Hox proteins drive cell segregation and non-autonomous apical remodelling during hindbrain segmentation

Abstract

Hox genes encode a conserved family of homeodomain transcription factors regulating development along the major body axis. During embryogenesis, Hox proteins are expressed in segment-specific patterns and control numerous different segment-specific cell fates. It has been unclear, however, whether Hox proteins drive the epithelial cell segregation mechanism that is thought to initiate the segmentation process. Here, we investigate the role of vertebrate Hox proteins during the partitioning of the developing hindbrain into lineage-restricted units called rhombomeres. Loss-of-function mutants and ectopic expression assays reveal that Hoxb4 and its paralogue Hoxd4 are necessary and sufficient for cell segregation, and for the most caudal rhombomere boundary (r6/r7). Hox4 proteins regulate Eph/ephrins and other cell-surface proteins, and can function in a non-cell-autonomous manner to induce apical cell enlargement on both sides of their expression border. Similarly, other Hox proteins expressed at more rostral rhombomere interfaces can also regulate Eph/ephrins, induce apical remodelling and drive cell segregation in ectopic expression assays. However, Krox20, a key segmentation factor expressed in odd rhombomeres (r3 and r5), can largely override Hox proteins at the level of regulation of a cell surface target, Epha4. This study suggests that most, if not all, Hox proteins share a common potential to induce cell segregation but in some contexts this is masked or modulated by other transcription factors.

Ephrin-Eph signaling in embryonic tissue separation

“…39 In mosaic experiments, forced expression of EphrinB2a, EphA4 or EphB4, caused cells to sort out from those segments where these molecules are normally not expressed and to integrate into the adjacent segments where they are expressed. 33,54,56 These results were mirror-imaged in loss-offunction experiments: depleted cells sorted from those rhombomeres that expressed this particular component (r3 and r5 for EphA4MO, r4 for ephrinB2a), but integrated the adjacent rhombomeres where it was naturally absent.…”

“…For instance, EphA4 expression in r3 and r5 depends on Krox20, EphA2 in r4 onHoxA1 and HoxB1, 53 while the continuous expression of EphA7 in most rhombomeres, but with a sharp r6/7 boundary, is due to repression by HoxB4/D4 in the more anterior segments. 54 Thus the overall pattern may be explained by the conjunction of activations and repressions controlled by the multiple HOX genes expressed in overlapping domains.…”

Repression of engrailed 2 inhibits the proliferation and invasion of human bladder cancer in vitro and in vivo

“…Zhou et al (51) indicated that Nkx3.1, a homeoprotein transcription factor, functions within the cells that produce them in a cell autonomous manner as well as can regulate genes and inhibit cell proliferation in a non-cell autonomous manner. Prin et al (52) reported that Hox4 proteins regulate Eph/ephrins and other cell-surface proteins during chick and mouse embryo development. These proteins can also function in a non-cell-autonomous manner to induce apical cell enlargement on both sides of their expression border.…”

Cellular and molecular insights into Hox protein action

“…Modulating cell shape is another way to control single or collective cell migration and tissue remodelling, and Hoxb1b has recently been shown to regulate microtubule dynamics during the process of neural tube formation in zebrafish (Zigman et al, 2014). Hox proteins also directly regulate cell shape, cell-to-cell communication and signalling to properly delimit functional and morphological borders between vertebrate hindbrain segments (Prin et al, 2014), and the Drosophila Hox protein Abdominal B (AbdB) has been shown to coordinate the modifications in cell adhesion and cytoskeleton required for cell shape changes and invagination during development of the posterior spiracle, which constitutes the posterior end of the larval respiratory system (Castelli Gair Hombria et al, 2009). Hox proteins can also couple differentiation with morphogenesis, controlling cell fate decisions along differentiation lineages.…”