p38 MAPK-dependent shaping of the keratin cytoskeleton in cultured cells      Stefan Wöll, Reinhard Windoffer and Rudolf E. Leube J. Cell Biol., 2007,  177:795-807. Abstract Plasticity of the resilient keratin intermediate filament cytoskeleton is an important prerequisite for epithelial tissue homeostasis. Here, the contribution of stress activated p38 MAPK to keratin network organization was examined in cultured cells. It was observed that phosphorylated p38 co-localized with keratin granules that were rapidly formed in response to orthovanadate. The same p38p recruitment was noted during mitosis, in various stress situations and in cells producing mutant keratins. In all these situations keratin 8 became phosphorylated on S73, a well-known p38 target site. To demonstrate that p38-dependent keratin phosphorylation determines keratin organization, p38 activity was pharmacologically and genetically modulated: Upregulation induced keratin granule formation whereas downregulation prevented keratin filament network disassembly. Furthermore, transient p38 inhibition also inhibited keratin filament precursor formation and mutant keratin granule dissolution. Taken together, the rapid and reversible effects of p38 activity on keratin phosphorylation and organization in diverse physiological, stress and pathological situations identify p38-dependent signalling as a major intermediate filament-regulating pathway. video1 video 2 video 3 Inhibition of p38 activity prevents formation of nascent keratin granules. Time-lapse recording of HK13-EGFP fluorescence (left, inverse presentation) and corresponding phase contrasts with overlaid HK13-EGFP fluorescence (right) in AK13-1 cells prior to and during treatment with the p38 inhibitor SB202190 (100 μM; corresponding Fig. 10 A, A'). ../../images were recorded every 20 s (display rate: 29 frames/s). Note the retraction of the KF network except for desmosome-anchored filaments in response to the drug while dynamic filopodia are still abundant and ruffling continues Inhibition of p38 activity prevents formation of nascent keratin granules and disassembly of keratin granules in cells producing mutant keratins. Time-lapse recording of HK14R125C-EYFP fluorescence at left (inverse presentation) and corresponding phase contrast ../../images together with overlaid EYFP-K14R125C- fluorescence micrographs at right of MT5K14-26 cells before, during and after a short treatment with the p38 inhibitor SB202190 (100 μM; corresponding Fig. 10 B - B''). ../../images were recorded every 20 s and are displayed at 30 frames/s. Note the transient cessation of keratin granule formation and stabilization of pre-existing keratin granules upon drug administration. Inhibition of p38 activity stabilizes mutant keratin granules. Time-lapse recording (3 ../../images/ min; display rate 25 frames/s) of HK14R125C-EYFP fluorescence at left (inverse presentation) and corresponding phase contrast ../../images at right before and after addition of the p38 inhibitor SB202190 (50 μM). The upper right corners extend beyond the image fields. Note the immediate cessation of granule disassembly upon drug addition of the drug.