The metalloprotease Tolloid-related and its TGF-β-like substrate Dawdle regulateDrosophilamotoneuron axon guidance

Neto proteins differentially modulate the gating properties of Drosophila NMJ glutamate receptors

The formation of functional synapses requires co-assembly of ion channels with their accessory proteins which controls where, when, and how neurotransmitter receptors function. The auxiliary protein Neto modulates the function of kainate-type glutamate receptors in vertebrates as well as at the Drosophila neuromuscular junction (NMJ), a glutamatergic synapse widely used for genetic studies on synapse development. We previously reported that Neto is essential for the synaptic recruitment and function of glutamate receptors. Here, using outside-out patch-clamp recordings and fast ligand application, we examine for the first time the biophysical properties of recombinant Drosophila NMJ receptors expressed in HEK293T cells and compare them with native receptor complexes of genetically controlled composition. The two Neto isoforms, Neto-α and Neto-β, differentially modulate the gating properties of NMJ receptors. Surprisingly, we found that deactivation is extremely fast and that the decay of synaptic currents resembles the rate of iGluR desensitization. The functional analyses of recombinant iGluRs that we report here should greatly facilitate the interpretation of compound in vivo phenotypes of mutant animals.

Drosophila Smad2 degradation occurs independently of linker phosphorylations

TGF-β signals are important for proliferation, differentiation, and cell fate determination during embryonic development and tissue homeostasis in adults. Drosophila Activin/TGF-β signals are transduced intracellularly when its transcription factor dSmad2 (also called Smad on X or Smox) is C-terminally phosphorylated by pathway receptors. Recently, it has been shown that receptor-activated dSmad2 undergoes bulk degradation, however, the mechanism of how this occurs is unknown. Here we investigated if two putative linker phosphorylation sites are involved in dSmad2 degradation. We demonstrate that degradation of activated-dSmad2 occurs independently of threonine phosphorylation at linker sites 252 and 277. We also show that dSmad2 degradation is not carried out by cellular proteasomes.

Psi promotes Drosophila wing growth via direct transcriptional activation of cell cycle targets and repression of growth inhibitors

The first characterised FUSE Binding Protein family member, FUBP1, binds single-stranded DNA to activate MYC transcription. Psi, the sole FUBP protein in Drosophila, binds RNA to regulate P-element and mRNA splicing. Our previous work revealed pro-growth functions for Psi, which depend, in part, on transcriptional activation of Myc. Genome-wide functions for FUBP family proteins in transcriptional control remain obscure. Here, through the first genome-wide binding and expression profiles obtained for a FUBP family protein, we demonstrate that, in addition to being required to activate Myc to promote cell growth, Psi also directly binds and activates stg to couple growth and cell division. Thus, Psi knockdown results in reduced cell division in the wing imaginal disc. In addition to activating these pro-proliferative targets, Psi directly represses transcription of the growth inhibitor tolkin (tok, a metallopeptidase implicated in TGFβ signalling). We further demonstrate tok overexpression inhibits proliferation, while tok loss of function increases mitosis alone and suppresses impaired cell division caused by Psi knockdown. Thus, Psi orchestrates growth through concurrent transcriptional activation of the pro-proliferative genes Myc and stg, in combination with repression of the growth inhibitor tok.

An exon junction complex-independent function of Barentsz in neuromuscular synapse growth

The exon junction complex controls the translation, degradation, and localization of spliced mRNAs, and three of its core subunits also play a role in splicing. Here, we show that a fourth subunit, Barentsz, has distinct functions within and separate from the exon junction complex in Drosophila neuromuscular development. The distribution of mitochondria in larval muscles requires Barentsz as well as other exon junction complex subunits and is not rescued by a Barentsz transgene in which residues required for binding to the core subunit eIF4AIII are mutated. In contrast, interactions with the exon junction complex are not required for Barentsz to promote the growth of neuromuscular synapses. We find that the Activin ligand Dawdle shows reduced expression in barentsz mutants and acts downstream of Barentsz to control synapse growth. Both barentsz and dawdle are required in motor neurons, muscles, and glia for normal synapse growth, and exogenous Dawdle can rescue synapse growth in the absence of barentsz. These results identify a biological function for Barentsz that is independent of the exon junction complex.

Proteolytic cleavage of Slit by the Tolkin protease converts an axon repulsion cue to an axon growth cue in vivo

Slit is a secreted protein that has a canonical function of repelling growing axons from the CNS midline. The full-length Slit (Slit-FL) is cleaved into Slit-N and Slit-C fragments, which have potentially distinct functions via different receptors. Here, we report that the BMP-1/Tolloid family metalloprotease Tolkin (Tok) is responsible for Slit proteolysis in vivo and in vitro. In Drosophilatok mutants lacking Slit cleavage, midline repulsion of axons occurs normally, confirming that Slit-FL is sufficient to repel axons. However, longitudinal axon guidance is highly disrupted in tok mutants and can be rescued by midline expression of Slit-N, suggesting that Slit is the primary substrate for Tok in the embryonic CNS. Transgenic restoration of Slit-N or Slit-C does not repel axons in Slit-null flies. Slit-FL and Slit-N are both biologically active cues with distinct axon guidance functions in vivo Slit signaling is used in diverse biological processes; therefore, differentiating between Slit-FL and Slit fragments will be essential for evaluating Slit function in broader contexts.

Interspecies Comparative Analyses Reveal Distinct Carbohydrate-Responsive Systems among Drosophila Species

During evolution, organisms have acquired variable feeding habits. Some species are nutritional generalists that adapt to various food resources, while others are specialists, feeding on specific resources. However, much remains to be discovered about how generalists adapt to diversified diets. We find that larvae of the generalists Drosophila melanogaster and D. simulans develop on three diets with different nutrient balances, whereas specialists D. sechellia and D. elegans cannot develop on carbohydrate-rich diets. The generalist D. melanogaster downregulates the expression of diverse metabolic genes systemically by transforming growth factor β (TGF-β)/Activin signaling, maintains metabolic homeostasis, and successfully adapts to the diets. In contrast, the specialist D. sechellia expresses those metabolic genes at higher levels and accumulates various metabolites on the carbohydrate-rich diet, culminating in reduced adaptation. Phenotypic similarities and differences strongly suggest that the robust carbohydrate-responsive regulatory systems are evolutionarily retained through genome-environment interactions in the generalists and contribute to their nutritional adaptabilities.

Selective Disruption of Synaptic BMP Signaling by a Smad Mutation Adjacent to the Highly Conserved H2 Helix

Bone morphogenetic proteins (BMPs) shape normal development and function via canonical and noncanonical signaling pathways. BMPs initiate canonical signaling by binding to transmembrane receptors that phosphorylate Smad proteins and induce their translocation into the nucleus and regulation of target genes. Phosphorylated Smads also accumulate at cellular junctions, but this noncanonical, local BMP signaling modality remains less defined. We have recently reported that phosphorylated Smad (pMad in Drosophila) accumulates at synaptic junctions in protein complexes with genetically distinct composition and regulation. Here, we examined a wide collection of DrosophilaMad alleles and searched for molecular features relevant to pMad accumulation at synaptic junctions. We found that strong Mad alleles generally disrupt both synaptic and nuclear pMad, whereas moderate Mad alleles have a wider range of phenotypes and can selectively impact different BMP signaling pathways. Interestingly, regulatory Mad mutations reveal that synaptic pMad appears to be more sensitive to a net reduction in Mad levels than nuclear pMad. Importantly, a previously uncharacterized allele, Mad8 , showed markedly reduced synaptic pMad but only moderately diminished nuclear pMad. The postsynaptic composition and electrophysiological properties of Mad8 neuromuscular junctions (NMJs) were also altered. Using biochemical approaches, we examined how a single point mutation in Mad8 could influence the Mad-receptor interface and identified a key motif, the H2 helix. Our study highlights the biological relevance of Smad-dependent, synaptic BMP signaling and uncovers a highly conserved structural feature of Smads, critical for normal development and function.

Two-factor specification of apoptosis: TGF-β signaling acts cooperatively with ecdysone signaling to induce cell- and stage-specific apoptosis of larval neurons during metamorphosis in Drosophila melanogaster

Developmentally regulated programmed cell death (PCD) is one of the key cellular events for precise controlling of neuronal population during postembryonic development of the central nervous system. Previously we have shown that a group of corazonin-producing peptidergic neurons (vCrz) undergo apoptosis in response to ecdysone signaling via ecdysone receptor (EcR)-B isoforms and Ultraspiracle during early phase of metamorphosis. Further utilizing genetic, transgenic, and mosaic analyses, we have found that TGF-β signaling mediated by a glia-produced ligand, Myoglianin, type-I receptor Baboon (particularly Babo-A isoform) and dSmad2, is also required autonomously for PCD of the vCrz neurons. Our studies show that TGF-β signaling is not acting epistatically to EcR or vice versa. We also show that ectopic expression of a constitutively active phosphomimetic form of dSmad2 (dSmad2^PM) is capable of inducing premature death of vCrz neurons in larva but not other larval neurons. Intriguingly, the dSmad2^PM-mediated killing is completely suppressed by coexpression of a dominant-negative form of EcR (EcR^DN), suggesting that EcR function is required for the proapoptotic dSmad2^PM function. Based on these data, we suggest that TGF-β and ecdysone signaling pathways act cooperatively to induce vCrz neuronal PCD. We propose that this type of two-factor authentication is a key developmental strategy to ensure the timely PCD of specific larval neurons during metamorphosis.

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.

TGFB-INHB/activin signaling regulates age-dependent autophagy and cardiac health through inhibition of MTORC2

Age-related impairment of macroautophagy/autophagy and loss of cardiac tissue homeostasis contribute significantly to cardiovascular diseases later in life. MTOR (mechanistic target of rapamycin kinase) signaling is the most well-known regulator of autophagy, cellular homeostasis, and longevity. The MTOR signaling consists of two structurally and functionally distinct multiprotein complexes, MTORC1 and MTORC2. While MTORC1 is well characterized but the role of MTORC2 in aging and autophagy remains poorly understood. Here we identified TGFB-INHB/activin signaling as a novel upstream regulator of MTORC2 to control autophagy and cardiac health during aging. Using Drosophila heart as a model system, we show that cardiac-specific knockdown of TGFB-INHB/activin-like protein daw induces autophagy and alleviates age-related heart dysfunction, including cardiac arrhythmias and bradycardia. Interestingly, the downregulation of daw activates TORC2 signaling to regulate cardiac autophagy. Activation of TORC2 alone through overexpressing its subunit protein rictor promotes autophagic flux and preserves cardiac function with aging. In contrast, activation of TORC1 does not block autophagy induction in daw knockdown flies. Lastly, either daw knockdown or rictor overexpression in fly hearts prolongs lifespan, suggesting that manipulation of these pathways in the heart has systemic effects on longevity control. Thus, our studies discover the TGFB-INHB/activin-mediated inhibition of TORC2 as a novel mechanism for age-dependent decreases in autophagic activity and cardiac health. Abbreviations: AI: arrhythmia index; BafA1: bafilomycin A₁; BMP: bone morphogenetic protein; CQ: chloroquine; CVD: cardiovascular diseases; DI: diastolic interval; ER: endoplasmic reticulum; HP: heart period; HR: heart rate; MTOR: mechanistic target of rapamycin kinase; NGS: normal goat serum; PBST: PBS with 0.1% Triton X-100; PDPK1: 3-phosphoinositide dependent protein kinase 1; RICTOR: RPTOR independent companion of MTOR complex 2; ROI: region of interest; ROUT: robust regression and outlier removal; ROS: reactive oxygen species; R-SMAD: receptor-activated SMAD; SI: systolic interval; SOHA: semi-automatic optical heartbeat analysis; TGFB: transformation growth factor beta; TSC1: TSC complex subunit 1.

Regulation of insulin and adipokinetic hormone/glucagon production in flies

Metabolic homeostasis is under strict regulation of humoral factors across various taxa. In particular, insulin and glucagon, referred to in Drosophila as Drosophila insulin-like peptides (DILPs) and adipokinetic hormone (AKH), respectively, are key hormones that regulate metabolism in most metazoa. While much is known about the regulation of DILPs, the mechanisms regulating AKH/glucagon production is still poorly understood. In this review, we describe the various factors that regulate the production of DILPs and AKH and emphasize the need for future studies to decipher how energy homeostasis is governed in Drosophila. This article is categorized under: Invertebrate Organogenesis > Flies Signaling Pathways > Global Signaling Mechanisms.

Endocrine Regulation of Energy Balance by Drosophila TGF-β/Activins

The Transforming growth factor beta (TGF-β) family of secreted proteins regulates a variety of key events in normal development and physiology. In mammals, this family, represented by 33 ligands, including TGF-β, activins, nodal, bone morphogenetic proteins (BMPs), and growth and differentiation factors (GDFs), regulate biological processes as diverse as cell proliferation, differentiation, apoptosis, metabolism, homeostasis, immune response, wound repair, and endocrine functions. In Drosophila, only 7 members of this family are present, with 4 TGF-β/BMP and 3 TGF-β/activin ligands. Studies in the fly have illustrated the role of TGF-β/BMP ligands during embryogenesis and organ patterning, while the TGF-β/activin ligands have been implicated in the control of wing growth and neuronal functions. In this review, we focus on the emerging roles of Drosophila TGF-β/activins in inter-organ communication via long-distance regulation, especially in systemic lipid and carbohydrate homeostasis, and discuss findings relevant to metabolic diseases in humans.

Tenectin recruits integrin to stabilize bouton architecture and regulate vesicle release at the Drosophila neuromuscular junction

Assembly, maintenance and function of synaptic junctions depend on extracellular matrix (ECM) proteins and their receptors. Here we report that Tenectin (Tnc), a Mucin-type protein with RGD motifs, is an ECM component required for the structural and functional integrity of synaptic specializations at the neuromuscular junction (NMJ) in Drosophila. Using genetics, biochemistry, electrophysiology, histology and electron microscopy, we show that Tnc is secreted from motor neurons and striated muscles and accumulates in the synaptic cleft. Tnc selectively recruits αPS2/βPS integrin at synaptic terminals, but only the cis Tnc/integrin complexes appear to be biologically active. These complexes have distinct pre- and postsynaptic functions, mediated at least in part through the local engagement of the spectrin-based membrane skeleton: the presynaptic complexes control neurotransmitter release, while postsynaptic complexes ensure the size and architectural integrity of synaptic boutons. Our study reveals an unprecedented role for integrin in the synaptic recruitment of spectrin-based membrane skeleton.

Nerve cells or neurons can communicate with each other by releasing chemical messengers into the gap between them, the synapse. Both neurons and synapses are surrounded by a network of proteins called the extracellular matrix, which anchors, protects and supports the synapse. The matrix also helps to regulate the dynamic communication across the synapses and consequently neurons.

Little is known about the proteins of the extracellular matrix, in particular about the ones involved in structural support. This is especially important for the so-called neuromuscular junctions, where neurons stimulate muscle contraction and trigger vigorous movement. Receptor proteins on cell surfaces, such as integrins, can bind to the extracellular matrix proteins to anchor the cells and are important for all cell junctions, including synaptic junctions. But because of their many essential roles during development, it was unclear how integrins modulate the activity of the synapse.

To investigate this further, Wang et al. studied the neuromuscular junctions of fruit flies. The experiments revealed that both muscle and neurons secrete a large protein called Tenectin, which accumulates into the small space between the neuron and the muscle, the synaptic cleft. This protein can bind to integrin and is necessary to support the neuromuscular junction structurally and functionally.

Wang et al. discovered that Tenectin works by gathering integrins on the surface of the neuron and the muscle. In the neuron, Tenectin forms complexes with integrin to regulate the release of neurotransmitters. In the muscle, the complexes provide support to the synaptic structures. However, when Tenectin was experimentally removed, it only disrupted the integrins at the neuromuscular junction, without affecting integrins in other regions of the cells, such as the site where the muscle uses integrins to attach to the tendon. Moreover, without Tenectin an important intracellular scaffolding meshwork that lines up and reinforces cell membranes was no longer organized properly at the synapse.

A next step will be to identify the missing components between Tenectin/integrin complexes on the surface of neurons and the neurotransmitter release machinery inside the cells. The extracellular matrix and its receptors play fundamental roles in the development and function of the nervous system. A better knowledge of the underlying mechanisms will help us to better understand the complex interplay between the synapse and the extracellular matrix.

The Drosophila TGF-beta/Activin-like ligands Dawdle and Myoglianin appear to modulate adult lifespan through regulation of 26S proteasome function in adult muscle

The Drosophila Activin signaling pathway employs at least three separate ligands – Activin-β (Actβ), Dawdle (Daw), and Myoglianin (Myo) – to regulate several general aspects of fruit fly larval development, including cell proliferation, neuronal remodeling, and metabolism. Here we provide experimental evidence indicating that both Daw and Myo are anti-ageing factors in adult fruit flies. Knockdown of Myo or Daw in adult fruit flies reduced mean lifespan, while overexpression of either ligand in adult muscle tissues but not in adipose tissues enhanced mean lifespan. An examination of ubiquitinated protein aggregates in adult muscles revealed a strong inverse correlation between Myo- or Daw-initiated Activin signaling and the amount of ubiquitinated protein aggregates. We show that this correlation has important functional implications by demonstrating that the lifespan extension effect caused by overexpression of wild-type Daw or Myo in adult muscle tissues can be completely abrogated by knockdown of a 26S proteasome regulatory subunit Rpn1 in adult fly muscle, and that the prolonged lifespan caused by overexpression of Daw or Myo in adult muscle could be due to enhanced protein levels of the key subunits of 26S proteasome. Overall, our data suggest that Activin signaling initiated by Myo and Daw in adult Drosophila muscles influences lifespan, in part, by modulation of protein homeostasis through either direct or indirect regulation of the 26S proteasome levels. Since Myo is closely related to the vertebrate muscle mass regulator Myostatin (GDF8) and the Myostatin paralog GDF11, our observations may offer a new experimental model for probing the roles of GDF11/8 in ageing regulation in vertebrates.

This article has an associated First Person interview with the first author of the paper.

Summary: This article has, for the first time, demonstrated that fruit fly TGF-beta, or Activin-type ligand Daw, or Myo-initiated Activin signaling in adult fruit fly muscle tissues works as an anti-ageing factor by regulating 26S proteasome activities in those tissues.

Regulation of neuroblast proliferation by surface glia in the Drosophila larval brain

Despite the importance of precisely regulating stem cell division, the molecular basis for this control is still elusive. Here, we show that surface glia in the developing Drosophila brain play essential roles in regulating the proliferation of neural stem cells, neuroblasts (NBs). We found that two classes of extracellular factors, Dally-like (Dlp), a heparan sulfate proteoglycan, and Glass bottom boat (Gbb), a BMP homologue, are required for proper NB proliferation. Interestingly, Dlp expressed in perineural glia (PG), the most outer layer of the surface glia, is responsible for NB proliferation. Consistent with this finding, functional ablation of PG using a dominant-negative form of dynamin showed that PG has an instructive role in regulating NB proliferation. Gbb acts not only as an autocrine proliferation factor in NBs but also as a paracrine survival signal in the PG. We propose that bidirectional communication between NBs and glia through TGF-β signaling influences mutual development of these two cell types. We also discuss the possibility that PG and NBs communicate via direct membrane contact or transcytotic transport of membrane components. Thus, our study shows that the surface glia acts not only as a simple structural insulator but also a dynamic regulator of brain development.

TGFβ/Activin signalling is required for ribosome biogenesis and cell growth in Drosophila salivary glands

Signalling by TGFβ superfamily factors plays an important role in tissue growth and cell proliferation. In Drosophila, the activity of the TGFβ/Activin signalling branch has been linked to the regulation of cell growth and proliferation, but the cellular and molecular basis for these functions are not fully understood. In this study, we show that both the RII receptor Punt (Put) and the R-Smad Smad2 are strongly required for cell and tissue growth. Knocking down the expression of Put or Smad2 in salivary glands causes alterations in nucleolar structure and functions. Cells with decreased TGFβ/Activin signalling accumulate intermediate pre-rRNA transcripts containing internal transcribed spacer 1 regions accompanied by the nucleolar retention of ribosomal proteins. Thus, our results show that TGFβ/Activin signalling is required for ribosomal biogenesis, a key aspect of cellular growth control. Importantly, overexpression of Put enhanced cell growth induced by Drosophila Myc, a well-characterized inducer of nucleolar hypertrophy and ribosome biogenesis.

Motor axon guidance in Drosophila

The Drosophila motor system starts to assemble during embryonic development. It is composed of 30 muscles per abdominal hemisegment and 36 motor neurons assembling into nerve branches to exit the CNS, navigate within the muscle field and finally establish specific connections with their target muscles. Several families of guidance molecules that play a role controlling this process as well as transcriptional regulators that program the behavior of specific motor neuron have been identified. In this review we summarize the role of both groups of molecules in the motor system as well as their relationship where known. It is apparent that partially redundant guidance protein families and membrane molecules with different functional output direct guidance decisions cooperatively. Some distinct transcriptional regulators seem to control guidance of specific nerve branches globally directing the expression of groups of pathfinding molecules in all motor neurons within the same motor branch.

Transforming Growth Factor β/Activin signaling in neurons increases susceptibility to starvation

Animals rely on complex signaling network to mobilize its energy stores during starvation. We have previously shown that the sugar-responsive TGFβ/Activin pathway, activated through the TGFβ ligand Dawdle, plays a central role in shaping the post-prandial digestive competence in the Drosophila midgut. Nevertheless, little is known about the TGFβ/Activin signaling in sugar metabolism beyond the midgut. Here, we address the importance of Dawdle (Daw) after carbohydrate ingestion. We found that Daw expression is coupled to dietary glucose through the evolutionarily conserved Mio-Mlx transcriptional complex. In addition, Daw activates the TGFβ/Activin signaling in neuronal populations to regulate triglyceride and glycogen catabolism and energy homeostasis. Loss of those neurons depleted metabolic reserves and rendered flies susceptible to starvation.

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.

Parallel Activin and BMP signaling coordinates R7/R8 photoreceptor subtype pairing in the stochastic Drosophila retina

Drosophila color vision is achieved by comparing outputs from two types of color-sensitive photoreceptors, R7 and R8. Ommatidia (unit eyes) are classified into two subtypes, known as 'pale' or 'yellow', depending on Rhodopsin expression in R7 and R8. Subtype specification is controlled by a stochastic decision in R7 and instructed to the underlying R8. We find that the Activin receptor Baboon is required in R8 to receive non-redundant signaling from the three Activin ligands, activating the transcription factor dSmad2. Concomitantly, two BMP ligands activate their receptor, Thickveins, and the transcriptional effector, Mad. The Amon TGFβ processing factor appears to regulate components of the TGFβ pathway specifically in pale R7. Mad and dSmad2 cooperate to modulate the Hippo pathway kinase Warts and the growth regulator Melted; two opposing factors of a bi-stable loop regulating R8 Rhodopsin expression. Therefore, TGFβ and growth pathways interact in postmitotic cells to precisely coordinate cell-specific output.

The Neuropeptide Corazonin Controls Social Behavior and Caste Identity in Ants

Social insects are emerging models to study how gene regulation affects behavior because their colonies comprise individuals with the same genomes but greatly different behavioral repertoires. To investigate the molecular mechanisms that activate distinct behaviors in different castes, we exploit a natural behavioral plasticity in Harpegnathos saltator, where adult workers can transition to a reproductive, queen-like state called gamergate. Analysis of brain transcriptomes during the transition reveals that corazonin, a neuropeptide homologous to the vertebrate gonadotropin-releasing hormone, is downregulated as workers become gamergates. Corazonin is also preferentially expressed in workers and/or foragers from other social insect species. Injection of corazonin in transitioning Harpegnathos individuals suppresses expression of vitellogenin in the brain and stimulates worker-like hunting behaviors, while inhibiting gamergate behaviors, such as dueling and egg deposition. We propose that corazonin is a central regulator of caste identity and behavior in social insects.

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.

The Drosophila Hox gene Ultrabithorax acts in both muscles and motoneurons to orchestrate formation of specific neuromuscular connections

Hox genes are known to specify motoneuron pools in the developing vertebrate spinal cord and to control motoneuronal targeting in several species. However, the mechanisms controlling axial diversification of muscle innervation patterns are still largely unknown. We present data showing that the Drosophila Hox gene Ultrabithorax (Ubx) acts in the late embryo to establish target specificity of ventrally projecting RP motoneurons. In abdominal segments A2 to A7, RP motoneurons innervate the ventrolateral muscles VL1-4, with VL1 and VL2 being innervated in a Wnt4-dependent manner. In Ubx mutants, these motoneurons fail to make correct contacts with muscle VL1, a phenotype partially resembling that of the Wnt4 mutant. We show that Ubx regulates expression of Wnt4 in muscle VL2 and that it interacts with the Wnt4 response pathway in the respective motoneurons. Ubx thus orchestrates the interaction between two cell types, muscles and motoneurons, to regulate establishment of the ventrolateral neuromuscular network.

Summary: Ultrabithorax controls correct innervation of ventrolateral muscles by coordinating Wnt4 ligand expression in muscles with the signalling pathway response in motoneurons.

Activins and Inhibins: Roles in Development, Physiology, and Disease

Since their original discovery as regulators of follicle-stimulating hormone (FSH) secretion and erythropoiesis, the TGF-β family members activin and inhibin have been shown to participate in a variety of biological processes, from the earliest stages of embryonic development to highly specialized functions in terminally differentiated cells and tissues. Herein, we present the history, structures, signaling mechanisms, regulation, and biological processes in which activins and inhibins participate, including several recently discovered biological activities and functional antagonists. The potential therapeutic relevance of these advances is also discussed.

C-terminal Src Kinase Gates Homeostatic Synaptic Plasticity and Regulates Fasciclin II Expression at the Drosophila Neuromuscular Junction

Forms of homeostatic plasticity stabilize neuronal outputs and promote physiologically favorable synapse function. A well-studied homeostatic system operates at the Drosophila melanogaster larval neuromuscular junction (NMJ). At the NMJ, impairment of postsynaptic glutamate receptor activity is offset by a compensatory increase in presynaptic neurotransmitter release. We aim to elucidate how this process operates on a molecular level and is preserved throughout development. In this study, we identified a tyrosine kinase-driven signaling system that sustains homeostatic control of NMJ function. We identified C-terminal Src Kinase (Csk) as a potential regulator of synaptic homeostasis through an RNAi- and electrophysiology-based genetic screen. We found that Csk loss-of-function mutations impaired the sustained expression of homeostatic plasticity at the NMJ, without drastically altering synapse growth or baseline neurotransmission. Muscle-specific overexpression of Src Family Kinase (SFK) substrates that are negatively regulated by Csk also impaired NMJ homeostasis. Surprisingly, we found that transgenic Csk-YFP can support homeostatic plasticity at the NMJ when expressed either in the muscle or in the nerve. However, only muscle-expressed Csk-YFP was able to localize to NMJ structures. By immunostaining, we found that Csk mutant NMJs had dysregulated expression of the Neural Cell Adhesion Molecule homolog Fasciclin II (FasII). By immunoblotting, we found that levels of a specific isoform of FasII were decreased in homeostatically challenged GluRIIA mutant animals–but markedly increased in Csk mutant animals. Additionally, we found that postsynaptic overexpression of FasII from its endogenous locus was sufficient to impair synaptic homeostasis, and genetically reducing FasII levels in Csk mutants fully restored synaptic homeostasis. Based on these data, we propose that Csk and its SFK substrates impinge upon homeostatic control of NMJ function by regulating downstream expression or localization of FasII.

Homeostasis is a fundamental topic in biology. Individual cells and systems of cells constantly monitor their environments and adjust their outputs in order to maintain physiological properties within ranges that can support life. The nervous system is no exception. Synapses and circuits are endowed with a capacity to respond to environmental challenges in a homeostatic fashion. As a result, synaptic output stays within an appropriate physiological range. We know that homeostasis is a fundamental form of regulation in animal nervous systems, but we have very little information about how it works. In this study, we examine the fruit fly Drosophila melanogaster and its ability to maintain normal levels of synaptic output over long periods of developmental time. We identify new roles in this process for classical signaling molecules called C-terminal Src kinase, Src family kinases, as well as a neuronal cell adhesion molecule called Fasciclin II, which was previously shown to stabilize synaptic contacts between neurons and muscles. Our work contributes to a broader understanding of how neurons work to maintain stable outputs. Ultimately, this type of knowledge could have important implications for neurological disorders in which stability is lost, such as forms of epilepsy or ataxia.

A Novel, Noncanonical BMP Pathway Modulates Synapse Maturation at the Drosophila Neuromuscular Junction

At the Drosophila NMJ, BMP signaling is critical for synapse growth and homeostasis. Signaling by the BMP7 homolog, Gbb, in motor neurons triggers a canonical pathway—which modulates transcription of BMP target genes, and a noncanonical pathway—which connects local BMP/BMP receptor complexes with the cytoskeleton. Here we describe a novel noncanonical BMP pathway characterized by the accumulation of the pathway effector, the phosphorylated Smad (pMad), at synaptic sites. Using genetic epistasis, histology, super resolution microscopy, and electrophysiology approaches we demonstrate that this novel pathway is genetically distinguishable from all other known BMP signaling cascades. This novel pathway does not require Gbb, but depends on presynaptic BMP receptors and specific postsynaptic glutamate receptor subtypes, the type-A receptors. Synaptic pMad is coordinated to BMP’s role in the transcriptional control of target genes by shared pathway components, but it has no role in the regulation of NMJ growth. Instead, selective disruption of presynaptic pMad accumulation reduces the postsynaptic levels of type-A receptors, revealing a positive feedback loop which appears to function to stabilize active type-A receptors at synaptic sites. Thus, BMP pathway may monitor synapse activity then function to adjust synapse growth and maturation during development.

Synaptic activity and synapse development are intimately linked, but our understanding of the coupling mechanisms remains limited. Anterograde and retrograde signals together with trans-synaptic complexes enable intercellular communications. How synapse activity status is monitored and relayed across the synaptic cleft remains poorly understood. The Drosophila NMJ is a very powerful genetic system to study synapse development. BMP signaling modulates NMJ growth via a canonical, Smad-dependent pathway, but also synapse stability, via a noncanonical, Smad-independent pathway. Here we describe a novel, noncanonical BMP pathway, which is genetically distinguishable from all other known BMP pathways. This pathway does not contribute to NMJ growth and instead influences synapse formation and maturation in an activity-dependent manner. Specifically, phosphorylated Smad (pMad in flies) accumulates at active zone in response to active postsynaptic type-A glutamate receptors, a specific receptor subtype. In turn, synaptic pMad functions to promote the recruitment of type-A receptors at synaptic sites. This positive feedback loop provides a molecular switch controlling which flavor of glutamate receptors will be stabilized at synaptic locations as a function of synapse status. Since BMP signaling also controls NMJ growth and stability, BMP pathway offers an exquisite means to monitor the status of synapse activity and coordinate NMJ growth with synapse maturation and stabilization.

What does it take to get the job done?

I am extremely honored to be the recipient of the 2015 Women in Cell Biology Junior Award. When I reflect on my journey in science, many great people and memorable experiences come to mind. Some of these encounters were truly career-defining moments. Others provided priceless lessons. In this essay, I recount some of the moments and experiences that influenced my scientific trajectory with the hope that they may inspire others.

Mondo-Mlx Mediates Organismal Sugar Sensing through the Gli-Similar Transcription Factor Sugarbabe

The ChREBP/Mondo-Mlx transcription factors are activated by sugars and are essential for sugar tolerance. They promote the conversion of sugars to lipids, but beyond this, their physiological roles are insufficiently understood. Here, we demonstrate that in an organism-wide setting in Drosophila, Mondo-Mlx controls the majority of sugar-regulated genes involved in nutrient digestion and transport as well as carbohydrate, amino acid, and lipid metabolism. Furthermore, human orthologs of the Mondo-Mlx targets display enrichment among gene variants associated with high circulating triglycerides. In addition to direct regulation of metabolic genes, Mondo-Mlx maintains metabolic homeostasis through downstream effectors, including the Activin ligand Dawdle and the Gli-similar transcription factor Sugarbabe. Sugarbabe controls a subset of Mondo-Mlx-dependent processes, including de novo lipogenesis and fatty acid desaturation. In sum, Mondo-Mlx is a master regulator of other sugar-responsive pathways essential for adaptation to a high-sugar diet.

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.

Anterograde Activin signaling regulates postsynaptic membrane potential and GluRIIA/B abundance at the Drosophila neuromuscular junction

Members of the TGF-β superfamily play numerous roles in nervous system development and function. In Drosophila, retrograde BMP signaling at the neuromuscular junction (NMJ) is required presynaptically for proper synapse growth and neurotransmitter release. In this study, we analyzed whether the Activin branch of the TGF-β superfamily also contributes to NMJ development and function. We find that elimination of the Activin/TGF-β type I receptor babo, or its downstream signal transducer smox, does not affect presynaptic NMJ growth or evoked excitatory junctional potentials (EJPs), but instead results in a number of postsynaptic defects including depolarized membrane potential, small size and frequency of miniature excitatory junction potentials (mEJPs), and decreased synaptic densities of the glutamate receptors GluRIIA and B. The majority of the defective smox synaptic phenotypes were rescued by muscle-specific expression of a smox transgene. Furthermore, a mutation in actβ, an Activin-like ligand that is strongly expressed in motor neurons, phenocopies babo and smox loss-of-function alleles. Our results demonstrate that anterograde Activin/TGF-β signaling at the Drosophila NMJ is crucial for achieving normal abundance and localization of several important postsynaptic signaling molecules and for regulating postsynaptic membrane physiology. Together with the well-established presynaptic role of the retrograde BMP signaling, our findings indicate that the two branches of the TGF-β superfamily are differentially deployed on each side of the Drosophila NMJ synapse to regulate distinct aspects of its development and function.

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.

SoxNeuro orchestrates central nervous system specification and differentiation in Drosophila and is only partially redundant with Dichaete

Background

Sox proteins encompass an evolutionarily conserved family of transcription factors with critical roles in animal development and stem cell biology. In common with vertebrates, the Drosophila group B proteins SoxNeuro and Dichaete are involved in central nervous system development, where they play both similar and unique roles in gene regulation. Sox genes show extensive functional redundancy across metazoans, but the molecular basis underpinning functional compensation mechanisms at the genomic level are currently unknown.

Results

Using a combination of genome-wide binding analysis and gene expression profiling, we show that SoxNeuro directs embryonic neural development from the early specification of neuroblasts through to the terminal differentiation of neurons and glia. To address the issue of functional redundancy and compensation at a genomic level, we compare SoxNeuro and Dichaete binding, identifying common and independent binding events in wild-type conditions, as well as instances of compensation and loss of binding in mutant backgrounds.

Conclusions

We find that early aspects of group B Sox functions in the central nervous system, such as stem cell maintenance and dorsoventral patterning, are highly conserved. However, in contrast to vertebrates, we find that Drosophila group B1 proteins also play prominent roles during later aspects of neural morphogenesis. Our analysis of the functional relationship between SoxNeuro and Dichaete uncovers evidence for redundant and independent functions for each protein, along with unexpected examples of compensation and interdependency, thus providing new insights into the general issue of transcription factor functional redundancy.

Intertissue control of the nucleolus via a myokine-dependent longevity pathway

Recent evidence indicates that skeletal muscle influences systemic aging, but little is known about the signaling pathways and muscle-released cytokines (myokines) responsible for this intertissue communication. Here, we show that muscle-specific overexpression of the transcription factor Mnt decreases age-related climbing defects and extends lifespan in Drosophila. Mnt overexpression in muscle autonomously decreases the expression of nucleolar components and systemically decreases rRNA levels and the size of the nucleolus in adipocytes. This nonautonomous control of the nucleolus, a regulator of ribosome biogenesis and lifespan, relies on Myoglianin, a myokine induced by Mnt and orthologous to human GDF11 and Myostatin. Myoglianin overexpression in muscle extends lifespan and decreases nucleolar size in adipocytes by activating p38 mitogen-activated protein kinase (MAPK), whereas Myoglianin RNAi in muscle has converse effects. Altogether, these findings highlight a key role for myokine signaling in the integration of signaling events in muscle and distant tissues during aging.

Systemic Activin signaling independently regulates sugar homeostasis, cellular metabolism, and pH balance in Drosophila melanogaster

The ability to maintain cellular and physiological metabolic homeostasis is key for the survival of multicellular organisms in changing environmental conditions. However, our understanding of extracellular signaling pathways that modulate metabolic processes remains limited. In this study we show that the Activin-like ligand Dawdle (Daw) is a major regulator of systemic metabolic homeostasis and cellular metabolism in Drosophila. We find that loss of canonical Smad signaling downstream of Daw leads to defects in sugar and systemic pH homeostasis. Although Daw regulates sugar homeostasis by positively influencing insulin release, we find that the effect of Daw on pH balance is independent of its role in insulin signaling and is caused by accumulation of organic acids that are primarily tricarboxylic acid (TCA) cycle intermediates. RNA sequencing reveals that a number of TCA cycle enzymes and nuclear-encoded mitochondrial genes including genes involved in oxidative phosphorylation and β-oxidation are up-regulated in the daw mutants, indicating either a direct or indirect role of Daw in regulating these genes. These findings establish Activin signaling as a major metabolic regulator and uncover a functional link between TGF-β signaling, insulin signaling, and metabolism in Drosophila.

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.

Activin signaling targeted by insulin/dFOXO regulates aging and muscle proteostasis in Drosophila

Reduced insulin/IGF signaling increases lifespan in many animals. To understand how insulin/IGF mediates lifespan in Drosophila, we performed chromatin immunoprecipitation-sequencing analysis with the insulin/IGF regulated transcription factor dFOXO in long-lived insulin/IGF signaling genotypes. Dawdle, an Activin ligand, is bound and repressed by dFOXO when reduced insulin/IGF extends lifespan. Reduced Activin signaling improves performance and protein homeostasis in muscles of aged flies. Activin signaling through the Smad binding element inhibits the transcription of Autophagy-specific gene 8a (Atg8a) within muscle, a factor controlling the rate of autophagy. Expression of Atg8a within muscle is sufficient to increase lifespan. These data reveal how insulin signaling can regulate aging through control of Activin signaling that in turn controls autophagy, representing a potentially conserved molecular basis for longevity assurance. While reduced Activin within muscle autonomously retards functional aging of this tissue, these effects in muscle also reduce secretion of insulin-like peptides at a distance from the brain. Reduced insulin secretion from the brain may subsequently reinforce longevity assurance through decreased systemic insulin/IGF signaling.

It is widely known that reduced insulin/IGF signaling slows aging in many contexts. This process requires the forkhead transcription factor (FOXO). FOXO modulates the expression of many genes, and the list of those associated with slow aging is impressive. But there are few data indicating the mechanisms or genes through which FOXO actually slows aging. Here, we identify a novel FOXO target, dawdle, the Activin-like ligand in fruit flies. We show that down-regulation of Activin signaling in muscle, but not in adipose tissue, leads to extended lifespan. In part it does so when it alleviates the negative transcriptional repression of its Smox transcription factor (a Smad transcription factor) upon a keystone autophagy gene, Atg8a. This double signaling cascade autonomously improves muscle performance (measured at cellular and functional levels) and nonautonomously extends lifespan as it reduces the secretion of insulin peptides from the brain. The work develops the emerging model for interacting autonomous-nonautonomous roles of insulin/IGF signaling as a systems integrative mechanism of aging control.

Building a synapse: a complex matter

L-glutamate is the primary neurotransmitter at excitatory synapses in the vertebrate CNS and at arthropod neuromuscular junctions (NMJs). However, the molecular mechanisms that trigger the recruitment of glutamate receptors at the onset of synaptogenesis and promote their stabilization at postsynaptic densities remain poorly understood. We have reported the discovery of a novel, evolutionary conserved molecule, Neto, essential for clustering of ionotropic glutamate receptors (iGluRs) at Drosophila NMJ. Neto is the first auxiliary subunit described in Drosophila and is the only non-channel subunit absolutely required for functional iGluRs. Here we review the role of Drosophila Neto in synapse assembly, its similarities with other Neto proteins and a new perspective on how glutamatergic synapses are physically assembled and stabilized.

Activin receptor inhibition by Smad2 regulates Drosophila wing disc patterning through BMP-response elements

Imaginal disc development in Drosophila requires coordinated cellular proliferation and tissue patterning. In our studies of TGFβ superfamily signaling components, we found that a protein null mutation of Smad2, the only Activin subfamily R-Smad in the fruit fly, produces overgrown wing discs that resemble gain of function for BMP subfamily signaling. The wing discs are expanded specifically along the anterior-posterior axis, with increased proliferation in lateral regions. The morphological defect is not observed in mutants for the TGFβ receptor baboon, and epistasis tests showed that baboon is epistatic to Smad2 for disc overgrowth. Rescue experiments indicate that Baboon binding, but not canonical transcription factor activity, of Smad2 is required for normal disc growth. Smad2 mutant discs generate a P-Mad stripe that is narrower and sharper than the normal gradient, and activation targets are correspondingly expressed in narrowed domains. Repression targets of P-Mad are profoundly mis-regulated, with brinker and pentagone reporter expression eliminated in Smad2 mutants. Loss of expression requires a silencer element previously shown to be controlled by BMP signaling. Epistasis experiments show that Baboon, Mad and Schnurri are required to mediate the ectopic silencer output in the absence of Smad2. Taken together, our results show that loss of Smad2 permits promiscuous Baboon activity, which represses genes subject to control by Mad-dependent silencer elements. The absence of Brinker and Pentagone in Smad2 mutants explains the compound wing disc phenotype. Our results highlight the physiological relevance of substrate inhibition of a kinase, and reveal a novel interplay between the Activin and BMP pathways.

Axonal growth towards Xenopus skin in vitro is mediated by matrix metalloproteinase activity

We have previously demonstrated that the growth of peripheral nervous system axons is strongly attracted towards limb buds and skin explants in vitro. Here, we show that directed axonal growth towards skin explants of Xenopus laevis in matrigel is associated with expression of matrix metalloproteinase (MMP)-18 and also other MMPs, and that this long-range neurotropic activity is inhibited by the broad-spectrum MMP inhibitors BB-94 and GM6001. We also show that forced expression of MMP-18 in COS-7 cell aggregates enhances axonal growth from Xenopus dorsal root ganglia explants. Nidogen is the target of MMPs released by cultured skin in matrigel, whereas other components remain intact. Our results suggest a novel link between MMP activity and extracellular matrix breakdown in the control of axonal growth.

Drosophila Neto is essential for clustering glutamate receptors at the neuromuscular junction

Neurotransmitter receptor recruitment at postsynaptic specializations is key in synaptogenesis, since this step confers functionality to the nascent synapse. The Drosophila neuromuscular junction (NMJ) is a glutamatergic synapse, similar in composition and function to mammalian central synapses. Various mechanisms regulating the extent of postsynaptic ionotropic glutamate receptor (iGluR) clustering have been described, but none are known to be essential for the initial localization and clustering of iGluRs at postsynaptic densities (PSDs). We identified and characterized the Drosophila neto (neuropilin and tolloid-like) as an essential gene required for clustering of iGluRs at the NMJ. Neto colocalizes with the iGluRs at the PSDs in puncta juxtaposing the active zones. neto loss-of-function phenotypes parallel the loss-of-function defects described for iGluRs. The defects in neto mutants are effectively rescued by muscle-specific expression of neto transgenes. Neto clustering at the Drosophila NMJ coincides with and is dependent on iGluRs. Our studies reveal that Drosophila Neto is a novel, essential component of the iGluR complexes and is required for iGluR clustering, organization of PSDs, and synapse functionality.

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.

Metalloproteinases in Drosophila to humans that are central players in developmental processes

Many secreted proteins are synthesized as precursors with propeptides that must be cleaved to yield the mature functional form of the molecule. In addition, various growth factors occur in extracellular latent complexes with protein antagonists and are activated upon cleavage of such antagonists. Research in the separate fields of embryonic patterning and extracellular matrix formation has identified members of the BMP1/Tolloid-like family of metalloproteinases as key players in these types of biosynthetic processing events in species ranging from Drosophila to humans.

Evolution of the TGF-β signaling pathway and its potential role in the ctenophore, Mnemiopsis leidyi

The TGF-β signaling pathway is a metazoan-specific intercellular signaling pathway known to be important in many developmental and cellular processes in a wide variety of animals. We investigated the complexity and possible functions of this pathway in a member of one of the earliest branching metazoan phyla, the ctenophore Mnemiopsis leidyi. A search of the recently sequenced Mnemiopsis genome revealed an inventory of genes encoding ligands and the rest of the components of the TGF-β superfamily signaling pathway. The Mnemiopsis genome contains nine TGF-β ligands, two TGF-β-like family members, two BMP-like family members, and five gene products that were unable to be classified with certainty. We also identified four TGF-β receptors: three Type I and a single Type II receptor. There are five genes encoding Smad proteins (Smad2, Smad4, Smad6, and two Smad1s). While we have identified many of the other components of this pathway, including Tolloid, SMURF, and Nomo, notably absent are SARA and all of the known antagonists belonging to the Chordin, Follistatin, Noggin, and CAN families. This pathway likely evolved early in metazoan evolution as nearly all components of this pathway have yet to be identified in any non-metazoan. The complement of TGF-β signaling pathway components of ctenophores is more similar to that of the sponge, Amphimedon, than to cnidarians, Trichoplax, or bilaterians. The mRNA expression patterns of key genes revealed by in situ hybridization suggests that TGF-β signaling is not involved in ctenophore early axis specification. Four ligands are expressed during gastrulation in ectodermal micromeres along all three body axes, suggesting a role in transducing earlier maternal signals. Later expression patterns and experiments with the TGF-β inhibitor SB432542 suggest roles in pharyngeal morphogenesis and comb row organization.

Drosophila MMP2 regulates the matrix molecule faulty attraction (Frac) to promote motor axon targeting in Drosophila

Matrix metalloproteinases (MMPs) are widely hypothesized to regulate signaling events through processing of extracellular matrix (ECM) molecules. We previously demonstrated that membrane-associated Mmp2 is expressed in exit glia and contributes to motor axon targeting. To identify possible substrates, we undertook a yeast interaction screen for Mmp2-binding proteins and identified the novel ECM protein faulty attraction (Frac). Frac encodes a multidomain extracellular protein rich in epidermal growth factor (EGF) and calcium-binding EGF domains, related to the vertebrate Fibrillin and Fibulin gene families. It is expressed in mesodermal domains flanking Mmp2-positive glia. The juxtaposition of Mmp2 and Frac proteins raises the possibility that Frac is a proteolytic target of Mmp2. Consistent with this hypothesis, levels of full-length Frac are increased in Mmp2 loss-of-function (LOF) and decreased in Mmp2 gain-of-function (GOF) embryos, indicating that Frac cleavage is Mmp2 dependent. To test whether frac is necessary for axon targeting, we characterized guidance in frac LOF mutants. Motor axons in frac LOF embryos are loosely associated and project ectopically, a phenotype essentially equivalent to that of Mmp2 LOF. The phenotypic similarity between enzyme and substrate mutants argues that Mmp2 activates Frac. In addition, Mmp2 overexpression pathfinding phenotypes depend on frac activity, indicating that Mmp2 is genetically upstream of frac. Last, overexpression experiments suggest that Frac is unlikely to have intrinsic signaling activity, raising the possibility that an Mmp2-generated Frac fragment acts as a guidance cue cofactor. Indeed, we present genetic evidence that Frac regulates a non-canonical LIM kinase 1-dependent bone morphogenetic protein signaling pathway in motoneurons necessary for axon pathfinding during embryogenesis.

100 years of Drosophila research and its impact on vertebrate neuroscience: a history lesson for the future

Discoveries in fruit flies have greatly contributed to our understanding of neuroscience. The use of an unparalleled wealth of tools, many of which originated between 1910–1960, has enabled milestone discoveries in nervous system development and function. Such findings have triggered and guided many research efforts in vertebrate neuroscience. After 100 years, fruit flies continue to be the choice model system for many neuroscientists. The combinational use of powerful research tools will ensure that this model organism will continue to lead to key discoveries that will impact vertebrate neuroscience.

Evolution of extracellular Dpp modulators in insects: The roles of tolloid and twisted-gastrulation in dorsoventral patterning of the Tribolium embryo

The formation of the BMP gradient which patterns the DV axis in flies and vertebrates requires several extracellular modulators like the inhibitory protein Sog/Chordin, the metalloprotease Tolloid (Tld), which cleaves Sog/Chordin, and the CR domain protein Twisted gastrulation (Tsg). While flies and vertebrates have only one sog/chordin gene they possess several paralogues of tld and tsg. A simpler and probably ancestral situation is observed in the short-germ beetle Tribolium castaneum (Tc), which possesses only one tld and one tsg gene. Here we show that in T. castaneum tld is required for early BMP signalling except in the head region and Tc-tld function is, as expected, dependent on Tc-sog. In contrast, Tc-tsg is required for all aspects of early BMP signalling and acts in a Tc-sog-independent manner. For comparison with Drosophila melanogaster we constructed fly embryos lacking all early Tsg activity (tsg;;srw double mutants) and show that they still establish a BMP signalling gradient. Thus, our results suggest that the role of Tsg proteins for BMP gradient formation has changed during insect evolution.

Canonical TGF-beta signaling is required for the balance of excitatory/inhibitory transmission within the hippocampus and prepulse inhibition of acoustic startle

Smad4 is a unique nuclear transducer for all TGF-beta signaling pathways and regulates gene transcription during development and tissue homeostasis. To elucidate the postnatal role of TGF-beta signaling in the mammalian brain, we generated forebrain-specific Smad4 knock-out mice. Surprisingly, the mutants showed no alteration in long-term potentiation and water maze, suggesting that Smad4 is not required for spatial learning and memory. However, these mutant mice did show enhancement of paired-pulse facilitation in excitatory synaptic transmission and stronger paired-pulse depression of GABA(A) currents in the hippocampus. The alteration of hippocampal electrophysiology correlated with mouse hyperactivity in homecage and open field tests. Mutant mice also showed overgrooming as well as deficits of prepulse inhibition, a widely used endophenotype of schizophrenia. With a specific real-time PCR array focused on TGF-beta signaling pathway, we identified a novel regulation mechanism of the pathway in the hippocampal neurons, in which Smad4-mediated signaling suppresses the level of extracellular antagonism of TGF-beta ligands through transcriptional regulation of follistatin, a selective inhibitor to activin/TGF-beta signaling in the hippocampus. In summary, we suggest that the canonical TGF-beta signaling pathway is critical for use-dependent modulation of GABA(A) synaptic transmission and dendritic homeostasis; furthermore, a disruption in the balance of the excitatory and inhibitory hippocampal network can result in psychiatric-like behavior.

Activin signaling functions upstream of Gbb to regulate synaptic growth at the Drosophila neuromuscular junction

Activins are members of the TGF-ss superfamily of secreted growth factors that control a diverse array of processes in vertebrates including endocrine function, cell proliferation, differentiation, immune response and wound repair. In Drosophila, the Activin ligand Dawdle (Daw) has been shown to regulate several aspects of neuronal development such as embryonic axonal pathfinding, neuroblast proliferation in the larval brain and growth cone motility in the visual system. Here we identify a novel role for Activin signaling in regulating synaptic growth at the larval neuromuscular junction (NMJ). Mutants for Daw, the Activin type I receptor Baboon (Babo), and the signal transducer dSmad2, display reduced NMJ size suggesting that Daw utilizes a canonical Activin signal-transduction pathway in this context. Additionally, loss of Daw/Babo activity affects microtubule stability, axonal transport and distribution of Futsch, the Drosophila microtubule associated protein 1B (MAP1B) homolog. We find that Babo signaling is required postsynaptically in the muscle, in contrast to the well-characterized retrograde BMP/Gbb signal that is required for synaptic growth and function in presynaptic cells. Finally, we show that the Daw/Babo pathway acts upstream of gbb, and is involved in maintenance of muscle gbb expression, suggesting that Activins regulate NMJ growth by modulating BMP activity through transcriptional regulation of ligand expression.