Centrosomal proteins and lactate dehydrogenase possess a common epitope in human cell lines

NR5A1 prevents centriole splitting by inhibiting centrosomal DNA-PK activation and β-catenin accumulation

Background

Adrenogonadal cell growth and differentiation are controlled by nuclear receptor NR5A1 (Ad4BP/SF-1) that regulates the expression of adrenal and gonadal genes. In addition, SF-1 also resides in the centrosome and controls centrosome homeostasis by restricting the activity of centrosomal DNA-PK and CDK2/cyclin A.

Results

Here we show that SF-1 depletion resulted in centriole splitting and amplification due to aberrant activation of DNA-PK in the centrosome of mouse adrenocortical Y1 cells. In the absence of SF-1, GSK3β was aberrantly phosphorylated during G1 phase and β-catenin was accumulated in the centrosome, but not in the nucleus. DNA-PK inhibitor vanillin reversed these phenomena. SF-1 overexpression led to inhibition of centrosomal DNA-PK activation caused by SF-1 depletion. Both full-length SF-1 and truncated SF-1 devoid of its DNA-binding domain rescued the multiple centrosome phenotype caused by SF-1 depletion, indicating that the effect of SF-1 in the centrosome is not contributed by its DNA-binding domain. Furthermore, SF-1 interacted with cyclin A in the centrosome, but not in the nucleus. Depletion of SF-1 also resulted in centriole splitting, genomic instability and reduced growth of mouse testicular Leydig MA10 cells.

Conclusion

Centrosomal DNA-PK signaling triggers the accumulation of β-catenin, leading to centrosome over-duplication and centriole splitting. This cascade of centrosomal events results in genomic instability and reduced cell numbers.

Integrating centrosome structure with protein composition and function in animal cells

The centrosome found in animal cells is a complex and dynamic organelle that functions as the major microtubule organizing center. Structural studies over the past several decades have defined the primary structural features of the centrosome but recent studies are now beginning to reveal structural detail previously unknown. Concurrent with these studies has been an explosion in the identification of the proteins that reside within the centrosome. Our growing understanding of how protein composition integrates with centrosome structure and hence with function is the focus of this review.

A protein related to brain microtubule-associated protein MAP1B is a component of the mammalian centrosome

The centrosome is the main microtubule organizing center of mammalian cells. Structurally, it is composed of a pair of centrioles surrounded by a fibro-granular material (the pericentriolar material) from which microtubules are nucleated. However, the nature of centrosomal molecules involved in microtubules nucleation is still obscure. Since brain microtubule-associated proteins (MAPs) lower the critical tubulin concentration required for microtubule nucleation in tubulin solution in vitro, we have examined their possible association with centrosomes. By immunofluorescence, monoclonal and polyclonal antibodies raised against MAP1B stain the centrosome in cultured cells as well as purified centrosomes, whereas antibodies raised against MAP2 give a completely negative reaction. The MAP1B-related antigen is localized to the pericentriolar material as revealed by immunoelectron microscopy. In preparations of purified centrosomes analyzed on poly-acrylamide gels, a protein that migrates as brain MAP1B is present. After blotting on nitrocellulose, it is decorated by anti-MAP1B antibodies and the amino acid sequence of proteolytic fragments of this protein is similar to brain MAP1B. Moreover, brain MAP1B and its centrosomal counterpart share the same phosphorylation features and have similar peptide maps. These data strongly suggest that a protein homologue to MAP1B is present in centrosomes and it is a good candidate for being involved in the nucleating activity of the pericentriolar material.

Identification of a 102 kDa protein (cytocentrin) immunologically related to keratin 19, which is a cytoplasmically derived component of the mitotic spindle pole

The mAb RK7, previously shown to recognize keratin 19, was also found to cross-react with a biologically unrelated 102 kDa protein, which becomes associated with the poles of the mitotic apparatus. This newly identified protein, called cytocentrin, is a stable cellular component, may be at least in part phosphorylated, and displays a cell cycle-dependent cellular localization. In interphase cells, it is diffusely distributed in the cytosol and shows no affinity for cytoplasmic microtubules. It becomes localized to the centrosome in early prophase, prior to nuclear envelope breakdown, separation of replicated centrosomes, and nucleation of mitotic apparatus microtubules. During metaphase, cytocentrin is located predominately at the mitotic poles, often appearing as an aggregate of small globular sub-components; it also associates with some polar microtubules. In late anaphase/early telophase cytocentrin dissociates entirely from the mitotic apparatus and becomes temporarily localized with microtubules in the midbody, from which it disappears by late telophase. In taxol-treated cells cytocentrin was associated with the center of the miniasters but also showed affinity for some cytoplasmic microtubules. Studies employing G2-synchronized cells and nocodazole demonstrated that cytocentrin can become associated with mitotic centrosomes independently of tubulin polymerization and that microtubules regrow from antigen-containing foci. We interpret these results to suggest that cytocentrin is a cytoplasmic protein that becomes specifically activated or modified at the onset of mitosis so that it can affiliate with the mitotic poles where it may provide a link between the pericentriolar material and other components of the mitotic apparatus.

Comparison of stability properties of lactate dehydrogenase B4/epsilon-crystallin from different species

epsilon-Crystallin occurs as an abundant lens protein in many birds and in crocodiles and has been identified as heart-type lactate dehydrogenase (LDH-B4). Lens proteins have, due to their longevity and environmental conditions, extraordinary requirements for structural stability. To study lens-protein stability, we compared various parameters of LDH-B4/epsilon-crystallin from lens and/or heart of duck, which has abundant amounts of this enzyme in its lenses, and of chicken and pig, which have no epsilon-crystallin. Measuring the thermostability of LDH-B4 from the different sources, the t50 values (temperature at which 50% of the enzyme activity remains after a 20-min period) for LDH-B4 from duck heart, duck lens and chicken heart were all found to be around 76 degrees C, whereas pig heart LDH-B4 was less thermostable, having a t50 value of 62.5 degrees C. A similar tendency was found with urea inactivation studies. Plotting the first-order rate constants obtained from inactivation kinetic plots against urea concentration, it was clear that LDH-B4 from pig heart was less stable in urea than the homologous enzymes from duck heart, chicken heart and duck lens. The duck and chicken enzymes were also much more resistant against proteolysis than the porcine enzyme. Therefore, it is concluded that avian LDH-B4 is structurally more stable than the homologous enzyme in mammals. This greater stability might make it suitable to function as a crystallin, as in duck, but is not necessarily associated with high lens expression, as in chicken.

Human centrosomal epitope is shared specifically with human lactate dehydrogenase-B isozyme

A rabbit serum (0013) used to identify pericentriolar proteins from isolated centrosomes (Gosti-Testu, F., Marty, M.C., Berges, J., Maunoury, R. and Bornens, M. (1986) EMBO J. 5, 2545-2550) was shown also to react through the same epitope with several non-centrosomal proteins including a major 36 kDa cytosolic antigen. This protein was identified to be human lactate dehydrogenase and the co-distribution of 0013 epitope on the centrosomal protein and on lactate dehydrogenase (LDH) was shown to be specific for human cells (Gosti, F., Marty, M.C., Courvalin, J.C., Maunoury, R. and Bornens, M. (1987) Proc. Natl. Acad. Sci. USA 84, 1000-1004). Human hepatic cells constitute, so far, the only exception to this co-distribution rule. By using this cell type which expresses only the LDH-A4 isozyme, we demonstrate that 0013 epitope is specific for the human LDH-B subunit, making serum 0013 the strongest anti-LDH-B available so far. The evolutionary and physiological significance of this situation is discussed.

Centrophilin: a novel mitotic spindle protein involved in microtubule nucleation

A novel protein has been identified which may serve a key function in nucleating spindle microtubule growth in mitosis. This protein, called centrophilin, is sequentially relocated from the centromeres to the centrosomes to the midbody in a manner dependent on the mitotic phase. Centrophilin was initially detected by immunofluorescence with a monoclonal, primate-specific antibody (2D3) raised against kinetochore-enriched chromosome extract from HeLa cells (Valdivia, M. M., and B. R. Brinkley. 1985. J. Cell Biol. 101:1124-1134). Centrophilin forms prominent crescents at the poles of the metaphase spindle, gradually diminishes during anaphase, and bands the equatorial ends of midbody microtubules in telophase. The formation and breakdown of the spindle and midbody correlates in time and space with the aggregation and disaggregation of centrophilin foci. Immunogold EM reveals that centrophilin is a major component of pericentriolar material in metaphase. During recovery from microtubule inhibition, centrophilin foci act as nucleation sites for the assembly of spindle tubules. The 2D3 probe recognizes two high molecular mass polypeptides, 180 and 210 kD, on immunoblots of whole HeLa cell extract. Taken together, these data and the available literature on microtubule dynamics point inevitably to a singular model for control of spindle tubule turnover.

The cortical microfilament system of lymphoblasts displays a periodic oscillatory activity in the absence of microtubules: implications for cell polarity

For an understanding of the role of microtubules in the definition of cell polarity, we have studied the cell surface motility of human lymphoblasts (KE37 cell line) using video microscopy, time-lapse photography, and immunofluorescent localization of F-actin and myosin. Polarized cell surface motility occurs in association with a constriction ring which forms on the centrosome side of the cell: the cytoplasm flows from the ring zone towards membrane veils which keep protruding in the same general direction. This association is ensured by microtubules: in their absence the ring is conspicuous and moves periodically back and forth across the cell, while a protrusion of membrane occurs alternately at each end of the cell when the ring is at the other. This oscillatory activity is correlated with a striking redistribution of myosin towards a cortical localization and appears to be due to the alternate flow of cortical myosin associated with the ring and to the periodic assembly of actin coupled with membrane protrusion. The ring cycle involves the progressive recruitment of myosin from a polar accumulation, or cap, its transportation across the cell and its accumulation in a new cap at the other end of the cell, suggesting an assembly-disassembly process. Inhibition of actin assembly induces, on the other hand, a dramatic microtubule-dependent cell elongation with definite polarity, likely to involve the interaction of microtubules with the cell cortex. We conclude that the polarized cell surface motility in KE37 cells is based on the periodic oscillatory activity of the actin system: a myosin-powered equatorial contraction and an actin-based membrane protrusion are concerted at the cell level and occur at opposite ends of the cell in absence of microtubules. This defines a polarity which reverses periodically as the ring moves across the cell. Microtubules impose a stable cell polarity by suppressing the ring movement. A permanent association of the myosin-powered contraction and the membrane protrusion is established which results in the unidirectional activity of the actin system. Microtubules exert their effect by controlling the recruitment of cytoplasmic myosin into the cortex, probably through their direct interaction with the cortical microfilament system.

Duck lens epsilon-crystallin and lactate dehydrogenase B4 are identical: a single-copy gene product with two distinct functions

To investigate whether or not duck lens epsilon-crystallin and duck heart lactate dehydrogenase (LDH) B4 are the product of the same gene, we have isolated and sequenced cDNA clones of duck epsilon-crystallin. By using these clones we demonstrate that there is a single-copy Ldh-B gene in duck and in chicken. In the duck lens this gene is overexpressed, and its product is subject to posttranslational modification. Reconstruction of the evolutionary history of the LDH protein family reveals that the mammalian Ldh-C gene most probably originated from an ancestral Ldh-A gene and that the amino acid replacement rate in LDH-C is approximately 4 times the rate in LDH-A. Molecular modeling of LDH-B sequences shows that the increased thermostability of the avian tetramer might be explained by mutations that increase the number of ion pairs. Furthermore, the replacement of bulky side chains by glycines on the corners of the duck protein suggests an adaptation to facilitate close packing in the lens.

Microtubules are required for centrosome expansion and positioning while microfilaments are required for centrosome separation in sea urchin eggs during fertilization and mitosis

Centrosomes undergo cell cycle-dependent changes in shape and separations, changes that govern the organization of the cytoskeleton. The cytoskeleton is largely organized by the centrosome; however, this investigation explores the importance of cytoskeletal elements in directing centrosome shape. Since the sea urchin egg during fertilization and mitosis displays dramatic and synchronous changes in centrosome shape, the effects of cytoskeletal inhibitors on centrosome compaction, expansion, and separation were explored by the use of anticentrosome immunofluorescence microscopy. Centrosome expansion and separation was studied during two phases: the transition after sperm incorporation, when the compact sperm centrosome enlarges and the sperm aster develops, and from prometaphase to telophase, when the compact spindle poles enlarge. Compaction was investigated when the dispersed centrosome at interphase condenses into the two spindle poles at prometaphase. Although centrosome expansion and separation typically occur concurrently, beta-mercaptoethanol results in centrosome separation independent of expansion. Microtubule inhibitors prevent centrosome expansion and separation, and expanded centrosomes collapse. Since pronuclear union is arrested by microtubule inhibitors, this treatment also affords the opportunity to explore the relative attractiveness of the male and female pronuclei for these centrosomal antigens. Both pronuclei acquire centrosomal material; though only the male centrosome is capable of organizing a functional bipolar mitotic apparatus at first division, the female centrosome nucleates a monaster. Microfilament inhibition (cytochalasin D) prevents centrosome separation but not expansion or compaction. These results demonstrate that as the centrosome shapes the cytoskeleton, the cytoskeleton alters centrosome shape.

Centrosome detection in sea urchin eggs with a monoclonal antibody against Drosophila intermediate filament proteins: characterization of stages of the division cycle of centrosomes

A mouse monoclonal antibody generated against Drosophila intermediate filament proteins (designated Ah6/5/9 and referred to herein as Ah6) is found to cross-react specifically with centrosomes in sea urchin eggs and with a 68-kDa antigen in eggs and isolated mitotic apparatus. When preparations stained with Ah6 are counterstained with a human autoimmune serum whose anti-centrosome activity has been established, the immunofluorescence images superimpose exactly. A more severe test of the specificity of the antibody demands that it display all of the stages of the centrosome cycle in the cell cycle: the flattening and spreading of the compact centrosomes followed by their division and the establishment of two compact poles. The test was made by an experimental design that uses a period of exposure of the eggs to 2-mercaptoethanol. This treatment allows observation of the stages of the centrosome cycle--separation, division, and bipolarization--while the chromosomes are arrested in metaphase. Mitosis is arrested in the presence of 0.1 M 2-mercaptoethanol. Chromosomes remain in a metaphase configuration while the centrosomes divide, producing four poles perpendicular to the original spindle axis. Microtubules are still present in the mitotic apparatus, as indicated by immunofluorescence and transmission electron microscopy. When 2-mercaptoethanol is removed, the chromosomes reorient to the poles of a tetrapolar (sometimes tripolar) mitotic apparatus. During the following cycle, the blastomeres form a monopolar mitotic apparatus. The observations of the centrosome cycle with the Ah6 antibody display very clearly all the stages that have been seen or deduced from work with other probes. The 68-kDa antigen that reacts with the Ah6 monoclonal antibody to Drosophila intermediate filament proteins must be a constant component of sea urchin centrosomes because it is present at all stages of the centrosome cycle.