Organization of highly acetylated chromatin around sites of heterogeneous nuclear RNA accumulation

Nuclear Compartments: An Incomplete Primer to Nuclear Compartments, Bodies, and Genome Organization Relative to Nuclear Architecture

This work reviews nuclear compartments, defined broadly to include distinct nuclear structures, bodies, and chromosome domains. It first summarizes original cytological observations before comparing concepts of nuclear compartments emerging from microscopy versus genomic approaches and then introducing new multiplexed imaging approaches that promise in the future to meld both approaches. I discuss how previous models of radial distribution of chromosomes or the binary division of the genome into A and B compartments are now being refined by the recognition of more complex nuclear compartmentalization. The poorly understood question of how these nuclear compartments are established and maintained is then discussed, including through the modern perspective of phase separation, before moving on to address possible functions of nuclear compartments, using the possible role of nuclear speckles in modulating gene expression as an example. Finally, the review concludes with a discussion of future questions for this field.

The solid and liquid states of chromatin

The review begins with a concise description of the principles of phase separation. This is followed by a comprehensive section on phase separation of chromatin, in which we recount the 60 years history of chromatin aggregation studies, discuss the evidence that chromatin aggregation intrinsically is a physiologically relevant liquid-solid phase separation (LSPS) process driven by chromatin self-interaction, and highlight the recent findings that under specific solution conditions chromatin can undergo liquid-liquid phase separation (LLPS) rather than LSPS. In the next section of the review, we discuss how certain chromatin-associated proteins undergo LLPS in vitro and in vivo. Some chromatin-binding proteins undergo LLPS in purified form in near-physiological ionic strength buffers while others will do so only in the presence of DNA, nucleosomes, or chromatin. The final section of the review evaluates the solid and liquid states of chromatin in the nucleus. While chromatin behaves as an immobile solid on the mesoscale, nucleosomes are mobile on the nanoscale. We discuss how this dual nature of chromatin, which fits well the concept of viscoelasticity, contributes to genome structure, emphasizing the dominant role of chromatin self-interaction.

Mapping 3D genome organization relative to nuclear compartments using TSA-Seq as a cytological ruler

Chen et al. present TSA-Seq, a new mapping method that measures cytological distances relative to spatially distinct nuclear subcompartments. From novel nuclear organization maps of human cells, they identify transcription hot zones of high gene density that are near nuclear speckles and enriched in highly expressed genes, housekeeping genes, and genes with low transcriptional pausing.

While nuclear compartmentalization is an essential feature of three-dimensional genome organization, no genomic method exists for measuring chromosome distances to defined nuclear structures. In this study, we describe TSA-Seq, a new mapping method capable of providing a “cytological ruler” for estimating mean chromosomal distances from nuclear speckles genome-wide and for predicting several Mbp chromosome trajectories between nuclear compartments without sophisticated computational modeling. Ensemble-averaged results in K562 cells reveal a clear nuclear lamina to speckle axis correlated with a striking spatial gradient in genome activity. This gradient represents a convolution of multiple spatially separated nuclear domains including two types of transcription “hot zones.” Transcription hot zones protruding furthest into the nuclear interior and positioning deterministically very close to nuclear speckles have higher numbers of total genes, the most highly expressed genes, housekeeping genes, genes with low transcriptional pausing, and super-enhancers. Our results demonstrate the capability of TSA-Seq for genome-wide mapping of nuclear structure and suggest a new model for spatial organization of transcription and gene expression.

Transmission Electron Microscopy Imaging to Analyze Chromatin Density Distribution at the Nanoscale Level

Transmission electron microscopy (TEM) is used to study the fine ultrastructural organization of cells. Delicate specimen preparation is required for results to reflect the "native" ultrastructural organization of subcellular features such as the nucleus. Despite the advent of high-resolution, fluorescent imaging of chromatin components, TEM still provides a unique and complementary level of resolution capturing chromatin organization at the nanoscale level. Here, we describe the workflow, from tissue preparation, TEM image acquisition and image processing, for obtaining a quantitative description of chromatin density distribution in plant cells, informing on local fluctuations and periodicity. Comparative analyses then allow to elucidate the structural changes induced by developmental or environmental cues, or by mutations affecting specific chromatin modifiers at the nanoscale level. We argue that this approach remains affordable and merits a renewed interest by the plant chromatin community.

Chromatin fibers: from classical descriptions to modern interpretation

The first description of intrachromosomal fibers was made by Baranetzky in 1880. Since that time, a plethora of fibrillar substructures have been described inside the mitotic chromosomes, and published data indicate that chromosomes may be formed as a result of the hierarchical folding of chromatin fibers. In this review, we examine the evolution and the current state of research on the morphological organization of mitotic chromosomes.

Sos1 disruption impairs cellular proliferation and viability through an increase in mitochondrial oxidative stress in primary MEFs

Using a 4-hydroxytamoxifen (4OHT)-inducible, conditional Sos1-null mutation, we analyzed wild-type (WT), single Sos1-KO, Sos2-KO and double Sos1/2 KO primary mouse embryonic fibroblasts (MEF) with an aim at evaluating the functional specificity or redundancy of the Sos1 and Sos2 alleles at the cellular level. The 4OHT-induced Sos1-KO and Sos1/2-DKO MEFs exhibited distinct flat morphology, enlarged cell perimeter and altered cytoskeletal organization that were not observed in the WT and Sos2-KO counterparts. The Sos1-KO and Sos1/2-DKO MEFs also displayed significant accumulation, in comparison with WT and Sos2-KO MEFs, of cytoplasmic vesicular bodies identified as autophagosomes containing degraded mitochondria by means of electron microscopy and specific markers. Cellular proliferation and migration were impaired in Sos1-KO and Sos1/2-DKO MEFs in comparison with WT and Sos2-KO MEFs, whereas cell adhesion was only impaired upon depletion of both Sos isoforms. RasGTP formation was practically absent in Sos1/2-DKO MEFs as compared with the other genotypes and extracellular signal-regulated kinase phosphorylation showed only significant reduction after combined Sos1/2 depletion. Consistent with a mitophagic phenotype, in vivo labeling with specific fluorophores uncovered increased levels of oxidative stress (elevated intracellular reactive oxygen species and mitochondrial superoxide and loss of mitochondrial membrane potential) in the Sos1-KO and the Sos1/2-DKO cells as compared with Sos2-KO and WT MEFs. Interestingly, treatment of the MEF cultures with antioxidants corrected the altered phenotypes of Sos1-KO and Sos1/2-DKO MEFs by restoring their altered perimeter size and proliferative rate to levels similar to those of WT and Sos2-KO MEFs. Our data uncover a direct mechanistic link between Sos1 and control of intracellular oxidative stress, and demonstrate functional prevalence of Sos1 over Sos2 with regards to cellular proliferation and viability.

Correlative fluorescence and EFTEM imaging of the organized components of the mammalian nucleus

The cell nucleus contains many distinct subnuclear compartments, domains, and bodies that vary in their composition, structure, and function. While the cellular constituents that occupy the subnuclear regions may be well known, defining the structural details of the molecular assembly of the constituents has been more difficult. A correlative fluorescence and energy-filtering transmission electron microscopy (EFTEM) imaging method has the ability to provide these details. The correlative microscopy method described here allows the tracking of subnuclear structures from specific cells by fluorescence microscopy and then, using electron energy loss imaging in the transmission electron microscope, reveals the ultrastructural features of the nuclear components along with endogenous elemental information that relates directly to the biochemical composition of the structure. The ultrastructural features and composition of well-characterized PML bodies and interchromatin granule clusters are compared to those of ligand-activated glucocorticoid receptor (GR) foci, with GR foci containing fibrogranular nucleic acid-containing features and PML bodies being devoid of nucleic acid.

Nonlinear optical imaging and Raman microspectrometry of the cell nucleus throughout the cell cycle

Fundamental understanding of cellular processes at molecular level is of considerable importance in cell biology as well as in biomedical disciplines for early diagnosis of infection and cancer diseases, and for developing new molecular medicine-based therapies. Modern biophotonics offers exclusive capabilities to obtain information on molecular composition, organization, and dynamics in a cell by utilizing a combination of optical spectroscopy and optical imaging. We introduce here a combination of Raman microspectrometry, together with coherent anti-Stokes Raman scattering (CARS) and two-photon excited fluorescence (TPEF) nonlinear optical microscopy, to study macromolecular organization of the nucleus throughout the cell cycle. Site-specific concentrations of proteins, DNA, RNA, and lipids were determined in nucleoli, nucleoplasmic transcription sites, nuclear speckles, constitutive heterochromatin domains, mitotic chromosomes, and extrachromosomal regions of mitotic cells by quantitative confocal Raman microspectrometry. A surprising finding, obtained in our study, is that the local concentration of proteins does not increase during DNA compaction. We also demonstrate that postmitotic DNA decondensation is a gradual process, continuing for several hours. The quantitative Raman spectroscopic analysis was corroborated with CARS/TPEF multimodal imaging to visualize the distribution of protein, DNA, RNA, and lipid macromolecules throughout the cell cycle.

Cajal's contribution to the knowledge of the neuronal cell nucleus

In 1906, the Spanish neurobiologist Santiago Ramón y Cajal was awarded the Nobel Prize in Physiology or Medicine in recognition of his work on the structure of neurons and their connections. Cajal is commonly regarded as the father of modern neuroscience. What is less well known is that Cajal also had a great interest in intracellular neuronal structures and developed the reduced silver nitrate method for the study of neurofibrils (neurofilaments) and nuclear subcompartments. It was in 1903 that Cajal discovered the "accessory body" ("Cajal body") and seven years later, published an article on the organization of the cell nucleus in mammalian neurons that represents a masterpiece of nuclear structure at the light microscopy level. In addition to the accessory body, it includes the analysis of several nuclear components currently recognized as fibrillar centers of the nucleolus, nuclear speckles of splicing factors, transcription foci, nuclear matrix, and the double nuclear membrane. The aim of this article is to revisit Cajal's contributions to the knowledge of the neuronal nucleus in light of our current understanding of nuclear structure and function.

Elucidating chromatin and nuclear domain architecture with electron spectroscopic imaging

Electron microscopy has been the 'gold standard' of spatial resolution for studying the structure of the cell nucleus. Electron spectroscopic imaging (ESI) offers advantages over conventional transmission electron microscopy by eliminating the need for heavy-atom contrast agents. ESI also provides mass-dependent and element-specific information at high resolution, permitting the distinguishing of structures that are primarily composed of protein, DNA, or RNA. The technique can be applied to understand the structural consequences of epigenetic modifications, such as modified histones, on chromatin fiber morphology. ESI can also be applied to elucidate the multifunctional behavior of subnuclear 'organelles' such as the nucleolus and promyelocytic leukemia nuclear bodies.

Cajal body number and nucleolar size correlate with the cell body mass in human sensory ganglia neurons

This paper studies the cell size-dependent organization of the nucleolus and Cajal bodies (CBs) in dissociated human dorsal root ganglia (DRG) neurons from autopsy tissue samples of patients without neurological disease. The quantitative analysis of nucleoli with an anti-fibrillarin antibody showed that all neurons have only one nucleolus. However, the nucleolar volume and the number of fibrillar centers per nucleolus significantly increase as a function of cell body size. Immunostaining for coilin demonstrated the presence of numerous CBs in DRG neurons (up to 20 in large size neurons). The number of CBs per neuron correlated positively with the cell body volume. Light and electron microscopy immunocytochemical analysis revealed the concentration of coilin, snRNPs, SMN and fibrillarin in CBs of DRG neurons. CBs were frequently associated with the nucleolus, active chromatin domains and PML bodies, but not with telomeres. Our results support the view that the nucleolar volume and number of both fibrillar centers and CBs depend on the cell body mass, a parameter closely related to transcriptional and synaptic activity in mammalian neurons. Moreover, the unusual large number of CBs could facilitate the transfer of RNA processing components from CBs to nucleolar and nucleoplasmic sites of RNA processing.

The transcriptional regulator CBP has defined spatial associations within interphase nuclei

It is becoming increasingly clear that nuclear macromolecules and macromolecular complexes are compartmentalized through binding interactions into an apparent three-dimensionally ordered structure. This ordering, however, does not appear to be deterministic to the extent that chromatin and nonchromatin structures maintain a strict 3-D arrangement. Rather, spatial ordering within the cell nucleus appears to conform to stochastic rather than deterministic spatial relationships. The stochastic nature of organization becomes particularly problematic when any attempt is made to describe the spatial relationship between proteins involved in the regulation of the genome. The CREB-binding protein (CBP) is one such transcriptional regulator that, when visualised by confocal microscopy, reveals a highly punctate staining pattern comprising several hundred individual foci distributed within the nuclear volume. Markers for euchromatic sequences have similar patterns. Surprisingly, in most cases, the predicted one-to-one relationship between transcription factor and chromatin sequence is not observed. Consequently, to understand whether spatial relationships that are not coincident are nonrandom and potentially biologically important, it is necessary to develop statistical approaches. In this study, we report on the development of such an approach and apply it to understanding the role of CBP in mediating chromatin modification and transcriptional regulation. We have used nearest-neighbor distance measurements and probability analyses to study the spatial relationship between CBP and other nuclear subcompartments enriched in transcription factors, chromatin, and splicing factors. Our results demonstrate that CBP has an order of spatial association with other nuclear subcompartments. We observe closer associations between CBP and RNA polymerase II-enriched foci and SC35 speckles than nascent RNA or specific acetylated histones. Furthermore, we find that CBP has a significantly higher probability of being close to its known in vivo substrate histone H4 lysine 5 compared with the closely related H4 lysine 12. This study demonstrates that complex relationships not described by colocalization exist in the interphase nucleus and can be characterized and quantified. The subnuclear distribution of CBP is difficult to reconcile with a model where chromatin organization is the sole determinant of the nuclear organization of proteins that regulate transcription but is consistent with a close link between spatial associations and nuclear functions.

Androgen receptor regulates nuclear trafficking and nuclear domain residency of corepressor HDAC7 in a ligand-dependent fashion

In addition to chromosomal proteins, histone deacetylases (HDACs) target transcription factors in transcriptional repression. Here, we show that the class II HDAC family member HDAC7 is an efficient corepressor of the androgen receptor (AR). HDAC7 resided in the cytoplasm in the absence of AR or a cognate ligand, but hormone-occupancy of AR induced nuclear transfer of HDAC7. Nuclear colocalization pattern of AR and HDAC7 was dependent on the nature of the ligand. In the presence of testosterone, a portion of HDAC7 localized to pearl-like nuclear domains, whereas AR occupied with antagonistic ligands cyproterone acetate- or casodex (bicalutamide) recruited HDAC7 from these domains to colocalize with the receptor in speckles and nucleoplasm in a more complete fashion. Ectopic expression of PML-3 relieved the repressive effect of HDAC7 on AR function by sequestering HDAC7 to PML-3 domains. AR acetylation at Lys630/632/633 was not the target of HDAC7 repression, since repression of AR function was independent of these acetylation sites. Moreover, the deacetylase activity of HDAC7 was in part dispensable in the repression of AR function. In sum, our results identify HDAC7 as a novel AR corepressor whose subcellular and subnuclear compartmentalization can be regulated in an androgen-selective manner.

Pre-neurodegeneration of mitral cells in the pcd mutant mouse is associated with DNA damage, transcriptional repression, and reorganization of nuclear speckles and Cajal bodies

DNA damage and impairment of its repair underlie several neurodegenerative diseases. The Purkinje cell degeneration (pcd) mutation causes the loss of Nna1 expression and is associated with a selective and progressive degeneration of specific neuronal populations, including mitral cells in the olfactory bulb. Using an in situ transcription assay, molecular markers for both nuclear compartments and components of the DNA damage/repair pathway, and ultrastructural analysis, here we demonstrate that the pcd mutation induces the formation of DNA damage/repair foci in mitral cells. Furthermore, this effect is associated with transcriptional inhibition, heterochromatinization, nucleolar segregation and the reorganization of nuclear speckles of splicing factors and Cajal bodies. The most significant cytoplasmic alteration observed was a partial replacement of rough endoplasmic reticulum cisternae by a larger amount of free ribosomes, while other organelles were structurally preserved. The tools employed in this work may be of use for the early detection of predegenerative processes in neurodegenerative disorders and for validating rescue strategies.

Nuclear localization of cytokeratin 8 and the O-linked N-acetylglucosamine-containing epitope H in epithelial cells of infiltrating ductal breast carcinomas: a combination of immunogold and EDTA regressive staining methods

In a previous study, the authors have shown cytokeratin 8 (CK8) and epitope H ultrastructural localization in breast cancer cell nuclei. Epitope H contains an O-linked N-acetylglucosamine (O-GlcNAc) residue in a specific conformation and/or environment recognized by monoclonal antibody H. In this study, double immunogold labeling of CK8 and epitope H combined with the EDTA regressive staining method was applied in biopsy material from infiltrating ductal breast carcinomas and fibroadenomas, to localize both antigens in correlation to RNPs distribution in the nuclear subcompartments of cancer cells. CK8 and epitope H were localized mostly over condensed chromatin, whereas staining was weaker over interchromatin granule clusters and perichromatin fibers. These results revealed, the distribution of CK8 in the nucleus as MAR-binding protein, contributing in the organization of the nuclear DNA in the neoplastic cell, as well as the distribution of O-GlcNAc glycosylated polypeptides bearing the epitope H. The latter finding indicates that these polypeptides might play a significant role in the neoplastic behavior of breast cancer cells because they colocalize in the same nuclear subcompartments with proteins modified by O-GlcNAc, such as hnRNPs G and A1, RNA polymerase II, its transcription factors, and the oncogene product of c-myc. These proteins are known to participate in coordinated transcription/RNA processing events, contributing in the neoplastic behavior of breast cancer cells.

Chromatin modification of the trefoil factor 1 gene in human breast cancer cells by the Ras/mitogen-activated protein kinase pathway

Histone H3 phosphorylation is a downstream response to activation of the Ras/mitogen-activated protein kinase (MAPK) pathway. This modification is thought to have a role in chromatin remodeling and in the initiation of gene transcription. In MCF-7 breast cancer cells, we observed that phosphorylated histone H3 (phospho-H3) at Ser(10) but not Ser(28) increased with phorbol ester (12-O-tetradecanoylphorbol-13-acetate, TPA) treatment. Although phosphorylated extracellular signal-regulated kinase 1/2 levels in these cells cultured under estradiol deplete and replete conditions displayed no change, a significant induction was observed after TPA treatment. Furthermore, whereas both estradiol and TPA increased trefoil factor 1 (TFF1) mRNA levels in these cells, only TPA-induced and not estradiol-induced TFF1 expression was inhibited by the H3 kinase mitogen and stress activated protein kinase (MSK) inhibitor H89 and MAPK kinase inhibitor UO126, showing the involvement of the Ras/MAPK following TPA induction. Mutation of the activator protein 1 (AP-1) binding site abrogated the TPA-induced transcriptional response of the luciferase reporter gene under the control of the TFF1 promoter, showing the requirement for the AP-1 site. In chromatin immunoprecipitation assays, estradiol treatment resulted in the association of the estrogen receptor-alpha (ERalpha) and acetylated H3 with the TFF1 promoter. The levels of phospho-H3 and MSK1 associated with the TFF1 promoter were moderately increased. In the presence of TPA, whereas ERalpha was not bound to the promoter, a strong association of acetylated and/or phospho-H3, MSK1, and c-Jun was observed. These results show that although both stimuli lead to TFF1 gene activation, estradiol and TPA exert their effects on TFF1 gene expression by different mechanisms.

Use of a chemically modified antisense oligonucleotide library to identify and validate Eg5 (kinesin-like 1) as a target for antineoplastic drug development

A library of 2'-methoxyethyl-modified antisense oligonucleotides (2'MOE ASO) targeting 1,510 different genes has been developed, validated, and used to identify cell cycle regulatory genes. The most effective molecular target identified was Eg5 (kinesin-like-1), which when inhibited gave the largest increase in 4N DNA in various tumor cells. The Eg5 ASO reduced Eg5 levels, inhibited proliferation, increased apoptosis, and altered the expression of other cell cycle proteins, including survivin and Aurora-A. To examine the therapeutic utility of the Eg5 ASO, the compound was also evaluated in xenograft models. Treatment with Eg5 ASO produced a statistically significant reduction of tumor growth, reduction in Eg5 expression in the tumors, and changes in histone phosphorylation, consistent with a loss of Eg5 protein expression. These data show, for the first time, the utility of a 2'MOE ASO library for high-throughput cell culture-based functional assays and suggest that an Eg5 ASO also has potential in a therapeutic strategy.

Modelling the compartmentalization of splicing factors

Splicing factor (SF) compartments, also known as speckles, are heterogeneously distributed compartments within the nucleus of eukaryotic cells that are enriched in pre-mRNA SFs. We derive a fourth-order aggregation-diffusion model that describes a possible mechanism underlying the organization of SFs into speckles. The model incorporates two hypotheses, namely (1) that self-organization of dephosphorylated SFs, modulated by a phosphorylation-dephosphorylation cycle, is responsible for the formation and disappearance of speckles, and (2) that an underlying nuclear structure plays a major role in the organization of SFs. A linear stability analysis about homogeneous steady-state solutions of the model reveals how the self-interaction among dephosphorylated SFs can result in the onset of spatial patterns. A detailed bifurcation analysis of the model describes how phosphorylation and dephosphorylation modulate the onset of the compartmentalization of SFs.

Histone acetylation increases chromatin accessibility

In eukaryotes, the interaction of DNA with proteins and supramolecular complexes involved in gene expression is controlled by the dynamic organization of chromatin inasmuch as it defines the DNA accessibility. Here, the nuclear distribution of microinjected fluorescein-labeled dextrans of 42 kDa to 2.5 MDa molecular mass was used to characterize the chromatin accessibility in dependence on histone acetylation. Measurements of the fluorescein-dextran sizes were combined with an image correlation spectroscopy analysis, and three different interphase chromatin condensation states with apparent pore sizes of 16-20 nm, 36-56 nm and 60-100 nm were identified. A reversible change of the chromatin conformation to a uniform 60-100 nm pore size distribution was observed upon increased histone acetylation. This result identifies histone acetylation as a central factor in the dynamic regulation of chromatin accessibility during interphase. In mitotic chromosomes, the chromatin exclusion limit was 10-20 nm and independent of the histone acetylation state.

Telomere-surrounding regions are transcription-permissive 3D nuclear compartments in human cells

Positioning of genes relative to nuclear heterochromatic compartments is thought to help regulate their transcriptional activity. Given that human subtelomeric regions are rich in highly expressed genes, we asked whether human telomeres are related to transcription-permissive nuclear compartments. To address this question, we investigated in the nuclei of normal human lymphocytes the spatial relations of two constitutively expressed genes (ACTB and RARA) and three nuclear transcripts (ACTB, IL2RA and TCRB) to telomeres and centromeres, as a function of gene activity and transcription levels. We observed that genes and gene transcripts locate close to telomere clusters and away from chromocenters upon activation of transcription. These findings, together with the observation that SC35 domains, which are enriched in pre-mRNA processing factors, are in close proximity to telomeres, indicate that telomere-neighboring regions are permissive to gene expression in human cells. Therefore, the associations of telomeres observed in the interphase nucleus might contribute, as opposed to chromocenters, for the establishment of transcription-permissive 3D nuclear compartments.

Distinct dynamics and distribution of histone methyl-lysine derivatives in mouse development

Histone methylation acts as an epigenetic regulator of chromatin activity through the modification of arginine and lysine residues on histones H3 and H4. In the case of lysine, this includes the formation of mono-, di-, or trimethyl groups, each of which is presumed to represent a distinct functional state at the cellular level. To examine the potential developmental roles of these modifications, we determined the global patterns of lysine methylation involving K9 on histone H3 and K20 on histone H4 in midgestation mouse embryos. For each lysine target site, we observed distinct subnuclear distributions of the mono- and trimethyl versions in 10T1/2 cells that were conserved within primary cultures and within the 3D-tissue architecture of the embryo. Interestingly, three of these modifications, histone H3 trimethyl K9, histone H4 monomethyl K20, and histone H4 trimethyl K20 exhibited marked differences in their distribution within the neuroepithelium. Specifically, both histone H3 trimethyl K9 and H4 monomethyl K20 were elevated in proliferating cells of the neural tube, which in the case of the K9 modification was limited to mitotic cells on the luminal surface. In contrast, histone H4 trimethyl K20 was progressively lost from these medial regions and became enriched in differentiating neurons in the ventrolateral neural tube. The inverse relationship of histone H4 K20 methyl derivatives is even more striking during skeletal and cardiac myogenesis where the accumulation of the trimethyl modification in pericentromeric heterochromatin suggests a role in gene silencing in postmitotic muscle cells. Importantly, our results establish that histone lysine methylation occurs in a highly dynamic manner that is consistent with their function in an epigenetic program for cell division and differentiation.

Nuclear Matrix Interactions at the Human Protamine Domain

The compact eukaryotic genome must be selectively opened to grant trans-factor access to cis-regulatory elements to overcome the primary barrier to gene transcription. The mechanism that governs the selective opening of chromatin domains (i.e. potentiation) remains poorly understood. In the absence of a well defined locus control region, the nuclear matrix is considered the primary candidate regulating the opening of the multigenic PRM1 --> PRM2 --> TNP2 human protamine domain. To directly examine its role, four lines of transgenic mice with different configurations of flanking nuclear matrix attachment regions (MARs) encompassing the protamine domain were created. We show that upon removal of the MARs, the locus becomes subject to position effects. The 3' MAR alone may be sufficient to protect against silencing. In concert, the MARs bounding this domain likely synergize to regulate the expression of the various members of this gene cluster. Interestingly, the MARs may convey a selective reproductive advantage, such that constructs bearing both 5' and 3' MARs are passed to their offspring with greater frequency. Thus, the MARs bounding the PRM1 --> PRM2 --> TNP2 protamine domain have many and varied functions.

STAT3 is enriched in nuclear bodies

Signal transducer and activator of transcription 3 (STAT3) is a transcription factor that is involved in a variety of biological functions. It is essential for the signal transduction of interleukin-6 (IL-6) and related cytokines. In response to IL-6 stimulation STAT3 becomes phosphorylated and translocates into the nucleus where it binds to enhancer sequences of target genes. We found that activated STAT3 is enriched in dot-like structures within the nucleus, which we termed STAT3 nuclear bodies. To examine the dynamics of STAT3 nuclear body formation, a fusion protein of STAT3 and yellow fluorescent protein (YFP) was constructed. Studies in living cells have shown that the appearance of STAT3 nuclear bodies is transient, correlating with the timecourse of tyrosine-phosphorylation of STAT3. Furthermore, we show by fluorescence recovery after photobleaching (FRAP) analysis that STAT3 within nuclear bodies consists of a highly mobile and an immobile fraction. Colocalization studies provided evidence that these bodies are accompanied with CREB binding protein (CBP) and acetylated histone H4, which are markers for transcriptionally active chromatin. Moreover, STAT3 nuclear bodies in HepG2 cells are not colocalized with promyelocytic leukemia oncoprotein (PML)-containing bodies; neither is a sumoylation of activated STAT3 detectable. Taken together, our data suggest that STAT3 nuclear bodies are either directly involved in active gene transcription or they serve as reservoirs of activated STAT3.

Diffusion-limited compartmentalization of mammalian cell nuclei assessed by microinjected macromolecules

In order to investigate the accessibility of the nucleoplasm for macromolecules with different physical properties, we microinjected FITC-conjugated dextrans of different sizes as well as anionic FITC-dextrans and FITC-poly-L-lysine into mammalian cell nuclei. Small dextrans displayed a homogeneous nuclear distribution. With increasing molecular mass (42 to 2500 kDa), FITC-dextrans were progressively excluded from chromatin regions, accumulating in and thereby outlining an apparently extended interchromatin space. Anionic FITC-dextrans (500 kDa) showed complete exclusion from labeled chromatin regions, while the positively charged FITC-poly-L-lysine was to some extent present within the chromatin regions. Moreover, the FITC-poly-L-lysine preferentially localized at the nuclear periphery. We also found a size-dependent exclusion of FITC-dextrans from nucleoli regions, while the FITC-poly-L-lysine accumulated in the nucleoli. Thus, the distinct and restricted nuclear accessibility for macromolecules is dependent on molecule size and electrical charge.

Quantitative analysis of CBP- and P300-induced histone acetylations in vivo using native chromatin

In vivo, histone tails are involved in numerous interactions, including those with DNA, adjacent histones, and other, nonhistone proteins. The amino termini are also the substrates for a number of enzymes, including histone acetyltransferases (HATs), histone deacetylases, and histone methyltransferases. Traditional biochemical approaches defining the substrate specificity profiles of HATs have been performed using purified histone tails, recombinant histones, or purified mononucleosomes as substrates. It is clear that the in vivo presentation of the substrate cannot be accurately represented by using these in vitro approaches. Because of the difficulty in translating in vitro results into in vivo situations, we developed a novel single-cell HAT assay that provides quantitative measurements of endogenous HAT activity. The HAT assay is performed under in vivo conditions by using the native chromatin structure as the physiological substrate. The assay combines the spatial resolving power of laser scanning confocal microscopy with simple statistical analyses to characterize CREB binding protein (CBP)- and P300-induced changes in global histone acetylation levels at specific lysine residues. Here we show that CBP and P300 exhibit unique substrate specificity profiles, consistent with the developmental and functional differences between the two HATs.

Clustering of multiple specific genes and gene-rich R-bands around SC-35 domains: evidence for local euchromatic neighborhoods

Typically, eukaryotic nuclei contain 10-30 prominent domains (referred to here as SC-35 domains) that are concentrated in mRNA metabolic factors. Here, we show that multiple specific genes cluster around a common SC-35 domain, which contains multiple mRNAs. Nonsyntenic genes are capable of associating with a common domain, but domain "choice" appears random, even for two coordinately expressed genes. Active genes widely separated on different chromosome arms associate with the same domain frequently, assorting randomly into the 3-4 subregions of the chromosome periphery that contact a domain. Most importantly, visualization of six individual chromosome bands showed that large genomic segments ( approximately 5 Mb) have striking differences in organization relative to domains. Certain bands showed extensive contact, often aligning with or encircling an SC-35 domain, whereas others did not. All three gene-rich reverse bands showed this more than the gene-poor Giemsa dark bands, and morphometric analyses demonstrated statistically significant differences. Similarly, late-replicating DNA generally avoids SC-35 domains. These findings suggest a functional rationale for gene clustering in chromosomal bands, which relates to nuclear clustering of genes with SC-35 domains. Rather than random reservoirs of splicing factors, or factors accumulated on an individual highly active gene, we propose a model of SC-35 domains as functional centers for a multitude of clustered genes, forming local euchromatic "neighborhoods."

Preferential nuclear localization of the human papillomavirus type 16 E6 oncoprotein in cervical carcinoma cells

The E6 protein of the high-risk human papillomavirus type 16 (HPV-16) is involved in the tumorigenesis of human cervical cells by targeting numerous cellular proteins. We characterized new anti-E6 monoclonal antibodies and used them for precise localization of the E6 oncoprotein within carcinoma cells. Overexpressed E6 protein was predominantly detected in the nucleus of transiently transfected HaCaT cells. While mostly localized at the periphery of condensed chromatin, E6 was also associated with nuclear ribonucleoproteic ultrastructures and with some ribosomal areas in the cytoplasm of SiHa and CaSki cells. The chimeric beta-galactosidase-E6 protein expressed in transfected HeLa cells was essentially localized in the nuclear compartment. Together, these data indicate that the E6 sequence of HPV-16 may encode a nuclear localization signal. The preferential nuclear distribution of this viral oncoprotein in HPV-transformed cells correlates with its activities at the transcriptional level.

The Dynamics of Chromosome Organization and Gene Regulation

With the sequence of the human genome now complete, studies must focus on how the genome is functionally organized within the confines of the cell nucleus and the dynamic interplay between the genome and its regulatory factors to effectively control gene expression and silencing. In this review I describe our current state of knowledge with regard to the organization of chromosomes within the nucleus and the positioning of active versus inactive genes. In addition, I discuss studies on the dynamics of chromosomes and specific genetic loci within living cells and its relationship to gene activity and the cell cycle. Furthermore, our current understanding of the distribution and dynamics of RNA polymerase II transcription factors is discussed in relation to chromosomal loci and other nuclear domains.

Visualizing chromatin and chromosomes in living cells

The dynamic organization of eukaryotic genomes in cell nuclei recently came into the focus of research interest. The kinetics of genome dynamics can be addressed only by approaches involving live cell microscopy. Different methods are available to visualize chromatin, specific chromatin fractions, or individual chromosome territories within nuclei of living mammalian cells. Appropriate labeling procedures as well as cell chamber systems and important controls for live cell microscopy are described.

Differential acetylation of histones H3 and H4 in paternal and maternal germline chromosomes during development of sciarid flies

A classic example of chromosome elimination and genomic imprinting is found in sciarid flies (Diptera. Sciaridae), where whole chromosomes of exclusively paternal origin are discarded from the genome at different developmental stages. Two types of chromosome elimination event occur in the germline. In embryos of both sexes, the extrusion of a single paternal X chromosome occurs in early germ nuclei and in male meiotic cells the whole paternal complement is discarded. In sciarids, early germ nuclei remain undivided for a long time and exhibit a high degree of chromatin compaction, so that chromosomes are cytologically individualized. We investigated chromatin differences between parental chromosomes in Sciara ocellaris and S. coprophila by analyzing histone acetylation modifications in early germ nuclei. We examined germ nuclei from early embryonic stages to premeiotic larval stages, male meiotic cell and early somatic nuclei following fertilization. In early germ cells, only half of the regular chromosome complement is highly acetylated for histones H4 and H3. The chromosomes that are highly acetylated are paternally derived. An exception is the paternal X chromosome that is eliminated from germ nuclei. At later stages preceding the initiation of mitotic gonial divisions, all chromosomes of the germline complement show similar high levels of histone H4/H3 acetylation. In male meiosis, maternal chromosomes are highly acetylated for histones H4 and H3, whereas the entire paternal chromosome set undergoing elimination appears under-acetylated. The results suggest that histone acetylation contributes towards specifying the imprinted behavior of germline chromosomes in sciarids.

Nuclear organization in differentiating oligodendrocytes

Many studies have suggested that the 3D organization of chromatin and proteins within the nucleus contributes to the regulation of gene expression. We tested multiple aspects of this nuclear organization model within a primary cell culture system. Oligodendrocyte lineage cells were examined to facilitate analysis of nuclear organization relative to a highly expressed tissue-specific gene, proteolipid protein (PLP), which exhibits transcriptional upregulation during differentiation from the immature progenitor stage to the mature oligodendrocyte stage. Oligodendrocyte lineage cells were isolated from brains of neonatal male rodents, and differentiation from oligodendrocyte progenitors to mature oligodendrocytes was controlled with culture conditions. Genomic in situ hybridization was used to detect the single copy of the X-linked PLP gene within each interphase nucleus. The PLP gene was not randomly distributed within the nucleus, but was consistently associated with the nuclear periphery in both progenitors and differentiated oligodendrocytes. PLP and a second simultaneously upregulated gene, the myelin basic protein (MBP) gene, were spatially separated in both progenitors and differentiated oligodendrocytes. Increased transcriptional activity of the PLP gene in differentiated oligodendrocytes corresponded with local accumulation of SC35 splicing factors. Differentiation did not alter the frequency of association of the PLP gene with domains of myelin transcription factor 1 (Myt1), which binds the PLP promoter. In addition to our specific findings related to the PLP gene, these data obtained from primary oligodendrocyte lineage cells support a nuclear organization model in which (1). nuclear proteins and genes can exhibit specific patterns of distribution within nuclei, and (2). activation of tissue-specific genes is associated with changes in local protein distribution rather than spatial clustering of coordinately regulated genes. This nuclear organization may be critical for complex nucleic-acid-protein interactions controlling normal cell development, and may be an important factor in aberrant regulation of cell differentiation and gene expression in transformed cells.

Large-scale chromatin organization and the localization of proteins involved in gene expression in human cells

Compartmentalization of the interphase nucleus is an important element in the regulation of gene expression. Here we investigated the functional organization of the interphase nucleus of HeLa cells and primary human fibroblasts. The spatial distribution of proteins involved in transcription (TFIIH and RNA polymerase II) and RNA processing and packaging (hnRNP-U) were analyzed in relation to chromosome territories and large-scale chromatin organization. We present evidence that these proteins are present predominantly in the interchromatin space, inside and between chromosome territories, and are largely excluded by domains of condensed chromatin. We show that they are present throughout the active and inactive X-chromosome territories in primary female fibroblasts, indicating that these proteins can freely diffuse throughout the interchromatin compartment in the interphase nucleus. Furthermore, we established that the in vivo spatial distribution of condensed chromatin in the interphase nucleus does not depend on ongoing transcription. Our data support a conceptually simple model for the functional organization of interphase nuclei.

CEP110 and ninein are located in a specific domain of the centrosome associated with centrosome maturation

The mammalian centrosome consists of a pair of centrioles surrounded by pericentriolar material (PCM). The architecture and composition of the centrosome, especially the PCM, changes during the cell cycle. Recently, a subset of PCM proteins have been shown to be arranged in a tubular conformation with an open and a closed end within the centrosome. The presence of such a specific configuration can be used as a landmark for mapping proteins in both a spatial and a temporal fashion. Such mapping studies can provide information about centrosome organization, protein dynamics,protein-protein interactions as well as protein function. In this study, the centrosomal proteins CEP110 and ninein were mapped in relationship to the tubular configuration. Both proteins were found to exhibit a similar distribution pattern. In the mother centrosome, they were found at both ends of the centrosome tube, including the site of centrosome duplication. However,in the daughter centrosome they were present only at the closed end. At the closed end of the mother and daughter centrosome tube, both CEP110 and ninein co-localized with the centriolar protein CEP250/c-Nap1, which confirms ninein's centriole association and places CEP110 in association with this structure. Importantly, the appearance of CEP110 and ninein at the open end of the daughter centrosome occurred during the telophase-G1 transition of the next cell cycle, concomitant with the maturation of the daughter centrosome into a mother centrosome. Microinjection of antibodies against either CEP110 or ninein into metaphase HeLa cells disrupted the reformation of the tubular conformation of proteins within the centrosome following cell division and consequently led to dispersal of centrosomal material throughout the cytosol. Further, microinjection of antibodies to either CEP110 or ninein into metaphase PtK2 cells not only disrupted the tubular configuration within the centrosome but also affected the centrosome's ability to function as a microtubule organizing center (MTOC). This MTOC function was also disrupted when the antibodies were injected into postmitotic cells. Taken together, our results indicate that: (1) a population of CEP110 and ninein is located in a specific domain within the centrosome, which corresponds to the open end of the centrosome tube and is the site of protein addition associated with maturation of a daughter centrosome into a mother centrosome; and (2) the addition of CEP110 and ninein are essential for the reformation of specific aspects of the interphase centrosome architecture following mitosis as well as being required for the centrosome to function as a MTOC.

FBI-1 can stimulate HIV-1 Tat activity and is targeted to a novel subnuclear domain that includes the Tat-P-TEFb-containing nuclear speckles

FBI-1 is a cellular POZ-domain-containing protein that binds to the HIV-1 LTR and associates with the HIV-1 transactivator protein Tat. Here we show that elevated levels of FBI-1 specifically stimulate Tat activity and that this effect is dependent on the same domain of FBI-1 that mediates Tat-FBI-1 association in vivo. FBI-1 also partially colocalizes with Tat and Tat's cellular cofactor, P-TEFb (Cdk9 and cyclin T1), at the splicing-factor-rich nuclear speckle domain. Further, a less-soluble population of FBI-1 distributes in a novel peripheral-speckle pattern of localization as well as in other nuclear regions. This distribution pattern is dependent on the FBI-1 DNA binding domain, on the presence of cellular DNA, and on active transcription. Taken together, these results suggest that FBI-1 is a cellular factor that preferentially associates with active chromatin and that can specifically stimulate Tat-activated HIV-1 transcription.

Enzymatic activity associated with class II HDACs is dependent on a multiprotein complex containing HDAC3 and SMRT/N-CoR

Histone deacetylases (HDACs) play a key role in regulating eukaryotic gene expression. The HDAC domain, homologous to the yeast repressors RPD3 and HDA1, is considered necessary and sufficient for enzymatic activity. Here, we show that the catalytic domain of HDAC4 interacts with HDAC3 via the transcriptional corepressor N-CoR/SMRT. All experimental conditions leading to the suppression of HDAC4 binding to SMRT/N-CoR and to HDAC3 result in the loss of enzymatic activity associated with HDAC4. In vitro reconstitution experiments indicate that HDAC4 and other class II HDACs are inactive in the context of the SMRT/N-CoR-HDAC3 complex and do not contribute to its enzymatic activity. These observations indicate that class II HDACs regulate transcription by bridging the enzymatically active SMRT/N-CoR-HDAC3 complex and select transcription factors independently of any intrinsic HDAC activity.

CCAAT/enhancer-binding protein alpha alters histone H3 acetylation at large subnuclear domains

Transcriptional regulation is commonly associated with local levels of histone acetylation, which controls chromatin structure at specific genes or within contiguous chromosomal domains. Less well understood are the higher order determinants of histone acetylation. The transcription factor, CCAAT/enhancer-binding protein alpha (C/EBPalpha), concentrates at one higher order structure, the peri-centromeric chromatin, and regulates differentiation in many cell types, including pituitary cells. We used quantitative fluorescence microscopy to show that immunostained acetylated histone H3 is relatively absent from peri-centromeric domains visible as large structures in mouse pituitary progenitor GHFT1-5 cells. GHFT1-5 cells do not contain C/EBPalpha. We observed that expression of C/EBPalpha in GHFT1-5 cells leads to an increased level of acetylated histone H3, but not acetylated histone H4, at the peri-centromeric domains. Only transcriptionally active forms of C/EBPalpha altered histone acetylation at the peri-centromeric domain. The altered state of histone acetylation at large intranuclear domains may complement, counteract, or supersede the more gene-local activities of other transcription factors to coordinate C/EBPalpha-induced cellular differentiation.

Regulation of global acetylation in mitosis through loss of histone acetyltransferases and deacetylases from chromatin

Histone acetylation, a reversible modification of the core histones, is widely accepted to be involved in remodeling chromatin organization for genetic reprogramming. Histone acetylation is a dynamic process that is regulated by two classes of enzymes, the histone acetyltransferases (HATs) and histone deacetylases (HDACs). Although promoter-specific acetylation and deacetylation has received most of the recent attention, it is superimposed upon a broader acting and dynamic acetylation that profoundly affects many nuclear processes. In this study, we monitored this broader histone acetylation as cells enter and exit mitosis. In contrast to the hypothesis that HATs and HDACs remain bound to mitotic chromosomes to provide an epigenetic imprint for postmitotic reactivation of the genome, we observed that HATs and HDACs are spatially reorganized and displaced from condensing chromosomes as cells progress through mitosis. During mitosis, HATs and HDACs are unable to acetylate or deacetylate chromatin in situ despite remaining fully catalytically active when isolated from mitotic cells and assayed in vitro. Our results demonstrate that HATs and HDACs do not stably bind to the genome to function as an epigenetic mechanism of selective postmitotic gene activation. Our results, however, do support a role for spatial organization of these enzymes within the cell nucleus and their relationship to euchromatin and heterochromatin postmitotically in the reactivation of the genome.

Human STAGA complex is a chromatin-acetylating transcription coactivator that interacts with pre-mRNA splicing and DNA damage-binding factors in vivo

GCN5 is a histone acetyltransferase (HAT) originally identified in Saccharomyces cerevisiae and required for transcription of specific genes within chromatin as part of the SAGA (SPT-ADA-GCN5 acetylase) coactivator complex. Mammalian cells have two distinct GCN5 homologs (PCAF and GCN5L) that have been found in three different SAGA-like complexes (PCAF complex, TFTC [TATA-binding-protein-free TAF(II)-containing complex], and STAGA [SPT3-TAF(II)31-GCN5L acetylase]). The composition and roles of these mammalian HAT complexes are still poorly characterized. Here, we present the purification and characterization of the human STAGA complex. We show that STAGA contains homologs of most yeast SAGA components, including two novel human proteins with histone-like folds and sequence relationships to yeast SPT7 and ADA1. Furthermore, we demonstrate that STAGA has acetyl coenzyme A-dependent transcriptional coactivator functions from a chromatin-assembled template in vitro and associates in HeLa cells with spliceosome-associated protein 130 (SAP130) and DDB1, two structurally related proteins. SAP130 is a component of the splicing factor SF3b that associates with U2 snRNP and is recruited to prespliceosomal complexes. DDB1 (p127) is a UV-damaged-DNA-binding protein that is involved, as part of a complex with DDB2 (p48), in nucleotide excision repair and the hereditary disease xeroderma pigmentosum. Our results thus suggest cellular roles of STAGA in chromatin modification, transcription, and transcription-coupled processes through direct physical interactions with sequence-specific transcription activators and with components of the splicing and DNA repair machineries.

Human HDAC7 histone deacetylase activity is associated with HDAC3 in vivo

Histone deacetylases (HDACs) are part of transcriptional corepressor complexes and play key roles in regulating chromatin structure. Three different classes of human HDACs have been defined based on their homology to HDACs found in Saccharomyces cerevisiae: RPD3 (class I), HDA1 (class II), and SIR2 (class III). Here we describe the identification and functional characterization of HDAC7, a new member of the human class II HDAC family. Although HDAC7 is localized mostly to the cell nucleus, it is also found in the cytoplasm, suggesting nucleocytoplasmic shuttling. The HDAC activity of HDAC7 maps to a carboxyl-terminal domain and is dependent on the interaction with the class I HDAC, HDAC3, in the cell nucleus. Cytoplasmic HDAC7 that is not bound to HDAC3 is enzymatically inactive. We provide evidence that the transcriptional corepressors SMRT and N-CoR could serve as critical mediators of HDAC7 activity by binding class II HDACs and HDAC3 by two distinct repressor domains. Different class II HDACs reside in the cell nucleus in stable and autonomous complexes with enzymatic activity, but the enzymatic activities associated with HDAC7 and HDAC4 rely on shared cofactors, including HDAC3 and SMRT/N-CoR.

Signal transduction pathways and chromatin structure in cancer cells

Molecular mechanisms controlling gene expression include cell shape, mechanical and chemical signal transduction pathways, chromatin remodeling, and DNA methylation. In this article, we will review the contribution of these molecular mechanisms and structural alterations in the malignant transformation of cells. The mechanical signaling pathway consists of the tissue matrix system that links together the three-dimensional skeletal networks, the extracellular matrix, cytoskeleton, and nuclear matrix. The cytoskeleton array is a dynamic system that transmits signals from the cell exterior to nuclear DNA. The composition and function of this mechanical signaling pathway is altered in cancer cells. Chemical signaling pathways such as the Ras/mitogen-activated protein kinase (MAPK) pathway stimulate the activity of kinases that modify transcription factors, histones, and chromatin remodeling factors. Oncoproteins deregulating this signaling pathway set in motion a series of events that cumulate to chromatin remodeling and aberrant gene expression. J. Cell. Biochem. Suppl. 35:27-35, 2000.

The integration of tissue structure and nuclear function

Living cells can filter the same set of biochemical signals to produce different functional outcomes depending on the deformation of the cell. It has been suggested that the cell may be "hard-wired" such that external forces can mediate internal nuclear changes through the modification of established, balanced, internal cytoskeletal tensions. This review will discuss the potential of subnuclear structures and nuclear chromatin to participate in or respond to transduction of mechanical signals originating outside the nucleus. The mechanical interactions of intranuclear structure with the nuclear lamina will be examined. The nuclear lamina, in turn, provides a structural link between the nucleus and the cytoplasmic and cortical cytoskeleton. These mechanical couplings may provide a basis for regulating gene expression through changes in cell shape.Key words: gene expression, cell structure, nuclear structure, mechanotransduction, chromatin.

CBP, a transcriptional coactivator and acetyltransferase

The CREB binding protein (CBP) was first identified as a protein that specifically binds to the active phosphorylated form of the cyclic-AMP response element binding protein (CREB). CBP was initially defined as a transcriptional coactivator that, as a result of its large size and multiple protein binding domain modules, may function as a molecular scaffold. More recently, an acetyltransferase activity, both of histones and nonhistones, has been found to be essential for transactivation. In this review, we will discuss the current understanding of the acetyltransferase specificity and activity of the CBP protein and how it may function to coactivate transcription. We will also examine the regulation of the CBP histone acetyltransferase activity in the cell cycle, by signal-transduction pathways and throughout development.Key words: CBP, acetyltransferase, chromatin, acetylation, p300.

The emerging role of class II histone deacetylases

Histone acetylation and deacetylation play essential roles in modifying chromatin structure and regulating gene expression in all eukaryotes. Several histone acetyltransferases have been identified that act as transcriptional coactivators. In contrast, histone deacetylases (HDACs) are part of transcriptional corepressor complexes. Based on their similarity to known yeast factors, the human HDACs are grouped into three classes. Class I HDACs are similar to the yeast transcriptional repressor yRPD3, while class II HDACs are related to yHDA1 and class III HDACs to ySIR2. In this review, we focus on the biology of class II HDACs. These newly discovered enzymes have been implicated in cell differentiation and development, and many molecular details are emerging that shed light on class II HDAC function and regulation. We discuss the biological role of these factors in the context of physiological processes.Key words: transcriptional regulation, histone deacetylases, class II HDACs, nucleocytoplasmic shuttling, MEF2.

The cells of the rabbit meniscus: their arrangement, interrelationship, morphological variations and cytoarchitecture

Four major morphologically distinct classes of cells were identified within the adult rabbit meniscus using antibodies to cytoskeletal proteins. Two classes of cell were present in the fibrocartilage region of the meniscus. These meniscal cells exhibited long cellular processes that extended from the cell body. A third cell type found in the inner hyaline-like region of the meniscus had a rounded form and lacked projections. A fourth cell type with a fusiform shape and no cytoplasmic projections was found along the superficial regions of the meniscus. Using a monoclonal antibody to connexin 43, numerous gap junctions were observed in the fibrocartilage region, whereas none were seen in cells either from the hyaline-like or the superficial zones of the meniscus. The majority of the cells within the meniscus exhibited other specific features such as primary cilia and 2 centrosomes. The placement of the meniscal cell subtypes as well as their morphology and architecture support the supposition that their specific characteristics underlie the ability of the meniscus to respond to different types of environmental mechanical loads.