Self-limiting stoichiometry in SnSe thin films.

Unique functionalities can arise when 2D materials are scaled down near the monolayer limit. However, in 2D materials with strong van der Waals bonds between layers, such as SnSe, maintaining stoichiometry while limiting vertical growth is difficult. Here, we describe how self-limiting stoichiometry can promote the growth of SnSe thin films deposited by molecular beam epitaxy. The Pnma phase of SnSe was stabilized over a broad range of Sn : Se flux ratios from 1 : 1 to 1 : 5. Changing the flux ratio does not affect the film stoichiometry, but influences the predominant crystallographic orientation. ReaxFF molecular dynamics (MD) simulation demonstrates that, while a mixture of Sn/Se stoichiometries forms initially, SnSe stabilizes as the cluster size evolves. The MD results further show that the excess selenium coalesces into Se clusters that weakly interact with the surface of the SnSe particles, leading to the limited stoichiometric change. Raman spectroscopy corroborates this model showing the initial formation of SnSe2 transitioning into SnSe as experimental film growth progresses. Transmission electron microscopy measurements taken on films deposited with growth rates above 0.25 Å s-1 show a thin layer of SnSe2 that disrupts the crystallographic orientation of the SnSe films. Therefore, using the conditions for self-limiting SnSe growth while avoiding the formation of SnSe2 was found to increase the lateral scale of the SnSe layers. Overall, self-limiting stoichiometry provides a promising avenue for maintaining growth of large lateral-scale SnSe for device fabrication.

Cholesterol-responsive metabolic proteins are required for larval development in Caenorhabditis elegans.

Caenorhabditis elegans, a cholesterol auxotroph, showed defects in larval development upon cholesterol starvation (CS) in a previous study. To identify cholesterol-responsive proteins likely responsible for the larval arrest upon CS, a comparative proteomic analysis was performed between C. elegans grown in normal medium supplemented with cholesterol (CN) and those grown in medium not supplemented with cholesterol (cholesterol starvation, CS). Our analysis revealed significant change (more than 2.2-fold, p < 0.05) in nine proteins upon CS. Six proteins were down-regulated [CE01270 (EEF-1A.1), CE08852 (SAMS-1), CE11068 (PMT-2), CE09015 (ACDH-1), CE12564 (R07H5.8), and CE09655 (RLA-0)], and three proteins were up-regulated [CE29645 (LEC-1), CE16576 (LEC-5), and CE01431 (NEX-1)]. RNAi phenotypes of two of the down-regulated genes, R07H5.8 (adenosine kinase) and rla-0 (ribosomal protein), in CN were similar to that of larval arrest in CS, and RNAi of a down-regulated gene, R07H5.8, in CS further enhanced the effects of CS, suggesting that down-regulation of these genes is likely responsible for the larval arrest in CS. All three up-regulated genes contain putative DAF-16 binding sites and mRNA levels of these three genes were all decreased in daf-16 mutants in CN, suggesting that DAF-16 activates expression of these genes.

Transcriptional activity and expression of liver X receptor in the ascidian Halocynthia roretzi.

Liver X receptors, LXRs, are ligand-activated transcription factors that belong to the group H nuclear receptor (NR) superfamily. In this study, an LXR (HrLXR) cDNA was cloned from the ascidian Halocynthia roretzi hepatopancreas and characterized to examine the functional conservation of ancestral LXRs in chordates. A phylogenetic analysis of HrLXR showed that it belongs to the tunicate (urochordate) LXR subgroup, which is distinct from vertebrate LXRs. Quantitative real-time PCR analysis revealed that HrLXR mRNA was expressed predominantly in the gills, and highly expressed in unfertilized eggs followed by decrease at later embryonic and larval stages. Unexpectedly, HrLXR was not activated by GW3965, whereas a synthetic ligand for a farnesoid X receptor, GW4064, activated HrLXR. This activation was abolished by the deletion of 51 amino acids from the N-terminus. In a mammalian two-hybrid system, HrLXR interacted with HrRXR in the presence of GW4064 or 9-cis retinoic acid. The injection of GW3965 and GW4064 in vivo increased the ATPbinding cassette sub-family G member 4 and HrLXR mRNA levels in the hepatopancreas and gills. These results suggest that the mRNA expression and transcriptional properties of HrLXR are different from those of vertebrate LXRs, although HrLXR is likely responsive to the related NR ligand, GW4064.

Molecular and expression analysis of the farnesoid X receptor in the urochordate Halocynthia roretzi.

The farnesoid X receptors (FXRs) are the major transcriptional regulators of bile salt synthesis in vertebrates. However, the structural conservation of invertebrate FXRs has only been studied for the major model organisms and studies on additional invertebrate FXRs are clearly required to obtain better resolution of FXR phylogeny and comparative developmental insights in chordates. In the present study, the cDNA encoding the farnesoid X receptor, HrFXR, was cloned from a marine invertebrate Halocynthia roretzi. The open reading frame of HrFXR encoded 688 amino acids including a longer N-terminal region and showed overall sequence identities of 28-41% to vertebrate and Ciona intestinalis FXRs. The N-terminal activation function 1 (AF-1) and hinge domains of HrFXR displayed relatively low identities (<20%), whereas the DNA-binding and ligand-binding domains showed relatively high (>73%) and intermediate (21-50%) identities, respectively. Based on a phylogenetic analysis, HrFXR belonged to a urochordate group, which was placed differently from vertebrate FXRα and FXRß subgroups. Real-time quantitative PCR analysis revealed that the HrFXR mRNA originated maternally and was highly expressed in adult gonads. Additionally, HrFXR mRNA levels in the gills and hepatopancreas showed significantly higher values in animals with soft tunic syndrome compared to those of normal individuals. Furthermore, direct injection of cholic acid significantly increased HrFXR transcript levels in vivo, although an expression vector containing HrFXR cDNA did not show a significant transactivation function in response to a well-known ligand for vertebrate FXR, GW4064, in HepG2 cells. These results suggest that the tunicate FXR has different structural and expressional characteristics compared to those of vertebrate FXRs.