NEUROTROPHIN-3-ENHANCED NERVE REGENERATION SELECTIVELY IMPROVES RECOVERY OF MUSCLE-FIBERS EXPRESSING MYOSIN HEAVY-CHAINS 2B

The purpose of this study was to evaluate the effect of neurotrophin 3 (NT-3) enhanced nerve regeneration on the reinnervation of a target m uscle. Muscle fibers can be classified according to their mechanical p roperties and myosin heavy chain (MHC) isoform composition. MHC1 conta ining slow-type and MHC2a or 2b fast-type fibers are normally distribu ted in a mosaic pattern, their phenotype dictated by motor innervation . After denervation, all fibers switch to fast-type MHC2b expression a nd also undergo atrophy resulting in loss of muscle mass. After regene ration, discrimination between fast and slow fibers returns, but the d istribution and fiber size change according to the level of reinnervat ion. In this study, rat gastrocnemius muscles (ipsilateral and contral ateral to the side of nerve injury) were collected up to 8 mo after ne rve repair, with or without local delivery of NT-3. The phenotype chan ges of MHC1, 2a, and 2b were analyzed by immunohistochemistry, and fib er type proportion, diameter, and grouping were assessed by computeriz ed image analysis. At 8 mo, the local delivery of NT-3 resulted in sig nificant improvement in gastrocnemius muscle weight compared with cont rols (NT-3 group 47%, controls 39% weight of contralateral normal musc le; P < 0.05). NT-3 delivery resulted in a significant increase in the proportion (NT-3 43.3%, controls 35.7%; P < 0.05) and diameter (NT-3 87.8 mu m, controls 70.8 mu m; P < 0.05) of fast type 2b fibers after reinnervation. This effect was specific to type 2b fibers; no normaliz ation was seen in other fiber types. This study indicates that NT-3-en hanced axonal regeneration has a beneficial effect on the motor target organ. Also, NT-3 may be specifically affecting a subset of motoneuro ns that determine type 2b muscle fiber phenotype. As NT-3 was topicall y applied to cut nerves, our data suggest a discriminating effect of t he neurotrophin on neuro-muscular interaction. These results would imp ly that muscle fibers may be differentially responsive to other neurot rophic factors and indicate the potential clinical role of NT-3 in the prevention of muscle atrophy after nerve injury.