Incretin Hormones GLP-1 and GIP Normalize Energy Utilization and Reduce Inflammation in the Brain in Alzheimer's Disease and Parkinson's Disease: From Repurposed GLP-1 Receptor Agonists to Novel Dual GLP-1/GIP Receptor Agonists as Potential Disease-Modifying Therapies.

Alzheimer's disease (AD) and Parkinson's disease (PD) are chronic neurodegenerative disorders with few effective drug treatments available. An underrated element of these diseases is that glucose uptake and energy utilization is much reduced in neurons. In the brains of patients, signaling of insulin, insulin-like growth factor 1, and other growth factors is downregulated early on. This leads to reduced glucose utilization and impaired mitochondrial function. In an attempt to compensate for the loss, other pathways are upregulated, e.g., the increased use of ketones produced from fatty acids by astrocytes that are shuttled to neurons. In addition, amino acids are increasingly used to generate energy. Despite this, neurons generate less and less energy over time, leading to impaired synaptic activity, reduced cell repair, mitogenesis, autophagy, the accumulation of misfolded proteins, and finally, to cell death. At the same time, the chronic inflammation response in the brain that is part of these diseases continues to damage neurons. Glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are peptide hormones and growth factors that have shown neuroprotective effects in animal studies and in clinical trials. GLP-1 and GIP receptor agonists were able to reduce inflammation while normalizing growth factor signaling and energy utilization in the brain. Insulin signaling was improved and energy utilization, glucose uptake, mitogenesis, and mitochondrial functionality was brought back to physiological levels. In addition, the chronic inflammation response and the levels of proinflammatory cytokines in the brain were much reduced. Clinical trials testing GLP-1 receptor agonists in patients with AD or PD have been conducted and have shown first successes, serving as proof of concept that activating GLP-1 receptor is a sensible strategy to treat AD/PD. A phase II study testing liraglutide in patients with AD showed first improvements, and two phase II trials testing exendin-4 (exenatide, Bydureon^®) or lixisenatide showed improvements in patients with PD. A recent phase III trial testing exendin-4 did not show an improvement, which may be linked to the lack of insulin desensitization in the study participants. Semaglutide (Rybelsus^®; Wegovy^®; Ozempic^®) is currently in two phase III trials for AD. Current drugs that are on the market have a long half-life in the blood and do not readily cross the blood-brain barrier (BBB). Newer dual GLP-1/GIP receptor agonists have been developed that can more easily cross the BBB and that show improved protection in animal models of AD and PD. Therefore, GLP-1 and GIP receptor agonists that can cross the BBB show promise as treatments for chronic neurodegenerative disorders.