Mechanism Underlying Phagocytic Activity Of Neuronal Progenitors Regulates Neurogenesis

Neurogenesis in the brain of adult mammals occurs throughout life, and has been clearly demonstrated at two locations under normal conditions: the subventricular zone(SVZ) of the lateral ventricles and the subgranular zone(SGZ) of the dentate gyrus in the hippocampus. These cells may be required for certain forms of brain function involving the olfactory bulb and the hippocampus, which is important for some forms of learning and memory. Doublecortin(DCX)-positive neural precursor cells(NPCs) comprise a significant phagocytic population within the neurogenic zones during adult neurogenesis, and their phagocytic activity critically contributes to neurogenesis in the adult brain. Ingestion of apoptotic cells increase AMP-activated protein kinase(AMPK) activity, a serine/threonine protein kinase acts as an energy sensor that is rapidly activated after phagocytosis as a key regulator which involved in sensing cellular ATP levels. GABA, the major inhibitory neurotransmitter in the adult brain, initially exerts an excitatory action on newborn neurons due to their high cytoplasmic chloride content. It has demonstrated that GABAB receptors are intimately associated with AMPK. GABAB receptors are heterodimeric G-protein coupled receptors composed of R1 and R2 subunits that mediate slow synaptic inhibition in the brain by activating inwardly-rectifying K+ channels(GIRKs) and inhibiting Ca2+ channels. They can also function as an important synaptic maturation signal early in life. AMPK is phosphorylated on threonine 172(T172) by enhanced metabolic activity and binds the R1 subunit and directly phosphorylates S783 in the R2 subunit of GABAB receptor to enhance its activation of GIRKs. The activation of GABAB receptors triggers secretion of brain-derived neurotrophic factor(BDNF) and promotes the functional maturation of GABAergic synapses in the hippocampus. Our study aimed to elucidate the molecular mechanisms that how NPCs phagocytosis affects adult-born neurons plasticity, we choose AMPK and GABA receptors and detect their role in newborn neurons microtubule development and membrane receptors subunit metabolisms. The neurogenesis of hippocampus was significantly reduced when Adult-born C57BL/6J were treated with annexin V, which can block the uptake of apoptotic cells of DCX+ cells. The phagocytose ability of DCX+ NPCs was measured by their engulfment of 3μm fluorescent microsphere. Our results showed that the number of fluorescent beads engulfed by the DCX+ NPCs was decreased after the differentiation. Neural precursor cells(NPCs) were cultured in vitro with ultraviolet-radiated NPCs as apoptotic cell for 4h, the phosphorylation of AMPK was measured by immunofluorescence of phospho-AMPKα(Thr172). The results showed that the phosphorylation of AMPK were increased after the engulfment of apoptotic cells. Then we treated the NPCs with 1mM AICAR(AMPK activator) or/and 10μM compound C(AMPK inhibitor). Our results showed that phosphorylation of AMPK was increased with the administration of AICAR, decreased by the inhibition of AMPK. The activation of AMPK can promote the phosphorylation of GABAB receptors and their expression on the membrane using immunofluorescence. The fluorescence intensity of BDNF was also enhanced by the activation of AMPK. The present study elucidate the molecular mechanisms that how NPCs phagocytosis affects adult-born neurons plasticity. These results reveal a potential metabolism-dependent pathway during the phagocytosis of NPCs, which play an important role in adult-born neurons neurogenesis and plasticity.