Cleft palate represents one of the most common congenital birth defects in human. During embryonic development, palatal shelves display oronasal (O-N) and anteroposterior polarity before the onset of fusion, but how the O-N pattern is established and how it relates to the expansion and fusion of the palatal shelves are unknown. Here we address these questions and show that O-N patterning is associated with the expansion and fusion of the palatal shelves and that Dlx5 is required for the O-N patterning of palatal mesenchyme. Loss of Dlx5 results in downregulation of Fgf7 and expanded Shh expression from the oral to the nasal side of the palatal shelf. This expanded Shh signaling is sufficient to restore palatal expansion and fusion in mice with compromised palatal mesenchymal cell proliferation, such as Msx1-null mutants. Exogenous Fgf7 inhibits Shh signaling and reverses the cranial neural crest (CNC) cell proliferation rescue in the Msx1/Dlx5 double knockout palatal mesenchyme. Thus, Dlx5-regulated Fgf7 signaling inhibits the expression of Shh, which in turn controls the fate of CNC cells through tissue-tissue interaction and plays a crucial role during palatogenesis. Our study shows that modulation of Shh signaling may be useful as a potential therapeutic approach for rescuing cleft palate.

arl13b was initially cloned as the novel cystic kidney gene scorpion (sco) in zebrafish and was shown to be required for cilia formation in the kidney duct. In mouse, a null mutant of Arl13b shows abnormal ultrastructure of the cilium and defective sonic hedgehog (Shh) signaling. Importantly, a recent study linked mutations in ARL13B to a classical form of Joubert syndrome (JS), an autosomal recessive disorder characterized by a distinctive cerebellar malformation. In this study, we analyzed the zebrafish arl13b (sco) mutant and gene products in detail. We first demonstrate that Arl13b is a protein that is highly enriched in the cilium and is required for cilia formation in multiple organs in zebrafish, and that knockdown of arl13b leads to multiple cilia-associated phenotypes. We additionally show that multiple regions of Arl13b are required for its localization to the cilium. By means of rescuing experiments with a series of deletion and point mutants, we further demonstrate that the ciliary localization is crucial for the in vivo function of Arl13b. Together, these results strongly support the hypothesis that JS-related disease (JSRD) is a ciliopathy, or a disease caused by ciliary defects, and that Arl13b functions mainly through the cilium.

Sprouting angiogenesis and lymphatic-blood vessel segregation both involve the migration of endothelial cells, but the precise migratory molecules that govern the decision of blood vascular endothelial cells to segregate into lymphatic vasculature are unknown. Here, we deleted endothelial Rac1 in mice (Tie1-Cre⁺;Rac1^fl/fl) and revealed, unexpectedly, that whereas blood vessel morphology appeared normal, lymphatic-blood vessel separation was impaired, with corresponding edema, haemorrhage and embryonic lethality. Importantly, normal levels of Rac1 were essential for directed endothelial cell migratory responses to lymphatic-inductive signals. Our studies identify Rac1 as a crucial part of the migratory machinery required for endothelial cells to separate and form lymphatic vasculature.

The directed differentiation of forebrain neuronal types from human embryonic stem cells (hESCs) has not been achieved. Here, we show that hESCs differentiate to telencephalic progenitors with a predominantly dorsal identity in a chemically defined medium without known morphogens. This is attributed to endogenous Wnt signaling, which upregulates the truncated form of GLI3, a repressor of sonic hedgehog (SHH). A high concentration of SHH, or the inhibition of Wnt by dickkopf 1 (DKK1) together with a low concentration of SHH, almost completely converts the primitive dorsal precursors to ventral progenitors, which is partially achieved through both downregulation of the truncated GLI3 and upregulation of full-length GLI3 expression. These dorsal and ventral telencephalic progenitors differentiate to functional glutamatergic and GABAergic neurons, respectively. Thus, although hESCs generate dorsal telencephalic cells, as opposed to ventral progenitors in other vertebrates, in the absence of exogenous morphogens, human cells use a similar molecular mechanism to control the dorsal versus ventral fate. The coordination of Wnt and SHH signaling through GLI3 represents a novel mechanism that regulates ventral-dorsal patterning in the development of forebrain neuronal subtypes.