Understanding glucose metabolism and insulin action at the blood–brain barrier: Implications for brain health and neurodegenerative diseases

The blood–brain barrier (BBB) is a highly selective, semipermeable barrier critical for maintaining brain homeostasis. The BBB regulates the transport of essential nutrients, hormones, and signaling molecules between the bloodstream and the central nervous system (CNS), while simultaneously protecting the brain from potentially harmful substances and pathogens. This selective permeability ensures that the brain is nourished and shielded from toxins. An exception to this are brain regions, such as the hypothalamus and circumventricular organs, which are irrigated by fenestrated capillaries, allowing rapid and direct response to various blood components. We overview the metabolic functions of the BBB, with an emphasis on the impact of altered glucose metabolism and insulin signaling on BBB in the pathogenesis of neurodegenerative diseases. Notably, endothelial cells constituting the BBB exhibit distinct metabolic characteristics, primarily generating ATP through aerobic glycolysis. This occurs despite their direct exposure to the abundant oxygen in the bloodstream, which typically supports oxidative phosphorylation. The effects of insulin on astrocytes, which form the glial limitans component of the BBB, show a marked sexual dimorphism. BBB nutrient sensing in the hypothalamus, along with insulin signaling, regulates systemic metabolism. Insulin modifies BBB permeability by regulating the expression of tight junction proteins, angiogenesis, and vascular remodeling, as well as modulating blood flow in the brain. The disruptions in glucose and insulin signaling are particularly evident in neurodegenerative diseases, such as Alzheimer's disease and Parkinson's disease, where BBB breakdown accelerates cognitive decline. This review highlights the critical role of normal glucose metabolism and insulin signaling in maintaining BBB functionality and investigates how disruptions in these pathways contribute to the onset and progression of neurodegenerative diseases.