Novel B-cell precursors blocked at the stage of DJH recombination

Master transcription factors determine cell-type-specific responses to TGF-β signaling

Transforming growth factor beta (TGF-β) signaling, mediated through the transcription factors Smad2 and Smad3 (Smad2/3), directs different responses in different cell types. Here we report that Smad3 co-occupies the genome with cell-type-specific master transcription factors. Thus, Smad3 occupies the genome with Oct4 in embryonic stem cells (ESCs), Myod1 in myotubes, and PU.1 in pro-B cells. We find that these master transcription factors are required for Smad3 occupancy and that TGF-β signaling largely affects the genes bound by the master transcription factors. Furthermore, we show that induction of Myod1 in nonmuscle cells is sufficient to redirect Smad3 to Myod1 sites. We conclude that cell-type-specific master transcription factors determine the genes bound by Smad2/3 and are thus responsible for orchestrating the cell-type-specific effects of TGF-β signaling.

V(D)J recombination of chromosomally integrated, wild-type deletional and inversional substrates occur at similar frequencies with no preference for orientation

Efficient and correct recombination of V(D)J substrates results in the generation of antibodies. The RSS substrates are oriented in two directions with respect to each other: deletional and inversional. Deletional recombination results in the formation of the coding joint and excision of the intervening sequences. Inversional recombination retains all the genomic sequences and forms both a coding joint and a signal joint. A bias for deletional recombination has been characterized with specific loci in vivo and recapitulated in experiments using extrachromosomal substrates. We constructed retroviral substrates of RSS in the deletional and inversional orientation. We introduced the substrates into wild-type and scid pre-B cells and measured the frequency of functional recombination in addition to open/shut recombination. We also mutated the RSSs to determine whether mutated sequences influenced orientation bias. We show that pre-B cells recombine the wild-type substrates at a 1.6 ratio of deletion:inversion. Nonamer mutated substrates recombined with a deletional bias whereas heptamer mutated substrates recombined with an inversional bias. A spacer length mutation and drastic mutations in the RSS abolish all recombination. These results suggest that there is no orientation bias with wild-type RSSs but that orientation bias occurs when RSSs are mutated.

B lineage-specific regulation of V(D)J recombinase activity is established in common lymphoid progenitors

Expression of V(D)J recombinase activity in developing lymphocytes is absolutely required for initiation of V(D)J recombination at antigen receptor loci. However, little is known about when during hematopoietic development the V(D)J recombinase is first active, nor is it known what elements activate the recombinase in multipotent hematopoietic progenitors. Using mice that express a fluorescent transgenic V(D)J recombination reporter, we show that the V(D)J recombinase is active as early as common lymphoid progenitors (CLPs) but not in the upstream progenitors that retain myeloid lineage potential. Evidence of this recombinase activity is detectable in all four progeny lineages (B, T, and NK, and DC), and rag2 levels are the highest in progenitor subsets immediately downstream of the CLP. By single cell PCR, we demonstrate that V(D)J rearrangements are detectable at IgH loci in ∼5% of splenic natural killer cells. Finally, we show that recombinase activity in CLPs is largely controlled by the Erag enhancer. As activity of the Erag enhancer is restricted to the B cell lineage, this provides the first molecular evidence for establishment of a lineage-specific transcription program in multipotent progenitors.

The viral and cellular forms of the Abelson (abl) oncogene

The precision of molecular biology has allowed a better definition of the components of the Abelson system. We know the gene structures and gene products for the cellular and viral forms of this family of related tyrosine kinases. However, many basic issues first identified in the early biological observations of Abelson, Rabstein, and others remain unanswered. The precise pathway for transformation in biochemical terms remains unknown for Ab-MLV and all of its relatives. Relatively little can be said to explain the preferential growth stimulation for certain hematopoietic cell types by the viral and other altered forms of the oncogene, and no clear insights into the function of the normal cellular forms of the abl oncogene are available. Future progress will certainly depend on the intensive efforts by many workers in the broader field of cellular growth control mechanisms.