Hox Paralog Group 2 Genes Control the Migration of Mouse Pontine Neurons through Slit-Robo Signaling

Abstract

The pontine neurons (PN) represent a major source of mossy fiber projections to the cerebellum. During mouse hindbrain development, PN migrate tangentially and sequentially along both the anteroposterior (AP) and dorsoventral (DV) axes. Unlike DV migration, which is controlled by the Netrin-1/Dcc attractive pathway, little is known about the molecular mechanisms guiding PN migration along the AP axis. Here, we show that Hoxa2 and Hoxb2 are required both intrinsically and extrinsically to maintain normal AP migration of subsets of PN, by preventing their premature ventral attraction towards the midline. Moreover, the migration defects observed in Hoxa2 and Hoxb2 mutant mice were phenocopied in compound Robo1;Robo2, Slit1;Slit2, and Robo2;Slit2 knockout animals, indicating that these guidance molecules act downstream of Hox genes to control PN migration. Indeed, using chromatin immunoprecipitation assays, we further demonstrated that Robo2 is a direct target of Hoxa2 in vivo and that maintenance of high Robo and Slit expression levels was impaired in Hoxa2 mutant mice. Lastly, the analysis of Phox2b-deficient mice indicated that the facial motor nucleus is a major Slit signaling source required to prevent premature ventral migration of PN. These findings provide novel insights into the molecular control of neuronal migration from transcription factor to regulation of guidance receptor and ligand expression. Specifically, they address the question of how exposure to multiple guidance cues along the AP and DV axes is regulated at the transcriptional level and in turn translated into stereotyped migratory responses during tangential migration of neurons in the developing mammalian brain.

Epigenetic regulations in hematopoietic Hox code

“…So far, there is no direct evidence to support this possibility. However, many studies have indicated that Hox genes (mostly from cluster A) control cell adhesion and migration in hematopoietic cells (Leroy et al, 2004), epithelial cells (Mace et al, 2005;Arderiu et al, 2007) and pontine neurons (Geisen et al, 2008). If Hox expression patterns define the cell positional identity and behavior, how menin-MLL or RA-Fgf-Wnt signaling manipulate Hox networks and how Hox networks regulate cell-cell and cell-extracellular matrix interactions?…”

Axonal Patterns and Targets of dA1 Interneurons in the Chick Hindbrain

“…Hence, other transcription factors should cooperate with Lim-HD proteins in patterning their growth. Hox code was shown to control axonal pathfinding in the spinal cord and hindbrain (Dasen et al, 2005;Geisen et al, 2008;Narita and Rijli, 2009;Watari-Goshima and Chisaka, 2011). It is possible that the diverse dA1 axonal tracts relay on different Hox genes.…”

“…Thus, the different turning points of dA1 r6 -7 and dA1 r2-5 might be determined by Eph/Ephrin cues. Interestingly, the expression of these guiding cues is regulated by Hox, Lim-HD, and Zic genes (García-Frigola et al, 2008;Geisen et al, 2008;Wilson et al, 2008). Thus, elucidating the network between the transcriptional codes and the guiding cues in governing dA1 axonal trajectories is the focus of future studies.…”

Moving away from the midline: new developments for Slit and Robo

“…2D). Migrating precerebellar neurons express all Robo receptors (except Robo4) (Marillat et al, 2002;Marillat et al, 2004;Geisen et al, 2008). Slits are expressed in the floor plate, the rhombic lip and in several cranial motor nuclei, such as the facial nucleus (Geisen et al, 2008;Gilthorpe et al, 2002;Hammond et al, 2005).…”

“…Interestingly, the migration of PN neurons is also perturbed in these mutants but in a different way. Upon leaving the rhombic lip, PN neurons migrate rostrally across several rhombomeres in a compact stream before turning ventrally towards the midline (Geisen et al, 2008). In Robo1/Robo2 and Slit1/Slit2 double knockouts, the stream of PN neurons splits and many neurons prematurely migrate towards the floor plate (Geisen et al, 2008).…”