The IgSF protein MDGA1 regulates morphology 
during a defined stage of placode-derived neuron maturation in developing chick cranial sensory ganglia.

The developing distal cranial sensory ganglia of the chick present an interesting and tractable model for the study of general processes of neural development. While the early stages of placodal neurogenesis, including induction of the placodes and initiation of the neurogenic programme, have been extensively studied, little is known about the molecular mechanisms that regulate migration of placode-derived neuroblasts and their aggregation to form ganglia. These questions have been addressed in the context of the trigeminal ganglion, however it remains unclear whether these principles apply to the epibranchial ganglia, on which the work presented here is focussed.
Molecules potentially involved in controlling placodal neuroblast migration in the epibranchial ganglia were identified through a comparative microarray screen carried out in the Begbie lab. A list of candidate genes implicated in a variety of different cellular was validated by determining expression patterns in the region of the epibranchial CSG by in situ hybridisation.
These expression patterns showed that different genes were expressed by different populations within the migratory stream. This question was further addressed through the detailed analysis of the expression patterns of a panel of neuronal and neurogenic markers, leading to the finding that placodal neuroblasts appear to sequentially upregulate different groups of genes as they migrate away from the placode. Neuroblasts within the migratory stream can further be subdivided according to cell morphology, which was assessed through high resolution imaging of GFP-labelled placodal cells. Multipolar and bipolar cells were concentrated around two different regions of the migratory stream with multipolar cells localised near the placode and bipolar cells localised closer to the neural tube. Together these findings support the hypothesis that placodal neuroblasts mature as they migrate towards the site of ganglion aggregation.
With this detailed description of the system in mind, the question of molecular control was addressed through the functional characterisation of a candidate gene identified in the original microarray screen. MDGA1, a GPI-anchored IgSF molecule that has been implicated in controlling radial migration of cortical neurons, was specifically expressed in the chick CSG at the relevant stages. RNAi-mediated knockdown and overexpression were used to test the function of MDGA1 in migrating placodal neuroblasts. These experiments showed that MDGA1 negatively regulates the formation and extension of neuronal projections in bipolar neuroblasts.
With the mechanisms of MDGA1 function relying entirely on protein-protein interactions at the cell-surface, we then set out to identify and characterise potential MDGA1 binding partners. SPR binding experiments carried out in collaboration with the Aricescu lab revealed that MDGA1 interacts with the Neuroligin family of synaptic proteins. Recent evidence has shown that MDGA1 interacts in cis with NLGN2 in rat hippocampal neurons where it disrupts its interaction in trans with Neurexin1. Neuroligins and Neurexins function to stabilise dendritic filopodia by creating trans-synaptic adhesions and recruiting the synaptic apparatus. Having determined that both NLGN2 and NRXN1 are expressed in placode-derived neuroblasts of the CSG, we propose that these molecules play a role in the stabilisation and extension of neuronal projections in this system and that this function in modulated by MDGA1 function.