New neurons incorporate into the granular cell layer of the dentate gyrus throughout life. Neurogenesis is modulated by behavior and plays a major role in hippocampal plasticity. Along with older mature neurons, new neurons structure the dentate gyrus, and determine its function. Recent data suggest that the level of hippocampal neurogenesis is substantial in the human brain, suggesting that neurogenesis may have important implications for human cognition. In support of that, impaired neurogenesis compromises hippocampal function and plays a role in cognitive deficits in Alzheimer's disease mouse models. We review current work suggesting that neuronal differentiation is defective in Alzheimer's disease, leading to dysfunction of the dentate gyrus. Additionally, alterations in critical signals regulating neurogenesis, such as presenilin-1, Notch 1, soluble amyloid precursor protein, CREB, and β-catenin underlie dysfunctional neurogenesis in Alzheimer's disease. Lastly, we discuss the detectability of neurogenesis in the live mouse and human brain, as well as the therapeutic implications of enhancing neurogenesis for the treatment of cognitive deficits and Alzheimer's disease.

Cyclic-AMP response element-binding protein (CREB) signaling has a critical role in the formation of memories. CREB signaling is dysfunctional in the brains of mouse models of Alzheimer's disease (AD), and evidence suggests that CREB signaling may be disrupted in human AD brains as well. Here, we show that both CREB and its activated form pCREB-Ser133 (pCREB) are reduced in the prefrontal cortex of AD patients. Similarly, the transcription cofactors CREB-binding protein (CBP) and p300 are reduced in the prefrontal cortex of AD patients, indicating additional dysfunction of CREB signaling in AD. Importantly, we show that pCREB expression is reduced in peripheral blood mononuclear cells (PBMC) of AD subjects. In addition, pCREB levels in PBMC positively correlated with pCREB expression in the postmortem brain of persons with AD. These results suggest that pCREB expression in PBMC may be indicative of its expression in the brain, and thus offers the intriguing possibility of pCREB as a biomarker of cognitive function and disease progression in AD.

The mechanism underlying impaired learning and memory in Alzheimer's disease is not fully elucidated. The phosphorylation of cyclic-AMP response element binding protein (pCREB) in the hippocampus is thought to be a critical initiating step in the formation of long-term memories. Here, we tested CRE-driven gene expression following learning in mice harboring the familial Alzheimer's disease-linked APPswe/PS1ΔE9 mutations using CRE-β galactosidase reporter. We show that young adult APPswe/PS1ΔE9 mice exhibit impaired recognition memory and reduced levels of pCREB, and its cofactors CREB binding protein (CBP) and p-300 following a learning task, compared to their wild type littermate counterparts. Impairments in learning-induced activation of CREB in these mice are manifested by reduced CRE-driven gene transcription. Importantly, expression of the CRE-driven immediate early gene, Egr-1 (Zif268) is decreased in the CA1 region of the hippocampus. These studies implicate defective CREB-dependent plasticity in the mechanism underlying learning and memory deficits in Alzheimer's disease.

Presenilin-1 (PS1), the catalytic core of the aspartyl protease γ-secretase, regulates adult neurogenesis. However, it is not clear whether the role of neurogenesis in hippocampal learning and memory is PS1-dependent, or whether PS1 loss of function in adult hippocampal neurogenesis can cause learning and memory deficits. Here we show that downregulation of PS1 in hippocampal neural progenitor cells causes progressive deficits in pattern separation and novelty exploration. New granule neurons expressing reduced PS1 levels exhibit decreased dendritic branching and dendritic spines. Further, they exhibit reduced survival. Lastly, we show that PS1 effect on neurogenesis is mediated via β-catenin phosphorylation and notch signaling. Together, these observations suggest that impairments in adult neurogenesis induce learning and memory deficits and may play a role in the cognitive deficits observed in Alzheimer’s disease.

To date, there is no reliable biomarker for the assessment or determination of cognitive dysfunction in Alzheimer's disease and related dementia. Such a biomarker would not only aid in diagnostics, but could also serve as a measure of therapeutic efficacy. It is widely acknowledged that the hallmarks of Alzheimer's disease, namely, amyloid deposits and neurofibrillary tangles, as well as their precursors and metabolites, are poorly correlated with cognitive function and disease stage and thus have low diagnostic or prognostic value. A lack of biomarkers is one of the major roadblocks in diagnosing the disease and in assessing the efficacy of potential therapies. The phosphorylation of cAMP Response Element Binding protein (pCREB) plays a major role in memory acquisition and consolidation. In the brain, CREB activation by phosphorylation at Ser133 and the recruitment of transcription cofactors such as CREB binding protein (CBP) is a critical step for the formation of memory. This set of processes is a prerequisite for the transcription of genes thought to be important for synaptic plasticity, such as Egr-1. Interestingly, *

Plant derivatives offer a novel and natural source of therapeutics. The desert plant Achillea fragrantissima (Forssk) Sch. Bip (Af) is characterized by protective antioxidative and anti-inflammatory properties. Here, we examined the effect of two Af-derived phytochemicals on learning and memory, amyloid-β protein precursor (AβPP) metabolism, and tau phosphorylation in the familial Alzheimer’s disease-linked APPswe/PS1∆E9 mouse model. We observed that mice that were injected with the phytochemicals showed a trend of improvement, albeit statistically insignificant, in the Novel Object Recognition task. However, we did not observe improvement in contextual fear conditioning, suggesting that the benefits of treatment may be either indirect or task-specific. In addition, we observed an increase in the full-length form of AβPP in the brains of mice treated with Af–derived phytochemicals. Interestingly, both in vivo and in vitro, there was no change in levels of soluble Aβ, oligomeric Aβ, or the carboxyl terminus fragments of AβPP (APP-CTFs), suggesting that the increase in full length AβPP does not exacerbate AβPP pathology, but may stabilize the full-length form of the molecule. Together, our data suggest that phytochemicals present in Af may have a modest positive impact on the progression of Alzheimer’s disease.

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