Free cholesterol determines reassembled high-density lipoprotein phospholipid phase structure and stability

Area of Science:

• Lipid Metabolism
• Protein Biochemistry
• Structural Biology

Background:

• High-density lipoproteins (HDL) play crucial roles in reverse cholesterol transport.
• The composition and structure of HDL, particularly reassembled HDL (rHDL), influence their function.
• Understanding the interactions between apolipoproteins, lipids, and cholesterol is key to HDL metabolism.

Purpose of the Study:

• To characterize the stability and structural properties of rHDL with varying free cholesterol (FC) content.
• To investigate the influence of different apolipoproteins (apo A-I vs. apo A-II) on rHDL structure and stability.
• To elucidate the phase behavior of rHDL components and their relationship to size and composition.

Main Methods:

• Isolation and analysis of reassembled HDL (rHDL) species with defined protein and lipid compositions.
• Differential scanning calorimetry (DSC) to determine thermal stability and phase transitions.
• Circular dichroism (CD) to assess the helical content and its temperature dependence.

Main Results:

• rHDL species exhibit quantized increases in size with increasing FC mole percent.
• DSC reveals distinct phases: virtual DMPC, boundary phase, and mixed FC/DMPC phase.
• FC stabilizes rHDL, with apo A-II conferring greater stability than apo A-I despite fewer helical regions.

Conclusions:

• FC content and apolipoprotein type significantly modulate rHDL structure, stability, and phase behavior.
• Apo A-II's higher lipophilicity enhances rHDL stability by strengthening hydrophobic interactions.
• These findings provide insights into the differential roles of FC and apolipoproteins in HDL assembly and remodeling.