In this two-part study, I investigated whether the cyclic-adenosine monophosphate response element-binding (CREB) protein has the potential to be clinically modulated as a therapeutic target for the treatment of neurodegenerative diseases. Part one consisted of a systematic review that was conducted on select articles gathered through a stepwise method to explore (1) the relationship between diseased, neurodegenerative brains and levels of active, phosphorylated CREB (pCREB), (2) increased activation of CREB as a treatment for neurodegenerative symptoms, and (3) a potential therapeutic drug for neurodegenerative diseases that can target CREB signaling. The results of the systematic review showed evidence that suggested excitotoxic concentrations of N-methyl-D-aspartate (NMDA) results in decreased pCREB levels, while decreased pCREB levels were associated with impaired cognition and behavior, increased cell death, as well as decreased CRE-gene transcription and long-term potentiation (LTP). Part two consisted of a systematic review and meta-analysis on clinical trials that used the phosphodiesterase type IV inhibitor, roflumilast, on healthy and schizophrenic patients. It was found that 100 µM roflumilast was able to improve verbal learning in healthy and schizophrenic subjects (ES = 64). Initial evidence indicates that future research on neurodegenerative diseases should further investigate CREB’s potential to be clinically modulated and research investigating PDE4 inhibitor drug therapy for the treatment of neurodegeneration should be expanded upon further in subsequent studies.

Deep brain stimulation (DBS) is currently being investigated as a therapy for treatment-resistant depression, with promising results. However, it is not clear whether or not DBS works via the same mechanisms as those induced by antidepressant medications. Processes currently implicated in antidepressant effects include neuroplastic changes and promotion of neurogenesis. We investigated the effects of chronic treatment with three different classes of antidepressants and DBS on markers of neuroplasticity (brain-derived neurotrophic factor, (BDNF), and phosphorylated cyclic-AMP regulatory element binding protein, (pCREB)) and neurogenesis (Ki-67, bromodeoxyuridine (BrdU) and doublecortin) in the rat hippocampus. No clear treatment effects were seen on BDNF, pCREB and Ki-67 levels. However all treatments caused increased levels of BrdU (range: 46%-96%) and doublecortin (8%-61%), although these effects were statistically significant only for DBS and amitriptyline, respectively. This overall pattern of results may suggest that diverse antidepressant treatments could possibly share common mechanisms involving cell survival and neuronal differentiation. Potentiated effects of DBS on cell survival may underlie its efficacy in treatment-resistant depression.

Deep brain stimulation (DBS) is currently being investigated as a therapy for treatment-resistant depression, with promising results. However, it is not clear whether or not DBS works via the same mechanisms as those induced by antidepressant medications. Processes currently implicated in antidepressant effects include neuroplastic changes and promotion of neurogenesis. We investigated the effects of chronic treatment with three different classes of antidepressants and DBS on markers of neuroplasticity (brain-derived neurotrophic factor, (BDNF), and phosphorylated cyclic-AMP regulatory element binding protein, (pCREB)) and neurogenesis (Ki-67, bromodeoxyuridine (BrdU) and doublecortin) in the rat hippocampus. No clear treatment effects were seen on BDNF, pCREB and Ki-67 levels. However all treatments caused increased levels of BrdU (range: 46%-96%) and doublecortin (8%-61%), although these effects were statistically significant only for DBS and amitriptyline, respectively. This overall pattern of results may suggest that diverse antidepressant treatments could possibly share common mechanisms involving cell survival and neuronal differentiation. Potentiated effects of DBS on cell survival may underlie its efficacy in treatment-resistant depression.

La dépression majeure peut être très dommageable pour le 8% des Nord-Américains qui en souffriront au moins une fois durant leur vie. Les traitements actuels de la dépression ont été créés grâce à la compréhension de l'hypothèse de la monoamine; où les transporteurs de la sérotonine ou les enzymes sont bloqués ou inhibés afin de maintenir le neurotransmetteur (NT) dans la fente synaptique. Sur une période de plusieurs semaines, cette augmentation de NT encourage la plasticité synaptique, ce qui augmente la sensibilité de réponse de 5-HT dans la fente synaptique. Cette forme de traitement est efficace chez 40% des patients, tandis que les 60% restants ont une réponse partielle ou nulle. Dans les modèles de dépression chez la souris des changements dans les niveaux de phosphorylation de CREB sont corrélés aux effets antidépresseurs (AD). Dans le cas RS67333, des effets AD apparaissent après 2 à 3 jours de traitement. Les effets anti-anhédoniques sont démontrés par la réponse au traitement. RS67333 est un agoniste spécifique du récepteur 5-HT4, un récepteur couplé à la protéine G (GPCR) qui est généralement situé, après synapses, dans les régions du système limbique. Il existe d'autres ligands qui sont également spécifiques au récepteur 5-HT4: 5-HT, Zacopride, Prucalopride et ML10302, mais les mêmes effets anti-anhédoniques constatés lors de RS67333 n’ont pas été observés. Comment RS67333 est-il différent des autres en termes de signalisation? Nous avons utilisé le transfert d’énergie par résonnance de bioluminescence (BRET) pour étudier les interactions 5-HT4R avec les sous-unités, α β γ de la protéine G et nous avons comparé ces associations avec d'autres agonistes connus de 5-HT4. Nous avons constaté que RS67333 est un agoniste partiel et qu’il active un changement conformationnel de la protéine Gαs, mais n’active pas un changement conformationnel de la protéine Gαo. Prucaloride, 5-HT et Zacopride induisent aussi un changement de conformation du complexe Gαs, mais pas ML10302. Pour la formation de Gαo, Prucalopride, ML10302 et 5-HT agissent comme des agonistes, mais pas zacopride ni RS67333. Nous avons utilisé un test de dosage immuno-enzymatique sur support solide (ELISA) pour mesurer l’intériorisation induite par les ligands pour étudier la régulation des récepteurs. Nous avons constaté que RS67333 et ML10302 n’induisent pas l'internalisation du récepteur, tandis que zacopride, prucalopride et 5-HT sont très efficaces. L'efficacité sous-optimale des ADs actuels met l’accent sur la nécessité de développer des médicaments avec d'autres mécanismes d'action. RS67333 a des effets persistants anti-anhédoniques après une courte période de temps, et d'autres études de ses propriétés de signalisation pourraient donner lieu à une nouvelle génération d’antidépresseurs rapides de longue durée d’action. / Major Depression can be very damaging on the 8% of North Americans that will have it at least once within their lifetime. Current treatments of depression have been developed through the understanding of the monoamine hypothesis; whereby serotonin (5-HT) transporters or enzymes are blocked or inhibited in order to maintain the neurotransmitter in the synaptic cleft. Over a period of several weeks this increase in NT induces synaptic plasticity which increases the response sensitivity of 5-HT in the synaptic cleft. This form of treatment is effective on 40% of patients, while the remaining 60% have partial or no response. In mouse models of depression, changes in phosphorylation levels of CREB correlated to antidepressants (AD) effects. In the case of RS67333 AD effects appear within 2-3 days of treatment. The response can be evidenced as anti- anhedonic. RS67333 is specific agonist for 5-HT4 receptor, a G protein coupled receptor (GPCR) that is typically located post synaptically, in limbic regions. There are other ligands that are also specific to receptor 5-HT4: 5-HT, Zacopride, prucalopride and ML10302, but have not been observed to have the same anti-anhedonia effects of RS67333. How does RS67333 differ from the others in terms of signaling? We used bioluminescence resonance energy transfer (BRET) to study the interactions of 5-HT4R with the α β γ subunits of the G protein and we compared these associations with other known 5-HT4 agonists. We found that RS67333 was a partial agonist and activated a conformational change of the Gαs protein but did not activate a conformational change of Gαo protein. Prucaloride, 5-HT and Zacopride also induced a conformational change of the Gαs complex but not ML10302. For the formation of Gαo, Prucalorpide, ML10302 and 5-HT acted as agonists, but not Zacopride. We used an ELISA assay to measure ligand induced internalization to study receptor regulation. We found that RS67333 and ML10302 did not induce receptor internalization, while Zacopride, Prucalopride and 5-HT were very effective. The suboptimal efficacy of current ADs stresses the need to develop drugs with other mechanisms of action. RS67333 has persistent anti-anhedonia effects after a short period of time and further studies of its signalling properties may open up a new generation of fast and long acting antidepressants.