Middle-out methods for spatiotemporal tissue engineering of organoids

Organoids recapitulate many aspects of the complex three-dimensional (3D) organization found within native tissues and even display tissue and organ-level functionality. Traditional approaches to organoid culture have largely employed a top-down tissue engineering strategy, whereby cells are encapsulated in a 3D matrix, such as Matrigel, alongside well-defined biochemical cues that direct morphogenesis. However, the lack of spatiotemporal control over niche properties renders cellular processes largely stochastic. Therefore, bottom-up tissue engineering approaches have evolved to address some of these limitations and focus on strategies to assemble tissue building blocks with defined multi-scale spatial organization. However, bottom-up design reduces the capacity for self-organization that underpins organoid morphogenesis. Here, we introduce an emerging framework, which we term middle-out strategies, that relies on existing design principles and combines top-down design of defined synthetic matrices that support proliferation and self-organization with bottom-up modular engineered intervention to limit the degrees of freedom in the dynamic process of organoid morphogenesis. We posit that this strategy will provide key advances to guide the growth of organoids with precise geometries, structures and function, thereby facilitating an unprecedented level of biomimicry to accelerate the utility of organoids to more translationally relevant applications.

In situ modulation of intestinal organoid epithelial curvature through photoinduced viscoelasticity directs crypt morphogenesis

Spatiotemporally coordinated transformations in epithelial curvature are necessary to generate crypt-villus structures during intestinal development. However, the temporal regulation of mechanotransduction pathways that drive crypt morphogenesis remains understudied. Intestinal organoids have proven useful to study crypt morphogenesis in vitro, yet the reliance on static culture scaffolds limits the ability to assess the temporal effects of changing curvature. Here, a photoinduced hydrogel cross-link exchange reaction is used to spatiotemporally alter epithelial curvature and study how dynamic changes in curvature influence mechanotransduction pathways to instruct crypt morphogenesis. Photopatterned curvature increased membrane tension and depolarization, which was required for subsequent nuclear localization of yes-associated protein 1 (YAP) observed 24 hours following curvature change. Curvature-directed crypt morphogenesis only occurred following a delay in the induction of differentiation that coincided with the delay in spatially restricted YAP localization, indicating that dynamic changes in curvature initiate epithelial curvature-dependent mechanotransduction pathways that temporally regulate crypt morphogenesis.

Engineering multicellular living systems-a Keystone Symposia report

The ability to engineer complex multicellular systems has enormous potential to inform our understanding of biological processes and disease and alter the drug development process. Engineering living systems to emulate natural processes or to incorporate new functions relies on a detailed understanding of the biochemical, mechanical, and other cues between cells and between cells and their environment that result in the coordinated action of multicellular systems. On April 3-6, 2022, experts in the field met at the Keystone symposium "Engineering Multicellular Living Systems" to discuss recent advances in understanding how cells cooperate within a multicellular system, as well as recent efforts to engineer systems like organ-on-a-chip models, biological robots, and organoids. Given the similarities and common themes, this meeting was held in conjunction with the symposium "Organoids as Tools for Fundamental Discovery and Translation".

Stress Relaxation and Composition of Hydrazone-Crosslinked Hybrid Biopolymer-Synthetic Hydrogels Determine Spreading and Secretory Properties of MSCs

The extracellular matrix plays a critical role in mechanosensing and thereby influences the secretory properties of bone-marrow-derived mesenchymal stem/stromal cells (MSCs). As a result, interest has grown in the development of biomaterials with tunable properties for the expansion and delivery of MSCs that are used in cell-based therapies. Herein, stress-relaxing hydrogels are synthesized as hybrid networks containing both biopolymer and synthetic macromer components. Hyaluronic acid is functionalized with either aldehyde or hydrazide groups to form covalent adaptable hydrazone networks, which are stabilized by poly(ethylene glycol) functionalized with bicyclononyne and heterobifunctional small molecule crosslinkers containing azide and benzaldehyde moieties. Tuning the composition of these gels allows for controlled variation in the characteristic timescale for stress relaxation and the amount of stress relaxed. Over this compositional space, MSCs are observed to spread in formulations with higher degrees of adaptability, with aspect ratios of 1.60 ± 0.18, and YAP nuclear:cytoplasm ratios of 6.5 ± 1.3. Finally, a maximum MSC pericellular protein thickness of 1.45 ± 0.38 µm occurred in highly stress-relaxing gels, compared to 1.05 ± 0.25 µm in non-adaptable controls. Collectively, this study contributes a new understanding of the role of compositionally defined stress relaxation on MSCs mechanosensing and secretion.

Programming hydrogels to probe spatiotemporal cell biology

The recapitulation of complex microenvironments that regulate cell behavior during development, disease, and wound healing is key to understanding fundamental biological processes. In vitro, multicellular morphogenesis, organoid maturation, and disease modeling have traditionally been studied using either non-physiological 2D substrates or 3D biological matrices, neither of which replicate the spatiotemporal biochemical and biophysical complexity of biology. Here, we provide a guided overview of the recent advances in the programming of synthetic hydrogels that offer precise control over the spatiotemporal properties within cellular microenvironments, such as advances in the control of cell-driven remodeling, bioprinting, or user-defined manipulation of properties (e.g., via light irradiation).

In Situ Super-Resolution Imaging of Organoids and Extracellular Matrix Interactions via Phototransfer by Allyl Sulfide Exchange-Expansion Microscopy (PhASE-ExM)

3D organoid models have recently seen a boom in popularity, as they can better recapitulate the complexity of multicellular organs compared to other in vitro culture systems. However, organoids are difficult to image because of the limited penetration depth of high-resolution microscopes and depth-dependent light attenuation, which can limit the understanding of signal transduction pathways and characterization of intimate cell-extracellular matrix (ECM) interactions. To overcome these challenges, phototransfer by allyl sulfide exchange-expansion microscopy (PhASE-ExM) is developed, enabling optical clearance and super-resolution imaging of organoids and their ECM in 3D. PhASE-ExM uses hydrogels prepared via photoinitiated polymerization, which is advantageous as it decouples monomer diffusion into thick organoid cultures from the hydrogel fabrication. Apart from compatibility with organoids cultured in Matrigel, PhASE-ExM enables 3.25× expansion and super-resolution imaging of organoids cultured in synthetic poly(ethylene glycol) (PEG) hydrogels crosslinked via allyl-sulfide groups (PEG-AlS) through simultaneous photopolymerization and radical-mediated chain-transfer reactions that complete in <70 s. Further, PEG-AlS hydrogels can be in situ softened to promote organoid crypt formation, providing a super-resolution imaging platform both for pre- and post-differentiated organoids. Overall, PhASE-ExM is a useful tool to decipher organoid behavior by enabling sub-micrometer scale, 3D visualization of proteins and signal transduction pathways.

4D Materials with Photoadaptable Properties Instruct and Enhance Intestinal Organoid Development

Intestinal organoids are self-organized tissue constructs, grown in vitro, that resemble the structure and function of the intestine and are often considered promising as a prospective platform for drug testing and disease modeling. Organoid development in vitro is typically instructed by exogenous cues delivered from the media, but cellular responses also depend on properties of the surrounding microenvironmental niche, such as mechanical stiffness and extracellular matrix (ECM) ligands. In recent years, synthetic hydrogel platforms have been engineered to resemble the in vivo niche, with the goal of generating physiologically relevant environments that can promote mature and reproducible organoid development. However, a few of these approaches consider the importance of intestinal organoid morphology or how morphology changes during development, as cues that may dictate organoid functionality. For example, intestinal organoids grown in vitro often lack the physical boundary conditions found in vivo that are responsible for shaping a collection of cells into developmentally relevant morphologies, resulting in organoids that often differ in structure and cellular organization from the parent organ. This disconnect relates, in part, to a lack of appropriate adaptable and programmable materials for cell culture, especially those that enable control over colony growth and differentiation in space and time (i.e., 4D materials). We posit that the future of organoid culture platforms may benefit from advances in photoadaptable chemistries and integration into biomaterials scaffolds, thereby allowing greater user-directed control over both the macro- and microscale material properties. In this way, synthetic materials can begin to better replicate changes in the ECM during development or regeneration in vivo. Recapitulation of cellular and tissue morphological changes, along with an appreciation for the appropriate developmental time scales, should help instruct the next generation of organoid models to facilitate predictable outcomes.

Discretizing Three-Dimensional Oxygen Gradients to Modulate and Investigate Cellular Processes

With the increased realization of the effect of oxygen (O2 ) deprivation (hypoxia) on cellular processes, recent efforts have focused on the development of engineered systems to control O2 concentrations and establish biomimetic O2 gradients to study and manipulate cellular behavior. Nonetheless, O2 gradients present in 3D engineered platforms result in diverse cell behavior across the O2 gradient, making it difficult to identify and study O2 sensitive signaling pathways. Using a layer-by-layer assembled O2 -controllable hydrogel, the authors precisely control O2 concentrations and study uniform cell behavior in discretized O2 gradients, then recapitulate the dynamics of cluster-based vasculogenesis, one mechanism for neovessel formation, and show distinctive gene expression patterns remarkably correlate to O2 concentrations. Using RNA sequencing, it is found that time-dependent regulation of cyclic adenosine monophosphate signaling enables cell survival and clustering in the high stress microenvironments. Various extracellular matrix modulators orchestrate hypoxia-driven endothelial cell clustering. Finally, clustering is facilitated by regulators of cell-cell interactions, mainly vascular cell adhesion molecule 1. Taken together, novel regulators of hypoxic cluster-based vasculogenesis are identified, and evidence for the utility of a unique platform is provided to study dynamic cellular responses to 3D hypoxic environments, with broad applicability in development, regeneration, and disease.

Hydrogel vehicles for sequential delivery of protein drugs to promote vascular regeneration

In recent years, as the mechanisms of vasculogenesis and angiogenesis have been uncovered, the functions of various pro-angiogenic growth factors (GFs) and cytokines have been identified. Therefore, therapeutic angiogenesis, by delivery of GFs, has been sought as a treatment for many vascular diseases. However, direct injection of these protein drugs has proven to have limited clinical success due to their short half-lives and systemic off-target effects. To overcome this, hydrogel carriers have been developed to conjugate single or multiple GFs with controllable, sustained, and localized delivery. However, these attempts have failed to account for the temporal complexity of natural angiogenic pathways, resulting in limited therapeutic effects. Recently, the emerging ideas of optimal sequential delivery of multiple GFs have been suggested to better mimic the biological processes and to enhance therapeutic angiogenesis. Incorporating sequential release into drug delivery platforms will likely promote the formation of neovasculature and generate vast therapeutic potential.

Senescent Cells with Augmented Cytokine Production for Microvascular Bioengineering and Tissue Repairs

Controlled delivery of cytokines and growth factors has been an area of intense research interest for molecular and cellular bioengineering, immunotherapy, and regenerative medicine. In this study, we show that primary human lung fibroblasts chemically induced to senescence (cell cycle arrest) can act as a living source to transiently produce factors essential for promoting vasculogenesis or angiogenesis, such as VEGF, HGF, and IL-8. Co-culture of senescent fibroblasts with HUVECs in a fibrin gel demonstrated accelerated formation and maturation of microvessel networks in as early as three days. Unlike the usage of non-senescent fibroblasts as the angiogenesis-promoting cells, this approach eliminates drawbacks related to the overproliferation of fibroblasts and the subsequent disruption of tissue architecture, integrity, or function. Co-culture of pancreatic islets with senescent fibroblasts and endothelial cells in a gel matrix maintains the viability and function of islets ex vivo for up to five days. Applying senescent fibroblasts to wound repair in vivo led to increased blood flow in a diabetic mouse model. Together, this work points to a new direction for engineering the delivery of cytokines and growth factors that promote microvascular tissue engineering and tissue repairs.

Acellular and cellular approaches to improve diabetic wound healing

Chronic diabetic wounds represent a huge socioeconomic burden for both affected individuals and the entire healthcare system. Although the number of available treatment options as well as our understanding of wound healing mechanisms associated with diabetes has vastly improved over the past decades, there still remains a great need for additional therapeutic options. Tissue engineering and regenerative medicine approaches provide great advantages over conventional treatment options, which are mainly aimed at wound closure rather than addressing the underlying pathophysiology of diabetic wounds. Recent advances in biomaterials and stem cell research presented in this review provide novel ways to tackle different molecular and cellular culprits responsible for chronic and nonhealing wounds by delivering therapeutic agents in direct or indirect ways. Careful integration of different approaches presented in the current article could lead to the development of new therapeutic platforms that can address multiple pathophysiologic abnormalities and facilitate wound healing in patients with diabetes.

Hypoxia and matrix viscoelasticity sequentially regulate endothelial progenitor cluster-based vasculogenesis

Vascular morphogenesis is the formation of endothelial lumenized networks. Cluster-based vasculogenesis of endothelial progenitor cells (EPCs) has been observed in animal models, but the underlying mechanism is unknown. Here, using O₂-controllabe hydrogels, we unveil the mechanism by which hypoxia, co-jointly with matrix viscoelasticity, induces EPC vasculogenesis. When EPCs are subjected to a 3D hypoxic gradient ranging from <2 to 5%, they rapidly produce reactive oxygen species that up-regulate proteases, most notably MMP-1, which degrade the surrounding extracellular matrix. EPC clusters form and expand as the matrix degrades. Cell-cell interactions, including those mediated by VE-cadherin, integrin-β2, and ICAM-1, stabilize the clusters. Subsequently, EPC sprouting into the stiffer, intact matrix leads to vascular network formation. In vivo examination further corroborated hypoxia-driven clustering of EPCs. Overall, this is the first description of how hypoxia mediates cluster-based vasculogenesis, advancing our understanding toward regulating vascular development as well as postnatal vasculogenesis in regeneration and tumorigenesis.

Tissue matrix arrays for high-throughput screening and systems analysis of cell function

Cell and protein arrays have demonstrated remarkable utility in the high-throughput evaluation of biological responses; however, they lack the complexity of native tissue and organs. Here, we describe tissue extracellular matrix (ECM) arrays for screening biological outputs and systems analysis. We spotted processed tissue ECM particles as two-dimensional arrays or incorporated them with cells to generate three-dimensional cell-matrix microtissue arrays. We then investigated the response of human stem, cancer, and immune cells to tissue ECM arrays originating from 11 different tissues, and validated the 2D and 3D arrays as representative of the in vivo microenvironment through quantitative analysis of tissue-specific cellular responses, including matrix production, adhesion and proliferation, and morphological changes following culture. The biological outputs correlated with tissue proteomics, and network analysis identified several proteins linked to cell function. Our methodology enables broad screening of ECMs to connect tissue-specific composition with biological activity, providing a new resource for biomaterials research and translation.

The design of dextran-based hypoxia-inducible hydrogels via in situ oxygen-consuming reaction

Hypoxia plays a critical role in the development and wound healing process, as well as a number of pathological conditions. Here, dextran-based hypoxia-inducible (Dex-HI) hydrogels formed with in situ oxygen consumption via a laccase-medicated reaction are reported. Oxygen levels and gradients were accurately predicted by mathematical simulation. It is demonstrated that Dex-HI hydrogels provide prolonged hypoxic conditions up to 12 h. The Dex-HI hydrogel offers an innovative approach to delineate not only the mechanism by which hypoxia regulates cellular responses, but may facilitate the discovery of new pathways involved in the generation of hypoxic and oxygen gradient environments.

REF4 and RFR1, subunits of the transcriptional coregulatory complex mediator, are required for phenylpropanoid homeostasis in Arabidopsis

The plant phenylpropanoid pathway produces an array of metabolites that impact human health and the utility of feed and fiber crops. We previously characterized several Arabidopsis thaliana mutants with dominant mutations in REDUCED EPIDERMAL FLUORESCENCE 4 (REF4) that cause dwarfing and decreased accumulation of phenylpropanoids. In contrast, ref4 null plants are of normal stature and have no apparent defect in phenylpropanoid biosynthesis. Here we show that disruption of both REF4 and its paralog, REF4-RELATED 1 (RFR1), results in enhanced expression of multiple phenylpropanoid biosynthetic genes, as well as increased accumulation of numerous downstream products. We also show that the dominant ref4-3 mutant protein interferes with the ability of the PAP1/MYB75 transcription factor to induce the expression of PAL1 and drive anthocyanin accumulation. Consistent with our experimental results, both REF4 and RFR1 have been shown to physically associate with the conserved transcriptional coregulatory complex, Mediator, which transduces information from cis-acting DNA elements to RNA polymerase II at the core promoter. Taken together, our data provide critical genetic support for a functional role of REF4 and RFR1 in the Mediator complex, and for Mediator in the maintenance of phenylpropanoid homeostasis. Finally, we show that wild-type RFR1 substantially mitigates the phenotype of the dominant ref4-3 mutant, suggesting that REF4 and RFR1 may compete with one another for common binding partners or for occupancy in Mediator. Determining the functions of diverse Mediator subunits is essential to understand eukaryotic gene regulation, and to facilitate rational manipulation of plant metabolic pathways to better suit human needs.

Volatile disinfection by-product analysis from chlorinated indoor swimming pools

Chlorination of indoor swimming pools is practiced for disinfection and oxidation of reduced compounds that are introduced to water by swimmers. However, there is growing concern associated with formation for chlorinated disinfection by-products (DBPs) in these settings. Volatile DBPs are of particular concern because they may promote respiratory ailments and other adverse health effects among swimmers and patrons of indoor pool facilities. To examine the scope of this issue, water samples were collected from 11 pools over a 6month period and analyzed for free chlorine and their volatile DBP content. Eleven volatile DBPs were identified: monochloramine (NH(2)Cl), dichloramine (NHCl(2)), trichloramine (NCl(3)), chloroform (CHCl(3)), bromoform (CHBr(3)), dichlorobromomethane (CHBrCl(2)), dibromochloromethane (CHBr(2)Cl), cyanogen chloride (CNCl), cyanogen bromide (CNBr), dichloroacetonitrile (CNCHCl(2)), and dichloromethylamine (CH(3)NCl(2)). Of these 11 DBPs, 10 were identified as regularly occurring, with CHBrCl(2) only appearing sporadically. Pool water samples were analyzed for residual chlorine compounds using the DPD colorimetric method and by membrane introduction mass spectrometry (MIMS). These two methods were chosen as complementary measures of residual chlorine, and to allow for comparisons between the methods. The DPD method was demonstrated to consistently overestimate inorganic chloramine content in swimming pools. Pairwise correlations among the measured volatile DBPs allowed identification of dichloromethylamine and dichloroacetonitrile as potential swimming pool water quality indicator compounds.