Activin A binds to perlecan through its pro-region that has heparin/heparan sulfate binding activity

Heparan sulfate proteoglycans regulate responses to oocyte paracrine signals in ovarian follicle morphogenesis

In the ovarian follicle, oocyte-secreted factors induce cumulus-specific genes and repress mural granulosa cell specific genes to establish these functionally distinct cell lineages. The mechanism establishing this precise morphogenic pattern of oocyte signaling within the follicle is unknown. The present study investigated a role for heparan sulphate proteoglycans (HSPG) as coreceptors mediating oocyte secreted factor signaling. In vitro maturation of cumulus oocyte complexes in the presence of exogenous heparin, which antagonizes HSPG signaling, prevented cumulus expansion and blocked the induction of cumulus-specific matrix genes, Has2 and Tnfaip6, whereas conversely, the mural granulosa-specific genes, Lhcgr and Cyp11a1, were strongly up-regulated. Heparin also blocked phosphorylation of SMAD2. Exogenous growth differentiation factor (GDF)-9 reversed these heparin effects; furthermore, GDF9 strongly bound to heparin sepharose. These observations indicate that heparin binds endogenous GDF9 and disrupts interaction with heparan sulphate proteoglycan coreceptor(s), important for GDF9 signaling. The expression of candidate HSPG coreceptors, Syndecan 1-4, Glypican 1-6, and Betaglycan, was examined. An ovulatory dose of human chorionic gonadotropin down-regulated Betaglycan in cumulus cells, and this regulation required GDF9 activity; conversely, Betaglycan was significantly increased in luteinizing mural granulosa cells. Human chorionic gonadotropin caused very strong induction of Syndecan 1 and Syndecan 4 in mural granulosa as well as cumulus cells. Glypican 1 was selectively induced in cumulus cells, and this expression appeared dependent on GDF9 action. These data suggest that HSPG play an essential role in GDF9 signaling and are involved in the patterning of oocyte signaling and cumulus cell function in the periovulatory follicle.

New insights into the mechanisms of activin action and inhibition

Like other members of the transforming growth factor-β (TGF-β) superfamily, activins are synthesised as precursor molecules comprising an N-terminal prodomain and C-terminal mature region. During synthesis, the prodomain interacts non-covalently with mature activin, maintaining the molecule in a conformation competent for dimerisation. Dimeric precursors are cleaved by proprotein convertases and activin is secreted from the cell non-covalently associated with its propeptide. Extracellularly, the propeptide interacts with heparan sulfate proteoglycans to regulate activin localization within tissues. The mature activin dimer exhibits the classic 'open-hand' structure of TGF-β ligands with 'finger-like' domains projecting outward from the cysteine knot core of the molecule. These finger domains form the binding epitopes for type I and II serine/threonine kinase receptors. Activins ability to access its signalling receptors is regulated by the extracellular binding proteins, follistatin, follistatin-like-3, and by inhibins, which, in the presence of betaglycan, sequester type II receptors.

Injectable perlecan domain 1-hyaluronan microgels potentiate the cartilage repair effect of BMP2 in a murine model of early osteoarthritis

The goal of this study was to use bioengineered injectable microgels to enhance the action of bone morphogenetic protein 2 (BMP2) and stimulate cartilage matrix repair in a reversible animal model of osteoarthritis (OA). A module of perlecan (PlnD1) bearing heparan sulfate (HS) chains was covalently immobilized to hyaluronic acid (HA) microgels for the controlled release of BMP2 in vivo. Articular cartilage damage was induced in mice using a reversible model of experimental OA and was treated by intra-articular injection of PlnD1-HA particles with BMP2 bound to HS. Control injections consisted of BMP2-free PlnD1-HA particles, HA particles, free BMP2 or saline. Knees dissected following these injections were analyzed using histological, immunostaining and gene expression approaches. Our results show that knees treated with PlnD1-HA/BMP2 had lesser OA-like damage compared to control knees. In addition, the PlnD1-HA/BMP2-treated knees had higher mRNA levels encoding for type II collagen, proteoglycans and xylosyltransferase 1, a rate-limiting anabolic enzyme involved in the biosynthesis of glycosaminoglycan chains, relative to control knees (PlnD1-HA). This finding was paralleled by enhanced levels of aggrecan in the articular cartilage of PlnD1-HA/BMP2-treated knees. Additionally, decreases in the mRNA levels encoding for cartilage-degrading enzymes and type X collagen were seen relative to controls. In conclusion, PlnD1-HA microgels constitute a formulation improvement compared to HA for efficient in vivo delivery and stimulation of proteoglycan and cartilage matrix synthesis in mouse articular cartilage. Ultimately, PlnD1-HA/BMP2 may serve as an injectable therapeutic agent for slowing or inhibiting the onset of OA after knee injury.

Prodomains regulate the synthesis, extracellular localisation and activity of TGF-β superfamily ligands

All transforming growth factor-β (TGF-β) ligands are synthesised as precursor molecules consisting of a signal peptide, an N-terminal prodomain and a C-terminal mature domain. During synthesis, prodomains interact non-covalently with mature domains, maintaining the molecules in a conformation competent for dimerisation. Dimeric precursors are cleaved by proprotein convertases, and TGF-β ligands are secreted from the cell non-covalently associated with their prodomains. Extracellularly, prodomains localise TGF-β ligands within the vicinity of their target cells via interactions with extracellular matrix proteins, including fibrillin and perlecan. For some family members (TGF-β1, TGF-β2, TGF-β3, myostatin, GDF-11 and BMP-10), prodomains bind with high enough affinity to suppress biological activity. The subsequent mechanism of activation of these latent TGF-β ligands varies according to cell type and context, but all activating mechanisms directly target prodomains. Thus, prodomains control many aspects of TGF-β superfamily biology, and alterations in prodomain function are often associated with disease.

Generation of a specific activin antagonist by modification of the activin A propeptide

Elevated activin A levels in inhibin-deficient mice promote the development of gonadal tumors and induce cachexia by reducing muscle, liver, stomach, and fat mass. Because activin A is an important regulator of tissue growth, inhibiting the actions of this TGFβ family ligand may halt or reverse pathology in diseased tissues. In this study, we modified the activin A propeptide to generate a specific activin antagonist. Propeptides mediate the synthesis and secretion of all TGFβ ligands and, for some family members (e.g. TGFβ1), bind the mature growth factor with high enough affinity to confer latency. By linking the C-terminal region of the TGFβ1 propeptide to the N-terminal region of the activin A propeptide, we generated a chimeric molecule [activin/TGFβ1 propeptide (AT propeptide)] with increased affinity for activin A. The AT propeptide was 30-fold more potent than the activin A propeptide at suppressing activin-induced FSH release by LβT2 pituitary gonadotrope cells. Binding of the AT propeptide to activin A shields the type II receptor binding site, thereby reducing Smad2 phosphorylation and downstream signaling. In comparison with the commonly used activin antagonists, follistatin (IC(50) 0.42 nM), soluble activin type II receptor A-Fc (IC(50) 0.47 nM), and soluble activin type II receptor B-Fc (IC(50) 0.91 nM), the AT propeptide (IC(50) 2.6 nM) was slightly less potent. However, it was more specific, inhibiting activin A and activin B (IC(50) 10.26 nM) but not the closely related ligands, myostatin and growth differentiation factor-11. As such, the AT propeptide represents the first specific activin antagonist, and it should be an effective reagent for blocking activin actions in vivo.

Endorepellin, the angiostatic module of perlecan, interacts with both the α2β1 integrin and vascular endothelial growth factor receptor 2 (VEGFR2): a dual receptor antagonism

Endorepellin, the C-terminal module of perlecan, negatively regulates angiogenesis counter to its proangiogenic parental molecule. Endorepellin (the C-terminal domain V of perlecan) binds the α2β1 integrin on endothelial cells and triggers a signaling cascade that leads to disruption of the actin cytoskeleton. Here, we show that both perlecan and endorepellin bind directly and with high affinity to both VEGF receptors 1 and 2, in a region that differs from VEGFA-binding site. In both human and porcine endothelial cells, this interaction evokes a physical down-regulation of both the α2β1 integrin and VEGFR2, with concurrent activation of the tyrosine phosphatase SHP-1 and downstream attenuation of VEGFA transcription. We demonstrate that endorepellin requires both the α2β1 integrin and VEGFR2 for its angiostatic activity. Endothelial cells that express α2β1 integrin but lack VEGFR2, do not respond to endorepellin treatment. Thus, we provide a new paradigm for the activity of an antiangiogenic protein and mechanistically explain the specificity of endorepellin for endothelial cells, the only cells that simultaneously express both receptors. We hypothesize that a mechanism such as dual receptor antagonism could operate for other angiostatic fragments.

Apolipoprotein B binding domains: evidence that they are cell-penetrating peptides that efficiently deliver antigenic peptide for cross-presentation of cytotoxic T cells

Low-density lipoproteins (LDLs) are a good source of cholesterol, which is important in cellular homeostasis and production of steroids. Apolipoprotein B-100 (ApoB-100), the sole protein component of LDL, is known to bind to cell surface LDL receptor (LDLR) or cell surface-bound proteoglycans and to be internalized into cells. We found that APCs, consisting of macrophages and dendritic cells, upregulate LDLR on culture in vitro without obvious stimulation. In contrast, T cell populations only upregulate LDLR on activation. Thus, we strategized that tagging immunogens to ApoB-100 might be a useful means to target Ag to APCs. We generated fusion proteins consisting of receptor binding sites in ApoB-100, coupled to OVA peptide (ApoB-OVA), as Ag delivery vehicles and demonstrated that this novel delivery method successfully cross-presented OVA peptides in eliciting CTL responses. Surprisingly, internalization of ApoB-OVA peptide occurred via cell surface proteoglycans rather than LDLRs, consistent with evidence that structural elements of ApoB-100 indicate it to have cell-penetrating peptide properties. Finally, we used this strategy to assess therapeutic vaccination in a tumor setting. OVA-expressing EL-4 tumors grew progressively in mice immunized with ApoB-100 alone but regressed in mice immunized with ApoB-OVA fusion protein, coinciding with development of OVA-specific CTLs. Thus, to our knowledge, this is the first article to describe the cell-penetrating properties of a conserved human origin cell penetrating peptide that may be harnessed as a novel vaccination strategy as well as a therapeutics delivery device.