Intercellular propagation of extracellular signal-regulated kinase activation revealed by in vivo imaging of mouse skin

• Published in: eLife Vol. 4; p. e05178
• Main Authors: Hiratsuka, Toru, Fujita, Yoshihisa, Naoki, Honda, Aoki, Kazuhiro, Kamioka, Yuji, Matsuda, Michiyuki
• Format: Journal Article
• Language: English
• Published: England eLife Sciences Publications Ltd 10.02.2015; eLife Sciences Publications, Ltd
• Subjects: Acetic acid; Animals; Biosensors; Cell Biology; Cell cycle; Cell Cycle - drug effects; Cell division; Cell Division - drug effects; Cell proliferation; Data analysis; Ear; Enzyme Activation - drug effects; Epidermal growth factor; Epidermis; Epidermis - drug effects; Epidermis - enzymology; ERK; Extracellular signal-regulated kinase; Extracellular Signal-Regulated MAP Kinases - metabolism; Extracellular Space - drug effects; Extracellular Space - enzymology; Fluorescence resonance energy transfer; Frequency dependence; Humans; Hypotheses; Imaging, Three-Dimensional; in vivo imaging; Ligands; Matrix Metalloproteinase Inhibitors - pharmacology; Mice, Transgenic; Microscopy; Propagation; Receptor, Epidermal Growth Factor - metabolism; Signal transduction; Single-Cell Analysis; Skin; Skin - enzymology; Spatial distribution; Tetradecanoylphorbol Acetate - pharmacology; Wound Healing - drug effects; Wounds; mouse; in vivo imaging; cell biology; cell cycle; ERK; epidermis
• DOI: 10.7554/eLife.05178
• ISSN: 2050-084X

Extracellular signal-regulated kinase (ERK) is a key effector of many growth signalling pathways. In this study, we visualise epidermal ERK activity in living mice using an ERK FRET biosensor. Under steady-state conditions, the epidermis occasionally revealed bursts of ERK activation patterns where ERK activity radially propagated from cell to cell. The frequency of this spatial propagation of radial ERK activity distribution (SPREAD) correlated with the rate of epidermal cell division. SPREADs and proliferation were stimulated by 12-O-tetradecanoylphorbol 13-acetate (TPA) in a manner dependent on EGF receptors and their cognate ligands. At the wounded skin, ERK activation propagated as trigger wave in parallel to the wound edge, suggesting that ERK activation propagation can be superimposed. Furthermore, by visualising the cell cycle, we found that SPREADs were associated with G2/M cell cycle progression. Our results provide new insights into how cell proliferation and transient ERK activity are synchronised in a living tissue. Our skin is our largest organ; it provides a barrier that protects the underlying tissues and internal organs from the external environment and acts as one of our first lines of defense against infection. Both of these roles subject the skin to wear and tear and so it must constantly create new skin cells to replace those lost or damaged. However, if this renewal process goes awry it can lead to excessive cell growth or skin cancer. To avoid this, cells tightly regulate the pathways that stimulate skin renewal. Skin renewal involves growth signals activating an enzyme called ERK. When and where the ERK enzyme is activated is normally tightly regulated, and many kinds of cancer have been linked to ERK becoming active at the wrong time or in the wrong place. Despite the importance of ERK in skin cells, a number of technical challenges have made it difficult to study how these signals are passed from cell to cell. Hiratsuka et al. have now examined genetically altered mice that produce a fluorescent sensor molecule that makes it possible to see ERK activity in living skin cells. The skin of anesthetized mice was observed under a microscope, and time-lapse videos revealed occasional ‘firework-like’ bursts of ERK activity. At first the ERK enzyme was active in a small cluster of skin cells, then ERK activity was seen in the surrounding cells—appearing to spread outwards over the course of several minutes—before the activity stopped. Hiratsuka et al. named this pattern of activity a ‘Spatial Propagation of Radial ERK Activity Distribution’, or SPREAD for short. By studying SPREADs in the skin on the ears and the back of these mice, Hiratsuka et al. learned that these bursts of ERK activity coincided with skin cell growth; the bursts happened more frequently in the areas where the skin cells were dividing. Applying a chemical that stimulates cell division to the skin of the mice triggered more bursts of ERK activity; whereas fewer bursts were observed if Hiratsuka et al. used other chemicals to block the activity of some of the signaling proteins that work upstream of ERK. Further experiments suggested that SPREADs encourage cells to progress through the cycle of events that leads a cell to divide; blocking these bursts caused the cell to pause at the stage just before it would normally divide. Hiratsuka et al. also observed similar patterns of ERK activity moving out like waves from the edges of skin wounds. Further research using similar methods will reveal how growth signals are triggered and propagated in healthy and diseased tissues, not only in the skin but also other organs such as the liver, intestine, and muscles.