Activated MST2 kinase is free of kinetic regulation

Canonically, MST1/2 functions as a core kinase of the Hippo pathway and non-canonically is both activated during apoptotic signaling and acts in concert with RASSFs in T-cells. Faithful signal transduction relies on both appropriate activation and regulated substrate phosphorylation by the activated kinase. Considerable progress has been made understanding the molecular mechanisms regulating activation of MST1/2 and identifying downstream signaling events. Here we present a kinetic analysis analyzing how the ability of MST1/2 to phosphorylate substrates is regulated. Using a steady state kinetic system, we parse the contribution of different factors including the domains of MST2, phosphorylation, caspase cleavage, and complex formation to MST2 activity. In the unphosphorylated state, we find the SARAH domain stabilizes substrate binding. Phosphorylation, we also determine, drives activation of MST2 and that once activated the kinase domain is free of regulation. The binding partners SAV1, MOB1A, and RASSF5 do not alter the kinetics of phosphorylated MST2. We also show that the caspase cleaved MST2 fragment is as active as full-length suggesting that the linker region of MST2 does not inhibit the catalytic activity of the kinase domain but instead regulates MST2 activity through non-catalytic mechanisms. This kinetic analysis helps establish a framework for interpreting how signaling events, mutations, and post-translational modifications contribute to signaling of MST2 in vivo.

Disruption of NBS1/MRN Complex Formation by E4orf3 Supports NF-κB That Licenses E1B55K-Deleted Adenovirus-Infected Cells to Accumulate DNA>4n

Genome instability, a hallmark of cancer, exists as part of a cycle that leads to DNA damage and DNA > 4n that further enhances genome instability. Ad E4orf3 is a viral oncogene. Here, we describe E4orf3 mediated signaling events that support DNA > 4n in Δ E1B Ad-infected cells. These signaling events may be linked to the oncogenic potential of E4orf3 and may provide a basis for how some cells survive with DNA > 4n.

Phospho-proteomics reveals that RSK signaling is required for proliferation of natural killer cells stimulated with IL-2 or IL-15

Natural killer (NK) cells are cytotoxic lymphocytes that play a critical role in the innate immune system. Although cytokine signaling is crucial for the development, expansion, and cytotoxicity of NK cells, the signaling pathways stimulated by cytokines are not well understood. Here, we sought to compare the early signaling dynamics induced by the cytokines interleukin (IL)-2 and IL-15 using liquid chromatography-mass spectrometry (LC-MS)-based phospho-proteomics. Following stimulation of the immortalized NK cell line NK-92 with IL-2 or IL-15 for 5, 10, 15, or 30 minutes, we identified 8,692 phospho-peptides from 3,023 proteins. Comparing the kinetic profiles of 3,619 fully quantified phospho-peptides, we found that IL-2 and IL-15 induced highly similar signaling in NK-92 cells. Among the IL-2/IL-15-regulated phospho-sites were both well-known signaling events like the JAK/STAT pathway and novel signaling events with potential functional significance including LCP1 Ser5, PAK2 Ser141, and STK17B Ser12. Using bioinformatic approaches, we sought to identify kinases regulated by IL-2/IL-15 stimulation and found that the p90 ribosomal S6 kinase (p90RSK) family was activated by both cytokines. Using pharmacological inhibitors, we then discovered that RSK signaling is required for IL-2 and IL-15-induced proliferation in NK-92 cells. Taken together, our analysis represents the first phospho-proteomic characterization of cytokine signaling in NK cells and increases our understanding of how cytokine signaling regulates NK cell function.

Progressive enhancement of kinetic proofreading in T cell antigen discrimination from receptor activation to DAG generation

T cells use kinetic proofreading to discriminate antigens by converting small changes in antigen binding lifetime into large differences in cell activation, but where in the signaling cascade this computation is performed is unknown. Previously, we developed a light-gated immune receptor to probe the role of ligand kinetics in T cell antigen signaling. We found significant kinetic proofreading at the level of the signaling lipid diacylglycerol (DAG) but lacked the ability to determine where the multiple signaling steps required for kinetic discrimination originate in the upstream signaling cascade (Tischer and Weiner, 2019). Here we uncover where kinetic proofreading is executed by adapting our optogenetic system for robust activation of early signaling events. We find the strength of kinetic proofreading progressively increases from Zap70 recruitment to LAT clustering to downstream DAG generation. These data suggest a distributed kinetic proofreading mechanism, with proofreading steps both at the receptor and at downstream signaling events. Leveraging the ability of our system to rapidly disengage ligand binding, we measure slower reset rates for downstream signaling events. Our observations of distributed kinetic proofreading and slowed resetting of downstream steps suggest a basis of cooperativity between multiple active receptors with implications in tissue homeostasis, autoimmunity, and immunotherapy off-target effects.

Angiotensin II Type 1 Receptor Tachyphylaxis Is Defined by Agonist Residence Time

Several GPCRs (G-protein–coupled receptors) have been reported to exhibit tachyphylaxis, which is an acute loss of functional receptor response after repeated stimuli with an agonist. GPCRs are important clinical targets for a wide range of disorders. Therefore, elucidation of the ligand features that contribute to receptor tachyphylaxis and signaling events underlying this phenomenon is important for drug discovery and development. In this study, we examined the role of ligand-binding kinetics in the tachyphylaxis of AT 1 R (angiotensin II type 1 receptor) using bioluminescence resonance energy transfer assays to monitor signaling events under both kinetic and equilibrium conditions. We investigated AT 1 R signal transduction and translocation promoted by the endogenous tachyphylactic agonist Ang II (angiotensin II) and its analogs, described previously for inducing reduced receptor tachyphylaxis. Estimation of binding kinetic parameters of the ligands revealed that the residence time of Ang II was higher than that of the analogs, resulting in more sustained G q protein activation and recruitment of β-arrestin than that promoted by the analogs. Furthermore, we observed that Ang II led to more sustained internalization of the receptor, thereby retarding its recycling to the plasma membrane and preventing further receptor responses. These results show that the apparent lack of tachyphylaxis in the studied analogs resulted from their short residence time at the AT 1 R. In addition, our data highlight the relevance of complete characterization of novel GPCR drug candidates, taking into account their receptor binding kinetics as well.

PSVIII-25 Transcriptional reprogramming in rumen epithelium during the developmental transition of pre-ruminant to the ruminant in cattle

The rumen is a critical organ mediating nutrient uptake and use in cattle. Healthy rumen development is essential to ensure animal feed efficiency. In this work, we present an analysis of transcriptomic dynamics in rumen epithelium during the transition from pre-rumination to rumination in cattle-fed hay or concentrated diets at weaning (eighteen Holstein bull calves, 3 X 6 groups). These two distinct phases of rumen development and function in cattle are tightly regulated by a series of signaling events and clusters of effectors on key pathways. Our analysis identifies putative signaling events and effectors. Gene activity shifts indicated the transcriptomic reprogramming required to induce developmental changes in ruminal epithelium and functional transitions. A principal component analysis distinguished the temporal expression patterns that clustered separately between pre- and post-weaning groups. A GO-term enrichment analysis reflected functional (physical and metabolic) development of ruminal epithelium and revealed the greatest number of DEGs were enriched in biological processes related to energy metabolism. Canonical pathway and upstream regulator analyses revealed transcription reprogramming with clusters of critical pathways and upstream regulators controlling functional and developmental transitions with no significant differences between hay- and concentrate-fed groups at weaning. The most highly activated transcription factors expressed during the weaning transition were PPARGC1A, INSR, NFE2L2, MYC, MYCN, and PPARA. Overall, the dietary shift from liquid to solid feeds prompted transcriptional reprogramming in rumen epithelial tissue reflecting critical nutrient-gene interactions occurring during the developmental progression of ruminant digestion.

Integrated omics networks reveal the temporal signaling events of brassinosteroid response in Arabidopsis

Brassinosteroids (BRs) are plant steroid hormones that regulate cell division and stress response. Here we use a systems biology approach to integrate multi-omic datasets and unravel the molecular signaling events of BR response in Arabidopsis. We profile the levels of 26,669 transcripts, 9,533 protein groups, and 26,617 phosphorylation sites from Arabidopsis seedlings treated with brassinolide (BL) for six different lengths of time. We then construct a network inference pipeline called Spatiotemporal Clustering and Inference of Omics Networks (SC-ION) to integrate these data. We use our network predictions to identify putative phosphorylation sites on BES1 and experimentally validate their importance. Additionally, we identify BRONTOSAURUS (BRON) as a transcription factor that regulates cell division, and we show that BRON expression is modulated by BR-responsive kinases and transcription factors. This work demonstrates the power of integrative network analysis applied to multi-omic data and provides fundamental insights into the molecular signaling events occurring during BR response.

Intracellular Delivery of Antibodies for Selective Cell Signaling Interference

Many intracellular signaling events remain poorly characterized due to a general lack of tools to interfere with “undruggable” targets. Antibodies have the potential to elucidate intracellular mechanisms via targeted disruption of cell signaling cascades because of their ability to bind to a target with high specificity and affinity. However, due to their size and chemical composition, antibodies cannot innately cross the cell membrane, and thus access to the cytosol with these macromolecules has been limited. Here, we describe strategies for accessing the intracellular space with recombinant antibodies mediated by cationic lipid nanoparticles to selectively disrupt intracellular signaling events. To enable such investigations, we first produced a series of antibody constructs, known as scFv-Fcs, containing additional, genetically encoded negative charges located at the C-termini of the constructs. Preparing proteins with negatively charged motifs has previously been shown to enhance intracellular protein delivery with cationic lipids, but usually for the purpose of genome editing or targeted cell death. We started by generating derivatives of scFv-Fc17, an antibody construct previously reported to bind specifically to signal transducer and activator of transcription 3 (STAT3) phosphorylated at Tyr705 (pYSTAT3). We screened a small number of lipids from our combinatorial lipid library with flow cytometry and found that PBA-Q76-O16B facilitated the most efficient delivery of scFv-Fcs under the conditions tested. In HepG2 cells, we observed up to 60.5% delivery efficacy, while in a STAT3-luciferase reporter cell line up to 71.5% delivery efficacy was observed. These results demonstrated the feasibility of accessing the intracellular space with scFv-Fcs. However, we also note that no more than modest changes were observed upon changing the numbers of negative charges in these constructs during delivery. Characterization of the cytotoxicity, size, and encapsulation efficiency of scFv-Fcs with PBA-Q76-O16B revealed that the constructs were generally well-behaved, with addition of differing quantities of negative charge resulting in at most modest effects. Importantly, functional assays monitoring transcriptional activity in luciferase reporter cell lines and HepG2 cells demonstrated significant reduction of gene expression downstream of pYSTAT3 following delivery of scFv-Fc17 constructs. Together, our results establish the use of recombinantly produced antibodies to selectively interfere with cell signaling events driven by a single posttranslational modification. Efficient intracellular delivery of engineered antibodies opens up possibilities for modulation of previously “undruggable” targets, including for potential therapeutic applications.

ROS Signaling-Mediated Novel Biological Targets: Brf1 and RNA Pol III Genes

Biomolecule metabolism produces ROS (reactive oxygen species) under physiological and pathophysiological conditions. Dietary factors (alcohol) and carcinogens (EGF, DEN, and MNNG) also induce the release of ROS. ROS often causes cell stress and tissue injury, eventually resulting in disorders or diseases of the body through different signaling pathways. Normal metabolism of protein is critically important to maintain cellular function and body health. Brf1 (transcript factor II B-related factor 1) and its target genes, RNA Pol III genes (RNA polymerase III-dependent genes), control the process of protein synthesis. Studies have demonstrated that the deregulation of Brf1 and its target genes is tightly linked to cell proliferation, cell transformation, tumor development, and human cancers, while alcohol, EGF, DEN, and MNNG are able to induce the deregulation of these genes through different signaling pathways. Therefore, it is very important to emphasize the roles of these signaling events mediating the processes of Brf1 and RNA Pol III gene transcription. In the present paper, we mainly summarize our studies on signaling events which mediate the deregulation of these genes in the past dozen years. These studies indicate that Brf1 and RNA Pol III genes are novel biological targets of ROS.