The target phenomenon in rat muscle following tenotomy and neurotomy. A comparative light microscopic and histochemical study

Target formation in muscle fibres indicates reinnervation - A proteomic study in muscle samples from peripheral neuropathies

AIMS

Target skeletal muscle fibres - defined by different concentric areas in oxidative enzyme staining - can occur in patients with neurogenic muscular atrophy. Here, we used our established hypothesis-free proteomic approach with the aim of deciphering the protein composition of targets. We also searched for potential novel interactions between target proteins.

METHODS

Targets and control areas were laser microdissected from skeletal muscle sections of 20 patients with neurogenic muscular atrophy. Samples were analysed by a highly sensitive mass spectrometry approach, enabling relative protein quantification. The results were validated by immunofluorescence studies. Protein interactions were investigated by yeast two-hybrid assays, coimmunoprecipitation experiments and bimolecular fluorescence complementation.

RESULTS

More than 1000 proteins were identified. Among these, 55 proteins were significantly over-represented and 40 proteins were significantly under-represented in targets compared to intraindividual control samples. The majority of over-represented proteins were associated with the myofibrillar Z-disc and actin dynamics, followed by myosin and myosin-associated proteins, proteins involved in protein biosynthesis and chaperones. Under-represented proteins were mainly mitochondrial proteins. Functional studies revealed that the LIM domain of the over-represented protein LIMCH1 interacts with isoform A of Xin actin-binding repeat-containing protein 1 (XinA).

CONCLUSIONS

In particular, proteins involved in myofibrillogenesis are over-represented in target structures, which indicate an ongoing process of sarcomere assembly and/or remodelling within this specific area of the muscle fibres. We speculate that target structures are the result of reinnervation processes in which filamin C-associated myofibrillogenesis is tightly regulated by the BAG3-associated protein quality system.

Disturbed trafficking of dystrophin and associated proteins in targetoid phenomena after chronic muscle denervation

Dystrophin and associated proteins form a complex with an important role at the sarcolemma. Expression of this protein complex is highly regulated during development and regeneration. In order to better understand assembling patterns of these proteins, we have studied their expression in targetoid-like phenomena found in human muscle after chronic denervation, a situation known to give rise to abnormal protein trafficking. In eight biopsies of patients with chronic denervation, mainly resulting from amyotrophic lateral sclerosis, we found a number of targetoid phenomena. Selective accumulation of a number of sarcolemmal and sarcoplasmatic proteins occurred in targetoid phenomena. The larger majority of them contained gamma-sarcoglycan (gammaSG), but none contained the developmental heavy chain myosin isoform. In a series of 166 targetoid phenomena which could be studied with 17 different antibodies recognizing sarcolemmal and cytoplasmatic proteins, a high level of colocalization of gammaSG with desmin and alpha-actinin was found. Colocalization rate was much lower with other proteins, including other members of the dystrophin-associated protein complex. These data show that selective changes in expression of otherwise closely related proteins occur during disturbed trafficking leading to target formation. Because members of the dystrophin-associated protein complex do not accumulate in a similar fashion within targets, we suggest that a complex molecular control of gene expression and trafficking of this complex is involved after chronic muscle denervation.

Filamin C accumulation is a strong but nonspecific immunohistochemical marker of core formation in muscle

Filamin C is the muscle isoform of a group of large actin-crosslinking proteins. On the one hand, filamin C is associated with the Z-disk of the myofibrillar apparatus and binds to myotilin; on the other hand, it interacts with the sarcoglycan complex at the sarcolemma. Filamin C may be involved in reorganizing the cytoskeleton in response to signalling events and in muscle it may, in addition, fulfill structural functions at the Z-disk. An examination of biopsies from patients with multi-minicore myopathy, central core myopathy and neurogenic target fibers with core-like target formations (TF) revealed strong reactivity of all the cores and target formations with two different anti-filamin C antibodies. In all three conditions, the immunoreactivity in the cores for filamin C was considerably stronger than that for desmin. Only for alphaB-crystallin were comparable levels of immunoreactivity detected. There was no difference in intensity for filamin C between the three pathological conditions. Thus, filamin C along with alphaB-crystallin is a strong and robust, but nonspecific marker of core formation. The reason why filamin C accumulates in cores is unclear at present, but we postulate that it may be critically involved in the chain of events eventually leading to myofibrillar degeneration.

Skeletal muscle response to tenotomy

Tenotomy is a commonly encountered clinical entity, whether traumatic or iatrogenic. This article reviews the response of skeletal muscle to tenotomy. The changes are subdivided into molecular, architectural, and functional categories. Architectural disruption of the muscle includes myofiber disorganization, central core necrosis, Z-line streaming, fibrosis of fibers and Golgi tendon organs, changes in sarcomere number, and alterations in the number of membrane particles. Molecular changes include transient changes in myosin heavy chain composition and expression of neural cell adhesion molecule (NCAM). Functionally, tenotomized muscle produces decreased maximum tetanic and twitch tension. Alterations in normal skeletal muscle structure and function are clinically applicable to the understanding of pathological states that follow tendon rupture and iatrogenic tenotomy.

Tenotomy prevents the functional improvement of a muscle reinnervated with a chronically severed nerve

During the early stages of nerve implantation, we followed the dynamic properties of the lateral gastrocnemius muscle of the rat, reinnervated with an acutely or chronically severed peroneal nerve. The aim of this study was to ascertain whether 1) the better functional recovery of a muscle reinnervated by a chronically severed foreign nerve is present from the onset of reinnervation, and 2) whether such functional improvement is due to the conditioning lesion effect. Our results indicate that better functional recovery is already apparent one week after nerve implantation, and it is due to the conditioning lesion effect, since tenotomy prevents such improvement. The tenotomy effect underlines the fact that some environmental factors concerning the target tissue, and not only the predegenerated nerve, are involved in the conditioning effect.

The morphological sequences in man of de- and reinnervation in free muscle transfer with microneurovascular anastomoses

Muscle biopsies from two patients treated by free gracilis muscle transfer and micro-neurovascular anastomoses for long standing facial palsy due to previous damage to the facial nerve during removal of tumours were examined by histochemistry and electron microscopy. Sequential muscle biopsies of one case taken at various intervals after transferring the muscle permitted the study of events of de- and reinnervation of human skeletal muscle. The changes in the early stages of denervation included atrophy of type 2B fibres. This was followed by progressive atrophy of all fibre types, distortion of the internal structure of the muscle fibres with the incidence of various cytoplasmic inclusions. Massive necrosis of the muscle did not occur. A prominent increase in satellite cells and their maturation to myotubes and new muscle fibres were evident. Reinnervation occurred with the formation of neuromuscular junctions some of them being at preformed sole plates. Fibre type grouping was apparent in addition to other changes such as the occurrence of targetoid hypertrophied muscle fibres. These changes are similar to those described previously during de- and reinnervation studies in experimental animals.

Influence of isaxonine on reinnervation of the rat gastrocnemius muscle

Isaxonine is reported to have a favourable effect on nerve regeneration. The histological and histochemical changes were compared in the gastrocnemius muscles of 20 rats, in which the left sciatic nerve was frozen-crushed for 10 s at the level of the buttock. The animals received a daily dose of intraperitoneal isaxonine and ten others the solvent only. After 8 weeks small type grouping was seen in rats receiving solvent only and large type grouping in rats treated with isaxonine. After 10 weeks a checkerboard pattern of the fibre types was observed in the muscles of animals receiving solvent only but fibre type grouping was present in those treated with isaxonine. This experiment indicates that isaxonine prevents synaptic remodelling by suppression of the redundant nerve endings from additional axons, which have regenerated more slowly, in making contact with motor endplates.

Calcium-activated protease activity in tenotomized muscle

The purpose of this study was to investigate the possible role of calcium-activated neutral protease in the disorganization and dissolution of the myofibrils of the rat soleus that occurs following tenotomy. Rats were killed 3, 5, 7, 14, 21, and 42 days after tenotomy of the soleus, and the muscles were removed and assayed for calcium-activated protease activity. Maximal protease activity occurred 1 week after tenotomy, at the time when myofibril organization is completely disrupted. Activity was still high 2 and 3 weeks after the operation, but returned to normal levels by 6 weeks, when muscle histology had returned to normal. The time course of the calcium-activated protease activity corresponded closely to the time course of the morphological changes. Thus, calcium-activated neutral protease may play a major role in myofibrillar proteolysis following tenotomy and in making the myofibril susceptible to proteolytic attack by other, less specific proteases.

Influence of isaxonine on the target phenomenon, muscle fibre size and neuromuscular junction in the tenotomized and denervated gastrocnemius muscle of the rat

Isaxonine is reported to accelerate the rate of peripheral nerve regeneration, stimulate axonal sprouting and promote motor and sensory function recovery. The target phenomenon in muscle fibers is the morphological expression of increased stretch activity in a diseased muscle. It can be produced in the gastrocnemius muscle of the rat by tenotomy and is inhibited by simultaneous denervation. The influence of isaxonine on this experimental model was studied. Twenty Wistar rats were submitted to bilateral section of the Achilles tendon and section of the left sciatic nerve. In ten rats, treated with isaxonine, the qualitative and quantitative changes in the gastrocnemius muscle were compared with the control group over a period of 3 weeks. No statistical differences were observed in the occurrence and inhibition of the target phenomenon, the muscle fibre size and the neuromuscular junction. Isaxonine has no direct effect on a diseased muscle with an intact nerve supply, nor in the early stages after complete denervation.

The development of central cores in both fiber types in tenotomized muscle

Using the histochemical reaction for myosin adenosine triphosphatase (ATPase), a study of the rat soleus muscle following tenotomy was undertaken. It was demonstrated that both type 2A and 2C fibers undergo degenerative changes following tenotomy. All previous studies have stated that only type 1 fibers were affected and developed central core lesions and that type 2 fibers were somehow protected from the degenerative process. The results of this experiment illustrate that central core lesions will develop in all three fiber types (types 1, 2A, and 2C) of the soleus following tenotomy.

Influence of stretch on the target phenomenon in the tenotomized and immobilized gastrocnemius muscle of the rat

The influence of traction on the target phenomenon was analysed in the tenotomized gastrocnemius muscle of Wistar rats. Target fibres were constantly observed at the medial side of the heads of the non-immobilized and tenotomized gastrocnemius muscles used as control. In the muscles fixed in flexion, no target fibres could be detected, while in the muscles fixed in hyperextension some target fibres were seen. The latter were less numerous than those present in the non-immobilized muscles. This experiment proves the importance of stretch mechanisms in the pathogenesis of the target phenomenon.

Response of the diaphragm muscle to electrical stimulation of the phrenic nerve. A histochemical and ultrastructural study

The histological, histochemical, and ultrastructural features of canine diaphragms subjected to pacing by high-frequency electrical stimulation (27 to 33 Hz) of the phrenic nerve are compared with unstimulated diaphragms and with diaphragms subjected to pacing by low-frequency stimulation (11 to 13 Hz). The high-frequency group showed a reduced tidal volume (fatigue) after long-term stimulation, and myopathic changes which included enlarged internal and sarcolemmal nuclei, ring fibers, moth-eaten fibers with irregular histochemical staining, core/targetoid fibers, and smearing and aggregation of Z-band material with electron microscopy. The low-frequency group did not develop a significant degree of fatigue or pathological changes, and showed histochemical evidence of transformation to fast-twitch (type II) fibers. Possible pathogenic mechanisms and their similarity to those in certain human neuromuscular diseases are discussed. The application of the findings resulting from high- and low-frequency stimulation to long-term diaphragm pacing in humans with chronic ventilatory insufficiency is also discussed.

Inhibition of the target phenomenon in tenotomized rabbit muscle by medullary lesions

Target fibres were demonstrated in the gastrocnemius muscles following section of the Achilles tendon of two rabbits. In five other animals with tenotomized gastrocnemius muscles, a subarachnoid phenol block of the lumbar and sacral spinal cord was performed. Large and small medullary lesions inhibited the occurrence of target fibres in the tenotomized muscles, the smallest one being neurolysis of the dorsal roots. This final experiment suggests that the target phenomenon is the morphological expression of an increased stretch sensitivity of the tenotomized muscle.

Changes in sarcomere length following tenotomy in the rat

The medial belly of the gastrocnemius and the extensor digitorum longus muscles of rats were tenotomized. One day following tenotomy, the mean sarcomere length of the fast medial gastrocnemius was 1.8 microns, a value comparable to that of tenotomized slow soleus. The mean sarcomere length of the tenotomized extensor digitorum longus, however, was 2.0 microns, a figure which differed significantly from the values obtained for both the soleus and the gastrocnemius. Histological preparations showed the presence of central core degeneration in slow fatigue-resistant fibers of the tenotomized gastrocnemius comparable to that seen in the soleus. No changes were found in the fibers of the tenotomized extensor digitorum longus. The fact that central core lesions were produced in the fibers of soleus and medial gastrocnemius but not in the extensor digitorum longus may be related to the lesser reduction in sarcomere length following tenotomy of the latter muscle.

Type I fiber atrophy in the vastus lateralis muscle in dogs with femoral fractures treated by hyperextension

A syndrome characterized by limb hyperextension, generalized muscle atrophy, abducted gait, and a limited range of joint motion is reported in five dogs, four of which were immature. Distal femoral fractures, of traumatic origin, were found in all dogs; four dogs were subjected to limb immobilization in extension for three to seven weeks. Lesions in muscle biopsies included fiber size variability, increased prominence of subsarcolemmal nuclei, increased perimysial fibrosis and focal necrosis. Histochemical and morphometric studies demonstrated a significant (p less than 0.05), Type I fiber atrophy in the vastus lateralis muscles in the limbs with femoral fractures treated by hyperextension. The shortest time period between onset of fracture and the presence of type I fiber atrophy was seven weeks.

Influence of 2,4 dichlorophenoxyacetate and of dantrolene sodium on the target phenomenon in tenotomized rat gastrocnemius muscle

The effect of 2,4 dichlorophenoxyacetate and of dantrolene sodium on the occurrence of target fibers in tenotomized rat gastrocnemius muscle was studied. Neither of the drugs influenced the appearance of target fibers or its inhibition by simultaneous neurotomy.

Quantitative analysis of the size distribution of target- and targetoid fibres employing the method of Daeves and Beckel for mixed distributions

An analysis of frequency distribution is performed in 250 target and targetoid fibres each from the anterior tibial muscle of a case with rapidly proceeding denervation atrophy. Following plane measurements on cross sections the size data were analysed according to the method of Daeves and Beckel [1] for mixed distributions. Three homogeneous normally distributed populations could thus be extracted from the mixed distributed sample. The largest population represents target fibres with a normal size range, while the second one encloses hypertrophic fibres, and the third and smallest one consists of fibres ranging within atrophic size limits. These findings support the previous presumption that targets predominantly occur in fibres of normal size and can therefore be regarded as manifestations of an early stage of denervation atrophy. The targetoid fibres, on the contrary, are of an atrophic size by 84%, while only a small population of about 15% contains targetoid fibres of a normal size on cross section.

Morphological properties of experimentally produced target fibres in tenotomized rat gastrocnemius muscle

The gastrocnemius muscles of 3 groups of 10 rats, sacrified 5, 7, and 12 days respectively, following tenotomy, were submitted to different types of fixation, fixative and embedding. The occurrence of target fibres is shown not to be an artefact due to the histological procedures. Further examination demonstrates that the target phenomenon occurs in the shortest fibres on the medial side of both heads of the gastrocnemius muscle and that it consists mainly of a disarrangement of the contractile elements of the muscle fibres. This study argues that this phenomenon represents some kind of myotonic state of a pathological muscle.

Development and inhibition of the target phenomenon in tenotomized rat muscle

The sequence of development of the target phenomenon in tenotomized gastrocnemius muscle was studied: the presence of target fibres was preceded by the occurrence of contraction bands and of "moth eaten" appearance of the fibres. This phenomenon was far more pronounced and occurred earlier in type II than in type I fibres. This target phenomenon and the contraction artefacts could be inhibited in the tenotomized muscles by simultaneous neurotomy or immobilization of the muscle with a plaster cast. Delayed denervation inhibited also the target phenomenon, if performed less than 5 days after the tenotomy. These series of experiments seem to indicate that the target phenomenon occurs in more irritable muscle fibres and that muscular activity is needed for its development.