Deconstructing and reconstructing the human brain with regionally specified brain organoids

Abstract

Brain organoids, three-dimensional neural cultures recapitulating the spatiotemporal organization and function of the brain in a dish, offer unique opportunities for investigating the human brain development and diseases. To model distinct parts of the brain, various region-specific human brain organoids have been developed. In this article, we review current approaches to produce human region-specific brain organoids, developed through the endeavor of many researchers. We highlight the applications of human region-specific brain organoids, especially in reconstructing regional interactions in the brain through organoid fusion. We also outline the existing challenges to drive forward further the brain organoid technology and its applications for future studies.

Keywords: ATAC-seq; Axonal connection; Brain organoid; Cell migration; Development; Differentiation; Disease modeling; Embryonic stem cells; Human pluripotent stem cells; Induced pluripotent stem cells; Interneuron; Organoid fusion; Region-specific; Thalamus.

Figure 1.

Summary of unguided and guided human brain organoids. Cerebral organoids generated from the unguided approach broadly resemble the whole human brain, including forebrain, midbrain, and hindbrain regions. Region-specific brain organoids generated from the guided strategies represent discrete parts of the human brain. Besides various brain regions, organoids resemble the spinal cord also can be produced.

Figure 2.

Guided strategies to generate human region-specific brain organoids. (A) Morphogen distributions in the developing neural tube. Shh is essential for the development of ventral axis. Wnts and retinoic acid play crucial roles in patterning the caudal axis of the neural tube. The intersection of Shh, Wnt, and Fgf8 is critical for midbrain development. RA: retinoic acid. (B) Gradients of Shh, Wnts, and BMPs in developing forebrain along the dorsal-ventral axis. In the forebrain region, Shh is concentrated in the ventral part, whereas Wnts and BMPs are concentrated in the dorsal part. CX: Cortex; LGE: Lateral ganglionic eminence; MGE: Medial ganglionic eminence; PoA: Preoptic area. (C) Schematic view of methods for generating human region-specific brain organoids from hPSCs. *The majority of cells undergo neuroectoderm differentiation in the self-guiding system, while differentiation towards other lineages may also exist. **The involvement of WNT inhibition in the forebrain organoid generation is optional. #The activation of SHH signaling can be achieved by supplementing either recombinant SHH or smoothened agonist.

Figure 3.

Fusion of region-specific brain organoids model regional interactions in the brain. (A) The cell migration models. The cortex contains two types of neurons, glutamatergic pyramidal neurons and GABAergic interneurons. During brain development, interneurons are derived from the ventral forebrain, chiefly from the medial ganglionic eminence, then migrate tangentially to the cortex. This migratory stream can be recapitulated in several organoid fusion systems, including the fusion of cortical spheroid with subpallium spheroid, the fusion of dorsal forebrain organoid with ventral forebrain organoid, and the fusion of cortical organoid with medial ganglionic eminence organoid. CX: Cortex; LGE: Lateral ganglionic eminence; MGE: Medial ganglionic eminence; PoA: Preoptic area. Note CGE (caudal ganglionic eminence) is not shown here. (B) The long-range connection model. In the developing brain, extensive thalamocortical and corticothalamic axon projections are established between the cortex and thalamus. This reginal connection can be recapitulated in a dish by fusion of human thalamus organoid with cortical organoid. CX: Cortex; TH: Thalamus.

Figure 4.

Applications of human brain organoids. Patient or healthy donor-derived iPSCs can be utilized to generate multiple brain organoid models, including cerebral organoids resembling the whole human brain, various region-specific brain organoids, and fused human brain organoids. These models can be applied to study a wide range of questions in both biological and medical perspectives. To facilitate disease modeling and other medical studies such as drug discovery, personalized medicine, and tissue replacement, genome editing technology will be employed to either introduce disease-risk alterations in the genome of healthy donor-derived iPSCs or to repair potential disease mutations in patient-derived iPSCs. In the long term, the findings and products basing on these brain organoid systems will eventually be applied to cure or alleviate symptoms in patients. Note during these processes, the recruit of complementary models such as rodents may be necessary as well.