Integrin and phosphotyrosine expression in normal and migrating newt keratinocytes

Macrophage-associated wound healing contributes to African green monkey SIV pathogenesis control

Natural hosts of simian immunodeficiency virus (SIV) avoid AIDS despite lifelong infection. Here, we examined how this outcome is achieved by comparing a natural SIV host, African green monkey (AGM) to an AIDS susceptible species, rhesus macaque (RM). To asses gene expression profiles from acutely SIV infected AGMs and RMs, we developed a systems biology approach termed Conserved Gene Signature Analysis (CGSA), which compared RNA sequencing data from rectal AGM and RM tissues to various other species. We found that AGMs rapidly activate, and then maintain, evolutionarily conserved regenerative wound healing mechanisms in mucosal tissue. The wound healing protein fibronectin shows distinct tissue distribution and abundance kinetics in AGMs. Furthermore, AGM monocytes exhibit an embryonic development and repair/regeneration signature featuring TGF-β and concomitant reduced expression of inflammatory genes compared to RMs. This regenerative wound healing process likely preserves mucosal integrity and prevents inflammatory insults that underlie immune exhaustion in RMs.

Re-epithelialization of large wound in paedomorphic and metamorphic axolotls

Axolotls (Ambystoma mexicanum) may heal their skin wounds scar-free in both paedomorphs and metamorphs. In previous studies on small punch skin wounds, rapid re-epithelialisation was noted in these two axolotl morphs. However, large wound size in mammals may affect wound healing. In this study, large circumferential full thickness excision wounds on the hind limbs were created on juvenile paedomorphic and metamorphic axolotls. The results showed re-epithelialisation was more quickly initiated in paedomorphs than in metamorphs after wounding. The migrating rate of epidermis on the wound bed was faster in paedomorphs than in metamorphs and thus completion of re-epithelialisation was faster in paedomorphs than in metamorphs. Within these re-epithelialisation periods, neither basement membrane nor dermis was reformed. Epidermal cell proliferation was detected by EdU-labelling technique. In the normal unwounded skin, epidermal proliferation rate was higher in paedomorphs than in metamorphs. After wounding, the epidermal proliferation rate was significantly lower in the migrating front on the wound bed than in the normal skin in paedomorphs. The EdU-labelling rate between normal skin and migration front was not different in metamorphs. Lacking of more proliferating epidermal cells on the wound bed indicated that the new epidermis here derived rather from migrating epidermal cells than from cell proliferation in situ. In conclusion, re-epithelialisation in the large wound might be fully completed in both morphs despite it was initiated earlier and with faster rate in paedomorphs than in metamorphs. The new epidermis on the wound bed derived mainly from cell migration than by cell proliferation in the re-epithelialisation period. J. Morphol. 278:228-235, 2017. © 2016 Wiley Periodicals,Inc.

Cooperative regulation of substrate stiffness and extracellular matrix proteins in skin wound healing of axolotls

Urodele amphibians (Ambystoma mexicanum), unique among vertebrates, can regenerate appendages and other body parts entirely and functionally through a scar-free healing process. The wound epithelium covering the amputated or damaged site forms early and is essential for initiating the subsequent regenerative steps. However, the molecular mechanism through which the wound reepithelializes during regeneration remains unclear. In this study, we developed an in vitro culture system that mimics an in vivo wound healing process; the biomechanical properties in the system were precisely defined and manipulated. Skin explants that were cultured on 2 to 50 kPa collagen-coated substrates rapidly reepithelialized within 10 to 15 h; however, in harder (1 GPa) and other extracellular matrices (tenascin-, fibronectin-, and laminin-coated environments), the wound epithelium moved slowly. Furthermore, the reepithelialization rate of skin explants from metamorphic axolotls cultured on a polystyrene plate (1 GPa) increased substantially. These findings afford new insights and can facilitate investigating wound epithelium formation during early regeneration using biochemical and mechanical techniques.

New insights into vertebrate skin regeneration

Regeneration biology has experienced a renaissance as clinicians, scientists, and engineers have combined forces to drive the field of regenerative medicine. Studies investigating the mechanisms that regulate wound healing in adult mammals have led to a good understanding of the stereotypical processes that lead to scarring. Despite comparative studies of fetal wound healing in which no scar is produced, the fact remains that insights from this work have failed to produce therapies that can regenerate adult human skin. In this review, we analyze past and contemporary accounts of wound healing in a variety of vertebrates, namely, fish, amphibians, and mammals, in order to demonstrate how examples of skin regeneration in adult organisms can impact traditional wound-healing research. When considered together, these studies suggest that inflammation and reepithelialization are necessary events preceding both scarring and regeneration. However, the extent to which these processes may direct one outcome over another is likely weaker than currently accepted. In contrast, the extent to which newly deposited extracellular matrix in the wound bed can be remodeled into new skin, and the intrinsic ability of new epidermis to regenerate appendages, appears to underlie the divergence between scar-free healing and the persistence of a scar. We discuss several ideas that may offer areas of overlap between researchers using these different model organisms and which may be of benefit to the ultimate goal of scar-free human wound healing.

Skin regeneration in adult axolotls: a blueprint for scar-free healing in vertebrates

While considerable progress has been made towards understanding the complex processes and pathways that regulate human wound healing, regenerative medicine has been unable to develop therapies that coax the natural wound environment to heal scar-free. The inability to induce perfect skin regeneration stems partly from our limited understanding of how scar-free healing occurs in a natural setting. Here we have investigated the wound repair process in adult axolotls and demonstrate that they are capable of perfectly repairing full thickness excisional wounds made on the flank. In the context of mammalian wound repair, our findings reveal a substantial reduction in hemostasis, reduced neutrophil infiltration and a relatively long delay in production of new extracellular matrix (ECM) during scar-free healing. Additionally, we test the hypothesis that metamorphosis leads to scarring and instead show that terrestrial axolotls also heal scar-free, albeit at a slower rate. Analysis of newly forming dermal ECM suggests that low levels of fibronectin and high levels of tenascin-C promote regeneration in lieu of scarring. Lastly, a genetic analysis during wound healing comparing epidermis between aquatic and terrestrial axolotls suggests that matrix metalloproteinases may regulate the fibrotic response. Our findings outline a blueprint to understand the cellular and molecular mechanisms coordinating scar-free healing that will be useful towards elucidating new regenerative therapies targeting fibrosis and wound repair.

Mechanical stretch induces clustering of ?1-integrins and facilitates adhesion

Human epithelial cells are permanently stimulated by external mechanical forces. The present in vitro study suggests that keratinocytes respond to mechanical strain by a coordinated spatial and functional utilization of beta1-integrins and the epidermal growth factor receptor (EGFR) with impact to the adhesion properties. It was found that a single mechanical stretch applied to HaCaT keratinocytes elevates the substrate adhesion, in particular to fibronectin and collagen type IV but not to laminin indicating the relevance of beta1-integrins in this process. This was confirmed using a functional blocking antibody directed against beta1-integrins which reversed the stretch-induced adhesion. Furthermore, mechanical stretch gives rise to a rapid redistribution of beta1-integrins in clusters on the basal cell membrane, without changing the overall amount of this particular integrin subset. Concomitantly, the EGFR co-localizes with beta1-integrin suggesting a functional cooperation of both membrane proteins in mechano-signaling. This is corroborated by data showing that stretch-induced activation of the EGFR and the downstream element extracellular regulated kinase 1/2 (ERK1/2) is reversed by preincubation with beta1-integrin antibodies. Vice versa, blocking the EGFR using a specific inhibitor abrogates stretch-induced ERK1/2 activation. In summary, these results show a functional cooperation of beta1-integrins and EGFR in the adhesion complex supporting the transmission of stretch-induced signals.

Dynamic characterization of the molecular events during in vitro epidermal wound healing

The aim of this study was to characterize some of the molecular events stimulated in vitro in response to injury within a confluent culture of normal epidermal keratinocytes as a model to understand the mechanisms of wound healing. To this end, an original device was developed specifically designed to perform calibrated injuries of great lengths within mono-stratified or pluri-stratified keratinocyte cultures. The experiments performed in this study validate this device as an appropriate tool for studying epidermal wound healing; this is because it performs mechanical injuries that stimulate the expression of multiple healing markers also known to be upregulated during wound healing in vivo (growth factors, cytokines, proteinases, extracellular matrix proteins). Using this device, it was demonstrated in human keratinocytes: mechanical injuries (i) immediately stimulate the tyrosine phosphorylation of numerous cellular proteins; (ii) induce molecular cascades leading to the activation of p21ras, mitogen-activated protein kinases, extracellular signal-regulated kinases 1/2, c-Jun NH2 terminal kinase, and p38 mitogen-activated protein kinase; and (iii) increase the phosphorylation of their respective substrates, c-jun and activator transcription factor 1. Wounding of these cells also results in increases in the DNA binding activities of several jun/fos activator protein-1 transcription factor complexes. It is important to note that the development of an appropriate wounding system was essential for performing this study, as use of a classical wounding procedure did not enable the detection of the biologic parameters reported above. In conclusion, these data indicate that using the appropriate system, it is possible to identify the signaling pathways activated in normal human keratinocyte cells after injury. In this study, it was shown that the mitogen-activated protein kinase pathways and activator protein-1 are stimulated in response to physical injury, and may be involved in regulating the expression of healing markers.

Phosphorylation of focal adhesion kinase (pp125(FAK)) is increased in human keratinocytes induced to migrate by extracellular matrices

During the healing process of skin wounds, human keratinocytes migrate across a provisional matrix of the wound bed. The mechanisms by which keratinocytes migrate on connective tissue are not known. In this study, we examined the role of focal adhesion kinase (FAK), an 125 kDa protein that co-localizes with focal adhesions in cells plated on extracellular matrix. We induced human keratinocytes into various states of migration by plating them on extracellular matrices that minimally, moderately, or strongly induce cellular migration, and then examined the expression of FAK at the protein level and its degree of tyrosine phosphorylation using Western immunoblotting and immunoprecipitation. In highly migratory human keratinocytes, we found that three proteins were predominantly tyrosine phosphorylated, one of them being FAK. Tyrosine phosphorylation of FAK tightly correlated with the level of cellular motility but not cell attachment to the matrix. Time course experiments demonstrated that in highly motile keratinocytes, tyrosine phosphorylation of FAK peaked at 12 h, the time when maximal migration on the matrix ensues. In contrast to FAK, the beta1 integrin subunit of human keratinocytes that configures with the alpha2, alpha3, and alpha5 integrin subunits to form integrin receptors for matrix, did not display tyrosine phosphorylation linked to motility. Using anti-sense oligonucleotides to FAK, we demonstrate that FAK is required for human keratinocyte migration, but not for focal adhesion formation.

Integrin involvement in keratocyte locomotion

Keratocytes are useful in the study of locomotion because they move rapidly (up to 1 micron/second) while maintaining an almost uniform shape, speed and direction. The smooth gliding motion of the keratocyte requires a precise coordination between adhesion, contractility, and retraction. To ask what role integrins play in keratocyte adhesion and locomotion, either RGD peptides or an anti-beta1 integrin mAb that binds to an ectodomain epitope and inhibits adhesion formation was added to the culture media of moving keratocytes. The response to these reagents depended on three interrelated factors: the dose of RGD/mAb, the apparent adhesion strength of the keratocyte to the substratum and the cell speed. High doses cause keratocytes to quickly and irreversibly round up. At intermediate RGD/mAb doses, keratocytes reestablish adhesion after treatment and briefly resume locomotion until partial detachment recurs. At the lowest doses, disruption of beta1 integrin-mediated adhesion formation destabilizes the lamella, temporarily preventing lamellar extension and forward movement of the cell. With increasing culture time, there is an increase in apparent adhesion and a corresponding marked decrease in locomotory velocity. Under these conditions, high doses of RGD/mAb do not cause keratocytes to detach or even produce detectable lamellar instabilities. We postulate that RGD/mAb competitively inhibits new beta1 integrin mediated adhesion formation that is required to support the rates of lamellar extension necessary for rapid locomotion.