Intracellular trafficking and signaling in development

Craniofacial Diseases Caused by Defects in Intracellular Trafficking

Cells use membrane-bound carriers to transport cargo molecules like membrane proteins and soluble proteins, to their destinations. Many signaling receptors and ligands are synthesized in the endoplasmic reticulum and are transported to their destinations through intracellular trafficking pathways. Some of the signaling molecules play a critical role in craniofacial morphogenesis. Not surprisingly, variants in the genes encoding intracellular trafficking machinery can cause craniofacial diseases. Despite the fundamental importance of the trafficking pathways in craniofacial morphogenesis, relatively less emphasis is placed on this topic, thus far. Here, we describe craniofacial diseases caused by lesions in the intracellular trafficking machinery and possible treatment strategies for such diseases.

Inhibitors of endocytosis prevent Wnt/Wingless signalling by reducing the level of basal β-catenin/Armadillo

A key step in the canonical Wnt signalling pathway is the inhibition of GSK3β, which results in the accumulation of nuclear β-catenin (also known as CTNNB1), and hence regulation of target genes. Evidence suggests that endocytosis is required for signalling, yet its role and the molecular understanding remains unclear. A recent and controversial model suggests that endocytosis contributes to Wnt signalling by causing the sequestration of the ligand-receptor complex, including LRP6 and GSK3 to multivesicular bodies (MVBs), thus preventing GSK3β from accessing β-catenin. Here, we use specific inhibitors (Dynasore and Dyngo-4a) to confirm the essential role of endocytosis in Wnt/Wingless signalling in human and Drosophila cells. However, we find no evidence that, in Drosophila cells or wing imaginal discs, LRP6/Arrow traffics to MVBs or that MVBs are required for Wnt/Wingless signalling. Moreover, we show that activation of signalling through chemical blockade of GSK3β is prevented by endocytosis inhibitors, suggesting that endocytosis impacts on Wnt/Wingless signalling downstream of the ligand-receptor complex. We propose that, through an unknown mechanism, endocytosis boosts the resting pool of β-catenin upon which GSK3β normally acts.

Notch signaling and cardiac repair

Notch signaling is critical for proper heart development and recently has been reported to participate in adult cardiac repair. Notch resides at the cell surface as a single pass transmembrane receptor, transits through the cytoplasm following activation, and acts as a transcription factor upon entering the nucleus. This dynamic and widespread cellular distribution allows for potential interactions with many signaling and binding partners. Notch displays temporal as well as spatial versatility, acting as a strong developmental signal, controlling cell fate determination and lineage commitment, and playing a pivotal role in embryonic and adult stem cell proliferation and differentiation. This review serves as an update of recent literature addressing Notch signaling in the heart, with attention to findings from noncardiac research that provide clues for further interpretation of how the Notch pathway influences cardiac biology. Specific areas of focus include Notch signaling in adult myocardium following pathologic injury, the role of Notch in cardiac progenitor cells with respect to differentiation and cardiac repair, crosstalk between Notch and other cardiac signaling pathways, and emerging aspects of noncanonical Notch signaling in heart.

The microenvironment patterns the pluripotent mouse epiblast through paracrine Furin and Pace4 proteolytic activities

The fate of pluripotent cells in early mouse embryos is controlled by graded Nodal signals that are activated by the endoproteases Furin and Pace4. Soluble forms of Furin and Pace4 cleave proNodal in vitro and after secretion in transfected cells, but direct evidence for paracrine activity in vivo is elusive. Here, we show that Furin and Pace4 are released by the extraembryonic microenvironment, and that they cleave a membrane-bound reporter substrate in adjacent epiblast cells and activate Nodal to maintain pluripotency. Secreted Pace4 and Furin also stimulated mesoderm formation, whereas endoderm was only induced by Pace4, correlating with a difference in the spatiotemporal distribution of these proteolytic activities. Our analysis of paracrine Furin and Pace4 activities and their in vivo functions significantly advances our understanding of how the epiblast is patterned by its microenvironment. Adding cell-cell communication to the pleiotropic portfolio of these proteases provides a new framework to study proprotein processing also in other relevant contexts.