Mammalian palatogenesis is a highly regulated morphogenetic process during which the embryonic primary and secondary palatal shelves develop as outgrowths from the medial nasal and maxillary ...prominences, respectively, remodel and fuse to form the intact roof of the oral cavity. The complexity of control of palatogenesis is reflected by the common occurrence of cleft palate in humans. Although the embryology of the palate has long been studied, the past decade has brought substantial new knowledge of the genetic control of secondary palate development. Here, we review major advances in the understanding of the morphogenetic and molecular mechanisms controlling palatal shelf growth, elevation, adhesion and fusion, and palatal bone formation.
In vertebrates, the Eph/ephrin family of signaling molecules is a large group of membrane-bound proteins that signal through a myriad of mechanisms and effectors to play diverse roles in almost every ...tissue and organ system. Though Eph/ephrin signaling has functions in diverse biological processes, one core developmental function is in the regulation of cell position and tissue morphology by regulating cell migration and guidance, cell segregation, and boundary formation. Often, the role of Eph/ephrin signaling is to translate patterning information into physical movement of cells and changes in morphology that define tissue and organ systems. In this review, we focus on recent advances in the regulation of these processes, and our evolving understanding of the in vivo signaling mechanisms utilized in distinct developmental contexts.
•Ephs and ephrins are critical for a variety of vertebrate morphogenetic functions.•Recent discoveries cast light on the in vivo relevance of bidirectional signaling.•Eph/ephrin signaling often acts at the interface of patterning and morphogenesis.•Ongoing discoveries clarify the self-organizing properties of Eph/ephrin signaling.
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.
Mutations in the X-linked human EPHRIN-B1 gene result in cleft palate and other craniofacial anomalies as part of craniofrontonasal syndrome (CFNS), but the molecular and developmental mechanisms by ...which ephrin-B1 controls the underlying developmental processes are not clear. Here we demonstrate that ephrin-B1 plays an intrinsic role in palatal shelf outgrowth in the mouse by regulating cell proliferation in the anterior palatal shelf mesenchyme. In ephrin-B1 heterozygous mutants, X inactivation generates ephrin-B1-expressing and -nonexpressing cells that sort out, resulting in mosaic ephrin-B1 expression. We now show that this process leads to mosaic disruption of cell proliferation and post-transcriptional up-regulation of EphB receptor expression through relief of endocytosis and degradation. The alteration in proliferation rates resulting from ectopic Eph-ephrin expression boundaries correlates with the more severe dysmorphogenesis of ephrin-B1(+/-) heterozygotes that is a hallmark of CFNS. Finally, by integrating phosphoproteomic and transcriptomic approaches, we show that ephrin-B1 controls proliferation in the palate by regulating the extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) signal transduction pathway.
The fusion of two distinct prominences into one continuous structure is common during development and typically requires integration of two epithelia and subsequent removal of that intervening ...epithelium. Using confocal live imaging, we directly observed the cellular processes underlying tissue fusion, using the secondary palatal shelves as a model. We find that convergence of a multi-layered epithelium into a single-layer epithelium is an essential early step, driven by cell intercalation, and is concurrent to orthogonal cell displacement and epithelial cell extrusion. Functional studies in mice indicate that this process requires an actomyosin contractility pathway involving Rho kinase (ROCK) and myosin light chain kinase (MLCK), culminating in the activation of non-muscle myosin IIA (NMIIA). Together, these data indicate that actomyosin contractility drives cell intercalation and cell extrusion during palate fusion and suggest a general mechanism for tissue fusion in development.
Abstract
Progression through the mitotic and meiotic cell cycle is driven by fluctuations in the levels of cyclins, the regulatory subunits controlling the localization and activity of CDK1 kinases. ...Cyclin levels are regulated through a precise balance of synthesis and degradation. Here we demonstrate that the synthesis of Cyclin B1 during the oocyte meiotic cell cycle is defined by the selective translation of mRNA variants generated through alternative cleavage and polyadenylation (APA). Using gene editing in mice, we introduced mutations into the proximal and distal polyadenylation elements of the 3′ untranslated region (UTR) of the Ccnb1 mRNA. Through in vivo loss-of-function experiments, we demonstrate that the translation of mRNA with a short 3′ UTR specifies Cyclin B1 protein levels that set the timing of meiotic re-entry. In contrast, translation directed by a long 3′ UTR is necessary to direct Cyclin B1 protein accumulation during the MI/MII transition. These findings establish that the progression through the cell cycle is dependent on the selective translation of multiple mRNA variants generated by APA.
Graphical Abstract
Graphical Abstract
The Wnt1-Cre transgenic mouse line is extensively used in the study of the development of the neural crest and its derivatives and the midbrain. The Wnt1 gene has important developmental roles in ...formation of the midbrain–hindbrain boundary, regulation of midbrain size, and neurogenesis of ventral midbrain dopaminergic (mDA) neurons. Here, we report that Wnt1-Cre transgenic mice exhibit phenotypes in multiple aspects of midbrain development. Significant expansion of the midbrain and increased proliferation in the developing inferior colliculus is associated with ectopic expression of Wnt1. Marked elevation of Wnt1 expression in the ventral midbrain is correlated with disruption of the differentiation program of ventral mDA neurons. We find that these phenotypes can be attributed to ectopic expression of Wnt1 from the Wnt1-Cre transgene leading to the ectopic activation of canonical Wnt/β-catenin signaling. Since these caveats could complicate the utility of Wnt1-Cre in some developmental circumstances, we report a new Wnt1-Cre2 transgenic mouse line that can serve the same purposes as the original without the associated phenotypic complications. These studies reveal an important caveat to a widely-used reagent, provide an improved version of this reagent, and indicate that the original Wnt1-Cre transgenic mouse line may be useful as a gain of function model for interrogating Wnt signaling mechanisms in multiple aspects of midbrain development.
•The Wnt1-Cre transgene activates Wnt signaling and causes developmental phenotypes.•Increased Wnt signaling blocks the first step of dopaminergic neuron differentiation.•We report a new Wnt1-Cre2 transgene that avoids these phenotypes.
Nkx2‐1 has long been known to be essential for proper respiratory development, as its loss in mice results in an unseparated trachea with underdeveloped cystic lungs. However, how NKX2‐1 regulates ...distinct cell fates in the proximal airways is unknown. While Nkx2‐1 has been extensively studied for its role in lung adenocarcinomas, few studies have described the effects of Nkx2‐1 loss on the trachea beyond its role in early tracheal separation morphogenesis. In previously published work we have shown that the majority of genes differentially expressed in the trachea and esophagus, during early foregut development are not regulated by NKX2‐1 and maintain their expected dorsoventral expression patterns in the unseparated foregut upon NKX2‐1 loss. These results suggested that Nkx2‐1 may not be a master regulator of early respiratory fate, but may instead regulate specific cell fate programs during tracheal development. To address this question, we examined E16.5 and E18.5 Nkx2‐1Δ/Δand control mouse tracheas by immunofluorescent staining for tracheal cell types. Secretory cells (MUC5AC), multiciliated cells (acetylated α‐TUBULIN and FOXJ1), and Club cells (SCGB1A1) were all found in Nkx2‐1Δ/Δventral foreguts by immunofluorescence. While all expected proximal airway cell types were present in Nkx2‐1Δ/Δ tracheas, a disproportionate number of multiciliated cells were present in the Nkx2‐1Δ/Δforegut compared to control. This was not a consequence of effects on early respiratory induction, because tamoxifen‐induced loss of NKX2‐1 in Nkx2‐1CreER/loxembryos at E12.5 also resulted in overabundance of FOXJ1 positive cells at E16.5. Based on these results, while we conclude that NKX2‐1 is not required to define the early proximal respiratory epithelial lineage, it may instead be required for specification of proper proportions and distribution of respiratory cell types.
Mutations in X-linked ephrin-B1 in humans cause craniofrontonasal syndrome (CFNS), a disease that affects female patients more severely than males. Sorting of ephrin-B1-positive and -negative cells ...following X-inactivation has been observed in ephrin-B1(+/-) mice; however, the mechanisms by which mosaic ephrin-B1 expression leads to cell sorting and phenotypic defects remain unknown. Here we show that ephrin-B1(+/-) mice exhibit calvarial defects, a phenotype autonomous to neural crest cells that correlates with cell sorting. We have traced the causes of calvarial defects to impaired differentiation of osteogenic precursors. We show that gap junction communication (GJC) is inhibited at ectopic ephrin boundaries and that ephrin-B1 interacts with connexin43 and regulates its distribution. Moreover, we provide genetic evidence that GJC is implicated in the calvarial defects observed in ephrin-B1(+/-) embryos. Our results uncover a novel role for Eph/ephrins in regulating GJC in vivo and suggest that the pleiotropic defects seen in CFNS patients are due to improper regulation of GJC in affected tissues.
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