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Lipids in cell membranes and subcellular compartments play essential roles in numerous cellular processes, such as energy production, cell signaling and inflammation. A specific organelle lipidome is characterized by lipid synthesis and metabolism, intracellular trafficking, and lipid homeostasis in the organelle. Over the years, considerable effort has been directed to the identification of the lipid fingerprints of cellular organelles. However, these fingerprints are not fully characterized due to the large variety and structural complexity of lipids and the great variability in the abundance of different lipid species. The process becomes even more challenging when considering that the lipidome differs in health and disease contexts. This review summarizes the information available on the lipid composition of mammalian cell organelles, particularly the lipidome of the nucleus, mitochondrion, endoplasmic reticulum, Golgi apparatus, plasma membrane and organelles in the endocytic pathway. The lipid compositions of extracellular vesicles and lamellar bodies are also described. In addition, several examples of subcellular lipidome dynamics under physiological and pathological conditions are presented. Finally, challenges in mapping organelle lipidomes are discussed., (© 2023. The Author(s).)

Aquatic invertebrates are a major source of biomaterials and bioactive natural products that can find applications as pharmaceutics, nutraceutics, cosmetics, antibiotics, antifouling products and biomaterials. Symbiotic microorganisms are often the real producers of many secondary metabolites initially isolated from marine invertebrates; however, a certain number of them are actually synthesized by the macro-organisms. In this review, we analysed the literature of the years 2010-2019 on natural products (bioactive molecules and biomaterials) from the main phyla of marine invertebrates explored so far, including sponges, cnidarians, molluscs, echinoderms and ascidians, and present relevant examples of natural products of interest to public and private stakeholders. We also describe omics tools that have been more relevant in identifying and understanding mechanisms and processes underlying the biosynthesis of secondary metabolites in marine invertebrates. Since there is increasing attention on finding new solutions for a sustainable large-scale supply of bioactive compounds, we propose that a possible improvement in the biodiscovery pipeline might also come from the study and utilization of aquatic invertebrate stem cells.

Adult stem cells (ASCs) in vertebrates and model invertebrates (e.g. Drosophila melanogaster) are typically long-lived, lineage-restricted, clonogenic and quiescent cells with somatic descendants and tissue/organ-restricted activities. Such ASCs are mostly rare, morphologically undifferentiated, and undergo asymmetric cell division. Characterized by 'stemness' gene expression, they can regulate tissue/organ homeostasis, repair and regeneration. By contrast, analysis of other animal phyla shows that ASCs emerge at different life stages, present both differentiated and undifferentiated phenotypes, and may possess amoeboid movement. Usually pluri/totipotent, they may express germ-cell markers, but often lack germ-line sequestering, and typically do not reside in discrete niches. ASCs may constitute up to 40% of animal cells, and participate in a range of biological phenomena, from whole-body regeneration, dormancy, and agametic asexual reproduction, to indeterminate growth. They are considered legitimate units of selection. Conceptualizing this divergence, we present an alternative stemness metaphor to the Waddington landscape: the 'wobbling Penrose' landscape. Here, totipotent ASCs adopt ascending/descending courses of an 'Escherian stairwell', in a lifelong totipotency pathway. ASCs may also travel along lower stemness echelons to reach fully differentiated states. However, from any starting state, cells can change their stemness status, underscoring their dynamic cellular potencies. Thus, vertebrate ASCs may reflect just one metazoan ASC archetype., (© 2021 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.)

Scope: Inadequate intake of choline commonly leads to liver diseases. Methionine- and choline-deficient diets (MCDD) induce fatty liver in mice which is partly mediated by triglyceride (TG) lipolysis in white adipose tissues (WATs). Because Fatp4 knockdown has been shown to increase adipocyte lipolysis in vitro, here, the effects of MCDD on WAT lipolysis in aP2-Cre Fatp4-knockout (Fatp4 A-/- ) mice are determined., Methods and Results: Isolated WATs of Fatp4 A-/- mice exposed to MCD medium show an increase in lipolysis, and the strongest effect is noted on glycerol release from subcutaneous fat. Fatp4 A-/- mice fed with MCDD for 4 weeks show an increase in serum glycerol, TG, and leptin levels associated with the activation of hormone-sensitive lipase in subcutaneous fat. Chow-fed Fatp4 A-/- mice also show an increase in serum leptin and very-low-density lipoproteins as well as liver phosphatidylcholine and sphingomyelin levels. Both chow- and MCDD-fed Fatp4 A-/- mice show a decrease in serum ketone and WAT sphingomyelin levels which supports a metabolic shift to TG for subsequent WAT lipolysis CONCLUSIONS: Adipose Fatp4 deficiency leads to TG lipolysis and leptin release, which are exaggerated by MCDD. The data imply hyperlipidemia risk by a low dietary choline intake and gene mutations that increase adipose TG levels., (© 2020 The Authors. Molecular Nutrition & Food Research published by Wiley-VCH GmbH.)

Current studies addressing the influence of interleukin-33 or its receptor (IL-33R/ST2) on development of atopic dermatitis-like inflammation in mice have reported conflicting results. We compared the response in single- and double-deficient IL-33 -/- /ST2 -/- C57BL/6J BomTac mice in the well-established calcipotriol-induced model of atopic dermatitis. All genotypes (groups of up to 14 mice) developed atopic dermatitis-like inflammation yet we observed no biologically relevant difference between groups in gross anatomy or ear thickness. Moreover, histological examination of skin revealed no differences in mononuclear leukocyte and granulocyte infiltration nor Th2 cytokine levels (IL-4 and IL-13). Finally, skin CD45+ cells and CD3+ cells were found at similar densities across all groups. Our findings indicate that lack of interleukin-33 and its receptor ST2 does not prevent the development of AD-like skin inflammation.

The corneocyte lipid envelope (CLE), a monolayer of ω-hydroxyceramides whose function(s) remain(s) uncertain, is absent in patients with autosomal recessive congenital ichthyoses with mutations in enzymes that regulate epidermal lipid synthesis. Secreted lipids fail to transform into lamellar membranes in certain autosomal recessive congenital ichthyosis epidermis, suggesting the CLE provides a scaffold for the extracellular lamellae. However, because cornified envelopes are attenuated in these autosomal recessive congenital ichthyoses, the CLE may also provide a scaffold for subjacent cornified envelope formation, evidenced by restoration of cornified envelopes after CLE rescue. We provide multiple lines of evidence that the CLE originates as lamellar body-limiting membranes fuse with the plasma membrane: (i) ABCA12 patients and Abca12 -/- mice display normal CLEs; (ii) CLEs are normal in Netherton syndrome, despite destruction of secreted LB contents; (iii) CLEs are absent in VSP33B-negative patients; (iv) limiting membranes of lamellar bodies are defective in lipid-synthetic autosomal recessive congenital ichthyoses; and (v) lipoxygenases, lipase activity, and LIPN co-localize within putative lamellar bodies., (Copyright © 2018 The Authors. Published by Elsevier Inc. All rights reserved.)

Although inflammation has traditionally been considered a response to either exogenous pathogen-associated signals or endogenous signals of cell damage, other perturbations of homeostasis, generally referred to as stress, may also induce inflammation. The relationship between stress and inflammation is, however, not well defined. Here, we describe a mechanism of IL-33 induction driven by hypo-osmotic stress in human keratinocytes and also report interesting differences when comparing the responsiveness of other inflammatory mediators. The induction of IL-33 was completely dependent on EGFR and calcium signaling, and inhibition of calcium signaling not only abolished IL-33 induction but also dramatically changed the transcriptional pattern of other cytokines upon hypo-osmotic stress. IL-33 was not secreted but instead showed nuclear sequestration, conceivably acting as a failsafe mechanism whereby it is induced by potential danger but released only upon more extreme homeostatic perturbations that result in cell death. Finally, stress-induced IL-33 was also confirmed in an ex vivo human skin model, translating this mechanism to a potential tissue-relevant signal in the human epidermis. In conclusion, we describe hypo-osmotic stress as an inducer of IL-33 expression, linking cellular stress to nuclear accumulation of a strong proinflammatory cytokine., (Copyright © 2018 The Authors. Published by Elsevier Inc. All rights reserved.)

Antibody array analysis of labeled proteomes has high throughput and is simple to perform, but validation remains challenging. Here, we used differential detergent fractionation and size exclusion chromatography in sequence for high-resolution separation of biotinylated proteins from human primary keratinocytes and leukocytes. Ninety-six sample fractions from each cell type were analyzed with microsphere-based antibody arrays and flow cytometry (microsphere affinity proteomics; MAP). Monomeric proteins and multi-molecular complexes in the cytosol, cytoplasmic organelles, membranes and nuclei were resolved as discrete peaks of antibody reactivity across the fractions. The fractionation also provided a two-dimensional matrix for assessment of specificity. Thus, antibody reactivity peaks were considered to represent specific binding if the position in the matrix was in agreement with published information about i) subcellular location, ii) size of the intended target, and iii) cell type-dependent variation in protein expression. Similarities in the reactivity patterns of either different antibodies to the same protein or antibodies to similar proteins were used as additional supporting evidence. This approach provided validation of several hundred proteins and identification of monomeric proteins and protein complexes. High-resolution MAP solves many of the problems associated with obtaining specificity with immobilized antibodies and a protein label. Thus, laboratories with access to chromatography and flow cytometry can perform large-scale protein analysis on a daily basis. This opens new possibilities for cell biology research in dermatology and validation of antibodies., Competing Interests: The authors have declared that no competing interests exist.

Mutations in several lipid synthetic enzymes that block fatty acid and ceramide production produce autosomal recessive congenital ichthyoses (ARCIs) and associated abnormalities in permeability barrier homeostasis. However, the basis for the phenotype in patients with NIPAL4 (ichthyin) mutations (among the most prevalent ARCIs) remains unknown. Barrier function was abnormal in an index patient and in canines with homozygous NIPAL4 mutations, attributable to extensive membrane stripping, likely from detergent effects of nonesterified free fatty acid. Cytotoxicity compromised not only lamellar body secretion but also formation of the corneocyte lipid envelope (CLE) and attenuation of the cornified envelope (CE), consistent with a previously unrecognized, scaffold function of the CLE. Together, these abnormalities result in failure to form normal lamellar bilayers, accounting for the permeability barrier abnormality and clinical phenotype in NIPA-like domain-containing 4 (NIPAL4) deficiency. Thus, NIPAL4 deficiency represents another lipid synthetic ARCI that converges on the CLE (and CE), compromising their putative scaffold function. However, the clinical phenotype only partially improved after normalization of CLE and CE structure with topical ω-O-acylceramide because of ongoing accumulation of toxic metabolites, further evidence that proximal, cytotoxic metabolites contribute to disease pathogenesis., (Copyright © 2018 American Society for Investigative Pathology. Published by Elsevier Inc. All rights reserved.)

Contact sensitization is common and affects up to 20% of the general population. The clinical manifestation of contact sensitization is allergic contact dermatitis. This is a clinical expression that is sometimes difficult to distinguish from other types of dermatitis, for example irritant and atopic dermatitis. Several studies have examined the pathogenesis and severity of allergic contact dermatitis by measuring the absence or presence of various biomarkers. In this review, we provide a non-systematic overview of biomarkers that have been studied in allergic contact dermatitis. These include genetic variations and mutations, inflammatory mediators, alarmins, proteases, immunoproteomics, lipids, natural moisturizing factors, tight junctions, and antimicrobial peptides. We conclude that, despite the enormous amount of data, convincing specific biomarkers for allergic contact dermatitis are yet to be described., (© 2017 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

IL-33 is a novel IL-1 family member with a putative role in inflammatory skin disorders and a complex biology. Therefore, recent conflicting data regarding its function in experimental models justify a close assessment of its tissue expression and regulation. Indeed, we report here that there are strong species differences in the expression and regulation of epidermal IL-33. In murine epidermis, IL-33 behaved similar to an alarmin, being constitutively expressed in keratinocyte nuclei and rapidly lost during acute inflammation. By contrast, human and porcine IL-33 were weakly expressed or absent in keratinocytes of noninflamed skin but induced during acute inflammation. To this end, we observed that expression of IL-33 in human keratinocytes but not murine keratinocytes was strongly induced by IFN-γ, and this upregulation completely depended on the presence of EGFR ligands. Accordingly, IFN-γ increased the expression of IL-33 in the basal layers of the epidermis in human ex vivo skin cultures only, despite good evidence of IFN-γ activity in cultures from both species. Together these findings demonstrate that a full understanding of IL-33 function in clinical settings must take species-specific differences into account.

Fatty acids in the epidermis can be incorporated into complex lipids or exist in a free form, and they are crucial to proper functions of the epidermis and its appendages, such as sebaceous glands. Epidermal fatty acids can be synthesized de novo by keratinocytes or taken up from extracutaneous sources in a process that likely involves protein transporters. Several proteins that are expressed in the epidermis have been proposed to facilitate the uptake of long-chain fatty acids (LCFA) in mammalian cells, including fatty acid translocase/CD36, fatty acid binding protein, and fatty acid transport protein (FATP)/very long-chain acyl-CoA synthetase. In this review, we will discuss the mechanisms by which these candidate transporters facilitate the uptake of fatty acids. We will then discuss the clinical implications of defects in these transporters and relevant animal models, including the FATP4 animal models and ichthyosis prematurity syndrome, a congenital ichthyosis caused by FATP4 deficiency. This article is part of a Special Issue entitled The Important Role of Lipids in the Epidermis and their Role in the Formation and Maintenance of the Cutaneous Barrier. Guest Editors: Kenneth R. Feingold and Peter Elias., (© 2013.)

Serum response factor (SRF) is a transcription factor that regulates the expression of growth-related immediate-early, cytoskeletal, and muscle-specific genes to control growth, differentiation, and cytoskeletal integrity in different cell types. To investigate the role for SRF in epidermal development and homeostasis, we conditionally knocked out SRF in epidermal keratinocytes. We report that SRF deletion disrupted epidermal barrier function leading to early postnatal lethality. Mice lacking SRF in epidermis displayed morphogenetic defects, including an eye-open-at-birth phenotype and lack of whiskers. SRF-null skin exhibited abnormal morphology, hyperplasia, aberrant expression of differentiation markers and transcriptional regulators, anomalous actin organization, enhanced inflammation, and retarded hair follicle (HF) development. Transcriptional profiling experiments uncovered profound molecular changes in SRF-null E17.5 epidermis and revealed that many previously identified SRF target CArG box-containing genes were markedly upregulated in SRF-null epidermis, indicating that SRF may function to repress transcription of a subset of its target genes in epidermis. Remarkably, when transplanted onto nude mice, engrafted SRF-null skin lacked hair but displayed normal epidermal architecture with proper expression of differentiation markers, suggesting that although keratinocyte SRF is essential for HF development, a cross-talk between SRF-null keratinocytes and the surrounding microenvironment is likely responsible for the barrier-deficient mutant epidermal phenotype.

Skin epidermis is an active site of lipid synthesis. The intercellular lipids of human stratum corneum (SC) are unique in composition and quite different from the lipids found in most biological membranes. The three major lipids in the SC are free fatty acids, cholesterol and ceramides. Fatty acids can be synthesized by keratinocytes de novo and, in addition, need to be taken up from the circulation. The latter process has been shown to be protein mediated, and several fatty acid transporters are expressed in skin. Recent studies of transgenic and knockout animal models for fatty acid transporters and the identification of fatty acid transport protein 4 (FATP4 or SLC27A4) mutations as causative for Ichthyosis Prematurity Syndrome highlight the vital roles of fatty acid transport and metabolism in skin homeostasis. This review provides an overview of our current understanding of the role of fatty acids and their transporters in cutaneous biology, including their involvement in epidermal barrier generation and skin inflammation.

Members of the fibroblast growth factor family bind to one or more of the four closely related membrane-spanning FGF receptors. In addition to signaling through the receptors, exogenous FGF-1 and FGF-2 are endocytosed and translocated to the cytosol and nucleus where they stimulate RNA and DNA synthesis. Here we have studied the ability of the four FGF receptors to facilitate translocation of exogenous FGF-1 to the cytosol and nucleus. FGFR1 and FGFR4 were able to mediate translocation, whereas FGFR2 and FGFR3 completely lacked this ability. By analyzing mutant FGFRs we found that the tyrosine kinase domain could be deleted from FGFR1 without abolishing translocation, whereas the C-terminal tail of the FGFRs, constituted by approximately 50 amino acids downstream of the kinase domain, plays a crucial role in FGF-1 translocation. Three amino acids residues within the C-terminal tail were found to be of particular importance for translocation. For FGFR2, the two amino acid substitutions Q774M and P800H were sufficient to enable the receptor to support FGF-1 translocation. The results demonstrate a striking diversity in function of the four FGFRs determined by their C-terminal domain.

The activating mutation FGFR3-R248C in the D2-D3 linker region of fibroblast growth factor receptor 3 leads as germline mutation to the neonatal lethal syndrome thanatophoric dysplasia type I (TD1). As somatic mutation it has been found in cancer. We introduced into the murine FGFR3 the mutation R242C that is orthologoues to the human mutation R248C. A strong reduction in binding of the 16 and 18 kDa forms of FGF1 to the mutant receptor was found, highlighting the importance of D2-D3 linker region of FGFR3 in determination of binding affinity to ligands. Another mutant, G374R, introduced into the murine FGFR3, is orthologoues to the human mutant FGFR3-G380R, and leads to achondroplasia (ACH). The binding of the 16 kDa and 18 kDa forms of FGF1 to this mutant receptor was the same as for wild-type FGFR3 in a cell-free system, but it was reduced in living cells. The data indicate a minor changes in conformation of FGFR3-G374R receptors at the cell surface that lead to reduced binding to FGF1.

The expression of fibroblast growth factors (FGF1 and FGF2) and their receptors has been reported in a variety of human neoplasms, including haematological neoplasia. Fibroblast growth factors can promote tumour growth directly, or indirectly through promoting the growth of vessels. In the present study, we evaluated the expression of FGF1 and FGF2 as well as FGF receptors 1-4 (FGFR1-FGFR4) in 39 cases of Hodgkin's lymphoma, including all subtypes, as well as Hodgkin's lymphoma cell lines. FGF1 and FGF2 and their receptors FGFR2-FGFR4, but not FGFR1, were found to be expressed by the malignant cells in the majority of cases. Interestingly, only FGFR3, but none of the FGFs or the other FGFRs, was expressed by the Hodgkin's lymphoma cell lines. This indicates that only FGFR3 is constitutively expressed by Hodgkin's lymphoma cells, whereas FGFs and the other FGFRs are induced in vivo. Culture of the Hodgkin's cell lines under conditions of hypoxic stress could induce vascular endothelial growth factor (VEGF) but not FGF expression. FGFs, in contrast to VEGF, might be expressed in response to paracrine stimuli. In situ hybridization did not reveal FGFR3 gene amplification or translocation as the cause of constitutive FGFR3 expression, as has been shown in a subset of multiple myeloma. FGFR3 might be expressed as part of the Hodgkin's cell phenotype. The demonstration of widespread expression of FGFs and some of their receptors in Hodgkin's lymphoma suggests that FGFs are important for sustaining growth of the lymphoma and suggests that anti-FGF antibodies could be used as an adjuvant treatment.

The expression of several deletion mutants of fibroblast growth factor receptor 4 (FGFR4) was studied in COS-1 cells. FGFR4-mutants lacking most of the extracellular region did not efficiently reach the plasma membrane but accumulated in the endoplasmic reticulum (ER) and Golgi body. A mutant FGFR4 lacking the kinase domain as well as most of the extracellular region (DeltaExt/R4Tth) had a distinct intracellular distribution. It localized in part to the nucleus, where it exhibited a striking spotted pattern. Ultrastructural studies showed that the nuclear spots consisted of several layers of membrane that were folded into onion-like structures at the nucleoplasmic side of the nuclear envelope. These intranuclear structures did not contain nuclear pores but were positive for the ER proteins calreticulin and protein disulfide isomerase, in addition to abundant DeltaExt/R4Tth. Formation of the intranuclear structures was sensitive to inhibition of protein kinase C. Live microscopy of a green-fluorescent-protein/DeltaExt/R4Tth fusion protein showed that the intranuclear structures were stable and immobile, suggesting that they function as deposits of the overexpressed mutant and associated membrane. The DeltaExt/R4Tth protein also induced formation of densely packed membrane stacks in the cytosol and we suggest a model were the intranuclear structures are formed by invagination of ER-derived membrane stacks into the nucleus.

Endocytic uptake and intracellular transport of acidic fibroblast growth factor (aFGF) was studied in cells transfected with FGF receptor 4 with mutations in the cytoplasmic part. Endocytic uptake in HeLa cells was reduced but not abolished when the tyrosine kinase of the receptor was inactivated by mutations or deletions. The tyrosine kinase-dependent endocytosis of aFGF was prevented by the expression of a dominant negative dynamin mutant that blocks endocytosis from coated pits and caveolae. However, more than half of the total endocytic uptake of aFGF was not affected under these conditions, indicating an endocytic uptake mechanism not involving coated pits or caveolae. Mutation or deletion of a putative caveolin-binding sequence did not prevent the localization of part of the receptors to a low density, caveolin-containing subcellular fraction. Whereas wild-type receptor transfers the growth factor from early endosomes to the recycling compartment, kinase negative, full length receptors were inefficient in this respect and the growth factor instead accumulated in lysosomes. By contrast, when most of the intracellular part of the receptor, including the kinase domain, was removed, aFGF was transported to the recycling compartment, as in cells that express wild-type receptors, suggesting the presence of a kinase-regulated targeting signal in the cytoplasmic tail.

The ability of COS cells to bind and internalise acidic fibroblast growth factor (aFGF) was studied after transient transfection of the cells with wild-type and mutated fibroblast growth factor receptor 4. In one case the tyrosine kinase of the receptor was inactivated by a point mutation in the active site, whereas in other cases parts of the receptor were deleted to remove various parts of the cytoplasmic domain. In all cases the receptors were expressed at the cell surface at a high level and the cells bound labelled growth factor efficiently and internalised it by endocytosis. Translocation of externally added aFGF across cellular membranes to reach the cytosol and nucleus was measured as transport of labelled growth factor to the nuclear fraction obtained by centrifugation, by farnesylation of growth factor modified to carry a CAAX motif, and by phosphorylation of the growth factor at a site specific for protein kinase C. Whereas both full-length receptors (with and without an active kinase domain) facilitated translocation of the growth factor to the cytosol and nucleus, as assessed by these methods, the mutants of the receptor where the C terminus was deleted, were unable to do so. In contrast, a receptor containing only the 57 most C-terminal amino acids of the cytoplasmic domain in addition to the juxtamembrane, transmembrane and extracellular domains, was in fact able to mediate translocation of aFGF to the cytosol. These data indicate that information contained in the C terminus of the receptor is required for translocation.