Oocyte Meiotic Spindle Assembly and Function

Abstract

Gametogenesis in animal oocytes reduces the diploid genome content of germline precursors to a haploid state in gametes by discarding ¾ of the duplicated chromosomes through a sequence of two meiotic cell divisions called meiosis I and II. The assembly of the microtubule-based spindle structure that mediates this reduction in genome content remains poorly understood compared to our knowledge of mitotic spindle assembly and function. In this review, we consider the diversity of oocyte meiotic spindle assembly and structure across animal phylogeny, review recent advances in our understanding of how animal oocytes assemble spindles in the absence of the centriole-based microtubule-organizing centers that dominate mitotic spindle assembly, and discuss different models for how chromosomes are captured and moved to achieve chromosome segregation during oocyte meiotic cell division.

Keywords: Chromosome; Chromosome congression; Chromosome segregation; Kinetochore; Meiosis; Microtubule; Oocyte; Spindle.

Figure 1

Distinct patterns of chromosome segregation occur during meiosis and mitosis. The progression of a single pair of homologous chromosomes (blue and red) through mitosis (A) and meiosis (B) is shown. Before either division, DNA replication duplicates both homologs. Subsequently, each homolog is composed of two sister chromatids tethered by sister chromatid cohesion (yellow ovals). (A) In many organisms, cohesion along chromosome arms is released during mitotic prophase. At metaphase, sisters remain linked by centromeric cohesion. Bipolar attachment of sister kinetochores to microtubules from opposite spindle poles aligns each homolog on the metaphase plate. In anaphase, centromeric cohesion is released and sister chromatids segregate toward opposite poles. (B) During prophase of meiosis I, homologous chromosomes become linked by reciprocal exchange of DNA during crossover recombination. Both sister kinetochores of one homolog capture microtubules from the same spindle pole; the sister kinetochores of the other homolog attach to microtubules from the opposite spindle pole (for simplicity, microtubule contact with both sister chromatid kinetochores is not depicted). In anaphase of meiosis I, release of cohesion between chromosome arms allows homologs to separate; sisters remain tethered by centromeric cohesion. In anaphase II, centromeric cohesion is released, allowing sisters to separate.

Figure 2

Microtubule dynamics and spindle structures during oocyte meiosis and mitosis. (A) Microtubules are hollow tubes 25 nm in diameter formed by lateral association of 13 protofilaments, linear polymers composed α- and β-tubulin heterodimers. Due to the ordered assembly of α/β-tubulin heterodimers, microtubules have intrinsic polarity. One end (the minus end) has α-tubulin as the terminal subunit. This end is relatively stable. The other end (the plus end) has β-tubulin as the terminal subunit. This highly dynamic end switches between rapid growth and shrinkage depending on whether the exposed β-tubulin subunits are bound to GTP (growth) or GDP (shrinkage). During growth, GTP-tubulin adds to the plus end, forming a GTP cap. GTP is hydrolyzed to GDP, and if GDP-β-tubulin is exposed on the plus end of a microtubule, catastrophic disassembly can occur. (B) Cartoons of spindle structure during meiosis and mitosis in C. elegans. During mitosis, long, radial microtubule arrays are nucleated in the pericentriolar material surrounding the centrioles. Some microtubules (blue) from each spindle pole are captured by kinetochores (red). Ultimately, these kinetochore/microtubule attachments align each chromosome on the metaphase plate. Other overlapping, antiparallel microtubules (green) become bundled together in a region called the midzone, a process that stabilizes mitotic spindle structure. In contrast, centrioles are degraded prior to assembly of the oocyte meiotic spindle. Short, often tiled, microtubules form near chromosomes and ultimately coalesce into a bipolar structure.

Figure 3

Assembly of meiotic spindles in mouse, worm, and fly oocytes. Live imaging of microtubules and chromosomes during oocyte meiosis in these three model organisms has revealed similarities and differences in spindle assembly. When spindle assembly begins, homolog pairs have undergone crossover recombination to form bivalents, with the exception of Drosophila chromosome 4, which does not recombine. (A) In mouse oocytes, small microtubule-organizing centers (MTOCs) form near the nuclear envelope prior to NEB. Subsequently, MTOCs are stretched into thin ribbons, often becoming fragmented. This stretching process requires dynein and BicD2, which anchors dynein at the nuclear envelope. After NEB, the Eg5 homolog KIF11 promotes further MTOC fragmentation, yielding an average of approximately 26 MTOCs per oocyte. These MTOCs then coalesce as bivalents congress, forming a bipolar spindle by metaphase. (B) At NEB in C. elegans oocytes, most bivalents are located near the nuclear envelope. Following NEB, a cloud of short microtubules assembles around the bivalents, which subsequently cluster together. Concurrently, several foci of the microtubule-scaffolding protein ASPM-1 appear, presumably present at small MTOCs. The ASPM-1 foci coalesce into two poles as bivalents congress onto the metaphase plate. Initially, the bipolar metaphase spindle is oriented parallel to the oocyte cortex, but then rotates to be perpendicular to the cortex. During rotation, the MTOCs widen as the spindle shortens dramatically. (C) In fly oocytes, the recombined bivalents form a compact structure called a karyosome. After NEB, microtubules accumulate around the karyosome and several dynamic, pole-like structures form. These microtubule foci coalesce to form the poles of the bipolar meiotic spindle. Augmin, γ-tubulin, and D-TACC are enriched at the poles.

Figure 4

Models for bipolar spindle assembly and chromosome congression and segregation in C. elegans. Three models are shown that address pole assembly (A) and chromosome congression and segregation (B and C) in C. elegans oocytes. (A) Proper microtubule-kinetochore attachments permit the coalescence of early pole foci to form a bipolar oocyte spindle. In this model, bivalent alignment and spindle bipolarity require that the two homologs of each bivalent attach specifically to microtubules from opposite spindle poles. Incorrect attachments (see text) are eliminated by the microtubule depolymerase KLP-7/MCAK. Failure to disrupt these incorrect attachments in mutants with reduced KLP-7 function results in an imbalance in spindle tension that interferes with the coalescence of early pole foci and thus results in the assembly of multipolar spindles. (B) Microtubule-kinetochore interactions orient and align bivalents on the metaphase spindle, but at the metaphase-to-anaphase transition, the polar microtubule arrays disassemble, and a new population of microtubules forms between homologs and pushes them apart independently of kinetochores. The assembly of these microtubules is mediated by CLASP and other factors present in ring-shaped structures between each homolog pair at metaphase. When homologs separate, the rings are left behind at the metaphase plate. (C) Lateral interactions between microtubules and microtubule motor proteins mediate both chromosome congression to the metaphase plate and anaphase segregation to the poles. The chromokinesin KLP-19, localized to midbivalent rings, interacts with spindle microtubules to produce a polar ejection force that aligns bivalents at the metaphase plate. Subsequently, during anaphase, polar microtubules interact with dynein, a minus-end directed microtubule motor that accumulates at increasing levels on the poleward/lateral regions of bivalents as meiosis progresses. Dynein-directed motility mediates the poleward movement of chromosomes during anaphase.