M. Clarke et Rh. Gomer, PSF AND CMF, AUTOCRINE FACTORS THAT REGULATE GENE-EXPRESSION DURING GROWTH AND EARLY DEVELOPMENT OF DICTYOSTELIUM, Experientia, 51(12), 1995, pp. 1124-1134
Throughout growth and development, Dictyostelium cells secrete autocri
ne factors that accumulate in proportion to cell density. At sufficien
t concentration, these factors cause changes in gene expression. Veget
ative Dictyostelium cells continuously secrete prestarvation factor (P
SF). The bacteria upon which the cells feed inhibit their response to
PSF, allowing the cells to monitor their own density in relation to th
at of their food supply. At high PSF/bacteria ratios, which occur duri
ng late exponential growth, PSF induces the expression of several gene
s whose products are needed for cell aggregation. When the food supply
has been depleted, PSF production declines, and a second density-sens
ing pathway is activated. Starving cells secrete conditioned medium fa
ctor (CMF), a glycoprotein of Mr 80 kDa that is essential for the deve
lopment of differentiated cell types. Antisense mutagenesis has shown
that cells lacking CMF cannot aggregate, and preliminary data suggest
that CMF regulates cAMP signal transduction. Calculations indicate tha
t a mechanism of simultaneously secreting and recognizing a signal mol
ecule, as used by Dictyostelium to monitor cell density, could also be
used to determine the total number of cells in a tissue.