Dysregulation of Calcium Handling in Duchenne Muscular Dystrophy-Associated Dilated Cardiomyopathy: Mechanisms and Experimental Therapeutic Strategies

Abstract

: Duchenne muscular dystrophy (DMD) is an X-linked recessive disease resulting in the loss of dystrophin, a key cytoskeletal protein in the dystrophin-glycoprotein complex. Dystrophin connects the extracellular matrix with the cytoskeleton and stabilizes the sarcolemma. Cardiomyopathy is prominent in adolescents and young adults with DMD, manifesting as dilated cardiomyopathy (DCM) in the later stages of disease. Sarcolemmal instability, leading to calcium mishandling and overload in the cardiac myocyte, is a key mechanistic contributor to muscle cell death, fibrosis, and diminished cardiac contractile function in DMD patients. Current therapies for DMD cardiomyopathy can slow disease progression, but they do not directly target aberrant calcium handling and calcium overload. Experimental therapeutic targets that address calcium mishandling and overload include membrane stabilization, inhibition of stretch-activated channels, ryanodine receptor stabilization, and augmentation of calcium cycling via modulation of the Serca2a/phospholamban (PLN) complex or cytosolic calcium buffering. This paper addresses what is known about the mechanistic basis of calcium mishandling in DCM, with a focus on DMD cardiomyopathy. Additionally, we discuss currently utilized therapies for DMD cardiomyopathy, and review experimental therapeutic strategies targeting the calcium handling defects in DCM and DMD cardiomyopathy.

Keywords: Serca2a; calcium; dilated cardiomyopathy; gene therapy; heart; mdx; membrane stabilization; muscular dystrophy; oxidative stress; phospholamban.

Figure 1

Common genetic and acquired causes of dilated cardiomyopathy in humans. ZASP: z-band alternatively spliced PDZ-motif.

Figure 2

Normal excitation-contraction coupling in cardiac myocytes. Membrane depolarization leads to a small influx of calcium through the L-type calcium channel (LTCC/DHPR) (1), which triggers a larger release of calcium from the sarcoplasmic reticulum (SR) through ryanodine receptor 2 (RyR2) (2). Calcium then binds to the myofilaments, triggering myocyte contraction (3). During the relaxation phase, calcium reuptake occurs by pumping calcium out of the cytoplasm back into the SR via Serca2a or through the Na⁺/Ca²⁺ exchanger (NCX) (4). Phospholamban negatively regulates Serca2a activity. β-adrenergic signaling leads to phospholamban (PLN) phosphorylation and dissociation from Serca2a, increasing the rate of calcium reuptake into the SR. Normal physiological stretch leads to NADPH oxidase 2 (NOX-2) production of reactive oxygen species (ROS), which increases calcium entry through stretch-activated channels (SACs). Dystrophin serves to stabilize the sarcolemma during the repeated stress of myocyte contraction and relaxation. Inset shows increased detail of the dystrophin glycoprotein complex (DCG) and myofilament proteins. NOS: nitric oxide synthase; MCU: mitochondrial calcium uniporter; NCLX: mitochondrial sodium calcium exchanger.

Figure 3

Mechanisms of calcium overload in dystrophin-deficient cardiac myocytes. The absence of dystrophin destabilizes the sarcolemma and leads to stress-induced membrane damage/micro-tears and calcium influx (a). Excessive reactive oxygen species (ROS) production in cardiac myocytes leads to further membrane damage and increased calcium influx via stretch-activated channels (SACs) and ryanodine receptor 2 (RyR2) (b). Increased L-type calcium channel (LTCC/DHPR) current also contributes to increased intracellular calcium (c). Calcium leak from RyR2 (d), decreased Serca2a expression (e) and increased phospholamban (PLN) inhibition of Serca2a decrease sarcoplasmic reticulum (SR) calcium load, subsequently decreasing calcium transient peak height and decay rate and inhibiting contractile function in later stages of Duchenne muscular dystrophy (DMD) cardiomyopathy (dark lines in transients). Increased cytosolic calcium leads to mitochondrial cell death pathways (f). * Indicates points of abnormal calcium entry into the myocyte. NCX: Sodium calcium exchanger; NOX-2: NADPH oxidase 2; NOS: Nitric oxide synthase; MCU: Mitochondrial calcium uniporter; NCLX: Mitochondrial sodium calcium exchanger.

Figure 4

Effects of increased calcium cycling on cardiac myocytes with dystrophin deficiency. Loss of dystrophin destabilizes the sarcolemma and leads to calcium mishandling and overload. Increasing calcium cycling via modulation of Serca2a/PLN function increases calcium uptake into the SR, which could decrease cytosolic calcium concentration. However, increased calcium cycling also increases contractility, which could subsequently cause increased membrane damage and exacerbate calcium overload. In the context of dystrophin deficiency, phospholamban (PLN) ablation led to increased membrane damage and worsened cardiomyopathy [19].