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Glypican Is a Modulator of Netrin-Mediated Axon Guidance

Netrin is a key axon guidance cue that orients axon growth during neural circuit formation. However, the mechanisms regulating netrin and its receptors in the extracellular milieu are largely unknown. Here we demonstrate that in Caenorhabditis elegans, LON-2/glypican, a heparan sulfate proteoglycan, modulates UNC-6/netrin signaling and may do this through interactions with the UNC-40/DCC receptor. We show that developing axons misorient in the absence of LON-2/glypican when the SLT-1/slit guidance pathway is compromised and that LON-2/glypican functions in both the attractive and repulsive UNC-6/netrin pathways. We find that the core LON-2/glypican protein, lacking its heparan sulfate chains, and secreted forms of LON-2/glypican are functional in axon guidance. We also find that LON-2/glypican functions from the epidermal substrate cells to guide axons, and we provide evidence that LON-2/glypican associates with UNC-40/DCC receptor–expressing cells. We propose that LON-2/glypican acts as a modulator of UNC-40/DCC-mediated guidance to fine-tune axonal responses to UNC-6/netrin signals during migration.


Introduction
Directed migrations of developing axons are essential for the proper wiring of the nervous system. A host of guidance cues and their receptors instruct axon guidance decisions. However, how these cues and the growth cone's responses to them are spatially and temporally regulated in vivo remains largely unknown. Answering this question is central to our understanding of how growing axons navigate in complex environments to reach their targets during development and regeneration.
UNC-6/netrin is a highly conserved secreted guidance cue with structural similarity to the extracellular matrix protein laminin [1][2][3]. UNC-6/netrin directs attractive guidance through receptors of the UNC-40/DCC family and repulsive guidance through both UNC-40/DCC and UNC-5/UNC5 receptors [4][5][6]. Notably, whereas netrin receptors and downstream transduction pathways have been well characterized, how netrin signals are regulated extracellularly remains largely unknown. UNC-6/netrin was identified through genetic analysis in Caenorhabditis elegans [1] and biochemically purified and cloned from vertebrate embryos [2]. A second biochemical component that synergized with netrin to elicit axon outgrowth was termed "netrin synergizing activity" (NSA) [3] and remains unidentified. Vertebrate netrin-1 and its receptor DCC can bind heparin, a fully sulfated version of heparan sulfate (HS), in vitro [3,7,8], and a general disruption of HS chain synthesis is detrimental to netrin-1-mediated axon outgrowth in vitro [9,10]. While heparan sulfate proteoglycans (HSPGs) might be intriguing candidates for NSA, it is not yet known whether a specific HSPG is required for netrin signaling or how interactions with HSPGs might regulate netrin signals to direct axons during nervous system development.
We addressed these questions using the nematode C. elegans, which has been instrumental for discovering major conserved axon guidance pathways. During larval development, the axon of the mechanosensory neuron AVM migrates ventrally as its growth cone integrates signals from two complementary guidance cues ( Fig 1A) [1,[4][5][6][11][12][13]: (1) UNC-6/netrin is secreted at the ventral midline and attracts the growth cone ventrally via the receptor UNC-40/ DCC [5,14], and (2) SLT-1/Slit is secreted by the dorsal muscles and repels the growth cone away from the dorsal side via the receptor SAX-3/Robo [12,13]. Animals null for the guidance cues unc-6/netrin or slt-1/Slit exhibit partial AVM ventral axon guidance defects, and loss of both cues in unc-6 slt-1 double mutants results in fully penetrant guidance defects (S1 Fig, [13]). AVM axons defective in guidance fail to extend ventrally and instead migrate laterally in the anterior direction (Fig 1). In this study, we use the AVM axon as a model to elucidate mechanisms that regulate UNC-6/netrin signaling.
Here we provide a missing link in understanding the modulation of UNC-6/netrin signaling in the extracellular milieu. We demonstrate that LON-2/glypican, a HSPG secreted from epidermal cells, acts as a modulator of the UNC-6/netrin signaling pathways to guide migrating cells and axons. We show that LON-2/glypican modulates UNC-6/netrin signaling in both attractive guidance mediated by the UNC-40/DCC receptor and repulsive guidance mediated by the UNC-40/DCC and UNC-5/UNC5 receptors. We provide evidence that LON-2/ glypican associates with UNC-40/DCC-receptor-expressing cells. We show that the N-terminal globular region of LON-2/glypican, lacking the three HS chain attachment sites, is functional in UNC-6/netrin-mediated guidance. Our studies unravel a novel mechanism by which LON-2/glypican is produced by substrate epidermal cells and released from the membrane to likely associate with UNC-40/DCC-expressing neurons, enabling the modulation of their responses to UNC-6/netrin during axon migrations.

Results
lon-2/Glypican and sdn-1/Syndecan Cooperate to Guide unc-6/Netrinand slt-1/Slit-Mediated Axon Migration To address whether a specific HSPG interacts with the netrin signaling system to guide axons, we first examined axon guidance in mutants lacking core HSPGs. HSPGs are composed of a core protein with covalently attached long unbranched HS chains [15]. HSPGs can be associated with the plasma membrane through either a transmembrane domain (e.g., syndecans) or a glycerophosphatidylinositide (GPI) anchor (e.g., glypicans) or be secreted into the extracellular milieu (e.g., perlecans and agrins). We examined the axon morphology of AVM in single, double, and triple mutants for several core HSPG proteins (see S1 Table for alleles). These included the sole C. elegans syndecan (sdn-1), the two glypicans (lon-2 and gpn-1), perlecan (unc-52), and agrin (agr-1). We found that the mild AVM axon guidance defects of sdn-1/syndecan mutants, including a null, [16] were enhanced by the complete loss of lon-2/glypican in double mutants lon-2 sdn-1 (Fig 1B), revealing a role for lon-2/glypican in AVM axon guidance. Similarly, loss of lon-2/glypican enhances sdn-1/syndecan mutants in motorneuron guidance [17]. Although the C. elegans genome encodes two glypicans, loss of function of the second glypican, gpn-1, using two likely null mutant alleles (see S2 Fig), did not enhance the defects of lon-2/glypican or sdn-1/syndecan null mutants in double or triple mutants ( Fig 1B). Moreover, we did not observe abnormal phenotypes in the single mutants for agr-1/agrin or unc-52/perlecan. These observations highlight the specificity of lon-2/glypican function in this axon guidance process and raise the possibility that lon-2/glypican might be a component of the pathways guiding the AVM axon towards the ventral midline.

lon-2/Glypican Acts in the Attractive and Repulsive unc-6/Netrin Guidance Pathways
Considering that AVM axon guidance occurs via the unc-6/netrin and slt-1/Slit pathways, mutations in genes such as lon-2/glypican and sdn-1/syndecan that affect AVM axon guidance may point towards interactions with either of these two guidance systems. Since the AVM axon guidance defects in lon-2 sdn-1 double mutants are qualitatively similar to those of mutants lacking unc-6/netrin or slt-1/Slit, we determined how lon-2/glypican and sdn-1/syndecan impact unc-6/netrin and slt-1/Slit signaling. In animals that completely lack slt-1/Slit function, the complete loss of a gene functioning independently of slt-1/Slit is expected to enhance the AVM guidance defects, such as in the double null mutants unc-6/netrin slt-1/Slit (see S1 Fig).
We tested the interactions of lon-2/glypican with the slt-1/Slit pathway in AVM axon guidance and found that the complete loss of lon-2/glypican enhanced a presumptive null allele of slt-1/ Slit in lon-2 slt-1 double mutants (Fig 1C), suggesting that lon-2/glypican functions in a lon-2/glypican functions in the attractive unc-6/netrin guidance pathway. (A) During the first larval stage of C. elegans, the pioneer neuron AVM extends ventrally along the body wall until it reaches the ventral nerve cord. Its migration results from the combined attractive response to UNC-6/netrin (secreted at the ventral midline) via the UNC-40/DCC receptor and the repulsive response to SLT-1/Slit (secreted by the dorsal muscles) via its SAX-3/Robo receptor. We visualized the morphology of the AVM axon using the transgene Pmec-4::gfp. (B) The heparan sulfate proteoglycans lon-2/glypican and sdn-1/ syndecan cooperate to guide the axon of AVM, as their simultaneous loss enhances guidance defects. The role of lon-2/glypican in axon guidance is specific, as the loss of lon-2/glypican, but not the loss of the other C. elegans glypican, gpn-1, enhances the defects of sdn-1/syndecan mutants. (C) Complete loss of lon-2/glypican enhances the axon guidance defects resulting from disrupted slt-1/Slit signaling in mutants for slt-1/Slit or its receptor sax-3/ Robo, as well as in animals misexpressing slt-1 in all body wall muscles (using a Pmyo-3::slt-1 transgene). Data for wild type and lon-2 are the same as in (B). (D) Complete loss of lon-2/glypican does not enhance the AVM guidance defects of unc-6/netrin mutants or of mutants for its receptor unc-40/DCC, suggesting that lon-2/glypican functions in the same genetic pathway as unc-6/netrin. Data for wild type and lon-2 are the same as in (B). (E) Loss of sdn-1/ syndecan function does not enhance the defects of slt-1/Slit or sax-3/Robo mutants but enhances the defects of unc-40/DCC mutants. Data for wild type, sdn-1, slt-1, sax-3, and unc-40 are the same as in (B-D). Error bars are standard error of the proportion. Asterisks denote significant difference: *** p 0.001,** p 0.01, and * p 0.05 (z-tests, p-values were corrected by multiplying by the number of comparisons). ns, not significant. pathway separate from slt-1/Slit. Loss of lon-2/glypican also enhanced guidance defects when signaling through sax-3/Robo, the slt-1/Slit receptor, was disrupted in lon-2 sax-3 double null mutants, providing further evidence that lon-2/glypican functions in a pathway separate from that of slt-1/Slit ( Fig 1C). As an additional method to investigate the impact of lacking lon-2/ glypican function when slt-1/Slit signaling is perturbed, we used a transgene that ectopically expresses slt-1/Slit from both ventral and dorsal body wall muscles (using Pmyo-3::slt-1) and misguides the axon of AVM [18]. Loss of lon-2/glypican enhanced the defects caused by slt-1/Slit misexpression (Fig 1C), consistent with the above findings that lon-2/glypican mediates its axon guidance effects independently of slt-1/Slit.
The unc-6/netrin pathway functions independently of slt-1/Slit to guide AVM. To address whether lon-2/glypican functions in the unc-6/netrin axon guidance pathway, we examined the AVM axon in double mutants of lon-2/glypican and unc-6/netrin. In animals that completely lack unc-6/netrin function, the complete loss of a gene functioning in the same unc-6/netrin pathway is expected to not enhance the AVM guidance defects, such as in the double null mutants unc-6; unc-40 (see S1 Fig). We found that the complete loss of lon-2/glypican did not enhance the guidance defects displayed by unc-6/netrin null mutants ev400 (Fig 1D). Given that loss of lon-2 enhances the defects of other guidance mutants (see doubles with sdn-1, slt-1, sax-3, and Pmyo-3::slt-1 in Fig 1B and 1C and sqv-5 in S3 Fig), the lack of enhancement when combined with the unc-6 null mutation suggests that lon-2/glypican functions in the same pathway as unc-6/netrin. Consistent with this idea, we also found that complete loss of lon-2/glypican did not enhance the AVM guidance defects of two null mutant alleles of the netrin receptor unc-40/DCC in the double mutants unc-40; lon-2 ( Fig 1D), suggesting that lon-2/glypican functions in the same pathway as unc-40/DCC in AVM ventral guidance. These observations raise the interesting possibility that lon-2/glypican may be the HSPG dedicated to modulate unc-6/ netrin signaling through unc-40/DCC during axon guidance.
Since lon-2/glypican functions independently of slt-1/Slit ( Fig 1C) and partly separate from sdn-1/syndecan (Fig 1B), we tested whether sdn-1/syndecan and slt-1/Slit function together to guide the axon of AVM. We found that defects in slt-1 sdn-1 double null mutants were not enhanced compared to the single mutants (Fig 1E), consistent with findings in Drosophila [19,20] and C. elegans [16]. We also found that double null mutants for sdn-1/syndecan and the slt-1/ Slit receptor sax-3/Robo were not enhanced compared to the single mutants ( Fig 1E). Our results support the notion that sdn-1/syndecan acts in the same genetic pathway as slt-1/Slit to guide AVM. Consistent with this, we found that the double null mutants for sdn-1/syndecan and the netrin receptor unc-40/DCC were enhanced, indicating that sdn-1/syndecan functions in a pathway separate from unc-6/netrin. The analysis of axon guidance in double mutants of unc-6/netrin and sdn-1/syndecan was precluded by their lethality. Our results are consistent with the notion that unc-6/netrin and sdn-1/syndecan act in different pathways of axon guidance.
In addition to unc-6/netrin acting as an attractive cue for cells expressing the unc-40/DCC receptor in ventral guidance, unc-6/netrin also acts as a repulsive cue for cells expressing both the unc-5/UNC5 and unc-40/DCC receptors, which together mediate dorsal guidance away from unc-6/netrin [4][5][6]. To address whether lon-2/glypican functions in unc-6/netrin-mediated repulsive guidance as well, we examined the dorsal migration of the distal tip cells (DTCs) and of the GABAergic motorneuron axons [4,11]. We found that lon-2/glypican single null mutants are defective in dorsal DTC migrations (Fig 2A and 2B) and that the complete loss of lon-2/glypican did not enhance the dorsal DTC migration defects of unc-6/netrin, unc-40/ DCC, or unc-5/UNC5 null mutants (Fig 2B), indicating that lon-2/glypican functions in the unc-6/netrin-repulsive guidance pathway as well. Similarly, complete loss of lon-2/glypican did not enhance the defects of unc-40/DCC mutants in the dorsal guidance of motorneuron axons ( Fig 2C). Given that loss of lon-2/glypican enhances the motorneuron axon guidance defects of sdn-1 mutants as shown in [17], lon-2/glypican plays a role in the dorsal guidance of motorneuron axons. The lack of enhancement of the defects in the dorsal guidance of motorneuron axons of unc-40/DCC mutants by loss of lon-2/glypican further supports that lon-2/glypican functions in the unc-6/netrin pathway mediating dorsal guidance. Thus, lon-2/glypican may In wild-type animals, the DTCs migrate away from the vulva along the anteroposterior axis [1], then turn dorsally [2], and turn again to migrate towards the midbody region [3]. Loss of lon-2/glypican leads to defective DTC guidance, including a failure to migrate dorsally, premature dorsal turning, or a failure to remain dorsal. (B) Quantification of the DTC migration defects in lon-2/glypican mutants and rescue by lon-2(+) (see S7 Table). For each transgenic line, transgenic animals were compared to nontransgenic sibling controls. Complete loss of lon-2/glypican does not enhance the defects of the unc-6/netrin null mutants or those of the null mutants for unc-5/UNC5 and unc-40/DCC, suggesting that lon-2/glypican functions in the same guidance pathway as unc-5/UNC5, unc-40/DCC, and unc-6/netrin (see S6 Table). (C) The axons of the GABAergic motorneurons project dorsally from the ventral midline towards the dorsal nerve cord. unc-6/netrin, unc-5/UNC5, and unc-40/DCC are required for this dorsal guidance of GABAergic axons. Complete loss of lon-2/glypican does not enhance the partially penetrant defects of unc-40/DCC null mutants, suggesting that lon-2/glypican functions in the same pathway as unc-40/DCC and unc-6/netrin to guide axons dorsally (see S5 Table). Error bars are standard error of the proportion. Asterisks denote significant difference: *** p 0.001 and * p 0.05 (z-tests, pvalues were corrected by multiplying by the number of comparisons). ns, not significant. modulate unc-6/netrin signaling not only during attractive guidance but also during repulsive guidance.
To investigate whether lon-2/glypican functions in the same genetic pathway as known downstream mediators of unc-6/netrin signaling, we tested for genetic interactions between lon-2/glypican and unc-34/enabled. unc-34/enabled is a regulator of actin polymerization for axonal filopodia outgrowth [18,[22][23][24][25][26], and its role in both unc-6/netrin and slt-1/Slit guidance pathways renders the analysis of genetic interactions in the context of normal AVM axon guidance challenging. Therefore, we used the unc-6/netrin-specific gain-of-function approach as above, in which the dorsal migration of the PVM axon upon ectopic expression of unc-5/ UNC5 is unc-34/enabled dependent (Fig 3B, [21,27]). We asked whether loss of lon-2/glypican could enhance the extent of suppression of PVM dorsal migration induced by loss of unc-34/ enabled. We found that the PVM dorsal migration was suppressed to the same degree in the double null mutants lon-2; unc-34 and the single mutant unc-34/enabled upon expression of unc-5/UNC5 in PVM (Pmec-7::unc-5, Fig 3B). These results support that lon-2/glypican functions with unc-6/netrin and unc-34/enabled during axon guidance.

Epidermal lon-2/Glypican Functions in Axon Guidance
The AVM growth cone extends along a basement membrane that is located between the epidermis, which is referred to as the hypodermis, and body wall muscles [11]. lon-2/glypican is expressed in the hypodermis and the intestine [28]. We asked in which cell type lon-2/glypican needs to be produced to guide AVM. We found that wild-type lon-2(+) transgenes expressed under the heterologous epidermal promoters Pdpy-7 and Pelt-3 (that drive expression in the hypodermis underlying the AVM growth cone, hyp7) rescued lon-2 slt-1 double mutants back to slt-1 single mutant levels, as efficiently as when expressed under the endogenous promoter Plon-2 ( Fig 4A, S3 Table). Rescue was not observed when we expressed lon-2/glypican in other epidermal cells (seam cells, Pgrd-10), in the migrating neuron itself (Pmec-7), in the intestine (Pelt-2), or in body wall muscles (Pmyo-3) (Fig 4A, S3 Table). Our results suggest that lon-2/ glypican is produced by the hypodermis underlying the growth cone of AVM to function in axon guidance.

sdn-1/Syndecan Functions Cell Autonomously
We found that expressing wild-type copies of sdn-1(+) in the AVM neuron (using the heterologous promoter Pmec-7) rescued axon defects of lon-2 sdn-1 double mutants ( Fig 4B). Accordingly, our examination of a transgene reporting sdn-1/syndecan expression (sdn-1::gfp [16]) revealed that sdn-1/syndecan is indeed expressed in the AVM neuron (S4 Fig), at the time of its ventral migration during the first larval stage. Thus, sdn-1/syndecan appears to function in the migrating neuron in the slt-1/Slit-sax-3/Robo guidance pathway, whereas lon-2/glypican appears to function nonautonomously, as it is produced by the hypodermis underlying the migrating neuron to modulate the unc-6/netrin guidance pathway. Consistent with this, we found that sdn-1(+) cannot replace the function of lon-2/glypican; expressing sdn-1/syndecan in either the cells that normally express lon-2/glypican (using Plon-2::sdn-1) or the migrating neuron itself (Pmec-7::sdn-1) did not rescue the loss of lon-2/glypican (S5 Fig), supporting that lon-2/glypican and sdn-1/syndecan have specific roles in axon guidance. For each rescued transgenic line, transgenic animals were compared to nontransgenic sibling controls (see S2 and S3 Tables). Data for wild type, lon-2, slt-1, and lon-2 slt-1 are the same as in Fig 1B and 1C. (B) Expression of sdn-1/syndecan in the migrating neuron is sufficient for function. Providing wild-type copies of sdn-1(+) in AVM (expressed under the heterologous promoter Pmec-7) rescues the axon guidance function of sdn-1 in a lon-2 sdn-1 double mutant. We assayed rescue of sdn-1 function using the double mutant lon-2 sdn-1 since it is easier to rescue defects that are 33% penetrant (as in the double lon-2(e678) sdn-1(zh20)) than to rescue defects that are 12% penetrant (as in the single mutant sdn-1(zh20)). For each transgenic line, transgenic animals were compared to nontransgenic sibling controls (see S2 Table). Data for wild type, lon-2, sdn-1, and lon-2 sdn-1 are the same as in Fig 1B-1D. Scale bar, 5 μm. Error bars are standard error of the proportion. Asterisks denote significant difference: *** p 0.001, ** p 0.01 and * p 0.05 (z-tests, p-values were corrected by multiplying by the number of comparisons). ns, not significant.

LON-2/Glypican Lacking Its Heparan Sulfate Chain Attachment Sites Functions in Axon Guidance
Glypicans are composed of a core protein moiety with covalently linked HS chains attached via a tetrasaccharide linker at specific Serine residues (Fig 5A, [15]). Prior studies on the role of HSPGs in other developmental pathways indicate that both the identity of the HSPG core proteins and the heterogeneity of their HS chains modified by epimerization and sulfations [15] contribute to the specificity of the interactions between particular HSPGs and the proteins that they bind [15,29,30].
To address the importance of the HS chains linked to LON-2/glypican during axon guidance, we tested whether a mutated form of LON-2/glypican lacking its HS chains could still function in axon guidance. For this experiment, the three Serine residues serving as HS chain attachment sites were mutated to Alanine residues, generating the mutant LON-2ΔGAG [31]. Western blot analysis confirmed that LON-2ΔGAG severely reduced HS chains associated with LON-2, in both worms and S2 cells (Fig 5B and S6 Fig). We then expressed LON-2ΔGAG under the Plon-2 endogenous promoter and found that the AVM guidance defects of lon-2 slt-1 double mutants were rescued back to the level of slt-1 single mutants ( Fig 5C). Similarly, the DTC migration defects of lon-2/glypican mutants were rescued by LON-2ΔGAG expression ( Fig 5D). Our results indicate that LON-2/glypican devoid of its HS-chain attachment sites can function in unc-6/ netrin-mediated guidance, suggesting that the core protein is the critical part of LON-2/glypican for its function in unc-6/netrin-mediated guidance of cell and axon migrations.

LON-2/Glypican Associates with Cells Expressing UNC-40/DCC
Our above observations provide evidence that the HSPG lon-2/glypican functions in the same genetic pathway as unc-6/netrin to guide migrating axons. It has been shown in several models that HSPGs play multifaceted roles across various signaling pathways, such as facilitating ligand-receptor interactions and transporting morphogens, as well as localizing and stabilizing ligands [32,33]. We asked if the LON-2/glypican molecules might interact with either UNC-6/ netrin or its receptor UNC-40/DCC, suggesting a potential mechanism of action for LON-2/ glypican in unc-6/netrin-mediated guidance. To test these interactions, we generated epitopetagged versions of LON-2/glypican, UNC-6/netrin, and UNC-40/DCC proteins, with human influenza hemagglutinin (HA), superfolder-GFP (SfGFP), and FLAG, respectively (Fig 6A), and used cell-mixing experiments. We independently expressed each of these labeled proteins in separate populations of Drosophila S2 cells for 2 d, then cocultured them overnight, and detected the tagged proteins by western blot analysis (see S7 Fig) and by immunostaining ( Fig 6A).
We tested whether the HS chains of LON-2/glypican were necessary for its association with UNC-40-expressing cells. We used a mutated form of LON-2/glypican lacking its three HS back to the level of slt-1 single mutants. Secreted globular LON-2/glypican is functional in axon guidance. LON-2/glypican was engineered to be secreted by deleting its GPI anchor (LON-2ΔGPI) or by deleting the C-terminus, thus lacking the GPI anchor and the HS attachment sites (N-LON-2). Both LON-2ΔGPI and N-LON-2 function in axon guidance, as assayed by their ability to rescue axon guidance defects of lon-2 slt-1 back down to the level of slt-1 single mutants. In contrast, a form of LON-2/glypican containing its C-terminus including the three HS attachment sites, but lacking its N-terminal globular domain (C-LON-2), is not functional (see S2 and S3 Tables), indicating that the N-terminal globular domain of the core protein is key to the function of LON-2/glypican in axon guidance. For each rescued transgenic line, transgenic animals were compared to nontransgenic sibling controls (see S2 and S3 Tables). Data for wild type, lon-2, slt-1, and lon-2 slt-1 are the same as in Fig 1B and 1C. (D) A form of LON-2/glypican lacking HS chain attachment sites is functional in DTC guidance. The DTC migration of lon-2 mutants carrying the transgene Plon-2::LON-2ΔGAG is rescued back to wild-type levels. Secreted N-terminus globular LON-2/glypican (N-LON-2) is functional in DTC guidance, as DTC guidance defects of lon-2 mutants are rescued by N-LON-2. Transgenic animals were compared to nontransgenic sibling controls (see S7 Table). Data for wild type and lon-2 are the same as in Fig 2B. Error bars are standard error of the proportion. Asterisks denote significant difference: *** p 0.001, ** p 0.01 (z-tests, p-values were corrected by multiplying by the number of comparisons).  (Fig 6B and 6C experiment 6). These results suggest that if LON-2/glypican interacted directly or indirectly with UNC-40/DCC, then the interactions of LON-2/glypican and UNC-6/netrin would occur with different regions of UNC-40/DCC. Consistent with this possibility, we found that LON-2/glypican still associated with cells expressing UNC-40ΔFn4/5:: FLAG, a mutated form of UNC-40/DCC that lacks the UNC-6/netrin-binding sites (FnIII domains 4 and 5) (Fig 6B and 6C experiment 7, [34,35]). Our results indicate that for LON-2/glypican to associate with UNC-40/DCC-expressing cells, the FnIII domains 4 and 5 of UNC-40/ DCC are dispensable and UNC-6/netrin does not need to be bound to UNC-40/DCC.

LON-2/Glypican Increases Membrane Outgrowths Triggered by UNC-40/DCC
Previous work has suggested that overexpression of DCC in cells overactivates DCC downstream signaling pathways, leading to cytoskeletal rearrangements that result in increased membrane extensions and cell surface area [36]. Similarly, expression of UNC-40/DCC leads to changes in cellular morphology in our cell assays (Fig 6D). To test whether the association of LON-2/glypican with UNC-40/DCC-expressing cells results in an activation of signaling downstream of UNC-40/DCC, we examined the impact of LON-2/glypican on the morphology of UNC-40/DCC-expressing cells. For these experiments, we mixed mCherry-expressing cells with either untransfected control cells or LON-2/glypican-expressing cells, and we also mixed UNC-40/mCherry-expressing cells with either untransfected control cells or LON-2/glypicanexpressing cells. Examination of the morphology of these cells 1 d after mixing revealed that UNC-40/mCherry-expressing cells mixed with LON-2/glypican exhibited an increased frequency of irregular shapes and membrane extensions, compared to UNC-40/mCherry cells mixed with control cells (Fig 6D). Thus, consistent with a model in which LON-2/glypican functions in the UNC-6/netrin signaling pathway to guide developing axons, the association of LON-2/glypican with UNC-40/DCC-expressing cells leads to increased membrane extensions, suggestive of increased signaling downstream of the UNC-40/DCC receptor.

LON-2/Glypican Is Released Extracellularly and Its N-terminal Domain Is Functional
While LON-2/glypican possesses a signature GPI anchor that mediates its attachment to plasma membranes (Fig 5A), our experiments indicate that LON-2/glypican is released into the extracellular milieu through cleavage where it can diffuse to associate with UNC-40/DCC-expressing cells. This is consistent with prior work demonstrating that many glypicans are shed or cleaved into a soluble form [37]. To verify that LON-2/glypican is indeed released into the extracellular medium, we collected cell-free media from HA::LON-2 cultures (HA::LON-2-conditioned medium) and added it to cells expressing UNC-40::FLAG. We found that HA::LON-2-conditioned medium contained HA::LON-2 that associated with UNC-40::FLAG-expressing cells. As above, this interaction was specific, as no HA::LON-2 signal was found on adjacent untransfected cells (Fig 6B and 6C experiment 8). This result provides compelling evidence that LON-2/glypican can be released from the membrane of LON-2/glypican-expressing cells, diffuses, and associates with UNC-40/DCC-expressing cells. We propose that using a similar mechanism, LON-2/ glypican may be shed from epidermal cells and may interact with migrating axons that express UNC-40/DCC. This is consistent with our finding that LON-2/glypican is produced by the hypodermis to function nonautonomously in unc-6/netrin-mediated AVM axon guidance.
To provide evidence for the model that LON-2/glypican can function in axon guidance when detached from the plasma membrane, we used a form of LON-2/glypican lacking the GPI anchor, LON-2ΔGPI, which should be directly secreted into the extracellular milieu [31]. LON-2ΔGPI rescued the AVM guidance defects of lon-2 slt-1 double mutants back to the level of slt-1 single mutants (Fig 5C). We also used a truncated form of LON-2/glypican (N-LON-2) containing the N-terminal globular domain, but lacking the C-terminal region, thus removing the three HS attachment sites and the GPI membrane anchor. N-LON-2 also rescued the AVM guidance defects of lon-2 slt-1 double mutants back to the level of slt-1 single mutants (Fig 5C). In contrast, a reciprocal construct containing only the C-terminus with the three HS attachment sites and the GPI anchor (C-LON-2) did not rescue the AVM axon guidance defects of lon-2 slt-1, consistent with the model that the N-terminal globular domain of LON-2/glypican is the key functional domain during guidance (Fig 5C). A secreted form of LON-2/glypican is also functional in DTC guidance, as we found that DTC guidance defects of lon-2/glypican mutants could be rescued by expression of N-LON-2, containing only the N-terminal globular domain ( Fig 5D). These findings also support the hypothesis that LON-2/glypican may normally be released from the hypodermis to interact with the unc-6/netrin pathway to direct the migrating growth cone during development (Fig 7).

Discussion
Growth cone responses to guidance cues require precise regulation as developing axons traverse complex extracellular environments in order to reach their targets. The mechanisms by which guidance cue signals are regulated in the extracellular milieu are still poorly understood [38]. Here, we demonstrate that the unc-6/netrin-unc-40/DCC guidance system is modulated by the HSPG lon-2/glypican.

lon-2/Glypican Is a Component of the unc-6/Netrin Signaling Pathways
Our studies identify the HSPG LON-2/glypican as a component of the unc-6/netrin attractive and repulsive signaling pathways that guide axons during development. We show that LON-2/ glypican specifically acts on unc-6/netrin signaling independently of slt-1/Slit. We demonstrate that lon-2/glypican functions from the hypodermis, the epidermal cells that secrete the substrate along which growth cones extend [11], and that a secreted form of LON-2/glypican, containing only its N-terminal globular region and lacking its HS chains, guides cells and axons in vivo. In addition, we provide evidence that LON-2/glypican is released from cells producing it and associates with cells expressing UNC-40/DCC receptors. Taken together, our observations support a hypothetical model in which GPI-linked LON-2/glypican is produced by substrate epidermal cells, is released into the extracellular milieu, and binds growth cones expressing UNC-40/DCC receptors to regulate attractive and repulsive responses of the growth cone to UNC-6/netrin.
The impact of lon-2/glypican on the unc-6/netrin signaling pathway is highly specific. First, loss of lon-2/glypican, but not of sdn-1/syndecan, suppresses the guidance phenotypes elicited by the gain-of-function condition in which unc-5/UNC5 was misexpressed. Second, the complete loss of lon-2/glypican does not enhance the guidance defects observed in null mutants for unc-6/netrin or its receptors unc-40/DCC and unc-5/UNC5, whereas it does enhance the defects of several other axon guidance mutants, including sdn-1/syndecan, slt-1/Slit, misexpressed slt-1/Slit (Pmyo-3::slt-1), sax-3/Robo, and sqv-5, suggesting that lon-2/glypican functions specifically in the unc-6/netrin pathway. Third, sdn-1/syndecan, cannot replace lon-2/glypican function, highlighting a requirement for lon-2/glypican that cannot be achieved by any HSPG. Given that the core protein of LON-2/glypican, devoid of its HS chains, is fully functional in guidance, the specificity of action of LON-2/glypican in netrin-mediated guidance appears to reside in the core protein itself. As a note, whereas lon-2/glypican mutants are defective in DTC migration, the lon-2/glypican mutant by itself does not show drastic alterations in AVM axon guidance as is observed with other modulators [32]. It is possible that in the absence of lon-2/glypican, another HSPG may provide compensation or that our scoring of strong alterations in pathfinding did not include more subtle phenotypes, as could be expected from a modulator of the signal [32].

The Core Protein of LON-2/Glypican Is Functional in UNC-6/Netrin-Mediated Guidance
We show that the LON-2/glypican core protein, devoid of HS attachment sites, is able to associate with UNC-40-expressing cells and is functional in unc-6/netrin-mediated guidance. Thus, the core protein is the critical region of LON-2/glypican for netrin-mediated axon guidance. This is in line with previous studies showing a contextual dependence of HS chains for glypican function. For instance, the core protein of C. elegans LON-2/glypican and of Drosophila glypican Dally do not require HS chains to function in the transforming growth factor beta (TGFβ) pathway [31,39]. Similarly, Drosophila glypican Dally-like interacts with Wg and Hh through their protein core in a HS-independent manner [33,40,41], and mammalian Glypican-3 does not require HS chains for its role in Wnt and Hh signaling [42][43][44][45]. While the HS chains are not critical for the role of LON-2/glypican in guidance, a contribution of HS chains to modulate functionality, as observed for other glypicans in the context of BMP4, Wnt3, Wg, and Hh signaling [33,39,41,42], cannot be ruled out. For instance, it is conceivable that the normal endogenous HS chains of LON-2/glypican may impact its trafficking, levels, release from the membrane, recruitment of binding partners, or recycling.

LON-2/Glypican Is Released from the Cell Surface to Guide Axons
LON-2/glypican is predicted to localize at the cell surface via its GPI anchor [31]. However, in our cell culture studies, we demonstrate that LON-2/glypican can be released as a soluble molecule from producing cells. We also show that two truncated forms of LON-2/glypican, LON-2ΔGPI and N-LON-2, which are no longer associated with the plasma membrane and are secreted into the extracellular milieu, can function to guide axons in vivo. This indicates that LON-2/glypican is likely released from the epidermal cells to reach the growth cone to modulate its guidance. This finding raises the question of how LON-2/glypican is released from the cell membrane and how this process might be regulated during development. The release of LON-2/glypican from the surface of cells could involve phospholipases that cleave the GPI anchor and/or proteases that cleave its extracellular domain, such as at a predicted furin-cleavage site (Fig 5A, [31]).
Glypican cleavage by lipases and proteases has been demonstrated to occur and to be functionally important in other contexts, such as in regulating fibroblast growth factor (FGF) and Wnt signaling during morphogenesis [37,46]. For instance, the Drosophila glypican Dally-like protein is cleaved at the GPI anchor by the lipase Notum, to negatively regulate Wnts [47]. Similarly, several mammalian glypicans, including glypican-3, are cleaved by Notum [48]. The functional importance of glypican proteolytic cleavage is illustrated by the processing of glypican-3 by a furin-like convertase to modulate Wnt signaling in zebrafish [49]. In addition, glypican-1 and glypican-4 are proteolytically cleaved to stimulate long-range FGF signaling in the Xenopus embryo [50] and increase the efficiency of myogenic differentiation in the presence of FGF in mammalian cells [51], respectively. Our studies show that glypican processing also functions during axon guidance.

LON-2/Glypican Associates with the Surface of UNC-40/ DCC-Expressing Cells
We demonstrate that LON-2/glypican is secreted into the extracellular medium and decorates the outline of UNC-40/DCC-expressing cells. Deleting the extracellular domain of UNC-40 (UNC-40ΔNt) abrogated the association of LON-2/glypican with UNC-40/DCC-expressing cells, indicating that LON-2/glypican may interact with UNC-40/DCC at the cell surface. The association of LON-2/glypican with UNC-40/DCC may be direct or indirect through interactions with other molecules (Fig 7). Our experiments demonstrate that UNC-6/netrin binding to UNC-40/DCC was undisturbed by the association of UNC-40/DCC with LON-2/glypican, suggesting that the possible interaction of LON-2/glypican with UNC-40/DCC likely involves a region of UNC-40/DCC other that the netrin binding sites. Indeed, we found that LON-2/ glypican associates with UNC-40/DCC-expressing cells even when the UNC-40/DCC receptors lack the UNC-6/netrin binding domains.
We found that LON-2/glypican leads to increased irregular morphology of UNC-40/ DCC-expressing cells. Ectopic expression of DCC in mammalian cells activates downstream signaling via Cdc42 and Rac1, producing cytoskeletal rearrangements that lead to filopodia outgrowth and cell surface extensions [36]. Our finding that the presence of LON-2/glypican enhances the UNC-40/DCC-induced irregular cell morphology and filopodia-like extensions suggests that the association of LON-2/glypican with UNC-40/DCC-expressing cells may increase signaling downstream of UNC-40/DCC. Consistent with this notion, we show that lon-2/glypican functions in the same signaling pathway as the UNC-40/DCC downstream mediator unc-34/enabled during axon guidance.
Our results suggest a possible regulatory mechanism in the extracellular space in which secreted LON-2/glypican modulates the activity of the receptor UNC-40/DCC. LON-2/glypican may directly bind UNC-40/DCC, or alternatively, LON-2/glypican may instead interact with other molecules to impact UNC-40/DCC to modulate its stability, distribution, or activity. Alternatively, LON-2/glypican could potentially function as a co-receptor for UNC-6/netrin, where it may facilitate the formation of UNC-6/netrin-UNC-40/DCC-LON-2/glypican signaling complexes, similar to the situation in FGF signaling [15]. It is also conceivable that LON-2/glypican could bind UNC-6/netrin directly as well, even if undetected in our assays, as netrin has been found to bind heparin in vitro [3,52,53]. Previous studies have also documented the binding of DCC to heparin in vitro [7,8], and while we have found that the core protein is the critical portion of LON-2/glypican in netrin-mediated axon guidance, it remains possible that the endogenous HS chains contribute to the function of LON-2/glypican in axon guidance.
In summary, our studies uncover a novel mechanism by which UNC-6/netrin signaling through its UNC-40/DCC receptor is modulated by the HSPG LON-2/glypican during axon pathfinding. Given the evolutionary conservation of the UNC-6/netrin pathway components (UNC-6/netrin and its receptors UNC-40/DCC and UNC-5/UNC5) and of glypicans (LON-2 is most similar to mammalian glypican-3) and that synthesis of HS chains is required for mammalian axons to respond to netrin-1 in vitro [9,10], glypicans are likely to play a role in netrinmediated axon pathfinding in mammals as well. Our findings provide a general mechanism for the extracellular regulation of growth cone responses to netrin during the development of nervous systems.

Nematode Strains and Genetics
Nematode cultures were maintained at 20°C on NGM plates seeded with OP50 bacteria as described [54]. Strains were constructed using standard genetic procedures and are all listed in S8 Table. Genotypes were confirmed by genotyping PCR or by sequencing when needed, using primers listed in S9 Table. Real-Time PCR (RT-PCR) for gpn-1 Alleles Total RNA was extracted from worm samples using Trizol (Invitrogen) according to the manufacturer's instructions. 500 ng RNA was used to reverse transcribe using the High Capacity cDNA Reverse Transcription Kit (Applied Biosystems) and random primers. PCR reactions were carried out with first-strand cDNA template, primers oCB834 (ATCAAGACCGAGTGATAGTG) and oCB1321 (TGGCGAGTATTCCCGTTTAG) were used for gpn-1 cDNA amplification, and primers oCB992 (TCGCTTCAAATCAGTTCAGC) and oCB993 (GCGAGCATTGAACAGTGAAG) were used for the control gene Y45F10D.4 [55] cDNA amplification.

Neuroanatomical and Distal Tip Cell Observations
Animals were mounted on agarose pads, anaesthetized with 100 mM sodium azide, and examined under a Zeiss Axio Scope.A1 or a Zeiss Axioskop 2 Plus.
AVM and PVM axon guidance analysis. Axons of neurons AVM and PVM were examined in L4 larvae and adult animals using zdIs5, an integrated Pmec-4::gfp reporter [56]. Animals with the cell body of AVM posterior to the vulva (cell migration defect) were excluded from axon guidance analysis.
AVM analysis. Worms were counted as mutant for AVM ventral axon guidance if (a) AVM failed to send an axon ventrally and instead projected laterally to the anterior or (b) the AVM axon projected laterally, in the anterior or the posterior direction, for at least~15 μm (>3 AVM cell body lengths) before projecting to the ventral side. The angle between the anterior/posterior axon projection and the ventral axon projection had to be >45°to be counted as mutant, therefore excluding animals with a slight curve in the ventral axon of AVM from the mutant count.
PVM analysis criteria. Worms were counted as having their PVM axon misoriented dorsally if the axon of PVM was projecting to the dorsal side of the animal. The vulva was used as a reference for the ventral side. Worms whose zdIs5 labeled neurons were too misplaced to be identified were excluded from analysis.
Distal tip cell guidance analysis. The path of migration of the DTCs brings about the shape of the mature gonad arms (Fig 2A, [11]). In the wild type, the DTC migrates away from the vulva location along the anteroposterior axis of the animal. The DTC then turns dorsally to reach the dorsal side of the animal, where it then migrates towards the vulva along the anteroposterior axis of the animal. To infer the path of DTC migration, gonad arms were examined in late L4 and young adult animals using DIC microscopy. Animals were counted as having abnormal gonad arm shapes when (a) the distal arm of the gonad was located ventrally instead of dorsally, indicating a failure of the DTC to migrate dorsally; (b) the proximal gonad arm was too short, resulting from a premature turn of the DTC towards the dorsal side; or (c) the gonad arm was twisted over itself.
GABAergic motorneuron axon analysis. Commissures of the GABAergic motorneurons were analyzed in L4 larvae and adults using ufIs34, an integrated Punc-47::mCherry reporter [57]. For each animal, all GABAergic commissures (except DD1 and VD2) were counted and categorized as either reaching the dorsal cord or failing to reach the dorsal cord. The fraction of commissures failing to reach the dorsal cord among all the commissures extending from the ventral cord was determined for each animal, averaged for the genotype, and expressed as a percentage.

C. elegans Constructs and Microinjections to Generate Transgenic Animals
All inserts of finalized clones were verified by sequencing.
Constructs to Express Variants of LON-2, UNC-40, UNC-6, SDN-1, and Evi in S2 Cells All inserts of finalized clones were verified by sequencing.
HA::LON-2 (pCB285). The C. elegans lon-2 full-length cDNA (yk1346g07) was amplified by PCR with primers adding the EcoRI and ApaI restriction enzyme sites and cloned into pBlueScript II (Life Technologies). HA::LON-2 was made by synthesizing a DNA fragment, carrying the restriction sites EcoRI and HincII and containing the 5ʹ end of the lon-2 cDNA with the HA tag added after the signal peptide sequence (at cDNA position 61) and cloned into pBlueScript/lon-2. The HA::LON-2 EcoRI/ApaI fragment was then cloned into the pActin5.1/ V5-His vector (Life Technologies).
HA::LON-2::myc (pCB313). The HA::LON-2::myc construct was made by synthesizing the 3ʹ end of the lon-2 cDNA (bases 739-1527) with the myc tag DNA sequence before the GPI anchor domain sequence (at cDNA position 1375) and an ApaI restriction site at the 3ʹ end. The PmlI/ApaI synthesized fragment was then cloned into the pActin5.1/HA::LON-2 (pCB285) construct cut with the same restriction enzymes.
SfGFP::UNC-6 (pCB292). The C. elegans unc-6 full-length cDNA (yk603d12) was amplified by PCR with primers adding the EcoRI and ApaI restriction enzyme sites and cloned into pBlueScript II. The superfolder-GFP::UNC-6 construct was made by synthesizing a DNA fragment, carrying the restriction sites EcoRI and HincII, and containing the 5ʹ end of the unc-6 cDNA with the superfolder-GFP sequence after the signal peptide sequence (cDNA position 70) and cloned into pBlueScript/unc-6. The superfolder-GFP::UNC-6 EcoRI/ApaI fragment was then cloned into the pActin5.1/V5-His vector.
UNC-40::FLAG (pCB301). The C. elegans full length unc-40 cDNA (yk449d8) was amplified by two sequential PCR reactions that added the XhoI and ApaI restriction enzyme sites, as well as the FLAG tag at the 3ʹ end before the stop codon (adding the FLAG tag at the C-terminus of UNC-40 after the intracellular domain). This UNC-40::FLAG cDNA was then cloned into the pActin5.1/V5-His vector.
UNC-40ΔNt::FLAG (pCB310). The unc-40 intracellular fragment (cDNA bases 3022-4245 that include the coding sequence for the transmembrane and intracellular domains of UNC-40), along with the FLAG tag, was amplified by PCR using the UNC-40::FLAG cDNA as template and primers carrying the EcoRI and ApaI sites. A start codon (ATG) was added to the forward primer. The UNC-40ΔNt::FLAG fragment was then cloned into the pActin5.1/ V5-His vector.
UNC-40ΔFn4/5::FLAG (pCB334). Using nested primers, the UNC-40ΔFn4/5 construct was made by PCR amplifying a 2-kb DNA fragment from genomic DNA from the strain NK821 qyIs155, which contains a deletion of fibronectin III domains 4 and 5 of the unc-40 cDNA ( [34]). The nested PCR added the FLAG tag to the 3ʹ end of the UNC-40ΔFn4/5 cDNA fragment. This final fragment was cloned into the pActin5.1/unc-40 vector using DraIII and ApaI restriction enzymes, thus generating an unc-40 full-length cDNA with a deletion of the fibronectin III domains 4 and 5.
SDN-1::myc (pCB336). The C. elegans sdn-1 cDNA, excluding the coding region for the transmembrane and intracellular C-terminal tail (cDNA bases 1-678), was amplified by two sequential PCR reactions adding the EcoRI and ApaI restriction sites and the myc tag sequence at the 3ʹ end before the stop codon. The sdn-1::myc EcoRI/ApaI fragment was then cloned into the pActin5.1/V5-His vector (Life Technologies).
The pAc/Evi::EGFP [60] and pAc/GFP constructs were a gift from V. Budnik (University of Massachusetts Medical School).
Western Blot Analysis of HA::LON-2::myc, UNC-40::FLAG and SfGFP:: UNC-6 Expressed in S2 Cells S2 cells were independently transfected with HA::LON-2::myc (pCB313), UNC-40::FLAG (pCB301), or SfGFP::UNC-6 (pCB292) constructs. 48 h after transfection, old culture medium was removed, and new medium was added to resuspend the cells. Equal volumes of resuspended cells that had been transfected with individual constructs were mixed and cocultured overnight. Cells were harvested, centrifuged, and combined with their corresponding supernatant from each of these cell mixes. 100 μL of supernatant of each mixture was saved and kept on ice. Cell pellets were washed once with 1X Phosphate Buffered Saline and lysed for 30 min at 4°C in 100 μL of ice-cold RIPA buffer (50 mM Tris HCl pH 7.5, 150 mM NaCl, 1% Triton-X100, 0.5% sodium deoxycholate, 0.1% SDS, and 1mM EDTA pH 8.0) supplemented with Protease Inhibitor Cocktail (Roche) and PMSF. Cell lysates were combined with their corresponding supernatant and mixed with 2X Laemmli sample buffer (BioRad). Each sample was split into three in order to run three protein gels in parallel. Proteins were separated by SDS-PAGE and transferred to PVDF membrane. Membranes were incubated with rabbit anti-myc (Santa Cruz #sc-789), mouse anti-FLAG (Sigma #F3165), and rabbit anti-GFP (Millipore AB3080) primary antibodies as well as HRP-linked goat anti-rabbit (Bio-Rad #166-2408EDU) and HRP-linked horse anti-mouse (Vector Labs PI-2000) secondary antibodies. Signals were revealed by chemiluminescence with Clarity Western ECL Substrate (BioRad) and imaged using the ChemiDoc System (BioRad).
S2 Cell Culture, Transfection, Mixing, and Immunostaining S2 cells were maintained in SFX Insect Media (HyClone) containing 10% Fetal Bovine Serum (HyClone) and Penicillin-Streptomycin (50 units-50 μg/mL) (Sigma). 70%-90% confluent S2 cells were transfected with 500 ng of each construct using Effectene (Qiagen) according to the manufacturer's protocol. 48 h after transfection, old culture medium was removed and new medium was added to resuspend the cells. Equal volumes of resuspended cells that had been transfected with individual constructs were plated onto coverslips and cocultured overnight. Cells were then fixed with 4% paraformaldehyde and immunostained with rabbit anti-HA (Life Technologies #715500) and mouse anti-FLAG (Sigma #F3165) primary antibodies and Alexa594 donkey anti-rabbit (Life Technologies #R37119) and Alexa647 goat anti-mouse (Life Technologies #A21235) secondary antibodies. Confocal analysis was performed on a Zeiss LSM 5 Pascal confocal microscope. Confocal images were processed using ImageJ. Each experiment was repeated at least three times.
For the experiment in which we use HA::LON-2-conditioned medium (supernatant) of cells expressing HA::LON-2, the culture medium was also changed 48 h after transfection, fresh medium was added, and the cells were incubated for another 48 h. This medium was collected and centrifuged at 1,500 rpm to remove cells and debris. This supernatant was added onto cells expressing UNC-40::FLAG, incubated overnight, and as above, fixed, stained, and imaged. Supporting Information S1 Data. Excel file containing, in separate sheets, the underlying numerical data for the graphs in Figs 1B, 1C, 1D, 1E, 2B, 2C, 3B, 4A, 4B, 5C and 5D and S1, S3 and S5 Figs. (XLSX) S2 Data. Excel file containing, in separate sheets, the underlying numerical data for the graphs in Fig 6C and 6D [13]. Mutants displayed here never exhibit a dorsally migrated AVM axon. Error bars are standard error of the proportion. Asterisks denote significant difference: ÃÃÃ p 0.001 (z-tests, p-values were corrected by multiplying by the number of comparisons). ns, not significant. (TIF) S2 Fig. gpn-1(ok377) and gpn-1(tm595) are likely null alleles. (A) gpn-1(ok377) and gpn-1 (tm595) are deletions (brackets) in the gpn-1 locus. (B) RT-PCR using primers oCB834 and oCB1321 yields truncated products in gpn-1(ok377) and gpn-1(tm595). Y45F10D.4 is a housekeeping gene used as an RT-PCR control [55]. Sequencing of the gpn-1 RT-PCR products for ok377 (blue) reveals that the transcript lacks most of exon 3 and has several in-frame stop codons ( Ã ), as intronic sequence (hatch pattern) gets incorporated into the mature transcript. Sequencing of the gpn-1 RT-PCR product for tm595 (red) reveals that the transcript lacks exons 2 and 3 and has several in-frame stop codons ( Ã ), as intronic sequence (hatch pattern) gets incorporated into the mature transcript. No alternatively spliced products were detected in the mutants ok377 and tm595. (C) gpn-1(ok377) and gpn-1(tm595) are strong loss-of-function mutations, likely nulls, in which, at most, small truncated proteins would get produced.  Table). (TIF) S4 Fig. SDN-1::GFP expression in the neuron AVM. Using the translational fusion sdn-1::gfp (opIs171), we found that SDN-1::GFP is expressed in hypodermal cells and neurons, as previously reported [16]. Importantly, we observed expression in the AVM neuron, including during the L1 stage, when the AVM growth cone migrates ventrally. This expression pattern is consistent with our finding that sdn-1/syndecan expression in AVM (Pmec-7::sdn-1) rescues the defects of sdn-1 mutants (Fig 4C), supporting a cell-autonomous role for sdn-1/syndecan in AVM. (TIF) S5 Fig. lon-2/glypican cannot be replaced by sdn-1/syndecan. lon-2 slt-1 double mutants exhibit enhanced AVM guidance defects as compared to slt-1 single mutants. The defects of the double mutants can be rescued back down to slt-1 single mutant levels with expression of wildtype Plon-2::lon-2(+). In contrast, expression of sdn-1(+) in which lon-2/glypican is normally expressed (using the Plon-2 promoter) or in the AVM neuron (using the heterologous promoter Pmec-7) cannot rescue the axon guidance defects of lon-2 slt-1 double mutants. Data for wild type, lon-2, slt-1, lon-2 slt-1, and Plon-2::lon-2 in lon-2 slt-1 are as in Fig 1B and 1C and Fig 4A. Fig 6B and 6C, while LON-2/glypican associates with cells expressing UNC-40/ DCC, LON-2/glypican does not associate with cells expressing Evi::GFP (experiments 1 and 2). Experiment 3 shows that SDN-1::myc/syndecan, another HSPG, does not associate with UNC-40/DCC-expressing cells. This SDN-1::myc was engineered to be secreted, as it lacks its transmembrane and intracellular C-terminal domains. These results indicate that the association of LON-2/glypican with UNC-40/DCC-expressing cells is specific and not a general feature of any HSPG. (B) Quantification of the association of HA::LON-2 with cells expressing UNC-40:: FLAG, Evi::GFP, and untransfected cells. Ten different optical fields containing~300 cells from three independent experiments were quantified and averaged. Error bars are standard error of the mean. Asterisks denote significant difference: ÃÃÃ p 0.001 (t-test versus untransfected cells). ns, not significant. (C) Quantification of the association of SDN-1::myc with cells expressing UNC-40::FLAG, HA::LON-2, and untransfected cells. Ten different optical fields containing~300 cells from three independent experiments were quantified. (TIF) S1  Table. PVM dorsal guidance defects quantified in wild-type and mutant strains with or without misexpression of unc-5 in the PVM neuron using transgene evIs25 Pmec-7::unc-5.