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Biomedical subjects

A G Engel

Publications and source records attributed to A G Engel.

At least 163 records · Page 9Linked to original sources
PubMed1984

Myasthenia gravis and myasthenic syndromes.

More than a decade ago myasthenic symptoms were observed in rabbits immunized with acetylcholine receptor (AChR) [119] and AChR deficiency was found at the neuromuscular junction in human myasthenia gravis (MG) [36]. By 1977 the autoimmune character of MG and the pathogenic role of AChR antibodies had been established by several measures. These included the demonstration of circulating AChR antibodies in nearly 90% of patients with MG [87], passive transfer with IgG of several features of the disease from human to mouse [149], localization of immune complexes (IgG and complement) on the postsynaptic membrane [30], and the beneficial effects of plasmapheresis [20, 123]. Substantial subsequent progress has occurred in understanding the structure and function of AChR and its interaction with AChR antibodies. The relationships of the concentration, specificities, and functional properties of the antibodies to the clinical state in MG have been carefully analyzed, and the mechanisms by which AChR antibodies impair neuromuscular transmission have been further investigated. The clinical classification of MG has been refined, the role of the thymus gland in the disease has been further clarified, and new information has become available on transient neonatal MG. The prognosis for generalized MG is improving, but there is still no consensus on its optimal management. Novel therapeutic approaches to MG are now being explored in animal models. Recognition of the autoimmune origin of acquired MG also implied that myasthenic disorders occurring in a genetic or congenital setting had a different cause. As a result, a number of congenital myasthenic syndromes have come to be recognized and investigated. Finally, an acquired disorder of neuromuscular transmission different from MG, the Lambert-Eaton myasthenic syndrome, has also been shown to have an autoimmune basis. In this syndrome, active zone particles of the presynaptic membrane are direct or indirect targets of the pathogenic autoantibodies.

Acetylcholinesterase↗
PubMed1984

Carnitine metabolism and inborn errors.

Current knowledge of the metabolic role, biosynthesis, cellular uptake, excretion and turnover of carnitine is reviewed. The clinical spectrum and possible aetiology of the primary muscle and primary systemic carnitine deficiency syndromes are considered and the various genetic defects of intermediary metabolism which can give rise to secondary carnitine deficiency are indicated.

Carnitine↗
PubMed1984

Kinetic compartmental analysis of carnitine metabolism in the human carnitine deficiency syndromes. Evidence for alterations in tissue carnitine transport.

The human primary carnitine deficiency syndromes are potentially fatal disorders affecting children and adults. The molecular etiologies of these syndromes have not been determined. In this investigation, we considered the hypothesis that these syndromes result from defective transport of carnitine into tissues, particularly skeletal muscle. The problem was approached by mathematical modeling, by using the technique of kinetic compartmental analysis. A tracer dose of L-[methyl-3H]carnitine was administered intravenously to six normal subjects, one patient with primary muscle carnitine deficiency (MCD), and four patients with primary systemic carnitine deficiency (SCD). Specific radioactivity was followed in plasma for 28 d. A three-compartment model (extracellular fluid, muscle, and "other tissues") was adopted. Rate constants, fluxes, pool sizes, and turnover times were calculated. Results of these calculations indicated reduced transport of carnitine into muscle in both forms of primary carnitine deficiency. However, in SCD, the reduced rate of carnitine transport was attributed to reduced plasma carnitine concentration. In MCD, the results are consistent with an intrinsic defect in the transport process. Abnormal fluctuations of the plasma carnitine, but of a different form, occurred in MCD and SCD. The significance of these are unclear, but in SCD they suggest abnormal regulation of the muscle/plasma carnitine concentration gradient. In 8 of 11 subjects, carnitine excretion was less than dietary carnitine intake. Carnitine excretion rates calculated by kinetic compartmental analysis were higher than corresponding rates measured directly, indicating degradation of carnitine. However, we found no radioactive metabolites of L-[methyl-3H]carnitine in urine. These observations suggest that dietary carnitine was metabolized in the gastrointestinal tract.

Adult↗
PubMed1983

Kinetic compartmental analysis of carnitine metabolism in the dog.

This study was undertaken to quantitate the dynamic parameters of carnitine metabolism in the dog. Six mongrel dogs were given intravenous injections of L-[methyl-3H]carnitine and the specific radioactivity of carnitine was followed in plasma and urine for 19-28 days. The data were analyzed by kinetic compartmental analysis. A three-compartment, open-system model [(a) extracellular fluid, (b) cardiac and skeletal muscle, (c) other tissues, particularly liver and kidney] was adopted and kinetic parameters (carnitine flux, pool sizes, kinetic constants) were derived. In four of six dogs the size of the muscle carnitine pool obtained by kinetic compartmental analysis agreed (+/- 5%) with estimates based on measurement of carnitine concentrations in different muscles. In three of six dogs carnitine excretion rates derived from kinetic compartmental analysis agreed (+/- 9%) with experimentally measured values, but in three dogs the rates by kinetic compartmental analysis were significantly higher than the corresponding rates measured directly. Appropriate chromatographic analyses revealed no radioactive metabolites in muscle or urine of any of the dogs. Turnover times for carnitine were (mean +/- SEM): 0.44 +/- 0.05 h for extracellular fluid, 232 +/- 22 h for muscle, and 7.9 +/- 1.1 h for other tissues. The estimated flux of carnitine in muscle was 210 pmol/min/g of tissue. Whole-body turnover time for carnitine was 62.9 +/- 5.6 days (mean +/- SEM). Estimated carnitine biosynthesis ranged from 2.9 to 28 mumol/kg body wt/day. Results of this study indicate that kinetic compartmental analysis may be applicable to study of human carnitine metabolism.

Animals↗
PubMed1983

Passive transfer of Lambert-Eaton myasthenic syndrome with IgG from man to mouse depletes the presynaptic membrane active zones.

In the Lambert-Eaton myasthenic syndrome (LEMS), there is a decreased release of acetylcholine quanta from the nerve terminal by nerve impulse. Recently, an autoimmune origin of LEMS was documented by passive transfer of its electrophysiologic features from man to mouse with IgG. Freeze-fracture electron microscopy of LEMS neuromuscular junctions has revealed a paucity of presynaptic membrane active zones. Thus, the active zones might be the targets of the pathogenic autoantibodies in LEMS. To test this assumption, freeze-fracture electron microscopic studies were done in mice injected with 10 mg of IgG daily from each of three LEMS patients and in control mice treated with normal human IgG or no IgG. IgG from patients 1 and 2 impaired neuromuscular transmission in mice, but IgG from patient 3 failed to do so. After 52-69 days of treatment, diaphragm or anterior tibial muscles were removed and coded. Paired muscles from control mice and mice receiving LEMS IgG were studied "blindly." Satisfactory freeze-fracture replicas of 185 presynaptic membrane P-faces were analyzed by stereometric methods. In mice treated with LEMS IgG that was pathogenic by electrophysiologic criteria, there was a selective depletion of active zones and active-zone particles but not of other membrane particles and there was a concomitant increase of large membrane particles aggregated into clusters. These findings provide additional evidence that the active zones facilitate quantal transmitter release by nerve impulse, lend further support to the assumption that the active-zone particles are Ca2+ channels, and establish mediation of the membrane lesions in LEMS by IgG.

Action Potentials↗
PubMed1983

Hypoglycemia, hepatic dysfunction, muscle weakness, cardiomyopathy, free carnitine deficiency and long-chain acylcarnitine excess responsive to medium chain triglyceride diet.

Fraternal twins who had fasting hypoglycemia, hypoketonemia, muscle weakness, and hepatic dysfunction are reported. The hepatic dysfunction occurred only during periods of caloric deprivation. The surviving patient developed a cardiomyopathy. In this sibling, muscle weakness and cardiomyopathy were markedly improved by a diet high in medium chain triglycerides. There was a marked deficiency of muscle total carnitine and a mild deficiency of hepatic total carnitine. Unlike patients with systemic carnitine deficiency, serum and muscle long-chain acylcarnitine were elevated and renal reabsorption of carnitine was normal. It was postulated that the defect in long-chain fatty acid oxidation in this disorder is caused by an abnormality in the mitochondrial acylcarnitine transport. Detailed studies of the cause of the hypoglycemia revealed that insulin, growth hormone, cortisol, and glucagon secretion were appropriate and that it is unlikely that there was a major deficiency of a glycolytic or gluconeogenic enzyme. Glucose production and alanine conversion to glucose were in the low normal range when compared to normal children in the postabsorptive state. The hypoglycemia in our patients was probably due to a modest increase in glucose consumption, secondary to the decreased oxidation of fatty acids and ketones, alternate fuels which spare glucose utilization, plus a modest decrease in hepatic glucose production secondary to decreased available hepatic energy substrates.

Cardiomyopathies↗
PubMed1983

Carnitine metabolism and deficiency syndromes.

L-Carnitine is an essential cofactor in transfer of long-chain fatty acids across the inner mitochondrial membrane. L-Carnitine is present in living systems in free form and as short-chain and long-chain fatty acylcarnitine esters. In recent years, several clinical syndromes due to or associated with carnitine deficiency have been described. They include 2 primary types--systemic and muscle (or myopathic) carnitine deficiency--and at least 15 syndromes in which carnitine deficiency seems to be secondary to genetic defects of intermediary metabolism or to other conditions. Possible beneficial effects of exogenous carnitine in ischemic heart disease have been the focus of intensive research in recent years. Free carnitine and esterified carnitine are measured by a sensitive enzymatic-radiochemical method. In some cases, the diagnosis of carnitine deficiency can be made by assay of total (free plus esterified) carnitine in plasma or serum. Proper diagnosis, however, often depends on determination of total carnitine in skeletal muscle or liver (or both). Since the first clinical description of carnitine deficiency in 1973, considerable progress has been made in defining and classifying the carnitine deficiency syndromes. Recent efforts in basic and clinical research have provided important clues about the molecular causes of these syndromes.

Adolescent↗
PubMed1982

Mechanisms of acetylcholine receptor loss from the neuromuscular junction.

At the normal mammalian neuromuscular junction the half-life of the acetylcholine receptor (AChR) ranges from 6 to 13 days (estimates from seven different laboratories). Indirect evidence suggests that the internalized receptor is degraded by a lysosomal mechanism. We have now traced the fate of the AChR labelled in vivo with peroxidase-alpha-bungarotoxin. Segments of junctional folds bearing AChRs are internalized by endocytosis. The endocytosed vesicles are engulfed by tubules and larger vesicles which, by electron cytochemical criteria, represent secondary lysosomes. Pathological mechanisms increased AChR loss from the end-plate. These include destruction of junctional folds, formation of immature junctions with a few or no junctional folds, accelerated internalization of AChR, impaired membrane insertion of new AChR and, possibly decreased AChR synthesis. The common mechanism for destruction of the junctional folds is an altered subsynaptic ionic milieu, and especially focal calcium excess. This can be induced by antibody and complement, too frequent or prolonged openings of the acetylcholine (ACh)-induced ion channel, and other membrane defects. In acquired autoimmune myasthenia gravis there is (a) antibody-dependent complement-mediated lysis of the junctional folds, (b) accelerated internalization of AChR cross-linked by antibody and (c) decreased insertion of AChR into the postsynaptic membrane. The last mechanism is attributed to lack of membrane patches available for tight packing and secure anchoring of the receptor. In acute, but not in chronic, experimental autoimmune myasthenia gravis, and infrequently in human myasthenia gravis, macrophages destroy junctional folds opsonized by antibody and C3. In a recently recognized congenital syndrome attributed to a prolonged open time of the ACh-induced ion channel, and to a lesser extent in congenital end-plate acetylcholinesterase deficiency, AChR is lost with degradation of junctional folds. In other, less well-defined, congenital syndromes there is deficiency or abnormal function of AChR. This could arise from decreased synthesis or membrane insertion or accelerated degradation of AChR, or from a structurally abnormal AChR with reduced affinity for ACh or with a diminished conductance or open time of its ion channel.

Animals↗
PubMed1982

A newly recognized congenital myasthenic syndrome attributed to a prolonged open time of the acetylcholine-induced ion channel.

Five familial cases (in two families) and one sporadic case of a new congenital myasthenic syndrome were investigated. Symptoms arise in infancy or later life. Typically, one finds selective involvement of cervical, scapular, and finger extensor muscles, ophthalmoparesis, and variable involvement of other muscles. There is a repetitive muscle action potential to single nerve stimulus in all muscles and a decremental response at 2 to 3 Hz stimulation in clinical affected muscles. Microelectrode studies reveal markedly prolonged end-plate potential (epp), miniature end-plate potential (mepp), and miniature end-plate current; normal quantum content of the epp; and a smaller than normal or low-normal mepp amplitude. Light microscopy demonstrates predominance of type I fibers, small groups of atrophic fibers, tubular aggregates and vacuoles near end-plates, abnormal end-plate configuration, and nonspecific myopathic changes. Abundant acetylcholinesterase activity is present at all end-plates, and the activity and kinetic properties of this enzyme in muscle are normal. Calcium accumulated at the end-plate in one patient. Quantitative electron microscopy shows decrease in the size of nerve terminals, increase in the density of synaptic vesicles, and reduction in the length of postsynaptic membranes. There is focal degeneration of junctional folds with corresponding loss of acetylcholine receptor, most marked in cases with the lowest mepp amplitude. There are no immune complexes at the end-plate. Fiber regions near end-plates display dilation, proliferation, and degeneration of the sarcoplasmic reticulum; nuclear, mitochondrial, and myofibrillar degeneration; and vacuoles resembling those found in periodic paralysis. A prolonged open time of the acetylcholine-induced ion channel is considered to be the basic abnormality and may account for the physiological, morphological, and clinical alterations.

Acetylcholine↗
PubMed1982

Complement activation in muscle fiber necrosis: demonstration of the membrane attack complex of complement in necrotic fibers.

The membranolytic C5b-9 complement membrane attach complex (MAC) is assembled after activation of either the classic or the alternative complement pathway. The quaternary configuration of the MAC macromolecule presents neoantigenic determinants not present on precursor molecules. Consequently, antibodies specific for these neoantigen(s) do not detect nonspecifically bound native complement precursors of MAC. By means of antibodies rendered specific for MAC neoantigen(s), MAC was localized by the immunoperoxidase reaction in cryostat sections of human muscle. In 66 biopsy specimens containing necrotic muscle fibers (Duchenne dystrophy, 13; other dystrophies, 15; inflammatory myopathies, 31; miscellaneous myopathies, 7) all of the necrotic fibers reacted for MAC neoantigen(s). C3 and C9 were also consistently localized in necrotic fibers, but localization of C1q, C4, and IgG was variable and often did not exceed background staining. None of the nonnecrotic fibers reacted for immunoglobulin or complement. Detection of MAC neoantigen(s) in necrotic fibers in a wide variety of muscle disease unambiguously shows that (1) the lytic complement pathway is consistently activated and participates in muscles fiber necrosis in vivo, and (2) complement reaction products are generated than can stimulate cellular infiltration and phagocytosis of the necrotic fiber. The findings also suggest that cell necrosis in general may involve participation of complement.

Antigens↗
PubMed1982

Carnitine transport in cultured muscle cells and skin fibroblasts from patients with primary systemic carnitine deficiency.

L-Carnitine transport was studied in cultured muscle cells and skin fibroblasts of patients with primary systemic carnitine deficiency and control subjects. In both cell culture types, two systems for carnitine transport were identified. The kinetic parameters for carnitine transport were remarkably similar in cultured muscle cells and skin fibroblasts. Normal rates and kinetic properties of carnitine transport were observed for both cell lines from patients with systemic carnitine deficiency. These studies do not rule out a defect in carnitine transport in vivo.

Biological Transport, Active↗
PubMed1982

Synthesizing enzymes for four neuroactive substances in motor neurons and neuromuscular junctions: light and electron microscopic immunocytochemistry.

Immunocytochemical evidence is presented for the existence of choline acetyltransferase (ChoAcTase), cysteine sulfinic acid decarboxylase (CSADCase), tyrosine hydroxylase (TyrOHase), and glutamic acid decarboxylase (GluDCase) in large motor neurons of the hypoglossal nucleus and the spinal cord and in nerve terminals of motor end plates in tongue and skeletal muscle of five mammalian species, including man. These enzymes, which are responsible for the synthesis of acetylcholine (AcCho), taurine, dopamine, and gamma-aminobutyrate (GABA), respectively, were detected by immunocytochemical studies with monoclonal or polyclonal antibodies raised against the enzymes. Electron microscopy of the neuromuscular junctions showed that the immunoreactivity in each case was confined to the cytoplasmic matrix of presynaptic nerve terminals. Immunoreactivity obtained for each enzyme antibody varied with the species. It was highest in fresh, unfixed muscle and lowest in aldehyde-fixed specimens. Negative controls were obtained with preimmune sera and antisera preabsorbed with pure ChoAcTase, CSADCase, or GluDCase antigen. Double-labeling studies with ChoAcTase antibodies and acetylcholinesterase (AcChoEase) antibodies, AcChoEase enzyme activity, or alpha-bungarotoxin binding indicated that ChoAcTase, AcChoEase, and AcCho receptors were colocalized at the same end plates.

Animals↗
PubMed1982

Coexistence in human and primate neuromuscular junctions of enzymes synthesizing acetylcholine, catecholamine, taurine, and gamma-aminobutyric acid.

Coexistence of neurotransmitter-synthesizing enzymes choline acetyltransferase, cysteine sulfinic acid decarboxylase, tyrosine hydroxylase, and L-glutamic acid decarboxylase was demonstrated at human and primate neuromuscular junctions with specific antibodies directed against these enzymes. Motor end plates were identified in unfixed cryostat sections by standard cholinergic markers for acetylcholinesterase and the acetylcholine receptor. Each of the four transmitter-synthesizing enzymes was localized at end plates displaying these markers. The presence of any two of the four enzymes at a given end plate was established by (i) showing immunoreaction for one enzyme followed by elution and demonstration of immunoreaction for a second enzyme, and (ii) paired immunofluorescence with simultaneous demonstration of one enzyme with a rhodamine-labeled second antibody and of the other enzyme with a fluorescein-labeled second antibody. These findings imply that motor nerve terminals have the capacity for synthesizing not only acetylcholine but also taurine, catecholamines, and gamma-aminobutyric acid. These substances, in turn, may participate in the normal regulation of nerve-muscle interaction or be significant in specific disorders involving the motor unit.

Acetylcholine↗
PubMed1982

Acetylcholinesterase of human erythrocytes and neuromuscular junctions: homologies revealed by monoclonal antibodies.

Human erythrocyte acetylcholinesterase was used to immunize mice, and hybridomas were generated by fusion of mouse spleen cells with cells of the Sp 2/0 myeloma cell line. Five independently derived hybridoma clones produced antibodies that bound to purified erythrocyte acetylcholinesterase. All of these antibodies crossreacted with human and monkey neuromuscular junctions; immunocytochemical staining patterns corresponded to the distribution of junctional acetylcholinesterase. The monoclonal antibodies fell into at least four categories based on differences in crossreactivity with neuromuscular acetylcholinesterase of rabbit, dog, calf, and guinea pig, and competition tests indicated that the antibodies defined five different antigenic sites on the acetylcholinesterase molecule. It is concluded that there is a high level of homology between the acetylcholinesterases of erythrocytes and neuromuscular junctions.

Acetylcholinesterase↗
PubMed1982

Ultrastructural aspects of acetylcholine receptor turnover at the normal end-plate and in autoimmune myasthenia gravis.

Acetylcholine receptor (AChR) deficiency at the myasthenic end-plate has been attributed to complement-mediated lysis of the junctional folds and to increased fractional degradation rate of AChR cross-linked by myasthenic immunoglobulin. This paper addresses the manner in which AChR is internalized and degraded at the normal end-plate and provides morphologic evidence for accelerated AChR degradation at the end-plate of rats with experimental autoimmune myasthenia gravis (EAMG). We sequentially traced the fate of end-plate AChR labeled in vivo with intramuscularly-injected peroxidase-alpha-bungarotoxin (PBGT) in control rats and rats with chronic EAMG. At both control and EAMG end-plates, AChR is internalized by endocytosis. The endocytosed vesicles containing AChR are transferred into the lysosomal compartment which extends from the junctional folds into the junctional sarcoplasm. Regardless of whether the initial intensity of the reaction for AChR at the EAMG end-plate appeared normal or reduced. AChR disappeared more rapidly from the EAMG than from the control end-plates. Despite the accelerated fractional turnover rate of end-plate AChR in EAMG, the postsynaptic membrane surface which could be labeled with PBGT for AChR remained unchanged over a 120-hour period. These data suggest that end-plate AChR is at a steady state in chronic EAMG.

Animals↗
PubMed1982

Estimation of degradation rate of acetylcholine receptor by external gamma counting in vivo.

We describe a novel method for estimating the rate of destruction of the acetylcholine receptor (AChR) associated with the motor end-plate in the living animal. After the intramuscular injection of a nontoxic dose of alpha-bungarotoxin that had been monoiodinated with 125I (125I-BGT), the loss of radioactivity from the injected site is monitored by an externally positioned gamma counter. Two days after labeling, the decline of radioactivity in the injected muscle parallels the decline of end-plate specific radioactivity and can be used for the estimation of the degradation rate and half-life of end-plate AChR. Using this method, we measured AChR half-life in normal and in myasthenic rats and found a 2.5-fold decrease of AChR half-life in the myasthenic animals. This in vivo method has potential clinical applications.

Animals↗
PubMed1981

The immunopathology of acquired myasthenia gravis.

Specific probes (alpha-bungarotoxin for acetylcholine receptor (AChR), staphylococcal protein A for IgG, monospecific antibodies against C3 and C9) labelled with peroxidase were applied to study of the ultrastructure of the MG end plate. In each case of MG there was postsynaptic AChR deficiency, usually greatest at end plates with marked degeneration of junctional folds. Morphometric estimates of postsynaptic AChR correlated linearly with the MEPP amplitude. In each case of MG, IgG was localized on the postsynaptic membrane where AChR is known to be located and on debris in the synaptic space. The abundance of antibody was proportionate to the amount of AChR remaining at the end plate. The localization of C3 was essentially identical with that of IgG. For most cases of MG it can be inferred that binding of IgG and C3 to AChR does not interfere with receptor function. C9, the terminal lytic complement component, was localized on debris in the synaptic space and on remnants of junctional folds. This proves that complement mediated destruction of junctional folds occurs in human MG. Studies in experimental auto-immune MG indicate that antibody-dependent internalization of AChR occurs in subclinical, mild and more severe diseases but increased AChR synthesis can compensate for this in subclinical and mild myasthenia. Complement-mediated injury of the postsynaptic membrane appears to be a requirement for induction of more severe MG.

Complement Activation↗