Muscle and nerve in muscular dysgenesis in the mouse at birth: sprouting and multiple innervation

Development of the mouse neuromuscular junction in the absence of regulated secretion

To investigate the role of neurotransmitter secretion in the development and stabilization of synapses, the innervation of the diaphragm and intercostal muscles was studied in munc18-1 null mutant mice, which lack regulated secretion. We found that this mutant is completely devoid of both spontaneous and evoked neuromuscular transmission throughout embryonic development. At embryonic day (E) 14, axonal targeting and main branching of the phrenic nerve were normal in this mutant, but tertiary branches were elongated and no terminal branches were observed at this stage, in contrast to control littermates. Acetylcholinesterase staining was observed in the endplate region of mutant muscle from E14 onwards, but not as dense and confined to spots as in controls. Acetylcholine receptor staining was also present in the endplate region of the mutant muscle. In this case, the staining density and the concentration in spots (clusters) were similar to controls, but the distribution of these clusters was less organized. Starting at E15, some receptor clusters co-localized with nerve terminal staining, suggesting synapses, but most clusters remained a-neural. Electron microscopical analysis confirmed the presence of synaptic structures in the mutant. Between E14 and birth, the characteristic staining pattern of nerve branches gradually disappeared in the mutant until, at E18, an elaborate meshwork of nerve fibers with no apparent organization remained. In the same period, most of the motor neuronal cell bodies in the spinal cord degenerated. In contrast, sensory ganglia in the dorsal root showed no obvious degeneration. These data suggest that regulated secretion is not essential for initial axon path finding, clustering of acetylcholine receptors, acetylcholinesterase or the formation of synapses. However, in the absence of regulated secretion, the maintenance of the motor neuronal system, organization of nerve terminal branches and stabilization of synapses is impaired and a-neural postsynaptic elements persist.

Neuromuscular synapses mediate motor axon branching and motoneuron survival during the embryonic period of programmed cell death

The embryonic period of motoneuron programmed cell death (PCD) is marked by transient motor axon branching, but the role of neuromuscular synapses in regulating motoneuron number and axonal branching is not known. Here, we test whether neuromuscular synapses are required for the quantitative association between reduced skeletal muscle contraction, increased motor neurite branching, and increased motoneuron survival. We achieved this by comparing agrin and rapsyn mutant mice that lack acetylcholine receptor (AChR) clusters. There were significant reductions in nerve-evoked skeletal muscle contraction, increases in intramuscular axonal branching, and increases in spinal motoneuron survival in agrin and rapsyn mutant mice compared with their wild-type littermates at embryonic day 18.5 (E18.5). The maximum nerve-evoked skeletal muscle contraction was reduced a further 17% in agrin mutants than in rapsyn mutants. This correlated to an increase in motor axon branch extension and number that was 38% more in agrin mutants than in rapsyn mutants. This suggests that specializations of the neuromuscular synapse that ensure efficient synaptic transmission and muscle contraction are also vital mediators of motor axon branching. However, these increases in motor axon branching did not correlate with increases in motoneuron survival when comparing agrin and rapsyn mutants. Thus, agrin-induced synaptic specializations are required for skeletal muscle to effectively control motoneuron numbers during embryonic development.

Development of cholinergic terminals around rat spinal motor neurons and their potential relationship to developmental cell death

Neuron death seems to be regulated mainly by postsynaptic target cells. In chicks, nicotinic antagonists such as alpha-bungarotoxin (alphaBT) can prevent normal cell death of somatic motor neurons (SMNs). For this effect, however, alphaBT could be acting at peripheral neuromuscular junctions and/or central cholinergic synapses. To investigate this issue, we first studied the development of cholinergic terminals in the rat spinal cord by using vesicular acetylcholine transporter immunocytochemistry. Labeled terminals were seen in the ventral horn as early as embryonic day 15 (E15), the beginning of the cell death period. Thus, central cholinergic synapses form at the correct time and place to be able to influence SMN death. We next added alphaBT to organotypic, spinal slice cultures made at E15. After 5 days in vitro, the number of SMNs in treated cultures was substantially greater than in control cultures, indicating that alphaBT can reduce SMN cell death in rats as it does in chicks. Moreover, peripheral target removal led to extensive loss of SMNs, and such a loss occurred even in the presence of alphaBT, indicating the necessity of peripheral target for the alphaBT effect. Finally, to determine whether central cholinergic terminals also may be involved in SMN death, we delayed the alphaBT treatment until after central cholinergic terminals had disappeared from the slice cultures. The increased number of surviving SMNs, even in the absence of central terminals, argued that alphaBT acts at peripheral, not central, cholinergic synapses to rescue SMNs from developmental cell death.

Promotion of motoneuron survival and branching in rapsyn-deficient mice

Inhibition of programmed cell death of motoneurons during embryonic development requires the presence of their target muscle and coincides with the initial stages of synaptogenesis. To evaluate the role of synapse formation on motoneuron survival during embryonic development, we counted the number of motoneurons in rapsyn-deficient mice. Rapsyn is a 43 kDa protein needed for the formation of postsynaptic specialisations at vertebrate neuromuscular synapses. Here we show that the rapsyn-deficient mice have a significant increase in the number of motoneurons in the brachial lateral motor column during the period of naturally occurring programmed cell death compared to their wild-type littermates. In addition, we observed an increase in intramuscular axonal branching in the rapsyn-deficient diaphragms compared to their wild-type littermates at embryonic day 18.5. These results suggest that deficits in the formation of the postsynaptic specialisation at the neuromuscular synapse, brought about by the absence of rapsyn, are sufficient to induce increases in both axonal branching and the survival of the innervating motoneuron. Moreover, these results support the idea that skeletal muscle activity through effective synaptic transmission and intramuscular axonal branching are major mechanisms that regulate motoneuron survival during development.

Neuromuscular activity blockade induced by muscimol and d-tubocurarine differentially affects the survival of embryonic chick motoneurons

To understand better how spontaneous motoneuron activity and intramuscular nerve branching influence motoneuron survival, we chronically treated chicken embryos in ovo with either d-tubocurarine (dTC) or muscimol during the naturally occurring cell death period, assessing their effects on activity by in ovo motility measurement and muscle nerve recordings from isolated spinal cord preparations. Because muscimol, a GABA(A) agonist, blocked both spontaneous motoneuron bursting and that elicited by descending input but did not rescue motoneurons, we conclude that spontaneous bursting activity is not required for the process of normal motoneuron cell death. dTC, which rescues motoneurons and blocks neuromuscular transmission, blocked neither spontaneous nor descending input-elicited bursting and early in the cell death period actually increased burst amplitude. These changes in motoneuron activation could alter the uptake of trophic molecules or gene transcription via altered patterns of [Ca(2+)](i), which in turn could affect motoneuron survival directly or indirectly by altering intramuscular nerve branching. A good correlation was found between nerve branching and motoneuron survival under various experimental conditions: (1) dTC, but not muscimol, greatly increased branching; (2) the removal of PSA from NCAM partially reversed the effects of dTC on both branching and survival, indicating that branching is a critical variable influencing motoneuron survival; (3) muscimol, applied with dTC, prevented the effect of dTC on survival and motoneuron bursting and, to a large extent, its effect on branching. However, the central effects of dTC also appear to be important, because muscimol, which prevented motoneuron activity in the presence of dTC, also prevented the dTC-induced rescue of motoneurons.

Neuromuscular development in the avian paralytic mutant crooked neck dwarf (cn/cn): further evidence for the role of neuromuscular activity in motoneuron survival

Neuromuscular transmission and muscle activity during early stages of embryonic development are known to influence the differentiation and survival of motoneurons and to affect interactions with their muscle targets. We have examined neuromuscular development in an avian genetic mutant, crooked neck dwarf (cn/cn), in which a major phenotype is the chronic absence of the spontaneous, neurally mediated movements (motility) that are characteristic of avian and other vertebrate embryos and fetuses. The primary genetic defect in cn/cn embryos responsible for the absence of motility appears to be the lack of excitation-contraction coupling. Although motility in mutant embryos is absent from the onset of activity on embryonic days (E) 3-4, muscle differentiation appears histologically normal up to about E8. After E8, however, previously separate muscles fuse or coalesce secondarily, and myotubes exhibit a progressive series of histological and ultrastructural degenerative changes, including disarrayed myofibrils, dilated sarcoplasmic vesicles, nuclear membrane blebbing, mitochondrial swelling, nuclear inclusions, and absence of junctional end feet. Mutant muscle cells do not develop beyond the myotube stage, and by E18-E20 most muscles have almost completely degenerated. Prior to their breakdown and degeneration, mutant muscles are innervated and synaptic contacts are established. In fact, quantitative analysis indicates that, prior to the onset of muscle degeneration, mutant muscles are hyperinnervated. There is increased branching of motoneuron axons and an increased number of synaptic contacts in the mutant muscle on E8. Naturally occurring cell death of limb-innervating motoneurons is also significantly reduced in cn/cn embryos. Mutant embryos have 30-40% more motoneurons in the brachial and lumbar spinal cord by the end of the normal period of cell death. Electrophysiological recordings (electromyographic and direct records form muscle nerves) failed to detect any differences in the activity of control vs. mutant embryos despite the absence of muscular contractile activity in the mutant embryos. The alpha-ryanodine receptor that is genetically abnormal in homozygote cn/cn embryos is not normally expressed in the spinal cord. Taken together, these data argue against the possibility that the mutant phenotype described here is caused by the perturbation of a central nervous system (CNS)-expressed alpha-ryanodine receptor. The hyperinnervation of skeletal muscle and the reduction of motoneuron death that are observed in cn/cn embryos also occur in genetically paralyzed mouse embryos and in pharmacologically paralyzed avian and rat embryos. Because a primary common feature in all three of these models is the absence of muscle activity, it seems likely that the peripheral excitation of muscle by motoneurons during normal development is a major factor in regulating retrograde muscle-derived (or muscle-associated) signals that control motoneuron differentiation and survival.

Barium currents in developing skeletal muscle cells of normal and mutant mice foetuses with 'muscular dysgenesis'

The ontogenesis of Ca channel activities was studied in the developing myotubes of normal mice and mutant mice foetuses with 'Muscular Dysgenesis'. The ionic current through Ca channels was measured with Ba2+ as charge carrier using the whole cell clamp technique. All dissociated myotubes from foetuses (14th to 18th day of gestation) showed two distinct inward Ba currents: a low threshold, transient current (T-type) and a high threshold sustained current. In normal myotubes, T-type current density increased from the 14th day to the 16th day of gestation. After day 16, T-type current density decreased gradually until birth. Similar changes in T-type current density were observed in developing dysgenic myotubes where the current density was about 40% of that measured in normal myotubes throughout the prenatal period studied. The high threshold sustained current (L-type current) density increased gradually with age in normal myotubes while absent in dysgenic muscle. The latter, regardless of age, showed a high threshold current (Idys) which is distinct from the L-type current. Idys density did not change during the prenatal myogenesis period studied.

Developmental modulation of physicochemical variants of the tailed asymmetric (16S) acetylcholinesterase by neuromuscular activity and innervation in the mouse embryo

We have studied the physicochemical properties of acetylcholinesterase (AChE) during embryonic development of normal and functionally impaired mouse skeletal muscle, focusing on the tailed asymmetric (16S) form of the enzyme. The muscle-specific 16S AChE exists in two different variants. One is associated with extracellular matrix and is high-salt soluble (HSS, also termed hydrophilic AChE), whereas the other form is anchored to cell membranes and is detergent extractable (DE, or hydrophobic AChE). Before innervation during normal embryonic development, both hydrophilic and hydrophobic 16S AChE exist in equal amounts. After muscle innervation, there was an increase (amounting three-fold on E18) in the levels of hydrophilic vs. hydrophobic 16S AChE. This alteration of the relative proportions of the two variants of 16S AChE did not occur in chronically inactive muscles either from the mouse mutant, muscular dysgenesis, or from tetrodotoxin-treated mouse embryos. Taken together with previous reports, the present results suggest that postsynaptic membrane depolarization-induced Ca2+ fluxes are important in modulating not only the synthesis of 16S AChE, but also the relative proportions of both physicochemical variants of this molecular form of AChE.

Neuromuscular development following tetrodotoxin-induced inactivity in mouse embryos

Developmental aspects of the neuromuscular system in mouse embryos chronically paralyzed in utero with tetrodotoxin (TTX) between embryonic days 14 and 18 were studied using biochemical and histological methods. The number of lumbar spinal motoneurons (MNs) was higher in inactive embryos than in controls suggesting a decreased motoneuron cell death. In association with the increase in MN number, choline acetyltransferase activity was significantly increased in both spinal cord and peripheral synaptic sites. Paralyzed muscles exhibited a decreased number of mature myofibers and the nuclei were centrally located. Creatine kinase activity was greatly decreased and total acetylcholine receptor and receptor cluster numbers per myofiber were significantly increased in paralyzed muscles. A similar pattern of changes occurs in the neuromuscular system of the mutant mouse muscular dysgenesis (mdg). However, in contrast to the mdg mutant, tetrodotoxin-treated muscles were similar to controls in their innervation pattern, in the ultrastructural aspects of the excitation-contraction coupling system (i.e., dyads and triads) and in the extent of dihydropyridine binding. Thus, neuromuscular inactivity is not sufficient to impair the pattern of muscle innervation or the appearance of either the triadic junctions or dihydropyridine receptors. These results indicate that alterations of dihydropyridine binding sites and triads in muscular dysgenesis cannot be accounted for by inactivity but rather must reflect a more primary defect involving the structural gene(s) regulating the development of one or more aspects of muscle differentiation.

Appearance of the slow Ca conductance in myotubes from mutant mice with "muscular dysgenesis"

Voltage gated Ca conductance in skeletal muscle cells from mice with muscular dysgenesis (mdg/mdg) and from normal mice was studied using the whole cell recording technique. The physiological properties of the myotubes from the mutant mice (uncoupling of excitation-contraction, deficiency in the voltage gated slow Ca conductance) were changed to normal when the mdg/mdg myotubes were cocultured with spinal cord cells from normal mice. Spinal cord cells from mutant mice failed to induce normal muscle activity in the mutant myotubes. In aged mutant myotubes cultured without spinal cord cells, the slow Ca conductance sometimes developed, although with smaller amplitude. The number of mdg/mdg myotubes with partial development of the slow Ca conductance increased with the age of the culture. E-C coupling was never established in aged mutant myotubes. The phenotypic reversion did not require functional synaptic transmission since it was also obtained when neuromuscular transmission was chronically blocked with alpha-bungarotoxin (4-40 micrograms/ml).

The muscular dysgenesis mutation in mice leads to arrest of the genetic program for muscle differentiation

Muscular dysgenesis (mdg) is a mutation in mice which causes the failure of excitation-contraction coupling in skeletal muscle. Although the sarcolemma, the sarcoplasmic reticulum, and the contractile apparatus all maintain nearly normal function, sarcolemmal depolarization fails to cause calcium release from the sarcoplasmic reticulum. Recently, the primary genetic defect in this mutation was shown to be located in the structural gene for the dihydropyridine receptor. We have examined the developmental expression from Fetal Day 15 onward, in normal and mutant muscle, of several unidentified genes as well as genes which are known markers of muscle differentiation. We find that the majority of mRNA sequences are found at similar concentrations in normal and dysgenic muscles at birth. Many differentiation-related genes also are expressed at normal levels early during myogenesis in mutant mice. However, as late fetal development progresses in dysgenic muscle, the mRNA concentrations for these genes fail to undergo the rapid rise which is characteristic of normal muscle. Several additional, unidentified genes, which normally would be down-regulated during development, remain expressed at a high level in dysgenic muscle. Thus, the primary absence of a functional dihydropyridine receptor appears to prevent the changes in gene expression which are necessary for maturation of skeletal muscle.

The regulation of intramuscular nerve branching during normal development and following activity blockade

In vertebrates, approximately 50% of the lumbosacral motoneurons die during a short period of development that coincides with synaptogenesis in the limb. Although it has been postulated that these motoneurons die because they fail to obtain adequate trophic support from the muscles, it is not clear how this factor is supplied. The mechanism by which activity blockade prevents motoneurons cell death is also unknown. In order to begin to understand the nature of these proposed trophic interactions, we have examined the temporal sequence of axonal invasion and ramification within two muscles of the chick hindlimb, the predominantly slow iliofibularis and the fast posterior iliotibialis, during the cell death period. We found striking differences in intramuscular nerve ingrowth and branching between fast and slow muscle. We also observed differences in the molecular composition of fast and slow myotubes that may contribute to the nerve pattern differences. In addition, we observed a progressive increase in the degree of intramuscular nerve fasciculation as well as a precise temporal sequence of nerve branching. The earliest detectable response to chronic curarization was a dramatic decrease in the degree of intramuscular nerve fasciculation. Activity blockade also greatly enhanced nerve branching within the muscles from the time that nerve branches normally formed, and, additionally, interfered with the normal cessation of axon growth. Our results support the idea that nerve endings are the sites of trophic uptake. Furthermore, although our results do not allow us to exclude other activity-dependent influences on motoneuron survival, they suggest the following testable hypotheses: (1) the normal regulation of motoneuron survival may result from the precise control of intramuscular nerve branching, (2) activity blockade may increase motoneuron survival by enhancing intramuscular nerve branching, and (3) anything which affects this complex process of nerve branching may also alter motoneuron survival.

Abnormal enwrapment of intramuscular axons by distal Schwann cells with defective basal lamina in the muscular dysgenic mouse embryo

In the muscular dysgenic (mdg/mdg) mouse embryo, both muscle and nerve are affected early during embryogenesis, from Embryonic Day 13 (E13). We now find that the mutation affects not only the degree of differentiation of the muscle and the pattern of motor innervation but also the relationship between Schwann cell and axon. We studied the sciatic nerve of normal and mdg/mdg embryos between E13 and E18 at the ultrastructural level. We found that in mdg/mdg nerve, (1) Schwann cells do not totally enwrap the growing axons in their most distal part, close to the growth cone, and (2) the terminal Schwann cells do not correctly surround the nerve endings and seal the corresponding synaptic contacts. Moreover, both types of mutant Schwann cell lack a normal electron-dense basal lamina. We found that there is an excess of axons relative to the Schwann cell population in the intramuscular portions of the mdg/mdg sciatic nerve. Our observations point toward a possible defect of the mechanism of migration and maturation of Schwann cells. Such a defect may in turn affect primarily or secondarily the mutual influences between Schwann cell and axon and lead to some or all of the major abnormalities observed in the mdg/mdg neuromuscular system, namely, multifocal polyinnervation, immature axon-myotube contacts, and abnormal T-tubule-sarcoplasmic reticulum junctions.

An electrophysiological study of skeletal muscle fibres in the 'muscular dysgenesis' mutation of the mouse

Experiments were performed on muscles of 18-19 day mice fetuses affected with muscular dysgenesis (mdg). Action potentials generated by electrical stimulation or potassium depolarization failed to trigger muscle contraction in mdg muscle fibres. By contrast, muscle contraction could be obtained by caffeine (15 mM) and, to a lesser degree, by nerve stimulation. We conclude that a defect in excitation-contraction (E-C) coupling is the cause of muscle paralysis. An early after potential (EAP) was present in the decay phase of the action potential and a potential 'creep' occurred in response to hyperpolarizing current pulses which can be taken as evidence for the presence of T-tubules in mdg muscle fibres. Data obtained from square pulse analysis and EAP measurements indicate larger input impedance and membrane time constant in mdg as compared to controls, which contrasts with similar surface membrane time constant (as estimated from the foot of the action potential) in both types of muscle. The excitability of the T-tubule system was tested by recording action potentials at early stages of TTX (5 X 10(-7) M) perfusion or washout in mdg and control muscles. In both cases, the action potentials decreased in amplitude and rate of rise and displayed two peaks, the second of which was suppressed by detubulation using the formamide treatment. This indicates action potential generation in the T-tubule membrane of mdg muscles. In all the impaled muscle fibers, nerve stimulation evoked epps which were accompanied by a weak local contraction in relation with Ca2+ influx through postsynaptic channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Excitation-contraction uncoupling in the developing skeletal muscle of the muscular dysgenesis mouse embryo

The muscular dysgenesis recessive autosomal mutation is characterized by a total lack of muscular contraction and a myofibrillar non-organization. Many abnormalities involved in the excitation-contraction coupling are found in mdg/mdg myotubes: 1) the internal structural organization of the membrane coupling between the sarcoplasmic reticulum (SR) and the transverse (T)-tubule forming the triadic association is defective: the triad number is decreased in the muscle and there are a lack of periodic densities between the SR and T-tubule apposed membranes. 2) the voltage-dependent Ca2+ channel contents, identified by binding with the specific blocker PN 200-110, are decreased. The two fast (30 ms) and slow (100 ms) Ca2+ currents present in normal myotubes are absent in mdg/mdg myotubes in vitro. 3) the Ca2+-dependent K+ conductance triggering an action potential followed by a long lasting after hyperpolarization (ahp) is absent in mdg/mdg myotubes. This indicates a lack of the free intracellular Ca2+ increased by the action potential. These results suggest that: 1) the lack of differentiated triadic junctions is directly correlated with very low amounts of voltage-dependent Ca2+ channels; 2) the low amount of Ca2+ channels results directly in decreased Ca2+ currents; 3) the decreased Ca2+ currents are the consequence of the low intracellular Ca2+ concentration which is not sufficient to trigger a contraction. However, the addition of normal motoneurones to mdg/mdg myotubes in culture induces, few days later, an increase in Ca2+ currents.

The development of motoneurons in the embryonic spinal cord of the mouse mutant, muscular dysgenesis (mdg/mdg): survival, morphology, and biochemical differentiation

Motoneuron development was studied in the spinal cord of the mouse mutant, muscular dysgenesis, between embryonic days (E) 13 and 18. Dysgenic embryos are characterized by the absence of neuromuscular activity (motility) and exhibit a number of other striking changes in neuromuscular development. Many of these changes have also been observed in chick embryos chronically treated with neuromuscular blocking agents that suppress motility. Motoneuron survival, as well as several other aspects of neuronal development, was examined in the thoracic and lumbar spinal cords of mutant and control embryos. There was a significant decrease in motoneuron numbers in control embryos indicating the presence of naturally occurring cell death in the mouse spinal cord. At all ages examined, the dysgenic embryos had significantly more healthy and significantly fewer degenerating motoneurons than controls. There were no differences in the number of dorsal root ganglion neurons or in any of the other morphometric parameters examined between mutant and control embryos. Creatine kinase activity, a marker for myofiber maturation, was significantly reduced in the limb musculature of mutant embryos. Choline acetyltransferase activity was significantly increased in the spinal cord of mutant embryos. No significant differences were observed in spinal cord levels of acetylcholinesterase activity between control and mutant embryos. The absence of muscle contractions in the dysgenic mouse is associated with a number of changes in neuromuscular development, including a substantial reduction of naturally occurring motoneuron death.

Globular and asymmetric acetylcholinesterase in frog muscle basal lamina sheaths

After denervation in vivo, the frog cutaneus pectoris muscle can be led to degenerate by sectioning the muscle fibers on both sides of the region rich in motor endplate, leaving, 2 wk later, a muscle bridge containing the basal lamina (BL) sheaths of the muscle fibers (28). This preparation still contains various tissue remnants and some acetylcholine receptor-containing membranes. A further mild extraction by Triton X-100, a nonionic detergent, gives a pure BL sheath preparation, devoid of acetylcholine receptors. At the electron microscope level, this latter preparation is essentially composed of the muscle BL with no attached plasmic membrane and cellular component originating from Schwann cells or macrophages. Acetylcholinesterase is still present in high amounts in this BL sheath preparation. In both preparations, five major molecular forms (18, 14, 11, 6, and 3.5 S) can be identified that have either an asymmetric or a globular character. Their relative amount is found to be very similar in the BL and in the motor endplate-rich region of control muscle. Thus, observations show that all acetylcholinesterase forms can be accumulated in frog muscle BL.

Disease expression in +-/+- ----mdg/mdg mouse chimeras: evidence for an extramuscular component in the pathogenesis of both dysgenic abnormal diaphragm innervation and skeletal muscle 16 S acetylcholinesterase deficiency

Homozygous mdg/mdg mice die at birth and express a syndrome of abnormalities, the most striking of which is a gross failure of skeletal muscle development. Recently, additional abnormalities in the development of nerve-muscle relationships have been recognized; in particular, on muscle fibers within the diaphragm, motor end plates are inappropriately dispersed and, in all muscles, there is a paucity of the 16 S form of acetylcholinesterase (AChE). These abnormalities could result entirely as secondary consequences of the primary muscle defect or from expression of the mdg defect in additional cell types, e.g., motor neurons. To determine if the muscle genotype alone is responsible for these defects in dysgenic mice, chimeras composed of both dysgenic and normal cells have been investigated. Different glucosephosphate isomerase variants existed in the mdg/mdg and normal cells comprising these chimeras and the mutant, normal, or mosaic genotypes of chimera diaphragm and skeletal muscle was estimated by measuring the relative proportions of each isozyme. In two chimeras, the diaphragm innervation pattern was revealed by AChE cytochemistry and in both, discrete regions of abnormally dispersed and normally restricted motor end-plate zones were observed. No correlation between these patterns of innervation and the assessed genotype of the muscle fibers existing in each area was observed. The relative 16 S AChE content in the limbs of four chimeras was found to range from 2.5 to 42.0%. Here also, no correlation between 16 S AChE content and the muscle genotype was observed. The results of these investigations are not consistent with a model of mdg/mdg pathogenesis in which only the skeletal muscle is primarily affected; an extramuscular deficiency responsible for at least part of the full mdg/mdg syndrome is therefore suggested.

Congenital canine myasthenia gravis: I. Deficient junctional acetylcholine receptors

The neuromuscular junction was studied during growth in two breeds of dog with a congenital familial form of myasthenia gravis (CMG): the Jack Russell terrier and the springer spaniel. Light microscopy revealed no difference in endplate size or nerve terminal morphology in age-matched CMG dogs and unaffected littermates. At all ages there was in individual muscle fibers of CMG dogs an approximately 75% lower postsynaptic membrane density of acetylcholine receptors (AChR). There was no evidence that this abnormality had an autoimmune basis. Measurements of miniature endplate potential amplitude in relation to fiber diameter revealed that the low density of AChR in the postsynaptic membrane of CMG dogs remained constant and did not change with the marked progression of muscle weakness during growth. Antigenic determinants of AChR were deficient in CMG muscle to the same extent as alpha-bungarotoxin (alpha BT) binding sites. Thus, a low density of AChR in the muscle's postsynaptic membrane appears to be the principal abnormality of CMG. The data suggest that the normal increase in number of acetylcholine (ACh) quanta released by nerve impulse may not fully compensate for the reduction in depolarization produced by a single quantum as the muscle fiber diameter increases. This could cause progression of weakness during growth. In addition to being a useful animal model of a rare form of CMG that resembles one form of human CMG, canine CMG offers a unique model for investigating events of synaptogenesis at the neuromuscular junction.

Relationship of genotype and in vitro contractility in mdg/mdg in equilibrium +/+ "mosaic" myotubes

Muscular dysgenesis (mdg) in the mouse is an autosomal recessive lethal disorder that is manifested by a gross failure of skeletal muscle development. In vitro mdg/mdg myoblasts proliferate normally and fuse successfully into myotubes, but these myotubes fail to contract either spontaneously or in response to physiological stimuli despite the presence of effective contractile elements and an ability to propagate action potentials normally. We have determined that mdg/mdg and +/+ myoblasts are capable of fusing in vitro to form "mosaic" myotubes which typically express an apparently normal contractile phenotype. Electrophoretic analysis of the relative activities of myotube glucosephosphate isomerase (GPI-1) isozymes provided a means of estimating the proportions of myonuclei of each genotype within individual myotubes. Only a very small proportion of genotypically normal myonuclei were required for expression of an apparently normal contractile phenotype.

Early effects in vitro of the muscular dysgenesis mutation on nervous tissue in the mouse

Muscular dysgenesis (mdg), a disease expressed in embryonic mice, severely affects the formation and differentiation of skeletal musculature. The focus of this investigation was an analysis of dysgenic nervous tissue with special attention centered on interactions between muscle and nerve cells in vitro. Results indicate that mdg/mdg spinal cord cells can form functional neuromuscular junctions in nerve-muscle cocultures and induce contractions in dysgenic muscle. However, dysgenic spinal cord cells induce fewer myotubes to contract and result in a delayed induction of dysgenic myotube contractile activity. Furthermore, mdg/mdg nervous tissue, or its conditioned medium, is associated with a higher incidence of morphologically abnormal myotube contractures. The results from this investigation demonstrate that there are functional abnormalities in both dysgenic muscle and nervous tissues which are stable and expressed for up to 3 weeks in vitro.

Extensive nerve overgrowth and paucity of the tailed asymmetric form (16 S) of acetylcholinesterase in the developing skeletal neuromuscular system of the dysgenic (mdg/mdg) mouse

In muscular dysgenesis (mdg) both skeletal muscle and motor innervation present an abnormal differentiation at birth (Rieger and Pinçon-Raymond. 1981). We establish in this report that the abnormalities of the motor innervation, extension of the innervation territory and nerve branching, and the low level of tailed asymmetric (16 S) acetylcholinesterase (AChE) are observed in the diaphragm as soon as the mutation is phenotypically expressed in the mdg/mdg embryo, at embryonic Day 13 1/2 or 14. The nerve-muscle interaction is thus abnormal at the very beginning of the formation of the neuromuscular system. The comparative electron microscopic study of normal and mutant muscle basal lamina shows that the newly formed mdg/mdg myotube lacks a basal lamina at Day 14. However, the biosynthesis of the basal lamina is not totally defective as, at Day 18, mdg/mdg myotubes possess basal lamina sheaths. Such delayed formation of extracellular material in the mutant may partly explain the abnormally low induction of 16 S AChE which is thought to be localized in the extracellular matrix and has potential important implications in nerve-muscle recognition and synapse stabilization.

Distribution and quantification of ACh receptors and innervation in diaphragm muscle of normal and mdg mouse embryos

Muscular dysgenesis (mdg) in the mouse is an autosomal recessive mutation, expressed in the homozygous state (in vivo and in vitro) as an absence of skeletal muscle contraction. The distribution of acetylcholine receptors (ACh R) in the diaphragms of phenotypically normal and dysgenic (mdg/mdg) embryos was studied from the 14th to 19th day of gestation by binding of 125I-alpha-bungarotoxin to the muscle, followed by autoradiography of longitudinally sectioned hemidiaphragms and/or of isolated muscle fibers. Localization of ACh R at putative motor end-plate regions begins 14 to 15 days in utero in both normal and dysgenic diaphragms. The distribution of high ACh R density patches is aberrantly scattered beyond the normal innervation pattern in dysgenic diaphragms. Isolated mutant fibers possess (1) multiple ACh R clusters, up to five per single fiber, (2) larger clusters of more variable morphology and variable receptor density than normal clusters, and (3) higher levels of extrajunctional receptors than normal fibers. These autoradiographic results correlate well with higher total level of toxin binding sites per diaphragm and per milligram protein in dysgenic vs normal muscle, as quantified from gamma counting of sucrose density gradient isolation of 125I-toxin-ACh R complexes. The dispersed distribution of ACh R patches on dysgenic muscle may be correlated with extensive phrenic nerve branching as demonstrated by silver impregnation technique. We suggest that the aberrant ACh R cluster distribution is a result of multiple innervation of single fibers from the branched nerve terminals. Possible causes of the excessive nerve branching in the mutant are discussed in light of generalized nerve sprouting found in paralyzed muscle.

Acetylcholinesterase of mammalian neuromuscular junctions: presence of tailed asymmetric acetylcholinesterase in synaptic basal lamina and sarcolemma

A sarcolemma-rich fraction can be isolated after subcellular fractionation of mouse intercostal muscles by sedimentation on a discontinuous sucrose gradient. The quantitative recovery of the acetylcholine receptor in this fraction is about 50%, which indicates the presence of a high proportion of postsynaptic membranes. Acetylcholinesterase (AcChoEase; EC 3.1.1.7) is found mainly in three different layers: the top layer, which contains soluble AcChoEase, the intermediate layer (fraction A), and the last, AcChoR-rich, layer (fraction C). The relative proportions of the molecular forms of AcChoEase are different in the three layers. The "16S" AcChoEase is in a higher proportion in both types of membrane fractions (A and C) compared to soluble AcChoEase. Both total AcChoEase and 16S AcChoEase are enriched in the A and C fractions. In the C fraction, the sequential use of homogenizations in the presence of detergent and high ionic strength allows the "solubilization" of two distinct AcChoEase pools. One is detergent-soluble and mainly composed of slow-sedimenting forms; the other one is detergent-insoluble, high-ionic strength-soluble, and composed mainly of collagen-like, tailed, asymmetric (16S) AcChoEase. Thus, most of the asymmetric AcChoEase is specifically localized in the synaptic extracellular matrix of the mammalian muscle fiber. However, in the A fraction, most of the 16S AcChoEase found is solubilized by detergent alone, suggesting an association with microsomal membranes. It may mean that at least some of the basal lamina-embedded 16S AcChoEase is preassembled intracellularly in the sarcoplasmic reticulum.

Evidence for a motor nerve growth factor

We review the evidence that a motor nerve growth factor released from muscle has wide ranging effects on the development and maintenance of muscle innervation. The actions of this putative factor on motor neurons are analogous to the actions of the well known nerve growth factor (NGF) on sympathetic and sensory neurons.