In patients with acute coronary syndromes, cardiac troponin I levels provide useful prognostic information and permit the early identification of patients with an increased risk of death.

No abstract

P2X receptors are membrane ion channels gated by extracellular ATP1,2 that are found widely in vertebrates, but not previously in microbes. Here we identify a weakly related gene in the genome of the social amoeba Dictyostelium discoideum, and show, with the use of heterologous expression in human embryonic kidney cells, that it encodes a membrane ion channel activated by ATP (30-100 μM). Site-directed mutagenesis revealed essential conservation of structure-function relations with P2X receptors of higher organisms. The receptor was insensitive to the usual P2X antagonists3 but was blocked by nanomolar concentrations of Cu 2+ ions. In D. discoideum, the receptor was found on intracellular membranes, with prominent localization to an osmoregulatory organelle, the contractile vacuole. Targeted disruption of the gene in D. discoideum resulted in cells that were unable to regulate cell volume in hypotonic conditions. Cell swelling in these mutant cells was accompanied by a marked inhibition of contractile vacuole emptying. These findings demonstrate a new functional role for P2X receptors on intracellular organelles, in this case in osmoregulation.The D. discoideum genome contains five sequences predicted to encode proteins homologous to vertebrate P2X receptors. The protein most closely related to the seven human receptors is DDB0168616 (Supplementary Fig. 1). HEK cells expressing a humanized version of this complementary DNA responded to ATP with robust inward currents (Fig. 1). ATP evoked unitary currents (conductance 8.2 pS at -100 mV) in outsideout patches (Fig. 1a, b), establishing that this gene (D. discoideum p2xA) encodes an ATPgated ion channel (DdP2X). Whole-cell currents evoked by ATP were concentrationdependent (10-300 μM) with kinetic properties most similar to those observed for human P2X 2 or P2X 4 receptors1 (Fig. 1d). With sodium as the only cation, the current-voltage relation reversed at zero and showed moderate inward rectification (Fig. 1c). By measuring the current reversal potentials in different extracellular ions, we found that the channels were freely permeable to Ca 2+ ions but less so to larger cations (the relative permeability P X /P Na was 1.5 ± 0.11 (n = 8 cells), 0.54 ± 0.01 (5), 0.48 ± 0.21 (5), 0.38 ± 0.16 (5) and 0.21 ± 0.05 (8), where X represents Ca 2+ , choline, Tris, tetraethylammonium and N-methyl-D-glucamine, respectively; Fig. 1c). These values are in the range of vertebrate P2X receptors1. Western blotting of the expressed receptor showed two bands at 48 and 55 kDa (Fig. 1e). The

Cooperation is central to many major transitions in evolution, including the emergence of eukaryotic cells, multicellularity and eusociality. Cooperation can be destroyed by the spread of cheater mutants that do not cooperate but gain the benefits of cooperation from others. However, cooperation can be preserved if cheaters are facultative, cheating others but cooperating among themselves. Several cheater mutants have been studied before, but no study has attempted a genome-scale investigation of the genetic opportunities for cheating. Here we describe such a screen in a social amoeba and show that cheating is multifaceted by revealing cheater mutations in well over 100 genes of diverse types. Many of these mutants cheat facultatively, producing more than their fair share of spores in chimaeras, but cooperating normally when clonal. These findings indicate that phenotypically stable cooperative systems may nevertheless harbour genetic conflicts. The opportunities for evolutionary moves and countermoves in such conflicts may select for the involvement of multiple pathways and numerous genes.

Understanding the maintenance of cooperation requires an understanding of the nature of cheaters and the strategies used to mitigate their effects. However, it is often difficult to determine how cheating or differential social success has arisen. For example, cheaters may employ different strategies (e.g., fixed and facultative), whereas other causes of unequal fitness in social situations can result in winners and losers without cheating. To address these problems, we quantified the social success of naturally occurring genotypes of Dictyostelium discoideum during the formation of chimeric fruiting bodies, consisting of dead stalk cells and viable spores. We demonstrate that an apparent competitive dominance hierarchy of spore formation in chimera is partly due to a fixed strategy where genotypes exhibit dramatically different spore allocations. However, we also find complex, variable facultative strategies, where genotypes change their allocation in chimera. By determining the magnitude and direction of these changes, we partition facultative cheating into two forms: (1) promotion of individual fitness through selfish behaviour ("self-promotion") and (2) coercion of other genotypes to act cooperatively. Our results demonstrate and define social interactions between D. discoideum isolates, thus providing a conceptual framework for the study of the genetic mechanisms that underpin social evolution.