Fibroblast growth factor 6 regulates sizing of the muscle stem cell pool.

Skeletal muscle stem cells, i.e., satellite cells (SCs), are the essential source of new myonuclei for skeletal muscle regeneration following injury or chronic degenerative myopathies. Both SC number and regenerative capacity diminish during aging. However, molecular regulators that govern sizing of the initial SC pool are unknown. We demonstrate that fibroblast growth factor 6 (FGF6) is critical for SC pool scaling. Mice lacking FGF6 have reduced SCs of early postnatal origin and impaired regeneration. By contrast, increasing FGF6 during the early postnatal period is sufficient for SC expansion. Together, these data support that FGF6 is necessary and sufficient to modulate SC numbers during a critical postnatal period to establish the quiescent adult muscle stem cell pool. Our work highlights postnatal development as a time window receptive for scaling a somatic stem cell population via growth factor signaling, which might be relevant for designing new biomedical strategies to enhance tissue regeneration.

Correction: Generation and selection of pluripotent stem cells for robust differentiation to insulin-secreting cells capable of reversing diabetes in rodents.

[This corrects the article DOI: 10.1371/journal.pone.0203126.].

Generation and selection of pluripotent stem cells for robust differentiation to insulin-secreting cells capable of reversing diabetes in rodents.

Induced pluripotent stem cell (iPSC) technology enables the creation and selection of pluripotent cells with specific genetic traits. This report describes a pluripotent cell line created specifically to form replacement pancreatic cells as a therapy for insulin-dependent diabetes. Beginning with primary pancreatic tissue acquired through organ donation, cells were isolated, re-programmed using non-integrating vectors and exposed to a four day differentiation protocol to generate definitive endoderm, a developmental precursor to pancreas. The best performing iPSC lines were then subjected to a 12-day basic differentiation protocol to generate endocrine pancreas precursors. The line that most consistently generated highly pure populations was selected for further development. This approach created an iPSC-variant cell line, SR1423, with a genetic profile correlated with preferential differentiation toward endodermal lineage at the loss of mesodermal potential. This report further describes an improved differentiation protocol that, coupled with SR1423, generated populations of greater than 60% insulin-expressing cells that secrete insulin in response to glucose and are capable of reversing diabetes in rodents. Created and banked following cGMP guidelines, SR1423 is a candidate cell line for the production of insulin-producing cells useful for the treatment of diabetes.

Cells lacking Pcp1p/Ugo2p, a rhomboid-like protease required for Mgm1p processing, lose mtDNA and mitochondrial structure in a Dnm1p-dependent manner, but remain competent for mitochondrial fusion.

The dynamin-related GTPase, Mgm1p, is critical for the fusion of the mitochondrial outer membrane, maintenance of mitochondrial DNA (mtDNA), formation of normal inner membrane structures, and inheritance of mitochondria. Although there are two forms of Mgm1p, 100 and 90 kDa, their respective functions and the mechanism by which these two forms are produced are not clear. We previously isolated ugo2 mutants in a genetic screen to identify components involved in mitochondrial fusion [J. Cell Biol. 152 (2001) 1123]. In this paper, we show that ugo2 mutants are defective in PCP1, a gene encoding a rhomboid-related serine protease. Cells lacking Pcp1p are defective in the processing of Mgm1p and produce only the larger (100 kDa) form of Mgm1p. Similar to mgm1delta cells, pcp1delta cells contain partially fragmented mitochondria, instead of the long tubular branched mitochondria of wild-type cells. In addition, pcp1delta cells, like mgm1delta cells, lack mtDNA and therefore are unable to grow on nonfermentable medium. Mutations in the catalytic domain lead to complete loss of Pcp1p function. Similar to mgm1delta cells, the fragmentation of mitochondria and loss of mtDNA of pcp1delta cells were rescued when mitochondrial division was blocked by inactivating Dnm1p, a dynamin-related GTPase. Surprisingly, in contrast to mgm1delta cells, which are completely defective in mitochondrial fusion, pcp1delta cells can fuse their mitochondria after yeast cell mating. Our study demonstrates that Pcp1p is required for the processing of Mgm1p and controls normal mitochondrial shape and mtDNA maintenance by producing the 90 kDa form of Mgm1p. However, the processing of Mgm1p is not strictly required for mitochondrial fusion, indicating that the 100 kDa form is sufficient to promote fusion.

Mgm1p, a dynamin-related GTPase, is essential for fusion of the mitochondrial outer membrane.

In Saccharomyces cerevisiae, mitochondrial fusion requires at least two outer membrane proteins, Fzo1p and Ugo1p. We provide direct evidence that the dynamin-related Mgm1 protein is also required for mitochondrial fusion. Like fzo1 and ugo1 mutants, cells disrupted for the MGM1 gene contain numerous mitochondrial fragments instead of the few long, tubular organelles seen in wild-type cells. Fragmentation of mitochondria in mgm1 mutants is rescued by disrupting DNM1, a gene required for mitochondrial division. In zygotes formed by mating mgm1 mutants, mitochondria do not fuse and mix their contents. Introducing mutations in the GTPase domain of Mgm1p completely block mitochondrial fusion. Furthermore, we show that mgm1 mutants fail to fuse both their mitochondrial outer and inner membranes. Electron microscopy demonstrates that although mgm1 mutants display aberrant mitochondrial inner membrane cristae, mgm1 dnm1 double mutants restore normal inner membrane structures. However, mgm1 dnm1 mutants remain defective in mitochondrial fusion, indicating that mitochondrial fusion requires Mgm1p regardless of the morphology of mitochondria. Finally, we find that Mgm1p, Fzo1p, and Ugo1p physically interact in the mitochondrial outer membrane. Our results raise the possibility that Mgm1p regulates fusion of the mitochondrial outer membrane through its interactions with Fzo1p and Ugo1p.