Dictyostelium discoideum

Description:  A sensitive signalling system constantly measures the density of the Dictyostelium population in relation to the bacterial food source.
Author:W. Nellen
ISBN: 0486241386   ISBN: 0486241386   ISBN: 0486241386   ISBN: 0486241386 
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Dictyostelium discoideum
A social amoeba as a biological model system

The amoeboid cells of Dictyostelium live in the forest soil and feed on bacteria. During this vegetative part of the life cycle, cells multiply by mitotic division. Since their radius of motility is limited and the uptake of prey is rather large, the food source will be depleted and cells will starve. Under these conditions, an alternative genetic program is initiated which allows the cells to ultimately find a new, richer environment (see the life cycle of Dictyostelium discoideum).

A sensitive signalling system constantly measures the density of the Dictyostelium population in relation to the bacterial food source. When the population/food source factor reaches a certain threshold level, cells begin to prepare for the developmental part of their life cycle (see also signal transduction project).

The initially independent cells activate an interconnected signalling and sensing system by which they co-ordinate a pulsatile, directed movement within a population: cAMP is produced and secreted, neighbouring cells sense the signal by cell surface cAMP receptors (cARs) which are G-protein coupled seven-transmembrane-receptors similar to e.g. the b-adrenergic receptor in mammals. The signal is transferred to the cytoskeleton and results in directed movement along the cAMP gradient. Simultaneously, the signal mediates activation of adenylyl cyclase and secretion of cAMP thus leading to a relay of cAMP release. In the medium, cAMP is degraded by an extracellular phosphodiesterase. The signal is extinguished and the environment is cleared for the next cAMP pulse. Pulses and chemotaxis towards the signal occurs with a frequency of 1 per 6 minutes. Approximately 106 cells aggregate and form tight cell to cell contacts mediated by cell surface adhesion molecules which are expressed upon cAMP signalling. The chemoattractant thus serves at least three functions (1) it regulates gene expression, (2) mediates directed cell movement and (3) supports the cAMP relay system.


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