The BMP ligand Gbb gates the expression of synaptic homeostasis independent of synaptic growth control

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.

Syncrip/hnRNP Q influences synaptic transmission and regulates BMP signaling at the Drosophila neuromuscular synapse

Synaptic plasticity involves the modulation of synaptic connections in response to neuronal activity via multiple pathways. One mechanism modulates synaptic transmission by retrograde signals from the post-synapse that influence the probability of vesicle release in the pre-synapse. Despite its importance, very few factors required for the expression of retrograde signals, and proper synaptic transmission, have been identified. Here, we identify the conserved RNA binding protein Syncrip as a new factor that modulates the efficiency of vesicle release from the motoneuron and is required for correct synapse structure. We show that syncrip is required genetically and its protein product is detected only in the muscle and not in the motoneuron itself. This unexpected non-autonomy is at least partly explained by the fact that Syncrip modulates retrograde BMP signals from the muscle back to the motoneuron. We show that Syncrip influences the levels of the Bone Morphogenic Protein ligand Glass Bottom Boat from the post-synapse and regulates the pre-synapse. Our results highlight the RNA-binding protein Syncrip as a novel regulator of synaptic output. Given its known role in regulating translation, we propose that Syncrip is important for maintaining a balance between the strength of presynaptic vesicle release and postsynaptic translation.

Endostatin is a trans-synaptic signal for homeostatic synaptic plasticity

At synapses in organisms ranging from fly to human, a decrease in postsynaptic neurotransmitter receptor function elicits a homeostatic increase in presynaptic release that restores baseline synaptic efficacy. This process, termed presynaptic homeostasis, requires a retrograde, trans-synaptic signal of unknown identity. In a forward genetic screen for homeostatic plasticity genes, we identified multiplexin. Multiplexin is the Drosophila homolog of Collagen XV/XVIII, a matrix protein that can be proteolytically cleaved to release Endostatin, an antiangiogenesis signaling factor. Here we demonstrate that Multiplexin is required for normal calcium channel abundance, presynaptic calcium influx, and neurotransmitter release. Remarkably, Endostatin has a specific activity, independent of baseline synapse development, that is required for the homeostatic modulation of presynaptic calcium influx and neurotransmitter release. Our data support a model in which proteolytic release of Endostatin signals trans-synaptically, acting in concert with the presynaptic CaV2.1 calcium channel, to promote presynaptic homeostasis.

Retrograde BMP signaling modulates rapid activity-dependent synaptic growth via presynaptic LIM kinase regulation of cofilin

The Drosophila neuromuscular junction (NMJ) is capable of rapidly budding new presynaptic varicosities over the course of minutes in response to elevated neuronal activity. Using live imaging of synaptic growth, we characterized this dynamic process and demonstrated that rapid bouton budding requires retrograde bone morphogenic protein (BMP) signaling and local alteration in the presynaptic actin cytoskeleton. BMP acts during development to provide competence for rapid synaptic growth by regulating the levels of the Rho-type guanine nucleotide exchange factor Trio, a transcriptional output of BMP-Smad signaling. In a parallel pathway, we find that the BMP type II receptor Wit signals through the effector protein LIM domain kinase 1 (Limk) to regulate bouton budding. Limk interfaces with structural plasticity by controlling the activity of the actin depolymerizing protein Cofilin. Expression of constitutively active or inactive Cofilin in motor neurons demonstrates that increased Cofilin activity promotes rapid bouton formation in response to elevated synaptic activity. Correspondingly, the overexpression of Limk, which inhibits Cofilin, inhibits bouton budding. Live imaging of the presynaptic F-actin cytoskeleton reveals that activity-dependent bouton addition is accompanied by the formation of new F-actin puncta at sites of synaptic growth. Pharmacological disruption of actin turnover inhibits bouton budding, indicating that local changes in the actin cytoskeleton at pre-existing boutons precede new budding events. We propose that developmental BMP signaling potentiates NMJs for rapid activity-dependent structural plasticity that is achieved by muscle release of retrograde signals that regulate local presynaptic actin cytoskeletal dynamics.

The less things change, the more they are different: contributions of long-term synaptic plasticity and homeostasis to memory

An important cellular mechanism contributing to the strength and duration of memories is activity-dependent alterations in the strength of synaptic connections within the neural circuit encoding the memory. Reversal of the memory is typically correlated with a reversal of the cellular changes to levels expressed prior to the stimulation. Thus, for stimulus-induced changes in synapse strength and their reversals to be functionally relevant, cellular mechanisms must regulate and maintain synapse strength both prior to and after the stimuli inducing learning and memory. The strengths of synapses within a neural circuit at any given moment are determined by cellular and molecular processes initiated during development and those subsequently regulated by the history of direct activation of the neural circuit and system-wide stimuli such as stress or motivational state. The cumulative actions of stimuli and other factors on an already modified neural circuit are attenuated by homeostatic mechanisms that prevent changes in overall synaptic inputs and excitability above or below specific set points (synaptic scaling). The mechanisms mediating synaptic scaling prevent potential excitotoxic alterations in the circuit but also may attenuate additional cellular changes required for learning and memory, thereby apparently limiting information storage. What cellular and molecular processes control synaptic strengths before and after experience/activity and its reversals? In this review we will explore the synapse-, whole cell-, and circuit level-specific processes that contribute to an overall zero sum-like set of changes and long-term maintenance of synapse strengths as a consequence of the accommodative interactions between long-term synaptic plasticity and homeostasis.

How voltage-gated calcium channels gate forms of homeostatic synaptic plasticity

Throughout life, animals face a variety of challenges such as developmental growth, the presence of toxins, or changes in temperature. Neuronal circuits and synapses respond to challenges by executing an array of neuroplasticity paradigms. Some paradigms allow neurons to up- or downregulate activity outputs, while countervailing ones ensure that outputs remain within appropriate physiological ranges. A growing body of evidence suggests that homeostatic synaptic plasticity (HSP) is critical in the latter case. Voltage-gated calcium channels gate forms of HSP. Presynaptically, the aggregate data show that when synapse activity is weakened, homeostatic signaling systems can act to correct impairments, in part by increasing calcium influx through presynaptic CaV2-type channels. Increased calcium influx is often accompanied by parallel increases in the size of active zones and the size of the readily releasable pool of presynaptic vesicles. These changes coincide with homeostatic enhancements of neurotransmitter release. Postsynaptically, there is a great deal of evidence that reduced network activity and loss of calcium influx through CaV1-type calcium channels also results in adaptive homeostatic signaling. Some adaptations drive presynaptic enhancements of vesicle pool size and turnover rate via retrograde signaling, as well as de novo insertion of postsynaptic neurotransmitter receptors. Enhanced calcium influx through CaV1 after network activation or single cell stimulation can elicit the opposite response-homeostatic depression via removal of excitatory receptors. There exist intriguing links between HSP and calcium channelopathies-such as forms of epilepsy, migraine, ataxia, and myasthenia. The episodic nature of some of these disorders suggests alternating periods of stable and unstable function. Uncovering information about how calcium channels are regulated in the context of HSP could be relevant toward understanding these and other disorders.

Retrograde BMP signaling at the synapse: a permissive signal for synapse maturation and activity-dependent plasticity

At the Drosophila neuromuscular junction (NMJ), the loss of retrograde, trans-synaptic BMP signaling causes motoneuron terminals to have fewer synaptic boutons, whereas increased neuronal activity results in a larger synapse with more boutons. Here, we show that an early and transient BMP signal is necessary and sufficient for NMJ growth as well as for activity-dependent synaptic plasticity. This early critical period was revealed by the temporally controlled suppression of Mad, the SMAD1 transcriptional regulator. Similar results were found by genetic rescue tests involving the BMP4/5/6 ligand Glass bottom boat (Gbb) in muscle, and alternatively the type II BMP receptor Wishful Thinking (Wit) in the motoneuron. These observations support a model where the muscle signals back to the innervating motoneuron's nucleus to activate presynaptic programs necessary for synaptic growth and activity-dependent plasticity. Molecular genetic gain- and loss-of-function studies show that genes involved in NMJ growth and plasticity, including the adenylyl cyclase Rutabaga, the Ig-CAM Fasciclin II, the transcription factor AP-1 (Fos/Jun), and the adhesion protein Neurexin, all depend critically on the canonical BMP pathway for their effects. By contrast, elevated expression of Lar, a receptor protein tyrosine phosphatase found to be necessary for activity-dependent plasticity, rescued the phenotypes associated with the loss of Mad signaling. We also find that synaptic structure and function develop using genetically separable, BMP-dependent mechanisms. Although synaptic growth depended on Lar and the early, transient BMP signal, the maturation of neurotransmitter release was independent of Lar and required later, ongoing BMP signaling.

Homeostatic signaling and the stabilization of neural function

The brain is astonishing in its complexity and capacity for change. This has fascinated scientists for more than a century, filling the pages of this journal for the past 25 years. But a paradigm shift is underway. It seems likely that the plasticity that drives our ability to learn and remember can only be meaningful in the context of otherwise stable, reproducible, and predictable baseline neural function. Without the existence of potent mechanisms that stabilize neural function, our capacity to learn and remember would be lost in the chaos of daily experiential change. This underscores two great mysteries in neuroscience. How are the functional properties of individual neurons and neural circuits stably maintained throughout life? And, in the face of potent stabilizing mechanisms, how can neural circuitry be modified during neural development, learning, and memory? Answers are emerging in the rapidly developing field of homeostatic plasticity.

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.

A presynaptic ENaC channel drives homeostatic plasticity

An electrophysiology-based forward genetic screen has identified two genes, pickpocket11 (ppk11) and pickpocket16 (ppk16), as being necessary for the homeostatic modulation of presynaptic neurotransmitter release at the Drosophila neuromuscular junction (NMJ). Pickpocket genes encode Degenerin/Epithelial Sodium channel subunits (DEG/ENaC). We demonstrate that ppk11 and ppk16 are necessary in presynaptic motoneurons for both the acute induction and long-term maintenance of synaptic homeostasis. We show that ppk11 and ppk16 are cotranscribed as a single mRNA that is upregulated during homeostatic plasticity. Acute pharmacological inhibition of a PPK11- and PPK16-containing channel abolishes the expression of short- and long-term homeostatic plasticity without altering baseline presynaptic neurotransmitter release, indicating remarkable specificity for homeostatic plasticity rather than NMJ development. Finally, presynaptic calcium imaging experiments support a model in which a PPK11- and PPK16-containing DEG/ENaC channel modulates presynaptic membrane voltage and, thereby, controls calcium channel activity to homeostatically regulate neurotransmitter release.

Homeostatic plasticity at the Drosophila neuromuscular junction

In biology, homeostasis refers to how cells maintain appropriate levels of activity. This concept underlies a balancing act in the nervous system. Synapses require flexibility (i.e. plasticity) to adjust to environmental challenges. Yet there must also exist regulatory mechanisms that constrain activity within appropriate physiological ranges. An abundance of evidence suggests that homeostatic regulation is critical in this regard. In recent years, important progress has been made toward identifying molecules and signaling processes required for homeostatic forms of neuroplasticity. The Drosophila melanogaster third instar larval neuromuscular junction (NMJ) has been an important experimental system in this effort. Drosophila neuroscientists combine genetics, pharmacology, electrophysiology, imaging, and a variety of molecular techniques to understand how homeostatic signaling mechanisms take shape at the synapse. At the NMJ, homeostatic signaling mechanisms couple retrograde (muscle-to-nerve) signaling with changes in presynaptic calcium influx, changes in the dynamics of the readily releasable vesicle pool, and ultimately, changes in presynaptic neurotransmitter release. Roles in these processes have been demonstrated for several molecules and signaling systems discussed here. This review focuses primarily on electrophysiological studies or data. In particular, attention is devoted to understanding what happens when NMJ function is challenged (usually through glutamate receptor inhibition) and the resulting homeostatic responses. A significant area of study not covered in this review, for the sake of simplicity, is the homeostatic control of synapse growth, which naturally, could also impinge upon synapse function in myriad ways. This article is part of the Special Issue entitled 'Homeostatic Synaptic Plasticity'.

The Drosophila BMPRII, wishful thinking, is required for eggshell patterning

The Drosophila eggshell is an elaborate structure that is derived from a monolayer of follicular epithelium surrounding the developing oocyte within the female ovary. The bone morphogenetic protein (BMP) signaling pathway is essential for controlling the patterning and morphogenesis of the eggshell. During oogenesis, the roles of patterning and morphogenesis by the BMP type I receptor thickveins (tkv) have been studied extensively. However, signaling through this pathway requires both type I and II receptors, and the latter has yet to be established in oogenesis. We focus on wishful thinking (wit), the Drosophila homolog to the mammalian BMP type II receptor, BMPRII. We found that wit is expressed dynamically in the FCs of D. melanogaster in an evolutionary conserved pattern. The expression patterns are highly correlated with the dynamics of the BMP signaling, which is consistent with our finding that wit is a target of BMP signaling. Furthermore, we established that WIT is necessary for BMP signaling, and loss of WIT is associated with cell autonomous loss of BMP responses. Of importance, we demonstrated that perturbations in WIT led to changes in eggshell morphologies in domains that are patterned by BMP signaling. Previous studies have shown a role for WIT in BMP signaling during neurogenesis; however, our results reveal a role for WIT in epithelial cells' development.

Synaptic retinoic acid signaling and homeostatic synaptic plasticity

One of the defining features of the nervous system is its ability to modify synaptic strength in an experience-dependent manner. Chronic elevation or reduction of network activity activates compensatory mechanisms that modulate synaptic strength in the opposite direction (i.e. reduced network activity leads to increased synaptic strength), a process called homeostatic synaptic plasticity. Among the many mechanisms that mediate homeostatic synaptic plasticity, retinoic acid (RA) has emerged as a novel signaling molecule that is critically involved in homeostatic synaptic plasticity induced by blockade of synaptic activity. In neurons, silencing of synaptic transmission triggers RA synthesis. RA then acts at synapses by a non-genomic mechanism that is independent of its well-known function as a transcriptional regulator, but operates through direct activation of protein translation in neuronal dendrites. Protein synthesis is activated by RA-binding to its receptor RARα, which functions locally in dendrites in a non-canonical manner as an RNA-binding protein that mediate RA's effect on translation. The present review will discuss recent progress in our understanding of the novel role of RA, which led to the identification of RA as a critical synaptic signaling molecule that mediates activity-dependent regulation of protein synthesis in neuronal dendrites. This article is part of the Special Issue entitled 'Homeostatic Synaptic Plasticity'.

A targeted glycan-related gene screen reveals heparan sulfate proteoglycan sulfation regulates WNT and BMP trans-synaptic signaling

A Drosophila transgenic RNAi screen targeting the glycan genome, including all N/O/GAG-glycan biosynthesis/modification enzymes and glycan-binding lectins, was conducted to discover novel glycan functions in synaptogenesis. As proof-of-product, we characterized functionally paired heparan sulfate (HS) 6-O-sulfotransferase (hs6st) and sulfatase (sulf1), which bidirectionally control HS proteoglycan (HSPG) sulfation. RNAi knockdown of hs6st and sulf1 causes opposite effects on functional synapse development, with decreased (hs6st) and increased (sulf1) neurotransmission strength confirmed in null mutants. HSPG co-receptors for WNT and BMP intercellular signaling, Dally-like Protein and Syndecan, are differentially misregulated in the synaptomatrix of these mutants. Consistently, hs6st and sulf1 nulls differentially elevate both WNT (Wingless; Wg) and BMP (Glass Bottom Boat; Gbb) ligand abundance in the synaptomatrix. Anterograde Wg signaling via Wg receptor dFrizzled2 C-terminus nuclear import and retrograde Gbb signaling via synaptic MAD phosphorylation and nuclear import are differentially activated in hs6st and sulf1 mutants. Consequently, transcriptional control of presynaptic glutamate release machinery and postsynaptic glutamate receptors is bidirectionally altered in hs6st and sulf1 mutants, explaining the bidirectional change in synaptic functional strength. Genetic correction of the altered WNT/BMP signaling restores normal synaptic development in both mutant conditions, proving that altered trans-synaptic signaling causes functional differentiation defects.

Jelly Belly trans-synaptic signaling to anaplastic lymphoma kinase regulates neurotransmission strength and synapse architecture

In Drosophila, the secreted signaling molecule Jelly Belly (Jeb) activates anaplastic lymphoma kinase (Alk), a receptor tyrosine kinase, in multiple developmental and adult contexts. We have shown previously that Jeb and Alk are highly enriched at Drosophila synapses within the CNS neuropil and neuromuscular junction (NMJ) and postulated a conserved intercellular signaling function. At the embryonic and larval NMJ, Jeb is localized in the motor neuron presynaptic terminal whereas Alk is concentrated in the muscle postsynaptic domain surrounding boutons, consistent with anterograde trans-synaptic signaling. Here, we show that neurotransmission is regulated by Jeb secretion by functional inhibition of Jeb-Alk signaling. Jeb is a novel negative regulator of neuromuscular transmission. Reduction or inhibition of Alk function results in enhanced synaptic transmission. Activation of Alk conversely inhibits synaptic transmission. Restoration of wild-type postsynaptic Alk expression in Alk partial loss-of-function mutants rescues NMJ transmission phenotypes and confirms that postsynaptic Alk regulates NMJ transmission. The effects of impaired Alk signaling on neurotransmission are observed in the absence of associated changes in NMJ structure. Complete removal of Jeb in motor neurons, however, disrupts both presynaptic bouton architecture and postsynaptic differentiation. Nonphysiologic activation of Alk signaling also negatively regulates NMJ growth. Activation of Jeb-Alk signaling triggers the Ras-MAP kinase cascade in both pre- and postsynaptic compartments. These novel roles for Jeb-Alk signaling in the modulation of synaptic function and structure have potential implications for recently reported Alk functions in human addiction, retention of spatial memory, cognitive dysfunction in neurofibromatosis, and pathogenesis of amyotrophic lateral sclerosis.

Activity-dependent retrograde laminin A signaling regulates synapse growth at Drosophila neuromuscular junctions

Retrograde signals induced by synaptic activities are derived from postsynaptic cells to potentiate presynaptic properties, such as cytoskeletal dynamics, gene expression, and synaptic growth. However, it is not known whether activity-dependent retrograde signals can also depotentiate synaptic properties. Here we report that laminin A (LanA) functions as a retrograde signal to suppress synapse growth at Drosophila neuromuscular junctions (NMJs). The presynaptic integrin pathway consists of the integrin subunit βν and focal adhesion kinase 56 (Fak56), both of which are required to suppress crawling activity-dependent NMJ growth. LanA protein is localized in the synaptic cleft and only muscle-derived LanA is functional in modulating NMJ growth. The LanA level at NMJs is inversely correlated with NMJ size and regulated by larval crawling activity, synapse excitability, postsynaptic response, and anterograde Wnt/Wingless signaling, all of which modulate NMJ growth through LanA and βν. Our data indicate that synaptic activities down-regulate levels of the retrograde signal LanA to promote NMJ growth.

DBL-1, a TGF-β, is essential for Caenorhabditis elegans aversive olfactory learning

The TGF-β superfamily is conserved throughout metazoan, and its members play essential roles in development and disease. TGF-β has also been implicated in adult neural plasticity. However, the underlying mechanisms are not well understood. Here we report that DBL-1, a Caenorhabditis elegans TGF-β homolog known to control body morphology and immunity, is essential for aversive olfactory learning of potentially harmful bacteria food. We show that DBL-1 generated by the AVA command interneurons, which are critical for sensorimotor responses, regulates aversive olfactory learning, and that the activity of the type I TGF-β receptor SMA-6 in the hypodermis is needed during adulthood to generate olfactory plasticity. These spatial and temporal mechanisms are critical for the DBL-1 signaling to achieve its diverse functions in development and adult neural plasticity. Interestingly, aversive training decreases AVA calcium response, leading to an increase in the DBL-1 signal secreted from AVA, revealing an experience-dependent change that can underlie the role of TGF-β signaling in mediating plasticity.

Wnt signaling in neuromuscular junction development

Wnt proteins are best known for their profound roles in cell patterning, because they are required for the embryonic development of all animal species studied to date. Besides regulating cell fate, Wnt proteins are gaining increasing recognition for their roles in nervous system development and function. New studies indicate that multiple positive and negative Wnt signaling pathways take place simultaneously during the formation of vertebrate and invertebrate neuromuscular junctions. Although some Wnts are essential for the formation of NMJs, others appear to play a more modulatory role as part of multiple signaling pathways. Here we review the most recent findings regarding the function of Wnts at the NMJ from both vertebrate and invertebrate model systems.

BMP receptor-activated Smads confer diverse functions during the development of the dorsal spinal cord

Bone Morphogenetic Proteins (BMPs) have multiple activities in the developing spinal cord: they specify the identity of the dorsal-most neuronal populations and then direct the trajectories of dorsal interneuron (dI) 1 commissural axons. How are these activities decoded by dorsal neurons to result in different cellular outcomes? Our previous studies have shown that the diverse functions of the BMPs are mediated by the canonical family of BMP receptors and then regulated by specific inhibitory (I) Smads, which block the activity of a complex of Smad second messengers. However, the extent to which this complex translates the different activities of the BMPs in the spinal cord has remained unresolved. Here, we demonstrate that the receptor-activated (R) Smads, Smad1 and Smad5 play distinct roles mediating the abilities of the BMPs to direct cell fate specification and axon outgrowth. Smad1 and Smad5 occupy spatially distinct compartments within the spinal cord, with Smad5 primarily associated with neural progenitors and Smad1 with differentiated neurons. Consistent with this expression profile, loss of function experiments in mouse embryos reveal that Smad5 is required for the acquisition of dorsal spinal neuron identities whereas Smad1 is critical for the regulation of dI1 axon outgrowth. Thus the R-Smads, like the I-Smads, have discrete roles mediating BMP-dependent cellular processes during spinal interneuron development.

BMPs are mediators in tissue crosstalk of the regenerating musculoskeletal system

The musculoskeletal system is a tight network of many tissues. Coordinated interplay at a biochemical level between tissues is essential for development and repair. Traumatic injury usually affects several tissues and represents a large challenge in clinical settings. The current demand for potent growth factors in such applications thus accompanies the keen interest in molecular mechanisms and orchestration of tissue formation. Of special interest are multitasking growth factors that act as signals in a variety of cell types, both in a paracrine and in an autocrine manner, thereby inducing cell differentiation and coordinating not only tissue assembly at specific sites but also maturation and homeostasis. We concentrate here on bone morphogenetic proteins (BMPs), which are important crosstalk mediators known for their irreplaceable roles in vertebrate development. The molecular crosstalk during embryonic musculoskeletal tissue formation is recapitulated in adult repair. BMPs act at different levels from the initiation to maturation of newly formed tissue. Interestingly, this is influenced by the spatiotemporal expression of different BMPs, their receptors and co-factors at the site of repair. Thus, the regenerative potential of BMPs needs to be evaluated in the context of highly connected tissues such as muscle and bone and might indeed be different in more poorly connected tissues such as cartilage. This highlights the need for an understanding of BMP signaling across tissues in order to eventually improve BMP regenerative potential in clinical applications. In this review, the distinct members of the BMP family and their individual contribution to musculoskeletal tissue repair are summarized by focusing on their paracrine and autocrine functions.

Experimental methods for examining synaptic plasticity in Drosophila

The Drosophila neuromuscular junction (NMJ) ranks as one of the preeminent model systems for studying synaptic development, function, and plasticity. In this article, we review the experimental genetic methods that include the use of mutated or reengineered ion channels to manipulate the synaptic connections made by motor neurons onto larval body-wall muscles. We also provide a consideration of environmental and rearing conditions that phenocopy some of the genetic manipulations.

Homeostatic synaptic plasticity: local and global mechanisms for stabilizing neuronal function

Neural circuits must maintain stable function in the face of many plastic challenges, including changes in synapse number and strength, during learning and development. Recent work has shown that these destabilizing influences are counterbalanced by homeostatic plasticity mechanisms that act to stabilize neuronal and circuit activity. One such mechanism is synaptic scaling, which allows neurons to detect changes in their own firing rates through a set of calcium-dependent sensors that then regulate receptor trafficking to increase or decrease the accumulation of glutamate receptors at synaptic sites. Additional homeostatic mechanisms may allow local changes in synaptic activation to generate local synaptic adaptations, and network-wide changes in activity to generate network-wide adjustments in the balance between excitation and inhibition. The signaling pathways underlying these various forms of homeostatic plasticity are currently under intense scrutiny, and although dozens of molecular pathways have now been implicated in homeostatic plasticity, a clear picture of how homeostatic feedback is structured at the molecular level has not yet emerged. On a functional level, neuronal networks likely use this complex set of regulatory mechanisms to achieve homeostasis over a wide range of temporal and spatial scales.

Glutamate receptors in synaptic assembly and plasticity: case studies on fly NMJs

The molecular and cellular mechanisms that control the composition and functionality of ionotropic glutamate receptors may be considered as most important "set screws" for adjusting excitatory transmission in the course of developmental and experience-dependent changes within neural networks. The Drosophila larval neuromuscular junction has emerged as one important invertebrate model system to study the formation, maintenance, and plasticity-related remodeling of glutamatergic synapses in vivo. By exploiting the unique genetic accessibility of this organism combined with diverse tools for manipulation and analysis including electrophysiology and state of the art imaging, considerable progress has been made to characterize the role of glutamate receptors during the orchestration of junctional development, synaptic activity, and synaptogenesis. Following an introduction to basic features of this model system, we will mainly focus on conceptually important findings such as the selective impact of glutamate receptor subtypes on the formation of new synapses, the coordination of presynaptic maturation and receptor subtype composition, the role of nonvesicularly released glutamate on the synaptic localization of receptors, or the homeostatic feedback of receptor functionality on presynaptic transmitter release.

Homeodomain interacting protein kinase 2 regulates postnatal development of enteric dopaminergic neurons and glia via BMP signaling

Trophic factor signaling is important for the migration, differentiation, and survival of enteric neurons during development. The mechanisms that regulate the maturation of enteric neurons in postnatal life, however, are poorly understood. Here, we show that transcriptional cofactor HIPK2 (homeodomain interacting protein kinase 2) is required for the maturation of enteric neurons and for regulating gliogenesis during postnatal development. Mice lacking HIPK2 display a spectrum of gastrointestinal (GI) phenotypes, including distention of colon and slowed GI transit time. Although loss of HIPK2 does not affect the enteric neurons in prenatal development, a progressive loss of enteric neurons occurs during postnatal life in Hipk2(-/-) mutant mice that preferentially affects the dopaminergic population of neurons in the caudal region of the intestine. The mechanism by which HIPK2 regulates postnatal enteric neuron development appears to involve the response of enteric neurons to bone morphogenetic proteins (BMPs). Specifically, compared to wild type mice, a larger proportion of enteric neurons in Hipk2(-/-) mutants have an abnormally high level of phosphorylated Smad1/5/8. Consistent with the ability of BMP signaling to promote gliogenesis, Hipk2(-/-) mutants show a significant increase in glia in the enteric nervous system. In addition, numbers of autophagosomes are increased in enteric neurons in Hipk2(-/-) mutants, and synaptic maturation is arrested. These results reveal a new role for HIPK2 as an important transcriptional cofactor that regulates the BMP signaling pathway in the maintenance of enteric neurons and glia, and further suggest that HIPK2 and its associated signaling mechanisms may be therapeutically altered to promote postnatal neuronal maturation.

Crimpy inhibits the BMP homolog Gbb in motoneurons to enable proper growth control at the Drosophila neuromuscular junction

The BMP pathway is essential for scaling of the presynaptic motoneuron arbor to the postsynaptic muscle cell at the Drosophila neuromuscular junction (NMJ). Genetic analyses indicate that the muscle is the BMP-sending cell and the motoneuron is the BMP-receiving cell. Nevertheless, it is unclear how this directionality is established as Glass bottom boat (Gbb), the known BMP ligand, is active in motoneurons. We demonstrate that crimpy (cmpy) limits neuronal Gbb activity to permit appropriate regulation of NMJ growth. cmpy was identified in a screen for motoneuron-expressed genes and encodes a single-pass transmembrane protein with sequence homology to vertebrate Cysteine-rich transmembrane BMP regulator 1 (Crim1). We generated a targeted deletion of the cmpy locus and find that loss-of-function mutants exhibit excessive NMJ growth. In accordance with its expression profile, tissue-specific rescue experiments indicate that cmpy functions neuronally. The overgrowth in cmpy mutants depends on the activity of the BMP type II receptor Wishful thinking, arguing that Cmpy acts in the BMP pathway upstream of receptor activation and raising the possibility that it inhibits Gbb activity in motoneurons. Indeed, the cmpy mutant phenotype is strongly suppressed by RNAi-mediated knockdown of Gbb in motoneurons. Furthermore, Cmpy physically interacts with the Gbb precursor protein, arguing that Cmpy binds Gbb prior to the secretion of mature ligand. These studies demonstrate that Cmpy restrains Gbb activity in motoneurons. We present a model whereby this inhibition permits the muscle-derived Gbb pool to predominate at the NMJ, thus establishing the retrograde directionality of the pro-growth BMP pathway.

Rapid active zone remodeling during synaptic plasticity

How can synapses change the amount of neurotransmitter released during synaptic plasticity? Although release in general is intensely investigated, its determinants during plasticity are still poorly understood. As a model for plastic strengthening of synaptic release, we here use the well-established presynaptic homeostatic compensation during interference with postsynaptic glutamate receptors at the Drosophila neuromuscular junction. Combining short-term plasticity analysis, cumulative EPSC analysis, fluctuation analysis, and quantal short-term plasticity modeling, we found an increase in the number of release-ready vesicles during presynaptic strengthening. High-resolution light microscopy revealed an increase in the amount of the active zone protein Bruchpilot and an enlargement of the presynaptic cytomatrix structure. Furthermore, these functional and structural alterations of the active zone were not only observed after lifelong but already after minutes of presynaptic strengthening. Our results demonstrate that presynaptic plasticity can induce active zone remodeling, which regulates the number of release-ready vesicles within minutes.

Inhibitory Smads differentially regulate cell fate specification and axon dynamics in the dorsal spinal cord

The roof plate resident BMPs have sequential functions in the developing spinal cord, establishing cell fate and orienting axonal trajectories. These activities are, however, restricted to the dI1-dI3 neurons in the most dorsal region of the spinal cord. What limits the extent of the action of the BMPs to these neurons? To address this question, we have examined both the distribution of the inhibitory Smads (I-Smads), Smad6 and Smad7 in the spinal cord and the consequence of ectopically expressing the I-Smads in chicken embryos. Our studies suggest that the I-Smads function in vivo to restrict the action of BMP signaling in the dorsal spinal cord. Moreover, the I-Smads have distinct roles in regulating the diverse activities of the BMPs. Thus, the ectopic expression of Smad7 suppresses the dI1 and dI3 neural fates and concomitantly increases the number of dI4-dI6 spinal neurons. In contrast, Smad6 most potently functions to block dI1 axon outgrowth. Taken together, these experiments suggest that the I-Smads have distinct roles in spatially limiting the response of cells to BMP signaling.

Development of axon-target specificity of ponto-cerebellar afferents

The function of neuronal networks relies on selective assembly of synaptic connections during development. We examined how synaptic specificity emerges in the pontocerebellar projection. Analysis of axon-target interactions with correlated light-electron microscopy revealed that developing pontine mossy fibers elaborate extensive cell-cell contacts and synaptic connections with Purkinje cells, an inappropriate target. Subsequently, mossy fiber-Purkinje cell connections are eliminated resulting in granule cell-specific mossy fiber connectivity as observed in mature cerebellar circuits. Formation of mossy fiber-Purkinje cell contacts is negatively regulated by Purkinje cell-derived BMP4. BMP4 limits mossy fiber growth in vitro and Purkinje cell-specific ablation of BMP4 in mice results in exuberant mossy fiber-Purkinje cell interactions. These findings demonstrate that synaptic specificity in the pontocerebellar projection is achieved through a stepwise mechanism that entails transient innervation of Purkinje cells, followed by synapse elimination. Moreover, this work establishes BMP4 as a retrograde signal that regulates the axon-target interactions during development.

Proper formation of whisker barrelettes requires periphery-derived Smad4-dependent TGF-beta signaling

Mammalian somatosensory topographic maps contain specialized neuronal structures that precisely recapitulate the spatial pattern of peripheral sensory organs. In the mouse, whiskers are orderly mapped onto several brainstem nuclei as a set of modular structures termed barrelettes. Using a dual-color iontophoretic labeling strategy, we found that the precise topography of barrelettes is not a result of ordered positions of sensory neurons within the ganglion. We next explored another possibility that formation of the whisker map is influenced by periphery-derived mechanisms. During the period of peripheral sensory innervation, several TGF-β ligands are exclusively expressed in whisker follicles in a dynamic spatiotemporal pattern. Disrupting TGF-β signaling, specifically in sensory neurons by conditional deletion of Smad4 at the late embryonic stage, results in the formation of abnormal barrelettes in the principalis and interpolaris brainstem nuclei and a complete absence of barrelettes in the caudalis nucleus. We further show that this phenotype is not derived from defective peripheral innervation or central axon outgrowth but is attributable to the misprojection and deficient segregation of trigeminal axonal collaterals into proper barrelettes. Furthermore, Smad4-deficient neurons develop simpler terminal arbors and form fewer synapses. Together, our findings substantiate the involvement of whisker-derived TGF-β/Smad4 signaling in the formation of the whisker somatotopic maps.

Identification of downstream targets of the bone morphogenetic protein pathway in the Drosophila nervous system

Bone Morphogenetic Protein (BMP) signaling mediated by the receptor Wishful thinking (Wit) is essential for nervous system development in Drosophila. Mutants lacking wit function show defects in neuromuscular junction development and function, specification of neurosecretory phenotypes, and eclosion behavior that result in lethality. The ligand is Glass bottom boat, the Drosophila ortholog of mammalian BMP-7, which acts as a retrograde signal through the Wit receptor. In order to identify transcriptional targets of the BMP pathway in the Drosophila nervous system, we have analyzed the gene expression profile of wit mutant larval central nervous system. Genes differentially expressed identified by microarray analysis have been verified by quantitative PCR and studied by in situ hybridization. Among the genes thus identified, we find solute transporters, neuropeptides, mitochondrial proteins, and novel genes. In addition, several genes are regulated by wit in an isoform-specific manner that suggest regulation of alternative splicing by BMP signaling.

The Cdc42-selective GAP rich regulates postsynaptic development and retrograde BMP transsynaptic signaling

Inhibition of Cdc42 by dRich induces postsynaptic release of the BMP ligand Glass bottom boat.

Retrograde bone morphogenetic protein signaling mediated by the Glass bottom boat (Gbb) ligand modulates structural and functional synaptogenesis at the Drosophila melanogaster neuromuscular junction. However, the molecular mechanisms regulating postsynaptic Gbb release are poorly understood. In this study, we show that Drosophila Rich (dRich), a conserved Cdc42-selective guanosine triphosphatase–activating protein (GAP), inhibits the Cdc42–Wsp pathway to stimulate postsynaptic Gbb release. Loss of dRich causes synaptic undergrowth and strongly impairs neurotransmitter release. These presynaptic defects are rescued by targeted postsynaptic expression of wild-type dRich but not a GAP-deficient mutant. dRich inhibits the postsynaptic localization of the Cdc42 effector Wsp (Drosophila orthologue of mammalian Wiskott-Aldrich syndrome protein, WASp), and manifestation of synaptogenesis defects in drich mutants requires Wsp signaling. In addition, dRich regulates postsynaptic organization independently of Cdc42. Importantly, dRich increases Gbb release and elevates presynaptic phosphorylated Mad levels. We propose that dRich coordinates the Gbb-dependent modulation of synaptic growth and function with postsynaptic development.

Single-cell optogenetic excitation drives homeostatic synaptic depression

Homeostatic processes have been proposed to explain the discrepancy between the dynamics of synaptic plasticity and the stability of brain function. Forms of synaptic plasticity such as long-term potentiation alter synaptic activity in a synapse- and cell-specific fashion. Although network-wide excitation triggers compensatory homeostatic changes, it is unknown whether neurons initiate homeostatic synaptic changes in response to cell-autonomous increases in excitation. Here we employ optogenetic tools to cell-autonomously excite CA1 pyramidal neurons and find that a compensatory postsynaptic depression of both AMPAR and NMDAR function results. Elevated calcium influx through L-type calcium channels leads to activation of a pathway involving CaM kinase kinase and CaM kinase 4 that induces synaptic depression of AMPAR and NMDAR responses. The synaptic depression of AMPARs but not of NMDARs requires protein synthesis and the GluA2 AMPAR subunit, indicating that downstream of CaM kinase activation divergent pathways regulate homeostatic AMPAR and NMDAR depression.

Retrograde neural circuit specification by target-derived neurotrophins and growth factors

Neural circuit assembly during development involves a series of highly regulated steps. While genetically pre-determined programs play key roles in the early steps including neurogenesis, migration, and initial growth and guidance of axons; increasing evidence indicates that as the axons reach their targets, the late steps of neuronal differentiation and connectivity formation may be influenced or even specified by target-derived signals. Here we attempt to provide a brief synthesized review on the roles of retrograde neurotrophin and growth factor signaling in regulating the final stages of neural circuit specificity such as axonal projection, dendritic patterning, neurotransmitter phenotype acquisition, and synapse formation.

TGF-beta2 alters the characteristics of the neuromuscular junction by regulating presynaptic quantal size

The amount of neurotransmitter released from a presynaptic terminal is the product of the quantal content (number of vesicles) and the presynaptic quantal size (QSpre, amount of transmitter per vesicle). QSpre varies with synaptic use, but its regulation is poorly understood. The motor nerve terminals at the neuromuscular junction (NMJ) contain TGF-beta receptors. We present evidence that TGF-beta2 regulates QSpre at the NMJ. Application of TGF-beta2 to the rat diaphragm NMJ increased the postsynaptic response to both spontaneous and evoked release of acetylcholine, whereas antibodies to TGF-beta2 or its receptor had the converse effect. L-vesamicol and bafilomycin blocked the actions of TGF-beta2, indicating that TGF-beta2 acts by altering the extent of vesicular filling. Recordings of the postsynaptic currents from the diaphragm were consistent with TGF-beta2 having this presynaptic action and a lesser postsynaptic effect on input resistance. TGF-beta2 also decreased quantal content by an atropine-sensitive pathway, indicating that this change is secondary to cholinergic feedback on vesicular release. Consequently, the net actions of TGF-beta2 at the NMJ were to amplify the postsynaptic effects of spontaneous transmission and to diminish the number of vesicles used per evoked stimulus, without diminishing the amount of acetylcholine released.

Synaptic homeostasis is consolidated by the cell fate gene gooseberry, a Drosophila pax3/7 homolog

In a large-scale screening effort, we identified the gene gooseberry (gsb) as being necessary for synaptic homeostasis at the Drosophila neuromuscular junction. The gsb gene encodes a pair-rule transcription factor that participates in embryonic neuronal cell fate specification. Here, we define a new postembryonic role for gooseberry. We show that gsb becomes widely expressed in the postembryonic CNS, including within mature motoneurons. Loss of gsb does not alter neuromuscular growth, morphology, or the distribution of essential synaptic proteins. However, gsb function is required postembryonically for the sustained expression of synaptic homeostasis. In GluRIIA mutant animals, miniature EPSP (mEPSP) amplitudes are significantly decreased, and there is a compensatory homeostatic increase in presynaptic release that restores normal muscle excitation. Loss of gsb significantly impairs the homeostatic increase in presynaptic release in the GluRIIA mutant. Interestingly, gsb is not required for the rapid induction of synaptic homeostasis. Furthermore, gsb seems to be specifically involved in the mechanisms responsible for a homeostatic increase in presynaptic release, since it is not required for the homeostatic decrease in presynaptic release observed following an increase in mEPSP amplitude. Finally, Gsb has been shown to antagonize Wingless signaling during embryonic fate specification, and we present initial evidence that this activity is conserved during synaptic homeostasis. Thus, we have identified a gene (gsb) that distinguishes between rapid induction versus sustained expression of synaptic homeostasis and distinguishes between the mechanisms responsible for homeostatic increase versus decrease in synaptic vesicle release.

Retrograde BMP signaling controls synaptic growth at the NMJ by regulating trio expression in motor neurons

Retrograde signaling is essential for coordinating the growth of synaptic structures; however, it is not clear how it can lead to modulation of cytoskeletal dynamics and structural changes at presynaptic terminals. We show that loss of retrograde bone morphogenic protein (BMP) signaling at the Drosophila larval neuromuscular junction (NMJ) leads to a significant reduction in levels of Rac GEF Trio and a diminution of transcription at the trio locus. We further find that Trio is required in motor neurons for normal structural growth. Finally, we show that transgenic expression of Trio in motor neurons can partially restore NMJ defects in larvae mutant for BMP signaling. Based on our findings, we propose a model in which a retrograde BMP signal from the muscle modulates GTPase activity through transcriptional regulation of Rac GEF trio, thereby regulating the homeostasis of synaptic growth at the NMJ.

Ménage à Trio during BMP-Mediated Retrograde Signaling at the NMJ

In flies, retrograde BMP signaling is an important mechanism by which postsynaptic cells regulate the structure and function of presynaptic terminals, ostensibly through changes in gene expression. Transcriptional targets, however, have remained mysterious. In this issue of Neuron, Haghighi and colleagues begin to unravel this puzzle by identifying the cytoskeletal regulator Trio.

A hierarchy of cell intrinsic and target-derived homeostatic signaling

Homeostatic control of neural function can be mediated by the regulation of ion channel expression, neurotransmitter receptor abundance, or modulation of presynaptic release. These processes can be implemented through cell autonomous or intercellular signaling. It remains unknown whether different forms of homeostatic regulation can be coordinated to achieve constant neural function. One way to approach this question is to confront a simple neural system with conflicting perturbations and determine whether the outcome reflects a coordinated, homeostatic response. Here, we demonstrate that two A-type potassium channel genes, shal and shaker, are reciprocally, transcriptionally coupled to maintain A-type channel expression. We then demonstrate that this homeostatic control of A-type channel expression prevents target-dependent, homeostatic modulation of synaptic transmission. Thus, we uncover a homeostatic mechanism that reciprocally regulates A-type potassium channels, and we define a hierarchical relationship between cell-intrinsic control of ion channel expression and target-derived homeostatic control of synaptic transmission.

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.

Importin-beta11 regulates synaptic phosphorylated mothers against decapentaplegic, and thereby influences synaptic development and function at the Drosophila neuromuscular junction

Importin proteins act both at the nuclear pore to promote substrate entry and in the cytosol during signal trafficking. Here, we describe mutations in the Drosophila gene importin-beta11, which has not previously been analyzed genetically. Mutants of importin-beta11 died as late pupae from neuronal defects, and neuronal importin-beta11 was present not only at nuclear pores but also in the cytosol and at synapses. Neurons lacking importin-beta11 were viable and properly differentiated but exhibited discrete defects. Synaptic transmission was defective in adult photoreceptors and at larval neuromuscular junctions (NMJs). Mutant photoreceptor axons formed grossly normal projections and synaptic terminals in the brain, but synaptic arbors on larval muscles were smaller while still containing appropriate synaptic components. Bone morphogenic protein (BMP) signaling was the apparent cause of the observed NMJ defects. Importin-beta11 interacted genetically with the BMP pathway, and at mutant synaptic boutons, a key component of this pathway, phosphorylated mothers against decapentaplegic (pMAD), was reduced. Neuronal expression of an importin-beta11 transgene rescued this phenotype as well as the other observed neuromuscular phenotypes. Despite the loss of synaptic pMAD, pMAD persisted in motor neuron nuclei, suggesting a specific impairment in the local function of pMAD. Restoring levels of pMAD to mutant terminals via expression of constitutively active type I BMP receptors or by reducing retrograde transport in motor neurons also restored synaptic strength and morphology. Thus, importin-beta11 function interacts with the BMP pathway to regulate a pool of pMAD that must be present at the presynapse for its proper development and function.

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.

Lack of nAChR activity depresses cochlear maturation and up-regulates GABA system components: temporal profiling of gene expression in alpha9 null mice

Background

It has previously been shown that deletion of chrna9, the gene encoding the α9 nicotinic acetylcholine receptor (nAChR) subunit, results in abnormal synaptic terminal structure. Additionally, all nAChR-mediated cochlear activity is lost, as characterized by a failure of the descending efferent system to suppress cochlear responses to sound. In an effort to characterize the molecular mechanisms underlying the structural and functional consequences following loss of α9 subunit expression, we performed whole-transcriptome gene expression analyses on cochleae of wild type and α9 knockout (α9^−/−) mice during postnatal days spanning critical periods of synapse formation and maturation.

Principal Findings

Data revealed that loss of α9 receptor subunit expression leads to an up-regulation of genes involved in synaptic transmission and ion channel activity. Unexpectedly, loss of α9 receptor subunit expression also resulted in an increased expression of genes encoding GABA receptor subunits and the GABA synthetic enzyme, glutamic acid decarboxylase. These data suggest the existence of a previously unrecognized association between the nicotinic cholinergic and GABAergic systems in the cochlea. Computational analyses have highlighted differential expression of several gene sets upon loss of nicotinic cholinergic activity in the cochlea. Time-series analysis of whole transcriptome patterns, represented as self-organizing maps, revealed a disparate pattern of gene expression between α9^−/− and wild type cochleae at the onset of hearing (P13), with knockout samples resembling immature postnatal ages.

Conclusions

We have taken a systems biology approach to provide insight into molecular programs influenced by the loss of nicotinic receptor-based cholinergic activity in the cochlea and to identify candidate genes that may be involved in nicotinic cholinergic synapse formation, stabilization or function within the inner ear. Additionally, our data indicate a change in the GABAergic system upon loss of α9 nicotinic receptor subunit within the cochlea.

The regulation of TGFbeta signal transduction

Transforming growth factor beta (TGFbeta) pathways are implicated in metazoan development, adult homeostasis and disease. TGFbeta ligands signal via receptor serine/threonine kinases that phosphorylate, and activate, intracellular Smad effectors as well as other signaling proteins. Oligomeric Smad complexes associate with chromatin and regulate transcription, defining the biological response of a cell to TGFbeta family members. Signaling is modulated by negative-feedback regulation via inhibitory Smads. We review here the mechanisms of TGFbeta signal transduction in metazoans and emphasize events crucial for embryonic development.

Postsynaptic regulation of synaptic plasticity by synaptotagmin 4 requires both C2 domains

Analogous to synaptotagmin 1, a calcium-sensitive regulator of presynaptic vesicle fusion, synaptotagmin 4 needs both of its calcium-binding sites to regulate synaptic plasticity via postsynaptic retrograde signaling.

Ca²⁺ influx into synaptic compartments during activity is a key mediator of neuronal plasticity. Although the role of presynaptic Ca²⁺ in triggering vesicle fusion though the Ca²⁺ sensor synaptotagmin 1 (Syt 1) is established, molecular mechanisms that underlie responses to postsynaptic Ca²⁺ influx remain unclear. In this study, we demonstrate that fusion-competent Syt 4 vesicles localize postsynaptically at both neuromuscular junctions (NMJs) and central nervous system synapses in Drosophila melanogaster. Syt 4 messenger RNA and protein expression are strongly regulated by neuronal activity, whereas altered levels of postsynaptic Syt 4 modify synaptic growth and presynaptic release properties. Syt 4 is required for known forms of activity-dependent structural plasticity at NMJs. Synaptic proliferation and retrograde signaling mediated by Syt 4 requires functional C2A and C2B Ca²⁺–binding sites, as well as serine 284, an evolutionarily conserved substitution for a key Ca²⁺-binding aspartic acid found in other synaptotagmins. These data suggest that Syt 4 regulates activity-dependent release of postsynaptic retrograde signals that promote synaptic plasticity, similar to the role of Syt 1 as a Ca²⁺ sensor for presynaptic vesicle fusion.

Activity-dependent regulation of synapses by retrograde messengers

Throughout the brain, postsynaptic neurons release substances from their cell bodies and dendrites that regulate the strength of the synapses they receive. Diverse chemical messengers have been implicated in retrograde signaling from postsynaptic neurons to presynaptic boutons. Here, we provide an overview of the signaling systems that lead to rapid changes in synaptic strength. We consider the capabilities, specializations, and physiological roles of each type of signaling system.

Nemo kinase interacts with Mad to coordinate synaptic growth at the Drosophila neuromuscular junction

Bone morphogenic protein (BMP) signaling is essential for the coordinated assembly of the synapse, but we know little about how BMP signaling is modulated in neurons. Our findings indicate that the Nemo (Nmo) kinase modulates BMP signaling in motor neurons. nmo mutants show synaptic structural defects at the Drosophila melanogaster larval neuromuscular junction, and providing Nmo in motor neurons rescues these defects. We show that Nmo and the BMP transcription factor Mad can be coimmunoprecipitated and find a genetic interaction between nmo and Mad mutants. Moreover, we demonstrate that Nmo is required for normal distribution and accumulation of phosphorylated Mad in motor neurons. Finally, our results indicate that Nmo phosphorylation of Mad at its N terminus, distinct from the BMP phosphorylation site, is required for normal function of Mad. Based on our findings, we propose a model in which phosphorylation of Mad by Nmo ensures normal accumulation and distribution of Mad and thereby fine tunes BMP signaling in motor neurons.

Importin 13 regulates neurotransmitter release at the Drosophila neuromuscular junction

In an unbiased genetic screen designed to isolate mutations that affect synaptic transmission, we have isolated homozygous lethal mutations in Drosophila importin 13 (imp13). Imp13 is expressed in and around nuclei of both neurons and muscles. At the larval neuromuscular junction (NMJ), imp13 affects muscle growth and formation of the subsynaptic reticulum without influencing any presynaptic structural features. In the absence of imp13, the probability of release of neurotransmitter and quantal content is increased, yet the abundance of the postsynaptic receptors and the amplitude of miniature excitatory junctional potentials are not affected. Interestingly, imp13 is required in the muscles to control presynaptic release. Thus, imp13 is a novel factor that affects neurotransmitter release at the fly NMJ. Its role in the context of synaptic homeostasis is discussed.

The probability of neurotransmitter release: variability and feedback control at single synapses

Information transfer at chemical synapses occurs when vesicles fuse with the plasma membrane and release neurotransmitter. This process is stochastic and its likelihood of occurrence is a crucial factor in the regulation of signal propagation in neuronal networks. The reliability of neurotransmitter release can be highly variable: experimental data from electrophysiological, molecular and imaging studies have demonstrated that synaptic terminals can individually set their neurotransmitter release probability dynamically through local feedback regulation. This local tuning of transmission has important implications for current models of single-neuron computation.

A presynaptic homeostatic signaling system composed of the Eph receptor, ephexin, Cdc42, and CaV2.1 calcium channels

The molecular mechanisms underlying the homeostatic modulation of presynaptic neurotransmitter release remain largely unknown. In a screen, we isolated mutations in Drosophila ephexin (Rho-type guanine nucleotide exchange factor) that disrupt the homeostatic enhancement of presynaptic release following impairment of postsynaptic glutamate receptor function at the Drosophila neuromuscular junction. We show that Ephexin is sufficient presynaptically for synaptic homeostasis and localizes in puncta throughout the nerve terminal. However, ephexin mutations do not alter other aspects of neuromuscular development, including morphology or active zone number. We then show that, during synaptic homeostasis, Ephexin functions primarily with Cdc42 in a signaling system that converges upon the presynaptic CaV2.1 calcium channel. Finally, we show that Ephexin binds the Drosophila Eph receptor (Eph) and Eph mutants disrupt synaptic homeostasis. Based on these data, we propose that Ephexin/Cdc42 couples synaptic Eph signaling to the modulation of presynaptic CaV2.1 channels during the homeostatic enhancement of presynaptic release.

Synapse formation in developing neural circuits

The nervous system consists of hundreds of billions of neurons interconnected into the functional neural networks that underlie behaviors. The capacity of a neuron to innervate and function within a network is mediated via specialized cell junctions known as synapses. Synapses are macromolecular structures that regulate intercellular communication in the nervous system, and are the main gatekeepers of information flow within neural networks. Where and when synapses form determines the connectivity and functionality of neural networks. Therefore, our knowledge of how synapse formation is regulated is critical to our understanding of the nervous system and how it goes awry in neurological disorders. Synapse formation involves pairing of the pre- and postsynaptic partners at a specific neurospatial coordinate. The specificity of synapse formation requires the precise execution of multiple developmental events, including cell fate specification, cell migration, axon guidance, dendritic growth, synaptic target selection, and synaptogenesis (Juttner and Rathjen in Cell. Mol. Life Sci. 62:2811, 2005; Salie et al., in Neuron 45:189, 2005; Waites et al., in Annu. Rev. Neurosci. 28:251, 2005). Remarkably, during the development of the vertebrate nervous system, these developmental processes occur almost simultaneously in billions of neurons, resulting in the formation of trillions of synapses. How this remarkable specificity is orchestrated during development is one of the outstanding questions in the field of neurobiology, and the focus of discussion of this chapter. We center the discussion of this chapter on the early developmental events that orchestrate the process of synaptogenesis prior to activity-dependent mechanisms. We have therefore limited the discussion of important activity-dependent synaptogenic events, which are discussed in other chapters of this book. Moreover, our discussion is biased toward lessons we have learned from invertebrate systems, in particular from C. elegans and Drosophila. We did so to complement the discussions from other chapters in this book, which focus on the important findings that have recently emerged from the vertebrate literature. The chapter begins with a brief history of the field of synaptic biology. This serves as a backdrop to introduce some of the historically outstanding questions of synaptic development that have eluded us during the past century, and which are the focus of this review. We then discuss some general features of synaptic structure as it relates to its function. In particular, we will highlight evolutionarily conserved traits shared by all synaptic structures, and how these features have helped optimize these ancient cellular junctions for interneural communication. We then discuss the regulatory signals that orchestrate the precise assembly of these conserved macromolecular structures. This discussion will be framed in the context of the neurodevelopmental process. Specifically, much of our discussion will focus on how the seemingly disparate developmental processes are intimately linked at a molecular level, and how this relationship might be crucial in the developmental orchestration of circuit assembly. We hope that the discussion of the multifunctional cues that direct circuit development provides a conceptual framework into understanding how, with a limited set of signaling molecules, precise neural wiring can be coordinated between synaptic partners.