Fibrin is a widely used biological scaffold in tissue engineering and regenerative medicine. While the polymerization dynamics from its soluble precursor fibrinogen has been studied for decades, few attempts have been made to modulate fibrin network structure through the addition of external agents that actively engage this process. We propose the use of polyethylene glycol (PEG)-based linkers that interact with fibrinogen via knob:hole affinity interactions as a means of controlling thrombin-mediated fibrin polymerization dynamics and resulting network structure. Using bivalent and tetravalent knob-PEG conjugates with sizes ranging from 2 to 20 kDa, we demonstrate that the clotting characteristics of fibrinogen solutions can be altered in a dose-dependent manner, with conjugate size playing a major role in altering fibrin network structure. Interestingly, factor XIIIa-catalyzed fibrin(ogen) crosslinking and plasmin-mediated degradation were not significantly impacted. This work demonstrates the feasibility of modulating fibrin network structure through the addition of knob-PEG conjugates that perturb the polymerization process through non-covalent knob:hole interactions.

Antisense oligodeoxynucleotides targeting the mRNA of the gap junction protein Cx43 promote tissue repair in a variety of different wounds. Delivery of the antisense drug has most often been achieved by a thermoreversible hydrogel, Pluronic F-127, which is very effective in the short term but does not allow for sustained delivery over several days. For chronic wounds that take a long time to heal, repeated dosing with the drug may be desirable but is not always compatible with conventional treatments such as the weekly changing of compression bandages on venous leg ulcers. Here the coating of collagen scaffolds with antisense oligonucleotides is investigated and a way to provide protection of the oligodeoxynucleotide drug is found in conjunction with sustained release over a 7 d period. This approach significantly reduces the normal foreign body reaction to the scaffold, which induces an increase of Cx43 protein and an inhibition of healing. As a result of the antisense integration into the scaffold, inflammation is reduced with the rate of wound healing and contracture is significantly improved. This coated scaffold approach may be very useful for treating venous leg ulcers and also for providing a sustained release of any other types of oligonucleotide drugs that are being developed.

The ability to design artificial extracellular matrices as cell instructive scaffolds has opened the door to technologies capable of studying cell fate in vitro and to guide tissue repair in vivo. One main component of the design of artificial extracellular matrices is the incorporation of biochemical cues to guide cell phenotype and multicellular organization. The extracellular matrix is composed of a heterogeneous mixture of proteins that present a variety of spatially discrete signals to residing cell populations. In contrast, most engineered ECMs do not mimic this heterogeneity. In recent years the use of photodeprotection has been used to achieve spatial immobilization of signals. However, these approaches have been limited mostly to small peptides. Here we combine photodeprotection with enzymatic reaction to achieve spatially controlled immobilization of active bioactive signals that range from small molecules to large proteins. A peptide substrate for transglutaminase factor XIII (FXIIIa) is caged with a photodeprotectable group, which is then immobilized to the bulk of a cell compatible hydrogel. With the use of focused light the substrate can be deprotected and used to immobilize patterned bioactive signals. This approach offers an innovative strategy to immobilize delicate bioactive signals, such as growth factors, without loss of activity and enables In situ cell manipulation of encapsulated cells.

Engineering extracellular matrices that utilize the body's natural healing capacity enable the progression of regenerative therapies. Fibrin, widely used as a surgical sealant, is one such matrix that may be augmented by the addition of protein factors to promote cell infiltration and differentiation. The thrombin-catalyzed conversion of fibrinogen to fibrin exposes N-terminal fibrin knobs that bind to C-terminal pockets to form the fibrin network. Here, we have created a platform system for the production of therapeutic proteins that capitalize on these native knob:pocket interactions for protein delivery within fibrin matrices. This system enables the retention of therapeutic proteins within fibrin without additional enzymatic or synthetic crosslinking factors. Using an integrin-binding fibronectin fragment as a model protein, we demonstrate that engineered knob-protein fusions bind consistently and specifically to fibrin(ogen). Equilibrium dissociation constants (KD) obtained using surface plasmon resonance indicate that these fusions have μM binding affinities, comparable to the native knob-containing fibrin fragments. The specificity of these interactions was verified by ELISA in the presence of molar excess of competing knob mimics. Release profiles and real-time confocal imaging demonstrate that the fusions were retained within fibrin matrices, even under the stringent continuous perfusion conditions used in the latter. In summary, this work explores the benefits and limitations of engaging native, biologically-inspired, non-covalent knob-pocket interactions within fibrin(ogen) for the retention of therapeutic proteins in fibrin matrices and provides insight into the stability of native knob:pocket interactions within fibrin networks.

Elastin-like polypeptides (ELPs) are polypentapeptides that undergo hydrophobic collapse and aggregation above a specific transition temperature, Tt. ELP diblocks sharing a common “core” block (I60) but varying “outer” blocks (A80, P40) were designed, where Tt,I < Tt,A < Tt,P. The formation of ~55 nm diameter mixed micelles from these ELP diblocks was verified using dynamic light scattering (DLS), multiangle light scattering (MALS) and fluorescence resonance energy transfer (FRET). To confer affinity to the blood circulating protein fibrinogen, a fibrinogen-binding tetrapeptide sequence (GPRP) was fused to A80-I60, while P40-I60 was fused to a non-binding control (GPSP). The self-assembling, peptide-displaying, mixed micelles exhibit temperature-modulated avidities for immobilized and soluble fibrinogen at 32 °C and 42 °C. In this initial proof-of-concept design, the engineered mixed micelles were shown to disengage fibrinogen at elevated temperatures. The modular nature of this system can be used for developing in vivo depot systems that will only be triggered to release in situ upon specific stimuli.

Current anticoagulants target coagulation Factors upstream from fibrin assembly and polymerization (i.e. formation of fibrin clot). While effective, this approach requires constant patient monitoring since pharmacokinetics and pharmacodynamics vary from patient to patient. To address these limitations, we developed an alternative anticoagulant that effectively inhibits fibrin polymerization. Specifically, we investigated PEGylated fibrin knob `A' peptides, evaluating the effect of both PEG chain length (0, 2, 5, 10, 20, and 30 kDa) and knob peptide sequence (GPRPAAC, GPRPFPAC, and GPRPPERC) on inhibiting fibrin polymerization (i.e. clot formation). Thrombin-initiated clotting assays with purified fibrinogen were performed to compare clot formation with each peptide-PEG conjugate. Results indicated a biphasic effect of PEG chain length whereby active-PEG conjugates demonstrated increasingly enhanced inhibition of fibrin polymerization from 0 to 5 kDa PEG. However, the anticoagulant activity diminished to control levels for PEG chains above 5 kDa. Ultimately, we observed a 10-fold enhancement of anticoagulant activity with active peptides PEGylated with 5 kDa PEG compared to non-PEGylated knob peptides. The sequence of the active peptide significantly influenced the anticoagulant properties only at the highest 1:100 molar ratio where GPRPFPAC-5 kDa PEG and GPRPPERC-5 kDa PEG demonstrated significantly lower percent clottable protein than GPRPAAC-5 kDa PEG. Moreover, human plasma treated with the active 5 kDa PEG conjugate exhibited delayed the prothrombin time to within the therapeutic range specified for oral anticoagulants. Collectively, this study demonstrated the utility of PEGylated fibrin knob peptides as potential anticoagulant therapeutics.

SummaryProlonged vascular hyperpermeability is a common feature of many diseases. Vascular hyperpermeability is typically associated with changes in the expression patterns of adherens and tight junction proteins. Here, we focus on the less-appreciated contribution of gap junction proteins (connexins) to basal vascular permeability and endothelial dysfunction. First, we assess the association of connexins with endothelial barrier integrity by introducing tools used in connexin biology and relating the findings to customary readouts in vascular biology. Second, we explore potential mechanistic ties between connexins and junction regulation. Third, we review the role of connexins in microvascular organisation and development, focusing on interactions of the endothelium with mural cells and tissue-specific perivascular cells. Last, we see how connexins contribute to the interactions between the endothelium and components of the immune system, by using neutrophils as an example. Mounting evidence of crosstalk between connexins and other junction proteins suggests that we rethink the way in which different junction components contribute to endothelial barrier function. Given the multiple points of connexin-mediated communication arising from the endothelium, there is great potential for synergism between connexin-targeted inhibitors and existing immune-targeted therapeutics. As more drugs targeting connexins progress through clinical trials, it is hoped that some might prove effective at countering vascular hyperpermeability.

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