Yeast provides insight into human development
In this week's issue of the journal Cell, scientists from the IMP present the surprising results of their investigations in yeast. They uncovered a transport mechanism with which the information content of a cell can be unequally distributed among its offspring. Such processes also play an important role in human embryonic development.
The body of an adult human being consists of about 10 trillion (1013) cells of more than 200 types. They all originate from a single cell, the fertilised egg, and almost every cell is equipped with a complete set of genetic information. However, depending on the tissue type, only certain genes are expressed in a mature cell. Already at an early stage in embryonic development (from the third or fourth cell division in humans), the cells specialise and lose the ability to produce a complete organism. This process is called differentiation. The mechanisms that control differentiation pathways are the subject of research in developmental biology.
Scientists working in this field have long been concerned with one question: How does a cell manage to divide into two daughter cells whose properties differ from one another? One possibility is that each of the two offspring is exposed to a slightly different environment. In this case, the cocktail of growth factors, hormones and other signalling substances in the surrounding fluid determines the further fate of the cell. Another possibility is that certain substances within the mother cell are not evenly distributed to the two offspring. The daughter cells then contain different amounts of these differentiation factors which determines their respective development path. In fact, both mechanisms, the influence of the "environment" and asymmetric cell division, play a role in the embryonic development of living organisms. This has been proven by studies on fruit flies that were awarded the Nobel Prize for Medicine last year. However, this did not solve the great mystery that scientists had been trying to solve since the turn of the century. How a cell manages to specifically concentrate certain substances at certain points of the plasma and thus pass them on unequally to its daughter remains a mystery.
A group of yeast geneticists led by Kim Nasmyth at the Research Institute for Molecular Pathology (IMP) in Vienna now provides the first indication of the long sought transport mechanism. In a labor-intensive and time-consuming research approach - the scientists call the method "saturating mutagenesis"- they uncovered an amazing process. Before a yeast cell divides, components of the cell's own supporting skeleton ensure that differentiation factors are accumulated at one end of the cell. The protein myosin takes over the transport and glides along the cytoskeleton as if on rails. A very similar process is already known from the contractile cells in the muscle. What is new is the realization that the cytoskeleton not only fulfills support and movement functions, but is also involved in the mediation of information to daughter cells. It is highly probable that similar mechanisms are also effective in humans. Such elementary processes have changed surprisingly little in the course of evolution, they are strongly "conserved". For this reason, simple organisms such as baker's yeast can be used as model systems. For Kim Nasmyth, the results presented in the latest issue of the journal Cell (Vol. 84, 8.3.1996) are a major breakthrough. "We have gained some more insight into how multicellular organisms develop from a single cell," he says. "We finally understand what distinguishes us humans from bacteria".
Bobola et al.: Asymmetric accumulation of Ash1p in postanaphase nuclei depends on a myosin and restricts yeast mating-type switching to mother cells. Cell, 8 March 1996. DOI: 10.1016/s0092-8674(00)81048-x