👤 Timothy Hla

Lipids, which constitute half of the brain's solid matter, are essential for forming specialized membranes of neural cells, providing energy sources, and facilitating cell-to-cell communication. Although the blood-brain barrier restricts lipid movement between peripheral circulation and the brain, multiple mechanisms supply the building blocks necessary to synthesize the diverse lipid species present in the central nervous system (CNS). In this issue of the JCI, Song et al. characterize specialized microvascular niches that metabolize circulating triglyceride-rich lipoproteins (TRLs) to deliver fatty acids into the brain. They located GPIHBP1, an essential chaperone for lipoprotein lipase (LPL) in the fenestrated endothelial cells of the choroid plexus (ChP) and circumventricular organs (CVOs), demonstrating lipolytic processing of peripheral TRLs and brain uptake of fatty acids. This advance implicates the GPIHBP1/LPL lipid metabolic hub in supporting the roles of the ChP and CVO in cerebrospinal fluid composition, immunity, satiety, thirst, and metabolic homeostasis. Show less

HDL-bound ApoM and albumin are protein chaperones for the circulating bioactive lipid, sphingosine 1-phosphate (S1P); in this role, they support essential extracellular S1P signaling functions in the vascular and immune systems. We previously showed that ApoM- and albumin-bound S1P exhibit differences in receptor activation and biological functions. Whether the physiological functions of S1P require chaperones is not clear. We examined ApoM-deficient, albumin-deficient, and double-KO (DKO) mice for circulatory S1P and its biological functions. In albumin-deficient mice, ApoM was upregulated, thus enabling S1P functions in embryonic development and postnatal adult life. The Show less

GPCR inhibitors are highly prevalent in modern therapeutics. However, interference with complex GPCR regulatory mechanisms leads to both therapeutic efficacy and adverse effects. Recently, the sphingosine-1-phosphate (S1P) receptor inhibitor FTY720 (also known as Fingolimod), which induces lymphopenia and prevents neuroinflammation, was adopted as a disease-modifying therapeutic in multiple sclerosis. Although highly efficacious, dose-dependent increases in adverse events have tempered its utility. We show here that FTY720P induces phosphorylation of the C-terminal domain of S1P receptor 1 (S1P₁) at multiple sites, resulting in GPCR internalization, polyubiquitinylation, and degradation. We also identified the ubiquitin E3 ligase WWP2 in the GPCR complex and demonstrated its requirement in FTY720-induced receptor degradation. GPCR degradation was not essential for the induction of lymphopenia, but was critical for pulmonary vascular leak in vivo. Prevention of receptor phosphorylation, internalization, and degradation inhibited vascular leak, which suggests that discrete mechanisms of S1P receptor regulation are responsible for the efficacy and adverse events associated with this class of therapeutics. Show less