Defective axonal transport in motor neuron disease

Several recent studies have highlighted the role of axonal transport in the pathogenesis of motor neuron diseases. Mutations in genes that control microtubule regulation and dynamics have been shown to cause motor neuron degeneration in mice and in a form of human motor neuron disease. In addition, mutations in the molecular motors dynein and kinesins and several proteins associated with the membranes of intracellular vesicles that undergo transport cause motor neuron degeneration in humans and mice. Paradoxically, evidence from studies on the legs at odd angles (Loa) mouse and a transgenic mouse model for human motor neuron disease suggest that partial limitation of the function of dynein may in fact lead to improved axonal transport in the transgenic mouse, leading to delayed disease onset and increased life span.     

Impaired slow axonal transport in wobbler mouse motor neuron disease

We studied slow axonal transport and morphometry of forelimb axons in wobbler mice and controls. In wobbler mice, the total radioactivity migrating with the slow transport was decreased by 50%. The velocity of transport also appeared to be reduced; 15 days following administration of a radioisotope, polypeptides migrating with slow component a of transport did not form a peak and remained mostly 2 mm from the spinal cord, while in controls slow component a was distributed as a peak which was located 4 mm from the cord. The ratios of the 68-kDa neurofilament subunit to tubulin and actin were significantly decreased (p less than 0.01 and p less than 0.005, respectively). This finding is consistent with a preferential reduction of the radioactivity migrating with neurofilament proteins in wobbler mice. Moreover, both the size and number of myelinated axons were markedly diminished, but their length was not significantly different, indicating that dying-back does not take place in axons of wobbler mice up to 12 mm from the spinal cord. The reduction in axonal transport may be due to the reduction in number and caliber of the axons and/or to reduced protein synthesis in cervical lower motor neurons; however, the abnormal distribution of the radioactive substance definitely results from impairment of the slow transport in the axons of the forelimb roots of wobbler mice. The transport impairment is not related to the presence of morphological changes in the perikaryon of wobbler mouse lower motor neurons, as it is much more widespread than would be expected if only altered neurons were involved.(ABSTRACT TRUNCATED AT 250 WORDS)     

Impairment of retrograde axonal transport in wobbler mouse motor neuron disease

The earliest horseradish peroxidase (HRP) neuronal labeling (the fastest retrograde transport) was determined by histochemical techniques at various intervals after intramuscular HRP injection in wobbler mice and normal littermates. In the clinically impaired forelimb system, the retrograde transport rate was 150-170 mm/day in wobbler mice and 170-230 mm/day in controls. However, there was no statistical difference between the two groups. The neuronal HRP accumulation at the early intervals was significantly less in wobbler mice than controls, suggesting that the amount of HRP transport was diminished in each axon. For the clinically intact hindlimb nerves, the rate was normal in wobbler mice, but the amount of neuronal HRP was significantly increased. Retrograde axonal transport appeared to be affected in a differential fashion, depending on the extent of disease.     

Axonal regeneration in wobbler motor neuron disease: quantitative histologic and axonal transport studies

The regenerative capacity of the cervical anterior horn cells was studied at 4 and 7 days following forelimb nerve crush in 19 wobbler mice and 18 normal littermates. Quantitative histologic and radiolabeled axonal transport techniques showed that the axotomized neurons of the wobbler mouse supported active axonal elongation. However, the average axon outgrowth rate determined by histologic technique was diminished by 25% and the fastest axon outgrowth rate determined by axonal transport technique was also decreased by 30% in wobbler mice as compared to controls. The distal labeled peak was absent in the wobbler mouse at 7 days, indicating that the regeneration rate of individual axons was widely dispersed. Histologic studies also showed that the wobbler axons grew slowly. This study suggests that axonal regeneration does occur in motor neurons undergoing a primary neuronopathy. However, the regenerative capacity was reduced and this appears to reflect an impairment of functional integrity in the anterior horn cells of the wobbler mouse.     

Voluntary activation and fiber density of fasciculations in motor neuron disease.

Two hundred voluntarily activated motor units and 211 fasciculations were recorded in the biceps of 10 patients with motor neuron disease with the Macro EMG technique. Twenty-two fasciculations, in nine of 10 muscles, had a potential of closely similar shape, amplitude, and area to that of a voluntary unit. Fasciculating units that could not be activated voluntarily had a higher mean number of spikes in their triggering single fiber potentials than units that could only be activated voluntarily, but statistically similar Macro EMG parameters. The mean number of single fiber spikes, and Macro EMG parameters, of fasciculations activated voluntarily, were similar to those of units that were only activated voluntarily. A positive correlation between fiber density and Macro EMG median amplitude and area in individual patients, and between number of single fiber spikes and Macro EMG amplitudes and areas in the pooled data, was found for fasciculations but not for voluntary units. At least 10% of fasciculations in patients with motor neuron disease may originate near or above the point of axonal branching and a proportion of those without evidence of voluntary activation may have a higher number of smaller muscle fibers, or more closely packed muscle fibers, of similar or greater size, than voluntarily activated motor units. Differences in the peripheral microanatomy of a number of fasciculation units not activated voluntarily may underlie ectopic impulse generation in the terminal axonal arborization, endplate zone, or muscle fibers of these units.     

Giant Aplysia neuron R2 has distal dendrites: evidence for protein sorting and a second spike initiation site

Because of its anatomy, the neuron R2 of Aplysia has been used to study how proteins are distributed to their appropriate destinations within the cell. The R2 cell body resides in the abdominal ganglion, while its axons terminate on glands in the skin. Using intracellular injection of HRP and intraxonal recordings, we found that R2 has a dendritic (receptive) arborization in the pleural ganglion. The structure of these dendrites was examined after injecting the soma with 3H-L-fucose, thereby labeling glycoproteins that are transported to all regions of the cell. Light- and electron-microscope autoradiography show that the openings to the dendrites are not on the periphery, but are suspended inside the axon by glial cell infoldings. All of the organelles seen in the axon are found in the dendrites, including 2 types of vesicles. Neither the axon nor the dendrites contain ribosomes. Thus, R2 has 3 functionally distinct regions--cell body, dendrites, presynaptic terminals--that are separated from each other by at least 4 cm. This implies that pre- and postsynaptic proteins made in the cell body are transported along the axon to the pleural ganglion, where they are sorted. To investigate this idea, we exposed the abdominal ganglion to 35S-methionine to label R2's proteins. Analyses by SDS-PAGE of the rapidly transported labeled proteins from R2 consistently showed a 78 kDa band that accumulated in the pleural ganglion and did not move into the peripheral nerves. This then is a putative dendritic constituent.     

运动神经元病血清特异抗原成分的检测

目的检测运动神经元病（ＭＮＤ）病人血清中是否存在运动神经元特异抗原成分，并探索ＭＮＤ潜在的诊断标志物。方法制备５株抗运动神经元单克隆抗体，并证明其对大鼠脊髓前角运动神经元具有高度特异的免疫组织化学反应。应用抗运动神经元单克隆抗体２４Ｂ０－ＭｃＡｂ，用ＥＬＩＳＡ法对２５例运动神经元病病人血清中的特异抗原成分进行检测。根据临床表现将２５例病人分为肌萎缩侧索硬化（ＡＬＳ）、脊肌萎缩症（ＳＭＡ）及进行性球麻痹（ＰＢＰ）３组，再按年龄段分３个亚组（＜２０岁组、２０～３９岁组、＞４０岁组）。结果发现８５％（２２／２５）临床确诊的ＭＮＤ病人存在较高浓度的特异抗原成分，ＭＮＤ病人与正常对照组对２４Ｂ０－ＭｃＡｂ的反应性差异有显著性意义（Ｐ＜０．０５），ＡＬＳ、ＳＭＡ及ＰＢＰ亚型之间差异也有显著性意义（Ｐ＜０．０５），而年龄组之间差异虽有显著性意义，其临床意义尚需进一步研究。性别组之间的差异无显著性意义。结论ＭＮＤ病人血清中存在运动神经元特异抗原成分。用抗运动神经元单克隆抗体以ＥＬＩＳＡ法检测运动神经元特异抗原可以作为诊断ＭＮＤ的辅助检查。     

Risk factors in motor neuron disease: a case-control study.

A case-control study of risk factors in 512 cases of motor neuron disease (MND) and 512 controls, affected by other neurological diseases, was performed. Clinical history showed a significant increase in gastric ulcer and mechanical injuries among MND cases, whereas the frequencies of operations, poliomyelitis, malignancies and autoimmune diseases were similar for cases and controls. The women affected by MND had a later menarche and an earlier menopause; therefore, the reproductive period was significantly shorter, supporting a possible role of sexual hormones in the pathogenesis of MND. The analysis of occupations showed an increased number of farmers and also of subjects exposed to chemical products among MND cases.     

Ocular motor function in motor neuron disease.

We studied ocular motor function in 34 patients with motor neuron disease (MND) and in 18 age-matched controls. This included the latency, accuracy, and amplitude-velocity relationships of saccades. We also examined ocular pursuit, the slow phases of optokinetic nystagmus, and the ability to suppress the vestibulo-ocular reflex (VOR) with visual fixation of a head-mounted target. Five of the subjects with MND had pronounced parkinsonian features on neurologic examination. The nonparkinsonian MND subjects had normal ocular motor function for all measures. Most subjects suppressed the VOR completely. The parkinsonian-MND patients had impairment of both saccadic and pursuit eye movements, and one parkinsonian-MND patient with poor pursuit was unable to suppress the VOR. We conclude that ocular motor function is generally spared in MND. The occasional appearance of ocular motor dysfunction probably reflects the incidence of secondary abnormalities such as parkinsonism.     

Olfactory impairment in motor neuron disease: a pilot study.

The ability to identify smells was tested in nine males and six females with motor neuron disease (MND) of varying severity, using the University of Pennsylvania Smell Identification Test (UPSIT). The olfactory impairment found in MND patients compared with age and sex matched controls is statistically significant at the 0.005 level. The relationship with Parkinson's disease, with Alzheimer's dementia and the possible aetiological implications of this new aspect of the MND are discussed.     

Transcranial magnetic stimulation identifies upper motor neuron involvement in motor neuron disease.

OBJECTIVE: To evaluate the sensitivity of transcranial magnetic stimulation (TMS) to identify upper motor neuron involvement in patients with motor neuron disease. BACKGROUND: Diagnosis of ALS depends on upper and lower motor neuron involvement. Lower motor neuron involvement may be documented with electromyography, whereas definite evidence of upper motor neuron involvement may be elusive. A sensitive, noninvasive test of upper motor neuron function would be useful. METHODS: TMS and clinical assessment in 121 patients with motor neuron disease. RESULTS: TMS revealed evidence of upper motor neuron dysfunction in 84 of 121 (69%) patients, including 30 of 40 (75%) patients with only probable upper motor neuron signs and unsuspected upper motor neuron involvement in 6 of 22 (27%) patients who had purely lower motor neuron syndromes clinically. In selected cases, upper motor neuron involvement identified with TMS was verified in postmortem examination. Increased motor evoked potential threshold was the abnormality observed most frequently and was only weakly related to peripheral compound muscle action potential amplitude. In a subset of 12 patients reexamined after 11+/-6 months, TMS showed progression of abnormalities, including progressive inexcitability of central motor pathways and loss of the normal inhibitory cortical stimulation silent period. CONCLUSIONS: TMS provides a sensitive means for the assessment and monitoring of excitatory and inhibitory upper motor neuron function in motor neuron disease.     

Divergent co-transmitter actions underlie motor pattern activation by a modulatory projection neuron

Co-transmission is a common means of neuronal communication, but its consequences for neuronal signaling within a defined neuronal circuit remain unknown in most systems. We are addressing this issue in the crab stomatogastric nervous system by characterizing how the identified modulatory commissural neuron (MCN)1 uses its co-transmitters to activate the gastric mill (chewing) rhythm in the stomatogastric ganglion (STG). MCN1 contains gamma-aminobutyric acid (GABA) plus the peptides proctolin and Cancer borealis tachykinin-related peptide Ia (CabTRP Ia), which it co-releases during the retractor phase of the gastric mill rhythm to influence both retractor and protractor neurons. By focally applying each MCN1 co-transmitter and pharmacologically manipulating each co-transmitter action during MCN1 stimulation, we found that MCN1 has divergent co-transmitter actions on the gastric mill central pattern generator (CPG), which includes the neurons lateral gastric (LG) and interneuron 1 (Int1), plus the STG terminals of MCN1 (MCN1(STG)). MCN1 used only CabTRP Ia to influence LG, while it used only GABA to influence Int1 and the contralateral MCN1(STG). These MCN1 actions caused a slow excitation of LG, a fast excitation of Int1 and a fast inhibition of MCN1(STG). MCN1-released proctolin had no direct influence on the gastric mill CPG, although it likely indirectly regulates this CPG via its influence on the pyloric rhythm. MCN1 appeared to have no ionotropic actions on the gastric mill follower motor neurons, but it did use proctolin and/or CabTRP Ia to excite them. Thus, a modulatory projection neuron can elicit rhythmic motor activity by using distinct co-transmitters, with different time courses of action, to simultaneously influence different CPG neurons.     

Pain in motor neuron disease.

Twenty-seven of 42 patients with motor neuron disease had significant pain. The nature and duration of the pain are described along with an illustrative case-report. The aetiology and most effective treatment of this common complication of motor neuron disease remain unclear.