Understanding the Hippo (Hpo) Kinase and its function in tissue growth and organ size control.

Open Access
Author:
Oboudiyat, Sahba
Area of Honors:
Biology
Degree:
Bachelor of Science
Document Type:
Thesis
Thesis Supervisors:
  • Zhi Chun Lai, Thesis Supervisor
  • Bernhard Luscher, Honors Advisor
Keywords:
  • Hippo
  • Hpo
  • Kinase
  • function
  • tissue growth
  • organ size control
  • Drosophila
Abstract:
In development, one of the most pressing questions is to understand how tissue growth and organ size are normally regulated during animal development and how interference with such regulation could lead to uncontrollable growth. With respect to my research, the growth inhibition mediated by Hippo signaling is a topic of interest. The Hippo signaling pathway is important for tissue growth and organ size control within Drosophila melanogaster. This pathway cascade consists of many proteins that are all responsible for restraining cell proliferation and promoting apoptosis. Mutations in the Hippo pathway, specifically in the Hpo gene, lead to tissue overgrowth, or a “hippopotamus-like” phenotype. The Hippo pathway is composed of a kinase cascade where Hpo phosphorylates the protein kinase Warts (Wts), activating Wts. The proteins responsible for the activation of Wts are Salvador (Sav) and Mob as a tumor suppressor (Mats). Sav functions as a scaffold protein, promoting the phosphorylation of Wts while Mats associates with the kinase activity of Wts, therefore activating it. Activated Wts then phosphorylates and inactivates the transcriptional co-activator Yorkie (Yki), which in turn inhibits growth. In contrast, active Yki is bound to the transcription factor Scalloped (Sd), which allows for the expression of organ growth genes that promote cell cycle progression. The aspect of the Hippo pathway that most interested me is the effect of hippo and salvador on cell proliferation, specifically with the change in area of Drosophila melanogaster wings. The Drosophila embryos were injected with the genes of interest, specifically HpoVN, HpoVC, HpoVN/HpoVC, HpoVC/SavVN and HpoVN KD, using a pUAST vector. After the Drosophila grew to maturation, the wing sizes for each set of genetically modified Drosophila were measured. The mutant Hpo, HpoVN KD, has the largest average total wing area of 1.34e6 pixels. Being the mutant, Wts was not phosphorylated and therefore Yki was activated, leading to cell proliferation and, therefore, greater wing area. It would then be expected for HpoVC and HpoVN to have wing growth, but they should both be less than the mutant. All of the data has been run through a one-way ANOVA statistical analysis using the Tukey-Kramer method through the Minitab© program. This was the case since HpoVC and HpoVN both had areas of 1.22e6 pixels and 1.20e6 pixels, respectively. It was concluded that the areas were less than the other wing types because Hpo phosphorylated Wts, which inactivated Yki, preventing overexpression.