During pregnancy, a remarkable 250,000 brain cells are generated every minute, culminating in an intricate tapestry of 100 billion interconnected neurons in a newborn's brain. Yet, the process by which this complex network assembles is largely enshrouded in mystery, partly due to the limited access neuroscientists have to samples of human embryonic brain tissue.
Microglia, the brain’s resident immune cells, have garnered significant interest due to their critical role in orchestrating many facets of brain development. These cells regulate the quantity and fate of neural stem cells, clear away dead cells, and release immune chemicals known as cytokines that communicate with other developing brain cells.
Traditionally, small animal models have provided some insights into these processes, but considerable biological differences have presented challenges in precisely understanding the role of microglia in early human development.
Advancements in organoid culture have, however, sparked a new sense of optimism. These self-organising clusters of brain tissue, engineered by first genetically ‘rewinding’ adult cells into an embryonic stem cell state called induced pluripotent stem cells (iPSCs) and then instructing them to become brain cells, are intended to capture the architectural and cellular characteristics of the developing brain.
Florent Ginhoux, Senior Principal Investigator at the Singapore Immunology Network (SIgN), noted: “Yet, current brain organoid models lack non-neuronal cells that play important roles in brain development and homeostasis, such as blood vessels and microglia.”
A*STAR researchers from SigN, Institute of Molecular and Cell Biology (IMCB), Translational Laboratory in Genetic Medicine (TLGM) and A*STAR Microscopy Platform (AMP); worked with researchers from KK Women’s and Children’s Hospital, National University of Singapore and SingHealth Duke-NUS Academic Medical Centre, Singapore; Gustave Roussy Hospital, Institut National de la Santé et de la Recherche Médicale (INSERM), Institut de Biologie de l’Ecole Normale Supérieure (IBENS), University of Bordeaux, and Oncoprot Platform, France; the University of Cambridge, UK; and Shanghai Jiao Tong University School of Medicine, China. In this international collaboration, Ginhoux and team spearheaded the creation of the first brain organoids containing microglia-like cells.
The endeavour began with generating macrophages from human induced pluripotent stem cells (iMac) from IPSCs. To mimic the initial flux of microglial progenitors into the developing human brain, the researchers cultured iMac and brain organoids together. In this environment, iMac cells differentiated into microglia-like cells, known as iMicro.
Remarkably, iMicro mimicked the behaviour of native microglia, actively traversing the organoid surface, eating dying cells and even consuming peptide debris linked to Alzheimer’s disease. Furthermore, iMicro spurred neuronal maturation by modulating the brain organoids’ neuronal progenitors called NPCs. Mirroring rodent studies, iMicro facilitated the transformation of NPCs in the organoids into mature neurons. Transcriptomic analyses revealed that iMicro were also highly active in cellular processes involving cholesterol transport and storage.
“We discovered a new role in which microglia promote the differentiation of NPCs into neurons by transferring cholesterol to NPCs,” said Ginhoux.
The team observed that iMac cells efficiently absorbed fluorescent cholesterol esters and stored them in lipid droplets before passing them to NPCs within the organoids. “Our work shows that human microglia-sufficient brain organoids could help uncover new functions of human microglia in brain development and brain diseases,” Ginhoux added.
Ginhoux’s team is now deploying their brain organoids to explore how dysfunctional microglia contribute to brain diseases such as Alzheimer’s and Parkinson’s disease, potentially clearing the path for novel treatments in the future.
The A*STAR-affiliated researchers contributing to this research are from the Singapore Immunology Network (SIgN), Institute of Molecular and Cell Biology (IMCB), Translational Laboratory in Genetic Medicine (TLGM) and A*STAR Microscopy Platform (AMP).
