**5. Cardiomyocyte intracellular remodeling**

## **5.1 Remodeling of cytoskeletal and sarcomeric proteins**

Cytoskeletal proteins are essential for the structure and function of the cardiac myocyte. Stetson *et al.* reported ventricular unloading in humans dynamically changes not only myocardial TNF-α, total collagen, and myocyte size, but also remodels the expression of structural proteins [129]. To understand if myocardial recovery was associated with changes in sarcomeric, nonsarcomeric, and membraneassociated proteins, microarray analysis has been performed on the paired HF samples before and after VAD [16]. Significant increase of lamin A/C, spectrin and integrins (α5 and β5), and decrease of integrins β1, β6, and α7 has been observed at VAD explantation compared to pre-LVAD. Expression of sarcomeric proteins such as β-actin, α-tropomyosin, actinin-α1, and filamin A increased, while troponin T3 and actinin-α2 decreased. Vinculin expression decreased 4.1-fold in the recovered group. Despite decreased cardiomyocyte size post-VAD, severe structural damage in cardiomyocytes persisted with partial improvement in the expression of actin, tropomyosin, troponin C, troponin T, and titin [130]. In pediatric HF, MCS increased the expression of structural proteins, including dystrophin and actin [35]. Furthermore, expression of genes involved in calcium homeostasis, cell differentiation, and growth, including *CNNA1, CDK2B, CSF2, E2F1, EGR1,* and *EGR2*, were normalized after VAD therapy, suggesting an active reverse remodeling process after MCS in pediatric HF.

#### **5.2 Dystrophin remodeling**

Dystrophin is a rod-shaped protein encoded by the *DMD* gene located on the X chromosome, the largest gene of 2.4 megabases (Mb) in the human genome [131]. Dystrophin connects the actin and cytoskeleton of muscle fibers to the myocyte membrane at its N-terminus. At the C-terminus, it connects the sarcolemmal complex known as the dystrophin-associated protein complex (DAPC) to the ECM, providing structural support for myocytes. Mutations in *DMD* cause Duchenne and Becker muscular dystrophies [132, 133]. Mutations in genes encoding cytoskeletal and sarcolemmal proteins provide the genetic basis for dilation and contractile dysfunction *via* "final common pathway." Abnormalities in *DMD* such as mutations in the N-terminus of dystrophin or in the cardiac-specific promoter, preferentially affecting cardiac function are associated with X-linked cardiomyopathy [134]. Vatta *et al.* investigated the integrity and response of dystrophin in end-stage dilated or ischemic cardiomyopathy HF patients to VAD therapy and identified disruption of N-terminal dystrophin in 18 HF patients studied [135]. This disruption was shown to be reversible in four patients after VAD support.

### **5.3 Remodeling in calcium cycling**

Regulation of Ca2+ cycling is a versatile signaling process that regulates cellular homeostasis in different cell types, including cardiac myocytes [136]. Reduced rates of relaxation and impaired contractile reserve are the major abnormalities seen in the failing heart as a result of disturbances in Ca2+ transients [137]. The proteins that regulate cardiomyocyte Ca2+ cycling include sarcoplasmic reticulum (SR) Ca2+ ATPase (SERCA), ryanodine receptor 2 (RyR2), phospholamban (PLB), and the sarcolemmal Na+ /Ca2+ exchanger (NCX) [138–140]. Chaudhary *et al.* demonstrated that improvement in cardiac function during LVAD support was associated with a favorable balance between SERCA and NCX, resulting from the isolated decrease in NCX without an increase in SERCA [141]. Reverse remodeling of SERCA2a expression has been shown to be completed by about 20 days of VAD support, while hearts supported by VAD for longer than 40 days have significantly increased relative collagen content [142]. Post-VAD recovery increased SR calcium content and shortened action potential duration due to rapid inactivation in L-type Ca2+ current [15]. Short-term VAD support recovered post-rest potentiation (PRP) response to a level close to that in nonfailing hearts, but recovery of impaired SR Ca2+ cycling was dependent on duration MCS [143]. Chronic unloading with recovery of contractile function demonstrated upregulation of *SERCA2a*, *RyR2,* and *NCX* genes after MCS [144]. Recovery of rate-dependent contractility in failing human hearts during early VAD support was associated with faster decay of Ca2+ transients, while long-term MCS triggered abnormal Ca2+ cycling [101, 143]. Moreover, long-term MCS resulted in significantly increased SMAD2 activity with downstream phosphorylation of Ca2+/calmodulin-dependent protein kinase type-IIδ (CaMKIIδ), myocyte enhancer factor 2 (MEF2), and myostatin. Improvements in the Ca2+ handling also depended on the severity of myocardial fibrosis, and ECM pathologies and excessive fibrosis limited the ability to recover [13].

## **5.4 Mitochondria and metabolism remodeling**

Unloading with VAD has been shown to contribute to reverse remodeling of mitochondria and recovery of energy metabolism of the failing heart. In healthy adult hearts, the generation of ATP as a source of energy relies on the oxidation of fatty acids, glucose and lactate in mitochondria, and fatty acid oxidation provides the majority (> 70%) of total ATP [145]. The balance between lactate production and consumption by lactate dehydrogenase (LDH) that converts it to pyruvate, which is then transported by mitochondrial pyruvate carrier (MPC) into the mitochondrial tricarboxylic acid (TCA) cycle is important in producing plentiful ATP. The MPC expression is lower in patients with HF compared to those of non-failing cohorts [146]. Thus, the failing heart runs

#### *Myocardial Remodeling with Ventricular Assist Devices DOI: http://dx.doi.org/10.5772/intechopen.110814*

increased glycolysis and decreased fatty acid oxidation for ATP production and the proportion of glucose oxidation to fatty acid oxidation depends on the severity of HF [147]. The generation of ATP is disturbed in HF with an increased glycolytic pyruvate-derived lactate and a simultaneous decrease in lactate utilization [148]. In addition, the opening of mitochondrial permeability transition pore (mPTP) in HF disrupts the mitochondrial membrane potential and disturbs oxidative phosphorylation pathways for ATP production, causing mitochondrial swelling and inducing apoptotic and necrotic cell death.

MCS improves systemic and cardiac metabolism *via* improvements in fatty acid oxidation, insulin resistance, and reductions in myocardial lipotoxicity through improved activation of the insulin/PI3K/AKT signaling cascade [149]. Significant decrease in long-chain acylcarnitines levels was consistent with improved fatty acid oxidation and utilization during long-term VAD support [150]. Diakos *et al*. reported induction of glycolysis through TCA without a subsequent increase in pyruvate oxidation in post-VAD patients [148], which may be attributed to the poor post-VAD recovery of mitochondrial oxidative capacity. Recently, the same group reported the beneficial cardioprotective effects of induced glycolysis as a result of an increase in rate-limiting enzymes of the pentose-phosphate pathway and 1-carbon metabolism in post-VAD patients [151]. All these have been associated with significantly reduced reactive oxygen species (ROS) and improved mitochondrial density [151, 152]. These metabolic improvements enhanced the glycosylation of α-dystroglycan, which maintains integrity between cytoskeleton and ECM [18]. Moreover, using high-resolution respirometry, a reduction in mitochondrial ROS up to 40% [153] and increased MPC1 abundance and glucose and glucose-6-phosphate levels, particularly, in mechanically unloaded hearts of ischemic HF patients has been demonstrated [154].

Levels of Ca2+ in the mitochondrial matrix regulate the activity of kinases and phosphatases involved in ATP production and mitochondrial quality control [155, 156]. In HF, the opening of mPTP not only disrupts the mitochondrial membrane potential but also reduces Ca2+ uptake, alters pH, and induces inflammation, leading to necrosis and death of cardiac myocytes [157]. Impaired mitochondrial Ca2+ uptake is the result of reduced Ca2+ release from SR and stimulates Ca2+ −sensitive dehydrogenases of the Krebs cycle [158].

About 20% of the total lipid composition on the mitochondrial inner membrane is constituted by cardiolipin and loss of cardiolipin and tetralinoleoyl-cardiolipin in HF is linked to excessive ROS production and cardiomyocyte apoptosis [159]. During mechanical unloading with LVAD, cardiolipin arrangement normalizes, which in turn, improves mitochondrial coupling [160]. Cellular proteases, such as cathepsins, are involved in the progression of HF. Parallel activation of cathepsins and their inhibitors was observed after VAD support. The expression of cathepsins and their inhibitors was significantly higher in pre-VAD compared to the heart transplant group and VAD induced a further increase in the cathepsin system. Significant positive correlations were observed between cardiac expression of cathepsins and their inhibitors as well as inflammatory cytokines [59, 161].

#### **5.5 Cardiomyocyte signal transduction pathways and signaling**

### *5.5.1 Mitogen-activated protein kinases*

There are several cell signal-transduction pathways regulated in the heart in direct response to changes in mechanical loading and stress. The family of MAPKs, such as ERKs, p38, and JNK1/2, are well-characterized signal-transduction pathways [162].

These kinases are involved in the regulation of cell growth, cardiac hypertrophy, and cell death [163, 164]. They are upregulated in patients with HF secondary to ischemic heart disease and cardiomyopathy [165, 166]. The ERKs activity regulates adaptive hypertrophy and prevention of cell death during the early phase of chronic pressure overload in response to stimulation of GPCRs and integrin activation [167]. Mechanical unloading with VAD support resulted in differential regulation of MAPK activity with a significant decrease in the activity of p44/42 ERK and JNK1/2 along with a subsequent increase in p38 activity after LVAD support [91]. The authors explained a decrease in ERK activity is likely due to its decreased phosphorylation at p44/42, while a combination of decreased phosphorylation and expression of JNK1/2 is responsible for decreased JNK1/2 activity in VAD-supported hearts. Activation of AKT regulates cardiac physiological hypertrophy, glucose metabolism, cell death, and angiogenesis [168]. In failing human hearts, a high grade of kinase phosphorylation in all 3 MAPKs and AKT have been observed [166]. After VAD support, ERKs and AKT activities were dramatically decreased in failing hearts, while GSK-3β activities were increased [89].
