CSM News
Electronic Edition
Volume 1, number 8
July 3, 1993

Please submit abstracts of your papers as soon as they have been
accepted for publication by sending them to CSM-News@worms.cmsbio.nwu.edu.


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Induction of terminal differentiation of Dictyostelium by cAMP
dependent protein kinase and opposing effects of intracellular and
extracellular cAMP on stalk cell differentiation.

Neil A. Hopper, Christophe Anjard*, Christophe D. Reymond* and Jeffrey G.
Williams

Imperial Cancer Research Fund, Clare Hall Laboratories, south Mimms,
Herts EN6 3LD, U.K. *Institut d'histologie et d'embryologie, Faculti
de midecine, Universiti de Lausanne, Rue du Bugnon 9 CH-1005 LAUSANNE,
Switzerland  

DEVELOPMENT, in press

ABSTRACT

   Expression of the catalytic (C) subunit of the cAMP dependent protein
kinase (PKA) of Dictyostelium under the control of heterologous, cell-type
specific promoters causes ectopic terminal differentiation. When expressed
under the control of a prespore specific promoter development is
accelerated, to yield highly aberrant fruiting bodies that contain a basal
mass of spore cells surrounding a central stalk-like structure. When
expressed under the control of a prestalk specific promoter development
arrests much earlier, at the tight mound stage. Prestalk cells move to the
apices of these mounds, apparently normally, but no tip is formed. Most of
the prestalk cells remain arrested in their development but there are
scattered within such mounds a few isolated stalk cells.
   We show that extracellular cAMP represses stalk cell specific gene
expression in cells where the kinase is constitutively active, suggesting
taht inhibition of stalk cell differentiation by cAMP in normal cells
(Berks and Kay, 1988) occurs because of an effect of extracellular cAMP on
an intracellular signalling pathway independent of PKA. We propose a scheme
whereby two separate events, a rise in intracellular cAMP levels and a fall
in extracellular cAMP concentration, are required to induce stalk cell
differentiation.

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Cell Type Regulation in Response to Expression of Ricin A in Dictyostelium

Gad Shaulsky and William F. Loomis

Center for Molecular Genetics, Department of Biology, University of
California, San Diego, La Jolla, California

Developmental Biology  (in press)

Abstract

   Expression of ricin A in either prespore or prestalk cells of Dictyostelium
discoideum results in cell autonomous lethality.  Strains expressing the
toxic gene under the control of a prestalk specific regulatory region fail
to culminate or form stalks, but form spores normally.  Strains expressing
ricin A under the control of a prespore specific regulatory region form
neither spores nor stalks.  Regulation of the cell types results in
conversion of prestalk cells to prespore cells when the prespore cells are
poisoned.  The newly converted cells then express ricin A and die.  In
contrast, we could not detect any significant conversion of prespore cells
to prestalk cells when the prestalk cells are poisoned under our
experimental conditions.  This regulation of cell types suggests that the
tendency of prestalk cells to regulate and become prespore cells is
inhibited by the already established prespore cells.  It appears that 
prespore cells control prestalk cell regulation by producing an inhibitor
of prespore differentiation to which they themselves are insensitive.