Neuroprotection by dipeptidyl-peptidase-4 inhibitors and glucagon-like peptide-1 analogs via the modulation of AKT-signaling pathway in Alzheimer's disease

Alzheimer's disease (AD) is the most common reason for progressive dementia in the elderly. It has been shown that disorders of the mammalian/mechanistic target of rapamycin (mTOR) signaling pathways are related to the AD. On the other hand, diabetes mellitus (DM) is a risk factor for the cognitive dysfunction. The pathogenesis of the neuronal impairment caused by diabetic hyperglycemia is intricate, which contains neuro-inflammation and/or neurodegeneration and dementia. Glucagon-like peptide-1 (GLP1) is interesting as a possible link between metabolism and brain impairment. Modulation of GLP1 activity can influence amyloid-beta peptide aggregation the phosphoinositide-3 kinase/AKT/mTOR signaling pathway in AD. The GLP1 receptor agonists have been shown to have favorable actions on the brain such as the improvement of neurological deficit. They might also exert a beneficial effect with refining learning and memory on the cognitive impairment induced by diabetes. Recent experimental and clinical evidence indicates that dipeptidyl-peptidase-4 (DPP4) inhibitors, being currently used for DM therapy, may also be effective for AD treatment. The DPP-4 inhibitors have demonstrated neuroprotection and cognitive improvements in animal models. Although further studies for mTOR, GLP1, and DPP4 signaling pathways in humans would be intensively required, they seem to be a promising approach for innovative AD-treatments. We would like to review the characteristics of AD pathogenesis, the key roles of mTOR in AD and the preventive and/ or therapeutic suggestions of directing the mTOR signaling pathway.