A small number of signalling pathways are used iteratively to regulate cell fates, cell proliferation and cell death in development. Notch is the receptor in one such pathway, and is unusual in that most of its ligands are also transmembrane proteins; therefore signalling is restricted to neighbouring cells. Although the intracellular transduction of the Notch signal is remarkably simple, with no secondary messengers, this pathway functions in an enormous diversity of developmental processes and its dysfunction is implicated in many cancers.

Notch signalling: a simple pathway becomes complex

The mammalian Rel/NF-κB family of transcription factors, including RelA, c-Rel, RelB, NF-κB1 (p50 and its precursor p105), and NF-κB2 (p52 and its precursor p100), plays a central role in the immune system by regulating several processes ranging from the development and survival of lymphocytes and lymphoid organs to the control of immune responses and malignant transformation. The five members of the NF-κB family are normally kept inactive in the cytoplasm by interaction with inhibitors called IκBs or the unprocessed forms of NF-κB1 and NF-κB2. A wide variety of signals emanating from antigen receptors, pattern-recognition receptors, receptors for the members of TNF and IL-1 cytokine families, and others induce differential activation of NF-κB heterodimers. Although work over the past two decades has shed significant light on the regulation of NF-κB transcription factors and their functions, much progress has been made in the past two years revealing new insights into the regulation and functions of NF-κB...

Regulation and Function of NF-κB Transcription Factors in the Immune System

Pax7 is required for the specification of myogenic satellite cells.

Stem cells are currently in the news for two reasons: the successful cultivation of human embryonic stem cell lines and reports that adult stem cells can differentiate into developmentally unrelated cell types, such as nerve cells into blood cells Both intrinsic and extrinsic signals regulate stem cell fate and some of these signals have now been identified Certain aspects of the stem cell microenvironment, or niche, are conserved between tissues, and this can be exploited in the application of stem cells to tissue replacement therapy

https://science.sciencemag.org/content/sci/287/5457/1427.full.pdf

Out of Eden: Stem Cells and Their Niches

Chromosomal aberrations are a hallmark of acute lymphoblastic leukaemia (ALL) but alone fail to induce leukaemia. To identify cooperating oncogenic lesions, we performed a genome-wide analysis of leukaemic cells from 242 paediatric ALL patients using high-resolution, single-nucleotide polymorphism arrays and genomic DNA sequencing. Our analyses revealed deletion, amplification, point mutation and structural rearrangement in genes encoding principal regulators of B lymphocyte development and differentiation in 40% of B-progenitor ALL cases. The PAX5 gene was the most frequent target of somatic mutation, being altered in 31.7% of cases. The identified PAX5 mutations resulted in reduced levels of PAX5 protein or the generation of hypomorphic alleles. Deletions were also detected in TCF3 (also known as E2A), EBF1, LEF1, IKZF1 (IKAROS) and IKZF3 (AIOLOS). These findings suggest that direct disruption of pathways controlling B-cell development and differentiation contributes to B-progenitor ALL pathogenesis. Moreover, these data demonstrate the power of high-resolution, genome-wide approaches to identify new molecular lesions in cancer.

Genome-wide analysis of genetic alterations in acute lymphoblastic leukaemia

The Pax5 gene encoding the B-cell-specific activator protein (BSAP) is expressed within the haematopoietic system exclusively in the B-lymphoid lineage, where it is required in vivo for progression beyond the pro-B-cell stage. However, Pax5 is not essential for in vitro propagation of pro-B cells in the presence of interleukin-7 and stromal cells. Here we show that pro-B cells lacking Pax5 are also incapable of in vitro B-cell differentiation unless Pax5 expression is restored by retroviral transduction. Pax5-/- pro-B cells are not restricted in their lineage fate, as stimulation with appropriate cytokines induces them to differentiate into functional macrophages, osteoclasts, dendritic cells, granulocytes and natural killer cells. As expected for a clonogenic haematopoietic progenitor with lymphomyeloid developmental potential, the Pax5-/- pro-B cell expresses genes of different lineage-affiliated programmes, and restoration of Pax5 activity represses this lineage-promiscuous transcription. Pax5 therefore plays an essential role in B-lineage commitment by suppressing alternative lineage choices.

Commitment to the B-lymphoid lineage depends on the transcription factor Pax5.

The transcription factor Pax5 is essential for initiating B cell lineage commitment, but its role in maintaining commitment is unknown. Using conditional Pax5 inactivation in committed pro-B cells, we demonstrate that Pax5 is required not only to initiate its B lymphoid transcription program, but also to maintain it in early B cell development. As a consequence of Pax5 inactivation, previously committed pro-B cells regained the capacity to differentiate into macrophages in vitro and to reconstitute T cell development in vivo in RAG2-/- mice. Hence, Pax5 expression is continuously required to maintain B cell lineage commitment, because its loss converts committed pro-B cells into hematopoietic progenitors with multilineage potential.

https://science.sciencemag.org/content/sci/297/5578/110.full.pdf

Reversion of B Cell Commitment upon Loss of Pax5 Expression

A central challenge in embryonic stem (ES) cell biology is to understand how to impose direction on primary lineage commitment In basal culture conditions, the majority of ES cells convert asynchronously into neural cells However, many cells resist differentiation and others adopt nonneural fates Mosaic activation of the neural reporter Sox-green fluorescent protein suggests regulation by cell-cell interactions We detected expression of Notch receptors and ligands in mouse ES cells and investigated the role of this pathway Genetic manipulation to activate Notch constitutively does not alter the stem cell phenotype However, upon withdrawal of self-renewal stimuli, differentiation is directed rapidly and exclusively into the neural lineage Conversely, pharmacological or genetic interference with Notch signalling suppresses the neural fate choice Notch promotion of neural commitment requires parallel signalling through the fibroblast growth factor receptor Stromal cells expressing Notch ligand stimulate neural specification of human ES cells, indicating that this is a conserved pathway in pluripotent stem cells These findings define an unexpected and decisive role for Notch in ES cell fate determination Limiting activation of endogenous Notch results in heterogeneous lineage commitment Manipulation of Notch signalling is therefore likely to be a key factor in taking command of ES cell lineage choice

/pdf/notch-promotes-neural-lineage-entry-by-pluripotent-embryonic-1750sd001g.pdf

Notch Promotes Neural Lineage Entry by Pluripotent Embryonic Stem Cells

Pax5 (BSAP) functions as both a transcriptional activator and repressor during midbrain patterning, B-cell development and lymphomagenesis. Here we demonstrate that Pax5 exerts its repression function by recruiting members of the Groucho corepressor family. In a yeast two-hybrid screen, the groucho-related gene product Grg4 was identified as a Pax5 partner protein. Both proteins interact cooperatively via two separate domains: the N-terminal Q and central SP regions of Grg4, and the octapeptide motif and C-terminal transactivation domain of Pax5. The phosphorylation state of Grg4 is altered in vivo upon Pax5 binding. Moreover, Grg4 efficiently represses the transcriptional activity of Pax5 in an octapeptide-dependent manner. Similar protein interactions resulting in transcriptional repression were also observed between distantly related members of both the Pax2/5/8 and Groucho protein families. In agreement with this evolutionary conservation, the octapeptide motif of Pax proteins functions as a Groucho-dependent repression domain in Drosophila embryos. These data indicate that Pax proteins can be converted from transcriptional activators to repressors through interaction with corepressors of the Groucho protein family.

/pdf/transcriptional-repression-by-pax5-bsap-through-interaction-14967xqz9x.pdf

Barry Heavey

Papers

Commitment to the B-lymphoid lineage depends on the transcription factor Pax5.

Reversion of B Cell Commitment upon Loss of Pax5 Expression

Notch Promotes Neural Lineage Entry by Pluripotent Embryonic Stem Cells

Transcriptional repression by Pax5 (BSAP) through interaction with corepressors of the Groucho family

Transcriptional control of B-cell development