Spinal cord limb motor neurons in dystrophia myotonica

CaMKIIβ deregulation contributes to neuromuscular junction destabilization in Myotonic Dystrophy type I

BACKGROUND

Myotonic Dystrophy type I (DM1) is the most common muscular dystrophy in adults. Previous reports have highlighted that neuromuscular junctions (NMJs) deteriorate in skeletal muscle from DM1 patients and mouse models thereof. However, the underlying pathomechanisms and their contribution to muscle dysfunction remain unknown.

METHODS

We compared changes in NMJs and activity-dependent signalling pathways in HSALR and Mbnl1ΔE3/ΔE3 mice, two established mouse models of DM1.

RESULTS

Muscle from DM1 mouse models showed major deregulation of calcium/calmodulin-dependent protein kinases II (CaMKIIs), which are key activity sensors regulating synaptic gene expression and acetylcholine receptor (AChR) recycling at the NMJ. Both mouse models exhibited increased fragmentation of the endplate, which preceded muscle degeneration. Endplate fragmentation was not accompanied by changes in AChR turnover at the NMJ. However, the expression of synaptic genes was up-regulated in mutant innervated muscle, together with an abnormal accumulation of histone deacetylase 4 (HDAC4), a known target of CaMKII. Interestingly, denervation-induced increase in synaptic gene expression and AChR turnover was hampered in DM1 muscle. Importantly, CaMKIIβ/βM overexpression normalized endplate fragmentation and synaptic gene expression in innervated Mbnl1ΔE3/ΔE3 muscle, but it did not restore denervation-induced synaptic gene up-regulation.

CONCLUSIONS

Our results indicate that CaMKIIβ-dependent and -independent mechanisms perturb synaptic gene regulation and muscle response to denervation in DM1 mouse models. Changes in these signalling pathways may contribute to NMJ destabilization and muscle dysfunction in DM1 patients.

Peripheral neuropathy is linked to a severe form of myotonic dystrophy in transgenic mice

Myotonic dystrophy type 1 (DM1) is a multisystem disorder with a variable phenotype. The involvement of peripheral nerves in DM1 disease is controversial. The DM1 animal model DM300 transgenic mice that carry 350 to 500 CTG repeats express a mild DM1 phenotype but do not exhibit motor or sensory pathology. Here, we investigated the presence or absence of peripheral neuropathy in transgenic mice (DMSXL) that carry more than 1,300 CTG repeats and display a severe form of DM1. Electrophysiologic, histologic, and morphometric methods were used to investigate the structure and function of peripheral nerves. We observed lower compound muscle action potentials recorded from hind limb muscles and slowing of sciatic nerve conduction velocity in DMSXL versus control mice. Morphometric analyses showed an axonopathy and neuronopathy in the DMSXL mice characterized by a decrease in numbers of myelinated motor axons in sciatic nerve and in spinal cord motor neurons. Pathologic alterations in the structure of hind limb neuromuscular junctions were also detected in the DMSXL mice. These results suggest that peripheral neuropathy can be linked to a large CTG expansion and a severe form of DM1.

The expansion of 300 CTG repeats in myotonic dystrophy transgenic mice does not induce sensory or motor neuropathy

Although many studies have been carried out to verify the involvement of the peripheral nervous system (PNS) in dystrophia myotonica (DM1) patients, the results remain controversial. The generation of DM1 transgenic mice displaying the human DM1 phenotype provides a useful tool to investigate the type and incidence of structural abnormalities in the PNS. In the present study, the morphological and morphometric analysis of semi-thin sections of sciatic and sural nerves, lumbar dorsal root ganglia (DRG) and lumbar spinal cords revealed that in DM1 transgenic mice carrying 300 CTG repeats, there is no change in the number and diameter of myelinated axons compared to wild type. Only a non-significant reduction in the percentage of thin myelinated axons was detected in electron micrographs of ultra-thin sciatic nerve sections. Analysis of the number of neurons did not reveal a loss in number of either sensory neurons in the lumbar DRG or motor neurons in the lumbar spinal cord in these DM1 mice. Furthermore, in hind limb muscle sections, stained with a neurofilament antibody and alpha-bungarotoxin, the intramuscular axon arborization appeared normal in DM1 mice and undistinguishable from that in wild-type mice. Moreover, in DM1 mice, there was no irregularity in the structure or an increase in the endplate area. Also statistical analysis did not show an increase in endplate density or in the concentration of acetylcholine receptors. Altogether, these results suggest that 300 CTG repeats are not sufficient to induce axonopathy, demyelination or neuronopathies in this transgenic mouse model.

Intracytoplasmic inclusion bodies of the substantia nigra in myotonic dystrophy. Immunohistochemical observations

We recently reported a significantly higher incidence of intracytoplasmic inclusion bodies (IIBs) of the substantia nigra in patients with myotonic dystrophy (MyD) than in age-matched controls. The changes are, per se, not specific, since a small percentage of disease and normal controls also showed similar inclusions. To elucidate the pathological significance of the inclusion in MyD, we studied immunohistochemical characteristics of IIBs of the substantia nigra in eight patients with MyD. Many IIBs showed moderately intense immunoreactivity for ubiquitin, microtubule-associated protein (MAP) 1 and MAP 2. However, the IIBs did not react with any of the following: anti-neurofilament protein antibodies (Abs) (68, 160 and 200 kDa), anti-neuron-specific enolase antibody (Ab), anti-tau Ab, anti-tubulin Abs (alpha and beta), anti-paired helical filament Ab, anti-actin Ab, anti-phosphorylated epitope of neurofilaments Ab, anti-synaptophysin Ab, anti-myelin basic protein Ab, anti-actin Ab and anti-glial fibrillary acidic protein Ab. Our results suggest that IIBs of the substantia nigra in MyD are related to an alteration of neuronal cytoskeleton metabolism affecting microtubular proteins in conjunction with activation of ubiquitin proteolytic systems.

Multimodality evoked potentials in myotonic dystrophy

Multimodality evoked potentials were performed in 18 patients affected by myotonic dystrophy (8 males and 10 females); the aim was to make an electrophysiological evaluation of the central nervous system involvement in this disease. We observed brainstem, somatosensory and visual evoked potential abnormalities respectively in 53%, 62.5% and 71.4% of cases, with no apparent relationship to the severity of the disease. Our abnormal findings provide further confirmation of CNS involvement in myotonic dystrophy and occur in various combinations suggesting an aspecific involvement at different levels, reflecting the multisystemic character of this disease.

Immaturity of muscle fibers in the congenital form of myotonic dystrophy: its consequences and its origin

Skeletal muscle maturation is impaired in children with congenital myotonic dystrophy. This immaturity is characterized at the light microscopy level by an abnormal presence of myotubes, small fascicles of muscle fibers, thin myofibers, and delayed muscle fiber type differentiation with a peripheral halo lacking mitochondrial oxidative enzyme activity. At an ultrastructural level, the characteristics are a paucity of myofibrils with a peripheral rim devoid of mitochondria and myofibrils in the fibers. In time the muscle is able to gain a certain degree of maturity as shown in one of our cases who had two successive muscle biopsies. The muscle, however, never becomes normal but retains discrepancies in fiber size and fiber type distribution and shows some fiber necrosis. Maturation of the motoneurons is normal, which may explain necrosis of immature muscle fibers. In an experimental study carried out to look for evidence of a circulatory factor in mothers of children with congenital myotonic dystrophy, it was found that sera from these mothers administered intra-peritoneally to newborn rats does in fact impair muscle maturation, whereas rats injected similarly with sera from control women showed normal muscle maturation.

Short-latency somatosensory evoked potentials in dystrophia myotonica

Somatosensory evoked potentials (SEPs) were recorded in a group of 21 patients with dystrophia myotonica and in a group of controls. Those with dystrophia myotonica had longer absolute peak latencies due to slower peripheral conduction. SEP abnormalities revealed peripheral and/or central conduction delays in 33% of the dystrophia myotonica subjects. There was no apparent relationship between the clinical severity of the disease and SEP abnormality.

Measurement of motor conduction velocity with Hopf's technique in myotonic dystrophy

Hopf's technique was used to measure maximal and minimal motor nerve conduction velocities, and the percentage of fibres with intermediate velocity, in the posterior tibial nerve in patients with myotonic dystrophy. A reduction of maximal and minimal conduction velocities was found. The distribution of fibres with intermediate velocity was nearly identical to that of the control group and the dispersion values were normal. These data do not support the hypothesis that a primary disturbance of the motor neurons is responsible for the muscle changes in myotonic dystrophy. The reduction of the motor nerve conduction velocity, which was an inconstant finding, should not be considered an indication of a neurogenic aetiology of myotonic dystrophy, but only one of the many disorders of a multisystem disease.

Morphometric quantification of the cervical limb motor cells in various neuromuscular diseases

The number of motor cells was significantly reduced in the C8 segment of the cervical spinal cord in all 12 cases of amyotrophic lateral sclerosis (ALS), in the C6 and/or C8 segments in 1 case of adult onset spinal muscular atrophy, 2 cases of Werdnig-Hoffmann (W-H) disease, 3 of 4 cases of chronic polyneuropathy and in 1 case of poliomyelitis and 1 of ossification of the posterior longitudinal ligament in the cervical spine (OPLL). The numbers of motor cells were normal in the C6 or C8 segment in 6 cases of muscular dystrophy, except in one case of congenital muscular dystrophy, who showed reduced numbers of the motor cells. Examination of the distribution of motor cells per 500 micrometers thickness in serial sections revealed that reduction in numbers of the motor cells was diffuse and symmetrical in half the cases of ALS and W-H disease and in the cases of chronic polyneuropathy and congenital dystrophy; diffuse but asymmetrical in the other ALS and W-H disease cases and in a case of adult spinal muscular atrophy, and localized and asymmetrical in the cases of poliomyelitis and OPLL. In muscular dystrophy the distribution of motor cells showed segmental variations similar to controls.

Incidence, severity, and time-course of motoneurone dysfunction in myotonic dystrophy: their significance for an understanding of anticipation

The numbers of functioning motor units and the amplitudes of the maximum evoked muscle responses have been measured in 198 muscles of 102 patients with myotonic dystrophy. Losses of units could be demonstrated in most of the extensor digitorum brevis and thenar muscles but less commonly in the hypothenar groups. A more proximal limb muscle, the soleus, was also shown to be involved frequently. Investigation of two premature infants withmyotonic dystrophy also revealed reductions of functiong units; in one infant clinical improvement was associated with increased muscle innervation. Repeated examinations of 10 adult patients disclosed an abnormal decline in neuromuscular function below the age of 60 years. The reduction in functioning units amounted to approximately 3% of the mean control value per annum. Analysis of 19 families showed that the severity of neuromuscular involvement was nearly always greater in members of later generations. If the predicted deterioration was also taken into account, the results strongly suggested that anticipation was a true genetic phenomenon rather than an artefact of selection. The combined results are considered to strengthen the concept of motoneurone dysfunction as the major pathogenetic factor in this form of dystrophy.