Bone-marrow-derived cells can contribute nuclei to skeletal muscle fibers. However, serial sectioning of muscle in mdx mice implanted with GFP-labeled bone marrow reveals that only 20% of the donor nuclei chronically incorporated in muscle fibers show dystrophin (or GFP) expression, which is still higher than the expected frequency of ''revertant'' fibers, but there is no overall increase above controls over time. Obviously, the vast majority of incorporated nuclei either never or only temporarily turn on myogenic genes; also, incorporated nuclei eventually loose the activation of the ␤-actin::GFP transgene. Consequently, we attempted to enhance the expression of dystrophin. In vivo application of the chromatinmodifying agents 5-azadeoxycytidine and phenylbutyrate as well as local damage by cardiotoxin injections caused a small increase in dystrophin-positive fibers without abolishing the appearance of ''silent'' nuclei. The results thus confirm that endogenous repair processes and epigenetic modifications on a small-scale lead to dystrophin expression from donor nuclei. Both effects, however, remain below functionally significant levels.bone marrow transplantation ͉ dystrophin ͉ chromatin-modifying agents

Human skeletal muscle stem cells from healthy donors aged 2-82 years (n = 13) and from three children suffering from Duchenne Muscular Dystrophy (DMD) were implanted into soleus muscles of immunoincompetent mice and were also expanded in vitro until senescence. Growth of implanted cells was quantified by structural features and by the amount of human DNA present in a muscle. Proliferative capacity in vitro and in vivo was inversely related to age of the donor. In vitro, a decline of about two mean population doublings (MPDs) per 10 years of donor's age was observed. Muscle stem cells from DMD children were prematurely aged. In general, cell preparations with low or decreasing content in desmin-positive cells produced more MPDs than age-matched high-desmin preparations and upon implantation more human DNA and more nonmyogenic than myogenic tissue. Thus, a "Desmin Factor" was derived which predicts "quality" of the human muscle tissue growing in vivo. This factor may serve as a prognostic tool.

The heterotrophic unicellular alga Prototheca wickerhamii is closely related to the photoautotrophic Chlorella vulgaris but has a 54,100-bp plastid DNA (ptDNA) that is much smaller than the chloroplast DNA of C. vulgaris (150,613 bp). The nucleotide sequence of 28,093 bp of the Prototheca ptDNA has been determined. No genes for photosynthetic functions have been found, except for sequences encoding six subunits of the ATP synthase ( atpA, atpB, atpE, atpF, atpH, and atpI). Transcripts of these atp genes have also been detected. Whether the leucoplasts of Prototheca contain a functional ATP synthase has still to be elucidated. Identified genes further include tufA, minD, cysT, and genes coding for three rRNAs, 22 tRNAs, and 12 ribosomal proteins. The results support the idea that, in the reduced plastid genome of Prototheca, genes coding for components of the plastid translational apparatus have been preferentially retained, and might be needed for the expression of the atp genes and some unassigned ORFs.

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The proliferative capacity of organotypic muscle stem cells, the satellite cells, from nine healthy human donors aged between 2 and 78 years was investigated. There was a loss in proliferative capacity with age, but the oldest donors (76, 78 years) would still be able to replace their musculature several times. Depending on frequency of desmin-positive (i.e., myogenic) cells during prolonged expansion, myoblast cultures could be designated as stable or unstable. There was a weak correlation between mean telomere lengths (estimated by flow-FISH) and remaining mean population doublings until senescence. A bimodal distribution of mean telomere lengths was apparent in both stable and unstable myoblast cultures and could be assigned to populations of differently sized cells. Furthermore, due to the presence of nonmyogenic cells with longer telomeres, unstable cultures tended to show an increasing rather than decreasing mean telomeric length on expansion. Bimodal distributions in myoblast cultures could be due to hitherto undefined myoblast populations.