Germ cells are remarkable cells. In our body, only germ cells undergo a complex division series to reduce their chromosomal copy number (meiosis). At the same time, germ cells differentiate into highly specialized cell types (sperm & oocyte) that can fuse with one another to produce a zygote. Last but not least, they are the sole cellular contributors to the developing embryo proper, providing an ‘immortal’ link between successive generations; termed the germ line.

The lineage of germ cells begins when primordial germ cells are set aside early in embryonic development. In juvenile life stages, germ cells undergo many cell fate decisions to populate the gonad, a somatic tissue specialized in housing and supporting the germ line. Germline development is tightly correlated with the development of the entire organism and unlike other tissues, the germ line reaches full functional capacity only in the adult stage.

We use the development of the C. elegans germ line as an in vivo model to ask three main questions:

1. What are the genetic and molecular principles of gene expression programs in germ cells?

2. How do the emerging regulatory RNA networks control cytoplasmic gene expression?

3. How are these networks collectively wired and utilized in a developing tissue?

A widely conserved feature of germ cell gene expression programs is their strong dependency on cytoplasmic mRNA regulation, in particular, translational control. Despite its unique regulatory benefits and broad usage in other cell types, the developmental mechanisms of translational control are still at large. The principal reasons for this gap in our understanding is our limited knowledge of the identity of the mRNA regulators, their biochemical functions, and their target mRNA molecules.

Our genetic work identified multiple pathways that can act either antagonistically or synergistically to promote a specific germ cell fate decision. These pathways are composed of many RNA-regulatory proteins that belong to conserved protein families; e.g. the multi KH-domain protein GLD-3/Bicaudal C, the PUF-domain proteins FBF, the RRM & Zn-finger proteins CPBs/CPEB and the non-canonical polymerases GLD-2 and GLD-4. In molecular terms, these RNA-regulators are classified as translational activators and translational repressors, directly regulating the synthesis of key cell fate determinants via poly(A) tail length control.

At the systems level, components of the individual genetic pathways form unique RNA-regulatory networks with distinct biochemical properties, i.e. poly(A) polymerases elongate the RNA's poly(A)tail while deadenylases shorten it. To direct different germ cell fate decisions these properties are fine-tuned and stabilized by positive/negative feedback loops, and converge on mRNA targets. We are currently modeling these networks and testing their predictions at the genetic, molecular and structural level. Furthermore, we are performing biochemical experiments to define the mRNA targets of the network at the systems level.

mRNAs are not naked molecules in the cell. At any given time, mRNAs are decorated with RNA-binding proteins, forming larger mRNA/protein (mRNP) complexes. These mRNPs decide over the fate of the mRNA (degraded, stored, translationally active) by recruiting mRNA-modifying enzymes. The formation and structure of mRNPs are key questions in the field of RNA biogenesis. Our molecular inroads into these problems was our discovery of GLD-3 and GLS-1, proteins that can bridge RNA-binding proteins to cytoplasmic poly(A) polymerases (cytoPAPs) and stimulate their activities. In collaboration with the Conti group (MPI-Biochemistry, Martinsried), we have recently solved the crystal structure of GLD-3 KH-domains. In the future we will continue to define the structural and biochemical properties of cytoplasmic poly(A) polymerase complexes.

mRNPs can assemble in larger structures that are visible by fluorescence light microscopy. The cytoplasm of germ cells contains a unique class of granules, which are not present in somatic cells (i.e. P granules in C. elegans, or polar granules in Drosophila). Such germplasm granules are associated with germ cell identity and segregate with the developing germline precursor cells during early embryogenesis. Although P granules are present throughout the lifetime of a germ cell, they have distinct behaviors in different germ cell stages. Comparing the cell biology of distinct mRNP granules and defining their properties is a future goal of ours.

By combining life cell imaging with theory in collaboration with the Hyman (MPI-CBG) and Juelicher Groups (MPI-Physics of Complex Systems, Dresden), we found P granules to possess a liquid droplet-like property, allowing them to fuse or shrink. This behavior is of fundamental importance for their asymmetric distribution in the 1-cell embryo and led us to propose a physiochemical model for P granule segregation. Currently, we are studying novel germplasm components to address their connection to translational control of mRNAs and germplasm granule behavior.

Germ cells dedicate a large portion of their lifetime to the process of meiosis. In contrast to the mitotic cell cycle, meiosis is a genome haploidization program that is initiated by one round of DNA replication followed by two consecutive rounds of chromosome segregation (only one in mitosis). Although mitosis can lead to different cell identities (asymmetric cell division), the two products are genetically identical and not yet differentiated. The four products of meiosis are genetically distinct (due to homologous recombination) and highly differentiated (sperm/oocytes).

Many steps throughout the meiotic program require the synthesis of new proteins from stored mRNAs. Previous work in Drosophila and Xenopus oocytes revealed that cytoplasmic poly(A) tail length control is important for late oogenesis and oocyte maturation. Our work on GLD-2 and GLD-4 cytoPAPs strongly expanded the general knowledge of cytoPAPs during female meiotic progression. Aslo, our recent findings highlight that both cytoPAP activities are essential for male gametogenesis and that the RNA-regulatory network is under sex specific controls. Hence, the remaining frontier that we would like to tackle is how RNA-regulatory complexes and networks are themselves controlled by sexual background and integrated into cell cycle and the cell differentiation program.

Lastly, we uncovered a surprising molecular link to Ste20-like kinases in meiotic progression. In particular, our work identified a highly conserved germinal center kinase, GCK-3, as a novel and essential regulator of male meiosis and proper chromosome segregation. GCK-3 is part of a new signal pathway that appears conserved throughout evolution. Further work is directed to dissect this pathway and how it connects to proper chromosome segregation.

We are grateful to the following institutions for generously funding our work:
Max Planck Society (MPG), German Research Foundation (DFG), and German Academic Exchange Program (DAAD).
We also like to acknowledge the DFG Forschergruppe 855 for sharing their passion for RNA-biology.