Hox11 Acts Cell Autonomously in Spleen Development and Its Absence Results in Altered Cell Fate of Mesenchymal Spleen Precursors

Foxn1 is not essential for T-cell development in teleosts

In mammals, T-cell development depends on the activity of the Foxn1 transcription factor in the thymic epithelium; mutations in the vertebrate-specific Foxn1 gene are associated with profound T-cell lymphopenia and fatal immunodeficiency. Here, we examined the extent of T-cell development in teleosts lacking a functional foxn1 gene. In zebrafish carrying a deleterious internal deletion of foxn1, reduced but robust lymphopoietic activity is maintained in the mutant thymus. Moreover, pseudogenization or loss of foxn1 in the genomes of deep-sea anglerfishes is independent of the presence or absence of the canonical signatures of the T-cell lineage. Thus, in contrast to the situation in mammals, the teleost thymus can support foxn1-independent lymphopoiesis, most likely through the activity of the Foxn4, an ancient metazoan paralog of Foxn1. Our results imply that during the early stages of vertebrate evolution, genetic control of thymopoiesis was functionally redundant and thus robust; in mammals, the genetic network was reorganized to become uniquely dependent on the FOXN1 transcription factor.

Congenital asplenia due to a tlx1 mutation reduces resistance to Aeromonas hydrophila infection in zebrafish

It is documented that tlx1, an orphan homeobox gene, plays critical roles in the regulation of early spleen developmental in mammalian species. However, there is no direct evidence supporting the functions of tlx1 in non-mammalian species, especially in fish. In this study, we demonstrated that tlx1 is expressed in the splenic primordia as early as 52 hours post-fertilization (hpf) in zebrafish. A tlx1^-/- homozygous mutant line was generated via CRISPR/Cas9 to elucidate the roles of tlx1 in spleen development in zebrafish. In the tlx1^-/- background, tlx1^-/- cells persisted in the splenic primordia until 52 hpf but were no longer detectable after 53 hpf, suggesting perturbation of early spleen development. The zebrafish also exhibited congenital asplenia caused by the tlx1 mutation. Asplenic zebrafish can survive and breed normally under standard laboratory conditions, but the survival rate of animals infected with Aeromonas hydrophila was significantly lower than that of wild-type (WT) zebrafish. In asplenic zebrafish, the mononuclear phagocyte system was partially impaired, as demonstrated by retarded b7r expression and reduced ccr2 expression after injection with an inactivated A. hydrophila vaccine. Furthermore, the expression of MHCII/IgM was significantly reduced in the congenitally asplenic fish compared with that of the WT zebrafish. Taken together, our data suggest that tlx1 is a crucial regulator of spleen development in fish, as it is in mammals. We have also provided a new perspective for studying the role of the spleen during pathogen challenge in fish.

Comparative analysis of spleen transcriptome detects differences in evolutionary adaptation of immune defense functions in bighead carp and silver carp

The evolutionary divergence of the immune defense functions in bighead carp (A. nobilis) and silver carp (H. molitrix) is still not understood at the molecular level. Here, we obtained 48,821,754 and 55,054,480 clean reads from spleen tissue libraries prepared for bighead carp and silver carp using Illumina paired-end sequencing technology, respectively, and identified 4976 orthologous genes from the transcriptome data sets by comparative analysis. Adaptive evolutionary analysis showed that 212 orthologous genes and 255 Gene Ontology (GO) terms were subjected to positive selection(Ka/Ks values > 1) only in bighead carp, and 195 orthologous genes and 309 GO terms only in silver carp. Among immune defense functions with significant evolutionary divergence, the positively selected biological processes in bighead carp mainly included B cell-mediated immunity, chemokine-mediated signaling pathway, and immunoglobulin mediated immune response, whereas those in silver carp mainly included the antigen processing and presentation, defense response to fungus, and detection of bacteria. Moreover, we found 2974 genes expressed only in spleen of bighead carp and 3494 genes expressed only in spleen of silver carp, where these genes were mostly enriched in the same biological processes or pathways. These results provide a better understanding of the differences in resistance to some diseases by bighead carp and silver carp, and also facilitate the identification of candidate genes related to disease resistance.

Mesodermal induction of pancreatic fate commitment

The pancreas is a compound gland comprised of both exocrine acinar and duct cells as well as endocrine islet cells. Most notable amongst the latter are the insulin-synthesizing β-cells, loss or dysfunction of which manifests in diabetes mellitus. All exocrine and endocrine cells derive from multipotent pancreatic progenitor cells arising from the primitive gut epithelium via inductive interactions with adjacent mesodermal tissues. Research in the last two decades has revealed the identity of many of these extrinsic cues and they include signaling molecules used in many other developmental contexts such as retinoic acid, fibroblast growth factors, and members of the TGF-β superfamily. As important as these inductive cues is the absence of other signaling molecules such as hedgehog family members. Much has been learned about the interactions of extrinsic factors with fate regulators intrinsic to the pancreatic endoderm. This new knowledge has had tremendous impact on the development of directed differentiation protocols for converting pluripotent stem cells to β-cells in vitro.

Transcriptome profiling of developing spleen tissue and discovery of immune-related genes in grass carp (Ctenopharyngodon idella)

Grass carp Ctenopharyngodon idella is an important freshwater aquaculture species. However, studies regarding transcriptomic profiling of developing spleen tissue in the grass carp are lacking. Here, the transcriptome sequencing from the spleen tissue of one-year-old (cis1) and three-year-old (cis3) grass carp was performed using Illumina paired-end sequencing technology. The de novo assemblies yielded 48,970 unigenes with average lengths of 1264.51 bp from the two libraries. The assembled unigenes were evaluated and functionally annotated by comparing with sequences in major public databases including Nr, COG, Swiss-Prot, KEGG, Pfam and GO. Comparative analysis of expression levels revealed that a total of 38,254 unigenes were expressed in both the cis1 and cis3 libraries, while 4356 unigenes were expressed only in the cis1 library, and 3312 unigenes were expressed only in the cis3 library. Meanwhile, 1782 unigenes (including 903 down-regulated and 879 up-regulated unigenes) were differentially expressed between the two developmental stages of the grass carp spleen. Based on GO and KEGG enrichment analysis, these differentially expressed genes widely participated in the regulation of immunity and response in the grass carp. Moreover, the main components of six immune-related pathways were identified, including complement and coagulation cascades, Toll-like receptor signaling, B-cell receptor signaling, T-cell receptor signaling, antigen processing and presentation, and chemokine signaling. Finally, two identified transcripts including TLR 8 and complement component C8 were validated for reliability by RT-PCR. Collectively, the results obtained in this study will provide a basis for the study of molecular mechanisms in grass carp spleen development.

Transcription factor TLX1 controls retinoic acid signaling to ensure spleen development

The molecular mechanisms that underlie spleen development and congenital asplenia, a condition linked to increased risk of overwhelming infections, remain largely unknown. The transcription factor TLX1 controls cell fate specification and organ expansion during spleen development, and Tlx1 deletion causes asplenia in mice. Deregulation of TLX1 expression has recently been proposed in the pathogenesis of congenital asplenia in patients carrying mutations of the gene-encoding transcription factor SF-1. Herein, we have shown that TLX1-dependent regulation of retinoic acid (RA) metabolism is critical for spleen organogenesis. In a murine model, loss of Tlx1 during formation of the splenic anlage increased RA signaling by regulating several genes involved in RA metabolism. Uncontrolled RA activity resulted in premature differentiation of mesenchymal cells and reduced vasculogenesis of the splenic primordium. Pharmacological inhibition of RA signaling in Tlx1-deficient animals partially rescued the spleen defect. Finally, spleen growth was impaired in mice lacking either cytochrome P450 26B1 (Cyp26b1), which results in excess RA, or retinol dehydrogenase 10 (Rdh10), which results in RA deficiency. Together, these findings establish TLX1 as a critical regulator of RA metabolism and provide mechanistic insights into the molecular determinants of human congenital asplenia.

Splenocytes seed bone marrow of myeloablated mice: implication for atherosclerosis

Extramedullary hematopoiesis has been shown to contribute to the pathogenesis of a variety of diseases including cardiovascular diseases. In this process, the spleen is seeded with mobilized bone marrow cells that augment its hematopoietic ability. It is unclear whether these immigrant cells that are produced/reprogrammed in spleen are similar or different from those found in the bone marrow. To begin to understand this, we investigated the relative potency of adult splenocytes per se to repopulate bone marrow of lethally-irradiated mice and its functional consequences in atherosclerosis. The splenocytes were harvested from GFP donor mice and transplanted into myeloablated wild type recipient mice without the inclusion of any bone marrow helper cells. We found that adult splenocytes repopulated bone marrow of myeloablated mice and the transplanted cells differentiated into a full repertoire of myeloid cell lineages. The level of monocytes/macrophages in the bone marrow of recipient mice was dependent on the cell origin, i.e., the donor splenocytes gave rise to significantly more monocytes/macrophages than the donor bone marrow cells. This occurred despite a significantly lower number of hematopoietic stem cells being present in the donor splenocytes when compared with donor bone marrow cells. Atherosclerosis studies revealed that donor splenocytes displayed a similar level of atherogenic and atheroprotective activities to those of donor bone marrow cells. Cell culture studies showed that the phenotype of macrophages derived from spleen is different from those of bone marrow. Together, these results demonstrate that splenocytes can seed bone marrow of myeloablated mice and modulate atherosclerosis. In addition, our study shows the potential of splenocytes for therapeutic interventions in inflammatory disease.

Generation of a Tlx1(CreER-Venus) knock-in mouse strain for the study of spleen development

The spleen is a lymphoid organ that serves as a unique niche for immune reactions, extramedullary hematopoiesis, and the removal of aged erythrocytes from the circulation. While much is known about the immunological functions of the spleen, the mechanisms governing the development and organization of its stromal microenvironment remain poorly understood. Here we report the generation and analysis of a Tlx1(Cre) (ER) (-Venus) knock-in mouse strain engineered to simultaneously express tamoxifen-inducible CreER(T2) and Venus fluorescent protein under the control of regulatory elements of the Tlx1 gene, which encodes a transcription factor essential for spleen development. We demonstrated that Venus as well as CreER expression recapitulates endogenous Tlx1 transcription within the spleen microenvironment. When Tlx1(Cre) (ER) (-Venus) mice were crossed with the Cre-inducible reporter strain, Tlx1-expressing cells as well as their descendants were specifically labeled following tamoxifen administration. We also showed by cell lineage tracing that asplenia caused by Tlx1 deficiency is attributable to altered contribution of mesenchymal cells in the spleen anlage to the pancreatic mesenchyme. Thus, Tlx1(Cre) (ER) (-Venus) mice represent a new tool for lineage tracing and conditional gene manipulation of spleen mesenchymal cells, essential approaches for understanding the molecular mechanisms of spleen development.

Congenital asplenia in mice and humans with mutations in a Pbx/Nkx2-5/p15 module

The molecular determinants of spleen organogenesis and the etiology of isolated congenital asplenia (ICA), a life-threatening human condition, are unknown. We previously reported that Pbx1 deficiency causes organ growth defects including asplenia. Here, we show that mice with splenic mesenchyme-specific Pbx1 inactivation exhibit hyposplenia. Moreover, the loss of Pbx causes downregulation of Nkx2-5 and derepression of p15Ink4b in spleen mesenchymal progenitors, perturbing the cell cycle. Removal of p15Ink4b in Pbx1 spleen-specific mutants partially rescues spleen growth. By whole-exome sequencing of a multiplex kindred with ICA, we identify a heterozygous missense mutation (P236H) in NKX2-5 showing reduced transactivation in vitro. This study establishes that a Pbx/Nkx2-5/p15 regulatory module is essential for spleen development.

NKL homeobox genes in leukemia

NK-like (NKL) homeobox genes code for transcription factors, which can act as key regulators in fundamental cellular processes. NKL genes have been implicated in divergent types of cancer. In this review, we summarize the involvement of NKL genes in cancer and leukemia in particular. NKL genes can act as tumor-suppressor genes and as oncogenes, depending on tissue type. Aberrant expression of NKL genes is especially common in T-cell acute lymphoblastic leukemia (T-ALL). In T-ALL, 8 NKL genes have been reported to be highly expressed in specific T-ALL subgroups, and in ~30% of cases, high expression is caused by chromosomal rearrangement of 1 of 5 NKL genes. Most of these NKL genes are normally not expressed in T-cell development. We hypothesize that the NKL genes might share a similar downstream effect that promotes leukemogenesis, possibly due to mimicking a NKL gene that has a physiological role in early hematopoietic development, such as HHEX. All eight NKL genes posses a conserved Eh1 repressor motif, which has an important role in regulating downstream targets in hematopoiesis and possibly in leukemogenesis as well. Identification of a potential common leukemogenic NKL downstream pathway will provide a promising subject for future studies.

Cooperative DNA-binding and sequence-recognition mechanism of aristaless and clawless

To achieve accurate gene regulation, some homeodomain proteins bind cooperatively to DNA to increase those site specificities. We report a ternary complex structure containing two homeodomain proteins, aristaless (Al) and clawless (Cll), bound to DNA. Our results show that the extended conserved sequences of the Cll homeodomain are indispensable to cooperative DNA binding. In the Al-Cll-DNA complex structure, the residues in the extended regions are used not only for the intermolecular contacts between the two homeodomain proteins but also for the sequence-recognition mechanism of DNA by direct interactions. The residues in the extended N-terminal arm lie within the minor groove of DNA to form direct interactions with bases, whereas the extended conserved region of the C-terminus of the homeodomain interacts with Al to stabilize and localize the third alpha helix of the Cll homeodomain. This structure suggests a novel mode for the cooperativity of homeodomain proteins.

Familial isolated congenital asplenia: case report and literature review

Congenital asplenia is a rare life-threatening condition, often presenting with pneumococcal sepsis. It may arise as part of situs abnormalities or result from an unrelated specific defect of spleen development. The mode of inheritance is usually autosomal dominant, though sporadic cases are also reported. In affected individuals, the use of appropriate antibiotic prophylaxis and immunisations could save lives. In our report, we describe a family of three siblings with isolated congenital asplenia and unaffected parents, suggestive of recessive inheritance. The diagnosis in the proband was made post mortem following overwhelming pneumococcal sepsis. We also review the literature and compare the eight families previously reported with congenital isolated asplenia.

Independent functions and mechanisms for homeobox gene Barx1 in patterning mouse stomach and spleen

Homeobox genes convey positional information in embryos and their role in patterning the mammalian gut is a topic of considerable interest. Barx1 is expressed selectively in fetal stomach mesenchyme and directs differentiation of overlying endoderm. Recombinant tissue cultures and study of young mouse embryos previously suggested that Barx1 controls expression of secreted Wnt antagonists, which suppress endodermal Wnt signaling, to enable stomach epithelial differentiation. We overcame mid-gestational lethality of Barx1(-/-) mouse embryos and report here the spectrum of anomalies in a distinctive and unprecedented model of gastrointestinal homeotic transformation. Using various mouse models, we confirm the importance of attenuated Wnt signaling in stomach development and the role of Barx1 in suppressing endodermal Wnt activity. Absence of Barx1 also results in fully penetrant defects in positioning and expansion of the spleen, an organ that originates within the mesothelial lining of the stomach. Barx1 is absent from the spleen primordium but highly expressed in the mesogastrium, indicating an indirect effect on spleen development. However, our results argue against a role for Wnt antagonism in genesis of the spleen. Mouse spleen development relies on several homeodomain transcriptional regulators that are expressed in the spleen primordium. Loss of Barx1 does not affect expression of any of these genes but notably reduces expression of Wt1, a transcription factor implicated in spleen morphogenesis and expressed in the mesothelium. These observations place Barx1 proximally within a Wt1 pathway of spleen development and reveal how a homeotic regulator employs different molecular mechanisms to mold neighboring organs.

TLX1/HOX11 transcription factor inhibits differentiation and promotes a non-haemopoietic phenotype in murine bone marrow cells

The TLX/HOX11 subfamily of divergent homeobox genes are involved in various aspects of embryogenesis and, in the case of TLX1/HOX11 and TLX3/HOX11L2, feature prominently as oncogenes in human T-cell acute lymphoblastic leukaemia. TLX1 possesses immortalising activity in a wide variety of blood cell lineages, however, the effect of this oncogene on haemopoietic cell differentiation has not been fully investigated. We therefore constitutively expressed TLX1 in murine bone marrow or fetal liver cells using retroviral transfer followed by transplantation and/or in vitro culture. TLX1 was found to dramatically alter haemopoiesis, promoting the emergence of a non-haemopoietic CD45(-) CD31(+) cell population while markedly inhibiting erythroid and granulocytic cell differentiation. To identify genetic programs perturbed by TLX1, a comparison of transcript profiles from J2E erythroid cells with and without enforced TLX1 expression was undertaken. This revealed a pattern of gene expression indicative of enhanced proliferation coupled to differentiation arrest. Of the genes identified, two, KIT and VEGFC, were found to be potential TLX1 targets based on transcriptional assays. These results demonstrate that TLX1 can act broadly to impair haemopoiesis and divert differentiation to an alternative fate. This may account for its ability to promote the pre-leukaemic state via perturbation of specific gene expression programs.

Development and function of the mammalian spleen

The vertebrate spleen has important functions in immunity and haematopoiesis, many of which have been well studied. In contrast, we know much less about the mechanisms governing its early embryonic development. However, as a result of work over the past decade-mostly using knockout mice--significant progress has been made in unravelling the genetic processes governing the spleen's early development. Key genetic regulators, such as Tlx1 and Pbx1, have been identified, and we know some of the early transcriptional hierarchies that control the early patterning and proliferation of the splenic primordium. In mouse and humans, asplenia can arise as a result of laterality defects, or the spleen can be absent with no other discernible abnormalities. Surprisingly, given the spleen's diverse functions, asplenic individuals suffer no major haematopoietic or immune defects apart from a susceptibility to infection with encapsulated bacteria. Recent evidence has shed light on a previously unknown role of the spleen in the development and maintenance of specific B cell populations that are involved in the initial response to infection caused by encapsulated bacteria. The lack of these populations in asplenic mice and humans may go some way to explaining this susceptibility.

A promoter with bidirectional activity is located between TLX1/HOX11 and a divergently transcribed novel human gene

The chromosomal region 10q24 is involved in reciprocal translocations with one of the T-cell receptor loci in a significant proportion of human T-cell acute lymphoblastic leukemias. The breakpoints of these rearrangements cluster immediately upstream of the TLX1 homeobox gene and lead to its transcriptional activation. Genomic analysis using sequences located on the opposite side of the breakpoint cluster region identified a novel gene composed of three exons that is oriented in a head-to-head manner with TLX1. The novel gene, named TDI (TLX1 divergent) codes for a 1.9 kb transcript with an atypically long 5' leader sequence. Although predicted to be a transcriptional regulator of 13.4 kDa, the TDI protein has no significant sequence similarity to any known protein. The TLX1 and TDI genes are separated by a short spacer of only 161 bp that contains numerous GC boxes and a centrally located CCAAT box embedded within a CpG island. Using luciferase as the reporter in transient transfection assays, the intergenic region was found to be a functional promoter with robust bidirectional activity. TLX1 and TDI thus appear to represent another example of a divergently transcribed gene pair whose expression is regulated by a common promoter. Our finding that TDI is transcriptionally co-activated in leukemic cells that aberrantly express TLX1, additionally suggests that it may have the potential to act as a co-operating oncogene in leukemogenesis.

Molecular Networks Orchestrating GALT Development

During evolution, the development of secondary lymphoid organs has evolved as a strategy to promote adaptive immune responses at sites of antigen sequestration. Mesenteric lymph nodes (LNs) and Peyer's patches (PPs) are localized in proximity to mucosal surfaces, and their development is coordinated by a series of temporally and spatially regulated molecular events involving the collaboration between hematopoietic, mesenchymal, and, for PPs, epithelial cells. Transcriptional control of cellular differentiation, production of cytokines as well as adhesion molecules are mandatory for organogenesis, recruitment of mature leukocytes, and lymphoid tissue organization. Similar to fetal and neonatal organogenesis, lymphoid tissue neoformation can occur in adult individuals at sites of chronic stimulation via cytokines and TNF-family member molecules. These molecules represent new therapeutic targets to manipulate the microenvironment during autoimmune diseases.

Spleen versus pancreas: strict control of organ interrelationship revealed by analyses of Bapx1-/- mice

During early stages of pancreatic development, the mesenchyme that contributes to the spleen overlies the dorsal pancreatic endoderm. Here, we show that interactions between splenic mesenchyme and pancreas proceed via a highly orchestrated morphogenetic program. Disruption of morphogenesis, as occurs in the Bapx1(Nkx3.2)(-/-) embryo, results in transformation of these tissues into well-organized, ectopic gut-like structures. Bapx1 plays a crucial organizing role effecting position and separation of the spleen and pancreas to prevent this metaplastic transformation. Similar transformations occur in organ cultures employing wild-type pancreatic endoderm and spleen mesenchyme, revealing the developmental plasticity of the pancreas and that precise spatial and temporal control of tissue interactions are required for development of both organs.

Identification of novel genes and transcription factors involved in spleen, thymus and immunological development and function

We constructed and analyzed six serial analysis of gene expression (SAGE) libraries to identify genes with previously uncharacterized roles in spleen or thymus development. A total of 625 070 tags were sequenced from the three spleen (embryonic day (E)15.5, E16.5 and adult) and three thymus (E15.5, E18.5 and adult) libraries. These tags corresponded to 83 182 tag types, which mapped unambiguously to 36 133 different genes. Genes over-represented in these libraries, compared to 115 mouse SAGE libraries (www.mouseatlas.org), included genes of known and unknown immunological or developmental relevance. The expression profiles of 11 genes with unknown roles in spleen and thymus development were validated using reverse transcription-qPCR. We further characterized the expression of one of these candidates, RIKEN cDNA 9230105E10 that encodes a murine homolog of Trim5alpha, in numerous adult tissues and immune cell types. In addition, we demonstrate that transcript levels are upregulated in response to TLR stimulation of plasmacytoid dendritic cells and macrophages. This work provides the first evidence of regulated and cell type-specific expression of this gene. In addition, these observations suggest that the SAGE libraries provide an important resource for further investigations into the molecular mechanisms regulating spleen and thymus organogenesis, as well as the development of immunological competence.

New insights into the cell biology of the marginal zone of the spleen

In the marginal zone of the spleen the bloodstream passes through an open system of reticular cells and fibers in which various myeloid and lymphoid cells are located. Macrophages in this region are well equipped to recognize pathogens and filter the blood by virtue of unique combinations of pattern recognition receptors. They interact with a specific set of B cells that can be found only in the marginal zone and that are able to react rapidly to bacterial antigens in particular. This combination of strategically located cells is an important factor in our defense against blood-borne pathogens. New data on the development of the marginal zone itself and the marginal zone B cells are reviewed and discussed in light of the function of the spleen in host defense.

The nuclear oncoprotein TLX1/HOX11 associates with pericentromeric satellite 2 DNA in leukemic T-cells

TLX1/HOX11, a DNA-binding homeodomain protein, was originally identified by virtue of its aberrant expression in T-cell leukemia and subsequently found to be crucial for normal spleen development. The precise mechanism of TLX1 function remains poorly understood, although it is known that it can act as both a transcriptional activator and repressor and can downregulate the Aldh1a1 gene in embryonic mouse spleen. Using a whole-genome PCR approach, we show here that TLX1 protein directly interacts with pericentromeric human satellite 2 DNA sequences. Such DNA is known to localize to heterochromatin, which among other roles has been implicated in gene silencing. The interaction was confirmed in vitro and in vivo by gel retardation and chromatin immunoprecipitation assays involving satellite 2 DNA, which contained sequences resembling TLX1 binding sites. Using immunofluorescence microscopy, TLX1 demonstrated a punctate pattern of staining in the nuclei of leukemic T-cells (ALL-SIL). Double labelling indicated that TLX1 colocalized with the centromeric protein CENP-B, demonstrating that the TLX1 foci corresponded to clusters of centromeric DNA. The novel interaction of TLX1 with constitutive heterochromatin adds an additional level of complexity to the intracellular functions of this transcriptional regulator and may have relevance to its roles in transcriptional repression and T-cell immortalization.

Regenerative medicine: a radical reappraisal of the spleen

The spleen has long been considered a dispensable organ. Recent research, however, has found that the spleen of adult mice holds a reservoir of stem cells that can rapidly and robustly differentiate into functional cells of diverse lineages. Splenic stem cells express Hox11, a key embryonic transcription factor that regulates organogenesis. The presence of multi-lineage stem cells in the spleen might represent lifelong persistence of cells from a primitive embryonic region called the aorta-gonad-mesonephros. By bringing together findings from diverse disciplines, we propose that the adult spleen is an important source of multi-lineage stem cells for future cellular therapies for diabetes and other diseases.

A Pbx1-dependent genetic and transcriptional network regulates spleen ontogeny

The genetic control of cell fate specification, morphogenesis and expansion of the spleen, a crucial lymphoid organ, is poorly understood. Recent studies of mutant mice implicate various transcription factors in spleen development, but the hierarchical relationships between these factors have not been explored. In this report, we establish a genetic network that regulates spleen ontogeny, by analyzing asplenic mice mutant for the transcription factors Pbx1, Hox11 (Tlx1), Nkx3.2 (Bapx1) and Pod1 (capsulin, Tcf21). We show that Hox11 and Nkx2.5, among the earliest known markers for splenic progenitor cells, are absent in the splenic anlage of Pbx1 homozygous mutant (-/-) embryos, implicating the TALE homeoprotein Pbx1 in splenic cell specification. Pbx1 and Hox11 genetically interact in spleen formation and loss of either is associated with a similar reduction of progenitor cell proliferation and failed expansion of the splenic anlage. Chromatin immunoprecipitation assays show that Pbx1 binds to the Hox11 promoter in spleen mesenchymal cells, which co-express Pbx1 and Hox11. Furthermore, Hox11 binds its own promoter in vivo and acts synergistically with TALE proteins to activate transcription, supporting its role in an auto-regulatory circuit. These studies establish a Pbx1-Hox11-dependent genetic and transcriptional pathway in spleen ontogeny. Additionally, we demonstrate that while Nkx3.2 and Pod1 control spleen development via separate pathways, Pbx1 genetically regulates key players in both pathways, and thus emerges as a central hierarchical co-regulator in spleen genesis.

A microarray model system identifies potential new target genes of the proto-oncogene HOX11

HOX11 is a homeobox gene originally identified at a chromosomal breakpoint in T-cell acute lymphoblastic leukemia (T-ALL). It is one of the most frequently deregulated genes in T-ALL, although the precise role of HOX11 in leukemogenesis as well as in normal development remains obscure. To gain more insight into the functional role of HOX11, we utilized a microarray model system to characterize the gene expression network that it directs. Using one of our T-ALL cell lines that had been stably transfected to express HOX11 and high-density oligonucleotide HG-U95A arrays, we identified a large number of differentially expressed genes in response to the enforced expression of HOX11. We focused on examining genes found to be up-regulated according to the microarray analysis and selected three putative target genes, NFKB2, SMARCD3, and NR4A3, for further investigation. We could not only confirm the up-regulation of NR4A3 by an independent method in all clones expressing HOX11, but luciferase reporter assays demonstrated that the effect that HOX11 exerted on the proximal promoter of NR4A3 was dependent on the presence of an intact homeodomain, providing support for the idea that HOX11 manifests its regulatory function via its action as a transcription factor.

The existence of a putative regulatory element in 3'-untranslated region of proto-oncogene HOX11's mRNA

HOX11 encodes a homeodomain-containing transcription factor which directs the development of the spleen during embryogenesis. While HOX11 expression is normally silenced through an unknown mechanism in all tissues by adulthood, the deregulation of HOX11 expression is associated with leukemia, such as T-cell acute lymphoblastic leukemia. The elucidation of regulatory elements contributing to the molecular mechanism underlying the regulation of HOX11 gene expression is of great importance. Previous reports of HOX11 regulatory elements mainly focused on the 5\'-flanking region of HOX11 on the chromosome related to transcriptional control. To expand the search of putative cis-elements involved in HOX11 regulation at the post-transcriptional level, we analyzed HOX11 mRNA 3\'-untranslated region (3\'UTR) and found an AU-rich region. To characterize this AU-rich region, in vitro analysis of HOX11 mRNA 3\'UTR was performed with human RNAbinding protein HuR, which interacts with AU-rich element (ARE) existing in the 3\'UTR of many growth factors' and cytokines' mRNAs. Our results showed that the HOX11 mRNA 3\'UTR can specifically bind with human HuR protein in vitro. This specific binding could be competed effectively by typical ARE containing RNA. After the deletion of the AU-rich region present in the HOX11 mRNA 3\'UTR, the interaction of HOX11 mRNA 3\'UTR with HuR protein was abolished. These findings suggest that HOX11 mRNA 3\'UTR contains cis-acting element which shares similarity in the action pattern with ARE-HuR interactions and may involve in the posttranscriptional regulation of the HOX11 gene.

Diabetes and Stem Cell Researchers Turn to the Lowly Spleen

The spleen gets no respect. Long seen as superfluous, the adult spleen of the mouse has recently been shown to hold stem cells that, in diabetic mice or genetically altered mice that lack a pancreas, effectively regenerate insulin-producing islet cells of the pancreas. Stem cells of the spleen express Hox11, a highly conserved transcription factor that plays a major role in the development of organs in vertebrate and invertebrate embryos. Hox11 and other members of the Hox family of genes may give stem cells of the spleen the capacity to mature into cell types other than islet cells, including neurons and bone cells. Multilineage splenic stem cells may trace to the embryogenesis and possible persistence into adulthood of a fetal stem cell region called the aorta-gonad-mesonephros (AGM). This Perspective calls for reappraisal of the lowly spleen for treating diabetes and other diseases of aging.

Dysregulation of ferroportin 1 interferes with spleen organogenesis in polycythaemia mice

Regulatory interferences at the iron transporter ferroportin 1 (Fpn1) cause transient defects in iron homeostasis and erythropoiesis in polycythaemia(Pcm) mutant mice. The present study identified decreased Fpn1 expression in placental syncytiotrophoblast cells at late gestation as the mechanism of neonatal iron deficiency in Pcm mutants. Tissue specificity of embryonic Fpn1 dysregulation was evident from concomitant decreases in Fpn1 mRNA and protein expression in placenta and liver, as opposed to upregulation of Fpn1 protein despite decreased transcript levels in spleen, implicating post-transcriptional regulation of Fpn1. Dysregulation of Fpn1 and decreased iron levels in Pcm mutant spleens correlated with apoptotic cell death in the stroma, resulting in a semidominant spleen regression. At 7 weeks of age, a transient increase in spleen size in Pcm heterozygotes reflected a transient erythropoietin-mediated polycythemia. Structurally, Pcm mutant spleens displayed a severe defect in red pulp formation, including disruption of the sinusoidal endothelium, as well as discrete defects in white pulp organization during postnatal development. Reduced functional competence of the Pcmmutant spleen was manifested by an impaired response to chemically induced hemolytic anemia. Thus, aberrant Fpn1 regulation and iron homeostasis interferes with development of the spleen stroma during embryogenesis,resulting in a novel defect in spleen architecture postnatally.

The splanchnic mesodermal plate directs spleen and pancreatic laterality, and is regulated by Bapx1/Nkx3.2

The mechanism by which left-right (LR) information is interpreted by organ primordia during asymmetric morphogenesis is largely unknown. We show that spleen and pancreatic laterality is dependent on a specialised, columnar mesodermal-derived cell layer referred to here as the splanchnic mesodermal plate (SMP). At early embryonic stages, the SMP is bilateral, surrounding the midline-located stomach and dorsal pancreatic bud. Under control of the LR asymmetry pathway, the left SMP is maintained and grows laterally. Mice carrying the dominant hemimelia (Dh) mutation lack the SMP. Significantly, the mice are asplenic and the pancreas remains positioned along the embryonic midline. In the absence of Fgf10 expression, the spleno-pancreatic mesenchyme and surrounding SMP grow laterally but contain no endodermal component, showing that leftward growth is autonomous and independent of endoderm. In the Bapx1(-/-) mutants, the SMP is defective. Normally, the SMP is a source for both Fgf9 and Fgf10; however, in the Bapx1 mutant, Fgf10 expression is downregulated and the dorsal pancreas remains at the midline. We conclude that the SMP is an organiser responsible for the leftward growth of the spleno-pancreatic region and that Bapx1 regulates SMP functions required for pancreatic laterality.

Defect in dorso-ventral patterning, asplenia, and conotruncus in a spontaneously aborted fetus

We describe a very unusual combination, and previously unreported, of malformations in an 18-week, spontaneously aborted male fetus. The fetus had a reversed dorsoventral positioning of the head and upper limbs relative to the body axis with the head and both upper limbs directed dorsally, and an abrupt rotation of the vertebral bones at the level of CZ The fetus also had asplenia, single ventricle, and conotruncus. The fetus also had flexion deformities at the wrist, reduction deformity of the left second digit, anomalies in ossification of the bones of the left hand, and bilateral talipes calcaneovalgus. The major malformations in this fetus were all blastogenetic in origin and consisted of dorsoventral patterning defect involving structures cranial to C7, a laterality, and a septation defect of the ventricle and outflow tract of the heart. The defects are interpreted as being the result of abnormal coordination of the molecular signaling involved in dorsoventral axis formation and laterality of the limbs and trunk, and possibly also in cardiac septation.

The functional mapping of long-range transcription control elements of the HOX11 proto-oncogene

Mapping of transcriptional control elements normally depends on the generation of a series of deletion mutants. The consequences of particular deletions are then functionally assessed by their ability to alter gene expression. The information derived from such investigations provides a general regulatory profile of the gene of interest, as well as generating a focus for future experiments. Due to the limitations of conventional DNA cloning methods, it has previously not been possible to use such an approach to rapidly assess the role of long-range regulatory elements that frequently lie further than 20 kb away from the coding region. In order to identify regulatory elements of the proto-oncogene HOX11 that may be mutated in a subset of childhood T-cell acute lymphoblastic leukaemia specimens, we generated nested deletions from a P1 artificial chromosome (PAC). This clone contained 95 kilobases (kb) of the HOX11 locus at 10q24; including 63 kb of 5' regulatory DNA. The deletion series was produced by the use of a recombination based cloning system and clones were subsequently transfected into mammalian cells. We have identified several long-range regulatory elements that mediate transcriptional control of HOX11. This approach is simple, rapid, and inexpensive. Furthermore, it generates multiple deletion clones in a single experiment. This novel approach opens up a new avenue for investigating long-range transcription control. Additionally, by allowing analysis of these elements in the natural context of large integrants the approach does not require the use of artificial extrachromosomal elements. This methodology can be applied to any gene cloned into a PAC or BAC vector and could also be useful in identifying appropriately sized deletion mutants for functional testing in transgenic models.

Lymphoid organ development and cell migration

As we travel into a new century, confronted with new infectious diseases and bioweapon threats, with surgeons continuing to push the boundaries of what is transplantable, and with gene therapists working on ways to remedy a myriad of genetic diseases, the need for improved methods to augment and suppress immune function is paramount. The recent discovery that a novel immunosuppressant works by blocking lymphocyte egress from lymphoid organs provides a compelling example of how improved understanding of lymphoid organ function will contribute to future drug development and human health. This volume brings together reviews from leaders in the field of thymus and secondary lymphoid organ biology, including discussions on the roles of transcriptional regulators Foxn1, retinoid-related orphan receptor gamma and nuclear factor-kappaB in lymphoid organ development, the function of lymphotoxin and other cytokines in lymphoid tissue organization, the guidance activity of chemokines in a multitude of immune cell-positioning events, the mechanism of action of the immunosuppressant FTY720, and the application of two-photon laser scanning microscopy to reveal the dynamic behavior of lymphoid cells in the depths of these essential tissues.

B cells develop in the zebrafish pancreas

The zebrafish, with its transparent free-living embryo, is a useful organism for investigating early stages in lymphopoiesis. Previously, we showed that T cells differentiate in the thymus by day 4, but no sites for B cell differentiation were seen until 3 weeks. We report here that on day 4, we detect rearrangements of genes encoding B cell receptors in DNA extracted from whole fish. Also by day 4, rag1 transcripts are seen in the pancreas, an organ not previously associated with lymphopoiesis; by day 10, Igmu transcripts are detected here. Thus, in zebrafish, the pancreas assumes the role of both the liver in fetal mice and the spleen in neonatal mice.

Onset of Tlx-3 expression in the chick cerebellar cortex correlates with the morphological development of fissures and delineates a posterior transverse boundary

Recent studies have shown that the mammalian cerebellar cortex can be subdivided into a reproducible array of zones and stripes. In particular, discontinuous patterns of gene expression together with mutational analysis suggest that there are at least four distinct transverse zones along the rostrocaudal axis in mouse: the anterior zone (lobules I-V), the central zone (lobules VI and VII), the posterior zone (lobules VIII and IX), and the nodular zone (lobule X). Here we show that the divergent homeobox-containing transcription factor, Tlx- 3 (also known as Hox11L2 or Rnx) is transiently expressed in external granule cells in a distinct transverse domain of the developing chick cerebellar cortex. Expression is first detected at Hamburger and Hamilton (HH) stage 35. Interestingly, Tlx-3 mRNA expression is initially confined to, and coincident with, the morphological development of fissures. Slightly later, at HH stage 38, expression extends throughout the developing external granular layer (EGL) of lobules I-IXab. Notably, no Tlx-3 expression was detected in lobules IXc and X at any developmental time point examined. Expression is noticeably stronger in nonproliferating cells located in the deep layer of the EGL. Tlx-3 expression is downregulated as granule cells migrate inward to form the internal granule layer and is undetectable shortly after birth. These results suggest that Tlx-3 is expressed as granule cells become postmitotic and suggest that Tlx-3 may play a role in the differentiation of distinct neuronal populations in the cerebellum.

A complex containing PBX2 contributes to activation of the proto-oncogene HOX11

Ectopic expression of the homeobox gene HOX11 is associated with a significant proportion of childhood T-cell acute lymphoblastic leukaemias (T-ALLs). We hypothesise that one mechanism of gene deregulation involves overcoming the silencing mechanism(s) of gene expression present in normal cells. Here, we describe a search for trans-acting factors that control transcriptional activity from a distal 5' region of the HOX11 promoter. We have identified a region of this promoter which contributes significantly to HOX11 activation and two distinct regulatory elements are involved. First, a PBX2 Regulatory Element PRE-1048 has been identified which contains a novel DNA-binding sequence and mediates significant activation of the HOX11 gene in K562 cells. This is the first report of a homeobox gene being specifically regulated by PBX2 and the second report of a vertebrate homeobox target gene of a PBX protein. The PREP1 protein was also shown to be part of the PRE-1048-binding complex. The other regulatory element we describe here RE-1019 contains little sequence conservation to known transcription control elements. It appears that this element is a novel sequence that binds an as yet unidentified factor, mediating significant activation of the HOX11 gene in K562 cells. This is the first detailed report of elements that mediate regulation of the proto-oncogene HOX11.