Figure 1: The C. elegans nematode is transparent (A), with a fully mapped nervous system composed of 302 neurons and 50 ectoderm-derived glia (here expressing RFP and GFP, B). We dissect nervous system assembly by analysing interactions of glia and neurons in the brain circuit (C), and its high-resolution structure in normal animals and genetic mutants (D). The C. elegans brain circuit assembles in the embryo (E): pioneer axons (red, arrow) coalesce and interact with glia (green) to guide circuit components through a glia–neuron molecular crosstalk of conserved genes (F).
Video 1: C. elegans individuals of different developmental stages forage on food through a characteristic crawling locomotion. Genetic mutants with defective circuit assembly show abnormal locomotion. Studying mutant circuit structure and locomotion allows us to dissect in vivo mechanisms of nervous system assembly.
Video 2: Developmental morphogenesis of C. elegans embryos, by bright field (left) and fluorescent live time-lapse imaging (right). Axons (arrows) of pioneer neurons (here expressing GFP) grow and navigate early to initiate brain-circuit assembly in vivo.
+++ At EMBL from December 2019 +++
The Rapti group dissects cellular and molecular mechanisms of nervous system assembly and the underlying glia–neuron crosstalk, using advanced genetics, genomics and live imaging approaches.
Previous and current research
How do embryos assemble their nervous system?
During an embryo’s life, cells with diverse fates and morphologies interact in space and time to give rise to tissues of well-defined architecture. Assembly of neural circuits is fundamental for life; yet understanding the events and mechanisms initiating this assembly remains tantalising. How do neural cells interact spatio-temporally, in environments void of circuits, while concurrently diversifying their fates? Pioneer cells are thought to initiate circuit assembly, but their molecular identities, development and interactions remain elusive. How do neural components establish specialised morphologies to coordinate in vivo assembly, towards functional connectivity? Which overarching principles pattern circuit assembly across species? Novel tools, technologies, and research directions are required to address these questions.
We address these questions using real-time imaging, refined molecular and cross-species investigation. The nematode Caenorhabditis elegans is a system for this multidisciplinary approach (Fig. 1), with transparent embryos, traceable lineage, morphogenesis tractable at single-cell resolution, mapped nervous system anatomy and connectivity, sequenced genome and sophisticated genetics. Major C. elegans neural cell types and conserved genes are shared across invertebrate and vertebrate species. Thus, C. elegans gene discovery can reveal conserved mechanisms regulating neurodevelopment.
Nervous systems consist mainly of neurons and glia, lineally related cell types in about equal numbers. Neurons transmit electrical currents, often through specialised axons. For more than a century, glia were thought to passively support neurons’ nutrition; however, recently they were implicated in neuronal development and function, in physiology and pathology. We identified that C. elegans glia, similar to vertebrate glia, initiate hierarchical brain assembly. We uncovered pioneer glia and glia-guided axons of specific molecular signature that cooperate functionally to drive assembly; they grow coalescing processes and molecularly guide circuit components (Fig. 1). A set of diverse, conserved molecular cues drive circuit assembly synergistically through a glia–neuron crosstalk that we are only beginning to understand.
Future projects and goals
We aim to dissect dynamic circuit assembly, at cellular and molecular level. Leveraging C. elegans tractable development and genetics, we study normal animals and mutants with defective circuit structure (Fig. 1D) and locomotion (Video 1), tracing back real-time emergence of their circuits in live, developing embryos (Video 2). We aim to combine high-resolution cellular and molecular studies with novel multidisciplinary approaches of high-speed and throughput imaging, in close collaboration with fellow EMBL groups.
- We will characterise the glia–neuron crosstalk underlying circuit assembly by identifying molecular mechanisms that control pathfinding synergistically, and thus were previously missed.
- We will study dynamic morphogenesis of pioneer glia and neurons, analysing how their molecular identities are established to drive pioneer morphogenesis that shapes functional connectivity.
- We aim to uncover overarching principles driving assembly. Caenorhabditis elegans pioneers resemble vertebrate counterparts, which are challenging to study. We will assess functional conservation of circuit assembly mechanisms with studies across species, including the mouse.