Substrate-Dependent Modulation of SIRT2 by a Fluorescent Probe, 1-Aminoanthracene.

Sirtuin isoform 2 (SIRT2) is an enzyme that catalyzes the removal of acyl groups from lysine residues. SIRT2's catalytic domain has a hydrophobic tunnel where its substrate acyl groups bind. Here, we report that the fluorescent probe 1-aminoanthracene (AMA) binds within SIRT2's hydrophobic tunnel in a substrate-dependent manner. AMA's interaction with SIRT2 was characterized by its enhanced fluorescence upon protein binding (>10-fold). AMA interacted weakly with SIRT2 alone in solution (Kd = 37 µM). However, when SIRT2 was equilibrated with a decanoylated peptide substrate, AMA's affinity for SIRT2 was enhanced ∼10-fold (Kd = 4 µM). The peptide's decanoyl chain and AMA co-occupied SIRT2's hydrophobic tunnel when bound to the protein. In contrast, binding of AMA to SIRT2 was competitive with a myristoylated substrate whose longer acyl chain occluded the entire tunnel. AMA competitively inhibited SIRT2 demyristoylase activity with an IC50 of 21 µM, which was significantly more potent than its inhibition of other deacylase activities. Finally, binding and structural analysis suggests that the AMA binding site in SIRT2's hydrophobic tunnel was structurally stabilized when SIRT2 interacted with a decanoylated or 4-oxononanoylated substrate, but AMA's binding site was less stable when SIRT2 was bound to an acetylated substrate. Our use of AMA to explore changes in SIRT2's hydrophobic tunnel that are induced by interactions with specific acylated substrates has implications for developing ligands that modulate SIRT2's substrate specificity.

Gram-Negative Bacteria Are Internalized Into Osteocyte-Like Cells.

While Gram-positive organisms are the most common causative agent of initial bone infections, the percentage of Gram-negative species increases in reoccurring bone infections. As bacterial internalization has been suggested as one cause of reoccurring bone infection, we tested the hypothesis that Gram-negative species of bacteria can be internalized into bone cells. Using the MLO-A5 and the MLO-Y4 cell lines as our cell models, we demonstrated that the Gram-negative species, Proteus mirabilis and Serratia marcescens, can be internalized in these cells using an internalization assay. This rate at which these two species were internalized was both time- and initial concentration-dependent. Confocal analysis demonstrated the presence of internalized bacteria within both cell types. Inhibition of the cellular uptake with methyl-ß-cyclodextrin and chloroquine both reduced internalized bacteria, indicating that this process is, at least in part, cell mediated. Finally, we demonstrated that the presence of internalized P. mirabilis did not impact cell viability, measured either by lactate dehydrogenase (LDH) release or 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) activity, while the presence of S. marcescens, on the other hand, both increased LDH release and reduced MTT activity, indicating a loss of cell viability in response to the organism. These results indicated that both species of Gram-negative bacteria can be internalized by bone cells and that these internalized bacteria could potentially result in reoccurring bone infections. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:861-870, 2020.

Depletion of Myo/Nog Cells in the Lens Mitigates Posterior Capsule Opacification in Rabbits.

Purpose: Posterior capsule opacification (PCO) is a vision-impairing disease that occurs in some adults and most children after cataract surgery. Contractile myofibroblasts contribute to PCO by producing wrinkles in the lens capsule that scatter light. Myofibroblasts in the lens originate from Myo/Nog cells named for their expression of the MyoD transcription factor and bone morphogenetic protein inhibitor noggin. In this study we tested the effects of depleting Myo/Nog cells on development of PCO. Methods: Myo/Nog cells were eliminated by injecting the G8 antibody conjugated to 3DNA nanocarriers for the cytotoxin doxorubicin (G8:3DNA:Dox) during cataract surgery in rabbits. The severity of PCO was scored by slit lamp analysis, gross and histologic observation, and immunofluorescence localization of α-smooth muscle actin. Results: G8:3DNA:Dox specifically induced cell death in Myo/Nog cells in the lens. None of the lenses administered G8:3DNA containing 9 to 36 µM doxorubicin developed greater than trace levels of central PCO and few myofibroblasts were present on the capsule. Less than 9% of these lenses exhibited greater than mild levels of peripheral PCO. Doxorubucin itself reduced PCO; however, myofibroblasts and wrinkles were abundant in the lens, and off-target effects were observed in the ciliary processes and cornea. Conclusions: Myo/Nog cells are the primary source of myofibroblasts in the lens after cataract surgery. Targeted depletion of Myo/Nog cells has potential for preventing PCO and preserving vision.

Rhabdomyosarcoma and Wilms tumors contain a subpopulation of noggin producing, myogenic cells immunoreactive for lens beaded filament proteins.

Myo/Nog cells are identified by their expression of the skeletal muscle specific transcription factor MyoD and the bone morphogenetic protein inhibitor noggin, and binding of the G8 monoclonal antibody. Their release of noggin is critical for morphogenesis and skeletal myogenesis. In the adult, Myo/Nog cells are present in normal tissues, wounds and skin tumors. Myo/Nog cells in the lens give rise to myofibroblasts that synthesize skeletal muscle proteins. The purpose of this study was to screen human lens tissue, rhabdomyosarcoma cell lines, and tissue sections from rhabdomyosarcoma, Wilms and tumors lacking features of skeletal muscle for co-localization of antibodies to Myo/Nog cell markers and the lens beaded filament proteins filensin and CP49. Immunofluorescence localization experiments revealed that Myo/Nog cells of the lens bind antibodies to beaded filament proteins. Co-localization of antibodies to G8, noggin, filensin and CP49 was observed in most RC13 and a subpopulation of RD human rhabdomyosarcoma cell lines. Western blotting with beaded filament antibodies revealed bands of similar molecular weights in RC13 and murine lens cells. Human alveolar, embryonal, pleomorphic and spindle cell rhabdomyosarcomas and Wilms tumors contained a subpopulation of cells immunoreactive for G8, noggin, MyoD and beaded filaments. G8 was also co-localized with filensin mRNA. Staining for beaded filament proteins was not detected in G8 positive cells in leiomyosarcomas, squamous and basal cell carcinomas, syringocarciomas and malignant melanomas. Lens beaded filament proteins were thought to be present only in the lens. Myo/Nog-like cells immunoreactive for beaded filaments may be diagnostic of tumors related to the skeletal muscle lineage.