Engineered Perineural Vascular Plexus for Modeling Developmental Toxicity

Human brain organoid: trends, evolution, and remaining challenges

Advanced brain organoids provide promising platforms for deciphering the cellular and molecular processes of human neural development and diseases. Although various studies and reviews have described developments and advancements in brain organoids, few studies have comprehensively summarized and analyzed the global trends in this area of neuroscience. To identify and further facilitate the development of cerebral organoids, we utilized bibliometrics and visualization methods to analyze the global trends and evolution of brain organoids in the last 10 years. First, annual publications, countries/regions, organizations, journals, authors, co-citations, and keywords relating to brain organoids were identified. The hotspots in this field were also systematically identified. Subsequently, current applications for brain organoids in neuroscience, including human neural development, neural disorders, infectious diseases, regenerative medicine, drug discovery, and toxicity assessment studies, are comprehensively discussed. Towards that end, several considerations regarding the current challenges in brain organoid research and future strategies to advance neuroscience will be presented to further promote their application in neurological research.

Human induced pluripotent stem cell-derived planar neural organoids assembled on synthetic hydrogels

The tailorable properties of synthetic polyethylene glycol (PEG) hydrogels make them an attractive substrate for human organoid assembly. Here, we formed human neural organoids from iPSC-derived progenitor cells in two distinct formats: (i) cells seeded on a Matrigel surface; and (ii) cells seeded on a synthetic PEG hydrogel surface. Tissue assembly on synthetic PEG hydrogels resulted in three dimensional (3D) planar neural organoids with greater neuronal diversity, greater expression of neurovascular and neuroinflammatory genes, and reduced variability when compared with tissues assembled upon Matrigel. Further, our 3D human tissue assembly approach occurred in an open cell culture format and created a tissue that was sufficiently translucent to allow for continuous imaging. Planar neural organoids formed on PEG hydrogels also showed higher expression of neural, vascular, and neuroinflammatory genes when compared to traditional brain organoids grown in Matrigel suspensions. Further, planar neural organoids contained functional microglia that responded to pro-inflammatory stimuli, and were responsive to anti-inflammatory drugs. These results demonstrate that the PEG hydrogel neural organoids can be used as a physiologically relevant in vitro model of neuro-inflammation.

Bioengineered perfused human brain microvascular networks enhance neural progenitor cell survival, neurogenesis, and maturation

Neural progenitor cells (NPCs) have the capability to self-renew and differentiate into neurons and glial cells. In the adult brain, NPCs are found near brain microvascular networks (BMVNs) in specialized microenvironments called the neurovascular niche (NVN). Although several in vitro NVN models have been previously reported, most do not properly recapitulate the intimate cellular interactions between NPCs and perfused brain microvessels. Here, we developed perfused BMVNs composed of primary human brain endothelial cells, pericytes, and astrocytes within microfluidic devices. When induced pluripotent stem cell-derived NPCs were introduced into BMVNs, we found that NPC survival, neurogenesis, and maturation were enhanced. The application of flow during BMVN coculture was also beneficial for neuron differentiation. Collectively, our work highlighted the important role of BMVNs and flow in NPC self-renewal and neurogenesis, as well as demonstrated our model's potential to study the biological and physical interactions of human NVN in vitro.

Advantages and Potential Benefits of Using Organoids in Nanotoxicology

Organoids are microtissues that recapitulate the complex structural organization and functions of tissues and organs. Nanoparticles have several specific properties that must be considered when replacing animal models with in vitro studies, such as the formation of a protein corona, accumulation, ability to overcome tissue barriers, and different severities of toxic effects in different cell types. An increase in the number of articles on toxicology research using organoid models is related to an increase in publications on organoids in general but is not related to toxicology-based publications. We demonstrate how the quantitative assessment of toxic changes in the structure of organoids and the state of their cell collections provide more valuable results for toxicological research and provide examples of research methods. The impact of the tested materials on organoids and their differences are also discussed. In conclusion, we highlight the main challenges, the solution of which will allow researchers to approach the replacement of in vivo research with in vitro research: biobanking and standardization of the structural characterization of organoids, and the development of effective screening imaging techniques for 3D organoid cell organization.

Reproducibility of developmental neuroplasticity in in vitro brain tissue models

The current prevalence of neurodevelopmental, neurodegenerative diseases, stroke and brain injury stimulates studies aimed to identify new molecular targets, to select the drug candidates, to complete the whole set of preclinical and clinical trials, and to implement new drugs into routine neurological practice. Establishment of protocols based on microfluidics, blood-brain barrier- or neurovascular unit-on-chip, and microphysiological systems allowed improving the barrier characteristics and analyzing the regulation of local microcirculation, angiogenesis, and neurogenesis. Reconstruction of key mechanisms of brain development and even some aspects of experience-driven brain plasticity would be helpful in the establishment of brain in vitro models with the highest degree of reliability. Activity, metabolic status and expression pattern of cells within the models can be effectively assessed with the protocols of system biology, cell imaging, and functional cell analysis. The next generation of in vitro models should demonstrate high scalability, 3D or 4D complexity, possibility to be combined with other tissues or cell types within the microphysiological systems, compatibility with bio-inks or extracellular matrix-like materials, achievement of adequate vascularization, patient-specific characteristics, and opportunity to provide high-content screening. In this review, we will focus on currently available and prospective brain tissue in vitro models suitable for experimental and preclinical studies with the special focus on models enabling 4D reconstruction of brain tissue for the assessment of brain development, brain plasticity, and drug kinetics.

Engineering Epithelial-Mesenchymal Microtissues to Study Cell-Cell Interactions in Development

Intercellular signaling drives human development, but there is a paucity of in vitro models that recapitulate important tissue architecture while remaining operationally simple and scalable. As an example, formation of the upper lip and palate requires the orchestrated proliferation and fusion of embryonic facial growth centers and is dependent on paracrine epithelial-mesenchymal signaling through multiple pathways including the Sonic Hedgehog (SHH), transforming growth factor-beta (Tgf-β), bone morphogenic protein (BMP), and epidermal growth factor (EGF) pathways. We have developed a robust, throughput-compatible microphysiological system to model intercellular signaling including epithelial-mesenchymal interactions that is useful for studying both normal and abnormal orofacial development. We describe the construction and operation of an engineered microplate created using CNC micromilling of 96-well microtiter plates capable of containing up to 20 epithelial-mesenchymal microtissues. A dense three-dimensional mesenchyme is created by embedding cells (O9-1, 3T3) in a biomimetic hydrogel. An epithelial layer is then overlayed on the microtissue by loading cells in engineered microchannels that flank the microtissue. The result is an engineering epithelial-mesenchymal interface that is both on and perpendicular to the imaging plane making it suitable for high-content imaging and analysis. The resulting microtissues and device are compatible with diverse analytical techniques including fluorescent and luminescent cell health and enzymatic reporter assays, gene expression analyses, and protein staining. This tractable model and approach promise to shed light on critical processes in intercellular signaling events in orofacial development and beyond.

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.

Human Blood Vessel Organoids Penetrate Human Cerebral Organoids and Form a Vessel-Like System

Vascularization of tissues, organoids and organ-on-chip models has been attempted using endothelial cells. However, the cultured endothelial cells lack the capacity to interact with other somatic cell types, which is distinct from developing vascular cells in vivo. Recently, it was demonstrated that blood vessel organoids (BVOs) recreate the structure and functions of developing human blood vessels. However, the tissue-specific adaptability of BVOs had not been assessed in somatic tissues. Herein, we investigated whether BVOs infiltrate human cerebral organoids and form a blood-brain barrier. As a result, vascular cells arising from BVOs penetrated the cerebral organoids and developed a vessel-like architecture composed of CD31⁺ endothelial tubes coated with SMA⁺ or PDGFR⁺ mural cells. Molecular markers of the blood-brain barrier were detected in the vascularized cerebral organoids. We revealed that BVOs can form neural-specific blood-vessel networks that can be maintained for over 50 days.

Bioengineering the neurovascular niche to study the interaction of neural stem cells and endothelial cells

The ability of mammalian neural stem cells (NSCs) to self-renew and differentiate throughout adulthood has made them ideal to study neurogenesis and attractive candidates for neurodegenerative disease therapies. In the adult mammalian brain, NSCs are maintained in the neurovascular niche (NVN) where they are found near the specialized blood vessels, suggesting that brain endothelial cells (BECs) are prominent orchestrators of NSC fate. However, most of the current knowledge of the mammalian NVN has been deduced from nonhuman studies. To circumvent the challenges of in vivo studies, in vitro models have been developed to better understand the reciprocal cellular mechanisms of human NSCs and BECs. This review will cover the current understanding of mammalian NVN biology, the effects of endothelial cell-derived signals on NSC fate, and the in vitro models developed to study the interactions between NSCs and BECs.

A Microphysiological Approach to Evaluate Effectors of Intercellular Hedgehog Signaling in Development

Paracrine signaling in the tissue microenvironment is a central mediator of morphogenesis, and modeling this dynamic intercellular activity in vitro is critical to understanding normal and abnormal development. For example, Sonic Hedgehog (Shh) signaling is a conserved mechanism involved in multiple developmental processes and strongly linked to human birth defects including orofacial clefts of the lip and palate. SHH ligand produced, processed, and secreted from the epithelial ectoderm is shuttled through the extracellular matrix where it binds mesenchymal receptors, establishing a gradient of transcriptional response that drives orofacial morphogenesis. In humans, complex interactions of genetic predispositions and environmental insults acting on diverse molecular targets are thought to underlie orofacial cleft etiology. Consequently, there is a need for tractable in vitro approaches that model this complex cellular and environmental interplay and are sensitive to disruption across the multistep signaling cascade. We developed a microplate-based device that supports an epithelium directly overlaid onto an extracellular matrix-embedded mesenchyme, mimicking the basic tissue architecture of developing orofacial tissues. SHH ligand produced from the epithelium generated a gradient of SHH-driven transcription in the adjacent mesenchyme, recapitulating the gradient of pathway activity observed in vivo. Shh pathway activation was antagonized by small molecule inhibitors of epithelial secretory, extracellular matrix transport, and mesenchymal sensing targets, supporting the use of this approach in high-content chemical screening of the complete Shh pathway. Together, these findings demonstrate a novel and practical microphysiological model with broad utility for investigating epithelial-mesenchymal interactions and environmental signaling disruptions in development.

Notch1-mediated inflammation is associated with endothelial dysfunction in human brain microvascular endothelial cells upon particulate matter exposure

Exposure to atmospheric particulate matter (PM) is an emerging risk factor for the pathogenesis of several diseases in humans, including cerebrovascular diseases. However, the mechanisms underlying PM-induced endothelial dysfunction are currently unclear. In this study, we examined how PM leads to endothelial dysfunction in human brain microvascular endothelial cells (HBMECs). We demonstrated that PM₁₀ exposure (up to 25 μg/mL) increase Notch1 cleavage, and it regulates endothelial dysfunction through NICD-mediated inflammation and senescence. PM₁₀-induced NICD signaling causes increased expression of interleukin-1 beta (IL-1β) and enhances characteristics of cellular senescence, which leads to increased endothelial permeability in HBMECs. Knockdown of Notch1 by siRNA blocks PM₁₀-induced endothelial dysfunction via the suppression of inflammation and senescence. Furthermore, we found that Notch1-mediated inflammation accelerates endothelial senescence, which eventually leads to endothelial dysfunction. Altogether, our data suggest that Notch1 and NICD are potential target regulators for the prevention of cerebrovascular endothelial dysfunction induced by ambient air pollutants such as PM.