Protein Modifications Involved in Cell Signaling

National Institute Of Allergy And Infectious Diseases

Investigators

Linked publications, trials & patents

Abstract

1. Quantification of the modification dynamics in TLR signaling pathways. The TLRs are a family of pathogen recognition receptors that alert the host to the presence of pathogens by recognizing molecular signatures, termed pathogen-associated molecular patterns (PAMPs). These sensors act as the first step in the induction of protective innate and adaptive immune responses. There are 11 human TLR homologues and they are each activated by one or more PAMP ligands. TLRs are all transmembrane proteins and their signaling is mediated by association of their internal domains with intracellular components. Classically, the TLR signaling cascade involves the myeloid differentiation primary response gene 88 (MyD88), interleukin-1 receptor-activated kinase (IRAK), and tumor-necrosis factor receptor-associated factor 6 (TRAF6), leading to the activation of Nuclear Factor kappaB (NF-kB). Among the most important genes to be regulated by TLR signaling are those encoding cytokines. Given the key role of cytokines in the orchestration of the inflammatory response, mechanisms of modulating their production has garnered substantial interest, in particular in the area of the development of therapies for the treatment of chronic inflammatory diseases. A clearer understanding of the TLR pathway leading to the cytokine production is required for a successful pharmacological intervention. A) We investigated differences in the phosphoprotein signaling cascades triggered by TLR2, TLR4, and TLR7 ligands using as a responding population a well-characterized murine macrophage cell line. We performed a global, quantitative, early post-stimulation kinetic analysis of the global mouse macrophage phosphoproteome using stable isotope labeling with amino acids coupled to phosphopeptide enrichment and high-resolution mass spectrometry. These results advanced our understanding of how macrophages sense and respond to a diverse set of TLR stimuli. The data were deposited in the publicly available online database Proteome Exchange (1). We are characterizing the changes in tyrosine phosphorylation in the same conditions and obtained quantitative mass spectrometry data showing changes in tyrosine phosphorylation following LPS stimulation. We are studying the changes in phosphorylation-dependent signaling in cells where the TLR signaling pathway components (MyD88, IRAK family proteins, CD14) are knocked down or knocked out. We currently use the phosphorylation datasets as well as datasets quantifying other PTMs (ADP-ribosylation, GlcNAc, ubiquitination) as additional constraints for a computational model of the TLR signaling network (project AI001085-07: Absolute Quantification of Molecular Representation and Interaction). The candidate proteins whose phosphorylation changed significantly during the investigated time course are being further examined in follow up biological experiments. We have characterized in detail the changes in phosphorylation of specific sites of MARCKS upon LPS stimulation and we are now exploring the biological significance of these sites. We have performed site-directed mutagenesis of the individual sites and the mutant MARCKS has been expressed in the cells where we knocked out the wild-type protein using CRISPR technology, and characterized by mass spectrometry. B). We have optimized another relative quantification method based on post-processing peptide labeling - the dimethylation method, extremely useful for proteomic and phosphoproteomic studies of primary cells and tissues not amenable to the SILAC protocol. The method was tested on murine macrophage lysates and the results were comparable to the SILAC- quantified study. We have applied dimethyl labeling to studies of the PTMs in the TLR pathway in primary murine macrophages and human elutriated monocytes. C). To complete the TLR signaling network and assess the proteome dynamics, we have conducted parallel studies of the proteome and secretome changes using the same cells and ligands as for the phosphoproteome analysis, but collecting data after longer periods of time to allow for changes in protein expression and secretion (2). We have validated the data using ELISA-based assays of cytokine production. We have performed data correlation with the transcriptome (in collaboration with Iain Fraser). We identified differences in signaling between individual TLRs and revealed specifics of pathway regulation at the protein level (3). The data will provide more stringent constraints for the TLR signaling model. 2. Analysis of post-translational modifications of CENP-A (collaboration with Yamini Dalal, NCI): In eukaryotes DNA is packaged into chromatin by essential histone proteins. Specialized histone variants such as centromere-specific histone H3 (CENP-A) provide a structural and epigenetic basis for chromosome segregation by marking centromeres. To maintain centromere parity after replication, CENP-A must segregate equally to nascent daughter DNA strands. How cells prevent unequal distribution of CENP-A to daughter strands after replication fork passage is unknown. We have characterized novel modifications within the histone fold domains of CENP-A and H4 that occur at G1-S cell cycle transition, which coincide with loss of the chaperone HJURP binding, suggesting a mechanistic basis for CENP-A structural conversion. We have established that accurate post-translational modification mapping is possible from minute amounts of protein, using ultra-sensitive mass spectrometry, and bioinformatic tools combined with careful manual spectra validation. We are continuing the detailed characterization of the modification states of CENP-A dependent on the cell cycle stage. Time-resolved, stimulus-dependent quantitative changes of modification at individual sites are being obtained using sensitive nanospray-based mass spectrometry (LTQ Orbitrap Velos) combined with Single Reaction Monitoring. We have recently demonstrated that while native CENP-A K124 is acetylated at G1/S, it switches to monomethylation during early S and mid-S phases (4). We are now using TAU gels for the optimal separation of histone variants to obtain comprehensive modification map of CENP-A in the primary tumor cells. The TAU gels and mass spectrometry are a novel combination of methods for histone post-translational modification analysis (5). 3. We have performed a quantitative analysis of the proteome of the cells from the terminal ileum (chosen as a site of intense host-microbe interactions) of germ-free and normal mice (collaboration with Drs. Natalia Shulzhenko and Andrey Morgun). The data showed large changes in the immune processes-related protein expression as well as in certain metabolic pathways. The correlation of the proteome and transcriptome data revealed several differentially regulated pathways and significant transcriptomeproteome discordance in the adaptation of the host to the microbiota. This discovery leads to a definite conclusion that transcript level analysis is not sufficient to predict protein levels and their influence on the function of many specific cellular pathways, so only the combination of the quantitative data at different levels will lead to the complete understanding of the complex relationships between the host and the microbiota. References: 1. Sjoelund V, Smelkinson M, and Nita-Lazar A. (2014) J Proteome Res. 2014 Nov 7;13(11):5185-97. doi: 10.1021/pr5002466. 2. Koppenol-Raab M, Sjoelund V, Manes NP, Gottschalk RA, Dutta B, Benet ZL, Fraser ID,Nita-LazarA. (2017) Mol Cell Proteomics 16(4 suppl 1):S172-S186. 3. Koppenol-Raab M., and Nita-Lazar A. (2017) Methods Mol Biol. 1636:301-312 4. Bui M, Pitman M, Nuccio A, Roque S, Donlin-Asp PG,Nita-LazarA, Papoian GA, Dalal Y.(2017) Epigenetics Chromatin 10:17. 5. Nuccio A., Bui M., Dalal Y., and Nita-Lazar A. (2017) Methods Enzymol. 586:275-290