Cold Spring Harbor Laboratory Press on May 11, 2018 -Published by genesdev.cshlp.org Downloaded from

Allelic exclusion of immunoglobulin genes ensures the expression of a single antibody molecule in B cells through mostly unknown mechanisms. Large-scale contraction of the immunoglobulin heavy-chain (Igh) locus facilitates rearrangements between Igh variable (V H ) and diversity gene segments in pro-B cells. Here we show that these long-range interactions are mediated by 'looping' of individual Igh subdomains. The Igk locus also underwent contraction by looping in small pre-B and immature B cells, demonstrating that immunoglobulin loci are in a contracted state in rearranging cells. Successful Igh recombination induced the rapid reversal of locus contraction in response to pre-B cell receptor signaling, which physically separated the distal V H genes from the proximal Igh domain, thus preventing further rearrangements. In the absence of locus contraction, only the four most proximal V H genes escaped allelic exclusion in immature μ-transgenic B lymphocytes. Pre-B cell receptor signaling also led to rapid repositioning of one Igh allele to repressive centromeric domains in response to downregulation of interleukin 7 signaling. These data link both locus 'decontraction' and centromeric recruitment to the establishment of allelic exclusion at the Igh locus.The diverse antigen receptor repertoire of lymphocytes is generated by V(D)J recombination, which assembles the variable regions of immunoglobulin and T cell receptor genes from discontinuous variable (V), diversity (D) and joining (J) gene segments during B and T cell development1,2. These gene segments are flanked by recombination signal sequences that function as recognition sites for the V(D)J recombinase consisting of recombination activating gene 1 (RAG1) and RAG2 proteins. After pairing of two compatible recombination signal sequences, the RAG1-RAG2 complex introduces doublestrand DNA breaks between the recombination signal sequences and flanking gene segments, followed by processing and religation of the DNA ends by repair factors of the nonhomologous end-joining machinery1,2. V(D)J recombination is tightly controlled in a lineage-and stage-specific way. Immunoglobulin and T cell receptor genes are rearranged only in B and T lymphocytes, respectively1,2. In the B lymphoid lineage, the immunoglobulin heavy-chain (Igh) locus

Loss of expression of neural cell-adhesion molecule (N-CAM) is implicated in the progression of tumour metastasis. Here we show that N-CAM modulates neurite outgrowth and matrix adhesion of beta-cells from pancreatic tumours by assembling a fibroblast-growth-factor receptor-4 (FGFR-4) signalling complex, which consists of N-cadherin, FGFR-4, phospholipase C gamma (PLC-gamma), the adaptor protein FRS2, pp60(c-src), cortactin and growth-associated protein-43 (GAP-43). Dominant-negative FGFR-4, inhibitors of FGFR signalling and anti-beta(1)-integrin antibodies repress matrix adhesion induced by N-CAM. FGF ligands can replace N-CAM in promoting matrix adhesion but not neurite outgrowth. The results indicate that N-CAM stimulates beta1-integrin-mediated cell-matrix adhesion by activating FGFR signalling. This is a potential mechanism for preventing the dissemination of metastatic tumour cells.

Signaling through the Notch1 receptor is essential for T cell development in the thymus. Stromal OP9 cells ectopically expressing the Notch ligand Delta-like1 mimic the thymic environment by inducing hemopoietic stem cells to undergo in vitro T cell development. Notch1 is also expressed on Pax5−/− pro-B cells, which are clonable lymphoid progenitors with a latent myeloid potential. In this study, we demonstrate that Pax5−/− progenitors efficiently differentiate in vitro into CD4+CD8+ αβ and γδ T cells upon coculture with OP9-Delta-like1 cells. In vitro T cell development of Pax5−/− progenitors strictly depends on Notch1 function and progresses through normal developmental stages by expressing T cell markers and rearranging TCRβ, γ, and δ loci in the correct temporal sequence. Notch-stimulated Pax5−/− progenitors efficiently down-regulate the expression of B cell-specific genes, consistent with a role of Notch1 in preventing B lymphopoiesis in the thymus. At the same time, Notch signaling rapidly induces cell surface expression of the c-Kit receptor and transcription of the target genes Deltex1 and pre-Tα concomitant with the activation of TCR Vβ germline transcription and the regulatory genes GATA3 and Tcf1. These data suggest that Notch1 acts upstream of GATA3 and Tcf1 in early T cell development and regulates Vβ-DJβ rearrangements by controlling the chromatin accessibility of Vβ genes at the TCRβ locus.

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The transcription factor Pax5 is essential for B cell commitment and development. Although the detailed Pax5 expression pattern within the hemopoietic system is still largely unknown, we previously reported that Pax5 is monoallelically transcribed in pro-B and mature B cells. In this study, we have investigated the expression of Pax5 at single-cell resolution by inserting a GFP or human Cd2 indicator gene under the translational control of an internal ribosomal entry sequence into the 3′ untranslated region of Pax5. These insertions were noninvasive, as B cell development was normal in Pax5ihCd2/ihCd2 and Pax5iGFP/iGFP mice. Transheterozygous Pax5ihCd2/iGFP mice coexpressed GFP and human CD2 at similar levels from pro-B to mature B cells, thus demonstrating biallelic expression of Pax5 at all stages of B cell development. No reporter gene expression could be detected in plasma cells and non-B cells of the hemopoietic system. Moreover, the vast majority of common lymphoid progenitors and pre-pro-B cells in the bone marrow of Pax5iGFP/iGFP mice did not yet express GFP, indicating that Pax5 expression is fully switched on only during the transition from uncommitted pre-pro-B cells to committed pro-B cells. Hence, the transcriptional initiation and B cell-specific expression of Pax5 is entirely consistent with its B cell lineage commitment function.