Altered cyclic-AMP receptor activity and morphogenesis in a chemosensory mutant of Dictyostelium discoideum

Sensory transduction in eukaryotes. A comparison between Dictyostelium and vertebrate cells

The organization of multicellular organisms depends on cell-cell communication. The signal molecules are often soluble components in the extracellular fluid, but also include odors and light. A large array of surface receptors is involved in the detection of these signals. Signals are then transduced across the plasma membrane so that enzymes at the inner face of the membrane are activated, producing second messengers, which by a complex network of interactions activate target proteins or genes. Vertebrate cells have been used to study hormone and neurotransmitter action, vision, the regulation of cell growth and differentiation. Sensory transduction in lower eukaryotes is predominantly used for other functions, notably cell attraction for mating and food seeking. By comparing sensory transduction in lower and higher eukaryotes general principles may be recognized that are found in all organisms and deviations that are present in specialised systems. This may also help to understand the differences between cell types within one organism and the importance of a particular pathway that may or may not be general. In a practical sense, microorganisms have the advantage of their easy genetic manipulation, which is especially advantageous for the identification of the function of large families of signal transducing components.

Phorbol 12-myristate 13-acetate modulates the cAMP-induced light-scattering response of a Dictyostelium discoïdeum cell population

The effect of phorbol 12-myristate 13-acetate upon the light-scattering response to cAMP of a D. discoïdeum cell suspension was investigated. It was found that the first spike of the cAMP-mediated light-scattering change (peaking at about 15-20 s after stimulation) was inhibited by the phorbol ester. This effect was concentration dependent with an half-maximum value for the inhibition of 4 nM. The inhibition was found to be maximal after a 10-20 min incubation time. The phorbol ester was shown to affect the dose-response relationship between the cAMP concentration and the relative amplitude of the light-scattering change, more by decreasing the number of cAMP receptors than by decreasing their apparent affinity for cAMP.

Reduced cAMP secretion in Dictyostelium discoideum mutant HB3

Extracellular cAMP induces the intracellular synthesis and subsequent secretion of cAMP in Dictyostelium discoideum (relay). cAMP relay was strongly diminished in mutant HB3 which shows abnormal development by making very small fruiting bodies. Extracellular cAMP binds to receptors on the surface of mutant cells and induces the rapid activation of adenylate cyclase. Intracellular cAMP rises to a concentration as high as that in wild-type cells but only a very small amount of cAMP is secreted. cAMP secretion in wild-type cells starts immediately after cAMP production, and is proportional to the intracellular cAMP concentration. In the mutant cells cAMP secretion starts a few minutes after cAMP production; by that time most of the intracellular cAMP is already degraded by phosphodiesterase and little cAMP is available for secretion. We conclude that mutant HB3 has a defect in the mechanism by which Dictyostelium cells secrete cAMP.

Genetics of early Dictyostelium discoideum development

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