‘Mini-Brains’ Provide Clues About Early Life Origins of Schizophrenia

Summary: Brain organoid study reveals multiple changes in brain cells during early embryonic development may contribute to schizophrenia later in life.

Source: Weill Cornell University

Multiple changes in brain cells during the first month of embryonic development may contribute to schizophrenia later in life, according to a new study by Weill Cornell Medicine investigators.

The researchers, whose study was published in Molecular Psychiatry on Nov. 17, used stem cells collected from patients with schizophrenia and people without the disease to grow 3-dimensional “mini-brains” or organoids in the laboratory. By comparing the development of both sets of organoids, they discovered that a reduced expression of two genes in the cells stymies early development and causes a shortage of brain cells in organoids grown from patient stem cells.

“This discovery fills an important gap in scientists’ understanding of schizophrenia,” said senior author Dr. Dilek Colak, assistant professor of neuroscience at the Feil Family Brain and Mind Institute and the Center for Neurogenetics at Weill Cornell Medicine. Symptoms of schizophrenia typically develop in adulthood, but postmortem studies of the brains of people with the disease found enlarged cavities called ventricles and differences in the cortical layers that likely occurred early in life.

“There were hints schizophrenia started during early development, but we didn’t have proof,” Dr. Colak said.

By growing organoids from stem cells collected from up to 21 human stem cell donors, the team, led by first author Dr. Michael Notaras, a former NHMRC CJ Martin Fellow in Dr. Colak’s lab, was able to grow brain tissue with each patient’s exact genetic make-up. Then, they used single-cell RNA sequencing to compare gene expression in individual cells in patient’s tissue and in tissue grown from people without schizophrenia.

“We found a common pathology among all the patients with schizophrenia despite each of the patients having distinct disease presentations,” Dr. Colak said.

Schizophrenia patient samples had reduced expression of two genes essential for brain development, a regulator of gene expression called BRN2 and a cell growth promoter called pleiotrophin. This led to reduced new brain cell production and increased brain cell death. Replacing the missing BRN2 in the cells restored brain cell production, while adding pleiotrophin reduced brain cell death. If more studies confirm these results, it could lead to the development of targeted therapies that help correct these genetic differences in specific brain cell types.

“We’ve made a fundamental discovery providing what we think is the first evidence in human tissue that multiple cell-specific mechanisms exist and likely contribute to risk of schizophrenia,” said Dr. Notaras. “This forces us, as a field, to reconsider when disease truly begins and how we should think about developing the next generation of schizophrenia therapeutics.”

Dr. Colak and her colleagues are currently using the mini-brains to start to tease out the role of individual cell types and to better understand how genetic factors may interact with the environment to cause schizophrenia. They are focusing on the role of endothelial cells, cells that typically line the blood vessels and release important immune molecules called cytokines. The mini-brains grown from patients with schizophrenia had an excess of early endothelial-related cells, which could lead to an excessive immune response to infection.

“This may explain the link between maternal infections during pregnancy and schizophrenia seen in mouse studies,” Dr. Colak said.

In addition to helping scientists better understand schizophrenia, Dr. Colak said she thinks that mini-brains grown from patient’s stem cells may be a useful tool for studying other brain diseases.

“The technique could be used to study the early life pathology of late-onset neuropsychiatric or neurodegenerative diseases like Alzheimer’s disease or Huntington disease,” Dr. Colak said.

Abstract

Schizophrenia is defined by cell-specific neuropathology and multiple neurodevelopmental mechanisms in patient-derived cerebral organoids

Due to an inability to ethically access developing human brain tissue as well as identify prospective cases, early-arising neurodevelopmental and cell-specific signatures of Schizophrenia (Scz) have remained unknown and thus undefined. To overcome these challenges, we utilized patient-derived induced pluripotent stem cells (iPSCs) to generate 3D cerebral organoids to model neuropathology of Scz during this critical period.

We discovered that Scz organoids exhibited ventricular neuropathology resulting in altered progenitor survival and disrupted neurogenesis. This ultimately yielded fewer neurons within developing cortical fields of Scz organoids. Single-cell sequencing revealed that Scz progenitors were specifically depleted of neuronal programming factors leading to a remodeling of cell-lineages, altered differentiation trajectories, and distorted cortical cell-type diversity.

While Scz organoids were similar in their macromolecular diversity to organoids generated from healthy controls (Ctrls), four GWAS factors (PTN, COMT, PLCL1, and PODXL) and peptide fragments belonging to the POU-domain transcription factor family (e.g., POU3F2/BRN2) were altered.

This revealed that Scz organoids principally differed not in their proteomic diversity, but specifically in their total quantity of disease and neurodevelopmental factors at the molecular level. Single-cell sequencing subsequently identified cell-type specific alterations in neuronal programming factors as well as a developmental switch in neurotrophic growth factor expression, indicating that Scz neuropathology can be encoded on a cell-type-by-cell-type basis.

Furthermore, single-cell sequencing also specifically replicated the depletion of BRN2 (POU3F2) and PTN in both Scz progenitors and neurons. Subsequently, in two mechanistic rescue experiments we identified that the transcription factor BRN2 and growth factor PTN operate as mechanistic substrates of neurogenesis and cellular survival, respectively, in Scz organoids.

Collectively, our work suggests that multiple mechanisms of Scz exist in patient-derived organoids, and that these disparate mechanisms converge upon primordial brain developmental pathways such as neuronal differentiation, survival, and growth factor support, which may amalgamate to elevate intrinsic risk of Scz.