Repair of excisional wounds in the embryo

Zones of cellular damage around pulsed-laser wounds

After a tissue is wounded, cells surrounding the wound adopt distinct wound-healing behaviors to repair the tissue. Considerable effort has been spent on understanding the signaling pathways that regulate immune and tissue-resident cells as they respond to wounds, but these signals must ultimately originate from the physical damage inflicted by the wound. Tissue wounds comprise several types of cellular damage, and recent work indicates that different types of cellular damage initiate different types of signaling. Hence to understand wound signaling, it is important to identify and localize the types of wound-induced cellular damage. Laser ablation is widely used by researchers to create reproducible, aseptic wounds in a tissue that can be live-imaged. Because laser wounding involves a combination of photochemical, photothermal and photomechanical mechanisms, each with distinct spatial dependencies, cells around a pulsed-laser wound will experience a gradient of damage. Here we exploit this gradient to create a map of wound-induced cellular damage. Using genetically-encoded fluorescent proteins, we monitor damaged cellular and sub-cellular components of epithelial cells in living Drosophila pupae in the seconds to minutes following wounding. We hypothesized that the regions of damage would be predictably arrayed around wounds of varying sizes, and subsequent analysis found that all damage radii are linearly related over a 3-fold range of wound size. Thus, around laser wounds, the distinct regions of damage can be estimated after measuring any one. This report identifies several different types of cellular damage within a wounded epithelial tissue in a living animal. By quantitatively mapping the size and placement of these different types of damage, we set the foundation for tracing wound-induced signaling back to the damage that initiates it.

Fos metamorphoses: Lessons from mutants in model organisms

The Fos oncogene gene family is evolutionarily conserved throughout Eukarya. Fos proteins characteristically have a leucine zipper and a basic region with a helix-turn-helix motif that binds DNA. In vertebrates, there are several Fos homologs. They can homo- or hetero-dimerize via the leucine zipper domain. Fos homologs coupled with other transcription factors, like Jun oncoproteins, constitute the Activator Protein 1 (AP-1) complex. From its original inception as an oncogene, the subsequent finding that they act as transcription factors binding DNA sequences known as TRE, to the realization that they are activated in many different scenarios, and to loss-of-function analysis, the Fos proteins have traversed a multifarious path in development and physiology. They are instrumental in 'immediate early genes' responses, and activated by a seemingly myriad assemblage of different stimuli. Yet, the majority of these studies were basically gain-of-function studies, since it was thought that Fos genes would be cell lethal. Loss-of-function mutations in vertebrates were recovered later, and were not cell lethal. In fact, c-fos null mutations are viable with developmental defects (osteopetrosis and myeloid lineage abnormalities). It was then hypothesized that vertebrate genomes exhibit partial redundancy, explaining the 'mild' phenotypes, and complicating assessment of complete loss-of-function phenotypes. Due to its promiscuous activation, fos genes (especially c-fos) are now commonly used as markers for cellular responses to stimuli. fos homologs high sequence conservation (including Drosophila) is advantageous as it allows critical assessment of fos genes functions in this genetic model. Drosophila melanogaster contains only one fos homolog, the gene kayak. kayak mutations are lethal, and allow study of all the processes where fos is required. The kayak locus encodes several different isoforms, and is a pleiotropic gene variously required for development involving cell shape changes. In general, fos genes seem to primarily activate programs involved in cellular architectural rearrangements and cell shape changes.

Epithelial bridges maintain tissue integrity during collective cell migration

The ability of skin to act as a barrier is primarily determined by the efficiency of skin cells to maintain and restore its continuity and integrity. In fact, during wound healing keratinocytes migrate collectively to maintain their cohesion despite heterogeneities in the extracellular matrix. Here, we show that monolayers of human keratinocytes migrating along functionalized micropatterned surfaces comprising alternating strips of extracellular matrix (fibronectin) and non-adherent polymer form suspended multicellular bridges over the non-adherent areas. The bridges are held together by intercellular adhesion and are subjected to considerable tension, as indicated by the presence of prominent actin bundles. We also show that a model based on force propagation through an elastic material reproduces the main features of bridge maintenance and tension distribution. Our findings suggest that multicellular bridges maintain tissue integrity during wound healing when cell-substrate interactions are weak and may prove helpful in the design of artificial scaffolds for skin regeneration.

Stress-activated protein kinases are negatively regulated by cell density

Stimulation by UV irradiation, TNFalpha, as well as PDGF or EGF activates the JNK/SAPK signalling pathway in mouse fibroblasts. This results in the phosphorylation of the N-terminal domain of c-Jun, increasing its transactivation potency. Using an antibody that specifically recognizes c-Jun phosphorylated at Ser63, we show that culture confluency drastically inhibited c-Jun N-terminal phosphorylation due to the inhibition of the JNK/SAPK pathway. Transfection experiments demonstrate that the inhibition occurs at the same level as, or upstream of, the small G-proteins cdc42 and Rac1. In contrast, the classical MAPK pathway was insensitive to confluency. The inhibition of JNK/SAPK activation depended on the integrity of the actin microfilament network. These results were confirmed and extended in monolayer wounding experiments. After PDGF, EGF or UV stimulation, c-Jun was predominantly phosphorylated in cells bordering the wound, which are the cells that move to occupy the wounded area. Thus, modulation of the stress-dependent signal cascade by confluency will restrict c-Jun N-terminal phosphorylation in response to mitogenic or chemotactic agents to cells that border a wounded area.

The hyaluronan receptor RHAMM regulates extracellular-regulated kinase

We have identified two RHAMM (receptor for hyaluronan-mediated motility) isoforms that encode an alternatively spliced exon 4 (Hall, C. L., Yang, B., Yang, X., Zhang, S., Turley, M., Samuel, S., Lange, L. A., Wang, C., Curpen, G. D., Savani, R. C., Greenberg, A. H., and Turley, E. A. (1995) Cell 82, 19-26 and Wang, C., Entwistle, J., Hou, G., Li, Q., and Turley, E. A. (1996) Gene 174, 299-306). One of these, RHAMM variant 4 (RHAMMv4), is transforming when overexpressed and regulates Ras signaling (Hall et al.). Here we note using flow cytometry and confocal analysis that RHAMM isoforms encoding exon 4 occur both on the cell surface and in the cytoplasm. Epitope-tagging experiments indicate that RHAMMv4 occurs only in the cytoplasm. Several observations suggest that both cell surface RHAMM isoforms and RHAMMv4 are involved in regulating extracellular-regulated kinase (ERK) activity. Affinity-purified anti-RHAMM exon 4 antibodies block the ability of platelet-derived growth factor to activate ERK, and these reagents modify the protein tyrosine phosphorylation profile of proteins resulting from treatment with platelet-derived growth factor. A dominant negative form of RHAMMv4 inhibits mutant active Ras activation of ERK and coimmunoprecipitates with both mitogen-activated protein kinase kinase and ERK, suggesting that the intracellular RHAMMv4 acts downstream of Ras, possibly at the level of mitogen-activated protein kinase kinase-ERK interactions. Consistent with this, overexpression of RHAMMv4 constitutively activates ERK. These results identify a novel mechanism for the regulation of the Ras-ERK signaling pathway and suggest that RHAMM plays multiple roles in this regulation.

Surface ectodermal wound healing in the chick embryo

Wound healing has been studied in the surface ectoderm overlying the midbrain region of stages 16-20 chick embryos by light microscopy, scanning and transmission electron microscopy, and immunofluorescent techniques. The embryos were divided into 6 groups, i.e. stages 16-17 for groups I, V and VI, and stages 19-20 for groups II, III and IV. For groups I and II embryos, a longitudinal incision about 0.6 mm was made close to the dorsal midline and the embryos incubated for varying periods of time up to 24 h. To determine the role of actin in the process of healing, selected groups I and II embryos were stained with FITC phalloidin and the wound margins examined using a confocal microscope. Wounds of all embryos in group I and about 20% in group II healed completely within 24 h of reincubation. The process of healing involved a change in the shapes of the ectodermal cells at the wound ends. This appeared as a zipping-up of the wound from both ends. In about 80% of group II embryos where healing did not occur, wound gaping was marked. Intense actin staining (actin cable) was observed at the wound margins of groups I and II embryos suggesting that the actin purse-string mechanism may play a role during wound healing in this epithelial model. The role of tension in wound healing was also determined by placing 2 secondary wounds about 0.5-0.7 mm long close to, and at right angles to the ends of the primary wound in groups III and V embryos. The procedure decreased the tension within the ectodermal cells at the wound ends. Groups IV and VI embryos served as controls for groups III and V embryos, respectively. Healing of both primary and secondary wounds after reduction of tension was rapid. Most primary wounds in group V embryos healed completely within 3 h of reincubation and the rate of reepithelialisation after the reduction of tension was about 160% more than that in group VI (control) embryos. Similarly, most primary wounds in group III embryos were almost closed within 6 h of reincubation. Here, the rate of reepithelialisation was 80 % more than that in group IV (controls). Thus tension is an important factor in wound healing in this model.

The mechanism of excisional fetal wound repair in vitro is responsive to growth factors

We have investigated the ability of fetal rat skin to heal an excisional wound in vitro. Skin from the backs of E17-E19 rats was wounded using a 1-mm diameter cutting needle and suspended in culture on a 6-pin cradle for 72 h. Neither contraction nor epithelial closure was observed within wounds created in skin from E19 embryos. In contrast, wounds in E17 skin contracted to 35-50% of their original area over 72 h, although, in the absence of serum, complete wound closure was not observed. Addition of FBS at the time of culture resulted in the movement of the epithelium over the dermal margins of the wound to effect complete closure. Histological sections through these healed wounds revealed an epithelial bridge spanning the dermal margins of the wound. A similar mechanism of repair was observed in the presence of day 14 adult wound fluid. The response of wounds in E17 skin to a range of growth factors was then assessed in an attempt to reproduce the serum response under defined conditions. Insulin-like growth factor I or epidermal growth factor did not significantly affect wound closure. Basic fibroblast growth factor, transforming growth factor-beta, or platelet-derived growth factor did promote wound closure although, in contrast to the serum-induced response, wound histology revealed repair had been achieved by dermal fibroblasts that occupied the space between the epithelial margins of the healed wound. We have therefore shown that the epithelial component of fetal wound repair proceeds in organ-cultured fetal skin in the absence of an adhesive substrate over which to migrate and is dependent on the source of trophic factors. The inability of skin taken from the E19 embryo to heal in vitro suggests a developmental switch in the mechanism of wound epithelialization.

Scar wars: implications of fetal wound healing for the pediatric burn patient

Scar formation and fibrosis often cause devastating disabilities in children suffering severe burn injury. In contrast to the child, the fetus has the ability to heal skin injury without scar formation, and instead with regeneration of epithelial and mesenchymal tissues and restoration of normal skin architecture. In this paper we review those unique features of the fetus and fetal wound healing that may contribute to the scarless repair process. It is hoped that an understanding of these remarkable reparative capabilities may lead to the development of new wound healing therapies that reduce or prevent scar formation and fibrosis in the management of children with burns.