Comparative expression analysis of Pax3 and Pax7 during mouse myogenesis.

Pax3 and Pax7 are closely related transcription factors involved in the commitment of myogenic precursors in the developing trunk. However, it is not yet clear whether these genes are required for myogenic cell specification in the head and for post-somitic myogensis per se. In part, this uncertainty is due to the scarce information about their normal time course and pattern of expression. Here, we present a systematic immunohistochemical in situ analysis of spatiotemporal characteristics of Pax3 and Pax7 protein expression in comparison to that of MyoD and myogenin in the developing trunk and head muscles. The observed patterns of expression suggest that Pax3 is not involved in myogenesis in the head and its post-somitic expression in the trunk and limb muscles is mostly repressed after stage E13.5. In contrast, Pax7 expression is shared among all striated muscles and exhibits a uniform pattern. Pax7 is expressed only in mononucleated cells that either differentiate into myotubes or later form satellite cells. During development of head muscles, expression of Pax7 follows expression of MyoD and myogenin, implying that Pax7 is not required to induce the initial steps of the myogenic program in the head. In Pax7 homozygous mutants, in which muscle development proceeds normally, expression of Pax3 is indistinguishable from its wild-type pattern (i. e. absent), suggesting that after stage E13.5 myogenesis does not require Pax3 and Pax7. These data challenge the concept that Pax3 and Pax7 determine a persistent lineage of myogenic precursors in pre-natal and post-natal muscle development.

Expression of two protein isoforms of PAX7 is controlled by competing cleavage-polyadenylation and splicing.

The PAX7 gene encodes an evolutionary conserved transcription factor that is involved in the determination of the myogenic cell lineage during the development of vertebrates. In the postnatal period, the function of PAX7 is ultimately required for the specification of muscle satellite cells. The fact that PAX7 is expressed in fast proliferating embryonal myoblasts and in quiescent satellite cells of adults raised the question whether different PAX7 protein isoforms may have distinct roles in these myogenic precursors. Previously, we identified a human PAX7 mRNA encoding a C-terminus which did not show any sequence similarity to the PAX7 proteins of other organisms. So far, there was no further information available concerning the biological nature and significance of this form of PAX7. Here, we show that expression of PAX7 can be regulated by differential transcriptional termination either in exon 9 or in exon 8. Thereby, differential mRNA cleavage-polyadenylation and splicing of PAX7 may result in production of two alternative protein forms that contain or exclude the evolutionary conserved carboxy-terminal domain, respectively. The existence of both protein isoforms in vivo was confirmed by Western blot analysis. These data imply that the alternative C-termini of PAX7 may convey different functions to the corresponding protein isoforms.

Getting the proto-Pax by the tail.

Pax genes encode transcription factors governing the determination of different cell types and even organs in the development of multicellular animals. Pax proteins are characterized by the presence of three evolutionarily conserved elements: two DNA-binding domains, the paired domain (PD) and paired-type homeodomain (PtHD), and the short octopeptide sequence (OP) located between PD and PtHD. PD is the defining feature of this class of genes, while OP and/or PtHD may be divergent or absent in some members of the family. Phylogenetic analyses of the PD and PtHD sequences do not distinguish which particular type of the extant Pax genes more resembles the ancestral type. Here we present evidence for the existence of a fourth evolutionarily conserved domain in the Pax proteins, the paired-type homeodomain tail (PHT). Our data also imply that the hypothetical proto-Pax protein most probably exhibited a complex structure, PD-OP-PtHD-PHT, which has been retained in the extant proteins Pax3/7 of the ascidia and lancelet, and Pax7 of the vertebrates. Finally, based on structural considerations, a scenario for the evolutionary emergence of the proto-Pax gene is proposed.

An anthropoid-specific segmental duplication on human chromosome 1q22.

Segmental duplications (SDs) play a key role in genome evolution by providing material for gene diversification and creation of variant or novel functions. They also mediate recombinations, resulting in microdeletions, which have occasionally been associated with human genetic diseases. Here, we present a detailed analysis of a large genomic region (about 240 kb), located on human chromosome 1q22, that contains a tandem SD, SD1q22. This duplication occurred about 37 million years ago in a lineage leading to anthropoid primates, after their separation from prosimians but before the Old and New World monkey split. We reconstructed the hypothetical unduplicated ancestral locus and compared it with the extant SD1q22 region. Our data demonstrate that, as a consequence of the duplication, new anthropoid-specific genetic material has evolved in the resulting paralogous segments. We describe the emergence of two new genes, whose new functions could contribute to the speciation of anthropoid primates. Moreover, we provide detailed information regarding structure and evolution of the SD1q22 region that is a prerequisite for future studies of its anthropoid-specific functions and possible linkage to human genetic disorders.