Advance in neuroprotective effects of proanthocyanidins (PCs): Structure, absorption, bioactivities, mechanism, and perspectives

With the global population growing and aging, along with increasing environmental, metabolic, and lifestyle-related risk factors, the worldwide incidence of stroke, Alzheimer's disease (AD) and other dementias, meningitis, and other neurological disorders-along with associated mortality-has risen significantly. Proanthocyanidins (PCs), which are oligomers and polymers of flavan-3-ols, are widely distributed across the plant kingdom, including in grape seeds, cinnamon, apples, cranberries, lotus seeds, and pine bark. They represent the second most abundant class of polyphenols in nature, after lignin. A substantial body of preclinical evidence indicates that PCs exert significant neuroprotective effects through multiple mechanisms. This review provides a systematic overview of the sources, structural characteristics, and bioavailability of PCs, with a focus on their pharmacological mechanisms in nervous system disease. Specifically, it examines their roles in regulating oxidative stress, neuroinflammation, protein homeostasis, apoptosis, autophagy, and key signaling pathways, including Nrf2/HO-1, CREB/BDNF, PI3K/Akt, MAPK, and NF-κB. Furthermore, this review systematically summarized the distinct structural forms of PCs, including monomers, dimers, trimers, and polymers, and explores their structure-activity relationships (SARs) in modulating the gut-brain axis. Additionally, recent advances in PCS-based nano-delivery systems and clinical studies related to neurological disorders are summarized. Growing evidence indicates that microbial metabolism in the gut serves as a key mechanism underlying their neuroprotective effects. Finally, the potential applications of PCs as promising dietary supplements or therapeutic agents for the prevention and treatment of nervous system diseases are discussed, along with existing challenges and future perspectives.