VEGF signaling through neuropilin 1 guides commissural axon crossing at the optic chiasm

Abstract

During development, the axons of retinal ganglion cell (RGC) neurons must decide whether to cross or avoid the midline at the optic chiasm to project to targets on both sides of the brain. By combining genetic analyses with in vitro assays, we show that neuropilin 1 (NRP1) promotes contralateral RGC projection in mammals. Unexpectedly, the NRP1 ligand involved is not an axon guidance cue of the class 3 semaphorin family, but VEGF164, the neuropilin-binding isoform of the classical vascular growth factor VEGF-A. VEGF164 is expressed at the chiasm midline and is required for normal contralateral growth in vivo. In outgrowth and growth cone turning assays, VEGF164 acts directly on NRP1-expressing contralateral RGCs to provide growth-promoting and chemoattractive signals. These findings have identified a permissive midline signal for axons at the chiasm midline and provide in vivo evidence that VEGF-A is an essential axon guidance cue.

Figure 1

Mouse RGCs Express NRP1, but Not NRP2, When Their Axons Cross the Optic Chiasm (A–L) Immunofluorescence labeling (A–D) and in situ hybridization (E–L) of horizontal sections through wild-type eyes at E12.5–17.5, the time when RGCs differentiate and extend axons through the optic chiasm. ISL1 staining (A–D) illustrates the position of RGC neurons (white arrows). Nrp1 (E–H) is expressed in the RGC layer (solid arrows) and by hyaloid and choroidal vessels (solid and clear arrowheads, respectively). In contrast, Nrp2 (I–L) is expressed in mesenchyme surrounding the eye (curved arrow in I), but not in blood vessels; expression in the RGC layer begins only at E17.5 (clear arrow). (M–R) Double immunofluorescence staining of horizontal sections through the eye with antibodies specific for NRP1 (red) and neurofilaments (NF; green in M–P) or IB4 (green in Q and R). Yellow staining indicates colocalization. NRP1-positive RGC axons are indicated with feathered arrows; hyaloid vessels, with solid arrowheads; and choroidal vessels, with clear arrowheads. (O) and (O′), (P) and (P′), and (R) and (R′) are higher magnifications of (M), (N), and (Q), respectively. (S) Schematic relationship of NRP1-positive blood vessels (BV) and RGC axons in the developing eye. (T) Double immunofluorescence staining of a horizontal section through the optic chiasm with antibodies specific for NRP1 (red) and neurofilaments (NF; green); the section was counterstained with the nuclear marker DAPI (blue). Feathered arrows indicate RGC axons; wavy arrows, capillaries in the diencephalon (outlined with a white dotted line). Scale bars: 100 μm.

Figure 2

NRP1 Is Essential for Normal Optic Tract Organization and Contralateral Projection at the Optic Chiasm (A and B) Wholemount views of RGC axons at the optic chiasm, labeled anterogradely with DiI at E14.0 (A) and E14.5 (B) in littermates expressing or lacking NRP1; ventral view, anterior up (see Figure S2A). The optic nerve (on), contralateral optic tract (ot_c), and ipsilateral optic tract (ot_i) are labeled in the first wild-type panel. Boxed regions are shown at higher magnification below each panel. Red arrowheads indicate the normal position of the ipsilateral projection; red arrows, the secondary tract and axon defasciculation in the mutants. (C) Coronal sections through the optic chiasm (top panels) and the site where the optic tracts begin to diverge (bottom panels) of anterogradely labeled E14.5 Nrp1^+/+ and Nrp1^−/− brains. (D) Ipsilateral index in Nrp1 null mutants. The method used to determine the ipsilateral index is shown on the left-hand side (see Supplemental Experimental Procedures for details). The mean (±SEM) ipsilateral index of E14.5 Nrp1^+/+ and Nrp1^−/− littermates is shown on the right-hand side; n = 10 each; ^∗∗∗p < 0.001 compared to wild-types. (E) Immunofluorescence labeling of radial glia and in situ hybridization (ISH) for ephrinb2 in coronal sections through the optic chiasm (oc) of E14.5 littermates expressing or lacking NRP1; dorsal is up. (F) ISH of coronal sections through stage-matched eyes expressing or lacking NRP1. Ephb1 identifies early ipsilaterally projecting RGCs in the dorsocentral retina (clear arrowhead). Zic2 identifies permanent ipsilaterally projecting RGCs in the ventrotemporal retina; the area outlined with a dotted square is shown at higher magnification in the insets; arrows indicate Zic2-positive RGCs; arrowheads, the ciliary margin. d, dorsal; v, ventral. Scale bars: 250 μm (A–C); 120 μm (E and F).

Figure 3

Expression of Class 3 SEMAs and Vegfa at the Developing Optic Chiasm (A) Schematic representation of the NRP1 regions that are essential for VEGF164 binding versus binding of the SEMA domain of class 3 SEMAs. (B) Domain structure of the three major mouse VEGF-A isoforms; the exon 7-encoded domain in VEGF164 mediates NRP1 binding. (C) Plane of sections through the optic chiasm and representative images of RGC axons at the chiasmatic midline at E12.5 and E14.5; RGC axons were labeled anterogradely with DiI, and the DiI photoconverted to a brown reaction product. (D and E) In situ hybridization of horizontal sections of wild-type embryos at the level of the optic chiasm with probes specific for Sema3a–3f (D) and of horizontal and coronal sections with a probe specific for Vegfa (E). Asterisks indicate the position in the E12.5 diencephalon where the optic chiasm will form; dotted lines indicate the position of the optic chiasm at older stages. Horizontal sections: anterior, up; coronal sections: dorsal, up. Scale bars: 200 μm.

Figure 4

Loss of VEGF164, but Not SEMA Signaling, Impairs RGC Axon Guidance at the Optic Chiasm (A, C, and E) Wholemount views of RGC axons, labeled anterogradely with DiI in E14.5 littermates expressing or lacking Sema3a (A), with or without SEMA signaling through neuropilins (Nrp1^Sema−/− Nrp2^−/−; C) or expressing or lacking VEGF164 (Vegfa^120/120; E); ventral view, anterior, up. In Vegfa^120/120 mutants, both optic tracts are defasciculated; red arrow indicates the normal position of the ipsilateral projection; red arrowheads, the secondary tract and axon defasciculation in the mutants. (B, D, and F) Mean (±SEM) ipsilateral index at E14.5 (Sema3a^+/+, n = 3; Sema3a^−/−, n = 4; Nrp1^Sema+/+ Nrp2^+/+, n = 5; Nrp1^Sema−/− and Nrp2^−/−, n = 7 each; Nrp1^Sema−/− Nrp2^−/−, n = 2; Vegfa^+/+ and Vegfa^120/120, n = 14 each); ^∗∗p < 0.01. (G and H) Coronal sections through the optic chiasm (top panels) and site where the optic tracts begin to diverge (bottom panels), after anterograde DiI labeling (G) or immunolabeling with neurofilament antibodies (H). Scale bars: 250 μm.

Figure 5

VEGF164 Is Essential for Contralateral Projection at the Optic Chiasm (A) Schematic illustration of the method used to retrogradely label and quantify the relative size of the ipsilateral and contralateral projections. DiI crystals were placed into the dorsal thalamus to label RGC axons in the optic tract on one side of the embryo. After dye diffusion into the ipsilateral and contralateral retinas, eyes were sectioned horizontally to quantify the number of labeled RGCs (B–D) or flatmounted to visualize the distribution of labeled cells within the retina (E). (B) Horizontal sections through the ventral ipsilateral retina in an E15.5 Vegfa^120/120 mutant and stage-matched wild-type following retrograde labeling from the optic tract; n, nasal; t, temporal. (C and D) Mean (± SEM) proportion of ipsilateral RGCs relative to total number of RGCs in both eyes (C) and proportion of ipsilateral RGCs originating in temporal versus nasal half of the ipsilateral retina (D) in E15.5 stage-matched wild-types, Vegfa^120/120 mutants, and mutants lacking NRP1 in blood vessel endothelium (Tie2^cre Nrp1^fl/–); ^∗ = p < 0.05 compared to wild-type or Tie2^cre Nrp1^fl/– conditional mutants (wild-type, n = 8; Vegfa^120/120, n = 6; Tie2^Cre Nrp1^fl/–, n = 5). (E) Flatmounted ipsilateral and contralateral retinas from E15.5 Vegfa^+/+ and Vegfa^120/120 embryos after retrograde labeling from the optic tract. The boxed regions are shown at higher magnification in the adjacent panels. DT, dorsotemporal; VN, ventronasal; DN, dorsonasal; VT, ventrotemporal. Scale bars: 125 μm.

Figure 6

VEGF164, but Not VEGF120, Promotes Outgrowth of Contralateral RGCs (A) Schematic illustration of the retinal areas placed into culture. Explants from peripheral ventrotemporal (VT) retina contain predominately ipsilaterally projecting RGCs, whereas peripheral dorsotemporal (DT), ventronasal (VN), and dorsonasal (DN) explants contain mainly contralaterally projecting RGCs. (B) Retinal explants from E14.5 wild-type dorsotemporal retina cultured for 24 hr in collagen gels in control culture medium or medium containing VEGF164 (50 ng/ml) or VEGF120 (50 ng/ml), fixed and stained for β-tubulin. (C) Mean (±SEM) total axon outgrowth from explants cultured in the presence of VEGF164 or VEGF120 (10 or 50 ng/ml), normalized to outgrowth in control cultures containing no exogenous VEGF (indicated with a dashed line). Number of control explants, 27–29 per quadrant; number of explants cultured with VEGF is indicated on the bars. ^∗p < 0.05, ^∗∗∗p < 0.001 compared to controls. (D) In situ hybridization with probes specific for Zic2 or Nrp1 on adjacent 20 μm sections through the E15.5 ventrotemporal wild-type retina. Images in right-hand panels were pseudocolored and overlaid to demonstrate the mutually exclusive expression pattern of both genes. (E) Retinal explants from E14.5 wild-type dorsotemporal retina cultured for 24 hr in medium containing or lacking VEGF164 (50 ng/ml) plus control goat IgG (1 μg/ml) or αNRP1 (0.5 μg/ml) and immunolabeled for β-tubulin. (F) Mean (±SEM) total axon outgrowth from explants cultured in the presence of control IgG (1 μg/ml) or αNRP1 (0.5 μg/ml) in the presence or absence of VEGF164 (50 ng/ml), normalized to the outgrowth in cultures containing control IgG alone (indicated with a dashed line). The number of explants per condition is indicated on the bars. ^∗∗∗p < 0.001 compared to control IgG. Scale bar, 200 μm.

Figure 7

VEGF164 Is a Chemoattractant for RGC Growth Cones (A) RGC growth cones at 0 min and 30 min after exposure to a gradient of vehicle (PBS), VEGF164, or VEGF164 in the presence αNRP1; the gradient emanated from a pipette (p), placed at a distance of 100 μm and a 45° angle relative to the growth cone; white arrows indicate the direction of growth cone extension. (B) Superimposed RGC axon trajectories over the 30 min observation period; black arrows indicate the direction of the gradient. (C and D) Mean (±SEM) turning angle (C) and cumulative frequency curves (D) of RGC growth cones from the ventrotemporal retina. The turning induced by VEGF120 or VEGF164 was not significantly different from the turning induced by PBS (C). For cumulative frequency curves, the turning angle of each growth cone was plotted against the percentage of growth cones turning to that angle or less. (E and F) Mean (±SEM) turning angle (E) and cumulative frequency curves (F) of RGC growth cones from the dorsotemporal retina. VEGF164 induced significant attraction relative to PBS or VEGF120 (^∗∗p < 0.01); the response was abrogated by αNRP1, but not control IgG. Scale bar: 25 μm.

Figure 8

Working Model for Axon Guidance at the Developing Mouse Optic Chiasm (A) In wild-type mice, VEGF164 at the chiasmatic midline counteracts inhibitory cues to promote the contralateral growth of NRP1-expressing axons, while repulsive ephrin B2 signals to EPHB1-expressing, NRP1-deficient axons to promote ipsilateral projection. Repulsive SLIT1 and SLIT2 signals cooperate to narrow the VEGF164-positive corridor through which RGC axons travel. (B) In the absence of VEGF164 signaling through NRP1, some RGC axons destined for the contralateral tract cannot overcome the inhibitory midline environment and form ectopic ipsilateral projections; in addition, the optic tracts defasciculate. (C) In the absence of ephrin B2 signaling through EPHB1, ipsilateral axons are no longer repelled from the midline and project contralaterally. (D) In the absence of SLIT signaling through Robo receptors, RGC axons are not constrained to the normal optic path and cross the VEGF164-positive midline region in a broader domain.