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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
J Neurosci. Author manuscript; available in PMC 2013 July 23.
Published in final edited form as:
PMCID: PMC3584569
NIHMSID: NIHMS438823

Serotonergic neurosecretory synapse targeting is controlled by Netrin-releasing guidepost neurons in C. elegans

Abstract

Neurosecretory release sites lack distinct post-synaptic partners, yet target to specific circuits. This targeting specificity regulates local release of neurotransmitters and modulation of adjacent circuits. How neurosecretory release sites target to specific regions is not understood. Here we identify a molecular mechanism that governs the spatial specificity of extrasynaptic neurosecretory terminal formation in the serotonergic NSM neurons of C. elegans. We show that post-embryonic arborization and neurosecretory terminal targeting of the C. elegans NSM neuron is dependent on the Netrin receptor UNC-40/DCC. We observe that UNC-40 localizes to specific neurosecretory terminals at the time of axon arbor formation. This localization is dependent on UNC-6/Netrin, which is expressed by nerve ring neurons that act as guideposts to instruct local arbor and release site formation. We find that both UNC-34/Enabled and MIG-10/Lamellipodin are required downstream of UNC-40 to link the sites of ENT formation to nascent axon arbor extensions. Our findings provide a molecular link between release site development and axon arborization, and introduce a novel mechanism that governs the spatial specificity of serotonergic extrasynaptic neurosecretory terminals in vivo.

Introduction

Neurons can communicate through both junctional and non-junctional pre-synaptic specializations. The relative frequencies of these two modes of neurotransmission vary between brain regions, however non-junctional release sites (also called neurosecretory terminals) are particularly common among monoaminergic neurons (Descarries et al., 1990; Descarries and Mechawar, 2000; Chase et al., 2004; Fuxe et al., 2010; Parent et al., 2010; Jafari et al., 2011). These non-junctional release sites lack distinct post-synaptic partners to encourage pre-synaptic maturation. Nonetheless, neuroanatomical studies have revealed that neurons elaborate arbors containing extrasynaptic neurosecretory terminals (herein termed ENTs) onto specific targets (Lidov and Molliver, 1982; Descarries et al., 1990; Voutsinos et al., 1994).

The precise targeting of these extrasynaptic release sites is crucial for their roles in locally modulating the responses of other neurons to junctional inputs. Across phyla, these extrasynaptic signals modulate vital functions such as locomotion and arousal, as well as responses to salient or rewarding stimuli such as food (Sawin et al., 2000; Chase et al., 2004; Fuxe et al., 2010). The physiological importance of achieving precise targeting of ENTs is perhaps best reflected by the fact that disruption in these systems is associated with a broad range of disorders, from drug addiction to movement disorders (Fuxe et al., 2010). Still, how target specificity arises for these neurosecretory terminals that lack postsynaptic partners is not understood.

The existence of non-junctional release sites is conserved in the nematode C. elegans. For instance, both dopaminergic and serotonergic neurons in C. elegans are capable of communicating through non-junctional terminals (Chase et al., 2004; Jafari et al., 2011). In particular, the main serotonergic neuron in C. elegans (called NSM) extends axonal arbors decorated with ENTs onto specific target regions (Albertson and Thomson, 1976; Axang et al., 2008; Jafari et al., 2011). The C. elegans NSM neuron provides an opportunity to examine targeted arborization and neurosecretory release site formation, as we can interrogate these conserved processes in vivo, and with single-cell resolution.

Here we took advantage of the facile genetics of the C. elegans system to conduct an unbiased screen to identify the molecular mechanism that governs the spatial specificity of extrasynaptic neurosecretory terminal formation. We observe that post-embryonic arborization is dependent on the Netrin receptor UNC-40/DCC, which localizes to specific neurosecretory terminals at the time of axon arbor formation. This localization is dependent on UNC-6/Netrin, which is expressed by nerve ring neurons that act as guideposts to instruct local arbor and neurosecretory terminal formation. Our findings provide a molecular link between neurosecretory release site development and axon arborization, and introduce a novel mechanism that governs the spatial specificity of extrasynaptic neurosecretory terminals in vivo.

Materials & Methods

Strains and genetics

Worms were raised on NGM plates at 20°C using OP50 E. coli as a food source. N2 Bristol was utilized as the wild-type reference strain. The following mutant strains were obtained through the Caenorhabditis Genetics Center: unc-40(e271)I, unc-6(ev400)X, sax-3(ky123)X, zdIs13 [tph-1p::gfp], jsIs682[rab-3p::gfp::rab-3]; rab-3(ju49), unc-34(e566), unc-34(lq17), mig-10(ct41), ced-5(tm1950), unc-73(e936), ced-10(n3246), mig-2(mu28), rac-2(ok326), unc-104(e1265), madd-2(ok2226), madd-2(ky602), madd-4(ok2854). The unc-40(e271) mutation is a null allele. The nucleotide polymorphism is c7968t and results in a R824* in the ectodomain (Peter Roy, University of Toronto, personal communication). ghIs9 [unc-6p::venus::unc-6]; unc-6(ev400) was received from Yoshio Goshima (Yokohama City University)(Asakura et al., 2010). vsIs45 [tph-1p::gfp] was a gift from Michael Koelle (Yale University). “3-day-old adults” are 1 day post-L4 animals. “4-day-old adults” are 2 days post-L4 animals. All analyses were performed using hermaphrodite nematodes. Unless otherwise indicated, “adult” nematodes are 3-day-old adults.

Molecular biology and transgenic lines

Expression clones were made in the pSM vector, a derivative of pPD49.26 (A. Fire) with extra cloning sites (S. McCarroll and C.I. Bargmann, unpublished data). The plasmids and transgenic strains (0.5–30ng µl-1) were generated using standard techniques (Mello and Fire, 1995) and coinjected with markers unc-122p::gfp or unc-122p::dsRed. olaIs1 [tph-1p::mCherry; tph-1p::cat-1::gfp], olaEx292 [tph-1p::mCherry], olaEx390 [unc-40p::unc-40; tph-1p::mCherry], olaEx188 [tph-1p::unc-40], olaEx799 [tph-1p::unc-40::gfp; tph-1p::mCherry], olaEx570 [tph-1p::unc-40::gfp; tph-1p::mCherry::rab-3], olaEx861 [unc-6p::gfp; tph-1p::mCherry], olaEx192 [tph-1p::unc-34a], olaEx253 [tph-1p::mig-10a], olaEx1113 [tph-1p::snb-1::yfp; tph-1p::mCherry::rab-3]; olaEx1117 [tph-1p::gfp]; olaEx1106 [tph-1p::gfp::syd-2; tph-1p::mCherry::rab-3].

Detailed subcloning information will be provided upon request.

EMS Mutagenesis and Mutant Cloning

unc-40(ola53) was isolated from a visual forward genetic screen designed to identify mutants with abnormal arborization in NSM. vsIs45 animals were mutagenized with EMS as previously described (Brenner, 1974).

Complementation tests were performed by generating ola53/unc-40(e271) trans-heterozygotes. The ola53 allele failed to complement unc-40(e271). The ola53 allele was sequenced using Sanger sequencing techniques which revealed a single C to T nucleotide substitution in exon 3 of unc-40 that results in a nonsense mutation R77*.

Fluorescence microscopy and confocal image acquisition and analysis

Images of fluorescently tagged fusion proteins were captured in live C. elegans nematodes using a 60× CFI Plan Apo VC, NA 1.4, oil objective on an UltraView VoX spinning disc confocal microscope (PerkinElmer). Worms were immobilized using 10mM levamisole (Sigma) and oriented anterior to the left and dorsal up. Images were analyzed using Volocity software (Improvision). Ratiometric images were generated using Volocity software as a ratio between CAT-1::GFP signal and cytosolic mCherry signal.

Cell-Autonomy and Mutant Rescue

The ola53 mutant phenotype was rescued using an unc-40 mini-gene construct as previously described (Colon-Ramos et al., 2007). Cell-specific rescue was achieved by expressing unc-40 cDNA under the control of the tph-1 promoter (Sze et al., 2002).

Quantification

Quantification of the NSM arborization pattern was performed using two criteria to denote wild-type phenotype. These include: 1) Enrichment of axon arbors within the region of NSM neurite that traverses the middle of the pharyngeal isthmus between the first and second pharyngeal bulbs, 2) When quantified by examining confocal micrographs, average arbor length for a given animal is 3μm in length or more, as measured from the intersection from the main neurite to the arbor tip. These two criteria were based on the WT phenotype as characterized by published studies (Axang et al., 2008). Mutant phenotypes and rescue were assessed using an UltraView VoX spinning disc confocal microscope and a Leica DM5000 B microscope. Ventral guidance was scored as WT if the neurite reached the second pharyngeal bulb, dorsal guidance was scored as WT if the neurite terminated halfway between the first and second pharyngeal bulbs. ENT quantifications were performed by generating line scans of CAT-1::GFP along the main NSM neurite. These line scan plots were then given a score of 0 or 1 describing the degree of ENT clustering, with “1” describing WT levels of ENT clustering, and 0 describing diffuse localization. These data were then averaged for each genotype.

Quantification of the relationship between arbor position and shaft ENT position was performed by inspecting confocal micrographs of animals co-expressing CAT-1::GFP and cytosolic mCherry. Arbors were scored as associated with a shaft ENT if CAT-1::GFP puncta were observed at the intersection between the axon arbor and the main neurite shaft. 70 total arbors were scored across 13 animals, and 78% of arbors were determined to be associated with a shaft ENT with a 95% Confidence Interval extending from 66.4% to 85.7%.

Quantification of UNC-40::GFP localization relative to mCherry::RAB-3 puncta was performed by visually inspecting confocal micrographs of animals simultaneously expressing UNC-40::GFP and mCherry::RAB-3. UNC-40::GFP puncta were scored as partially overlapping with mCherry::RAB-3 clusters if GFP and mCherry signal was continuous (i.e., no gaps were visible). 78.3% of UNC-40::GFP clusters partially overlap with mCherry::RAB-3 clusters (with a 95% Confidence interval that extends from 57.7% to 90.8% of UNC-40::GFP clusters partially overlapping with mCherry::RAB-3 clusters). Of the 21.7% of clusters that did not partially overlap with mCherry::RAB-3 clusters, all were localized to axon arbors, and averaged a distance of 1.20μm from the nearest shaft RAB-3 cluster (n=5 animals).

EM Analyses

EM micrographs were obtained from www.wormimage.org. The EM micrographs presented were obtained from the pharyngeal isthmus set from animal N2W, which was imaged by microscopist Nichol Thomson, annotated by Donna Albertson and Marilyn Anness, and curated by the laboratory of David Hall at Albert Einstein College of Medicine through their repository, www.wormimage.com.

To determine the presence of arbors and release sites in EM micrographs, we analyzed EM micrograph series for three animals: JSA, N2W and N2T. We examined serial sections obtained in the anatomical region extending from the posterior part of the first pharyngeal bulb to the anterior part of the second pharyngeal bulb. In the case of animal N2W, from which micrographs in Figure 1A and 1B were obtained, this region included images 136 to 575. Images were inspected for arbor-like extensions continuous with the main NSM neurite and bounded by a visible lipid bilayer. Arbors were then inspected for the presence of synaptic vesicle accumulations and dense projections as previously described and with the assistance of David Hall (White et al., 1986). The micrographs presented in Figures 1A and B are representative of other arbors that were also visualized in micrographs obtained for this region in animals JSA, N2W, and N2T.

Figure 1
The serotonergic NSM neuron forms ENT-containing axon arbors in a specific neuroanatomical coordinate

Statistical Analyses

P values for categorical rescue data were calculated using Fischer's exact test. Error bars for categorical data were calculated using 95% confidence intervals. Statistical significance for UNC-6-expressing guidepost neuron relationship to NSM arbors was determined using the Chi-Squared Test. For continuous data, P values were calculated by performing T-Tests. Error bars for continuous data were calculated using SEM.

Results

NSM exhibits stereotyped post-embryonic arborization and ENT Formation

We generated single-cell fluorescent markers to simultaneously visualize NSM morphology and ENT development. The bilaterally symmetrical NSMs extend their neurites during embryogenesis (data not shown)(Axang et al., 2008). As previously reported, the main neurite bifurcates just posterior to the cell body, forming two axons and one sensory dendrite (Figure 1D–E,H and (Albertson and Thomson, 1976; Axang et al., 2008)). The guidance of these neurites is completed during embryogenesis before the animal hatches (Fig. 1E and (Axang et al., 2008)). Days later, after the animal has hatched and completed early larval stages, we observe that the ventral neurite arborizes within a well-defined neuroanatomical coordinate that corresponds to the location of the nematode nerve ring (Figure 1C–D, H (Axang et al., 2008)). These findings are consistent with previous reports, and suggest a temporal uncoupling of the processes of axon guidance and axon arborization in NSM, with arborization occurring days after axon guidance has terminated (Axang et al., 2008).

Interestingly, the main serotonergic neuron in the parasitic nematode Ascaris suum, which is also called NSM, also arborizes over the nerve ring region. A. suum NSM neurons differ from C. elegans neurons both in terms of soma position and guidance. A. suum NSM cell bodies are positioned posterior to the nerve ring, unlike the C. elegans NSM, which is positioned anteriorly. As a result, A. suum NSMs guide anteriorly toward the nerve ring, while C. elegans NSMs guide posteriorly to reach the nerve ring. In spite of these differences, in both species these neurosecretory neurons arborize specifically over the nerve ring (Johnson et al., 1996). This conserved relationship suggests that targeting mechanisms exist in nematodes to specify neurosecretory neuron arborization over the nerve ring.

NSM’s axon arbors are highly varicose, and have been suggested to contain release sites (Axang et al., 2008). To determine whether these axon arbors form synaptic-like release sites, we first inspected serial EM micrographs of the pharyngeal region from the C. elegans EM repository, WormImage (see Materials and Methods). In particular, we examined EM sections taken from the center portion of the pharyngeal isthmus, where axon arbors typically form (see dashed line in cartoon, Figure 1D). As previously reported, we observed that NSM extends axon arbors adjacent to the basement membrane of the pharynx (Figure 1A–B, (Axang et al., 2008)). Furthermore, we observe dense projections and synaptic vesicles both in the axon arbors and in the main shaft of the NSM neurite (Figure 1A–B). Our observations indicate that the NSM neuron can form release sites both in the main axon shaft and in the axon arbors.

To image these release sites in vivo we expressed proteins that localize to synaptic vesicles (RAB-3 and SNB-1) or active zones (SYD-2) cell-specifically in NSM. Consistent with the EM data, we observed RAB-3, SNB-1, and SYD-2 co-localized in a punctate pattern both in the main axon shaft, as well as in axon arbors (Figure 1K–S). Importantly, we observed that the vesicular serotonin transporter CAT-1 (Duerr et al., 1999) colocalizes with RAB-3, suggesting that the observed release sites correspond to serotonergic vesicle clusters (Figure 1K–M). We also observed that CAT-1:GFP localization to the arbors was dependent on UNC-104/kinesin (Figure 1T-U). Together, our findings indicate that synaptic vesicles are transported to and cluster at release sites in the arbors of NSM.

Using these in vivo markers, we next examined the developmental dynamics of the ENTs in NSM. Examination of ENT development across hundreds of individual animals demonstrated that although the number, shape, and pattern of arbors varies between individuals, where and when arbors and ENTs form is highly stereotyped across animals (n>500 animals; Figure 1E–J).

Interestingly, we observe a spatial correlation between the position of the vesicle clusters and axon arbor branch points. While vesicle clusters, particularly those in the extreme distal and proximal portions of the neurite, are not all associated with arbors, the majority of arbors are associated with vesicle clusters (Figure 1J, arrows). Specifically, 78% of axon arbors contain synaptic vesicle clusters at their base (n=70) (Figure 1J, see Materials and Methods for quantification strategy and variance). These in vivo data are consistent with EM micrographs that reveal release sites at the base of axon arbors (Figure 1A). A correlation between presynaptic release sites and arbor branch points has also been observed in vertebrates, where it has been suggested that release site positions could instruct the emergence of branches and arbors (Alsina et al., 2001; Javaherian and Cline, 2005; Ruthazer et al., 2006).

In summary, our markers allow us to visualize arborization and ENT development with single cell resolution and in vivo. Consistent with previous reports, we observe that NSM forms neurosecretory release sites in the arbors directly apposed to the basement membrane of the pharynx, proximal to the nerve ring (Axang et al., 2008).

unc-40 instructs axon arborization and synaptic vesicle clustering

To identify molecular signals that regulate the precise targeting of serotonergic ENTs in vivo, we performed a forward genetic EMS screen. From this screen we identified a mutant, ola53, with a highly penetrant defect in arbor and ENT formation. Specifically, ENT-containing arbors in ola53 animals are largely absent from the nerve ring terminal field (Figure 2B, ,3C;3C; in WT, 95.5% of animals display ENT-containing arbors in the terminal field, n=22 animals; while in ola53 mutants, 4.3% of animals display arbors, n=47 animals). This defect is not likely a developmental delay as we did not detect arbors in the target field even in 4-day-old adult mutants (Compare Figure 2C and 2D).

Figure 2
UNC-40 is required for terminal arbor formation and synaptic vesicle clustering in the NSM neuron
Figure 3
UNC-40 acts cell-autonomously in the NSM neurons to instruct axon arborization

We also observe that in the mutant animals, vesicles within the ventral neurite fail to cluster properly. Instead, mutants display a more diffuse distribution of serotonin-containing vesicles along the length of the ventral neurite (Figure 2 F–K). Specifically, we observed that while synapses are clustered in 85.0% of adult wild type animals (n=20 animals), they fail to cluster in 50.0% of mutants (n=22 animals).

We then conducted genetic analyses to uncover the molecular lesion responsible for the ola53 mutant phenotype. Three lines of evidence indicate that ola53 is a novel allele of the canonical axon guidance receptor, unc-40/DCC. First, our novel mutant allele, ola53, phenocopies and fails to complement the canonical unc-40(e271) allele (Figure 3A,C and data not shown). Second, sequencing of the unc-40 genetic locus in ola53 mutants revealed an early stop codon in the unc-40 gene (Figure 2E). Third, axon arborization defects in ola53 mutant animals are rescued by an unc-40 mini-gene construct (data not shown). Together, our findings indicate that ola53 is an allele of unc-40, and reveal a novel role for this receptor in promoting local development of extrasynaptic neurosecretory terminals in serotonergic neurons.

UNC-40/DCC is an UNC-6/Netrin receptor, and is best known for its role in axon guidance (Chan et al., 1996; Keino-Masu et al., 1996). In unc-40(ola53) animals, and in animals carrying the canonical null allele unc-40(e271), we observe outgrowth defects of the dorsal neurite as previously reported (Axang et al., 2008). However the ventral axons which bear the synaptic vesicle-containing arbors display normal guidance and outgrowth (Figure 2B; no significant difference in length between WT and unc-40 ventral neurites was observed, data not shown). Thus, the observed requirement for local development of extrasynaptic neurosecretory terminals is not a result of ventral axon guidance defects in NSM. These data are consistent with observations that axon guidance and axonal arborization are temporally uncoupled processes in NSM development (Axang et al., 2008). Interestingly, a similar temporal uncoupling of axon guidance and terminal arborization has been observed for serotonergic neurons in the rat CNS, where it has been suggested that these distinct developmental steps for these vertebrate serotonergic neurons may depend on separate sets of factors (Jacobs and Azmitia, 1992).

UNC-40 acts cell autonomously in NSM, where it localizes to extrasynaptic release sites and instructs local axon arborization

To identify where UNC-40 acts to instruct arborization and vesicle clustering, we first examined cell-specific rescue of unc-40(e271) in NSM. To achieve this, we generated a transgene that expresses unc-40 cDNA using the cell-specific NSM promoter, tph-1p (Sze et al., 2002). Expression of this transgene in unc-40(e271) animals results in rescue of the unc-40(e271) arborization defects (Figure 3A–C), indicating that UNC-40 acts cell-autonomously in NSM to instruct local arborization.

To understand how UNC-40 instructs local arborization and ENT formation, we visualized its subcellular localization in NSM by generating transgenic animals that express UNC-40::GFP cell-specifically in NSM. We observed that UNC-40::GFP is diffusely localized in NSM neurites in Larval Stage 1 animals prior to arborization (Figure 4A–C, n>10). Interestingly, we observed that in Larval Stage 4 animals undergoing arborization, UNC-40 localizes to discrete puncta. UNC-40 subcellular localization during this stage corresponds to areas of the main NSM neurite adjacent to the nerve ring target field, or to nascent arbors within the target field (Figure 4D–F, n>15). We also observed that in adult animals that have completed arborization, UNC-40 became diffusely localized (Figure 4G–I, n>10). Our data indicate that UNC-40 subcellular localization is dynamically regulated during development, and suggest that UNC-40 localizes to subcellular compartments within the nerve ring target region to instruct local arbor formation.

Figure 4
UNC-40 dynamically localizes to ENTs at the time of axon arborigenesis

Given the requirement for UNC-40 both in vesicle clustering and local arbor formation in NSM, we hypothesized that UNC-40 might localize to specific release sites near the nerve ring to instruct local arborization. To examine this hypothesis, we simultaneously expressed UNC-40::GFP and the synaptic vesicle marker mCherry::RAB-3 and visualized the NSM neurons of L4 animals. We observed that all UNC-40::GFP puncta were localized in one of two places: clustered at the tips of nascent axon arbors or clustered adjacent to mCherry::RAB-3 puncta in the main axon shaft (Figure 4J–L’, see Materials and Methods for quantification strategy and variance). Our findings suggest that UNC-40 localizes to vesicle clusters in the main axon shaft, where it then instructs the outgrowth of nascent arbors. This model is also consistent with the observation that a majority of axon arbors within the nerve ring terminal field have synaptic vesicle clusters at their bases.

Recent studies conducted in the vertebrate optic tectum demonstrated that Netrin promotes arbor outgrowth and presynaptic assembly at junctional synapses (Manitt et al., 2009). Although NSM release sites are morphologically distinct from junctional synapses, our findings now provide evidence that could help explain how the Netrin receptor UNC-40 links release sites to arborization. We observe that UNC-40 localizes to vesicle clusters, where it promotes the outgrowth of arbors adjacent to the release sites. It has been observed that presynaptic sites promote the formation of nascent branches in a number of developmental contexts (Alsina et al., 2001; Javaherian and Cline, 2005; Ruthazer et al., 2006; Manitt et al., 2009). Our observations now provide a molecular link that may explain the association between the position of release sites and axon arbor extension.

UNC-40-mediated ENT targeting is genetically separable from UNC-40-mediated branching and synaptogenesis

Our observations regarding a requirement for UNC-40 in vesicle clustering and arborization in NSM are reminiscent of recently reported roles for the Netrin receptor in axon and dendrite branching, and in the formation of junctional synapses (Manitt et al., 2009; Hao et al., 2010; Park et al., 2011; Smith et al., 2012; Stavoe and Colon-Ramos, 2012; Timofeev et al., 2012). To determine if the molecular mechanisms underlying ENT targeting are shared with branching and synaptogenesis, we examined if molecules required for branching and synaptogenesis downstream of UNC-40 in other neurons were also required for arborization in NSM.

We first examined the role of madd-2/trim-9, a tripartite motif protein that was recently shown to act in the UNC-40 pathway in axon branching (Hao et al., 2010). We observe that madd-2(ok2226) mutant animals display defective outgrowth of the ventral neurite of NSM, with one or more ventral neurites failing to extend in 80% of madd-2(ok2226) mutant animals (n=30). Because the arbor-containing ventral neurites do not develop correctly in madd-2(ok2226) mutant animals, we were unable to examine the arbors in this mutant background. However, our findings indicate that madd-2 is required for extension of the ventral neurite, and suggest that madd-2 is involved primarily in unc-40 independent outgrowth events in NSM.

We next examined whether signaling molecules required for UNC-40-mediated synaptogenesis were also required for arborization in NSM. UNC-40 plays a conserved role in instructing presynaptic assembly of junctional synapses (Colon-Ramos et al., 2007; Manitt et al., 2009; Park et al., 2011; Timofeev et al., 2012). The molecular mechanisms required for UNC-40-mediated vesicle clustering at presynaptic sites were recently identified, and shown to depend on the Rac GEF, CED-5/DOCK-180 and CED-10/RAC1 (Stavoe and Colon-Ramos, 2012). To determine if the mechanisms that underpin UNC-40-dependent synaptogenesis (in AIY interneuron) and UNC-40-dependent arborization (in NSM) are shared, we examined NSM in ced-5(tm1950) and ced-10(n3246) mutant animals. We observe that ced-5(tm1950) mutant animals display wild-type axon arborization in NSM (n>30 animals), suggesting that it is not required for NSM axon arborization. For ced-10(n3246) mutant animals, we observe defective outgrowth of the ventral neurite of NSM, with one or more ventral neurites failing to extend in over 60% of ced-10(n3246) mutant animals (n=27). This phenotype, which was similar to that observed for madd-2(ok2226) mutants, prevented us from examining the ced-10 requirement in arborization, and suggested that ced-10, like madd-2, is involved in unc-40-independent outgrowth events in NSM. Together, our findings indicate that the molecular factors involved downstream of UNC-40 in presynaptic assembly and branching are either not required for NSM arborization (as in the case of ced-5) or play other roles in NSM ventral neurite outgrowth (as in the case of madd-2 and ced-10).

We then examined if other downstream components of the Netrin pathway involved in guidance and cell migration could be required for NSM axon arborization. We first examined if the Rac GEF unc-73/Trio or the Racs, mig-2 and rac-2, were required for arborization in NSM (Zipkin et al., 1997; Lundquist et al., 2001). We observed that 64% of unc-73(e936) mutants display a defect in axon arborization in NSM (n=25 animals), indicating that the Rac GEF unc-73 is required for axon arborization in NSM. However, we did not observe a dramatic arborization phenotype for downstream Racs rac-2 or mig-2. rac-2(ok326) mutants were not significantly different from wild type (n=47), while mig-2(mu28) mutant animals display low-penetrance arborization defects (35% of mig-2(mu28) animals display a defect in axon arborization, n=40). These data indicate that the Rac pathway is at least partially required for NSM arborization.

UNC-40 exerts its functions through the cytoskeletal adaptor protein, MIG-10/Lamellipodin and through UNC-34/Enabled (Gitai et al., 2003; Adler et al., 2006; Chang et al., 2006). We observed that mig-10(ct41) mutant animals display highly penetrant defects in axon arborization in NSM (Figure 5E, 81.8% of animals display defective axon arborization, n=44) (Stavoe et al., 2012). We also observe a highly penetrant axon arborization defect in unc-34(e566) mutants (Figure 5C, 92.3% of animals display defective axon arborization, n=26). Cell autonomy studies revealed that, like UNC-40, these two downstream molecules act cell-autonomously in NSM to effect their function during arborization (Figure 5D n=25, Figure 5F n=36).

Figure 5
UNC-34 and MIG-10 function cell-autonomously in NSM to instruct axon arborization

In vertebrates, the unc-34 homolog Ena/VASP directly interacts with the mig-10 homolog, Lamellipodin during axonal morphogenesis (Krause et al., 2004; Michael et al., 2010). It has also been reported that unc-34 and mig-10 have non-overlapping and cooperative functions downstream of UNC-40 to mediate outgrowth (Chang et al., 2006). To understand how MIG-10 and UNC-34 act to instruct arborization, we generated mig-10(ct41);unc-34(lq17) double mutants. We observed that mig-10(ct41);unc-34(lq17) double mutants display an enhanced defect in axon arborization as compared to mig-10(ct41) or unc-34(lq17) single mutants (Figure 5G–H, K). These findings suggest that these two genes act cooperatively downstream of UNC-40 to instruct axon arborization in NSM. Consistent with this model, we observe that mig-10(ct41);unc-40(e271) and unc-34(lq17);unc-40(e271) double mutant phenocopy the unc-40(e271) single mutants. Our findings indicate that axon arborization in NSM depends on signaling pathways that are different from those reported for branching and presynaptic assembly, and more similar to those used to organize the actin cytoskeleton during outgrowth. Together with our previous findings, our data suggest that UNC-40 localizes to release sites in the shaft, where it acts to organize the actin cytoskeleton to instruct axon arborization onto target regions.

UNC-6-expressing guidepost neurons instruct local serotonergic axon arborization

The local development of ENTs and the subcellular localization of UNC-40 suggest the existence of local signals that instruct these processes. UNC-40/DCC is a receptor for UNC-6/Netrin (Chan et al., 1996; Keino-Masu et al., 1996). Therefore, we next examined if UNC-6/Netrin was required for arborization and ENT development in NSM.

We found that mutant animals lacking unc-6/Netrin phenocopied unc-40 mutant animals (Figure 6A–D). In 67.6% of unc-6(ev400) animals (n=34), axon arbors were absent from the nerve ring region. Consistent with Netrin acting as the ligand for UNC-40 in instructing arborization, we also observed that unc-6(ev400);unc-40(ola53) double mutant animals phenocopied the single mutants, with no enhancement in penetrance with respect to the downstream unc-40(ola53) phenotype (Figure 6D, arbors absent in 95.7% of unc-40(e271) mutants (n=47); 93.1% in unc-40(e271); unc-6(ev400) double mutants (n=29)). We did not observe a dramatic phenotype in mutants lacking the UNC-40 ligand, MADD-4 (data not shown)(Seetharaman et al., 2011).

Figure 6
UNC-6 instructs UNC-40 localization and axon arbor formation

We then examined if UNC-6 was required for the subcellular localization of UNC-40::GFP in NSM. We observed that in unc-6(ev400) mutant animals, UNC-40::GFP never clustered during the L4 stage, instead remaining diffusely localized throughout the development of the animal (Figure 6E–G, n>15). Our findings indicate that UNC-6/Netrin is required for UNC-40 clustering during NSM axon arborization and terminal ENT formation.

Consistent with UNC-6 acting as a local cue, we observed that broad expression of UNC-6/Netrin in unc-6(ev400) mutants using a heat-shock promoter did not result in rescue of the unc-6 phenotype in NSM (data not shown). To examine if UNC-6 acts as a local cue, we first visualized the endogenous sites of UNC-6/Netrin expression during the time of NSM arborization. We achieved this by imaging L4 animals expressing an UNC-6::VENUS fusion protein under the unc-6 endogenous regulatory elements (Asakura et al., 2010). As previously reported, we observed that the expression of UNC-6/Netrin persists post-embryonically in few neurons (Wadsworth et al., 1996). Interestingly, we observed that UNC-6 expression persists in nerve ring neurons during the time of NSM arborization. Moreover, UNC-6/Netrin is transported to and clusters at the nerve ring (Figure 7A–C and (Wadsworth et al., 1996; Asakura et al., 2010)). Careful inspection of UNC-6/Netrin clusters revealed that they are in close apposition to the sites of NSM arborization (Figure 7C–C”, n>10).

Figure 7
UNC-6 is expressed by guidepost neurons that provide a local signal to instruct NSM arborization at target regions

NSM forms extrasynaptic release sites and does not have junctional synaptic partners at the nerve ring (Figure 1A–B, (Albertson and Thomson, 1976; Axang et al., 2008)). We hypothesized that these Netrin-expressing neurons, which are not postsynaptic partners to NSM, could serve as guideposts to direct localized arborization and extrasynaptic neurosecretory terminal development near the nerve ring.

To examine this hypothesis, we altered the positions of the NSMs and of Netrin-expressing neurons and assessed local arborization. We achieved this by visualizing Netrin-expressing neurons and NSM arbors in the sax-3(ky123) mutant background. SAX-3/ROBO is a guidance molecule required for axon pathfinding in many C. elegans neurons, including NSM (Axang et al., 2008). Moreover, SAX-3/ROBO mutants display altered nerve ring positioning (Zallen et al., 1999). Therefore, in sax-3(ky123) mutants, the relative position of NSM with respect to the nerve ring is disrupted. Because the expressivity of the phenotype is variable, this provided an opportunity to evaluate animals in which NSM and the Netrin-expressing neurons intersected at aberrant coordinates in the animal, or not at all. Analyses of the phenotypes revealed that proximity of the ventral NSM neurite to the Netrin-expressing neurons was associated with arborization (Figure 7 D–E’; n=32 animals). This occurred even when misguided NSM neurites were in proximity to misguided Netrin-expressing neurons outside the nerve ring, and resulted in ectopic NSM arborization at the sites of proximity (such as the anterior procorpus, Figure 7E–E’). Conversely, if NSM neurites did not intersect with the Netrin-expressing neurons, arbors did not form (Figure 7D, n = 20). Together, our data indicate that UNC-6-expressing guidepost neurons instruct local serotonergic axon arborization and extrasynaptic neurosecretory terminal targeting.

Discussion

The development of neurosecretory terminals onto specific target areas enables neurotransmitters and neuropeptides to exert local roles (Hornung, 2003; Vitalis and Parnavelas, 2003; Bonnin et al., 2007). Although the specificity of neurosecretory terminals is well-documented in vertebrates and invertebrates, we do not yet understand how this precise architecture is specified (Descarries et al., 1990; Jacobs and Azmitia, 1992; Moukhles et al., 1997; Parent et al., 2010). Here we undertook an unbiased approach to identify the cellular and molecular mechanisms required for this process, and uncovered a novel role for the unc-6/unc-40 signaling pathway in instructing local neurosecretory release site targeting. Our findings represent the first description of a mechanism underlying the spatial specificity of extrasynaptic neurosecretory terminals in vivo.

ENT development represents a unique challenge in synaptic specificity. Unlike junctional synapses, the neurosecretory release sites of NSM are unopposed by post-synaptic partners to encourage pre-synapse maturation and specificity. How neurosecretory release sites achieve specificity in targeting is therefore not known. Here we show that nerve ring neurons act as guideposts in specifying the positions of release sites in NSM. Guidepost neurons have been reported to coordinate the innervation of junctional synapses during embryonic development (McConnell et al., 1989; Ghosh et al., 1990; Del Rio et al., 1997). We now identify that Netrin-expressing neurons in the nerve ring can serve a postembryonic role as guideposts specifying local neurosecretory release site formation.

Guidepost neurons instruct local arborization and release site formation of the NSMs through the expression of Netrin. Netrin is a chemotrophic factor best known for its role as a long range cue instructing axon guidance and cell migrations (Hedgecock et al., 1990; Ishii et al., 1992; Serafini et al., 1994; Yee et al., 1999). Our work suggests that Netrin acts as a short-range signaling cue to specify the site of serotonergic arbor formation. This is perhaps best demonstrated by the fact that shifting the position of the guidepost neurons results in a shift in the position of the NSM axon arbors.

Previous studies have shown that Netrin can act as a short range signaling cue in several developmental contexts (Keleman and Dickson, 2001; Srinivasan et al., 2003; Baker et al., 2006; Brankatschk and Dickson, 2006). Recent work in C. elegans, Drosophila, and vertebrates have also uncovered roles for Netrin in synapse formation that are consistent with this chemotrophic factor acting as a short-range cue (Colon-Ramos et al., 2007; Manitt et al., 2009; Park et al., 2011; Timofeev et al., 2012). For example, a recent study in Xenopus describes a role for Netrin in encouraging presynaptic development of DCC-expressing retinal ganglion cells onto Netrin-expressing tectal neurons (Manitt et al., 2009). We now present evidence that Netrin acts as a short-range cue in nerve ring neurons that are not post-synaptic to NSM’s ENTs. Rather, these guideposts coordinate the local development of arbors and neurosecretory release sites proximal to the nerve ring target field.

Whether at long or short range, Netrin typically exerts its influence early in development (Hedgecock et al., 1990; Ishii et al., 1992; Serafini et al., 1994; Rajasekharan and Kennedy, 2009). However, studies of Netrin expression in both vertebrates and invertebrates have demonstrated that Netrin expression persists beyond embryonic development (Wadsworth et al., 1996; Livesey and Hunt, 1997; Manitt et al., 2001). Post-embryonic roles for Netrin are not well understood, but it has been proposed that Netrin could play post-developmental roles in circuit maintenance and plasticity (Shatzmiller et al., 2008; Manitt et al., 2011). Our work identifies a novel post-embryonic role for Netrin in specifying neurosecretory terminal differentiation in C. elegans.

We observe that Netrin is required for proper arborization of the NSM neuron at the specific target field proximal to the nerve ring. Our data suggest that UNC-40-mediated ENT targeting is genetically separable from UNC-40-mediated branching and synaptogenesis. For instance, the NSM neurite forms three major branches, and its ventral branch arborizes in the nerve ring region (Figure 1). The processes of branch extension and arborization occur at different times in development. Interestingly, dorsal branch extension is also dependent on UNC-40, indicating that UNC-40 can play multifunctional roles within the same neuron to instruct different developmental events at various stages (Axang et al., 2008). It appears that branching and arborization events are not only temporally uncoupled, but are also molecularly uncoupled. For example, Netrin-dependent branching is dependent on the tripartite motif protein MADD-2 (Hao et al., 2010). We observe that in NSM, madd-2 is primarily required for the outgrowth of the ventral neurite. unc-40 does not play a major role in the outgrowth of the ventral neurite, suggesting that in NSM, madd-2 pays an unc-40-independent role. Conversely, unc-34 and mig-10, which do not display obvious outgrowth defects of the ventral branch, are required for arborization. Together, our developmental and molecular data indicate that the process of arborization reported here, and the process of branch outgrowth, are distinct. Similarly, we observe that molecules required for Netrin-mediated presynaptic assembly are dispensable, or play minor roles, in NSM arborization.

Despite these important distinctions, unc-40-dependent outgrowth, branching, arborization and presynaptic assembly all require specific patterns of UNC-40 subcellular localization (Adler et al., 2006; Hao et al., 2010; Park et al., 2011; Stavoe and Colon-Ramos, 2012). Indeed, in our system, Netrin exerts its role in axon arborization and terminal ENT formation by directing the localization of its receptor, UNC-40. UNC-40’s transient localization to putative ENTs along the main neurite represents a link between the processes of axon arborization and the position of release sites. Consistent with this link, we observe a specific correlation between the positions of synaptic vesicle accumulations and axon arbor branch points. Such a correlation has been previously reported in vertebrate neurons (Alsina et al., 2001; Javaherian and Cline, 2005; Ruthazer et al., 2006; Manitt et al., 2009). Moreover, it was recently demonstrated that in vertebrates Netrin promotes arbor outgrowth and presynaptic assembly at junctional synapses (Manitt et al., 2009). Our study now supports a role for UNC-40 as the molecular link between the release sites and axon arborization. Specifically, our findings suggest that UNC-40 localizes to pre-existing release sites in the shaft, where it instructs outgrowth of arbors which will then contain terminal ENTs.

In the absence of UNC-40, we observe defects in NSM both in local arborization as well as in vesicle clustering. We hypothesize that underpinning both of these defects is the actin cytoskeleton. Consistent with this, we observe that unc-34/Enabled and mig-10/Lamellipodin are required to instruct arborization. Both mig-10 and unc-34 are known to regulate the actin cytoskeleton during axon outgrowth (Gitai et al., 2003; Chang et al., 2006; Drees and Gertler, 2008; Quinn and Wadsworth, 2008). mig-10 is also required to organize the actin cytoskeleton during synaptogenesis (Stavoe and Colon-Ramos, 2012). Therefore, our findings suggest that unc-40 subcellular localization is required to organize the actin cytoskeleton to promote both vesicle clustering at release sites and axon arborization.

Evidence from vertebrate studies supports a role for the Netrin pathway in the development of monoaminergic circuits. For example, studies in dopamine neurons suggest that the UNC-40 vertebrate homologue, DCC, regulates terminal arborization and synaptic organization of dopaminergic systems (Xu et al., 2010; Flores, 2011; Manitt et al., 2011). While, to our knowledge, there is no evidence for Netrin regulating serotonergic release sites in vertebrates, Netrin and serotonin signaling pathways do intersect in several vertebrate contexts. First, high levels of DCC expression have been detected in developing embryonic murine serotonin neurons, suggesting a role for DCC during serotonin neurodevelopment in vertebrates (Wylie et al., 2010). Furthermore, it has been demonstrated that extrasynaptically released serotonin modifies the Netrin responses of axons originating from the posterior region of the dorsal thalamus, converting attraction to repulsion, and demonstrating a developmental interplay between Netrin-responsive neurons and serotonin-expressing neurons (Bonnin et al., 2007). We now uncover a mechanism by which Netrin governs the spatial specificity of serotonergic extrasynaptic neurosecretory terminals in vivo. Given the evolutionary conservation of the described mechanisms, we speculate that our findings may represent a novel and conserved mechanism for the specification of neurosecretory release sites in vivo.

ACKNOWLEDGEMENTS

This work was funded by the following grants to D.C.-R.: R01 NS076558, R00 NS057931, March of Dimes Research Grant, fellowships from the Klingenstein Foundation and the Alfred P. Sloan Foundation. J. Nelson was also supported by the Interdepartmental Neuroscience Program Training Grant, 5 T32 NS 41228. We thank Y. Goshima, K. Shen, M. Koelle and the Caenorhabditis Genetic Center for strains and reagents. We acknowledge www.wormimage.org for making available the C. elegans EM images. We thank the Hall lab for producing www.wormimage.org and D. Hall for his expertise and aid in analyzing and annotating EM sections. We also acknowledge the work of D. Albertson and M. Anness in annotating the EM prints, and microscopist N. Thomson for obtaining images. We thank Z. Altun (www.wormatlas.org) for the NSM diagram used in figures 1 and and7.7. We also thank C. Gao, J. Belina, E. Strittmatter, and G. Chatterjee for technical assistance and M. Hammarlund, C. Smith, M. Koelle, K. Shen and members of the Colón-Ramos lab for discussion and sharing of advice.

Footnotes

Author Contribution J.C.N and D.C.R. designed experiments, J.C.N. performed experiments, J.C.N. and D.C.R. analyzed and interpreted the data, and wrote the paper.

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