In the adult peripheral nervous system axonal growth may occur following axotomy or in the absence of a direct injury. In the latter situation, axonal plasticity may take place as collateral sprouting or structural remodelling of the terminal arborization. While it is commonly accepted that the neurite growth-associated protein B-50 is consistently upregulated during regeneration after nerve injury, it is not clear whether it is also overexpressed during the axonal remodelling which occurs in the absence of any direct injury to the motoneurone following muscle target modification. This situation is present in the mdx mouse, an animal model of Duchenne Muscular Dystrophy, in which a continuous degeneration-regeneration pattern of muscle fibres is accompanied by remodelling of muscular innervation. We studied B-50 immunoreactivity in several muscles (gastrocnemius, quadriceps, diaphragm) of 12 mdx and 6 control mice aged 1 to 12 months. In normal mice B-50 is expressed in a few nerve fibres mostly associated with blood vessels. In mdx mice B-50 positive axons were mostly localised within areas of muscle degeneration-regeneration. Among such fibres, some were in the vicinity of blood vessels, whereas others were associated with endplates, indicating that in the mutant mice B-50 was also expressed by some motoneurones. These results show that B-50 may be upregulated during axonal remodelling in the absence of a direct injury to the motoneurones. It is postulated that such an upregulation is important for nerve fibre growth and synaptogenesis and also for a better maturation and survival of the newly formed muscle fibres.
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