Jumonji domain containing protein 6 (Jmjd6) modulates splicing and specifically interacts with arginine-serine-rich (RS) domains of SR- and SR-like proteins.

The Fe(II) and 2-oxoglutarate dependent oxygenase Jmjd6 has been shown to hydroxylate lysine residues in the essential splice factor U2 auxiliary factor 65 kDa subunit (U2AF65) and to act as a modulator of alternative splicing. We describe further evidence for the role of Jmjd6 in the regulation of pre-mRNA processing including interactions of Jmjd6 with multiple arginine-serine-rich (RS)-domains of SR- and SR-related proteins including U2AF65, Luc7-like protein 3 (Luc7L3), SRSF11 and Acinus S', but not with the bona fide RS-domain of SRSF1. The identified Jmjd6 target proteins are involved in different mRNA processing steps and play roles in exon dependent alternative splicing and exon definition. Moreover, we show that Jmjd6 modifies splicing of a constitutive splice reporter, binds RNA derived from the reporter plasmid and punctually co-localises with nascent RNA. We propose that Jmjd6 exerts its splice modulatory function by interacting with specific SR-related proteins during splicing in a RNA dependent manner.

Genetic manipulation of isoprene emissions in poplar plants remodels the chloroplast proteome.

Biogenic isoprene (2-methyl-1,3-butadiene) improves the integrity and functionality of thylakoid membranes and scavenges reactive oxygen species (ROS) in plant tissue under stress conditions. On the basis of available physiological studies, we hypothesized that the suppression of isoprene production in the poplar plant by genetic engineering would cause changes in the chloroplast protein pattern, which in turn would compensate for changes in chloroplast functionality and overall plant performance under abiotic stress. To test this hypothesis, we used a stable isotope-coded protein-labeling technique in conjunction with polyacrylamide gel electrophoresis and liquid chromatography tandem mass spectrometry. We analyzed quantitative and qualitative changes in the chloroplast proteome of isoprene-emitting and non isoprene-emitting poplars. Here we demonstrate that suppression of isoprene synthase by RNA interference resulted in decreased levels of chloroplast proteins involved in photosynthesis and increased levels of histones, ribosomal proteins, and proteins related to metabolism. Overall, our results show that the absence of isoprene triggers a rearrangement of the chloroplast protein profile to minimize the negative stress effects resulting from the absence of isoprene. The present data strongly support the idea that isoprene improves/stabilizes thylakoid membrane structure and interferes with the production of ROS.

Proteomic survey reveals altered energetic patterns and metabolic failure prior to retinal degeneration.

Inherited mutations that lead to misfolding of the visual pigment rhodopsin (Rho) are a prominent cause of photoreceptor neuron (PN) degeneration and blindness. How Rho proteotoxic stress progressively impairs PN viability remains unknown. To identify the pathways that mediate Rho toxicity in PNs, we performed a comprehensive proteomic profiling of retinas from Drosophila transgenics expressing Rh1(P37H), the equivalent of mammalian Rho(P23H), the most common Rho mutation linked to blindness in humans. Profiling of young Rh1(P37H) retinas revealed a coordinated upregulation of energy-producing pathways and attenuation of energy-consuming pathways involving target of rapamycin (TOR) signaling, which was reversed in older retinas at the onset of PN degeneration. We probed the relevance of these metabolic changes to PN survival by using a combination of pharmacological and genetic approaches. Chronic suppression of TOR signaling, using the inhibitor rapamycin, strongly mitigated PN degeneration, indicating that TOR signaling activation by chronic Rh1(P37H) proteotoxic stress is deleterious for PNs. Genetic inactivation of the endoplasmic reticulum stress-induced JNK/TRAF1 axis as well as the APAF-1/caspase-9 axis, activated by damaged mitochondria, dramatically suppressed Rh1(P37H)-induced PN degeneration, identifying the mitochondria as novel mediators of Rh1(P37H) toxicity. We thus propose that chronic Rh1(P37H) proteotoxic stress distorts the energetic profile of PNs leading to metabolic imbalance, mitochondrial failure, and PN degeneration and therapies normalizing metabolic function might be used to alleviate Rh1(P37H) toxicity in the retina. Our study offers a glimpse into the intricate higher order interactions that underlie PN dysfunction and provides a useful resource for identifying other molecular networks that mediate Rho toxicity in PNs.

Retinal proteome alterations in a mouse model of type 2 diabetes.

AIMS/HYPOTHESIS: Diabetic retinopathy is a major complication of type 2 diabetes and the leading cause of blindness in adults of working age. Neuronal defects are known to occur early in disease, but the source of this dysfunction is unknown. The aim of this study was to examine differences in the retinal membrane proteome among non-diabetic mice and mouse models of diabetes either with or without metformin treatment. METHODS: Alterations in the retinal membrane proteome of 10-week-old diabetic db/db mice, diabetic db/db mice orally treated with the anti-hyperglycaemic metformin, and congenic wild-type littermates were examined using label-free mass spectrometry. Pathway enrichment analysis was completed with Genomatix and Ingenuity. Alterations in Slc17a7 mRNA and vesicular glutamate transporter 1 (VGLUT1) protein expression were evaluated using real-time quantitative PCR and IMMUNOFLUORESCENCE. RESULTS: A total of 98 proteins were significantly differentially abundant between db/db and wild-type animals. Pathway enrichment analysis indicated decreases in levels of proteins related to synaptic transmission and cell signalling. Metformin treatment produced 63 differentially abundant proteins compared with untreated db/db mice, of which only 43 proteins were found to occur in both datasets, suggesting that treatment only partially normalises the alterations induced by diabetes. VGLUT1, which is responsible for loading glutamate into synaptic vesicles, was found to be differentially abundant in db/db mice and was not normalised by metformin. The decrease in Slc17a7/VGLUT1 was confirmed by transcriptomic and immunocytochemical analysis. CONCLUSIONS/INTERPRETATION: These findings expand the knowledge of the protein changes in diabetic retinopathy and suggest that membrane-associated signalling proteins are susceptible to changes that are partially ameliorated by treatment

Expression changes and novel interaction partners of talin 1 in effector cells of autoimmune uveitis.

Autoimmune uveitis is characterized by crossing of blood-retinal barrier (BRB) by autoaggressive immune cells. Equine recurrent uveitis (ERU) is a valuable spontaneous model for autoimmune uveitis and analyses of differentially expressed proteins in ERU unraveled changed protein clusters in target tissues and immune system. Healthy eyes are devoid of leukocytes. In ERU, however, leukocytes enter the inner eye and subsequently destroy it. Molecular mechanisms enabling cell migration through BRB still remain elusive. Previously, we detected decreased talin 1 expression in blood-derived granulocytes of ERU cases, linking the innate immune system to ERU. Because changes in leukocyte protein expression pattern may play a role in pathological abnormalities leading to migration ability, we aimed at identifying interactors of talin 1 in leukocytes with immunoprecipitation, followed by LC-MS/MS for candidate identification. This enabled us to identify CD90 (Thy1) as novel interactor of talin 1 besides several other interactors. In blood-derived granulocytes from healthy individuals, CD90 was highly abundant and significantly reduced in ERU, especially in effector cells. Connection between talin 1 and CD90 and their expression differences in inflammation is an interesting novel finding allowing deeper insight into immune response of innate immune system and granulocyte migration ability in this organ-specific autoimmune disease.

Identification of autoantigens in body fluids by combining pull-downs and organic precipitations of intact immune complexes with quantitative label-free mass spectrometry.

Most autoimmune diseases are multifactorial diseases and are caused by the immunological reaction against a number of autoantigens. Key for understanding autoimmune pathologies is the knowledge of the targeted autoantigens, both initially and during disease progression. We present an approach for autoantigen identification based on isolation of intact autoantibody-antigen complexes from body fluids. After organic precipitation of high molecular weight proteins and free immunoglobulins, released autoantigens were identified by quantitative label-free liquid chromatography mass spectrometry. We confirmed feasibility of target enrichment and identification from highly complex body fluid proteomes by spiking of a predefined antibody-antigen complex at low level of abundance. As a proof of principle, we studied the blinding disease autoimmune uveitis, which is caused by autoreactive T-cells attacking the inner eye and is accompanied by autoantibodies. We identified three novel autoantigens in the spontaneous animal model equine recurrent uveitis (secreted acidic phosphoprotein osteopontin, extracellular matrix protein 1, and metalloproteinase inhibitor 2) and confirmed the presence of the corresponding autoantibodies in 15-25% of patient samples by enzyme-linked immunosorbent assay. Thus, this workflow led to the identification of novel autoantigens in autoimmune uveitis and may provide a versatile and useful tool to identify autoantigens in other autoimmune diseases in the future.

Galectin-3 induces clustering of CD147 and integrin-ß1 transmembrane glycoprotein receptors on the RPE cell surface.

Proliferative vitreoretinopathy (PVR) is a blinding disease frequently occurring after retinal detachment surgery. Adhesion, migration and matrix remodeling of dedifferentiated retinal pigment epithelial (RPE) cells characterize the onset of the disease. Treatment options are still restrained and identification of factors responsible for the abnormal behavior of the RPE cells will facilitate the development of novel therapeutics. Galectin-3, a carbohydrate-binding protein, was previously found to inhibit attachment and spreading of retinal pigment epithelial cells, and thus bares the potential to counteract PVR-associated cellular events. However, the identities of the corresponding cell surface glycoprotein receptor proteins on RPE cells are not known. Here we characterize RPE-specific Gal-3 containing glycoprotein complexes using a proteomic approach. Integrin-ß1, integrin-α3 and CD147/EMMPRIN, a transmembrane glycoprotein implicated in regulating matrix metalloproteinase induction, were identified as potential Gal-3 interactors on RPE cell surfaces. In reciprocal immunoprecipitation experiments we confirmed that Gal-3 associated with CD147 and integrin-ß1, but not with integrin-α3. Additionally, association of Gal-3 with CD147 and integrin-ß1 was observed in co-localization analyses, while integrin-α3 only partially co-localized with Gal-3. Blocking of CD147 and integrin-ß1 on RPE cell surfaces inhibited binding of Gal-3, whereas blocking of integrin-α3 failed to do so, suggesting that integrin-α3 is rather an indirect interactor. Importantly, Gal-3 binding promoted pronounced clustering and co-localization of CD147 and integrin-ß1, with only partial association of integrin-α3. Finally, we show that RPE derived CD147 and integrin-ß1, but not integrin-α3, carry predominantly ß-1,6-N-actyl-D-glucosamine-branched glycans, which are high-affinity ligands for Gal-3. We conclude from these data that extracellular Gal-3 triggers clustering of CD147 and integrin-ß1 via interaction with ß1,6-branched N-glycans on RPE cells and hypothesize that Gal-3 acts as a positive regulator for CD147/integrin-ß1 clustering and therefore modifies RPE cell behavior contributing to the pathogenesis of PVR. Further investigations at this pathway may aid in the development of specific therapies for PVR.

The PI3K/Akt/mTOR pathway is implicated in the premature senescence of primary human endothelial cells exposed to chronic radiation.

The etiology of radiation-induced cardiovascular disease (CVD) after chronic exposure to low doses of ionizing radiation is only marginally understood. We have previously shown that a chronic low-dose rate exposure (4.1 mGy/h) causes human umbilical vein endothelial cells (HUVECs) to prematurely senesce. We now show that a dose rate of 2.4 mGy/h is also able to trigger premature senescence in HUVECs, primarily indicated by a loss of growth potential and the appearance of the senescence-associated markers ß-galactosidase (SA-ß-gal) and p21. In contrast, a lower dose rate of 1.4 mGy/h was not sufficient to inhibit cellular growth or increase SA-ß-gal-staining despite an increased expression of p21. We used reverse phase protein arrays and triplex Isotope Coded Protein Labeling with LC-ESI-MS/MS to study the proteomic changes associated with chronic radiation-induced senescence. Both technologies identified inactivation of the PI3K/Akt/mTOR pathway accompanying premature senescence. In addition, expression of proteins involved in cytoskeletal structure and EIF2 signaling was reduced. Age-related diseases such as CVD have been previously associated with increased endothelial cell senescence. We postulate that a similar endothelial aging may contribute to the increased rate of CVD seen in populations chronically exposed to low-dose-rate radiation.

Long-term effects of acute low-dose ionizing radiation on the neonatal mouse heart: a proteomic study.

Epidemiological studies establish that children and young adults are especially susceptible to radiation-induced cardiovascular disease (CVD). The biological mechanisms behind the elevated CVD risk following exposure at young age remain unknown. The present study aims to elucidate the long-term effects of ionizing radiation by studying the murine cardiac proteome after exposure to low and moderate radiation doses. NMRI mice received single doses of total body (60)Co gamma-irradiation on postnatal day 10 and were sacrificed 7 months later. Changes in cardiac protein expression were quantified using isotope-coded protein label and tandem mass spectrometry. We identified 32, 31, 66, and 34 significantly deregulated proteins after doses of 0.02, 0.1, 0.5, and 1.0 Gy, respectively. The four doses shared 9 deregulated proteins. Bioinformatics analysis showed that most of the deregulated proteins belonged to a limited set of biological categories, including metabolic processes, inflammatory response, and cytoskeletal structure. The transcription factor peroxisome proliferator-activated receptor alpha was predicted as a common upstream regulator of several deregulated proteins. This study indicates that both adaptive and maladaptive responses to the initial radiation damage persist well into adulthood. It will contribute to the understanding of the long-term consequences of radiation-induced injury and developmental alterations in the neonatal heart.

Rrp5p, Noc1p and Noc2p form a protein module which is part of early large ribosomal subunit precursors in S. cerevisiae.

Eukaryotic ribosome biogenesis requires more than 150 auxiliary proteins, which transiently interact with pre-ribosomal particles. Previous studies suggest that several of these biogenesis factors function together as modules. Using a heterologous expression system, we show that the large ribosomal subunit (LSU) biogenesis factor Noc1p of Saccharomyces cerevisiae can simultaneously interact with the LSU biogenesis factor Noc2p and Rrp5p, a factor required for biogenesis of the large and the small ribosomal subunit. Proteome analysis of RNA polymerase-I-associated chromatin and chromatin immunopurification experiments indicated that all members of this protein module and a specific set of LSU biogenesis factors are co-transcriptionally recruited to nascent ribosomal RNA (rRNA) precursors in yeast cells. Further ex vivo analyses showed that all module members predominantly interact with early pre-LSU particles after the initial pre-rRNA processing events have occurred. In yeast strains depleted of Noc1p, Noc2p or Rrp5p, levels of the major LSU pre-rRNAs decreased and the respective other module members were associated with accumulating aberrant rRNA fragments. Therefore, we conclude that the module exhibits several binding interfaces with pre-ribosomes. Taken together, our results suggest a co- and post-transcriptional role of the yeast Rrp5p-Noc1p-Noc2p module in the structural organization of early LSU precursors protecting them from non-productive RNase activity.

Direct comparison of MS-based label-free and SILAC quantitative proteome profiling strategies in primary retinal Müller cells.

To better understand the involvement of retinal Müller glial (RMG) cells in retinal diseases, we phenotyped primary porcine RMGs in dependence of cultivation time using different quantitative proteomic strategies. A well-established LC-MS/MS-based quantification method was employed: stable isotope labeling by amino acids in cell culture (SILAC) and directly compared to label-free (LF) quantifications, based on total peak intensities using two different programs (MaxQuant and Progenesis LC-MS). The overall numbers of detected proteins were largely similar (overlap of 1324 proteins), only a total of 173 proteins were significantly altered between the different culture conditions. However, among these, only 21 proteins were shared between the three analytical strategies. Hence, the majority of altered proteins only reached significance thresholds in one of the applied analyses with a larger overlap between the two LF approaches. Among the shared, differentially abundant proteins were known RMG markers as well as new proteins associated with glial cell transition. However, proteins correlated to cellular transitions and dedifferentiation were also found among the proteins only significant in one or two of the applied strategies. Consequently, the application of different quantification and analytical strategies could increase the analytical depths of proteomic phenotyping.

Analysis of ribosome biogenesis factor-modules in yeast cells depleted from pre-ribosomes.

Formation of eukaryotic ribosomes requires more than 150 biogenesis factors which transiently interact with the nascent ribosomal subunits. Previously, many pre-ribosomal intermediates could be distinguished by their protein composition and rRNA precursor (pre-rRNA) content. We purified complexes of ribosome biogenesis factors from yeast cells in which de novo synthesis of rRNA precursors was down-regulated by genetic means. We compared the protein composition of these largely pre-rRNA free assemblies with the one of analogous pre-ribosomal preparations by semi-quantitative mass spectrometry. The experimental setup minimizes the possibility that the analysed pre-rRNA free protein modules were derived from (partially) disrupted pre-ribosomal particles and provides thereby strong evidence for their pre-ribosome independent existence. In support of the validity of this approach (i) the predicted composition of the analysed protein modules was in agreement with previously described rRNA-free complexes and (ii) in most of the cases we could identify new candidate members of reported protein modules. An unexpected outcome of these analyses was that free large ribosomal subunits are associated with a specific set of ribosome biogenesis factors in cells where neo-production of nascent ribosomes was blocked. The data presented strengthen the idea that assembly of eukaryotic pre-ribosomal particles can result from transient association of distinct building blocks.

The Noc-domain containing C-terminus of Noc4p mediates both formation of the Noc4p-Nop14p submodule and its incorporation into the SSU processome.

Noc1p, Noc3p and Noc4p are eukaryotic proteins which play essential roles in yeast ribosome biogenesis and contain a homologous stretch of about 45 aminoacids (Noc-domain) of unknown function. Yeast Noc4p is a component of the small ribosomal subunit (SSU) processome, can be isolated as a stable Noc4p-Nop14p SSU-processome submodule from yeast cells, and is required for nuclear steps of small ribosomal subunit rRNA maturation. We expressed a series of mutated alleles of NOC4 in yeast cells and analysed whether the corresponding protein variants support vegetative growth, interact with Nop14p, and are incorporated into the SSU-processome. The data reveal that the essential C-terminus of Noc4p which contains 237 aminoacids including the Noc-domain represents a protein-protein interaction module. It is required and sufficient for its association with Nop14p and several nuclear precursors of the small ribosomal subunit. The N-terminal Noc4-part seems to be targeted to pre-ribosomes via the C-terminus of Noc4p and plays there an essential role in SSU-processome function. Replacement of the Noc4p-Noc-domain by its homologues Noc1p-counterpart results in a hybrid Noc4p variant which fails to associate with Nop14p and pre-ribosomes. On the other hand, exchange of 6 amino acids in the Noc1-Noc-domain of this hybrid Noc4p protein is sufficient to restore its essential in vivo functions. These data suggest that Noc-domains of Noc1p and Noc4p share a common structural backbone in which diverging amino acids play crucial roles in mediating specific regulated interactions. Our analysis allows us to distinguish between different functions of certain domains within Noc4p and contribute to the understanding of how incorporation of Noc4p into ribosomal precursors is coupled to rRNA processing and maturation of the small ribosomal subunit.