Final Report Summary - TELOMERES IN MEIOSIS (Telomere function in meiosis)
While telomeres play key roles in safeguarding genome stability by protecting chromosome ends from degradation and fusion, they also play conserved and as yet poorly characterized roles in meiosis. The starting point for this project was our observation that fission yeast telomeres, which gather together near the centrosome (called the SPB) in early meiosis to form the highly conserved ‘bouquet’, control the formation of the meiotic spindle. This ERC project aimed to determine how the bouquet could affect spindle formation; it also aimed to explore why chromosomes often failed to attach to the spindles that do form in the absence of the bouquet. We found that telomeres not only modify the nuclear membrane beneath the SPB to allow insertion and spindle nucleation, but also telomeres promote centromere assembly. These studies reveal unanticipated levels of interchangeabilty and interdependence between nuclear domains formerly thought of as separate and autonomous.
Aim 1: How does the spindle organizing center differ in the presence and absence of the bouquet? We found that while the duplication of the SPB occurs properly in the absence of the bouquet, SPB separation, insertion into the nuclear membrane, and recruitment of the spindle-initiating -tubulin complex are defective. Moreover, the conserved inner nuclear membrane protein Sad1, which underlies the SPB. fails to accumulate to normal levels in the absence of the bouquet. Hence, telomeres control local Sad1 levels and in turn, the remodelling of the nuclear membrane necessary for SPB insertion and spindle formation.
Aim 2: What feature of the bouquet confers its ability to control the spindle? We find that in strains lacking telomerase whose chromosomes have lost their telomeres and circularized, spindle formation is rescued by the introduction of a single internal telomere repeat stretch to one circular chromosome, or the introduction of a plasmid harbouring such an internal telomere stretch. These results place useful constraints on models for how telomere control spindles, for instance indicating that a conformational/chemical modification underlies the bouquet’s activity rather than mechanical pulling. We also find that in bouquet-deficient cells in which centromeres contact the SPB during early meiosis, spindle formation is rescued. Therefore, centromeres and telomeres have interchangeable roles in controlling spindle formation and indeed, centromeres may regulate mitotic spindle formation in a manner analogous to telomeric control of meiotic spindle formation.
Aim 3: How can the telomere bouquet affect the ability of chromosomes to attach to spindles? In the roughly 50% of bouquet-defective meiotic cells that manage to form proper bipolar spindles, we find that there is often a chromosome that fails to attach to those spindles. Moreover, we find that centromere and kinetochore assembly is defective on those chromosomes that fail in spindle attachment. Our data suggests that this unanticipated function of telomeres in promoting centromere assembly stems from the ability of telomeres to organize nuclear microenvironments conducive to centromere assembly.
Aim 1: How does the spindle organizing center differ in the presence and absence of the bouquet? We found that while the duplication of the SPB occurs properly in the absence of the bouquet, SPB separation, insertion into the nuclear membrane, and recruitment of the spindle-initiating -tubulin complex are defective. Moreover, the conserved inner nuclear membrane protein Sad1, which underlies the SPB. fails to accumulate to normal levels in the absence of the bouquet. Hence, telomeres control local Sad1 levels and in turn, the remodelling of the nuclear membrane necessary for SPB insertion and spindle formation.
Aim 2: What feature of the bouquet confers its ability to control the spindle? We find that in strains lacking telomerase whose chromosomes have lost their telomeres and circularized, spindle formation is rescued by the introduction of a single internal telomere repeat stretch to one circular chromosome, or the introduction of a plasmid harbouring such an internal telomere stretch. These results place useful constraints on models for how telomere control spindles, for instance indicating that a conformational/chemical modification underlies the bouquet’s activity rather than mechanical pulling. We also find that in bouquet-deficient cells in which centromeres contact the SPB during early meiosis, spindle formation is rescued. Therefore, centromeres and telomeres have interchangeable roles in controlling spindle formation and indeed, centromeres may regulate mitotic spindle formation in a manner analogous to telomeric control of meiotic spindle formation.
Aim 3: How can the telomere bouquet affect the ability of chromosomes to attach to spindles? In the roughly 50% of bouquet-defective meiotic cells that manage to form proper bipolar spindles, we find that there is often a chromosome that fails to attach to those spindles. Moreover, we find that centromere and kinetochore assembly is defective on those chromosomes that fail in spindle attachment. Our data suggests that this unanticipated function of telomeres in promoting centromere assembly stems from the ability of telomeres to organize nuclear microenvironments conducive to centromere assembly.