Activation Of Glycogen Synthase Kinase-3 Stimulates Adult Hippocampal Neurogenesis And The Underlying Mechanisms

Neurogenesis persists in the adult mammalian brain and remains restricted to the subgranular zone (SGZ) of the dentate gyrus in hippocampus and the subventricular zone (SVZ) of the lateral ventricles in forebrain. Alzheimer's disease (AD) is the most common neurodegenerative disease. Increased hippocampal neurogenesis has been observed in AD brain, but little is known about the mechanisms that result in enhanced neurogenesis in AD. Until now, none of current medications has been proved to cure AD efficiently. Stimulation of endogenous adult neurogenesis, which could produce more newborn neurons to replace old and dead ones to play normal functions, is a promising therapeutic target for this disease. Therefore, it is of great value to know about which factors could influence adult neurogenesis and elucidate the mechanisms that lead to enhanced neurogenesis in AD brain.One of the major pathological features of AD brain is the presence of neurofibrillary tangles which is mainly composed of hyperphosphorylated microtubule associated protein tau. Deregulation of glycogen synthase kinase-3 (GSK-3) activity has been postulated to play an important role in AD, on the basis of its association with neurofibrillary tangles. Besides, GSK-3βand tau protein have been reported to have a role in the development of neurons. In the present study, we investigate the effect of GSK-3 activation on adult neurogenesis and examine the relationship between phosphorylated tau protein and immature neurons in the SGZ and SVZ.To explore the possible involvement of GSK-3βin neurogenesis, we first performed double staining of GSK-3βwith immature neuronal markers in the SGZ and SVZ. Next, we injected WT into the left ventricle of rat brain to activate GSK-3βto investigate the effect of GSK-3βactivation on adult neurogenesis by immunohistochemical methods. To further confirm the role of GSK-3 on adult hippocampal neurogenesis, we injected different amounts of WT and used SB, a specific inhibitor of GSK-3, alone or together with WT to treat animals. At 3 days after the brain injection, animals were sacrificed to examine the level of neurogenesis by western blot analysis. To explore the effect of GSK-3 activation on the proliferation, survival and differentiation of newborn cells in the adult hippocampus, we injected intraperitoneally the rats with BrdU for 3 days, and then injected WT or NS, as a control, through lateral ventricle. The number of BrdU-labeled cells and phenotype of newborn cells in the SGZ region were estimated at different time points after the ventricle injection.The results are shown as follows:1. GSK-3βis widely expressed in the SGZ of dentate gyrus and the SVZ of forebrain. Total GSK-3βis colocalized with immature neurons in the SGZ and SVZ, while inactivate GSK-3βis dissociated from immature neurons in these two areas.2. Activation of GSK-3 stimulates neurogenesis specifically in the dentate gyrus of hippocampus.3. GSK-3 induced enhanced hippocampal neurogenesis mainly by facilitating cell proliferation.To explore the relationship between tau phosphorylation and adult neurogenesis, we compared the level of tau phosphorylation in NS- and WT-treated animals. Next, we performed immunohistochemistry in the brain slices of newborn (P1), 8 days (P8), 30 days (P30) and 3-4 months old (adult) rats to examine the correlation between tau phosphorylation and neurogenesis during brain development. Antibodies DCX and NeuroD were used to examine the distribution of immature neurons in the dentate gyrus and SVZ, and PS396, PT205 and PT231 / Tau-1 were used to examine the expression of phosphorylated / unphosphorylated tau protein in these two regions. Finally, we performed double immunofluorescent staining of tau protein with immature neuronal markers in the SGZ and SVZ of adut rat brain to observe the relationship between tau protein and adult neurogenesis. The results are presented as below:1. Activation of GSK-3 induced elevated level of tau phosphorylation in the dentate gyrus of rats. 2. Distribution of DCX- and NeuroD-positive cells is correlated with the expression of phosphorylated tau protein in the dentate gyrus during brain development. Though the distribution of DCX decreases significantly with aging, which differs from that of tau protein, the expression of both DCX and tau protein increases during development and remains at moderate levels when matures into adulthood.3. Phosphorylated tau is colocalized with immature neurons in the SGZ and SVZ of adult rat brain.These results suggest that activation of GSK-3 facilitates adult neurogenesis in the SGZ of rat hippocampus and phosphorylated tau protein plays a role in the development of adult born immature neurons. Our finding provides a possible mechanism that leads to the enhanced neurogenesis observed in the AD brains.