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Morphogenesis is a physical process that requires the generation of mechanical forces to achieve dynamic changes in cell position, tissue shape, and size as well as biochemical signals to coordinate these events. Mechanical forces are also used by the embryo to transmit detailed information across space and detected by target cells, leading to downstream changes in cellular properties and behaviors. Indeed, forces provide signaling information of complementary quality that can both synergize and diversify the functional outputs of biochemical signaling. Here, we discuss recent findings that reveal how mechanical signaling and biochemical signaling are integrated during morphogenesis and the possible context-specific advantages conferred by the interactions between these signaling mechanisms.

Craniofacial morphogenesis is a highly dynamic process that requires changes in the behaviors and physical properties of cells in order to achieve the proper organization of different craniofacial structures. Boundary formation is a critical process in cellular organization, patterning, and ultimately tissue separation. There are several recurring cellular mechanisms through which boundary formation and cellular organization occur including, transcriptional patterning, cell segregation, cell adhesion and migratory guidance. Disruption of normal boundary formation has dramatic morphological consequences, and can result in human craniofacial congenital anomalies. In this review we discuss boundary formation during craniofacial development, specifically focusing on the cellular behaviors and mechanisms underlying the self-organizing properties that are critical for craniofacial morphogenesis.

Ephrin-B2, a member of the Eph/ephrin family of cell signaling molecules, has been implicated in the guidance of cranial and trunk neural crest cells (NCC) and development of the branchial arches(BA), but detailed examination in mice has been hindered by embryonic lethality of Efnb2 null loss of function due to a requirement in angiogenic remodeling. To elucidate the developmental roles for Efnb2, we generated a conditional rescue knock-in allele that allows rescue of ephrin-B2 specifically in the vascular endothelium (VE), but is otherwise ephrin-B2 deficient. Restoration of ephrin-B2 expression specifically to the VE completely circumvents angiogenic phenotypes, indicating that the requirement of ephrin-B2 in angiogenesis is limited to the VE. Surprisingly, we find that expression of ephrin-B2 specifically in the VE is also sufficient for normal NCC migration and that conversely, embryos in which ephrin-B2 is absent specifically from the VE exhibit NCC migration and survival defects. Disruption of vascular development independent of loss of ephrin-B2 function also leads to defects in NCC and BA development. Together, these data indicate that direct ephrin-B2 signaling to NCCs is not required for NCC guidance, which instead depends on proper organization of the embryonic vasculature.

The expression of members of the Eph family of receptor tyrosine kinases and their ephrin ligands is frequently dysregulated in medulloblastomas. We assessed the expression and functional role of EphB1 in medulloblastoma cell lines and engineered mouse models. mRNA and protein expression profiling showed expression of EphB1 receptor in the human medulloblastoma cell lines DAOY and UW228. EphB1 downregulation reduced cell growth and viability, decreased the expression of important cell cycle regulators, and increased the percentage of cells in G1 phase of the cell cycle. It also modulated the expression of proliferation, and cell survival markers. In addition, EphB1 knockdown in DAOY cells resulted in significant decrease in migration, which correlated with decreased β1-integrin expression and levels of phosphorylated Src. Furthermore, EphB1 knockdown enhanced cellular radiosensitization of medulloblastoma cells in culture and in a genetically engineered mouse medulloblastoma model. Using genetically engineered mouse models, we established that genetic loss of EphB1 resulted in a significant delay in tumor recurrence following irradiation compared to EphB1-expressing control tumors. Taken together, our findings establish that EphB1 plays a key role in medulloblastoma cell growth, viability, migration, and radiation sensitivity, making EphB1 a promising therapeutic target.

BackgroundEph receptors, comprising the A- and B-subfamilies, are the largest family of receptor tyrosine kinases in the mammalian genome, and their function is critical for morphogenesis in a variety of contexts. Whereas signaling through B-type Ephs has been demonstrated to play a role in cleft lip and palate (CL/P), the involvement of A-type Ephs has not been examined in this context notwithstanding a recent genome-wide association study that identified the EPHA3 locus as a candidate for non-syndromic CL/P.ResultsHere, we present a systematic analysis of the gene expression patterns for the nine EphA receptors at progressive stages of mouse development and find that EphA3, EphA4, and EphA7 exhibit restricted overlapping patterns of expression during palate development. We find that homozygous mutation of EphA3 or compound homozygous mutation of EphA3 and EphA4 in mice does not result in defective midfacial development, supporting the possibility of redundant function with EphA7. We also document previously undescribed expression patterns in other tissues of the craniofacial complex including the lacrimal duct and salivary glands.ConclusionsTogether, these results are consistent with the hypothesis that mutations in EPHA family genes may cause CL/P and also suggest that functional redundancy between family members may be at play.