Two major allergens--the house dust mite Dermatophagoides pteronyssinus (Der p 1) and cockroach allergens--are proteolytically active and stimulate the protease-activated receptor 2 (PAR-2). Jeong et al. (2008, this issue) exposed mouse and human epidermis to both allergens and correlated the observed delay in permeability barrier recovery to PAR-2 activation/signaling. This article exposes the secretive boundaries between barrier homeostasis and immunity.

Cannabinoid receptors (CBR) 1 and 2 have been implicated in keratinocyte differentiation/proliferation. How CB receptors affect epidermal permeability barrier and stratum corneum structure and function remains unclear. Permeability barrier abrogation was induced by sequential tape-stripping of the SC and assessed in both CB1R and CB2R knockout (-/-) mice in comparison with wild-type (+/+) littermates. Absence of CB1R delays permeability barrier recovery, while the latter was found to be accelerated in CB2R -/- mice. While increased lamellar body (LB) secretion is observed in CB2R -/- mice accounting for the enhanced recovery, CB1R -/- animals display strong alterations in lipid bilayer structures. Markers for epidermal differentiation (i.e. filaggrin, loricrin and involucrin) and terminal differentiation (i.e. TUNEL assay and caspase-14 activation) were respectively decreased and increased in CB1R and CB2R -/- mice. Surprisingly, CB1R agonist treatment of human cultured keratinocytes increases mRNA of p21 and cytokeratin 1 and 10 and decreases cyclin D1 but protein levels remained unchanged. Such paradox could partially be explained by the increase in non-phosphorylated-4E-BP1, an inhibitor of mRNA translation, following CB1R agonist treatment. Altogether, these observations put forward the importance and the complexity of cannabinoid signalling for the regulation of permeability barrier and epidermal differentiation.

Epidermal permeability barrier formation depends upon lamellar body (LB) secretion/fusion with the apical plasma membrane (APM) of outermost stratum granulosum (SG) cell, creating cholesterol/glycosphingolipid-enriched lipid rafts-like domains. We found that the dimensions of these domains are comparable to lipid raft in other cell types; and that acute barrier disruption regulates their size and dynamics. To assess the function of these LB-derived raft-like domains, we assessed APM dynamics and barrier recovery in methyl-beta-cyclodextrin (MbetaCD)-treated hairless mice and caveolin-1 knockouts (cav-1(-/-)). MbetaCD treatment impaired APM raft-like domain formation and barrier recovery. Accelerated barrier recovery is observed in cav-1(-/-) in parallel with expansion of raft-like domains. Barrier abrogation of normal epidermis resulted in translocation of cav-1 from the cytoplasm to raft-like membrane domains, restricting further raft-like domain formation and initiating terminal differentiation. Inhibition of LB secretion by monensin and absence of cav-1 delayed terminal differentiation. Furthermore, cav-1(-/-) mice exhibited an increased propensity to develop experimentally induced epidermal hyperplasia correlating with lipid raft persistence. Finally, the epidermal hyperplasia in psoriasis and Netherton syndrome is paralleled by increased lipid raft formation. These studies demonstrate that cav-1 delivery to the APM by LB trafficking to APM "brakes" further LB secretion, signals terminal differentiation, and regulates epidermal hyperproliferation.

The stratum corneum (SC) is a biosensor that mediates responses to a variety of exogenous insults through various signalling mechanisms, including the activation of SC serine proteases (SP) kallikrein cascade. The SPINK5 gene encodes an SP inhibitor, the lympho-epithelial-Kazal-type-1 inhibitor (LEKTI-1), which in turn will buffer the excess of SP cascade initiation, key in the maintenance of permeability barrier homeostasis. We demonstrate that LEKTI processing can occur within the SC after secretion from stratum granulosum keratinocytes at least partially by klk7, an SC-specific chymotryptic SP. Unlike the recently described LEKTI-2, neither recombinant full-length LEKTI-1 nor recombinant LEKTI-1 fragments exhibit antimicrobial activity. Finally, we discuss the pathophysiological implications of LEKTI-1 in skin biology as well as its contribution to the pathogenesis of Netherton Syndrome and its potential involvement in atopic dermatitis.