Chemoattraction and chemotaxis in Dictyostelium discoideum: myxamoeba cannot read spatial gradients of cyclic adenosine monophosphate.

Abstract

Myxamoebae of the morphogenetic cellular slime mold Dictyostelium discoideum are thought to be able to accurately read and respond to directional information in spatial gradients of cyclic AMP. We examined the spatial and temporal mechanisms proposed for chemotaxis by comparing the behavior of spreading or evenly distributed cell populations after exposure to well-defined spatial gradients . The effects of gradient generation on cells were avoided by using predeveloped gradients . Qualitatively different responses were obtained using (a) isotropic, (b) static spatial, or (c) temporal (impulse) gradients in a simple chamber of penetrable micropore filters . We simulated models of chemotaxis and chemokinesis to aid our interpretations . The attractive and locomotory responses of populations were maximally stimulated by 0.05 AM cyclic AMP, provided that cellular phosphodiesterase was inhibited . But a single impulse of cyclic AMP during gradient development caused a greater and qualitatively different attraction . Attraction in spatial gradients was only transient, in that populations eventually developed a random distribution when confined to a narrow territory . Populations never accumulated nor lost their random distribution even in extremely steep spatial gradients . Attraction in spatial gradients was inducible only in spreading populations, not randomly distributed ones. Thus, spatial gradients effect biased-random locomotion : i.e ., chemokinesis without adaptation . Cells cannot read gradients; the reaction of the cells is stochastic. Spatial gradients do not cause chemotaxis, which probably requires a sharp stimulant concentration increase (a temporal gradient) as a pulse or impulse . The results also bear on concepts of how embryonic cells might be able to decipher the positional information in a morphogen spatial gradient during development .The morphogenesis of free-living myxamoebae of Dictyostelium discoideum into a differentiated metazoan is characterized by the intermingling effects of cyclic AMP (cAMP), including increased cohesiveness, chemokinesis, chemotaxis, cell polarization, CAMP and cAMP phosphodiesterase (PDE)' synthesis and emission, aggregation, and differentiation (1-8). The aggregation of dispersed populations follows starvation and depends partly on the reception and excretion of cAMP by each cell. Emissions first appear in a pulsatile form and cause the orientation of individual cells (3, 9), but they later coalesce into streams, which efficiently guide them to-'Abbreviations used in this paper: BBSS, Bonner's buffered salt solution ; DTT, dithiothreitol; PDE, phosphodiestease; SM medium, Sussman's standard medium ; 0, diameter .ward the aggregation center. Such morphogenic fields (10) might be indispensable determinants of cell positioning, patterning, and organismal form during embryogenesis (11,12) . However, the mechanisms of directed locomotion and their relation to the effect of morphogens on cells remain largely unclear.The possible mechanisms of chemotaxis are prevale...

Quantitative analysis of cell motility and chemotaxis in Dictyostelium discoideum by using an image processing system and a novel chemotaxis chamber providing stationary chemical gradients.

“…Our results are not easily accommodated with those of Vicker et al (1984) who used chemotaxis chambers with source and sink reservoirs to provide essentially stable linear gradients. Instead of measuring the movement of individual cells, they measured the asymmetric distribution of amebae resulting from chemotaxis.…”

“…The accuracy of chemotaxis by amoebae in the two cases is similar (Varnum and Soil, 1984). Vicker et al (1984) found optimum chemotaxis at slightly higher concentrations during gradient formation in their chambers, but did not detect chemotaxis in stable gradients (see Discussion).…”

Gradient sensing in defined chemotactic fields

“…These devices established a stable linear gradient of cAMP across the stream. Chemotaxis was observed when the gradient, Δ C /Δ x , was steeper than 10 −3 nM/μm and was optimal at Δ C /Δ x between 0.01 nM/μm and 0.1nM/μm, in excellent agreement with previous measurements under stationary conditions 18,19,20. These results established that developed Dictyostelium cells can respond to a stable spatial gradient by persistent chemotaxis.…”

Chemotaxis in Eukaryotic Cells: A Focus on Leukocytes and Dictyostelium

“…DDA are motile in the absence of chemoattractant (Po tel & MacKay 1979); however, acceleration in response to increasing concentrations applied in vitro and in vivo (by approaching cAMP waves) is readily measurable (Varnum et a1 19 85;Vicker et al 1984Vicker et al , 1986MacKay 19 78). Chemokinesis is most obvious in PMN, which are immobile in the absence of a stimulant.…”