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S G Webster

Publications and source records attributed to S G Webster.

At least 37 records · Page 2Linked to original sources
PubMed1994

Involvement of cAMP and cGMP in the mode of action of molt-inhibiting hormone (MIH) a neuropeptide which inhibits steroidogenesis in a crab.

In crustaceans, production of molting hormones (or ecdysteroids) by the molting glands (Y-organs; YO), is under negative control exerted by a neuropeptide, the molt-inhibiting hormone (MIH). MIH of the crab Carcinus maenas inhibits in vitro steroidogenesis of basal (intermolt crab) or activated (premolt crab) YO. MIH inhibits secretion of the two ecdysteroids synthesized by crab YO, ecdysone (E) secreted throughout the molting cycle, and 25-deoxyecdysone (25dE), secreted during the premolt period. At a MIH concentration of 10(-8) M, E is reduced about 50% and 25dE 94%. Regardless of the molting stage, this inhibition of steroidogenesis is reversible, dose dependent and measurable after 5 min. On intermolt YO, MIH induced cGMP increase and 8BrcGMP mimics the effect of MIH: at this stage cGMP seems to be involved with MIH inhibition of steroidogenesis. On premolt YO MIH induced a transient increase of cAMP (2-fold) and a long-lasting enhancement of cGMP (60-fold). On active YO, we demonstrated that a low concentration (10(-5) M) of dbcAMP, 8BrcAMP, 8BrcGMP, or agents increasing intracellular cAMP, mimic MIH effects and inhibit steroidogenesis. From these observations it is concluded that both cyclic nucleotides are involved in the mode of action of MIH on activated YO. At this premolt period, MIH/cAMP may act cooperatively with MIH/cGMP in the inhibitory control of steroidogenesis by crab YO.

1-Methyl-3-isobutylxanthine↗
PubMed1993

Ovarian and hemolymph ecdysteroid titers during vitellogenesis in Macrobrachium rosenbergii.

Changes in ovarian and hemolymph ecdysteroid concentration and composition during vitellogenesis have been investigated in the freshwater prawn Macrobrachium rosenbergii. Free ecdysteroids (20-hydroxyecdysone and ecdysone) in hemolymph increased in concentration during vitellogenesis from zero at stage 0 to 1.5 ng/ml at stage I to 7.3 ng/ml in mature, stage IV animals. 20-Hydroxyecdysonoic acid (1.2 ng/ml) was detected in the stage IV hemolymph. Ovarian-free ecdysteroid concentration, expressed as nanograms per gram of tissue, fell during vitellogenesis from 83.2 ng/g at stage 0, non-pigmented tissue to 14.2 ng/g at stage IV tissue, being minimal at stage I (6.3 ng/g). However, expression of ovarian free ecdysteroid content as nanograms per ovary revealed a rise from 7.7 ng/ovary at stage 0, nonpigmented tissue to 28.3 ng/ovary at stage IV, again being minimal at stage I (2.0 ng/ovary). 20-Hydroxyecdysonoic acid and ecdysonoic acid were identified at ovarian stages II-III and stage IV.

Animals↗
PubMed1993

Quantification, immunoaffinity purification and sequence analysis of a pigment-dispersing hormone of the shore crab, Carcinus maenas (L.).

1. A sensitive sandwich-ELISA for pigment-dispersing hormone (PDH) with a detection limit of approximately 5 fmol per well has been developed using primary IgG and secondary biotinylated IgG fractions from an antiserum against beta-PDH and streptavidin-peroxidase conjugate for detection. 2. ELISA determinations in different parts of the central nervous system of the shore crab Carcinus maenas revealed maximum levels of 15.1 pmol immunoreactive PDH in the two eyestalk ganglia of one crab, 3.6 pmol in the sinus glands, 2.8 pmol in the brain and 0.8 pmol in the thoracic ganglia. 3. Carcinus PDH was purified from whole eyestalk ganglia and sinus glands by use of a simple two-step purification procedure consisting of immunoaffinity-prepurification on an anti-PDH IgG-protein A-Sepharose column and HPLC. 4. Automated gas-phase sequencing of the purified peptide and FAB-mass spectroscopy unambiguously revealed the sequence of Carcinus PDH as NSELINSILGLPKVMNDAamide (M(r) 1927.2 Da), which is identical to the beta-PDH of other brachyuran crustaceans.

Amino Acid Sequence↗
PubMed1993

Evidence for myoblast-extrinsic regulation of slow myosin heavy chain expression during muscle fiber formation in embryonic development.

Vertebrate muscles are composed of an array of diverse fast and slow fiber types with different contractile properties. Differences among fibers in fast and slow MyHC expression could be due to extrinsic factors that act on the differentiated myofibers. Alternatively, the mononucleate myoblasts that fuse to form multinucleated muscle fibers could differ intrinsically due to lineage. To distinguish between these possibilities, we determined whether the changes in proportion of slow fibers were attributable to inherent differences in myoblasts. The proportion of fibers expressing slow myosin heavy chain (MyHC) was found to change markedly with time during embryonic and fetal human limb development. During the first trimester, a maximum of 75% of fibers expressed slow MyHC. Thereafter, new fibers formed which did not express this MyHC, so that the proportion of fibers expressing slow MyHC dropped to approximately 3% of the total by midgestation. Several weeks later, a subset of the new fibers began to express slow MyHC and from week 30 of gestation through adulthood, approximately 50% of fibers were slow. However, each myoblast clone (n = 2,119) derived from muscle tissues at six stages of human development (weeks 7, 9, 16, and 22 of gestation, 2 mo after birth and adult) expressed slow MyHC upon differentiation. We conclude from these results that the control of slow MyHC expression in vivo during muscle fiber formation in embryonic development is largely extrinsic to the myoblast. By contrast, human myoblast clones from the same samples differed in their expression of embryonic and neonatal MyHCs, in agreement with studies in other species, and this difference was shown to be stably heritable. Even after 25 population doublings in tissue culture, embryonic stage myoblasts did not give rise to myoblasts capable of expressing MyHCs typical of neonatal stages, indicating that stage-specific differences are not under the control of a division dependent mechanism, or intrinsic "clock." Taken together, these results suggest that, unlike embryonic and neonatal MyHCs, the expression of slow MyHC in vivo at different developmental stages during gestation is not the result of commitment to a distinct myoblast lineage, but is largely determined by the environment.

Cell Differentiation↗
PubMed1991

Amino acid sequence of putative moult-inhibiting hormone from the crab Carcinus maenas.

Putative moult-inhibiting hormone (MIH) was isolated from sinus glands of the shore crab Carcinus maenas, and its primary structure determined by automated Edman degradation of endoproteinase derived peptide fragments. MIH is a 78 residue neuropeptide (deduced molecular mass 9181 Da) with three disulphide bridges and unblocked N- and C-termini. MIH shows some homology to the crustacean hyperglycemic hormone (CHH) neuropeptide family. However, consideration of the roles of various members of this group, together with sequence information recently reported, strongly suggests that these neuropeptides may be multifunctional.

Amino Acid Sequence↗
PubMed1989

Localization of muscle gene products in nuclear domains.

The localization of gene products is central to the development of cell polarity and pattern specification during embryogenesis. To monitor the distribution of gene products encoded by different nuclei in the same cell in tissue culture, we fused cells of different species to form multinucleated non-dividing heterokaryons. In previous fusion studies, cell-surface antigens and organelles contributed by disparate cell types intermixed within minutes. Using heterokaryons produced with differentiated muscle cells, we demonstrate here that a muscle membrane component, the Golgi apparatus mediating its transport, and a sarcomeric myosin heavy chain are localized in the vicinity of the nuclei responsible for their synthesis. These results provide direct evidence that products (organelle, membrane and structural proteins) derived from individual nuclei can remain localized in myotubes, a finding with implications both for neuromuscular synapse formation and for the carrier state of Duchenne muscular dystrophy.

Animals↗
PubMed1986

Developmental progression of myosin gene expression in cultured muscle cells.

Myosin heavy chains are encoded by distinct members of a multigene family at different stages of muscle development. Study of the underlying regulatory mechanisms has been hindered because transitions in myosin expression have not been readily attained in tissue culture. Here we show a transition from early (fetal) to late (perinatal/adult) myosins defined by two monoclonal antibodies, F1.652 and N3.36, in the myotubes of mouse C2C12 cells. On day 1 of differentiation, essentially all myosin was early myosin. By day 8, early myosin dropped to 25% of its day 1 value and was replaced by late myosin. The transition occurred without neural contact, connective tissue components, or complex substrates, suggesting that its regulation may be intrinsic to the muscle cell. Our results demonstrate that a developmental progression in myosin gene expression, which occurs rapidly, with high frequency, and under relatively simple conditions, is now amenable to molecular analysis in cultured muscle cells.

Animals↗
PubMed1986

Neurohormonal control of ecdysteroid biosynthesis by Carcinus maenas Y-organs in vitro, and preliminary characterization of the putative molt-inhibiting hormone (MIH).

Using simple culture techniques, the effects of neurosecretory tissue, sinus gland-conditioned media, and sinus gland extracts upon the biosynthesis of ecdysteroids by Carcinus maenas Y-organs in vitro were investigated. The sinus glands were found to be a major source of a factor which profoundly repressed ecdysteroid synthesis and which did not appear to be species-specific within other brachyurans examined (Liocarcinus, Cancer). It is suggested that the inhibitory factor is produced by the neurosecretory tissues of the medulla terminalis. It is argued that the inhibitory factor is the putative molt-inhibiting hormone (MIH). Partial characterization revealed that MIH is a heat-stable, trypsin-sensitive neuropeptide, eluting on a Sephadex G-50 gel in a range of approximately 6-14 kDa. By consideration of the dose-response characteristics, it is estimated that MIH may be active in the subpicomolar range.

Animals↗
PubMed1985

Plasticity of the differentiated state.

Heterokaryons provide a model system in which to examine how tissue-specific phenotypes arise and are maintained. When muscle cells are fused with nonmuscle cells, muscle gene expression is activated in the nonmuscle cell type. Gene expression was studied either at a single cell level with monoclonal antibodies or in mass cultures at a biochemical and molecular level. In all of the nonmuscle cell types tested, including representatives of different embryonic lineages, phenotypes, and developmental stages, muscle gene expression was induced. Differences among cell types in the kinetics, frequency, and gene dosage requirements for gene expression provide clues to the underlying regulatory mechanisms. These results show that the expression of genes in the nuclei of differentiated cells is remarkably plastic and susceptible to modulation by the cytoplasm. The isolation of the genes encoding the tissue-specific trans-acting regulators responsible for muscle gene activation should now be possible.

Aged↗
PubMed1983

Effects of exogenous ecdysterone upon moulting, proecdysial development, and limb regeneration in the prawn Palaemon elegans.

Injection of small doses of ecdysterone accelerated moulting and proecdysis in the prawn Palaemon elegans. Injection of large doses of ecdysterone (1-10 micrograms) markedly accelerated proecdysis, but death always occurred prior to or during moulting and was accompanied by abnormal setal development and retarded cuticle formation. Dose-response curves were obtained for a range of hormone doses from 10 to 0.01 micrograms by administering ecdysterone during postmoult (stages A-B) and early premoult (stages DE0-DL0). Accelerated proecdysis and viable moulting were more marked in the group injected during early premoult (stages DE0-DL0). The sensitivity threshold for prawns injected during this time was less than 40 ng g-1 but could not be determined more precisely in view of the range of ecdysterone concentrations used. In contrast, the sensitivity threshold for ecdysterone administered during postmoult (stages A-B) was much higher, between 0.4 and 2 micrograms g-1. The possible mechanisms controlling sensitivity thresholds for ecdysterone are discussed. The effect of ecdysterone on limb regeneration was also studied. Within the concentration range used, ecdysterone was found to have no effect on the rate of limb regeneration.

Animals↗
PubMed1981

Isotopic scanning of bone in the diagnosis of osteomalacia.

Results of a study performed over 18 months in 21 elderly patients showed that isotopic bone scanning is a sensitive technique for the detection of osteomalacia. It can be used not only to indicate the presence of the disease but the response to treatment.

Aged↗
PubMed1979

Erythrocyte folate levels in young and old.

Erythrocyte folate levels were compared among 25 young healthy subjects (mean age, 27 years), 29 healthy subjects aged over 75 living in the community, 62 subjects (mean age, 75) admitted to a geriatric assessment ward (acute illnesses), and 32 subjects (aged over 65) in a ward for long-term physical or mental illnesses. Overall, the female/male sex ratio varied from 2:1 to 3:1. For the three elderly groups, the incidence of low erythrocyte folate levels (less than 100 mmicrogram/100 ml) were 24 percent, 16 percent, and 18 percent, respectively. For the young group, the mean value would be over 296 mmicrogram/100 ml. Dietary folate deficiency may often account for low folate blood levels in the elderly, but other factors should also be implicated, e.g., the ability to absorb folate.

Adult↗
PubMed1977

A comparison of fat absorption in young and old subjects.

Using an oral fat load, it is demonstrated that the pattern of absorption in elderly subjects is significantly different from that found in younger controls. This change may be due to the slower gastric emptying or reduced pancreatic function observed in this study. The possibility of impaired small bowel absorption has not been excluded.

Adolescent↗