To investigate whether changes in platelet condition during platelet storage correlate with an altered expression of platelet membrane proteins, the binding of monoclonal antibodies (MoAbs) to fresh platelets was compared with MoAbs' binding to thrombin-activated platelets and to platelets stored as platelet concentrates. The MoAbs included antibodies against the platelet glycoprotein (GP) IIb/IIIa complex and against two activation-dependent antigens, one of which was a component of the internal platelet alpha-granule membrane (GMP 140) and the other of which was a 53-kD protein derived from platelet lysosomes. The binding of MoAbs to platelets fixed with 1 percent paraformaldehyde was measured by flow cytometry. In thrombin-activated platelets, a threefold increase was found in the expression of GP IIb/IIIa over that in fresh platelets. The binding of the activation-dependent MoAbs increased from 2 to 3 percent to 70 to 80 percent of the platelets. Storage of platelet concentrates for 5 days resulted in a 60 percent increase in GP IIb/IIIa expression compared to Day 0 and increased binding of the MoAbs directed against GMP-140 from 3 to 16 percent and against the 53-kD protein from 2 to 8 percent of the platelets, respectively. These changes correlated with modifications in platelet morphology (decrease in swirling), leakage of lactate dehydrogenase, and release of beta-thromboglobulin. These data indicate that platelets become activated and are damaged during the storage of platelet concentrates.(ABSTRACT TRUNCATED AT 250 WORDS)

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The aim of this study was to establish the time scale of developmental changes in innervation of skeletal muscle fibers in man. Specimens of thigh and intercostal muscle from 19 embryos and 18 infants were examined with histological methods which enabled the discrimination between fetal (gamma) and adult (epsilon) types of acetylcholine receptors (AChRs). At 8 weeks of development, AChRs were distributed diffusely in the myotube membranes. Following onset of innervation in approximately the ninth week the length of the AChR positive area diminished and reached its shortest size at the sixteenth developmental week. At the sixteenth and eighteenth week some nerve terminals opposed the muscle membrane outside the AChR positive area. Decrease in the number of nerve terminals, strongly suggesting elimination of polyneuronal innervation, started in the sixteenth week and was completed in the twenty-fifth week. This fetal (gamma) type of AChR could no longer be demonstrated after the thirty-first week. The length of the end-plates as determined by the presence of AChRs increased again in the last week before birth and reached a plateau size by the end of the first year after birth. It is concluded that in man the transition from poly- to mononeuronal innervation takes place between the sixteenth and twenty-fifth weeks of development. The evidence available suggests that the retraction of nerve terminals is preceded by loss of AChRs from the muscle membrane facing the terminals. There is no relationship between retraction of nerve terminals and the switch from fetal to adult type of AChR. The size of the presynaptic apparatus changes little after the first year of life.

Myasthenia gravis is usually caused by autoantibodies to the acetylcholine receptor (AChR). The AChR is clustered and anchored in the postsynaptic membrane of the neuromuscular junction (NMJ) by a cytoplasmic protein called rapsyn. We previously showed that resistance to experimental autoimmune myasthenia gravis (EAMG) in aged rats correlates with increased rapsyn concentration at the NMJ. It is possible, therefore, that endogenous rapsyn expression may be an important determinant of AChR loss and neuromuscular transmission failure in the human disease, and that upregulation of rapsyn expression could be used therapeutically. To examine first a potential therapeutic application of rapsyn upregulation, we induced acute EAMG in young rats by passive transfer of AChR antibody, mAb 35, and used in vivo electroporation to over-express rapsyn unilaterally in one tibialis anterior. We looked at the compound muscle action potentials (CMAPs) in the tibialis anterior, at rapsyn and AChR expression by quantitative radioimmunoassay and immunofluorescence, and at the morphology of the NMJs, comparing the electroporated and untreated muscles, as well as the control and EAMG rats. In control rats, transfected muscle fibres had extrasynaptic rapsyn aggregates, as well as slightly increased rapsyn and AChR concentrations at the NMJ. In EAMG rats, despite deposits of the membrane attack complex, the rapsyn-overexpressing muscles showed no decrement in the CMAPs, no loss of AChR, and the majority had normal postsynaptic folds, whereas endplates of untreated muscles showed typical AChR loss and morphological damage. These data suggest not only that increasing rapsyn expression could be a potential treatment for selected muscles of myasthenia gravis patients, but also lend support to the hypothesis that individual differences in innate rapsyn expression could be a factor in determining disease severity.

Autoantibodies to muscle-specific kinase (MuSK) can cause myasthenia gravis (MG). The pathophysiological mechanism remains unknown. We report in vitro electrophysiological and histological studies of the neuromuscular junction in a MuSK MG patient. Low levels of presynaptic acetylcholine release and small miniature endplate potentials were found. This combination of pre-and postsynaptic abnormalities was supported by histology, revealing partially denervated postsynaptic areas, and some degeneration of postsynaptic folds. Results suggest that anti-MuSK antibodies reduce the stability of muscle-nerve contact.