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Gong T, McNally KL, Konanoor S, Peraza A, Bailey C, Redemann S, McNally FJ. Mechanisms of Meiotic Spindle Initiation in Caenorhabditis elegans Oocytes. bioRxiv 2024:2024.04.19.590357. [PMID: 38659754 PMCID: PMC11042349 DOI: 10.1101/2024.04.19.590357] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/26/2024]
Abstract
Microtubule-based spindle formation is essential to faithful chromosome segregation during cell division. In many animal species, the oocyte meiotic spindle forms without centrosomes, unlike most mitotic cells. Even in mitotic cells, centrosomes are sometimes dispensable for bipolar spindle formation. In some systems, Ran-GEF on chromatin initiates spindle assembly. We found that in C. elegans oocytes, endogenously-tagged Ran-GEF dissociates from chromatin during spindle assembly but re-associates during meiotic anaphase. Meiotic spindle assembly was normal after auxin-induced degradation of Ran-GEF but anaphase I was faster than controls and extrusion of the first polar body frequently failed. In search of a possible alternative pathway for spindle assembly, we found that soluble tubulin concentrates in the nuclear volume during germinal vesicle breakdown as well as in the spindle region during metaphase I and metaphase II. Through light and electron microscopy we found that the concentration of soluble tubulin in the metaphase II spindle region is enclosed by ER sheets which exclude cytoplasmic organelles including mitochondria and yolk granules from the meiotic spindle. We suggest that this concentration of soluble tubulin may be a redundant mechanism promoting spindle assembly near chromosomes. We present data supporting a model in which cytoplasmic organelles exclude cytoplasmic volume to drive concentration of tubulin within the nuclear/spindle envelope.
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Affiliation(s)
- Ting Gong
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616, USA
| | - Karen L McNally
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616, USA
| | - Siri Konanoor
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616, USA
| | - Alma Peraza
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616, USA
| | - Cynthia Bailey
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616, USA
| | - Stefanie Redemann
- Department of Cell Biology, University of Virginia, School of Medicine, Charlottesville, VA, USA
| | - Francis J McNally
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616, USA
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Beath E, Bailey C, Magadum MM, Qiu S, McNally KL, McNally FJ. Katanin, kinesin-13 and ataxin-2 inhibit premature interaction between maternal and paternal genomes in C. elegans zygotes. bioRxiv 2024:2024.03.12.584242. [PMID: 38559153 PMCID: PMC10979973 DOI: 10.1101/2024.03.12.584242] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Indexed: 04/04/2024]
Abstract
Fertilization occurs before completion of oocyte meiosis in the majority of animal species and sperm contents move long distances within zygotes of mouse and C. elegans. If incorporated into the meiotic spindle, paternal chromosomes could be expelled into a polar body resulting in lethal monosomy. Through live imaging of fertilization in C. elegans, we found that the microtubule disassembling enzymes, katanin and kinesin-13 limit long range movement of sperm contents and that maternal ataxin-2 maintains paternal DNA and paternal mitochondria as a cohesive unit that moves together. Depletion of katanin or double depletion of kinesin-13 and ataxin-2 resulted in capture of the sperm contents by the meiotic spindle. Thus limiting movement of sperm contents and maintaining cohesion of sperm contents within the zygote both contribute to preventing premature interaction between maternal and paternal genomes.
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Affiliation(s)
- Elizabeth Beath
- Department of Molecular and Cellular Biology, University of California, Davis Davis, CA 95618, USA
| | - Cynthia Bailey
- Department of Molecular and Cellular Biology, University of California, Davis Davis, CA 95618, USA
| | | | - Shuyan Qiu
- Department of Molecular and Cellular Biology, University of California, Davis Davis, CA 95618, USA
| | - Karen L McNally
- Department of Molecular and Cellular Biology, University of California, Davis Davis, CA 95618, USA
| | - Francis J McNally
- Department of Molecular and Cellular Biology, University of California, Davis Davis, CA 95618, USA
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Li W, Crellin HA, Cheerambathur D, McNally FJ. Redundant microtubule crosslinkers prevent meiotic spindle bending to ensure diploid offspring in C. elegans. PLoS Genet 2023; 19:e1011090. [PMID: 38150489 PMCID: PMC10775986 DOI: 10.1371/journal.pgen.1011090] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/14/2023] [Revised: 01/09/2024] [Accepted: 12/05/2023] [Indexed: 12/29/2023] Open
Abstract
Oocyte meiotic spindles mediate the expulsion of ¾ of the genome into polar bodies to generate diploid zygotes in nearly all animal species. Failures in this process result in aneuploid or polyploid offspring that are typically inviable. Accurate meiotic chromosome segregation and polar body extrusion require the spindle to elongate while maintaining its structural integrity. Previous studies have implicated three hypothetical activities during this process, including microtubule crosslinking, microtubule sliding and microtubule polymerization. However, how these activities regulate spindle rigidity and elongation as well as the exact proteins involved in the activities remain unclear. We discovered that C. elegans meiotic anaphase spindle integrity is maintained through redundant microtubule crosslinking activities of the Kinesin-5 family motor BMK-1, the microtubule bundling protein SPD-1/PRC1, and the Kinesin-4 family motor, KLP-19. Using time-lapse imaging, we found that single depletion of KLP-19KIF4A, SPD-1PRC1 or BMK-1Eg5 had minimal effects on anaphase B spindle elongation velocity. In contrast, double depletion of SPD-1PRC1 and BMK-1Eg5 or double depletion of KLP-19KIF4A and BMK-1Eg5 resulted in spindles that elongated faster, bent in a myosin-dependent manner, and had a high rate of polar body extrusion errors. Bending spindles frequently extruded both sets of segregating chromosomes into two separate polar bodies. Normal anaphase B velocity was observed after double depletion of KLP-19KIF4A and SPD-1PRC1. These results suggest that KLP-19KIF4A and SPD-1PRC1 act in different pathways, each redundant with a separate BMK-1Eg5 pathway in regulating meiotic spindle elongation. Depletion of ZYG-8, a doublecortin-related microtubule binding protein, led to slower anaphase B spindle elongation. We found that ZYG-8DCLK1 acts by excluding SPD-1PRC1 from the spindle. Thus, three mechanistically distinct microtubule regulation modules, two based on crosslinking, and one based on exclusion of crosslinkers, power the mechanism that drives spindle elongation and structural integrity during anaphase B of C.elegans female meiosis.
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Affiliation(s)
- Wenzhe Li
- Department of Molecular and Cellular Biology, University of California, Davis, California, United States of America
| | - Helena A. Crellin
- Wellcome Centre for Cell Biology & Institute of Cell Biology, School of Biological Sciences, University of Edinburgh, Edinburgh, United Kingdom
| | - Dhanya Cheerambathur
- Wellcome Centre for Cell Biology & Institute of Cell Biology, School of Biological Sciences, University of Edinburgh, Edinburgh, United Kingdom
| | - Francis J. McNally
- Department of Molecular and Cellular Biology, University of California, Davis, California, United States of America
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Gong T, McNally FJ. Caenorhabditis elegans spermatocytes can segregate achiasmate homologous chromosomes apart at higher than random frequency during meiosis I. Genetics 2023; 223:iyad021. [PMID: 36792551 PMCID: PMC10319977 DOI: 10.1093/genetics/iyad021] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/19/2022] [Revised: 02/05/2023] [Accepted: 02/07/2023] [Indexed: 02/17/2023] Open
Abstract
Chromosome segregation errors during meiosis are the leading cause of aneuploidy. Faithful chromosome segregation during meiosis in most eukaryotes requires a crossover which provides a physical attachment holding homologs together in a "bivalent." Crossovers are critical for homologs to be properly aligned and partitioned in the first meiotic division. Without a crossover, individual homologs (univalents) might segregate randomly, resulting in aneuploid progeny. However, Caenorhabditis elegans zim-2 mutants, which have crossover defects on chromosome V, have fewer dead embryos than that expected from random segregation. This deviation from random segregation is more pronounced in zim-2 males than that in females. We found three phenomena that can explain this apparent discrepancy. First, we detected crossovers on chromosome V in both zim-2(tm574) oocytes and spermatocytes, suggesting a redundant mechanism to make up for the ZIM-2 loss. Second, after accounting for the background crossover frequency, spermatocytes produced significantly more euploid gametes than what would be expected from random segregation. Lastly, trisomy of chromosome V is viable and fertile. Together, these three phenomena allow zim-2(tm574) mutants with reduced crossovers on chromosome V to have more viable progeny. Furthermore, live imaging of meiosis in spo-11(me44) oocytes and spermatocytes, which exhibit crossover failure on all 6 chromosomes, showed 12 univalents segregating apart in roughly equal masses in a homology-independent manner, supporting the existence of a mechanism that segregates any 2 chromosomes apart.
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Affiliation(s)
- Ting Gong
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616, USA
| | - Francis J McNally
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616, USA
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Juanico IY, Meyer CM, McCarthy JE, Gong T, McNally FJ. Paternal mitochondria from an rmd-2, rmd-3, rmd-6 triple mutant are properly positioned in the C. elegans zygote. MicroPubl Biol 2021; 2021:10.17912/micropub.biology.000422. [PMID: 34296068 PMCID: PMC8290264 DOI: 10.17912/micropub.biology.000422] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 06/25/2021] [Revised: 07/09/2021] [Accepted: 07/12/2021] [Indexed: 11/13/2022]
Abstract
RMD-1,2,3,6 (regulator of microtubule dynamics) is a family of homologous proteins conserved between humans and C. elegans. Human RMD-3/PTPIP51 is a mitochondrial protein that tethers mitochondria to the endoplasmic reticulum. C. elegans RMD-2, 3, and 6 are expressed in sperm. To test whether paternal RMD-2, 3, 6 might redundantly tether paternal mitochondria to maternal ER at fertilization, we generated an rmd-2, rmd-3, rmd-6 triple mutant. Paternal mitochondria derived from control or triple mutant worms were concentrated in a cloud around the paternal DNA at the future posterior end of zygotes during meiosis. No significant difference was detected in the position of paternal mitochondria within the zygote nor in the position of paternal mitochondria relative to paternal DNA within the zygote. There was also no reduction in progeny viability between control and triple mutant worms.
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Affiliation(s)
- Iris Y Juanico
- Dept. of Molecular and Cellular Biology, University of California, Davis, Davis, CA USA
| | - Christina M Meyer
- Dept. of Molecular and Cellular Biology, University of California, Davis, Davis, CA USA
| | - John E McCarthy
- Dept. of Molecular and Cellular Biology, University of California, Davis, Davis, CA USA
| | - Ting Gong
- Dept. of Molecular and Cellular Biology, University of California, Davis, Davis, CA USA
| | - Francis J McNally
- Dept. of Molecular and Cellular Biology, University of California, Davis, Davis, CA USA,
Correspondence to: Francis J McNally ()
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Danlasky BM, Panzica MT, McNally KP, Vargas E, Bailey C, Li W, Gong T, Fishman ES, Jiang X, McNally FJ. Evidence for anaphase pulling forces during C. elegans meiosis. J Cell Biol 2020; 219:211469. [PMID: 33064834 PMCID: PMC7577052 DOI: 10.1083/jcb.202005179] [Citation(s) in RCA: 20] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/24/2020] [Revised: 07/20/2020] [Accepted: 09/16/2020] [Indexed: 02/06/2023] Open
Abstract
Anaphase chromosome movement is thought to be mediated by pulling forces generated by end-on attachment of microtubules to the outer face of kinetochores. However, it has been suggested that during C. elegans female meiosis, anaphase is mediated by a kinetochore-independent pushing mechanism with microtubules only attached to the inner face of segregating chromosomes. We found that the kinetochore proteins KNL-1 and KNL-3 are required for preanaphase chromosome stretching, suggesting a role in pulling forces. In the absence of KNL-1,3, pairs of homologous chromosomes did not separate and did not move toward a spindle pole. Instead, each homolog pair moved together with the same spindle pole during anaphase B spindle elongation. Two masses of chromatin thus ended up at opposite spindle poles, giving the appearance of successful anaphase.
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Vargas E, McNally KP, Cortes DB, Panzica MT, Danlasky BM, Li Q, Maddox AS, McNally FJ. Spherical spindle shape promotes perpendicular cortical orientation by preventing isometric cortical pulling on both spindle poles during C. elegans female meiosis. Development 2019; 146:dev.178863. [PMID: 31575646 DOI: 10.1242/dev.178863] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/01/2019] [Accepted: 09/16/2019] [Indexed: 12/13/2022]
Abstract
Meiotic spindles are positioned perpendicular to the oocyte cortex to facilitate segregation of chromosomes into a large egg and a tiny polar body. In C. elegans, spindles are initially ellipsoid and parallel to the cortex before shortening to a near-spherical shape with flattened poles and then rotating to the perpendicular orientation by dynein-driven cortical pulling. The mechanistic connection between spindle shape and rotation has remained elusive. Here, we have used three different genetic backgrounds to manipulate spindle shape without eliminating dynein-dependent movement or dynein localization. Ellipsoid spindles with flattened or pointed poles became trapped in either a diagonal or a parallel orientation. Mathematical models that recapitulated the shape dependence of rotation indicated that the lower viscous drag experienced by spherical spindles prevented recapture of the cortex by astral microtubules emanating from the pole pivoting away from the cortex. In addition, maximizing contact between pole dynein and cortical dynein stabilizes flattened poles in a perpendicular orientation, and spindle rigidity prevents spindle bending that can lock both poles at the cortex. Spindle shape can thus promote perpendicular orientation by three distinct mechanisms.
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Affiliation(s)
- Elizabeth Vargas
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616, USA
| | - Karen P McNally
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616, USA
| | - Daniel B Cortes
- Biology Department, University of North Carolina, Chapel Hill, NC 27599, USA
| | - Michelle T Panzica
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616, USA
| | - Brennan M Danlasky
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616, USA
| | - Qianyan Li
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616, USA
| | - Amy Shaub Maddox
- Biology Department, University of North Carolina, Chapel Hill, NC 27599, USA
| | - Francis J McNally
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616, USA
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McNally FJ, Roll-Mecak A. Microtubule-severing enzymes: From cellular functions to molecular mechanism. J Cell Biol 2018; 217:4057-4069. [PMID: 30373906 PMCID: PMC6279391 DOI: 10.1083/jcb.201612104] [Citation(s) in RCA: 104] [Impact Index Per Article: 17.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/12/2018] [Revised: 08/13/2018] [Accepted: 10/04/2018] [Indexed: 12/24/2022] Open
Abstract
McNally and Roll-Mecak review the molecular mechanism of microtubule-severing enzymes and their diverse roles in processes ranging from cell division to ciliogensis and morphogenesis. Microtubule-severing enzymes generate internal breaks in microtubules. They are conserved in eukaryotes from ciliates to mammals, and their function is important in diverse cellular processes ranging from cilia biogenesis to cell division, phototropism, and neurogenesis. Their mutation leads to neurodegenerative and neurodevelopmental disorders in humans. All three known microtubule-severing enzymes, katanin, spastin, and fidgetin, are members of the meiotic subfamily of AAA ATPases that also includes VPS4, which disassembles ESCRTIII polymers. Despite their conservation and importance to cell physiology, the cellular and molecular mechanisms of action of microtubule-severing enzymes are not well understood. Here we review a subset of cellular processes that require microtubule-severing enzymes as well as recent advances in understanding their structure, biophysical mechanism, and regulation.
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Affiliation(s)
- Francis J McNally
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA
| | - Antonina Roll-Mecak
- Cell Biology and Biophysics Unit, Porter Neuroscience Research Center, National Institute of Neurological Disorders and Stroke, Bethesda, MD .,Biochemistry and Biophysics Center, National Heart, Lung and Blood Institute, Bethesda, MD
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Nithianantham S, McNally FJ, Al-Bassam J. Structural basis for disassembly of katanin heterododecamers. J Biol Chem 2018; 293:10590-10605. [PMID: 29752405 DOI: 10.1074/jbc.ra117.001215] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/04/2017] [Revised: 04/20/2018] [Indexed: 11/06/2022] Open
Abstract
The reorganization of microtubules in mitosis, meiosis, and development requires the microtubule-severing activity of katanin. Katanin is a heterodimer composed of an ATPase associated with diverse cellular activities (AAA) subunit and a regulatory subunit. Microtubule severing requires ATP hydrolysis by katanin's conserved AAA ATPase domains. Whereas other AAA ATPases form stable hexamers, we show that katanin forms only a monomer or dimers of heterodimers in solution. Katanin oligomers consistent with hexamers of heterodimers or heterododecamers were only observed for an ATP hydrolysis-deficient mutant in the presence of ATP. X-ray structures of katanin's AAA ATPase in monomeric nucleotide-free and pseudo-oligomeric ADP-bound states revealed conformational changes in the AAA subdomains that explained the structural basis for the instability of the katanin heterododecamer. We propose that the rapid dissociation of katanin AAA oligomers may lead to an autoinhibited state that prevents inappropriate microtubule severing or that cyclical disassembly into heterodimers may critically contribute to the microtubule-severing mechanism.
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Affiliation(s)
- Stanley Nithianantham
- From the Department of Molecular Cellular Biology University of California, Davis, California 95616
| | - Francis J McNally
- From the Department of Molecular Cellular Biology University of California, Davis, California 95616
| | - Jawdat Al-Bassam
- From the Department of Molecular Cellular Biology University of California, Davis, California 95616
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Abstract
Meiosis produces haploid gametes by accurately reducing chromosome ploidy through one round of DNA replication and two subsequent rounds of chromosome segregation and cell division. The cell divisions of female meiosis are highly asymmetric and give rise to a large egg and two very small polar bodies that do not contribute to development. These asymmetric divisions are driven by meiotic spindles that are small relative to the size of the egg and have one pole juxtaposed against the cell cortex to promote polar body extrusion. An additional unique feature of female meiosis is that fertilization occurs before extrusion of the second polar body in nearly all animal species. Thus sperm-derived chromosomes are present in the egg during female meiosis. Here, we explore the idea that the asymmetry of female meiosis spatially separates the sperm from the meiotic spindle to prevent detrimental interactions between the spindle and the paternal chromosomes.
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Affiliation(s)
- Michelle T Panzica
- a Department of Molecular and Cellular Biology , University of California , Davis , Davis , CA , USA
| | - Francis J McNally
- a Department of Molecular and Cellular Biology , University of California , Davis , Davis , CA , USA
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Vargas E, McNally K, Friedman JA, Cortes DB, Wang DY, Korf IF, McNally FJ. Autosomal Trisomy and Triploidy Are Corrected During Female Meiosis in Caenorhabditis elegans. Genetics 2017; 207:911-922. [PMID: 28882988 PMCID: PMC5676225 DOI: 10.1534/genetics.117.300259] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/03/2017] [Accepted: 09/05/2017] [Indexed: 11/18/2022] Open
Abstract
Trisomy and triploidy, defined as the presence of a third copy of one or all chromosomes, respectively, are deleterious in many species including humans. Previous studies have demonstrated that Caenorhabditis elegans with a third copy of the X chromosome are viable and fertile. However, the extra X chromosome was shown to preferentially segregate into the first polar body during oocyte meiosis to produce a higher frequency of euploid offspring than would be generated by random segregation. Here, we demonstrate that extra autosomes are preferentially eliminated by triploid C. elegans and trisomy IV C. elegans Live imaging of anaphase-lagging chromosomes and analysis of REC-8 staining of metaphase II spindles revealed that, in triploids, some univalent chromosomes do not lose cohesion and preferentially segregate intact into the first polar body during anaphase I, whereas other autosomes segregate chromatids equationally at anaphase I and eliminate some of the resulting single chromatids during anaphase II. We also demonstrate asymmetry in the anaphase spindle, which may contribute to the asymmetric segregation. This study reveals a pathway that allows aneuploid parents to produce euploid offspring at higher than random frequency.
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Affiliation(s)
- Elizabeth Vargas
- Department of Molecular and Cellular Biology, University of California, Davis, California 95616
| | - Karen McNally
- Department of Molecular and Cellular Biology, University of California, Davis, California 95616
| | - Jacob A Friedman
- Department of Molecular and Cellular Biology, University of California, Davis, California 95616
| | - Daniel B Cortes
- Department of Molecular and Cellular Biology, University of California, Davis, California 95616
| | - David Y Wang
- Department of Molecular and Cellular Biology, University of California, Davis, California 95616
| | - Ian F Korf
- Department of Molecular and Cellular Biology, University of California, Davis, California 95616
| | - Francis J McNally
- Department of Molecular and Cellular Biology, University of California, Davis, California 95616
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Flynn JR, McNally FJ. A casein kinase 1 prevents expulsion of the oocyte meiotic spindle into a polar body by regulating cortical contractility. Mol Biol Cell 2017; 28:2410-2419. [PMID: 28701347 PMCID: PMC5576904 DOI: 10.1091/mbc.e17-01-0056] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/23/2017] [Revised: 05/25/2017] [Accepted: 07/05/2017] [Indexed: 01/20/2023] Open
Abstract
Casein kinase 1 gamma inhibits the rho pathway to slow polar body contractile ring ingression. The polar body ring ingresses too far and engulfs the meiotic spindle in CSNK-1–depleted embryos. During female meiosis, haploid eggs are generated from diploid oocytes. This reduction in chromosome number occurs through two highly asymmetric cell divisions, resulting in one large egg and two small polar bodies. Unlike mitosis, where an actomyosin contractile ring forms between the sets of segregating chromosomes, the meiotic contractile ring forms on the cortex adjacent to one spindle pole, then ingresses down the length of the spindle to position itself at the exact midpoint between the two sets of segregating chromosomes. Depletion of casein kinase 1 gamma (CSNK-1) in Caenorhabditis elegans led to the formation of large polar bodies that contain all maternal DNA, because the contractile ring ingressed past the spindle midpoint. Depletion of CSNK-1 also resulted in the formation of deep membrane invaginations during meiosis, suggesting an effect on cortical myosin. Both myosin and anillin assemble into dynamic rho-dependent cortical patches that rapidly disassemble in wild-type embryos. CSNK-1 was required for disassembly of both myosin patches and anillin patches. Disassembly of anillin patches was myosin independent, suggesting that CSNK-1 prevents expulsion of the entire meiotic spindle into a polar body by negatively regulating the rho pathway rather than through direct inhibition of myosin.
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Affiliation(s)
- Jonathan R Flynn
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616
| | - Francis J McNally
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616
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Abstract
After fertilization, interactions between sperm and egg DNA must be prevented before the completion of female meiosis. Panzica et al. show that cortical tethering by F-actin prevents contact between the paternal DNA and the meiotic spindle. Fertilization occurs during female meiosis in most animals, which raises the question of what prevents the sperm DNA from interacting with the meiotic spindle. In this study, we find that Caenorhabditis elegans sperm DNA stays in a fixed position at the opposite end of the embryo from the meiotic spindle while yolk granules are transported throughout the embryo by kinesin-1. In the absence of F-actin, the sperm DNA, centrioles, and organelles were transported as a unit with the yolk granules, resulting in sperm DNA within 2 µm of the meiotic spindle. F-actin imaging revealed a cytoplasmic meshwork that might restrict transport in a size-dependent manner. However, increasing yolk granule size did not slow their velocity, and the F-actin moved with the yolk granules. Instead, sperm contents connect to the cortical F-actin to prevent interaction with the meiotic spindle.
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Affiliation(s)
- Michelle T Panzica
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA
| | - Harold C Marin
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA
| | | | - Francis J McNally
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA
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McNally KP, Panzica MT, Kim T, Cortes DB, McNally FJ. A novel chromosome segregation mechanism during female meiosis. Mol Biol Cell 2016; 27:2576-89. [PMID: 27335123 PMCID: PMC4985259 DOI: 10.1091/mbc.e16-05-0331] [Citation(s) in RCA: 29] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/26/2016] [Accepted: 06/14/2016] [Indexed: 01/28/2023] Open
Abstract
During conventional anaphase A, chromosomes move outward toward spindle poles. Caenorhabditis elegans meiotic spindle poles move inward toward chromosomes to achieve the same end. In a wide range of eukaryotes, chromosome segregation occurs through anaphase A, in which chromosomes move toward stationary spindle poles, anaphase B, in which chromosomes move at the same velocity as outwardly moving spindle poles, or both. In contrast, Caenorhabditis elegans female meiotic spindles initially shorten in the pole-to-pole axis such that spindle poles contact the outer kinetochore before the start of anaphase chromosome separation. Once the spindle pole-to-kinetochore contact has been made, the homologues of a 4-μm-long bivalent begin to separate. The spindle shortens an additional 0.5 μm until the chromosomes are embedded in the spindle poles. Chromosomes then separate at the same velocity as the spindle poles in an anaphase B–like movement. We conclude that the majority of meiotic chromosome movement is caused by shortening of the spindle to bring poles in contact with the chromosomes, followed by separation of chromosome-bound poles by outward sliding.
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Affiliation(s)
- Karen Perry McNally
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616
| | - Michelle T Panzica
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616
| | - Taekyung Kim
- Ludwig Institute for Cancer Research, San Diego, CA 92093 Department of Cellular and Molecular Medicine, University of California, San Diego, La Jolla, CA 92093
| | - Daniel B Cortes
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616
| | - Francis J McNally
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616
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Crowder ME, Flynn JR, McNally KP, Cortes DB, Price KL, Kuehnert PA, Panzica MT, Andaya A, Leary JA, McNally FJ. Dynactin-dependent cortical dynein and spherical spindle shape correlate temporally with meiotic spindle rotation in Caenorhabditis elegans. Mol Biol Cell 2015; 26:3030-46. [PMID: 26133383 PMCID: PMC4551317 DOI: 10.1091/mbc.e15-05-0290] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/19/2015] [Accepted: 06/24/2015] [Indexed: 11/11/2022] Open
Abstract
Cytoplasmic dynein accumulates on the cortex of Caenorhabditis elegans female meiotic spindles just before they rotate in a dynein-dependent manner. These spindles also shorten to a spherical shape that might reduce the drag that opposes cortical pulling by dynein. Oocyte meiotic spindles orient with one pole juxtaposed to the cortex to facilitate extrusion of chromosomes into polar bodies. In Caenorhabditis elegans, these acentriolar spindles initially orient parallel to the cortex and then rotate to the perpendicular orientation. To understand the mechanism of spindle rotation, we characterized events that correlated temporally with rotation, including shortening of the spindle in the pole-to pole axis, which resulted in a nearly spherical spindle at rotation. By analyzing large spindles of polyploid C. elegans and a related nematode species, we found that spindle rotation initiated at a defined spherical shape rather than at a defined spindle length. In addition, dynein accumulated on the cortex just before rotation, and microtubules grew from the spindle with plus ends outward during rotation. Dynactin depletion prevented accumulation of dynein on the cortex and prevented spindle rotation independently of effects on spindle shape. These results support a cortical pulling model in which spindle shape might facilitate rotation because a sphere can rotate without deforming the adjacent elastic cytoplasm. We also present evidence that activation of spindle rotation is promoted by dephosphorylation of the basic domain of p150 dynactin.
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Affiliation(s)
- Marina E Crowder
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616
| | - Jonathan R Flynn
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616
| | - Karen P McNally
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616
| | - Daniel B Cortes
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616
| | - Kari L Price
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616
| | - Paul A Kuehnert
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616
| | - Michelle T Panzica
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616
| | - Armann Andaya
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616
| | - Julie A Leary
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616
| | - Francis J McNally
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616
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17
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Cortes DB, McNally KL, Mains PE, McNally FJ. The asymmetry of female meiosis reduces the frequency of inheritance of unpaired chromosomes. eLife 2015; 4:e06056. [PMID: 25848744 PMCID: PMC4412107 DOI: 10.7554/elife.06056] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/12/2014] [Accepted: 04/03/2015] [Indexed: 11/17/2022] Open
Abstract
Trisomy, the presence of a third copy of one chromosome, is deleterious and results in inviable or defective progeny if passed through the germ line. Random segregation of an extra chromosome is predicted to result in a high frequency of trisomic offspring from a trisomic parent. Caenorhabditis elegans with trisomy of the X chromosome, however, have far fewer trisomic offspring than expected. We found that the extra X chromosome was preferentially eliminated during anaphase I of female meiosis. We utilized a mutant with a specific defect in pairing of the X chromosome as a model to investigate the apparent bias against univalent inheritance. First, univalents lagged during anaphase I and their movement was biased toward the cortex and future polar body. Second, late-lagging univalents were frequently captured by the ingressing polar body contractile ring. The asymmetry of female meiosis can thus partially correct pre-existing trisomy.
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Affiliation(s)
- Daniel B Cortes
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, United States
| | - Karen L McNally
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, United States
| | - Paul E Mains
- Department of Biochemistry and Molecular Biology, University of Calgary, Calgary, Canada
| | - Francis J McNally
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, United States
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18
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McNally K, Berg E, Cortes DB, Hernandez V, Mains PE, McNally FJ. Katanin maintains meiotic metaphase chromosome alignment and spindle structure in vivo and has multiple effects on microtubules in vitro. Mol Biol Cell 2014; 25:1037-49. [PMID: 24501424 PMCID: PMC3967969 DOI: 10.1091/mbc.e13-12-0764] [Citation(s) in RCA: 43] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022] Open
Abstract
Caenorhabditis elegans bivalents are positioned between dense bundles of microtubules within female meiotic spindles. Rapid inactivation of katanin after meiotic spindle assembly causes loss of organized microtubule bundles and displacement of bivalents from the metaphase plate. Purified katanin can preferentially sever at intersections between microtubules. Assembly of Caenorhabditis elegans female meiotic spindles requires both MEI-1 and MEI-2 subunits of the microtubule-severing ATPase katanin. Strong loss-of-function mutants assemble apolar intersecting microtubule arrays, whereas weaker mutants assemble bipolar meiotic spindles that are longer than wild type. To determine whether katanin is also required for spindle maintenance, we monitored metaphase I spindles after a fast-acting mei-1(ts) mutant was shifted to a nonpermissive temperature. Within 4 min of temperature shift, bivalents moved off the metaphase plate, and microtubule bundles within the spindle lengthened and developed a high degree of curvature. Spindles eventually lost bipolar structure. Immunofluorescence of embryos fixed at increasing temperature indicated that MEI-1 was lost from spindle microtubules before loss of ASPM-1, indicating that MEI-1 and ASPM-1 act independently at spindle poles. We quantified the microtubule-severing activity of purified MEI-1/MEI-2 complexes corresponding to six different point mutations and found a linear relationship between microtubule disassembly rate and meiotic spindle length. Previous work showed that katanin is required for severing at points where two microtubules intersect in vivo. We show that purified MEI-1/MEI-2 complexes preferentially sever at intersections between two microtubules and directly bundle microtubules in vitro. These activities could promote parallel/antiparallel microtubule organization in meiotic spindles.
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Affiliation(s)
- Karen McNally
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616 Genes and Development Research Group, Department of Biochemistry and Molecular Biology, University of Calgary, Calgary, AB T2N 4N1, Canada
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19
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Abstract
Accurate positioning of spindles is essential for asymmetric mitotic and meiotic cell divisions that are crucial for animal development and oocyte maturation, respectively. The predominant model for spindle positioning, termed "cortical pulling," involves attachment of the microtubule-based motor cytoplasmic dynein to the cortex, where it exerts a pulling force on microtubules that extend from the spindle poles to the cell cortex, thereby displacing the spindle. Recent studies have addressed important details of the cortical pulling mechanism and have revealed alternative mechanisms that may be used when microtubules do not extend from the spindle to the cortex.
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Affiliation(s)
- Francis J McNally
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616, USA.
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20
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Loughlin R, Wilbur JD, McNally FJ, Nédélec FJ, Heald R. Katanin contributes to interspecies spindle length scaling in Xenopus. Cell 2012; 147:1397-407. [PMID: 22153081 DOI: 10.1016/j.cell.2011.11.014] [Citation(s) in RCA: 153] [Impact Index Per Article: 12.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/02/2011] [Revised: 08/01/2011] [Accepted: 11/07/2011] [Indexed: 11/19/2022]
Abstract
Bipolar spindles must separate chromosomes by the appropriate distance during cell division, but mechanisms determining spindle length are poorly understood. Based on a 2D model of meiotic spindle assembly, we predicted that higher localized microtubule (MT) depolymerization rates could generate the shorter spindles observed in egg extracts of X. tropicalis compared to X. laevis. We found that katanin-dependent MT severing was increased in X. tropicalis, which, unlike X. laevis, lacks an inhibitory phosphorylation site in the katanin p60 catalytic subunit. Katanin inhibition lengthened spindles in both species. In X. tropicalis, k-fiber MT bundles that connect to chromosomes at their kinetochores extended through spindle poles, disrupting them. In both X. tropicalis extracts and the spindle simulation, a balance between k-fiber number and MT depolymerization is required to maintain spindle morphology. Thus, mechanisms have evolved in different species to scale spindle size and coordinate regulation of multiple MT populations in order to generate a robust steady-state structure.
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Affiliation(s)
- Rose Loughlin
- Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720, USA
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21
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Abstract
Before chromosome expulsion into polar bodies during female meiosis, the APC inhibits CDK-1 to allow dynein-driven spindle rotation. In animals, the female meiotic spindle is positioned at the egg cortex in a perpendicular orientation to facilitate the disposal of half of the chromosomes into a polar body. In Caenorhabditis elegans, the metaphase spindle lies parallel to the cortex, dynein is dispersed on the spindle, and the dynein activators ASPM-1 and LIN-5 are concentrated at spindle poles. Anaphase-promoting complex (APC) activation results in dynein accumulation at spindle poles and dynein-dependent rotation of one spindle pole to the cortex, resulting in perpendicular orientation. To test whether the APC initiates spindle rotation through cyclin B–CDK-1 inactivation, separase activation, or degradation of an unknown dynein inhibitor, CDK-1 was inhibited with purvalanol A in metaphase-I–arrested, APC-depleted embryos. CDK-1 inhibition resulted in the accumulation of dynein at spindle poles and dynein-dependent spindle rotation without chromosome separation. These results suggest that CDK-1 blocks rotation by inhibiting dynein association with microtubules and with LIN-5–ASPM-1 at meiotic spindle poles and that the APC promotes spindle rotation by inhibiting CDK-1.
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Affiliation(s)
- Marina L Ellefson
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616, USA
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22
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Abstract
Katanin is a heterodimeric microtubule-severing protein that is conserved among eukaryotes. Loss-of-function mutations in the Caenorhabditis elegans katanin catalytic subunit, MEI-1, cause specific defects in female meiotic spindles. To determine the relationship between katanin's microtubule-severing activity and its role in meiotic spindle formation, we analyzed the MEI-1(A338S) mutant. Unlike wild-type MEI-1, which mediated disassembly of microtubule arrays in Xenopus fibroblasts, MEI-1(A338S) had no effect on fibroblast microtubules, indicating a lack of microtubule-severing activity. In C. elegans, MEI-1(A338S) mediated assembly of extremely long bipolar meiotic spindles. In contrast, a nonsense mutation in MEI-1 caused assembly of meiotic spindles without any poles as assayed by localization of the spindle-pole protein, ASPM-1. These results indicated that katanin protein, but not katanin's microtubule-severing activity, is required for assembly of acentriolar meiotic spindle poles. To understand the nonsevering activities of katanin, we characterized the N-terminal domain of the katanin catalytic subunit. The N-terminal domain was necessary and sufficient for binding to the katanin regulatory subunit. The katanin regulatory subunit in turn caused a dramatic change in the microtubule-binding properties of the N-terminal domain of the catalytic subunit. This unique bipartite microtubule-binding structure may mediate the spindle-pole assembly activity of katanin during female meiosis.
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Affiliation(s)
- Karen Perry McNally
- Department of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616, USA.
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23
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Fabritius AS, Ellefson ML, McNally FJ. Nuclear and spindle positioning during oocyte meiosis. Curr Opin Cell Biol 2010; 23:78-84. [PMID: 20708397 DOI: 10.1016/j.ceb.2010.07.008] [Citation(s) in RCA: 50] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/06/2010] [Revised: 07/16/2010] [Accepted: 07/19/2010] [Indexed: 10/19/2022]
Abstract
Female meiosis is unique in that an asymmetrically positioned meiotic spindle expels chromosomes into tiny, non-developing polar bodies. The extrusion of chromosomes into polar bodies is always mediated by meiotic spindles that are attached to the oocyte cortex by one pole. The asymmetric, cortical positioning of the oocyte meiotic spindle preserves the volume and contents of the oocyte. Recent work in C. elegans and mouse has provided mechanistic details of spindle positioning in oocytes.
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Affiliation(s)
- Amy S Fabritius
- Department of Molecular and Cellular Biology, University of California-Davis, Davis, CA 95616, United States
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24
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McNally KL, Martin JL, Ellefson M, McNally FJ. Kinesin-dependent transport results in polarized migration of the nucleus in oocytes and inward movement of yolk granules in meiotic embryos. Dev Biol 2009; 339:126-40. [PMID: 20036653 DOI: 10.1016/j.ydbio.2009.12.021] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/13/2009] [Revised: 12/11/2009] [Accepted: 12/16/2009] [Indexed: 01/28/2023]
Abstract
During female meiosis, meiotic spindles are positioned at the oocyte cortex to allow expulsion of chromosomes into polar bodies. In C. elegans, kinesin-dependent translocation of the entire spindle to the cortex precedes dynein-dependent rotation of one spindle pole toward the cortex. To elucidate the role of kinesin-1 in spindle translocation, we examined the localization of kinesin subunits in meiotic embryos. Surprisingly, kinesin-1 was not associated with the spindle and instead was restricted to the cytoplasm in the middle of the embryo. Yolk granules moved on linear tracks, in a kinesin-dependent manner, away from the cortex, resulting in their concentration in the middle of the embryo where the kinesin was concentrated. These results suggest that cytoplasmic microtubules might be arranged with plus ends extending inward, away from the cortex. This microtubule arrangement would not be consistent with direct transport of the meiotic spindle toward the cortex by kinesin-1. In maturing oocytes, the nucleus underwent kinesin-dependent migration to the future site of spindle attachment at the anterior cortex. Thus the spindle translocation defect observed in kinesin-1 mutants may be a result of failed nuclear migration, which places the spindle too far from the cortex for the spindle translocation mechanism to function.
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Affiliation(s)
- Karen L McNally
- Section of Molecular and Cellular Biology, 149 Briggs Hall, University of California, Davis, Davis, CA 95616, USA
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25
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Meyerzon M, Fridolfsson HN, Ly N, McNally FJ, Starr DA. UNC-83 is a nuclear-specific cargo adaptor for kinesin-1-mediated nuclear migration. Development 2009; 136:2725-33. [PMID: 19605495 DOI: 10.1242/dev.038596] [Citation(s) in RCA: 79] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Intracellular nuclear migration is essential for many cellular events including fertilization, establishment of polarity, division and differentiation. How nuclei migrate is not understood at the molecular level. The C. elegans KASH protein UNC-83 is required for nuclear migration and localizes to the outer nuclear membrane. UNC-83 interacts with the inner nuclear membrane SUN protein UNC-84 and is proposed to connect the cytoskeleton to the nuclear lamina. Here, we show that UNC-83 also interacts with the kinesin-1 light chain KLC-2, as identified in a yeast two-hybrid screen and confirmed by in vitro assays. UNC-83 interacts with and recruits KLC-2 to the nuclear envelope in a heterologous tissue culture system. Additionally, analysis of mutant phenotypes demonstrated that both KLC-2 and the kinesin-1 heavy chain UNC-116 are required for nuclear migration. Finally, the requirement for UNC-83 in nuclear migration could be partially bypassed by expressing a synthetic outer nuclear membrane KLC-2::KASH fusion protein. Our data support a model in which UNC-83 plays a central role in nuclear migration by acting to bridge the nuclear envelope and as a kinesin-1 cargo-specific adaptor so that motor-generated forces specifically move the nucleus as a single unit.
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Affiliation(s)
- Marina Meyerzon
- Department of Molecular and Cellular Biology, University of California, Davis, CA 95616, USA
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26
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Johnson JLFA, Lu C, Raharjo E, McNally K, McNally FJ, Mains PE. Levels of the ubiquitin ligase substrate adaptor MEL-26 are inversely correlated with MEI-1/katanin microtubule-severing activity during both meiosis and mitosis. Dev Biol 2009; 330:349-57. [PMID: 19361490 DOI: 10.1016/j.ydbio.2009.04.004] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/08/2008] [Revised: 03/04/2009] [Accepted: 04/02/2009] [Indexed: 10/20/2022]
Abstract
The MEI-1/MEI-2 microtubule-severing complex, katanin, is required for oocyte meiotic spindle formation and function in C. elegans, but the microtubule-severing activity must be quickly downregulated so that it does not interfere with formation of the first mitotic spindle. Post-meiotic MEI-1 inactivation is accomplished by two parallel protein degradation pathways, one of which requires MEL-26, the substrate-specific adaptor that recruits MEI-1 to a CUL-3 based ubiquitin ligase. Here we address the question of how MEL-26 mediated MEI-1 degradation is triggered only after the completion of MEI-1's meiotic function. We find that MEL-26 is present only at low levels until the completion of meiosis, after which protein levels increase substantially, likely increasing the post-meiotic degradation of MEI-1. During meiosis, MEL-26 levels are kept low by the action of another type of ubiquitin ligase, which contains CUL-2. However, we find that the low levels of meiotic MEL-26 have a subtle function, acting to moderate MEI-1 activity during meiosis. We also show that MEI-1 is the only essential target for MEL-26, and possibly for the E3 ubiquitin ligase CUL-3, but the upstream ubiquitin ligase activating enzyme RFL-1 has additional essential targets.
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Affiliation(s)
- Jacque-Lynne F A Johnson
- Genes and Development Research Group, Department of Biochemistry and Molecular Biology, University of Calgary, Calgary, AB, Canada T2N 4N1
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27
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Ellefson ML, McNally FJ. Kinesin-1 and cytoplasmic dynein act sequentially to move the meiotic spindle to the oocyte cortex in Caenorhabditis elegans. Mol Biol Cell 2009; 20:2722-30. [PMID: 19357192 DOI: 10.1091/mbc.e08-12-1253] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023] Open
Abstract
During female meiosis in animals, the meiotic spindle is attached to the egg cortex by one pole during anaphase to allow selective disposal of half the chromosomes in a polar body. In Caenorhabditis elegans, this anaphase spindle position is achieved sequentially through kinesin-1-dependent early translocation followed by anaphase-promoting complex (APC)-dependent spindle rotation. Partial depletion of cytoplasmic dynein heavy chain by RNA interference blocked spindle rotation without affecting early translocation. Dynein depletion also blocked the APC-dependent late translocation that occurs in kinesin-1-depleted embryos. Time-lapse imaging of green fluorescent protein-tagged dynein heavy chain as well as immunofluorescence with dynein-specific antibodies revealed that dynein starts to accumulate at spindle poles just before the initiation of rotation or late translocation. Accumulation of dynein at poles was kinesin-1 independent and APC dependent, just like dynein driven spindle movements. This represents a case of kinesin-1/dynein coordination in which these two motors of opposite polarity act sequentially and independently on a cargo to move it in the same direction.
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Affiliation(s)
- Marina L Ellefson
- Department of Molecular and Cellular Biology, University of California Davis, Davis, CA 95616, USA
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28
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Abstract
Accurate control of spindle length is a conserved feature of eukaryotic cell division. Lengthening of mitotic spindles contributes to chromosome segregation and cytokinesis during mitosis in animals and fungi. In contrast, spindle shortening may contribute to conservation of egg cytoplasm during female meiosis. Katanin is a microtubule-severing enzyme that is concentrated at mitotic and meiotic spindle poles in animals. We show that inhibition of katanin slows the rate of spindle shortening in nocodazole-treated mammalian fibroblasts and in untreated Caenorhabditis elegans meiotic embryos. Wild-type C. elegans meiotic spindle shortening proceeds through an early katanin-independent phase marked by increasing microtubule density and a second, katanin-dependent phase that occurs after microtubule density stops increasing. In addition, double-mutant analysis indicated that gamma-tubulin-dependent nucleation and microtubule severing may provide redundant mechanisms for increasing microtubule number during the early stages of meiotic spindle assembly.
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Affiliation(s)
- Karen McNally
- Section of Molecular and Cellular Biology, University of California, Davis, CA 95616, USA
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29
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Abstract
In animals, female meiotic spindles are attached to the egg cortex in a perpendicular orientation at anaphase to allow the selective disposal of three haploid chromosome sets into polar bodies. We have identified a complex of interacting Caenorhabditis elegans proteins that are involved in the earliest step in asymmetric positioning of anastral meiotic spindles, translocation to the cortex. This complex is composed of the kinesin-1 heavy chain orthologue, UNC-116, the kinesin light chain orthologues, KLC-1 and -2, and a novel cargo adaptor, KCA-1. Depletion of any of these subunits by RNA interference resulted in meiosis I metaphase spindles that remained stationary at a position several micrometers from the cell cortex during the time when wild-type spindles translocated to the cortex. After this prolonged stationary period, unc-116(RNAi) spindles moved to the cortex through a partially redundant mechanism that is dependent on the anaphase-promoting complex. This study thus reveals two sequential mechanisms for translocating anastral spindles to the oocyte cortex.
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Affiliation(s)
- Hsin-ya Yang
- Section of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616, USA
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30
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McNally KL, McNally FJ. Fertilization initiates the transition from anaphase I to metaphase II during female meiosis in C. elegans. Dev Biol 2005; 282:218-30. [PMID: 15936342 DOI: 10.1016/j.ydbio.2005.03.009] [Citation(s) in RCA: 60] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/08/2004] [Revised: 03/02/2005] [Accepted: 03/15/2005] [Indexed: 11/15/2022]
Abstract
Oocytes from most animals arrest twice during the meiotic cell cycle. The universally conserved prophase I arrest is released by a maturation hormone that allows progression to a second arrest point, typically metaphase I or II. This second arrest allows for short-term storage of fertilization-competent eggs and is released by signaling that occurs during fertilization. Nematodes are unique in that the maturation hormone is secreted by sperm rather than by the mother's somatic tissues. We have investigated the nature of the second arrest in matured but unfertilized Caenorhabditis elegans embryos using time-lapse imaging of GFP-tubulin or GFP-histone. Unfertilized embryos completed anaphase I but did not form polar bodies or assemble meiosis II spindles. Nevertheless, unfertilized embryos assembled female pronuclei at the same time as fertilized embryos. Analysis of embryos fertilized by sperm lacking the SPE-11 protein indicated that fertilization promotes meiotic cytokinesis through the SPE-11 protein but assembly of the meiosis II spindle is initiated through an SPE-11-independent pathway.
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Affiliation(s)
- Karen L McNally
- Section of Molecular and Cellular Biology, 149 Briggs Hall, University of California, Davis, CA 95616, USA
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31
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Abstract
In most animals, successful segregation of female meiotic chromosomes involves sequential associations of the meiosis I and meiosis II spindles with the cell cortex so that extra chromosomes can be deposited in polar bodies. The resulting reduction in chromosome number is essential to prevent the generation of polyploid embryos after fertilization. Using time-lapse imaging of living Caenorhabditis elegans oocytes containing fluorescently labeled chromosomes or microtubules, we have characterized the movements of meiotic spindles relative to the cell cortex. Spindle assembly initiated several microns from the cortex. After formation of a bipolar structure, the meiosis I spindle translocated to the cortex. When microtubules were partially depleted, translocation of the bivalent chromosomes to the cortex was blocked without affecting cell cycle timing. In oocytes depleted of the microtubule-severing enzyme, MEI-1, spindles moved to the cortex, but association with the cortex was unstable. Unlike translocation of wild-type spindles, movement of MEI-1-depleted spindles was dependent on FZY-1/CDC20, a regulator of the metaphase/anaphase transition. We observed a microtubule and FZY-1/CDC20-dependent circular cytoplasmic streaming in wild-type and mei-1 mutant embryos during meiosis. We propose that, in mei-1 mutant oocytes, this cytoplasmic streaming is sufficient to drive the spindle into the cortex. Cytoplasmic streaming is not the normal spindle translocation mechanism because translocation occurred in the absence of cytoplasmic streaming in embryos depleted of either the orbit/CLASP homolog, CLS-2, or FZY-1. These results indicate a direct role of microtubule severing in translocation of the meiotic spindle to the cortex.
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Affiliation(s)
- Hsin-ya Yang
- Section of Molecular and Cellular Biology, University of California, Davis, CA 95616, USA
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32
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Abstract
Microtubules are essential for a wide range of cellular processes that vary between cell types. Katanin is a microtubule-severing protein that carries out an essential role in meiotic spindles in Caenorhabditis elegans and a non-essential role in mitotic spindles of vertebrates. In contrast to these M-phase associated roles, katanin is also essential for post-mitotic differentiation events in vertebrate neurons and in Arabidopsis. This diversity of function suggests that katanin's activity might be regulated by multiple mechanisms. Because katanin is active in M-phase Xenopus extracts but not in interphase extracts, we assayed for regulators of katanin's activity in these extracts. The microtubule-severing activity of purified katanin was inhibited by interphase Xenopus extracts. Fractionation revealed that this inhibition was due to at least 4 separable components, one of which contains the MAP4 homolog, XMAP230. Inhibition of katanin-mediated microtubule-disassembly activity by the XMAP230-containing fraction was reversible by cyclinB/cdk1, suggesting one possible mechanism for the increased severing activity observed in M-phase Xenopus extracts. In a previous study, spindle pole association by katanin was essential for its activity during mitosis suggesting that katanin's activity might also be regulated by co-localization with an activator. The polo-like kinase, Plx1, co-localized with katanin at spindle poles in vivo and purified Plx1 increased the microtubule-severing activity of katanin in vitro. These in vitro experiments illustrate the potential complexity of the regulation of katanin's activity in vivo and may explain how katanin can carry out widely different functions in different cell types.
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Affiliation(s)
- Karen Perry McNally
- Section of Molecular and Cellular Biology, University of California, Davis 95616, USA
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33
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Abstract
Katanin is a microtubule-severing protein that is concentrated at mitotic spindle poles but katanin's function in the mitotic spindle has not been previously reported. Inhibition of katanin with either of two dominant-negative proteins or a subunit-specific antibody prevented the redistribution of γ-tubulin from the centrosome to the spindle in prometaphase CV-1 cells as assayed by immunofluorescence microscopy. Becauseγ-tubulin complexes can bind to pre-existing microtubule minus ends,these results could be explained by a model in which the broad distribution ofγ-tubulin in the mitotic spindle is in part due to cytosolicγ-tubulin ring complexes binding to microtubule minus ends generated by katanin-mediated microtubule severing. Because microtubules depolymerize at their ends, we hypothesized that a greater number of microtubule ends generated by severing in the spindle would result in an increased rate of spindle disassembly when polymerization is blocked with nocodazole. Indeed,katanin inhibition slowed the rate of spindle microtubule disassembly in the presence of nocodazole. However, katanin inhibition did not affect the rate of exchange between polymerized and unpolymerized tubulin as assayed by fluorescence recovery after photobleaching. These results support a model in which katanin activity regulates the number of microtubule ends in the spindle.
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Affiliation(s)
- Dan Buster
- Section of Molecular and Cellular Biology, University of California, Davis, CA 95616, USA
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34
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Abstract
Microtubules play a critical role in establishing asymmetry in many cell types. Recent work suggests how conserved protein complexes that bind specifically to the growing ends of microtubules may establish polarized links to special regions of the cell cortex.
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Affiliation(s)
- F J McNally
- Section of Molecular and Cellular Biology, University of California at Davis, 95616, USA.
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35
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Srayko M, Buster DW, Bazirgan OA, McNally FJ, Mains PE. MEI-1/MEI-2 katanin-like microtubule severing activity is required for Caenorhabditis elegans meiosis. Genes Dev 2000. [DOI: 10.1101/gad.14.9.1072] [Citation(s) in RCA: 80] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Abstract
The Caenorhabditis elegans meiotic spindle is morphologically distinct from the first mitotic spindle, yet both structures form in the same cytoplasm ∼20 minutes apart. Themei-1 and mei-2 genes of C. elegans are required for the establishment of the oocyte meiotic spindle but are not required for mitotic spindle function. mei-1 encodes an AAA ATPase family member with similarity to the p60 catalytic subunit of the heterodimeric sea urchin microtubule-severing protein, katanin. We report that mei-2 encodes a 280-amino acid protein containing a region similar to the p80-targeting subunit of katanin. MEI-1 and MEI-2 antibodies decorate the polar ends of meiotic spindle microtubules and meiotic chromatin. We find that the subcellular location of MEI-2 depends on wild-type mei-1 activity and vice versa. These experiments, combined with MEI-1 and MEI-2's similarity to p60 and p80 katanin, suggest that the C. elegans proteins function as a complex. In support of this idea, MEI-1 and MEI-2 physically associate in HeLa cells. Furthermore, co-expression of MEI-1 and MEI-2 in HeLa cells results in the disassembly of microtubules. These data lead us to conclude that MEI-1/MEI-2 microtubule-severing activity is required for meiotic spindle organization in C. elegans.
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36
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Abstract
The assembly and function of the mitotic spindle requires the activity of a number of microtubule-binding proteins. Some microtubule-binding proteins bind microtubules in vitro but do not co-localize with microtubules in interphase cells. Instead these proteins associate with specific subregions of the mitotic spindle. Katanin, a heterodimeric microtubule-severing ATPase, is found localized at mitotic spindle poles. In this paper we demonstrate that human p60 katanin and the C-terminal domain of human p80 katanin both bind microtubules in vitro. Association of these two proteins results in an increased microtubule affinity and increased microtubule-severing activity in vitro. Association of these subunits in transfected HeLa cells increases microtubule disassembly activity and targeting to spindle poles. The N-terminal WD40 domain of p80 katanin acts as a negative regulator of microtubule disassembly activity and is also required for spindle pole localization, possibly through interactions with another spindle-pole protein. These results support a model in which katanin is targeted to spindle poles through a combination of direct microtubule binding by the p60 subunit and through interactions between the WD40 domain and an unknown protein. We propose that both domains of p80 are essential in precisely regulating katanin's activity in vivo.
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Affiliation(s)
- K P McNally
- Section of Molecular and Cellular Biology, University of California, Davis, CA 95616, USA.
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37
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Srayko M, Buster DW, Bazirgan OA, McNally FJ, Mains PE. MEI-1/MEI-2 katanin-like microtubule severing activity is required for Caenorhabditis elegans meiosis. Genes Dev 2000; 14:1072-84. [PMID: 10809666 PMCID: PMC316576] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/16/2023]
Abstract
The Caenorhabditis elegans meiotic spindle is morphologically distinct from the first mitotic spindle, yet both structures form in the same cytoplasm approximately 20 minutes apart. The mei-1 and mei-2 genes of C. elegans are required for the establishment of the oocyte meiotic spindle but are not required for mitotic spindle function. mei-1 encodes an AAA ATPase family member with similarity to the p60 catalytic subunit of the heterodimeric sea urchin microtubule-severing protein, katanin. We report that mei-2 encodes a 280-amino acid protein containing a region similar to the p80-targeting subunit of katanin. MEI-1 and MEI-2 antibodies decorate the polar ends of meiotic spindle microtubules and meiotic chromatin. We find that the subcellular location of MEI-2 depends on wild-type mei-1 activity and vice versa. These experiments, combined with MEI-1 and MEI-2's similarity to p60 and p80 katanin, suggest that the C. elegans proteins function as a complex. In support of this idea, MEI-1 and MEI-2 physically associate in HeLa cells. Furthermore, co-expression of MEI-1 and MEI-2 in HeLa cells results in the disassembly of microtubules. These data lead us to conclude that MEI-1/MEI-2 microtubule-severing activity is required for meiotic spindle organization in C. elegans.
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Affiliation(s)
- M Srayko
- Department of Biochemistry, Development Research Group, University of Calgary, Calgary, Alberta, T2N 4N1 Canada
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38
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Abstract
The rapid switching between growth and shrinkage at microtubule ends is important for many cellular processes. Recent studies on the structure of the microtubule and on the mechanism of action of the microtubule regulators XKCM1 and OP18 have revealed how these switching events are regulated.
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Affiliation(s)
- F J McNally
- Section of Molecular and Cellular Biology, University of California at Davis, Davis, California 95616, USA.
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39
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Abstract
Several lines of evidence suggest that microtubules are nucleated at the neuronal centrosome, and then released for transport into axons and dendrites. Here we sought to determine whether the microtubule-severing protein known as katanin mediates microtubule release from the neuronal centrosome. Immunomicroscopic analyses on cultured sympathetic neurons show that katanin is present at the centrosome, but is also widely distributed throughout the neuron. Microinjection of an antibody that inactivates katanin results in a dramatic accumulation of microtubules at the centrosome, indicating that katanin is indeed required for microtubule release from the centrosome. However, the antibody also causes an inhibition of axon outgrowth that is more immediate than expected on this basis alone. It may be that katanin severs microtubules throughout the cell body to keep them sufficiently short to be efficiently transported into developing processes. Consistent with this idea, there were significantly fewer free ends of microtubules in the cell bodies of neurons that had been injected with the katanin antibody compared with controls. These results indicate that microtubule-severing by katanin is essential for releasing microtubules from the neuronal centrosome, and also for regulating the length of the microtubules after their release.
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Affiliation(s)
- F J Ahmad
- Department of Anatomy, The University of Wisconsin Medical School, Madison, Wisconsin 53706, USA
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40
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Abstract
Microtubules are dynamic structures whose proper rearrangement during the cell cycle is essential for the positioning of membranes during interphase and for chromosome segregation during mitosis. The previous discovery of a cyclin B/cdc2-activated microtubule-severing activity in M-phase Xenopus egg extracts suggested that a microtubule-severing protein might play an important role in cell cycle-dependent changes in microtubule dynamics and organization. However, the isolation of three different microtubule-severing proteins, p56, EF1alpha, and katanin, has only confused the issue because none of these proteins is directly activated by cyclin B/cdc2. Here we use immunodepletion with antibodies specific for a vertebrate katanin homologue to demonstrate that katanin is responsible for the majority of M-phase severing activity in Xenopus eggs. This result suggests that katanin is responsible for changes in microtubules occurring at mitosis. Immunofluorescence analysis demonstrated that katanin is concentrated at a microtubule-dependent structure at mitotic spindle poles in Xenopus A6 cells and in human fibroblasts, suggesting a specific role in microtubule disassembly at spindle poles. Surprisingly, katanin was also found in adult mouse brain, indicating that katanin may have other functions distinct from its mitotic role.
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Affiliation(s)
- F J McNally
- Section of Molecular and Cellular Biology, University of California, Davis, Davis, California 95616, USA.
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41
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Lohret TA, McNally FJ, Quarmby LM. A role for katanin-mediated axonemal severing during Chlamydomonas deflagellation. Mol Biol Cell 1998; 9:1195-207. [PMID: 9571249 PMCID: PMC25341 DOI: 10.1091/mbc.9.5.1195] [Citation(s) in RCA: 66] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/06/1997] [Accepted: 02/20/1998] [Indexed: 02/07/2023] Open
Abstract
Deflagellation of Chlamydomonas reinhardtii, and other flagellated and ciliated cells, is a highly specific process that involves signal-induced severing of the outer doublet microtubules at a precise site in the transition region between the axoneme and basal body. Although the machinery of deflagellation is activated by Ca2+, the mechanism of microtubule severing is unknown. Severing of singlet microtubules has been observed in vitro to be catalyzed by katanin, a heterodimeric adenosine triphosphatase that can remove tubulin subunits from the walls of stable microtubules. We found that purified katanin induced an ATP-dependent severing of the Chlamydomonas axoneme. Using Western blot analysis and indirect immunofluorescence, we demonstrate that Chlamydomonas expresses a protein that is recognized by an anti-human katanin antibody and that this protein is localized, at least in part, to the basal body complex. Using an in vitro severing assay, we show that the protein(s) responsible for Ca2+-activated outer doublet severing purify with the flagellar-basal body complex. Furthermore, deflagellation of purified flagellar-basal body complexes is significantly blocked by the anti-katanin antibody. Taken together, these data suggest that a katanin-like mechanism may mediate the severing of the outer doublet microtubules during Chlamydomonas deflagellation.
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Affiliation(s)
- T A Lohret
- Department of Cell Biology, Emory University School of Medicine, Atlanta, Georgia 30322-3030, USA
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42
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Hartman JJ, Mahr J, McNally K, Okawa K, Iwamatsu A, Thomas S, Cheesman S, Heuser J, Vale RD, McNally FJ. Katanin, a microtubule-severing protein, is a novel AAA ATPase that targets to the centrosome using a WD40-containing subunit. Cell 1998; 93:277-87. [PMID: 9568719 DOI: 10.1016/s0092-8674(00)81578-0] [Citation(s) in RCA: 260] [Impact Index Per Article: 10.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
Microtubule disassembly at centrosomes is involved in mitotic spindle function. The microtubule-severing protein katanin, a heterodimer of 60 and 80 kDa subunits, was previously purified and shown to localize to centrosomes in vivo. Here we report the sequences and activities of the katanin subunits. p60 is a new member of the AAA family of ATPases, and we show that expressed p60 has microtubule-stimulated ATPase and microtubule-severing activities in the absence of p80. p80 is a novel protein containing WD40 repeats, which are frequently involved in protein-protein interactions. The p80 WD40 domain does not participate in p60 dimerization, but localizes to centrosomes in transfected mammalian cells. These results indicate katanin's activities are segregated into a subunit (p60) that possesses enzymatic activity and a subunit (p80) that targets the enzyme to the centrosome.
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Affiliation(s)
- J J Hartman
- Department of Cellular and Molecular Pharmacology, University of California, San Francisco 94143, USA
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43
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Abstract
The assembly and function of the mitotic spindle involve specific changes in the dynamic properties of microtubules. One such change results in the poleward flux of tubulin in which spindle microtubules polymerize at their kinetochore-attached plus ends while they shorten at their centrosome-attached minus ends. Since free microtubule minus ends do not depolymerize in vivo, the poleward flux of tubulin suggests that spindle microtubules are actively disassembled at or near their centrosomal attachment points. The microtubule-severing ATPase, katanin, has the ability actively to sever and disassemble microtubules and is thus a candidate for the role of a protein mediating the poleward flux of tubulin. Here we determine the subcellular localization of katanin by immunofluorescence as a preliminary step in determining whether katanin mediates the poleward flux of tubulin. We find that katanin is highly concentrated at centrosomes throughout the cell cycle. Katanin's localization is different from that of gamma-tubulin in that microtubules are required to maintain the centrosomal localization of katanin. Direct comparison of the localization of katanin and gamma-tubulin reveals that katanin is localized in a region surrounding the gamma-tubulin-containing pericentriolar region in detergent-extracted mitotic spindles. The centrosomal localization of katanin is consistent with the hypothesis that katanin mediates the disassembly of microtubule minus ends during poleward flux.
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Affiliation(s)
- F J McNally
- Section of Molecular and Cellular Biology, University of California at Davis 95616, USA
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44
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Abstract
Microtubule dynamics change dramatically during the cell cycle, but the mechanisms by which these changes occur are unknown. Recent progress has been made in four areas: firstly, in the determination of changes in microtubule turnover and net tubulin polymer levels in vivo; secondly, in the elucidation of mechanisms of regulation of microtubule dynamics by microtubule-associated protein 4; thirdly, in the determination of the mechanisms by which Xenopus microtubule-associated protein regulates microtubule dynamics; and fourthly, in the elucidation of the structural basis of microtubule nucleation by gamma tubulin.
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Affiliation(s)
- F J McNally
- Section of Molecular and Cellular Biology, University of California, Davis 95616, USA.
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45
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Abstract
In Saccharomyces cerevisiae, the HMR-E silencer blocks site-specific interactions between proteins and their recognition sequences in the vicinity of the silencer. Silencer function is correlated with the firing of an origin of replication at HMR-E. An essential gene with a role in transcriptional silencing was identified by means of a screen for mutations affecting expression of HMR. This gene, known as ORC2, was shown to encode a component of the origin recognition complex that binds yeast origins of replication. A temperature-sensitive mutation in ORC2 disrupted silencing in cells grown at the permissive temperature. At the restrictive temperature, the orc2-1 mutation caused cell cycle arrest at a point in the cell cycle indicative of blocks in DNA replication. The orc2-1 mutation also resulted in the enhanced mitotic loss of a plasmid, suggestive of a defect in replication. These results provide strong evidence for an in vivo role of ORC in both chromosomal replication and silencing, and provide a link between the mechanism of silencing and DNA replication.
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Affiliation(s)
- M Foss
- Department of Molecular and Cellular Biology, University of California, Berkeley 94720
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46
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Abstract
Eukaryotic cells rapidly reorganize their microtubule cytoskeleton during the cell cycle, differentiation, and cell migration. In this study, we have purified a heterodimeric protein, katanin, that severs and disassembles microtubules to tubulin dimers. The disassembled tubulin can repolymerize, indicating that it is not irreversibly modified or denatured in the reaction. Katanin is a microtubule-stimulated ATPase and requires ATP hydrolysis to sever microtubules. Katanin represents a novel type of enzyme that utilizes energy from nucleotide hydrolysis to break tubulin-tubulin bonds within a microtubule polymer, a process that may aid in disassembling complex microtubule arrays within cells.
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Affiliation(s)
- F J McNally
- Department of Pharmacology, University of California, San Francisco 94143
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47
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Abstract
Copies of the mating-type genes are present at three loci on chromosome III of the yeast Saccharomyces cerevisiae. The genes at the MAT locus are transcribed, whereas the identical genes at the silent loci, HML and HMR, are not transcribed. Several genes, including the four SIR genes, and two sites, HMR-E and HMR-I, are required for repression of transcription at the HMR locus. Three elements have been implicated in the function of the HMR-E silencer: a binding site for the RAP1 protein, a binding site for the ABF1 protein, and an 11-bp consensus sequence common to nearly all autonomously replicating sequence (ARS) elements (putative origins of DNA replication). RAP1 and ABF1 binding sites of different sequence than those found at HMR-E were joined with an 11-bp ARS consensus sequence to form a synthetic silencer. The synthetic silencer was able to repress transcription of the HMRa1 gene, confirming that binding sites for RAP1 and ABF1 and the 11-bp ARS consensus sequence were the functional components of the silencer in vivo. Mutations in the ABF1 binding site or in the ARS consensus sequence of the synthetic silencer caused nearly complete derepression of transcription at HMR. The ARS consensus sequence mutation also eliminated the ARS activity of the synthetic silencer. These data suggested that replication initiation at the HMR-E silencer was required for establishment of the repressed state at the HMR locus.
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Affiliation(s)
- F J McNally
- Department of Molecular and Cellular Biology, University of California, Berkeley 94720
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48
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Abstract
The establishment of embryonic polarity is a crucial step in pattern formation and morphogenesis. In the fruitfly Drosophila melanogaster, embryonic polarity depends primarily on genes expressed in the female during oogenesis. Mutations in these 'maternal effect' genes can lead to major disruptions in normal pattern formation. Two classes of maternal genes essential for the establishment of polarity in the embryo have been identified. Lesions in one class, the 'bicaudal' genes, disrupt the anterior-posterior axis; lesions in the other class disrupt dorsal-ventral polarity, and in the most extreme cases embryos fail to form any ventral or lateral structures. Genetic studies suggest that the anterior-posterior and dorsal-ventral axes may be independent as the defects observed in mutants from each class seem to be restricted to one axis only. The dorsal (dl) locus is one of the maternal effect genes involved in the establishment of dorsal-ventral polarity. Homozygous dl females produce embryos exhibiting the mutant phenotype--complete lack of dorsal-ventral polarity in the strongest alleles--irrespective of the genotype of the father. Although dl is a maternal effect locus and must be expressed during oogenesis, the gene product, or a substance depending on the normal function of the dl gene, seems to be active early in embryogenesis, as the dl phenotype can be partially rescued by injection of cytoplasm from wild-type cleavage-stage embryos. Here we report the molecular cloning of the dorsal locus and a study of its expression.
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