Wnt signaling during synaptic development and plasticity

The formation of synaptic connections requires a dialogue between pre and postsynaptic cells to coordinate the assembly of the presynaptic release machinery and the postsynaptic receptive complexes. Signaling molecules of the Wnt family of proteins are central to this trans-synaptic dialogue. At the neuromuscular junction and central synapses, Wnts promote synaptic assembly by signaling to the developing pre and postsynaptic compartments. In addition, new studies reveal that expression of Wnt proteins and localization of their Fz receptors are regulated by neuronal activity. Importantly, Wnts mediates the synaptic changes induced by patterned neuronal activity or sensory experience in mature neurons. Here we review recent findings into the function of Wnt signaling at the synapse and its link to activity-dependent synaptic growth and function.     

Rapid bidirectional switching of synaptic NMDA receptors

Synaptic NMDA-type glutamate receptors (NMDARs) play important roles in synaptic plasticity, brain development, and pathology. In the last few years, the view of NMDARs as relatively fixed components of the postsynaptic density has changed. A number of studies have now shown that both the number of receptors and their subunit compositions can be altered. During development, the synaptic NMDARs subunit composition changes, switching from predominance of NR2B-containing to NR2A-containing receptors, but little is known about the mechanisms involved in this developmental process. Here, we report that, depending on the pattern of NMDAR activation, the subunit composition of synaptic NMDARs is under extremely rapid, bidirectional control at neonatal synapses. This switching, which is at least as rapid as that seen with AMPARs, will have immediate and dramatic consequences on the integrative capacity of the synapse.     

Brain-derived neurotrophic factor-tropomyosin-related kinase B signaling contributes to activity-dependent changes in synaptic proteins

The ability of synapses to undergo changes in structure and function in response to alterations of neuronal activity is an essential property of neural circuits. One way that this is achieved is through global changes in the molecular composition of the synapse; however, it is not clear how these changes are coupled to the dynamics of neuronal activity. Here we found that, in cultured rat cortical neurons, bidirectional changes of neuronal activity led to corresponding alterations in the expression of brain-derived neurotrophic factor (BDNF) and phosphorylation of its receptor tropomyosin-related kinase B (TrkB), as well as in the level of synaptic proteins. Exogenous BDNF reversed changes in synaptic proteins induced by chronic activity blockade, while inhibiting Trk kinase activity or depleting endogenous BDNF abolished the concentration changes induced by chronic activity elevation. Both tetrodotoxin and bicuculline had significant, but opposite, effects on synaptic protein ubiquitination in a time-dependent manner. Furthermore, exogenous BDNF was sufficient to increase ubiquitination of synaptic proteins, whereas scavenging endogenous BDNF or inhibiting Trk kinase activity prevented the ubiquitination of synaptic proteins induced by chronic elevation of neuronal activity. Inhibiting the proteasome or blocking protein polyubiquitination mimicked the effect of tetrodotoxin on the levels of synaptic proteins and canceled the effects of BDNF. Our study indicates that BDNF-TrkB signaling acts upstream of the ubiquitin proteasome system, linking neuronal activity to protein turnover at the synapse.     

Calcium- and activity-dependent synaptic plasticity.

Calcium ions play crucial signaling roles in many forms of activity-dependent synaptic plasticity. Recent presynaptic [Ca2+]i measurements and manipulation of presynaptic exogenous buffers reveal roles for residual [Ca2+]i following conditioning stimulation in all phases of short-term synaptic enhancement. Pharmacological manipulations implicate mitochondria in post-tetanic potentiation. New evidence supports an influence of Ca2+ in replacing depleted vesicles after synaptic depression. In addition, high-resolution measurements of [Ca2+]i in dendritic spines show how Ca2+ can encode the precise relative timing of presynaptic input and postsynaptic activity and generate long-term synaptic modifications of opposite polarity.     

Wnt signaling regulates experience-dependent synaptic plasticity in the adult nervous system

Comment on: Jensen M, et al. Cell 2012; 149:173-87.     

Wnt signaling regulates experience-dependent synaptic plasticity in the adult nervous system

Comment on: Jensen M, et al. Cell 2012; 149:173-87.     

Narp and NP1 form heterocomplexes that function in developmental and activity-dependent synaptic plasticity

Narp is a neuronal immediate early gene that plays a role in excitatory synaptogenesis. Here, we report that native Narp in brain is part of a pentraxin complex that includes NP1. These proteins are covalently linked by disulfide bonds into highly organized complexes, and their relative ratio in the complex is dynamically dependent upon the neuron's activity history and developmental stage. Complex formation is dependent on their distinct N-terminal coiled-coil domains, while their closely homologous C-terminal pentraxin domains mediate association with AMPA-type glutamate receptors. Narp is substantially more effective in assays of cell surface cluster formation, coclustering of AMPA receptors, and excitatory synaptogenesis, yet their combined expression results in supraadditive effects. These studies support a model in which Narp can regulate the latent synaptogenic activity of NP1 by forming mixed pentraxin assemblies. This mechanism appears to contribute to both activity-independent and activity-dependent excitatory synaptogenesis.     

NMDA受体激活引起的突触活动对Wnt非经典通路的影响

目的探讨NMDA受体激活引起的突触活动诱导Wnt非经典通路的活化。方法构建C57BL/6J胎鼠大脑皮层神经元原代培养体系,用NMDA处理神经元细胞,并结合Western blotting、双免疫荧光染色等技术,检测神经元细胞内Wnt非经典通路的相关蛋白的变化。结果免疫荧光染色显示成功建立了C57BL/6J胎鼠大脑皮层神经元体外培养体系,原代神经元细胞在体外培养10d生长良好,且纯度达90%;体外培养的神经元细胞内存在Wnt5a神经递质,经NMDA的刺激,发现Wnt非经典通路的两个标志性蛋白CaMKII和JNK的磷酸化水平显著增加,且Wnt非经典通路的一种受体Frizzled-5的蛋白表达水平也显著增加。进一步的研究显示,用NMDA竞争性抑制剂DAP5能够阻断NMDA引起的CaMKII和JNK蛋白的磷酸化水平的提高。结论 NMDA受体的激活会诱导Wnt非经典通路的活化。     

Wnt signaling and the regulation of stem cell function

Canonical Wnt signaling plays a crucial role in controlling cell expansion in many types of stem cells. Recent studies, however, demonstrated that Wnt is not only a general stem cell growth factor but can also influence cell lineage decisions in certain stem cell types by promoting specific fates at the expense of others. Thus, Wnt signaling elicits multiple functions in stem cells. Wnt activity appears to depend on cell-intrinsic properties that might change with time during development, thereby altering the cellular response to Wnt. Moreover, the spatial context of a stem cell also determines how the cell interprets Wnt signal activity, in that synergistic or antagonistic signaling pathways can modulate Wnt signaling. How a stem cell integrates Wnt and other signals and how such signaling networks regulate stem cell function on the molecular level remains to be elucidated.     

Wnt signaling regulates the function of MyoD and myogenin

The myogenic regulatory factors (MRFs), MyoD and myogenin, can induce myogenesis in a variety of cell lines but not efficiently in monolayer cultures of P19 embryonal carcinoma stem cells. Aggregation of cells expressing MRFs, termed P19[MRF] cells, results in an approximately 30-fold enhancement of myogenesis. Here we examine molecular events occurring during P19 cell aggregation to identify potential mechanisms regulating MRF activity. Although myogenin protein was continually present in the nuclei of >90% of P19[myogenin] cells, only a fraction of these cells differentiated. Consequently, it appears that post-translational regulation controls myogenin activity in a cell lineage-specific manner. A correlation was obtained between the expression of factors involved in somite patterning, including Wnt3a, Wnt5b, BMP-2/4, and Pax3, and the induction of myogenesis. Co-culturing P19[Wnt3a] cells with P19[MRF] cells in monolayer resulted in a 5- to 8-fold increase in myogenesis. Neither BMP-4 nor Pax3 was efficient in enhancing MRF activity in unaggregated P19 cultures. Furthermore, BMP-4 abrogated the enhanced myogenesis induced by Wnt signaling. Consequently, signaling events resulting from Wnt3a expression but not BMP-4 signaling or Pax3 expression, regulate MRF function. Therefore, the P19 cell culture system can be used to study the link between somite patterning events and myogenesis.     

Wnt signaling regulates acetylcholine receptor translocation and synaptic plasticity in the adult nervous system

The adult nervous system is plastic, allowing us to learn, remember, and forget. Experience-dependent plasticity occurs at synapses--the specialized points of contact between neurons where signaling occurs. However, the mechanisms that regulate the strength of synaptic signaling are not well understood. Here, we define a Wnt-signaling pathway that modifies synaptic strength in the adult nervous system by regulating the translocation of one class of acetylcholine receptors (AChRs) to synapses. In Caenorhabditis elegans, we show that mutations in CWN-2 (Wnt ligand), LIN-17 (Frizzled), CAM-1 (Ror receptor tyrosine kinase), or the downstream effector DSH-1 (disheveled) result in similar subsynaptic accumulations of ACR-16/α7 AChRs, a consequent reduction in synaptic current, and predictable behavioral defects. Photoconversion experiments revealed defective translocation of ACR-16/α7 to synapses in Wnt-signaling mutants. Using optogenetic nerve stimulation, we demonstrate activity-dependent synaptic plasticity and its dependence on ACR-16/α7 translocation mediated by Wnt signaling via LIN-17/CAM-1 heteromeric receptors.     

MPTP modulates hippocampal synaptic transmission and activity-dependent synaptic plasticity via dopamine receptors

Parkinson's disease (PD)-like symptoms and cognitive deficits are inducible by 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP). Since cognitive abilities, including memory formations rely also on hippocampus, we set out to clarify the effects of MPTP on hippocampal physiology. We show that bath-application of MPTP (25?μM) to acute hippocampal slices enhanced AMPA receptor-mediated field excitatory postsynaptic potentials (AMPAr-fEPSPs) transiently, whereas N-methyl-D-aspartate (NMDA) receptor-mediated fEPSPs (NMDAr-fEPSPs) were facilitated persistently. The MPTP-mediated transient AMPAr-fEPSP facilitation was antagonized by the dopamine D2-like receptor antagonists, eticlopride (1?μM) and sulpiride (1 and 40?μM). In contrast, the persistent enhancement of NMDAr-fEPSPs was prevented by the dopamine D1-like receptor antagonist SCH23390 (10?μM). In addition, we show that MPTP decreased paired-pulse facilitation of fEPSPs and mEPSCs frequency. Regarding activity-dependent synaptic plasticity, 25?μM MPTP transformed short-term potentiation (STP) into a long-term potentiation (LTP) and caused a slow onset potentiation of a non-tetanized synaptic input after induction of LTP in a second synaptic input. This heterosynaptic slow onset potentiation required activation of dopamine D1-like and NMDA-receptors. We conclude that acute MPTP application affects basal synaptic transmission by modulation of presynaptic vesicle release and facilitates NMDAr-fEPSPs as well as activity-dependent homo- and heterosynaptic plasticity under participation of dopamine receptors.     

The immunoglobulin super family protein RIG-3 prevents synaptic potentiation and regulates Wnt signaling

Cell surface Ig superfamily proteins (IgSF) have been implicated in several aspects of neuron development and function. Here, we describe the function of a?Caenorhabditis elegans IgSF protein, RIG-3. Mutants lacking RIG-3 have an exaggerated paralytic response to a cholinesterase inhibitor, aldicarb. Although RIG-3 is expressed in motor neurons, heightened drug responsiveness was caused by an aldicarb-induced increase in muscle ACR-16 acetylcholine receptor (AChR) abundance, and a corresponding potentiation of postsynaptic responses at neuromuscular junctions. Mutants lacking RIG-3 also had defects in the anteroposterior polarity of the ALM mechanosensory neurons. The effects of RIG-3 on synaptic transmission and ALM polarity were both mediated by changes in Wnt signaling, and in particular by inhibiting CAM-1, a Ror-type receptor tyrosine kinase that binds Wnt ligands. These results identify RIG-3 as a regulator of Wnt signaling, and suggest that RIG-3 has an anti-plasticity function that prevents activity-induced changes in postsynaptic receptor fields.     

TrkB receptor signaling and activity-dependent inhibitory synaptogenesis

When mouse organotypic cerebellar cultures were exposed to anti-GABA agents that increased neuronal activity early in development, there was a doubling of the ratio of inhibitory axosomatic synapse profiles to Purkinje cell somatic profiles after two weeks in vitro, which correlated with a decrease in spontaneous cortical discharges. When similar cultures were maintained in medium with activity blocking agents, Purkinje cell axosomatic synapses were reduced to approximately half of control values and, after recovery from activity blockade, the cultures discharged hyperactively. By contrast, the full complement of excitatory cortical synapses developed in the absence of neuronal activity. These results support the concept that neuronal activity is necessary for the complete development of inhibitory circuitry. When cerebellar cultures were simultaneously exposed to activity blocking agents and to neurotrophins BDNF or NT-4, both of which bound to the TrkB receptor, the numbers of inhibitory Purkinje cell axosomatic synapses were similar to those of untreated control cultures, and control rates of spontaneous cortical discharges were recorded. The TrkC receptor ligand, NT-3, did not promote inhibitory synapse development in the absence of neuronal activity, and such cultures exhibited hyperactive cortical discharges. These results are consistent with a role for TrkB receptor ligands in activity-dependent inhibitory synaptogenesis. Subsequent exposure of cerebellar cultures to antibody to the extracellular domain of TrkB induced an increased development of Purkinje cell axosomatic synapses, while similar antibody activation of TrkC had no effect on inhibitory synaptogenesis. The promotion of inhibitory synapse development by specific antibody activation of TrkB supports the concept that signaling for activity-dependent inhibitory synaptogenesis is via the TrkB receptor.     

Expression profile and function of Wnt signaling mechanisms in malignant mesothelioma cells

Malignant mesothelioma (MM) is an uncommon and particularly aggressive cancer associated with asbestos exposure, which currently presents an intractable clinical challenge. Wnt signaling has been reported to play a role in the neoplastic properties of mesothelioma cells but has not been investigated in detail in this cancer. We surveyed expression of Wnts, their receptors, and other key molecules in this pathway in well established in vitro mesothelioma models in comparison with primary mesothelial cultures. We also tested the biological response of MM cell lines to exogenous Wnt and secreted regulators, as well as targeting β-catenin. We detected frequent expression of Wnt3 and Wnt5a, as well as Fzd 2, 4 and 6. The mRNA of Wnt4, Fzd3, sFRP4, APC and axin2 were downregulated in MM relative to mesothelial cells while LEF1 was overexpressed in MM. Functionally, we observed that Wnt3a stimulated MM proliferation while sFRP4 was inhibitory. Furthermore, directly targeting β-catenin expression could sensitise MM cells to cytotoxic drugs. These results provide evidence for altered expression of a number of Wnt/Fzd signaling molecules in MM. Modulation of Wnt signaling in MM may prove a means of targeting proliferation and drug resistance in this cancer.