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The Journal of Experimental Biology 206, 2191-2200 (2003)
doi: 10.1242/jeb.00412

Neurotrophin 4/5 is required for the normal development of the slow muscle fiber phenotype in the rat soleus

Dario I. Carrasco^* and Arthur W. English

Department of Cell Biology, Emory University School of Medicine, Atlanta, GA 30322, USA

^* Author for correspondence (e-mail: dcarras{at}emory.edu)

Accepted 26 March 2003

During normal postnatal development, rat soleus (SOL) muscle fibers undergo a dramatic fast-to-slow myosin heavy chain (MyHC) isoform transformation. We exploited this phenomenon to evaluate the role of neurotrophin 4/5 (NT-4/5) in the regulation of muscle fiber phenotype. Intramuscular injections of recombinant NT-4/5 into the SOL muscle of rat neonates significantly accelerated the normal fast-to-slow MyHC isoform transformation. Sequestration of endogenous NT-4/5 with TrkB–IgG prevented this transformation from occurring. Administration of the other TrkB ligand, brain-derived neurotrophic factor (BDNF), did not affect the normal course of the MyHC isoform transformation in this muscle, indicating that the observed effect is NT-4/5 specific. Botulinum toxin blockade of synaptic transmission significantly disrupted the normal fast-to-slow MyHC isoform switch. Because administration of NT-4/5 to paralyzed muscles failed to restore the normal course of this MyHC transformation, we believe that the effect of NT-4/5 is not directly on the muscle fibers but that it probably activates or forms a type of retrograde signal to motoneurons. The developmental upregulation of NT-4/5 mRNA in rat SOL muscle fibers occurred earlier than the upregulation of MyHC I/b mRNA associated with muscle fiber transformation. This timing is consistent with the idea that NT-4/5 is involved in early events that lead to the upregulation of the slow MyHC isoform in this muscle.

Key words: neurotrophin, development, skeletal muscle, myosin heavy chain, synaptic transmission, rat

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