Dynamic Click Hydrogels for Xeno-Free Culture of Induced Pluripotent Stem Cells

Impact of Annealing Chemistry on the Properties and Performance of Microporous Annealed Particle Hydrogels

Microporous annealed particle (MAP) hydrogels are a promising class of in situ-forming scaffolds for tissue repair and regeneration. While an expansive toolkit of annealing chemistries has been described, the effects of different annealing chemistries on MAP hydrogel properties and performance have not been studied. In this study, we address this gap through a controlled head-to-head comparison of poly(ethylene glycol) (PEG)-based MAP hydrogels that were annealed using tetrazine-norbornene and thiol-norbornene click chemistry. Characterization of material properties revealed that tetrazine click annealing significantly increases MAP hydrogel shear storage modulus and results in slower in vitro degradation kinetics when microgels with a higher cross-link density are used. However, these effects are muted when the MAP hydrogels are fabricated from microgels with a lower cross-link density. In contrast, in vivo testing in murine critical-sized calvarial defects revealed that these differences in physicochemical properties do not translate to differences in bone volume or calvarial defect healing when growth-factor-loaded MAP hydrogel scaffolds are implanted into mouse calvarial defects. Nonetheless, the impact of tetrazine click annealing could be important in other applications and should be investigated further.

Soft Polyethylene Glycol Hydrogels Support Human PSC Pluripotency and Morphogenesis

Lumenogenesis within the epiblast represents a critical step in early human development, priming the embryo for future specification and patterning events. However, little is known about the specific mechanisms that drive this process due to the inability to study the early embryo in vivo. While human pluripotent stem cell (hPSC)-based models recapitulate many aspects of the human epiblast, most approaches for generating these 3D structures rely on ill-defined, reconstituted basement membrane matrices. Here, we designed synthetic, nonadhesive polyethylene glycol (PEG) hydrogel matrices to better understand the role of matrix mechanical cues in iPSC morphogenesis, specifically elastic modulus. First, we identified a narrow range of hydrogel moduli that were conducive to the hPSC viability, pluripotency, and differentiation. We then used this platform to investigate the effects of the hydrogel modulus on lumenogenesis, finding that matrices of intermediate stiffness yielded the most epiblast-like aggregates. Conversely, stiffer matrices impeded lumen formation and apico-basal polarization, while the softest matrices yielded polarized but aberrant structures. Our approach offers a simple, modular platform for modeling the human epiblast and investigating the role of matrix cues in its morphogenesis.

Degradable and Multifunctional PEG-Based Hydrogels Formed by iEDDA Click Chemistry with Stable Click-Induced Supramolecular Interactions

The inverse electron demand Diels-Alder (iEDDA) reactions are highly efficient click chemistry increasingly utilized in bioconjugation, live cell labeling, and the synthesis and modification of biomaterials. iEDDA click reactions have also been used to cross-link tetrazine (Tz) and norbornene (NB) modified macromers [e.g., multiarm poly(ethylene glycol) or PEG]. In these hydrogels, Tz-NB adducts exhibit stable supramolecular interactions with a high hydrolytic stability. Toward engineering a new class of PEG-based click hydrogels with highly adaptable properties, we previously reported a new group of NB-derivatized PEG macromers via reacting hydroxyl-terminated PEG with carbic anhydride (CA). In this work, we show that hydrogels cross-linked by PEGNBCA or its derivatives exhibited fast and tunable hydrolytic degradation. Here, we show that PEGNBCA (either mono- or octafunctional) and its dopamine or tyramine conjugated derivatives (i.e., PEGNB-D and PEGNB-T) readily cross-link with 4-arm PEG-Tz to form a novel class of multifunctional iEDDA click hydrogels. Through modularly adjusting the macromers with unstable and stable iEDDA click-induced supramolecular interactions (iEDDA-CSI), we achieved highly tunable degradation, with full degradation in less than 2 weeks to over two months. We also show that secondary enzymatic reactions could dynamically stiffen these hydrogels. These hydrogels could also be spatiotemporally photopatterned through visible light-initiated photochemistry. Finally, the iEDDA-CSI hydrogels post ester hydrolysis displayed shear-thinning and self-healing properties, enabling injectable delivery.

Photo-click hydrogels for 3D in situ differentiation of pancreatic progenitors from induced pluripotent stem cells

BACKGROUND

Induced pluripotent stem cells (iPSC) can be differentiated to cells in all three germ layers, as well as cells in the extraembryonic tissues. Efforts in iPSC differentiation into pancreatic progenitors in vitro have largely been focused on optimizing soluble growth cues in conventional two-dimensional (2D) culture, whereas the impact of three-dimensional (3D) matrix properties on the morphogenesis of iPSC remains elusive.

METHODS

In this work, we employ gelatin-based thiol-norbornene photo-click hydrogels for in situ 3D differentiation of human iPSCs into pancreatic progenitors (PP). Molecular analysis and single-cell RNA-sequencing were utilized to elucidate on the distinct identities of subpopulations within the 2D and 3D differentiated cells.

RESULTS

We found that, while established soluble cues led to predominately PP cells in 2D culture, differentiation of iPSCs using the same soluble factors led to prominent branching morphogenesis, ductal network formation, and generation of diverse endoderm populations. Through single-cell RNA-sequencing, we found that 3D differentiation resulted in enrichments of pan-endodermal cells and ductal cells. We further noted the emergence of a group of extraembryonic cells in 3D, which was absent in 2D differentiation. The unexpected emergence of extraembryonic cells in 3D was found to be associated with enrichment of Wnt and BMP signaling pathways, which may have contributed to the emergence of diverse cell populations. The expressions of PP signature genes PDX1 and NKX6.1 were restored through inhibition of Wnt signaling at the beginning of the posterior foregut stage.

CONCLUSIONS

To our knowledge, this work established the first 3D hydrogel system for in situ differentiation of human iPSCs into PPs.

Modeling development using hydrogels

The development of multicellular complex organisms relies on coordinated signaling from the microenvironment, including both biochemical and mechanical interactions. To better understand developmental biology, increasingly sophisticated in vitro systems are needed to mimic these complex extracellular features. In this Primer, we explore how engineered hydrogels can serve as in vitro culture platforms to present such signals in a controlled manner and include examples of how they have been used to advance our understanding of developmental biology.

Photo-click hydrogels for 3D in situ differentiation of pancreatic progenitors from induced pluripotent stem cells

Background Induced pluripotent stem cells (iPSC) can be differentiated to cells in all three germ layers, as well as cells in the extraembryonic tissues. Efforts in iPSC differentiation into pancreatic progenitors in vitro have largely been focused on optimizing soluble growth cues in conventional two-dimensional (2D) culture, whereas the impact of three-dimensional (3D) matrix properties on the morphogenesis of iPSC remains elusive. Methods In this work, we employ gelatin-based thiol-norbornene photo-click hydrogels for in situ 3D differentiation of human iPSCs into pancreatic progenitors (PP). Molecular analysis and single cell RNA-sequencing were utilized to elucidate on the distinct identities of subpopulations within the 2D and 3D differentiated cells. Results We found that, while established soluble cues led to predominately PP cells in 2D culture, differentiation of iPSCs using the same soluble factors led to prominent branching morphogenesis, ductal network formation, and generation of diverse endoderm populations. Through single-cell RNA-sequencing, we found that 3D differentiation resulted in enrichments of pan-endodermal cells and ductal cells. We further noted the emergence of a group of extraembryonic cells in 3D, which was absent in 2D differentiation. The unexpected emergence of extraembryonic cells in 3D was found to be associated with enrichment of Wnt and BMP signaling pathways, which may have contributed to the emergence of diverse cell populations. The expressions of PP signature genes PDX1 and NKX6.1 were restored through inhibition of Wnt signaling at the beginning of the posterior foregut stage. Conclusions To our knowledge, this work established the first 3D hydrogel system for in situ differentiation of human iPSCs into PPs. Ongoing work focuses on enhancing pancreatic differentiation efficiency through modulating physicochemical properties of the iPSC-laden matrices.

Poly(ethylene glycol)-Norbornene as a Photoclick Bioink for Digital Light Processing 3D Bioprinting

Digital light processing (DLP) bioprinting is an emerging technology for three-dimensional bioprinting (3DBP) owing to its high printing fidelity, fast fabrication speed, and higher printing resolution. Low-viscosity bioinks such as poly(ethylene glycol) diacrylate (PEGDA) are commonly used for DLP-based bioprinting. However, the cross-linking of PEGDA proceeds via chain-growth photopolymerization that displays significant heterogeneity in cross-linking density. In contrast, step-growth thiol-norbornene photopolymerization is not oxygen inhibited and produces hydrogels with an ideal network structure. The high cytocompatibility and rapid gelation of thiol-norbornene photopolymerization have lent itself to the cross-linking of cell-laden hydrogels but have not been extensively used for DLP bioprinting. In this study, we explored eight-arm PEG-norbornene (PEG8NB) as a bioink/resin for visible light-initiated DLP-based 3DBP. The PEG8NB-based DLP resin showed high printing fidelity and cytocompatibility even without the use of any bioactive motifs and high initial stiffness. In addition, we demonstrated the versatility of the PEGNB resin by printing solid structures as cell culture devices, hollow channels for endothelialization, and microwells for generating cell spheroids. This work not only expands the selection of bioinks for DLP-based 3DBP but also provides a platform for dynamic modification of the bioprinted constructs.

Heparinized Gelatin-Based Hydrogels for Differentiation of Induced Pluripotent Stem Cells

Chemically defined hydrogels are increasingly utilized to define the effects of extracellular matrix (ECM) components on cellular fate determination of human embryonic and induced pluripotent stem cell (hESC and hiPSCs). In particular, hydrogels cross-linked by orthogonal click chemistry, including thiol-norbornene photopolymerization and inverse electron demand Diels–Alder (iEDDA) reactions, are explored for 3D culture of hESC/hiPSCs owing to the specificity, efficiency, cytocompatibility, and modularity of the cross-linking reactions. In this work, we exploited the modularity of thiol-norbornene photopolymerization to create a biomimetic hydrogel platform for 3D culture and differentiation of hiPSCs. A cell-adhesive, protease-labile, and cross-linkable gelatin derivative, gelatin-norbornene (GelNB), was used as the backbone polymer for constructing hiPSC-laden biomimetic hydrogels. GelNB was further heparinized via the iEDDA click reaction using tetrazine-modified heparin (HepTz), creating GelNB-Hep. GelNB or GelNB-Hep was modularly cross-linked with either inert macromer poly(ethylene glycol)-tetra-thiol (PEG4SH) or another bioactive macromer-thiolated hyaluronic acid (THA). The formulations of these hydrogels were modularly tuned to afford biomimetic matrices with similar elastic moduli but varying bioactive components, enabling the understanding of each bioactive component on supporting hiPSC growth and ectodermal, mesodermal, and endodermal fate commitment under identical soluble differentiation cues.

Hydrolytically Degradable PEG-Based Inverse Electron Demand Diels-Alder Click Hydrogels

Hydrogels cross-linked by inverse electron demand Diels–Alder (iEDDA) click chemistry are increasingly used in biomedical applications. With a few exceptions in naturally derived and chemically modified macromers, iEDDA click hydrogels exhibit long-term hydrolytic stability, and no synthetic iEDDA click hydrogels can undergo accelerated and tunable hydrolytic degradation. We have previously reported a novel method for synthesizing norbornene (NB)-functionalized multiarm poly(ethylene glycol) (PEG), where carbic anhydride (CA) was used to replace 5-norbornene-2-carboxylic acid. The new PEGNB_CA-based thiol-norbornene hydrogels exhibited unexpected fast yet highly tunable hydrolytic degradation. In this contribution, we leveraged the new PEGNB_CA macromer for forming iEDDA click hydrogels with [methyl]tetrazine ([m]Tz)-modified macromers, leading to the first group of synthetic iEDDA click hydrogels with highly tunable hydrolytic degradation kinetics. We further exploited Tz and mTz dual conjugation to achieve tunable hydrolytic degradation with an in vitro degradation time ranging from 2 weeks to 3 months. Finally, we demonstrated the excellent in vitro cytocompatibility and in vivo biocompatibility of the new injectable PEGNB_CA-based iEDDA click cross-linked hydrogels.

Endoderm and mesoderm derivatives in embryonic stem cell differentiation and their use in developmental toxicity testing

Embryonic stem cell differentiation models have increasingly been applied in non-animal test systems for developmental toxicity. After the initial focus on cardiac differentiation, attention has also included an array of neuro-ectodermal differentiation routes. Alternative differentiation routes in the mesodermal and endodermal germ lines have received less attention. This review provides an inventory of achievements in the latter areas of embryonic stem cell differentiation, with a view to possibilities for their use in non-animal test systems in developmental toxicology. This includes murine and human stem cell differentiation models, and also gains information from the field of stem cell use in regenerative medicine. Endodermal stem cell derivatives produced in vitro include hepatocytes, pancreatic cells, lung epithelium, and intestinal epithelium, and mesodermal derivatives include cardiac muscle, osteogenic, vascular and hemopoietic cells. This inventory provides an overview of studies on the different cell types together with biomarkers and culture conditions that stimulate these differentiation routes from embryonic stem cells. These models may be used to expand the spectrum of embryonic stem cell based new approach methodologies in non-animal developmental toxicity testing.

Dual Functionalization of Gelatin for Orthogonal and Dynamic Hydrogel Cross-Linking

Gelatin-based hydrogels are widely used in biomedical fields because of their abundance of bioactive motifs that support cell adhesion and matrix remodeling. Although inherently bioactive, unmodified gelatin exhibits temperature-dependent rheology and solubilizes at body temperature, making it unstable for three-dimensional (3D) cell culture. Therefore, the addition of chemically reactive motifs is required to render gelatin-based hydrogels with highly controllable cross-linking kinetics and tunable mechanical properties that are critical for 3D cell culture. This article provides a series of methods toward establishing orthogonally cross-linked gelatin-based hydrogels for dynamic 3D cell culture. In particular, we prepared dually functionalized gelatin macromers amenable for sequential, orthogonal covalent cross-linking. Central to this material platform is the synthesis of norbornene-functionalized gelatin (GelNB), which forms covalently cross-linked hydrogels via orthogonal thiol-norbornene click cross-linking. Using GelNB as the starting material, we further detail the methods for synthesizing gelatin macromers susceptible to hydroxyphenylacetic acid (HPA) dimerization (i.e., GelNB-HPA) and hydrazone bonding (i.e., GelNB-CH) for on-demand matrix stiffening. Finally, we outline the protocol for synthesizing a gelatin macromer capable of adjusting hydrogel stress relaxation via boronate ester bonding (i.e., GelNB-BA). The combination of these orthogonal chemistries affords a wide range of gelatin-based hydrogels as biomimetic matrices in tissue engineering and regenerative medicine applications.

Biomimetic stiffening of cell-laden hydrogels via sequential thiol-ene and hydrazone click reactions

Hydrogels with dynamically tunable crosslinking are invaluable for directing stem cell fate and mimicking a stiffening matrix during fibrosis or tumor development. The increases in matrix stiffness during tissue development are often accompanied by the accumulation of extracellular matrices (e.g., collagen, hyaluronic acid (HA)), a phenomenon that has received little attention in the development of dynamic hydrogels. In this contribution, we present a gelatin-based cell-laden hydrogel system capable of being dynamically stiffened while accumulating HA, a key glycosaminoglycans (GAG) increasingly deposited by stromal cells during tumor progression. Central to this strategy is the synthesis of a dually-modified gelatin macromer - gelatin-norbornene-carbohydrazide (GelNB-CH), which is susceptible to both thiol-norbornene photopolymerization and hydrazone click chemistry. We demonstrate that the crosslinking density of cell-laden thiol-norbornene hydrogels can be dynamically tuned via simple incubation with aldehyde-bearing macromers (e.g., oxidized dextran (oDex) or oHA). The GelNB-CH hydrogel system is highly cytocompatible, as demonstrated by in situ encapsulation of pancreatic cancer cells (PCC) and cancer-associated fibroblasts (CAF). This unique dynamic stiffening scheme provides a platform to study tandem accumulation of HA and elevation in matrix stiffness in the pancreatic tumor microenvironment. STATEMENT OF SIGNIFICANCE: Hydrogels permitting on-demand and secondary crosslinking are ideal for mimicking a stiffening tumor microenvironment (TME). However, none of the current dynamic hydrogels account for both stiffening and accumulation of hyaluronic acid (HA), a major extracellular matrix component increasingly deposited in tumor stromal tissues, including pancreatic ductal adenocarcinoma (PDAC). The current work addresses this gap by developing a dynamic hydrogel system capable of simultaneously increasing stiffness and HA accumulation. This is achieved by a new gelatin macromer permitting sequential thiol-norbornene (for primary network crosslinking) and hydrazone click chemistry (for bioinert or biomimetic stiffening with oxidized dextran (oDex) or oHA, respectively). The results of this study provide new insights into how dynamically changing physicochemical matrix properties guide cancer cell fate processes.

Structurally Dynamic Hydrogels for Biomedical Applications: Pursuing a Fine Balance between Macroscopic Stability and Microscopic Dynamics

Owing to their unique chemical and physical properties, hydrogels are attracting increasing attention in both basic and translational biomedical studies. Although the classical hydrogels with static networks have been widely reported for decades, a growing number of recent studies have shown that structurally dynamic hydrogels can better mimic the dynamics and functions of natural extracellular matrix (ECM) in soft tissues. These synthetic materials with defined compositions can recapitulate key chemical and biophysical properties of living tissues, providing an important means to understanding the mechanisms by which cells sense and remodel their surrounding microenvironments. This review begins with the overall expectation and design principles of dynamic hydrogels. We then highlight recent progress in the fabrication strategies of dynamic hydrogels including both degradation-dependent and degradation-independent approaches, followed by their unique properties and use in biomedical applications such as regenerative medicine, drug delivery, and 3D culture. Finally, challenges and emerging trends in the development and application of dynamic hydrogels are discussed.

Supramolecular Click Product Interactions Induce Dynamic Stiffening of Extracellular Matrix-Mimetic Hydrogels

Progressive stiffening of the extracellular matrix (ECM) is observed in tissue development as well as in pathologies such as cancer, cardiovascular disease, and fibrotic disease. However, methods to recapitulate this phenomenon in vitro face critical limitations. Here, we present a poly(ethylene glycol)-based peptide-functionalized ECM-mimetic hydrogel platform capable of facile, user-controlled dynamic stiffening. This platform leverages supramolecular interactions between inverse-electron demand Diels-Alder tetrazine-norbornene click products (TNCP) to create pendant moieties that undergo non-covalent crosslinking, stiffening a pre-existing network formed via thiol-ene click chemistry over the course of 6 h. Pendant TNCP moieties have a concentration-dependent effect on gel stiffness while still being cytocompatible and permissive of cell-mediated gel degradation. The robustness of this approach as well as its simplicity and ease of translation give it broad potential utility.

Facile Synthesis of Rapidly Degrading PEG-Based Thiol-Norbornene Hydrogels

An alternate synthesis route was developed to prepare norbornene-functionalized poly(ethylene glycol) (PEG) from reacting multiarm PEG with carbic anhydride. The macromer, PEGNB_CA, permits photo-cross-linking of thiol-norbornene hydrogels with kinetics comparable to conventional PEGNB macromer. In addition, PEGNB_CA provides an additional carboxylate group for further conjugation with amine-bearing molecules. Interestingly, PEGNB_CA thiol-norbornene hydrogels are highly susceptible to hydrolytic degradation through enhanced ester hydrolysis. The ester linkage is further weakened after the secondary conjugation, resulting in extremely rapid degradation of PEGNB hydrogels. More importantly, the degradation can be readily adjusted via tuning macromer compositions, with complete degradation time ranging from hours to weeks. The PEGNB_CA hydrogels are also highly cytocompatible toward various cell types, providing opportunities for future applications in tissue engineering and advanced biofabrication.

Hydrogel Models with Stiffness Gradients for Interrogating Pancreatic Cancer Cell Fate

Pancreatic ductal adenocarcinoma (PDAC) is the most common type of pancreatic cancer and has seen only modest improvements in patient survival rate over the past few decades. PDAC is highly aggressive and resistant to chemotherapy, owing to the presence of a dense and hypovascularized fibrotic tissue, which is composed of stromal cells and extracellular matrices. Increase deposition and crosslinking of matrices by stromal cells lead to a heterogeneous microenvironment that aids in PDAC development. In the past decade, various hydrogel-based, in vitro tumor models have been developed to mimic and recapitulate aspects of the tumor microenvironment in PDAC. Advances in hydrogel chemistry and engineering should provide a venue for discovering new insights regarding how matrix properties govern PDAC cell growth, migration, invasion, and drug resistance. These engineered hydrogels are ideal for understanding how variation in matrix properties contributes to the progressiveness of cancer cells, including durotaxis, the directional migration of cells in response to a stiffness gradient. This review surveys the various hydrogel-based, in vitro tumor models and the methods to generate gradient stiffness for studying migration and other cancer cell fate processes in PDAC.