Visualization of filamentous actin (F-actin) in living cells is critical for the study of morphogenetic processes such as cell division, migration or polarization. We recently described in these pages the 17-amino-acid Lifeact peptide, which can be used to specifically label F-actin without adverse effects on actin dynamics 1. Although this marker has since been used in many cell types and experimental models it is still of limited utility when studying primary cells or when looking at tissues and whole animals.

To address this limitation for mammalian systems, we generated transgenic mice that express Lifeact fused to mRFPruby 2 or EGFP. Expression of fusion proteins was driven by a chicken actin promoter under the influence of the CMV enhancer to ensure strong and ubiquitous expression 3 (Supplementary Fig. 1 and Supplementary Methods). Transgenic mice were viable, fertile and phenotypically normal, confirming our previous findings that Lifeact expression does not interfere with cellular processes. For additional characterization, we selected transgenic lines with the most ubiquitous expression patterns. In these lines, which we called Lifeact mice, we could detect strong fluorescence throughout embryogenesis and in six organs (Fig. 1a and Supplementary Figs. 2, 3). To characterize Lifeact-fusion expression, we counterstained cryosections of 14 organs with fluorescent phalloidin. Staining in all samples perfectly overlapped with Lifeact fluorescence (Fig. 1b, c and Supplementary Figs. 4, 5). Flow-cytometric analysis of hematopoietic cells revealed staining of all cells, except for erythrocytes and their precursors (Fig. 1d–g and Supplementary Fig. 6). We then analyzed actin organization of different cell types in more detail. Total internal reflection microscopy (TIRFM) of skin fibroblasts, neurons, platelets and T cells revealed evenly distributed expression of Lifeact and labeling of typical actin structures such as lamellipodia, filopodia or podosomes (Fig. 1h–k, and Supplementary Video 1).