In skeletal muscle, the transcription factors Foxo1 and Foxo3A control expression of proteins that mediate muscle atrophy, making the nuclear concentration and nuclear-cytoplasmic movements of Foxo1 and Foxo3A of therapeutic interest in conditions of muscle wasting. Here, we use Foxo-GFP fusion proteins adenovirally expressed in cultured adult mouse skeletal muscle fibers to characterize the time course of nuclear efflux of Foxo1-GFP in response to activation of the insulin-like growth factor-1 (IGF-1)/phosphatidylinositol-3-kinase (PI3K)/Akt pathway to determine the time course of nuclear influx of Foxo1-GFP during inhibition of this pathway and to show that Akt mediates the efflux of nuclear Foxo1-GFP induced by IGF-1. Localization of endogenous Foxo1 in muscle fibers, as determined via immunocytochemistry, is consistent with that of Foxo1-GFP. Inhibition of the nuclear export carrier chromosome region maintenance 1 by leptomycin B (LMB) traps Foxo1 in the nucleus and results in a relatively rapid rate of Foxo1 nuclear accumulation, consistent with a high rate of nuclear-cytoplasmic shuttling of Foxo1 under control conditions before LMB application, with near balance of unidirectional influx and efflux. Expressed Foxo3A-GFP shuttles ∼20-fold more slowly than Foxo1-GFP. Our approach allows quantitative kinetic characterization of Foxo1 and Foxo3A nuclear-cytoplasmic movements in living muscle fibers under various experimental conditions.

Wimmer RJ, Liu Y, Schachter TN, Stonko DP, Peercy BE, Schneider MF. Mathematical modeling reveals modulation of both nuclear influx and efflux of Foxo1 by the IGF-I/PI3K/Akt pathway in skeletal muscle fibers. Am J Physiol Cell Physiol 306: C570-C584, 2014. First published January 22, 2014; doi:10.1152/ajpcell.00338.2013.-Foxo family transcription factors contribute to muscle atrophy by promoting transcription of the ubiquitin ligases muscle-specific RING finger protein and muscle atrophy F-box/atrogin-1. Foxo transcriptional effectiveness is largely determined by its nuclear-cytoplasmic distribution, with unphosphorylated Foxo1 transported into nuclei and phosphorylated Foxo1 transported out of nuclei. We expressed the fluorescent fusion protein Foxo1-green fluorescent protein (GFP) in cultured adult mouse flexor digitorum brevis muscle fibers and tracked the time course of the nuclear-to-cytoplasmic Foxo1-GFP mean pixel fluorescence ratio (N/C) in living fibers by confocal imaging. We previously showed that IGF-I, which activates the Foxo kinase Akt/PKB, caused a rapid marked decline in N/C, whereas inhibition of Akt caused a modest increase in N/C. Here we develop a two-state mathematical model for Foxo1 nuclear-cytoplasmic redistribution, where Foxo phosphorylation/dephosphorylation is assumed to be fast compared with nuclear influx and efflux. Cytoplasmic Foxo1-GFP mean pixel fluorescence is constant due to the much larger cytoplasmic than nuclear volume. Analysis of N/C time courses reveals that IGF-I strongly increased unidirectional nuclear efflux, indicating similarly increased fractional phosphorylation of Foxo1 within nuclei, and decreased unidirectional nuclear influx, indicating increased cytoplasmic fractional phosphorylation of Foxo1. Inhibition of Akt increased Foxo1 unidirectional nuclear influx, consistent with block of Foxo1 cytoplasmic phosphorylation, but did not decrease Foxo1 unidirectional nuclear efflux, indicating that Akt may not be involved in Foxo1 nuclear efflux under control conditions. New media change experiments show that cultured fibers release IGF-I-like factors, which maintain low nuclear Foxo1 in the medium. This study demonstrates the power of quantitative modeling of observed nuclear fluxes.Akt1; Foxo1; IGF-I; skeletal muscle TRANSCRIPTION OF A GIVEN GENE is controlled by numerous regulatory proteins, including transcription factors, which are primary activators of transcription, as well as numerous other activators, coactivators, repressors, and corepressors (12, 15a). These factors combine to form macromolecular transcriptional regulatory complexes assembled on the promoter regions of the gene in question (16). To participate in such regulatory complexes, the regulatory molecules must be resident in the nucleus. Thus an important aspect of transcriptional regulation is control of the nuclear-cytoplasmic distribution of these regulatory molecules, and the mechanisms controlling nuclearcytoplasmic movements of transcriptional regulatory molecules can thus be important dete...

Forkhead box O 1 (Foxo1) controls the expression of proteins that carry out processes leading to skeletal muscle atrophy, making Foxo1 of therapeutic interest in conditions of muscle wasting. The transcription of Foxo1-regulated proteins is dependent on the translocation of Foxo1 to the nucleus, which can be repressed by insulin-like growth factor-1 (IGF-1) treatment. The role of Foxo1 in muscle atrophy has been explored at length, but whether Foxo1 nuclear activity affects skeletal muscle excitation-contraction (EC) coupling has not yet been examined. Here, we use cultured adult mouse skeletal muscle fibers to investigate the effects of Foxo1 overexpression on EC coupling. Fibers expressing Foxo1-green fluorescent protein (GFP) exhibit an inability to contract, impaired propagation of action potentials, and ablation of calcium transients in response to electrical stimulation compared with fibers expressing GFP alone. Evaluation of the transverse (T)-tubule system morphology, the membranous system involved in the radial propagation of the action potential, revealed an intact T-tubule network in fibers overexpressing Foxo1-GFP. Interestingly, long-term IGF-1 treatment of Foxo1-GFP fibers, which maintains Foxo1-GFP outside the nucleus, prevented the loss of normal calcium transients, indicating that Foxo1 translocation and the atrogenes it regulates affect the expression of proteins involved in the generation and/or propagation of action potentials. A reduction in the sodium channel Nav1.4 expression in fibers overexpressing Foxo1-GFP was also observed in the absence of IGF-1. We conclude that increased nuclear activity of Foxo1 prevents the normal muscle responses to electrical stimulation and that this indicates a novel capability of Foxo1 to disable the functional activity of skeletal muscle.

No abstract

The transcription factor Foxo1 is integral to the regulation of expression of proteins which promote muscle atrophy. Phosphorylation of Foxo1 causes its translocation to the cytoplasm and thus prevents Foxo1‐DNA binding and consequent transcription of genes that cause muscle atrophy and cell death. Thus, phosphorylation of Foxo1 leads to cell survival and muscle hypertrophy. Maintenance of Foxo1 phosphorylation and its resulting cytoplasmic retention could be used to suppress muscle atrophy and thereby shift the atrophy/hypertrophy balance in favor of hypertrophy. Akt and serum‐ and glucocorticoid‐inducible kinase (SGK) are important regulators of the phosphorylation status of Foxo1. These pathways have been well characterized and the effects on Foxo1 localization have been reported. However, the mechanisms which regulate nuclear influx and nuclear efflux have not been separately evaluated. Here, we determine the effects of Akt kinase activity specifically on nuclear influx of Foxo1. To accomplish th/is goal, we quantified nuclear and cytoplasmic levels of adenovirally expressed Foxo1‐GFP in cultured flexor digitorum brevis. The nuclear influx during treatment with kinase inhibitor Akt IV alone and in combination with the nuclear efflux inhibitor lemptomycin B provides insight into the activity of Akt as a Foxo1 kinase. Surprisingly, Akt inhibition reveals Akt to have little effect on the rate of nuclear influx of Foxo1. To evaluate translocation of endogenous Foxo1 in a similar manner immunocytochemistry and western blotting techniques were used. These results indicate that Akt either phosphorylates Foxo1 primarily in the nucleus and not in the cytoplasm, or that there is a mechanism for cytoplasmic phosphorylation of Foxo1 other than via Akt. Supported by training grants T32 AR0075592 and T32 HL072751 and research grant R01 AR056477.