1. Overview of brain-like organs
Brain organoids (BOs) are three-dimensional organoids formed in the in vitro environment by the spontaneous assembly of human pluripotent stem cells to form globular mimics, which in turn differentiate into three-dimensional organoids.
These organoids contain cell types similar to those of the human brain and are able to mimic the growth and development process of the early human brain in terms of molecular, cellular and structural aspects, and possess physiological functions similar to those of the human brain.
Currently, brain-like organs cultured in various laboratories around the world can be broadly categorized into two groups:
The first group are those anamorphs that have not been subjected to targeted induction during maturation and differentiate independently into neural ectoderm, eventually forming brain-like organs that contain multiple brain regions;
The second category is the directional differentiation of anthropoid embryos toward specific brain regions under the action of morphogenetic factors, and the eventual formation of brain-like organs capable of mimicking the morphology and function of specific brain regions, such as cortical-like organs, thalamic-like organs, midbrain-like organs, and striatal-like organs.
2. Brain-like organs in neurodevelopmental disorders
1) Autism Spectrum Disorder (ASD)
Autism Spectrum Disorder (ASD) is a disorder caused by a combination of genetic and environmental factors, and hundreds of risk genes associated with ASD have been identified.
Patients with ASD often present with neurodevelopmental abnormalities during childhood, and patho-anatomical studies have shown a tendency for ASD patients to have an overgrowth of their cortical volume in the early postnatal period, although the molecular mechanisms remain unclear.
A team of Harvard and MIT researchers collaborated to culture cortical brain-like organs with mutations in the SUV420H1, ARID1B, and CHD8 genes, which are associated with abnormal head volume in ASD patients.
By sequencing the single-cell transcriptomes of these three cerebral organoids, the study found a significant increase in the proportion of inhibitory neurons.
This suggests that although mutations in different ASD-associated genes may lead to different clinical symptoms, these genes may trigger common pathological changes, such as the development of macrocephaly, through similar mechanisms.
2) Microcephaly
Microcephaly is a condition in which the patient's head size is smaller compared to normal and usually becomes apparent at birth or during early development.
It is currently believed that this condition is caused by abnormal gene expression in the fetus during pregnancy, but studies on this are difficult to verify because the fetus is in the mother's body.
CDK5RAP2 is thought to be the key gene responsible for microcephaly, which is a human-specific segment of the gene sequence, and therefore relevant experiments could not be carried out in animal models.
The team first obtained dermal fibroblasts from patients with microcephaly who had a mutation in the CDK5RAP2 gene, and then induced them into pluripotent stem cells by reprogramming techniques, and further cultured them to induce brain-like organs.
It was found that the brain-like organs of these patients developed with a significantly lower proportion of neural progenitor cells and a premature increase in the number of neurons.
In patients with microcephaly, the brain enters the differentiation stage prematurely during development, but the number of neural progenitor cells is insufficient. This study provides a concrete explanation for the mechanism of microcephaly.
3. Brain-like organs in neurodegenerative diseases
1) Alzheimer's Disease (AD)
Currently, there are more than 40 million people with Alzheimer's disease (AD) worldwide, but no effective clinical treatment has been found.
There are two main reasons for this predicament:
The etiology of AD is complex, and studies have shown that amyloid deposition, neuronal fibrillary tangles, and hyperphosphorylation of Tau proteins may be triggers for AD;
Due to genetic differences between humans and other experimental animals, many drugs that are effective in animal models are difficult to translate into clinical therapy, leading to significant challenges in drug development.
It was found that brain-like organs showed increased levels of β-amyloid and elevated levels of phosphorylation of Tau proteins after exposure to serum, along with synaptic deficits.
Brain-like organs have also been used in the screening of Alzheimer's disease drugs, making them an important tool in the study of this disease.
4) Parkinson's Disease (PD)
Parkinson's disease (PD) is a neurodegenerative disorder characterized by a constant loss of dopaminergic neurons in the midbrain and substantia nigra regions.
Experimental studies have revealed distinct PD pathologic features in midbrain-like organs with DNAJC6 mutations, including degradation of dopaminergic neurons, aggregation of alpha-synaptic nuclear proteins, increased frequency of neuronal firing, and mitochondrial lysosomal dysfunction.
The experimental results showed that DNAJC6 deletion impaired the WNT-LMX1A signaling pathway, which further led to pathological changes.
Midbrain-like organs have not only been used to explore the pathomechanisms of PD, but researchers have also applied them to therapeutic studies of PD.
4. Substance addiction
1) Alcohol dependence
Maternal intake of alcohol during pregnancy can negatively affect fetal development, possibly leading to fetal alcohol syndrome, and the fetus may develop cognitive dysfunction and neurological disorders by the age of 9 to 10 years.
With the development of brain organoid technology, the mechanisms behind this phenomenon are better understood.
The research team discovered a new mechanism by which alcohol intake during pregnancy affects fetal development.
They induced human pluripotent stem cells into cortical brain-like organs and exposed them to alcohol.
Experiments have shown that alcohol causes changes in the expression of genes related to the development of the nervous system, disrupts the structure and function of mitochondria in cortical brain-like organs, and ultimately triggers apoptosis.
2) Cocaine addiction
Fetal prenatal exposure to cocaine affects the development of the cerebral cortex. Although animal experiments have yielded some research results, the exact molecular mechanisms remain unclear.
By inducing cortical brain-like organs from human pluripotent stem cells, the team discovered CYP3A5, the target of cocaine's action in affecting brain development.
Knockdown of CYP3A5 was able to reverse the negative effects of cocaine on brain development, which opens up new possibilities for future mitigation of neurodevelopmental damage caused by prenatal cocaine exposure by acting on this target.
3) Marijuana addiction
Marijuana contains a variety of components, with tetrahydrocannabinol (THC) being the one that primarily causes the addictive effects.
It was found that exposure of cortical brain-like organs to THC inhibits the maturation and differentiation of neural stem cells and decreases the levels of endogenous cannabinoid receptor 1, which inhibits axonal growth.
In addition, another single-cell sequencing study showed a significant increase in broken DNA in brain-like organs after 7 days of action of the endogenous cannabinoid receptor agonist WIN55,212-2, and that activation of endogenous cannabinoid receptors leads to apoptosis in the brain-like organs, which in turn affects the number of neurons.
4) Methamphetamine
Methamphetamine has been shown to affect cognitive functioning, cause neurotoxic reactions, and impede brain development.
Although there is a research base on the first two aspects, there is still a lack of clarity on the mechanisms of how methamphetamine affects brain development.
By combining brain organoid technology with single-cell transcriptome sequencing, the team explored the effects of methamphetamine on prenatal brain development.
Differential gene analysis showed that genes regulating neural stem cell proliferation and neuronal differentiation were significantly downregulated in the methamphetamine-exposed group, whereas genes associated with inflammation and oxidative stress were significantly upregulated.
Prenatal methamphetamine exposure may impede brain development by modulating neuronal cell proliferation and differentiation with concomitant neuroinflammation.
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