Proliferation-independent role of NF2 (merlin) in limiting biliary morphogenesis.

The architecture of individual cells and cell collectives enables functional specification, a prominent example being the formation of epithelial tubes that transport fluid or gas in many organs. The intrahepatic bile ducts (IHBDs) form a tubular network within the liver parenchyma that transports bile to the intestine. Aberrant biliary 'neoductulogenesis' is also a feature of several liver pathologies including tumorigenesis. However, the mechanism of biliary tube morphogenesis in development or disease is not known. Elimination of the neurofibromatosis type 2 protein (NF2; also known as merlin or neurofibromin 2) causes hepatomegaly due to massive biliary neoductulogenesis in the mouse liver. We show that this phenotype reflects unlimited biliary morphogenesis rather than proliferative expansion. Our studies suggest that NF2 normally limits biliary morphogenesis by coordinating lumen expansion and cell architecture. This work provides fundamental insight into how biliary fate and tubulogenesis are coordinated during development and will guide analyses of disease-associated and experimentally induced biliary pathologies.

Quantitative functions of Argonaute proteins in mammalian development.

Argonaute proteins (Ago1-4) are essential components of the microRNA-induced silencing complex and play important roles in both microRNA biogenesis and function. Although Ago2 is the only one with the slicer activity, it is not clear whether the slicer activity is a universally critical determinant for Ago2's function in mammals. Furthermore, functional specificities associated with different Argonautes remain elusive. Here we report that microRNAs are randomly sorted to individual Argonautes in mammals, independent of the slicer activity. When both Ago1 and Ago2, but not either Ago1 or Ago2 alone, are ablated in the skin, the global expression of microRNAs is significantly compromised and it causes severe defects in skin morphogenesis. Surprisingly, Ago3 is able to load microRNAs efficiently in the absence of Ago1 and Ago2, despite a significant loss of global microRNA expression. Quantitative analyses reveal that Ago2 interacts with a majority of microRNAs (60%) in the skin, compared with Ago1 (30%) and Ago3 (<10%). This distribution is highly correlated with the abundance of each Argonaute, as quantified by shotgun proteomics. The quantitative correlation between Argonautes and their associated microRNAs is conserved in human cells. Finally, we measure the absolute expression of Argonaute proteins and determine that their copy number is ~1.4 × 10(5) to 1.7 × 10(5) molecules per cell. Together, our results reveal a quantitative picture for microRNA activity in mammals.

Rapid and widespread suppression of self-renewal by microRNA-203 during epidermal differentiation.

MicroRNAs (miRNAs) play important roles in differentiation of stem cells. However, the precise dynamics of miRNA induction during stem cell differentiation have not been visualized and molecular mechanisms through which miRNAs execute their function remain unclear. Using high-resolution in situ hybridization together with cell lineage and proliferation markers in mouse skin, we show that miR-203 is transcriptionally activated in the differentiating daughter cells upon the asymmetric cell division of interfollicular progenitor cells. Once induced, miR-203 rapidly promotes the cell cycle exit within 6 hours and abolishes self-renewal of the progenitor cells. With an inducible mouse model, we identify numerous miR-203 in vivo targets that are highly enriched in regulation of cell cycle and cell division, as well as in response to DNA damage. Importantly, co-suppression of individual targets, including p63, Skp2 and Msi2 by miR-203 is required for its function of promoting the cell cycle exit and inhibiting the long-term proliferation. Together, our findings reveal the rapid and widespread impact of miR-203 on the self-renewal program and provide mechanistic insights into the potent role of miR-203 during the epidermal differentiation. These results should also contribute to understanding the role of miR-203 in the development of skin cancer.

TGF-ß Signaling Is Necessary and Sufficient for Pharyngeal Arch Artery Angioblast Formation.

The pharyngeal arch arteries (PAAs) are transient embryonic blood vessels that mature into critical segments of the aortic arch and its branches. Although defects in PAA development cause life-threating congenital cardiovascular defects, the molecular mechanisms that orchestrate PAA morphogenesis remain unclear. Through small-molecule screening in zebrafish, we identified TGF-ß signaling as indispensable for PAA development. Specifically, chemical inhibition of the TGF-ß type I receptor ALK5 impairs PAA development because nkx2.5+ PAA progenitor cells fail to differentiate into tie1+ angioblasts. Consistent with this observation, we documented a burst of ALK5-mediated Smad3 phosphorylation within PAA progenitors that foreshadows angioblast emergence. Remarkably, premature induction of TGF-ß receptor activity stimulates precocious angioblast differentiation, thereby demonstrating the sufficiency of this pathway for initiating the PAA progenitor to angioblast transition. More broadly, these data uncover TGF-ß as a rare signaling pathway that is necessary and sufficient for angioblast lineage commitment.

MicroRNA-205 controls neonatal expansion of skin stem cells by modulating the PI(3)K pathway.

Skin stem cells (SCs) are specified and rapidly expanded to fuel body growth during early development. However, the molecular mechanisms that govern the amplification of skin SCs remain unclear. Here we report an essential role for miR-205, one of the most highly expressed microRNAs in skin SCs, in promoting neonatal expansion of these cells. Unlike most mammalian miRNAs, genetic deletion of miR-205 causes neonatal lethality with severely compromised epidermal and hair follicle growth. In the miR-205 knockout skin SCs, phospho-Akt is significantly downregulated, and the SCs prematurely exit the cell cycle. In the hair follicle, this accelerates the transition of the neonatal SCs towards quiescence. We identify multiple miR-205-targeted negative regulators of PI(3)K signalling that mediate the repression of phospho-Akt and restrict the proliferation of SCs. Our findings reveal an essential role for miR-205 in maintaining the expansion of skin SCs by antagonizing negative regulators of PI(3)K signalling.

Heart field origin of great vessel precursors relies on nkx2.5-mediated vasculogenesis.

The pharyngeal arch arteries (PAAs) are transient embryonic blood vessels that make indispensable contributions to the carotid arteries and great vessels of the heart, including the aorta and pulmonary arteries. During embryogenesis, the PAAs appear in a craniocaudal sequence to connect pre-existing segments of the primitive circulation after de novo vasculogenic assembly from angioblast precursors. Despite the unique spatiotemporal characteristics of PAA development, the embryonic origins of PAA angioblasts and the genetic factors regulating their emergence remain unknown. Here, we identify the embryonic source of PAA endothelium as nkx2.5(+) progenitors in lateral plate mesoderm long considered to adopt cell fates within the heart exclusively. Further, we report that PAA endothelial differentiation relies on Nkx2.5, a canonical cardiac transcription factor not previously implicated in blood vessel formation. Together, these studies reveal the heart field origin of PAA endothelium and attribute a new vasculogenic function to the cardiac transcription factor Nkx2.5 during great vessel precursor development.