Identifying key markers and mechanisms underlying adult neural stem and progenitor cells and neurogenesis
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The adult mammalian brain is a dynamic structure, capable of undergoing cellular and molecular remodeling in response to an animal’s interactions with the outside world. One dramatic example of brain plasticity is the birth and addition of functional newborn cells to an existing circuit. This process, arising from adult neural stem cells, recapitulates many hallmarks of neural development events into adulthood in two specialized environments: the subventricular zone along the lateral ventricle and the subgranular zone in the dentate gyrus of the hippocampus. The hippocampus draws much interest as a model system because adult-born neurons adapt the brain to temporal events in external space including spatial learning and retention, pattern discrimination, and clearance of memory traces, regulating cognitive and affective behaviors as well as neural plasticity. Deficits in adult hippocampal neurogenesis have been implicated in various brain disorders and psychiatric diseases. Importantly, adult neurogenesis is also present in the human hippocampus and may serve as a cellular mechanism that mediates or augments recovery from mental disorders and neural damage. In the adult hippocampus, quiescent radial glia-like neural stem cells (RGLs) continuously give rise to newborn dentate granule neurons and astrocytes. It is well established that both extrinsic environmental signals and intrinsic signaling pathways regulate the sequential process of neurogenesis ranging from quiescent RGL activation, persistence, division, fate specification, to newborn neuron development, migration, and integration. Therefore, understanding how adult neural stem cells and their progeny are regulated during development and in adulthood has implications for brain plasticity and regenerative medicine. My thesis research presented here examines how adult neurogenesis occurs at a cellular level throughout neural development from stem cell to nerve cell. The first part of my thesis examines how neural stem cells are maintained to sustain adult neurogenesis. Stem cell quiescence is highly regulated by their local microenvironment to sustain continuous neurogenesis in the adult brain, whereas dysregulation leads to stem cell depletion. Quiescence has also been suggested to be essential for establishing the adult neural stem cell pool during development. However, how the stem cell population is maintained throughout early development to ensure a sufficient number of adult neural stem cells to support life-long neurogenesis is not well understood. I identified for the first time, a novel stem cell-derived niche factor that maintains quiescence and prevents developmental exhaustion of neural stem cells to sustain continuous neurogenesis in the adult mammalian brain. My finding highlights the complexity of prospective regulatory mechanisms to maintain a viable stem cell pool over the lifespan. The second part of my thesis focuses on imaging technology development that enables real-time observation on the dentate gyrus at a single-cell resolution. Stem cells exhibit two defining characteristics, the capacity to simultaneously give rise to the differentiated progeny through asymmetric cell differentiation, and to make more copies of themselves through symmetric cell division. How neural stem cells divide are carefully balanced to ensure correct number of cells located in the neurogenic niche. Moreover, brain injuries were known to largely alter the neural stem cell division pattern, potentially to replace cells lost through injury. Accurate proliferation control of adult neural stem cells raises hope for their potential in regenerative therapies for brain repair. However, the dynamics of hippocampal neural stem cell division in adult mammals remain unclear; the hippocampus in the mammalian brain is located hundreds of millimeters from the brain surface, rendering it technically difficult for direct observations. I developed a live imaging system, with easy access to the hippocampus, which allows for the integrative view on the changes in behavior of adult neural stem cells and their progeny at a single-cell resolution. The third part of my thesis characterizes how newborn neurons migrate within the adult hippocampus. Mammalian brain development is a complex, ordered process whereby newborn neurons follow stereotyped migration modes to organize into specific patterns required for complicated neural circuit formation. Classically, principal excitatory neurons are thought to organize into radial columns that underlie the basic brain circuits, whereas inhibitory neurons disperse tangentially across these columns to modulate the principal circuits. These principles are thought to be fundamental to the genesis of the complex mammalian brain. Surprisingly, I, in collaboration with Gerry Sun, found that precursors for excitatory principal neurons exhibit tangential migration in the adult mammalian brain, which utilizes direct contacts with niche vasculature as a migration substrate. This finding represents a novel form of glutamatergic cell migration in the adult mammalian nervous system. In summary, the work I accomplished during graduate school, including the thesis work presented here, has contributed significant novel discoveries in neural development in the adult brain (see summarized in Figure 19). I have found novel molecular and cellular mechanisms regulating adult neural stem cell quiescence and persistence (Zhou et al., 2018), division, fate decision (Sun et al., 2015a), and newborn neuron migration (Sun et al., 2015b). I developed live imaging technology allowing real-time observation on single-cell behaviors, as well as the ongoing work (not included in this thesis) where I utilize single-cell RNA sequencing technology with bioinformatics pipeline to identify novel key molecular signature during human hippocampal neurogenesis. Overall, my findings have great relevance to better understanding brain development and regenerative medicine.