Proteoglycans Make the Grade-ient

Proteases as therapeutics

Proteases are an expanding class of drugs that hold great promise. The U.S. FDA (Food and Drug Administration) has approved 12 protease therapies, and a number of next generation or completely new proteases are in clinical development. Although they are a well-recognized class of targets for inhibitors, proteases themselves have not typically been considered as a drug class despite their application in the clinic over the last several decades; initially as plasma fractions and later as purified products. Although the predominant use of proteases has been in treating cardiovascular disease, they are also emerging as useful agents in the treatment of sepsis, digestive disorders, inflammation, cystic fibrosis, retinal disorders, psoriasis and other diseases. In the present review, we outline the history of proteases as therapeutics, provide an overview of their current clinical application, and describe several approaches to improve and expand their clinical application. Undoubtedly, our ability to harness proteolysis for disease treatment will increase with our understanding of protease biology and the molecular mechanisms responsible. New technologies for rationally engineering proteases, as well as improved delivery options, will expand greatly the potential applications of these enzymes. The recognition that proteases are, in fact, an established class of safe and efficacious drugs will stimulate investigation of additional therapeutic applications for these enzymes. Proteases therefore have a bright future as a distinct therapeutic class with diverse clinical applications.

Uptake of the necrotic serpin in Drosophila melanogaster via the lipophorin receptor-1

The humoral response to fungal and Gram-positive infections is regulated by the serpin-family inhibitor, Necrotic. Following immune-challenge, a proteolytic cascade is activated which signals through the Toll receptor. Toll activation results in a range of antibiotic peptides being synthesised in the fat-body and exported to the haemolymph. As with mammalian serpins, Necrotic turnover in Drosophila is rapid. This serpin is synthesised in the fat-body, but its site of degradation has been unclear. By “freezing” endocytosis with a temperature sensitive Dynamin mutation, we demonstrate that Necrotic is removed from the haemolymph in two groups of giant cells: the garland and pericardial athrocytes. Necrotic uptake responds rapidly to infection, being visibly increased after 30 mins and peaking at 6–8 hours. Co-localisation of anti-Nec with anti-AP50, Rab5, and Rab7 antibodies establishes that the serpin is processed through multi-vesicular bodies and delivered to the lysosome, where it co-localises with the ubiquitin-binding protein, HRS. Nec does not co-localise with Rab11, indicating that the serpin is not re-exported from athrocytes. Instead, mutations which block late endosome/lysosome fusion (dor, hk, and car) cause accumulation of Necrotic-positive endosomes, even in the absence of infection. Knockdown of the 6 Drosophila orthologues of the mammalian LDL receptor family with dsRNA identifies LpR1 as an enhancer of the immune response. Uptake of Necrotic from the haemolymph is blocked by a chromosomal deletion of LpR1. In conclusion, we identify the cells and the receptor molecule responsible for the uptake and degradation of the Necrotic serpin in Drosophila melanogaster. The scavenging of serpin/proteinase complexes may be a critical step in the regulation of proteolytic cascades.

Serpin inhibitors control a wide range of rapid physiological responses that are activated by proteolytic cascades, such as blood coagulation, inflammation, the complement pathway, and angiogenesis. They interact with their target proteinases by a “suicide inhibition” mechanism, which generates an inert, denatured, serpin/proteinase complex. In mammals, humoral serpins are secreted from the liver into the blood plasma. The denatured complex is later endocytosed back into the liver and degraded. In Drosophila, the Necrotic serpin is secreted from the fat-body into the haemolymph, where it controls the humoral immune response. We show here, however, that Necrotic is not endocytosed in the fat-body, but in the garland and pericardial athrocytes. These cells clear serpins from the haemolymph extremely rapidly. The Necrotic-binding receptor for this process is LpR1, a member of the LDLR family. The endocytosed serpin is targeted for lysosomal degradation, with none being recycled to the haemolymph. More importantly, we show that mutations in LpR1 cause a profound effect on the immune response. Thus, our results indicate that the scavenging of serpin/proteinase complexes might be a critical step in the regulation of proteolytic cascades.

Interactions between heparan sulfate and proteins-design and functional implications

Heparan sulfate (HS) proteoglycans at cell surfaces and in the extracellular matrix of most animal tissues are essential in development and homeostasis, and variously implicated in disease processes. Functions of HS polysaccharide chains depend on ionic interactions with a variety of proteins including growth factors and their receptors. Negatively charged sulfate and carboxylate groups are arranged in various types of domains, generated through strictly regulated biosynthetic reactions and with enormous potential for structural variability. The level of specificity of HS-protein interactions is assessed through binding experiments in vitro using saccharides of defined composition, signaling assays in cell culture, and targeted disruption of genes for biosynthetic enzymes followed by phenotype analysis. While some protein ligands appear to require strictly defined HS structure, others bind to variable saccharide domains without any apparent dependence on distinct saccharide sequence. These findings raise intriguing questions concerning the functional significance of regulation in HS biosynthesis.

How does the ureteric bud branch?

Many genes that modulate kidney development have been identified; however, the molecular interactions that direct arborization of the ureteric bud (UB) remain incompletely understood. This article discusses how "systems" approaches may shed light on the structure of the gene network during UB branching morphogenesis and the mechanisms involved in the formation of a branched collecting system from a straight epithelial tube in the context of a stage model. In vitro and genetic studies suggest that the stages seem to be governed by a conserved network of genes that establish a "tip-stalk generator"; these genes sustain iterative UB branching tubulogenesis through minimal alterations in the network architecture as a budding system shifts to one that autocatalytically branches through budding. The differential expression of stage-specific positive and inhibitory factors in the mesenchyme, likely presented in the context of heparan sulfate proteoglycans, and effector molecules in the epithelium seems to regulate advancement between stages; similar principles may apply to other branching epithelia such as the lung, salivary gland, pancreas, mammary gland, and prostate. Active mesenchymal interactions with the UB seem to govern vectorial arborization and tapering of the collecting system and its terminal differentiation. Cessation of branching correlates with induction of mesenchyme as well as local extracellular matrix changes. Perturbations of these mechanisms and/or single-nucleotide polymorphisms in genes regulating UB branching may predispose to a variety of renal diseases (e.g., hypertension and chronic kidney disease) by altering nephron number. Decentralization of the gene-protein interaction network may explain the relative paucity of branching phenotypes in mutant mice and in human disease.

Microarray analysis reveals differences in expression of cell surface and extracellular matrix components during development of the trout ovary and testis

Trout normally spawn at 3 years of age, however, a small percentage mature a year early. This provides an opportunity to study reproductive timing and developmental processes. The ovarian and testicular extracellular matrix (ECM) participates in processes such as growth, adhesion, differentiation, cell migration and patterning. The composition of the ECM defines the interactions of specific regulatory ligands with their receptors and modulates and regulates gonadal function. To identify some of the genes involved in these processes, a 16,006-gene salmonid cDNA microarray was used to compare three-year-old normal with two-year-old normal (maturing) and with two-year-old precocious (pre-spawn) ovarian and testicular transcriptomes. We provide evidence for differences in expression of some of the genes during vasculogenesis, angiogenesis, fibrillogenesis and other processes involving ECM remodeling. Sex-specific gene expression differences of ECM components were documented between the trout ovary and testis in each developmental state. Significant differences in the expression of genes involved in translation, transcription, cell-cycling and differentiation were identified. We also report, for the first time, unequivocal evidence for the transcription of high levels of adult and embryonic hemoglobins in the developed ovary; and for the expression of transcripts that encode zona pellucida glycoproteins in both the ovary and testis of trout.

HSPG modulation of BMP signaling in fibrodysplasia ossificans progressiva cells

Cell surface heparan sulfate proteoglycans (HSPGs) play important roles in morphogen gradient formation and cell signaling. Bone morphogenetic protein (BMP) signaling is dysregulated in fibrodysplasia ossificans progressiva (FOP), a disabling disorder of progressive heterotopic bone formation. Here, we investigated the role of HSPG glycosaminoglycan (GAG) side chains on BMP signaling and found increased total and HSPG-specific GAG chain levels and dysregulation in HSPG modulation of BMP signaling in FOP lymphoblastoid cells (LCLs). Specifically, HSPG profiling demonstrated abundant mRNA and protein levels of glypican 1 and syndecan 4 on control and FOP LCLs, with elevated core protein levels on FOP cells. Targeted downregulation of glypican 1 core protein synthesis by siRNA enhanced BMP signaling in control and FOP cells, while reduction of syndecan 4-core protein synthesis decreased BMP signaling in control, but not FOP cells. These results suggest that FOP cells are resistant to the stimulatory effects of cell surface HSPG GAG chains, but are susceptible to the inhibitory effects, as shown by downregulation of glypican 1. These data support that HSPG modulation of BMP signaling is altered in cells from patients with FOP and that altered HSPG-related BMP signaling may play a role in the pathogenesis of the disease.

The anti-angiogenic His/Pro-rich fragment of histidine-rich glycoprotein binds to endothelial cell heparan sulfate in a Zn2+-dependent manner

The plasma protein histidine-rich glycoprotein (HRGP), which has been identified as an angiogenesis inhibitor, binds to heparan sulfate (HS) in a Zn(2+)-dependent manner. We wished to test whether this interaction is mechanistically important in mediation of the anti-angiogenic effect of HRGP. Inhibition of angiogenesis by HRGP is exerted through its central His/Pro-rich domain, which is proteolytically released. A 35-amino-acid residue synthetic peptide, HRGP330, derived from the His/Pro-rich domain retains the inhibitory effect on blood vessel formation in vitro and in vivo, an effect dependent on the presence of Zn(2+). We now show that HRGP330 binds heparin/HS with the same capacity as full-length HRGP, and the binding is Zn(2+)-dependent. Peptides derived from the His/Pro-rich domain of HRGP downstream of HRGP330 fail to inhibit endothelial cell migration and display a significantly reduced heparin-binding capacity. An even shorter peptide, HRGP335, covering a 26-amino-acid sequence within HRGP330 retains full heparin/HS-binding capacity. Characterization of the HS interaction shows that there is a tissue-specific HS pattern recognized by HRGP335 and that the minimal length of heparin/HS required for binding to HRGP335 is an 8-mer oligosaccharide. Saturation of the HS binding sites in HRGP330 by pre-incubation with heparin abrogates the HRGP330-induced rearrangement of endothelial cell focal adhesions, suggesting that interaction with cell surface HS is needed for HRGP330 to exert its anti-angiogenic effect.