CD1a on Langerhans cells controls inflammatory skin disease.

CD1a is a lipid-presenting molecule that is abundantly expressed on Langerhans cells. However, the in vivo role of CD1a has remained unclear, principally because CD1a is lacking in mice. Through the use of mice with transgenic expression of CD1a, we found that the plant-derived lipid urushiol triggered CD1a-dependent skin inflammation driven by CD4(+) helper T cells that produced the cytokines IL-17 and IL-22 (TH17 cells). Human subjects with poison-ivy dermatitis had a similar cytokine signature following CD1a-mediated recognition of urushiol. Among various urushiol congeners, we identified diunsaturated pentadecylcatechol (C15:2) as the dominant antigen for CD1a-restricted T cells. We determined the crystal structure of the CD1a-urushiol (C15:2) complex, demonstrating the molecular basis of urushiol interaction with the antigen-binding cleft of CD1a. In a mouse model and in patients with psoriasis, CD1a amplified inflammatory responses that were mediated by TH17 cells that reacted to self lipid antigens. Treatment with blocking antibodies to CD1a alleviated skin inflammation. Thus, we propose CD1a as a potential therapeutic target in inflammatory skin diseases.

The x-ray crystal structure of mannose-binding lectin-associated serine proteinase-3 reveals the structural basis for enzyme inactivity associated with the Carnevale, Mingarelli, Malpuech, and Michels (3MC) syndrome.

The mannose-binding lectin associated-protease-3 (MASP-3) is a member of the lectin pathway of the complement system, a key component of human innate and active immunity. Mutations in MASP-3 have recently been found to be associated with Carnevale, Mingarelli, Malpuech, and Michels (3MC) syndrome, a severe developmental disorder manifested by cleft palate, intellectual disability, and skeletal abnormalities. However, the molecular basis for MASP-3 function remains to be understood. Here we characterize the substrate specificity of MASP-3 by screening against a combinatorial peptide substrate library. Through this approach, we successfully identified a peptide substrate that was 20-fold more efficiently cleaved than any other identified to date. Furthermore, we demonstrated that mutant forms of the enzyme associated with 3MC syndrome were completely inactive against this substrate. To address the structural basis for this defect, we determined the 2.6-Å structure of the zymogen form of the G666E mutant of MASP-3. These data reveal that the mutation disrupts the active site and perturbs the position of the catalytic serine residue. Together, these insights into the function of MASP-3 reveal how a mutation in this enzyme causes it to be inactive and thus contribute to the 3MC syndrome.

Molecular determinants of the substrate specificity of the complement-initiating protease, C1r.

The serine protease, C1r, initiates activation of the classical pathway of complement, which is a crucial innate defense mechanism against pathogens and altered-self cells. C1r both autoactivates and subsequently cleaves and activates C1s. Because complement is implicated in many inflammatory diseases, an understanding of the interaction between C1r and its target substrates is required for the design of effective inhibitors of complement activation. Examination of the active site specificity of C1r using phage library technology revealed clear specificity for Gln at P2 and Ile at P1', which are found in these positions in physiological substrates of C1r. Removal of one or both of the Gln at P2 and Ile at P1' in the C1s substrate reduced the rate of C1r activation. Substituting a Gln residue into the P2 of the activation site of MASP-3, a protein with similar domain structure to C1s that is not normally cleaved by C1r, enabled efficient activation of this enzyme. Molecular dynamics simulations and structural modeling of the interaction of the C1s activation peptide with the active site of C1r revealed the molecular mechanisms that particularly underpin the specificity of the enzyme for the P2 Gln residue. The complement control protein domains of C1r also made important contributions to efficient activation of C1s by this enzyme, indicating that exosite interactions were also important. These data show that C1r specificity is well suited to its cleavage targets and that efficient cleavage of C1s is achieved through both active site and exosite contributions.

Mannose-binding lectin serine proteases and associated proteins of the lectin pathway of complement: two genes, five proteins and many functions?

The lectin pathway of the complement system is activated following the binding of carbohydrate-based ligands by recognition molecules such as mannose-binding lectin (MBL) or ficolins. Engagement of the recognition molecules causes activation of associated MBL-associated serine proteases or MASPs, which in turn activate downstream complement molecules to activate the system. Two MASP genes are alternatively spliced during expression to yield 5 proteins, including three proteases (MASP-1, -2 and -3) and two truncated proteins, MAp19 and MAp44. Here we discuss what is currently known about these proteins in terms of their structure and function. MASP-2 is autoactivated following the initial binding events of the pathway and is able to subsequently activate the C4 and C2 substrates required to activate the rest of the pathway. MASP-1 is able to augment MASP-2 activation, but also appears to play other roles, although the physiological significance of these is not yet clear. The roles of the truncated Map19 and Map44 proteins and the MASP-3 protease are currently unknown. The proteases form an interesting sub-family of proteins that clearly should be the focus of future research in order to establish their biological roles. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

A nonfibrin macromolecular cofactor for tPA-mediated plasmin generation following cellular injury.

Tissue-type plasminogen activator (tPA) is an extracellular protease that converts plasminogen into plasmin. For tPA to generate plasmin under biologic conditions, a cofactor must first bring tPA and plasminogen into physical proximity. Fibrin provides this cofactor for tPA-mediated plasmin generation in blood. Despite being naturally devoid of fibrin(ogen), tPA-mediated plasmin formation also occurs in the brain. The fibrin-like cofactor(s) that facilitates plasmin formation in the injured brain has remained unknown. Here we show that protein aggregates formed during neuronal injury provide a macromolecular, nonfibrin cofactor that promotes tPA-mediated plasmin formation and subsequent cell breakdown. The binding of plasminogen and tPA to these protein aggregates occurs via distinct mechanisms. Importantly, nonneuronal cell types also exhibit this cofactor effect upon injury, indicating a general phenomenon. This novel cofactor identified in nonviable cells has ramifications for ischemic stroke where tPA is used clinically and where plasmin activity within the injured brain is unwanted. A means of selectively inhibiting the binding of tPA to nonviable cells while preserving its association with fibrin may be of benefit for the treatment of ischemic stroke.

The lysine-specific gingipain of Porphyromonas gingivalis : importance to pathogenicity and potential strategies for inhibition.

Periodontitis is a disease affecting the supporting structures of the teeth. The most severe forms of the disease result in tooth loss and have recently been strongly associated with systemic diseases, including cardiovascular and lung diseases and cancer. The disease is caused by biofilms of predominantly anaerobic bacteria. A major pathogen associated with severe, adult forms of the disease is Porphyromonas gingivalis. This organism produces potent cysteine proteases known as gingipains, which have specificity for cleavage after arginine or lysine residues. The lysine-specific gingipain, Kgp, appears to be the major virulence factor of this organism and here we describe its structure and function. We also discuss the inhibitors of the enzyme produced to date and the potential pathways to newer versions of such molecules that will be required to combat periodontitis.

Modulation of the proteolytic activity of the complement protease C1s by polyanions: implications for polyanion-mediated acceleration of interaction between C1s and SERPING1.

The complement system plays crucial roles in the immune system, but incorrect regulation causes inflammation and targeting of self-tissue, leading to diseases such as systemic lupus erythematosus, rheumatoid arthritis and age-related macular degeneration. In vivo, the initiating complexes of the classical complement and lectin pathways are controlled by SERPING1 [(C1 inhibitor) serpin peptidase inhibitor, clade G, member 1], which inactivates the components C1s and MASP-2 (mannan-binding lectin serine peptidase 2). GAGs (glycosaminoglycan) and DXS (dextran sulfate) are able to significantly accelerate SERPING1-mediated inactivation of C1s, the key effector enzyme of the classical C1 complex, although the mechanism is poorly understood. In the present study we have shown that C1s can bind to DXS and heparin and that these polyanions enhanced C1s proteolytic activity at low concentrations and inhibited it at higher concentrations. The recent determination of the crystal structure of SERPING1 has given rise to the hypothesis that both the serpin (serine protease inhibitor)-polyanion and protease-polyanion interactions might be required to accelerate the association rate of SERPING1 and C1s. To determine what proportion of the acceleration was due to protease-polyanion interactions, a chimaeric mutant of alpha1-antitrypsin containing the P4-P1 residues from the SERPING1 RCL (reactive-centre loop) was produced. Like SERPING1, this molecule is able to effectively inhibit C1s, but is unable to bind polyanions. DXS exerted a biphasic effect on the association rate of C1s which correlated strongly with the effect of DXS on C1s proteolytic activity. Thus, whereas polyanions are able to bind C1s and modulate its activity, polyanion interactions with SERPING1 must also play a vital role in the mechanism by which these cofactors accelerate the C1s-SERPING1 reaction.

Determination of the crystal structure and substrate specificity of ananain.

Ananain (EC 3.4.22.31) accounts for less than 10% of the total enzyme in the crude pineapple stem extract known as bromelain, yet yields the majority of the proteolytic activity of bromelain. Despite a high degree of sequence identity between ananain and stem bromelain, the most abundant bromelain cysteine protease, ananain displays distinct chemical properties, substrate preference and inhibitory profile compared to stem bromelain. A tripeptidyl substrate library (REPLi) was used to further characterize the substrate specificity of ananain and identified an optimal substrate for cleavage by ananain. The optimal tripeptide, PLQ, yielded a high kcat/Km value of 1.7 x 106 M-1s-1, with cleavage confirmed to occur after the Gln residue. Crystal structures of unbound ananain and an inhibitory complex of ananain and E-64, solved at 1.73 and 1.98 Å, respectively, revealed a geometrically flat and open S1 subsite for ananain. This subsite accommodates diverse P1 substrate residues, while a narrow and deep hydrophobic pocket-like S2 subsite would accommodate a non-polar P2 residue, such as the preferred Leu residue observed in the specificity studies. A further illustration of the atomic interactions between E-64 and ananain explains the high inhibitory efficiency of E-64 toward ananain. These data reveal the first in depth structural and functional data for ananain and provide a basis for further study of the natural properties of the enzyme.

Dual Modifications of α-Galactosylceramide Synergize to Promote Activation of Human Invariant Natural Killer T Cells and Stimulate Anti-tumor Immunity.

Glycosylceramides that activate CD1d-restricted invariant natural killer T (iNKT) cells have potential therapeutic applications for augmenting immune responses against cancer and infections. Previous studies using mouse models identified sphinganine variants of α-galactosylceramide as promising iNKT cell activators that stimulate cytokine responses with a strongly proinflammatory bias. However, the activities of sphinganine variants in mice have generally not translated well to studies of human iNKT cell responses. Here, we show that strongly proinflammatory and anti-tumor iNKT cell responses were achieved in mice by a variant of α-galactosylceramide that combines a sphinganine base with a hydrocinnamoyl ester on C6″ of the sugar. Importantly, the activities observed with this variant were largely preserved for human iNKT cell responses. Structural and in silico modeling studies provided a mechanistic basis for these findings and suggested basic principles for capturing useful properties of sphinganine analogs of synthetic iNKT cell activators in the design of immunotherapeutic agents.