"Megaconial" mitochondria observed in a case of chronic polymyositis

The non-glycosylated isoform of MIC26 is a constituent of the mammalian MICOS complex and promotes formation of crista junctions

Mitochondrial membrane architecture is important for organelle function. Alterations thereof are linked to a number of human disorders including diabetes and cardiomyopathy. The MICOS complex was recently reported to be a central player determining cristae structure and formation of crista junctions. Here we investigated the functional role of MIC26, a lipoprotein formerly termed APOO. Its levels are increased in diabetic heart tissue and in blood plasma of patients suffering from acute coronary syndrome. We demonstrate that human MIC26 exists in three distinct forms: (1) a glycosylated and secreted 55kDa protein, (2) an ER/Golgi-resident form thereof, and (3) a non-glycosylated 22kDa mitochondrial protein. The latter isoform spans the mitochondrial inner membrane and physically interacts with several MICOS complex subunits such as MIC60, MIC27, and MIC10. We further demonstrate that MIC26 and MIC27, a homologous protein formerly termed APOOL, regulate their levels in an antagonistic manner. Both proteins are positively correlated with the levels of MIC10 as well as tafazzin, an enzyme required for cardiolipin remodeling. Overexpression of MIC26 induced fragmentation of mitochondria, promoted ROS formation and resulted in impaired mitochondrial respiration. Downregulation of MIC26 induced a decrease in mitochondrial oxygen consumption, whereas mitochondrial network morphology and ROS levels remained unaffected. MIC26 depletion led to alterations in mitochondrial ultrastructure and caused a significant reduction in the number of crista junctions. In summary, we show that the human apolipoprotein MIC26 is a bona fide subunit of the MICOS complex and that MIC26 is linked to cardiolipin metabolism and promotes crista junction formation.

Multiple mitochondrial alterations in a case of myopathy

Mitochondrial alterations are the most common feature of human myopathies. A biopsy of quadriceps muscle from a 50-year-old woman exhibiting myopathic symptoms was examined by transmission electron microscopy. Biopsied fibers from quadriceps muscle displayed numerous subsarcolemmal mitochondria that contained crystalloids. Numbering 1-6 per organelle, these consisted of rows of punctuate densities measuring ∼0.34 nm; the parallel rows of these dots had a periodicity of ∼0.8 nm. The crystalloids were ensconced within cristae or in the outer compartment. Some mitochondria without crystalloids had circumferential cristae, leaving a membrane-free center that was filled with a farinaceous material. Other scattered fibrocyte defects included disruption of the contractile apparatus or its sporadic replacement by a finely punctuate material in some myofibers. Intramitochondrial crystalloids, although morphologically striking, do not impair organelle physiology to a significant degree, so the muscle weakness of the patient must originate elsewhere.

Multiple muscle cell alterations in a case of encephalomyopathy

Skeletal muscle from an encephalomyopathy was examined by morphological and biochemical modalities. Mitochondria displayed variability in size, numbers per myocyte, and morphology. Certain organelles had stacks of dense cristae, others contained variable numbers of crystalloids or several lipid droplets. In isolated skeletal muscle mitochondria, oxidative phosphorylation was reduced, but activities of the electron transport chain components were unaffected. This is the second case of adult onset encephalomyopathy with a phenotype overlapping MERRF and Kearns-Sayre syndrome associated with a heteroplasmic mtDNA 3255G > A mutation in the tRNA(UUR(LEU)). This study emphasizes the desirability of a multidisciplinary approach in the diagnosis of complex myopathies.

Electron microscopic findings in levator muscle biopsies of patients with isolated congenital or acquired ptosis

OBJECTIVE

Systemic mitochondriopathies as chronic progressive external ophthalmoplegia (CPEO) are frequently associated with ptosis. We investigated whether mitochondrial abnormalities in the levator muscle are also found in patients with isolated congenital or acquired ptosis showing no other signs of mitochondrial cytopathy.

METHODS

Biopsies of levator muscle were taken during surgery from 24 patients with isolated congenital (group 1) or early-onset acquired ptosis (group 2). All patients were given a thorough clinical examination before and after surgery. Ultrathin muscle sections were examined by transmission electron microscopy. The findings were compared with biopsies from five patients with CPEO (positive control) and two patients with traumatic ptosis or pseudoptosis (negative control).

RESULTS

The mean levator function equalled 7.3 mm (range 4-10 mm) in group 1 and 12.8 mm (range 9-15 mm) in group 2. Eight out of 11 patients in group 1 and eight out of 13 patients in group 2 were found to have mitochondrial alterations such as megamitochondria, mitochondrial matrix alterations and abnormal cristae, similar to CPEO. Within group 1 and 2, no significant clinical differences were found between patients with and without mitochondrial abnormalities.

CONCLUSION

Mitochondrial alterations were found in a surprisingly large proportion of levator biopsies from patients with isolated congenital or early-onset acquired ptosis. There was no statistically significant correlation between mitochondrial alterations and levator function. Our findings suggest that the ultrastructural assessment of mitochondria in the eyelid muscle is a valuable tool, and may guide further biochemical and mutation screening tests that will help to understand the etiopathology of this disease.

Les maladies mitochondriales : mécanismes moléculaires, principaux cadres cliniques et approches diagnostiques

Mitochondrial diseases are relatively common inherited metabolic diseases due to mitochondrial respiratory chain dysfunction. Their clinical presentation is extremely diverse, multisystemic or confined to a single tissue, sporadic or transmitted, by maternal or mendelian inheritance. The diagnosis of mitochondrial disorders is difficult. It is based upon several types of clues both clinical (family history, type of symptoms but also their association in syndromic presentation,...) and biological (alteration of the lactate metabolism, brain imaging, morphological alterations especially of muscle tissue). The diagnosis relies upon the demonstration of a defect of the respiratory chain activities and/or upon the identification of the underlying genetic alteration. Molecular diagnosis remains quite difficult and up to-date concerns essentially mitochondrial DNA mutations. On one hand, clinical and biological presentations as well as enzymatic defects lack specificity. On the other hand, candidate genes are very numerous and part of them are probably still unknown.

Megamitochondria formation - physiology and pathology

Mitochondria undergo structural changes simultaneously with their functional changes in both physiological and pathological conditions. These structural changes of mitochondria are classified into two categories: simple swelling and the formation of megamitochondria (MG). Data have been accumulated to indicate that free radicals play a crucial role in the mechanism of the MG formation induced by various experimental conditions which are apparently various. These include ethanol-, chloramphenicol- and hydrazine-induced MG formation. Involvement of free radicals in the mechanism of MG formation is showed by the fact that MG formation is successfully suppressed by free radical scavengers such as alpha-tocopherol, coenzyme Q(10), and 4-OH-TEMPO. Detailed mechanisms and pathophysiological meanings of MG formation still remain to be investigated. However, a body of evidence strongly suggests that enormous changes in physicochemical and biochemical properties of the mitochondrial membranes during MG formation take place and these changes are favorable for membrane fusion. A recent report showed that continous exposure of cells with MG to free radicals induces apoptosis, finding which suggests that MG formation is an adaptative process to unfavorable environments at the level of intracellular organelles. Mitochondria try to decrease intracellular reactive oxygen species (ROS) levels by decreasing the consume of oxygen via MG formation. If mitochondria succeed to suppress intracellular ROS levels, MG return to normal both structurally and functionally, and they restore the ability to actively synthesize ATP. If cells are additionally exposed to excess amounts of free radicals, MG become swollen, membrane potential of mitochondria (DeltaPsim) decreases, cytochrome c is released from mitochondria, leading to activation of caspases and apoptosis is induced.

Diagnostic yield muscle biopsy in patients with clinical evidence of mitochondrial cytopathy

We retrospectively reviewed 118 muscle biopsy specimens from 113 patients with clinical and/or biochemical evidence of mitochondrial cytopathy. Light microscopic evaluation revealed histologic abnormalities in 65 specimens. The most common histologic findings included angular atrophic esterase-positive muscle fibers, type II muscle atrophy, regenerating muscle fibers, and scattered cytochrome-oxidase deficient fibers. Ragged red fibers were noted in 3 specimens on a Gomori trichrome stain. Electron microscopic evaluation was performed in 113 muscle specimens, and in 34, no abnormalities were identified. Increased numbers of mitochondria, particularly in the subsarcolemmal region, were identified in 54 specimens. Increased mitochondrial size was seen in 8 specimens and paracrystalline mitochondrial inclusions in 3. Other ultrastructural findings included focally increased glycogen deposition, focal Z-band streaming, and focally increased lipid accumulation. For 39 cases, concomitant skin biopsy specimens were available; abnormalities were identified by electron microscopy in 12. The majority of biopsy specimens demonstrated some light or electron microscopic abnormality. Specific histologic findings suggestive of mitochondrial abnormalities (partial cytochrome oxidase deficiency, ragged red fibers) were noted in a minority of cases. Ultrastructural evidence of mitochondrial abnormalities was noted in the majority of cases.

Histochemical and ultrastructural characteristics of leg muscle fibres in patients with repairative abdominal aortic aneurysm (AAA)

Tibialis anterior (ta) muscle biopsies before and after elective abdominal aortic aneurysm (AAA) repair operation were obtained, in order to observe possible changes after the aortic declamping reperfusion. Open muscle biopsies were taken from each of eight patients (60-75 years old) which were processed for enzyme histochemistry, and for transmission electron microscopy (EM). Morphometric analysis was applied to estimate the number and the area of muscle fibres of each fibre type. Rectus abdominis muscle biopsies were served as controls. Before the operation the predominant elements found were the presence of atrophic muscle fibres, fibre size diversity, localised cellular reactions, increased extent of connective tissue, disappearance, in many cases, of the mosaic pattern, predominance of type I and oxidative fibres, and existence of fibres with core-like structures in the sarcoplasm. Type I fibres consisted of 66.95 +/- 9% of all muscle fibres, the mean cross sectional area of which was 3,372.8 +/- 1,016 microm(2) and of type II fibres was 3,786.5 +/- 6,046 microm(2). After the aortic clamping was performed mitochondrial swelling was found, as well as disorganisation of sarcomeres. After declamping of the aorta, there were also severe edema, local fibre necrosis, and adhesion of leucocytes, whereas muscle fibre areas became 3,935.18 micro 531 microm(2) for type I and 5,804 +/- 1,075 microm(2) for type II. The short ischemic period during aortic clamping and the subsequent reperfusion resulted mainly in ultrastructural changes.

Polymyalgia rheumatica and mitochondrial myopathy: clinicopathologic and biochemical studies in five cases

PURPOSE

The coexistence of mitochondrial myopathy and polymyalgia rheumatica without giant cell arteritis is an interesting association. The frequency of this association was assessed in a prospective study.

PATIENTS AND METHODS

Muscle biopsy specimens were obtained from 15 patients with polymyalgia rheumatica. When ragged red fibers (RRF) were observed, histochemical, ultrastructural, and biochemical studies were performed.

RESULTS

In five cases, we found the typical appearance of mitochondrial myopathy, with the presence of numerous RRF. Histochemical and biochemical results confirmed these mitochondrial myopathies, showing miscellaneous deficiencies of mitochondrial respiratory chain enzymes.

CONCLUSION

Persistence of histologic and biochemical abnormalities after steroid treatment in two patients seems to indicate that a subclinical mitochondrial myopathy preceded polymyalgia rheumatica. How a mitochondrial myopathy could induce or facilitate the emergence of a polymyalgia rheumatica remains unknown.

Low succinate dehydrogenase (SDH) activity in a patient with a hereditary myopathy with paroxysmal myoglobinuria

An unusual hereditary myopathy with paroxysmal myoglobinuria has been described previously. We have studied muscle biopsy specimens taken before and after exercise to exhaustion (24 min at 20-25 W) in a young woman with this condition. Marked glycogenolysis with lactate production and marked phosphagen breakdown (ATP + CP) were observed after exercise, and almost all type I fibres were found to be depleted of glycogen. Succinate dehydrogenase (SDH) activity was low, while the activities of 3-OH-acyl-CoA-dehydrogenase, phosphofructokinase, phosphorylase and lactate dehydrogenase were normal. On electron microscopy, the mitochondria showed abnormalities typical of mitochondrial myopathy. The findings in our patient suggest a limitation of mitochondrial function, probably related to SDH in the tricarboxylic acid cycle and complex II in the electron transport chain. This may explain the inability of ATP regeneration to keep pace with ATP utilization during exercise. Other metabolic defects may coexist.

Recent advances in the morphology of myositis

Myositis in man may be divided into infectious and non-infectious forms. The myopathologist more often deals with the latter forms which comprise dermatomyositis/polymyositis, inclusion body myositis, mixed connective tissue disease/collagenoses, and granulomatous myopathies. Modern morphological techniques as enzyme-histochemistry, electron microscopy, immunohistology, and morphometry are of different value in various forms of myositis, but are often indispensable techniques in up-to-date diagnostic work up of a myositis.

Mitochondrial myopathies

Mitochondrial myopathies are clinically heterogeneous disorders that can affect multiple systems besides skeletal muscle (mitochondrial encephalomyopathies or cytopathies) and are usually defined by morphological abnormalities of muscle mitochondria. There are a few distinctive syndromes, such as the Kearns-Sayre syndrome; myoclonus epilepsy with ragged-red fibers; and mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes. Biochemically, mitochondrial myopathies can be divided into defects of substrate utilization, oxidation-phosphorylation coupling, and the respiratory chain. Because mitochondria have their own DNA and their own translation and transcription apparatuses, mitochondrial myopathies can be due to defects of either a nuclear or mitochondrial genome and can be transmitted by mendelian or maternal inheritance.

Ultrastructural examination of the skeletal muscles in polymyalgia rheumatica

Skeletal muscle biopsies of 13 patients with polymyalgia rheumatica (PMR) were examined systematically with the electron microscope. Ultrastructural changes in the PMR specimens have been classified according to 14 criteria including nucleus, myofilaments, mitochondria, T-system, as well as glycogen deposition, lipid, lipofuscin, and myelin figures. The described muscle changes are of both regressive and progressive character and non-specific in themselves. The systematic recording of these criteria shows an extraordinaryly high incidence in PMR. This constellation of characteristics gives the ultrastructural picture of the skeletal musculature in PMR a definite profile. Similarities with the changes in other muscular diseases suggest that in PMR also neurogenous mechanisms may play a pathogenetic role.

[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.

Myoclonus epilepsy associated with ragged-red fibres (mitochondrial abnormalities ): disease entity or a syndrome? Light-and electron-microscopic studies of two cases and review of literature

A report is given of an association of dyssynergia cerebellaris myoclonica associated with Friedreich's ataxia and mitochondrial myopathy in 2 patients. They had suffered from gradually increasing bursts of myoclonus since the wage of 14 and childhood, respectively. The other striking clinical features included generalized convulsions, mental deterioration, intention tremor, ataxia, muscular atrophy and deformity of feet. Muscle biopsies revealed ragged-red fibres in both cases. On electron microscopy these fibres contained subsarcolemnal aggregations of abundant abnormal mitochondria with proliferation of inner membranes or paracrystalline inclusions. One of these patients showed elevated blood lactate and pyruvate with an increased lactate/pyruvate ration, apparently of primary origin. These 2 cases resemble those reported briefly by Tsairis et al. (1974). An association of dyssynergia cerebellaris myoclonica associated with Friedreich's ataxia and mitochondrial myopathy in these 2 patients is unlikely to be coincidental but may represent one nosological entity. This myoclonus epilepsy syndrome associated with ragged-red fibres is compared with other possibly related mitochondrial encephalomyopathies.

Ultrastructural study of globular inclusions in human skeletal muscle mitochondria

Limb muscle biopsies from a patient with Luft's disease and a patient with a slowly progressive neuromuscular disorder since infancy revealed by conventional electron microscopy the presence of globular inclusions in the mitochondria as one of the most prominent morphological findings. Electron cytochemical studies on fresh tissue blocks showed no cytochrome c oxidase activity within the globular inclusions. The study of strontium uptake supported by either NAD and flavo-protein linked substrates in freshly isolated mitochondria fractions showed no electron-dense needles within the globular inclusions. Attempts to remove the inclusions with pepsin and with pronase on ultrathin sections failed but they were partially and totally removed by treatment of the sections with hydrogen peroxide. Freeze fracture studies showed the globular inclusions consisted of amorphous and lamellar material. The results suggest that the globular inclusions in muscle mitochondria may consist primarily of lipid.

Mitochondrial abnormalities of late motor neuron degeneration following poliomyelitis and other neurogenic muscular atrophies

A case of late motor neuron degeneration following poliomyelitis with abnormal mitochondria in muscle fibers is presented with two additional cases of systemic neurogenic muscular atrophy (Charcot-Marie-Tooth disease). Muscle biopsy revealed a neurogenic pattern of variable severity in all cases. Subsarcolemmal zones of hyperactivity and hyperpositive intermyofibrillar collections of granular material present in a variable proportion of type I fibers were demonstrated by oxidative enzymes. Ultrastructurally they corresponded to abnormal mitochondria, with paracrystalline inclusions in one case. The finding is discussed in the light of the previous literature on mitochondrial myopathies. Mitochondrial alterations are not specific and their significance in neurogenic conditions is debated.

On the pathogenesis of mitochondrial myopathies. An experimental study

The intra-arterial injection of 2-4 dinitrophenol, an uncoupler of oxidative phosphorylation, resulted in the production of ragged red fibers. The ultrastructure of these fibers showed intramitochondrial paracrystalline inclusions, laminar and fingerprint bodies. Antimycin A and oligomycin injection (which inhibit mitochondrial respiration) only caused swelling and disruption of the mitochondria. An increase in muscle lactic acid, decrease in ATP, glycogen and phosphocreatine was observed after the injection of all these agents. This indicates that lactic acidosis has no significant role in the pathogenesis of mitochondrial pathology. It is concluded that mitochondrial changes are a morphological expression of uncoupled but intact mitochondrial respiration.

The early effects of ischemia upon skeletal muscle mitochondria

The effects of early ischemia were studied in the anterior tibial muscle of Sprague-Dawley rats after 2--24 hr of tourniquet compression at the thigh. Ragged-red fibers, moth-eaten fibers, cores and targets were seen in tissue examined by enzyme histochemistry and electron microscopy. Giant mitochondria, abnormalities of cristal arrangement, crystalloids, osmiophilic inclusion bodies and myeloid figures were dominant features of the mitochondrial reaction. The results of this experiment indicate that early ischemia induces a variety of changes described in other neuromuscular conditions such as dystrophy and the "mitochondrial myopathies". The pathogenesis of these changes and their relationship to human disease of muscle is discussed.

The significance of ragged-red fibres in neuromuscular disease

The pathological significance of ragged-red fibres is uncertain. We have studied ragged-red fibres in the muscle biopsies of 3 adults; one with polymyositis and two with progressive external ophthalmoplegia. All the ragged-red fibres were Type 1 fibres. In two patients the mean diameter of the ragged-red fibres was significantly smaller than the unaffected Type 1 fibres. Some of these fibres showed features of regeneration, and others of degeneration. In the patient with polymyositis the mitochondria were proliferated and contained osmiophilic dense bodies; in the other two patients paracrystalline mitochondrial inclusions were prominent. These findings suggest that ragged-red fibres do not represent a single pathological process.

[Intracristal linear inclusions in mitochondria of human rhabdomyoma cells (author's transl)]

Electron microscopic study of a rhabdomyoma of the soft palate of a 50-year-old woman revealed linear structures within the intracristal spaces of mitochondria. In the center of most cristae, an electron-dense line was found located in parallel arrangement to the limiting cristal membrane. The linear inclusion was separated from the cristal membrane by a narrow space of lower electron-density. Thus, when sectioned longitudinally, the abnormal cristae exhibited a pentalaminar appearance. Additionally, they were characterized by a greater diameter than that of normal cristae and a "rigid" configuration. Higher magnification showed that the intracristal line consisted of a row of periodically arranged, electron-dense dots. Sometimes, these seemed to be connected with the wall of the intracristal space by fine, electron-dense cross-striations. In some mitochondria of rhabdomyoma cells, crystals with a linear substructure were observed in both intra- and extracristal spaces. The crystalline bodies located within cristae had probably been formed by accumulation of linear inclusions. The extracristal bodies seemed to have arisen from fusion of pentalaminar cristae. It is supposed that the intracristal linear inclusions in mitochondria of rhabdomyoma cells represent crystallized proteins of mitochondrial enzymes.

Morphological changes in skeletal muscle after transplantation. A light and electron-microscopic study of the initial phases of degeneration and regeneration

The morphological changes in subcutaneously implanted muscle homografts in mice were studied by light and electron microscopy 1-23 days after transplantation. The initial degenerative changes were identical in isografts and allografts and were essentially thesame as those found in muscle explants in tissue culture and aftervarious forms of muscle injury. Regnerative changes were prominent at the periphery of graftsby 48-72 hr beforeevidence of graft revascularization could be demonstated by India ink perfusion. Active regeneration occurred in isografts and in allografts during the first week resulting in the formation of a new population of muscle fibres. Rejection subsequently occurredin allografts between days 7-14. Regeneration was retarded or completely inhibitied by exposure of the donor animal to 1,500 rad of X-irradiation 1-2 hours prior to transplantationsuggesting that regeneration is brough about by cells dervied from the graft. Eectron-microscopic observations in 48-72 hr grafts suggested that primitive mononucleated myogenic cellsmay form within degeneratiang muscle fibres by a process of myonuclear sequestration. Inactive satellite cells were present in the muscle from which grafts were taken in some donoranimals but no definite conclusions could be reached as to the role of these cells in graftsregeneration.

[Mitochondrial changes of the skeletal muscle in the peroneal muscular atrophy (Charcot-Marie-Tooth disease). Histological and electron microscopic studies (author's transl)]

This report deals with two sisters (38 and 44 years old) suffering from Charcot-Marie-Tooth disease. Muscle biopsies were taken from the deltoid and the rectus femoris. In one of the cases a sural nerve biopsy was also made. Light microscopy showed only slight myopathic changes. The histochemical reactions disclosed an increase in lipid deposition and in NADH-TR activity of type 1 fibres. Electron microscopy showed abnormal mitochondrial aggregates, which were surrounded inconstantly by glycogen deposits, especially in subsarcolemmal space. Many of the atypical mitochondria showed paracristalline inclusions within the cristae. The significance of these findings is discussed and compared with similar reports in the literature.

Mitochondrial functions in five cases of human neuromuscular disorders

We determined the respiration, respiratory control, and Pi:O ratios with different substrates in mitochondria isolated from five cases of human neuromuscular disorders (two cases of central core disease, two cases of neuropathy of Dejerine-Sottas, and one case of Kugelberg-Welander's disease) and compared them with normal human muscle. In all the myopathies studied, a severe derangement of the respiratory control with variable derangement of oxidative phosphorylation was found. This supports the idea that a group of neuromyopathies shares the same biochemical lesion as the so-called mitochondrial myopathies, forming with them a group of myopathies which may be related through a similar biochemical lesion of varying degree. Alternatively, disturbance of mitochondrial functions in a number of myopathies could be considered as a non-specific finding.