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Levels of mRNA coding for motoneuron growth-promoting factors are increased in denervated muscle
F A Rassendren1, E Bloch-Gallego, H Tanaka
1Centre de Recherche de Biochimie Macromoléculaire, UPR 8402, Centre National de la Recherche Scientifique/Institut National de la Santé et de la Recherche Médicale U249, Montpellier, France.
Abstract:
Partial denervation of skeletal muscle induces sprouting of axons remaining within the muscle, possibly as a result of increased synthesis by denervated muscle fibers of motoneuron growth-promoting factors. Direct verification of this hypothesis has not been possible because the molecules responsible are not unambiguously characterized. We used Xenopus oocytes as a functional assay for mRNAs coding for secreted growth factors: preparations of mRNA from innervated and denervated neonatal muscle were injected into oocytes. Three days later, oocytes injected with denervated muscle mRNA expressed increased levels of nicotinic acetylcholine receptor and voltage-dependent sodium channels at their membrane. Proteins secreted by the same oocytes were tested for their effects on (i) neurite outgrowth from embryonic chicken ventral spinal cord neurons; (ii) survival in mixed culture of embryonic chicken motoneurons identified using the SC1 antibody; and (iii) survival of embryonic motoneurons purified by panning on SC1 antibody. In all three assays, media conditioned by oocytes injected with mRNA from denervated muscle contained significantly higher levels of biological activity than did those from oocytes injected with innervated muscle mRNA or water. mRNA was prepared from muscle at different times after denervation: a maximal increase was obtained already after 1 day, consistent with an involvement in sprouting. Synthesis of motoneuron growth-promoting factors is thus regulated by denervation in a parallel fashion to that of other key components of the neuromuscular junction.
Insights
Denervation of skeletal muscle increases the synthesis of motoneuron growth factors. These factors promote nerve regrowth and survival, crucial for neuromuscular junction repair.
Area of Science:
- Neuroscience
- Molecular Biology
- Muscle Physiology
Background:
- Partial denervation of skeletal muscle can trigger axonal sprouting.
- This sprouting may be due to increased synthesis of motoneuron growth factors by denervated muscle fibers.
- The specific molecules responsible have not been clearly identified.
Purpose of the Study:
- To functionally assay mRNAs encoding secreted growth factors from innervated and denervated muscle.
- To investigate the role of denervation-induced factors in promoting motoneuron growth and survival.
Main Methods:
- Xenopus oocytes were used as a functional assay system.
- mRNA from neonatal rat innervated and denervated muscle was injected into oocytes.
- Secreted proteins from oocytes were tested for effects on neuronal neurite outgrowth and motoneuron survival.
Main Results:
- Oocytes injected with denervated muscle mRNA showed increased expression of nicotinic acetylcholine receptors and sodium channels.
- Media conditioned by these oocytes significantly enhanced neurite outgrowth and motoneuron survival.
- Maximal increases in growth-promoting activity were observed as early as 1 day post-denervation.
Conclusions:
- Denervation of skeletal muscle upregulates the synthesis of motoneuron growth-promoting factors.
- These factors play a role in axonal sprouting and neuromuscular junction regeneration.
- The synthesis of these factors is regulated in parallel with other neuromuscular junction components upon denervation.