PubMed Health⌕ Search

Biomedical subjects

Susan Chan

Publications and source records attributed to Susan Chan.

13 recordsLinked to original sources
PubMed2006

Ikaros is required for plasmacytoid dendritic cell differentiation.

Plasmacytoid dendritic cells (pDCs) are specialized DCs that produce high levels of type I IFN upon viral infection. Despite their key immunoregulatory role, little is known about pDC ontogeny or how developmental events regulate their function. We show that mice expressing low levels of the transcription factor Ikaros (Ik(L/L)) lack peripheral pDCs, but not other DC subsets. Loss of pDCs is associated with an inability to produce type I IFN after challenge with Toll-like receptor-7 and -9 ligands, or murine cytomegalovirus (MCMV) infection. In contrast, conventional DCs are present in normal numbers and exhibit normal responses in vivo after challenge with MCMV or inactivated toxoplasma antigen. Interestingly, Ik(L/L) bone marrow (BM) cells contain a pDC population that appears blocked at the Ly-49Q- stage of differentiation and fails to terminally differentiate in response to Flt-3L, a cytokine required for pDC differentiation. This differentiation block is strictly dependent on a cell-intrinsic requirement for Ikaros in pDC-committed precursors. Global gene expression profiling of Ik(L/L) BM pDCs reveals an up-regulation of genes not normally expressed, or expressed at low levels, in WT pDCs. These studies suggest that Ikaros controls pDC differentiation by silencing a large array of genes.

Animals↗
PubMed2006

Absence of the steroid receptor coactivator-3 induces B-cell lymphoma.

Steroid receptor coactivator 3 (SRC-3/ACTR/AIB-1/pCIP/RAC3/TRAM-1) is a member of the p160 family of nuclear receptor coactivators that plays an important role in mammary gland growth, development, and tumorigenesis. We show that deletion of SRC-3 gene decreases platelet and increases lymphocytes numbers, leading to the development of malignant B-cell lymphomas upon aging. The expansion of the lymphoid lineage in SRC-3(-/-) mice is cell autonomous, correlates with an induction of proliferative and antiapoptotic genes secondary to constitutive NF-kappaB activation, and can be reversed by restoration of SRC-3 expression. NF-kappaB activation is explained by the degradation of IkappaB, consequent to increases in free IkappaB kinase, which is no longer inhibited by SRC-3. These results demonstrate that SRC-3 regulates lymphopoiesis and in combination with previous studies indicate that SRC-3 has vastly diverging effects on cell proliferation depending on the cellular context, ranging from proliferative and tumorigenic (breast) to antiproliferative (lymphoid cells) effects.

Animals↗
PubMed2006

Notch activation is an early and critical event during T-Cell leukemogenesis in Ikaros-deficient mice.

The Ikaros transcription factor is both a key regulator of lymphocyte differentiation and a tumor suppressor in T lymphocytes. Mice carrying a hypomorphic mutation (Ik(L/L)) in the Ikaros gene all develop thymic lymphomas. Ik(L/L) tumors always exhibit strong activation of the Notch pathway, which is required for tumor cell proliferation in vitro. Notch activation occurs early in tumorigenesis and may precede transformation, as ectopic expression of the Notch targets Hes-1 and Deltex-1 is detected in thymocytes from young Ik(L/L) mice with no overt signs of transformation. Notch activation is further amplified by secondary mutations that lead to C-terminal truncations of Notch 1. Strikingly, restoration of Ikaros activity in tumor cells leads to a rapid and specific downregulation of Notch target gene expression and proliferation arrest. Furthermore, Ikaros binds to the Notch-responsive element in the Hes-1 promoter and represses Notch-dependent transcription from this promoter. Thus, Ikaros-mediated repression of Notch target gene expression may play a critical role in defining the tumor suppressor function of this factor.

Amino Acid Sequence↗
PubMed2005

MyD88-dependent and -independent murine cytomegalovirus sensing for IFN-alpha release and initiation of immune responses in vivo.

Antiviral immunity requires early and late mechanisms in which IFN-alpha and IL-12 play major roles. However, the initial events leading to their production remain largely unclear. Given the crucial role of TLR in innate recognition, we investigated their role in antiviral immunity in vivo. Upon murine CMV (MCMV) infection, both MyD88-/- and TLR9-/- mice were more susceptible and presented increased viral loads compared with C57BL/6, TLR2-/-, TLR3-/-, or TLR4-/- mice. However, in terms of resistance to infection, IFN-alpha production and in many other parameters of early inflammatory responses, the MyD88-/- mice showed a more defective response than TLR9-/- mice. In the absence of the TLR9/MyD88 signaling pathway, cytokine production was dramatically impaired with a complete abolition of bioactive IL-12p70 serum release contrasting with a high flexibility for IFN-alpha release, which is initially (36 h) plasmacytoid dendritic cell- and MyD88-dependent, and subsequently (44 h) PDC-, MyD88-independent and, most likely, TLR-independent. NK cells from MCMV-infected MyD88-/- and TLR9-/- mice displayed a severely impaired IFN-gamma production, yet retained enhanced cytotoxic activity. In addition, dendritic cell activation and critical inflammatory cell trafficking toward the liver were still effective. In the long term, except for isotype switching to MCMV-specific IgG1, the establishment of Ab responses was not significantly altered. Thus, our results demonstrate a critical requirement of TLR9 in the process of MCMV sensing to assure rapid antiviral responses, coordinated with other TLR-dependent and -independent events that are sufficient to establish adaptive immunity.

Adaptor Proteins, Signal Transducing↗
PubMed2005

Distinctive and indispensable roles of PU.1 in maintenance of hematopoietic stem cells and their differentiation.

The PU.1 transcription factor is a key regulator of hematopoietic development, but its role at each hematopoietic stage remains unclear. In particular, the expression of PU.1 in hematopoietic stem cells (HSCs) could simply represent "priming" of genes related to downstream myelolymphoid lineages. By using a conditional PU.1 knock-out model, we here show that HSCs express PU.1, and its constitutive expression is necessary for maintenance of the HSC pool in the bone marrow. Bone marrow HSCs disrupted with PU.1 in situ could not maintain hematopoiesis and were outcompeted by normal HSCs. PU.1-deficient HSCs also failed to generate the earliest myeloid and lymphoid progenitors. PU.1 disruption in granulocyte/monocyte-committed progenitors blocked their maturation but not proliferation, resulting in myeloblast colony formation. PU.1 disruption in common lymphoid progenitors, however, did not prevent their B-cell maturation. In vivo disruption of PU.1 in mature B cells by the CD19-Cre locus did not affect B-cell maturation, and PU.1-deficient mature B cells displayed normal proliferation in response to mitogenic signals including the cross-linking of surface immunoglobulin M (IgM). Thus, PU.1 plays indispensable and distinct roles in hematopoietic development through supporting HSC self-renewal as well as commitment and maturation of myeloid and lymphoid lineages.

Animals↗
PubMed2005

Visualizing PU.1 activity during hematopoiesis.

OBJECTIVE: PU.1 is a critical transcription factor for hematopoietic development that is required for the early differentiation of myeloid, erythroid, and B lineage cells. To gain a better insight into PU.1 function, we performed a comprehensive analysis of PU.1 gene activity in the hematopoietic system, using a green fluorescent protein reporter mouse line. METHODS: We used flow cytometry to analyze green fluorescent protein (GFP) expression, along with various cell surface markers, in heterozygote mice that harbor a GFP reporter knocked into exon1 of the PU.1 gene. Phenotypic and functional properties of GFP+ and GFP- precursors were studied. RESULTS: We show that PU.1 is dynamically and heterogeneously expressed in many hematopoietic lineages, from the stem cell stage to terminally differentiated cells, suggesting that PU.1 is not only important in early differentiation events but also may play a role in mature hematopoietic cell function. Further, examination of GFP+ vs GFP- populations shows that differentiation, but not commitment, to the myeloid lineage requires PU.1. In contrast, B cell commitment is associated with low levels of PU.1 expression. CONCLUSION: Our study provides a detailed visualization of PU.1 gene activity in hematopoietic cells, and shows that highly dynamic regulation of PU.1 accompanies cell fate decisions during hematopoiesis.

Animals↗
PubMed2004

Enhanced bone formation in lipodystrophic PPARgamma(hyp/hyp) mice relocates haematopoiesis to the spleen.

The peroxisome proliferator-activated receptor gamma (PPARgamma) controls adipogenesis and metabolism. We demonstrate here that the absence of PPARgamma in fat has potent osteogenic activities, which affect haematopoiesis. The congenital absence of PPARgamma in fat of lipodystrophic PPARgamma(hyp/hyp) mice, strongly enhanced bone mass and consequentially reduced the bone-marrow cavity. Consistent with this, PPARgamma(hyp/hyp) mice had a significant decrease in bone marrow cellularity and resorted to extramedullary haematopoiesis in the spleen to maintain haematopoiesis. Our data indicate that antagonizing PPARgamma activity in fat could be an effective way to combat osteoporosis and suggest that haematopoietic function should be scrutinized in lipodystrophic subjects.

Animals↗
PubMed2004

PU.1 determines the self-renewal capacity of erythroid progenitor cells.

PU.1 is a hematopoietic-specific transcriptional activator that is absolutely required for the differentiation of B lymphocytes and myeloid-lineage cells. Although PU.1 is also expressed by early erythroid progenitor cells, its role in erythropoiesis, if any, is unknown. To investigate the relevance of PU.1 in erythropoiesis, we produced a line of PU.1-deficient mice carrying a green fluorescent protein reporter at this locus. We report here that PU.1 is tightly regulated during differentiation-it is expressed at low levels in erythroid progenitor cells and down-regulated upon terminal differentiation. Strikingly, PU.1-deficient fetal erythroid progenitors lose their self-renewal capacity and undergo proliferation arrest, premature differentiation, and apoptosis. In adult mice lacking one PU.1 allele, similar defects are detected following stress-induced erythropoiesis. These studies identify PU.1 as a novel and critical regulator of erythropoiesis and highlight the versatility of this transcription factor in promoting or preventing differentiation depending on the hematopoietic lineage.

Animals↗
PubMed2004

Evaluation of the whole genome DNA-DNA hybridization technique to identify bacteria in histological sections of periradicular lesions.

Whole genome DNA-DNA hybridization has been used to identify bacteria in periradicular lesions partly because there is no amplification of the bacteria, therefore, minor contaminants are not detected. There are, however, potential pitfalls with this technique, including inability to distinguish dead bacteria, cross-reactions of species within a genus, and inability to detect species present in low numbers because of loss of DNA during extraction and purification. Alternatively, inadequate extraction and purification of DNA could result in false positives. Therefore, controls are required to monitor DNA loss, DNA cross-reactions, and DNA of pure cultures mixed with bacteria-free tissue to monitor for false positives. We determined that the quality of DNA extracted from histological sections of periradicular lesions is excellent for DNA-DNA hybridization. Although lesions contain large numbers of bacteria, histological sections through lesions barely contain sufficient quantity of bacteria for such analysis. This was confirmed by histological observation of sparsely distributed bacteria within lesions. Furthermore, we found that the bacteria are not distributed evenly throughout periradicular lesions, in numbers or species.

Animals↗
PubMed2002

Ikaros regulates neutrophil differentiation.

The Ikaros gene encodes a zinc finger transcription factor that is selectively expressed by all hematopoietic cells. Although Ikaros is required for lymphocyte differentiation, its role in the myeloid lineage is unclear. We show here that Ikaros expression is temporally regulated during neutrophil differentiation: Ikaros is primarily expressed at immature stages and significantly less so in mature neutrophils. Furthermore Ik(L/L) mice, harboring a hypomorphic mutation at the Ikaros locus, exhibit several defects during neutrophil differentiation. (1) Ik(L/L) fetal livers contain high numbers of neutrophil lineage cells, and this increase is reflected in the number of GM-CSF-dependent progenitor cells. (2) The migratory potential and survival of neutrophil progenitors is altered in vitro. (3) Expression of the Gr-1 marker is delayed and repressed. In contrast, neutrophil function appears normal. These data demonstrate that Ikaros regulates early neutrophil differentiation but is dispensable in mature neutrophils.

Animals↗
PubMed2002

Ikaros is critical for B cell differentiation and function.

The Ikaros gene encodes a zinc-finger transcription factor required during early B cell development, as B-lineage cells are absent in mice lacking Ikaros. Here we describe a novel Ikaros-targeted mouse line carrying a beta-galactosidase reporter in which low amounts of Ikaros proteins remain expressed. In homozygote animals, B cells are absent during fetal development, but develop postnatally from a reduced pool of precursors. In vitro, the proliferation and differentiation of B-lineage progenitors are severely impaired. These defects are attenuated in vivo, but bone marrow B cells display an unusual pattern of cell surface marker expression and show decreased transcript levels for TdT, Rag-1, Rag-2 and lambda 5. These abnormalities suggest a partial block at the proB cell stage of differentiation. In the periphery, mature B cells exhibit a lower activation threshold but form fewer germinal centers in response to antigenic stimulation. Our results show that Ikaros controls multiple aspects of B cell differentiation and function.

Animals↗