Semester

Fall

Date of Graduation

2010

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Biology

Committee Chair

Ashok Bidwai

Abstract

The Notch signaling pathway is highly conserved in vertebrates and invertebrates and regulates binary cell fate specifications in diverse developmental contexts. In a process termed lateral inhibition, Notch signaling restricts a particular cell fate to a single cell from a group of multi-potential cells. This mechanism ensures precision in cell fate assignations, which are vital for histogenesis and organogenesis. Studies during Drosophila neurogenesis, specifically development of the compound eye and the mechanosensory bristles, have provided the most detailed insights into the underlying mechanisms. During this process, Notch signaling results in the selection of single R8 photoreceptors or bristle sensory organ precursors (SOP's) from proneural clusters that express the transcriptional activators encoded by atonal ( ato) or genes of the achaete scute Complex ( ASC), respectively. This selection is mediated through the E(spl) repressors, a group of evolutionarily conserved bHLH proteins that antagonize Ato or ASC. The emerging view is that repression by E(spl) proteins is a regulated process, one involving phosphorylation. The work described in this dissertation is aimed at characterizing the role of protein kinase CK2 and the phosphatase PP2A in lateral inhibition. The work of Chapter 2 provides in vivo evidence that loss of CK2 compromises lateral inhibition and results in the specification of supernumerary R8's and SOP's from proneural clusters. These effects appear to be sensitive to E(spl) dosage, consistent with the direct interaction of CK2 with a subset of E(spl) members. The eye and bristle phenotypes associated with loss of CK2 indicate that the activity of this kinase is essential for inhibitory Notch signaling. The work of Chapter 3 provides in vivo evidence that Widerborst (Wdb), the regulatory subunit of the phosphatase PP2A, is a participatory component of Notch signaling. Using gain- and loss-of-function studies in wild type flies, or those mutant for Notch and E(spl), it is demonstrated that Wdb is essential for PP2A activity, and that PP2A plays a role that is opposite to that of CK2. These opposing activities of CK2 and PP2A may involve direct interactions, and indicate that a regulatory nexus influencing E(spl) activity lies at the heart of Notch signaling. The studies of Chapter 4 characterize a novel hypomorphic allele of CK2 to directly demonstrate a role for this kinase in eye and bristle development. In addition, the lethality of this hypomorphic allele has been rescued by the expression of a variant of the catalytic (alpha) subunit that exhibits temperature-sensitive (ts) behavior in vivo. This is the first ts -allele of CK2. Given the roles of CK2 in diverse developmental programs, compounded by the early lethality of null alleles of this kinase, flies rescued by ts-alleles of CK2 should represent a resource of wide utility to the Drosophila community.

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