Congratulations Laura Peterson Brown, PhD

On Tuesday, June 2, 2020 Laura Peterson Brown (John McCarthy Lab) successfully defended her dissertation and earned her PhD. Congratulations Dr. Laura Peterson Brown!

Doctoral Committee

Dr. John McCarthy
Department of Physiology, Mentor

Dr. Brian Delisle
Department of Physiology

Dr. Steve Estus
Department of Physiology

Dr. Tim Butterfield
Department of Athletic Training and Clinical Nutrition

Dr. Charlotte Peterson
Department of Physical Therapy

Dr. Doug Harrison (Outside Examiner)
Departmentof Biology

Abstract

Ribosomes are the molecular machinery of the cell that catalyzes synthesis of peptides from amino acids. The eukaryotic ribosome is made up of 4 strands of ribosomal RNA (rRNA) and ~80 ribosomal proteins. While many tissues routinely exhibit variations of ribosomal protein stoichiometry, tissue specific ribosomal proteins are rare. The ribosomal protein with the highest tissue specificity of any ribosomal protein is found in striated muscle, ribosomal protein L3-like (RPL3L). Other than its tissue specificity, association with atrial fibrillation, and chromosomal location, there is little known about the function of RPL3L. However, its ubiquitously expressed paralog, RPL3, has been well documented to be essential for ribosome biogenesis, aid in peptidyl transfer, and increase translational fidelity.
This thesis, therefore, seeks to address the critical gap in knowledge on the function of RPL3L in striated muscle and specifically, the effect of RPL3L knockout (KO) on cardiac function and protein translation in vivo. To that end, a RPL3L KO mouse was generated that, instead of striated muscle-specific RPL3L, expresses the ubiquitous RPL3 in striated muscles.
The first aim of this dissertation was to test the hypothesis that RPL3L KO would induce cardiac arrhythmias by expression in the atria. First the expression pattern of RPL3 and RPL3L in the wild-type (WT) heart were established by both RT-PCR and Western blot. Both indicated that while the ventricle has high expression of RPL3L, RPL3 is found at much lower levels (~10% that of RPL3L). The atria however, had the opposite expression pattern with RPL3 being high and RPL3L not expressed. In order to determine if the RPL3L KO mice recapitulated the fibrillation phenotype seen in humans with Rpl3l variants, we performed echocardiography and electrocardiography on WT and KO mice. No changes were observed in heart rate, ejection fraction, wall thickness during systole or diastole, fractional shortening or stroke volume under resting conditions. When telemetry fitted mice were treated with the β2 adrenergic receptor agonist, isoproterenol, both WT and KO mice showed a significant increase in heart rate after treatment (p=0.02 and 0.0007 respectively) but the rate of response was significantly more rapid in KO mice (p=<0.0001). Due to the increase in rate of response to isoproterenol in the KO, we hypothesized that loss of expression of RPL3L in the pace-making center of the heart, the sinoatrial node, was responsible for the rapid increase in heart rate To that end, single-cell RNA sequencing data from nuclei of the sinoatrial node, and proteomic data from the sinoatrial node were queried. Analysis revealed that RPL3L is expressed at a very low level at the mRNA level in the sinoatrial node but that it is not detected at the protein level. These results do not support the hypothesis that loss of RPL3L in the atria causes atrial fibrillation, rather this evidence suggests that if RPL3L plays a role in atrial fibrillation, it is likely secondary to a ventricular pathology.
The second aim of this dissertation was to test the hypothesis that RPL3L plays a functionally specialized role in the ribosome causing enhanced translation of a subset of mRNAs, thereby conferring preferential recruitment to mRNAs which are specific to striated muscle. Actively translating ribosomes of cardiac tissue were isolated via polysome fractionation and were subsequently subjected to RNA sequencing (RNA-seq). Analysis revealed that there were 216 mRNAs that were differentially translated (but not differentially transcribed). Of these genes, 68 were more highly translated in WT (RPL3L-ribosomes) whereas 148 were more highly translated in the KO (RPL3-containing ribosomes). Gene ontology of differentially translated mRNAs showed highest enrichment for genes involved in RNA binding and splicing. These results support the hypothesis that there is differential translation of a subset of mRNAs. However, in contrast to our hypothesis, the mRNAs that were enriched in the translating fraction were not specific to striated muscle.
This study demonstrates that KO of RPL3L is not lethal, and while it does cause changes in cardiac response to isoproterenol, its loss is not sufficient to induce atrial fibrillation in mice. This study also demonstrates that RPL3L expression is robust and highly specific to the ventricles of the heart but that its expression exhibits only minor alterations on the cardiac translatome. The findings here help to further our understanding of translation in the heart and its effects on cardiac physiology.

Acknowledgements

The following thesis, while an individual work, benefited from the insights and direction of several people. First, my Thesis Chair, John J. McCarthy who was a constant source of encouragement, who challenged me and helped me develop into a scientist. I would also like to thank my committee, Charlotte Peterson, Seven Estus, Timothy Butterfield, Brian Delisle, and my outside examiner, Douglas Harrison.
I would also like to thank those who helped mold me into a scientist on a daily basis – my lab mates: Yuan Wen, Ivan Vechetti, Alexander Alimov, Taylor Valentino, Vandre Casagrande-Figueiredo and Brooks Mobely. I would also like to thank Ivan Vechetti Jr. for his patience with me, his unending willingness to help, hear my ideas, and give feedback. I would like to thank Yuan Wen for always listening to my constantly new hypotheses, challenging my thinking, and his constant, encouraging support. I could not have done it without you two. I would also like to thank Shelby Meier for years of emotional and scientific support, as well as help preparing for my defense.
I never intended my educational journey to go this far, nor did I think I was even capable of it, but I would like to thank all the people who always believed in me. Firstly, my family, my parents Celeste and David Peterson, my parents in law, Jane and Ron Brown, and all my siblings, especially my brother Joshua Peterson who was always unabashedly proud of me from start to finish. I would like to thank my husband, Christopher Brown who has supported me in every way possible and has always believed in me even when I did not believe in myself. When challenges arose, he was always the one to assure me that I could overcome it. When experiments would fail, his was the shoulder that I cried on. I cannot thank him enough for putting up with me, helping me, and encouraging me through this journey.
Lastly, I would like to thank United States taxpayers. Without their support this research would have never been possible. I was supported both by a National Institute of Health grant as well as a National Science Foundation Graduate Research Fellowship. The NIH grant kick-started this whole project, and the NSF grant gave me the liberty to study what I felt were the most important things to know about the beguiling protein, RPL3L. Thank you to each and every taxpayer. Your support of basic research does not go unnoticed or unappreciated.