Vimentin binding to phosphorylated Erk sterically hinders enzymatic dephosphorylation of the kinase

Signalling over a distance: gradient patterns and phosphorylation waves within single cells

Recent discoveries of phosphorylation gradients and microdomains with different protein activities have revolutionized our perception of information transfer within single cells. The different spatial localization of opposing reactions in protein-modification cycles has been shown to bring about heterogeneous stationary patterns and travelling waves of protein activities. We review spatial patterns and modes of signal transfer through phosphorylation/dephosphorylation and GDP/GTP exchange cycles and cascades. We show how switches between low-activity and high-activity states in a bistable activation–deactivation cycle can initiate the propagation of travelling protein-modification waves in the cytoplasm. Typically, an activation wave is initiated at the plasma membrane and propagates through the cytoplasm until it reaches the nucleus. An increase in deactivator activity is followed by the initiation of an inactivation wave that moves in the reverse direction from the nucleus. We show that the ratio of opposing enzyme rates is a key parameter that controls both the spread of activation through cascades and travelling waves.

Anaplasma phagocytophilum AptA modulates Erk1/2 signalling

Anaplasma phagocytophilum causes human granulocytic anaplasmosis, one of the most common tick-borne diseases in North America. This unusual obligate intracellular pathogen selectively persists within polymorphonuclear leucocytes. In this study, using the yeast surrogate model we identified an A. phagocytophilum virulence protein, AptA (A. phagocytophilum toxin A), that activates mammalian Erk1/2 mitogen-activated protein kinase. This activation is important for A. phagocytophilum survival within human neutrophils. AptA interacts with the intermediate filament protein vimentin, which is essential for A. phagocytophilum-induced Erk1/2 activation and infection. A. phagocytophilum infection reorganizes vimentin around the bacterial inclusion, thereby contributing to intracellular survival. These observations reveal a major role for the bacterial protein, AptA, and the host protein, vimentin, in the activation of Erk1/2 during A. phagocytophilum infection.

Signaling to transcription networks in the neuronal retrograde injury response

Retrograde signaling from axon to soma activates intrinsic regeneration mechanisms in lesioned peripheral sensory neurons; however, the links between axonal injury signaling and the cell body response are not well understood. Here, we used phosphoproteomics and microarrays to implicate approximately 900 phosphoproteins in retrograde injury signaling in rat sciatic nerve axons in vivo and approximately 4500 transcripts in the in vivo response to injury in the dorsal root ganglia. Computational analyses of these data sets identified approximately 400 redundant axonal signaling networks connected to 39 transcription factors implicated in the sensory neuron response to axonal injury. Experimental perturbation of individual overrepresented signaling hub proteins, including Abl, AKT, p38, and protein kinase C, affected neurite outgrowth in sensory neurons. Paradoxically, however, combined perturbation of Abl together with other hub proteins had a reduced effect relative to perturbation of individual proteins. Our data indicate that nerve injury responses are controlled by multiple regulatory components, and suggest that network redundancies provide robustness to the injury response.

Placental growth factor (PlGF) enhances breast cancer cell motility by mobilising ERK1/2 phosphorylation and cytoskeletal rearrangement

Background:

During metastasis, cancer cells migrate away from the primary tumour and invade the circulatory system and distal tissues. The stimulatory effect of growth factors has been implicated in the migration process. Placental growth factor (PlGF), expressed by 30–50% of primary breast cancers, stimulates measurable breast cancer cell motility in vitro within 3 h. This implies that PlGF activates intracellular signalling kinases and cytoskeletal remodelling necessary for cellular migration. The PlGF-mediated motility is prevented by an Flt-1-antagonising peptide, BP-1, and anti-PlGF antibody. The purpose of this study was to determine the intracellular effects of PlGF and the inhibiting peptide, BP-1.

Methods:

Anti-PlGF receptor (anti-Flt-1) antibody and inhibitors of intracellular kinases were used for analysis of PlGF-delivered intracellular signals that result in motility. The effects of PlGF and BP-1 on kinase activation, intermediate filament (IF) protein stability, and the actin cytoskeleton were determined by immunohistochemistry, cellular migration assays, and immunoblots.

Results:

Placental growth factor stimulated phosphorylation of extracellular-regulated kinase (ERK)1/2 (pERK) in breast cancer cell lines that also increased motility. In the presence of PlGF, BP-1 decreased cellular motility, reversed ERK1/2 phosphorylation, and decreased nuclear and peripheral pERK1/2. ERK1/2 kinases are associated with rearrangements of the actin and IF components of the cellular cytoskeleton. The PlGF caused rearrangements of the actin cytoskeleton, which were blocked by BP-1. The PlGF also stabilised cytokeratin 19 and vimentin expression in MDA-MB-231 human breast cancer cells in the absence of de novo transcription and translation.

Conclusions:

The PlGF activates ERK1/2 kinases, which are associated with cellular motility, in breast cancer cells. Several of these activating events are blocked by BP-1, which may explain its anti-tumour activity.

Neuroproteomics approaches to decipher neuronal regeneration and degeneration

Given the complexity of brain and nerve tissues, systematic approaches are essential to understand normal physiological conditions and functional alterations in neurological diseases. Mass spectrometry-based proteomics is increasingly used in neurosciences to determine both basic and clinical differential protein expression, protein-protein interactions, and post-translational modifications. Proteomics approaches are especially useful to understand the mechanisms of nerve regeneration and degeneration because changes in axons following injury or in disease states often occur without the contribution of transcriptional events in the cell body. Indeed, the current understanding of axonal function in health and disease emphasizes the role of proteolysis, local axonal protein synthesis, and a broad range of post-translational modifications. Deciphering how axons regenerate and degenerate has thus become a postgenomics problem, which depends in part on proteomics approaches. This review focuses on recent proteomics approaches designed to uncover the mechanisms and molecules involved in neuronal regeneration and degeneration. It emerges that the principal degenerative mechanisms converge to oxidative stress, dysfunctions of axonal transport, mitochondria, chaperones, and the ubiquitin-proteasome systems. The mechanisms regulating nerve regeneration also impinge on axonal transport, cytoskeleton, and chaperones in addition to changes in signaling pathways. We also discuss the major challenges to proteomics work in the nervous system given the complex organization of the brain and nerve tissue at the anatomical, cellular, and subcellular levels.

Retrograde axonal transport: pathways to cell death?

Active transport along the axon is crucial to the neuron. Motor-driven transport supplies the distal synapse with newly synthesized proteins and lipids, and clears damaged or misfolded proteins. Microtubule motors also drive long-distance signaling along the axon via signaling endosomes. Although positive signaling initiated by neurotrophic factors has been well-studied, recent research has focused on stress-signaling along the axon. Here, the connections between axonal transport alterations and neurodegeneration are discussed, including evidence for defective transport of vesicles, mitochondria, degradative organelles, and signaling endosomes in models of amyotrophic lateral sclerosis, Huntington's, Parkinson's and Alzheimer's disease. Defects in transport are sufficient to induce neurodegeneration, but recent progress suggests that changes in retrograde signaling pathways correlate with rapidly progressive neuronal cell death.

Vimentin-mediated signalling is required for IbeA+ E. coli K1 invasion of human brain microvascular endothelial cells

IbeA in meningitic Escherichia coli K1 strains has been described previously for its role in invasion of BMECs (brain microvascular endothelial cells). Vimentin was identified as an IbeA-binding protein on the surface of HBMECs (human BMECs). In the present study, we demonstrated that vimentin is a primary receptor required for IbeA+ E. coli K1-induced signalling and invasion of HBMECs, on the basis of the following observations. First, E44 (IbeA+ E. coli K1 strain) invasion was blocked by vimentin inhibitors (withaferin A and acrylamide), a recombinant protein containing the vimentin head domain and an antibody against the head domain respectively. Secondly, overexpression of GFP (green fluorescent protein)-vimentin and GFP-VDM (vimentin head domain deletion mutant) significantly increased and decreased bacterial invasion respectively. Thirdly, bacterial invasion was positively correlated with phosphorylation of vimentin at Ser82 by CaMKII (Ca2+/calmodulin-dependent protein kinase II) and IbeA+ E. coli-induced phosphorylation of ERK (extracellular-signal-regulated kinase). Blockage of CaMKII by KN93 and inhibition of ERK1/2 phosphorylation by PD098059 resulted in reduced IbeA+ E. coli invasion. Fourthly, IbeA+ E. coli and IbeA-coated beads induced the clustering of vimentin that was correlated with increased entry of bacteria and beads. Lastly, IbeA+ E. coli K1 invasion was inhibited by lipid-raft-disrupting agents (filipin and nystatin) and caveolin-1 siRNA (small interfering RNA), suggesting that caveolae/lipid rafts are signalling platforms for inducing IbeA-vimentin-mediated E. coli invasion of HBMECs. Taken together, the present studies suggest that a dynamic and function-related interaction between IbeA and its primary receptor vimentin at HBMEC membrane rafts leads to vimentin phosphorylation and ERK-mediated signalling, which modulate meningitic E. coli K1 invasion.

The subcellular localization of MEK and ERK--a novel nuclear translocation signal (NTS) paves a way to the nucleus

The ERK cascade is a central signaling pathway that regulates a large number of intracellular processes including proliferation, differentiation, development and also survival or apoptosis. The induction of so many distinct and even opposing cellular processes raises the question as to how the signaling specificity of the cascade is regulated. In the past few years, subcellular localization of components of the ERK cascade was shown to play an important role in specificity determination. Here we describe the dynamic subcellular localization of Raf kinases, MEKs, and particularly ERKs, which translocate into the nucleus during many cellular processes to induce transcription. We also describe in details the recent identification of a novel nuclear translocation mechanism for ERKs, which is based on a nuclear translocation sequence (NTS) within their kinase insert domain (KID). Phosphorylation of this domain, mainly upon stimulation, allows ERKs to interact with the nuclear importing protein - importin7, which mediates the penetration of the interacting ERKs into the nucleus via nuclear pores. Interestingly, the NTS is not specific to ERKs, and seems to be a general signal for regulating nuclear accumulation of various proteins, including MEKs, upon their stimulation. Better understanding of this mechanism may clarify the role of the massive nuclear translocation of many regulatory proteins shortly after their stimulation.

Vimentin induces changes in cell shape, motility, and adhesion during the epithelial to mesenchymal transition

Vimentin is used widely as a marker of the epithelial to mesenchymal transitions (EMTs) that take place during embryogenesis and metastasis, yet the functional implications of the expression of this type III intermediate filament (IF) protein are poorly understood. Using form factor analysis and quantitative Western blotting of normal, metastatic, and vimentin-null cell lines, we show that the level of expression of vimentin IFs (VIFs) correlates with mesenchymal cell shape and motile behavior. The reorganization of VIFs caused by expressing a dominant-negative mutant or by silencing vimentin with shRNA (neither of which alter microtubule or microfilament assembly) causes mesenchymal cells to adopt epithelial shapes. Following the microinjection of vimentin or transfection with vimentin cDNA, epithelial cells rapidly adopt mesenchymal shapes coincident with VIF assembly. These shape transitions are accompanied by a loss of desmosomal contacts, an increase in cell motility, and a significant increase in focal adhesion dynamics. Our results demonstrate that VIFs play a predominant role in the changes in shape, adhesion, and motility that occur during the EMT.

Spatially distributed cell signalling

Emerging evidence indicates that complex spatial gradients and (micro)domains of signalling activities arise from distinct cellular localization of opposing enzymes, such as a kinase and phosphatase, in signal transduction cascades. Often, an interacting, active form of a target protein has a lower diffusivity than an inactive form, and this leads to spatial gradients of the protein abundance in the cytoplasm. A spatially distributed signalling cascade can create step-like activation profiles, which decay at successive distances from the cell surface, assigning digital positional information to different regions in the cell. Feedback and feedforward network motifs control activity patterns, allowing signalling networks to serve as cellular devices for spatial computations.

Axonal transport proteomics reveals mobilization of translation machinery to the lesion site in injured sciatic nerve

Investigations of the molecular mechanisms underlying responses to nerve injury have highlighted the importance of axonal transport systems. To obtain a comprehensive view of the protein ensembles associated with axonal transport in injured axons, we analyzed the protein compositions of axoplasm concentrated at ligatures following crush injury of rat sciatic nerve. LC-MS/MS analyses of iTRAQ-labeled peptides from axoplasm distal and proximal to the ligation sites revealed protein ensembles transported in both anterograde and retrograde directions. Variability of replicates did not allow straightforward assignment of proteins to functional transport categories; hence, we performed principal component analysis and factor analysis with subsequent clustering to determine the most prominent injury-related transported proteins. This strategy circumvented experimental variability and allowed the extraction of biologically meaningful information from the quantitative neuroproteomics experiments. 299 proteins were highlighted by principal component analysis and factor analysis, 145 of which correlate with retrograde and 154 of which correlate with anterograde transport after injury. The analyses reveal extensive changes in both anterograde and retrograde transport proteomes in injured peripheral axons and emphasize the importance of RNA binding and translational machineries in the axonal response to injury.

Direct effects of osteoprotegerin on human bone cell metabolism

PURPOSE

Osteoprotegerin (OPG) affects bone metabolism by intercepting the RANK-RANKL interaction which prevents osteoclastic differentiation and consequently reduces bone resorption. Different bone phenotypes of mice overexpressing OPG and of mice with knockdown of receptor activator of NF-kappaB (RANK) or RANK-ligand (RANKL) suggest that the mechanism of action of the OPG-RANKL-RANK system in regulating bone remodeling is not completely understood. Furthermore, OPG increases bone mass and density independently from reduced osteoclastogenesis which is consistent with the possibility that OPG may directly affect bone metabolism beyond its known role as decoy receptor for RANKL.

METHODS

We treated primary human osteoblastic cells with OPG and inhibitory anti-RANKL antibodies and measured cellular ALP activity, in vitro mineralization, vitronectin receptor protein expression and ERK phosphorylation. We also analyzed the mRNA co-expression of ALP and OPG ex vivo in bone biopsies from acute and old stable vertebral fractures.

RESULTS

OPG directly increased ALP activity and in vitro mineralization of HOC, enhanced expression of the vitronectin receptor thereby increasing adherence of HOC to vitronectin and stimulated ERK phosphorylation. All OPG-mediated effects could be prevented by RANKL antibodies or RANKL-siRNA transfection and MAPK inhibitor PD98059 reduced the stimulatory effect of OPG on integrin alphav expression. In acutely fractured vertebrae OPG and ALP mRNA expression was significantly increased compared to stable vertebral fractures. In conclusion, OPG exerts direct osteoanabolic effects on HOC metabolism via RANKL in addition to its well described role as decoy receptor for RANKL.

The dynamic properties of intermediate filaments during organelle transport

Intermediate filament (IF) dynamics during organelle transport and their role in organelle movement were studied using Xenopus laevis melanophores. In these cells, pigment granules (melanosomes) move along microtubules and microfilaments, toward and away from the cell periphery in response to alpha-melanocyte stimulating hormone (alpha-MSH) and melatonin, respectively. In this study we show that melanophores possess a complex network of vimentin IFs which interact with melanosomes. IFs form an intricate, honeycomb-like network that form cages surrounding individual and small clusters of melanosomes, both when they are aggregated and dispersed. Purified melanosome preparations contain a substantial amount of vimentin, suggesting that melanosomes bind to IFs. Analyses of individual melanosome movements in cells with disrupted IF networks show increased movement of granules in both anterograde and retrograde directions, further supporting the notion of a melanosome-IF interaction. Live imaging reveals that IFs, in turn, become highly flexible as melanosomes disperse in response to alpha-MSH. During the height of dispersion there is a marked increase in the rate of fluorescence recovery after photobleaching of GFP-vimentin IFs and an increase in vimentin solubility. These results reveal a dynamic interaction between membrane bound pigment granules and IFs and suggest a role for IFs as modulators of granule movement.

Endocytosis and signalling: a meeting with mathematics

Although endocytosis has traditionally been understood as a signal attenuation mechanism, an emerging view considers endocytosis as an integral part of signal propagation and processing. On the short time scale, trafficking of endocytic vesicles contributes to signal propagation from the surface to distant targets, with bi-directional communication between signalling and trafficking. Mathematical modelling helps combine the mechanistic, molecular knowledge with rigorous analysis of the complex output dynamics of endocytosis in time and space. Simulations reveal novel roles for endocytosis, including the control of cell polarity, enhancing the spatial signal propagation, and controlling the signal magnitudes, kinetics, and synchronization with stimulus dynamics.

Axonal mRNAs: characterisation and role in the growth and regeneration of dorsal root ganglion axons and growth cones

We have developed a compartmentalised culture model for the purification of axonal mRNA from embryonic, neonatal and adult rat dorsal root ganglia. This mRNA was used un-amplified for RT-qPCR. We assayed for the presence of axonal mRNAs encoding molecules known to be involved in axon growth and guidance. mRNAs for beta-actin, beta-tubulin, and several molecules involved in the control of actin dynamics and signalling during axon growth were found, but mRNAs for microtubule-associated proteins, integrins and cell surface adhesion molecules were absent. Quantification of beta-actin mRNA by means of qPCR showed that the transcript is present at the same level in embryonic, newborn and adult axons. Using the photoconvertible reporter Kaede we showed that there is local translation of beta-actin in axons, the rate being increased by axotomy. Knock down of beta-actin mRNA by RNAi inhibited the regeneration of new axon growth cones after in vitro axotomy, indicating that local translation of actin-related molecules is important for successful axon regeneration.

The role of local protein synthesis and degradation in axon regeneration

In axotomised regenerating axons, the first step toward successful regeneration is the formation of a growth cone. This requires a variety of dynamic morphological and biochemical changes in the axon, including the appearance of many new cytoskeletal, cell surface and signalling molecules. These changes suggest the activation of coordinated complex cellular processes. A recent development has been the demonstration that the regenerative ability of some axons depends on their capacity to locally synthesise new proteins and degrade others at the injury site autonomously from the cell body. There are also events involving the degradation of cytoskeletal and other molecules, and activation of signalling pathways, with axotomy-induced calcium changes probably being an initiating event. A future challenge will be to understand how this complex network of processes interacts in order to find therapeutic ways of promoting the regeneration of CNS axons.

Involvement of the cytoskeletal elements in articular cartilage homeostasis and pathology

The cytoskeleton of all cells is a three-dimensional network comprising actin microfilaments, tubulin microtubules and intermediate filaments. Studies in many cell types have indicated roles for these cytoskeletal proteins in many diverse cellular processes including alteration of cell shape, movement of organelles, migration, endocytosis, secretion, cell division and extracellular matrix assembly. The cytoskeletal networks are highly organized in structure enabling them to fulfil their biological functions. This review will primarily focus on the organization and function of the three major cytoskeletal networks in articular cartilage chondrocytes. Articular cartilage is a major load-bearing tissue of the synovial joint; it is well known that the cytoskeleton acts as a physical interface between the chondrocytes and the extracellular matrix in 'sensing' mechanical stimuli. The effect of mechanical load on cytoskeletal element expression and organization will also be reviewed. Abnormal mechanical load is widely believed to be a risk factor for the development of osteoarthritis. Several studies have intimated that the major cytoskeletal networks are disorganized or often absent in osteoarthritic cartilage chondrocytes. The implications and possible reasoning for this are more widely discussed and placed into context with their potential relevance to disease and therapeutic strategies.

Nuclear transport factors in neuronal function

Active nucleocytoplasmic transport of macromolecules requires soluble transport carriers of the importin/karyopherin superfamily. Although the nuclear transport machinery is essential in all eukaryotic cells, neurons must also mobilise importins and associated proteins to overcome unique spatiotemporal challenges. These include switches in importin alpha subtype expression during neuronal differentiation, localized axonal synthesis of importin beta1 to coordinate a retrograde injury signaling complex on axonal dynein, and trafficking of regulatory and signaling molecules from synaptic terminals to cell bodies. Targeting of RNAs encoding critical components of the importins complex and the Ran system to axons allows sophisticated local regulation of the system for mobilization upon need. Finally, a number of importin family members have been associated with mental or neurodegenerative diseases. The extended roles recently discovered for importins in the nervous system might also be relevant in non-neuronal cells, and the localized modes of importin regulation in neurons offer new avenues to interrogate their cytoplasmic functions.

Nerve injury signaling

Although neurons within the peripheral nervous system (PNS) have a remarkable ability to repair themselves after injury, neurons within the central nervous system (CNS) do not spontaneously regenerate. This problem has remained recalcitrant despite a century of research on the reaction of axons to injury. The balance between inhibitory cues present in the environment and the intrinsic growth capacity of the injured neuron determines the extent of axonal regeneration following injury. The cell body of an injured neuron must receive accurate and timely information about the site and extent of axonal damage in order to increase its intrinsic growth capacity and successfully regenerate. One of the mechanisms contributing to this process is retrograde transport of injury signals. For example, molecules activated at the injury site convey information to the cell body leading to the expression of regeneration-associated genes and increased growth capacity of the neuron. Here we discuss recent studies that have begun to dissect the injury-signaling pathways involved in stimulating the intrinsic growth capacity of injured neurons.

Positional information generated by spatially distributed signaling cascades

The temporal and stationary behavior of protein modification cascades has been extensively studied, yet little is known about the spatial aspects of signal propagation. We have previously shown that the spatial separation of opposing enzymes, such as a kinase and a phosphatase, creates signaling activity gradients. Here we show under what conditions signals stall in the space or robustly propagate through spatially distributed signaling cascades. Robust signal propagation results in activity gradients with long plateaus, which abruptly decay at successive spatial locations. We derive an approximate analytical solution that relates the maximal amplitude and propagation length of each activation profile with the cascade level, protein diffusivity, and the ratio of the opposing enzyme activities. The control of the spatial signal propagation appears to be very different from the control of transient temporal responses for spatially homogenous cascades. For spatially distributed cascades where activating and deactivating enzymes operate far from saturation, the ratio of the opposing enzyme activities is shown to be a key parameter controlling signal propagation. The signaling gradients characteristic for robust signal propagation exemplify a pattern formation mechanism that generates precise spatial guidance for multiple cellular processes and conveys information about the cell size to the nucleus.

Retrograde injury signaling in lesioned axons

The cell body of a lesioned neuron must receive accurate and timely information on the site and extent of axonal damage, in order to mount an appropriate response. Specific mechanisms must therefore exist to transmit such information along the length of the axon from the lesion site to the cell body. Three distinct types of signals have been postulated to underlie this process, starting with injury-induced discharge of axon potentials, and continuing with two distinct types of retrogradely transported macromolecular signals. The latter includes, on the one hand, an interruption of the normal supply of retrogradely transported trophic factors from the target, and, on the other hand, activated proteins originating from the injury site. This chapter reviews the progress on understanding the different mechanistic aspects of the axonal response to injury, and how the information is conveyed from the injury site to the cell body to initiate regeneration.

Proteomic analysis identifies proteins that continue to grow hepatic stem-like cells without differentiation

To understand the molecular mechanism underlying vigorous proliferative activity of hepatic stem-like (HSL) cells, we performed two-dimensional electrophoresis to identify the proteins statistically more abundant in rapidly growing undifferentiated HSL cells than in sodium butyrate-treated differentiated HSL cells. Matrix-assisted laser desorption/ionization-time of flight mass spectrometry and Mascot search identified 6 proteins including prohibitin, vimentin, ezrin, annexin A3, acidic ribosomal phosphoprotein P0 and Grp75. Prohibitin and vimentin control the mitogen-activated protein (MAP) kinase pathway. Ezrin is phosphorylated by various protein-tyrosine kinases and modulates interactions between cytoskeletal and membrane proteins. Annexin A3 has a role in DNA synthesis. Acidic ribosomal phosphoprotein P0 and Grp75 play in protein synthesis. These results suggest that the proteins related to the MAP kinase cascade had some role in continuous proliferation of HSL cells without differentiation.

Localized regulation of axonal RanGTPase controls retrograde injury signaling in peripheral nerve

Peripheral sensory neurons respond to axon injury by activating an importin-dependent retrograde signaling mechanism. How is this mechanism regulated? Here, we show that Ran GTPase and its associated effectors RanBP1 and RanGAP regulate the formation of importin signaling complexes in injured axons. A gradient of nuclear RanGTP versus cytoplasmic RanGDP is thought to be fundamental for the organization of eukaryotic cells. Surprisingly, we find RanGTP in sciatic nerve axoplasm, distant from neuronal cell bodies and nuclei, and in association with dynein and importin-alpha. Following injury, localized translation of RanBP1 stimulates RanGTP dissociation from importins and subsequent hydrolysis, thereby allowing binding of newly synthesized importin-beta to importin-alpha and dynein. Perturbation of RanGTP hydrolysis or RanBP1 blockade at axonal injury sites reduces the neuronal conditioning lesion response. Thus, neurons employ localized mechanisms of Ran regulation to control retrograde injury signaling in peripheral nerve.

Intermediate filaments: versatile building blocks of cell structure

Cytoskeletal intermediate filaments (IF) are organized into a dynamic nanofibrillar complex that extends throughout mammalian cells. This organization is ideally suited to their roles as response elements in the subcellular transduction of mechanical perturbations initiated at cell surfaces. IF also provide a scaffold for other types of signal transduction that together with molecular motors ferries signaling molecules from the cell periphery to the nucleus. Recent insights into their assembly highlight the importance of co-translation of their precursors, the hierarchical organization of their subunits in the formation of unit-length filaments (ULF) and the linkage of ULF into mature apolar IF. Analyses by atomic force microscopy reveal that mature IF are flexible and can be stretched to over 300% of their length without breaking, suggesting that intrafilament subunits can slide past one another when exposed to mechanical stress and strain. IF also play a role in the organization of organelles by modulating their motility and providing anchorage sites within the cytoplasm.

Study of protein targets for covalent modification by the antitumoral and anti-inflammatory prostaglandin PGA1: focus on vimentin

Prostaglandins with cyclopentenone structure (cyPG) display potent antiproliferative actions that have elicited their study as potential anticancer agents. Several natural and synthetic analogs of the cyPG prostaglandin A(1) (PGA(1)) have proven antitumoral efficacy in cancer cell lines and animal models. In addition, PGA(1) has been used as an inhibitor of transcription factor NF-kappaB-mediated processes, including inflammatory gene expression and viral replication. An important determinant for these effects is the ability of cyPG to form Michael adducts with free thiol groups. The chemical nature of this interaction implies that PGA(1) could covalently modify cysteine residues in a large number of cellular proteins potentially involved in its beneficial effects. However, only a few targets of PGA(1) have been identified. In previous work, we have observed that a biotinylated analog of PGA(1) that retains the cyclopentenone moiety (PGA(1)-B) binds to multiple targets in fibroblasts. Here, we have addressed the identification of these targets through a proteomic approach. Cell fractionation followed by avidin affinity chromatography yielded a fraction enriched in proteins modified by PGA(1)-B. Analysis of this fraction by SDS-PAGE and LC-MS/MS allowed the identification of the chaperone Hsp90, elongation and initiation factors for protein synthesis and cytoskeletal proteins including actin, tubulin and vimentin. Furthermore, we have characterized the modification of vimentin both in vitro and in intact cells. Our observations indicate that cysteine 328 is the main site for PGA(1) addition. These results may contribute to a better understanding of the mechanism of action of PGA(1) and the potential of cyPG-based therapeutic strategies.

Nociceptors--noxious stimulus detectors

In order to deal effectively with danger, it is imperative to know about it. This is what nociceptors do--these primary sensory neurons are specialized to detect intense stimuli and represent, therefore, the first line of defense against any potentially threatening or damaging environmental inputs. By sensing noxious stimuli and contributing to the necessary reactions to avoid them--rapid withdrawal and the experience of an intensely unpleasant or painful sensation, nociceptors are essential for the maintenance of the body's integrity. Although nociceptive pain is clearly an adaptive alarm system, persistent pain is maladaptive, essentially an ongoing false alarm. Here, we highlight the genesis of nociceptors during development and the intrinsic properties of nociceptors that enable them to transduce, conduct, and transmit nociceptive information and also discuss how their phenotypic plasticity contributes to clinical pain.

Intermediate filament scaffolds fulfill mechanical, organizational, and signaling functions in the cytoplasm

Intermediate filaments (IFs) are cytoskeletal polymers whose protein constituents are encoded by a large family of differentially expressed genes. Owing in part to their properties and intracellular organization, IFs provide crucial structural support in the cytoplasm and nucleus, the perturbation of which causes cell and tissue fragility and accounts for a large number of genetic diseases in humans. A number of additional roles, nonmechanical in nature, have been recently uncovered for IF proteins. These include the regulation of key signaling pathways that control cell survival, cell growth, and vectorial processes including protein targeting in polarized cellular settings. As this discovery process continues to unfold, a rationale for the large size of this family and the context-dependent regulation of its members is finally emerging.

Role of Non-phosphorylated Activation Loop Residues in Determining ERK2 Dephosphorylation, Activity, and Subcellular Localization

Extracellular signal-regulated kinases (ERKs) activity is regulated by MAPK/ERK kinases (MEKs), which phosphorylate the regulatory Tyr and Thr residues in ERKs activation loop, and by various phosphatases that remove the incorporated phosphates. Although the role of the phosphorylated residues in the activation loop of ERKs is well studied, much less is known about the role of other residues within this loop. Here we substituted several residues within amino acids 173-177 of ERK2 and studied their role in ERK2 phosphorylation, substrate recognition, and subcellular localization. We found that substitution of residues 173-175 and particularly Pro(174) to alanines reduces the EGF-induced ERK2 phosphorylation, without modifying its in vitro phosphorylation by MEK1. Examining the ability of these mutants to be dephosphorylated revealed that 173-5A mutants are hypersensitive to phosphatases, indicating that these residues are important for setting the phosphorylation/dephosphorylation balance of ERKs. In addition, 173-5A mutants reduced ERK2 activity toward Elk-1, without affecting the activity of ERK2 toward MBP, while substitution of residues 176-8 decreased ERK2 activity toward both substrates. Substitution of Asp(177) to alanine increased nuclear localization of the construct in MEK1-overexpressing cells, suggesting that this residue together with His(176) is involved in the dissociation of ERK2 from MEKs. Combining CRS/CD motif and the activation loop mutations revealed that these two regions cooperate in determining the net phosphorylation of ERK2, but the role of the CRS/CD motif predominates that of the activation loop residues. Thus, we show here that residues 173-177 of ERK2 join other regulatory regions of ERKs in governing ERK activity.

Novel functions of vimentin in cell adhesion, migration, and signaling

Vimentin is the major intermediate filament (IF) protein of mesenchymal cells. It shows dynamically altered expression patterns during different developmental stages and high sequence homology throughout all vertebrates, suggesting that the protein is physiologically important. Still, until recently, the real tasks of vimentin have been elusive, primarily because the vimentin-deficient mice were originally characterized as having a very mild phenotype. Recent studies have revealed several key functions for vimentin that were not obvious at first sight. Vimentin emerges as an organizer of a number of critical proteins involved in attachment, migration, and cell signaling. The highly dynamic and complex phosphorylation of vimentin seems to be a likely regulator mechanism for these functions. The implicated novel vimentin functions have broad ramifications into many different aspects of cell physiology, cellular interactions, and organ homeostasis.