Increased expression of neurofilament subunit NF-L produces morphological alterations that resemble the pathology of human motor neuron disease

Slowing of axonal transport is a very early event in the toxicity of ALS-linked SOD1 mutants to motor neurons

Mutations in copper/zinc superoxide dismutase 1 (SOD1), primary causes of human amyotrophic lateral sclerosis (ALS), provoke motor neuron death through an unidentified toxic property. The known neurofilament-dependent slowing of axonal transport, combined with the prominent misaccumulation of neurofilaments in ALS, suggests that an important aspect of toxicity may arise from damage to transport. Here we verify this hypothesis for two SOD1 mutations linked to familial ALS. Reduced transport of selective cargoes of slow transport, especially tubulin, arises months before neurodegeneration. For one mutant, this represents the earliest detectable abnormality. Thus, damage to the cargoes or machinery of slow transport is an early feature of toxicity mediated by mutant SOD1.

Amyotrophic lateral sclerosis associated with mutations in superoxide dismutase: a putative mechanism of degeneration

Amyotrophic lateral sclerosis (ALS) is a devastating neurologic disease that rapidly progresses from mild motor symptoms to severe motor paralysis and premature death. Until recently, there were few substantive studies conducted on the pathogenesis of the disease. With the genetic linkage of mutations in the superoxide dismutase (SOD-1) gene with familial ALS patients, new avenues for study have become available including transgenic mice and culture models. Although not yet providing a complete picture of the disease mechanism, studies utilizing these model systems have greatly advanced our understanding of the mechanism of degeneration and should eventually lead to putative therapeutic agents. In this review, we will present the important findings from these model systems, provide a framework in which to evaluate these findings, and speculate on the mechanism of degeneration initiated by the mutations in SOD-1.

Oxidative stress and motor neurone disease

The effects of oxidative stress within post mitotic cells such as neurones may be cumulative, and injury by free radical species is a major potential cause of the age-related deterioration in neuronal function seen in several neurodegenerative diseases. There is strong evidence that oxidative stress plays an important role in the pathogenesis of motor neurone disease (MND). Point mutations in the antioxidant enzyme Cu,Zn superoxide dismutase (SOD1) are found in some pedigrees with the familial form of MND. How mutations in this ubiquitous enzyme cause the relatively selective cell death of specific groups of motor neurones is not clear, although a number of hypotheses have been forwarded. These include (1) the formation of hydroxyl radicals, (2) the catalysis of reactions of the nitrogen centred oxidant species peroxynitrite, (3) toxicity of copper or zinc and (4) protein aggregation. Some experimental support for these different hypotheses has been produced by manipulating cells in culture to express the mutant SOD1 proteins and by generating transgenic mice which over-express mutant SOD1. Observations in these model systems are, in some cases at least, supported by observations made on pathological material from patients with similar SOD1 mutations. Furthermore, there are reports of evidence of free radical mediated damage to neurones in the sporadic form of MND. Several lines of evidence suggest that alterations in the glutamatergic neurotransmitter system may also play a key role in the injury to motor neurones in sporadic MND. There are several important subcellular targets, which may be preferentially impaired within motor neurones, including neurofilament proteins and mitochondria. Future research will need to identify the aspects of the molecular and physiological phenotype of human motor neurones that makes them susceptible to degeneration in MND, and to identify those genetic and environmental factors which combine to cause this disease in individuals and in familial pedigrees.

Molecular analyses of the Cu/Zn superoxide dismutase gene in patients with familial amyotrophic lateral sclerosis (ALS) in Japan

1. Amyotrophic lateral sclerosis (ALS) is a degenerative disorder characterized by selective damage to the neural system that mediates voluntary movement. Although the pathophysiologic process of ALS remains unknown, about 5 to 10% of cases are familial. According to genetic linkage studies, the familial ALS (FALS) gene has been mapped on chromosome 21 in some families and recent work identified some different missense mutations in the Cu/Zn superoxide dismutase gene in FALS families. 2. We recently identified five mutations in six FALS families. The mutations identified in our FALS families are H46R, L84V, I104F, S134N, and V148I. The H46R mutation that locates in the active site of Cu/Zn SOD gene is associated with two Japanese families with very slow progression of ALS. On the other hand, the L84V mutation associated with a rapidly progressive loss of motor function with predominant lower motor neuron manifestations. 3. In the family with the V148I, the phenotype of the patient varied very much among the affected members. One case had weakness of the lower extremities at first and died without bulbar paresis. The second case first noticed wasting of the upper limbs with bulbar symptoms, but the third had weakness of upper extremities without developing dysarthria nor dysphagia until death. These mutations account for 50% of all FALS families screened, although Cu/Zn SOD gene mutations are responsible for less than about 13-21% in the Western population. 4. Our results indicate that the progression of disease with mutations of Cu/Zn SOD is well correlated with each mutation. The exact mechanism by which the abnormal Cu/Zn SOD molecules selectively affect the function of motor neurons is still unknown.

Genetic neurodegenerative diseases: the human illness and transgenic models

Review The neurodegenerative disorders, a heterogeneous group of chronic progressive diseases, are among the most puzzling and devastating illnesses in medicine. Some of these disorders, such as Alzheimer's disease, amyotrophic lateral sclerosis, the prion diseases, and Parkinson's disease, can occur sporadically and, in some instances, are caused by inheritance of gene mutations. Huntington's disease is acquired in an entirely genetic manner. Transgenic mice that express disease-causing genes recapitulate many features of these diseases. This review provides an overview of transgenic mouse models of familial amyotrophic lateral sclerosis, familial Alzheimer's disease, and Huntington's disease and the emerging insights relevant to the underlying molecular mechanisms of these diseases.

Identification of septins in neurofibrillary tangles in Alzheimer's disease

Septins are evolutionarily conserved cytoskeletal GTPases that can form heteropolymer complexes involved in cytokinesis and other cellular processes. We detected expression of the human septin genes Nedd5, H5, Diff6, and hCDC100 in postmortem brain tissues using the reverse transcription-coupled polymerase chain reaction and their products by immunoblot analysis. Four antibodies directed against three septins, Nedd5, H5, and Diff6, consistently labeled neurofibrillary tangles, neuropil threads, and dystrophic neurites in the senile plaques in brains affected by Alzheimer's disease but did not label obvious structures in young control brains. Immunoelectron microscopy revealed that Nedd5 localized to the paired helical filaments. Pre-tangles, the precursory granular deposits that accumulate in the neuronal cytoplasm, also were labeled with the antibodies. These findings suggest that at least the three septins are associated with tau-based paired helical filament core, and may contribute to the formation of neurofibrillary tangle as integral constituents of paired helical filaments.

Disruption of the NF-H gene increases axonal microtubule content and velocity of neurofilament transport: relief of axonopathy resulting from the toxin beta,beta'-iminodipropionitrile

To investigate the role of the neurofilament heavy (NF-H) subunit in neuronal function, we generated mice bearing a targeted disruption of the gene coding for the NF-H subunit. Surprisingly, the lack of NF-H subunits had little effect on axonal calibers and electron microscopy revealed no significant changes in the number and packing density of neurofilaments made up of only the neurofilament light (NF-L) and neurofilament medium (NF-M) subunits. However, our analysis of NF-H knockout mice revealed an approximately 2.4-fold increase of microtubule density in their large ventral root axons. This finding was further corroborated by a corresponding increase in the ratio of assembled tubulin to NF-L protein in insoluble cytoskeletal preparations from the sciatic nerve. Axonal transport studies carried out by the injection of [35S]methionine into spinal cord revealed an increased transport velocity of newly synthesized NF-L and NF-M proteins in motor axons of NF-H knockout mice. When treated with beta,beta'-iminodipropionitrile (IDPN), a neurotoxin that segregates microtubules and retards neurofilament transport, mice heterozygous or homozygous for the NF-H null mutation did not develop neurofilamentous swellings in motor neurons, unlike normal mouse littermates. These results indicate that the NF-H subunit is a key mediator of IDPN-induced axonopathy.

Neurofilament-dependent radial growth of motor axons and axonal organization of neurofilaments does not require the neurofilament heavy subunit (NF-H) or its phosphorylation

Neurofilaments are essential for establishment and maintenance of axonal diameter of large myelinated axons, a property that determines the velocity of electrical signal conduction. One prominent model for how neurofilaments specify axonal growth is that the 660-amino acid, heavily phosphorylated tail domain of neurofilament heavy subunit (NF-H) is responsible for neurofilament-dependent structuring of axoplasm through intra-axonal crossbridging between adjacent neurofilaments or to other axonal structures. To test such a role, homologous recombination was used to generate NF-H-null mice. In peripheral motor and sensory axons, absence of NF-H does not significantly affect the number of neurofilaments or axonal elongation or targeting, but it does affect the efficiency of survival of motor and sensory axons. Loss of NF-H caused only a slight reduction in nearest neighbor spacing of neurofilaments and did not affect neurofilament distribution in either large- or small-diameter motor axons. Since postnatal growth of motor axon caliber continues largely unabated in the absence of NF-H, neither interactions mediated by NF-H nor the extensive phosphorylation of it within myelinated axonal segments are essential features of this growth.

Using knockout and transgenic mice to study neurophysiology and behavior

Reverse genetics, in which detailed knowledge of a gene of interest permits in vivo modification of its expression or function, provides a powerful method for examining the physiological relevance of any protein. Transgenic and knockout mouse models are particularly useful for studies of complex neurobiological problems. The primary aims of this review are to familiarize the nonspecialist with the techniques and limitations of mouse mutagenesis, to describe new technologies that may overcome these limitations, and to illustrate, using representative examples from the literature, some of the ways in which genetically altered mice have been used to analyze central nervous system function. The goal is to provide the information necessary to evaluate critically studies in which mutant mice have been used to study neurobiological problems.

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.

Aberrant expression of a cytokeratin in a subset of hepatocytes during chronic WHV infection

Chronic infection of woodchucks with woodchuck hepatitis virus (WHV) invariably leads, within 2-4 years, to the appearance of hepatocellular carcinoma (HCC). HCC is preceded by an extended period of chronic liver damage, probably resulting from the immune response to viral antigens. It may be that infection itself also induces changes in the hepatocyte population. To begin to identify some of the changes in the liver prior to the appearance of HCC, monoclonal antibodies (MAbs) were generated from mice immunized with hepatocytes from a woodchuck chronically infected with WHV or with a tumor lysate. Immunofluorescence microscopy was used to select MAbs that reacted with host markers whose patterns of expression would distinguish chronically infected from uninfected liver or from liver tumors. One of these MAbs (2F2) reacted strongly with a subset of hepatocytes in chronically infected liver; a similar staining pattern was not detected in uninfected or transiently infected liver. Evidence is presented that this strong staining reaction reflects the overexpression or accumulation of the hepatocyte-specific intermediate filament protein, cytokeratin K18, a protein previously implicated in cryptogenic cirrhosis of the liver in humans (Ku, N. O. , Wright, T. L., Terrault, N. A., Gish, R., and Omary, M. B. J. Clin. Invest. 99: 19-23, 1997). Double immunofluorescent staining with antibodies to K18 and M-envelope protein of WHV suggested that strong reactivity to K18 was limited to cells expressing high levels of one or both of the large viral-envelope proteins, M and L; however, high expression of these viral proteins was not always associated with a strong K18 staining reaction.

Protective effect of neurofilament heavy gene overexpression in motor neuron disease induced by mutant superoxide dismutase

To investigate the role of neurofilaments in motor neuron disease caused by superoxide dismutase (SOD1) mutations, transgenic mice expressing a amyotrophic lateral sclerosis-linked SOD1 mutant (SOD1(G37R)) were mated with transgenic mice expressing human neurofilament heavy (NF-H) subunits. Unexpectedly, expression of human NF-H transgenes increased by up to 65%, the mean lifespan of SOD1(G37R) mice. Microscopic examination corroborated the protective effect of NF-H protein against SOD1 toxicity. Although massive neurodegeneration occurred in 1-yr-old mice expressing SOD1(G37R) alone, spinal root axons and motor neurons were remarkably spared in doubly SOD1(G37R);NF-H-transgenic littermates.

Absence of neurofilaments reduces the selective vulnerability of motor neurons and slows disease caused by a familial amyotrophic lateral sclerosis-linked superoxide dismutase 1 mutant

Mutations in superoxide dismutase 1 (SOD1), the only proven cause of amyotrophic lateral sclerosis (ALS), provoke disease through an unidentified toxic property. Neurofilament aggregates are pathologic hallmarks of both sporadic and SOD1-mediated familial ALS. By deleting NF-L, the major neurofilament subunit required for filament assembly, onset and progression of disease caused by familial ALS-linked SOD1 mutant G85R are significantly slowed, while selectivity of mutant-mediated toxicity for motor neurons is reduced. In NF-L-deleted animals, levels of the two remaining neurofilament subunits, NF-M and NF-H, are markedly reduced in axons but are elevated in motor neuron cell bodies. Thus, while neither perikaryal nor axonal neurofilaments are essential for SOD1-mediated disease, the absence of assembled neurofilaments both diminishes selective vulnerability and slows SOD1(G85R) mutant-mediated toxicity to motor neurons.

Dendritic changes in Alzheimer's disease and factors that may underlie these changes

It seems likely that the Alzheimer disease (AD)-related dendritic changes addressed in this article are induced by two principally different processes. One process is linked to the plastic response associated with deafferentation, that is, long-lasting transneuronally induced regressive changes in dendritic geometry and structure. The other process is associated with severe alterations of the dendritic- and perikaryal cytoskeleton as seen in neurons with the neurofibrillary pathology of AD, that is, the formation of paired helical filaments formed by hyperphosphorylated microtubule-associated protein tau. As the development of dendritic and cytoskeletal abnormalities are at least mediated by alterations in signal transduction, this article also reviews changes in signal pathways in AD. We also discuss transgenic approaches developed to model and understand cytoskeletal abnormalities.

A rod end deletion in the intermediate filament protein nestin alters its subcellular localization in neuroepithelial cells of transgenic mice

Neuroepithelial and radial glial cells span between the ventricular and the pial surfaces of the neural tube and express two intermediate filaments (IFs), nestin and vimentin, which form a filamentous network throughout the length of the cells. In this report we study the polymerization characteristics of nestin and examine how mutations affect the assembly and localization of the nestin protein in cultured cells and in the developing CNS of transgenic mice. A wild-type rat nestin gene transfected into the IF-free SW13 cell line failed to assemble into a filamentous network but was incorporated into the existing IF network of a subclone expressing vimentin, demonstrating that nestin requires vimentin for proper assembly. In transgenic mice, rat nestin formed a network indistinguishable from that formed by endogenous nestin and vimentin, but a mutant form lacking five amino acids at the carboxy terminus of the rod domain was largely restricted to the pial endfeet. Since nestin mRNA is localized to the pial endfoot region we propose that both transgenes are translated there, but that the wild-type protein is preferentially incorporated into the IF network. These observations provide evidence for hierarchical assembly and a complex organization of the IF network along the ventricular-pial axis in the early CNS.

Kinesin and cytoplasmic dynein in spinal spheroids with motor neuron disease

Kinesin and cytoplasmic dynein are two major molecular motors responsible for fast axonal transport. As visualized by immunohistochemistry with monoclonal antibodies, both motors were found to be distributed throughout the cell bodies, dendrites and axons of motor neurons in normal human spinal cords. Large axonal swellings, spheroids, in the spinal cords of patients with motor neuron disease showed massive accumulation of kinesin co-localized with highly phosphorylated neurofilaments. Of 114 spheroids in five spinal cords, 87% were stained heavily with the three anti-kinesin antibodies used in this study. Cytoplasmic dynein was scarce or absent in most of the spheroids. These findings suggest that kinesin selectively accumulates in the spheroids of motor neuron axons, causing disturbance of the machinery for anterograde fast axonal transport in motor neuron disease.

Kinesin accumulation in chick spinal axonal swellings with beta,beta'-iminodipropionitrile (IDPN) intoxication

Kinesin is a major molecular motor responsible for anterograde axonal transport. Chicks were injected with beta,beta'-iminodipropionitrile (IDPN) to induce axonal swellings in spinal motor neurons and spinal sensory ganglion neurons. Cylindrical swollen axons were found in the anterior horn and anterior funiculus of the spinal cord, anterior root, and spinal ganglia. All of the axonal swellings were heavily stained with two anti-kinesin monoclonal antibodies. The swellings were mildly stained with an anti-cytoplasmic dynein and anti-tubulin antibodies, and weakly stained with an anti-tau antibody. These suggest the isolated disturbance of kinesin transport with neurofilament accumulation in IDPN intoxication.

Characterization of the mouse neurofilament light (NF-L) gene promoter by in vitro transcription

We have used in vitro transcription to access the basic sequences and factors required for the transcription of the mouse neurofilament light promoter (pNF-L) in the absence of chromatin structure. Deletion from -1.7 to -154 results in little change in NF-L promoter activity using nuclear extracts from either brain (expressing) or liver (non-expressing) tissues. Further deletion to -29 results in a gradual five-fold drop in promoter activity in both extracts. Only replacement of the entire -148 to -29 region results in a drop in NF-L promoter activity to basal levels. Thus, the NF-L promoter differs from the mouse NF heavy (NF-H) and mid-sized (NF-M) promoters in that no specific sequence within the immediate upstream NF-L promoter region (-154 to -29) appears to be responsible for enhancement or brain-specific transcription. We show that the order of strength of the three NF promoters is NF-H>NF-M>NF-L and identify sequences that can increase or reduce transcription when placed in front of heterologous NF promoters. We conclude that the NF-L promoter is a modular, weak and promiscuous promoter whose regulation differs from NF-H or NF-M. Our data suggest that chromatin structure may play an important role in the regulation of the NF-L promoter.

Stability determinants are localized to the 3'-untranslated region and 3'-coding region of the neurofilament light subunit mRNA using a tetracycline-inducible promoter

The tetracycline-responsive expression system of Bujard was used to compare rates of decay of wild-type and mutant neurofilament (NF) light subunit (NF-L) mRNAs. Optimal conditions for activation and inactivation of the target transgene were determined using a luciferase reporter gene. Analyses of mRNA stability were thereupon conducted on cells that were doubly transfected with transactivator and inducible target genes and derived from pooled clones of transfected cells. Rates of mRNA decay were compared upon inactivation of the transgenes after high levels of mRNA had been induced. Deletion of the 445-nucleotide (nt) 3'-untranslated region (3'-UTR) (L/++(+)-) or 527 nt of the 3'-coding region (3'-CR) (L/++-+) increased the stability of NF-L mRNA compared with the full-length (L/++(++)) transcript in neuronal (N2a and P19 cells) and non-neuronal (L cells) lines. Deletion of both the 3'-UTR and 3'-CR (L/++--) led to a further stabilization of the transcript. A major stability determinant was then localized to a 68-nt sequence that forms the junction between the 3'-CR and 3'-UTR of NF-L and is the binding site of a unique ribonucleoprotein complex (Cañete-Soler, R., Schwartz, M. L., Hua, Y., and Schlaepfer, W. W. (1998) J. Biol. Chem. 273, 12655-12661). The studies establish a novel system for mapping determinants of mRNA stability and have applied the system to localize determinants that regulate the stability of the NF-L mRNA.

Characterization of ribonucleoprotein complexes and their binding sites on the neurofilament light subunit mRNA

Levels of neurofilament (NF) gene expression are important determinants of basic neuronal properties, but overexpression can lead to motoneuron degeneration in transgenic mice. In a companion study (Cañete-Soler, R., Schwartz, M. L., Hua, Y., and Schlaepfer, W. W. (1998) J. Biol. Chem. 273, 12650-12654), we show that levels of NF expression are regulated by altering mRNA stability and that stability determinants are present in the 3'-coding region (3'-CR) and 3'-untranslated region (3'-UTR) of the NF light subunit (NF-L) transcript. This study characterizes the ribonucleoprotein complexes that bind to the NF-L mRNA when cytoplasmic brain extracts are incubated with radioactive probes. Gel retardation assays reveal ribonucleoprotein complexes that are selectively competed with poly(C) or poly(U))/poly(A) homoribopolymers and are referred to as C-binding and U/A-binding complexes, respectively. The C-binding complex forms on the proximal 45 nucleotides of 3'-UTR, but its assembly is markedly enhanced by 23 nucleotides of flanking 3'-CR sequence. U/A-binding complexes form at multiple binding sites in the 3'-CR and 3'-UTR. A pattern of reciprocal binding suggests that the C-binding and U/A-binding complexes interact and may compete for common components or binding sites. Cross-linking studies reveal unique polypeptides in the C-binding and U/A-binding complexes. The findings provide the basis for probing mechanisms regulating NF-L mRNA stability and the relationship between NF overexpression and motoneuron degeneration in transgenic mice.

Massive mitochondrial degeneration in motor neurons triggers the onset of amyotrophic lateral sclerosis in mice expressing a mutant SOD1

Amyotrophic lateral sclerosis (ALS) involves motor neuron degeneration, skeletal muscle atrophy, paralysis, and death. Mutations in Cu,Zn superoxide dismutase (SOD1) are one cause of the disease. Mice transgenic for mutated SOD1 develop symptoms and pathology similar to those in human ALS. To understand the disease mechanism, we developed a simple behavioral assay for disease progression in mice. Using this assay, we defined four stages of the disease in mice expressing G93A mutant SOD1. By studying mice with defined disease stages, we tied several pathological features into a coherent sequence of events leading to motor neuron death. We show that onset of the disease involves a sharp decline of muscle strength and a transient explosive increase in vacuoles derived from degenerating mitochondria, but little motor neuron death. Most motor neurons do not die until the terminal stage, approximately 9 weeks after disease onset. These results indicate that mutant SOD1 toxicity is mediated by damage to mitochondria in motor neurons, and this damage triggers the functional decline of motor neurons and the clinical onset of ALS. The absence of massive motor neuron death at the early stages of the disease indicates that the majority of motor neurons could be rescued after clinical diagnosis.

Expression of amino-terminally truncated PrP in the mouse leading to ataxia and specific cerebellar lesions

The physiological role of prion protein (PrP) remains unknown. Mice devoid of PrP develop normally but are resistant to scrapie; introduction of a PrP transgene restores susceptibility to the disease. To identify the regions of PrP necessary for this activity, we prepared PrP knockout mice expressing PrPs with amino-proximal deletions. Surprisingly, PrP lacking residues 32-121 or 32-134, but not with shorter deletions, caused severe ataxia and neuronal death limited to the granular layer of the cerebellum as early as 1-3 months after birth. The defect was completely abolished by introducing one copy of a wild-type PrP gene. We speculate that these truncated PrPs may be nonfunctional and compete with some other molecule with a PrP-like function for a common ligand.

Dystonin Is Essential for Maintaining Neuronal Cytoskeleton Organization

The mouse neurological mutant dystonia musculorum (dt) suffers from a hereditary sensory neuropathy. We have previously described the cloning and characterization of the dt gene, which we named dystonin (Dst). We had shown that dystonin is a neural isoform of bullous pemphigoid antigen 1 (Bpag1) with an N-terminal actin-binding domain. It has been shown previously that dystonin is a cytoskeletal linker protein, forming a bridge between F-actin and intermediate filaments. Here, we have used two different antibody preparations against dystonin and detected a high-molecular-weight protein in immunoblot analysis of spinal cord extracts. We also show that this high-molecular-weight protein was not detectable in the nervous system of all dt alleles tested. Immunohistochemical analysis revealed that dystonin was present in different compartments of neurons-cell bodies, dendrites, and axons, regions which are rich in the three elements of the cytoskeleton (F-actin, neurofilaments, and microtubules). Ultrastructural analysis of dt dorsal root axons revealed disorganization of the neurofilament network and surprisingly also of the microtubule network. In this context it is of interest that we observed altered levels of the microtubule-associated proteins MAP2 and tau in spinal cord neurons of different dt alleles. Finally, dt dorsal root ganglion neurons formed neurites in culture, but the cytoskeleton was disorganized within these neurites. Our results demonstrate that dystonin is essential for maintaining neuronal cytoskeleton integrity but is not required for establishing neuronal morphology. Copyright 1998 Academic Press.

GFAP-deficient astrocytes are capable of stellation in vitro when cocultured with neurons and exhibit a reduced amount of intermediate filaments and an increased cell saturation density

Glial fibrillary acidic protein (GFAP) is an intermediate filament protein predominantly expressed in cells of astroglial origin. To allow for the study of the biological functions of GFAP we have previously generated GFAP-negative mice by gene targeting [Pekny et al. (1995) EMBO J. 14, 1590-1598]. Astrocytes in culture, similar to reactive astrocytes in vivo, express three intermediate filament proteins: GFAP, vimentin, and nestin. Using primary astrocyte-enriched cultures from GFAP-negative mice, we now report on the effect of GFAP absence on (i) the synthesis of other intermediate filament proteins in astrocytes, (ii) intermediate filament formation, (iii) astrocyte process formation (stellation) in response to neurons in mixed cerebellar astrocyte/neuron cultures, and (iv) saturation cell density in vitro. GFAP-/- astrocytes were found to produce both nestin and vimentin. At the ultrastructural level, the amount of intermediate filaments as revealed by transmission electron microscopy was reduced in GFAP-/- astrocytes compared to that in GFAP+/+ astrocytes. GFAP-/- astrocytes retained the ability to form processes in response to neurons in mixed astrocyte/neuron cultures from the cerebellum. GFAP-/- astrocyte-enriched primary cultures exhibited an increased final cell saturation density. The latter leads us to speculate that the loss of GFAP expression observed focally in a proportion of human malignant gliomas may reflect tumor progression toward a more rapidly growing and malignant phenotype.

Antagonistic roles of neurofilament subunits NF-H and NF-M against NF-L in shaping dendritic arborization in spinal motor neurons

Dendrites play important roles in neuronal function. However, the cellular mechanism for the growth and maintenance of dendritic arborization is unclear. Neurofilaments (NFs), a major component of the neuronal cytoskeleton, are composed of three polypeptide subunits, NF-H, NF-M, and NF-L, and are abundant in large dendritic trees. By overexpressing each of the three NF subunits in transgenic mice, we altered subunit composition and found that increasing NF-H and/or NF-M inhibited dendritic arborization, whereas increasing NF-L alleviated this inhibition. Examination of cytoskeletal organization revealed that increasing NF-H and/or NF-M caused NF aggregation and dissociation of the NF network from the microtubule (MT) network. Increasing NF-H or NF-H together with NF-M further reduced NFs from dendrites. However, these changes were reversed by elevating the level of NF-L with either NF-H or NF-M. Thus, NF-L antagonizes NF-H and NF-M in organizing the NF network and maintaining a lower ratio of NF-H and NF-M to NF-L is critical for the growth of complex dendritic trees in motor neurons.

Manipulation of gene expression in the mammalian nervous system: application in the study of neurite outgrowth and neuroregeneration-related proteins

A fundamental issue in neurobiology entails the study of the formation of neuronal connections and their potential to regenerate following injury. In recent years, an expanding number of gene families has been identified involved in different aspects of neurite outgrowth and regeneration. These include neurotrophic factors, cell-adhesion molecules, growth-associated proteins, cytoskeletal proteins and chemorepulsive proteins. Genetic manipulation technology (transgenic mice, knockout mice, viral vectors and antisense oligonucleotides) has been instrumental in defining the function of these neurite outgrowth-related proteins. The aim of this paper is to provide an overview of the above-mentioned four approaches to manipulate gene expression in vivo and to discuss the progress that has been made using this technology in helping to understand the molecular mechanisms that regulate neurite outgrowth. We will show that work with transgenic mice and knockout mice has contributed significantly to the dissection of the function of several proteins with a key role in neurite outgrowth and neuronal survival. Recently developed viral vectors for gene transfer in postmitotic neurons have opened up new avenues to analyze the function of a protein following local expression in naive adult rodents. The initial results with viral vector-based gene transfer provide a conceptual framework for further studies on genetic therapy of neuroregeneration and neurodegenerative diseases.

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.

Time course of neuropathology in the spinal cord of G86R superoxide dismutase transgenic mice

Transgenic mice with a G86R mutation in the mouse superoxide dismutase (SOD-1) gene, which corresponds to a mutation observed in familial amyotrophic lateral sclerosis (ALS), display progressive motor dysfunction leading to paralysis and premature death. In endstage SOD-1 transgenic mice, there is marked loss of spinal motor neurons and interneurons, accumulation of phosphorylated neurofilament inclusions, and reactive astrocytosis. The present study details the time course and ultrastructural appearance of these pathologic changes and correlates the timing of these events with the behavioral symptoms. There is no significant reduction in the number of total neurons, motor neurons, or interneurons in the ventral spinal cord of presymptomatic mice, as compared to age-matched control mice. In contrast, there is a significant reduction in the number of total neurons (-23.5%), motor neurons (-28.9%), and interneurons (-23.5%) in symptomatic SOD-1 transgenic mice. This neuron loss correlates temporally with the onset of reactive astrocytosis and the appearance of phosphorylated neurofilament inclusions. The identical timing of motor neuron and interneuron degeneration in this model of ALS strongly suggests that degeneration in the spinal cord of patients with ALS is not specifically directed at motor neurons, but rather more generally at several populations of neurons in the spinal cord. In addition, the late onset and rapid progression of neuron loss suggest that a toxic property is accumulating while the SOD-1 transgenic mice are presymptomatic, and that this toxic property must reach a threshold level before the onset of neuronal degeneration.

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.

Specific interaction of HTLV tax protein and a human type IV neuronal intermediate filament protein

The human T-cell leukemia virus (HTLV) is associated with adult T cell leukemia and neurological disorders (TSP/HAM). The HTLV transcriptional transactivator, Tax, is known to exert its effect through protein-protein interaction with several transcription factors that activate genes in T cell proliferation. The pathogenic mechanism in the CNS is less defined. Using the yeast two-hybrid system, we have identified a specific Tax-binding protein as the neuronal specific intermediate filament protein, alpha-internexin. Tax binds to the domain corresponding to the rod region of alpha-internexin, which is essential for neurofilament assembly. The Tax domains involved in binding are separable from those involved in transactivation. TxBP-1/alpha-internexin and Tax are expressed in the cytoplasm and nucleus, respectively, when expressed alone, but in coexpressing cells, colocalization of both proteins was observed in a perinuclear, punctate distribution. This in vivo interaction also resulted in a dramatic reduction in Tax transactivation and the network formation by alpha-internexin. The specific interaction of Tax and a neuronal specific intermediate filament protein may provide a clue to the pathogenesis of TSP/HAM.

Study of proline-directed protein kinases involved in phosphorylation of the heavy neurofilament subunit

The high-molecular-mass neurofilament subunit (NFH) is normally hypophosphorylated in the neuronal perikaryon and undergoes extensive phosphorylation after entering the initial axon segment. Aberrant hyperphosphorylation of perikaryal NFH is a common feature of many neurological diseases. In a previous study (), we demonstrated a correlation between phosphorylation of perikaryal NFH and induction of stress-activated protein kinase (SAPK)-gamma. In this report, we present direct evidence showing that the in vivo activation of SAPKs by an upstream activator (MEKK-1) caused extensive NFH phosphorylation. We also show that stress-activated p38 kinases were not involved in the phosphorylation of perikaryal NFH in cultured dorsal root ganglion neurons and that this process was reversible. SAPKgamma was shown to be located in both the cell body and the neurites of the cultured neurons, suggesting that it is likely to be involved in the phosphorylation of cytoplasmic substrates. These could include neuritic NFH, which is highly phosphorylated despite the demonstrated lack of cyclin-dependent kinase-5 activity in these neurons. Neuritic NFH was also highly phosphorylated in neuronal cultures devoid of Schwann cells, indicating that this form of post-translational modification does not require cues stemming from Schwann cell-axon contacts. Collectively, these findings provide significant new insights into mechanisms involved in NFH phosphorylation in normal neurons and in disease states characterized by aberrant phosphorylation of neurofilaments.

Neurobiological problems in long-term deep space flights

Future missions in space may involve long-term travel beyond the magnetic field of the Earth, subjecting astronauts to radiation hazards posed by solar flares and galactic cosmic rays, altered gravitation fields and physiological stress. Thus, it is critical to determine if there will be any reversible or irreversible, detrimental neurological effects from this prolonged exposure to space. A question of particular importance focuses on the long-term effects of the space environment on the central nervous system (CNS) neuroplasticity, with the potential acute and/or delayed effects that such perturbations might entail. Although the short-term effects of microgravity on neural control were studied on previous low earth orbit missions, the late consequences of stress in space, microgravity and space radiation have not been addressed sufficiently at the molecular, cellular and tissue levels. The possibility that space flight factors can interact influencing the neuroplastic response in the CNS looms critical issue not only to understand the ontogeny of the CNS and its functional integrity, but also, ultimately the performance of astronauts in extended space forays. The purpose of this paper is to review the neurobiological modifications that occur in the CNS exposed to the space environment, and its potential consequences for extended deep space flight.

Neurofilaments and orthograde transport are reduced in ventral root axons of transgenic mice that express human SOD1 with a G93A mutation

Mice engineered to express a transgene encoding a human Cu/Zn superoxide dismutase (SOD1) with a Gly93 --> Ala (G93A) mutation found in patients who succumb to familial amyotrophic lateral sclerosis (FALS) develop a rapidly progressive and fatal motor neuron disease (MND) similar to amyotrophic lateral sclerosis (ALS). Hallmark ALS lesions such as fragmentation of the Golgi apparatus and neurofilament (NF)-rich inclusions in surviving spinal cord motor neurons as well as the selective degeneration of this population of neurons were also observed in these animals. Since the mechanism whereby mutations in SOD1 lead to MND remains enigmatic, we asked whether NF inclusions in motor neurons compromise axonal transport during the onset and progression of MND in a line of mice that contained approximately 30% fewer copies of the transgene than the original G93A (Gurney et al., 1994). The onset of MND was delayed in these mice compared to the original G93A mice, but they developed the same neuropathologic abnormalities seen in the original G93A mice, albeit at a later time point with fewer vacuoles and more NF inclusions. Quantitative Western blot analyses showed a progressive decrease in the level of NF proteins in the L5 ventral roots of G93A mice and a concomitant reduction in axon caliber with the onset of motor weakness. By approximately 200 d, both fast and slow axonal transports were impaired in the ventral roots of these mice coincidental with the appearance of NF inclusions and vacuoles in the axons and perikarya of vulnerable motor neurons. This is the first demonstration of impaired axonal transport in a mouse model of ALS, and we infer that similar impairments occur in authentic ALS. Based on the temporal correlation of these impairments with the onset of motor weakness and the appearance of NF inclusions and vacuoles in vulnerable motor neurons, the latter lesions may be the proximal cause of motor neuron dysfunction and degeneration in the G93A mice and in FALS patients with SOD1 mutations.

Heterogeneous expression of neurofilament proteins in forebrain and cerebellum during development: clinical implications for spinocerebellar ataxia

Using quantitative immunoblotting, we have measured the level of two mammalian neurofilament proteins, the 68-kDa NF-L and the 66-kDa NF-66 (alpha-internexin), in the rat CNS during development. NF-66 is localized in neurons and neuronal processes in both embryonic and postnatal brain. Importantly, NF-66 is more abundant than NF-L in both forebrain and cerebellum during development. The prevalence of NF-66 over NF-L is most pronounced in brain gray matter. The expression of both NF-66 and NF-L increases continuously during the first month after birth. In situ hybridization demonstrated that NF-66, but not NF-L is, expressed in the cerebellar granule cells. Our findings suggest that the neurofilaments are heterogeneous in developmental expression, among neuronal subtypes and in composition. Human NF-66 neurofilament has recently been mapped to chromosome 10q24. Careful analysis of the human genome map indicates NF-66 gene lies within the critical region of infantile-onset spinocerebellar ataxia (IOSCA). The characteristic developmental expression and spatial localization of the NF-66 gene suggests it as a candidate gene for the disease.

Delayed maturation of regenerating myelinated axons in mice lacking neurofilaments

Using the technique of homologous recombination in embryonic stem cells, we generated mice bearing a targeted disruption of the gene encoding the neurofilament light (NF-L) protein. The absence of NF-L protein in mice resulted in dramatic declines of approximately 20-fold in the levels of neurofilament medium and heavy proteins in the brain and sciatic nerve while increases were detected for other cytoskeletal proteins such as tubulin and GAP-43. Despite a lack of neurofilaments and hypotrophy of axons, the NF-L knockout mice develop normally and do not exhibit overt phenotypes. However, in both NF-L -/- and NF-L +/- mice, the regeneration of myelinated axons following crush injury of peripheral nerves was found to be abnormal. In the second week after axotomy, the number of newly regenerated myelinated axons in the sciatic nerve and facial nerve of NF-L -/- mice corresponded to only approximately 25 and approximately 5% of the number of myelinated axons found in normal mice, respectively. At this early postaxotomy stage, electron microscopy of nerve segments distal to the crush site in NF-L -/- mice revealed abundant clusters of axonal sprouts that were indicative of retarded maturation of regenerating fibers. The analysis of the distal sciatic nerve at 2 months after crush indicated that neurofilament-deficient axons have the capacity to regrow for a long distance and to remyelinate, albeit at a slower rate. These results provide the first direct evidence for a role of neurofilaments in the maturation of regenerating myelinated axons.

Transgenic analyses reveal developmentally regulated neuron- and muscle-specific elements in the murine neurofilament light chain gene promoter

We report here the developmental activity of regulatory elements that reside within 1.7 kilobases of the murine neurofilament light chain (NF-L) gene promoter. NF-L promoter activity is first detected at embryonic day 8.5 in neuroepithelial cells. Neuron-specific gene expression is maintained in the spinal cord until embryonic day 12.5 and at later developmental stages in the brain and sensory neuroepithelia. After day 14.5, the promoter becomes active in myogenic cells. Transgene expression in both neurons and muscle is consistent with the detection of endogenous NF-L transcript in both neuronal and myogenic tissues of neonates by reverse transcriptase-polymerase chain reaction. Neuron- and muscle-specific activities of the NF-L promoter decrease and are nearly undetectable after birth. Thus, the 1.7-kilobase NF-L promoter contains regulatory elements for initiation but not maintenance of transcription from the NF-L locus. Deletion analyses reveal that independent regulatory elements control the observed tissue-specific activities and implicate a potential MyoD binding site as the muscle-specific enhancer. Our results demonstrate that the NF-L promoter contains distinct regulatory elements for both neuron- and muscle-specific gene expression and that these activities are temporally separated during embryogenesis.

The use of transgenic mouse models of amyotrophic lateral sclerosis in preclinical drug studies

The discovery of mutations in the human SOD1 gene encoding Cu,Zn superoxide dismutase (Cu,Zn SOD) in patients with familial amyotrophic lateral sclerosis (ALS) has made possible the development of etiological models of the disease. Expression of mutant SOD1 genes in transgenic mice causes a progressive paralytic disease whose general features resemble ALS in humans. We have used the transgenic model to explore etiological mechanisms and to screen potential therapeutics. Our results and those of others show that familial ALS mutations cause a gain-of-function in Cu,Zn SOD that enhances the generation of damaging oxygen radicals. This may render motor neurons sensitive to the excitotoxic effects of ambient glutamate, as a putative glutamatergic inhibitor such as riluzole has therapeutic efficacy both in the transgenic model and in human ALS. This finding highlights the utility of the SOD1-G93A transgenic mouse model for preclinical drug studies in ALS.

Increased 3-nitrotyrosine in both sporadic and familial amyotrophic lateral sclerosis

The pathogenesis of neuronal degeneration in both sporadic and familial amyotrophic lateral sclerosis (ALS) associated with mutations in superoxide dismutase may involve oxidative stress. A leading candidate as a mediator of oxidative stress is peroxynitrite, which is formed by the reaction of superoxide with nitric oxide. 3-Nitrotyrosine is a relatively specific marker for oxidative damage mediated by peroxynitrite. In the present study, biochemical measurements showed increased concentrations of 3-nitrotyrosine and 3-nitro-4-hydroxyphenylacetic acid in the lumbar and thoracic spinal cord of ALS patients. Increased 3-nitrotyrosine immunoreactivity was observed in motor neurons of both sporadic and familial ALS patients. Neurologic control patients with cerebral ischemia also showed increased 3-nitrotyrosine immunoreactivity. These findings suggest that peroxynitrite-mediated oxidative damage may play a role in the pathogenesis of both sporadic and familial ALS.

Clinical and molecular aspects of motoneurone diseases: animal models, neurotrophic factors and Bcl-2 oncoprotein

Animal models of motor neurone disease (MND) are being increasingly used for screening molecules with clinical potential. A number of different treatments to decrease the progression of neuronal cell loss have been proposed; these include: Bcl-2 (B-cell leukaemia oncogene-2), neurotrophic factors, glutamate receptor inhibitors and Ca2+ channel antagonists. In this review Yves Sagot, Richard Vejsada and Ann C. Kato focus on the effects of neurotrophic factors and Bcl-2, both of which have been shown to prevent cell death in various experimental paradigms. Studies performed in animal models of MND have confirmed the potential of these molecules to support motoneurone survival. Some of them have been shown to act in synergy and these results are discussed in the context of molecular mechanisms leading to collaborative and synergistic activities, and also with respect to presumptive subpopulations of motoneurones, which express diverse receptors for neurotrophic factors. Finally, the current status of clinical trials for amyotrophic lateral sclerosis using neurotrophic factors will be discussed, as well as recent reports that neurotrophic factors can exert adverse effects on neuronal survival.

Coordinate induction of the three neurofilament genes by the Brn-3a transcription factor

The POU domain transcription factor Brn-3a is able to stimulate neurite outgrowth when overexpressed in the neuronal ND7 cell line, whereas the closely related Brn-3b factor does not have this effect. We show that Brn-3a overexpression also enhances the expression of the three neurofilament genes at both the mRNA and protein levels, whereas Brn-3b overexpression has no effect. In addition Brn-3a activates the three neurofilament gene promoters in co-transfection assays in both neuronal and non-neuronal cells. As observed for enhanced neurite outgrowth, the stimulation of neurofilament gene expression and activation of the neurofilament gene promoters is observed with the isolated POU domain of Brn-3a. A single amino acid change in the POU homeodomain of Brn-3a to the equivalent amino acid in Brn-3b abolishes its ability to activate the neurofilament promoters, whereas the reciprocal change converts Brn-3b to an activator of these promoters.

Phosphorylation of human high molecular weight neurofilament protein (hNF-H) by neuronal cyclin-dependent kinase 5 (cdk5)

Neurofilaments (NFs), the neuron-specific intermediate (i.e. approximately 10-nm diameter) filaments are major cytoskeletal components of most neurons. In a mature mammalian neuron, NFs are co-assembled from three subunits, NF-L (low), NF-M (medium), and NF-H (high), with molecular masses of 68, 95, and 115 kDa, respectively. Neurofilament proteins (NF-Ps), particularly, NF-H, are most extensively phosphorylated in large myelinated axons under normal conditions. This phosphorylation occurs on the serine residues of the lysine (Lys)-serine (Ser)-proline (Pro) (KSP) multiple amino acid repeats of the carboxy-terminal tail domain. Phosphorylation of KSP motifs affects physical, biochemical, and immunological properties of NF-H. For example, phosphorylation is thought to play a pivotal role in the maintenance of the neuronal cytoskeletal structure which influences the conduction velocity of the nerve fiber. The key components responsible for phosphorylation are not known. In this study, an identified cyclin-dependent kinase 5 (cdk5), isolated from nervous tissue, has been shown to phosphorylate the human NF-H (hNF-H) and affects its electrophoretic mobility. On the basis of the following observations, it is suggested that neuronal cdk5 (cdk5) phosphorylates KSPXK motifs in the human high molecular weight neurofilament (hNF-H) and affects its electrophoretic mobility. (1) A 14-mer synthetic peptide (KSPEKAKSPVKEEA) derived from hNF-H; (2) a bacterially expressed protein containing 14 KSPXK multiple repeats of hNF-H in C-terminal tail domain; and (3) a dephosphorylated hNF-H in neurofilament preparation are phosphorylated by cdk5. The decrease in molecular mass of hNF-H caused by dephosphorylated was completely recovered upon cdk5 phosphorylation. It is proposed that neuronal cdk5 regulates phosphorylation of the KSPXK motif in hNF-H and other cytoskeletal proteins with similar motifs in the nervous system.

Acquired motor neuron disorders

The acquired motor neuron disorders are a heterogeneous group of conditions in which motor neuron degeneration or dysfunction produces the predominant manifestation of weakness, while the sensory system is clinically spared. The disorders most commonly seen in clinical practice are amyotrophic lateral sclerosis, late manifestations of poliomyelitis, and lower motor neuron syndromes, including motor neuropathy. Less often, acquired motor neuron disorders may complicate metabolic, toxic, or systemic disorders. The pathogenesis of most acquired motor neuron disorders is poorly understood, and treatment is mainly supportive; however clues to understanding the pathogenesis of amyotrophic lateral sclerosis are emerging, and new pharmacologic therapies are available. There is a growing sense of hope that combinations of drugs that are currently being tested may impact the survival of amyotrophic lateral sclerosis.

Transgenic animals as models of neurodegenerative diseases in humans

Neurodegenerative diseases are of major socioeconomic importance and represent an enormous challenge for the scientific and medical communities. Advances in molecular genetics during the past decade have begun to provide approaches for the establishment of animal models for these disorders using transgenic technology. Their analysis will lead to better understanding of disease pathogenesis and will be invaluable for the identification of novel diagnostic and therapeutic agents. With the current pace of genomic research, the generation of transgenic animal models, reproducing in full the pathology and symptoms of even complex disorders such as Alzheimer's disease, must now be considered achievable.