Evidence for endocytotic uptake of cobra neurotoxin in mouse skeletal muscle

Protein secretion from mouse skeletal muscle: coupling of increased exocytotic and endocytotic activities in denervated muscle

The secretion of proteins labelled by incorporation of radioactive amino acids was studied in innervated and 10 to 13-day-denervated mouse skeletal muscle. The secretion of 3H-leucine-labelled proteins, expressed per mg muscle wet weight, increased after denervation, and the kinetics of the secretory process was also altered in denervated muscle. Separation of secreted 35S-methionine-labelled proteins by sodium dodecyl sulphate polyacrylamide gel electrophoresis followed by autoradiography revealed some denervation-induced alterations in the pattern of secreted proteins. The secretion from both innervated and denervated muscle was highly temperature sensitive and was reversibly inhibited by brefeldin A, a drug that blocks forward membrane transport from the endoplasmic reticulum/Golgi apparatus. This drug was also found to inhibit the uptake of fluorescein isothiocyanate-labelled dextran in denervated muscle but had no effect on the endocytotic activity of innervated muscle. This lends support to the hypothesis that the increased endocytotic activity in denervated muscle is coupled to a high secretory activity.

Protamine alters apical membrane K+ and Cl- permeability in gallbladder epithelium

Protamine addition to the solution bathing the mucosal side of Necturus gallbladder epithelium (25-100 mg/l) caused depolarization of both cell membranes, a mucosa-negative change in transepithelial voltage, an increase in the apical membrane resistance (Ra) followed by a decrease, and a monotonic increase in transepithelial resistance (Rt). In protamine (25 mg/l), the change in apical membrane voltage elicited by elevating mucosal solution [K+] from 2.5 to 92.5 mM was reduced from 66 +/-2 to 38 +/- 5 mV (P less than 0.001). The K+-induced fall in Ra was also reduced in protamine. These effects could also be elicited by elevating mucosal solution [K+] simultaneously with the addition of protamine and by transient addition of protamine during exposure to the high K+ medium. The effect of protamine on the electrodiffusive Cl- permeability of the apical membrane (PCl) was studied both in control and forskolin-treated tissues. In the absence of forskolin, the hyperpolarization of Vmc produced by lowering mucosal [Cl-] to 10 mM was reversed to a small depolarization; in forskolin, the initial depolarization produced by lowering [Cl-] was significantly increased. Finally, exposure to protamine in the absence of forskolin produced an initial fall in intracellular Cl- activity. Our results indicate that protamine decreases apical membrane K+ permeability and increases apical membrane PCl. The time course of the effects of protamine suggests the possibility of an initial effect on surface potential, followed by secondary actions mediated by intracellular events.

Effects of botulinum toxin induced muscle paralysis on endocytosis and lysosomal enzyme activities in mouse skeletal muscle

The effects of botulinum toxin (type A) induced muscle paralysis on endocytosis and lysosomal enzyme activities in skeletal muscle were compared with the effects of surgical denervation. Muscle atrophy, measured as decrease in total muscle protein content, was as large or larger after botulinum toxin treatment as after denervation. Endocytic activity, measured as the in vitro uptake of horseradish peroxidase, and the specific activities of the lysosomal enzymes N-acetyl-beta-D-glucosaminidase and cathepsin D were all increased six days after denervation. Only the specific activity of cathepsin D was increased six days after botulinum toxin poisoning. The uptake of horseradish peroxidase and the specific activity of N-acetyl-beta-D-glucosaminidase were also increased eleven days after poisoning. Transverse sections of eleven days botulinum poisoned muscles from animals injected with horseradish peroxidase showed fibres with dense peroxidase staining similar to those seen in denervated muscle although they seemed to occur less frequently. The results show that increases in endocytic activity and lysosomal enzyme activities may occur in skeletal muscle without the presence of degenerating axons. The differences in effects of surgical denervation and botulinum toxin induced paralysis are discussed in terms of what is known about the mechanism of action of botulinum toxin and the possible functional roles of the two lysosomal enzymes studied.

An ultrastructural study of the segmental uptake of horseradish peroxidase in the endplate region of denervated skeletal muscle fibres

The segmental uptake of horseradish peroxidase (HRP) in the endplate region of denervated skeletal muscle fibres has been studied ultrastructurally using a method for selecting single muscle fibres with high segmental peroxidase staining from denervated mouse tibialis anterior muscle. Segments containing large peroxidase positive phagosomes could already be seen 10-15 min after i.v. injection of HRP. Such segments were still present 24 h after HRP injection. The localization of phagosomes, deep in the fibres rather than immediately under the sarcolemma, suggests that the uptake occurs from t-tubuli. Vivid proliferation of t-tubuli, consisting of vesiculation, enlargement and encircling of cytoplasmic components, was also observed. The HRP accumulates in phagosomes of varying size and shape. Similar membrane-limited bodies without or with very weak peroxidase staining were also observed. The peroxidase-positive phagosomes participate in autophagic processes as suggested by their content of undegraded cellular material. Golgi profiles, which occurred deep in the muscle fibres, and enlarged components of the sarcoplasmic reticulum were frequently encountered in the segments. Myofibrillar degeneration occurs in the segments and progresses with time after denervation. The described segments may be related to the increased membrane turnover in denervated muscle fibres and/or they may be related to processes aimed at establishing new synaptic contacts.

Myotoxic activity of the crude venom and the principal neurotoxin, taipoxin, of the Australian taipan, Oxyuranus scutellatus

1 The crude venom of the Australian taipan. Oxyuranus scutellatus and its principal neurotoxin, taipoxin, were injected into the anterolateral aspect of one hind limb of the rat. 2 The effects of the venom and toxin on the morphology and physiology on the underlying soleus muscles were examined. 3 Both the crude venom and the toxin caused necrosis and degeneration of the muscle. Damage to the peripheral muscle fibres could be seen at the light microscopic level as early as 3 h after injection of the toxic compounds. 4 The necrotic response was accompanied by an infiltration of phagocytic cells and an extensive oedema. The wet weight of the damaged muscles was almost doubled by 6 h. 5 In individual muscle fibres, necrosis was associated with the disruption of the plasma membrane and the disorganization of the myofibrils. The basal lamina of the muscle fibres was left intact. 6 Denervated mammalian muscles and innervated avian muscles were also destroyed by tiapoxin, but immature avian muscle growing in tissue culture was resistant. 7 Of the 3 subunits of taipoxin, only the basic alpha-taipoxin was itself myotoxic. However, its potency was enhanced by the presence of the acid gamma subunit. The role of the neutral beta-subunit is unclear. 8 The period of necrosis and degeneration lasted for approximately 48 h, after which the muscle fibres began to regenerate. Regeneration took place within the surviving basal lamina, with the formation of myotubes by three days, and small, immature muscle fibres by five days. Regeneration was virtually complete by 21 days.

Neuromuscular junctions and alpha-bungarotoxin-binding sites in denervated and contralateral cat skeletal muscles

1. The distributions of cholinesterase (ChE) activity and acetylcholine (ACh) receptors were studied in normal and denervated cat hind-limb fast-twitch skeletal muscles and in muscles contralateral to denervated muscles. 2. On normal muscle fibres almost all receptors were confined to the immediately post-junctional membrane, although a perijunctional gradient of increased ACh receptor density was found on both fast- and slow-twitch fibres. After denervation, the extrajunctional ACh receptor density increased greatly and remained high for at least 10 months. Ectopic regions staining for ChE activity and having a high density of ACh receptors appeared in denervated muscle. The number of junctional ACh receptors decreased slowly after denervation, with a half-time of about 140 days. 3. Fast-twitch muscles contralateral to denervated muscles also showed changes, including an increase in junctional size and a small but significant increase in extra-junctional ACh receptor density.

T-tubule endocytosis in dystrophic chicken muscle and its relation to muscle fiber degeneration

Pectoralis muscles from normal and dystrophic chickens were investigated 2 h after an i.v. injection of horseradish peroxidase, by cytochemical and biochemical techniques to demonstrate peroxidase activity. Light microscopic examination of dystrophic muscles showed that peroxidase activity could be detected inside a population of fibers, in deliminated bodies often restricted to segments of the muscle fiber. Such bodies containing peroxidase were not observed in normal muscle fibers. Electron microscopy of dystrophic muscle fibers revealed that numerous vesicles containing peroxidase were frequently present in fiber regions with signs of cytoplasmic degradation. These vesicles, which occasionally were found to be coated, were 50--100 nm in size and appeared to be derived from t-tubules. Larger (up to 1.7 micrometers) inclusions containing peroxidase and delimited by a single membrane were also present at degenerating areas of dystrophic muscle fibers. These bodies seemed to be formed by fusion between several primary t-tubule vesicles and probably also lysosomes. Vacuoles containing the peroxidase were frequently encountered. Biochemical determination of horseradish peroxidase activity, performed after extensive washing of the muscle tissue, showed that dystrophic muscles contained about twice as much peroxidase as normal control muscles. It is suggested that endocytosis from t-tubules is an early and essential pathological phenomenon in dystrophic muscle fibers, which may be related to lysosomal function and muscle fiber degeneration

Acetylcholine receptor degradation measured by density labeling: effects of cholinergic ligands and evidence against recycling

The methodology of density labeling of proteins by biosynthetic incorporation of 2H, 13C, 15N-amino acids into newly synthesized polypeptide chains allows the direct measurement of the turnover rate of the acetylcholine receptor in cultured chick skeletal muscle. In this study, receptors synthesized in medium containing 2H, 13C, 15N-amino acids were resolved from 1H, 12C, 14N-receptors by velocity sedimentation in sucrose-deuterium oxide gradients, and their proportions were determined by computer analysis of the gradient profiles. The kinetics of turnover of acetylcholine receptors are identical for developing chick muscle fibers grown in medium containing 2H, 13C, 15N-amino acids or 1H, 12C, 14N-amino acids, and the high degree of substitution of normal aminoacyl residues by 2H, 13C, 15N-residues does not affect the turnover rate of the denser receptor. Comparison of the turnover rates in continuous and pulse-labeling experiments gave independent confirmation of these results. The application of a potent, essentially irreversible blocking agent, alpha-bungarotoxin, increases the median lifetime of receptors from 17 hr for the native unbound receptor to 22 hr for the alpha-bungarotoxin-receptor complex. As predicted, the total number of alpha-bungarotoxin binding sites increased in the continued presence of alpha-bungarotoxin due to extension of receptor lifetime. To determine whether other cholinergic agents affect the turnover rate of the receptor, measurements were performed on cultures grown in the presence of 10(-4) M d-tubocurare or 10(-4) M carbachol, a reversible antagonist and a reversible agonist, respectively, of the nicotinic acetylcholine receptor. The receptor degradation rates of the drug-treated cells were identical to control values. The total number of alpha-bungarotoxin binding sites was reduced by 30% in the presence of carbachol, indicating that this agent affects the rate of synthesis of the acetylcholine receptor. Data formerly interpreted as suggesting a cycling of receptor-containing plasma membrane out of and back into the sarcolemma are now understood to reflect the alteration in receptor lifetime upon complexing with alpha-bungarotoxin. The intracellular "hidden" receptor sites were found to remain inside the myotubes and thus do not signify an intracellular pool of recycling plasma membrane.

Further studies on the binding properties of cobra neurotoxin to cholinergic receptors in mouse skeletal muscle

Preparations of 3H-monoacetylated derivatives of Naja naja siamensis neurotoxin siamensis 3 were found to be resistant to degradation and deacetylation during in vitro muscle incubation. A low rate of free 3H-acetate (less than 0.4%) was present in the preparations, but should not interfere with the binding of toxin to cholinergic receptors in mouse extensor digitorum longus muscles in vitro. The binding of toxin to cholinergic receptors in mouse extensor digitorum longus muscle was essentially irreversible. d-Tubocurarine antagonized the binding of toxin in both innervated and denervated muscles. Administration of actinomycin D one day after denervation prevented the appearance of new toxin binding sites in the muscles.