Diffusion of low density lipoprotein-receptor complex on human fibroblasts

Area of Science:

• Cell Biology
• Biophysics

Background:

• Low-density lipoprotein (LDL) receptor-mediated endocytosis is crucial for cholesterol homeostasis.
• Understanding the mobility of the LDL-receptor complex on the cell surface is key to elucidating its internalization mechanism.

Purpose of the Study:

• To quantify the diffusion of the LDL-receptor complex on human fibroblasts.
• To investigate the effect of temperature and cell membrane properties on LDL-receptor diffusion.
• To determine if impaired LDL-receptor diffusion contributes to internalization defects.

Main Methods:

• Utilized a fluorescently labeled LDL derivative (dil(3)-LDL) for live-cell imaging.
• Employed fluorescence photobleaching recovery and direct observation of Brownian motion.
• Assessed membrane fluidity with a lipid analog (NBD-PC) and cytoskeletal components (F-actin) via NBD-phallacidin staining.

Main Results:

• LDL-receptor complex diffusion was significantly temperature-dependent, increasing from <20% at 10°C to ~75% at 21°C.
• Diffusion coefficients on membrane blebs (approx. 10(-9) cm(2)/s) were much higher than on native cell surfaces (approx. two orders of magnitude slower).
• Loss of F-actin cytoskeleton correlated with increased diffusion on blebs, suggesting release from lateral constraints.

Conclusions:

• The LDL-receptor complex exhibits restricted diffusion on native fibroblast membranes due to cytoskeletal interactions.
• Impaired LDL-receptor diffusion does not explain the internalization defect in J.D. cells.
• While diffusion is enhanced on blebs, it remains significantly slower than theoretical limits on native membranes, indicating lateral constraints on LDL-receptor mobility.