Katrina Cooper, PhD, assistant professor, Department of Molecular Biology, UMDNJ-School of Osteopathic Medicine

The long-term goal of my research is to elucidate the mechanism by which a cell decides its fate in response to environmental cues. Misinterpretation of these signals can result in the cell choosing the incorrect fate, which, in turn, can lead to neoplasia, i.e. tumor formation. To study this in the laboratory, I use two model experimental systems, Saccharomyces cerevisiae (bakers yeast) and the mouse. The combination of using yeast and mammalian model systems is very powerful. Experiments performed in yeast commonly identify the molecular pathways that are triggered in response to stress. As many biological processes are highly conserved, the information gained in yeast often translates directly to mammalian pathways. Here the consequence to the whole animal of making these cell fate decisions can be studied. Significantly, the mouse model system is excellent in determining which cell fate decision can trigger tumor formation.

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All cells—from those that make up the smallest bacteria to the trillions that compose a human—need to interpret internal and external signals for survival. External signals are extensive and can range from temperature shifts to UV exposure. The interpretation of these signals allows humans to survive, for example, brief bouts of oxygen deprivation during birth, high temperatures as experienced during a fever, oxidative damage associated with aging, or the environmental toxins that everyone faces in their daily lives. On a cellular basis, the interpretation of these signals directs a cell towards three fates; it can either stop growing and repair the damage, divide or die (called programmed cell death or apoptosis). Misinterpretation of these external signals can result in a cell choosing the wrong fate, resulting in adverse outcomes such as cancers.

Figure 1

Figure 1: Comparison of events that occur during mitotic and the 1st meiotic division in S. cerevisiae. The yellow circles represent diploid cells, the red and blue lines chromosomes.

My research is focused on understanding two aspects of cellular interpretation of external signals. The first focuses on cell division in atypical cells, the second on programmed cell death. Cell division is characterized by DNA replication where an identical copy of each chromosome is synthesized. Next, the newly synthesized chromosomes separate and are partitioned equally into the new daughter cells. To ensure that these processes occur without mistakes, there are proofreading mechanisms called checkpoints. An increasing number of cancers have been attributed to mutations in genes that encode checkpoint proteins. Thus, understanding how these checkpoint pathways operate is important in deciphering the molecular basis of many cancer types.

Cancerous cells are considered atypical. One of their hallmarks is their ability to override checkpoint mechanisms that control normal cell divisions. Accordingly, one of my research goals is to understand the molecular mechanisms that regulate checkpoint mechanisms in atypically dividing cells. To study this, we focus on the highly conserved process of meiosis. Meiosis is the process found in germ cells (sperm and eggs) that undergo a reduction of the genome from two copies of every chromosome to only one. This reduction is accomplished by executing two chromosome divisions without an intervening round of DNA replication. The first meiotic division (M1) is an excellent example of atypical cell division (Figure 1). Rather than the newly replicated chromosomes separating as they do during normal mitotic cell division, they remain paired during the first meiotic nuclear division. Therefore, checkpoint pathways that normally function to ensure that this very event does not happen must be co-opted to allow this meiosis-specific event to occur.

Exploiting the conservation in the pathways that control all cell divisions, it has been demonstrated that the transition from one stage of cell division to the next requires the destruction of key regulatory proteins. These proteins are targeted for destruction by the attachment of a small protein called ubiquitin by an enzyme termed an ubiquitin ligase. We have discovered a protein called Ama1p that directs the degradation of key regulators during meiotic progression. Ama1p is a member of a highly conserved protein family called Cdc20 that activates, and provides specificity, for the multi subunit ubiquitin ligase termed the anaphase promoting complex/cyclosome (APC/C). Significantly, Ama1p is the first member of this family that functions exclusively outside mitotic cell division. Like other Cdc20 family members, Ama1p acts as a bridging protein, bringing the APC/C in direct contact with its substrates. As a result the APC/C presents the targeted substrates to the 26S proteasome (termed the molecular garbage can) for destruction. Underscoring its importance in controlling meiotic development, Ama1p is required for the successful completion of the meiotic program. In other words, the destruction of Ama1p’s substrates is important for transitioning through the meiotic divisions. This adaptation of the APC/C by Ama1p represents an excellent example of how specialized regulators can recruit basic cell cycle machinery for new functions. Understanding how Ama1p performs these tasks as well as elucidating its role in meiotic checkpoint control is thus one of the goals of my research program.

The second focus of my laboratory is to understand the relationship between stress and programmed cell death. For a cancer cell to divide a sufficient number of times to form a tumor, it must survive adverse conditions such as a poor nutrient and oxygen supply due to the lack of established blood vessels. In addition, disseminated malignancies are commonly treated with cytotoxic agents (e. g., chemotherapy, radiation) that target the unregulated growth associated with tumors. Mounting evidence suggests that one underlying mechanism by which malignancies become established and/or become protected from cytotoxic agents is through aberrant activation of a pathway generally referred to as the “stress response.” This system, which is found in all organisms from procaryotes to man, elicits the expression of several conserved gene families (heat shock proteins e. g., Hsp70, Hsp27) that protect the cell from adverse environmental conditions. In human breast cancer, overexpression of Hsp’s has been associated with tumors that are both more invasive and/or resistant to chemotherapeutic drugs. Taken together, these studies suggest a correlation between up regulation of the stress response and tumor survival. Therefore, the ability to selectively suppress the stress response may allow more productive use of current chemotherapy regimens and/or prevent the loss of tumor suppressor function. Work originally conducted in budding yeast identified a protein (cyclin C) that negatively regulates the stress response genes. Deletion of cyclinC renders cells more resistant to stress through inactivation of the programmed cell death (PCD) pathway (Figure 2). Many pieces of data suggest that the human and yeast cyclin C are playing similar roles in the cell. For example, the yeast and human cyclin C occupy the same subcellular localization, are regulated in a similar manner, and interact with the same proteins. We are now developing a mouse model system that mimics our yeast work to determine if the role of cyclin C is conserved and whether cyclin C functions as a tumor suppressor.

Figure 2

Figure 2. Cyclin C regulates PCD in yeast. Wild type and mutant cultures were examined by TUNEL assays before and after H2O2 treatment (0.4 mM). The green signal indicates cells undergoing PCD. Mutating cyclin C (CycC) protects the cell from PCD. Mutating a factor (BCK1) that inhibits cyclin C activity makes the cells hypersensitive to stress.

Katrina Cooper is an assistant professor in the Department of Molecular Biology at UMDNJ-School of Osteopathic Medicine. She received her PhD in 1993 from Oxford University, England. Armed with a NATO fellowship, she pursued her post-doctoral training at The Fox Chase Cancer Center in Philadelphia. She joined the faculty of Drexel University Medical School in 2001, and assumed her current position in 2005. She is a recipient of grants from the American Cancer Society and the W. W. Smith Charitable Trust.