Cell proliferation in normal epidermis

Ephrin-Eph signaling: an important regulator of epithelial integrity and barrier function

Eph receptor-interacting proteins (ephrin) ligands and their erythropoietin-producing human hepatocellular (Eph) receptors elicit bidirectional signals that regulate cell migration, angiogenesis, neuronal plasticity, and other developmental processes in the embryo. In adulthood, ephrin-Eph signaling regulates numerous homeostatic events, including epithelial cell proliferation and differentiation. Epithelial surfaces, including those of skin and vagina, are lined by layers of stratified squamous epithelium (SSE) that protect against mechanical stress and microbial pathogen invasion. Ephrin-Eph signaling is known to promote cutaneous epithelial barrier function by regulating the expression of specialized cell-cell adhesion junctions termed desmosomes, but the role of this signaling system in maintaining epithelial integrity and barrier function in the vagina is less explored. This review summarizes current understanding of ephrin-Eph signaling that regulates desmosome expression and barrier function in the skin and considers evidence that suggests ephrin-Eph signaling similarly regulates these processes in vaginal SSE.

The purpose and ubiquity of turnover

Turnover-constant component production and destruction-is ubiquitous in biology. Turnover occurs across organisms and scales, including for RNAs, proteins, membranes, macromolecular structures, organelles, cells, hair, feathers, nails, antlers, and teeth. For many systems, turnover might seem wasteful when degraded components are often fully functional. Some components turn over with shockingly high rates and others do not turn over at all, further making this process enigmatic. However, turnover can address fundamental problems by yielding powerful properties, including regeneration, rapid repair onset, clearance of unpredictable damage and errors, maintenance of low constitutive levels of disrepair, prevention of stable hazards, and transitions. I argue that trade-offs between turnover benefits and metabolic costs, combined with constraints on turnover, determine its presence and rates across distinct contexts. I suggest that the limits of turnover help explain aging and that turnover properties and the basis for its levels underlie this fundamental component of life.

Antiproliferative and Anti-Inflammatory Effects of the Polyphenols Phloretin and Balsacone C in a Coculture of T Cells and Psoriatic Keratinocytes

Plaque psoriasis is a chronic inflammatory skin disease causing red inflamed lesions covered by scales. Leukocytes, including dendritic cells and T cells, participate in the inflammation of the skin by producing multiple cytokines, thus contributing to the hyperproliferation of keratinocytes. Lack of effectiveness and toxic side effects are the main concerns with conventional treatments, and research involving new antipsoriatic molecules is essential. In this study, the anti-inflammatory and antiproliferative effects of two natural polyphenols, phloretin and balsacone C, were investigated using the coculture of T cells and psoriatic keratinocytes. Phloretin exerted antiproliferative activity by regulating the expression of antigen Ki67 and proliferating cell nuclear antigen (PCNA). These effects were comparable to those of methotrexate, a reference treatment for moderate to severe psoriasis. With balsacone C, the expression of Ki67 was also reduced. Additionally, phloretin decreased the levels of multiple pro-inflammatory cytokines: monocyte chemoattractant protein-1 (MCP-1/CCL2), macrophage inflammatory protein-1α (MIP-1α), granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-1 alpha (IL-1α), interleukin-1 beta (IL-1β), interleukin-6 (IL-6), interleukin-17A (IL-17A), and tumor necrosis factor alpha (TNF-α). The increased interleukin-2 (IL-2) levels with phloretin and methotrexate also represented anti-inflammatory activity. Balsacone C and methotrexate decreased the levels of IL-1α and IL-1β, but methotrexate exerted a higher reduction. In summary, the anti-inflammatory effects of phloretin were more pronounced than those of methotrexate and balsacone C. In addition, the expression of lymphocyte common antigen (CD45) was more similar to that of the healthy condition after using phloretin or methotrexate. Finally, phloretin stood out from the other compounds and appears promising for psoriasis treatment.

Dynamics and Epigenetics of the Epidermal Differentiation Complex

Epidermis is the outer skin layer built of specialized cells called keratinocytes. Keratinocytes undergo a unique differentiation process, also known as cornification, during which their gene expression pattern, morphology and other properties change remarkably to the effect that the terminally differentiated, cornified cells can form a physical barrier, which separates the underlying tissues from the environment. Many genes encoding proteins that are important for epidermal barrier formation are located in a gene cluster called epidermal differentiation complex (EDC). Recent data provided valuable information on the dynamics of the EDC locus and the network of interactions between EDC gene promoters, enhancers and other regions, during keratinocytes differentiation. These data, together with results concerning changes in epigenetic modifications, provide a valuable insight into the mode of regulation of EDC gene expression.

"Small Hepatocytes" in the Liver

Mature hepatocytes (MHs) in an adult rodent liver are categorized into the following three subpopulations based on their proliferative capability: type I cells (MH-I), which are committed progenitor cells that possess a high growth capability and basal hepatocytic functions; type II cells (MH-II), which possess a limited proliferative capability; and type III cells (MH-III), which lose the ability to divide (replicative senescence) and reach the final differentiated state. These subpopulations may explain the liver's development and growth after birth. Generally, small-sized hepatocytes emerge in mammal livers. The cells are characterized by being morphologically identical to hepatocytes except for their size, which is substantially smaller than that of ordinary MHs. We initially discovered small hepatocytes (SHs) in the primary culture of rat hepatocytes. We believe that SHs are derived from MH-I and play a role as hepatocytic progenitors to supply MHs. The population of MH-I (SHs) is distributed in the whole lobules, a part of which possesses a self-renewal capability, and decreases with age. Conversely, injured livers of experimental models and clinical cases showed the emergence of SHs. Studies demonstrate the involvement of SHs in liver regeneration. SHs that appeared in the injured livers are not a pure population but a mixture of two distinct origins, MH-derived and hepatic-stem-cell-derived cells. The predominant cell-derived SHs depend on the proliferative capability of the remaining MHs after the injury. This review will focus on the SHs that appeared in the liver and discuss the significance of SHs in liver regeneration.

Methods for Inferring Cell Cycle Parameters Using Thymidine Analogues

Tritiated thymidine autoradiography, 5-bromo-2'-deoxyuridine (BrdU) 5-chloro-2'-deoxyuridine (CldU), 5-iodo-2'-deoxyuridine (IdU), and 5-ethynyl-2'-deoxyiridine (EdU) labeling have been used for identifying the fraction of cells undergoing the S-phase of the cell cycle and to follow the fate of these cells during the embryonic, perinatal, and adult life in several species of vertebrate. In this current review, I will discuss the dosage and times of exposition to the aforementioned thymidine analogues to label most of the cells undergoing the S-phase of the cell cycle. I will also show how to infer, in an asynchronous cell population, the duration of the G₁, S, and G₂ phases, as well as the growth fraction and the span of the whole cell cycle on the base of some labeling schemes involving a single administration, continuous nucleotide analogue delivery, and double labeling with two thymidine analogues. In this context, the choice of the optimal dose of BrdU, CldU, IdU, and EdU to label S-phase cells is a pivotal aspect to produce neither cytotoxic effects nor alter cell cycle progression. I hope that the information presented in this review can be of use as a reference for researchers involved in the genesis of tissues and organs.

Optimized High-Content Imaging Screening Quantifying Micronuclei Formation in Polymer-Treated HaCaT Keratinocytes

Research on nano- and micro-plastic particles (NMPPs) suggests their potential threat to human health. Some studies have even suggested genotoxic effects of NMPP exposure, such as micronuclei (MN) formation, while others found the opposite. To clarify the ability of NMPP to induce MN formation, we used non-malignant HaCaT keratinocytes and exposed these to a variety of polystyrene (PS) and poly methyl methacrylate (PMMA) particle types at different concentrations and three different sizes. Investigations were performed following acute (one day) and chronic exposure (five weeks) against cytotoxic (amino-modified NMPPs) and genotoxic (methyl methanesulfonate, MMS) positive controls. An optimized high-content imaging workflow was established strictly according to OECD guidelines for analysis. Algorithm-based object segmentation and MN identification led to computer-driven, unsupervised quantitative image analysis results on MN frequencies among the different conditions and thousands of cells per condition. This could only be realized using accutase, allowing for partial cell detachment for optimal identification of bi-nucleated cells. Cytotoxic amino-modified particles were not genotoxic; MMS was both. During acute and long-term studies, PS and PMMA particles were neither toxic nor increased MN formation, except for 1000 nm PS particles at the highest concentration of unphysiological 100 µg/mL. Interestingly, ROS formation was significantly decreased in this condition. Hence, most non-charged polymer particles were neither toxic nor genotoxic, while aminated particles were toxic but not genotoxic. Altogether, we present an optimized quantitative imaging workflow applied to a timely research question in environmental toxicity.

Individualised computational modelling of immune mediated disease onset, flare and clearance in psoriasis

Despite increased understanding about psoriasis pathophysiology, currently there is a lack of predictive computational models. We developed a personalisable ordinary differential equations model of human epidermis and psoriasis that incorporates immune cells and cytokine stimuli to regulate the transition between two stable steady states of clinically healthy (non-lesional) and disease (lesional psoriasis, plaque) skin. In line with experimental data, an immune stimulus initiated transition from healthy skin to psoriasis and apoptosis of immune and epidermal cells induced by UVB phototherapy returned the epidermis back to the healthy state. Notably, our model was able to distinguish disease flares. The flexibility of our model permitted the development of a patient-specific “UVB sensitivity” parameter that reflected subject-specific sensitivity to apoptosis and enabled simulation of individual patients’ clinical response trajectory. In a prospective clinical study of 94 patients, serial individual UVB doses and clinical response (Psoriasis Area Severity Index) values collected over the first three weeks of UVB therapy informed estimation of the “UVB sensitivity” parameter and the prediction of individual patient outcome at the end of phototherapy. An important advance of our model is its potential for direct clinical application through early assessment of response to UVB therapy, and for individualised optimisation of phototherapy regimes to improve clinical outcome. Additionally by incorporating the complex interaction of immune cells and epidermal keratinocytes, our model provides a basis to study and predict outcomes to biologic therapies in psoriasis.

We present a new computer model for psoriasis, an immune-mediated disabling skin disease which presents with red, raised scaly plaques that can appear over the whole body. Psoriasis affects millions of people in the UK alone and causes significant impairment to quality of life, and currently has no cure. Only a few treatments (including UVB phototherapy) can induce temporary remission. Despite our increased understanding about psoriasis, treatments are still given on a ‘trial and error’ basis and there are no reliable computer models that can a) elucidate the mechanisms behind psoriasis onset or flare and b) predict a patient’s response to a course of treatment (e.g., phototherapy) and the likelihood of inducing a period of remission. Our computer model addresses both these needs. First, it explicitly describes the interaction between the immune system and skin cells. Second, our model captures response to therapy at the individual patient level and enables personalised prediction of clinical outcomes. Notably, our model also supports prediction of amending individual UVB phototherapy regimes based on the patient’s initial response that include for example personalised delivery schedules (i.e., 3x weekly vs. 5x weekly phototherapy). Therefore, our work is a crucial step towards precision medicine for psoriasis treatment.

Nephrin expression in human epidermal keratinocytes and its implication in poor wound closure

Nephrin is a type-1 transmembrane protein and a component of the slit diaphragm renal-filtration barrier. It has several functions in actin remodeling and cell-cell adhesion. Nephrin is principally located in the kidney glomerulus, but several studies have reported that nephrin is found in the pancreas, brain, and placenta. However, nephrin expression and its role in human skin have not yet been reported. First, using single-cell RNA sequencing, immunohistochemistry, and immuno-electron microscopy, nephrin expression was confirmed in human-skin epidermal keratinocytes. Nephrin expression colocalized with the expression of zonula occludens-1 in keratinocytes and was closely related to keratinocyte cell density, proliferation, and migration. High glucose treatment decreased nephrin expression and compromised keratinocyte cell migration without yes-associated protein nuclear entry. This reduced cell migration under high glucose conditions was improved in nephrin-overexpressing keratinocytes. Nephrin was highly expressed on the margins of re-epithelized epidermis based on in vivo mice and ex vivo human skin wound models. The results demonstrate that nephrin is expressed in human-skin keratinocytes and functions in cell adhesion, proliferation, and migration. In conclusion, this study suggests that nephrin may have a variety of physiological roles in human skin.

Molecular and cellular regulation of psoriatic inflammation

This review highlights the molecular and cellular mechanisms underlying psoriatic inflammation with an emphasis on recent developments which may impact on treatment approaches for this chronic disease. We consider both the skin and the musculoskeletal compartment and how different manifestations of psoriatic inflammation are linked. This review brings a focus to the importance of inflammatory feedback loops that exist in the initiation and chronic stages of the condition, and how close interaction between the epidermis and both innate and adaptive immune compartments drives psoriatic inflammation. Furthermore, we highlight work done on biomarkers to predict the outcome of therapy as well as the transition from psoriasis to psoriatic arthritis.

Updated Perspectives on Keratinocytes and Psoriasis: Keratinocytes are More Than Innocent Bystanders

Psoriasis is a complex disease triggered by genetic, immunologic, and environmental stimuli. Many genes have been linked to psoriasis, like the psoriasis susceptibility genes, some of which are critical in keratinocyte biology and epidermal barrier function. Still, the exact pathogenesis of psoriasis is unknown. In the disease, the balance between the proliferative and differentiative processes of keratinocytes becomes altered. Multiple studies have highlighted the role of dysregulated immune cells in provoking the inflammatory responses seen in psoriasis. In addition to immune cells, accumulating evidence shows that keratinocytes are involved in psoriasis pathogenesis, as discussed in this review. Although certain immune cell-derived factors stimulate keratinocyte hyperproliferation, activated keratinocytes can also produce anti-microbial peptides, cytokines, and chemokines that can promote their proliferation, as well as recruit immune cells to help initiate and reinforce inflammatory feedback loops. Psoriatic keratinocytes also show intrinsic differences from normal keratinocytes even after removal from the in vivo inflammatory environment; thus, psoriatic keratinocytes have been found to exhibit abnormal calcium metabolism and possible epigenetic changes that contribute to psoriasis. The Koebner phenomenon, in which injury promotes the development of psoriatic lesions, also provides evidence for keratinocytes' contributions to disease pathogenesis. Furthermore, transgenic mouse studies have confirmed the importance of keratinocytes in the etiology of psoriasis. Finally, in addition to immune cells and keratinocytes, data in the literature support roles for other cell types, tissues, and systems in psoriasis development. These other contributors are all potential targets for therapies, suggesting the importance of a holistic approach when treating psoriasis.

Comparative Efficacy and Safety of Tirbanibulin for Actinic Keratosis of the Face and Scalp in Europe: A Systematic Review and Network Meta-Analysis of Randomized Controlled Trials

Actinic keratosis (AK) is a chronic skin condition that may progress to cutaneous squamous cell carcinoma. We conducted a systematic review of efficacy and safety for key treatments for AK of the face and scalp, including the novel 5-day tirbanibulin 1% ointment. MEDLINE, PubMed, Embase, Cochrane Library, clinical trial registries and regulatory body websites were searched. The review included 46 studies, of which 35 studies included interventions commonly used in Europe and were sufficiently homogenous to inform a Bayesian network meta-analysis of complete clearance against topical placebo or vehicle. The network meta-analysis revealed the following odds ratios and 95% credible intervals: cryosurgery 13.4 (6.2-30.3); diclofenac 3% 2.9 (1.9-4.3); fluorouracil 0.5% + salicylic acid 7.6 (4.6-13.5); fluorouracil 4% 30.3 (9.1-144.7); fluorouracil 5% 35.0 (10.2-164.4); imiquimod 3.75% 8.5 (3.5-22.4); imiquimod 5% 17.9 (9.1-36.6); ingenol mebutate 0.015% 12.5 (8.1-19.9); photodynamic therapy with aminolevulinic acid 24.1 (10.9-52.8); photodynamic therapy with methyl aminolevulinate 11.7 (6.0-21.9); tirbanibulin 1% 11.1 (6.2-20.9). Four sensitivity analyses, from studies assessing efficacy after one treatment cycle only, for ≤25 cm2 treatment area, after 8 weeks post-treatment, and with single placebo/vehicle node confirmed the findings from the base case. Safety outcomes were assessed qualitatively. These results suggest that tirbanibulin 1% offers a novel treatment for AK, with a single short treatment period, favourable safety profile and efficacy, in line with existing topical treatments available in Europe.

Identification of Keratinocyte Mitogens: Implications for Hyperproliferation in Psoriasis and Atopic Dermatitis

Psoriasis and atopic dermatitis are chronic inflammatory skin diseases characterized by keratinocyte (KC) hyperproliferation and epidermal acanthosis (hyperplasia). The milieu of disease-associated cytokines and soluble factors is considered a mitogenic factor; however, pinpointing the exact mitogens in this complex microenvironment is challenging. We employed organotypic human epidermal equivalents, faithfully mimicking native epidermal proliferation and stratification, to evaluate the proliferative effects of a broad panel of (literature-based) potential mitogens. The KC GF molecule, the T-helper 2 cytokines IL-4 and IL-13, and the psoriasis-associated cytokine IL-17A caused acanthosis by hyperplasia through a doubling in the number of proliferating KCs. In contrast, IFN-γ lowered proliferation, whereas IL-6, IL-20, IL-22, and oncostatin M induced acanthosis not by hyperproliferation but by hypertrophy. The T-helper 2‒cytokine‒mediated hyperproliferation was Jak/signal transducer and activator of transcription 3 dependent, whereas IL-17A and KC GF induced MAPK/extracellular signal‒regulated kinase kinase/extracellular signal‒regulated kinase‒dependent proliferation. This discovery that key regulators in atopic dermatitis and psoriasis are direct KC mitogens not only adds evidence to their crucial role in the pathophysiological processes but also highlights an additional therapeutic pillar for the mode of action of targeting biologicals (e.g., dupilumab) or small-molecule drugs (e.g., tofacitinib) by the normalization of KC turnover within the epidermal compartment.

Maintaining the proliferative cell niche in multicellular models of epithelia

The maintenance of the proliferative cell niche is critical to epithelial tissue morphology and function. In this paper we investigate how current modelling methods can result in the erroneous loss of proliferative cells from the proliferative cell niche. Using an established model of the inter-follicular epidermis we find there is a limit to the proliferative cell densities that can be maintained in the basal layer (the niche) if we do not include additional mechanisms to stop the loss of proliferative cells from the niche. We suggest a new methodology that enables maintenance of a desired homeostatic population of proliferative cells in the niche: a rotational force is applied to the two daughter cells during the mitotic phase of division to enforce a particular division direction. We demonstrate that this new methodology achieves this goal. This methodology reflects the regulation of the orientation of cell division.

Experiments in the EpiDerm 3D Skin In Vitro Model and Minipigs In Vivo Indicate Comparatively Lower In Vivo Skin Sensitivity of Topically Applied Aneugenic Compounds

Risk management of in vitro aneugens for topically applied compounds is not clearly defined because there is no validated methodology to accurately measure compound concentration in proliferating stratum basale keratinocytes of the skin. Here, we experimentally tested several known aneugens in the EpiDerm reconstructed human skin in vitro micronucleus assay and compared the results to flow cytometric mechanistic biomarkers (phospho-H3; MPM2, DNA content). We then evaluated similar biomarkers (Ki-67, nuclear area) using immunohistochemistry in skin sections of minipigs following topical exposure the potent aneugens, colchicine, and hesperadin. Data from the EpiDerm model showed positive micronucleus responses for all aneugens tested following topical or direct media dosing with similar sensitivity when adjusted for applied dose. Quantitative benchmark dose-response analysis exhibited increases in the mitotic index biomarkers phospho-H3 and MPM2 for tubulin binders and polyploidy for aurora kinase inhibitors are at least as sensitive as the micronucleus endpoint. By comparison, the aneugens tested did not induce histopathological changes, increases in Ki-67 immunolabeling or nuclear area in skin sections from the in vivo minipig study at doses in significant excess of those eliciting a response in vitro. Results indicate the EpiDerm in vitro micronucleus assay is suitable for the hazard identification of aneugens. The lack of response in the minipig studies indicates that the barrier function of the minipig skin, which is comparable to human skin, protects from the effects of aneugens in vivo. These results provide a basis for conducting additional studies in the future to further refine this understanding.

Blue Light Irradiation Induces Human Keratinocyte Cell Damage via Transient Receptor Potential Vanilloid 1 (TRPV1) Regulation

Although blue light has been reported to affect skin cells negatively, little is known about its action mechanisms in skin cells. Therefore, we investigated the role of the transient receptor potential vanilloid 1 (TRPV1) in blue light-induced effects on human keratinocytes and its underlying mechanisms. Blue light decreased cell proliferation and upregulated TRPV1 expression. Blue light also suppressed the epidermal growth factor receptor- (EGFR-) mediated signaling pathway by reducing the protein levels of EGFR and suppressing the EGFR/PI3K/AKT/GSK3β/FoxO3a pathway. The blue light-induced effect in cell proliferation was reversed by TRPV1 siRNA, but not capsazepine, a TRPV1-specific antagonist. In addition, blue light irradiation increased the production of reactive oxygen species (ROS) and tumor necrosis factor-α (TNF-α). Blue light irradiation also increased both phosphorylation levels of TRPV1 and calcium influx. The blue light-induced increase in production of ROS and TNF-α was reversed by capsazepine. Furthermore, the blue light-induced increase in production of TNF-α was attenuated by SP600125 or PDTC. These findings show that blue light regulates cell survival and production of ROS and TNF-α; its effects are mediated via TRPV1. Specifically, the effects of blue light on cell proliferation are mediated by upregulating TRPV1, a negative regulator of EGFR-FoxO3a signaling. Blue light-induced production of ROS and TNF-α is also mediated through increased calcium influx via TRPV1 activation.

Association between genetically predicted telomere length and facial skin aging in the UK Biobank: a Mendelian randomization study

Are shorter telomeres causal risk factors for facial aging on a large population level? To examine if longer, genetically predicted telomeres were causally associated with less facial aging using Mendelian randomization analysis. Two-sample Mendelian randomization methods were applied to the summary statistics of a genome-wide association study (GWAS) for self-reported facial aging from 417, 772 participants of the UK Biobank data. Twenty single-nucleotide polymorphisms (SNPs) that were of genome-wide significance were selected as instrumental variables for leukocyte telomere length. The main analyses were performed primarily using the random-effects inverse-variance weighted method and were complemented with the MR-Egger regression, weighted median, and weighted mode approaches. The intercept of MR-Egger regression was used to assess horizontal pleiotropy. Longer genetically predicted telomeres were associated with a lower likelihood of facial aging (β = − 0.02, 95% confidence interval: − 0.04, − 0.002). Comparable results were obtained using MR-Egger regression, weighted median, and weighted mode approaches. The intercept of MR-Egger regression was close to zero (0.002) that was not suggestive of horizontal pleiotropy. Our findings provided evidence to support a potential causal relationship between longer genetically predicted telomeres and less facial aging.

Vulnerability of the skin barrier to mechanical rubbing

Skin barrier function is the battlefront for preventing permeation of harmful substances and infectious diseases. However, it can be destroyed by mechanical forces, as shown in many studies. Excess rubbing may increase the permeability of the skin to aqueous material. Although the skin barrier plays an important physiological role in humans, the vulnerability of skin to mechanical rubbing is poorly understood. Therefore, we investigated the effects of rubbing on the skin in vitro; skin damage was quantified by laser-induced fluorescence. Microscopic observation showed that keratinocytes in the stratum corneum sustained traumatic damage, which reduced the barrier function in that region. The permeability of the skin to an aqueous solution increased with rubbing frequency and load, and rubbing markedly reduced the barrier function of the stratum corneum. To understand the mechanisms underlying the skin damage, we developed a simple mathematical model assuming that the skin is a viscoelastic material. We hypothesized that the increased skin permeability was caused by the damage sustained by keratinocytes in the stratum corneum, and that the permeability was proportional to the time-averaged strain. Our theoretical results showed quantitative agreement with the experimental results and illustrated that rubbing and strain relaxation play key roles in rubbing-induced permeation.

Long-term Effects of 222-nm ultraviolet radiation C Sterilizing Lamps on Mice Susceptible to Ultraviolet Radiation

Germicidal lamps that emit primarily 254 nm ultraviolet radiation (UV) are routinely utilized for surface sterilization but cannot be used for human skin because they cause genotoxicity. As an alternative, 222‐nm UVC has been reported to exert sterilizing ability comparable to that of 254‐nm UVC without producing cyclobutane pyrimidine dimers (CPDs), the major DNA lesions caused by UV. However, there has been no clear evidence for safety in chronic exposure to skin, particularly with respect to carcinogenesis. We therefore investigated the long‐term effects of 222‐nm UVC on skin using a highly photocarcinogenic phenotype mice that lack xeroderma pigmentosum complementation group A (Xpa‐) gene, which is involved in repairing of CPDs. CPDs formation was recognized only uppermost layer of epidermis even with high dose of 222‐nm UVC exposure. No tumors were observed in Xpa‐knockout mice and wild‐type mice by repetitive irradiation with 222‐nm UVC, using a protocol which had shown to produce tumor in Xpa‐knockout mice irradiated with broad‐band UVB. Furthermore, erythema and ear swelling were not observed in both genotype mice following 222‐nm UVC exposure. Our data suggest that 222‐nm UVC lamps can be safely used for sterilizing human skin as far as the perspective of skin cancer development.

The Mite-Gallery Unit: A New Concept for Describing Scabies through Entodermoscopy

Scabies has always represented a diagnostic challenge for dermatologists, especially in subclinical cases or in atypical ones due to the coexistence of other diseases. Fortunately, dermatoscopy has enabled easier and faster in situ diagnosis. The aim of this study is to examine old and new dermatoscopic signs that Sarcoptes scabiei produces on the skin during its whole life cycle through entodermoscopy (dermatoscopy with an entomological focus) which, unlike traditional optical microscope examination, allows the local micro-environment to be preserved intact. Patients were enrolled during outbreaks of scabies from hospitals or nursing homes for the elderly in Bari (Italy). The study was performed applying both immersion and polarized dry dermatoscopy. The systematic use of dermatoscopy highlighted the morphological complexity of the Sarcoptes tunnel that had been described previously as a simple unitary structure. On the contrary, it is possible to distinguish three separate segments of the burrow that introduce a new anatomo-functional concept called the Mite-Gallery Unit (MGU). This approach, based on the mite life cycle and local skin turnover (the latter usually being ignored), allows the dermatologist to recognize not only Sarcoptes using the gallery, but also new descriptors including tunnels without Sarcoptes, those with acari alone, and those with associated signs of inflammation. The diagnosis of scabies using optical microscopy until recently has always involved demonstrating the mite and its products outside the human body (on a glass slide) without taking into account exactly what happens within the epidermis. Entodermoscopy is a term used to encapsulate both the presence of the parasite, the usual target of microscopy, and the changes produced in the superficial layers of the epidermis in situ. Thus, the scabies tunnel or burrow can be shown to be composed of three parts, the Head, Body, and Tail, in which different events affecting both mite and host develop. The Mite-Gallery Unit provides a new anatomical and functional explanation of scabies because it provides a more comprehensive in vivo and in situ dermatoscopic diagnosis. In this respect, dermatoscopy takes into account the behavior of the mite in addition to its interaction with its habitat, the human skin.

Postmitotic cell longevity-associated genes: a transcriptional signature of postmitotic maintenance in neural tissues

Different cell types have different postmitotic maintenance requirements. Nerve cells, however, are unique in this respect as they need to survive and preserve their functional complexity for the entire lifetime of the organism, and failure at any level of their supporting mechanisms leads to a wide range of neurodegenerative conditions. Whether these differences across tissues arise from the activation of distinct cell type-specific maintenance mechanisms or the differential activation of a common molecular repertoire is not known. To identify the transcriptional signature of postmitotic cellular longevity (PMCL), we compared whole-genome transcriptome data from human tissues ranging in longevity from 120 days to over 70 years and found a set of 81 genes whose expression levels are closely associated with increased cell longevity. Using expression data from 10 independent sources, we found that these genes are more highly coexpressed in longer-living tissues and are enriched in specific biological processes and transcription factor targets compared with randomly selected gene samples. Crucially, we found that PMCL-associated genes are downregulated in the cerebral cortex and substantia nigra of patients with Alzheimer's and Parkinson's disease, respectively, as well as Hutchinson-Gilford progeria-derived fibroblasts, and that this downregulation is specifically linked to their underlying association with cellular longevity. Moreover, we found that sexually dimorphic brain expression of PMCL-associated genes reflects sexual differences in lifespan in humans and macaques. Taken together, our results suggest that PMCL-associated genes are part of a generalized machinery of postmitotic maintenance and functional stability in both neural and non-neural cells and support the notion of a common molecular repertoire differentially engaged in different cell types with different survival requirements.

3D bioprinting of skin tissue: From pre-processing to final product evaluation

Bioprinted skin tissue has the potential for aiding drug screening, formulation development, clinical transplantation, chemical and cosmetic testing, as well as basic research. Limitations of conventional skin tissue engineering approaches have driven the development of biomimetic skin equivalent via 3D bioprinting. A key hope for bioprinting skin is the improved tissue authenticity over conventional skin equivalent construction, enabling the precise localization of multiple cell types and appendages within a construct. The printing of skin faces challenges broadly associated with general 3D bioprinting, including the selection of cell types and biomaterials, and additionally requires in vitro culture formats that allow for growth at an air-liquid interface. This paper provides a thorough review of current 3D bioprinting technologies used to engineer human skin constructs and presents the overall pipelines of designing a biomimetic artificial skin via 3D bioprinting from the design phase (i.e. pre-processing phase) through the tissue maturation phase (i.e. post-processing) and into final product evaluation for drug screening, development, and drug delivery applications.

Engineered Epidermal Progenitor Cells Can Correct Diet-Induced Obesity and Diabetes

Somatic gene therapy is a promising approach for treating otherwise terminal or debilitating diseases. The human skin is a promising conduit for genetic engineering, as it is the largest and most accessible organ, epidermal autografts and tissue-engineered skin equivalents have been successfully deployed in clinical applications, and skin epidermal stem/progenitor cells for generating such grafts are easy to obtain and expand in vitro. Here, we develop skin grafts from mouse and human epidermal progenitors that were engineered by CRISPR-mediated genome editing to controllably release GLP-1 (glucagon-like peptide 1), a critical incretin that regulates blood glucose homeostasis. GLP-1 induction from engineered mouse cells grafted onto immunocompetent hosts increased insulin secretion and reversed high-fat-diet-induced weight gain and insulin resistance. Taken together, these results highlight the clinical potential of developing long-lasting, safe, and versatile gene therapy approaches based on engineering epidermal progenitor cells.

Structuring of Hydrogels across Multiple Length Scales for Biomedical Applications

The development of new materials for clinical use is limited by an onerous regulatory framework, which means that taking a completely new material into the clinic can make translation economically unfeasible. One way to get around this issue is to structure materials that are already approved by the regulator, such that they exhibit very distinct physical properties and can be used in a broader range of clinical applications. Here, the focus is on the structuring of soft materials at multiple length scales by modifying processing conditions. By applying shear to newly forming materials, it is possible to trigger molecular reorganization of polymer chains, such that they aggregate to form particles and ribbon-like structures. These structures then weakly interact at zero shear forming a solid-like material. The resulting self-healing network is of particular use for a range of different biomedical applications. How these materials are used to allow the delivery of therapeutic entities (cells and proteins) and as a support for additive layer manufacturing of larger-scale tissue constructs is discussed. This technology enables the development of a range of novel materials and structures for tissue augmentation and regeneration.

Delivery systems of current biologicals for the treatment of chronic cutaneous wounds and severe burns

While wound therapy remains a clinical challenge in current medical practice, much effort has focused on developing biological therapeutic approaches. This paper presents a comprehensive review of delivery systems for current biologicals for the treatment of chronic wounds and severe burns. The biologicals discussed here include proteins such as growth factors and gene modifying molecules, which may be delivered to wounds free, encapsulated, or released from living systems (cells, skin grafts or skin equivalents) or biomaterials. Advances in biomaterial science and technologies have enabled the synthesis of delivery systems such as scaffolds, hydrogels and nanoparticles, designed to not only allow spatially and temporally controlled release of biologicals, but to also emulate the natural extracellular matrix microenvironment. These technologies represent an attractive field for regenerative wound therapy, by offering more personalised and effective treatments.

Advances in keratinocyte delivery in burn wound care

This review gives an updated overview on keratinocyte transplantation in burn wounds concentrating on application methods and future therapeutic cell delivery options with a special interest in hydrogels and spray devices for cell delivery. To achieve faster re-epithelialisation of burn wounds, the original autologous keratinocyte culture and transplantation technique was introduced over 3 decades ago. Application types of keratinocytes transplantation have improved from cell sheets to single-cell solutions delivered with a spray system. However, further enhancement of cell culture, cell viability and function in vivo, cell carrier and cell delivery systems remain themes of interest. Hydrogels such as chitosan, alginate, fibrin and collagen are frequently used in burn wound care and have advantageous characteristics as cell carriers. Future approaches of keratinocyte transplantation involve spray devices, but optimisation of application technique and carrier type is necessary.

Immunohistochemical distribution of Ki67 in epidermis of thick glabrous skin of human digits

The glabrous skin on the flexor sides of hands and feet, compared to other integument regions, has thicker epidermis and more complex pattern of epidermal ridges, wherefore in microscopy is denominated as thick skin. The epidermis of this skin type has individually unique and permanent superficial patterns, called dermatoglyphics, which are maintained by regenerative potential of deep epidermal rete ridges, that interdigitate with adjacent dermis. Using light microscopy, we analyzed cadaveric big toes thick skin samples, described histology of deep epidermal ridges (intermediate, limiting, and transverse), and quantitatively evidenced their pattern of proliferation by immunohistochemical assessment of Ki67. Immunohistochemical distribution of Ki67 was confined to basal and suprabasal layers, with pattern of distribution specific for intermediate, limiting and transverse ridges that gradually transform within epidermal height. Deep epidermal ridges, interdigitating with dermal papillae, participate in construction of intricate epidermal base, whose possible role in epidermal regeneration was also discussed. Having a prominent morphology, this type of epidermis offers the best morphological insight in complexities of skin organization, and its understanding could challenge and improve currently accepted models of epidermal organization.

Differential expression of cyclin D1, Ki‑67, pRb, and p53 in psoriatic skin lesions and normal skin

Psoriasis is a hyperproliferative inflammatory skin disease; therefore, it is highly likely that psoriatic skin lesions may transform into malignancies. However, malignant transformation is not common. We performed immunohistochemical studies using anti‑cyclin D1, anti‑cyclin E, anti‑pRb, anti‑p53, anti‑p16INK4a, and anti‑Ki‑67 antibodies in normal skin, psoriatic epidermal tissue, and squamous cell carcinoma (SCC) tissue. Furthermore, western blot analysis and immunohistochemical staining were performed to ascertain differences in cyclin D1, cyclin E, pRb, and Ki‑67 expression before and after treatment for psoriasis. Cyclin D1 expression was higher in chronic psoriatic lesions than that in normal epidermis. Psoriasis lesions showed a strong intensity of positive nuclear staining for cyclin D1 among several normally stained nuclei in the basal layer. Cyclin E expression in psoriasis was stronger in the granular and spinous layer than in the normal epidermis. Expression levels of pRb and p53 were found to be higher in the psoriasis group compared with the normal epidermis. Total basal layer cell counts for p53WT expression were found to be significantly higher in the psoriasis group compared with the normal group. However, p16 expression was very weak in the normal and psoriasis groups compared with that in the SCC group. Ki‑67 immunoreactivity was significantly higher in psoriasis compared with normal epidermis and was similar with that in the SCC group. According to immunohistochemistry and immunoblot analysis, the expression levels of cyclin D1, cyclin E, pRb, and Ki‑67 in psoriasis lesions decreased after treatment and were similar with those in the normal group. Thus, increased expression of cyclin D1 and cyclin E may be involved in cell cycle progression in psoriatic epidermis, and pRb and p53 may play important roles in the prevention of malignant transformation under the hyperproliferative state in psoriasis.

Hepatic Progenitor Cells: An Update

Liver regeneration is a fascinating and complex process with many medical implications. An important component of this regenerative process is the hepatic progenitor cell (HPC). These appealing cells are able to participate in the renewal of hepatocytes and cholangiocytes when the normal homeostatic regeneration is exhausted. Moreover, the HPC niche is of vital importance toward the activation, differentiation, and proliferation of the HPC. This niche provides a rich microenvironment for the regulation of the HPC, thanks to the intercellular secretion of molecules. New findings indicate that the regenerative possibilities in the liver could provide a diverse basis for therapeutic targets.

The Endothelial Protein C Receptor Is a Potential Stem Cell Marker for Epidermal Keratinocytes

Endothelial protein C receptor (EPCR) is a specific receptor for anticoagulant protein C and expressed by human epidermis and cultured keratinocytes. Here we investigated whether: (a) the level of EPCR in keratinocytes is associated with their growth potential; and (b) EPCR is a potential marker for human epidermal stem cells. Human keratinocytes isolated from foreskins or adult skin tissues were transfected with EPCR siRNA or EPCR overexpressing plasmids. Cell proliferation, long term proliferation potential, colony forming efficiency (CFE), and in vitro epidermal regeneration ability of EPCR^high and EPCR^l °^w cells were assessed. The expression and colocalization of EPCR with stem cell markers p63, integrin β1, and activation of MAP kinases were detected by flow cytometry, immunofluorescence staining, or Western blot. Results showed that EPCR was highly expressed by the basal layer of skin epidermis. EPCR^high cells were associated with the highest levels of p63 and integrin β1. Most EPCR^high cells were smaller in size, formed larger colonies and had a greater long term growth potential, CFE, holoclone formation, and in vitro epidermal regeneration ability when compared to EPCR^l °^w cells. Blocking EPCR resulted in keratinocyte apoptosis, particularly in nondifferentiated conditions. Cell proliferation and p63 expression were reduced by blocking EPCR and enhanced by overexpressing this receptor. These data indicate that EPCR can regulate p63, is associated with highly proliferative keratinocytes, and is a potential human epidermal stem cell marker. Stem Cells 2017;35:1786-1798.

Stem cells in psoriasis

Psoriasis is a complex chronic relapsing inflammatory disease. Although the exact mechanism remains unknown, it is commonly accepted that the development of psoriasis is a result of multi-system interactions among the epidermis, dermis, blood vessels, immune system, neuroendocrine system, metabolic system, and hematopoietic system. Many cell types have been confirmed to participate in the pathogenesis of psoriasis. Here, we review the stem cell abnormalities related to psoriasis that have been investigated recently.

Recent advances in understanding psoriasis

T helper (Th) cells producing interleukin (IL)-17, IL-22, and tumor necrosis factor (TNF) form the key T cell population driving psoriasis pathogenesis. They orchestrate the inflammation in the skin that results in the proliferation of keratinocytes and endothelial cells. Besides Th17 cells, other immune cells that are capable of producing IL-17-associated cytokines participate in psoriatic inflammation. Recent advances in psoriasis research improved our understanding of the cellular and molecular players that are involved in Th17 pathology and inflammatory pathways in the skin. The inflammation-driving actions of TNF in psoriasis are already well known and antibodies against TNF are successful in the treatment of Th17-mediated psoriatic skin inflammation. A further key cytokine with potent IL-17-/IL-22-promoting properties is IL-23. Therapeutics directly neutralizing IL-23 or IL-17 itself are now extending the therapeutic spectrum of antipsoriatic agents and further developments are on the way. The enormous progress in psoriasis research allows us to control this Th17-mediated inflammatory skin disease in many patients.

Biomolecular bases of the senescence process and cancer. A new approach to oncological treatment linked to ageing

Human ageing is associated with a gradual decline in the physiological functions of the body at multiple levels and it is a key risk factor for many diseases, including cancer. Ageing process is intimately related to widespread cellular senescence, characterised by an irreversible loss of proliferative capacity and altered functioning associated with telomere attrition, accumulation of DNA damage and compromised mitochondrial and metabolic function. Tumour and senescent cells may be generated in response to the same stimuli, where either cellular senescence or transformation would constitute two opposite outcomes of the same degenerative process. This paper aims to review the state of knowledge on the biomolecular relationship between cellular senescence, ageing and cancer. Importantly, many of the cell signalling pathways that are found to be altered during both cellular senescence and tumourigenesis are regulated through shared epigenetic mechanisms and, therefore, they are potentially reversible. MicroRNAs are emerging as pivotal players linking ageing and cancer. These small RNA molecules have generated great interest from the point of view of future clinical therapy for cancer because successful experimental results have been obtained in animal models. Micro-RNA therapies for cancer are already being tested in clinical phase trials. These findings have potential importance in cancer treatment in aged people although further research-based knowledge is needed to convert them into an effective molecular therapies for cancer linked to ageing.

Modelling epidermis homoeostasis and psoriasis pathogenesis

We present a computational model to study the spatio-temporal dynamics of epidermis homoeostasis under normal and pathological conditions. The model consists of a population kinetics model of the central transition pathway of keratinocyte proliferation, differentiation and loss and an agent-based model that propagates cell movements and generates the stratified epidermis. The model recapitulates observed homoeostatic cell density distribution, the epidermal turnover time and the multilayered tissue structure. We extend the model to study the onset, recurrence and phototherapy-induced remission of psoriasis. The model considers psoriasis as a parallel homoeostasis of normal and psoriatic keratinocytes originated from a shared stem cell (SC) niche environment and predicts two homoeostatic modes of psoriasis: a disease mode and a quiescent mode. Interconversion between the two modes can be controlled by interactions between psoriatic SCs and the immune system and by normal and psoriatic SCs competing for growth niches. The prediction of a quiescent state potentially explains the efficacy of multi-episode UVB irradiation therapy and recurrence of psoriasis plaques, which can further guide designs of therapeutics that specifically target the immune system and/or the keratinocytes.

Novel Functions of the Anticoagulant Activated Protein C in Maintaining Skin Barrier Integrity to Impact on Skin Disease

The epidermis is the outermost skin layer and provides the first line of defence against the external environment. Keratinocytes are the most predominant cells in the epidermis and play a critical role in maintaining epidermal barrier function. When the barrier is disrupted any of a number of diseases, such as chronic wounds, psoriasis, pemphigus, atopic dermatitis or toxic epidermal necrolysis, can take hold. Activated protein C (APC) or its precursor, protein C, is abundantly expressed by skin epidermal keratinocytes and stimulates their proliferation and migration, and inhibits apoptosis and inflammation, leading to a healing phenotype. Importantly, APC also increases the barrier function of keratinocytes by promoting expression and cell-cell contact redistribution of tight junction proteins. These cytoprotective properties of APC on epidermal keratinocytes place it as an exciting new therapy for skin disorders associated with the disruption of barrier function and inflammation.

Prodifferentiation, anti-inflammatory and antiproliferative effects of delphinidin, a dietary anthocyanidin, in a full-thickness three-dimensional reconstituted human skin model of psoriasis

BACKGROUND

Psoriasis is a chronic inflammatory disorder of skin and joints for which conventional treatments that are effective in clearing the moderate-to-severe disease are limited due to long-term safety issues. This necessitates exploring the usefulness of botanical agents for treating psoriasis. We previously showed that delphinidin, a diet-derived anthocyanidin endowed with antioxidant and anti-inflammatory properties, induces normal epidermal keratinocyte differentiation and suggested its possible usefulness for the treatment of psoriasis [1].

OBJECTIVES

To investigate the effect of delphinidin (0-20 μM; 2-5 days) on psoriatic epidermal keratinocyte differentiation, proliferation and inflammation using a three-dimensional reconstructed human psoriatic skin equivalent (PSE) model.

METHODS

PSEs and normal skin equivalents (NSEs) established on fibroblast-contracted collagen gels with respective psoriatic and normal keratinocytes and treated with/without delphinidin were analyzed for histology, expression of markers of differentiation, proliferation and inflammation using histomorphometry, immunoblotting, immunochemistry, qPCR and cultured supernatants for cytokine with a Multi-Analyte ELISArray Kit.

RESULTS

Our data show that treatment of PSE with delphinidin induced (1) cornification without affecting apoptosis and (2) the mRNA and protein expression of markers of differentiation (caspase-14, filaggrin, loricrin, involucrin). It also decreased the expression of markers of proliferation (Ki67 and proliferating cell nuclear antigen) and inflammation (inducible nitric oxide synthase and antimicrobial peptides S100A7-psoriasin and S100A15-koebnerisin, which are often induced in psoriatic skin). ELISArray showed increased release of psoriasis-associated keratinocyte-derived proinflammatory cytokines in supernatants of the PSE cultures, and this increase was significantly suppressed by delphinidin.

CONCLUSIONS

These observations provide a rationale for developing delphinidin for the management of psoriasis.

Skin as a living coloring book: how epithelial cells create patterns of pigmentation

The pigmentation of mammalian skin and hair develops through the interaction of two basic cell types - pigment donors and recipients. The pigment donors are melanocytes, which produce and distribute melanin through specialized structures. The pigment recipients are epithelial cells, which acquire melanin and put it to use, collectively yielding the pigmentation visible to the eye. This review will focus on the pigment recipients, the historically less understood cell type. These end-users of pigment are now known to exert a specialized control over the patterning of pigmentation, as they identify themselves as melanocyte targets, recruit pigment donors, and stimulate the transfer of melanin. As such, this review will discuss the evidence that the skin is like a coloring book: the pigment recipients create a 'picture,' a blueprint for pigmentation, which is colorless initially but outlines where pigment should be placed. Melanocytes then melanize the recipients and 'color in' the picture.

Cycling up the epidermis: reconciling 100 years of debate

There is likely general consensus within the skin research community that cell cycle control is critical to epidermal homeostasis and disease. The current predominant model proposes that keratinocytes switch off DNA replication and undergo cell cycle and cell growth arrest as they initiate terminal differentiation. However, this model cannot explain key physiological features of the skin, mainly why squamous differentiation prevails over proliferation in benign hyperproliferative disorders. In recent years, we have proposed an alternative model that involves mitotic slippage and endoreplication. This new model is controversial and has encountered resistance within the field. However, looking back at history, the epidermal cell cycle has been a matter of controversy and debate for around 100 years now. The accumulated data are confusing and contradictory. Our present model can explain and reconcile both old and new paradoxical observations. Here, we explain and discuss the endoreplicative cell cycle, the evidence for and against its existence in human epidermis and the important implications for skin homeostasis and disease. We show that regardless of the strengths or weaknesses of the Endoreplication Model, the existing evidence in support of the Cell Cycle Arrest Model is very weak.

Epidermal dynamics and patterns of intraepidermal melanoma

BACKGROUND

Dermatopathologists know that the epidermis represents a dynamic compartment and that its cells mature from the basal layer to the skin surface in approximately 45 days. What may seem intuitive - but not obvious - is that the dynamics of the epidermis can affect the patterns of melanoma cells within the epidermis. Here this conjecture is explored with an abstract, theoretical model.

METHODS

To control the independent effects of epidermal replacement velocity and thickness as well as rate of melanoma cell penetration of the epidermis, an abstraction of the epidermis was created and simulated via computer.

RESULTS

Simulated plots of the epidermis show that the number and pattern of melanoma cells in the epidermis is affected by the probability of melanoma cells entering the epidermis, by the velocity of epidermal replacement and by epidermal thickness.

CONCLUSION

This analysis suggests that the dynamics of the epidermis are sufficient to affect the patterns of melanoma cells within the epidermis.

Circadian rhythm connections to oxidative stress: implications for human health

SIGNIFICANCE

Oxygen and circadian rhythmicity are essential in a myriad of physiological processes to maintain homeostasis, from blood pressure and sleep/wake cycles, down to cellular signaling pathways that play critical roles in health and disease. If the human body or cells experience significant stress, their ability to regulate internal systems, including redox levels and circadian rhythms, may become impaired. At cellular as well as organismal levels, impairment in redox regulation and circadian rhythms may lead to a number of adverse effects, including the manifestation of a variety of diseases such as heart diseases, neurodegenerative conditions, and cancer.

RECENT ADVANCES

Researchers have come to an understanding as to the basics of the circadian rhythm mechanism, as well as the importance of the numerous species of oxidative stress components. The effects of oxidative stress and dysregulated circadian rhythms have been a subject of intense investigations since they were first discovered, and recent investigations into the molecular mechanisms linking the two have started to elucidate the bases of their connection.

CRITICAL ISSUES

While much is known about the mechanics and importance of oxidative stress systems and circadian rhythms, the front where they interact has had very little research focused on it. This review discusses the idea that these two systems are together intricately involved in the healthy body, as well as in disease.

FUTURE DIRECTIONS

We believe that for a more efficacious management of diseases that have both circadian rhythm and oxidative stress components in their pathogenesis, targeting both systems in tandem would be far more successful.

The synaptic maintenance problem: membrane recycling, Ca2+ homeostasis and late onset degeneration

Most neurons are born with the potential to live for the entire lifespan of the organism. In addition, neurons are highly polarized cells with often long axons, extensively branched dendritic trees and many synaptic contacts. Longevity together with morphological complexity results in a formidable challenge to maintain synapses healthy and functional. This challenge is often evoked to explain adult-onset degeneration in numerous neurodegenerative disorders that result from otherwise divergent causes. However, comparably little is known about the basic cell biological mechanisms that keep normal synapses alive and functional in the first place. How the basic maintenance mechanisms are related to slow adult-onset degeneration in different diseasesis largely unclear. In this review we focus on two basic and interconnected cell biological mechanisms that are required for synaptic maintenance: endomembrane recycling and calcium (Ca²⁺) homeostasis. We propose that subtle defects in these homeostatic processes can lead to late onset synaptic degeneration. Moreover, the same basic mechanisms are hijacked, impaired or overstimulated in numerous neurodegenerative disorders. Understanding the pathogenesis of these disorders requires an understanding of both the initial cause of the disease and the on-going changes in basic maintenance mechanisms. Here we discuss the mechanisms that keep synapses functional over long periods of time with the emphasis on their role in slow adult-onset neurodegeneration.