Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder characterized by the selective death of motor neurons in the motor cortex, brainstem, and spinal cord. A large number of rodent models are available that show motor neuron death and a progressive motor phenotype that is more or less reminiscent of what occurs in patients. These rodent models contain genes with spontaneous or induced mutations or (over) express different (mutant) genes. Some of these models have been of great value to delineate potential pathogenic mechanisms that cause and/or modulate selective motor neuron degeneration. In addition, these genetic rodent models play a crucial role in testing and selecting potential therapeutics that can be used to treat ALS and/or other motor neuron disorders. In this paper, we give a systematic overview of the most important genetic rodent models that show motor neuron degeneration and/or develop a motor phenotype. In addition, we discuss the value and limitations of the different models and conclude that it remains a challenge to find more and better rodent models based on mutations in new genes causing ALS.
Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by the selective death of motor neurons in the motor cortex, brainstem, and spinal cord. Typically, ALS is an adult-onset disorder with a fast progression. About 5% to 10% of ALS patients have a family history (FALS), and most of these patients inherit the disease in an autosomal dominant way. The large majority of patients (90% to 95%) have no clear family history and are considered as sporadic ALS (SALS). FALS and SALS are clinically indistinguishable, and all patients experience muscle weakness, atrophy, and spasticity. This is the consequence of the loss of both upper and lower motor neurons. Ultimately, patients become paralyzed and denervation of respiratory muscles causes the death of the patient, on average three to five years after the first clinical signs. As studying a sporadic disease is very difficult, most research focused on the genetic causes of FALS (for reviews see [
The most important and most studied cause of FALS are mutations in the gene encoding superoxide dismutase 1 (SOD1). On average, SOD1 mutations are responsible for about 20% of FALS cases [
All other genetic causes of FALS are rare and/or cause an atypical form of ALS. These include mutations in the genes encoding alsin (ALS2; [
For years, a number of mouse models exist that contain spontaneous or induced mutations leading to motor neuron death (Table
Overview of spontaneous or induced mouse models showing motor neuron degeneration.
Name | Mutated gene | Gene product | Inheritance | Human disease | Reference |
---|---|---|---|---|---|
Wobbler | Subunit of the GARP complex | recessive | NA | [ | |
Nmd | Immunoglobulin | recessive | SMARD1 | [ | |
Pmn | tubulin-specific chaperone E | recessive | motor neuropathy HRD/SSS | [ | |
Loa | dynactin | dominant | sensory neuropathy | [ | |
Cra | dynactin | dominant | sensory neuropathy | [ |
SMARD: spinal muscular atrophy with respiratory distress, HRD: hypoparathyroidism-retardation dysmorphism syndrome, SSS: Sanjad-Sakati syndrome, and NA: not available.
Overview of transgenic rodent models for motor neuron degeneration (Mendelian and typical FALS).
Disease | Gene product | Inheritance | Animal | Genetic modification | Reference |
---|---|---|---|---|---|
ALS1 | Superoxide dismutase 1 | Dominant | Mouse | genomic hSOD1 G37R | [ |
genomic hSOD1 G85R | [ | ||||
genomic mSOD1 G86R | [ | ||||
genomic hSOD1 G93A | [ | ||||
genomic hSOD1 L126Z(stop) | [ | ||||
genomic hSOD1 L126delTT | [ | ||||
genomic hSOD1 Quad | [ | ||||
PrP; cDNA SOD1 G37R | [ | ||||
Thy-1; cDNA hSOD1 G93A | [ | ||||
Rat | genomic hSOD1 H46R | [ | |||
genomic hSOD1 G93A | [ | ||||
genomic hSOD1 G93A | [ | ||||
Dominant/recessive | Mouse | genomic hSOD1 D90A | [ | ||
ALS6 | FUS/TLS | Dominant (recessive) | Mouse | FUS/TLS KO | [ |
ALS10 | TDP-43 | Dominant | Mouse | Prp; hTDP-43 A315T | [ |
Thy-1; hTDP-43 WT | [ | ||||
PrP; hTDP-43 WT | [ | ||||
PrP; hTDP-43 A315T | [ | ||||
PrP; hTDP-43 M337V | [ | ||||
PrP; hTDP-43 WT | [ | ||||
Rat | TRE; hTDP-43 M337V and WT | [ |
hSOD1: human superoxide dismutase 1, mSOD1: mouse superoxide dismutase 1, FUS: fused in sarcoma, TLS: translocated in liposarcoma, KO: knockout, PrP: prion promoter, TRE: tTA-activated promoter.
Overview of transgenic mouse models for motor neuron degeneration (atypical or rare FALS and candidate genes).
Disease | Gene product | Inheritance | Animal | Genetic modification | Reference |
---|---|---|---|---|---|
ALS2 | Alsin | Recessive | Mouse | KO (exon 3) | [ |
KO (stop codon in exon 3) | [ | ||||
KO (exon 3 and 4) | [ | ||||
KO (exon 4) | [ | ||||
ALS8 | VAPB | Dominant | Mouse | PrP; VAPB P56S | [ |
ALS | Dynactin | Dominant | Mouse | Knock-in G59S p150Glued | [ |
Thy-1; G59 p150Glued | [ | ||||
CMT2E/1F | Neurofilament-L | Dominant | Mouse | NF-L L394P | [ |
NA | Peripherin | NA | Mouse | overexpression | [ |
NA | Vascular endothelial growth factor | NA | Mouse | VEGF | [ |
FTDP-tau | tau | Dominant | Mouse | 4R human tau | [ |
R406W human tau | [ | ||||
P301L human tau | [ | ||||
G272V, P301S human tau | [ | ||||
V337M human tau | [ | ||||
P301S human tau | [ |
PrP: prion promoter, CMT: Charcot-Marie-Tooth, FTDP: frontotemporal dementia with parkinsonism, and NA: not available.
The Wobbler mouse arose as the result of a spontaneous mutation at the “Institute of Animal Genetics” in Edinburgh. These mice have an unsteady gait with progressive weakness. The most characteristic abnormality found in the Wobbler mouse is the degeneration of nerve cells of the motor system in the brainstem and in the spinal cord. This results in progressive denervation of skeletal muscles, leading to muscle atrophy especially in the head, neck, and forelimbs while the hind limbs are less severely affected [
Originally, the Wobbler mouse was considered as a good model of motor neuron degeneration. However, this mouse model does not develop a typical motor neuron disease. The degeneration of motor neurons and the onset of astrogliosis and microgliosis in the spinal cord seem to be preceded by a more widespread neurodegeneration [
The gene mutated in this autosomal recessive disease model is
The “Neuromuscular degeneration” (Nmd) mouse contains a spontaneous autosomal recessive mutation that leads to neuromuscular degeneration [
A mutation that creates a cryptic donor splice site was found in intron 4 of the gene encoding the immunoglobulin
The “Progressive motor neuronopathy” (Pmn) mutant mouse develops hindlimb paralysis and displays progressive degeneration of motor axons while the cell bodies of the motor neurons in the ventral spinal cord are unaffected [
The underlying genetic cause of the disease in the Pmn mouse is a point mutation in the
Two independent
Two different point mutations in the gene encoding dynein were found in the Loa and Cra mice [
However, the validity of the Loa mouse as a model for ALS was questioned by the observation that no selective loss of motor neurons was detected [
Also the Cra mice do not show motor neuron loss, not even in aged animals [
After the discovery of mutations in the SOD1 gene, a transgenic mouse overexpressing mutant (G93A) SOD1 was created by insertion of multiple copies of human genomic SOD1 into the mouse genome [
In addition to the original mutant (G93A) SOD1 mice, a large number of other transgenic mice that (over)express human SOD1 containing other mutations (G37R, G85R or D90A) or mutant (G86R) mouse SOD1 were created (Table
The different mutant SOD1 mouse models have been extensively studied and strongly indicate that mutant SOD1 causes selective motor neuron death by a “gain of function” Moreover, these mice were used to study the pathogenic changes occurring during the disease process, and they were also crossbred with many other transgenic mice to get insight about the pathogenic mechanism(s) involved. From one of these crossbreedings, it was concluded that mutant SOD1-mediated oxidative abnormalities are not the primary cause of mutant SOD1 toxicity. SOD1 is an enzyme that requires copper to catalyze the conversion of toxic superoxide radicals into hydrogen peroxide and oxygen. Copper plays a crucial role in the normal and/or aberrant enzymatic activity of the enzyme, and copper loading of SOD1 is performed by a specific copper chaperone (CCS). Crossbreeding of transgenic mutant (G93A) SOD1 mice with knockout mice lacking the CCS, does not influence the life span of mutant (G93A) SOD1 mice [
The mutant SOD1 models were also used to clarify the contribution of different cell types in the disease process. Selective (over)expression of mutant SOD1 in motor neurons [
The mutant SOD1 mouse models were also extensively used to test new therapeutic strategies. One famous example is minocycline, a drug that inhibits microglial activation and that shows a positive effect in three independent studies using two different mutant SOD1 models [
Transgenic rats overexpressing mutant (H46R and G93A) SOD1 were also created and these animals also develop an age-dependent degeneration of motor neurons leading to paralysis and death [
Recently, two studies reported the discovery of mutations in FUS (fused in sarcoma)/TLS (translocated in liposarcoma) as a new genetic cause of ALS in Cape Verde, the USA, Australia and the UK [
The FUS/TLS protein is involved in RNA metabolism and regulation of transcription. FUS/TLS knockout mice die immediately after birth [
Transactivation response DNA-binding protein with molecular weight 43 kD (TDP-43) was first identified as one of the major constituents of the intraneuronal inclusions observed in ALS and frontotemporal lobar degeneration with ubiquitin-positive inclusions (FTLD-U) [
TDP-43 has two RNA binding domains and a glycine-rich domain in the C-terminal part, with which it binds to various heterogeneous nuclear nucleoproteins (hnRNPs). It is more abundantly present in the nucleus than in the cytoplasm. Although the exact role of TDP-43 is incompletely understood, it is thought to play a role in RNA processing, stabilisation, and transport [
Transgenic mice were created by overexpression of mutant (A315T) TDP-43 under the control of the mouse prion promoter [
The latter seems to be the case as overexpression of human wild-type TDP-43 driven by the Thy-1 promotor also results in toxicity. These transgenic mice develop gait abnormalities and show degeneration of motor neurons and neurons in layer 5 of the frontal cortex [
In addition to the different mouse models, a conditional rat model was created in which wild-type TDP-43 or mutant (M337V) TDP-43 is expressed under the control of a promoter that can be silenced by treating the transgenic rats with doxycycline [
ALS2 is a rare autosomal, recessive form of ALS that is characterized by a juvenile onset of progressive spasticity in the limbs, facial, and pharyngeal muscles. In families of Arabic origin, mutations in the ALS2 gene on chromosome 2 were discovered [
Several groups generated an alsin knockout mouse [
A missense mutation in the VAPB gene was found in a Brazilian family [
A G59S mutation located in the microtubule-binding domain of dynactin p150Glued was described as the cause of an autosomal dominant, late-onset motor neuron disease in a large family of European descent [
Neurofilaments (NFs) are the most abundant intermediate filaments in neurons and consist of three subunits: NF-L, NF-M, and NF-H. NF accumulations are found in both familial and sporadic ALS cases [
Knockout mice for these different subunits alone or double transgenic mice deficient in two NF subunits do not show a clear phenotype, although some of these mice show a loss of motor axons. Also overexpression of the different NF subunits does not induce motor neuron death. In some of these transgenic mice NF accumulations in neuronal cell bodies were found, but this does not induce motor neuron death. However, NF abnormalities can induce selective motor neuron death
More recently and because of the proposed role of RNA metabolism in the pathogenesis of ALS, a transgenic mouse overexpressing the 3′-UTR of NF-L received renewed interest [
A frameshift deletion and a mutation in the peripherin gene encoding another intermediate filament protein were discovered in ALS patients [
Survival of motor neurons is dependent on the presence of growth factors. Absence of these neurotrophic factors can cause motor neuron death, as is illustrated by the phenotype of the transgenic mice in which the gene for ciliary neurotrophic factor (CNTF) is deleted [
Another example of motor neuron death induced by insufficient growth factors is the VEGF
Manipulations involving the gene encoding the microtubule-associated protein, tau, also resulted in a number of mouse models showing a clear motor phenotype. Tau is an axonal phosphoprotein that establishes short cross-bridges between axonal microtubules and, thereby, supports intracellular trafficking, including axonal transport. In neurons affected by a tauopathy, tau is hyperphosphorylated and is located not only in axons but also in cell bodies and dendrites. Tau is the major component of the intracellular filamentous deposits found in a number of neurodegenerative diseases including Alzheimer’s disease, while mutations in tau are associated with frontotemporal dementia with parkinsonism (FTDP) (for a review see [
For more than a decade, the mutant SOD1 mice and rats have been the prototype of an ideal model to study (selective) motor neuron death, the hallmark of ALS. However, the major frustration is that this almost perfect model did not lead to a major breakthrough on the therapeutic level. All positive therapeutic interventions in this model failed in subsequent human clinical trials. As a consequence, the ALS field is eagerly awaiting new rodent models that are generated starting from other mutated genes. Until recently, these attempts were based on genetic defects causing atypical or very rare forms of familial ALS. Since the discovery of mutations in the genes encoding TDP-43 and FUS/TLS, hope arises that (over) expression of these mutated genes will lead to the generation of new ALS models. Despite a large number of attempts, the rodent models based on (mutant) TDP-43 are not yet perfect. These models lack specificity in relation to the mutation and/or the cell type affected. The value of the rodent models that are the result of modification of genes that unexpectedly cause a motor neuron phenotype is unclear. In the best case, these models point to a new pathogenic mechanism. None of these models is currently used for routine drug screening.
In conclusion, the mutant SOD1 rodent models remains the best models currently available to study the pathogenesis of ALS and to test new therapies, although the question remains how representative this model is for human sporadic and familial ALS. As a consequence, searching for more and better rodent models based on mutations in new genes remains an important challenge in the ALS field.
Research of the author is supported by grants from the “Fund for Scientific Research Flanders” (FWO-Vlaanderen), the University of Leuven, the Belgian Government (Interuniversity Attraction Poles, programme P6/43 of the Belgian Federal Science Policy Office), the “Association Belge contre les Maladies neuro-Musculaires” (ABMM), the “Association Française contre les Myopathies” (AFM), the “Frick Foundation for ALS Research” and the “Fondation Thierry Latran.”