Genetic inactivation of synaptosomal‐associated protein 25 (SNAP‐25) in adult hippocampal neural progenitors impairs pattern discrimination learning but not survival or structural maturation of newborn dentate granule cells

Abstract

Adult neurogenesis is necessary for proper cognition and behavior, however, the mechanisms that underlie the integration and maturation of newborn neurons into the pre-existing hippocampal circuit are not entirely known. In this study, we sought to determine the role of action potential (AP)-dependent synaptic transmission by adult-generated dentate granule cells (DGCs) in their survival and function within the existing circuitry. We used a triple transgenic mouse (NestinCreER :Snap25 : tdTomato) to inducibly inactivate AP-dependent synaptic transmission within adult hippocampal progenitors and their progeny. Behavioral testing in a hippocampal-dependent A/B contextual fear-discrimination task revealed impaired discrimination learning in mice harboring SNAP-25-deficient adult-generated dentate granule cells (DGCs). Despite poor performance on this neurogenesis-dependent task, the production and survival of newborn DGCs was quantitatively unaltered in tamoxifen-treated NestinCreER :Snap25 : tdTomato SNAP compared to tamoxifen-treated NestinCreER :Snap25 : tdTomato control mice. Although SNAP-25-deficient adult DGCs displayed a small but statistically significant enhancement in proximal dendritic branching, their overall dendritic length and distal branching complexity was unchanged. SNAP-25-deficient newborn DGCs also displayed robust efferent mossy fiber output to CA3, with normal linear density of large mossy fiber terminals (LMTs). These studies suggest that AP-dependent neurotransmitter release by newborn DGCs is not essential for their survival or rudimentary structural maturation within the adult hippocampus.

Genetic inactivation of hypoxia inducible factor 1-alpha (HIF-1α) in adult hippocampal progenitors impairs neurogenesis and pattern discrimination learning

“…Contextual fear conditioning is a widely used hippocampal-dependent task strongly influenced by adult hippocampal neurogenesis. Loss of neurogenesis functions following either x-irradiation or genetic ablation of adult hippocampal progenitors (Saxe et al, 2006), or following silencing of newborn DGCs via genetic or optogenetic methods (Gu et al, 2012; Gustus et al, 2018; Huckleberry et al, 2018), results in impaired contextual fear-discrimination learning. Conversely, increasing adult hippocampal neurogenesis by genetic inhibition of apoptosis improves performance on this task (Sahay, Scobie, et al, 2011).…”

“…In addition, we utilized impaired pattern discrimination learning in either contextual and/or spatial domains as potential behavioral readouts for impaired neurogenesis. Specifically, we utilized the A-B contextual fear discrimination paradigm, previously demonstrated to be dependent upon adult hippocampal neurogenesis (Gustus et al, 2018; Kheirbek, Tannenholz, & Hen, 2012; McHugh et al, 2007; Niibori et al, 2012; Sahay, Scobie, et al, 2011; Tronel et al, 2012), and a more recently developed hippocampal-dependent novel trial-unique nonmatching- to-location (TUNL) paradigm using a touch-screen operant chamber (Josey & Brigman, 2015). Our findings demonstrate impaired adult hippocampal neurogenesis following genetic activation of NSPC-encoded HIF-1α, which is directly correlated with impaired discrimination learning.…”

Resistance of Postnatal Hippocampal Neurogenesis to Alcohol Toxicity in a Third Trimester‐Equivalent Mouse Model of Gestational Alcohol Exposure

“…These effects are not due to impaired progenitor proliferation or size of the progenitor pool, but are most likely due to impaired activity‐dependent survival and incorporation of early postmitotic DGCs into the existing hippocampal circuitry under conditions of EE (Choi et al., ; Kajimoto et al., ). Impaired EE‐mediated neurogenesis in this model of moderate gestational EtOH exposure correlates with impaired A‐B contextual fear discrimination learning (Kajimoto et al., ), a behavioral task previously demonstrated by us and others to be dependent upon the activity of adult‐generated dentate granule cells (aDGCs; Gustus et al., ; Kheirbek et al., ; Niibori et al., ; Tronel et al., ). Interestingly, gestational EtOH exposure also results in a compensatory increase (~2‐fold) in the frequency of spontaneous excitatory postsynaptic currents within surviving aDGCs under conditions of EE, as assessed by patch clamp recordings in hippocampal slice preparations, concomitant with impaired EE‐mediated dendritic branching and excitatory synaptic activity in older pre‐existing, developmentally generated dentate granule cells (dDGCs; Kajimoto et al., ).…”

“…The number of tdTom + /NeuN + co‐labeled neurons was counted within serial coronal sections using the Optical Fractionator probe in Stereoinvestigator software (MicroBrightField Bioscience, Williston, VT) linked to an Olympus IX‐81 DSU spinning disk confocal microscope with a 40× objective (Shinjuku, Tokyo, Japan) as previously described (Gustus et al., ). A region‐of‐interest (ROI) contour was manually outlined within each section using a 10× objective.…”

“…We utilized a well‐characterized Cre‐loxP genetic labeling approach in NestinCreER T2 :tdTomato bitransgenic mice to identify newborn DGCs. These mice harbor a tamoxifen‐inducible tdTomato reporter for indelible labeling of newborn DGCs in vivo (Cunningham et al., ; Gustus et al., ). Surprisingly, we found that moderate EtOH exposure during the early postnatal period impacts neither the production nor the survival of dDGCs, and has no effect on the production of aDGCs or the ability to mount a neurogenic response to EE in adulthood.…”