The role of G protein-coupled receptors in the early evolution of neurotransmission and the nervous system

The origin and evolution of the nervous system is one of the most intriguing and enigmatic events in biology. The recent sequencing of complete genomes from early metazoan organisms provides a new platform to study the origins of neuronal gene families. This review explores the early metazoan expansion of the largest integral transmembrane protein family, the G protein-coupled receptors (GPCRs), which serve as molecular targets for a large subset of neurotransmitters and neuropeptides in higher animals. GPCR repertories from four pre-bilaterian metazoan genomes were compared. This includes the cnidarian Nematostella vectensis and the ctenophore Mnemiopsis leidyi, which have primitive nervous systems (nerve nets), the demosponge Amphimedon queenslandica and the placozoan Trichoplax adhaerens, which lack nerve and muscle cells. Comparative genomics demonstrate that the rhodopsin and glutamate receptor families, known to be involved in neurotransmission in higher animals are also widely found in pre-bilaterian metazoans and possess substantial expansions of rhodopsin-family-like GPCRs. Furthermore, the emerging knowledge on the functions of adhesion GPCRs in the vertebrate nervous system provides a platform to examine possible analogous roles of their closest homologues in pre-bilaterians. Intriguingly, the presence of molecular components required for GPCR-mediated neurotransmission in pre-bilaterians reveals that they exist in both primitive nervous systems and nerve-cell-free environments, providing essential comparative models to better understand the origins of the nervous system and neurotransmission.

Tribolium embryo morphogenesis: may the force be with you

Development of multicellular organisms depends on patterning and growth mechanisms encoded in the genome, but also on the physical properties and mechanical interactions of the constituent cells that interpret these genetic cues. This fundamental biological problem requires integrated studies at multiple levels of biological organization: from genes, to cell behaviors, to tissue morphogenesis. We have recently combined functional genetics with live imaging approaches in embryos of the insect Tribolium castaneum, in order to understand their remarkable transformation from a uniform single-layered blastoderm into a condensed multi-layered embryo covered by extensive extra-embryonic tissues. We first developed a quick and reliable methodology to fluorescently label various cell components in entire Tribolium embryos. Live imaging of labeled embryos at single cell resolution provided detailed descriptions of cell behaviors and tissue movements during normal embryogenesis. We then compared cell and tissue dynamics between wild-type and genetically perturbed embryos that exhibited altered relative proportions of constituent tissues. This systematic comparison led to a qualitative model of the molecular, cellular and tissue interactions that orchestrate the observed epithelial rearrangements. We expect this work to establish the Tribolium embryo as a powerful and attractive model system for biologists and biophysicists interested in the molecular, cellular and mechanical control of tissue morphogenesis.