Robin E. C. Lee, University of Pittsburgh, Computational and Systems Biology

Title:
“Leveraging cell-to-cell variability to understand TNF-induced signal transduction circuits”

Abstract:
In the human body, many protein and small molecule ligands choreograph cellular behavior by regulating proliferation, migration, differentiation and cell death. One such ligand, tumor necrosis factor (TNF), is particularly interesting because it can activate NF-κB survival and caspase-dependent death pathways in the same cell. Inappropriate cellular responses to TNF can be carcinogenic or lead to chronic inflammatory disease; therefore, in response to TNF, each cell must carefully decide whether to live or die by weighing these pro-survival and pro-death signals. My group’s research combines molecular biology, live-cell imaging and computational models to determine how single cells interpret TNF-induced signals, and commit to cell fate decisions. In one project, we developed a novel live-cell to fixed-cell workflow to associate NF-κB translocation and transcription of a target gene in the same cell. This technique revealed the unexpected ability of cells to discern, and decode, TNF-induced changes in protein localization through use of ‘memory’. Using a deterministic model, we have explored motifs within the network architecture that allow each cell to retain ‘memory’ of its recent states, and use this as a baseline from which to measure change of nuclear NF-κB abundance. The model highlights the importance of an incoherent feed-forward loop involving negative regulators of the NF-κB transcriptional system. NF-κB-driven expression of these negative regulators provides ‘memory’, and effectively shapes the transcriptional response of other TNF-induced genes. In another collaborative project, we developed a simple microfluidic device to deliver TNF to human cancer cells with extremely high spatial and temporal precision. This work revealed that extremely short pulses of TNF are capable of eliciting full activation of NF-κB in some cells but not others. Through this work we determined the threshold of NF-κB translocation required for a transcriptional response in single cells and made an intriguing observation that early cell death is enhanced in cells exposed to shorter pulses of TNF.