The role of proNGF, the precursor of nerve growth factor (NGF), in the biology of adult neural stem cells (aNSCs) is still unclear. Here, we analyzed adult hippocampal neurogenesis in AD11 transgenic mice, in which the constitutive expression of anti-NGF antibody leads to an imbalance of proNGF over mature NGF. We found increased proliferation of progenitors but a reduced neurogenesis in the AD11 dentate gyrus (DG)-hippocampus (HP). Also in vitro, AD11 hippocampal neural stem cells (NSCs) proliferated more, but were unable to differentiate into morphologically mature neurons. By treating wild-type hippocampal progenitors with the uncleavable form of proNGF (proNGF-KR), we demonstrated that proNGF acts as mitogen on aNSCs at low concentration. The mitogenic effect of proNGF was specifically addressed to the radial glia-like (RGL) stem cells through the induction of cyclin D1 expression. These cells express high levels of p75 NTR , as demonstrated by immunofluorescence analyses performed ex vivo on RGL cells isolated from freshly dissociated HP-DG or selected in vitro from NSCs by leukemia inhibitory factor. Clonogenic assay performed in the absence of mitogens showed that RGLs respond to proNGF-KR by reactivating their proliferation and thus leading to neurospheres formation. The mitogenic effect of proNGF was further exploited in the expansion of mouse-induced neural stem cells (iNSCs). Chronic exposure of iNSCs to proNGF-KR increased their proliferation. Altogether, we demonstrated that proNGF acts as mitogen on hippocampal and iNSCs. STEM CELLS 2019;37:1223-1237
SIGNIFICANCE STATEMENTTo the authors' knowledge, this work identifies for the first time a new role of proNGF as a specific mitogenic factor for the proliferation of adult hippocampal neural stem cells and in induced neural stem cells. The relevance of this finding is that it adds new knowledge to the complex picture of neural stem cells biology and indicates proNGF/NGF balance critical for adult hippocampal neurogenesis. In addition, the results open new perspectives in the view of developing future therapeutic approaches based on the stimulation of endogenous adult neurogenesis or on cell-reprogramming protocols.Stereological analysis of the number of cells was performed on series of 40-μm free-floating coronal sections of the entire DG of the HP, which were analyzed by confocal microscopy to www.StemCells.com