A multiscale hybrid mathematical model of epidermal-dermal interactions during skin wound healing.

Following injury, skin activates a complex wound healing programme. While cellular and signalling mechanisms of wound repair have been extensively studied, the principles of epidermal-dermal interactions and their effects on wound healing outcomes are only partially understood. To gain new insight into the effects of epidermal-dermal interactions, we developed a multiscale, hybrid mathematical model of skin wound healing. The model takes into consideration interactions between epidermis and dermis across the basement membrane via diffusible signals, defined as activator and inhibitor. Simulations revealed that epidermal-dermal interactions are critical for proper extracellular matrix deposition in the dermis, suggesting these signals may influence how wound scars form. Our model makes several theoretical predictions. First, basal levels of epidermal activator and inhibitor help to maintain dermis in a steady state, whereas their absence results in a raised, scar-like dermal phenotype. Second, wound-triggered increase in activator and inhibitor production by basal epidermal cells, coupled with fast re-epithelialization kinetics, reduces dermal scar size. Third, high-density fibrin clot leads to a raised, hypertrophic scar phenotype, whereas low-density fibrin clot leads to a hypotrophic phenotype. Fourth, shallow wounds, compared to deep wounds, result in overall reduced scarring. Taken together, our model predicts the important role of signalling across dermal-epidermal interface and the effect of fibrin clot density and wound geometry on scar formation. This hybrid modelling approach may be also applicable to other complex tissue systems, enabling the simulation of dynamic processes, otherwise computationally prohibitive with fully discrete models due to a large number of variables

C-type lectin receptor expression is a hallmark of neutrophils infiltrating the skin in epidermolysis bullosa acquisita

IntroductionInflammatory epidermolysis bullosa acquisita (EBA) is characterized by a neutrophilic response to anti-type VII collagen (COL7) antibodies resulting in the development of skin inflammation and blistering. The antibody transfer model of EBA closely mirrors this EBA phenotype.MethodsTo better understand the changes induced in neutrophils upon recruitment from peripheral blood into lesional skin in EBA, we performed single-cell RNA-sequencing of whole blood and skin dissociate to capture minimally perturbed neutrophils and characterize their transcriptome.ResultsThrough this approach, we identified clear distinctions between circulating activated neutrophils and intradermal neutrophils. Most strikingly, the gene expression of multiple C-type lectin receptors, which have previously been reported to orchestrate host defense against fungi and select bacteria, were markedly dysregulated. After confirming the upregulation of Clec4n, Clec4d, and Clec4e in experimental EBA as well as in lesional skin from patients with inflammatory EBA, we performed functional studies in globally deficient Clec4e−/− and Clec4d−/− mice as well as in neutrophil-specific Clec4n−/− mice. Deficiency in these genes did not reduce disease in the EBA model.DiscussionCollectively, our results suggest that while the upregulation of Clec4n, Clec4d, and Clec4e is a hallmark of activated dermal neutrophil populations, their individual contribution to the pathogenesis of EBA is dispensable

Delineating mechanisms of cutaneous wound healing and regeneration in adults

Regeneration of hair follicles (HFs) and dermal adipocytes (DAs) occurs in mouse skin wounds upon large excisional wounding. Although HF regeneration is observed in African spiny mice of the genus Acomys and northern elephant seals after apex predator-inflicted wounding, laboratory rats do not display such regenerative phenotype. Such regeneration defect was observed in large excisional wound healing models in several rat strains, which undergo otherwise normal wound re-epithelialization. Inter-species transcriptome analyses between laboratory mouse and rat wound tissues attributed such lack of HF regeneration to differences in expression of inflammation markers, epigenetic remodelers and pleiotropic signaling molecules, including Satb1, Setd1b, Setdb1, and Whsc1l1. In mice, the origin of de novo HF regeneration has been partially elucidated, whereas the origin of DAs, a complex tissue that proceeds HF regeneration, remained elusive. Functional lineage tracing revealed the origin of DAs to be myofibroblastic. Bulk RNA-sequencing of genetically-labeled, FACS-purified myofibroblasts across a wound healing time course identified Zfp423 to be markedly up-regulated at a time-point coincident with initiation of DA regeneration. Pharmacological and genetic ablation/down-modulation of BMP signaling resulted in a significant DA regeneration defect. Because the origin of myofibroblasts appears to be tissue- and injury context-specific, the origin of myofibroblasts that contribute to DA regeneration in skin wounds was interrogated. Droplet-enabled single cell transcriptome analyses on unsorted, viable cells from wound dermal tissues collected prior the onset of HF regeneration was performed. Dimensionality reduction analyses revealed a large degree of cellular heterogeneity in the dermal compartment of early stage wounds. Furthermore, sub-clustering of wound fibroblasts further revealed a large degree of fibroblast heterogeneity. Pseudotime analyses revealed a putative fibroblast-myofibroblast differentiation trajectory and identified genes, including transcription factors, that may be important in myofibroblast differentiation in skin wounds in vivo. A subset of myofibroblasts expressed hematopoetic markers, most notably Lyz2, suggesting a common monocytic-origin. Full-length single cell RNA-sequencing and immunoblotting analyses of genetically labeled myofibroblasts confirmed these in silico observations. Bone marrow transplantation and functional lineage tracing using pan-hematopoetic Cre drivers demonstrated labeling of DA in regenerated skin wounds, suggesting that a population of hematopoetic-derived myofibroblasts contributes to regeneration of mouse skin wounds

Anatomical, Physiological, and Functional Diversity of Adipose Tissue.

Adipose tissue depots can exist in close association with other organs, where they assume diverse, often non-traditional functions. In stem cell-rich skin, bone marrow, and mammary glands, adipocytes signal to and modulate organ regeneration and remodeling. Skin adipocytes and their progenitors signal to hair follicles, promoting epithelial stem cell quiescence and activation, respectively. Hair follicles signal back to adipocyte progenitors, inducing their expansion and regeneration, as in skin scars. In mammary glands and heart, adipocytes supply lipids to neighboring cells for nutritional and metabolic functions, respectively. Adipose depots adjacent to skeletal structures function to absorb mechanical shock. Adipose tissue near the surface of skin and intestine senses and responds to bacterial invasion, contributing to the bodys innate immune barrier. As the recognition of diverse adipose depot functions increases, novel therapeutic approaches centered on tissue-specific adipocytes are likely to emerge for a range of cancers and regenerative, infectious, and autoimmune disorders

Hair Follicle Signaling Networks: A Dermal Papilla–Centric Approach

Functional testing of dermal papilla (DP) signaling inputs into hair follicle (HF) morphogenesis and regeneration is becoming possible with the advent of new Cre lines. Targeted deletion of the signature genes in early DP precursors has revealed significant signaling redundancy during HF morphogenesis. Furthermore, the DP lineage commitment program can be exploited for generating highly inductive DP cells to be used in HF bioengineering assays

Hair Follicle Signaling Networks: A Dermal Papilla–Centric Approach

Functional testing of dermal papilla (DP) signaling inputs into hair follicle (HF) morphogenesis and regeneration is becoming possible with the advent of new Cre lines. Targeted deletion of the signature genes in early DP precursors has revealed significant signaling redundancy during HF morphogenesis. Furthermore, the DP lineage commitment program can be exploited for generating highly inductive DP cells to be used in HF bioengineering assays

Light-Emitting Hair Follicles: Studying Skin Regeneration With In Vivo Imaging

Cutting-edge imaging technologies and new luminescent and fluorescent genetic tools now make it possible to study hair regeneration in vivo in real time at the microscopic single-cell level and at the macroscopic level of hair follicle populations. These technologies also allow for noninvasive assessment of the skin's clinically relevant homeostatic parameters, such as oxidative stress levels and pH