Integration of anteroposterior and dorsoventral regulation ofPhox2btranscription in cranial motoneuron progenitors by homeodomain proteins

Abstract

Little is known about the molecular mechanisms that integrate anteroposterior (AP) and dorsoventral (DV) positional information in neural progenitors that specify distinct neuronal types within the vertebrate neural tube. We have previously shown that in ventral rhombomere (r)4 of Hoxb1 and Hoxb2 mutant mouse embryos, Phox2bexpression is not properly maintained in the visceral motoneuron progenitor domain (pMNv), resulting in a switch to serotonergic fate. Here, we show that Phox2b is a direct target of Hoxb1 and Hoxb2. We found a highly conserved Phox2b proximal enhancer that mediates rhombomere-restricted expression and contains separate Pbx-Hox (PH) and Prep/Meis (P/M) binding sites. We further show that both the PH and P/M sites are essential for Hox-Pbx-Prep ternary complex formation and regulation of the Phox2b enhancer activity in ventral r4. Moreover, the DV factor Nkx2.2 enhances Hox-mediated transactivation via a derepression mechanism. Finally, we show that induction of ectopic Phox2b-expressing visceral motoneurons in the chick hindbrain requires the combined activities of Hox and Nkx2 homeodomain proteins. This study takes an important first step to understand how activators and repressors, induced along the AP and DV axes in response to signaling pathways, interact to regulate specific target gene promoters, leading to neuronal fate specification in the appropriate developmental context.

Temporal control of neuronal diversity: common regulatory principles in insects and vertebrates?

“…Interestingly, in one segment of the hindbrain (rhombomere 4), the Phox2b r Foxa2 transition is normally suppressed, such that VM production is prolonged and 5HT neurons are absent. This is because the resident Hox protein in rhombomere 4, Hoxb1, maintains progenitor expression of Phox2b for longer than in other regions (Pattyn et al, 2003;Samad et al, 2004). Hoxb1 expression in progenitors, in turn, depends upon the combined activities of NK6 homeobox protein (Nkx6) and another Hox protein, Hoxb2 (Pattyn et al, 2003).…”

Temporal requirement ofHoxa2in cranial neural crest skeletal morphogenesis

“…In situ hybridisation on whole-mount embryos and cryosections was performed as previously described (Samad et al, 2004). The following RNA probes were used: Hoxa2 , Alx4 (Qu et al, 1997), Bapx1 (Nkx3.2) (Lettice et al, 2001), Msx1 (Robert et al, 1989), Six2 (Oliver et al, 1995).…”

The columnar genevndis required for tritocerebral neuromere formation during embryonic brain development ofDrosophila

“…In particular, the sequential generation of visceral motoneurons and serotonergic neurons from a common pool of neural progenitors located in the ventral hindbrain crucially depend on the integrated activities of Nkx2.2-and Hox1/2-class homeodomain proteins (Pattyn et al, 2003a;Pattyn et al, 2003b). An important function of these proteins is to coordinate the spatial and temporal activation of the homeodomain protein Phox2b, which in turn acts as a binary switch in the selection of motor neuron or serotonergic neuronal fate (Pattyn et al, 2003a;Samad et al, 2004). De-repressive activity of Nkx2.2 at or in vicinity of Pbx/Hox-binding sites proximal to the Phox2b enhancer enhances transcriptional activation of Phox2b by Hox1 and Pbx factors (Samad et al, 2004).…”

“…An important function of these proteins is to coordinate the spatial and temporal activation of the homeodomain protein Phox2b, which in turn acts as a binary switch in the selection of motor neuron or serotonergic neuronal fate (Pattyn et al, 2003a;Samad et al, 2004). De-repressive activity of Nkx2.2 at or in vicinity of Pbx/Hox-binding sites proximal to the Phox2b enhancer enhances transcriptional activation of Phox2b by Hox1 and Pbx factors (Samad et al, 2004). These data suggest that comparable with the integrated activity of vnd and lab in Drosophila brain neuromere specification, integrated activity of the Nkx2.2 and Hox1/2 proteins is involved in the specification of segmental neural lineages.…”