SMAD6-mediated modulation of BMP/TGF-β signaling disrupts midbrain growth in chick embryos

Bone morphogenetic proteins (BMPs) play important roles in dorsal spinal cord patterning. Their presence in the roof plate of the midbrain suggests that BMP-related signaling may also contribute to dorsal midbrain development. However, whether BMP/TGF-β-associated SMAD-dependent signaling contributes to dorsal midbrain growth remains unclear. We examined this question in chick embryos by misexpressing components of BMP/TGF-β-associated signaling pathways. BMP4 overexpression did not affect midbrain growth, whereas GDF7 overexpression reduced midbrain growth. Expression of a truncated dominant-negative BMP receptor type 1b had no detectable effect on midbrain size. Ectopic expression of SMAD6, an intracellular inhibitor of BMP/TGF-β-associated SMAD-dependent signaling, significantly reduced midbrain size, which correlated with reduced mitotic activity in SMAD6-overexpressing cells. SMAD6 overexpression was also associated with low-penetrance changes in mesencephalic trigeminal nucleus (MTN) axon trajectories. These results support a contribution of SMAD-dependent signaling pathways to early dorsal midbrain growth.