Identification of the p16-Arc subunit of the Arp 2/3 complex as a substrate of MAPK-activated protein kinase 2 by proteomic analysis

A Negative Feedback Loop in Ultraviolet A-Induced Senescence in Human Dermal Fibroblasts Formed by SPCA1 and MAPK

Secretory pathway calcium ATPase 1 (SPCA1) is a calcium pump localized specifically to the Golgi. Its effects on UVA-induced senescence have never been examined. In our study, expression of SPCA1 was increased in UVA-irradiated human dermal fibroblasts (HDFs) by activating mitogen-activated protein kinase (MAPK) and its downstream transcription factor, c-jun. Dual-luciferase reporter and chromatin immunoprecipitation assays revealed that c-jun regulated SPCA1 by binding to its promoter. Furthermore, downregulating SPCA1 with siRNA transfection aggravated UVA-induced senescence due to an elevation of intracellular calcium concentrations and a subsequent increase in reactive oxygen species (ROS) and MAPK activity. In contrast, overexpression of SPCA1 reduced calcium overload, consequently lowering the ROS level and suppressing MAPK activation. This alleviated the cellular senescence caused by UVA irradiation. These results indicated that SPCA1 might exert a protective effect on UVA-induced senescence in HDFs via forming a negative feedback loop. Specifically, activation of MAPK/c-jun triggered by UVA transcriptionally upregulated SPCA1. In turn, the increased SPCA1 lowered the intracellular Ca²⁺ level, probably through pumping Ca²⁺ into the Golgi, leading to a reduction of ROS, eventually decreasing MAPK activity and diminishing UVA-induced senescence.

Articular Chondrocyte Phenotype Regulation through the Cytoskeleton and the Signaling Processes That Originate from or Converge on the Cytoskeleton: Towards a Novel Understanding of the Intersection between Actin Dynamics and Chondrogenic Function

Numerous studies have assembled a complex picture, in which extracellular stimuli and intracellular signaling pathways modulate the chondrocyte phenotype. Because many diseases are mechanobiology-related, this review asked to what extent phenotype regulators control chondrocyte function through the cytoskeleton and cytoskeleton-regulating signaling processes. Such information would generate leverage for advanced articular cartilage repair. Serial passaging, pro-inflammatory cytokine signaling (TNF-α, IL-1α, IL-1β, IL-6, and IL-8), growth factors (TGF-α), and osteoarthritis not only induce dedifferentiation but also converge on RhoA/ROCK/Rac1/mDia1/mDia2/Cdc42 to promote actin polymerization/crosslinking for stress fiber (SF) formation. SF formation takes center stage in phenotype control, as both SF formation and SOX9 phosphorylation for COL2 expression are ROCK activity-dependent. Explaining how it is molecularly possible that dedifferentiation induces low COL2 expression but high SF formation, this review theorized that, in chondrocyte SOX9, phosphorylation by ROCK might effectively be sidelined in favor of other SF-promoting ROCK substrates, based on a differential ROCK affinity. In turn, actin depolymerization for redifferentiation would “free-up” ROCK to increase COL2 expression. Moreover, the actin cytoskeleton regulates COL1 expression, modulates COL2/aggrecan fragment generation, and mediates a fibrogenic/catabolic expression profile, highlighting that actin dynamics-regulating processes decisively control the chondrocyte phenotype. This suggests modulating the balance between actin polymerization/depolymerization for therapeutically controlling the chondrocyte phenotype.

Role of p38/MAPKs in Alzheimer's disease: implications for amyloid beta toxicity targeted therapy

A myriad of environmental and genetic factors, as well as the physiologic process of aging, contribute to Alzheimer's disease (AD) pathology. Neuroinflammation is and has been a focus of interest, as a common gateway for initiation of many of the underlying pathologies of AD. Amyloid beta (Aβ) toxicity, increasing RAGE expression, tau hyperphosphorylation, induction of apoptosis, and deregulated autophagy are among other mechanisms, partly entangled and being explained by activation of mitogen-activated protein kinase (MAPK) and MAPK signaling. p38 MAPK is the most essential regulator of Aβ induced toxicity from this family. p38 induces NF-κB activation, glutamate excitotoxicity, and disruption of synaptic plasticity, which are other implications of all justifying the p38 MAPK as a potential target to break the vicious Aβ toxicity cycle. Until recently, many in vivo and in vitro studies have investigated the effects of p38 MAPK inhibitors in AD. The pyridinyl imidazole compounds SB202190 and SB203580 have shown promising anti-apoptotic results in vivo. MW108 inhibits activation of p38 and is able to postpone cognitive decline in animal models. The PD169316, with anti-inflammatory, anti-oxidative, and anti-apoptotic features, has improved spatial memory in vivo. Natural compounds from Camellia sinensis (green tea), polyphenols from olive oil, pinocembrin from propolis, and the puerarine extract isoflavones, have shown strong anti-apoptotic features, mediated by p38 MAPK inhibition. Use of these drug targets is limited due to central nervous system side effects or cross-reactivity with other kinases, predicting the low efficacy of these drugs in clinical trials.

Weak power frequency magnetic fields induce microtubule cytoskeleton reorganization depending on the epidermal growth factor receptor and the calcium related signaling

We have shown previously that a weak 50 Hz magnetic field (MF) invoked the actin-cytoskeleton, and provoked cell migration at the cell level, probably through activating the epidermal growth factor receptor (EGFR) related motility pathways. However, whether the MF also affects the microtubule (MT)-cytoskeleton is still unknown. In this article, we continuously investigate the effects of 0.4 mT, 50 Hz MF on the MT, and try to understand if the MT effects are also associated with the EGFR pathway as the actin-cytoskeleton effects were. Our results strongly suggest that the MF effects are similar to that of EGF stimulation on the MT cytoskeleton, showing that 1) the MF suppressed MT in multiple cell types including PC12 and FL; 2) the MF promoted the clustering of the EGFR at the protein and the cell levels, in a similar way of that EGF did but with higher sensitivity to PD153035 inhibition, and triggered EGFR phosphorylation on sites of Y1173 and S1046/1047; 3) these effects were strongly depending on the Ca²⁺ signaling through the L-type calcium channel (LTCC) phosphorylation and elevation of the intracellular Ca²⁺ level. Strong associations were observed between EGFR and the Ca²⁺ signaling to regulate the MF-induced-reorganization of the cytoskeleton network, via phosphorylating the signaling proteins in the two pathways, including a significant MT protein, tau. These results strongly suggest that the MF activates the overall cytoskeleton in the absence of EGF, through a mechanism related to both the EGFR and the LTCC/Ca²⁺ signaling pathways.

Hippocampal metabotropic glutamate receptor long-term depression in health and disease: focus on mitogen-activated protein kinase pathways

Group I metabotropic glutamate receptor (mGluR) dependent long-term depression (LTD) is a major form of synaptic plasticity underlying learning and memory. The molecular mechanisms involved in mGluR-LTD have been investigated intensively for the last two decades. In this 60th anniversary special issue article, we review the recent advances in determining the mechanisms that regulate the induction, transduction and expression of mGluR-LTD in the hippocampus, with a focus on the mitogen-activated protein kinase (MAPK) pathways. In particular we discuss the requirement of p38 MAPK and extracellular signal-regulated kinase 1/2 (ERK 1/2) activation. The recent advances in understanding the signaling cascades regulating mGluR-LTD are then related to the cognitive impairments observed in neurological disorders, such as fragile X syndrome and Alzheimer's disease. mGluR-LTD is a form of synaptic plasticity that impacts on memory formation. In the hippocampus mitogen-activated protein kinases (MAPKs) have been found to be important in mGluR-LTD. In this 60th anniversary special issue article, we review the independent and complementary roles of two classes of MAPK, p38 and ERK1/2 and link this to the aberrant mGluR-LTD that has an important role in diseases. This article is part of the 60th Anniversary special issue.

Consensus brain-derived protein, extraction protocol for the study of human and murine brain proteome using both 2D-DIGE and mini 2DE immunoblotting

Two-dimensional gel electrophoresis (2DE) is a powerful tool to uncover proteome modifications potentially related to different physiological or pathological conditions. Basically, this technique is based on the separation of proteins according to their isoelectric point in a first step, and secondly according to their molecular weights by SDS polyacrylamide gel electrophoresis (SDS-PAGE). In this report an optimized sample preparation protocol for little amount of human post-mortem and mouse brain tissue is described. This method enables to perform both two-dimensional fluorescence difference gel electrophoresis (2D-DIGE) and mini 2DE immunoblotting. The combination of these approaches allows one to not only find new proteins and/or protein modifications in their expression thanks to its compatibility with mass spectrometry detection, but also a new insight into markers validation. Thus, mini-2DE coupled to western blotting permits to identify and validate post-translational modifications, proteins catabolism and provides a qualitative comparison among different conditions and/or treatments. Herein, we provide a method to study components of protein aggregates found in AD and Lewy body dementia such as the amyloid-beta peptide and the alpha-synuclein. Our method can thus be adapted for the analysis of the proteome and insoluble proteins extract from human brain tissue and mice models too. In parallel, it may provide useful information for the study of molecular and cellular pathways involved in neurodegenerative diseases as well as potential novel biomarkers and therapeutic targets.

Proteomics and phosphoproteomics provide insights into the mechanism of action of a novel pyrazolo[3,4-d]pyrimidine Src inhibitor in human osteosarcoma

Osteosarcoma (OS) is a highly malignant bone tumour, affecting mainly children and young adults between 10 and 20 years of age. It represents the most frequent primitive malignant tumour of the skeletal system and is characterized by an extremely aggressive clinical course, with rapid development of lung metastases. In the last few years, targeting Src in the treatment of OS has become one of the major challenges in the development of new drugs, since an elevated Src kinase activity has been associated with the development and the maintenance of the OS malignant phenotype. Recently, SI-83, a novel pyrazolo[3,4-d]pyrimidine derivate Src inhibitor, was selected as a promising OS therapeutic drug because of its elevated anti-tumour effects toward human OS. In the present study, gel-based proteomics and phosphoproteomics revealed significant changes in proteins involved in many cancer related processes. We got insight into SI-83 proapoptotic and antiproliferative properties (overrepresentation of GRIA1, GRP78, and CALR and underrepresentation of NPM1, RCN, and P4HB). Nevertheless, the most significant findings of our work are the SI-83 induced dephosphorylation of ARPC5L, a subunit of the actin related Arp2/3 complex, and the decrease of other cytoskeleton proteins. These data, together with a dramatic impairment of SaOS-2 cell migration and adhesion, suggest that SI-83 may have antimetastatic features that enhance its use as a potent OS chemotherapeutic drug.

Proteomic analysis identifies an NADPH oxidase 1 (Nox1)-mediated role for actin-related protein 2/3 complex subunit 2 (ARPC2) in promoting smooth muscle cell migration

A variety of vascular pathologies, including hypertension, restenosis and atherosclerosis, are characterized by vascular smooth muscle cell (VSMC) hypertrophy and migration. NADPH oxidase 1 (Nox1) plays a pivotal role in these phenotypes via distinct downstream signaling. However, the mediators differentiating these distinct phenotypes and their precise role in vascular disease are still not clear. The present study was designed to identify novel targets of VSMC Nox1 signaling using 2D Differential In-Gel Electrophoresis and Mass Spectrometry (2D-DIGE/MS). VSMC treatment with scrambled (Scrmb) or Nox1 siRNA and incubation with the oxidant hydrogen peroxide (H₂O₂; 50 μM, 3 h) followed by 2D-DIGE/MS on cell lysates identified 10 target proteins. Among these proteins, actin-related protein 2/3 complex subunit 2 (ARPC2) with no previous link to Nox isozymes, H₂O₂, or other reactive oxygen species (ROS), was identified and postulated to play an intermediary role in VSMC migration. Western blot confirmed that Nox1 mediates H₂O₂-induced ARPC2 expression in VSMC. Treatment with a p38 MAPK inhibitor (SB203580) resulted in reduced ARPC2 expression in H₂O₂-treated VSMC. Additionally, wound-healing “scratch” assay confirmed that H₂O₂ stimulates VSMC migration via Nox1. Importantly, gene silencing of ARPC2 suppressed H₂O₂-stimulated VSMC migration. These results demonstrate for the first time that Nox1-mediated VSMC migration involves ARPC2 as a downstream signaling target.

WASH inhibits autophagy through suppression of Beclin 1 ubiquitination

Autophagy degrades cytoplasmic proteins and organelles to recycle cellular components that are required for cell survival and tissue homeostasis. However, it is not clear how autophagy is regulated in mammalian cells. WASH (Wiskott-Aldrich syndrome protein (WASP) and SCAR homologue) plays an essential role in endosomal sorting through facilitating tubule fission via Arp2/3 activation. Here, we demonstrate a novel function of WASH in modulation of autophagy. We show that WASH deficiency causes early embryonic lethality and extensive autophagy of mouse embryos. WASH inhibits vacuolar protein sorting (Vps)34 kinase activity and autophagy induction. We identified that WASH is a new interactor of Beclin 1. Beclin 1 is ubiquitinated at lysine 437 through lysine 63 linkage in cells undergoing autophagy. Ambra1 is an E3 ligase for lysine 63-linked ubiquitination of Beclin 1 that is required for starvation-induced autophagy. The lysine 437 ubiquitination of Beclin 1 enhances the association with Vps34 to promote Vps34 activity. WASH can suppress Beclin 1 ubiquitination to inactivate Vps34 activity leading to suppression of autophagy.

Phosphorylation of actin-related protein 2 (Arp2) is required for normal development and cAMP chemotaxis in Dictyostelium

Phosphorylation of the actin-related protein 2 (Arp2) subunit of the Arp2/3 complex on evolutionarily conserved threonine and tyrosine residues was recently identified and shown to be necessary for nucleating activity of the Arp2/3 complex and membrane protrusion of Drosophila cells. Here we use the Dictyostelium diploid system to replace the essential Arp2 protein with mutants that cannot be phosphorylated at Thr-235/6 and Tyr-200. We found that aggregation of the resulting mutant cells after starvation was substantially slowed with delayed early developmental gene expression and that chemotaxis toward a cAMP gradient was defective with loss of polarity and attenuated F-actin assembly. Chemotaxis toward cAMP was also diminished with reduced cell speed and directionality and shorter pseudopod lifetime when Arp2 phosphorylation mutant cells were allowed to develop longer to a responsive state similar to that of wild-type cells. However, clathrin-mediated endocytosis and chemotaxis under agar to folate in vegetative cells were only subtly affected in Arp2 phosphorylation mutants. Thus, phosphorylation of threonine and tyrosine is important for a subset of the functions of the Arp2/3 complex, in particular an unexpected major role in regulating development.

The Role of p38 MAPK and Its Substrates in Neuronal Plasticity and Neurodegenerative Disease

A significant amount of evidence suggests that the p38-mitogen-activated protein kinase (MAPK) signalling cascade plays a crucial role in synaptic plasticity and in neurodegenerative diseases. In this review we will discuss the cellular localisation and activation of p38 MAPK and the recent advances on the molecular and cellular mechanisms of its substrates: MAPKAPK 2 (MK2) and tau protein. In particular we will focus our attention on the understanding of the p38 MAPK-MK2 and p38 MAPK-tau activation axis in controlling neuroinflammation, actin remodelling and tau hyperphosphorylation, processes that are thought to be involved in normal ageing as well as in neurodegenerative diseases. We will also give some insight into how elucidating the precise role of p38 MAPK-MK2 and p38 MAPK-tau signalling cascades may help to identify novel therapeutic targets to slow down the symptoms observed in neurodegenerative diseases such as Alzheimer's and Parkinson's disease.

Role of β-catenin in post-meiotic male germ cell differentiation

Though roles of β-catenin signaling during testis development have been well established, relatively little is known about its role in postnatal testicular physiology. Even less is known about its role in post-meiotic germ cell development and differentiation. Here, we report that β-catenin is highly expressed in post-meiotic germ cells and plays an important role during spermiogenesis in mice. Spermatid-specific deletion of β-catenin resulted in significantly reduced sperm count, increased germ cell apoptosis and impaired fertility. In addition, ultrastructural studies show that the loss of β-catenin in post-meiotic germ cells led to acrosomal defects, anomalous release of immature spermatids and disruption of adherens junctions between Sertoli cells and elongating spermatids (apical ectoplasmic specialization; ES). These defects are likely due to altered expression of several genes reportedly involved in Sertoli cell-germ cell adhesion and germ cell differentiation, as revealed by gene expression analysis. Taken together, our results suggest that β-catenin is an important molecular link that integrates Sertoli cell-germ cell adhesion with the signaling events essential for post-meiotic germ cell development and maturation. Since β-catenin is also highly expressed in the Sertoli cells, we propose that binding of germ cell β-catenin complex to β-catenin complex on Sertoli cell at the apical ES surface triggers a signaling cascade that regulates post-meiotic germ cell differentiation.

Activation and function of the MAPKs and their substrates, the MAPK-activated protein kinases

The mitogen-activated protein kinases (MAPKs) regulate diverse cellular programs by relaying extracellular signals to intracellular responses. In mammals, there are more than a dozen MAPK enzymes that coordinately regulate cell proliferation, differentiation, motility, and survival. The best known are the conventional MAPKs, which include the extracellular signal-regulated kinases 1 and 2 (ERK1/2), c-Jun amino-terminal kinases 1 to 3 (JNK1 to -3), p38 (α, β, γ, and δ), and ERK5 families. There are additional, atypical MAPK enzymes, including ERK3/4, ERK7/8, and Nemo-like kinase (NLK), which have distinct regulation and functions. Together, the MAPKs regulate a large number of substrates, including members of a family of protein Ser/Thr kinases termed MAPK-activated protein kinases (MAPKAPKs). The MAPKAPKs are related enzymes that respond to extracellular stimulation through direct MAPK-dependent activation loop phosphorylation and kinase activation. There are five MAPKAPK subfamilies: the p90 ribosomal S6 kinase (RSK), the mitogen- and stress-activated kinase (MSK), the MAPK-interacting kinase (MNK), the MAPK-activated protein kinase 2/3 (MK2/3), and MK5 (also known as p38-regulated/activated protein kinase [PRAK]). These enzymes have diverse biological functions, including regulation of nucleosome and gene expression, mRNA stability and translation, and cell proliferation and survival. Here we review the mechanisms of MAPKAPK activation by the different MAPKs and discuss their physiological roles based on established substrates and recent discoveries.

Rear polarization of the microtubule-organizing center in neointimal smooth muscle cells depends on PKCα, ARPC5, and RHAMM

Directed migration of smooth muscle cells (SMCs) from the media to the intima in arteries occurs during atherosclerotic plaque formation and during restenosis after angioplasty or stent application. The polarized orientation of the microtubule-organizing center (MTOC) is a key determinant of this process, and we therefore investigated factors that regulate MTOC polarity in vascular SMCs. SMCs migrating in vivo from the medial to the intimal layer of the rat carotid artery following balloon catheter injury were rear polarized, with the MTOC located posterior of the nucleus. In tissue culture, migrating neointimal cells maintained rear polarization, whereas medial cells were front polarized. Using phosphoproteomic screening and mass spectrometry, we identified ARPC5 and RHAMM as protein kinase C (PKC)-phosphorylated proteins associated with rear polarization of the MTOC in neointimal SMCs. RNA silencing of ARPC5 and RHAMM, PKC inhibition, and transfection with a mutated nonphosphorylatable ARPC5 showed that these proteins regulate rear polarization by organizing the actin and microtubule cytoskeletons in neointimal SMCs. Both ARPC5 and RHAMM, in addition to PKC, were required for migration of neointimal SMCs.

Activation and Function of the MAPKs and Their Substrates, the MAPK-Activated Protein Kinases

The mitogen-activated protein kinases (MAPKs) regulate diverse cellular programs by relaying extracellular signals to intracellular responses. In mammals, there are more than a dozen MAPK enzymes that coordinately regulate cell proliferation, differentiation, motility, and survival. The best known are the conventional MAPKs, which include the extracellular signal-regulated kinases 1 and 2 (ERK1/2), c-Jun amino-terminal kinases 1 to 3 (JNK1 to -3), p38 (α, β, γ, and δ), and ERK5 families. There are additional, atypical MAPK enzymes, including ERK3/4, ERK7/8, and Nemo-like kinase (NLK), which have distinct regulation and functions. Together, the MAPKs regulate a large number of substrates, including members of a family of protein Ser/Thr kinases termed MAPK-activated protein kinases (MAPKAPKs). The MAPKAPKs are related enzymes that respond to extracellular stimulation through direct MAPK-dependent activation loop phosphorylation and kinase activation. There are five MAPKAPK subfamilies: the p90 ribosomal S6 kinase (RSK), the mitogen- and stress-activated kinase (MSK), the MAPK-interacting kinase (MNK), the MAPK-activated protein kinase 2/3 (MK2/3), and MK5 (also known as p38-regulated/activated protein kinase [PRAK]). These enzymes have diverse biological functions, including regulation of nucleosome and gene expression, mRNA stability and translation, and cell proliferation and survival. Here we review the mechanisms of MAPKAPK activation by the different MAPKs and discuss their physiological roles based on established substrates and recent discoveries.

The p38/MK2/Hsp25 pathway is required for BMP-2-induced cell migration

Background

Bone morphogenetic proteins (BMPs) have been shown to participate in the patterning and specification of several tissues and organs during development and to regulate cell growth, differentiation and migration in different cell types. BMP-mediated cell migration requires activation of the small GTPase Cdc42 and LIMK1 activities. In our earlier report we showed that activation of LIMK1 also requires the activation of PAKs through Cdc42 and PI3K. However, the requirement of additional signaling is not clearly known.

Methodology/Principal Findings

Activation of p38 MAPK has been shown to be relevant for a number of BMP-2′s physiological effects. We report here that BMP-2 regulation of cell migration and actin cytoskeleton remodelling are dependent on p38 activity. BMP-2 treatment of mesenchymal cells results in activation of the p38/MK2/Hsp25 signaling pathway downstream from the BMP receptors. Moreover, chemical inhibition of p38 signaling or genetic ablation of either p38α or MK2 blocks the ability to activate the downstream effectors of the pathway and abolishes BMP-2-induction of cell migration. These signaling effects on p38/MK2/Hsp25 do not require the activity of either Cdc42 or PAK, whereas p38/MK2 activities do not significantly modify the BMP-2-dependent activation of LIMK1, measured by either kinase activity or with an antibody raised against phospho-threonine 508 at its activation loop. Finally, phosphorylated Hsp25 colocalizes with the BMP receptor complexes in lamellipodia and overexpression of a phosphorylation mutant form of Hsp25 is able to abolish the migration of cells in response to BMP-2.

Conclusions

These results indicate that Cdc42/PAK/LIMK1 and p38/MK2/Hsp25 pathways, acting in parallel and modulating specific actin regulatory proteins, play a critical role in integrating responses during BMP-induced actin reorganization and cell migration.

Treatment of cultured human astrocytes and vascular endothelial cells with protein kinase CK2 inhibitors induces early changes in cell shape and cytoskeleton

Ubiquitous protein kinase CK2 is a key regulator of cell migration, proliferation and tumor growth. CK2 is abundant in retinal astrocytes, and its inhibition suppresses retinal neovascularization in a mouse retinopathy model. In human astrocytes, CK2 co-distributes with GFAP-containing intermediate filaments, which implies its association with cytoskeleton. Contrary to astrocytes, CK2 is co-localized in microvascular endothelial cells (HBMVEC) with microtubules and actin stress fibers, but not with vimentin-containing intermediate filaments. Specific CK2 inhibitors (TBB, TBI, TBCA and DMAT) and nine novel CK2 inhibiting compounds (TID43, TID46, Quinolone-7, Quinolone-39, FNH28, FNH62, FNH64, FNH68 and FNH74) were tested at 10-200 μM for their ability to induce morphological alterations in cultured human astrocytes (HAST-40), and HBMVEC (For explanation of the inhibitor names, see "Methods" section). CK2 inhibitors caused dramatic changes in shape of cultured cells with effective inhibitor concentrations between 50 and 100 μM. Attached cells retracted, acquired shortened processes, and eventually rounded up and detached. CK2 inhibitor-induced morphological alterations were completely reversible and were not blocked by caspase inhibition. However, longer treatment or higher inhibitor concentration did cause apoptosis. The speed and potency of the CK2 inhibitors effects on cell shape and adhesion were inversely correlated with serum concentration. Western analyses showed that TBB and TBCA elicited a significant (about twofold) increase in the activation of p38 and ERK1/2 MAP kinases that may be involved in cytoskeleton regulation. This novel early biological cell response to CK2 inhibition may underlie the anti-angiogenic effect of CK2 suppression in the retina.

Accelerators, Brakes, and Gears of Actin Dynamics in Dendritic Spines

Dendritic spines are actin-rich structures that accommodate the postsynaptic sites of most excitatory synapses in the brain. Although dendritic spines form and mature as synaptic connections develop, they remain plastic even in the adult brain, where they can rapidly grow, change, or collapse in response to normal physiological changes in synaptic activity that underlie learning and memory. Pathological stimuli can adversely affect dendritic spine shape and number, and this is seen in neurodegenerative disorders and some forms of mental retardation and autism as well. Many of the molecular signals that control these changes in dendritic spines act through the regulation of filamentous actin (F-actin), some through direct interaction with actin, and others via downstream effectors. For example, cortactin, cofilin, and gelsolin are actin-binding proteins that directly regulate actin dynamics in dendritic spines. Activities of these proteins are precisely regulated by intracellular signaling events that control their phosphorylation state and localization. In this review, we discuss how actin-regulating proteins maintain the balance between F-actin assembly and disassembly that is needed to stabilize mature dendritic spines, and how changes in their activities may lead to rapid remodeling of dendritic spines.

Differential proteome analysis of the preeclamptic placenta using optimized protein extraction

The human placenta is a difficult tissue to work with using proteomic technology since it contains large amounts of lipids and glycogen. Both lipids and glycogen are known to interfere with the first step in the two-dimensional polyacrylamide gel electrophoresis (2D-PAGE), the isoelectric focusing. In order to gain the best possible protein separation on 2D-PAGE, an optimized sample preparation protocol for placental proteins was developed. Two different buffers, urea/CHAPS and Hepes, were used for solubilization in combination with six different precipitation methods. The removal of glycogen from the samples by centrifugation was crucial for the final proteome maps. Solubilization with urea/CHAPS in combination with dichloromethane/methanol or acidified acetone proved to be the best precipitation procedures. When applied to clinical placenta samples apolipoprotein A1 was found to be accumulated in the preeclamptic placenta, where it may either have a nutritional effect or act as a modifier of signal transduction.

Application of proteomics to neutrophil biology

Polymorphonuclear leukocytes or neutrophils are a primary effector cell of the innate immune system and contribute to the development of adaptive immunity. Neutrophils participate in both the initiation and resolution of inflammatory responses through a series of highly coordinated molecular and phenotypic changes. To accomplish these changes, neutrophils express numerous receptors and use multiple overlapping and redundant signal transduction pathways. Dysregulation of the activation or resolution pathways plays a role in a number of human diseases. A comprehensive understanding of the regulation of neutrophil responses can be provided by high throughput proteomic technologies and sophisticated computational analysis. The first steps in the application of proteomics to understanding neutrophil biology have been taken. Here we review the application of expression, structural, and functional proteomic studies to neutrophils. Although defining the complex molecular events associated with neutrophil activation is in the early stages, the data generated to date suggest that proteomic technologies will dramatically enhance our understanding of neutrophil biology.

Sample prep for proteomics of breast cancer: proteomics and gene ontology reveal dramatic differences in protein solubilization preferences of radioimmunoprecipitation assay and urea lysis buffers

BACKGROUND

An important step in the proteomics of solid tumors, including breast cancer, consists of efficiently extracting most of proteins in the tumor specimen. For this purpose, Radio-Immunoprecipitation Assay (RIPA) buffer is widely employed. RIPA buffer's rapid and highly efficient cell lysis and good solubilization of a wide range of proteins is further augmented by its compatibility with protease and phosphatase inhibitors, ability to minimize non-specific protein binding leading to a lower background in immunoprecipitation, and its suitability for protein quantitation.

RESULTS

In this work, the insoluble matter left after RIPA buffer extraction of proteins from breast tumors are subjected to another extraction step, using a urea-based buffer. It is shown that RIPA and urea lysis buffers fractionate breast tissue proteins primarily on the basis of molecular weights. The average molecular weight of proteins that dissolve exclusively in urea buffer is up to 60% higher than in RIPA.Gene Ontology (GO) and Directed Acyclic Graphs (DAG) are used to map the collective biological and biophysical attributes of the RIPA and urea proteomes. The Cellular Component and Molecular Function annotations reveal protein solubilization preferences of the buffers, especially the compartmentalization and functional distributions.It is shown that nearly all extracellular matrix proteins (ECM) in the breast tumors and matched normal tissues are found, nearly exclusively, in the urea fraction, while they are mostly insoluble in RIPA buffer. Additionally, it is demonstrated that cytoskeletal and extracellular region proteins are more soluble in urea than in RIPA, whereas for nuclear, cytoplasmic and mitochondrial proteins, RIPA buffer is preferred.Extracellular matrix proteins are highly implicated in cancer, including their proteinase-mediated degradation and remodelling, tumor development, progression, adhesion and metastasis. Thus, if they are not efficiently extracted by RIPA buffer, important information may be missed in cancer research.

CONCLUSION

For proteomics of solid tumors, a two-step extraction process is recommended. First, proteins in the tumor specimen should be extracted with RIPA buffer. Second, the RIPA-insoluble material should be extracted with the urea-based buffer employed in this work.

Phosphorylation of the Arp2/3 complex is necessary to nucleate actin filaments

The actin-related protein 2/3 (Arp2/3) complex is the primary nucleator of new actin filaments in most crawling cells. Nucleation-promoting factors (NPFs) of the Wiskott-Aldrich syndrome protein (WASP)/Scar family are the currently recognized activators of the Arp2/3 complex. We now report that the Arp2/3 complex must be phosphorylated on either threonine or tyrosine residues to be activated by NPFs. Phosphorylation of the Arp2/3 complex is not necessary to bind NPFs or the sides of actin filaments but is critical for binding the pointed end of actin filaments and nucleating actin filaments. Mass spectrometry revealed phosphorylated Thr237 and Thr238 in Arp2, which are evolutionarily conserved residues. In cells, phosphorylation of only the Arp2 subunit increases in response to growth factors, and alanine substitutions of Arp2 T237 and T238 or Y202 inhibits membrane protrusion. These findings reveal an additional level of regulation of actin filament assembly independent of WASP proteins, and show that phosphorylation of the Arp2/3 complex provides a logical "or gate" capable integrating diverse upstream signals.

The wheat germ cell-free based screening of protein substrates of calcium/calmodulin-dependent protein kinase II delta

Calcium/calmodulin-dependent protein kinase II (CaMKII) plays a crucial role in mediating calcium signaling. Here, we demonstrate a method for screening substrates phosphorylated by human CaMKII delta using a wheat cell-free system. The cell-free mixture expressing CaMKII delta was incubated with HeLa extracts and radiolabeled ATP. From analysis of two-dimensional electrophoresis gels and mass spectrometry, two proteins were found. The cell-free based in vitro kinase assay revealed that CaMKII delta phosphorylates eukaryotic translation initiation factor 4B and stress-induced phosphoprotein 1 (STIP1), the latter on Ser189. Furthermore, constitutively-active CaMKII delta phosphorylated STIP1 in HeLa cells and dramatically promoted nuclear localization of STIP1, suggesting that calcium signals via CaMKII delta may regulate subcellular localization of STIP1. This approach may be a useful tool for target screening of protein kinases.

Involvement of p38 mitogen-activated protein kinase in the regulation of platelet-derived growth factor -induced cell migration

The aim of this study was to investigate the role of p38 mitogen-activated protein kinase (MAPK) in cell migration induced by platelet-derived growth factor (PDGF). Western blot was performed to detect the phosphorylation of p38 in NIH3T3 cells treated with PDGF. A Transwell cell migration system was used to determine the effects of PDGF treatment on the migration of NIH3T3 cells and the influence of p38 deficiency on this process in a p38 gene knockout (p38(-/-)) mouse embryonic fibroblast cell line. On the stimulation of PDGF, the migration of NIH3T3 cells was significantly increased (P < 0.001) compared to the control and p38 MAP kinase was simultaneously phosphorylated. Furthermore, the PDGF-induced cell migration was significantly blocked in p38 gene knockout (p38(-/-)) mouse embryonic fibroblasts (MEFs) (P < 0.001) as compared with the wild type cells (p38(+/+)). p38 MAPK plays an important role in the regulation of cell migration induced by PDGF.

Proteomics of Blood-Based Therapeutics

Blood-based therapeutics are cellular or plasma components derived from human blood. Their production requires appropriate selection and treatment of the donor and processing of cells or plasma proteins. In contrast to clearly defined, chemically synthesized drugs, blood-derived therapeutics are highly complex mixtures of plasma proteins or even more complex cells. Pathogen transmission by the product as well as changes in the integrity of blood constituents resulting in loss of function or immune modulation are currently important issues in transfusion medicine. Protein modifications can occur during various steps of the production process, such as acquisition, enrichment of separate components (e.g. coagulation factors, cell populations), virus inactivation, conservation, and storage. Contemporary proteomic strategies allow a comprehensive assessment of protein modifications with high coverage, offer capabilities for qualitative and even quantitative analysis, and for high-throughput protein identification. Traditionally, proteomics approaches predominantly relied on two-dimensional gel electrophoresis (2-DE). Even if 2-DE is still state of the art, it has inherent limitations that are mainly based on the physicochemical properties of the proteins analyzed; for example, proteins with extremes in molecular mass and hydrophobicity (most membrane proteins) are difficult to assess by 2-DE. These limitations have fostered the development of mass spectrometry centered on non-gel-based separation approaches, which have proven to be highly successful and are thus complementing and even partially replacing 2-DE-based approaches. Although blood constituents have been extensively analyzed by proteomics, this technology has not been widely applied to assess or even improve blood-derived therapeutics, or to monitor the production processes. As proteomic technologies have the capacity to provide comprehensive information about changes occurring during processing and storage of blood products, proteomics can potentially guide improvement of pathogen inactivation procedures and engineering of stem cells, and may also allow a better understanding of factors influencing the immunogenicity of blood-derived therapeutics. An important development in proteomics is the reduction of inter-assay variability. This now allows the screening of samples taken from the same product over time or before and after processing. Optimized preparation procedures and storage conditions will reduce the risk of protein alterations, which in turn may contribute to better recovery, reduced exposure to allogeneic proteins, and increased transfusion safety.

Active stress kinases in proliferating endothelial cells associated with cytoskeletal structures

It has become increasingly clear that stress-activated protein kinases have cytoplasmic substrates in addition to well-established transcription factor substrates in cell nuclei. The present study documented specific cytoplasmic locations of these enzymes in proliferating vascular cells. Immunofluorescent staining for active c-jun NH2-terminal kinase (JNK), the precipitation of JNK with microfilaments, and the loss of fiber-associated active JNK after cytochalasin treatment, but not nocodazole treatment, together indicate that active JNK is associated with stress fibers. The lack of complete scaffold colocalization and the total lack of immediate upsteam kinase colocalization along with the inability of JNK inhibitors to alter JNK-microfilament associations suggest that the microfilament association is not simply involved in enzyme activation. In addition, active p38 was found along with vinculin in focal adhesions. Although the p38 in focal adhesions could also be disrupted by cytochalasin treatment, it remained stable after nocodazole treatment. These results support the hypothesis that vascular cell stress kinase enzymes are important for signal transduction in the cytoplasm. The localization of active stress-activated protein kinases to specific cytoskeletal structures in proliferating cells suggests that subsets of these enzymes are involved in signal transduction to and/or from the cytoskeleton under conditions that include vascular cell proliferation.

Nitration of p38 MAPK in the placenta: association of nitration with reduced catalytic activity of p38 MAPK in pre-eclampsia

Peroxynitrite, a potent pro-oxidant formed from the interaction of superoxide and nitric oxide, has been widely reported to be nitrating tyrosine residues in proteins resulting in the formation of nitrotyrosine. Biological nitration of tyrosine, a footprint of oxidative injury, has been found to occur in various pathological states including pre-eclampsia, a leading cause of maternal mortality and increased perinatal mortality. Oxidative stress is a major contributor to endothelial dysfunction in pre-eclampsia. Previously, we have demonstrated increased nitrotyrosine immunostaining in placental villous vascular endothelium, surrounding vascular smooth muscle and villous stroma from pre-eclamptic or diabetic pregnancies. Immunoprecipitation (IP) with antinitrotyrosine antibodies followed by immunoblot analysis identified increased nitration of phospho-p38 mitogen-activated protein kinase (MAPK) in the pre-eclamptic placenta. The catalytic activity of p38 MAPK and concentration of phospho-p38 MAPK was also found to be reduced in placentae from pre-eclamptic pregnancies. Comparison of peptide masses of a 42-kDa protein obtained by mass spectrometry with masses of a theoretical tryptic digest of p38 MAPK that was modified by phosphorylation and nitration identified the protein to be p38 MAPK.

Analysis of Properties of Small Heat Shock Protein Hsp25 in MAPK-activated Protein Kinase 2 (MK2)-deficient Cells

Small heat shock proteins (sHsps) exist in dynamic oligomeric complexes and display diverse biological functions ranging from chaperone properties to modulator of apoptosis. So far, the role of stress-dependent phosphorylation of mammalian sHsps for its structure and function has been analyzed by using various phosphorylation site mutants overexpressed in different cell types as well as by non-exclusive inhibitors of the p38 MAPK cascade. Here we investigate the role of phosphorylation of endogenous sHsp in a genetic model lacking the major Hsp25 kinase, the MAP kinase-activated protein kinase MK2. We demonstrate that in MK2-deficient fibroblasts, where no stress-dependent phosphorylation of Hsp25 at Ser86 and no in vitro binding to 14-3-3 was detectable, stress-dependent disaggregation of endogenous Hsp25 complexes is impared and kinetics of arsenite-dependent, H2O2-dependent, and sublethal heat shock-induced insolubilization of Hsp25 is delayed. Similarly, green fluorescent protein-tagged Hsp25 shows retarded subcellular accumulation into stress granules in MK2-deficient cells after arsenite treatment. Decreased insolubilization of Hsp25 in MK2-deficient cells correlates with increased resistance against arsenite, H2O2, and sublethal heat shock treatment and with decreased apoptosis. In contrast, after severe, lethal heat shock MK2-deficient embryonic fibroblasts cells show fast and complete insolubilization of Hsp25 independent of MK2 and no increased stress resistance. Hence, MK2-dependent formation of insoluble stress granules and irreversible cell damage by oxidative stresses and sublethal heat shock correlate and only upon severe, lethal heat shock MK2-independent processes could determine insolubilization of Hsp25 and are more relevant for cellular stress damage.

A proteomic screen identified stress-induced chaperone proteins as targets of Akt phosphorylation in mesangial cells

The serine-threonine kinase Akt regulates mesangial cell apoptosis, proliferation, and hypertrophy. To define Akt signaling pathways in mesangial cells, we performed a functional proteomic screen for rat mesangial cell proteins phosphorylated by Akt. A group of chaperone proteins, heat shock protein (Hsp) 70, Hsp90alpha, Hsp90beta, Glucose-regulated protein (Grp) Grp78, Grp94, and protein disulfide isomerase (PDI) were identified as potential Akt substrates by two techniques: (a) in vitro phosphorylation of mesangial cell lysate by recombinant active Akt followed by protein separation by SDS-PAGE or 2-DE and phosphoprotein identification by peptide mass fingerprinting using MALDI-MS, or (b) immunoblot analysis of proteins from PDGF-stimulated mesangial cells using an anti-Akt phospho-motif antibody. In vitro kinase reactions using recombinant proteins confirmed that Akt phosphorylates Hsp70, Hsp90alpha and beta, Grp94, and PDI. Immunoprecipitation of Akt from mesangial cell lysate coprecipitated Grp78 and Hsp70. PDGF stimulation of mesangial cells caused an acidic shift in the isoelectric point of Hsp70, Hsp90, and PDI that was dependent on PI-3K activity for Hsp70 and Hsp90. The data suggest that Akt-mediated phosphorylation of stress-induced chaperones represents a mechanism for regulation of chaperone function during mesangial cell responses to physiologic and pathologic stimuli.

MAPKAP kinases - MKs - two's company, three's a crowd

Downstream of mitogen-activated protein kinases (MAPKs), three structurally related MAPK-activated protein kinases (MAPKAPKs or MKs) - MK2, MK3 and MK5 - signal to diverse cellular targets. Although there is no known common function for all three MKs, these kinases are involved in important processes: MKs regulate gene expression at the transcriptional and post-transcriptional level, control cytoskeletal architecture and cell-cycle progression, and are implicated in inflammation and cancer.

Defining mitogen-activated protein kinase pathways with mass spectrometry-based approaches

Mitogen-activated protein kinases are a group of ubiquitously expressed kinase pathways that have been conserved from yeast through humans. They control a large number of critical cell functions. Identification of targets of those kinases is necessary to define signal transduction pathways that lead to cell responses. The application of a number of mass spectrometry-based techniques to the identification of phosphoproteins is reviewed. A new proteomic approach is described for the identification of the downstream targets of specific kinases that combines phosphorylation of cell lysates in in vitro kinase reactions by active recombinant kinase with protein separation by two-dimensional (2D) gel electrophoresis or SDS-PAGE and phosphoprotein identification by MALDI-TOF mass spectrometry or by phosphopeptide enrichment and tandem mass spectrometry. The results suggested that a combination of multiple approaches will be required to fully identify phosphoproteomes.

MAP kinases and cell migration

Recent studies have demonstrated that mitogen-activated protein kinases (MAPKs), including Jun N-terminus kinase (JNK), p38 and Erk, play crucial roles in cell migration. JNK, for example, regulates cell migration by phosphorylating paxillin, DCX, Jun and microtubule-associated proteins. Studies of p38 show that this MAPK modulates migration by phosphorylating MAPK-activated protein kinase 2/3 (MAPKAP 2/3), which appears to be important for directionality of migration. Erk governs cell movement by phosphorylating myosin light chain kinase (MLCK), calpain or FAK. Thus, the different kinases in the MAPK family all seem able to regulate cell migration but by distinct mechanisms.

Recent advances in blood-related proteomics

Blood is divided in two compartments, namely, plasma and cells. The latter contain red blood cells, leukocytes, and platelets. From a descriptive medical discipline, hematology has evolved towards a pioneering discipline where molecular biology has permitted the development of prognostic and diagnostic indicators for disease. The recent advance in MS and protein separation now allows similar progress in the analysis of proteins. Proteomics offers great promise for the study of proteins in plasma/serum, indeed a number of proteomics databases for plasma/serum have been established. This is a very complex body fluid containing lipids, carbohydrates, amino acids, vitamins, nucleic acids, hormones, and proteins. About 1500 different proteins have recently been identified, and a number of potential new markers of diseases have been characterized. Here, examples of the enormous promise of plasma/serum proteomic analysis for diagnostic/prognostic markers and information on disease mechanism are given. Within the blood are also a large number of different blood cell types that potentially hold similar information. Proteomics of red blood cells, until now, has not improved our knowledge of these cells, in contrast to the major progresses achieved while studying platelets and leukocytes. In the future, proteomics will change several aspects of hematology.

Proteomic Analysis of Human Neutrophil Granules

Stimulated exocytosis of intracellular granules plays a critical role in conversion of inactive, circulating neutrophils to fully activated cells capable of chemotaxis, phagocytosis, and bacterial killing. The functional changes induced by exocytosis of each of the granule subsets, gelatinase (tertiary) granules, specific (secondary) granules, and azurophil (primary) granules, are poorly defined. To improve the understanding of the role of exocytosis of these granule subsets, a proteomic analysis of the azurophil, specific, and gelatinase granules from human neutrophils was performed. Two different methods for granule protein identification were applied. First, two-dimensional (2D) gel electrophoresis followed by MALDI-TOF MS analysis of peptides obtained by in-gel trypsin digestion of proteins was performed. Second, peptides from tryptic digests of granule membrane proteins were separated by two-dimensional microcapillary chromatography using strong cation exchange and reverse phase microcapillary high pressure liquid chromatography and analyzed with electrospray ionization tandem mass spectrometry (2D HLPC ESI-MS/MS). Our analysis identified 286 proteins on the three granule subsets, 87 of which were identified by MALDI MS and 247 were identified by 2D HPLC ESI-MS/MS. The increased sensitivity of 2D HPLC ESI-MS/MS, however, resulted in identification of over 500 proteins from subcellular organelles contaminating isolated granules. Defining the proteome of neutrophil granule subsets provides a basis for understanding the role of exocytosis in neutrophil biology. Additionally, the described methods may be applied to mobilizable compartments of other secretory cells.

Akt-mediated valosin-containing protein 97 phosphorylation regulates its association with ubiquitinated proteins

Hypoxia is a common environmental stress that influences signaling pathways and cell function. Previous studies from our laboratory have identified significant differences in cellular responses to sustained or intermittent hypoxia with the latter proving more cytotoxic. We hypothesized that differences in susceptibility of neurons to intermittent (IH) and sustained hypoxia (SH) are mediated by altered Akt signaling. SH, but not IH, induced a significant increase in Akt activation in rat CA1 hippocampal region extracts compared with room air controls. Akt immunoprecipitations followed by proteomic analysis identified valosin-containing protein (VCP) as an Akt-binding protein. In addition, VCP expression and association with Akt was enhanced during SH, and this association was decreased upon phosphoinositide 3-kinase/Akt pathway blockade with LY294002. Active recombinant Akt phosphorylated recombinant VCP in vitro. Site-directed mutagenesis studies identified Ser352, Ser746, and Ser748 as Akt phosphorylation sites on VCP. In addition, rat CA1 hippocampal tissue exposed to SH exhibited an acidic pI shift of VCP. Protein phosphatase 2A treatment inhibited this acidic shift consistent with SH-induced phosphorylation of VCP in vivo. PC-12 cells transfected with active Akt, but not dominant negative Akt or vector, induced VCP expression and an acidic shift in VCP pI, which was inhibited by protein phosphatase 2A treatment. Furthermore, VCP association with ubiquitinated proteins was demonstrated in vector-transfected PC-12 cell lysates, whereas active Akt-transfected cells demonstrated a marked decrease in association of VCP with ubiquitinated proteins. We concluded that Akt phosphorylates VCP in vitro and in vivo, and VCP phosphorylation releases it from ubiquitinated substrate protein(s) possibly allowing ubiquitinated protein(s) to be degraded by the proteosome.

Myeloid-Related Protein-14 Is a p38 MAPK Substrate in Human Neutrophils

The targets of the p38 MAPK pathway that mediate neutrophil functional responses are largely unknown. To identify p38 MAPK targets, a proteomic approach was applied in which recombinant active p38 MAPK and [(32)P]ATP were added to lysates from unstimulated human neutrophils. Proteins were separated by two-dimensional gel electrophoresis, and phosphoproteins were visualized by autoradiography and identified by MALDI-TOF. Myeloid-related protein-14 (MRP-14) was identified as a candidate p38 MAPK substrate. MRP-14 phosphorylation by p38 MAPK was confirmed by an in vitro kinase reaction using purified MRP-14/MRP-8 complexes. The site of MRP-14 phosphorylation by p38 MAPK was identified by tandem mass spectrometry and site-directed mutagenesis to be Thr(113). MRP-14 phosphorylation by p38 MAPK in intact neutrophils was confirmed by [(32)P]orthophosphate loading, followed by fMLP stimulation in the presence and absence of a p38 MAPK inhibitor, SB203580. Confocal microscopy of Triton X-100 permeabilized neutrophils showed that a small amount of MRP-14 was associated with cortical F-actin in unstimulated cells. fMLP stimulation resulted in a p38 MAPK-dependent increase in MRP-14 staining at the base of lamellipodia. By immunoblot analysis, MRP-14 was present in plasma membrane/secretory vesicle fractions and gelatinase and specific granules, but not in azurophil granules. The amount of MRP-14 associated with plasma membrane/secretory vesicle and gelatinase granule fractions increased after fMLP stimulation in a p38 MAPK-dependent manner. Direct phosphorylation of the MRP-14/MRP-8 complex by p38 MAPK increased actin binding in vitro by 2-fold. These results indicate that MRP-14 is a potential mediator of p38 MAPK-dependent functional responses in human neutrophils.

Methodologic advancements in the study of airway smooth muscle

The study of isolated airway myocytes has provided important information relative to specific processes that regulate contraction, proliferation, and synthetic properties of airway smooth muscle (ASM). To place this information in physiological context, however, improved methods to examine airway biology in vivo are needed. Advances in genetic, biochemical, and optical methods provide unprecedented opportunities to improve our understanding of in vivo physiology and pathophysiology. This article describes 4 important methodologic advances in the study of ASM: (1) the development of transgenic mice that could be used to investigate ASM proliferation and phenotype switching during the development of hypersensitivity, and to investigate excitation-contraction coupling; (2) the use of CD38-deficient mice to confirm the role of CD38-dependent, cyclic adenosine diphosphate-ribose-mediated calcium release in airway responsiveness; (3) investigation of the role of actin filament length and p38 mitogen-activated protein kinase activity in regulating the mechanical plasticity-elasticity balance in contracted ASM; and (d) the use of bronchial biopsies to study ASM structure and phenotype in respiratory science.

Proteomic identification of Bcl2-associated athanogene 2 as a novel MAPK-activated protein kinase 2 substrate

The p38 MAPK cascade is activated by various stresses or cytokines. Downstream of p38 MAPKs, there are diversification and extensive branching of signaling pathways. Fluorescent two-dimensional difference gel electrophoresis of phosphoprotein-enriched samples from HeLa cells in which p38 MAPK activity was either suppressed or activated enabled us to detect approximately 90 candidate spots for factors involved in p38-dependent pathways. Among these candidates, here we identified four proteins including Bcl-2-associated athanogene 2 (BAG2) by peptide mass fingerprintings. BAG family proteins are highly conserved throughout eukaryotes and regulate Hsc/Hsp70-mediated molecular chaperone activities and apoptosis. The results of two-dimensional immunoblots suggested that the phosphorylation of BAG2 was specifically controlled in a p38 MAPK-dependent manner. Furthermore, BAG2 was directly phosphorylated at serine 20 in vitro by MAPK-activated protein kinase 2 (MAPKAP kinase 2), which is known as a primary substrate of p38 MAPK and mediates several p38 MAPK-dependent processes. We confirmed that MAPKAP kinase 2 is also required for phosphorylation of BAG2 in vivo. Thus, p38 MAPK-MAPKAP kinase 2-BAG2 phosphorylation cascade may be a novel signaling pathway for response to extracellular stresses.

ERK and p38 MAPK-activated protein kinases: a family of protein kinases with diverse biological functions

Conserved signaling pathways that activate the mitogen-activated protein kinases (MAPKs) are involved in relaying extracellular stimulations to intracellular responses. The MAPKs coordinately regulate cell proliferation, differentiation, motility, and survival, which are functions also known to be mediated by members of a growing family of MAPK-activated protein kinases (MKs; formerly known as MAPKAP kinases). The MKs are related serine/threonine kinases that respond to mitogenic and stress stimuli through proline-directed phosphorylation and activation of the kinase domain by extracellular signal-regulated kinases 1 and 2 and p38 MAPKs. There are currently 11 vertebrate MKs in five subfamilies based on primary sequence homology: the ribosomal S6 kinases, the mitogen- and stress-activated kinases, the MAPK-interacting kinases, MAPK-activated protein kinases 2 and 3, and MK5. In the last 5 years, several MK substrates have been identified, which has helped tremendously to identify the biological role of the members of this family. Together with data from the study of MK-knockout mice, the identities of the MK substrates indicate that they play important roles in diverse biological processes, including mRNA translation, cell proliferation and survival, and the nuclear genomic response to mitogens and cellular stresses. In this article, we review the existing data on the MKs and discuss their physiological functions based on recent discoveries.

MAPKAP kinase 2 phosphorylates tristetraprolin on in vivo sites including Ser178, a site required for 14-3-3 binding

MAPKAP kinase 2 (MK2) is required for tumor necrosis factor synthesis. Tristetraprolin (TTP) binds to the 3'-untranslated region of tumor necrosis factor mRNA and regulates its fate. We identified in vitro and in vivo phosphorylation sites in TTP using nanoflow high pressure liquid chromatography microelectrospray ionization tandem mass spectrometry and novel methods for direct digestion of TTP bound to affinity matrices (GSH-beads or anti-Myc linked to magnetic beads). MK2Delta3B, activated in Escherichia coli by p38alpha, phosphorylates TTP in vitro at major sites Ser(52) and Ser(178) (>10-fold in abundance) as well as at several minor sites that were detected after enriching for phosphopeptides with immobilized metal affinity chromatography. MK2 phosphorylation of TTP creates a functional 14-3-3 binding site. In cells, TTP was phosphorylated at Ser(52), Ser(178), Thr(250), and Ser(316) and at SP sites in a cluster (Ser(80)/Ser(82)/Ser(85)). Anisomycin treatment of NIH 3T3 cells increased phosphorylation of Ser(52) and Ser(178). Overexpression of MK2 sufficed to increase phosphorylation of Ser(52) and Ser(178) but not Ser(80)/Ser(82)/Ser(85) or Thr(250). Thus, Ser(52) and Ser(178) are putative MK2 sites in vivo. Identified phosphosite(s) may be biologic switches controlling mRNA stability and translation.

Arp2/3 and 'the shape of things to come'

The shape of plant cells is reliant upon an organised actin network. A series of recent publications have emphasised the central role played by the actin-nucleating Arp2/3 complex in plant growth. In animals and fungi, the Arp2/3 complex influences cell motility and morphogenesis through its ability to nucleate actin filaments locally in response to cell signalling pathways. There are potential parallels in the action and control of the Arp2/3 complex in plants and animals.