The kyphoscoliosis (ky) mouse is deficient in hypertrophic responses and is caused by a mutation in a novel muscle-specific protein.

The ky mouse mutant exhibits a primary degenerative myopathy preceding chronic thoraco-lumbar kyphoscoliosis. The histopathology of the ky mutant suggests that Ky protein activity is crucial for normal muscle growth and function as well as the maturation and stabilization of the neuromuscular junction. Muscle hypertrophy in response to increasing demand is deficient in the ky mutant, whereas adaptive fibre type shifts take place. The ky locus has previously been localized to a small region of mouse chromosome 9 and we have now identified the gene and the mutation underlying the kyphoscoliotic mouse. The ky transcript encodes a novel protein that is detected only in skeletal muscle and heart. The identification of the ky gene will allow detailed analysis of the impact of primary myopathy on idiopathic scoliosis in mice and man.

Regenerating muscle fibers induce directional sprouting from nearby nerve terminals: studies in living mice.

The principal aim of this work was to better understand how regenerating muscle fibers become innervated in adult animals. To induce muscle regeneration, individual identified muscle fibers in a mouse were damaged with a laser focused through a microscope. The muscle fiber that degenerated and the muscle fiber that was formed in its place were followed by viewing the same site repeatedly over a period of 2 d to 40 weeks. Commonly, the nerve terminal innervating the irradiated muscle fiber partially retracted during muscle fiber degeneration, and then sprouted to innervate the regenerating muscle fiber at the same site it had previously innervated the muscle fiber that was damaged. During the early phase of muscle regeneration we also observed sprouts originating from nerve terminals on adjacent muscle fibers. The new nerve growth was a response to the regenerating muscle fiber rather than to the degenerated fiber it replaced because repeated damage of the same site every 2-3 d over a 10 d period (to prevent regeneration) did not cause any sprouting. The direction of the sprouts on adjacent muscle fibers showed a bias toward the regenerating muscle fiber, although they avoided the region occupied by the original nerve terminal. Forty percent of the sprouts managed to reach the regenerated fiber. Nonetheless, by 11 d after muscle fiber damage, all sprouts had regressed, leaving the new fiber innervated by the same motor axon that innervated the fiber that was damaged. On the other hand, when the overlying nerve terminal as well as the muscle fiber was damaged, the sprouts from nearby muscle fibers were both more numerous and more stable, and in five cases we observed two or more new synaptic junctions on the regenerating fiber originating from different axons. In one case we witnessed a protracted competition between the original motor axon as it sprouted back and the sprouts from nearby junctions for sole innervation of the regenerate. Ultimately, the surviving sprouts myelinated and became the permanent and exclusive input to the new fiber. These results indicate that regenerating muscle fibers emit a signal that induces directional sprouting from nearby undamaged nerve terminals. Reinnervation of the regenerating muscle fiber by one axon apparently prevents the maintenance of such neurites. Because the process of muscle regeneration shares many features in common with myogenesis during embryonic development, it is likely that developing muscle fibers present an analogous stimulus to ingrowing motor axons.(ABSTRACT TRUNCATED AT 400 WORDS)

The calcium homeostasis and the membrane potential of cultured muscle cells from patients with myotonic dystrophy.

Using the fluorescence indicator, quin2, we compared the cytoplasmic Ca2+ concentration ([Ca2+]i) of cultured myotubes obtained from control subjects and myotonic dystrophy (MyD) patients. In Ca2(+)-free buffer the [Ca2+]i of the cultured MyD muscle cells was not significantly different from that of the control cells. In the presence of 1 mM external Ca2+ the cultured MyD muscle cells showed a significantly higher [Ca2+]i, which was due to the influx of Ca2+ through voltage-operated nifedipine-sensitive Ca2+ channels. In the presence of external Ca2+, MyD myotubes did not respond to acetylcholine, whereas control myotubes showed a transient increase in [Ca2+]i after addition of acetylcholine. This increase was inhibited by the addition of nifedipine. The differences in Ca2(+)-homeostasis between cultured MyD muscle cells and control cells were not due to differences in the resting membrane potential or the inability of the MyD cells to depolarize as a response to acetylcholine. Therefore, cultured MyD muscle cells exhibit altered nifedipine-sensitive voltage-operated channels which are active under conditions in which they are normally present in the inactive state, and which are unable to respond to depolarization caused by acetylcholine.

Development of early swimming in Xenopus laevis embryos: myotomal musculature, its innervation and activation.

The development of the axial musculature, its innervation and early locomotion in Xenopus laevis embryos are described. Between stages 17 and 40 some 45 myotomes are formed on each side of the body. During this period the animals develop from non-motile to free swimming embryos. Using fluorescein-conjugated bungarotoxin the acquisition of acetylcholine receptor-sites was studied. At stage 25 (early flexure stage) bound bungarotoxin was confined to the first seven intermyotomal clefts, in free swimming embryos (stage 33) to the first 20 clefts. Application of horseradish peroxidase to the intermyotomal clefts in embryos ranging from stages 25 to 37/38 revealed that primary motoneurons were usually situated 100-400 microns, i.e. 0.5-1.5 myotomes, rostral to the cleft they innervated. The motor axons left the spinal cord at the caudal side of each spinal segment where neural crest was present between the cord and the myotomes. At stage 25 ventral root activity could be recorded extracellularly from only the first three intermyotomal clefts, at stage 32/33 from the first 16 clefts. The first spontaneous rhythmic swimming-like activity could be recorded around stage 28. Between stages 27 and 32/33 the initial swimming frequency and the swimming episode duration increased at least three-fold. Comparable results were obtained with high-speed cinematography and measurements with a photoelectric transducer. Between stages 17 and 40 the number of myotomes increased by 0.9 myotome h, approximately 11.4 h later followed by the innervation of the myotomes at 0.7 cleft/h. About 3.6 h after this, ventral root activity appeared at the rate of 0.6 cleft h. This study shows that the early swimming pattern generating neuronal network, located within the rostral spinal cord, reaches a state of "critical mass" around stage 27, at which the first rhythmic swimming activity occurs. At least 6-10 functional spinal segments and adjacent myotomes are required for early swimming.

Structural and functional properties of reticulospinal neurons in the early-swimming stage Xenopus embryo.

This study presents direct evidence that in Xenopus laevis embryos ipsi- and contralaterally descending reticulospinal fibers from the caudal brain stem project to the spinal cord, where they directly contact primary motoneurons. At stage 30, occasional contacts between primary motoneurons and descending axons are present. These contacts are possibly already functional since presynaptic vesicles were sometimes observed. Furthermore, the physiological data obtained in this study suggest that reticulospinal neurons in the caudal brain stem are involved in the central generation of early swimming. The first ingrowth of reticulospinal axons was observed in the rostral spinal cord after application of HRP to the caudal brain stem of stage 27/28 embryos. By stage 32, many supraspinal axons could be found in the spinal cord at the level of the 12/13th myotome, near the time of the first rhythmic swimming. Both lamellipodial and varicose growth cones were found. Intracellular recordings from the brain stem and extracellular recordings from the myotomal muscles in curarized embryos around stage 30 revealed neurons in the caudal brain stem which were active during early fictive swimming. After intracellular staining with Lucifer yellow neurons with descending axons were found in the brain-stem reticular formation. These reticulospinal neurons showed "motoneuron-like" phasic activity, producing one spike each swimming cycle. Rhythmically occurring spikes with swimming periodicity were superimposed on a sustained depolarization level of some 5-30 mV. Reticulospinal neurons in the brain stem resemble descending interneurons in the spinal cord by their morphology, projection pattern, and activity during early swimming. Reticulospinal neurons and descending interneurons might therefore form one continuous population of projecting interneurons with a different location but a similar function. In support of this we propose that the embryonic brain-stem reticular formation forms part of the swimming pattern generator.

The anatomical substrate for telencephalic function.

The basic thesis for this study was that the telencephalon is needed to make decisions in new situations. Subsidiary hypotheses were that the telencephalon consists of: (a) a sensorimotor system which generates motor activity from sensory input and (b) a selection system which makes choices from possible motor programs. It was postulated that the selection system should fulfil the following requirements: be accessible for past and present events, have the capacity to process this information in a nondetermined way with a possibility for ordering, and have access to motor-affecting systems (the sensorimotor system). The ability of the selection system to correlate information in a nonpredetermined way was considered most important. In short: The selection system should be able to associate any information in any combination, and have the capability for internal control of neuronal activity and external selection of motor programs (see Fig. 1A.) Xenopus laevis was chosen as a subject, since it has a relatively simple telencephalon, with characteristics that it shares with "primitive" species of different vertebrate classes, and because it is easy to maintain as a laboratory animal. The main method used was the determination of connections with HRP. The pallium was in the focus of attention, since it was considered to be the core of the selection system. Immunohistochemistry was used as an additional parameter to compare Xenopus laevis forebrain with those of other vertebrates. The results showed that the pallium can be subdivided into a rostral (third) and a caudal (two-thirds) entity. The rostral third is the main recipient for thalamic and olfactory input. The caudal two-thirds are linked up to the rostral third and have a refined microcircuitry. Efferents from the pallium remain restricted to the forebrain. The entire pallium consists of a network of intrinsic reciprocal connections and can be considered to be positioned between the medial pallium (hippocampus), septum, and amygdaloid complex (amygdala). As a whole this system targets the hypothalamus. The hypothalamus in turn projects into the striatum complex (striatum with anterior entopeduncular nucleus). The rostral dorsal pallium and the amygdaloid complex also project into the striatum complex. The striatum is positioned between the sensory input from the thalamus and olfactory bulbs, and the motor output to the medulla. It is concluded, on the basis of its straightforward input-output relations and uniform appearance, that the striatum complex fulfils the requirements for a sensorimotor system. The pallium together with the septum, amygdaloid complex, and hypothalamus fulfils the requirements for a selection system.(ABSTRACT TRUNCATED AT 400 WORDS)

Reticulospinal neurons, locomotor control and the development of tailswimming in Xenopus.

The development of early swimming in Xenopus occurs early during the embryonic period and within a few hours. Between stages 25 and 33 the central nervous system reaches a state of 'critical mass' at which the for swimming necessary body structures have partly developed, thus enabling the embryo to move through the water. The pattern of undulatory body movements is formed within the pattern generators in the central nervous system (CNS) involving different types of neurons in the spinal cord and brainstem. Tailswimming in Xenopus embryos can be evoked by tactile stimuli, light or vibrations. Here the development of brainstem-spinal connections and their possible role in swimming caused by external stimuli will be discussed. It is now clear that reticulospinal neurons are among the first neurons that differentiate within the CNS, their axons enter the spinal cord when the first swimming movements occur, that they are active in a motoneuron-like fashion, during--and involved in the control of early tailswimming. Among the reticulospinal neurons only the Mauthner cell seems to serve a command function.

The development of primary afferents to the lumbar spinal cord in Xenopus laevis.

The development of dorsal root ganglia (DRG) and the primary afferent system of the hindlimb was studied during metamorphosis in Xenopus laevis larvae. The first DRG cells appeared at stage 40 and at stage 48 the first primary afferent fibers entered the lumbar spinal cord where they bifurcated into ascending and descending branches. Primary afferent fibers and the dendrites of the secondary motoneurons contacted first in a lateral (state 56) and later (stage 58) in a dorsomedial terminal field. Reflexogenic hindlimb behaviour at stage 56 concurred with the presence of the lateral terminal field and many unipolar dorsal root ganglion cells.

The development of serotonergic raphespinal projections in Xenopus laevis.

The development of serotonin-immunoreactive neurons in the central nervous system of Xenopus laevis larvae has been studied with special emphasis on the development of the raphe nuclei and raphespinal projections. The first serotonergic neurons were observed in the rostral part of the brain stem at stage 25, only 28 hr after fertilization. By stage 28 some 20 serotonin-immunoreactive neurons were found in the rostral part of the brain stem, bearing small protrusions on the ventromedial side of the soma. These initial axonal outgrowths reach the rostral part of the spinal cord at stage 32. By stage 35/36 the growth cones of the descending serotonergic axons in the spinal cord have reached the level of the anus (10th to 15th myotome). Up to stage 45 the majority of the descending serotonergic axons was found in the dorsolateral part of the marginal zone of the spinal cord. After stage 45 some serotonergic axons were also found scattered over other parts of the spinal marginal zone. Collateral branches were first observed in the caudal part of the brain stem at stage 35/36. Later they occurred also in the rostral (stage 43) and caudal (stage 50) spinal cord, usually on fibers in the ventral half of the spinal cord. The number of serotonergic neurons in the central nervous system (brain stem and hypothalamus) increased steadily throughout development until stage 45. After that the total number of serotonergic neurons in the central nervous system increased about two times faster than the number of serotonergic neurons in the raphe nuclei, due to a massive increase of serotonergic neurons in the hypothalamus. The present study shows that young, just differentiated raphe neurons already contain serotonin. The generation of these neurons appears to take place in the ventricular zone (matrix) of the brain stem between the caudal border of the mesencephalon and the entrance of the nervus octavus. From here these neurons seem to migrate to their final destination. The distribution of serotonin-immunoreactive neurons in the brain stem suggests that a superior (not described so far in Anura) and an inferior raphe nucleus can be distinguished in Xenopus. A rostrocaudal gradient seems to be present in the production of serotonergic neurons which project to the spinal cord. Spinal projections from the raphe nuclei are particularly extensive from the nucleus raphes inferior and gradually decrease rostralwards. In the rostral part of the nucleus raphes superior almost no neurons projecting to the spinal cord are found.

The development of the dendritic organization of primary and secondary motoneurons in the spinal cord of Xenopus laevis. An HRP study.

During embryonic and larval development of the clawed toad, Xenopus laevis, two different populations of motoneurons appear in the spinal cord. In this study the development of primary motoneurons which innervate the axial musculature (used during embryonic locomotion) and of secondary motoneurons which innervate the extremity musculature (used for locomotion during metamorphosis and thereafter) was analyzed with horseradish peroxidase (HRP) as a neuronal marker. After application of HRP to the axial musculature (rostral five postotic myotomes) the first labeled primary motoneurons were found at stage 24/25. During development gradually more labeled neurons were observed. These primary motoneurons send their dendrites into the marginal zone (white matter). At first only dorsal and lateral dendrites develop (stages 25-33), followed by ventral dendrites (stage 37/38). Up till stage 48 the developing dendrites extend throughout the marginal zone. Hereafter the marginal zone increases particularly at the dorsolateral edge, a development which is not followed by the dendrites of the primary motoneurons. The dendrites of mature primary motoneurons (stages 58-62) occupy the ventral and ventrolateral parts of the marginal zone. At stage 48, shortly after the hindlimb bud arises (stage 46, early metamorphosis), the first neurons related to this developing extremity could be labeled in the ventrolateral part of the lumbar spinal cord. At first these secondary motoneurons bear only a few dorsal dendrites of which only the tips reache out in the adjacent white matter. Already at stage 50 these dorsal dendrites have invaded the whole dorsolateral part of the marginal zone. Also the first ventral dendrites were observed at this stage. Later, at stage 53/54 also some ventral dendrites have reached the white matter together with a few lateral dendrites. At these early metamorphic stages already some primary afferent fibers were found making contact with the dorsomedial dendrites. At stage 58 for the first time recurrent axon collaterals were found, which extend into the ventromedial part of the marginal zone. The development of motoneurons in the spinal cord seems to be characterized by two phases: (1) establishment of contacts between motoneurons and target muscles, and (2) subsequent formation of connections of these motoneurons with other nerve cells within the central nervous system. The dendrites of primary motoneurons follow the development of the marginal zone, while dendrites of secondary motoneurons develop into an already well developed marginal zone.(ABSTRACT TRUNCATED AT 400 WORDS)

The innervation of some proboscis structures involved in feeding behavior of the blowfly (Calliphora vicina).

The microtopology of the motoneurons involved in protraction and retraction of the proboscis of the blowfly (Calliphora vicina) has been studied. In addition, taste input from the labellar hairs was investigated. As a result of this study it appears that protraction movements are controlled by two while retraction movements are guided by three motoneurons on each side. The neurons in each group apear to be in ipsicontralateral communication with each other. The musculi protractores fulcri (MPF) probably contain a proprioceptive cell group which projects to the MPF motoneurons. It is proposed that the proboscis motor system can be modulated by proprioception as well as by chemosensory labellar input. Neurosecretory cells may be involved in adjusting muscle power.

Early development of descending pathways from the brain stem to the spinal cord in Xenopus laevis.

The early development of descending pathways from the brain stem to the spinal cord has been studied in Xenopus laevis tadpoles. The relatively protracted development of this permanently aquatic amphibian as well as its transparency during development make this animal particularly attractive for experimental studies. Between the 5th and 10th myotome the spinal cord was crushed with a thin needle and dry horseradish peroxidase (HRP) crystals were applied. After a survival time of one day the tadpoles were fixed and the brain and spinal cord were stained as a whole according to a modification of the heavy metal intensification of the DAB-reaction, cleared in cedarwood oil and examined as wholemounts. At stage 28 (the neural tube has just closed) the first brain stem neurons projecting to the spinal cord were found in what appear to be the nucleus reticularis inferior and -medius. At this stage of development the first, uncoordinated swimming movements can be observed. At stage 30/31 (the tailbud is visible) both Mauthner cells project to the spinal cord as well as the interstitial nucleus of the fasciculus longitudinalis medialis situated in the mesencephalon. Towards stage 35/36 (the tail is now clearly visible), a more extensive reticulospinal innervation of the spinal cord appears, now including cells of the nucleus reticularis superior. At this stage also the first vestibulospinal and raphespinal projections were found. At stage 43/44 (the tadpoles have now a well-developed tail) the pattern of reticulospinal projections appears to be completed with the presence of labeled neurons in the nucleus reticularis isthmi. From stage 43/44 on, the number of HRP-positive cells is steadily increasing. At stage 47/48, when the hindlimb buds appear, the descending projections to the spinal cord are comparable with the adult situation except for the absence of a rubrospinal and a hypothalamospinal projection. The observations demonstrate that already very early in development reticulospinal fibers and, somewhat later, Mauthner cell axons and vestibulospinal fibers innervate the spinal cord. Furthermore, a caudorostral gradient appears to exist with regard to the development of descending projections to the spinal cord. However, the interstitial nucleus of the fasciculus longitudinalis medialis forms an exception to this rule.