endogenous one in these transgenics, the overall increase in expression is significantly higher compared to a non-transgenic. It has been shown in both FUS and TDP models of disease that the age of onset seems related to the level of overexpression rather than the mutation (both wild-type FUS and wild-type TDP overexpression mouse models show neurodegenerative-like phenotypes).

    It is also interesting that in these animals there is no sign of downregulation of the endogenous protein, which might result from the mutation because FUS is known to self-regulate (by splicing its own mRNA, which leads it to be degraded) and does so in other models (Mitchell et al., 2013). What is not clear is whether this model can tell us anything about FTLD-FUS. It might be that FTLD-FUS is not caused primarily by a FUS deficit but by a disruption of pathways in which it is involved. Certainly there are other proteins, such as TAF15 and EWS and TNPO1, which are found in FTLD-FUS inclusions but are not found in ALS-FUS inclusions.  

    The authors report that the mutant and wild-type FUS form complexes. That agrees with a variety of papers that suggest that this is the case. It is perhaps surprising that they did not find an interaction between the WT protein and itself, as has been previously published. I think that the formation of these stable complexes is very interesting, as it fits with the hypothesis that the mutant protein can sequester the wild-type protein within cytoplasmic aggregates in ALS. Given that a single base pair change can lead to such a devastating disease, it is not surprising that it might have such an effect in cell and animal models. 

    In this case I suspect that the lack of accumulation of FUS in the cytoplasm has more to do with longevity, because the level of FUS seems fairly toxic to the mice and they only live a short while. I suspect that the expression is high in these animals and that a lower sustained expression of FUS would more likely result in aggregates. Previous models showed that increasing the overall expression level of WT FUS by 1.4- to 1.9-fold was sufficient to result in an aggressive, fatal phenotype. I feel that this model, like most models, tells us something about the mechanism of disease but does not fully recapitulate the disease. After all, we are comparing a disease that takes a few decades to manifest (even in the FUS families) to a very rapid onset in the models. I think that all overexpression models, regardless of whether they are WT or mutant forms of the protein, tell us that FUS, like TDP43, is a heavily regulated protein and that under- or overexpression of it results in cell death. This suggests that there are critical cell functions that are carried out by FUS for which any disruption results in problems for the cell. 

    I think that the RNA work that has been done in this paper and the neuronal changes seen here, linked with recent publications on both TDP and FUS in the nervous system, suggests that a disruption of RNA metabolism in the neurons leading to altered synaptic function and ultimately cell death is a likely disease mechanism. 

    References:

    . Overexpression of human wild-type FUS causes progressive motor neuron degeneration in an age- and dose-dependent fashion. Acta Neuropathol. 2012 Sep 9; PubMed.

    View all comments by Caroline Vance

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References

Paper Citations

  1. . ALS-associated mutation FUS-R521C causes DNA damage and RNA splicing defects. J Clin Invest. 2014 Mar 3;124(3):981-99. Epub 2014 Feb 10 PubMed.

External Citations

  1. The Jackson Lab: Stock# 026406

Further Reading