The ventral hindbrain of the vertebrate embryo contains a population of neuronal progenitor cells that gives rise to visceral motor neurons (vMNs) and serotonergic (5-HT) neurons. Each hindbrain segment, or rhombomere, initially generates vMNs, but all the rhombomeres except for r4 switch to producing 5-HT neurons at a defined time point (around embryonic day 10.5 in the mouse). These observations raise two crucial questions — how do progenitors switch from vMN to 5-HT neuron production, and how is r4 prevented from making this switch? In Genes and Development, Pattyn and colleagues show that the answers might lie with a set of homeobox genes that are also important for the spatial patterning of the hindbrain.

Cross section of a mouse hindbrain at embryonic day 11, just after the switch from vMN to 5-HT neuron production. The zone that gives rise to these neurons is labelled in blue. The vMNs (green) are migrating away from this zone, which is now generating 5-HT neurons (pink). Courtesy of J. Ericson, Karolinska Institute, Stockholm, Sweden.

The Phox2b gene is required for the generation of vMNs in the hindbrain, and Pattyn et al. found that in r2–r3 and r5–r7, the switch from vMN to 5-HT neuron generation coincides with downregulation of Phox2b in the ventral portion of each rhombomere. In r4, on the other hand, Phox2b expression is maintained at all dorsoventral levels. Another feature that distinguishes r4 from the other rhombomeres is its expression of Hoxb1 , and the authors showed that r4 can generate 5-HT neurons in Hoxb1 mutant embryos. Nkx6.1 and 6.2 are required to maintain Hoxb1 expression in the ventral part of r4 — in their absence, the Hoxb1 expression domain recedes dorsally, and r4 begins to generate 5-HT neurons at around the same time. This indicates that Hoxb1 suppresses the switch to 5-HT neuron production. Its importance was underlined in Hoxb2 mutant embryos, where Hoxb1 is downregulated in the ventral part of r4, but later than in the Nkx6 mutants. In these embryos, r4 generates fewer 5-HT neurons than in the Nkx6 mutants, implying that production of these neurons begins later, commensurate with the downregulation of Hoxb1.

To bring the story full circle, Pattyn et al. showed that Hoxb1 suppresses 5-HT neuron production by prolonging the expression of Phox2b in the ventral part of r4. So, the downregulation of Phox2b seems to be the key molecular switch that converts cells in the ventral hindbrain from vMN to 5-HT neuronal progenitors. These findings represent an important step towards understanding how the properties of neuronal progenitors in the hindbrain change over time, and future studies should uncover whether homeobox genes act to couple the spatial and temporal aspects of neuronal specification in other regions of the nervous system.