determine cell fate?
tem cells are critically important for human development. These cells are unique in that they can divide to yield functionally distinct daughter cells. This process, known as differentiation, is guided by the environmental context that is presented to the stem cell. When stem cells differentiate they must express a different subset of genes. A molecular signature that is present in all cells, known as epigenetics, guides the expression of these gene subsets. If stem cells respond improperly to their environment and do not undergo the appropriate epigenetic switch, they may grow uncontrollably resulting in cancer. Due to its implications for cancer treatments, the epigenetic switch that regulates the decision to grow or differentiate is an area of intense research. While this switch is difficult to monitor in stem cells, yeast provide an ideal model system to study differentiation. Yeast are simple organisms whose molecular functions are highly conserved with human cells. Despite their simplicity, they are capable of undergoing differentiation in a process known as meiosis. Using meiosis as a model for differentiation, my focus is to understand the epigenetic switch that guides a yeast cell to stop proliferating and differentiate.
Stem cells are unique in that when they divide, one daughter remains stem cell like while the other daughter adopts a new cell fate by initiating a differentiation program. The switch between proliferation and differentiation is a critical decision a cell must make several times during development. This switch requires the cell to exit the cell cycle and initiate an intricate gene expression program that produces the correct protein at the correct time. Errors in this process can lead to embryonic lethality or several disease states including cancers. A current model for cancer development is based on the premise that stem cells ignore cues to differentiate and begin to divide uncontrollably. Understanding the exogenous and intrinsic cues that control this cell fate decision may provide new targets for rational anti-cancer drug design. Along these lines, one promising avenue is the epigenetic signature that is present in each type of cell.
Epigenetics refers to changes in gene expression that do not alter the primary DNA sequence. These changes in gene expression are influenced by environmental cues and are driven by modifications to proteins responsible for packaging DNA, the histones, which are required for organizing DNA into a higher order structure known as chromatin. Histones are capable of undergoing a cornucopia of covalent modifications, collectively known as the histone code, which can act in isolation or in combination with one another and have a direct effect on gene transcription. While there are an exponential number of combinations that these modifications can result in, some generalizations that correlate modifications to function can be made. For example, histone acetylation and deacetylation are associated with transcriptional activation and repression, respectively. Since precursor stem cells have a different gene expression pattern than their terminally differentiated cells, they also have a different histone acetylation pattern. Frequently, cancer cells display aberrant patterns of acetylation, which is a signature of their inappropriate gene expression patterns. Due to the reversible nature of the histone code, these epigenetic marks have become new exciting targets for cancer therapies. For example, HDAC inhibitors have been used with success in treating a variety of cancer subtypes.
My work is centered on understanding the epigenetic mechanisms controlling meiotic differentiation in the budding yeast, Saccharomyces cerevisiae. We use yeast as a model system because it is highly conserved with other eukaryotic organisms, but does not require complex manipulations to identify gene functions. Similar to terminal differentiation by stem cells, meiotic differentiation involves a parental cell dividing to yield distinct daughter cells. Also similar to stem cell differentiation, meiotic differentiation can only occur if yeast cells are a specific type (diploid) and are grown in specific environmental conditions. While differences in yeast meiosis and stem cell differentiation exist, similar epigenetic mechanisms are maintained that govern the differentiation process.
Meiotic differentiation requires three temporal waves of transcription, generally referred to as early, middle, and late. My work centers on a negative regulator of the early meiotic genes (EMGs), a protein known as Ume6p, which interacts with DNA in a sequence specific manner at the promoter of EMGs. Here it recruits a histone deacetylase (HDAC) known as Rpd3p that maintains chromatin in a deacetylated state, allowing gene repression. We have recently found that as cells enter meiosis, EMG induction requires Ume6p degradation. This finding reversed the notion that meiotic EMG induction required the transition of Ume6p from a gene repressor to an activator. As Ume6p is destroyed, Rpd3p dissociates from EMG promoters, which facilitates histone acetylation and gene induction. This allows cells to progress through the meiotic differentiation program. The importance of Ume6p destruction and histone acetylation to meiotic differentiation is perhaps best highlighted in mutant yeast strains. In yeast that contain a stabilized form of Ume6p, meiosis is severely delayed. Similarly, mutants lacking enzymes responsible for maintaining proper acetylation or methylation status of chromatin fail to properly execute meiotic development. In addition, Ume6p destruction is controlled by a system that is able to read the meiosis-specific signal that tells the cell to stop proliferating and differentiate. Using Ume6p as an entry point, my current work focuses on elaborating the signaling pathway that controls the switch between proliferation and differentiation in yeast.
This research has critical implications for current cancer treatments. Recently, HDAC inhibitors have reached clinical trials in a number of different tumor subtypes. While these drugs have seen success, very little is known about the targets of these inhibitors and about the mechanism of these drugs’ actions. How do these treatments work in cancer cells? How do they tip the balance of the cell from deacetylation to acetylation? How does this transition allow cancer cell death? In the stem cell model for cancer development, cancer cells remain in their progenitor cell stage, failing to differentiate into their terminal cells. These progenitor cells become immortalized, and can develop into solid tumors. My work suggests that HDAC inhibitors may allow differentiation by targeting histones and inducing genes required for the differentiation program. However, they are not targeted to specific genomic regions and influence all histones non-discriminatorily. Using my work, we hope to identify new drug targets that act on specific genes controlling the critical decision between proliferation and differentiation.
Michael Law earned a BS in biology from the Richard Stockton College of NJ in 2000. He earned his PhD from the University of Southern California where he was awarded an NIH fellowship while working with Dr. Ite Laird-Offringa. He is currently a postdoctoral fellow at UMDNJ-School of Osteopathic Medicine. His work is funded by an NIH National Research Service Award.