Rapid acidification of endocytic vesicles containing alpha 2-macroglobulin

Area of Science:

• Cell Biology
• Biochemistry

Background:

• Receptor-mediated endocytosis is a key cellular process for internalizing ligands.
• Understanding the dynamic pH changes within endocytic vesicles is crucial for elucidating cellular trafficking pathways.

Purpose of the Study:

• To quantify pH changes in the microenvironment of internalized ligands during receptor-mediated endocytosis.
• To determine the pH of endocytic vesicles prior to lysosomal fusion.

Main Methods:

• Utilized fluorescein-labeled alpha 2-macroglobulin (F-alpha 2M) as a pH indicator.
• Employed microscope spectrofluorometry to measure fluorescence intensities in single BALB/c 3T3 mouse fibroblasts.
• Determined pH from the ratio of fluorescein fluorescence intensities at specific excitation wavelengths (450 nm and 490 nm).
• Validated endosomal localization using colloidal gold-labeled alpha 2M and electron microscopy.

Main Results:

• The pH of the F-alpha 2M microenvironment in endocytic vesicles was measured at 5.0 +/- 0.2.
• Electron microscopy confirmed alpha 2M localization in uncoated vesicles negative for acid phosphatase.
• Lysosomal pH was determined to be 4.6 +/- 0.2 using fluorescein dextran.

Conclusions:

• Endocytic vesicles exhibit an acidic pH of approximately 5.0.
• This acidification occurs before the vesicles fuse with lysosomes.
• These findings provide insights into the early stages of the endocytic pathway and lysosomal biogenesis.