Ph.D Thesis

Ph.D StudentKeren-Politansky Anat
SubjectMAPK Stress Signals and Cardiac Development of Xenopus
DepartmentDepartment of Medicine
Supervisor ASSOCIATE PROF. Eyal Bengal
Full Thesis textFull thesis text - English Version


Vertebrate cardiac development is derived from paired primordia of the anterior dorsolateral mesoderm that expresses the Nkx2.5 and GATA4 transcription factors. Although signals promoting cardiac progenitors cell specification have been extensively studied in many organisms, there is no clear distinction between early axis patterning signals, that affect cardiac cell specification and differentiation, and direct inducers of the expression of cardiac-specific genes, such as Nkx and GATA family members. Even less is known about the intracellular pathways inducing their expression. We investigated the earliest signaling events leading to the expression of the key transcription factors Nkx2.5 and GATA4 in cardiac progenitors of Xenopus laevis.

We report that anterior neural ectoderm induces early expression of Nkx2.5 in neighboring cells at late gastrula. Since Fibroblast growth factor 3 (Fgf3) is expressed in this tissue at that stage of development, we analyze if it is the inducer of Nkx2.5 expression. Our results describe an intracellular signaling pathway involving p38 MAPK and the CREB transcription factor, which are necessary and sufficient to induce Nkx2.5 expression independently of their function in patterning. Furthermore, the intracellular p38 MAPK and CREB pathway functions downstream of Fgf to initiate Nkx2.5 expression without affecting GATA4 expression. Therefore, we suggest that Fgf expressed in anterior neural ectoderm is a major inducer of Nkx2.5 expression in neighboring cells. In these cells, Fgf activates an intracellular p38 MAPK signaling pathway and its downstream target, the CREB transcription factor, which participate in the expression of Nkx2.5 in cardiac progenitors.

Our results reveal the existence of an independent signaling pathway affecting GATA4 expression. We show that Wnt11, which is secreted from the dorsal mesoderm lip, functions to initiate GATA4 expression. We also find that the intracellular mediator of Wnt11 signaling is the c-Jun N-terminal kinas (JNK). JNK is both necessary and sufficient to induce GATA4 expression. The initiation of GATA4 and Nkx2.5 expression is mutual exclusive since JNK and p38 MAPK pathways are antagonistic. 

Finally, we show that while Fgf and Wnt11 are required to initiate cardiac genes expression, BMP signaling is necessary to maintain their expression at later developmental stages.