EMBRYONIC POLARITY AND THE SOMA-GERMLINE DICHOTOMY

Our lab studies the earliest stages of embryogenesis to understand how single-celled eggs develop into complex multicellular embryos. We focus on the choice between soma and germline, one of the first developmental decisions faced by embryos. Our goal is to identify and characterize the molecular mechanisms that activate embryonic development, polarize embryos, and distinguish between somatic and germline cells, using Caenorhabditis elegans as a model system. Our research program is divided into three areas:

Oocyte-to-embryo transition

FIGURE 1: Localization of MBK-2 changes  during the oocyte-to-embryo transition.

The beginning of development is marked by a remarkable transition: the quiescent oocyte is transformed into a dynamic embryo ready to differentiate into many cell types. We recently found that the oocyte-to-embryo transition requires the coordinate degradation of several oocyte proteins. We identified minibrain kinase 2 (MBK-2), a member of the evolutionarily conserved DYRK family of kinases, as a candidate master regulator of oocyte protein degradation.  MBK-2 phosphorylates oocytes proteins shortly after fertilization during the meiotic divisions. Surprisingly, progression through the meiotic divisions, rather than fertilization per se, is what is required to activate MBK-2 and initiate protein degradation. Premature entry into meiotic M phase  in unfertilized oocytes is sufficient to relocalize MBK-2 from the cortex to the cytoplasm and trigger the degradation of oocyte proteins.  Our findings suggest that, in addition to its well-known role in regulating chromosome dynamics, the meiotic cell cycle triggers egg-wide developmental changes essential for the initiation of embryonic development. We are currently investigating the mechanisms that link the meiotic cell cycle to MBK-2 and the transition from oocyte to embryo.

FIGURE 2: PAR-2 dynamics during polarization of the zygote.

Embryonic polarity

This part of our research seeks to understand how embryos become polarized along the anterior/posterior axis and how this spatial information is used to delineate distinct somatic (anterior) and germline (posterior) domains.  A/P polarity is initiated by a microtubule-organizing center (MTOC) brought in by the sperm at fertilization. In collaboration with the Kemphues Lab (Cornell U.), we have found that the primary effect of the MTOC is to displace  polarity regulators PAR-3, PAR-6 and PKC-3 away from the sperm, allowing PAR-1 and PAR-2 to accumulate on the cortex nearest the sperm. Using live imaging of GFP-tagged proteins and biochemistry, we have begun to identify the molecular mechanisms underlying PAR dynamics. A critical step is phosphorylation by PKC-3 of PAR-1 and PAR-2 on residues essential for cortical localization. Our findings support a model where reciprocal inhibitory interactions between PAR proteins polarize the zygote by reinforcing an initial asymmetry in PKC-3.  

Soma-germline dichotomy

FIGURE 3: Somatic (3 cells on left) and germline (1 cell on right) blastomeres contain different types of RNP granules.

The zygote undergo a series of asymmetric divisions to generate somatic and germline blastomeres. Somatic blastomeres degrade maternal RNAs and activate transcription soon after their separation from the germ lineage. In contrast, germline blastomeres maintain maternal RNAs and delay the activation of mRNA transcription until after gastrulation. The difference in transcriptional activity is due to PIE-1, a global transcriptional repressor that segregates with the germline blastomeres. PIE-1 interferes with phosphorylation of the carboxy terminal domain (CTD) repeats of RNA polymerase II.  In collaboration with the Blackwell lab (Harvard U.), we have shown that a  PIE-1 inhibits transcription  in part via a motif that resembles the CTD repeats. 

The mechanisms that lead to a difference in maternal RNA stability between somatic and germline blastomeres are less understood. We have found that this difference correlates with distinct classes of cytoplasmic ribonucleoprotein particles (RNPs) found in the two cell types.  Somatic blastomeres contain granules similar to the RNA degradation centers of yeast and mammalian cells (P-bodies). Germline blastomeres contain related, but compositionally distinct, larger granules (Fig. 3). We are currently investigating the function of these germline-specific structures.  RNA binding proteins are common among regulators of germ cell development in many organisms, raising the possibility that germ cells preferentially use post-transcriptional mechanisms to regulate gene expression.  To address this question, we are developing methods to systematically characterize the expression of germline genes. 

We gratefully acknowledge support from the National Institutes of Health and the Howard Hughes Medical Institute.