Stroma-Mediated Dysregulation of Myelopoiesis in Mice Lacking IκBα

Notch signaling in the malignant bone marrow microenvironment: implications for a niche-based model of oncogenesis

Fueled by the growing interest in stem cell biology and the promise of regenerative medicine, study of the hematopoietic stem cell (HSC) microenvironment has provided critical insights into normal and malignant hematopoiesis. Notch receptor signaling in this microenvironment is a critical regulator of HSC fate and differentiation. Notch signaling also has the potential to modulate the growth of various malignant cell types, as evidenced by the growing list of hematologic cancers and other malignancies associated with either mutations in Notch genes or alterations in Notch signaling. In both health and disease, activation of Notch signaling predominantly exerts influence through stromal cell interactions with the tumor or stem cell microenvironments. Definitive evidence from transgenic mouse models has shown that alterations in stromal cell signaling from the bone marrow niche can induce malignant outgrowth of preleukemic clones and leukemia. Understanding how Notch receptor signals in the bone marrow microenvironment govern stem cell behavior will advance our understanding of cancer pathogenesis in hematologic malignancies and may have implications for treating metastatic solid tumors involving bone. These microenvironmental interactions are potential therapeutic targets for treating and preventing a variety of diseases, including bone marrow failure disorders, myelodysplastic syndromes, leukemia, and lymphoma.

Regulation of hematopoiesis by activators and inhibitors of Wnt signaling from the niche

Hematopoietic stem cells (HSCs) are a rare population of somatic stem cells that have the ability to regenerate the entire mature blood system in a hierarchical way for the duration of an adult life. Adult HSCs reside in the bone marrow niche. Different niche cell types and molecules regulate the balance of HSC dormancy and activation as well as HSC behavior in both normal and malignant hematopoiesis. Here, we describe the interplay of HSCs and their niche, in particular the involvement of the Wnt signaling pathway. Although the prevailing notion has been that malignant transformation of HSCs is the main cause of leukemia, evidence is mounting that disruption of niche regulation by transformed hematopoietic cells, which may overexpress Wnt signaling or intrinsic stromal defects in gene expression, is at least a collaborative factor in leukemogenesis. Thus, insights into the normal and altered functions of niche components will help to obtain a better understanding of normal and malignant hematopoiesis and how environmental factors affect these processes.

Advances in understanding the leukaemia microenvironment

Dynamic interactions between leukaemic cells and cells of the bone marrow are a feature of haematological malignancies. Two distinct microenvironmental niches in the bone marrow, the 'osteoblastic (endosteal)' and 'vascular' niches, provide a sanctuary for subpopulations of leukaemic cells to evade chemotherapy-induced death and allow acquisition of drug resistance. Key components of the bone marrow microenvironment as a home for normal haematopoietic stem cells and the leukaemia stem cell niches, and the molecular pathways critical for microenvironment/leukaemia interactions via cytokines, chemokines and adhesion molecules as well as hypoxic conditions, are described in this review. Finally, the genetic abnormalities of leukaemia-associated stroma are discussed. Further understanding of the contribution of the bone marrow niche to the process of leukaemogenesis may provide new targets that allow destruction of leukaemia stem cells without adversely affecting normal stem cell self-renewal.

Notch signaling in acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) is initiated by the PML-RARA (PR) fusion oncogene and has a characteristic expression profile that includes high levels of the Notch ligand Jagged-1 (JAG1). In this study, we used a series of bioinformatic, in vitro, and in vivo assays to assess the role of Notch signaling in human APL samples, and in a PML-RARA knock-in mouse model of APL (Ctsg-PML-RARA). We identified a Notch expression signature in both human primary APL cells and in Kit+Lin-Sca1+ cells from pre-leukemic Ctsg-PML-RARA mice. Both genetic and pharmacologic inhibition of Notch signaling abrogated the enhanced self-renewal seen in hematopoietic stem/progenitor cells from pre-leukemic Ctsg-PML-RARA mice, but had no influence on cells from age-matched wild-type mice. In addition, six of nine murine APL tumors tested displayed diminished growth in vitro when Notch signaling was inhibited pharmacologically. Finally, we found that genetic inhibition of Notch signaling with a dominant-negative Mastermind-like protein reduced APL growth in vivo in a subset of tumors. These findings expand the role of Notch signaling in hematopoietic diseases, and further define the mechanistic events important for PML-RARA-mediated leukemogenesis.

Intestinal tumorigenesis initiated by dedifferentiation and acquisition of stem-cell-like properties

Cell-type plasticity within a tumor has recently been suggested to cause a bidirectional conversion between tumor-initiating stem cells and nonstem cells triggered by an inflammatory stroma. NF-κB represents a key transcription factor within the inflammatory tumor microenvironment. However, NF-κB's function in tumor-initiating cells has not been examined yet. Using a genetic model of intestinal epithelial cell (IEC)-restricted constitutive Wnt-activation, which comprises the most common event in the initiation of colon cancer, we demonstrate that NF-κB modulates Wnt signaling and show that IEC-specific ablation of RelA/p65 retards crypt stem cell expansion. In contrast, elevated NF-κB signaling enhances Wnt activation and induces dedifferentiation of nonstem cells that acquire tumor-initiating capacity. Thus, our data support the concept of bidirectional conversion and highlight the importance of inflammatory signaling for dedifferentiation and generation of tumor-initiating cells in vivo.

Current insights into neutrophil homeostasis

Neutrophil granulocytes represent the first immunologic barrier against invading pathogens, and neutropenia predisposes to infection. However, neutrophils may also cause significant collateral inflammatory damage. Therefore, neutrophil numbers are tightly regulated by an incompletely understood homeostatic feedback loop adjusting the marrow's supply to peripheral needs. Granulocyte colony-stimulating factor (G-CSF) is accepted to be the major determinant of neutrophil production, and G-CSF levels have, soon after its discovery, been described to be inversely correlated with neutrophil counts. A neutrophil sensor, or "neutrostat," has, therefore, been postulated. The prevailing feedback hypothesis was established in adhesion molecule-deficient mice; it includes macrophages and Th17 cells, which determine G-CSF levels in response to the number of peripherally transmigrated, apoptosing neutrophils. Recent work has deepened our understanding of homeostatic regulation of neutrophil granulopoiesis, but there are still inconsistent findings and unresolved questions when it comes to a plausible hypothesis, similar to the feedback control models of red cell or platelet homeostasis.

From the transcription of genes involved in ectodermal dysplasias to the understanding of associated dental anomalies

Orodental anomalies are one aspect of rare diseases and are increasingly identified as diagnostic and predictive traits. To understand the rationale behind gene expression during tooth or other ectodermal derivative development and the disruption of odontogenesis or hair and salivary gland formation in human syndromes we analyzed the expression patterns of a set of genes (Irf6, Nfkbia, Ercc3, Evc2, Map2k1) involved in human ectodermal dysplasias in mouse by in situ hybridization. The expression patterns of Nfkbia, Ercc3 and Evc2 during odontogenesis had never been reported previously. All genes were indeed transcribed in different tissues/organs of ectodermal origin. However, for Nfkbia, Ercc3, Evc2, and Map2k1, signals were also present in the ectomesenchymal components of the tooth germs. These expression patterns were consistent in timing and localization with the known dental anomalies (tooth agenesis, microdontia, conical shape, enamel hypoplasia) encountered in syndromes resulting from mutations in those genes. They could also explain the similar orodental anomalies encountered in some of the corresponding mutant mouse models. Translational approaches in development and medicine are relevant to gain understanding of the molecular events underlying clinical manifestations.

IκBα deficiency in brain leads to elevated basal neuroinflammation and attenuated response following traumatic brain injury: implications for functional recovery

Background

The transcription factor NFκB is an important mediator of cell survival and inflammation in the immune system. In the central nervous system (CNS), NFκB signaling has been implicated in regulating neuronal survival following acute pathologic damage such as traumatic brain injury (TBI) and stroke. NFκB is normally bound by the principal inhibitory protein, IκBα, and sequestered in the cytoplasm. Activation of NFκB requires the degradation of IκBα, thereby freeing NFκB to translocate to the nucleus and activate the target genes. Mice deficient in IκBα display deregulated and sustained NFκB activation and early postnatal lethality, highlighting a critical role of IκBα in NFκB regulation.

Results

We investigated the role of IκBα in regulating NFκB activity in the brain and the effects of the NFκB/IκBα pathway in mediating neuroinflammation under both physiological and brain injury conditions. We report that astrocytes, but not neurons, exhibit prominent NFκB activity, and that basal NFκB activity in astrocytes is elevated in the absence of IκBα. By generating mice with brain-specific deletion of IκBα, we show that IκBα deficiency does not compromise normal brain development. However, basal neuroinflammation detected by GFAP and Iba1 immunoreactivity is elevated. This leads to impaired inflammatory responses following TBI and worsened brain damage including higher blood brain barrier permeability, increased injury volumes and enlarged ventricle volumes.

Conclusions

We conclude that, in the CNS, astrocyte is the primary cell type subject to NFκB regulation. We further demonstrate that IκBα plays an important role in regulating NFκB activity in the brain and a robust NFκB/IκBα-mediated neuroinflammatory response immediately following TBI is beneficial.

Noncanonical NF-κB signaling regulates hematopoietic stem cell self-renewal and microenvironment interactions

RelB and nuclear factor κB (NF-κB2) are the main effectors of NF-κB noncanonical signaling and play critical roles in many physiological processes. However, their role in hematopoietic stem/progenitor cell (HSPC) maintenance has not been characterized. To investigate this, we generated RelB/NF-κB2 double-knockout (dKO) mice and found that dKO HSPCs have profoundly impaired engraftment and self-renewal activity after transplantation into wild-type recipients. Transplantation of wild-type bone marrow cells into dKO mice to assess the role of the dKO microenvironment showed that wild-type HSPCs cycled more rapidly, were more abundant, and had developmental aberrancies: increased myeloid and decreased lymphoid lineages, similar to dKO HSPCs. Notably, when these wild-type cells were returned to normal hosts, these phenotypic changes were reversed, indicating a potent but transient phenotype conferred by the dKO microenvironment. However, dKO bone marrow stromal cell numbers were reduced, and bone-lining niche cells supported less HSPC expansion than controls. Furthermore, increased dKO HSPC proliferation was associated with impaired expression of niche adhesion molecules by bone-lining cells and increased inflammatory cytokine expression by bone marrow cells. Thus, RelB/NF-κB2 signaling positively and intrinsically regulates HSPC self-renewal and maintains stromal/osteoblastic niches and negatively and extrinsically regulates HSPC expansion and lineage commitment through the marrow microenvironment.

Constitutive activity of NF-kappa B in myeloid cells drives pathogenicity of monocytes and macrophages during autoimmune neuroinflammation

The NF-κB/REL-family of transcription factors plays a central role in coordinating the expression of a wide variety of genes controlling immune responses including autoimmunity of the central nervous system (CNS). The inactive form of NF-κB consists of a heterodimer which is complexed with its inhibitor, IκB. Conditional knockout-mice for IκBα in myeloid cells (lysMCreIκBα^fl/fl) have been generated and are characterized by a constitutive activation of NF-κB proteins allowing the study of this transcription factor in myelin-oligodendrocyte-glycoprotein induced experimental autoimmune encephalomyelitis (MOG-EAE), a well established experimental model for autoimmune demyelination of the CNS.

In comparison to controls, lysMCreIκBα^fl/fl mice developed a more severe clinical course of EAE. Upon histological analysis on day 15 p.i., there was an over two fold increased infiltration of T-cells and macrophages/microglia. In addition, lysMCreIκBα^fl/fl mice displayed an increased expression of the NF-κB dependent factor inducible nitric oxide synthase in inflamed lesions. These changes in the CNS are associated with increased numbers of CD11b positive splenocytes and a higher expression of Ly6c on monocytes in the periphery. Well in accordance with these changes in the myeloid cell compartment, there was an increased production of the monocyte cytokines interleukin(IL)-12 p70, IL-6 and IL-1beta in splenocytes. In contrast, production of the T-cell associated cytokines interferon gamma (IFN-gamma) and IL-17 was not influenced.

In summary, myeloid cell derived NF-κB plays a crucial role in autoimmune inflammation of the CNS and drives a pathogenic role of monocytes and macrophages independently from T-cells.

The triterpenoid CDDO-Me promotes hematopoietic progenitor expansion and myelopoiesis in mice

The synthetic triterpenoid CDDO-Me has been shown to directly inhibit the growth of myeloid leukemias and lends itself to a wide array of therapeutic indications, including inflammatory conditions, because of its inhibition of NF-κB. We have previously demonstrated protection from acute graft-versus-host disease after CDDO-Me administration in an allogeneic bone marrow transplantation model. In the current study, we observed that CDDO-Me promoted myelopoiesis in both naive and transplanted mice. This effect was dose dependent, as high doses of CDDO-Me inhibited myeloid growth in vitro. All lineages (granulocyte macrophage colony-forming unit, BFU-E) were promoted by CDDO-Me. We then compared the effects with granulocyte colony-stimulating factor, a known inducer of myeloid expansion and mobilization from the bone marrow. Whereas both drugs induced terminal myeloid expansion in the spleen, peripheral blood, and bone marrow, granulocyte colony-stimulating factor only induced granulocyte macrophage colony-forming unit precursors in the spleen, while CDDO-Me increased these precursors in the spleen and bone marrow. After sublethal total-body irradiation, mice pretreated with CDDO-Me further displayed an accelerated recovery of myeloid progenitors and total nucleated cells in the spleen. A similar expansion of myeloid and myeloid progenitors was noted with CDDO-Me treatment after syngeneic bone marrow transplantation. Combined, these data suggest that CDDO-Me may be of use posttransplantation to accelerate myeloid recovery in addition to the prevention of graft-versus-host disease.

AML1/RUNX1 functions as a cytoplasmic attenuator of NF-κB signaling in the repression of myeloid tumors

Functional deregulation of transcription factors has been found in many types of tumors. Transcription factor AML1/RUNX1 is one of the most frequent targets of chromosomal abnormalities in human leukemia and altered function of AML1 is closely associated with malignant transformation of hematopoietic cells. However, the molecular basis and therapeutic targets of AML1-related leukemia are still elusive. Here, we explored immediate target pathways of AML1 by in vitro synchronous inactivation in hematopoietic cells. We found that AML1 inhibits NF-κB signaling through interaction with IκB kinase complex in the cytoplasm. Remarkably, AML1 mutants found in myeloid tumors lack the ability to inhibit NF-κB signaling, and human cases with AML1-related leukemia exhibits distinctly activated NF-κB signaling. Furthermore, inhibition of NF-κB signaling in leukemic cells with mutated AML1 efficiently blocks their growth and development of leukemia. These findings reveal a novel role for AML1 as a cytoplasmic attenuator of NF-κB signaling and indicate that NF-κB signaling is one of the promising therapeutic targets of hematologic malignancies with AML1 abnormality.

Myeloid IκBα deficiency promotes atherogenesis by enhancing leukocyte recruitment to the plaques

Activation of the transcription factor NF-κB appears to be involved in different stages of atherogenesis. In this paper we investigate the role of NF-κB inhibitor IκBα in atherosclerosis. Myeloid-specific deletion of IκBα results in larger and more advanced lesions in LDL-R-deficient mice without affecting the compositional phenotype of the plaques or systemic inflammatory markers in the plasma. We show that IκBα-deleted macrophages display enhanced adhesion to an in vitro endothelial cell layer, coinciding with an increased expression of the chemokine CCL5. Also, in vivo we found that IκBα^del mice had more leukocytes adhering to the luminal side of the endothelial cell layers that cover the atherosclerotic plaques. Moreover, we introduce ER-MP58 in this paper as a new immunohistochemical tool for quantifying newly recruited myeloid cells in the atherosclerotic lesion. This staining confirms that in IκBα^del mice more leukocytes are attracted to the plaques. In conclusion, we show that IκBα deletion in myeloid cells promotes atherogenesis, probably through an induced leukocyte recruitment to plaques.

Leukemia stem cells and microenvironment: biology and therapeutic targeting

Acute myelogenous leukemia is propagated by a subpopulation of leukemia stem cells (LSCs). In this article, we review both the intrinsic and extrinsic components that are known to influence the survival of human LSCs. The intrinsic factors encompass regulators of cell cycle and prosurvival pathways (such as nuclear factor kappa B [NF-κB], AKT), pathways regulating oxidative stress, and specific molecular components promoting self-renewal. The extrinsic components are generated by the bone marrow microenvironment and include chemokine receptors (CXCR4), adhesion molecules (VLA-4 and CD44), and hypoxia-related proteins. New strategies that exploit potentially unique properties of the LSCs and their microenvironment are discussed.

Effects of the bone marrow microenvironment on hematopoietic malignancy

The bone marrow (BM) is contained within the bone cavity and is the main site of hematopoiesis, the continuous development of blood cells from immature hematopoietic stem and progenitor cells. The bone marrow consists of developing hematopoietic cells and non-hematopoietic cells, the latter collectively termed the bone marrow microenvironment. These non-hematopoietic cells include cells of the osteoblast lineage, adipocytes and endothelial cells. For many years these bone marrow microenvironment cells were predicted to play active roles in regulating hematopoiesis, and recent studies have confirmed such roles. Importantly, more recent data has indicated that cells of the BM microenvironment may also contribute to hematopoietic diseases. In this review we provide an overview of the roles of the data suggesting that the cells of the bone marrow microenvironment may play an active role in the initiation and progression of hematopoietic malignancy.

Innate immune signaling in the myelodysplastic syndromes

Myelodysplastic syndromes (MDS) are heterogeneous clonal hematologic malignancies characterized by cytopenias caused by ineffective hematopoiesis and propensity to progress to acute myeloid leukemia. Innate immunity provides immediate protection against pathogens by coordinating activation of signaling pathways in immune cells. Given the prominent role of the innate immune pathway in regulating hematopoiesis, it is not surprising that aberrant signaling of this pathway is associated with hematologic malignancies. Increased activation of the innate immune pathway may contribute to dysregulated hematopoiesis, dysplasia, and clonal expansion in myelodysplastic syndromes.

Atopic dermatitis-like disease and associated lethal myeloproliferative disorder arise from loss of Notch signaling in the murine skin

Background

The Notch pathway is essential for proper epidermal differentiation during embryonic skin development. Moreover, skin specific loss of Notch signaling in the embryo results in skin barrier defects accompanied by a B-lymphoproliferative disease. However, much less is known about the consequences of loss of Notch signaling after birth.

Methodology and Principal Findings

To study the function of Notch signaling in the skin of adult mice, we made use of a series of conditional gene targeted mice that allow inactivation of several components of the Notch signaling pathway specifically in the skin. We demonstrate that skin-specific inactivation of Notch1 and Notch2 simultaneously, or RBP-J, induces the development of a severe form of atopic dermatitis (AD), characterized by acanthosis, spongiosis and hyperkeratosis, as well as a massive dermal infiltration of eosinophils and mast cells. Likewise, patients suffering from AD, but not psoriasis or lichen planus, have a marked reduction of Notch receptor expression in the skin. Loss of Notch in keratinocytes induces the production of thymic stromal lymphopoietin (TSLP), a cytokine deeply implicated in the pathogenesis of AD. The AD-like associated inflammation is accompanied by a myeloproliferative disorder (MPD) characterized by an increase in immature myeloid populations in the bone marrow and spleen. Transplantation studies revealed that the MPD is cell non-autonomous and caused by dramatic microenvironmental alterations. Genetic studies demontrated that G-CSF mediates the MPD as well as changes in the bone marrow microenvironment leading to osteopenia.

Significance

Our data demonstrate a critical role for Notch in repressing TSLP production in keratinocytes, thereby maintaining integrity of the skin and the hematopoietic system.

How the niche regulates hematopoietic stem cells

The hematopoietic stem cell (HSC) forms all types of blood cells of the hematopoietic system. In the adult, HSC are mainly quiescent, being mostly in G0/G1 phase of cell cycle during steady-state conditions. However, during hematopoietic stress, the stem cells respond quickly to regenerate the damaged hematopoietic system. To understand how environmental signals affect HSC and its progeny, it is essential to know the lineage relationships and transcriptional mechanisms controlling self-renewal, proliferation and differentiation. Because of the high possible output of blood cells from a single HSC, a tight regulation of these processes is extremely important. An essential component for this control is the marrow microenvironment, in this context also referred to as the HSC niche. The niche is heterogeneous and regulates stem cell metabolism through both surface-bound and soluble factors. Several signaling pathways have been shown to take part in these regulation processes, with Notch and especially Wnt signaling being the best studied ones. Dysregulation of the niche, for instance by environmental exposure, has recently been shown to lead to hematopoietic abnormalities. Thus, to understand the effect of the environment on hematopoiesis, it is of importance to study both HSC, its direct progeny and the cellular components of the niche. Detailed knowledge of the regulatory mechanisms operating between hematopoietic cells and their direct surroundings facilitates the study of how such signaling may be disrupted by environmental exposure.

A novel role for the marrow microenvironment in initiating and sustaining hematopoietic disease

BACKGROUND

The marrow microenvironment is composed of a complex network of cells and extra cellular matrix that cooperate to regulate normal hematopoiesis. There is growing evidence that microenvironmental defects can contribute to the pathogenesis of hematological malignancies.

OBJECTIVE/METHODS

We review the role of the microenvironment in inducing and sustaining hematological malignancies.

RESULTS/CONCLUSIONS

Two basic mechanisms could explain the role of microenvironmental defects in the evolution of hematopoietic neoplasms. There is significant data to support the first mechanism, in which the malignant hematopoietic clone induces reversible functional changes in the microenvironment that result in improved growth conditions for the malignant cells. More recent studies from mouse models have indicated that a second mechanism involving primary microenvironmental defects can also result in malignancy.

Therapeutic targeting of microenvironmental interactions in leukemia: mechanisms and approaches

In hematological malignancies, there are dynamic interactions between leukemic cells and cells of the bone marrow microenvironment. Specific niches within the bone marrow microenvironment provide a sanctuary for subpopulations of leukemic cells to evade chemotherapy-induced death and allow acquisition of a drug-resistant phenotype. This review focuses on molecular and cellular biology of the normal hematopoietic stem cell and the leukemia stem cell niche, and of the molecular pathways critical for microenvironment/leukemia interactions. The key emerging therapeutic targets include chemokine receptors (CXCR4), adhesion molecules (VLA4 and CD44), and hypoxia-related proteins HIF-1alpha and VEGF. Finally, the genetic and epigenetic abnormalities of leukemia-associated stroma will be discussed. This complex interplay provides a rationale for appropriately tailored molecular therapies targeting not only leukemic cells but also their microenvironment to ensure improved outcomes in leukemia.

Myeloid cells control termination of lung inflammation through the NF-kappaB pathway

Although acute lung inflammation in response to local or systemic infection involves myeloid and nonmyeloid cells, the interplay between different cell types remains poorly defined. Since NF-kappaB is a key transcription factor for innate immunity, we investigated whether dysregulated NF-kappaB activation in myeloid cells impacts inflammatory signaling in nonmyeloid cells and generation of neutrophilic lung inflammation in response to systemic endotoxemia. We generated bone marrow chimeras by fetal liver transplantation of cells deficient in IkappaBalpha or p50 into lethally irradiated NF-kappaB reporter transgenic mice. No differences were apparent between bone marrow chimeras in the absence of an inflammatory stimulus; however, following intraperitoneal injection of Escherichia coli lipopolysaccharide (LPS), IkappaBalpha- or p50-deficient bone marrow chimeras showed increased NF-kappaB activation in nonhematopoietic cells, exaggerated neutrophilic inflammation, and higher mortality compared with untransplanted reporter mice and wild-type bone marrow chimeras. Primary bone marrow-derived macrophages (BMDM) from IkappaBalpha(-/-) or p50(-/-) exhibited increased NF-kappaB activation and macrophage inflammatory protein-2 production after LPS treatment compared with wild-type cells, and coculture of BMDM with lung epithelial (A549) cells resulted in increased NF-kappaB activation in A549 cells and excess IL-8 production by these epithelial cells. These studies indicate an important role for inhibitory members of the NF-kappaB family acting specifically within myeloid cells to limit inflammatory responses in the lungs.

Lack of conventional dendritic cells is compatible with normal development and T cell homeostasis, but causes myeloid proliferative syndrome

Dendritic cells are critically involved in the promotion and regulation of T cell responses. Here, we report a mouse strain that lacks conventional CD11c(hi) dendritic cells (cDCs) because of constitutive cell-type specific expression of a suicide gene. As expected, cDC-less mice failed to mount effective T cell responses resulting in impaired viral clearance. In contrast, neither thymic negative selection nor T regulatory cell generation or T cell homeostasis were markedly affected. Unexpectedly, cDC-less mice developed a progressive myeloproliferative disorder characterized by prominent extramedullary hematopoiesis and increased serum amounts of the cytokine Flt3 ligand. Our data identify a critical role of cDCs in the control of steady-state hematopoiesis, revealing a feedback loop that links peripheral cDCs to myelogenesis through soluble growth factors, such as Flt3 ligand.

Stroma-dependent apoptosis in clonal hematopoietic precursors correlates with expression of PYCARD

The role of the marrow microenvironment in the pathophysiology of myelodysplastic syndromes (MDSs) remains controversial. Using stromal/hematopoietic cell cocultures, we investigated the effects of stroma-derived signals on apoptosis sensitivity in hematopoietic precursors. The leukemia-derived cell line KG1a is resistant to proapoptotic ligands. However, when cocultured with the human stromal cell line HS5 (derived from normal marrow) and exposed to tumor necrosis factor-alpha (TNF-alpha), KG1a cells showed caspase-3 activation and induction of apoptosis. Apoptosis was contact dependent. Identical results were obtained in coculture with primary stroma. Gene-expression profiling of KG1a cells identified coculture-induced up-regulation of various genes involved in apoptosis, including PYCARD. Suppression of PYCARD expression in KG1a by miRNA interfered with apoptosis. Knockdown of the TNF receptor 1 (TNFR1) or TNFR2 in HS5 cells had no effect. However, knockdown of R1 in KG1a cells prevented TNF-alpha-induced apoptosis, while apoptosis was still induced by TNF-alpha-related apoptosis-inducing ligand. Primary CD34(+) cells from MDS marrow, when cocultured with HS5 and TNF-alpha, also underwent apoptosis. In contrast, no apoptosis was observed in CD34(+) cells from the marrow of healthy donors. These data indicate that stroma may convey not only protective effects on hematopoietic cells, but, dependent upon the milieu, may also facilitate apoptosis.

IkappaBalpha is required for marginal zone B cell lineage development

Inactivation of members of the nuclear factor-kappaB (NF-kappaB) family results in the decrease or defect of marginal zone B (MZB) cells. It is not known which inhibitors of the NF-kappaB family (IkappaB) are required for MZB cell development. Here, we show that mice with B cell-specific inactivation of the main NF-kappaB inhibitor IkappaBalpha have a marked decrease of MZB cells and their presumed precursors. They exhibited increased mortality rates after blood-borne bacterial infection, indicating the importance of MZB cells for bacterial clearance. In contrast, response to T cell-dependent and -independent antigens resulted only in minor changes in immunoglobulin production. Our data demonstrate the importance of the intact NF-kappaB/IkappaBalpha pathway for proper MZB cell development.

Crosstalk between keratinocytes and adaptive immune cells in an IkappaBalpha protein-mediated inflammatory disease of the skin

Inflammatory diseases at epithelial borders develop from aberrant interactions between resident cells of the tissue and invading immunocytes. Here, we unraveled basic functions of epithelial cells and immune cells and the sequence of their interactions in an inflammatory skin disease. Ubiquitous deficiency of the IkappaBalpha protein (Ikba(Delta)(/Delta)) as well as concomitant deletion of Ikba specifically in keratinocytes and T cells (Ikba(K5Delta/K5Delta lckDelta/lckDelta)) resulted in an inflammatory skin phenotype that involved the epithelial compartment and depended on the presence of lymphocytes as well as tumor necrosis factor and lymphotoxin signaling. In contrast, mice with selective ablation of Ikba in keratinocytes or lymphocytes showed inflammation limited to the dermal compartment or a normal skin phenotype, respectively. Targeted deletion of RelA from epidermal keratinocytes completely rescued the inflammatory skin phenotype of Ikba(Delta)(/Delta) mice. This finding emphasizes the important role of aberrant NF-kappaB activation in both keratinocytes and lymphocytes in the development of the observed inflammatory skin changes.

A microenvironment-induced myeloproliferative syndrome caused by retinoic acid receptor gamma deficiency

Myeloproliferative syndromes (MPS) are a heterogeneous subclass of nonlymphoid hematopoietic neoplasms which are considered to be intrinsic to hematopoietic cells. The causes of MPS are largely unknown. Here, we demonstrate that mice deficient for retinoic acid receptor gamma (RARgamma), develop MPS induced solely by the RARgamma-deficient microenvironment. RARgamma(-/-) mice had significantly increased granulocyte/macrophage progenitors and granulocytes in bone marrow (BM), peripheral blood, and spleen. The MPS phenotype continued for the lifespan of the mice and was more pronounced in older mice. Unexpectedly, transplant studies revealed this disease was not intrinsic to the hematopoietic cells. BM from wild-type mice transplanted into mice with an RARgamma(-/-) microenvironment rapidly developed the MPS, which was partially caused by significantly elevated TNFalpha in RARgamma(-/-) mice. These data show that loss of RARgamma results in a nonhematopoietic cell-intrinsic MPS, revealing the capability of the microenvironment to be the sole cause of hematopoietic disorders.

GGTase-I deficiency reduces tumor formation and improves survival in mice with K-RAS-induced lung cancer

Protein geranylgeranyltransferase type I (GGTase-I) is responsible for the posttranslational lipidation of CAAX proteins such as RHOA, RAC1, and cell division cycle 42 (CDC42). Inhibition of GGTase-I has been suggested as a strategy to treat cancer and a host of other diseases. Although several GGTase-I inhibitors (GGTIs) have been synthesized, they have very different properties, and the effects of GGTIs and GGTase-I deficiency are unclear. One concern is that inhibiting GGTase-I might lead to severe toxicity. In this study, we determined the effects of GGTase-I deficiency on cell viability and K-RAS-induced cancer development in mice. Inactivating the gene for the critical beta subunit of GGTase-I eliminated GGTase-I activity, disrupted the actin cytoskeleton, reduced cell migration, and blocked the proliferation of fibroblasts expressing oncogenic K-RAS. Moreover, the absence of GGTase-I activity reduced lung tumor formation, eliminated myeloproliferative phenotypes, and increased survival of mice in which expression of oncogenic K-RAS was switched on in lung cells and myeloid cells. Interestingly, several cell types remained viable in the absence of GGTase-I, and myelopoiesis appeared to function normally. These findings suggest that inhibiting GGTase-I may be a useful strategy to treat K-RAS-induced malignancies.

Conditional mutagenesis reveals immunological functions of widely expressed genes: activation thresholds, homeostatic mechanisms and disease models

Evolutionarily conserved, widely expressed genes provide the functional backbone of most, if not all, cell types. Although mouse mutants created by germ line gene inactivation are instrumental in establishing the importance of such genes in vivo, distortion of embryonic development or multiple body systems often preclude detailed functional studies. To overcome this limitation, DNA recombination systems such as Cre/loxP of bacteriophage P1, have been adapted for use in mammalian cells. The mutagenic event is restricted to the tissue or cell type in question leaving other body systems undisturbed. Conditional inactivation of Csk or Socs3, for example, established their key role in the prevention of inappropriate inflammation, while unexpected immunoregulatory activities emerged from studies of the NF-kappaB and AP-1 pathways. Also, cell types responsible for protective or pathogenic TNFalpha production have been identified. Inactivation of immunoregulatory receptors in leukocyte subsets can provide robust experimental systems revealing the conceptual simplicity underlying the modulation of complex signaling pathways during homeostatic responses. As illustrated for TGF-beta receptor, such system-guided approaches can provide a comprehensive picture of the regulatory events driving in vivo phenotype and specific responses of primary cells. This in turn facilitates the identification of novel regulatory mechanisms, targets for therapeutic intervention and prediction of side effects. With the increasing evidence for a role of somatic mutations in a wider range of human diseases, conditional mouse models are set to play a continuing part in the identification of pathogenic mechanisms for restoration of normal cellular processes in diseases including cancer, inflammation and autoimmunity.

Unravelling the complexities of the NF-κB signalling pathway using mouse knockout and transgenic models

The nuclear factor-kappaB (NF-kappaB) signalling pathway serves a crucial role in regulating the transcriptional responses of physiological processes that include cell division, cell survival, differentiation, immunity and inflammation. Here we outline studies using mouse models in which the core components of the NF-kappaB pathway, namely the IkappaB kinase subunits (IKKalpha, IKKbeta and NEMO), the IkappaB proteins (IkappaBalpha, IkappaBbeta, IkappaBvarepsilon and Bcl-3) and the five NF-kappaB transcription factors (NF-kappaB1, NF-kappaB2, c-Rel, RelA and RelB), have been genetically manipulated using transgenic and knockout technology.

Loss of Ikkbeta promotes migration and proliferation of mouse embryo fibroblast cells

The IkappaB kinase complex (IKK) is central to the activation of NF-kappaB, a critical transcription factor governing expression of genes involved in cell proliferation and anti-apoptotic responses. Mice with genetic disruptions of the Ikkbeta or Ikkgamma gene loci die during embryogenesis because of severe hepatic apoptosis. We now show that Ikkbeta gene deficiency promotes migration and proliferation of mouse embryo fibroblast cells. Morphological analyses revealed an unusual protrusion of the cytoplasm in Ikkbeta(-/-) cells when cultured at a lower density. In a Boyden chamber assay, Ikkbeta(-/-) cells exhibited a high rate of invasion and migration. Enhanced formation of actin stress fibers was also observed in the Ikkbeta(-/-) cells. Mechanistic studies indicated that IKKbeta affects the expression of proteins involved in the assembly of cytoskeleton and cell movement. Furthermore, re-expression of Ikkbeta and antioxidant treatment in Ikkbeta(-/-) cells caused a reversal of protrusive phenotype and high motility, respectively. Furthermore, elimination of reactive oxygen species (ROS) blocked expression of snail and subsequently derepressed E-cadherin expression. Although the underlying mechanism is likely entangled and complicated, the data presented indicate that generation of ROS played a key role in the morphological and mobility changes in Ikkbeta(-/-) cells. These data thus suggest that IKKbeta provides inhibitory signals for cell mobility and growth. Deficiency in the Ikkbeta gene promotes cell mobilization, at least partially, through a ROS-dependent mechanism.

Dissection of the NF-kappaB signalling cascade in transgenic and knockout mice

Studies in transgenic and knockout mice have made a major contribution to our current understanding of the physiological functions of the NF-kappaB signalling cascade. The generation and analysis of mice with targeted modifications of individual components of the NF-kappaB pathway tremendously advanced our knowledge of the roles of the NF-kappaB proteins themselves, and also of the many activators and negative regulators of NF-kappaB. These studies have highlighted the complexity of the NF-kappaB system, by revealing the multiple interactions, redundancies, but also diverse functions, performed by the different molecules participating in the regulation of NF-kappaB signalling. Furthermore, inhibition or enforced activation of NF-kappaB in transgenic mice has uncovered the critical roles that NF-kappaB plays in the pathogenesis of various diseases such as liver failure, diabetes and cancer.