Melanocortins promote neurogenesis and counteract cognitive decline in a transgenic mouse model of moderate Alzheimer’s disease

Alzheimer's disease (AD), both sporadic and genetic, is a chronic disorder characterized by activation of the amyloid/tau cascade in the hippocampus and isocortex. Besides neuroprotective approaches, also neurorestorative strategies for AD are under intensive investigations. [1] The melanocortin system consists of endogenous neuropeptides of the adrenocorticotropin/melanocyte-stimulating hormone (ACTH/MSH) family, acting via five different metabotropic melanocortin receptor subtypes (MC1-MC5). Melanocortins also induce neuroprotection associated with long-lasting functional recovery and counteraction of cognitive decline, as found in acute experimental neurodegenerative conditions and more recently in a chronic neurodegenerative disease as AD. [2] Further, these endogenous peptides have been by us reported to stimulate neurogenesis in an acute neurodegenerative disorder as ischemic stroke. [3] Here we investigated the possible neuroprotective and neurogenic effect of melanocortins in AD with a medium level of severity by using 24 week-old (at the start of the study) APPSwe transgenic mice (Tg2576). METHODS: Tg2576 mice were treated (once daily on days 1-50) with a nanomolar dose of the melanocortin analog [Nle4,D-Phe7]α-melanocyte-stimulating hormone (NDP-α-MSH). Animals were prepared for 5-bromo-2’-deoxyuridine (BrdU) labeling of proliferating cells at days 1-11 of the study, and histological and immunohistochemical studies of the brain were performed for the assessment of neurogenesis. Further, the mouse ability to learn and recall was evaluated by means of the Morris water-maze test at the twenty-seventh week (starting 14 days after the first BrdU injection) and thirty-first week of age. Within 90 min the end of the last behavioural test (day 50 of the study; 31 week-old mice) animals were killed and the brains were removed and processed for histological examination. The whole hippocampi were dissected from brains of some animals to perform western blot analysis of the Zif268 protein (Zif268 protein is transiently expressed after synaptic activation). All values were analyzed by means of two-way repeated measures ANOVA (behavioral data) or one-way ANOVA (all other data), both followed by the Student-Newman-Keuls’ test. A value of p < 0.05 was considered significant. RESULTS: Treatment of Tg2576 mice with the melanocortin analog [Nle4,D-Phe7]α-melanocyte-stimulating hormone (NDP-α-MSH) reduced cerebral cortex/hippocampus level of Aβ deposit (p < 0.001), increased hippocampus Zif268 expression (p <0.001), improved brain histological picture and cognitive functions (p <0.001), relative to saline-treated Tg2576 animals, and no signs of toxicity were recorded. Further, immunohistochemical examination of the hippocampus on day 50 (end of the study) showed, in the dentate gyrus of NDP-α-MSH-treated Tg2576 mice, a very elevated number of BrdU immunoreactive cells colocalized with NeuN (indicator of mature neurons) and Zif268 (indicator of functionally integrated neurons), in comparison with saline-treated Tg2576 animals (p <0.001); no newly formed astrocytes were found. Animal pretreatment (before each administration of NDP-α-MSH) with the selective melanocortin MC4 receptor antagonist HS024 prevented all favourable effects of NDP-α-MSH (p <0.001). CONCLUSIONS: Our data suggest that MC4 receptor-stimulating melanocortins are able to counteract cognitive decline in experimental AD not only by affording neuroprotection, but also by inducing intense neurogenesis. These agents could be candidates for an innovative and safe strategy to counteract AD progression in humans.