Bimodal distribution of motility and cell fate in Dictyostelium discoideum

Pre-starvation amoebae of Dictyostelium discoideum exhibit random movements. Starved cells aggregate by directed movements (chemotaxis) towards cyclic AMP and differentiate into live spores or dead stalk cells. Many differences between presumptive spore and stalk cells precede differentiation. We have examined whether cell motility-related factors are also among them. Cell speeds and localisation of motility-related signalling molecules were monitored by live cell imaging and immunostaining (a) in nutrient medium during growth, (b) immediately following transfer to starvation medium and (c) in nutrient medium that was re-introduced after a brief period of starvation. Cells moved randomly under all three conditions but mean speeds increased following transfer from nutrient medium to starvation medium; the transition occurred within 15 min. The distribution of speeds in starvation medium was bimodal: about 20% of the cells moved significantly faster than the remaining 80%. The motility-related molecules F-actin, PTEN and PI3 kinase were distributed differently in slow and fast cells. Among starved cells, the calcium content of slower cells was lower than that of the faster cells. All differences reverted within 15 min after restoration of the nutrient medium. The slow/fast distinction was missing in Polysphondylium pallidum, a cellular slime mould that lacks the presumptive stalk and spore cell classes, and in the trishanku (triA(-)) mutant of D. discoideum, in which the classes exist but are unstable. The transition from growth to starvation triggers a spontaneous and reversible switch in the distribution of D. discoideum cell speeds. Cells whose calcium content is relatively low (known to be presumptive spore cells) move slower than those whose calcium levels are higher (known to be presumptive stalk cells). Slow and fast cells show different distributions of motility-related proteins. The switch is indicative of a bistable mechanism underlying cell motility.     

C3-Heteroaroyl cannabinoids as photolabeling ligands for the CB2 cannabinoid receptor

A series of tricyclic cannabinoids incorporating a heteroaroyl group at C3 were prepared as probes to explore the binding site(s) of the CB1 and CB2 receptors. This relatively unexplored structural motif is shown to be CB2 selective with K(i) values at low nanomolar concentrations when the heteroaromatic group is 3-benzothiophenyl (41) or 3-indolyl (50). When photoactivated, the lead compound 41 was shown to successfully label the CB2 receptor through covalent attachment at the active site while 50 failed to label. The benzothiophenone moiety may be a photoactivatable moiety suitable for selective labeling.     

Chemotaxis, signal relaying and aggregation morphology

It is shown that a system of cells which are attracted by chemotaxis to a signal released by themselves can aggregate (in the form of delta function concentrations). This aggregation is initiated by random fluctuations which occur naturally. When the signalling is cooperative, such an aggregation presents a characteristic streaming morphology, observed in some of the cellular slime moulds.     

A differential chemotactic response of slime mould amoebae to regions of the early amphibian embryo

Slime mould amoebae show a chemotactic response to cells of the early axolotl embryo. This response is stronger to dorsal cells than to ventral cells. It is suggested that the chemotactant is cAMP.     

Identification of neoblast- and regeneration-specific miRNAs in the planarian Schmidtea mediterranea

In recent years, the planarian Schmidtea mediterranea has emerged as a tractable model system to study stem cell biology and regeneration. MicroRNAs are small RNA species that control gene expression by modulating translational repression and mRNA stability and have been implicated in the regulation of various cellular processes. Though recent studies have identified several miRNAs in S. mediterranea, their expression in neoblast subpopulations and during regeneration has not been examined. Here, we identify several miRNAs whose expression is enriched in different neoblast subpopulations and in regenerating tissue at different time points in S. mediterranea. Some of these miRNAs were enriched within 3 h post-amputation and may, therefore, play a role in wound healing and/or neoblast migration. Our results also revealed miRNAs, such as sme-miR-2d-3p and the sme-miR-124 family, whose expression is enriched in the cephalic ganglia, are also expressed in the brain primordium during CNS regeneration. These results provide new insight into the potential biological functions of miRNAs in neoblasts and regeneration in planarians.     

The scale-invariance of spatial patterning in a developing system

Summary Regulating systems, that is, those which exhibit scale-invariant patterns in the adult, are supposed, to do so on account of interactions between cells during development. The nature of these interactions has to be such that the system of positional information (map) in the embryo also regulates. To our knowledge, this supposition regarding a regulating map has not been subjected to a direct test in any embryonic system. Here we do so by means of a simple and novel criterion and use it to examine tip regeneration in the mulicellular stage (slug) ofDictyostelium discoideum. When anterior, tip-containing fragments of slugs are amputated, a new tip spontaneously regenerates at the cut surface of the (remaining) posterior fragment. The time needed for regeneration to occur depends on the relative size of the amputated fragment but is independent of the total size of the slug. We conclude from this finding that there is at least one system underlying positional information in the slug which regulates.