Molecular Basis for RNA Cytidine Acetylation by NAT10.

Human NAT10 acetylates the N4 position of cytidine in RNA, predominantly on rRNA and tRNA, to facilitate ribosome biogenesis and protein translation. NAT10 has been proposed as a therapeutic target in cancers as well as aging-associated pathologies such as Hutchinson-Gilford Progeria Syndrome (HGPS). The ∼120 kDa NAT10 protein uses its acetyl-CoA-dependent acetyltransferase, ATP-dependent helicase, and RNA binding domains in concert to mediate RNA-specific N4-cytidine acetylation. While the biochemical activity of NAT10 is well known, the molecular basis for catalysis of eukaryotic RNA acetylation remains relatively undefined. To provide molecular insights into the RNA-specific acetylation by NAT10, we determined the single particle cryo-EM structures of Chaetomium thermophilum NAT10 ( Ct NAT10) bound to a bisubstrate cytidine-CoA probe with and without ADP. The structures reveal that NAT10 forms a symmetrical heart-shaped dimer with conserved functional domains surrounding the acetyltransferase active sites harboring the cytidine-CoA probe. Structure-based mutagenesis with analysis of mutants in vitro supports the catalytic role of two conserved active site residues (His548 and Tyr549 in Ct NAT10), and two basic patches, both proximal and distal to the active site for RNA-specific acetylation. Yeast complementation analyses and senescence assays in human cells also implicates NAT10 catalytic activity in yeast thermoadaptation and cellular senescence. Comparison of the NAT10 structure to protein lysine and N-terminal acetyltransferase enzymes reveals an unusually open active site suggesting that these enzymes have been evolutionarily tailored for RNA recognition and cytidine-specific acetylation.

Lipid-driven interleaflet coupling of plasma membrane order regulates FcεRI signaling in mast cells.

Interleaflet coupling-the influence of one leaflet on the properties of the opposing leaflet-is a fundamental plasma membrane organizational principle. This coupling is proposed to participate in maintaining steady-state biophysical properties of the plasma membrane, which in turn regulates some transmembrane signaling processes. A prominent example is antigen (Ag) stimulation of signaling by clustering transmembrane receptors for immunoglobulin E (IgE), FcεRI. This transmembrane signaling depends on the stabilization of ordered regions in the inner leaflet for sorting of intracellular signaling components. The resting inner leaflet has a lipid composition that is generally less ordered than the outer leaflet and that does not spontaneously phase separate in model membranes. We propose that interleaflet coupling can mediate ordering and disordering of the inner leaflet, which is poised in resting cells to reorganize upon stimulation. To test this in live cells, we first established a straightforward approach to evaluate induced changes in membrane order by measuring inner leaflet diffusion of lipid probes by imaging fluorescence correlation spectroscopy, by imaging fluorescence correlation spectroscopy (ImFCS), before and after methyl-α-cyclodexrin (mαCD)-catalyzed exchange of outer leaflet lipids (LEX) with exogenous order- or disorder-promoting phospholipids. We examined the functional impact of LEX by monitoring two Ag-stimulated responses: recruitment of cytoplasmic Syk kinase to the inner leaflet and exocytosis of secretory granules (degranulation). Based on the ImFCS data in resting cells, we observed global increase or decrease of inner leaflet order when outer leaflet is exchanged with order- or disorder-promoting lipids, respectively. We find that the degree of both stimulated Syk recruitment and degranulation correlates positively with LEX-mediated changes of inner leaflet order in resting cells. Overall, our results show that resting-state lipid ordering of the outer leaflet influences the ordering of the inner leaflet, likely via interleaflet coupling. This imposed lipid reorganization modulates transmembrane signaling stimulated by Ag clustering of IgE-FcεRI.

Trimer-to-Monomer Disruption Mechanism for a Potent, Protease-Resistant Antagonist of Tumor Necrosis Factor-α Signaling.

Aberrant tumor necrosis factor-α (TNFα) signaling is associated with many inflammatory diseases. The homotrimeric quaternary structure of TNFα is essential for receptor recognition and signal transduction. Previously, we described an engineered α/ß-peptide inhibitor that potently suppresses TNFα activity and resists proteolysis. Here, we present structural evidence that both the α/ß-peptide inhibitor and an all-α analogue bind to a monomeric form of TNFα. Calorimetry data support a 1:1 inhibitor/TNFα stoichiometry in solution. In contrast, previous cocrystal structures involving peptide or small-molecule inhibitors have shown the antagonists engaging a TNFα dimer. The structural data reveal why our inhibitors favor monomeric TNFα. Previous efforts to block TNFα-induced cell death with peptide inhibitors revealed that surfactant additives to the assay conditions cause a more rapid manifestation of inhibitory activity than is observed in the absence of additives. We attributed this effect to a loose surfactant TNFα association that lowers the barrier to trimer dissociation. Here, we used the new structural data to design peptide inhibitors bearing a surfactant-inspired appendage intended to facilitate TNFα trimer dissociation. The appendage modified the time course of protection from cell death.

Expression of lncRNA NEAT1 in endometriosis and its biological functions in ectopic endometrial cells as mediated via miR-124-3p.

BACKGROUND: Endometriosis (EM) is a gynecological disease that poses severe health risks to women, although its pathogenesis has yet to be fully elucidated. It has been shown that long non-coding RNAs (lncRNAs) are closely associated with EM initiation and have a role in the development of this disease. Previous studies exploring the expression of the lncRNA nuclear paraspeckle assembly transcript 1 (NEAT1) have shown that this lncRNA functions as a tumor promoter in endometrial cancer. However, its exact mechanism of action in EM remains unclear. OBJECTIVE: This report was designed to illustrate the potential molecular mechanisms of lncRNA NEAT1 on EM. METHODS: Endometrial tissues were extracted from EM model rats and patients with EM. Hematoxylin and eosin staining was applied to detect the morphological changes that occurred in rats after construction of the model. Endometrial stromal cells (ESCs) were extracted from either ectopic endometrium (EC) or eutopic endometrium (EU) tissues from patients with EM. LncRNA NEAT1 and miR-124-3p expression in EM tissues and cells were subsequently evaluated by reverse transcription-quantitative (RT-q)PCR analysis. MTT assay, flow cytometric analysis, western blot assay and Transwell assay were then employed to examine the effect of NEAT1 and miR-124-3p on EC-ESC proliferation, apoptosis, migration and invasion, respectively. The targeted relationship between lncRNA NEAT1 and miR-124-3p was subsequently confirmed by dual-luciferase and co-transfection assays. RESULTS: MiR-124-3p was identified as a target of NEAT1, and could be negatively regulated by NEAT1 in EC-ESCs. The expression level of NEAT1 was evidently increased, whereas that of miR-124-3p was decreased, in the EM in vivo model, EM tissues and EC-ESCs from patients with EM. The loss-of-function assays further established that silencing of NEAT1 could inhibit EC-ESC proliferation, migration, and invasion, but it led to the promotion of apoptosis via targeting miR-124-3p. CONCLUSIONS: NEAT1 is significantly upregulated in EM, promoting malignant behavior in EM through targeting miR-124-3p expression.