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Angiogenesis, the fundamental process by which new blood vessels form from existing ones, depends on precise spatial and temporal gene expression within specific compartments of the endothelium. However, the molecular links between proangiogenic signals and downstream gene expression remain unclear. During sprouting angiogenesis, the specification of endothelial cells into the tip cells that lead new blood vessel sprouts is coordinated by vascular endothelial growth factor A (VEGFA) and Delta-like ligand 4 (Dll4)/Notch signaling and requires high levels of Notch ligand DLL4. Here, we identify MEF2 transcription factors as crucial regulators of sprouting angiogenesis directly downstream from VEGFA. Through the characterization of a Dll4 enhancer directing expression to endothelial cells at the angiogenic front, we found that MEF2 factors directly transcriptionally activate the expression of Dll4 and many other key genes up-regulated during sprouting angiogenesis in both physiological and tumor vascularization. Unlike ETS-mediated regulation, MEF2-binding motifs are not ubiquitous to all endothelial gene enhancers and promoters but are instead overrepresented around genes associated with sprouting angiogenesis. MEF2 target gene activation is directly linked to VEGFA-induced release of repressive histone deacetylases and concurrent recruitment of the histone acetyltransferase EP300 to MEF2 target gene regulatory elements, thus establishing MEF2 factors as the transcriptional effectors of VEGFA signaling during angiogenesis.

Angiogenesis, the fundamental process by which new blood vessels form from existing ones, depends on precise spatial and temporal gene expression within specific compartments of the endothelium. However, the molecular links between pro-angiogenic signals and downstream gene expression remain unclear. During sprouting angiogenesis, the specification of endothelial cells into the tip cells that lead new blood vessel sprouts is coordinated by VEGFA and Dll4/Notch signaling, and requires high levels of Notch ligand Delta-like ligand 4 (DLL4). Here, we identify MEF2 transcription factors as crucial regulators of sprouting angiogenesis directly downstream of VEGFA. Through the characterization of a Dll4 enhancer directing expression to endothelial cells at the angiogenic front, we find that MEF2 factors directly transcriptionally activate the expression of Dll4 and many other key genes upregulated during sprouting angiogenesis in both physiological and tumour vascularization. Unlike ETS-mediated regulation, MEF2 binding motifs are not ubiquitous to all endothelial gene enhancers and promoters, but are instead over-represented around genes associated with sprouting angiogenesis. MEF2-target gene activation is directly linked to VEGFA-induced release of repressive histone deacetylases and concurrent recruitment of the histone acetyl-transferase EP300 to MEF2-target gene regulatory elements, thus establishing MEF2 factors as the transcriptional effectors of VEGFA signaling during angiogenesis.

Normal strains near a growing fatigue crack have been studied in situ using the Digital Image Correlation (DIC) technique in a compact tension specimen of stainless steel 316L under tension-tension cyclic loading. An error analysis of the measured displacements and strains has been carried out, and the results show that the precisions of displacements and strains in the direction perpendicular to the crack plane and ahead of the crack are better than those parallel to the crack plane and in the wake of the crack. The errors in the measured displacements are in the range of 0.06 to 0.1 m and between 0.2% and 0.4% for the measured strains, the latter are well below the critical strain at the onset of crack growth about 8%. Strain ratchetting was found ahead of the growing fatigue crack tip, albeit very close to the crack tip.

Novel solar cell materials consisting of Si nanoparticles embedded in SiO2 layers have been studied using positron annihilation spectroscopy in Doppler broadening mode and photoluminescence. Two positron-trapping interface states are observed after high temperature annealing at 1100 °C. One of the states is attributed to the (SiO2/Si bulk) interface and the other to the interface between the Si nanoparticles and SiO2. A small reduction in positron trapping into these states is observed after annealing the samples in N2 atmosphere with 5% H2. Enhanced photoluminescence is also observed from the samples following this annealing step. Novel solar cell materials consisting of Si nanoparticles embedded in SiO layers have been studied using positron annihilation spectroscopy in Doppler broadening mode and photoluminescence. Two positron-trapping interface states are observed after high temperature annealing at 1100 °C. One of the states is attributed to the (SiO/Si bulk) interface and the other to the interface between the Si nanoparticles and SiO. A small reduction in positron trapping into these states is observed after annealing the samples in N atmosphere with 5% H . Enhanced photoluminescence is also observed from the samples following this annealing step.

Erbium diffusion in silicon dioxide layers prepared by magnetron sputtering, chemical vapor deposition, and thermal growth has been investigated by secondary ion mass spectrometry, and diffusion coefficients have been extracted from simulations based on Fick’s second law of diffusion. Erbium diffusion in magnetron sputtered silicon dioxide from buried erbium distributions has in particular been studied, and in this case a simple Arrhenius law can describe the diffusivity with an activation energy of 5.3±0.1 eV. Within a factor of two, the erbium diffusion coefficients at a given temperature are identical for all investigated matrices.