Distribution of dystrophin and neurofilament protein in muscle spindles of normal and Mdx-dystrophic mice: an immunocytochemical study

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.

Impaired muscle spindle function in murine models of muscular dystrophy

KEY POINTS

Muscular dystrophy patients suffer from progressive degeneration of skeletal muscle fibres, sudden spontaneous falls, balance problems, as well as gait and posture abnormalities. Dystrophin- and dysferlin-deficient mice, models for different types of muscular dystrophy with different aetiology and molecular basis, were characterized to investigate if muscle spindle structure and function are impaired. The number and morphology of muscle spindles were unaltered in both dystrophic mouse lines but muscle spindle resting discharge and their responses to stretch were altered. In dystrophin-deficient muscle spindles, the expression of the paralogue utrophin was substantially upregulated, potentially compensating for the dystrophin deficiency. The results suggest that muscle spindles might contribute to the motor problems observed in patients with muscular dystrophy.

ABSTRACT

Muscular dystrophies comprise a heterogeneous group of hereditary diseases characterized by progressive degeneration of extrafusal muscle fibres as well as unstable gait and frequent falls. To investigate if muscle spindle function is impaired, we analysed their number, morphology and function in wildtype mice and in murine model systems for two distinct types of muscular dystrophy with very different disease aetiology, i.e. dystrophin- and dysferlin-deficient mice. The total number and the overall structure of muscle spindles in soleus muscles of both dystrophic mouse mutants appeared unchanged. Immunohistochemical analyses of wildtype muscle spindles revealed a concentration of dystrophin and β-dystroglycan in intrafusal fibres outside the region of contact with the sensory neuron. While utrophin was absent from the central part of intrafusal fibres of wildtype mice, it was substantially upregulated in dystrophin-deficient mice. Single-unit extracellular recordings of sensory afferents from muscle spindles of the extensor digitorum longus muscle revealed that muscle spindles from both dystrophic mouse strains have an increased resting discharge and a higher action potential firing rate during sinusoidal vibrations, particularly at low frequencies. The response to ramp-and-hold stretches appeared unaltered compared to the respective wildtype mice. We observed no exacerbated functional changes in dystrophin and dysferlin double mutant mice compared to the single mutant animals. These results show alterations in muscle spindle afferent responses in both dystrophic mouse lines, which might cause an increased muscle tone, and might contribute to the unstable gait and frequent falls observed in patients with muscular dystrophy.

Distribution of TTX-sensitive voltage-gated sodium channels in primary sensory endings of mammalian muscle spindles

Knowledge of the molecular mechanisms underlying signaling of mechanical stimuli by muscle spindles remains incomplete. In particular, the ionic conductances that sustain tonic firing during static muscle stretch are unknown. We hypothesized that tonic firing by spindle afferents depends on sodium persistent inward current (INaP) and tested for the necessary presence of the appropriate voltage-gated sodium (NaV) channels in primary sensory endings. The NaV_1.6 isoform was selected for both its capacity to produce INaP and for its presence in other mechanosensors that fire tonically. The present study shows that NaV_1.6 immunoreactivity (IR) is concentrated in heminodes, presumably where tonic firing is generated, and we were surprised to find NaV_1.6 IR strongly expressed also in the sensory terminals, where mechanotransduction occurs. This spatial pattern of NaV_1.6 IR distribution was consistent for three mammalian species (rat, cat, and mouse), as was tonic firing by primary spindle afferents. These findings meet some of the conditions needed to establish participation of INaP in tonic firing by primary sensory endings. The study was extended to two additional NaV isoforms, selected for their sensitivity to TTX, excluding TTX-resistant NaV channels, which alone are insufficient to support firing by primary spindle endings. Positive immunoreactivity was found for NaV_1.1, predominantly in sensory terminals together with NaV_1.6 and for NaV_1.7, mainly in preterminal axons. Differential distribution in primary sensory endings suggests specialized roles for these three NaV isoforms in the process of mechanosensory signaling by muscle spindles.NEW & NOTEWORTHY The molecular mechanisms underlying mechanosensory signaling responsible for proprioceptive functions are not completely elucidated. This study provides the first evidence that voltage-gated sodium channels (NaVs) are expressed in the spindle primary sensory ending, where NaVs are found at every site involved in transduction or encoding of muscle stretch. We propose that NaVs contribute to multiple steps in sensory signaling by muscle spindles as it does in other types of slowly adapting sensory neurons.

Mechanotransduction in the muscle spindle

The focus of this review is on the principal sensory ending of the mammalian muscle spindle, known as the primary ending. The process of mechanosensory transduction in the primary ending is examined under five headings: (i) action potential responses to defined mechanical stimuli—representing the ending's input–output properties; (ii) the receptor potential—including the currents giving rise to it; (iii) sensory-terminal deformation—measurable changes in the shape of the primary-ending terminals correlated with intrafusal sarcomere length, and what may cause them; (iv) putative stretch-sensitive channels—pharmacological and immunocytochemical clues to their identity; and (v) synaptic-like vesicles—the physiology and pharmacology of an intrinsic glutamatergic system in the primary and other mechanosensory endings, with some thoughts on the possible role of the system. Thus, the review highlights spindle stretch-evoked output is the product of multi-ionic receptor currents plus complex and sophisticated regulatory gain controls, both positive and negative in nature, as befits its status as the most complex sensory organ after the special senses.

Corticotropin-releasing factor 2 receptor localization in skeletal muscle

Our objective in this study was to localize the corticotropin-releasing factor 2 receptor (CRF2R) in rodent and human skeletal muscle. We found CRF2R protein to be abundant in neural tissues in skeletal muscle, including large nerve fibers and bundles, neural tissue associated with mechanoreceptors, muscle spindles, and the Golgi tendon organ. CRF2R protein was also abundant in blood vessels in skeletal muscle. CRF2R protein was also observed, although with less abundance, in the endo/perimysial regions in skeletal muscle. The localization of the CRF2R to blood vessels is consistent with the CRF2R-mediated vascular phenomena observed previously, but the observation of CRF2R in neural tissue in skeletal muscle is a novel finding with an unknown function.

Morphological study of two human facial muscles: orbicularis oculi and corrugator supercilii

Human facial muscles are unique in that they do not cross joints and they function either to open and close the apertures of the face or to tug the skin into intricate movements producing facial expressions. Compared to other skeletal muscles of the body, little is known about the microscopic architecture and organization of facial muscles. It was hypothesized that facial muscles with different roles would possess differences in their cellular organization and morphology that would reflect their unique function. The palpebral orbicularis oculi (oo) and the corrugator supercilii (cs) were studied because they are in close topographical proximity to one another and share the same nerve supply and embryonic origin. This study compared the two muscles which were procured as biopsies from cosmetic surgery procedures. Architectural and morphological features were elucidated using a combination of conventional histological stains, immunocytochemistry and histochemistry. Quantitative measures of fiber sizes, shapes, and fiber-type distributions were performed along with measures of capillary area per unit of contractile area (capillary index). Fiber-type profiles and motor end-plates were demonstrated by using antibodies to fast and slow myosins, as well as to neurofilament protein. The oo was shown to differ significantly from the cs on the basis of fiber shapes, sizes, and types. The oo muscle fibers were small, rounded, and 89% of them were of the fast-twitch (Type II) variety. The muscle fibers in the cs were larger, polygonal, and only 49% of them were of the fast-twitch variety. The capillary index of the cs was 2.4 times that of the oo.

Quantitative morphology of mast cells in skeletal muscle of normal and genetically dystrophic mice

BACKGROUND

Mast cells are indigenous connective tissue cells that function in the process of inflammation and edema. Their numbers were studied in a quantitative morphological study of the soleus muscles from 32-week-old and 56-week-old normal and genetically dystrophic dy2J and mdx mice to determine the incidence of mast cells in muscle to increasing age and to normal and myopathic conditions.

METHODS

Soleus muscles from normal C57B1/J and from dystrophic C57B1/SnJ (dy2J/dy2J) and C57BL/10ScSn mdx mice were processed for examination by light and electron microscopy. Quantitation of mast cells was performed on semi-thick sections and expressed as an average of cells per millimeter squared of muscle tissue.

RESULTS

Mast cells were observed in the connective tissue interstitium that normally separates skeletal muscle into fascicles. Their cytoplasmic granules stained metachromatically with toluidine blue and often obscured the single, centrally placed nucleus. They occurred singly or in small groups and were most frequently seen adjacent to neurovascular elements within the muscle, and in many cases were closely associated with the outer capsular regions of muscle spindles. Between the 32- and 56-week-old groups in each strain, an age-related increase in mast cell numbers was observed. In the dystrophic conditions, the dy2J and mdx skeletal muscles exhibited a two- to four-fold increase in mast cells when compared to normals in both age groups. Extensive connective tissue proliferation and sites of necrotic and regenerating muscle were common features in both myopathies.

CONCLUSIONS

Results of this study indicate that a significantly higher number of mast cells which exist in dy2J and mdx murine skeletal muscles may be related to the high amount of connective tissue infiltration and extensive muscle fiber remodelling in these conditions. Moreover, the close proximity of mast cells to muscle spindles and nerve fascicles suggests that these cells may play a role in modulating their activities.

Nitric oxide synthase and cyclic GMP-dependent protein kinase concentrated at the neuromuscular endplate

Nitric oxide mediates diverse functions in development and physiology of vertebrate skeletal muscle. Neuronal type nitric oxide synthase-mu is enriched in fast-twitch fibers and binds to syntrophin, a component of the sarcolemmal dystrophin glycoprotein complex. Here, we show that cyclic GMP-dependent protein kinase type I, a primary effector for nitric oxide, occurs selectively at the neuromuscular junction, in mice and rats, and both neuronal type nitric oxide synthase-mu and cyclic GMP-dependent protein kinase type I remain at skeletal muscle endplates at least two weeks following muscle denervation. Expression of neuronal type nitric oxide synthase-mu and cyclic GMP-dependent protein kinase type I are up-regulated following fusion of cultured primary myotubes. Interestingly, the highest levels of neuronal type nitric oxide synthase-mu in muscle are found complexed with dystrophin at the sarcolemma of intrafusal fibers in muscle spindles. Localization of neuronal type nitric oxide synthase-mu and cyclic GMP-dependent protein kinase type I at the neuromuscular junction suggests functions for nitric oxide and cyclic GMP in the regulation of synaptic actions of intra- and extrafusal muscle fibers.

Extracellular matrix and transmembrane linkages at the termination of intrafusal fibers and the outer capsule in chicken muscle spindles

Attachments of intrafusal fibers and of the outer spindle capsule at the far polar region were examined by immunohistochemistry in serially sectioned chicken leg muscles. Patterns of distribution of connective tissues and intracellular filaments suggest that, in this segment of the muscle spindle, intrafusal fibers bind laterally with the capsule. Contrary to extrafusal fibers at myotendinous junctions, folded plasmalemmas at the ends of intrafusal fibers were rare. Thus, there was little end-to-end interlocking between intrafusal fibers and the extracellular matrix. The tapered contours of terminating intrafusal fibers resembled those of extrafusal fibers which end in fascicles without tendinous connections. At points where the distal portions of intrafusal fibers closely adjoined and overlapped extrafusal fibers, alpha-actinin, vinculin, filamin, talin, beta 1 integrin, spectrin, and dystrophin occurred with moderate to great frequency. It is generally accepted that these compounds are links in molecular chains that extend from the intracellular space across cell membranes to the extracellular matrix. Their location along substantial lengths of extrafusal fibers, distal capsule, and terminating intrafusal fibers suggests the presence of numerous transverse connections between elements of the terminal portion of the spindle and nonspindle tissues. Hence, it is likely that forces monitored by chicken spindles in muscles undergoing length changes are transferred from extrafusal fibers and extracellular matrix to the receptors in large part via lateral shear instead of by longitudinal tension.