Monopolin

Lateral and End-On Kinetochore Attachments Are Coordinated to Achieve Bi-orientation in Drosophila Oocytes

In oocytes, where centrosomes are absent, the chromosomes direct the assembly of a bipolar spindle. Interactions between chromosomes and microtubules are essential for both spindle formation and chromosome segregation, but the nature and function of these interactions is not clear. We have examined oocytes lacking two kinetochore proteins, NDC80 and SPC105R, and a centromere-associated motor protein, CENP-E, to characterize the impact of kinetochore-microtubule attachments on spindle assembly and chromosome segregation in Drosophila oocytes. We found that the initiation of spindle assembly results from chromosome-microtubule interactions that are kinetochore-independent. Stabilization of the spindle, however, depends on both central spindle and kinetochore components. This stabilization coincides with changes in kinetochore-microtubule attachments and bi-orientation of homologs. We propose that the bi-orientation process begins with the kinetochores moving laterally along central spindle microtubules towards their minus ends. This movement depends on SPC105R, can occur in the absence of NDC80, and is antagonized by plus-end directed forces from the CENP-E motor. End-on kinetochore-microtubule attachments that depend on NDC80 are required to stabilize bi-orientation of homologs. A surprising finding was that SPC105R but not NDC80 is required for co-orientation of sister centromeres at meiosis I. Together, these results demonstrate that, in oocytes, kinetochore-dependent and -independent chromosome-microtubule attachments work together to promote the accurate segregation of chromosomes.

In acentrosomal oocytes, spindle assembly depends on the chromosomes. The nature of the chromosome-microtubule interactions in oocytes that organize spindle bipolarity and orientation of the homologs has been unclear. We have found that several types of functional chromosome-microtubule interactions exist in oocytes, and that each type participates in unique aspects of chromosome orientation and spindle assembly. We present here a model for chromosome-based spindle assembly and chromosome movements in oocytes that highlights the multiple and unappreciated roles played by the kinetochores and has implications for how homologous chromosomes bi-orient during meiosis.

Targeting oocyte maturation to improve fertility in older women

Reproductive aging is an increasingly pressing problem facing women in modern society, due to delay in child bearing. According to Statistics Canada, 52% of all Canadian births in 2011 were by women aged 30 years and older, up from 24% in 1981 ( http://www.statcan.gc.ca/pub/91-209-x/2013001/article/11784-eng.htm ). Women older than 35 years of age experience significantly increased risks of infertility, miscarriage and congenital birth defects, mostly due to poor quality of the eggs. Increasingly sophisticated, and often invasive, assisted reproductive technologies (ARTs) have helped millions of women to achieve reproductive success. However, by and large, ARTs do not address the fundamental issue of reproductive aging in women: age-related decline in egg quality. More importantly, ARTs are not, and will never be, the main solution for the general population. Here, I attempt to review the scientific literature on age-related egg quality decline, based mostly on studies in mice and in humans. Emphasis is given to the brief period of time called oocyte maturation, which occurs just prior to ovulation. The rationale for this emphasis is that oocyte maturation represents a critical window where unfavorable ovarian conditions in older females contribute significantly to the decline of egg quality, and that science-based intervention during oocyte maturation represents the best chance of improving egg quality in older women. Finally, I summarize our own work in recent years on peri-ovulatory putrescine supplementation as a possible remedy for reproductive aging.

Molecular mechanisms of homologous chromosome pairing and segregation in plants

In most eukaryotic species, three basic steps of pairing, recombination and synapsis occur during prophase of meiosis I. Homologous chromosomal pairing and recombination are essential for accurate segregation of chromosomes. In contrast to the well-studied processes such as recombination and synapsis, many aspects of chromosome pairing are still obscure. Recent progress in several species indicates that the telomere bouquet formation can facilitate homologous chromosome pairing by bringing chromosome ends into close proximity, but the sole presence of telomere clustering is not sufficient for recognizing homologous pairs. On the other hand, accurate segregation of the genetic material from parent to offspring during meiosis is dependent on the segregation of homologs in the reductional meiotic division (MI) with sister kinetochores exhibiting mono-orientation from the same pole, and the segregation of sister chromatids during the equational meiotic division (MII) with kinetochores showing bi-orientation from the two poles. The underlying mechanism of orientation and segregation is still unclear. Here we focus on recent studies in plants and other species that provide insight into how chromosomes find their partners and mechanisms mediating chromosomal segregation.

Chromosome segregation: disarming the protector

One of the key features of meiosis is that shugoshin in complex with protein phosphatase 2A (PP2A) protects centromeric cohesin during meiosis I, but not during meiosis II. A new model suggests that a PP2A inhibitor mediates deprotection of centromeric cohesin during meiosis II.