In mammals, new neurons are added to the olfactory bulb (OB) throughout life. Most of these new neurons, granule and periglomerular cells originate from the subventricular zone (SVZ) lining the lateral ventricles and migrate via the rostral migratory stream toward the OB. Thousands of new neurons appear each day, but the function of this ongoing neurogenesis remains unclear. In this study, we irradiated adult mice to impair constitutive OB neurogenesis, and explored the functional impacts of this irradiation on the sense of smell. We found that focal irradiation of the SVZ greatly decreased the rate of production of new OB neurons, leaving other brain areas intact. This effect persisted for up to seven months after exposure to 15 Gray. Despite this robust impairment, the thresholds for detecting pure odorant molecules and short-term olfactory memory were not affected by irradiation. Similarly, the ability to distinguish between odorant molecules and the odorant-guided social behavior of irradiated mice were not affected by the decrease in the number of new neurons. Only long-term olfactory memory was found to be sensitive to SVZ irradiation. These findings suggest that the continuous production of adult-generated neurons is involved in consolidating or restituting long-lasting olfactory traces.

Neurogenesis decreases during aging and following cranial radiotherapy, causing a progressive cognitive decline that is currently untreatable. However, functional neural stem cells remained present in the subventricular zone of high dose‐irradiated and aged mouse brains. We therefore investigated whether alterations in the neurogenic niches are perhaps responsible for the neurogenesis decline. This hypothesis was supported by the absence of proliferation of neural stem cells that were engrafted into the vascular niches of irradiated host brains. Moreover, we observed a marked increase in TGF‐β1 production by endothelial cells in the stem cell niche in both middle‐aged and irradiated mice. In co‐cultures, irradiated brain endothelial cells induced the apoptosis of neural stem/progenitor cells via TGF‐β/Smad3 signalling. Strikingly, the blockade of TGF‐β signalling in vivo using a neutralizing antibody or the selective inhibitor SB‐505124 significantly improved neurogenesis in aged and irradiated mice, prevented apoptosis and increased the proliferation of neural stem/progenitor cells. These findings suggest that anti‐TGF‐β‐based therapy may be used for future interventions to prevent neurogenic collapse following radiotherapy or during aging.

Although neural stem cells (NSCs) sustain continuous neurogenesis throughout the adult lifespan of mammals, they progressively exhibit proliferation defects that contribute to a sharp reduction in subventricular neurogenesis during aging. However, little is known regarding the early age-related events in neurogenic niches. Using a fluorescence-activated cell sorting technique that allows for the prospective purification of the main neurogenic populations from the subventricular zone (SVZ), we demonstrated an early decline in adult neurogenesis with a dramatic loss of progenitor cells in 4 month-old young adult mice. Whereas the activated and quiescent NSC pools remained stable up to 12 months, the proliferative status of activated NSCs was already altered by 6 months, with an overall extension of the cell cycle resulting from a specific lengthening of G₁. Whole genome analysis of activated NSCs from 2- and 6-month-old mice further revealed distinct transcriptomic and molecular signatures, as well as a modulation of the TGFβ signalling pathway. Our microarray study constitutes a cogent identification of new molecular players and signalling pathways regulating adult neurogenesis and its early modifications.

Deciphering the mechanisms that regulate the quiescence of adult neural stem cells (NSCs) is crucial for the development of therapeutic strategies based on the stimulation of their endogenous regenerative potential in the damaged brain. We show that LeX^bright cells sorted from the adult mouse subventricular zone exhibit all the characteristic features of quiescent NSCs. Indeed, they constitute a subpopulation of slowly dividing cells that is able to enter the cell cycle to regenerate the irradiated niche. Comparative transcriptomic analyses showed that they express hallmarks of NSCs but display a distinct molecular signature from activated NSCs (LeX⁺EGFR⁺ cells). Particularly, numerous membrane receptors are expressed on quiescent NSCs. We further revealed a different expression pattern of Syndecan-1 between quiescent and activated NSCs and demonstrated its role in the proliferation of activated NSCs. Our data highlight the central role of the stem cell microenvironment in the regulation of quiescence in adult neurogenic niches.

Nowak, E., Etienne, O., Millet, P., Silva Lages, C., Mathieu, C., Mouthon, M. A. and Boussin, F. D. Radiation-Induced H2AX Phosphorylation and Neural Precursor Apoptosis in the Developing Brain of Mice. Radiat. Res. 165, 155–164 (2006).We showed that γ irradiation of the developing mouse brain with 2 Gy induced a massive apoptosis of neural precursors but not of neurons within 24 h. Successive phosphorylation and dephosphorylation of histone H2AX have been linked to DNA breaks and repair. Similar numbers of nuclear foci of phosphorylated H2AX (γ-H2AX) were found 1 h postirradiation in neural precursors and in neurons, suggesting that differences in radiosensitivity were not related to variations in the numbers of DNA double-strand breaks induced by radiation. Surviving neural precursors like neurons totally lost γ-H2AX within 24 h after irradiation, but they had a slower kinetics of loss of γ-H2AX foci. This suggests that the DNA repair machinery processed damage more slowly in these neural precursors in relation to their greater radiosensitivity. We also found a bright and diffuse γ-H2AX staining of nuclei of cells at an early stage of apoptosis, whereas cells at later stages of apoptosis were unstained. This was probably related to phosphorylation and subsequent degradation of H2AX in the course of DNA fragmentation during apoptosis. Detection of γ-H2AX-bright nuclei may thus be a useful marker of neural cells at an early stage of apoptosis.