Because fertilization occurs during female meiosis and because sperm-derived centrioles form sperm asters equipped to capture the female pronucleus, all animals must actively prevent sperm asters from interfering with meiotic spindles until after extrusion of the second polar body. C. elegans uses at least two mechanisms to accomplish this: (1) the meiotic spindle and sperm are positioned at opposite ends of the zygote and (2) recruitment of pericentriolar proteins to the sperm centrioles is suppressed so that asters do not form until after extrusion of the second polar body. Kinesin-1 and its binding partner, KCA-1, are involved in both of these mechanisms.
Early in diakinesis, the prophase nucleus or germinal vesicle is centered within the oocyte. The germinal vesicle then migrates away from the end of the oocyte that is closest to the sperm in the spermatheca. This directed migration requires kinesin-1/ KCA-1 (McNally et al., 2010
). Fertilization then occurs at the end of the oocyte that first enters the spermatheca (Samuel et al., 2001
). This system results in the rough placement of the meiotic spindle and sperm at opposite ends of the oval embryo long before establishment of embryonic polarity, which is induced by the sperm aster after completion of meiosis (Gönczy and Rose, 2005
). During meiotic metaphase I and II, the spindle is maintained at the future anterior cortex by kinesin-1/KCA-1 (Yang et al., 2005
), whereas during anaphase I and II the spindle is anchored at the cortex by cytoplasmic dynein (Ellefson and McNally, 2009
; 2011). Positioning of the sperm DNA during female meiosis is more enigmatic. Kinesin-1/KCA-1 appears to transport the sperm DNA inward from the site of fertilization at the plasma membrane but the sperm DNA is maintained in the future posterior third of the embryo by an unknown mechanism. This migration of the sperm DNA is counterintuitive because it places the sperm DNA closer to the meiotic spindle. In fact, the average minimum distance between sperm DNA and meiotic spindle is not significantly different (p = .16, unpaired t test) between wild-type (27.8 ± 1.7 µm, n = 24) and kca-1(RNAi)
(24.2 ± 1.6 µm, n = 15). Even though the meiotic spindle is further from the future anterior cortex in kca-1(RNAi)
embryos, the sperm DNA is also closer to the future posterior cortex. Because cortical contact by the sperm aster initiates establishment of embryonic polarity in C. elegans
(Gönczy and Rose, 2005
), the inward transport of the sperm might be important in preventing premature polarity establishment. Two time-lapse studies of human fertilization reported that the male pronucleus always forms in the center of the zygote (Mio, 2006
; Payne et al., 1997
). This result indicates that the sperm DNA must be transported inward from the site of fertilization in human zygotes, which do not use the sperm as a cue for establishing embryonic polarity (Cooke et al., 2003
). Depending on the sizes of the oocyte, spindle and sperm aster, centering the sperm while anchoring the spindle at the cortex might serve the general purpose of keeping the spindle and sperm apart.
Kinesin-1/KCA-1 is also essential for blocking recruitment of the maternal pericentriolar proteins, SPD-5 and γ-tubulin, to the sperm-derived centrioles. Unlike other kinesin-1/KCA-1-dependent processes like germinal vesicle positioning (Yang et al., 2003
; McNally et al., 2010
), meiotic spindle positioning (Yang et al., 2003
), and sperm DNA positioning (this study), inhibition of pericentriolar protein recruitment to sperm centrioles does not require cytoplasmic microtubules and therefore cannot involve transport by kinesin on cytoplasmic microtubules. The requirement for the centriolar protein, SPD-2, in assembly of a shell of kinesin-1/KCA-1 around the sperm DNA and centrioles suggests a direct or indirect interaction between kinesin-1/KCA-1 and SPD-2. The shell of kinesin-1/KCA-1 might directly block diffusion of large complexes of pericentriolar material (PCM) from the cytoplasm to the centrioles or act as a scaffold for phosphatases that oppose PCM recruitment. Alternatively kinesin-1/KCA-1 might act more globally on the large cytoplasmic pool of SPD-2. Disruption of this pathway only resulted in spindle capture by the sperm aster in cases where the sperm aster and spindle ended up in close proximity.
The importance of sperm aster suppression in human zygotes is difficult to discern from the literature. The time between sperm-egg fusion and second polar body extrusion after in vitro fertilization is difficult to interpret from published studies (Mio, 2006
) in part because of the long and heterogeneous time required for the sperm to traverse the zona pellucida. Intracytoplasmic injection of sperm eliminates these uncertainties and the second polar body is extruded on average 2 hr after injection of human sperm into human oocytes (Mio, 2006
; Payne et al., 1997
). This would appear to be sufficient time for a sperm aster to form and interfere with anaphase II. The invasive methods required to unambiguously score sperm aster formation are not compatible with assisted reproduction in humans. However, when human sperm is injected into bovine oocytes, sperm aster formation is suppressed during anaphase II and does not occur until decondensation of both male and female pronuclei (Nakamura et al., 2001
). It is likely that the use of advanced polarization microscopy (Kilani et al., 2011
) will allow noninvasive analysis of sperm aster formation in human zygotes.
The finding that KCA-1 is degraded during meiosis suggests that normal centrosome maturation after meiosis is initiated by regulated proteolysis of KCA-1. Numerous proteins are degraded by different ubiquitin ligases in a highly ordered temporal sequence during or shortly after meiosis in C. elegans
. For example, CAV-1 is degraded during meiosis in an endocytosis-dependent manner (Sato et al., 2006
). MEI-1 is degraded at the exit from meiosis by a CUL-3-dependent mechanism (Pintard et al., 2003
). OMA-1 is degraded after the first mitotic cleavage (Nishi and Lin, 2005
). Although each of these proteins is degraded at a different time by a different mechanism, degradation of CAV-1 (Sato et al., 2006
), MEI-1 and OMA-1 (Pellettieri et al., 2003
) are also blocked in embryos arrested in meiotic metaphase I by depletion of anaphase promoting complex subunits (Pellettieri et al., 2003
). The surprising finding that KCA-1 is degraded and that centrosomes mature in metaphase-arrested, APC-depleted embryos indicates that multiple, synchronized but independent clocks control the transition from oocyte to embryo.