Folate receptors targeted to clathrin-coated pits cannot regulate vitamin uptake

Area of Science:

• Cell biology
• Molecular biology
• Endocytosis mechanisms

Background:

• Potocytosis is an endocytic pathway for small molecule internalization.
• Folate receptor-mediated uptake of 5-methyltetrahydrofolate (5-MTHF) has been linked to caveolae.
• The glycosyl-phosphatidylinositol (GPI) anchor of the folate receptor may mediate clustering and caveolar internalization.

Purpose of the Study:

• To directly test the hypothesis that the GPI anchor directs folate receptor internalization via caveolae.
• To investigate the role of caveolae versus clathrin-coated pits in 5-MTHF uptake and cytoplasmic delivery.

Main Methods:

• Construction of a chimeric folate receptor replacing the GPI anchor with the low-density lipoprotein receptor's transmembrane domain and cytoplasmic tail.
• Comparison of 5-MTHF uptake and cytoplasmic delivery in cells expressing wild-type versus chimeric folate receptors.
• Assessment of vitamin uptake regulation in cells utilizing clathrin-coated pits.

Main Results:

• Cells expressing the chimeric receptor showed slower 5-MTHF delivery to the cytoplasm compared to cells with wild-type receptors.
• Efficient cytoplasmic delivery of 5-MTHF appears dependent on caveolae-mediated endocytosis.
• Cells internalizing folate via clathrin-coated pits failed to regulate vitamin uptake when folate-replete or confluent.

Conclusions:

• Caveolae are crucial for the efficient cytoplasmic delivery of 5-methyltetrahydrofolate mediated by the folate receptor.
• The GPI anchor of the folate receptor plays a key role in directing its internalization via caveolae.
• Clathrin-coated pit-mediated endocytosis of folate is not subject to the same regulatory mechanisms as caveolae-mediated uptake.