Degeneration of cocultures of spinal muscular atrophy muscle cells and rat spinal cord explants is not due to secreted factors and cannot be prevented by neurotrophins.

We have shown recently that cocultures of muscle cells from infantile spinal muscular atrophy (SMA) patients innervated by motoneurons of normal rat spinal cord explants undergo a degeneration process, suggesting that muscle may play a role in this atrophy, which previously has been considered to be a pure motoneuron disease. Conditional media of SMA cocultures did not affect control healthy nerve muscle cocultures. Conversely, conditioned media of control cocultures were unable to prevent degeneration of SMA cocultures. Moreover, neurotrophic factors, thought to be of help in motoneuron disease treatment, did not protect SMA cocultures from premature death. Our results suggest that the abnormal phenotype observed in nerve-muscle coculture (1) is not due to the release of a toxic factor nor to the lack of a secreted survival factor, and (2) does not respond to neurotrophin treatment.

Neurotrophins increase motoneurons' ability to innervate skeletal muscle fibers in rat spinal cord--human muscle cocultures.

Neurotrophins, nerve growth factor (NGF), neurotrophin-3 (NT-3), neurotrophin-5 (NT-5) and brain-derived neurotrophic factor (BDNF), were studied in vitro in a coculture model of human skeletal muscle myotubes and rat embryo spinal cord explants, which enables the different steps of functional innervation to be followed, including neurite outgrowth, synapse formation and induction of contractile activity. We found that NT-3, NT-5, BDNF, but not NGF simultaneously induced a significant increase in the number and length of neurites emerging from spinal cord explants, the number of endplates per muscle fiber, and the area of innervated muscle fibers around each spinal cord explant. These results suggest that neurotrophins NT-3, NT-5 and BDNF enhance spinal cord motoneurons potential of innervation.

Constitutive muscular abnormalities in culture in spinal muscular atrophy.

To explore the cause of spinal muscular atrophy (SMA), we used an in-vitro model of nerve-muscle co-cultures in which motoneurons were normal and satellite cells were obtained from SMA patients. In co-cultures initiated with satellite cells from type I and type II SMA patients only, we observed degeneration of the innervated fibres after 1-3 weeks of nerve-muscle co-culture. This process involved vacuolisation, disorganisation, and death of the innervated muscle fibres. This observation points to a muscular implication in the severe forms of SMAs.

[Changes of some proteins expression of the spinal cord and muscles in infantile spinal muscular atrophy]. / Variations de l'expression de certaines protéines de la moelle épinière et du muscle dans l'amyotrophie spinale infantile.

The various types of childhood spinal muscular atrophy (SMA) represent a spectrum of clinical disorders resulting from the degeneration of motor neurons (MN). The genetic defect has been recently localized to chromosome 5q in the region 11.2-13.3. Under normal conditions, half of the motor neurons die during embryonic development, while the remaining 50% survive to innervate muscle fibers and form neuromuscular junctions. Numerous studies using in vivo and in vitro models have shown that survival of MNs depends on the presence of trophic factors of neuronal and muscular origin. However, at the present time, no molecular mechanisms can be proposed to account for the nature and the sequence of the interactions leading to the formation and maintenance of a functional neuromuscular junction. To gain a better understanding of the SMA disorders, an alternative to genetic studies consists in analyzing the molecular mechanisms underlying this pathology. Variations in the expression of proteins, for instance, might reflect the pathological phenotype. We thought it possible to detect differences in the protein(s) which would correlate with the molecular deficit of childhood SMA. We, therefore, compared the patterns of human protein expression from normal controls and SMA spinal cord and muscle. Significant variations in the expression of some proteins, which have been quantified by a computerized Bio-Image electrophoresis system, have been found. In particular, two proteins, a and b (126 kDa and 112 kDa) which are very probably common to spinal cord and muscle show a marked increase of their expression in children with SMA.(ABSTRACT TRUNCATED AT 250 WORDS)