T-call development: is notch a key player in lineage decisions?

Zebrafish Models of Human Leukemia: Technological Advances and Mechanistic Insights

Insights concerning leukemic pathophysiology have been acquired in various animal models and further efforts to understand the mechanisms underlying leukemic treatment resistance and disease relapse promise to improve therapeutic strategies. The zebrafish (Danio rerio) is a vertebrate organism with a conserved hematopoietic program and unique experimental strengths suiting it for the investigation of human leukemia. Recent technological advances in zebrafish research including efficient transgenesis, precise genome editing, and straightforward transplantation techniques have led to the generation of a number of leukemia models. The transparency of the zebrafish when coupled with improved lineage-tracing and imaging techniques has revealed exquisite details of leukemic initiation, progression, and regression. With these advantages, the zebrafish represents a unique experimental system for leukemic research and additionally, advances in zebrafish-based high-throughput drug screening promise to hasten the discovery of novel leukemia therapeutics. To date, investigators have accumulated knowledge of the genetic underpinnings critical to leukemic transformation and treatment resistance and without doubt, zebrafish are rapidly expanding our understanding of disease mechanisms and helping to shape therapeutic strategies for improved outcomes in leukemic patients.

Ly6d-L, a cell surface ligand for mouse Ly6d

The mouse Ly6 gene family encodes proteins found in lymphocytes and other cells. Some are involved in cell activation; no ligands have been found. A ligand for Ly6d (ThB) was identified on lymphocytes using microspheres loaded with Ly6d and the cDNA isolated from a spleen/thymus library by panning on Ly6d. The Ly6d ligand (Ly6d-L) is a nonglycosylated protein of 9 kDa of broad distribution, rich in cysteine, with no discernable transmembrane sequence. Its N and C termini are on the cell surface, where it associates with a 30 kDa protein. Ly6d-L is homologous with an EGF repeat of Notch.

Neoplastic transformation by Notch is independent of transcriptional activation by RBP-J signalling

Signalling through the transmembrane receptor Notch is triggered by ligand binding, which induces the proteolytic cleavage of the Notch protein. This cleavage generates an intracellular fragment of the Notch protein (Notch-IC), which translocates into the nucleus and modifies transcription of target genes through its association with the RBP-J protein. Thus, the isolated Notch-IC protein represents the constitutively activated receptor. We have performed a deletion analysis of Notch IC in order to identify the transferable transactivation domain of Notch-IC and the minimal domain of Notch-IC required for RBP-J dependent transactivational activation. Functionally, Notch-IC has been linked to cell fate decision in development and oncogenesis in vivo. In vitro, Notch-IC can cooperate in neoplastic transformation of baby rat kidney cells with the adenoviral E1A protein. We have defined the minimal domain of Notch-IC required for E1A cotransformation. This domain, consisting of the ankyrin repeats of Notch-IC only, can neither activate RBP-J dependent transcription nor does it carry a transactivation domain. Therefore, the ankyrin repeat domain of Notch-IC might trigger novel pathways relevant for transformation but unrelated to RBP-J signalling.

Arbiter of differentiation and death: Notch signaling meets apoptosis

Notch-ligand interactions are a highly conserved mechanism that regulates cell fate decisions. Over the past few years, numerous observations have shown that this mechanism operates to regulate cell differentiation in an enormous variety of developmental and cell maturation processes. Recent studies indicate that in addition to cell differentiation, Notch signaling has direct effects on proliferation and programmed cell death. The picture emerging from these findings suggests that, depending on cellular and developmental context, Notch signaling may function as a general "arbiter" of cell fate, regulating differentiation potential, rate of proliferation, and apoptotic cell death. In this review, we briefly summarize the current knowledge of the structure and function of Notch receptors and discuss the recent evidence that Notch signaling regulates apoptotic cell death. The possible mechanisms of this effect and its potential implications for developmental biology, immunobiology, neuropathology, and tumor biology are discussed.

Macrophage-Derived Chemokine Is Localized to Thymic Medullary Epithelial Cells and Is a Chemoattractant for CD3+, CD4+, CD8low Thymocytes

Macrophage-derived chemokine (MDC) is a recently identified CC chemokine that is a potent chemoattractant for dendritic cells, natural killer (NK) cells, and the Th2 subset of peripheral blood T cells. In normal tissues, MDC mRNA is expressed principally in the thymus. Immunohistochemical analysis performed on 5 human postnatal thymuses showed high MDC immunoreactivity, which was selectively localized to epithelial cells within the medulla. To examine the effects of MDC on immature T cells, we have identified cDNA clones for mouse and rat MDC. Expression of MDC in murine tissues is also highly restricted, with significant levels of mRNA found only in the thymus. Thymocytes express high-affinity binding sites for MDC (kd = 0.7 nmol/L), and, in vitro, MDC is a chemoattractant for these cells. MDC-responsive murine thymocytes express mRNA for CCR4, a recently identified receptor for MDC. Phenotypic analysis of MDC-responsive cells shows that they are enriched for a subset of double-positive cells that express high levels of CD3 and CD4 and that have reduced levels of CD8. This subset of MDC-responsive cells is consistent with the observed expression of MDC within the medulla, because more mature cells are found there. MDC may therefore play a role in the migration of T-cell subsets during development within the thymus.

Macrophage-Derived Chemokine Is Localized to Thymic Medullary Epithelial Cells and Is a Chemoattractant for CD3+, CD4+, CD8low Thymocytes

Macrophage-derived chemokine (MDC) is a recently identified CC chemokine that is a potent chemoattractant for dendritic cells, natural killer (NK) cells, and the Th2 subset of peripheral blood T cells. In normal tissues, MDC mRNA is expressed principally in the thymus. Immunohistochemical analysis performed on 5 human postnatal thymuses showed high MDC immunoreactivity, which was selectively localized to epithelial cells within the medulla. To examine the effects of MDC on immature T cells, we have identified cDNA clones for mouse and rat MDC. Expression of MDC in murine tissues is also highly restricted, with significant levels of mRNA found only in the thymus. Thymocytes express high-affinity binding sites for MDC (kd = 0.7 nmol/L), and, in vitro, MDC is a chemoattractant for these cells. MDC-responsive murine thymocytes express mRNA for CCR4, a recently identified receptor for MDC. Phenotypic analysis of MDC-responsive cells shows that they are enriched for a subset of double-positive cells that express high levels of CD3 and CD4 and that have reduced levels of CD8. This subset of MDC-responsive cells is consistent with the observed expression of MDC within the medulla, because more mature cells are found there. MDC may therefore play a role in the migration of T-cell subsets during development within the thymus.

R-esp1, a rat homologue of drosophila groucho, is differentially expressed after optic nerve crush and mediates NGF-induced survival of PC12 cells

The differential display reverse transcription polymerase chain reaction method was used to detect alterations in gene expression in the superior colliculus after optic nerve crush in adult rats. One of the most prominent changes observed was the selective induction of R-esp1, a homologue of the Drosophila enhancer of split locus (Groucho). Therefore, we studied the influence of R-esp1 on nerve growth factor (NGF)-induced cell survival of PC12 cells. Overexpression of R-esp1 promotes cell survival even in the absence of NGF and, conversely, it is reduced by antisense-mediated inhibition of R-esp1 expression. In conclusion, we propose a novel model in which R-esp1 protein mediates the NGF-signaling pathway.

Endogenous presenilin 1 redistributes to the surface of lamellipodia upon adhesion of Jurkat cells to a collagen matrix

Most familial early-onset Alzheimer's disease cases are caused by mutations in the presenilin 1 (PS1) gene. Subcellular localization of the endogenous PS1 is essential for understanding its function, interactions with proteins, and role in Alzheimer's disease. Although numerous studies revealed predominant localization of PS1 to endoplasmic reticulum and Golgi, there are conflicting reports on the localization of PS1 to the cell surface. We found that endogenous PS1 is highly expressed in T lymphocytes (Jurkat cells). Using a variety of methods, we present evidence that endogenous PS1 is localized to the cell surface in addition to intracellular membrane compartments. Moreover, PS1 appeared in high levels on the surface of lamellipodia upon adhesion of the cells to a collagen matrix. The redistribution of PS1 in adhered cells was strikingly similar to that of the well characterized adhesion protein CD44. Cell surface PS1 formed complexes in vivo with actin-binding protein filamin (ABP-280), which is known to form bridges between cell surface receptors and cytoskeleton and mediate cell adhesion and cell motility. Taken together, our results suggest a role of PS1 in cell adhesion and/or cell-matrix interaction.

The transcription factor NFAT4 is involved in the generation and survival of T cells

Nuclear factor of activated T cells (NFAT) is a family of four related transcription factors implicated in cytokine and early response gene expression in activated lymphocytes. Here we report that NFAT4, in contrast to NFATp and NFATc, is preferentially expressed in DP thymocytes. Mice lacking NFAT4 have impaired development of CD4 and CD8 SP thymocytes and peripheral T cells as well as hyperactivation of peripheral T cells. The thymic defect is characterized by increased apoptosis of DP thymocytes. The increased apoptosis and hyperactivation may reflect heightened sensitivity to TcR-mediated signaling. Further, mice lacking NFAT4 have impaired production of Bcl-2 mRNA and protein. NFAT4 thus plays an important role in the successful generation and survival of T cells.

Macrophage-derived chemokine is a functional ligand for the CC chemokine receptor 4

Macrophage-derived chemokine (MDC) is a recently identified member of the CC chemokine family. MDC is not closely related to other chemokines, sharing most similarity with thymus- and activation-regulated chemokine (TARC), which contains 37% identical amino acids. Both chemokines are highly expressed in the thymus, with little expression seen in other tissues. In addition, the genes for MDC and TARC are encoded by human chromosome 16. To explore this relationship in greater detail, we have more precisely localized the MDC gene to chromosome 16q13, the same position reported for the TARC gene. We have also examined the interaction of MDC with CC chemokine receptor 4 (CCR4), recently shown to be a receptor for TARC. Using a fusion protein of MDC with secreted alkaline phosphatase, we observed high affinity binding of MDC-secreted alkaline phosphatase to CCR4-transfected L1.2 cells (Kd = 0.18 nM). MDC and TARC competed for binding to CCR4, while no binding competition was observed for six other chemokines (MCP-1, MCP-3, MCP-4, RANTES (regulated on activation normal T cell expressed and secreted), macrophage inflammatory protein-1 alpha, macrophage inflammatory protein-1 beta). MDC was tested for calcium mobilization in L1.2 cells tranfected with seven different CC chemokine receptors. MDC induced a calcium flux in CCR4-transfected cells, but other receptors did not respond to MDC. TARC, which also induced calcium mobilization in CCR4 transfectants, was unable to desensitize the response to MDC. In contrast, MDC fully desensitized a subsequent response to TARC. Both MDC and TARC functioned as chemoattractants for CCR4 transfectants, confirming that MDC is also a functional ligand for CCR4. Since MDC and TARC are both expressed in the thymus, one role for these chemokines may be to attract CCR4-bearing thymocytes in the process of T cell education and differentiation.