Changes in the surface membrane during myoblast fusion

Modifications of the adenylate cyclase complex during differentiation of cultured myoblasts

Alterations in receptor-independent activation of adenylate cyclase during proliferation and differentiation of L6E9 myoblasts were studied using Mn2+, forskolin, and Gpp(NH)p. Analyses were performed 3, 6, and 10 days following subculture, corresponding to onset of proliferation, end of proliferation with start of differentiation, and completion of differentiation, respectively. The apparent activation constant for Mn2+ decreases with the age of the culture; the apparent activation constant for Mg2+ does not. Bimodal activation by Mn2+, i.e., at concentrations greater than 10 mM, results in total adenylate cyclase activity less than the Vmax and occurs exclusively in differentiated cultures. Independent of the presence of Mg2+, forskolin activation occurs with low-and high-affinity constants in differentiated cultures and with a low affinity constant in youngest cultures; intermediate cultures (day 6) demonstrate low- and high-affinity activation only in the presence of high Mg2+. In contrast, the Vmax for forskolin increases with increasing Mg2+ in all culture ages. Although Gpp(NH)p-dependent adenylate cyclase activation occurs with an apparent activation constant independent of culture age and Mg2+, low Mg2+ fosters bimodal activation by Gpp(NH)p, i.e., above 100 microM nucleotide, total adenylate cyclase activity is less than the Vmax. The loss of stimulatory capacity by high Gpp(NH)p is greatest in differentiated cultures. Additional experiments are presented to substantiate that bimodal activation by Gpp(NH)p is specific. Cholera- and pertussis toxin-dependent ADP ribosylation patterns demonstrate a marked decrease in both Ns and Ni in differentiated cultures. The data suggest that alterations in postreceptor activation of adenylate cyclase during the course of differentiation and proliferation are mediated by guanine nucleotide binding proteins as well as by allosteric cation regulatory units.

Dependence of Na+,K+-ATPase and electrogenic component of Em in cultured myotubes on cell fusion

This study was undertaken in order to determine the relation among cell fusion, [3H]ouabain binding and the membrane potential (Em) of cultured rat skeletal muscle. The amount of ouabain bound and the Em both increased with age, the increases being most dramatic following fusion. Inhibition of fusion prevented the developmental increases in both properties of cultured muscle. After fusion, the size of the electrogenic component of Em, determined by the decrease in Em produced by ouabain within 5-10 min, increased independent of the age at which fusion occurred. It is concluded that the increase in Em with age depends on postfusion appearance and activity of Na,K-ATPase.

The differentiation of avian skeletal muscle in culture: changes in responsiveness of adenylyl cyclase to prostaglandin E1 and adrenergic agonists

The responsiveness of adenylyl cyclase during avian myogenesis in vitro has been examined. Measurements of cyclic AMP generation in intact cells revealed that the precursor myoblast is highly responsive to prostaglandin E1 (11-fold maximum stimulation); whereas its response to isoproterenol is much smaller (2-fold). From the onset of terminal differentiation, responsiveness to the beta-adrenergic agonist increases progressively, reaching a 5.5-fold maximum response by 96 hr of culture. In contrast, there is little change in the cell population's responsiveness to prostaglandin E1. The rise in catecholamine responsiveness is consistent with previously reported increases in beta-receptors accompanying differentiation. DL-propranolol blocks the response of myoblasts and myotubes to 10(-6) M isoproterenol with the same half maximal inhibition value of 1 X 10(-8) mol. The results also suggest a change in the adrenergic character of the receptors and/or coupling to adenylyl cyclase as myoblasts differentiate. First the alpha-adrenergic antagonist phentolamine (10(-7)-10(-4) mol) inhibits the myoblast's response but enhances that of the myotube. Second, the potency ratios for the responses to isoproterenol, epinephrine, and norepinephrine shift from 1.1:1.0:1.0 in the myoblast to 3.3:2.1:1.0 in the myotube. The findings are discussed with reference to the role of prostaglandins in the positive control of muscle differentiation and the changes in the catecholamine-responsive adenylyl cyclase system as an aspect of the expression of the muscle phenotype.

Permeability changes resulting from virus-cell fusion: temperature-dependence of the contributing processes

A new assay for membrane fusion, using the fluorescent probe pyrene-sulphonyl-phosphatidyl ethanolamine, has been developed. Fusion between the envelope of Sendai virus and human erythrocytes or Lettre cells has a Q10 of approximately 4 at 37 degrees C, increasing to approximately 7 at 7 degrees C; there is no lag to onset of fusion. Viral neuraminidase has a Q10 of 2.3 between 37 degrees C and 4 degrees C. Its action limits the extent of fusion by causing the elution of virus; this effect is particularly marked at low temperature because of the difference in Q10 for fusion and neuraminidase. The temperature-dependence of the initiation of permeability changes following the removal of inhibitory amounts of Ca2+ is approximately 2; thus membrane fusion is the principal temperature-sensitive step during the permeabilization of cells by Sendai virus. A recovery process, by which cells become insensitive to the removal of Ca2+ and which therefore limits the extent of permeabilization, has a Q10 of 7.4 between 37 degrees C and 21 degrees C. It is concluded that the lag to onset of permeability changes is not due to a lag in virus-cell membrane fusion, but to the gradual acquisition of a threshold level of membrane damage; the extent of permeabilization depends on the rate of fusion relative to the rates of neuraminidase and recovery.

Myxoviruses do not induce non-specific alterations in membrane permeability early on in infection

The permeability characteristics of cells infected with myxoviruses have been studied by measuring the concentrative uptake of nutrients, the concentration of intracellular K+, and the maintenance of the Na+ gradient across the plasma membrane. Cells either show no change at all (Sendai virus-infected BHK cells and measles virus-infected Vero cells) or they show a decreased ability to concentrate nutrients, while intracellular K+ and the Na+ gradient remain unchanged (Sendai and influenza virus-infected L-1210 cells, measles virus-infected lymphocytes and mumps virus-infected L-41 cells). In no case, therefore, was a change observed that resembles the non-specific increase in membrane permeability induced by haemolytic paramyxoviruses (35, 42) or the non-specific membrane leakiness postulated to take place in infected cells (8, 9). A preliminary account of some of these findings has been presented (39).