Frederic Bonte

Skin hydration: a review on its molecular mechanisms

Water is absolutely essential for the normal functioning of the skin and especially its outer layer, the stratum corneum (SC). Loss of water from the skin must be carefully regulated, a function dependent on the complex nature of the SC. The retention of water in the SC is dependent on two major components: (1) the presence of natural hygroscopic agents within the corneocytes (collectively referred to as natural moisturizing factor) and (2) the SC intercellular lipids orderly arranged to form a barrier to transepidermal water loss (TEWL). The water content of the SC is necessary for proper SC maturation and skin desquamation. Increased TEWL impairs enzymatic functions required for normal desquamation resulting in the visible appearance of dry, flaky skin. There have been recent discoveries regarding the complex mechanisms of skin hydration. In particular, it has been discovered that glycerol, a well‐known cosmetic ingredient, exists in the SC as a natural endogenous humectant. Hyaluronan, which has been regarded mainly as dermal component, is found in the epidermis and is important for maintaining normal SC structure and epidermal barrier function. More importantly, the discovery of the existence of the water‐transporting protein aquaporin‐3 in the viable epidermis and the presence of tight junction structures at the junction between the stratum granulosum and SC have brought new insights into the mechanisms of skin water distribution and barrier function.

Fibroblasts and myofibroblasts in wound healing

(Myo)fibroblasts are key players for maintaining skin homeostasis and for orchestrating physiological tissue repair. (Myo)fibroblasts are embedded in a sophisticated extracellular matrix (ECM) that they secrete, and a complex and interactive dialogue exists between (myo)fibroblasts and their microenvironment. In addition to the secretion of the ECM, (myo)fibroblasts, by secreting matrix metalloproteinases and tissue inhibitors of metalloproteinases, are able to remodel this ECM. (Myo)fibroblasts and their microenvironment form an evolving network during tissue repair, with reciprocal actions leading to cell differentiation, proliferation, quiescence, or apoptosis, and actions on growth factor bioavailability by binding, sequestration, and activation. In addition, the (myo)fibroblast phenotype is regulated by mechanical stresses to which they are subjected and thus by mechanical signaling. In pathological situations (excessive scarring or fibrosis), or during aging, this dialogue between the (myo)fibroblasts and their microenvironment may be altered or disrupted, leading to repair defects or to injuries with damaged and/or cosmetic skin alterations such as wrinkle development. The intimate dialogue between the (myo)fibroblasts and their microenvironment therefore represents a fascinating domain that must be better understood in order not only to characterize new therapeutic targets and drugs able to prevent or treat pathological developments but also to interfere with skin alterations observed during normal aging or premature aging induced by a deleterious environment.

The silver locus product (Silv⁄gp100⁄Pmel17) as a new tool for the analysis of melanosome transfer in human melanocyte-keratinocyte co-culture

Abstract:  Melanosomes are melanocyte‐specific lysosome‐related organelles that are transferred to keratinocytes of the epidermis and anagen hair bulb. Transferred melanin forms supra‐nuclear caps that protect epidermal keratinocytes against UV irradiation. The mechanism(s) responsible for melanosome transfer into keratinocytes and their subsequent intra‐keratinocyte distribution has long remained one of the most enigmatic of heterotypic cell interactions. Although there have been many attempts to study this process, significant progress has been hindered by the absence of an adequate in vitro model. During our ongoing study of melanocyte–keratinocyte interactions in skin and hair follicle, we have developed a novel in vitro assay that exploits the specificity of Silv/Pmel17/gp100 expression for melanosome/melanin granules. Using matched cultures of keratinocytes and melanocytes isolated from normal healthy epidermis together with double immunofluorescence, we have determined that gp100 is a surprisingly useful tracker of transferred melanin. Moreover, transferred gp100 stained melanin granules emit a bright fluorescence signal, facilitating ready quantification of melanin transfer levels between melanocytes and keratinocytes. This quantitative approach was validated using known inducers and inhibitors of the melanocyte phenotype. This assay further confirmed that cytophagocytosis of melanocyte components (e.g. dendrite tips) by keratinocytes is one route for melanin incorporation into keratinocytes. Lastly, a role for the recently proposed filopodium as a direct conduit for melanin transfer was substantiated using this novel approach. In conclusion, this assay promises to significantly aid our investigations of the molecular basis of melanosome transfer and offers a new tool for the clinical evaluation of melanocyte modulators.

Decreased expression of keratinocyte β1 integrins in chronically sun-exposed skin in vivo

Summary Background Chronic exposure to ultraviolet (UV) radiation induces changes in the skin structure which are mostly found in the superficial dermis and at the dermal–epidermal junction. Keratinocytes and fibroblasts contribute both to the synthesis and to the degradation of the molecules important for the integrity of this skin site. While several studies have reported on alterations of dermal components and of the functions of fibroblasts in vivo and in vitro after UV exposure, recent data suggested that keratinocytes could be the main skin cell type involved in the photoageing process.

Skin Aquaporins: Function in Hydration, Wound Healing, and Skin Epidermis Homeostasis

Several aquaporins (AQPs) are expressed in mammalian skin. Some are directly involved in water transport, such as AQP5, which is involved in sweat secretion. In contrast, the physiological role of skin aquaglyceroporins, which permeate both water and glycerol, appears more and more complex. AQP3 is the most abundant skin aquaglyceroporin. Both water and glycerol transport by AQP3 appear to play an important role in hydration of mammalian skin epidermis. In addition, recent data suggest that glycerol transport by AQP3 is involved in the metabolism of lipids in skin as well as in the regulation of proliferation and differentiation of keratinocytes. Finally, AQP3 is also believed to be important in wound healing, as a water channel by facilitating cell migration, and as a glycerol transporter by enhancing keratinocyte proliferation and differentiation.

Human epidermal cells progressively lose their cardiolipins during ageing without change in mitochondrial transmembrane potential

Mitochondria dysfunction is considered to be a major cause of the modifications that occur during cell ageing. For this reason, cardiolipin, a suitable marker of the chondriome, as well as the mitochondrial transmembrane potential were examined in keratinocytes obtained from 9- to 75-year-old women. The study was carried out by flow cytometry using two fluorescent mitochondria probes: nonyl acridine orange, which binds specifically to cardiolipin, and rhodamine 123, which is incorporated mainly in response to transmembrane potential. Cardiolipin levels in cells from elderly donors (75 years old) would be 57% lower (r = 0.540; P = 0.0002) than those in children (9 years old), while the inner transmembrane potential remained unchanged (r = 0.0394; P = 0.8017). The stability of the membrane potential may be explained by either or both of the following hypotheses: (i) the same pool of organelles able to maintain membrane potential is conserved even when cardiolipin levels decrease (ii) mitochondria membrane potential does indeed decrease with age but is compensated by glycolysis energy production. Finally, it may be stated that the fluorescent probes nonyl acridine orange and rhodamine 123 might be of interest in testing the phenotype of senescent cells and would be useful in screening the role of certain specific genes in cell ageing.

In vitro biosynthesis of type I and III collagens by human dermal fibroblasts from donors of increasing age

A quantitative study of type I and type III collagen production was carried out on primary cultures of human dermal fibroblasts. Cultures were initiated from facial and mammary skin of 29 women aged between 19 and 68 years. Secreted and cell-associated collagen levels were determined by an enzyme linked immunosorbent assay (ELISA). We found that the secretion of type I and type III collagen decreased linearly with age (r = 0.432; P = 0.0193 and r = 0.502; P = 0.0147, respectively). There was a 29% loss in secretion ability for type I and type III collagen over the 49-year period studied. Furthermore, no significant linear age-related decrease was observed for type I and type III collagen associated with the cellular fraction. The influence of body site was also analysed. We observed a significant linear age-related decrease in type I collagen secretion by mammary skin cells (P = 0.0183 and r = 0.618) as well as facial skin cells (P = 0.0037 and r = 0.699). Furthermore, only mammary skin fibroblasts showed a significant linear age-related decrease in secreted type III collagen (P = 0.106 and r = 0.513). No age-related variations in cell-associated collagen were found.

Thermotropic Phase Behavior of in vivo Extracted Human Stratum Corneum Lipids

The thermotropic phase behavior of lipids extracted either in vivo from inner forearm (SCLE) or plantar callus (PC) was investigated by differential scanning calorimetry and small angle X-ray diffraction. PC composition was chromatographically modified (MPC) by eliminating the more polar lipids in order to evaluate their role. Analysis of composition confirms the potential use of PC as a source of stratum corneum lipids. MPC and SCLE exhibit similar differential scanning calorimetry (DSC) profiles with a main transition around 50°C attributed to the solid-to-liquid phase transition of the ceramides. The absence of a transition around 50°C for PC suggests the possible perturbation of ceramide packing by the significantly high proportion of phospholipids. X-ray data suggest a high miscibility of sebum components in stratum corneum lipids with possible modification of chain packing. The MPC patterns show a lipid phase separation which underscores the role of polar lipids in cholesterol/free fatty acids/sterol esters/ceramides structural cohesion.

Simarouba amara extract increases human skin keratinocyte differentiation

An aqueous extract of Simarouba amara was studied for its activity on the differentiation of human skin keratinocytes. Submerged and air-exposed treated keratinocyte cultures displayed a more highly differentiated histoarchitecture, with presence of ultrastructural differentiated elements, than untreated controls. Immunohistochemistry of involucrin and activation of transglutaminase activity provided further evidence for the increase in corneocyte envelope formation observed ultrastructurally. Lipid analysis of air-exposed cultures revealed an increase in the cholesterol sulphate, cholesterol and ceramide contents. After 4 weeks of treatment on the hemiface of volunteers, the capacitance and transepidermal water loss evaluation revealed the potential interest of this extract for improvement of skin hydration. Electron microscopic examination of the corneocyte envelope on tape strips confirmed its actions. Taken together these data demonstrated that an aqueous extract of S. amara increases human keratinocyte differentiation.

Expression of Bcl‐2 and Bax in cultured normal human keratinocytes and melanocytes: relationship to differentiation and melanogenesis

Keratinocyte differentiation and melanogenesis are two major cellular processes by which the epidermal compartment of the skin acquires its protective properties. Bcl‐2, an oncoprotein involved in the regulation of apoptosis, has been shown to be expressed by keratinocytes and melanocytes. To determine whether Bcl‐2 and Bax, a protein which heterodimerizes with Bcl‐2, may control these epidermal functions, we investigated the expression of these two oncogenes in cultivated human keratinocytes and melanocytes from the same donors, respectively induced to differentiate and to produce melanin. As determined by cytometry, we observed that these two cell types constitutively express the two proto‐oncogenes. Quantification of Bcl‐2 antige n sites per cell showed that Bcl‐2 expression is higher in keratinocytes than in melanocytes. An increase in transglutaminase activity, a marker of keratinocyte terminal differentiation initiating cornified envelope formation, was accompained by a decrease in Bcl‐2 levels without significant modification of Bax expression. In paralleled Bcl‐2 down‐regulation and Bax up‐regulatin. This led us to conclude that the expression of these two oncogenes and their cellular ratio are closely involved in keratinocyte differentiation and melanogenesis.