How have human organoids been used in research to study SARS-CoV-2?
In a recent review published in the journal Organoidsscientists presented an overview of the use of human stem cell-derived organoids to understand the functionality and cell tropism of severe acute respiratory syndrome coronavirus 2 (SARS-Cov-2), a deadly beta-coronavirus responsible for the ongoing coronavirus disease 2019 (covid19 pandemic.
Study: Organoid models of SARS-CoV-2 infection: what have we learned about COVID-19?
Organoids are three-dimensional, multicellular tissue structures that mimic the structure and function of corresponding endogenous organs. Organ-specific cells derived from human stem cells can self-organize to form organoids. Two types of stem cells are mainly used for the preparation of organoids. These include embryonic pluripotent stem cells or induced pluripotent stem cells and organ-restricted stem cells.
Organoids have shown promising results in understanding the pathogenesis of respiratory and intestinal infections. For example, during the Zika virus outbreak, brain organoids developed from induced pluripotent stem cells were widely used to understand disease pathogenesis.
Schematic structure of SARS-CoV-2 virion and viral life cycle. The SARS-CoV-2 virion contains an envelope with three proteins: the spike (S) protein, the membrane (M) and envelope (E) proteins. The viral RNA genome and the nucleocapsid (N) protein are contained inside the particle. The schematic of the virion shows in more detail the S protein subunit 1 (S1) containing the receptor binding domain (RBD). The SARS-CoV-2 replication cycle shows two modes of viral entry (endocytic and TMPRSS2-mediated pathways). Both require docking to ACE2 on the cell surface and release of the viral genome into the cytoplasm via membrane fusion. The endocytic pathway is inhibited by (hydroxy)chloroquine and E64d, while TMPRSS2-mediated entry is inhibited by camostat. After synthesis of the viral genome and proteins, the viral particles are assembled and leave the infected cell.
Organoids in COVID-19
SARS-CoV-2 is an enveloped, single-stranded, positive-sense RNA virus belonging to the human beta-coronavirus family. Being a respiratory virus, it mainly infects the epithelial cells of the respiratory tract, causing infections in the upper and lower respiratory tract. However, the virus also induces infections in the blood vessels, heart, intestine, brain, liver, and kidneys in severe cases.
Organoid technology has played an extremely vital role in understanding the life cycle and cell tropism of SARS-CoV-2, host-virus interactions, and the pathogenesis of the COVID-19 multisystem. Various organ-specific organoids have been used in COVID-19-related studies to understand disease pathology in specific tissues.
Organoid models used in studies of SARS-CoV-2 infection.
Airway organoid models
Respiratory organoids developed from human airway epithelium derived from progenitor cells have been used to understand the cell tropism of SARS-CoV-2 in the airways. These organoids can be differentiated at an air-liquid interface or embedded in a matrix. Several cell types are included in the respiratory organoids including basal cells, hair cells, and goblet cells.
Studies using respiratory organoids have confirmed that SARS-CoV-2 primarily infects hair cells and that viral entry and release occurs from the apical surface of the airways. Moreover, it was observed that the expressions of angiotensin converting enzyme 2 (ACE2) and TMPRSS2 (proteins of the host cell facilitating viral entry) are significantly elevated in the epithelium of the nose, which gradually decrease towards the proximal airways. Studies using lung bud organoids have demonstrated that type II pneumocytes support SARS-CoV-2 infection in the alveolar region.
Several organoid cultures established from respiratory tissues are now commercially available, including organoids from the nose, bronchi, alveoli, and lung. Although expensive, commercial organoids play an extremely important role in the development of drugs and vaccines against COVID-19.
Organoid models of intestinal epithelium
Adult intestinal epithelial stem cells have been used to grow human intestinal organoids to study the pathogenesis of SARS-CoV-2 infection in the gastrointestinal tract. Using human small intestine organoids with alveolar organoids, the difference in replication capacity between the original SARS-CoV-2 strain and the alpha variant was established.
Due to the high expression of ACE2 and TMPRSS2, small intestinal organoids have been widely used to understand the wide range of transmission and infection dynamics of SARS-CoV-2 as well as virus-induced changes in the gut microbiota. Additionally, these organoids have been used to screen for potential antiviral agents.
Organoid models of the cardiovascular system
Cardiomyocytes derived from induced pluripotent stem cells have been used to develop cardiac organoids. Studies using these organoids have shown that entry of SARS-CoV-2 into cardiomyocytes requires interaction with ACE2; however, entry can be via the endocytic or TMPRSS2-dependent pathway. These organoids have also been used to identify SARS-CV-2 inhibitors and understand the heart damage caused by the virus.
Hair organoids derived from induced pluripotent stem cells have been used to confirm productive SARS-CoV-2 infection in endothelial cells. Moreover, in COVID-19 patients, a significant association was observed between circulating endothelial cells and disease severity.
To understand the wide range of cardiovascular pathologies related to COVID-19, more complex organoids, in which the vasculature is integrated into the organ, are needed.
Organoid models of the brain
Brain organoids derived from human embryonic stem cells or induced pluripotent stem cells have been used to understand the mechanism of SARS-CoV-2 entry into the central nervous system as well as virus-induced neurological complications, including headache. head, dizziness and loss of smell. and taste, seizures, encephalopathy and cerebrovascular disease. Various region-specific brain organoids, including cortical, hippocampal, hypothalamic, and midbrain organoids, have been developed. However, these organoids failed to confirm productive SARS-CoV-2 infection in the brain.
A production brain infection was confirmed using brain organoids containing choroid plexus epithelium. This model showed that SARS-CoV-2-induced epithelial damage results in a loss of barrier function, facilitating viral entry into the brain.
Kidney organoid models
Modified kidney organoids expressing ACE2 and TMPRSS2 have been developed to understand SARS-CoV-2-induced kidney pathologies. Similarly, normal human kidney proximal tubule epithelial cells have been used to develop matrix-embedded renal epithelial organoids. To grow kidney organoids, conditional reprogramming conditions were used to initially create long-term two-dimensional cultures, followed by their differentiation into three-dimensional organoids.
Organoid models of the eye
Ocular surfaces, including the cornea, sclera, and limbus, are considered alternate sites of SARS-CoV-2 infection. Retinal and “whole eye” organoids induced by human embryonic stem cell-induced pluripotent stem cells have been used to study ocular infection with SARS-CoV-2. These studies confirmed the expression of ACE2 and TMPRSS2 and SARS-CoV-2 infection in limbus cells.
Collectively, a wide range of preclinical human organoid models developed during the COVID-19 pandemic provide a valuable platform to study viral life cycle, transmissibility, and pathogenesis.[if–>