Dermal Adipocytes Research Articles

States and Fates of Skin Fibroblasts Revealed through Chromatin Accessibility

Adult mammalian skin harbors a myriad of stromal populations, but it is only recently that we have been able to untangle their developmental origins, lineage-specific functions, and fluctuations in influence during skin development, aging, and injury/repair. Thompson et al. (2022) now unveil the epigenetic status underlying the heterogeneity of skin stromal cells. They also show that neonatal skin stromal cells exhibit chromatin landscapes that shape two epigenetically distinct lineages: dermal papilla and adipocyte fates.

Microphysiological endothelial models to characterize subcutaneous drug absorption.

The high variability in subcutaneous bioavailability of protein therapeutics is poorly understood, contributing to critical delays in patient access to new therapies. Preclinical animal and in vitro models fail to provide a physiologically relevant testbed to parse potential contributors to human bioavailability, therefore new strategies are necessary. Here, we present a microphysiological model of the human hypodermal vasculature at the injection site to study the interactions of administered protein therapeutics within the microenvironment that influence subcutaneous bioavailability. Our model combines human dermal endothelial cells, fibroblasts, and adipocytes, self-assembled into three-dimensional, perfusable microvessels that express relevant extracellular matrix. We demonstrate the utility of the model for measurement of biophysical parameters within the hypodermal microenvironment that putatively impact protein kinetics and distribution at the injection site. We propose that microphysiological models of the subcutaneous space have applications in preclinical development of protein therapeutics intended for subcutaneous administration with optimal bioavailability.

Dermal White Adipose Tissue (dWAT) Is Regulated by Foxn1 and Hif-1α during the Early Phase of Skin Wound Healing.

Dermal white adipose tissue (dWAT) is involved in the maintenance of skin homeostasis. However, the studies concerning its molecular regulation are limited. In the present paper, we ask whether the introduction of two transcription factors, Foxn1 and Hif-1α, into the post-wounded skin of Foxn1−/− mice regulates dWAT during wound healing (days 3 and 6). We have chosen lentivirus vectors (LVs) as a tool to deliver Foxn1 and Hif-1α into the post-wounded skin. We documented that combinations of both transgenes reduces the number, size and diameter of dermal adipocytes at the wound bed area. The qRT-PCR analysis of pro-adipogenic genes, revealed that LV-Hif-1α alone, or combined with LV-Foxn1, increases the mRNA expression of Pparγ, Glut 4 and Fasn at post-wounding day 6. However, the most spectacular stimulatory effect of Foxn1 and/or Hif-1α was observed for Igf2, the growth factor participating in adipogenic signal transduction. Our data also shows that Foxn1/Hif-1α, at post-wounding day 3, reduces levels of CD68 and MIP-1γ mRNA expression and the percentage of CD68 positive cells in the wound site. In conclusion, the present data are the first to document that Foxn1 and Hif-1α cooperatively (1) regulate dWAT during the proliferative phase of skin wound healing through the Igf2 signaling pathway, and (2) reduce the macrophages content in the wound site.

Parallel Single-Cell Multiomics Analysis of Neonatal Skin Reveals the Transitional Fibroblast States that Restrict Differentiation into Distinct Fates

One of the keys to achieving skin regeneration lies within understanding the heterogeneity of neonatal fibroblasts, which support skin regeneration. However, the molecular underpinnings regulating the cellular states and fates of these cells are not fully understood. To investigate this, we performed a parallel multiomics analysis by processing neonatal murine skin for single-cell Assay for Transposase-Accessible Chromatin sequencing and single-cell RNA sequencing separately. Our approach revealed that fibroblast clusters could be sorted into papillary and reticular lineages on the basis of transcriptome profiling, as previously reported. However, single-cell Assay for Transposase-Accessible Chromatin sequencing analysis of neonatal fibroblast lineage markers, such as Dpp4/Cd26, Corin, and Dlk1 along with markers of myofibroblasts, revealed accessible chromatin in all fibroblast populations despite their lineage-specific transcriptome profiles. These results suggest that accessible chromatin does not always translate to gene expression and that many fibroblast lineage markers reflect a fibroblast state, which includes neonatal papillary fibroblasts, reticular fibroblasts, and myofibroblasts. This analysis also provides a possible explanation as to why these marker genes can be promiscuously expressed in different fibroblast populations under different conditions. Our single-cell Assay for Transposase-Accessible Chromatin sequencing analysis also revealed that the functional lineage restriction between dermal papilla and adipocyte fates is regulated by distinct chromatin landscapes. Finally, we have developed a webtool for our multiomics analysis: https://skinregeneration.org/scatacseq-and-scrnaseq-data-from-thompson-et-al-2021-2/.

Skin Fibrosis and Recovery Is Dependent on Wnt Activation via DPP4

Fibrosis is the life-threatening, excessive accumulation of the extracellular matrix and is sometimes associated with a loss of lipid-filled cells in the skin and other organs. Understanding the mechanisms of fibrosis and associated lipodystrophy and their reversal may reveal new targets for therapeutic intervention. Invivo genetic models are needed to identify key targets that induce recovery from established fibrosis. Wnt signaling is activated in animal and human fibrotic diseases across organs. Here, we developed a genetically inducible and reversible Wnt activation model and showed that it is sufficient to cause fibrotic dermal remodeling, including extracellular matrix expansion and shrinking of dermal adipocytes. Upon withdrawal from Wnt activation, Wnt-induced fibrotic remodeling was reversed in mouse skin-fully restoring skin architecture. Next, we demonstrated CD26/ DPP4 is a Wnt/β-catenin-responsive gene and a functional mediator of fibrotic transformation. We provide genetic evidence that the Wnt/DPP4 axis is required to drive fibrotic dermal remodeling and is associated with human skin fibrosis severity. Remarkably, DPP4 inhibitors can be repurposed to accelerate recovery from established Wnt-induced fibrosis. Collectively, this study identifies Wnt/DPP4 axis as a key driver of extracellular matrix homeostasis and dermal fat loss, providing therapeutic avenues to manipulate the onset and reversal of tissue fibrosis.

Skin aging: Dermal adipocytes metabolically reprogram dermal fibroblasts.

Emerging data connects the aging process in dermal fibroblasts with metabolic reprogramming, provided by enhanced fatty acid oxidation and reduced glycolysis. This switch may be caused by a significant expansion of the dermal white adipose tissue (dWAT) layer in aged, hair-covered skin. Dermal adipocytes cycle through de-differentiation and re-differentiation. As a result, there is a strongly enhanced release of free fatty acids into the extracellular space during the de-differentiation of dermal adipocytes in the catagen phase of the hair follicle cycle. Both caveolin-1 and adiponectin are critical factors influencing these processes. Controlling the expression levels of these two factors also offers the ability to manipulate the metabolic preferences of the different cell types within the microenvironment of the skin, including dermal fibroblasts. Differential expression of adiponectin and caveolin-1 in the various cell types may also be responsible for the cellular metabolic heterogeneity within the cells of the skin.

Generation of Skin Organoids: Potential Opportunities and Challenges.

Although several types of human skin substitutes are currently available, they usually do not include important skin appendages such as hair follicles and sweat glands, or various skin-related cells, such as dermal adipocytes and sensory neurons. This highlights the need to improve the in vitro human skin generation model for use as a tool for investigating skin diseases and as a source of cells or tissues for skin regeneration. Skin organoids are generated from stem cells and are expected to possess the complexity and function of natural skin. Here, we summarize the current literatures relating to the “niches” of the local skin stem cell microenvironment and the formation of skin organoids, and then discuss the opportunities and challenges associated with multifunctional skin organoids.

634 Single cell transcriptomics identifies a two way conversion program between dermal progenitors and adipocytes during skin development and regeneration

Dermal white adipose tissue (dWAT) is a unique layer of adipocytes and their fibroblast precursors within the reticular dermis of the skin, and recent studies have found that dWAT plays an essential role in skin development and regeneration. However, little is known how adipocytes are developed from their dermal progenitor and how adipogenesis is controlled during skin regeneration. Here, we performed single-cell RNA-sequencing (scRNA-seq) analyses of dermal fibroblasts (dFBs) isolated from mice at various post-developmental ages. Pseudotime analyses of these dFB clusters identified a Pdgfra+CD24hiThy1loSca1lo population as developmental progenitors for preadipocytes (pAD) and this progenitor population was highly abundant in neonatal skin early in life but declined in adulthood. In contrast, a Pdgfra+Sca1hiDPP4hi adult reticular interstitial (RI) adipocyte progenitor (AP) population that developed postnatally was found in adult skin. ScRNAseq analyses were then performed on dFBs from wounded skin to determine the cell fate and role of these adult RI-APs during skin regeneration. Pseudotime analysis and immunofluorescence analysis of wounded skin showed that RI-APs rapidly infiltrated the wound center and gave rise to pAD/AD and/or myofibroblasts during early stages of wound healing, contributing to dermal regeneration. Pathway analyses revealed that this two way conversion between dermal progenitors and pAD/AD was dynamically regulated by an interplay between the WNT, TGFβ and NFKB pathways. Together, our results have identified and defined the developmental pathways that shape adipogenesis from dermal progenitors during skin development and regeneration. These results provide insights into how defective adipocyte progenitor function and adipogenesis may be associated with defective wound healing associated with aging and diabetes and/or fibrotic diseases such as keloid scarring.

632 Adipocyte-derived fatty acids induce metabolic activation of macrophage differentiation in the wound bed

After injury, a timely inflammatory response that involves the recruitment of blood-derived circulating monocytes is essential for tissue repair. Once in the wound bed, these cells differentiate into macrophages during the inflammatory phase, a process that goes awry during aging in chronic non-healing wounds. Cell differentiation is a highly energy-demanding process, and metabolic substrates present in the wound bed niche are important for the wound healing outcome. We previously demonstrated that dermal adipocytes release fatty acids into wound beds after injury to promote inflammation, yet the function of adipocyte-derived fatty acids in the initiation of the inflammatory response after injury is not known. To unveil the role of adipocytes as providers of fatty acids used to fuel the metabolic requirements of immune cell differentiation, we set out to evaluate their role on monocyte to macrophage differentiation in the skin wound bed. Here, we utilize in vivo mouse models of skin injury and metabolic assays to reveal that monocytes utilize fatty acids to fuel metabolic programs that induce macrophage differentiation. We show that extracellular vesicles (EVs) loaded with lipids are actively taken up by monocytes in vitro and in mice in vivo. Using real-time respirometry, we show that these fatty acids activate the b-oxidation metabolic pathway in monocytes and its inhibition leads to the abrogation of macrophage differentiation. Furthermore, we show that with age, not only adipocyte-derived EV production was impaired, but more importantly, these particles were less effective in rewiring monocyte metabolism towards oxidative phosphorylation. In summary, our findings reveal an essential adipocyte-monocyte metabolic axis that controls inflammation in the wound bed niche.

612 Wnt signaling induces fibrotic fat loss via DPP4 in skin fibrosis

Fibrotic diseases involve a loss of fat or lipid-filled cells in several organs including the lungs, liver, and skin in addition to expansion of extracellular matrix (ECM). Dermal fibrosis including excess scarring, scleroderma, and eczema, affects about 40 million people, yet there are no therapies for its reversal. Skin is unique among fibrotic organs due to its accessibility and distinct fat compartment, called dermal white adipose tissue (DWAT). Reduced lipid among mature adipocytes which comprise the DWAT has implications for their many functions. The mechanisms governing fibrotic DWAT lipid depletion and the effects of lipid depletion are not known. Wnt signaling is dysregulated among fibrotic tissues and sustained Wnt signaling in mouse dermis is sufficient to cause dermal fibrosis including DWAT lipid depletion through previously unknown mediators. Here we test the following hypothesis: Induced Wnt signaling stimulates lipid breakdown via dipeptidyl peptidase 4 (DPP4) in dermal adipocytes impacting ECM expansion. Using an inducible and reversible genetic mouse model of dermal Wnt activation, we identify cellular mechanisms of lipid depletion and recovery. Wnt activation leads to breakdown of intracellular lipid in DWAT, stimulation of the lipolytic axis, and increased ECM remodeling, all of which recover during reversal. Genetic ablation of candidate Wnt-responsive factor, DPP4, in our model leads to DWAT preservation, attenuated lipid breakdown, diminished ECM remodeling, and reduced dermal thickening. Finally, chemical inhibition of DPP4 in our mouse model leads to accelerated recovery of DWAT and dermis. Together these data demonstrate that Wnt-DPP4 modulates lipid homeostasis in adipocytes, impacting neighboring dermal fibroblasts. Thus, our results suggest that treatment of dermal fibrosis with FDA-approved DPP4 inhibitors may effectively target lipid depletion, a new cellular player in fibrosis.

Diet-induced obesity promotes infection by impairment of the innate antimicrobial defense function of dermal adipocyte progenitors.

Infections are a major complication of obesity, but the mechanisms responsible for impaired defense against microbes are not well understood. Here, we found that adipocyte progenitors were lost from the dermis during diet-induced obesity (DIO) in humans and mice. The loss of adipogenic fibroblasts from mice resulted in less antimicrobial peptide production and greatly increased susceptibility to Staphylococcus aureus infection. The decrease in adipocyte progenitors in DIO mice was explained by expression of transforming growth factor-β (TGFβ) by mature adipocytes that then inhibited adipocyte progenitors and the production of cathelicidin in vitro. Administration of a TGFβ receptor inhibitor or a peroxisome proliferator-activated receptor-γ agonist reversed this inhibition in both cultured adipocyte progenitors and in mice and subsequently restored the capacity of obese mice to defend against S. aureus skin infection. Together, these results explain how obesity promotes dysfunction of the antimicrobial function of reactive dermal adipogenesis and identifies potential therapeutic targets to manage skin infection associated with obesity.

The Role of Adipose Tissue in Hair Regeneration: A Potential Tool for Management?

Human adipose tissue (AT) is a rich and easily harvestable source of stem cells and various growth factors (GFs). It has been widely used hitherto for facial rejuvenation and volumization. Increasing evidence shows that dermal adipocytes are intricately associated with hair follicles (HFs) and may be necessary to drive follicular stem cell activation. Early published data have shown encouraging preliminary results for the use of adipocytes and their stem cells as a treatment option for hair growth. The aim of this review study is to analyze published literature on the effect of fat on hair growth and to summarize the current evidence.

Dermal adipocytes contribute to the metabolic regulation of dermal fibroblasts.

Dermal fibroblasts are an essential population of skin cells. They are not only responsible for synthesis and remodelling of the extracellular matrix of the dermis, but also communicate with other skin cells via autocrine and paracrine interactions. Skin-associated dermal adipocytes reside below the reticular dermis. Strong lipolysis is observed during the regression of dermal adipocytes. However, the nature of the local intercellular crosstalk in which lipids released by dermal adipocytes affecting the metabolism of adjacent skin fibroblasts has not yet been examined. With the use of a series of novel mouse models that allow us to manipulate adipocytes, we demonstrate that dermal adipocytes can modulate the structure of the dermis through regulating extracellular matrix production in dermal fibroblasts. Fatty acids released by dermal adipocytes are involved in this process. Our observations offer new in vivo insights into the role of dermal adipocyte-derived lipids in influencing metabolism of adjacent local cells in the skin through a paracrine effect in the microenvironment of the dermal adipocyte.

Fat cells help wounds heal

Dermal adipocytes promote wound healing by recruiting macrophages and transdifferentiating into myofibroblasts.

336 Diet-induced obesity impairs the antimicrobial defense function of dermal adipocyte progenitors

Obesity affects more than 20% of the world population and is a major risk factor for several skin disorders such as infection, delayed wound healing and psoriasis. Dermal white adipose tissue (DWAT) has been recognized as an important antimicrobial defense layer of the skin, but how obesity impacts this important element of skin innate immunity is unclear. Here, we used a diet-induced obesity (DIO) mouse model, single-cell RNA-seq analyses and adipocyte lineage-tracing to better understand how obesity impacts the innate immune functions of DWAT and dermal adipocyte precursors. Mice fed a 60% high-fat diet for 6-months had marked adipocyte hypertrophy compared to standard diet and lineage-tracing showed that >95% of adipocytes at 6 m of DIO were new adipocytes formed 1 week after start of HFD feeding. DIO mice also showed a drastic change in dermal fibroblasts (dFB) subclusters including depletion of 3 adipogenic progenitor populations. Loss of these populations was associated with a lack of pathogen-triggered reactive adipogenesis, impaired cathelicidin antimicrobial peptide production and increased susceptibility to S. aureus infection. Culture of primary dermal adipocyte progenitors with mature adipocytes promoted a loss of antimicrobial function in the precursor population. This negative feedback from mature adipocytes was due to secretion of TGFβ, and administration of a TGFBR inhibitor or a PPARγ agonist reversed this inhibition in cultured adipocyte progenitors and restored the capacity in vitro and in vivo to initiate reactive adipogenesis and to kill S. aureus. Together, these results suggest that dysfunction of dermal innate immune defense function may explain several disorders associated with obesity and highlights TGFβ and PPARγ as targets for intervention.

Skinny Fat Cells Stimulate Wound Healing.

In this issue of Cell Stem Cell, Shook etal. (2020) reveal that dermal adipocytes regulate skin wound repair via release of fatty acids that promote macrophage recruitment and accelerated revascularization. Furthermore, mature dermal adipocytes dedifferentiate into migratory extracellularmatrix-producing myofibroblasts.

Activation of skin-resident group 2 innate lymphoid cells by cold-sensing TRPM8+ neuron-derived signals in thermo-homeostatic regulation and tissue inflammation

Abstract Innate lymphoid cells (ILCs) in the skin are a heterogeneous population of innate lymphocytes that function in local homeostatic regulation but could also cause inflammation. How homeostatic versus inflammatory functions of skin ILCs are regulated is poorly understood. To explore this question, we analyzed gene expression profiles of skin ILCs of wild-type and Rag1−/− mice and discovered that the chemokine receptor CCR10+ skin ILCs dominant in the health skin were of homeostatic properties while CCR10− skin ILCs are activated effector cells with elevated expression of various stimulatory cytokine receptors and signaling molecules. In stearoyl-CoA desaturase 1-knockout (KO) mice whose skin was defective in thermohomeostatic maintenance due to impaired fatty acid synthesis and lipid barrier formation, there was predominant activation of CCR10−CD81+ST2+ group 2 ILCs (ILC2s) while raising these mice at a thermo-neutral room temperature or feeding them with high-fat diet reduced the ILC2 activation, suggesting that skin ILC2s are activated by cold-induced signals to help thermo-homeostatic maintenance. Consistent with this, topical application of menthol, an agonist ligand for the cold-sensing receptor TRPM8, led to activation of CCR10−CD81+ST2+ skin ILC2s and increased dermal adipocyte maturation and uncoupling protein 1 production. Furthermore, compared to wild-type controls, menthol-treated TRPM8-KO and IL-18-KO mice had reduced CCR10−CD81+ activated skin ILC2s, suggesting that IL-18 mediated the TRPM8 activation of skin ILC2s. Our study reveals a neuron-derived cold-sensing pathway in activation of skin ILC2s that helps thermo-regulation but could also cause tissue inflammation if chronically activated.

Dermal Adipocyte Lipolysis and Myofibroblast Conversion Are Required for Efficient Skin Repair.

Mature adipocytes store fatty acids and are a common component of tissue stroma. Adipocyte function in regulating bone marrow, skin, muscle, and mammary gland biology is emerging, but the role of adipocyte-derived lipids in tissue homeostasis and repair is poorly understood. Here, we identify an essential role for adipocyte lipolysis in regulating inflammation and repair after injury in skin. Genetic mouse studies revealed that dermal adipocytes are necessary to initiate inflammation after injury and promote subsequent repair. We find through histological, ultrastructural, lipidomic, and genetic experiments in mice that adipocytes adjacent to skin injury initiate lipid release necessary for macrophage inflammation. Tamoxifen-inducible genetic lineage tracing of mature adipocytes and single-cell RNA sequencing revealed that dermal adipocytes alter their fate and generate ECM-producing myofibroblasts within wounds. Thus, adipocytes regulate multiple aspects of repair and may be therapeutic for inflammatory diseases and defective wound healing associated with aging and diabetes.

Regulated in Development and DNA Damage Responses 1 Prevents Dermal Adipocyte Differentiation and Is Required for Hair Cycle–Dependent Dermal Adipose Expansion

Dermal white adipose tissue (dWAT) expansion is associated with important homeostatic and pathologic processes in skin. Even though mTOR/protein kinase B signaling is important for adipogenesis, the role of regulated development of DNA damage responses 1 (REDD1), a negative regulator of mTOR/protein kinase B, is poorly understood. Loss of REDD1 in mice resulted in reduction of body mass, total fat, size of gonadal white adipose tissue, and interscapular brown adipose tissue. Inguinal subcutaneous white adipose tissue and dWAT in REDD1 knockouts were expanded compared with wild type mice. Size and number of mature adipocytes in dWAT were also increased in adult REDD1 knockouts. This dWAT phenotype was established around postnatal day 18 and did not depend on the hair growth cycle. Numbers of adipocyte precursor cells were lower in REDD1 knockout skin. Invitro analysis revealed increased differentiation of skin-derived REDD1 knockout adipocyte precursor cells as indicated by higher lipid accumulation and increased adipogenic marker expression. 3T3L1 cells overexpressing REDD1 had decreased sensitivity to differentiation. Overall, our findings indicate that REDD1 silencing induced expansion of dWAT through hypertrophy and hyperplasia. This REDD1-dependent mechanism of adipogenesis could be used to preferentially target skin-associated adipose tissue for therapeutic purposes.

Cytological and functional characteristics of fascia adipocytes in rats: A unique population of adipocytes

The dermal adipocytes, superficial fascia and subcutaneous adipose tissue (SAT) exist in the interspaces between the dermis and muscular fascia. They are adjacent to each other and traditionally recognized as one SAT. Recently, the dermal adipocyte was redefined as a unique population independent from the SAT. Also, we identified a novel type of adipogenic progenitors in rat superficial fascia. This study aimed to examine cytological and functional characteristics of fascial adipocytes in rats. Superficial fascia had no adipocytes in neonatal rats but gradually appeared numbers of adipocytes in growing rats. Adipogenic progenitors were found to reside in fascia and had strong ability in spontaneous and induced adipogenic differentiation in vitro. Differentiated fascial adipocytes versus subcutaneous or visceral adipocytes expressed increased adipose triglyceride lipase but decreased beta-adrenoreceptor, perilipin-1 and hormone-sensitive lipase (HSL), thus having high basal lipolysis but low lipolysis response to catecholamines. Phosphorylation of perilipin-1 and HSL and translocation of HSL to lipid droplets were attenuated in response to catecholamines rather than post-adrenoreceptoral lipolytic stimulators. The results suggested that superficial fascia was an origin of adipocytes with distinct developmental, cytological and functional characteristics. We proposed that fascial adipocytes could be considered as a unique population of adipocytes in the body. The fascia origin of adipocytes as an adipogenic model might logically explain fat neogenesis occurred at anatomical locations where originally exist no adipose tissues and thereby no adipose-derived stromal precursors. Also, the special histoanatomical relations and overlaps between the dermis, superficial fascia, SAT, and their adipocytes were discussed.