Analysis of Foxc1 Function During Somite Formation and Chondrogenesis of the Axial Skeleton

• Author / Creator

  Lavy, Rotem

• Development of the axial skeleton in vertebrates begins with the segmentation of the paraxial mesoderm into somites. Somites are transient structures in the embryos that ultimately give rise to the skeletal muscle, dermis, tendons, cartilage, and bony structures of the axial skeleton. Once somite-derived sclerotome migrates to the appropriate position within the embryo body, it undergoes chondrogenesis and ossification, giving rise to the ribs and vertebrae through endochondral ossification. Defects in the formation of the somites or the generation or ossification of the cartilage template of the skeleton can lead to abnormal skeletal development. While the molecular mechanisms driving somitogenesis and skeletal development and differentiation have been extensively studied much is still unknown. FOXC1 is a forkhead box transcription factor with an essential role in skeletal development. Foxc1 is expressed in the mesodermal tissues giving rise to the axial skeleton. Disruptions in Foxc1 are associated with abnormalities in somitogenesis as well as in the formation of the ribs, vertebrae, long bones, and bones of the skull vault. Here I assess the role of FOXC1 in somitogenesis and chondrogenesis, two of the critical developmental process crucial for proper formation of the axial skeleton. Loss of foxc1a function in zebrafish results in reduced expression of mesp-ba and a lack of formation of the anterior somites. The transcription factor mesp-ba has an important role in determining somite boundary formation. The expression of mesp-ba in the presomitic mesoderm is regulated by the ripply1-tbx6 negative feedback network. I examined how foxc1a interacts with the ripply1-tbx6 network to regulate the expression of mesp-ba in the presomitic mesoderm and anterior somite formation. The results herein demonstrate that foxc1a interacts with the ripply1-tbx6 regulatory network to regulate the expression gradient of raldh2, a member of the retinoic acid signaling network. The interaction between the opposing gradients of raldh2 and fgf8a in the presomitic mesoderm is critical for somite boundary formation and the expression of mesp-ba. When Foxc1a is absent establishment of these gradients is disrupted resulting in reduced mesp-ba expression. Anterior somite formation is more sensitive to fluctuations in retinoic acid expression, and they are therefore more severely affected in the foxc1a morphants. Chondrogenic differentiation is highly dependent on the expression of Sox9, a SRY-related transcription factor. Using published ChIP-Seq data I have identified a Foxc1 enhancer region responsive to SOX9 activation. I have also demonstrated that FOXC1 overexpression in the initial stages of chondrogenesis leads to acceleration of the chondrogenic differentiation and elevation of chondrogenic marker expression. Finally, I have shown that while FOXC1 and SOX9 do not appear to be physically interacting with one another, they do come in close vicinity, suggesting they have cooperative function regulating downstream targets in chondrogenesis and endochondral bone formation. My results demonstrate that FOXC1 has a crucial role in correct development of the axial skeleton and that it is directly involved in the regulation of many processes during skeletal growth. These observations shed some light on the complexities and tight regulation involving embryonic development and help gain better understanding of the role that FOXC1 plays in many of these events.

• Subjects / Keywords

  • Sox9
  • Endochondral ossification
  • Foxc1
  • Somitogenesis
  • Chondrogenesis
  • Ripply1
  • Zebrafish

• Graduation date

  Spring 2018

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3PK07H0J

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.