Cell volume regulates terminal differentiation of cultured human epidermal keratinocytes
Differentiation of cultured human epidermal stem cells is regulated by interactions with the underlying substrate. Whereas differentiation is typically stimulated when keratinocytes are prevented from spreading, we previously identified two micron-scale topographical substrates that regulate differentiation of spread cells. On one substrate (S1), individual cells interact with small circular topographies, and differentiation is stimulated; on the other (S2), cells interact with larger triangular topographies, and differentiation is inhibited. By scanning electron microscopy we visualised substrate interactions at higher resolution than previously and using live cell imaging we established that induction of the differentiation marker involucrin did not involve transient cell rounding on S1. Bulk gene expression profiling did not reveal any differences between cells on S1 and S2 prior to the selective upregulation of differentiation markers at 12h on S1 and cell stiffness was lower on both S1 and S2 than on flat substrates. Nevertheless, cells on S2 differed from cells on flat and S1 substrates because they exhibited reduced cell volume, prompting us to explore whether cell volume could regulate differentiation independent of culture substrate. Treatment with polyethylene glycol (PEG) reduced cell volume and inhibited differentiation regardless of whether keratinocytes were seeded on flat, S1 or S2 substrates, micropatterned islands or in suspension. Conversely, treatment with deionised water increased cell volume and stimulated differentiation of substrate adherent keratinocytes. On flat substrates treatment with the Ca2+ chelator 1,2-bis-(2-aminophenoxy)ethane-N,N,N,N-tetraacetic acid acetoxymethyl ester or an inhibitor of the water channel aquaporin 3 blocked induction of differentiaton by deionised water, whereas the gadolinium3+, a stretch-activated calcium channel blocker, did not. Our studies identify a new mechanism by which keratinocyte-niche interactions regulate initiation of differentiation.