Syndecan-1 Stimulates Adult Neurogenesis in the Mouse Ventricular-Subventricular Zone after Injury.

The production of neurons from neural stem cells (NSCs) persists throughout life in the mouse ventricular-subventricular zone (V-SVZ). We have previously reported that NSCs from adult V-SVZ are contained in cell populations expressing the carbohydrate SSEA-1/LeX, which exhibit either characteristics of quiescent NSCs (qNSCs) or of actively dividing NSCs (aNSCs) based on the absence or the presence of EGF-receptor, respectively. Using the fluorescence ubiquitination cell cycle indicator-Cdt1 transgenic mice to mark cells in G0/G1 phase of the cell cycle, we uncovered a subpopulation of qNSCs which were primed to enter the cell cycle in vitro. Besides, we found that treatment with Syndecan-1, a heparan sulfate proteoglycan involved in NSC proliferation, hastened the division of qNSCs and increased proliferation of aNSCs shortening their G1 phase in vitro. Furthermore, administration of Syndecan-1 ameliorated the recovery of neurogenic populations in the V-SVZ after radiation-induced injury providing potential cure for neurogenesis decline during brain aging or after injury.

The HIF1α/JMY pathway promotes glioblastoma stem-like cell invasiveness after irradiation.

Human glioblastoma (GBM) is the most common primary malignant brain tumor. A minor subpopulation of cancer cells, known as glioma stem-like cells (GSCs), are thought to play a major role in tumor relapse due to their stem cell-like properties, their high resistance to conventional treatments and their high invasion capacity. We show that ionizing radiation specifically enhances the motility and invasiveness of human GSCs through the stabilization and nuclear accumulation of the hypoxia-inducible factor 1α (HIF1α), which in turn transcriptionally activates the Junction-mediating and regulatory protein (JMY). Finally, JMY accumulates in the cytoplasm where it stimulates GSC migration via its actin nucleation-promoting activity. Targeting JMY could thus open the way to the development of new therapeutic strategies to improve the efficacy of radiotherapy and prevent glioma recurrence.

Distinct Molecular Signatures of Quiescent and Activated Adult Neural Stem Cells Reveal Specific Interactions with Their Microenvironment.

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 LeXbright 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.

Hedgehog Controls Quiescence and Activation of Neural Stem Cells in the Adult Ventricular-Subventricular Zone.

Identifying the mechanisms controlling quiescence and activation of neural stem cells (NSCs) is crucial for understanding brain repair. Here, we demonstrate that Hedgehog (Hh) signaling actively regulates different pools of quiescent and proliferative NSCs in the adult ventricular-subventricular zone (V-SVZ), one of the main brain neurogenic niches. Specific deletion of the Hh receptor Patched in NSCs during adulthood upregulated Hh signaling in quiescent NSCs, progressively leading to a large accumulation of these cells in the V-SVZ. The pool of non-neurogenic astrocytes was not modified, whereas the activated NSC pool increased after a short period, before progressively becoming exhausted. We also showed that Sonic Hedgehog regulates proliferation of activated NSCs in vivo and shortens both their G1 and S-G2/M phases in culture. These data demonstrate that Hh orchestrates the balance between quiescent and activated NSCs, with important implications for understanding adult neurogenesis under normal homeostatic conditions or during injury.

Age-related neurogenesis decline in the subventricular zone is associated with specific cell cycle regulation changes in activated neural stem cells.

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 TGFß signalling pathway. Our microarray study constitutes a cogent identification of new molecular players and signalling pathways regulating adult neurogenesis and its early modifications.

Cell Sorting of Neural Stem and Progenitor Cells from the Adult Mouse Subventricular Zone and Live-imaging of their Cell Cycle Dynamics.

Neural stem cells (NSCs) in the subventricular zone of the lateral ventricles (SVZ) sustain olfactory neurogenesis throughout life in the mammalian brain. They successively generate transit amplifying cells (TACs) and neuroblasts that differentiate into neurons once they integrate the olfactory bulbs. Emerging fluorescent activated cell sorting (FACS) techniques have allowed the isolation of NSCs as well as their progeny and have started to shed light on gene regulatory networks in adult neurogenic niches. We report here a cell sorting technique that allows to follow and distinguish the cell cycle dynamics of the above-mentioned cell populations from the adult SVZ with a LeX/EGFR/CD24 triple staining. Isolated cells are then plated as adherent cells to explore in details their cell cycle progression by time-lapse video microscopy. To this end, we use transgenic Fluorescence Ubiquitination Cell Cycle Indicator (FUCCI) mice in which cells are red-fluorescent during G1 phase due to a G1 specific red-Cdt1 reporter. This method has recently revealed that proliferating NSCs progressively lengthen their G1 phase during aging, leading to neurogenesis impairment. This method is easily transposable to other systems and could be of great interest for the study of the cell cycle dynamics of brain cells in the context of brain pathologies.

TGFß lengthens the G1 phase of stem cells in aged mouse brain.

Neurogenesis decreases during aging causing a progressive cognitive decline but it is still controversial whether proliferation defects in neurogenic niches result from a loss of neural stem cells or from an impairment of their progression through the cell cycle. Using an accurate fluorescence-activated cell sorting technique, we show that the pool of neural stem cells is maintained in the subventricular zone of middle-aged mice while they have a reduced proliferative potential eventually leading to the subsequent decrease of their progeny. In addition, we demonstrate that the G1 phase is lengthened during aging specifically in activated stem cells, but not in transit-amplifying cells, and directly impacts on neurogenesis. Finally, we report that inhibition of TGFß signaling restores cell cycle progression defects in stem cells. Our data highlight the significance of cell cycle dysregulation in stem cells in the aged brain and provide an attractive foundation for the development of anti-TGFß regenerative therapies based on stimulating endogenous neural stem cells.

Quiescent neural stem cells exit dormancy upon alteration of GABAAR signaling following radiation damage.

Quiescent neural stem cells (NSCs) are considered the reservoir for adult neurogenesis, generating new neurons throughout life. Until now, their isolation has not been reported, which has hampered studies of their regulatory mechanisms. We sorted by FACS quiescent NSCs and their progeny from the subventricular zone (SVZ) of adult mice according to the expression of the NSC marker LeX/CD15, the EGF receptor (EGFR) and the CD24 in combination with the vital DNA marker Hoechst 33342. Characterization of sorted cells showed that the LeX(bright)/EGFR-negative population was enriched in quiescent cells having an NSC phenotype. In contrast to proliferating NSCs and progenitors, the LeX(bright)/EGFR-negative cells, i.e. quiescent NSCs, resisted to a moderate dose of gamma-radiation (4Gy), entered the cell cycle two days after irradiation prior to EGFR acquisition and ultimately repopulated the SVZ. We further show that the GABAAR signaling regulates their cell cycle entry by using specific GABAAR agonists/antagonists and that the radiation-induced depletion of neuroblasts, the major GABA source, provoked their proliferation in the irradiated SVZ. Our study demonstrates that quiescent NSCs are specifically enriched in the LeX(bright)/EGFR-negative population, and identifies the GABAAR signaling as a regulator of the SVZ niche size by modulating the quiescence of NSCs.

Vascular-derived TGF-ß increases in the stem cell niche and perturbs neurogenesis during aging and following irradiation in the adult mouse brain.

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-ß1 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-ß/Smad3 signalling. Strikingly, the blockade of TGF-ß 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-ß-based therapy may be used for future interventions to prevent neurogenic collapse following radiotherapy or during aging.

In vivo importance of homologous recombination DNA repair for mouse neural stem and progenitor cells.

We characterized the in vivo importance of the homologous recombination factor RAD54 for the developing mouse brain cortex in normal conditions or after ionizing radiation exposure. Contrary to numerous homologous recombination genes, Rad54 disruption did not impact the cortical development without exogenous stress, but it dramatically enhanced the radiation sensitivity of neural stem and progenitor cells. This resulted in the death of all cells irradiated during S or G2, whereas the viability of cells irradiated in G1 or G0 was not affected by Rad54 disruption. Apoptosis occurred after long arrests at intra-S and G2/M checkpoints. This concerned every type of neural stem and progenitor cells, showing that the importance of Rad54 for radiation response was linked to the cell cycle phase at the time of irradiation and not to the differentiation state. In the developing brain, RAD54-dependent homologous recombination appeared absolutely required for the repair of damages induced by ionizing radiation during S and G2 phases, but not for the repair of endogenous damages in normal conditions. Altogether our data support the existence of RAD54-dependent and -independent homologous recombination pathways.

Impaired functionality and homing of Fancg-deficient hematopoietic stem cells.

Fanconi anemia (FA) is a human rare genetic disorder characterized by congenital defects, bone marrow (BM) failure and predisposition to leukemia. The progressive aplastic anemia suggests a defect in the ability of hematopoietic stem cells (HSC) to sustain hematopoieis. We have examined the role of the nuclear FA core complex gene Fancg in the functionality of HSC. In Fancg-/- mice, we observed a decay of long-term HSC and multipotent progenitors that account for the reduction in the LSK compartment containing primitive hematopoietic cells. Fancg-/- lymphoid and myeloid progenitor cells were also affected, and myeloid progenitors show compromised in vitro functionality. HSC from Fancg-/- mice failed to engraft and to reconstitute at short and long term the hematopoiesis in a competitive transplantation assay. Fancg-/- LSK cells showed a loss of quiescence, an impaired migration in vitro in response to the chemokine CXCL12 and a defective homing to the BM after transplantation. Finally, the expression of several key genes involved in self-renewal, quiescence and migration of HSC was dysregulated in Fancg-deficient LSK subset. Collectively, our data reveal that Fancg should play a role in the regulation of physiological functions of HSC.

Lack of a p21waf1/cip -dependent G1/S checkpoint in neural stem and progenitor cells after DNA damage in vivo.

The cyclin-dependent kinase inhibitor p21(waf1/cip) mediates the p53-dependent G1/S checkpoint, which is generally considered to be a critical requirement to maintain genomic stability after DNA damage. We used staggered 5-ethynyl-2'deoxyuridine/5-bromo-2'-deoxyuridine double-labeling in vivo to investigate the cell cycle progression and the role of p21(waf1/cip) in the DNA damage response of neural stem and progenitor cells (NSPCs) after exposure of the developing mouse cortex to ionizing radiation. We observed a radiation-induced p21-dependent apoptotic response in migrating postmitotic cortical cells. However, neural stem and progenitor cells (NSPCs) did not initiate a p21(waf1/cip1) -dependent G1/S block and continued to enter S-phase at a similar rate to the non-irradiated controls. The G1/S checkpoint is not involved in the mechanisms underlying the faithful transmission of the NSPC genome and/or the elimination of critically damaged cells. These processes typically involve intra-S and G2/M checkpoints that are rapidly activated after irradiation. p21 is normally repressed in neural cells during brain development except at the G1 to G0 transition. Lack of activation of a G1/S checkpoint and apoptosis of postmitotic migrating cells after DNA damage appear to depend on the expression of p21 in neural cells, since substantial cell-to-cell variations are found in the irradiated cortex. This suggests that repression of p21 during brain development prevents the induction of the G1/S checkpoint after DNA damage.

Alternative lengthening of telomeres in human glioma stem cells.

Cancer stem cells are increasingly recognized as major therapeutic targets. We report here the isolation of glioma stem cells (GSCs) maintaining telomere length through a telomerase-independent mechanism known as alternative lengthening of telomeres (ALTs). TG20 cells were isolated from a glioblastoma multiforme, which had the ALT phenotype. They have no detectable telomerase activity and extremely long and heterogeneous telomeres colocalizing with promyelocytic leukemia bodies. The cancer stem cell potential of TG20 cells was confirmed based on their expression of neural stem cell markers, their capacity of in vitro long-term proliferation and to form intracranial tumors in immune-deficient mice. Interestingly, we found that both in vitro and in vivo TG20 cells were significantly more resistant to ionizing radiation than GSCs with telomerase activity. Analysis of DNA damage foci, DNA double-strand breaks repair, and chromosome instability suggest that radiation resistance was related to interference of ALT pathway with DNA damage response. Therefore, our data show for the first time that the ALT pathway can confer to cancer stem cells the capacity to sustain long-term proliferation as telomerase activity and importantly may also affect treatment efficiency. TG20 cells are thus the first cellular model of GSCs displaying ALT and should prove to be useful for the development of specific treatment strategies.

Cellular and behavioral effects of cranial irradiation of the subventricular zone in adult mice.

BACKGROUND: 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. METHODOLOGY/PRINCIPAL FINDINGS: 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. CONCLUSION/SIGNIFICANCE: These findings suggest that the continuous production of adult-generated neurons is involved in consolidating or restituting long-lasting olfactory traces.

Role of Fanconi DNA repair pathway in neural stem cell homeostasis.

Defects in DNA repair pathways have been involved in collapse of early neurogenesis leading to brain development abnormalities and embryonic lethality. However, consequences of DNA repair defects in adult neural stem and progenitor cells and their potential contribution in ageing phenotype are poorly understood. The Fanconi anaemia (FA) pathway, which functions primarily as a DNA damage response system, has been examined in neural stem and progenitor cells during developmental and adult neurogenesis. We have shown that loss of fanca and fancg specifically provokes neural progenitor apoptosis during forebrain development, related to DNA repair defects, which persists in adulthood leading to depletion of the neural stem cell pool with ageing. In addition, neural stem cells from FA mice had a reduced capacity to self-renew in vitro. Here, we expand upon our recent work and give further data examining possible implication of oxidative stress. Therefore, FA phenotype might be interpreted as a premature ageing of stem cells, DNA damages being among the driving forces of ageing.

Endothelial cell-derived bone morphogenetic proteins control proliferation of neural stem/progenitor cells.

Neurogenesis persists in the adult brain subventricular zone where neural stem/progenitor cells (NSPCs) lie close to brain endothelial cells (BECs). We show in mouse that BECs produce bone morphogenetic proteins (BMPs). Coculture of embryonic and adult NSPCs with BECs activated the canonical BMP/Smad pathway and reduced their proliferation. We demonstrate that coculture with BECs in the presence of EGF and FGF2 induced a reversible cell cycle exit of NSPCs (LeX+) and an increase in the amount of GFAP/LeX-expressing progenitors thought to be stem cells. Levels of the phosphatidylinositol phosphatase PTEN were upregulated in NSPCs after coculture with BECs, or treatment with recombinant BMP4, with a concomitant reduction in Akt phosphorylation. Silencing Smad5 with siRNA or treatment with Noggin, a BMP antagonist, demonstrated that upregulation of PTEN in NSPCs required BMP/Smad signaling and that this pathway regulated cell cycle exit of NSPCs. Therefore, BECs may provide a feedback mechanism to control the proliferation of NSPCs.

Fanconi DNA repair pathway is required for survival and long-term maintenance of neural progenitors.

Although brain development abnormalities and brain cancer predisposition have been reported in some Fanconi patients, the possible role of Fanconi DNA repair pathway during neurogenesis is unclear. We thus addressed the role of fanca and fancg, which are involved in the activation of Fanconi pathway, in neural stem and progenitor cells during brain development and adult neurogenesis. Fanca(-/-) and fancg(-/-) mice presented with microcephalies and a decreased neuronal production in developing cortex and adult brain. Apoptosis of embryonic neural progenitors, but not that of postmitotic neurons, was increased in the neocortex of fanca(-/-) and fancg(-/-) mice and was correlated with chromosomal instability. In adult Fanconi mice, we showed a reduced proliferation of neural progenitor cells related to apoptosis and accentuated neural stem cells exhaustion with ageing. In addition, embryonic and adult Fanconi neural stem cells showed a reduced capacity to self-renew in vitro. Our study demonstrates a critical role for Fanconi pathway in neural stem and progenitor cells during developmental and adult neurogenesis.

Neural stem cells from mouse forebrain are contained in a population distinct from the 'side population'.

Developing and adult forebrains contain neural stem cells (NSCs) but no marker is available to highly purify them. When analysed by flow cytometry, stem cells from various tissues are enriched in a 'side population' (SP) characterized by the exclusion of the fluorescent dye Hoechst 33342. Here, we characterize the SP in embryonic, neonatal and adult forebrains, as well as in neurosphere cultures and we have determined whether this SP could be a source of enriched NSCs. By using specific inhibitors, we found that the SP from embryonic forebrain results from the activity of the ABCG2 transporter, a characteristic of other stem cells, whereas the SP from adult forebrain probably results from the ABCB1 transporter. SP cells from embryonic and adult forebrains, however, expressed a range of cell surface markers more consistent with a haematopoietic/endothelial origin than with a neural origin; NSC markers were mostly expressed on cells outside the SP (in the main population, MP). Moreover, assays for NSC growth in vitro showed that SP cells from embryonic and adult forebrains did not generate NSC-derived colonies, whereas the MP did. We thus conclude that NSCs from developing and adult forebrains are not contained in the SP contrary to stem cells from other tissues.

Radiation-induced H2AX phosphorylation and neural precursor apoptosis in the developing brain of mice.

We showed that gamma 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 (gamma-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 gamma-H2AX within 24 h after irradiation, but they had a slower kinetics of loss of gamma-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 gamma-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 gamma-H2AX-bright nuclei may thus be a useful marker of neural cells at an early stage of apoptosis.