In vivo tracking of histone H3 lysine 9 acetylation in Xenopus laevis during tail regeneration

Tail and Spinal Cord Regeneration in Urodelean Amphibians

Urodelean amphibians can regenerate the tail and the spinal cord (SC) and maintain this ability throughout their life. This clearly distinguishes these animals from mammals. The phenomenon of tail and SC regeneration is based on the capability of cells involved in regeneration to dedifferentiate, enter the cell cycle, and change their (or return to the pre-existing) phenotype during de novo organ formation. The second critical aspect of the successful tail and SC regeneration is the mutual molecular regulation by tissues, of which the SC and the apical wound epidermis are the leaders. Molecular regulatory systems include signaling pathways components, inflammatory factors, ECM molecules, ROS, hormones, neurotransmitters, HSPs, transcriptional and epigenetic factors, etc. The control, carried out by regulatory networks on the feedback principle, recruits the mechanisms used in embryogenesis and accompanies all stages of organ regeneration, from the moment of damage to the completion of morphogenesis and patterning of all its structures. The late regeneration stages and the effects of external factors on them have been poorly studied. A new model for addressing this issue is herein proposed. The data summarized in the review contribute to understanding a wide range of fundamentally important issues in the regenerative biology of tissues and organs in vertebrates including humans.

Real-time single-molecule imaging of transcriptional regulatory networks in living cells

Gene regulatory networks drive the specific transcriptional programmes responsible for the diversification of cell types during the development of multicellular organisms. Although our knowledge of the genes involved in these dynamic networks has expanded rapidly, our understanding of how transcription is spatiotemporally regulated at the molecular level over a wide range of timescales in the small volume of the nucleus remains limited. Over the past few decades, advances in the field of single-molecule fluorescence imaging have enabled real-time behaviours of individual transcriptional components to be measured in living cells and organisms. These efforts are now shedding light on the dynamic mechanisms of transcription, revealing not only the temporal rules but also the spatial coordination of underlying molecular interactions during various biological events.

Epigenetic processes associated with neonatal spinal transection

In early development, the spinal cord in healthy or disease states displays remarkable activity-dependent changes in plasticity, which may be in part due to the increased activity of brain derived neurotrophic factor (BDNF). Indeed, BDNF delivery has been efficacious in partially ameliorating many of the neurobiological and behavioral consequences of spinal cord injury (SCI), making elucidating the role of BDNF in the normative developing and injured spinal cord a critical research focus. Recent work in our laboratory provided evidence for aberrant global and locus-specific epigenetic changes in methylation of the Bdnf gene as a consequence of SCI. In the present study, animals underwent thoracic lesions on P1, with cervical and lumbar tissue being later collected on P7, P14, and P21. Levels of Bdnf expression and methylation (exon IX and exon IV), in addition to global methylation levels were quantified at each timepoint. Results indicated locus-specific reductions of Bdnf expression that was accompanied by a parallel increase in methylation caudal to the injury site, with animals displaying increased Bdnf expression at the P14 timepoint. Together, these findings suggest that epigenetic activity of the Bdnf gene may act as biomarker in the etiology and intervention effort efficacy following SCI.

Regenerative Potential of Injured Spinal Cord in the Light of Epigenetic Regulation and Modulation

A spinal cord injury is a form of physical harm imposed on the spinal cord that causes disability and, in many cases, leads to permanent mammalian paralysis, which causes a disastrous global issue. Because of its non-regenerative aspect, restoring the spinal cord's role remains one of the most daunting tasks. By comparison, the remarkable regenerative ability of some regeneration-competent species, such as some Urodeles (Axolotl), Xenopus, and some teleost fishes, enables maximum functional recovery, even after complete spinal cord transection. During the last two decades of intensive research, significant progress has been made in understanding both regenerative cells' origins and the molecular signaling mechanisms underlying the regeneration and reconstruction of damaged spinal cords in regenerating organisms and mammals, respectively. Epigenetic control has gradually moved into the center stage of this research field, which has been helped by comprehensive work demonstrating that DNA methylation, histone modifications, and microRNAs are important for the regeneration of the spinal cord. In this review, we concentrate primarily on providing a comparison of the epigenetic mechanisms in spinal cord injuries between non-regenerating and regenerating species. In addition, we further discuss the epigenetic mediators that underlie the development of a regeneration-permissive environment following injury in regeneration-competent animals and how such mediators may be implicated in optimizing treatment outcomes for spinal cord injurie in higher-order mammals. Finally, we briefly discuss the role of extracellular vesicles (EVs) in the context of spinal cord injury and their potential as targets for therapeutic intervention.

Studying Chromatin Epigenetics with Fluorescence Microscopy

Epigenetic modifications of histones (methylation, acetylation, phosphorylation, etc.) are of great importance in determining the functional state of chromatin. Changes in epigenome underlay all basic biological processes, such as cell division, differentiation, aging, and cancerous transformation. Post-translational histone modifications are mainly studied by immunoprecipitation with high-throughput sequencing (ChIP-Seq). It enables an accurate profiling of target modifications along the genome, but suffers from the high cost of analysis and the inability to work with living cells. Fluorescence microscopy represents an attractive complementary approach to characterize epigenetics. It can be applied to both live and fixed cells, easily compatible with high-throughput screening, and provide access to rich spatial information down to the single cell level. In this review, we discuss various fluorescent probes for histone modification detection. Various types of live-cell imaging epigenetic sensors suitable for conventional as well as super-resolution fluorescence microscopy are described. We also focus on problems and future perspectives in the development of fluorescent probes for epigenetics.

CRISPR/Cas9-based simple transgenesis in Xenopus laevis

Transgenic techniques have greatly increased our understanding of the transcriptional regulation of target genes through live reporter imaging, as well as the spatiotemporal function of a gene using loss- and gain-of-function constructs. In Xenopus species, two well-established transgenic methods, restriction enzyme-mediated integration and I-SceI meganuclease-mediated transgenesis, have been used to generate transgenic animals. However, donor plasmids are randomly integrated into the Xenopus genome in both methods. Here, we established a new and simple targeted transgenesis technique based on CRISPR/Cas9 in Xenopus laevis. In this method, Cas9 ribonucleoprotein (RNP) targeting a putative harbor site (the transforming growth factor beta receptor 2-like (tgfbr2l) locus) and a preset donor plasmid DNA were co-injected into the one-cell stage embryos of X. laevis. Approximately 10% of faithful reporter expression was detected in F0 crispants in a promoter/enhancer-specific manner. Importantly, efficient germline transmission and stable transgene expression were observed in the F1 offspring. The simplicity of this method only required preparation of a donor vector containing the tgfbr2l genome fragment and Cas9 RNP targeting this site, which are common experimental procedures used in Xenopus laboratories. Our improved technique allows the simple generation of transgenic X. laevis, so is expected to become a powerful tool for reporter assay and gene function analysis.

Molecular probes for cellular imaging of post-translational proteoforms

Specific post-translational modification (PTM) states of a protein affect its property and function; understanding their dynamics in cells would provide deep insight into diverse signaling pathways and biological processes. However, it is not trivial to visualize post-translational modifications in a protein- and site-specific manner, especially in a living-cell context. Herein, we review recent advances in the development of molecular imaging tools to detect diverse classes of post-translational proteoforms in individual cells, and their applications in studying precise roles of PTMs in regulating the function of cellular proteins.

An early Shh-H2O2 reciprocal regulatory interaction controls the regenerative program during zebrafish fin regeneration

Reactive oxygen species (ROS), originally classified as toxic molecules, have attracted increasing interest given their actions in cell signaling. Hydrogen peroxide (H2O2), the major ROS produced by cells, acts as a second messenger to modify redox-sensitive proteins or lipids. After caudal fin amputation, tight spatiotemporal regulation of ROS is required first for wound healing and later to initiate the regenerative program. However, the mechanisms carrying out this sustained ROS production and their integration with signaling pathways are still poorly understood. We focused on the early dialog between H2O2 and Sonic Hedgehog (Shh) during fin regeneration. We demonstrate that H2O2 controls Shh expression and that Shh in turn regulates the H2O2 level via a canonical pathway. Moreover, the means of this tight reciprocal control change during the successive phases of the regenerative program. Dysregulation of the Hedgehog pathway has been implicated in several developmental syndromes, diabetes and cancer. These data support the existence of an early positive crosstalk between Shh and H2O2 that might be more generally involved in various processes paving the way to improve regenerative processes, particularly in vertebrates.

Characterization of cells expressing MRI reporters for the analysis of epigenetics

Epigenetics is thought to be involved in highly advanced life phenomena, and its regulation has created new opportunities in regenerative medicine. Mintbody (modification-specific intracellular antibody) can track a posttranslational protein modification in epigenetics using a genetic system encoded within a single chain of variable fragments tagged with a fluorescent protein. Magnetic resonance imaging (MRI) is a technique that allows observation of specific molecules in living organisms. The ferritin heavy chain (FTH1) is one of the MRI reporters used in mammals. The combination of FTH1 with mintbody may show remarkable ability as a reporter for MRI to investigate epigenetics in the deep part of a living organism. This article discusses the suitability and safety of FTH1 for use in the analysis of epigenetics by MRI. Cells expressing the FTH1 hybrid of mintbody showed insufficiently increased sensitivity by MRI even in the presence of excess iron. After incubation with ferric ammonium citrate, DNA damage was found in cells expressing the FTH1 hybrid of mintbody. The use of FTH1 as a genetically encoded reporter for MRI appears to be limited by the requirement of metal and its relatively low sensitivity. These results suggest future directionality and the possibility of studying epigenetics in vivo.

Live-Cell Imaging Probes to Track Chromatin Modification Dynamics

The spatiotemporal organization of chromatin is regulated at different levels in the nucleus. Epigenetic modifications such as DNA methylation and histone modifications are involved in chromatin regulation and play fundamental roles in genome function. While the one-dimensional epigenomic landscape in many cell types has been revealed by chromatin immunoprecipitation and sequencing, the dynamic changes of chromatin modifications and their relevance to chromatin organization and genome function remain elusive. Live-cell probes to visualize chromatin and its modifications have become powerful tools to monitor dynamic chromatin regulation. Bulk chromatin can be visualized by both small fluorescent dyes and fluorescent proteins, and specific endogenous genomic loci have been detected by adapting genome-editing tools. To track chromatin modifications in living cells, various types of probes have been developed. Protein domains that bind weakly to specific modifications, such as chromodomains for histone methylation, can be repeated to create a tighter binding probe that can then be tagged with a fluorescent protein. It has also been demonstrated that antigen-binding fragments and single-chain variable fragments from modification-specific antibodies can serve as binding probes without disturbing cell division, development and differentiation. These modification-binding modules are used in modification sensors based on fluorescence/Förster resonance energy transfer to measure the intramolecular conformational changes triggered by modifications. Other probes can be created using a bivalent binding system, such as fluorescence complementation or luciferase chemiluminescence. Live-cell chromatin modification imaging using these probes will address dynamic chromatin regulation and will be useful for assaying and screening effective epigenome drugs in cells and organisms.

Hydrogen peroxide is necessary during tail regeneration in juvenile axolotl

BACKGROUND

Hydrogen peroxide (H₂ O₂ ) is a key reactive oxygen species (ROS) generated during appendage regeneration among vertebrates. However, its role during tail regeneration in axolotl as redox signaling molecule is unclear.

RESULTS

Treatment with exogenous H₂ O₂ rescues inhibitory effects of apocynin-induced growth suppression in tail blastema cells leading to cell proliferation. H₂ O₂ also promotes recruitment of immune cells, regulate the activation of AKT kinase and Agr2 expression during blastema formation. Additionally, ROS/H₂ O₂ regulates the expression and transcriptional activity of Yap1 and its target genes Ctgf and Areg.

CONCLUSIONS

These results show that H₂ O₂ is necessary and sufficient to promote tail regeneration in axolotls. Additionally, Akt signaling and Agr2 were identified as ROS targets, suggesting that ROS/H₂ O₂ is likely to regulate epimorphic regeneration through these signaling pathways. In addition, ROS/H₂ O₂ -dependent-Yap1 activity is required during tail regeneration.

Xenopus as a platform for discovery of genes relevant to human disease

Congenital birth defects result from an abnormal development of an embryo and have detrimental effects on children's health. Specifically, congenital heart malformations are a leading cause of death among pediatric patients and often require surgical interventions within the first year of life. Increased efforts to navigate the human genome provide an opportunity to discover multiple candidate genes in patients suffering from birth defects. These efforts, however, fail to provide an explanation regarding the mechanisms of disease pathogenesis and emphasize the need for an efficient platform to screen candidate genes. Xenopus is a rapid, cost effective, high-throughput vertebrate organism to model the mechanisms behind human disease. This review provides numerous examples describing the successful use of Xenopus to investigate the contribution of patient mutations to complex phenotypes including congenital heart disease and heterotaxy. Moreover, we describe a variety of unique methods that allow us to rapidly recapitulate patients' phenotypes in frogs: gene knockout and knockdown strategies, the use of fate maps for targeted manipulations, and novel imaging modalities. The combination of patient genomics data and the functional studies in Xenopus will provide necessary answers to the patients suffering from birth defects. Furthermore, it will allow for the development of better diagnostic methods to ensure early detection and intervention. Finally, with better understanding of disease pathogenesis, new treatment methods can be tailored specifically to address patient's phenotype and genotype.

Simple Fluorogenic Cellular Assay for Histone Deacetylase Inhibitors Based on Split-Yellow Fluorescent Protein and Intrabodies

Histone deacetylase (HDAC) inhibitors that regulate the posttranslational modifications of histone tails are therapeutic drugs for many diseases such as cancers, neurodegenerative diseases, and asthma; however, convenient and sensitive methods to measure the effect of HDAC inhibitors in cultured mammalian cells remain limited. In this study, a fluorogenic assay was developed to detect the acetylation of lysine 9 on histone H3 (H3K9ac), which is involved in several cancers, Alzheimer's disease, and autism spectrum disorder. To monitor the changes in H3K9ac levels, an H3K9ac-specific intrabody fused with a small fragment FP11 of the split-yellow fluorescent protein (YFP) (scFv-FP11) was expressed in mammalian cells, together with a larger YFP fragment FP1-10 fused with a nuclear localization signal. When the intranuclear level of H3K9ac is increased, the scFv-FP11 is more enriched in the nucleus via passive diffusion through the nuclear pores from the cytoplasm, which increases the chance of forming a fluorescent complex with the nuclear YFP1-10. The results showed that the YFP fluorescence increased when the cells were treated with HDAC inhibitors. Moreover, the sensitivity of the split YFP reporter system to three HDAC inhibitors was higher than that of a conventional cell viability test. The assay system will be a simple and sensitive detection method to evaluate HDAC inhibitor activities at the levels of both single cells and cell populations.

Immunohistochemical Analysis of Histone H3 Modification in Newt Tail Tissue Cells following Amputation

Background

Newts have impressive regenerative capabilities, but it remains unclear about the role of epigenetic regulation in regeneration process. We herein investigated histone modifications in newt tail tissue cells following amputation.

Methods and Results

Iberian ribbed male newts (6-8 months old) were suffered to about 1.5 cm length of amputation of their tails for initiating regeneration process, and the residual stump of tail tissues was collected for immunohistochemical analysis 3 days later. Compared to the tissue cells of intact tails, c-kit-positive stem cells and PCNA-positive proliferating cells were significantly higher in tails suffered to amputation (P < 0.001). Amputation also significantly induced the acetylation of H3K9, H3K14, and H3K27 in cells of the tails with amputation (P < 0.001), but did not significantly change the methylation of H3K27 (P = 0.063).

Conclusion

These results suggest that epigenetic regulation likely involves in newt tail regeneration following amputation.

Monitoring Histone Methylation (H3K9me3) Changes in Live Cells

H3K9me3 (methylation of lysine 9 of histone H3) is an epigenetic modification that acts as a repressor mark. Several diseases, including cancers and neurological disorders, have been associated with aberrant changes in H3K9me3 levels. Different tools have been developed to enable detection and quantification of H3K9me3 levels in cells. Most techniques, however, lack live cell compatibility. To address this concern, we have engineered recombinant protein sensors for probing H3K9me3 in situ. A heterodimeric sensor containing a chromodomain and chromo shadow domain from HP1a was found to be optimal in recognizing H3K9me3 and exhibited similar spatial resolution to commercial antibodies. Our sensor offers similar quantitative accuracy in characterizing changes in H3K9me3 compared to antibodies but claims single cell resolution. The sensor was applied to evaluate changes in H3K9me3 responding to environmental chemical atrazine (ATZ). ATZ was found to result in significant reductions in H3K9me3 levels after 24 h of exposure. Its impact on the distribution of H3K9me3 among cell populations was also assessed and found to be distinctive. We foresee the application of our sensors in multiple toxicity and drug-screening applications.

A Simple Knock-In System for Xenopus via Microhomology Mediated End Joining Repair

Following completion of the genome sequences of Xenopus tropicalis and X. laevis, gene targeting techniques have become increasingly important for the further development of Xenopus research in the life sciences. Gene knockout using programmable nucleases, such as TALEN and CRISPR/Cas9, has reached a level whereby we can readily and routinely perform loss-of-function analysis of genes of interest in these species. However, there is still room for improvement in gene knock-in techniques owing to some technical problems. To overcome these problems, several knock-in techniques have been developed. Among them, we introduce in this chapter a simple knock-in system mediated by microhomology mediated end joining repair. This protocol allows us to produce knock-in animals for in vivo tagging, promoter/enhancer traps, and transgenesis in both of these Xenopus species.

Chromatin dynamics underlying the precise regeneration of a vertebrate limb - Epigenetic regulation and cellular memory

Wound healing, tissue regeneration, and organ regrowth are all regeneration phenomena observed in vertebrates after an injury. However, the ability to regenerate differs greatly among species. Mammals can undergo wound healing and tissue regeneration, but cannot regenerate an organ; for example, they cannot regrow an amputated limb. In contrast, amphibians and fish have much higher capabilities for organ-level regeneration. In addition to medical studies and those in conventional mammalian models such as mice, studies in amphibians and fish have revealed essential factors for and mechanisms of regeneration, including the regrowth of a limb, tail, or fin. However, the molecular nature of the cellular memory needed to precisely generate a new appendage from an amputation site is not fully understood. Recent reports have indicated that organ regeneration is closely related to epigenetic regulation. For example, the methylation status of genomic DNA is related to the expression of regeneration-related genes, and histone-modification enzymes are required to control the chromatin dynamics for regeneration. A proposed mechanism of cellular memory involving an inheritable system of epigenetic modification led us to hypothesize that epigenetic regulation forms the basis for cellular memory in organ regeneration. Here we summarize the current understanding of the role of epigenetic regulation in organ regeneration and discuss the relationship between organ regeneration and epigenetic memory.

Xenopus Resources: Transgenic, Inbred and Mutant Animals, Training Opportunities, and Web-Based Support

Two species of the clawed frog family, Xenopus laevis and X. tropicalis, are widely used as tools to investigate both normal and disease-state biochemistry, genetics, cell biology, and developmental biology. To support both frog specialist and non-specialist scientists needing access to these models for their research, a number of centralized resources exist around the world. These include centers that hold live and frozen stocks of transgenic, inbred and mutant animals and centers that hold molecular resources. This infrastructure is supported by a model organism database. Here, we describe much of this infrastructure and encourage the community to make the best use of it and to guide the resource centers in developing new lines and libraries.

More Than Just a Bandage: Closing the Gap Between Injury and Appendage Regeneration

The remarkable regenerative capabilities of amphibians have captured the attention of biologists for centuries. The frogs Xenopus laevis and Xenopus tropicalis undergo temporally restricted regenerative healing of appendage amputations and spinal cord truncations, injuries that are both devastating and relatively common in human patients. Rapidly expanding technological innovations have led to a resurgence of interest in defining the factors that enable regenerative healing, and in coupling these factors to human therapeutic interventions. It is well-established that early embryonic signaling pathways are critical for growth and patterning of new tissue during regeneration. A growing body of research now indicates that early physiological injury responses are also required to initiate a regenerative program, and that these differ in regenerative and non-regenerative contexts. Here we review recent insights into the biophysical, biochemical, and epigenetic processes that underlie regenerative healing in amphibians, focusing particularly on tail and limb regeneration in Xenopus. We also discuss the more elusive potential mechanisms that link wounding to tissue growth and patterning.

Hydrogen Peroxide and Redox Regulation of Developments

Reactive oxygen species (ROS), which were originally classified as exclusively deleterious compounds, have gained increasing interest in the recent years given their action as bona fide signalling molecules. The main target of ROS action is the reversible oxidation of cysteines, leading to the formation of disulfide bonds, which modulate protein conformation and activity. ROS, endowed with signalling properties, are mainly produced by NADPH oxidases (NOXs) at the plasma membrane, but their action also involves a complex machinery of multiple redox-sensitive protein families that differ in their subcellular localization and their activity. Given that the levels and distribution of ROS are highly dynamic, in part due to their limited stability, the development of various fluorescent ROS sensors, some of which are quantitative (ratiometric), represents a clear breakthrough in the field and have been adapted to both ex vivo and in vivo applications. The physiological implication of ROS signalling will be presented mainly in the frame of morphogenetic processes, embryogenesis, regeneration, and stem cell differentiation. Gain and loss of function, as well as pharmacological strategies, have demonstrated the wide but specific requirement of ROS signalling at multiple stages of these processes and its intricate relationship with other well-known signalling pathways.

Homeotic transformation of tails into limbs in anurans

Anuran tadpoles can regenerate their tails after amputation. However, they occasionally form ectopic limbs instead of the lost tail part after vitamin A treatment. This is regarded as an example of a homeotic transformation. In this phenomenon, the developmental fate of the tail blastema is apparently altered from that of a tail to that of limbs, indicating a realignment of positional information in the blastema. Morphological observations and analyses of the development of skeletal elements during the process suggest that positional information in the blastema is rewritten from tail to trunk specification under the influence of vitamin A, resulting in limb formation. Despite the extensive information gained from morphological observations, a comprehensive understanding of this phenomenon also requires molecular data. We review previous studies related to anuran homeotic transformation. The findings of these studies provide a basis for evaluating major hypotheses and identifying molecular data that should be prioritized in future studies. Finally, we argue that positional information for the tail blastema changes to that for a part of the trunk, leading to homeotic transformations. To suggest this hypothesis, we present published data that favor the rewriting of positional information.

A transgenic reporter under control of an es1 promoter/enhancer marks wound epidermis and apical epithelial cap during tail regeneration in Xenopus laevis tadpole

Rapid wound healing and subsequent formation of the apical epithelial cap (AEC) are believed to be required for successful appendage regeneration in amphibians. Despite the significant role of AEC in limb regeneration, its role in tail regeneration and the mechanisms that regulate the wound healing and AEC formation are not well understood. We previously identified Xenopus laevis es1, which is preferentially expressed in wounded regions, including the AEC after tail regeneration. In this study we established and characterized transgenic Xenopus laevis lines harboring the enhanced green fluorescent protein (EGFP) gene under control of an es1 gene regulatory sequence (es1:egfp). The EGFP reporter expression was clearly seen in several regions of the embryo and then declined to an undetectable level in larvae, recapitulating the endogenous es1 expression. After amputation of the tadpole tail, EGFP expression was re-activated at the edge of the stump epidermis and then increased in the wound epidermis (WE) covering the amputation surface. As the stump started to regenerate, the EGFP expression became restricted to the most distal epidermal region, including the AEC. EGFP was preferentially expressed in the basal or deep cells but not in the superficial cells of the WE and AEC. We performed a small-scale pharmacological screening for chemicals that affected the expression of EGFP in the stump epidermis after tail amputation. The EGFP expression was attenuated by treatment with an inhibitor for ERK, TGF-β or reactive oxygen species (ROS) signaling. These treatments also impaired wound closure of the amputation surface, suggesting that the three signaling activities are required for es1 expression in the WE and successful wound healing after tail amputation. These findings showed that es1:egfp Xenopus laevis should be a useful tool to analyze molecular mechanisms regulating wound healing and appendage regeneration.

Nerves, H2O2 and Shh: Three players in the game of regeneration

The tight control of reactive oxygen species (ROS) levels is required during regeneration. H₂O₂ in particular assumes clear signalling functions at different steps in this process. Injured nerves induce high levels of H₂O₂ through the activation of the Hedgehog (Shh) pathway, providing an environment that promotes cell plasticity, progenitor recruitment and blastema formation. In turn, high H₂O₂ levels contribute to growing axon attraction. Once re-innervation is completed, nerves subsequently downregulate H₂O₂ levels to their original state. A similar regulatory loop between H₂O₂ levels and nerves also exists during development. This suggests that redox signalling is a major actor in cell plasticity.

Live imaging of H3K9 acetylation in plant cells

Proper regulation of histone acetylation is important in development and cellular responses to environmental stimuli. However, the dynamics of histone acetylation at the single-cell level remains poorly understood. Here we established a transgenic plant cell line to track histone H3 lysine 9 acetylation (H3K9ac) with a modification-specific intracellular antibody (mintbody). The H3K9ac-specific mintbody fused to the enhanced green fluorescent protein (H3K9ac-mintbody-GFP) was introduced into tobacco BY-2 cells. We successfully demonstrated that H3K9ac-mintbody-GFP interacted with H3K9ac in vivo. The ratio of nuclear/cytoplasmic H3K9ac-mintbody-GFP detected in quantitative analysis reflected the endogenous H3K9ac levels. Under chemically induced hyperacetylation conditions with histone deacetylase inhibitors including trichostatin A, Ky-2 and Ky-14, significant enhancement of H3K9ac was detected by H3K9ac-mintbody-GFP dependent on the strength of inhibitors. Conversely, treatment with a histone acetyltransferase inhibitor, C646 caused a reduction in the nuclear to cytoplasmic ratio of H3K9ac-mintbody-GFP. Using this system, we assessed the environmental responses of H3K9ac and found that cold and salt stresses enhanced H3K9ac in tobacco BY-2 cells. In addition, a combination of H3K9ac-mintbody-GFP with 5-ethynyl-2'-deoxyuridine labelling confirmed that H3K9ac level is constant during interphase.

A Genetically Encoded Probe for Live-Cell Imaging of H4K20 Monomethylation

Eukaryotic gene expression is regulated in the context of chromatin. Dynamic changes in post-translational histone modification are thought to play key roles in fundamental cellular functions such as regulation of the cell cycle, development, and differentiation. To elucidate the relationship between histone modifications and cellular functions, it is important to monitor the dynamics of modifications in single living cells. A genetically encoded probe called mintbody (modification-specific intracellular antibody), which is a single-chain variable fragment tagged with a fluorescent protein, has been proposed as a useful visualization tool. However, the efficacy of intracellular expression of antibody fragments has been limited, in part due to different environmental conditions in the cytoplasm compared to the endoplasmic reticulum where secreted proteins such as antibodies are folded. In this study, we have developed a new mintbody specific for histone H4 Lys20 monomethylation (H4K20me1). The specificity of the H4K20me1-mintbody in living cells was verified using yeast mutants and mammalian cells in which this target modification was diminished. Expression of the H4K20me1-mintbody allowed us to monitor the oscillation of H4K20me1 levels during the cell cycle. Moreover, dosage-compensated X chromosomes were visualized using the H4K20me1-mintbody in mouse and nematode cells. Using X-ray crystallography and mutational analyses, we identified critical amino acids that contributed to stabilization and/or proper folding of the mintbody. Taken together, these data provide important implications for future studies aimed at developing functional intracellular antibodies. Specifically, the H4K20me1-mintbody provides a powerful tool to track this particular histone modification in living cells and organisms.