Stem Cell Mechanisms Governing Discrete Waves of Gliogenesis in the Childhood Brain
White matter is the infrastructure of the brain, providing conduits for communication between neural regions. White matter continues to mature from birth until early adulthood, particularly in regions of brain critical for higher cognitive functions. However, the precise timing of white matter maturation in the various neural circuits is not well described, and the mechanisms controlling white matter developmental/maturation are poorly understood. White matter is conceptually like wires and their insulating sheath is a substance called myelin. It is clear that neural stem and precursor cells contribute significantly to white matter maturation by forming the cells that generate myelin. In the proposed experiments, we will map the precise timing of myelination in the human brain and changes in the populations of neural precursor cells that generate myelin from birth to adulthood and define mechanisms that govern the process of white matter maturation. The resulting findings about how white matter develops may provide insights for white matter regeneration to aid in therapy for diseases such as cerebral palsy, multiple sclerosis and chemotherapy-induced cognitive dysfunction.
Diseases of white matter account for significant neurological morbidity in both children and adults in California. Understanding the cellular and molecular mechanisms that govern white matter development the may unlock clues to the regenerative potential of endogeneous stem and precursor cells in the childhood and adult brain. Although the brain continues robust white matter development throughout childhood, adolescence and young adulthood, relatively little is known about the mechanisms that orchestrate proliferation, differentiation and functional maturation of neural stem and precursor cells to generate myelin-forming oligodendrocytes during postnatal brain development. In the present proposal, we will define how white matter precursor cell populations vary with age throughout the brain and determine if and how neuronal activity instructs neural stem and precursor cell contributions to human white matter myelin maturation.
Disruption of white matter myelination is implicated in a range of neurological diseases, including cerebral palsy, multiple sclerosis, cognitive dysfunction from chemotherapy exposure, attention deficit and hyperactivity disorder (ADHD) and even psychiatric diseases such as schizophrenia. The results of these studies have the potential to elucidate clues to white matter regeneration that may benefit hundreds of thousands of Californians.
Formation of the insulated fiber infrastructure of the human brain (called "myelin") depends upon the function of a precursor cell type called "oligodendrocyte precursor cells (OPC)". The first Aim of this study seeks to determine how OPCs differ from each other in different regions of the brain, and over different ages. Understanding this heterogeneity is important as we explore the regenerative capacity of this class of precursor cells. We have, in the past year, isolated OPCs from various regions of the human brain from individuals at various ages and are studying the molecular characteristics of these precursor cells at the single cell level in order to define distinct OPC subpopulations. We have also begun to study the functional capabilities of OPCs isolated from different brain regions. The second Aim of this study seeks to understand how interactions between electrically active neurons and OPCs affect OPC function and myelin formation. We have found that when mouse motor cortex neurons "fire" signals in such a way as to elicit a complex motor behavior, much as would happen when one practices a motor task, OPCs within that circuit respond and myelination increases. This affects the function of that circuit in a lasting way. These results indicate that neurons and OPCs interact in important ways to modulate myelination and supports the hypothesis that OPC function may play a role in learning.
Sending neural impulses quickly down a long nerve fiber requires a specialized type of insulation called myelin, made by a cell called an oligodendrocyte that wraps itself around neuronal projections. Myelin-insulated nerve fibers make up the “white matter” of the brain, the vast tracts that connect one information-processing area of the brain to another. We have now shown that neuronal activity prompts oligodendrocyte precursor cell (OPC) proliferation and differentiation into myelin-forming oligodendrocytes. Neuronal activity also causes an increase in the thickness of the myelin sheaths within the active neural circuit, making signal transmission along the neural fiber more efficient. This was found to be true in both juvenile and in adult brains Metaphorically, it’s much like a system for improving traffic flow along roadways that are heavily used. And as with a transportation system, improving the routes that are most productive makes the whole system more efficient.
Interestingly, some parts of the neural circuit studied showed evidence of myelin-forming precursor cell response to neuronal activity, while other parts of the active circuit did not. In related work, we are making progress towards understanding how OPCs differ in various regions of the brain, examining the molecular heterogeneity of human OPCs at a single cell level.