Transduction of the chemotactic signal to the actin cytoskeleton of Dictyostelium discoideum

Dictyostelium discoideum amebae chemotax toward folate during vegetative growth and toward extracellular cAMP during the aggregation phase that follows starvation. Stimulation of starving amebae with extracellular cAMP leads to both actin polymerization and pseudopod extension (Hall et al., 1988, J. Cell. Biochem. 37, 285-299). We have identified an actin nucleation activity (NA) from starving amebae that is regulated by cAMP receptors and controls actin polymerization (Hall et al., 1989, J. Cell Biol., in press). We show here that NA from vegetative cells is also regulated by chemotactic receptors for folate. Our studies indicate that NA is an essential effector in control of the actin cytoskeleton by chemotactic receptors. Guided by a recently proposed model for signal transduction from the cAMP receptor (Snaar-Jagalska et al., 1988, Dev. Genet. 9, 215-225), we investigated which of three signaling pathways activates the NA effector. Treatment of whole cells with a commercial pertussis toxin preparation (PT) inhibited cAMP-stimulated NA. However, endotoxin contamination of the PT appears to account for this effect. The synag7 mutation and caffeine treatment do not inhibit activation of NA by cAMP. Thus, neither activation of adenylate cyclase nor a G protein sensitive to PT treatment of whole cells is necessary for the NA response. Actin nucleation activity stimulated with folate is normal in vegetative fgdA cells. However, cAMP suppresses rather than activates NA in starving fgdA cells. This indicates that the components of the actin nucleation effector are present and that a pathway regulating the inhibitor(s) of nucleation remains functional in starving fgdA cells. The locus of the fgdA defect, a G protein implicated in phospholipase C activation, is directly or indirectly responsible for transduction of the stimulatory chemotactic signal from cAMP receptors to the nucleation effector in Dictyostelium.     

Plate 457. Mutisia Subspinosa Compositae

Mutisia subspinosa Cav. is described and illustrated and its relationships with other members of Mutisia sect. Guariruma (Cass.) Cabrera discussed. Notes on its cultivation are provided along with a historical insight into the collections of Née, made on the Malaspina expedition.     

Conserving livestock breed biodiversity

Growing awareness of the importance of conserving the biodiversity of livestock breeds is paralleled by genetic advances that will help objective planning of conservation. Inventories of breeds, long advocated, are now being established and concepts originally formulated for the quantification of species diversity are being applied. The breeds thus conserved will provide valuable resources for the future of agriculture, especially in the developing world.     

Rac GTPase interacts with GAPs and target proteins through multiple effector sites.

Rac, a small GTPase in the ras superfamily, regulates at least two biological processes in animal cells: (i) the polymerization of actin and the assembly of integrin complexes to produce lamellipodia and ruffles; and (ii) the activity of an NADPH oxidase in phagocytic cells. NADPH oxidase activation is mediated through a rac effector protein, p67phox, and using chimeras made between rac and the closely related GTPase, rho, we have identified two distinct effector sites in rac, one N-terminal and one C-terminal, both of which are required for activation of p67phox. The same two effector sites are essential for rac-induced actin polymerization in fibroblasts. p65PAK, a ubiquitous serine/threonine kinase, interacts with rac at both the N- and C-terminal effector sites, but the GTPase-activating protein, bcr interacts with rac at a different region. This makes p65PAK, but not bcr, a candidate effector of rac-induced lamellipodium formation.     

Properties of microfiltration membranes: Mechanisms of flux loss in the recovery of an enzyme

The transmission and rate of filtration of the enzyme yeast alcohol dehydrogenase (YADH) has been studied at capillary pore microfiltration membranes. Photon correlation spectroscopy (PCS) with nanometer resolution showed that the enzyme existed as discreate molecules only for a narrow range of pH and ionic strength. Under such conditions, the transmission of the enzyme was high. However, the rate of filtration still decreased continuously with time. Analyssis of the time dependence of the rate of filtration indicated that this decrease was due to in-pore enzyme deposition at low concentration ("standard blocking model") and suface depositon at high concentration ("cake filtration model"). Use of atomic force microscopy (AFM) gave unequivocal and quantitative confirmation of these inferences. The work shows the great advantage of using advanced physical characterization techniques, both for the identification of the optimum conditions for filtration (PCS) and for the elucidation of mechanisms giving rise to inefficiencies in the filtration process (AFM). (c) 1995 John Wiley & Sons, Inc.     

Endocrine cells of the human gastrointestinal tract have no proliferative capacity

Summary There is compelling evidence that the epithelial cell lineages of the gastrointestinal tract are derived from a common stem cell precursor, but the details of the subsequent cellular hierarchies remain uncertain. In this context, it is important to know the arrangement of cell proliferation that gives rise to the final cell populations. In rodents, a number of studies have been performed examining the possible proliferative capacity of endocrine cells, but a wide range of technical problems makes interpretation of these data difficult. Continuous labelling studies suggest that there is potential for proliferation in endocrine cells but flash labelling studies have not been conclusive. In man there are no data on this issue. We have taken advantage of the ability to perform double immunostaining for operational markers of proliferation (Ki67 antigen) and endocrine cell phenotype (chromogranin expression). We demonstrate that there are no double-labelled cells in the normal stomach, small intestine or colon of fetal, neonatal or adult humans. Moreover, no double-labelled cells are found in pathological states associated with endocrine cell hyperplasia (gastritis, ulcerative colitis). These data indicate that the normal endocrine cells of the human gut have no proliferative capacity and that, in this cell lineage, population expansion precedes differentiation.