Final Report Summary - PROTOBRAIN (Sensory-motor circuits in marine zooplankton and early evolution of the nervous system)
Over the past five years we focused on developing a new animal model for behavioural and circuit neuroscience, the marine annelid Platynereis dumerilii. We focused on the larval nervous system, since due to its small size, stereotypic neuron complement and stereotypic connectivity it allows the study of circuit function at single cell resolution. The Platynereis larva is also amenable to complete connectome reconstruction. We have developed this system to a level where we can now address the role of single identified neurons in completely mapped neuronal circuits during larval behaviour. We develop several tools and resources, including whole-body gene expression registration, transcriptome and neuropeptidome resources, and 30 neuropeptide-specific antibodies. We also performed a large-scale in vitro screen to identify the G-protein coupled receptors (GPCR) of neuropeptides and successfully deorphanized 20 Platynereis GPCRs. Another main focus of the project was to reconstruct the neuronal connectome of larval circuits. We have reconstructed and analysed the visual phototactic circuit of the larvae, representing the first full complete circuit diagram of a visual circuit in any animal. This circuit allows larvae to turn away or towards a light source by contracting the longitudinal trunk muscles. The circuit consisting of approximately 70 neurons, including the photoreceptor cells of the pigmented eyes, three layers of interneurons, and motorneurons that connect to the longitudinal muscles. The connectome allowed us to dissect the logic of this circuit and to experimentally analyse the role of a reciprocal circuit motif in contrast enhancement. We also showed that this circuit is very similar between individuals, demonstrating that neuronal connectivity in Platynereis larvae is hard-wired. We also worked on the regulation of Platynereis larval behaviour by neuropeptides, and identified the neuropeptide MIP as a conserved regulator of larval settlement. Using serial multiplex immunogold labelling using the newly established method siGOLD, we could directly map 11 different neuropeptides to the larval connectome using immunogold labelling.