Neurotrophin-3 and trkC in muscle are non-essential for the development of mouse muscle spindles

Abstract

Neurotrophin-3 (NT3) or TrkC null mutant mice were examined for the presence of muscle spindles. Muscles of mastication, but not limbs, contained spindles in newborn and adolescent mutants. The intramuscular distribution and morphological properties of spindles in mutant masticatory muscles were indistinguishable from those of wild-type spindles. Intrafusal fibers of NT3- or trkC-deficient spindles-expressed the slow-tonic isoform of myosin heavy chains, characteristic of wild-type spindles. Sensory nerve endings were observed in spindles of mutants by electron microscopy. Thus, NT3 or trkC, which is expressed in wild-type spindles, may serve functions other than those related to spindle assembly. Presumably, proprioceptive neurons innervating jaw muscles are dependent on factors other than NT3 for survival and maintenance.

Formation of a full complement of cranial proprioceptors requires multiple neurotrophins

“…Note the presence of spindles (arrowheads) in both the mutant and wild‐type muscles. That NT3‐/‐ jaw muscles also contain spindles expressing the S46 antigen similar to wild‐type and BDNF‐/‐ mice was shown previously (Kucera et al, 1998). Scale bar = 20 μm.…”

“…Neurotrophins other than NT3 might support developing TMN proprioceptive neurons, in contrast to dorsal root ganglion (DRG) proprioceptive neurons. Fewer proprioceptive afferents and muscle spindles are present in jaw muscles of mice lacking NT3 or its tyrosine kinase (Trk) receptor (Kucera et al, 1998), but they are not entirely absent as in the limbs of NT3 null mutants (Ernfors et al, 1994a). The presence of masticatory spindles in NT3‐/‐ mice suggests that the corresponding proprioceptive TMN neurons depend on a neurotrophin(s) other than NT3.…”

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

“…Finally, the extracellular matrix molecule type IV collagen, observed using scanning electron microscopy, has been proposed as a molecule important in intrafusal fiber differentiation [11,12]. While significant advances have been made using in vivo techniques, a clear definition of all the factors required for intrafusal fiber differentiation has not been documented [13][14][15][16][17]. Therefore, one approach to gain additional insight into intrafusal fiber myogenesis is to utilize a defined in vitro [18].…”

Differential responses to the application of exogenous NT‐3 are observed for subpopulations of motor and sensory neurons depending on the presence of skeletal muscle

“…For example, the muscles of mastication, but not of limbs, contained spindles in newborn and adolescent NT-3 and TrkC null mutant mice, suggesting that, at least in the muscles of mastication, the muscle secreted NT-3, or associated TrkC, may serve in functions other than those related to spindle assembly and that proprioceptive neurons innervating the jaw muscles, as suggested above, are dependent on factors other than NT-3 for maintenance and survival (Kucera et al, 1998).…”