To investigate the role of CREB-mediated gene expression on the excitability of CA1 pyramidal neurons, we obtained intracellular recordings from pyramidal neurons of transgenic mice expressing a constitutively active form of CREB, VP16 -CREB, in a regulated and restricted manner. We found that transgene expression increased the neuronal excitability and inhibited the slow and medium afterhyperpolarization currents. These changes may contribute to the reduced threshold for LTP observed in these mice. When strong transgene expression was turned on for prolonged period of time, these mice also showed a significant loss of hippocampal neurons and sporadic epileptic seizures. These deleterious effects were dose dependent and could be halted, but not reversed by turning off transgene expression. Our experiments reveal a new role for hippocampal CREB-mediated gene expression, identify the slow afterhyperpolarization as a primary target of CREB action, provide a new mouse model to investigate temporal lobe epilepsy and associated neurodegeneration, and illustrate the risks of cell death associated to a sustained manipulation of this pathway. As a result, our study has important implications for both the understanding of the cellular bases of learning and memory and the consideration of therapies targeted to the CREB pathway.

Regulated expression of a constitutively active form of cAMP response element-binding protein (CREB), VP16-CREB, lowers the threshold for the late phase of long-term potentiation in the Schaffer collateral pathway in a de novo gene expression-independent manner, and increases the excitability and reduces afterhyperpolarization of neurons at the amygdala and the hippocampus. We explore the consequences of these changes on the consolidation of fear conditioning and find that the expression of VP16-CREB can bypass the requirement for de novo gene expression associated with long-term memory formation, suggesting that CREB-dependent gene expression is sufficient for fear memory consolidation.

The cAMP-responsive element-binding protein (CREB) pathway has been involved in 2 major cascades of gene expression regulating neuronal function. The first one presents CREB as a critical component of the molecular switch that controls long-lasting forms of neuronal plasticity and learning. The second one relates CREB to neuronal survival and protection. To investigate the role of CREB-dependent gene expression in neuronal plasticity and survival in vivo, we generated bitransgenic mice expressing A-CREB, an artificial peptide with strong and broad inhibitory effect on the CREB family, in forebrain neurons in a regulatable manner. The expression of A-CREB in hippocampal neurons impaired L-LTP, reduced intrinsic excitability and the susceptibility to induced seizures, and altered both basal and activity-driven gene expression. In the long-term, the chronic inhibition of CREB function caused severe loss of neurons in the CA1 subfield as well as in other brain regions. Our experiments confirmed previous findings in CREB-deficient mutants and revealed new aspects of CREB-dependent gene expression in the hippocampus supporting a dual role for CREB-dependent gene expression regulating intrinsic and synaptic plasticity and promoting neuronal survival.

We compare here the neurodegenerative processes observed in the hippocampus of bitransgenic mice with chronically altered levels of cAMP-response element-binding protein (CREB) function. The combination of genome-wide transcriptional profiling of degenerating hippocampal tissue with microscopy analyses reveals that the sustained inhibition of CREB function in A-CREB mice is associated with dark neuron degeneration, whereas its strong chronic activation in VP16-CREB mice primarily causes excitotoxic cell death and inflammation. Furthermore, the meta-analysis with gene expression profiles available in public databases identifies relevant common markers to other neurodegenerative processes and highlights the importance of the immune response in neurodegeneration. Overall, these analyses define the ultrastructural and transcriptional signatures associated with these two forms of hippocampal neurodegeneration, confirm the importance of fine-tuned regulation of CREBdependent gene expression for CA1 neuron survival and function, and provide novel insight into the function of CREB in the etiology of neurodegenerative processes. The cAMP-signaling pathway is activated in neurons in response to a wide array of stimuli. Synaptic activity, hormones, growth factors released during development, hypoxia, and stress, among other stimuli, can trigger the phosphorylation of the cAMP-response element-binding protein (CREB), causing its activation and the subsequent induction of a transient wave of CREB-dependent gene expression.1 This activity is necessary for the survival of different neuronal subtypes both in vitro and in vivo.2-6 The requirement for CREB is very stringent in the peripheral nervous system, 7 whereas in the central nervous system most neuronal types are only affected when both CREB and the cAMP-response element modulator (CREM) are eliminated or inhibited. 8-10Our earlier research on bitransgenic mice expressing either a strong dominant-negative CREB/CREM inhibitor (A-CREB) or a constitutively active CREB variant (VP16-CREB) revealed markedly different initial effects of repressing or boosting the activity of CREB in neuronal gene expression and physiology. A-CREB reduced intrinsic excitability and impaired long-term potentiation (LTP), 10 whereas VP16-CREB enhanced excitability 11 and LTP. 12 Strikingly, these divergent early effects led to apparently similar late deleterious effects, as both manipulations caused in the long term a severe loss of neurons in the CA1 subfield of the hippocampus in a dose-and time-dependent manner. 10,11 Do both the blockade and the enhancement of CREB function trigger the same cell death program? Is CREB misregulation, as earlier proposed, 13-18 involved in neurodegenerative diseases? To answer these questions, we performed parallel histological and cell death assays as well as gene profiling in the hippocampus of A-CREB and VP16-CREB mice. We define the ultrastructural transcriptional signatures associated with these two forms of neurodegeneration, and show that a sustained block...