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Mechanism of CREB dysregulation in Alzheimer brain

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Alzheimer's disease (AD) is characterized by cognitive dysfunction and progressive neurodegeneration. Hallmarks of AD pathology include accumulation of A?-containing plaques, formation of neurofibrillary tangles and synaptic dysfunction. Microglia, the resident macrophages of the brain, play a protective role by reducing A? load. However, cytokines and free radicals released from hyperactivated microglia cause neuronal damage. Injured neurons facilitate their own demise by sending out distress signals to microglia. This proposal takes a close look at the molecular events in neurons exposed to activated microglia by in vitro and in vivo studies. Altered signaling pathways and orchestrated gene expression patterns in neurons are known to play significant roles in causing AD pathology. In this model, transcription factors provide a crucial link between signaling pathways and gene expression. Cyclic AMP response element binding protein (CREB), a nuclear transcription factor, enhances cognition, memory formation and neuronal survival. CREB is known to be downregulated in the AD brain. However, therapeutically targeting CREB is a challenge because of its broad spectrum actions. Transcription factors are directed to appropriate promoters in a context and cell-type dependent manner by upstream signals. We have characterized the growth factor-stimulated signaling pathways that activate CREB and the mechanism through which oxidative stress interferes with CREB function in cultured neurons. In a recent study with AD post-mortem samples and Alzheimer's transgenic mice, we identified the decrease of hippocampal CREB expression by A?-generated oxidative stress as a late event. But the decrease in CREB content is preceded by persistent downregulation of CREB function. The following two critical findings from our preliminary studies suggest that inflammation, an early event in the progression of AD, interferes with CREB function. (i) CREB-regulated BDNF expression in human neuroprogenitor cell (NPC)- derived neurons was decreased by conditioned medium from A?-activated human microglia. (ii) Restoration of CREB function protected neurons cocultured with A?-activated microglia. Protective and neurodegenerative pathways of inflammation can be delineated at the transcriptional level. Inflammation-mediated CREB dysregulation takes place at the following sites: (i) Cytokines and reactive oxygen species decrease CREB phosphorylation/activation. (ii) Inflammation activates c-jun, STAT-1 and NF-?B, the transcriptions factors that compete with CREB for the limited pool of coactivators, CBP and p300. (iii) Inflammation can direct CREB to pathways other than those needed for neuronal function. (iv) Proteins produced in response to inflammation functionally antagonize CREB target proteins that promote neuronal survival. Our hypothesis is that the pathogenic component of chronic neuroinflammation, acting synergistically with oxidative stress, downregulates CREB-mediated transcription of neuroprotective genes in the AD brain. This hypothesis will be tested in human neuroprogenitor cell derived-neurons, cocultured with microglia and in a triple transgenic Alzheimer's mice using novel methodologies including laser capture microdissection, network motif-based analysis and design-based stereology with the following Specific Aims: Aim 1. To determine the mechanism of CREB downregulation in cultured human neuroprogenitor cell-derived neurons exposed to A?-activated microglia and astrocytes. Aim 2. To identify neuronal CREB downregulation as a pathogenic component of neuroinflammation in triple transgenic Alzheimer's (3XTg-AD) mouse brain: Therapeutic strategies can target transcription factors and their network for profound beneficial effects. Our lab has 15 years of experience and the expertise to examine CREB function in the context of parallel activation of competing transcription factors during inflammation and oxidative stress.
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