Role of BMP signaling and the homeoprotein Iroquois in the specification of the cranial placodal field.
Different types of placodes originate at the anterior border of the neural plate but it is still an unresolved question whether individual placodes arise as distinct ectodermal specializations in situ or whether all or a subset of the placodes originate from a common preplacodal field. We have analyzed the expression and function of the homeoprotein Iro1 in Xenopus and zebrafish embryos, and we have compared its expression with several preplacodal and placodal markers. Our results indicate that the iro1 genes are expressed in the preplacodal region, being one of the earliest markers for this area. We show that an interaction between the neural plate and the epidermis is able to induce the expression of several preplacodal markers, including Xiro1, by a similar mechanism to that previously shown for neural crest induction. In addition, we analyzed the role of BMP in the specification of the preplacodal field by studying the expression of the preplacodal markers Six1, Xiro1, and several specific placodal markers. We experimentally modified the level of BMP activity by three different methods. First, we implanted beads soaked with noggin in early neurula stage Xenopus embryos; second, we injected the mRNA that encodes a dominant negative of the BMP receptor into Xenopus and zebrafish embryos; and third, we grafted cells expressing chordin into zebrafish embryos. The results obtained using all three methods show that a reduction in the level of BMP activity leads to an expansion of the preplacodal and placodal region similar to what has been described for neural crest regions. By using conditional constructs of Xiro1, we performed gain and loss of function experiments. We show that Xiro1 play an important role in the specification of both the preplacodal field as well as individual placodes. We have also used inducible dominant negative and activator constructs of Notch signaling components to analyze the role of these factors on placodal development. Our results indicate that the a precise level of BMP activity is required to induce the neural plate border, including placodes and neural crest cells, that in this border the iro1 gene is activated, and that this activation is required for the specification of the placodes.
Glavic
Alvaro
A
Millennium Nucleus in Developmental Biology, Laboratory of Developmental Biology, Facultad de Ciencias, Universidad de Chile, Santiago.
Maris Honoré
Stella
S
Gloria Feijóo
Carmen
C
Bastidas
Francisco
F
Allende
Miguel L
ML
Mayor
Roberto
R
D000818:Animals; D019521:Body Patterning / Q000502:physiology; D052004:Bone Morphogenetic Protein Receptors; D019485:Bone Morphogenetic Proteins / Q000235:genetics / Q000378:metabolism*; D004625:Embryo, Nonmammalian; D004817:Epidermis / Q000196:embryology / Q000378:metabolism; D018507:Gene Expression Regulation, Developmental; D006257:Head / Q000196:embryology*; D018398:Homeodomain Proteins / Q000235:genetics / Q000378:metabolism*; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism; D009419:Nerve Tissue Proteins; D009420:Nervous System / Q000196:embryology / Q000378:metabolism; D017978:Receptors, Growth Factor / Q000235:genetics / Q000378:metabolism; D051880:Receptors, Notch; D015398:Signal Transduction; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism; D014981:Xenopus / Q000196:embryology; D029867:Xenopus Proteins / Q000235:genetics / Q000378:metabolism; D015027:Zebrafish / Q000196:embryology; D029961:Zebrafish Proteins / Q000235:genetics / Q000378:metabolism*
D019485:Bone Morphogenetic Proteins; D018398:Homeodomain Proteins; C110460:IRX2 protein, Xenopus; D008565:Membrane Proteins; D009419:Nerve Tissue Proteins; D017978:Receptors, Growth Factor; D051880:Receptors, Notch; D014157:Transcription Factors; D029867:Xenopus Proteins; C484675:Xiro1 protein, Xenopus; D029961:Zebrafish Proteins; C459599:irx1b protein, zebrafish; D052004:Bone Morphogenetic Protein Receptors