Growing evidence highlighted that metabolic defects are a risk factor for the development of Alzheimer disease (AD). This striking association spurs the necessity to further explore the molecular mechanisms underlying brain metabolic regulation that fuels neuronal activity. Within this picture, alterations of brain insulin signaling, known as brain insulin resistance, were reported to foster the development of AD. Indeed, insulin signaling regulates the maintenance of synaptic plasticity, the cell stress response and the neuronal metabolism, which are processes central to cognitive and learning functions. It comes that development of brain insulin resistance impairs all these functions. Recovering brain insulin signaling activation is challenging because once brain insulin resistance developed administration of insulin (through the intranasal route) promotes only wake effects. Indeed, from a molecular point of view insulin resistance is characterized by the uncoupling between IR and IRS1, meaning that even if insulin binds to IR, the following IRS1 activation does not occur. To overcome this limitation, published studies suggest the use glucagon-like peptide 1 (GLP1)-mimetics. GLP1 pathway, shares the same molecular targets with the insulin signaling cascade, named the MAPK and the PI3K/Akt pathway, downstream form GLP1 receptor (GLP-1R). Hence, through the activation of GLP1 pathway it is possible to bypass the uncoupling between IR and IRS1. Since GLP1 is rapidly degraded by DPP4 enzyme, maintaining elevated levels of GLP1 in the brain represents a promising strategy. Our project will evaluate age-associated changes of DPP4 levels and activity along with alterations of brain insulin signaling and AD neuropathology in the brain of 3xTg-AD mice and WT. Furthermore, we will test the neuroprotective effects of intranasal sitagliptin (a well-known DPP4 inhibitor) administration in improving brain insulin signaling and AD neuropathology in 3xTg-AD mice.
We expect to find a tight correlation between alterations of insulin signaling and neuropathological markers of AD. The novel idea underlying this project is that increased DPP4 activity represents a key molecular alteration driving brain insulin resistance and cognitive decline.
Behavioral and functional analyses will provide invaluable information about the effectiveness of "metabolic" drugs for age-related brain disease. In addition, our findings will provide novel and suitable biomarkers (DPP4, IR and IRS1) associated with increased susceptibility to neurodegenerative and metabolic diseases. We expect that sitagliptin treatment by inhibiting hyper-active DPP4 (1) will be able to restore insulin signaling in the brain of 3xTg-AD mice and through this mechanism (2) will prevent/delay the aberrant metabolism of APP and Tau, therefore (3) slowing/inhibiting overproduction of Ab and hyperphosphorylation of Tau. Indeed, we expect that in sitagliptin-treated 3xTg-AD mice Ab and pTau levels will be reduced along with an improvement of cognitive functions.
The impact of the proposed work is therefore substantial, especially in the light of the increasing number of people expected to be affected by dementia. It is also important to emphasize that the drug selected to conduct this research is already approved for clinical use in the treatment of T2DM. So, the evidence of the effectiveness could offer an opportunity for an immediate assessment as a drug for the treatment of age-related cognitive decline and possibly promote the clinical "re-purpusing".
The use of intranasal route for drug administration represent an innovation of this project. Craft and co-workers showed that insulin given by intranasal route (Frey WH II, US Patent 6313093, 2001) was able to reach the brain and exert its neuroprotective effects avoiding systemic effects.
Preliminary findings collected by our group strongly encourage to perform the current project since the results that will be generated, each focusing on targets different but all equally crucial and strictly converging, may offer significant advances in the understanding pathophysiological mechanisms involved in age-associate cognitive decline. Interestingly the results may suggest a novel and promising therapy that hopefully could have the potential not just to alleviate the symptoms but also to modify the course of the neuropathology in AD.
Finally, we want to underlie that our proposal if successful has the potential to be a solid base to design future studies in AD. DPP4 enzyme other than being a novel target in AD for its role on GLP1, appears of great interest due to its pleiotropic functions. In particular, DPP4 mediates the cleavage of a number of pro-inflammatory cytokines [25]. Furthermore, DPP4 was reported to promote the formation of the N-terminally truncated Aß isoform, which can be rapidly transformed into the pyroglutamate isoform (pGlu-Aß) known to be more toxic and more prone to accumulate into plaques than full-length Aß [39]. Both increased inflammatory processes and accumulation of (pGlu-Aß) are features of DS brain neuropathology [40], which could be mitigated by using DPP4 inhibitors.
Considering that we are investigating a biological mechanism largely unknown, the 2 aims of the project are high risk/high gain. The PI and his group have a strong track record of published work on AD and insulin signaling, and the PI proved expertise in working with both mice and human with a multidisciplinary approach is a guaranty for the success of this project. Considering the need to find a treatment to prevent/slow the development of AD, the high gain deriving from the collected results will be to offer an instrument to manage such condition.