Kinetochore structure, duplication, and distribution in mammalian cells: analysis by human autoantibodies from scleroderma patients

Autoantibodies from a patient with scleroderma CREST recognized kinetochores of the higher plant Haemanthus

Human autoantibodies from a patient with scleroderma CREST (calcinosis, Raynaud phenomenon, esophageal dismotility, sclerodactyly, telangiectasia) were used to immunostain kinetochores on chromosomes in endosperm of the seed of the monocot Haemanthus katherinae Bak. Kinetochores of mitotic chromosomes and prekinetochores of interphase cells were specifically stained using conventional indirect immunofluorescence procedures as well as a nonfading immunogold-silver-enhanced technique and analyzed by fluorescence and video microscopy. In interphase, prekinetochores were either single or double structures depending on the stage of the cell cycle but became quadruple (two distinct stained dots on each chromatid) in mid-to-late prophase. In favorable preparations of prometaphase chromosomes, multiple subunits could be resolved within each sister kinetochore suggesting a compound organization. Western blot analysis demonstrated common epitopes in centromeric peptides of HeLa and Haemanthus cell extracts. Although the molecular mass of individual polypeptides differed in the two species, the presence of shared epitopes indicates striking conservation of centromere/kinetochore components throughout evolution.

CENP-B: a major human centromere protein located beneath the kinetochore

The family of three structurally related autoantigens CENP-A (17 kD), CENP-B (80 kD), and CENP-C (140 kD) are the best characterized components of the human centromere, and they have been widely assumed to be components of the kinetochore. Kinetochore components are currently of great interest since this structure, which has long been known to be the site of microtubule attachment to the chromosome, is now believed to be a site of force production for anaphase chromosome movement. In the present study we have mapped the distribution of CENP-B in mitotic chromosomes by immunoelectron microscopy using two monospecific polyclonal antibodies together with a newly developed series of ultra-small 1-nm colloidal gold probes. We were surprised to find that greater than 95% of CENP-B is distributed throughout the centromeric heterochromatin beneath the kinetochore. This strongly supports other emerging evidence that CENP-B is specifically associated with alpha-satellite heterochromatin. Although in certain instances CENP-B can be seen to be concentrated immediately adjacent to the lower surface of the kinetochore, the outer plate remains virtually unlabeled. Similar analysis with a human autoimmune serum that recognizes all three CENP antigens reveals an additional unsuspected feature of kinetochore structure. In addition to recognizing antigens in the centromeric heterochromatin, the autoantiserum recognizes a concentration of antigens lateral to the kinetochore. This difference in staining pattern may reflect the presence of a "collar" of chromatin rich in CENP-C and/or CENP-A encircling the kinetochore plates.

Structure of the human centromere at metaphase

Until recently the centromere was thought to be a relatively homogeneous region of densely packed heterochromatin with a single differentiated domain--the kinetochore--at its surface, representing the point of attachment of the mitotic spindle. We now know that the centromere of higher eukaryotes is composed of several domains that have been identified using antibody probes. Somewhere within the domains are located both the factor(s) that control the disjunction of sister chromatids and the molecular motor responsible for chromosome movement towards the spindle poles.

An antigen located in the kinetochore region in metaphase and on polar microtubule ends in the midbody region in anaphase, characterised using a monoclonal antibody

We describe a new component of the kinetochore region of Chinese hamster ovary cells, which was characterised using a monoclonal antibody (mAb). This antigen was localised on the kinetochore regions of purified metaphase chromosomes, but in anaphase it was instead located on the polar microtubules in the midbody region, where they terminate in the stembody. It was not detectable in prophase or interphase cells by immunofluorescence, but was present in the interphase nucleus as shown by immunoblotting after SDS-polyacrylamide gel electrophoresis. The mAb recognised two polypeptides of Mr 140,000 and 155,000. The localisation of this antigen in metaphase on the kinetochore region, where the plus ends of the kinetochore microtubules are temporarily stabilised when they attach, and later in the stembody and midbody where the plus ends of the polar microtubules are stabilised in anaphase and telophase, suggests that it could play a role in stabilising the plus ends of microtubules and thus in the control of microtubule dynamics during mitosis.

Paired arrangement of nonhomologous centromeres during vertebrate spermiogenesis

Indirect immunofluorescence staining with human anti-kinetochore antibodies was used to study the position of centromeres during vertebrate spermiogenesis. Many species of Amphibia have a low chromosome number and very large spermatids and spermatozoa. The number of kinetochore dots correlates exactly with the haploid chromosome number. This implies that kinetochore duplication occurs in the interval between meiosis I and meiosis II. The nonhomologous centromeres are arranged in tandem during the entire course of spermiogenesis and in mature spermatozoa. A higher order centromere arrangement was found in spermiogenic cells of Anura and Urodela. In mammals, immunofluorescence analysis is complicated by the extreme condensation of chromatin during spermiogenesis and the high chromosome numbers. Nevertheless, centromere-centromere associations were observed in mammalian round spermatids and sporadically in testicular spermatozoa. This indicates that pair-wise association of centromeres is a universal principle of centromere arrangement at the postmeiotic stage.

Ciliary microtubule capping structures contain a mammalian kinetochore antigen

Structures that cap the plus ends of microtubules may be involved in the regulation of their assembly and disassembly. Growing and disassembling microtubules in the mitotic apparatus are capped by kinetochores and ciliary and flagellar microtubules are capped by the central microtubule cap and distal filaments. To compare the ciliary caps with kinetochores, isolated Tetrahymena cilia were stained with CREST (Calcinosis/phenomenon esophageal dysmotility, sclerodactyly, telangiectasia) antisera known to stain kinetochores. Immunofluorescence microscopy revealed that a CREST antiserum stained the distal tips of cilia that contained capping structures but did not stain axonemes that lacked capping structures. Both Coomassie blue-stained gels and Western blots probed with CREST antiserum revealed that a 97-kD antigen copurifies with the capping structures. Affinity-purified antibodies to the 97-kD ciliary protein stained the tips of cap-containing Tetrahymena cilia and the kinetochores in HeLa, Chinese hamster ovary, and Indian muntjak cells. These results suggest that at least one polypeptide found in the kinetochore is present in ciliary microtubule capping structures and that there may be a structural and/or functional homology between these structures that cap the plus ends of microtubules.

Formation of the astral mitotic spindle: ultrastructural basis for the centrosome-kinetochore interaction

The formation of the astral mitotic spindle is initiated at the time of nuclear envelope breakdown from an interaction between the replicated spindle poles (i.e. centrosomes) and the chromosomes. As a result of this interaction bundles of microtubules are generated which firmly attach the kinetochores on each chromosome to opposite spindle poles. Since these kinetochore fibers are also involved in moving the chromosomes, the mechanism by which they are formed is of paramount importance to understanding the etiology of force production within the spindle. As a prelude to outlining such a mechanism, the dynamics of spindle formation and chromosome behavior are examined in the living cell. Next, the properties of centrosomes and kinetochores are reviewed with particular emphasis on the structural and functional changes that occur within these organelles as the cell transits from interphase to mitosis. Finally, a number of recent observations relevant to the mechanism by which these organelles interact are detailed and discussed. From these diverse data it can be concluded that kinetochore fiber microtubules are derived from dynamically unstable astral microtubules that grow into, or grow by and then interact laterally with, the kinetochore. Moreover, the data clearly demonstrate that the interaction of a single astral microtubule with one of the kinetochores on an unattached chromosome is sufficient to attach the chromosome to the spindle, orient it towards a pole, and initiate poleward motion. As the chromosomes move into the region of the forming spindle more astral microtubules become incorporated into the nascent kinetochore fibers and chromosome velocity decreases dramatically. During this time the distribution of spindle microtubules changes from two overlapping radial arrays to the fusiform array characteristic of metaphase cells.

Immunocytochemistry of the cell nucleus

This electron microscopic review addresses in situ immunocytochemistry of the mammalian cell nucleus with special reference to the use of autoantibodies, which are the major source of antinuclear antibodies. The localization of many key nuclear antigens is documented and immunocytochemical data are related to the major functional processes of transcription and processing of RNA and to replication of DNA.

Centromere structure and function in neoplasia

The mammalian centromere plays an essential role in maintenance of diploidy in the cell. It is therefore imperative that we understand the structure and function of the mammalian centromere in order to plan strategy to control the incidence of aneuploidy and resultant malformations of the nonneoplastic as well as neoplastic tissues. Even though considerable information is available about the structure and some functional aspects of centromeres of lower eukaryotes such as yeast, the structure of the mammalian centromere is still a matter of conjecture limited to an understanding of the base composition of the alphoid sequences putatively located in the centromeric DNA of higher apes. We do, however, have a better understanding of the structure and role of the kinetochore. In all eukaryotes analyzed so far, the centromeres in a given genome separate in a sequential manner dependent upon the time of replication of pericentric and centromeric DNA. Some chromosomes, generally found in neoplastic cells, that carry more than one centromere show premature separation of the accessory centromeres. These centromeres and the associated pericentric regions replicate their DNA in an earlier part of the S phase than those that show kinetochore activity; both, however, carry DNA of the same composition. The active centromeres in these chromosomes show kinetochore protein binding as detected by antikinetochore antibody; the inactive centromeres are usually devoid of these proteins. The double minutes in neoplastic cells also lack kinetochore proteins, perhaps due to a lack of any centromere. Some dicentric and multicentric chromosomes in cancer cells and transformed cell lines do not display premature centromere separation. In these chromosomes, all centromeric sites show kinetochore proteins and all centromeric regions replicate their DNA simultaneously. These chromosomes also exhibited meiotic-like behavior of some centromeres and show postanaphase separation of some centromeres, resulting in bridges. These bridges, upon breakage and rejoining of sister chromatids, generate new multicentric chromosomes. The resulting chromosomes also exhibit formation of compound kinetochores. Some of these phenomena are novel descriptions of the centromere behavior in cancer cells. This review also discusses the role of aberrant centromere separation in human biology, providing correlates between errors of centromere separation and neoplasia.

Mitosis

Data that describe both the structure and the physiology of the mitotic spindle are reviewed. Some of the molecules that have been shown to play a role in mitosis are tabulated, and how mitosis might work is considered.

A human centromere antigen (CENP-B) interacts with a short specific sequence in alphoid DNA, a human centromeric satellite

We report the interaction between a human centromere antigen and an alphoid DNA, a human centromeric satellite DNA, which consists of 170-bp repeating units. A cloned alphoid DNA fragment incubated with a HeLa cell nuclear extract is selectively immunoprecipitated by the anticentromere sera from scleroderma patients. Immunoprecipitation of the DNA made by primer extension defines the 17-bp segment on the alphoid DNA that is required for formation of DNA-antigen complex. On the other hand, when proteins bound to the biotinylated alphoid DNA carrying the 17-bp motif are recovered by streptavidin agarose and immunoblotted, the 80-kD centromere antigen (CENP-B) is detected. DNA binding experiments for proteins immunoprecipitated with anticentromere serum, separated by gel electrophoresis, and transferred to a membrane strongly suggest that the 80-kD antigen specifically binds to the DNA fragment with the 17-bp motif. The 17-bp motif is termed the "CENP-B box." Alphoid monomers with the CENP-B box are found in all the known alphoid subclasses, with varying frequencies, except the one derived from the Y chromosome so far cloned. These results imply that the interaction of the 80-kD centromere antigen with the CENP-B box in the alphoid repeats may play some crucial role in the formation of specified structure and/or function of human centromere.

Sequence of centromere separation: characterization of multicentric chromosomes in a rat cell line

The B1 cell line of rat cerebral endothelium origin exhibits several dicentric and multicentric chromosomes. These chromosomes, unlike multicentrics in mouse (Vig and Zinkowski 1986) do not show premature centromere separation. All centromeres deposit kinetochore proteins and appear to be functional. Even the centromeres which fail to migrate to the poles during anaphase and make side arm bridges bind to spindle microtubules. Some multicentric chromosomes show kinetochores spaced apart with intervening stretches of euchromatin while others are located adjacent to each other thus exhibiting tandem repeats and forming a "compound" kinetochore (Brinkeley et al. 1984). Also, unlike mouse multicentric chromosomes in which different pericentric regions and the centromeres replicate at different times, the rat chromosomes appear to replicate all pericentric and centric regions in a given multicentric simultaneously. The present studies indicate that centromeres in rat and mouse replicate during the last part of the S-phase and in continuation with the pericentric heterochromatin.

Visualization of centromere proteins CENP-B and CENP-C on a stable dicentric chromosome in cytological spreads

We have screened for the presence of two centromere autoantigens, CENP-B (80 kDa) and CENP-C (140 kDa) at the inactive centromere of a naturally occurring stable dicentric chromosome using specific antibodies that do not cross-react with any other chromosomal proteins. In order to discriminate between the active and inactive centromeres on this chromosome we have developed a modification of the standard methanol/acetic acid fixation procedure that allows us to obtain high-quality cytological spreads that retain antigenicity with the anti-centromere antibodies. We have noted three differences in the immunostaining patterns with specific anti-CENP-B and CENP-C antibodies. (1) The amount of detectable CENP-B varies from chromosome to chromosome. The amount of CENP-C appears to be more or less the same on all chromosomes. (2) CENP-B is present at both active and inactive centromeres of stable dicentric autosomes. CENP-C is not detectable at the inactive centromeres. (3) While immunofluorescence with anti-CENP-C antibodies typically gives two discrete spots, staining with anti-CENP-B often appears as a single bright bar connecting both sister centromeres. This suggests that while CENP-C may be confined to the outer centromere in the kinetochore region, CENP-B may be distributed throughout the entire centromere. Our data suggest that CENP-C is likely to be a component of some invariant chromosomal substructure, such as the kinetochore. CENP-B may be involved in some other aspect of centromere function, such as chromosome movement or DNA packaging.

Kinetochore development in two dicentric chromosomes in man. A light and electron microscopic study

Two dicentric human chromosomes were investigated with light and electron microscopic techniques. One chromosome, with a translocation tdic(5;13)(p12;p12), behaved as a dicentric in about half the cells: it had two primary constrictions; C- and Cd-banding showed two centromeres; and the CREST antikinetochore antibody reacted with the two centromeres with equal affinity. Electron microscopic analysis of sectioned metaphases showed that the dicentric could develop kinetochores at both centromeres simultaneously. The other dicentric chromosome, tdic(21;21)(q22;q22), occasionally showed two primary constrictions, but both C- and Cd-banding distinguished between an active and an inactive centromere, and the CREST antibody reacted only weakly with the inactive centromere. Electron microscopy showed kinetochore development at only one centromere.

Alphoid satellite DNA is tightly associated with centromere antigens in human chromosomes throughout the cell cycle

In this study, we have examined a DNA element specific to the centromere domain of human chromosomes. Purified HeLa chromosomes were digested with the restriction enzyme Sau3AI and fractionated by sedimentation through a sucrose gradient. Fractions showing antigenecity to anticentromere (kinetochore) serum obtained from a scleroderma CREST patient were used to construct a DNA library. From this library we found one clone which has specifically hybridized to the centromere domain of metaphase chromosomes using a biotinylated probe DNA and FITC-conjugated avidin. The clone contained a stretch of alphoid DNA dimer. To determine precisely the relative location of the alphoid DNA stretch and the centromere antigen, a method was developed to carry out in situ hybridization of DNA and indirect immunofluorescent staining of antigen on the same cell preparation. Using this method, we have found perfect overlapping of the alphoid DNA sites with the centromere antigen sites in both metaphase chromosomes and nuclei at various stages in the cell cycle. We have also observed this exact correlation at the attachment sites of artificially extended sister chromatids. These results suggest the possibility that alphoid DNA repeats are a key component of kinetochore structure.

Centromeric association and non-random distribution of centromeres in human tumour cells

Centromere arrangement in interphase and metaphase cells of two human tumour cell lines was analysed using anti-kinetochore antibodies as immunofluorescent probes. In GLC1 interphase nuclei, kinetochores were non-randomly positioned around the nucleolus and close to the nuclear membrane. During S and early G2 phase, necklace-like strands of kinetochores were formed in the centre of the nucleus. The duplication of sister kinetochores during the G2 phase was not synchronized. At late G2 phase, a relatively random topological distribution of centromeres was observed with short linear arrays of sister kinetochores. Carefully spread metaphase plates of MDA-MB231 cells generally exhibited a linear alignment of centromeres and large centromeric clusters. In completely pulverized MDA-MB231 cells, centromeres showed a strong tendency to associate with each other.

Kinetochore immunofluorescence in micronuclei: a rapid method for the in situ detection of aneuploidy and chromosome breakage in human fibroblasts

We have developed a rapid and simple immunodetection assay for the in situ identification of aneuploidy in mitotic fibroblasts. Kinetochore (centromere)-containing micronuclei can be detected easily and rapidly by immunofluorescence. The action of colchicine and its derivatives on the mitotic spindle apparatus of mammalian cells induces chromosome lag and aneuploidy. The treatment of normal human fibroblasts with Colcemid resulted in increased levels of micronuclei. Using an immunofluorescence stain (scleroderma CREST antiserum, biotinylated goat antihuman IgG and streptavidin-Texas Red) to detect the presence of kinetochores, it was observed that 90% of the Colcemid-induced micronuclei contained one or more fluorescent bodies (kinetochores). Cultured skin fibroblasts from a patient with ataxia telangiectasia (AT), which is a chromosome breakage syndrome, were used as a control. The AT fibroblasts exhibited elevated levels of spontaneous micronuclei when compared with normal fibroblasts, and 85% of these micronuclei were kinetochore-negative. This finding supports the hypothesis that the majority of spontaneous micronuclei in AT cells arise from chromosome breakage. The spontaneous micronucleus frequencies for 8 strains of human fibroblasts were in the order of 0.5-2%. Spontaneous levels of kinetochore-positive micronuclei were measured for these 8 strains; in 5 of the strains, about 25% of the micronuclei were kinetochore-positive, and in the other 3 strains approximately 50% of the micronuclei were kinetochore-positive. These data suggest that genetic factors may play a role in the control of the spontaneous levels of chromosome breakage and/or segregation errors which result in aneuploidy.

Immunofluorescent staining of kinetochores in micronuclei: a new assay for the detection of aneuploidy

The immunofluorescent staining of kinetochores in micronuclei with antikinetochore antibodies was used to develop an in vitro assay for aneuploidy-inducing agents. The results show that about 80% of micronuclei induced by either colchicine or chloral hydrate contained kinetochores; only 9% of X-ray-induced micronuclei reacted positively to the antibody. These findings indicate that the in vitro micronucleus assay coupled with immunofluorescent staining of kinetochores can be a useful method for assessing the ability of chemicals to induce aneuploidy and/or chromosome aberrations.

Differential isotype recognition of two centromere associated polypeptides by immunoblotting in connective tissue disease

On investigating the immunoblotting profile of 65 systemic sclerosis patients, a 140 kD polypeptide was recognised by sera from 16, when immunoblotted against a nuclear-enriched K562 cell sonicate. All 16 sera contained anticentromere antibodies (ACA) detected by immunofluorescence (IF) and 15 of 16 also recognized a 19 kD polypeptide on immunoblotting. Two ACA positive sera failed to recognize the 140 kD polypeptide but one of these recognized the 19 kD polypeptide. The 140 kD polypeptide identified a group with more limited skin involvement (P less than 0.05) and all 16 had Raynaud's phenomenon. The sera from three of 100 systemic lupus erythematosus (SLE) patients also recognized both polypeptides. On investigating the isotype specificity, the 140 kD polypeptide was strongly detected by an IgM autoantibody and the 19 kD polypeptide by an IgG autoantibody.

Detection of distinct structural domains within the primary constriction using autoantibodies

We report the immunological differentiation of structures within the primary constriction. These include the kinetochore and the connecting strand, a structure which connects sister kinetochores. The location and temporal appearance of the connecting strand antigen suggest that it could play a role in the maintenance of sister chromatid pairing. In addition, we report the identification of a novel epitope that is localized to discrete patches along the entire length of the junction between sister chromatids at metaphase (the junction patch antigen). The patches on the inner surface of the euchromatic arms can be disrupted by Colcemid treatment while those found in the primary constriction remain intact. The apparent heterogeneity of the patches suggests that they may play different roles in the regulation of sister chromatid pairing. Because of their cytological localization and possible functional role, the junction patch and connecting strand antigens have provisionally been collectively termed CLiPs (Chromatid Linking Proteins). All of these antigenic sites are shown to be distinct from centromeric heterochromatin, which can itself be immunologically differentiated from the euchromatic arms. The relationship between the antigenicity of the primary constriction and the unique manner in which chromatin is organized in this region is discussed.

Kinetochore appearance during meiosis, fertilization and mitosis in mouse oocytes and zygotes

The events of mammalian fertilization overlap with the completion of meiosis and first mitosis; the pro-nuclei never fuse, instead the parental genomes first intermix at the mitotic spindle equator at metaphase. Since kinetochores are essential for the attachment of chromosomes to spindle microtubules, this study explores their appearance and behavior in mouse oocytes, zygotes and embryos undergoing the completion of meiosis, fertilization and mitoses. Kinetochores are traced with immunofluorescence microscopy using autoimmune sera from patients with CREST (CREST = calcinosis, Raynaud's phenomenon, esophageal dysmotility, sclerodactyly, telangiectasia) scleroderma. These sera cross-react with the 17 kDa centromere protein (CENP-A) and the 80 kDa centromere protein (CENP-B) found at the kinetochores in human cell cultures. The unfertilized oocyte is ovulated arrested at second meiotic metaphase and kinetochores are detectable as paired structures aligned at the spindle equator. At meiotic anaphase, the kinetochores separate and remain aligned at the distal sides of the chromosomes until telophase, when their alignment perpendicular to the spindle axis is lost. The female pronucleus and the second polar body nucleus each receive a detectable complement of kinetochores. Mature sperm have neither detectable centrosomes nor detectable kinetochores, and shortly after sperm incorporation kinetochores become detectable in the decondensing male pronucleus. In pronuclei, the kinetochores are initially distributed randomly and later found in apposition with nucleoli. At mitosis, the kinetochores behave in a pattern similar to that observed at meiosis or mitosis in somatic cells: irregular distribution at prophase, alignment at metaphase, separation at anaphase and redistribution at telophase. They are also detectable in later stage embryos. Colcemid treatment disrupts the meiotic spindle and results in the dispersion of the meiotic chromosomes along the oocyte cortex; the chromosomes remain condensed with detectable kinetochores. Fertilization of Colcemid-treated oocytes results in the incorporation of a sperm which is unable to decondense into a male pronucleus. Remarkably kinetochores become detectable at 5 h post-insemination, suggesting that the emergence of the paternal kinetochores is not strictly dependent on male pronuclear decondensation.

Correlates between autoantibodies to nucleolar antigens and clinical features in patients with systemic sclerosis (scleroderma)

Immunofluorescence on rat liver sections was used to select high-titer antinucleolar antibodies (ANoA) in the sera of patients with systemic sclerosis (scleroderma). In 646 patients, 53 ANoA sera (8%) were identified, and of these, 46 were available in sufficient quantities for further analysis. The complex of RNA polymerase I was immunoprecipitated by 7 sera (15%), which uniformly produced punctate nucleolar staining. The PM-Scl antigen, a particle consisting of 11 polypeptides, was immunoprecipitated by 8 sera (17%), all of which displayed homogeneous nucleolar staining. A 34-kd nucleolar protein (fibrillarin) of the U3 RNP complex was positive in immunoblotting of 22 sera (48%), which characteristically produced clumpy nucleolar staining. Antibodies against RNA polymerase I were associated with diffuse scleroderma of short duration, which was characterized by a high prevalence of internal organ involvement, including renal crisis. Anti-U3 RNP antibodies had a high prevalence in men with significantly less joint involvement, compared with ANoA-negative patients. Anti-PM-Scl antibodies identified a group of scleroderma patients with a high prevalence of concomitant myositis and renal involvement.

Mutational and in vitro protein-binding studies on centromere DNA from Saccharomyces cerevisiae

Centromeres on chromosomes in the yeast Saccharomyces cerevisiae contain approximately 140 base pairs (bp) of DNA. The functional centromere (CEN) region contains three important sequence elements (I, PuTCACPuTG; II, 78 to 86 bp of high-AT DNA; and III, a conserved 25-bp sequence with internal bilateral symmetry). Various point mutations or deletions in the element III region have a profound effect on CEN function in vivo, indicating that this DNA region is a key protein-binding site. This has been confirmed by the use of two in vitro assays to detect binding of yeast proteins to DNA fragments containing wild-type or mutationally altered CEN3 sequences. An exonuclease III protection assay was used to demonstrate specific binding of proteins to the element III region of CEN3. In addition, a gel DNA fragment mobility shift assay was used to characterize the binding reaction parameters. Sequence element III mutations that inactivate CEN function in vivo also prevent binding of proteins in the in vitro assays. The mobility shift assay indicates that double-stranded DNAs containing sequence element III efficiently bind proteins in the absence of sequence elements I and II, although the latter sequences are essential for optimal CEN function in vivo.

Sequence of centromere separation: differential replication of pericentric heterochromatin in multicentric chromosomes

The dicentric and multicentric chromosomes in L cells and a brain tumor cell line of mouse display only one site of kinetochore formation associated with the 'active' centromere. The accessory or 'inactive' centromeres show premature separation. These cell lines were treated with 10(-6) M 5-bromodeoxyuridine (BrdUrd) followed by anti-BrdUrd antibody to study the pattern of replication of pericentric heterochromatin flanking the active vs inactive centromeres. Regardless of its quantity, heterochromatin around the inactive centromere replicates earlier than that associated with the active centromere. There appears to be a relationship between the timing of separation of a centromere and the timing of replication of pericentric heterochromatin. The premature replication of heterochromatin associated with an inactive centromere may be responsible for its premature separation and, hence, inactivity.

Systemic scleroderma. Clinical and pathophysiologic aspects

Systemic scleroderma is a generalized disease of connective tissue involving mainly the skin, the gastrointestinal tract, the lungs, the heart, and the kidneys. It can be present in different forms, of which acroscleroderma, with limited cutaneous and extracutaneous involvement, and diffuse scleroderma within a more rapid progression are most characteristic. Circulating antibodies against antinucleolar antigens are present in most patients with systemic scleroderma. They are helpful for establishing a classification and for determining the prognosis of the disease; their involvement in the pathogenesis, however, is still unclear. Alterations of the blood vessels and induction of fibroblasts by potent mediators are thought to play an important role in the early phase of scleroderma. Therefore early diagnosis is required, which then can initiate vasoactive therapy. In patients with systemic scleroderma, who also suffer from additional myositis, interstitial lung diseases, or arthritis, anti-inflammatory treatment with prednisolone and azathioprine is suggested. Development and progression of fibrosis cannot yet be influenced sufficiently. Only D-penicillamine affecting cross-linking of collagen has been widely used in scleroderma and has some beneficial effect.

Analysis of double minutes and double minute-like chromatin in human and murine tumor cells using antikinetochore antibodies

Antikinetochore antibodies from patients with the calcinosis, Raynaud's phenomenon, esophageal dismobility, sclerodactyly, telangiectasia-(CREST)-syndrome of scleroderma were used as immunofluorescent probes to discriminate between the presence and absence of kinetochores in minute chromosomes not previously seen by conventional banding methods. Double minute chromosomes (DM) consistently lack the antigenic component of the kinetochore, which is direct evidence for the fact that they do not have a centromere. Although somatically stable in malignant cell populations, DM are unable to attach to the mitotic spindle. Conversely, despite their structural similarity to DM, chromosome fragments and supernumerary marker chromosomes exhibit intensely fluorescing kinetochores and, thus, are subject to a precise anaphasic distribution.

Mutational and in vitro protein-binding studies on centromere DNA from Saccharomyces cerevisiae

Centromeres on chromosomes in the yeast Saccharomyces cerevisiae contain approximately 140 base pairs (bp) of DNA. The functional centromere (CEN) region contains three important sequence elements (I, PuTCACPuTG; II, 78 to 86 bp of high-AT DNA; and III, a conserved 25-bp sequence with internal bilateral symmetry). Various point mutations or deletions in the element III region have a profound effect on CEN function in vivo, indicating that this DNA region is a key protein-binding site. This has been confirmed by the use of two in vitro assays to detect binding of yeast proteins to DNA fragments containing wild-type or mutationally altered CEN3 sequences. An exonuclease III protection assay was used to demonstrate specific binding of proteins to the element III region of CEN3. In addition, a gel DNA fragment mobility shift assay was used to characterize the binding reaction parameters. Sequence element III mutations that inactivate CEN function in vivo also prevent binding of proteins in the in vitro assays. The mobility shift assay indicates that double-stranded DNAs containing sequence element III efficiently bind proteins in the absence of sequence elements I and II, although the latter sequences are essential for optimal CEN function in vivo.

Centromeric proteins recognized by CREST sera and meiotic chromosome segregation

Analysis of the peptides recognized by CREST sera was carried out in different mouse tissues and cells, including spermatozoa. In all cases, a polypeptide of Mr = 18,000 was recognized by the sera and occasionally two other proteins of Mr = 80,000 and Mr = 140,000 were observed after immunoblotting of nuclear proteins. In both early and late spermatids, centromeric staining was observed after incubation and immunofluorescence with CREST sera. After detergent treatment, it was even possible to detect centromeric staining in mature spermatozoa. In spermatid cells, the immunofluorescent pattern presented a binomial distribution of the number of fluorescent spots, with a mean value around half of the haploid number of chromosomes. Since this pattern is the result of chromosome segregation after meiosis II, our data suggest that this centromeric peptide is not directly implicated in the chromosome segregation process. On the other hand, the distribution of spots after immunofluorescence suggests a different organization of centromeric components in meiosis I and meiosis II.

A new method for the cytological analysis of autoantibody specificities using whole-mount, surface-spread meiotic nuclei

A new method for the cytological analysis of antinuclear antibody binding offers several advantages over conventional techniques. Nuclei in meiosis, prepared by surface-spreading spermatocytes, provide a detailed examination of the constituents of the nucleus--euchromatin, heterochromatin, sex chromatin, nucleoli, centromeres, and dense patches that seem related to RNA metabolism--and each of these structures can be seen to change in morphology and antibody labeling during the course of meiotic prophase. Results using sera from humans and mice with autoimmune disease, and using several mouse monoclonal antibodies, demonstrate the potential of this method for clinical and research applications, both for more common antibody types and for those that bind epitopes which are unique to germ cells.

The inner centromere protein (INCENP) antigens: movement from inner centromere to midbody during mitosis

We describe a novel set of polypeptide antigens that shows a dramatic change in structural localization during mitosis. Through metaphase these antigens define a new chromosomal substructure that is located between the sister chromatids. Because the antigens are concentrated in the pericentromeric region, we have provisionally termed them the INCENPs (inner centromere proteins). The INCENPs (two polypeptides of 155 and 135 kD) were identified with a monoclonal antibody that was raised against the bulk proteins of the mitotic chromosome scaffold fraction. These two polypeptides are the most tightly bound chromosomal proteins known. When scaffolds are prepared, 100% of the detectable INCENPs remain scaffold associated. We were therefore unprepared for the fate of the INCENPs at anaphase. As the sister chromatids separate, the INCENPs dissociate fully from them, remaining behind at the metaphase plate as the chromatids migrate to the spindle poles. During anaphase the INCENPs are found on coarse fibers in the central spindle, and also in close apposition to the cell membrane in the region of the forming contractile ring. During telophase, the INCENPs gradually become focused onto the forming midbody, together with which they are ultimately discarded. Several possible in vivo roles for the INCENPs are suggested by these data: regulation of sister chromatid pairing, stabilization of the plane of cleavage, and separation of spindle poles at anaphase.

Tubulin interaction with kinetochore proteins: analysis by in vitro assembly and chemical cross-linking

The sera from patients with the CREST (calcinosis, Raynaud's phenomenon, esophageal dysmotility, sclerodactyly, telangiectasia) variation of the autoimmune disease scleroderma contain autoantibodies that specifically recognize the kinetochore by immunofluorescence. Two major antigens of molecular masses 18 and 80 kD are consistently identified by Western blotting of proteins of isolated chromosomes using CREST sera. In this paper, the possible roles that these two proteins play in the interaction of metaphase chromosomes with tubulin and microtubules are examined using two different procedures. In one set of experiments. Chinese hamster ovary (CHO) chromosomes were extracted with 1-2 M NaCl before incubating with phosphocellulose-purified tubulin under in vitro microtubule assembly conditions. After this treatment, the kinetochores of the residual chromosome scaffolds can still initiate the in vitro assembly of microtubules. Immunoblots of the chromosome scaffold proteins demonstrate that the 18-kD protein has been solubilized by the 1-2 M NaCl extraction, suggesting that this protein is not essential for microtubule assembly at the kinetochore. In a second approach, tubulin was covalently cross-linked to kinetochores of CHO chromosomes using the reversible cross-linking reagent dithiobis (succinimidyl propionate). After DNase I digestion, the chromosomes were solubilized and subjected to anti-tubulin affinity chromatography. Tubulin-kinetochore protein complexes were specifically eluted and analyzed by PAGE and immunoblotting with scleroderma CREST serum. Only a small number of proteins were eluted from the antitubulin affinity column as shown by Coomassie Blue-stained gels. In addition to tubulin, an 80-kD polypeptide, bands at 110 and 24 kD, as well as a faint band at 54 kD, can be resolved. Several minor bands can also be seen in silver-stained gels. The 80-kD protein band from whole metaphase chromosomes reacted with scleroderma CREST serum by immunoblotting and therefore probably represents the major centromere antigen CENP-B. This report provides evidence for a specific protein complex on metaphase chromosomes that is contiguous with kinetochore-bound tubulin and may be involved in microtubule-kinetochore interactions during mitosis.

Synaptonemal complex antigen location and conservation

The axial cores of chromosomes in the meiotic prophase nuclei of most sexually reproducing organisms play a pivotal role in the arrangement of chromatin, in the synapsis of homologous chromosomes, in the process of genetic recombination, and in the disjunction of chromosomes. We report an immunogold analysis of the axial cores and the synaptonemal complexes (SC) using two mouse monoclonal antibodies raised against isolated rat SCs. In Western blots of purified SCs, antibody II52F10 recognizes a 30- and a 33-kD peptide (Heyting, C., P. B. Moens, W. van Raamsdonk, A. J. J. Dietrich, A. C. G. Vink, and E. J. W. Redeker, 1987, Eur. J. Cell Biol., 43: 148-154). In spreads of rat spermatocyte nuclei it produces gold grains over the cores of autosomal and sex chromosomes. The cores label lightly during the chromosome pairing stage (zygotene) of early meiotic prophase and they become more intensely labeled when they are parallel aligned as the lateral elements of the SC during pachytene (55 grains/micron SC). Statistical analysis of electronically recorded gold grain positions shows that the two means of the bimodal gold grain distribution coincide with the centers of the lateral elements. At diplotene, when the cores separate, the antigen is still detected along the length of the core and the enlarged ends are heavily labeled. Shadow-cast SC preparations show that recombination nodules are not labeled. The continued presence suggests that the antigens serve a continuing function in the cores, such as chromatin binding, and/or structural integrity. Antibody III15B8, which does not recognize the 30- and 33-kD peptides, produces gold grains predominantly between the lateral elements. The grain distribution is bimodal with the mean of each peak just inside the pairing face of the lateral element. The antigen is present where and while the cores of the homologous chromosomes are paired. From the location and the timing, it is assumed that the antigen recognized by III15B8 functions in chromosome pairing at meiotic prophase. The two anti-rat SC antibodies label rat and mouse SCs but not rabbit or dog SCs. A positive control using human CREST (calcinosis, Raynaud's phenomenon, esophageal dysmotility, sclerodactyly, telangiectasia) anti-centromere serum gives equivalent labeling of SC centromeres in the rat, mouse, rabbit, and dog. It is concluded that the SC antigens recognized by II52F10 and III15B8 are not widely conserved. The two antibodies do not bind to cellular or nuclear components of somatic cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunocytochemical demonstration of kinetochores in human sperm heads

CREST sera have been used to identify kinetochores in mature mammalian sperm heads. It is necessary to decondense the sperm heads artificially to permit access of the reagents before the kinetochores can be demonstrated immunocytochemically. The distribution of kinetochores in the sperm heads appears to be random. These results show that the kinetochore antigen recognized by the CREST sera used here is retained during spermiogenesis and is passed on to the zygote at fertilization.

A technique for simultaneous antikinetochore immunofluorescence staining and Q-banding in chromosomes from human lymphocytes

Antikinetochore immunofluorescence staining has been used in several studies to determine whether a second kinetochore is present, active, or both, in multicentric chromosomes. All of these studies have used tissue culture cells, and contended with the problem of obtaining well spread, banded metaphase chromosomes without affecting the kinetochore staining. We have adapted hypotonic, centrifugation and chromosome staining techniques to obtain simultaneous Q-banding and bright kinetochore staining of chromosomes from human lymphocytes.

Centrosome-kinetochore interaction in multinucleate cells

The interaction between centrosomes and kinetochores was studied in multinucleate cells induced by Colcemid treatment or by random cell fusion. Except for prematurely condensed chromosomes (PCC) of the G2-phase, PCCs do not develop their own spindle area. Perhaps the maturation promoting factor (MPF) fails to activate these centrosomes. In such PCCs, the kinetochore-centrosome interaction was found to be non-specific: sometimes only a few chromosomes of a group could establish connections with centrosomes, sometimes chromosomes from the same PCC group developed microtubule (MT) attachment with different centrosomes (not the pair), and sometimes kinetochores of PCC groups failed to interact with MTs. These findings explain the abnormal mitotic behaviour of PCCs as seen in the light microscope. These PCCs develop micronuclei or normal nuclei by nuclear re-formation in telophase. All the different PCC groups revealed kinetochores with kinetochore plates. It was shown that transformation of presumptive kinetochores to a trilaminar kinetochore does not depend on nuclear envelope breakdown or on the degree of chromosome condensation. This may be induced by the MPF which may initiate different events like chromosome condensation, nuclear envelope breakdown and kinetochore transformation by secondary factors. Other observations like establishment of connections by different chromosome groups to a common centrosome, kinetochore attachment of PCCs to different centrosomes, interaction of one kinetochore with two centrosomes, kinetochores being stretched and bent to receive microtubules and finally the failure of some kinetochores to develop MT attachment, all strongly suggest that the kinetochores serve as the point of termination rather than the nucleation sites of kinetochore MTs.

A 17-kD centromere protein (CENP-A) copurifies with nucleosome core particles and with histones

We have detected and begun to characterize a 17-kD centromere-specific protein, CENP-A (Earnshaw, W. C., and N. Rothfield, 1985, Chromosoma., 91:313-321). Sera from several humans with CREST scleroderma autoimmune disease (CREST: calcinosis, Raynaud's phenomenon, esophageal dsymotility, sclerodactyly, and telangiectasia) bind this protein in immunoblot assays of HeLa whole cell or nuclear extracts. We have affinity purified the anti-17-kD centromere protein (anti-CENP-A) specific antibodies from immunoblots of HeLa nuclear protein. The antibodies react with epitopes present on CENP-A derived from humans but apparently do not recognize specific epitopes in either rat or chicken nuclei. Only human nuclear protein is CENP-A positive by immunoblot. Furthermore, human cells show localization of anti-CENP-A antibody to centromeres by immunofluorescence microscopy, whereas rat cells do not. On extraction from the nucleus, CENP-A copurifies with core histones and with nucleosome core particles. We conclude that this centromere-specific protein is a histone-like component of chromatin. The data suggest that CENP-A functions as a centromere-specific core histone.

Molecular cloning of cDNA for CENP-B, the major human centromere autoantigen

We have isolated a series of overlapping cDNA clones for approximately 95% of the mRNA that encodes CENP-B, the 80-kD human centromere autoantigen recognized by patients with anticentromere antibodies. The cloned sequences encode a polypeptide with an apparent molecular mass appropriate for CENP-B. This polypeptide and CENP-B share three non-overlapping epitopes. The first two are defined by monoclonal antibodies elicited by injection of cloned fusion protein. Epitope 1 corresponds to a major antigenic site recognized by the anticentromere autoantibody used to obtain the original clone. Epitope 2 is a novel one not recognized by the autoantibody. These epitopes were shown to be distinct both by competitive binding experiments and by their presence or absence on different subcloned portions of the fusion protein. The third independent epitope, recognized by a subset of anticentromere-positive patient sera, maps to a region substantially closer to the amino terminus of the fusion protein. DNA and RNA blot analyses indicate that CENP-B is unrelated to CENP-C, a 140-kD centromere antigen also recognized by these antisera. CENP-B is the product of a 2.9-kb mRNA that is encoded by a single genetic locus. This mRNA is far too short to encode a polypeptide the size of CENP-C. The carboxy terminus of CENP-B contains two long domains comprised almost entirely of glutamic and aspartic acid residues. These domains may be responsible for anomalous migration of CENP-B on SDS-polyacrylamide gels, since the true molecular mass of CENP-B is approximately 65 kD, 15 kD less than the apparent molecular mass deduced from gel electrophoresis. Quite unexpectedly, immunofluorescence analysis using antibodies specific for CENP-B reveals that the levels of antigen vary widely between chromosomes.

Chromosome organisation in polyploid mouse trophoblast nuclei

At least one-third of mouse trophoblast cells undergo endoreduplication during the first half of gestation. It has been suggested that the endoreduplicated chromosomes may be polytenised. Here it is shown, using in situ hybridisation to the alpha-1 antitrypsin genes, which map at a unique site, that while there is a tendency for duplicated chromosomes to cluster, this does not involve the complete fusion of replicated chromatids found in fully polytene chromosomes, and in a substantial proportion of homologues the sites on the chromosome arms corresponding to these genes are widely separated. The centromeres do not fuse into a single chromocentre but the possibility is not ruled out that individual chromosomes may be polytenised in the centromeric region. Evidence is also presented showing that endoreduplication in trophoblast nuclei is not accompanied by the formation of new prekinetochore structures, in contrast to the situation in polyploid mouse liver and C127 cells.

Size variation in kinetochores of human chromosomes

Aneuploidy, the loss or gain of chromosomes from cells, is likely in many cases to involve the kinetochore, the site of attachment of spindle microtubules. We analyzed human fibroblast cells with antikinetochore-antibody indirect immunofluorescence, and noted an apparent heterogeneity in the sizes of kinetochores among different chromosomes. The Y chromosome in particular always showed minute kinetochores, an observation which was quantified and substantiated using computer-assisted image analysis. This finding, combined with literature reports about in vivo and in vitro involvement of the Y chromosome in aneuploidy, was used to frame a novel hypothesis about the generation of chromosome imbalance.