Drosophila S6 Kinase like inhibits neuromuscular junction growth by downregulating the BMP receptor thickveins

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.

Drosophila Atlastin regulates synaptic vesicle mobilization independent of bone morphogenetic protein signaling

BACKGROUND

The endoplasmic reticulum (ER) contacts endosomes in all parts of a motor neuron, including the axon and presynaptic terminal, to move structural proteins, proteins that send signals, and lipids over long distances. Atlastin (Atl), a large GTPase, is required for membrane fusion and the structural dynamics of the ER tubules. Atl mutations are the second most common cause of Hereditary Spastic Paraplegia (HSP), which causes spasticity in both sexes' lower extremities. Through an unknown mechanism, Atl mutations stimulate the BMP (bone morphogenetic protein) pathway in vertebrates and Drosophila. Synaptic defects are caused by atl mutations, which affect the abundance and distribution of synaptic vesicles (SV) in the bouton. We hypothesize that BMP signaling, does not cause Atl-dependent SV abnormalities in Drosophila.

RESULTS

We show that atl knockdown in motor neurons (Atl-KD) increases synaptic and satellite boutons in the same way that constitutively activating the BMP-receptor Tkv (thick veins) (Tkv-CA) increases the bouton number. The SV proteins Cysteine string protein (CSP) and glutamate vesicular transporter are reduced in Atl-KD and Tkv-CA larvae. Reducing the activity of the BMP receptor Wishful thinking (wit) can rescue both phenotypes. Unlike Tkv-CA larvae, Atl-KD larvae display altered activity-dependent distributions of CSP staining. Furthermore, Atl-KD larvae display an increased FM 1-43 unload than Control and Tkv-CA larvae. As decreasing wit function does not reduce the phenotype, our hypothesis that BMP signaling is not involved is supported. We also found that Rab11/CSP colocalization increased in Atl-KD larvae, which supports the concept that late recycling endosomes regulate SV movements.

CONCLUSIONS

Our findings reveal that Atl modulates neurotransmitter release in motor neurons via SV distribution independently of BMP signaling, which could explain the observed SV accumulation and synaptic dysfunction. Our data suggest that Atl is involved in membrane traffic as well as formation and/or recycling of the late endosome.

Gbb glutathionylation promotes its proteasome-mediated degradation to inhibit synapse growth

Glutathionylation is a posttranslational modification involved in various molecular and cellular processes. However, it remains unknown whether and how glutathionylation regulates nervous system development. To identify critical regulators of synapse growth and development, we performed an RNAi screen and found that postsynaptic knockdown of glutathione transferase omega 1 (GstO1) caused significantly more synaptic boutons at the Drosophila neuromuscular junctions. Genetic and biochemical analysis revealed an increased level of glass boat bottom (Gbb), the Drosophila homolog of mammalian bone morphogenetic protein (BMP), in GstO1 mutants. Further experiments showed that GstO1 is a critical regulator of Gbb glutathionylation at cysteines 354 and 420, which promoted its degradation via the proteasome pathway. Moreover, the E3 ligase Ctrip negatively regulated the Gbb protein level by preferentially binding to glutathionylated Gbb. These results unveil a novel regulatory mechanism in which glutathionylation of Gbb facilitates its ubiquitin-mediated degradation. Taken together, our findings shed new light on the crosstalk between glutathionylation and ubiquitination of Gbb in synapse development.

Madm/NRBP1 mediates synaptic maintenance and neurodegeneration-induced presynaptic homeostatic potentiation

The precise regulation of synaptic connectivity and function is essential to maintain neuronal circuits. Here, we show that the Drosophila pseudo-kinase Madm/NRBP1 (Mlf-1-adapter-molecule/nuclear-receptor-binding protein 1) is required presynaptically to maintain synaptic stability and to coordinate synaptic growth and function. Presynaptic Madm mediates these functions by controlling cap-dependent translation via the target of rapamycin (TOR) effector 4E-BP/Thor (eukaryotic initiation factor 4E binding protein/Thor). Strikingly, at degenerating neuromuscular synapses, postsynaptic Madm induces a compensatory, transsynaptic signal that utilizes the presynaptic homeostatic potentiation (PHP) machinery to offset synaptic release deficits and to delay synaptic degeneration. Madm is not required for canonical PHP but induces a neurodegeneration-specific form of PHP and acts via the regulation of the cap-dependent translation regulators 4E-BP/Thor and S6-kinase. Consistently, postsynaptic induction of canonical PHP or TOR activation can compensate for postsynaptic Madm to alleviate functional and structural synaptic defects. Our results provide insights into the molecular mechanisms underlying neurodegeneration-induced PHP with potential neurotherapeutic applications.

AP2 Regulates Thickveins Trafficking to Attenuate NMJ Growth Signaling in Drosophila

Compromised endocytosis in neurons leads to synapse overgrowth and altered organization of synaptic proteins. However, the molecular players and the signaling pathways which regulate the process remain poorly understood. Here, we show that σ2-adaptin, one of the subunits of the AP2-complex, genetically interacts with Mad, Medea and Dad (components of BMP signaling) to control neuromuscular junction (NMJ) growth in Drosophila Ultrastructural analysis of σ2-adaptin mutants show an accumulation of large vesicles and membranous structures akin to endosomes at the synapse. We found that mutations in σ2-adaptin lead to an accumulation of Tkv receptors at the presynaptic membrane. Interestingly, the level of small GTPase Rab11 was significantly reduced in the σ2-adaptin mutant synapses. However, expression of Rab11 does not restore the synaptic defects of σ2-adaptin mutations. We propose a model in which AP2 regulates Tkv internalization and endosomal recycling to control synaptic growth.

Local BMP signaling: A sensor for synaptic activity that balances synapse growth and function

Synapse development is coordinated by intercellular communication between the pre- and postsynaptic compartments, and by neuronal activity itself. In flies as in vertebrates, neuronal activity induces input-specific changes in the synaptic strength so that the entire circuit maintains stable function in the face of many challenges, including changes in synapse number and strength. But how do neurons sense synapse activity? In several studies carried out using the Drosophila neuromuscular junction (NMJ), we demonstrated that local BMP signaling provides an exquisite sensor for synapse activity. Here we review the main features of this exquisite sensor and discuss its functioning beyond monitoring the synapse activity but rather as a key controller that operates in coordination with other BMP signaling pathways to balance synapse growth, maturation and function.

Glia-derived temporal signals orchestrate neurogenesis in the Drosophila mushroom body

Intrinsic mechanisms such as temporal series of transcription factors orchestrate neurogenesis from a limited number of neural progenitors in the brain. Extrinsic regulations, however, remain largely unexplored. Here we describe a two-step glia-derived signal that regulates neurogenesis in the Drosophila mushroom body (MB). In a temporal manner, glial-specific ubiquitin ligase dSmurf activates non-cell-autonomous Hedgehog signaling propagation by targeting the receptor Patched to suppress and promote the exit of MB neuroblast (NB) proliferation, thereby specifying the correct α/β cell number without affecting differentiation. Independent of NB proliferation, dSmurf also stabilizes the expression of the cell-adhesion molecule Fasciclin II (FasII) via its WW domains and regulates FasII homophilic interaction between glia and MB axons to refine α/β-lobe integrity. Our findings provide insights into how extrinsic glia-to-neuron communication coordinates with NB proliferation capacity to regulate MB neurogenesis; glial proteostasis is likely a generalized mechanism in orchestrating neurogenesis.

Ttm50 facilitates calpain activation by anchoring it to calcium stores and increasing its sensitivity to calcium

Calcium-dependent proteolytic calpains are implicated in a variety of physiological processes, as well as pathologies associated with calcium overload. However, the mechanism by which calpain is activated remains elusive since intracellular calcium levels under physiological conditions do not reach the high concentration range required to trigger calpain activation. From a candidate screening using the abundance of the calpain target glutamate receptor GluRIIA at the Drosophila neuromuscular junction as a readout, we uncovered that calpain activity was inhibited upon knockdown of Ttm50, a subunit of the Tim23 complex known to be involved in the import of proteins across the mitochondrial inner membrane. Unexpectedly, Ttm50 and calpain are co-localized at calcium stores Golgi and endoplasmic reticulum (ER), and Ttm50 interacts with calpain via its C-terminal domain. This interaction is required for calpain localization at Golgi/ER, and increases calcium sensitivity of calpain by roughly an order of magnitude. Our findings reveal the regulation of calpain activation by Ttm50, and shed new light on calpain-associated pathologies.

Target-dependent retrograde signaling mediates synaptic plasticity at the Drosophila neuromuscular junction

Neurons that innervate multiple targets often establish synapses with target-specific strengths, and local forms of synaptic plasticity. We have examined the molecular-genetic mechanisms that allow a single Drosophila motoneuron, the ventral Common Exciter (vCE), to establish connections with target-specific properties at its various synaptic partners. By driving transgenes in a subset of vCE's targets, we found that individual target cells are able to independently control the properties of vCE's innervating branch and synapses. This is achieved by means of a trans-synaptic growth factor secreted by the target cell. At the larval neuromuscular junction, postsynaptic glutamate receptor activity stimulates the release of the BMP4/5/6 homolog Glass bottom boat (Gbb). As larvae mature and motoneuron terminals grow, Gbb activates the R-Smad transcriptional regulator phosphorylated Mad (pMad) to facilitate presynaptic development. We found that manipulations affecting glutamate receptors or Gbb within subsets of target muscles led to local effects either specific to the manipulated muscle or by a limited gradient within the presynaptic branches. While presynaptic development depends on pMad transcriptional activity within the motoneuron nucleus, we find that the Gbb growth factor may also act locally within presynaptic terminals. Local Gbb signaling and presynaptic pMad accumulation within boutons may therefore participate in a "synaptic tagging" mechanism, to influence synaptic growth and plasticity in Drosophila.

AWD regulates timed activation of BMP signaling in intestinal stem cells to maintain tissue homeostasis

Precise control of stem cell (SC) proliferation ensures tissue homeostasis. In the Drosophila intestine, injury-induced regeneration involves initial activation of intestinal SC (ISC) proliferation and subsequent return to quiescence. These two phases of the regenerative response are controlled by differential availability of the BMP type I receptor Thickveins (Tkv), yet how its expression is dynamically regulated remains unclear. Here we show that during homeostasis, the E3 ubiquitin ligase Highwire and the ubiquitin-proteasome system maintain low Tkv protein expression. After ISC activation, Tkv is stabilized by proteasome inhibition and undergoes endocytosis due to the induction of the nucleoside diphosphate kinase Abnormal Wing Disc (AWD). Tkv internalization is required for the activation of the Smad protein Mad, and for the return to quiescence after a regenerative episode. Our data provide insight into the mechanisms ensuring tissue homeostasis by dynamic control of somatic stem cell activity.

Regeneration after injury in the Drosophila intestine involves early activation of intestinal stem cells (ISCs) and subsequent return to quiescence. Here the authors show that return to quiescence by ISCs involves BMP Type I receptor Tkv protein stabilization along with AWD mediated internalization into endocytic vesicles.

Tao Negatively Regulates BMP Signaling During Neuromuscular Junction Development in Drosophila

The coordinated growth and development of synapses is critical for all aspects of neural circuit function and mutations that disrupt these processes can result in various neurological defects. Several anterograde and retrograde signaling pathways, including the canonical Bone Morphogenic Protein (BMP) pathway, regulate synaptic development in vertebrates and invertebrates. At the Drosophila larval neuromuscular junction (NMJ), the retrograde BMP pathway is a part of the machinery that controls NMJ expansion concurrent with larval growth. We sought to determine whether the conserved Hippo pathway, critical for proportional growth in other tissues, also functions in NMJ development. We found that neuronal loss of the serine-threonine protein kinase Tao, a regulator of the Hippo signaling pathway, results in supernumerary boutons which contain a normal density of active zones. Tao is also required for proper synaptic function, as reduction of Tao results in NMJs with decreased evoked excitatory junctional potentials. Surprisingly, Tao function in NMJ growth is independent of the Hippo pathway. Instead, our experiments suggest that Tao negatively regulates BMP signaling as reduction of Tao leads to an increase in pMad levels in motor neuron nuclei and an increase in BMP target gene expression. Taken together, these results support a role for Tao as a novel inhibitor of BMP signaling in motor neurons during synaptic development and function.

The autophagy-related gene Atg101 in Drosophila regulates both neuron and midgut homeostasis

Atg101 is an autophagy-related gene identified in worms, flies, mice, and mammals, which encodes a protein that functions in autophagosome formation by associating with the ULK1-Atg13-Fip200 complex. In the last few years, the critical role of Atg101 in autophagy has been well-established through biochemical studies and the determination of its protein structure. However, Atg101's physiological role, both during development and in adulthood, remains less understood. Here, we describe the generation and characterization of an Atg101 loss-of-function mutant in Drosophila and report on the roles of Atg101 in maintaining tissue homeostasis in both adult brains and midguts. We observed that homozygous or hemizygous Atg101 mutants were semi-lethal, with only some of them surviving into adulthood. Both developmental and starvation-induced autophagy processes were defective in the Atg101 mutant animals, and Atg101 mutant adult flies had a significantly shorter lifespan and displayed a mobility defect. Moreover, we observed the accumulation of ubiquitin-positive aggregates in Atg101 mutant brains, indicating a neuronal defect. Interestingly, Atg101 mutant adult midguts were shorter and thicker and exhibited abnormal morphology with enlarged enterocytes. Detailed analysis also revealed that the differentiation from intestinal stem cells to enterocytes was impaired in these midguts. Cell type-specific rescue experiments disclosed that Atg101 had a function in enterocytes and limited their growth. In summary, the results of our study indicate that Drosophila Atg101 is essential for tissue homeostasis in both adult brains and midguts. We propose that Atg101 may have a role in age-related processes.

Calcium-Activated Calpain Specifically Cleaves Glutamate Receptor IIA But Not IIB at the Drosophila Neuromuscular Junction

Calpains are calcium-dependent, cytosolic proteinases active at neutral pH. They do not degrade but cleave substrates at limited sites. Calpains are implicated in various pathologies, such as ischemia, injuries, muscular dystrophy, and neurodegeneration. Despite so, the physiological function of calpains remains to be clearly defined. Using the neuromuscular junction of Drosophila of both sexes as a model, we performed RNAi screening and uncovered that calpains negatively regulated protein levels of the glutamate receptor GluRIIA but not GluRIIB. We then showed that calpains enrich at the postsynaptic area, and the calcium-dependent activation of calpains induced cleavage of GluRIIA at Q788 of its C terminus. Further genetic and biochemical experiments revealed that different calpains genetically and physically interact to form a protein complex. The protein complex was required for the proteinase activation to downregulate GluRIIA. Our data provide a novel insight into the mechanisms by which different calpains act together as a complex to specifically control GluRIIA levels and consequently synaptic function.SIGNIFICANCE STATEMENT Calpain has been implicated in neural insults and neurodegeneration. However, the physiological function of calpains in the nervous system remains to be defined. Here, we show that calpain enriches at the postsynaptic area and negatively and specifically regulates GluRIIA, but not IIB, level during development. Calcium-dependent activation of calpain cleaves GluRIIA at Q788 of its C terminus. Different calpains constitute an active protease complex to cleave its target. This study reveals a critical role of calpains during development to specifically cleave GluRIIA at synapses and consequently regulate synaptic function.

Distinct functions of a cGMP-dependent protein kinase in nerve terminal growth and synaptic vesicle cycling

Sustained neurotransmission requires the tight coupling of synaptic vesicle (SV) exocytosis and endocytosis. The mechanisms underlying this coupling are poorly understood. We tested the hypothesis that a cGMP-dependent protein kinase (PKG), encoded by the foraging (for) gene in Drosophila melanogaster, is critical for this process using a for null mutant, genomic rescues, and tissue specific rescues. We uncoupled FOR's exocytic and endocytic functions in neurotransmission using a temperature-sensitive shibire mutant in conjunction with fluorescein-assisted light inactivation of FOR. We discovered a dual role for presynaptic FOR, where FOR inhibits SV exocytosis during low frequency stimulation by negatively regulating presynaptic Ca2+ levels and maintains neurotransmission during high frequency stimulation by facilitating SV endocytosis. Additionally, glial FOR negatively regulated nerve terminal growth through TGF-β signaling and this developmental effect was independent from FOR's effects on neurotransmission. Overall, FOR plays a critical role in coupling SV exocytosis and endocytosis, thereby balancing these two components to maintain sustained neurotransmission.

BMP signaling downstream of the Highwire E3 ligase sensitizes nociceptors

A comprehensive understanding of the molecular machinery important for nociception is essential to improving the treatment of pain. Here, we show that the BMP signaling pathway regulates nociception downstream of the E3 ubiquitin ligase highwire (hiw). hiw loss of function in nociceptors caused antagonistic and pleiotropic phenotypes with simultaneous insensitivity to noxious heat but sensitized responses to optogenetic activation of nociceptors. Thus, hiw functions to both positively and negatively regulate nociceptors. We find that a sensory reception-independent sensitization pathway was associated with BMP signaling. BMP signaling in nociceptors was up-regulated in hiw mutants, and nociceptor-specific expression of hiw rescued all nociception phenotypes including the increased BMP signaling. Blocking the transcriptional output of the BMP pathway with dominant negative Mad suppressed nociceptive hypersensitivity that was induced by interfering with hiw. The up-regulated BMP signaling phenotype in hiw genetic mutants could not be suppressed by mutation in wallenda suggesting that hiw regulates BMP in nociceptors via a wallenda independent pathway. In a newly established Ca²⁺ imaging preparation, we observed that up-regulated BMP signaling caused a significantly enhanced Ca²⁺ signal in the axon terminals of nociceptors that were stimulated by noxious heat. This response likely accounts for the nociceptive hypersensitivity induced by elevated BMP signaling in nociceptors. Finally, we showed that 24-hour activation of BMP signaling in nociceptors was sufficient to sensitize nociceptive responses to optogenetically-triggered nociceptor activation without altering nociceptor morphology. Overall, this study demonstrates the previously unrevealed roles of the Hiw-BMP pathway in the regulation of nociception and provides the first direct evidence that up-regulated BMP signaling physiologically sensitizes responses of nociceptors and nociception behaviors.

Although pain is a universally experienced sensation that has a significant impact on human lives and society, the molecular mechanisms of pain remain poorly understood. Elucidating these mechanisms is particularly important to gaining insight into the clinical development of currently incurable chronic pain diseases. Taking an advantage of the powerful genetic model organism Drosophila melanogaster (fruit flies), we unveil the Highwire-BMP signaling pathway as a novel molecular pathway that regulates the sensitivity of nociceptive sensory neurons. Highwire and the molecular components of the BMP signaling pathway are known to be widely conserved among animal phyla, from nematode worms to humans. Since abnormal sensitivity of nociceptive sensory neurons can play a critical role in the development of chronic pain conditions, a deeper understanding of the regulation of nociceptor sensitivity has the potential to advance effective therapeutic strategies to treat difficult pain conditions.

Neurexin-Neuroligin 1 regulates synaptic morphology and functions via the WAVE regulatory complex in Drosophila neuromuscular junction

Neuroligins are postsynaptic adhesion molecules that are essential for postsynaptic specialization and synaptic function. But the underlying molecular mechanisms of neuroligin functions remain unclear. We found that Drosophila Neuroligin 1 (DNlg1) regulates synaptic structure and function through WAVE regulatory complex (WRC)-mediated postsynaptic actin reorganization. The disruption of DNlg1, DNlg2, or their presynaptic partner neurexin (DNrx) led to a dramatic decrease in the amount of F-actin. Further study showed that DNlg1, but not DNlg2 or DNlg3, directly interacts with the WRC via its C-terminal interacting receptor sequence. That interaction is required to recruit WRC to the postsynaptic membrane to promote F-actin assembly. Furthermore, the interaction between DNlg1 and the WRC is essential for DNlg1 to rescue the morphological and electrophysiological defects in dnlg1 mutants. Our results reveal a novel mechanism by which the DNrx-DNlg1 trans-synaptic interaction coordinates structural and functional properties at the neuromuscular junction.

The Rap activator Gef26 regulates synaptic growth and neuronal survival via inhibition of BMP signaling

In Drosophila, precise regulation of BMP signaling is essential for normal synaptic growth at the larval neuromuscular junction (NMJ) and neuronal survival in the adult brain. However, the molecular mechanisms underlying fine-tuning of BMP signaling in neurons remain poorly understood. We show that loss of the Drosophila PDZ guanine nucleotide exchange factor Gef26 significantly increases synaptic growth at the NMJ and enhances BMP signaling in motor neurons. We further show that Gef26 functions upstream of Rap1 in motor neurons to restrain synaptic growth. Synaptic overgrowth in gef26 or rap1 mutants requires BMP signaling, indicating that Gef26 and Rap1 regulate synaptic growth via inhibition of BMP signaling. We also show that Gef26 is involved in the endocytic downregulation of surface expression of the BMP receptors thickveins (Tkv) and wishful thinking (Wit). Finally, we demonstrate that loss of Gef26 also induces progressive brain neurodegeneration through Rap1- and BMP signaling-dependent mechanisms. Taken together, these results suggest that the Gef26-Rap1 signaling pathway regulates both synaptic growth and neuronal survival by controlling BMP signaling.

Neuroligin 4 regulates synaptic growth via the bone morphogenetic protein (BMP) signaling pathway at the Drosophila neuromuscular junction

The neuroligin (Nlg) family of neural cell adhesion molecules is thought to be required for synapse formation and development and has been linked to the development of autism spectrum disorders in humans. In Drosophila melanogaster, mutations in the neuroligin 1–3 genes have been reported to induce synapse developmental defects at neuromuscular junctions (NMJs), but the role of neuroligin 4 (dnlg4) in synapse development has not been determined. Here, we report that the Drosophila neuroligin 4 (DNlg4) is different from DNlg1–3 in that it presynaptically regulates NMJ synapse development. Loss of dnlg4 results in reduced growth of NMJs with fewer synaptic boutons. The morphological defects caused by dnlg4 mutant are associated with a corresponding decrease in synaptic transmission efficacy. All of these defects could only be rescued when DNlg4 was expressed in the presynapse of NMJs. To understand the basis of DNlg4 function, we looked for genetic interactions and found connections with the components of the bone morphogenetic protein (BMP) signaling pathway. Immunostaining and Western blot analyses demonstrated that the regulation of NMJ growth by DNlg4 was due to the positive modulation of BMP signaling by DNlg4. Specifically, BMP type I receptor thickvein (Tkv) abundance was reduced in dnlg4 mutants, and immunoprecipitation assays showed that DNlg4 and Tkv physically interacted in vivo. Our study demonstrates that DNlg4 presynaptically regulates neuromuscular synaptic growth via the BMP signaling pathway by modulating Tkv.

Secreted tissue inhibitor of matrix metalloproteinase restricts trans-synaptic signaling to coordinate synaptogenesis

Synaptogenesis is coordinated by trans-synaptic signals that traverse the specialized synaptomatrix between presynaptic and postsynaptic cells. Matrix metalloproteinase (Mmp) activity sculpts this environment, balanced by secreted tissue inhibitors of Mmp (Timp). Here, we use the simplified Drosophila melanogaster matrix metalloproteome to test the consequences of eliminating all Timp regulatory control of Mmp activity at the neuromuscular junction (NMJ). Using in situ zymography, we find Timp limits Mmp activity at the NMJ terminal and shapes extracellular proteolytic dynamics surrounding individual synaptic boutons. In newly generated timp null mutants, NMJs exhibit architectural overelaboration with supernumerary synaptic boutons. With cell-targeted RNAi and rescue studies, we find that postsynaptic Timp limits presynaptic architecture. Functionally, timp null mutants exhibit compromised synaptic vesicle cycling, with activity that is lower in amplitude and fidelity. NMJ defects manifest in impaired locomotor function. Mechanistically, we find that Timp limits BMP trans-synaptic signaling and the downstream synapse-to-nucleus signal transduction. Pharmacologically restoring Mmp inhibition in timp null mutants corrects bone morphogenetic protein (BMP) signaling and synaptic properties. Genetically restoring BMP signaling in timp null mutants corrects NMJ structure and motor function. Thus, Timp inhibition of Mmp proteolytic activity restricts BMP trans-synaptic signaling to coordinate synaptogenesis.

Presynaptic morphogenesis, active zone organization and structural plasticity in Drosophila

Effective adaptation of neural circuit function to a changing environment requires many forms of plasticity. Among these, structural plasticity is one of the most durable, and is also an intrinsic part of the developmental logic for the formation and refinement of synaptic connectivity. Structural plasticity of presynaptic sites can involve the addition, remodeling, or removal of pre- and post-synaptic elements. However, this requires coordination of morphogenesis and assembly of the subcellular machinery for neurotransmitter release within the presynaptic neuron, as well as coordination of these events with the postsynaptic cell. While much progress has been made in revealing the cell biological mechanisms of postsynaptic structural plasticity, our understanding of presynaptic mechanisms is less complete.

Angelman Syndrome Protein Ube3a Regulates Synaptic Growth and Endocytosis by Inhibiting BMP Signaling in Drosophila

Altered expression of the E3 ubiquitin ligase UBE3A, which is involved in protein degradation through the proteasome-mediated pathway, is associated with neurodevelopmental and behavioral defects observed in Angelman syndrome (AS) and autism. However, little is known about the neuronal function of UBE3A and the pathogenesis of UBE3A-associated disorders. To understand the in vivo function of UBE3A in the nervous system, we generated multiple mutations of ube3a, the Drosophila ortholog of UBE3A. We found a significantly increased number of total boutons and satellite boutons in conjunction with compromised endocytosis in the neuromuscular junctions (NMJs) of ube3a mutants compared to the wild type. Genetic and biochemical analysis showed upregulation of bone morphogenetic protein (BMP) signaling in the nervous system of ube3a mutants. An immunochemical study revealed a specific increase in the protein level of Thickveins (Tkv), a type I BMP receptor, but not other BMP receptors Wishful thinking (Wit) and Saxophone (Sax), in ube3a mutants. Ube3a was associated with and specifically ubiquitinated lysine 227 within the cytoplasmic tail of Tkv, and promoted its proteasomal degradation in Schneider 2 cells. Negative regulation of Tkv by Ube3a was conserved in mammalian cells. These results reveal a critical role for Ube3a in regulating NMJ synapse development by repressing BMP signaling. This study sheds new light onto the neuronal functions of UBE3A and provides novel perspectives for understanding the pathogenesis of UBE3A-associated disorders.

The Crossroads of Synaptic Growth Signaling, Membrane Traffic and Neurological Disease: Insights from Drosophila

Neurons require target-derived autocrine and paracrine growth factors to maintain proper identity, innervation, homeostasis and survival. Neuronal growth factor signaling is highly dependent on membrane traffic, both for the packaging and release of the growth factors themselves, and for regulation of intracellular signaling by their transmembrane receptors. Here, we review recent findings from the Drosophila larval neuromuscular junction (NMJ) that illustrate how specific steps of intracellular traffic and inter-organelle interactions impinge on signaling, particularly in the bone morphogenic protein, Wingless and c-Jun-activated kinase pathways, regulating elaboration and stability of NMJ arbors, construction of synapses and synaptic transmission and homeostasis. These membrane trafficking and signaling pathways have been implicated in human motor neuron diseases including amyotrophic lateral sclerosis and hereditary spastic paraplegia, highlighting their importance for neuronal health and survival.