The gene encoding alsin, a protein with three guanine-nucleotide exchange factor domains, is mutated in a form of recessive amyotrophic lateral sclerosis

Management of motor neurone disease

Motor neurone disease is a progressive neurodegenerative disorder leading to severe disability and death. It is clinically characterised by mixed upper and lower motor neurone involvement affecting bulbar, limb, and respiratory musculature. Recent guidelines have established diagnostic criteria and defined management of the condition. In a proportion of familial amyotrophic lateral sclerosis there is a mutation in the gene encoding the enzyme copper/zinc superoxide dismutase 1; this has allowed mutation screening and generated considerable laboratory based research. The diagnosis must be given with care and consideration and close follow up is essential. Management involves a multidisciplinary team based in the hospital and the community. Riluzole is the only drug shown to have a disease modifying effect and has been approved by the National Institute for Clinical Excellence. The essence of care is good symptomatic management, including nutritional support with percutaneous endoscopic gastrostomy and ventilatory care with non-invasive ventilation. Palliative care should be introduced before the terminal stages after careful discussion with the patient and carers. Knowledge of this condition has grown dramatically recently with a parallel improvement in treatment and ability to deal with the most troublesome problems.

Missense mutation in the tubulin-specific chaperone E (Tbce) gene in the mouse mutant progressive motor neuronopathy, a model of human motoneuron disease

Progressive motor neuronopathy (pmn) mutant mice have been widely used as a model for human motoneuron disease. Mice that are homozygous for the pmn gene defect appear healthy at birth but develop progressive motoneuron disease, resulting in severe skeletal muscle weakness and respiratory failure by postnatal week 3. The disease starts at the motor endplates, and then leads to axonal loss and finally to apoptosis of the corresponding cell bodies. We localized the genetic defect in pmn mice to a missense mutation in the tubulin-specific chaperone E (Tbce) gene on mouse chromosome 13. The human orthologue maps to chromosome 1q42.3. The Tbce gene encodes a protein (cofactor E) that is essential for the formation of primary alpha-tubulin and beta-tubulin heterodimeric complexes. Isolated motoneurons from pmn mutant mice exhibit shorter axons and axonal swelling with irregularly structured beta-tubulin and tau immunoreactivity. Thus, the pmn gene mutation provides the first genetic evidence that alterations in tubulin assembly lead to retrograde degeneration of motor axons, ultimately resulting in motoneuron cell death.

Binding of p190RhoGEF to a destabilizing element on the light neurofilament mRNA is competed by BC1 RNA

The enhancement of RNA-mediated motor neuron degeneration in transgenic mice by mutating a major mRNA instability determinant in a light neurofilament (NF-L) transgene implicates cognate RNA binding factors in the pathogenesis of motor neuron degeneration. p190RhoGEF is a neuron-enriched guanine exchange factor (GEF) that binds to the NF-L-destabilizing element, to c-Jun N-terminal kinase-interactive protein-1 (JIP-1), and to 14-3-3 and may link neurofilament expression to pathways affecting neuronal homeostasis. This study was undertaken to identify additional RNA species that bind p190RhoGEF and could affect interactions of the exchange factor with NF-L transcripts. The C-terminal domain of p190RhoGEF, containing the RNA-binding site, was expressed as a glutathione S-transferase fusion protein and was used as an affinity probe to isolate interactive RNAs in rat brain extracts. As expected, NF-L mRNA was identified as an RNA specie eluted from the affinity column. In addition, BC1 RNA was also found enriched in the bound RNA fraction. BC1 is a 152-nucleotide RNA that is highly expressed but untranslated in differentiated neurons. We show that BC1 and NF-L mRNA bind to a similar site in the C-terminal domain of p190RhoGEF, and their bindings to p190RhoGEF are readily cross-competed. Moreover, we identify a novel binding site in BC1 to account for its interaction with p190RhoGEF. The findings suggest a novel role of BC1 in differentiated neurons involving RNA-protein interactions of p190RhoGEF.

Pathways to motor neuron degeneration in transgenic mouse models

Amyotrophic lateral sclerosis (ALS) is an adult-onset neurological disorder characterized by the selective loss of motor neurons. A pathological hallmark of both sporadic and familial ALS is the presence of abnormal accumulations of neurofilament and peripherin proteins in motor neurons. In the past decade, transgenic mouse approaches have been used to address the role of such cytoskeletal abnormalities in motor neuron disease and also to unravel the pathogenesis caused by mutations in the gene coding for superoxide dismutase 1 (SOD1) that account for ~20% of familial ALS cases. In mouse models, disparate effects could result from different types of intermediate filament (IF) aggregates. Perikaryal IF accumulations induced by the overexpression of any of the three wild-type neurofilament proteins were quite well tolerated by motor neurons. Indeed, perikaryal swellings provoked by NF-H overexpression can even confer protection against toxicity of mutant SOD1. Other types of IF aggregates seem neurotoxic, such as those found in transgenic mice overexpressing either peripherin or an assembly-disrupting NF-L mutant. Moreover, understanding the toxicity of SOD1 mutations has been surprisingly difficult. The analysis of transgenic mice expressing mutant SOD1 has yielded complex results, suggesting that multiple pathways may contribute to disease that include the involvement of non-neuronal cells.

Is the transportation highway the right road for hereditary spastic paraplegia?

The term "hereditary spastic paraplegia" (HSP) refers to a genetically and clinically diverse group of disorders whose primary feature is progressive spasticity of the lower extremities. The condition arises because of degeneration of the longest motor and sensory axons on the spinal cord, which appear to be most sensitive to the underlying mutations. The marked genetic heterogeneity in HSP, with 20 loci chromosomally mapped and eight genes now identified, suggests that a number of defective cellular processes may be shown to result in the disease. Although previous studies have suggested a mitochondrial basis for at least one form of the disease, a mechanism common to a number of the other genes mutated in HSP has remained elusive until now. The identification of the most recent genes for the condition suggests that aberrant cellular-trafficking dynamics may be a common process responsible for the specific pattern of neurodegeneration seen in HSP.

Therapeutic developments in the treatment of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis is a progressive neurodegenerative disease characterised by the selective death of motor neurones. The mechanisms and processes responsible for the selective loss of motor neurones are still unknown, however several hypotheses have been put forward, including oxidative damage and/or toxicity from intracellular aggregates due to mutant superoxide dismutase-1 activity, axonal strangulation from cytoskeletal abnormalities, loss of trophic factor support and glutamate-mediated excitotoxicity. These theories are based on a better understanding of the genetics of amyotrophic lateral sclerosis and on biochemical and pathological analysis of post-mortem tissue. They have led to the development of appropriate animal and cell culture models, allowing the sequence of events in motor neuronal degeneration to be unravelled and potential therapeutic agents to be screened. Unfortunately, the majority of therapeutics found to be efficacious in the animal and cell culture models have failed in human trials. Riluzole is still the only proven therapy in humans, shown to extend survival of amyotrophic lateral sclerosis patients by approximately 3 months, but it has no effect on muscle strength. Other potential therapeutic approaches are being identified, including inhibition of caspase-mediated cell death, maintenance of mitochondrial integrity and energy production, regulation of glutamate homeostasis, reduction of inflammation and control of neurofilament synthesis. Hopefully, in the near future some new agents will be found that can alter the course of this devastating and fatal disease.

Inflammatory processes in amyotrophic lateral sclerosis

Neuroinflammation is a characteristic of pathologically affected tissue in several neurodegenerative disorders. These changes can be observed in the brainstem and spinal cord of amyotrophic lateral sclerosis (ALS) cases and in mouse models of the disease. They include an accumulation of large numbers of activated microglia and astrocytes, as well as small numbers of T cells, mostly adhering to postcapillary venules. Accompanying biochemical alterations include the appearance of numerous molecules characteristic of free-radical attack, the occurrence of proteins associated with activation of the complement cascade, and a sharp upregulation of the enzyme cyclooxygenase 2 (COX-2). Anti-inflammatory agents may have a role to play in treating ALS. COX-2 is a particularly attractive target because of its marked increase in ALS spinal cord.

Untranslated element in neurofilament mRNA has neuropathic effect on motor neurons of transgenic mice

Studies of experimental motor neuron degeneration attributable to expression of neurofilament light chain (NF-L) transgenes have raised the possibility that the neuropathic effects result from overexpression of NF-L mRNA, independent of NF-L protein effects (Cañete-Soler et al., 1999). The present study was undertaken to test for an RNA-mediated pathogenesis. Transgenic mice were derived using either an enhanced green fluorescent protein reporter construct or modified chimeric constructs that differ only in their 3' untranslated regions (UTRs). Motor function and spinal cord histology were normal in mice expressing the unmodified reporter transgene. In mice expressing a chimeric transgene in which sequence of NF-L 3' UTR was inserted into the 3' UTR of the reporter transgene, we observed growth retardation and reduced kinetic activity during postnatal development. Older mice developed impairment of motor function and atrophy of nerve fibers in the ventral roots. A similar but more severe phenotype was observed when the chimeric transgene contained a 36 bp c-myc insert in an mRNA destabilizing element of the NF-L sequence. Our results suggest that neuropathic effects of overexpressing NF-L can occur at the level of transgene RNA and are mediated by sequences in the NF-L 3' UTR.

Infantile-onset ascending hereditary spastic paralysis is associated with mutations in the alsin gene

We studied 15 patients, from 10 families, who presented with severe spastic paralysis with an infantile onset and an ascending progression. Spastic paraplegia began during the first 2 years of life and extended to upper limbs within the next few years. During the first decade of life, the disease progressed to tetraplegia, anarthria, dysphagia, and slow eye movements. Overall, the disease was compatible with long survival. Signs of lower motor-neuron involvement were never observed, whereas motor-evoked potentials and magnetic resonance imaging demonstrated a primitive, pure degeneration of the upper motor neurons. Genotyping and linkage analyses demonstrated that this infantile-onset ascending hereditary spastic paralysis (IAHSP) is allelic to the condition previously reported as juvenile amyotrophic lateral sclerosis at the ALS2 locus on chromosome 2q33-35 (LOD score 6.66 at recombination fraction 0). We analyzed ALS2, recently found mutated in consanguineous Arabic families presenting either an ALS2 phenotype or juvenile-onset primary lateral sclerosis (JPLS), as a candidate gene. In 4 of the 10 families, we found abnormalities: three deletions and one splice-site mutation. All the mutations lead to a truncated alsin protein. In one case, the mutation affected both the short and the long alsin transcript. In the six remaining families, absence of cDNA ALS2 mutations suggests either mutations in regulatory ALS2 regions or genetic heterogeneity, as already reported in JPLS. Alsin mutations are responsible for a primitive, retrograde degeneration of the upper motor neurons of the pyramidal tracts, leading to a clinical continuum from infantile (IAHSP) to juvenile forms with (ALS2) or without (JPLS) lower motor-neuron involvement. Further analyses will determine whether other hereditary disorders with primitive involvement of the central motor pathways, as pure forms of spastic paraplegia, could be due to alsin dysfunction.

Motor neurone disease

Motor neurone disease (MND), or amyotrophic lateral sclerosis (ALS), is a neurodegenerative disorder of unknown aetiology. Progressive motor weakness and bulbar dysfunction lead to premature death, usually from respiratory failure. Confirming the diagnosis may initially be difficult until the full clinical features are manifest. For all forms of the disease there is a significant differential diagnosis to consider, including treatable conditions, and therefore specialist neurological opinion should always be sought. Clear genetic inheritance has been demonstrated in a minority of patients with familial ALS but elucidation of the biological basis of genetic subtypes is also providing important information which may lead to treatments for sporadic forms of the disease. In the absence of curative or disease modifying therapy, management is supportive and requires a multidisciplinary approach. If, as seems likely, complex inherited and environmental factors contribute to the pathogenesis of MND, future treatment may involve a combination of molecular based treatments or restoration of cellular integrity using stem cell grafts.

SPG20 is mutated in Troyer syndrome, an hereditary spastic paraplegia

Troyer syndrome (TRS) is an autosomal recessive complicated hereditary spastic paraplegia (HSP) that occurs with high frequency in the Old Order Amish. We report mapping of the TRS locus to chromosome 13q12.3 and identify a frameshift mutation in SPG20, encoding spartin. Comparative sequence analysis indicates that spartin shares similarity with molecules involved in endosomal trafficking and with spastin, a molecule implicated in microtubule interaction that is commonly mutated in HSP.

Minocycline Slows Disease Progression in a Mouse Model of Amyotrophic Lateral Sclerosis

There is currently no effective pharmacological treatment for amyotrophic lateral sclerosis (ALS). Because recent evidence suggests that secondary inflammation and caspase activation may contribute to neurodegeneration in ALS, we tested the effects of minocycline, a second-generation tetracycline with anti-inflammatory properties, in mice expressing a mutant superoxide dismutase (SOD1(G37R)) linked to human ALS. Administration of minocycline into the diet, beginning at late presymptomatic stage (7 or 9 months of age), delayed the onset of motor neuron degeneration, muscle strength decline, and it increased the longevity of SOD1(G37R) mice by approximately 5 weeks for approximately 70% of tested mice. Moreover, less activation of microglia was detected at early symptomatic stage (46 weeks) and at the end stage of disease in the spinal cord of SOD1(G37R) mice treated with minocycline. These results indicate that minocycline, which is clinically well tolerated, may represent a novel and effective drug for treatment of ALS.

Genetically engineered mouse models of neurodegenerative diseases

Recent research has significantly advanced our understanding of the molecular mechanisms of neurodegenerative diseases, including Alzheimer's disease (AD) and motor neuron disease. Here we emphasize the use of genetically engineered mouse models that are instrumental for understanding why AD is a neuronal disease, and for validating attractive therapeutic targets. In motor neuron diseases, Cu/Zn superoxide dismutase and survival motor neuron mouse models are useful in testing disease mechanisms and therapeutic strategies for amyotrophic lateral sclerosis (ALS) and spinal motor atrophy, respectively, but the mechanisms that account for selective motor neuron loss remain uncertain. We anticipate that, in the future, therapies based on understanding disease mechanisms will be identified and tested in mouse model systems.

Disruption of dynein/dynactin inhibits axonal transport in motor neurons causing late-onset progressive degeneration

To test the hypothesis that inhibition of axonal transport is sufficient to cause motor neuron degeneration such as that observed in amyotrophic lateral sclerosis (ALS), we engineered a targeted disruption of the dynein-dynactin complex in postnatal motor neurons of transgenic mice. Dynamitin overexpression was found to disassemble dynactin, a required activator of cytoplasmic dynein, resulting in an inhibition of retrograde axonal transport. Mice overexpressing dynamitin demonstrate a late-onset progressive motor neuron degenerative disease characterized by decreased strength and endurance, motor neuron degeneration and loss, and denervation of muscle. Previous transgenic mouse models of ALS have shown abnormalities in microtubule-based axonal transport. In this report, we describe a mouse model that confirms the critical role of disrupted axonal transport in the pathogenesis of motor neuron degenerative disease.

Human CCS gene: genomic organization and exclusion as a candidate for amyotrophic lateral sclerosis (ALS)

BACKGROUND

Amyotrophic lateral sclerosis (ALS) is a progressive lethal disorder of large motor neurons of the spinal cord and brain. In approximately 20% of the familial and 2% of sporadic cases the disease is due to a defect in the gene encoding the cytosolic antioxidant enzyme Cu, Zn-superoxide dismutase (SOD1). The underlying molecular defect is known only in a very small portion of the remaining cases and therefore involvement of other genes is likely. As SOD1 receives copper, essential for its normal function, by the copper chaperone, CCS (Copper Chaperone for SOD), we considered CCS as a potential candidate gene for ALS.

RESULTS

We have characterized the genomic organization of CCS and determined exon-intron boundaries. The 823 bp coding region of the CCS is organized in 8 exons. We have evaluated involvement of the CCS in ALS by sequencing the entire coding region for mutations in 20 sporadic ALS patients.

CONCLUSIONS

No causative mutations for the ALS have been detected in the CCS gene in 20 sporadic ALS patients analyzed, but an intragenic single nucleotide polymorphism has been identified.

The role of Rho GTPases in disease development

The functionality and efficacy of Rho GTPase signaling is pivotal for a plethora of biological processes. Due to the integral nature of these molecules, the dysregulation of their activities can result in diverse aberrant phenotypes. Dysregulation can, as will be described below, be based on an altered signaling strength on the level of a specific regulator or that of the respective GTPase itself. Alternatively, effector pathways emanating from a specific Rho GTPase may be under- or overactivated. In this review, we address the role of the Rho-type GTPases as a subfamily of the Ras-superfamily of small GTP-binding proteins in the development of various disease phenotypes. The steadily growing list of genetic alterations that specifically impinge on proper Rho GTPase function corresponds to pathological categories such as cancer progression, mental disabilities and a group of quite diverse and unrelated disorders. We will provide an overview of disease-rendering mutations in genes that have been positively correlated with Rho GTPase signaling and will discuss the cellular and molecular mechanisms that may be affected by them.

Genetic inroads in familial ALS

Amyotrophic lateral sclerosis (ALS) is a common neurodegenerative disease causing cell death of motor neurons and progressive muscle weakness. The disease is familial in ten percent of cases, of which one-fifth are due to mutations in the gene encoding Cu/Zn superoxide dismutase (SOD1). Two papers in this issue of Nature Genetics describe homozygous mutations in a new gene on chromosome 2q33 in 4 families of Arabian origin with a rare form of juvenile onset ALS (ALS2). The predicted protein structure has domains homologous to GTPase regulatory proteins, and both the types of mutation and the pattern of inheritance suggest that motor neuron degeneration is the result of a loss of function. Further work will determine the relevance of this breakthrough to other, more common forms of ALS.