Retinoblastoma promotes definitive erythropoiesis by repressing Id2 in fetal liver macrophages

An E2F binding-deficient Rb1 protein partially rescues developmental defects associated with Rb1 nullizygosity

Rb1 is essential for normal embryonic development, as null mice die in midgestation with widespread unscheduled cell proliferation. Rb1 protein (pRb) mediates cell cycle control by binding E2F transcription factors and repressing expression from E2F-dependent promoters. An increasing amount of evidence suggests that pRb loss also compromises cellular differentiation. Since differentiation is often dependent on cell cycle exit, it is currently unclear whether the effects of pRb on differentiation are an indirect consequence of pRb/E2F-mediated cell cycle control or whether they reflect direct cell-type-specific pRb functions. We have mutated Rb1 in the mouse to express a protein (R654W) specifically deficient in binding E2F1, E2F2, and E2F3. R654W mutant embryos exhibit cell cycle defects the same as those of Rb1 null embryos, reinforcing the importance of the interactions of pRb with E2F1, E2F2, and E2F3 for cell cycle control. However, R654W embryos survive at least 2 days longer than Rb1 null embryos, and increased life span is associated with improved erythrocyte and fetal liver macrophage differentiation. In contrast, R654W pRb does not rescue differentiation defects associated with pRb-deficient retinae. These data indicate that Rb1 makes important cell-type-specific contributions to cellular differentiation that are genetically separable from its general ability to stably bind E2F1, E2F2, and E2F3 and regulate the cell cycle.

The protein ENH is a cytoplasmic sequestration factor for Id2 in normal and tumor cells from the nervous system

Id2 is a natural inhibitor of the basic helix-loop-helix transcription factors and the retinoblastoma tumor suppressor protein. Active Id2 prevents differentiation and promotes cell-cycle progression and tumorigenesis in the nervous system. A key event that regulates Id2 activity during differentiation is translocation from the nucleus to the cytoplasm. Here we show that the actin-associated protein enigma homolog (ENH) is a cytoplasmic retention factor for Id2. ENH contains three LIM domains, which bind to the helix-loop-helix domain of Id proteins in vitro and in vivo. ENH is up-regulated during neural differentiation, and its ectopic expression in neuroblastoma cells leads to translocation of Id2 from the nucleus to the cytoplasm, with consequent inactivation of transcriptional and cell-cycle-promoting functions of Id2. Conversely, silencing of ENH by RNA interference prevents cytoplasmic relocation of Id2 in neuroblastoma cells differentiated with retinoic acid. Finally, the differentiated neural crest-derived tumor ganglioneuroblastoma coexpresses Id2 and ENH in the cytoplasm of ganglionic cells. These data indicate that ENH contributes to differentiation of the nervous system through cytoplasmic sequestration of Id2. They also suggest that ENH is a restraining factor of the oncogenic activity of Id proteins in neural tumors.

A retinoblastoma ortholog controls stalk/spore preference in Dictyostelium

We describe rblA, the Dictyostelium ortholog of the retinoblastoma susceptibility gene Rb. In the growth phase, rblA expression is correlated with several factors that lead to 'preference' for the spore pathway. During multicellular development, expression increases 200-fold in differentiating spores. rblA-null strains differentiate stalk cells and spores normally, but in chimeras with wild type, the mutant shows a strong preference for the stalk pathway. rblA-null cells are hypersensitive to the stalk morphogen DIF, suggesting that rblA normally suppresses the DIF response in cells destined for the spore pathway. rblA overexpression during growth leads to G1 arrest, but as growing Dictyostelium are overwhelmingly in G2 phase, rblA does not seem to be important in the normal cell cycle. rblA-null cells show reduced cell size and a premature growth-development transition; the latter appears anomalous but may reflect selection pressures acting on social ameba.

E2F-Rb complexes regulating transcription of genes important for differentiation and development

Inactivation of the retinoblastoma tumour suppressor protein (pRb) is a hallmark of most human cancers. Accordingly, pRb is serving as a paradigm in our quest to understand tumour suppressor function. The role played by pRb and the related 'pocket proteins', p107 and p130, in regulating cell cycle progression has been extensively studied over the past two decades. The function of pRb in regulating transcriptional programmes in differentiating cells is less well understood. Recently, the use of a variety of different cell, animal and plant model systems has allowed us a first glimpse at some of the molecular mechanisms underlying pRb-mediated transcriptional regulation during differentiation and development.

Id helix-loop-helix proteins negatively regulate TRANCE-mediated osteoclast differentiation

Tumor necrosis factor (TNF)-related activation-induced cytokine (TRANCE) induces osteoclast formation from monocyte/macrophage lineage cells via various transcription factors, including the Mi transcription factor (Mitf). Here, we show that inhibitors of differentiation/DNA binding (Ids), helix-loop-helix (HLH) transcription factors, negatively regulate TRANCE-induced osteoclast differentiation. Expression levels of Id1, Id2, and Id3 genes are significantly reduced by TRANCE during osteoclastogenesis. Interestingly, overexpression of the 3 Id genes in bone marrow-derived monocyte/macrophage lineage cells (BMMs) inhibits the formation of tartrate-resistant acid phosphatase (TRAP)-positive multinuclear osteoclasts, but it does not alter the ability of BMMs to either phagocytose or differentiate into dendritic cells (DCs). Overexpression of Id2 in BMMs attenuates the gene induction of nuclear factor of activated T cells c1 (NFATc1) and osteoclast-associated receptor (OSCAR) during TRANCE-mediated osteoclastogenesis. Furthermore, Id proteins interact with Mitf, a basic HLH (bHLH) transcription factor, and inhibit its transactivation of OSCAR, which is a costimulatory receptor expressed by osteoclast precursors, by attenuating the DNA binding ability of Mitf to the E-box site of the OSCAR promoter. Taken together, our results reveal both a new facet of negative regulation, mediated by Id proteins, as well as the mechanism whereby TRANCE signaling overcomes it, allowing osteoclastogenesis to proceed.

Differential involvement of PU.1 and Id2 downstream of TGF-beta1 during Langerhans-cell commitment

Langerhans cells (LCs) are highly abundant dendritic cells (DCs) in epidermal and mucosal tissues. The transcription factors PU.1 and Id2 have been implicated as positive regulators of LC development from hematopoietic progenitor cells. LC differentiation from progenitors is absolutely dependent on transforming growth factor beta 1 (TGF-beta1) in vitro as well as in vivo; however, downstream mechanisms are poorly defined. We found that both PU.1 and Id2 are induced by TGF-beta1 in human CD34+ monocyte/LC (M/LC) progenitor cells, and that neither ectopic PU.1 or Id2 alone, nor both together, could replace TGF-beta1 in its instructive function on LC commitment. However, both factors critically contributed to LC differentiation by acting at 2 distinct intersection points. Ectopic PU.1 strongly enhanced TGF-beta1-dependent LC development. Additionally, Notch-induced generation of interstitial-type DCs was associated with PU.1 up-regulation. Thus, PU.1 is generally increased during myeloid DC development. Ectopic Id2 inhibits the acquisition of early monocytic characteristics by cells generated in the absence of TGF-beta1 and also inhibits monocyte induction by alternative stimuli. Since TGF-beta1 represses a default monocyte pathway of common progenitor cells, PU.1 and Id2 seem to modulate lineage options of M/LC precursors, downstream of TGF-beta1.

Tumour suppressor retinoblastoma protein Rb: a transcriptional regulator

Rb was the first tumour suppressor identified through human genetic studies. The most significant achievement after almost twenty years since its cloning is the revelation that Rb possesses functions of a transcription regulator. Rb serves as a transducer between the cell cycle machinery and promoter-specific transcription factors. In this capacity, Rb is best known as a repressor of the E2F/DP family of transcription factors, which regulate expression of genes involved in cell proliferation and survival. An equally important aspect of Rb as a transcription regulator is that Rb also activates certain differentiation transcription factors to promote cellular differentiation. The molecular mechanisms behind the repressive effects of Rb on E2Fs have come to light in significant details, while those relating to Rb activation of differentiation transcription factors are much less understood. Finally, it has become clear that there are other aspects to Rb function that are not immediately related to transcription regulation.

A hanging drop culture method to study terminal erythroid differentiation

OBJECTIVE

To design a culture method allowing the quantitative and qualitative analysis of terminal erythroid differentiation.

METHODS

Primary erythroid progenitors derived either from mouse tissues or from human umbilical cord blood were differentiated using hanging drop cultures and compared to methylcellulose cultures. Cultured cells were analyzed by FACS to assess differentiation.

RESULTS

We describe a practical culture method by adapting the previously described hanging drop culture system to conditions allowing terminal differentiation of primary erythroid progenitors. Using minimal volumes of media and small numbers of cells, we obtained quantitative terminal erythroid differentiation within two days of culture in the case of murine cells and 4 days in the case of human cells.

CONCLUSIONS

The established methods for ex vivo culture of primary erythroid progenitors, such as methylcellulose-based burst-forming unit-erythroid (BFU-E) and colony-forming unit-erythroid (CFU-E) assays, allow the detection of committed erythroid progenitors but are of limited value to study terminal erythroid differentiation. We show that the application of hanging drop cultures is a practical alternative that, in combination with clonogenic assays, enables a comprehensive assessment of the behavior of primary erythroid cells ex vivo in the context of genetic and drug-induced perturbations.

MEK kinase 1 activity is required for definitive erythropoiesis in the mouse fetal liver

Mitogen-activated protein kinase/extracellular signal to regulated kinase (MEK) kinase 1 (MEKK1) is a c-Jun N-terminal kinase (JNK) activating kinase known to be implicated in proinflammatory responses and cell motility. Using mice deficient for MEKK1 kinase activity (Mekk1(DeltaKD)) we show a role for MEKK1 in definitive mouse erythropoiesis. Although Mekk1(DeltaKD) mice are alive and fertile on a 129 x C57/BL6 background, the frequency of Mekk1(DeltaKD) embryos that develop past embryonic day (E) 14.5 is dramatically reduced when backcrossed into the C57/BL6 background. At E13.5, Mekk1(DeltaKD) embryos have normal morphology but are anemic due to failure of definitive erythropoiesis. When Mekk1(DeltaKD) fetal liver cells were transferred to lethally irradiated wild-type hosts, mature red blood cells were generated from the mutant cells, suggesting that MEKK1 functions in a non-cell-autonomous manner. Based on immunohistochemical and hemoglobin chain transcription analysis, we propose that the failure of definitive erythropoiesis is due to a deficiency in enucleation activity caused by insufficient macrophage-mediated nuclear DNA destruction.

Id family of helix-loop-helix proteins in cancer

Over the past few decades, biologists have identified key molecular signatures associated with a wide range of human cancers. Recently, animal models have been particularly useful in establishing whether such signatures have functional relevance; the overexpression of pro-oncogenic or loss of anti-oncogenic factors have been evaluated for their effects on various tumour models. The aim of this review is to analyze the potential role of the inhibitor of DNA binding (Id) proteins in cancer and examine whether deregulated Id activity is tumorigenic and contributes to hallmarks of malignancy, such as loss of differentiation (anaplasia), unrestricted proliferation and neoangiogenesis.

Id2 mediates tumor initiation, proliferation, and angiogenesis in Rb mutant mice

The inhibitor of differentiation Id2 is a target of the retinoblastoma (Rb) protein during mouse embryogenesis. In Rb(+/-) mice, LOH at the wild-type Rb allele initiates pituitary adenocarcinoma, a tumor derived from embryonic melanotropes. Here we identify a critical role for Id2 in initiation, growth, and angiogenesis of pituitary tumors from Rb(+/-) mice. We show that proliferation and differentiation are intimately coupled in Rb(+/-) pituitary cells before tumor initiation. In Id2-null pituitaries, premature activation of basic helix-loop-helix-mediated transcription and expression of the cdk inhibitor p27(Kip1) impairs the proliferation of melanotropes and tumor initiation. Without Id2, Rb(+/-) mice have fewer early tumor lesions and a markedly decreased proliferation rate of the tumor foci. Expression of Id2 by pituitary tumor cells promotes growth and angiogenesis by functioning as a master regulator of vascular endothelial growth factor (VEGF). In human neuroblastoma, the N-Myc-driven expression of Id2 is sufficient and necessary for expression of VEGF. These results establish that aberrant Id2 activity directs initiation and progression of embryonal cancer.

The specification of early hematopoiesis in the mammal

PURPOSE OF REVIEW

A number of interesting surprises has emerged during the past year in the field of early hematopoietic development. This review highlights recent studies that have challenged the prevailing view of embryonic and fetal hematopoiesis in mammals, with a focus on the mouse as a model system. The authors apologize to the many colleagues whose work could not be cited because of space limitations.

RECENT FINDINGS

Advances in our understanding of the embryonic origins of hematopoiesis in mammals and in the regulation of primitive and definitive hematopoietic development are discussed.

SUMMARY

The ontological relation between primitive (embryonic) and definitive (fetal and adult) hematopoiesis still holds some mysteries for the biologist. Both technical and conceptual breakthroughs have refined our view of how blood cells form at different stages of development. What we learn from the embryo is not only of fundamental interest but may have future applications in the clinic.