Research Article: Sperm Chromatin-Induced Ectopic Polar Body Extrusion in Mouse Eggs after ICSI and Delayed Egg Activation

Date Published: September 29, 2009

Publisher: Public Library of Science

Author(s): Manqi Deng, Rong Li, Joanna Mary Bridger.

Abstract: Meiotic chromosomes in an oocyte are not only a maternal genome carrier but also provide a positional signal to induce cortical polarization and define asymmetric meiotic division of the oocyte, resulting in polar body extrusion and haploidization of the maternal genome. The meiotic chromosomes play dual function in determination of meiosis: 1) organizing a bipolar spindle formation and 2) inducing cortical polarization and assembly of a distinct cortical cytoskeleton structure in the overlying cortex for polar body extrusion. At fertilization, a sperm brings exogenous paternal chromatin into the egg, which induces ectopic cortical polarization at the sperm entry site and leads to a cone formation, known as fertilization cone. Here we show that the sperm chromatin-induced fertilization cone formation is an abortive polar body extrusion due to lack of spindle induction by the sperm chromatin during fertilization. If experimentally manipulating the fertilization process to allow sperm chromatin to induce both cortical polarization and spindle formation, the fertilization cone can be converted into polar body extrusion. This suggests that sperm chromatin is also able to induce polar body extrusion, like its maternal counterpart. The usually observed cone formation instead of ectopic polar body extrusion induced by sperm chromatin during fertilization is due to special sperm chromatin compaction which restrains it from rapid spindle induction and therefore provides a protective mechanism to prevent a possible paternal genome loss during ectopic polar body extrusion.

Partial Text: Sexual reproduction is the formation of a new individual following the union of two haploid gametes, one from a male and the other from a female, through a process of fertilization. It is known that male and female germ cells undergo different processes of meiotic divisions and post-meiotic remodeling to produce gametes with striking differences in size, shape, chromatin packaging state, cell cycle, and etc. An oocyte undergoes two rounds of extreme asymmetric meiotic division following one round of DNA replication, producing a large sized haploid egg and discarding the other half of the chromosomes into two small polar bodies designated for degeneration. Female meiosis in many animal specieses is not complete at ovulation but is arrested at metaphase of the second meiosis (MII), waiting for fertilization to reinitiate the meiosis II. At fertilization, a sperm not only delivers a haploid paternal genome to the egg but also triggers resumption and completion of meiosis II by inducing Ca2+ spikes [1], culminating in the extrusion of the second polar body (PbII). In contrast to female meiosis, a spermatocyte produces 4 haploid spermatids after completion of meiosis. However, successful haploidization of spermatocytes is only the first step towards gamete production and the produced haploid spermatids must undergo extensive post-meiotic chromatin reorganization and morphological changes before fertilization. One of the most striking post-meiotic changes is to repackage the paternal haploid genome with the testis-specific nuclear basic proteins, e.g. protamines, forming a tightly compacted chromatin structure [2], [3], [4], [5], [6] coupled with dramatic morphological changes from round spermatids into tadpole-shaped spermatozoa with motility [7]. The protamine-packed DNA makes the sperm chromatin six-fold more compacted than the somatic histone-chromatin [3], [8] and the compaction is further enhanced by crosslinking (via disulfide bonds) while passing through epididymis before ejaculation [9], [10].

Sperm-induced fertilization cone has been observed for decades but little is known about the mechanism of formation and particularly its relevance to polar body extrusion. Our results suggest that the sperm chromatin-induced fertilization cone and meiotic chromosome-induced polar body extrusion may share the same cytokinetic mechanism. The sperm chromatin-induced cortical cone can result in ectopic polar body extrusion if sperm chromatin is allowed to assemble a bipolar spindle prior to anaphase onset. Conversely, the meiotic chromosome-induced polar body extrusion can be directed to cone formation if the meiotic spindle is not formed or disrupted. This suggests that the chromatin-induced either cortical cone formation or polar body extrusion can be viewed as at different stages of cytokinesis. During polar body extrusion, cortical cone formation is spatially coordinated with anaphase spindle midzone-induced membrane furrowing and abscission which drives cytokinesis to completion. While in the absence of spindle, the cortical cone can not be abscissed in the absence of midzone signals.

The experimental animals were handled in accordance with good animal practice as defined by the National Institute of Health of the United States and guidelines of Institutional Animal Care and Use Committee (IACUC) and all the animal work was approved by the IACUC committee at the Stowers Institute for Medical Research, protocol #2007-0013.