to contribute to the barrier’s natural development and degradation.1,27–29 Indeed, in human epidermis, occlusive TJ are present in the upper stratum granulosum (SG) but appear to be expressed in a patchy pattern, usually not fully circumventing the flattened cell outlines. Nevertheless, these TJ strands, most frequently encountered in the last three living cell layers are able to hinder the outward penetration of tracers experimentally applied to the dermal side of the skin. As TJ expression coincides with the apically oriented migration of lamellar bodies, it may be speculated that epidermal TJ contribute to the SG cell polarization.30 All the transmembrane and cytoplasmic proteins necessary for formation of functional TJ are present in the SG and the functional junctions may be created instantly, eg, in case of acute abrogation of the principal permeability barrier in the SC.31 In human skin, TJ may thus participate in a regulatory mechanism of SC barrier formation and constitute an instantly available backup system when the SC barrier fails. The fact that these riveting structures become immobilized at the cell periphery during the process of cornification further underlines their importance for the SC barrier homeostasis. Increased number of TJ-like contacts may be observed in the SC after chemical challenge or in pathologies provoking abnormal SC formation, thus indicating a possible compensatory effect.32
Most recently, various signaling pathways involved in the epidermal development and maturation continue to be studied and still new molecular mechanisms contributing to normal and pathological barrier function are being discovered.33–37 A new exciting field of investigation concerns epigenetic regulation of the homeostatic mechanisms of epidermal proliferation/ differentiation leading to the barrier formation. Involvement of the non-coding micro-RNAs and lncRNAs in stabilization of these processes through modulation of the gene transcription adds a supplementary level to the complex mechanisms of the barrier control.38 Together, these findings highlight the dependence and synergy between different processes and behaviors within the epidermis to create a healthy, intact skin barrier. As such, irregularities to intrinsic mechanisms of the epidermis (eg, keratinocyte differentiation or tight junction formation), as well as SC disruptions through external means can trigger a chain of events that lead to prolonged barrier disorders.
Many of the data on molecular mechanisms underlying epidermal barrier function have been obtained using rodent models, either submitted to acute barrier disruption and/or bearing laboratory-induced genetic modifications. In many instances, conclusions drawn from these experiments remain fully valid as far as human skin is concerned. Nevertheless, the existing notable differences in skin morphology and physiology between the species make rather controversial some animal-derived observations. Human pathology, instead, provides a wide spectrum of situations where defined gene mutations result in abnormal expression of skin barrier’s constitutive or regulatory elements.8 These correlations may be advantageously exploited for a better understanding of the permeability barrier function and be a source of ideas for therapeutic intervention.37,39
Epidermis, Compromised Barrier, and Disease
Epidermal impairment can result from acute injury or exposure, or be linked to lifelong, chronic conditions that require daily attention. Virtually all dysfunctions of the epidermis, whether inborn or acquired, are associated with notable modifications of the permeability barrier. It is particularly evident in dermatoses with an important inflammatory component.40,41 In many cases, barrier dysfunction may be at the origin of a skin disease, like it is the case in atopic dermatitis (AD), and contributes to the vicious circle of a given pathology via induction of an inflammatory response.35,40 Deficient expression of an epidermal protein filaggrin, due to the loss-of-function gene mutations, has been found responsible for AD occurrence in up to 50% of the northern European cases.42 Filaggrin is elaborated in the granular layer keratinocytes and its catabolic processing in the SC leads to the abundance of hydrophilic amino acids constituting the bulk of so-called natural moisturizing factor (NMF).43 Absence or a marked reduction of the NMF compromises SC hydration and, thus, barrier function. Interestingly, the same filaggrin mutations present on both gene alleles result in ichthyosis vulgaris phenotype, most frequently associated with atopy. In the case of ichthyosis, the epidermis must compensate for the leaky barrier by hyperkeratosis. Accumulation of the corneocytes is likely promoted by a particularly low degree of SC hydration, possibly impeding activity of hydrolytic SC enzymes.25 This putative mechanism could overdrive the desquamation -favorable context of serine protease activation due to a more basic (optimal) intracellular pH in the amino acid-deficient tissue.44 Nano-mechanical and ultrastructural investigations of elastic properties of filaggrin deficient corneocytes demonstrate a significant reduction in the cell stiffness and a delayed degradation of corneodesmosomes, both being potential indicators of SC functionality.45,46 In addition to an alteration of filaggrin expression, AD epidermis also exhibits a significant reduction in key TJ proteins and, most importantly, ceramides, including CER1[EOS].13,14 Regarding the changes to ceramides, their decreased levels and shortening of their acyl chains have been observed in non-involved skin of AD, independent of filaggrin mutations, which may have etiologic significance. Altered ceramide expression levels and both their lamellar and lateral organization correlate with the disease activity (SCORAD).14 Even more depressed ceramide levels, mainly CER[EOS], CER[NP], and free sterols, have been reported in AD lesions, with concomitant increase of sphingosine (CER[S]) and sphinganine (CER[DS])-based ceramides.13 The observed changes may be due to modifications in pH and inflammatory cytokine-sensitive enzymes involved in lipid biosynthesis
Most recently, various signaling pathways involved in the epidermal development and maturation continue to be studied and still new molecular mechanisms contributing to normal and pathological barrier function are being discovered.33–37 A new exciting field of investigation concerns epigenetic regulation of the homeostatic mechanisms of epidermal proliferation/ differentiation leading to the barrier formation. Involvement of the non-coding micro-RNAs and lncRNAs in stabilization of these processes through modulation of the gene transcription adds a supplementary level to the complex mechanisms of the barrier control.38 Together, these findings highlight the dependence and synergy between different processes and behaviors within the epidermis to create a healthy, intact skin barrier. As such, irregularities to intrinsic mechanisms of the epidermis (eg, keratinocyte differentiation or tight junction formation), as well as SC disruptions through external means can trigger a chain of events that lead to prolonged barrier disorders.
Many of the data on molecular mechanisms underlying epidermal barrier function have been obtained using rodent models, either submitted to acute barrier disruption and/or bearing laboratory-induced genetic modifications. In many instances, conclusions drawn from these experiments remain fully valid as far as human skin is concerned. Nevertheless, the existing notable differences in skin morphology and physiology between the species make rather controversial some animal-derived observations. Human pathology, instead, provides a wide spectrum of situations where defined gene mutations result in abnormal expression of skin barrier’s constitutive or regulatory elements.8 These correlations may be advantageously exploited for a better understanding of the permeability barrier function and be a source of ideas for therapeutic intervention.37,39
Epidermis, Compromised Barrier, and Disease
Epidermal impairment can result from acute injury or exposure, or be linked to lifelong, chronic conditions that require daily attention. Virtually all dysfunctions of the epidermis, whether inborn or acquired, are associated with notable modifications of the permeability barrier. It is particularly evident in dermatoses with an important inflammatory component.40,41 In many cases, barrier dysfunction may be at the origin of a skin disease, like it is the case in atopic dermatitis (AD), and contributes to the vicious circle of a given pathology via induction of an inflammatory response.35,40 Deficient expression of an epidermal protein filaggrin, due to the loss-of-function gene mutations, has been found responsible for AD occurrence in up to 50% of the northern European cases.42 Filaggrin is elaborated in the granular layer keratinocytes and its catabolic processing in the SC leads to the abundance of hydrophilic amino acids constituting the bulk of so-called natural moisturizing factor (NMF).43 Absence or a marked reduction of the NMF compromises SC hydration and, thus, barrier function. Interestingly, the same filaggrin mutations present on both gene alleles result in ichthyosis vulgaris phenotype, most frequently associated with atopy. In the case of ichthyosis, the epidermis must compensate for the leaky barrier by hyperkeratosis. Accumulation of the corneocytes is likely promoted by a particularly low degree of SC hydration, possibly impeding activity of hydrolytic SC enzymes.25 This putative mechanism could overdrive the desquamation -favorable context of serine protease activation due to a more basic (optimal) intracellular pH in the amino acid-deficient tissue.44 Nano-mechanical and ultrastructural investigations of elastic properties of filaggrin deficient corneocytes demonstrate a significant reduction in the cell stiffness and a delayed degradation of corneodesmosomes, both being potential indicators of SC functionality.45,46 In addition to an alteration of filaggrin expression, AD epidermis also exhibits a significant reduction in key TJ proteins and, most importantly, ceramides, including CER1[EOS].13,14 Regarding the changes to ceramides, their decreased levels and shortening of their acyl chains have been observed in non-involved skin of AD, independent of filaggrin mutations, which may have etiologic significance. Altered ceramide expression levels and both their lamellar and lateral organization correlate with the disease activity (SCORAD).14 Even more depressed ceramide levels, mainly CER[EOS], CER[NP], and free sterols, have been reported in AD lesions, with concomitant increase of sphingosine (CER[S]) and sphinganine (CER[DS])-based ceramides.13 The observed changes may be due to modifications in pH and inflammatory cytokine-sensitive enzymes involved in lipid biosynthesis
