Anatomical and functional significance of F-spondin and the role of circadian factors in adult neurogenesis in zebrafish
Alvarez, Veronica Akle
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Adult neurogenesis occurs in specific niches in the brain. Despite major interest in this process, many questions remain regarding the factors that regulate neurogenesis and maintain the stem cell niches. This dissertation investigates the contribution of two different factors: the extracellular matrix (ECM), with a focus on F-spondin, and the circadian system, with a focus on the hormone melatonin. Studies on the localization of F-spondin mRNA expression throughout zebrafish lifespan were conducted using a transgenic zebrafish line, in which GFP expression is under control of the f-spondin promoter. The results revealed that (1) F-spondin is expressed in critical areas involved in CNS patterning during development (e.g., notochord and floorplate) and in all sixteen adult neurogenic niches of zebrafish brain, suggesting a role in maintaining or modulating optimal microenvironment for developing neurons; (2) F-spondin is expressed by CNS neurons with long projections (e.g., habenula neurons), potentially releasing the protein at the axon terminals and stabilizing their distal synaptic connections through perineuronal nets; (3) F-spondin is expressed by diverse CSF-contacting neurons that sense and/or contribute to the contents of the CSF, thus potentially serving as a long-range signaling molecule, and (4) F-spondin is present in constantly renewing peripheral structures (e.g., pharyngeal teeth) and in actively regenerating tissue (e.g., caudal fin, following amputation). Assessment of the temporal 24-h pattern of F-spondin mRNA expression revealed a nighttime peak. Together, these data suggest an important role for F-spondin, and its periodic variation, in patterning and maintenance of neuronal and peripheral tissues during development and adulthood in the zebrafish. Studies addressing the circadian control of neurogenesis in adult zebrafish brain used both BrdU incorporation and quantitative RT-PCR approaches to study daily variation in cell division and expression of genes regulating the cell-cycle. The results demonstrated that (1) neurogenesis has a daily cycle, with an increased number of cells in S-phase during the night; (2) the exact timing of peak proliferation is location-dependent, and (3) melatonin promotes cell proliferation. The BrdU-based data was consistent with the expression pattern of several cell-cycle regulatory genes (eye/ins A2, B2, D, E), which undergo daily variation, with high mRNA levels present at night. This work, for the first time, demonstrates that F-spondin is important beyond developmental stages and further study is needed to elucidate its function in neurogenic niches of vertebrates. It also demonstrates that the cell-cycle regulatory genes and the rate of adult neurogenesis in zebrafish are controlled by circadian factors, including melatonin.
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