Aster Proteins Regulate the Accessible Cholesterol Pool in the Plasma Membrane.

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• Mammalian plasma membranes contain a distinct accessible cholesterol pool.
• Excess plasma membrane cholesterol moves to the endoplasmic reticulum, regulating the SREBP2 pathway.
• The Aster/Gramd1 family mediates nonvesicular cholesterol transport from plasma membrane to endoplasmic reticulum.

Purpose of the Study:

• Investigate the role of Aster/Gramd1 transporters in regulating accessible cholesterol pools.
• Determine the impact of Aster deficiency on cholesterol transport and SREBP2 pathway activity.
• Characterize the molecular interactions of Aster proteins with membranes.

Main Methods:

• Utilized nanoscale secondary ion mass spectrometry (NanoSIMS) to analyze accessible cholesterol.
• Examined cholesterol-loaded fibroblasts with Aster-A knockdown and Aster-deficient macrophages.
• Performed cosedimentation experiments with liposomes to study Aster-B GRAM domain binding.

Main Results:

• Aster deficiency led to an expanded accessible cholesterol pool in plasma membranes.
• Reduced cholesterol transport to the endoplasmic reticulum was observed in Aster-deficient cells.
• Increased expression of SREBP2-regulated genes indicated impaired cholesterol homeostasis.
• The Aster-B GRAM domain binds cholesterol in a concentration-dependent manner, enhanced by phosphatidylserine.

Conclusions:

• The Aster-mediated pathway is crucial for controlling accessible plasma membrane cholesterol levels.
• Aster transporters regulate the activity of the SREBP pathway by managing cholesterol flux.
• Understanding Aster function provides insights into cellular cholesterol homeostasis and related diseases.