Glial wingless/Wnt regulates glutamate receptor clustering and synaptic physiology at the Drosophila neuromuscular junction.

Glial cells are emerging as important regulators of synapse formation, maturation, and plasticity through the release of secreted signaling molecules. Here we use chromatin immunoprecipitation along with Drosophila genomic tiling arrays to define potential targets of the glial transcription factor Reversed polarity (Repo). Unexpectedly, we identified wingless (wg), a secreted morphogen that regulates synaptic growth at the Drosophila larval neuromuscular junction (NMJ), as a potential Repo target gene. We demonstrate that Repo regulates wg expression in vivo and that local glial cells secrete Wg at the NMJ to regulate glutamate receptor clustering and synaptic function. This work identifies Wg as a novel in vivo glial-secreted factor that specifically modulates assembly of the postsynaptic signaling machinery at the Drosophila NMJ.

Integration of a retrograde signal during synapse formation by glia-secreted TGF-ß ligand.

Glial cells are crucial regulators of synapse formation, elimination, and plasticity [1, 2]. In vitro studies have begun to identify glial-derived synaptogenic factors [1], but neuron-glia signaling events during synapse formation in vivo remain poorly defined. The coordinated development of pre- and postsynaptic compartments at the Drosophila neuromuscular junction (NMJ) depends on a muscle-secreted retrograde signal, the TGF-ß/BMP Glass bottom boat (Gbb) [3, 4]. Muscle-derived Gbb activates the TGF-ß receptors Wishful thinking (Wit) and either Saxophone (Sax) or Thick veins (Tkv) in motor neurons [3, 4]. This induces phosphorylation of Mad (P-Mad) in motor neurons, its translocation into the nucleus with a co-Smad, and activation of transcriptional programs controlling presynaptic bouton growth [5]. Here we show that NMJ glia release the TGF-ß ligand Maverick (Mav), which likely activates the muscle activin-type receptor Punt to potently modulate Gbb-dependent retrograde signaling and synaptic growth. Loss of glial Mav results in strikingly reduced P-Mad at NMJs, decreased Gbb transcription in muscle, and in turn reduced muscle-to-motor neuron retrograde TGF-ß/BMP signaling. We propose that by controlling Gbb release from muscle, glial cells fine tune the ability of motor neurons to extend new synaptic boutons in correlation to muscle growth. Our work identifies a novel glia-derived synaptogenic factor by which glia modulate synapse formation in vivo.

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.

Glia and muscle sculpt neuromuscular arbors by engulfing destabilized synaptic boutons and shed presynaptic debris.

Synapse remodeling is an extremely dynamic process, often regulated by neural activity. Here we show during activity-dependent synaptic growth at the Drosophila NMJ many immature synaptic boutons fail to form stable postsynaptic contacts, are selectively shed from the parent arbor, and degenerate or disappear from the neuromuscular junction (NMJ). Surprisingly, we also observe the widespread appearance of presynaptically derived "debris" during normal synaptic growth. The shedding of both immature boutons and presynaptic debris is enhanced by high-frequency stimulation of motorneurons, indicating that their formation is modulated by neural activity. Interestingly, we find that glia dynamically invade the NMJ and, working together with muscle cells, phagocytose shed presynaptic material. Suppressing engulfment activity in glia or muscle by disrupting the Draper/Ced-6 pathway results in a dramatic accumulation of presynaptic debris, and synaptic growth in turn is severely compromised. Thus actively growing NMJ arbors appear to constitutively generate an excessive number of immature boutons, eliminate those that are not stabilized through a shedding process, and normal synaptic expansion requires the continuous clearance of this material by both glia and muscle cells.

Cell-type-specific and developmental regulation of heterogeneous nuclear ribonucleoprotein K mRNA in the rat nervous system.

Heterogeneous nuclear ribonucleoprotein K (hnRNP K) was originally identified as being part of the hnRNP particle. hnRNP K has subsequently been shown to be involved in a number of fundamental biological processes such as RNA transport and processing as well as transcription and translation. In addition, hnRNP K is an integral player in a variety of intracellular signal transduction pathways. Not surprisingly given this broad array of cellular functions, hnRNP K is a highly interactive protein binding directly to both single- and double-stranded nucleic acids as well as numerous signaling proteins. Interestingly, earlier studies demonstrated that hnRNP K protein is not ubiquitously expressed and does not exist in a fixed stoichiometry with other hnRNP proteins. We have extended this earlier work and report here the spatially- and developmentally-regulated expression of hnRNP K mRNA during development of the rat nervous system. In the central nervous system, hnRNP K mRNA expression gradually decreases during development until it is restricted to a very limited number of structures including most notably the hippocampus and the retina. Immunohistochemical data indicate that hnRNP K protein expression closely parallels hnRNP K mRNA expression. In contrast to the central nervous system, hnRNP K in the peripheral nervous system remains high throughout embryonic development with dramatic expression in several peripheral ganglia.