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Marlow FL. Maternal Control of Development in Vertebrates: My Mother Made Me Do It! San Rafael (CA): Morgan & Claypool Life Sciences; 2010.

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Maternal Control of Development in Vertebrates: My Mother Made Me Do It!

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Cleavage/Mitosis: Going Multicellular

So far, we have discussed maternal-effect genes that are necessary to produce a fertilization competent egg. Once fertilized, the maternal and paternal genetic material (the pronuclei) will fuse, and the now single-celled embryo must divide to produce the cells required to form the multicellular animal. While the precise number varies depending on the animal, the first several mitotic cell cycles occur before the zygotic or embryonic genome is expressed; thus, these cell cleavages depend on maternal products as well as contribution from the fertilizing sperm (Kim and Roy, 2006; Yabe et al., 2007; Zhong et al., 2005). The maternal factors involved are largely unknown but are expected to include proteins that mediate the destruction of meiotic factors and post-translational regulation of mitotic cell cycle regulators. In this chapter, we will discuss the maternal genes required for the early cell divisions of the embryo.

In some vertebrates, such as humans and mouse, the mitotic cycle begins prior to or concomitant with maternal and paternal pronuclei fusion, while in other animals (e.g., zebrafish, Xenopus), the pronuclei fuse before mitosis. In zebrafish, mitotic cycle initiation does not absolutely require pronuclear fusion as is evident from futile cycle maternal-effect mutants in which haploid nuclei fusion is blocked; nonetheless, cytokinesis persists (Dekens et al., 2003). The cleavages of early vertebrate embryos are distinct from the cell divisions that occur later in development. The early embryonic cleavages are more rapid, and the cycles consist of synthesis and division phases without gap or intervening phases (Figure 16). Consequently, each cell division amplifies the total number of cells and the DNA content, but the resulting cells become increasingly smaller with each division during the cleavage phase. In animals that develop rapidly, more cleavage divisions are completed before the zygotic genome is activated (e.g., 12 synchronous cleavage cycles in Xenopus and zebrafish; Newport and Kirschner, 1982; Kane and Kimmel, 1993) and 4 in bovine embryos]. In these animals, there is a prolonged temporal window separating regulation of cellular cleavages and zygotic genome activation. Thus, the genes that are essential to regulate early cleavages can be distinguished from those genes that are not required for cleavage per se, but instead are necessary for zygotic or embryonic genome activation (ZGA, EGA). At or after zygotic genome activation, the cell cycle lengthens as gap, and intervening phases are introduced between synthesis and division (Figure 16).

FIGURE 16. Schematic depicting the maternal to zygotic transition.

FIGURE 16

Schematic depicting the maternal to zygotic transition. The cleavages of early embryos lack gap phases, are rapid, generate increasingly smaller cells, and are under maternal control. Upon zygotic genome activation, the cell cycle lengthens as gap phases (more...)

Genetic Uncoupling of Karyokinesis and Cytokinesis in Zebrafish

Cell division or cleavage requires duplication of the chromosomes and centrioles followed by equal partitioning of the genetic material (karyokinesis) and division of the cytoplasm (cytokinesis). Distinct classes of maternal-effect mutants in which karyokinesis and cytokinesis are genetically uncoupled have been identified through genetic screens (Figure 16) (Dosch et al., 2004; Kishimoto et al., 2004; Pelegri et al., 2004; Pelegri et al., 1999). One class of zebrafish maternal-effect mutants disrupt genes required to initiate cleavage; indivisible and atomos mutants are fertilized, but do not divide (Dosch et al., 2004). In these mutants, both karyokinesis and cytokinesis are blocked (Dosch et al., 2004). The spindle and the nuclear envelope have not been examined nor has the possibility that defective pronuclear fusion contributes to these mutant phenotypes been ruled out. However, the zebrafish futile cycle mutants discussed above are defective in pronuclear fusion yet undergo cytokinesis indicating that fusion of the maternal and paternal genetic material is not prerequisite for cytokinesis, but is essential for karyokinesis (Dekens et al., 2003).

In the phenotypic class comprised of golden gate ( gdg), kwai, bo peep, and waldo mutants, cytokinesis proceeds despite impaired karyokinesis (Pelegri et al., 2004). In contrast, mitosis occurs without cytokinesis in ack mutants (Kishimoto et al., 2004). Finally, weeble and barrette are required for both cytokinesis and mitosis, as is cobblestone, but only in a subset of cells (Pelegri et al., 2004). This collection of mutants disrupts genes that initiate or coordinate both karyokinesis and cytokinesis and provide genetic evidence for distinct regulation of the nuclear and cellular division aspects of early cleavages. Although the molecular identities of the disrupted genes required for early cleavage have yet to be determined, it is possible that some of the genes encode regulators of meiotic factor clearance or mitosis activators.

Contributions of the Cytoskeleton to Cytokinesis and Karyokinesis

Evidence that the cytoskeleton sustains cleavages comes from interference studies using antibodies and cytoskeletal inhibitors of actin, tubulin, and cytokeratin during embryonic cleavage stages in model systems, including zebrafish, frog, and mouse. These studies uncovered maternal contributions of the cytoskeleton to regulation of the spindle, of furrow formation, of endocytosis, of membrane remodeling, and of cellular cohesiveness via delivery of adhesion molecules to the cell surface in cleavage stage embryos (Danilchik et al., 1998; Feng et al., 2002; Jesuthasan, 1998; Zhong et al., 2005). More recently, zebrafish maternal-effect screens have contributed genetic support for essential and distinct contributions of cytoskeletal components to controlling the early cleavages. Depending on the cell, cleavage furrows form at the cortex in the vicinity of the spindle apparatus in response to signals emanating from the spindle itself or from the associated astral microtubules (Bringmann, 2008; Bringmann et al., 2007; Bringmann and Hyman, 2005; Bringmann, Skiniotis et al., 2004). The zebrafish maternal-effect cellular island and futile cycle mutants support a role for signals from both (Dekens et al., 2003; Yabe et al., 2009). Cellular Island encodes AuroraB Kinase, previously implicated in regulating furrow formation (reviewed by Ruchaud et al., 2007). In zebrafish, cellular island is required to form the astral microtubule-associated furrows, but not the spindle-induced furrows (Yabe et al., 2009). Microtubule nucleation and spindle assembly require futile cycle ( fue) function while furrow formation and cell partitioning are independently regulated in the zebrafish (Dekens et al., 2003). Although the spindles are not properly formed in fue mutants, those spindles that are intact are sufficient to initiate furrow formation (Dekens et al., 2003). Thus, both mechanisms seem to contribute nonredundantly in the zebrafish to regulation of distinct subsets of the cleavage furrows present in the early embryo.

After furrow initiation, the product of the zebrafish nebel gene is required for cleavage furrow microtubule array formation (Pelegri et al., 1999). Mutants disrupting the zebrafish aura gene indicate that aura promotes membrane recruitment to the furrow in order to accomplish cleavage by a mechanism that is not understood (Pelegri et al., 2004). Regulation of actin dynamics during cleavage furrow formation is mediated by acytokinesis function, which is required to accomplish successful cytokinesis and karyokinesis during early cleavage stages in zebrafish embryos (Kishimoto et al., 2004). Determining the molecular identity of these disrupted genes promises to provide insight into the molecular mechanisms regulating microtubule and actin dynamics during separation of the chromosomes and cellular partitioning.

Cellular atoll/Sas6: Centrosome Duplication and Karyokinesis in Zebrafish

Centrosomes contribute to preservation of ploidy through their function in organizing the spindle poles during cell division. The eggs of many animals are devoid of centrioles until fertilization when the sperm provides the centrioles necessary for development of the embryo. The centrioles are duplicated prior to the first cell division to generate the centrosomes that will mediate chromosome separation during cell division. The zebrafish cellular atoll mutant disrupts a conserved residue within the centriolar protein Spindle assembly 6, Sas6. Evidence for an essential maternal role for Sas6 in formation and duplication of centrosomes during embryonic cleavages comes from a hypomorphic allele disrupting Sas6 (Yabe et al., 2007). The first cell division is normal in the progeny of mutant mothers; however, during the second division cycle, a subset of cellular atoll progeny develops with a monopolar spindle and a single centriolar pair (Yabe et al., 2007). The consequence of failed centrosome duplication during the second cell division cycle is defective karyokinesis and furrow formation (Yabe et al., 2007). Not all animals rely on centrosome-dependent spindle assembly during early cleavages, notably, in the mouse, centriolar assembly is regulated independent of early cleavage (Wadsworth and Khodjakov, 2004).

Pms2: A Role for Mismatch Repair in Preventing Aneuploidy

The DNA mismatch repair pathway corrects base pair errors and small deletions or insertions. During the initial rapid cleavages of mouse embryos maternal post-meiotic segregation increased 2 (Pms2), a functional homolog of the mismatch repair protein MutL, functions to limit the accumulation of mutations during replication as is evidenced by the replication errors that accumulate in embryos lacking maternal Pms2 at the one-cell stage (Gurtu et al., 2002). The pms2 maternal-effect mutant phenotype indicates that, even after the zygotic genome has been activated, the embryo seems to have no mechanism to remedy the aneuploidy caused in the absence of maternal PMS2 function, while surviving Fmn2 mutants discussed earlier indicate that a mechanism may exist to correct aneuploidy (Dumont et al., 2007; Gurtu et al., 2002; Leader et al., 2002). The reasons for the differences in ability of zygotic gene function to compensate or correct the error may lie in the developmental timing, meiosis versus first cleavage. Alternatively, it may be that an excess of chromosomes, as occurs in Fmn2 maternal mutants can be repaired, while the widespread deletions, insertions, and rearrangements causing inappropriate copy numbers observed in MutL mutants are beyond the capacity of the repair machinery.

Geminin: Limiting Replication with Licensing Components

Geminin is a component of the licensing machinery that acts to limit DNA replication to a single round during each cell division cycle. Geminin deficiency in the mouse results in developmental arrest at the four- to eight-cell stage when maternally supplied Geminin protein is thought to be exhausted or degraded (Hara et al., 2006; Gonzalez et al., 2006). Based on the correlation between degradation of the maternal protein, the onset of the SI-phase block, and ensuing excessive endoreplication, which causes premature differentiation of trophoblast cells in zygotic Geminin mutants, maternal Geminin is predicted to prevent the cells of the early embryo from adopting trophoblast fate at the expense of the pluripotent ICM cells until zygotic Geminin protein is produced (Hara et al., 2006; Gonzalez et al., 2006). A maternal mutant disrupting Geminin is needed to definitively demonstrate an essential role for maternal Geminin in preventing precocious trophoblast differentiation.

Microtubules and Motors: Regulating the Spindle and Polar Body

A plus-ended kinesin-related motor protein, Eg5, regulates proliferation during early cleavage stages in the mouse (Castillo and Justice, 2007). Although not directly examined in the Eg5 knockout, the proliferation defects have been ascribed to Eg5 function in spindle assembly. Maternal eg5 mRNA and protein peaks in the oocytes of mouse and Xenopus are consistent with a maternal role for Eg5; this maternal contribution has been proposed to sustain normal development of zygotic mutants until the maternal Eg5 protein stores become limiting at zygotic genome activation (Houliston et al., 1994; Sawin and Mitchison, 1995; Winston et al., 2000; Zeng and Schultz, 2003).

Mouse γ-tubulin is also essential for spindle formation and proper nuclear division. In γ-tubulin mutants, defects in nuclear division correlate with, and thus have been attributed to the temporal exhaustion of maternal γ-tubulin protein stores (Yuba-Kubo et al., 2005). Depletion and gain of function studies in oocytes support a maternal contribution of γ-tubulin to spindle regulation. Depleting γ-tubulin diminishes spindle and polar body size, while supplying an excess of maternal γ-tubulin during meiotic divisions increases spindle and polar body size (Barrett and Albertini, 2007). Interestingly, polar bodies that receive additional γ-tubulin are not only larger, but also undergo cytokinesis rather than degradation after they are extruded (Barrett and Albertini, 2007). How the total pool of γ-tubulin available to the oocyte is partitioned to ensure that sufficient γ-tubulin is allocated to complete meiosis, yet enough is reserved to permit normal progression through the early cleavage cycles of the embryo is not understood.

Tcl1: Contribution of a Cell Survival Factor to Embryonic Cleavages

T cell leukemia/lymphoma 1, tcl1, encodes a protein with globular domains that functions as a cofactor of Akt1 in cell culture. Tcl1 knockout mice develop as fertile males or as females with compromised fertility, but maternal Tcl1 is not essential for meiosis or fertilization (Narducci et al., 2002). When cultured, the progeny of mutant mothers fail to develop to blastocyst stages; arrest of the majority of embryos at the four- to eight-cell stage indicates a necessary role for maternal Tcl1 during preimplantation stages (Narducci et al., 2002). Narducci and colleagues showed that Tcl1 protein shuttles between the cell cortex and the nucleus through the eight-cell stage of development in a cell cycle-dependent manner. Consistent with the cell cycle-regulated shuttling of the protein, maternal Tcl1 is required for cleavage (Narducci et al., 2002). Normal expression of zygotic genome activation reporters and initiation of compaction in the progeny of Tcl1 maternal mutants provided evidence that zygotic genome activation and differentiation are Tcl1-independent processes. Though dispensable for compaction, reduced numbers of cells were observed in the progeny of maternal Tcl1 mutants (Narducci et al., 2002). Therefore, it is possible that the embryos arrest because they lack sufficient cells to form a proper inner cell mass. However, the mechanism by which maternal Tcl1 regulates early embryonic cleavages and the transition to the blastocyst stage remains to be elucidated.

Most candidate genes postulated to function maternally during early cleavage stages were chosen based on their expression profiles. For example, transcripts with robust expression at, or prior to, the first cleavages and subsequent rapid elimination thereafter are candidate regulators of early cleavages or of embryonic genome activation (EGA). In mouse, the maternal to zygotic transition occurs after only one cell cycle (Figure 16) (Mager et al., 2006; Mathavan et al., 2005; Melton, 1991; Memili and First, 2000; Misirlioglu et al., 2006; Zimmermann and Schultz, 1994). In general, embryos that activate the zygotic genome during the initial cleavage stages would be expected to arrest at a similarly early developmental stage when maternal function is depleted. Accordingly, in the mouse, many maternal-effect mutants arrest at around the two-cell stage. The switch from cleavage to mitosis cycles and from maternal to zygotic control of development both occur around the time of the first cell cycle. Therefore, in the mouse, mutations that disrupt genes required for mitosis or genes required for zygotic genome activation will cause developmental arrest at the two-cell stage and fail to implant (Figure 16). On the other hand, mutations that disrupt mitosis regulators may not show a phenotype due to compensation by the zygotic counterpart. If an embryo does arrest before EGA, it is not likely to express zygotic genes. Conversely, if the zygotic genome is not activated, the ability of the embryo to continue to cleave will be compromised. The temporal proximity of these developmental events makes it extremely challenging to distinguish whether a gene has a specific and essential role in regulating the cell cycle, other aspects of the first embryonic cleavages, or is required to activate the embryonic genome.

Copyright © 2010 by Morgan & Claypool Life Sciences.
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