Ascorbic acid enables reversible dopamine receptor 3H-agonist binding

The effects of ascorbic acid on dopaminergic ³H-agonist receptor binding were studied in membrane homogenates of bovine anterior pituitary and caudate, and rat striatum. In all tissues virtually no stereospecific binding (defined using luM (+)butaclamol) of the ³H-agonists N-propylnorapomorphine (NPA), apomorphine, or dopamine could be demonstrated in the absence of ascorbic acid. Although levels of total ³H-agonist binding were three to five times greater in the absence than in the presence of 0.1% ascorbic acid, the increased binding was entirely non-stereospecific. Greater amounts of dopamine-inhibitable ³H-NPA binding could be demonstrated in the absence of 0.1% ascorbic acid, but this measure of “specific binding” was demonstrated not to represent dopamine receptor binding since several other catecholamines and catechol were equipotent with dopamine and more potent than the dopamine agonist (±)amino-6,7-dihydroxy-1,2,3,4-tetrahydronapthalene (ADTN) in inhibiting this binding. High levels of dopamine-displaceable ³H-agonist binding were detected in fresh and boiled homogenates of cerebellum, an area of brain which receives no dopaminergic innervation, further demonstrating the non-specific nature of ³H-agonist binding in the absence of ascorbic acid. These studies emphasize that under typical assay conditions ascorbic acid is required in order to demonstrate reversible and specific ³H-agonist binding to dopamine receptors.     

The control of IgM expression in a murine B cell lymphoma

The regulation of IgM expression was studied in clones derived from a murine B lymphocyte cell line, WEHI279.1. During normal B cell development IgM heavy chain synthesis increases concomitantly with heightened IgM secretion and reduced cell-surface IgM. However, in these subclones, the levels of membrane-bound and secreted IgM were regulated independently of one another. The amount of IgM secreted by the cells was tightly coupled to the amount of heavy chain synthesis, suggesting that the major control of secretion is pretranslational. Surface IgM exhibited a more complex regulation, with both pre- and posttranslational components. Variation in the expression of both forms of IgM occurred at high frequency. Although IgM expression follows a unidirectional pathway in nontransformed cells, the variability in these tumor cells was reversible and cellautonomous. High levels of phenotypic variability may be important in the ability of transformed cells to escape the immune response.     

Taurine transport in human placental trophoblast is important for regulation of cell differentiation and survival

The outer epithelial cell layer of human placenta, the syncytiotrophoblast, is a specialised terminally differentiated multinucleate tissue. It is generated and renewed from underlying cytotrophoblast cells that undergo proliferation, differentiation and fusion with syncytiotrophoblast. Acquisition of fresh cellular components is thought to be balanced by apoptosis and shedding of aged nuclei. This process of trophoblast cell turnover maintains a functional syncytiotrophoblast, capable of sufficient nutrient transfer from mother to foetus. Foetal growth restriction (FGR) is a pregnancy complication associated with aberrant trophoblast turnover and reduced activity of certain amino acid transporters, including the taurine transporter (TauT). Taurine is the most abundant amino acid in human placenta implying an important physiological role within this tissue. Unlike other amino acids, taurine is not incorporated into proteins and in non-placental cell types represents an important osmolyte involved in cell volume regulation, and is also cytoprotective. Here, we investigated the role of taurine in trophoblast turnover using RNA interference to deplete primary human trophoblast cells of TauT and reduce intracellular taurine content. Trophoblast differentiation was compromised in TauT-deficient cells, and susceptibility of these cells to an inflammatory cytokine that is elevated in FGR was increased, evidenced by elevated levels of apoptosis. These data suggest an important role for taurine in trophoblast turnover and cytoprotection.     

Modification of host cell phagosomes by Toxoplasma gondii involves redistribution of surface proteins and secretion of a 32 kDa protein

Toxoplasma gondii resists endocytic processing within host cell phagosomes that are modified by a prominent membranous network which forms the interface between host cell and the enclosed parasite. The formation of this intravacuolar network involves redistribution of the major outer membrane proteins of the Toxoplasma cell, consisting of 41, 35, 29, 22 kDa species, as shown by radioimmunoprecipitation, sodium dodecyl sulfate polyacrylamide gel electrophoresis, and immunogold EM labeling using monoclonal antibodies (mAbs) to T. gondii. In addition, the major 32 kDa protein found in the purified intravacuolar networks was recognized by mAb 1G5 which does not react with the surface of intact Toxoplasma cells. Immunoperoxidase EM using mAb 1G5 indicated that the 32 kDa protein is a constituent of electron-dense vacuoles within the Toxoplasma cell, in addition to being a prominent component of the intravacuolar network. Thus, assembly of the intravacuolar network appears to involve regulated release of the 32 kDa protein in conjunction with shedding of surface membrane proteins by the parasite. Our results suggest that the structural modifications of host cell phagosomes by T. gondii are precisely regulated events that follow invasion and consequently may contribute to intracellular survival.     

Predation by longfin smelt ( Spirinchus thaleichthys ) on the mysid Neomysis mercedis in Lake Washington

1. Longfin smelt and mysid samples collected in Lake Washington from 1988 to 1992 were examined, and historic data on smelt and mysid abundance were reviewed to assess the impacts of smelt predation on Neomysis mercedis. The feeding habits of smelt were also examined for differences between odd and even years, and seasonal and ontogenetic variations in smelt diet were described. 2. A reciprocal relationship in the abundance of 1+ smelt and Neomysis suggested that smelt regulate mysid abundance in the lake. 3. The proportion of smelt diet contributed by mysids was higher for less abundant year classes of smelt. 4. These results indicate that intraspecific competition for scarce mysids occurs in the even-year classes of smelt.