Engineered Exosomes: A Promising Drug Delivery Strategy for Brain Diseases.

Exosomes are a heterogeneous group of nano-sized natural membrane vesicles released from various cells and exist in body fluids. Different from the previous understanding of the function of exosomes as "garbage bins", exosomes act as carriers with many kinds of bioactive molecules (e.g., proteins, lipids, and nucleic acids) to play an important role in cell-cell communication. Growing evidence in recent years has suggested that exosomes also play some roles in the pathogenesis, diagnosis, and treatment modalities of some brain diseases, including ischemic stroke, Alzheimer's disease, Parkinson's disease, multiple sclerosis, and brain cancers. Exosomes as therapeutic drug carriers for brain drug delivery have received extensive attention as well as exosomes can overcome the blood-brain barrier (BBB). However, the low targeting ability and size-dependent cellular uptake of native exosomes could profoundly affect the delivery performance of exosomes. Recent studies have indicated that engineered exosomes can increase the drug uptake efficiency and the subsequent drug efficacy. In the present paper, we will briefly introduce the engineering methods and applications of engineered exosomes in the treatment of brain diseases, and then focus on discussing the advantages and challenges of exosome- based drug delivery platforms to further enrich and boost the development of exosomes as a promising drug delivery strategy for brain diseases.