Staphylococcus aureus is responsible for the majority of skin and soft tissue infections, and aging and the growing obesity epidemic have further increased this risk. We have recently discovered that dermal white adipose tissue (dWAT) protects against invasive S. aureus infection by producing the antimicrobial peptide cathelicidin (CAMP), and this function is lost with advancing age. Here we investigated if obesity may disturb this beneficial immune response from dWAT. Using diet-induced obesity (DIO) mouse model, we found that obese dWAT was not only characterized by hypertrophic growth of adipocytes and but also characterized by a drastic depletion of adipogenic dermal fibroblasts (dFB), compared to lean controls. As a result, pathogen-triggered adipogenesis responses, including preadipocyte activation and CAMP production, were impaired in obese compared to normal diet mice. In vitro co-culture of adipogenic dFB and mature adipocytes revealed that mature adipocytes rapidly suppressed the adipogenic function of dFB. Further studies found that mature adipocytes secreted TGFβ1 and TGFβ2, which then activated the TGFBR-SMAD2/3 pathway and suppressed the expression of Pparg in dFB, leading to loss of the adipogenic potential of dFB upon co-culture with mature adipocytes. Treatments with TGFBR inhibitor (SB) and PPARγ agonist Rosiglitazone (ROSI) during co-culture synergistically restored the adipogenic function of dFB. In vivo, combination of SB and ROSI had the best effect in improving the reactive adipogenesis response and increasing skin resistance against S. aureus in HFD mice. Together, obesity disturbs the homeostatic balance between dFB and adipocytes, leading to loss of the antimicrobial defense function of dermal fat. Our results suggest that dWAT dysfunction may be partially responsible for the loss of skin defense in obesity, and TGFBR and PPARγ may be potential therapeutic targets to treat S. aureusskin infection associated with obesity.

Local differentiation of dermal adipogenic stem cells to mature adipocytes is necessary for cutaneous antimicrobial defense against invasive bacterial infection. However, the increase in mature adipocytes seen in obesity is paradoxically associated with increased rates of skin infection. In this study, we investigated if the antimicrobial-adipogenic response is impaired in obesity and how this may occur. Using mouse models of diet-induced obesity, we found that obese mice fed a high-fat diet lost local antimicrobial activity as seen by an increased susceptibility to S. aureus cutaneous infection. This was associated with a loss of adipogenic stem cells isolated from the dermis and a gain in mature adipocytes. In vitro studies of primary mouse dermal adipogenic stem cells showed that expression of cathelicidin mRNA (Camp) peaked 6-fold (p<0.05) in early adipogenesis and was subsequently lost with further differentiation. Functional assays confirmed that newly differentiating immature adipocytes produced antimicrobial activity in their culture supernatant that led to a greater than 3-log decrease in methicillin-resistant S. aureus growth. During the later phase of maturation, this antimicrobial activity was weakened and Camp mRNA expression decreased 9-fold (p<0.05). Furthermore, we found that when adipogenic stem cells were co-cultured with mature adipocytes, or exposed to mature adipocyte conditioned media, this resulted in inhibition of the capacity of adipogenic stem cells to express Camp mRNA, produce cathelicidin protein, and inhibit S. aureus growth. These results suggest that undifferentiated adipogenic stem cells are an antimicrobial reserve that can be locally induced to fight infection. However, mature adipocytes lose antimicrobial activity and may also diminish the antimicrobial capacity of adjacent adipogenic stem cells. These findings provide an explanation for why cutaneous defense is impaired in the skin of obese patients.

Psoriasis is a chronic inflammatory disease that affects 2% of the global population. Hyperproliferation and angiogenesis are some of the common biological phenomena shared by normal wound healing and psoriasis. In order to examine this more closely, RNA sequencing (RNA-seq) was used to identify differentially expressed genes (DEGs) after wounding of normal healthy skin and compared to lesional psoriatic skin. Three psoriatic subjects underwent a single 2mm punch biopsy to their lesional and nonlesional skin, and three controls had a single 2 mm punch biopsy of healthy skin. Four days later, these subjects had 4mm post-wounding punch biopsies, which were centered directly over the initial biopsy sites. We identified 779 genes significantly involved in control wound healing, and 269 genes significantly involved in psoriatic lesional skin (>+/- 1.95 fold change, p < 0.05). Gene ontology analysis identified 35 significant biological processes enriched in control wounding, and 16 biological processes enriched in psoriatic lesional skin. Response to interferon beta was upregulated in both wounded normal skin and psoriatic lesional skin in comparison to normal skin, and keratinization was most increased (at least 20 fold), in psoriasis lesional skin in comparison to normal skin. In wounded normal skin, leukocyte migration involved in inflammatory response and positive regulation of monocyte chemotaxis was also increased at least 15 fold. Interestingly, biological processes enriched in psoriasis but not in wounded normal skin were primarily limited to phosphatidylethanolamine acyl-chain remodeling and phosphatidylcholine acyl-chain remodeling. Further studies with larger sample sizes and qRT-PCR will be done in order to confirm these preliminary findings.

Transcriptome differences in wound healing between psoriatic nonlesional and healthy skin In psoriasis, the Koebner phenomenon describes the appearance of isomorphic skin lesions after local trauma in a previously unaffected area. While the pathogenesis remains unclear, this phenomenon suggests that differences exist between psoriatic and healthy wound healing. In this study, we used RNA-sequencing (RNA-seq) to compare wound healing in psoriasis nonlesional and healthy skin. Three psoriasis subjects and three control subjects each received one 2 mm punch biopsy in nonlesional or healthy skin, respectively. Three days later, a 2 mm punch biopsy over the initial biopsy site was collected for RNA-seq. We identified 833 genes and 779 genes significantly involved in psoriatic nonlesional and control wound healing, respectively (>+/–1.95 fold change, p < 0.05). Gene ontology analysis showed 35 significant biological processes enriched in control wounding and 27 biological processes enriched in psoriatic nonlesional wounding. Control wounding uniquely involved antiviral defense processes such as interferon-beta response, type I interferon signaling pathway, negative regulation of viral genome replication, and response to lipopolysaccharide. Psoriatic nonlesional wounding uniquely involved response to reactive oxygen species, cell redox homeostasis, negative regulation of endopeptidase activity, and positive regulation of cell proliferation. These findings suggest that antiviral and interferon immune responses vary in psoriatic nonlesional wound healing compared to healthy wound healing, which may play a role in koebnerization. This is the first study comparing wound healing in psoriasis nonlesional and control skin. Further studies are being done to identify genes in pathways of interest and confirm these preliminary findings using RT-qPCR and Western blotting.

Significant similarities between psoriasis and wound healing have been demonstrated in vitro and in vivo, but no literature exists at the transcriptome level. Here, we use RNA-sequencing (RNA-seq) to compare psoriasis and wound healing to further elucidate the pathogenesis of psoriasis. Two psoriasis subjects received one 2 mm punch biopsy of a psoriatic plaque and two control subjects received one 2 mm punch biopsy of normal skin. Three days later, each control subject received a 2 mm punch biopsy over the initial biopsy site. RNA-seq of post-wounded skin showed a transcriptome profile distinct from pre-wounded skin that more closely matched psoriasis expression in both heat map and hierarchical cluster analysis. To confirm this quantitatively, differentially expressed genes (DEGs) with >4 fold change and p<0.05 were identified. 52% of 810 psoriasis genes were implicated in healthy wounding. However, 1616 genes were identified in post-wounded skin. Gene ontology analysis for normal wounded skin and psoriatic lesional skin demonstrated greatest enrichment in keratinization. However, normal wounded skin showed unique enrichment for cell activation, leukocyte activation and adhesion, and inflammatory response. In contrast, psoriasis lesional skin was uniquely enriched in epithelial cell differentiation, and epidermis and ectoderm development. These findings suggest that psoriasis and normal wounding share similarities at the transcriptome level, but psoriasis may lack genes required for normal wound healing. Further investigations with larger sample sizes are necessary to confirm these preliminary findings.

The inflammasome is a complex of proteins that has a critical role in mounting an inflammatory response in reply to a harmful stimulus that compromises the homeostatic state of the tissue. The NLRP3 inflammasome, which is found in a wound-like environment, is comprised of three components: the NLRP3, the adaptor protein ASC and caspase-1. Interestingly, although ASC levels do not fluctuate, caspase-1 levels are elevated in both physiological and pathological conditions. Despite the observation that merely raising caspase-1 levels is sufficient to induce inflammation, the crucial question regarding the mechanism governing its expression is unexplored. We found that, in an inflammatory microenvironment, caspase-1 is regulated by NF-κB. Consistent with this association, the inhibition of caspase-1 activity parallels the effects on wound healing caused by the abrogation of NF-κB activation. Surprisingly, not only does inhibition of the NF-κB/caspase-1 axis disrupt the inflammatory phase of the wound-healing program, but it also impairs the stimulation of cutaneous epithelial stem cells of the proliferative phase. These data provide a mechanistic basis for the complex interplay between different phases of the wound-healing response in which the downstream signaling activity of immune cells can kindle the amplification of local stem cells to advance tissue repair.

Atopic dermatitis is an inflammatory skin disease that affects approximately 20% of children worldwide. Left untreated, the barrier function of the skin is compromised, increasing susceptibility to dehydration and infection. Despite its prevalence, its multifactorial nature has complicated the unraveling of its etiology. We found that chronic loss of epidermal caspase-8 recapitulates many aspects of atopic dermatitis, including a spongiotic phenotype whereby intercellular adhesion between epidermal keratinocytes is disrupted, adversely affecting tissue architecture and function. Although spongiosis is generally thought to be secondary to edema, we found that suppression of matrix metalloproteinase-2 activity is sufficient to abrogate this defect. p38 MAPK induces matrix metalloproteinase-2 expression to cleave E-cadherin, which mediates keratinocyte cohesion in the epidermis. Thus, the conditional loss of caspase-8, which we previously found to mimic a wound response, can be used to gain insights into how these same wound-healing processes are commandeered in inflammatory skin diseases.

This paper provides evidence for a role of caspase 8 in skin homeostasis. Loss of epidermal caspase 8, an important mediator of apoptosis, promotes several phases of the wound healing response in mice, including epidermal cell proliferation and inflammation. The findings illustrate how the loss of caspase 8 can have an impact beyond programmed cell death to affect the local microenvironment and elicit processes common to wound repair and many neoplastic skin disorders. This paper provides evidence for a role of caspase 8 in skin homeostasis. Loss of epidermal caspase 8 promotes epidermal cell proliferation and inflammation, and authors propose that caspase 8 expression by keratinocytes in the granular layer limits the release of preformed IL1α. Tissue homeostasis and regeneration are regulated by an intricate balance of seemingly competing processes—proliferation versus differentiation, and cell death versus survival1. Here we demonstrate that the loss of epidermal caspase 8, an important mediator of apoptosis2, recapitulates several phases of a wound healing response in the mouse. The epidermal hyperplasia in the caspase 8 null skin is the culmination of signals exchanged between epidermal keratinocytes, dermal fibroblasts and leukocytic cells. This reciprocal interaction is initiated by the paracrine signalling of interleukin 1α (IL1α), which activates both skin stem cell proliferation and cutaneous inflammation. The non-canonical secretion of IL1α is induced by a p38-MAPK-mediated upregulation of NALP3 (also known as NLRP3), leading to inflammasome assembly and caspase 1 activation. Notably, the increased proliferation of basal keratinocytes is counterbalanced by the growth arrest of suprabasal keratinocytes in the stratified epidermis by IL1α-dependent NFκB signalling. Altogether, our findings illustrate how the loss of caspase 8 can affect more than programmed cell death to alter the local microenvironment and elicit processes common to wound repair and many neoplastic skin disorders.