High endocytotic and lysosomal activities in segments of rat myotubes differentiated in vitro

Novel clearance of muscle proteins by muscle cells

Blood levels of cardiac troponins (cTn) and myoglobin are analysed when myocardial infarction (MI) is suspected. Here we describe a novel clearance mechanism for muscle proteins by muscle cells. The complete plasma clearance profile of cTn and myoglobin was followed in rats after intravenous or intermuscular injections and analysed by PET and fluorescence microscopy of muscle biopsies and muscle cells. Compared with intravenous injections, only 5 % of cTnT, 0.6 % of cTnI and 8 % of myoglobin were recovered in the circulation following intramuscular injection. In contrast, 47 % of the renal filtration marker FITC-sinistrin and 81 % of cTn fragments from MI-patients were recovered after intramuscular injection. In addition, PET and biopsy analysis revealed that cTn was taken up by the quadriceps muscle and both cTn and myoglobin were endocytosed by cultured muscle cells. This local clearance mechanism could possibly be the dominant clearance mechanism for cTn, myoglobin and other muscle damage biomarkers released by muscle cells.

Measurement of sub-membrane [Ca2+] in adult myofibers and cytosolic [Ca2+] in myotubes from normal and mdx mice using the Ca2+ indicator FFP-18

The hypothesis that intracellular Ca(2+) is elevated in dystrophic (mdx) skeletal muscle due to increased Ca(2+) influx is controversial. As the sub-sarcolemmal Ca(2+) ([Ca(2+)](mem)) should be even higher than the global cytosolic Ca(2+) in the presence of increased Ca(2+) influx, we investigated [Ca(2+)](mem) levels in collagenase-isolated adult flexor digitorum brevis (FDB) myofibres and myotubes of mdx and normal mice with the near-membrane Ca(2+) indicator FFP-18. Confocal imaging showed strong localization of FFP-18 to the sarcolemma only. No significant difference in [Ca(2+)](mem) was found in FDB myofibres of normal (77.3+/-3.8 nM, n=68) and mdx (79.3+/-5.6 nM, n=21, p=0.89) mice using FFP-18. Increasing external Ca(2+) to 18 mM did not significantly affect [Ca(2+)](mem) in either the normal or mdx myofibres. In the myotubes, the FFP-18 was non-selectively incorporated, distributing throughout the cytoplasm, and FFP-18-derived [Ca(2+)] values were similar to values obtained with Fura-2. Nevertheless, in the mdx myotubes, the [Ca(2+)] measured with FFP-18 increased linearly to a level approximately 2.75 times that of controls as the time of culture was prolonged. In older mdx myotubes (>or=8 days in culture), 18 mM extracellular Ca(2+) increased the steady state cytosolic [Ca(2+)] to approximately 22 times greater level than controls. This study suggests that the sub-sarcolemmal Ca(2+) homeostasis is well maintained in isolated adult mdx myofibers and also further supports the hypothesis that cytosolic Ca(2+) handling is compromised in mdx myotubes.

Rabies virus entry at the neuromuscular junction in nerve-muscle cocultures

Early events in rabies virus entry into neurons were investigated in chick spinal cord-muscle cocultures. Rabies virus (CVS strain) was adsorbed to the surface of cells in the cold. At times up to 10 min of warming to 37 degrees C, virus was most intensely localized to dense swellings on the myotube surface. Texas Red-labeled alpha-bungarotoxin, which binds to nicotinic acetylcholine receptors, colocalized precisely with virus at the densities identifying these regions as neuromuscular junctions. Rabies virus also colocalized in the junctions with synapsin I, a marker for synaptic vesicles. The endosome tracers Lucifer Yellow, Texan Red-dextran, and rhodamine-wheat germ agglutinin were added to the cultures at the end of the virus adsorption period and the cultures were warmed. At 10 min, rabies virus and tracers colocalized at neuromuscular junctions and nerve terminals. At 30 min, rabies virus and tracers showed more intense fluorescence over nerve fibers and nerve cell bodies. At 60 min, nerve terminals, nerve fibers, and nerve cell bodies showed intense fluorescence and colocalization for rabies virus and tracers. LysoTracker Red, a marker for acidic compartments, colocalized with rabies virus at nerve-muscle contacts. These findings show that in nerve-muscle cocultures, the neuromuscular junction is the major site of entry into neurons. Colocalization of virus and endosome tracers within nerve terminals indicates that virus resides in an early endosome compartment, some of which are acidified. The progressive increase of virus and tracers in nerve fibers and nerve cell bodies over time is consistent with retrograde transport of endocytosed virus from the motor nerve terminal.

Secretion of plasminogen activator and lysosomal enzymes from mouse skeletal muscle: effect of denervation

Levels of hydrolytic enzymes increase in skeletal muscle after denervation and their activities in the extracellular matrix appear to be important for interaction between muscle and nerve. Using enzymatic assays for beta-glucuronidase, beta-galactosidase, and plasminogen activator, we show that secretion of these enzymes from mouse skeletal muscle increases after denervation and that drugs interfering with the secretory pathway or the reuptake of enzymes modulate this release. Thus, brefeldin A inhibited secretion of plasminogen activator activity and mannan increased secreted amounts of beta-glucuronidase, but not of beta-galactosidase, in denervated muscle. In innervated muscle, brefeldin A decreased secreted activity of plasminogen activator, but mannan had no effect on secretion of either beta-glucuronidase or beta-galactosidase. Furthermore, secretion of plasminogen activator was temperature dependent. These observations, together with previous studies, suggest that secretion of hydrolytic enzymes from adult skeletal muscle may be of physiological significance in nerve-muscle communication.