Single-cell reporters for inflammatory caspase activity

Thesis: S.M., Massachusetts Institute of Technology, Computational and Systems Biology Program, 2014.Cataloged from PDF version of thesis.Includes bibliographical references (page 29).Caspases are a 12-member family of human proteases that regulate apoptosis and inflammation. They serve as key effectors downstream of diverse signaling receptors and shape cell fate. Inflammatory caspases mediate the proteolytic processing of inflammatory cytokines and are essential in maintaining immune function, but also lead to disease when deregulated. In order to examine the activity of inflammatory caspases, we generated 2 inflammatory caspase reporters: a fluorescence resonance energy transfer (FRET) inflammatory caspase activity reporter as well as a fluorescent translocation inflammatory caspase reporter. These reporters were then used to study inflammatory caspase activity in vitro using recombinant caspases and in vivo using a simplified cell culture model. The inflammatory caspase activity reporters have the potential to capture inflammatory caspase activation under a variety of stimuli. They also have several advantages compared to existing methods: they are non-destructive and can be used for live single cell measurements; they do not require the addition of exogenous chemicals or cofactors; and they do not covalently modify the inflammatory caspases. Inflammatory caspase activation is a rapid, asynchronous process, and detecting the activity of the mature inflammatory caspase molecules is made difficult due to the short half-life of the enzyme. The reporters we have developed can fill this need.by Arshed Al-Obeidi.S.M

Single-cell cloning of human T-cell lines reveals clonal variation in cell death responses to chemotherapeutics

Genetic modification of human leukemic cell lines using CRISPR-Cas9 has become a staple of gene-function studies. Single-cell cloning of modified cells is frequently used to facilitate studies of gene function. Inherent in this approach is an assumption that the genetic drift, amplified in some cell lines by mutations in DNA replication and repair machinery, as well as non-genetic factors will not introduce significant levels of experimental cellular heterogeneity in clones derived from parental populations. In this study, we characterize the variation in cell death of fifty clonal cell lines generated from human Jurkat and MOLT-4 T-cells edited by CRISPR-Cas9. We demonstrate a wide distribution of sensitivity to chemotherapeutics between non-edited clonal human leukemia T-cell lines, and also following CRISPR-Cas9 editing at the NLRP1 locus, or following transfection with non-targeting sgRNA controls. The cell death sensitivity profile of clonal cell lines was consistent across experiments and failed to revert to the non-clonal parental phenotype. Whole genome sequencing of two clonal cell lines edited by CRISPR-Cas9 revealed unique and shared genetic variants, which had minimal read support in the non-clonal parental population and were not suspected CRISPR-Cas9 off-target effects. These variants included genes related to cell death and drug metabolism. The variation in cell death phenotype of clonal populations of human T-cell lines may be a consequence of T-cell line genetic instability, and to a lesser extent clonal heterogeneity in the parental population or CRISPR-Cas9 off-target effects not predicted by current models. This work highlights the importance of genetic variation between clonal T-cell lines in the design, conduct, and analysis of experiments to investigate gene function after single-cell cloning

The pseudokinase MLKL activates PAD4-dependent NET formation in necroptotic neutrophils

Neutrophil extracellular trap (NET) formation can generate short-term, functional anucleate cytoplasts and trigger loss of cell viability. We demonstrated that the necroptotic cell death effector mixed lineage kinase domain-like (MLKL) translocated from the cytoplasm to the plasma membrane and stimulated downstream NADPH oxidase-independent ROS production, loss of cytoplasmic granules, breakdown of the nuclear membrane, chromatin decondensation, histone hypercitrullination, and extrusion of bacteriostatic NETs. This process was coordinated by receptor-interacting protein kinase-1 (RIPK1), which activated the caspase-8-dependent apoptotic or RIPK3/MLKL-dependent necroptotic death of mouse and human neutrophils. Genetic deficiency of RIPK3 and MLKL prevented NET formation but did not prevent cell death, which was because of residual caspase-8-dependent activity. Peptidylarginine deiminase 4 (PAD4) was activated downstream of RIPK1/RIPK3/MLKL and was required for maximal histone hypercitrullination and NET extrusion. This work defines a distinct signaling network that activates PAD4-dependent NET release for the control of methicillin-resistant Staphylococcus aureus (MRSA) infection