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
\nunderstanding of proprioceptive neural circuit formation in the developing embryo. The function of
\nsensory interneurons, such as dI3, and the Dorsal Root Entry Zone (DREZ), which are components of
\nproprioceptive neural circuits, are well characterised, but the developmental processes and molecular
\nenvironment needed for their formation are not well understood. We investigated the Bone
\nMorphogenic Pathway (BMP) which is important for the development of dorsal interneurons.
\nHowever, its involvement in the birth of dI3 neurons and the formation of the DREZ is not well
\nunderstood. In this study, we used the chick embryo model to investigate the effect of dorsomorphin
\n(DMN) which is a BMP pathway inhibitor, and isoliquiritigenin (ISL) which is a BMP pathway
\nactivator on dI3 interneuron generation and DREZ anatomy. Our objective was to inject these small
\nmolecule drugs sub-blastodermally (10μl for DMN, 10μl for ISL) at defined stages (16, 18) of chick
\nembryo development and use immunohistochemistry to investigate dorsal interneuron and DREZ
\nformation. We found that DMN broadens and dampens levels of Smad1/5/9, a downstream effector of
\nBMP signalling within a specific time window before the birth of dI3 interneurons in the dorsal neural
\ntube which correlates with significant increases in the number of dI3 neurons that are generated at the
\nexpense of dI1 and dI2 neurons and ventrally positioned motor neurons. ISL has no significant impact
\non the dI3 neuron population but does significantly increase the number of dI1 interneurons as well as
\nthe number of dorsal rootlets innervating the DREZ. These observations suggest that both the level
\nand the duration of BMP signalling are important for dI3 interneuron generation and provide new
\ninsight into the role of BMP signalling in DREZ formation. Future work will involve the investigation
\nof other downstream signalling markers to confirm disruption of the BMP signalling by DMN and ISL,
\nas well as the investigation of BMP receptor and BMP inhibitor expression.