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The level of BMP4 signaling is critical for the regulation of distinct T-box gene expression domains and growth along the dorso-ventral axis of the optic cup.

Behesti H, Holt JK, Sowden JC - BMC Dev. Biol. (2006)

Bottom Line: Increased levels of BMP4 signaling caused decreased proliferation, reduced retinal volume and altered the shape of the optic cup.Our findings suggest the existence of a dorsal-high, ventral-low BMP4 signaling gradient across which distinct domains of Tbx2, Tbx3, Tbx5 and Vax2 transcription factor gene expression are set up.Furthermore we show that the correct level of BMP4 signaling is critical for normal growth of the mammalian embryonic eye.

View Article: PubMed Central - HTML - PubMed

Affiliation: Developmental Biology Unit, Institute of Child Health, University College London, 30 Guilford Street, London, WC1N 1EH, UK. h.behesti@ich.ucl.ac.uk <h.behesti@ich.ucl.ac.uk>

ABSTRACT

Background: Polarised gene expression is thought to lead to the graded distribution of signaling molecules providing a patterning mechanism across the embryonic eye. Bone morphogenetic protein 4 (Bmp4) is expressed in the dorsal optic vesicle as it transforms into the optic cup. Bmp4 deletions in human and mouse result in failure of eye development, but little attempt has been made to investigate mammalian targets of BMP4 signaling. In chick, retroviral gene overexpression studies indicate that Bmp4 activates the dorsally expressed Tbx5 gene, which represses ventrally expressed cVax. It is not known whether the Tbx5 related genes, Tbx2 and Tbx3, are BMP4 targets in the mammalian retina and whether BMP4 acts at a distance from its site of expression. Although it is established that Drosophila Dpp (homologue of vertebrate Bmp4) acts as a morphogen, there is little evidence that BMP4 gradients are interpreted to create domains of BMP4 target gene expression in the mouse.

Results: Our data show that the level of BMP4 signaling is critical for the regulation of distinct Tbx2, Tbx3, Tbx5 and Vax2 gene expression domains along the dorso-ventral axis of the mouse optic cup. BMP4 signaling gradients were manipulated in whole mouse embryo cultures during optic cup development, by implantation of beads soaked in BMP4, or the BMP antagonist Noggin, to provide a local signaling source. Tbx2, Tbx3 and Tbx5, showed a differential response to alterations in the level of BMP4 along the entire dorso-ventral axis of the optic cup, suggesting that BMP4 acts across a distance. Increased levels of BMP4 caused expansion of Tbx2 and Tbx3, but not Tbx5, into the ventral retina and repression of the ventral marker Vax2. Conversely, Noggin abolished Tbx5 expression but only shifted Tbx2 expression dorsally. Increased levels of BMP4 signaling caused decreased proliferation, reduced retinal volume and altered the shape of the optic cup.

Conclusion: Our findings suggest the existence of a dorsal-high, ventral-low BMP4 signaling gradient across which distinct domains of Tbx2, Tbx3, Tbx5 and Vax2 transcription factor gene expression are set up. Furthermore we show that the correct level of BMP4 signaling is critical for normal growth of the mammalian embryonic eye.

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Repression of Tbx2 and Tbx5 expression by exogenous Noggin. (A) Post-culture embryo showing Tbx5 expression in the non-treated dorsal optic cup. (B) Contralateral Noggin-treated eye, showing absence of Tbx5 expression in the optic cup. (C) Post-culture embryo showing Tbx2 expression in the non-treated dorsal optic cup. (D) Contralateral Noggin-treated eye showing a reduced Tbx2 expression domain. Tbx5 expression (E, F) and Tbx2 expression (G, H) are not altered in non-operated and BSA-treated eyes respectively of post-culture embryos. A'-H' show higher magnifications of eyes in A-H. Arrows indicate boundaries of gene expression domains. Implanted beads are marked by asterisks. (I-P) Serial transverse vibratome sections through the optic cup showing altered morphology after Noggin treatment, with the dorsal-most sections in upper panels. (I-L) Post-culture non-treated eye showing invagination of the presumptive neural retina (P-NR) in the ventral optic cup. (M-P) Contralateral Noggin-treated eye showing dorsal extension of the optic stalk region as compared to the non-treated eye. (Q) Schematic representation of normal Tbx2 and Tbx5 expression domains in a lateral view of the optic cup, and the dorsal shift of the expression domains induced by exogenous Noggin. Scale bars: A-H, 0.5 mm; I-P, 0.1 mm. Abbreviations; MN, mandibular process of the first branchial arch; MX, maxillary process of the first branchial arch; N, nasal process; P-NR, presumptive neural retina; OS, optic stalk.
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Figure 4: Repression of Tbx2 and Tbx5 expression by exogenous Noggin. (A) Post-culture embryo showing Tbx5 expression in the non-treated dorsal optic cup. (B) Contralateral Noggin-treated eye, showing absence of Tbx5 expression in the optic cup. (C) Post-culture embryo showing Tbx2 expression in the non-treated dorsal optic cup. (D) Contralateral Noggin-treated eye showing a reduced Tbx2 expression domain. Tbx5 expression (E, F) and Tbx2 expression (G, H) are not altered in non-operated and BSA-treated eyes respectively of post-culture embryos. A'-H' show higher magnifications of eyes in A-H. Arrows indicate boundaries of gene expression domains. Implanted beads are marked by asterisks. (I-P) Serial transverse vibratome sections through the optic cup showing altered morphology after Noggin treatment, with the dorsal-most sections in upper panels. (I-L) Post-culture non-treated eye showing invagination of the presumptive neural retina (P-NR) in the ventral optic cup. (M-P) Contralateral Noggin-treated eye showing dorsal extension of the optic stalk region as compared to the non-treated eye. (Q) Schematic representation of normal Tbx2 and Tbx5 expression domains in a lateral view of the optic cup, and the dorsal shift of the expression domains induced by exogenous Noggin. Scale bars: A-H, 0.5 mm; I-P, 0.1 mm. Abbreviations; MN, mandibular process of the first branchial arch; MX, maxillary process of the first branchial arch; N, nasal process; P-NR, presumptive neural retina; OS, optic stalk.

Mentions: To provide further evidence supporting the role of BMP4 signaling in regulating T-box gene expression, we used the BMP antagonist, Noggin, to block BMP4 signaling in the eye (Fig. 4). Noggin has high affinity for BMP4 and prevents BMP receptor-ligand association, hence disrupting the signaling pathway [59].


The level of BMP4 signaling is critical for the regulation of distinct T-box gene expression domains and growth along the dorso-ventral axis of the optic cup.

Behesti H, Holt JK, Sowden JC - BMC Dev. Biol. (2006)

Repression of Tbx2 and Tbx5 expression by exogenous Noggin. (A) Post-culture embryo showing Tbx5 expression in the non-treated dorsal optic cup. (B) Contralateral Noggin-treated eye, showing absence of Tbx5 expression in the optic cup. (C) Post-culture embryo showing Tbx2 expression in the non-treated dorsal optic cup. (D) Contralateral Noggin-treated eye showing a reduced Tbx2 expression domain. Tbx5 expression (E, F) and Tbx2 expression (G, H) are not altered in non-operated and BSA-treated eyes respectively of post-culture embryos. A'-H' show higher magnifications of eyes in A-H. Arrows indicate boundaries of gene expression domains. Implanted beads are marked by asterisks. (I-P) Serial transverse vibratome sections through the optic cup showing altered morphology after Noggin treatment, with the dorsal-most sections in upper panels. (I-L) Post-culture non-treated eye showing invagination of the presumptive neural retina (P-NR) in the ventral optic cup. (M-P) Contralateral Noggin-treated eye showing dorsal extension of the optic stalk region as compared to the non-treated eye. (Q) Schematic representation of normal Tbx2 and Tbx5 expression domains in a lateral view of the optic cup, and the dorsal shift of the expression domains induced by exogenous Noggin. Scale bars: A-H, 0.5 mm; I-P, 0.1 mm. Abbreviations; MN, mandibular process of the first branchial arch; MX, maxillary process of the first branchial arch; N, nasal process; P-NR, presumptive neural retina; OS, optic stalk.
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Figure 4: Repression of Tbx2 and Tbx5 expression by exogenous Noggin. (A) Post-culture embryo showing Tbx5 expression in the non-treated dorsal optic cup. (B) Contralateral Noggin-treated eye, showing absence of Tbx5 expression in the optic cup. (C) Post-culture embryo showing Tbx2 expression in the non-treated dorsal optic cup. (D) Contralateral Noggin-treated eye showing a reduced Tbx2 expression domain. Tbx5 expression (E, F) and Tbx2 expression (G, H) are not altered in non-operated and BSA-treated eyes respectively of post-culture embryos. A'-H' show higher magnifications of eyes in A-H. Arrows indicate boundaries of gene expression domains. Implanted beads are marked by asterisks. (I-P) Serial transverse vibratome sections through the optic cup showing altered morphology after Noggin treatment, with the dorsal-most sections in upper panels. (I-L) Post-culture non-treated eye showing invagination of the presumptive neural retina (P-NR) in the ventral optic cup. (M-P) Contralateral Noggin-treated eye showing dorsal extension of the optic stalk region as compared to the non-treated eye. (Q) Schematic representation of normal Tbx2 and Tbx5 expression domains in a lateral view of the optic cup, and the dorsal shift of the expression domains induced by exogenous Noggin. Scale bars: A-H, 0.5 mm; I-P, 0.1 mm. Abbreviations; MN, mandibular process of the first branchial arch; MX, maxillary process of the first branchial arch; N, nasal process; P-NR, presumptive neural retina; OS, optic stalk.
Mentions: To provide further evidence supporting the role of BMP4 signaling in regulating T-box gene expression, we used the BMP antagonist, Noggin, to block BMP4 signaling in the eye (Fig. 4). Noggin has high affinity for BMP4 and prevents BMP receptor-ligand association, hence disrupting the signaling pathway [59].

Bottom Line: Increased levels of BMP4 signaling caused decreased proliferation, reduced retinal volume and altered the shape of the optic cup.Our findings suggest the existence of a dorsal-high, ventral-low BMP4 signaling gradient across which distinct domains of Tbx2, Tbx3, Tbx5 and Vax2 transcription factor gene expression are set up.Furthermore we show that the correct level of BMP4 signaling is critical for normal growth of the mammalian embryonic eye.

View Article: PubMed Central - HTML - PubMed

Affiliation: Developmental Biology Unit, Institute of Child Health, University College London, 30 Guilford Street, London, WC1N 1EH, UK. h.behesti@ich.ucl.ac.uk <h.behesti@ich.ucl.ac.uk>

ABSTRACT

Background: Polarised gene expression is thought to lead to the graded distribution of signaling molecules providing a patterning mechanism across the embryonic eye. Bone morphogenetic protein 4 (Bmp4) is expressed in the dorsal optic vesicle as it transforms into the optic cup. Bmp4 deletions in human and mouse result in failure of eye development, but little attempt has been made to investigate mammalian targets of BMP4 signaling. In chick, retroviral gene overexpression studies indicate that Bmp4 activates the dorsally expressed Tbx5 gene, which represses ventrally expressed cVax. It is not known whether the Tbx5 related genes, Tbx2 and Tbx3, are BMP4 targets in the mammalian retina and whether BMP4 acts at a distance from its site of expression. Although it is established that Drosophila Dpp (homologue of vertebrate Bmp4) acts as a morphogen, there is little evidence that BMP4 gradients are interpreted to create domains of BMP4 target gene expression in the mouse.

Results: Our data show that the level of BMP4 signaling is critical for the regulation of distinct Tbx2, Tbx3, Tbx5 and Vax2 gene expression domains along the dorso-ventral axis of the mouse optic cup. BMP4 signaling gradients were manipulated in whole mouse embryo cultures during optic cup development, by implantation of beads soaked in BMP4, or the BMP antagonist Noggin, to provide a local signaling source. Tbx2, Tbx3 and Tbx5, showed a differential response to alterations in the level of BMP4 along the entire dorso-ventral axis of the optic cup, suggesting that BMP4 acts across a distance. Increased levels of BMP4 caused expansion of Tbx2 and Tbx3, but not Tbx5, into the ventral retina and repression of the ventral marker Vax2. Conversely, Noggin abolished Tbx5 expression but only shifted Tbx2 expression dorsally. Increased levels of BMP4 signaling caused decreased proliferation, reduced retinal volume and altered the shape of the optic cup.

Conclusion: Our findings suggest the existence of a dorsal-high, ventral-low BMP4 signaling gradient across which distinct domains of Tbx2, Tbx3, Tbx5 and Vax2 transcription factor gene expression are set up. Furthermore we show that the correct level of BMP4 signaling is critical for normal growth of the mammalian embryonic eye.

Show MeSH
Related in: MedlinePlus