Cellular injury induces activation of MAPK via P2Y receptors

Rational design and synthesis of lumican stapled peptides for promoting corneal wound healing

PURPOSE

Lumican is a major extracellular matrix (ECM) component in the cornea that is upregulated after injury and promotes corneal wound healing. We have recently shown that peptides designed based on the 13 C-terminal amino acids of lumican (LumC13 and LumC13C-A) are able to recapitulate the effects of lumican on promoting corneal wound healing. Herein we used computational chemistry to develop peptide mimetics derived from LumC13C-A with increased stability and half-life that are biologically active and non-toxic, thereby promoting corneal wound healing with increased pharmacological potential.

METHODS

Different peptides staples were rationalized using LumC13C-A sequence by computational chemistry, docked to TGFβRI and the interface binding energies compared. Lowest scoring peptides were synthesized, and the toxicity of peptides tested using CCK8-based cell viability assay. The efficacy of the stapled peptides at promoting corneal wound healing was tested using a proliferation assay, an in vitro scratch assay using human corneal epithelial cells and an in vivo murine corneal debridement wound healing model.

RESULTS

Binding free energies were calculated using MMGBSA algorithm, and peptides LumC13C and LumC13S5 displayed superior binding to ALK5 compared to the non-stapled peptide LumC13C-A. The presence of the hydrocarbon staple in LumC13C enhances the stability of the α-helical conformation, thereby facilitating more optimal interactions with the ALK5 receptor. The stapled peptides do not present cytotoxic effects on human corneal epithelial cells at a 300 nM concentration. Similar to lumican and LumC13C-A, both C13C and LumC13S5 significantly promote corneal wound healing both in vitro and in vivo.

CONCLUSIONS

Highly stable and non-toxic stapled peptides designed based on LumC13, significantly promote corneal wound healing. As a proof of principle, our data shows that more stable and pharmacologically relevant peptides can be designed based on endogenous peptide sequences for treating various corneal pathologies.

Time-dependent conformational analysis of ALK5-lumican complex in presence of graphene and graphene oxide employing molecular dynamics and MMPBSA calculation

Lumican, an extracellular matrix protein avails wound healing by binding to ALK5 membrane receptor (TGF-beta receptor I). Their interaction enables epithelialization and substantiates rejuvenation of injured tissue. To enrich permanence of ALK5-lumican interaction, we employed graphene and graphene oxide co-factors. Herein, this study explicates concomitancy of graphene and graphene oxide with ALK5-lumican. We performed an in silico approach involving molecular modelling, molecular docking, molecular dynamics for 200 ns, DSSP analysis and MMPBSA calculations. Results of molecular dynamics indicate cofactors influential in altering bioactive site of lumican than ALK5. Similarly, MMPBSA calculations unveiled binding energy of apoenzyme as -108.09 kcal/mol, holoenzyme (G) as -79.20 kcal/mol and holoenzyme (GO) as -114.33 kcal/mol. This concludes graphene oxide lucrative in enhancing binding energy of ALK5-lumican in holoenzyme (GO) via coil formation of Lum C₁₃ domain. In contrast, graphene reduced binding energy of ALK5-lumican in holoenzyme (G) modifying Lum C₁₃ into beta sheets. MMPBSA residual contribution analysis of Lum C₁₃ residues revealed binding energy of -13.9 kcal/mol for apoenzyme, -6.8 kcal/mol for holoenzyme (G) and -19.5 kcal/mol for holoenzyme (GO). This supports coil formation propitious for better ALK5-Lum interaction. Highest SASA energy of -21.05 kcal/mol of holoenzyme (G) assures graphene reasonable for improved ALK5-lumican hydrophobicity. As per the motive of the study, graphene oxide enriches permanence of ALK5-lumican. This provides counsel for plausible exploitation of lumican and graphene oxide as targeted/nano drug delivery system to reinstate acute wounds, chronic wounds, corneal wounds, hypertrophic scars and keloids in near future. Communicated by Ramaswamy H. Sarma.

Extracellular ATP Mediates Cancer Cell Migration and Invasion Through Increased Expression of Cyclooxygenase 2

The tumor microenvironment plays a major role in the ability of the tumor cells to undergo metastasis. A major player of tumors gaining metastatic property is the inflammatory protein, cyclooxygenase 2 (COX-2). Several tumors show upregulation of this protein, which has been implicated in mediating metastasis in various cancer types such as of colon, breast and lung. In this report, we show that the concentration of extracellular ATP (eATP) is increased in response to cell death mediated by chemotherapeutic agents such as doxorubicin. By using three different cell-lines-HeLa (cervical), IMR-32 (neuronal) and MCF-7 (breast)-we show that this eATP goes on to act on purinergic (P2) receptors. Among the various P2 receptors expressed in these cells we identified P2X7, in IMR-32 and MCF-7 cells, and P2Y12, in HeLa cells, as important in modulating cell migration and invasion. Downstream of the P2 receptor activation, both p42/44 mitogen-activated protein kinase (MAPK) and the p38 MAPK are activated in these cells. These result in an increase in the expression of COX-2 mRNA and protein. We also observe an increase in the activity of matrix metalloproteinase 2 (MMP-2) enzyme in these cells. Blocking the P2 receptors not only blocks migration and invasion, but also COX-2 synthesis and MMP-2 activity. Our results show the link between purinergic receptors and COX-2 expression. Increased levels of ATP in the tumor microenvironment, therefore, leads to increased COX-2 expression, which, in turn, affords migratory and invasive properties to the tumor. This provides P2 receptor-based anti-inflammatory drugs (PBAIDs) a potential opportunity to be explored as cancer therapeutics.

Abnormal neovascular and proliferative conjunctival phenotype in limbal stem cell deficiency is associated with altered microRNA and gene expression modulated by PAX6 mutational status in congenital aniridia

PURPOSE

To evaluate conjunctival cell microRNA (miRNAs) and mRNA expression in relation to observed phenotype of progressive limbal stem cell deficiency in a cohort of subjects with congenital aniridia with known genetic status.

METHODS

Using impression cytology, bulbar conjunctival cells were sampled from 20 subjects with congenital aniridia and 20 age and sex-matched healthy control subjects. RNA was extracted and miRNA and mRNA analyses were performed using microarrays. Results were related to severity of keratopathy and genetic cause of aniridia.

RESULTS

Of 2549 miRNAs, 21 were differentially expressed in aniridia relative to controls (fold change ≤ -1.5 or ≥ +1.5). Among these miR-204-5p, an inhibitor of corneal neovascularization, was downregulated 26.8-fold in severely vascularized corneas. At the mRNA level, 539 transcripts were differentially expressed (fold change ≤ -2 or ≥ +2), among these FOSB and FOS were upregulated 17.5 and 9.7-fold respectively, and JUN by 2.9-fold, all being components of the AP-1 transcription factor complex. Pathway analysis revealed enrichment of PI3K-Akt, MAPK, and Ras signaling pathways in aniridia. For several miRNAs and transcripts regulating retinoic acid metabolism, expression levels correlated with keratopathy severity and genetic status.

CONCLUSION

Strong dysregulation of key factors at the miRNA and mRNA level suggests that the conjunctiva in aniridia is abnormally maintained in a pro-angiogenic and proliferative state, and these changes are expressed in a PAX6 mutation-dependent manner. Additionally, retinoic acid metabolism is disrupted in severe, but not mild forms of the limbal stem cell deficiency in aniridia.

Intracellular signaling dynamics and their role in coordinating tissue repair

Tissue repair is a complex process that requires effective communication and coordination between cells across multiple tissues and organ systems. Two of the initial intracellular signals that encode injury signals and initiate tissue repair responses are calcium and extracellular signal-regulated kinase (ERK). However, calcium and ERK signaling control a variety of cellular behaviors important for injury repair including cellular motility, contractility, and proliferation, as well as the activity of several different transcription factors, making it challenging to relate specific injury signals to their respective repair programs. This knowledge gap ultimately hinders the development of new wound healing therapies that could take advantage of native cellular signaling programs to more effectively repair tissue damage. The objective of this review is to highlight the roles of calcium and ERK signaling dynamics as mechanisms that link specific injury signals to specific cellular repair programs during epithelial and stromal injury repair. We detail how the signaling networks controlling calcium and ERK can now also be dissected using classical signal processing techniques with the advent of new biosensors and optogenetic signal controllers. Finally, we advocate the importance of recognizing calcium and ERK dynamics as key links between injury detection and injury repair programs that both organize and execute a coordinated tissue repair response between cells across different tissues and organs. This article is categorized under: Models of Systems Properties and Processes > Mechanistic Models Biological Mechanisms > Cell Signaling Laboratory Methods and Technologies > Imaging Models of Systems Properties and Processes > Organ, Tissue, and Physiological Models.

Signaling pathways involved in adaptive responses to cell membrane disruption

Plasma membrane disruption occurs frequently in many animal tissues. Cell membrane disruption induces not only a rapid and massive influx of Ca²⁺ into the cytosol but also an efflux or release of various signaling molecules, such as ATP, from the cytosol; in turn, these signaling molecules stimulate a variety of pathways in both wounded and non-wounded neighboring cells. These signals first trigger cell membrane repair responses in the wounded cell but then induce an adaptive response, which results in faster membrane repair in the event of future wounds in both wounded and non-wounded neighboring cells. In addition, signaling pathways stimulated by membrane disruption induce other adaptive responses, including cell survival, regeneration, migration, and proliferation. This chapter summarizes the role of intra- and intercellular signaling pathways in adaptive responses triggered by cell membrane disruption.

Uridine-5'-Triphosphate Partially Blocks Differentiation Signals and Favors a more Repair State in Cultured rat Schwann Cells

Schwann cells (SCs) play a key role in peripheral nerve regeneration. After damage, they respond acquiring a repair phenotype that allows them to proliferate, migrate and redirect axonal growth. Previous studies have shown that Uridine-5'-Triphosphate (UTP) and its purinergic receptors participate in several pathophysiological responses in the nervous system. Our group has previously described how UTP induces the migration of a Schwannoma cell line and promotes wound healing. These data suggest that UTP participates in the signaling involved in the regeneration process. In the present study we evaluated UTP effects in isolated rat SCs and cocultures of SCs and dorsal root ganglia neurons. UTP reduced cAMP-dependent Krox-20 induction in SCs. UTP also reduced the N-cadherin re-expression that occurs when SCs and axons make contact. In myelinating cocultures, a non-significant tendency to a lower expression of P0 and MAG proteins in presence of UTP was observed. We also demonstrated that UTP induced SC migration without affecting cell proliferation. Interestingly, UTP was found to block neuregulin-induced phosphorylation of the ErbB3 receptor, a pathway involved in the regeneration process. These results indicate that UTP could acts as a brake to the differentiation signals, promoting a more migratory state in the repair-SCs.

Purinergic Signaling in Corneal Wound Healing: A Tale of 2 Receptors

Nucleotide release and purinergic signaling make up the earliest response to corneal injury and are vital for proper wound healing. In this study, we review the importance of nucleotide release in the injury response and focus on the contribution of 2 receptors that mediate purinergic signaling, P2Y2 and P2X7. These receptors mediate the early response to injury and activate downstream signaling to promote cytoskeletal rearrangement and cell migration. The contribution of corneal nerves to the purinergic injury response is also discussed. Finally, we look at implications of altered purinergic signaling in diabetic wound healing and important targets for future research.

Adenosine A₂A receptors permit mGluR5-evoked tyrosine phosphorylation of NR2B (Tyr1472) in rat hippocampus: a possible key mechanism in NMDA receptor modulation

A great body of evidence points toward a functional interaction between metabotropic glutamate 5 receptors (mGluR5) and NMDA receptors (NMDAR) that enhances synaptic plasticity and cognition. However, the molecular mechanism underlying this interaction remains unclear. Here, we show that co-activation of mGluR5 and NMDAR in hippocampal slices synergistically leads to a robust phosphorylation of NR2B (Tyr1472), which is Src kinase dependent and is enabled by endogenous adenosine acting on A2A receptors. As it is well known, NR2B (Tyr1472) phosphorylation anchors NR2B-containing NMDARs to the surface of post-synaptic membranes, preventing their internalization. This is supported by our electrophysiological experiments showing that co-activation of mGluR5 and NMDARs robustly enhances NMDAR-dependent neuronal excitability recorded in CA1 hippocampal region, which temporally coincides with the robust increase in NR2B (Tyr1472) phosphorylation, depends on Src kinases and is also permitted by A2A receptors. Thus, we strongly suggest that NR2B (Tyr1472) phosphorylation constitutes, at least to some extent, the molecular mechanism underlying the mGluR5-mediated enhancement of NMDAR-dependent responses, which is modulated by A2A receptors. A better understanding of the molecular basis of mGluR5/NMDAR interaction would elucidate their role in synaptic plasticity processes as well as in pathological conditions. We propose the following molecular mechanism by which metabotropic Glutamate Receptor 5 (mGluR5) potentiate ionotropic Glutamate N-Methyl-D-Aspartate Receptor (NMDAR) responses in rat hippocampus. Co-activation of mGLUR5/NMDAR activates Src kinases, leading to NR2B(Tyr1472) phosphorylation, which anchors NR2B-containing NMDAR to the plasma membrane, thus inducing a robust increase in the NMDA-dependent excitability. Interestingly, adenosine A2A receptors license the mGluR5-induced NR2B(Tyr1472) phosphorylation.

Corneal Epithelial Wound Healing

The cornea is important for a clear vision by refracting light onto the lens, which in turn focusing on the retina. To maintain a smooth optical surface, corneal epithelium has to continuously renew itself to function as a barrier so that it protects the eye from various environmental insults. The adult corneal epithelium is maintained homeostatically by an integrated process of cell proliferation, migration, differentiation, stratification, and desquamation/apoptosis. Impairment of this process results in persistent corneal defect, leading to the blindness. Researches throughout the years revealed that appropriate integration and coordination of cell signaling events are responsible for corneal epithelial renewal and wound healing. In this chapter, we will review works done on cell culture, animal models, and human trials to focus on the signaling network during corneal wound healing process which will have potential for the discovery of novel drug to improve corneal wound healing.

Physiological impact of abnormal lipoxin A₄ production on cystic fibrosis airway epithelium and therapeutic potential

Lipoxin A4 has been described as a major signal for the resolution of inflammation and is abnormally produced in the lungs of patients with cystic fibrosis (CF). In CF, the loss of chloride transport caused by the mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel gene results in dehydration, mucus plugging, and reduction of the airway surface liquid layer (ASL) height which favour chronic lung infection and neutrophil based inflammation leading to progressive lung destruction and early death of people with CF. This review highlights the unique ability of LXA4 to restore airway surface hydration, to stimulate airway epithelial repair, and to antagonise the proinflammatory program of the CF airway, circumventing some of the most difficult aspects of CF pathophysiology. The report points out novel aspects of the cellular mechanism involved in the physiological response to LXA4, including release of ATP from airway epithelial cell via pannexin channel and subsequent activation of and P2Y11 purinoreceptor. Therefore, inadequate endogenous LXA4 biosynthesis reported in CF exacerbates the ion transport abnormality and defective mucociliary clearance, in addition to impairing the resolution of inflammation, thus amplifying the vicious circle of airway dehydration, chronic infection, and inflammation.

Activation of P2RY11 and ATP release by lipoxin A4 restores the airway surface liquid layer and epithelial repair in cystic fibrosis

In cystic fibrosis (CF), the airway surface liquid (ASL) height is reduced as a result of impaired ion transport, which favors bacterial colonization and inflammation of the airway and leads to progressive lung destruction. Lipoxin (LX)A4, which promotes resolution of inflammation, is inadequately produced in the airways of patients with CF. We previously demonstrated that LXA4 stimulates an ASL height increase and epithelial repair. Here we report the molecular mechanisms involved in these processes. We found that LXA4 (1 nM) induced an apical ATP release from non-CF (NuLi-1) and CF (CuFi-1) airway epithelial cell lines and CF primary cultures. The ATP release induced by LXA4 was completely inhibited by antagonists of the ALX/FPR2 receptor and Pannexin-1 channels. LXA4 induced an increase in intracellular cAMP and calcium, which were abolished by the selective inhibition of the P2RY11 purinoreceptor. Pannexin-1 and ATP hydrolysis inhibition and P2RY11 purinoreceptor knockdown all abolished the increase of ASL height induced by LXA4. Inhibition of the A2b adenosine receptor did not affect the ASL height increase induced by LXA4, whereas the PKA inhibitor partially inhibited this response. The stimulation of NuLi-1 and CuFi-1 cell proliferation, migration, and wound repair by LXA4 was inhibited by the antagonists of Pannexin-1 channel and P2RY11 purinoreceptor. Taken together, our results provide evidence for a novel role of LXA4 in stimulating apical ATP secretion via Pannexin-1 channels and P2RY11 purinoreceptors activation leading to an ASL height increase and epithelial repair.

Shock wave treatment enhances cell proliferation and improves wound healing by ATP release-coupled extracellular signal-regulated kinase (ERK) activation

Shock wave treatment accelerates impaired wound healing in diverse clinical situations. However, the mechanisms underlying the beneficial effects of shock waves have not yet been fully revealed. Because cell proliferation is a major requirement in the wound healing cascade, we used in vitro studies and an in vivo wound healing model to study whether shock wave treatment influences proliferation by altering major extracellular factors and signaling pathways involved in cell proliferation. We identified extracellular ATP, released in an energy- and pulse number-dependent manner, as a trigger of the biological effects of shock wave treatment. Shock wave treatment induced ATP release, increased Erk1/2 and p38 MAPK activation, and enhanced proliferation in three different cell types (C3H10T1/2 murine mesenchymal progenitor cells, primary human adipose tissue-derived stem cells, and a human Jurkat T cell line) in vitro. Purinergic signaling-induced Erk1/2 activation was found to be essential for this proliferative effect, which was further confirmed by in vivo studies in a rat wound healing model where shock wave treatment induced proliferation and increased wound healing in an Erk1/2-dependent fashion. In summary, this report demonstrates that shock wave treatment triggers release of cellular ATP, which subsequently activates purinergic receptors and finally enhances proliferation in vitro and in vivo via downstream Erk1/2 signaling. In conclusion, our findings shed further light on the molecular mechanisms by which shock wave treatment exerts its beneficial effects. These findings could help to improve the clinical use of shock wave treatment for wound healing.

Uridine 5'-triphosphate promotes in vitro Schwannoma cell migration through matrix metalloproteinase-2 activation

In response to peripheral nerve injury, Schwann cells adopt a migratory phenotype and modify the extracellular matrix to make it permissive for cell migration and axonal re-growth. Uridine 5′-triphosphate (UTP) and other nucleotides are released during nerve injury and activate purinergic receptors expressed on the Schwann cell surface, but little is known about the involvement of purine signalling in wound healing. We studied the effect of UTP on Schwannoma cell migration and wound closure and the intracellular signaling pathways involved. We found that UTP treatment induced Schwannoma cell migration through activation of P2Y₂ receptors and through the increase of extracellular matrix metalloproteinase-2 (MMP-2) activation and expression. Knockdown P2Y₂ receptor or MMP-2 expression greatly reduced wound closure and MMP-2 activation induced by UTP. MMP-2 activation evoked by injury or UTP was also mediated by phosphorylation of all 3 major mitogen-activated protein kinases (MAPKs): JNK, ERK1/2, and p38. Inhibition of these MAPK pathways decreased both MMP-2 activation and cell migration. Interestingly, MAPK phosphorylation evoked by UTP exhibited a biphasic pattern, with an early transient phosphorylation 5 min after treatment, and a late and sustained phosphorylation that appeared at 6 h and lasted up to 24 h. Inhibition of MMP-2 activity selectively blocked the late, but not the transient, phase of MAPK activation. These results suggest that MMP-2 activation and late MAPK phosphorylation are part of a positive feedback mechanism to maintain the migratory phenotype for wound healing. In conclusion, our findings show that treatment with UTP stimulates in vitro Schwannoma cell migration and wound repair through a MMP-2-dependent mechanism via P2Y₂ receptors and MAPK pathway activation.

Epithelial wounds induce differential phosphorylation changes in response to purinergic and EGF receptor activation

Protein phosphorylation is a dynamic post-translational modification. Mass spectrometry-based quantitation was performed to determine the phosphoproteome profile of epithelial cells in response to injury, nucleotide, or epidermal growth factor. Phosphotyrosine enrichment used immunoprecipitation and immobilized metal affinity chromatography. Nucleotides released after scratch wounding activate purinergic receptors, leading to a distinct phosphorylation profile on epidermal growth factor receptor (EGFR) compared with its natural ligand. ATP induced a 2- to 15-fold phosphorylation increase over control on EGFR Y974, Y1086, and Y1148, with minimal phosphorylation intensity on EGFR Y1173 compared with the level measured in response to epidermal growth factor. Differential phosphorylation induced by epidermal growth factor or ATP was site specific on Src, Shc, phospholipase Cγ, protein kinase C, focal adhesion kinase, paxillin, and mitogen-activated protein kinases 1, 12, and 13. After wounding, the P2Y2 receptor mRNA expression increased, and after knockdown, migration and Ca(2+) mobilization were impaired. To examine phosphorylation mediated by P2Y2, cells were cultured in media containing stable isotope-labeled amino acids, the receptor was knocked down, and the cells were stimulated. Mass spectrometry-based comparison of the phosphorylation profiles of control versus transfected cells revealed a 50-fold decrease in phosphorylation of EGFR Y974 and 1086, with no decrease in Y1173 phosphorylation. A similarfold decrease in Src Y421 and Y446 and paxillin Y118 was detected, indicating the far-reaching importance of the P2Y2 receptor in mediating migration.

Protease-activated receptor-2 induces migration of pancreatic cancer cells in an extracellular ATP-dependent manner

BACKGROUND

Protease-activated receptor 2 (PAR-2) is a G protein-coupled receptor suggested to play an important role in the proliferation and migration of tumor cells of epithelial origin. However, the role of PAR-2 in the setting of pancreatic cancer remains largely unexplored.

OBJECTIVES

To understand the importance of PAR-2 in pancreatic cancer cell migration.

METHODS AND RESULTS

The present study shows that PAR-2 does not affect pancreatic cancer cell proliferation but significantly induces the migration of pancreatic cancer cells in scratch assays. Interestingly, PAR-2 does not affect migration in a trans-well setting. This apparent discrepancy depends on extracellular ATP release in the scratch assays and the addition of exogenous (ATP)-induced PAR-2-dependent migration in trans-well assays, whereas a specific P2Y11 receptor antagonist prevents PAR-2-driven migration in scratch assays. In the scratch assays, inhibitors of Src, Rac, protein kinase C, mitogen-activated protein kinase kinase, p38, and epidermal growth factor (EGF) receptor blocked PAR-2-driven migration, whereas they did not affect fetal calf serum-driven wound closure.

CONCLUSION

Taken together, PAR-2 activation drives pancreatic cancer cell migration via an EGF-Src-Rac-p38/mitogen-activated protein kinase kinase/EGF1/2 signaling pathway, which is facilitated by extracellular ATP. Targeting the PAR-2/ATP-driven signaling pathway may therefore limit cell migration, which could inhibit pancreatic cancer metastasis.

Hydrolysis of extracellular ATP by ectonucleoside triphosphate diphosphohydrolase (ENTPD) establishes the set point for fibrotic activity of cardiac fibroblasts

The establishment of set points for cellular activities is essential in regulating homeostasis. Here, we demonstrate key determinants of the fibrogenic set point of cardiac fibroblasts (CFs) by focusing on the pro-fibrotic activity of ATP, which is released by CFs. We tested the hypothesis that the hydrolysis of extracellular ATP by ectonucleoside triphosphate diphosphohydrolases (ENTPDs) regulates pro-fibrotic nucleotide signaling. We detected two ENTPD isoforms, ENTPD-1 and -2, in adult rat ventricular CFs. Partial knockdown of ENTPD-1 and -2 with siRNA increased basal extracellular ATP concentration and enhanced the pro-fibrotic effect of ATP stimulation. Sodium polyoxotungstate-1, an ENTPD inhibitor, not only enhanced the pro-fibrotic effects of exogenously added ATP but also increased basal expression of α-smooth muscle actin, plasminogen activator inhibitor-1 and transforming growth factor (TGF)-β, collagen synthesis, and gel contraction. Furthermore, we found that adenosine, a product of ATP hydrolysis by ENTPD, acts via A2B receptors to counterbalance the pro-fibrotic response to ATP. Removal of extracellular adenosine or inhibition of A2B receptors enhanced pro-fibrotic ATP signaling. Together, these results demonstrate the contribution of basally released ATP in establishing the set point for fibrotic activity in adult rat CFs and identify a key role for the modulation of this activity by hydrolysis of released ATP by ENTPDs. These findings also imply that cellular homeostasis and fibrotic response involve the integration of signaling that is pro-fibrotic by ATP and anti-fibrotic by adenosine and that is regulated by ENTPDs.

New insights in wound response and repair of epithelium

Epithelial wounds usually heal relatively quickly, but repair may be impaired by environmental stressors, such as hypoxic or diabetic states, rendering patients vulnerable to a number of corneal pathologies. Though this response appears simple, at first, years of research have uncovered the complicated biochemical pathways coordinating the wound healing response. Here, we investigate signaling cascades and individual proteins involved in the corneal epithelium's self-repair. We will explore how an epithelial cell migrates across the wound bed and attaches itself to its new post-injury surroundings, including its neighboring cells and the basement membrane, through focal adhesions and hemidesmosomes. We will also discuss how the cell coordinates this motion physiologically, through calcium signaling and protein phosphorylation, focusing on the communication through purinergic, glutamatergic, and growth factor receptors. Many of these aspects reflect and can be extended to similar epithelial surfaces, and can be used to facilitate wound healing in patients with various underlying pathologies. The collective library of laboratory and clinical research done around the world has demonstrated how important precise regulation of these processes is in order for the injured corneal epithelium to properly heal.

Gap junctions and Bystander Effects: Good Samaritans and executioners

The "Bystander" and "Good Samaritan" effects involve the transfer of toxic or beneficial compounds from one cell to a generally adjacent other through gap junction channels and through extracellular routes. The variety of injuries in which bystander cell killing or protection occurs has greatly expanded in the last decade to include infectious agents and therapeutic compounds, radiation injury, chaperones in cell therapy and apoptosis in development. This has been accompanied by the appreciation that both gap junction mediated and paracrine routes are used for the signaling of the "kiss of life" and the "kiss of death" and that manipulations of these pathways and the molecules that use them may find therapeutic utility in treatment of a variety of pathological conditions.

Communication between corneal epithelial cells and trigeminal neurons is facilitated by purinergic (P2) and glutamatergic receptors

Previously, we demonstrated that nucleotides released upon mechanical injury to corneal epithelium activate purinergic (P2) receptors resulting in mobilization of a Ca²⁺ wave. However, the tissue is extensively innervated and communication between epithelium and neurons is critical and not well understood. Therefore, we developed a co-culture of primary trigeminal neurons and human corneal limbal epithelial cells. We demonstrated that trigeminal neurons expressed a repertoire of P2Yand P2X receptor transcripts and responded to P2 agonists in a concentration-dependent manner. Mechanical injuries to epithelia in the co-cultures elicited a Ca²⁺ wave that mobilized to neurons and was attenuated by Apyrase, an ectonucleotidase. To elucidate the role of factors released from each cell type, epithelial and neuronal cells were cultured, injured, and the wound media from one cell type was collected and added to the other cell type. Epithelial wound media generated a rapid Ca²⁺ mobilization in neuronal cells that was abrogated in the presence of Apyrase, while neuronal wound media elicited a complex response in epithelial cells. The rapid Ca²⁺ mobilization was detected, which was abrogated with Apyrase, but it was followed by Ca²⁺ waves that occurred in cell clusters. When neuronal wound media was preincubated with a cocktail of N-methyl-D-aspartate (NMDA) receptor inhibitors, the secondary response in epithelia was diminished. Glutamate was detected in the neuronal wound media and epithelial expression of NMDA receptor subunit transcripts was demonstrated. Our results indicate that corneal epithelia and neurons communicate via purinergic and NMDA receptors that mediate the wound response in a highly orchestrated manner.

The acute phase reactant orosomucoid-1 is a bimodal regulator of angiogenesis with time- and context-dependent inhibitory and stimulatory properties

Background

Tissues respond to injury by releasing acute phase reaction (APR) proteins which regulate inflammation and angiogenesis. Among the genes upregulated in wounded tissues are tumor necrosis factor-alpha (TNFα) and the acute phase reactant orosomucoid-1 (ORM1). ORM1 has been shown to modulate the response of immune cells to TNFα, but its role on injury- and TNFα-induced angiogenesis has not been investigated. This study was designed to characterize the role of ORM1 in the angiogenic response to injury and TNFα.

Methods and Results

Angiogenesis was studied with in vitro, ex vivo, and in vivo angiogenesis assays. Injured rat aortic rings cultured in collagen gels produced an angiogenic response driven by macrophage-derived TNFα. Microarray analysis and qRT-PCR showed that TNFα and ORM1 were upregulated prior to angiogenic sprouting. Exogenous ORM1 delayed the angiogenic response to injury and inhibited the proangiogenic effect of TNFα in cultures of aortic rings or isolated endothelial cells, but stimulated aortic angiogenesis over time while promoting VEGF production and activity. ORM1 inhibited injury- and TNFα-induced phosphorylation of MEK1/2 and p38 MAPK in aortic rings, but not of NFκB. This effect was injury/TNFα-specific since ORM1 did not inhibit VEGF-induced signaling, and cell-specific since ORM1 inhibited TNFα-induced phosphorylation of MEK1/2 and p38 MAPK in macrophages and endothelial cells, but not mural cells. Experiments with specific inhibitors demonstrated that the MEK/ERK pathway was required for angiogenesis. ORM1 inhibited angiogenesis in a subcutaneous in vivo assay of aortic ring-induced angiogenesis, but stimulated developmental angiogenesis in the chorioallantoic membrane (CAM) assay.

Conclusion

ORM1 regulates injury-induced angiogenesis in a time- and context-dependent manner by sequentially dampening the initial TNFα-induced angiogenic response and promoting the downstream stimulation of the angiogenic process by VEGF. The context-dependent nature of ORM1 angioregulatory function is further demonstrated in the CAM assay where ORM1 stimulates developmental angiogenesis without exerting any inhibitory activity.

ATP released from cardiac fibroblasts via connexin hemichannels activates profibrotic P2Y2 receptors

Cardiac fibroblasts (CFs) play an essential role in remodeling of the cardiac extracellular matrix. Extracellular nucleotide signaling may provoke a profibrotic response in CFs. We tested the hypothesis that physical perturbations release ATP from CFs and that ATP participates in profibrotic signaling. ATP release was abolished by the channel inhibitor carbenoxolone and inhibited by knockdown of either connexin (Cx)43 or Cx45 (47 and 35%, respectively), implying that hypotonic stimulation induces ATP release via Cx43 and Cx45 hemichannels, although pannexin 1 may also play a role. ATP released by hypotonic stimulation rapidly (<10 min) increased phosphorylated ERK by 5-8 fold, an effect largely eliminated by P2Y(2) receptor knockdown or ATP hydrolysis with apyrase. ATP stimulation of P2Y(2) receptors increased α-smooth muscle actin (α-SMA) production, and in an ERK-dependent manner, ATP increased collagen accumulation by 60% and mRNA expression of profibrotic markers: plasminogen activator inhibitor-1 and monocyte chemotactic protein-1 by 4.5- and 4.0-fold, respectively. Apyrase treatment substantially reduced the basal profibrotic phenotype, decreasing collagen and α-SMA content and increasing matrix metalloproteinase expression. Thus, ATP release activates P2Y(2) receptors to mediate profibrotic responses in CFs, implying that nucleotide release under both basal and activated states is likely an important mechanism for fibroblast homeostasis.

Using imaging methods to interrogate radiation-induced cell signaling

There is increasing emphasis on the use of systems biology approaches to define radiation-induced responses in cells and tissues. Such approaches frequently rely on global screening using various high throughput 'omics' platforms. Although these methods are ideal for obtaining an unbiased overview of cellular responses, they often cannot reflect the inherent heterogeneity of the system or provide detailed spatial information. Additionally, performing such studies with multiple sampling time points can be prohibitively expensive. Imaging provides a complementary method with high spatial and temporal resolution capable of following the dynamics of signaling processes. In this review, we utilize specific examples to illustrate how imaging approaches have furthered our understanding of radiation-induced cellular signaling. Particular emphasis is placed on protein colocalization, and oscillatory and transient signaling dynamics.

Corneal epithelium expresses a variant of P2X(7) receptor in health and disease

Improper wound repair of the corneal epithelium can alter refraction of light resulting in impaired vision. We have shown that ATP is released after injury, activates purinergic receptor signaling pathways and plays a major role in wound closure. In many cells or tissues, ATP activates P2X(7) receptors leading to cation fluxes and cytotoxicity. The corneal epithelium is an excellent model to study the expression of both the full-length P2X(7) form (defined as the canonical receptor) and its truncated forms. When Ca(2+) mobilization is induced by BzATP, a P2X(7) agonist, it is attenuated in the presence of extracellular Mg(2+) or Zn(2+), negligible in the absence of extracellular Ca(2+), and inhibited by the competitive P2X7 receptor inhibitor, A438079. BzATP enhanced phosphorylation of ERK. Together these responses indicate the presence of a canonical or full-length P2X(7) receptor. In addition BzATP enhanced epithelial cell migration, and transfection with siRNA to the P2X(7) receptor reduced cell migration. Furthermore, sustained activation did not induce dye uptake indicating the presence of truncated or variant forms that lack the ability to form large pores. Reverse transcription-polymerase chain reaction and Northern blot analysis revealed a P2X(7) splice variant. Western blots identified a full-length and truncated form, and the expression pattern changed as cultures progressed from monolayer to stratified. Cross-linking gels demonstrated the presence of homo- and heterotrimers. We examined epithelium from age matched diabetic and non-diabetic corneas patients and detected a 4-fold increase in P2X(7) mRNA from diabetic corneal epithelium compared to non-diabetic controls and an increased trend in expression of P2X(7)variant mRNA. Taken together, these data indicate that corneal epithelial cells express full-length and truncated forms of P2X(7), which ultimately allows P2X(7) to function as a multifaceted receptor that can mediate cell proliferation and migration or cell death.

The role of electrical signals in murine corneal wound re-epithelialization

Ion flow from intact tissue into epithelial wound sites results in lateral electric currents that may represent a major driver of wound healing cell migration. Use of applied electric fields (EF) to promote wound healing is the basis of Medicare-approved electric stimulation therapy. This study investigated the roles for EFs in wound re-epithelialization, using the Pax6(+/-) mouse model of the human ocular surface abnormality aniridic keratopathy (in which wound healing and corneal epithelial cell migration are disrupted). Both wild-type (WT) and Pax6(+/-) corneal epithelial cells showed increased migration speeds in response to applied EFs in vitro. However, only Pax6(+/+) cells demonstrated consistent directional galvanotaxis towards the cathode, with activation of pSrc signaling, polarized to the leading edges of cells. In vivo, the epithelial wound site normally represents a cathode, but 43% of Pax6(+/-) corneas exhibited reversed endogenous wound-induced currents (the wound was an anode). These corneas healed at the same rate as WT. Surprisingly, epithelial migration did not correlate with direction or magnitude of endogenous currents for WT or mutant corneas. Furthermore, during healing in vivo, no polarization of pSrc was observed. We found little evidence that Src-dependent mechanisms of cell migration, observed in response to applied EFs in vitro, normally exist in vivo. It is concluded that endogenous EFs do not drive long-term directionality of sustained healing migration in this mouse corneal epithelial model. Ion flow from wounds may nevertheless represent an important component of wound signaling initiation.

Distinct activation of epidermal growth factor receptor by UTP contributes to epithelial cell wound repair

The release of nucleotides after injury activates purinergic receptors, leading to phosphorylation of site-specific residues on epidermal growth factor receptor (EGFR). To elucidate the differences between the injury-induced response and that induced by exogenous EGF, we examined recruitment of docking proteins, internalization of EGFR, and migration after injury. Injury induced by scratch wounds or stimulation by addition of UTP caused a brief internalization of EGFR, which paralleled the lesser association with growth factor receptor-bound protein 2 (Grb2) and phosphorylation of EGFR. The internalization caused by EGF was sustained and detected for longer than 60 minutes and correlated with phosphorylation of the receptor. The EGF caused recruitment of Grb2, phospholipase C-γ-1 (PLCγ1), Shc, and Src to EGFR. Glutathione S-transferase pull downs were performed, and glutathione S-transferase-PLCγ1 showed binding of Grb2 when stimulated with EGF but not with UTP or injury. Furthermore, UTP did not induce PLCγ1 phosphorylation, and the phosphorylation induced by EGF was attenuated by costimulation with UTP. The response to heparin-binding EGF was equivalent to that of EGF. Site-directed mutagenesis showed that phosphorylation of Y1068 and Y1086 of EGFR is required for repair. Together, our results show that injury and activation of purinergic receptors and direct activation of EGFR via EGF induce distinct downstream pathways.

Cell-cell propagation of NF-κB transcription factor and MAP kinase activation amplifies innate immunity against bacterial infection

The enteroinvasive bacterium Shigella flexneri uses multiple secreted effector proteins to downregulate interleukin-8 (IL-8) expression in infected epithelial cells. Yet, massive IL-8 secretion is observed in Shigellosis. Here we report a host mechanism of cell-cell communication that circumvents the effector proteins and strongly amplifies IL-8 expression during bacterial infection. By monitoring proinflammatory signals at the single-cell level, we found that the activation of the transcription factor NF-κB and the MAP kinases JNK, ERK, and p38 rapidly propagated from infected to uninfected adjacent cells, leading to IL-8 production by uninfected bystander cells. Bystander IL-8 production was also observed during Listeria monocytogenes and Salmonella typhimurium infection. This response could be triggered by recognition of peptidoglycan and is mediated by gap junctions. Thus, we have identified a mechanism of cell-cell communication that amplifies innate immunity against bacterial infection by rapidly spreading proinflammatory signals via gap junctions to yet uninfected cells.

Zinc released from injured cells is acting via the Zn2+-sensing receptor, ZnR, to trigger signaling leading to epithelial repair

A role for Zn(2+) in accelerating wound healing is established, yet, the signaling pathways linking Zn(2+) to tissue repair are not well known. We show that in the human HaCaT keratinocytes extracellular Zn(2+) induces a metabotropic Ca(2+) response that is abolished by silencing the expression of the G-protein-coupled receptor GPR39, suggesting that this Zn(2+)-sensing receptor, ZnR, is mediating the response. Keratinocytic-ZnR signaling is highly selective for Zn(2+) and can be triggered by nanomolar concentrations of this ion. Interestingly, Zn(2+) was also released following cellular injury, as monitored by a specific non-permeable fluorescent Zn(2+) probe, ZnAF-2. Chelation of Zn(2+) and scavenging of ATP from conditioned medium, collected from injured epithelial cultures, was sufficient to eliminate the metabotropic Ca(2+) signaling. The signaling triggered by Zn(2+), via ZnR, or by ATP further activated MAP kinase and induced up-regulation of the sodium/proton exchanger NHE1 activity. Finally, activation of ZnR/GPR39 signaling or application of ATP enhanced keratinocytes scratch closure in an in vitro model. Thus our results indicate that extracellular Zn(2+), which is either applied or released following injury, activates ZnR/GPR39 to promote signaling leading to epithelial repair.

The P2Y2 receptor mediates the epithelial injury response and cell migration

Injury to epithelial cells results in the release of ATP and stimulation of purinergic receptors and is thought to alter cell migration and wound repair. Medium from the injured cells triggers Ca(2+) mobilization and phosphorylation of ERK, both of which are inhibited if the medium is pretreated with apyrase. To understand the wound repair mechanism that occurs with injury, our goal was to determine which purinergic receptor(s) was the critical player in the wound response. We hypothesize that the P2Y(2) receptor is the key player in the response of corneal epithelial cells to cell damage and subsequent repair events. Cells transfected with short interfering RNA to either P2Y(2) or P2Y(4) were stimulated either by injury or addition of UTP and imaged using fluo 3-AM to monitor changes in fluorescence. When cells with downregulated P2Y(2) receptors were injured or stimulated with UTP, the intensity of the Ca(2+) release was reduced significantly. However, when cells with downregulated P2Y(4) receptors were stimulated, only the UTP-induced Ca(2+) response was reduced significantly. In addition, downregulation of the P2Y(2) receptor inhibited wound closure compared with unstimulated cells or cells transfected with nontargeting sequence. This downregulation resulted also in an attenuation in phosphorylation of Src and ERK. Together, these data indicate that the P2Y(2) receptor plays a major biological role in the corneal injury response and repair mechanisms.

Gq-coupled purinergic receptors inhibit insulin-like growth factor-I/phosphoinositide 3-kinase pathway-dependent keratinocyte migration

After skin wound, released growth factors and extracellular nucleotides regulate the different phases of healing, including re-epithelialization. Here, we show that, in keratinocytes, purinergic P2Y2 receptors inhibit the motogenic IGF-I/PI3K pathway. Therefore, extracellular nucleotides may play key roles during skin remodelling after wound.

Insulin-like growth factor-I (IGF-I) activation of phosphoinositol 3-kinase (PI3K) is an essential pathway for keratinocyte migration that is required for epidermis wound healing. We have previously reported that activation of Gα_(q/11)-coupled-P2Y₂ purinergic receptors by extracellular nucleotides delays keratinocyte wound closure. Here, we report that activation of P2Y₂ receptors by extracellular UTP inhibits the IGF-I–induced p110α-PI3K activation. Using siRNA and pharmacological inhibitors, we demonstrate that the UTP antagonistic effects on PI3K pathway are mediated by Gα_(q/11)—and not G_(i/o)—independently of phospholipase Cβ. Purinergic signaling does not affect the formation of the IGF-I receptor/insulin receptor substrate-I/p85 complex, but blocks the activity of a membrane-targeted active p110α mutant, indicating that UTP acts downstream of PI3K membrane recruitment. UTP was also found to efficiently attenuate, within few minutes, the IGF-I–induced PI3K-controlled translocation of the actin-nucleating protein cortactin to the plasma membrane. This supports the UTP ability to alter later migratory events. Indeed, UTP inhibits keratinocyte spreading and migration promoted by either IGF-I or a membrane-targeted active p110α mutant, in a Gα(q/11)-dependent manner both. These findings provide new insight into the signaling cross-talk between receptor tyrosine kinase and Gα_(q/11)-coupled receptors, which mediate opposite effects on p110α-PI3K activity and keratinocyte migration.

Functional coupling expression of COX-2 and cPLA2 induced by ATP in rat vascular smooth muscle cells: role of ERK1/2, p38 MAPK, and NF-kappaB

AIMS

Vascular smooth muscle cells (VSMCs) that function as synthetic units play important roles in inflammatory diseases such as atherosclerosis and angiogenesis. As extracellular nucleotides such as ATP have been shown to act via activation of P(2) purinoceptors implicated in various inflammatory diseases, we hypothesized that extracellular nucleotides contribute to vascular diseases via upregulated expression of inflammatory proteins, such as cyclooxygenase (COX-2) and cytosolic phospholipase A2 (cPLA2) in VSMCs.

METHODS AND RESULTS

Western blotting, promoter assay, RT-PCR, and PGE2 immunoassay revealed that ATPgammaS induced expression of COX-2 and prostaglandin (PGE2) synthesis through the activation of p42/p44 MAPK (mitogen-activated protein kinase), p38 MAPK, and nuclear factor-kappaB (NF-kappaB). These responses were attenuated by inhibitors of MAPK/ERK kinase (MEK1/2; U0126), p38 MAPK (SB202190), and NF-kappaB (helenalin), or by tranfection with dominant negative mutants of p42, p38, IkappaB kinase (IKK)alpha, and IKKbeta. Furthermore, the ATPgammaS-stimulated translocation of NF-kappaB into the nucleus and degradation of IkappaBalpha was blocked by U0126 and helenalin. In addition, the ATPgammaS-stimulated cPLA2 expression was inhibited by U0126, SB202190, helenalin, celecoxib (a selective COX-2 inhibitor), and PGE2 receptor antagonists (AH6809, GW627368X, and SC-19220). However, the inhibitory effect of celecoxib on cPLA2 expression was reversed by addition of exogenous PGE2.

CONCLUSION

Our results suggest that in VSMCs, activation of p42/p44 MAPK, p38 MAPK, and NF-kappaB is essential for ATPgammaS-induced COX-2 expression and PGE2 synthesis. Newly synthesized PGE2 was observed to act as an autocrine signal contributing to cPLA2 expression, which may be implicated in inflammatory responses. Collectively, our findings provide insights into the correlation between COX-2 and cPLA2 expression in ATPgammaS-stimulated VSMCs with similar molecular mechanisms and functional coupling to amplify the occurrence of vessel disease-related vascular inflammation.

The time-course analysis of gene expression during wound healing in mouse skin

RNA analysis has been applied to forensic work to determine wound age. We investigated mRNA expression using quantitative RT-PCR of ten genes, including c-fos, fosB, mitogen-activated protein kinase phosphatase-1 (MKP-1), CD14, chemokine (C-C motif) ligand 9 (CCL9), placenta growth factor (PlGF), mast cell protease-5 (MCP-5), growth arrest specific 5 (Gas5), beta-2 microglobulin (B2M) and major urinary protein-1 (MUP-1), in terms of repair response in adult mice. The expression level of c-fos, fosB and MKP-1 transcripts increased drastically, peaked within 1h, and that of the CD14 and CCL9 transcripts peaked from 12 to 24h. An increase in PlGF and MCP-5 mRNA appeared on about day 5. Gas5, B2M and MUP-1 transcripts showed no significant change. Each gene had differentially expressional patterns with time-course. Our result implied that the observation of the 7 genes in wounded skin could serve to aid in the accurate diagnosis of wound age.

Wounding sheets of epithelial cells activates the epidermal growth factor receptor through distinct short- and long-range mechanisms

Wounding epithelia induces activation of the epidermal growth factor receptor (EGFR), which is absolutely required for induction of motility. ATP is released from cells after wounding; it binds to purinergic receptors on the cell surface, and the EGFR is subsequently activated. Exogenous ATP activates phospholipase D, and we show here that ATP activates the EGFR through the phospholipase D2 isoform. The EGFR is activated in cells far (>0.3 cm) from wounds, which is mediated by diffusion of extracellular ATP because activation at a distance from wounds is abrogated by eliminating ATP in the medium with apyrase. In sharp contrast, activation of the EGFR near wounds is not sensitive to apyrase. Time-lapse microscopy revealed that cells exhibit increased motilities near edges of wounds; this increase in motility is not sensitive to apyrase, and apyrase does not detectably inhibit healing of wounds in epithelial sheets. This novel ATP/PLD2-independent pathway activates the EGFR by a transactivation process through ligand release, and it involves signaling by a member of the Src family of kinases. We conclude that wounding activates two distinct signaling pathways that induce EGFR activation and promote healing of wounds in epithelial cells. One pathway signals at a distance from wounds through release of ATP, and another pathway acts locally and is independent on ATP signaling.

Damage-induced activation of ERK1/2 in cochlear supporting cells is a hair cell death-promoting signal that depends on extracellular ATP and calcium

Acoustic overstimulation and ototoxic drugs can cause permanent hearing loss as a result of the damage and death of cochlear hair cells. Relatively little is known about the signaling pathways triggered by such trauma, although a significant role has been described for the c-Jun N-terminal kinase [one of the mitogen-activated protein kinases (MAPKs)] pathway. We investigated the role of another MAPK family, the extracellularly regulated kinases 1 and 2 (ERK1/2) during hair cell damage in neonatal cochlear explants. Within minutes of subjecting explants to mechanical damage, ERK1/2 were transiently activated in Deiters' and phalangeal cells but not in hair cells. The activation of ERK1/2 spread along the length of the cochlea, reaching its peak 5-10 min after damage onset. Release of extracellular ATP and the presence of functional connexin proteins were critical for the activation and spread of ERK1/2. Damage elicited an intercellular Ca(2+) wave in the hair cell region in the first seconds after damage. In the absence of Ca(2+) influx, the intercellular Ca(2+) wave and the magnitude and spread of ERK1/2 activation were reduced. Treatment with the aminoglycoside neomycin produced a similar pattern of ERK1/2 activation in supporting cells surrounding pyknotic hair cells. When ERK1/2 activation was prevented, there was a reduction in the number of pyknotic hair cells. Thus, activation of ERK1/2 in cochlear supporting cells in vitro is a common damage signaling mechanism that acts to promote hair cell death, indicating a direct role for supporting cells in regulating hair cell death.

Regulation by P2X7: epithelial migration and stromal organization in the cornea

PURPOSE

Previously, the authors demonstrated that BzATP, a P2X(7) receptor agonist, enhanced corneal epithelial migration in vitro. The goal here was to characterize the role of the P2X(7) receptor in the repair of in vivo corneal epithelial debridement wounds and in the structural organization of the corneal stroma.

METHODS

Epithelial debridement was performed on P2X(7) knockout (P2X(7)(-/-)) and wild-type (WT) mice, and eyes were harvested after 16 hours. Corneas were stained with Richardson vital stain, and the wound area was recorded. Corneas were fixed and prepared for light microscopic, immunohistochemical, and electron microscopic analysis. Cuprolinic blue staining was performed to analyze stromal proteoglycans (PGs). Real-time PCR was performed to examine the expression of stromal collagens.

RESULTS

P2X(7) was present in the WT corneal epithelium but was not detected in P2X(7)(-/-) mice. Pannexin-1, a protein demonstrated to interact with P2X(7), was absent from the wound edge in P2X(7)(-/-). This was associated with a trend toward delayed corneal reepithelialization. Stromal ultrastructure and collagen alignment were altered in P2X(7)(-/-), and collagen fibrils had smaller diameters with a larger interfibrillar distances. Expression of collagen alpha1(I) and alpha3(v) was reduced. There were 30% fewer sulfated PGs along fibrils in the P2X(7)(-/-) stroma.

CONCLUSIONS

In the absence of the P2X(7) receptor, the expression of proteins in the corneal epithelium was altered and wound healing was compromised. Loss of receptor resulted in morphologic changes in the stroma, including changes in alignment of collagen fibrils, decreased expression of collagen, and smaller fibrils with fewer PGs per fibril.

Nucleotides in ocular secretions: their role in ocular physiology

The eye is the sense organ that permits the detection of light owing to the existence of a sophisticated neuronal array, called the retina, which is responsive to photons. The correct functioning of this complex system requires the coordination of several intraocular structures that ultimately permit the perfect focusing of images on the neural retina. Light has to pass through different media: the tear, the cornea, aqueous humour, lens, and vitreous humour before it reaches the retina. Moreover, the composition and structure of some of these media can change due to several physiological mechanisms. Nucleotides are active components of the humours bathing relevant ocular structures. The tear contains nucleotides and dinucleotides that control the process of tearing, wound healing and protects of superficial infections. In the inner eye, the aqueous humour also presents a collection of mono and dinucleotides that affect pupil contraction, aqueous humour production and accommodation. Behind the lens and between this structure and the retina the vitreous humour can modify the physiology of the retinal cells, mostly the ganglion cells. By investigating the actions of nucleotides and dinucleotide present in the ocular humours we will be able not only to understand the functioning of the ocular structures but also to develop new pharmacological therapies for pathologies such as dry eye, glaucoma or retinal detachment.

Calcium signalling in wound-responsive normal human urothelial cell monolayers

Epithelial tissue repair requires coordination of migratory and proliferative activity both adjacent to and remote from the wound edge. Although calcium signalling is implicated, the specific mechanisms are poorly understood. This study characterises the calcium signal invoked in response to scratch wounding of normal human urothelial (NHU) cells and relates it to the localised cellular response. Immediately after wounding of confluent NHU cell monolayers, cells adjacent to the wound edge showed a sustained (>30 min) rise in [Ca(2+)](i), while there was an independent, but simultaneous calcium wave that propagated out from the wound edge. The transient signal involved release of calcium from intracellular stores and was not mediated via gap junctions, but by diffusion of extracellular agonists. We demonstrated that ATP was partially responsible for the initiation and propagation of the calcium wave and showed that the calcium release mechanism was mediated in part via activation of inositol-1,4,5-triphosphate (IP(3)) receptors. By contrast, the sustained calcium signal originated from the extracellular milieu and correlated with an increased rate of migration by these cells. The work presented here provides supportive evidence that the calcium signature, defined by its temporal and amplitude characteristics, is important in co-ordinating the response of cells within an epithelial cell monolayer after wounding.

P2Y2 nucleotide receptors inhibit trauma-induced death of astrocytic cells

Nucleotides as well as other neurotransmitters are known to be released to the extracellular space upon injury. To determine whether nucleotides acting on P2Y(2) nucleotide receptors promote protective or degenerative events after trauma in astrocytic cells, a well-established model of in vitro brain trauma was applied to 1321N1 cells expressing recombinant P2Y(2) nucleotide receptors (P2Y(2)R-1321N1). Cellular death was examined by measuring DNA fragmentation and caspase activation. Fragmented DNA was observed 48 h post-injury in 1321N1 cells, while P2Y(2) nucleotide receptor expressing cells did not show DNA fragmentation. A laddering pattern of fragmented DNA following injury was observed upon inhibition of P2Y(2) nucleotide receptors with suramin. Time-dependent increases of cleaved caspase-9, a mitochondrial-associated caspase, correlated with injury-induced cellular death. A decreased bax/bcl-2 gene expression ratio was observed in P2Y(2)R-1321N1 cells after traumatic injury, while untransfected 1321N1 cells showed a significant time-dependent increase of the bax/bcl-2 gene expression ratio. Activation of protein kinases was assessed to determine the signaling pathways involved in cell death and survival responses following traumatic injury. In P2Y(2)R-1321N1 and 1321N1 cells p38 phosphorylation was stimulated in a time-dependent manner but the phosphatidylinositol 3-kinase-dependent activation of extracellular signal-regulated kinase 1/2 and protein kinase B (PKB)/Akt was only observed in P2Y(2)R-1321N1 cells after injury. The stress-activated protein kinase/c-Jun NH2-terminal kinase (SAPK/JNK) signaling pathway was not activated by traumatic injury in either astrocytic cell line. Inhibition of p38 kinase signaling pathway by treatment with PD1693, a MKK3/6 inhibitor, abolished the expression of cleaved caspase-9, the increase in the bax/bcl-2 gene expression ratio, as well as the fragmentation of DNA that followed injury of 1321N1 cells. Taken together, our results demonstrate a novel role for P2Y(2) nucleotide receptors and extracellular nucleotides in mediating survival responses to glial cells undergoing cellular death induced by trauma.

Extracellular nucleotides regulate CCL20 release from human primary airway epithelial cells, monocytes and monocyte-derived dendritic cells

Extracellular nucleotides regulate ion transport and mucociliary clearance in human airway epithelial cells (HAECs) via the activation of P2 receptors, especially P2Y(2). Therefore, P2Y(2) receptor agonists represent potential pharmacotherapeutic agents to treat cystic fibrosis (CF). Nucleotides also modulate inflammatory properties of immune cells like dendritic cells (DCs), which play an important role in mucosal immunity. Using DNA-microarray experiments, quantitative RT-PCR and cytokine measurements, we show here that UTP up-regulated approximately 2- to 3-fold the antimicrobial chemokine CCL20 expression and release in primary HAECs cultured on permeable supports at an air-liquid interface (ALI). Both P2Y(2) (ATPgammaS, UTP, INS365) and P2Y(6) (UDP, INS48823) agonists increased CCL20 release. UTP-induced CCL20 release was insensitive to NF-kappaB pathway inhibitors but sensitive to inhibitors of ERK1/2 and p38/MAPK pathways. Furthermore, UTP had no effect on interleukin-(IL)-8 release and reduced the release of both CCL20 and IL-8 induced by TNF-alpha and LPS. Accordingly, UTP reduced the capacity of basolateral supernatants of HAECs treated with TNF-alpha or LPS to induce the chemoattraction of both CD4(+) T lymphocytes and neutrophils. In addition, we show that, in monocyte-derived DCs, ATPgammaS, and UDP but not UTP/INS365-stimulated CCL20 release. Likewise, UDP but not ATPgammaS was also able to increase CCL20 release from monocytes. Pharmacological experiments suggested an involvement of P2Y(11) or P2Y(6) receptors through NF-kappaB, ERK1/2, and p38/MAPK pathways. Altogether, our data demonstrate that nucleotides may modulate chemokine release and leukocyte recruitment in inflamed airways by acting on both epithelial and immune cells. Our results could be relevant for further clinical investigations in CF.

Extracellular adenosine triphosphate induces glutamate transporter-1 expression in hippocampus

ATP can be significantly released following various brain insults and activates the extracellular signal-regulated protein kinase (ERK) pathway in astrocytes. Glutamate transporter-1 (GLT1) is the major forebrain astroglial glutamate transporter and its expression is stimulated also via ERK1/2 phosphorylation. We thus hypothesized that extracellular ATP could be a signal to GLT1 modulation in hippocampal slices obtained from rat. We indeed observed by western blot analysis that, after 1 mM ATP exposure, GLT1 expression, but not the glutamate-aspartate transporter, was enhanced. At the same time, high ATP induced significant rates of cell death in piramidal and granule cell layers, as shown by propidium iodide uptake, and increased glutamate uptake through GLT1 transporter. Also using confocal laser-scanning microscopy, we observed that ATP induced a vigorous and extensive GLT1-labeling on glial fibrillary acidic protein-positive cells. This stimulation was abolished by purine/pyrimidine nucleotide receptor antagonists and by MEK1/2 inhibitor. The present study demonstrates a novel mechanism of GLT1 regulation by extracellular ATP, reinforcing the evidence of cross talk between glutamatergic and purinergic systems.

Injury and nucleotides induce phosphorylation of epidermal growth factor receptor: MMP and HB-EGF dependent pathway

The early events that occur rapidly after injury trigger signal cascades that are essential for proper wound closure of corneal epithelial cells. We hypothesize that injury releases ATP, which stimulates purinergic receptors and elicits the phosphorylation of epidermal growth factor receptor (EGFR) tyrosine residues and subsequent cell migration by a MMP and HB-EGF dependent pathway. We demonstrated that the inhibition of purinergic receptors with the antagonist, Reactive Blue 2, abrogated the phosphorylation of EGFR and ERK. Pre-incubation of cells with the EGFR kinase inhibitor, AG1478, and subsequent stimulation by injury or ATP resulted in a decrease in phosphorylation of EGFR and migration. Furthermore, downregulation of EGFR by siRNA, inhibited the EGF-induced intracellular Ca(2+) wave. However, the response to injury and ATP was retained indicating the presence of two signaling pathways. Inhibition with either CRM197 or TIMP-3 decreased injury and nucleotide-induced phosphorylation of both EGFR and ERK. Incubation in the presence of a functional blocking antibody to HB-EGF also resulted in a decrease in the phosphorylation of EGFR. In addition, cell migration was inhibited by CRM197 and rescued when cells were incubated with HB-EGF. We showed that injury-induced phosphorylation of specific tyrosine residues and found that a similar pattern of phosphorylation was induced by trinucleotides. These studies indicate that injury-induced purinergic receptor activation leads to phosphorylation of EGFR, ERK and migration.

Wound-induced ATP release and EGF receptor activation in epithelial cells

We have shown previously that wounding of human corneal epithelial (HCE) cells resulted in epidermal growth factor receptor (EGFR) transactivation through ectodomain shedding of heparin-binding EGF-like growth factor (HB-EGF). However, the initial signal to trigger these signaling events in response to cell injury remains elusive. In the present study, we investigated the role of ATP released from the injured cells in EGFR transactivation in HCE cells as well as in BEAS 2B cells, a bronchial epithelial cell line. Wounding of epithelial monolayer resulted in the release of ATP into the culture medium. The wound-induced rapid activation of phosphatidylinositol-3-kinase (PI3K) and extracellular signal-regulated kinase (ERK) pathways in HCE cells was attenuated by eliminating extracellular ATP, ADP and adenosine. The nonhydrolyzable ATP analog ATP-gamma-S induced rapid and sustained EGFR activation that depended on HB-EGF shedding and ADAM (a disintegrin and metalloproteinase). Targeting pathways leading to HB-EGF shedding and EGFR activation attenuated ATP-gamma-S-enhanced closure of small scratch wounds. The purinoceptor antagonist reactive blue 2 decreased wound closure and attenuated ATP-gamma-S induced HB-EGF shedding. Taken together, our data suggest that ATP, released upon epithelial injury, acts as an early signal to trigger cell responses including an increase in HB-EGF shedding, subsequent EGFR transactivation and its downstream signaling, resulting in wound healing.

Airway epithelial cell migration and wound repair by ATP-mediated activation of dual oxidase 1

The airway epithelium is continuously subjected to environmental pollutants, airborne pathogens, and allergens and relies on several intrinsic mechanisms to maintain barrier integrity and to promote epithelial repair processes following injury. Here, we report a critical role for dual oxidase 1 (Duox1), a newly identified NADPH oxidase homolog within the tracheobronchial epithelium, in airway epithelial cell migration and repair following injury. Activation of Duox1 during epithelial injury is mediated by cellular release of ATP, which signals through purinergic receptors expressed on the epithelial cell surface. Purinergic receptor stimulation by extracellular ATP is a critical determinant of epithelial cell migration and repair following injury and is associated with activation of extracellular signal-regulated kinases (ERK1/2) and matrix metalloproteinase-9 (MMP-9). Stimulation of these integral features of epithelial cell migration and repair processes was found to require the activation of Duox1. Our findings demonstrate a novel role for Duox1 in the tracheobronchial epithelium, in addition to its proposed role in antimicrobial host defense, by participating in epithelial repair processes to maintain epithelial integrity and barrier function in the face of environmental stress.

Phospholipase D and extracellular signal-regulated kinase in hepatic stellate cells: effects of platelet-derived growth factor and extracellular nucleotides

We have previously provided evidence suggesting that phosphatidic acid, possibly derived from the hydrolysis of phosphatidylcholine by phospholipase D (PLD), is involved in platelet-derived growth factor (PDGF)-mediated increases in extracellular signal-regulated kinase (ERK) activity and DNA synthesis in rat hepatic stellate cells (HSC), the primary fibrogenic cells of the liver. A recent study has shown the presence of P2Y nucleotide receptors on HSC that are coupled to contraction and synthesis of the matrix component, alpha1-procollagen, leading to the suggestion that they may represent a new therapeutic target in the treatment of liver fibrosis. However, although extracellular nucleotides have been shown to stimulate both PLD and ERK, and to elicit proliferation of fibrogenic cells outside the liver, their effect on these parameters in HSC have not yet been investigated. PLD activity was determined by [3H]choline release and [3H]phosphatidylbutanol production, ERK activity by Western blotting, and DNA synthesis by [3H]thymidine incorporation. We report here, for the first time in HSC, that extracellular nucleotides stimulate PLD activity and a sustained activation of ERK. However, in contrast to PDGF, nucleotides had negligible effects on DNA synthesis. Moreover, the effects of PDGF and nucleotides on PLD and ERK were not additive, suggesting activation of the same PLD isoform and pool of ERK. The data demonstrate that nucleotide-stimulated PLD and ERK activities are not coupled to DNA synthesis in HSC. Instead, these responses may be linked to other phenotypic changes associated with activated HSC such as increases in contraction, motility, or extracellular matrix deposition.

In vivo detection of secreted proteins from wounded skin using capillary ultrafiltration probes and mass spectrometric proteomics

The identification of in vivo secreted proteins is a major challenge in systems biology. Here we report a novel technique using capillary ultrafiltration (CUF) probes to identify the secreted proteins involved in wound healing. CUF probes, which use semipermeable membrane hollow fibers to continuously capture secreted proteins, were used to sample skin wound fluids. To identify low-abundance proteins, we digested the CUF probe-collected wound fluid with trypsin and then directly subjected it to MS without using 2-DE separation. Two protein fragments with masses of 1565.7 and 1694.8 Da were identified by MS as peptides of thymosin beta10 and beta4, respectively. This is the first identification of thymosin beta10 as an in vivo constituent of the skin wound fluid. The LKKTETQ peptide, a common actin-binding domain of thymosin beta4 and beta10, significantly enhanced skin wound healing in vitro and in vivo. Our data suggest that the enhancement of wound healing by LKKTETQ may be mediated by purinergic receptors. The technique of using CUF probes linked to mass spectrometric proteomics represents a powerful method to identify in vivo secreted proteins, and may be applicable for identification of proteins relevant in various human diseases.

The roles of calcium signaling and ERK1/2 phosphorylation in a Pax6+/- mouse model of epithelial wound-healing delay

Background

Congenital aniridia caused by heterozygousity at the PAX6 locus is associated with ocular surface disease including keratopathy. It is not clear whether the keratopathy is a direct result of reduced PAX6 gene dosage in the cornea itself, or due to recurrent corneal trauma secondary to defects such as dry eye caused by loss of PAX6 in other tissues. We investigated the hypothesis that reducing Pax6 gene dosage leads to corneal wound-healing defects. and assayed the immediate molecular responses to wounding in wild-type and mutant corneal epithelial cells.

Results

Pax6^+/- mouse corneal epithelia exhibited a 2-hour delay in their response to wounding, but subsequently the cells migrated normally to repair the wound. Both Pax6^+/+ and Pax6^+/- epithelia activated immediate wound-induced waves of intracellular calcium signaling. However, the intensity and speed of propagation of the calcium wave, mediated by release from intracellular stores, was reduced in Pax6^+/- cells. Initiation and propagation of the calcium wave could be largely decoupled, and both phases of the calcium wave responses were required for wound healing. Wounded cells phosphorylated the extracellular signal-related kinases 1/2 (phospho-ERK1/2). ERK1/2 activation was shown to be required for rapid initiation of wound healing, but had only a minor effect on the rate of cell migration in a healing epithelial sheet. Addition of exogenous epidermal growth factor (EGF) to wounded Pax6^+/- cells restored the calcium wave, increased ERK1/2 activation and restored the immediate healing response to wild-type levels.

Conclusion

The study links Pax6 deficiency to a previously overlooked wound-healing delay. It demonstrates that defective calcium signaling in Pax6^+/- cells underlies this delay, and shows that it can be pharmacologically corrected. ERK1/2 phosphorylation is required for the rapid initiation of wound healing. A model is presented whereby minor abrasions, which are quickly healed in normal corneas, transiently persist in aniridic patients, compromising the corneal stroma.

The extracellular zinc-sensing receptor mediates intercellular communication by inducing ATP release

Taste and salivary secretion disorders have been linked to zinc deficiency, indeed zinc is found in secretory granules in the salivary gland. The signaling role for the zinc release in this tissue, however, is poorly understood. Here, we address the signaling pathways and physiological role of the zinc-sensing receptor, ZnR, in the ductal salivary gland cell line, HSY. Exposure of these cells to zinc triggered intracellular Ca2+ release from thapsigargin-sensitive stores. The G alpha q inhibitor, YM-254890 (1 microM), eliminated the Zn2+-dependent Ca2+ response, demonstrating that ZnR is a G alpha q-coupled receptor. Dose-response curves yielded an apparent K0.5 of 36 microM and a Hill coefficient of 7 in the absence of extracellular Ca2+, and K0.5 of 55 microM with a Hill coefficient of 3 in its presence. This indicates that although Zn2+ is essential for ZnR activation, Ca2+ may affect the receptor co-operativity. The homologous desensitization pattern of ZnR was characterized by pre-exposure of cells to Zn2+ at concentrations found to activate the receptor. Re-exposure of cells to Zn2+ elicited an attenuated Zn2+-dependent Ca2+ response for at least 3 h, indicating that the ZnR is strongly desensitized by Zn2+. Finally, we studied the paracrine affects of ZnR using a co-culture consisting of the HSY cells and vascular smooth muscle cells (VSMCs). While no Zn2+-dependent Ca2+ release was observed in VSMC alone, application of Zn2+ to the co-culture induced a Ca2+ rise in both HSY cells and VSMC. This Ca2+ rise was inhibited by the ATP scavenger, apyrase. Taken together, our results demonstrate that ZnR activity is monitored in salivary cells and is modulated by extracellular Ca2+. We further show that ZnR enhances secretion of ATP, thereby linking zinc to key signaling pathways involved in modification of salivary secretions by the ductal cells.

Tri-nucleotide receptors play a critical role in epithelial cell wound repair

The cornea plays a major role in the refraction of light to the retina. Therefore, the integrity and transparency of the corneal epithelium are critical to vision. Following injury, a combination of rapid signal transduction events and long-term cell migration are essential for wound closure. We have demonstrated previously that injury resulted in the release of nucleotides that induce the propagation of a Ca²⁺ wave to neighboring cells. This suggests that nucleotides and their receptors are critical components of wound healing. Epidermal growth factor (EGF) and integrins also have been shown to play a role in injury. In this study, we demonstrate that pretreatment of cells with ATP and UTP inhibited the immediate wound response, while BzATP, ADP, and UDP did not affect this response. Tri-nucleotide pretreatment also reduced the EGF induced Ca²⁺ response. Additionally, lower EC₅₀ concentrations of ATP and UTP triggered migration of cells that was enhanced further with EGF and was inhibited by the tripeptide, RGD. Results indicate that the desensitization induced by ATP and UTP was specific. While ADP and UDP cause a homologous desensitization of their own signal, they did not cause an inhibition of the wound response nor does BzATP. Neither Ca²⁺ wave propagation nor cell migration occurred in response to β,γ-MeATP. Together these results lead us to hypothesize that corneal epithelial wound repair is mediated by both P2Y₂ and P2Y₄ receptors.

P2Y receptors play a critical role in epithelial cell communication and migration

Cellular injury induces a complex series of events that involves Ca2+ signaling, cell communication, and migration. One of the first responses following mechanical injury is the propagation of a Ca2+ wave (Klepeis et al. [2001] J Cell Sci 114(Pt 23):4185-4195). The wave is generated by the extracellular release of ATP, which also induces phosphorylation of ERK (Yang et al. [2004] J Cell Biochem 91(5):938-950). ATP and other nucleotides, which bind to and activate specific purinergic receptors were used to mimic injury. Our goal was to determine which of the P2Y purinergic receptors are expressed and stimulated in corneal epithelial cells and which signaling pathways are activated leading to changes in cell migration, an event critical for wound closure. In this study, we demonstrated that the P2Y1, P2Y2, P2Y4, P2Y6, and P2Y11 receptors were present in corneal epithelial cells. A potency profile was determined by Ca2+ imaging for nucleotide agonists as follows: ATP > or = UTP > ADP > or = UDP. In contrast, negligible responses were seen for beta,gamma-meATP, a general P2X receptor agonist and adenosine, a P1 receptor agonist. Homologous desensitization of the Ca2+ response was observed for the four nucleotides. However, P2Y receptor internalization and degradation was not detected following stimulation with ATP, which is in contrast to EGFR internalization observed in response to EGF. ATP induced cell migration was comparable to that of EGF and was maximal at 1 microM. Cells exposed to ATP, UTP, ADP, and UDP demonstrated a rapid twofold increase in phosphorylation of paxillin at Y31 and Y118, however, there was no activation elicited by beta,gamma-meATP or adenosine. Additional studies demonstrated that wound closure was inhibited by reactive blue 2. These results indicate that P2Y receptors play a critical role in the injury repair process.