Publications by authors named "Melina Schuh"

41 Publications

Two mechanisms drive pronuclear migration in mouse zygotes.

Nat Commun 2021 02 5;12(1):841. Epub 2021 Feb 5.

Department of Meiosis, Max Planck Institute for Biophysical Chemistry, Göttingen, Germany.

A new life begins with the unification of the maternal and paternal chromosomes upon fertilization. The parental chromosomes first become enclosed in two separate pronuclei near the surface of the fertilized egg. The mechanisms that then move the pronuclei inwards for their unification are only poorly understood in mammals. Here, we report two mechanisms that act in concert to unite the parental genomes in fertilized mouse eggs. The male pronucleus assembles within the fertilization cone and is rapidly moved inwards by the flattening cone. Rab11a recruits the actin nucleation factors Spire and Formin-2 into the fertilization cone, where they locally nucleate actin and further accelerate the pronucleus inwards. In parallel, a dynamic network of microtubules assembles that slowly moves the male and female pronuclei towards the cell centre in a dynein-dependent manner. Both mechanisms are partially redundant and act in concert to unite the parental pronuclei in the zygote's centre.
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http://dx.doi.org/10.1038/s41467-021-21020-xDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7864974PMC
February 2021

Phase Separation during Germline Development.

Trends Cell Biol 2021 Jan 14. Epub 2021 Jan 14.

Department of Meiosis, Max Planck Institute for Biophysical Chemistry, 37077 Göttingen, Germany. Electronic address:

Phase separation has emerged as a new key principle of intracellular organization. Phase-separated structures play diverse roles in various biological processes and pathogenesis of protein aggregation diseases. Recent work has revealed crucial functions for phase separation during germline development. Phase separation controls the assembly and segregation of germ granules that determine which embryonic cells become germ cells. Phase separation promotes the formation of the Balbiani body, a structure that stores organelles and RNAs during the prolonged prophase arrest of oocytes. Phase separation also facilitates meiotic recombination that prepares homologous chromosomes for segregation, and drives the formation of a liquid-like spindle domain that promotes spindle assembly in mammalian oocytes. We review how phase separation drives these essential steps during germline development.
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http://dx.doi.org/10.1016/j.tcb.2020.12.004DOI Listing
January 2021

Aneuploidy in human eggs: contributions of the meiotic spindle.

Biochem Soc Trans 2021 Feb;49(1):107-118

Department of Meiosis, Max Planck Institute for Biophysical Chemistry, 37077 Göttingen, Germany.

Human eggs frequently contain an incorrect number of chromosomes, a condition termed aneuploidy. Aneuploidy affects ∼10-25% of eggs in women in their early 30s, and more than 50% of eggs from women over 40. Most aneuploid eggs cannot develop to term upon fertilization, making aneuploidy in eggs a leading cause of miscarriages and infertility. The cellular origins of aneuploidy in human eggs are incompletely understood. Aneuploidy arises from chromosome segregation errors during the two meiotic divisions of the oocyte, the progenitor cell of the egg. Chromosome segregation is driven by a microtubule spindle, which captures and separates the paired chromosomes during meiosis I, and sister chromatids during meiosis II. Recent studies reveal that defects in the organization of the acentrosomal meiotic spindle contribute to human egg aneuploidy. The microtubules of the human oocyte spindle are very frequently incorrectly attached to meiotic kinetochores, the multi-protein complexes on chromosomes to which microtubules bind. Multiple features of human oocyte spindles favour incorrect attachments. These include spindle instability and many age-related changes in chromosome and kinetochore architecture. Here, we review how the unusual spindle assembly mechanism in human oocytes contributes to the remarkably high levels of aneuploidy in young human eggs, and how age-related changes in chromosome and kinetochore architecture cause aneuploidy levels to rise even higher as women approach their forties.
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http://dx.doi.org/10.1042/BST20200043DOI Listing
February 2021

The BCL-2 pathway preserves mammalian genome integrity by eliminating recombination-defective oocytes.

Nat Commun 2020 05 25;11(1):2598. Epub 2020 May 25.

Sex Chromosome Biology Laboratory, The Francis Crick Institute, London, NW1 1AT, UK.

DNA double-strand breaks (DSBs) are toxic to mammalian cells. However, during meiosis, more than 200 DSBs are generated deliberately, to ensure reciprocal recombination and orderly segregation of homologous chromosomes. If left unrepaired, meiotic DSBs can cause aneuploidy in gametes and compromise viability in offspring. Oocytes in which DSBs persist are therefore eliminated by the DNA-damage checkpoint. Here we show that the DNA-damage checkpoint eliminates oocytes via the pro-apoptotic BCL-2 pathway members Puma, Noxa and Bax. Deletion of these factors prevents oocyte elimination in recombination-repair mutants, even when the abundance of unresolved DSBs is high. Remarkably, surviving oocytes can extrude a polar body and be fertilised, despite chaotic chromosome segregation at the first meiotic division. Our findings raise the possibility that allelic variants of the BCL-2 pathway could influence the risk of embryonic aneuploidy.
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http://dx.doi.org/10.1038/s41467-020-16441-zDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7248069PMC
May 2020

Meiotic Kinetochores Fragment into Multiple Lobes upon Cohesin Loss in Aging Eggs.

Curr Biol 2019 11 31;29(22):3749-3765.e7. Epub 2019 Oct 31.

Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, Göttingen 37077, Germany. Electronic address:

Chromosome segregation errors during female meiosis are a leading cause of pregnancy loss and human infertility. The segregation of chromosomes is driven by interactions between spindle microtubules and kinetochores. Kinetochores in mammalian oocytes are subjected to special challenges: they need to withstand microtubule pulling forces over multiple hours and are built on centromeric chromatin that in humans is decades old. In meiosis I, sister kinetochores are paired and oriented toward the same spindle pole. It is well established that they progressively separate from each other with advancing female age. However, whether aging also affects the internal architecture of centromeres and kinetochores is currently unclear. Here, we used super-resolution microscopy to study meiotic centromere and kinetochore organization in metaphase-II-arrested eggs from three mammalian species, including humans. We found that centromeric chromatin decompacts with advancing maternal age. Kinetochores built on decompacted centromeres frequently lost their integrity and fragmented into multiple lobes. Fragmentation extended across inner and outer kinetochore regions and affected over 30% of metaphase-II-arrested (MII) kinetochores in aged women and mice, making the lobular architecture a prominent feature of the female meiotic kinetochore. We demonstrate that a partial cohesin loss, as is known to occur in oocytes with advancing maternal age, is sufficient to trigger centromere decompaction and kinetochore fragmentation. Microtubule pulling forces further enhanced the fragmentation and shaped the arrangement of kinetochore lobes. Fragmented kinetochores were frequently abnormally attached to spindle microtubules, suggesting that kinetochore fragmentation could contribute to the maternal age effect in mammalian eggs.
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http://dx.doi.org/10.1016/j.cub.2019.09.006DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6868511PMC
November 2019

Chromosome errors in human eggs shape natural fertility over reproductive life span.

Science 2019 09;365(6460):1466-1469

DNRF Center for Chromosome Stability, Department of Cellular and Molecular Medicine, Faculty of Health and Medical Sciences, University of Copenhagen, Denmark.

Chromosome errors, or aneuploidy, affect an exceptionally high number of human conceptions, causing pregnancy loss and congenital disorders. Here, we have followed chromosome segregation in human oocytes from females aged 9 to 43 years and report that aneuploidy follows a U-curve. Specific segregation error types show different age dependencies, providing a quantitative explanation for the U-curve. Whole-chromosome nondisjunction events are preferentially associated with increased aneuploidy in young girls, whereas centromeric and more extensive cohesion loss limit fertility as women age. Our findings suggest that chromosomal errors originating in oocytes determine the curve of natural fertility in humans.
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http://dx.doi.org/10.1126/science.aav7321DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7212007PMC
September 2019

A liquid-like spindle domain promotes acentrosomal spindle assembly in mammalian oocytes.

Science 2019 06;364(6447)

Department of Meiosis, Max Planck Institute for Biophysical Chemistry, 37077 Göttingen, Germany.

Mammalian oocytes segregate chromosomes with a microtubule spindle that lacks centrosomes, but the mechanisms by which acentrosomal spindles are organized and function are largely unclear. In this study, we identify a conserved subcellular structure in mammalian oocytes that forms by phase separation. This structure, which we term the liquid-like meiotic spindle domain (LISD), permeates the spindle poles and forms dynamic protrusions that extend well beyond the spindle. The LISD selectively concentrates multiple microtubule regulatory factors and allows them to diffuse rapidly within the spindle volume. Disruption of the LISD via different means disperses these factors and leads to severe spindle assembly defects. Our data suggest a model whereby the LISD promotes meiotic spindle assembly by serving as a reservoir that sequesters and mobilizes microtubule regulatory factors in proximity to spindle microtubules.
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http://dx.doi.org/10.1126/science.aat9557DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6629549PMC
June 2019

Publisher Correction: Acute and rapid degradation of endogenous proteins by Trim-Away.

Nat Protoc 2019 Aug;14(8):2596

Laboratory of Molecular Biology, Medical Research Council, Cambridge, UK.

In the version of this paper originally published, the present address of W.A. McEwan was accidentally omitted. This address (UK Dementia Research Institute, Department of Clinical Neurosciences, University of Cambridge, Cambridge, UK) has now been added as affiliation 3, and the equal-contributions note has been updated to affiliation 4. These changes are reflected in the PDF and HTML versions of the protocol.
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http://dx.doi.org/10.1038/s41596-018-0092-8DOI Listing
August 2019

Functions of actin in mouse oocytes at a glance.

J Cell Sci 2018 11 22;131(22). Epub 2018 Nov 22.

Department of Meiosis, Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, 37077 Göttingen, Germany

Gametes undergo a specialized and reductional cell division termed meiosis. Female gametes (oocytes) undergo two rounds of meiosis; the first meiotic division produces the fertilizable egg, while the second meiotic division occurs upon fertilization. Both meiotic divisions are highly asymmetric, producing a large egg and small polar bodies. Actin takes over various essential function during oocyte meiosis, many of which commonly rely on microtubules in mitotic cells. Specifically, the actin network has been linked to long-range vesicle transport, nuclear positioning, spindle migration and anchorage, polar body extrusion and accurate chromosome segregation in mammalian oocytes. In this Cell Science at a Glance article and the accompanying poster, we summarize the many functions of the actin cytoskeleton in oocytes, with a focus on findings from the mouse model system.
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http://dx.doi.org/10.1242/jcs.218099DOI Listing
November 2018

Acute and rapid degradation of endogenous proteins by Trim-Away.

Nat Protoc 2018 10;13(10):2149-2175

Laboratory of Molecular Biology, Medical Research Council, Cambridge, UK.

Protein depletion is a key approach to understanding the functions of a protein in a biological system. We recently developed the Trim-Away approach in order to rapidly degrade endogenous proteins without prior modification. Trim-Away is based on the ubiquitin ligase and Fc receptor TRIM21, which recognizes antibody-bound proteins and targets them for degradation by the proteasome. In a typical Trim-Away experiment, protein degradation is achieved in three steps: first, introduction of an antibody against the target protein; second, recruitment of endogenous or exogenous/overexpressed TRIM21 to the antibody-bound target protein; and third, proteasome-mediated degradation of the target protein, antibody and TRIM21 complex. Protein degradation by Trim-Away is acute and rapid, with half-lives of ~10-20 min. The major advantages of Trim-Away over other protein degradation methods are that it can be applied to any endogenous protein without prior modification; that it uses conventional antibodies that are widely available; and that it can be applied to a wide range of cell types, including nondividing primary human cells, for which other loss-of-function assays are challenging. In this protocol, we describe the detailed procedures for antibody preparation and delivery in mouse oocytes and cultured cells via microinjection and electroporation. In addition, we provide recommendations for antibody selection and validation, and for the generation of TRIM21-overexpressing cell lines for cases in which endogenous TRIM21 is limited. A typical Trim-Away experiment takes just a few hours.
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http://dx.doi.org/10.1038/s41596-018-0028-3DOI Listing
October 2018

Taking a confident leap into uncertainty.

Authors:
Melina Schuh

Nat Cell Biol 2018 Sep;20(9):1007

Max Planck Institute for Biophysical Chemistry, Göttingen, Germany.

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http://dx.doi.org/10.1038/s41556-018-0177-1DOI Listing
September 2018

Assembly and Positioning of the Oocyte Meiotic Spindle.

Annu Rev Cell Dev Biol 2018 10 20;34:381-403. Epub 2018 Jul 20.

Department of Meiosis, Max Planck Institute for Biophysical Chemistry, 37077 Göttingen, Germany; email:

Fertilizable eggs develop from diploid precursor cells termed oocytes. Once every menstrual cycle, an oocyte matures into a fertilizable egg in the ovary. To this end, the oocyte eliminates half of its chromosomes into a small cell termed a polar body. The egg is then released into the Fallopian tube, where it can be fertilized. Upon fertilization, the egg completes the second meiotic division, and the mitotic division of the embryo starts. This review highlights recent work that has shed light on the cytoskeletal structures that drive the meiotic divisions of the oocyte in mammals. In particular, we focus on how mammalian oocytes assemble a microtubule spindle in the absence of centrosomes, how they position the spindle in preparation for polar body extrusion, and how the spindle segregates the chromosomes. We primarily focus on mouse oocytes as a model system but also highlight recent insights from human oocytes.
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http://dx.doi.org/10.1146/annurev-cellbio-100616-060553DOI Listing
October 2018

Double trouble at the beginning of life.

Science 2018 07;361(6398):128-129

Max Planck Institute for Biophysical Chemistry, Göttingen 37077, Germany.

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http://dx.doi.org/10.1126/science.aau3216DOI Listing
July 2018

Preface.

Methods Cell Biol 2018 ;145:xvii

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http://dx.doi.org/10.1016/S0091-679X(18)30085-2DOI Listing
December 2018

A microscopy-based approach for studying meiosis in live and fixed human oocytes.

Methods Cell Biol 2018 26;145:315-333. Epub 2018 May 26.

Max Planck Institute for Biophysical Chemistry, Department of Meiosis, Göttingen, Germany. Electronic address:

Human eggs frequently carry an incorrect number of chromosomes, which is a leading cause of pregnancy loss and congenital disorders. The origins of high aneuploidy rates in human eggs have remained largely unclear. This is due to two main reasons: first, the availability of human eggs is limited so that studies of fixed human eggs typically involve very small numbers and limited quantifications. Second, methods for studying meiosis in live human eggs have been missing. The ever rising prevalence of Assisted Reproductive Technologies has facilitated a recent breakthrough in the field. The mechanistic basis of meiosis in humans can now be examined directly in live eggs. Here, we present a robust method for culturing human eggs in vitro and describe how meiotic processes in human eggs can be studied in real time using fluorescent reporters. We further describe methods for the in-depth analysis of immunolabeled eggs by super-resolution light microscopy.
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http://dx.doi.org/10.1016/bs.mcb.2018.03.039DOI Listing
December 2018

Preface.

Methods Cell Biol 2018 ;144:xix

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http://dx.doi.org/10.1016/S0091-679X(18)30049-9DOI Listing
December 2018

Actin Disassembly: How to Contract without Motors?

Curr Biol 2018 03;28(6):R275-R277

Max Planck Institute for Biophysical Chemistry, Department of Meiosis, Am Fassberg 11, 37077 Göttingen, Germany. Electronic address:

Contractile actin networks take on various functions in cells. How disordered actin networks contract is still poorly understood. A recent study proposes a contractile mechanism that is driven by actin disassembly and required to prevent chromosome losses in starfish oocytes.
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http://dx.doi.org/10.1016/j.cub.2018.02.019DOI Listing
March 2018

A Method for the Acute and Rapid Degradation of Endogenous Proteins.

Cell 2017 Dec 16;171(7):1692-1706.e18. Epub 2017 Nov 16.

Medical Research Council, Laboratory of Molecular Biology, Cambridge CB2 0QH, UK; Max Planck Institute for Biophysical Chemistry, 37077 Göttingen, Germany. Electronic address:

Methods for the targeted disruption of protein function have revolutionized science and greatly expedited the systematic characterization of genes. Two main approaches are currently used to disrupt protein function: DNA knockout and RNA interference, which act at the genome and mRNA level, respectively. A method that directly alters endogenous protein levels is currently not available. Here, we present Trim-Away, a technique to degrade endogenous proteins acutely in mammalian cells without prior modification of the genome or mRNA. Trim-Away harnesses the cellular protein degradation machinery to remove unmodified native proteins within minutes of application. This rapidity minimizes the risk that phenotypes are compensated and that secondary, non-specific defects accumulate over time. Because Trim-Away utilizes antibodies, it can be applied to a wide range of target proteins using off-the-shelf reagents. Trim-Away allows the study of protein function in diverse cell types, including non-dividing primary cells where genome- and RNA-targeting methods are limited.
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http://dx.doi.org/10.1016/j.cell.2017.10.033DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5733393PMC
December 2017

Actin protects mammalian eggs against chromosome segregation errors.

Science 2017 08;357(6353)

Department of Meiosis, Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, 37077 Göttingen, Germany.

Chromosome segregation is driven by a spindle that is made of microtubules but is generally thought to be independent of actin. Here, we report an unexpected actin-dependent mechanism that drives the accurate alignment and segregation of chromosomes in mammalian eggs. Prominent actin filaments permeated the microtubule spindle in eggs of several mammalian species, including humans. Disrupting actin in mouse eggs led to significantly increased numbers of misaligned chromosomes as well as lagging chromosomes during meiosis I and II. We found that actin drives accurate chromosome segregation by promoting the formation of functional kinetochore fibers, the microtubule bundles that align and segregate the chromosomes. Thus, actin is essential to prevent chromosome segregation errors in eggs, which are a leading cause of miscarriages, infertility, and Down syndrome.
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http://dx.doi.org/10.1126/science.aal1647DOI Listing
August 2017

Two pathways regulate cortical granule translocation to prevent polyspermy in mouse oocytes.

Nat Commun 2016 12 19;7:13726. Epub 2016 Dec 19.

Medical Research Council Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge Biomedical Campus, Cambridge CB2 0QH, UK.

An egg must be fertilized by a single sperm only. To prevent polyspermy, the zona pellucida, a structure that surrounds mammalian eggs, becomes impermeable upon fertilization, preventing the entry of further sperm. The structural changes in the zona upon fertilization are driven by the exocytosis of cortical granules. These translocate from the oocyte's centre to the plasma membrane during meiosis. However, very little is known about the mechanism of cortical granule translocation. Here we investigate cortical granule transport and dynamics in live mammalian oocytes by using Rab27a as a marker. We show that two separate mechanisms drive their transport: myosin Va-dependent movement along actin filaments, and an unexpected vesicle hitchhiking mechanism by which cortical granules bind to Rab11a vesicles powered by myosin Vb. Inhibiting cortical granule translocation severely impaired the block to sperm entry, suggesting that translocation defects could contribute to miscarriages that are caused by polyspermy.
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http://dx.doi.org/10.1038/ncomms13726DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5187413PMC
December 2016

The Phosphatase Dusp7 Drives Meiotic Resumption and Chromosome Alignment in Mouse Oocytes.

Cell Rep 2016 10;17(5):1426-1437

Medical Research Council, Laboratory of Molecular Biology, Cambridge Biomedical Campus, Francis Crick Ave., Cambridge CB2 0QH, UK; Max Planck Institut for Biophysical Chemistry, Am Faßberg 11, 37077 Göttingen, Germany. Electronic address:

Mammalian oocytes are stored in the ovary, where they are arrested in prophase for prolonged periods. The mechanisms that abrogate the prophase arrest in mammalian oocytes and reinitiate meiosis are not well understood. Here, we identify and characterize an essential pathway for the resumption of meiosis that relies on the protein phosphatase DUSP7. DUSP7-depleted oocytes either fail to resume meiosis or resume meiosis with a significant delay. In the absence of DUSP7, Cdk1/CycB activity drops below the critical level required to reinitiate meiosis, precluding or delaying nuclear envelope breakdown. Our data suggest that DUSP7 drives meiotic resumption by dephosphorylating and thereby inactivating cPKC isoforms. In addition to controlling meiotic resumption, DUSP7 has a second function in chromosome segregation: DUSP7-depleted oocytes that enter meiosis show severe chromosome alignment defects and progress into anaphase prematurely. Altogether, these findings establish the phosphatase DUSP7 as an essential regulator of multiple steps in oocyte meiosis.
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http://dx.doi.org/10.1016/j.celrep.2016.10.007DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5215830PMC
October 2016

Mechanisms of Aneuploidy in Human Eggs.

Trends Cell Biol 2017 01 20;27(1):55-68. Epub 2016 Oct 20.

Department of Meiosis, Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, D-37077, Göttingen, Germany. Electronic address:

Eggs and sperm develop through a specialized cell division called meiosis. During meiosis, the number of chromosomes is reduced by two sequential divisions in preparation for fertilization. In human female meiosis, chromosomes frequently segregate incorrectly, resulting in eggs with an abnormal number of chromosomes. When fertilized, these eggs give rise to aneuploid embryos that usually fail to develop. As women become older, errors in meiosis occur more frequently, resulting in increased risks of infertility, miscarriage, and congenital syndromes, such as Down's syndrome. Here, we review recent studies that identify the mechanisms causing aneuploidy in female meiosis, with a particular emphasis on studies in humans.
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http://dx.doi.org/10.1016/j.tcb.2016.09.002DOI Listing
January 2017

The BTG4 and CAF1 complex prevents the spontaneous activation of eggs by deadenylating maternal mRNAs.

Open Biol 2016 09;6(9)

Medical Research Council, Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge Biomedical Campus, Cambridge CB2 0QH, UK Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, 37077 Göttingen, Germany

Once every menstrual cycle, eggs are ovulated into the oviduct where they await fertilization. The ovulated eggs are arrested in metaphase of the second meiotic division, and only complete meiosis upon fertilization. It is crucial that the maintenance of metaphase arrest is tightly controlled, because the spontaneous activation of the egg would preclude the development of a viable embryo (Zhang et al. 2015 J. Genet. Genomics 42, 477-485. (doi:10.1016/j.jgg.2015.07.004); Combelles et al. 2011 Hum. Reprod. 26, 545-552. (doi:10.1093/humrep/deq363); Escrich et al. 2011 J. Assist. Reprod. Genet. 28, 111-117. (doi:10.1007/s10815-010-9493-5)). However, the mechanisms that control the meiotic arrest in mammalian eggs are only poorly understood. Here, we report that a complex of BTG4 and CAF1 safeguards metaphase II arrest in mammalian eggs by deadenylating maternal mRNAs. As a follow-up of our recent high content RNAi screen for meiotic genes (Pfender et al. 2015 Nature 524, 239-242. (doi:10.1038/nature14568)), we identified Btg4 as an essential regulator of metaphase II arrest. Btg4-depleted eggs progress into anaphase II spontaneously before fertilization. BTG4 prevents the progression into anaphase by ensuring that the anaphase-promoting complex/cyclosome (APC/C) is completely inhibited during the arrest. The inhibition of the APC/C relies on EMI2 (Tang et al. 2010 Mol. Biol. Cell 21, 2589-2597. (doi:10.1091/mbc.E09-08-0708); Ohe et al. 2010 Mol. Biol. Cell 21, 905-913. (doi:10.1091/mbc.E09-11-0974)), whose expression is perturbed in the absence of BTG4. BTG4 controls protein expression during metaphase II arrest by forming a complex with the CAF1 deadenylase and we hypothesize that this complex is recruited to the mRNA via interactions between BTG4 and poly(A)-binding proteins. The BTG4-CAF1 complex drives the shortening of the poly(A) tails of a large number of transcripts at the MI-MII transition, and this wave of deadenylation is essential for the arrest in metaphase II. These findings establish a BTG4-dependent pathway for controlling poly(A) tail length during meiosis and identify an unexpected role for mRNA deadenylation in preventing the spontaneous activation of eggs.
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http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5043581PMC
http://dx.doi.org/10.1098/rsob.160184DOI Listing
September 2016

Sister kinetochore splitting and precocious disintegration of bivalents could explain the maternal age effect.

Elife 2015 Dec 15;4:e11389. Epub 2015 Dec 15.

Medical Research Council Laboratory of Molecular Biology, Cambridge, United Kingdom.

Aneuploidy in human eggs is the leading cause of pregnancy loss and Down's syndrome. Aneuploid eggs result from chromosome segregation errors when an egg develops from a progenitor cell, called an oocyte. The mechanisms that lead to an increase in aneuploidy with advanced maternal age are largely unclear. Here, we show that many sister kinetochores in human oocytes are separated and do not behave as a single functional unit during the first meiotic division. Having separated sister kinetochores allowed bivalents to rotate by 90 degrees on the spindle and increased the risk of merotelic kinetochore-microtubule attachments. Advanced maternal age led to an increase in sister kinetochore separation, rotated bivalents and merotelic attachments. Chromosome arm cohesion was weakened, and the fraction of bivalents that precociously dissociated into univalents was increased. Together, our data reveal multiple age-related changes in chromosome architecture that could explain why oocyte aneuploidy increases with advanced maternal age.
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http://dx.doi.org/10.7554/eLife.11389DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4755749PMC
December 2015

A three-step MTOC fragmentation mechanism facilitates bipolar spindle assembly in mouse oocytes.

Nat Commun 2015 Jul 6;6:7217. Epub 2015 Jul 6.

Medical Research Council, Laboratory of Molecular Biology, Cambridge CB2 0QH, UK.

Assembly of a bipolar microtubule spindle is essential for accurate chromosome segregation. In somatic cells, spindle bipolarity is determined by the presence of exactly two centrosomes. Remarkably, mammalian oocytes do not contain canonical centrosomes. This study reveals that mouse oocytes assemble a bipolar spindle by fragmenting multiple acentriolar microtubule-organizing centres (MTOCs) into a high number of small MTOCs to be able to then regroup and merge them into two equal spindle poles. We show that MTOCs are fragmented in a three-step process. First, PLK1 triggers a decondensation of the MTOC structure. Second, BicD2-anchored dynein stretches the MTOCs into fragmented ribbons along the nuclear envelope. Third, KIF11 further fragments the MTOCs following nuclear envelope breakdown so that they can be evenly distributed towards the two spindle poles. Failure to fragment MTOCs leads to defects in spindle assembly, which delay chromosome individualization and congression, putting the oocyte at risk of aneuploidy.
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http://dx.doi.org/10.1038/ncomms8217DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4501430PMC
July 2015

Live imaging RNAi screen reveals genes essential for meiosis in mammalian oocytes.

Nature 2015 Aug 6;524(7564):239-242. Epub 2015 Jul 6.

Medical Research Council, Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge Biomedical Campus, Cambridge CB2 0QH, United Kingdom.

During fertilization, an egg and a sperm fuse to form a new embryo. Eggs develop from oocytes in a process called meiosis. Meiosis in human oocytes is highly error-prone, and defective eggs are the leading cause of pregnancy loss and several genetic disorders such as Down's syndrome. Which genes safeguard accurate progression through meiosis is largely unclear. Here we develop high-content phenotypic screening methods for the systematic identification of mammalian meiotic genes. We targeted 774 genes by RNA interference within follicle-enclosed mouse oocytes to block protein expression from an early stage of oocyte development onwards. We then analysed the function of several genes simultaneously by high-resolution imaging of chromosomes and microtubules in live oocytes and scored each oocyte quantitatively for 50 phenotypes, generating a comprehensive resource of meiotic gene function. The screen generated an unprecedented annotated data set of meiotic progression in 2,241 mammalian oocytes, which allowed us to analyse systematically which defects are linked to abnormal chromosome segregation during meiosis, identifying progression into anaphase with misaligned chromosomes as well as defects in spindle organization as risk factors. This study demonstrates how high-content screens can be performed in oocytes, and allows systematic studies of meiosis in mammals.
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http://dx.doi.org/10.1038/nature14568DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4538867PMC
August 2015

Human oocytes. Error-prone chromosome-mediated spindle assembly favors chromosome segregation defects in human oocytes.

Science 2015 Jun;348(6239):1143-7

Medical Research Council, Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge Biomedical Campus, Cambridge CB2 0QH, UK.

Aneuploidy in human eggs is the leading cause of pregnancy loss and several genetic disorders such as Down syndrome. Most aneuploidy results from chromosome segregation errors during the meiotic divisions of an oocyte, the egg's progenitor cell. The basis for particularly error-prone chromosome segregation in human oocytes is not known. We analyzed meiosis in more than 100 live human oocytes and identified an error-prone chromosome-mediated spindle assembly mechanism as a major contributor to chromosome segregation defects. Human oocytes assembled a meiotic spindle independently of either centrosomes or other microtubule organizing centers. Instead, spindle assembly was mediated by chromosomes and the small guanosine triphosphatase Ran in a process requiring ~16 hours. This unusually long spindle assembly period was marked by intrinsic spindle instability and abnormal kinetochore-microtubule attachments, which favor chromosome segregation errors and provide a possible explanation for high rates of aneuploidy in human eggs.
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http://dx.doi.org/10.1126/science.aaa9529DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4477045PMC
June 2015

Nuclear envelope breakdown: actin' quick to tear down the wall.

Curr Biol 2014 Jul;24(13):R605-7

MRC Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge Biomedical Campus, Cambridge CB2 0QH, UK. Electronic address:

Nuclear envelope breakdown in metazoan cells is thought to be facilitated by microtubules, which pull on the nuclear membranes. Unexpectedly, an F-actin meshwork helps to tear down the large nucleus of starfish oocytes and to prevent chromosome loss in meiosis.
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http://dx.doi.org/10.1016/j.cub.2014.05.059DOI Listing
July 2014

Melina Schuh: first comes the egg.

J Cell Biol 2014 Mar;204(7):1080-1

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http://dx.doi.org/10.1083/jcb.2047piDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3971742PMC
March 2014

Spire and Formin 2 synergize and antagonize in regulating actin assembly in meiosis by a ping-pong mechanism.

PLoS Biol 2014 Feb 25;12(2):e1001795. Epub 2014 Feb 25.

Laboratoire d'Enzymologie et Biochimie Structurale, CNRS, Gif-sur-Yvette, France.

In mammalian oocytes, three actin binding proteins, Formin 2 (Fmn2), Spire, and profilin, synergistically organize a dynamic cytoplasmic actin meshwork that mediates translocation of the spindle toward the cortex and is required for successful fertilization. Here we characterize Fmn2 and elucidate the molecular mechanism for this synergy, using bulk solution and individual filament kinetic measurements of actin assembly dynamics. We show that by capping filament barbed ends, Spire recruits Fmn2 and facilitates its association with barbed ends, followed by rapid processive assembly and release of Spire. In the presence of actin, profilin, Spire, and Fmn2, filaments display alternating phases of rapid processive assembly and arrested growth, driven by a "ping-pong" mechanism, in which Spire and Fmn2 alternately kick off each other from the barbed ends. The results are validated by the effects of injection of Spire, Fmn2, and their interacting moieties in mouse oocytes. This original mechanism of regulation of a Rho-GTPase-independent formin, recruited by Spire at Rab11a-positive vesicles, supports a model for modulation of a dynamic actin-vesicle meshwork in the oocyte at the origin of asymmetric positioning of the meiotic spindle.
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http://dx.doi.org/10.1371/journal.pbio.1001795DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3934834PMC
February 2014