Vertebrate Neural Stem Cells: Development, Plasticity, and Regeneration

Adult Neurogenesis of Teleost Fish Determines High Neuronal Plasticity and Regeneration

Studying the properties of neural stem progenitor cells (NSPCs) in a fish model will provide new information about the organization of neurogenic niches containing embryonic and adult neural stem cells, reflecting their development, origin cell lines and proliferative dynamics. Currently, the molecular signatures of these populations in homeostasis and repair in the vertebrate forebrain are being intensively studied. Outside the telencephalon, the regenerative plasticity of NSPCs and their biological significance have not yet been practically studied. The impressive capacity of juvenile salmon to regenerate brain suggests that most NSPCs are likely multipotent, as they are capable of replacing virtually all cell lineages lost during injury, including neuroepithelial cells, radial glia, oligodendrocytes, and neurons. However, the unique regenerative profile of individual cell phenotypes in the diverse niches of brain stem cells remains unclear. Various types of neuronal precursors, as previously shown, are contained in sufficient numbers in different parts of the brain in juvenile Pacific salmon. This review article aims to provide an update on NSPCs in the brain of common models of zebrafish and other fish species, including Pacific salmon, and the involvement of these cells in homeostatic brain growth as well as reparative processes during the postraumatic period. Additionally, new data are presented on the participation of astrocytic glia in the functioning of neural circuits and animal behavior. Thus, from a molecular aspect, zebrafish radial glia cells are seen to be similar to mammalian astrocytes, and can therefore also be referred to as astroglia. However, a question exists as to if zebrafish astroglia cells interact functionally with neurons, in a similar way to their mammalian counterparts. Future studies of this fish will complement those on rodents and provide important information about the cellular and physiological processes underlying astroglial function that modulate neural activity and behavior in animals.

Increased endogenous reactive oxygen species normalizes proliferation defects of Bmi1 heterozygous knockout neural stem cells

OBJECTIVES

The Bmi1gene, one of transcriptional suppressor genes in multi-comb family, maintains proliferation of neural stem cells (NSCs) and redox homeostasis. However, heterozygous deletion of the Bmi1 gene (Bmi1+/-) does not reduce the proliferative ability of NSCs. The aim of the present study was to reveal the underlying mechanism of this phenotype.

METHODS

NSCs derived from the cortex of newborn Bmi1+/- and wild-type (WT) mice were treated with different concentrations of hydrogen peroxide (H2O2) and antioxidant N-acetyl-L-cysteine (NAC) for 24 h followed by analyses of NSC proliferation and oxidative stress-related indexes.

RESULTS

The levels of reactive oxygen species (ROS) of Bmi1+/--NSCs were slightly higher than that of WT-NSCs at baseline. H2O2 increased ROS and NAC reduced ROS in a concentration-dependent pattern, but the change was significantly greater in Bmi1+/--NSCs than WT-NSCs. The proliferation and self-renewal ability of Bmi1+/--NSCs and WT-NSCs were comparable in a basic state. After 1 μM H2O2 treatment, Brdu incorporation ratio, cell viability, total antioxidant capacity (T-AOC) and total superoxide dismutase activity were increased slightly in WT-NSCs, but decreased in Bmi1+/--NSCs. H2O2 at 10 μM decreased proliferation and self-renewal ability of both genotype NSCs, with greater effect in Bmi1+/-. After treatment with 1 mM NAC, the number and diameter of neurospheres, Brdu incorporation rate, cell viability, T-AOC and total superoxide dismutase activity of Bmi1+/--NSCs were lower than those of WT-NSCs.

CONCLUSION

These results suggest that Bmi1+/--NSCs exhibit normal proliferation and self-renewal due to a slight increase in ROS, but are more vulnerable to changes in redox status.

The role of neuro-epithelial-like and radial-glial stem and progenitor cells in development, plasticity, and repair

Neural stem and progenitor cells (NSPCs) are the primary source of new neurons in the brain and serve critical roles in tissue homeostasis and plasticity throughout life. Within the vertebrate brain, NSPCs are located within distinct neurogenic niches differing in their location, cellular composition, and proliferative behaviour. Heterogeneity in the NSPC population is hypothesized to reflect varying capacities for neurogenesis, plasticity and repair between different neurogenic zones. Since the discovery of adult neurogenesis, studies have predominantly focused on the behaviour and biological significance of adult NSPCs (aNSPCs) in rodents. However, compared to rodents, who show lifelong neurogenesis in only two restricted neurogenic niches, zebrafish exhibit constitutive neurogenesis across multiple stem cell niches that provide new neurons to every major brain division. Accordingly, zebrafish are a powerful model to probe the unique cellular and molecular profiles of NSPCs and investigate how these profiles govern tissue homeostasis and regenerative plasticity within distinct stem cell populations over time. Amongst the NSPC populations residing in the zebrafish central nervous system (CNS), proliferating radial-glia, quiescent radial-glia and neuro-epithelial-like cells comprise the majority. Here, we provide insight into the extent to which these distinct NSPC populations function and mature during development, respond to experience, and contribute to successful CNS regeneration in teleost fish. Together, our review brings to light the dynamic biological roles of these individual NSPC populations and showcases their diverse regenerative modes to achieve vertebrate brain repair later in life.

Genetic deletion of Rnd3 in neural stem cells promotes proliferation via upregulation of Notch signaling

Rnd3, a Rho GTPase, is involved in the inhibition of actin cytoskeleton dynamics through the Rho kinase-dependent signaling pathway. We previously demonstrated that mice with genetic deletion of Rnd3 developed a markedly larger brain compared with wild-type mice. Here, we demonstrate that Rnd3 knockout mice developed an enlarged subventricular zone, and we identify a novel role for Rnd3 as an inhibitor of Notch signaling in neural stem cells. Rnd3 deficiency, both in vivo and in vitro, resulted in increased levels of Notch intracellular domain protein. This led to enhanced Notch signaling and promotion of aberrant neural stem cell growth, thereby resulting in a larger subventricular zone and a markedly larger brain. Inhibition of Notch activity abrogated this aberrant neural stem cell growth.