Cathepsin D is present in human eccrine sweat and involved in the postsecretory processing of the antimicrobial peptide DCD-1L

Cathepsin D is released after severe tissue trauma in vivo and is capable of generating C5a in vitro

In response to severe tissue trauma several danger sensing and signalling cascades are activated, including the complement and the apoptosis systems. In polytrauma patients, both the early activation of the complement cascade with an excessive generation of the potent anaphylatoxin C5a and the induction of apoptosis have been shown to modulate the post-traumatic immune response. However, little is known about a direct interaction between the complement and apoptosis systems after severe tissue trauma. Therefore the focus of the present study was to elucidate the interplay between the central complement component C5 and the pro-apoptotic aspartic protease cathepsin D. In vivo, the cathepsin D plasma concentration of multiple injured patients was markedly increased when compared to healthy volunteers. In vitro incubation of C5 with cathepsin D resulted in a concentration- and time-dependent generation of C5a, which was inhibited by the aspartate protease inhibitor pepstatin A. Immunoblotting and sequencing analysis indicated that the C5 cleavage product represents the native form of human C5a, also exhibiting chemotactic activity for human neutrophils. In conclusion, these data show for the first time that cathepsin D is increased in plasma early after severe tissue injury. Furthermore, the results provide in vitro evidence of cleavage of C5 by an aspartic protease with subsequent generation of functional C5a, which represents a new path of complement activation.

Identification and expressional analysis of two cathepsins from half-smooth tongue sole (Cynoglossus semilaevis)

Cathepsins are a family of lysosomal proteases that play an important role in protein degradation, antigen presentation, apoptosis, and inflammation. Cathepsins are divided into three groups, i.e., cysteine protease, serine protease, and aspartic protease. Cathepsin D and cathepsin L, which are aspartic protease and cysteine protease respectively, have been identified in a number of teleosts; however, the immunological relevance of fish cathepsins is largely unknown. In this study, we cloned and analyzed the expression profiles of a cathepsin D (CsCatD) and a cathepsin L (CsCatL) homologs from half-smooth tongue sole (Cynoglossus semilaevis). CsCatD is composed of 396 amino acid residues and shares 67.6-88.4% overall sequence identities with fish and human cathepsin D. Structurally CsCatD possesses an aspartic endopeptidase domain, which contains two conserved aspartic acid residues that form the catalytic site. CsCatL is 336 residues in length and shares 64.7-90.2% overall sequence identities with fish and human cathepsin L. CsCatL has an N-terminal cathepsin propeptide inhibitor domain followed by a Papain family cysteine protease domain, the latter containing four conserved catalytic residues: Gln-133, Cys-139, His-279, and Asn-303. Recombinant CsCatL purified from Escherichia coli exhibited apparent protease activity. Quantitative real time RT-PCR analysis detected constitutive expression of CsCatD and CsCatL in multiple tissues, with the lowest level found in heart and the highest level found in liver. Experimental challenge of tongue sole with the bacterial pathogen Vibrio anguillarum and megalocytivirus caused significant inductions of both CsCatD and CsCatL expression in kidney and spleen in time-dependent manners. Immunization of the fish with a subunit vaccine also enhanced CsCatD and CsCatL expression in the first week post-vaccination. These results suggest involvement of CsCatD and CsCatL in host immune reactions to bacterial and viral infections and in the process of antigen-induced immune response.

Processing of HEBP1 by cathepsin D gives rise to F2L, the agonist of formyl peptide receptor 3

The peptide F2L was previously characterized as a high-affinity natural agonist for the human formyl peptide receptor (FPR) 3. F2L is an acetylated 21-aa peptide corresponding with the N terminus of the intracellular heme-binding protein 1 (HEBP1). In the current work, we have investigated which proteases were able to generate the F2L peptide from its precursor HEBP1. Structure-function analysis of F2L identified three amino acids, G(3), N(7), and S(8), as the most important for interaction of the peptide with FPR3. We expressed a C-terminally His-tagged form of human HEBP1 in yeast and purified it to homogeneity. The purified protein was used as substrate to identify proteases generating bioactive peptides for FPR3-expressing cells. A conditioned medium from human monocyte-derived macrophages was able to generate bioactivity from HEBP1, and this activity was inhibited by pepstatin A. Cathepsin D was characterized as the protease responsible for HEBP1 processing, and the bioactive product was identified as F2L. We have therefore determined how F2L, the specific agonist of FPR3, is generated from the intracellular protein HEBP1, although it is unknown in which compartment the processing by cathepsin D occurs in vivo.

Procathepsin D and cancer: From molecular biology to clinical applications

Procathepsin D (pCD) is overexpressed and secreted by cells of various tumor types including breast and lung carcinomas. pCD affects multiple features of tumor cells including proliferation, invasion, metastases and apoptosis. Several laboratories have previously shown that the mitogenic effect of pCD on cancer cells is mediated via its propeptide part (APpCD). However, the exact mechanism of how pCD affects cancer cells has not been identified. Recent observations have also revealed the possible use of pCD/APpcD as a marker of cancer progression. The purpose of this review is to summarize the three major potentials of pCD-tumor marker, potential drug, and screening agent.

Spotlight on Human LL-37, an Immunomodulatory Peptide with Promising Cell-Penetrating Properties

Cationic antimicrobial peptides are major components of innate immunity and help control the initial steps of the infectious process. They are expressed not only by immunocytes, but also by epithelial cells. They share an amphipathic secondary structure with a polar cationic site, which explains their tropism for prokaryote membranes and their hydrophobic site contributing to the destructuration of these membranes. LL-37 is the only cationic antimicrobial peptide derived from human cathelicidin. LL-37 can also cross the plasma membrane of eukaryotic cells, probably through special domains of this membrane called lipid rafts. This transfer could be beneficial in the context of vaccination: the activation of intracellular toll-like receptors by a complex formed between CpG oligonucleotides and LL-37 could conceivably play a major role in the building of a cellular immunity involving NK cells.

Temporal profiling of the secretome during adipogenesis in humans

Adipose tissue plays a key role as a fat-storage depot and as an endocrine organ. Although mouse adipogenesis has been studied extensively, limited studies have been conducted to characterize this process in humans. We carried out a temporal proteomic analysis to interrogate the dynamic changes in the secretome of primary human preadipocytes as they differentiate into mature adipocytes. Using iTRAQ-based quantitative proteomics, we identified and quantified 420 proteins from the secretome of differentiated human adipocytes. Our results revealed that the majority of proteins showed differential expression during the course of differentiation. In addition to adipokines known to be differentially secreted in the course of adipocyte differentiation, we identified a number of proteins whose dynamic expression in this process has not been previously documented. They include collagen triple helix repeat containing 1, cytokine receptor-like factor 1, glypican-1, hepatoma-derived growth factor, SPARC related modular calcium binding protein 1, SPOCK 1, and sushi repeat-containing protein. A bioinformatics analysis using Human Protein Reference Database and Human Proteinpedia revealed that of the 420 proteins identified, 164 proteins possess signal peptides and 148 proteins are localized to the extracellular compartment. Additionally, we employed antibody arrays to quantify changes in the levels of 182 adipokines during human adipogenesis. This is the first large-scale quantitative proteomic study that combines two platforms, mass spectrometry and antibody arrays, to analyze the changes in the secretome during the course of adipogenesis in humans.

The secretome of adipocytes is regulated during the differentiation of preadipocytes into adipocytes. Using iTRAQ-based quantitative proteomics, we identified and quantified 420 proteins. Additionally, we employed antibody arrays to quantify changes in the levels of 182 adipokines. This is the first large-scale quantitative proteomic study that combines two platforms, mass spectrometry and antibody arrays, to analyze the changes in the secretome during the course of adipogenesis in humans.

Trophallaxis and prophylaxis: social immunity in the carpenter ant Camponotus pennsylvanicus

In social insects, group behaviour can increase disease resistance among nest-mates and generate social prophylaxis. Stomodeal trophallaxis, or mutual feeding through regurgitation, may boost colony-level immunocompetence. We provide evidence for increased trophallactic behaviour among immunized workers of the carpenter ant Camponotus pennsylvanicus, which, together with increased antimicrobial activity of the regurgitate droplet, help explain the improved survival of droplet recipient ants relative to controls following an immune challenge. We have identified a protein related to cathepsin D, a lysosomal protease, as a potential contributor to the antimicrobial activity. The combined behavioural and immunological responses to infection in these ants probably represent an effective mechanism underlying the social facilitation of disease resistance, which could potentially produce socially mediated colony-wide prophylaxis. The externalization and sharing of an individual's immune responses via trophallaxis could be an important component of social immunity, allowing insect colonies to thrive under high pathogenic pressures.

CNS-expressed cathepsin D prevents lymphopenia in a murine model of congenital neuronal ceroid lipofuscinosis

Deficiency in Cathepsin D (CtsD), the major cellular lysosomal aspartic proteinase, causes the congenital form of neuronal ceroid lipofuscinoses (NCLs). CtsD-deficient mice show severe visceral lesions like lymphopenia in addition to their central nervous system (CNS) phenotype of ceroid accumulation, microglia activation, and seizures. Here we demonstrate that re-expression of CtsD within the CNS but not re-expression of CtsD in visceral organs prevented both central and visceral pathologies of CtsD(-/-) mice. Our results suggest that CtsD was substantially secreted from CNS neurons and drained from CNS to periphery via lymphatic routes. Through this drainage, CNS-expressed CtsD acts as an important modulator of immune system maintenance and peripheral tissue homeostasis. These effects depended on enzymatic activity and not on proposed functions of CtsD as an extracellular ligand. Our results furthermore demonstrate that the prominent accumulation of ceroid/lipofuscin and activation of microglia in brains of CtsD(-/-) are not lethal factors but can be tolerated by the rodent CNS.

Dermcidin-derived peptides show a different mode of action than the cathelicidin LL-37 against Staphylococcus aureus

Dermcidin (DCD) is an antimicrobial peptide which is constitutively expressed in eccrine sweat glands. By postsecretory proteolytic processing in sweat, the DCD protein gives rise to anionic and cationic DCD peptides with a broad spectrum of antimicrobial activity. Many antimicrobial peptides induce membrane permeabilization as part of their killing mechanism, which is accompanied by a loss of the bacterial membrane potential. In this study we show that there is a time-dependent bactericidal activity of anionic and cationic DCD-derived peptides which is followed by bacterial membrane depolarization. However, DCD-derived peptides do not induce pore formation in the membranes of gram-negative and gram-positive bacteria. This is in contrast to the mode of action of the cathelicidin LL-37. Interestingly, LL-37 as well as DCD-derived peptides inhibit bacterial macromolecular synthesis, especially RNA and protein synthesis, without binding to microbial DNA or RNA. Binding studies with components of the cell envelope of gram-positive and gram-negative bacteria and with model membranes indicated that DCD-derived peptides bind to the bacterial envelope but show only a weak binding to lipopolysaccharide (LPS) from gram-negative bacteria or to peptidoglycan, lipoteichoic acid, and wall teichoic acid, isolated from Staphylococcus aureus. In contrast, LL-37 binds strongly in a dose-dependent fashion to these components. Altogether, these data indicate that the mode of action of DCD-derived peptides is different from that of the cathelicidin LL-37 and that components of the bacterial cell envelope play a role in the antimicrobial activity of DCD.

Antimicrobial peptides, skin infections, and atopic dermatitis

The innate immune system evolved more than 2 billion years ago to first recognize pathogens then eradicate them. Several distinct defects in this ancient but rapidly responsive element of human immune defense account for the increased incidence of skin infections in atopics. These defects include abnormalities in the physical barrier of the epidermis, alterations in microbial pattern recognition receptors such as toll receptors and nucleotide binding oligomerization domains, and a diminished capacity to increase the expression of antimicrobial peptides during inflammation. Several antimicrobial peptides are affected including; cathelicidin, HBD-2, and HBD-3, which are lower in lesional skin of atopics compared with other inflammatory skin diseases, and dermcidin, which is decreased in sweat. Other defects in the immune defense barrier of atopics include a relative deficiency in plasmacytoid dendritic cells. In the future, understanding the cause of these defects may allow therapeutic intervention to reduce the incidence of infection in atopic individuals and potentially decrease the severity of this disorder.

Cathepsin D--many functions of one aspartic protease

For years, it has been held that cathepsin D (CD) is involved in rather non-specific protein degradation in a strongly acidic milieu of lysosomes. Studies with CD knock-out mice revealed that CD is not necessary for embryonal development, but it is indispensable for postnatal tissue homeostasis. Mutation that abolishes CD enzymatic activity causes neuronal ceroid lipofuscinosis (NCL) characterized by severe neurodegeneration, developmental regression, visual loss and epilepsy in both animals and humans. In the last decade, however, an increasing number of studies demonstrated that enzymatic function of CD is not restricted solely to acidic milieu of lysosomes with important consequences in regulation of apoptosis. In addition to CD enzymatic activity, it has been shown that apoptosis is also regulated by catalytically inactive mutants of CD which suggests that CD interacts with other important molecules and influences cell signaling. Moreover, procathepsin D (pCD), secreted from cancer cells, acts as a mitogen on both cancer and stromal cells and stimulates their pro-invasive and pro-metastatic properties. Numerous studies found that pCD/CD level represents an independent prognostic factor in a variety of cancers and is therefore considered to be a potential target of anti-cancer therapy. Studies dealing with functions of cathepsin D are complicated by the fact that there are several simultaneous forms of CD in a cell-pCD, intermediate enzymatically active CD and mature heavy and light chain CD. It became evident that these forms may differently regulate the above-mentioned processes. In this article, we review the possible functions of CD and its various forms in cells and organisms during physiological and pathological conditions.

Proteomic analysis of skin invasion by blood fluke larvae

BACKGROUND

During invasion of human skin by schistosome blood fluke larvae (cercariae), a multicellular organism breaches the epidermis, basement membrane, and dermal barriers of skin. To better understand the pathobiology of this initial event in schistosome infection, a proteome analysis of human skin was carried out following invasion by cercariae of Schistosoma mansoni.

METHODOLOGY AND RESULTS

Human skin samples were exposed to cercariae for one-half hour to two hours. Controls were exposed to water used to collect cercariae in an identical manner, and punctured to simulate cercarial tunnels. Fluid from both control and experimental samples was analyzed by LC/MS/MS using a linear ion trap in "triple play" mode. The coexistence of proteins released by cercariae and host skin proteins from epidermis and basement membrane confirmed that cercarial tunnels in skin were sampled. Among the abundant proteins secreted by cercariae was the cercarial protease that has been implicated in degradation of host proteins, secreted proteins proposed to mediate immune invasion by larvae, and proteins implicated in protection of parasites against oxidative stress. Components of the schistosome surface tegument, previously identified with immune serum, were also released. Both lysis and apoptosis of epidermal cells took place during cercarial invasion of the epidermis. Components of lysed epidermal cells, including desmosome proteins which link cells in the stratum granulosum and stratum spinosum, were identified. While macrophage-derived proteins were present, no mast cell or lymphocyte cytokines were identified. There were, however, abundant immunoglobulins, complement factors, and serine protease inhibitors in skin. Control skin samples incubated with water for the same period as experimental samples ensured that invasion-related proteins and host protein fragments were not due to nonspecific degeneration of the skin samples.

CONCLUSIONS

This analysis identified secreted proteins from invasive larvae that are released during invasion of human skin. Analysis of specific host proteins in skin invaded by cercariae served to highlight both the histolytic events facilitating cercarial invasion, and the host defenses that attempt to arrest or retard invasion. Proteins abundant in psoriatic skin or UV and heat-stressed skin were not abundant in skin invaded by cercariae, suggesting that results did not reflect general stress in the surgically removed skin specimen. Abundant immunoglobulins, complement factors, and serine protease inhibitors in skin form a biochemical barrier that complements the structural barrier of the epidermis, basement membrane, and dermis. The fragmentation of some of these host proteins suggests that breaching of host defenses by cercariae includes specific degradation of immunoglobulins and complement, and either degradation of, or overwhelming the host protease inhibitor repertoire.

Antimicrobial peptides in human skin disease

The skin continuously encounters microbial pathogens. To defend against this, cells of the epidermis and dermis have evolved several innate strategies to prevent infection. Antimicrobial peptides are one of the primary mechanisms used by the skin in the early stages of immune defense. In general, antimicrobial peptides have broad antibacterial activity against gram-positive and negative bacteria and also show antifungal and antiviral activity. The antimicrobial activity of most peptides occurs as a result of unique structural characteristics that enable them to disrupt the microbial membrane while leaving human cell membranes intact. However, antimicrobial peptides also act on host cells to stimulate cytokine production, cell migration, proliferation, maturation, and extracellular matrix synthesis. The production by human skin of antimicrobial peptides such as defensins and cathelicidins occurs constitutively but also greatly increases after infection, inflammation or injury. Some skin diseases show altered expression of antimicrobial peptides, partially explaining the pathophysiology of these diseases. Thus, current research suggests that understanding how antimicrobial peptides modify susceptibility to microbes, influence skin inflammation, and modify wound healing, provides greater insight into the pathophysiology of skin disorders and offers new therapeutic opportunities.

Naturally processed dermcidin-derived peptides do not permeabilize bacterial membranes and kill microorganisms irrespective of their charge

Dermcidin (DCD) is a recently described antimicrobial peptide, which is constitutively expressed in eccrine sweat glands and transported via sweat to the epidermal surface. By postsecretory proteolytic processing in sweat the dermcidin protein gives rise to several truncated DCD peptides which differ in length and net charge. In order to understand the mechanism of antimicrobial activity, we analyzed the spectrum of activity of several naturally processed dermcidin-derived peptides, the secondary structure in different solvents, and the ability of these peptides to interact with or permeabilize the bacterial membrane. Interestingly, although all naturally processed DCD peptides can adopt an alpha-helical conformation in solvents, they have a diverse and partially overlapping spectrum of activity against gram-positive and gram-negative bacteria. This indicates that the net charge and the secondary structure of the peptides are not important for the toxic activity. Furthermore, using carboxyfluorescein-loaded liposomes, membrane permeability studies and electron microscopy we investigated whether DCD peptides are able to permeabilize bacterial membranes. The data convincingly show that irrespective of charge the different DCD peptides are not able to permeabilize bacterial membranes. However, bacterial mutants lacking specific cell envelope modifications exhibited different susceptibilities to killing by DCD peptides than wild-type bacterial strains. Finally, immunoelectron microscopy studies indicated that DCD peptides are able to bind to the bacterial surface; however, signs of membrane perturbation were not observed. These studies indicate that DCD peptides do not exert their activity by permeabilizing bacterial membranes.

Interactions of an anionic antimicrobial peptide with Staphylococcus aureus membranes

The antimicrobial activity of the anionic peptide, AP1 (GEQGALAQFGEWL), was investigated. AP1 was found to kill Staphylococcus aureus with an MLC of 3mM and to induce maximal surface pressure changes of 3.8 mN m(-1) over 1200s in monolayers formed from lipid extract of S. aureus membranes. FTIR spectroscopy showed the peptide to be alpha-helical (100%) in the presence of vesicles formed from this lipid extract and to induce increases in their fluidity (Deltanu circa 0.5 cm(-1)). These combined data show that AP1 is able to function as an alpha-helical antimicrobial peptide against Gram-positive bacteria and suggest that the killing mechanism used by the peptide involves interactions with the membrane lipid headgroup region. Moreover, this killing mechanism differs strongly from that previously reported for AP1 against Gram-negative bacteria, indicating the importance of considering the effects of membrane lipid composition when investigating the structure/function relationships of antimicrobial peptides.