Cell-projection pumping: A hydrodynamic cell-stiffness dependent mechanism for cytoplasmic transfer between mammalian cells

We earlier reported cytoplasmic fluorescence exchange between cultured human fibroblasts and malignant cells (MC). Current time-lapse microscopy showed most exchange was from fibroblasts into MC, with less in the reverse direction. We were surprised transfer was not via tunneling nanotubes as described by others, but was instead via fine and often branching cell projections that defied direct visual resolution because of their size and rapid movement. Their structure was revealed nonetheless, by their organellar cargo and the grooves they formed indenting MC, while transfer events coincided with rapid retraction of the cell-projections. This suggested a hydrodynamic mechanism that could be tested by mathematical modelling. Increased hydrodynamic pressure in retracting cell-projections, normally returns cytoplasm to the cell body. We hypothesize 9cell-projection pumping9 (CPP), where cytoplasm in retracting cell-projections partially equilibrates into adjacent recipient cells via temporary inter-cellular cytoplasmic continuities. Plausibility for CPP was explored via a mathematical model, which predicted preferential CPP into cells with lower cell stiffness, since pressure equilibrates towards least resistance. Predictions from the model were satisfied when fibroblasts were co-cultured with MC, and fluorescence exchange related with cell stiffness measured by atomic force microscopy. When transfer into 5000 simulated recipient MC or fibroblasts was studied in computer simulations, inputting experimental cell stiffness and donor cell fluorescence values generated transfers to simulated recipient cells similar to those seen by experiment, including an expected inverse relationship between receptor cell stiffness and fluorescence uptake. We propose CPP as a novel mechanism in mammalian inter-cellular cytoplasmic transfer and communication.