A family of mammalian Na+-dependent L-ascorbic acid transporters.

Vitamin C (L-ascorbic acid) is essential for many enzymatic reactions, in which it serves to maintain prosthetic metal ions in their reduced forms (for example, Fe2+, Cu+), and for scavenging free radicals in order to protect tissues from oxidative damage. The facilitative sugar transporters of the GLUT type can transport the oxidized form of the vitamin, dehydroascorbic acid, but these transporters are unlikely to allow significant physiological amounts of vitamin C to be taken up in the presence of normal glucose concentrations, because the vitamin is present in plasma essentially only in its reduced form. Here we describe the isolation of two L-ascorbic acid transporters, SVCT1 and SVCT2, from rat complementary DNA libraries, as the first step in investigating the importance of L-ascorbic acid transport in regulating the supply and metabolism of vitamin C. We find that SVCT1 and SVCT2 each mediate concentrative, high-affinity L-ascorbic acid transport that is stereospecific and is driven by the Na+ electrochemical gradient. Despite their close sequence homology and similar functions, the two isoforms of the transporter are discretely distributed: SVCT1 is mainly confined to epithelial systems (intestine, kidney, liver), whereas SVCT2 serves a host of metabolically active cells and specialized tissues in the brain, eye and other organs.     

Aggregates of acetylcholinesterase induced by acetylcholine receptor-aggregating factor

Basal lamina-rich extracts of Torpedo californica electric organ contain a factor that causes acetylcholine receptors (AChRs) on cultured myotubes to aggregate into patches. Our previous studies have indicated that the active component of these extracts is similar to the molecules in the basal lamina which direct the aggregation of AChRs in the muscle fibre plasma membrane at regenerating neuromuscular junctions in vivo. Because it can be obtained in large amounts and assayed in controlled conditions in cell culture, the AChR-aggregating factor from electric organ may be especially useful for examining in detail how the postsynaptic apparatus of regenerating muscle is assembled. Here we demonstrate that the electric organ factor causes not only the formation of AChR aggregates on cultured myotubes, but also the formation of patches of acetylcholinesterase (AChE). This finding, together with the observation that basal lamina directs the formation of both AChR and AChE aggregates at regenerating neuromuscular junctions in vivo, leads us to hypothesize that a single component of the synaptic basal lamina causes the formation of both these synaptic specializations on regenerating myofibres.     

Identification of angiogenic activity and the cloning and expression of platelet-derived endothelial cell growth factor

Cloning and sequencing of the complementary DNA for platelet-derived endothelial cell growth factor indicates that it is a novel factor distinct from previously characterized proteins. The factor, a protein with a relative molecular mass of about 45,000, stimulates endothelial cell growth and chemotaxis in vitro and angiogenesis in vivo.     

Selective activation of Lyt 2+ precursor T cells by ligation of the antigen receptor

Resting T lymphocytes may be activated either physiologically, by the specific recognition of antigen in association with molecules encoded by the major histocompatibility complex (MHC), or non-physiologically using mitogens such as concanavalin A (Con A). The former activation process is difficult to analyse because resting precursor T cells specific for a particular antigen-MHC combination can only be isolated in the presence of a large excess of bystander cells of irrelevant specificity; clonal populations of uniform specificity are not useful for studying the activation of naive T cells because there is no reason to believe that such cloned cells ever return to the state of resting precursors. Mitogens may activate a large fraction of resting T cells, but analysis is again complicated because the target molecule(s) of most mitogens is unknown and the relationship of this kind of activation to physiological induction by antigen plus MHC molecules remains unclear. By using a monoclonal antibody specific for the antigen receptors on approximately 25% of all T cells of both Lyt 2+ and Lyt 2- subsets, we have studied the induction of lymphokine responsiveness in resting normal T cells. This antibody, immobilized on Sepharose beads, is sufficient to activate Lyt 2+ T cells, but not Lyt 2- T cells, to clonal expansion in the presence of a mixture of lymphokines (10% rat spleen Con A supernatant). We report here that clonal growth of the T cells obeys single-hit kinetics in limiting-dilution microcultures, suggesting that a single cell type is limiting. We conclude that cytotoxic T-lymphocyte (Tc) precursors require only ligation of the antigen receptor before they become responsive to lymphokines, whereas helper T-lymphocyte (Th) precursors require additional signals.