Unique role for dentate gyrus microglia in neuroblast survival and in VEGF-induced activation.

Neurogenic roles of microglia (MG) are thought to include an active role in adult hippocampal neurogenesis in addition to their established roles in pruning surplus dendrites and clearing dead neuroblasts. However, identification of such a role and its delineation in the neurogenic cascade is yet to be established. Using diphtheria toxin-aided MG ablation, we show that MG reduction in the DG-the site where neuronal stem cells (NSCs) reside-is sufficient to impede overall hippocampal neurogenesis due to reduced survival of newly formed neuroblasts. To examine whether MG residing in the hippocampal neurogenic zone are inherently different from MG residing elsewhere in the hippocampus, we compared growth factor responsiveness of DG MG with that of CA1 MG. Strikingly, transgenic induction of the potent neurogenic factor VEGF elicited robust on-site MG expansion and activation exclusively in the DG and despite eliciting a comparable angiogenic response in the CA1 and elsewhere. Temporally, DG-specific MG expansion preceded both angiogenic and neurogenic responses. Remarkably, even partial MG reduction during the process of VEGF-induced neurogenesis led to reducing the number of newly formed neuroblasts to the basal level. Transcriptomic analysis of MG retrieved from the naïve DG and CA1 uncovered a set of genes preferentially expressed in DG MG. Notably the tyrosine kinase Axl is exclusively expressed in naïve and VEGF-induced DG MG and its inhibition prevented neurogenesis augmentation by VEGF. Taken together, findings uncover inherent unique properties of DG MG of supporting both basal- and VEGF-induced adult hippocampal neurogenesis.

VEGF preconditioning leads to stem cell remodeling and attenuates age-related decay of adult hippocampal neurogenesis.

Several factors are known to enhance adult hippocampal neurogenesis but a factor capable of inducing a long-lasting neurogenic enhancement that attenuates age-related neurogenic decay has not been described. Here, we studied hippocampal neurogenesis following conditional VEGF induction in the adult brain and showed that a short episode of VEGF exposure withdrawn shortly after the generation of durable new vessels (but not under conditions where newly made vessels failed to persist) is sufficient for neurogenesis to proceed at a markedly elevated level for many months later. Continual neurogenic increase over several months was not accompanied by accelerated exhaustion of the neuronal stem cell (NSC) reserve, thereby allowing neurogenesis to proceed at a markedly elevated rate also in old mice. Neurogenic enhancement by VEGF preconditioning was, in part, attributed to rescue of age-related NSC quiescence. Remarkably, VEGF caused extensive NSC remodelling manifested in transition of the enigmatic NSC terminal arbor onto long cytoplasmic processes engaging with and spreading over even remote blood vessels, a configuration reminiscent of early postnatal "juvenile" NSCs. Together, these findings suggest that VEGF preconditioning might be harnessed for long-term neurogenic enhancement despite continued exposure to an "aged" systemic milieu.