Stem cells push back the frontiers of psychiatric research
The human brain is notoriously difficult to study. The organ is home to billions of cells that come in hundreds of flavors, woven into a network of trillions of dynamic cellular connections that make it one of the most complex structures in the body. It is the seat of decidedly human traits like language, creativity, and higher cognition that set us apart from other organisms, making animal models less than ideal for studying human illnesses like psychiatric disease.
In addition, the brain is ethically and practically inviolable. Unlike studies of cancer or immune disorders, in which diseased tissue can be sampled relatively easily, obtaining neurons from living people is unfeasible. Research volunteers are unlikely to consent to the biopsy of their brain tissue and even if they did, growing them in a dish would be no easy task.
The ideal experimental model for studying psychiatric disease would be an easily replenished supply of human brain cells – for example, by generating them from so-called “pluripotent” stem cells that can theoretically be coaxed into neurons or other brain cell types – but until very recently, such a crucial research tool didn’t exist.
In recent years, scientists have turned to the human genome for insight on the genes and biological processes that underlie mental illness, in hopes of shedding light on the brain’s inner workings in health and disease and charting new therapeutic and diagnostic avenues. Over the past decade, researchers at the Stanley Center for Psychiatric Research at the Broad Institute have helped assemble large international collections of DNA from people with and without psychiatric disease and performed large-scale analyses to identify genetic risk factors. These efforts have been hugely successful; for example, more than 100 genetic regions have been linked to schizophrenia, and many more are likely to come.
But this success has also painted a daunting picture of the complexity associated with the genetics of psychiatric disorders. Many of these illnesses are “polygenic,” meaning they are influenced by the cumulative, subtle effects of a great number of genetic variations, rather than caused by a single mutation of large effect. To make sense of the many genetic risk factors coming out of human genomic studies, scientists need new, high-throughput ways to study how those genetic variations impact the function of brain cells.
Recognizing that unique approaches are needed to make headway in mental illness, a growing team of scientists in the Stanley Center’s Stem Cell Program aims to build an innovative resource for cellular studies of neurobiology and psychiatric disease: a “biobank” of hundreds of stem cell lines that will enable the analysis of these disorders at a resolution and scale unheard of in neuroscience.
Led by Kevin Eggan, an institute member of the Broad and a professor in the Department of Stem Cell and Regenerative Biology at Harvard University, the group harnesses the latest advances in stem cell science, cellular reprogramming, and gene-editing technology to generate functioning human neurons in the dish. These cellular models will be crucial to unraveling the biological roots of psychiatric diseases, to charting new therapeutic avenues, and to shedding light on normal brain physiology and development.
“One of the great challenges in studying psychiatric diseases is that they’re so polygenic. Our understanding of the genetics underlying these disorders is growing, yet it’s still far behind other diseases like cancer,” said Lindy Barrett, a group leader in the Stem Cell Program. “To understand what’s going wrong in the brain, we need large collections of cell lines with diverse genetic makeup to cover all the genome variation in disease.”
To create this new biobank, Eggan, Barrett, and their colleagues turned to “pluripotent” stem cells, which have the capacity to become many different cell types in the body. With the right coaxing in the lab, pluripotent cells could spawn a nearly endless supply of human brain cells for research.
Since the Stem Cell Program’s founding in 2014, the researchers have been building infrastructure in their lab spaces at both the Broad’s Stanley Center and at Harvard University, and refining protocols to generate neurons from pluripotent stem cells.
The process begins with fibroblasts – most often, from skin biopsies – taken from healthy volunteers or from patients with psychiatric diseases that are collected at partner institutions such as McLean Hospital. Those fibroblasts are then sent to collaborators at the New York Stem Cell Foundation, where they spend several months being “reprogrammed” into less specialized cells, known as induced pluripotent stem (iPS) cells. The iPS cells are sent to the group’s labs at Harvard and Broad, where they can be treated with a particular cocktail of factors to “differentiate,” or mature, into neuronal cells.
In addition to studying the effects of natural genetic variation in patient populations, the researchers want the ability to engineer precise genetic variants in neurons, an incredibly useful tool for investigating the role of genes and variants identified in large-scale genomic studies. To do so, they harness the power of the cutting-edge gene-editing technology known as CRISPR-Cas9. Using this genetic “cut-and-paste” method, they can introduce single or multiple genetic changes into iPS cells or embryonic stem cells (pluripotent cells isolated from human embryos, rather than derived from specialized cells like fibroblasts) and isolate the effects of just those changes in the resulting differentiated neurons.
With these custom-made cell lines, scientists within the Stanley Center and beyond can investigate the functional effects of particular patterns of genetic variation in a high-throughput manner, using a variety of experimental assays in the lab. “For the Stanley Center, this is the first real opportunity to approach cellular studies with such statistical power,” said Eggan. “We can investigate the underlying cell biology of these polygenic illnesses at a larger scale than what’s been done before, and with genetic diversity that more accurately reflects what we see in the population.”
For now, the team is focused on generating excitatory cortical neurons because of evidence for changes in the cortex (the brain’s outer layer, responsible for many facets of cognitive function) in psychiatric disease. The researchers hope to eventually generate other brain cell types, including astrocytes and interneurons, to explore the involvement of the brain’s many cell types in disease and the effects of disease-linked variants on those cells. So far, they’ve been able to reliably produce neurons that resemble brain cells at an early stage of maturation, which are useful for studying disorders of early neurodevelopment. They are also working with the Broad’s Center for the Development of Therapeutics to explore options for automating some steps of the differentiation process, to allow them to scale up even further.
The Stem Cell Program plans to bank human cell lines that can be shared with others to conduct original research. “We want to share these tools and empower other researchers so they can make discoveries about the molecules and pathways involved in psychiatric disease and, hopefully, one day identify new treatments for these illnesses based on the underlying biology,” said Barrett.