Transmembrane signal transduction in oocyte maturation and fertilization: focusing on Xenopus laevis as a model animal

Mechanical characterization of Xenopus laevis oocytes using atomic force microscopy

Mechanical properties are essential for the biological activities of cells, and they have been shown to be affected by diseases. Therefore, accurate mechanical characterization is important for studying the cell lifecycle, cell-cell interactions, and disease diagnosis. While the cytoskeleton and actin cortex are typically the primary structural stiffness contributors in most live cells, oocytes possess an additional extracellular layer known as the vitelline membrane (VM), or envelope, which can significantly impact their overall mechanical properties. In this study, we utilized nanoindentation via an atomic force microscope to measure the Young's modulus of Xenopus laevis oocytes at different force setpoints and explored the influence of the VM by conducting measurements on oocytes with the membrane removed. The findings revealed that the removal of VM led to a significant decrease in the apparent Young's modulus of the oocytes, highlighting the pivotal role of the VM as the main structural component responsible for the oocyte's shape and stiffness. Furthermore, the mechanical behavior of VM was investigated through finite element (FE) simulations of the nanoindentation process. FE simulations with the VM Young's modulus in the range 20-60 MPa resulted in force-displacement curves that closely resemble experimental in terms of shape and maximum force for a given indentation depth.

Mitochondrial morphology, distribution and activity during oocyte development

Mitochondria have a crucial role in cellular function and exhibit remarkable plasticity, adjusting both their structure and activity to meet the changing energy demands of a cell. Oocytes, female germ cells that become eggs, undergo unique transformations: the extended dormancy period, followed by substantial increase in cell size and subsequent maturation involving the segregation of genetic material for the next generation, present distinct metabolic challenges necessitating varied mitochondrial adaptations. Recent findings in dormant oocytes challenged the established respiratory complex hierarchies and underscored the extent of mitochondrial plasticity in long-lived oocytes. In this review, we discuss mitochondrial adaptations observed during oocyte development across three vertebrate species (Xenopus, mouse, and human), emphasising current knowledge, acknowledging limitations, and outlining future research directions.

Toward the understanding of biology of oocyte life cycle in Xenopus Laevis: No oocytes left behind

BACKGROUND

For the past more than 25 years, we have been focusing on the developmental and reproductive biology of the female gametes, oocytes, and eggs, of the African clawed frog Xenopus laevis.

METHODS

The events associated with the life cycle of these cells can be classified into the four main categories: first, oogenesis and cell growth in the ovary during the first meiotic arrest; second, maturation and ovulation that occur simultaneously and result in the acquisition of fertilization competence and the second meiotic arrest; third, fertilization, that is sperm-induced transition from egg to zygote; and fourth, egg death after spontaneous activation in the absence of fertilizing sperm.

MAIN FINDINGS

Our studies have demonstrated that signal transduction system involving tyrosine kinase Src and other oocyte/egg membrane-associated molecules such as uroplakin III and some other cytoplasmic proteins such as mitogen-activated protein kinase (MAPK) play important roles for successful ovulation, maturation, fertilization, and initiation of embryonic development.

CONCLUSION

We summarize recent advances in understanding cellular and molecular mechanisms underlying life cycle events of the oocytes and eggs. Our further intention is to discuss and predict potentially promising impact of the recent findings on the challenges facing reproductive biology and medicine, as well as societal contexts.

Membrane Microdomains as Platform to Study Membrane-Associated Events During Oogenesis, Meiotic Maturation, and Fertilization in Xenopus laevis

Studies on the egg plasma membrane-associated tyrosine kinase Src have shed light on the identity of the molecular machinery that is responsible for gamete interaction and possibly fusion in African clawed frog Xenopus laevis. Here we describe our protocol for identifying and analyzing molecular and cellular machinery that contributes to a variety of biological processes in the course of oogenesis, oocyte maturation, egg fertilization, and early embryogenesis in Xenopus. Our current special interest is to evaluate the hypothesis that the oocyte/egg membrane microdomain (MD)-associated uroplakin III-Src system is responsible for mediating sperm-egg membrane interaction/fusion signal to the oocyte/egg cytoplasm to initiate embryonic and zygotic development in this species. Therefore, this chapter contains a brief introduction to biology of oocytes and eggs in Xenopus and addresses the following questions: (1) What is oocyte/egg MD? (2) Why do we study oocyte/egg MD? (3) How to manipulate oocyte/egg MD? (4) What has been achieved by oocyte/egg MD studies? (5) What are the next steps in oocyte/egg MD studies?

Deficiency in Sperm-Egg Protein Interaction as a Major Cause of Fertilization Failure

Complete elucidation of fertilization process at molecular level is one of the unresolved challenges in sexual reproduction studies, and understanding the molecular mechanism is crucial in overcoming difficulties in infertility and unsuccessful in vitro fertilization. Sperm-oocyte interaction is one of the most remarkable events in fertilization process, and deficiency in protein-protein interactions which mediate this interaction is a major cause of unexplained infertility. Due to detection of how the various defects of sperm-oocyte interaction can affect fertilization failure, different experimental methods have been applied. This review summarizes the current understanding of sperm-egg interaction mechanism during fertilization and also accumulates the different types of sperm-egg interaction abnormalities and their association with infertility. Several detection approaches regarding sperm-egg protein interactions and the associated defects are reviewed in this paper.

Prophase I: Preparing Chromosomes for Segregation in the Developing Oocyte

Formation of an oocyte involves a specialized cell division termed meiosis. In meiotic prophase I (the initial stage of meiosis), chromosomes undergo elaborate events to ensure the proper segregation of their chromosomes into gametes. These events include processes leading to the formation of a crossover that, along with sister chromatid cohesion, forms the physical link between homologous chromosomes. Crossovers are formed as an outcome of recombination. This process initiates with programmed double-strand breaks that are repaired through the use of homologous chromosomes as a repair template. The accurate repair to form crossovers takes place in the context of the synaptonemal complex, a protein complex that links homologous chromosomes in meiotic prophase I. To allow proper execution of meiotic prophase I events, signaling processes connect different steps in recombination and synapsis. The events occurring in meiotic prophase I are a prerequisite for proper chromosome segregation in the meiotic divisions. When these processes go awry, chromosomes missegregate. These meiotic errors are thought to increase with aging and may contribute to the increase in aneuploidy observed in advanced maternal age female oocytes.

Moderate increase of reactive oxygen species triggers meiotic resumption in rat follicular oocytes

AIM

The mammalian ovary generates reactive oxygen species (ROS) on an extraordinary scale; however, the role of ROS during meiotic cell cycle progression in follicular oocytes remains poorly understood. The present study was aimed to determine whether a moderate increase of ROS level in the ovary is beneficial for meiotic resumption from diplotene arrest in follicular oocytes.

METHODS

Cumulus oocyte complexes were collected from the ovaries of female rats that had been treated with either: (i) pregnant mare's serum gonadotrophin; or (ii) pregnant mare's serum gonadotrophin + human chorionic gonadotrophin. We analyzed morphological changes, ROS and hydrogen peroxide levels, catalase activity, 3',5'-cyclic adenosine monophosphate and 3',5'-cyclic guanosine monophosphate levels, Thr14/Tyr15, Th-161, total cyclin-dependent kinase 1 (Cdk1) and cyclin B1 levels.

RESULTS

Human chorionic gonadotrophin treatment induced meiotic resumption from diplotene arrest and extrusion of first polar body in cumulus oocyte complexes collected from ovaries and cultured for 3 h in vitro. Meiotic resumption from diplotene arrest was associated with increased ROS and hydrogen peroxide levels but decreased 3',5'-cyclic adenosine monophosphate as well as 3',5'-cyclic guanosine monophosphate levels. The reduced cyclic nucleotide levels were associated with decreased Thr161 phosphorylated Cdk1 and cyclin B1 level but increased Thr14/Tyr15 phosphorylated Cdk1 level leading to maturation promoting factor destabilization. Destabilized maturation-promoting factor triggered meiotic resumption from diplotene arrest and progression to metaphase-I as well as metaphase-II stage in follicular oocytes.

CONCLUSION

Our findings suggest that a moderate increase of ROS in the ovary is beneficial for meiotic resumption from diplotene arrest and extrusion of first polar body in follicular oocytes.