Sequence variants in human neurofilament proteins: absence of linkage to familial amyotrophic lateral sclerosis

Stathmin-2 loss leads to neurofilament-dependent axonal collapse driving motor and sensory denervation

The mRNA transcript of the human STMN2 gene, encoding for stathmin-2 protein (also called SCG10), is profoundly impacted by TAR DNA-binding protein 43 (TDP-43) loss of function. The latter is a hallmark of several neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS). Using a combination of approaches, including transient antisense oligonucleotide-mediated suppression, sustained shRNA-induced depletion in aging mice, and germline deletion, we show that stathmin-2 has an important role in the establishment and maintenance of neurofilament-dependent axoplasmic organization that is critical for preserving the caliber and conduction velocity of myelinated large-diameter axons. Persistent stathmin-2 loss in adult mice results in pathologies found in ALS, including reduced interneurofilament spacing, axonal caliber collapse that drives tearing within outer myelin layers, diminished conduction velocity, progressive motor and sensory deficits, and muscle denervation. These findings reinforce restoration of stathmin-2 as an attractive therapeutic approach for ALS and other TDP-43-dependent neurodegenerative diseases.

Sequencing of neurofilament genes identified NEFH Ser787Arg as a novel risk variant of sporadic amyotrophic lateral sclerosis in Chinese subjects

BACKGROUND

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease with neuronal cell inclusions composed of neurofilaments and other abnormal aggregative proteins as pathological hallmarks. Approximately 90% of patients have sporadic cases (sALS), and at least 4 genes, i.e. C9orf72, SOD1, FUS and TARDBP, have been identified as the main causative genes, while many others have been proposed as potential risk genes. However, these mutations could explain only ~ 10% of sALS cases. The neurofilament polypeptides encoded by NEFH, NEFM, and NEFL are promising protein biomarkers for ALS and other degenerative diseases. However, whether the genetic variants of these genes were associated with ALS remain ambiguous.

METHODS

Here, we used PCR-Sanger to sequence the exons of these three genes in a cohort of 371 sALS patients and 711 healthy controls (Phase I) and validated the risk variant in another 300 sALS patients and 1076 controls (Phase II).

RESULTS

A total of 92 variants were identified, including 36 rare heterozygous variants in NEFH, 27 in NEFM, and 16 in NEFL, and only rs568759161 (p.Ser787Arg) in NEFH reached nominal statistical power (P = 0.02 at Phase I, P = 0.009 at Phase II) in the case-control comparison. Together, the Phase I and II studies showed the significantly higher frequency of the variant in cases (9/1342, 0.67%) than in controls (2/3574, 0.07%) (OR 12.06; 95% CI 2.60-55.88; P = 0.0003). No variants passed multiple testing in the discovery cohort, but rs568759161 was associated with ALS in a replication cohort.

CONCLUSIONS

Our results confirmed that NEFH Ser787Arg is a novel sALS risk variant in Chinese subjects, but NEFM and NEFL were not associated with sALS. These data may have implications for genetic counselling and for understanding the pathogenesis of sALS.

Neurofilaments: neurobiological foundations for biomarker applications

Interest in neurofilaments has risen sharply in recent years with recognition of their potential as biomarkers of brain injury or neurodegeneration in CSF and blood. This is in the context of a growing appreciation for the complexity of the neurobiology of neurofilaments, new recognition of specialized roles for neurofilaments in synapses and a developing understanding of mechanisms responsible for their turnover. Here we will review the neurobiology of neurofilament proteins, describing current understanding of their structure and function, including recently discovered evidence for their roles in synapses. We will explore emerging understanding of the mechanisms of neurofilament degradation and clearance and review new methods for future elucidation of the kinetics of their turnover in humans. Primary roles of neurofilaments in the pathogenesis of human diseases will be described. With this background, we then will review critically evidence supporting use of neurofilament concentration measures as biomarkers of neuronal injury or degeneration. Finally, we will reflect on major challenges for studies of the neurobiology of intermediate filaments with specific attention to identifying what needs to be learned for more precise use and confident interpretation of neurofilament measures as biomarkers of neurodegeneration.

Therapeutic targets for amyotrophic lateral sclerosis: current treatments and prospects for more effective therapies

Although amyotrophic lateral sclerosis (ALS) was described more than 130 years ago, the cause(s) of most cases of this adult motor neuron disease remains a mystery. With the discovery of mutations in one gene (Cu/Zn superoxide dismutase) as a primary cause of some forms of ALS, model systems have been developed that have helped us begin to understand mechanisms involved in motor neuron death and enabled testing of potential new therapies. Several other genes have been implicated as risk factors in motor neuron diseases, including neurofilaments, cytoplasmic dynein and dynactin, vascular endothelial growth factor, and angiogenin. With advances in the basic research of the disease, many hypotheses accounting for motor neuron death are being explored, including loss of trophic support, protein mishandling, mitochondrial dysfunction, excitotoxicity, axonal abnormalities and inflammation. Many of these mechanisms are the focus of research in other neurodegenerative disorders, such as Parkinson's, Alzheimer's and Huntington's disease.

Clinical genetics of amyotrophic lateral sclerosis: what do we really know?

Hereditary amyotrophic lateral sclerosis (ALS) encompasses a group of genetic disorders characterized by adult-onset loss of the lower and upper motor neuron systems, often with involvement of other parts of the nervous system. Cases of hereditary ALS have been attributed to mutations in 12 different genes, the most common being SOD1, FUS and TARDBP-mutations in the other genes are rare. The identified genes explain 25-35% of cases of familial ALS, but identifying the remaining genes has proved difficult. Only a few genes seem to account for significant numbers of ALS cases, with many others causing a few cases each. Hereditary ALS can be inherited in an autosomal dominant, autosomal recessive or X-linked manner, and families with low disease penetrance are frequently observed. In such families, the genetic predisposition may remain unnoticed, so many patients carry a diagnosis of isolated or sporadic ALS. The only clinical feature that distinguishes recognized hereditary from apparently sporadic ALS is a lower mean age of onset in the former. All the clinical features reported in hereditary cases (including signs of extrapyramidal, cerebellar or cognitive involvement) have also been observed in sporadic cases. Genetic counseling and risk assessment in relatives depend on establishing the specific gene defect and the disease penetrance in the particular family.

Variation of the neurofilament medium KSP repeat sub-domain across mammalian species: implications for altering axonal structure

The evolution of larger mammals resulted in a corresponding increase in peripheral nerve length. To ensure optimal nervous system functionality and survival, nerve conduction velocities were likely to have increased to maintain the rate of signal propagation. Increases of conduction velocities may have required alterations in one of the two predominant properties that affect the speed of neuronal transmission: myelination or axonal diameter. A plausible mechanism to explain faster conduction velocities was a concomitant increase in axonal diameter with evolving axonal length. The carboxy terminal tail domain of the neurofilament medium subunit is a determinant of axonal diameter in large caliber myelinated axons. Sequence analysis of mammalian orthologs indicates that the neurofilament medium carboxy terminal tail contains a variable lysine-serine-proline (KSP) repeat sub-domain flanked by two highly conserved sub-domains. The number of KSP repeats within this region of neurofilament medium varies among species. Interestingly, the number of repeats does not change within a species, suggesting that selective pressure conserved the number of repeats within a species. Mapping KSP repeat numbers onto consensus phylogenetic trees reveals independent KSP expansion events across several mammalian clades. Linear regression analyses identified three subsets of mammals, one of which shows a positive correlation in the number of repeats with head-body length. For this subset of mammals, we hypothesize that variations in the number of KSP repeats within neurofilament medium carboxy terminal tail may have contributed to an increase in axonal caliber, increasing nerve conduction velocity as larger mammals evolved.

Neuronal intermediate filaments and neurodegenerative disorders

Intermediate filaments represent the most abundant cytoskeletal element in mature neurons. Mutations and/or accumulations of neuronal intermediate filament proteins are frequently observed in several human neurodegenerative disorders. Although it is now admitted that disorganization of the neurofilament network may be directly involved in neurodegeneration, certain type of perikaryal intermediate filament aggregates confer protection in motor neuron disease. The use of various mouse models provided a better knowledge of the role played by the disorganization of intermediate filaments in the pathogenesis of neurodegenerative disorders, but the mechanisms leading to the formation of these aggregates remain elusive. Here, we will review some neurodegenerative diseases involving intermediate filaments abnormalities and possible mechanisms susceptible to provoke them.

Amyotrophic lateral sclerosis: from current developments in the laboratory to clinical implications

Amyotrophic lateral sclerosis (ALS) is a late-onset progressive degeneration of motor neurons occurring both as a sporadic and a familial disease. The etiology of ALS remains unknown, but one fifth of instances are due to specific gene defects, the best characterized of which is point mutations in the gene coding for Cu/Zn superoxide dismutase (SOD1). Because sporadic and familial ALS affect the same neurons with similar pathology, it is hoped that understanding these gene defects will help in devising therapies effective in both forms. A wealth of evidence has been collected in rodents made transgenic for mutant SOD1, which represent the best available models for familial ALS. Mutant SOD1 likely induces selective vulnerability of motor neurons through a combination of several mechanisms, including protein misfolding, mitochondrial dysfunction, oxidative damage, cytoskeletal abnormalities and defective axonal transport, excitotoxicity, inadequate growth factor signaling, and inflammation. Damage within motor neurons is enhanced by noxious signals originating from nonneuronal neighboring cells, where mutant SOD1 induces an inflammatory response that accelerates disease progression. The clinical implication of these findings is that promising therapeutic approaches can be derived from multidrug treatments aimed at the simultaneous interception of damage in both motor neurons and nonmotor neuronal cells.

New movements in neurofilament transport, turnover and disease

Revealing the mechanisms by which neurofilament transport and turnover are regulated has proven difficult over the years but recent studies have given new insight into these processes. Mature neurofilament fibers may incorporate a fourth functional subunit, alpha-internexin, as new evidence suggests. Recent findings have made the role of phosphorylation in regulating neurofilament transport velocity controversial. Kinesin and dynein may transport neurofilaments in slow axonal transport as they have been found to associate with neurofilaments. Neurofilament transport and turnover rates may be reduced depending on the existing stationary neurofilament network. Finally, mutations in neurofilament light that have been linked to Charcot-Marie-Tooth disease as well as other neurofilament abnormalities in human disease are discussed.

Focused Proteomics in Post-Mortem Human Spinal Cord

With a highly sensitive electrospray ionization-Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR MS) system, proteins were identified in minimal amounts of spinal cord from patients with the neurodegenerative disease amyotrophic lateral sclerosis (ALS) and compared to proteins in spinal cord from control subjects. The results show 18 versus 16 significantly identified (p < 0.05) proteins, respectively, all known to be found in the central nervous system. The most abundant protein in both groups was the glial fibrillary acidic protein, GFAP. Other proteins were, for example, hemoglobin alpha- and beta chain, myelin basic protein, thioredoxin, alpha enolase, and choline acetyltransferase. This study also includes the technique of laser microdissection in combination with pressure catapulting (LMPC) for the dissection of samples and specific neurons. Furthermore, complementary experiments with nanoLC-matrix assisted laser desorption ionization time-of-flight tandem mass spectrometry (MALDI-TOF-TOF MS) confirmed the results of the ESI-FTICR MS screening and provided additional results of further identified proteins.

Amyotrophic lateral sclerosis as a complex genetic disease

In complex diseases like ALS, there are multiple genetic and environmental factors all contributing to disease liability. The genetic factors causing susceptibility to developing ALS can be considered a spectrum from single genes with large effect sizes causing classical Mendelian ALS, to genes of smaller effect, producing apparently sporadic disease. We examine the statistical genetic principles that underpin this model and review what is known about ALS as a disease with complex genetics.

The molecular genetics of non-ALS motor neuron diseases

Hereditary disorders of voluntary motor neurons are individually relatively uncommon, but have the potential to provide significant insights into motor neuron function in general and into the mechanisms underlying the more common form of sporadic Amyotrophic Lateral Sclerosis. Recently, mutations in a number of novel genes have been associated with Lower Motor Neuron (HSPB1, HSPB8, GARS, Dynactin), Upper Motor Neuron (Spastin, Atlastin, Paraplegin, HSP60, KIF5A, NIPA1) or mixed ALS-like phenotypes (Alsin, Senataxin, VAPB, BSCL2). In comparison to sporadic ALS these conditions are usually associated with slow progression, but as experience increases, a wide variation in clinical phenotype has become apparent. At the molecular level common themes are emerging that point to areas of specific vulnerability for motor neurons such as axonal transport, endosomal trafficking and RNA processing. We review the clinical and molecular features of this diverse group of genetically determined conditions and consider the implications for the broad group of motor neuron diseases in general.

Mutations in neurofilament genes are not a significant primary cause of non-SOD1-mediated amyotrophic lateral sclerosis

While 1 to 2% of amyotrophic lateral sclerosis (ALS) is caused by mutations in the SOD1 gene, the basis of the remaining instances of inherited disease is unknown. Neuropathology, mouse modeling, and human genetics have implicated neurofilaments in the pathogenesis of motor neuron diseases such as ALS and Charcot-Marie-Tooth disease (CMT). A systematic analysis of the coding region and intron-exon boundaries of all three neurofilament genes is now reported from DNA samples derived from more than 200 non-SOD1 linked familial and sporadic ALS patients, along with >400 non-disease control individuals. Rare variants within each of the three neurofilament subunits that are predicted to affect neurofilament assembly properties were identified at higher frequency in non-SOD1 mutant ALS samples. However, none could be unambiguously linked to dominantly inherited disease. Thus, mutations in neurofilaments are possible risk factors that may contribute to pathogenesis in ALS in conjunction with one or more additional genetic or environmental factors, but are not significant primary causes of ALS.

Alsin is partially associated with centrosome in human cells

Mutations in the ALS2 gene has recently been linked to cases of juvenile amyotrophic lateral sclerosis, juvenile primary lateral sclerosis and ascending hereditary spastic paralysis. All reported mutations predict the production of truncated forms of Alsin suggesting a loss of function mechanism for these motor neuron disorders. Here we used the tetracycline-regulated expression system to overexpress the full-length and truncated forms of Alsin in different cell lines. Alsin overexpression caused severe phenotypic changes in monkey COS-7 cells including the enlargement and accumulation of early endosomes, impairment of mitochondria trafficking and fragmentation of the Golgi apparatus. Our results further demonstrate the requirement of the Alsin VPS9 domain for occurrence of the vacuolation process and the role of Alsin as a guanine nucleotide exchange factor for Rab5. Transfected human SW13 cells exhibited an unexpected centrosomal localization for Alsin that was linked to the presence of the c-terminal part of the protein. Immunofluorescence staining revealed a colocalization of Alsin with the centrosomal markers gamma-tubulin and A kinase anchoring protein (AKAP-450). Similar results were obtained with human LA-N-2 and SK-N-SH neuronal cells. Moreover endogenous Alsin was detected in a centrosome preparation purified from human cortical brain. Considering the crucial role of centrosome in the production of microtubules required for intracellular transport, these findings are of potential relevance for unravelling the disease mechanisms linked to Alsin mutations.

Calpain mediates calcium-induced activation of the erk1,2 MAPK pathway and cytoskeletal phosphorylation in neurons: relevance to Alzheimer's disease

Aberrant phosphorylation of the neuronal cytoskeleton is an early pathological event in Alzheimer's disease (AD), but the underlying mechanisms are unclear. Here, we demonstrate in the brains of AD patients that neurofilament hyperphosphorylation in neocortical pyramidal neurons is accompanied by activation of both Erk1,2 and calpain. Using immunochemistry, Western blot analysis, and kinase activity measurements, we show in primary hippocampal and cerebellar granule (CG) neurons that calcium influx activates calpain and Erk1,2 and increases neurofilament phosphorylation on carboxy terminal polypeptide sites known to be modulated by Erk1,2 and to be altered in AD. Blocking Erk1,2 activity either with antisense oligonucleotides to Erk1,2 mRNA sequences or by specifically inhibiting its upstream activating kinase MEK1,2 markedly reduced neurofilament phosphorylation. Calpeptin, a cell-permeable calpain inhibitor, blocked both Erk1,2 activation and neurofilament hyperphosphorylation at concentrations that inhibit calpain-mediated cleavage of brain spectrin. By contrast, inhibiting Erk1,2 with U-0126, a specific inhibitor of Mek1,2, had no appreciable effect on ionomycin-induced calpain activation. These findings demonstrate that, under conditions of calcium injury in neurons, calpains are upstream activators of Erk1,2 signaling and are likely to mediate in part the hyperphosphorylation of neurofilaments and tau seen at early stages of AD as well as the neuron survival-related functions of the MAP kinase pathway.

Functions of intermediate filaments in neuronal development and disease

Five major types of intermediate filament (IF) proteins are expressed in mature neurons: the three neurofilament proteins (NF-L, NF-M, and NF-H), alpha-internexin, and peripherin. While the differential expression of IF genes during embryonic development suggests potential functions of these proteins in axogenesis, none of the IF gene knockout experiments in mice caused gross developmental defects of the nervous system. Yet, deficiencies in neuronal IF proteins are not completely innocuous. Substantial developmental loss of motor axons was detected in mice lacking NF-L and in double knockout NF-M;NF-H mice, supporting the view of a role for IFs in axon stabilization. Moreover, the absence of peripherin resulted in approximately 30% loss of small sensory axons. Mice lacking NF-L had a scarcity of IF structures and exhibited a severe axonal hypotrophy, causing up to 50% reduction in conduction velocity, a feature that would be very detrimental for large animal species. Unexpectedly, the NF-M rather than NF-H protein turned out to be required for proper radial growth of large myelinated axons. Studies with transgenic mice suggest that some types of IF accumulations, reminiscent of those found in amyotrophic lateral sclerosis (ALS), can have deleterious effects and even cause neurodegeneration. Additional evidence for the involvement of IFs in pathogenesis came from the recent discovery of neurofilament gene mutations linked to ALS and Charcot-Marie-Tooth disease (CMT2E). Conversely, we discuss how certain types of perikaryal neurofilament aggregates might confer protection in motor neuron disease.

Mutation analysis of the neurofilament M gene in Parkinson's disease

Neurofilament M, a major component of Lewy bodies, represents an interesting candidate in the pathogenesis of Parkinson's disease (PD). We performed detailed mutation analyses of the NF-M gene in 322 familial and sporadic PD patients. Two polymorphisms (Ala475Thr and Gly697Arg) occurred at similar frequencies in PD patients and controls. A Pro725Gln substitution and a deletion of valine in position 829 were identified in two PD patients. These substitutions affect residues of the NF-M protein that are highly conserved among different species. None of our patients carried the Gly336Ser substitution, which has been described in familial PD. Our results argue against a major role of NF-M in PD. However, rare variants of the NF-M gene may act as susceptibility factors for PD and functional analyses of the identified variations are warranted to decipher possible mechanisms in neurodegeneration.

Neurofilaments and neurological disease

Neurofilaments are one of the major components of the neuronal cytoskeleton and are responsible for maintaining the calibre of axons. They are modified by post-translational changes that are regulated in complex fashions including by the interaction with neighbouring glial cells. Neurofilament accumulations are seen in several neurological diseases and neurofilament mutations have now been associated with Charcot-Marie-Tooth disease, Parkinson's disease and amyotrophic lateral sclerosis. In this review, we discuss the structure, normal function and molecular pathology of neurofilaments.

Effects of Charcot-Marie-Tooth-linked mutations of the neurofilament light subunit on intermediate filament formation

Neurofilaments (NFs) are the major intermediate filaments (IFs) of mature neurons. They play important roles in the structure and function of axons. Recently, two mutations in the neurofilament light (NFL) subunit have been identified in families affected by Charcot-Marie-Tooth (CMT) neuropathy type 2. We have characterized the effects of these NFL mutations on the formation of IF networks using a transient transfection system. Both mutations disrupted the self-assembly of human NFL. The Q333P mutant in the rod domain of NFL also disrupted the formation of rat and human NFL/NFM heteropolymers. The phenotypes produced by the P8R mutation in the head domain of NFL were less severe. The P8R mutant NFL co-polymerized with NFM to form bundled filaments and, less often, aggregates. Our results suggest that alterations in the formation of a normal IF network in neurons elicited by these NFL mutations may contribute to the development of Charcot-Marie-Tooth neuropathy.

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.

Neurofilament L gene is not a genetic factor of sporadic and familial Parkinson's disease

Mutations in two genes, alpha-synuclein and parkin, have been identified as some rare causes for familial Parkinson's disease (PD). alpha-Synuclein and parkin protein have subsequently been identified in Lewy bodies (LB). To gain further insight into the pathogenesis of PD we investigated the role of neurofilament light (NF-L), another component of LB aggregation. A detailed mutation search of the NF-L gene in 328 sporadic and familial PD patients of German ancestry revealed three silent DNA changes (G163A, C224T, C487T) in three unrelated patients. Analysis of the promoter region of the NF-L gene identified a total of three base pair substitutions defining five haplotypes. Association studies based on these haplotypes revealed no significant differences between PD patients and 344 control individuals. Therefore, NF-L is unlikely to play a major role in the pathogenesis of PD.

Familial amyotrophic lateral sclerosis

The increasing complexity of the pathways implicated in the pathogenesis of familial amyotrophic lateral sclerosis (ALS) has stimulated intensive research in many directions. Genetic analysis of familial ALS has yielded six loci and one disease gene (SOD1), initially suggesting a role for free radicals in the disease process, although the mechanisms through which the mutant exerts toxicity and results in selective motor neuron death remain uncertain. Numerous studies have focused on structural elements of the affected cell, emphasizing the role of neurofilaments and peripherin and their functional disruption in disease. Other topics examined include cellular homeostasis of copper and calcium, particularly in the context of oxidative stress and the processes of protein aggregation, glutamate excitotoxicity, and apoptosis. It has become evident that there is considerable interplay between these mechanisms and, as the role of each is established, a common picture may emerge, enabling the development of more targeted therapies. This study discusses the main areas of investigation and reviews the findings.

Cellular changes in motoneurons in a transgenic mouse model for amyotrophic lateral sclerosis as revealed by monoclonal antibody Py

Transgenic mice (G93A) carrying the human amyotrophic lateral sclerosis (ALS) linked superoxide dismutase 1 (SOD1) mutations develop a motoneuron disease resembling human ALS. The affected motoneurons are characterized by the presence of cellular alterations. The antigen recognized by the monoclonal antibody Py is suggested to be associated with the neurofilamentous and microtubular elements of the cytoskeleton of specific neuron populations including the spinal motoneurons. The aim of the present study was to measure changes in the relative Py-immunoreactivity per identified Choline-Acetyl-Transferase (ChAT)-immunoreactive motoneuron during the disease progression. The relative Py-immunoreactivity of identified spinal motoneurons was measured on double stained (Py and ChAT) motoneurons using a digital imaging system coupled to an inverse microscope. A significant decrease of Py-immunoreactivity was already noted in the pre-symptomatic stages of the disease even before the onset of massive motoneuron degeneration. It is concluded that the Py-antibody detects early intracellular abnormalities related to neurodegenerative changes in spinal motoneurons of transgenic SOD1-(G93A) mice.

Apoptosis as a general cell death pathway in neurodegenerative diseases

Neurodegenerative processes are generally characterized by the long-lasting course of neuronal death and the selectivity of the neuronal population or brain structure involved in the lesion. Two main common forms of cell death that have been described in neurons as in other vertebrate tissues i.e., necrosis and apoptosis. Necrosis is the result of cellular "accidents", such as those occurring in tissues subjected to chemical trauma. The necrotizing cells swell, rupture and provoke an inflammatory response. Apoptosis, on the other hand, is dependent on the cell's "decision" to commit suicide and die, and therefore is referred to as "programmed cell death" (PCD). The course of apoptotic death is characterized by a massive morphological change, including cell shrinkage, nuclear (chromosome) condensation and DNA degradation. Activation of PCD in an individual cell is based on its own internal metabolism, environment, developmental background and its genetic information. Such a situation occurs in most of the neurodegenerative disorders such as Alzheimer's, Parkinson's and Huntington's diseases and amyotrophic lateral sclerosis (ALS). In these pathological situations, specific neurons undergo apoptotic cell death characterized by DNA fragmentation, increased levels of pro-apoptotic genes and "apoptotic proteins" both, in human brain and in experimental models. It is of utmost importance to conclusively determine the mode of cell death in neurodegenerative diseases, because new "anti-apoptotic" compounds may offer a means of protecting neurons from cell death and of slowing the rate of cell degeneration and illness progression.

Phosphorylation state of the native high-molecular-weight neurofilament subunit protein from cervical spinal cord in sporadic amyotrophic lateral sclerosis

The intraneuronal aggregation of phosphorylated high-molecular-weight neurofilament protein (NFH) in spinal cord motor neurons is considered to be a key pathological marker of amyotrophic lateral sclerosis (ALS). In order to determine whether this observation is due to the aberrant or hyper-phosphorylation of NFH, we have purified and characterized NFH from the cervical spinal cords of ALS patients and controls. We observed no differences between ALS and normal controls in the physicochemical properties of NFH in Triton X-100 insoluble protein fractions, with respect to migration patterns on 2D-iso electrofocusing (IEF) gels, the rate of Escherichia coli alkaline phosphatase mediated dephosphorylation, or the rate of calpain-mediated proteolysis. The rate of calpain-mediated proteolysis was unaffected by either exhaustive NFH dephosphorylation or by the addition of calmodulin to the reaction. Phosphopeptides and the phosphorylated motifs characterized by liquid chromatography tandem mass spectroscopy (LC/MS/MS) analysis demonstrated that all the phosphorylated residues found in ALS NFH were also found to be phosphorylated in normal human NFH samples. Hence, we have observed no difference in the physicochemical properties of normal and ALS NFH extracted from cervical spinal cords, suggesting that the perikaryal aggregation of highly phosphorylated NF in ALS neurons reflects the aberrant somatotopic localization of normally phosphorylated NFH.

Genetics of sporadic ALS

The only known gene to be involved in ALS is the CuZn-superoxide dismutase (CuZn-SOD) gene. Since 1993, 89 disease-associated mutations have been found in this gene, 14 of them in cases with apparently sporadic ALS. Most frequent are the D90A (most often with recessive inheritance, but a few with dominant inheritance) and the I113T (dominant inheritance with variable penetrance). Statistical and genealogical evidence suggest that quite a number of diagnosed sporadic cases may in fact be familial cases in pedigrees with very low disease penetrance.

Neurofilament homeostasis and motoneurone degeneration

Neurofilament disorganisation is a hallmark of various neurodegenerative diseases. We review here current knowledge of neurofilament structure, gene expression and function. Neurofilament involvement in motoneurone neurological diseases is discussed in view of recent data from transgenic and spontaneous mouse mutants. In the mammalian neurone, the three neurofilament subunits are assembled into intermediate filaments as obligate heteropolymers. The subunits are expressed differentially during development and adult life according to the cell type and its physiological state. In addition to the well-established role of neurofilaments in the control of axonal calibre, there is increasing evidence that neurofilaments can interact with other cytoskeletal components and can modulate the axoplasmic flow. Although the extent to which neurofilament abnormalities contribute to the pathogenesis in human diseases remains unknown, emerging evidence suggests that disorganised neurofilaments can provoke degeneration and death of neurones. BioEssays 23:24-33, 2001.

Cytoskeletal abnormalities in amyotrophic lateral sclerosis: beneficial or detrimental effects?

Cytoskeletal abnormalities have been reported in cases of amyotrophic lateral sclerosis (ALS) including abnormal inclusions containing neurofilaments (NFs) and/or peripherin, reduced mRNA levels for the NF light (NF-L) protein and mutations in the NF heavy (NF-H) gene. Recently, transgenic mouse approaches have been used to address whether cytoskeletal changes may contribute to motor neuron disease. Mice lacking one of the three NF subunits are viable and do not develop motor neuron disease. Nonetheless, mice with null mutations for NF-L or for both NF-M and NF-H genes developed severe atrophy of ventral and dorsal root axons. The atrophic process is associated with hind limb paralysis during aging in mice deficient for both NF-M and NF-H proteins. The overexpression in mice of transgenes coding for wild-type or mutant NF proteins can provoke abnormal NF accumulations, axonal atrophy and sometimes motor dysfunction. However, the perikaryal NF accumulations are generally well tolerated by motor neurons and, except for expression of a mutant NF-L transgene, they did not provoke massive motor neuron death. Increasing the levels of perikaryal NF proteins may even confer protection in motor neuron disease caused by ALS-linked mutations in the superoxide dismutase (SOD1). In contrast, the overexpression of wild-type peripherin, a type of IF gene upregulated by inflammatory cytokines, provoked the formation of toxic IF inclusions with the high-molecular-weight NF proteins resulting in the death of motor neurons during aging. These results together with the detection of peripherin inclusions at early stage of disease in mice expressing mutant SOD1 suggest that IF inclusions containing peripherin may play a contributory role in ALS pathogenesis.

A new variant of Charcot-Marie-Tooth disease type 2 is probably the result of a mutation in the neurofilament-light gene

Charcot-Marie-Tooth (CMT) disease is the most common inherited motor and sensory neuropathy. The axonal form of the disease is designated as "CMT type 2" (CMT2). Although four loci known to be implicated in autosomal dominant CMT2 have been mapped thus far (on 1p35-p36, 3q13. 1, 3q13-q22, and 7p14), no one causative gene is yet known. A large Russian family with CMT2 was found in the Mordovian Republic (Russia). Affected members had the typical CMT2 phenotype. Additionally, several patients suffered from hyperkeratosis, although the association, if any, between the two disorders is not clear. Linkage with the CMT loci already known (CMT1A, CMT1B, CMT2A, CMT2B, CMT2D, and a number of other CMT-related loci) was excluded. Genomewide screening pinpointed the disease locus in this family to chromosome 8p21, within a 16-cM interval between markers D8S136 and D8S1769. A maximum two-point LOD score of 5.93 was yielded by a microsatellite from the 5' region of the neurofilament-light gene (NF-L). Neurofilament proteins play an important role in axonal structure and are implicated in several neuronal disorders. Screening of affected family members for mutations in the NF-L gene and in the tightly linked neurofilament-medium gene (NF-M) revealed the only DNA alteration linked with the disease: a A998C transversion in the first exon of NF-L, which converts a conserved Gln333 amino acid to proline. This alteration was not found in 180 normal chromosomes. Twenty unrelated CMT2 patients, as well as 26 others with an undetermined form of CMT, also were screened for mutations in NF-L, but no additional mutations were found. It is suggested that Gln333Pro represents a rare disease-causing mutation, which results in the CMT2 phenotype.

Motor neurons are rich in non-phosphorylated neurofilaments: cross-species comparison and alterations in ALS

The localization and distribution of non-phosphorylated neurofilaments (NP-NF) in the upper and lower motor neurons was investigated in the rat, the common marmoset, the rhesus monkey and man using the SMI-32 antibody. Within the spinal cord of all species studied, the most intense NP-NF immunoreactivity was observed within the ventral horn alpha-motor neurons. Concurrent staining for the cholinergic marker choline acetyltransferase (ChAT) demonstrated that virtually all of the ChAT-positive alpha-motor neurons contain NP-NF immunoreactivity. Although NP-NF staining was also observed in other neurons within the ventral and intermediate horns, these neurons were loosely scattered and contained a considerably lower staining intensity. The only other prominent NP-NF staining in the spinal cord occurred within the neurons of the dorsal nucleus of Clark and the intermediolateral cell column. Phosphorylated neurofilament (P-NF) immunoreactivity was found primarily in neuronal processes. Occasionally, a solitary motor neuron contained weak P-NF immunoreactivity. Within the brainstem, neurons in all cranial nerve motor nuclei contained intense NP-NF immunoreactivity. The distribution and apparent density of NP-NF immunoreactive neurons in these nuclei was virtually identical to that observed for neurons immunoreactive for ChAT. NP-NF immunoreactive neurons of relatively lower intensity were found in many other regions of the brainstem. All of the giant Betz cells of layer (L) V in the motor cortex contained dark NP-NF immunoreactivity. Within the spinal cord of amyotrophic lateral sclerosis (ALS) patients, both Nissl and NP-NF staining demonstrated the dramatic loss of alpha-motor neurons characteristic of this disorder. Some of the remaining motor neurons contained intense P-NF immunoreactivity. These observations suggest that NP-NF immunoreactivity is a good marker for motor neurons in health and disease and may be a useful tool for studies of motor neuron degeneration (MND).

Neurofilament functions in health and disease

Transgenic approaches have recently been used to investigate the functions of neuronal intermediate filaments. Gene knockout studies have demonstrated that neurofilaments are not required for axogenesis and that individual neurofilament proteins play distinct roles in filament assembly and in the radial growth of axons. The involvement of neurofilaments in disease is supported by the discovery of novel mutations in the neurofilament heavy gene from cases of amyotrophic lateral sclerosis and by reports of neuronal death in mouse models expressing neurofilament and alpha-internexin transgenes. However, mouse studies have shown that axonal neurofilaments are not required for pathogenesis caused by mutations in superoxide dismutase and that increasing perikaryal levels of neurofilament proteins may even confer protection in this disease.

Glutamatergic receptors regulate expression, phosphorylation and accumulation of neurofilaments in spinal cord neurons

Glutamatergic regulation of neurofilament expression, phosphorylation and accumulation in cultured spinal cord neurons was studied. At seven days in culture, 0.15% of the neurons were immunoreactive for non-phosphorylated neurofilaments, but essentially no cells immunoreactive for phosphorylated neurofilaments were seen. The number and size of the immunoreactive cells in culture corresponded well to those of rat and human spinal cord neurons in vivo. In spinal cord cultures, sublethal, long-lasting stimulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate or metabotrophic receptors, but not N-methyl-D-aspartate receptors, dose-dependently increased the number of non-phosphorylated neurofilament-immunoreactive cells, which was blocked by nifedipine, an antagonist of voltage-sensitive Ca2+ channels. Stimulation of kainate or all non-N-methyl-D-aspartate receptors decreased the expression of medium-molecular-weight neurofilament messenger RNA. Blockade of AMPA/kainate receptors, but not of N-methyl-D-aspartate receptors, increased the amount of phosphorylated neurofilament protein and the number of phosphorylated neurofilament-immunoreactive cell bodies. The phosphorylated neurofilament-immunoreactive cell population was different from the non-phosphorylated neurofilament-immunoreactive neurons, which lost their axonal non-phosphorylated neurofilament immunoreactivity but showed intense cytoplasmic labeling in response to the blockade of AMPA/ kainate receptors. Immunoreactivity for phosphoserine did not change upon glutamate receptor stimulation and blockade. The results show that activation of AMPA/kainate receptors decreases the expression of neurofilament messenger RNA and neurofilament phosphorylation in spinal cord neurons by a mechanism involving active voltage-sensitive Ca2+ channels. Blockade of these receptors seems to disturb axonal neurofilament transport. Because AMPA/kainate receptors mediate chronic glutamatergic death of spinal motor neurons and these receptors have been suggested to be involved in the pathogenesis of amyotrophic lateral sclerosis, the observed alteration in neurofilament phosphorylation and distribution may contribute to the pathogenesis of chronic motor neuron diseases.

Molecular mechanisms regulating motor neuron development and degeneration

Motor neurons are a well-defined, although heterogeneous group of cells responsible for transmitting information from the central nervous system to the locomotor system. Spinal motor neurons are specified by soluble factors produced by structures adjacent to the primordial spinal cord, signaling through homeodomain proteins. Axonal pathfinding is regulated by cell-surface receptors that interact with extracellular ligands and once synaptic connections have formed, the survival of the somatic motor neuron is dependent on the provision of target-derived growth factors, although nontarget-derived factors, produced by either astrocytes or Schwann cells, are also potentially implicated. Somatic motor neuron degeneration leads to profound disability, and multiple pathogenetic mechanisms including aberrant growth factor signaling, abnormal neurofilament accumulation, excitotoxicity, and autoimmunity have been postulated to be responsible. Even when specific deficits have been identified, for example, mutations of the superoxide dismutase-1 gene in familial amyotrophic sclerosis and polyglutamine expansion of the androgen receptor in spinal and bulbar muscular atrophy, the mechanisms by which somatic motor neuronal degeneration occurs remain unclear. In order to treat motor system degeneration effectively, we will need to understand these mechanisms more thoroughly.

Selective impairment of fast anterograde axonal transport in the peripheral nerves of asymptomatic transgenic mice with a G93A mutant SOD1 gene

Transgenic mice that express a mutant Cu/Zn superoxide dismutase (SOD1) gene have been provided a valuable model for human amyotrophic lateral sclerosis (ALS). We studied a possible impairment of fast axonal transport in transgenic mice carrying a Gly93-->Ala (G93A) mutant SOD1 gene found in human familial ALS (FALS). Left sciatic nerve was ligated for 6 h in transgenic (Tg) and age-matched wild-type (WT) mice. Immunohistochemical analyses were performed for accumulations of kinesin and cytoplasmic dynein on both sides of the ligation site. Clinical function and histology in the spinal cords, sciatic nerves and gastrocnemius muscles were also assessed. The mice were examined at an early asymptomatic stage (aged 19 weeks) and a late stage (30 weeks) just before the development of the symptoms. WT mice showed an apparent increase in immunoreactivities for kinesin and cytoplasmic dynein at proximal and distal of the ligation, respectively. In contrast, the young Tg mice showed a selective decrease of kinesin accumulation in the proximal of the ligation. The mice were asymptomatic with a mild histological change only in muscles. The old Tg mice showed a marked reduction of the immunoreactivity for kinesin and cytoplasmic dynein on both sides of the ligation. They had a significant loss of spinal motor neurons, relatively small myelinated fiber densities of sciatic nerves, and severe muscular changes. These results provide direct evidence that the SOD1 mutation leads to impaired fast axonal transport, particularly in the anterograde direction at an early, asymptomatic stage preceding loss of spinal motor neurons and peripheral axons. This impairment may contribute to subsequent selective motor neuron death in the present model implicated for human FALS.

The genetic and molecular mechanisms of motor neuron disease

Significant progress has been made in the identification of genes and chromosomal loci associated with several types of motor neuron disease. Of particular interest is recent work on the pathogenic mechanisms underlying these diseases, especially studies in in vitro model systems and in transgenic and gene-targeted mice.

Transgenic mice in the study of ALS: the role of neurofilaments

Amyotrophic lateral sclerosis (ALS) is an adult-onset neurological disorder of multiple etiologies that affects primarily motor neurons in the brain and spinal cord. Abnormal accumulations of neurofilaments (NFs) in motor neurons and a down-regulation of mRNA for the NF light subunit (NF-L) are associated with ALS, but it remains unclear to what extent these NF perturbations contribute to human disease. Transgenic mouse studies demonstrated that overexpression of normal and mutant NF proteins can sometimes provoke a motor neuronopathy characterized by the presence of abnormal NF accumulations resembling those found in ALS. Remarkably, the motor neuronopathy in transgenic mice overexpressing human NF heavy (NF-H) subunits was rescued by the co-expression of a human NF-L transgene at levels that restored a correct stoichiometry of NF-L to NF-H subunits. Transgenic approaches have also been used to investigate the role of NFs in disease caused by Cu/Zn superoxide dismutase (SOD1) mutations, which is responsible for approximately 2% cases of ALS. Studies with transgenic mice expressing low levels of a fusion NF-H/lacZ protein, in which NFs are withheld from the axonal compartment, suggested that axonal NFs are not toxic intermediates required for SOD1-mediated disease. On the contrary, overexpression of human NF-H proteins was found to confer an effective protection against mutant SOD1 toxicity in transgenic mice, a phenomenon that may be due to the ability of NF proteins to chelate calcium. In conclusion, transgenic studies showed that disorganized NFs can sometimes have noxious effects resulting in neuronopathy. However, in the context of motor neuron disease caused by mutant SOD1, there is emerging evidence that NF proteins rather play a protective role.

Neurofilaments in health and disease

This article reviews current knowledge of neurofilament structure, phosphorylation, and function and neurofilament involvement in disease. Neurofilaments are obligate heteropolymers requiring the NF-L subunit together with either the NF-M or the NF-H subunit for polymer formation. Neurofilaments are very dynamic structures; they contain phosphorylation sites for a large number of protein kinases, including protein kinase A (PKA), protein kinase C (PKC), cyclin-dependent kinase 5 (Cdk5), extracellular signal regulated kinase (ERK), glycogen synthase kinase-3 (GSK-3), and stress-activated protein kinase gamma (SAPK gamma). Most of the neurofilament phosphorylation sites, located in tail regions of NF-M and NF-H, consist of the repeat sequence motif, Lys-Ser-Pro (KSP). In addition to the well-established role of neurofilaments in the control of axon caliber, there is growing evidence based on transgenic mouse studies that neurofilaments can affect the dynamics and perhaps the function of other cytoskeletal elements, such as microtubules and actin filaments. Perturbations in phosphorylation or in metabolism of neurofilaments are frequently observed in neurodegenerative diseases. A down-regulation of mRNA encoding neurofilament proteins and the presence of neurofilament deposits are common features of human neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS), Parkinson's disease, and Alzheimer's disease. Although the extent to which neurofilament abnormalities contribute to pathogenesis in these human diseases remains unknown, emerging evidence, based primarily on transgenic mouse studies and on the discovery of deletion mutations in the NF-H gene of some ALS eases, suggests that disorganized neurofilaments can provoke selective degeneration and death of neurons. An interference of axonal transport by disorganized neurofilaments has been proposed as one possible mechanism of neurofilament-induced pathology. Other factors that can potentially lead to the accumulation of neurofilaments will be discussed as well as the emerging evidence for neurofilaments as being possible targets of oxidative damage by mutations in the superoxide dismutase enzyme (SOD1); such mutations are responsible for approximately 20% of familial ALS cases.

Amyotrophic lateral sclerosis: current issues in classification, pathogenesis and molecular pathology

The classification of amyotrophic lateral sclerosis (ALS) is reconsidered in the light of developments in the molecular pathogenesis and histopathology of the condition. A current view is encapsulated in the El Escorial World Federation of Neurology criteria for the diagnosis of ALS. While intended for research purposes, use of these criteria for entry into clinical trials may result in the exclusion of some patient groups with related disorders that are likely to share aetiological mechanisms but which are not classified as 'definite ALS' or 'probable ALS'. The relationship between ALS and the more restricted motor disorders of progressive lateral sclerosis and progressive muscular atrophy, together with cerebral degenerations including ALS-dementia and ALS-related frontal lobe dementia, are reviewed. The possibility is raised that they all represent syndromic manifestations of a similar pathogenetic cascade whose clinical phenotype depends upon the anatomical selectivity of involvement in each individual. The new evidence regarding the central role of oxidative stress and abnormal glutamatergic neurotransmission in familial and sporadic ALS seem applicable across these disorders. New evidence regarding the molecular pathology of inclusion bodies in these various syndromes, including ubiquitinated inclusions and hyaline conglomerate inclusions, shows striking similarities between them. Marked differences in the anatomical distribution of lesions determine the predominance and type of motor and cognitive features in each syndrome. This concept of a clinicopathological spectrum is potentially of equal relevance to other late onset neurodegenerative disorders including multisystem atrophies, the Lewy body disorders and various manifestations of Alzheimer's disease. It will gain increasing importance as therapies evolve from the symptomatic to those directed at underlying pathogenetic events.

Pathogenesis of two axonopathies does not require axonal neurofilaments

Neurofilaments are a major component of the axonal cytoskeleton and their abnormal accumulation is a prominent feature of the cytopathology encountered in several neurodegenerative diseases. Thus, an attractive and widely held model of pathogenesis involves the participation of disrupted neurofilaments as a common toxic intermediate. Here, in direct contrast to this hypothesis, we show that two neurodegenerative disease models in the mouse, dystonia musculorum (dt) and a superoxide dismutase 1 (SOD1)-mediated form of human motor neuron disease (amyotrophic lateral sclerosis, ALS), progress with little or no abatement on a transgenic background in which neurofilaments are withheld from the axonal compartment. By specifically excluding a necessary role for axonal neurofilaments, our observations redefine the components of the pathogenic pathway leading to axon disruption in these two degenerative diseases.

Intermediate filaments and their associated proteins: multiple dynamic personalities

A fusion of mouse and human genetics has now proven that intermediate filaments form a flexible scaffold essential for structuring cytoplasm in a variety of cell contexts. In some cases, the formation of this scaffold is achieved through a newly identified family of intermediate-filament-associated proteins that form cross-bridges between intermediate filaments and other cytoskeletal elements, including actin and microtubules.

A structural scaffolding of intermediate filaments in health and disease

The cytoplasm of animal cells is structured by a scaffolding composed of actin microfilaments, microtubules, and intermediate filaments. Intermediate filaments, so named because their 10-nanometer diameter is intermediate between that of microfilaments (6 nanometers) and microtubules (23 nanometers), assemble into an anastomosed network within the cytoplasm. In combination with a recently identified class of cross-linking proteins that mediate interactions between intermediate filaments and the other cytoskeletal networks, evidence is reviewed here that intermediate filaments provide a flexible intracellular scaffolding whose function is to structure cytoplasm and to resist stresses externally applied to the cell. Mutations that weaken this structural framework increase the risk of cell rupture and cause a variety of human disorders.

Elevated free nitrotyrosine levels, but not protein-bound nitrotyrosine or hydroxyl radicals, throughout amyotrophic lateral sclerosis (ALS)-like disease implicate tyrosine nitration as an aberrant in vivo property of one familial ALS-linked superoxide dismutase 1 mutant

Mutations in superoxide dismutase 1 (SOD1; EC 1.15.1.1) are responsible for a proportion of familial amyotrophic lateral sclerosis (ALS) through acquisition of an as-yet-unidentified toxic property or properties. Two proposed possibilities are that toxicity may arise from imperfectly folded mutant SOD1 catalyzing the nitration of tyrosines [Beckman, J. S., Carson, M., Smith, C. D. & Koppenol, W. H. (1993) Nature (London) 364, 584] through use of peroxynitrite or from peroxidation arising from elevated production of hydroxyl radicals through use of hydrogen peroxide as a substrate [Wiedau-Pazos, M., Goto, J. J., Rabizadeh, S., Gralla, E. D., Roe, J. A., Valentine, J. S. & Bredesen, D. E. (1996) Science 271, 515-518]. To test these possibilities, levels of nitrotyrosine and markers for hydroxyl radical formation were measured in two lines of transgenic mice that develop progressive motor neuron disease from expressing human familial ALS-linked SOD1 mutation G37R. Relative to normal mice or mice expressing high levels of wild-type human SOD1, 3-nitrotyrosine levels were elevated by 2- to 3-fold in spinal cords coincident with the earliest pathological abnormalities and remained elevated in spinal cord throughout progression of disease. However, no increases in protein-bound nitrotyrosine were found during any stage of SOD1-mutant-mediated disease in mice or at end stage of sporadic or SOD1-mediated familial human ALS. When salicylate trapping of hydroxyl radicals and measurement of levels of malondialdehyde were used, there was no evidence throughout disease progression in mice for enhanced production of hydroxyl radicals or lipid peroxidation, respectively. The presence of elevated nitrotyrosine levels beginning at the earliest stages of cellular pathology and continuing throughout progression of disease demonstrates that tyrosine nitration is one in vivo aberrant property of this ALS-linked SOD1 mutant.

Neurofilaments and motor neuron disease

Amyotrophic lateral sclerosis (ALS) is an adult-onset and heterogeneous neurological disorder that affects primarily motor neurons in the brain and spinal cord. Although multiple genetic and environmental factors might be implicated in ALS, the striking similarities in the clinical and pathological features of sporadic ALS and familial ALS suggest that similar mechanisms of disease may occur. A common and perhaps universal pathological finding in ALS is the presence of abnormal accumulations of neurofilaments (often called spheroids or Lewy body-like deposits) in the cell body and proximal axon of surviving motor neurons. Such neurofilament deposits have been widely viewed as a consequence of neuronal dysfunction, perhaps reflecting axonal transport defects. This review discusses the emerging evidence, based primarily on transgenic mouse studies and on the discovery of deletion mutations in a neurofilament gene associated with ALS, that neurofilament proteins can play a causative role in motor neuron disease.