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The lack of suitable in vitro models that accurately represent specific tissues and disease states is a significant hindrance to basic and translational research. This has led to the development of 3D organoids, which provide greater complexity than 2D models and make stable, physiologically relevant models that can be cultivated over long-time periods. Organoids have been generated to model numerous tissue types, including pancreas, liver, kidney, retina, brain and tumors, and have demonstrated the broad potential of these systems in advancing our understanding of the biology of complex systems. Organoids have potential for use in drug screening, toxicity testing, disease modeling and to study embryonic development.
Organoids are stem cell-derived self-assembling 3D structures, which reproduce certain characteristics of an organ. Organoids are generated from adult stem cells or induced pluripotent stem cells (iPSCs) which are induced to differentiate, arranging themselves dependent on distinct expression profiles of cellular adhesion molecules and spatially restricted lineage commitment. Spatially constraining cells in tissue or by using biological scaffolds promotes further differentiation of stem cells and is crucial in the generation of organoids. Biological scaffolds derived from Engelbreth-Holm-Swarm (EHS) mouse sarcoma cells, such as Cultrex® Basement Membrane Extracts, are most commonly used in the laboratory and provide environmental cues, including growth factors, which encourage cells to attach and form organoid structures. Small molecules are also widely used in the culture media for directing the growth and differentiation of organoids.
Drug discovery is an area where organoids are likely to be of particular use. Organoids generated from patient-derived iPSCs have been found to reproduce disease characteristics and are useful in the preclinical screening of new therapeutics to establish efficacy at the cellular level. A recent paper from Pellegrini et al., however, offers a different perspective on how organoids may be of use in drug screening.
The choroid plexus (ChP) comprises a layer of epithelial cells surrounding capillaries and connective tissue and is responsible for producing cerebrospinal fluid (CSF). It also forms a barrier between the blood and the CSF, which prevents toxic substances in the circulation from reaching the brain. The ChP lies deep within the brain, which has until now made its structure and function difficult to study.
Pellegrini and the team established ChP organoids by adapting a protocol for generating cerebral organoids from human iPSCs, adding BMP-4 and CHIR 99021 to the maturation medium. The resulting organoids were enriched in cuboidal epithelium and developed fluid-filled compartments containing colorless liquid and resembled the choroid plexus in structure and function. Analysis of the colorless liquid revealed that it closely resembled CSF. Structurally the ChP organoids have tight junctions, primary cilia, extensive microvilli, multivesicular bodies, as well as extracellular vesicles, all characteristic of the ChP.
These organoids have the potential to predict the CNS permeability of new therapies in development, in order to determine a compound’s potential to treat neurological diseases or its possible toxicity.In vivo the choroid plexus shows differing permeability to L-DOPA and Dopamine. The ChP organoid also shows differing permeability to these compounds, with the former being transported into the organoid, but not the latter, demonstrating the proof-of-principle that this 3D system could be used to model CNS permeability of drugs.
In 2016 a clinical trial of BIA 10-2474, a fatty acid amide hydrolase inhibitor and potential treatment for various neurological conditions, was under way in France. Five participants in the trial suffered serious acute neurotoxicity and one died. The compound had not exhibited neurotoxic effects in animals, but ChP organoids derived from human iPSCs exhibit toxic accumulation of BIA 10-2474, revealing the potential utility of this system in toxicity testing of new therapies.
Tumor organoids for multiple different cancer types, such as breast, prostate, colon and endometrial cancers, have been generated from patient cancer biopsies. Recently Maenhoudt et al. reported the generation of ovarian cancer (OC) organoids from patients with high-grade serious ovarian cancer (HGSOC) using a method adapted from Boretto et al. (2019) to improve organoid establishment and growth.
These biopsy-derived tumor organoids exhibit the same phenotype as the originating tumor and recapitulate disease. They provide a useful model for the study of the development of OC and can additionally be used to screen new therapies for their effectiveness against this type of cancer. However, they have another potential use, in personalized medicine. OC organoids derived from different patients exhibit distinct sensitivities to conventional chemotherapeutic drugs, such as paclitaxel, carboplatin, gemcitabine and doxorubicin. Therefore, they could have utility in clinical practice to enable clinicians to select the most effective treatment for an individual patient.
Since publishing their original paper, Pellegrini and the team have used the ChP organoid to learn more about COVID-19. This viral infection is characterized by severe respiratory symptoms, but some patients also develop neurological symptoms, such as headache, confusion and seizures. Pellegrini’s team therefore examined the effect of SARS-CoV-2 or pseudovirions carrying SARS-CoV-2 spike proteins on ChP organoids and found that the virus principally infects choroid plexus epithelial barrier cells, but not neurons or glia. This results in damage to the epithelial cells and consequent leakage of the blood-CSF barrier.
Boretto et al. (2019) Patient-derived organoids from endometrial disease capture clinical heterogeneity and are amenable to drug screening. Nat Cell Biol 21, 1041. PMID: 31371824
Maenhoudt et al. (2020) Developing organoids from ovarian cancer as experimental and preclinical models. Stem Cell Rep 14, 717. PMID: 32243841
Pellegrini et al. (2020) Human CNS barrier-forming organoids with cerebrospinal fluid production. Science 369, eaaz5626. PMID: 32527923
Pellegrini et al. (2020) SARS-CoV-2 infects the brain choroid plexus and disrupts the blood-CSF barrier in human brain organoids. Cell Stem Cell 27 951. PMID: 33113348