Tenascin is present in human muscle spindles and neuromuscular junctions

Tissue engineering intrafusal fibers: dose- and time-dependent differentiation of nuclear bag fibers in a defined in vitro system using neuregulin 1-beta-1

While much is known about muscle spindle structure, innervation and function, relatively few factors have been identified that regulate intrafusal fiber differentiation and spindle development. Identification of these factors will be a crucial step in tissue engineering functional muscle systems. In this study, we investigated the role of the growth factor, neuregulin 1-beta-1 (Nrg 1-beta-1) EGF, for its ability to influence myotube fate specification in a defined culture system utilizing the non-biological substrate N-1[3-(trimethoxysilyl)propyl]-diethylenetriamine (DETA). Based on morphological and immunocytochemical criteria, Nrg 1-beta-1 treatment of developing myotubes increases the ratio of nuclear bag fibers to total myotubes from 0.019 to 0.100, approximately a five-fold increase. The myotube cultures were evaluated for expression of the intrafusal fiber-specific alpha cardiac-like myosin heavy chain and for the expression of the non-specific slow myosin heavy chain. Additionally, the expression of ErbB2 receptors on all myotubes was observed, while phosphorylated ErbB2 receptors were only observed in Nrg 1-beta-1-treated intrafusal fibers. After Nrg 1-beta-1 treatment, we were able to observe the expression of the intrafusal fiber-specific transcription factor Egr3 only in fibers exhibiting the nuclear bag phenotype. Finally, nuclear bag fibers were characterized electrophysiologically for the first time in vitro. This data shows conclusively, in a serum-free system, that Nrg 1-beta-1 is necessary to drive specification of forming myotubes to the nuclear bag phenotype.

Somatic and intramuscular distribution of muscle spindles and their relation to muscular angiotypes

The distribution pattern of muscle spindles in the skeletal musculature has been reviewed in a large number of muscles (using the literature data especially from cat and man), and the relation of spindle content to muscle mass was quantitatively examined in 36 cat and 140 human muscles. In both species, the number of spindles increases with increasing muscle mass in a power law fashion of the form y=bx+a, whereby y denotes the logarithm of spindle content within a muscle, and x is the logarithm of muscle mass. For the cat, slope b and intercept a were estimated as 0.39 and 1.53, and for man as 0.48 and 1.33, respectively. The results show that the spindle content of a muscle may be related to its mass, confirming a similar analysis made previously by Banks and Stacey (Mechano receptors, Plenum Press, New York, 1988, pp. 263-269) in a different data set. With regard to the histological profile of muscle fibers, (as it is already well documented by many groups) muscle spindles tend to be located in deeper muscle regions where oxidative fibers predominate, and are far scarcer in superficial and flat muscle regions where glycolytic fibers predominate. These discrete muscle regions differ also in the properties of the vessel tree supplying them, for which the term oxidative and glycolytic "angiotype" has been used. The results from these three aspects of analysis (relation to muscle mass, relation to muscle regions with high oxidative index and relation to muscle regions with dense vascular supply) were combined with histological findings showing that spindles may be in systematic anatomical contact to intramuscular vessels. Based on these data a hypothesis is proposed according to which, both the number and intramuscular placement of muscle spindles are related to the oxidative angiotype supplying the muscle territories rich in oxidative fibers. The hypothesis is discussed.

Immunolocalization of tenascin-C in human type II fiber atrophy

Tenascin-C is a multifunctional extracellular matrix glycoprotein with stimulatory and anti-adhesive or inhibitory properties for axon growth. Its location and discontinuous expression are restricted in innervated muscle tissues. Tenascin-C accumulated interstitially among human denervated muscle fibers and close to normal-sized fibers. To expand our knowledge of the expression of tenascin-C in human neuromuscular disorders, we investigated immunohistologically 20 human muscle specimens with type II myofiber atrophy of children and adults. Tenascin-C immunoreactivity in adult type II atrophy was frequent, and accumulation in children was sparse and weak. In both groups, tenascin-C immunoreactivity was found: 1. Interstitially around normal-sized type II muscle fibers. 2. Around atrophic type II muscle fibers. 3. Around small-caliber myofibers with centrally located nuclei. These results indicate that tenascin-C immunoreactivity: (1) is detectable around early denervated and reinnervated muscle fibers and, therefore, (2) may reflect in part the molecularly ongoing process of denervation and reinnervation in human type II fiber atrophy.

Organization and reorganization of neuromuscular junctions in mice lacking neural cell adhesion molecule, tenascin-C, or fibroblast growth factor-5

Many proteins have been hypothesized to mediate intercellular interactions that regulate the formation, maturation, and maintenance of the skeletal neuromuscular junction. Three of the best characterized of these are a membrane-associated adhesion molecule, neural cell adhesion molecule (N-CAM), an extracellular matrix component, tenascin-C, and a soluble growth factor, fibroblast growth factor-5 (FGF-5). To assess the roles of these molecules in synaptogenesis in vivo, we examined neuromuscular junctions in homozygous mutant mice lacking N-CAM, tenascin-C, FGF-5, or both N-CAM and tenascin-C. End plates were 14% smaller in N-CAM-deficient mice than in controls, and formation of junctional folds was delayed in this mutant. In all other respects tested, however, the structure and molecular architecture of neuromuscular junctions were normal in all three single mutants and in the double mutant. We also tested the abilities of damaged motor axons to reinnervate mutant muscle after axotomy and of intact motor axons to sprout after partial denervation. Again, no significant differences among genotypes were observed. Together, these results demonstrate that N-CAM, tenascin-C, and FGF-5 are dispensable for major aspects of synaptic development and regeneration.