An ultrastructure investigation of intrafusal muscle fibers in myotonic dystrophy

In Myotonic Dystrophy Type 1 Head Repositioning Errors Suggest Impaired Cervical Proprioception

Background: Myotonic dystrophy type 1 (DM1) is a rare multisystemic genetic disorder with motor hallmarks of myotonia, muscle weakness and wasting. DM1 patients have an increased risk of falling of multifactorial origin, and proprioceptive and vestibular deficits can contribute to this risk. Abnormalities of muscle spindles in DM1 have been known for years. This observational cross-sectional study was based on the hypothesis of impaired cervical proprioception caused by alterations in the neck spindles. Methods: Head position sense was measured in 16 DM1 patients and 16 age- and gender-matched controls. A head-to-target repositioning test was requested from blindfolded participants. Their head was passively rotated approximately 30° leftward or rightward and flexed or extended approximately 25°. Participants had to replicate the imposed positions. An optoelectronic system was adopted to measure the angular differences between the reproduced and the imposed positions (joint position error, JPE, °) concerning the intended (sagittal, horizontal) and unintended (including the frontal) planar projections. In DM1 patients, JPEs were correlated with clinical and balance measures. Static balance in DM1 patients was assessed through dynamic posturography. Results: The accuracy and precision of head repositioning in the intended sagittal and horizontal error components did not differ between DM1 and controls. On the contrary, DM1 patients showed unintended side-bending to the left and the right: the mean [95%CI] of frontal JPE was -1.29° [-1.99°, -0.60°] for left rotation and 0.98° [0.28°, 1.67°] for right rotation. The frontal JPE of controls did not differ significantly from 0° (left rotation: 0.17° [-0.53°, 0.87°]; right rotation: -0.22° [-0.91°, 0.48°]). Frontal JPE differed between left and right rotation trials (p < 0.001) only in DM1 patients. No correlation was found between JPEs and measures from dynamic posturography and clinical scales. Conclusions: Lateral head bending associated with head rotation may reflect a latent impairment of neck proprioception in DM1 patients.

Balance impairment in myotonic dystrophy type 1: Dynamic posturography suggests the coexistence of a proprioceptive and vestibular deficit

Falls are frequent in Myotonic Dystrophy type 1 (DM1), but the pathophysiology of the balance impairment needs further exploration in this disease. The current work aims to provide a richer understanding of DM1 imbalance. Standing balance in 16 patients and 40 controls was tested in two posturographic tests (EquiTest™). In the Sensory Organization Test (SOT), standstill balance was challenged by combining visual (eyes open vs. closed) and environmental conditions (fixed vs. sway-tuned platform and/or visual surround). In the “react” test, reflexes induced by sudden shifts in the support base were studied. Oscillations of the body centre of mass (COM) were measured. In the SOT, COM sway was larger in patients than controls in any condition, including firm support with eyes open (quiet standing). On sway-tuned support, COM oscillations when standing with closed eyes were larger in patients than controls even after taking into account the oscillations with eyes open. In the “react” paradigm, balance reflexes were delayed in patients. Results in both experimental paradigms (i.e., SOT and react test) are consistent with leg muscle weakness. This, however, is not a sufficient explanation. The SOT test highlighted that patients rely on vision more than controls to maintain static balance. Consistently enough, evidence is provided that an impairment of proprioceptive and vestibular systems contributes to falls in DM1. Rehabilitation programs targeted at reweighting sensory systems may be designed to improve safe mobility in DM1.

Muscle spindle function in healthy and diseased muscle

Almost every muscle contains muscle spindles. These delicate sensory receptors inform the central nervous system (CNS) about changes in the length of individual muscles and the speed of stretching. With this information, the CNS computes the position and movement of our extremities in space, which is a requirement for motor control, for maintaining posture and for a stable gait. Many neuromuscular diseases affect muscle spindle function contributing, among others, to an unstable gait, frequent falls and ataxic behavior in the affected patients. Nevertheless, muscle spindles are usually ignored during examination and analysis of muscle function and when designing therapeutic strategies for neuromuscular diseases. This review summarizes the development and function of muscle spindles and the changes observed under pathological conditions, in particular in the various forms of muscular dystrophies.

Novel Drosophila model of myotonic dystrophy type 1: phenotypic characterization and genome-wide view of altered gene expression

Myotonic dystrophy type 1 (DM1) is a multisystemic RNA-dominant disorder characterized by myotonia and muscle degeneration. In DM1 patients, the mutant DMPK transcripts containing expanded CUG repeats form nuclear foci and sequester the Muscleblind-like 1 splicing factor, resulting in mis-splicing of its targets. However, several pathological defects observed in DM1 and their link with disease progression remain poorly understood. In an attempt to fill this gap, we generated inducible transgenic Drosophila lines with increasing number of CTG repeats. Targeting the expression of these repeats to the larval muscles recapitulated in a repeat-size-dependent manner the major DM1 symptoms such as muscle hypercontraction, splitting of muscle fibers, reduced fiber size or myoblast fusion defects. Comparative transcriptional profiling performed on the generated DM1 lines and on the muscleblind (mbl)-RNAi line revealed that nuclear accumulation of toxic CUG repeats can affect gene expression independently of splicing or Mbl sequestration. Also, in mblRNAi contexts, the largest portion of deregulated genes corresponded to single-transcript genes, revealing an unexpected impact of the indirect influence of mbl on gene expression. Among the single-transcript Mbl targets is Muscle protein 20 involved in myoblast fusion and causing the reduced number of nuclei in muscles of mblRNAi larvae. Finally, by combining in silico prediction of Mbl targets with mblRNAi microarray data, we found the calcium pump dSERCA as a Mbl splice target and show that the membrane dSERCA isoform is sufficient to rescue a DM1-induced hypercontraction phenotype in a Drosophila model.

Functional sparing of intrafusal muscle fibers in muscular dystrophies

In a previous study, we showed that patients with muscular dystrophies (MDs) perceive passive movements, experience sensations of illusory movement induced by muscle tendon vibration, and have proprioceptive-regulated sways in response to vibratory stimulation applied to the neck and ankle muscle tendons. These findings argue for preserved proprioceptive functions of muscle spindles. However, it is unclear whether the function of intrafusal muscle fibers is spared, i.e., whether they retain their ability to contract when submitted to a fusimotor drive. To answer this question, we analyzed the effects of reinforcement maneuvers (mental computation and the Jendrassik maneuver) that are known to increase muscle spindle sensitivity via fusimotor drive in healthy subjects. Nine patients with different MDs participated in the study. Reinforcement maneuvers increased both the mean amplitude of the Achilles tendon reflex (187 +/- 52.9% of the mean control amplitude) and the sensitivity of muscle spindle afferents to imposed movements of the ankle. The same reinforcement maneuvers failed to alter the amplitude of the Hoffmann reflex in the triceps surae muscle. These results suggest that the intrafusal muscle fibers preserve their contractile abilities in slowly progressive MDs. The reasons for a differential impairment of intrafusal and extrafusal muscle fibers and the clinical implications of the present results are discussed.

Lacunar dilatations of intrafusal and extrafusal terminal cisternae, annulate lamellae, confronting cisternae and tubulofilamentous inclusions within the spectrum of muscle and nerve fiber changes in myotonic dystrophy

In 3 out of 5 muscle spindles available in skeletal muscle biopsy specimens from 30 patients with myotonic dystrophy (MD) unusually large lacunar dilatations of terminal cisternae were observed that had thus far only been reported in extrafusal muscle fibers. Cytoplasmic annulate lamellae, confronting cisternae and regularly proliferated terminal cisternae, as well as intranuclear tubulovesicular inclusions were found in extrafusal muscle fibers that in combination with concentric membranous bodies seen in perineurial cells and Schwann cells generally emphasize an involvement of the endoplasmic reticulum in the pathogenesis of MD. In addition, a nuclear inclusion body was observed composed of tubulofilamentous structures with close similarity to those thought to be rather specific for inclusion body myositis. Vesicles filled with amorphous material originating from outer spindle capsule cells were suggested to indicate matrical lipidic debris leading to "ghost bodies" and calcifying globules. Light microscopical evaluation of 8 sural nerve specimens revealed a neuropathy in only 2 patients that was predominantly axonal in type and of slight to moderate severity with a secondary demyelinating component in 1 patient. These findings add to the large spectrum of muscle and nerve fiber changes in MD underlining the phenotypic multiplicity of a well defined genetic defect.

The effect of tenotomy and immobilization on muscle spindles and tendon organs of the rat calf muscles. A histochemical and morphometrical study

The morphological and histochemical alterations in the muscle spindles and Golgi tendon organs of tenotomized or immobilized calf muscles of rats (m. soleus, m. gastrocnemius) were studied in 54 animals. The intact contralateral feet served as controls. There was no change in the number of mechanoreceptors in either the tenotomized or the immobilized muscles. The diameter of the intrafusal fibers was increased somewhat after 1 week, but decreased by 14% to 40% in 3 weeks. The periaxial space of muscle spindle had diminished or disappeared by 2 weeks from the beginning of the procedures. The thickness of the outer capsule of the spindles had markedly increased, especially in the polar region. In the Golgi tendon organs the capsule had also thickened significantly and the internal space diminished. The alterations were similar following tenotomy or immobilization, but more marked after tenotomy than after immobilization.

A comparative study of muscle spindles in slow and fast neonatal muscles of normal and dystrophic mice

Muscle spindles from the slow-twitch soleus and the fast-twitch extensor digitorum longus (EDL) muscles of genetically dystrophic mice of the dy2J/dy2J strain were compared with age-matched normal animals at neonatal ages of 1-3 weeks according to histochemical, quantitative, and ultrastructural parameters. Intrafusal fibers in both the soleus and EDL exhibited similar regional differences in myosin ATPase activity, and conformed to those noted previously in various adult species. In distal polar regions, all nuclear bag fibers resembled extrafusal fibers of the type 1 variety, whereas in capsular zones they could be divided into two subtypes. Nuclear chain fibers possessed a staining pattern similar to type 2 extrafusal fibers, and in contrast to the bag fibers they exhibited no regional variations. These features were consistently observed in both the normal and dystrophic muscles at all ages. Spindles varied only slightly in their number and distribution in the two types of muscle, and their location followed the neurovascular branching pattern in each. Irrespective of age or genotype, spindles in the soleus were more homogeneously dispersed, but those in the EDL were concentrated along the dorsal aspect of the muscle. No significant differences were noted in the total number of spindles between normal and dystrophic muscles. In addition, no dramatic differences were observed in the muscle spindle index for soleus and EDL. The first obvious disease-related changes were noted in extrafusal fibers of the soleus of 3-week-old mice, and spindles were often located close to these areas of fiber degeneration. Despite alterations in the surrounding tissue, however, spindles appeared morphologically unaltered in dystrophy. These observations indicate that intrafusal fibers of spindles in neonatal mice appear enzymatically and histologically unaffected in incipient stages of progressive muscular dystrophy.

[Afferent innervation of musculus sphincter ani externus of men]

Investigations of the existence and distribution of muscle spindles and tendon organs in the human voluntary anal sphincter muscle in 53 cases of autopsy material from both female and male persons (age: 27th week of gestation to 81 years) brought forward the following results: Similar to animals, muscle spindles are formed in the last trimenon of gestation. They are more numerous as has been expected hitherto and there are different types with a different content of intrafusal fibers. Lamellar corpuscular organs are situated around the ending of the muscle fibers in the anal skin. In our opinion they are to substitute muscle tendon organs which could not be detected.

Ultrastructure of muscle spindle in congenital myotonic dystrophy. A study of preterm infant muscle spindles

The muscle spindle in the preterm infants with congenital myotonic dystrophy consisted of numerous, unfused intrafusal fibers. These fibers showed immature myofilament arrangement at 27 weeks but had the nuclear arrangement of bag and chain type. The motor endings were very sparse while prominent sensory endings were seen. At 34-37 weeks, even though the muscle fibers were unfused, the nuclear bag and chain fibers could be differentiated by the presence and absence of M line. Immature motor endings were seen at this stage. In conclusion, the muscle spindle in the preterm infants is immature.

Myotonia dystrophica with heart involvement: an electron microscopic study of skeletal, cardiac, and smooth muscle

The electron microscopic features of the striated skeletal muscle, the striated cardiac muscle, and the smooth muscle from a woman who had been suffering for many years from myotonia dystrophica with cardiac involvement are described. The skeletal muscle was studied at two different stages of the disease. In the first material the main changes consisted of centrally situated nuclei, disorganisation of the sarcomeres, and focal disruption of the Z-line. The satellite cells were well represented. Three years later atrophy and degenerative, necrotic changes of the skeletal muscle were evident. The satellite cells were absent. Few changes were seen in the striated cardiac muscle. These consisted of slight interstitial fibrosis and large accumulations of mitochondria with intramitochondrial dense granules. The smooth muscle cells of the oesophagus showed disorientated filaments and mild degenerative changes. It is concluded that the skeletal muscle was more severely affected than the other types of muscle.

Ultrastructure of the muscle spindle in dystrophia myotonica. II. The sensory and motor nerve terminals

The normal pattern of innervation was lost in the four dystrophia myotonica muscle spindles examined. There had been a proliferation of both sensory and motor nerve terminals, and many endings showed varying degrees of structural abnormality. The changes seen in the sensory nerve endings were an incomplete contact relationship between the terminal and the muscle fibre, the association of Schwann cell processes with the nerve ending, abnormal shape of the terminal, multiple sensory endings, and changes in the ultrastructural organization of the terminal axoplasm. Remnants of degenerated sensory terminals, and isolated sensory endings were also seen. Motor nerve terminals varied in size and shape. In general, they contained a normal complement of vesicles and mitochondria. Junctional folding was often absent or simplified, and sarcoplasmic specialization was, at the most, rudimentary. The relationship between these changes and intrafusal muscle fibre fragmentation is discussed.

Ultrastructure of the muscle spindle in dystrophia myotonica. I. The intrafusal muscle fibres

The abnormal intrafusal muscle fibres of four spindles located in extensor indicis biopsies from two patients with dystrophia myotonica have been examined by electron microscopy. In all cases, the fibres had undergone severe longitudinal fragmentation in their equatorial, proximal-polar and mid-polar zones. It was not possible to differentiate the fibres into nuclear bag and nuclear chain types. Many of the fragments were arranged in clusters, each cluster being bounded by a single basement membrane. Numerous partially divided fibres were seen in mid-polar sections, and a morphological picture of the fragmentation process has been presented. The ultrastructural appearance of individual fibre fragments ranged from normality through to severe atrophy. Intrafusal satellite cells showed an increase in numbers, presumably in response to the pathological processes affecting the muscle fibres. The nature of this response is discussed.