Dysbindin links presynaptic proteasome function to homeostatic recruitment of low release probability vesicles.

Here we explore the relationship between presynaptic homeostatic plasticity and proteasome function at the Drosophila neuromuscular junction. First, we demonstrate that the induction of homeostatic plasticity is blocked after presynaptic proteasome perturbation. Proteasome inhibition potentiates release under baseline conditions but not during homeostatic plasticity, suggesting that proteasomal degradation and homeostatic plasticity modulate a common pool of vesicles. The vesicles that are regulated by proteasome function and recruited during homeostatic plasticity are highly EGTA sensitive, implying looser Ca2+ influx-release coupling. Similar to homeostatic plasticity, proteasome perturbation enhances presynaptic Ca2+ influx, readily-releasable vesicle pool size, and does not potentiate release after loss of specific homeostatic plasticity genes, including the schizophrenia-susceptibility gene dysbindin. Finally, we provide genetic evidence that Dysbindin levels regulate the access to EGTA-sensitive vesicles. Together, our data suggest that presynaptic protein degradation opposes the release of low-release probability vesicles that are potentiated during homeostatic plasticity and whose access is controlled by dysbindin.

Potent degradation of neuronal miRNAs induced by highly complementary targets.

MicroRNAs (miRNAs) regulate target mRNAs by silencing them. Reciprocally, however, target mRNAs can also modulate miRNA stability. Here, we uncover a remarkable efficacy of target RNA-directed miRNA degradation (TDMD) in rodent primary neurons. Coincident with degradation, and while still bound to Argonaute, targeted miRNAs are 3' terminally tailed and trimmed. Absolute quantification of both miRNAs and their decay-inducing targets suggests that neuronal TDMD is multiple turnover and does not involve co-degradation of the target but rather competes with miRNA-mediated decay of the target. Moreover, mRNA silencing, but not TDMD, relies on cooperativity among multiple target sites to reach high efficacy. This knowledge can be harnessed for effective depletion of abundant miRNAs. Our findings bring insight into a potent miRNA degradation pathway in primary neurons, whose TDMD activity greatly surpasses that of non-neuronal cells and established cell lines. Thus, TDMD may be particularly relevant for miRNA regulation in the nervous system.

mSYD1A, a mammalian synapse-defective-1 protein, regulates synaptogenic signaling and vesicle docking.

Structure and function of presynaptic terminals are critical for the transmission and processing of neuronal signals. Trans-synaptic signaling systems instruct the differentiation and function of presynaptic release sites, but their downstream mediators are only beginning to be understood. Here, we identify the intracellular mSYD1A (mouse Synapse-Defective-1A) as a regulator of presynaptic function in mice. mSYD1A forms a complex with presynaptic receptor tyrosine phosphatases and controls tethering of synaptic vesicles at synapses. mSYD1A function relies on an intrinsically disordered domain that interacts with multiple structurally unrelated binding partners, including the active zone protein liprin-α2 and nsec1/munc18-1. In mSYD1A knockout mice, synapses assemble in normal numbers but there is a significant reduction in synaptic vesicle docking at the active zone and an impairment of synaptic transmission. Thus, mSYD1A is a regulator of presynaptic release sites at central synapses.

Pro-neurotrophins secreted from retinal ganglion cell axons are necessary for ephrinA-p75NTR-mediated axon guidance.

BACKGROUND: Retinotectal map formation develops via topographically specific guidance and branching of retinal axons in their target area. This process is controlled, in part, by reverse signalling of ephrinAs expressed on retinal axons. As glycosylphosphatidylinositol-anchored molecules, ephrinAs require transmembrane co-receptors to exert this function, for which the two neurotrophin receptors, p75NTR and TrkB, were recently proposed. RESULTS: We show here that the ligands for these receptors, the brain-derived neurotrophic factor precursor (proBDNF) and its processed form, BDNF, respectively, control the branching of retinal axons antagonistically, which they mediate by inducing the corresponding neurotrophin receptor-ephrinA complexes. Moreover, scavenging proneurotrophins, by adding antibodies specific for the pro-domain of proBNDF or a soluble extracellular domain of p75NTR, abolish repellent ephrinA reverse signalling in the stripe assay. CONCLUSIONS: This indicates that retinal cells secrete proneurotrophins, inducing the ephrinA-p75NTR interaction and enabling repellent axon guidance. The antagonistic functions of proBDNF and BDNF raise the possibility that topographic branching is controlled by local control of processing of proneurotrophins.

Cadherin-7 and cadherin-6B differentially regulate the growth, branching and guidance of cranial motor axons.

Cadherin-7 (Cad7) and cadherin-6B (Cad6B) are expressed in early and late phases of cranial motoneuron development, respectively. Cad7 is expressed by cranial motoneurons soon after they are generated, as well as in the environment through which their axons extend. By contrast, Cad6B is expressed by mature cranial motoneurons. We demonstrate in chick that these cadherins play distinct roles in cranial motor axon morphology, branching and projection. Using in vitro approaches, we show that Cad7 enhances motor axon outgrowth, suppresses the formation of multiple axons and restricts interstitial branching, thus promoting the development of a single unbranched axon characteristic of differentiating motoneurons. Conversely, Cad6B in vitro promotes motor axon branching, a characteristic of mature motoneurons. In vivo gain- and loss-of-function experiments for these cadherins yielded phenotypes consistent with this interpretation. In particular, a loss of cadherin-mediated interactions in vivo led to dysregulation of the cranial motoneuron normal branching programme and caused axon navigation defects. We also demonstrate that Cad6B functions via the phosphatidylinositol 3-kinase pathway. Together, these data show that Cad7 and Cad6B differentially regulate cranial motoneuron growth, branching and axon guidance.

A TrkB/EphrinA interaction controls retinal axon branching and synaptogenesis.

Toward understanding topographically specific branching of retinal axons in their target area, we have studied the interaction between neurotrophin receptors and members of the Eph family. TrkB and its ligand BDNF are uniformly expressed in the retina and tectum, respectively, and exert a branch-promoting activity, whereas EphAs and ephrinAs are expressed in gradients in retina and tectum and can mediate a suppression of axonal branching. We have identified a novel cis interaction between ephrinA5 and TrkB on retinal ganglion cell axons. TrkB interacts with ephrinA5 via its second cysteine-rich domain (CC2), which is necessary and sufficient for binding to ephrinA5. Their functional interaction is twofold: ephrinA5 augments BDNF-promoted retinal axon branching in the absence of its activator EphA7-Fc, whereas EphA7-Fc application abolishes branching in a local and concentration-dependent manner. The importance of TrkB in this process is shown by the fact that overexpression of an isolated TrkB-CC2 domain interfering with the ephrinA/TrkB interaction abolishes this regulatory interplay, whereas knockdown of TrkB via RNA interference diminishes the ephrinA5-evoked increase in branching. The ephrinA/Trk interaction is neurotrophin induced and specifically augments the PI-3 kinase/Akt pathway generally known to be involved in the promotion of branching. In addition, ephrinAs/TrkB modulate axon branching and also synapse formation of hippocampal neurons. Our findings uncover molecular mechanisms of how spatially restricted axon branching can be achieved by linking globally expressed branch-promoting with differentially expressed branch-suppressing activities. In addition, our data suggest that growth factors and the EphA-ephrinA system interact in a way that affects axon branching and synapse development.