Time Heals All Wounds: Probing Skin Injuries with Spatial Biology
Introduction
The skin provides a critical barrier that protects an organism’s internal tissues and organs from physical damage, infection, and desiccation.1 Mammalian skin contains two distinct layers: the epidermis and the dermis. The epidermis is primarily composed of keratinocytes, which terminally differentiate and cornify as they migrate towards the external environment.2 The body replenishes these cells through epidermal stem cells (EpSCs) in the basal layer of the epidermis. The dermis is enriched in extracellular matrix (ECM) proteins, fibroblasts, blood vessels, and immune cells.
Upon injury, many cell types collaborate to heal the wound and repair the barrier. This dynamic process is often divided into four phases: clot formation, inflammation, proliferation, and remodeling.1 After skin damage, platelets become activated and seal the wound by forming a fibrin clot. Subsequently, tissue-resident and circulation-derived immune cells, such as macrophages and neutrophils, infiltrate the wound bed to clear any invading microbes or apoptotic host cells.2 After several days, EpSCs and fibroblasts proliferate, differentiate, and migrate to the injury site to regenerate the epidermis and deposit new ECM components, respectively. Weeks later, the body tries to restore the skin’s structure to its pre-injury state by reducing the density of cells at the site and remodeling the ECM.3 However, these actions ultimately generate scar tissue.
Exploring Wound Healing with Spatial Biology
Spatial biology techniques allow scientists to characterize a cell population’s genome, transcriptome, proteome, epigenome, or metabolome while retaining the cells’ spatial context within the tissue.4 These methods improve their understanding of the cellular diversity, interactions, and functions occurring during complex biological processes. Recently, researchers have examined wound healing using spatial transcriptomic and proteomic approaches.
T Cells and Keratinocytes
The open wound is a harsh microenvironment, exhibiting low oxygen and nutrient levels, but high concentrations of reactive oxygen species and cellular debris.5 Scientists originally thought that epithelial cells could directly sense the lack of oxygen and respond by activating the transcription factor hypoxia-inducible factor 1α (HIF1α) to induce wound healing. However, recent results have challenged this long-standing hypothesis. Using single-cell and spatial transcriptomics and functional assays, researchers determined that skin-resident retinoic acid-related orphan receptor γt+ (RORγt+) γδ T cells secrete interleukin-17A (IL-17A).5 This cytokine activates the HIF1α signaling pathway in wound-edge keratinocytes to drive their migration into the wound bed and promote re-epithelialization.
EpSCs and the ECM
Specialized ECM proteins in the basement membrane (BM) physically link EpSCs and basal keratinocytes to the dermis below.6 This interaction maintains homeostasis in the skin and regulates EpSCs during wound healing, but researchers know little about the mechanism underlying these processes. By employing a spatial proteomics workflow, which combined tissue decellularization, laser capture microdissection, and mass spectrometry, scientists analyzed the proteome of healthy human skin and skin lesions from patients with secondary syphilis (SS).6 They observed reduced ECM protein levels, such as transforming growth factor-β-induced (TGFBI), in the zone around the BM of SS samples compared to control samples. Additionally, they demonstrated that this ECM glycoprotein increases EpSC proliferation and enhances wound healing, suggesting that TGFBI regulates EpSC function.
Fibroblast Diversity
Fibroblasts respond to tissue injury by producing and secreting ECM proteins to form the scar tissue at the wound site.7 However, scientists need a better understanding of fibroblasts during wound healing to start developing therapies that could stimulate tissue regeneration rather than fibrotic remodeling. Using single-cell and spatial transcriptomics and single-cell epigenomics, researchers analyzed tissue-resident fibroblasts involved in skin repair spatially and temporally.7 They detected four subpopulations over the two-week healing period: mechanofibrotic, activated responder, proliferator, and remodeling fibroblasts. These subpopulations have different transcriptional profiles and follow diverse trajectories in terms of migration, differentiation, and proliferation. During the inflammation stage, a subset of mechanofibrotic cells differentiate as they migrate from the outer wound to the wound edge and form the activated-responder subpopulation. Later in the proliferation phase, mechanotransduction signals induce some of the mechanofibrotic fibroblasts to move to the wound center and differentiate into proliferator fibroblasts. Simultaneously, a remodeling fibroblast subpopulation, also derived from the mechanofibrotic cells, emerges in the deep dermal region.
[References]
1. Peña OA, Martin P. Cellular and molecular mechanisms of skin wound healing. Nat Rev Mol Cell Biol. 2024;25(8):599-616.
2. Boothby IC, et al. Regulatory T cells in skin injury: At the crossroads of tolerance and tissue repair. Sci Immunol. 2020;5(47):eaaz9631.
3. Schultz GS, et al. Principles of wound healing. In: Mechanisms of Vascular Disease: A Reference Book for Vascular Specialists. University of Adelaide Press; 2011.
4. Bressan D, et al. The dawn of spatial omics. Science. 2023;381(6657):eabq4964.
5. Konieczny P, et al. Interleukin-17 governs hypoxic adaptation of injured epithelium. Science. 2022;377(6602):eabg9302.
6. Li J, et al. Spatially resolved proteomic map shows that extracellular matrix regulates epidermal growth. Nat Commun. 2022;13:4012.
7. Foster DS, et al. Integrated spatial multiomics reveals fibroblast fate during tissue repair. PNAS. 2021;118(41):e2110025118.
