In a groundbreaking achievement, a research team led by experts at Cincinnati Children’s has developed the world’s first human mini-brain that incorporates a fully functional blood-brain barrier (BBB). This significant advancement, published on May 15, 2024, in Cell Stem Cell, promises to accelerate the understanding and treatment of various brain disorders, including stroke, cerebral vascular disorders, brain cancer, Alzheimer’s disease, Huntington’s disease, Parkinson’s disease, and other neurodegenerative conditions.

“Lack of an authentic human BBB model has been a major hurdle in studying neurological diseases,” says lead corresponding author Ziyuan Guo, PhD. “Our breakthrough involves generating human BBB organoids from human pluripotent stem cells, mimicking human neurovascular development to produce a faithful representation of the barrier in growing, functioning brain tissue. This is an important advance because animal models we currently use in research do not accurately reflect human brain development and BBB functionality.”

Understanding the Blood-Brain Barrier

The blood-brain barrier is a unique feature of the brain’s blood vessels, characterized by an extra lining of tightly packed cells that sharply limit the size of molecules that can pass from the bloodstream into the central nervous system (CNS). This barrier maintains brain health by preventing harmful substances from entering while allowing essential nutrients to reach the brain. However, it also hinders many potentially beneficial medicines from reaching the brain. Several neurological disorders are caused or worsened when the blood-brain barrier forms improperly or breaks down. Significant differences between human and animal brains have led to many promising new drugs failing in human trials after being tested on animal models.

A Major Leap in Research

“Through stem cell bioengineering, we have developed an innovative platform based on human stem cells that allows us to study the intricate mechanisms governing BBB function and dysfunction. This provides unprecedented opportunities for drug discovery and therapeutic intervention,” Guo says.

Research teams worldwide have been racing to develop brain organoids—tiny, growing 3D structures that mimic the early stages of brain formation. Unlike cell types grown flat in a lab dish, organoid cells are connected, self-assembling into spherical forms and “talking” to each other as human cells do during fetal development.

Cincinnati Children’s has led the way in developing other organoids, including functional intestine, stomach, and esophagus organoids. However, until now, no research center had succeeded in making a brain organoid that features the special barrier lining found in human brain blood vessels.

The research team calls their new model “BBB assembloids.” These assembloids combine two distinct types of organoids: brain organoids that replicate human brain tissue and blood vessel organoids that mimic vascular structures. The combination process began with brain organoids measuring 3 to 4 millimeters in diameter and blood vessel organoids about 1 millimeter in diameter. Over about a month, these separate structures fused into a single sphere measuring slightly more than 4 millimeters in diameter (about 1/8 of an inch, or roughly the size of a sesame seed).

These integrated organoids recreate many complex neurovascular interactions observed in the human brain, but they are not complete brain models. For example, the tissue does not contain immune cells and lacks connections to the rest of the body’s nervous system.

Initial Proof of Concept

To demonstrate the potential utility of the new assembloids, the researchers used a line of patient-derived stem cells to make assembloids that accurately replicated key features of a rare brain condition called cerebral cavernous malformation. This genetic disorder, characterized by dysfunctional blood-brain barrier integrity, results in clusters of abnormal blood vessels in the brain that often resemble raspberries. The disorder significantly increases the risk of stroke.

“Our model accurately recapitulated the disease phenotype, offering new insights into the underlying molecular and cellular pathology of cerebral vascular disorders,” Guo says.

Potential Applications

The co-authors envision a variety of potential uses for BBB assembloids:

  • Personalized Drug Screening: Patient-derived BBB assembloids could serve as avatars to tailor therapies based on patients’ unique genetic and molecular profiles.
  • Disease Modeling: Success in making BBB assembloids could accelerate the development of human brain tissue models for more neurovascular disorders, including rare and genetically complex conditions.
  • High-Throughput Drug Discovery: Scaling up assembloid production could allow for more accurate and rapid analysis of whether potential brain medications can effectively cross the BBB.
  • Environmental Toxin Testing: BBB assembloids could help evaluate the toxic effects of environmental pollutants, pharmaceuticals, and other chemical compounds.
  • Immunotherapy Development: The new assembloids could support the delivery of immune-based therapies to the brain by investigating the role of the BBB in neuroinflammatory and neurodegenerative diseases.
  • Bioengineering and Biomaterials Research: Biomedical engineers and materials scientists will benefit from having a lab model of the BBB to test novel biomaterials, drug delivery vehicles, and tissue engineering strategies.

“Overall, BBB assembloids represent a game-changing technology with broad implications for neuroscience, drug discovery, and personalized medicine,” Guo says.

By Impact Lab