Degradation rates of acetylcholine receptors can be modified in the postjunctional plasma membrane of the vertebrate neuromuscular junction

Denervation of vertebrate muscle causes an acceleration of acetylcholine receptor turnover at the neuromuscular junction. This acceleration reflects the composite behavior of two populations of receptors: "original receptors" present at the junction at the time of denervation, and "new receptors" inserted into the denervated junction to replace the original receptors as they are degraded (Levitt, T. A., and M. M. Salpeter, 1981, Nature (Lond.), 291:239-241). The present study examined the degradation rate of original receptors to determine whether reinnervation could reverse the effect of denervation. Sternomastoid muscles in adult mice were denervated by either cutting or crushing the nerve, and the nerves either allowed to regenerate or ligated to prevent regeneration. The original receptors were labeled with 125I-alpha-bungarotoxin at the time of denervation, and their degradation rate followed by gamma counting. We found that when the nerve was not allowed to regenerate, the degradation decreased from a t1/2 of approximately 8-10 d to one of approximately 3 d (as reported earlier for denervated original receptors) and remained at that half-life throughout the experiment (approximately 36 d). If the axons were allowed to regenerate (which occurred asynchronously between day 14 and day 30 after nerve cut and between day 7 and 13 after nerve crush), the accelerated degradation rate of the original receptors reverted to a t1/2 of approximately 8 d. Our data lead us to conclude that the effect of denervation on the degradation rate of original receptors can be reversed by reinnervating. The nerve can thus slow the degradation rate of receptors previously inserted into the postsynaptic membrane.     

Organization and structure of two mixed micellar phases of the sphingomyelin/Triton X-305 system

Properties of mixed dispersions of sphingomyelin and the nonionic detergent, Triton X-305, were investigated by analytical ultracentrifugation and by autocorrelation spectroscopy of scattered laser light. These properties were compared with those of the sphingomyelin/Triton X-100 mixed micellar system reported previously [S. Yedgar, Y. Barenholz, and V. G. Cooper (1974) Biochim. Biophys. Acta 363, 98-111]. The substitution of the 30-unit ethylene oxide chain of Triton X-305 for the 10-unit chain of the Triton X-100 resulted in the appearance of two micellar phases at all detergent/lipid mixture ratios studied, whereas only a single mixed micellar phase was observed using Triton X-100. Despite this difference, the properties of the mixed lipid/detergent micelles obtained using Triton X-100 have been verified in the following respects: The detergent aggregation numbers in the mixed micelles are quite constant over a wide range of detergent molar fractions, being about 70 and 400 for the lighter and heavier mixed micellar phases, respectively. The detergent aggregation numbers are larger in the mixed micelle than in the pure detergent micelle. Very large sphingomyelin aggregation numbers can be accommodated within the mixed micelles, apparently by the critical intervention of the detergent molecules to produce a stable micellar structure.     

Human lymphocyte differentiation antigens HB-10 and HB-11. II. Differential production of B cell growth and differentiation factors by distinct helper T cell subpopulations

Two monoclonal antibodies (HB-10 and HB-11), which react with human T, B, and NK cells, identify approximately 50% of the Leu-3+ T helper (TH) cells in adult blood. In the present studies, the functional capabilities of the HB-11+ and HB-11-TH cell subpopulations were examined after purification by fluorescence-activated cell sorting. Both subpopulations proliferated in response to PHA, Con A, PWM, and OKT-3 antibodies. The HB-11+ TH cells gave a minimal proliferative response to soluble tetanus toxoid antigen, whereas HB-11-TH cells responded well. After mitogen activation, both HB-11+ and HB-11-TH cells and to produce soluble factors which induce large B cells to proliferate. However, PWM-stimulated HB-11+TH cells were incapable of inducing B cells to differentiate into antibody-secreting plasma cells, whereas HB-11-TH cells were efficient in this regard. The results suggest that the HB-11 antigen is expressed on a subpopulation of virgin TH cells that can produce B cell growth factors but are deficient in the ability to produce B cell differentiation factors.     

The synergistic influence of human interferon-gamma and interferon-alpha on suppression of hematopoietic progenitor cells is additive with the enhanced sensitivity of these cells to inhibition by interferons at low oxygen tension in vitro

The influences of human interferons--natural gamma (2 X 10(7) NIH reference U/mg), recombinant gamma (approximately 5 X 10(6) U/mg), natural alpha (1.4 X 10(8) international reference U/mg), and natural beta (10(6) international reference U/mg)--were evaluated alone or in combination for their effects in vitro on colony formation by low density human bone marrow granulocyte-macrophage (CFU-GM), erythroid (BFU-E), and multipotential (CFU-GEMM) progenitor cells incubated at 5% CO2 in normal incubator (approximately 20%) O2 tension or low (5%) O2 tension. Alone, these interferons demonstrated the same dose response inhibitory curves, as we reported previously, when cells were grown at 20% O2. Recombinant IFN-gamma gave the same dose response curve as natural IFN-gamma. Natural or recombinant interferon synergized with IFN-alpha to suppress colony formation at concentrations that were approximately 2 log units lower than that required by either interferon alone. Equal concentrations of these interferons were not needed for the synergistic effect and were still apparent when one was present at concentrations of 2 log units less than the other. IFN-gamma synergized to a lesser extent with IFN-beta, but IFN-alpha did not synergize with IFN-beta. Cells grown at 5% O2 were more sensitive to inhibition by 2 log units less IFN-gamma or IFN-alpha, and this effect was additive with the synergistic effects of IFN-gamma and IFN-alpha together. These results may have physiological, pathological, and/or clinical relevance.     

Human B cell responsiveness to B cell growth factor after activation by phorbol ester and monoclonal anti-mu antibody

The effect of phorbol ester on human B cell activation was examined. Picomolar to nanomolar concentrations of phorbol ester induced a high level of proliferation in small IgM-positive B cells isolated from peripheral blood by fluorescence-activated cell sorting. The addition of optimal doses of anti-mu antibody resulted in enhanced proliferation of phorbol ester-activated B cells. The addition of B cell growth factor (BCGF) to phorbol ester-activated B cells also resulted in a dose-dependent synergistic effect and maximal enhancement on day 3. BCGF activity could be absorbed with either phorbol ester- or anti-mu-activated B cells, but not with resting B cells, thus confirming the induction of functional BCGF receptor expression. Cell proliferation was not necessary for the induction of functional BCGF receptors. Phorbol ester was a more efficient inducer of BCGF receptor expression than was anti-mu antibody; gamma-interferon treatment had no effect. BCGF enhanced transferrin receptor expression by phorbol ester-activated B cells. The results suggest that phorbol ester-activated small B cells can be used to monitor BCGF activity, and this synergistic combination may be useful in establishing BCGF-dependent B cell clones in culture.     

Condensed tannins deter feeding by browsing ruminants in a South African savanna

Summary The palatability of 14 species of woody plant was assessed for three species of browsing ruminant, namely kudus, impalas and goats. Results show that palatability was most clearly related to leaf contents of condensed tannins. The effect was a threshold one, with all plants containing more than 5% condensed tannins being rejected as food during the wet season period. In contrast palatability was not influenced by concentrations of protein-precipitating polyphenols, and only weakly related to contents of nitrogen, phosphorus, cations, fibre components and other secondary metabolites. Insect herbivory shows a different pattern. These findings support the hypotheses that (i) condensed tannins function to protect plant cell walls against microbial attack; (ii) hydrolyzable tannins function to inactivate the digestive enzymes of insect herbivores. Large mammalian herbivores are influenced by condensed tannins due to their dependance upon microbial fermentation of plant cell walls for part of their energy needs.     

A developmentally regulated hydroxyproline-rich glycoprotein from the cell walls of soybean seed coats

In soybean seeds the level of hydroxyproline is regulated in a developmental and tissue-specific manner. The seed coat contains approximately 77% of the total hydroxyproline in the seed at all stages of development. We determined the ratio of hydroxyproline to dry weight in a number of tissues within the seed; however, only the seed coat shows an increase in this ratio during development. Within the many cell layers of the seed coat, hydroxyproline is most abundant in the external layer. The hydroxyproline is present as an hydroxyproline-rich cell wall glycoprotein. The protein is rich in hydroxyproline (36%), lysine (11%), proline (10%), histidine (9%), tyrosine (9%), and serine (8%). The carbohydrate portion is 90 mole% arabinose and 10 mole% galactose. The arabinose residues are attached to hydroxyproline mostly in the form of trisaccharides. The apparent molecular weight of this glycoprotein is 100,000 daltons.