Megapinocytosis: a novel endocytic pathway

Area of Science:

• Cell Biology
• Immunology
• Macrophage Biology

Background:

• Macrophages are key immune cells with diverse functions.
• M2 macrophages are involved in tissue repair and immune modulation.
• Endocytosis is a crucial cellular process for nutrient uptake and signaling.

Purpose of the Study:

• To characterize novel large endocytotic structures in M2 macrophages.
• To investigate the formation and ultrastructure of these structures.
• To determine if these structures are associated with specific stimuli like human cytomegalovirus.

Main Methods:

• Scanning electron microscopy (SEM) for surface morphology.
• Transmission electron microscopy (TEM) with high-pressure freezing for ultrastructure.
• Comparative analysis between M1 and M2 macrophages.

Main Results:

• M2 macrophages form large endocytotic structures named megapinosomes (several micrometers in diameter).
• Megapinosome formation involves plasma membrane invagination and closure by microvilli-like structures.
• Ultrastructural analysis revealed a trabecular meshwork within megapinosomes, topologically linked to the cytosol.
• Megapinosome formation increased upon human cytomegalovirus inoculation in M2 macrophages.
• Megapinosomes were not observed in M1 macrophages.

Conclusions:

• Megapinosomes represent a distinct form of endocytosis in M2 macrophages.
• Megapinosome formation is specific to M2 macrophages and can be induced by human cytomegalovirus.
• These findings offer new insights into M2 macrophage endocytic mechanisms and their role in immune responses.