Abstract
The adult subventricular zone (SVZ) of the mammalian brain contains neural progenitor cells (NPCs) that continuously produce neuroblasts throughout life. These neuroblasts migrate towards the olfactory bulb where they differentiate into local interneurons. The neurogenic niche of the SVZ includes, in addition to NPCs and neuroblasts, astrocytes, ependymal cells, blood vessels and the molecules released by these cell types. In the last few years, microglial cells have also been included as a key component of the SVZ neurogenic niche. Microglia in the SVZ display unique phenotypic features, and are more densely populated and activated than in non-neurogenic regions. In this article we will review literature reporting microglia-NPC interactions in the SVZ and the role of this bilateral communication in microglial function and in NPC biology. This interaction can take place through the release of soluble factors, extracellular vesicles or gap junctional communication. In addition, as NPCs are used for cell replacement therapies, they can establish therapeutically relevant crosstalks with host microglia which will also be summarized throughout the article.
Highlights
Microglia are the resident immune cells in the central nervous system (CNS)
Microglia are more densely populated in these two neurogenic regions (Mosher et al, 2012), and, second, they exhibit a more activated phenotype than in non-neurogenic regions (Goings et al, 2006). These data point to additional relevant roles of microglia in the control of adult neurogenesis, a complex process that involves the proliferation of neural stem and neural progenitor cells (NPCs) and their subsequent migration, differentiation and functional integration into pre-existing circuitry
In addition to signals mediated by cytokines, growth factors, neurotransmitters and hormones, a novel type of intercellular messenger involved in the regulation of NPC self-renewal and differentiation has been identified recently: extracellular vesicles (Bátiz et al, 2016; Morton et al, 2018)
Summary
Microglia are the resident immune cells in the central nervous system (CNS). They comprise a population of about 5%–20% of glial cells, from which they differ, among other properties, in their embryonic origin. It is well accepted that surveillant microglia, in addition to their monitoring role in the healthy CNS, can influence neuronal structure and function contributing to the correct maintenance of neural circuits (Wake et al, 2009; Bechade et al, 2013). Microglia are more densely populated in these two neurogenic regions (Mosher et al, 2012), and, second, they exhibit a more activated phenotype than in non-neurogenic regions (Goings et al, 2006). These data point to additional relevant roles of microglia in the control of adult neurogenesis, a complex process that involves the proliferation of neural stem and neural progenitor cells (NPCs) and their subsequent migration, differentiation and functional integration into pre-existing circuitry. SVZ type B cells can generate oligodendrocyte precursors that contribute to the maintenance of the oligodendrocyte population in the neighboring corpus callosum, striatum and fimbria-fornix (Menn et al, 2006; Gonzalez-Perez et al, 2009)
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