Retinoic acid regulates programmed cell death through BMP signalling

Influence of DNA-methylation at multiple stages of limb chondrogenesis

Precise regulation of gene expression is of utmost importance during cell fate specification. DNA methylation is a key epigenetic mechanism that plays a significant role in the regulation of cell fate by recruiting repression proteins or inhibiting the binding of transcription factors to DNA to regulate gene expression. Limb development is a well-established model for understanding cell fate decisions, and the formation of skeletal elements is coordinated through a sequence of events that control chondrogenesis spatiotemporally. It has been established that epigenetic control participates in cartilage maturation. However, further investigation is required to determine its role in the earliest stages of chondrocyte differentiation. This study investigates how the DNA methylation environment affects cell fate divergence during the early chondrogenic events. Our research has shown for the first time that inhibiting DNA methylation in interdigital tissue with 5-azacytidine results in the formation of an ectopic digit. This discovery suggested that DNA methylation dynamics could regulate the fate of cells between chondrogenesis and cell death during autopod development. Our in vitro findings indicate that DNA methylation at the early stages of chondrogenesis is integral in regulating condensation by controlling cell adhesion and proapoptotic genes. As a result, the dynamics of methylation and demethylation are crucial in governing chondrogenesis and cell death during different stages of limb chondrogenesis.

Bone morphogenetic protein (BMP) signaling determines neuroblastoma cell fate and sensitivity to retinoic acid

Retinoic acid (RA) is a standard-of-care neuroblastoma drug thought to be effective by inducing differentiation. Curiously, RA has little effect on primary human tumors during upfront treatment but can eliminate neuroblastoma cells from the bone marrow during post-chemo consolidation therapy-a discrepancy that has never been explained. To investigate this, we treated a large cohort of neuroblastoma cell lines with RA and observed that the most RA-sensitive cells predominantly undergo apoptosis or senescence, rather than differentiation. We conducted genome-wide CRISPR knockout screens under RA treatment, which identified BMP signaling as controlling the apoptosis/senescence vs differentiation cell fate decision and determining RA's overall potency. We then discovered that BMP signaling activity is markedly higher in neuroblastoma patient samples at bone marrow metastatic sites, providing a plausible explanation for RA's ability to clear neuroblastoma cells specifically from the bone marrow, seemingly mimicking interactions between BMP and RA during normal development.

Cell death in the developing vertebrate limb: A locally regulated mechanism contributing to musculoskeletal tissue morphogenesis and differentiation

Our aim is to critically review current knowledge of the function and regulation of cell death in the developing limb. We provide a detailed, but short, overview of the areas of cell death observed in the developing limb, establishing their function in morphogenesis and structural development of limb tissues. We will examine the functions of this process in the formation and growth of the limb primordia, formation of cartilaginous skeleton, formation of synovial joints, and establishment of muscle bellies, tendons, and entheses. We will analyze the plasticity of the cell death program by focusing on the developmental potential of progenitors prior to death. Considering the prolonged plasticity of progenitors to escape from the death process, we will discuss a new biological perspective that explains cell death: this process, rather than secondary to a specific genetic program, is a consequence of the tissue building strategy employed by the embryo based on the formation of scaffolds that disintegrate once their associated neighboring structures differentiate.

We examine the functions of cell death in the formation and growth of the limb primordia.

We analyze the plasticity of the cell death program by focusing on the developmental potential of progenitors prior to death.

Considering the prolonged plasticity of progenitors to escape from the death process and the absence of defined genetic program in their regulation we propose that cell death is a consequence of the tissue building strategy employed by the embryo regulated by epigenetic factors .

Cooperation of BMP and IHH signaling in interdigital cell fate determination

The elaborate anatomy of hands and feet is shaped by coordinated formation of digits and regression of the interdigital mesenchyme (IM). A failure of this process causes persistence of interdigital webbing and consequently cutaneous syndactyly. Bone morphogenetic proteins (BMPs) are key inductive factors for interdigital cell death (ICD) in vivo. NOGGIN (NOG) is a major BMP antagonist that can interfere with BMP-induced ICD when applied exogenously, but its in vivo role in this process is unknown. We investigated the physiological role of NOG in ICD and found that Noggin null mice display cutaneous syndactyly and impaired interdigital mesenchyme specification. Failure of webbing regression was caused by lack of cell cycle exit and interdigital apoptosis. Unexpectedly, Noggin null mutants also exhibit increased Indian hedgehog (Ihh) expression within cartilage condensations that leads to aberrant extension of IHH downstream signaling into the interdigital mesenchyme. A converse phenotype with increased apoptosis and reduced cell proliferation was found in the interdigital mesenchyme of Ihh mutant embryos. Our data point towards a novel role for NOG in balancing Ihh expression in the digits impinging on digit-interdigit cross talk. This suggests a so far unrecognized physiological role for IHH in interdigital webbing biology.

Regulation of the expression of tumor necrosis factor‑related genes by abnormal histone H3K27 acetylation: Implications for neural tube defects

The association between apoptosis and neural tube defects (NTDs) is recognized as important, however, the precise link remains to be elucidated. Epigenetic modifications in human NTDs have been detected previously. In the present study, the occurrence of epigenetic modifications in apoptosis-related genes was investigated in a retinoic acid (RA)-induced mouse NTD model. Among 84 key genes involved in programmed cell death, 13 genes, including tumor necrosis factor (Tnf), annexin A5, apoptosis inhibitor 5, Bcl2-associated athanogene 3, baculoviral IAP repeat-containing 3, caspase (Casp)12, Casp4, Casp8, lymphotoxin β receptor, NLR family, apoptosis inhibitory protein 2, TNF receptor superfamily (Tnfrsf)1a, TNF superfamily (Tnfs)f10 and Tnfsf12, were downregulated, whereas nucleolar protein 3 was upregulated in the RA-induced NTD mice. Chromatin immunoprecipitation assays revealed that the regulatory regions of these differentially expressed TNF-related genes showed reduced histone H3K27 acetylation in NTDs, compared with control mice without NTDs. Reverse transcription-quantitative polymerase chain reaction revealed that H3K27ac-binding to the differentially regulated genes was markedly decreased in the NTD mice, whereas binding to the unchanged genes Casp3 and Nfkb1 was unaffected. In conclusion, certain TNF-related genes appeared to be downregulated in NTDs, possibly as a result of abnormal histone H3K27 acetylation. These results shed new light on the epigenetic dysregulation of apoptosis-related genes in NTDs.

Bones Morphogenic Protein-4 and retinoic acid combined treatment comparative analysis for in vitro differentiation potential of murine mesenchymal stem cells derived from bone marrow and adipose tissue into germ cells

Nowadays, infertility is no longer considered as an unsolvable disorder due to progresses in germ cells derived from stem lineage with diverse origins. Technical and ethical challenges push researchers to investigate various tissue sources to approach more efficient gametes. The purpose of the current study is to investigate the efficacy of a combined medium, retinoic acid (RA) together with Bone Morphogenic Protein-4 (BMP4), on differentiation of Bone Marrow Mesenchymal Stem Cells (BMMSCs) and adipose-derived mesenchymal stem cells (ADMSCs) into germ cells. Murine MSCs were obtained from both Bone Marrow (BM) and Adipose Tissue (AT) samples and were analyzed for surface markers to get further verification of their nature. BMMSCs and ADMSCs were induced into osteogenic and adipogenic lineage cells respectively, to examine their multipotency. They were finally differentiated into germ cells using media enriched with BMP4 for 4 days followed by addition of RA for 7 days (11 days in total). Analyzing of differentiation potential of BMMSCs- and ADMSCs were performed via Immunofluorescence, Flowcytometry and Real time-PCR techniques for germ cell-specific markers (Mvh, Dazl, Stra8 and Scp3). Mesenchymal surface markers (CD90 and CD44) were expressed on both BMMSCs and ADMSCs, while endothelial and hematopoietic cell markers (CD31 and CD45) had no expression. Finally, all germ-specific markers were expressed in both BM and AT. Although germ cells differentiated from ADMSCs showed faster growth and proliferation as well as easy collection, they significantly expressed germ-specific markers lower than BMMSCs. This suggests stronger differentiation potential of murine BMMSCs than ADMSCs.

Sox9 Expression in Amniotes: Species-Specific Differences in the Formation of Digits

In tetrapods the digit pattern has evolved to adapt to distinct locomotive strategies. The number of digits varies between species or even between hindlimb and forelimb within the same species. These facts illustrate the plasticity of embryonic limb autopods. Sox9 is a precocious marker of skeletal differentiation of limb mesenchymal cells. Its pattern of expression in the developing limb has been widely studied and reflects the activity of signaling cascades responsible for skeletogenesis. In this assay we stress previously overlooked differences in the pattern of expression of Sox9 in limbs of avian, mouse and turtle embryos which may reflect signaling differences associated with distinct limb skeletal morphologies observed in these species. Furthermore, we show that Sox9 gene expression is higher and maintained in the interdigital region in species with webbed digits in comparison with free digit animals.

Aberrant Meiotic Modulation Partially Contributes to the Lower Germination Rate of Pollen Grains in Maize (Zea mays L.) Under Low Nitrogen Supply

Pollen germination is an essential step towards successful pollination during maize reproduction. How low niutrogen (N) affects pollen germination remains an interesting biological question to be addressed. We found that only low N resulted in a significantly lower germination rate of pollen grains after 4 weeks of low N, phosphorus or potassium treatment in maize production. Importantly, cytological analysis showed 7-fold more micronuclei in male meiocytes under the low N treatment than in the control, indicating that the lower germination rate of pollen grains was partially due to numerous chromosome loss events resulting from preceding meiosis. The appearance of 10 bivalents in the control and low N cells at diakinesis suggested that chromosome pairing and recombination in meiosis I was not affected by low N. Further gene expression analysis revealed dramatic down-regulation of Nuclear Division Cycle 80 (Ndc80) and Regulator of Chromosome Condensation 1 (Rcc1-1) expression and up-regulation of Cell Division Cycle 20 (Cdc20-1) expression, although no significant difference in the expression level of kinetochore foundation proteins Centromeric Histone H3 (Cenh3) and Centromere Protein C (Cenpc) and cohesion regulators Recombination 8 (Rec8) and Shugoshin (Sgo1) was observed. Aberrant modulation of three key meiotic regulators presumably resulted in a high likelihood of erroneous chromosome segregation, as testified by pronounced lagging chromosomes at anaphase I or cell cycle disruption at meiosis II. Thus, we proposed a cytogenetic mechanism whereby low N affects male meiosis and causes a higher chromosome loss frequency and eventually a lower germination rate of pollen grains in a staple crop plant.

BMP4 Cooperates with Retinoic Acid to Induce the Expression of Differentiation Markers in Cultured Mouse Spermatogonia

Spermatogenesis is sustained by the proliferation and differentiation of spermatogonial stem cells (SSCs). However, the molecules controlling these processes remain largely unknown. Here, we developed a simplified high concentration serum-containing system for the culture of mouse SSCs. Analysis of SSCs markers and transplantation results revealed that the cultured spermatogonia retained stem cell characteristics after long-term in vitro propagation. Using this culture system, the expression and function of bone morphogenetic protein 4 (BMP4) were explored. Immunostaining showed that BMP4 was predominantly expressed in germ cells and that its level increased as spermatogenesis progresses. BMP4 receptors BMPR1A and BMPRII were present in spermatogonia, spermatocytes, and round spermatids. Moreover, despite the mRNAs of these two genes being present in mouse Sertoli cells, only BMPRII was detected by using Western blotting assays. While exogenous BMP4 by itself did not induce the expression of Stra8 and c-Kit, two marker genes of differentiating spermatogonia, a significant cooperative effect of BMP4 and retinoic acid (RA) was observed. Moreover, pretreatment of cultured spermatogonia with the BMP4 antagonist Noggin could inhibit RA-induced expression of these two marker genes. In conclusion, BMP4 may exert autocrine effects and act cooperatively with RA to induce the differentiation of spermatogonia in vivo.

Turning Bone Morphogenetic Protein 2 (BMP2) on and off in Mesenchymal Cells

The concentration, location, and timing of bone morphogenetic protein 2 (BMP2, HGNC:1069, GeneID: 650) gene expression must be precisely regulated. Abnormal BMP2 levels cause congenital anomalies and diseases involving the mesenchymal cells that differentiate into muscle, fat, cartilage, and bone. The molecules and conditions that influence BMP2 synthesis are diverse. Understandably, complex mechanisms control Bmp2 gene expression. This review includes a compilation of agents and conditions that can induce Bmp2. The currently known trans-regulatory factors and cis-regulatory elements that modulate Bmp2 expression are summarized and discussed. Bone morphogenetic protein 2 (BMP2, HGNC:1069, GeneID: 650) is a classical morphogen; a molecule that acts at a distance and whose concentration influences cell behavior. In mesenchymal cells, the concentration of BMP2 influences myogenesis, adipogenesis, chondrogenesis, and osteogenesis. Because the amount, timing, and location of BMP2 synthesis influence the allocation of cells to muscle, fat, cartilage, and bone, the mechanisms that regulate the Bmp2 gene are crucial. Key early mesodermal events that require precise Bmp2 regulation include heart specification and morphogenesis. Originally named for its osteoinductive properties, healing fractures requires BMP2. The human Bmp2 gene also has been linked to osteoporosis and osteoarthritis. In addition, all forms of pathological calcification in the vasculature and in cardiac valves involve the pro-osteogenic BMP2. The diverse tissues, mechanisms, and diseases influenced by BMP2 are too numerous to list here (see OMIM: 112261). However, in all BMP2-influenced pathologies, changes in the behavior and differentiation of pluripotent mesenchymal cells are a recurring theme. Consequently, much effort has been devoted to identifying the molecules and conditions that influence BMP2 synthesis and the complex mechanisms that control Bmp2 gene expression. This review begins with an overview of the Bmp2 gene's chromosomal neighborhood and then summarizes and evaluates known regulatory mechanisms and inducers.

BMPs are direct triggers of interdigital programmed cell death

During vertebrate embryogenesis the interdigital mesenchyme is removed by programmed cell death (PCD), except in species with webbed limbs. Although bone morphogenetic proteins (BMPs) have long been known to be players in this process, it is unclear if they play a direct role in the interdigital mesenchyme or if they only act indirectly, by affecting fibroblast growth factor (FGF) signaling. A series of genetic studies have shown that BMPs act indirectly by regulating the withdrawal of FGF activity from the apical ectodermal ridge (AER); this FGF activity acts as a cell survival factor for the underlying mesenchyme. Other studies using exogenous factors to inhibit BMP activity in explanted mouse limbs suggest that BMPs do not act directly in the mesenchyme. To address the question of whether BMPs act directly, we used an interdigit-specific Cre line to inactivate several genes that encode components of the BMP signaling pathway, without perturbing the normal downregulation of AER-FGF activity. Of three Bmps expressed in the interdigital mesenchyme, Bmp7 is necessary for PCD, but Bmp2 and Bmp4 both have redundant roles, with Bmp2 being the more prominent player. Removing BMP signals to the interdigit by deleting the receptor gene, Bmpr1a, causes a loss of PCD and syndactyly, thereby unequivocally proving that BMPs are direct triggers of PCD in this tissue. We present a model in which two events must occur for normal interdigital PCD: the presence of a BMP death trigger and the absence of an FGF survival activity. We demonstrate that neither event is required for formation of the interdigital vasculature, which is necessary for PCD. However, both events converge on the production of reactive oxygen species that activate PCD.

Vascular patterning regulates interdigital cell death by a ROS-mediated mechanism

Blood vessels serve as key regulators of organogenesis by providing oxygen, nutrients and molecular signals. During limb development, programmed cell death (PCD) contributes to separation of the digits. Interestingly, prior to the onset of PCD, the autopod vasculature undergoes extensive patterning that results in high interdigital vascularity. Here, we show that in mice, the limb vasculature positively regulates interdigital PCD. In vivo, reduction in interdigital vessel number inhibited PCD, resulting in syndactyly, whereas an increment in vessel number and distribution resulted in elevation and expansion of PCD. Production of reactive oxygen species (ROS), toxic compounds that have been implicated in PCD, also depended on interdigital vascular patterning. Finally, ex vivo incubation of limbs in gradually decreasing oxygen levels led to a correlated reduction in both ROS production and interdigital PCD. The results support a role for oxygen in these processes and provide a mechanistic explanation for the counterintuitive positive role of the vasculature in PCD. In conclusion, we suggest a new role for vascular patterning during limb development in regulating interdigital PCD by ROS production. More broadly, we propose a double safety mechanism that restricts PCD to interdigital areas, as the genetic program of PCD provides the first layer and vascular patterning serves as the second.

Impact of retinoic acid exposure on midfacial shape variation and manifestation of holoprosencephaly in Twsg1 mutant mice

Holoprosencephaly (HPE) is a developmental anomaly characterized by inadequate or absent midline division of the embryonic forebrain and midline facial defects. It is believed that interactions between genes and the environment play a role in the widely variable penetrance and expressivity of HPE, although direct investigation of such effects has been limited. The goal of this study was to examine whether mice carrying a mutation in a gene encoding the bone morphogenetic protein (BMP) antagonist twisted gastrulation (Twsg1), which is associated with a low penetrance of HPE, are sensitized to retinoic acid (RA) teratogenesis. Pregnant Twsg1(+/-) dams were treated by gavage with a low dose of all-trans RA (3.75 mg/kg of body weight). Embryos were analyzed between embryonic day (E)9.5 and E11.5 by microscopy and geometric morphometric analysis by micro-computed tomography. P19 embryonal carcinoma cells were used to examine potential mechanisms mediating the combined effects of increased BMP and retinoid signaling. Although only 7% of wild-type embryos exposed to RA showed overt HPE or neural tube defects (NTDs), 100% of Twsg1(-/-) mutants exposed to RA manifested severe HPE compared to 17% without RA. Remarkably, up to 30% of Twsg1(+/-) mutants also showed HPE (23%) or NTDs (7%). The majority of shape variation among Twsg1(+/-) mutants was associated with narrowing of the midface. In P19 cells, RA induced the expression of Bmp2, acted in concert with BMP2 to increase p53 expression, caspase activation and oxidative stress. This study provides direct evidence for modifying effects of the environment in a genetic mouse model carrying a predisposing mutation for HPE in the Twsg1 gene. Further study of the mechanisms underlying these gene-environment interactions in vivo will contribute to better understanding of the pathogenesis of birth defects and present an opportunity to explore potential preventive interventions.

Dietary vitamin A regulates wingless-related MMTV integration site signaling to alter the hair cycle

Alopecia areata (AA) is an autoimmune hair loss disease caused by a cell-mediated immune attack of the lower portion of the cycling hair follicle. Feeding mice 3-7 times the recommended level of dietary vitamin A accelerated the progression of AA in the graft-induced C3H/HeJ mouse model of AA. In this study, we also found that dietary vitamin A, in a dose dependent manner, activated the hair follicle stem cells (SCs) to induce the development and growth phase of the hair cycle (anagen), which may have made the hair follicle more susceptible to autoimmune attack. Our purpose here is to determine the mechanism by which dietary vitamin A regulates the hair cycle. We found that vitamin A in a dose-dependent manner increased nuclear localized beta-catenin (CTNNB1; a marker of canonical wingless-type Mouse Mammary Tumor Virus integration site family (WNT) signaling) and levels of WNT7A within the hair follicle bulge in these C3H/HeJ mice. These findings suggest that feeding mice high levels of dietary vitamin A increases WNT signaling to activate hair follicle SCs.

Dimeric combinations of MafB, cFos and cJun control the apoptosis-survival balance in limb morphogenesis

Apoptosis is an important mechanism for sculpting morphology. However, the molecular cascades that control apoptosis in developing limb buds remain largely unclear. Here, we show that MafB was specifically expressed in apoptotic regions of chick limb buds, and MafB/cFos heterodimers repressed apoptosis, whereas MafB/cJun heterodimers promoted apoptosis for sculpting the shape of the limbs. Chromatin immunoprecipitation sequencing in chick limb buds identified potential target genes and regulatory elements controlled by Maf and Jun. Functional analyses revealed that expression of p63 and p73, key components known to arrest the cell cycle, was directly activated by MafB and cJun. Our data suggest that dimeric combinations of MafB, cFos and cJun in developing chick limb buds control the number of apoptotic cells, and that MafB/cJun heterodimers lead to apoptosis via activation of p63 and p73.

Formation of normal interdigital web spaces in the hand revisited: implications for the pathogenesis of syndactyly in humans and experimental animals

The creation of the normal web spaces has been attributed to apoptosis. This paper presents evidence that lysosomal-mediated cell death and extracellular matrix degradation are important events in addition to cell death by apoptosis. The author proposes the use of the term interdigital cell death- extracellular matrix degradation instead of interdigital apoptosis. Furthermore, the concept of web creation by differential growth is introduced along with the discussion of the latest research in molecular biology and genetics on the topic.

Deregulated cell death and lymphocyte homeostasis cause premature lethality in mice lacking the BH3-only proteins Bim and Bmf

BH3 domain-only proteins (BH3-only) proteins are members of the Bcl-2 family that play crucial roles in embryogenesis and the maintenance of tissue homeostasis by triggering apoptotic cell death. The BH3-only protein Bim is critical for developmental apoptosis of lymphocytes, securing establishment of tolerance and for the termination of immune responses. Bim is believed to act in concert with other BH3-only proteins or members of the tumor necrosis factor receptor family in getting rid of unwanted cells. Bmf, a related BH3-only protein, was shown to play a role in B-cell homeostasis and to mediate cell death in response to certain apoptotic triggers, including glucocorticoid, histone deacetylase inhibitors, and overexpression of the c-Myc proto-oncogene. Here we show that Bim and Bmf have overlapping functions during mouse development and coregulate lymphocyte homeostasis and apoptosis in a nonredundant manner. Double deficiency of Bim and Bmf caused more B lymphadenopathy than loss of either BH3-only protein alone, and this was associated with autoimmune glomerulonephritis and a range of malignancies in aged mice. Thus, our results demonstrate that Bim and Bmf act in concert to prevent autoimmunity and malignant disease, strengthening the rational for the development of BH3-only protein mimicking therapeutics for the treatment of such disorders.

Detection of cells programmed to die in mouse embryos

Programmed Cell Death (PCD) is a broad term used to describe a series of events that culminate in the death of specific cells. In the embryo it occurs at predictable stages and tissues. During mouse development, PCD is a mechanism to preserve the homeostasis of the growing organism, and also is needed for the morphogenesis of a variety of structures. Apoptosis or PCD type I shows a sequence of morphological and biochemical changes such as plasma membrane blebbing, increase in mitochondrial membrane permeability, caspase activation, chromatin condensation, and phagocytosis. Many of these changes can be used to determine the occurrence of apoptosis in different type of samples. For example, apoptosis has been visualized in whole embryos and tissue sections using vital dyes, and by detection of degraded DNA or active caspases. In the present report, we compare these methods during the course of interdigital cell death in the mouse limbs. We discuss which method is the most suitable to detect a particular stage of apoptosis, which in some cases may be relevant for the interpretation of data. We detail combined protocols to observe mRNA expression or protein and cell death in the same tissue sample. Furthermore, we discuss some of the methodological problems to analyze autophagic cell death or PCD type II during embryo development.

Interdigital cell death in the embryonic limb is associated with depletion of Reelin in the extracellular matrix

Interdigital cell death is a physiological regression process responsible for sculpturing the digits in the embryonic vertebrate limb. Changes in the intensity of this degenerative process account for the different patterns of interdigital webbing among vertebrate species. Here, we show that Reelin is present in the extracellular matrix of the interdigital mesoderm of chick and mouse embryos during the developmental stages of digit formation. Reelin is a large extracellular glycoprotein which has important functions in the developing nervous system, including neuronal survival; however, the significance of Reelin in other systems has received very little attention. We show that reelin expression becomes intensely downregulated in both the chick and mouse interdigits preceding the establishment of the areas of interdigital cell death. Furthermore, fibroblast growth factors, which are cell survival signals for the interdigital mesoderm, intensely upregulated reelin expression, while BMPs, which are proapototic signals, downregulate its expression in the interdigit. Gene silencing experiments of reelin gene or its intracellular effector Dab-1 confirmed the implication of Reelin signaling as a survival factor for the limb undifferentiated mesoderm. We found that Reelin activates canonical survival pathways in the limb mesoderm involving protein kinase B and focal adhesion kinase. Our findings support that Reelin plays a role in interdigital cell death, and suggests that anoikis (apoptosis secondary to loss of cell adhesion) may be involved in this process.

Irx1 and Irx2 are coordinately expressed and regulated by retinoic acid, TGFβ and FGF signaling during chick hindlimb development

The Iroquois homeobox (Irx) genes play a crucial role in the regionalization and patterning of tissues and organs during metazoan development. The Irx1 and Irx2 gene expression pattern during hindlimb development has been investigated in different species, but its regulation during hindlimb morphogenesis has not been explored yet. The aim of this study was to evaluate the gene expression pattern of Irx1 and Irx2 as well as their regulation by important regulators of hindlimb development such as retinoic acid (RA), transforming growth factor β (TGFβ) and fibroblast growth factor (FGF) signaling during chick hindlimb development. Irx1 and Irx2 were coordinately expressed in the interdigital tissue, digital primordia, joints and in the boundary between cartilage and non-cartilage tissue. Down-regulation of Irx1 and Irx2 expression at the interdigital tissue coincided with the onset of cell death. RA was found to down-regulate their expression by a bone morphogenetic protein-independent mechanism before any evidence of cell death. Furthermore, TGFβ protein regulated Irx1 and Irx2 in a stage-dependent manner at the interdigital tissue, it inhibited their expression when it was administered to the interdigital tissue at developing stages before their normal down-regulation. TGFβ administered to the interdigital tissue at developing stages after normal down-regulation of Irx1 and Irx2 evidenced that expression of these genes marked the boundary between cartilage tissue and non-cartilage tissue. It was also found that at early stages of hindlimb development FGF signaling inhibited the expression of Irx2. In conclusion, the present study demonstrates that Irx1 and Irx2 are coordinately expressed and regulated during chick embryo hindlimb development as occurs in other species of vertebrates supporting the notion that the genomic architecture of Irx clusters is conserved in vertebrates.

Molecular control of cell differentiation and programmed cell death during digit development

During the hand plate development, the processes of cell differentiation and control of cell death are relevant to ensure a correct shape of the limb. The progenitor cell pool that later will differentiate into cartilage to form the digits arises from undifferentiated mesenchymal cells beneath the apical ectodermal ridge (AER). Once these cells abandon the area of influence of signals from AER and ectoderm, some cells are committed to chondrocyte lineage forming the digital rays. However, if the cells are not committed to chondrocyte lineage, they will form the prospective interdigits that in species with free digits will subsequently die. In this work, we provide the overview of the molecular interactions between different signaling pathways responsible for the formation of digit and interdigit regions. In addition, we briefly describe some experiments concerning the most important signals responsible for promoting cell death. Finally, on the basis that the interdigital tissue has chondrogenic potential, we discuss the hypothesis that apoptotic-promoting signals might also act as antichondrogenic factors and chondrogenic factors might operate as anti-apoptotic factors.

Smad8 is expressed in the anterior necrotic zone: evidence for a role of bone morphogenetic proteins/SMAD signaling in the activation of a molecular cascade that culminates in cell death

Bone morphogenetic proteins (BMPs) play a crucial role in programmed cell death (PCD), a biological process required for the sculpturing of the embryonic limbs. However, it is unknown if BMP signaling directly promotes cell death, or if it induces a molecular cascade that culminates in cell death. Given that Smad8, which encodes one component of BMP signaling, is expressed during the regression of interdigital tissue and responds to BMPs, we presumed that it may be expressed in other cell death areas during chick limb development such as the anterior and posterior necrotic zones (ANZ and PNZ). The present study found that the Smad8 expression pattern in the anterior mesoderm of the hindlimb is very similar to that observed in limbs stained to detect cell death. Also, BMPs and retinoic acid, which act as apoptosis-promoting factors, induced expression of Smad8 before the onset of cell death, while sonic hedgehog protein, acting as a survival factor, inhibited Smad8 expression in the ANZ. However, although there was correlation between Smad8 expression patterns and PCD in the ANZ, phosphorylated forms of SMAD1/5/8 and TUNEL staining did not co-localize in dying cells. Interestingly, a short pulse of BMP was sufficient to trigger cell death. On the other hand, most dying cells were located in the avascular region, while many cells expressing Smad8 were located in the vascular region of the ANZ. These results suggest that BMPs mediated by SMAD signaling activate a molecular cascade that culminates in PCD.

Vitamin A in reproduction and development

The requirement for vitamin A in reproduction was first recognized in the early 1900's, and its importance in the eyes of developing embryos was realized shortly after. A greater understanding of the large number of developmental processes that require vitamin A emerged first from nutritional deficiency studies in rat embryos, and later from genetic studies in mice. It is now generally believed that all-trans retinoic acid (RA) is the form of vitamin A that supports both male and female reproduction as well as embryonic development. This conclusion is based on the ability to reverse most reproductive and developmental blocks found in vitamin A deficiency induced either by nutritional or genetic means with RA, and the ability to recapitulate the majority of embryonic defects in retinoic acid receptor compound null mutants. The activity of the catabolic CYP26 enzymes in determining what tissues have access to RA has emerged as a key regulatory mechanism, and helps to explain why exogenous RA can rescue many vitamin A deficiency defects. In severely vitamin A-deficient (VAD) female rats, reproduction fails prior to implantation, whereas in VAD pregnant rats given small amounts of carotene or supported on limiting quantities of RA early in organogenesis, embryos form but show a collection of defects called the vitamin A deficiency syndrome or late vitamin A deficiency. Vitamin A is also essential for the maintenance of the male genital tract and spermatogenesis. Recent studies show that vitamin A participates in a signaling mechanism to initiate meiosis in the female gonad during embryogenesis, and in the male gonad postnatally. Both nutritional and genetic approaches are being used to elucidate the vitamin A-dependent pathways upon which these processes depend.

Recent advances and prospects in the differentiation of pancreatic cells from human embryonic stem cells

Recent studies with human embryonic stem (hES) cells have established new protocols for substantial generation of pancreatic progenitors from definitive endoderm. These findings add to the efficient derivation of definitive endoderm, which is controlled by Wnt and Nodal pathways, and delineate a step forward in the quest for alternative beta-cell sources. It also indicates that critical refining of the available strategies might help define a universal protocol for pancreatic differentiation applicable to several cell lines, therefore offering the possibility for transplantation of immune-matched or patient-specific hES-derived beta-cells. We appraise here the fundamental role that bone morphogenetic protein, fibroblast growth factor, and retinoid signaling play during pancreas development, and describe a fundamental emergence of their combination in recent studies that generated pancreatic cells from hES cells. We finally enumerate some prospects that might improve further differentiation of the progenitor cells into functional beta-cells needed in diabetes cell therapy.

Sculpturing digit shape by cell death

Physiological cell death is a key mechanism that ensures appropriate development and maintenance of tissues and organs in multicellular organisms. Most structures in the vertebrate embryo exhibit defined areas of cell death at precise stages of development. In this regard the areas of interdigital cell death during limb development provide a paradigmatic model of massive cell death with an evident morphogenetic role in digit morphogenesis. Physiological cell death has been proposed to occur by apoptosis, cellular phenomena genetically controlled to orchestrate cell suicide following two main pathways, cytochrome C liberation from the mitochondria or activation of death receptors. Such pathways converge in the activation of cysteine proteases known as caspases, which execute the cell death program, leading to typical morphologic changes within the cell, termed apoptosis. According to these findings it would be expected that caspases loss of function experiments could cause inhibition of interdigital cell death promoting syndactyly phenotypes. A syndactyly phenotype is characterized by absence of digit freeing during development that, when caused by absence of interdigital cell death, is accompanied by the persistence of an interdigital membrane. However this situation has not been reported in any of the KO mice or chicken loss of function experiments ever performed. Moreover histological analysis of dying cells within the interdigit reveals the synchronic occurrence of different types of cell death. All these findings are indicative of caspase alternative and/or complementary mechanisms responsible for physiological interdigital cell death. Characterization of alternative cell death pathways is required to explain vertebrate morphogenesis. Today there is great interest in cell death via autophagy, which could substitute or act synergistically to the apoptotic pathway. Here we discuss what is known about physiological cell death in the developing interdigital tissue of vertebrate embryos, paying special attention to the avian species.

Wnt/beta-catenin and retinoic acid receptor signaling pathways interact to regulate chondrocyte function and matrix turnover

Activation of the Wnt/beta-catenin and retinoid signaling pathways is known to tilt cartilage matrix homeostasis toward catabolism. Here, we investigated possible interactions between these pathways. We found that all-trans-retinoic acid (RA) treatment of mouse epiphyseal chondrocytes in culture did increase Wnt/beta-catenin signaling in the absence or presence of exogenous Wnt3a, as revealed by lymphoid enhancer factor/T-cell factor/beta-catenin reporter activity and beta-catenin nuclear accumulation. This stimulation was accompanied by increased gene expression of Wnt proteins and receptors and was inhibited by co-treatment with Dickkopf-related protein-1, an extracellular inhibitor of Wnt/beta-catenin signaling, suggesting that RA modulates Wnt signaling at Wnt cell surface receptor level. RA also enhanced matrix loss triggered by Wnt/beta-catenin signaling, whereas treatment with a retinoid antagonist reduced it. Interestingly, overexpression of retinoic acid receptor gamma (RARgamma) strongly inhibited Wnt/beta-catenin signaling in retinoid-free cultures, whereas small interfering RNA-mediated silencing of endogenous RARgamma expression strongly increased it. Small interfering RNA-mediated silencing of RARalpha or RARbeta had minimal effects. Co-immunoprecipitation and two-hybrid assays indicated that RARgamma interacts with beta-catenin and induces dissociation of beta-catenin from lymphoid enhancer factor in retinoid-free cultures. The N-terminal domain (AF-1) of RARgamma but not the C-terminal domain (AF-2) was required for association with beta-catenin, whereas both AF-1 and AF-2 were necessary for inhibition of beta-catenin transcriptional activity. Taken together, our data indicate that the Wnt and retinoid signaling pathways do interact in chondrocytes, and their cross-talks and cross-regulation play important roles in the regulation of cartilage matrix homeostasis.

Progressive interdigital cell death: regulation by the antagonistic interaction between fibroblast growth factor 8 and retinoic acid

The complete cohort of molecules involved in interdigital cell death (ICD) and their interactions are yet to be defined. Bmp proteins, retinoic acid (RA) and Fgf8 have been previously identified as relevant factors in the control of ICD. Here we determined that downregulation of Fgf8 expression in the ectoderm overlying the interdigital areas is the event that triggers ICD, whereas RA is the persistent cell death-inducing molecule that acts on the distal mesenchyme by a mechanism involving the induction of Bax expression. Inhibition of the mitogen-activated protein kinase (Mapk) pathway prevents the survival effect of Fgf8 on interdigital cells and the accompanying Erk1/2 phosphorylation and induction of Mkp3 expression. Fgf8 regulates the levels of RA by both decreasing the expression of Raldh2 and increasing the expression of Cyp26b1, whereas RA reduces Fgfr1 expression and Erk1/2 phosphorylation. In the mouse limb, inhibition of Bmp signaling in the mesenchyme does not affect ICD. However, noggin in the distal ectoderm induces Fgf8 expression and reduces interdigit regression. In the chick limb, exogenous noggin reduces ICD, but, when applied to the distal mesenchyme, this reduction is associated with an increase in Fgf8 expression. In agreement with the critical decline in Fgf8 expression for the activation of ICD, distal interdigital cells acquire a proximal position as interdigit regression occurs. We identified proliferating distal mesenchymal cells as those that give rise to the interdigital cells fated to die. Thus, ICD is determined by the antagonistic regulation of cell death by Fgf8 and RA and occurs through a progressive, rather than massive, cell death mechanism.

Hematopoiesis and Retinoids: Development and Disease

Retinoids function as activating ligands for a class of nuclear receptors that control gene expression programs for a wide range of tissues and organs during embryogenesis and throughout life. Over the years, three sets of observations have spurred interest in the function of retinoids with respect to development and disease of hematopoietic cells. Since the 1920s, epidemiological studies indicated altered hematopoiesis in vitamin A-deficient (VAD) human populations. More recently, the ability of retinoids to affect various aspects of hematopoietic development has been demonstrated in vitro. Finally, it was discovered that the gene encoding a retinoid receptor is a key target for chromosomal translocations that cause acute promyelocytic leukemia (APL). More recent investigations using targeted gene disruptions, VAD animal models, and mouse models of leukemia have continued to shed light on the function of the retinoid pathway in blood cells. It is now clear that retinoids are required for normal hematopoiesis during both yolk sac and fetal liver stages of hematopoiesis, while the pathway has at least modulatory functions for bone marrow derived progenitors. Studies of normal development and APL have provided complementary insight into the molecular control of blood cell differentiation. Here we review the evidence for retinoid requirements in hematopoiesis and also summarize current ideas regarding how this pathway is subverted in leukemia.

Genetic control of programmed cell death during animal development

The elimination of unwanted cells by programmed cell death is a common feature of animal development. Genetic studies in the nematode Caenorhabditis elegans, the fruit fly Drosophila melanogaster, and the mouse have not only revealed the molecular machineries that cause the programmed demise of specific cells, but have also allowed us to get a glimpse of the types of pathways that regulate these machineries during development. Rather than serving as a broad overview of programmed cell death during development, this review focuses on recent advances in our understanding of the regulation of specific programmed cell death events during nematode, fly, and mouse development. Recent studies have revealed that many of the regulatory pathways involved play additional important roles in development, which confirms that the programmed cell death fate is an integral aspect of animal development.

Expression and regulation of the decoy bone morphogenetic protein receptor BAMBI in the developing avian face

Here, we examine the expression and regulation of the gene BAMBI, a kinase-deficient decoy receptor capable of interacting with type I bone morphogenetic protein (BMP) receptors in avian embryos. Initially, expression was limited to the endoderm during neurula and pharyngula stages. From embryonic day 3.5 (stage 20) and onward, BAMBI expression almost perfectly overlapped with known expression patterns for BMP4, particularly in the face and limbs. We performed bead implant experiments in the face to see which signals could be repressing or promoting expression of BAMBI. Our data point to retinoids and BMPs as being major positive regulators of BAMBI expression; however, fibroblast growth factor 2 acts to repress BAMBI. Furthermore, retinoic acid is likely to act directly on BAMBI as induction occurs in the presence of cycloheximide. The data suggested that BAMBI could be used to regulate Bmp signaling during tissue interactions that are an integral part of facial morphogenesis.

Chronically administered retinoic acid has trophic effects in the rat small intestine and promotes adaptation in a resection model of short bowel syndrome

Following the loss of functional small bowel surface area, the intestine undergoes a compensatory adaptive response. The observation that adaptation is inhibited in vitamin A-deficient rats following submassive intestinal resection suggested that vitamin A is required for this response and raised the possibility that exogenous vitamin A could augment adaptation. Therefore, to directly assess whether chronically administered retinoic acid could stimulate gut adaptation in a model of short bowel syndrome and to address the mechanisms of any such effects, Sprague-Dawley rats were implanted with controlled release retinoic acid or control pellets and then subjected to mid-small bowel or sham resections. At 2 wk postoperation, changes in gut morphology, crypt cell proliferation and apoptosis, enterocyte migration, the extracellular matrix, and gene expression were assessed. Retinoic acid had significant trophic effects in resected and sham-resected rats. Retinoic acid markedly inhibited apoptosis and stimulated crypt cell proliferation and enterocyte migration postresection. Data presented indicate that these proadaptive effects of retinoic acid may be mediated via changes in the extracellular matrix (e.g., by increasing collagen IV synthesis, decreasing E-cadherin expression, and reducing integrin beta(3) levels), via affects on Hedgehog signaling (e.g., by reducing expression of the Hedgehog receptors Ptch and Ptch2 and the Gli1 transcription factor), by increasing expression of Reg1 and Pap1, and by modulation of retinoid and peroxisome proliferator-activated receptor signaling pathways. These studies are the first to demonstrate that retinoic acid can significantly enhance intestinal adaptation and suggest it may be beneficial in patients with short bowel syndrome.

On the origin and evolutionary diversification of beetle horns

Many scarab beetles produce rigid projections from the body called horns. The exaggerated sizes of these structures and the staggering diversity of their forms have impressed biologists for centuries. Recent comparative studies using DNA sequence-based phylogenies have begun to reconstruct the historical patterns of beetle horn evolution. At the same time, developmental genetic experiments have begun to elucidate how beetle horns grow and how horn growth is modulated in response to environmental variables, such as nutrition. We bring together these two perspectives to show that they converge on very similar conclusions regarding beetle evolution. Horns do not appear to be difficult structures to gain or lose, and they can diverge both dramatically and rapidly in form. Although much of this work is still preliminary, we use available information to propose a conceptual developmental model for the major trajectories of beetle horn evolution. We illustrate putative mechanisms underlying the evolutionary origin of horns and the evolution of horn location, shape, allometry, and dimorphism.

Tendon-muscle crosstalk controls muscle bellies morphogenesis, which is mediated by cell death and retinoic acid signaling

Vertebrate muscle morphogenesis is a complex developmental process, which remains quite yet unexplored at cellular and molecular level. In this work, we have found that sculpturing programmed cell death is a key morphogenetic process responsible for the formation of individual foot muscles in the developing avian limb. Muscle fibers are produced in excess in the precursor dorsal and ventral muscle masses of the limb bud and myofibers lacking junctions with digital tendons are eliminated via apoptosis. Microsurgical experiments to isolate the developing muscles from their specific tendons are consistent with a role for tendons in regulating survival of myogenic cells. Analysis of the expression of Raldh2 and local treatments with retinoic acid indicate that this signaling pathway mediates apoptosis in myogenic cells, appearing also involved in tendon maturation. Retinoic acid inhibition experiments led to defects in muscle belly segmentation and myotendinous junction formation. It is proposed that heterogeneous local distribution of retinoids controlled through Raldh2 and Cyp26A1 is responsible for matching the fleshy and the tendinous components of each muscle belly.

Modulation of Bone Morphogenetic Protein (BMP) 2 gene expression by Sp1 transcription factors

Changes in Bone Morphogenetic Protein (BMP) 2 gene expression and activity have been linked to many pathological conditions including cancer, osteoarthritis, and birth defects. BMP2 gene polymorphisms have been linked to osteoporosis and osteoarthritis. Sp1 and related proteins are widely expressed regulators of gene expression whose transcription activating abilities vary in different cells and on different genes. We present data indicating that the ratio of Sp1 and Sp3 isoforms varies in cells that express or do not express BMP2. Furthermore, the orientation of Sp1 sites conserved between four orders of mammals influences BMP2 expression. Together our data indicate that the stoichiometry and orientation of Sp1 and Sp3 complexes on the BMP2 promoter influence BMP2 expression.

BMP action in skeletogenesis involves attenuation of retinoid signaling

The bone morphogenetic protein (BMP) and growth and differentiation factor (GDF) signaling pathways have well-established and essential roles within the developing skeleton in coordinating the formation of cartilaginous anlagen. However, the identification of bona fide targets that underlie the action of these signaling molecules in chondrogenesis has remained elusive. We have identified the gene for the retinoic acid (RA) synthesis enzyme Aldh1a2 as a principal target of BMP signaling; prochondrogenic BMPs or GDFs lead to attenuation of Aldh1a2 expression and, consequently, to reduced activation of the retinoid signaling pathway. Consistent with this, antagonism of retinoid signaling phenocopies BMP4 action, whereas RA inhibits the chondrogenic stimulatory activity of BMP4. BMP4 also down-regulates Aldh1a2 expression in organ culture and, consistent with this, Aldh1a2 is actively excluded from the developing cartilage anlagens. Collectively, these findings provide novel insights into BMP action and demonstrate that BMP signaling governs the fate of prechondrogenic mesenchyme, at least in part, through regulation of retinoid signaling.

Novel retinoid targets in the mouse limb during organogenesis

Bioactive retinoids are potent limb teratogens, upregulating apoptosis, decreasing chondrogenesis, and producing limb-reduction defects. To target the origins of these effects, we examined gene expression changes in the developing murine limb after 3 h of culture with teratogenic concentrations of vitamin A. Embryonic day 12 CD-1 limbs were cultured in the absence or presence of vitamin A (retinol acetate) at 1.25 and 62.5muM (n = 5). Total RNA was used to probe Atlas 1.2 cDNA arrays. Eighty-one genes were significantly upregulated by retinol exposure; among these were key limb development signaling molecules, extracellular matrix and adhesion proteins, oncogenes, and a large number of transcriptional regulators, including Eya2, Id3, Snail, and Hes1. To relate these expression changes to teratogenic outcome, the response of these four genes was assessed after culture with vitamin A and retinoid receptor antagonists that are able to rescue retinoid-induced malformations; expression levels were correlated with limb malformations. Lastly, pathways analysis revealed that a large number of the genes significantly affected by retinoid treatment are functionally linked through direct interactions. Several regulatory gene cascades emerged relevant to morphogenesis, cell-fate, and chondrogenesis; moreover, members of these cascades crosstalk with one other. These results indicate that retinoids act in a coordinated fashion to disrupt development at multiple levels. In sum, this work proposes several unifying mechanisms for retinoid-induced limb malformations, identifies novel retinoid targets, and highlights Eya2, Id3, Snail, and Hes1 as potential key teratogenic effectors.

Tid1 is a Smad-binding protein that can modulate Smad7 activity in developing embryos

In a search for binding partners to Smad8, we identified the chicken homologue of the mammalian Tid1 protein (cTid1), which is a regulator of apoptosis related to the Drosophila tumour suppressor Tid56. The cTid1 coding sequence is highly conserved with mammalian Tid1, including the DnaJ domain that interacts with Hsp70 (heat-shock protein 70). The cTid1 gene is widely expressed with transcripts enriched in the developing blood islands of the embryonic-yolk sac. We show that cTid1 can bind to other members of the Smad family and that highest binding activity occurs with the negative regulatory Smad7, through the conserved MH2 domain. This interaction can have functional relevance in vivo, since co-expression of Tid1 blocks the dorsalizing and BMP (bone morphogenetic protein)-dependent regulatory activity of Smad7 in developing Xenopus embryos. The finding that these proteins can interact suggests the potential for linking two important cell survival/apoptosis pathways.

Expression and regulation of antioxidant enzymes in the developing limb support a function of ROS in interdigital cell death

Vertebrate limb development is a well-studied model of apoptosis; however, little is known about the intracellular molecules involved in activating the cell death machinery. We have shown that high levels of reactive oxygen species (ROS) are present in the interdigital 'necrotic' tissue of mouse autopod, and that antioxidants can reduce cell death. Here, we determined the expression pattern of several antioxidant enzymes in order to establish their role in defining the areas with high ROS levels. We found that the genes encoding the superoxide dismutases and catalase are expressed in autopod, but they are downregulated in the interdigital regions at the time ROS levels increased and cell death was first detected. The possible role of superoxide and/or peroxide in activating cell death is supported by the protective effect of a superoxide dismutase/catalase mimetic. Interestingly, we found that peroxidase activity and glutathione peroxidase-4 gene (Gpx4) expression were restricted to the non-apoptotic tissue (e.g., digits) of the developing autopod. Induction of cell death with retinoic acid caused an increase in ROS and decrease in peroxidase activity. Even more inhibition of glutathione peroxidase activity leads to cell death in the digits, suggesting that a decrease in antioxidant activity, likely due to Gpx4, caused an increase in ROS levels, thus triggering apoptosis.

A polymorphism in a conserved posttranscriptional regulatory motif alters bone morphogenetic protein 2 (BMP2) RNA:protein interactions

The bone morphogenetic protein (BMP)2 gene has been genetically linked to osteoporosis and osteoarthritis. We have shown that the 3'-untranslated regions (UTR) of BMP2 genes from mammals to fishes are extraordinarily conserved. This indicates that the BMP2 3'-UTR is under stringent selective pressure. We present evidence that the conserved region is a strong posttranscriptional regulator of BMP2 expression. Polymorphisms in cis-regulatory elements have been proven to influence susceptibility to a growing number of diseases. A common single nucleotide polymorphism (SNP) disrupts a putative posttranscriptional regulatory motif, an AU-rich element, within the BMP2 3'-UTR. The affinity of specific proteins for the rs15705 SNP sequence differs from their affinity for the normal human sequence. More importantly, the in vitro decay rate of RNAs with the SNP is higher than that of RNAs with the normal sequence. Such changes in mRNA:protein interactions may influence the posttranscriptional mechanisms that control BMP2 gene expression. The consequent alterations in BMP2 protein levels may influence the development or physiology of bone or other BMP2-influenced tissues.

Programmed cell death in the regenerating deer antler

Antlers are the only mammalian appendages capable of epimorphic regeneration and thus provide a unique model for investigating the mechanisms that underlie mammalian regeneration. Antlers elongate by a modified endochondral ossification process while intramembranous ossification takes place concurrently around the antler shaft. In this study, sites of apoptosis in the growing antler tip were identified by TUNEL staining and related to cell proliferation, as determined by PCNA staining. Bcl-2 and bax were identified by RT-PCR and bax was also immunolocalized in tissue sections. The apoptotic index was high in perichondrium, undifferentiated mesenchymal cells and cellular periosteum but was low in skin. The proliferation index was high in mesenchyme, skin (specifically in hair follicles) and cellular periosteum; it was low in fibrous perichondrium and periosteum, and barely detectable in cartilage. Both bcl-2 and bax were found to be more highly expressed in the perichondrium/mesenchyme and non-mineralized cartilage than in skin and mineralized cartilage. Bax was immunolocalized in mesenchyme cells, chondroprogenitors, chondrocytes, osteoblasts, osteocytes and osteoclasts. In conclusion, this study shows that programmed cell death plays a necessary role in regenerating antlers, as it does during skeletal development, bone growth and bone remodelling. The high level of apoptosis and proliferation in mesenchymal progenitor cells confirms that this represents the antler 'growth zone'. In fact, the percentage of TUNEL-positive cells in the mesenchymal growth zone (up to 64%) is higher than that recorded in any other adult tissue. This extensive cell death probably reflects the phenomenal rate of morphogenesis and tissue remodelling that takes place in a growing antler. The local and/or systemic factors that control the balance between cell growth and apoptosis in antler tissues now need to be determined.

Bmp2 and Gata4 function additively to rescue heart tube development in the absence of retinoids

We used the vitamin A-deficient (VAD) quail model to investigate the retinoid-dependent mechanism that regulates heart tube development. We showed previously that decreased levels of Gata4 in cardiogenic mesoderm and endoderm correlate with the cardiomyopathy caused by VAD, but that this could be rescued by transplanting normal anterior endoderm. Bmp2 is a known cardiogenic factor that is expressed normally in lateral plate mesoderm and cardiac-associated pharyngeal endoderm. Here we show that (like Gata4) transcripts encoding Bmp2 and BMP-dependent signaling activity are decreased throughout the heart-forming region of the VAD embryo. Addition of Bmp2 protein or forced expression of Gata4 in cultured VAD embryos leads to a partial rescue of the cardiomyopathy, and addition of both Bmp2 and Gata4 has an additive positive effect. Our data are consistent with a requirement for retinoid signaling to maintain expression of Bmp2, which regulates Gata4, and in addition acts with Gata4 to regulate genes important for normal morphogenesis of the primitive heart tube.

Regulation of hematopoiesis by retinoid signaling

The discovery that retinoic acid efficiently stimulates the terminal differentiation of granulocytic leukemia cells had a major impact on clinical hematology, but has also inspired research into the normal function of the retinoid signaling pathway during hematopoiesis. New animal models and loss-of-function approaches have successfully revealed requirements for the pathway at defined embryonic stages that are relevant for distinct hematopoietic cell populations. For example, novel insight has been gained regarding the function of retinoids in yolk sac hematovascular development, fetal erythropoiesis, T-cell homing, and hematopoietic stem and progenitor cell biology. The lessons learned so far indicate that future development of sophisticated animal models will be needed to fully understand the intricacy and specificity of this complex signaling pathway, but that this effort will be productive and continue to inform both basic and clinical research on many fronts.

Vitamin A regulation of BMP4 expression in the male germ line

The molecular mechanisms leading to male infertility in vitamin A deficient (VAD) rodents have never been fully elucidated. Here, we report an interaction between BMP4 and retinoid signaling pathways in germ cells that may help clarify the biochemical basis of VAD. Adult germ cells, in particular spermatogonia, expressed BMP4 at both the mRNA and protein levels. BMP4 expression was significantly up-regulated in the testes of VAD mice and was down-regulated in freshly isolated germ cells and VAD testes by retinol, but not retinoic acid. The retinoid-responsive gene, RARbeta, was not induced in germ cells following retinoid treatment. Examination of BMP4 promoter usage in spermatogonia and the VAD testis revealed that germ cells utilize the recently characterized BMP4 intron 2 promoter, in addition to the classical 1A and 1B promoters. The observed decrease in BMP4 in response to retinol was mediated by the 1A and intron 2 promoters of the BMP4 gene. Our results reflect a direct requirement for retinoids by germ cells for the resumption of spermatogenesis in VAD animals via mechanisms that involve the suppression of BMP4 expression.

The transforming growth factor-beta family members bone morphogenetic protein-2 and macrophage inhibitory cytokine-1 as mediators of the antiangiogenic activity of N-(4-hydroxyphenyl)retinamide

PURPOSE

Tumor growth appears to be an angiogenesis-dependent process. N-(4-hydroxyphenyl)retinamide (fenretinide; 4HPR) has been found to inhibit and/or prevent tumor growth under diverse conditions. Although 4HPR is antiangiogenic, the molecular mechanisms of this effect remain largely unknown.

EXPERIMENTAL DESIGN

Endothelial cells were treated with 4HPR in vitro to study the effects on migration, invasion, and organization, as well as gene expression by microarray and quantitative PCR studies. In vivo angiogenesis was evaluated in the Matrigel model.

RESULTS

4HPR treatment substantially modified the biological activities of endothelial cells, repressing their capacity to migrate, invade, and organize into capillary-like structures. The inhibition of invasion induced by 4HPR was also associated with decreased activities of the metalloproteases matrix metalloproteinase-2 and CD13/APN. Using oligonucleotide microarrays, we observed that bone morphogenetic protein-2 and macrophage inhibitory cytokine-1, two multifunctional cytokines of the transforming growth factor-beta family that regulate the growth, differentiation, apoptosis, and matrix accumulation of a variety of cells, are up-regulated in vitro by 4HPR. Both these molecules specifically inhibited endothelial cell growth, migration, and invasion in vitro and suppressed angiogenesis in the Matrigel plug assay in vivo. Blocking antibodies to bone morphogenetic protein-2 were able to reverse the suppressive effects of 4HPR in vitro and in vivo.

CONCLUSIONS

These data support the conclusion that 4HPR inhibits tumor growth by repression of new vessel growth and identify novel points of regulation of angiogenesis in transforming growth factor-beta family proteins.

Alcohol in hepatocellular cancer

Hepatocellular cancer accounts for almost half a million cancer deaths a year, with an escalating incidence in the Western world. Alcohol has long been recognized as a major risk factor for cancer of the liver and of other organs including oropharynx, larynx, esophagus, and possibly the breast and colon. There is compelling epidemiologic data confirming the increased risk of cancer associated with alcohol consumption, which is supported by animal experiments. Cancer of the liver associated with alcohol usually occurs in the setting of cirrhosis. Alcohol may act as a cocarcinogen, and has strong synergistic effects with other carcinogens including hepatitis B and C, aflatoxin, vinyl chloride, obesity, and diabetes mellitus. Acetaldehyde, the main metabolite of alcohol, causes hepatocellular injury, and is an important factor in causing increased oxidant stress, which damages DNA. Alcohol affects nutrition and vitamin metabolism, causing abnormalities of DNA methylation. Abnormalities of DNA methylation, a key pathway of epigenetic gene control, lead to cancer. Other nutritional and metabolic effects, for example on vitamin A metabolism, also play a key role in hepatocarcinogenesis. Alcohol enhances the effects of environmental carcinogens directly and by contributing to nutritional deficiency and impairing immunological tumor surveillance. This review summarizes the epidemiologic evidence for the role of alcohol in hepatocellular cancer, and discusses the mechanisms involved in the promotion of cancer.

Programmed cell death in the embryonic vertebrate limb

The developing limb bud provides one of the best examples in which programmed cell death exerts major morphogenetic functions. In this work, we revise the distribution and the developmental significance of cell death in the embryonic vertebrate limb and its control by the BMP signalling pathway. In addition, paying special attention to the interdigital apoptotic zones, we review current data concerning the intracellular death machinery implicated in mesodermal limb apoptosis.