Alteration of tight junction proteins is an early event in psoriasis: putative involvement of proinflammatory cytokines.

Psoriasis is an inflammatory skin disease characterized by hyperproliferation of keratinocytes, impaired barrier function, and pronounced infiltration of inflammatory cells. Tight junctions (TJs) are cell-cell junctions that form paracellular barriers for solutes and inflammatory cells. Altered localization of TJ proteins in the epidermis was described in plaque-type psoriasis. Here we show that localization of TJ proteins is already altered in early-stage psoriasis. Occludin, ZO-1, and claudin-4 are found in more layers than in normal epidermis, and claudin-1 and -7 are down-regulated in the basal and in the uppermost layers. In plaque-type psoriasis, the staining patterns of occludin and ZO-1 do not change, whereas the claudins are further down-regulated. Near transmigrating granulocytes, all TJ proteins except for junctional adhesion molecule-A are down-regulated. Treatment of cultured keratinocytes with interleukin-1beta and tumor necrosis factor-alpha, which are present at elevated levels in psoriatic skin, results in an increase of transepithelial resistance at early time points and a decrease at later time points. Injection of interleukin-1beta into an ex vivo skin model leads to an up-regulation of occludin and ZO-1, resembling TJ protein alteration in early psoriasis. Our results show for the first time that alteration of TJ proteins is an early event in psoriasis and is not the consequence of the more profound changes found in plaque-type psoriasis. Our data indicate that cytokines are involved in alterations of TJ proteins observed in psoriasis.

Targeted disruption of the protein kinase SGK3/CISK impairs postnatal hair follicle development.

Members of the serum- and glucocorticoid-regulated kinase (SGK) family are important mediators of growth factor and hormone signaling that, like their close relatives in the Akt family, are regulated by lipid products of phosphatidylinositol-3-kinase. SGK3 has been implicated in the control of cell survival and regulation of ion channel activity in cultured cells. To begin to dissect the in vivo functions of SGK3, we generated and characterized Sgk3 null mice. These mice are viable and fertile, and in contrast to mice lacking SGK1 or Akt2, respectively, display normal sodium handling and glucose tolerance. However, although normal at birth, by postpartum day 4 they have begun to display an unexpected defect in hair follicle morphogenesis. The abnormality in hair follicle development is preceded by a defect in proliferation and nuclear accumulation of beta-catenin in hair bulb keratinocytes. Furthermore, in cultured keratinocytes, heterologous expression of SGK3 potently modulates activation of beta-catenin/Lef-1-mediated gene transcription. These data establish a role for SGK3 in normal postnatal hair follicle development, possibly involving effects on beta-catenin/Lef-1-mediated gene transcription.

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.

Actin reorganization is abnormal and cellular ATP is decreased in Hailey-Hailey keratinocytes.

Actin reorganization and the formation of adherens junctions are necessary for normal cell-to-cell adhesion in keratinocytes. Hailey-Hailey disease (HHD) is blistering skin disease, resulting from mutations in the Ca2+ ATPase ATP2C1, which controls Ca2+ concentrations in the cytoplasm and Golgi of human keratinocytes. Because actin reorganization is among the first responses to raised cytoplasmic Ca2+, we examined Ca2+-induced actin reorganization in normal and HHD keratinocytes. Even though HHD keratinocytes display raised baseline cytoplasmic Ca2+, we found that actin reorganization in response to Ca2+ was impaired in HHD keratinocytes. Defects in actin reorganization were linked to a marked decrease in cellular ATP in HHD keratinocytes, which persists, in vivo, in HHD epidermis. Defective actin reorganization was reproduced in normal keratinocytes in which the intracellular ATP concentration had been lowered pharmacologically. ATP concentrations in undifferentiated keratinocytes markedly declined after extracellular Ca2+ was increased, but then recovered to a new baseline that was approximately 150% of the previous baseline. In contrast, ATP concentrations in HHD keratinocytes did not change in response to increased extracellular Ca2+. This report provides new insights into how the ATP2C1-controlled ATP metabolism mediates Ca2+-induced cell-to-cell adhesion in normal keratinocytes. In addition, these findings implicate inadequate ATP stores as an additional cause in the pathogenesis of HHD and suggest novel therapeutic options.

Human keratinocyte ATP2C1 localizes to the Golgi and controls Golgi Ca2+ stores.

Hailey-Hailey disease (MIM16960) is a blistering skin disease caused by mutations in the Ca2+ ATPase ATP2C1. We found that the abnormal Ca2+ signaling seen in Hailey-Hailey disease keratinocytes correlates with decreased protein levels of ATP2C1. Human ATP2C1 protein approximated 115 kDa in size. The ATP2C1 is localized to the Golgi apparatus in human keratinocytes, similar to its localization in yeast and Caenorhabditis elegans. To test whether the ATP2C1 controls Golgi Ca2+ stores, we measured intraorganelle Ca2+ concentrations using specifically targeted aequorins. Whereas normal keratinocytes display Golgi Ca2+ levels comparable to other epithelial cells, Hailey-Hailey disease keratinocyte Golgi Ca2+ refill is slower, and the maximum Ca2+ concentration reached is significantly lower. These findings were replicated in vivo, because clinically normal Hailey-Hailey disease epidermis contained lower Ca2+ stores and displayed an abnormal Ca2+ gradient. In this report we localize the ATP2C1, demonstrate its physiologic relevance in mammalian cells, and measure intraorganelle Golgi Ca2+ in keratinocytes.

Neonatal development of the stratum corneum pH gradient: localization and mechanisms leading to emergence of optimal barrier function.

Although basal permeability barrier function is established at birth, the higher risk for infections, dermatitis, and percutaneous absorption of toxic agents may indicate incomplete permeability barrier maturation in the early neonatal period. Since stratum corneum (SC) acidification in adults is required for normal permeability barrier homeostasis, and lipid processing occurs via acidic pH dependent enzymes, we hypothesized that, in parallel with the less acidic surface pH, newborn SC would exhibit signs of incomplete barrier formation. Fluorescence lifetime imaging reveals that neonatal rat SC acidification first becomes evident by postnatal day 3, in extracellular "microdomains" at the SC- stratum granulosum (SG) interface, where pH-sensitive lipid processing is known to occur. This localized acidification correlated temporally with efficient processing of secreted lamellar body contents to mature extracellular lamellar bilayers. Since expression of the key acidifying mechanism NHE1 is maximal just prior to birth, and gradually declines over the first postnatal week, suboptimal SC acidification at birth cannot be attributed to insufficient NHE1 expression, but could instead reflect reduced NHE1 activity. Expression of the key lipid processing enzyme, beta-glucocerebrosidase (beta-GlcCer'ase), develops similar to NHE1, excluding a lack of beta-GlcCer'ase protein as rate limiting for efficient lipid processing. These results define a postnatal development consisting of initial acidification in the lower SC followed by outward progression, which is accompanied by formation of mature extracellular lamellar membranes. Thus, full barrier competence appears to require the extension of acidification in microdomains from the SC/SG interface outward toward the skin surface in the immediate postnatal period.

Parallelized TCSPC for dynamic intravital fluorescence lifetime imaging: quantifying neuronal dysfunction in neuroinflammation.

Two-photon laser-scanning microscopy has revolutionized our view on vital processes by revealing motility and interaction patterns of various cell subsets in hardly accessible organs (e.g. brain) in living animals. However, current technology is still insufficient to elucidate the mechanisms of organ dysfunction as a prerequisite for developing new therapeutic strategies, since it renders only sparse information about the molecular basis of cellular response within tissues in health and disease. In the context of imaging, Förster resonant energy transfer (FRET) is one of the most adequate tools to probe molecular mechanisms of cell function. As a calibration-free technique, fluorescence lifetime imaging (FLIM) is superior for quantifying FRET in vivo. Currently, its main limitation is the acquisition speed in the context of deep-tissue 3D and 4D imaging. Here we present a parallelized time-correlated single-photon counting point detector (p-TCSPC) (i) for dynamic single-beam scanning FLIM of large 3D areas on the range of hundreds of milliseconds relevant in the context of immune-induced pathologies as well as (ii) for ultrafast 2D FLIM in the range of tens of milliseconds, a scale relevant for cell physiology. We demonstrate its power in dynamic deep-tissue intravital imaging, as compared to multi-beam scanning time-gated FLIM suitable for fast data acquisition and compared to highly sensitive single-channel TCSPC adequate to detect low fluorescence signals. Using p-TCSPC, 256×256 pixel FLIM maps (300×300 µm(2)) are acquired within 468 ms while 131×131 pixel FLIM maps (75×75 µm(2)) can be acquired every 82 ms in 115 µm depth in the spinal cord of CerTN L15 mice. The CerTN L15 mice express a FRET-based Ca-biosensor in certain neuronal subsets. Our new technology allows us to perform time-lapse 3D intravital FLIM (4D FLIM) in the brain stem of CerTN L15 mice affected by experimental autoimmune encephalomyelitis and, thereby, to truly quantify neuronal dysfunction in neuroinflammation.

Major translocation of calcium upon epidermal barrier insult: imaging and quantification via FLIM/Fourier vector analysis.

Calcium controls an array of key events in keratinocytes and epidermis: localized changes in Ca(2+) concentrations and their regulation are therefore especially important to assess when observing epidermal barrier homeostasis and repair, neonatal barrier establishment, in differentiation, signaling, cell adhesion, and in various pathological states. Yet, tissue- and cellular Ca(2+) concentrations in physiologic and diseased states are only partially known, and difficult to measure. Prior observations on the Ca(2+) distribution in skin were based on Ca(2+) precipitation followed by electron microscopy, or proton-induced X-ray emission. Neither cellular and/or subcellular localization could be determined through these approaches. In cells in vitro, fluorescent dyes have been used extensively for ratiometric measurements of static and dynamic Ca(2+) concentrations, also assessing organelle Ca(2+) concentrations. For lack of better methods, these findings together build the basis for the current view of the role of Ca(2+) in epidermis, their limitations notwithstanding. Here we report a method using Calcium Green 5N as the calcium sensor and the phasor-plot approach to separate raw lifetime components. Thus, fluorescence lifetime imaging (FLIM) enables us to quantitatively assess and visualize dynamic changes of Ca(2+) at light-microscopic resolution in ex vivo biopsies of unfixed epidermis, in close to in vivo conditions. Comparing undisturbed epidermis with epidermis following a barrier insult revealed major shifts, and more importantly, a mobilization of high amounts of Ca(2+) shortly following barrier disruption, from intracellular stores. These results partially contradict the conventional view, where barrier insults abrogate a Ca(2+) gradient towards the stratum granulosum. Ca(2+) FLIM overcomes prior limitations in the observation of epidermal Ca(2+) dynamics, and will allow further insights into basic epidermal physiology.

CD44 regulates tight-junction assembly and barrier function.

Upon barrier disturbance, adult CD44 knockout (KO) mice show delayed recovery of epidermal barrier function. This correlates with the loss of apical polarization of lamellar body (LB) secretion. As tight junctions (TJs) are crucial for barrier function and regulate polarized targeting of vesicles, we hypothesized that CD44 regulates TJs and associated cell polarity complexes, which in turn contributes to altered skin barrier function in CD44 KO mice. We show a delay in embryonic barrier formation associated with a loss of apical LB localization in CD44 KO mice, which correlates with alterations in TJ proteins and Par3. Simultaneously, the activity of Rac1, a major regulator of TJ barrier function, was reduced. Importantly, normalization of barrier function at E18.5 coincided with the recovery of these proteins. Tape-stripping experiments revealed that the loss of CD44 also affected TJ proteins upon induced disturbance of the barrier in adult mice. In CD44 KO keratinocytes, cell polarization and TJ barrier function were impaired. An alteration of differentiation markers was also observed, but was less pronounced than alterations of TJ proteins. Taken together, the results reveal an important function for CD44 in the assembly and function of TJs, suggesting their involvement in the skin barrier phenotype of CD44 KO mice.

Regulation of epidermal tight-junctions (TJ) during infection with exfoliative toxin-negative Staphylococcus strains.

Tight Junction (TJ) proteins have been shown to exert a barrier function within the skin. Here, we study the fate of TJ proteins during the challenge of the skin by bacterial colonization and infection. We investigated the influence of various exfoliative toxin-negative Staphylococcus strains on TJ, adherens junction (AJ), desmosomal proteins, and actin in a human keratinocyte infection culture and in a porcine skin infection model. We found that the pathogen Staphylococcus aureus downregulates TJ and subsequently AJ and desmosomal proteins, including atypical protein kinase C, an essential player in TJ formation, at the cell-cell borders of keratinocytes in a time and concentration dependent manner. Little changes in protein and RNA levels were seen, indicating redistribution of proteins. In cultured keratinocytes, a reduction of transepithelial resistance was observed. Staphylococcus epidermidis shows only minor effects. All strains induced enhanced expression of occludin and ZO-1 at the beginning of colonization/infection. Thus, we demonstrate that TJ are likely to be involved in skin infection of exfoliative toxin-negative S. aureus. As we did not find a change in actin, and as changes of TJ preceded alterations of AJs and desmosomes, we suggest that S. aureus targets TJ.

Involucrin expression is decreased in Hailey-Hailey keratinocytes owing to increased involucrin mRNA degradation.

Hailey-Hailey disease (HHD) (MIM 16960) is an autosomal-dominant blistering skin disease caused by a mutation in the Ca2+-ATPase ATP2C1 (protein SPCA1), responsible for controlling Ca2+ concentrations in the cytoplasm and Golgi in human keratinocytes. Cytosolic Ca2+ concentrations, in turn, play a major role in the regulation of keratinocyte differentiation. To study how ATP2C1 function impacts keratinocyte differentiation, we assessed involucrin expression in HHD keratinocytes. Involucrin is a protein that makes up the cornified envelope of keratinocytes and is expressed in response to increased intracellular Ca2+ concentrations. Even though HHD keratinocytes suffer from abnormally high cytosolic Ca2+, we found that these cells expressed lower involucrin protein levels at both low and high extracellular Ca2+ concentrations when compared with normal control keratinocytes. Decreased involucrin protein levels were caused by lower involucrin mRNA levels in HHD keratinocytes. Decreased involucrin mRNA, in turn, was caused by increased rates of involucrin mRNA degradation. Ca2+-sensitive involucrin AP-1 promotor activity was increased, both in HHD keratinocytes and in an small interfering RNA (siRNA) experimental model, suggesting compensatory promoter upregulation in the face of increased mRNA degradation. This report provides new insights into differentiation defects in HHD and its relationship to Ca2+ signaling.

Defective cyclic guanosine monophosphate-gated calcium channels and the pathogenesis of psoriasis.

A positive association between intake of calcium channel blockers and psoriasis has been observed recently. Intake of blockers of voltage-gated calcium ion channels is associated with outbreaks of psoriasis after a latent period in patients with and without a previous family history of psoriasis. This suggests that interfering with calcium influx may trigger psoriasis. Calcium influx also occurs via cyclic guanosine monophosphate-gated channels; human keratinocytes contain functional and non-functional (splice variants) versions of these channels. We show here that keratinocytes and skin from psoriatic individuals express higher levels of mRNA encoding a non-functional cyclic guanosine monophosphate-gated calcium channel and that high expression of the splice variant by transfection of cells in culture leads to loss of protein expression for the functional cyclic guanosine monophosphate-gated Ca2+ channels.

Two-photon fluorescence lifetime imaging of the skin stratum corneum pH gradient.

Two-photon fluorescence lifetime imaging is used to identify microdomains (1-25 microm) of two distinct pH values within the uppermost layer of the epidermis (stratum corneum). The fluorophore used is 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF), whose lifetime tau (pH 4.5, tau = 2.75 ns; pH 8.5, tau = 3.90 ns) is pH dependent over the pH range of the stratum corneum (pH 4.5 to pH 7.2). Hairless mice (SKH1-hrBR) are used as a model for human skin. Images (< or =50 microm x 50 microm) are acquired every 1.7 microm from the stratum corneum surface to the first viable layer (stratum granulosum). Acidic microdomains (average pH 6.0) of variable size (~1 microm in diameter with variable length) are detected within the extracellular matrix of the stratum corneum, whereas the intracellular space of the corneocytes in mid-stratum corneum (25 microm diameter) approaches neutrality (average pH 7.0). The surface is acidic. The average pH of the stratum corneum increases with depth because of a decrease in the ratio of acidic to neutral regions within the stratum corneum. The data definitively show that the stratum corneum acid mantle results from the presence of aqueous acidic pockets within the lipid-rich extracellular matrix.

Glucosylceramide synthesis and synthase expression protect against ceramide-induced stress.

Ceramides (Cers), critical for epidermal barrier function, also can inhibit keratinocyte proliferation, while glucosylceramides (GlcCers) exert pro-mitogenic effects. Since alterations in Cer-to-GlcCer ratios appear to modulate cellular growth versus apoptosis, we assessed whether keratinocytes up-regulate GlcCer synthesis as a protective mechanism against Cer-induced stress. Exogenous sphingomyelinase (SMase) treatment of cultured human keratinocytes (CHK) initially decreased proliferation and cellular sphingomyelin (50-60% decrease; P < 0.001), and increased Cer levels (6.1- to 6.8-fold; P < 0.001). Proliferation recovered to normal rates by 24 h, in parallel with increased cellular GlcCer. Both GlcCer synthesis and GlcCer synthase activity increased significantly by 8 h following SMase (8.2- and 2.4-fold, respectively; P < 0.01 each vs. control), attributed to antecedent increases in GlcCer synthase mRNA and protein expression. Further evidence that GlcCer production is responsible for normalized CHK proliferation includes: a) attenuation of SMase-induced inhibition of proliferation by exogenous GlcCer; and b) enhancement of the SMase effect in cells cotreated with the GlcCer synthase inhibitor, PDMP (D-threo-1-phenyl-2(decanoylamino)-3-morpholino-1-propanol). Finally, although proliferation in immortalized, nontransformed keratinocytes (HaCaT) also was inhibited by SMase, HaCaT cells that overexpress GlcCer synthase were resistant to this effect. Thus, SMase-induced stress initiates a response in keratinocytes that includes upregulation of GlcCer synthesis which may protect against the deleterious effects of excess Cer.

NHE1 regulates the stratum corneum permeability barrier homeostasis. Microenvironment acidification assessed with fluorescence lifetime imaging.

The outermost epidermal layer, the stratum corneum (SC), exhibits an acidic surface pH, whereas the pH at its base approaches neutrality. NHE1 is the only Na(+)/H(+) antiporter isoform in keratinocytes and epidermis, and has been shown to regulate intracellular pH. We now demonstrate a novel function for NHE1, as we find that it also controls acidification of extracellular "microdomains" in the SC that are essential for activation of pH-sensitive enzymes and the formation of the epidermal permeability barrier. NHE1 expression in epidermis is most pronounced in granular cell layers, and although the surface pH of NHE1 knockout mice is only slightly more alkaline than normal using conventional pH measurements, a more sensitive method, fluorescence lifetime imaging, demonstrates that the acidic intercellular domains at the surface and of the lower SC disappear in NHE1 -/- animals. Fluorescence lifetime imaging studies also reveal that SC acidification does not occur through a uniform gradient, but through the progressive accumulation of acidic microdomains. These findings not only visualize the spatial distribution of the SC pH gradient, but also demonstrate a role for NHE1 in the generation of acidic extracellular domains of the lower SC, thus providing the acidification of deep SC interstices necessary for lipid processing and barrier homeostasis.