Foxo1 nucleo-cytoplasmic distribution and unidirectional nuclear influx are the same in nuclei in a single skeletal muscle fiber but vary between fibers

Foxo transcription factors promote protein breakdown and atrophy of skeletal muscle fibers. Foxo transcriptional effectiveness is largely determined by phosphorylation-dependent nucleo-cytoplasmic shuttling. Imaging Foxo1-green fluorescent protein (GFP) over time in 124 nuclei in 68 multinucleated adult skeletal muscle fibers under control culture conditions reveals large variability between fibers in Foxo1-GFP nucleo-cytoplasmic concentration ratio (N/C) and in the apparent rate coefficient ( kI′) for Foxo1-GFP unidirectional nuclear influx (measured with efflux blocked by leptomycin B). Pairs of values of N/C or of kI′ from different nuclei in the same fiber were essentially the same, but only weakly correlated in nuclei from different fibers in the same culture well. Thus, fiber to fiber variability of cellular factors, but not extracellular factors, determines Foxo1 distribution. Over all nuclei, N/C and kI′ were closely proportional, indicating that kI′ is the major determinant of Foxo1 distribution. IGF-I activation of Foxo kinase Akt reduces variability by decreasing kI′ and N/C in all fibers. However, inhibiting Akt did not drive kI′ uniformly high, indicating other pathways in Foxo1 regulation.