Differentiation of supernumerary fibres in neonatally deefferented rat muscle spindles

Fiber Content and Myosin Heavy Chain Composition of Muscle Spindles in Aged Human Biceps Brachii

The present study investigated potential age-related changes in human muscle spindles with respect to the intrafusal fiber-type content and myosin heavy chain (MyHC) composition in biceps brachii muscle. The total number of intrafusal fibers per spindle decreased significantly with aging, due to a significant reduction in the number of nuclear chain fibers. Nuclear chain fibers in old spindles were short and some showed novel expression of MyHC α-cardiac. The expression of MyHC α-cardiac in bag1and bag2fibers was greatly decreased in the A region. The expression of slow MyHC was increased in nuclear bag1fibers and that of fetal MyHC decreased in bag2fibers whereas the patterns of distribution of the remaining MyHC isoforms were generally not affected by aging. We conclude that aging appears to have an important impact on muscle spindle composition. These changes in muscle spindle phenotype may reflect an age-related deterioration in sensory and motor innervation and are likely to have an impact in motor control in the elderly.

Neural precursor cells from a fatal human motoneuron disease differentiate despite aberrant gene expression

Precursor cells of the human central nervous system can be cultured in vitro to reveal pathogenesis of diseases or developmental disorders. Here, we have studied the biology of neural precursor cells (NPCs) from patients of lethal congenital contracture syndrome (LCCS), a severe motoneuron disease leading to prenatal death before the 32nd gestational week. LCCS fetuses are immobile because of a motoneuron defect, seen as degeneration of the anterior horn and descending tracts of the developing spinal cord. The genetic defect for the syndrome is unknown. We show that NPCs isolated postmortem from LCCS fetuses grow and are maintained in culture, but display increased cell cycle activity. Global transcript analysis of undifferentiated LCCS precursor cells present with changes in EGF-related signaling when compared with healthy age-matched human controls. Further, we show that LCCS-derived NPCs differentiate into cells of neuronal and glial lineage and that the initial differentiation is not accompanied by overt apoptosis. Cells expressing markers Islet-1 and Hb9 are also generated from the LCCS NPCs, suggesting that the pathogenic mechanism of LCCS does not directly affect the differentiation capacity or survival of the cells, but the absence of motoneurons in LCCS may be caused by a noncell autonomous mechanism.

Expression of myosin heavy chain isoforms along intrafusal fibers of rat soleus muscle spindles after 14 days of hindlimb unloading

Morphological, contractile, histochemical, and electrophoretical characteristics of slow postural muscles are altered after hindlimb unloading (HU). However, very few data on intrafusal fibers (IFs) are available. Our aim was to determine the effects of 14 days of hindlimb unloading on the morphological and immunohistochemical characteristics of IF in rat soleus muscle. Thirty-three control and 32 unloaded spindles were analyzed. The number and distribution of muscle spindles did not appear to be affected after unloading. There was no significant difference in number, cross-sectional area, and histochemical properties of IF between the two groups. However, after unloading, a significant decrease in slow type 1 MHC isoform and a slight increase in slow-tonic MHC expression were observed in the B and C regions of the bag1 fibers. The alpha-cardiac MHC expression was significantly decreased along the entire length of the bag2 fibers and in the B and C regions of the bag1 fibers. In 12 muscle spindles, the chain fibers expressed the slow type 1 and alpha-cardiac MHC isoforms over a short distance of the A region, although these isoforms are not normally expressed. The most striking finding of the study was the relative resistance of muscle spindles to perturbation induced by HU.

Afferent-inherent regulation of myosin heavy chain isoforms in rat muscle spindles

Whether afferents exert their morphogenetic influence on spindles through release of trophic factors at intrafusal fiber junctions or via participation in proprioceptive pathways which modulate the motor activity to muscles was investigated by comparing myosin heavy chain (MHC) expression in intrafusal fibers after ablation of afferents (deafferentation, or DA) to the extensor digitorum longus (EDL) of adult rats or after ablation of the corresponding central processes of afferents to the spinal cord (central-process ablation, or CPA). DA and CPA elicited an exaggerated pedal plantarflexion, and hypertrophy of the EDL concomitant with atrophy of the soleus in the affected hindlimb. Frequencies and patterns of expression of seven MHCs expressed by intrafusal fibers in CPA muscles were indistinguishable from normal rats. However, frequencies and patterns of expression of several MHCs were abnormal following DA. Thus factors transported anterogradely from afferents to intrafusal fibers may regulate MHC expression in intrafusal fibers.

Expression of myosin heavy chain isoforms in regenerated muscle spindle fibres after muscle grafting in young and adult rats--plasticity of intrafusal satellite cells

Satellite cells are the myogenic precursor cells of postnatal skeletal muscles. After muscle injury they can proliferate, differentiate, fuse and form myofibres. We have analysed regeneration of distinctly different types of intrafusal fibres in rat muscle spindles. We have introduced the new technique of heterochronous allotransplantation and compared it with the previously used standard autografting method. The allotransplantation method enables one to graft muscles from very young animals; we have used the extensor digitorum longus (EDL) muscles from 2- to 28-day-old rats, which were grafted into EDL muscles of adult inbred recipients. The regenerated "intrafusal" fibres did not express the spindle-specific slow tonic and alpha cardiac-like myosin heavy chain (MyHC) isoforms and they did not exhibit the dual mATPase reaction typical of the nuclear bag2 fibres and the characteristic regional differences in MyHC expression and in the mATPase reaction of nuclear bag1 and nuclear bag2 fibres. On the other hand, they expressed either fast twitch or slow twitch/beta cardiac MyHC isoforms and exhibited an alkali or acid stable mATPase reaction along their whole length, like extrafusal fast type 2 and slow type 1 muscle fibres, respectively. In all regenerated muscle spindles only motor, but no sensory axons were found. More than 85% of muscle spindles in our sample contained regenerated spindle fibres of the same extrafusal fibre type (either type 2 or type 1), in contrast to control muscle spindles, which always contained intrafusal fibres of three different intrafusal fibre types (nuclear bag1, nuclear bag2 and nuclear chain fibres). There were no differences in MyHC expression and mATPase activity between spindle fibres regenerated in grafts taken from young rats of various ages or between allotransplanted and autotransplanted EDL muscles. The present results demonstrate that regenerated "intrafusal" fibres resemble, according to MyHC expression, extrafusal fast or slow muscle fibres. It can thus be concluded that intrafusal satellite cells derived from distinctly different nuclear bag1, nuclear bag2 and nuclear chain fibres show great plasticity, as their MyHC expression can be respecified towards the extrafusal muscle fibre phenotype by foreign alpha-motor innervation.

Stability of myosin heavy chain isoforms in selectively denervated adult rat muscle spindles

BACKGROUND

Rat intrafusal fibers consist of multiple isoforms of myosin heavy chains (MHCs) whose expression involves complex interactions among motor neurons, sensory neurons, and muscle cells during spindle development. Little is known about the roles of sensory and motor innervation in regulating and maintaining expression of MHC isoforms in adult rat muscle spindles.

METHODS

MHC expression was investigated in deafferented or deefferented adult rat muscle spindles by reacting transverse sections of spindles with a panel of monoclonal antibodies specific for different MHC isoforms.

RESULTS

Deefferentation or deafferentation did not alter the number of intrafusal fibers expressing most MHC isoforms. However, the numbers of fibers expressing two MHC isoforms were altered in deefferented muscle spindles. Nuclear bag1 fibers ceased to express alpha-cardiac MHC and upregulated embryonic MHC after ablation of motor innervation. Likewise, bag2 and chain fibers downregulated avian neonatal/fast MHC following deafferentation, but chain fibers upregulated type 2A MHC and became more extrafusal-like in their pattern of MHC expression.

CONCLUSIONS

These data indicate that (1) perturbations in spindle sensory and motor nerve supplies produce less severe alterations in MHC expression in mature intrafusal fibers than do similar lesions in developing intrafusal fibers and (2) MHC expression in intrafusal fibers reflects a combination of inductive and suppressive effects of motor and sensory neurons.

Familial fetal akinesia deformation sequence with a skeletal muscle maturation defect

Two female siblings with the fetal akinesia deformation sequence (FADS) are described. Both showed facial anomalies, arthrogrypotic extremities, hypoplastic lungs, and fetal growth retardation. The central nervous system of the second sibling, including the spinal cord, was normal. The skeletal muscle was studied by immunohistochemistry for the expression of several myosin heavy chain isoforms, M-band proteins and intermediate filament proteins. The skeletal muscle was immature and atypical muscle spindles containing up to 31 intrafusal fibers were found. These findings suggest that a lethal FADS phenotype may involve a maturation defect of the skeletal muscle, and, in this family, may be inherited in a recessive fashion.

Expression of myosin heavy chain isoforms and myogenesis of intrafusal fibres in rat muscle spindles

This review concerns the pattern of expression and regulation of myosin heavy chain (MHC) isoforms in intrafusal fibres of rat muscle spindles detected by immunocytochemistry. The three types of intrafusal fibres--nuclear bag1, nuclear bag2, and nuclear chain fibres--are unique in co-expressing several MHCs including special isoforms such as slow tonic and alpha cardiac-like MHC and isoforms typical of muscle development, such as embryonic and neonatal MHC. The distinct intrafusal fibre types appear sequentially during rat hind limb development, the nuclear bag2 precursors being first identifiable at 17-18 days in utero as the only primary myotubes expressing slow tonic MHC. Sensory innervation is required for the expression of "spindle-specific" MHC isoforms. Motor innervation contributes to the diversity in distribution of the different MHCs along the length of the nuclear bag fibres. It is suggested that unique populations of myoblasts are destined to become intrafusal fibres during development in the rat hind limb muscles and that the regional heterogeneity in MHC expression is related both to sensory and motor innervation and to the properties of the myoblast lineages. These distinct features make intrafusal fibres an attractive in situ model for investigating myogenesis, myofibrillogenesis, and the mechanisms regulating MHC expression.

Neomyogenesis in neonatally de-efferented and postnatally denervated rat muscle spindles

The ultrastructure of muscle spindles de-efferented by the extirpation of the lumbosacral spinal cord at the age of 2 days and subsequently deprived of their sensory innervation by the section of the sciatic nerve at 3-4 weeks of age was studied in serial sections of 2-month-old rat hindlimb muscles. De-efferentation leaves the primary sensory neurons and their peripheral axons intact and capable of inducing the muscle spindle morphogenesis during the critical period of their development. In de-efferented and subsequently denervated muscle spindles, new supernumerary intrafusal muscle profiles (SIPs) appeared in the muscle spindle A region. They were formed in intimate spatial relation with the original intrafusal muscle fibres (IMFs) predominantly from activated satellite cells derived from both nuclear bag (larger diameter) and nuclear chain fibres. SIPs, however, lacked the typical nuclear accumulations, as well as other ultrastructural distinctions present in control IMFs. The majority of differentiated SIPs separated from original IMFs, whereas the less differentiated SIPs were usually closely apposed to the surface of the parent IMFs and both were covered by the common basal lamina. In some spindles, the original IMFs and/or new SIPs at different stages of their differentiation were found together and they formed clusters of variable shape and composition. In the majority of clusters, all profiles seemed to be isolated along their entire length, although in few clusters, occasional cytoplasmic connections of variable length between intrafusal profiles were found. This result is important for the interpretation of the forthcoming study of expression of muscle spindle-specific myosin heavy chain isoforms in denervated SIPs in rat muscle spindles gradually deprived of their motor and sensory innervation.

Increase in the number of intrafusal muscle fibres in rat muscles after neonatal motor denervation

Rat muscle spindles disintegrate after total neonatal muscle denervation in which both sensory and motor axons are cut, but develop almost normally during the first three weeks after neonatal de-efferentation, attaining the usual complement of four intrafusal fibres, two bag and two chain fibres. Thereafter additional intrafusal fibres differentiate in most of the de-efferented spindles. We have examined the development of supernumerary fibres in muscle spindles of the rat hind limb muscles one to four months after neonatal de-efferentation using transmission electron microscopy. Additional intrafusal fibres originate both from activated intrafusal myosatellites and by different forms of fibre division. In a sample of 27 muscle spindles examined along the A zone five weeks after de-efferentation, the mean number of intrafusal fibre profiles increased almost two-fold to 7.9 +/- 3.3 (S.D.). Up to 20 intrafusal fibre profiles per spindle cross-section were found in muscles de-efferented for four months. The supernumerary fibres were apparently derived from all three intrafusal fibre types, but predominantly from the chain fibres. It is noteworthy that de-efferentation which causes wasting of extrafusal muscle initiates myogenesis and maintains additional intrafusal fibres within the spindle capsules.