The Role of Phosphorylated CREB in the Preferential Recruitment of Immature Dentate Granule Neurons into Learning and Memory Circuitry

Adult hippocampal neurogenesis has been linked with learning and memory in mammalian species (including humans). Recent work has demonstrated a critical period following the birth of new hippocampal neurons during which they display a competitive advantage over their neighbors and integrate preferentially into spatial memory circuitry. The current study represents a preliminary investigation into this effect, with the overarching goal of elucidating the molecular mechanism that underlies the accelerated integration of immature neurons into spatial memory circuitry. Based on its role in regulating both neurogenic processes (e.g., proliferation, maturation, and survival) and intrinsic neuronal excitability, it was hypothesized that the intracellular protein CREB might play an important role in the neurogenic basis of memory. To mark the birth of adult-generated neurons, rats were injected with the neural proliferation marker BrdU at several time-points (1, 3, 6, and 10 weeks) prior to training in the Morris Water Maze. Double-label (BrdU + pCREB) immunofluorescence in conjunction with high-powered confocal microscopy was used to visualize the extent of pCREB expression in both BrdU+ and BrdU- cells in the granule cell layer of the hippocampus. Three week old dentate granule neurons showed greater pCREB expression in response to the MWM than did their one week old counterparts. Although qualitative analysis indicates that pCREB is more strongly expressed in 6 week old dentate granule neurons relative to neighbouring neurons, this effect failed to reach statistical significance. Based on the small sample size used in the histological portion of this study in addition to the large effect size discovered, this failure to achieve statistical significance likely represents a type II error. That is, a significant effect is present, but the current study lacked the power to detect it statistically. The potential role played by CREB in mediating the preferential recruitment of immature neurons into hippocampal-dependent learning and memory circuitry warrants further investigation

Engrams, Neurogenesis, and Forgetting

In this thesis, three projects are described that add to our understanding of how forgetting is represented in the brain. In project 1, age-dependent changes in forgetting of spatial information is characterized in mice. Infant mice displayed significant forgetting when tested one month following training (i.e., infantile amnesia). Overtraining infant mice did not overcome the forgetting phenotype, indicating that the observed forgetting is not due to a deficit in memory encoding. Presentation of an environmental reminder one month following training led to memory recovery, suggesting that infantile forgetting is caused, at least in part, by a deficit in memory retrieval. Given the evidence that infantile amnesia is a deficit in memory retrieval, project 2 asked whether infantile forgetting can be overcome via a dentate gyrus engram ‘tag-and-activate’ strategy. Optogenetic stimulation of dentate gyrus encoding neural ensembles recovered ‘lost’ infant memories even 90 days following the initial learning event. In a process akin to pattern completion, memory recovery was associated with reinstatement of both hippocampal and cortical engram neurons. Recent work from our lab has shown that hippocampal neurogenesis promotes forgetting in both infants and adults. In project 3, the mechanism underlying neurogenesis-mediated forgetting was explored. Multiple transgenic mouse lines were used that granted bidirectionally control over the extent to which adult-generated dentate gyrus neurons remodel surrounding neural circuitry. Using this strategy, we found evidence that newborn neurons promote forgetting by reconfiguring the circuitry within which hippocampal memories are embedded. Importantly, neurogenesis-induced forgetting was negatively correlated with engram reinstatement in the hippocampus. Together, this thesis adds to the growing literature on the neural basis of forgetting.Ph.D

Conditional Deletion of alpha-CaMKII Impairs Integration of Adult-Generated Granule Cells into Dentate Gyrus Circuits and Hippocampus-Dependent Learning

New granule cells are continuously integrated into hippocampal circuits throughout adulthood, and the fine-tuning of this process is likely important for efficient hippocampal function. During development, this integration process is critically regulated by the alpha-calcium/calmodulin-dependent protein kinase II (alpha-CaMKII), and here we ask whether this role is conserved in the adult brain. To do this, we developed a transgenic strategy to conditionally delete alpha-CaMKII from neural progenitor cells and their progeny in adult mice. First, we found that the selective deletion of alpha-CaMKII from newly generated dentate granule cells led to an increase in dendritic complexity. Second, alpha-CaMKII deletion led to a reduction in number of mature synapses and cell survival. Third, consistent with altered morphological and synaptic development, acquisition of one-trial contextual fear conditioning was impaired after deletion of alpha-CaMKII from newly generated dentate granule cells. Previous work in Xenopus identified alpha-CaMKII as playing a key role in the stabilization of dendritic and synaptic structure during development. The current study indicates that alpha-CaMKII plays a plays a similar, cell-autonomous role in the adult hippocampus and, in addition, reveals that the loss of alpha-CaMKII from adult-generated granule cells is associated with impaired hippocampus-dependent learning

A Glo1-Methylglyoxal Pathway that Is Perturbed in Maternal Diabetes Regulates Embryonic and Adult Neural Stem Cell Pools in Murine Offspring

Maternal diabetes is known to adversely influence brain development in offspring. Here, we provide evidence that this involves the circulating metabolite methylglyoxal, which is increased in diabetes, and its detoxifying enzyme, glyoxalase 1 (Glo1), which when mutated is associated with neurodevelopmental disorders. Specifically, when Glo1 levels were decreased in embryonic mouse cortical neural precursor cells (NPCs), this led to premature neurogenesis and NPC depletion embryonically and long-term alterations in cortical neurons postnatally. Increased circulating maternal methylglyoxal caused similar changes in embryonic cortical precursors and neurons and long-lasting changes in cortical neurons and NPCs in adult offspring. Depletion of embryonic and adult NPCs was also observed in murine offspring exposed to a maternal diabetic environment. Thus, the Glo1-methylglyoxal pathway integrates maternal and NPC metabolism to regulate neural development, and perturbations in this pathway lead to long-lasting alterations in adult neurons and NPC pools