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

Abstract

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.

Effect of chloroquine-induced myopathy on rat soleus muscle sarcoplasm and expression of clathrin

“…7,12,14 High endocytotic activity mainly is associated with the coated pits/endosomes, whereas elevated exocytotic activity involves primary lysosomes derived from the endoplasmic reticulum/Golgi apparatus. 9,12,14 Clathrin is a well-known constituent of the coat of such vesicles as the coated pits/endosomes and primary lysosomes. 11 We found specific up-regulation of clathrin in denervated muscles of chloroquine-as well as salinetreated rats as compared with the innervated control muscles of both groups.…”

Golgi Complex, Endoplasmic Reticulum Exit Sites, and Microtubules in Skeletal Muscle Fibers Are Organized by Patterned Activity

“…At the resolution of light microscopy we cannot determine whether GLUT4 is in the postsynaptic membrane or in vesicles docked at the membrane. The accumulation of GLUT4 may be linked to increased endocytosis and exocytosis around the endplates of denervated fibers (Vult von Steyern et al, 1993), although the endocytic markers do not label the NMJ itself. Whatever its origin, this staining depends on evoked electrical activity but not so much on its specific pattern.…”

The nifk gene is widely expressed in mouse tissues and is up‐regulated in denervated hind limb muscle

“…brain-derived neurotrophic factor (BDNF; Funakoshi et al, 1993;Koliatsos et al, 1993), glial cell line-derived neurotrophic factor (GDNF; Lie and Weis, 1998;Naveilhan et al, 1997) and neural cell adhesion molecule (N-CAM; Tews et al, 1997). Increased endocytosis occurs in the perisynaptic region of denervated muscle (Elmquist et al, 1992;Libelius and Tågerud, 1984) that may reflect exocytotic activity resulting from secretion of neurotrophic factor(s) (Vult von Steyern et al, 1993). A proposed function of these factors is the guiding of regenerating nerves to the original synapse region, which is the preferred site for reinnervation (Bennett and Pettigrew, 1976;Taxt, 1983).…”