Authors' Perspectives: Cell Size Sparks Embryonic Genome Awakening

In an early embryo undergoing cell division, maternally loaded RNA and proteins regulate the cell cycle. However, at some point in the early life of an embryo, zygotic nuclei "wake up" from sleep mode, and zygotic gene expression takes control over subsequent embryo development. The principal determinant of this transition has been a topic of debate for decades.

Recently, a group from the Perelman School of Medicine at the University of Pennsylvania published a paper that revealed fresh insights on how an embryo "hands over" control of development from mother to zygote.

The paper was featured as the cover story in an issue of Developmental Cell, and caught our attention. Here, two of the co-authors share their insights on the findings and other topics covered in their lab.

Both are researchers at the University of Pennsylvania; Matthew Good is an Assistant Professor in the Department of Cell and Developmental Biology, and in the Department of Bioengineering. Hui Chen is the first author and a postdoctoral fellow in Good's lab. Their paper is summarized in this press release, and more information about their research can be found on their website.

Michele Wilson (MW): In the latest issue of Developmental Cell, you report that the size of a cell is the principal determinant of zygotic genome activation. Can you expand on this finding?

Matthew Good (MG) and Hui Chen (HC): After fertilization of the egg, the embryo undergoes multiple rounds of cell division in the absence of cell growth. Because the size of the blastula embryo is unchanged, cell division causes cell volume to decrease by many orders of magnitude; 100,000-fold decrease in Xenopus laevis. Each cell contains an equivalent amount of DNA and therefore the ratio of DNA to cytoplasmic volume increases dramatically during early development.

One model for ZGA regulation is that the embryo overcomes transcriptional repression by achieving a threshold DNA:cytoplasm ratio, in effect titrating out an inhibitor by reducing cell volume. Our findings are consistent with this view. We also expanded this model to show that single cells make the decision to initiate large-scale ZGA by reaching a threshold cell size, and that changes in cell size are sufficient to alter the timing of ZGA. Importantly, we were able to rule out other competing models, including the theory that an embryo counts time or counts number of rounds of cell divisions to determine the onset of ZGA.

MW: Why is there so much interest in identifying the factors that govern zygotic genome activation?

MG & HC: The maternal-to-zygotic transition (MZT) is an essential event in the early life of an embryo; it is necessary for subsequent development in animals and plants. Within vertebrate model organisms such as Xenopus and zebrafish, the genes expressed at ZGA are required for gastrulation and specification of three primary germ layers, ectoderm, mesoderm and endoderm.

Dysregulation of ZGA can have severe developmental consequences. Additionally, because the timing of ZGA varies significantly between species, a fundamental question in cell and developmental biology is how embryos or the cells within it decide to initiate genome activation.

We, along with other labs would like to identify the molecular factors that regulate this decision-making.

MW: Can you explain what led you to select the African clawed frog as your model? Do you expect this process to hold true in other species?

MG & HC: We selected the African clawed frog, Xenopus laevis, for its unique features. At the onset of ZGA, it contains a very large gradient of cell sizes within a single blastula embryo. In contrast, other model embryos, including those from mouse and zebrafish, contain cells whose sizes are relatively homogenous. The cell size variation in Xenopus allowed us to distinguish between a timer model and a cell sizer model for ZGA regulation.

A second important feature is that the embryos are amenable to physical manipulation at 1-cell stage, following fertilization. This feature allowed us to alter the size of embryo - generate half-volume and quarter-volume embryos – to distinguish between a cell cycle counter model and cell sizer model. In most other species that have relatively homogenous cell sizes in the embryo at the onset of ZGA, it would be very difficult to disentangle the effects of time, cell division count, and cell size, all of which change in concert during cleavage stages. Xenopus laevis embryos provided us an ideal system in which to separate the effects of these parameters.

MW: In your lab, you also study the idea that organelle sizes are flexible and coupled to cell size. Can you comment on the current line of thinking on this, and any other related hypotheses you are testing?

MG & HC: It is now widely believed that the sizes of intracellular organelles are coupled to the volume or dimensions of a cell. My lab is particularly interested in how the sizes of large structures such as the interphase nucleus and mitotic spindle are regulated, and the consequences to a cell caused by dysregulation of their sizes.

MW: What other aspects of cell biology are you studying in your lab, in relation to cell size regulation?

MG & HC: We are interested in identifying disease conditions in which cells and organelles either lose size homeostasis or shift their size scaling relationships.

MW: How are microfluidic technologies leveraged in your research? What are the main techniques driving your work?

MG & HC: We use microfluidic technologies to encapsulate purified components or complex cytoplasmic mixtures to study how the size and shape of a cell affects the assembly of intracellular structures, signaling and gene expression.

Matt Good and Hui Chen were speaking to Michele Wilson, Science Writer for Technology Networks.