Biomedical subjects

M E Gurney

Publications and source records attributed to M E Gurney.

At least 37 records · Page 2Linked to original sources

PubMed1998

Protein oxidative damage in a transgenic mouse model of familial amyotrophic lateral sclerosis.

The Gly93-->Ala mutation in the Cu,Zn superoxide dismutase (Cu,Zn-SOD) gene (SOD1) found in some familial amyotrophic lateral sclerosis (FALS) patients has been shown to result in an aberrant increase in hydroxyl radical production by the mutant enzyme that may cause oxidative injury to spinal motor neurons. In the present study, we analyzed the extent of oxidative injury to lumbar and cervical spinal cord proteins in transgenic FALS mice that overexpress the SOD1 mutation [TgN(SOD1-G93A)G1H] in comparison with nontransgenic mice. Total protein oxidation was examined by spectrophotometric measurement of tissue protein carbonyl content by the dinitrophenylhydrazine (DNPH) assay. Four ages were investigated: 30 (pre-motor neuron pathology and clinical disease), 60 (after initiation of pathology, but pre-disease), 100 (approximately 50% loss of motor neurons and function), and 120 (near complete hindlimb paralysis) days. Protein carbonyl content in 30-day-old TgN(SOD1-G93A)G1H mice was twice as high as the level found in age-matched nontransgenic mice. However, at 60 and 100 days of age, the levels were the same. Then, between 100 and 120 days of age, the levels in the TgN(SOD1-G93A)G1H mice increased dramatically (557%) compared with either the nontransgenic mice or transgenic animals that overexpress the wild-type human Cu,Zn-SOD [TgN(SOD1)N29]. The 100-120-day increase in spinal cord protein carbonyl levels was confirmed by sodium dodecyl sulfate-polyacrylamide gel electrophoretic separation and western blot immunoassay, which enabled the identification of heavily oxidized individual proteins using a monoclonal antibody against DNPH-derivatized proteins. One of the more heavily oxidized protein bands (14 kDa) was identified by immunoprecipitation as largely Cu,Zn-SOD. Western blot comparison of the extent of Cu,Zn-SOD protein carbonylation revealed that the level in spinal cord samples from 120-day-old TgN(SOD1-G93A)G1H mice was significantly higher than that found in age-matched nontransgenic or TgN(SOD1)N29 mice. These results suggest that the increased hydroxyl radical production associated with the G93A SOD1 mutation and/or lipid peroxidation-derived radical species (peroxyl or alkoxyl) causes extensive protein oxidative injury and that the Cu,Zn-SOD itself is a key target, which may compromise its antioxidant function.

Aging↗

PubMed1998

Intracellular calcium parallels motoneuron degeneration in SOD-1 mutant mice.

Transgenic mice with Cu,Zn superoxide dismutase (SOD-1) mutations provide a unique model to examine altered Ca homeostasis in selectively vulnerable and resistant motoneurons. In degenerating spinal motoneurons of G93 A SOD-1 mice, developing vacuoles were filled with calcium, while calcium was gradually depleted from the cytoplasm and intact mitochondria. In oculomotor neurons, no degenerative changes, vacuolization, or increased calcium were noted. Motor axon terminals of interosseus muscle gradually degenerated and intracellular calcium was depleted. Oculomotor terminals of mutant SOD-1 mice were smaller and exhibited no degenerative changes, but did exhibit unique membrane-enclosed organelles containing calcium. Spinal motoneurons of SOD-1 mice were shown to have fewer calcium binding proteins, such as parvalbumin, compared with oculomotor neurons. These data suggest that the SOD-1 mutation is associated with impaired calcium homeostasis in motoneurons in vivo, with increased likelihood of degeneration associated with higher levels of intracellular calcium and lower to absent levels of calbindin-D28K and/or parvalbumin, and decreased likelihood of degeneration associated with minimally changed calcium and ample calbindin-D28K and/or parvalbumin.

Animals↗

PubMed1998

Riluzole preserves motor function in a transgenic model of familial amyotrophic lateral sclerosis.

Riluzole was tested in a dose-ranging study for preservation of motor function in a transgenic mouse model of familial ALS. The model is based on expression of mutant human Cu,Zn superoxide dismutase in mouse brain and spinal cord. In contrast with the human ALS trials, in the mouse model, riluzole significantly preserved motor function as assessed by nightly running in a wheel. The effect of riluzole on motor performance was greater earlier in disease than later, suggesting that riluzole may have benefit for "quality-of-life" measures in ALS. Treatment with riluzole was initiated earlier in the transgenic model than in the human ALS trials, which may account for the significantly better outcome.

Amyotrophic Lateral Sclerosis↗

PubMed1997

Midbrain dopaminergic neuronal degeneration in a transgenic mouse model of familial amyotrophic lateral sclerosis.

Familial amyotrophic lateral sclerosis has been linked in 15% of families to mutations in the gene encoding for copper-zinc superoxide dismutase (Cu/Zn-SOD), a key enzyme in the cellular defense mechanisms against free radical attack. We used a transgenic mouse model of familial amyotrophic lateral sclerosis (transgenic G1H mice) based on expression of mutant human Cu/Zn-SOD to examine the influence of the transgene expression on midbrain dopaminergic neurons, cells that contain conspicuous amounts of this enzyme. At the time that 50% of motor neurons of the spinal cord were lost, we observed concurrent reductions in dopamine levels in the caudate-putamen and the nucleus accumbens of transgenic G1H mice. In addition, numbers of tyrosine hydroxylase-immunostained neurons were significantly reduced in both the substantia nigra (26%) and the ventral tegmental area (16%) compared to those in their nontransgenic littermates. Similar abnormalities were not observed in the brains of transgenic mice overexpressing wild-type Cu/Zn-SOD. These findings indicate that overexpression of the mutated Cu/Zn-SOD protein caused a significant loss of midbrain dopaminergic neurons in addition to the loss of spinal motor neurons. The potential of the mutated enzyme to induce cell death extending beyond the motor neurons is consistent with the description of substantia nigra degeneration in some patients with familial amyotrophic lateral sclerosis. Furthermore, if mutated Cu/Zn-SOD is conclusively shown to kill cells by oxidative stress, such an observation would be in keeping with the known sensitivity of dopaminergic neurons to free radical attack.

Amyotrophic Lateral Sclerosis↗

PubMed1997

Increased 3-nitrotyrosine and oxidative damage in mice with a human copper/zinc superoxide dismutase mutation.

Mutations in copper/zinc superoxide dismutase (SOD1) cause a subset of cases of autosomal dominant familial amyotrophic lateral sclerosis (FALS). Transgenic mice that express these point mutations develop progressive paralysis and motor neuron loss thought to be caused by a gain-of-function of the enzyme. The gain-of-function may be an enhanced ability of the mutant SOD1 to generate .OH radicals or to facilitate peroxynitrite-mediated nitration of proteins. We found significant increases in concentrations of 3-nitrotyrosine, a marker of peroxynitrite-mediated nitration, in upper and lower spinal cord and in cerebral cortex of transgenic mice with the FALS-associated G93A mutation. Malondialdehyde, a marker of lipid peroxidation, was increased in cerebral cortex. 3-Nitrotyrosine-, heme oxygenase-1-, and malondialdehyde-modified protein immunoreactivities were increased throughout SOD1 transgenic mice spinal cord but particularly within motor neurons. These results suggest that the gain-of-function of at least one mutant SOD1 associated with FALS involves increased protein nitration and oxidative damage, which may play a role in neuronal degeneration.

Analysis of Variance↗

PubMed1997

Transgenic animal models of familial amyotrophic lateral sclerosis.

Amyotrophic lateral sclerosis (ALS) occurs in both sporadic and familial forms, which have very similar clinical presentation and course. Approximately 20% of the familial cases of ALS are caused by mutation of the SODI gene encoding Cu, Zn superoxide dismutase (SOD). Over 30 different SODI gene mutations have been found in patients. Most are missense mutations that cause the substitution of one amino acid for another. The failure to find deletions in familial ALS suggests that the mutant protein is required for pathogenesis. Studies in transgenic mice indicate that familial ALS is caused by gain-of-function mutations in the SODI gene. These enhance formation of free radicals by the mutant enzyme. When expressed at high levels in transgenic mice, mutant human Cu,Zn SOD causes a clinical disease that resembles human ALS. Selective degeneration of motor neurones in the spinal cord and brainstem is accompanied by progressive motor impairment. Pathogenesis in the transgenic model of familial ALS is a sequential, two-step process in which damage mediated by free radicals accumulates to a threshold that triggers catastrophic motor neurone loss through glutamate-mediated, excitotoxic mechanisms. Evidence in support of this hypothesis comes from therapeutic studies with antioxidants and inhibitors of glutamatergic neurotransmission.

Amyotrophic Lateral Sclerosis↗

PubMed1997

A low expressor line of transgenic mice carrying a mutant human Cu,Zn superoxide dismutase (SOD1) gene develops pathological changes that most closely resemble those in human amyotrophic lateral sclerosis.

About 15-20% of patients with familial amyotrophic lateral sclerosis (ALS) carry one of several missense mutations in the gene for Cu,Zn superoxide dismutase (SOD1). We have previously reported on an animal model of this disease produced by the transgenic expression of a mutant form of human SOD1 containing a Gly93-->Ala amino acid substitution. Several lines of transgenic mice were produced, characterized by a differing tempo and severity of disease that generally correlated with the number of mutant gene copies that these lines expressed. We reported that mice expressing high copy numbers (18-25) developed a disease with a relatively short course and with a pathology mainly characterized by severe vacuolar degeneration of motor neurons and their process. Lewy-like bodies and swollen axons were also present. The exquisite localization to motor neurons was the feature that made the pathology in these overexpressors germane to the human disease. Severe vacuolar degeneration, however, was considered to be at variance with human ALS, in which similar changes have not been described. In the present study, we have made a temporal characterization of microscopic and immunohistochemical changes in a line of transgenic mice expressing lower copy numbers of the mutant gene. These mice, designated G5/G5, survive more than 400 days and present pathological changes which are virtually identical to those in the human disease. In fact, in these animals, anterior horn cell depletion, atrophy, astrocytosis, and the presence of numerous ubiquitinated Lewy-like bodies and axonal swellings are the main pathological features, while vacuolar pathology is minimal. This study underscores the importance of the level of expression of the mutant enzyme in the resulting clinical and pathological disease, and supports the value of this transgenic model as an excellent tool for investigating both pathogenesis of human ALS and possible therapeutic avenues.

Amyotrophic Lateral Sclerosis↗

PubMed1997

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.

Amyotrophic Lateral Sclerosis↗

PubMed1997

The copper chelator d-penicillamine delays onset of disease and extends survival in a transgenic mouse model of familial amyotrophic lateral sclerosis.

A subpopulation of familial cases of amyotrophic lateral sclerosis has been linked to mutations in the gene encoding Cu/Zn superoxide dismutase (SOD1). There is in vitro evidence that certain SOD1 mutants, in addition to their normal dismutation function, show increased ability of the enzyme to act as a peroxidase. This reaction is sensitive to inhibition by copper chelators. To test this hypothesis in vivo, we administered the copper chelator d-penicillamine to a transgenic mouse model of familial amyotrophic lateral sclerosis overexpressing a mutated form of human SOD1. We demonstrate that oral administration of d-penicillamine is able to delay the onset of the disease and extend the survival of these mice. Histological studies also showed a decreased loss of facial motor neurons in d-penicillamine-treated transgenic mice, corroborating the slower evolution of the disease in these animals. These results suggest that copper chelators may benefit patients with familial amyotrophic lateral sclerosis linked to mutations in the SOD1 gene.

Amyotrophic Lateral Sclerosis↗

PubMed1997

Oxidative stress, mutant SOD1, and neurofilament pathology in transgenic mouse models of human motor neuron disease.

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that primarily affects motor neurons in the spinal cord and brain stem. About 10% of all ALS cases are familial (FALS), inherited in an autosomal dominant manner. One fifth of FALS patients carry mutations in the Cu/Zn superoxide dismutase (SOD1) gene, and several lines of transgenic mice have been engineered to express mutant forms of the SOD1 gene that are linked to FALS. Significantly, many of these transgenic lines of mice develop a motor neuron disease (MND) that resembles human FALS. Oxidative stress induced by human SOD1 mutations is believed to play an important role in the pathogenesis of FALS and the FALS-like MND seen in the mutant SOD1 transgenic mice. For example, two lines of these mice showed prominent degeneration of mitochondria and endoplasmic reticulum in spinal cord neurons. Furthermore, recent studies have shown that neurofilament (NF)-rich spheroids. Lewy body-like NF inclusions, altered ubiquitin immunoreactivity, and Golgi fragmentation occur in the spinal cord motoneurons of these mutant SOD1 transgenic mice. Because these lesions recapitulate hallmark abnormalities of human ALS, mutant SOD1 transgenic mice provide a useful model for studies designed to elucidate the pathogenesis of ALS. Furthermore, transgenic mice that overexpress NF proteins also develop a clinical and pathologic phenotype similar to human MND, and polymorphisms in an NF gene have been linked to patients with ALS. Collectively, these observations implicate NF protein abnormalities in the pathogenesis of this disorder. Accordingly, this review summarizes recent insights into mechanisms of motor neuron degeneration in ALS that have emerged from studies of these new animal models of this neurodegenerative disease.

Amyotrophic Lateral Sclerosis↗

PubMed1996

The Golgi apparatus of spinal cord motor neurons in transgenic mice expressing mutant Cu,Zn superoxide dismutase becomes fragmented in early, preclinical stages of the disease.

Dominant mutations of the SOD1 gene encoding Cu,Zn superoxide dismutase have been found in members of certain families with familial amyotrophic lateral sclerosis (ALS). To better understand the contribution of SOD1 mutations in the pathogenesis of familial ALS, we developed transgenic mice expressing one of the mutations found in familial ALS. These animals display clinical and pathological features closely resembling human ALS. Early changes observed in these animals were intra-axonal and dendritic vacuoles due to dilatation of the endoplasmic reticulum and vacuolar degeneration of mitochondria. We have reported that the Golgi apparatus of spinal cord motor neurons in patients with sporadic ALS is fragmented and atrophic. In this study we show that spinal cord motor neurons of transgenic mice for an SOD1 mutation display a lesion of the Golgi apparatus identical to that found in humans with sporadic ALS. In these mice, the stacks of the cisternae of the fragmented Golgi apparatus are shorter than in the normal organelle, and there is a reduction in Golgi-associated vesicles and adjacent cisternae of the rough endoplasmic reticulum. Furthermore, the fragmentation of the Golgi apparatus occurs in an early, presymptomatic stage and usually precedes the development of the vacuolar changes. Transgenic mice overexpressing the wild-type human superoxide dismutase are normal. In familial ALS, an early lesion of the Golgi apparatus of motor neurons may have adverse functional effects, because newly synthesized proteins destined for fast axoplasmic transport pass through the Golgi apparatus.

Aging↗

PubMed1996

Transgenic mice carrying a human mutant superoxide dismutase transgene develop neuronal cytoskeletal pathology resembling human amyotrophic lateral sclerosis lesions.

Mutations in the human Cu,Zn superoxide dismutase gene (SOD1) are found in 20% of kindreds with familial amyotrophic lateral sclerosis. Transgenic mice (line G1H) expressing a human SOD1 containing a mutation of Gly-93 --> Ala (G93A) develop a motor neuron disease similar to familial amyotrophic lateral sclerosis, but transgenic mice (line N1029) expressing a wild-type human SOD1 transgene do not. Because neurofilament (NF)-rich inclusions in spinal motor neurons are characteristic of amyotrophic lateral sclerosis, we asked whether mutant G1H and/or N1029 mice develop similar NF lesions. NF inclusions (i.e., spheroids, Lewy body-like inclusions) were first detected in spinal cord motor neurons of the G1H mice at 82 days of age about the time these mice first showed clinical evidence of disease. Other neuronal intermediate filament proteins (alpha-internexin, peripherin) also accumulated in these spheroids. The onset of accumulations of ubiquitin immunoreactivity in the G1H mice paralleled the emergence of vacuoles and NF-rich spheroids in neurons, but they did not colocalize exclusively with spheroids. In contrast, NF inclusions were not seen in the N1029 mice until they were 132 days old, and ubiquitin immunoreactivity was not increased in the N1029 mice even at 199 days of age. Astrocytosis in spinal cord was associated with a marked increase in glial fibrillary acidic protein immunoreactivity in the G1H mice, but not in the N1029 mice. Finally, comparative studies revealed a striking similarity between the cytoskeletal pathology in the G1H transgenic mice and in patients with amyotrophic lateral sclerosis. These findings link a specific SOD1 mutation with alterations in the neuronal cytoskeleton of patients with amyotrophic lateral sclerosis. Thus, neuronal cytoskeletal abnormalities may be implicated in the pathogenesis of human familial amyotrophic lateral sclerosis.

Aging↗

PubMed1996

Brain-derived neurotrophic factor transiently stabilizes silent synapses on developing neuromuscular junctions.

A general feature of the developing nervous system is the activity-dependent rearrangement of genetically defined, synaptic connections. A parallel process occurs at the developing neuromuscular junction as activity-dependent synapse withdrawal reduces the initial polyneuronal innervation of individual muscle fibers to a mononeuronal innervation within the first few weeks after birth. Because members of the neurotrophin gene family influence motor neuron differentiation and survival, we examined whether or not they also influence synaptic rearrangements in neonatal muscles. We found that treatment with brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), or neurotrophin-4/5 (NT-4/5) causes the transient retention of multiple synaptic contacts on neonatal myofibers. However, the combination of both electrophysiological and histological assays revealed that the majority of such supernumerary synaptic contacts are functionally inactive or "silent." There also occurs an increase in the number of retracting axons. Because BDNF mRNA is expressed in developing muscle and the trkB tyrosine kinase receptor for BDNF is expressed by neonatal motor neurons, our results suggest that BDNF may play an endogenous role in the refinement of synaptic connectivity that occurs in skeletal muscles after birth.

Animals↗

PubMed1996

Benefit of vitamin E, riluzole, and gabapentin in a transgenic model of familial amyotrophic lateral sclerosis.

Familial amyotrophic lateral sclerosis (FALS) has been linked in some families to dominant mutations of the SOD1 gene encoding Cu,Zn superoxide dismutase (Cu,ZnSOD). We have used a transgenic model of FALS based on expression of mutant human Cu,ZnSOD to explore the etiology and therapy of the genetic disease. Expression of mutant, but not wild-type, human Cu,ZnSOD in mice places the brain and spinal cord under oxidative stress. This causes depletion of vitamin E, rather than the typical age-dependent increase in vitamin E content as occurs in nontransgenic mice and in mice expressing wild-type human Cu,ZnSOD. Dietary supplementation with vitamin E delays onset of clinical disease and slows progression in the transgenic model but does not prolong survival. In contrast, two putative inhibitors of the glutamatergic system, riluzole and gabapentin, prolong survival. However, riluzole did not delay disease onset. Thus, there was clear separation of effects on onset, progression, and survival by the three therapeutics tested. This suggests the hypothesis that oxidative damage produced by the expression of mutant Cu,ZnSOD causes slow or weak excitotoxicity that can be inhibited in part by alerting glutamate release or biosynthesis presynaptically.

Acetates↗

PubMed1996

Pathogenic mechanisms in familial amyotrophic lateral sclerosis due to mutation of Cu, Zn superoxide dismutase.

Oxidative mechanisms of damage have been implicated indirectly in the damage to brain tissue caused acutely by ischemia or chronically by neurodegenerative diseases. A direct link between pathogenesis and antioxidant enzyme systems has come from studies of a genetic form of amyotrophic lateral sclerosis (ALS). ALS causes the degeneration of motor neurons in cortex, brainstem and spinal cord with consequent progressive paralysis and death. The disease occurs in both sporadic and familial forms. Some 20% of kindreds in which ALS is inherited in an autosomal dominant fashion have mutations in the gene (SOD1) encoding Cu, Zn superoxide dismutase (SOD). Several SOD1 mutations have been shown by ourselves and others to cause motor neuron disease when expressed at high levels in transgenic mice, whereas transgenic mice expressing comparable amounts of wild-type human SOD do not show clinical disease. Thus, we have argued that motor neuron disease is caused by gain-of-function mutations in the human SOD1 gene. Our current experiments investigate the link between mutation of SOD1 and oxidative pathways of damage.

Amyotrophic Lateral Sclerosis↗

PubMed1995

Inhibition of anti-V3 domain antibody binding to human immunodeficiency virus type-1-infected cells by sulfated polysaccharides.

The third variable domain (V3 domain) of the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein gp120 is an immunodominant region. Anti-V3 domain antibodies neutralize both HIV-1 infection and syncytium formation. The V3 domain has a high density of positive charge which is a potential binding site for anti-HIV-1 sulfated polysaccharides. To investigate the inhibitory effect of sulfated polysaccharides on the binding of anti-V3 domain antibody, fluorescence-activating cell sorting analysis was performed using two kinds of antibodies, NEA9284 (purified, 0.25 micrograms/ml) and 0.5 beta (ascite, 2.0 mg/ml), and HIV-1-infected CEM cells. When the binding assay with a 1:100 dilution of each antibody was performed in the presence of dextran sulfate, heparin, and inositol hexasulfate at concentrations which are antiviral, the compounds did not inhibit the binding of either antibody. As the antibody concentration was decreased with higher dilution, dextran sulfate was able to reduce antibody binding by 50-60%. Thus, antagonism of anti-V3 domain antibody binding by sulfated polysaccharides is not as extensive as reported previously by several groups.

Amino Acid Sequence↗

PubMed1995

Neuropathological changes in two lines of mice carrying a transgene for mutant human Cu,Zn SOD, and in mice overexpressing wild type human SOD: a model of familial amyotrophic lateral sclerosis (FALS).

Two different lines of mice, G1 and G20, carrying a transgene for a mutant form of Cu,Zn SOD, found in a family with familial amyotrophic lateral sclerosis (FALS), develop clinical and pathological changes which are, in their late stages, strikingly similar to those in human disease. We have analyzed the distribution and characteristics of lesions in the central and peripheral nervous systems of such mice. The most affected structure was the spinal cord, followed by the medulla, pons and midbrain. The early stages of the disease were characterized by vascular degeneration of anterior horn neurons and their processes, while, in the late stages, the main changes consisted of neuronal loss and atrophy of the anterior horns and the deposition in these areas of multiple filamentous inclusions resembling Lewy bodies. In the late stages of the disease, the white matter of the spinal cord was also involved, particularly in the anterior and lateral columns. Posterior columns were also involved, but to a much lesser degree. The brainstem structures also showed vacuolar degeneration of several motor nuclei and of several groups neurons in the reticular formation. Anterior roots and peripheral nerves showed the classical features of Wallerian degeneration. The dorsal root ganglia, with rare exceptions, were unremarkable. The posterior roots showed mild changes in the most severely affected mice. Changes in these two affected lines were compared to changes in mice overexpressing wild type, rather than mutant human Cu,Zn SOD. These mice never developed clinical disease, although, pathologically, they developed very mild vacuolar changes in the anterior horns of the spinal cord and in motor axons. This study shows that although simple overexpression of SOD may be injurious to motor neurons, albeit very mildly, the mutant form is necessary to produce both clinical disease and severe pathological changes which, in the chronic stage of the disease, have striking similarities to human familial ALS. A dominant gain of function, therefore, is the most likely pathogenesis of tissue injury induced by mutations in Cu,Zn SOD.

Age of Onset↗

PubMed1995

Leukemia inhibitory factor influences the timing of programmed synapses withdrawal from neonatal muscles.

We show that leukemia inhibitory factor (LIF) plays a physiological role in the programmed withdrawal of synapses from neonatal muscles. First, LIF mRNA is present in embryonic skeletal muscle and is developmentally regulated. We detect high levels of LIF mRNA at embryonic day 17 (E17) in mouse hind leg muscles. The content of LIF mRNA falls 10-fold between E17 and birth and then remains low in the neonate and adult. The decrease in LIF mRNA in skeletal muscle coincides with the end of secondary myogenesis and the completion of the adult number of myofibers. Second, treatment of the mouse tensor fascia latae (TFL), a superficial muscle of the hind leg, with LIF from birth (100 ng/day), transiently delays the withdrawal of excess inputs from polyneuronally innervated myofibers by approximately 3 days. The midpoint of the process is shifted from 7.5 +/- 10.2 +/- 0.6 days of age. LIF treatment delays synapse withdrawal by altering its timing without an appreciable effect on its rate. Third, in mice homozygous for a disruption of the LIF gene, the midpoint in the reduction of multiply innervated TFL myofibers occurs 1 day earlier, at 6.5 +/- 0.5 days of age. Muscle fiber number is unchanged in LIF null mice. Treatment with LIF does not alter the rate of neonatal growth, the number of muscle fibers in the TFL, or the reappearance of inputs that have been eliminated. Instead, LIF appears to delay maturation of the motor unit by transiently delaying the onset of synapse withdrawal. We hypothesize that this is a necessary component of a selective process that will operate simultaneously and equally on multiple, competing motor units.

Animals↗