The blood-brain barrier consists of specialized cells that line central nervous system blood vessels, allowing only certain cells and molecules to pass to and from the brain. This barrier becomes leaky with age, and this results in growing inflammation and dysfunction in the brain. Inappropriate molecules find their way through and provoke the immune system of the brain into a damaging, lasting inflammatory reaction. This is an important early stage in the progression towards neurodegeneration and consequent cognitive decline. Researchers here report on their investigations of the biochemistry of blood-brain barrier dysfunction, focusing on TSP1 and its ability to disrupt the blood-brain barrier by breaking down proteins involved in the tight junction structures that link cells together. MicroRNA-195 can block some of this disruption when delivered intravenously, which makes it a potentially interesting basis for treatment.

Blood-brain barrier (BBB) disruption contributes to neurodegenerative diseases. Loss of tight junction (TJ) proteins in cerebral endothelial cells (ECs) is a leading cause of BBB breakdown. We recently reported that miR-195 provides vasoprotection, which urges us to explore the role of miR-195 in BBB integrity. Here, we found cerebral miR-195 levels decreased with age, and BBB leakage was significantly increased in miR-195 knockout mice. Furthermore, exosomes from miR-195-enriched astrocytes increased endothelial TJ proteins and improved BBB integrity.

To decipher how miR-195 promoted BBB integrity, we first demonstrated that TJ proteins were metabolized via autophagic-lysosomal pathway and the autophagic adaptor p62 was necessary to promote TJ protein degradation in cerebral ECs. Next, proteomic analysis of exosomes revealed miR-195-suppressed thrombospondin-1 (TSP1) as a major contributor to BBB disruption. Moreover, TSP1 was demonstrated to activate selective autophagy of TJ proteins by increasing the formation of claudin-5-p62 and ZO1-p62 complexes in cerebral ECs while TSP1 impaired general autophagy.

Delivering TSP1 antibody into the circulation showed dose-dependent reduction of BBB leakage by 20%-40% in 25-month-old mice. Intravenous or intracerebroventricular injection of miR-195 rescued TSP1-induced BBB leakage. Dementia patients with BBB damage had higher levels of serum TSP1 compared to those without BBB damage, while the normal subjects had the lowest TSP1. Taken together, the study implies that TSP1-regulated selective autophagy facilitates the degradation of TJ proteins and weakens BBB integrity. An adequate level of miR-195 can suppress the autophagy-lysosome pathway via a reduction of TSP1, which may be important for maintaining BBB function.