Direct visualization of a stratified epithelium reveals that wounds heal by unified sliding of cell sheets

The association between corneal immune cell dynamics and comfort in silicone hydrogel contact lens wearers

PURPOSE

To study the association between corneal immune cell dynamics and contact lens (CL) comfort, as well as tear film inflammatory mediators in established CL wearers.

METHODS

A prospective, cross-sectional study including existing CL wearers was conducted. Symptoms were assessed with a comfort visual analog scale (VAS) and the Contact Lens Dry Eye Questionnaire-8. In-vivo corneal confocal microscopy was used to track immune cell dynamics over 3 timepoints at 5-minute intervals. Trajectory speed was calculated as the total length travelled by cells divided by time; displacement speed as the straight-line distance between the start and end positions of a cell divided by time; and persistence ratio as the ratio of displacement speed and trajectory speed. Measurements were performed for 1-3 cells per participant, and the minimum, maximum, and average values were analyzed. Tear film samples were collected to assess matrix metalloproteinase (MMP)-9, tissue inhibitor of metalloproteinase (TIMP)-1, and the MMP-9/TIMP-1 ratio. Correlations between immune cell dynamics, symptoms, and tear inflammatory mediators, were analyzed.

RESULTS

Nineteen CL wearers (4 men and 15 women) aged 30 ± 5 years were included. Participants wore silicone hydrogel CLs for 8-10 h before clinical assessment. There was a significant association between comfort VAS and the maximum (rho = 0.533; p = 0.019) and average (rho = 0.506; p = 0.027) values of immune cell displacement speed. The concentration of MMP-9 was associated with the minimum value of trajectory speed (rho = 0.621; p = 0.031). Finally, the MMP-9/TIMP-1 ratio was associated with the maximum value of trajectory speed (rho = -0.717; p = 0.030), and the maximum (rho = -0.720; p = 0.008) and average (rho = -0.678; p = 0.015) values of displacement speed.

CONCLUSION

Corneal immune cell dynamics is related to sensations of discomfort in silicone hydrogel CL wearers, with cell speed possibly being regulated by inflammatory mediators assessed from the tear film. These observations may aid in understanding the mechanisms underlying the discomfort response.

New Opportunities for Electric Fields in Promoting Wound Healing: Collective Electrotaxis

Significance: It has long been hypothesized that naturally occurring electric fields (EFs) aid wound healing by guiding cell migration. Consequently, the application of EFs has significant potential for promoting wound healing. However, the mechanisms underlying the cellular response to EFs remain unclear. Recent Advances: Although the directed migration of isolated single cells under EFs has been studied for decades, only recently has experimental evidence demonstrated the distinct collective migration of large sheets of keratinocytes and corneal epithelial cells in response to applied EFs. Accumulating evidence suggests that the emergent properties of cell groups in response to EF guidance offer new opportunities for EF-assisted directional migration. Critical Issues: In this review, we provide an overview of the field of collective electrotaxis, highlighting key advances made in recent years. We also discuss advanced engineering strategies utilized to manipulate collective electrotaxis. Future Directions: We outline a series of unanswered questions in this field and propose potential applications of collective electrotaxis in developing electrical stimulation technologies for wound healing.

Protocol for electrotaxis of large epithelial cell sheets

Here, we present a protocol for electrotaxis of large epithelial cell sheets without compromising the integrity of cell epithelia in a high-throughput customized directed current electrotaxis chamber. We describe the fabrication and use of polydimethylsiloxane stencils to control the size and shape of human keratinocyte cell sheets. We detail cell tracking, cell sheet contour assay, and particle image velocimetry to reveal the spatial and temporal motility dynamics of cell sheets. This approach is applicable to other collective cell migration studies. For complete details on the use and execution of this protocol, please refer to Zhang et al. (2022).¹.

The uPA/uPAR system in astrocytic wound healing

The repair of injured tissue is a highly complex process that involves cell proliferation, differentiation, and migration. Cell migration requires the dismantling of intercellular contacts in the injured zone and their subsequent reconstitution in the wounded area. Urokinase-type plasminogen activator (uPA) is a serine proteinase found in multiple cell types including endothelial cells, smooth muscle cells, monocytes, and macrophages. A substantial body of experimental evidence with different cell types outside the central nervous system indicates that the binding of uPA to its receptor (uPAR) on the cell surface prompts cell migration by inducing plasmin-mediated degradation of the extracellular matrix. In contrast, although uPA and uPAR are abundantly found in astrocytes and uPA binding to uPAR triggers astrocytic activation, it is unknown if uPA also plays a role in astrocytic migration. Neuronal cadherin is a member of cell adhesion proteins pivotal for the formation of cell-cell contacts between astrocytes. More specifically, while the extracellular domain of neuronal cadherin interacts with the extracellular domain of neuronal cadherin in neighboring cells, its intracellular domain binds to β-catenin, which in turn links the complex to the actin cytoskeleton. Glycogen synthase kinase 3β is a serine-threonine kinase that prevents the cytoplasmic accumulation of β-catenin by inducing its phosphorylation at Ser33, Ser37, and Ser41, thus activating a sequence of events that lead to its proteasomal degradation. The data discussed in this perspective indicate that astrocytes release uPA following a mechanical injury, and that binding of this uPA to uPAR on the cell membrane induces the detachment of β-catenin from the intracellular domain of neuronal cadherin by triggering its extracellular signal-regulated kinase 1/2-mediated phosphorylation at Tyr650. Remarkably, this is followed by the cytoplasmic accumulation of β-catenin because uPA-induced extracellular signal-regulated kinase 1/2 activation also phosphorylates lipoprotein receptor-related protein 6 at Ser1490, which in turn, by recruiting glycogen synthase kinase 3β to its intracellular domain abrogates its effect on β-catenin. The cytoplasmic accumulation of β-catenin is followed by its nuclear translocation, where it induces the expression of uPAR, which is required for the migration of astrocytes from the injured edge into the wounded area.

Propagation dynamics of electrotactic motility in large epithelial cell sheets

Directional migration initiated at the wound edge leads epithelia to migrate in wound healing. How such coherent migration is achieved is not well understood. Here, we used electric fields to induce robust migration of sheets of human keratinocytes and developed an in silico model to characterize initiation and propagation of epithelial collective migration. Electric fields initiate an increase in migration directionality and speed at the leading edge. The increases propagate across the epithelial sheets, resulting in directional migration of cell sheets as coherent units. Both the experimental and in silico models demonstrated vector-like integration of the electric and default directional cues at free edge in space and time. The resultant collective migration is consistent in experiments and modeling, both qualitatively and quantitatively. The keratinocyte model thus faithfully reflects key features of epithelial migration as a coherent tissue in vivo, e.g. that leading cells lead, and that epithelium maintains cell-cell junction.

Bioelectric Signaling: Role of Bioelectricity in Directional Cell Migration in Wound Healing

In wound healing, individual cells' behaviors coordinate movement toward the wound center to restore small or large barrier defects. The migration of epithelial cells as a continuous sheet structure is one of the most important processes by which the skin barrier is restored. How such multicellular and tissue level movement is initiated upon injury, coordinated during healing, and stopped when wounds healed has been a research focus for decades. When skin is wounded, the compromised epithelial barrier generates endogenous electric fields (EFs), produced by ion channels and maintained by cell junctions. These EFs are present across wounds, with the cathodal pole at the wound center. Epithelial cells detect minute EFs and migrate directionally in response to electrical signals. It has long been postulated that the naturally occurring EFs facilitate wound healing by guiding cell migration. It is not until recently that experimental evidence has shown that large epithelial sheets of keratinocytes or corneal epithelial cells respond to applied EFs by collective directional migration. Although some of the mechanisms of the collective cell migration are similar to those used by isolated cells, there are unique mechanisms that govern the coordinated movement of the cohesive sheet. We will review the understanding of wound EFs and how epithelial cells and other cells important to wound healing respond to the electric signals individually as well as collectively. Mounting evidence suggests that wound bioelectrical signaling is an important mechanism in healing. Critical understanding and proper exploitation of this mechanism will be important for better wound healing and regeneration.

Mechanical coupling of supracellular stress amplification and tissue fluidization during exit from quiescence

Most cells in the human body reside in a dormant state characterized by slow growth and minimal motility. During episodes such as wound healing, stem cell activation, and cancer growth, cells adapt to a more dynamic behavior characterized by proliferation and migration. However, little is known about the mechanical forces controlling the transition from static to motile following exit from dormancy. We demonstrate that keratinocyte monolayers install a mechanical system during dormancy that produces a coordinated burst of intercellular mechanical tension only minutes after dormancy exit. The activated forces are essential for large-scale displacements of otherwise motility-restricted cell sheets. Thus, cells sustain a mechanical system during dormancy that idles in anticipation of cell cycle entry and prompt activation of motion.

Cellular quiescence is a state of reversible cell cycle arrest that is associated with tissue dormancy. Timely regulated entry into and exit from quiescence is important for processes such as tissue homeostasis, tissue repair, stem cell maintenance, developmental processes, and immunity. However, little is known about processes that control the mechanical adaption to cell behavior changes during the transition from quiescence to proliferation. Here, we show that quiescent human keratinocyte monolayers sustain an actinomyosin-based system that facilitates global cell sheet displacements upon serum-stimulated exit from quiescence. Mechanistically, exposure of quiescent cells to serum-borne mitogens leads to rapid amplification of preexisting contractile sites, leading to a burst in monolayer tension that subsequently drives large-scale displacements of otherwise motility-restricted monolayers. The stress level after quiescence exit correlates with the level of quiescence depth at the time of activation, and a critical stress magnitude must be reached to overcome the cell sheet displacement barrier. The study shows that static quiescent cell monolayers are mechanically poised for motility, and it identifies global stress amplification as a mechanism for overcoming motility restrictions in confined confluent cell monolayers.

Investigating wound healing characteristics of gingival and skin keratinocytes in organotypic cultures

OBJECTIVES

The gingiva heals at an accelerated rate with reduced scarring when compared to skin. Potential well-studied factors include immune cell number, angiogenesis disparities and fibroblast gene expression. Differential keratinocyte gene expression, however, remains relatively understudied. This study explored the contrasting healing efficiencies of gingival and skin keratinocytes, alongside their differential gene expression patterns.

METHODS

3D organotypic culture models of human gingiva and skin were developed using temporarily immortalised primary keratinocytes. Models were wounded for visualisation of re-epithelialisation and analysis of keratinocyte migration to close the wound gap. Concurrently, differentially expressed genes between primary gingival and skin keratinocytes were identified, validated, and functionally assessed.

RESULTS

Characterisation of the 3D cultures of gingiva and skin showed differentiation markers that recapitulated organisation of the corresponding in vivo tissue. Upon wounding, gingival models displayed a significantly higher efficiency in re-epithelialisation and stratification versus skin, repopulating the wound gap within 24 hours. This difference was likely due to distinct patterns of migration, with gingival cells demonstrating a form of sheet migration, in contrast to skin, where the leading edge was typically 1-2 cells thick. A candidate approach was used to identify several genes that were differentially expressed between gingival and skin keratinocytes. Knockdown of PITX1 resulted in reduced migration capacity of gingival cells.

CONCLUSION

Gingival keratinocytes retain in vivo superior wound healing capabilities in in vitro 2D and 3D environments. Intrinsic gene expression differences could result in gingival cells being 'primed' for healing and play a role in faster wound resolution.

CLINICAL SIGNIFICANCE STATEMENT

The successful development of organotypic models, that recapitulate re-epithelialisation, will underpin further studies to analyse the oral response to wound stimuli, and potential therapeutic interventions, in an in vitro environment.

A machine learning based model accurately predicts cellular response to electric fields in multiple cell types

Many cell types migrate in response to naturally generated electric fields. Furthermore, it has been suggested that the external application of an electric field may be used to intervene in and optimize natural processes such as wound healing. Precise cell guidance suitable for such optimization may rely on predictive models of cell migration, which do not generalize. Here, we present a machine learning model that can forecast directedness of cell migration given a timeseries of previous directedness and electric field values. This model is trained using time series galvanotaxis data of mammalian cranial neural crest cells obtained through time-lapse microscopy of cells cultured at 37 °C in a galvanotaxis chamber at ambient pressure. Next, we show that our modeling approach can be used for a variety of cell types and experimental conditions with very limited training data using transfer learning methods. We adapt the model to predict cell behavior for keratocytes (room temperature, ~ 18–20 °C) and keratinocytes (37 °C) under similar experimental conditions with a small dataset (~ 2–5 cells). Finally, this model can be used to perform in silico studies by simulating cell migration lines under time-varying and unseen electric fields. We demonstrate this by simulating feedback control on cell migration using a proportional–integral–derivative (PID) controller. This data-driven approach provides predictive models of cell migration that may be suitable for designing electric field based cellular control mechanisms for applications in precision medicine such as wound healing.

Quantifying the impact of electric fields on single-cell motility

Cell motility in response to environmental cues forms the basis of many developmental processes in multicellular organisms. One such environmental cue is an electric field (EF), which induces a form of motility known as electrotaxis. Electrotaxis has evolved in a number of cell types to guide wound healing and has been associated with different cellular processes, suggesting that observed electrotactic behavior is likely a combination of multiple distinct effects arising from the presence of an EF. To determine the different mechanisms by which observed electrotactic behavior emerges, and thus to design EFs that can be applied to direct and control electrotaxis, researchers require accurate quantitative predictions of cellular responses to externally applied fields. Here, we use mathematical modeling to formulate and parameterize a variety of hypothetical descriptions of how cell motility may change in response to an EF. We calibrate our model to observed data using synthetic likelihoods and Bayesian sequential learning techniques and demonstrate that EFs bias cellular motility through only one of a selection of hypothetical mechanisms. We also demonstrate how the model allows us to make predictions about cellular motility under different EFs. The resulting model and calibration methodology will thus form the basis for future data-driven and model-based feedback control strategies based on electric actuation.

Understanding cornea epithelial stem cells and stem cell deficiency: Lessons learned using vertebrate model systems

Animal models have contributed greatly to our understanding of human diseases. Here, we focus on cornea epithelial stem cell (CESC) deficiency (commonly called limbal stem cell deficiency, LSCD). Corneal development, homeostasis and wound healing are supported by specific stem cells, that include the CESCs. Damage to or loss of these cells results in blindness and other debilitating ocular conditions. Here we describe the contributions from several vertebrate models toward understanding CESCs and LSCD treatments. These include both mammalian models, as well as two aquatic models, Zebrafish and the amphibian, Xenopus. Pioneering developments have been made using stem cell transplants to restore normal vision in patients with LSCD, but questions still remain about the basic biology of CESCs, including their precise cell lineages and behavior in the cornea. We describe various cell lineage tracing studies to follow their patterns of division, and the fates of their progeny during development, homeostasis, and wound healing. In addition, we present some preliminary results using the Xenopus model system. Ultimately, a more thorough understanding of these cornea cells will advance our knowledge of stem cell biology and lead to better cornea disease therapeutics.

Capturing Stem Cell Behavior Using Intravital and Live Cell Microscopy

Stem cells maintain tissue homeostasis by driving cellular turnover and regeneration upon damage. They reside within specialized niches that provide the signals required for stem cell maintenance. Stem cells have been identified in many tissues and cancer types, but their behavior within the niche and their reaction to microenvironmental signals were inferred from limited static observations. Recent advances in live imaging techniques, such as live cell imaging and intravital microscopy, have allowed the visualization of stem cell behavior and dynamics over time in their (near) native environment. Through these recent technological advances, it is now evident that stem cells are much more dynamic than previously anticipated, resulting in a model in which stemness is a state that can be gained or lost over time. In this review, we will highlight how live imaging and intravital microscopy have unraveled previously unanticipated stem cell dynamics and plasticity during development, homeostasis, regeneration, and tumor formation.

In vivo immune cell dynamics in the human cornea

In vivo confocal microscopy (IVCM) allows the evaluation of the living human cornea at the cellular level. The non-invasive nature of this technique longitudinal, repeated examinations of the same tissue over time. Image analysis of two-dimensional time-lapse sequences of presumed immune cells with and without visible dendrites at the corneal sub-basal nerve plexus in the eyes of healthy individuals was performed. We demonstrated evidence that cells without visible dendrites are highly dynamic and move rapidly in the axial directions. A number of dynamic cells were observed and measured from three eyes of different individuals. The total average displacement and trajectory speeds of three cells without visible dendrites (N = 9) was calculated to be 1.12 ± 0.21 and 1.35 ± 0.17 μm per minute, respectively. One cell with visible dendrites per cornea was also analysed. Tracking dendritic cell dynamics in vivo has the potential to significantly advance the understanding of the human immune adaptive and innate systems. The ability to observe and quantify migration rates of immune cells in vivo is likely to reveal previously unknown insights into corneal and general pathophysiology and may serve as an effective indicator of cellular responses to intervention therapies.

Flexibility sustains epithelial tissue homeostasis

Epithelia surround our bodies and line most of our organs. Intrinsic homeostatic mechanisms replenish and repair these tissues in the face of wear and tear, wounds, and even the presence of accumulating mutations. Recent advances in cell biology, genetics, and live-imaging techniques have revealed that epithelial homeostasis represents an intrinsically flexible process at the level of individual epithelial cells. This homeostatic flexibility has important implications for how we think about the more dramatic cell plasticity that is frequently thought to be associated with pathological settings. In this review, we will focus on key emerging mechanisms and processes of epithelial homeostasis and elaborate on the known molecular mechanisms of epithelial cell interactions to illuminate how epithelia are maintained throughout an organism's lifetime.

Wound Healing Coordinates Actin Architectures to Regulate Mechanical Work

How cells with diverse morphologies and cytoskeletal architectures modulate their mechanical behaviors to drive robust collective motion within tissues is poorly understood. During wound repair within epithelial monolayers in vitro, cells coordinate the assembly of branched and bundled actin networks to regulate the total mechanical work produced by collective cell motion. Using traction force microscopy, we show that the balance of actin network architectures optimizes the wound closure rate and the magnitude of the mechanical work. These values are constrained by the effective power exerted by the monolayer, which is conserved and independent of actin architectures. Using a cell-based physical model, we show that the rate at which mechanical work is done by the monolayer is limited by the transformation between actin network architectures and differential regulation of cell-substrate friction. These results and our proposed mechanisms provide a robust physical model for how cells collectively coordinate their non-equilibrium behaviors to dynamically regulate tissue-scale mechanical output.

Visualizing the Contribution of Keratin-14+ Limbal Epithelial Precursors in Corneal Wound Healing

It is thought that corneal epithelial injuries resolve by leading-edge cells “sliding” or “rolling” into the wound bed. Here, we challenge this notion and show by real-time imaging that corneal wounds initially heal by “basal cell migration.” The K14CreER^T2-Confetti multi-colored reporter mouse was employed to spatially and temporally fate-map cellular behavior during corneal wound healing. Keratin-14⁺ basal epithelia are forced into the wound bed by increased population pressure gradient from the limbus to the wound edge. As the defect resolves, centripetally migrating epithelia decelerate and replication in the periphery is reduced. With time, keratin-14⁺-derived clones diminish in number concomitant with their expansion, indicative that clonal evolution aligns with neutral drifting. These findings have important implications for the involvement of stem cells in acute tissue regeneration, in key sensory tissues such as the cornea.

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Basal limbal epithelial cell proliferation is increased following a corneal injury

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Corneal epithelial wounds initially heal by K14⁺ basal cell migration

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STICS accurately measures clonal dynamics during wound closure

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Computational modeling confirms the pivotal role of LESCs in wound repair

Coordinated collective migration and asymmetric cell division in confluent human keratinocytes without wounding

Epithelial sheet spreading is a fundamental cellular process that must be coordinated with cell division and differentiation to restore tissue integrity. Here we use consecutive serum deprivation and re-stimulation to reconstruct biphasic collective migration and proliferation in cultured sheets of human keratinocytes. In this system, a burst of long-range coordinated locomotion is rapidly generated throughout the cell sheet in the absence of wound edges. Migrating cohorts reach correlation lengths of several millimeters and display dependencies on epidermal growth factor receptor-mediated signaling, self-propelled polarized migration, and a G₁/G₀ cell cycle environment. The migration phase is temporally and spatially aligned with polarized cell divisions characterized by pre-mitotic nuclear migration to the cell front and asymmetric partitioning of nuclear promyelocytic leukemia bodies and lysosomes to opposite daughter cells. This study investigates underlying mechanisms contributing to the stark contrast between cells in a static quiescent state compared to the long-range coordinated collective migration seen in contact with blood serum.

Epithelial sheet migration requires polarized and coordinated cell movement. Here, the authors demonstrate serum-activated collective migration followed by polarized asymmetric cell divisions in otherwise quiescent human keratinocyte monolayers in the absence of wound edges.

In vivo imaging of epithelial wound healing in the cnidarian Clytia hemisphaerica demonstrates early evolution of purse string and cell crawling closure mechanisms

Background

All animals have mechanisms for healing damage to the epithelial sheets that cover the body and line internal cavities. Epithelial wounds heal either by cells crawling over the wound gap, by contraction of a super-cellular actin cable (“purse string”) that surrounds the wound, or some combination of the two mechanisms. Both cell crawling and purse string closure of epithelial wounds are widely observed across vertebrates and invertebrates, suggesting early evolution of these mechanisms. Cnidarians evolved ~600 million years ago and are considered a sister group to the Bilateria. They have been much studied for their tremendous regenerative potential, but epithelial wound healing has not been characterized in detail. Conserved elements of wound healing in bilaterians and cnidarians would suggest an evolutionary origin in a common ancestor. Here we test this idea by characterizing epithelial wound healing in live medusae of Clytia hemisphaerica.

Results

We identified cell crawling and purse string-mediated mechanisms of healing in Clytia epithelium that appear highly analogous of those seen in higher animals, suggesting that these mechanisms may have emerged in a common ancestor. Interestingly, we found that epithelial wound healing in Clytia is 75 to >600 times faster than in cultured cells or embryos of other animals previously studied, suggesting that Clytia may provide valuable clues about optimized healing efficiency. Finally, in Clytia, we show that damage to the basement membrane in a wound gap causes a rapid shift between the cell crawling and purse string mechanisms for wound closure. This is consistent with work in other systems showing that cells marginal to a wound choose between a super-cellular actin cable or lamellipodia formation to close wounds, and suggests a mechanism underlying this decision.

Conclusions

1. Cell crawling and purse string mechanisms of epithelial wound healing likely evolved before the divergence of Cnidaria from the bilaterian lineage ~ 600mya 2. In Clytia, the choice between cell crawling and purse string mechanisms of wound healing depends on interactions between the epithelial cells and the basement membrane.

Electronic supplementary material

The online version of this article (10.1186/s12861-017-0160-2) contains supplementary material, which is available to authorized users.

Rebamipide suppresses TNF-α production and macrophage infiltration in the conjunctiva

PURPOSE

To evaluate the anti-inflammatory effect of rebamipide during corneal epithelial wound healing using a mouse wound repair model.

METHODS

A 2-mm circular disc of the central cornea was demarcated in the right eye of C57BL/6 mice and the epithelium removed. Rebamipide 2% eyedrop was instilled onto the wounded eye 5 times a day (n = 26). Phosphate-buffered saline (PBS) was used in the control group (n = 26). Corneal and conjunctival IL-1β and TNF-α levels were measured at 6 h and 24 h postinjury by ELISA. The wounded area was evaluated by fluorescein staining at 24 h postinjury. Macrophage infiltration was assessed immunohistochemically, and TNF-α secretion from macrophages was examined in vitro.

RESULTS

Conjunctival IL-1β and corneal IL-1β levels were not significantly different between PBS-treated and rebamipide-treated groups. However, conjunctival TNF-α level was significantly lower in the rebamipide-treated group compared with the PBS-treated group. Macrophage migration into the conjunctiva, but not into the cornea, was suppressed by rebamipide treatment. In addition, TNF-α secretion from cultured macrophages was suppressed by rebamipide in a concentration-dependent manner. Rebamipide treatment significantly accelerated corneal epithelial wound healing at 24 h postinjury.

CONCLUSIONS

In a mouse corneal epithelial wound model, rebamipide suppressed TNF-α secretion and macrophage infiltration in the conjunctiva, which might have contributed to accelerated corneal epithelial wound healing in the first 24 h following injury.

Tissue-scale coordination of cellular behaviour promotes epidermal wound repair in live mice

Tissue repair is fundamental to our survival as tissues are challenged by recurrent damage. During mammalian skin repair, cells respond by migrating and proliferating to close the wound. However, the coordination of cellular repair behaviours and their effects on homeostatic functions in a live mammal remains unclear. Here we capture the spatiotemporal dynamics of individual epithelial behaviours by imaging wound re-epithelialization in live mice. Differentiated cells migrate while the rate of differentiation changes depending on local rate of migration and tissue architecture. Cells depart from a highly proliferative zone by directionally dividing towards the wound while collectively migrating. This regional coexistence of proliferation and migration leads to local expansion and elongation of the repairing epithelium. Finally, proliferation functions to pattern and restrict the recruitment of undamaged cells. This study elucidates the interplay of cellular repair behaviours and consequent changes in homeostatic behaviours that support tissue-scale organization of wound re-epithelialization.

Cellular mechanisms of skin repair in humans and other mammals

The increased incidence of non-healing skin wounds in developed societies has prompted tremendous research efforts on the complex process known as "wound healing". Unfortunately, the weak relevance of modern wound healing research to human health continues to be a matter of concern. This review summarizes the current knowledge of the cellular mechanisms that mediate wound closure in the skin of humans and laboratory animals. The author highlights the anatomical singularities of human skin vs. the skin of other mammals commonly used for wound healing research (i.e. as mice, rats, rabbits, and pigs), and discusses the roles of stem cells, myofibroblasts, and the matrix environment in the repair process. The majority of this review focuses on reepithelialization and wound closure. Other aspects of wound healing (e.g. inflammation, fibrous healing) are referred to when relevant to the main topic. This review aims at providing the reader with a clear understanding of the similarities and differences that have been reported over the past 100 years between the healing of human wounds and that of other mammals.

Fate Mapping Mammalian Corneal Epithelia

The anterior aspect of the cornea consists of a stratified squamous epithelium, thought to be maintained by a rare population of stem cells (SCs) that reside in the limbal transition zone. Although migration of cells that replenish the corneal epithelium has been studied for over a century, the process is still poorly understood and not well characterized. Numerous techniques have been employed to examine corneal epithelial dynamics, including visualization by light microscopy, the incorporation of vital dyes and DNA labels, and transplantation of genetically marked cells that have acted as cell and lineage beacons. Modern-day lineage tracing utilizes molecular methods to determine the fate of a specific cell and its progeny over time. Classically employed in developmental biology, lineage tracing has been used more recently to track the progeny of adult SCs in a number of organs to pin-point their location and understand their movement and influence on tissue regeneration. This review highlights key discoveries that have led researchers to develop cutting-edge genetic tools to effectively and more accurately monitor turnover and displacement of cells within the mammalian corneal epithelium. Collating information on the basic biology of SCs will have clinical ramifications in furthering our knowledge of the processes that govern their role in homeostasis, wound-healing, transplantation, and how we can improve current unsatisfactory SC-based therapies for patients suffering blinding corneal disease.

Establishment of a novel in vitro model of stratified epithelial wound healing with barrier function

The repair of wounds through collective movement of epithelial cells is a fundamental process in multicellular organisms. In stratified epithelia such as the cornea and skin, healing occurs in three steps that include a latent, migratory, and reconstruction phases. Several simple and inexpensive assays have been developed to study the biology of cell migration in vitro. However, these assays are mostly based on monolayer systems that fail to reproduce the differentiation processes associated to multilayered systems. Here, we describe a straightforward in vitro wound assay to evaluate the healing and restoration of barrier function in stratified human corneal epithelial cells. In this assay, circular punch injuries lead to the collective migration of the epithelium as coherent sheets. The closure of the wound was associated with the restoration of the transcellular barrier and the re-establishment of apical intercellular junctions. Altogether, this new model of wound healing provides an important research tool to study the mechanisms leading to barrier function in stratified epithelia and may facilitate the development of future therapeutic applications.

Ephrin-Bs Drive Junctional Downregulation and Actin Stress Fiber Disassembly to Enable Wound Re-epithelialization

For a skin wound to successfully heal, the cut epidermal-edge cells have to migrate forward at the interface between scab and healthy granulation tissue. Much is known about how lead-edge cells migrate, but very little is known about the mechanisms that enable active participation by cells further back. Here we show that ephrin-B1 and its receptor EphB2 are both upregulated in vivo, just for the duration of repair, in the first 70 or so rows of epidermal cells, and this signal leads to downregulation of the molecular components of adherens and tight (but not desmosomal) junctions, leading to loosening between neighbors and enabling shuffle room among epidermal cells. Additionally, this signaling leads to the shutdown of actomyosin stress fibers in these same epidermal cells, which may act to release tension within the wound monolayer. If this signaling axis is perturbed, then disrupted healing is a consequence in mouse and man.

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Ephrin-B/EphBs are upregulated in the migrating wound epidermis in mouse and man

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Ephrin-B/EphB signaling drives junction loosening, thus enabling re-epithelialization

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Ephrin-B/EphB signaling also leads to dissolution of stress fibers and tension release

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In human chronic wounds ephrin-Bs are misregulated and may be a therapeutic target

Collective cell migration: Implications for wound healing and cancer invasion

During embryonic morphogenesis, wound repair and cancer invasion, cells often migrate collectively via tight cell-cell junctions, a process named collective migration. During such migration, cells move as coherent groups, large cell sheets, strands or tubes rather than individually. One unexpected finding regarding collective cell migration is that being a “multicellular structure” enables cells to better respond to chemical and physical cues, when compared with isolated cells. This is important because epithelial cells heal wounds via the migration of large sheets of cells with tight intercellular connections. Recent studies have gained some mechanistic insights that will benefit the clinical understanding of wound healing in general. In this review, we will briefly introduce the role of collective cell migration in wound healing, regeneration and cancer invasion and discuss its underlying mechanisms as well as implications for wound healing.

4,8-Sphingadienine and 4-hydroxy-8-sphingenine activate ceramide production in the skin

Background

Ingestion of glucosylceramide improves transepidermal water loss (TEWL) from the skin, but the underlying mechanism by which a small amount of dietary glucosylceramide can vastly improve skin conditions remains unclear. In a previous report, glucosylceramides were shown to be digested to sphingoids, which were shown to be absorbed through the intestinal epithelium. Based on these observations, we hypothesized that sphingoids are the key molecules facilitating endogenous ceramide production. In this study, we assessed the effect of 4,8-sphingadienine (d18:2) and 4-hydroxy-8-sphingenine (t18:1), derived from konjac glucosylceramide, on stimulating ceramide production.

Methods

Konjac glucosylceramide acidolysis was performed using hydrochloric acid; the resulting d18:2 and t18:1 were fractionated by column chromatography. Real-time quantitative RT-PCR was performed to assess the effect of d18:2 and t18:1 on gene expression in normal human epidermal keratinocytes, while their effect on the nuclear receptor, peroxisome proliferator-activated receptor (PPAR)γ, was measured using a receptor-cofactor assay system. The effect of d18:2 and t18:1 on stimulating ceramide production was evaluated using HPTLC analysis in a 3-dimensional human skin model.

Results

We noted the upregulation of genes related to de novo ceramide synthesis as well as of those encoding the elongases of very long-chain fatty acids by d18:2 and t18:1, but not by glucosylceramide and 4-sphingenine. Both these sphingoids also facilitated the expression of PPARβ/δ and PPARγ; moreover, they also demonstrated ligand activity for PPARγ. These results indicated that d18:2 and t18:1 promote the differentiation of keratinocytes. Analysis of the lipids within the 3-dimensional human skin model indicated that treatment with d18:2 and t18:1 not only upregulated gene expression but also increased ceramide production.

Conclusions

The sphingoids d18:2 and t18:1 activated genes related to de novo ceramide synthesis and increased ceramide production, whereas glucosylceramide and 4-sphingenine could not. These results suggest that the effect of dietary glucosylceramides on the skin is mediated by d18:2 and t18:1.

The dimeric platelet collagen receptor GPVI-Fc reduces platelet adhesion to activated endothelium and preserves myocardial function after transient ischemia in mice

Platelets play a critical role in the pathophysiology of reperfusion, sepsis, and cardiovascular diseases. In a multiple step process, they adhere to activated endothelium and release proinflammatory cytokines thereby promoting the inflammatory process. Glycoprotein VI (GPVI) is the major collagen receptor on the platelet surface and triggers platelet activation and primary hemostasis. Activation of GPVI leads to stable platelet adhesion and degranulation of platelet granules. However, GPVI is critically involved in platelet adhesion to activated endothelium without exposure of subendothelial matrix. Earlier studies show that the soluble GPVI-Fc binds to collagen and protects mice from atherosclerosis and decreases neointima proliferation after arterial injury. Here, we show for the first time that recombinant GPVI-Fc binds to activated endothelium mainly via vitronectin and prevents platelet/endothelial interaction. Administration of GPVI-Fc reduced infarct size and preserved cardiac function in a mouse model of myocardial infarction. This process was associated with reduced GPVI-induced platelet degranulation and release of proinflammatory cytokines in vitro and in vivo. Taken together, administration of GPVI-Fc offers a novel strategy to control platelet-mediated inflammation and to preserve myocardial function following myocardial infarction.

Substrate viscosity enhances correlation in epithelial sheet movement

The movement of the epithelium plays vital roles in the development and renewal of complex tissues, from the separation of tissues in the early embryo, to turnover in the homeostasis of the gastrointestinal mucosa. Yet, despite its importance, a clear interpretation of the mechanism for collective motion in epithelial sheets remains elusive. This interpretation is prohibited by the lack of understanding of the relationship between motion and cell-cell contact, and their mediation by the mechanical properties of the underlying substrate. To better mimic physiological substrates that have inherent viscosity, we probe this relationship using polydimethylsiloxane, a substrate whose mechanical properties can be tuned from predominantly elastic to viscous by altering its cross-linking content. We therefore characterize the comparative spatiotemporal correlations in cell velocity during the movement of an epithelial monolayer as a function of the viscoelasticity of the substrate. Our results show that high correlation in cell velocity is achieved when the substrate G''(ω) is ~0.4 × G'(ω). This correlation is driven by a balance between cell-cell contact and the adhesion and contraction of the extracellular matrix. For G'(ω) > G'(ω), this balance shifts, and contraction of the tissue drives the substrate to flow, further elevating the correlation in movement.

Cystic fibrosis transmembrane conductance regulator is involved in airway epithelial wound repair

The role of cystic fibrosis (CF) transmembrane conductance regulator (CFTR) in airway epithelial wound repair was investigated using normal human bronchial epithelial (NHBE) cells and a human airway epithelial cell line (Calu-3) of serous gland origin. Measurements of wound repair were performed using continuous impedance sensing to determine the time course for wound closure. Control experiments showed that the increase in impedance corresponding to cell migration into the wound was blocked by treatment with the actin polymerization inhibitor, cytochalasin D. Time lapse imaging revealed that NHBE and Calu-3 cell wound closure was dependent on cell migration, and that movement occurred as a collective sheet of cells. Selective inhibition of CFTR activity with CFTR(inh)-172 or short hairpin RNA silencing of CFTR expression produced a significant delay in wound repair. The CF cell line UNCCF1T also exhibited significantly slower migration than comparable normal airway epithelial cells. Inhibition of CFTR-dependent anion transport by treatment with CFTR(inh)-172 slowed wound closure to the same extent as silencing CFTR protein expression, indicating that ion transport by CFTR plays a critical role in migration. Moreover, morphologic analysis of migrating cells revealed that CFTR inhibition or silencing significantly reduced lamellipodia protrusion. These findings support the conclusion that CFTR participates in airway epithelial wound repair by a mechanism involving anion transport that is coupled to the regulation of lamellipodia protrusion at the leading edge of the cell.

Coherent movement of cell layers during wound healing by image correlation spectroscopy

We have determined the complex sequence of events from the point of injury until reepithelialization in axolotl skin explant model and shown that cell layers move coherently driven by cell swelling after injury. We quantified three-dimensional cell migration using correlation spectroscopy and resolved complex dynamics such as the formation of dislocation points and concerted cell motion. We quantified relative behavior such as velocities and swelling of cells as a function of cell layer during healing. We propose that increased cell volume ( approximately 37% at the basal layer) is the driving impetus for the start of cell migration after injury where the enlarged cells produce a point of dislocation that foreshadows and dictates the initial direction of the migrating cells. Globally, the cells follow a concerted vortex motion that is maintained after wound closure. Our results suggest that cell volume changes the migration of the cells after injury.

Activation and damage of endothelial cells upon hypoxia/reoxygenation. Effect of extracellular pH

Disturbances of blood flow upon vascular occlusions and spasms result in hypoxia and acidosis, while its subsequent restoration leads to reoxygenation and pH normalization (re-alkalization) in ischemic sites of the vascular bed. The effect of hypoxia/reoxygenation on activation and stimulation of apoptosis in cultured human endothelial cells was studied. The cells were subjected to hypoxia (2% O2, 5% CO2, 93% N(2)) for 24 h followed by reoxygenation (21% O2, 5% CO2, 74% N(2)) for 5 h. Reoxygenation was carried out at different pH-6.4 (preservation of acidosis after hypoxia), 7.0, and 7.4 (partial and complete re-alkalization, respectively). Hypoxia only slightly (by approximately 30%) increased the cell adhesion molecule ICAM-1 content on the cell surface, whereas reoxygenation more than doubled its expression. The reoxygenation effect depended on the medium acidity, and ICAM-1 increase was more pronounced at pH 7.0 compared to that at pH 6.4 and 7.4. Neither hypoxia nor reoxygenation induced expression of two other cell adhesion molecules, VCAM and E-selectin. Incubation of cells under hypoxic conditions but not reoxygenation stimulated secretion of von Willebrand factor and increased its concentration in the culture medium by more than 4 times. The percentage of cells containing apoptosis marker, activated caspase-3, was increased by approximately 1.5 times upon hypoxia as well as hypoxia/reoxygenation. Maximal values were achieved when reoxygenation was performed at pH 7.0. These data show that hypoxia/reoxygenation stimulate pro-inflammatory activation (ICAM-1 expression) and apoptosis (caspase-3 activation) of endothelial cells, and the extracellular pH influences both processes.

Novel cultured porcine corneal irritancy assay with reversibility endpoint

Several alternative assays exist to assess ocular irritancy without the use of live animals. However, these assays cannot address ocular injury reversibility. Reversibility is an issue critical to regulatory authorities and manufactures of commercial products, as ocular irritation caused by misuse or accidental exposure to a product may cause irreversible eye damage. Here we report the development and initial characterization of a novel ocular irritation assay that addresses ocular injury reversibility. This assay, the Porcine Corneal Ocular Reversibility Assay (PorCORA), uses an air-interface porcine corneal culture system to sustain ex vivo porcine corneas as a model system. These corneas are maintained in culture for 21 days to determine if cornea injury, once inflicted, will reverse. Corneal injury reversibility is measured using Sodium Fluorescein (NaFl) stain to detect compromised epithelial barrier function. In this study, we examined the effects of five compounds on the cultured corneas: phosphate-buffered saline (PBS), 100% Ethanol (EtOH), 3% Sodium Dodecyl Sulfate (SDS), 1% Benzalkonium Chloride (BAK), and 10% Sodium Hydroxide (NaOH). Overall, the persistence of corneal effects between historical Draize rabbit eye data and PorCORA indicates a correlation coefficient of 0.98 (for the five compounds tested) and a correlation coefficient of 0.97 with the Draize modified maximal average score (MMAS). Finally, both fluorescence confocal microscopy and histopathology evidence demonstrates that the PorCORA and NaFl measurements are indicative of actual cellular and tissue damage. PorCORA shows promise as a potential non-animal replacement assay capable of predicting ocular damage reversibility.

Ultrastructure of tracheal epithelial cells migrating in an in vivo environment

The tracheal epithelium can be induced to move as a cellular sheet by heterotopic transplantation, which offers the opportunity to observe migrating cells as a group in an in vivo environment. We therefor investigated the ultrastructural characteristics of migrating tracheal epithelial cells with special reference to the moving front using this transplantation. The migrating epithelial cells underwent squamous metaplasia and lost their differentiated characteristics such as cilia or secretory granules. Several unique observations were made concerning the mechanism of mobility: one is that epithelial cells in the front were elongated in a direction perpendicular to the course of movement, different from previous reports in vitro. The second is that lamellipodia, which are regarded as the major locomotive machinery in the adult wound epithelium, did not make up the major part of the front; the major portion of the anterior fringe of the moving front was usually smooth and gently curved, and actin cables parallel to the elongated cells were observed by confocal laser microscopy, indicating that the purse-string mechanism of epithelial wound healing takes place. The third finding is that the cells in the front had irregular bleb-like structures on their antero-basal surface, which were formed even in the portion where the cells did not attach to the matrix. Few organelles were recognized in these structures. From their location, one might propose that these bleb-like structures play a role in the recognition of the substrate and thus the movement of the cell sheet.

Pulmonary fibrosis: pathogenesis, etiology and regulation

Pulmonary fibrosis and architectural remodeling of tissues can severely disrupt lung function, often with fatal consequences. The etiology of pulmonary fibrotic diseases is varied, with an array of triggers including allergens, chemicals, radiation and environmental particles. However, the cause of one of the most common pulmonary fibrotic conditions, idiopathic pulmonary fibrosis (IPF), is still unclear. This review examines common mechanisms of pulmonary wound-healing responses following lung injury, and highlights the pathogenesis of some of the most widespread pulmonary fibrotic diseases. A three phase model of wound repair is reviewed that includes; (1) injury; (2) inflammation; and (3) repair. In most pulmonary fibrotic conditions dysregulation at one or more of these phases has been reported. Chronic inflammation can lead to an imbalance in the production of chemokines, cytokines, growth factors, and disrupt cellular recruitment. These changes coupled with excessive pro-fibrotic IL-13 and/or TGFbeta1 production can turn a well-controlled healing response into a pathogenic fibrotic response. Endogenous regulatory mechanisms are discussed including novel areas of therapeutic intervention. Restoring homeostasis to these dysregulated healing responses, or simply neutralizing the key pro-fibrotic mediators may prevent or slow the progression of pulmonary fibrosis.

Electrotaxis and wound healing: experimental methods to study electric fields as a directional signal for cell migration

Electric fields were measured at human skin wounds over one and half centuries ago. Modern techniques have verified and greatly extended our understanding of the existence of endogenous wound electric fields. In virtually all wounds studied, disruption of an epithelial layer instantaneously generates endogenous electric fields. As electric fields have the intrinsic property of being vectorial, it has long been proposed that these fields may serve as a directional signal guiding cell migration in wound healing. We have established several experimental systems to study the guidance effects and mechanisms of electric fields on cell migration. Most types of cells migrate directionally in a small electric field, a phenomenon called galvanotaxis/electrotaxis. Remarkably, electric fields of strength equal to those detected at in vivo wounds direct cell migration and override some other well-accepted coexistent guidance cues such as contact inhibition. The naturally occurring endogenous electric fields therefore may be an important signaling mechanism that regulates directional cell movement in vivo. Applied electric fields may have a potential clinical role in guiding cell migration in wound healing. The magnitude and direction of the electric field can be more precisely and quickly changed than most other guidance cues such as chemical cues. Application of electric fields thus offers a robust experimental system for study of directional cell migration with extensive flexibility. We present a brief review of the background and describe the experimental system for studying electrotaxis.

Electrical fields in wound healing-An overriding signal that directs cell migration

Injury that disrupts an epithelial layer instantaneously generates endogenous electric fields (EFs), which were detected at human skin wounds over 150 years ago. Recent researches combining molecular, genetic and imaging techniques have provided significant insights into cellular and molecular responses to this "unconventional" signal. One unexpected finding is that the EFs play an overriding guidance role in directing cell migration in epithelial wound healing. In experimental models where other directional cues (e.g., contact inhibition release, population pressure etc.) are present, electric fields of physiological strength override them and direct cell migration. The electrotaxis or galvanotaxis is mediated by polarized activation of multiple signaling pathways that include PI3 kinases/Pten, membrane growth factor receptors and integrins. Genetic manipulation of PI3 kinase/Pten (Phosphoinositide 3-kinases/phosphatase and tensin homolog) and integrin beta4 demonstrated the importance of those molecules. The electric fields are therefore a fundamental signal that directs cell migration in wound healing. One of the most challenging question is: How do cells sense the very weak electric signals? Clinically, it is highly desirable to develop practical and reliable technologies for wound healing management exploiting the electric signaling.

Tissue inhibitor of metalloproteinase-1 moderates airway re-epithelialization by regulating matrilysin activity

Obliterative bronchiolitis (OB) is the histopathological finding in chronic lung allograft rejection. Mounting evidence suggests that epithelial damage drives the development of airway fibrosis in OB. Tissue inhibitor of metalloproteinase (TIMP)-1 expression increases in lung allografts and is associated with the onset of allograft rejection. Furthermore, in a mouse model of OB, airway obliteration is reduced in TIMP-1-deficient mice. Matrilysin (matrix metallproteinase-7) is essential for airway epithelial repair and is required for the re-epithelialization of airway wounds by facilitating cell migration; therefore, the goal of this study was to determine whether TIMP-1 inhibits re-epithelialization through matrilysin. We found that TIMP-1 and matrilysin co-localized in the epithelium of human lungs with OB and both co-localized and co-immunoprecipitated in wounded primary airway epithelial cultures. TIMP-1-deficient cultures migrated faster, and epithelial cells spread to a greater extent compared with wild-type cultures. TIMP-1 also inhibited matrilysin-mediated cell migration and spreading in vitro. In vivo, TIMP-1 deficiency enhanced airway re-epithelialization after naphthalene injury. Furthermore, TIMP-1 and matrilysin co-localized in airway epithelial cells adjacent to the wound edge. Our data demonstrate that TIMP-1 interacts with matrix metalloproteinases and regulates matrilysin activity during airway epithelial repair. Furthermore, we speculate that TIMP-1 overexpression restricts airway re-epithelialization by inhibiting matrilysin activity, contributing to a stereotypic injury response that promotes airway fibrosis via bronchiole airway epithelial damage and obliteration.

Visualization of fast-moving cells in vivo using digital holographic video microscopy

Digital in-line holography offers some significant advantages over conventional optical holography and microscopy to image biological specimens. By combining holography with digital video microscopy, an in-line holographic video microscope is developed and is capable of recording spatial 3D holographic images of biological specimens, while preserving the time dimension. The system enables high-speed video recording of fast cell movement, such as the rapid movement of blood cells in the blood stream in vivo. This capability is demonstrated with observations of fast 3-D movement of live cells in suspension cultures in response to a gentle shake to the Petri dish. The experimental and numerical procedures are incorporated with a fast reconstruction algorithm for reconstruction of holographic video frames at various planes (z axis) from the hologram and along the time axis. The current system enables both lateral and longitudinal resolutions down to a few micrometers. Postreconstruction processing of background subtraction is utilized to eliminate noise caused by scattered light, thereby enabling visualization of, for example, blood streams of live Xenopos tadpoles. The combination of digital holography and microscopy offers unique advantages for imaging of fast moving cells and other biological particles in three dimensions in vivo with high spatial and temporal resolution.

Prioritising guidance cues: directional migration induced by substratum contours and electrical gradients is controlled by a rho/cdc42 switch

Coordinated cell migration is a fundamental feature of embryogenesis but the intracellular mechanism by which cells integrate co-existing extracellular cues to yield appropriate vectoral migration is unknown. Cells in the cornea are guided by a naturally occurring DC electric field (EF) (electrotaxis) as they navigate non-planar substrata but the relative potencies of electrotaxis and guidance by substratum shape (contact guidance) have never been determined. We tested the hypothesis that vectoral migration was controlled by selective activation of rac, cdc42 or rho in response to a 150 mV/mm EF or to a series of parallel substratum nanogrooves (NGs) 130 nm deep. EFs and NGs were presented singly or in combination. Electrotaxis of dissociated bovine corneal epithelial cells (CECs) on planar quartz required signalling by cdc42 and rho but not rac. Contact guidance by substratum NGs required rho but not cdc42 or rac activities. When an EF and NGs were superimposed in parallel, cathodal electrotaxis along NGs was enhanced compared to that on planar quartz but when they were superimposed orthogonally (vertical NGs with horizontal EF) cells were recruited from contact guidance to electrotaxis, suggesting that the EF was more potent. However, increasing the EF to 250 mV/mm was insufficient to recruit the majority to electrotaxis. Consistent for the cues in isolation, when an EF (150 mV/mm) and NGs were superimposed orthogonally, rac activity was not essential for either contact guidance or electrotaxis. However, attenuation of cdc42 signalling abolished electrotaxis and enhanced contact guidance relative to controls (no drug), whereas inhibiting rho signalling enhanced electrotaxis and rho stimulation enhanced contact guidance. Our data are consistent with the idea that migrating CECs use a cdc42/rho "switch" to sort vectoral cues, with cdc42 controlling electrotaxis and rho controlling contact guidance.

Thymosin beta 4: A novel corneal wound healing and anti-inflammatory agent

Thymosin beta 4 (Tbeta(4)) is a low molecular weight protein present in all cells except erythrocytes. Although Tbeta(4) is the major monomeric actin-sequestering peptide in cells and can depolymerize F-actin, evidence is mounting to support the idea that it has multiple, seemingly diverse, cellular functions. In cornea, as in other tissues, Tbeta(4) promotes cell migration and wound healing, has anti-inflammatory properties, and suppresses apoptosis. In this review we discuss the current state of knowledge regarding the effects of Tbeta(4) in maintaining the healthy, functional cornea. The clinical implications of the use of Tbeta(4) as a wound healing and anti-inflammatory agent are discussed.

gamma delta T cells are necessary for platelet and neutrophil accumulation in limbal vessels and efficient epithelial repair after corneal abrasion

Corneal epithelial abrasion in C57BL/6 mice induces an inflammatory response with peak accumulation of neutrophils in the corneal stroma within 12 hours. Platelets localize in the limbal vessels throughout the same time course as neutrophils and contribute to wound healing because antibody-dependent depletion of platelets retards epithelial division and wound closure. In the present study, T cells in the limbal epithelium were found to predominantly express the gammadelta T-cell receptor (TCR). Corneal abrasion in wild-type, CD11a(-/-), and P-sel(-/-) mice increased the numbers of gammadelta T cells in the limbal and peripheral corneal epithelium and in the corneal stroma adjacent to the limbal blood vessels. Intercellular adhesion molecule (ICAM)-1(-/-) mice exhibited a reduction in gammadelta T-cell accumulation. TCRdelta(-/-) mice exhibited reduced inflammation and delayed epithelial wound healing as evidenced by delayed wound closure, reduced epithelial cell division, reduced neutrophil infiltration, and reduced epithelial cell density at 96 hours after wounding. TCRdelta(-/-) mice also exhibited >60% reduction in platelet localization in the limbus despite similar platelet counts and platelet function assessed with an in vivo thrombosis model. These results are consistent with the conclusion that gammadelta T cells are necessary for efficient inflammation, platelet localization in the limbus, and epithelial wound healing after corneal abrasion.

Thymosin beta-4 and the eye: I can see clearly now the pain is gone

The cornea epithelium responds to injury by synthesizing several cytokines, growth factors, and tissue remodeling molecules. Proinflammatory cytokines have been implicated in the inflammation that follows corneal epithelial injury and cytokine-mediated processes play a significant role in corneal epithelial wound healing. Poorly regulated corneal inflammatory reactions that occur after injury can retard healing. In turn, persistent corneal epithelial defects and inflammation may lead to ocular morbidity and permanent visual loss. Therefore, treatments with agents that enhance corneal reepithelialization and regulate the inflammatory response without the deleterious side effects of currently used agents, such as corticosteroids, would result in improved clinical outcome and would represent a major advance in the field. Evidence is mounting to support the idea that thymosin beta-4 (Tbeta-4) has multiple, seemingly diverse, cellular functions. In the cornea, as in other tissues, Tbeta-4 promotes cell migration and wound healing, has anti-inflammatory properties, and suppresses apoptosis. Prior studies from our laboratory have demonstrated the potent wound healing and anti-inflammatory effects of Tbeta-4 in numerous models of corneal injury. Recently, we demonstrated that Tbeta-4 suppresses the activation of the transcription factor, nuclear factor-kappa b (NF-kappaB) in TNF-alpha-stimulated cells. TNF-alpha initiates cell signaling pathways that converge on the activation of NF-kappaB, thus both are known mediators of the inflammatory process. These results have important clinical implications for the potential role of Tbeta-4 as a corneal anti-inflammatory and wound-healing agent.

Beta-adrenergic receptor agonists delay while antagonists accelerate epithelial wound healing: evidence of an endogenous adrenergic network within the corneal epithelium

Wound healing is a complex and well-orchestrated biological process. Corneal epithelial cells (CECs) must respond quickly to trauma to rapidly restore barrier function and protect the eye from noxious agents. They express a high level of beta2-adrenergic receptors but their function is unknown. Here, we report the novel finding that they form part of a regulatory network in the corneal epithelium, capable of modulating corneal epithelial wound repair. Beta-adrenergic receptor agonists delay CEC migration via a protein phosphatase 2A-mediated mechanism and decrease both electric field-directed migration and corneal wound healing. Conversely, beta-adrenergic receptor antagonists accelerate CEC migration, enhance electric field-mediated directional migration, and promote corneal wound repair. We demonstrate that CECs express key enzymes required for epinephrine (beta-adrenergic receptor agonist) synthesis in the cytoplasm and can detect epinephrine in cell extracts. We propose that the mechanism for the pro-motogenic effect of the beta-adrenergic antagonist is blockade of the beta2-adrenergic receptor preventing autocrine catecholamine binding. Further investigation of this network will improve our understanding of one of the most frequently prescribed class of drugs.

Effects of applied DC electric field on ligament fibroblast migration and wound healing

Applied electric fields (static and pulsing) are widely used in orthopedic practices to treat nonunions and spine fusions and have been shown to improve ligament healing in vivo. Few studies, however, have addressed the effect of electric fields (EFs) on ligament fibroblast migration and biosynthesis. In the current study, we applied static and pulsing direct current (DC) EFs to calf anterior cruciate ligament (ACL) fibroblasts. ACL fibroblasts demonstrated enhanced migration speed and perpendicular alignment to the applied EFs. The motility of ligament fibroblasts was further modulated on type I collagen. In addition, type I collagen expression increased in ACL fibroblasts after exposure to pulsing EFs. In vitro wound-healing studies showed inhibitory effects of static EFs, which were alleviated with a pulsing EF. Our results demonstrate that applied EFs augment ACL fibroblast migration and biosynthesis and provide potential mechanisms by which EFs may be used for enhancing ligament healing and repair.

Lymphocyte function-associated antigen-1-dependent inhibition of corneal wound healing

Abrasion of murine corneal epithelium induces neutrophil emigration through limbal vessels into the avascular corneal stroma, peaking within 12 to 18 hours after wounding. A central corneal wound closes within 24 hours by epithelial cell migration and division, and during wound closure corneal epithelial cells express intercellular adhesion molecule (ICAM)-1 (CD54). We investigated the contributions of lymphocyte function-associated antigen (LFA)-1 (CD11a/CD18) and Mac-1 (CD11b/CD18) by analyzing wound closure in mice with targeted deletions of CD11a (CD11a-/-) or CD11b (CD11b-/-). In contrast to CD11a-/- mice, CD11b deficiency revealed a much greater delay in epithelial wound closure with >90% inhibition of epithelial cell division at a time when neutrophil accumulation in the cornea was approximately threefold higher than normal. Treating CD11b-/- mice with anti-CD11a monoclonal antibody at the time of epithelial abrasion resulted in significant reductions in neutrophils and significant increases in corneal epithelial cell division and migration. Treating CD11b-/- mice with anti-ICAM-1 significantly increased measures of healing but marginally reduced neutrophil influx. In conclusion, wound healing after corneal epithelial abrasion is disrupted by the absence of CD11b. The disruption is apparently linked to excessive neutrophil accumulation at a time when epithelial division is essential to wound repair, and neutrophils appear to be detrimental through processes involving LFA-1 and ICAM-1.

The spark of life: the role of electric fields in regulating cell behaviour using the eye as a model system

Endogenous electric fields (EF) have long been known to influence cell behaviour during development, neural cell tropism, wound healing and cell behaviour generally. The effect is based on short circuiting of electrical potential differences across cell and tissue boundaries generated by ionic segregation. Recent in vitro and in vivo studies have shown that EF regulate not only cell movement but orientation of cells during mitosis, an effect which may underlie shaping of tissues and organs. The molecular basis of this effect is founded on receptor-mediated cell signalling events and alterations in cytoskeletal function as revealed in studies of gene deficient cells. Remarkably, not all cells respond directionally to EF in the same way and this has consequences, for instance, for lens development and vascular remodelling. The physical basis of EF effect may be related to changes induced in 'bound water' at the cell surface, whose organisation in association with trans-membrane proteins (e.g. receptors) is disrupted when EF are generated.