Cell-Compatible, Site-Specific Covalent Modification of Hydrogel Scaffolds Enables User-Defined Control over Cell-Material Interactions

Chemical and Biological Engineering Strategies to Make and Modify Next-Generation Hydrogel Biomaterials

There is a growing interest in the development of technologies to probe and direct in vitro cellular function for fundamental organoid and stem cell biology, functional tissue and metabolic engineering, and biotherapeutic formulation. Recapitulating many critical aspects of the native cellular niche, hydrogel biomaterials have proven to be a defining platform technology in this space, catapulting biological investigation from traditional two-dimensional (2D) culture into the 3D world. Seeking to better emulate the dynamic heterogeneity characteristic of all living tissues, global efforts over the last several years have centered around upgrading hydrogel design from relatively simple and static architectures into stimuli-responsive and spatiotemporally evolvable niches. Towards this end, advances from traditionally disparate fields including bioorthogonal click chemistry, chemoenzymatic synthesis, and DNA nanotechnology have been co-opted and integrated to construct 4D-tunable systems that undergo preprogrammed functional changes in response to user-defined inputs. In this Review, we highlight how advances in synthetic, semisynthetic, and bio-based chemistries have played a critical role in the triggered creation and customization of next-generation hydrogel biomaterials. We also chart how these advances stand to energize the translational pipeline of hydrogels from bench to market and close with an outlook on outstanding opportunities and challenges that lay ahead.

Spontaneous and site-specific immobilization of PNGase F via spy chemistry

Protein N-glycosylation plays a critical role in a wide range of biological processes, and aberrant N-glycosylation is frequently associated with various pathological states. For global N-glycosylation analysis, N-glycans are typically released from glycoproteins mediated by endoglycosidases, primarily peptide N-glycosidase F (PNGase F). However, conventional N-glycan release by in-solution PNGase F is time-consuming and nonreusable. Although some immobilization methods can save time and reduce the enzyme dosage, including affinity interaction and covalent binding, the immobilized PNGase F by these traditional methods may compromises the immobilized enzyme's stability and biofunction. Therefore, a new approach is urgently needed to firmly and steadily immobilize PNGase F. To meet this demand, we have developed a spontaneous and site-specific way to immobilize PNGase F onto magnetic nanoparticles via Spy chemistry. The magnetic nanoparticles were synthesized and modified with SpyTag as a solid surface. The PNGase F fused with SpyCatcher can then be site-specifically and covalently immobilized onto this solid phase, forming a firm isopeptide bond via self-catalysis between the SpyTag peptide and SpyCatcher. Importantly, the immobilization process mediated by mild spy chemistry does not result in PNGase F inactivation; and allows immobilized PNGase F to rapidly release various types of glycans (high-mannose, sialylated, and hybrid) from glycoproteins. Moreover, the immobilized PNGase F exhibited good deglycosylation activity and facilitated good reusability in consecutive reactions. Deglycosylation of clinical samples was completed by the immobilized PNGase F as fast as several minutes.

Elastic Properties of Poly(ethylene glycol) Nanomembranes and Respective Implications

Free-standing poly(ethylene glycol) (PEG) membranes were prepared from amine- and epoxy-terminated four-arm STAR-PEG precursors in a thickness range of 40–320 nm. The membranes feature high stability and an extreme elasticity, as emphasized by the very low values of Young’s modulus, varying from 2.08 MPa to 2.6 MPa over the studied thickness range. The extreme elasticity of the membranes stems from the elastomer-like character of the PEG network, consisting of the STAR-PEG cores interconnected by crosslinked PEG chains. This elasticity is only slightly affected by a moderate reduction in the interconnections at a deviation from the standard 1:1 composition of the precursors. However, both the elasticity and stability of the membranes are strongly deteriorated by a strong distortion of the network imposed by electron irradiation of the membranes. In contrast, exposure of the membranes to ultraviolet (UV) light (254 nm) does not affect their elastic properties, supporting the assumption that the only effect of such treatment is the decomposition of the PEG material with subsequent desorption of the released fragments. An analysis of the data allowed for the exclusion of so called “hot electrons” as a possible mechanism behind the modification of the PEG membranes by UV light.

Bioprinted microvasculature: progressing from structure to function

Three-dimensional (3D) bioprinting seeks to unlock the rapid generation of complex tissue constructs, but long-standing challenges with efficientin vitromicrovascularization must be solved before this can become a reality. Microvasculature is particularly challenging to biofabricate due to the presence of a hollow lumen, a hierarchically branched network topology, and a complex signaling milieu. All of these characteristics are required for proper microvascular-and, thus, tissue-function. While several techniques have been developed to address distinct portions of this microvascularization challenge, no single approach is capable of simultaneously recreating all three microvascular characteristics. In this review, we present a three-part framework that proposes integration of existing techniques to generate mature microvascular constructs. First, extrusion-based 3D bioprinting creates a mesoscale foundation of hollow, endothelialized channels. Second, biochemical and biophysical cues induce endothelial sprouting to create a capillary-mimetic network. Third, the construct is conditioned to enhance network maturity. Across all three of these stages, we highlight the potential for extrusion-based bioprinting to become a central technique for engineering hierarchical microvasculature. We envision that the successful biofabrication of functionally engineered microvasculature will address a critical need in tissue engineering, and propel further advances in regenerative medicine andex vivohuman tissue modeling.

Adding Dynamic Biomolecule Signaling to Hydrogel Systems via Tethered Photolabile Cell-Adhesive Proteins

Sequential biochemical signaling events direct key native tissue processes including disease progression, wound healing and angiogenesis, and tissue regeneration. While in vitro modeling of these processes is critical to understanding endogenous tissue behavior and improving therapeutic outcomes, current models inadequately recapitulate the dynamism of these signaling events. Even the most advanced current synthetic tissue culture constructs are restricted in their capability to sequentially add and remove the same molecule to model transient signaling. Here, we developed a genetically encoded method for reversible biochemical signaling within poly(ethylene glycol) (PEG)-based hydrogels for greater accuracy of modeling tissue regeneration within a reductionist environment. We designed and implemented a recombinant protein with a SpyCatcher domain connected to a cell-adhesive RGDS peptide domain by a light-cleavable domain known as PhoCl. This protein was shown to bind to SpyTag-functionalized PEG-matrices via SpyTag-SpyCatcher isopeptide bonding to present RGDS adhesive ligands to cells. Upon 405 nm light exposure, the PhoCl domain was cleaved to subsequently release the RGDS peptide, which diffused out of the matrix. This system was implemented to confer reversible adhesion of 3T3 fibroblasts to the PEG-based hydrogel surface in 2D culture (73.36 ± 21.47% cell removal upon cell-compatible light exposure) and temporal control over cell spreading over time in 3D culture within cell-degradable PEG-based hydrogels, demonstrating the capability of this system to present dynamic signaling events to cells toward modeling native tissue processes within in a controlled, ECM-mimetic matrix.

Chemically Orthogonal Protein Ligation Domains for Independent Control of Hydrogel Modification with Adhesive Ligands and Growth Factors

The twin, chemically orthogonal protein ligation domains, SpyCatcher and SnoopCatcher, were used to link two engineered proteins into poly(ethylene glycol) (PEG) hydrogels in order to control both endothelial cell adhesion and material-mediated pro-mitotic stimulation. SpyCatcher was appended with an N-terminal adhesion ligand RGDS to form RGDS-SC, and SnoopCatcher was appended with the vascular endothelial growth factor (VEGF)-mimetic peptide QK to form QK-SnpC. QK-SnpC formed a spontaneous covalent bond with SnoopTag peptide with 40% reaction efficiency, both in solution, in a PEG gel containing SnoopTag peptide, and in a PEG gel with both SnoopTag and SpyTag sites. QK-SnpC added to cell culture media enhanced endothelial cell proliferation compared to a negative control, and was statistically indistinguishable from the positive control of 130 pM VEGF₁₆₅. Endothelial cells seeded onto PEG gels presenting both RGDS-SC and QK-SnpC showed ∼50% of cells actively proliferating (defined as Ki67+), compared to ∼31% of cells seeded on gels presenting RGDS-SC alone. These results show that complementary nondiffusing biochemical signals can be linked into PEG-DA hydrogels simultaneously using 'Catcher-based ligation strategies, thereby inducing more nuanced cell-material interactions.

Recent advances in bio-orthogonal and dynamic crosslinking of biomimetic hydrogels

In recent years, dynamic, 'click' hydrogels have been applied in numerous biomedical applications. Owing to the mild, cytocompatible, and highly specific reaction kinetics, a multitude of orthogonal handles have been developed for fabricating dynamic hydrogels to facilitate '4D' cell culture. The high degree of tunability in crosslinking reactions of orthogonal 'click' chemistry has enabled a bottom-up approach to install specific biomimicry in an artificial extracellular matrix. In addition to click chemistry, highly specific enzymatic reactions are also increasingly used for network crosslinking and for spatiotemporal control of hydrogel properties. On the other hand, covalent adaptable chemistry has been used to recapitulate the viscoelastic component of biological tissues and for formulating self-healing and shear-thinning hydrogels. The common feature of these three classes of chemistry (i.e., orthogonal click chemistry, enzymatic reactions, and covalent adaptable chemistry) is that they can be carried out under ambient and aqueous conditions, a prerequisite for maintaining cell viability for in situ cell encapsulation and post-gelation modification of network properties. Due to their orthogonality, different chemistries can also be applied sequentially to provide additional biochemical and mechanical control to guide cell behavior. Herein, we review recent advances in the use of orthogonal click chemistry, enzymatic reactions, and covalent adaptable chemistry for the development of dynamically tunable and biomimetic hydrogels.

Power to the protein: enhancing and combining activities using the Spy toolbox

Proteins span an extraordinary range of shapes, sizes and functionalities. Therefore generic approaches are needed to overcome this diversity and stream-line protein analysis or application. Here we review SpyTag technology, now used in hundreds of publications or patents, and its potential for detecting and controlling protein behaviour. SpyTag forms a spontaneous and irreversible isopeptide bond upon binding its protein partner SpyCatcher, where both parts are genetically-encoded. New variants of this pair allow reaction at a rate approaching the diffusion limit, while reversible versions allow purification of SpyTagged proteins or tuned dynamic interaction inside cells. Anchoring of SpyTag-linked proteins has been established to diverse nanoparticles or surfaces, including gold, graphene and the air/water interface. SpyTag/SpyCatcher is mechanically stable, so is widely used for investigating protein folding and force sensitivity. A toolbox of scaffolds allows SpyTag-fusions to be assembled into defined multimers, from dimers to 180-mers, or unlimited 1D, 2D or 3D networks. Icosahedral multimers are being evaluated for vaccination against malaria, HIV and cancer. For enzymes, Spy technology has increased resilience, promoted substrate channelling, and assembled hydrogels for continuous flow biocatalysis. Combinatorial increase in functionality has been achieved through modular derivatisation of antibodies, light-emitting diodes or viral vectors. In living cells, SpyTag allowed imaging of protein trafficking, retargeting of CAR-T cell killing, investigation of heart contraction, and control of nucleosome position. The simple genetic encoding and rapid irreversible reaction provide diverse opportunities to enhance protein function. We describe limitations as well as future directions.

Using Tools from Optogenetics to Create Light-Responsive Biomaterials: LOVTRAP-PEG Hydrogels for Dynamic Peptide Immobilization

Hydrogel materials have become a versatile platform for in vitro cell culture due to their ability to simulate many aspects of native tissues. However, precise spatiotemporal presentation of peptides and other biomolecules has remained challenging. Here we report the use of light-sensing proteins (LSPs), more commonly used in optogenetics research, as light-activated reversible binding sites within synthetic poly(ethylene glycol) (PEG) hydrogels. We used LOVTRAP, a two component LSP system consisting of LOV2, a protein domain that can cycle reversibly between "light" and "dark" conformations in response to blue light, and a z-affibody, Zdark (Zdk), that binds the dark state of LOV2, to spatiotemporally control the presentation of a recombinant protein within PEG hydrogels. By immobilizing LOV2 within PEG gels, we were able to capture a recombinant fluorescent protein (used as a model biomolecule) containing a Zdk domain, and then release the Zdk fusion protein using blue light. Zdk was removed from LOV2-containing PEG gels using focused blue light, resulting in a 30% reduction of fluorescence compared to unexposed regions of the gel. Additionally, the reversible binding capability of LOVTRAP was observed in our system, enabling our LOV2 gels to capture and release Zdk at least three times. By adding a Zdk domain to a recombinant peptide or protein, dynamic, spatially constrained displays of non-diffusing ligands within a PEG gel could feasibly be achieved using LOV2.

Development of SpyTag/SpyCatcher-Bacmid Expression Vector System (SpyBEVS) for Protein Bioconjugations Inside of Silkworms

Protein conjugations at post-translational levels are known to be essential to protein stability and function. Recently, it has been proven that the split protein CnaB2 (SpyTag/SpyCatcher, ST/SC) from Streptococcus pyogenes can induce covalent conjugation rapidly and efficiently under various conditions. The protein of interest fused with the split protein SC/ST could be assembled spontaneously. In light of this finding, we introduced the ST/SC protein coupling concept into the silkworm-bacmid protein expression system (SpyBEVS). As a proof of concept, we first examined and confirmed that a competent ligation occurred between ST/SC-fused protein partners in vitro in cultured silkworm cells and in vivo in silkworm larvae by co-infection of several recombinant baculoviruses. The protein conjugation could be also achieved sufficiently by a simple one-step mixture of purified ST/SC-tagged peptide-protein pairs in vitro. Given the flexibility and robustness of silkworm-BEVS, our results on SpyBEVS show an alternative method for enabling the production of protein decorations in vitro and inside of silkworms.