Signaling (MAPK) Pathways that Control Cell Polarity and Cell Differentiation
The CUllen Lab is interested in signal transduction pathways. All cells sense and respond to changes in the environment. Sensing and relaying changes in the extracellular milieu is mediated by signal transduction pathways. One type of evolutionary conserved signaling pathway are Mitogen-Activated Protein Kinase (MAPK) pathways. MAPK pathways are found in all eukaryotes and function in diverse ways to regulate cell differentiation, cell cycle progression, and the response to stress. An interesting feature of MAPK pathways is that they can share components with other pathways in the cell. How pathways that function in integrated networks induce the ‘right’ response is not well understood. Furthermore, inappropriate regulation of MAPK pathways can lead to improper signaling, which is an underlying cause of diseases including cancers, immune diseases, inflammation, and neurodegenerative disorders.
Our laboratory uses the model organism budding yeast to study MAPK pathways. In yeast, MAPK pathways control the response to stress and orchestrate cell differentiation to specialized cell types. Using this model, we have been able to identify new regulators of MAPK pathways and new mechanisms for their regulation. For example, we showed that mucins that regulate MAPK pathways in yeast undergo processing and release of an inhibitory extracellular glycodomain. More recently, we have identified an adaptor that regulates and might help to insulate the pathway (the pathway we study shares components with other MAPK pathways). We are currently working on how positional cues impact MAPK pathway signaling through the Rho GTPase Cdc42.
The specific MAPK that we study in our laboratory regulates a cell differentiation response called filamentous growth. Filamentous growth is a fungal-specific growth mode that occurs in yeast and many other fungal species. During filamentous growth, cells change their shape and grow as branched filaments. In some species of pathogenic microorganisms, filamentous growth is required for virulence. Among the signaling pathways that regulate filamentous growth is an ERK-type MAPK pathway called the filamentous growth (fMAPK) pathway. Studying how MAPK pathways regulate filamentous growth in yeast can shine light on MAPK-dependent differentiation responses in general as well as the molecular basis of fungal pathogenesis.
The proteins that regulate the fMAPK pathway in yeast have been identified by our laboratory and other laboratories. A signalling mucin-type glycoprotein (Msb2), together with polar landmarks, regulates the cell’s major polarity GTPase Cdc42p, which activates a canonical p21-MAPKKK-MAPKK-MAPK kinase cascade. Key questions surrounding the regulation of the fMAPK pathway remain unaddressed. One question is how does the mucin Msb2 regulate the GTPase module? A second question is how is a specific signal sent by a pathway that shares components with other MAPK pathways in the cell? Our lab is approaching these questions in different ways:
1. By the development and utilization of genomic and proteomic approaches, we have discovered and are continuing to identify proteins that regulate the filamentous growth pathway. For example, we used the fact that the extracellular domain of Msb2 is shed to develop a high throughput screening approach called secretion profiling to identify new regulators of Msb2/fMAPK.
2. By building new assays to measure and quantitate filamentous growth, we are learning more about the molecular basis of the response. We are currently utilizing genetics, microscopy, and bioinformatics analysis to define aspects of filamentous growth regulation.
3. Biochemistry, molecular biology, and in vivo protein-interaction approaches (e.g. two-hybrid analysis and FRET) are being employed, which are critical to determine how proteins that regulate MAPK pathways function to induce a response.