Acetylation State of Lysine 14 of Histone H3.3 Affects Mutant Huntingtin Induced Pathogenesis.

Huntington's Disease (HD) is a fatal neurodegenerative disorder caused by the expansion of a polyglutamine-coding CAG repeat in the Huntingtin gene. One of the main causes of neurodegeneration in HD is transcriptional dysregulation that, in part, is caused by the inhibition of histone acetyltransferase (HAT) enzymes. HD pathology can be alleviated by increasing the activity of specific HATs or by inhibiting histone deacetylase (HDAC) enzymes. To determine which histone's post-translational modifications (PTMs) might play crucial roles in HD pathology, we investigated the phenotype-modifying effects of PTM mimetic mutations of variant histone H3.3 in a Drosophila model of HD. Specifically, we studied the mutations (K→Q: acetylated; K→R: non-modified; and K→M: methylated) of lysine residues K9, K14, and K27 of transgenic H3.3. In the case of H3.3K14Q modification, we observed the amelioration of all tested phenotypes (viability, longevity, neurodegeneration, motor activity, and circadian rhythm defects), while H3.3K14R had the opposite effect. H3.3K14Q expression prevented the negative effects of reduced Gcn5 (a HAT acting on H3K14) on HD pathology, while it only partially hindered the positive effects of heterozygous Sirt1 (an HDAC acting on H3K14). Thus, we conclude that the Gcn5-dependent acetylation of H3.3K14 might be an important epigenetic contributor to HD pathology.

Interaction of huntingtin with PRMTs and its subsequent arginine methylation affects HTT solubility, phase transition behavior and neuronal toxicity.

Huntington's disease (HD) is an incurable neurodegenerative disorder caused by a CAG expansion in the huntingtin gene (HTT). Post-translational modifications of huntingtin protein (HTT), such as phosphorylation, acetylation and ubiquitination, have been implicated in HD pathogenesis. Arginine methylation/dimethylation is an important modification with an emerging role in neurodegeneration; however, arginine methylation of HTT remains largely unexplored. Here we report nearly two dozen novel arginine methylation/dimethylation sites on the endogenous HTT from human and mouse brain and human cells suggested by mass spectrometry with data-dependent acquisition. Targeted quantitative mass spectrometry identified differential arginine methylation at specific sites in HD patient-derived striatal precursor cell lines compared to normal controls. We found that HTT can interact with several type I protein arginine methyltransferases (PRMTs) via its N-terminal domain. Using a combination of in vitro methylation and cell-based experiments, we identified PRMT4 (CARM1) and PRMT6 as major enzymes methylating HTT at specific arginines. Alterations of these methylation sites had a profound effect on biochemical properties of HTT rendering it less soluble in cells and affected its liquid-liquid phase separation and phase transition patterns in vitro. We found that expanded HTT 1-586 fragment can form liquid-like assemblies, which converted into solid-like assemblies when the R200/205 methylation sites were altered. Methyl-null alterations increased HTT toxicity to neuronal cells, while overexpression of PRMT 4 and 6 was beneficial for neuronal survival. Thus, arginine methylation pathways that involve specific HTT-modifying PRMT enzymes and modulate HTT biochemical and toxic properties could provide targets for HD-modifying therapies.

The histone replacement gene His4r is involved in heat stress induced chromatin rearrangement.

His4r is the only known variant of histone H4 in Drosophila. It is encoded by the His4r single-copy gene that is located outside of the histone gene cluster and expressed in a different pattern than H4, although the encoded polypeptides are identical. We generated a null mutant (His4rΔ42) which is homozygous viable and fertile without any apparent morphological defects. Heterozygous His4rΔ42 is a mild suppressor of position-effect variegation, suggesting that His4r has a role in the formation or maintenance of condensed chromatin. Under standard conditions loss of His4r has a modest effect on gene expression. Upon heat-stress the induction of the Heat shock protein (HSP) genes Hsp27 and Hsp68 is stronger in His4rΔ42 mutants with concordantly increased survival rate. Analysis of chromatin accessibility after heat shock at a Hsp27 regulatory region showed less condensed chromatin in the absence of His4r while there was no difference at the gene body. Interestingly, preconditioning before heat shock led to increased chromatin accessibility, HSP gene transcription and survival rate in control flies while it did not cause notable changes in His4rΔ42. Thus, our results suggest that His4r might play a role in fine tuning chromatin structure at inducible gene promoters upon environmental stress conditions.

The Arabidopsis RLCK VI_A2 Kinase Controls Seedling and Plant Growth in Parallel with Gibberellin.

The plant-specific receptor-like cytoplasmic kinases (RLCKs) form a large, poorly characterized family. Members of the RLCK VI_A class of dicots have a unique characteristic: their activity is regulated by Rho-of-plants (ROP) GTPases. The biological function of one of these kinases was investigated using a T-DNA insertion mutant and RNA interference. Loss of RLCK VI_A2 function resulted in restricted cell expansion and seedling growth. Although these phenotypes could be rescued by exogenous gibberellin, the mutant did not exhibit lower levels of active gibberellins nor decreased gibberellin sensitivity. Transcriptome analysis confirmed that gibberellin is not the direct target of the kinase; its absence rather affected the metabolism and signalling of other hormones such as auxin. It is hypothesized that gibberellins and the RLCK VI_A2 kinase act in parallel to regulate cell expansion and plant growth. Gene expression studies also indicated that the kinase might have an overlapping role with the transcription factor circuit (PIF4-BZR1-ARF6) controlling skotomorphogenesis-related hypocotyl/cotyledon elongation. Furthermore, the transcriptomic changes revealed that the loss of RLCK VI_A2 function alters cellular processes that are associated with cell membranes, take place at the cell periphery or in the apoplast, and are related to cellular transport and/or cell wall reorganisation.

Integrated evolutionary analysis reveals antimicrobial peptides with limited resistance.

Antimicrobial peptides (AMPs) are promising antimicrobials, however, the potential of bacterial resistance is a major concern. Here we systematically study the evolution of resistance to 14 chemically diverse AMPs and 12 antibiotics in Escherichia coli. Our work indicates that evolution of resistance against certain AMPs, such as tachyplesin II and cecropin P1, is limited. Resistance level provided by point mutations and gene amplification is very low and antibiotic-resistant bacteria display no cross-resistance to these AMPs. Moreover, genomic fragments derived from a wide range of soil bacteria confer no detectable resistance against these AMPs when introduced into native host bacteria on plasmids. We have found that simple physicochemical features dictate bacterial propensity to evolve resistance against AMPs. Our work could serve as a promising source for the development of new AMP-based therapeutics less prone to resistance, a feature necessary to avoid any possible interference with our innate immune system.

Rapid decline of bacterial drug-resistance in an antibiotic-free environment through phenotypic reversion.

Antibiotic resistance typically induces a fitness cost that shapes the fate of antibiotic-resistant bacterial populations. However, the cost of resistance can be mitigated by compensatory mutations elsewhere in the genome, and therefore the loss of resistance may proceed too slowly to be of practical importance. We present our study on the efficacy and phenotypic impact of compensatory evolution in Escherichia coli strains carrying multiple resistance mutations. We have demonstrated that drug-resistance frequently declines within 480 generations during exposure to an antibiotic-free environment. The extent of resistance loss was found to be generally antibiotic-specific, driven by mutations that reduce both resistance level and fitness costs of antibiotic-resistance mutations. We conclude that phenotypic reversion to the antibiotic-sensitive state can be mediated by the acquisition of additional mutations, while maintaining the original resistance mutations. Our study indicates that restricting antimicrobial usage could be a useful policy, but for certain antibiotics only.

Mutant huntingtin disturbs circadian clock gene expression and sleep patterns in Drosophila.

Deficiency of the sleep-wake cycle can accelerate the progression of Huntington's disease (HD) and exacerbate symptoms making it a target of investigation to better understand the molecular pathology of the disorder. In this study we analyzed sleep defects in a Drosophila model of HD and investigated whether disturbed sleep coincides with alterations in the molecular mechanism controlling circadian rhythm. To analyze sleep defects we recorded the daily activity of flies in 12:12 hours light:dark entrainment and in regard to the underlying molecular mechanism measured circadian "clock" gene expression. In HD flies we observed reduced amount of sleep, sleep fragmentation and prolonged sleep latency. We found changes in gene expression patterns of both transcriptional feedback loops of circadian regulation. We detected prolonged expression of the core feedback loop components period and timeless, whilst the secondary feedback loop member vrille had lower expression rates in general. Our results show that the Drosophila HD model recapitulates most of the sleep related symptoms reported in patients therefore it can be a potential tool to study the molecular background of sleep defects in HD. Altered expression of circadian "clock" genes suggests that disturbed sleep pattern in HD might be the consequence of disturbed circadian regulation.

Drosophila small ovary gene is required for transposon silencing and heterochromatin organization, and ensures germline stem cell maintenance and differentiation.

Self-renewal and differentiation of stem cells is one of the fundamental biological phenomena relying on proper chromatin organization. In our study, we describe a novel chromatin regulator encoded by the Drosophila small ovary (sov) gene. We demonstrate that sov is required in both the germline stem cells (GSCs) and the surrounding somatic niche cells to ensure GSC survival and differentiation. sov maintains niche integrity and function by repressing transposon mobility, not only in the germline, but also in the soma. Protein interactome analysis of Sov revealed an interaction between Sov and HP1a. In the germ cell nuclei, Sov colocalizes with HP1a, suggesting that Sov affects transposon repression as a component of the heterochromatin. In a position-effect variegation assay, we found a dominant genetic interaction between sov and HP1a, indicating their functional cooperation in promoting the spread of heterochromatin. An in vivo tethering assay and FRAP analysis revealed that Sov enhances heterochromatin formation by supporting the recruitment of HP1a to the chromatin. We propose a model in which sov maintains GSC niche integrity by regulating transposon silencing and heterochromatin formation.

Systematic genetic interaction studies identify histone demethylase Utx as potential target for ameliorating Huntington's disease.

Huntington's disease (HD) is a dominantly inherited neurodegenerative disease caused by alterations in the huntingtin gene (htt). Transcriptional dysregulation is an early event in HD progression. Protein acetylation and methylation particularly on histones regulates chromatin structure thereby preventing or facilitating transcription. Although protein acetylation has been found to affect HD symptoms, little is known about the potential role of protein methylation in HD pathology. In recent years, a series of proteins have been described that are responsible for methylating and demethylating histones as well as other proteins. We carried out systematic genetic interaction studies testing lysine and arginine methylases and demethylases in a Drosophila melanogaster HD model. We found that modulating methylation enzymes that typically affect histone positions H3K4, H3K36 or H3K79 had varying effects on HD pathology while modulating ones that typically affect constitutive heterochromatin marks at H3K9 and H4K20 generally had limited impact on HD pathology. In contrast, modulating enzymes acting on the facultative heterochromatin mark at H3K27 had specific effects on HD pathology, with reduction of the demethylase Utx rescuing HTT-induced pathology while reducing Polycomb Repressive Complex2 core methylase components led to more aggressive pathology. Further exploration of the mechanism underlying the methylation-specific interactions suggest that these lysine and arginine methylases and demethylases are likely exerting their influence through non-histone targets. These results highlight a novel therapeutic approach for HD in the form of Utx inhibition.