Publication Date

2011-10-06

Availability

Embargoed

Embargo Period

2013-10-05

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Cell Biology and Anatomy (Medicine)

Date of Defense

2011-09-01

First Committee Member

Richard L. Rotundo

Second Committee Member

Mary L. King

Third Committee Member

Gregory Conner

Fourth Committee Member

Terace Fletcher

Abstract

In skeletal muscle acetylcholinesterase AChE is highly expressed at sites of nerve-muscle contact where it is regulated at both the transcriptional and post-transcriptional levels. Scientists have elucidated many aspects of synaptic AChE structure, function, and localization during the past 80 years. However our understanding of the molecular mechanisms underlying its regulation is incomplete, but it appears to involve both translational and post-translational events as well. We found that Pumilio-2 (PUM2), an RNA binding translational repressor, is highly localized at the neuromuscular junction where AChE mRNA concentrates and that PUM2 binds to the AChE transcripts when immoprecipitation studies were performed. A direct binding between a recombinant PUM2-HD and the Pumilio Binding Site (PBE) in a segment of the AChE 3’UTR was demonstrated by Gel shift assays. Transfecting skeletal muscle cells with shRNAs specific for PUM2 upregulated AChE expression, whereas overexpression of PUM2 decreased AChE activity. We conclude that PUM2 binds to AChE mRNA and regulates AChE expression translationally at the neuromuscular synapse. We found that PUM2 is regulated by the motor nerve suggesting a trans-synaptic mechanism for locally regulating translation of specific synaptic proteins involved in modulating synaptic transmission, analogous to CNS synapses. PUM2 expression is critically important in many cell types, virtually nothing is known about the regulation of PUM2 expression itself. Analyzing the PUM2 mRNA 3’UTR we found fifteen possible PBEs in the 3 Kb 3’ UTR. We show that PUM2 binds in vivo to its own mRNA. Overexpression of PUM2 in several cell types transfected with a green fluorescent protein (GFP) reporter construct linked to the full length PUM2 3’UTR (GFP-PUM2-3’UTRFL) suppresses GFP expression suggesting that PUM2 downregulates its own expression by binding to its own 3’UTR. Mutations of the first five PBEs yield the expression of the reporter gene indicating that at least one PBE is functional in the autoregulation of PUM2. These observations suggest a novel model for the localized regulation of protein translation through a negative feedback loop. Much is known about PUM2 as a translational regulative protein but little is known about PUM2 cell localization and possible mechanism of translational regulation. In this work we found PUM2 to be highly localized to the cell rough endoplasmic reticulum and that PUM2 is associated with ribosomal RNA. In addition, we found that the GFP protein itself, together with its mRNA and ribosomal RNA (rRNA), were localized in the PUM2 positive complexes when GFP-PUM2-3’UTRFL was transfected into muscle cells. These observations further suggest a mechanism of regulation where translation of the protein occurs but the protein remains associated with the ribonucleoprotein complex, possibly to be transported together with its mRNA to specific domains inside the cell. Thus when needed, more protein is produced in those specific cell regions.

Keywords

Pumilio; Acetylcholinesterase; RNA binding protein; synapse; neuromuscular junction

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