FGF/ERK (fibroblast growth factor/extracellular signal-regulated kinase) signaling is crucial for the patterning and proliferation of cell lineages that comprise the mouse blastocyst. It plays a central role for the coordinated cell fate specification and tissue growth across the embryo before implantation in the maternal uterus. HprtERK-KTR (HprtCAG:ERK-KTR-mClover) targeted mutant mice express a rapidly responsive ERK-kinase translocation reporter (KTR) from a CAG promoter, enabling bright, live imaging and ratiometric quantification of ERK signaling activity at single cell resolution. ERK-KTR consists of a docking site (a fragment of human ELK1, an ERK substrate), nuclear localization (NLS) and export (NES) signals, and mClover green/yellow fluorescent protein. When the ERK signaling pathway is inactive, the ERK-KTR sensor is unphosphorylated and nuclear import predominates, resulting in nuclear enrichment/accumulation of mClover fluorescence. Conversely, when the ERK signaling pathway is active, activated p-ERK phosphorylates the sensor and nuclear export predominates, resulting in cytoplasmic enrichment. The cytoplasmic to nuclear ratio of ERK-KTR serves as proxy for ERK activity. Expression of the reporter is faithful to FGF/ERK signaling activity in cells of the mouse preimplantation embryo. 3D time-lapse imaging of this biosensor in embryos reveals spatially graded ERK activity in the trophectoderm prior to overt polar-vs-mural differentiation. Within the inner cell mass (ICM), all cells relay FGF/ERK signals with varying durations and magnitude. Primitive endoderm cells display higher overall levels of ERK activity, while pluripotent epiblast cells exhibit lower basal activity with sporadic pulses. An inducible, ERK-KTR-LSL line is available as Stock No. <a href="https://www.jax.org/strain/035566">035566</a>. That line contains a vector randomly inserted into a non-coding region of chromosome 13 and also contains the H2B/Ruby fusion protein nuclear marker.

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HprtERK-KTR (HprtCAG:ERK-KTR-mClover) targeted mutant mice express a rapidly responsive extracellular signal-regulated kinase (ERK)-kinase translocation reporter (KTR) that enables live imaging and ratiometric quantification of ERK signaling activity at single cell resolution.

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