Intraocular biomarker identification in uveitis associated with juvenile idiopathic arthritis.

PURPOSE: To investigate the presence of biomarkers in aqueous humor (AH) from patients with uveitis associated with juvenile idiopathic arthritis (JIA). METHODS: AH (N = 73) AND SERUM (N = 105) SAMPLES FROM 116 CHILDREN WERE ANALYZED USING SURFACE ENHANCED LASER DESORPTION/IONIZATION TIME OF FLIGHT MASS SPECTROMETRY (SELDI-TOF MS). THE SAMPLES WERE DIVIDED INTO THE FOLLOWING GROUPS: JIA, silent chronic anterior uveitis (AU), other uveitis entities, and noninflammatory controls. Statistical biomarker identification was performed using the SELDI-ToF Biomarker Analysis Cluster Wizard followed by multivariate statistical analysis. Biochemical identification of biomarkers was performed by polyacrylamide gel protein separation, followed by liquid chromatography tandem mass spectrometry. ELISA was performed in a number of AH samples representing all four study groups. RESULTS: In the JIA group, one AH protein peak at mass/charge (m/z) 13,762 had qualitative and quantitative differences in expression compared with the other uveitis entities and the controls, but not to the group of silent chronic AU. Its quantitative expression in AH of patients with JIA and other silent chronic AU was positively associated with uveitis activity. The protein at m/z 13,762 in AH was identified as transthyretin (TTR). The TTR concentration in AH differed significantly between the study groups (P = 0.006) with considerably higher TTR concentrations in JIA and silent chronic AU samples positive for m/z 13,762 than those of the other uveitis and control groups. CONCLUSIONS: TTR is a potential intraocular biomarker of JIA- associated uveitis. Its role in the pathogenesis of silent chronic AU with and without arthritis needs further investigation.

Tyrosine phosphorylation of tau by the SRC family kinases lck and fyn.

BACKGROUND: Tau protein is the principal component of the neurofibrillary tangles found in Alzheimer's disease, where it is hyperphosphorylated on serine and threonine residues, and recently phosphotyrosine has been demonstrated. The Src-family kinase Fyn has been linked circumstantially to the pathology of Alzheimer's disease, and shown to phosphorylate Tyr18. Recently another Src-family kinase, Lck, has been identified as a genetic risk factor for this disease. RESULTS: In this study we show that Lck is a tau kinase. In vitro, comparison of Lck and Fyn showed that while both kinases phosphorylated Tyr18 preferentially, Lck phosphorylated other tyrosines somewhat better than Fyn. In co-transfected COS-7 cells, mutating any one of the five tyrosines in tau to phenylalanine reduced the apparent level of tau tyrosine phosphorylation to 25-40% of that given by wild-type tau. Consistent with this, tau mutants with only one remaining tyrosine gave poor phosphorylation; however, Tyr18 was phosphorylated better than the others. CONCLUSIONS: Fyn and Lck have subtle differences in their properties as tau kinases, and the phosphorylation of tau is one mechanism by which the genetic risk associated with Lck might be expressed pathogenically.

Plasma gelsolin is decreased and correlates with rate of decline in Alzheimer's disease.

Alzheimer's disease (AD) is a progressive neurodegenerative disorder where definite diagnosis can only be made postmortem, and for which the most promising peripheral markers of disease state and severity have been found in the cerebrospinal fluid. However, recent results suggest that differences in the levels of certain plasma proteins do exist between AD patients and non-demented controls (NDC). Herein, we undertook an untargeted discovery study using isobaric mass tagging to compare the plasma protein levels between slow cognitive declining AD patients, rapid cognitive declining AD patients (RCD) and NDC subjects. Subsequent relative quantification and statistical analysis identified a list of candidate proteins able to distinguish RCD from NDC groups based on multivariate analysis. Selected proteins were then validated by western blot analysis in an independent sample set of 60 AD and 35 NDC subjects. In this cohort, AD patients displayed significantly lower plasma gelsolin levels compared to NDC subjects. Additionally, gelsolin levels correlated with disease progression rate estimated by Mini-Mental Status Examination decline per year. In order to further investigate gelsolin expression, three different brain regions from an additional cohort of 23 subjects and their respective plasma samples were analysed. No significant change in brain gelsolin levels could be established between AD and control subjects. Interestingly, this study reveals yet another condition where plasma gelsolin levels are decreased and our findings, together with the reported interaction of gelsolin and amyloid-beta, makes plasma gelsolin an attractive candidate for further studies targeted at better understanding disease progression in AD.

Candidate verification of iron-regulated Neisseria meningitidis proteins using isotopic versions of tandem mass tags (TMT) and single reaction monitoring.

Tandem Mass Tags (TMT) are suited to both global and targeted quantitation approaches of proteins and peptides. Different versions of these tags allow for the generation of both isobaric and isotopic sets of reagents sharing the same common structure. This feature allows for a straightforward transfer of data obtained during discovery studies into targeted investigations. In prior discovery studies, an isobaric set of these reagents was used to identify Neisseria meningitidis proteins expressed under iron-limitation. Here, we apply isotopic versions of those reagents in combination with single reaction monitoring to verify selected candidates found to be differentially regulated in these discovery studies, representing both well-known and novel iron-regulated proteins, such as the MtrCDE drug efflux pump. In this targeted approach (TMT-SRM), the selectivity of SRM is maintained while allowing the incorporation of an internal reference standard into the experiment. By monitoring 184 transitions, TMT-SRM resulted in the quantitation of 33 peptides representing 12 proteins. The acquired data corroborated the results obtained during the discovery phase. Furthermore, these data obtained by MS-based quantitation of peptides were independently confirmed by western blotting results, an orthogonal approach based on quantitation at the protein level.

Phosphorylation of tau regulates its axonal transport by controlling its binding to kinesin.

Defective axonal transport has been proposed as an underlying mechanism that may give rise to neurodegeneration. We investigated the effect of phosphorylation on the axonal transport of tau, a neuronal protein that stabilizes microtubules and is hyperphosphorylated and mislocalized in Alzheimer's disease. We report here that specific inhibition of glycogen synthase kinase-3 (GSK-3) reduces tau phosphorylation and significantly decreases the overall rate of axonal transport of tau in rat cortical neurons. Tau mutants, with serine/threonine targets of GSK-3 mutated to glutamate to mimic a permanent state of phosphorylation, were transported at a significantly increased rate compared to wild-type tau. Conversely, tau mutants, in which alanine replaced serine/threonine to mimic permanent dephosphorylation, were transported at a decreased rate compared to wild-type tau. We also found that tau interacts with the light chain of kinesin-1 and that this is dependent on the phosphorylation state of tau. Tau phosphorylation by GSK-3 increased binding, and dephosphorylated tau exhibited a reduced association with kinesin-1. We conclude that GSK-3 phosphorylation of tau modulates its axonal transport by regulating binding to kinesin-1. Hyperphosphorylated tau in Alzheimer's disease appearing first in distal portions of axons may result from aberrant axonal transport of phosphorylated tau reported here.

Comparison of cellular ribonucleoprotein complexes associated with the APOBEC3F and APOBEC3G antiviral proteins.

The human apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like 3F (APOBEC3F [A3F]) and A3G proteins are effective inhibitors of infection by various retroelements and share approximately 50% amino acid sequence identity. We therefore undertook comparative analyses of the protein and RNA compositions of A3F- and A3G-associated ribonucleoprotein complexes (RNPs). Like A3G, A3F is found associated with a complex array of cytoplasmic RNPs and can accumulate in RNA-rich cytoplasmic microdomains known as mRNA processing bodies or stress granules. While A3F RNPs display greater resistance to disruption by RNase digestion, the major protein difference is the absence of the Ro60 and La autoantigens. Consistent with this, A3F RNPs also lack a number of small polymerase III RNAs, including the RoRNP-associated Y RNAs, as well as 7SL RNA. Alu RNA is, however, present in A3F and A3G RNPs, and both proteins suppress Alu element retrotransposition. Thus, we define a number of subtle differences between the RNPs associated with A3F and A3G and speculate that these contribute to functional differences that have been described for these proteins.

Novel phosphorylation sites in tau from Alzheimer brain support a role for casein kinase 1 in disease pathogenesis.

Tau in Alzheimer disease brain is highly phosphorylated and aggregated into paired helical filaments comprising characteristic neurofibrillary tangles. Here we have analyzed insoluble Tau (PHF-tau) extracted from Alzheimer brain by mass spectrometry and identified 11 novel phosphorylation sites, 10 of which were assigned unambiguously to specific amino acid residues. This brings the number of directly identified sites in PHF-tau to 39, with an additional six sites indicated by reactivity with phosphospecific antibodies to Tau. We also identified five new phosphorylation sites in soluble Tau from control adult human brain, bringing the total number of reported sites to nine. To assess which kinases might be responsible for Tau phosphorylation, we used mass spectrometry to determine which sites were phosphorylated in vitro by several kinases. Casein kinase 1delta and glycogen synthase kinase-3beta were each found to phosphorylate numerous sites, and each kinase phosphorylated at least 15 sites that are also phosphorylated in PHF-tau from Alzheimer brain. A combination of casein kinase 1delta and glycogen synthase kinase-3beta activities could account for over three-quarters of the serine/threonine phosphorylation sites identified in PHF-tau, indicating that casein kinase 1delta may have a role, together with glycogen synthase kinase-3beta, in the pathogenesis of Alzheimer disease.

ALS2/Alsin regulates Rac-PAK signaling and neurite outgrowth.

Rac and its downstream effectors p21-activated kinase (PAK) family kinases regulate actin dynamics within growth cones to control neurite outgrowth during development. The activity of Rac is stimulated by guanine nucleotide exchange factors (GEFs) that promote GDP release and GTP binding. ALS2/Alsin is a recently described GEF that contains a central domain that is predicted to regulate the activities of Rac and/or Rho and Cdc42 activities. Mutations in ALS2 cause some recessive familial forms of amyotrophic lateral sclerosis (ALS) but the function of ALS2 is poorly understood. Here we demonstrate that ALS2 is present within growth cones of neurons, in which it co-localizes with Rac. Furthermore, ALS2 stimulates Rac but not Rho or Cdc42 activities, and this induces a corresponding increase in PAK1 activity. Finally, we demonstrate that ALS2 promotes neurite outgrowth. Defects in these functions may therefore contribute to motor neuron demise in ALS.

A novel strategy using MASCOT Distiller for analysis of cleavable isotope-coded affinity tag data to quantify protein changes in plasma.

A novel strategy consisting of cleavable Isotope-Coded Affinity Tag (cICAT) combined with MASCOT Distiller was evaluated as a tool for the quantification of proteins in "abnormal" patient plasma, prepared by pooling samples from patients with acute stroke. Quantification of all light and heavy cICAT-labelled peptide ion pairs was obtained using MASCOT Distiller combined with a proprietary software. Peptides displaying differences were selected for identification by MS. These preliminary results show the promise of our approach to identify potential biomarkers.

Tyrosine 394 is phosphorylated in Alzheimer's paired helical filament tau and in fetal tau with c-Abl as the candidate tyrosine kinase.

Tau is a major microtubule-associated protein of axons and is also the principal component of the paired helical filaments (PHFs) that comprise the neurofibrillary tangles found in Alzheimer's disease and other tauopathies. Besides phosphorylation of tau on serine and threonine residues in both normal tau and tau from neurofibrillary tangles, Tyr-18 was reported to be a site of phosphorylation by the Src-family kinase Fyn. We examined whether tyrosine residues other than Tyr-18 are phosphorylated in tau and whether other tyrosine kinases might phosphorylate tau. Using mass spectrometry, we positively identified phosphorylated Tyr-394 in PHF-tau from an Alzheimer brain and in human fetal brain tau. When wild-type human tau was transfected into fibroblasts or neuroblastoma cells, treatment with pervanadate caused tau to become phosphorylated on tyrosine by endogenous kinases. By replacing each of the five tyrosines in tau with phenylalanine, we identified Tyr-394 as the major site of tyrosine phosphorylation in tau. Tyrosine phosphorylation of tau was inhibited by PP2 (4-amino-5-(4-chlorophenyl-7-(t-butyl)pyrazolo[3,4-d]pyrimidine), which is known to inhibit Src-family kinases and c-Abl. Cotransfection of tau and kinases showed that Tyr-18 was the major site for Fyn phosphorylation, but Tyr-394 was the main residue for Abl. In vitro, Abl phosphorylated tau directly. Abl could be coprecipitated with tau and was present in pretangle neurons in brain sections from Alzheimer cases. These results show that phosphorylation of tau on Tyr-394 is a physiological event that is potentially part of a signal relay and suggest that Abl could have a pathogenic role in Alzheimer's disease.

p38alpha stress-activated protein kinase phosphorylates neurofilaments and is associated with neurofilament pathology in amyotrophic lateral sclerosis.

Neurofilament middle and heavy chains (NFM and NFH) are heavily phosphorylated on their carboxy-terminal side-arm domains in axons. The mechanisms that regulate this phosphorylation are complex. Here, we demonstrate that p38alpha, a member of the stress-activated protein kinase family, will phosphorylate NFM and NFH on their side-arm domains. Aberrant accumulations of neurofilaments containing phosphorylated NFM and NFH side-arms are a pathological feature of amyotrophic lateral sclerosis (ALS) and we also demonstrate that p38alpha and active forms of p38 family kinases are associated with these accumulations. This is the case for sporadic and familial forms of ALS and also in a transgenic mouse model of ALS caused by expression of mutant superoxide dismutase-1 (SOD1). Thus, p38 kinases may contribute to the aberrant phosphorylation of NFM and NFH side-arms in ALS.

The c-Abl tyrosine kinase phosphorylates the Fe65 adaptor protein to stimulate Fe65/amyloid precursor protein nuclear signaling.

The amyloid precursor protein (APP) is proteolytically processed to release a C-terminal domain that signals to the nucleus to regulate transcription of responsive genes. The APP C terminus binds to a number of phosphotyrosine binding (PTB) domain proteins and one of these, Fe65, stimulates APP nuclear signaling. Fe65 is an adaptor protein that contains a number of protein-protein interaction domains. These include two PTB domains, the second of which binds APP, and a WW domain that binds proline-rich ligands. One ligand for the Fe65WW domain is the tyrosine kinase c-Abl. Here, we show that active c-Abl stimulates APP/Fe65-mediated gene transcription and that this effect is mediated by phosphorylation of Fe65 on tyrosine 547 within its second PTB domain. The homologous tyrosine within the motif Tyr-(Leu/Met)-Gly is conserved in a variety of PTB domains, and this suggests that PTB tyrosine phosphorylation occurs in other proteins. As such, PTB domain phosphorylation may represent a novel mechanism for regulating the function of this class of protein.

The neuronal adaptor protein Fe65 is phosphorylated by mitogen-activated protein kinase (ERK1/2).

Fe65 is a neuronal adaptor protein that binds a number of ligands and which functions in both gene transcription/nuclear signalling and in the regulation of cell migration and motility. These different functions within the nucleus and at the cell surface are mediated via Fe65's different binding partners. An Fe65/APP/TIP60 complex is transcriptionally active within the nucleus and an Fe65/APP/Mena complex probably regulates actin dynamics in lamellipodia. The mechanisms that regulate these different Fe65 functions are unclear. Here, we demonstrate that Fe65 is a phosphoprotein and, using mass spectrometry sequencing, identify for the first time in vivo phosphorylation sites in Fe65. We also show that Fe65 is a substrate for phosphorylation by the mitogen-activated protein kinases ERK1/2. Our results provide a mechanism by which Fe65 function may be modulated to fulfil its various roles.

Reversible cysteine-targeted oxidation of proteins during renal oxidative stress.

Biotin-cysteine was used to study protein S-thiolation in isolated rat kidneys subjected to ischemia and reperfusion. After 40 min of ischemia, total protein S-thiolation increased significantly (P < 0.05), by 311%, and remained significantly elevated (P < 0.05), 221% above control, after 5 min of postischemic reperfusion. Treatment of protein samples with 2-mercaptoethanol abolished the S-thiolation signals detected, consistent with the dependence of the signal on the presence of a disulfide bond. With the use of gel filtration chromatography followed by affinity purification with streptavidin-agarose, S-thiolated proteins were purified from CHAPS-soluble kidney homogenate. The proteins were then separated by SDS-PAGE and stained with Coomassie blue. With a combination of matrix-assisted laser desorption ionization time of flight mass spectrometry and LC/MS/MS analysis of protein bands digested with trypsin, a number of S-thiolation substrates were identified. These included the LDL receptor-related protein 2, ATP synthase alpha chain, heat shock protein 90 beta, hydroxyacid oxidase 3, serum albumin precursor, triose phosphate isomerase, and lamin. These represent proteins that may be functionally regulated by S-thiolation and thus could undergo a change in activity or function after renal ischemia and reperfusion.

Detection, quantitation, purification, and identification of cardiac proteins S-thiolated during ischemia and reperfusion.

We have developed methods that allow detection, quantitation, purification, and identification of cardiac proteins S-thiolated during ischemia and reperfusion. Cysteine was biotinylated and loaded into isolated rat hearts. During oxidative stress, biotin-cysteine forms a disulfide bond with reactive protein cysteines, and these can be detected by probing Western blots with streptavidin-horseradish peroxidase. S-Thiolated proteins were purified using streptavidin-agarose. Thus, we demonstrated that reperfusion and diamide treatment increased S-thiolation of a number of cardiac proteins by 3- and 10-fold, respectively. Dithiothreitol treatment of homogenates fully abolished the signals detected. Fractionation studies indicated that the modified proteins are located within the cytosol, membrane, and myofilament/cytoskeletal compartments of the cardiac cells. This shows that biotin-cysteine gains rapid and efficient intracellular access and acts as a probe for reactive protein cysteines in all cellular locations. Using Western blotting of affinity-purified proteins we identified actin, glyceraldehyde-3-phosphate dehydrogenase, HSP27, protein-tyrosine phosphatase 1B, protein kinase Calpha, and the small G-protein ras as substrates for S-thiolation during reperfusion of the ischemic rat heart. MALDI-TOF mass fingerprint analysis of tryptic peptides independently confirmed actin and glyceraldehyde-3-phosphate dehydrogenase S-thiolation during reperfusion. This approach has also shown that triosephosphate isomerase, aconitate hydratase, M-protein, nucleoside diphosphate kinase B, and myoglobin are S-thiolated during post-ischemic reperfusion.