Author ORCID Identifier

https://orcid.org/0000-0003-0850-7319

Date of Award

8-11-2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Neuroscience Institute

First Advisor

Daniel N. Cox

Second Advisor

Charles Derby

Third Advisor

Victor Faundez

Fourth Advisor

Angela Mabb

Abstract

Discovering the molecular mechanisms that regulate dendritic diversification is essential to understanding the formation and modulation of functional neural circuitry. Recent studies have demonstrated that the TFs Cut and Knot combinatorially regulate cell-type specific dendritogenesis in Drosophila multidendritic (md) sensory neurons via cellular pathways that converge on cytoskeletal architecture. Neurogenomic analyses identified PP2A serine/threonine phosphatase complex as a downstream effector of both Cut and Knot. The PP2A complex is composed of a catalytic subunit microtubule star (mts), a scaffolding subunit PP2A-29B, and one of four alternate regulatory subunits widerborst (wdb), twins, well-rounded (wrd) and CG4733. Mutant analyses of mts and PP2A-29B reveal severe reductions in dendritic arborization with wdb appearing to function as the relevant regulatory subunit in Class IV (CIV) md sensory neurons. In contrast, mutations in mts and PP2A-29B leads to increased dendritic complexity in Class I (CI) md sensory neurons. Cellularly, live imaging reveals that mts mutations lead to microtubule (MT) destabilization. In addition, dynamic EB1::GFP imaging of MTs reveals Mts is required to maintain dendritic MT polarity. In contrast to its effect on MTs, mts knockdown leads to F-actin reorganization with a proximal shift towards the soma. Loss of mts leads to significant reductions in dendritic localization of organelles including mitochondria and satellite Golgi outposts in CIV neurons, while in CI neurons, mts mutants show increased Golgi outpost trafficking along the dendritic arbor. Further, mts mutant neurons exhibit defects in neuronal polarity as revealed by Golgi outposts appearing in the proximal axon whereas in controls Golgi outposts are primarily restricted to the soma and at satellite locations on dendrites. Furthermore, in mts mutants selective markers of dendritic and axonal compartments are detected in the opposing compartments. At a regulatory level, the translational repressor FMR1 may function as a PP2A target in CIV neurons, whereas the TF FoxO may function as a target in CI neurons. In contrast, phenotypic analyses suggest that the β-tubulin subunit 85D represents a common PP2A target across these md neuron subtypes. Collectively, these studies provide insights into the functional roles of the PP2A phosphatase in promoting dendritic diversity.

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