Hox genes regulate muscle founder cell pattern autonomously and regulate morphogenesis through motor neurons.

The differentiation of myoblasts to form functional muscle fibers is a consequence of interactions between the mesoderm and ectoderm. The authors examine the role of segment identity in directing these interactions by studying the role of Hox genes in patterning adult muscles in Drosophila. Using the 'four-winged fly' to remove Ultrabithorax function in the developing adult, the authors alter the identity of the ectoderm of the third thoracic segment towards the second and show that this is sufficient to inductively alter most properties of the mesoderm-myoblast number, molecular diversity, and migration pattern-to that of the second thoracic segment. Not all aspects of myogenesis are determined by the segment identity of the ectoderm. The autonomous identity of the mesoderm is important for choosing muscle founder cells in the correct segmental pattern. The authors show this by removal of the function of Antennapedia, the Hox gene expressed in the mesoderm of the third thoracic segment. This results in the transformation of founder cells to a second-thoracic pattern. The authors also report a role for the nervous system in later aspects of muscle morphogenesis by specifically altering Ultrabithorax gene expression in motor neurons. Thus, ectoderm and mesoderm segment identities collaborate to direct muscle differentiation by affecting distinct aspects of the process.

Drosophila Heartless acts with Heartbroken/Dof in muscle founder differentiation.

The formation of a multi-nucleate myofibre is directed, in Drosophila, by a founder cell. In the embryo, founders are selected by Notch-mediated lateral inhibition, while during adult myogenesis this mechanism of selection does not appear to operate. We show, in the muscles of the adult abdomen, that the Fibroblast growth factor pathway mediates founder cell choice in a novel manner. We suggest that the developmental patterns of Heartbroken/Dof and Sprouty result in defining the domain and timing of activation of the Fibroblast growth factor receptor Heartless in specific myoblasts, thereby converting them into founder cells. Our results point to a way in which muscle differentiation could be initiated and define a critical developmental function for Heartbroken/Dof in myogenesis.

A deletion at the mouse Xist gene exposes trans-effects that alter the heterochromatin of the inactive X chromosome and the replication time and DNA stability of both X chromosomes.

The inactive X chromosome of female mammals displays several properties of heterochromatin including late replication, histone H4 hypoacetylation, histone H3 hypomethylation at lysine-4, and methylated CpG islands. We show that cre-Lox-mediated excision of 21 kb from both Xist alleles in female mouse fibroblasts led to the appearance of two histone modifications throughout the inactive X chromosome usually associated with euchromatin: histone H4 acetylation and histone H3 lysine-4 methylation. Despite these euchromatic properties, the inactive X chromosome was replicated even later in S phase than in wild-type female cells. Homozygosity for the deletion also caused regions of the active X chromosome that are associated with very high concentrations of LINE-1 elements to be replicated very late in S phase. Extreme late replication is a property of fragile sites and the 21-kb deletions destabilized the DNA of both X chromosomes, leading to deletions and translocations. This was accompanied by the phosphorylation of p53 at serine-15, an event that occurs in response to DNA damage, and the accumulation of gamma-H2AX, a histone involved in DNA repair, on the X chromosome. The Xist locus therefore maintains the DNA stability of both X chromosomes.

Asynchronous replication and autosome-pair non-equivalence in human embryonic stem cells.

A number of mammalian genes exhibit the unusual properties of random monoallelic expression and random asynchronous replication. Such exceptional genes include genes subject to X inactivation and autosomal genes including odorant receptors, immunoglobulins, interleukins, pheromone receptors, and p120 catenin. In differentiated cells, random asynchronous replication of interspersed autosomal genes is coordinated at the whole chromosome level, indicative of chromosome-pair non-equivalence. Here we have investigated the replication pattern of the random asynchronously replicating genes in undifferentiated human embryonic stem cells, using fluorescence in situ hybridization based assay. We show that allele-specific replication of X-linked genes and random monoallelic autosomal genes occur in human embryonic stem cells. The direction of replication is coordinated at the whole chromosome level and can cross the centromere, indicating the existence of autosome-pair non-equivalence in human embryonic stem cells. These results suggest that epigenetic mechanism(s) that randomly distinguish between two parental alleles are emerging in the cells of the inner cell mass, the source of human embryonic stem cells.

Founder myoblasts and fibre number during adult myogenesis in Drosophila.

We have examined the mechanisms underlying the setting of myotubes and choice of myotube number in adult Drosophila. We find that the pattern of adult myotubes is prefigured by a pattern of duf-lacZ-expressing myoblasts at appropriate locations. Selective expression of duf-lacZ in single myoblasts emerges from generalized, low-level expression in all adult myoblasts during the third larval instar. The number of founders, thus chosen, corresponds to the number of fibres in a muscle. In contrast to the embryo, the selection of individual adult founder cells during myogenesis does not depend on Notch-mediated lateral inhibition. Our results suggest a general mechanism by which multi-fibre muscles can be patterned.