Final Report Summary - HRMCB (Hidden role of the meiotic chromosomal bouquet)
Meiosis is an essential process for sexual reproduction. Errors in the process of meiosis can lead to miscarriage and genetic disorders. Many regulations are involved in successful gametogenesis and one of the main causes of errors is defective chromosome segregation during meiosis. Such mis-segregations are known to increase with age. Telomere attrition in oocytes also occurs with age. Telomeres are non-coding DNA sequences located at the ends of chromosomes. Whereas telomeres are known to protect and maintain chromosomes, they also have crucial role in meiosis. Thus, telomere deficiency might have an impact on meiosis.
In early meiotic prophase, telomeres move to the nuclear membrane and migrate toward a limited area of the nuclear envelope, often near the microtubule-organising centre (MTOC). This dynamic movement of telomeres folds chromosomes to form a so-called ‘chromosomal bouquet’. This widely conserved chromosomal rearrangement is observed in many organisms including yeasts, plants and mammals. However, significance of this chromosome arrangement remains to be established. Our previous work revealed that the chromosomal bouquet plays a crucial role in regulating the meiotic MTOC and spindle (Tomita and Cooper, 2007). We have also demonstrated that telomeres dissociate in a concerted manner from the MTOC, an event we call “telomere fireworks”, suggesting that the association, and subsequent dissociation, of telomeres triggers meiotic progression.
In this project, we focused on how telomeres contribute to meiotic progression. To understand how telomeres monitor the progression of meiotic prophase and control spindles, I proposed to determine structural changes of telomeres through meiosis using fission yeast as a model organism. We found that the telomeric protein becomes heavily phosphorylated throughout meiotic prophase, and this negative charge was diminished after telomere fireworks. We identified the negative charged residues of the telomeric protein is required for the bouquet formation and its meiotic function.
We also set up live single cell imagining analysis to determine individual progression of meiosis. Our system revealed that, without bouquet formation, meiotic prophase progresses like wildtype however exits from meiotic prophase was severely delayed. We observed that the cyclin-dependent kinase (CDK) that is essential for spindle formation and cell cycle progression localises to the telomeres before the exit from meiotic prophase. We found that spindle defects observed in the telomere mutants were correlated to delayed accumulation of CDK, highlighting importance of the bouquet configuration in timely meiosis progression.
During meiotic prophase, homologous chromosomes undergo programmed DNA breaks and recombination between homologous. We found that the bouquet is formed during this meiotic recombination process and any delay in the completion of the recombination stage activate Rad3 (ATR in humans) and Chk1, the DNA damage checkpoint pathways, to restrain meiotic prophase and the bouquet configuration. Forced termination of the bouquet during the meiotic recombination stage led to spindle defects, indicating that the bouquet needs to be formed until the end of meiotic prophase. Thus, we suggest that the chromosomal bouquet monitors the completion of meiotic recombination to synchronize MTOC maturation with the progression of prophase chromosomal events.
Our research sheds light on a previously unanticipated communication; the telomere coordinates completion of chromosome events during meiotic prophase with meiosis entry. As the bouquet configuration is conserved to mammals, similar activity of meiotic telomeres is expected. Further investigation of the mechanism underlying the end of the bouquet stage will provide a high-resolution picture of chromosome dynamics during meiotic prophase, and may reveal how telomere deficiency or shortening causes meiotic defects.
In early meiotic prophase, telomeres move to the nuclear membrane and migrate toward a limited area of the nuclear envelope, often near the microtubule-organising centre (MTOC). This dynamic movement of telomeres folds chromosomes to form a so-called ‘chromosomal bouquet’. This widely conserved chromosomal rearrangement is observed in many organisms including yeasts, plants and mammals. However, significance of this chromosome arrangement remains to be established. Our previous work revealed that the chromosomal bouquet plays a crucial role in regulating the meiotic MTOC and spindle (Tomita and Cooper, 2007). We have also demonstrated that telomeres dissociate in a concerted manner from the MTOC, an event we call “telomere fireworks”, suggesting that the association, and subsequent dissociation, of telomeres triggers meiotic progression.
In this project, we focused on how telomeres contribute to meiotic progression. To understand how telomeres monitor the progression of meiotic prophase and control spindles, I proposed to determine structural changes of telomeres through meiosis using fission yeast as a model organism. We found that the telomeric protein becomes heavily phosphorylated throughout meiotic prophase, and this negative charge was diminished after telomere fireworks. We identified the negative charged residues of the telomeric protein is required for the bouquet formation and its meiotic function.
We also set up live single cell imagining analysis to determine individual progression of meiosis. Our system revealed that, without bouquet formation, meiotic prophase progresses like wildtype however exits from meiotic prophase was severely delayed. We observed that the cyclin-dependent kinase (CDK) that is essential for spindle formation and cell cycle progression localises to the telomeres before the exit from meiotic prophase. We found that spindle defects observed in the telomere mutants were correlated to delayed accumulation of CDK, highlighting importance of the bouquet configuration in timely meiosis progression.
During meiotic prophase, homologous chromosomes undergo programmed DNA breaks and recombination between homologous. We found that the bouquet is formed during this meiotic recombination process and any delay in the completion of the recombination stage activate Rad3 (ATR in humans) and Chk1, the DNA damage checkpoint pathways, to restrain meiotic prophase and the bouquet configuration. Forced termination of the bouquet during the meiotic recombination stage led to spindle defects, indicating that the bouquet needs to be formed until the end of meiotic prophase. Thus, we suggest that the chromosomal bouquet monitors the completion of meiotic recombination to synchronize MTOC maturation with the progression of prophase chromosomal events.
Our research sheds light on a previously unanticipated communication; the telomere coordinates completion of chromosome events during meiotic prophase with meiosis entry. As the bouquet configuration is conserved to mammals, similar activity of meiotic telomeres is expected. Further investigation of the mechanism underlying the end of the bouquet stage will provide a high-resolution picture of chromosome dynamics during meiotic prophase, and may reveal how telomere deficiency or shortening causes meiotic defects.