A plasma membrane template for macropinocytic cups

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• Macropinocytosis is a cellular process for internalizing large volumes of extracellular fluid via actin-dependent plasma membrane invaginations.
• The precise molecular organization and regulation of the actin cytoskeleton during macropinocytosis remain incompletely understood.

Purpose of the Study:

• To investigate the spatial organization of signaling molecules and actin regulators at the plasma membrane during macropinocytosis in Dictyostelium.
• To elucidate the role of specific signaling pathways, such as Ras and Rac, in the formation and regulation of macropinocytic cups.

Main Methods:

• Live-cell imaging of fluorescently tagged proteins in Dictyostelium discoideum.
• Analysis of signaling molecule localization (PIP3, Ras, Rac) and actin dynamics (SCAR/WAVE, F-actin).
• Genetic manipulation, including mutations in RasGAP NF1, to probe signaling pathways.

Main Results:

• Macropinocytic cups are organized around co-localized patches of PIP3, active Ras, and active Rac.
• A ring of active SCAR/WAVE is consistently found at the periphery of these signaling patches and other related structures.
• Patch formation is independent of the F-actin ring, and Ras signaling plays an instructive role, as evidenced by enlarged patches in NF1 mutants.
• New macropinocytic cups frequently arise from the splitting of existing cups.

Conclusions:

• Plasma membrane cup structures, including those in macropinocytosis and phagocytosis, arise from self-organizing signaling patches (Ras/PIP3).
• These signaling patches recruit actin nucleators to their periphery, driving the formation of the characteristic cup shape.
• The study proposes a model where signaling patch dynamics, rather than solely actin polymerization, dictate the formation of cellular uptake structures.