Cellular Basis of Diastolic Dysfunction - Term Paper Example

Applications of Biomedical Science-Muscle Fitness and Failure Background The heart is a pump made up of muscle tissue. It has four chambers: the upper two chambers are the right and left atria, while the lower chambers are the right (RV) and left ventricles (LV). Circulating blood passes through the four chambers in only one direction with the aid of the tricuspid, pulmonary, mitral, and aortic valves. Each of these valves has a set of flaps that open and close to regulate the entry of blood. After circulating through the body, oxygen-poor blood flows back to the heart through the veins, enters the right atrium, and empties into the right ventricle through the tricuspid valve. Blood then flows through the pulmonary valve into the pulmonary artery, going into the lungs to get oxygen. Blood returns to the heart by passing the pulmonary veins again, enters the left atrium, and passes through the mitral valve into left ventricle. The left ventricle pumps out the oxygen rich blood through the aorta into the body’s general circulation. This is the complete cardiac cycle. The process of systole starts from the closure of the mitral valve to the closure of the aortic valve. The rest of the cardiac cycle is called the diastole, although the diastole could also include portions of the systole if basing on myocardial physiology (Brutsaert & De Keulenaer, 2006). During the diastole, the heart relaxes after contraction as it prepares to refill with circulating blood. The ventricular diastole occurs when the ventricles are relaxing, while atrial diastole means that the atria are relaxing. The pressure in the left and right ventricles decreases from the peak observed during the systole. The blood pressure measured in the arm is actually the blood pressure in the left ventricle. When the left ventricle drops to a pressure lower than that in the left atrium, the mitral valve will open and fill with the LV with blood. The same holds true for the right ventricle and right atrium. These events are regulated and coordinated by a series of electrical impulses from specialized heart cells. The cardiac muscles or myocytes initiate their own contraction independently of external nerves. Heart disease and heart failure have been correlated with deficiencies in the diastole (ability to fill) and systole (ability to eject). In 30 to 50 percent of patients with heart failure, most have a normal or almost normal ejection fraction in the left ventricle. The heart failure is attributed to left ventricular diastolic dysfunction, a condition that has received less attention compared to systolic dysfunction (Vasan & Benjamin, 2001). Heart failure with normal ejection fraction is currently known to be the most common form of heart disorder in the community (Kitzman, 2008). Diastolic dysfunction (DD) is observed in several diseases of the heart. Arterial hypertension has been identified as the main cause of DD. Changes in diastolic isovolumic relaxation are brought about by change in intracellular calcium, as observed in previous studies in insulin resistant hypertensives, and is induced by an abnormal re-uptake of calcium by sarcoplasmic reticulum (Galderis, 2005). Increased incidence of DD has made it necessary to understand the mechanisms underlying the function and pathogenesis. Cellular mechanisms The cellular mechanism for the diastole has been discussed in several reviews (Borlaug & Kass, 2006) (Kass, Bronzwaer, & Paulus, 2004). Briefly, in cardiac cells, relaxation commences when Ca2+ dissociates from the myocardial thin filaments and extrudes from the cytoplasm into the sarcoplasmic reticulum (SR) via an ATP-dependent process. Entry into the SR is through the action of SERCA (sarcoplasmic/endoplasmic calcium ATPase), and outside the cell is via the NCX or the sodium-calcium exchanger. The processing of calcium is further influenced by its diastolic release from the ryanodine receptor. Relaxation is hastened by decreases in Ca2+ through the action of protein kinase A (PKA) phosphorylation of, and increase in SERCA2a activity, which is enhanced by increased PKA phosphorylation of phospholamban (PLB). These signals are coupled to activities of β-adrenergic receptors, nitric oxide synthase and natriuretic peptide receptor agonists (Borlaug & Kass, 2006). Role and regulation of calcium in diastolic dysfunction The capacity of the heart to relax depends on its capacity to return to its resting phase. After ejection of oxygen-rich blood into the circulatory system, heart muscles must relax, in preparation for filling-up again. The myocytes can only relax when the cross-bridges of the actin-myosin filaments have detached or loosened and cytosolic calcium concentration are reduced. Anything that interferes with these two events will delay relaxation (Kass, Bronzwaer, & Paulus, 2004). In normal myocytes, calcium enters the cells through L-type ion channels. This event incites the release of more calcium from the sarcoplasmic reticulum, the calcium storage facility of the myocyte, through the ryanodine receptor–calcium-release channel (RyR or RyR2). The release of calcium signals actin–myosin cross-bridging, and contraction of the myocytes. Synchronously releasing calcium through clusters of RyR2 produces the “calcium spark,” the bigger magnitude of the spark, the larger the delay the relaxation of the cardiac muscle. RyR2 is highly regulated by the calstabin2 (also known as FKBP12.6), a channel-stabilizing protein. During the diastole, calstabin2 binds to RyR2 and maintains calcium channel closure to prevent further calcium increase in the cytosol. Phosphorylating RyR2 by cyclic AMP–dependent protein kinase A will dissociate calstabin2 from RyR2. Therefore, RyR2 is more likely to open. Mutations in RyR2 can cause the chronic adrenergic stimulation of cardiac myocytes in failing hearts (Prior, Napolitano, Tiso, & al, 2001). Sustained stimulation of calcium release, leads to the down-regulation of the expression of the β-adrenergic receptors, and hyperphosphorylation of RyR2 by protein kinase A. Consequently, calstabin2 dissociates from RyR2, and sarcoplasmic calcium leaks into the cytoplasm during diastole. This calcium is dangerous because it can generate delayed after-depolarization (DAD), by enhancing the entry of sodium ions via the Na+-H+ exchanger, which could trigger ventricular tachycardia and sudden death. In a recent study, transgenic mice hearts over expressing high levels of human Na+-H+ exchanger developed contractile dysfunction, hypertrophy, and heart failure (Nakamura, Iwata, Arai, Komamura, & Wakabayashi, 2008). High Na+ levels induced the release and accumulation of diastolic and systolic Ca2+. Loading of sarcoplasmic Ca2+ was also increased due to calmodulin-dependent protein kinase (CaMK) II phosphorylation of phospholamban, which increases the activity of SERCA. Transport of calcium ions in the myofibers is essential in the muscle contraction coordination, and therefore, to the mechanisms of congestive heart failure. Under normal conditions, cyclic AMP mediates the activation of PKA resulting in the phosphorylation of the RyR2 channel and the dissociation of calstabin2 from RyR2. Calcium is released from the SR, myocytes contract during the systole, and the actin-myosin cross-bridging is tightened. In weak hearts, excessive phosphorylation of RyR2 lessens the binding by calstabin2, which results in RyR2 channels that are partially open during diastole. Calcium leakage will prevail, depleting the sarcoplasmic reticulum calcium stores. When adrenergic stimuli are produced during physical activity, there will be less calcium released to the cytoplasm, thus reducing the contraction of the systolic myocytes (Fart & Basson, 2004). However, some studies have shown that hyperphosphorylation alone cannot be responsible for increased cellular calcium concentrations during the diastole. Defects in interactions between the N-terminal (1-600) and central (2000-2500) domains of RyR2 can cause calcium leak and reduced SR-Ca2+ load, which can increase contractile dysfunction (Oda, Yano, Yamamoto, Tokuhisa, Okuda, & Doi, 2005). These domains harbor mutation sites in catecholaminergic polymorphic ventricular tachycardia (CPVT). It was postulated that these domains interact with each other to form a zipped state, stabilizing the closed RyR2 channels. However, in failing hearts, the interactions loosen, resulting in a leaky channel. Interactions of the N and central domains with I-domain and their roles in channel regulation verified that defective interactions cause Ca2+ leakage (Tateishi, et al., 2009). The knock-in model of the CPVT RyR2 mutant (R4496C) showed bi-directional tachycardia, suggesting that the C-terminal mutation induced cellular Ca2+ increases, or DAD that resulted in deadly arrhythmia. However, since the channel gating was unaffected (the calcium leak was unabated) by the addition of closure-promoting compounds, the RyR2 defect due to the mutation at the C-terminal may not be due to N-terminal and central region interactions (Liu, et al., 2006). A recent study on untreated cardiomyocytes of RyR2 mutants showed that Ca2+ was highly and spontaneously released during the diastole (Fernández-Velasco, et al., 2009). The RyR2 mutants were highly sensitive to Ca2+, while DAD and decreased SR Ca2+ loading were observed at high pacing rates. RyR2 is therefore a key factor in the development of arrhythmia, and could be a target in developing novel drugs that can prevent heart failure. Control of relaxation by myocytes A low force generating state is necessary for bound actin-myosin fibres before heart muscle can relaxed. Hampering the preceding step, which is calcium removal from the cytosol, will delay myocardial relaxation. Delay in calcium removal includes the prolongation of the Ca2+ transient because resequestration into the sarcoplasmic reticulum (SR) was reduced and the sodium/calcium exchanger (NCX) has an abnormal extrusion. Other factors that delay calcium removal are cross-bridge uncoupling by abnormal high-energy phosphate metabolism, and there are abnormalities of the contractile proteins that affect their interactions, or Ca2+ sensitivity. Intramyocyte calcium regulation abnormalities are associated with diabetes-induced diastolic dysfunction with preserved global systolic performance. Prolonged Ca2+ transient decay, reduction calcium in intra-sarcoplasmic (SR) reticulum stores, lower Ca2+ sparks, and decreased SERCA2a protein content, were all found to consistently relate with decreased SR-Ca2+ reuptake when the cardiac myocytes are in the relaxation phase (Lacombe, et al., 2007) . However, this can be reversed by stabilizing the binding of calstabin2 to RyR2 by using small compounds (Wehrens, Lehnart, & Reiken, 2004). Changes in myocyte–calcium-handling proteins, which includes the SR–Ca2+ release channel, calstabin2, sarcoplasmic reticular Ca2+-ATPase (SERCA2a) pump, and its modulator phospholamban (PLB), and NCX, are implicated in altering the calcium flow in failing hearts, and contribute to delayed relaxation. During heart failure, the SR–Ca2+ uptake activity decreases, PKA and PLB levels are decreased (Frank, Bolck, Brixius, Kranias, & Schwinger, 2002).The proteins show decreased activity due to serine -phosphorylation or decreased phosphorylation by protein kinase A. Increased levels of phosphatase also lengthened relaxation delay (Neumann, 2002). Phosphorylation may have been reduced by increasing levels of protein kinase C that can activate protein phosphatase I (Braz, et al., 2004). How SERCA2a phosphorylation by PKA affects relaxation delay is not so clear and but recent studies on the molecular interaction suggest the presence of additional sites for the interaction between SERCA2a and PLB (Toyoshima, Asahi, Sugita, Khanna, Tsuda, & MacLennan, 2003). This interaction and their role in muscle relaxation have been shown in genetic engineering studies with PLB mutants that have improved muscle relaxation. PLB with no inhibitor and potential facilitator functions (Luo, et al., 1994) acts in a manner that is dependent on the physiologic conditions. Restoring the wild-type PLB to minus- PLB mice improves rate-dependent relaxation but not basal rates (Champion, Georgakopoulos, Haldar, Wang, Wang, & Kass, 2003). The up regulation of NCX levels in diseased myocardium work to extrude intracellular Ca2+ and offset depressed SR–Ca2+ uptake. However, the entry of calcium ions via the NCX is mediated by higher sodium, reduced-peak Ca2+, and prolonged action potential; all these factors will increase relaxation (Weisser-Thomas, Piacentino, Gaughan, Margulies, & Houser, 2003). However, it is not certainly clear how the balance of NCX–Ca2+ flux affects diastolic relaxation in failing hearts. Modification in the interaction of Ca2+ with the regulatory thin filaments, and in heart muscle proteins, also reduces relaxation. When alterations in troponin I (Tn1) to another isoform were made, PKA can no longer phosphorylate it (Wolska, et al., 2001). Overexpressing Tn1, that acts as if phosphorylated by PKA, lengthens and shortens relaxation. Molecular motors can also delay relaxation. The β- myosin heavy chain (MHC) isoform, prolongs force rise and relaxation, when compared to the α- MHC isoform. In humans, the β- MHC isoform prevails under normal conditions. Mutations in myosin also directly affect actin binding. More research is necessary to understand the structure–function relationships in myofilament and thin filament proteins that determine relaxation (Kass, Bronzwaer, & Paulus, 2004). The contraction of hearts below their unstressed volume allows them to store elastic energy that is released during the next diastolic period. This is elastic recoil, which is a negative pressure if cardiac filling is suddenly aborted. In cardiac failure, this elastic recoil is reduced, which can be due to changes in structure and biochemistry of the extracellular matrix, the heart’s incapacity to contract below equilibrium volume or structural changes in sarcomeric proteins, including titin (Fukuda, Sasaki, Ishiwata, & Kurihara, 2001). Titin is increasingly recognized as a determinant of myocyte stiffness and elastic recoil. Calcium and phosphorylation by PKA can evoke posttranslational modification in titin, which can dynamically affect the restoring force. When the myocytes are shortened below the equilibrium, forces in the titin molecule provide a speedy return to the unstressed length. Thus, quantitative and qualitative changes in titin can affect early relaxation. Stretching of the myocytes during diastolic filling can result in deformation without the elastic resistance provided by titin (Wu, Cazorla, & Labeit, 2000). Protein kinase G has also been shown to phosphorylate titin, similar to PKA (Kruger, et al., 2009). The phosphorylation of a serine residue in titin’s N2B fragment results in a very sharp decrease in cardiomyofibrillar stiffness. A deficit in basal concentration of phosphorylated titin in failing hearts compared to healthy hearts confirms the significant contribution of titin to diastolic dysfunction in heart failure. This observation invalidates the previous concept that distinct mediators are responsible for acute changes in myocardial stiffness during the diastole. Multiple mechanisms apparently affect diastolic stiffness and among these are titin-actin interactions, PKG or PKA phosphorylation and isoform shifts (Borbely, van Heerebeek, & Paulus, 2009). References Borbely, B, van Heerebeek, L, & Paulus, W 2009 ‘Transcriptional and posttranslational modifications of titin: implications for diastole’, Circulation Research vol 104, pp.12-14 Borlaug, BA & Kass, DA 2006, ‘Mechanisms of diastolic dysfunction in heart failure’, TCM , vol 16, no. 8, pp. 273-279. Braz, JC, Gregory, K, Pathak, A, Zhao, W, Sahin, B, Klevitsky, R, et al. 2004’, ‘PKC-alpha regulates cardiac contractility and propensity toward heart failure’, Nature Medicine , vol 10, pp. 248-254. Brutsaert, DL & De Keulenaer, GW 2006, ‘Diastolic heart failure: a myth’, Current Opinion in Cardiology , vol. 21, pp.240-248. Champion, HC, Georgakopoulos, D, Haldar, S, Wang, L, Wang, Y & Kass, DA 2003, ‘Robust adenoviral and adeno-associated viral gene transfer to the in vivo murine heart: application to study of phospholamban physiology’, Circulation, vol. 108, pp. 2790-2797. Fart, MA & Basson, CT 2004, ‘Sparking the failing heart’, New England Journal of Medicine, vol. 351, no. 2, pp. 185-186. Fernández-Velasco, M, Rueda, A, Rizzi, N, Benitah, J-P, Colombi, B, Napolitano, C 2009. ‘Increased Ca2+ sensitivity of the ryanodine receptor mutant RyR2R4496C underlies catecholaminergic polymorphic ventricular tachycardia’. Circulation Research , vol 104, pp. 201-209. Frank, KF, Bolck, B, Brixius, K, Kranias, EG & Schwinger, RH 2002, ‘Modulation of SERCA: implications for the failing human heart’, Basic Research in Cardiology, vol. 97, pp. 172-178. Fukuda, N, Sasaki, D, Ishiwata, S & Kurihara, S 2001, ‘Length dependence of tension generation in rat skinned cardiac muscle: role of titin in the Frank-Starling mechanism of the heart’, Circulation , vol. 104, pp. 1639-1634. Galderis, M. 2005, ‘Diastolic dysfunction and diastolic heart failure: diagnostic, prognostic and therapeutic aspects’, Cardiovascular Ultrasound, vol. 5, p. 3. Kass, DA, Bronzwaer, JG & Paulus, WJ 2004, ‘What mechanisms underlie diastolic dysfunction in heart failure?’ Circulation Research, vol. 94, pp.1533-1542. Kitzman, D. (2008). Diastolic dysfunction, one piece of the heart failure with normal ejection fraction puzzle. Circulation , vol 117, pp.2044-2046. Kruger, M, Kotter, S, Grutzner, A, Lang, P, Andresen, C, Redfield, M, et al. 2009, ‘Protein kinase G modulates human myocardial passive stiffness by phosphorylation of the titin springs’, Circulation Research, vol 104, pp. 87-94. Lacombe, VA, Viatchenko-Karpinski, S, Terentyev, D, Sridhar, A, Emani, S, Bonagura, JD, et al. 2007, ‘Mechanisms of impaired calcium handling underlying subclinical diastolic dysfunction in diabetes’, American Journal of Physiolology, Regulation and Integrative Computational Physiology , vol. 293, no. 5, pp. R1787-R1797. Liu, N, Colombi, B, Memmi, M, Zissimopoulos, S, Rizzi, N, Negri, S, et al 2006, ‘Arrhythmogenesis in catecholaminergic polymorphic ventricular tachycardia: insights from a RyR2 R4496C knock-in mouse model’, Circulation Research, vol 99, pp. 292–298. Luo, W, Grupp, IL, Harrer, J, Ponniah, S, Grupp, G, Duffy, JJ et al. 1994, ‘Targeted ablation of the phospholamban gene is associated with markedly enhanced myocardial contractility and loss of β-agonist stimulation’, Circulation Research, vol. 75, pp. 401-409. Nakamura, T, Iwata, Y, Arai, Y, Komamura, K & Wakabayashi, S 2008 ‘Activation of Na+/H+ exchanger 1 is sufficient to generate Ca2+ signals that induce cardiac hypertrophy and heart failure’, Circulation Research, vol 103, pp. 891-899. Neumann, J 2002, ‘Altered phosphatase activity in heart failure, influence on Ca2+ movement’, Basic Research in Cardiology, vol 97, pp. 191-195. Oda, T, Yano, M, Yamamoto, T, Tokuhisa, T, Okuda, S, Doi, M et al. 2005. ‘Defective regulation of inter-domain interactions within the ryanodine receptor plays a key role in the pathogenesis of heart failure’. Circulation, vol. 111, pp. 3244–3254. Prior, IS, Napolitano, C, Tiso, N et al 2001, ‘Mutations in the cardiac ryanodine receptor gene (hRyR2) underlie catecholaminergic polymorphic ventricular tachycardia’, Circulation, vol 103, pp. 196-200. Tateishi, M, Yano, M, Mochizuki, M, Suetomi, T, Ono, M, Xu, X, et al 2009, ‘Defective domain–domain interactions within the ryanodine receptor as a critical cause of diastolic Ca2+ leak in failing hearts’. Cardiovascular Research, vol. 81, pp.536-545. Toyoshima, C, Asahi, M, Sugita, Y, Khanna, R, Tsuda, T & MacLennan, DH 2003, ‘Modeling of the inhibitory interaction of phospholamban with the Ca2+ ATPase’, Proceedings of the National Academies of Science USA, vol 100, pp.467–472. Vasan, RS & Benjamin, AD 2001, ‘Diastolic heart failure- no time to relax’, New England Journal of Medicine , vol. 344, no. 1, pp. 56-58. Wehrens, XH, Lehnart, SE & Reiken, SR 2004, ‘Protection from cardiac arrhythmia through ryanodine eceptor-stabilizing protein calstabin2’ Science, vol. 304, pp. 292-296. Weisser-Thomas, J, Piacentino, VI, Gaughan, JP, Margulies, K & Houser, SR 2003, ‘Calcium entry via Na/Ca exchange during the action potential directly contributes to contraction of failing human ventricular myocytes’, Cardiovascular Research , vol.57, pp. 974-985. Wolska, BM, Vijayan, K, Arteaga, GM, Konhilas, JP, Phillips, RM, Kim, R, Naya, T & Leiden, JM 2001, ‘Expression of slow skeletal troponin I in adult transgenic mouse heart muscle reduces the force decline observed during acidic conditions’, Journal of Physiology, vol 536, pp. 863-870. Wu, Y, Cazorla, O & Labeit, D 2000, ‘Changes in titin and collagen underlie diastolic stiffness diversity of cardiac muscle’, Journal of Molecular and Cellular Cardiology, vol. 32, pp.2151-2161. Read More