Wnts and TGF beta in synaptogenesis: old friends signalling at new places

Abstract

The formation of mature synaptic connections involves the targeted transport and aggregation of synaptic vesicles, the gathering of presynaptic release sites and the clustering of postsynaptic neurotransmitter receptors and ion channels. Positional cues are required to orient the cytoskeleton in the direction of neuronal outgrowth, and also to direct the juxtaposition of synaptic protein complexes at the pre- and postsynaptic membranes. Both anterograde and retrograde factors are thought to contribute positional information during synaptic differentiation, and recent studies in vertebrates and invertebrates have begun to uncover a new role in this process for proteins that are essential for pattern formation in the early embryo.

Figure 1. The Wnt and TGFβ signalling pathways

Transforming growth factor-β (TGFβ) ligand binding causes heterodimerization of type I and type II receptors. The type II receptor then activates the type I receptor by phosphorylation. The type I receptor then phosphorylates R-Smads, which transduce the signal into the nucleus via a co-Smad, Smad4. The R-Smad/Smad4 complex binds to various developmentally regulated transcription factors to direct gene expression. Wnt binding to the Frizzled receptor leads to phosphorylation of Dishevelled. Phosphorylated Dishevelled prevents the formation of the Apc (adenomatous polyposis coli)-Axin-Conducin-Gsk3β (glycogen synthase kinase 3β) complex, which, in the absence of Wnt, signals the degradation of β-catenin. By contrast, binding of Wnt to Frizzled receptors stabilizes cytoplasmic β-catenin, which complexes with lymphoid enhancer-binding factor/T-cell factor (Lef/Tcf) transcription factors and initiates transcription of Wnt-responsive genes. The Wnt pathway can also modulate cytoskeletal dynamics. In the absence of Wnt signalling, Gsk3β phosphorylates the microtubule-associated protein 1b (Map1b), thereby destabilizing microtubule bundles. Wnt signalling inhibits Gsk3β to maintain a stable microtubular network. Ub, ubiquitination.

Figure 2. Wingless signalling is essential for the proper formation of pre- and postsynaptic specializations at the fly neuromuscular junction

a | At the wild type neuromuscular junction (NMJ), Wingless (Wg; green) is secreted by, and localizes to, synaptic boutons (red). Inset: A single bouton at higher magnification, showing that Wg immunostaining (green) is localized both pre- and postsynaptically. Anti-horseradish peroxidase (HRP; red) is the neuronal marker. Arrow points to a synaptic bouton. b and c | At the ultrastructural level, the pre- and postsynaptic specializations seen in the wild type synapse (b) such as the active zone T-bars (arrow) — electron dense structures at Drosophila NMJs that are associated with presynaptic densities, and around which synaptic vesicles cluster — and subsynaptic reticulum (SSR), are completely lacking in the wg mutant (c).

Figure 3. Events during new synapse formation at the Drosophila neuromuscular junction, and the role of Wnt and TGFβ pathway in these processes

a, b | During Drosophila neuromuscular (NMJ) development, precise apposition of the pre- and postsynaptic molecular components is required for bouton maturation and subsequent expansion. New boutons bud out from existing mature boutons to adjust for an increase in target muscle size. c, d | New buds expand and subsequently mature in tune with a synchronous folding of the postsynaptic membrane into the subsynaptic reticulum (SSR). Recent observations at the Drosophila NMJ demonstrate crucial roles for the Wnt and transforming growth factor-β (TGFβ) signal transduction pathways in these processes^–. e | In the absence of Wingless (Wg) function, fewer synapses form, and those that do form fail to develop pre- and postsynaptic densities, T-bars and postsynaptic SSR. The presynaptic cytoskeletal network appears to be destabilized, and postsynaptic glutamate receptors (GluR) are not correctly localized. f | wit mutant synapses have normal SSR and apparently normal GluR localization. However, they have presynaptic defects such as an increase in the number of T-bars per presynaptic density, abnormal apposition of pre- and postsynaptic densities, and an abnormal appearance of presynaptic T-bars. These studies indicate a presynaptic role for the TGFβ type II receptor, Wit, which functions as a part of a retrograde signalling mechanism to adjust the size of the presynaptic arborization to the postsynaptic muscle size. By contrast, Wg is secreted by presynaptic boutons, and seems to function in an anterograde fashion to control the correct positioning of active zones and postsynaptic SSR, as well as bouton number. Together, they coordinate synapse development by regulating bouton budding, cytoskeletal stability, number of functional active zones, folding of the postsynaptic muscle membrane, and levels of pre- and postsynaptic molecules, including GluRs and cell adhesion molecules. DFz2, Drosophila Frizzled 2; Spin, Spinster.