Glucocorticoids accelerate fetal maturation of the epidermal permeability barrier in the rat

The postnatal resolution of developmental toxicity induced by pharmacological diacylglycerol acyltransferase 2 (DGAT2) inhibition during gestation in rats

Ervogastat (PF-06865571) is a small molecule diacylglycerol acyltransferase 2 (DGAT2) inhibitor being developed for the oral treatment of non-alcoholic steatohepatitis (NASH) with liver fibrosis. DGAT2 is a key enzyme in triglyceride synthesis in tissues and in regulating energy metabolism. Fertility and developmental toxicity studies with ervogastat were conducted in female rats and rabbits. There were no effects on female rat fertility or rabbit embryo-fetal development. Administration of ervogastat to pregnant rats during organogenesis reduced fetal weight and caused higher incidences of bent bones in fetuses that were shown to resolve by postnatal day 28 and were therefore considered to be transient variations secondary to developmental delay. Extended dosing in rats through the end of gestation and lactation (pre- and post-natal development study) caused impaired skin development, reduced offspring viability and growth retardation. The spectrum of developmental effects in rats is consistent with the intended pharmacology (altered triglyceride metabolism) and the transient nature of the skeletal findings, along with the late gestational window of sensitivity for the effects on skin barrier development, reduce the concern for potential adverse developmental effects following unintended early gestational exposure to ervogastat in humans where treatment can be discontinued once pregnancy is determined.

Roles of the Glucocorticoid and Mineralocorticoid Receptors in Skin Pathophysiology

The nuclear hormone receptor (NR) superfamily comprises approximately 50 evolutionarily conserved proteins that play major roles in gene regulation by prototypically acting as ligand-dependent transcription factors. Besides their central role in physiology, NRs have been largely used as therapeutic drug targets in many chronic inflammatory conditions and derivatives of their specific ligands, alone or in combination, are frequently prescribed for the treatment of skin diseases. In particular, glucocorticoids (GCs) are the most commonly used compounds for treating prevalent skin diseases such as psoriasis due to their anti-proliferative and anti-inflammatory actions. However, and despite their therapeutic efficacy, the long-term use of GCs is limited because of the cutaneous adverse effects including atrophy, delayed wound healing, and increased susceptibility to stress and infections. The GC receptor (GR/NR3C1) and the mineralocorticoid receptor (MR/NR3C2) are members of the NR subclass NR3C that are highly related, both structurally and functionally. While the GR is ubiquitously expressed and is almost exclusively activated by GCs; an MR has a more restricted tissue expression pattern and can bind GCs and the mineralocorticoid aldosterone with similar high affinity. As these receptors share 95% identity in their DNA binding domains; both can recognize the same hormone response elements; theoretically resulting in transcriptional regulation of the same target genes. However, a major mechanism for specific activation of GRs and/or MRs is at the pre-receptor level by modulating the local availability of active GCs. Furthermore, the selective interactions of each receptor with spatio-temporally regulated transcription factors and co-regulators are crucial for the final transcriptional outcome. While there are abundant genome wide studies identifying GR transcriptional targets in a variety of tissue and cell types; including keratinocytes; the data for MR is more limited thus far. Our group and others have studied the role of GRs and MRs in skin development and disease by generating and characterizing mouse and cellular models with gain- and loss-of-function for each receptor. Both NRs are required for skin barrier competence during mouse development and also play a role in adult skin homeostasis. Moreover, the combined loss of epidermal GRs and MRs caused a more severe skin phenotype relative to single knock-outs (KOs) in developing skin and in acute inflammation and psoriasis, indicating that these corticosteroid receptors play cooperative roles. Understanding GR- and MR-mediated signaling in skin should contribute to deciphering their tissue-specific relative roles and ultimately help to improve GC-based therapies.

Effects of early-life malnutrition on neurodevelopment and neuropsychiatric disorders and the potential mechanisms

Lines of evidence have demonstrated that early-life malnutrition is highly correlated with neurodevelopment and adulthood neuropsychiatric disorders, while some findings are conflicting with each other. In addition, the biological mechanisms are less investigated. We systematically reviewed the evidence linking early-life nutrition status with neurodevelopment and clinical observations in human and animal models. We summarized the effects of special nutritious on neuropsychiatric disorders and explored the underlying potential mechanisms. The further understanding of the biological regulation of early-life nutritional status on neurodevelopment might shed light on precision nutrition at an integrative systems biology framework.

The effects of antenatal glucocorticoid exposure on fetal and neonatal skin maturation

AIMS

The use of antenatal glucocorticoids in women with preterm labor has dramatically improved outcomes for premature infants. The most commonly used antenatal glucocorticoids are betamethasone and dexamethasone. Glucocorticoids accelerate fetal lung growth by several mechanisms, including the maturation of type II pneumocytes enabling surfactant production. Furthermore, the lipids in the lung share similarity with those in the skin. Therefore, antenatal administration of glucocorticoids may have effects on the structure and function of the developing epidermal barrier in fetuses and neonates.

METHODS

We performed a systematic review to characterize these effects, identifying 11 studies (six animal and five human studies).

RESULTS

Five out of the six animal studies used a rodent model for investigating the effects of antenatally administered glucocorticoids, while the other used an ovine model. Antenatally administered glucocorticoids accelerated skin maturation in animal studies, but studies of human fetuses found conflicting results. None of the reviewed studies compared the effects of different types of glucocorticoids.

CONCLUSIONS

More human studies are needed to fully understand the effects of antenatal steroids. However, as the antenatal use of glucocorticoids in preterm pregnancies has become part of standard clinical practice, it would be unethical to carry out a large randomized controlled trial. We may have to rely on animal models to improve our understanding of the effects of antenatal glucocorticoid exposure on the fetal and neonatal skin maturation.

Dysfunctional Skin-Derived Glucocorticoid Synthesis Is a Pathogenic Mechanism of Psoriasis

Glucocorticoids (GC) are the primary steroids that regulate inflammation and have been exploited therapeutically in inflammatory skin diseases. Despite the broad-spectrum therapeutic use of GC, the biochemical rationale for locally treating inflammatory skin conditions is poorly understood, as systemic GC production remains largely functional in these patients. GC synthesis has been well characterized in healthy skin, but the pathological consequence has not been examined. Here we show de novo GC synthesis, and GC receptor expression is dysfunctional in both nonlesional and lesional psoriatic skin. Use of GC receptor epidermal knockout mice with adrenalectomy allowed for the distinction between local (keratinocyte) and systemic GC activity. Compensation exhibited by adult GC receptor epidermal knockout mice demonstrated that keratinocyte-derived GC synthesis protected skin from topical phorbol 12-myristate 13-acetate-induced inflammatory assault. Thus, localized de novo GC synthesis in skin is essential for controlling inflammation, and loss of the GC pathway in psoriatic skin represents an additional pathological process in this complex inflammatory skin disease.

Do Prenatal Corticosteroids Affect Brain Maturation of the Premature Infant? An Electroencephalography Study

OBJECTIVE

To assess whether prenatal treatment with betamethasone has a significant influence on cerebral maturation indices as measured by electroencephalographic (EEG) indices.

STUDY DESIGN

Infants born less than 35 weeks postmenstrual age (PMA) were prospectively enrolled if their mother received a full course of bethametasone prior to delivery (study group) or not (control group); infants with major intracranial abnormalities were excluded as well as those who were sedated or needed assisted ventilation. EEG was recorded during the first 10 days of life. Interburst intervals and maximal amplitudes of theta and delta bandwidths were calculated by a signal processing software. A multivariate general linear model was used to analyze the relationship between the 2 groups and the different electrophysiologic parameters, adjusting for PMA and mode of delivery.

RESULTS

Thirty-eight infants were included in the study group and 36 in the control group. Univariate analysis demonstrated a negative correlation between PMA at test and EEG indices (interburst interval and delta and theta frequencies). Multivariate analysis demonstrated a less robust correlation of PMA and EEG indices and a positive correlation of prenatal betamethasone treatment with Theta frequencies. Repeating the data analysis separately for each study group, the above results remained significant mainly in the study group.

CONCLUSIONS

Our findings suggest a possible stabilization effect of corticosteroids on the central nervous system and a possible delay of the maturation of cerebral activity related to prenatal corticosteroids use. These findings may relate to a better neurodevelopmental outcome of infants treated prenatally with corticosteroids.

Significance of antenatal glucocorticoid exposure for pressure injury prevalence in neonates

AIMS

Studies have highlighted that antenatal steroids could have an effect on neonatal skin maturation. This study examined if there was a relationship between the administration of antenatal glucocorticoids for mothers and the skin injuries in their neonates. Data from skin injury audit were extracted from the neonatal database and analyzed to determine differences in the prevalence of neonates with pressure injuries [cases] whose mothers had received antenatal steroids, compared to those without pressure injuries [control].

RESULTS

Of 247 neonates audited, 77 [31%], had documented pressure injuries, 170 [69%] had no documented injury. The median birth weight and gestation were 1400 g [IQR 893-2268 g] and 30.3 weeks [IQR 26.3-40.0 weeks] respectively. Of the neonates born less than 34 weeks, 80% were exposed to antenatal steroids and were equally distributed across patient genders. Within the 77 cases, 53 [66%] were exposed to antenatal steroids compared to controls in which 88 [53%] had not. The effect between cases and controls was not statistically significant [χ2 = 2.81, P = 0.09]. However a difference was noted between genders, as female neonates benefited from the exposure to steroids [OR = 0.317, 95% [CI 0.105-0.956], p value -0.041].

CONCLUSION

Antenatal glucocorticoids appear to be beneficial in reducing pressure injury prevalence in female neonates.

Glucocorticoid receptor and Klf4 co-regulate anti-inflammatory genes in keratinocytes

The glucocorticoid (GC) receptor (GR) and Kruppel-like factor Klf4 are transcription factors that play major roles in skin homeostasis. However, whether these transcription factors cooperate in binding genomic regulatory regions in epidermal keratinocytes was not known. Here, we show that in dexamethasone-treated keratinocytes GR and Klf4 are recruited to genomic regions containing adjacent GR and KLF binding motifs to control transcription of the anti-inflammatory genes Tsc22d3 and Zfp36. GR- and Klf4 loss of function experiments showed total GR but partial Klf4 requirement for full gene induction in response to dexamethasone. In wild type keratinocytes induced to differentiate, GR and Klf4 protein expression increased concomitant with Tsc22d3 and Zfp36 up-regulation. In contrast, GR-deficient cells failed to differentiate or fully induce Klf4, Tsc22d3 and Zfp36 correlating with increased expression of the epithelium-specific Trp63, a known transcriptional repressor of Klf4. The identified transcriptional cooperation between GR and Klf4 may determine cell-type specific regulation and have implications for developing therapies for skin diseases.

Efeitos da ingestão de cafeína durante a gestação e a lactação sobre a pele de ratas e de filhotes e sua relação com as concentrações séricas do cortisol materno

O objetivo deste trabalho foi verificar os efeitos da ingestão materna de diferentes doses de cafeína durante a gestação e a lactação, na pele de ratas-mães e filhotes, bem como sua relação com as concentrações séricas do cortisol materno. Vinte e quatro ratas Wistar adultas foram distribuídas em quatro grupos, representados pelo controle e tratados, com cafeína nas doses de 25, 50 e 100mg/kg. Os grupos tratados receberam cafeína por sonda orogástrica durante toda a gestação e a lactação. O controle recebeu água destilada como placebo. Foram avaliados e quantificados os diferentes tipos de folículos pilosos e a espessura da epiderme. A técnica de imuno-histoquímica, com o uso do anticorpo anti-CDC47, foi utilizada para avaliar a proliferação celular da epiderme e dos folículos pilosos das mães. Na mãe, também foram mensurados os níveis séricos de cortisol pela técnica da quimioluminescência. Os dados foram submetidos à análise de variância com comparação das médias pelos testes Kruskal-Wallis e SNK. Nos grupos tratados com cafeína nas doses de 25 e 50mg/kg, tanto as mães quanto seus filhotes apresentaram hipotricose e/ou alopecia focal. Apesar de a frequência de alterações macroscópicas das mães ter sido superior a dos filhotes, nestes as lesões, quando presentes, foram difusas. A análise histológica demonstrou calcinose de folículos pilosos nas mães e nos filhotes. Mas a morfometria somente revelou diferença significativa no número de folículos pilosos das mães, bem como redução significativa da proliferação celular dos folículos pilosos do grupo tratado com 50mg/kg de cafeína. Os níveis de cortisol materno somente foram significativamente elevados no grupo tratado com 100mg/kg de cafeína. Conclui-se que a cafeína ingerida pelas ratas gestantes e lactantes pode causar lesões cutâneas tanto nas mães quanto nos filhotes, caracterizadas por hipotricose e/ou alopecia, independentemente dos níveis séricos do cortisol materno.

Impact of maternal under nutrition on obstetric outcomes

Maternal malnutrition, ranging from under nutrition to over dietary intake before and in the pregnant state, is worldwide problem with significant consequences, not only for survival and increased risk for acute and chronic diseases both in mother and child, but also for economic productivity of individuals in the societies and additional costs on health system. Inter alia, pre-pregnancy underweight and insufficient gestational weight gain are considered as individual risk factors for the occurrence of spontaneous interruption, preterm birth, fetal growth restriction, and hypertensive disorders, strongly associated with poorer perinatal outcome. In a portion of this population, major eating disorders (anorexia and bulimia nervosa), once thought to be rare, but nowadays enlarged due to cultural pressure on the drive for thinness, have been identified as the etiology of an abnormal nutritional condition in developed countries, in contrast to long standing food deprivation in developing countries. Actually, even if without a complete weight management guidance for these selected pregnant women, an appropriate weight gain is recommended during pregnancy. Mainly, therapeutic approach is prevention using specific programs of improving weight before pregnant status. In this article, a review of the literature on selected obstetrical risks associated with maternal underweight has been performed and both the target prevention and management strategies have been described.

Control of steroid receptor dynamics and function by genomic actions of the cochaperones p23 and Bag-1L

Molecular chaperones encompass a group of unrelated proteins that facilitate the correct assembly and disassembly of other macromolecular structures, which they themselves do not remain a part of. They associate with a large and diverse set of coregulators termed cochaperones that regulate their function and specificity. Amongst others, chaperones and cochaperones regulate the activity of several signaling molecules including steroid receptors, which upon ligand binding interact with discrete nucleotide sequences within the nucleus to control the expression of diverse physiological and developmental genes. Molecular chaperones and cochaperones are typically known to provide the correct conformation for ligand binding by the steroid receptors. While this contribution is widely accepted, recent studies have reported that they further modulate steroid receptor action outside ligand binding. They are thought to contribute to receptor turnover, transport of the receptor to different subcellular localizations, recycling of the receptor on chromatin and even stabilization of the DNA-binding properties of the receptor. In addition to these combined effects with molecular chaperones, cochaperones are reported to have additional functions that are independent of molecular chaperones. Some of these functions also impact on steroid receptor action. Two well-studied examples are the cochaperones p23 and Bag-1L, which have been identified as modulators of steroid receptor activity in nuclei. Understanding details of their regulatory action will provide new therapeutic opportunities of controlling steroid receptor action independent of the widespread effects of molecular chaperones.

Krüppel-like factors are effectors of nuclear receptor signaling

Binding of steroid and thyroid hormones to their cognate nuclear receptors (NRs) impacts virtually every aspect of postembryonic development, physiology and behavior, and inappropriate signaling by NRs may contribute to disease. While NRs regulate genes by direct binding to hormone response elements in the genome, their actions may depend on the activity of other transcription factors (TFs) that may or may not bind DNA. The Krüppel-like family of transcription factors (KLF) is an evolutionarily conserved class of DNA-binding proteins that influence many aspects of development and physiology. Several members of this family have been shown to play diverse roles in NR signaling. For example, KLFs (1) act as accessory transcription factors for NR actions, (2) regulate expression of NR genes, and (3) as gene products of primary NR response genes function as key players in NR-dependent transcriptional networks. In mouse models, deletion of different KLFs leads to aberrant transcriptional and physiological responses to hormones, underscoring the importance of these proteins in the regulation of hormonal signaling. Understanding the functional relationships between NRs and KLFs will yield important insights into mechanisms of NR signaling. In this review we present a conceptual framework for understanding how KLFs participate in NR signaling, and we provide examples of how these proteins function to effect hormone action.

Deletion of mesenchymal glucocorticoid receptor attenuates embryonic lung development and abdominal wall closure

As a member of the nuclear hormone receptor superfamily of ligand-activated transcription factors, the glucocorticoid receptor (GR) is essential for normal embryonic development. To date, the role of mesenchymal glucocorticoid signaling during development has not been fully elucidated. In the present study, we investigated the role of the GR during embryogenesis specifically in mesenchymal tissues. To this aim, we crossed GRflox mice with Dermo1-Cre mice to generate GR(Dermo1) mice, where the GR gene was deleted within mesenchymal cells. Compared to their wild type littermates, GR(Dermo1) mice displayed severe pulmonary atelectasis, defects in abdominal wall formation resulting in intestinal herniation, abnormal extracellular matrix synthesis in connective tissues and high postnatal lethality. Lungs of GR(Dermo1) mice failed to progress from the canalicular to saccular stage, as evidenced by the presence of immature air sacs, thickened interstitial mesenchyme and an underdeveloped vascular network between E17.5 and E18.5. Furthermore, myofibroblasts and vascular smooth muscle cells, although present in normal numbers in GR(Dermo1) animals, were characterized by significantly reduced elastin synthesis, whilst epithelial lining cells of the immature saccules were poorly differentiated. A marked reduction in normal elastin and collagen deposits were also observed in connective tissues adjacent to the umbilical hernia. This study demonstrates that eliminating the GR in cells of the mesenchymal lineage results in marked effects on interstitial fibroblast function, including a significant decrease in elastin synthesis. This results in lung atelectasis and postnatal lethality, as well as additional and hitherto unrecognized developmental defects in abdominal wall formation. In addition, altered glucocorticoid signaling in the mesenchyme attenuates normal lung epithelial differentiation.

Epidermal inactivation of the glucocorticoid receptor triggers skin barrier defects and cutaneous inflammation

The glucocorticoid (GC) receptor (GR) mediates the effects of physiological and pharmacological GC ligands and has a major role in cutaneous pathophysiology. To dissect the epithelial versus mesenchymal contribution of GR in developing and adult skin, we generated mice with keratinocyte-restricted GR inactivation (GR epidermal knockout or GR(EKO) mice). Developing and early postnatal GR(EKO) mice exhibited impaired epidermal barrier formation, abnormal keratinocyte differentiation, hyperproliferation, and stratum corneum (SC) fragility. At birth, GR(EKO) epidermis showed altered levels of epidermal differentiation complex genes, proteases and protease inhibitors which participate in SC maintenance, and innate immunity genes. Many upregulated genes, including S100a8/a9 and Tslp, also have increased expression in inflammatory skin diseases. Infiltration of macrophages and degranulating mast cells were observed in newborn GR(EKO) skin, hallmarks of atopic dermatitis. In addition to increased extracellular signal-regulated kinase activation, GR(EKO) newborn and adult epidermis had increased levels of phosphorylated signal transducer and activator of transcription 3, a feature of psoriasis. Although adult GR(EKO) epidermis had a mild phenotype of increased proliferation, perturbation of skin homeostasis with detergent or phorbol ester triggered an exaggerated proliferative and hyperkeratotic response relative to wild type. Together, our results show that epidermal loss of GR provokes skin barrier defects and cutaneous inflammation.

2,3,7,8-Tetrachlorodibenzo-p-dioxin increases the expression of genes in the human epidermal differentiation complex and accelerates epidermal barrier formation

Chloracne is commonly observed in people exposed to dioxins, yet the mechanism of toxicity is not well understood. The pathology of chloracne is characterized by hyperkeratinization of the interfollicular squamous epithelium, hyperproliferation and hyperkeratinization of hair follicle cells as well as a metaplastic response of the ductular sebum secreting sebaceous glands. In vitro studies using normal human epidermal keratinocytes to model interfollicular human epidermis demonstrate a 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-mediated acceleration of differentiation and increase in gene expression of several prodifferentiation genes, including filaggrin (FLG). Here, we demonstrated that the TCDD-activated aryl hydrocarbon receptor (AHR) bound a small fragment of DNA upstream of the transcriptional start sites of the FLG gene, containing one of two candidate xenobiotic response elements (XREs). Reporter assays using the promoter region of FLG containing the two putative XREs indicated that the increase in this messenger RNA (mRNA) was due to TCDD-mediated enhanced transcription, which was lost when both XREs were mutated. As FLG is part of the human epidermal differentiation complex (EDC) found on chromosome 1, we measured mRNAs from an additional 18 EDC genes for their regulation by TCDD. Of these genes, 14 were increased by TCDD. Immunoblot assays demonstrated that the proteins of FLG as well as that of another prodifferentiation gene, small proline rich protein 2, were increased by TCDD. In utero exposure to TCDD accelerated the formation of the epidermal barrier in the developing mouse fetus by approximately 1 day. These results indicate that the epidermal permeability barrier is a functional target of the TCDD-activated AHR.

Glucocorticoid receptors, epidermal homeostasis and hair follicle differentiation

Glucocorticoids (GCs) exert their biological and therapeutical actions through the GC receptor (GR), a ligand-dependent transcription factor. Synthetic GC derivatives are widely prescribed for treating numerous cutaneous inflammatory and immune diseases due to their great efficacy. However, chronic treatment with GCs produces adverse side-effects including skin atrophy, delayed wound healing, and in certain cases, GC resistance. The mechanisms underlying the therapeutic actions of the GR in skin have been extensively studied; in contrast, the role of GR as a modulator of epidermal development and homeostasis has received less attention. The ubiquitous functional inactivation of GR results in defective epidermal formation although the underlying mechanisms have not been fully characterized. The use of transcriptomic approaches both in vitro and in vivo allowed the identification of genes that are regulated by GR in developing and adult skin. A main goal to understand the role of GR in skin biology is to identify primary transcriptional targets as well as the signaling pathways mediating GR action. Furthermore, it will be important to decipher the contribution of GR in the different cellular compartments of the skin, including keratinocytes of the interfollicular epidermis and hair follicles, and their respective stem cell progenitors. Additionally, recent findings indicating that the skin acts as a true peripheral endocrine organ implies greater complexity than originally thought. The local production of GCs and other steroid hormones should be considered as a modulator of skin function under homeostatic and diseased conditions. Finally, studying GR function in skin should take into account that the mineralocorticoid receptor may also mediate GC actions and/or regulate transcription either by itself or in combination with GR. Addressing these issues should help to elucidate the mechanisms by which Gr contributes to establishment of a competent epidermal barrier and may also have implications in the context of dermatological treatments based on GC-analogs.

Antenatal Corticosteroids and Postnatal Fluid Restriction Produce Differential Effects on AQP3 Expression, Water Handling, and Barrier Function in Perinatal Rat Epidermis

Loss of water through the immature skin can lead to hypothermia and dehydration in preterm infants. The water and glycerol channel aquaglyceroporin-3 (AQP3) is abundant in fetal epidermis and might influence epidermal water handling and transepidermal water flux around birth. To investigate the role of AQP3 in immature skin, we measured in vivo transepidermal water transport and AQP3 expression in rat pups exposed to clinically relevant fluid homeostasis perturbations. Preterm (E18) rat pups were studied after antenatal corticosteroid exposure (ANS), and neonatal (P1) rat pups after an 18 h fast. Transepidermal water loss (TEWL) and skin hydration were determined, AQP3 mRNA was quantified by RT-PCR, and in-situ hybridization and immunocytochemistry were applied to map AQP3 expression. ANS resulted in an improved skin barrier (lower TEWL and skin hydration), while AQP3 mRNA and protein increased. Fasting led to loss of barrier integrity along with an increase in skin hydration. These alterations were not paralleled by any changes in AQP3. To conclude, antenatal corticosteroids and early postnatal fluid restriction produce differential effects on skin barrier function and epidermal AQP3 expression in the rat. In perinatal rats, AQP3 does not directly determine net water transport through the skin.

Functional skin adaptation in infancy - almost complete but not fully competent

Early postnatal life is a period of active functional reorganization and cutaneous physiological adaptation to the extrauterine environment. Skin as the outermost organ of mammalians is endowed of multiple functions such as protection, secretion, absorption and thermoregulation. Birth stimulates the epidermal barrier maturation and the skin surface acidification especially in premature infants. In full-term infants the developed stratum corneum accomplishes competent barrier function, in contrast to prematures. Complete barrier maturation in preterm infants is fulfilled by 2-4 weeks of the postnatal life. However, in preterms with 23-25 weeks gestational age this process takes longer. Versatile regulatory mechanisms, namely skin surface acidity, calcium ion gradient and nuclear hormone receptors/ligands are interrelated in the complex postnatal newborn adaptation. The skin of newborns is adjusting quickly to the challenging environmental conditions of the postpartum. However, certain functions, for example, microcirculation, continue to develop even beyond the neonatal period, that is, up to the age of 14-17 weeks. Different environmental factors (for instance, dry and cold climate, diapers and cosmetic care procedures) influence the postnatal development of skin functional parameters such as stratum corneum hydration and the permeability barrier especially in premature infants. The aim of this article is to summarize the current knowledge on skin physiology in newborn and infants with a practical approach and to discuss the possible clinical consequences. This review offers the readership a critical and practical overview of skin physiology in newborns and infants. It emphasizes possible new research fields in neonatal and infantile skin physiology.

PPARδ activation promotes stratum corneum formation and epidermal permeability barrier development during late gestation

The goal of epidermal ontogenesis is to form a stratum corneum (SC), which is required for post-natal permeability barrier function. The regulation of epidermal ontogenesis is poorly understood, but nuclear hormone receptors have been shown to have an important function. As peroxisome proliferator-activated receptor-delta (PPARdelta) is very abundant in fetal epidermis and PPARdelta activation stimulates differentiation and permeability barrier formation in adults, we hypothesized that PPARdelta might regulate epidermal ontogenesis. Treatment of fetal rat explants with the PPARdelta ligand, GW 610742X, accelerates permeability barrier development, evidenced by a decrease in transepidermal water loss and an enhanced outside-in barrier function, attributable to the presence of more mature lamellar membranes in the SC and enhanced expression of loricrin and involucrin. Similarly, the intra-amniotic administration of GW 610742X also accelerates the formation of the SC and permeability barrier development. Finally, in PPARdelta-deficient mice the formation of the SC and the expression of differentiation-related proteins were delayed on days 16.5 and 17.5 of gestation. However, at later stages (day 18.5 and after birth), there were no differences between wild-type- and PPARdelta-deficient mice, indicating only a transient delay in epidermal ontogenesis. These studies show that PPARdelta has a role in SC formation and permeability barrier development.

In utero domoic acid toxicity: a fetal basis to adult disease in the California sea lion (Zalophus californianus)

California sea lions have been a repeated subject of investigation for early life toxicity, which has been documented to occur with increasing frequency from late February through mid-May in association with organochlorine (PCB and DDT) poisoning and infectious disease in the 1970’s and domoic acid poisoning in the last decade. The mass early life mortality events result from the concentrated breeding grounds and synchronization of reproduction over a 28 day post partum estrus cycle and 11 month in utero phase. This physiological synchronization is triggered by a decreasing photoperiod of 11.48 h/day that occurs approximately 90 days after conception at the major California breeding grounds. The photoperiod trigger activates implantation of embryos to proceed with development for the next 242 days until birth. Embryonic diapause is a selectable trait thought to optimize timing for food utilization and male migratory patterns; yet from the toxicological perspective presented here also serves to synchronize developmental toxicity of pulsed environmental events such as domoic acid poisoning. Research studies in laboratory animals have defined age-dependent neurotoxic effects during development and windows of susceptibility to domoic acid exposure. This review will evaluate experimental domoic acid neurotoxicity in developing rodents and, aided by comparative allometric projections, will analyze potential prenatal toxicity and exposure susceptibility in the California sea lion. This analysis should provide a useful tool to forecast fetal toxicity and understand the impact of fetal toxicity on adult disease of the California sea lion.

Glucocorticoid receptor is required for skin barrier competence

To investigate the contribution of the glucocorticoid receptor (GR) in skin development and the mechanisms underlying this function, we have analyzed two mouse models in which GR has been functionally inactivated: the knockout GR(-/-) mice and the dimerization mutant GR(dim/dim) that mediates defective DNA binding-dependent transcription. Because GR null mice die perinatally, we evaluated skin architecture of late embryos by histological, immunohistochemical, and electron microscopy studies. Loss of function of GR resulted in incomplete epidermal stratification with dramatically abnormal differentiation of GR(-/-), but not GR(+/-) embryos, as demonstrated by the lack of loricrin, filaggrin, and involucrin markers. Skin sections of GR(-/-) embryos revealed edematous basal and lower spinous cells, and electron micrographs showed increased intercellular spaces between keratinocytes and reduced number of desmosomes. The absent terminal differentiation in GR(-/-) embryos correlated with an impaired activation of caspase-14, which is required for the processing of profilaggrin into filaggrin at late embryo stages. Accordingly, the skin barrier competence was severely compromised in GR(-/-) embryos. Cultured mouse primary keratinocytes from GR(-/-) mice formed colonies with cells of heterogeneous size and morphology that showed increased growth and apoptosis, indicating that GR regulates these processes in a cell-autonomous manner. The activity of ERK1/2 was constitutively augmented in GR(-/-) skin and mouse primary keratinocytes relative to wild type, which suggests that GR modulates skin homeostasis, at least partially, by antagonizing ERK function. Moreover, the epidermis of GR(+/dim) and GR(dim/dim) embryos appeared normal, thus suggesting that DNA-binding-independent actions of GR are sufficient to mediate epidermal and hair follicle development during embryogenesis.

Maternal-fetal transfer of domoic acid in rats at two gestational time points

BACKGROUND AND OBJECTIVES

Prenatal exposure to asymptomatic doses of domoic acid (DA) causes learning and memory deficits later in life; therefore, we sought to measure distribution of DA in maternal plasma and brain, prenatal brain, and amniotic fluid 1 hr after exposure, a time frame that normally encompasses acute seizure behavior.

METHODS

Pregnant rats were given a single intravenous dose of DA (0.6 or 1.6 mg/kg body weight) at either gestational day (GD) 13 or GD20, which correspond to the beginning of rat embryo neurogenesis and the last day of gestation, respectively. Using a direct ELISA, dose-dependent levels of DA were detected in each sample matrix tested.

RESULTS

An average of 6.6 and 14 ng DA/g brain tissue was found in GD13 and GD20 prenatal rats, respectively. Brain concentrations of DA in the GD13 prenates were identical to amniotic fluid levels, consistent with no restriction for DA to enter the GD13 prenatal brain. At GD20 the prenatal brain contained half the concentration of DA in the amniotic fluid, and was approximately half that found in the brain of the dams. After 1 hr, fetal brain and amniotic fluid contained between 1 and 5% of DA found in the maternal circulation. The amniotic fluid levels of DA in this study were also within the same range measured in stranded California sea lions that showed reproductive failure.

CONCLUSIONS

DA crosses the placenta, enters brain tissue of prenates, and accumulates in the amniotic fluid. Amniotic fluid appears to be a useful fluid to monitor DA exposure.

Postnatal requirement of the epithelial sodium channel for maintenance of epidermal barrier function

In skin, the physiological consequence of an epithelial sodium channel (ENaC) deficiency is not obvious directly at birth. Nevertheless, within hours after birth, mice deficient for the alpha-subunit of the highly amiloride-sensitive epithelial sodium channel (alphaENaC/Scnn1a) suffer from a significant increased dehydration. This is characterized by a loss of body weight (by 6% in 6 h) and an increased transepidermal water loss, which is accompanied by a higher skin surface pH in 1-day-old pups. Although early and late differentiation markers, as well as tight junction protein distribution and function, seem unaffected, deficiency of alphaENaC severely disturbs the stratum corneum lipid composition with decreased ceramide and cholesterol levels, and increased pro-barrier lipids, whereas covalently bound lipids are drastically reduced. Ultrastructural analysis revealed morphological changes in the formation of intercellular lamellar lipids and the lamellar body secretion. Extracellular formation of the lamellar lipids proved to be abnormal in the knockouts. In conclusion, ENaC deficiency results in progressive dehydration and, consequently, weight loss due to severe impairment of lipid formation and secretion. Our data demonstrate that ENaC expression is required for the postnatal maintenance of the epidermal barrier function but not for its generation.

Targeted skin overexpression of the mineralocorticoid receptor in mice causes epidermal atrophy, premature skin barrier formation, eye abnormalities, and alopecia

The mineralocorticoid receptor (MR) is a transcription factor of the nuclear receptor family, activation of which by aldosterone enhances salt reabsorption in the kidney. The MR is also expressed in nonclassical aldosterone target cells (brain, heart, and skin), in which its functions are incompletely understood. To explore the functional importance of MR in mammalian skin, we have generated a conditional doxycycline-inducible model of MR overexpression, resulting in double-transgenic (DT) mice [keratin 5-tTa/tetO-human MR (hMR)], targeting the human MR specifically to keratinocytes of the epidermis and hair follicle (HF). Expression of hMR throughout gestation resulted in early postnatal death that could be prevented by antagonizing MR signaling. DT mice exhibited premature epidermal barrier formation at embryonic day 16.5, reduced HF density and epidermal atrophy, increased keratinocyte apoptosis at embryonic day 18.5, and premature eye opening. When hMR expression was initiated after birth to overcome mortality, DT mice developed progressive alopecia and HF cysts, starting 4 months after hMR induction, preceded by dystrophy and cycling abnormalities of pelage HF. In contrast, interfollicular epidermis, vibrissae, and footpad sweat glands in DT mice were normal. This new mouse model reveals novel biological roles of MR signaling and offers an instructive tool for dissecting nonclassical functions of MR signaling in epidermal, hair follicle, and ocular physiology.

cPGES/p23 is required for glucocorticoid receptor function and embryonic growth but not prostaglandin E2 synthesis

A number of studies have identified cytosolic prostaglandin E(2) synthase (cPGES)/p23 as a cytoplasmic protein capable of metabolism of prostaglandin E(2) (PGE(2)) from the cyclooxygenase metabolite prostaglandin endoperoxide (PGH(2)). However, this protein has also been implicated in a number of other pathways, including stabilization of the glucocorticoid receptor (GR) complex. To define the importance of the functions assigned to this protein, mice lacking detectible cPGES/p23 expression were generated. cPGES/p23(-/-) pups die during the perinatal period and display retarded lung development reminiscent of the phenotype of GR-deficient neonates. Furthermore, GR-sensitive gluconeogenic enzymes are not induced in the prenatal period. However, unlike GR-deficient embryos, cPGES/p23(-/-) embryos are small and a proliferation defect is observed in cPGES/p23(-/-) fibroblasts. Analysis of arachidonic acid metabolites in embryonic tissues and primary fibroblasts failed to support a function for this protein in PGE(2) biosynthesis. Thus, while the growth retardation of the cPGES/p23(-/-) pups and decreased proliferation of primary fibroblasts identify functions for this protein in addition to GR stabilization, it is unlikely that these functions include metabolism of PGH(2) to PGE(2).

Lipid defect underlies selective skin barrier impairment of an epidermal-specific deletion of Gata-3

Skin lies at the interface between the complex physiology of the body and the external environment. This essential epidermal barrier, composed of cornified proteins encased in lipids, prevents both water loss and entry of infectious or toxic substances. We uncover that the transcription factor GATA-3 is required to establish the epidermal barrier and survive in the ex utero environment. Analysis of Gata-3 mutant transcriptional profiles at three critical developmental stages identifies a specific defect in lipid biosynthesis and a delay in differentiation. Genomic analysis identifies highly conserved GATA-3 binding sites bound in vivo by GATA-3 in the first intron of the lipid acyltransferase gene AGPAT5. Skin from both Gata-3-/- and previously characterized barrier-deficient Kruppel-like factor 4-/- newborns up-regulate antimicrobial peptides, effectors of innate immunity. Comparison of these animal models illustrates how impairment of the skin barrier by two genetically distinct mechanisms leads to innate immune responses, as observed in the common human skin disorders psoriasis and atopic dermatitis.

Klf4 and corticosteroids activate an overlapping set of transcriptional targets to accelerate in utero epidermal barrier acquisition

Premature infants are at an increased risk for infections and dehydration because of incomplete development of the epidermis, which attains its essential function as a barrier only during the last stages of in utero development. When a premature birth is anticipated, antenatal corticosteroids are administered to accelerate lung epithelium differentiation. One pleiotropic, but beneficial, effect of antenatal corticosteroids is acceleration of skin barrier establishment by an unknown mechanism. In mice, the transcription factor Klf4 is both necessary and sufficient, within a developmental field of competence, to establish this skin barrier, as demonstrated by targeted ablation and transgenic expression of Klf4, respectively. Here, we report that Klf4 and corticosteroid treatment coordinately accelerate barrier acquisition in vivo. Transcriptional profiling reveals that the genes regulated by corticosteroids and Klf4 during the critical window of epidermal development significantly overlap. KLF4 activates the proximal promoters of a significant subset of these genes. Dissecting the intersection of the genetic and pharmacological pathways, regulated by KLF4 and corticosteroids, respectively, leads to a mechanistic understanding of the normal process of epidermal development in utero.

The Hsp90 cochaperone p23 is essential for perinatal survival

The functions of molecular chaperones have been extensively investigated biochemically in vitro and genetically in bacteria and yeast. We have embarked on a functional genomic analysis of the Hsp90 chaperone machine in the mouse by disrupting the p23 gene using a gene trap approach. p23 is an Hsp90 cochaperone that is thought to stabilize Hsp90-substrate complexes and, independently, to act as the cytosolic prostaglandin E2 synthase. Gene deletions in budding and fission yeasts and knock-down experiments with the worm have not revealed any clear in vivo requirements for p23. We find that p23 is not essential for overall prenatal development and morphogenesis of the mouse, which parallels the observation that it is dispensable for proliferation in yeast. In contrast, p23 is absolutely necessary for perinatal survival. Apart from an incompletely formed skin barrier, the lungs of p23 null embryos display underdeveloped airspaces and substantially reduced expression of surfactant genes. Correlating with the known function of glucocorticoids in promoting lung maturation and the role of p23 in the assembly of a hormone-responsive glucocorticoid receptor-Hsp90 complex, p23 null fibroblast cells have a defective glucocorticoid response. Thus, p23 contributes a nonredundant, temporally restricted, and tissue-specific function during mouse development.

Epidermal barrier formation and recovery in skin disorders

Skin is at the interface between the complex physiology of the body and the external, often hostile, environment, and the semipermeable epidermal barrier prevents both the escape of moisture and the entry of infectious or toxic substances. Newborns with rare congenital barrier defects underscore the skin's essential role in a terrestrial environment and demonstrate the compensatory responses evoked ex utero to reestablish a barrier. Common inflammatory skin disorders such as atopic dermatitis and psoriasis exhibit decreased barrier function, and recent studies suggest that the complex response of epidermal cells to barrier disruption may aggravate, maintain, or even initiate such conditions. Either aiding barrier reestablishment or dampening the epidermal stress response may improve the treatment of these disorders. This Review discusses the molecular regulation of the epidermal barrier as well as causes and potential treatments for defects of barrier formation and proposes that medical management of barrier disruption may positively affect the course of common skin disorders.

Connexin 26 regulates epidermal barrier and wound remodeling and promotes psoriasiform response

Inflammatory skin disorders result in significant epidermal changes, including keratinocyte hyperproliferation, incomplete differentiation, and impaired barrier. Here we test whether, conversely, an impaired epidermal barrier can promote an inflammatory response. Mice lacking the transcription factor Kruppel-like factor 4 (Klf4) have a severe defect in epidermal barrier acquisition. Transcription profiling of Klf4(-/-) newborn skin revealed similar changes in gene expression to involved psoriatic plaques, including a significant upregulation of the gap junction protein connexin 26 (Cx26). Ectopic expression of Cx26 from the epidermis-specific involucrin (INV) promoter (INV-Cx26) demonstrated that downregulation of Cx26 is required for barrier acquisition during development. In juvenile and adult mice, persistent Cx26 expression kept wounded epidermis in a hyperproliferative state, blocked the transition to remodeling, and led to an infiltration of immune cells. Mechanistically, ectopic expression of Cx26 in keratinocytes resulted in increased ATP release, which delayed epidermal barrier recovery and promoted an inflammatory response in resident immune cells. These results provide a molecular link between barrier acquisition in utero and epidermal remodeling after wounding. More generally, these studies suggest that the most effective treatments for inflammatory skin disorders might concomitantly suppress the immune response and enhance epidermal differentiation to restore the barrier.

Antenatal steroids and fluid balance in very low birthweight infants

OBJECTIVES

To determine if insensible water loss (IWL) differed between infants exposed or not exposed antenatally to corticosteroids and to explore possible mechanisms for the early postnatal diuresis associated with antenatal steroid exposure.

DESIGN

Retrospective analysis of prospectively collected data.

SETTING

Level three neonatal intensive care unit.

PATIENTS

Ninety six infants, median gestational age 27.5 weeks (range 23-33).

MAIN OUTCOME MEASURES

Comparison of the IWL, urine output and osmolality, fluid input, electrolyte imbalance, respiratory illness severity (as assessed by surfactant requirement, maximum peak inspiratory pressure, and inspired oxygen concentration), and cardiovascular status (as assessed by inotrope requirement) between infants with antenatal corticosteroid exposure and gestational age matched controls.

RESULTS

The infants exposed to antenatal steroids differed significantly from the controls in having both a lower IWL (p = 0.0135) and a higher urine output (p = 0.0036) on day 1, and fewer developed hyponatraemia (p = 0.027) on day 2. Fewer of those exposed to antenatal steroids required inotropes (p = 0.06), but their respiratory status was similar to that of the controls.

CONCLUSIONS

Infants exposed to antenatal corticosteroids have a lower IWL. The results suggest that greater skin maturation, but also better perfusion rather than less severe respiratory status, explains the early diuresis in infants exposed to antenatal steroids.

Complex redundancy to build a simple epidermal permeability barrier

To survive the transition from an aqueous in utero to a terrestrial ex utero environment, mice and humans must construct an epidermal permeability barrier in utero. Terminally differentiated epidermal cells, lipids and tight junctions are all essential to achieve this barrier. Recent analyses of mouse mutants with defects in structural components of the terminally differentiated epidermal cell, catalyzing enzymes, lipid processing, transcriptional regulators and the intercellular junctions have highlighted their essential function in establishing the epidermal permeability barrier. Particularly interesting examples include modulation of the expression of transglutaminase 1 enzyme, the transcription factor Klf4 and the claudin tight junction proteins. However, careful analysis of the various mutant phenotypes during embryonic development, as neonates and either as adults or transplanted skin, has revealed much more about the redundancy and compensatory mechanisms of the system. Molecular analysis of the various mouse mutants has demonstrated common pathways to compensate for loss of the epidermal barrier.

Stratum corneum acidification in neonatal skin: secretory phospholipase A2 and the sodium/hydrogen antiporter-1 acidify neonatal rat stratum corneum

At birth, human stratum corneum (SC) displays a near-neutral surface pH, which declines over several days to weeks to months to an acidic pH, comparable to that of adults. Recent studies suggest that an acidic pH is required for normal permeability barrier homeostasis and SC integrity/cohesion. We assessed here the basis for postnatal acidification in the neonatal rat, where SC pH, as measured with a flat surface electrode, declines progressively from near-neutral levels (pH 6.63) on postnatal days 0 to 1 to adult levels (pH 5.9) or even below over the subsequent 7 to 8 d. The postnatal decline in SC pH was paralleled by a progressive activation of a pH-dependent hydrolytic enzyme, beta-glucocerebrosidase. Because SC acidification could not be linked to commonly implicated exogenous factors, such as bacterial colonization, or the deposition of sebaceous gland products. We next assessed whether changes in one or more of three endogenous mechanisms demonstrate postnatal activity changes that contribute to the progressive development of an acidic SC pH. Although the histidine-to-urocanic acid pathway has been implicated in acidification of the adult SC, surface pH is completely normal in histidase-deficient (his/his, Peruvian) mice, ruling out a requirement for this mechanism. In contrast, when sodium/hydrogen antiporter-1 (NHE1), which predominantly acidifies membrane domains at the stratum granulosum-SC interface, is inhibited, postnatal acidification of the SC is partially blocked. Likewise, SC secretory phospholipase A2 (sPLA2) activity, measured with a fluorometric assay, is low at birth, but increases progressively (by 66%) over the first 5 d after birth, and inhibition of sPLA2 between days 0 to 1 and days 5 to 6 delays postnatal SC acidification. Together, these results describe a neonatal model, in which the development of an acidic surface pH can be ascribed, in part, to progressive SC acidification by two endogenous mechanisms, namely, sPLA2 and NHE1, which are known to be important for acidification of adult rodent SC. Conversely, the impaired acidification of neonatal SC, which has important functional and clinical consequences, can be explained by the relatively low activities of one or both of these mechanisms at birth.

Impaired skin wound healing in peroxisome proliferator-activated receptor (PPAR)alpha and PPARbeta mutant mice

We show here that the alpha, beta, and gamma isotypes of peroxisome proliferator-activated receptor (PPAR) are expressed in the mouse epidermis during fetal development and that they disappear progressively from the interfollicular epithelium after birth. Interestingly, PPARalpha and beta expression is reactivated in the adult epidermis after various stimuli, resulting in keratinocyte proliferation and differentiation such as tetradecanoylphorbol acetate topical application, hair plucking, or skin wound healing. Using PPARalpha, beta, and gamma mutant mice, we demonstrate that PPARalpha and beta are important for the rapid epithelialization of a skin wound and that each of them plays a specific role in this process. PPARalpha is mainly involved in the early inflammation phase of the healing, whereas PPARbeta is implicated in the control of keratinocyte proliferation. In addition and very interestingly, PPARbeta mutant primary keratinocytes show impaired adhesion and migration properties. Thus, the findings presented here reveal unpredicted roles for PPARalpha and beta in adult mouse epidermal repair.

Influence of antenatal steroids and sex on maturation of the epidermal barrier in the preterm infant

BACKGROUND

The epidermal barrier is well developed in term infants but defective in the immature infant with important clinical consequences. The development of the barrier shares similarities with production of pulmonary surfactant. Studies in the rat have shown that barrier maturation is accelerated by antenatal steroids, both structurally and functionally. Females have a more mature barrier than males at the same gestational age. These factors have not been studied in the human.

AIM

To examine the influence of antenatal steroids and sex on maturation of the epidermal barrier in the preterm infant.

SUBJECTS

A total of 137 infants born before 34 weeks gestation, 80 boys and 57 girls, were studied: 87 had been exposed to antenatal steroids, and 50 had not; 99 were studied prospectively, and 38 had been studied previously.

METHOD

Barrier function was measured as transepidermal water loss from abdominal skin by evaporimetry. Measurements were made within the first 48 hours and corrected to a standard relative humidity of 50% (TEWL(50)).

RESULTS

The relation between TEWL(50) and gestation was exponential with very high levels in the most immature infants. No influence of antenatal steroids or sex could be shown. When infants who were optimally exposed to antenatal steroids were considered alone, no effect could be shown.

CONCLUSION

Epidermal maturation in the preterm infant does not appear to be influenced by antenatal steroids or sex, suggesting that the mechanism of maturation differs from that of the rat.

Effect of prenatal steroids on potassium balance in extremely low birth weight neonates

OBJECTIVE

Potassium is the most abundant intracellular cation and plays an important role in a variety of cell functions. Potassium homeostasis and regulation are important aspects of fluid and electrolyte homeostasis in extremely low birth weight (ELBW) infants. Because prenatal steroid (PNS) treatment promotes maturation of many epithelial cell systems, we sought to determine whether PNS affects potassium homeostasis in ELBW infants (<1000 g) during the first week of life.

METHOD

Serum potassium (SK) concentration, potassium intake and output, and renal clearance were collected prospectively each day during the first week of life. Infants whose mothers received a full course of steroids before delivery (PNS group: n = 16) were compared with those infants whose mothers did not receive steroids (nonsteroid group [NSG]: n = 14). The decision to treat with PNS was made entirely by the obstetric staff in a nonrandomized manner. Potassium intake and excretion and serum and urine electrolytes were measured every 12 hours, and urine output was monitored every 2 to 3 hours. Hyperkalemia was defined as SK >6. 5 mmol/L in a nonhemolyzed sample on at least 1 measurement from a central line.

RESULTS

There were no significant differences between the groups in gestational age, Apgar score, and birth weight. SK increased initially after birth in the absence of exogenous K intake in all infants, then subsequently decreased and stabilized by day 4 of life. The peak SK was significantly lower in the PNS group than in the NSG group (5.2 +/-.2 mmol/L vs 6.2 +/-.4 mmol/L). Moreover, the peak SK was higher than 6.5 mmol/L in 70% of the NSG infants and in none of the PNS group. Hyperkalemia occurred in the NSG infants within the first 2 days when urine output was significantly lower than in PNS infants. SK peaked in the absence of potassium intake with similar potassium excretion in both groups. PNS infants had similar cumulative potassium intake with a lower cumulative potassium excretion than did NSG infants. PNS infants had a significantly less negative potassium balance than did NSG infants by day 7 of life (-1.0 mmol/kg vs -7.0 mmol/kg). There was no statistical difference in the daily serum creatinine levels, fractional excretion of potassium, and in the daily creatinine clearance between the 2 groups.

CONCLUSION

We conclude that treatment with PNS prevents the nonoliguric hyperkalemia known to occur in ELBW neonates. We speculate that PNS induces upregulation of cell membrane sodium, potassium-adenosinetriphosphatase activity in the fetus. The differences in negative potassium balance may be accounted for by stabilization of cell membranes that may result in a decrease in potassium shift from intracellular to extracellular compartments.

Fetal epidermal differentiation and barrier development In vivo is accelerated by nuclear hormone receptor activators

Nuclear receptors which interact with the retinoid X receptor are involved in the regulation of epidermal differentiation and development. We have recently shown that activators of the peroxisome proliferator-activated receptor and of the farnesoid X-activated receptor accelerate epidermal barrier maturation in fetal rat skin in vitro. In this study we asked whether cutaneous development in utero was affected by peroxisome proliferator-activated receptor or farnesoid X-activated receptor activators, or by an activator of another retinoid X receptor partner, liver X receptor. Activators of the peroxisome proliferator-activated receptor (clofibrate or linoleic acid), farnesoid X-activated receptor (farnesol or juvenile hormone III), or liver X receptor (22R-hydroxycholesterol), were injected into the amniotic fluid of fetal rats on gestational day 17. Fetal epidermal barrier function and morphology was assessed on day 19. Whereas vehicle-treated fetal rats displayed no measurable barrier (transepidermal water loss > 10 mg per cm2 per h), a measurable barrier was induced by the intra-amniotic administration of all activators tested (transepidermal water loss range 4.0-8.5 mg per cm2 per h). By light microscopy, control pups lacked a well-defined stratum corneum, whereas a distinct stratum corneum and a thickened stratum granulosum were present in treated pups. By electron microscopy, the extracellular spaces of the stratum corneum in control pups revealed a paucity of mature lamellar unit structures, whereas these structures filled the stratum corneum interstices in treated pups. Additionally, protein and mRNA levels of loricrin and filaggrin, two structural proteins of stratum corneum, were increased in treated epidermis, as were the activities of two lipid catabolic enzymes critical to stratum corneum function, beta-glucocerebrosidase and steroid sulfatase. Finally, peroxisome proliferator-activated receptor-alpha and -delta and liver X receptor-alpha and -beta mRNAs were detected in fetal epidermis by reverse transcriptase-polymerase chain reaction and northern analyses. The presence of these receptors and the ability of their activators to stimulate epidermal barrier and stratum corneum development suggest a physiologic role for peroxisome proliferator-activated receptor and liver X receptor and their endogenous ligands in the regulation of cutaneous development.

Effects of prenatal steroids on water and sodium homeostasis in extremely low birth weight neonates

OBJECTIVE

We sought to determine if prenatal steroid (PNS) treatment affects water and sodium (Na) balance in extremely low birth weight infants (<1000 g).

METHODS

PNS treatment enhances lung maturation in preterm infants and induces maturation of renal tubular function and adenylate cyclase activity in animals. We compared water and Na homeostasis for the first week of life in those infants whose mothers received steroids before delivery (PNS: n = 16) to those who did not (nonsteroid group [NSG]: n = 14). The data were collected prospectively, but PNS treatment was not given in a randomized manner. Fluids were initiated at 100 to 125 mL/kg/d and adjusted every 8 to 12 hours to allow a daily weight loss of </=4% of birth weight and to maintain normal serum electrolytes. Weight, serum and urine electrolytes, and urine output were frequently measured and fluid intake was adjusted by increasing the amount of free water to achieve these goals.

RESULTS

When using our fluid management protocol, the percent weight loss in both groups was equivalent during each of the 7 days (15% PNS vs 17% NSG maximum loss) as well as the cumulative urine output at 1 week of age (663 mL/kg/wk PNS vs 681 mL/kg/wk NSG). PNS infants had a higher urine output on the first 2 days of life and a lower daily fluid intake for the first week. PNS infants also had significantly less insensible water loss for each of the first 4 days of life. The PNS group had a significantly lower mean peak serum Na of 138 +/- 1 mmol/L vs 144 +/- 2 mmol/L and none had a peak serum Na >150 mmol/L compared with 36% of the NSG infants. PNS infants had a higher cumulative Na excretion at day 2 of life (10 +/- 2 mmol/kg vs 6 +/- 1 mmol/kg) but a less negative cumulative Na balance at 1 week (-10 mmol/kg vs -14 mmol/kg).

CONCLUSION

PNS treatment was associated with lower estimated insensible water loss, a decreased incidence of hypernatremia, and an earlier diuresis and natriuresis in extremely low birth weight neonates. We speculate that PNS effects these changes through enhancement of epithelial cell maturation improving skin barrier function. PNS treatment may also enhance lung Na, K-ATPase activity leading to an earlier postnatal reabsorption of fetal lung fluid increasing extracellular volume expansion to help prevent hypernatremia.

Ligands and activators of nuclear hormone receptors regulate epidermal differentiation during fetal rat skin development

Because a protective barrier is essential for life, the development of the epidermis and stratum corneum must be completed prior to birth. The epidermal permeability barrier is comprised of corneocytes embedded in a lipid enriched matrix. Recent studies from our laboratory, using an explant model of fetal rat skin development that closely parallels in utero development, have shown that hormones and other activators of members of the nuclear receptor family regulate permeability barrier ontogenesis by stimulating lipid metabolism and the formation of the extracellular lipid lamellae. Using this model we sought to determine whether these hormones and nuclear activators also regulate keratinocyte differentiation during fetal development. Profilaggrin/filaggrin and loricrin expression, assessed by in situ hybridization and by immunohistochemistry, were progressively increased during epidermal ontogenesis. Whereas profilaggrin/filaggrin and loricrin were not expressed at day 17 of gestation, by day 19 both were present in the upper layers of the epidermis and both became still more abundant by day 21. These developmental changes also occurred in fetal skin explants cultured in vitro for 4 d, although the expression levels did not appear as robust as in utero. Whereas neither profilaggrin/filaggrin nor loricrin were expressed in control explants cultured for 2 d, they were seen in explants treated with either thyroid hormone, glucocorticoids, or estrogens. In contrast, dihydrotestosterone treatment delayed the expression of profilaggrin/filaggrin and loricrin. Moreover, both clofibrate, a peroxisome proliferator-activated receptor-alpha ligand, and juvenile hormone III, a farnesoid X-activated receptor activator, markedly accelerated fetal epidermal differentiation, stimulating both profilaggrin/filaggrin and loricrin expression. Our results demonstrate that several hormones and activators of nuclear hormone receptors regulate epidermal differentiation during fetal development, affecting key constituents of both keratohyalin granules and the cornified envelope. Thus, a variety of ligands/activators of nuclear receptors accelerate not only permeability barrier ontogenesis, but also the expression of structural proteins essential for stratum corneum formation.

Glucocorticoid deficiency delays stratum corneum maturation in the fetal mouse

The stratum corneum (SC) matures during late gestation in man and other mammals. Using the fetal rat as an experimental model, we have previously shown that glucocorticoids given in pharmacologic doses accelerate fetal SC maturation and barrier formation. To determine whether glucocorticoids are required for normal SC maturation, we examined the epidermal morphology of glucocorticoid-deficient (C-) murine pups, derived from matings of mice homozygous for null mutations of the corticotropin-releasing hormone alleles. In control pups on day 17.5 of gestation (term is 19.5 d), a multilayered SC was present and neutral lipid deposition in a membrane pattern was observed using Nile red fluorescence histochemistry. Ultrastructurally, mature lamellar unit structures predominate in the SC intercellular domains. In contrast, in C-pups only a single layer of SC was evident on day 17.5, and secreted lamellar material was not organized into mature lamellar structures. Furthermore, the expression of structural proteins necessary for cornified envelope formation, involucrin, loricrin, and filaggrin, and the activity of the lipid synthetic enzymes beta-glucocerebrosidase and steroid sulfatase, markers of barrier maturation, were reduced in day 17.5 C-pups. C-pups derived from pregnancies supplemented with physiologic amounts of cortisone, however, display normal SC ultrastructure on day 17.5 of gestation. Furthermore, at birth, both control and C-pups exhibit a multilayered SC replete with mature lamellar membrane structures. These data demonstrate that fetal glucocorticoid deficiency delays SC maturation, and suggests that normal levels of glucocorticoids are not absolutely required for SC development.

Development of skin barrier function in premature infants

Histologic analysis suggests that epidermal development is complete in utero at approximately 34 wk gestational age. Infants born more prematurely have elevated rates of both transepidermal water loss and transcutaneous heat loss, and have difficulty maintaining homeostasis. The underdeveloped integument is also a portal of entry for infection and the percutaneous uptake of toxins. Previous measurements of transepidermal water loss have suggested that, regardless of gestational age, competent barrier function is attained within 2-4 wk postnatal age. In this study we have utilized another noninvasive biophysical technique, low frequency impedance spectroscopy, to complement transepidermal water loss measurements. We present longitudinal data from infants ranging from 23 to 32 wk gestational age. The results suggest that, for ultra-low birth weight infants (23-25 wk gestational age), the complete development of a fully functional stratum corneum can require significantly longer than 4 wk. In contrast, the data from the older infants suggest that a postnatal existence period of 2-4 wk may not be necessary to attain functional maturity, because infants born at 30 and 32 wk gestational age were found to have barrier function comparable with that of adults.

Patterned acquisition of skin barrier function during development

Skin barrier function is conferred by the outer layer of epidermis, the stratum corneum, and is essential for terrestrial life. Quantitative trans-epidermal water loss assays show that barrier forms late in embryogenesis, permitting the foetus to survive a terrestrial environment at birth. Using qualitative in situ assays for skin permeability, we show that barrier forms in a patterned manner late in mouse gestation. Barrier forms at specific epidermal sites, then spreads around the embryo as a moving front. The moving front of permeability change is accompanied by multiple changes in the outer, stratum corneum-precursor cells. We use the permeability assays to show that final stages of cornified envelope assembly are coordinated with initial stages of barrier formation. Hence the whole-mount permeability assays record developmental acquisition of a known, essential component of the adult barrier. We demonstrate the authenticity of the whole-mount assays after maternal glucocorticoid therapy (known to accelerate barrier formation) and in additional species including the rat where barrier formation is well characterized by TEWL assay (Aszterbaum, M., Menon, G. K., Feingold, K. R. and Williams, M. L. Pediatr. Res. 31, 308–317). The demonstration of patterned barrier formation in other species suggests patterned change as the universal mode of embryonic barrier acquisition. These results highlight the importance of patterning as a mode of epidermal maturation during development.

Keratinocyte differentiation is stimulated by activators of the nuclear hormone receptor PPARalpha

Peroxisome proliferator activated receptors (PPAR) belong to the superfamily of nuclear hormone receptors that heterodimerize with the retinoid X receptor and regulate transcription of several genes involved in lipid metabolism and adipocyte differentiation. Because of the role of 1,25-dihydroxyvitamin D3 and retinoic acid working through similar receptors (the vitamin D receptor and retinoic acid receptor, respectively) on keratinocyte differentiation, we have examined the effects of activators of PPARalpha on keratinocyte differentiation. The rate of cornified envelope formation was increased 3-fold in keratinocytes maintained in low calcium (0.03 mM) and incubated in the presence of clofibric acid, a potent PPARalpha activator. Involucrin, a cornified envelope precursor, and the cross-linking enzyme transglutaminase, were increased at both the message level (2-7-fold) and the protein level (4-12-fold) by clofibric acid. Furthermore, physiologic doses of the fatty acids oleic acid, linoleic acid, and eicosatetraynoic acid, which are also activators of PPARalpha, also induced involucrin and transglutaminase protein and mRNA. In contrast, the PPARgammaligand prostaglandin J2 had no effect on protein or mRNA levels of involucrin or transglutaminase. Levels of involucrin and transglutaminase mRNA and protein were induced by clofibric acid in keratinocytes incubated in 1.2 mM calcium, a concentration which by itself induces keratinocyte differentiation. Finally, PPARalpha activators inhibit DNA synthesis. This study demonstrates that PPARalpha activators, including putative endogenous ligands such as fatty acids, induce differentiation and inhibit proliferation in keratinocytes, and suggests a regulatory role for the PPARalpha in epidermal homeostasis.

Hypothyroidism delays fetal stratum corneum development in mice

The epidermal permeability barrier, required for terrestrial life, is localized to lipid-enriched lamellar membranes in the extracellular spaces of the stratum corneum (SC). Immaturity of the SC is a significant contributor to morbidity and mortality in premature infants. Previous studies have shown that supraphysiologic concentrations of thyroid hormone accelerate epidermis/SC ontogenesis. Here we studied SC development in Hyt/Hyt mice who are genetically hypothyroid due to a mutation in the TSH receptor. In control mice on d 18 of gestation (term 19.5 d), only focal areas displayed a mature SC membrane pattern. By 19 d of gestation there was a mature multilayered SC with lamellar unit structures filling the extracellular spaces similar to that seen in mature mice. In Hyt/Hyt mice SC development was delayed at both 18 and 19 d of gestation. In both strains of mice, within the first day after birth there were no differences in epidermal or SC appearance, and the SC was fully mature. These findings indicate that thyroid hormone plays a physiologic role during normal intrauterine development of the SC. However, normal SC maturation ultimately occurs, indicating that thyroid hormone is not absolutely essential. Previous studies have shown that glucocorticoids accelerate SC development in euthyroid rats, and in the present study we demonstrate that glucocorticoids also accelerate SC ontogenesis in euthyroid mice. In contrast, in Hyt/Hyt mice glucocorticoids did not accelerate or normalize SC development, indicating that the glucocorticoid effect on SC maturation requires a euthyroid state or that glucocorticoids act via thyroid hormone. These studies demonstrate that thyroid hormone status is an important regulator of fetal SC development.