The croonian lecture, 1976. Egg cytoplasm and gene control in development

Controlling the Messenger: Regulated Translation of Maternal mRNAs in Xenopus laevis Development

The selective translation of maternal mRNAs encoding cell-fate determinants drives the earliest decisions of embryogenesis that establish the vertebrate body plan. This chapter will discuss studies in Xenopus laevis that provide insights into mechanisms underlying this translational control. Xenopus has been a powerful model organism for many discoveries relevant to the translational control of maternal mRNAs because of the large size of its oocytes and eggs that allow for microinjection of molecules and the relative ease of manipulating the oocyte to egg transition (maturation) and fertilization in culture. Consequently, many key studies have focused on the expression of maternal mRNAs during the oocyte to egg transition (the meiotic cell cycle) and the rapid cell divisions immediately following fertilization. This research has made seminal contributions to our understanding of translational regulatory mechanisms, but while some of the mRNAs under consideration at these stages encode cell-fate determinants, many encode cell cycle regulatory proteins that drive these early cell cycles. In contrast, while maternal mRNAs encoding key developmental (i.e., cell-fate) regulators that function after the first cleavage stages may exploit aspects of these foundational mechanisms, studies reveal that these mRNAs must also rely on distinct and, as of yet, incompletely understood mechanisms. These findings are logical because the functions of such developmental regulatory proteins have requirements distinct from cell cycle regulators, including becoming relevant only after fertilization and then only in specific cells of the embryo. Indeed, key maternal cell-fate determinants must be made available in exquisitely precise amounts (usually low), only at specific times and in specific cells during embryogenesis. To provide an appreciation for the regulation of maternal cell-fate determinant expression, an overview of the maternal phase of Xenopus embryogenesis will be presented. This section will be followed by a review of translational mechanisms operating in oocytes, eggs, and early cleavage-stage embryos and conclude with a discussion of how the regulation of key maternal cell-fate determinants at the level of translation functions in Xenopus embryogenesis. A key theme is that the molecular asymmetries critical for forming the body axes are established and further elaborated upon by the selective temporal and spatial regulation of maternal mRNA translation.

Generation of Albino Cynops pyrrhogaster by Genomic Editing of the tyrosinase Gene

Albino animals are useful for in situ hybridization experiments that demonstrate gene expression in embryos and organs, for the immunological rejection of skin grafts transplanted to host animals, and to identify tissues with regenerative ability during limbs and retina regeneration processes. Cynops pyrrhogaster has extensive regenerating capacities. To facilitate regenerative research, in the present study, we produced albino C. pyrrhogaster using genomic editing. The DNA fragment containing part of the tyrosinase gene from C. pyrrhogaster was amplified using degenerate primers corresponding to evolutionarily conserved nucleotide sequences among several species, and the nucleotide sequence was determined. We designed a transcription activator-like effector nuclease (TALEN) that targets a candidate of the C. pyrrhogaster tyrosinase gene. Fertilized eggs were injected with TALEN mRNA, and albinos of C. pyrrhogaster were obtained. The results of the present study demonstrated that TALEN can be used effectively for genomic editing in C. pyrrhogaster and that the candidates of the tyrosinase gene that were cloned by us are essential for melanin synthesis. The albino newts created in the present study can be used as versatile experimental material.

Cloning: pathways to a pluripotent future

In this month's essay, Anne McLaren traces the winding and pitted pathways that connect the early days of the cell theory of biology in the 1830s to the new and unfolding era of cloning science and technology that came to worldwide attention in 1997 with the announcement of the birth of Dolly, the Scottish cloned sheep. The possibilities, including the potential for new medical treatments and perhaps even human cloning, are fantastic ... and ethically charged.

Expression and activity of p40MO15, the catalytic subunit of cdk-activating kinase, during Xenopus oogenesis and embryogenesis

Threonine 161 phosphorylation of p34cdc2 and its equivalent threonine 160 in p33cdk2 by cdk-activating kinase (CAK) is essential for the activation of these cyclin-dependent kinases. We have studied the expression and associated kinase activity of p40MO15, the catalytic subunit of CAK, during Xenopus oogenesis, meiotic maturation, and early development to understand in more detail how cdk kinases are regulated during these events. We find that p40MO15 is a stable protein with a half-life > 16 h that is accumulated during oogenesis. p40MO15 protein and its associated CAK activity are localized predominantly to the germinal vesicle; however, a small but significant proportion is found in the cytoplasm. The amount of p40MO15 detected in stage VI oocytes remains unchanged through meiotic maturation, fertilization, and early embryogenesis. Significantly, p40MO15 was found to be constitutively active during oogenesis, meiotic maturation, and the rapid mitotic cycles of early development. This suggests that regulation of p34cdc2 and p33cdk2 activity during cell cycle progression does not involve changes in the level or activity of p40MO15/CAK.

Induction of chromosome replication during maturation of amphibian oocytes

After fertilization amphibian embryos replicate their chromosomes faster than bacteria replicate their much smaller chromosomes. During oogenesis, materials are accumulated to sustain these rapid cycles of chromosome replication. Thus growth is uncoupled from nuclear division. Most of the machinery for DNA replication and chromatin assembly is present in the oocyte, which lacks only the ability to initiate on a DNA duplex. After maturation and activation a cell cycle clock is triggered which specifies initiation of DNA replication on endogenous chromosomes, injected nuclei or injected plasmid DNA. The ability to reinitiate replication of a replicated molecule is tightly coupled to the cell cycle clock. Each egg can replicate an amount of DNA equivalent to 500 diploid nuclei in only five hours. However, each egg can assemble an amount of purified DNA equivalent to 12 000 diploid nuclei into regularly spaced nucleosomes in only one hour. The molecular basis of these extraordinary rates of DNA replication and chromatin assembly is considered.

Mouse oocytes transcribe injected Xenopus 5S RNA gene

Transcripts produced after injection of the Xenopus 5S RNA gene into oocyte germinal vesicles of mice migrate electrophoretically with the 5S RNA marker, an indication that the gene is transcribed and processed with considerable accuracy. Approximately two 5S RNA molecules are transcribed per gene per hour. This system may be useful in studying DNA processing and gene regulation by the mammalian ovum and might be modified to allow permanent incorporation of specific genes into mice.