Wnts Promote Synaptic Assembly Through T-Cell Specific Transcription Factors in Caenorhabditis elegans

Synapses are specialized neuronal connections essential for neuronal function. Defects in synaptic assembly or maintenance usually lead to various neurological disorders. Synaptic assembly is regulated by secreted molecules such as Wnts. Wnts are a large family of conserved glycosylated signaling molecules involved in many aspects of neural development and maintenance. However, the molecular mechanisms by which Wnts regulate synaptic assembly remain elusive due to the large number of ligands/receptors, the diversity of signaling cascades and the complexity of the nervous system. In this study, through genetic manipulation, we uncover that C. elegans Wnt-2 (CWN-2) is required for synaptic development. The CWN-2 signal is required during both embryonic and postembryonic development, in the nervous system and intestine, for promoting synaptic assembly. Furthermore, we provide genetic evidence for CWN-2 promoting synaptogenesis through the Frizzled receptor (FZD) CFZ-2, the Dishevelled (DVL) DSH-2, the β-catenin SYS-1 and the only T-cell specific transcription factor POP-1/TCF. Importantly, it is the first time to report the requirement of a TCF for presynaptic assembly. These findings expand our understanding of the synaptogenic mechanisms and may provide therapeutic insights into Wnt-related neurological disorders.

Wnts play complex roles at multiple levels in synaptic development due to the large number of ligands/receptors and the diversity of signal cascades. Wnts activate different signaling cascades to regulate synaptic assembly. For examples, Wnt7a promotes presynaptic assembly through the canonical pathway (Lucas and Salinas, 1997;Hall et al., 2000;Cerpa et al., 2008;Davis et al., 2008), while it promotes postsynaptic PSD95 expression or spine growth by non-canonical Ca 2+ signal pathways (Ciani et al., 2011). The role of Wnts in synaptogenesis is conserved in metazoans (Packard et al., 2002;Inaki et al., 2007;Klassen and Shen, 2007;Jing et al., 2009;Jensen et al., 2012a;Park and Shen, 2012). Although it is well known that Wnts are required to regulate synaptic assembly, many questions remain. For example, systematic studies of Wnts in synaptic assembly are missing. Additionally, although the expression of T-cell specific transcription factor 1 (TCF1) and Lymphoid Enhancing Factor 1 (LEF1), the downstream components in the canonical Wnt pathway, was associated with memory consolidation in mice (Fortress et al., 2013), the requirement of TCF/LEF molecules for synaptic assembly or maintenance has not been reported in any system.
C. elegans has proven to be an excellent model for addressing molecular mechanisms underlying synaptogenesis in vivo at the single cellular level in live animals (Jin, 2005). Wnt signal pathways are conserved in the nematode C. elegans (Sawa and Korswagen, 2013), and regulate neuromuscular junction (NMJ) synaptic assembly and plasticity (Klassen and Shen, 2007;Jensen et al., 2012b;Mizumoto and Shen, 2013;Pandey et al., 2017). However, it remains unknown if Wnts are required for non-NMJ presynaptic formation in the nematode nerve ring, which is analogous to the vertebrate brain. To address this question, we systematically examined the requirement of all five Wnts, four Frizzled receptors (FZDs), three Dishevelled (DVLs), four β-catenin and only one POP-1/TCF for the presynaptic assembly in the Amphid interneurons (AIY). We found that genes encoding components in the canonical Wnt pathway, including cwn-2/Wnt, cfz-2/Fzd, dsh-2/Dvl, sys-1/β-catenin and the pop-1/Tcf, are required for promoting AIY synaptic assembly during both embryonic and postembryonic stages both in the nervous system and in the intestine.

RNA Interference
RNAi constructs were transformed into HT115. The RNAi bacteria and plates were prepared as described previously (Fraser et al., 2000). For each RNAi treatment, we collected eggs from 10 synchronized day 1 adults for 2 h and fed them with RNAi bacteria. Then AIY synaptic phenotype was scored 66 h later when they reached the day 1 adult stage. To quantified the effect of cwn-2 or pop-1 knockdown from different developmental stages, synchronized L1, L2, L4, adult (day 1) animals (12, 20, 42, 66 h after eggs, respectively) were fed with RNAi bacteria and scored for the AIY presynaptic phenotype 3 days later. The wrm-1, dsh-2, sys-1 and pop-1 RNAi efficiency is also verified by quantifying the lethality (Supplementary Figure S9).

Fluorescence Microscopy and Confocal Imaging
Animals were synchronized at specific stages. Larva and adult animals were immobilized using 50 mM muscimol on 3% agarose pad. For examining AIY Zone 2 morphology, we used the Nikon Ni-U fluorescent microscope with FITC filter and 40× objectives. All images presented in this study and for fluorescent intensity quantification were taken with Perkin Elmer UltraView VoX or Andor Dragonfly Spinning Disc Confocal Microscope with 40× objectives and 488 nm (for GFP) or 561 nm (for mCherry or RFP) laser. Images were displayed as extended focus projection. We used Adobe photoshop CC to process the rotation and brightness/contrast levels.

Quantification
Wild type AIY Zone 2 synaptic structure forms a large cluster. The AIY Zone 2 presynaptic fragmentation defined as the AIY zone 2 becomes more than one smaller clusters. For each analysis, at least 20 synchronized animals were scored blindly for each genotype for at least three biological replicates. For rescue analysis, data were collected from three independent transgenic lines. L1, L2, L3, L4, adult (day 1) animals were synchronized as 12, 20, 30, 42, 66 h after eggs laid, respectively. We use the velocity to quantify the fluorescence intensity in AIY Zone 2 and Zone 3. Zone 2 was morphologically defined at the AIY elbow region where the ventral AIY neurite process enters the nerve ring with 10 micrometers in length. Zone 3 is from the distal site of Zone 2 to the tip of the AIY neurites. In the process of analyzing images, we draw a dashed frame figure 1.6 times as long as it wide with the corner as the symmetric center point.

Statistical Analyses
We determined the P values using GraphPad Prism 6.0 (GraphPad Software). Statistical significance was analyzed using either t-students test or ANOVA as indicated in the figure legends.

CWN-2/Wnt Is Required for AIY Presynaptic Clustering
C. elegans AIYs are a pair of bilateral symmetric interneurons located in the nerve ring (Figures 1A,B; White et al., 1986). AIY neurites can be divided into three Zones based on the anatomic location: the ventral part proximal to the soma, called Zone 1; the distal axon in nerve ring, called Zone 3; and the middle elbow region called Zone 2 ( Figure 1B; White et al., 1986;Colón-Ramos et al., 2007). AIY forms large presynaptic clusters in Zone 2, which can be labeled with the synaptic vesicle marker GFP::RAB-3, and the synaptic clustering phenotype is highly reproducible across individual animals (87%, Figures 1D,G; Colón-Ramos et al., 2007).
To test whether Wnts are required for synaptic clustering, we examined the AIY synaptic vesicle marker GFP::RAB-3 clustering phenotype in all four viable Wnt loss of function mutants: cwn-1(ok546), cwn-2(ok895), egl-20(n585), lin-44(n1792; their genetic lesions are shown in Figure 1C, Supplementary Figures S2A-C), and in the essential Wnt mom-2 knockdown animals. Among those mutant alleles we examined, cwn-1(ok546), cwn-2(ok895) and lin-44(n1792) are most likely to be null alleles since the first two are big deletions and the third is an early stop (Herman et al., 1995;Zinovyeva and Forrester, 2005). The egl-20(n585) is probably a strong loss of function mutation since the altered a highly conserved cysteine at position 99 to a Serine (Maloof et al., 1999). We only found that cwn-2 is required for the synaptic clustering as revealed by the synaptic vesicle GFP::RAB-3 marker (Figures 1E,G,H and Supplementary Figures S2F-J). In cwn-2(ok895) mutants, the coherent Zone 2 GFP::RAB-3 clustering is fragmented in 74.7% animals (p < 0.0001, Figures 1E,G). Additionally, we quantified the relative Green fluorescent protein (GFP) intensity and found that the intensity of GFP::RAB-3 is reduced by 60.2% (p < 0.001, Figures 1E,H). To confirm the requirement of cwn-2 for AIY synaptic clustering, we knocked down cwn-2 by RNAi and found robust AIY Zone 2 synaptic fragmentation in cwn-2 RNAi treated animals as well (P < 0.001, Figure 1G). The requirement of cwn-2 for GFP::RAB-3 clustering is further confirmed by the fact that the AIY synaptic defect in cwn-2(ok895) mutants can be rescued by expressing the wild type cwn-2 transgene in cwn-2(ok895) mutants (P < 0.0001 for Zone 2 fragmentation, P < 0.05 for GFP intensity, Figures 1F-H). To examine if the AIY Zone 3 region is affected by cwn-2(ok895), we quantified the GFP intensity and found that the GFP intensity in the AIY Zone 3 region is normal in cwn-2(ok895) mutants (Supplementary Figure S3).
2 GFP is fragmented in cwn-2(ok895) mutants (P < 0.0001 for Zone 2 fragmentation, P < 0.01 for fluorescent intensity, Figures 2A -D ,E,F). Together, these data suggest that cwn-2 is required for both AIY synaptic vesicle and active zone protein assembly in the Zone 2 region. Since SYD-1 marker and RAB-3 marker are colocalized and the presynaptic defect is the same for both markers in cwn-2(ok895), we only use GFP::RAB-3 for our further analysis.

CWN-2 Promotes Synaptogenesis During Both Embryonic and Postembryonic Development
The previous described synaptic phenotype in cwn-2(ok895) could result from the defect of synaptic assembly during embryogenesis or synaptic maintenance during postembryonic stages. To differentiate those two, we examined the AIY synaptic marker GFP::RAB-3 at four larval stages (L1-L4) and the adult stage. In wild type animals, AIY forms a large synaptic cluster at Zone 2 region immediately upon hatching (6.8% abnormal, Figure 3A). The size of the GFP::RAB-3 cluster increases and the morphology remains consistent during growth (Figures 3A-G). However, in cwn-2(ok895) mutants, the Zone 2 fragmentation appears at the first larval stage L1 (76.8% abnormal) and continues into adulthood (Figures 3A -E ,F). Additionally, although the GFP::RAB-3 intensity is similar between wild type and cwn-2(ok895) mutants from hatching to L2 stage (P = 0.64 at L1, P = 0.38 at L2), it is significantly reduced beginning in L3 stage in cwn-2(ok895) mutants (P < 0.001 at L3, P < 0.001 at L4, P < 0.01 at adult, Figures 3A -E ,G). These results suggest that cwn-2 is required not only for synaptic formation during embryonic development, but also for synaptic expansion during postnatal growth.
To confirm the requirement of cwn-2 during the postembryonic development further, we treated wild type animals with cwn-2 RNAi beginning at different stages (L1, L2, L4 and day 1 adult) and examined the AIY presynaptic phenotype 3 days later. We found that postembryonic knockdown of cwn-2 from either L1 or L2, but not from L4 or adult day 1 stage, also led to a synaptic fragmentation in the AIY Zone 2 (P < 0.001 at L1, P < 0.01 at L2, Figure 3H).

CWN-2 Acts Both in the Nervous System and in the Intestine to Regulate AIY Synaptic Clustering
To determine where cwn-2 acts, we first determine the expression pattern by the transcription reporter Pcwn-2::GFP. Consistent with previous findings, Pcwn-2::GFP is expressed beginning in early embryonic stages, mainly in the digestive and nervous systems, with weak expression in the body wall muscles at adult stage (Supplementary Figure S4 and data not shown; Kennerdell et al., 2009;Song et al., 2010). The GFP reporter is only seen in the intestine before 2-fold stage (Supplementary Figures S4A-C). In late embryonic stage, Pcwn-2::GFP is expressed both in the intestine and the pharynx (Supplementary Figures S4D,E). After hatching, Pcwn-2::GFP is mainly seen in the pharynx, some head neurons, the body wall muscle and the intestine (Supplementary Figures S4F,G). However, Pcwn-2::GFP is not seen in the AIY (Supplementary Figures S4H-J).

CFZ-2/FZD Is Required for AIY Synaptic Clustering
Wnts bind to FZD receptors and activate downstream cascade signaling pathways. C. elegans has four genes encoding FZDs: mig-1, lin-17, cfz-2 and mom-5. To address the requirement  H) and fluorescence intensity (G) for synaptic vesicle marker. The Zone 2 fragmentation appears beginning at newly hatched L1 and remains through adulthood (F), while the fluorescence intensity is normal before L2 stage and significantly reduced beginning at L3 (G) in cwn-2(ok895) mutants. Postembryonic knockdown of cwn-2 from L1 or L2, but not from L4 or adult stage results robust Zone 2 fragmentation (H). Data are averaged from at least three biological replicates. ns: not significant, * * p < 0.01, * * * p < 0.001, * * * * p < 0.0001, analyzed by two-tailed Student's t-test. Error bars represent SEM.

DSH-2/DVL and SYS-1/β-Catenin Are Required for AIY Synaptic Clustering
The binding of Wnt to FZD receptors activates DVL. Three C. elegans DVLs are encoded by: dsh-1, dsh-2 and mig-5. To address if they are required for AIY synaptic clustering, we examined the synaptic marker GFP::RAB-3 in dsh-1(ok1445), a putative hypomorphic allele (Supplementary Figure S8A; Klassen and Shen, 2007), and dsh-2 and mig-5 knockdown animals. FIGURE 4 | cwn-2 acts in nervous system and intestine to regulate AIY synaptic clustering. (A-H) Confocal micrographs of the AIY presynaptic pattern of GFP::RAB-3. Presynaptic marker GFP::RAB-3 forms a large continuous cluster at AIY Zone 2 in wild type animals (A), and the cluster is fragmented in cwn-2(ok895) mutants (B). This Zone 2 fragmentation in cwn-2(ok895) is rescued by driving expression of cwn-2 with its own promoter (C), pan-neuronal rab-3 promoter (D), AIY-specific ttx-3 promoter (E) or intestinal-specific ges-1 promoter (F), but not with pharyngeal-specific myo-2 promoter (G) or body wall muscle-specific myo-3 promoter (H). Dashed boxes highlight AIY zone 2 and asterisks indicate the position of AIY soma. The scale bar represents 10 µm. (I,J) Quantification of the GFP::RAB-3 fragmentation (I) or fluorescence intensity (J) in AIY Zone 2. The Zone 2 fragmentation phenotype is rescued by expressing cwn-2 either in the nerve system (with pan-neuronal rab-3 promoter or with AIY-specific ttx-3 promoter) or in the intestine (with ges-1 promoter), while the GFP::RAB-3 intensity can only be rescued by expressing cwn-2 in the intestine, not in the nerve system. Data for each genotype are averaged from at least three biological replicates. Transgenic data are averaged from at least two independent lines. ns: not significance, * * * * p < 0.0001, analyzed by one-way ANOVA Dunnett's test. Error bars represent 95% confidence interval.

POP-1/TCF Is Required for AIY Presynaptic Assembly
The β-catenin interacts with the TCF transcription factors to activate the expression of downstream targets. C. elegans has only one TCF homolog encoded by pop-1 (Lin et al., 1998). To address the requirement of pop-1 for AIY synaptic assembly, we first examined the synaptic phenotype in the pop-1(hu9) mutants, which harbors a mutation at the β-catenin interaction site ( Figure 7A; Korswagen et al., 2002). We found that the AIY Zone 2 fragmentation in pop-1(hu9) mutants was similar to that in cwn-2(ok895) or cfz-2(ok1201) mutants or knockdown of dsh-2 or sys-1 animals (Figures 7B,C,E). To further confirm the requirement of pop-1 for AIY presynaptic assembly, we knocked down pop-1 by RNAi. We found that pop-1 knockdown also resulted in a robust AIY Zone 2 fragmentation (Figures 7D,F,G).
adult stages in pop-1(hu9) mutants. Similar to cwn-2(ok895) mutants, the AIY Zone 2 presynaptic fragmentation appears starting from L1 and continues into adult stage (P < 0.0001 for L1-L3 and adult stages, P < 0.001 for L4 stage, Figure 7E). Next, we treated wild type animals with pop-1 RNAi starting at L1, L2, L4 and day 1 adult stages and examined the GFP::RAB-3 in AIY 3 days later. Similar to the results from cwn-2 RNAi, we observed the synaptic fragmentation in animals treated with pop-1 RNAi starting at L1 or L2, but not after L4 (P < 0.001 for L1, P < 0.01 for L2, P = 0.21 for L4, and 0.14 for adult stages, Figure 7F), suggesting that pop-1 is required in larval stages for the presynaptic assembly.
Collectively, our data suggest that CWN-2 functions through the canonical Wnt signal pathway, which requires CFZ-2/FZD, DSH-2/DVL, SYS-1/β-catenin and the only POP-1/TCF to promote AIY presynaptic assembly. This CWN-2/Wnt signaling acts both cell-autonomously in the AIY and non-cell-autonomously in the intestine, both during embryonic and postembryonic development to promote the AIY presynaptic assembly ( Figure 7H).

DISCUSSION
Synapses are key structures for neuronal function, and synaptic assembly is precisely regulated. In this study, we reported a molecular mechanism by which CWN-2/Wnt regulates the presynaptic assembly in interneuron AIY in C. elegans. Our results demonstrate that CWN-2 regulates the presynaptic assembly during embryonic and postembryonic development through the canonical Wnt signaling pathway, requiring CFZ-2, DSH-2, and SYS-1, and the TCF transcription factor POP-1.
Our genetic data strongly support that specific components in the canonical Wnt signaling promotes C. elegans nerve ring interneuron presynaptic assembly. However, the limitation of this work is that we can not conclusively exclude the requirement of some components in the pathway for two reasons. First, we tested for the requirement of essential genes through RNAi, which could not deplete their expression; second, we scored the synaptic defect mainly based on the fragmentation at Zone 2 region, which will miss those genes that only affect the GFP::RAB-3 intensity or size.

CWN-2/Wnt Promotes Synaptic Assembly
In this study, we found that C. elegans CWN-2 promotes presynaptic assembly as supported by several lines of evidence. First, loss-of-function mutation cwn-2(ok895) results in reduction of both synaptic vesicle and active zone markers at AIY presynaptic sites; second, the synaptic defect in cwn-2(ok895) is rescued by transforming a wild type copy of cwn-2; third, knockdown of cwn-2 with RNAi decreases the AIY synaptic vesicle GFP::RAB-3 clustering.
C. elegans have five Wnts: CWN-1, CWN-2, EGL-20, LIN-44 and MOM-2 (Shackleford et al., 1993;Herman and Horvitz, 1994;Thorpe et al., 1997;Maloof et al., 1999). At presynaptic sites of NMJ, LIN-44 and EGL-20 inhibit synaptic assembly (Klassen and Shen, 2007;Mizumoto and Shen, 2013). The findings that CWN-2 promotes the interneuron AIY presynaptic assembly expand our understanding of the roles of Wnts in C. elegans presynaptic assembly. CWN-2 is the closest to Wnt5 in Drosophila and mammals (Prud'homme et al., 2002). Similar to the role of CWN-2 in AIY presynaptic assembly, Drosophila Wnt5 promotes synaptic formation (Liebl et al., 2008). However, mammalian Wnt5a has a more complex role in hippocampal synaptic development (Davis et al., 2008;Farias et al., 2009;Cuitino et al., 2010;Varela-Nallar et al., 2010;Thakar et al., 2017). Wnt5a was found to promote both glutamatergic spine morphogenesis and GABA receptor trafficking in rat cultured hippocampal neurons (Cuitino et al., 2010;Varela-Nallar et al., 2010), but to inhibit glutamatergic synaptic development in mouse hippocampal neurons (Davis et al., 2008;Thakar et al., 2017). In addition to Wnt5a, other Wnts have been found to either promote or inhibit synaptogenesis in different organisms, suggesting evolutionally conserved roles of Wnts in synaptic development (Budnik and Salinas, 2011;Park and Shen, 2012;Barik et al., 2014). The complex roles of Wnts are partly due to the diversity of Wnts and receptors, various signaling cascades and the complexity and dynamics of synapses in the nervous system. Combined with previous studies (Klassen and Shen, 2007;Mizumoto and Shen, 2013), our findings suggest that like in mammals, C. elegans Wnts have both positive and negative roles in regulating presynaptic assembly.
The POP-1/TCF Mediated Canonical Wnt Pathway Is Required for Presynaptic Assembly Wnts function through either canonical or noncanonical pathways (Ciani and Salinas, 2005). The canonical pathway is mediated by FZD receptors, DVLs, β-catenin and TCF transcription factors (Wisniewska, 2013). Our study found that CWN-2 promotes presynaptic assembly through the canonical Wnt signaling pathway supported by the following evidence. First, mutation in cfz-2/Fzd, or knockdown of dsh-2/Dvl or sys-1/β-catenin by RNAi mimics the AIY Zone 2 presynaptic fragmentation in either cwn-2 or cfz-2 mutants. Second, lossof-function mutation or knockdown of pop-1/Tcf resembles the AIY presynaptic fragmentation observed in either cwn-2 or cfz-2 mutants. Third, combination of mutations, or mutations and knockdown of two or three genes described above shows similar degree of the AIY presynaptic fragmentation to that in any single mutants. Collectively, these data suggest that CWN-2 regulates AIY presynaptic assembly through the canonical signal pathway.
In C. elegans, at the presynaptic sites, LIN-44 and EGL-20 inhibit synaptic assembly independent of the TCF/POP-1 (Klassen and Shen, 2007;Mizumoto and Shen, 2013). Although mutations of POP-1 enhanced the DD neuron presynaptic assembly defect in FSN-1 mutants, POP-1 single mutants showed normal presynaptic phenotype (Tulgren et al., 2014). In vertebrates or Drosophila, the TCF/LEF family of transcription factors can be activated by Wnts (Eastman and Grosschedl, 1999;Korswagen and Clevers, 1999), and is associated with memory consolidation (Fortress et al., 2013), but no evidence indicates its role in synaptic assembly thus far. Our findings showed for the first time that the β-catenin SYS-1 and the TCF transcription factor POP-1 are required for presynaptic assembly in the interneuron AIY.

Embryonic and Postembryonic
Requirement for the Cell-Autonomous and Non-Cell-Autonomous CWN-2 Signal to Promote Synaptic Clustering CWN-2 is expressed both during embryonic and postembryonic developmental stages, and our data suggests that CWN-2 has a role in synaptic assembly during both stages. First, we found that in cwn-2(ok895) or pop-1(hu9) mutants, the AIY Zone 2 presynaptic fragmentation appeared in newly hatched L1, suggesting that cwn-2 and pop-1 are required during embryonic development. Second, postnatal knockdown of cwn-2, dsh-2, or pop-1 with RNAi results in the AIY presynaptic assembly defect, indicating that the Wnt signal is required during larval stages for AIY presynaptic assembly.
AIY presynaptic assembly is largely established during embryonic stages and is maintained throughout the life of the animal (Colón-Ramos et al., 2007;Shao et al., 2013). However, during postembryonic development, as the animal grows, the nervous system architecture, including synaptic structure, scales up (Bénard and Hobert, 2009). We found that while the AIY synaptic distribution is maintained during postnatal development and adult stages, the size or intensity of synaptic marker increases as animals grow (Figure 3). At the NMJs, extracellular matrix (ECM) components such as type IV collagen EMB-9 and ECM remodeling ADAMT proteases such as GON-1 are required for maintaining the synaptic structures during the postnatal stages (Kurshan et al., 2014;Qin et al., 2014). The immunoglobulin superfamily (IgSF) protein ZIG-10 was recently identified for maintaining synaptic densities during development and adulthood (Cherra and Jin, 2016).
Wnts can act either locally or at long distances. Our studies showed that cwn-2 expressed either in the nervous system (or AIY) or in the intestine rescues the AIY Zone 2 fragmentation in the cwn-2 loss-of-function mutants, suggesting that both neuronal and intestinal CWN-2/Wnt regulates the AIY presynaptic assembly. However, the presynaptic GFP::RAB-3 intensity can only be rescued by expressing cwn-2 in the intestine, not in the nerve system, suggesting that the fragmentation effect and the reduction of the GFP::RAB-3 intensity are regulated independently. Alternatively, the fragmentation could be a more severe reduction of the GFP::RAB-3 intensity. Further experiments need to be done to differentiate those possibilities. The data also indicate that CWN-2 from the intestine is probably more important than that from nerve system. Similar to CWN-2, the Frizzled receptor CFZ-2 acts both cell-autonomously and non-cell-autonomously for the fragmentation phenotype. We speculate that the intestinal CWN-2/Wnt signaling is indirect and probably act through secreted signaling molecules. Consistent with this hypothesis, molecules involved in exocytosis or secretion in the intestine are found to regulate neuronal function (Doi and Iwasaki, 2002;Mahoney et al., 2008). Further studies are needed to determine how the CWN-2/Wnt signaling in the intestine regulates synaptogenesis in the AIY neurons.
Wnts are evolutionarily conserved signaling molecules playing critical roles in neural development, including synaptogenesis (Koles and Budnik, 2012;Park and Shen, 2012). Wnt signaling dysfunction is often associated with neurodevelopmental and neurodegenerative disorders such as autism, schizophrenia, bipolar disorder and Alzheimer's disease (Gould and Manji, 2002;Inestrosa et al., 2012;Kwan et al., 2016). The most common feature for those disorders is the defects of synaptic function. Wnt signaling blockade leads to synaptic disassembly in mature hippocampal neurons and probably some neurodegenerative disorders (Purro et al., 2014). Our finding that the presynaptic development requires the canonical Wnt signal and TCF transcriptional factors might provide cues to develop therapeutic strategies for related neurological disorders.

AUTHOR CONTRIBUTIONS
YS and ZS conceived, designed the project. YS, QL and ZS performed experiments and analyzed data, interpreted the results. YS and ZS wrote the manuscript.

FUNDING
This study was supported by the National Natural Science Foundation of China (Grant No. 31471026) and the Young 1000 Plan.