CA-074 Me br Results br Discussion Our present

Results
Discussion Our present data demonstrate four key events that are mediated by the aPKC-CBP pathway: (1) neurogenesis in young adult mice (3 months old) by, at least in part, preventing the death of newborn neurons; (2) maintenance of a stable rate of hippocampal neuronal differentiation and maturation in mature adult mice (6 months old); (3) formation of hippocampal-dependent fear memory and maintenance of spatial learning and memory in mature adult mice (6 months old); and (4) maintenance of the association of CBP with CREB in mature adult hippocampi (6 months old), when CREB activity/pS133-CREB is significantly reduced. Importantly, elevation of pS133-CREB expression in vivo rescues the impaired phenotypes at the cellular, behavioral, and molecular levels in mature adult CbpS436A-KI mice where the aPKC-CBP pathway is deficient. Hence, our study strongly argues that the aPKC-CBP pathway is a homeostatic intrinsic mechanism that maintains a sustained rate of hippocampal neurogenesis and hippocampal-dependent memory in response to reduced CREB activity during early adulthood (3–6 months). Originally, we identified the aPKC-CBP pathway as a pro-differentiation pathway during embryonic CA-074 Me development (Wang et al., 2010). Enriched neural developmental cues during cortex development converge on the aPKC-CBP pathway to promote the differentiation of embryonic NPCs into three neural cell lineages. Here, we ask a different question as to whether aPKC-mediated CBP phosphorylation/activation is a homeostatic signaling cascade that modulates adult hippocampal neurogenesis. Interestingly, recent research indicates that an early and dramatic decline in hippocampal neurogenesis during early adulthood (3–6 months) is primarily due to a decrease in neural progenitor proliferation and newborn neuron survival in the absence of any large changes in neuronal differentiation rates (Kuipers et al., 2015). Thus, a sustained rate of neuronal differentiation may produce sufficient amounts of newborn neurons that can be functionally integrated into neural circuits to support increased memory during adulthood. In the present study, we found that the aPKC-CBP pathway is essential to maintain the stable rate of hippocampal neuronal differentiation and maturation during early adulthood development (3–6 months), suggesting its role as a homeostatic intrinsic mechanism in response to cellular changes during early adulthood to sustain functional neurogenesis, a key player in hippocampal-dependent memory formation. In addition to phenotypic analyses, our study also provides insights into the molecular mechanisms that mediate the aPKC-CBP pathway in regulating hippocampal neurogenesis and hippocampal-dependent memory in an age-dependent manner. Previous work in liver cells shows that fully phosphorylated CBP at S436 eliminates the binding of CBP to CREB to regulate gluconeogenic gene expression (He et al., 2009), while we found that CBPS436 phosphorylation is required for CBP to bind to CREB in mature adult hippocampal extracts (6 months old) but not those of young adult mice (3 months old). The discrepancy between the hepatic and hippocampal tissues may be explained by the different CREB signals in the two tissue types. Specifically, CREB was constantly phosphorylated at S133 in hepatic tissue under the testing condition (He et al., 2009), while hippocampal tissues showed a significant reduction of pS133-CREB in mature adults (6 months old) regardless of the genotype. pS133-CREB is known to be a rate-limiting step in promoting the interaction between CREB and CBP (Parker et al., 1996). Our working model is that high levels of S133-phosphorylated CREB in young adult hippocampi play a dominant, stimulatory role in the regulation of the binding between CBP and CREB, whereas S436 phosphorylation in CBP is a compensatory regulator for the interaction between CBP and CREB in mature adult hippocampi when pS133-CREB is significantly reduced (Figure 7). This model is well supported by our data showing that aPKC activity was significantly enhanced in mature adult hippocampi where a significant reduction of pS133-CREB is evident. More interestingly, we observe that the activated form of CREB, pS133-CREB, is restrictively expressed in the hippocampal SGZ neurogenic region. Moreover, most of pS133-CREB-positive cells in the hippocampi are DCX-positive neuroblasts/newborn neurons, suggesting its role in the acquisition and maturation of DCX-positive cells. This idea has been explored in several previous studies (Herold et al., 2011; Merz et al., 2011; Nakagawa et al., 2002b). Importantly, we show here that the expression of pS133-CREB is robustly reduced in the adult hippocampi during early adulthood. We further demonstrate that elevation of CREB phosphorylation by rolipram treatment is able to rescue the hippocampal neuronal differentiation deficit and impaired pre-exposure fear memory, and restore diminished CBP binding to CREB in mature adult CbpS436A-KI mice (6 months old). Together, these data suggest that the aPKC-CBP pathway is a compensatory signaling cascade that is activated in response to reduced CREB activity in mature adult hippocampi to sustain the interaction between CBP and CREB, potentially contributing to hippocampal neurogenesis and hippocampal-dependent fear memory.