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Doctor of Philosophy (PhD)
Dr. Jonathan Satin
Heart failure is a major public health problem and a leading cause of mortality. This clinical condition affects populations of all ages, and is the result of various cardiomyopathies. Almost half of these patients suffer specifically from heart failure with reduced ejection fraction; these hearts have decreased performance due to a failure of the heart to contract with sufficient force to meet demand. While there are therapies available to increase contractility, none of these enhance contraction without also further promoting pathological signaling and remodeling.
Under normal physiological conditions, the body elevates cardiac output through the fight-or-flight response. This response activates b-adrenergic receptors (b-AR) at the level of individual cardiomyocytes, which leads to enhanced calcium handling in order to increase contraction. One of the major targets of b-AR downstream signaling is the L-type calcium channel (LTCC). The influx of calcium through the LTCC (ICa,L) provides the trigger for calcium induced calcium release from the sarcoplasmic reticulum in order to produce a contraction; LTCC activity is significantly increased when b-ARs are activated. However, b-ARs are chronically activated in heart failure, leading to pathological remodeling and further development of heart failure. This has served as a foundation to establish dogma that increasing ICa,L in a manner that reflects b-AR activation necessarily promotes pathology. Because b-AR signaling is a principle physiological mechanism to increase cardiac output, understanding this pathway and how to increase calcium safely is critical to successfully treating heart failure. Discovering a mechanism to increase cardiac output downstream of b-AR signaling would be ideal so as to preserve the fight-or-flight response while also boosting cardiac performance in order to meet demand.
The mechanism by which LTCC activity is increased under b-AR signaling, known as modulation, has been a major focus of study for many years; however, it remains unknown. The LTCC is a heteromultimeric protein complex, and is inhibited by an endogenous small monomeric GTPase called Rad. Studies in heterologous expression systems show overexpression of Rad blocks calcium current through the LTCC; absence of Rad yields a significant increase in calcium current. Whole-body Rad knock out mouse models demonstrate calcium current that mirrors calcium current stimulated by b-AR signaling; however, this also promoted significant growth of the heart. To investigate the effect of Rad deletion without contributions from non-cardiac tissue, a cardiomyocyte-restricted inducible Rad knock out mouse model was created. The work of this dissertation utilizes this mouse to better understand the mechanism by which Rad inhibits the LTCC by studying the effects on channel function, cellular calcium handling, and overall cardiac structure and function in the absence of Rad.
Using an array of methods and techniques, the studies in this dissertation establish Rad as a critical target of the LTCC to respond to b-AR stimulation. When Rad is depleted specifically from cardiomyocytes, ICa,L is increased in a safe, stable manner that mirrors LTCC modulation, both in sinoatrial node and in the ventricle. This regulation is governed specifically by the C-terminus of Rad. Elevated ICa,L in the absence of Rad promotes enhanced calcium handling and increased cardiac output without progression to heart failure, and occurs independently of b-AR activation. Enhanced calcium cycling in the absence of Rad is balanced by accelerated inactivation of the LTCC so as to promote positive inotropy without instigating arrhythmogenesis. This allows for cardiac protection under conditions of pressure-overload induced heart failure. In summary, the work of this dissertation supports Rad deletion specifically from cardiomyocytes as an ideal positive inotrope for heart failure treatment due to the novel mechanism to increase ICa,L in a manner that preserves structure, function, and the fight-or-flight response within the heart.
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This study was supported by the American Heart Association's Predoctoral Fellowship (no. 19PRE34380909) from 2018-2020.
This study was also supported by National Institute of General Medical Sciences Grant (T32GM118292) from 2017-2018.
This study was also supported by National Institute of Health (no. HL131782) from 2019-2024.
Ahern, Brooke Mildred, "Rad Modulation of the L-type Calcium Channel Confers Systolic Advantage in the Heart" (2021). Theses and Dissertations--Physiology. 50.