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Cuts by Caspase-14 Control the Proteolysis of Filaggrin

2011; Elsevier BV; Volume: 131; Issue: 11 Linguagem: Inglês

10.1038/jid.2011.282

1523-1747

Leopold Eckhart, Erwin Tschachler,

Microbial Natural Products and Biosynthesis

Although mutations in the filaggrin gene (FLG) have been shown to be associated with ichthyosis vulgaris and atopic dermatitis, the function and regulation of filaggrin remain incompletely understood. In this issue, Hoste et al. report that filaggrin is directly cleaved by caspase-14. Acting in concert with other proteases, caspase-14 controls the breakdown of filaggrin to free amino acids and amino acid derivatives that contribute to the hydration and UVB absorption capacity of the stratum corneum. These findings identify a new layer of complexity in the regulation of epidermal barrier function. Although mutations in the filaggrin gene (FLG) have been shown to be associated with ichthyosis vulgaris and atopic dermatitis, the function and regulation of filaggrin remain incompletely understood. In this issue, Hoste et al. report that filaggrin is directly cleaved by caspase-14. Acting in concert with other proteases, caspase-14 controls the breakdown of filaggrin to free amino acids and amino acid derivatives that contribute to the hydration and UVB absorption capacity of the stratum corneum. These findings identify a new layer of complexity in the regulation of epidermal barrier function. Processing of filaggrin in the epidermis Filaggrin became a focus of dermatological research when mutations in its gene (FLG) were found to be associated with ichthyosis vulgaris and atopic dermatitis (Irvine and McLean, 2006Irvine A.D. McLean W.H. Breaking the (un)sound barrier: filaggrin is a major gene for atopic dermatitis.J Invest Dermatol. 2006; 126: 1200-1202Abstract Full Text Full Text PDF PubMed Scopus (201) Google Scholar). Because filaggrin is expressed only in keratinocytes, defects in the epidermal barrier are now considered likely to precede immune activation and allergies in patients with atopic dermatitis (Irvine and McLean, 2006Irvine A.D. McLean W.H. Breaking the (un)sound barrier: filaggrin is a major gene for atopic dermatitis.J Invest Dermatol. 2006; 126: 1200-1202Abstract Full Text Full Text PDF PubMed Scopus (201) Google Scholar). Although the mechanism of FLG-associated barrier defects has not yet been fully elucidated, recent studies have revealed multiple interactions between filaggrin and other proteins that are critical for its physiological function. Like other members of the S100-fused-type protein family, filaggrin is expressed as a precursor protein consisting of an N-terminal S100 domain and a series of filaggrin repeats. Proteolytic processing yields filaggrin monomers that are important in the aggregation of intermediate filaments during cornification of keratinocytes (Steinert et al., 1981Steinert P.M. Cantieri J.S. Teller D.C. et al.Characterization of a class of cationic proteins that specifically interact with intermediate filaments.Proc Natl Acad Sci USA. 1981; 78: 4097-4101Crossref PubMed Scopus (228) Google Scholar). In the stratum corneum, complete proteolysis of filaggrin leads to the release of free amino acids that function as components of the natural moisturizing factor (NMF) (Scott and Harding, 1986Scott I.R. Harding C.R. Filaggrin breakdown to water binding compounds during development of the rat stratum corneum is controlled by the water activity of the environment.Dev Biol. 1986; 115: 84-92Crossref PubMed Scopus (278) Google Scholar) (Figure 1). Moreover, filaggrin-derived histidine is converted to urocanic acid, which protects the underlying layers of the skin against UVB radiation (Barresi et al., 2011Barresi C. Stremnitzer C. Mlitz V. et al.Increased sensitivity of histidinemic mice to UVB radiation suggests a crucial role of endogenous urocanic acid in photoprotection.J Invest Dermatol. 2011; 131: 188-194Abstract Full Text Full Text PDF PubMed Scopus (90) Google Scholar). In addition to proteolysis, protein phosphorylation, dephosphorylation, and deimination control the progressive conversion of profilaggrin to its amino acid constituents (Sandilands et al., 2009Sandilands A. Sutherland C. Irvine A.D. et al.Filaggrin in the frontline: role in skin barrier function and disease.J Cell Sci. 2009; 122: 1285-128594Crossref PubMed Scopus (558) Google Scholar). The report by Hoste et al., 2011Hoste E. Kemperman P. Devos M. et al.Caspase-14 is required for filaggrin degradation to natural moisturizing factors in the skin.J Invest Dermatol. 2011; 131: 2233-2241Abstract Full Text Full Text PDF PubMed Scopus (147) Google Scholar, this issue) uncovers a novel part of the complex processing of filaggrin and highlights the experimental challenges faced by investigators studying biochemical processes in the outermost layers of the skin. Caspase-14 is a filaggrin-processing enzyme The cysteine protease caspase-14 was linked to the breakdown of filaggrin in 2007 by Declercq and colleagues, who generated and characterized a caspase-14 knockout mouse model (Denecker et al., 2007Denecker G. Hoste E. Gilbert B. et al.Caspase-14 protects against epidermal UVB photodamage and water loss.Nat Cell Biol. 2007; 9: 666-674Crossref PubMed Scopus (232) Google Scholar). They found that caspase-14 deficiency was associated with the accumulation of incompletely degraded filaggrin fragments within the stratum corneum, a decrease in stratum corneum hydration, increased transepidermal water loss, and sensitivity to UVB photodamage. In an extension of this work, Hoste et al., 2011Hoste E. Kemperman P. Devos M. et al.Caspase-14 is required for filaggrin degradation to natural moisturizing factors in the skin.J Invest Dermatol. 2011; 131: 2233-2241Abstract Full Text Full Text PDF PubMed Scopus (147) Google Scholar now provide evidence for the molecular mechanisms underlying the phenotype of caspase-14 knockout mice. The investigators identify direct cleavage sites of caspase-14 within filaggrin monomers in vitro and report that lack of caspase-14 leads to a decrease in the concentration of filaggrin-derived components of the NMF and consequently to a decrease in the UVB photoprotectant urocanic acid in the stratum corneum (Hoste et al., 2011Hoste E. Kemperman P. Devos M. et al.Caspase-14 is required for filaggrin degradation to natural moisturizing factors in the skin.J Invest Dermatol. 2011; 131: 2233-2241Abstract Full Text Full Text PDF PubMed Scopus (147) Google Scholar). Caspase-14 is coexpressed with filaggrin in terminally differentiated keratinocytes; it also undergoes proteolytic maturation (Eckhart et al., 2000Eckhart L. Declercq W. Ban J. et al.Terminal differentiation of human keratinocytes and stratum corneum formation is associated with caspase-14 activation.J Invest Dermatol. 2000; 115: 1148-1151Crossref PubMed Scopus (178) Google Scholar; Lippens et al., 2000Lippens S. Kockx M. Knaapen M. et al.Epidermal differentiation does not involve the pro-apoptotic executioner caspases, but is associated with caspase-14 induction and processing.Cell Death Differ. 2000; 7: 1218-1224Crossref PubMed Scopus (202) Google Scholar; Figure 1). An as-yet-unknown protease cleaves procaspase-14 to generate a large and a small catalytic subunit that persist in the stratum corneum (Fischer et al., 2004Fischer H. Stichenwirth M. Dockal M. et al.Stratum corneum–derived caspase-14 is catalytically active.FEBS Lett. 2004; 577: 446-450Abstract Full Text Full Text PDF PubMed Scopus (46) Google Scholar; Hibino et al., 2010Hibino T. Fujita E. Tsuji Y. et al.Purification and characterization of active caspase-14 from human epidermis and development of the cleavage site-directed antibody.J Cell Biochem. 2010; 109: 487-497PubMed Google Scholar). The processing of caspase-14 is essential but not sufficient for catalytic activation (Mikolajczyk et al., 2004Mikolajczyk J. Scott F.L. Krajewski S. et al.Activation and substrate specificity of caspase-14.Biochemistry. 2004; 43: 10560-10569Crossref PubMed Scopus (48) Google Scholar). In vitro high concentrations of kosmotropic (i.e., structure-stabilizing) salts are required for caspase-14 to be active (Mikolajczyk et al., 2004Mikolajczyk J. Scott F.L. Krajewski S. et al.Activation and substrate specificity of caspase-14.Biochemistry. 2004; 43: 10560-10569Crossref PubMed Scopus (48) Google Scholar; Fischer et al., 2004Fischer H. Stichenwirth M. Dockal M. et al.Stratum corneum–derived caspase-14 is catalytically active.FEBS Lett. 2004; 577: 446-450Abstract Full Text Full Text PDF PubMed Scopus (46) Google Scholar; Hoste et al., 2011Hoste E. Kemperman P. Devos M. et al.Caspase-14 is required for filaggrin degradation to natural moisturizing factors in the skin.J Invest Dermatol. 2011; 131: 2233-2241Abstract Full Text Full Text PDF PubMed Scopus (147) Google Scholar). As kosmotropes exert their effects on proteins by competing for water molecules, the in vitro requirements of caspase-14 activity may be mimicked by the low water content known to exist close to the skin surface. Indeed, filaggrin is degraded when the hydration level in the outer layers of the stratum corneum decreases (Scott and Harding, 1986Scott I.R. Harding C.R. Filaggrin breakdown to water binding compounds during development of the rat stratum corneum is controlled by the water activity of the environment.Dev Biol. 1986; 115: 84-92Crossref PubMed Scopus (278) Google Scholar). Further studies should define more precisely the microenvironment that allows caspase-14 to cleave filaggrin in the epidermis. It is tempting to speculate that a unique dependency on a distinct milieu allows caspase-14 to act as a sensor for environmental conditions that regulate the degradation of filaggrin. Hoste et al., 2011Hoste E. Kemperman P. Devos M. et al.Caspase-14 is required for filaggrin degradation to natural moisturizing factors in the skin.J Invest Dermatol. 2011; 131: 2233-2241Abstract Full Text Full Text PDF PubMed Scopus (147) Google Scholar determined two sites of caspase-14-mediated cleavage of filaggrin in vitro, and they suggested the existence of other cleavage sites that could not be identified, largely for technical reasons. Both sites identified in this report conform to caspases' strict requirement for an aspartate residue on the amino-terminal side of the hydrolyzed peptide bond (Mikolajczyk et al., 2004Mikolajczyk J. Scott F.L. Krajewski S. et al.Activation and substrate specificity of caspase-14.Biochemistry. 2004; 43: 10560-10569Crossref PubMed Scopus (48) Google Scholar). The substrate specificity of caspases, including caspase-14, also depends on the three amino acid residues that precede the aspartate residue (Mikolajczyk et al., 2004Mikolajczyk J. Scott F.L. Krajewski S. et al.Activation and substrate specificity of caspase-14.Biochemistry. 2004; 43: 10560-10569Crossref PubMed Scopus (48) Google Scholar). Intriguingly, filaggrin shows little evolutionary conservation of its amino acid sequence, including the sequence of the caspase-14 cleavage sites identified by Hoste et al., 2011Hoste E. Kemperman P. Devos M. et al.Caspase-14 is required for filaggrin degradation to natural moisturizing factors in the skin.J Invest Dermatol. 2011; 131: 2233-2241Abstract Full Text Full Text PDF PubMed Scopus (147) Google Scholar. Interspecies differences in the caspase-14 cleavage pattern of filaggrin and the existence of caspase-14 substrates other than filaggrin should be investigated in future studies. Caspase-14-dependent markers of filaggrin catabolism The report of Hoste et al., 2011Hoste E. Kemperman P. Devos M. et al.Caspase-14 is required for filaggrin degradation to natural moisturizing factors in the skin.J Invest Dermatol. 2011; 131: 2233-2241Abstract Full Text Full Text PDF PubMed Scopus (147) Google Scholar shows that caspase-14 deficiency affects two molecular parameters of filaggrin catabolism: the pattern of filaggrin fragments and the concentrations of NMF components in the stratum corneum. Previously, the NMF content of stratum corneum has been suggested as a marker for FLG mutations (Kezic et al., 2008Kezic S. Kemperman P.M. Koster E.S. et al.Loss-of-function mutations in the filaggrin gene lead to reduced level of natural moisturizing factor in the stratum corneum.J Invest Dermatol. 2008; 128: 2117-2119Crossref PubMed Scopus (238) Google Scholar). Hoste et al., 2011Hoste E. Kemperman P. Devos M. et al.Caspase-14 is required for filaggrin degradation to natural moisturizing factors in the skin.J Invest Dermatol. 2011; 131: 2233-2241Abstract Full Text Full Text PDF PubMed Scopus (147) Google Scholar now demonstrate that, in addition to the mutation status of filaggrin, the efficiency of filaggrin catabolism is a major determinant of NMF concentrations. Complementary to the decreased concentrations of NMF components, the absence of caspase-14 is also associated with an increase in the abundance of filaggrin degradation intermediates (Hoste et al., 2011Hoste E. Kemperman P. Devos M. et al.Caspase-14 is required for filaggrin degradation to natural moisturizing factors in the skin.J Invest Dermatol. 2011; 131: 2233-2241Abstract Full Text Full Text PDF PubMed Scopus (147) Google Scholar). It is conceivable that distinct patterns of filaggrin fragments correlate with different states of stratum corneum homeostasis or even with subsets of human diseases. Interestingly, the level of active caspase-14 has been reported to be reduced in patients with atopic dermatitis (Yamamoto et al., 2011Yamamoto M. Kamata Y. Iida T. et al.Quantification of activated and total caspase-14 with newly developed ELISA systems in normal and atopic skin.J Dermatol Sci. 2011; 61: 110-117Abstract Full Text Full Text PDF PubMed Scopus (29) Google Scholar). Unfortunately, the filaggrin fragmentation pattern was not determined in that study. The results of Hoste et al., 2011Hoste E. Kemperman P. Devos M. et al.Caspase-14 is required for filaggrin degradation to natural moisturizing factors in the skin.J Invest Dermatol. 2011; 131: 2233-2241Abstract Full Text Full Text PDF PubMed Scopus (147) Google Scholar suggest an unconventional migration behavior of filaggrin fragments in polyacrylamide gel electrophoresis. Therefore, the evaluation of alternative assays and the specific adaption of the analytical methods to the unique properties of filaggrin, including a high content of charged amino acid residues, may be required to define diagnostic characteristics of filaggrin degradation. It is important to note that Hoste et al., 2011Hoste E. Kemperman P. Devos M. et al.Caspase-14 is required for filaggrin degradation to natural moisturizing factors in the skin.J Invest Dermatol. 2011; 131: 2233-2241Abstract Full Text Full Text PDF PubMed Scopus (147) Google Scholar have not proposed that caspase-14 is the only protease that cleaves filaggrin. Rather, their report shows that, in the absence of caspase-14, other enzymes can initiate proteolysis of filaggrin. In addition, proteases such as calpain 1 and bleomycin hydrolase (Kamata et al., 2009Kamata Y. Taniguchi A. Yamamoto M. et al.Neutral cysteine protease bleomycin hydrolase is essential for the breakdown of deiminated filaggrin into amino acids.J Biol Chem. 2009; 284: 12829-12836Crossref PubMed Scopus (130) Google Scholar) are required to complete the degradation of filaggrin. Therefore, to understand the regulation of filaggrin degradation, it is necessary to determine the interplay of caspase-14 with these other proteases as well as the order in which the proteolytic cuts occur. Concluding remarks New evidence demonstrates that not only mutations in the filaggrin gene but also alterations in filaggrin processing may result in skin barrier defects and that caspase-14 takes part in this process. Hoste et al., 2011Hoste E. Kemperman P. Devos M. et al.Caspase-14 is required for filaggrin degradation to natural moisturizing factors in the skin.J Invest Dermatol. 2011; 131: 2233-2241Abstract Full Text Full Text PDF PubMed Scopus (147) Google Scholar provide a basis for improving strategies to diagnose filaggrin-associated skin disorders and to modulate caspase-14-dependent barrier function of the stratum corneum.