Microenvironmental Regulation of Macropinocytosis Facilitates Extracellular Fluid Sampling

Cells sense chemical and mechanical signals within their local microenvironment. To sample for chemical signals, cells employ bulk extracellular fluid uptake through macropinocytosis in a receptor-independent manner. Although macropinocytosis occurs in microenvironments of multicellular organisms, it remains largely unknown whether and how the microenvironment itself regulates fluid sampling. Here, we reveal that the microenvironmental properties regulate fluid sampling by macropinocytosis. Using macrophages as constitutively sampling immune cells, we discover that macropinocytosis in three-dimensional (3D) microenvironments is more efficient than on flat 2D surfaces. Enhanced macropinocytosis in 3D is driven by low adhesiveness to the cellular substrate, releasing the mechanical linkage between the cell and its surroundings, thereby facilitating the formation of actin-based membrane protrusions that sample the local substrate-free space. Mechanistically, we identify that the microenvironment regulates the utilization of macropinocytosis-competent actin networks, as the actin nucleating Arp2/3 complex and its regulator Hem1 particularly drive macropinocytosis in 3D. Our results establish microenvironmental properties as stimulators and regulators of macropinocytosis, providing important implications for fluid sampling during physiology, immunity, and diseases. Additionally, our findings identify a reciprocal relationship between the microenvironment and fluid sampling, wherein cells acquire information from the microenvironment by macropinocytosis, while this process, in turn, is regulated by the microenvironment.