Regulating the scaling of growth and pattern during neural development

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Tissue formation requires the coordination of two fundamental cellular processes: growth and patterning. Within the nervous system the initial driver of growth and patterning are signaling factors called morphogens. They activate gene regulatory networks to specify distinct cell fates within the developing central nervous system (CNS) and sculpt the size and boundaries to tissues and organs during development. Thus, a clear understanding of how morphogens can accomplish these feats of tissue engineering is of central importance to developmental biology. Three key morphogenetic pathways that regulate tissue growth and survival are those regulated by the secreted proteins Sonic Hedgehog (Shh), Wingless/Int-related (Wnt), and Bone Morphogenetic Protein (BMP). Work from our group has revealed the intricate relationship between Shh and Wnt signaling during craniofacial and nerve development. We generated a series of unique mouse mutants that carefully titrated the levels Shh signaling during development. We discovered that excessive Shh signaling leads to a restriction Wnt signaling and growth promoting pathways in craniofacial primordia, resulting in hypoplasia. We also showed that if Shh levels are too high, cell death occurs in migrating neural crest cells and prevents their integration with placode cells during sensory ganglia formation. Therefore, the appropriate levels of Shh signaling is required to ensure the coordination of growth signals with cell fate acquisition in developing tissues. It is however unclear how Shh levels act to achieves this balance, and answering this question will shed light on how growth and specification can scale according to the widely divergent body sizes in vertebrates. The proposed research program will take advantage of our Shh pathway mutants to generate a 'scalable' spinal organoid model system to test the relationship between growth and patterning. Growth dynamics will be correlated with a mapping of the developmental trajectories of neural cell subtypes using single cell RNA sequencing technology. A complimentary approach involves removing a key growth promoting checkpoint control gene within the context of altered Shh signaling levels and evaluate its effect on the growth and pattering of the nervous system. This genetic approach will also label Shh-positive tissues with green fluorescent protein, allowing for an analysis of dynamic cellular and molecular events underpinning growth control and patterning by the Shh mophogen gradient. We will then identify the gene networks that regulate the decision branch points involved in coordinating growth control and cell specification in the neural tissues. In so doing our research program will address a long-standing problem in evolutionary developmental biology, the scaling of 'growth and form', and will also make contributions to the lives of Canadians by optimizing methodologies for the generation of functional tissues suitable for replacement therapies.