A cell-free electrochemiluminescence assay for measuring beta1-integrin-ligand interactions

Fibrin functionalization with synthetic adhesive ligands interacting with α6β1 integrin receptor enhance neurite outgrowth of embryonic stem cell-derived neural stem/progenitors

To enhance fibrin hydrogel affinity towards pluripotent stem cell-derived neural stem/progenitor cells (NSPCs) and its capacity to support NSPC migration and neurite extension, we explored the tethering of synthetic peptides engaging integrin α6β1, a cell receptor enriched in NSPCs. Six α6β1 integrin ligands were tested for their ability to support integrin α6β1-mediated adhesion of embryonic stem cell-derived NSPCs (ES-NSPs) and sustain ES-NSPC viability, migration, and neuronal differentiation. Due to their better performance, peptides T1, HYD1, and A5G81 were immobilized into fibrin and functionalized gels characterized in terms of peptide binding efficiency, structure and viscoelastic properties. Tethering of T1 or HYD1 successfully enhanced cell outgrowth from ES-NSPC neurospheres (up to 2.4-fold increase), which exhibited a biphasic response to peptide concentration. Inhibition assays evidenced the involvement of α6β1 and α3β1 integrins in mediating radial outgrowth on T1-/HYD1-functionalized gels. Fibrin functionalization also promoted neurite extension of single ES-NSPCs in fibrin, without affecting cell proliferation and neuronal differentiation. Finally, HYD1-functionalized gels were found to provide a permissive environment for axonal regeneration, leading up to a 2.0-fold increase in neurite extension from rat dorsal root ganglia explants as compared to unmodified fibrin, and to significant improved locomotor function after spinal cord injury (complete transection), along with a trend toward a higher area positive for growth associated protein 43 (marker for axonal growth cone formation). Our results suggest that conjugation of α6β1 integrin-binding motifs is of interest to increase the biofunctionality of hydrogels used in 3D platforms for ES-NSPC culture and potentially, in matrix-assisted ES-NSPC transplantation.

STATEMENT OF SIGNIFICANCE

Impact statement: The transplantation of NSPCs derived from pluripotent stem cells holds much promise for the treatment of central nervous system disorders. Moreover, the combinatorial use of biodegradable hydrogels with NSPCs was shown to contribute to the establishment of a more permissive environment for survival and integration of transplanted cells. In this study, fibrin hydrogels functionalized with a synthetic peptide engaging integrin α6β1 (HYD1) were shown to promote neurite extension of ES-NSPCs, which is fundamental for the formation of functional neuronal relay circuits after NSPC transplantation. Notably, HYD1-functionalized fibrin per se led to enhanced axonal growth ex vivo and to an improvement in locomotor function after implantation in a rat model of spinal cord injury. Conjugation of α6β1 integrin-binding motifs may therefore be of interest to confer bioactivity to NSPC hydrogel vehicles.

Dynamic structural changes are observed upon collagen and metal ion binding to the integrin α1 I domain

We have applied hydrogen-deuterium exchange mass spectrometry, in conjunction with differential scanning calorimetry and protein stability analysis, to examine solution dynamics of the integrin α1 I domain induced by the binding of divalent cations, full-length type IV collagen, or a function-blocking monoclonal antibody. These studies revealed features of integrin activation and α1I-ligand complexes that were not detected by static crystallographic data. Mg(2+) and Mn(2+) stabilized α1I but differed in their effects on exchange rates in the αC helix. Ca(2+) impacted α1I conformational dynamics without altering its gross thermal stability. Interaction with collagen affected the exchange rates in just one of three metal ion-dependent adhesion site (MIDAS) loops, suggesting that MIDAS loop 2 plays a primary role in mediating ligand binding. Collagen also induced changes consistent with increased unfolding in both the αC and allosteric C-terminal helices of α1I. The antibody AQC2, which binds to α1I in a ligand-mimetic manner, also reduced exchange in MIDAS loop 2 and increased exchange in αC, but it did not impact the C-terminal region. This is the first study to directly demonstrate the conformational changes induced upon binding of an integrin I domain to a full-length collagen ligand, and it demonstrates the utility of the deuterium exchange mass spectrometry method to study the solution dynamics of integrin/ligand and integrin/metal ion interactions. Based on the ligand and metal ion binding data, we propose a model for collagen-binding integrin activation that explains the differing abilities of Mg(2+), Mn(2+), and Ca(2+) to activate I domain-containing integrins.

Multiple beta 1 integrins mediate enhancement of human airway smooth muscle cytokine secretion by fibronectin and type I collagen

Altered airway smooth muscle (ASM) function and enrichment of the extracellular matrix (ECM) with interstitial collagen and fibronectin are major pathological features of airway remodeling in asthma. We have previously shown that these ECM components confer enhanced ASM proliferation in vitro, but their action on its newly characterized secretory function is unknown. Here, we examined the effects of fibronectin and collagen types I, III, and V on IL-1beta-dependent secretory responses of human ASM cells, and characterized the involvement of specific integrins. Cytokine production (eotaxin, RANTES, and GM-CSF) was evaluated by ELISA, RT-PCR, and flow cytometry. Function-blocking integrin mAbs and RGD (Arg-Gly-Asp)-blocking peptides were used to identify integrin involvement. IL-1beta-dependent release of eotaxin, RANTES, and GM-CSF was enhanced by fibronectin and by fibrillar and monomeric type I collagen, with similar changes in mRNA abundance. Collagen types III and V had no effect on eotaxin or RANTES release but did modulate GM-CSF. Analogous changes in intracellular cytokine accumulation were found, but in <25% of the total ASM cell population. Function-blocking Ab and RGD peptide studies revealed that alpha2beta1, alpha5beta1, alphavbeta1, and alphavbeta3 integrins were required for up-regulation of IL-1beta-dependent ASM secretory responses by fibronectin, while alpha2beta1 was an important transducer for type I collagen. Thus, fibronectin and type I collagen enhance IL-1beta-dependent ASM secretory responses through a beta1 integrin-dependent mechanism. Enhancement of cytokine release from ASM by these ECM components may contribute to airway wall inflammation and remodeling in asthma.

Design, synthesis, and analysis of a polyethelene glycol-modified (PEGylated) small molecule inhibitor of integrin {alpha}4{beta}1 with improved pharmaceutical properties

Integrin alpha4beta1 plays an important role in inflammatory processes by regulating the migration of leukocytes into inflamed tissues. Previously, we identified BIO5192 [2(S)-{[1-(3,5-dichloro-benzenesulfonyl)-pyrrolidine-2(S)-carbonyl]-amino}-4-[4-methyl-2(S)-(methyl-{2-[4-(3-o-tolyl-ureido)-phenyl]-acetyl}-amino)-pentanoylamino]-butyric acid], a highly selective and potent (K(D) of 9 pM) small molecule inhibitor of alpha4beta1. Although BIO5192 is efficacious in various animal models of inflammatory disease, high doses and daily treatment of the compound are needed to achieve a therapeutic effect because of its relatively short serum half-life. To address this issue, polyethylene glycol modification (PEGylation) was used as an approach to improve systemic exposure. BIO5192 was PEGylated by a targeted approach in which derivatizable amino groups were incorporated into the molecule. Two sites were identified that could be modified, and from these, five PEGylated compounds were synthesized and characterized. One compound, 2a-PEG (K(D) of 19 pM), was selected for in vivo studies. The pharmacokinetic and pharmacodynamic properties of 2a-PEG were dramatically improved relative to the unmodified compound. The PEGylated compound was efficacious in a rat model of experimental autoimmune encephalomyelitis at a 30-fold lower molar dose than the parent compound and required only a once-a-week dosing regimen compared with a daily treatment for BIO5192. Compound 2a-PEG was highly selective for alpha4beta1. These studies demonstrate the feasibility of PEGylation of alpha4beta1-targeted small molecules with retention of activity in vitro and in vivo. 2a-PEG, and related compounds, will be valuable reagents for assessing alpha4beta1 biology and may provide a new therapeutic approach to treatment of human inflammatory diseases.

beta1-Integrins mediate enhancement of airway smooth muscle proliferation by collagen and fibronectin

Airway smooth muscle (ASM) accumulation and enrichment of the extracellular matrix (ECM) with type I collagen and fibronectin are major pathologic features of airway remodeling in asthma. These ECM components confer enhanced ASM proliferation in vitro, but a requirement for specific integrin ECM receptors has not been examined. Here, we examined the mitogen platelet-derived growth factor (PDGF)-BB on beta1-integrin expression on human ASM cells cultured on these ECM substrates and defined the involvement of specific integrins in cell attachment and proliferation using integrin-neutralizing antibodies. PDGF-BB-dependent proliferation was enhanced two- to threefold by monomeric type I collagen or fibronectin and to a lesser extent by vitronectin; other interstitial ECM components (fibrillar type I and III collagen and tenascin-C) had no effect. Except for increased alpha3 expression induced by PDGF-BB and monomeric type I collagen or fibronectin, alpha1, alpha2, alpha4, alpha5, alphav, and alphavbeta3 integrins were unchanged compared with unstimulated cells on plastic. Blocking antibodies revealed alpha2beta1- and alphavbeta3-mediated attachment to monomeric type I collagen, whereas attachment to fibronectin required alpha5beta1. In contrast, enhancement of PDGF-BB-dependent proliferation by either monomeric type I collagen or fibronectin required alpha2beta1, alpha4beta1, and alpha5beta1 integrins. These data suggest multiple beta1-integrins regulate enhanced ASM proliferative responses.

Beta1-integrins in the primary cilium of MDCK cells potentiate fibronectin-induced Ca2+ signaling

Because beta(1)-integrin is involved in sensing of fluid flow rate in endothelial cells, a function that in Madin-Darby canine kidney (MDCK) cells is confined to the primary cilium, we hypothesized beta(1)-integrin to be an important part of the primary ciliary mechanosensory apparatus in MDCK cells. We observed that beta(1)-integrin, alpha(3)-integrin, and perhaps alpha(5)-integrin were localized to the primary cilium of MDCK cells by combining lectin and immunofluorescence confocal microscopy. beta(1)-Integrin was also colocalized with tubulin to the primary cilia of the rat renal collecting ducts, as well as to the cilia of proximal tubules and thick ascending limbs. Immunogold-electron microscopy confirmed the presence of beta(1)-integrin on primary cilia of MDCK cells and rat collecting ducts. Intracellular Ca(2+) levels, monitored by fluorescence microscopy on fluo 4-loaded MDCK cells, significantly increased on addition of fibronectin, a beta(1)-integrin ligand, to mature MDCK cells with an IC(50) of 0.02 mg/l. In immature, nonciliated cells or in deciliated mature cells, the IC(50) was 0.40 mg/l. Blocking the fibronectin-binding sites of beta(1)-integrin with RGD peptide prevented the Ca(2+) signal. Cross-linking of beta(1)-integrins by Sambucus nigra agglutinin produced a Ca(2+) response similar to the addition of fibronectin. Furthermore, the fibronectin-induced response was not dependent on flow or a flow-induced Ca(2+) response. Finally, the flow-induced Ca(2+) response was not prevented by the fibronectin-induced signal. Although beta(1)-integrin on the primary cilium greatly potentiates the fibronectin-induced Ca(2+) signaling in MDCK cells, the flow-dependent Ca(2+) signal is not mediated through activation of beta(1)-integrin.

Antagonism of the α4 integrin subunit attenuates the acute inflammatory response to stent implantation yet is insufficient to prevent late intimal formation

Mononuclear leukocytes infiltrate the artery wall via integrin-mediated mechanisms and play an integral role in intimal formation after stenting. We sought to determine if acute antagonism of the alpha4 subunit of very late antigen-4 is sufficient for the late attenuation of stent intimal area (IA). Twenty-four hypercholesterolemic rabbits underwent iliac artery balloon injury, followed 2 weeks later by stent implantation, and the animals were randomized to receive an anti-alpha4 antibody (HP1/2) or a nonspecific isotypic control immunoglobulin (1E6) intravenously 1 h before stenting. Compared with controls, HP1/2-treated rabbits showed 50%, 51%, and 44% reductions in the percentage on intimal cells that were macrophages on days 3, 7, and 28 after stenting and a 59% reduction in intimal proliferation on day 3. Although stent IA was reduced by 63% and 48% in the antibody-treated group compared with the control group on days 3 and 7, this difference was not present on day 28. These data highlight the need for sustained, anti-inflammatory therapies for the prevention of stent intimal formation.