E and F alpha series prostaglandins in developing muscles

Secreted Metabolome of ALS-Related hSOD1(G93A) Primary Cultures of Myocytes and Implications for Myogenesis

Amyotrophic lateral sclerosis (ALS) is a motor neuron (MN) disease associated with progressive muscle atrophy, paralysis, and eventually death. Growing evidence demonstrates that the pathological process leading to ALS is the result of multiple altered mechanisms occurring not only in MNs but also in other cell types inside and outside the central nervous system. In this context, the involvement of skeletal muscle has been the subject of a few studies on patients and ALS animal models. In this work, by using primary myocytes derived from the ALS transgenic hSOD1(G93A) mouse model, we observed that the myogenic capability of such cells was defective compared to cells derived from control mice expressing the nonpathogenic hSOD1(WT) isoform. The correct in vitro myogenesis of hSOD1(G93A) primary skeletal muscle cells was rescued by the addition of a conditioned medium from healthy hSOD1(WT) myocytes, suggesting the existence of an in trans activity of secreted factors. To define a dataset of molecules participating in such safeguard action, we conducted comparative metabolomic profiling of a culture medium collected from hSOD1(G93A) and hSOD1(WT) primary myocytes and report here an altered secretion of amino acids and lipid-based signaling molecules. These findings support the urgency of better understanding the role of the skeletal muscle secretome in the regulation of the myogenic program and mechanisms of ALS pathogenesis and progression.

The COX-2 pathway is essential during early stages of skeletal muscle regeneration

Skeletal muscle regeneration comprises several overlapping cellular processes, including inflammation and myogenesis. Prostaglandins (PGs) may regulate muscle regeneration, because they modulate inflammation and are involved in various stages of myogenesis in vitro. PG synthesis is catalyzed by different isoforms of cyclooxygenase (COX), which are inhibited by nonsteroidal anti-inflammatory drugs. Although experiments employing nonsteroidal anti-inflammatory drugs have implicated PGs in tissue repair, how PGs regulate muscle regeneration remains unclear, and the potentially distinct roles of different COX isoforms have not been investigated. To address these questions, a localized freeze injury was induced in the tibialis anterior muscles of mice chronically treated with either a COX-1- or COX-2-selective inhibitor (SC-560 and SC-236, respectively), starting before injury. The size of regenerating myofibers was analyzed at time points up to 5 wk after injury and found to be decreased by SC-236 and in COX-2−/−muscles, but unaffected by SC-560. In contrast, SC-236 had no effect on myofiber growth when administered starting 7 days after injury. The attenuation of myofiber growth by SC-236 treatment and in COX-2−/−muscles is associated with decreases in the number of myoblasts and intramuscular inflammatory cells at early times after injury. Together, these data suggest that COX-2-dependent PG synthesis is required during early stages of muscle regeneration and thus raise caution about the use of COX-2-selective inhibitors in patients with muscle injury or disease.

Expression of the extracellular fatty acid binding protein (Ex-FABP) during muscle fiber formation in vivo and in vitro

We report that Ex-FABP, an extracellular protein belonging to the lipocalin family and involved in the extracellular transport of long-chain fatty acids, is expressed in the forming myotubes both in vivo and in vitro. The presence of the protein and of the mRNA was observed in newly formed myotubes at early stages of chick embryo development by immunohistochemistry and by in situ hybridization. At later stages of development myofibers still expressed both the mRNA and the protein. Ex-FABP expression was observed also in the developing myocardium and the muscular layer of large blood vessels. In agreement with these findings, an initial expression of the mRNA and protein secretion by cultured chicken myoblasts were observed only after the onset of myoblast fusion. Double-immunofluorescence staining of these cultured cells revealed that multinucleate myotubes were stained by antibodies directed against both the Ex-FABP and the sarcomeric myosin, whereas immature myotubes and single myoblasts were not. When added to cultured myoblasts, antibodies against the Ex-FABP induced a strong enhancement of the production of the same protein. In all experiments some cell sufferance and a transient impairment of myotube formation were also observed. The finding that the continuous removal of the Ex-FABP from the culture medium of myoblasts, due to the formation of immune complexes, resulted in an overproduction of the protein suggests a feedback (autocrine) control during myotube differentiation and maturation. We propose that the requirement for increased transport and metabolism of free fatty acid released from the membrane phospholipids and storage lipids, mediated by Ex-FABP, may be essential during differentiation of multinucleated myotubes or that an increased local demand of fatty acids and metabolites may act as a local hormone in tissues differentiating and undergoing morphogenesis.

Increased PLA2 activity is not related to increase GLUT1 expression in L6 myotubes under hypoxic conditions

Incubation of L6 myotubes for 24 h under hypoxic conditions leads to a 5.8 +/- 1.2 fold increase in 2-deoxyglucose uptake. In those conditions phospholipase A2 is activated, leading to a 2.4 +/- 0.8 fold increased release of arachidonic acid (AA) to the medium, and to 95% increased synthesis of PGF2 alpha but not of PGE2 as compared to cells incubated in normoxic conditions. Under hypoxia, the PLA2 inhibitor bromophenacyl bromide (BPB) inhibited AA release and PGF2 alpha synthesis, yet it did not affect the increase in glucose uptake into L6 myotubes. The amount of GLUT1 immunoreactive proteins in total membranes of hypoxia treated cells was evaluated 5.1 +/- 1.2 fold compared to control cells. Neither 10 microM BPB nor 100 mM aspirin (ASA) prevented this increase in GLUT1 expression. Preincubation of myotubes for either 1 or 23 h with 50 microM exogenous AA, prevented insulin induced 2-deoxyglucose uptake stimulation, suggesting that although AA or one of its metabolites did not regulate the synthesis or stability of GLUT1, it may interfere with the signal transduction of insulin in muscle cells.

Prostaglandin F2 alpha stimulates hypertrophic growth of cultured neonatal rat ventricular myocytes

Prostaglandin F2 alpha (PGF2 alpha) stimulates protein synthesis of skeletal and smooth muscle cells in culture and is elevated in the heart during compensatory growth. We hypothesized that PGF2 alpha stimulates hypertrophic growth of neonatal rat cardiac myocytes. Prostaglandin F2 alpha increased [3H]phenylalanine incorporation by cultured ventricular myocytes in a dose-dependent manner (EC50 = 11 nM), suggesting action through a PGF-specific receptor. Semiquantitative reverse transcriptase polymerase chain reaction revealed that PGF receptor mRNA is expressed in ventricular myocytes > A7R5 vascular smooth muscle cells >> cardiac fibroblast-like cells. The protein content of cardiomyocyte cultures was increased by 10 nM PGF2 alpha and 11 beta-PGF2 alpha but was unchanged by 10 nM PGD2, PGE2, PGF1 alpha, carbaprostacyclin, U-46619, or 12- or 15-hydroxyeicosatrienoic acid. Stimulation of myofibrillar gene expression by PGF2 alpha was demonstrated by Northern and Western blot analysis for myosin light chain-2 (MLC-2) and by transient transfection experiments with MLC-2 luciferase expression plasmids. In addition, myofibrillogenesis was increased by PGF2 alpha as assessed by immunocytochemical staining with MLC-2 antisera. Prostaglandin F2 alpha did not affect myocyte proliferation or [3H]thymidine incorporation, thus myocyte growth occurred by hypertrophy. Proliferative and hypertrophic growth of cardiac fibroblast-like cells were unaffected by PGF2 alpha. We conclude that PFG2 alpha stimulates hypertrophic growth of neonatal rat ventricular myocytes in culture and speculate that PGF2 alpha plays a role in myocardial adaptation to chronic hypertrophic stimuli, recovery from injury, and cardiac ontogeny.