We are interested in the process of macropinocytosis, a mechanism by which cells can engulf large volumes of their surrounding fluid.

Cells need to ingest fluid from their environment for many reasons. The most fundamental is to feed, providing nutrients to sustain growth, but in multicellular organisms it is also critical for immune cells to continuously sample their environment, in order to detect foreign bodies. Taking in large quantities of extracellular fluid is however not without its risks, and many harmful viruses and bacteria can also exploit macropinocytosis as a way to enter host cells.

A major focus of the lab is to understand how macropinosomes form. This requires complex rearrangement of the cytoskeleton, in order to generate a cup-shaped projection that can efficiently entrap and internalise fluid.

Macropinocytic cup formation in Dictyostelium cells visualised using a PI(3,4,5)P3 reporter 

Despite their important roles in both normal cell behaviour and disease, surprisingly little is known about what happens to macropinosomes inside the cell. After the macropinosome has formed, the cell is left with a large bag of extracellular liquid (with whatever it contains) enclosed by a membrane. The cell then has the challenge of breaking this into smaller vesicles and concentrating the contents so the macropinosome can integrate with other cellular compartments such as the digestive machinery and recover any nutrients.

Visualisation of macropinosome dynamics after engulfment, using a probe for PI(3)P, which is present on the vesicles for 10mins post-internalisation.

Another problem the cell faces is that the membrane surrounding the macropinosome comes from the cell surface, and contains all the surface proteins that come with it. To prevent these from being digested along with the macropinosome contents, it is crucial to rescue these proteins and shuttle them back to the plasma membrane. Without such a mechanism cells would not maintain enough surface proteins to function properly.

Recruitment of WASH (green) and the vacuolar ATPase (red) to macropinosomes immediately following internalisation, driving cell surface protein recycling and acidification respectively

The need to remodel vesicles and remove selective proteins from their surface is not unique to macropinocytosis. We want to understand the basic, fundamental principles by which cells manipulate vesicles. We therefore hope to ultimately extend our work to other cellular pathways such as the engulfment of bacteria and the capture and digestion of damaged intracellular components.

University of Sheffield