PROPERTIES OF REISSNER FIBER ASSEMBLY AND SIGNALING DURING BODY AXIS STRAIGHTENING

Abstract

Spinal conditions like scoliosis are highly prevalent across the globe. However, not much is known about how the spine develops its symmetrical shape and maintains that shape throughout adulthood. In this thesis, I used zebrafish as a model organism to study early body straightening and how zebrafish maintain their straight spine throughout adulthood. Chapter I focuses on understanding the Reissner fiber, a proteinaceous fiber that sits in the zebrafish central canal of the spinal cord. The central canal is lined with motile cilia, which generate a fluid flow that promotes the formation of the Reissner fiber. Like motile cilia, the Reissner fiber is an important component for both early body straightening and maintaining a straight spine throughout life. It is primarily composed of the glycoprotein SCOspondin. However, other potential components of the Reissner fiber may act in axial straightening. One of these candidates are F-spondin proteins, Spon1a and Spon1b. To test if these proteins are important in early axial straightening, I generated somatic mutants using CRISPR/Cas9 to knock out the genes that encode Spon1a and Spon1b. I found that neither Spon1a nor Spon1b were required for axial straightening, so they are not likely relevant to developing a straight body axis. Mutation of these genes also did not disrupt the integrity of the Reissner fiber, adding further evidence that F-spondin proteins are not critical for spinal development. After examining F-spondin proteins, I wanted to further investigate the Reissner fiber. Therefore, my second experiment in Chapter I examined the dynamics of Reissner fiber assembly, a largely unknown topic. To investigate this, I generated SCOspondin mosaic mutants using CRISPR/Cas9. In these mutants, the central canal of the zebrafish had some cells that could produce SCOspondin while some that couldn’t, allowing me to examine whether assembly relied on local secretion of SCOspondin, or if the protein aggregated to form the Reissner fiber in a more distributed fashion. I found that Reissner fiber assembly relied on two factors. First, there had to be enough SCOspondin in the central canal above a threshold. I also found that the Reissner fiber could form over gaps of floor plate cells that couldn’t produce SCOspondin, supporting a distributed model of assembly Chapter II focuses on signaling downstream of the Reissner fiber, namely the role of Urotensin-II peptides (Urps). Urps have recently emerged as a factor that mediates spine morphology that acts downstream of the Reissner fiber; however, less is known about their role in maintaining a straight spine throughout adolescence and adulthood. Therefore, we generated germline mutants that lacked Urp 1, Urp 2, both Urp 1 and Urp 2, or the Urp receptor. We found that while Urps were not necessary for proper body straightening early in development, zebrafish lacking these peptides developed spinal curves during adolescent growth stages. The spinal curves in urotensin mutants worsened into adulthood, but showed a distinct phenotype compared to motile cilia mutants. Because the phenotypes of urotensin and motile cilia mutants do not match, this indicates that Urps are not the only factor acting downstream of motile cilia to control spine morphology. Overall, this thesis adds to the current model of early axial straightening and adult spine morphology. Chapter I ruled out potential proteins from the model and explored the dynamics of 4 Reissner fiber assembly. Chapter II explored the important role of Urps in maintaining a straight spine throughout life. Ultimately, this information can help us understand human spinal conditions like scoliosis.