Apolipoprotein D, a Novel Ligand for CD36, Is Essential for Blood-Brain Barrier Integrity

The disruption of the blood-brain barrier (BBB) is a central pathogenic event in many central nervous system disorders. However, the mechanisms regulating BBB function remain incompletely understood, and effective treatments are lacking. Brain mural cells differ significantly from their peripheral counterparts, a distinction likely critical for maintaining BBB integrity.

We combined proteomic profiling of human brain vs peripheral mural cells with multiple ischemic stroke models (global apolipoprotein D [ApoD] knockout, mural cell-specific ApoD knockout, and adeno-associated virus-mediated ApoD overexpression) to evaluate the role of ApoD in BBB integrity. Mechanistic studies (co-immunoprecipitation, binding assays, including surface plasmon resonance, bio-layer interferometry, cross-linking mass spectrometry, and CD36 loss-of-function approaches, both in vitro and in vivo) were performed to determine how ApoD interacts with CD36 and inhibits its signaling. Finally, we assessed the effect of ApoD glycosylation on CD36 binding and tested therapeutic delivery of hypoglycosylated ApoD in stroke.

Our study has shown an increased expression of ApoD in mural cells after ischemic stroke. We found that mural cell-derived ApoD functions as an inhibitory ligand of endothelial CD36, suppressing pathological endothelial proliferation, preserving BBB integrity, and promoting neurological recovery. Additionally, overexpression of ApoD in mural cells improved BBB integrity and enhanced functional recovery in ApoD-null mice. Mechanistically, ApoD competes with long-chain fatty acids for CD36 binding and directly attenuates downstream CD36 signaling. Furthermore, we reveal that peripheral hyperglycosylated ApoD (hyperglyco-ApoD) showed minimal effect on BBB integrity maintenance, whereas hypoglycosylation of ApoD enhances its binding affinity to CD36, amplifying its therapeutic efficacy. Exogenous administration of hypoglyco-ApoD via vein injection profoundly inhibited BBB disruption and improved neural function, especially in aging stroke.

Our work identifies a previously unrecognized paracrine mechanism in which mural cell-derived ApoD directly engages endothelial CD36 to restrain pathological endothelial proliferation, thereby preserving BBB integrity and promoting neurological recovery after stroke. These findings further suggest that hypoglycosylated ApoD, with its higher CD36-binding affinity, merits investigation as a potential strategy to enhance BBB repair in central nervous system disorders.