Bharat Reddy

Staphylococcus aureus protein A induces airway epithelial inflammatory responses by activating TNFR1

Staphylococcus aureus is a major human pathogen that is associated with diverse types of local and systemic infection characterized by inflammation dominated by polymorphonuclear leukocytes. Staphylococci frequently cause pneumonia, and these clinical isolates often have increased expression of protein A, suggesting that this protein may have a role in virulence. Here we show that TNFR1, a receptor for tumor-necrosis factor-α (TNF-α) that is widely distributed on the airway epithelium, is a receptor for protein A. We also show that the protein A–TNFR1 signaling pathway has a central role in the pathogenesis of staphylococcal pneumonia.

TLR2 is mobilized into an apical lipid raft receptor complex to signal infection in airway epithelial cells

Toll-like receptors (TLRs) mediate host responses to bacterial gene products. As the airway epithelium is potentially exposed to many diverse inhaled bacteria, TLRs involved in defense of the airways must be broadly responsive, available at the exposed apical surface of the cells, and highly regulated to prevent activation following trivial encounters with bacteria. We demonstrate that TLR2 is enriched in caveolin-1-associated lipid raft microdomains presented on the apical surface of airway epithelial cells after bacterial infection. These receptor complexes include myeloid differentiation protein (MyD88), interleukin-1 receptor-activated kinase-1, and TNF receptor-associated factor 6. The signaling capabilities of TLR2 are amplified through its association with the asialoganglioside gangliotetraosylceramide (Gal beta 1,2GalNAc beta 1,4Gal beta 1,4Glc beta 1,1Cer), which has receptor function itself for many pulmonary pathogens. Ligation of either TLR2 or asialoGM1 by ligands with specificity for either receptor, by Pseudomonas aeruginosa, or by Staphylococcus aureus stimulates IL-8 production through activation of NF-kappa B, as mediated by TLR2 and MyD88. Thus, TLR2 in association with asialo-glycolipids presented within the context of lipid rafts provides a broadly responsive signaling complex at the apical surfaces of airway cells to initiate the host response to potential bacterial infection.

Bacterial neuraminidase facilitates mucosal infection by participating in biofilm production

Many respiratory pathogens, including Hemophilus influenzae, Streptococcus pneumoniae, and Pseudomonas aeruginosa, express neuraminidases that can cleave alpha2,3-linked sialic acids from glycoconjugates. As mucosal surfaces are heavily sialylated, neuraminidases have been thought to modify epithelial cells by exposing potential bacterial receptors. However, in contrast to neuraminidase produced by the influenza virus, a role for bacterial neuraminidase in pathogenesis has not yet been clearly established. We constructed a mutant of P. aeruginosa PAO1 by deleting the PA2794 neuraminidase locus (Delta2794) and tested its virulence and immunostimulatory capabilities in a mouse model of infection. Although fully virulent when introduced i.p., the Delta2794 mutant was unable to establish respiratory infection by i.n. inoculation. The inability to colonize the respiratory tract correlated with diminished production of biofilm, as assessed by scanning electron microscopy and in vitro assays. The importance of neuraminidase in biofilm production was further demonstrated by showing that viral neuraminidase inhibitors in clinical use blocked P. aeruginosa biofilm production in vitro as well. The P. aeruginosa neuraminidase has a key role in the initial stages of pulmonary infection by targeting bacterial glycoconjugates and contributing to the formation of biofilm. Inhibiting bacterial neuraminidases could provide a novel mechanism to prevent bacterial pneumonia.

Regulation of Set9-Mediated H4K20 Methylation by a PWWP Domain Protein

Methylation of histone H4 lysine 20 (H4K20me) is essential for recruiting checkpoint proteins 53BP1/Crb2 to DNA lesions and subsequent activation of a DNA-damage checkpoint. In fission yeast, Set9 (spKMT5) catalyzes mono-, di-, and trimethylation of H4K20. However, the mechanisms that regulate Set9 function are poorly understood. Here, we identified a PWWP domain protein Pdp1 as a Set9-associated factor. Pdp1 binds to histones and is required for Set9 chromatin localization. Yeast cells without Pdp1 were deficient in all three states of H4K20me, sensitive to genotoxic treatments, and impaired in Crb2 recruitment. The PWWP domain of Pdp1 binds to H4K20me, and mutations within the PWWP domain that abrogated this interaction in vitro reduced both the association of Set9 with chromatin and the extent of H4K20me in vivo. These results demonstrate that the PWWP domain is a new methyl-lysine recognition motif that plays important roles in epigenetic regulation.

Histone H3 Lysine 14 Acetylation Is Required for Activation of a DNA Damage Checkpoint in Fission Yeast

Background: Histone acetylation regulates diverse cellular processes. Results: The fission yeast Mst2 complex is a specific histone H3 lysine 14 acetyltransferase. Conclusion: H3K14 acetylation is required for DNA damage checkpoint activation. Significance: These analyses define the in vivo functions of the acetylation of a single histone lysine residue. Histone lysine acetylation has emerged as a key regulator of genome organization. However, with a few exceptions, the contribution of each acetylated lysine to cellular functions is not well understood because of the limited specificity of most histone acetyltransferases and histone deacetylases. Here we show that the Mst2 complex in Schizosaccharomyces pombe is a highly specific H3 lysine 14 (H3K14) acetyltransferase that functions together with Gcn5 to regulate global levels of H3K14 acetylation (H3K14ac). By analyzing the effect of H3K14ac loss through both enzymatic inactivation and histone mutations, we found that H3K14ac is critical for DNA damage checkpoint activation by directly regulating the compaction of chromatin and by recruiting chromatin remodeling protein complex RSC.

Elimination of a specific histone H3K14 acetyltransferase complex bypasses the RNAi pathway to regulate pericentric heterochromatin functions

In Schizosaccharomyces pombe, the RNAi pathway is required for the formation of pericentric heterochromatin, proper chromosome segregation, and repression of pericentric meiotic recombination. Here we demonstrate that, when the activity of the histone H3 Lys 14 (H3K14) acetyltransferase Mst2 is eliminated, the RNAi machinery is no longer required for pericentric heterochromatin functions. We further reveal that reducing RNA polymerase II recruitment to pericentric regions is essential for maintaining heterochromatin in the absence of RNAi.

Rapid epigenetic adaptation to uncontrolled heterochromatin spreading

Heterochromatin, a highly compact chromatin state characterized by histone H3K9 methylation and HP1 protein binding, silences the underlying DNA and influences the expression of neighboring genes. However, the mechanisms that regulate heterochromatin spreading are not well understood. In this study, we show that the conserved Mst2 histone acetyltransferase complex in fission yeast regulates histone turnover at heterochromatin regions to control heterochromatin spreading and prevents ectopic heterochromatin assembly. The combined loss of Mst2 and the JmjC domain protein Epe1 results in uncontrolled heterochromatin spreading and massive ectopic heterochromatin, leading to severe growth defects due to the inactivation of essential genes. Interestingly, these cells quickly recover by accumulating heterochromatin at genes essential for heterochromatin assembly, leading to their reduced expression to restrain heterochromatin spreading. Our studies discover redundant pathways that control heterochromatin spreading and prevent ectopic heterochromatin assembly and reveal a fast epigenetic adaptation response to changes in heterochromatin landscape. DOI: http://dx.doi.org/10.7554/eLife.06179.001

Elimination of shelterin components bypasses RNAi for pericentric heterochromatin assembly.

The RNAi pathway is required for heterochromatin assembly at repetitive DNA elements in diverse organisms. In fission yeast, loss of RNAi causes pericentric heterochromatin defects, compromising gene silencing and chromosome segregation. Here we show that deletion of telomere shelterin components restores pericentric heterochromatin and its functions in RNAi mutants. We further isolated a separation-of-function mutant of Poz1 and revealed that defective telomere silencing, but not telomere length control, is critical for bypassing RNAi. Further analyses demonstrated that compromising shelterin-mediated heterochromatin assembly in RNAi mutants releases heterochromatin protein Swi6, which is redistributed to pericentric regions through RNAi-independent heterochromatin assembly pathways. Given the high mobility of Swi6 protein and that increased levels of Swi6 facilitates heterochromatin spreading as well as ectopic heterochromatin assembly, our results suggest that constitutive heterochromatin domains use multiple pathways to form high-affinity platforms to restrain Swi6, thus limiting its availability and avoiding promiscuous heterochromatin formation.