Caspase-14 protects against epidermal UVB photodamage and water loss

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.

No death without life: vital functions of apoptotic effectors

As a result of the genetic experiments performed in Caenorhabditis elegans, it has been tacitly assumed that the core proteins of the 'apoptotic machinery' (CED-3, -4, -9 and EGL-1) would be solely involved in cell death regulation/execution and would not exert any functions outside of the cell death realm. However, multiple studies indicate that the mammalian orthologs of these C. elegans proteins (i.e. caspases, Apaf-1 and multidomain proteins of the Bcl-2 family) participate in cell death-unrelated processes. Similarly, loss-of-function mutations of ced-4 compromise the mitotic arrest of DNA-damaged germline cells from adult nematodes, even in a context in which the apoptotic machinery is inoperative (for instance due to mutations of egl-1 or ced-3). Moreover, EGL-1 is required for the activation of autophagy in starved nematodes. Finally, the depletion of caspase-independent death effectors, such as apoptosis-inducing factor (AIF) and endonuclease G, provokes cell death-independent consequences, both in mammals and in yeast (Saccharomyces cerevisiae). These results corroborate the conjecture that any kind of protein that has previously been specifically implicated in apoptosis might have a phylogenetically conserved apoptosis-unrelated function, most likely as part of an adaptive response to cellular stress.

Histidase expression in human epidermal keratinocytes: regulation by differentiation status and all-trans retinoic acid

BACKGROUND

Histidase (histidine ammonia lyase) converts histidine into urocanic acid, the main ultraviolet (UV) light absorption factor of the stratum corneum. It is unknown if and how histidase is regulated in the epidermis.

OBJECTIVE

We have investigated the transcriptional regulation of histidase expression in epidermal keratinocytes.

METHODS

Human epidermal keratinocytes were cultured in vitro and exposed to UV irradiation, a number of cytokines and all-trans retinoic acid (ATRA) (1 microM). Keratinocyte differentiation was triggered by maintaining confluent cells in monolayer culture and by establishing three-dimensional skin equivalents. The mRNA expression level of histidase in keratinoytes as well as in the epidermis and other tissues was determined by quantitative real-time PCR. Protein expression was determined by Western blot analysis.

RESULTS

Human epidermis contained higher levels of histidase transcripts than all other tissues investigated. Expression of histidase strongly increased at the mRNA and protein levels during differentiation of primary keratinocytes in vitro. Treatment of keratinocytes with UVA and UVB did not significantly change the expression level of histidase. By contrast, ATRA suppressed histidase expression almost completely.

CONCLUSIONS

Our results show that histidase is upregulated during keratinocyte differentiation and that ATRA but not UV irradiation modulates the expression level of histidase. Suppression of histidase-mediated production of urocanic acid may contribute to the increase in UV sensitivity that is caused by treatment with retinoids.

Caspase-14 reveals its secrets

Caspase-14 is a unique member of the evolutionarily conserved family of cysteinyl aspartate-specific proteinases, which are mainly involved in inflammation and apoptosis. However, recent evidence also implicates these proteases in proliferation and differentiation. Although most caspases are ubiquitously expressed, caspase-14 expression is confined mainly to cornifying epithelia, such as the skin. Moreover, caspase-14 activation correlates with cornification, indicating that it plays a role in terminal keratinocyte differentiation. The determination of in vitro conditions for caspase-14 activity paved the way to identifying its substrates. The recent development of caspase-14-deficient mice underscored its importance in the correct degradation of (pro)filaggrin and in the formation of the epidermal barrier that protects against dehydration and UVB radiation. Here, we review the current knowledge on caspase-14 in skin homeostasis and disease.

Caspase assays: identifying caspase activity and substrates in vitro and in vivo

The measurement of general caspase activity and the quantification of purified recombinant caspases in vitro can be accomplished with relative ease. But the determination of which caspases are active in a cellular context is much more challenging. This is because commercially available small molecule substrates and inhibitors do not display sufficient specificity to dissect the complex interplay of caspase pathways. Here we describe procedures that can be used to validate which caspases are active in cell culture models and determine which caspases are responsible for specific cleavage events. We also recommend methods for working with recombinant initiator caspases in vitro and suggest ways to accurately assess the cleavage efficiency of natural caspase substrates.

Acute modulations in permeability barrier function regulate epidermal cornification: role of caspase-14 and the protease-activated receptor type 2

Stratum corneum comprises corneocytes, derived from outer stratum granulosum during terminal differentiation, embedded in a lipid-enriched extracellular matrix, secreted from epidermal lamellar bodies. Permeability barrier insults stimulate rapid secretion of preformed lamellar bodies from the outer stratum granulosum, regulated through modulations in ionic gradients and serine protease (SP)/protease-activated receptor type 2 (PAR2) signaling. Because corneocytes are also required for barrier function, we hypothesized that corneocyte formation could also be regulated by barrier function. Barrier abrogation by two unrelated methods initiated a wave of cornification, assessed as TdT-mediated dUTP nick end-labeling-positive cells in stratum granulosum and newly cornified cells by electron microscopy. Because cornification was blocked by occlusion, corneocytes formed specifically in response to barrier, rather than injury or cell replacement, requirements. SP inhibitors and hyperacidification (which decreases SP activity) blocked cornification after barrier disruption. Similarly, cornification was delayed in PAR2(-/-) mice. Although classical markers of apoptosis [poly(ADP-ribose)polymerase and caspase (Casp)-3] remained unchanged, barrier disruption activated Casp-14. Moreover, the pan-Casp inhibitor Z-VAD-FMK delayed cornification, and corneocytes were structurally aberrant in Casp14(-/-) mice. Thus, permeability barrier requirements coordinately drive both the generation of the stratum corneum lipid-enriched extracellular matrix and the transformation of granular cells into corneocytes, in an SP- and Casp-14-dependent manner, signaled by PAR2.

The filaggrin story: novel insights into skin-barrier function and disease

Recent reports have uncovered the key role of the protein filaggrin in maintaining an effective skin barrier against the external environment. Loss-of-function mutations in the profilaggrin gene (FLG) are common and are present in up to 10% of the population. These mutations are the cause of the semi-dominant skin-scaling disorder ichthyosis vulgaris and are a major risk factor for the development of atopic dermatitis. The discovery of these mutations also provides new data concerning the genetics of atopic asthma as well as intriguing insight into disease mechanisms of systemic allergies involving antigen exposure in skin with defective barrier function. Collectively, these novel findings have significant implications for the classification and future clinical management of patients with atopic and allergic diseases.