A large bioactive BMP ligand with distinct signaling properties is produced by alternative proconvertase processing

Maternal TGF-β ligand Panda breaks the radial symmetry of the sea urchin embryo by antagonizing the Nodal type II receptor ACVRII

In the highly regulative embryo of the sea urchin Paracentrotus lividus, establishment of the dorsal-ventral (D/V) axis critically depends on the zygotic expression of the TGF-β nodal in the ventral ectoderm. nodal expression is first induced ubiquitously in the 32-cell embryo and becomes progressively restricted to the presumptive ventral ectoderm by the early blastula stage. This early spatial restriction of nodal expression is independent of Lefty, and instead relies on the activity of Panda, a maternally expressed TGF-β ligand related to Lefty and Inhibins, which is required maternally for D/V axis specification. However, the mechanism by which Panda restricts the early nodal expression has remained enigmatic and it is not known if Panda works like a BMP ligand by opposing Nodal and antagonizing Smad2/3 signaling, or if it works like Lefty by sequestering an essential component of the Nodal signaling pathway. In this study, we report that Panda functions as an antagonist of the TGF-β type II receptor ACVRII (Activin receptor type II), which is the only type II receptor for Nodal signaling in the sea urchin and is also a type II receptor for BMP ligands. Inhibiting translation of acvrII mRNA disrupted D/V patterning across all 3 germ layers and caused acvrII morphants to develop with a typical Nodal loss-of-function phenotype. In contrast, embryos overexpressing acvrII displayed strong ectopic Smad1/5/8 signaling at blastula stages and developed as dorsalized larvae, a phenotype very similar to that caused by over activation of BMP signaling. Remarkably, embryos co-injected with acvrII mRNA and panda mRNA did not show ectopic Smad1/5/8 signaling and developed with a largely normal dorsal-ventral polarity. Furthermore, using an axis induction assay, we found that Panda blocks the ability of ACVRII to orient the D/V axis when overexpressed locally. Using co-immunoprecipitation, we showed that Panda physically interacts with ACVRII, as well as with the Nodal co-receptor Cripto, and with TBR3 (Betaglycan), which is a non-signaling receptor for Inhibins in mammals. At the molecular level, we have traced back the antagonistic activity of Panda to the presence of a single proline residue, conserved with all the Lefty factors, in the ACVRII binding motif of Panda, instead of a serine as in most of TGF-β ligands. Conversion of this proline to a serine converted Panda from an antagonist that opposed Nodal signaling and promoted dorsalization to an agonist that promoted Nodal signaling and triggered ventralization when overexpressed. Finally, using phylogenomics, we analyzed the emergence of the agonist and antagonist form of Panda in the course of evolution. Our data are consistent with the idea that the presence of a serine at that position, like in most TGF-β, was the ancestral condition and that the initial function of Panda was possibly in promoting and not in antagonizing Nodal signaling. These results highlight the existence of key functional and structural elements conserved between Panda and Lefty, allow to draw an intriguing parallel between sea urchin Panda and mammalian Inhibin α and raise the unexpected possibility that the original function of Panda may have been in activation of the Nodal pathway rather than in its inhibition.

BMP4 and GREM1 are targets of SHH signaling and downstream regulators of collagen in the penis

BACKGROUND

Cavernous nerve (CN) injury, caused by prostatectomy and diabetes, initiates a remodeling process (smooth muscle apoptosis and increased collagen) in the corpora cavernosa of the penis of patients and animal models that is an underlying cause of erectile dysfunction (ED), and the Sonic hedgehog (SHH) pathway plays an essential role in the response of the penis to denervation, as collagen increases with SHH inhibition and decreases with SHH treatment.

AIM

We examined if part of the mechanism of how SHH prevents penile remodeling and increased collagen with CN injury involves bone morphogenetic protein 4 (BMP4) and gremlin1 (GREM1) and examined the relationship between SHH, BMP4, GREM1, and collagen in penis of ED patients and rat models of CN injury, SHH inhibition, and SHH, BMP4, and GREM1 treatment.

METHODS

Corpora cavernosa of Peyronie's disease (control), prostatectomy, and diabetic ED patients were obtained (N = 30). Adult Sprague Dawley rats (n = 90) underwent (1) CN crush (1-7 days) or sham surgery; (2) CN injury and BMP4, GREM1, or mouse serum albumin (control) treatment via Affi-Gel beads or peptide amphiphile (PA) for 14 days; (3) 5E1 SHH inhibitor, IgG, or phosphate-buffered saline (control) treatment for 2 to 4 days; or (4) CN crush with mouse serum albumin or SHH for 9 days.

OUTCOMES

Immunohistochemical and Western analysis for BMP4 and GREM1, and collagen analysis by hydroxyproline and trichrome stain were performed.

RESULTS

BMP4 and GREM1 proteins were identified in corpora cavernosa smooth muscle of prostatectomy, diabetic, and Peyronie's patients, and in rat smooth muscle, sympathetic nerve fibers, perineurium, blood vessels, and urethra. Collagen decreased 25.4% in rats with CN injury and BMP4 treatment (P = .02) and increased 61.3% with CN injury and GREM1 treatment (P = .005). Trichrome stain showed increased collagen in rats treated with GREM1. Western analysis identified increased BMP4 and GREM1 in corpora cavernosa of prostatectomy and diabetic patients, and after CN injury (1-2 days) in our rat model. Localization of BMP4 and GREM1 changed with SHH inhibition. SHH treatment increased the monomer form of BMP4 and GREM1, altering their range of signaling.

CLINICAL IMPLICATIONS

A better understanding of penile remodeling and how fibrosis occurs with loss of innervation is essential for development of novel ED therapies.

STRENGTHS AND LIMITATIONS

The relationship between SHH, BMP4, GREM1, and collagen is complex in the penis.

CONCLUSION

BMP4 and GREM1 are downstream targets of SHH that impact collagen and may be useful in collaboration with SHH to prevent penile remodeling and ED.

SHH regulates penile morphology and smooth muscle through a mechanism involving BMP4 and GREM1

BACKGROUND

The cavernous nerve (CN) is frequently damaged in prostatectomy and diabetic patients with erectile dysfunction (ED), initiating changes in penile morphology including an acute and intense phase of apoptosis in penile smooth muscle and increased collagen, which alter penile architecture and make corpora cavernosa smooth muscle less able to relax in response to neurotransmitters, resulting in ED.

AIM

Sonic hedgehog (SHH) is a critical regulator of penile smooth muscle, and SHH treatment suppresses penile remodeling after CN injury through an unknown mechanism; we examine if part of the mechanism of how SHH preserves smooth muscle after CN injury involves bone morphogenetic protein 4 (BMP4) and gremlin1 (GREM1).

METHODS

Primary cultures of smooth muscle cells were established from prostatectomy, diabetic, hypertension and Peyronie's (control) (N = 18) patients. Cultures were characterized by ACTA2, CD31, P4HB, and nNOS immunohistochemical analysis. Patient smooth muscle cell growth was quantified in response to BMP4 and GREM1 treatment. Adult Sprague Dawley rats underwent 1 of 3 surgeries: (1) uninjured or CN-injured rats were treated with BMP4, GREM1, or mouse serum albumin (control) proteins via Affi-Gel beads (N = 16) or peptide amphiphile (PA) (N = 26) for 3 and 14 days, and trichrome stain was performed; (2) rats underwent sham (N = 3), CN injury (N = 9), or CN injury and SHH PA treatment for 1, 2, and 4 days (N = 9).

OUTCOMES

Western analysis for BMP4 and GREM1 was performed; (3) rats were treated with 5E1 SHH inhibitor (N = 6) or IgG (control; N = 6) for 2 and 4 days, and BMP4 and GREM1 localization was examined. Statistics were performed by analysis of variance with Scheffé's post hoc test.

RESULTS

BMP4 increased patient smooth muscle cell growth, and GREM1 decreased growth. In rats, BMP4 treatment via Affi-Gel beads and PA increased smooth muscle at 3 and 14 days of treatment. GREM1 treatment caused increased collagen and smooth muscle at 3 days, which switched to primarily collagen at 14 days. CN injury increased BMP4 and GREM1, while SHH PA altered Western band size, suggesting alternative cleavage and range of BMP4 and GREM1 signaling. SHH inhibition in rats increased BMP4 and GREM1 in fibroblasts.

CLINICAL IMPLICATIONS

Understanding how SHH PA preserves and regenerates penile morphology after CN injury will aid development of ED therapies.

STRENGTHS AND LIMITATIONS

SHH treatment alters BMP4 and GREM1 localization and range of signaling, which can affect penile morphology.

CONCLUSION

Part of the mechanism of how SHH regulates corpora cavernosa smooth muscle involves BMP4 and GREM1.

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.

Heterodimerization-dependent secretion of bone morphogenetic proteins in Drosophila

Combinatorial signaling is key to instruct context-dependent cell behaviors. During embryonic development, adult homeostasis, and disease, bone morphogenetic proteins (BMPs) act as dimers to instruct specific cellular responses. BMP ligands can form both homodimers or heterodimers; however, obtaining direct evidence of the endogenous localization and function of each form has proven challenging. Here, we make use of precise genome editing and direct protein manipulation via protein binders to dissect the existence and functional relevance of BMP homodimers and heterodimers in the Drosophila wing imaginal disc. This approach identified in situ the existence of Dpp (BMP2/4)/Gbb (BMP5/6/7/8) heterodimers. We found that Gbb is secreted in a Dpp-dependent manner in the wing imaginal disc. Dpp and Gbb form a gradient of heterodimers, whereas neither Dpp nor Gbb homodimers are evident under endogenous physiological conditions. We find that the formation of heterodimers is critical for obtaining optimal signaling and long-range BMP distribution.

Molecular Mechanisms of AMH Signaling

Anti-Müllerian Hormone (AMH) is a secreted glycoprotein hormone with critical roles in reproductive development and regulation. Its chemical and mechanistic similarities to members of the Transforming Growth Factor β (TGF-β) family have led to its placement within this signaling family. As a member of the TGF-β family, AMH exists as a noncovalent complex of a large N-terminal prodomain and smaller C-terminal mature signaling domain. To produce a signal, the mature domain will bind to the extracellular domains of two type I and two type II receptors which results in an intracellular SMAD signal. Interestingly, as will be discussed in this review, AMH possesses several unique characteristics which set it apart from other ligands within the TGF-β family. In particular, AMH has a dedicated type II receptor, Anti-Müllerian Hormone Receptor Type II (AMHR2), making this interaction intriguing mechanistically as well as therapeutically. Further, the prodomain of AMH has remained largely uncharacterized, despite being the largest prodomain within the family. Recent advancements in the field have provided valuable insight into the molecular mechanisms of AMH signaling, however there are still many areas of AMH signaling not understood. Herein, we will discuss what is known about the biochemistry of AMH and AMHR2, focusing on recent advances in understanding the unique characteristics of AMH signaling and the molecular mechanisms of receptor engagement.

Protection of the Prodomain α1-Helix Correlates with Latency in the Transforming Growth Factor-β Family

The 33 members of the transforming growth factor beta (TGF-β) family are fundamentally important for organismal development and homeostasis. Family members are synthesized and secreted as pro-complexes of non-covalently associated prodomains and growth factors (GF). Pro-complexes from a subset of family members are latent and require activation steps to release the GF for signaling. Why some members are latent while others are non-latent is incompletely understood, particularly because of large family diversity. Here, we have examined representative family members in negative stain electron microscopy (nsEM) and hydrogen deuterium exchange (HDX) to identify features that differentiate latent from non-latent members. nsEM showed three overall pro-complex conformations that differed in prodomain arm domain orientation relative to the bound growth factor. Two cross-armed members, TGF-β1 and TGF-β2, were each latent. However, among V-armed members, GDF8 was latent whereas ActA was not. All open-armed members, BMP7, BMP9, and BMP10, were non-latent. Family members exhibited remarkably varying HDX patterns, consistent with large prodomain sequence divergence. A strong correlation emerged between latency and protection of the prodomain α1-helix from exchange. Furthermore, latency and protection from exchange correlated structurally with increased α1-helix buried surface area, hydrogen bonds, and cation-pi bonds. Moreover, a specific pattern of conserved basic and hydrophobic residues in the α1-helix and aromatic residues in the interacting fastener were found only in latent members. Thus, this first comparative survey of TGF-β family members reveals not only diversity in conformation and dynamics but also unique features that distinguish latent members.

BMP/TGF-β signaling as a modulator of neurodegeneration in ALS

This commentary focuses on the emerging intersection between BMP/TGF-β signaling roles in nervous system function and the amyotrophic lateral sclerosis (ALS) disease state. Future research is critical to elucidate the molecular underpinnings of this intersection of the cellular processes disrupted in ALS and those influenced by BMP/TGF-β signaling, including synapse structure, neurotransmission, plasticity, and neuroinflammation. Such knowledge promises to inform us of ideal entry points for the targeted modulation of dysfunctional cellular processes in an effort to abrogate ALS pathologies. It is likely that different interventions are required, either at discrete points in disease progression, or across multiple dysfunctional processes which together lead to motor neuron degeneration and death. We discuss the challenging, but intriguing idea that modulation of the pleiotropic nature of BMP/TGF-β signaling could be advantageous, as a way to simultaneously treat defects in more than one cell process across different forms of ALS.

Starvation-induced regulation of carbohydrate transport at the blood-brain barrier is TGF-β-signaling dependent

During hunger or malnutrition, animals prioritize alimentation of the brain over other organs to ensure its function and, thus, their survival. This protection, also-called brain sparing, is described from Drosophila to humans. However, little is known about the molecular mechanisms adapting carbohydrate transport. Here, we used Drosophila genetics to unravel the mechanisms operating at the blood-brain barrier (BBB) under nutrient restriction. During starvation, expression of the carbohydrate transporter Tret1-1 is increased to provide more efficient carbohydrate uptake. Two mechanisms are responsible for this increase. Similar to the regulation of mammalian GLUT4, Rab-dependent intracellular shuttling is needed for Tret1-1 integration into the plasma membrane; even though Tret1-1 regulation is independent of insulin signaling. In addition, starvation induces transcriptional upregulation that is controlled by TGF-β signaling. Considering TGF-β-dependent regulation of the glucose transporter GLUT1 in murine chondrocytes, our study reveals an evolutionarily conserved regulatory paradigm adapting the expression of sugar transporters at the BBB.

Muscle-derived Myoglianin regulates Drosophila imaginal disc growth

Organ growth and size are finely tuned by intrinsic and extrinsic signaling molecules. In Drosophila, the BMP family member Dpp is produced in a limited set of imaginal disc cells and functions as a classic morphogen to regulate pattern and growth by diffusing throughout imaginal discs. However, the role of TGFβ/Activin-like ligands in disc growth control remains ill-defined. Here, we demonstrate that Myoglianin (Myo), an Activin family member, and a close homolog of mammalian Myostatin (Mstn), is a muscle-derived extrinsic factor that uses canonical dSmad2-mediated signaling to regulate wing size. We propose that Myo is a myokine that helps mediate an allometric relationship between muscles and their associated appendages.

A Neuron-Glial Trans-Signaling Cascade Mediates LRRK2-Induced Neurodegeneration

Pathogenic mutations in leucine-rich repeat kinase 2 (LRRK2) induce an age-dependent loss of dopaminergic (DA) neurons. We have identified Furin 1, a pro-protein convertase, as a translational target of LRRK2 in DA neurons. Transgenic knockdown of Furin1 or its substrate the bone morphogenic protein (BMP) ligand glass bottom boat (Gbb) protects against LRRK2-induced loss of DA neurons. LRRK2 enhances the accumulation of phosphorylated Mad (pMad) in the nuclei of glial cells in the vicinity of DA neurons but not in DA neurons. Consistently, exposure to paraquat enhances Furin 1 levels in DA neurons and induces BMP signaling in glia. In support of a neuron-glial signaling model, knocking down BMP pathway members only in glia, but not in neurons, can protect against paraquat toxicity. We propose that a neuron-glial BMP-signaling cascade is critical for mediating age-dependent neurodegeneration in two models of Parkinson's disease, thus opening avenues for future therapeutic interventions.

Muscle-derived Dpp regulates feeding initiation via endocrine modulation of brain dopamine biosynthesis

In animals, the brain regulates feeding behavior in response to local energy demands of peripheral tissues, which secrete orexigenic and anorexigenic hormones. Although skeletal muscle is a key peripheral tissue, it remains unknown whether muscle-secreted hormones regulate feeding. In Drosophila, we found that decapentaplegic (dpp), the homolog of human bone morphogenetic proteins BMP2 and BMP4, is a muscle-secreted factor (a myokine) that is induced by nutrient sensing and that circulates and signals to the brain. Muscle-restricted dpp RNAi promotes foraging and feeding initiation, whereas dpp overexpression reduces it. This regulation of feeding by muscle-derived Dpp stems from modulation of brain tyrosine hydroxylase (TH) expression and dopamine biosynthesis. Consistently, Dpp receptor signaling in dopaminergic neurons regulates TH expression and feeding initiation via the downstream transcriptional repressor Schnurri. Moreover, pharmacologic modulation of TH activity rescues the changes in feeding initiation due to modulation of dpp expression in muscle. These findings indicate that muscle-to-brain endocrine signaling mediated by the myokine Dpp regulates feeding behavior.

BMP7 functions predominantly as a heterodimer with BMP2 or BMP4 during mammalian embryogenesis

BMP7/BMP2 or BMP7/BMP4 heterodimers are more active than homodimers in vitro, but it is not known whether these heterodimers signal in vivo. To test this, we generated knock in mice carrying a mutation (Bmp7^R-GFlag) that prevents proteolytic activation of the dimerized BMP7 precursor protein. This mutation eliminates the function of BMP7 homodimers and all other BMPs that normally heterodimerize with BMP7. While Bmp7 null homozygotes are live born, Bmp7^R-GFlag homozygotes are embryonic lethal and have broadly reduced BMP activity. Furthermore, compound heterozygotes carrying the Bmp7^R-G allele together with a null allele of Bmp2 or Bmp4 die during embryogenesis with defects in ventral body wall closure and/or the heart. Co-immunoprecipitation assays confirm that endogenous BMP4/7 heterodimers exist. Thus, BMP7 functions predominantly as a heterodimer with BMP2 or BMP4 during mammalian development, which may explain why mutations in either Bmp4 or Bmp7 lead to a similar spectrum of congenital defects in humans.

Gene expression correlates of facultative predation in the blow fly Chrysomya rufifacies (Diptera: Calliphoridae)

Effects of intraguild predation (IGP) on omnivores and detritivores are relatively understudied when compared to work on predator guilds. Functional genetic work in IGP is even more limited, but its application can help answer a range of questions related to ultimate and proximate causes of this behavior. Here, we integrate behavioral assays and transcriptomic analysis of facultative predation in a blow fly (Diptera: Calliphoridae) to evaluate the prevalence, effect, and correlated gene expression of facultative predation by the invasive species Chrysomya rufifacies. Field work observing donated human cadavers indicated facultative predation by C. rufifacies on the native blow fly Cochliomyia macellaria was rare under undisturbed conditions, owing in part to spatial segregation between species. Laboratory assays under conditions of starvation showed predation had a direct fitness benefit (i.e., survival) to the predator. As a genome is not available for C. rufifacies, a de novo transcriptome was developed and annotated using sequence similarity to Drosophila melanogaster. Under a variety of assembly parameters, several genes were identified as being differentially expressed between predators and nonpredators of this species, including genes involved in cell-to-cell signaling, osmotic regulation, starvation responses, and dopamine regulation. Results of this work were integrated to develop a model of the processes and genetic regulation controlling facultative predation.

Specificity, versatility, and control of TGF-β family signaling

Encoded in mammalian cells by 33 genes, the transforming growth factor-β (TGF-β) family of secreted, homodimeric and heterodimeric proteins controls the differentiation of most, if not all, cell lineages and many aspects of cell and tissue physiology in multicellular eukaryotes. Deregulation of TGF-β family signaling leads to developmental anomalies and disease, whereas enhanced TGF-β signaling contributes to cancer and fibrosis. Here, we review the fundamentals of the signaling mechanisms that are initiated upon TGF-β ligand binding to its cell surface receptors and the dependence of the signaling responses on input from and cooperation with other signaling pathways. We discuss how cells exquisitely control the functional presentation and activation of heteromeric receptor complexes of transmembrane, dual-specificity kinases and, thus, define their context-dependent responsiveness to ligands. We also introduce the mechanisms through which proteins called Smads act as intracellular effectors of ligand-induced gene expression responses and show that the specificity and impressive versatility of Smad signaling depend on cross-talk from other pathways. Last, we discuss how non-Smad signaling mechanisms, initiated by distinct ligand-activated receptor complexes, complement Smad signaling and thus contribute to cellular responses.

BMP-dependent synaptic development requires Abi-Abl-Rac signaling of BMP receptor macropinocytosis

Retrograde BMP trans-synaptic signaling is essential for synaptic development. Despite the importance of endocytosis-regulated BMP receptor (BMPR) control of this developmental signaling, the mechanism remains unknown. Here, we provide evidence that Abelson interactor (Abi), a substrate for Abl kinase and component of the SCAR/WAVE complex, links Abl and Rac1 GTPase signaling to BMPR macropinocytosis to restrain BMP-mediated synaptic development. We find that Abi acts downstream of Abl and Rac1, and that BMP ligand Glass bottom boat (Gbb) induces macropinocytosis dependent on Rac1/SCAR signaling, Abl-mediated Abi phosphorylation, and BMPR activation. Macropinocytosis acts as the major internalization route for BMPRs at the synapse in a process driven by Gbb activation and resulting in receptor degradation. Key regulators of macropinocytosis (Rabankyrin and CtBP) control BMPR trafficking to limit BMP trans-synaptic signaling. We conclude that BMP-induced macropinocytosis acts as a BMPR homeostatic mechanism to regulate BMP-mediated synaptic development.

BMP ligands act as retrograde signalling molecules to regulate presynaptic development, and regulation of BMP receptors by endocytosis may be an important component of this signalling pathway. Here, the authors show that Abi-mediated macropinocytosis of BMP receptors in Drosophila larva and contributes to neuromuscular development.

Bone Morphogenetic Protein Glass Bottom Boat (BMP5/6/7/8) and its receptor Wishful Thinking (BMPRII) are required for injury-induced allodynia in Drosophila

Background Chronic pain affects millions of people worldwide; however, its cellular and molecular mechanisms have not been completely elucidated. It is thought that chronic pain is triggered by nociceptive sensitization, which produces elevated nocifensive responses. A model has been developed in Drosophila melanogaster to investigate the underlying mechanisms of chronic pain using ultraviolet-induced tissue injury to trigger thermal allodynia, a nociceptive hypersensitivity to a normally innocuous stimulus. Larvae were assayed for their behavioral latencies to produce a distinct avoidance response under different thermal conditions. Previously, Decapentaplegic, a member of the Bone Morphogenetic Protein (BMP) family and orthologous to mammalian BMP2/4, was shown to be necessary for the induction of allodynia. Here, we further investigate the BMP pathway to identify other essential molecules necessary to activate the nociceptive sensitization pathway. Results Using the GAL4-UAS-RNAi system to induce a cell-specific knockdown of gene expression, we further explored BMP pathway components to identify other key players in the induction of nociceptive sensitization by comparing the responses of manipulated animals to those of controls. Here, we show that a second BMP, Glass Bottom Boat, and its receptor Wishful Thinking are both necessary for injury-induced thermal allodynia since the formation of sensitization was found to be severely attenuated when either of these components was suppressed. The effects on pain perception appear to be specific to the sensitization system, as the ability to respond to a normally noxious stimulus in the absence of injury was left intact, and no nociceptor morphological defects were observed. Conclusion These results provide further support of the hypothesis that the BMP pathway plays a crucial role in the development of nociceptive sensitization. Because of its strong conservation between invertebrates and mammals, the BMP pathway may be worthy of future investigation for the development of targeted treatments to alleviate chronic pain.

Somatic clones heterozygous for recessive disease alleles of BMPR1A exhibit unexpected phenotypes in Drosophila

The majority of mutations studied in animal models are designated as recessive based on the absence of visible phenotypes in germline heterozygotes. Accordingly, genetic studies primarily rely on homozygous loss-of-function to determine gene requirements, and a conceptually-related ‘two-hit model’ remains the central paradigm in cancer genetics. Here we investigate pathogenesis due to somatic mutation in epithelial tissues, a process that predominantly generates heterozygous cell clones. To study somatic mutation in Drosophila, we generated inducible alleles that mimic human Juvenile polyposis-associated BMPR1A mutations. Unexpectedly, four of these mutations had no phenotype in heterozygous carriers but exhibited clear tissue-level effects when present in somatic clones of heterozygous cells. We conclude that these alleles are indeed recessive when present in the germline, but nevertheless deleterious when present in heterozygous clones. This unforeseen effect, deleterious heteromosaicism, suggests a ‘one-hit’ mechanism for disease initiation that may explain some instances of pathogenesis associated with spontaneous mutation.

Drosophila models of FOP provide mechanistic insight

Fibrodysplasia ossificans progressiva (FOP) is a rare bone disease characterized by episodic events of heterotopic ossification (HO). All cases of FOP have been attributed to mutations in the ACVR1 gene that render the encoded BMP type I ALK2 receptor hypersensitive, resulting in the activation of BMP signaling, at inappropriate times in inappropriate locations. The episodic or sporadic nature of HO associated with FOP rests with the occurrence of specific 'triggers' that push the hypersensitive ALK2-FOP receptor into full signaling mode. Identification of these triggers and their mechanism of action is critical for preventing HO and its devastating consequences in FOP patients. Models of FOP, generated in Drosophila, are shown to activate the highly conserved BMP signaling pathway in both Drosophila cell culture and in developing tissues in vivo. The most common FOP mutation, R206H, in ALK2 and its synonymous mutation, K262H, in the orthologous Drosophila receptor Sax, abolish the ability of wild type receptors to inhibit BMP ligand-induced signaling and lead to ubiquitous pathway activation in both cases but with important differences. When expressed in Drosophila, human ALK2R206H exhibits constitutive signaling. SaxK262H on the other hand can elicit excessive signaling similar to that observed for ALK2R206H in mammalian systems in vivo. For example, hyperactive signaling mediated by SaxK262H is triggered by an increase in ligand or in type II receptors. Interestingly, while the constitutive nature of ALK2R2026H in Drosophila requires activation by the type II receptor, it does not require its ligand binding domain. The differences exhibited by the two Drosophila FOP models enable a valuable comparative analysis poised to reveal critical regulatory mechanisms governing signaling output from these mutated receptors. Modifier screens using these Drosophila FOP models will be extremely valuable in identifying genes or compounds that reduce or prevent the hyperactive BMP signaling that initiates HO associated with FOP.

TGF-β Family Signaling in Connective Tissue and Skeletal Diseases

The transforming growth factor β (TGF-β) family of signaling molecules, which includes TGF-βs, activins, inhibins, and numerous bone morphogenetic proteins (BMPs) and growth and differentiation factors (GDFs), has important functions in all cells and tissues, including soft connective tissues and the skeleton. Specific TGF-β family members play different roles in these tissues, and their activities are often balanced with those of other TGF-β family members and by interactions with other signaling pathways. Perturbations in TGF-β family pathways are associated with numerous human diseases with prominent involvement of the skeletal and cardiovascular systems. This review focuses on the role of this family of signaling molecules in the pathologies of connective tissues that manifest in rare genetic syndromes (e.g., syndromic presentations of thoracic aortic aneurysm), as well as in more common disorders (e.g., osteoarthritis and osteoporosis). Many of these diseases are caused by or result in pathological alterations of the complex relationship between the TGF-β family of signaling mediators and the extracellular matrix in connective tissues.

Alternative cleavage of the bone morphogenetic protein (BMP), Gbb, produces ligands with distinct developmental functions and receptor preferences

The family of TGF-β and bone morphogenetic protein (BMP) signaling proteins has numerous developmental and physiological roles. They are made as proprotein dimers and then cleaved by proprotein convertases to release the C-terminal domain as an active ligand dimer. Multiple proteolytic processing sites in Glass bottom boat (Gbb), the Drosophila BMP7 ortholog, can produce distinct ligand forms. Cleavage at the S1 or atypical S0 site in Gbb produces Gbb15, the conventional small BMP ligand, whereas NS site cleavage produces a larger Gbb38 ligand. We hypothesized that the Gbb prodomain is involved not only in regulating the production of specific ligands but also their signaling output. We found that blocking NS cleavage increased association of the full-length prodomain with Gbb15, resulting in a concomitant decrease in signaling activity. Moreover, NS cleavage was required in vivo for Gbb-Decapentaplegic (Dpp) heterodimer-mediated wing vein patterning but not for Gbb15-Dpp heterodimer activity in cell culture. Gbb NS cleavage was also required for viability through its regulation of pupal ecdysis in a type II receptor Wishful thinking (Wit)-dependent manner. In fact, Gbb38-mediated signaling exhibits a preference for Wit over the other type II receptor Punt. Finally, we discovered that Gbb38 is produced when processing at the S1/S0 site is blocked by O-linked glycosylation in third instar larvae. Our findings demonstrate that BMP prodomain cleavage ensures that the mature ligand is not inhibited by the prodomain. Furthermore, alternative processing of BMP proproteins produces ligands that signal through different receptors and exhibit specific developmental functions.

TGF-β Family Signaling in Drosophila

The transforming growth factor β (TGF-β) family signaling pathway is conserved and ubiquitous in animals. In Drosophila, fewer representatives of each signaling component are present compared with vertebrates, simplifying mechanistic study of the pathway. Although there are fewer family members, the TGF-β family pathway still regulates multiple and diverse functions in Drosophila. In this review, we focus our attention on several of the classic and best-studied functions for TGF-β family signaling in regulating Drosophila developmental processes such as embryonic and imaginal disc patterning, but we also describe several recently discovered roles in regulating hormonal, physiological, neuronal, innate immunity, and tissue homeostatic processes.

Regulation of Drosophila hematopoietic sites by Activin-β from active sensory neurons

An outstanding question in animal development, tissue homeostasis and disease is how cell populations adapt to sensory inputs. During Drosophila larval development, hematopoietic sites are in direct contact with sensory neuron clusters of the peripheral nervous system (PNS), and blood cells (hemocytes) require the PNS for their survival and recruitment to these microenvironments, known as Hematopoietic Pockets. Here we report that Activin-β, a TGF-β family ligand, is expressed by sensory neurons of the PNS and regulates the proliferation and adhesion of hemocytes. These hemocyte responses depend on PNS activity, as shown by agonist treatment and transient silencing of sensory neurons. Activin-β has a key role in this regulation, which is apparent from reporter expression and mutant analyses. This mechanism of local sensory neurons controlling blood cell adaptation invites evolutionary parallels with vertebrate hematopoietic progenitors and the independent myeloid system of tissue macrophages, whose regulation by local microenvironments remain undefined.

Proprotein convertase Furin1 expression in the Drosophila fat body is essential for a normal antimicrobial peptide response and bacterial host defense

Invading pathogens provoke robust innate immune responses in Dipteran insects, such as Drosophila melanogaster In a systemic bacterial infection, a humoral response is induced in the fat body. Gram-positive bacteria trigger the Toll signaling pathway, whereas gram-negative bacterial infections are signaled via the immune deficiency (IMD) pathway. We show here that the RNA interference-mediated silencing of Furin1-a member of the proprotein convertase enzyme family-specifically in the fat body, results in a reduction in the expression of antimicrobial peptides. This, in turn, compromises the survival of adult fruit flies in systemic infections that are caused by both gram-positive and -negative bacteria. Furin1 plays a nonredundant role in the regulation of immune responses, as silencing of Furin2, the other member of the enzyme family, had no effect on survival or the expression of antimicrobial peptides upon a systemic infection. Furin1 does not directly affect the Toll or IMD signaling pathways, but the reduced expression of Furin1 up-regulates stress response factors in the fat body. We also demonstrate that Furin1 is a negative regulator of the Janus kinase/signal transducer and activator of transcription signaling pathway, which is implicated in stress responses in the fly. In summary, our data identify Furin1 as a novel regulator of humoral immunity and cellular stress responses in Drosophila-Aittomäki, S., Valanne, S., Lehtinen, T., Matikainen, S., Nyman, T. A., Rämet, M., Pesu, M. Proprotein convertase Furin1 expression in the Drosophila fat body is essential for a normal antimicrobial peptide response and bacterial host defense.

Structural Biology and Evolution of the TGF-β Family

We review the evolution and structure of members of the transforming growth factor β (TGF-β) family, antagonistic or agonistic modulators, and receptors that regulate TGF-β signaling in extracellular environments. The growth factor (GF) domain common to all family members and many of their antagonists evolved from a common cystine knot growth factor (CKGF) domain. The CKGF superfamily comprises six distinct families in primitive metazoans, including the TGF-β and Dan families. Compared with Wnt/Frizzled and Notch/Delta families that also specify body axes, cell fate, tissues, and other families that contain CKGF domains that evolved in parallel, the TGF-β family was the most fruitful in evolution. Complexes between the prodomains and GFs of the TGF-β family suggest a new paradigm for regulating GF release by conversion from closed- to open-arm procomplex conformations. Ternary complexes of the final step in extracellular signaling show how TGF-β GF dimers bind type I and type II receptors on the cell surface, and enable understanding of much of the specificity and promiscuity in extracellular signaling. However, structures suggest that when GFs bind repulsive guidance molecule (RGM) family coreceptors, type I receptors do not bind until reaching an intracellular, membrane-enveloped compartment, blurring the line between extra- and intracellular signaling. Modulator protein structures show how structurally diverse antagonists including follistatins, noggin, and members of the chordin family bind GFs to regulate signaling; complexes with the Dan family remain elusive. Much work is needed to understand how these molecular components assemble to form signaling hubs in extracellular environments in vivo.

Structural insights into BMP receptors: Specificity, activation and inhibition

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor-β family (TGFβ), which signal through hetero-tetrameric complexes of type I and type II receptors. In humans there are many more TGFβ ligands than receptors, leading to the question of how particular ligands can initiate specific signaling responses. Here we review structural features of the ligands and receptors that contribute to this specificity. Ligand activity is determined by receptor-ligand interactions, growth factor prodomains, extracellular modulator proteins, receptor assembly and phosphorylation of intracellular signaling proteins, including Smad transcription factors. Detailed knowledge about the receptors has enabled the development of BMP-specific type I receptor kinase inhibitors. In future these may help to treat human diseases such as fibrodysplasia ossificans progressiva.

Matters of context guide future research in TGFβ superfamily signaling

The highly conserved wiring of the SMAD-dependent transforming growth factor β (TGFβ) superfamily signaling pathway has been mapped over the last 20 years after molecular discovery of its component parts. Numerous alternative TGFβ-activated signaling pathways that elicit SMAD-independent biological responses also exist. However, the molecular mechanisms responsible for the renowned context dependency of TGFβ signaling output remains an active and often confounding area of research, providing a prototype relevant to regulation of other signaling pathways. Highlighting discoveries presented at the 9th FASEB meeting, The TGFβ Superfamily: Signaling in Development and Disease (July 12-17th 2015 in Snowmass, Colorado), this Review outlines research into the rich contextual nature of TGFβ signaling output and offers clues for therapeutic advances.

Anti-Müllerian hormone is a gonadal cytokine with two circulating forms and cryptic actions

Anti-Müllerian hormone (AMH) is a multi-faceted gonadal cytokine. It is present in all vertebrates with its original function in phylogeny being as a regulator of germ cells in both sexes, and as a prime inducer of the male phenotype. Its ancient functions appear to be broadly conserved in mammals, but with this being obscured by its overt role in triggering the regression of the Müllerian ducts in male embryos. Sertoli and ovarian follicular cells primarily release AMH as a prohormone (proAMH), which forms a stable complex (AMHN,C) after cleavage by subtilisin/kexin-type proprotein convertases or serine proteinases. Circulating AMH is a mixture of proAMH and AMHN,C, suggesting that proAMH is activated within the gonads and putatively by its endocrine target-cells. The gonadal expression of the cleavage enzymes is subject to complex regulation, and the preliminary data suggest that this influences the relative proportions of proAMH and AMHN,C in the circulation. AMH shares an intracellular pathway with the bone morphogenetic protein (BMP) and growth differentiation factor (GDF) ligands. AMH is male specific during the initial stage of development, and theoretically should produce male biases throughout the body by adding a male-specific amplification of BMP/GDF signalling. Consistent with this, some of the male biases in neuron number and the non-sexual behaviours of mice are dependent on AMH. After puberty, circulating levels of AMH are similar in men and women. Putatively, the function of AMH in adulthood maybe to add a gonadal influence to BMP/GDF-regulated homeostasis.

Mechanisms of BMP-Receptor Interaction and Activation

Bone morphogenetic proteins (BMPs), together with the eponymous transforming growth factor (TGF) β and the Activins form the TGFβ superfamily of ligands. This protein family comprises more than 30 structurally highly related proteins, which determine formation, maintenance, and regeneration of tissues and organs. Their importance for the development of multicellular organisms is evident from their existence in all vertebrates as well as nonvertebrate animals. From their highly specific functions in vivo either a strict relation between a particular ligand and its cognate cellular receptor and/or a stringent regulation to define a distinct temperospatial expression pattern for the various ligands and receptor is expected. However, only a limited number of receptors are found to serve a large number of ligands thus implicating highly promiscuous ligand-receptor interactions instead. Since in tissues a multitude of ligands are often found, which signal via a highly overlapping set of receptors, this raises the question how such promiscuous interactions between different ligands and their receptors can generate concerted and highly specific cellular signals required during embryonic development and tissue homeostasis.

Structure of bone morphogenetic protein 9 procomplex

Bone morphogenetic proteins (BMPs) belong to the TGF-β family, whose 33 members regulate multiple aspects of morphogenesis. TGF-β family members are secreted as procomplexes containing a small growth factor dimer associated with two larger prodomains. As isolated procomplexes, some members are latent, whereas most are active; what determines these differences is unknown. Here, studies on pro-BMP structures and binding to receptors lead to insights into mechanisms that regulate latency in the TGF-β family and into the functions of their highly divergent prodomains. The observed open-armed, nonlatent conformation of pro-BMP9 and pro-BMP7 contrasts with the cross-armed, latent conformation of pro-TGF-β1. Despite markedly different arm orientations in pro-BMP and pro-TGF-β, the arm domain of the prodomain can similarly associate with the growth factor, whereas prodomain elements N- and C-terminal to the arm associate differently with the growth factor and may compete with one another to regulate latency and stepwise displacement by type I and II receptors. Sequence conservation suggests that pro-BMP9 can adopt both cross-armed and open-armed conformations. We propose that interactors in the matrix stabilize a cross-armed pro-BMP conformation and regulate transition between cross-armed, latent and open-armed, nonlatent pro-BMP conformations.

Expression of the myostatin gene in the adductor muscle of the Pacific lion-paw scallop Nodipecten subnodosus in association with growth and environmental conditions

The cDNA sequence of the myostatin gene in the Pacific lion-paw Nodipecten subnodosus (Ns-mstn) was characterized, and the temporal expression during grow-out was analyzed for the first time in a scallop. Ns-mstn encodes a 459-amino-acid protein in which two propeptide proteolytic sites were identified, the previously recognized (RSKR) and a second one at position 266-269 aa (RRKR). The alternative furin cleavage site could be related with post-translational processing, or it could be a tissue-specific mechanism for signaling activity. The Ns-mstn transcript was located by in situ hybridization in sarcomeres and around the nucleus of muscle fibers. The temporal expression analysis by qPCR in the adductor muscle showed that Ns-mstn expression was significantly different (P < 0.05) between months during the grow-out period, increasing largely during the summer months when both biomass and muscle weight did not increase or even decreased; muscle fiber size and number were found to decrease significantly. Exogenous and endogenous factors such as high temperature and low food availability, as well as gametogenesis and reproduction, can be associated with the growth pattern and Ns-mstn expression changes. Our results indicate that MSTN is involved in adductor muscle growth regulation in N. subnodosus as it occurs in vertebrate skeletal muscle although Ns-mstn expression in non-muscle organs/tissues suggests additional functions.

Prodomain removal enables neto to stabilize glutamate receptors at the Drosophila neuromuscular junction

Stabilization of neurotransmitter receptors at postsynaptic specializations is a key step in the assembly of functional synapses. Drosophila Neto (Neuropillin and Tolloid-like protein) is an essential auxiliary subunit of ionotropic glutamate receptor (iGluR) complexes required for the iGluRs clustering at the neuromuscular junction (NMJ). Here we show that optimal levels of Neto are crucial for stabilization of iGluRs at synaptic sites and proper NMJ development. Genetic manipulations of Neto levels shifted iGluRs distribution to extrajunctional locations. Perturbations in Neto levels also produced small NMJs with reduced synaptic transmission, but only Neto-depleted NMJs showed diminished postsynaptic components. Drosophila Neto contains an inhibitory prodomain that is processed by Furin1-mediated limited proteolysis. neto null mutants rescued with a Neto variant that cannot be processed have severely impaired NMJs and reduced iGluRs synaptic clusters. Unprocessed Neto retains the ability to engage iGluRs in vivo and to form complexes with normal synaptic transmission. However, Neto prodomain must be removed to enable iGluRs synaptic stabilization and proper postsynaptic differentiation.

Synapse development is initiated by genetic programs, but is coordinated by neuronal activity, by communication between the pre- and postsynaptic compartments, and by cellular signals that integrate the status of the whole organisms and its developmental progression. The molecular mechanisms underlining these processes are poorly understood. In particular, how neurotransmitter receptors are recruited and stabilized at central synapses remain the subject of intense research. The Drosophila NMJ is a glutamatergic synapse similar in composition and physiology with mammalian central excitatory synapses. Like mammals, Drosophila utilizes auxiliary subunit(s) to modulate the formation and function of glutamatergic synapses. We have previously reported that Neto is an auxiliary protein essential for functional glutamate receptors and for organization of postsynaptic specializations. Here we report that synapse assembly and NMJ development are exquisitely sensitive to postsynaptic Neto levels. Furthermore, we show that Neto activity is controlled by Furin-type proteases, which regulate the processing and maturation of many developmentally important proteins, from growth factors and neuropeptides to extracellular matrix components. Such concerted control may serve to coordinate synapse assembly with synapse growth and developmental progression.

Morphogen transport: theoretical and experimental controversies

According to morphogen gradient theory, extracellular ligands produced from a localized source convey positional information to receiving cells by signaling in a concentration-dependent manner. How do morphogens create concentration gradients to establish positional information in developing tissues? Surprisingly, the answer to this central question remains largely unknown. During development, a relatively small number of morphogens are reiteratively deployed to ensure normal embryogenesis and organogenesis. Thus, the intracellular processing and extracellular transport of morphogens are tightly regulated in a tissue-specific manner. Over the past few decades, diverse experimental and theoretical approaches have led to numerous conflicting models for gradient formation. In this review, we summarize the experimental evidence for each model and discuss potential future directions for studies of morphogen gradients. For further resources related to this article, please visit the WIREs website.

CONFLICT OF INTEREST

The authors have declared no conflicts of interest for this article.

Crimpy enables discrimination of presynaptic and postsynaptic pools of a BMP at the Drosophila neuromuscular junction

Distinct pools of the bone morphogenetic protein (BMP) Glass bottom boat (Gbb) control structure and function of the Drosophila neuromuscular junction. Specifically, motoneuron-derived Gbb regulates baseline neurotransmitter release, whereas muscle-derived Gbb regulates neuromuscular junction growth. Yet how cells differentiate between these ligand pools is not known. Here we present evidence that the neuronal Gbb-binding protein Crimpy (Cmpy) permits discrimination of pre- and postsynaptic ligand by serving sequential functions in Gbb signaling. Cmpy first delivers Gbb to dense core vesicles (DCVs) for activity-dependent release from presynaptic terminals. In the absence of Cmpy, Gbb is no longer associated with DCVs and is not released by activity. Electrophysiological analyses demonstrate that Cmpy promotes Gbb's proneurotransmission function. Surprisingly, the Cmpy ectodomain is itself released upon DCV exocytosis, arguing that Cmpy serves a second function in BMP signaling. In addition to trafficking Gbb to DCVs, we propose that Gbb/Cmpy corelease from presynaptic terminals defines a neuronal protransmission signal.

Strategies for exploring TGF-β signaling in Drosophila

The TGF-β pathway is an evolutionarily conserved signal transduction module that mediates diverse biological processes in animals. In Drosophila, both the BMP and Activin branches are required for viability. Studies rooted in classical and molecular genetic approaches continue to uncover new developmental roles for TGF-β signaling. We present an overview of the secreted ligands, transmembrane receptors and cellular Smad transducer proteins that compose the core pathway in Drosophila. An assortment of tools have been developed to conduct tissue-specific loss- and gain-of-function experiments for these pathway components. We discuss the deployment of these reagents, with an emphasis on appropriate usage and limitations of the available tools. Throughout, we note reagents that are in need of further improvement or development, and signaling features requiring further study. A general theme is that comparison of phenotypes for ligands, receptors, and Smads can be used to map tissue interactions, and to separate canonical and non-canonical signaling activities. Core TGF-β signaling components are subject to multiple layers of regulation, and are coupled to context-specific inputs and outputs. In addition to fleshing out how TGF-β signaling serves the fruit fly, we anticipate that future studies will uncover new regulatory nodes and modes and will continue to advance paradigms for how TGF-β signaling regulates general developmental processes.

Nodal·Gdf1 heterodimers with bound prodomains enable serum-independent nodal signaling and endoderm differentiation

The TGFβ family member Nodal is central to control pluripotent stem cell fate, but its use as a stem cell differentiation factor is limited by low specific activity. During development, Nodal depends on growth and differentiation factor (Gdf)-1 and on the shared co-receptor Cryptic to specify visceral left-right axis asymmetry. We therefore asked whether the functionality of Nodal can be augmented by Gdf1. Because Nodal and Gdf1 coimmunoprecipitate each other, they were predicted to form heterodimers, possibly to facilitate diffusion or to increase the affinity for signaling receptors. Here, we report that Gdf1 suppresses an unexpected dependence of Nodal on serum proteins and that it is critically required for non-autonomous signaling in cells expressing Cryptic. Nodal, Gdf1, and their cleaved propeptides copurified as a heterodimeric low molecular weight complex that stimulated Activin receptor (Acvr) signaling far more potently than Nodal alone. Although heterodimerization with Gdf1 did not increase binding of Nodal to Fc fusions of co-receptors or Acvr extracellular domains, it was essential for soluble Acvr2 to inhibit Nodal signaling. This implies that Gdf1 potentiates Nodal activity by stabilizing a low molecular weight fraction that is susceptible to neutralization by soluble Acvr2. Finally, in differentiating human ES cells, endodermal markers were more efficiently induced by Nodal·Gdf1 than by Nodal, suggesting that Nodal·Gdf1 is an attractive new reagent to direct stem cell differentiation.

BMP growth factor signaling in a biomechanical context

Bone Morphogenetic Proteins (BMPs) are members of the transforming growth factor-β superfamily of secreted polypeptide growth factors and are important regulators in a multitude of cellular processes. To ensure the precise and balanced propagation of their pleiotropic signaling responses, BMPs and their corresponding signaling pathways are subject to tight control. A large variety of regulatory mechanisms throughout different biological levels combines into a complex network and provides the basis for physiological BMP function. This regulatory network not only includes biochemical factors but also mechanical cues. Both BMP signaling and mechanotransduction pathways are tightly interconnected and represent an elaborate signaling network active during development but also during organ homeostasis. Moreover, its dysregulation is associated with a number of human pathologies. A more detailed understanding of this crosstalk in respect to molecular interactions will be indispensable in the future, in particular to understand BMP-related diseases as well as with regard to an efficient clinical application of BMP ligands.

Cleavage of the Drosophila screw prodomain is critical for a dynamic BMP morphogen gradient in embryogenesis

Dorsoventral patterning of the Drosophila embryo is regulated by graded distribution of bone morphogenetic proteins (BMPs) composed of two ligands, decapentaplegic (Dpp) a BMP2/4 ortholog and screw (Scw) a BMP5/6/7/8 family member. scw(E1) encodes an unusual allele that was isolated as a dominant enhancer of partial loss-of-function mutations in dpp. However, the molecular mechanisms that underlie this genetic interaction remain to be addressed. Here we show that scw(E1) contains a mutation at the furin cleavage site within the prodomain that is crucial for ligand production. Furthermore, our data show that Scw(E1) preferentially forms heterodimers with Dpp rather than homotypic dimers, providing a possible explanation for the dominant negative phenotype of scw(E1) alleles. The unprocessed prodomain of Scw(E1) remains in a complex with the Dpp:Scw heterodimer, and thus could interfere with interaction of the ligand with the extracellular matrix, or the kinetics of processing/secretion of the ligand in vivo. These data reveal novel mechanisms by which post-translational regulation of Scw can modulate Dpp signaling activity.

Regulation of TGFβ and related signals by precursor processing

Secreted cytokines of the TGFβ family are found in all multicellular organisms and implicated in regulating fundamental cell behaviors such as proliferation, differentiation, migration and survival. Signal transduction involves complexes of specific type I and II receptor kinases that induce the nuclear translocation of Smad transcription factors to regulate target genes. Ligands of the BMP and Nodal subgroups act at a distance to specify distinct cell fates in a concentration-dependent manner. These signaling gradients are shaped by multiple factors, including proteases of the proprotein convertase (PC) family that hydrolyze one or several peptide bonds between an N-terminal prodomain and the C-terminal domain that forms the mature ligand. This review summarizes information on the proteolytic processing of TGFβ and related precursors, and its spatiotemporal regulation by PCs during development and various diseases, including cancer. Available evidence suggests that the unmasking of receptor binding epitopes of TGFβ is only one (and in some cases a non-essential) function of precursor processing. Future studies should consider the impact of proteolytic maturation on protein localization, trafficking and turnover in cells and in the extracellular space.

Postsynaptic glutamate receptors regulate local BMP signaling at the Drosophila neuromuscular junction

Effective communication between pre- and postsynaptic compartments is required for proper synapse development and function. At the Drosophila neuromuscular junction (NMJ), a retrograde BMP signal functions to promote synapse growth, stability and homeostasis and coordinates the growth of synaptic structures. Retrograde BMP signaling triggers accumulation of the pathway effector pMad in motoneuron nuclei and at synaptic termini. Nuclear pMad, in conjunction with transcription factors, modulates the expression of target genes and instructs synaptic growth; a role for synaptic pMad remains to be determined. Here, we report that pMad signals are selectively lost at NMJ synapses with reduced postsynaptic sensitivities. Despite this loss of synaptic pMad, nuclear pMad persisted in motoneuron nuclei, and expression of BMP target genes was unaffected, indicating a specific impairment in pMad production/maintenance at synaptic termini. During development, synaptic pMad accumulation followed the arrival and clustering of ionotropic glutamate receptors (iGluRs) at NMJ synapses. Synaptic pMad was lost at NMJ synapses developing at suboptimal levels of iGluRs and Neto, an auxiliary subunit required for functional iGluRs. Genetic manipulations of non-essential iGluR subunits revealed that synaptic pMad signals specifically correlated with the postsynaptic type-A glutamate receptors. Altering type-A receptor activities via protein kinase A (PKA) revealed that synaptic pMad depends on the activity and not the net levels of postsynaptic type-A receptors. Thus, synaptic pMad functions as a local sensor for NMJ synapse activity and has the potential to coordinate synaptic activity with a BMP retrograde signal required for synapse growth and homeostasis.

Human blood contains both the uncleaved precursor of anti-Mullerian hormone and a complex of the NH2- and COOH-terminal peptides

Anti-Müllerian hormone (AMH) in blood is a marker of ovarian status in women and the presence of cryptic testes in babies. Despite this, the molecular form of AMH in blood has not been verified. AMH is synthesized as an inert proprotein precursor (proAMH), which can be cleaved to yield NH2-terminal (AMHN) and COOH-terminal (AMHC) fragments, that can complex noncovalently (AMHN,C). Developing males have 10-fold more AMH than young adults. We report here that human blood is a mixture of inactive proAMH and receptor-binding AMHN,C. The AMH in the blood of boys, men, and premenopausal women was immunoprecipitated using antibodies to the NH2- and COOH-terminal peptides. The precipitated proteins were then analyzed by Western blots, using recombinant proteins as markers. The glycosylation status of AMH was verified using deglycosylating enzymes. The NH2-terminal antibody precipitated a major protein that migrated alongside rhproAMH and was detected by anti-AMHN and anti-AMHC. This antibody also precipitated significant levels of AMHN and AMHC from all participants. Antibodies specific to AMHC precipitated rhAMHC but did not precipitate AMHC from human blood. Hence, all the AMHC in human blood appears to be bound to AMHN. Both AMHN and proAMH were glycosylated, independent of age and sex. In conclusion, boys and young adults have the same form of AMH, with a significant proportion being the inactive precursor. This raises the possibility that the endocrine functions of AMH are partly controlled by its cleavage in the target organ. The presence of proAMH in blood may confound the use of AMH for diagnosis.

Fine-tuned shuttles for bone morphogenetic proteins

Bone morphogenetic proteins (BMPs) are potent secreted signaling factors that trigger phosphorylation of Smad transcriptional regulators through receptor complex binding at the cell-surface. Resulting changes in target gene expression impact critical cellular responses during development and tissue homeostasis. BMP activity is tightly regulated in time and space by secreted modulators that control BMP extracellular distribution and availability for receptor binding. Such extracellular regulation is key for BMPs to function as morphogens and/or in the formation of morphogen activity gradients. Here, we review shuttling systems utilized to control the distribution of BMP ligands in tissue of various geometries, developing under different temporal constraints. We discuss the biological advantages for employing specific strategies for BMP shuttling and roles of varied ligand forms.

Species differences in the expression and activity of bone morphogenetic protein 15

Oocyte-derived bone morphogenetic protein 15 (BMP15) regulates ovulation rate and female fertility in a species-specific manner, being important in humans and sheep and largely superfluous in mice. To understand these species differences, we have compared the expression and activity of human, murine, and ovine BMP15. In HEK293F cells, human BMP15 is highly expressed (120 ng/ml), ovine BMP15 is poorly expressed (15 ng/ml), and murine BMP15 is undetectable. Because BMP15 synthesis is dependent upon interactions between the N-terminal prodomain and the C-terminal mature domain, we used site-directed mutagenesis to identify four prodomain residues (Glu(46), Glu(47), Leu(49), and Glu(50)) that mediate the high expression of human BMP15. Substituting these residues into the prodomains of murine and ovine BMP15 led to significant increases in growth factor expression; however, maximal expression was achieved only when the entire human prodomain was linked to the mature domains of the other species. Using these chimeric constructs, we produced and purified murine and ovine BMP15 and showed that in a COV434 granulosa cell bioassay, these molecules displayed little activity relative to human BMP15 (EC(50) 0.2nM). Sequence analysis suggested that the disparity in activity could be due to species differences at the type I receptor binding interface. Indeed, murine BMP15 activity was restored when specific residues through this region (Pro(329)/Tyr(330)) were replaced with the corresponding residues (Arg(329)/Asp(330)) from human BMP15. The identified differences in the expression and activity of BMP15 likely underlie the relative importance of this growth factor between species.

The factor VII-activating protease (FSAP) enhances the activity of bone morphogenetic protein-2 (BMP-2)

Factor VII-activating protease (FSAP) is a circulating protease involved in the pathogenesis of atherosclerosis, calcification, and fibrotic processes. To understand how FSAP controls the balance of local growth factors, we have investigated its effect on the regulation of bone morphogenetic proteins (BMPs). BMP-2 is produced as a large pro-form and secreted as a mature heparin-binding growth factor after intracellular processing by pro-protein convertases (PCs). In this study, we discovered that FSAP enhances the biological activity of mature BMP-2 as well as its pro-form, as shown by osteogenic differentiation of C2C12 myoblasts. These findings were complemented by knockdown of FSAP in hepatocytes, which revealed BMP-2 processing by endogenous FSAP. N-terminal sequencing indicated that pro-BMP-2 was cleaved by FSAP at the canonical PC cleavage site, giving rise to mature BMP-2 (Arg(282)↓Gln(283)), as well as in the N-terminal heparin binding region of mature BMP-2, generating a truncated mature BMP-2 peptide (Arg(289)↓Lys(290)). Similarly, mature BMP-2 was also cleaved to a truncated peptide within its N-terminal region (Arg(289)↓Lys(290)). Plasmin exhibited a similar activity, but it was weaker compared with FSAP. Thrombin, Factor VIIa, Factor Xa, and activated protein C were not effective. These results were further supported by the observation that the mutation of the heparin binding region of BMP-2 inhibited the processing by FSAP but not by PC. Thus, the proteolysis and activation of pro-BMP-2 and mature BMP-2 by FSAP can regulate cell differentiation and calcification in vasculature and may explain why polymorphisms in the gene encoding for FSAP are related to vascular diseases.

You're going to need a bigger (glass bottom) boat

Signaling molecules of the transforming growth factor (TGF)-β family are generated from proprotein precursors containing prodomain sequences that are typically removed to allow signaling by the mature ligands. A form of a TGF-β family ligand that remains covalently attached to its prodomain but retains signaling activity has been identified. Glass bottom boat (Gbb), a Drosophila homolog of the bone morphogenetic protein 5/6/7/8 subfamily, is active as a carboxyl-terminal fragment of the proprotein (Gbb15) that is generated by a conventional processing event common to TGF-β ligands. Unexpectedly, a larger form (Gbb38) produced by processing at a newly identified furin site in the prodomain is also secreted and active. Contrary to the present paradigm in which TGF-β ligands require dissociation of the entire prodomain for activity, Gbb38 is active in cell culture and in vivo without additional processing at conventional sites. The large form can restore the viability of gbb mutant animals but has distinct signaling properties compared with the conventional form. Production of multiple functional ligands from one proprotein is a potential mechanism to fine-tune TGF-β signaling outputs. Mutations in TGF-β family members have been linked to human diseases, several of which affect potential furin cleavage sites in prodomains. However, given the diversity of potential furin processing sites and prodomain functions, direct experimentation will be required to determine whether production of active jumbo ligands is a general feature of TGF-β superfamily members.