Di-methyl H4 lysine 20 targets the checkpoint protein Crb2 to sites of DNA damage

Histone lysine methylation is an important chromatin modification that can be catalyzed to a mono-, di-, or tri-methyl state. An ongoing challenge is to decipher how these different methyllysine histone marks can mediate distinct aspects of chromatin function. The fission yeast checkpoint protein Crb2 is rapidly targeted to sites of DNA damage after genomic insult, and this recruitment requires methylation of histone H4 lysine 20 (H4K20). Here we show that the tandem tudor domains of Crb2 preferentially bind the di-methylated H4K20 residue. Loss of this interaction by disrupting either the tudor-binding motif or the H4K20 methylating enzyme Set9/Kmt5 ablates Crb2 localization to double-strand breaks and impairs checkpoint function. Further we show that dimethylation, but not tri-methylation, of H4K20 is required for Crb2 localization, checkpoint function, and cell survival after DNA damage. These results argue that the di-methyl H4K20 modification serves as a binding target that directs Crb2 to sites of genomic lesions and defines an important genome integrity pathway mediated by a specific methyl-lysine histone mark.     

The Association between Quality of HIV Care,Loss to Follow-Up and Mortality in Pediatric and Adolescent Patients Receiving Antiretroviral Therapy in Nigeria

Access to pediatric HIV treatment in resource-limited settings has risen significantly. However, little is known about the quality of care that pediatric or adolescent patients receive. The objective of this study is to explore quality of HIV care and treatment in Nigeria and to determine the association between quality of care, loss-to-follow-up and mortality. A retrospective cohort study was conducted including patients ≤18 years of age who initiated ART between November 2002 and December 2011 at 23 sites across 10 states. 1,516 patients were included. A quality score comprised of 6 process indicators was calculated for each patient. More than half of patients (55.5%) were found to have a high quality score, using the median score as the cut-off. Most patients were screened for tuberculosis at entry into care (81.3%), had adherence measurement and counseling at their last visit (88.7% and 89.7% respectively), and were prescribed co-trimoxazole at some point during enrollment in care (98.8%). Thirty-seven percent received a CD4 count in the six months prior to chart review. Mortality within 90 days of ART initiation was 1.9%. A total of 4.2% of patients died during the period of follow-up (mean: 27 months) with 19.0% lost to follow-up. In multivariate regression analyses, weight for age z-score (Adjusted Hazard Ratio (AHR): 0.90; 95% CI: 0.85, 0.95) and high quality indicator score (compared a low score, AHR: 0.43; 95% CI: 0.26, 0.73) had a protective effect on mortality. Patients with a high quality score were less likely to be lost to follow-up (Adjusted Odds Ratio (AOR): 0.42; 95% CI: 0.32, 0.56), compared to those with low score. These findings indicate that providing high quality care to children and adolescents living with HIV is important to improve outcomes, including lowering loss to follow-up and decreasing mortality in this age group.     

Amphipath-Induced Nanoscale Changes in Outer Hair Cell Plasma Membrane Curvature

Outer hair cell (OHC) electromotility enables frequency selectivity and sensitivity in mammalian audition. Electromotility is generated by the transmembrane protein prestin and is sensitive to amphipathic compounds including salicylate, chlorpromazine (CPZ), and trinitrophenol (TNP). Although these compounds induce observable membrane curvature changes in erythrocytes, their effects on OHC membrane curvature are unknown. In this work, fluorescence polarization microscopy was applied to investigate the effects of salicylate, CPZ, and TNP on di-8-ANEPPS orientation in the OHC plasma membrane. Our results demonstrate the ability of fluorescence polarization microscopy to measure amphipath-induced changes in di-8-ANEPPS orientation, consistent with nanoscale changes in membrane curvature between regularly spaced proteins connecting the OHC plasma membrane and cytoskeleton. Simultaneous application of oppositely charged amphipaths generally results in no net membrane bending, consistent with predictions of the bilayer couple hypothesis; however, the application of salicylate (10 mM), which inhibits electromotility, is not reversed by the addition of CPZ. This result supports other findings that suggest salicylate primarily influences electromotiliy and OHC nonlinear capacitance via a direct interaction with prestin. In contrast, we find that CPZ and TNP influence the voltage sensitivity of prestin via membrane bending, demonstrating the mechanosensitivity of this unique membrane motor protein.