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Dissertation
Uncovering a novel role for junctophilin-1 in cardiac dyad architecture and calcium handling
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Spring 2026
DOI: 10.25820/etd.008423
Abstract
Efficient excitation-contraction coupling (E-C coupling) in cardiac muscle depends on the precise and stable alignment of transverse tubules (T-tubules) with the terminal cisternae of the sarcoplasmic reticulum (jSR) at the level of the sarcomeric Z-lines. These highly specialized membrane junctions, known as cardiac muscle dyads, serve as the structural and functional foundation of rapid calcium signaling and coordinated muscle contraction. Despite their fundamental role, the mechanisms that govern the organization and maintenance of these junctions along Z-lines in cardiac muscle remain incompletely understood.
Members of the junctophilin family of proteins (JP1-4) provide structural support within T-tubule/jSR contacts. Canonically, JP1 is regarded as the primary paralog expressed in skeletal muscle triads and JP2 in cardiac muscle dyads. However, emerging evidence indicates that JP1 is also expressed in cardiac muscle, though its functional significance in this tissue is unresolved.
In chapter 2 of this thesis, I identify JP1 as a previously unrecognized regulator of cardiomyocyte structure, where it interacts with dystrophin and preserves dyad alignment at the Z-line, emphasizing its nonredundant role in maintaining dyad ultrastructure. I first explore the localization and mRNA and protein expression of JP1 in cardiomyocytes and then characterize the cardiac-specific JP1 knockout mouse model. Using immunoprecipitation followed by mass spectrometry, I identify that JP1 binds directly to dystrophin, a large protein that anchors the actin cytoskeleton to the extracellular matrix. JP1 binds dystrophin via residues at its N-terminus in an interaction that cannot be reproduced by the corresponding residues in JP2. Further, I show that in a mouse model of muscular dystrophy where dystrophin is not expressed in cardiomyocytes, JP1 protein is reduced by 40% and in JP1 knockout mice, dystrophin protein is reduced by more than half.
In chapter 3, I extend this work to examine how JP1 mRNA and protein expression or protein degradation and function are affected in cardiac disease. I show that JP1 mRNA and protein are significantly reduced in animal models of heart failure (HF) as well as in human hearts from patients with ischemic or dilated cardiomyopathy. Moreover, co-immunoprecipitation experiments reveal that HF conditions disrupt the JP1/dystrophin interaction, as the proportion of JP1 bound to dystrophin is reduced in failing hearts compared to controls. To investigate the mechanism underlying the loss of JP1 protein, I identify JP1 as a substrate for calpain-mediated proteolysis and map its cleavage site to residues 505-510, establishing a molecular pathway by which JP1 degradation may contribute to dyadic disorganization in HF.
Together, the findings presented in this thesis uncover a previously unrecognized role for JP1 in maintaining cardiac dyad architecture through its interaction with dystrophin. These results provide a new insight into the molecular mechanisms that govern dyad stability under physiological conditions and reveal how their disruption contributes to structural remodeling in HF.
Details
- Title: Subtitle
- Uncovering a novel role for junctophilin-1 in cardiac dyad architecture and calcium handling
- Creators
- Grace Ciampa
- Contributors
- Long-Sheng Song (Advisor)Kris DeMali (Advisor)Andrew Norris (Committee Member)Chad Grueter (Committee Member)Ryan Boudreau (Committee Member)Madeline Shea (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Biochemistry and Molecular Biology
- Date degree season
- Spring 2026
- DOI
- 10.25820/etd.008423
- Publisher
- University of Iowa
- Number of pages
- xiv, 128 pages
- Copyright
- Copyright 2026 Grace Ciampa
- Language
- English
- Date submitted
- 04/22/2026
- Description illustrations
- Illustrations, graphs, charts, tables
- Description bibliographic
- Includes bibliographical references (pages 114-128).
- Public Abstract (ETD)
Each heartbeat depends on a precise process called excitation-contraction coupling, which allows heart muscle cells to convert electrical signals into contraction. This process requires close alignment of two membrane systems, the transverse tubules (T-tubules) and the sarcoplasmic reticulum (SR), at structures known as dyads. How these dyads are organized and maintained in heart muscle has not been fully understood.
Junctophilins are a family of proteins that stabilize contact between T-tubules and SR. This thesis identifies junctophilin-1 (JP1) as a previously unrecognized structural regulator in the heart. While JP1 was thought to be specific to skeletal muscle and junctophilin-2 (JP2) the main cardiac isoform, this work shows that JP1 is also expressed in heart muscle where it plays a distinct and essential role.
I demonstrate that JP1 binds directly to dystrophin, a large structural protein that anchors the cell’s internal framework to its outer membrane. This interaction maintains the alignment of dyads along muscle fibers. In mice lacking JP1, dyad organization is disrupted, and dystrophin levels are reduced. In mice lacking dystrophin, dyad organization is disrupted in a similar way to mice lacking JP1, and JP1 levels are reduced. This shows that these proteins stabilize each other, and that this interaction is critical for dyad structure. I also show that JP1 levels decline in both human and animal models of heart failure and that heart failure weakens the JP1/dystrophin interaction. Finally, I identify that JP1 is degraded by the enzyme calpain in disease conditions, which cuts the protein at a specific site.
Together, these findings reveal a new mechanism for maintaining heart cell structure and offer insight into how its disruption contributes to heart failure.
- Academic Unit
- Biochemistry and Molecular Biology
- Record Identifier
- 9985176975202771
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