Origins of fission yeast and selection as a model organism
The fission yeast Schizosaccharomyces pombe, was first isolated from East African beer by the German biologist P. Linder. But the use of fission yeast as an experimental organism was initiated later by Urs Leupold in the 1940s. He developed the initial genetic studies and selected wt homotallic (h90) and heterotallic (h+ and h-) strains used in all the fission yeast labs around the world. From these strains, the rest of worldwide lab collections have been developed. In the 1950s Murdoch Mitchison realized the potential of this organism for studies of cell physiology and began to analyze the cell growth during the cell cycle.
Ever since fission yeast has attracted geneticists and biologists that were seduced by the easy genetic manipulations that could be done with this yeast. Most importantly the field of cell cycle research took off in the early 1970s, when Paul Nurse, after spending several months in the lab of Urs Leopold, went to Mitchison’s lab in Edinburgh and started the cdc screens (cdc for cell division cycle).
This work allowed Paul Nurse and his collaborators (Nurse, Thuriaux et al. 1976; Nurse and Thuriaux 1980) to identify the main components that control the biochemical clock of the cell cycle like the cyclin dependent kinase Cdc2 (Cdk1), the cyclin Cdc13 (Cycline B), or Cdc25 phosphatase, and develop the initial model of cell cycle control in fission yeast, which will be detailed later on.
Fission yeast growth pattern and division cycle
Fission yeast has become over the years a very successful model in cell biology for several reasons. Fission yeast is an easy to grow organism. It is a haploid single cell model which, in combination with a high rate of homologous recombination, makes its genome manipulations easy. In particular, with the availability of fluorescent proteins in the 90s, has allowed the fast development of powerful live cell imaging approaches.
Fission yeast cells also have a very simple morphology and a reproducible growth pattern and coordinate cell growth and cell cycle in a very robust manner (see next paragraph). This property has allowed the identification in genetic screens such as the cdc screen mentioned earlier, lots of mutants affecting cell morphogenesis and cell division, whose characterization has initiated the molecular understanding of these key cellular events.
An exponentially growing newborn fission yeast cell is a small rod that measures 4μm wide and around 7-8μm in length. Like bacteria and plants, fission yeast cells have a rigid cell wall. At the beginning of the cell cycle cell growth is restricted to one end, the old end preexisting in the mother cell. As the cell cycle progresses in early G2 and the cell reaches around 9.5μm, at 0.34 of the cell cycle, the activation of growth in the new end occurs in a process called NETO (for new end take-off). Wild type cells show two linear segments of cell growth during the first 75% of the cycle stopping growth during mitosis, which occupies about 25% of the cell cycle. There is a rate-change point (RCP) that coincides with NETO and that causes a 35% increase in cell rate growth. This increase is not only due to the start of cell growth, since the old end slows down its growth rate after NETO (Mitchison and Nurse 1985). A recent report challenges this view and claims that fission yeast growth is exponential and with no RCPs (Cooper 2013). More work to clarify the situation is required, but so far the linear model is the only one used in all cell cycle studies in S. pombe.
Polarity and morphogenesis in fission yeast
The rod shaped cell of fission yeast is encased in a cell wall. Its growth is restricted to the cell tips and requires cell wall remodeling helped by turgor pressure. As explained earlier in this manuscript, the growth zones are cell cycle regulated with newborn cells growing only from the old end until the new end is activated in a process called NETO in early G2. Later, when cells enter into mitosis, cell growth stops at the tips and the growth machinery relocalizes at the division site in order to build a septum. These cell cycle-dependent modifications of the growth pattern are linked to changes in cell polarity, under the control of the Rho GTPase Cdc42 which localization and activation dictates cell polarity by influencing the actin cytoskeleton to control cell morphogenesis (Hachet, Bendezu et al. 2012).
Cell cycle progression and control of mitotic entry.
Fission yeast is a well-recognized model for cell cycle studies were lots of the most important elements of the eukaryotic biochemical cell cycle clock have been discovered. In fission yeast the major player in controlling both the G1S and the G2 M transitions is the cyclin-dependent kinase Cdc2 kinase, known in all organisms as Cdk1. Different levels of activity of Cdc2 operate in different phases of the cell cycle. Cdk1 activity is low in G1, moderate during S-phase and G2 and high at the end of G2 and during most of M-phase.
Cdc2 associates with different cyclins during cell cycle progression. During G1, the G1-cyclin-Cdc2 takes the decision to commit into a new cell cycle. Later on, a low activity of Cyclin-B-Cdc2 triggers S-phase while a high activity is required for the G2/M transition. The Cyclin B Cdc2-complex is also known as MPF (mitosis promoting factor) as it was first discovered for its role as main mitotic inducer.
Table of contents :
A. The cell cycle
1. Interphase: getting ready to divide
2. Mitosis: segregating cellular components in two equal sets
3. Cytokinesis: making two independent cellular entities
4. Coordinating growth with division
B. Principles of cytoskeleton organization
1. F-actin networks
2. Microtubule networks
A. Schizosaccharomyces pombe as a model in cellular biology
1. Model presentation
2. Polarity and morphogenesis in S. pombe
B. Cell division in S. pombe
1. Cell cycle progression and control of mitotic entry.
2. Cytokinesis in S. pombe.
C. The AMPK family of kinases
1. Structural features of the AMPKs
A. Role of Blt1 in stabilizing the ring precursor nodes during their maturation and compaction
Article 1: Blt1 and Mid1 Provide Overlapping Membrane Anchors To Position the Division Plane in Fission Yeast
B. Cdr2 node organization and architecture.
Article 2: Molecular control of the Wee1 regulatory pathway by the SAD kinase Cdr2
IV. Discussion and perspectives
V. Synthèse en Français
B. RESULTATS ET CONCLUSIONS