The ventral midline of the neural tube adopts a floor plate identity along most of the antero–posterior axis, except for the anterior forebrain, where it differentiates as hypothalamic tissue. In a new study reported in Development, Kapsimali and colleagues used a zebrafish model to investigate the molecular mechanisms that determine cell fate at the ventral midline.

The zebrafish masterblind (mbl) mutation reduces the activity of Axin1, a negative regulator of the Wnt/β-catenin signalling pathway. Kapsimali et al. found that in mbl embryos the floor plate was expanded at the expense of hypothalamic tissue. However, the hypothalamic domain could be restored by transplanting axin1-overexpressing epiblast cells into the prospective floor plate of young mbl embryos. Transplantation of these same cells into the prospective floor plate of wild-type embryos induced ectopic expression of hypothalamic marker genes at midbrain and hindbrain levels. Together, these observations indicate that induction of hypothalamic tissue at the ventral midline requires inhibition of the Wnt/β-catenin pathway.

Intriguingly, not all Wnt inhibitors seem to be equally capable of promoting hypothalamic fate. The authors tested the effects of various intracellular and extracellular Wnt antagonists, and found that the intracellular antagonists, which included Axin1, induced ectopic expression of hypothalamic markers in the ventral CNS, whereas secreted inhibitors, such as Dickkopf1, Tlc and the frizzled-related protein Frzb1, did not. So, what other properties of the intracellular Wnt inhibitors might underlie their ability to induce hypothalamic tissue? One possible explanation comes from the finding that Axin1 seems to facilitate Nodal signalling — a pathway that is known to be required for the development of the floor plate and ventral hypothalamus. The authors found that both Nodal pathway activation and Axin1 expression promoted the insertion of cells into the ventral midline.

Kapsimali et al. propose that the principal role of Axin1 is to inhibit Wnt/β-catenin signalling. However, its additional effects on Nodal signalling might increase the likelihood that cells are in the right place at the right time to respond to Wnt inhibitors. The next challenge will be to identify the sources of the Wnt signals and inhibitors, so that we can obtain a clearer view of the precise sequence of events that leads to specification of hypothalamic tissue at the ventral midline.