Localized hypoxia within the subgranular zone determines the early survival of newborn hippocampal granule cells

The majority of adult hippocampal newborn cells die during early differentiation from intermediate progenitors (IPCs) to immature neurons. Neural stem cells in vivo are located in a relative hypoxic environment, and hypoxia enhances their survival, proliferation and stemness in vitro. Thus, we hypothesized that migration of IPCs away from hypoxic zones within the SGZ might result in oxidative damage, thus triggering cell death. Hypoxic niches were observed along the SGZ, composed of adult NSCs and early IPCs, and oxidative byproducts were present in adjacent late IPCs and neuroblasts. Stabilizing hypoxia inducible factor-1α with dimethyloxallyl glycine increased early survival, but not proliferation or differentiation, in neurospheres in vitro and in newly born SGZ cells in vivo. Rescue experiments in Baxfl/flmutants supported these results. We propose that localized hypoxia within the SGZ contributes to the neurogenic microenvironment and determines the early, activity-independent survival of adult hippocampal newborn cells. DOI: http://dx.doi.org/10.7554/eLife.08722.001


Introduction
In the subgranular zone (SGZ) of the adult hippocampal dentate gyrus (DG), one of the two neurogenic niches in the adult mammalian brain, new neurons are continuously generated throughout adulthood (Taupin and Gage, 2002;van Praag et al., 2002). Adult hippocampal neurogenesis is a highly dynamic and regulated process that evolves slowly over several weeks . Adult neural stem/ radial glia-like cells (Type I cells) reside at the interface between the SGZ and the hilus, and give rise to rapidly dividing intermediate progenitors (Type 2 cells). These cells migrate a short distance into the SGZ and progressively (via Type 2a and Type 2b intermediate progenitors) differentiate into neuroblasts (Type 3 cells) Fuentealba et al., 2012;Bonaguidi et al., 2012). Neuroblasts then exit the cell cycle and become immature granule cell neurons, which in turn migrate into the granule cell layer and incorporate into the pre-existing functional hippocampal circuits (Toni et al., 2008).
In the young adult mouse DG as many as 4000 new cells are born daily, but only a subset (~30%) survive at 4 weeks post-mitosis to become mature granule cell neurons (Dayer et al., 2003;Kempermann, 2003;Kempermann et al., 2006;Sierra et al., 2010). The survival of adult-generated granule cells exhibits two critical periods; an early one during the transition from transient amplifying progenitors to neuroblasts and a later one during the integration of the immature neurons (Dayer et al., 2003;Kempermann, 2003;Sierra et al., 2010;Mandyam et al., 2007). The early phase is associated with phagocytosis of apoptotic cells by microglia (Sierra et al., 2010). GABAmediated-depolarization and NMDA-receptor-mediated neuronal activity regulate the later phase of survival, at two and three weeks after mitosis respectively (Jagasia et al., 2009;Tashiro et al., 2006). However, less is known about the mechanisms responsible for the early death of newborn granule cells which occurs during the first days post mitosis, as cells exit the specialized microenvironment (NSC niche) of the adult SGZ.
There is increasing evidence that differential oxygen tensions may be an important component of the NSC niche and that oxygen sensing can regulate the proliferation, survival and differentiation of neural stem cells and progenitors (Panchision, 2009;de Filippis and Delia, 2011;Mazumdar et al., 2010). Thus, we investigated hypoxia within the SGZ, and whether stabilizing Hypoxia Inducible Factor-1 a (HIF-1a), the master regulator of oxygen homeostasis, influences the early survival of the adult hippocampal newborn cells.

Detection of hypoxic niches and oxidative damage in the adult SGZ
We assessed for hypoxia within the SGZ of the adult DG following intraperitoneal injection of the hypoxia marker pimonidazole hydrochloride (Hypoxyprobe, Figure 1B) . Pimonidazole (PH) is reductively activated and binds only to cells that have oxygen concentrations less than 14 mM, equivalent to a pO 2 of 10 mm Hg (1.3% O 2 ) . Pimonidazole labeled hypoxic cells or groups of cells along the inner border of the SGZ ( Figure 1B). Double labeling with selective markers revealed that the majority (71.7 ± 10.4%) of the hypoxic cells were radial glia (GFAP+) and a smaller percentage (28.3 ± 9.5%) were early intermediate progenitor cells (Tbr2+/ Dcx-) ( Figure 1C,D) in close proximity to the SGZ. In contrast, none of the hypoxic cells (0%) expressed the neuroblast marker doublecortin (DCX, Figure 1C,D).
Shortly after their generation, proliferating intermediate precursors translocate away from the proximal domain and into the intermediate domain (Fuentealba et al., 2012). Thus, we hypothesized that migration away from hypoxic zones might result in oxidative damage as diagrammed in eLife digest The hippocampus is a region in the mammalian brain that has been implicated in the formation of new memories. This process involves the birth of new neurons, which are created at a rate of~4000 a day in the hippocampus of a young adult mouse. Yet only a fraction of these cells survive to form mature neurons. These cells die in two main waves -the first occurs days after they form, and the second several weeks later when as immature neurons they integrate into the brain. During this later wave, new neurons become active and survive if they connect with other nerve cells and die if they don't. But little is known about what causes the earlier wave of cell death.
The tissues that contain the precursors of new neurons often have lower oxygen levels compared to other tissues. This means that when these cells start to become neurons and leave these sites, they have to face higher levels of oxygen and may undergo "oxidative" damage. This led Chatzi et al. to ask whether such oxidative damage might cause the early loss of new neurons in the hippocampus.
First, the part of the hippocampus that contains the precursor cells (called the subgranular zone or SGZ) was found to have patchy areas of low oxygen. Further experiments then revealed that chemicals that may cause oxidative damage were present in the nearby cells that had already started on the path to become new neurons.
Chatzi et al. then tested whether chemically stabilizing a protein called Hypoxia Inducible Factor-1a (or HIF1a for short), which naturally helps cells to adapt to low oxygen environments, might increase the survival of the cells in the SGZ. Higher levels of HIF1a did indeed increase the survival of these cells.
These findings suggest that newborn cells in the SGZ walk a tightrope between a low oxygen environment that supports the early precursors and the surrounding higher oxygen levels that can be toxic to those cells that start to become neurons. Further studies of the proteins and molecules that act downstream of HIF1a could shed light on ways to enhance the survival of these newlygenerated neurons. Figure 1A. Oxidative byproducts were highly localized in cells within the SGZ as assayed by 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-OHdG) labeling, an established biomarker of nucleic acid oxidative damage ( Figure 1E). Interestingly, phenotypic analysis revealed that 74.6 ± 5.8% of the 8-OHdG positive cells were late intermediate progenitors (Tbr2+/DCX+) and 29.2 ± 12.2% were neuroblasts (DCX+ only) ( Figure 1E,F). Thus, a subset of newborn cells in the SGZ undergoes oxidative damage during their early migration and differentiation.
We treated adult mice with DMOG (50 mg/kg once daily) or vehicle for 3 days. Consistent with stabilization of HIF-1a by DMOG, mRNA levels of HIF-1a as well as its downstream targets VEGF, EPO and LEF-1, were significantly higher after 3 days of DMOG treatment ( Figure 2A). As expected given the effect on downstream targets, DMOG treatment also increased HIF-1a protein levels ( Figure 2B, p = 0.03). The increase in HIF1a mRNA levels occurs because of autoregulation of transcription by hypoxia response elements in the HIF1a promoter (Iyer et al., 1998). Hypoxia also increases phosphorylation of Akt (Ser 473 ), a serine/threonine kinase that promotes cell survival and reduces apoptosis (Beitner-Johnson et al., 2001). Double immunohistochemistry with an antibody that specifically recognizes phosphorylated Akt together with an antibody against the neuroblast/ immature neuron marker doublecortin (DCX) ( Figure 2C left panels), revealed that DMOG treatment induced a two-fold increase in the number of phospho-AKT+/DCX+ cells relative to vehicle treated animals (p = 0.01, n = 3 animals, Figure 2C right panel), also consistent with the hypoxia mimetic action of DMOG.

Role of neuronal activity
Neuronal survival can be activity-dependent (Jagasia et al., 2009;Tashiro et al., 2006), thus we investigated whether newborn granule cells have received synaptic innervation by three days postmitosis, the time point at which DMOG maximally increased survival. Acute slices from wildtype animals were prepared 3 days after retroviral infection and GFP-expressing 3 day old cells were identified using fluorescence microscopy and recorded using whole-cell voltage clamp techniques. Labeled cells were predominantly located in the subgranular zone as expected, with passive membrane properties consistent with immature neuronal precursors (R input = 13.2 ± 1.9 GW, C m = 4.8 ± 0.5 pF, n = 11 cells), including a fast inward current in response to a depolarizing voltage step consistent with a Na + spike in 9 of 11 cells ( Figure 6A). Strikingly, none of these cells in wildtype animals had any spontaneous post-synaptic currents (PSCs) in >5 min of continuous recording per cell (n = 11), compared to an sPSC frequency of 0.55 ± 0.15 Hz in adjacent mature cells (n = 4). Furthermore, none of the 3 day-old cells had any evoked synaptic responses in the middle molecular layer ( >ten 10-second 8 Hz trains per cell, n = 11; Figure 6A), despite robust extracellular field potentials in these same slices at the same stimulation intensity (n = 7, data not shown) or large PSCs in neighboring mature cells (n = 4; Figure 6B). As expected for this feedforward circuit, the same middle molecular layer stimuli also produced repetitive spiking in hilar neurons (data now shown), indicating that hilar mossy cells or GABAergic interneurons were activated by our stimulation but were not functionally connected to 3 day-old cells. Together, these results indicate that there is minimal, if any, synaptic innervation at this maturational stage.

Effect of hypoxia on adult DG-derived neurospheres
To further test the role of hypoxia, we used the neurosphere assay to examine survival and proliferation of adult DG progenitors (Azari et al., 2010;Deleyrolle and Reynolds, 2009;Louis et al., 2013). Adult DG neurospheres were generated and cultured for 7 days at ambient oxygen tension (21% O 2 ), in the presence of DMOG (100 mM) or in hypoxic conditions (2.5% O 2 ). Neurospheres under all three conditions exhibited normal morphology, each containing committed progenitors that co-expressed nestin and doublecortin ( Figure 7A). Treatment with DMOG or reducing O 2 levels to 2.5% increased the number of neurospheres, a measure of cell survival (p = 0.01 and p = 0.002 respectively), but had no effect on the size of the neurospheres, a measure of cells' proliferation (p = 0.98, Figure 7B,C). Consistent with enhanced survival DMOG or hypoxia reduced the percentage of apoptotic cells in neurospheres as measured by caspase-3 immunoreactivity (normoxia: 28.1 ± 1.7%; DMOG: 17.7 ± 2.6%, p = 0.01; hypoxia: 11.4 ± 2.3%, p <0.0001, Figure 7D-G). The neurosphere assay also enabled us to test the sensitivity of the hypoxic marker pimonidazole hydrochloride (PH), which labeled groups of cells in the SGZ ( Figure 1B). No pimonidazole binding was detected in neurospheres cultured in standard atmospheric culture conditions or in the presence of DMOG, whereas only a small percentage of PH+ cells (10.8 ± 4.4%), were detected in neurospheres cultured in 2.5% O 2 ( Figure 7H-K), indicating that PH labeling occurs at O 2 2.5%. Consistent with our in vivo experiments, our neurosphere data results strongly suggest that hypoxia is an essential factor for the survival of DG-derived intermediate progenitors in vitro.

Discussion A hypoxic niche in the SGZ
The partial oxygen pressure (pO 2 ) and concentration in brain tissue is well below the 40 mm Hg in venous blood exiting the brain and is also heterogeneous (Erecińska and Silver, 2001). In rat hippocampus the pO 2 has been estimated to be 20.3 mmHg (Erecińska and Silver, 2001), whereas even more hypoxic regions have been detected in the SGZ of the adult mouse DG (Mazumdar et al., 2010). Using the hypoxia marker, pimonidazole hydrochloride, which labels cells with oxygen levels <10 mm Hg (1.3%), we found that SGZ stem-cell like-radial glia and early intermediate progenitors lie within hypoxic zones. Interestingly, not all cells in the SGZ were labeled, but were localized to narrow zones of a few cell diameters wide along the hilar border of the SGZ. Only a fraction of GFAP+ or Tbr2+ cells were pimonidazole-positive whereas the hypoxic cell did not label with the proliferating marker Ki67 (data not shown). Thus heterogeneous zones of tissue pO 2 may reflect some degree of cell-state specificity as well as local metabolic demand that changes constantly even within a small specific area/volume of the brain.
The two major determinants for tissue oxygen concentration are blood flow (oxygen supply) and oxygen consumption rate (cellularity). The latter may predominate in the SGZ with its high ratio of nuclei to blood vessels compared to other brain regions (Mazumdar et al., 2010). Although some neural stem cells are perivascular (Ottone et al., 2014;Tavazoie et al., 2008;Palmer et al., 2000), it remains to be determined whether these vessels carry highly oxygenated blood or how easily oxygen is distributed to the cell. Interestingly, a recent paper (Sun et al., 2015) revealed that within the SGZ zone only late amplifying progenitors (Tbr2+/DCX+) and horizontal early neuroblasts (Tbr2-/ DCX+), among the entire population of SGZ progenitors are in direct contact with blood vessels.
Numerous adult stem cells reside in hypoxic niches, where they maintain a quiescent state depending predominantly on anaerobic glycolysis (de Filippis and Delia, 2011; Mohyeldin et al.,  Platero-Luengo et al., 2014). Stem cells exhibit different metabolic properties than their differentiated progeny that may promote 'stemness'. However, whether anaerobic metabolism is an adaptation to low oxygen levels in the specific niches in vivo or is an intrinsic stem cell property is still unclear. In either case, our results are consistent with two-photon phosphorescence direct in vivo measurements of local oxygen tension in the bone marrow, which revealed a hypoxic adult stem cell niche with high heterogeneity in local pO 2 (Spencer et al., 2014). Thus our data supports the existence of steep pO 2 gradients between the neurogenic niche and the granule cell layer.

Tissue oxygen and apoptosis
Although a microenvironment with low oxygen concentration confers resistance to reactive oxygen species and cytotoxic stressors, stem cells and their progeny are susceptible to changes in redox status upon migration away from hypoxic zones (Mohyeldin et al., 2010;Platero-Luengo et al., 2014;Suda et al., 2011). Interestingly, we detected oxidative byproducts in intermediate progenitors and neuroblasts located adjacent to the hypoxic niches. This correlation suggests that migration of intermediate progenitors away from hypoxic zones leads to oxidative damage, and thus triggers an early phase of apoptosis. In vitro, hypoxic conditions (defined as <5% O 2 ) reduces apoptosis, promotes proliferation, and increases cultured embryonic, adult neural stem cells and neuronal progenitors (Sierra et al., 2010;Shingo et al., 2001;Studer et al., 2000;Felling, 2006;Gustafsson et al., 2005;Clarke and van der Kooy, 2009;Chen et al., 2007;Bürgers et al., 2008). Deletion of hypoxia-inducible factor-1 alpha subunit (HIF-1a) in vivo, a transcriptional activator mediating adaptive cellular responses to hypoxia, dramatically decreased adult hippocampal neurogenesis (Mazumdar et al., 2010). Additionally, conditional ablation of HIF1-a in adult mouse brain resulted in hydrocephalus, decreased neurogenesis (partly due to an increase in apoptosis) and deficits in spatial memory (Tomita et al., 2003). In our experiments, DMOG, an agent that stabilizes HIF1-a under normoxic conditions, reversed the early phase of cell death of newborn cells in the adult dentate gyrus. These results strongly support the idea that transition of progenitors from a hypoxic niche to normal tissue oxygen contributes to cell death.

The time course of survival in the SGZ
Prior studies using mitotic markers such as BrdU found that only 30-50% of adult-born newborn neurons survive by 30 days post-mitosis (Dayer et al., 2003;Kempermann, 2003;Mandyam et al., 2007;Jagasia et al., 2009;Tashiro et al., 2006), but these studies often did not investigate survival during the first few days post-mitosis. Thus the prevailing idea, until recently, had been that adultgenerated hippocampal granule cell survival is regulated mostly at the immature neuron stage (2-4 weeks post-mitosis), when is affected by neuronal activity and behavior (Jagasia et al., 2009;Tashiro et al., 2006;Gage et al., 1999). Our results support two critical periods for survival of newborn cells in the adult SGZ, early (1-4 days post-mitosis) and late (1-3 weeks post-mitosis) with most (2/3rds) of cell death occurring in the first week (Dayer et al., 2003;Kempermann, 2003;Sierra et al., 2010;Mandyam et al., 2007;Jagasia et al., 2009;Tashiro et al., 2006;Mazumdar et al., 2010). The early apoptosis is likely executed by members of the p53 and Bcl-2 protein families that are key players regulating the apoptotic machinery of adult DG neural precursors and newborn neurons (Sun, 2004;Sahay et al., 2011;Cancino et al., 2013;Fatt et al., 2014). Although programmed neuronal death is often activity-regulated, whether there is synaptic innervation during the first few days post-mitosis has been unclear (Song et al., 2013;Esposito, 2005). Our results show that at 3 days post-mitosis, adult-born cells demonstrate no detectable neural activity. Rather, we propose that the metabolic milieu of the adult SGZ, a previously underexplored variable of this highly specialized neurogenic niche, plays a critical role in the early survival of adult generated hippocampal granule cells.

Animals
All procedures were performed according to the National Institutes of Health Guidelines for the Care and Use of Laboratory Animals and were in compliance with approved IACUC protocols at Oregon Health & Science University. Subjects were young adult (six weeks old) C57BL/6J (wildtype) and B6;129-Bax tm2Sjk Bak1 tm1Thsn/J (Bax fl/fl ) transgenic mice (Takeuchi et al., 2005). Homozygotic Bax fl/fl mutants generated on a Bak1 null background have exons 2-4 of Bax deleted following Cremediated recombination. The conditional deletion of Bax combined with the Bak1 null allele greatly reduces apoptotic cell death and thus makes these mice useful in studies of apoptosis regulation (Takeuchi et al., 2005).

Reagents
Hypoxic zones in the adult hippocampus were detected using pimonidazole hydrochloride kit (Hypoxyprobe) according to manufacturer's protocol. Pimonidazole hydrichloride in water was injected intraperitoneally at a dosage of 60 mg/kg. Animals were sacrificed at 1 hr and the brain removed following cardiac perfusion. Given that the half-life of pimonidazole is 22 min in mice, >99% of free drug had been cleared by the time of sacrifice, thus the pimonidazole imaged in our experiments was already bound at time of perfusion (Walton et al., 1987;Williams et al., 1982).
To examine survival of newborn cells in the adult hippocampus, mice were injected intraperitoneally with Bromodeoxyuridine (BrdU, Sigma-Aldrich, St. Louis, MO) at 300 mg/kg twice with a 4 hr interval between doses, and sacrificed at different time points. This pulse-chase protocol was chosen to saturate mitotic cell labeling within a single cell cycle as determined previously (Cameron and Mckay, 2001). To stabilize hypoxia-inducible factor 1-a, mice were treated with dimethyloxallyl glycine (DMOG, Cayman Chemicals, Ann Arbor, MI) at 50 mg/kg intraperitoneally daily for 3 or 7 days and animals were sacrificed at different time points. The comparison group received vehicle (30% DMSO). The primary antibodies used were: anti-pimonidazole hydrochrolide (1:

Immunohistochemistry and quantitation
Mice were terminally anesthetized according to IACUC-approved protocols, transcardially perfused with saline (4 ml, <30 sec) followed by 20ml of 4% paraformaldehyde (PFA), and brains were postfixed overnight. Coronal sections (100mm thick) of the hippocampus were collected from each mouse and permeabilized in 0.4% Triton in PBS (PBST) for 30 min. Sections were then blocked for 30 min with 5% horse serum in PBST and incubated overnight (4˚C) with primary antibody in 5% horse serum/PBST. Sections incubated with anti-BrdU were first incubated in 2N hydrochloric acid in potassium PBST for 30 min (37˚C), washed twice and blocked with horse serum as described above. After extensive washing, sections were incubated with the appropriate secondary antibody conjugated with Alexa 488, 568 or 647 (Molecular Probes, Eugene, OR), for 2 hr at room temperature. They were then washed in PBST (2 Â 10 min) and mounted with Dapi Fluoromount-G (SouthernBiotech, Birmingham, AL).
For quantification of immunopositive cells, six sections offering dorsal to ventral coverage of the dentate gyrus were stained from each animal. For the BrdU survival analysis, 6-12 animals were analyzed for each time point and condition, while for the rest of the immunofluorescence quantification 3-5 animals were analyzed per group. Slides were coded and imaged by an investigator blinded to experimental condition with a Zeiss LSM780 confocal microscope under a 10x 0.45NA, 20x or 40x 0.8NA lens. A 49 mm z-stack (consisting of 7 optical sections of 7mm thickness) was obtained from every slice. For evaluation of cells densities, positive cells within the dentate gyrus per z-stack were counted (Image J) and divided by the volume of the dentate gyrus. To determine the volume of the DG quantified the cross sectional area of the dentate gyrus (Image J) was multiplied by the section thickness. Cells densities from every stack were averaged per mouse and the results were pooled to generate mean values. For normalization the density of cells in individual DMOG treated animals was normalized to the mean cell density of the animals in the control group.

Volumetric analysis of the DG
Measurements were taken in every sixth 49 mm coronal section stained with DAPI (Sigma-Aldrich). Sections were digitized at appropriate magnification and the areas of the dentate granule cell layer (GCL) were measured using ImageJ software. Volumes (V) were calculated as V = SA Á i Á d, according to Cavalieri´s principle, with A representing the sum of areas from both hemispheres of each section, i the interval between the sections, and d the section thickness, respectively.

Quantitative real-time PCR
Total RNA was isolated from dentate gyri microdissected from animals treated with either vehicle or DMOG for 3 days. RNA samples were DNAse treated using the DNAfree kit (Ambion, Carlsbad, CA) followed by an additional ethanol precipitation. cDNAs were synthesized from 200 ng of RNA using random hexamer primers from the First Strand cDNA Synthesis Kit (Fermentas, Waltham, MA). The real time reaction was performed in triplicate using FastStart SYBR Green Master (Roche, Switzerland). Quantification was performed using the efficiency-corrected DDCT method. The following primers were used for qRT-PCR: HIF-1a sense CCATTCCTCATCCGTCAAATA anti-sense AAGTTCTTCCGGCTCATAAC, EPO sense CAGAGACCCTTCAGCTTCATATAG, anti-sense TCTGG-AGGCGACATCAATTC, VEGF sense ACACCCACCCACATACA, anti-sense TCCAGTGAAGACACC-AATAAC, Lef-1 sense AGAACACCCTGATGAAGGAAAG, antisense GTACGGGTCGCTGTTCATATT, 18S sense CGCGGTTCTATTTTGTTGGT, anti-sense TCGTCTTCGAAACTCCGACT.

Western blot analyses
Microdissected dentate gyri from animals treated either with vehicle or DMOG for 3 days were mechanically homogenized in RIPA lysis buffer (Thermoscientific, Waltham, MA) and samples were spun for 10min at 4 o C. The protein concentration in the supernatant was determined using the Bradford Protein Assay and supernatants were boiled at 95 o C for 5min. Total protein samples were separated by SDS-PAGE and equal amounts of protein (30 mg) were loaded into a 10% gradient precast gel (Invitrogen). The separated proteins were transferred on a polyvinilydine difluoride membrane by electro transfer. The membrane was then incubated sequentially with the primary antibodies [mouse monoclonal HIF1a (Abcam, UK, Ab-1, 1:1000); mouse monoclonal b-tubulin (Developmental Studies Hybridoma Bank, Iowa City, IA, E7, 1:5000). Target proteins were detected using a chemiilluminescence ECL system (Genesys Technologies, Daly City, CA) after incubation with secondady antibody with conjugated horseradish peroxidase (anti-mouse IgG, HRP linked, Cell signalling). The blots were densitometrically analysed using Image J. The optical densities were calculated by normalising the ratio of target protein to b-tubulin.

Retrovirus production and stereotaxic injections
Moloney Murine Leukemia Virus-based retroviral vectors require cell mitosis for transduction and were used to identify and manipulate adult-born granule cells. Retroviruses were created using a pSie-based viral backbone, and expression was driven by a ubiquitin promoter and followed by a woodchuck posttranscriptional regulatory element (Tashiro et al., 2006;Luikart et al., 2011;Schnell et al., 2014).
To study cell survival, groups of WT and Bax fl/fl mice (4-7 animals per condition) were injected with a 1:1 mixture of two viruses, the nuclear cre-GFP-expressing virus and the the mCherry-expressing virus, to control for viral injection coordinates and baseline cell survival. Mice were anesthetized using an isoflurane gas system (Veterinary Anesthesia Systems Co.) and placed in a Kopf stereotaxic frame fitted with a gas nose cone. A skin incision was made and holes were drilled at x: ± 1.1 mm, y: À1.9 mm from bregma. Using a 10 ml Hamilton syringe with a 30 ga needle and the Quintessential Stereotaxic Injector (Stoelting, Kiel, WI), 2 ml mixed viral stock (1 ml of each virus) was delivered at 0.25 ml/min at z-depths of 2.5 and 2.3 mm. The syringe was left in place for 1 min after each injection before being slowly withdrawn. The skin above the injection site was closed using veterinary glue. Animals received post-operative lidocaine and drinking water containing children's Tylenol. The number of GFP-expressing cells was divided by number of mCherry expressing cells for each animal.

Electrophysiology
Live hippocampal slices were prepared from virus-injected animals and whole cell voltage-clamp recordings to asses synaptic activity were performed from retrovirus-labeled 3 day old granule cells using a cesium gluconate-based internal. Excitatory afferents and feed-forward inhibition were stimulated with a bipolar electrode (FHC Inc., Bowdoin, ME) placed into the middle molecular layer (MML) approximately 150 um from the cells of interest, using a 0.1 msec 100 mA stimulating current. Stimulation efficacy was verified by recording from immediately adjacent mature cells (unlabeled) or by recording extracellular field potentials with an ACSF-filled glass pipette placed in the MML in the dendritic field adjacent to the immature neuron, while keeping the stimulation position and intensity constant. All recordings from mature cells or dendritic fields demonstrated clear responses to stimulation. Solutions and acute slice protocols are as reported previously (Schnell et al., 2014).

Statistical analysis
Statistical analysis was performed with Prism Software (GraphPad Software, La Jolla, CA). All data were obtained from n!3 animals and expressed as mean ± standard deviation (SD). Experiments involving ! two groups were compared using unpaired two-tailed Students t-test or multiple-way analysis of variance (AVOVA) followed by post-hoc analysis with Sidak's multiple comparison test. Significance was defined as *p <0.05, **p <0.01, ***p <0.001.