Directed oxygen gradients initiate a robust early remodeling response in engineered vascular grafts

Targeting HIF-α for robust prevascularization of human cardiac organoids

Prevascularized 3D microtissues have been shown to be an effective cell delivery vehicle for cardiac repair. To this end, our lab has explored the development of self-organizing, prevascularized human cardiac organoids by co-seeding human cardiomyocytes with cardiac fibroblasts, endothelial cells, and stromal cells into agarose microwells. We hypothesized that this prevascularization process is facilitated by the endogenous upregulation of hypoxia-inducible factor (HIF) pathway in the avascular 3D microtissues. In this study, we used Molidustat, a selective PHD (prolyl hydroxylase domain enzymes) inhibitor that stabilizes HIF-α, to treat human cardiac organoids, which resulted in 150 ± 61% improvement in endothelial expression (CD31) and 220 ± 20% improvement in the number of lumens per organoids. We hypothesized that the improved endothelial expression seen in Molidustat treated human cardiac organoids was dependent upon upregulation of VEGF, a well-known downstream target of HIF pathway. Through the use of immunofluorescent staining and ELISA assays, we determined that Molidustat treatment improved VEGF expression of non-endothelial cells and resulted in improved co-localization of supporting cell types and endothelial structures. We further demonstrated that Molidustat treated human cardiac organoids maintain cardiac functionality. Lastly, we showed that Molidustat treatment improves survival of cardiac organoids when exposed to both hypoxic and ischemic conditions in vitro. For the first time, we demonstrate that targeted HIF-α stabilization provides a robust strategy to improve endothelial expression and lumen formation in cardiac microtissues, which will provide a powerful framework for prevascularization of various microtissues in developing successful cell transplantation therapies.

Sequential adaptation of perfusion and transport conditions significantly improves vascular construct recellularization and biomechanics

Recellularization of ex vivo-derived scaffolds remains a significant hurdle primarily due to the scaffolds subcellular pore size that restricts initial cell seeding to the scaffolds periphery and inhibits migration over time. With the aim to improve cell migration, repopulation, and graft mechanics, the effects of a four-step culture approach were assessed. Using an ex vivo-derived vein as a model scaffold, human smooth muscle cells were first seeded onto its ablumen (Step 1: 3 hr) and an aggressive 0-100% nutrient gradient (lumenal flow under hypotensive pressure) was created to initiate cell migration across the scaffold (Step 2: Day 0 to 19). The effects of a prolonged aggressive nutrient gradient created by this single lumenal flow was then compared with a dual flow (lumenal and ablumenal) in Step 3 (Day 20 to 30). Analyses showed that a single lumenal flow maintained for 30 days resulted in a higher proportion of cells migrating across the scaffold toward the vessel lumen (nutrient source), with improved distribution. In Step 4 (Day 31 to 45), the transition from hypotensive pressure (12/8 mmHg) to normotensive (arterial-like) pressure (120/80 mmHg) was assessed. It demonstrated that recellularized scaffolds exposed to arterial pressures have increased glycosaminoglycan deposition, physiological modulus, and Young's modulus. By using this stepwise conditioning, the challenging recellularization of a vein-based scaffold and its positive remodeling toward arterial biomechanics were obtained.

Oxygen Regulation in Development: Lessons from Embryogenesis towards Tissue Engineering

Oxygen is a vital source of energy necessary to sustain and complete embryonic development. Not only is oxygen the driving force for many cellular functions and metabolism, but it is also involved in regulating stem cell fate, morphogenesis, and organogenesis. Low oxygen levels are the naturally preferred microenvironment for most processes during early development and mainly drive proliferation. Later on, more oxygen and also nutrients are needed for organogenesis and morphogenesis. Therefore, it is critical to maintain oxygen levels within a narrow range as required during development. Modulating oxygen tensions is performed via oxygen homeostasis mainly through the function of hypoxia-inducible factors. Through the function of these factors, oxygen levels are sensed and regulated in different tissues, starting from their embryonic state to adult development. To be able to mimic this process in a tissue engineering setting, it is important to understand the role and levels of oxygen in each developmental stage, from embryonic stem cell differentiation to organogenesis and morphogenesis. Taking lessons from native tissue microenvironments, researchers have explored approaches to control oxygen tensions such as hemoglobin-based, perfluorocarbon-based, and oxygen-generating biomaterials, within synthetic tissue engineering scaffolds and organoids, with the aim of overcoming insufficient or nonuniform oxygen levels and nutrient supply.

Mesenchymal stem cell cultivation in electrospun scaffolds: mechanistic modeling for tissue engineering

Tissue engineering is a multidisciplinary field of research in which the cells, biomaterials, and processes can be optimized to develop a tissue substitute. Three-dimensional (3D) architectural features from electrospun scaffolds, such as porosity, tortuosity, fiber diameter, pore size, and interconnectivity have a great impact on cell behavior. Regarding tissue development in vitro, culture conditions such as pH, osmolality, temperature, nutrient, and metabolite concentrations dictate cell viability inside the constructs. The effect of different electrospun scaffold properties, bioreactor designs, mesenchymal stem cell culture parameters, and seeding techniques on cell behavior can be studied individually or combined with phenomenological modeling techniques. This work reviews the main culture and scaffold factors that affect tissue development in vitro regarding the culture of cells inside 3D matrices. The mathematical modeling of the relationship between these factors and cell behavior inside 3D constructs has also been critically reviewed, focusing on mesenchymal stem cell culture in electrospun scaffolds.

Laser micro-ablation of fibrocartilage tissue: Effects of tissue processing on porosity modification and mechanics

The temporomandibular joint disk (TMJd) is an extremely dense and avascular fibrocartilaginous extracellular matrix (ECM) resulting in a limited regenerative capacity. The use of decellularized TMJd as a biocompatible scaffold to guide tissue regeneration is restricted by innate subcellular porosity of the ECM that hinders cellular infiltration and regenerative events. Incorporation of an artificial microporosity through laser micro-ablation (LMA) can alleviate these cell and diffusion based limitations. In this study, LMA was performed either before or after decellularization to assess to effect of surfactant treatment on porosity modification as well as the resultant mechanical and physical scaffold properties. Under convective flow or agitation schemes, pristine and laser ablated disks were decellularized using either low (0.1% w/v) or high (1% w/v) concentrations of sodium dodecyl sulfate (SDS). Results show that lower concentrations of SDS minimized collagen degradation and tissue swelling while retaining its capacity to solubilize cellular content. Regardless of processing scheme, laser ablated channels incorporated after SDS treatment were relatively smaller and more uniform than those incorporated before SDS treatment, indicating an altered laser interaction with surfactant treated tissues. Smaller channels correlated with less disruption of native biomechanical properties indicating surfactant pre-treatment is an important consideration when using LMA to produce artificial porosity in ex vivo derived tissues. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 1858-1868, 2018.

Human Perinatal-Derived Biomaterials

Human perinatal tissues have been used for over a century as allogeneic biomaterials. Due to their advantageous properties including angiogenecity, anti-inflammation, anti-microbial, and immune privilege, these tissues are being utilized for novel applications across wide-ranging medical disciplines. Given continued clinical success, increased adoption of perinatal tissues as a disruptive technology platform has allowed for significant penetration into the multi-billion dollar biologics market. Here, we review current progress and future applications of perinatal biomaterials, as well as associated regulatory issues.

Imaging of oxygen gradients in giant umbrella cells: an ex vivo PLIM study

O2 plays a pivotal role in aerobic metabolism and regulation of cell and tissue function. Local differences and fluctuations in tissue O2 levels are well documented; however, the physiological significance of O2 microgradients, particularly at the subcellular level, remains poorly understood. Using the cell-penetrating phosphorescent O2 probe Pt-Glc and confocal fluorescence microscopy, we visualized O2 distribution in individual giant (>100-μm) umbrella cells located superficially in the urinary bladder epithelium. We optimized conditions for in vivo phosphorescent staining of the inner surface of the mouse bladder and subsequent ex vivo analysis of excised live tissue. Imaging experiments revealed significant (≤85 μM) and heterogeneous deoxygenation within respiring umbrella cells, with radial O2 gradients of up to 40 μM across the cell, or ∼0.6 μM/μm. Deeply deoxygenated (5–15 μM O2) regions were seen to correspond to the areas enriched with polarized mitochondria. Pharmacological activation of mitochondrial respiration decreased oxygenation and O2 gradients in umbrella cells, while inhibition with antimycin A dissipated the gradients and caused gradual reoxygenation of the tissue to ambient levels. Detailed three-dimensional maps of O2 distribution potentially can be used for the modeling of intracellular O2-dependent enzymatic reactions and downstream processes, such as hypoxia-inducible factor signaling. Further ex vivo and in vivo studies on intracellular and tissue O2 gradients using confocal imaging can shed light on the molecular mechanisms regulating O2-dependent (patho)physiological processes in the bladder and other tissues.

Cartilage repair and subchondral bone migration using 3D printing osteochondral composites: a one-year-period study in rabbit trochlea

Increasing evidences show that subchondral bone may play a significant role in the repair or progression of cartilage damage in situ. However, the exact change of subchondral bone during osteochondral repair is still poorly understood. In this paper, biphasic osteochondral composite scaffolds were fabricated by 3D printing technology using PEG hydrogel and β-TCP ceramic and then implanted in rabbit trochlea within a critical size defect model. Animals were euthanized at 1, 2, 4, 8, 16, 24, and 52 weeks after implantation. Histological results showed that hyaline-like cartilage formed along with white smooth surface and invisible margin at 24 weeks postoperatively, typical tidemark formation at 52 weeks. The repaired subchondral bone formed from 16 to 52 weeks in a “flow like” manner from surrounding bone to the defect center gradually. Statistical analysis illustrated that both subchondral bone volume and migration area percentage were highly correlated with the gross appearance Wayne score of repaired cartilage. Therefore, subchondral bone migration is related to cartilage repair for critical size osteochondral defects. Furthermore, the subchondral bone remodeling proceeds in a “flow like” manner and repaired cartilage with tidemark implies that the biphasic PEG/β-TCP composites fabricated by 3D printing provides a feasible strategy for osteochondral tissue engineering application.

The effect of terminal sterilization on structural and biophysical properties of a decellularized collagen-based scaffold; implications for stem cell adhesion

Terminal sterilization induces physical and chemical changes in the extracellular matrix (ECM) of ex vivo-derived biomaterials due to their aggressive mechanism of action. Prior studies have focused on how sterilization affects the mechanical integrity of tissue-based biomaterials but have rarely characterized effects on early cellular interaction, which is indicative of the biological response. Using a model fibrocartilage disc scaffold, these investigations compare the effect of three common sterilization methods [peracetic acid (PAA), gamma irradiation (GI), and ethylene oxide (EtO)] on a range of material properties and characterized early cellular interactions. GI and EtO produced unfavorable structural damage that contributed to inferior cell adhesion. Conversely, exposure to PAA resulted in limited structural alterations while inducing chemical modifications that favored cell attachment. Results suggest that the sterilization approach can be selected to modulate biomaterial properties to favor cellular adhesion and has relevance in tissue engineering and regenerative medicine applications. Furthermore, the study of cellular interactions with modified biomaterials in vitro provides information of how materials may react in subsequent clinical applications.

Microfluidic generation of polydopamine gradients on hydrophobic surfaces

Engineered surface-bound molecular gradients are of great importance for a range of biological applications. In this paper, we fabricated a polydopamine gradient on a hydrophobic surface. A microfluidic device was used to generate a covalently conjugated gradient of polydopamine (PDA), which changed the wettabilty and the surface energy of the substrate. The gradient was subsequently used to enable the spatial deposition of adhesive proteins on the surface. When seeded with human adipose mesenchymal stem cells, the PDA-graded surface induced a gradient of cell adhesion and spreading. The PDA gradient developed in this study is a promising tool for controlling cellular behavior and may be useful in various biological applications.

Cancer Stem Cells under Hypoxia as a Chemoresistance Factor in Breast and Brain

Over the last fifteen years, basic science and clinical studies have aimed to identify cancer stem cells (CSCs) in multiple types of cancer in order to unravel their mechanistic roles in cancer recurrence for therapeutic exploitation. Exposure of cells and tissues to hypoxia, or sub-atmospheric concentrations of oxygen (< 21% O₂), stimulates various stress response pathways that bias the cells towards a self-preserving, anti-apoptotic phenotype. Despite major advances in our understanding of hypoxia, CSCs, and their interrelated nature, some of the most promising cancer therapies have shown limited efficacy in clinic for the past few years, in part due to the inherently hypoxic nature of growing tumors. In the present article, we discuss recent findings regarding the behavior of breast and brain CSCs under hypoxia, as well as the mechanisms that have been shown to drive their chemo-/radioresistance and metastatic potential.