Sprenger

We are analyzing the control of the cell cycle at genetic, cellular and biochemical levels and are interested in the integration of the cell cycle with developmental processes. We are using the whole spectrum of the model organism Drosophila and cell culture systems.

In our studies, we have uncovered important mechanisms that regulate entry into and exit from mitosis. In addition, we are interested in the mechanisms that regulate the G1-S transition and would like to understand how special cell cycles like endocycles are controlled.

Our results will help to increase our knowledge of normal cell proliferation control mechanisms and thereby will help to understand what goes wrong in uncontrolled cell proliferation diseases like cancer.

We have identified the gene rca1 as an essential inhibitor of the APC/C. In the absence of Rca1 function, cells fail to divide because cyclins, that are required for entry into mitosis are untimely degraded before mitosis by the APC/C. Thus, Rca1 is required to inhibit the APC/C before mitosis. Rca1 shares limited sequence homology with the APC/C inhibitors of the vertebrate Emi-family. For these proteins, it was proposed that they work by inhibiting the APC/C as a pseudosubstrate inhibitor. We would like to establish if Rca1 works in a similar manner and how APC/C inhibition is achieved. In addition, Rca1 itself has to be regulated during the cell cycle to relieve APC/C inhibition when the ubiquitin ligase should be active. We could observe that Rca1 is itself degraded during G1 and now would like to understand how the degradation is achieved and regulated.

Besides its role during the cell cycle, the APC/C has been reported to be active and required in cells that have exited the cell cycle. By using APC/C sensors and APC/C mutations or RNAi knockdown procedures we would like to see if other, non cell-cycle functions of the APC/C are required.

The most common cell cycle type consists of 4 Phases: Mitosis when cell division takes place is usually followed by a so called “gap”-Phase, G1 before DNA-replication is initiated during S-phase that is followed by another gap-Phase, G2.
In multicellular organisms many “special” cell cycle types can be observed as well, especially during development. In Drosophila, the first cycles consist solely of S and M-Phases and slightly later a G1-phase is still not present in the division pattern. Endoreduplication cycles (see below) and meiosis, when germ cells are generated are fascinating deviations from the normal cell cycle pattern that we would like to understand.

The precise duplication of the genome is crucial for the survival of any organism. In multicellular organisms genome instability potentially gives rise to cancer and thus compromises the life of the whole organism. To maintain the integrity of the genome, DNA replication and mitosis must be coordinated during cell division cycles so that DNA replication occurs only once per cycle and mitosis only after complete duplication of the genome. To avoid re-replication events, a network of proteins ensures that cells acquire the license for DNA replication only in a specific phase of the cell cycle

While re-replication without an intervening mitosis is strictly prevented during cell division cycles, this process is enforced during endoreplication cycles. This cell cycle variant is prominently found during plant and invertebrate development, but some mammalian cell types, like megakaryocytes or trophoblast giant cells, are also known to undergo endoreplication cycles. Endoreplicating cells undergo repeated rounds of DNA synthesis that are separated by distinct Gap phases, but never enter mitosis.

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Despite the absence of mitosis, the Anaphase-Promoting Complex is required for endoreduplication cycles. However, the degradation of mitotic substrates like cyclins is of no importance here. Instead, APC/C is required for the degradation of an inhibitor, geminin that regulates the assembly of DNA-replication origns. When the APC/C is inhibited, Geminin accumulates and prevents reassembly of DNA-replication origins.

Repeated rounds of S- and Gap phases are regulated by distinct pathways thar result in oscialltions of cell cycle regulators. At the heart of the system is Cyclin E, an unstable protein that requires transcriptional input from the transcription factor E2F. Once E2F can accumulate, it triggers DNA-replication and this in turn results in E2F degradation, hence reduced CycE. The APC/C is inhibited by CycE and only after a drop in CycE levels, APC/C is able to degrade geminin relieving the inhibition in DNA-Origins assembly

The speed and synchrony as well as the relative big size of the embryo make this an ideal model to follow cell cycle events in real time. This is possible through the use of fluorescently marked proteins (see this years Nobel Prize in Chemisty) that are attached to proteins that bind to DNA (in Cyan) or Tubulin (red). In the movie on the right, three cycles from a part of an early Drosophila embryo are shown.

The earliest cycles during embryogenesis are not easily explained by standard cell cycle models. Usually, a cell cycle is characterized by oscillations in cell cycle components or their activity. For example, Cdk1, a kinase that triggers mitosis, is only active shortly before mitosis when it promotes mitosis and needs to be inactivated when cells exit mitosis. However, in the early nuclear cycles, no fluctuations in Cdk1 activity can be observed. It remains to be investigated how these cycles are coordinated.

Prof. Dr. rer. nat. Frank Sprenger
Professur für Genetik

Geb?ude Westliche Naturwissenschaften D3_2.323
Tel. 0941 943-3167
frank.sprenger@ur.de

Curriculum vitae

Arlett Hirsch
Sekretariat Lehrstuhl Genetik

Geb?ude Westliche Naturwissenschaften D3_2.311
Tel. 0941 943-3075
arlett.hirsch@ur.de

Dr. rer. nat. Manuel Saller
Postdoc

Geb?ude Westliche Naturwissenschaften D3_2.321
Tel. 0941 943-3168, Labor 3169
Manuel.Saller@ur.de

Sebastian Sigl
Doktorand

Geb?ude Westliche Naturwissenschaften D3_2.321
Tel. 0941 943-3168, Labor 3169
Sebastian.Sigl@ur.de

Matthias Kies

Thomas R?ssler

Jan Polz