Alzheimer's disease therapy: Neurotrophin-based regenerative approaches

Neurotrophins are a class of proteins that maintain the health and phenotype of neuronal cells under normal physiological conditions. Nerve growth factor was the first neurotrophin to be discovered, supporting the survival and cholinergic phenotype of basal forebrain cholinergic neurons, which are crucial in maintaining cognitive function in healthy individuals. Nerve growth factor metabolism is altered in Alzheimer's disease and, along with the degeneration of basal forebrain cholinergic neurons and loss of cholinergic pathways in the affected brain, contributes to cognitive problems. These findings initiated the application of nerve growth factor supplementation as a regenerative strategy against Alzheimer's disease in the late 20 th century. Later decades witnessed the development of drugs that support cholinergic activity, namely, cholinesterase inhibitors offering small but persisting cognitive benefits in Alzheimer's disease patients. Further developments in the Alzheimer's disease field have witnessed the rise of anti-amyloid immunotherapies that target the amyloid plaques in Alzheimer's disease brains in an attempt to reduce disease pathology. Over the years, several reports have appeared in support of or undermining the therapeutic claims of each strategy, while many other therapeutic approaches are being presently tested. In this narrative review, we present broader perspectives regarding cholinergic therapeutic strategies against Alzheimer's disease, highlighting aspects in the Alzheimer's disease field that need to be addressed, and propose future perspectives. We provide a special focus on neurotrophic molecules, especially on nerve growth factor, due to its close association with cognitive pathways and its relationship with cholinergic pathways, since cholinesterase inhibitors remain a widely used medication for Alzheimer's disease patients even after 30 years of research.