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. Author manuscript; available in PMC: 2009 Jun 8.
Published in final edited form as: Neuron. 2007 Dec 6;56(5):793–806. doi: 10.1016/j.neuron.2007.09.033

Tiling of R7 Axons in the Drosophila Visual System is Mediated Both by Transduction of an Activin Signal to the Nucleus and by Mutual Repulsion

Chun-Yuan Ting 1,+, Tory Herman 2,+,*, Shinichi Yonekura 1,#, Shuying Gao 1, Jian Wang 3, Mihaela Serpe 4, Michael B O’Connor 4, S Lawrence Zipursky 5, Chi-Hon Lee 1,*
PMCID: PMC2693211  NIHMSID: NIHMS35706  PMID: 18054857

Summary

The organization of neuronal wiring into layers and columns is a common feature of both vertebrate and invertebrate brains. In the Drosophila visual system, each R7 photoreceptor axon projects within a single column to a specific layer of the optic lobe. We refer to the restriction of terminals to single columns as tiling. In a genetic screen based on an R7-dependent behavior, we identified the Activin receptor Baboon and the nuclear import adaptor Importin-α3 as being required to prevent R7 axon terminals from overlapping with the terminals of R7s in neighboring columns. This tiling function requires the Baboon ligand, dActivin, the transcription factor, dSmad2, and retrograde transport from the growth cone to the R7 nucleus. We propose that dActivin is an autocrine signal that restricts R7 growth cone motility, and we demonstrate that it acts in parallel with a paracrine signal that mediates repulsion between R7 terminals.

Introduction

Topographic maps, in which the spatial relationships among neurons are preserved in the arrangement of their synaptic targets, are found throughout the nervous system. Restricting the information detected by equivalent sensory neurons to distinct columns results in a succession of topographic maps and provides a means of transmitting spatial information from the periphery to more central regions of the brain with high fidelity (Kaas, 1997). The establishment of gross topographic maps is particularly well understood in the vertebrate visual system, where gradients of Eph receptors and ephrin ligands play a crucial role (reviewed in McLaughlin et al. 2003). Much less is known about the mechanisms that act on a finer scale to restrict terminals to specific columns. The precise columnar arrangement of the Drosophila visual system provides an excellent model for exploring the genetic and molecular basis for this important patterning function (Kunes and Steller, 1993).

Each unit (ommatidium) of the Drosophila eye contains three types of photoreceptor neurons, R1-R6, R7, and R8, which express different rhodopsins and project axons retinotopically into three different layers of the optic lobe (Meinertzhagen, 1993; Ting and Lee, 2007). The R7 and R8 axons from a single ommatidium fasciculate, thereby bundling information from the same point in space into a single column. During development, the R8 axons project first, forming a retinotopic map in the medulla. The R7 axons follow the R8s but terminate in a more proximal medulla layer; the different color modalities are thereby segregated (Ting et al., 2005). The accurate processing of spatial and color information therefore presumably requires that R7 and R8 faithfully restrict their connections to the correct column and layer (Choe and Clandinin, 2005; Stavenga and Hardie, 1989).

R7 neurons are particularly amenable to genetic manipulation -- for example, individual homozygous mutant R7 axons can be analyzed in otherwise wild-type animals -- and therefore provide an attractive system in which to study connectivity (Lee et al., 2001). Previous work has identified several molecules required for R7 axons to terminate in the correct target layer (Choe et al., 2006; Hofmeyer et al., 2006; Maurel-Zaffran et al., 2001; Clandinin et al., 2001; Lee et al., 2001; Newsome et al., 2000). However, even mutant R7s that project to the incorrect layer remain restricted to a single column. The molecular basis for this tiling phenomenon is not known.

To identify the molecules required for axon tiling in the visual system, we performed a genetic screen for mutations affecting R7 connectivity. We found that loss of the nuclear import adaptor, Importin-α3 (Imp-α3), or of the Activin receptor, Baboon (Babo), caused R7 terminals to invade adjacent columns. We present evidence that Imp-α3 is required for nuclear import of dSmad2 in R7s in response to an Activin signal received by Babo; our results suggest further that this Activin signal is received by R7 axon terminals and originates from the R7s themselves. We demonstrate that the invasiveness of imp-α3 and babo mutant R7s is markedly increased by eliminating R7 neurons from neighboring columns. Based on these findings, we propose that R7 tiling is regulated in a redundant fashion by (1) a paracrine signal promoting repulsion between R7 terminals in adjacent columns; and (2) an autocrine signal inhibiting R7 growth cone motility through an Activin-dependent signaling pathway and nuclear import.

Results

imp-α3 and babo are required in R7 neurons to mediate a normal preference for UV light

To identify genes required for R7 connectivity, we conducted a behavioral screen for mutations that disrupt an R7-dependent behavior: phototaxis to UV in preference to visible light (Lee et al., 2001; Clandinin et al., 2001). As previously described, we screened mosaic animals in which R7 neurons were homozygous for a mutagenized chromosome arm while nearly all other cells were heterozygous.

In a screen of both randomly mutagenized 3rd chromosomes and a collection of known mutations, we isolated three new alleles of imp-α3 (16-8, 17-76, and w73; see Experimental Procedures for mapping and cloning information) and identified a babo null mutation (babo9). While only a small percentage of the wild-type mosaic flies failed to phototax to the UV light (18.9% of FRT82, n=412; 15.0% of FRT42, n=604), a significant proportion of mosaic flies containing FRT82 imp-α3 or FRT42 babo mutant R7s failed to do so: 44.9% (n=414) of imp-α316-8 flies, 51.5% (n=379) of imp-α317-76 files, and 49.8% (n=432) of babo9 flies phototaxed to visible light. A deletion allele of imp-α3, D93, created by Mason et al (2003) caused a similar defect (56.8% [n=287]). The corresponding performance indices are graphed in Figure 1; the differences between the mutants and their matching controls are statistically significant (p<0.00001), while the difference between the two controls (FRT42 and FRT82) is not (p>0.5). imp-α3 encodes an Importin-α, and babo encodes a type I TGF-β/Activin receptor, suggesting that nuclear import and TGF-β/Activin signaling are required autonomously for R7 function.

Figure 1.

Figure 1

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imp-α3 and babo are required in R7 neurons to mediate a normal preference for UV light.

(A) Targeted mitotic recombination generated mosaic flies with homozygous wild-type (FRT82 and FRT42) or mutant (FRT82 imp-α3 and FRT42 babo) R7s (see text). The mosaic flies were tested for their phototaxis toward UV in preference to visible light. The performance index for each genotype was calculated from the numbers of flies choosing visible (Nvis) or UV light (Nuv) using the following formula: P.I.=[Nuv-Nvis]/ [Nuv+Nvis]. The differences between the responses of imp-α3 or babo mosaic flies and those of the matching controls are highly statistically significant (asterisk or pound sign, p<0.00001, Fisher’s Exact test) while the differences between the two controls (FRT42 and FRT82) are not significant (double asterisks, p> 0.5). Error bars represent the variations among trials.

The tiling of imp-α3 and babo mutant R7 axon terminals is disrupted

To determine whether these behavioral defects reflected defects in R7 connectivity, we used MARCM (mosaic analysis with a repressible cell marker) to examine individual imp-α3 and babo mutant R7 axons. As previously described, we generated homozygous mutant R7s by expressing Flp recombinase under the control of the GMR promoter, and we positively labeled the mutant R7 terminals using MARCM together with the synaptic marker synaptotagmin-GFP or the membrane-tethered marker CD8-GFP (Clandinin et al., 2001; Lee et al., 2001; Lee and Luo, 1999). In adult animals, each wild-type R7 axon labeled by the GMR-Flp/MARCM system terminates at the M6 layer of the medulla, where it forms a spherical terminus, or synaptic bouton, that is restricted spatially to a single column (0% defect [n=181], Figures 2A,A’, arrows; D’). While imp-α3 mutant R7 axons terminated correctly in the M6 layer, a significant proportion extended laterally into columns occupied by neighboring wild-type R7s (Figures 2B,B’,D’; 29.2% [n=137] of imp-α317-76, 18.9% [n=217] of imp-α316-8, and 23.0% [n=213] of imp-α3D93 mutant R7s invaded neighboring columns). The orientation of these lateral extensions appeared to be random. These defects in R7 column-specific targeting were fully rescued by expression of a wild-type imp-α3 cDNA, indicating that the imp-α3 mutations are solely responsible for the phenotype (0.6% were defective, n=168; data not shown). We were also able to rescue the imp-α3 mutant R7s by expression of Drosophila importin-α1 (0.5% were defective, n=211) or importin-α2 (4.6% were defective, n=304), indicating that other Importin-α paralogs can fully or partially replace Imp-α3 function (data not shown).

Figure 2.

Figure 2

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The terminals of imp-α3 and babo mutant R7 axons invade adjacent columns.

(A-C) Individual homozygous mutant R7 cells were generated using GMR-Flp (imp-α3 and wild-type) or ey3.5-Flp (babo) mediated mitotic recombination; and their synaptic boutons or axons in the medulla were labeled with synaptotagmin-GFP (green) (A,A’,B,B’) or mCD8-GFP (green) (C,C’), using the MARCM system. R7 and R8 axons were visualized with Mab24B10 (red). (A,A’) wild-type; (B,B’) imp-α3 mutant; (C,C’) babo mutant. (A) In adult flies, wild-type R7 axons formed non-overlapping synaptic boutons (arrows) at the medulla M6 layer. The regularity of this array of R7 terminals is clear in an orthogonal section (A’) (arrows). By contrast, single imp-α3 or babo mutant R7 axons terminated at the appropriate layer but extended aberrant processes into neighboring columns (B,B’,C,C’) (arrows). (A’-C’) are orthogonal views of (A-C), respectively. Scale bar: in A, 5 μm for A-C’.

(D,D’) Schematic diagrams summarizing A-C’.

R7s homozygous for the babo9 null mutation also invaded neighboring columns, although much less frequently (6% [n=270] were defective) (data not shown). To test whether this weaker phenotype might result from perdurance of wild-type babo product already present in the heterozygous R7 precursor cells, we used ey3.5-Flp to induce mitotic recombination earlier in eye development (Nern et al., 2005; Newsome et al., 2000). The resulting babo mutant R7s invaded neighboring columns more frequently (12.7% [n=291] for babo9; 9.6 % [n=228] for babo52) (Figures 2C,C’), indicating that perdurance provides a partial explanation for the weak phenotype in GMR-Flp-induced clones. Expressing either of the two Babo isoforms completely rescued the babo R7 phenotypes (Brummel et al., 1999) (data not shown), confirming that the babo mutations are responsible for the observed defects and indicating that Babo’s function in R7s is not isoform-specific.

We have previously shown that R7 target selection occurs in two distinct stages (Ting et al., 2005). At 17 hours after puparium formation (hr APF) newly differentiated R7 axons project into the medulla and terminate at the R7-temporary layer. At 50 hr APF R7 growth cones simultaneously regain motility and project into their final destination layer, M6 (Figures 3A,A’). imp-α3 and babo mutant R7 axons invade adjacent columns by extending collaterals within M6, suggesting that this defect occurs during the second stage of target selection. To test this, we examined the mutant R7 axons at 40 hr and 50 hr APF and found that, indeed, both imp-α3 and babo mutant R7s targeted correctly at 40 hr APF (Figures 3B,C) but extended laterally into neighboring columns at 50 hr APF(20.0% [n=45] and 10.3% [n=78], respectively) (Figures 3B’,C’).

Figure 3.

Figure 3

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babo and imp-α3 mutant R7 growth cones invade neighboring columns at the second stage of target selection.

Wild-type, babo, or imp-α3 mutant R7 axons were examined before (40 hr APF [A,B,C]) and during (50 hr APF [A’,B’,C’]) the second stage of target selection. Mutant R7s were generated as described in Figure 2 and their axons were labeled with mCD8-GFP (green) using the MARCM system. R7 and R8 axons were visualized with Mab24B10 (red). (A,A’) wild type; (B, B’)imp-α316-8 mutant; (C, C’) babo9 mutant.

(A) At 40 hr APF, wild-type R7 growth cones terminated in the R7-temporary layer. (A’) At 50 hr APF, R7 growth cones regained motility and extended slightly deeper to their final target layer.

(B-C’) At 40 hr APF, single imp-α3 (B) or babo (C) mutant R7 axons terminated correctly in the R7-temporary layer. At 50 hr APF, imp-α3 (B’) or babo (C’) mutant R7 axons (arrow) made aberrant lateral extensions into neighboring columns.

Scale bar in A, 5 μm for A-C’.

It is unclear why defects in R7 tiling alone, particularly at low frequency, would disrupt the ability of R7s to drive phototaxis to UV light. It is possible that these mutations cause additional defects in R7 function. One possibility is that the R7 fails to differentiate normally. However, we have found that both imp-α3 and babo mutant R7s assume correct positions in their ommatidia and express the neuronal marker Elav (Figures 6E,G), the photoreceptor marker Chaoptin (Figures 2B,B’,C,C’), and the R7 marker Prospero (Supplementary Figures 1A,B). Finally, babo mutant R7s express the appropriate UV-sensitive rhodopsins, as assessed by an anti-Rh3 antibody (Supplementary Figure 1D). We therefore conclude that imp-α3 and babo are not required for R7 fate specification. Instead it is likely that these mutations cause subtle defects in synapse formation or function in addition to the defects in tiling. Indeed, we observe that the synaptic boutons of imp-α3 and babo mutant R7s appeared to be smaller and more irregular than those of wild-type R7s (Figures 2A,B,C).

Figure 6.

Figure 6

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Imp-α3 physically associates with dSmad2 and is required for nuclear accumulation of dSmad2 in R7s.

(A,B) Imp-α3 and dSmad2 form a complex in S2 cells. GFP-tagged Imp-α3 (double plus sign) and myc-tagged dSmad2 (asterisk) were expressed in S2 cells, and their physical association was assessed by reciprocal co-immunoprecipitation (coIP). In the S2 cells expressing Imp-α3-GFP and dSmad-myc (lanes 1, 3, 5, 7) but not in the control cells (lanes 2, 4, 6, 8), dSmad2-myc co-immunoprecipitated (co-IPed) with Imp-α3-GFP in the anti-GFP precipitates (lane 1). Conversely, Imp-α3-GFP coIPed with dSmad2-myc in the anti-myc precipitates (lane 5). Note that S2 cells express endogenous Imp-α3 (single plus sign), which was also precipitated by the anti-myc antibody in the presence, but not absence, of dSmad-myc (lanes 5, 6). By contrast, dSmad2-myc, Imp-α3-GFP, or endogenous Imp-α3 were not immunoprecipitated by a control serum (data not shown).

(C) Imp-α3 and dSmad2 are physically associated in R cells. Imp-α3-GFP and Flag- tagged dSmad2 (lanes 9, 11, 13) or Flag-dSmad2 alone (lanes 10, 12, 14) were expressed in the retina, using the GMR-Gal4 driver, and their physical association in the developing larval retina was assessed by coIP. Flag-dSmad2 was precipitated by the anti-GFP antibody in the presence of Imp-α3-GFP (lane 9) but not in its absence (lane 10).

Double asterisks denote a non-specific band present in both experimental and control groups (lanes 1, 2, 9, 10), and is most likely the IgG heavy chain.

(D-E’) Single wild-type (D,D’) or imp-α3 mutant (E,E’) R7s were generated in 50 hr APF eye discs using GMR-Flp-mediated mitotic recombination and labeled with Flag-dSmad2 (red) and mCD8-GFP (green) using the MARCM system. Photoreceptor nuclei were visualized using anti-Elav antibody (blue). (D,D’). In wild-type (FRT82 control) R7s, Flag-dSmad2 protein accumulated in the nuclei of R7 clones (arrows). (E,E’) In imp-α3 mutant R7 cells, nuclear dSmad2 staining was significantly reduced.

(F-G’) Mosaic clones of wild-type (F,F’) or babo mutant (G,G’) cells were generated using ey3.5-Flp and labeled as described above. R7s (arrows) were identified by their unique nuclear location and cell membrane pattern. Compared with wild type, Flag-dSmad2 staining in babo mutant R7 nuclei was reduced. Note that GMR-Flp and ey3.5-Flp also generated mosaic clones in pigment cells (arrowheads), in which nuclear accumulation of Flag-dSmad2 appeared to be unaffected by the absence of Imp-α3 or Babo.

(H) Quantitation of Flag-dSmad2 levels in wild-type and imp-α3 or babo mutant R7 and cone cell nuclei. Anti-Elav staining was used to identify the locations of the nuclei. The pixel intensity (0-255 AU [arbitrary unit]) of the anti-Flag staining in the nuclei was used to derive an average intensity for each nucleus. Each entry represents the mean and standard error (error bar). The cell types and genotypes are given below each bar and the numbers of data points are indicated above the bars. Imp-α3 or babo mutant R7s had significant lower levels (asterisk, p<0.001) of nuclear Flag-Smad2 than their matching wild-type controls (FRT82, FRT42, respectively). By contrast, the differences between wild-type and Imp-α3 or babo mutant cone cells are not statistically significant.

Redundant mechanisms restrict R7 terminals to the correct columns

The proportion of imp-α3 and babo mutant R7 axons that invade adjacent columns is relatively small. Previous studies by Ashley and Katz suggested that mutually repulsive interactions among R7 terminals might prevent overlap (Ashley and Katz, 1994). To determine whether imp-α3 and babo mutant R7s are still subject to repulsion by their neighbors, we tested the effect of removing the R7s adjacent to imp-α3 or babo mutant R7s. Males hemizygous for the temperature-sensitive sevenless allele, V1, lack most R7s when raised at the non-permissive temperature 29°C (sevenless encodes a receptor tyrosine kinase that signals through the Ras pathway to induce the R7 fate). In this background we used the GMR-Flp/MARCM method to create GFP-labeled homozygous wild-type or imp-α3 mutant R7s, and we used the ey3.5-Flp/MARCM method to create GFP-labeled homozygous wild-type or babo mutant R7s. We found that wild-type R7 axons formed morphologically normal synaptic boutons in retinotopically correct columns even in a largely empty R7 terminal field (Figures 4A,A’,C,C’). By contrast, removing neighboring R7s greatly increased the tendency of imp-α3 or babo mutant R7s to invade adjacent targets. Approximately 84.3% (n=32) of isolated imp-α3 R7 terminals extended laterally into neighboring columns (as compared to 23.0% of imp-α3 R7 terminals surrounded by fully innervated targets), and 22.2% of these lateral extensions spanned several columns (Figure 4B,B’). Similarly, 75.0% (n=76) of isolated babo R7 terminals invaded neighboring columns (as compared to 12.7% of babo R7 terminals surrounded by fully innervated targets) (Figures 4D,D’). These results suggest that imp-α3 and babo mutant R7 axons are still responsive to repulsion by neighboring R7s, accounting for their incomplete ability to invade adjacent occupied columns and indicating that at least two pathways act in a redundant fashion to restrict terminals to columns.

Figure 4.

Figure 4

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imp-α3 and babo mutant R7s are still responsive to repulsion from neighboring wild-type R7s.

(A-D’) Wild-type, imp-α3 or babo mutant R7 axons were examined in adults lacking most other R7s. In a sevts mutant background, single wild-type (A,A’) or imp-α3 (B,B’) mutant R7s were generated using GMR-FLP, and single wild-type (C,C’) or babo (D,D’) mutant R7s were generated using ey3.5-Flp, and their terminals were labeled with synaptotagmin-GFP or mCD8-GFP using the MARCM system (green). Mab24B10 labels both photoreceptor axons and a subset of medulla neurons that have processes in the R7 target layer (red). At 29°C, most of the R7 neurons are absent, but the R8 synaptic boutons serve as landmarks for medulla columns (arrowheads; small arrows indicate medullar neuron staining).

(A,A’) In the absence of neighboring R7s (arrowheads), wild-type R7 axons targeted to the correct layer (M6) and formed single synaptic boutons (arrow; green) that remained restricted to the correct columns.

(B,B’) Removing neighboring R7s strongly enhanced the tiling defect of imp-α3 mutant R7s. Mutant R7 terminals (arrow) traveled across several unoccupied columns and failed to form synaptic boutons (see text for quantitation).

(C,C’) Wild-type R7s created by ey3.5-Flp projected axons to the correct layer and columns in the absence of neighboring R7s. Note that ey3.5-Flp also generates R8 clones, whose axons (double small arrows) terminated correctly at the M3 layer.

(D-D”) Removing neighboring R7s strongly enhanced the tiling defect of babo mutant R7. Like imp-α3 mutant R7 terminals, babo mutant R7 terminals invaded neighboring unoccupied columns with high frequency and formed boutons of reduced size (D,D’). Some babo mutant R7 axons invaded adjacent empty columns even before arriving at the M6 layer (D”; arrow points to mutant R7; schematized in the final panel of E); this invasion occurred only after the M3 layer.

(A’-D’) are orthogonal views of (A-D), respectively. Scale bar: in A, 5 μm for A-D’. (E) Schematic diagrams summarizing A-D.

Both the Babo ligand, dActivin, and substrate, dSmad2, are required in R7s for tiling of their axons

The Babo receptor serine/threonine kinase has previously been shown to phosphorylate the transcription factor, dSmad2 in response to dActivin, resulting in translocation of dSmad2 to the nucleus (Zheng et al., 2003). To determine possible sources of Activin in the visual system, we performed in situ hybridization of retinas at 40 hr APF and found that dActivin mRNA was expressed in R7 and R8 neurons (Figures 5A,A’). We therefore tested whether dActivin might be required in these R neurons for tiling of R7 terminals. Because dActivin mutants are not available, we used dominantnegative and RNA interference approaches to disrupt dActivin function (Zheng et al., 2003). We found that expressing a dominant negative form (DN) or RNAi of dActivin specifically in the developing retina (using the GMR-Gal4 driver) resulted in R7 tiling defects (6.0%, n=401) resembling those of babo mutants (Figures 5C,C’). While modest, this phenotype was considerably enhanced when neighboring R7s were removed: 36% (n=50) of R7s lacking dActivin extended laterally into unoccupied columns (Figure 5D’). By contrast, GMR-Gal4 alone (Figure 5B,B’,D) or GMR-Gal4 driving RNAi constructs for the TGF-β ligands Myoglianin or Maverick (data not shown) did not cause any detectable R7 phenotypes. We also expressed dActivinDN or dActivin RNAi in medulla neurons using various medulla drivers (including Apterous-Gal4, and ey-Gal4) and found no significant effect on R7 connectivity (data not shown). These results indicate that R7 axons are restricted to the correct column by receipt of a dActivin signal and suggest that this signal originates from the R7 and/or R8 cells.

Figure 5.

Figure 5

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dActivin and dSmad2 are required in R cells for R7 tiling.

(A,A’) dActivin is expressed in R7 and R8. dActivin expression was assessed in 40 hr APF eye discs by in situ hybridization with a probe complementary to dActivin mRNA (green). R cells were visualized with GMR-mRFP (pseudo-colored in blue) and R7s with anti-Prospero antibodies (red). In (A) the equator is marked with white lines, and in (A’) R cell bodies are outlined in white (R1-R6), red (R7), or yellow (R8) dashed lines. (A’) is a high magnification view of (A). For clarity, the red channel was omitted in (A’).

(B-D’). Disrupting dActivin in R7s causes tiling defects (arrows). R7 connectivity was assessed in adult flies carrying GMR-Gal4 alone (B,B’,D) or GMR-Gal4 driving a dominant negative construct of dActivin (C,C’, D’) in wild-type (B-C’) or sevts animals (D,D’). Removal of neighboring R7s increased the invasiveness of R7s expressing ActivinDN (see text for quantitation). Photoreceptor axons were visualized using a GMR promoter driving a membrane-associated GFP (GFPras, green) as well as Mab24B10 (red).

(E,E’) Homozygous dSmad2 mutant R7 cells were generated with ey3.5-Flp and labeled with mCD8-GFP (green) using the MARCM system. R7 and R8 axons were visualized with Mab24B10 (red). Single dSmad2 mutant R7 axons terminated at the appropriate layer but extended aberrant processes (arrows) into neighboring columns.

(B’,C’,E’) are orthogonal views of (B,C,E), respectively. Scale bars: in A, 8 μm; in A’, 5 μm; in B’, 5 μm for B-E’.

We next tested whether the Babo substrate dSmad2 is required for R7 tiling. Using the ey3.5-Flp/MARCM method we found that dSmad2 mutant R7 axons invaded adjacent targets (7.6% [n=576]; Figures 5E,E’; the patterning of dSmad2 mutant eye discs was wild-type [data not shown]). Note that while the frequency of the phenotype is low, it is similar to that of babo and dActivin and is, again, likely to reflect redundancy; for technical reasons (both dSmad2 and sev are on the same chromosome), we did not assess the dSmad2 phenotype of R7s in a sevV1 background. The transcription factor Mad has been shown to transmit signaling of Dpp and Gbb, two TGF-β family ligands. In contrast to dSmad2, single Mad mutant R7s, generated by GMR-Flp/MARCM, did not exhibit any detectable phenotypes (data not shown).

Imp-α3 and dSmad2 form complexes in S2 cells and retinal tissue

Because of the resemblance among the dActivin, babo, dSmad2, and imp-α3 mutant R7 phenotypes and the known role of Importins in nuclear import, we set out to test whether imp-α3 might be involved in nuclear import of dSmad2 during R7 target selection. To determine whether Imp-α3 and dSmad2 form a complex, we examined whether they co-immunoprecipitated (coIPed) from cells expressing both proteins from transgenes. To facilitate detection and immunoprecipitation, we expressed GFP-tagged Imp-α3 (Imp-α3-GFP) and myc-tagged dSmad2 (myc-dSmad2) in S2 cells. The expression level of Imp-α3-GFP was close to endogenous levels (Figure 6B). We found that myc-dSmad2 coIPed with Imp-α3-GFP in the anti-GFP precipitates (Figure 6A). Conversely, Imp-α3-GFP, as well as the endogenous Imp-α3, coIPed with myc-dSmad2 in the anti-myc precipitates, and the coIP of endogenous Imp-α3 was dependent upon the presence of myc-dSmad2 (Figure 6B). We next examined whether dSmad2 and Imp-α3 form a physical complex in retinal tissue. We expressed Flag-tagged dSmad2 and Imp-α3-GFP in the retina using the GMR-Gal4 driver and found that Flag-dSmad2 co-IPed with Imp-α3-GFP in anti-GFP precipitates (Figure 6C). As anti-dSmad2 antibodies suitable for immunoprecipitation and western blotting are not available, we were unable to assess whether complexes form in retinal tissue when these proteins are expressed at physiological relevant levels. Nevertheless, these results indicate that Imp-α3 and dSmad2 can form a physical complex in vivo.

Imp-α3 and Babo are required for proper nuclear accumulation of dSmad2 in R7s

To test whether dSmad2 is localized to R7 nuclei during target selection and whether this localization requires imp-α3, we expressed Flag-tagged dSmad2 in MARCM clones. In this system, the expression of dSmad2 is driven by the constitutive actin promoter, thereby making it unlikely that imp-α3 affects transcription of Flag-dSmad2. In wild-type clones at 50 hr APF, Flag-tagged dSmad2 accumulated in R7 cell nuclei (Figures 6D,D’,F,F’, quantified in H). By contrast, dSmad2 staining was significantly reduced in the nuclei of imp-α3 mutant R7s (generated by the GMR-Flp/MARCM method) (Figures 6E,E’,H) and in the nuclei of babo mutant R7s (generated by the ey3.5-Flp/MARCM method) (Figures 6G,G’,H). The levels of nuclear dSmad2 staining were quantified in panel H, and the differences between wild-type and mutants are significant (p<0.001). By contrast, the nuclear accumulation of dSmad2 in imp-α3 or babo mutant pigment cells was largely unaffected (Figures 6E,E’,G,G’,H). Together these data indicate that dSmad2 accumulation in R7 nuclei depends on Imp-α3 and Babo, suggesting that the imp-α3 mutant R7 defect results at least in part from its disruption of the Activin/Babo/dSmad2 signaling pathway.

Imp-α3 and dSmad2 are found in R cell axons, and R7 tiling requires retrograde axonal transport

To determine whether Imp-α3 and dSmad2 might transduce a signal received by R7 growth cones, we determined their subcellular localizations using immunohistochemistry. First, we used an anti-Imp-α3 antibody to stain whole-mount developing eye-brain complexes. While the anti-Imp-α3 antibody specifically labeled endogenous Imp-α3 in the eye discs (Supplementary Figure 2), it failed to penetrate fully into the brain samples (data not shown). To resolve the penetration problem and to improve spatial resolution, we used an R-cell culture system (Li and Meinertzhagen, 1995, 1997). Dissociated photoreceptor neurons from developing eye discs extend axons and growth cones within two days of culture (Figures 7A-C’”; see Experimental Procedures for details). Using anti-Imp-α3 antibodies, we found that endogenous Imp-α3 is located in R cell growth cones as well as in axons (Figures 7A’,B’). Flag-tagged dSmad2 (Figure 7A”,B”,C”) largely overlapped with anti-Imp-α3 staining (Pearson’s correlation coefficient, r=0.401) (Figures 7A’”,B’”), unlike Nervana, a neuron-specific Na+/K+-ATPase (r= -0.32; Figure 7C’-C’”). Both proteins were also seen in the cell bodies. The localization of both proteins to axons and growth cones suggested that Imp-α3 and dSmad2 might transduce a signal from the developing R7 terminals to the nucleus. One prediction of this model is that retrograde axonal transport would be required to carry these molecules to the cell body and therefore that disruption of the Dynein/Dynactin complex should cause R7s to invade adjacent columns. To test this, we used actin-Gal4 and the GMR-Flp/MARCM system to express a previously characterized dominant negative form of the dynactin subunit Glued (GlDN) in R7s. While this had only a modest effect on R7 tiling (18.1% were defective, n=144; Figures 7D,D’), when GlDN was expressed in R7s whose neighbors had been removed, 71.4% of R7s (n=21) extended into adjacent columns (Figure 7E,E’). These results support a model in which the Activin signal is received by R7 growth cones and transduced to the nucleus (Figure 8).

Figure 7.

Figure 7

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Imp-α3 and dSmad2 colocalize in photoreceptor axons, and retrograde transport is required for R7 tiling. (A-C’’’) Flag-tagged dSmad2 was expressed in third instar larval eye discs using the GMR-Gal4 driver. The subcellular localization of Flag-dSmad2 and endogenous Imp-α3 was assessed in cultured photoreceptor neurons using anti-Flag (A”,B”,C”) (red) and anti-Imp-α3 (A’,B’) (green) antibodies, respectively. (A’”,B”’) Imp-α3 and Flag signals co-localized in the growth cones (large arrows) as well as vesicle-like structures (small arrows) along the axons. (C-C”’) By contrast, the neuron-specific Na+/K+-ATPase Nervana, detected by anti-HRP staining (blue), was found in the entire axon, exhibiting minimal overlap with the Flag signal. (A,B,C) are bright field images of the cultured photoreceptor neurons. (B-B”’) are high magnification views of (A-A”’), respectively. Scale bars: in A, 15 μm for A’-A’’’, C-C’’’; in B, 5 μm for B’-B”. (D-E’) Blocking retrograde axonal transport affects R7 tiling. A dominant negative form of the Dynactin subunit Glued (GlDN) was expressed in individual R7s using act-Gal4 and the GMR-Flp/MARCMsystem, and their axons were examined in adults. R7s expressing GlDN were labeled with mCD8-GFP (green), and all photoreceptor axons were visualized with Mab24B10 (red). (D, D’) R7s expressing GlDN infrequently invaded adjacent columns but (E, E’) did so more frequently when neighboring R7s were removed by sevts (see text for quantitation). (D’,E’) are orthogonal views of (D,E), respectively. Scale bar: in D, 5 μm for D-E’.

Figure 8.

Figure 8

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A schematic model shows the tiling of R7 terminals being regulated by two partially redundant pathways: (1) an unknown signal (blue double arrows) that mediates repulsive interactions between adjacent R7 growth cones, and (2) an Activin signal (red arrows) that regulates intrinsic R7 growth cone motility by the following mechanism. Activin, secreted from the R7 growth cone, activates its receptor Babo in R7, resulting in phosphorylation of the transcription factor, Smad2. The phosphorylated Smad2, in complexes with Imp-α3, then shuttles into the nucleus to reduce growth cone motility via transcriptional regulation of yet-to-be identified target genes.

Discussion

Previous anatomical studies have highlighted two prominent features of neuropil organization in the fly visual system: the axons of most neuron classes arborize in characteristic layers of the brain and remain restricted either to one column or a small number of adjacent columns (Fischbach and Dittrich 1989; Bausenwein et al., 1992). To gain insight into the developmental mechanisms that regulate these aspects of axon targeting we have focused on the R7 photoreceptor neuron. In previous work, we and others have characterized mechanisms controlling the precise layer termination of R7 growth cones. In this paper, we analyze the mechanisms that restrict R7 terminals to the correct columns. We demonstrate that the latter process is regulated by two partially redundant pathways: a paracrine signal that mediates repulsion between adjacent R7 axons and an autocrine Activin signal that is transduced by retrograde transport and import of the transcription factor Smad2 into the nucleus by a component of the classical nuclear import pathway, Importin-α3.

R7 terminals are restricted to the correct columns redundantly by Activin signaling and by repulsion between adjacent R7s

A prominent organizing feature of the medulla is the restriction of axons and their terminals, including those of R7, R8, and the lamina monopolar neurons L1-L5, to single columns (Takemura and Meinertzhagen, personal communication; Millard et al. 2007; Fischbach and Dittrich 1989). This phenomenon is similar to the tiling of processes observed in both the peripheral and central nervous systems (reviewed in [Grueber and Jan, 2004]). Ablation experiments in both fly and zebrafish support the view that repulsive interactions between processes of different cells of the same class prevent overlap of dendritic and axonal receptive fields (Gao et al., 2000; Sagasti et al., 2005). Consistent with this model, wild-type R7 terminals only invade adjacent columns from which R7s have been removed (Ashley and Katz, 1994). However, an R7 axon does not invade even empty columns unless it is under “competitive pressure” from additional R7 axons within its own column, suggesting that a second, intrinsic mechanism also restricts R7 terminals (Ashley and Katz, 1994).

We have here presented evidence that Activin signaling is required for tiling of R7 terminals: loss of babo, dSmad2, or dActivin causes R7 axons to invade adjacent occupied targets. Because these mutant axons invade even when they are not under competitive pressure, we hypothesize that Activin affects an intrinsic property of R7 terminals such as their motility or ability to initiate synaptogenesis (Figure 8). In support of this model, we have found that virtually all R7 axons lacking babo, dSmad2, or dActivin initially extend filopodia beyond the R7-temporary layer at 17 hr APF (Supplementary Figures 3B’,C’, 4B), although these retract by 40 hr APF, and that expression of a constitutively active babo receptor in R7s affects growth cone morphology (Supplementary Figure 4C,C’). While we cannot rule out models in which Activin signaling also mediates repulsion among R7 terminals, we have shown that R7s unable to respond to Activin are still partly repelled by their neighbors, indicating the existence of repulsive mechanisms that are redundant with Activin. Recent studies have demonstrated that Dscam2 mediates repulsion between L1 growth cones in a layer immediately distal to the R7 terminals (Millard et al. 2007). As Dscam2 is not expressed in R7, other cell surface proteins must mediate the repulsive interactions between adjacent R7 terminals. The identification of Activin’s involvement in R7 tiling paves the way to identifying such molecules by allowing the removal of a pathway that is functionally redundant with them.

Tiling of R7 terminals requires Imp-α3-mediated transport of dSmad2 to the R7 nucleus

Members of the TGF-β superfamily have been widely implicated in regulating axon guidance and synaptogenesis by both transcription-dependent and independent mechanisms (Packard et al., 2003; Parker et al., 2004); Butler and Dodd, 2003; Colavita et al., 1998; Nash et al., 2000; Eaton and Davis, 2005; Marques et al., 2003; McCabe et al., 2003; Marin et al., 2005; Parker et al., 2006; Serpe and O’Connor M, 2006; Zheng et al., 2003) In this study, we found that loss of dSmad2 from R7s resembles loss of Activin, suggesting that the tiling of R7 terminals requires changes in transcription. In support of this model, we found that restriction of R7 terminals also requires imp-α3, which we have shown is required for the accumulation of dSmad2 in R7 nuclei. While some previous vertebrate studies have suggested that individual Smads are imported by Importin-α (Xiao et al., 2003) or Importin-β Xiao et al, 2000; Kurisaki et al, 2001), others argue instead that active Smad complexes can enter the nucleus by an importinindependent mechanism (Xu et al, 2003; Chen et al, 2005). Our results provide the first genetic evidence that Smad function can require Importin-α-mediated nuclear import and may help reconcile previous results by demonstrating that different cell types import dSmad2 by different mechanisms (while R7s require imp-α3, pigment cells do not). We not that imp-α3 mutant R7s have more frequent defects in tiling than babo or dSmad2 mutant R7s, suggesting that imp-α3 may transport additional nuclear proteins that, redundantly with the Activin pathway, restrict R7 terminals.

In addition to their classical nuclear import function, Importins have been implicated in mediating retrograde transport of signals from growth cones to the nucleus (Ambron and Walters, 1996; Hanz and Fainzilber, 2004; Hanz et al., 2003). Both Imp-α3 and dSmad2 are found throughout the length of R7 axons, and we have found that, like loss of Activin signaling, disrupting retrograde axonal transport affects the intrinsic mechanism that restricts R7 terminals. These results are consistent with a model in which the Activin signal is received by R7 growth cones, and dSmad2 bound to Imp-α3 is transported through the axon and ultimately into the nucleus (Figure 8).

What might be the source of the Activin signal?

Surprisingly, Activin appears to be required in the R neurons and likely in R7s themselves: disrupting Activin in all R neurons causes R7 terminals to invade adjacent targets, and among the photoreceptors, only R7 and R8 express Activin. Our attempt to test whether Activin is specifically required in R7s met with only partial success: we used sevenless-Gal4 (sev-Gal4) to express UAS-ActivinDN in R7s but not R8s and found that the resulting R7s temporarily overshoot their initial target layer, a phenotype also caused by loss of Babo or dSmad2 (Supplementary Figure 4B). Thus, Activin can function as an autocrine effector. Unfortunately the sev-Gal4 driver is no longer expressed by 40-50 hr APF, the time at which Activin prevents R7s from invading adjacent columns, and sev-Gal4/ActivinDN R7s appear normal at this timepoint (data not shown).

Nonetheless, the finding that R7s and/or R8s are the source of Activin raises two mechanistic questions. First, if the R7 or R8 neurons themselves are providing the signal, and the signal is simply transduced into the R7 nucleus, why might Activin, as we argue, be secreted in the target region and received by the R7 growth cone (i.e. rather than being secreted and received by the cell body)? One possibility is that R7s use Activin to coordinate their developmental program with that of other cells within the medulla. For example, one could imagine that both R7 growth cones and their post-synaptic targets would encounter the Activin signal in the medulla at the same time, allowing them to coordinate their preparations for mutual synaptogenesis. A second question is, therefore, how might Activin signaling be coordinated with the R7 growth cones’ arrival in the medulla? We have found that the Activin-processing enzyme Tolloid-related (Tlr) is located both at the R7-temporary target layer (at 17 hr APF) and at the final R7 target layer (at 50 hr APF) (Supplementary Figures 5B,B’) and that Tlr mutants exhibit severe R7 retinotopic map defects (Supplementary Figures 5C,C’). One possibility is therefore that the medulla localization of Tlr might confer spatial and temporal specificity on Activin expressed by R7 and/or R8.

While Activin is also expressed in R8s, we have found no evidence that Activin affects R8 axons, as neither babo mutant R8s nor R8s expressing GMR-Gal4/UAS-ActivinDN exhibited connectivity defects (data not shown). However, we cannot rule out the possibility that redundancy obscures such a role (for example, there is no straightforward method of removing adjacent R8s).

The relevant transcriptional targets of Activin signaling in R7 remain to be identified

In the mushroom body, dActivin-signaling results in up-regulation of the ecdysone receptor gene, EcR-B1 (Zheng et al., 2003). While EcR-B1 is expressed in essentially all photoreceptor neurons, three lines of evidence indicate that EcR-B1 is not the target gene of Activin signaling in R7s: the expression level of EcR-B1, as judged by anti-EcR-B1 staining, was not altered in babo mutant R7 clones (Supplementary Figures 6A-A”); forced expression of EcR-B1 did not rescue babo mutant R7 defects (Supplementary Figures 6B-C”); and USP R7 mutants did not phenocopy babo (Supplementary Figures 6D-E”). In the dorsal cluster of Atonal-positive neurons, Babomediated signaling, via an EcR-independent pathway, mediates morphogenesis and axonal extension (Zheng et al., 2006). The versatility of Activin signaling likely reflects its ability to regulate the expression of different genes in a context-dependent manner (reviewed in [Sanyal et al., 2004]). We speculate that dActivin signaling activates a transcriptional program that not only restricts growth cone motility once R7s are within their target layer but also promotes synaptogenesis. Such a model could explain the observed strong defects in R7-mediated behavior despite the infrequent specific defects in R7 tiling. Identifying the transcriptional targets of Activin signaling in R7s will likely provide insight into these processes.

Experimental Procedures

UV/Vis light choice assay

The effects of mutations on R7 function were assayed by creating mosaic animals having homozygous mutant R7s (using GMR-Flp and PAN-R7-tetanus toxin as described in [Lee et al., 2001]) and testing their behavior in a UV/Vis choice test (also described in [Lee et al., 2001]). The behavioral set up is similar to that used in Reinke and Zipursky (1988) (Reinke and Zipursky, 1988) except that the light sources were replaced with UV and green light-emitting diodes. UV at 370 nm (0.15 μw/cm2) and green light at 525 nm(7.8 lux) were used in all the tests. Performance index (P.I.) was calculated using the following formula, P.I.=[Nuv-Nvis]/ [Nuv+Nvis].

Genetics

Fly stocks were maintained at 22°C on standard medium unless stated otherwise. Mutagenesis was performed using ethylmethane sulfonate following standard procedures (Ashburner, 1989; Grigliatti, 1986).

The following stocks were used to generate imp-α3 or babo R7 mosaic animals for the UV/Vis behavior assay: (1) FRT42, R7-TNTE/CyO; (2) GMR-Flp;FRT42, babo9/CyO; (3) FRT82, R7-TNTE/TM6B; (4) GMR-FLP; ; FRT82, impα316-8/TM6B; (5) GMR-Flp;;FRT82, impα3D93 /TM6B (a gift from David S. Goldfarb); (6) GMR-Flp; ;FRT82, impα317-76 /TM6B.

For generating eye-mosaic animals used in histological analyses, ey3.5-Flp (a generous gift from Iris Salecker) was used in combination with the MARCM system. ey3.5-Flp induces eye mosaic clones but not brain clones. Single mutant R7 cells were generated using the GMR-Flp/MARCM method as described previously (Lee et al., 2001). For analyzing single wild-type or mutant R7s at different developmental stages, we included the actin-Gal4 driver in the genetic scheme (Nern et al., 2005). Fly stocks that were used for these experiments are as follows:

For the creation of imp-α3 MARCM R7 clones and rescue experiments: (1) actin-GAL4, UAS-syt-eGFP (or UAS-mCD8GFP)/CyO;FRT82, tubP-GAL80/TM6B; (2) GMR-Flp;;FRT82,impα316-8/TM6B; (3) GMR-Flp;;FRT82,impα3D93/TM6B; (4) GMR-Flp;;FRT82,impα317-76; (5) ;UAS-impα1;FRT82,impα3D93 /TM6B; (6) ;UAS-impα2; FRT82D93 /TM6B; (7) ; UAS-impα3;FRT82,impα3D93 /TM6b; (8) UAS-Flag-dSmad2;; FRT82,impα3D93 /TM6B.

For the creation of babo R7 MARCM and eye clones, whole-eye mutants, and rescue experiments: (1) ey3.5-Flp;GlaBc/CyOG; (2) FRT42,tubP-GAL80/CyO;actin-GAL4, UAS-syt-eGFP (or UAS-mCD8GFP); (3) FRT42,babo9,UAS-mCD8-GFP/CyO; (4) ey3.5-Flp, GMR-myr-RFP;FRT42,ClRpw+/CyOG; (5) UAS-babo-a;FRT42,babo9, UAS-mCD8-GFP/CyOG; (6) UAS-babo-b;FRT42,babo9,UAS-mCD8-GFP/CyOG; (7) UAS-Flag-dSmad2; FRT42,babo9,UAS-mCD8-GFP/CyOG.

For the creation of dSmad2 and usp MARCM clones: (1) FRT19A, tubP-GAL80, hs-Flp;UAS-syt-eGFP; (2) FRT19A, l(X)MB388,UAS-mCD8-GFP; (3) FRT19A, I(X)48 USP; (4) ;; ey3.5-Flp/TM6B.

For ectopic expression of various transgenes: (1) UAS-EcR-B1; (2) ;UAS-CMdActivin; UAS-CMdActivin (3) UAS-HLdActivin; UAS-HLdActivin/CyO; (4) UAS-mav RNAi; (5) UAS-myo RNAi; (6) GMR-Gal4,GMR-GFPras /CyOG; (7) UAS-Flag-dSmad2; (8) pdActivin-GAL4; (9) pdSmad2-Gal4; (10) ;UAS-nlsGFP; (11) UAS-mCD8GFP; (12) ;;UAS-impα3-GFP; (13) ;;UAS-baboDA; (14) ;sevenless-Gal4. The transgenic flies used for analyzing imp-α3 rescue were generous gifts from David S. Goldfarb.

For the creation of wild-type, imp-α3 and babo MARCM clones in the absence of neighboring R7s: (1) sevv1, GMR-Flp;;FRT82,impα3D93/TM6B; (2) sevv1, GMR-Flp;;FRT82/TM6B; (3) ;actin-GAL4, UAS-syt-eGFP/CyO;FRT82, tubP-GAL80/TM6B; (4) ;FRT42,babo9,UAS-mCD8-GFP/CyO;ey3.5-Flp; (5) FRT42,tubP-GAL80/CyO;actin-GAL4, UAS-syt-eGFP.

For expressing a dominant negative form of Glued in individual R7s in the presence or absence of neighboring R7s: (1) GMR-Flp; UAS-GlDN /CyO; FRT82/TM6B (2) ;actin-GAL4, UAS-syt-eGFP; FRT82, tubP-GAL80/TM6B (3) sevv1, GMR-Flp; UAS-GlDN /CyO; FRT82/TM6B.

The trans-allelic combination of TlrE1 and Tlrex[2-41] was used for determining the specificity of anti-Tlr antibody and for analyzing tlr mutant phenotypes. TlrE1 and Tlrex[2-41] are null and strong hypomorphic tlr alleles, respectively (Serpe and O’connor 2006).

Molecular identification of the imp-α3 gene

The locus defined by the complementation group (16-8, w73, 17-76) was mapped to a region of <100kb by failure to complement available deficiencies and by male meiotic recombination. Predicted open reading frames within this region were sequenced, and only one, that of imp-α3, contained sequence changes associated with the 16-8, w73, and 17-76 mutations; each resulted in a stop codon at a distinct position in the imp-α3 reading frame (data not shown). Shortly after our identification, an imp-α3 allele, D93, generated by P element excision by D. A. Mason (Mason et al., 2003) became available to us. 16-8, 17-76, and w73 all fail to complement D93, and all alleles cause essentially identical phenotypes.

Molecular biology

The pUAS-dSmad2-myc, pUAS-impα3-GFP, and pUAS-Flag-dSmad2 vectors were constructed using PCR-based cloning techniques and confirmed by sequencing. Transgenic flies were generated using standard microinjection techniques. Cloning procedures are available upon request.

Histology

Immunohistochemistry, confocal imaging, image deconvolution, and 3D image rendering were performed as described previously (Ting et al., 2005). The Imaris coloc module and ImageJ package (NIH) were used to analyze co-localization and to calculate Pearson’s coefficient of correlation (r). For the quantification of nuclear Flag-dSmad2 level, wild-type and mutant eye discs were processed in parallel for immunohistochemistry and imaged using identical laser power settings. The average intensity (0-254 AU) was calculated for each nucleus using the ImageJ package.

The following concentrations of primary antibodies were used: mAb24B10, 1:100 dilution; mouse anti-Prospero MR1A, 1:50 dilution; mouse anti-EcR-B1 AD4.4, 1:50 dilution; rat anti-Elav 7E8A10, 1:200 dilution, mouse anti-Rh3 2B1, 1: 50 dilution (a generous gift from Steve Britt); rabbit anti-GFP, 1: 500 dilution (Torrey Pines Biolabs); mouse anti-Flag M5, 1:200 dilution (Sigma); goat anti-HRP-Cy5, 1:50 dilution (Jackson Immunoresearch); anti-Tlr, 1:500 (Serpe and O’connor 2006). The secondary antibodies including goat anti-rabbit or mouse IgG coupled to Alexa 488, Alexa 568, or Alexa 647 (Molecular Probes) were used in a 1:200 dilution.

In situ hybridization

Pupal eye brain complexes were dissected and fixed by treatment of 4% paraformadehyde (in 0.1M phosphate buffer, pH 7.2) for 1 hr at room temperature (RT). After 3 times washes by PBT (PBS, 0.5% Triton X-100), tissues were prehybridized, and hybridized at 60°C using DIG-labeled antisense or sense RNA probes in the Hybridization buffer (50% formamide, 5X SSC, 100mg/ml salmon sperm DNA, 100mg/ml heparin, 0.1% Tween20). A full length Activin cDNA clone (RE37047), obtained from BDGC. A Bam HI/ Kpn I fragment of the activin cDNA was subcloned into pBluescript, which contains the T3 and T7 promoters used for RNA probe synthesis. These RNA probes were prepared using a DIG probe synthesis kit (Roche) according to the manufacturer’s instructions. After overnight hybridization, tissues were washed twice with the hybridization buffer for 20 min at 60°C and twice with 1:1 ratio of PBT (PBS-0.1% Tween20) and hybridization buffer at 60°C and twice with PBT (PBS-0.1% Tween20) at RT for 20 min. After blocking non-specific binding with 10% Goat serum (in PBS and 0.5% Triton X-100), the tissues were stained with a sheep anti-DIG-POD antibody (1:300, Roche) and a mouse anti-prospero antibody (1:50) at 4°C overnight. The staining was developed using an Alexa-488 tyramide-based Tyramide signal amplification kit (Invitrogen) and counter-stained with anti-mouse Alexa Fluor 647 secondary antibody.

Biochemistry

S2-adhesion cells used for biochemistry were a generous gift from James Clemens. Cell culture and transfection with Effectene were performed according to the QIAGEN manuals. For expressing Imp-α3-GFP and dSmad2-myc proteins in S2 cells, pRmHa3-GAL4, pUAST-impα3-GFP and PUAST-dSmad2-myc constructs were co-transfected into the S2 cells. CuSO4 (0.7 μM) was added 24 h post-transfection to induce the protein expression. The S2 cells were induced for 24 h and then lysed with lysis buffer (50 mM Tris-Cl [pH 7.4], 150 mM NaCl, 1 mM dithiothreitol (DTT), protease inhibitors and 1% NP-40). For each lane, a protein sample equivalent to 200,000 S2 cells was loaded. Rabbit anti-GFP antibody (Torrey Pines Biolabs), mouse anti-myc monoclonal antibody (Roche), and mouse anti-Imp-α3 monoclonal antibody (a generous gift from Carl S. Parker) were used to detect the Imp-α3-GFP, dSmad2-myc, and endogenous Imp-α3 proteins, respectively. In our in vivo Co-IP assay, we used flies carrying the retina driver, GMR-GAL4, and the expression vectors, UAS-Flag-dSmad2 and UAS-impα3-GFP. Third instar larval eye discs (30 eye discs per experiment) were dissected and the resulting lysate was subjected to immunoprecipitation and Western blot analysis as described (Yonekura et al., 2006).

Primary culture of photoreceptor neurons

The primary culture of photoreceptor neurons was performed essentially as described previously (Li and Meinertzhagen, 1995, 1997) with minor modifications. Briefly, the 3rd instar larval eye-discs were dissected in Shield and Sang’s M3 modified medium (MM3) with the addition of 2% FBS and antibiotics (penicillin and streptomycin). After washing the dissected eye-discs in PBS, the eye-discs were dissociated by incubation in 10X trypsin-EDTA (Invitrogen) in PBS for 1 hr at room temperature and then triturated by pipetting. The dissociated cells were centrifuged at 150g for 5 min and washed with MM3 medium with 2%FBS. The cells were resuspended in 100 μl of MM3 medium with 2% FBS, 1 μg/ml 20-hydroxyecdysone (sigma), and 1 μg/ml bovine insulin (Gibco). The equivalent of ten eye-discs was plated onto Conconavalin A-coated 35 mm glass-bottom microwell dishes (MatTek Cultureware). Cells were incubated at 25°C for 24-48 hr to allow axonal growth before they were subjected to immunohistochemistry.

Supplementary Material

01

Acknowledgments

We thank Tzumin Lee and Claude Desplan for reagents and Zhigang He for communicating results prior to publication. We thank James Clemens, Steven Britt, CarlS. Parker, and Iris Salecker for S2 cells, anti-Rh3 antibody, anti-Importin-α3 antibody and ey3.5-Flp flies, respectively and Luisa Vasconcelos for in situ hybridization protocol. We thank, Benjamin White, Howard Nash, Kelsey Martin, Alan Hinnebusch, and Henry Levin for carefully reading this manuscript and for helpful discussion and Margaret Dieringer for manuscript handling and editing. This work was supported by the Intramural Research Program of the NIH, National Institute of Child Health and Human Development (grant HD008748-03 to C.-H.L.), and a Burroughs-Wellcome Career Development Award (to T.H.).

Footnotes

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