Zur Ultrastruktur der ?Target-Fasern?

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.

ALS-Associated Endoplasmic Reticulum Proteins in Denervated Skeletal Muscle: Implications for Motor Neuron Disease Pathology

Alpha-motoneurons and muscle fibres are structurally and functionally interdependent. Both cell types particularly rely on endoplasmic reticulum (ER/SR) functions. Mutations of the ER proteins VAPB, SigR1 and HSP27 lead to hereditary motor neuron diseases (MNDs). Here, we determined the expression profile and localization of these ER proteins/chaperons by immunohistochemistry and immunoblotting in biopsy and autopsy muscle tissue of patients with amyotrophic lateral sclerosis (ALS) and other neurogenic muscular atrophies (NMAs) and compared these patterns to mouse models of neurogenic muscular atrophy. Postsynaptic neuromuscular junction staining for VAPB was intense in normal human and mouse muscle and decreased in denervated Nmd^2J mouse muscle fibres. In contrast, VAPB levels together with other chaperones and autophagy markers were increased in extrasynaptic regions of denervated muscle fibres of patients with MNDs and other NMAs, especially at sites of focal myofibrillar disintegration (targets). These findings did not differ between NMAs due to ALS and other causes. G93A-SOD1 mouse muscle fibres showed a similar pattern of protein level increases in denervated muscle fibres. In addition, they showed globular VAPB-immunoreactive structures together with misfolded SOD1 protein accumulations, suggesting a primary myopathic change. Our findings indicate that altered expression and localization of these ER proteins and autophagy markers are part of the dynamic response of muscle fibres to denervation. The ER is particularly prominent and vulnerable in both muscle fibres and alpha-motoneurons. Thus, ER pathology could contribute to the selective build-up of degenerative changes in the neuromuscular axis in MNDs.

Expression of a cell death marker (Clusterin) in muscle target fibers

We report, for the first time, the expression of immunoreactivity to clusterin in skeletal muscle. Clusterin, a protein probably related to the process of programmed cell death (apoptosis), was specifically very highly expressed in target fibers. All target fibers found in 50 muscle biopsy samples from a variety of neuromuscular disorders expressed a high concentration of clusterin in the middle of the targets. Clusterin was not expressed in any targetoid fibers or cores. Acute denervation, where targets are mostly seen, may be the beginning of apoptosis. Hence our findings support the concept that targets are harbingers of acute denervation.

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.

Core/targetoid fibres and multiple cytoplasmic bodies in organophosphate neuropathy

From correlated histochemical and ultrastructural investigations of biopsy specimens of a rare case with delayed-onset neuropathy due to organophosphate (Dipterex) intoxication, several pathological characteristics were recognized in the muscle fibres. Most striking was concurrence of core/targetoids and multiple cytoplasmic bodies in the same fibres. Foci of markedly reduced or absent oxidative enzyme activity were revealed in 71.6% of type I fibres and in 3.4% of type II fibres. Most of those foci were two-zoned core/targetoids. Cytoplasmic bodies were also revealed selectively in type I fibres contrary to the previous reports. Targets, targetoids, cores and multicores were suggested to be strongly related to each other pathogenetically and to be due to the disturbance of some neurotrophic influences. In the biopsy specimen taken 220 days after exposure to the drug, numerous sarcoplasmic masses were revealed, and they were very likely to represent special forms of regeneration.

Target-targetoid phenomenon of the human muscle fibers. A histological, histochemical and ultrastructural study

Target and targetoid fibers in a muscle biopsy from a patient with paralysis of the deltoid and supraspinatus muscles were studied by light and electron microscopy. The probable cause of the neuropathy was tumor compression. Target and targetoid change was exclusively confined to hypertrophic or normal-sized fibers. Morphometric evaluation of the target and targetoid fibers showed no significant difference between them. With the electron microscope, up to 4 structural zones were seen in the typical target fiber but many were devoid of either zone 2(halo) or zone 3, or both. It was conceivable that focal irregularity and streaming of Z-bands were the primary alterations in the process of target-targetoid fiber formation, and that this phenomenon was induced both by partial residual innervation as well as re-innervation.

Focal abnormalities in mitochondrial distribution in muscle. Two atypical cases of so-called "central core disease"

Describing two new cases of so-called Central Core Disease, the authors revealed certain atypical features: The cores were formed in central as well as in peripheral position; they were observed in some apparently type II fibers; typical "structured" and "unstructured" cores coexisted with "reversed" core. Starting from this pictures a cycle of core formation was imaginated supposing to be initiated as the consequence of abnormal functional interrelationship between muscular and neural components in early myogenesis.

[Hypertrophy of the transversal tubular system in target fibres. An electron microscopic study (author's transl)]

In a muscle biopsy from a patient suffering about 2 years from an amyotrophic lateral sclerosis groups of atrophic fibres and target fibres were observed. By electron microscopic examination, amongst numerous parallel or spinal cisternae near the plasma membrane, single-membrane bound smooth tubular elements with a constant diameter of 200-300 A occur in the central and intermediary zone of some target fibres. The connexion between these tubules and the terminal cisternae (triads) of the sarcoplasmic reticulum, suggest that this phenomenon seems to be a proliferation of the transversal-tubular system due to the partial denervation. Only a mechanical displacement of the T-tubules as a result of the destruction of the muscle fibres is not plausible, because the T-system volumetrically amounts only 0.3-0.5% of the muscle fibre.

Considerations on the morphogenesis of target and targetoid fibres based on morphometric investigations

A man of 68, who died from a second heart stroke had neurogenic atrophy of the legs. The anterior tibial muscle showed abundant target and targetoid fibres. Half a year prior to death he had increasing weakness and wasting of the legs. The morphometric evaluation of the target and targetoid fibres showed a considerable independence of the plane size of the single zones from the total gauge of the fibres on cross section. Only a small part of the variation in size of zone 2 (4%) depended on the size of the target, indicating that zone 2 cannot just be a passive product of a shrinkage and disappearance of the target, but that there must be an active process, which leads to the change in zone 2. The regressive development of the centrally demarcated fibrillar bundle (target) is regarded as secondary degeneration following interruption of the nutritional supply.

The target phenomenon in human muscle: a comparative light microscopic histochemical and electron microscopic study

A large number of target fibres is observed in a muscle biopsy from a patient with an acute flaccid paraplegia of the lower limbs due to a polyneuroradiculopathy of unknown origin. With the electron microscope up to 4 structural zones are seen in the target fibers. The histochemical and ultrastructural observations in this case, suggest that the "target fiber" phenomenon is due to a disturbed trophic influence of the nervous system.

The relationship between target, targetoid, and targetoid/core fibers in severe neurogenic muscular atrophy

In the m. tibialis anterior of a 68-year-old man with rapidly developing denervation atrophy in the legs since 1/2 year prior to death from heart stroke, abundant unifocal concentric fiber changes, such as target, targetoid/core, and targetoid fibers could be observed. Besides, large vacuolized fibers with multiple changes resembling cytoplasmic bodies in the peripheral zone were present as well; they are interpreted as fibers with multicentric target or targetoid formations. The target fibers displayed a broad variation of their outer appearance suggesting a continuous transition to targetoid/core fibers (with a dense center) and targetoid fibers (with a central change to aquous sarcoplasm showing a paucity of fibrillar structures). Very few fibers with a central densification of fibrillar material with or without a thin intermediate zone were fairly akin to core fibers of central core disease; others were more alike the type of targetoid fibers, previously described in the literature, showing a dense target-like center; both were summarized under the term, inaugurated by Engel et al. (1966), "targetoid/core fibers". Simultaneous occurrence of the different kinds of concentric fiber changes suggested a strong relation between all of them in the sense of representing different developmental stages of the same pathogenetic process. Thus, the central core disease, for instance, might be a disorder with a generalization of concentric fiber changes having come to arrest in the earliest stage of development.