Expression of a constitutively active type I BMP receptor using a retroviral vector promotes the development of adrenergic cells in neural crest cultures

ZSWIM4 regulates embryonic patterning and BMP signaling by promoting nuclear Smad1 degradation

The dorsoventral gradient of BMP signaling plays an essential role in embryonic patterning. Zinc Finger SWIM-Type Containing 4 (zswim4) is expressed in the Spemann-Mangold organizer at the onset of Xenopus gastrulation and is then enriched in the developing neuroectoderm at the mid-gastrula stages. Knockdown or knockout of zswim4 causes ventralization. Overexpression of zswim4 decreases, whereas knockdown of zswim4 increases the expression levels of ventrolateral mesoderm marker genes. Mechanistically, ZSWIM4 attenuates the BMP signal by reducing the protein stability of SMAD1 in the nucleus. Stable isotope labeling by amino acids in cell culture (SILAC) identifies Elongin B (ELOB) and Elongin C (ELOC) as the interaction partners of ZSWIM4. Accordingly, ZSWIM4 forms a complex with the Cul2-RING ubiquitin ligase and ELOB and ELOC, promoting the ubiquitination and degradation of SMAD1 in the nucleus. Our study identifies a novel mechanism that restricts BMP signaling in the nucleus.

Recent advances in the developmental origin of neuroblastoma: an overview

Neuroblastoma (NB) is a pediatric tumor that originates from neural crest-derived cells undergoing a defective differentiation due to genomic and epigenetic impairments. Therefore, NB may arise at any final site reached by migrating neural crest cells (NCCs) and their progeny, preferentially in the adrenal medulla or in the para-spinal ganglia.

NB shows a remarkable genetic heterogeneity including several chromosome/gene alterations and deregulated expression of key oncogenes that drive tumor initiation and promote disease progression.

NB substantially contributes to childhood cancer mortality, with a survival rate of only 40% for high-risk patients suffering chemo-resistant relapse. Hence, NB remains a challenge in pediatric oncology and the need of designing new therapies targeted to specific genetic/epigenetic alterations become imperative to improve the outcome of high-risk NB patients with refractory disease or chemo-resistant relapse.

In this review, we give a broad overview of the latest advances that have unraveled the developmental origin of NB and its complex epigenetic landscape.

Single-cell RNA sequencing with spatial transcriptomics and lineage tracing have identified the NCC progeny involved in normal development and in NB oncogenesis, revealing that adrenal NB cells transcriptionally resemble immature neuroblasts or their closest progenitors. The comparison of adrenal NB cells from patients classified into risk subgroups with normal sympatho-adrenal cells has highlighted that tumor phenotype severity correlates with neuroblast differentiation grade.

Transcriptional profiling of NB tumors has identified two cell identities that represent divergent differentiation states, i.e. undifferentiated mesenchymal (MES) and committed adrenergic (ADRN), able to interconvert by epigenetic reprogramming and to confer intra-tumoral heterogeneity and high plasticity to NB.

Chromatin immunoprecipitation sequencing has disclosed the existence of two super-enhancers and their associated transcription factor networks underlying MES and ADRN identities and controlling NB gene expression programs.

The discovery of NB-specific regulatory circuitries driving oncogenic transformation and maintaining the malignant state opens new perspectives on the design of innovative therapies targeted to the genetic and epigenetic determinants of NB. Remodeling the disrupted regulatory networks from a dysregulated expression, which blocks differentiation and enhances proliferation, toward a controlled expression that prompts the most differentiated state may represent a promising therapeutic strategy for NB.

Bone morphogenetic protein induces bone invasion of melanoma by epithelial-mesenchymal transition via the Smad1/5 signaling pathway

Oral malignant melanoma, which frequently invades the hard palate or maxillary bone, is extremely rare and has a poor prognosis. Bone morphogenetic protein (BMP) is abundantly expressed in bone matrix and is highly expressed in malignant melanoma, inducing an aggressive phenotype. We examined the role of BMP signaling in the acquisition of an aggressive phenotype in melanoma cells in vitro and in vivo. In five cases, immunohistochemistry indicated the phosphorylation of Smad1/5 (p-Smad1/5) in the nuclei of melanoma cells. In the B16 mouse and A2058 human melanoma cell lines, BMP2, BMP4, or BMP7 induces morphological changes accompanied by the downregulation of E-cadherin, and the upregulation of N-cadherin and Snail, markers of epithelial-mesenchymal transition (EMT). BMP2 also stimulates cell invasion by increasing matrix metalloproteinase activity in B16 cells. These effects were canceled by the addition of LDN193189, a specific inhibitor of Smad1/5 signaling. In vivo, the injection of B16 cells expressing constitutively activated ALK3 enhanced zygoma destruction in comparison to empty B16 cells by increasing osteoclast numbers. These results suggest that the activation of BMP signaling induces EMT, thus driving the acquisition of an aggressive phenotype in malignant melanoma.

Transcriptomic analysis identifies novel targets for individual bone morphogenetic protein type 1 receptors in endothelial cells

Bone Morphogenetic Protein (BMP) signaling regulates diverse biological processes. Upon ligand binding, BMP receptors (BMPRs) phosphorylate SMAD1/5 and other noncanonical downstream effectors to induce transcription of downstream targets. However, the precise role of individual BMP receptors in this process remains largely unknown due to the complexity of downstream signaling and the innate promiscuity of ligand-receptor interaction. To delineate unique downstream effectors of individual BMPR1s, we analyzed the transcriptome of human umbilical endothelial cells (HUVECs) expressing three distinct constitutively active BMPR1s of which expression was detected in endothelial cells (ECs). From our analyses, we identified a number of novel downstream targets of BMPR1s in ECs. More importantly, we found that each BMPR1 possesses a distinctive set of downstream effectors, suggesting that each BMPR1 is likely to retain unique function in ECs. Taken together, our analyses suggest that each BMPR1 regulates downstream targets non-redundantly in ECs to create context-dependent outcomes of the BMP signaling.

Pitx1 regulates cement gland development in Xenopus laevis through activation of transcriptional targets and inhibition of BMP signaling

The cement gland in Xenopus laevis has long been used as a model to study the interplay of cell signaling and transcription factors during embryogenesis. It has been shown that an intermediate level of Bone Morphogenetic Protein (BMP) signaling is essential for cement gland formation. In addition, several transcription factors have been linked to cement gland development. One of these, the homeodomain-containing protein Pitx1, can generate ectopic cement gland formation; however, the mechanisms underlying this process remain obscure. We report here, for the first time, a requirement for Pitx proteins in cement gland formation, in vivo: knockdown of both pitx1 and the closely related pitx2c inhibit endogenous cement gland formation. Pitx1 transcriptionally activates cement gland differentiation genes through both direct and indirect mechanisms, and functions as a transcriptional activator to inhibit BMP signaling. This inhibition, required for the expression of pitx genes, is partially mediated by Pitx1-dependent follistatin expression. Complete suppression of BMP signaling inhibits induction of cement gland markers by Pitx1; furthermore, we find that Pitx1 physically interacts with Smad1, an intracellular transducer of BMP signaling. We propose a model of cement gland formation in which Pitx1 limits local BMP signaling within the cement gland primordium, and recruits Smad1 to activate direct downstream targets.

From proliferation to target innervation: signaling molecules that direct sympathetic nervous system development

The sympathetic division of the autonomic nervous system includes a variety of cells including neurons, endocrine cells and glial cells. A recent study (Furlan et al. 2017) has revised thinking about the developmental origin of these cells. It now appears that sympathetic neurons and chromaffin cells of the adrenal medulla do not have an immediate common ancestor in the form a "sympathoadrenal cell", as has been long believed. Instead, chromaffin cells arise from Schwann cell precursors. This review integrates the new findings with the expanding body of knowledge on the signalling pathways and transcription factors that regulate the origin of cells of the sympathetic division of the autonomic nervous system.

A Small Dibromotyrosine Derivative Purified From Pseudoceratina Sp. Suppresses TGF-β Responsiveness by Inhibiting TGF-β Type I Receptor Serine/Threonine Kinase Activity

For clinical application, there is a great need for small-molecule inhibitors (SMIs) that could control pathogenic effects of transforming growth factor (TGF-β) and/or modulate effects of TGF-β in normal responses. Selective SMIs of the TGF-β signaling pathway developed for therapeutics will also be powerful tools in experimentally dissecting this complex pathway, especially its cross-talk with other signaling pathways. In this study, we characterized (1'R,5'S,6'S)-2-(3',5'-dibromo-1',6'-dihydroxy-4'-oxocyclohex-2'-enyl) acetonitrile (DT), a member of a new class of small-molecule inhibitors related to bromotyrosine derivate from Pseudoceratina sp., which inhibits the TGF-β type I receptor serine/threonine kinase known as activin receptor-like kinase (ALK) 5. The inhibitory effects of DT on TGF-β-induced Smad signaling and epithelial-to-mesenchymal transition (EMT) were investigated in epithelial cells using in vitro kinase assay, luciferase reporter assays, immunoblotting, confocal microscopy, and wound healing assays. The novel ALK5 inhibitor, DT, inhibited the TGF-β-stimulated transcriptional activations of 3TP-Lux. In addition, DT decreased phosphorylated Smad2/3 levels and the nuclear translocation of Smad2/3 increased by TGF-β. In addition, DT inhibited TGF-β-induced EMT and wound healing of A549 cells. Our results suggest that DT is a potential therapeutic agent for fibrotic disease and cancer treatment. J. Cell. Biochem. 117: 2800-2814, 2016. © 2016 Wiley Periodicals, Inc.

BMP-7 Signaling and its Critical Roles in Kidney Development, the Responses to Renal Injury, and Chronic Kidney Disease

Chronic kidney disease (CKD) is a significant health problem that most commonly results from congenital abnormalities in children and chronic renal injury in adults. The therapeutic potential of BMP-7 was first recognized nearly two decades ago with studies demonstrating its requirement for kidney development and ability to inhibit the pathogenesis of renal injury in models of CKD. Since this time, our understanding of CKD has advanced considerably and treatment strategies have evolved with the identification of many additional signaling pathways, cell types, and pathologic processes that contribute to disease progression. The purpose of this review is to revisit the seminal studies that initially established the importance of BMP-7, highlight recent advances in BMP-7 research, and then integrate this knowledge with current research paradigms. We will provide an overview of the evolutionarily conserved roles of BMP proteins and the features that allow BMP signaling pathways to function as critical signaling nodes for controlling biological processes, including those related to CKD. We will discuss the multifaceted functions of BMP-7 during kidney development and the potential for alterations in BMP-7 signaling to result in congenital abnormalities and pediatric kidney disease. We will summarize the renal protective effects of recombinant BMP-7 in experimental models of CKD and then propose a model to describe the potential physiological role of endogenous BMP-7 in the innate repair mechanisms of the kidneys that respond to renal injury. Finally, we will highlight emerging clinical approaches for applying our knowledge of BMP-7 toward improving the treatment of patients with CKD.

BMP-Smad 1/5/8 signalling in the development of the nervous system

The transcription factors, Smad1, Smad5 and Smad8, are the pivotal intracellular effectors of the bone morphogenetic protein (BMP) family of proteins. BMPs and their receptors are expressed in the nervous system (NS) throughout its development. This review focuses on the actions of Smad 1/5/8 in the developing NS. The mechanisms by which these Smad proteins regulate the induction of the neuroectoderm, the central nervous system (CNS) primordium, and finally the neural crest, which gives rise to the peripheral nervous system (PNS), are reviewed herein. We describe how, following neural tube closure, the most dorsal aspect of the tube becomes a signalling centre for BMPs, which directs the pattern of the development of the dorsal spinal cord (SC), through the action of Smad1, Smad5 and Smad8. The direct effects of Smad 1/5/8 signalling on the development of neuronal and non-neuronal cells from various neural progenitor cell populations are then described. Finally, this review discusses the neurodevelopmental abnormalities associated with the knockdown of Smad 1/5/8.

Regional differences in neural crest morphogenesis

Neural crest cells are pluripotent cells that emerge from the neural epithelium, migrate extensively, and differentiate into numerous derivatives, including neurons, glial cells, pigment cells and connective tissue. Major questions concerning their morphogenesis include: 1) what establishes the pathways of migration and 2) what controls the final destination and differentiation of various neural crest subpopulations. These questions will be addressed in this review. Neural crest cells from the trunk level have been explored most extensively. Studies show that melanoblasts are specified shortly after they depart from the neural tube, and this specification directs their migration into the dorsolateral pathway. We also consider other reports that present strong evidence for ventrally migrating neural crest cells being similarly fate restricted. Cranial neural crest cells have been less analyzed in this regard but the preponderance of evidence indicates that either the cranial neural crest cells are not fate-restricted, or are extremely plastic in their developmental capability and that specification does not control pathfinding. Thus, the guidance mechanisms that control cranial neural crest migration and their behavior vary significantly from the trunk. The vagal neural crest arises at the axial level between the cranial and trunk neural crest and represents a transitional cell population between the head and trunk neural crest. We summarize new data to support this claim. In particular, we show that: 1) the vagal-level neural crest cells exhibit modest developmental bias; 2) there are differences in the migratory behavior between the anterior and the posterior vagal neural crest cells reminiscent of the cranial and the trunk neural crest, respectively; 3) the vagal neural crest cells take the dorsolateral pathway to the pharyngeal arches and the heart, but the ventral pathway to the peripheral nervous system and the gut. However, these pathways are not rigidly specified because of prior fate restriction. Understanding the molecular, cellular and behavioral differences between these three populations of neural crest cells will be of enormous assistance when trying to understand the evolution of the neck.

BMP signaling regulates sympathetic nervous system development through Smad4-dependent and -independent pathways

Induction of the sympathetic nervous system (SNS) from its neural crest (NC) precursors is dependent on BMP signaling from the dorsal aorta. To determine the roles of BMP signaling and the pathways involved in SNS development, we conditionally knocked out components of the BMP pathways. To determine if BMP signaling is a cell-autonomous requirement of SNS development, the Alk3 (BMP receptor IA) was deleted in the NC lineage. The loss of Alk3 does not prevent NC cell migration, but the cells die immediately after reaching the dorsal aorta. The paired homeodomain factor Phox2b, known to be essential for survival of SNS precursors, is downregulated, suggesting that Phox2b is a target of BMP signaling. To determine if Alk3 signals through the canonical BMP pathway, Smad4 was deleted in the NC lineage. Loss of Smad4 does not affect neurogenesis and ganglia formation; however, proliferation and noradrenergic differentiation are reduced. Analysis of transcription factors regulating SNS development shows that the basic helix-loop-helix factor Ascl1 is downregulated by loss of Smad4 and that Ascl1 regulates SNS proliferation but not noradrenergic differentiation. To determine if the BMP-activated Tak1 (Map3k7) pathway plays a role in SNS development, Tak1 was deleted in the NC lineage. We show that Tak1 is not involved in SNS development. Taken together, our results suggest multiple roles for BMP signaling during SNS development. The Smad4-independent pathway acts through the activation of Phox2b to regulate survival of SNS precursors, whereas the Smad4-dependent pathway controls noradrenergic differentiation and regulates proliferation by maintaining Ascl1 expression.

Development of satellite glia in mouse sympathetic ganglia: GDNF and GFR alpha 1 are not essential

The phenotypic development of satellite cells in mouse sympathetic ganglia was examined by localizing the transcription factors, Sox10 and Phox2b, the neuronal marker, tyrosine hydroxylase (TH), and brain-derived fatty acid binding protein (B-FABP), which identifies glial precursors and mature glia. In E10.5 mice, most cells in the sympathetic chain expressed both Sox10 and Phox2b, with a minority of cells expressing Sox10 only or Phox2b only. In E11.5 mice, the majority of cells expressed Sox10 only or Phox2b only. B-FABP was colocalized with Sox10 in satellite glial precursors, which were located on the periphery of the ganglion. There was no overlap between B-FABP and Phox2b or B-FABP and TH. During subsequent development, the number of B-FABP+ cells increased and they became more common deep within the ganglion. In E12.5 and E18.5 mice, there was no overlap between Sox10 and Phox2b, and 98% of Sox10 cells were also B-FABP+. Satellite glial precursors in E11.5-E15.5 mice also expressed the GDNF-binding molecule, GFRalpha1. B-FABP immunoreactive cells did not express Ret or NCAM, two potential signaling molecules for GDNF/GFRalpha1. In E12.5 and E18.5 mice lacking GFRalpha1 or GDNF, the development of B-FABP immunoreactive satellite cells was normal, and hence neither GDNF or GFRalpha1 are essential for the development of satellite glia in sympathetic ganglia.

Persistent expression of BMP-4 in embryonic chick adrenal cortical cells and its role in chromaffin cell development

Background

Adrenal chromaffin cells and sympathetic neurons both originate from the neural crest, yet signals that trigger chromaffin development remain elusive. Bone morphogenetic proteins (BMPs) emanating from the dorsal aorta are important signals for the induction of a sympathoadrenal catecholaminergic cell fate.

Results

We report here that BMP-4 is also expressed by adrenal cortical cells throughout chick embryonic development, suggesting a putative role in chromaffin cell development. Moreover, bone morphogenetic protein receptor IA is expressed by both cortical and chromaffin cells. Inhibiting BMP-4 with noggin prevents the increase in the number of tyrosine hydroxylase positive cells in adrenal explants without affecting cell proliferation. Hence, adrenal BMP-4 is likely to induce tyrosine hydroxylase in sympathoadrenal progenitors. To investigate whether persistent BMP-4 exposure is able to induce chromaffin traits in sympathetic ganglia, we locally grafted BMP-4 overexpressing cells next to sympathetic ganglia. Embryonic day 8 chick sympathetic ganglia, in addition to principal neurons, contain about 25% chromaffin-like cells. Ectopic BMP-4 did not increase this proportion, yet numbers and sizes of 'chromaffin' granules were significantly increased.

Conclusion

BMP-4 may serve to promote specific chromaffin traits, but is not sufficient to convert sympathetic neurons into a chromaffin phenotype.

Bone morphogenetic protein signalling and vertebrate nervous system development

Transforming growth factor-beta (TGFbeta) signalling, particularly signalling from the bone morphogenetic protein (BMP) members of this protein family, is crucial for the development of both the central and peripheral nervous systems in vertebrates. Experimental embryology and genetics performed in a range of organisms are providing insights into how BMPs establish the neural tissue and control the types and numbers of neurons formed. These studies also highlight the interactions between different developmental signals that are necessary to form a functional nervous system. The challenges ahead will be to uncover functions of TGFbeta signalling in later stages of CNS development, as well as to determine possible associations with neurological diseases.

Anabolic agents and the bone morphogenetic protein pathway

A major unmet need in the medical field today is the availability of suitable treatments for the ever-increasing incidence of osteoporosis and the treatment of bone deficit conditions. Although therapies exist which prevent bone loss, the options are extremely limited for patients once a substantial loss of skeletal bone mass has occurred. Patients who have reduced bone mass are predisposed to fractures and further morbidity. The FDA recently approved PTH (1-34) (Teriparatide) for the treatment of postmenopausal osteoporosis after both preclinical animal and clinical human studies indicated it induces bone formation. This is the only approved bone anabolic agent available but unfortunately it has limited use, it is relatively expensive and difficult to administer. Consequently, the discovery of low cost orally available bone anabolic agents is critical for the future treatment of bone loss conditions. The intricate process of bone formation is co-ordinated by the action of many different bone growth factors, some stored in bone matrix and others released into the bone microenvironment from surrounding cells. Although all these factors play important roles, the bone morphogenetic proteins (BMPs) clearly play a central role in both bone cartilage formation and repair. Recent research into the regulation of the BMP pathway has led to the discovery of a number of small molecular weight compounds as candidate bone anabolic agents. These agents may usher in a new wave of more innovative and versatile treatments for osteoporosis as well as orthopedic and dental indications.

Perspectives on integration of cell extrinsic and cell intrinsic pathways of signaling required for differentiation of noradrenergic sympathetic ganglion neurons

This review presents an analysis of current research aimed at deciphering the interplay of cell extrinsic and intrinsic signals required for specification and differentiation of noradrenergic sympathetic ganglion neurons. The development of noradrenergic sympathetic ganglion neurons depends upon expression of a core set of DNA regulatory molecules, including the Phox2 homeodomain proteins and the basic helix-loop-helix proteins, HAND2 and MASH1 whose expression is dependent upon cell extrinsic cues. Both bone morphogenetic protein(s) and cAMP have an integral role in the specification/differentiation of noradrenergic sympathetic ganglion neurons but how signaling downstream of these molecules is integrated and identification of their particular functions is just beginning to be elucidated. Data currently available suggests a model with BMP providing both instructive and permissive cues in a pathway integrated by cAMP and MAPK by activation of both canonical and non-canonical intracellular signaling cascades.

The basic helix-loop-helix factor Hand 2 regulates autonomic nervous system development

Mammalian autonomic nervous system (ANS) development requires the combinatorial action of a number of transcription factors, which include Mash 1, Phox 2b, and GATA 3. Here we show that the bHLH transcription factor, Hand 2 (dHAND), is expressed concurrently with Mash 1 during sympathetic nervous system (SNS) development and that the expression of Hand 2 is not dependent on Mash 1. This suggests that these two bHLH factors work in parallel during SNS development. We also show that ectopic expression of Hand 2 activates the neuronal program and promotes the acquisition of a phenotype corresponding to peripheral neurons including neurons of the SNS lineage in P19 embryonic carcinoma cells. We propose that Hand 2 works in parallel with other members of the transcriptional network to regulate ANS developmental but can ectopically activate the program by a cross-regulatory mechanism that includes the activation of Mash 1. We show that this function is dependent on its interaction with the histone acetyltransferase p300/CBP, indicating that Hand 2 functions to promote ANS development as part of a larger transcriptional complex.

Reiterated Wnt and BMP signals in neural crest development

Common signaling pathways such as those for Wnts and BMPs are used many times during embryogenesis. During the development of the neural crest, Wnt and BMP signals are used repeatedly at different stages to influence initial induction, segregation from the neuroepithelium and cell fate determination. This review considers how specificity is generated within the neural crest for these reiterated signals, discussing examples of how the outcomes of signaling events are modulated by context.

The cAMP pathway regulates both transcription and activity of the paired homeobox transcription factor Phox2a required for development of neural crest-derived and central nervous system-derived catecholaminergic neurons

Pluripotent neural crest (NC) cells differentiate to diverse lineages, including the neuronal, sympathoadrenal lineage. In primary NC cultures, bone morphogenetic protein 2 (BMP2) requires moderate activation of cAMP signaling for induction of the sympathoadrenal lineage. However, the mechanism by which cAMP signaling synergizes with BMP2 to induce the sympathodrenal lineage is unknown. Herein, we demonstrate that moderate activation of cAMP signaling induces both transcription and activity of proneural transcription factor Phox2a. In NC cultures inhibition of cAMP-response element-binding protein (CREB)-mediated transcription by expression of dominant-negative CREB suppresses Phox2a transcription and sympathoadrenal lineage development. Interestingly, the constitutively active CREB(DIEDML), despite inducing Phox2a transcription, is insufficient for sympathoadrenal lineage development, requiring activation of the cAMP pathway. Because CREB(DIEDML)-mediates cAMP-dependent transcription without requiring activation by the cAMP-dependent protein kinase A (PKA), these results identify PKA activation as necessary in sympathoadrenal lineage development. Treatment of NC cultures with the PKA inhibitor H89 or 1-10 nm okadaic acid (OA), a serine/threonine PP2A-like phosphatase inhibitor, suppresses sympathoadrenal lineage development. Likewise, OA treatment of the CNS-derived catecholaminergic CAD cell line inhibits cAMP-mediated neuronal differentiation. Specifically, OA inhibits cAMP-mediated Phox2a dephosphorylation, cAMP-dependent Phox2a DNA binding in vitro, and cAMP- and Phox2a-dependent dopamine-beta-hydroxylase-luciferase reporter expression. Together, these results support cAMP-dependent Phox2a dephosphorylation is required for its activation. We conclude that moderate activation of cAMP signaling has dual inputs in catecholaminergic, sympathoadrenal lineage development; that is, regulation of both Phox2a transcription and activity. These results provide the first mechanistic understanding of how moderate activation of the cAMP pathway in synergy with BMP2 promotes sympathoadrenal lineage development.

BMP4 supports noradrenergic differentiation by a PKA-dependent mechanism

Differentiation of neural crest-derived noradrenergic neurons depends upon signaling mediated downstream of BMP binding to cognate receptors and involving cAMP. Compiled data from many groups suggest that neurogenesis and cell type-specific noradrenergic marker gene regulation is coordinated through the expression and function of the basic helix-loop-helix DNA binding protein HAND2 and the homeodomain DNA binding protein Phox2a. However, information detailing how BMP-mediated signaling and signaling through cAMP are coordinated has been lacking. We now provide compelling data suggesting that differentiation of noradrenergic sympathetic ganglion neurons depends upon both canonical and non-canonical pathways of BMP-mediated signaling. The non-canonical pathway involves the activation of protein kinase A (PKA) independent of cAMP. This is a novel mechanism in neural crest-derived cells and is necessary to support neurogenesis as well as aspects of DBH promoter regulation involving HAND2 phosphorylation and dimerization. The expression of transcripts encoding HAND2 and Phox2a is regulated via canonical BMP signaling and thus affects both neurogenesis and cell type-specific gene expression. Interestingly, cAMP- and MapK-mediated signaling modulate specific target sites in both the canonical and non-canonical BMP pathways. Activity of MapK is required for HAND2 transcription and thus affects neurogenesis. Signaling affected by cAMP is necessary for the transcription of Phox2a as well as regulation of DBH promoter transactivation by Phox2a and HAND2. We suggest a comprehensive model that shows how BMP- and cAMP-mediated intracellular signaling integrate neurogenesis and cell type-specific noradrenergic marker gene expression and function.

Adenosine signaling promotes neuronal, catecholaminergic differentiation of primary neural crest cells and CNS-derived CAD cells

In neural crest (NC) cultures cAMP signaling is an instructive signal in catecholaminergic, sympathoadrenal cell development. However, the extracellular signals activating the cAMP pathway during NC cell development have not been identified. We demonstrate that in avian NC cultures, evidenced by tyrosine hydroxylase expression and catecholamine biosynthesis, adenosine and not adrenergic signaling, together with BMP2, promotes sympathoadrenal cell development. In NC cultures, addition of the adenosine receptor agonist NECA in the presence of BMP2 promotes sympathoadrenal cell development, whereas the antagonist CGS 15943 or the adenosine degrading enzyme adenosine deaminase (ADA) suppresses TH expression. Importantly, NC cells express A2A and A2B receptors which couple with Gsalpha increasing intracellular cAMP. Employing the CNS-derived catecholaminergic CAD cell line, we also demonstrate that neuronal differentiation mediated by serum withdrawal is further enhanced by treatment with IBMX, a cAMP-elevating agent, or the adenosine receptor agonist NECA, acting via cAMP. By contrast, the adenosine receptor antagonist CGS 15943 or the adenosine degrading enzyme ADA inhibits CAD cell neuronal differentiation mediated by serum withdrawal. These results support that adenosine is a physiological signal in neuronal differentiation of the CNS-derived catecholaminergic CAD cell line and suggest that adenosine signaling is involved in NC cell development in vivo.

High-level activation of cyclic AMP signaling attenuates bone morphogenetic protein 2-induced sympathoadrenal lineage development and promotes melanogenesis in neural crest cultures

The intensity of cyclic AMP (cAMP) signaling is a differential instructive signal in neural crest (NC) cell specification. By an unknown mechanism, sympathoadrenal lineage specification is suppressed by high-level activation of cAMP signaling. In NC cultures, high-level activation of cAMP signaling mediates protein kinase A (PKA)-dependent Rap1-B-Raf-ERK1/2 activation, leading to cytoplasmic accumulation of phospho-Smad1, thus terminating bone morphogenetic protein 2 (BMP2)-induced sympathoadrenal cell development. Concurrently, cAMP signaling induces transcription of the melanocyte-determining transcription factor Mitf and melanogenesis. dnACREB and E1A inhibit Mitf expression and melanogenesis, supporting the notion that CREB activation is necessary for melanogenesis. However, constitutively active CREB(DIEDML) without PKA activation is insufficient for Mitf expression and melanogenesis, indicating PKA regulates additional aspects of Mitf transcription. Thus, high-level activation of cAMP signaling plays a dual role in NC cell differentiation: attenuation of BMP2-induced sympathoadrenal cell development and induction of melanogenesis. We conclude the intensity of activation of signal transduction cascades determines cell lineage segregation mechanisms.

Mechanisms and perspectives on differentiation of autonomic neurons

Neurons share many features in common but are distinguished by expression of phenotypic characteristics that define their specific function, location, or connectivity. One aspect of neuronal fate determination that has been extensively studied is that of neurotransmitter choice. The generation of diversity of neuronal subtypes within the developing nervous system involves integration of extrinsic and intrinsic instructive cues resulting in the expression of a core set of regulatory molecules. This review focuses on mechanisms of growth and transcription factor regulation in the generation of peripheral neural crest-derived neurons. Although the specification and differentiation of noradrenergic neurons are the focus, I have tried to integrate these into a larger picture providing a general roadmap for development of autonomic neurons. There is a core of DNA binding proteins required for the development of sympathetic, parasympathetic, and enteric neurons, including Phox2 and MASH1, whose specificity is regulated by the recruitment of additional transcriptional regulators in a subtype-specific manner. For noradrenergic neurons, the basic helix-loop-helix DNA binding protein HAND2 (dHAND) appears to serve this function. The studies reviewed here support the notion that neurotransmitter identity is closely linked to other aspects of neurogenesis and reveal a molecular mechanism to coordinate expression of pan-neuronal genes with cell type-specific genes.

Molecular cloning and characterization of the promoter region of the human Phox2b gene

The closely related homeodomain transcription factors, Phox2a and Phox2b, are restrictively expressed in central and peripheral noradrenergic (NA) neurons in an overlapping but distinct manner, and critically regulate the differentiation and neurotransmitter identity of NA neurons. The structure and function of the human Phox2a (hPhox2a) promoter has recently been reported. Towards the long-term goal of delineating the regulatory cascade of NA neuron differentiation, we isolated a human Phox2b (hPhox2b) genomic clone encompassing approximately 7.8 kb of the 5' upstream promoter region, the entire exon-intron structure and 4.5 kb of the 3' flanking region. Two transcription start sites are identified to reside 115 and 110 nucleotides upstream of the start codon, based on both primer extension and 5'-rapid amplification of the cDNA ends analyses. In addition, transient transfection assays indicate that 1.1 kb or longer upstream sequences of the hPhox2b gene may confer cell type-specific gene expression in certain, but not all cell lines. The promoter activity of the hPhox2b gene is modestly transactivated by forced co-expression of Phox2b and the hPhox2b gene promoter contains a high-affinity binding site at -320 to -295 bp. This study provides a frame to further elucidate the molecular mechanisms underlying the regulation of Phox2a and Phox2b gene expression and its relation to NA differentiation.

BMP pathways are involved in otic capsule formation and epithelial-mesenchymal signaling in the developing chicken inner ear

The vertebrate inner ear consists of a complex labyrinth of epithelial cells that is surrounded by a bony capsule. The molecular mechanisms coordinating the development of the membranous and bony labyrinths are largely unknown. Previously, using avian retrovirus encoding Noggin (RCAS-Noggin) or beads soaked with Noggin protein, we have shown that bone morphogenetic proteins (BMPs) are important for the development of the otic epithelium in the chicken inner ear. Here, using two additional recombinant avian retroviruses, dominant negative and constitutively active forms of BMP receptors IB (BMPRIB), we show that BMPs, possibly acting through BMPRIB, are important for otic capsule formation. We also show that Bmp2 is strongly expressed in the prospective semicircular canals starting from the canal outpouch stage, suggesting that BMP2 plays an important role in canal formation. In addition, by correlating expression patterns of Bmps, their receptors, and localization of phosphorylated R-Smad (phospho R-Smad) immunoreactivity, an indicator of BMP activation, we show that BMPs emanating from the otic epithelium influence chondrogenesis of the otic capsule including the cartilage surrounding the semicircular canals.

Specification of catecholaminergic and serotonergic neurons

The specification of neurotransmitter phenotype is an important aspect of neuronal fate determination. Substantial progress has been made in uncovering key extracellular signals and transcriptional regulators that control the mode of neurotransmission in several model systems, among which catecholaminergic and serotonergic neurons feature prominently. Here, we review our current knowledge of the regulatory circuits that direct neurotransmitter choice, and discuss the development of well-studied types of catecholaminergic and serotonergic neurons. One emerging concept is that different types of neuron use a similar core programme to control shared modes of neurotransmission, but recruit different factors that are specific for each neuronal type. Another is that most factors that specify neurotransmitter identity also control other features of the neuronal phenotype.

Activin and bone morphogenetic proteins are present in perinatal sensory neuron target tissues that induce neuropeptides

Previous studies have shown that sensory target tissues induce neuropeptides in naïve sensory neurons, and that activin and bone morphogenetic proteins (BMPs) are capable of inducing neuropeptides associated with nociception in embryonic sensory neurons in vitro. The goal of the present study was to learn if these ligands were available in native sensory neuron target tissues at correct developmental periods to play this inductive role in vivo. Sensory neurons initially contact their peripheral target tissues and begin to express neuropeptides during late embryogenesis, and we demonstrate that activin and BMPs are present in the embryo and neonate to regulate sensory neuron differentiation. Native embryonic and neonatal target tissues were analyzed by immunoblot and immunohistochemical studies using ligand-specific antibodies. Although activin was easily solubilized, BMPs were detected only after high salt extraction, suggesting that BMPs were bound to extracellular moieties and were capable of acting only locally in native tissues. One inhibitor, noggin, was present in both embryonic skin and muscle. In combination, these data suggest that neuronal differentiation is unlikely to be regulated by simple expression of ligand, but that the functional availability of ligand is a critical component confering biological activity.

Localized BMP4-noggin interactions generate the dynamic patterning of noggin expression in somites

Interactions between BMP4 and its inhibitor, noggin, regulate patterning of somites and neural crest. During mesoderm development, noggin mRNA is expressed in the intermediate mesoderm. Upon segmentation, it is detected in the lateral portion of epithelial somites becoming progressively medialized as they mature. In dissociated segments, noggin becomes transiently confined to the dorsomedial lip of the dermomyotome. Here, we investigated the factor(s) that control this lateral-to-medial shift in transcription of somitic noggin. Inhibition of BMP activity in the caudal lateral plate/intermediate mesoderm prevented noggin transcription in the lateral somite. Further rostrally (or later in development), inhibition of tube-derived BMP, but not of Wnt activity, prevented initial noggin expression in the dorsomedial lip of the dermomyotome. Moreover, BMP4 was sufficient to trigger initial expression of noggin even in the absence of ectoderm and/or neural tube, suggesting a direct action on the dorsomedial somite. Thus, the patterns of noggin transcription in somites are directly regulated by BMP4 activities emanating first from the mesoderm and later from the neural tube. Expression patterns of BMP4 and of type IA BMP receptors are spatiotemporally compatible with this lateral-to-medial shift. These results highlight the existence in the neural tube-mesoderm complex of a regulatory loop by which BMP positively regulates transcription of noggin, which in turn represses further ligand activity.

Structural and functional characterization of the 5' upstream promoter of the human Phox2a gene: possible direct transactivation by transcription factor Phox2b

The specification of neurotransmitter identity is a critical step in neural development. Recent progresses have indicated that the closely related homeodomain factors Phox2a and 2b are essential for development of noradrenergic (NA) neuron differentiation, and may directly determine the neurotransmitter identity. With a long-term goal of understanding the regulatory cascade of NA phenotype determination, we isolated and characterized a hPhox2a genomic clone encompassing approximately 7.5 kb of the 5' upstream promoter region, the entire exon-intron structure, and approximately 4 kb of the 3' flanking region. Using mRNAs isolated from the Phox2a-expressing human cell line, both primer extension and 5'-rapid amplification of cDNA ends analyses identified a single transcription start site that resides 172 nucleotides upstream of the start codon. The transcription start site was preceded by a TATA-like sequence motif and transcripts from this site contained an additional G residue at the 5' position, supporting the authenticity of this site as the transcriptional start site of hPhox2a. We assembled hPhox2a-luciferase reporter constructs containing different lengths of the 5' upstream sequences. Transient transfection assays of these reporter constructs in both hPhox2a-positive and -negative cell lines show that 1.3-kb or longer upstream sequences of the hPhox2a gene may confer NA cell-specific reporter gene expression. Furthermore, cotransfection assays in the Phox2a-negative HeLa cell line show that forced expression of Phox2b, but not that of Phox2a or MASH1, significantly transactivates the transcriptional activity of hPhox2a. This study will provide a frame to further delineate the regulatory cascade of NA neuron differentiation.

Two signal transduction pathways involved in the catecholaminergic differentiation of avian neural crest-derived cells in vitro

Molecules derived from the neural tube and found in chick embryo extract (CEE) and bone morphogenetic proteins (BMP) support the differentiation of neural crest-derived catecholaminergic (CA) neurons. We now report that intracellular signaling resulting in the activation of Map kinase (MapK) or translocation of Smad1 mediate the differentiation of CA neurons in response to CEE or BMP 4, respectively. The differentiation of CA neurons was significantly reduced by inhibiting MapK using PD98059 or by pan-specific blockade of tyrosine kinases using Herbimycin A. In the presence of BMP 4 and inhibitors of MapK signaling, differentiation of CA neurons was only moderately reduced. Independent of MapK, BMP 4 induced translocation of Smad1 from the cytosol to the nucleus and induced transcription of dHAND, a DNA binding protein required for the differentiation of CA neurons. The data suggest that CEE-derived factors and BMP4 support the differentiation of CA neurons via independent signaling pathways.

Segmental organization of neural crest migration

Avian neural crest cells migrate on precise pathways to their target areas where they form a wide variety of cellular derivatives, including neurons, glia, pigment cells and skeletal components. In one portion of their pathway, trunk neural crest cells navigate in the somitic mesoderm in a segmental fashion, invading the rostral, while avoiding the caudal, half-sclerotome. This pattern of cell migration, imposed by the somitic mesoderm, contributes to the metameric organization of the peripheral nervous system, including the sensory and sympathetic ganglia. At hindbrain levels, neural crest cells also travel from the neural tube in a segmental manner via three migratory streams of cells that lie adjacent to even-numbered rhombomeres. In this case, the adjacent mesoderm does not possess an obvious segmental organization, compared to the somitic mesoderm at trunk levels. Thus, the mechanisms by which the embryo controls segmentally-organized cell migrations have been a fascinating topic over the past several years. Here, I discuss findings from classical and recent studies that have delineated several of the tissue, cellular and molecular elements that contribute to the segmental organization of neural crest migration, primarily in the avian embryo. One common theme is that neural crest cells are prohibited from entering particular territories in the embryo due to the expression of inhibitory factors. However, permissive, migration-promoting factors may also play a key role in coordinating neural crest migration.

Wnt and BMP signaling govern lineage segregation of melanocytes in the avian embryo

Recent studies show that specification of some neural crest lineages occurs prior to or at the time of migration from the neural tube. We investigated what signaling events establish the melanocyte lineage, which has been shown to migrate from the trunk neural tube after the neuronal and glial lineages. Using in situ hybridization, we find that, although Wnts are expressed in the dorsal neural tube throughout the time when neural crest cells are migrating, the Wnt inhibitor cfrzb-1 is expressed in the neuronal and glial precursors and not in melanoblasts. This expression pattern suggests that Wnt signaling may be involved in specifying the melanocyte lineage. We further report that Wnt-3a-conditioned medium dramatically increases the number of pigment cells in quail neural crest cultures while decreasing the number of neurons and glial cells, without affecting proliferation. Conversely, BMP-4 is expressed in the dorsal neural tube throughout the time when neural crest cells are migrating, but is decreased coincident with the timing of melanoblast migration. This expression pattern suggests that BMP signaling may be involved in neural and glial cell differentiation or repression of melanogenesis. Purified BMP-4 reduces the number of pigment cells in culture while increasing the number of neurons and glial cells, also without affecting proliferation. Our data suggest that Wnt signaling specifies melanocytes at the expense of the neuronal and glial lineages, and further, that Wnt and BMP signaling have antagonistic functions in the specification of the trunk neural crest.

Bone morphogenetic proteins (BMP6 and BMP7) enhance the protective effect of neurotrophins on cultured septal cholinergic neurons during hypoglycemia

The effects of two bone morphogenetic proteins (BMP6, BMP7), alone and in combination with neurotrophins, were tested on cultures of embryonic day 15 rat septum. A week-long exposure to BMP6 or BMP7 in the optimal concentration range of 2-5 n M increased the activity of choline acetyltransferase (ChAT) by 1.6-2-fold, in both septal and combined septal-hippocampal cultures. The increase in ChAT activity reached significance after 4 days and continued to increase over an 11-day exposure. Under control culture conditions neither BMP significantly altered the number of cholinergic neurons, and BMP effects on ChAT activity were less than linearly additive with those of nerve growth factor. The effects of BMPs and BMP + neurotrophin combinations were also assayed under two stress conditions: low-density culture and hypoglycemia. In low-density cultures BMPs and BMP + neurotrophin combinations preserved ChAT activity more effectively than neurotrophins alone. During 24 h hypoglycemic stress, BMPs alone did not preserve ChAT activity, but BMP + neurotrophin combinations preserved ChAT activity much more effectively than neurotrophins alone. These results demonstrate that BMP6 and BMP7 enhance ChAT activity under control and low-density stress conditions, and that during a hypoglycemic stress their trophic effect requires and complements that exerted by neurotrophins.

Skin cell induction of calcitonin gene-related peptide in embryonic sensory neurons in vitro involves activin

Target skin cells induce the neuropeptide calcitonin gene-related peptide (CGRP) in naïve embryonic dorsal root ganglion (DRG) neurons in vitro, but the molecular basis of that induction is not known. Recombinant activin or bone morphogenetic proteins (BMPs) dramatically increase the number of sensory neurons with CGRP and substance P in vitro (X. Ai et al., 1999, Mol. Cell. Neurosci. 14, 506-518). These experiments were designed to test if activin or BMPs accounted for the CGRP-inductive activity by skin cells. To identify factors from skin that induce CGRP, we developed a bioassay in which embryonic DRG neurons isolated before peripheral target contact in vivo are challenged in vitro with specific factors. Conditioned medium from an embryonic rat skin cell line induced neuronal CGRP expression, and induction was blocked by follistatin, implicating transforming growth factor family members. Immunoblot analysis revealed that the skin cell line medium contained several activin and bone morphogenetic protein moieties. Antibody specific to activin neutralized most of the CGRP-inductive activity in skin conditioned medium. These data indicate that the CGRP-inductive action of skin cells involves activin and establish activin as a candidate regulator of this sensory neuropeptide phenotype during development.

DIFFERENTIAL EXPRESSION OF SYMPATHOADRENAL LINEAGE–DETERMINING GENES AND PHENOTYPIC MARKERS IN CULTURED PRIMARY NEURAL CREST CELLS

Bone morphogenetic protein-2 (BMP-2) promotes the development of primary neural crest cells grown in tissue culture to the sympathoadrenal (SA) lineage. Independent studies have characterized the expression patterns of SA-lineage genes in developing chicken embryo; however, studies using cultured primary neural crest cells have characterized only the expression patterns of the catecholaminergic markers, tyrosine hydroxylase (TH) and catecholamines (CAs). To further explore the molecular mechanisms that control SA-cell development using the in vitro model system, it is crucial to define the expression patterns of both the catecholaminergic markers and the genes regulating SA-lineage determination. Accordingly, we defined, in the absence and presence of BMP-2, the temporal expression patterns of TH and CA, the SA lineage-determining genes ASH-1, Phox2a, and Phox2b, the GATA-2 gene, and the pan-neuronal SCG10 gene. Comparison of these data with the reported temporal and spatial patterns of expression in vivo demonstrate that the inductive steps of SA-lineage determination, including the specification of neurotransmitter identity and neuronal fate, are recapitulated in the neural-crest culture system.

Utilization of bone morphogenetic protein receptors during chondrocyte maturation

Cartilage from the upper, cephalic portion of embryonic chick sternums undergoes hypertrophy, while the lower, caudal portion of the sternum remains as cartilage. Bone morphogenetic proteins (BMPs) induce type X collagen (colX) in cultured upper but not lower sternal chondrocytes (LSCs). We have examined the utilization of BMP receptors (BMPRs) by upper sternal chondrocytes (USCs) and LSCs both by analyzing receptor expression and by overexpressing mutant BMPRs. Reverse-transcription polymerase chain reaction (RT-PCR) analyses indicate that both upper and lower chondrocytes produce messenger RNA (mRNA) for all three receptors: BMPR type IA (BMPR-IA), BMPR type IB (BMPR-IB), and BMPR type II (BMPR-II). Infection of USC with retroviral vectors expressing constitutively active (CA) BMPRs showed that CA-BMPR-IB, like exogenous BMP-4, induced both colX mRNA and elevated alkaline phosphatase (AP), while CA-BMPR-IA was markedly less potent. However, expression of activated receptors in LSC cultures resulted in only minimal induction of hypertrophic markers. Consistent with the results seen for CA receptors, dominant negative (DN) BMPR-IB blocked BMP-induced hypertrophy in USCs more effectively than DN-BMPR-IA. These results imply that the major BMPR required for BMP induction of chondrocyte hypertrophy is BMPR-IB, and that difference between permanent and prehypertrophic chondrocytes is not caused by absence of receptors required for BMP signaling.

Induction and maintenance of the neuronal cholinergic phenotype in the central nervous system by BMP-9

Bone morphogenetic proteins (BMPs) have multiple functions in the developing nervous system. A member of this family, BMP-9, was found to be highly expressed in the embryonic mouse septum and spinal cord, indicating a possible role in regulating the cholinergic phenotype. In cultured neurons, BMP-9 directly induced the expression of the cholinergic gene locus encoding choline acetyltransferase and the vesicular acetylcholine transporter and up-regulated acetylcholine synthesis. The effect was reversed upon withdrawal of BMP-9. Intracerebroventricular injection of BMP-9 increased acetylcholine levels in vivo. Although certain other BMPs also up-regulated the cholinergic phenotype in vitro, they were less effective than BMP-9. These data indicate that BMP-9 is a differentiating factor for cholinergic central nervous system neurons.

Expression and regulation of type I BMP receptors during early avian sympathetic ganglion development

We have investigated the expression and regulation of the mRNAs for the type I BMP receptors, BMPR-IA and BMPR-IB, in quail embryos in vivo and in neural crest cultures in vitro. BMPR-IB mRNA was expressed in the primordial sympathetic ganglia at stage 17, soon after the first expression of Cash-1 mRNA, the avian homolog of the Drosophila transcription factor achaete-scute. BMP-4 mRNA was detected in the dorsal aorta at stage 17, coincident with BMPR-IB mRNA expression in the sympathetic ganglia. BMPR-IA mRNA was first expressed in the sympathetic ganglia at stage 18. Moreover, BMP-4 ligand mRNA was detected in the sympathetic ganglia starting at stage 18. BMPR-IA and BMPR-IB were differentially regulated in cultured neural crest cells. BMPR-IB was expressed in primary outgrowths of neural crest cells but was downregulated after primary outgrowths were harvested and replated in secondary cultures. In secondary cultures of neural crest cells, exogenous BMP-2 and BMP-4 increased the expression of BMPR-IA but decreased the expression of BMPR-IB. The expression of both type I BMP receptors was inhibited by exogenous TGF-beta1. Our results suggest distinct roles for BMPR-IA and BMPR-IB in the development of the sympathoadrenal phenotype from cells of the neural crest.

Cyclic AMP signaling functions as a bimodal switch in sympathoadrenal cell development in cultured primary neural crest cells

Cells of the vertebrate neural crest (crest cells) are an invaluable model system to address cell fate specification. Crest cells are amenable to tissue culture, and they differentiate to a variety of neuronal and nonneuronal cell types. Earlier studies have determined that bone morphogenetic proteins (BMP-2, -4, and -7) and agents that elevate intracellular cyclic AMP (cAMP) stimulate the development of the sympathoadrenal (SA, adrenergic) lineage in neural crest cultures. To investigate whether interactive mechanisms between signaling pathways influence crest cell differentiation, we characterized the combinatorial effects of BMP-2 and cAMP-elevating agents on the development of quail trunk neural crest cells in primary culture. We report that the cAMP signaling pathway modulates both positive and negative signals influencing the development of SA cells. Specifically, we show that moderate activation of cAMP signaling promotes, in synergy with BMP-2, SA cell development and the expression of the SA lineage-determining gene Phox2a. By contrast, robust activation of cAMP signaling opposes, even in the presence of BMP-2, SA cell development and the expression of the SA lineage-determining ASH-1 and Phox2 genes. We conclude that cAMP signaling acts as a bimodal regulator of SA cell development in neural crest cultures.

The effects of bone morphogenetic protein 2 and 4 (BMP2 and BMP4) on gap junctions during neurodevelopment

Nervous system deficits account for the third largest group of fatal birth defects (after heart and respiratory problems) in North America. Although considerable advance has been made in neuroscience research, the early events involved in neurogenesis remain to be elucidated. More specifically, the effects of signaling molecules on intercellular communication during neurodevelopment have not yet been studied. The development of the central nervous system is regulated, at least in part, by signaling molecules such as bone morphogenetic proteins (BMPs). In this study, we have used the embryonal mouse P19 cell line to examine the effects of BMP2 and BMP4 on gap junctional communication as well as neuronal and astrocytic differentiation. The undifferentiated P19 cells show high levels of the gap junction protein, connexin43 (Cx43), and functional intercellular coupling. However, Cx43 expression and dye coupling decrease as these cells differentiate into neurons and astrocytes. In contrast, cells treated with BMP2 or BMP4 lose their capacity to differentiate into neurons but not astrocytes, while they maintain extensive gap junctional communication. The very few neurons that remain in the BMP-treated cultures are coupled (a characteristic not seen in the control neurons). Together, our data suggest that BMPs may play a critical role in morphogenesis of P19 cells while they affect gap junctions.

Patterning of dopaminergic neurotransmitter identity among Caenorhabditis elegans ray sensory neurons by a TGFbeta family signaling pathway and a Hox gene

We have investigated the mechanism that patterns dopamine expression among Caenorhabditis elegans male ray sensory neurons. Dopamine is expressed by the A-type sensory neurons in three out of the nine pairs of rays. We used expression of a tyrosine hydroxylase reporter transgene as well as direct assays for dopamine to study the genetic requirements for adoption of the dopaminergic cell fate. In loss-of-function mutants affecting a TGFbeta family signaling pathway, the DBL-1 pathway, dopaminergic identity is adopted irregularly by a wider subset of the rays. Ectopic expression of the pathway ligand, DBL-1, from a heat-shock-driven transgene results in adoption of dopaminergic identity by rays 3–9; rays 1 and 2 are refractory. The rays are therefore prepatterned with respect to their competence to be induced by a DBL-1 pathway signal. Temperature-shift experiments with a temperature-sensitive type II receptor mutant, as well as heat-shock induction experiments, show that the DBL-1 pathway acts during an interval that extends from two to one cell generation before ray neurons are born and begin to differentiate. In a mutant of the AbdominalB class Hox gene egl-5, rays that normally express EGL-5 do not adopt dopaminergic fate and cannot be induced to express DA when DBL-1 is provided by a heat-shock-driven dbl-1 transgene. Therefore, egl-5 is required for making a subset of rays capable of adopting dopaminergic identity, while the function of the DBL-1 pathway signal is to pattern the realization of this capability.

Expression of HAND gene products may be sufficient for the differentiation of avian neural crest-derived cells into catecholaminergic neurons in culture

Members of the basic helix-loop-helix family of DNA binding proteins have important roles in the development of subpopulations of neural crest-derived neurons. We have cloned the chicken homologues of dHAND (HAND2) and eHAND (HAND1), basic helix-loop-helix DNA binding proteins whose neuronal expression is restricted to sympathetic and enteric neural crest-derived ganglia. Transcripts encoding dHAND and eHAND are expressed in sympathetic ganglia beginning at Hamburger and Hamilton stage 17-18. Antisense blockade of transcripts encoding HAND genes in neural crest-derived cells in vitro results in a significant reduction in neurogenesis. Differentiation of catecholaminergic neurons is also reduced by 52% if the expression of transcripts encoding dHAND and eHAND is reduced using antisense oligonucleotide blockade. The effect on neurogenesis and phenotypic expression of neural crest-derived neurons is specific; blockade of HAND gene expression has no apparent influence on the differentiation in vitro of neural tube-derived neurons. Use of a replication-competent avian retrovirus to constitutively express HAND genes in neural crest-derived cells in vitro, under nonpermissive growth conditions in medium supplemented with 2% chick embryo extract (CEE), induced precocious catecholaminergic differentiation. Constitutive expression of HAND gene products resulted in a significant increase in catecholaminergic differentiation of cells grown in medium supplemented with 10% CEE, a permissive growth condition for catecholaminergic development. These results suggest that the expression by neural crest cells of dHAND and eHAND may be both sufficient and necessary for catecholaminergic phenotypic expression.

Bone morphogenetic proteins-2 and -4 attenuate apoptosis in a cerebellar primitive neuroectodermal tumor cell line

Similarities between primitive neuroectodermal tumors and central nervous system (CNS) progenitor cells have evoked interest in the response of these tumors to endogenous growth factors. The bone morphogenetic proteins (BMPs) have recently been found to regulate survival and differentiation of CNS progenitor cell populations. In this study, we investigated the effects of BMP-2, BMP-4, and BMP-6 on the undifferentiated cerebellar primitive neuroectodermal tumor or medulloblastoma cell line DAOY. Analysis by reverse transcriptase-polymerase chain reaction showed that mRNAs for type IA and type II BMP receptors were present in control cultures. In cultures treated with BMP-2, mRNAs for BMP receptor type IB and the activin R-I receptor became evident. Cultures were analyzed for total cell counts, proliferating cell nuclear antigen (PCNA), and apoptotic DNA fragmentation. There was a significant increase in total cell number in the BMP-2 and BMP-4 treatment groups, without any change in PCNA reactivity, and a dramatic decrease in the proportion of apoptotic nuclei at concentrations of BMP-2 and BMP-4 above 5 ng/ml (P<0.001). These effects were not observed with BMP-6, TGF-beta1 or GDNF. These results suggest that the increase in total cell number is due to the attenuation of apoptosis by BMP-2 and BMP-4. The anti-apoptotic effect of BMP-2 and BMP-4 on this neuroectodermal cell line has potential clinical implications for neuroectodermal tumors.

Transcriptional control of neurotransmitter phenotype

The specification of neurotransmitter phenotype is an important aspect of neuronal fate determination. Recent studies have begun to define essential transcriptional regulators involved in controlling the mode of neurotransmission in vertebrates and invertebrates, and to examine their regulation by cell-extrinsic factors. An emerging concept is that the control of transmitter choice is intimately linked to that of other aspects of the neuronal phenotype.