Mutating a conserved cysteine in GPIHBP1 reduces amounts of GPIHBP1 in capillaries and abolishes LPL binding

Area of Science:

• Biochemistry
• Genetics
• Molecular Biology

Background:

• Conserved cysteines in Ly6 family proteins are crucial for function, with mutations causing diseases like chylomicronemia (GPIHBP1) and paroxysmal nocturnal hemoglobinuria (CD59).
• Previous studies showed human GPIHBP1 cysteine mutations had minimal impact on cell surface trafficking in cultured cells, questioning in vivo relevance.

Purpose of the Study:

• To investigate the in vivo effects of a GPIHBP1 cysteine mutation (p.C63Y) on protein trafficking, LPL binding, and chylomicronemia.
• To determine if CHO cell studies accurately reflect GPIHBP1 behavior in a living organism.

Main Methods:

• Generated mice with a specific GPIHBP1 cysteine mutation (p.C63Y).
• Assessed LPL binding ability of the mutant GPIHBP1.
• Utilized immunohistochemistry to evaluate GPIHBP1 expression levels on endothelial cells in vivo.
• Analyzed the quaternary structure (monomeric vs. oligomeric) of GPIHBP1 in mouse tissues.

Main Results:

• The p.C63Y mutation abolished GPIHBP1's ability to bind LPL.
• Mice with the mutation exhibited severe chylomicronemia.
• Mutant GPIHBP1 was detected on endothelial cell surfaces but at approximately 70% lower levels compared to wild-type.
• Mutant GPIHBP1 predominantly existed as a monomer in mouse tissues.

Conclusions:

• Mutation of a conserved cysteine in GPIHBP1 disrupts LPL binding, leading to LPL mislocalization and severe chylomicronemia.
• The p.C63Y mutation reduces, but does not eliminate, GPIHBP1 surface expression on endothelial cells in vivo.
• In vivo studies are essential for accurately modeling the effects of GPIHBP1 mutations on protein function and associated diseases.