RUI: The role of PP2A in TCR-mediated Cell Fate Decisions of Developing T Cells

Haverford College, Haverford PA

Investigators

Abstract

Intellectual merit: Each new T lymphocyte produced in the body expresses a novel T cell receptor (TCR) that has the potential to react to self-proteins. Fortunately, organisms have devised a way to avoid autoimmunity, by 'testing' immature T cells (thymocytes) for self-reactivity as they mature in the protected environment of the thymus. If the T cell receptors expressed by an immature thymocyte engage self-antigens encountered in the thymus with high avidity, the TCRs will generate signals that kill the cell. A proportion of cells that pass this test mature into CD4+ and CD8+ single-positive T cells that populate the lymph nodes and spleen (the 'periphery'). Once in the periphery, T cells respond very differently to strong T cell receptor engagement. Instead of dying, they proliferate and differentiate into helper and killer lymphocytes. It is still a mystery how immature thymocytes and their immediate descendants, mature T cells, respond so differently to the identical TCR signal. Published findings funded by a previous NSF-RUI grant revealed a provocative distinction between the signaling cascades initiated by T cell receptors in immature and mature T cells. Whereas both cell populations respond to TCR stimulation by upregulating the expression of Nur77 - a transcription factor known to induce cell death - only mature T cells heavily phosphorylate Nur77, an event that banishes Nur77 to the cytosol of a cell. The investigators have also shown that the failure of immature T cells to phosphorylate Nur77 in response to TCR signals is in part due to their failure to activate (phosphorylate) a key upstream regulator of cell survival, Akt. The goal of the project is to move closer to the origin of the differences in TCR signaling between immature and mature T cells. The PI describes an unexpected finding, that immature T cells fail to phosphorylate Akt not because of a problem with the activity of upstream kinases, but because of an enhancement in the activity of an upstream phosphatase. The project will explore a novel hypothesis, that this phosphatase activity and holoenzyme composition differ between immature and mature T cells, and that these differences are the basis for the distinct response of immature and mature T cells to TCR stimulation. Broader impact: The work will be performed entirely by undergraduates at Haverford College, and the experiments that led to the hypothesis were motivated directly by their original pursuits. The NSF RUI (Research at Undergraduate Institutions) funding not only allows students to be exposed to the most current research performed in the immunological discipline, but also permits them to be direct contributors to it. Funding of this project directly supports the efforts of senior Biology majors, all of whom are expected to perform an original research project. It will also support the development of younger students (many of whom are in Haverford's Multicultural Scholar Program) who join the lab in the summer or as work-study students during the academic year. All students are exposed to an educational approach that asks them to raise a question of importance, test a hypothesis experimentally, and rigorously critique data. NSF funds allow the students to engage in bench research at a sophisticated level and enhances the expectations that students and faculty have of each other as they collaborate in an effort that inspires the development a serious intellectual commitment, a willingness to engage literature, critique evidence, and the courage to speculate. Funds also permit students to follow their experimental 'noses' and wander off narrow trajectories to test an original thought. Some students produce publishable data and participate in writing papers, most will present their work at national and local scientific meetings, and some will discover something novel that will inspire the next grant and the next 'generation' of students. And those who may not pursue research as a career will still have experienced what it is like to be a participant rather than an observer in the generation of knowledge - and will leave with a more developed sense of responsibility for the quality of information that they will one day shape and share.