Migration of germ cells
How a single cell gives rise to a complex multicellular embryo is a fascinating question in biology. This process involves co-ordination of cell division, differentiation and migration. We use the embryonic development of the fruit fly, Drosophila melanogaster, to study cell migration and in particular that of the germ cells, the cells that will later give rise to sperm and eggs. Unlike many other cell types whose movement is regulated by protein mediators, the survival and migration of Drosophila germ cells is dependent on lipids.
Germ cells are those cells that will give rise in the adult to eggs and sperm. So in a sense germ cells are an immortal cell line that link one generation to the next. In many organisms the germ cells are formed early in embryogenesis whereas the cells that will give rise to the remaining cells of the gonad are formed later and in a different region of the embryo. Therefore the germ cells must migrate within the embryo to find and associate with the cells of the gonad.
In the fruitfly, the embryo begins as a single cell and upon fertilization rapid nuclear divisions followed by cellularization generates an embryo of approximately 6000 somatic cells and 30 germ cells. The somatic cells undergo a series of complex movements called gastrulation, generating a more complex embryo with multiple cell layers and the cells in different layers destined to specific fates.
Following gastrulation the germ cells start their migratory journey. They begin as a tightly packed cluster of cells inside the pocket of the midgut (stage 9). They rapidly individualize, dispersing in all directions, moving as single cells between the midgut cells and into the overlying mesoderm (stage 10). During this process the cluster splits into two groups as there are two embryonic gonads each generating one of the two ovaries or testes of adult females and males respectively. It is in the mesoderm that the somatic cells of the gonad are formed. Once the germ cells meet their somatic counterparts their migration ends and they remain associated with these cells for the rest of embryogenesis. In the adult the germ cells form a stem cell pool and divide to both replenish the pool but also generate a virtually unlimited supply of sperm or eggs.
Genes regulating germ cell behaviour
Genetic screens have been used to identify genes that regulate the behaviour of the germ cells during their migration. Two critical genes that were uncovered are called wunen and its neighboring and related gene wunen2. Named after a wayward character in the Chinese epic novel Journey to the West, these genes are responsible for keeping the germ cells on course. They are expressed in a number of somatic tissues and prevent the germ cells from migrating into these tissues. They not only affect where the germ cells can move but they also control their survival. Over-expressing either of these genes in the embryo causes the germ cells to die during their migration. So germ cells are kept on track by a mixture of guidance through repulsion away from peripheral tissues and elimination.
In addition to their expression in certain somatic tissues these genes are also expressed in the germ cells themselves. In a rather surprising finding, the germ cell expression of wunen and wunen2 is required to keep the germ cells alive during their migration. So the same genes that are acting to promote germ cell death when expressed in somatic tissues, actually promote germ cell survival when expressed in another.
So what happens if you remove wunen and wunen2 from both the germ cells and somatic cells at the same time? In this case the germ cells don’t migrate at all, instead deciding to remain tightly associated with each other inside the midgut pocket. Thus germ cell dispersal that occurs at the onset of migration fails to occur. Given that somatic wunen expressing cells repel germ cells, could it be that germ cell wunens are responsible for repelling germ cells away from each other and causing the initial dispersal? Though finding direct evidence for this is tricky, we used the distribution of germ cells in the two gonads as a read out of potential germ cell-germ cell repulsion. In wild type embryos the germ cells are always evenly shared between the two gonads. However when we look in embryos in which the germ cells lack wunen and wunen2, the few germ cells that do survive and make it successfully to the gonads tend to be in the same gonad and are not shared evenly. Thus wunen mediated germ cell-germ cell repulsion is necessary for germ cell dispersal at the onset of migration and for the equal sorting of the germ cells between the two embryonic gonads. This dispersal function may serve to optimize reproductive potential by assuring maximal germ cell occupancy of both gonads.
Lipids and germ cells
So what is happening at the molecular level? wunen and wunen2 both encode lipid phosphatase phosphatases, enzymes which remove a phosphate group from their substrate, in this case a lipid. Dephosphorylation has the potential to abrogate the signaling capability of the lipid phosphate and to initiate its breakdown. Interestingly lipid phosphatase phosphatases contain several membrane spanning domains meaning they can sit in the membranes of intracellular organelles or on the outermost membrane of the cell, the plasma membrane. In the later case, the phosphatase domain would face towards the outside of the cell, meaning these enzymes are capable of dephosphorylating extracellular substrates.
One model to explain the effects of Wunens on germ cells proposes that Wunens on the surface of somatic cells depletes an extracellular lipid phosphate attractant, leading to repulsion of germ cells from wunen-expressing somatic cells. Germ cells also deplete the lipid phosphate attractant leading to germ cell-germ cell repulsion. This leads germ cells to cross the midgut in all directions, ensuring that the germ cells become partitioned equally to the embryonic gonads. Germ cells die when they are unable to dephosphorylate the lipid phosphate attractant, either because they lack wunens or because wunen expressing somatic cells have locally depleted the lipid phosphate.
The genetic control of Drosophila germ cell migration illustrates several interesting points. Firstly, how destruction of signals can play important roles in directing cells. Secondly that cell-to-cell repulsion can be used as a mechanism to disperse cells, and finally that signals that couple both survival and directionality can be particularly effective in producing faithful migration. A key goal for current research is the identification of the proposed lipid phosphate attractant. Information gained by studying lipids in an organism such as Drosophila will not only increase our understanding of the different types of signaling molecules that nature uses to control cells but will hopefully lead to the identification of disease states in which mis-regulation of lipid signals is an underlying cause.