During early mouse development the homeobox gene Hesx1 is expressed in prospective forebrain tissue, but later becomes restricted to Rathke's pouch, the primordium of the anterior pituitary gland. Mice lacking Hesx1 exhibit variable anterior CNS defects and pituitary dysplasia. Mutants have a reduced prosencephalon, anopthalmia or micropthalmia, defective olfactory development and bifurcations in Rathke's pouch. Neonates exhibit abnormalities in the corpus callosum, the anterior and hippocampal commissures, and the septum pellucidum. A comparable and equally variable phenotype in humans is septo-optic dysplasia (SOD). We have cloned human HESX1 and screened for mutations in affected individuals. Two siblings with SOD were homozygous for an Arg53Cys missense mutation within the HESX1 homeodomain which destroyed its ability to bind target DNA. These data suggest an important role for Hesx1/HESX1 in forebrain, midline and pituitary development in mouse and human.

The pre-B cell-specific expression of the lambda5 gene is regulated at the level of transcription. The 5' region of the lambda5 gene has been shown to contain an enhancer that activates heterologous promoters. Here, we show that this enhancer, B(lambda5), also acts as a lineage- and tissue-restricted enhancer on its own promoter. We define the enhancer core, b(lambda5), that carries around 50% of the total enhancer activity. We also demonstrate that the transcription factor early B cell factor (EBF) binds to a DNA motif in the lambda5 core enhancer which is crucial for enhancer activity, suggesting that lambda5 is a second target gene of EBF.

The pre-B-cell receptor (pre-BCR), composed of Ig heavy and surrogate light chain (SLC), signals pre-BII-cell proliferative expansion. We have investigated whether the pre-BCR also signals downregulation of the SLC genes (VpreB and lambda5), thereby limiting this expansion. We demonstrate that, as BM cells progress from the pre-BI to large pre-BII-cell stage, there is a shift from bi- to mono-allelic lambda5 transcription, while the second allele is silenced in small pre-BII cells. A VpreB1-promoter-driven transgene shows the same pattern, therefore suggesting that VpreB1 is similarly regulated and thereby defines the promoter as a target for transcriptional silencing. Analyses of pre-BCR-deficient mice show a temporal delay in lambda5 downregulation, thereby demonstrating that the pre-BCR is essential for monoallelic silencing at the large pre-BII-cell stage. Our data also suggest that SLP-65 is one of the signaling components important for this process. Furthermore, the VpreB1/lambda5 alleles undergo dynamic changes with respect to nuclear positioning and heterochromatin association, thereby providing a possible mechanism for their transcriptional silencing.

The cytokine IL-7 and its receptor are essential for normal B and T lymphopoiesis. We have analyzed the role of this receptor in B cell development throughout ontogeny in IL-7 receptor a-deficient mice. We demonstrate that the IL-7 receptor becomes progressively more important with age. B lymphopoiesis takes place, albeit at reduced levels, in fetal liver and bone marrow of young mice, but is arrested in adults. The outcome is a severe reduction, from an early age, in peripheral B cells including follicular, marginal zone and B-1 B cells as well as perturbed splenic B cell structures, which are restored after adoptive transfer of normal spleen cells. We conclude that in the absence of the IL-7 receptor, the residual B lymphopoiesis occurring early in ontogeny must be facilitated by another component, whereas the IL-7 receptor is the key factor in adults. The impairment of marginal zone and B-1 B cells in IL-7 receptor-but not IL-7-deficient mice suggests non-redundant functions for the IL-7 receptor ligands, IL-7 and thymic stromal lymphopoietin.

The process of allelic exclusion ensures that each B cell expresses a B-cell receptor encoded by only one of its Ig heavy (IgH) and light (IgL) chain alleles. Although its precise mechanism is unknown, recruitment of the nonfunctional IgH allele to centromeric heterochromatin correlates with the establishment of allelic exclusion. Similarly, recruitment in activated splenic B cells correlates with cell division. In the latter, the recruited IgH allele was reported to be transcriptionally silent. However, it is not known whether monoallelic recruitment during establishment of allelic exclusion correlates with transcriptional silencing. To investigate this, we assessed the transcriptional status of both IgH alleles in single primary cells over the course of B-cell development, using RNA fluorescence in situ hybridization. Before allelic exclusion both alleles are transcribed. Thereafter, in pre-BII and subsequent developmental stages both functional and nonfunctional VDJ- and DJ-transcription is observed. Thus, after the establishment of IgH allelic exclusion, monoallelic recruitment to heterochromatin does not silence VDJ- or DJ-transcription, but serves another purpose.