The occurrence of paracrystalline mitochondrial inclusions in normal human skeletal muscle

Diagnosis of primary mitochondrial disorders -Emphasis on myopathological aspects

Mitochondrial disorders are one of the most common neurometabolic disorders affecting all age groups. The phenotype-genotype heterogeneity in these disorders can be attributed to the dual genetic control on mitochondrial functions, posing a challenge for diagnosis. Though the advancement in the high-throughput sequencing and other omics platforms resulted in a "genetics-first" approach, the muscle biopsy remains the benchmark in most of the mitochondrial disorders. This review focuses on the myopathological aspects of primary mitochondrial disorders. The utility of muscle biopsy is not limited to analyse the structural abnormalities; rather it also proves to be a potential tool to understand the deranged sub-cellular functions.

Ultrastructural characterization of peripheral denervation in a mouse model of Type III spinal muscular atrophy

Spinal muscular atrophy (SMA) is a heritable, autosomal recessive neuromuscular disorder characterized by a loss of the survival of motor neurons (SMN) protein, which leads to degeneration of lower motor neurons, and muscle atrophy. Despite SMA being nosographically classified as a motor neuron disease, recent advances indicate that peripheral alterations at the level of the neuromuscular junction (NMJ), involving the muscle, and axons of the sensory-motor system, occur early, and may even precede motor neuron loss. In the present study, we used a mouse model of slow progressive (type III) SMA, whereby the absence of the mouse SMN protein is compensated by the expression of two human genes (heterozygous SMN1A2G, and SMN2). This leads to late disease onset and prolonged survival, which allows for dissecting slow degenerative steps operating early in SMA pathogenesis. In this purely morphological study carried out at transmission electron microscopy, we extend the examination of motor neurons and proximal axons towards peripheral components, including distal axons, muscle fibers, and also muscle spindles. We document remarkable ultrastructural alterations being consistent with early peripheral denervation in SMA, which may shift the ultimate anatomical target in neuromuscular disease from the spinal cord towards the muscle. This concerns mostly mitochondrial alterations within distal axons and muscle, which are quantified here through ultrastructural morphometry. The present study is expected to provide a deeper knowledge of early pathogenic mechanisms in SMA.

Increased intra-mitochondrial lipofuscin aggregates with spherical dense body formation in mitochondrial myopathy

Lipofuscin aggregation may result from incomplete degradation of damaged mitochondria by autophagy-lysosome pathway, and intra-mitochondrial lipofuscin aggregation may exacerbate mitochondrial abnormalities in mitochondrial myopathy (MM) and mitochondrial disease. We examined vastus lateralis muscle biopsies from 24 patients with pathologically diagnosed MM and clinically diagnosed chronic progressive external ophthalmoplegia, in comparison to the biopsies from 3 other groups:10 patients with inclusion body myositis (IBM), 11 younger adults, and 10 older subjects with no to minimal myopathic changes. Lipofuscin aggregation in muscle fibres was assessed on autofluorescence microscopy, some histochemical stains, and electron microscopy (EM). EM analyses demonstrated intra-mitochondrial lipofuscin aggregates, spherical dense bodies (SDBs), and paracrystalline inclusions (PCIs) which were semi-quantitatively assessed. Intra-mitochondrial lipofuscin aggregates showed no significant differences between groups of MM patients and older subjects or IBM patients, but significant differences between groups of younger adults and others with associated age-related changes. Intra-mitochondrial SDBs were significantly more in MM patients than in older subjects, IBM patients, and younger adults. There was a significant positive correlation between intra-mitochondrial lipofuscin aggregates and SDBs. These findings suggest that intra-mitochondrial formation of lipofuscin SDBs is more in MM and contributing to the pathophysiology of mitochondrial disease.

Protein crystallization in living cells

Protein crystallization in living cells has been observed surprisingly often as a native assembly process during the past decades, and emerging evidence indicates that this phenomenon is also accessible for recombinant proteins. But only recently the advent of high-brilliance synchrotron sources, X-ray free-electron lasers, and improved serial data collection strategies has allowed the use of these micrometer-sized crystals for structural biology. Thus, in cellulo crystallization could offer exciting new possibilities for proteins that do not crystallize applying conventional approaches. In this review, we comprehensively summarize the current knowledge of intracellular protein crystallization. This includes an overview of the cellular functions, the physical properties, and, if known, the mode of regulation of native in cellulo crystal formation, complemented with a discussion of the reported crystallization events of recombinant proteins and the current method developments to successfully collect X-ray diffraction data from in cellulo crystals. Although the intracellular protein self-assembly mechanisms are still poorly understood, regulatory differences between native in cellulo crystallization linked to a specific function and accidently crystallizing proteins, either disease associated or recombinantly introduced, become evident. These insights are important to systematically exploit living cells as protein crystallization chambers in the future.

The Spectrum of Mitochondrial Ultrastructural Defects in Mitochondrial Myopathy

Mitochondrial functions are intrinsically linked to their morphology and membrane ultrastructure. Characterizing abnormal mitochondrial structural features may thus provide insight into the underlying pathogenesis of inherited and acquired mitochondrial diseases. Following a systematic literature review on ultrastructural defects in mitochondrial myopathy, we investigated skeletal muscle biopsies from seven subjects with genetically defined mtDNA mutations. Mitochondrial ultrastructure and morphology were characterized using two complimentary approaches: transmission electron microscopy (TEM) and serial block face scanning EM (SBF-SEM) with 3D reconstruction. Six ultrastructural abnormalities were identified including i) paracrystalline inclusions, ii) linearization of cristae and abnormal angular features, iii) concentric layering of cristae membranes, iv) matrix compartmentalization, v) nanotunelling, and vi) donut-shaped mitochondria. In light of recent molecular advances in mitochondrial biology, these findings reveal novel aspects of mitochondrial ultrastructure and morphology in human tissues with implications for understanding the mechanisms linking mitochondrial dysfunction to disease.

Ultrastructural changes in paravertebral muscles associated with degenerative spondylolisthesis

STUDY DESIGN

The paravertebral muscle of 30 patients with spondylolisthesis and 30 control patients were investigated histologically.

OBJECTIVE

To propose myopathologic paravertebral muscle changes in cases of degenerative lumbar spondylolisthesis.

SUMMARY OF BACKGROUND DATA

The stability of the vertebral column is based on both active and passive systems. The passive system is composed of the vertebrae, the intervertebral discs, and the ligaments. Surrounding muscles and tendons constitute the active system. The autochthonous back muscles take over support functions if the passive system is ineffective. In some cases, muscles are overstrained for a long period, ultimately leading to muscular changes. This study was performed to determine the histopathologic correlates of this permanent strain.

METHODS

Between July 1998 and July 1999, paravertebral muscle biopsies were performed for 30 patients with monosegmental degenerative spondylolisthesis undergoing posterior lumbar interbody fusion. The tissue samples were submitted to histologic analysis including immune and enzyme histochemistry and electron microscopy. In addition, the muscle fibers were submitted to morphometry.

RESULTS

Severe pathologic alterations were found. The findings showed that 22 patients (73.3%) had ragged red fibers with evident ultrastructural mitochondrial anomalies. The cristae appeared irregular in 12 patients (40%) Type 1 paracrystalline inclusions were detected in five samples (16.6%) and dense bodies in eight (26.6%). Fibers with ubiquitin-positive inclusions were detected by immunohistochemistry in 13 patients (43.3%). As shown by the electron microscope, these corresponded to granulofilamentous inclusions and polyglucosan bodies. The samples were submitted to genetical analysis because biochemical studies showed reduced activity of the respiratory chain enzymes. Normal mitochondrial deoxyribonucleic acids of unchanged length were detected.

CONCLUSIONS

Apart from nonspecific myopathic changes such as those observed in rimmed vacuoles and rods, increased numbers of polyglucosan bodies were detected. This increase in polyglucosan bodies currently has not been described in patients with otherwise normal muscles.

Mitochondrial myopathies and the role of the pathologist in the molecular era

Mitochondrial encephalomyopathies are under increasing consideration in the differential diagnosis of diverse metabolic diseases from infancy to late adulthood. This is to be expected considering the vital importance of mitochondria to cellular respiration in all eukaryotes. the vulnerability of the mitochondrial genome to injury, and the expanding appreciation of the role of mitochondria as a common denominator in cell death in ischemia/anoxia, sepsis, and neurodegenerative diseases. Primary disease of the mitochondrial respiratory chain is estimated to occur with an incidence of between 6 and 16/100,000 individuals. Virtually all tissues have been shown to be involved in diverse mitochondriopathies, but none is more appropriate for diagnosis in most cases than skeletal muscle. The conventional histological and ultrastructural diagnosis of mitochondrial disease in muscle has been increasingly supplanted by the biochemical assessment of respiratory chain enzyme deficiencies and definitive genetic diagnosis. The use of such techniques has afforded a greater understanding for the relative lack of specificity of both light and electron microscopic observations. A review of the current situation by placing muscle pathology in the context of biochemical and genetic diagnosis serves as a paradigm for the role of the pathologist in the molecular era.

Cerebrotendinous xanthomatosis. Controversies about nerve and muscle: observations in ten patients

Neuromuscular characteristics were documented in ten patients with biochemically and genetically confirmed cerebrotendinous xanthomatosis. An array of genotypes was found in these patients. Only one patient complained of muscle weakness, while clinical signs of peripheral neuropathy were present in six patients. Electromyogram showed predominantly axonal neuropathy in seven patients. Neurogenic changes were seen in muscle biopsies of nine patients. Sural nerve biopsies of three patients showed features of axonal neuropathy. In addition, in one patient, extensive onion bulb formation was seen, which is indicative of a primarily demyelinating process. Five patients had normal mitochondrial respiratory chain enzyme activity. It is concluded that myopathy is not a feature of cerebrotendinous xanthomatosis and that the most prominent neuromuscular abnormality is sensorimotor axonal polyneuropathy.

Adult glycogenosis II with paracrystalline mitochondrial inclusions and Hirano bodies in skeletal muscle

Hirano bodies constitute eosinophilic intracytoplasmic inclusions, typically seen in the central nervous system, where they are related to senility and certain dementias such as Alzheimer's disease or the Parkinson-dementia complex. They have been found in different tissues of experimental animals and, on rare occasions, in extraocular muscles of elderly individuals. However, to our knowledge they have not been described in skeletal muscle in locations other than extraocular muscles or associated with muscle pathology. Glycogenosis II or Pompe's disease, is a metabolic disorder caused by acid maltase deficiency and is characterized by glycogen accumulation in lysosomes in various tissues, including skeletal muscle. There are three clinical forms depending on age at onset, the most frequent being the childhood form. We present the histopathological and ultrastructural findings of a muscle biopsy performed in a case of the adult form of glycogenosis II which showed, in addition to characteristic lysosomal glycogen storage, paracrystalline mitochondrial inclusions and, as an exceptional finding, intracytoplasmic Hirano bodies in some muscle fibres.

Biology and pathology of the mitochondrion

The mitochondrion is a highly efficient organelle that is essential for the function of cells. It is currently accepted that mitochondria originated from primitive nonphotosynthetic bacteria that were engulfed by early eucharyotic cells. Most of the normal and pathologic interactions between mitochondria and the cells that contain them can be viewed as symbiotic processes. The main features of mitochondrial structure and function and some of the pathological disorders that involve this organelle are reviewed.

Mitochondrial diseases and myopathies: a series of muscle biopsy specimens with ultrastructural changes in the mitochondria

From 1986 to 1991, 472 muscle biopsy specimens from patients from different hospitals in Norway were examined. Of these, 364 were embedded for electron microscopy, and 194 were examined with electron microscopy. Ultrastructural alterations in the mitochondria were detected in 49 of these specimens. Characteristic electron microscopic findings included subsarcolemmal accumulation of abnormal mitochondria of various shapes and sizes, often containing electron-dense granules and sometimes lipid vacuoles in the mitochondria and diffusely electron-lucent matrix space. Paracrystalline inclusion bodies were seldom seen in specimens from young patients, but in some cases mitochondrial electron-dense granules at the cristae were found. These amorphous densities are consistent with lipoproteins, suggesting that they may represent an early stage of paracrystalline inclusions. Biochemical and genetic exploration of the patients with biopsy specimens suggesting mitochondrial disease indicated maternally genetic inheritance and an enzyme defect in the respiratory chain in 21 patients in two families. Three patients had MELAS syndrome, 7 Marinesco-Sjögren syndrome, and 2 Kearns-Sayre syndrome. Five family members had ptosis, cardiomyopathy, mild myopathy, and increased lactate in cerebrospinal fluid and serum. In addition to the diseases mentioned above, changes in the mitochondria were detected in other conditions such as Rett's syndrome (n = 1), ornithine transcarbamylase deficiency (n = 2), and hypothyroidism (n = 2) as well as in 3 patients with clinical and laboratory results indicative of inflammatory myopathy and 3 patients with clinical and laboratory findings consistent with peripheral neuropathy. It is concluded that, although ultrastructural changes in the mitochondria may represent unspecific findings, electron microscopic examination of muscle biopsy specimens is a useful screening method to select specimens for further biochemical analysis and to obtain an early and more precise diagnosis of the disease.

Mitochondria and ageing

This work reviews the role of mitochondria in the ageing process and summarizes pathomorphological biochemical and molecular genetic data. The pathophysiological mechanisms underlying the phenomenon of ageing are only partly understood. There is, however, increasing evidence that mitochondria are essentially involved. In various tissues of various species a decline in the respiratory chain capacity is seen with ageing. Enzyme histochemistry of cytochrome c oxidase (complex IV of the respiratory chain) has revealed an age-related increase of randomly distributed defective fibres/cells in the skeletal and heart muscle the random pattern probably indicating cellular heterogeneity of the ageing process. Observed deletions of mitochondrial DNA during ageing may represent one causative factor. Similar to primary mitochondrial myopathies point mutations and depletion of the mtDNA are probably also involved. There is some evidence that damage of the mitochondrial genome and of other mitochondrial structures might be due to increased oxygen radical production during ageing. The role of nuclear influences on the degeneration of mitochondrial function remains, however, also to be determined. Nevertheless, the decline of respiratory chain function with ageing represents an important factor for the decline of functional organ reserve capacity in senescence.

Mitochondrial abnormalities in the postviral fatigue syndrome

We have examined the muscle biopsies of 50 patients who had postviral fatigue syndrome (PFS) for from 1 to 17 years. We found mild to severe atrophy of type II fibres in 39 biopsies, with a mild to moderate excess of lipid. On ultrastructural examination, 35 of these specimens showed branching and fusion of mitochondrial cristae. Mitochondrial degeneration was obvious in 40 of the biopsies with swelling, vacuolation, myelin figures and secondary lysosomes. These abnormalities were in obvious contrast to control biopsies, where even mild changes were rarely detected. The findings described here provide the first evidence that PFS may be due to a mitochondrial disorder precipitated by a virus infection.

Ragged-red fibres detected in paraffin sections by a monoclonal antibody to inner mitochondrial membrane

An immunohistochemical method is reported using the M-II68 monoclonal antibody, which detects mitochondrial accumulations ("ragged-red fibres") in routinely processed (formalin-fixed, paraffin-embedded) muscle tissue. Ten cases with electron-microscopically and histochemically proven mitochondrial myopathy featured 4% to 24% ragged-red fibres. In a series of 50 muscle biopsies without mitochondrial myopathy, scattered ragged-red fibres (less than 0.1%) were present in a few normal and pathological muscles. The immunohistochemical method is specific for mitochondria, does not require frozen tissue and permits rapid examination of large areas.

Variation in retinal changes and muscle pathology in mitochondriopathies

A variety of retinal changes that have so far not been classified under mitochondriopathies can now be included in this group, since muscle biopsy has identified ragged-red fibers with pathological mitochondriae. The ophthalmological findings in our relatively large group of 12 patients with mitochondrial myopathies are compared with the spectrum of myopathic findings. No obvious correlation exists between the severity of the pathological retinal changes and the characteristic of the myopathic alterations. In addition to fine pigmentation and depigmentation, severe dystrophic changes of the retina, pigment epithelium, and the choroid were observed. In two patients with severe chorioretinal dystrophy the correlation with generalized mitochondriopathy was not suspected prior to muscle biopsy.

Mitochondria-related encephalomyopathies

Owing to advances in morphological and biochemical techniques, the mitochondria-related myopathies and encephalomyopathies have emerged as a still rapidly growing group of primary and secondary metabolic disorders, which may extend from infancy to late adulthood. Impairment of the biochemically diversified mitochondria is reflected in an enormous number of deficiencies, often affecting several mitochondrial enzymes in the same patient; morphologically abnormal mitochondria are common and are thus not specific to individual mitochondrial enzyme deficiencies. Skeletal muscle biopsies have provided a wealth of data through histological and histochemical studies and from isolated mitochondria. As a similar abundance of biochemical and morphological findings has not been obtained from brain tissue in mitochondrial encephalomyopathies, investigation of these disorders is still in its infancy; interpretation of these conditions and their encephalopathic components has largely been based on comparison of data not derived from brain tissues. Therefore, it has been, and still is, largely the link between an encephalopathy and an associated mitochondrial myopathy that identifies the brain lesions as clinical and morphological expressions of a mitochondrial defect. As enzyme histochemical and electron microscopic investigations of mitochondrial encephalopathies have not yielded a comparable rich spectrum of morphological findings, it is conceivable that the spectrum of mitochondrial encephalopathies may be much larger than defined by the hitherto identified encephalomyopathies. This may be especially so when the myopathic component is of minor nosological significance.

Two types of mitochondrial crystals in diseased human skeletal muscle fibers

Mitochondrial crystalline inclusions, frequently found in mitochondrial myopathies, were analyzed by crystallographic techniques and computer-aided image processing. It could be shown that these structures were real crystals. There are two distinct types of crystal, which can be distinguished by shape, size, and pattern. So-called type I crystals are usually present in the intracristal space, whereas the type II crystals are preferentially located in the intermembrane space between outer and inner mitochondrial membranes. The unit cell dimensions were found to be 38 x 34 x 8 nm for the type I crystals and 20 x 17 x 8 nm for the type II crystals. These results strongly suggest that the crystals are composed of macromolecules, presumably proteins. Arguments are presented that indicate that type I crystals occur only in type 1 muscle fibers and type II crystals in type 2 muscle fibers.

Morphological observations in skeletal muscle from patients with a mitochondrial myopathy

Mitochondrial metabolic dysfunction is considered to be the cause of certain congenital myopathies and a number of multisystem disorders in humans. The morphological hallmark of these diseases is the 'ragged red' fibre, which shows abnormally intensive oxidative enzyme reactions. Electron microscopy reveals that the numerically increased mitochondria in these fibres are often markedly enlarged and possess aberrant configurations of cristae. The mitochondrial matrix often contains lipid-like inclusions or shows vacuolation. The most characteristic mitochondrial abnormality is the occurrence of highly ordered inclusions in the intracristal or intermembrane space. These inclusions appear to be true crystals, composed of proteinaceous material. It is argued that the activity of accumulation of proteins in the mitochondria is related to the nuclear and nucleolar hypertrophy noticeable in the ragged red fibres. Since protein crystals in mitochondria in particular occur when an increased capillary density around the ragged red fibres is present, it is suggested that oxygen free radicals and lipid peroxidation processes are involved in the ragged red fibre pathology.

Ultrastructural modifications of muscle in three types of compartment syndrome

The histological and ultrastructural aspects of three cases representing different types of compartment syndromes are analyzed. In the acute syndrome edema is the prominent feature. The two chronic cases are characterized by an accumulation of intermyofibrillar lipid globules. All three have in common mitochondrial enlargement, disorganization of the cristae and paracrystalline inclusions. These pathological findings are discussed in the light of lesions seen in striated muscle ischaemia or in certain metabolic myopathies.

Myopathy with mitochondrial abnormalities and rimmed vacuoles

A man of 44 years suffering from an exercise-induced neuromuscular disease with mitochondrial abnormalities and rimmed vacuoles is reported. The mitochondrial abnormalities and rimmed vacuoles (autophagic vacuoles) are interpreted as sequential changes of the same pathogenetic process depending on the degree of energy deficiency.

Chronic relapsing myalgia (? Post viral): clinical, histological, and biochemical studies

Two patients with persistent myalgia characterised by onset after an ill-defined systemic illness, marked fluctuations in the severity of the symptoms, and normal neuromuscular examination with the exception of variable muscle tenderness on deep palpation, may have a forme fruste of myalgic encephalomyelitis. Differentiation from psychogenic muscle pain is important in management. Muscle histology revealed non-specific Type II fibre atrophy. Mitochondrial respiration was assayed polarographically in intact organelles in vitro and revealed a mild depression of State 3 respiration rates with Site I and Site II substrates.

Unusual ultrastructural findings in neuroblastoma

A morphologic study was made of the cell population which had infiltrated the bone marrow of a five-year-old boy. These cells showed a tendency to form rosette-like structures. These structures as well as the presence (at ultrastructural level) of neurosecretory granules, cell processes, and microtubules in the neoplastic cells led to a diagnosis of neuroblastoma. Certain characteristics, not previously reported in neuroblastoma, were identified, such as gap junction type intercellular contacts, paracrystalline arrays in mitochondrial atpyical cristae and nucleolus-like bodies (nematosomes). Gap junctions are involved in the intercellular transfer of ions and low molecular weight metabolites and may explain the tendency to form cellular cluster in "rosettes" which are characteristic of this neoplasm. The presence of nematosomes in the tumor cell cytoplasm is one more piece of evidence which substantiates the nervous origin of these cells.

[An autosomal recessive syndrome with myopathy and central and peripheral nervous system involvement (author's transl)]

Three of 11 children, offspring of a consanguineous marriage, presented a progressive myopathy and seizures, associated with symptoms suggesting both central and peripheral nervous system involvement. The ultrastructural muscular lesions were not specific. The association of severe impairment of muscle tissue and of central nervous system is rare, being described in centronuclear myopathy, cerebromuscular dystrophy, Kearns-Sayre syndrome and in a few isolated cases. Clinically only these isolated observations and especially the Kearns-Sayre syndrome demonstrate analogies to our observations. These data lead us to the discussion of the specificity of ultrastructural lesions, especially mitochondrial abnormalities. Some authors consider these abnormalities to be the biochemical hallmark for ophthalmoplegia plus, whereas for others, especially Drachman, they are an inconstant and nonspecific finding, merely the consequence and not the cause of this disease. These observations argue for the relationship between muscular pathology and nervous system dysfunction.