Tissue repair is often hampered during aging. Worldwide, chronic wounds in elderly present a major challenge to the medical and socioeconomic infrastructure of societies. A comprehensive understanding of how the aging innate immune system impacts wound homeostasis is lacking. Here we employed the approach of immune modulation to restore disrupted wound repair in aged mice skin. We found that a short pulse of bacterial lipopolysaccharide (LPS) before wounding markedly accelerate tissue repair in aged mice, which - if non-primed - exhibit a defective epidermal wound closure. LPS priming induces rapid sealing of wounds, immune cell activity, keratinocyte responsiveness and their differentiation towards a newly reconstituted wound epithelium. Structural elements such as NETs composed of DNA and membrane protrusions derived from LPS-activated neutrophils and macrophages, respectively, reinforce physical skin barrier in aged wounds. The physical barrier established by LPS-primed innate immune cells subsequently facilitates epithelial tongue migration and adhesion of ECM-producing mesenchymal cells. Collectively, this not only prevents the invasion of pathogens into the restoring skin tissue after injury, but also averts the persistence of low-grade inflammation associated with aged wounds. These findings underscore the benefit of immune cell priming in promoting cellular interactions between innate immune cells and epithelial cells that consequently restores physical skin barrier and promote tissue repair.

ABCB5+ dermal mesenchymal stem cells (DMSCs) regulate macrophage activation via interleukin-1 receptor antagonist (IL-1Ra), but upstream control mechanisms remain unclear. Here, we define a dual-signal model integrating inflammatory and type 2 cytokine pathways. IFNγ/LPS priming initiates IL-1Ra expression, while IL-4 signaling through IL-4Rα amplifies this response via STAT6 in both ABCB5+ DMSCs and macrophages. In coculture, ABCB5+ DMSCs drive macrophage polarization toward a CD206+/CD163+ phenotype with enrichment of CD163-expressing subsets. IL-4Rα blockade with dupilumab inhibits STAT6 activation, suppresses IL-1Ra amplification, and attenuates expansion of this IL-4Rα-dependent macrophage population. Collectively, these findings identify IL-4/IL-4Rα/STAT6 signaling as a conserved amplifier of IL-1Ra-mediated stromal-immune crosstalk.

Senescent fibroblasts accumulate in the connective tissue of all organs and promote organ aging and aging-related diseases. The underlying mechanisms for the accumulation of senescent fibroblasts are poorly understood. Natural killer (NK) cells of innate immunity play a critical role in the removal of tissue resident senescent cells. We here show that NK cells from old adults and old mice fail to efficiently remove senescent fibroblasts. This is due to severely reduced perforin and granzyme B release from aged NK cells where perforin is responsible for inducing holes in the membrane of senescent fibroblasts through which granzyme B enters enforcing cell death of senescent fibroblasts. We demonstrate elevated activation of the small Cdc42 Rho GTPase in aged NK cells to be responsible for the disruption of the microtubular organization which is essential for the proper release of perforin and granzyme B and for energy homeostasis. Attenuation of the elevated activity of Cdc42 in aged human NK cells with CASIN, a small molecule Cdc42 inhibitor, rebalances Cdc42 activity to a young level. Rebalancing of Cdc42 restores proper perforin and granzyme B release and attenuates reduced ATP levels in aged NK cells resulting in an attenuated "youthful" cytotoxicity of aged NK cells against senescent cells. Collectively, we identified a previously unreported molecular mechanism underlying functional impairment of NK cells from older adults. In perspective, our data hold promise to develop novel strategies against age-related disorders driven by tissue-resident senescent fibroblasts.

Dysregulation of macrophage populations at the wound site is responsible for the non-healing state of chronic wounds. The underlying mechanisms in diabetic conditions at single cell resolution and therapeutic advances remain, however, largely unexplored. Here, it is reported that acetyl histone-H3 (Lys27), an epigenetic mark regulating the macrophage transcriptome, is lost in the hostile tissue microenvironment of diabetes. Diabetic conditions suppress the acetylation of histone, a critical regulator of trained immunity, by activating histone deacetylase (HDAC)-dependent deacetylation pathways. This, in consequence, suppresses STAT1 signaling in macrophages under diabetic conditions. Interestingly, butyrate, a potent HDAC inhibitor, restores the acetyl histone-H3 (Lys27)-dependent transcriptome and thereby effectively rescues macrophage functions even in a persisting diabetic microenvironment. Butyrate not only reinstalls the physiologic STAT1 mediated immune program in macrophages at early phases of diabetic skin repair, but also harmonizes macrophage interactions with keratinocytes and fibroblasts, depicting a unique fingerprint of tissue regeneration. Most interestingly, butyrate breaks the vicious cycle of inflammation in chronic wounds by restoring classical stages of wound healing. This study offers novel pathogenic insight and the unique opportunity to reverse perturbed immune function, holding promise to successfully treat diabetic and other chronic wounds with unresolved inflammation.

Aging constitutes the largest risk factor for melanoma progression. While a contribution of factors secreted from senescent skin fibroblasts to the progression of melanoma has been proposed, the nature of such factors and subsequent underlying mechanisms remains elusive. Here we show that the chemokine GCP-2 is excessively released by senescent fibroblasts in vitro and the skin of old melanoma patients. GCP-2 regulates, via phosphorylation of the transcription factor CREB at serine 133, defense-, cell cycle control-, and glycolysis-enhancing genes in melanoma cell lines. GCP-2 promotes oncogenic properties in vitro and in vivo in murine melanoma models. Inhibition of CREB phosphorylation in melanoma cells represses glycolytic target genes and induces a switch from glycolysis to oxidative phosphorylation that translates into a significant decline in tumor size in vivo in murine melanoma models. This study identifies a senescent fibroblast to chemokine to CREB to metabolic axis that drives melanoma progression. Targeting this axis may hold promise for novel therapeutic approaches in difficult-to-treat melanoma in older adults.

Skin is the largest organ of the human body and undergoes both intrinsic (chronological) and extrinsic aging. While intrinsic skin aging is driven by genetic and epigenetic factors, extrinsic aging is mediated by external threats such as UV irradiation or fine particular matters, the sum of which is referred to as exposome. The clinical manifestations and biochemical changes are different between intrinsic and extrinsic skin aging, albeit overlapping features exist, eg, increased generation of reactive oxygen species, extracellular matrix degradation, telomere shortening, increased lipid peroxidation, or DNA damage. As skin is a prominent target for many hormones, the molecular and biochemical processes underlying intrinsic and extrinsic skin aging are under tight control of classical neuroendocrine axes. However, skin is also an endocrine organ itself, including the hair follicle, a fully functional neuroendocrine "miniorgan." Here we review pivotal hormones controlling human skin aging focusing on IGF-1, a key fibroblast-derived orchestrator of skin aging, of GH, estrogens, retinoids, and melatonin. The emerging roles of additional endocrine players, ie, α-melanocyte-stimulating hormone, a central player of the hypothalamic-pituitary-adrenal axis; members of the hypothalamic-pituitary-thyroid axis; oxytocin, endocannabinoids, and peroxisome proliferator-activated receptor modulators, are also reviewed. Until now, only a limited number of these hormones, mainly topical retinoids and estrogens, have found their way into clinical practice as anti-skin aging compounds. Further research into the biological properties of endocrine players or its derivatives may offer the development of novel senotherapeutics for the treatment and prevention of skin aging.

Skin function depends on a meticulously regulated dynamic interaction of distinct skin compartments such as the epidermis and dermis. Adaptive responses at the molecular and cellular level are essential for these interactions - and if dysregulated - drive skin aging and other pathologies. After defining the role of redox homeodynamics in physiology and aging pathology, we focus on the redox distress-dependent aging of dermal fibroblasts including their progenitors. We here discuss the prime role of senescent fibroblasts in the control of their own endogenous niche and stem cell niches for epidermal stem cells, hair follicle stem cells, adipocyte precursors and muscle stem cells. We here review that redox imbalance induced reduction in Insulin-like Growth Factor-1 drives skin aging by the depletion of stem cell pools. This IGF-1 reduction is mediated via the redox-sensitive transcription factor JunB and also by the redox-dependent changes in sphingolipid-metabolism, among others. In addition, we will discuss the changes in the extracellular matrix of the skin affecting cellular senescence and the skin integrity and function in aging. The aim is a deeper understanding of the two main redox-dependent hubs such as JunB-induced depletion of IGF-1, and the sphingolipid-mediated remodeling of the cell membrane with its impact on IGF-1, fibroblast heterogeneity, function, senescence and plasticity in skin aging.

In contrast to mammals, adult zebrafish achieve complete heart regeneration via proliferation of cardiomyocytes. Surprisingly, we found that regenerating cardiomyocytes experience DNA replication stress, which represents one reason for declining tissue regeneration during aging in mammals. Pharmacological inhibition of ATM and ATR kinases revealed that DNA damage response signaling is essential for zebrafish heart regeneration. Manipulation of Bone Morphogenetic Protein (BMP)-Smad signaling using transgenics and mutants showed that BMP signaling alleviates cardiomyocyte replication stress. BMP signaling also rescues neonatal mouse cardiomyocytes, human fibroblasts and human hematopoietic stem and progenitor cells (HSPCs) from replication stress. DNA fiber spreading assays indicate that BMP signaling facilitates re-start of replication forks after replication stress-induced stalling. Our results identify the ability to overcome replication stress as key factor for the elevated zebrafish heart regeneration capacity and reveal a conserved role for BMP signaling in promotion of stress-free DNA replication.

Though Traumatic Brain Injury (TBI) and skin trauma often occur together, it is unresolved whether TBI changes the healing of skin wounds. We here explored whether TBI impacts the sequence of events during skin wound healing. Incisional skin wounds from mice subjected to TBI were assessed employing unbiased transcriptome analysis and immunostaining. Transcriptome analysis at day 1 after combined trauma detects a significant enrichment of genes involved in macrophage and T cell recruitment and activation in contrast to skin wounds without TBI. At day 7 after combined trauma, genes in pathways of re-epithelialisation including cornification and keratinisation and of anti-inflammatory responses were highly enriched. These findings were confirmed by immunostaining with increased re-epithelialisation and cornification and an increased number of macrophages and T cells resolving inflammation. Moreover, the number of dermal myofibroblasts is highly increased in skin wounds after combined trauma. Collectively, TBI induces a robust defence response characterised by early onset of enhanced immunity, faster epidermal barrier formation, and myofibroblast-driven acceleration of wound closure, which may together help counteract systemic infection.

Cockayne syndrome (CS) is an autosomal recessive disorder of developmental delay, multiple organ system degeneration and signs of premature ageing. We show here, using the RNA-seq data from two CS mutant cell lines, that the CS key transcriptional signature displays significant enrichment of neurodegeneration terms, including genes relevant in Huntington disease (HD). By using deep learning approaches and two published RNA-Seq datasets, the CS transcriptional signature highly significantly classified and predicted HD and control samples. Neurodegeneration is one hallmark of CS disease, and fibroblasts from CS patients with different causative mutations display disturbed ribosomal biogenesis and a consecutive loss of protein homeostasis - proteostasis. Encouraged by the transcriptomic data, we asked whether this pathomechanism is also active in HD. In different HD cell-culture models, we showed that mutant Huntingtin impacts ribosomal biogenesis and function. This led to an error-prone protein synthesis and, as shown in different mouse models and human tissue, whole proteome instability, and a general loss of proteostasis.

One hallmark of aging is a decline in tissue regeneration, which can be caused by DNA replication stress. Whether highly regenerative species like zebrafish are immune from such hindrances to replication is unknown. In contrast to most mammals, adult zebrafish achieve complete heart regeneration via cell cycle entry and proliferation of mature cardiomyocytes. We found that cycling cardiomyocytes experience replication stress, which is induced by the demands of regeneration, but does not occur during physiological heart growth. Since zebrafish cardiomyocyte regeneration is remarkably efficient, heart regeneration appears to depend on elevated capabilities to overcome replication stress. Indeed, pharmacological inhibition of ATM and ATR kinases revealed that DNA damage response signaling is essential for heart regeneration. Using inducible overexpression of ligands and inhibitors of the Bone Morphogenetic Protein (BMP)-Smad pathway, combined with analysis of genetic mutants, we found that BMP signaling alleviates cardiomyocyte replication stress. In the absence of BMP signaling, cardiomyocytes become arrested in the S-phase of the cell cycle, which prevents progression to mitosis and results in heart regeneration failure. Interestingly, BMP signaling can also rescue neonatal mouse cardiomyocytes and human fibroblasts from hydroxyurea-induced replication stress. DNA fiber spreading assays in human cancer cells and human hematopoietic stem and progenitor cells (HSPCs) indicate that BMP signaling acts directly on replication dynamics by accelerating DNA replication fork progression and by facilitating their re-start after replication stress-induced stalling. Our results identify the ability to overcome replication stress as key factor for the elevated heart regeneration capacity in zebrafish. Notably, the conserved capability of BMP signaling to promote stress-free DNA replication might unlock new avenues towards anti-aging and pro-regenerative applications in humans.### Competing Interest StatementThe authors have declared no competing interest.

Background: Cellular senescence is the main cause of skin and organ aging and is associated with a wide range of aging-related diseases. Objectives: To understand which senolytics, senomorphics, and cell-based therapies have been developed to alleviate and even rejuvenate skin aging and reduce cellular senescence. Methods: Basic literature for the mode of action of senolytics and senomorphics and their clinical perspectives in daily routine are discussed. Results: Various causes lead to mitochondrial dysfunction and the activation of pro-aging signaling pathways, which eventually lead to cellular senescence with degradation of structural proteins of the dermal connective tissue and severe suppression of regenerative stem cell niches of the skin. Conclusions: Depletion of senescent cells suppress skin aging and enforce rejuvenation of skin and other organs and their function. The removal of senescent cells by cells of the native immune system is severely disturbed during aging. Selected senolytics and senomorphics are approved and are already on the market.

TFIIH is a complex essential for transcription of protein-coding genes by RNA polymerase II, DNA repair of UV-lesions and transcription of rRNA by RNA polymerase I. Mutations in TFIIH cause the cancer prone DNA-repair disorder xeroderma pigmentosum (XP) and the developmental and premature aging disorders trichothiodystrophy (TTD) and Cockayne syndrome. A total of 50% of the TTD cases are caused by TFIIH mutations. Using TFIIH mutant patient cells from TTD and XP subjects we can show that the stress-sensitivity of the proteome is reduced in TTD, but not in XP. Using three different methods to investigate the accuracy of protein synthesis by the ribosome, we demonstrate that translational fidelity of the ribosomes of TTD, but not XP cells, is decreased. The process of ribosomal synthesis and maturation is affected in TTD cells and can lead to instable ribosomes. Isolated ribosomes from TTD patients show an elevated error rate when challenged with oxidized mRNA, explaining the oxidative hypersensitivity of TTD cells. Treatment of TTD cells with N-acetyl cysteine normalized the increased translational error-rate and restored translational fidelity. Here we describe a pathomechanism that might be relevant for our understanding of impaired development and aging-associated neurodegeneration.

Zusammenfassung Hintergrund Zelluläre Seneszenz ist die Hauptursache für die Haut- und Organalterung mit Ausprägung zahlreicher altersassoziierter Erkrankungen. Fragestellung Welche innovativen therapeutischen Strategien zum Einsatz von Senolytika, Senomorphika und Zelltherapien gibt es, um die Organalterung und die Hautalterung zu vermindern und eine Rejuvenierung zu erzielen. Material und Methode Es werden eine Auswertung und Literaturübersicht zur Wirkweise von Senolytika und Senomorphika, eine Diskussion von Grundlagenarbeiten und klinische Perspektiven gegeben. Ergebnisse Verschiedene Ursachen führen über mitochondriale Dysfunktion und Aktivierung von Alterungssignalwegen zur zellulären Seneszenz mit einem Abbau des dermalen Bindegewebes und Unterdrückung der regenerativen Stammzellnischen. Schlussfolgerungen Depletion von seneszenten Zellen hemmen die Alterung und können zur Rejuvenierung der Haut, anderer Organe und deren Funktion führen. Die Eliminierung der seneszenten Zellen durch Zellen des Immunsystems ist im Alter gestört. Einzelne Senolytika und Senomorphika sind bereits zugelassen.

Depletion of senescent cells suppress skin aging and enforce rejuvenation of skin and other organs and their function. The removal of senescent cells by cells of the native immune system is severely disturbed during aging. Selected senolytics and senomorphics are approved and are already on the market.

The naked mole-rat (NMR) Heterocephalus glaber (from the Greek/latin words ἕτερος, heteros = divergent, κεφαλή, kephalē = head and glabra = hairless) was first described by Rüppell (Fig. 1) and belongs to the Hystricognath (from the Greek words ὕστριξ, hystrix = porcupine and γνάθος, gnathos = jaw) as a suborder of rodents. NMR are characterized by the highest longevity among rodents and reveal a profound cancer resistance. Details of its skin-specific protective and resistance mechanisms against aging and carcinogenesis have so far not been adequately characterized. Recently, our knowledge of NMR skin biology was complemented and expanded by published data using state-of-the art histological and molecular techniques. Here we review and integrate novel published data regarding skin morphology and histology of the aging NMR and the underlying mechanisms at the cellular and molecular level. We relate this data to the longevity of the NMR and its resistance to neoplastic transformation and discuss further open questions to understand its extraordinary longevity. In addition, we will address the exposome, defined as "the total of all non-genetic, endogenous and exogenous environmental influences" on the skin, respiratory tract, stomach, and intestine. Finally, we will discuss in perspective further intriguing possibilities arising from the interaction of skin with other organs.

Mutations in a broad variety of genes can provoke the severe childhood disorder trichothiodystrophy (TTD) that is classified as a DNA repair disease or a transcription syndrome of RNA polymerase II. In an attempt to identify the common underlying pathomechanism of TTD we performed a knockout/knockdown of the two unrelated TTD factors TTDN1 and RNF113A and investigated the consequences on ribosomal biogenesis and performance. Interestingly, interference with these TTD factors created a nearly uniform impact on RNA polymerase I transcription with downregulation of UBF, disturbed rRNA processing and reduction of the backbone of the small ribosomal subunit rRNA 18S. This was accompanied by a reduced quality of decoding in protein translation and the accumulation of misfolded and carbonylated proteins, indicating a loss of protein homeostasis (proteostasis). As the loss of proteostasis by the ribosome has been identified in the other forms of TTD, here we postulate that ribosomal dysfunction is a common underlying pathomechanism of TTD.