Large-Scale Production of Wholly Cellular Bioinks via the Optimization of Human Induced Pluripotent Stem Cell Aggregate Culture in Automated Bioreactors

Mitophagy Facilitates Cytosolic Proteostasis to Preserve Cardiac Function

Background

Protein quality control (PQC) is critical for maintaining sarcomere structure and function in cardiac myocytes, and mutations in PQC pathway proteins, such as CRYAB (arginine to glycine at position 120, R120G) and BAG3 (proline to lysine at position 209, P209L) induce protein aggregate pathology with cardiomyopathy in humans. Novel observations in yeast and mammalian cells demonstrate mitochondrial uptake of cytosolic protein aggregates. We hypothesized that mitochondrial uptake of cytosolic protein aggregates and their removal by mitophagy, a lysosomal degradative pathway essential for myocardial homeostasis, facilitates cytosolic protein quality control in cardiac myocytes.

Methods

Mice with inducible cardiac myocyte specific ablation of TRAF2 (TRAF2icKO), which impairs mitophagy, were assessed for protein aggregates with biochemical fractionation and super-resolution imaging in comparison to floxed controls. Induced pluripotent stem cell (iPSC)-derived cardiac myocytes with R120G knock-in to the CRYAB locus were assessed for localization of the CRYAB protein. Transgenic mice expressing R120G CRYAB protein (R120G-TG) were subjected to both TRAF2 gain-of-function (with AAV9-cardiac Troponin T promoter-driven TRAF2 transduction) and TRAF2 loss-of-function (with tamoxifen-inducible ablation of one Traf2 allele) in cardiac myocytes to determine the effect of mitophagy modulation on cardiac structure, function, and protein aggregate pathology.

Results

Cardiomyocyte-specific ablation of TRAF2 results accumulation of mitochondrial and cytosolic protein aggregates and DESMIN mis-localization to protein aggregates. Isolated mitochondria take up cardiomyopathy-associated aggregate-prone cytosolic chaperone proteins, namely arginine to glycine (R120G) CRYAB mutant and proline to lysine (P209L) BAG3 mutant. R120G-CRYAB mutant protein increasingly localizes to mitochondria in human and mouse cardiomyocytes. R120G-TG mice demonstrate upregulation of TRAF2 in the mitochondrial fraction with increased mitophagy as compared with wild type. Adult-onset inducible haplo-insufficiency of TRAF2 resulted in accelerated mortality, impaired left ventricular systolic function and increased protein aggregates in R120G-TG mice as compared with controls. Conversely, AAV9-mediated TRAF2 transduction in R120G-TG mice reduced mortality and attenuated left ventricular systolic dysfunction, with reduced protein aggregates and restoration of normal localization of DESMIN, a cytosolic scaffolding protein chaperoned by CRYAB, as compared with control AAV9-GFP group.

Conclusions

TRAF2-mediated mitophagy in cardiac myocytes facilitates removal of cytosolic protein aggregates and can be stimulated to ameliorate proteotoxic cardiomyopathy.

MAGIC matrices: freeform bioprinting materials to support complex and reproducible organoid morphogenesis

Organoids are powerful models of tissue physiology, yet their applications remain limited due to their relatively simple morphology and high organoid-to-organoid structural variability. To address these limitations we developed a soft, composite yield-stress extracellular matrix that supports optimal organoid morphogenesis following freeform 3D bioprinting of cell slurries at tissue-like densities. The material is designed with two temperature regimes: at 4 °C it exhibits reversible yield-stress behavior to support long printing times without compromising cell viability. When transferred to cell culture at 37 °C, the material cross-links and exhibits similar viscoelasticity and plasticity to basement membrane extracts such as Matrigel. We first characterize the rheological properties of MAGIC matrices that optimize organoid morphogenesis, including low stiffness and high stress relaxation. Next, we combine this material with a custom piezoelectric printhead that allows more reproducible and robust self-organization from uniform and spatially organized tissue "seeds." We apply MAGIC matrix bioprinting for high-throughput generation of intestinal, mammary, vascular, salivary gland, and brain organoid arrays that are structurally similar to those grown in pure Matrigel, but exhibit dramatically improved homogeneity in organoid size, shape, maturation time, and efficiency of morphogenesis. The flexibility of this method and material enabled fabrication of fully 3D microphysiological systems, including perfusable organoid tubes that experience cyclic 3D strain in response to pressurization. Furthermore, the reproducibility of organoid structure increased the statistical power of a drug response assay by up to 8 orders-of-magnitude for a given number of comparisons. Combined, these advances lay the foundation for the efficient fabrication of complex tissue morphologies by canalizing their self-organization in both space and time.

Advancing Synthetic Hydrogels through Nature-Inspired Materials Chemistry

Synthetic extracellular matrix (ECM) mimics that can recapitulate the complex biochemical and mechanical nature of native tissues are needed for advanced models of development and disease. Biomedical research has heavily relied on the use of animal-derived biomaterials, which is now impeding their translational potential and convoluting the biological insights gleaned from in vitro tissue models. Natural hydrogels have long served as a convenient and effective cell culture tool, but advances in materials chemistry and fabrication techniques now present promising new avenues for creating xenogenic-free ECM substitutes appropriate for organotypic models and microphysiological systems. However, significant challenges remain in creating synthetic matrices that can approximate the structural sophistication, biochemical complexity, and dynamic functionality of native tissues. This review summarizes key properties of the native ECM, and discusses recent approaches used to systematically decouple and tune these properties in synthetic matrices. The importance of dynamic ECM mechanics, such as viscoelasticity and matrix plasticity, is also discussed, particularly within the context of organoid and engineered tissue matrices. Emerging design strategies to mimic these dynamic mechanical properties are reviewed, such as multi-network hydrogels, supramolecular chemistry, and hydrogels assembled from biological monomers.

Advances in Cell-Rich Inks for Biofabricating Living Architectures

Advancing biofabrication toward manufacturing living constructs with well-defined architectures and increasingly biologically relevant cell densities is highly desired to mimic the biofunctionality of native human tissues. The formulation of tissue-like, cell-dense inks for biofabrication remains, however, challenging at various levels of the bioprinting process. Promising advances have been made toward this goal, achieving relatively high cell densities that surpass those found in conventional platforms, pushing the current boundaries closer to achieving tissue-like cell densities. On this focus, herein the overarching challenges in the bioprocessing of cell-rich living inks into clinically grade engineered tissues are discussed, as well as the most recent advances in cell-rich living ink formulations and their processing technologies are highlighted. Additionally, an overview of the foreseen developments in the field is provided and critically discussed.

Organoid bioinks: construction and application

Organoids have emerged as crucial platforms in tissue engineering and regenerative medicine but confront challenges in faithfully mimicking native tissue structures and functions. Bioprinting technologies offer a significant advancement, especially when combined with organoid bioinks-engineered formulations designed to encapsulate both the architectural and functional elements of specific tissues. This review provides a rigorous, focused examination of the evolution and impact of organoid bioprinting. It emphasizes the role of organoid bioinks that integrate key cellular components and microenvironmental cues to more accurately replicate native tissue complexity. Furthermore, this review anticipates a transformative landscape invigorated by the integration of artificial intelligence with bioprinting techniques. Such fusion promises to refine organoid bioink formulations and optimize bioprinting parameters, thus catalyzing unprecedented advancements in regenerative medicine. In summary, this review accentuates the pivotal role and transformative potential of organoid bioinks and bioprinting in advancing regenerative therapies, deepening our understanding of organ development, and clarifying disease mechanisms.

Advances in the application of extracellular vesicles derived from three-dimensional culture of stem cells

Stem cells (SCs) have been used therapeutically for decades, yet their applications are limited by factors such as the risk of immune rejection and potential tumorigenicity. Extracellular vesicles (EVs), a key paracrine component of stem cell potency, overcome the drawbacks of stem cell applications as a cell-free therapeutic agent and play an important role in treating various diseases. However, EVs derived from two-dimensional (2D) planar culture of SCs have low yield and face challenges in large-scale production, which hinders the clinical translation of EVs. Three-dimensional (3D) culture, given its ability to more realistically simulate the in vivo environment, can not only expand SCs in large quantities, but also improve the yield and activity of EVs, changing the content of EVs and improving their therapeutic effects. In this review, we briefly describe the advantages of EVs and EV-related clinical applications, provide an overview of 3D cell culture, and finally focus on specific applications and future perspectives of EVs derived from 3D culture of different SCs.

A Visual, In-Expensive, and Wireless Capillary Rheometer for Characterizing Wholly-Cellular Bioinks

Rheological measurements with in situ visualization can elucidate the microstructural origin of complex flow behaviors of an ink. However, existing commercial rheometers suffer from high costs, the need for dedicated facilities for microfabrication, a lack of design flexibility, and cabling that complicates operation in sterile or enclosed environments. To address these limitations, a low-cost ($300) visual, in-expensive and wireless rheometer (VIEWR) using 3D-printed and off-the-shelf components is presented. VIEWR measurements are validated by steady-state and transient flow responses for different complex fluids, and microstructural flow profiles and evolution of yield-planes are revealed via particle image velocimetry. Using the VIEWR, a wholly-cellular bioink system comprised of compacted cell aggregates is characterized, and complex yield-stress and viscoelastic responses are captured via concomitantly visualizing the spatiotemporal evolution of aggregate morphology. A symmetric hyperbolic extensional-flow geometry is further constructed inside a capillary tube using digital light processing. Such geometries allow for measuring the extensional viscosity at varying deformation rates and further visualizing the alignment and stretching of aggregates under external flow. Synchronized but asymmetric evolution of aggregate orientation and strain through the neck is visualized. Using varying geometries, the jamming and viscoelastic deformation of aggregates are shown to contribute to the extensional viscosity of the wholly-cellular bioinks.

Unleashing the Power of Undifferentiated Induced Pluripotent Stem Cell Bioprinting: Current Progress and Future Prospects

Induced pluripotent stem cell (iPSC) technology has revolutionized various fields, including stem cell research, disease modeling, and regenerative medicine. The evolution of iPSC-based models has transitioned from conventional two-dimensional systems to more physiologically relevant three-dimensional (3D) models such as spheroids and organoids. Nonetheless, there still remain challenges including limitations in creating complex 3D tissue geometry and structures, the emergence of necrotic core in existing 3D models, and limited scalability and reproducibility. 3D bioprinting has emerged as a revolutionary technology that can facilitate the development of complex 3D tissues and organs with high scalability and reproducibility. This innovative approach has the potential to effectively bridge the gap between conventional iPSC models and complex 3D tissues in vivo. This review focuses on current trends and advancements in the bioprinting of iPSCs. Specifically, it covers the fundamental concepts and techniques of bioprinting and bioink design, reviews recent progress in iPSC bioprinting research with a specific focus on bioprinting undifferentiated iPSCs, and concludes by discussing existing limitations and future prospects.

Biomimetic cell culture for cell adhesive propagation for tissue engineering strategies

Biomimetic cell culture, which involves creating a biomimetic microenvironment for cells in vitro by engineering approaches, has aroused increasing interest given that it maintains the normal cellular phenotype, genotype and functions displayed in vivo. Therefore, it can provide a more precise platform for disease modelling, drug development and regenerative medicine than the conventional plate cell culture. In this review, initially, we discuss the principle of biomimetic cell culture in terms of the spatial microenvironment, chemical microenvironment, and physical microenvironment. Then, the main strategies of biomimetic cell culture and their state-of-the-art progress are summarized. To create a biomimetic microenvironment for cells, a variety of strategies has been developed, ranging from conventional scaffold strategies, such as macroscopic scaffolds, microcarriers, and microgels, to emerging scaffold-free strategies, such as spheroids, organoids, and assembloids, to simulate the native cellular microenvironment. Recently, 3D bioprinting and microfluidic chip technology have been applied as integrative platforms to obtain more complex biomimetic structures. Finally, the challenges in this area are discussed and future directions are discussed to shed some light on the community.