The functional identity of centromeres arises from a set of specific nucleoprotein particle subunits of the centromeric chromatin fibre. These include CENP-A and histone H3 nucleosomes and a novel nucleosome-like complex of CENPs -T, -W, -S and -X. Fluorescence cross-correlation spectroscopy and Förster resonance energy transfer (FRET) revealed that human CENP-S and -X exist principally in complex in soluble form and retain proximity when assembled at centromeres. Conditional labelling experiments show that they both assemble de novo during S phase and G2, increasing approximately three- to fourfold in abundance at centromeres. Fluorescence recovery after photobleaching (FRAP) measurements documented steady-state exchange between soluble and assembled pools, with CENP-X exchanging approximately 10 times faster than CENP-S (t1/2 ∼ 10 min versus 120 min). CENP-S binding to sites of DNA damage was quite distinct, with a FRAP half-time of approximately 160 s. Fluorescent two-hybrid analysis identified CENP-T as a uniquely strong CENP-S binding protein and this association was confirmed by FRET, revealing a centromere-bound complex containing CENP-S, CENP-X and CENP-T in proximity to histone H3 but not CENP-A. We propose that deposition of the CENP-T/W/S/X particle reveals a kinetochore-specific chromatin assembly pathway that functions to switch centromeric chromatin to a mitosis-competent state after DNA replication. Centromeres shuttle between CENP-A-rich, replication-competent and H3-CENP-T/W/S/X-rich mitosis-competent compositions in the cell cycle.
centromere; mitosis; constitutive centromere-associated network; kinetochore
Protein–protein interactions are the basis of all processes in living cells, but most studies of these interactions rely on biochemical in vitro assays. Here we present a simple and versatile fluorescent-three-hybrid (F3H) strategy to visualize and target protein–protein interactions. A high-affinity nanobody anchors a GFP-fusion protein of interest at a defined cellular structure and the enrichment of red-labelled interacting proteins is measured at these sites. With this approach, we visualize the p53–HDM2 interaction in living cells and directly monitor the disruption of this interaction by Nutlin 3, a drug developed to boost p53 activity in cancer therapy. We further use this approach to develop a cell-permeable vector that releases a highly specific peptide disrupting the p53 and HDM2 interaction. The availability of multiple anchor sites and the simple optical readout of this nanobody-based capture assay enable systematic and versatile analyses of protein–protein interactions in practically any cell type and species.
Screens for protein–protein interactions and for drugs that disrupt them typically use in vitro assays which fail to capture the complexity of the cell’s interior. By fixing proteins to distinct cellular locations, Herce et al. demonstrate a fluorescent-three-hybrid approach to probe such interactions in their cellular contexts.
Multi-atlas segmentation has been widely used to segment various anatomical structures. The success of this technique partly relies on the selection of atlases that are best mapped to a new target image after registration. Recently, manifold learning has been proposed as a method for atlas selection. Each manifold learning technique seeks to optimize a unique objective function. Therefore, different techniques produce different embeddings even when applied to the same data set. Previous studies used a single technique in their method and gave no reason for the choice of the manifold learning technique employed nor the theoretical grounds for the choice of the manifold parameters. In this study, we compare side-by-side the results given by 3 manifold learning techniques (Isomap, Laplacian Eigenmaps and Locally Linear Embedding) on the same data set. We assess the ability of those 3 different techniques to select the best atlases to combine in the framework of multi-atlas segmentation. First, a leave-one-out experiment is used to optimize our method on a set of 110 manually segmented atlases of hippocampi and find the manifold learning technique and associated manifold parameters that give the best segmentation accuracy. Then, the optimal parameters are used to automatically segment 30 subjects from the Alzheimer’s Disease Neuroimaging Initiative (ADNI). For our dataset, the selection of atlases with Locally Linear Embedding gives the best results. Our findings show that selection of atlases with manifold learning leads to segmentation accuracy close to or significantly higher than the state-of-the-art method and that accuracy can be increased by fine tuning the manifold learning process.
Methyl-CpG-binding protein 2 (MeCP2) is generally considered to act as a transcriptional repressor, whereas recent studies suggest that MeCP2 is also involved in transcription activation. To gain insight into this dual function of MeCP2, we assessed the impact of MeCP2 on higher-order chromatin structure in living cells using mammalian cell systems harbouring a lactose operator and reporter gene-containing chromosomal domain to assess the effect of lactose repressor-tagged MeCP2 (and separate MeCP2 domains) binding in living cells. Our data reveal that targeted binding of MeCP2 elicits extensive chromatin unfolding. MeCP2-induced chromatin unfolding is triggered independently of the methyl-cytosine-binding domain. Interestingly, MeCP2 binding triggers the loss of HP1γ at the chromosomal domain and an increased HP1γ mobility, which is not observed for HP1α and HP1β. Surprisingly, MeCP2-induced chromatin unfolding is not associated with transcriptional activation. Our study suggests a novel role for MeCP2 in reorganizing chromatin to facilitate a switch in gene activity.
Argininemia is a rare hereditary disease due to a deficiency of hepatic arginase, which is the last enzyme of the urea cycle and hydrolyzes arginine to ornithine and urea. The onset of the disease is usually in childhood, and clinical manifestations include progressive spastic paraparesis and mental retardation. Liver involvement is less frequent and usually not as severe as observed in other UCDs. For this reason, and because usually there is a major neurological disease at diagnosis, patients with argininemia are rarely considered as candidates for OLT despite its capacity to replace the deficient enzyme by an active one. We report on long-term follow-up of two patients with argininemia. Patient 1 was diagnosed by the age of 20 months and despite appropriate conventional treatment progressed to spastic paraparesis with marked limp. OLT was performed at 10 years of age with normalization of plasmatic arginine levels and guanidino compounds. Ten years post-OLT, under free diet, there is no progression of neurological lesions. The second patient (previously reported by our group) was diagnosed at 2 months of age, during a neonatal cholestasis workup study. OLT was performed at the age of 7 years, due to liver cirrhosis with portal hypertension, in the absence of neurological lesions and an almost-normal brain MRI. After OLT, under free diet, there was normalization of plasmatic arginine levels and guanidino compounds. Twelve years post-OLT, she presents a normal neurological examination. We conclude that OLT prevents progressive neurological impairment in argininemia and should be considered when appropriate conventional treatment fails.
There is considerable interest in designing therapeutic studies of individuals at risk of Alzheimer disease (AD) to prevent the onset of symptoms. Cortical β-amyloid plaques, the first stage of AD pathology, can be detected in vivo using positron emission tomography (PET), and several studies have shown that ∼1/3 of healthy elderly have significant β-amyloid deposition. Here we assessed whether asymptomatic amyloid-PET-positive controls have increased rates of brain atrophy, which could be harnessed as an outcome measure for AD prevention trials. We assessed 66 control subjects (age = 73.5±7.3 yrs; MMSE = 29±1.3) from the Australian Imaging Biomarkers & Lifestyle study who had a baseline Pittsburgh Compound B (PiB) PET scan and two 3T MRI scans ∼18-months apart. We calculated PET standard uptake value ratios (SUVR), and classified individuals as amyloid-positive/negative. Baseline and 18-month MRI scans were registered, and brain, hippocampal, and ventricular volumes and annualized volume changes calculated. Increasing baseline PiB-PET measures of β-amyloid load correlated with hippocampal atrophy rate independent of age (p = 0.014). Twenty-two (1/3) were PiB-positive (SUVR>1.40), the remaining 44 PiB-negative (SUVR≤1.31). Compared to PiB-negatives, PiB-positive individuals were older (76.8±7.5 vs. 71.7±7.5, p<0.05) and more were APOE4 positive (63.6% vs. 19.2%, p<0.01) but there were no differences in baseline brain, ventricle or hippocampal volumes, either with or without correction for total intracranial volume, once age and gender were accounted for. The PiB-positive group had greater total hippocampal loss (0.06±0.08 vs. 0.02±0.05 ml/yr, p = 0.02), independent of age and gender, with non-significantly higher rates of whole brain (7.1±9.4 vs. 4.7±5.5 ml/yr) and ventricular (2.0±3.0 vs. 1.1±1.0 ml/yr) change. Based on the observed effect size, recruiting 384 (95%CI 195–1080) amyloid-positive subjects/arm will provide 80% power to detect 25% absolute slowing of hippocampal atrophy rate in an 18-month treatment trial. We conclude that hippocampal atrophy may be a feasible outcome measure for secondary prevention studies in asymptomatic amyloidosis.
Thickness measurements of the cerebral cortex can aid diagnosis and provide valuable information about the temporal evolution of diseases such as Alzheimer's, Huntington's, and schizophrenia. Methods that measure the thickness of the cerebral cortex from in-vivo magnetic resonance (MR) images rely on an accurate segmentation of the MR data. However, segmenting the cortex in a robust and accurate way still poses a challenge due to the presence of noise, intensity non-uniformity, partial volume effects, the limited resolution of MRI and the highly convoluted shape of the cortical folds. Beginning with a well-established probabilistic segmentation model with anatomical tissue priors, we propose three post-processing refinements: a novel modification of the prior information to reduce segmentation bias; introduction of explicit partial volume classes; and a locally varying MRF-based model for enhancement of sulci and gyri. Experiments performed on a new digital phantom, on BrainWeb data and on data from the Alzheimer's Disease Neuroimaging Initiative (ADNI) show statistically significant improvements in Dice scores and PV estimation (p<10−3) and also increased thickness estimation accuracy when compared to three well established techniques.
Gaussian mixture model; Expectation-maximization; Markov Random Field; Cortical segmentation; Partial volume effect
Epigenetic marks like methylation of cytosines at CpG dinucleotides are essential for mammalian development and play a major role in the regulation of gene expression and chromatin architecture. The methyl-cytosine binding domain (MBD) protein family recognizes and translates this methylation mark. We have recently shown that the level of MeCP2 and MBD2, two members of the MBD family, increased during differentiation and their ectopic expression induced heterochromatin clustering in vivo. As oligomerization of these MBD proteins could constitute a factor contributing to the chromatin clustering effect, we addressed potential associations among the MBD family performing a series of different interaction assays in vitro as well as in vivo. Using recombinant purified MBDs we found that MeCP2 and MBD2 showed the stronger self and cross association as compared to the other family members. Besides demonstrating that these homo- and hetero-interactions occur in the absence of DNA, we could confirm them in mammalian cells using co-immunoprecipitation analysis. Employing a modified form of the fluorescent two-hybrid protein-protein interaction assay, we could clearly visualize these associations in single cells in vivo. Deletion analysis indicated that the region of MeCP2 comprising amino acids 163–309 as well the first 152 amino acids of MBD2 are the domains responsible for MeCP2 and MBD2 associations. Our results strengthen the possibility that MeCP2 and MBD2 direct interactions could crosslink chromatin fibers and therefore give novel insight into the molecular mechanism of MBD mediated global heterochromatin architecture.
The aim of this study was to compare the effect of an intermittent intense aerobic exercise session and a resistance exercise session on blood cell counts and oxidative stress parameters in middle-aged women. Thirty-four women were selected and divided into three groups: RE group (performing 60 min of resistance exercises, N = 12), spinning group (performing 60 min of spinning, N = 12), and control group (not exercising regularly, N = 10). In both exercise groups, lymphocytes and monocytes decreased after 1-h recuperation (post-exercise) compared to immediately after exercise (P < 0.05). Immediately after exercise, in both exercised groups, a significant increase in TBARS (from 16.5 ± 2 to 25 ± 2 for the spinning group and from 18.6 ± 1 to 28.2 ± 3 nmol MDA/mL serum for the RE group) and protein carbonyl (from 1.0 ± 0.3 to 1.6 ± 0.2 for the spinning group and from 0.9 ± 0.2 to 1.5 ± 0.2 nmol/mg protein for the RE group) was observed (P < 0.05). A decrease in antioxidant activities (non-protein sulfhydryl, superoxide dismutase, catalase) was also demonstrated with a negative correlation between damage markers and antioxidant body defenses (P < 0.05). These results indicate that an acute bout of intermittent or anaerobic exercise induces immune suppression and increases the production of reactive oxygen species, causing oxidative stress in middle-aged and trained women. Furthermore, we demonstrated that trained women show improved antioxidant capacity and lower oxidative damage than sedentary ones, demonstrating the benefits of chronic regular physical activity.
Intermittent exercise; Anaerobic exercise; Oxidative stress; Immune suppression; Middle-aged women
The X-linked Mecp2 is a known interpreter of epigenetic information and mutated in Rett syndrome, a complex neurological disease. MeCP2 recruits HDAC complexes to chromatin thereby modulating gene expression and, importantly regulates higher order heterochromatin structure. To address the effects of MeCP2 deficiency on heterochromatin organization during neural differentiation, we developed a versatile model for stem cell in vitro differentiation. Therefore, we modified murine Mecp2 deficient (Mecp2−/y) embryonic stem cells to generate cells exhibiting green fluorescent protein expression upon neural differentiation. Subsequently, we quantitatively analyzed heterochromatin organization during neural differentiation in wild type and in Mecp2 deficient cells. We found that MeCP2 protein levels increase significantly during neural differentiation and accumulate at constitutive heterochromatin. Statistical analysis of Mecp2 wild type neurons revealed a significant clustering of heterochromatin per nuclei with progressing differentiation. In contrast we found Mecp2 deficient neurons and astroglia cells to be significantly impaired in heterochromatin reorganization. Our results (i) introduce a new and manageable cellular model to study the molecular effects of Mecp2 deficiency, and (ii) support the view of MeCP2 as a central protein in heterochromatin architecture in maturating cells, possibly involved in stabilizing their differentiated state.
All cellular processes depend on the expression and repression of the right sets of genes at the right time. As each cell contains the same DNA, transcriptional and epigenetic factors have to maintain tight control over gene expression. Even a small divergence from the correct transcriptional program can lead to severe defects and even death. Having deciphered the complete linear genetic information, we need to clarify how this information is organized into the dynamic and highly heterogeneous three-dimensional space of the eukaryotic cell nucleus. Observations on the higher order organization of DNA into differentiated condensation levels date back to the early twentieth century, and potential implications of these structural features to gene expression were postulated shortly after. In particular, proximity of genes to condensed regions of heterochromatin was proposed to negatively influence their expression and, henceforward, the concept of heterochromatin as subnuclear silencing compartment emerged. Methodological advances fueled a flurry of recent studies, which only, in part, led support to this concept. In this review, we address how (hetero)chromatin structure and proximity might influence gene expression and discuss the challenges and means to unravel this fundamental biological question.
Kinetochores are multi-protein megadalton assemblies that are required for attachment of microtubules to centromeres and, in turn, the segregation of chromosomes in mitosis. Kinetochore assembly is a cell cycle regulated multi-step process. The initial step occurs during interphase and involves loading of the 15-subunit constitutive centromere associated complex (CCAN), which contains a 5-subunit (CENP-P/O/R/Q/U) sub-complex. Here we show using a fluorescent three-hybrid (F3H) assay and fluorescence resonance energy transfer (FRET) in living mammalian cells that CENP-P/O/R/Q/U subunits exist in a tightly packed arrangement that involves multifold protein-protein interactions. This sub-complex is, however, not pre-assembled in the cytoplasm, but rather assembled on kinetochores through the step-wise recruitment of CENP-O/P heterodimers and the CENP-P, -O, -R, -Q and -U single protein units. SNAP-tag experiments and immuno-staining indicate that these loading events occur during S-phase in a manner similar to the nucleosome binding components of the CCAN, CENP-T/W/N. Furthermore, CENP-P/O/R/Q/U binding to the CCAN is largely mediated through interactions with the CENP-N binding protein CENP-L as well as CENP-K. Once assembled, CENP-P/O/R/Q/U exchanges slowly with the free nucleoplasmic pool indicating a low off-rate for individual CENP-P/O/R/Q/U subunits. Surprisingly, we then find that during late S-phase, following the kinetochore-binding step, both CENP-Q and -U but not -R undergo oligomerization. We propose that CENP-P/O/R/Q/U self-assembles on kinetochores with varying stoichiometry and undergoes a pre-mitotic maturation step that could be important for kinetochores switching into the correct conformation necessary for microtubule-attachment.
UVA (320–400 nm) represents the main spectral component of solar UV radiation, induces pre-mutagenic DNA lesions and is classified as Class I carcinogen. Recently, discussion arose whether UVA induces DNA double-strand breaks (dsbs). Only few reports link the induction of dsbs to UVA exposure and the underlying mechanisms are poorly understood. Using the Comet-assay and γH2AX as markers for dsb formation, we demonstrate the dose-dependent dsb induction by UVA in G1-synchronized human keratinocytes (HaCaT) and primary human skin fibroblasts. The number of γH2AX foci increases when a UVA dose is applied in fractions (split dose), with a 2-h recovery period between fractions. The presence of the anti-oxidant Naringin reduces dsb formation significantly. Using an FPG-modified Comet-assay as well as warm and cold repair incubation, we show that dsbs arise partially during repair of bi-stranded, oxidative, clustered DNA lesions. We also demonstrate that on stretched chromatin fibres, 8-oxo-G and abasic sites occur in clusters. This suggests a replication-independent formation of UVA-induced dsbs through clustered single-strand breaks via locally generated reactive oxygen species. Since UVA is the main component of solar UV exposure and is used for artificial UV exposure, our results shine new light on the aetiology of skin cancer.
Heterochromatic regions represent a significant portion of the mammalian genome and have been implied in several important cellular processes, including cell division and genomic stability. However, its composition and dynamics remain largely unknown. To better understand how heterochromatin functions and how it is organized within the context of the cell nucleus, we have developed molecular tools allowing the targeting of virtually any nuclear factor specifically to heterochromatic regions and, thereby, the manipulation, also in a temporally controlled manner, of its composition. To validate our approach, we have ectopically targeted MeCP2 chromatin binding deficient Rett mutants to constitutive heterochromatic regions and analyze its functional consequences. We could show that, once bound to their endogenous target regions, their ability to re-organize higher order chromatin structure is restored. Furthermore, a temporally controlled targeting strategy allowed us to monitor MeCP2-mediated chromatin rearrangements in vivo and to visualize large-scale chromatin movements over several micrometers, as well as heterochromatic foci fusion events. This novel strategy enables specific tethering of any protein to heterochromatin and lays the ground for controlled manipulation of its composition and organization.
This study evaluated the use of an avirulent live Salmonella Choleraesuis vaccine to reduce the seroprevalence and number of Salmonella carrier pigs at slaughter. Seven batches of 500 pigs were included in each of the two study groups: the vaccinated group (VG) that was orally vaccinated and the control group (CG) that received a placebo on the first day of life. The groups were managed in a three-site system and followed up from birth to slaughter. Blood samples (n=378) were collected from each VG and CG to monitor the on-farm seroprevalence in both groups. Mesenteric lymph nodes and blood from animals (n=390) belonging to each group were collected at slaughter. At the first day of life, the seroprevalence in control batches ranged from 77.9 to 96.3 per cent, while in vaccinated batches, it ranged from 66.6 to 92.6 per cent. At weaning (21 days of age), the number of seropositives decreased in both groups (mean of 12 and 3.7 per cent for CG and VG, respectively). At slaughter, batches of VG had a significantly (P<0.0001) lower seroprevalence (46.6±5 per cent) and isolation of Salmonella from lymph nodes (33.1±5 per cent) compared with CG batches (79.7±4 per cent and 59.5±5 per cent, respectively). The results indicate that administration of a Salmonella choleraesuis-attenuated vaccine on the first day of life decreases Salmonella isolation and seroprevalence in pigs at slaughter.
The discovery of the DNA double helix structure half a century ago immediately suggested a mechanism for its duplication by semi-conservative copying of the nucleotide sequence into two DNA daughter strands. Shortly after, a second fundamental step toward the elucidation of the mechanism of DNA replication was taken with the isolation of the first enzyme able to polymerize DNA from a template. In the subsequent years, the basic mechanism of DNA replication and its enzymatic machinery components were elucidated, mostly through genetic approaches and in vitro biochemistry. Most recently, the spatial and temporal organization of the DNA replication process in vivo within the context of chromatin and inside the intact cell are finally beginning to be elucidated. On the one hand, recent advances in genome-wide high throughput techniques are providing a new wave of information on the progression of genome replication at high spatial resolution. On the other hand, novel super-resolution microscopy techniques are just starting to give us the first glimpses of how DNA replication is organized within the context of single intact cells with high spatial resolution. The integration of these data with time lapse microscopy analysis will give us the ability to film and dissect the replication of the genome in situ and in real time.
Integration of genomic studies and super-resolution microscopy indicates that, in vivo, DNA replication is self-propagating and increases in efficiency through S phase.
In addition to endocytosis-mediated cellular uptake, hydrophilic cell-penetrating peptides are able to traverse biological membranes in a non-endocytic mode termed transduction, resulting in immediate bioavailability. Here we analysed structural requirements for the non-endocytic uptake mode of arginine-rich cell-penetrating peptides, by a combination of live-cell microscopy, molecular dynamics simulations and analytical ultracentrifugation. We demonstrate that the transduction efficiency of arginine-rich peptides increases with higher peptide structural rigidity. Consequently, cyclic arginine-rich cell-penetrating peptides showed enhanced cellular uptake kinetics relative to their linear and more flexible counterpart. We propose that guanidinium groups are forced into maximally distant positions by cyclization. This orientation increases membrane contacts leading to enhanced cell penetration.
Cell-penetrating peptides can deliver molecular cargoes into living cells, and cross biological membranes by transduction—a non-endocytic mechanism. Here, the transduction efficiency of cyclic arginine-rich peptides is shown to be higher than that of more flexible linear peptides.
Methyl CpG binding protein 2 (MeCP2) binds DNA, and has a preference for methylated CpGs and, hence, in cells, it accumulates in heterochromatin. Even though it is expressed ubiquitously MeCP2 is particularly important during neuronal maturation. This is underscored by the fact that in Rett syndrome, a neurological disease, 80% of patients carry a mutation in the MECP2 gene. Since the MECP2 gene lies on the X chromosome and is subjected to X chromosome inactivation, affected patients are usually chimeric for wild type and mutant MeCP2. Here, we present the generation and characterization of the first rat monoclonal MeCP2 specific antibodies as well as mouse monoclonal antibodies and a rabbit polyclonal antibody. We demonstrate that our antibodies are suitable for immunoblotting, (chromatin) immunoprecipitation and immunofluorescence of endogenous and ectopically expressed MeCP2. Epitope mapping revealed that most of the MeCP2 monoclonal antibodies recognize the C-terminal domain and one the N-terminal domain of MeCP2. Using slot blot analysis, we determined a high sensitivity of all antibodies, detecting amounts as low as 1 ng of MeCP2 protein. Moreover, the antibodies recognize MeCP2 from different species, including human, mouse, rat and pig. Lastly, we have validated their use by analyzing and quantifying X chromosome inactivation skewing using brain tissue of MeCP2 heterozygous null female mice. The new MeCP2 specific monoclonal antibodies described here perform well in a large variety of immunological applications making them a very valuable set of tools for studies of MeCP2 pathophysiology in situ and in vitro.
The replication of the genome is a spatio-temporally highly organized process. Yet, its flexibility throughout development suggests that this process is not genetically regulated. However, the mechanisms and chromatin modifications controlling replication timing are still unclear. We made use of the prominent structure and defined heterochromatic landscape of pericentric regions as an example of late replicating constitutive heterochromatin. We manipulated the major chromatin markers of these regions, namely histone acetylation, DNA and histone methylation, as well as chromatin condensation and determined the effects of these altered chromatin states on replication timing. Here, we show that manipulation of DNA and histone methylation as well as acetylation levels caused large-scale heterochromatin decondensation. Histone demethylation and the concomitant decondensation, however, did not affect replication timing. In contrast, immuno-FISH and time-lapse analyses showed that lowering DNA methylation, as well as increasing histone acetylation, advanced the onset of heterochromatin replication. While dnmt1−/− cells showed increased histone acetylation at chromocenters, histone hyperacetylation did not induce DNA demethylation. Hence, we propose that histone hypoacetylation is required to maintain normal heterochromatin duplication dynamics. We speculate that a high histone acetylation level might increase the firing efficiency of origins and, concomitantly, advances the replication timing of distinct genomic regions.
Nuclear organization of chromatin is an important level of genome regulation with positional changes of genes occurring during reprogramming. Inherent variability of biological specimens, wide variety of sample preparation and imaging conditions, though pose significant challenges to data analysis and comparison. Here, we describe the development of a computational image analysis toolbox overcoming biological variability hurdles by a novel single cell randomizing normalization. We performed a comparative analysis of the relationship between spatial positioning of pluripotency genes with their genomic activity and determined the degree of similarity between fibroblasts, induced pluripotent stem cells and embryonic stem cells. Our analysis revealed a preferred positioning of actively transcribed Sox2, Oct4 and Nanog away from the nuclear periphery, but not from pericentric heterochromatin. Moreover, in the silent state, we found no common nuclear localization for any of the genes. Our results suggest that the surrounding gene density hinders relocation from an internal nuclear position. Altogether, our data do not support the hypothesis that the nuclear periphery acts as a general transcriptional silencer, rather suggesting that internal nuclear localization is compatible with expression in pluripotent cells but not sufficient for expression in mouse embryonic fibroblasts. Thus, our computational approach enables comparative analysis of topological relationships in spite of stark morphological variability typical of biological data sets.
Recent discovery of 5-hydroxymethylcytosine (5hmC) in genomic DNA raises the question how this sixth base is recognized by cellular proteins. In contrast to the methyl-CpG binding domain (MBD) of MeCP2, we found that the SRA domain of Uhrf1, an essential factor in DNA maintenance methylation, binds 5hmC and 5-methylcytosine containing substrates with similar affinity. Based on the co-crystal structure, we performed molecular dynamics simulations of the SRA:DNA complex with the flipped cytosine base carrying either of these epigenetic modifications. Our data indicate that the SRA binding pocket can accommodate 5hmC and stabilizes the flipped base by hydrogen bond formation with the hydroxyl group.
The processing of lagging-strand intermediates has not been demonstrated in vitro for herpes simplex virus type 1 (HSV-1). Human flap endonuclease-1 (Fen-1) was examined for its ability to produce ligatable products with model lagging-strand intermediates in the presence of the wild-type or exonuclease-deficient (exo−) HSV-1 DNA polymerase (pol). Primer/templates were composed of a minicircle single-stranded DNA template annealed to primers that contained 5′ DNA flaps or 5′ annealed DNA or RNA sequences. Gapped DNA primer/templates were extended but not significantly strand displaced by the wild-type HSV-1 pol, although significant strand displacement was observed with exo− HSV-1 pol. Nevertheless, the incubation of primer/templates containing 5′ flaps with either wild-type or exo− HSV-1 pol and Fen-1 led to the efficient production of nicks that could be sealed with DNA ligase I. Both polymerases stimulated the nick translation activity of Fen-1 on DNA- or RNA-containing primer/templates, indicating that the activities were coordinated. Further evidence for Fen-1 involvement in HSV-1 DNA synthesis is suggested by the ability of a transiently expressed green fluorescent protein fusion with Fen-1 to accumulate in viral DNA replication compartments in infected cells and by the ability of endogenous Fen-1 to coimmunoprecipitate with an essential viral DNA replication protein in HSV-1-infected cells.
While the etiology of Parkinson's disease remains largely elusive, there is accumulating evidence suggesting that mitochondrial dysfunction occurs prior to the onset of symptoms in Parkinson's disease. Mitochondria are remarkably primed to play a vital role in neuronal cell survival since they are key regulators of energy metabolism (as ATP producers), of intracellular calcium homeostasis, of NAD+/NADH ratio, and of endogenous reactive oxygen species production and programmed cell death. In this paper, we focus on mitochondrial dysfunction-mediated alpha-synuclein aggregation. We highlight some of the findings that provide proof of evidence for a mitochondrial metabolism control in Parkinson's disease, namely, mitochondrial regulation of microtubule-dependent cellular traffic and autophagic lysosomal pathway. The knowledge that microtubule alterations may lead to autophagic deficiency and may compromise the cellular degradation mechanisms that culminate in the progressive accumulation of aberrant protein aggregates shields new insights to the way we address Parkinson's disease. In line with this knowledge, an innovative window for new therapeutic strategies aimed to restore microtubule network may be unlocked.
The hypodactylous (hd) locus impairs limb development and spermatogenesis, leading to male infertility in rats. We show that the hd mutation is caused by an insertion of an endogenous retrovirus into intron 10 of the Cntrob gene. The retroviral insertion in hd mutant rats disrupts the normal splicing of Cntrob transcripts and results in the expression of a truncated protein. During the final phase of spermiogenesis, centrobin localizes to the manchette, centrosome, and the marginal ring of the spermatid acroplaxome, where it interacts with keratin 5-containing intermediate filaments. Mutant spermatids show a defective acroplaxome marginal ring and separation of the centrosome from its normal attachment site of the nucleus. This separation correlates with a disruption of head-tail coupling apparatus, leading to spermatid decapitation during the final step of spermiogenesis and the absence of sperm in the epididymis. Cntrob may represent a novel candidate gene for presently unexplained hereditary forms of teratozoospermia and the “easily decapitated sperm syndrome” in humans.
Truncation of centrobin in hd/hd rats causes sperm decapitation.
acroplaxome; centrobin; centrosome; spermatid; spermatogenesis; teratozoospermia