Revealing nascent proteomics in signaling pathways and cell differentiation.

Regulation of gene expression at the level of protein synthesis is a crucial element in driving how the genetic landscape is expressed. However, we are still limited in technologies that can quantitatively capture the immediate proteomic changes that allow cells to respond to specific stimuli. Here, we present a method to capture and identify nascent proteomes in situ across different cell types without disturbing normal growth conditions, using O-propargyl-puromycin (OPP). Cell-permeable OPP rapidly labels nascent elongating polypeptides, which are subsequently conjugated to biotin-azide, using click chemistry, and captured with streptavidin beads, followed by digestion and analysis, using liquid chromatography-tandem mass spectrometry. Our technique of OPP-mediated identification (OPP-ID) allows detection of widespread proteomic changes within a short 2-hour pulse of OPP. We illustrate our technique by recapitulating alterations of proteomic networks induced by a potent mammalian target of rapamycin inhibitor, MLN128. In addition, by employing OPP-ID, we identify more than 2,100 proteins and uncover distinct protein networks underlying early erythroid progenitor and differentiation states not amenable to alternative approaches such as amino acid analog labeling. We present OPP-ID as a method to quantitatively identify nascent proteomes across an array of biological contexts while preserving the subtleties directing signaling in the native cellular environment

Liquid droplet formation by HP1α suggests a role for phase separation in heterochromatin.

Gene silencing by heterochromatin is proposed to occur in part as a result of the ability of heterochromatin protein 1 (HP1) proteins to spread across large regions of the genome, compact the underlying chromatin and recruit diverse ligands. Here we identify a new property of the human HP1α protein: the ability to form phase-separated droplets. While unmodified HP1α is soluble, either phosphorylation of its N-terminal extension or DNA binding promotes the formation of phase-separated droplets. Phosphorylation-driven phase separation can be promoted or reversed by specific HP1α ligands. Known components of heterochromatin such as nucleosomes and DNA preferentially partition into the HP1α droplets, but molecules such as the transcription factor TFIIB show no preference. Using a single-molecule DNA curtain assay, we find that both unmodified and phosphorylated HP1α induce rapid compaction of DNA strands into puncta, although with different characteristics. We show by direct protein delivery into mammalian cells that an HP1α mutant incapable of phase separation in vitro forms smaller and fewer nuclear puncta than phosphorylated HP1α. These findings suggest that heterochromatin-mediated gene silencing may occur in part through sequestration of compacted chromatin in phase-separated HP1 droplets, which are dissolved or formed by specific ligands on the basis of nuclear context

Widespread Protein Aggregation as an Inherent Part of Aging in C. elegans

Several hundred proteins become insoluble and aggregation-prone as a consequence of aging in Caenorhabditis elegans. The data indicate that these proteins influence disease-related protein aggregation and toxicity

Pkh-kinases control eisosome assembly and organization

Eisosomes help sequester a subgroup of plasma membrane proteins into discrete membrane domains that colocalize with sites of endocytosis. Here we show that the major eisosome component Pil1 in vivo is a target of the long-chain base (LCB, the biosynthetic precursors to sphingolipids)-signaling pathway mediated by the Pkh-kinases. Eisosomes disassemble if Pil1 is hyperphosphorylated (i) upon overexpression of Pkh-kinases, (ii) upon reducing LCB concentrations by inhibiting serine-palmitoyl transferase in lcb1-mutant cells or by poisoning the enzyme with myriocin, and (iii) upon mimicking hyperphosphorylation in pil1-mutant cells. Conversely, more Pil1 assembles into eisosomes if Pil1 is hypophosphorylated (i) upon reducing Pkh-kinase activity in pkh1 pkh2-mutant cells, (ii) upon activating Pkh-kinases by addition of LCBs, and (iii) upon mimicking hypophosphorylation in pil1-mutant cells. The resulting enlarged eisosomes show altered organization. Other data suggest that Pkh signaling and sphingolipids are important for endocytosis. Taken together with our previous results that link eisosomes to endocytosis, these observations suggest that Pkh-kinase signaling relayed to Pil1 may help regulate endocytic events to modulate the organization of the plasma membrane

Glucose sensor O-GlcNAcylation coordinates with phosphorylation to regulate circadian clock.

Posttranslational modifications play central roles in myriad biological pathways including circadian regulation. We employed a circadian proteomic approach to demonstrate that circadian timing of phosphorylation is a critical factor in regulating complex GSK3β-dependent pathways and identified O-GlcNAc transferase (OGT) as a substrate of GSK3β. Interestingly, OGT activity is regulated by GSK3β; hence, OGT and GSK3β exhibit reciprocal regulation. Modulating O-GlcNAcylation levels alter circadian period length in both mice and Drosophila; conversely, protein O-GlcNAcylation is circadianly regulated. Central clock proteins, Clock and Period, are reversibly modified by O-GlcNAcylation to regulate their transcriptional activities. In addition, O-GlcNAcylation of a region in PER2 known to regulate human sleep phase (S662-S674) competes with phosphorylation of this region, and this interplay is at least partly mediated by glucose levels. Together, these results indicate that O-GlcNAcylation serves as a metabolic sensor for clock regulation and works coordinately with phosphorylation to fine-tune circadian clock

Molecular constituents of neuronal AMPA receptors

Dynamic regulation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) underlies aspects of synaptic plasticity. Although numerous AMPAR-interacting proteins have been identified, their quantitative and relative contributions to native AMPAR complexes remain unclear. Here, we quantitated protein interactions with neuronal AMPARs by immunoprecipitation from brain extracts. We found that stargazin-like transmembrane AMPAR regulatory proteins (TARPs) copurified with neuronal AMPARs, but we found negligible binding to GRIP, PICK1, NSF, or SAP-97. To facilitate purification of neuronal AMPAR complexes, we generated a transgenic mouse expressing an epitope-tagged GluR2 subunit of AMPARs. Taking advantage of this powerful new tool, we isolated two populations of GluR2 containing AMPARs: an immature complex with the endoplasmic reticulum chaperone immunoglobulin-binding protein and a mature complex containing GluR1, TARPs, and PSD-95. These studies establish TARPs as the auxiliary components of neuronal AMPARs

Quantitative proteomic analysis of Huh-7 cells infected with Dengue virus by label-free LC–MS

AbstractDengue is an important and growing public health problem worldwide with an estimated 100million new clinical cases annually. Currently, no licensed drug or vaccine is available. During natural infection in humans, liver cells constitute one of the main targets of dengue virus (DENV) replication. However, a clear understanding of dengue pathogenesis remains elusive. In order to gain a better reading of the cross talk between virus and host cell proteins, we used a proteomics approach to analyze the host response to DENV infection in a hepatic cell line Huh-7. Differences in proteome expression were assayed 24h post-infection using label-free LC–MS. Quantitative analysis revealed 155 differentially expressed proteins, 64 of which were up-regulated and 91 down-regulated. These results reveal an important decrease in the expression of enzymes involved in the glycolytic pathway, citrate cycle, and pyruvate metabolism. This study provides large-scale quantitative information regarding protein expression in the early stages of infection that should be useful for better compression of the pathogenesis of dengue.Biological significanceDengue infection involves alterations in the homeostasis of the host cell. Defining the interactions between virus and cell proteins should provide a better understanding of how viruses propagate and cause disease. Here, we present for the first time the proteomic analysis of hepatocytes (Huh-7 cells) infected with DENV-2 by label-free LC–MS.This article is part of a Special Issue entitled: Proteomics, mass spectrometry and peptidomics, Cancun 2013. Guest Editors: César López-Camarillo, Victoria Pando-Robles and Bronwyn Jane Barkla

Double impact of cigarette smoke and mechanical ventilation on the alveolar epithelial type II cell

INTRODUCTION: Ventilator-induced lung injury (VILI) impacts clinical outcomes in acute respiratory distress syndrome (ARDS), which is characterized by neutrophil-mediated inflammation and loss of alveolar barrier function. Recent epidemiological studies suggest that smoking may be a risk factor for the development of ARDS. Because alveolar type II cells are central to maintaining the alveolar epithelial barrier during oxidative stress, mediated in part by neutrophilic inflammation and mechanical ventilation, we hypothesized that exposure to cigarette smoke and mechanical strain have interactive effects leading to the activation of and damage to alveolar type II cells. METHODS: To determine if cigarette smoke increases susceptibility to VILI in vivo, a clinically relevant rat model was established. Rats were exposed to three research cigarettes per day for two weeks. After this period, some rats were mechanically ventilated for 4 hours. Bronchoalveolar lavage (BAL) and differential cell count was done and alveolar type II cells were isolated. Proteomic analysis was performed on the isolated alveolar type II cells to discover alterations in cellular pathways at the protein level that might contribute to injury. Effects on levels of proteins in pathways associated with innate immunity, oxidative stress and apoptosis were evaluated in alveolar type II cell lysates by enzyme-linked immunosorbent assay. Statistical comparisons were performed by t-tests, and the results were corrected for multiple comparisons using the false discovery rate. RESULTS: Tobacco smoke exposure increased airspace neutrophil influx in response to mechanical ventilation. The combined exposure to cigarette smoke and mechanical ventilation significantly increased BAL neutrophil count and protein content. Neutrophils were significantly higher after smoke exposure and ventilation than after ventilation alone. DNA fragments were significantly elevated in alveolar type II cells. Smoke exposure did not significantly alter other protein-level markers of cell activation, including Toll-like receptor 4; caspases 3, 8 and 9; and heat shock protein 70. CONCLUSIONS: Cigarette smoke exposure may impact ventilator-associated alveolar epithelial injury by augmenting neutrophil influx. We found that cigarette smoke had less effect on other pathways previously associated with VILI, including innate immunity, oxidative stress and apoptosis

Schwann Cell O-GlcNAc Glycosylation Is Required for Myelin Maintenance and Axon Integrity

Schwann cells (SCs), ensheathing glia of the peripheral nervous system, support axonal survival and function. Abnormalities in SC metabolism affect their ability to provide this support and maintain axon integrity. To further interrogate this metabolic influence on axon–glial interactions, we generated OGT-SCKO mice with SC-specific deletion of the metabolic/nutrient sensing protein O-GlcNAc transferase that mediates the O-linked addition of N-acetylglucosamine (GlcNAc) moieties to Ser and Thr residues. The OGT-SCKO mice develop tomaculous demyelinating neuropathy characterized by focal thickenings of the myelin sheath (tomacula), progressive demyelination, axonal loss, and motor and sensory nerve dysfunction. Proteomic analysis identified more than 100 O-GlcNAcylated proteins in rat sciatic nerve, including Periaxin (PRX), a myelin protein whose mutation causes inherited neuropathy in humans. PRX lacking O-GlcNAcylation is mislocalized within the myelin sheath of these mutant animals. Furthermore, phenotypes of OGT-SCKO and Prx-deficient mice are very similar, suggesting that metabolic control of PRX O-GlcNAcylation is crucial for myelin maintenance and axonal integrity. SIGNIFICANCE STATEMENT The nutrient sensing protein O-GlcNAc transferase (OGT) mediates post-translational O-linked N-acetylglucosamine (GlcNAc) modification. Here we find that OGT functions in Schwann cells (SCs) to maintain normal myelin and prevent axonal loss. SC-specific deletion of OGT (OGT-SCKO mice) causes a tomaculous demyelinating neuropathy accompanied with progressive axon degeneration and motor and sensory nerve dysfunction. We also found Periaxin (PRX), a myelin protein whose mutation causes inherited neuropathy in humans, is O-GlcNAcylated. Importantly, phenotypes of OGT-SCKO and Prx mutant mice are very similar, implying that compromised PRX function contributes to the neuropathy of OGT-SCKO mice. This study will be useful in understanding how SC metabolism contributes to PNS function and in developing new strategies for treating peripheral neuropathy by targeting SC function