Date Available

3-10-2019

Year of Publication

2017

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Arts and Sciences

Department/School/Program

Biology

First Advisor

Dr. Brian C. Rymond

Abstract

The Serf gene is evolutionarily highly conserved but its biological function is not known in any organism. In human, SERF1/H4F5 was first identified as a modifier of the disease Spinal Muscular Atrophy (SMA). SMA is caused by mutations in the Survival Motor Neuron 1(SMN1) gene leading to diminished levels of the Smn protein. More than 90% of patients with the most severe form of SMA have deletions that remove SERF1 as well as mutaions within SMN1. Hence, loss of Serf activity is hypothesized to exacerbate SMA disease progression. The primary motivation of this thesis was to test this intriguing but yet unverified hypothesis using a model organism, Drosophila melanogaster.

To genetically manipulate Serf activity I created deletion, overexpression and knockdown alleles of Serf. I found that Serf is non-essential for viability in Drosophila and that null mutants have no obvious developmental defects. However, the loss of Serf gene activity results in diminished adult locomotion. In addition, Serf null mutants show lower Smn protein abundance. As Smn mRNA levels do not change with Serf manipulation, regulation likely occurs at the level of Smn protein translation or stability.

I tested the impact of Serf in SMA by altering Serf expression in a fly SMA model harboring equivalent Smn point mutations as those that cause SMA in human patients. I found that diminished Serf levels exacerbate the observed mutant phenotype in growth, development and viability which correlates with decreased Smn protein abundance. Importantly, the simple overexpression of Serf in certain Smn mutant backgrounds increases the Smn protein abundance, which in some cases, correlates with a partial rescue of the associated phenotypic defects. In addition to being required for maximal Smn abundance, I found that Serf gene expression directly correlates with the abundance of toxic α-synuclein protein seen in a fly Parkinson’s disease model. These data support a role for Serf in protein homeostasis relevant to proteins active in at least two distinct neurodegenerative diseases.

My study has also revealed that Serf influences lifespan in Drosophila. Loss of Serf reduces lifespan by 20-30% whereas ubiquitous overexpression of Serf results in an equivalent extension of the normal lifespan. Lifespan extension occurs even when Serf overexpression is restricted to muscles, neurons or only adult tissues. Change in lifespan with Serf manipulation inversely correlates with the accumulation of poly-ubiquitinated protein aggregates, a marker of tissue aging. These aggregates are marked with Ref(2)p/p62, a target of autophagy. Analysis of expression of several genes in the autophagy pathway suggests that Serf expression may promote longevity, at least in part, by upregulating the life-extending autophagy pathway. Serf gene expression also correlated with a modest resistance to oxidative stress and changes in the abundance of a mitochondrial marker protein, mitofusin, suggesting the possibility that Serf activity may impact mitochondrial function. Taken together, these studies establish Serf as a modifier of the Smn-limited SMA phenotype and reveal previously unknown roles for the Serf gene in Drosophila mobility and lifespan.

Digital Object Identifier (DOI)

https://doi.org/10.13023/ETD.2017.034

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