Regulation of TGFβ-superfamily signalling during early embryonic patterning

Abstract

One of the earliest steps in development is the specification of the germ layers, the embryonic subdivision into endoderm, mesoderm and ectoderm. Members of the Transforming Growth Factor β (TGFβ) family are required for endoderm and mesoderm induction, whilst inhibition of this pathway is vital for ectodermal specification. TGFβ inhibitors in the ectodermal region prevent the endoderm and mesoderm inducing signals from acting animally. Coco, an inhibitor of TGFβ, Bone Morphogenetic Protein (BMP) and Wnt signalling, is expressed throughout the animal half of the Xenopus laevis embryo up to gastrula stages. Loss-of-function experiments using morpholino oligonucleotides (MO) to Coco investigated whether Coco is required to inhibit these TGFβ ligands from signalling animally. Morphant embryos had clear germ layers defects: a loss of mesoderm and an expansion of endoderm. To investigate whether these defects were caused by an over activation of TGFβ signalling in the animal pole, Coco morphant embryos were coinjected with MOs to candidate TGFβ ligands (Vg1, Activin, Nodal). ActivinMO rescued the Coco knockdown phenotypes demonstrating that, in normal development, Coco prevents Activin signals from acting animally, ensuring correct ectoderm specification.
Coco overexpression induces ectopic heads that contain forebrain and midbrain, due to inhibition of both BMP and Wnt signalling. To investigate whether induction of heads by Coco is mediated via the induction of other factors, epistatic experiments were performed; Coco overexpression was combined with a knock down of BMP inhibitors. A reduction of BMP antagonism led to induction of ectopic heads that lacked anterior tissue, demonstrating a requirement for other BMP inhibitors downstream of Coco. To investigate downstream targets of genes involved in germ layer specification (Coco) and neural induction (Noggin1) a microarray screen was performed. Differences were found between the two signalling molecules confirming their different roles in early embryonic patterning.

Date of Award	2012
Original language	English
Awarding Institution	King's College London
Supervisor	Esther Bell (Supervisor) & Clemens Kiecker (Supervisor)