Heterodimerization-dependent secretion of bone morphogenetic proteins in Drosophila

Mutations that prevent phosphorylation of the BMP4 prodomain impair proteolytic maturation of homodimers leading to lethality in mice

Bone morphogenetic protein4 (BMP4) plays numerous roles during embryogenesis and can signal either alone as a homodimer, or together with BMP7 as a more active heterodimer. BMPs are generated as inactive precursor proteins that dimerize and are cleaved to generate the bioactive ligand and inactive prodomain fragments. In humans, heterozygous mutations within the prodomain of BMP4 are associated with birth defects. We studied the effect of two of these mutations (p.S91C and p.E93G), which disrupt a conserved FAM20C phosphorylation motif, on ligand activity. We compared the activity of ligands generated from BMP4, BMP4S91C, or BMP4E93G in Xenopus embryos and found that these mutations reduce the activity of BMP4 homodimers but not BMP4/7 heterodimers. We generated Bmp4S91C and Bmp4E93G knock-in mice and found that Bmp4S91C/S91C mice die by E11.5 and display reduced BMP activity in multiple tissues including the heart. Most Bmp4E93G/E93G mice die before weaning and Bmp4-/E93G mutants die prenatally with reduced or absent eyes, heart, and ventral body wall closure defects. Mouse embryonic fibroblasts (MEFs) isolated from Bmp4S91C and Bmp4E93G embryos show accumulation of BMP4 precursor protein, reduced levels of cleaved BMP ligand and reduced BMP activity relative to MEFs from wild type littermates. Because Bmp7 is not expressed in MEFs, the accumulation of unprocessed BMP4 precursor protein in mice carrying these mutations most likely reflects an inability to cleave BMP4 homodimers, leading to reduced levels of ligand and BMP activity in vivo. Our results suggest that phosphorylation of the BMP4 prodomain is required for proteolytic activation of BMP4 homodimers, but not heterodimers.

MuSK Regulates Neuromuscular Junction Nav1.4 Localization and Excitability

The neuromuscular junction (NMJ) is the linchpin of nerve-evoked muscle contraction. Broadly, the NMJ transduces nerve action potentials into muscle fiber action potentials (MFAPs). Efficient neuromuscular transmission requires cholinergic signaling, which generates endplate potentials (EPPs), and excitation, the amplification of an EPP by postsynaptic voltage-gated sodium channels (Nav1.4) to generate the MFAP. Compared to the cholinergic component, the signaling pathways that organize Nav1.4 are poorly characterized. Muscle-specific kinase (MuSK), in addition to its Ig1 domain-dependent role as the main organizer of acetylcholine receptors (AChRs), also binds BMPs via its Ig3 domain and shapes BMP-induced signaling. Using mice lacking the MuSK Ig3 domain ("ΔIg3-MuSK"), we probed the role of this domain at the NMJ. NMJs formed in ΔIg3-MuSK animals with pre- and postsynaptic specializations aligned at all ages examined. However, the ΔIg3-MuSK postsynaptic apparatus was fragmented from an early age. Synaptic electrophysiology showed that spontaneous and nerve-evoked acetylcholine release, AChR density, and endplate currents were comparable at WT and ΔIg3-MuSK NMJs. However, single fiber electromyography revealed that nerve-evoked MFAPs in ΔIg3-MuSK muscle were abnormal, exhibiting jitter and blocking. Nerve-evoked compound muscle action potentials and muscle force were also diminished. Finally, Nav1.4 levels were reduced at ΔIg3-MuSK NMJs, but not extrasynaptically, indicating that the observed excitability defects result from impaired synaptic localization of this ion channel. We propose distinct, domain-specific roles for MuSK at the NMJ: the Ig1 domain mediates agrin-LRP4-mediated AChR localization, while the Ig3 domain maintains postsynaptic Nav1.4 density, conferring the muscle excitability required to amplify cholinergic signals and trigger action potentials.

Mutations that prevent phosphorylation of the BMP4 prodomain impair proteolytic maturation of homodimers leading to lethality in mice

Bone morphogenetic protein4 (BMP4) plays numerous roles during embryogenesis and can signal either alone as a homodimer, or together with BMP7 as a more active heterodimer. BMPs are generated as inactive precursor proteins that dimerize and are cleaved to generate the bioactive ligand and inactive prodomain fragments. In humans, heterozygous mutations within the prodomain of BMP4 are associated with birth defects. We studied the effect of two of these mutations (p.S91C and p.E93G), which disrupt a conserved FAM20C phosphorylation motif, on ligand activity. We compared the activity of ligands generated from BMP4, BMP4S91C or BMP4E93G in Xenopus embryos and found that these mutations reduce the activity of BMP4 homodimers but not BMP4/7 heterodimers. We generated Bmp4 S91C and Bmp4 E93G knock-in mice and found that Bmp4 S91C/S91C mice die by E11.5 and display reduced BMP activity in multiple tissues including the heart. Most Bmp4 E93G/E93G mice die before weaning and Bmp4 -/E93G mutants die prenatally with reduced or absent eyes, heart and ventral body wall closure defects. Mouse embryonic fibroblasts (MEFs) isolated from Bmp4 S91C and Bmp4 E93G embryos show accumulation of BMP4 precursor protein, reduced levels of cleaved BMP ligand and reduced BMP activity relative to MEFs from wild type littermates. Because Bmp7 is not expressed in MEFs, the accumulation of unprocessed BMP4 precursor protein in mice carrying these mutations most likely reflects an inability to cleave BMP4 homodimers, leading to reduced levels of ligand and BMP activity in vivo. Our results suggest that phosphorylation of the BMP4 prodomain is required for proteolytic activation of BMP4 homodimers, but not heterodimers.

Postsynaptic BMP signaling regulates myonuclear properties in Drosophila larval muscles

The syncytial mammalian muscle fiber contains a heterogeneous population of (myo)nuclei. At the neuromuscular junction (NMJ), myonuclei have specialized positioning and gene expression. However, it remains unclear how myonuclei are recruited and what regulates myonuclear output at the NMJ. Here, we identify specific properties of myonuclei located near the Drosophila larval NMJ. These synaptic myonuclei have increased size in relation to their surrounding cytoplasmic domain (size scaling), increased DNA content (ploidy), and increased levels of transcription factor pMad, a readout for BMP signaling activity. Our genetic manipulations show that local BMP signaling affects muscle size, nuclear size, ploidy, and NMJ size and function. In support, RNA sequencing analysis reveals that pMad regulates genes involved in muscle growth, ploidy (i.e., E2f1), and neurotransmission. Our data suggest that muscle BMP signaling instructs synaptic myonuclear output that positively shapes the NMJ synapse. This study deepens our understanding of how myonuclear heterogeneity supports local signaling demands to fine tune cellular function and NMJ activity.

Soma-to-germline BMP signal is essential for Drosophila spermiogenesis

In the Drosophila testis, developing germ cells are encapsulated by somatic support cells throughout development. Soma-germline interactions are essential for successful spermiogenesis. However, it is still not fully understood what signaling events take place between the soma and the germline. In this study, we found that a Bone Morphogenetic Protein (BMP) ligand, Glass bottom boat (Gbb), secreted from somatic cyst cells (CCs), signals to differentiating germ cells to maintain proper spermiogenesis. Knockdown of Gbb in CCs or the type I BMP receptor Saxophone (Sax) in germ cells leads to a defect in sperm head bundling and decreased fertility. Our Transmission Electron Microscopy (TEM) analyses revealed that the mutant germ cells have aberrant morphology of mitochondria throughout the stages of spermiogenesis and exhibit a defect in nebenkern formation. Elongating spermatids show uncoupled nuclei and elongating mitochondrial derivatives, suggesting that improper mitochondrial development may cause sperm bundling defects. Taken together, we propose a new role of soma-derived BMP signaling, which is essential for spermiogenesis.

Functionalized Protein Binders in Developmental Biology

Developmental biology has greatly profited from genetic and reverse genetic approaches to indirectly studying protein function. More recently, nanobodies and other protein binders derived from different synthetic scaffolds have been used to directly dissect protein function. Protein binders have been fused to functional domains, such as to lead to protein degradation, relocalization, visualization, or posttranslational modification of the target protein upon binding. The use of such functionalized protein binders has allowed the study of the proteome during development in an unprecedented manner. In the coming years, the advent of the computational design of protein binders, together with further advances in scaffold engineering and synthetic biology, will fuel the development of novel protein binder-based technologies. Studying the proteome with increased precision will contribute to a better understanding of the immense molecular complexities hidden in each step along the way to generate form and function during development.

The prodomain of bone morphogenetic protein 2 promotes dimerization and cleavage of BMP6 homodimers and BMP2/6 heterodimers

Bone morphogenetic protein 2 (BMP2) and BMP6 are key regulators of systemic iron homeostasis. All BMPs are generated as inactive precursor proteins that dimerize and are cleaved to generate the bioactive ligand and inactive prodomain fragments, but nothing is known about how BMP2 or BMP6 homodimeric or heterodimeric precursor proteins are proteolytically activated. Here, we conducted in vitro cleavage assays, which revealed that BMP2 is sequentially cleaved by furin at two sites, initially at a site upstream of the mature ligand, and then at a site adjacent to the ligand domain, while BMP6 is cleaved at a single furin motif. Cleavage of both sites of BMP2 is required to generate fully active BMP2 homodimers when expressed in Xenopus embryos or liver endothelial cells, and fully active BMP2/6 heterodimers in Xenopus. We analyzed BMP activity in Xenopus embryos expressing chimeric proteins consisting of the BMP2 prodomain and BMP6 ligand domain, or vice versa. We show that the prodomain of BMP2 is necessary and sufficient to generate active BMP6 homodimers and BMP2/6 heterodimers, whereas the BMP6 prodomain cannot generate active BMP2 homodimers or BMP2/6 heterodimers. We examined BMP2 and BMP6 homodimeric and heterodimeric ligands generated from native and chimeric precursor proteins expressed in Xenopus embryos. Whereas native BMP6 is not cleaved when expressed alone, it is cleaved to generate BMP2/6 heterodimers when co-expressed with BMP2. Furthermore, BMP2-6 chimeras are cleaved to generate BMP6 homodimers. Our findings reveal an important role for the BMP2 prodomain in dimerization and proteolytic activation of BMP6.

Protocol for generating in-frame seamless knockins in Drosophila using the SEED/Harvest technology

The generation of knockins is fundamental to dissect biological systems. SEED/Harvest, a technology based on CRISPR-Cas9, offers a powerful approach for seamless genome editing in Drosophila. Here, we present a protocol to tag any gene in the Drosophila genome using SEED/Harvest technology. We describe knockin design, plasmid preparation, injection, and insertion screening. We then detail procedures for germline harvesting. The technique combines straightforward cloning and robust screening of insertions, while still resulting in scarless gene editing. For complete details on the use and execution of this protocol, please refer to Aguilar et al.1.

Transsynaptic BMP Signaling Regulates Fine-Scale Topography between Adjacent Sensory Neurons

Sensory axons projecting to the central nervous system are organized into topographic maps that represent the locations of sensory stimuli. In some sensory systems, even adjacent sensory axons are arranged topographically, forming "fine-scale" topographic maps. Although several broad molecular gradients are known to instruct coarse topography, we know little about the molecular signaling that regulates fine-scale topography at the level of two adjacent axons. Here, we provide evidence that transsynaptic bone morphogenetic protein (BMP) signaling mediates local interneuronal communication to regulate fine-scale topography in the nociceptive system of Drosophila larvae. We first show that the topographic separation of the axon terminals of adjacent nociceptors requires their common postsynaptic target, the A08n neurons. This phenotype is recapitulated by knockdown of the BMP ligand, Decapentaplegic (Dpp), in these neurons. In addition, removing the Type 2 BMP receptors or their effector (Mad transcription factor) in single nociceptors impairs the fine-scale topography, suggesting the contribution of BMP signaling originated from A08n. This signaling is likely mediated by phospho-Mad in the presynaptic terminals of nociceptors to ensure local interneuronal communication. Finally, reducing Dpp levels in A08n reduces the nociceptor-A08n synaptic contacts. Our data support that transsynaptic BMP signaling establishes the fine-scale topography by facilitating the formation of topographically correct synapses. Local BMP signaling for synapse formation may be a developmental strategy that independently regulates neighboring axon terminals for fine-scale topography.

The BMP2 prodomain promotes dimerization and cleavage of BMP6 homodimers and BMP2/6 heterodimers in vivo

Bone morphogenetic protein 2 (BMP2) and BMP6 are key regulators of systemic iron homeostasis. All BMPs are generated as inactive precursor proteins that dimerize and are cleaved to generate the bioactive ligand and inactive prodomain fragments, but nothing is known about how BMP2 or BMP6 homodimeric or heterodimeric precursor proteins are proteolytically activated. Here, we conducted in vitro cleavage assays, which revealed that BMP2 is sequentially cleaved by furin at two sites, initially at a site upstream of the mature ligand, and then at a site adjacent to the ligand domain, while BMP6 is cleaved at a single furin motif. Cleavage of both sites of BMP2 is required to generate fully active BMP2 homodimers when expressed in Xenopus embryos or liver endothelial cells, and fully active BMP2/6 heterodimers in Xenopus . We analyzed BMP activity in Xenopus embryos expressing chimeric proteins consisting of the BMP2 prodomain and BMP6 ligand domain, or vice versa. We show that the prodomain of BMP2 is necessary and sufficient to generate active BMP6 homodimers and BMP2/6 heterodimers, whereas the BMP6 prodomain cannot generate active BMP2 homodimers or BMP2/6 heterodimers. We examined BMP2 and BMP6 homodimeric and heterodimeric ligands generated from native and chimeric precursor proteins expressed in Xenopus embryos. Whereas native BMP6 is not cleaved when expressed alone, it is cleaved to generate BMP2/6 heterodimers when co-expressed with BMP2. Furthermore, BMP2-6 chimeras are cleaved to generate BMP6 homodimers. Our findings reveal an important role for the BMP2 prodomain in dimerization and proteolytic activation of BMP6.

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.

Diffusible fraction of niche BMP ligand safeguards stem-cell differentiation

Drosophila male germline stem cells (GSCs) reside at the tip of the testis and surround a cluster of niche cells. Decapentaplegic (Dpp) is one of the well-established ligands and has a major role in maintaining stem cells located in close proximity. However, the existence and the role of the diffusible fraction of Dpp outside of the niche have been unclear. Here, using genetically-encoded nanobodies called Morphotraps, we physically block Dpp diffusion without interfering with niche-stem cell signaling and find that a diffusible fraction of Dpp is required to ensure differentiation of GSC daughter cells, opposite of its role in maintenance of GSC in the niche. Our work provides an example in which a soluble niche ligand induces opposed cellular responses in stem cells versus in differentiating descendants to ensure spatial control of the niche. This may be a common mechanism to regulate tissue homeostasis.

Dally is not essential for Dpp spreading or internalization but for Dpp stability by antagonizing Tkv-mediated Dpp internalization

Dpp/BMP acts as a morphogen to provide positional information in the Drosophila wing disc. Key cell-surface molecules to control Dpp morphogen gradient formation and signaling are heparan sulfate proteoglycans (HSPGs). In the wing disc, two HSPGs, the glypicans Division abnormally delayed (Dally) and Dally-like (Dlp) have been suggested to act redundantly to control these processes through direct interaction of their heparan sulfate (HS) chains with Dpp. Based on this assumption, a number of models on how glypicans control Dpp gradient formation and signaling have been proposed, including facilitating or hindering Dpp spreading, stabilizing Dpp on the cell surface, or recycling Dpp. However, how distinct HSPGs act remains largely unknown. Here, we generate genome-engineering platforms for the two glypicans and find that only Dally is critical for Dpp gradient formation and signaling through interaction of its core protein with Dpp. We also find that this interaction is not sufficient and that the HS chains of Dally are essential for these functions largely without interacting with Dpp. We provide evidence that the HS chains of Dally are not essential for spreading or recycling of Dpp but for stabilizing Dpp on the cell surface by antagonizing receptor-mediated Dpp internalization. These results provide new insights into how distinct HSPGs control morphogen gradient formation and signaling during development.

Is Drosophila Dpp/BMP morphogen spreading required for wing patterning and growth?

Secreted signaling molecules act as morphogens to control patterning and growth in many developing tissues. Since locally produced morphogens spread to form a concentration gradient in the surrounding tissue, spreading is generally thought to be the key step in the non-autonomous actions. Here, we review recent advances in tool development to investigate morphogen function using the role of decapentaplegic (Dpp)/bone morphogenetic protein (BMP)-type ligand in the Drosophila wing disc as an example. By applying protein binder tools to distinguish between the roles of Dpp spreading and local Dpp signaling, we found that Dpp signaling in the source cells is important for wing patterning and growth but Dpp spreading from this source cells is not as strictly required as previously thought. Given recent studies showing unexpected requirements of long-range action of different morphogens, manipulating endogenous morphogen gradients by synthetic protein binder tools could shed more light on how morphogens act in developing tissues.

The beauty of seeing the real thing: BMP heterodimer detection in vivo reveals dimer composition and activity

BMP family ligands can direct cells to divide, differentiate, or die, depending on cell context and specific hetero- or homodimer combinations. In this issue of Developmental Cell, Bauer et al. detect endogenous Drosophila ligand dimers in situ and show that BMP dimer composition affects both signaling range and activity.