Cloning of a Drosophila melanogaster homologue of the mouse type-I bone morphogenetic proteins-2/-4 receptor: a potential decapentaplegic receptor

Bone morphogenetic protein signaling: the pathway and its regulation

In the mid-1960s, bone morphogenetic proteins (BMPs) were first identified in the extracts of bone to have the remarkable ability to induce heterotopic bone. When the Drosophila gene decapentaplegic (dpp) was first identified to share sequence similarity with mammalian BMP2/BMP4 in the late-1980s, it became clear that secreted BMP ligands can mediate processes other than bone formation. Following this discovery, collaborative efforts between Drosophila geneticists and mammalian biochemists made use of the strengths of their respective model systems to identify BMP signaling components and delineate the pathway. The ability to conduct genetic modifier screens in Drosophila with relative ease was critical in identifying the intracellular signal transducers for BMP signaling and the related transforming growth factor-beta/activin signaling pathway. Such screens also revealed a host of genes that encode other core signaling components and regulators of the pathway. In this review, we provide a historical account of this exciting time of gene discovery and discuss how the field has advanced over the past 30 years. We have learned that while the core BMP pathway is quite simple, composed of 3 components (ligand, receptor, and signal transducer), behind the versatility of this pathway lies multiple layers of regulation that ensures precise tissue-specific signaling output. We provide a sampling of these discoveries and highlight many questions that remain to be answered to fully understand the complexity of BMP signaling.

Smad-Independent BMP Signaling in Somatic Cells Limits the Size of the Germline Stem Cell Pool

In developing organisms, proper tuning of the number of stem cells within a niche is critical for the maintenance of adult tissues; however, the involved mechanisms remain largely unclear. Here, we demonstrate that Thickveins (Tkv), a type I bone morphogenetic protein (BMP) receptor, acts in the Drosophila developing ovarian soma through a Smad-independent pathway to shape the distribution of BMP signal within the niche, impacting germline stem cell (GSC) recruitment and maintenance. Somatic Tkv promotes Egfr signaling to silence transcription of Dally, which localizes BMP signals on the cell surface. In parallel, Tkv promotes Hh signaling, which promotes escort cell cellular protrusions and upregulates expression of the Drosophila BMP homolog, Dpp, forming a positive feedback loop that enhances Tkv signaling and strengthens the niche boundary. Our results reveal a role for non-canonical BMP signaling in the soma during GSC establishment and generally illustrate how complex, cell-specific BMP signaling mediates niche-stem cell interactions.

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Tkv, a BMP receptor, in the developing ovarian soma controls fertility

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Knockdown Tkv in the developing soma causes ectopic germline stem cell (GSC) accumulation

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Tkv in the soma controls GSC number by limiting BMPs within the GSC niche

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BMP-Tkv signaling in the soma limits GSC number via Egfr and Hh signaling

fussel (fuss)--A negative regulator of BMP signaling in Drosophila melanogaster

The TGF-β/BMP signaling cascades control a wide range of developmental and physiological functions in vertebrates and invertebrates. In Drosophila melanogaster, members of this pathway can be divided into a Bone Morphogenic Protein (BMP) and an Activin-ß (Act-ß) branch, where Decapentaplegic (Dpp), a member of the BMP family has been most intensively studied. They differ in ligands, receptors and transmitting proteins, but also share some components, such as the Co-Smad Medea (Med). The essential role of Med is to form a complex with one of the two activating Smads, mothers against decapentaplegic (Mad) or dSmad, and to translocate together to the nucleus where they can function as transcriptional regulators of downstream target genes. This signaling cascade underlies different mechanisms of negative regulation, which can be exerted by inhibitory Smads, such as daughters against decapentaplegic (dad), but also by the Ski-Sno family. In this work we identified and functionally analyzed a new member of the Ski/Sno-family, fussel (fuss), the Drosophila homolog of the human functional suppressing element 15 (fussel-15). fuss codes for two differentially spliced transcripts with a neuronal expression pattern. The proteins are characterized by a Ski-Sno and a SAND homology domain. Overexpression studies and genetic interaction experiments clearly reveal an interaction of fuss with members of the BMP pathway, leading to a strong repression of BMP-signaling. The protein interacts directly with Medea and seems to reprogram the Smad pathway through its influence upon the formation of functional Mad/Medea complexes. This leads amongst others to a repression of downstream target genes of the Dpp pathway, such as optomotor blind (omb). Taken together we could show that fuss exerts a pivotal role as an antagonist of BMP signaling in Drosophila melanogaster.

The Ca2+-dependent protease Calpain A regulates Cactus/I kappaB levels during Drosophila development in response to maternal Dpp signals

Regulation of NF kappaB activity is central to many processes during development and disease. Activation of NF kappaB family members depends on degradation of inhibitory I kappaB proteins. In Drosophila, a nuclear gradient of the NF kappaB/c-rel protein Dorsal subdivides the embryonic dorsal-ventral axis, defining the extent and location of mesodermal and ectodermal territories. Activation of the Toll pathway directs Dorsal nuclear translocation by inducing proteosomal degradation of the I kappaB homologue Cactus. Another mechanism that impacts on Dorsal activation involves the Toll-independent pathway, which regulates constitutive Cactus degradation. We have shown that the BMP protein Decapentaplegic (Dpp) inhibits Cactus degradation independent of Toll. Here we report on a novel element of this pathway: the calcium-dependent protease Calpain A. Calpain A knockdowns increase Cactus levels, shifting the Dorsal gradient and dorsal-ventral patterning. As shown for mammalian I kappaB, this effect requires PEST sequences in the Cactus C-terminus, implying a conserved role for calpains. Alteration of Calpain A or dpp results in similar effects on Dorsal target genes. Epistatic analysis confirms Calpain A activity is regulated by Dpp, indicating that Dpp signals increase Cactus levels through Calpain A inhibition, thereby interfering with Dorsal activation. This mechanism may allow coordination of Toll, BMP and Ca(2+) signals, conferring precision to Dorsal-target expression domains.

Specialized extraembryonic cells connect embryonic and extraembryonic epidermis in response to Dpp during dorsal closure in Drosophila

Dorsal closure in Drosophila embryogenesis involves expansion of the dorsal epidermis, followed by closure of the opposite epidermal edges. This process is driven by contractile force generated by an extraembryonic epithelium covering the yolk syncytium known as the amnioserosa. The secreted signaling molecule Dpp is expressed in the leading edge of the dorsal epidermis and is essential for dorsal closure. We found that the outermost row of amnioserosa cells (termed pAS) maintains a tight basolateral cell-cell adhesion interface with the leading edge of dorsal epidermis throughout the dorsal closure process. pAS was subject to altered cell motility in response to Dpp emanating from the dorsal epidermis, and this response was essential for dorsal closure. alphaPS3 and betaPS integrin subunits accumulated in the interface between pAS and dorsal epidermis, and were both required for dorsal closure. Looking at alphaPS3, type I Dpp receptor, and JNK mutants, we found that pAS cell motility was altered and pAS and dorsal epidermis adhesion failed under the mechanical stress of dorsal closure, suggesting that a Dpp-mediated mechanism connects the squamous pAS to the columnar dorsal epidermis to form a single coherent epithelial layer.

Drosophila p24 homologues eclair and baiser are necessary for the activity of the maternally expressed Tkv receptor during early embryogenesis

p24 proteins are assumed to play an important role in the transport of secreted and transmembrane proteins into membranes. However, only few cargo proteins are known that partially, but in no case completely require p24 proteins for membrane transport. Here, we show that two p24 proteins are essential for dorsoventral patterning of Drosophila melanogaster embryo. Mutations in the genes, eclair (eca) and baiser (bai), encoding two p24 proteins reduce signalling by the TGF-beta homologue, Dpp, in early embryos. This effect is strictly maternal and specific to early embryogenesis, as Dpp signalling in other contexts is not notably affected. We provide genetic evidence that in the absence of eca or bai function in the oocyte, the maternally expressed type I TGF-beta receptor Tkv is not active. We propose that during early embryogenesis eca and bai are specifically required for the activity of the maternal Tkv, while the zygotic Tkv is not affected in the mutant embryos. Mutations in either eca or bai are sufficient for the depletion of Tkv activity and no enhancement of the phenotypes was observed in embryos derived from oocytes mutant for both genes. The dependence of maternal Tkv protein on the products of p24 genes may serve as an in vivo model for studying p24 proteins.

Insect Neuropeptide and Peptide Hormone Receptors: Current Knowledge and Future Directions

Peptides form a very versatile class of extracellular messenger molecules that function as chemical communication signals between the cells of an organism. Molecular diversity is created at different levels of the peptide synthesis scheme. Peptide messengers exert their biological functions via specific signal-transducing membrane receptors. The evolutionary origin of several peptide precursor and receptor gene families precedes the divergence of the important animal Phyla. In this chapter, current knowledge is reviewed with respect to the analysis of peptide receptors from insects, incorporating many recent data that result from the sequencing of different insect genomes. Therefore, detailed information is provided on six different peptide receptor families belonging to two distinct receptor categories (i.e., the heptahelical and the single transmembrane receptors). In addition, the remaining problems, the emerging concepts, and the future prospects in this area of research are discussed.

Morphogenetic apoptosis: a mechanism for correcting discontinuities in morphogen gradients

Smooth gradients of the morphogens Hh, Dpp, and Wg are required for proper development of Drosophila imaginal discs. Here, it is reported that, when a discontinuity is generated between two adjacent cells in the reception of either the Dpp or Wg signal, then cells on either side of the discontinuity boundary undergo apoptosis by activating the c-Jun N-terminal Kinase (JNK) pathway. Furthermore, in the medial region of the wing imaginal disc, the JNK pathway is also activated if cells do not receive the proper levels of Dpp and Hh signals. These observations suggest that cells within a developing field have the ability to access their spatial positions by comparing the level of morphogen signal they receive with that of their neighbors. This phenomenon is likely related to the process of cell competition, and we suggest that it is an evolutionarily important mechanism that helps prevent abnormal tissue specification and growth during development.

p38 mitogen-activated protein kinase can be involved in transforming growth factor beta superfamily signal transduction in Drosophila wing morphogenesis

p38 mitogen-activated protein kinase (p38) has been extensively studied as a stress-responsive kinase, but its role in development remains unknown. The fruit fly, Drosophila melanogaster, has two p38 genes, D-p38a and D-p38b. To elucidate the developmental function of the Drosophila p38's, we used various genetic and pharmacological manipulations to interfere with their functions: expression of a dominant-negative form of D-p38b, expression of antisense D-p38b RNA, reduction of the D-p38 gene dosage, and treatment with the p38 inhibitor SB203580. Expression of a dominant-negative D-p38b in the wing imaginal disc caused a decapentaplegic (dpp)-like phenotype and enhanced the phenotype of a dpp mutant. Dpp is a secretory ligand belonging to the transforming growth factor beta superfamily which triggers various morphogenetic processes through interaction with the receptor Thick veins (Tkv). Inhibition of D-p38b function also caused the suppression of the wing phenotype induced by constitutively active Tkv (TkvCA). Mosaic analysis revealed that D-p38b regulates the Tkv-dependent transcription of the optomotor-blind (omb) gene in non-Dpp-producing cells, indicating that the site of D-p38b action is downstream of Tkv. Furthermore, forced expression of TkvCA induced an increase in the phosphorylated active form(s) of D-p38(s). These results demonstrate that p38, in addition to its role as a transducer of emergency stress signaling, may function to modulate Dpp signaling.

Role of dpp signalling in prepattern formation of the dorsocentral mechanosensory organ in Drosophila melanogaster

A proneural cluster of dorsocentral bristles forms adjacent to the dorsal side of wg-expressing cells in the notum region of the wing imaginal disc. It has been shown that wg activity is required for these structures to form. However, the restriction of this proneural cluster to the dorsal posterior side of the wg expression domain in the anterior compartment of the wing imaginal disc has suggested that Wg signalling itself is insufficient to establish the dorsocentral proneural cluster. Some factor(s) from the posterior side must participate in this action in cooperation with Wg signalling. We have examined the role of Dpp signalling in dorsocentral bristle formation by either ectopically activating or conditionally reducing Dpp signalling. Ubiquitous activation of Dpp signalling in the notum region of the wing imaginal disc induced additional dorsocentral proneural cluster all along the dorsal side of the wg expression domain, and altered wg expression. Conditional loss-of-function of Dpp signalling during disc development resulted in the inhibition of dorsocentral proneural cluster formation and expansion of the wg expression domain. These results suggest that Dpp signalling has two indispensable roles in dorsocentral bristle formation: induction of the dorsocentral proneural cluster in cooperation with Wg signalling and restriction of the wg expression domain in the notum region of the wing imaginal disc.

Bone morphogenetic proteins: neurotrophic roles for midbrain dopaminergic neurons and implications of astroglial cells

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor beta (TGF-beta) superfamily that have been implicated in tissue growth and remodelling. Recent evidence suggests that several BMPs are expressed in the developing and adult brain. Specifically, we show that BMP 2 and BMP 6 are expressed in the developing midbrain floor of the rat. We studied potential neurotrophic effects of BMPs on the in vitro survival, transmitter uptake and protection against MPP+ toxicity of mesencephalic dopaminergic neurons cultured from the embryonic midbrain floor at embryonic day (E) 14. At 10 ng/ml and under serum-free conditions, most BMPs promoted the survival of dopaminergic neurons visualized by tyrosine hydroxylase immunocytochemistry during an 8-day culture period, but to varying extents (relative potencies: BMP 6 = 12 > 2, 4, 7). BMPs 6 and 12 were as effective as fibroblast growth factor-2 (FGF-2) and glial cell line-derived neurotrophic factor, promoting survival 1.7-fold compared with controls. BMPs 9 and 11 were not effective. Dose-response curves revealed an EC50 for BMPs 2, 6 and 12 of 2 ng/ml. BMPs 2, 4, 6, 7, 9 and 12 also promoted DNA synthesis and astroglial cell differentiation, visualized by 5-bromodeoxyuridine (BrdU) incorporation and glial fibrillary acidic protein (GFAP) immunocytochemistry respectively. Suppression of cell proliferation and subsequent maturation of GFAP-positive cells by 5-fluorodeoxyuridine or aminoadipic acid abolished the neuron survival-promoting effect of BMP 2. This suggests that BMPs, like other non-TGF-beta factors affecting dopaminergic neuron survival, act indirectly, probably by stimulating the synthesis and/or release of glial-derived trophic factors. BMP 6 and BMP 7 also increased the uptake of [3H]dopamine without affecting the uptake of [3H]5-hydroxytryptamine and [3H]GABA, underscoring the specificity of the trophic effect. We conclude that several BMPs share a neurotrophic capacity for dopaminergic midbrain neurons with other members of the TGF-beta superfamily, but act indirectly, possibly through glial cells.

Specification of the embryonic limb primordium by graded activity of Decapentaplegic

Two thoracic limbs of Drosophila, the leg and the wing, originate from a common cluster of cells that include the source of two secreted signaling molecules, Decapentaplegic and Wingless. We show that Wingless, but not Decapentaplegic, is responsible for initial specification of the limb primordia with a distal identity. Limb formation is restricted to the lateral position of the embryo by negative control of the early function of Decapentaplegic and the EGF receptor homolog that determine the global dorsoventral pattern. Late function of Decapentaplegic locally determines two additional cell identities in a dosage dependent manner. Loss of Decapentaplegic activity results in a deletion of the proximal structures of the limb, which is in contrast to the consequence of decapentaplegic mutations in the imaginal disc, which cause a deletion of distal structures. The results indicate that the limb pattern elements are added in a distal to proximal direction in the embryo, which is opposite to what is happening in the growing imaginal disc. We propose that Wingless and Decapentaplegic act sequentially to initiate the proximodistal axis.

Signal transduction and TGF-beta superfamily receptors

The TGF-beta superfamily includes a large number of related growth and differentiation factors expressed in virtually all phyla. Superfamily members bind to specific cell surface receptors that activate signal transduction mechanisms to elicit their effects. Candidate receptors fall into two primary groups, termed type I and type II receptors. Both types are serine/threonine kinases. Upon activation by the appropriate ligand, type I and type II receptors physically interact to form hetero-oligomers and subsequently activate intracellular signaling cascades, ultimately regulating gene transcription and expression. In addition, TGF-beta binds to a third receptor class, type III, a membrane-anchored proteoglycan lacking the kinase activity typical of signal transducing molecules. Type III receptors appear to regulate ligand availability to type I and type II receptors. Although a number of transduction mechanisms may be available to TGF-beta superfamily members, evidence gathered through the use of specific kinase and G-protein inhibitors and through assays measuring activation and levels of signaling intermediates suggests that at least one signaling pathway interacts with Ras and Raf proteins via a G-protein intermediate. Raf begins the cytoplasmic kinase cascade that leads to gene regulation. The myriad responses regulated by TGF-beta superfamily members makes the understanding of signal transduction mechanisms utilized by these proteins of great interest to a wide range of biological disciplines.

Bone morphogenetic protein-2 but not bone morphogenetic protein-4 and -6 stimulates chemotactic migration of human osteoblasts, human marrow osteoblasts, and U2-OS cells

Bone morphogenetic proteins (BMPs) have important functions for the differentiation of bone cells, but the exact role for bone remodeling and bone healing still needs to be defined. Migration of bone forming cells is an important physiological event both during bone healing and bone remodeling. The chemotatic properties of the bone morphogenetic protein family of growth factors have not been investigated. In this study the chemotactic effects of the bone morphogenetic proteins BMP-2, -4, and -6 have been quantitated toward human osteoblasts, human marrow stromal osteoblasts, and U2-OS human osteosarcoma cells. BMP-2 stimulated the migration of human stromal osteoblasts, human osteoblasts, and U2-OS cells with bell-shaped response curves in a dose-dependent manner with a 300% increase in cell migration at 1.0 ng/mL for human stromal osteoblasts and a 170-180% increase for human osteoblasts and U2-OS cells. At higher concentrations, migration decreased to background levels. BMP-4 and -6 did not show any effect on cellular migration. This study shows that BMP-2 can stimulate in vitro migration of human osteoblasts and human osteosarcoma cells. BMP-2 might play a role in the chemotactic recruitment of especially undifferentiated osteoblasts during bone remodeling and bone healing.

Bmpr encodes a type I bone morphogenetic protein receptor that is essential for gastrulation during mouse embryogenesis

Bone morphogenetic proteins (BMPs) are secreted proteins that interact with cell-surface receptors and are believed to play a variety of important roles during vertebrate embryogenesis. Bmpr, also known as ALK-3 and Brk-1, encodes a type I transforming growth factor-beta (TGF-beta) family receptor for BMP-2 and BMP-4. Bmpr is expressed ubiquitously during early mouse embryogenesis and in most adult mouse tissues. To study the function of Bmpr during mammalian development, we generated Bmpr-mutant mice. After embryonic day 9.5 (E9.5), no homozygous mutants were recovered from heterozygote matings. Homozygous mutants with morphological defects were first detected at E7.0 and were smaller than normal. Morphological and molecular examination demonstrated that no mesoderm had formed in the mutant embryos. The growth characteristics of homozygous mutant blastocysts cultured in vitro were indistinguishable from those of controls; however, embryonic ectoderm (epiblast) cell proliferation was reduced in all homozygous mutants at E6.5 before morphological abnormalities had become prominent. Teratomas arising from E7.0 mutant embryos contained derivatives from all three germ layers but were smaller and gave rise to fewer mesodermal cell types, such as muscle and cartilage, than controls. These results suggest that signaling through this type I BMP-2/4 receptor is not necessary for preimplantation or for initial postimplantation development but may be essential for the inductive events that lead to the formation of mesoderm during gastrulation and later for the differentiation of a subset of mesodermal cell types.