Phenotypic overlap between atopic dermatitis and autism

Background

Autism, a childhood behavioral disorder, belongs to a large suite of diseases, collectively referred to as autism spectrum disorders (ASD). Though multifactorial in etiology, approximately 10% of ASD are associated with atopic dermatitis (AD). Moreover, ASD prevalence increases further as AD severity worsens, though these disorders share no common causative mutations. We assessed here the link between these two disorders in the standard, valproic acid mouse model of ASD. In prior studies, there was no evidence of skin involvement, but we hypothesized that cutaneous involvement could be detected in experiments conducted in BALB/c mice. BALB/c is an albino, laboratory-bred strain of the house mouse and is among the most widely used inbred strains used in animal experimentation.

Methods

We performed our studies in valproic acid (VPA)-treated BALB/c hairless mice, a standard mouse model of ASD. Mid-trimester pregnant mice received a single intraperitoneal injection of either valproic acid sodium salt dissolved in saline or saline alone on embryonic day 12.5 and were housed individually until postnatal day 21. Only the brain and epidermis appeared to be affected, while other tissues remain unchanged. At various postnatal time points, brain, skin and blood samples were obtained for histology and for quantitation of tissue sphingolipid content and cytokine levels.

Results

AD-like changes in ceramide content occurred by day one postpartum in both VPA-treated mouse skin and brain. The temporal co-emergence of AD and ASD, and the AD phenotype-dependent increase in ASD prevalence correlated with early appearance of cytokine markers (i.e., interleukin [IL]-4, 5, and 13), as well as mast cells in skin and brain. The high levels of interferon (IFN)γ not only in skin, but also in brain likely account for a significant decline in esterified very-long-chain N-acyl fatty acids in brain ceramides, again mimicking known IFNγ-induced changes in AD.

Conclusion

Baseline involvement of both AD and ASD could reflect concurrent neuro- and epidermal toxicity, possibly because both epidermis and neural tissues originate from the embryonic neuroectoderm. These studies illuminate the shared susceptibility of the brain and epidermis to a known neurotoxin, suggesting that the atopic diathesis could be extended to include ASD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12868-021-00645-0.

Exosomes from Human Adipose Tissue-Derived Mesenchymal Stem Cells Promote Epidermal Barrier Repair by Inducing de Novo Synthesis of Ceramides in Atopic Dermatitis

Atopic dermatitis (AD) is a multifactorial, heterogeneous disease associated with epidermal barrier disruption and intense systemic inflammation. Previously, we showed that exosomes derived from human adipose tissue-derived mesenchymal stem cells (ASC-exosomes) attenuate AD-like symptoms by reducing multiple inflammatory cytokine levels. Here, we investigated ASC-exosomes' effects on skin barrier restoration by analyzing protein and lipid contents. We found that subcutaneous injection of ASC-exosomes in an oxazolone-induced dermatitis model remarkably reduced trans-epidermal water loss, while enhancing stratum corneum (SC) hydration and markedly decreasing the levels of inflammatory cytokines such as IL-4, IL-5, IL-13, TNF-α, IFN-γ, IL-17, and TSLP, all in a dose-dependent manner. Interestingly, ASC-exosomes induced the production of ceramides and dihydroceramides. Electron microscopic analysis revealed enhanced epidermal lamellar bodies and formation of lamellar layer at the interface of the SC and stratum granulosum with ASC-exosomes treatment. Deep RNA sequencing analysis of skin lesions demonstrated that ASC-exosomes restores the expression of genes involved in skin barrier, lipid metabolism, cell cycle, and inflammatory response in the diseased area. Collectively, our results suggest that ASC-exosomes effectively restore epidermal barrier functions in AD by facilitating the de novo synthesis of ceramides, resulting in a promising cell-free therapeutic option for treating AD.

Dominant de novo DSP mutations cause erythrokeratodermia-cardiomyopathy syndrome

Disorders of keratinization (DOK) show marked genotypic and phenotypic heterogeneity. In most cases, disease is primarily cutaneous, and further clinical evaluation is therefore rarely pursued. We have identified subjects with a novel DOK featuring erythrokeratodermia and initially-asymptomatic, progressive, potentially fatal cardiomyopathy, a finding not previously associated with erythrokeratodermia. We show that de novo missense mutations clustered tightly within a single spectrin repeat of DSP cause this novel cardio-cutaneous disorder, which we term erythrokeratodermia-cardiomyopathy (EKC) syndrome. We demonstrate that DSP mutations in our EKC syndrome subjects affect localization of desmosomal proteins and connexin 43 in the skin, and result in desmosome aggregation, widening of intercellular spaces, and lipid secretory defects. DSP encodes desmoplakin, a primary component of desmosomes, intercellular adhesion junctions most abundant in the epidermis and heart. Though mutations in DSP are known to cause other disorders, our cohort features the unique clinical finding of severe whole-body erythrokeratodermia, with distinct effects on localization of desmosomal proteins and connexin 43. These findings add a severe, previously undescribed syndrome featuring erythrokeratodermia and cardiomyopathy to the spectrum of disease caused by mutation in DSP, and identify a specific region of the protein critical to the pathobiology of EKC syndrome and to DSP function in the heart and skin.

Ablation of the calcium-sensing receptor in keratinocytes impairs epidermal differentiation and barrier function

The calcium-sensing receptor (CaR) plays an essential role in mediating Ca²⁺-induced keratinocyte differentiation in vitro. In this study, we generated keratinocyte-specific CaR knockout (^EpidCaR-/-) mice to investigate the function of the CaR in epidermal development in vivo. ^EpidCaR-/- mice exhibited a delay in permeability barrier formation during embryonic development. Ion capture cytochemistry detected the loss of the epidermal Ca²⁺ gradient in the ^EpidCaR-/- mice. The expression of terminal differentiation markers and key enzymes mediating epidermal sphingolipid transport and processing in the ^EpidCaR-/- epidermis was significantly reduced. The ^EpidCaR-/- epidermis displayed a marked decrease in the number of lamellar bodies and lamellar body secretion, thinner lipid-bound cornified envelopes and a defective permeability barrier. Consistent with in vivo results, epidermal keratinocytes cultured from ^EpidCaR-/- mice demonstrated abnormal Ca²⁺_I handling and diminished differentiation. The impairment in epidermal differentiation and permeability barrier in ^EpidCaR-/- mice maintained on a low calcium (0.02%) diet is more profound and persistent with age then in ^EpidCaR-/- mice maintained on a normal calcium (1.3%) diet.

Deleting CaR perturbs the epidermal Ca²⁺ gradient and impairs keratinocyte differentiation and permeability barrier homeostasis, indicating a key role for the CaR in normal epidermal development.

Ichthyosis in Sjögren-Larsson syndrome reflects defective barrier function due to abnormal lamellar body structure and secretion

Sjögren–Larsson syndrome is a genetic disease characterized by ichthyosis, mental retardation, spasticity and mutations in the ALDH3A2 gene coding for fatty aldehyde dehydrogenase, an enzyme necessary for oxidation of fatty aldehydes and fatty alcohols. We investigated the cutaneous abnormalities in 9 patients with Sjögren–Larsson syndrome to better understand how the enzymatic deficiency results in epidermal dysfunction. Histochemical staining for aldehyde oxidizing activity was profoundly reduced in the epidermis. Colloidal lanthanum perfusion studies showed abnormal movement of tracer into the extracellular spaces of the stratum corneum consistent with a leaky water barrier. The barrier defect could be attributed to the presence of abnormal lamellar bodies, many with disrupted limiting membranes or lacking lamellar contents. Entombed lamellar bodies were present in the cytoplasm of corneocytes suggesting blockade of lamellar body secretion. At the stratum granulosum–stratum corneum interface, non-lamellar material displaced or replaced secreted lamellar membranes, and in the stratum corneum, the number of lamellar bilayers declined and lamellar membrane organization was disrupted by foci of lamellar/non-lamellar phase separation. These studies demonstrate the presence of a permeability barrier abnormality in Sjögren–Larsson syndrome, which localizes to the stratum corneum interstices and can be attributed to abnormalities in lamellar body formation and secretion.

Stratum Corneum Acidification Is Impaired in Moderately Aged Human and Murine Skin

Aged skin commonly is afflicted by inflammatory skin diseases or xerosis/eczema that could be triggered or exacerbated by impaired epidermal permeability barrier homeostasis. This defect is linked to reduced epidermal lipid synthesis in humans and in mice of advanced age (i.e., >75 years in human or >18-24 months in mice). We now report that barrier defects in moderately aged humans (50-80 years) or analogously aged mice (12-15 months) are linked instead to defective stratum corneum (SC) acidity. In moderately aged mouse epidermis, we find that abnormal acidification, in turn, is linked to decreased Na+/H+ antiporter (NHE1) expression. Decreased NHE1 levels lead to increased SC pH, which results in defective lipid processing and delayed maturation of lamellar membranes, due to suboptimal activation of the pH-sensitive essential, lipid-processing enzyme, beta-glucocerebrosidase. Conversely, impaired SC integrity in moderately aged mice is due to increased pH-dependent activation of serine proteases, leading to premature degradation of corneodesmosomes. These abnormalities were normalized by exogenously acidifying the SC, suggesting a basis for the well-known acidification therapies that are widely used to treat the pathologic xerosis/eczema seen in moderately aged humans.

Fatty acid 2-hydroxylase, encoded by FA2H, accounts for differentiation-associated increase in 2-OH ceramides during keratinocyte differentiation

Ceramides in mammalian stratum corneum comprise a heterogeneous mixture of molecular species that subserve the epidermal permeability barrier, an essential function for survival in a terrestrial environment. In addition to a variation of sphingol species, hydroxylation of the amide-linked fatty acids contributes to the diversity of epidermal ceramides. Fatty acid 2-hydroxylase, encoded by the gene FA2H, the mammalian homologue of FAH1 in yeast, catalyzes the synthesis of 2-hydroxy fatty acid-containing sphingolipids. We assessed here whether FA2H accounts for 2-hydroxyceramide/2-hydroxyglucosylceramide synthesis in epidermis. Reverse transcription-PCR and Western immunoblots demonstrated that FA2H is expressed in cultured human keratinocytes and human epidermis, with FA2H expression and fatty acid 2-hydroxylase activity increased with differentiation. FA2H-siRNA suppressed 2-hydroxylase activity and decreased 2-hydroxyceramide/2-hydroxyglucosylceramide levels, demonstrating that FA2H accounts for synthesis of these sphingolipids in keratinocytes. Whereas FA2H expression and 2-hydroxy free fatty acid production increased early in keratinocyte differentiation, production of 2-hydroxyceramides/2-hydroxyglucosylceramides with longer chain amide-linked fatty acids (> or =C24) increased later. Keratinocytes transduced with FA2H-siRNA contained abnormal epidermal lamellar bodies and did not form the normal extracellular lamellar membranes required for the epidermal permeability barrier. These results reveal that 1) differentiation-dependent up-regulation of ceramide synthesis and fatty acid elongation is accompanied by up-regulation of FA2H; 2) 2-hydroxylation of fatty acid by FA2H occurs prior to generation of ceramides/glucosylceramides; and 3) 2-hydroxyceramides/2-hydroxyglucosylceramides are required for epidermal lamellar membrane formation. Thus, late differentiation-linked increases in FA2H expression are essential for epidermal permeability barrier homeostasis.

Hyaluronan-CD44 interaction stimulates keratinocyte differentiation, lamellar body formation/secretion, and permeability barrier homeostasis

In this study we investigated whether hyaluronan (HA)-CD44 interaction influences epidermal structure and function. Our data show that CD44 deficiency is accompanied by reduction in HA staining in CD44 knockout (k/o) mouse skin leading to a marked thinning of epidermis versus wild-type mouse skin. A significant delay in the early barrier recovery (following acute barrier disruption) occurs in CD44 k/o versus wild-type mouse skin. To assess the basis for these alterations in CD44 k/o mouse epidermis, we determined that differentiation markers are greatly reduced in the epidermis of CD44 k/o versus wild-type mice, while conversely HA binding to CD44 triggers differentiation in cultured human keratinocytes. CD44 downregulation (using CD44 small interfering RNAs) also inhibits HA-mediated keratinocyte differentiation. Slower barrier recovery in CD44 k/o mice could be further attributed to reduced lamellar body formation, loss of apical polarization of LB secretion, and downregulation of cholesterol synthesis. Accordingly, HA-CD44 binding stimulates both LB formation and secretion. Together, these observations demonstrate new roles for HA-CD44 interaction in regulating both epidermal differentiation and lipid synthesis/secretion, which in turn influence permeability barrier homeostasis. HA-CD44 signaling could comprise a novel approach to treat skin disorders characterized by abnormalities in differentiation, lipid synthesis, and/or barrier function.

Barrier dysfunction and pathogenesis of neutral lipid storage disease with ichthyosis (Chanarin-Dorfman syndrome)

Neutral lipid storage disease with ichthyosis (NLSDI; Chanarin-Dorfman syndrome) is an ichthyosiform syndrome, often associated with mutations in a lipid hydrolase, CGI-58. The presence of oil red O-positive, neutral lipid droplets in tissue biopsies, and/or in leukocytes on blood smears, coupled with a constellation of multisystem abnormalities and a pruritic ichthyosiform erythroderma, are together diagnostic of NLSDI. We investigated the pathogenesis of the ichthyosiform erythroderma in patients from three unrelated kindreds with a clinical diagnosis of NLSDI. Basal permeability barrier function and stratum corneum (SC) integrity were abnormal, but barrier recovery rates were faster than normal, as in atopic dermatitis. The basal barrier abnormality was linked to the secretion of lipid micro-inclusions, first segregated within lamellar bodies (LB), which then form a non-lamellar phase within the SC interstices, shown by combined ruthenium tetroxide post-fixation and lipid-retaining resin-white embedding. With colloidal lanthanum nitrate perfusion, excess water/solute movement was restricted to the SC interstices, and further localized to non-lamellar domains. Phase separation of excess stored lipid provides a unifying pathogenic mechanism not only for NLSDI, but also in several other inherited ichthyosiform disorders of lipid metabolism, such as recessive X-linked ichthyosis and type 2 Gaucher's disease.

Coordinate assembly of lipids and enzyme proteins into epidermal lamellar bodies

Formation of the epidermal permeability barrier requires delivery of lamellar body (LB) contents to the stratum corneum interstices. LB are enriched in a mixture of polar lipids and a family of hydrolytic enzymes, required for the extracellular processing of the secreted polar lipids into the more hydrophobic products which mediate barrier function. Prior non-quantitative studies show that acute barrier disruption leads to immediate secretion of the contents of performed LB from the outermost layer of granular cells, followed by the synthesis and accelerated secretion of newly-formed (= nascent) organelles over 0.5-4 h. We asked here whether lipids and hydrolytic enzymes are packaged into nascent organelles separately, or in a parallel, linked process. We first quantified the rate of appearance of lipids (by the content of internal lamellae within LB) and enzyme content (by cytochemistry of neutral lipase and acid sphingomyelinase); both are concentrated in LB, and in nascent organelles. Immediately after barrier disruption, the density of LB in the cytosol of the outermost granular cell decreased by > 50% reduction at 30 min, returning to near-normal densities by 4 h. Nascent organelles budded off a trans-Golgi-like reticulum, in the outermost granular cells as early as 30 min. In quantitative studies, LB progressively accumulated lipid and enzyme contents in parallel. However, when lipid/lamellae generation was inhibited with lipid synthesis inhibitors, enzymes did not accumulate in organelles. Likewise, when exogenous physiologic lipids were delivered to sites of LB generation in the face of brefeldin A blockade of organellogenesis, or when lipids were delivered in conjunction with treatment with lipid synthesis inhibitors, enzymes accumulated only in those organelles that displayed lipid content. These studies demonstrate: (a) quantitative changes in the density of LB in the outermost granular cell at various time points after acute barrier disruption; (b) the origin of nascent organelles in a trans-Golgi-like reticulum; (c) co-ordinate packaging of lipid and enzyme contents into nascent organelles; (d) that lipid deposition in nascent organelles is required for enzyme accumulation; and (e) that enzymes can be delivered to nascent organelles, even if the source of lipid is of exogenous rather than endogenous origin.

Microwave incubation improves lipolytic enzyme preservation for ultrastructural cytochemistry

Standard methods for the ultrastructural detection of lipase and sphingomyelinase activities in the skin result in considerable loss of structural preservation, often interfering with accurate delineation of enzyme localization in association with specific organelles. Moreover, poor preservation occurs, even after extensive aldehyde prefixation, owing to the prolonged incubation times needed to detect residual enzyme activity, which often require non-physiological conditions. A modified incubation protocol is described here, which uses microwave irradiation in conjunction with drastically shortened incubation times, resulting in both superior ultrastructural preservation and excellent localization in mammalian epidermis. This method should be useful generally not only for the study of lipase localization in skin, but also in conjunction with the cytochemical detection of a variety of enzymes in various types of tissue.

Evidence that the gonadotrophs are the likely site of production of angiotensin II in the anterior pituitary of the rat

To determine whether the angiotensin II-like immunoreactivity (AII-IR) previously reported in rat gonadotrophs is generated locally, we stained for AII-IR in neonatal rat pituitary explants and dispersed adult pituitary cells maintained in serum-free medium. In both preparations, AII-IR was found in cells with the morphological characteristics of gonadotrophs. In the anterior pituitaries of adult male rats, AII-IR and LH beta immunoreactivity were found by electron microscopy to be located in the same secretory granules. Pituitary tissue was also extracted, and the AII content was measured by RIA. The gland contained 300 times more AII than could be accounted for by the extracellular fluid in the gland. On HPLC, the pituitary AII-IR comigrated with synthetic Ile5-AII. Thus, it appears that the AII-IR in rat pituitary gonadotrophs is AII and that it is likely to be produced within these cells.

Lowicryl K4M embedding of brain tissue for immunogold electron microscopy

We present methods for embedding brain tissue in Lowicryl K4M embedding medium and localizing antigens using postembedding immunogold techniques. After perfusion fixation with 4% paraformaldehyde and 0.1% glutaraldehyde in 0.1 M cacodylate buffer, blocks of rat brain were placed in 2% aqueous uranyl acetate for 1 hour, dehydrated in 50%, 70%, and 95% ethanol, infiltrated with Lowicryl/ethanol mixtures (1:2 for 10 min, 1:1 for 15 min) and 100% Lowicryl (20 min and 25 min). Polymerization was carried out under UV light for 24-48 hours at room temperature. Several neural antigens, including three different synaptic vesicle proteins and an enzyme associated with the postsynaptic density, were localized by this technique, indicating that this procedure may have wide applicability.

Development of arthrospores of Trichophyton mentagrophytes

Arthrosporogenesis of the dermatophyte Trichophyton mentagrophytes was examined by light and by scanning and transmission electron microscopy. Sabouraud dextrose agar plates were inoculated with microconidia and incubated in an atmosphere of 8% CO2. Typical germination and hyphal branching continued to day 4, when hyphae began to be increasingly coated with granular-fibrillar material. Multiple replication of nuclei and formation of segregating septa followed. By day 6 the thick surface mesh sometimes was restricted to protruding rings, probably over septa. Between days 6 and 7, after thickening of outer and septal walls, units began to round and separate. Triangular gaps, which developed at the junction of septa and outer wall layers, enlarged so that spores were held together at their poles and along a tangential ring. With elongation of the spore to its barrel shape, the halves of the septum separated and the ring pulled apart, leaving a jagged, circular flange originating from the outer layer of cell wall extended toward the poles, covering the apparently exposed inner wall layer. Newly formed arthrospores, which measured 2.0 to 3.3 by 2.9 to 3.8 micronm and possessed walls of about 0.33-micronm thickness, has smooth sides but somewhat rough poles.