Cerebral Organoids in Neurodevelopment, Neurodegeneration & Virology Research
What are Cerebral Organoids?
Cerebral organoids, also called brain organoids, are miniature organs artificially grown from embryonic stem cells (ESCs) or human induced pluripotent stem cells (hIPSCs), and that resemble a brain in their functional organization. As they are derived from human cells, cerebral organoids enable the study of neurophysiology and disease using in vitro and in vivo methods that cannot be used in humans.
The development of brain organoids begins with the cultivation of embryoid bodies from stem cells, which have the same three developmental layers as embryos: endoderm, mesoderm and ectoderm. Differentiation of the neuroectoderm and expansion of the neuroepithelium are the next stages of organoid development. This protocol was originally developed by Lancaster et al and has since been refined to enable the generation of organoids that recapitulate specific brain areas by addition of extracellular signals, also known as patterning factors. Mature organoids can have areas that express genetic markers of hippocampal neurons for example, as well as the optic cup and cortical layers. Cortical neurons in mature organoids display spontaneous Ca2+ surges and Ca2+ oscillations, imaged with Ca2+ indicator dyes, both of which are features of functional neuronal signaling.
Applications of Cerebral Organoids
Cerebral organoids can be used to study neurophysiology and neurodevelopment, as well as to model various neurological diseases. Due to their origin in human cells, cerebral organoids span the gap between animal models and clinical research. Additionally, the ability to generate organoids from patient stem cells enables researchers to study mechanisms of disease directly related to cellular processes.
Cerebral Organoids for Neurodevelopment Research
Neurodevelopment and neurodevelopmental disorders are hard to study in rodent animal models, due to differences in the way the brain develops and in mature cortical surface area between mice/rats and humans. Cerebral organoid development mirrors fetal brain development, which enables the investigation of neurodevelopmental processes, including the development of specific brain areas, as well as neurodevelopmental disorders.
Microcephaly is a rare neurological condition where the brain and head don’t develop properly, and is thought to be the result of depletion of neural progenitor cells (NPC) during development. Using cerebral organoids generated from microcephaly patient stem cells, Lancaster et al (2013) and Gabriel et al (2016) have shown that NPCs undergo asymmetric cell division and premature differentiation in microcephaly, compared to NPCs in organoids from healthy donors.
Similarly, organoids have been used to study monogenic autism spectrum disorders, such as Timothy syndrome, which is caused by missense mutation in the Cav1.x (L-type) calcium channel gene CACNA1C. Dorsal and ventral forebrain cerebral organoids from patients with Timothy syndrome exhibit defects in Ca2+ signaling, as well as abnormal migration of GABAergic interneurons (Birey et al, 2017)
Cerebral Organoids for Neurodegeneration Research
Alzheimer’s disease (AD) is a progressive neurodegenerative disease characterized by protein aggregation and neuronal loss, leading to cognitive and functional impairment. Although most cases of AD are sporadic, a small percentage of cases are inherited. Organoids generated from early-onset, familial AD patient cells display both intracellular β-amyloid (Aβ) plaques, and extracellular neurofibrillary tangles, pathological disease hallmarks that animal models have poorly demonstrated.
Park et al developed an AD cerebral organoid model by co-culturing neurons and astrocytes differentiated from hNPCs derived from patient stem cells alongside a human microglial cell line in a microfluidic system. This organoid system successfully models key AD features including Aβ aggregation, phosphorylated Tau accumulation and associated neuroinflammation, and mirrors the microglial recruitment, neurotoxic activities, and nitric oxide (NO) release that damage astrocytes and neurons in AD. This organoid co-culture provides a more precise and comprehensive model for the investigation of AD pathology, when compared to most animal models.
Cerebral Organoids for Virology Research
Cerebral organoids have also been utilized in virology research. Zika virus is a mosquito-borne flavivirus associated with mild symptoms in adults but can cause fetal microcephaly and severe fetal brain defects in pregnancy. Using brain organoids infected with Zika virus, Qian et al (2017) and Long et al (2020) have shown that the virus causes reorganization of the endoplasmic reticulum to facilitate viral replication, using super-resolution microscopy.
Similarly, cerebral organoids have been used to investigate the neurological symptoms of COVID-19, caused by SARS-CoV-2. Symptoms including headache, loss of sense of taste and smell, and confusion and seizures in severe cases have been reported, as well as identification of SARS-CoV-2 in brain biopsies from fatal cases. Zhang et al showed that SARS-CoV-2 can infect cerebral organoids and that it co-localizes with neuronal and NPC markers, and that the virus can replicate in neurons with active release of viral progeny.
Header image taken from Lancaster et al (2013), showing neural progenitor cells stained for Sox2 (red) and neurons stained for Tuj1 (green). Nuclei stained blue with Hoechst 33342 (Cat. No. 5117).
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