Intracellular SMAD6 inhibition disrupts midbrain growth in chick embryos

Blocking a key intracellular signaling brake inside developing brain cells can dramatically shrink the growing midbrain, according to a new study in chick embryos. Researchers found that overexpressing SMAD6, an inhibitor of BMP/TGF-β-associated SMAD-dependent signaling, significantly reduced the size of the dorsal midbrain, an effect that correlated with fewer cells entering mitosis. The finding, published in Discover Neuroscience, points to a previously underappreciated role for SMAD-dependent signaling in the early expansion of the dorsal midbrain, even though the same pathway appears largely dispensable for early fate specification in this brain region.

The study was conducted by Dionysia Moschou, Annette Richter and Andrea Wizenmann, who used in ovo electroporation to misexpress components of BMP/TGF-β signaling in the midbrain neural tube of chick embryos at Hamburger-Hamilton stages 9 to 11. The embryos were then allowed to develop to stages 16 to 18, when the dorsal midbrain, which gives rise to the optic tectum, has begun its rapid lateral expansion. By measuring the area of the electroporated midbrain half and comparing it with the untouched contralateral side, the team could quantify how each manipulation affected tissue growth.

The results were strikingly dependent on where in the pathway the manipulation occurred. Overexpressing the ligand BMP4 had no effect on midbrain size, and neither did expressing a truncated dominant-negative form of the BMP receptor type 1b, which was expected to block signal transduction at the cell surface. In contrast, overexpressing GDF7, a growth differentiation factor of the TGF-β superfamily normally expressed in the roof plate, significantly reduced the size of the transfected midbrain hemisphere. SMAD6 overexpression produced a similar and statistically significant reduction, with the transfected side reaching an area ratio of 0.82 compared with a wildtype left-to-right ratio of 0.99.

The specificity of the SMAD6 effect was underlined by careful controls. Electroporating GFP alone did not alter midbrain growth, and the authors showed that the severity of the growth deficit appeared to depend on how much of the dorsal midbrain was transfected. In embryos with broader SMAD6-GFP expression, the reduction in midbrain size was significant, while in embryos with sparser transfection it was not. The authors emphasized that this subgroup analysis, with only three embryos per group, was descriptive and should be interpreted cautiously, but it suggests that the phenotype scales with the extent of pathway disruption.

Why did blocking a single receptor fail where intracellular inhibition succeeded? The authors propose that redundancy within the BMP receptor system may be responsible. Several BMP ligands, including BMP2, BMP4 and BMP7, are present in the developing dorsal midbrain and can signal through multiple type 1 receptors, including BMPR1A (ALK3), BMPR1B (ALK6) and, in some contexts, ACVR1 (ALK2). Blocking one receptor subtype may therefore leave pathway activity largely intact. SMAD6, by contrast, acts downstream of all type 1 receptors, interfering with SMAD1 signaling and preventing formation of the active SMAD1-SMAD4 transcriptional complex. It can also interact with components of the TGF-β-associated pathway, meaning its effects may extend beyond canonical BMP signaling.

The cellular basis of the growth reduction was probed using phospho-histone H3, a marker that labels cells in late G2 and the beginning of M-phase. In GFP-electroporated control embryos, 59 percent of PH3-positive cells were found on the electroporated side. With SMAD6 overexpression, this pattern reversed: only 39 percent of mitotic cells were on the SMAD6-expressing side, a statistically significant reduction confirmed by two-way ANOVA and post hoc comparisons. The authors interpret this as reduced mitotic activity in SMAD6-transfected tissue, while noting that PH3 labels mitotic cells rather than overall proliferative history, and that alternative mechanisms such as premature differentiation or increased apoptosis cannot be excluded without additional assays.

Importantly, the authors acknowledge that pathway activity was not directly measured after any of the manipulations. Without analyzing phospho-SMAD1/5/8 levels in the transfected tissue, it remains uncertain whether canonical BMP signaling was effectively increased or reduced in the manipulated cells. Because SMAD6 can interfere with broader SMAD-dependent signaling, the findings support a contribution of SMAD-dependent pathways to dorsal midbrain growth but do not provide definitive evidence of BMP-specific inhibition. The PH3 analysis also used GFP-electroporated control tissue rather than an empty vector matched to the SMAD6 construct backbone, so potential vector-backbone effects cannot be fully excluded.

Beyond growth, the study uncovered hints of a role in axonal organization. In a subset of embryos with broad SMAD6 transfection, 3 out of 11, the trajectories of axons from neurons of the mesencephalic trigeminal nucleus deviated from their typical ventral path, and the lateral longitudinal fascicle appeared broader or less organized. Similar low-penetrance changes were seen with dominant-negative BMPR1B, in 2 out of 7 embryos, and in a single GDF7-overexpressing embryo out of 11. Because these phenotypes appeared in only a subset of embryos and were not assessed by blinded scoring, the authors urge caution, noting that experiments with predefined scoring criteria would be needed to establish whether BMP/SMAD-dependent signaling directly contributes to MTN axon guidance.

The broader context makes these findings notable. BMP signaling is well established as a central player in dorsal spinal cord patterning, where roof plate-derived BMPs and WNTs specify dorsal interneuron populations. In the midbrain, however, previous work has shown that activating or blocking BMPR1 signaling does not change the expression of dorsal markers such as Pax3, Pax7, Meis2 and Efnb1, and deactivation of BMP signaling does not affect the development of MTN neurons. The new study suggests that even when fate specification is unaffected, signaling pathways may still regulate tissue expansion, mitotic activity and axonal organization during the rapid growth phase of the dorsal midbrain.

The observation that both GDF7 overexpression, which increases ligand availability, and SMAD6 overexpression, which inhibits intracellular signaling, reduced midbrain size suggests that dorsal midbrain growth may depend on tightly regulated levels of BMP/TGF-β superfamily signaling rather than a simple linear relationship in which more pathway activity means more growth. The authors propose that dorsal midbrain development requires signaling within an optimal range, with either excessive ligand or intracellular inhibition shifting cells away from conditions that support normal tissue expansion. Because PH3 analysis was not performed in GDF7-overexpressing embryos, it remains unknown whether the GDF7 phenotype, like the SMAD6 phenotype, is associated with reduced mitotic activity, and future work with mitotic, apoptosis and differentiation markers will be needed to resolve the cellular mechanisms behind these growth deficits.

Subject of Research: The role of BMP/TGF-β-associated SMAD-dependent signaling in early dorsal midbrain growth in chick embryos

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

Article References: Moschou, D., Richter, A., & Wizenmann, A. (2026). SMAD6-mediated modulation of BMP/TGF-β signaling disrupts midbrain growth in chick embryos. Discover Neuroscience, 21(1), Article 98. https://doi.org/10.1186/s13064-026-00328-8

Image Credits: AI Generated

DOI: 10.1186/s13064-026-00328-8

Keywords: SMAD6, BMP signaling, TGF-β, midbrain development, chick embryo, GDF7, BMPR1B, mitotic activity, mesencephalic trigeminal nucleus, optic tectum, electroporation, neural tube