Scratch-induced partial skin wounds re-epithelialize by sheets of independently migrating keratinocytes

Immediate but Temporal Response: The Role of Distal Epithelial Cells in Wound Healing

Efficient oral mucosal wound healing requires coordinated responses from epithelial progenitor cells, yet their spatiotemporal recruitment and activation remain unclear. Using a mouse model of palatal mucosal wound healing, we investigated the dynamics of epithelial cells during this process. Proliferation analysis revealed that, in addition to the expected proliferation center near the wound edge, distal cell populations rapidly activated post-injury by elevating their mitotic activity. These distal cells displayed predominant lateral expansion in the basal layer, suggesting roles beyond just tissue renewal. However, while proximal proliferation center cells sustained heightened proliferation until re-epithelialization was completed, distal cells restored basal turnover rates before wound closure, indicating temporally confined contributions. Lineage tracing of Wnt-responsive epithelial cells showed remarkable clone expansion in basal layers both proximally and distally after wounding, contrasting with gradual clone expansion in homeostasis. Although prioritizing tissue repair, epithelial progenitor cells maintained differentiation programs and barrier functions, with the exception of the leading edge. At the leading edge, we found accelerated cell turnover, but the differentiation program was suspended. In summary, our findings uncovered that oral wound re-epithelialization involves two phases: an initial widespread response with proliferation of proximal and distal cells, followed by proliferation confined to the wound proximal region. Uncovering these stage-specific healing mechanisms provides insights for developing targeted therapeutic strategies to improve wound care.

Cdc42 activity in the trailing edge is required for persistent directional migration of keratinocytes

Fibroblasts migrate discontinuously by generating transient leading-edge protrusions and irregular, abrupt retractions of a narrow trailing edge. In contrast, keratinocytes migrate persistently and directionally via a single, stable, broad protrusion paired with a stable trailing-edge. The Rho GTPases Rac1, Cdc42 and RhoA are key regulators of cell protrusions and retractions. However, how these molecules mediate cell-type specific migration modes is still poorly understood. In fibroblasts, all three Rho proteins are active at the leading edge, suggesting short-range coordination of protrusive Rac1 and Cdc42 signals with RhoA retraction signals. Here, we show that Cdc42 was surprisingly active in the trailing-edge of migrating keratinocytes. Elevated Cdc42 activity colocalized with the effectors MRCK and N-WASP suggesting that Cdc42 controls both myosin activation and actin polymerization in the back. Indeed, Cdc42 was required to maintain the highly dynamic contractile acto-myosin retrograde flow at the trailing edge of keratinocytes, and its depletion induced ectopic protrusions in the back, leading to decreased migration directionality. These findings suggest that Cdc42 is required to stabilize the dynamic cytoskeletal polarization in keratinocytes, to enable persistent, directional migration.

Epithelial wound healing in Clytia hemisphaerica provides insights into extracellular ATP signaling mechanisms and P2XR evolution

Epithelial wound healing involves the collective responses of many cells, including those at the wound margin (marginal cells) and those that lack direct contact with the wound (submarginal cells). How these responses are induced and coordinated to produce rapid, efficient wound healing remains poorly understood. Extracellular ATP (eATP) is implicated as a signal in epithelial wound healing in vertebrates. However, the role of eATP in wound healing in vivo and the cellular responses to eATP are unclear. Almost nothing is known about eATP signaling in non-bilaterian metazoans (Cnidaria, Ctenophora, Placozoa, and Porifera). Here, we show that eATP promotes closure of epithelial wounds in vivo in the cnidarian Clytia hemisphaerica (Clytia) indicating that eATP signaling is an evolutionarily ancient strategy in wound healing. Furthermore, eATP increases F-actin accumulation at the edges of submarginal cells. In Clytia, this indicates eATP is involved in coordinating cellular responses during wound healing, acting in part by promoting actin remodeling in cells at a distance from the wound. We also present evidence that eATP activates a cation channel in Clytia epithelial cells. This implies that the eATP signal is transduced through a P2X receptor (P2XR). Phylogenetic analyses identified four Clytia P2XR homologs and revealed two deeply divergent major branches in P2XR evolution, necessitating revision of current models. Interestingly, simple organisms such as cellular slime mold appear exclusively on one branch, bilaterians are found exclusively on the other, and many non-bilaterian metazoans, including Clytia, have P2XR sequences from both branches. Together, these results re-draw the P2XR evolutionary tree, provide new insights into the origin of eATP signaling in wound healing, and demonstrate that the cytoskeleton of submarginal cells is a target of eATP signaling.

Edge trenching: a case study of a novel debridement technique

Debridement is a vital part of wound bed preparation; however, even a perfectly debrided wound may stall if it has vertical edges or the tissue in the wound base does not merge with the wound edges. A novel debridement technique described in this case study allows wound remodelling to overcome these difficulties.

The basement membrane in epidermal polarity, stemness, and regeneration

The epidermis is a specialized epithelium that constitutes the outermost layer of the skin, and it provides a protective barrier against environmental assaults. Primarily consisting of multilayered keratinocytes, the epidermis is continuously renewed by proliferation of stem cells and the differentiation of their progeny, which undergo terminal differentiation as they leave the basal layer and move upward toward the surface, where they die and slough off. Basal keratinocytes rest on a basement membrane at the dermal-epidermal junction that is composed of specific extracellular matrix proteins organized into interactive and mechanically supportive networks. Firm attachment of basal keratinocytes, and their dynamic regulation via focal adhesions and hemidesmosomes, is essential for maintaining major skin processes, such as self-renewal, barrier function, and resistance to physical and chemical stresses. The adhesive integrin receptors expressed by epidermal cells serve structural, signaling, and mechanosensory roles that are critical for epidermal cell anchorage and tissue homeostasis. More specifically, the basement membrane components play key roles in preserving the stem cell pool, and establishing cell polarity cues enabling asymmetric cell divisions, which result in the transition from a proliferative basal cell layer to suprabasal cells committed to terminal differentiation. Finally, through a well-regulated sequence of synthesis and remodeling, the components of the dermal-epidermal junction play an essential role in regeneration of the epidermis during skin healing. Here too, they provide biological and mechanical signals that are essential to the restoration of barrier function.

Multiphoton intravital microscopy of rodents

Tissues are heterogeneous with respect to cellular and non-cellular components and in the dynamic interactions between these elements. To study the behaviour and fate of individual cells in these complex tissues, intravital microscopy (IVM) techniques such as multiphoton microscopy have been developed to visualize intact and live tissues at cellular and subcellular resolution. IVM experiments have revealed unique insights into the dynamic interplay between different cell types and their local environment, and how this drives morphogenesis and homeostasis of tissues, inflammation and immune responses, and the development of various diseases. This Primer introduces researchers to IVM technologies, with a focus on multiphoton microscopy of rodents, and discusses challenges, solutions and practical tips on how to perform IVM. To illustrate the unique potential of IVM, several examples of results are highlighted. Finally, we discuss data reproducibility and how to handle big imaging data sets.

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.

Recent Advances in Nano-Formulations for Skin Wound Repair Applications

Skin injuries caused by accidents and acute or chronic diseases place a heavy burden on patients and health care systems. Current treatments mainly depend on preventing infection, debridement, and hemostasis and on supplementing growth factors, but patients will still have scar tissue proliferation or difficulty healing and other problems after treatment. Conventional treatment usually focuses on a single factor or process of wound repair and often ignores the influence of the wound pathological microenvironment on the final healing effect. Therefore, it is of substantial research value to develop multifunctional therapeutic methods that can actively regulate the wound microenvironment and reduce the oxidative stress level at the wound site to promote the repair of skin wounds. In recent years, various bioactive nanomaterials have shown great potential in tissue repair and regeneration due to their properties, including their unique surface interface effect, small size effect, enzyme activity and quantum effect. This review summarizes the mechanisms underlying skin wound repair and the defects in traditional treatment methods. We focus on analyzing the advantages of different types of nanomaterials and comment on their toxicity and side effects when used for skin wound repair.

LABKIT: Labeling and Segmentation Toolkit for Big Image Data

We present LABKIT, a user-friendly Fiji plugin for the segmentation of microscopy image data. It offers easy to use manual and automated image segmentation routines that can be rapidly applied to single- and multi-channel images as well as to timelapse movies in 2D or 3D. LABKIT is specifically designed to work efficiently on big image data and enables users of consumer laptops to conveniently work with multiple-terabyte images. This efficiency is achieved by using ImgLib2 and BigDataViewer as well as a memory efficient and fast implementation of the random forest based pixel classification algorithm as the foundation of our software. Optionally we harness the power of graphics processing units (GPU) to gain additional runtime performance. LABKIT is easy to install on virtually all laptops and workstations. Additionally, LABKIT is compatible with high performance computing (HPC) clusters for distributed processing of big image data. The ability to use pixel classifiers trained in LABKIT via the ImageJ macro language enables our users to integrate this functionality as a processing step in automated image processing workflows. Finally, LABKIT comes with rich online resources such as tutorials and examples that will help users to familiarize themselves with available features and how to best use LABKIT in a number of practical real-world use-cases.

Integrin α5β1 nano-presentation regulates collective keratinocyte migration independent of substrate rigidity

Nanometer-scale properties of the extracellular matrix influence many biological processes, including cell motility. While much information is available for single-cell migration, to date, no knowledge exists on how the nanoscale presentation of extracellular matrix receptors influences collective cell migration. In wound healing, basal keratinocytes collectively migrate on a fibronectin-rich provisional basement membrane to re-epithelialize the injured skin. Among other receptors, the fibronectin receptor integrin α5β1 plays a pivotal role in this process. Using a highly specific integrin α5β1 peptidomimetic combined with nanopatterned hydrogels, we show that keratinocyte sheets regulate their migration ability at an optimal integrin α5β1 nanospacing. This efficiency relies on the effective propagation of stresses within the cell monolayer independent of substrate stiffness. For the first time, this work highlights the importance of extracellular matrix receptor nanoscale organization required for efficient tissue regeneration.

An Intravital Microscopy Toolbox to Study Mammary Gland Dynamics from Cellular Level to Organ Scale

The architecture of the mouse mammary gland is highly dynamic and constantly remodeled during pubertal development and estrous cycle-driven sprouting and regression of alveolar side branches. During each of these developmental stages, turnover is driven by distinct subsets of mammary epithelial cells. Extensive previous research has shed light on the unique morphological and cell biological characteristics of each stage. However, technological shortcomings failed to capture the dynamics and single-cell contributions to mammary remodeling. Here, we developed in vivo imaging strategies to follow the same mammary ducts over time and quantify the dynamics of mammary gland growth and remodeling from single-cell level to organ scale. Using a combination of intravital microscopy and genetic reporter systems we show how proliferative heterogeneity drives ductal morphogenesis during different developmental stages. To visualize pubertal growth at the cellular level, we performed long-term time-lapse imaging of extending terminal end buds through a mammary imaging window. We show that single-cells within the terminal end buds are extremely motile and continuously exchange position whilst the duct is elongating. To visualize short-term remodeling in the adult mammary gland at the single cell level, we performed multi-day intravital imaging in photoconvertible Kikume Green-Red mice and fluorescent ubiquitination-based cell cycle indicator mice. We demonstrate that the contribution of single-cells to estrous-driven remodeling is highly variable between cells in the same micro-environment. To assess the effects of this dynamic proliferative contribution on the long-term stability of tissue architecture, we developed a repeated skin flap method to assess mammary gland morphology by intravital microscopy over extended time spans for up to six months. Interestingly, in contrast to the short-term dynamic remodeling, the long-term morphology of the mammary gland remains remarkably stable. Together, our tool box of imaging strategies allows to identify and map transient and continuing dynamics of single cells to the architecture of the mammary gland.