ERK basal and pulsatile activity are differentially regulated in mammalian epidermis to control proliferation and exit from the stem cell compartment

Fluctuation in signal transduction pathways is frequently observed during mammalian development. However, its role in regulating stem cells has not been explored. Here we tracked spatiotemporal ERK MAPK dynamics in human epidermal stem cells. While stem cells and differentiated cells were distinguished by high and low stable basal ERK activity, respectively, we also found cells with pulsatile ERK activity. Transitions from Basal-Pulse (stem) to Basal-Pulse, Basal-Pulse, and Basal-Pulse (differentiated) cells occurred in expanding keratinocyte colonies and in response to a range of differentiation stimuli. Pharmacological inhibition of ERK induced differentiation only when cells were in the Basal-Pulse state. Basal ERK activity and pulses were differentially regulated by DUSP10 and DUSP6, leading us to speculate that DUSP6-mediated ERK pulse downregulation promotes initiation of differentiation whereas DUSP10-mediated downregulation of mean ERK activity promotes and stabilizes post-commitment differentiation. Quantification of MAPK1/3, DUSP6 and DUSP10 transcripts in individual cells demonstrated that ERK activity is controlled both transcriptionally and post-transcriptionally. When cells were cultured on a topography that mimics the epidermal-dermal interface, spatial segregation of mean ERK activity and pulses was observed. In vivo imaging of mouse epidermis revealed a patterned distribution of basal cells with pulsatile ERK activity and downregulation was linked to the onset of differentiation. Our findings demonstrate that ERK MAPK signal fluctuations link kinase activity to stem cell dynamics.