The skin is largely comprised of keratinocytes within the interfollicular epidermis. Over approximately two weeks these cells differentiate and traverse the thickness of the skin. The stage of differentiation is therefore reflected in the positions of cells within the tissue, providing a convenient axis along which to study the signaling events that occur in situ during keratinocyte terminal differentiation and over this extended two-week timescale.

We used this system to study the canonical ERK-MAPK signaling cascade (Raf-1, MEK-1/2 and ERK-1/2) which has been implicated in controlling diverse cellular behaviors, including proliferation and differentiation. While the molecular interactions involved in signal transduction through this cascade have been well characterized in cell culture experiments, our understanding of how this sequence of events unfolds to determine cell fate within a homeostatic tissue environment has not been fully characterized.

Thus, we measured the abundance of total and phosphorylated ERK-MAPK signaling proteins within interfollicular keratinocytes in transverse cross-sections of human epidermis using immunofluorescence microscopy. These data show coordinated variation in the abundance of phosphorylated ERK-MAPK components across the epidermis. To investigate these data further, we developed a mathematical model of the signaling cascade using a normalized-Hill differential equation formalism.

The model demonstrates that the spatial profile of activation for ERK-MAPK signaling components across the epidermis may be maintained in a cell-autonomous fashion by an underlying spatial gradient in calcium signaling. The approaches developed for this work may be useful for studying intracellular signaling within other tissue systems.