Examining the Role of Phospholamban Phosphorylation on Interaction with SERCA Using Fluorescence Microscopy

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Examining the Role of Phospholamban Phosphorylation on Interaction with SERCA Using Fluorescence Microscopy

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Regulation of the Sarco/Endoplasmic Reticulum Ca2+-ATPase (SERCA) by Phospholamban (PLB) plays a crucial role in normal cardiomyocyte function through controlling the speed and extent of myocyte relaxation. The interaction between PLB and SERCA is altered in many forms of heart failure (HF), making these proteins potential targets for the treatment of HF. Both proteins have been extensively studied in vitro, where their basic structure and function were determined, and in animal models, where their role in disease was examined. However, key information connecting the in vitro experiments and animal models is needed to better understand the PLB-SERCA interaction and to design effective HF treatment strategies. In particular, we wanted to examine two conflicting in vitro models of how the PLB-SERCA interaction changes after PLB phosphorylation: the dissociation model and the structural model. In the dissociation model, phosphorylation causes PLB to dissociate from SERCA, while in the structural model, phosphorylation causes a shift in the PLB binding position along SERCA. In order to determine the correct model of PLB-SERCA interaction in live cells, we expressed cyan fluorescent protein (CFP) and yellow fluorescent protein (YFP) fused to the N-termini of SERCA and PLB respectively, in HEK293 cells for fluorescence resonance energy transfer (FRET) microscopy experiments. We were able to use the native beta-adrenergic signaling system in the cells to control the state of PLB phosphorylation in a time-dependent manner. For the dissociation model to be true, we expected to see a significant reduction in FRET between CFP-SERCA and YFP-PLB after PLB phosphorylation. While significant increases in PLB phosphorylation were produced in the cells, FRET did not decrease. Instead, FRET increased with PLB phosphorylation at serine 16, indicating either a shorter distance between PLB and SERCA, or higher binding of PLB to SERCA. As the beta-adrenergic signal progressed through the cells, causing phosphorylation of PLB at threonine 17, FRET returned towards basal levels, but did not show the decrease below basal FRET levels that would indicate PLB dissociation from SERCA. Thus, we determined that there is a subtle change in the PLB-SERCA interaction due to PLB phosphorylation rather than a large scale dissociation. In order to differentiate changes in distance from changes in binding time-resolved (TR)-FRET experiments were required. Fluorescence lifetime imaging microscopy (FLIM) is a variant of TR-FRET that measures fluorescence decay curves with a fast-pulsed laser and photon counting board attached to a confocal microscope. These fluorescence decay curves provide more information than intensity measurements since they can be fit to multiple exponentials to test different interaction models. FLIM is a relatively new technique, thus we worked on developing appropriate experimental conditions for acquiring fluorescence decays that contained enough photons for multi-exponential fitting while still measuring individual cells. We were able to use FLIM to measure FRET values similar to those acquired on standard fluorescence microscopes and confirmed that phosphorylation of PLB did not cause dissociation from SERCA. However, further improvements to FLIM acquisition and analysis are needed for the multi-exponential fitting to provide a better model of PLB-SERCA interaction in live cells.


University of Minnesota Ph.D. dissertation. July 2018. Major: Biochemistry, Molecular Bio, and Biophysics. Advisor: David Thomas. 1 computer file (PDF); xiii, 130 pages.

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Haydon, Suzanne. (2018). Examining the Role of Phospholamban Phosphorylation on Interaction with SERCA Using Fluorescence Microscopy. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/200174.

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