Human matrix metalloproteinase specificity studies using collagen sequence-based synthetic peptides

Evaluating the physicochemical effects of conjugating peptides into thermogelling hydrogels for regenerative biomaterials applications

Thermogelling hydrogels, such as poly(N-isopropylacrylamide) [P(NiPAAm)], provide tunable constructs leveraged in many regenerative biomaterial applications. Recently, our lab developed the crosslinker poly(glycolic acid)-poly(ethylene glycol)-poly(glycolic acid)-di(but-2-yne-1,4-dithiol), which crosslinks P(NiPAAm-co-glycidyl methacrylate) via thiol-epoxy reaction and can be functionalized with azide-terminated peptides via alkyne-azide click chemistry. This study's aim was to evaluate the impact of peptides on the physicochemical properties of the hydrogels. The physicochemical properties of the hydrogels including the lower critical solution temperature, crosslinking times, swelling, degradation, peptide release and cytocompatibility were evaluated. The gels bearing peptides increased equilibrium swelling indicating hydrophilicity of the hydrogel components. Comparable sol fractions were found for all groups, indicating that inclusion of peptides does not impact crosslinking. Moreover, the inclusion of a matrix metalloproteinase-sensitive peptide allowed elucidation of whether release of peptides from the network was driven by hydrolysis or enzymatic cleavage. The hydrophilicity of the network determined by the swelling behavior was demonstrated to be the most important factor in dictating hydrogel behavior over time. This study demonstrates the importance of characterizing the impact of additives on the physicochemical properties of hydrogels. These characteristics are key in determining design considerations for future in vitro and in vivo studies for tissue regeneration.

Synthetic extracellular matrices with tailored adhesiveness and degradability support lumen formation during angiogenic sprouting

A major deficit in tissue engineering strategies is the lack of materials that promote angiogenesis, wherein endothelial cells from the host vasculature invade the implanted matrix to form new blood vessels. To determine the material properties that regulate angiogenesis, we have developed a microfluidic in vitro model in which chemokine-guided endothelial cell sprouting into a tunable hydrogel is followed by the formation of perfusable lumens. We show that long, perfusable tubes only develop if hydrogel adhesiveness and degradability are fine-tuned to support the initial collective invasion of endothelial cells and, at the same time, allow for matrix remodeling to permit the opening of lumens. These studies provide a better understanding of how cell-matrix interactions regulate angiogenesis and, therefore, constitute an important step towards optimal design criteria for tissue-engineered materials that require vascularization.

Current tissue engineering strategies lack materials that promote angiogenesis. Here the authors develop a microfluidic in vitro model in which chemokine-guided endothelial cell sprouting into a tunable hydrogel is followed by the formation of perfusable lumens to determine the material properties that regulate angiogenesis.

Development of a functional salivary gland tissue chip with potential for high-content drug screening

Radiation therapy for head and neck cancers causes salivary gland dysfunction leading to permanent xerostomia. Limited progress in the discovery of new therapeutic strategies is attributed to the lack of in vitro models that mimic salivary gland function and allow high-throughput drug screening. We address this limitation by combining engineered extracellular matrices with microbubble (MB) array technology to develop functional tissue mimetics for mouse and human salivary glands. We demonstrate that mouse and human salivary tissues encapsulated within matrix metalloproteinase-degradable poly(ethylene glycol) hydrogels formed in MB arrays are viable, express key salivary gland markers, and exhibit polarized localization of functional proteins. The salivary gland mimetics (SGm) respond to calcium signaling agonists and secrete salivary proteins. SGm were then used to evaluate radiosensitivity and mitigation of radiation damage using a radioprotective compound. Altogether, SGm exhibit phenotypic and functional parameters of salivary glands, and provide an enabling technology for high-content/throughput drug testing.

Matrix metalloproteinase (MMP)-degradable tissue engineered periosteum coordinates allograft healing via early stage recruitment and support of host neurovasculature

Despite serving as the clinical "gold standard" treatment for critical size bone defects, decellularized allografts suffer from long-term failure rates of ~60% due to the absence of the periosteum. Stem and osteoprogenitor cells within the periosteum orchestrate autograft healing through host cell recruitment, which initiates the regenerative process. To emulate periosteum-mediated healing, tissue engineering approaches have been utilized with mixed outcomes. While vascularization has been widely established as critical for bone regeneration, innervation was recently identified to be spatiotemporally regulated together with vascularization and similarly indispensable to bone healing. Notwithstanding, there are no known approaches that have focused on periosteal matrix cues to coordinate host vessel and/or axon recruitment. Here, we investigated the influence of hydrogel degradation mechanism, i.e. hydrolytic or enzymatic (cell-dictated), on tissue engineered periosteum (TEP)-modified allograft healing, especially host vessel/nerve recruitment and integration. Matrix metalloproteinase (MMP)-degradable hydrogels supported endothelial cell migration from encapsulated spheroids whereas no migration was observed in hydrolytically degradable hydrogels in vitro, which correlated with increased neurovascularization in vivo. Specifically, ~2.45 and 1.84-fold, and ~3.48 and 2.58-fold greater vessel and nerve densities with high levels of vessel and nerve co-localization was observed using MMP degradable TEP (MMP-TEP) -modified allografts versus unmodified and hydrolytically degradable TEP (Hydro-TEP)-modified allografts, respectively, at 3 weeks post-surgery. MMP-TEP-modified allografts exhibited greater longitudinal graft-localized vascularization and endochondral ossification, along with 4-fold and 2-fold greater maximum torques versus unmodified and Hydro-TEP-modified allografts after 9 weeks, respectively, which was comparable to that of autografts. In summary, our results demonstrated that the MMP-TEP coordinated allograft healing via early stage recruitment and support of host neurovasculature.

Engineered Collagen Matrices

Collagen is the most abundant protein in mammals, accounting for approximately one-third of the total protein in the human body. Thus, it is a logical choice for the creation of biomimetic environments, and there is a long history of using collagen matrices for various tissue engineering applications. However, from a biomaterial perspective, the use of collagen-only scaffolds is associated with many challenges. Namely, the mechanical properties of collagen matrices can be difficult to tune across a wide range of values, and collagen itself is not highly amenable to direct chemical modification without affecting its architecture or bioactivity. Thus, many approaches have been pursued to design scaffold environments that display critical features of collagen but enable improved tunability of physical and biological characteristics. This paper provides a brief overview of approaches that have been employed to create such engineered collagen matrices. Specifically, these approaches include blending of collagen with other natural or synthetic polymers, chemical modifications of denatured collagen, de novo creation of collagen-mimetic chains, and reductionist methods to incorporate collagen moieties into other materials. These advancements in the creation of tunable, engineered collagen matrices will continue to enable the interrogation of novel and increasingly complex biological questions.

Matrix Control of Periodontal Ligament Cell Activity Via Synthetic Hydrogel Scaffolds

Rebuilding the tooth-supporting tissues (periodontium) destroyed by periodontitis remains a clinical challenge. Periodontal ligament cells (PDLCs), multipotent cells within the periodontal ligament (PDL), differentiate and form new PDL and mineralized tissues (cementum and bone) during native tissue repair in response to specific extracellular matrix (ECM) cues. Thus, harnessing ECM cues to control PDLC activity ex vivo, and ultimately, to design a PDLC delivery vehicle for tissue regeneration is an important goal. In this study, poly(ethylene glycol) hydrogels were used as a synthetic PDL ECM to interrogate the roles of cell-matrix interactions and cell-mediated matrix remodeling in controlling PDLC activity. Results showed that PDLCs within matrix metalloproteinase (MMP)-degradable hydrogels expressed key PDL matrix genes and showed a six to eightfold increase in alkaline phosphatase (ALP) activity compared with PDLCs in nondegradable hydrogel controls. The increase in ALP activity, commonly considered an early marker of cementogenic/osteogenic differentiation, occurred independent of the presentation of the cell-binding ligand RGD or soluble media cues and remained elevated when inhibiting PDLC-matrix binding and intracellular tension. ALP activity was further increased in softer hydrogels regardless of degradability and was accompanied by an increase in PDLC volume. However, scaffolds that fostered PDLC ALP activity did not necessarily promote hydrogel ECM mineralization. Rather, matrix mineralization was greatest in stiffer, MMP-degradable hydrogels and required the presence of soluble media cues. These divergent outcomes illustrate the complexity of the PDLC response to ECM cues and the limitations of current scaffold materials. Nevertheless, key biomaterial design principles for controlling PDLC activity were identified for incorporation into scaffolds for periodontal tissue regeneration. Impact statement Engineered scaffolds are an attractive approach for delivering periodontal ligament cells (PDLCs) to rebuild the tooth-supporting tissues. Replicating key extracellular matrix (ECM) cues within tissue engineered scaffolds may maximize PDLC potential. However, the identity of important ECM cues and how they can be harnessed to control PDLC activity is still unknown. In this study, matrix degradability, cell-matrix binding, and stiffness were varied using synthetic poly(ethylene glycol) hydrogels for three-dimensional PDLC culture. PDLCs exhibited dramatic and divergent responses to these cues, supporting further investigation of ECM-replicating scaffolds for control of PDLC behavior and periodontal tissue regeneration.

Recent Advances and Challenges in Controlling the Spatiotemporal Release of Combinatorial Anticancer Drugs from Nanoparticles

To overcome cancer, various chemotherapeutic studies are in progress; among these, studies on nano-formulated combinatorial drugs (NFCDs) are being actively pursued. NFCDs function via a fusion technology that includes a drug delivery system using nanoparticles as a carrier and a combinatorial drug therapy using two or more drugs. It not only includes the advantages of these two technologies, such as ensuring stability of drugs, selectively transporting drugs to cancer cells, and synergistic effects of two or more drugs, but also has the additional benefit of enabling the spatiotemporal and controlled release of drugs. This spatial and temporal drug release from NFCDs depends on the application of nanotechnology and the composition of the combination drug. In this review, recent advances and challenges in the control of spatiotemporal drug release from NFCDs are provided. To this end, the types of combinatorial drug release for various NFCDs are classified in terms of time and space, and the detailed programming techniques used for this are described. In addition, the advantages of the time and space differences in drug release in terms of anticancer efficacy are introduced in depth.

Interplay between degradability and integrin signaling on mesenchymal stem cell function within poly(ethylene glycol) based microporous annealed particle hydrogels

Microporous annealed particle (MAP) hydrogels are promising materials for delivering therapeutic cells. It has previously been shown that spreading and mechanosensing activation of human mesenchymal stem cells (hMSCs) incorporated in these materials can be modulated by tuning the modulus of the microgel particle building blocks. However, the effects of degradability and functionalization with different integrin-binding peptides on cellular responses has not been explored. In this work, RGDS functionalized and enzymatically degradable poly(ethylene glycol) (PEG) microgels were annealed into MAP hydrogels via thiol-ene click chemistry and photopolymerization. During cell-mediated degradation, the microgel surfaces were remodeled to wrinkles or ridges, but the scaffold integrity was maintained. Moreover, cell spreading, proliferation, and secretion of extracellular matrix proteins were significantly enhanced in faster matrix metalloproteinase degrading (KCGPQGIWGQCK) MAP hydrogels compared to non-degradable controls after 8 days of culture. We subsequently evaluated paracrine activity by hMSCs seeded in the MAP hydrogels functionalized with either RGDS or c(RRETAWA), which is specific for α5β1 integrins, and evaluated the interplay between degradability and integrin-mediated signaling. Importantly, c(RRETAWA) functionalization upregulated secretion of bone morphogenetic protein-2 overall and on a per cell basis, but this effect was critically dependent on microgel degradability. In contrast, RGDS functionalization led to higher overall vascular endothelial growth factor secretion in degradable scaffolds due to the high cell number. These results demonstrate that integrin-binding peptides can modulate hMSC behavior in PEG-based MAP hydrogels, but the results strongly depend on the susceptibility of the microgel building blocks to cell-mediated matrix remodeling. This relationship should be considered in future studies aiming to further develop these materials for stem cell delivery and tissue engineering applications. STATEMENT OF SIGNIFICANCE: Microporous annealed particle (MAP) hydrogels are attracting increasing interest for tissue repair and regeneration and have shown superior results compared to conventional hydrogels in multiple applications. Here, we studied the impact of MAP hydrogel degradability and functionalization with different integrin-binding peptides on human mesenchymal stem cells (hMSCs) that were incorporated during particle annealing. Degradability was found to improve cell growth, spreading, and extracellular matrix production regardless of the integrin-binding peptide. Moreover, in degradable MAP hydrogels the integrin-binding peptide c(RRETAWA) was found to increase osteogenic protein expression by hMSCs compared to RGDS-functionalized MAP hydrogels. These results have important implications for the development of a MAP hydrogel-based hMSC delivery system for bone tissue engineering.

Tumor Imaging Using Radiolabeled Matrix Metalloproteinase-Activated Anthrax Proteins

Increased activity of matrix metalloproteinases (MMPs) is associated with worse prognosis in different cancer types. The wild-type protective antigen (PA-WT) of the binary anthrax lethal toxin was modified to form a pore in cell membranes only when cleaved by MMPs (to form PA-L1). Anthrax lethal factor (LF) is then able to translocate through these pores. Here, we used a 111In-radiolabeled form of LF with the PA/LF system for noninvasive in vivo imaging of MMP activity in tumor tissue by SPECT. Methods: MMP-mediated activation of PA-L1 was correlated to anthrax receptor expression and MMP activity in a panel of cancer cells (HT1080, MDA-MB-231, B8484, and MCF7). Uptake of 111In-radiolabeled PA-L1, 111In-PA-WTK563C, or 111In-LFE687A (a catalytically inactive LF mutant) in tumor and normal tissues was measured using SPECT/CT imaging in vivo. Results: Activation of PA-L1 in vitro correlated with anthrax receptor expression and MMP activity (HT1080 > MDA-MB-231 > B8484 > MCF7). PA-L1-mediated delivery of 111In-LFE687A was demonstrated and was corroborated using confocal microscopy with fluorescently labeled LFE687A Uptake was blocked by the broad-spectrum MMP inhibitor GM6001. In vivo imaging showed selective accumulation of 111In-PA-L1 in MDA-MB-231 tumor xenografts (5.7 ± 0.9 percentage injected dose [%ID]/g) at 3 h after intravenous administration. 111In-LFE687A was selectively delivered to MMP-positive MDA-MB-231 tumor tissue by MMP-activatable PA-L1 (5.98 ± 0.62 %ID/g) but not by furin-cleavable PA-WT (1.05 ± 0.21 %ID/g) or a noncleavable PA variant control, PA-U7 (2.74 ± 0.24 %ID/g). Conclusion: Taken together, our results indicate that radiolabeled forms of mutated anthrax lethal toxin hold promise for noninvasive imaging of MMP activity in tumor tissue.

Perlecan domain I gradients establish stable biomimetic heparin binding growth factor gradients for cell migration in hydrogels

Growth factor gradients orchestrate many biological processes including organogenesis, wound healing, cancer invasion, and metastasis. Heparin-binding growth factor (HBGF) gradients are established in living systems by proteoglycans including the extracellular matrix heparan sulfate proteoglycan, perlecan/HSPG2. Three potential HBGF-binding glycosaminoglycan attachment sites occur in N-terminal domain I of perlecan's five domains. Our overarching goal was to form stable, biomimetic non-covalently bound HBGF gradients surrounding cells encapsulated in hyaluronate-based hydrogels by first establishing perlecan domain I (PlnD1) gradients. A versatile multichannel gradient maker device (MGMD) was designed and 3D printed, then used to create desired gradients of microparticles in hydrogels. Next, we used the device to covalently incorporate gradients of PEGylated PlnD1 in hydrogels with high-low-high or high-medium-low concentrations across the hydrogel width. Fluorescently-labeled fibroblast growth factor-2 was delivered to hydrogels in phosphate-buffered saline and allowed to electrostatically bind to the covalently pre-incorporated PlnD1, producing stable non-covalent HBGF gradients. To test cell viability after flow through the MGMD, delicate primary human salivary stem/progenitor cells were encapsulated in gradient hydrogels where they showed high viability and continued to grow. Next, to test migratory behavior in response to HBGF gradients, two cell types, preosteoblastic MC3T3-E1 cell line and breast cancer cell line MDA-MB-231 were encapsulated in or adjacent to PlnD1-modified hydrogels. Both cell lines migrated toward HBGFs bound to PlnD1. We conclude that establishing covalently-bound PlnD1 gradients in hydrogels provides a new means to establish physiologically-relevant gradients of HBGFs that are useful for a variety of applications in tissue engineering and cancer biology. STATEMENT OF SIGNIFICANCE: Gradients of heparin binding growth factors (HBGFs) direct cell behavior in living systems. HBGFs bind electrostatically to gradients of HS proteoglycans in the extracellular matrix creating HBGF gradients. We recreated HBGF gradients in physiological hyaluronate-based hydrogels using a 3D-printed multichannel gradient maker device (MGMD) that created gradients of HS proteoglycan-derived perlecan/HSPG2 domain I. We demonstrated the ability of a variety of cells, including primary salivary stem/progenitor cells, pre-osteoblastic cells and an invasive breast cancer cell line, to be co-encapsulated in gradient hydrogels by flowing them together through the MGMD. The versatile device and the ability to create HBGF gradients in hydrogels for a variety of applications is innovative and of broad utility in both cancer biology and tissue engineering applications.

Peptide-Based Functional Biomaterials for Soft-Tissue Repair

Synthetically derived peptide-based biomaterials are in many instances capable of mimicking the structure and function of their full-length endogenous counterparts. Combine this with the fact that short mimetic peptides are easier to produce when compared to full length proteins, show enhanced processability and ease of modification, and have the ability to be prepared under well-defined and controlled conditions; it becomes obvious why there has been a recent push to develop regenerative biomaterials from these molecules. There is increasing evidence that the incorporation of peptides within regenerative scaffolds can result in the generation of structural recognition motifs that can enhance cell attachment or induce cell signaling pathways, improving cell infiltration or promote a variety of other modulatory biochemical responses. By highlighting the current approaches in the design and application of short mimetic peptides, we hope to demonstrate their potential in soft-tissue healing while at the same time drawing attention to the advances made to date and the problems which need to be overcome to advance these materials to the clinic for applications in heart, skin, and cornea repair.

The extracellular matrix in tumor progression and metastasis

The extracellular matrix (ECM) constitutes the scaffold of tissues and organs. It is a complex network of extracellular proteins, proteoglycans and glycoproteins, which form supramolecular aggregates, such as fibrils and sheet-like networks. In addition to its biochemical composition, including the covalent intermolecular cross-linkages, the ECM is also characterized by its biophysical parameters, such as topography, molecular density, stiffness/rigidity and tension. Taking these biochemical and biophysical parameters into consideration, the ECM is very versatile and undergoes constant remodeling. This review focusses on this remodeling of the ECM under the influence of a primary solid tumor mass. Within this tumor stroma, not only the cancer cells but also the resident fibroblasts, which differentiate into cancer-associated fibroblasts (CAFs), modify the ECM. Growth factors and chemokines, which are tethered to and released from the ECM, as well as metabolic changes of the cells within the tumor bulk, add to the tumor-supporting tumor microenvironment. Metastasizing cancer cells from a primary tumor mass infiltrate into the ECM, which variably may facilitate cancer cell migration or act as barrier, which has to be proteolytically breached by the infiltrating tumor cell. The biochemical and biophysical properties therefore determine the rates and routes of metastatic dissemination. Moreover, primed by soluble factors of the primary tumor, the ECM of distant organs may be remodeled in a way to facilitate the engraftment of metastasizing cancer cells. Such premetastatic niches are responsible for the organotropic preference of certain cancer entities to colonize at certain sites in distant organs and to establish a metastasis. Translational application of our knowledge about the cancer-primed ECM is sparse with respect to therapeutic approaches, whereas tumor-induced ECM alterations such as increased tissue stiffness and desmoplasia, as well as breaching the basement membrane are hallmark of malignancy and diagnostically and histologically harnessed.

Comparative genomic analysis of collagen gene diversity

Collagen gene family, comprising 30% of the total protein mass in mammals, is the major part of extracellular matrix. To understand the complexity of collagen gene family, detailed sequence, phylogenetic and synteny analyses of 44 collagen genes were performed. According to sequence analysis results, Fibril-associated collagen with interrupted triple helices (FACITs) were identified as the most recently evolved vertebrate-specific collagens while Fibril-forming collagens and Collagen VI, VII, XXVI, and XXVIII were the most ancient collagens, originating at the time of choanoflagellates. Network-forming collagens were entirely conserved from arthopods to homo sapiens, except one gene loss event. Of note, bird specific gene dispensability of COL1A1, COL3A1, COL5A3 and COL11A2 genes was observed in Fibril-forming collagens. According to phylogenetic analysis, gene duplications in collagen family occurred at variable time points during invertebrate to vertebrate evolution. However, majority of gene duplications in FACITs and network-forming collagens occurred at fish time point, suggesting large scale duplications at the root of vertebrate lineage. Lastly, synteny analysis identified 12 conserved blocks containing 27 collagen genes in vertebrate species. Interestingly, dysregulation of seven conserved blocks including block1 (COL11A1), block3 (COL3A1, COL5A2), block5 (COL6A5, COL6A6), block7 (COL1A2), block9 (COL4A1, COL4A2), block11 (COL6A1, COL6A2, COL18A1) and block12 (COL4A5, COL4A6) were also reported in different diseases including cancer. The current study revealed many critical insights into sequence, structural and functional diversity of collagen gene family. In future, by using this information we may be able to establish the clinical and pathological relevance of these conserved collagen blocks in different diseases.

Stromal PTEN Regulates Extracellular Matrix Organization in the Mammary Gland

The organization of the extracellular matrix has a profound impact on cancer development and progression. The matrix becomes aligned throughout tumor progression, providing “highways” for tumor cell invasion. Aligned matrix is associated with breast density and is a negative prognostic factor in several cancers; however, the underlying mechanisms regulating this reorganization remain poorly understood. Deletion of the tumor suppressor Pten in the stroma was previously shown to promote extracellular matrix expansion and tumor progression. However, it was unknown if PTEN also regulated matrix organization. To address this question, a murine model with fibroblast-specific Pten deletion was used to examine how PTEN regulates matrix remodeling. Using second harmonic generation microscopy, Pten deletion was found to promote collagen alignment parallel to the mammary duct in the normal gland and further remodeling perpendicular to the tumor edge in tumor-bearing mice. Increased alignment was observed with Pten deletion in vitro using fibroblast-derived matrices. PTEN loss was associated with fibroblast activation and increased cellular contractility, as determined by traction force microscopy. Inhibition of contractility abrogated the increased matrix alignment observed with PTEN loss. Murine mammary adenocarcinoma cells cultured on aligned matrices derived from Pten^−/− fibroblasts migrated faster than on matrices from wild-type fibroblasts. Combined, these data demonstrate that PTEN loss in fibroblasts promotes extracellular matrix deposition and alignment independently from cancer cell presence, and this reorganization regulates cancer cell behavior. Importantly, stromal PTEN negatively correlated with collagen alignment and high mammographic density in human breast tissue, suggesting parallel function for PTEN in patients.

Towards Developing Bioresponsive, Self-Assembled Peptide Materials: Dynamic Morphology and Fractal Nature of Nanostructured Matrices

(Arginine-alanine-aspartic acid-alanine)₄ ((RADA)₄) nanoscaffolds are excellent candidates for use as peptide delivery vehicles: they are relatively easy to synthesize with custom bio-functionality, and assemble in situ to allow a focal point of release. This enables (RADA)₄ to be utilized in multiple release strategies by embedding a variety of bioactive molecules in an all-in-one "construct". One novel strategy focuses on the local, on-demand release of peptides triggered via proteolysis of tethered peptide sequences. However, the spatial-temporal morphology of self-assembling nanoscaffolds may greatly influence the ability of enzymes to both diffuse into as well as actively cleave substrates. Fine structure and its impact on the overall effect on peptide release is poorly understood. In addition, fractal networks observed in nanoscaffolds are linked to the fractal nature of diffusion in these systems. Therefore, matrix morphology and fractal dimension of virgin (RADA)₄ and mixtures of (RADA)₄ and matrix metalloproteinase 2 (MMP-2) cleavable substrate modified (RADA)₄ were characterized over time. Sites of high (glycine-proline-glutamine-glycine+isoleucine-alanine-serine-glutamine (GPQG+IASQ), CP1) and low (glycine-proline-glutamine-glycine+proline-alanine-glycine-glutamine (GPQG+PAGQ), CP2) cleavage activity were chosen. Fine structure was visualized using transmission electron microscopy. After 2 h of incubation, nanofiber networks showed an established fractal nature; however, nanofibers continued to bundle in all cases as incubation times increased. It was observed that despite extensive nanofiber bundling after 24 h of incubation time, the CP1 and CP2 nanoscaffolds were susceptible to MMP-2 cleavage. The properties of these engineered nanoscaffolds characterized herein illustrate that they are an excellent candidate as an enzymatically initiated peptide delivery platform.

Chemoproteomics of matrix metalloproteases in a model of cartilage degeneration suggests functional biomarkers associated with posttraumatic osteoarthritis

Active matrix metalloproteases (MMPs) play a significant role in the pathogenesis of many diseases including osteoarthritis (OA), which involves progressive proteolytic degradation of cartilage. Clinical success of OA interventions that target MMPs has been limited by a lack of information about the presence and activity of specific disease-related proteases. We therefore developed a chemoproteomics approach based on MS to characterize the release and activity of MMPs in an in vitro model of the early inflammatory phase of posttraumatic OA (PTOA). We designed and synthesized chemical activity-based probes (ABPs) to identify active MMPs in bovine cartilage explants cultured for 30 days with the proinflammatory cytokine, interleukin-1α. Using these probes in an activity-based protein profiling-multidimensional identification technology (ABPP-MudPIT) approach, we identified active MMP-1, -2, -3, -7, -9, -12, and -13 in the medium after 10 days of culture, the time at which irreversible proteolysis of the collagen network in the explant was detected using proteolytic activation of FRET-quenched MMP substrates. Total MMP levels were quantified by shotgun proteomics, which, taken with ABPP-MudPIT data, indicated the presence of predominantly inactive MMPs in the culture medium. The selectivity of the ABPP-MudPIT approach was further validated by detection of specific endogenous MMPs activated de novo with 4-aminophenylmurcuric acetate. The utility of the new ABPP-MudPIT approach for detecting molecular biomarkers of PTOA disease initiation and potential targets for therapeutics motivates possible application in other diseases involving MMP activity.

Engineered modular biomaterial logic gates for environmentally triggered therapeutic delivery

The successful transport of drug- and cell-based therapeutics to diseased sites represents a major barrier in the development of clinical therapies. Targeted delivery can be mediated through degradable biomaterial vehicles that utilize disease biomarkers to trigger payload release. Here, we report a modular chemical framework for imparting hydrogels with precise degradative responsiveness by using multiple environmental cues to trigger reactions that operate user-programmable Boolean logic. By specifying the molecular architecture and connectivity of orthogonal stimuli-labile moieties within material cross-linkers, we show selective control over gel dissolution and therapeutic delivery. To illustrate the versatility of this methodology, we synthesized 17 distinct stimuli-responsive materials that collectively yielded all possible YES/OR/AND logic outputs from input combinations involving enzyme, reductant and light. Using these hydrogels we demonstrate the first sequential and environmentally stimulated release of multiple cell lines in well-defined combinations from a material. We expect these platforms will find utility in several diverse fields including drug delivery, diagnostics and regenerative medicine.

Photo-immobilized EGF chemical gradients differentially impact breast cancer cell invasion and drug response in defined 3D hydrogels

Breast cancer cell invasion is influenced by growth factor concentration gradients in the tumor microenvironment. However, studying the influence of growth factor gradients on breast cancer cell invasion is challenging due to both the complexities of in vivo models and the difficulties in recapitulating the tumor microenvironment with defined gradients using in vitro models. A defined hyaluronic acid (HA)-based hydrogel crosslinked with matrix metalloproteinase (MMP) cleavable peptides and modified with multiphoton labile nitrodibenzofuran (NDBF) was synthesized to photochemically immobilize epidermal growth factor (EGF) gradients. We demonstrate that EGF gradients can differentially influence breast cancer cell invasion and drug response in cell lines with different EGF receptor (EGFR) expression levels. Photopatterned EGF gradients increase the invasion of moderate EGFR expressing MDA-MB-231 cells, reduce invasion of high EGFR expressing MDA-MB-468 cells, and have no effect on invasion of low EGFR-expressing MCF-7 cells. We evaluate MDA-MB-231 and MDA-MB-468 cell response to the clinically tested EGFR inhibitor, cetuximab. Interestingly, the cellular response to cetuximab is completely different on the EGF gradient hydrogels: cetuximab decreases MDA-MB-231 cell invasion but increases MDA-MB-468 cell invasion and cell number, thus demonstrating the importance of including cell-microenvironment interactions when evaluating drug targets.

Tricine as a convenient scaffold for the synthesis of C-terminally branched collagen-model peptides

A novel and convenient method for the synthesis of C-terminally branched collagen-model peptides has been achieved using tricine (N-[tris(hydroxymethyl)methyl]glycine) as a branching scaffold and 1,2-diaminoethane or 1,4-diaminobutane as a linker. The peptide sequence was incorporated directly onto the linker and scaffold during solid-phase synthesis without additional manipulations. The resulting branched triple-helical peptides exhibited comparable thermal stabilities to the parent, unbranched sequence, and served as substrates for matrix metalloproteinase-1 (MMP-1). The tricine-based branch reported herein represents the simplest synthetic scaffold for the convenient synthesis of covalently linked homomeric collagen-model triple-helical peptides.

Uniform neural tissue models produced on synthetic hydrogels using standard culture techniques

The aim of the present study was to test sample reproducibility for model neural tissues formed on synthetic hydrogels. Human embryonic stem (ES) cell-derived precursor cells were cultured on synthetic poly(ethylene glycol) (PEG) hydrogels to promote differentiation and self-organization into model neural tissue constructs. Neural progenitor, vascular, and microglial precursor cells were combined on PEG hydrogels to mimic developmental timing, which produced multicomponent neural constructs with 3D neuronal and glial organization, organized vascular networks, and microglia with ramified morphologies. Spearman's rank correlation analysis of global gene expression profiles and a comparison of coefficient of variation for expressed genes demonstrated that replicate neural constructs were highly uniform to at least day 21 for samples from independent experiments. We also demonstrate that model neural tissues formed on PEG hydrogels using a simplified neural differentiation protocol correlated more strongly to in vivo brain development than samples cultured on tissue culture polystyrene surfaces alone. These results provide a proof-of-concept demonstration that 3D cellular models that mimic aspects of human brain development can be produced from human pluripotent stem cells with high sample uniformity between experiments by using standard culture techniques, cryopreserved cell stocks, and a synthetic extracellular matrix. Impact statement Pluripotent stem (PS) cells have been characterized by an inherent ability to self-organize into 3D "organoids" resembling stomach, intestine, liver, kidney, and brain tissues, offering a potentially powerful tool for modeling human development and disease. However, organoid formation must be quantitatively reproducible for applications such as drug and toxicity screening. Here, we report a strategy to produce uniform neural tissue constructs with reproducible global gene expression profiles for replicate samples from multiple experiments.

Local remodeling of synthetic extracellular matrix microenvironments by co-cultured endometrial epithelial and stromal cells enables long-term dynamic physiological function

Mucosal barrier tissues, comprising a layer of tightly-bonded epithelial cells in intimate molecular communication with an underlying matrix-rich stroma containing fibroblasts and immune cells, are prominent targets for drugs against infection, chronic inflammation, and other disease processes. Although human in vitro models of such barriers are needed for mechanistic studies and drug development, differences in extracellular matrix (ECM) needs of epithelial and stromal cells hinder efforts to create such models. Here, using the endometrium as an example mucosal barrier, we describe a synthetic, modular ECM hydrogel suitable for 3D functional co-culture, featuring components that can be remodeled by cells and that respond dynamically to sequester local cell-secreted ECM characteristic of each cell type. The synthetic hydrogel combines peptides with off-the-shelf reagents and is thus accessible to cell biology labs. Specifically, we first identified a single peptide as suitable for initial attachment of both endometrial epithelial and stromal cells using a 2D semi-empirical screen. Then, using a co-culture system of epithelial cells cultured on top of gel-encapsulated stromal cells, we show that inclusion of ECM-binding peptides in the hydrogel, along with the integrin-binding peptide, leads to enhanced accumulation of basement membrane beneath the epithelial layer and more fibrillar collagen matrix assembly by stromal cells over two weeks in culture. Importantly, endometrial co-cultures composed of either cell lines or primary cells displayed hormone-mediated differentiation as assessed by morphological changes and secretory protein production. A multiplex analysis of apical cytokine and growth factor secretion comparing cell lines and primary cells revealed strikingly different patterns, underscoring the importance of using primary cell models in analysis of cell-cell communication networks. In summary, we define a "one-size-fits-all" synthetic ECM that enables long-term, physiologically responsive co-cultures of epithelial and stromal cells in a mucosal barrier format.

Responsive Hybrid (Poly)peptide-Polymer Conjugates

(Poly)peptide-polymer conjugates continue to garner significant interest in the production of functional materials given their composition of natural and synthetic building blocks that confer select and synergistic properties. Owing to opportunities to design predefined architectures and structures with different morphologies, these hybrid conjugates enable new approaches for producing micro- or nanomaterials. Their modular design enables the incorporation of multiple responsive properties into a single conjugate. This review presents recent advances in (poly)peptide-polymer conjugates for drug-delivery applications, with a specific focus on the utility of the (poly)peptide component in the assembly of particles and nanogels, as well as the role of the peptide in triggered drug release.

Matrix degradability controls multicellularity of 3D cell migration

A major challenge in tissue engineering is the development of materials that can support angiogenesis, wherein endothelial cells from existing vasculature invade the surrounding matrix to form new vascular structures. To identify material properties that impact angiogenesis, here we have developed an in vitro model whereby molded tubular channels inside a synthetic hydrogel are seeded with endothelial cells and subjected to chemokine gradients within a microfluidic device. To accomplish precision molding of hydrogels and successful integration with microfluidics, we developed a class of hydrogels that could be macromolded and micromolded with high shape and size fidelity by eliminating swelling after polymerization. Using this material, we demonstrate that matrix degradability switches three-dimensional endothelial cell invasion between two distinct modes: single-cell migration and the multicellular, strand-like invasion required for angiogenesis. The ability to incorporate these tunable hydrogels into geometrically constrained settings will enable a wide range of previously inaccessible biomedical applications.

The fabrication of vascularized 3D tissues requires an understanding of how material properties govern endothelial cell invasion into the surrounding matrix. Here the authors integrate a non-swelling synthetic hydrogel with a microfluidic device to study chemokine gradient-driven angiogenic sprouting and find that matrix degradability modulates the collectivity of cell migration.

Diagnosing peri-implant disease using the tongue as a 24/7 detector

Our ability of screening broad communities for clinically asymptomatic diseases critically drives population health. Sensory chewing gums are presented targeting the tongue as 24/7 detector allowing diagnosis by "anyone, anywhere, anytime". The chewing gum contains peptide sensors consisting of a protease cleavable linker in between a bitter substance and a microparticle. Matrix metalloproteinases in the oral cavity, as upregulated in peri-implant disease, specifically target the protease cleavable linker while chewing the gum, thereby generating bitterness for detection by the tongue. The peptide sensors prove significant success in discriminating saliva collected from patients with peri-implant disease versus clinically asymptomatic volunteers. Superior outcome is demonstrated over commercially available protease-based tests in saliva. "Anyone, anywhere, anytime" diagnostics are within reach for oral inflammation. Expanding this platform technology to other diseases in the future features this diagnostic as a massive screening tool potentially maximizing impact on population health.Early detection of gum inflammation caused by dental implants helps prevent tissue damage. Here, the authors present a peptide sensor that generates a bitter taste when cleaved by proteases present in peri-implant disease, embed it in a chewing gum, and compare the probe to existing sensors using patient saliva.

Microcarriers with Synthetic Hydrogel Surfaces for Stem Cell Expansion

Microcarriers are scalable support surfaces for cell growth that enable high levels of expansion, and are particularly relevant for expansion of human mesenchymal stem cells (hMSCs). The goal of this study is to develop a poly(ethylene glycol) (PEG)-based microcarrier coating for hMSC expansion. Commercially available microcarriers do not offer customizability of microcarrier surface properties, including elastic modulus and surface cell adhesion ligands. The lab has previously demonstrated that tuning these material properties on PEG-based hydrogels can modulate important cellular growth characteristics, such as cell attachment and expansion, which are important in microcarrier-based culture. Eosin-Y is adsorbed to polystyrene microcarriers and used as a photoinitiator for thiol-ene polymerization under visible light. Resultant PEG coatings are over 100 µm thick and localized to microcarrier surfaces. This thickness is relevant for cells to react to mechanical properties of the hydrogel coating, and coated microcarriers support hMSC attachment and expansion. hMSC expansion is highly favorable on coated microcarriers in serum-free media, with doubling times under 25 h in the growth phase, and retained osteogenic and adipogenic differentiation capacity after culture on microcarriers. These microcarriers with defined, synthetic coatings enable tailorable surfaces for cell expansion that may be suitable for a variety of biomanufacturing applications.

Versatile synthetic alternatives to Matrigel for vascular toxicity screening and stem cell expansion

The physiological relevance of Matrigel as a cell-culture substrate and in angiogenesis assays is often called into question. Here, we describe an array-based method for the identification of synthetic hydrogels that promote the formation of robust in vitro vascular networks for the detection of putative vascular disruptors, and that support human embryonic stem cell expansion and pluripotency. We identified hydrogel substrates that promoted endothelial-network formation by primary human umbilical vein endothelial cells and by endothelial cells derived from human induced pluripotent stem cells, and used the hydrogels with endothelial networks to identify angiogenesis inhibitors. The synthetic hydrogels show superior sensitivity and reproducibility over Matrigel when evaluating known inhibitors, as well as in a blinded screen of a subset of 38 chemicals, selected according to predicted vascular disruption potential, from the Toxicity ForeCaster library of the US Environmental Protection Agency. The identified synthetic hydrogels should be suitable alternatives to Matrigel for common cell-culture applications.

Specific recruitment of circulating angiogenic cells using biomaterials as filters

Endogenous recruitment of circulating angiogenic cells (CACs) is an emerging strategy to induce angiogenesis within a defect site, and multiple recent strategies have deployed soluble protein releasing biomaterials for this purpose. However, the way in which the design of biomaterials affects CAC recruitment and invasion are poorly understood. Here we used an enhanced-throughput cell invasion assay to systematically examine the effects of biomaterial design on CAC recruitment. The screens co-optimized hydrogel presentation of a stromal-derived factor-1α (SDF-1α) gradient, hydrogel degradability, and hydrogel stiffness for maximal CAC invasion. We also examined the specificity of this invasion by assessing dermal fibroblast, mesenchymal stem cell, and lymphocyte invasion individually and in co-culture with CACs to identify hydrogels specific to CAC invasion. These screens suggested a subset of MMP-degradable hydrogels presenting a specific range of SDF-1α gradient slopes that induced specific invasion of CACs, and we posit that the design parameters of this subset of hydrogels may serve as instructive templates for the future design of biomaterials to specifically recruit CACs. We also posit that this design concept may be applied more broadly in that it may be possible to utilize these specific subsets of biomaterials as "filters" to control which types of cell populations invade into and populate the biomaterial.

STATEMENT OF SIGNIFICANCE

The recruitment of specific cell types for cell-based therapies in vivo is of great interest to the regenerative medicine community. Circulating angiogenic cells (CACs), CD133+ cells derived from the blood stream, are of particular interest for induction of angiogenesis in ischemic tissues, and recent studies utilizing soluble-factor releasing biomaterials to recruit these cells in vivo show great promise. However, these studies are largely "proof of concept" and are not systematic in nature. Thus, little is currently known about how biomaterial design affects the recruitment of CACs. In the present work, we use a high throughput cell invasion screening platform to systematically examine the effects of biomaterial design on circulating angiogenic cell (CAC) recruitment, and we successfully screened 263 conditions at 3 replicates each. Our results identify a particular subset of conditions that robustly recruit CACs. Additionally, we found that these conditions also specifically recruited CACs and excluded the other tested cells types of dermal fibroblasts, mesenchymal stem cells, and lymphocytes. This suggests an intriguing new role for biomaterials as "filters" to control the types of cells that invade and populate that biomaterial.

Ultrasensitive tumour-penetrating nanosensors of protease activity

The ability to identify cancer lesions with endogenous biomarkers is currently limited to tumours ~1 cm in diameter. We recently reported an exogenously administered tumour-penetrating nanosensor that sheds, in response to tumour-specific proteases, peptide fragments that can then be detected in the urine. Here, we report the optimization, informed by a pharmacokinetic mathematical model, of the surface presentation of the peptide substrates to both enhance on-target protease cleavage and minimize off-target cleavage, and of the functionalization of the nanosensors with tumour-penetrating ligands that engage active trafficking pathways to increase activation in the tumour microenvironment. The resulting nanosensor discriminated sub-5 mm lesions in human epithelial tumours and detected nodules with median diameters smaller than 2 mm in an orthotopic model of ovarian cancer. We also demonstrate enhanced receptor-dependent specificity of signal generation in the urine in an immunocompetent model of colorectal liver metastases, and in situ activation of the nanosensors in human tumour microarrays when re-engineered as fluorogenic zymography probes.

A Genome-wide Analysis of Human Pluripotent Stem Cell-Derived Endothelial Cells in 2D or 3D Culture

A defined protocol for efficiently deriving endothelial cells from human pluripotent stem cells was established and vascular morphogenesis was used as a model system to understand how synthetic hydrogels influence global biological function compared with common 2D and 3D culture platforms. RNA sequencing demonstrated that gene expression profiles were similar for endothelial cells and pericytes cocultured in polyethylene glycol (PEG) hydrogels or Matrigel, while monoculture comparisons identified distinct vascular signatures for each cell type. Endothelial cells cultured on tissue-culture polystyrene adopted a proliferative phenotype compared with cells cultured on or encapsulated in PEG hydrogels. The proliferative phenotype correlated to increased FAK-ERK activity, and knockdown or inhibition of ERK signaling reduced proliferation and expression for cell-cycle genes while increasing expression for “3D-like” vasculature development genes. Our results provide insight into the influence of 2D and 3D culture formats on global biological processes that regulate cell function.

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Defined, high-efficiency differentiation of human PSCs to endothelial cell

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Comprehensive genome-wide comparisons of 2D and 3D cell-culture formats

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Gene expression profiles for endothelial cells and pericytes in 3D cell culture

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Highly proliferative phenotypes on tissue-culture polystyrene surfaces

Engineering epithelial-stromal interactions in vitro for toxicology assessment

Crosstalk between epithelial and stromal cells drives the morphogenesis of ectodermal organs during development and promotes normal mature adult epithelial tissue homeostasis. Epithelial-stromal interactions (ESIs) have historically been examined using mammalian models and ex vivo tissue recombination. Although these approaches have elucidated signaling mechanisms underlying embryonic morphogenesis processes and adult mammalian epithelial tissue function, they are limited by the availability of tissue, low throughput, and human developmental or physiological relevance. In this review, we describe how bioengineered ESIs, using either human stem cells or co-cultures of human primary epithelial and stromal cells, have enabled the development of human in vitro epithelial tissue models that recapitulate the architecture, phenotype, and function of adult human epithelial tissues. We discuss how the strategies used to engineer mature epithelial tissue models in vitro could be extrapolated to instruct the design of organotypic culture models that can recapitulate the structure of embryonic ectodermal tissues and enable the in vitro assessment of events critical to organ/tissue morphogenesis. Given the importance of ESIs towards normal epithelial tissue development and function, such models present a unique opportunity for toxicological screening assays to incorporate ESIs to assess the impact of chemicals on mature and developing epidermal tissues.

Tissue engineering-based therapeutic strategies for vocal fold repair and regeneration

Vocal folds are soft laryngeal connective tissues with distinct layered structures and complex multicomponent matrix compositions that endow phonatory and respiratory functions. This delicate tissue is easily damaged by various environmental factors and pathological conditions, altering vocal biomechanics and causing debilitating vocal disorders that detrimentally affect the daily lives of suffering individuals. Modern techniques and advanced knowledge of regenerative medicine have led to a deeper understanding of the microstructure, microphysiology, and micropathophysiology of vocal fold tissues. State-of-the-art materials ranging from extracecullar-matrix (ECM)-derived biomaterials to synthetic polymer scaffolds have been proposed for the prevention and treatment of voice disorders including vocal fold scarring and fibrosis. This review intends to provide a thorough overview of current achievements in the field of vocal fold tissue engineering, including the fabrication of injectable biomaterials to mimic in vitro cell microenvironments, novel designs of bioreactors that capture in vivo tissue biomechanics, and establishment of various animal models to characterize the in vivo biocompatibility of these materials. The combination of polymeric scaffolds, cell transplantation, biomechanical stimulation, and delivery of antifibrotic growth factors will lead to successful restoration of functional vocal folds and improved vocal recovery in animal models, facilitating the application of these materials and related methodologies in clinical practice.

Modular GAG-matrices to promote mammary epithelial morphogenesis in vitro

Matrix systems used to study complex three-dimensional (3D) cellular processes like mammary epithelial tissue morphogenesis and tumorigenesis ex vivo often require ill-defined biological components, which lead to poor reproducibility and a lack of control over physical parameters. In this study, a well-defined, tunable synthetic biohybrid hydrogel composed of the glycosaminoglycan heparin, star-shaped poly(ethylene glycol) (starPEG), and matrix metalloproteinase- (MMP-) cleavable crosslinkers was applied to dissect the biophysical and biochemical signals promoting human mammary epithelial cell (MEC) morphogenesis. We show that compliant starPEG-heparin matrices promote the development of polarized MEC acini. Both the presence of heparin and MMP-cleavable crosslinks are essential in facilitating MEC morphogenesis without supplementation of exogenous adhesion ligands. In this system, MECs secrete and organize laminin in basement membrane-like assemblies to promote integrin signaling and drive acinar development. Therefore, starPEG-heparin hydrogels provide a versatile platform to study mammary epithelial tissue morphogenesis in a chemically defined and precisely tunable 3D in vitro microenvironment. The system allows investigation of biophysical and biochemical aspects of mammary gland biology and potentially a variety of other organoid culture studies.

Human iPSC-derived endothelial cell sprouting assay in synthetic hydrogel arrays

Activation of vascular endothelial cells (ECs) by growth factors initiates a cascade of events during angiogenesis in vivo consisting of EC tip cell selection, sprout formation, EC stalk cell proliferation, and ultimately vascular stabilization by support cells. Although EC functional assays can recapitulate one or more aspects of angiogenesis in vitro, they are often limited by undefined substrates and lack of dependence on key angiogenic signaling axes. Here, we designed and characterized a chemically-defined model of endothelial sprouting behavior in vitro using human induced pluripotent stem cell-derived endothelial cells (iPSC-ECs). We rapidly encapsulated iPSC-ECs at high density in poly(ethylene glycol) (PEG) hydrogel spheres using thiol-ene chemistry and subsequently encapsulated cell-dense hydrogel spheres in a cell-free hydrogel layer. The hydrogel sprouting array supported pro-angiogenic phenotype of iPSC-ECs and supported growth factor-dependent proliferation and sprouting behavior. iPSC-ECs in the sprouting model responded appropriately to several reference pharmacological angiogenesis inhibitors of vascular endothelial growth factor, NF-κB, matrix metalloproteinase-2/9, protein kinase activity, and β-tubulin, which confirms their functional role in endothelial sprouting. A blinded screen of 38 putative vascular disrupting compounds from the US Environmental Protection Agency's ToxCast library identified six compounds that inhibited iPSC-EC sprouting and five compounds that were overtly cytotoxic to iPSC-ECs at a single concentration. The chemically-defined iPSC-EC sprouting model (iSM) is thus amenable to enhanced-throughput screening of small molecular libraries for effects on angiogenic sprouting and iPSC-EC toxicity assessment.

STATEMENT OF SIGNIFICANCE

Angiogenesis assays that are commonly used for drug screening and toxicity assessment applications typically utilize natural substrates like Matrigel(TM) that are difficult to spatially pattern, costly, ill-defined, and may exhibit lot-to-lot variability. Herein, we describe a novel angiogenic sprouting assay using chemically-defined, bioinert poly(ethylene glycol) hydrogels functionalized with biomimetic peptides to promote cell attachment and degradation in a reproducible format that may mitigate the need for natural substrates. The quantitative assay of angiogenic sprouting here enables precise control over the initial conditions and can be formulated into arrays for screening. The sprouting assay here was dependent on key angiogenic signaling axes in a screen of angiogenesis inhibitors and a blinded screen of putative vascular disrupting compounds from the US-EPA.

The In Vitro and In Vivo Response to MMP-Sensitive Poly(Ethylene Glycol) Hydrogels

Enzyme-sensitive hydrogels are a promising class of materials for cell encapsulation and tissue engineering because their ability to be degraded by cell-secreted factors. However, it is well known that nearly all synthetic biomaterials elicit a foreign body response (FBR) upon implantation. Therefore, this study aimed to evaluate the in vitro and in vivo response to an enzyme-sensitive hydrogel. Hydrogels were formed from poly(ethylene glycol) with the peptide crosslinker, C-VPLS↓LYSG-C, which is susceptible to matrix metalloproteinases 2 and 9. We evaluated the hydrogel by exogenously delivered enzymes, encapsulated mesenchymal stem cells as a tissue engineering relevant cell type, and by macrophage-secreted factors in vitro and for the FBR through macrophage attachment in vitro and in a subcutaneous mouse model. These hydrogels rapidly degraded upon exposure to exogenous MMP-2 and to lesser degree with MMP-9. Encapsulated mesenchymal stem cells were capable of degrading the hydrogels via matrix metalloproteinases. Inflammatory macrophages were confirmed to attach to the hydrogels, but were not capable of rapidly degrading the hydrogels. In vivo, these hydrogels remained intact after 4 weeks and exhibited a classic FBR with inflammatory cells at the hydrogel surface and a fibrous capsule. In summary, these findings suggest that while this MMP-2/9 sensitive hydrogel is readily degraded in vitro, it does not undergo rapid degradation by the FBR. Thus, the long term stability of these hydrogels in vivo coupled with the ability for encapsulated cells to degrade the hydrogel makes them promising materials for tissue engineering.

Stable engineered vascular networks from human induced pluripotent stem cell-derived endothelial cells cultured in synthetic hydrogels

Here, we describe an in vitro strategy to model vascular morphogenesis where human induced pluripotent stem cell-derived endothelial cells (iPSC-ECs) are encapsulated in peptide-functionalized poly(ethylene glycol) (PEG) hydrogels, either on standard well plates or within a passive pumping polydimethylsiloxane (PDMS) tri-channel microfluidic device. PEG hydrogels permissive towards cellular remodeling were fabricated using thiol-ene photopolymerization to incorporate matrix metalloproteinase (MMP)-degradable crosslinks and CRGDS cell adhesion peptide. Time lapse microscopy, immunofluorescence imaging, and RNA sequencing (RNA-Seq) demonstrated that iPSC-ECs formed vascular networks through mechanisms that were consistent with in vivo vasculogenesis and angiogenesis when cultured in PEG hydrogels. Migrating iPSC-ECs condensed into clusters, elongated into tubules, and formed polygonal networks through sprouting. Genes upregulated for iPSC-ECs cultured in PEG hydrogels relative to control cells on tissue culture polystyrene (TCP) surfaces included adhesion, matrix remodeling, and Notch signaling pathway genes relevant to in vivo vascular development. Vascular networks with lumens were stable for at least 14days when iPSC-ECs were encapsulated in PEG hydrogels that were polymerized within the central channel of the microfluidic device. Therefore, iPSC-ECs cultured in peptide-functionalized PEG hydrogels offer a defined platform for investigating vascular morphogenesis in vitro using both standard and microfluidic formats.

STATEMENT OF SIGNIFICANCE

Human induced pluripotent stem cell-derived endothelial cells (iPSC-ECs) cultured in synthetic hydrogels self-assemble into capillary networks through mechanisms consistent with in vivo vascular morphogenesis.

Recombinant Resilin-Based Bioelastomers for Regenerative Medicine Applications

The outstanding elasticity, excellent resilience at high-frequency, and hydrophilic capacity of natural resilin have motivated investigations of recombinant resilin-based biomaterials as a new class of bio-elastomers in the engineering of mechanically active tissues. Accordingly, here the comprehensive characterization of modular resilin-like polypeptide (RLP) hydrogels is presented and their suitability as a novel biomaterial for in vivo applications is introduced. Oscillatory rheology confirmed that a full suite of the RLPs can be rapidly cross-linked upon addition of the tris(hydroxymethyl phosphine) cross-linker, achieving similar in situ shear storage moduli (20 k ± 3.5 Pa) across various material compositions. Uniaxial stress relaxation tensile testing of hydrated RLP hydrogels under cyclic loading and unloading showed negligible stress reduction and hysteresis, superior reversible extensibility, and high resilience with Young's moduli of 30 ± 7.4 kPa. RLP hydrogels containing MMP-sensitive domains are susceptible to enzymatic degradation by matrix metalloproteinase-1 (MMP-1). Cell culture studies revealed that RLP-based hydrogels supported the attachment and spreading (2D) of human mesenchymal stem cells and did not activate cultured macrophages. Subcutaneous transplantation of RLP hydrogels in a rat model, which to our knowledge is the first such reported in vivo analysis of RLP-based hydrogels, illustrated that these materials do not elicit a significant inflammatory response, suggesting their potential as materials for tissue engineering applications with targets of mechanically demanding tissues such as vocal fold and cardiovascular tissues.

Resilin-PEG Hybrid Hydrogels Yield Degradable Elastomeric Scaffolds with Heterogeneous Microstructure

Hydrogels derived from resilin-like polypeptides (RLPs) have shown outstanding mechanical resilience and cytocompatibility; expanding the versatility of RLP-based materials via conjugation with other polypeptides and polymers would offer great promise in the design of a range of materials. Here, we present an investigation of the biochemical and mechanical properties of hybrid hydrogels composed of a recombinant RLP and a multiarm PEG macromer. These hybrid hydrogels can be rapidly cross-linked through a Michael-type addition reaction between the thiols of cysteine residues on the RLP and vinyl sulfone groups on the multiarm PEG. Oscillatory rheology and tensile testing confirmed the formation of elastomeric hydrogels with mechanical resilience comparable to aortic elastin; hydrogel stiffness was easily modulated through the cross-linking ratio. Macromolecular phase separation of the RLP-PEG hydrogels offers the unique advantage of imparting a heterogeneous microstructure, which can be used to localize cells, through simple mixing and cross-linking. Assessment of degradation of the RLP by matrix metalloproteinases (MMPs) illustrated the specific proteolysis of the polypeptide in both its soluble form and when cross-linked into hydrogels. Finally, the successful encapsulation and viable three-dimensional culture of human mesenchymal stem cells (hMSCs) demonstrated the cytocompatibility of the RLP-PEG gels. Overall, the cytocompatibility, elastomeric mechanical properties, microheterogeneity, and degradability of the RLP-PEG hybrid hydrogels offer a suite of promising properties for the development of cell-instructive, structured tissue engineering scaffolds.

A synthetic modular approach for modeling the role of the 3D microenvironment in tumor progression

Here, we demonstrate the flexibility of peptide-functionalized poly(ethylene glycol) (PEG) hydrogels for modeling tumor progression. The PEG hydrogels were formed using thiol-ene chemistry to incorporate a matrix metalloproteinase-degradable peptide crosslinker (KKCGGPQG↓IWGQGCKK) permissive to proteolytic remodeling and the adhesive CRGDS peptide ligand. Tumor cell function was investigated by culturing WM239A melanoma cells on PEG hydrogel surfaces or encapsulating cells within the hydrogels, and either as monocultures or indirect (non-contact) cocultures with primary human dermal fibroblasts (hDFs). WM239A cluster size and proliferation rate depended on the shear elastic modulus for cells cultured on PEG hydrogels, while growth was inhibited by coculture with hDFs regardless of hydrogel stiffness. Cluster size was also suppressed by hDFs for WM239A cells encapsulated in PEG hydrogels, which is consistent with cells seeded on top of hydrogels. Notably, encapsulated WM239A clusters and single cells adopted invasive phenotypes in the hDF coculture model, which included single cell and collective migration modes that resembled invasion from human melanoma patient-derived xenograft tumors encapsulated in equivalent PEG hydrogels. Our combined results demonstrate that peptide-functionalized PEG hydrogels provide a useful platform for investigating aspects of tumor progression in 2D and 3D microenvironments, including single cell migration, cluster growth and invasion.

Poly(ethylene glycol) Hydrogel Scaffolds Containing Cell-Adhesive and Protease-Sensitive Peptides Support Microvessel Formation by Endothelial Progenitor Cells

The development of stable, functional microvessels remains an important obstacle to overcome for tissue engineered organs and treatment of ischemia. Endothelial progenitor cells (EPCs) are a promising cell source for vascular tissue engineering as they are readily obtainable and carry the potential to differentiate towards all endothelial phenotypes. The aim of this study was to investigate the ability of human umbilical cord blood-derived EPCs to form vessel-like structures within a tissue engineering scaffold material, a cell-adhesive and proteolytically degradable poly(ethylene glycol) (PEG) hydrogel. EPCs in co-culture with angiogenic mural cells were encapsulated in hydrogel scaffolds by mixing with polymeric precursors and using a mild photocrosslinking process to form hydrogels with homogeneously dispersed cells. EPCs formed 3D microvessels networks that were stable for at least 30 days in culture, without the need for supplemental angiogenic growth factors. These 3D EPC microvessels displayed aspects of physiological microvasculature with lumen formation, expression of endothelial cell proteins (connexin 32, VE-cadherin, eNOS), basement membrane formation with collagen IV and laminin, perivascular investment of PDGFR-β and α-SMA positive cells, and EPC quiescence (<1% proliferating cells) by 2 weeks of co-culture. Our findings demonstrate the development of a novel, reductionist system that is well-defined and reproducible for studying progenitor cell-driven microvessel formation.

Human Induced Pluripotent Stem Cell Derived Neuronal Cells Cultured on Chemically-Defined Hydrogels for Sensitive In Vitro Detection of Botulinum Neurotoxin

Botulinum neurotoxin (BoNT) detection provides a useful model for validating cell-based neurotoxicity screening approaches, as sensitivity is dependent on functionally competent neurons and clear quantitative endpoints are available for correlating results to approved animal testing protocols. Here, human induced pluripotent stem cell (iPSC)-derived neuronal cells were cultured on chemically-defined poly(ethylene glycol) (PEG) hydrogels formed by "thiol-ene" photopolymerization and tested as a cell-based neurotoxicity assay by determining sensitivity to active BoNT/A1. BoNT/A1 sensitivity was comparable to the approved in vivo mouse bioassay for human iPSC-derived neurons and neural stem cells (iPSC-NSCs) cultured on PEG hydrogels or treated tissue culture polystyrene (TCP) surfaces. However, maximum sensitivity for BoNT detection was achieved two weeks earlier for iPSC-NSCs that were differentiated and matured on PEG hydrogels compared to TCP. Therefore, chemically-defined synthetic hydrogels offer benefits over standard platforms when optimizing culture conditions for cell-based screening and achieve sensitivities comparable to an approved animal testing protocol.

Human pluripotent stem cell-derived neural constructs for predicting neural toxicity

Human pluripotent stem cell-based in vitro models that reflect human physiology have the potential to reduce the number of drug failures in clinical trials and offer a cost-effective approach for assessing chemical safety. Here, human embryonic stem (ES) cell-derived neural progenitor cells, endothelial cells, mesenchymal stem cells, and microglia/macrophage precursors were combined on chemically defined polyethylene glycol hydrogels and cultured in serum-free medium to model cellular interactions within the developing brain. The precursors self-assembled into 3D neural constructs with diverse neuronal and glial populations, interconnected vascular networks, and ramified microglia. Replicate constructs were reproducible by RNA sequencing (RNA-Seq) and expressed neurogenesis, vasculature development, and microglia genes. Linear support vector machines were used to construct a predictive model from RNA-Seq data for 240 neural constructs treated with 34 toxic and 26 nontoxic chemicals. The predictive model was evaluated using two standard hold-out testing methods: a nearly unbiased leave-one-out cross-validation for the 60 training compounds and an unbiased blinded trial using a single hold-out set of 10 additional chemicals. The linear support vector produced an estimate for future data of 0.91 in the cross-validation experiment and correctly classified 9 of 10 chemicals in the blinded trial.

Delivery of a Protease-Activated Cytolytic Peptide Prodrug by Perfluorocarbon Nanoparticles

Melittin is a cytolytic peptide derived from bee venom that inserts into lipid membranes and oligomerizes to form membrane pores. Although this peptide is an attractive candidate for treatment of cancers and infectious processes, its nonspecific cytotoxicity and hemolytic activity have limited its therapeutic applications. Several groups have reported the development of cytolytic peptide prodrugs that only exhibit cytotoxicity following activation by site-specific proteases. However, systemic administration of these constructs has proven difficult because of their poor pharmacokinetic properties. Here, we present a platform for the design of protease-activated melittin derivatives that may be used in conjunction with a perfluorocarbon nanoparticle delivery system. Although native melittin was substantially hemolytic (HD50: 1.9 μM) and cytotoxic (IC50: 2.4 μM), the prodrug exhibited 2 orders of magnitude less hemolytic activity (HD50: > 100 μM) and cytotoxicity (IC50: > 100 μM). Incubation with matrix metalloproteinase-9 (MMP-9) led to cleavage of the prodrug at the expected site and restoration of hemolytic activity (HD50: 3.4 μM) and cytotoxicity (IC50: 8.1 μM). Incubation of the prodrug with perfluorocarbon nanoparticles led to stable loading of 10,250 peptides per nanoparticle. Nanoparticle-bound prodrug was also cleaved and activated by MMP-9, albeit at a fourfold slower rate. Intravenous administration of prodrug-loaded nanoparticles in a mouse model of melanoma significantly decreased tumor growth rate (p = 0.01). Because MMPs and other proteases play a key role in cancer invasion and metastasis, this platform holds promise for the development of personalized cancer therapies directed toward a patient's individual protease expression profile.

Collagenolytic Matrix Metalloproteinase Activities toward Peptomeric Triple-Helical Substrates

Although collagenolytic matrix metalloproteinases (MMPs) possess common domain organizations, there are subtle differences in their processing of collagenous triple-helical substrates. In this study, we have incorporated peptoid residues into collagen model triple-helical peptides and examined MMP activities toward these peptomeric chimeras. Several different peptoid residues were incorporated into triple-helical substrates at subsites P3, P1, P1', and P10' individually or in combination, and the effects of the peptoid residues were evaluated on the activities of full-length MMP-1, MMP-8, MMP-13, and MMP-14/MT1-MMP. Most peptomers showed little discrimination between MMPs. However, a peptomer containing N-methyl Gly (sarcosine) in the P1' subsite and N-isobutyl Gly (NLeu) in the P10' subsite was hydrolyzed efficiently only by MMP-13 [nomenclature relative to the α1(I)772-786 sequence]. Cleavage site analysis showed hydrolysis at the Gly-Gln bond, indicating a shifted binding of the triple helix compared to the parent sequence. Favorable hydrolysis by MMP-13 was not due to sequence specificity or instability of the substrate triple helix but rather was based on the specific interactions of the P7' peptoid residue with the MMP-13 hemopexin-like domain. A fluorescence resonance energy transfer triple-helical peptomer was constructed and found to be readily processed by MMP-13, not cleaved by MMP-1 and MMP-8, and weakly hydrolyzed by MT1-MMP. The influence of the triple-helical structure containing peptoid residues on the interaction between MMP subsites and individual substrate residues may provide additional information about the mechanism of collagenolysis, the understanding of collagen specificity, and the design of selective MMP probes.

Multifunctional bioscaffolds for 3D culture of melanoma cells reveal increased MMP activity and migration with BRAF kinase inhibition

Matrix metalloproteinases (MMPs) are important for many different types of cancer-related processes, including metastasis. Understanding the functional impact of changes in MMP activity during cancer treatment is an important facet not typically evaluated as part of preclinical research. With MMP activity being a critical component of the metastatic cascade, we designed a 3D hydrogel system to probe whether pharmacological inhibition affected human melanoma cell proteolytic activity; metastatic melanoma is a highly aggressive and drug-resistant form of skin cancer. The relationship between MMP activity and drug treatment is unknown, and therefore we used an in situ fluorogenic MMP sensor peptide to determine how drug treatment affects melanoma cell MMP activity in three dimensions. We encapsulated melanoma cells from varying stages of progression within PEG-based hydrogels to examine the relationship between drug treatment and MMP activity. From these results, a metastatic melanoma cell line (A375) and two inhibitors that inhibit RAF (PLX4032 and sorafenib) were studied further to determine whether changes in MMP activity led to a functional change in cell behavior. A375 cells exhibited increased MMP activity despite an overall decrease in metabolic activity with PLX4032 treatment. The changes in proteolytic activity correlated with increased cell elongation and increased single-cell migration. In contrast, sorafenib did not alter MMP activity or cell motility, showing that the changes induced by PLX4032 were not a universal response to small-molecule inhibition. Therefore, we argue the importance of studying MMP activity with drug treatment and its possible implications for unwanted side effects.

Encoding Hydrogel Mechanics via Network Cross-Linking Structure

The effects of mechanical cues on cell behaviors in 3D remain difficult to characterize as the ability to tune hydrogel mechanics often requires changes in the polymer density, potentially altering the material’s biochemical and physical characteristics. Additionally, with most PEG diacrylate (PEGDA) hydrogels, forming materials with compressive moduli less than ∼10 kPa has been virtually impossible. Here, we present a new method of controlling the mechanical properties of PEGDA hydrogels independent of polymer chain density through the incorporation of additional vinyl group moieties that interfere with the cross-linking of the network. This modification can tune hydrogel mechanics in a concentration dependent manner from <1 to 17 kPa, a more physiologically relevant range than previously possible with PEG-based hydrogels, without altering the hydrogel’s degradation and permeability. Across this range of mechanical properties, endothelial cells (ECs) encapsulated within MMP-2/MMP-9 degradable hydrogels with RGDS adhesive peptides revealed increased cell spreading as hydrogel stiffness decreased in contrast to behavior typically observed for cells on 2D surfaces. EC-pericyte cocultures exhibited vessel-like networks within 3 days in highly compliant hydrogels as compared to a week in stiffer hydrogels. These vessel networks persisted for at least 4 weeks and deposited laminin and collagen IV perivascularly. These results indicate that EC morphogenesis can be regulated using mechanical cues in 3D. Furthermore, controlling hydrogel compliance independent of density allows for the attainment of highly compliant mechanical regimes in materials that can act as customizable cell microenvironments.

Evaluation of MMP substrate concentration and specificity for neovascularization of hydrogel scaffolds

Controlled vascular response in scaffolds following implantation remains a significant clinical challenge. A critical biomaterial design criterion is the synchronization of the rates of scaffold degradation and vascularized tissue formation. Matrix metalloproteinases (MMPs) are key enzymes that regulate neovascularization and extracellular matrix remodelling. Synthetic protease-sensitive hydrogels offer controllable environments for investigating the role of matrix degradation on neovascularization. In this study, PEG hydrogels containing MMP-sensitive peptides with increased catalytic activity for MMPs expressed during neovascularization were investigated. Scaffolds were functionalized with MMP-2-, MMP-14- or general collagenase-sensitive peptides and with varying peptide concentration using crosslinkers containing one (SSite) or multiple (TSite) repeats of each protease-sensitive sequence. Increasing peptide concentration enhanced the degradation kinetics of scaffolds functionalized with MMP-specific sequences while 80% of the collagenase-sensitive scaffolds remained upon exposure to MMP-2 and MMP-14. In vitro neovascularization was consistent with in vivo tissue invasion with significantly increased invasion occurring within SSite MMP-specific as compared to collagenase-sensitive hydrogels and with further invasion in TSite as compared to SSite hydrogels regardless of peptide specificity. All scaffolds supported in vivo neovascularization; however, this was not dependent on peptide specificity. These findings demonstrate that peptide concentration and specificity regulate in vivo scaffold degradation, neovascularization and matrix remodelling.

A peptide functionalized poly(ethylene glycol) (PEG) hydrogel for investigating the influence of biochemical and biophysical matrix properties on tumor cell migration

To address the challenges associated with defined control over matrix properties in 3D cell culture systems, we employed a peptide functionalized poly(ethylene glycol) (PEG) hydrogel matrix in which mechanical modulus and adhesive properties were tuned. An HT-1080 human fibrosarcoma cell line was chosen as a model for probing matrix influences on tumor cell migration using the PEG hydrogel platform. HT-1080 speed varied with a complex dependence on both matrix modulus and Cys-Arg-Gly-Asp-Ser (CRGDS) adhesion ligand concentration, with regimes in which motility increased, decreased, or was minimally altered being observed. We further investigated cell motility by forming matrix interfaces that mimic aspects of tissue boundaries that might be encountered during invasion by taking advantage of the spatial control of the thiol-ene photochemistry to form patterned regions of low and high cross-linking densities. HT-1080s in 100 Pa regions of patterned PEG hydrogels tended to reverse direction or aggregate at the interface when they encountered a 360 Pa boundary. In contrast, HT-1080s were apparently unimpeded when migrating from the stiff to the soft regions of PEG peptide hydrogels, which may indicate that cells are capable of "reverse durotaxis" within at least some matrix regimes. Taken together, our results identified matrix regimes in which HT-1080 motility was both positively and negatively influenced by cell adhesion or matrix modulus.

Skeletal tissue regeneration: where can hydrogels play a role?

The emerging field of tissue engineering reveals promising approaches for the repair and regeneration of skeletal tissues including the articular cartilage, bone, and the entire joint. Amongst the myriad of biomaterials available to support this strategy, hydrogels are highly tissue mimicking substitutes and thus of great potential for the regeneration of functional tissues. This review comprises an overview of the novel and most promising hydrogels for articular cartilage, osteochondral and bone defect repair. Chondro- and osteo-conductive and -instructive hydrogels are presented, highlighting successful combinations with inductive signals and cell sources. Moreover, advantages, drawbacks, and future perspectives of the role of hydrogels in skeletal regeneration are addressed, pointing out the current state of this rising approach.

The role of collagen charge clusters in the modulation of matrix metalloproteinase activity

Members of the matrix metalloproteinase (MMP) family selectively cleave collagens in vivo. Several substrate structural features that direct MMP collagenolysis have been identified. The present study evaluated the role of charged residue clusters in the regulation of MMP collagenolysis. A series of 10 triple-helical peptide (THP) substrates were constructed in which either Lys-Gly-Asp or Gly-Asp-Lys motifs replaced Gly-Pro-Hyp (where Hyp is 4-hydroxy-L-proline) repeats. The stabilities of THPs containing the two different motifs were analyzed, and kinetic parameters for substrate hydrolysis by six MMPs were determined. A general trend for virtually all enzymes was that, as Gly-Asp-Lys motifs were moved from the extreme N and C termini to the interior next to the cleavage site sequence, kcat/Km values increased. Additionally, all Gly-Asp-Lys THPs were as good or better substrates than the parent THP in which Gly-Asp-Lys was not present. In turn, the Lys-Gly-Asp THPs were also always better substrates than the parent THP, but the magnitude of the difference was considerably less compared with the Gly-Asp-Lys series. Of the MMPs tested, MMP-2 and MMP-9 most greatly favored the presence of charged residues with preference for the Gly-Asp-Lys series. Lys-Gly-(Asp/Glu) motifs are more commonly found near potential MMP cleavage sites than Gly-(Asp/Glu)-Lys motifs. As Lys-Gly-Asp is not as favored by MMPs as Gly-Asp-Lys, the Lys-Gly-Asp motif appears advantageous over the Gly-Asp-Lys motif by preventing unwanted MMP hydrolysis. More specifically, the lack of Gly-Asp-Lys clusters may diminish potential MMP-2 and MMP-9 collagenolytic activity. The present study indicates that MMPs have interactions spanning the P23-P23' subsites of collagenous substrates.

Differential effects of cell adhesion, modulus and VEGFR-2 inhibition on capillary network formation in synthetic hydrogel arrays

Efficient biomaterial screening platforms can test a wide range of extracellular environments that modulate vascular growth. Here, we used synthetic hydrogel arrays to probe the combined effects of Cys-Arg-Gly-Asp-Ser (CRGDS) cell adhesion peptide concentration, shear modulus and vascular endothelial growth factor receptor 2 (VEGFR2) inhibition on human umbilical vein endothelial cell (HUVEC) viability, proliferation and tubulogenesis. HUVECs were encapsulated in degradable poly(ethylene glycol) (PEG) hydrogels with defined CRGDS concentration and shear modulus. VEGFR2 activity was modulated using the VEGFR2 inhibitor SU5416. We demonstrate that synergy exists between VEGFR2 activity and CRGDS ligand presentation in the context of maintaining HUVEC viability. However, excessive CRGDS disrupts this synergy. HUVEC proliferation significantly decreased with VEGFR2 inhibition and increased modulus, but did not vary monotonically with CRGDS concentration. Capillary-like structure (CLS) formation was highly modulated by CRGDS concentration and modulus, but was largely unaffected by VEGFR2 inhibition. We conclude that the characteristics of the ECM surrounding encapsulated HUVECs significantly influence cell viability, proliferation and CLS formation. Additionally, the ECM modulates the effects of VEGFR2 signaling, ranging from changing the effectiveness of synergistic interactions between integrins and VEGFR2 to determining whether VEGFR2 upregulates, downregulates or has no effect on proliferation and CLS formation.