BMP signaling protects telencephalic fate by repressing eye identity and its Cxcr4-dependent morphogenesis

Abstract

Depletion of Wnt signaling is a major requirement for the induction of the anterior prosencephalon. However, the molecular events driving the differential regionalization of this area into eye-field and telencephalon fates are still unknown. Here we show that the BMP pathway is active in the anterior neural ectoderm during late blastula to early gastrula stage in zebrafish. Bmp2b mutants and mosaic loss-of-function experiments reveal that BMP acts as a repressor of eye-field fate through inhibition of its key transcription factor Rx3, thereby protecting the future telencephalon from acquiring eye identity. This BMP-driven mechanism initiates the establishment of the telencephalon prior to the involvement of Wnt antagonists from the anterior neural border. Furthermore, we demonstrate that Rx3 and BMP are respectively required to maintain and restrict the chemokine receptor cxcr4a, which in turn contributes to the morphogenetic separation of eye-field and telencephalic cells during early neurulation.

Figure 1. BMP, but Not Wnt, Subdivides the Anterior Forebrain Neural Fate into Eye and Telencephalon

(A-C) Foxg1/rx3 in situ hybridization of dkk1 RNA (A) and dkk1 morpholino-injected embryos (C) compared to uninjected control (B) bud stage embryos (end of gastrulation), dorsal view,anterior to the top. (D-F) Schematic illustration of fate changes in response to BMP depletion in swrsup>ta72 ^−/− (D) and elevated level of BMP signaling in chordin^{−/ −}(F), compared to wild-type (E) bud stage embryos, lateral view, dorsal to the right. fb, forebrain; mb,midbrain; hb, hindbrain; sc, spinal chord; meso, mesoderm. (G-O) Foxg1, rx3 double in situ hybridization in swrsup>ta72 ^−/− (G, H, and M), WT (I, J, and N), and chordin^−/− (K, L, and O). Asterisks in (G), (H), and (M) mark the center of the radialized axis. (G) Animal pole view, (H) lateral view, (I-L) dorsal view, anterior to the top, (M-O) sagittal section of the neural plate, anterior to the left. See also Figure S1

Figure 2. Patterning of the Anterior Neural Plate Requires BMP Signaling at Pre- and Early Gastrula Stages

(A-E) Embryos were treated with 10 μM of DM (B-E) or equivalent volume of DMSO (A) at time points indicated. All embryos were fixed at bud stage followed by double in situ against foxg1 and rx3. (A, D, and E) Dorsalview, (B and C) animalpole view, and (A’-E’) sagittal section. DM treatment from 60% epiboly until bud stage results in partial rescue of the telencephalon at most anterior position (arrows in D and D’). (F-J) In rx3^−/− mutants foxg1 expression is expanded across the eye field at one-somite stage (G), but not at bud stage (F). In DM-treated rx3^−/− embryos (H-J), the expanded eye field expresses foxg1 in the entire rx3-positive domain, except at its margin and a small stripe of cells at the medial/ventral position (arrows in J). (F and G) Dorsal view, anterior to the top, (H) animal pole view, (I) lateral view, (J) sagittal section, ventral to the left. BS, bud stage; 1som, one-somite stage. See also Figure S2.

Figure 3. BMP Signaling Is Active at the Rostral Margin of the Neural Plate during Early Gastrulation

(A-H) Whole-mount P-Smad1,5,8 antibody staining (green) and dlx3 transcript detection (red) in 60% epiboly (early gastrula, A-D) and bud stage (end gastrula, E-H).White lines mark the border of dlx3 expression. (A-D) Animal pole view, dorsal to the right. (E-H) Dorsal view, anterior to the left. (D) is a 40 × magnification of the yellow boxed area in (C). (H) is a close-up of the yellow boxed area in (G). All figures are z-projections of confocal sections. nne, nonneural ectoderm; preT, prospective telencephalon; Epi, epidermis; PP, preplacodal territory; Tel, telencephalon. See also Figure S3.

Figure 4. Repression of Anterior Neural Plate Markers by Elevated BMP Activity Is Limited to the Early Stage of Gastrulation

All embryos were injected with HSP70:caBMPr plasmid at one- to two-cell stage followed by heat shock at oblong/early blastula stage (A-F) or shield/early gastrula stage (G and H) and fixed at bud stage. Heat shock induction of caBMPr at oblong stage represses foxg1 expression non-cell-autonomously (arrows in A and B) and rx3 expression cell autonomously (arrows in C and D). Dlx3 is ectopically induced in caBMPr-expressing cells inside the neural plate (arrows in E and F). Heat shock at shield stage does not affect rx3 expression (arrowsinG and H).All figures are z-projections of confocal sections.See also Figure S4.

Figure 5. Mosaic BMP Loss of Function at Early Stages inside the Telencephalon Results in Ectopic Expression of rx3

Embryos were injected with HSP70:dnBMPr plasmid at one- to two-cell stage followed by heat shock at oblong stage/early blastula (A-L) and shield stage/early gastrulation (M and N), respectively. The arrows in (A)-(H) mark cells with ectopic rx3 expression at stages indicated. (E and F) Close-up of the boxed area marked in the inset. (I-L) Expression of telencephalic (Tel.) markers, tlc and emx3, is maintained in dnBMPr-expressing cells (arrows in I-L). (M and N) Heat shockat50% epiboly/early gastrulation does not result in ectopic rx3 expression (arrows in M and N). All views are dorsal, anterior at the top. White dotted lines in (C), (D), (M), and (N) mark the border of the eye field. All figures are z-projections of confocal sections. 1s, one-somite stage. See also Figure S5.

Figure 6. Ectopic Expression of Retinal Markers in BMP-Depleted Telencephalic Cells at 24 hpf

(A-G) Embryos were injected with HSP70:dnBMPr plasmid at one- to two-cell stage followed by heat shock at oblong/early blastula stage. The arrows indicate dnBMPr-positive cells in the telencephalon at 24hr, which ectopically express retinal markers mab21l2 (A and B) and vsx2 (C-G). The number of transiently transgenic dnBMPr cells is significantly lower at 24 hpf than at earlier stages, and the remaining transgenic cells frequently undergo apoptosis (arrows in D). (C-E) White dotted lines mark the vsx2-positive retina. All views are dorsal, anterior at the top. All figures are z-projections of confocal sections. MB, midbrain. See also Figure S6.

Figure 7. The Chemokine Receptor cxcr4a Demarcates the Border of the Eye Field and Telencephalon Downstream of BMP and Rx3

(A-D) Cxcr4a expression in rx3^−/− or sib at 80% epiboly (A and B) and at bud stage in sib (C) and rx3^−/− (D) embryos. Sdf1b/rx3 double in situ at 80% epiboly/midgastrula stage (E) and four-somite stage (4s) (F). Inset in (E) shows a sagittal section of the boxed area,anteriorto the left. (G-J) Cxcr4a transcript expression (red) in dnBMPr transgenic mosaics (green in G and H) and caBMPr transgenic mosaics (green in I and J) at bud stage. (G and H) Close-up of the boxed area marked in the inset. (I and J) Repression of cxcr4a expression in caBMPr-overexpressing cells. (K-N) Cross-section at med ial A-P level of the eye field (dashed linein the insets in K and L). Segregation ofrx3 (blue) and emx3(red) expression domainsin WT (K and M) and cxcr4a^um20+/- x cxcr4a^um20+/- crosses (Land N; n = 14/61) at four-somite stage. Dotted lines mark the border of emx3-expressing cells as defined by fluorescent images (M and N). (O) A model of the genetic network leading to formation of the eye and telencephalon. Blue arrows demonstrate the present work.Dotted-arrow line indicates that Rx3 is not essential for induction but for maintenance of cxcr4a expression. See also Figure S7.