Understanding the Role of BMP Signalling in the Formation of Neural Circuits in the Chick Embryo

Regenerative cell-based therapies for spinal cord injuries (SCI) will benefit from a deeper understanding of proprioceptive neural circuit formation in the developing embryo. The function of sensory interneurons, such as dI3, and the Dorsal Root Entry Zone (DREZ), which are components of proprioceptive neural circuits, are well characterised, but the developmental processes and molecular environment needed for their formation are not well understood. We investigated the Bone Morphogenic Pathway (BMP) which is important for the development of dorsal interneurons. However, its involvement in the birth of dI3 neurons and the formation of the DREZ is not well understood. In this study, we used the chick embryo model to investigate the effect of dorsomorphin (DMN) which is a BMP pathway inhibitor, and isoliquiritigenin (ISL) which is a BMP pathway activator on dI3 interneuron generation and DREZ anatomy. Our objective was to inject these small molecule drugs sub-blastodermally (10μl for DMN, 10μl for ISL) at defined stages (16, 18) of chick embryo development and use immunohistochemistry to investigate dorsal interneuron and DREZ formation. We found that DMN broadens and dampens levels of Smad1/5/9, a downstream effector of BMP signalling within a specific time window before the birth of dI3 interneurons in the dorsal neural tube which correlates with significant increases in the number of dI3 neurons that are generated at the expense of dI1 and dI2 neurons and ventrally positioned motor neurons. ISL has no significant impact on the dI3 neuron population but does significantly increase the number of dI1 interneurons as well as the number of dorsal rootlets innervating the DREZ. These observations suggest that both the level and the duration of BMP signalling are important for dI3 interneuron generation and provide new insight into the role of BMP signalling in DREZ formation. Future work will involve the investigation of other downstream signalling markers to confirm disruption of the BMP signalling by DMN and ISL, as well as the investigation of BMP receptor and BMP inhibitor expression.