Cells often eat solid particles, a process called phagocytosis. Researchers often use synthetic nanoparticles to understand the intricate details of this process, but these do not always resemble the foods that cells feast upon in nature. Now, researchers have engineered biological particles of controlled size and shape that mirror natural foods and combined these with high-resolution imaging to understand how cells deform to wrap their food. The findings could shed light on how cells take in nutrients and respond to foreign invaders such as bacteria.
When confronted with a particle to engulf, cells send out pseudopods (membrane extensions) to embrace it. Then they zip up the edges of the pseudopods to form a membrane-bound compartment that encloses the food particle.
To see this process in more detail, the team developed two types of synthetic particles that could be easily distinguished using a high-resolution microscopy technique known as 3D structured illumination microscopy. This technique enabled the researchers to not only visualize the membrane wrapping around the particles but also to quantify how much the membrane stretched to accommodate different sized and shaped particles. They found that spherical particles of the same size stretch the membrane more than elongated particles, likely because the curvature of the spherical particle requires more membrane for a complete wrap.
The team also observed another behavior that could provide clues to how cells discriminate between different types of food particles. Typically, cells use chemical receptors on their surface to identify particles targeted for phagocytosis. However, the team noticed that cells were more efficient at encircling the football-shaped particles over spherical particles, regardless of the presence of chemical signals that would normally trigger phagocytosis. This suggests that cells may use physical properties, such as shape, to target certain types of particles—information they could use to distinguish between edible molecules and potential threats like invading bacteria.
The researchers say their approach provides a new means of studying phagocytosis with more realistic particle shapes and sizes than were previously possible. They plan to use their methods to investigate how membrane-bound compartments traffic within cells and how they deliver their cargo.
