Down regulation of the L-type Ca2+ channel, GRK2, and phosphorylated phospholamban: protective mechanisms for the denervated failing heart

Focal reduction in left ventricular 123I-metaiodobenzylguanidine uptake and impairment in systolic function in patients with Anderson-Fabry disease

BACKGROUND

Abnormalities of cardiac sympathetic innervation have been demonstrated in Anderson-Fabry disease (AFD). We aimed to investigate the relationship between regional left ventricular (LV) denervation and regional function abnormalities.

METHODS

Twenty-four AFD patients (43.7 ± 12.8 years) were studied by 123I-metaiodobenzylguanidine (MIBG) cardiac imaging and speckle-tracking echocardiography. Segmental tracer uptake was estimated according to 0 to 4 score, and total defect score (TDS) was calculated for each patient.

RESULTS

Segmental longitudinal strain worsened as MIBG uptake score increased (P < 0.001). By ROC analysis, a segmental longitudinal strain > - 16.2% predicted a segmental MIBG uptake score ≥1, with 79.7% sensitivity and 65.3% specificity. Segmental MIBG uptake defects were found in 13 out 24 AFD patients. LV mass index (60.8 ± 10.1 vs. 41.4 ± 9.8 g/h2.7), relative wall thickness (0.51 ± 0.06 vs. 0.40 ± 0.06), systolic pulmonary artery pressure (35.2 ± 6.7 vs. 27.2 ± 4.2 mmHg), and longitudinal strain (- 14.3 ± 2.7 vs. -19.4 ± 1.8%) were significantly higher in patients with segmental defect (all P < 0.01). At multivariate linear regression analysis, global longitudinal strain was independently associated with TDS (B = 3.007, 95% confidence interval 1.384 to 4.630, P = 0.001).

CONCLUSIONS

Reduced cardiac MIBG uptake reflects the severity of cardiac involvement in AFD patients. LV longitudinal function impairment seems to be an earlier disease feature than regional myocardial denervation.

Adrenergic supersensitivity and impaired neural control of cardiac electrophysiology following regional cardiac sympathetic nerve loss

Myocardial infarction (MI) can result in sympathetic nerve loss in the infarct region. However, the contribution of hypo-innervation to electrophysiological remodeling, independent from MI-induced ischemia and fibrosis, has not been comprehensively investigated. We present a novel mouse model of regional cardiac sympathetic hypo-innervation utilizing a targeted-toxin (dopamine beta-hydroxylase antibody conjugated to saporin, DBH-Sap), and measure resulting electrophysiological and Ca²⁺ handling dynamics. Five days post-surgery, sympathetic nerve density was reduced in the anterior left ventricular epicardium of DBH-Sap hearts compared to control. In Langendorff-perfused hearts, there were no differences in mean action potential duration (APD₈₀) between groups; however, isoproterenol (ISO) significantly shortened APD₈₀ in DBH-Sap but not control hearts, resulting in a significant increase in APD₈₀ dispersion in the DBH-Sap group. ISO also produced spontaneous diastolic Ca²⁺ elevation in DBH-Sap but not control hearts. In innervated hearts, sympathetic nerve stimulation (SNS) increased heart rate to a lesser degree in DBH-Sap hearts compared to control. Additionally, SNS produced APD₈₀ prolongation in the apex of control but not DBH-Sap hearts. These results suggest that hypo-innervated hearts have regional super-sensitivity to circulating adrenergic stimulation (ISO), while having blunted responses to SNS, providing important insight into the mechanisms of arrhythmogenesis following sympathetic nerve loss.

Heart-Brain Axis: Effects of Neurologic Injury on Cardiovascular Function

A complex interaction exists between the nervous and cardiovascular systems. A large network of cortical and subcortical brain regions control cardiovascular function via the sympathetic and parasympathetic outflow. A dysfunction in one system may lead to changes in the function of the other. The effects of cardiovascular disease on the nervous system have been widely studied; however, our understanding of the effects of neurological disorders on the cardiovascular system has only expanded in the past 2 decades. Various pathologies of the nervous system can lead to a wide range of alterations in function and structure of the cardiovascular system ranging from transient and benign electrographic changes to myocardial injury, cardiomyopathy, and even cardiac death. In this article, we first review the anatomy and physiology of the central and autonomic nervous systems in regard to control of the cardiovascular function. The effects of neurological injury on cardiac function and structure will be summarized, and finally, we review neurological disorders commonly associated with cardiovascular manifestations.

Sympathetic modulation of electrical activation in normal and infarcted myocardium: implications for arrhythmogenesis

The influence of cardiac sympathetic innervation on electrical activation in normal and chronically infarcted ventricular myocardium is not understood. Yorkshire pigs with normal hearts (NL, n = 12) or anterior myocardial infarction (MI, n = 9) underwent high-resolution mapping of the anteroapical left ventricle at baseline and during left and right stellate ganglion stimulation (LSGS and RSGS, respectively). Conduction velocity (CV), activation times (ATs), and directionality of propagation were measured. Myocardial fiber orientation was determined using diffusion tensor imaging and histology. Longitudinal CV (CV_L) was increased by RSGS (0.98 ± 0.11 vs. 1.2 ± 0.14m/s, P < 0.001) but not transverse CV (CV_T). This increase was abrogated by β-adrenergic receptor and gap junction (GJ) blockade. Neither CV_L nor CV_T was increased by LSGS. In the peri-infarct region, both RSGS and LSGS shortened ARIs in sinus rhythm (423 ± 37 vs. 322 ± 30 ms, P < 0.001, and 423 ± 36 vs. 398 ± 36 ms, P = 0.035, respectively) and altered activation patterns in all animals. CV, as estimated by mean ATs, increased in a directionally dependent manner by RSGS (14.6 ± 1.2 vs. 17.3 ± 1.6 ms, P = 0.015), associated with GJ lateralization. RSGS and LSGS inhomogeneously modulated AT and induced relative or absolute functional activation delay in parts of the mapped regions in 75 and 67%, respectively, in MI animals, and in 0 and 15%, respectively, in control animals (P < 0.001 for both). In conclusion, sympathoexcitation increases CV in normal myocardium and modulates activation propagation in peri-infarcted ventricular myocardium. These data demonstrate functional control of arrhythmogenic peri-infarct substrates by sympathetic nerves and in part explain the temporal nature of arrhythmogenesis.NEW & NOTEWORTHY This study demonstrates regional control of conduction velocity in normal hearts by sympathetic nerves. In infarcted hearts, however, not only is modulation of propagation heterogeneous, some regions showed paradoxical conduction slowing. Sympathoexcitation altered propagation in all infarcted hearts studied, and we describe the temporal arrhythmogenic potential of these findings.Listen to this article's corresponding podcast at http://ajpheart.podbean.com/e/sympathetic-nerves-and-cardiac-propagation/.

Molecular and cellular neurocardiology: development, and cellular and molecular adaptations to heart disease

The nervous system and cardiovascular system develop in concert and are functionally interconnected in both health and disease. This white paper focuses on the cellular and molecular mechanisms that underlie neural-cardiac interactions during development, during normal physiological function in the mature system, and during pathological remodelling in cardiovascular disease. The content on each subject was contributed by experts, and we hope that this will provide a useful resource for newcomers to neurocardiology as well as aficionados.

The nervous heart

Many cardiac electrophysiological abnormalities are accompanied by autonomic nervous system dysfunction. Here, we review mechanisms by which the cardiac nervous system controls normal and abnormal excitability and may contribute to atrial and ventricular tachyarrhythmias. Moreover, we explore the potential antiarrhythmic and/or arrhythmogenic effects of modulating the autonomic nervous system by several strategies, including ganglionated plexi ablation, vagal and spinal cord stimulations, and renal sympathetic denervation as therapies for atrial and ventricular arrhythmias.

Mesenteric lymph from rats with trauma-hemorrhagic shock causes abnormal cardiac myocyte function and induces myocardial contractile dysfunction

Myocardial contractile dysfunction develops following trauma-hemorrhagic shock (T/HS). We have previously shown that, in a rat fixed pressure model of T/HS (mean arterial pressure of 30-35 mmHg for 90 min), mesenteric lymph duct ligation before T/HS prevented T/HS-induced myocardial contractile depression. To determine whether T/HS lymph directly alters myocardial contractility, we examined the functional effects of physiologically relevant concentrations of mesenteric lymph collected from rats undergoing trauma-sham shock (T/SS) or T/HS on both isolated cardiac myocytes and Langendorff-perfused whole hearts. Acute application of T/HS lymph (0.1-2%), but not T/SS lymph, induced dual inotropic effects on myocytes with an immediate increase in the amplitude of cell shortening (1.4 ± 0.1-fold) followed by a complete block of contraction. Similarly, T/HS lymph caused dual, positive and negative effects on cellular Ca²⁺ transients. These effects were associated with changes in the electrophysiological properties of cardiac myocytes; T/HS lymph initially prolonged the action potential duration (action potential duration at 90% repolarization, 3.3 ± 0.4-fold), and this was followed by a decrease in the plateau potential and membrane depolarization. Furthermore, intravenous infusion of T/HS lymph, but not T/SS lymph, caused myocardial contractile dysfunction at 24 h after injection, which mimicked actual T/HS-induced changes; left ventricular developed pressure (LVDP) and the maximal rate of LVDP rise and fall (±dP/dt(max)) were decreased and inotropic response to Ca²⁺ was blunted. However, the contractile responsiveness to β-adrenergic receptor stimulation in the T/HS lymph-infused hearts remained unchanged. These results suggest that T/HS lymph directly causes negative inotropic effects on the myocardium and that T/HS lymph-induced changes in myocyte function are likely to contribute to the development of T/HS-induced myocardial dysfunction.

Impaired β-adrenergic responsiveness accentuates dysfunctional excitation-contraction coupling in an ovine model of tachypacing-induced heart failure

Reduced inotropic responsiveness is characteristic of heart failure (HF). This study determined the cellular Ca2+ homeostatic and molecular mechanisms causing the blunted β-adrenergic (β-AR) response in HF.We induced HF by tachypacing in sheep; intracellular Ca2+ concentration was measured in voltage-clamped ventricular myocytes. In HF, Ca2+ transient amplitude and peak L-type Ca2+ current (ICa-L) were reduced (to 70 ± 11% and 50 ± 3.7% of control, respectively, P <0.05) whereas sarcoplasmic reticulum (SR) Ca2+ content was unchanged. β-AR stimulation with isoprenaline (ISO) increased Ca2+ transient amplitude, ICa-L and SRCa2+ content in both cell types; however, the response of HF cells was markedly diminished (P <0.05).Western blotting revealed an increase in protein phosphatase levels (PP1, 158 ± 17% and PP2A, 188 ± 34% of control, P <0.05) and reduced phosphorylation of phospholamban in HF (Ser16, 30 ± 10% and Thr17, 41 ± 15% of control, P <0.05). The β-AR receptor kinase GRK-2 was also increased in HF (173 ± 38% of control, P <0.05). In HF, activation of adenylyl cyclase with forskolin rescued the Ca2+ transient, SR Ca2+ content and SR Ca2+ uptake rate to the same levels as control cells in ISO. In conclusion, the reduced responsiveness of the myocardium to β-AR agonists in HF probably arises as a consequence of impaired phosphorylation of key intracellular proteins responsible for regulating the SR Ca2+ content and therefore failure of the systolic Ca2+ transient to increase appropriately during β-AR stimulation.

Autoregulation of cardiac l-type calcium channels

l-Type calcium channels (LTCCs) are major contributors to electrical and contractile function of the heart. They regulate action potential duration, enable calcium entry into cardiac myocytes for contraction, and regulate growth-related signaling in the heart. In cardiac development and in mature heart disease, LTCCs are regulated at levels of acute function and transcription. In addition, LTCCs are clinically relevant therapeutic targets for antihypertensive medications. In this review, we discuss LTCC homeostasis whereby cardiac myocytes maintain LTCC expression via a novel transcriptionally regulated pathway that includes a segment of the LTCC that moves between surface membrane and nucleus.

L-type calcium channel C terminus autoregulates transcription

Calcium homeostasis is critical for cardiac myocyte function and must be tightly regulated. The guiding hypothesis of this study is that a carboxyl-terminal cleavage product of the cardiac L-type calcium channel (Ca(V)1.2) autoregulates expression. First, we confirmed that the Ca(V)1.2 C terminus (CCt) is cleaved in murine cardiac myocytes from mature and developing ventricle. Overexpression of full-length CCt caused a 34+/-8% decrease of Ca(V)1.2 promoter activity, and truncated CCt caused an 80+/-3% decrease of Ca(V)1.2 promoter (n=12). The full-length CCt distributes into cytosol and nucleus. A deletion mutant of CCt has a greater relative affinity for the nucleus than full-length CCt, and this is consistent with increased repression of Ca(V)1.2 promoter activity by truncated CCt. Chromatin immunoprecipitation analysis revealed that CCt interacts with the Ca(V)1.2 promoter in adult ventricular cardiac myocytes at promoter modules containing Nkx2.5/Mef2, C/EBp, and a cis regulatory module. The next hypothesis tested was that CCt contributes to transcriptional signaling associated with cellular hypertrophy. We explored whether fetal cardiac myocyte Ca(V)1.2 was regulated by serum in vitro. We tested atrial natriuretic factor promoter activity as a positive control and measured the serum response of Ca(V)1.2 promoter, protein, and L-type current (I(Ca,L)) from fetal mouse ventricular myocytes. Serum increased atrial natriuretic factor promoter activity and cell size as expected. Serum withdrawal increased Ca(V)1.2 promoter activity, mRNA, and I(Ca,L). Moreover, serum withdrawal decreased the relative nuclear localization of CCt. A combination of promoter deletion mutant analyses, and the response of promoter mutants to serum withdrawal support the conclusion that CCt, a proteolytic fragment of Ca(V)1.2, autoregulates Ca(V)1.2 expression in cardiac myocytes. These data support the novel mechanism that a mobile segment of Ca(V)1.2 links Ca handling to nuclear signaling.

Development of contractile dysfunction in rat heart failure: hierarchy of cellular events

The cellular mechanisms underlying the development of congestive heart failure (HF) are not well understood. Accordingly, we studied myocardial function in isolated right ventricular trabeculae from rats in which HF was induced by left ventricular myocardial infarction (MI). Both early-stage (12 wk post-MI; E-pMI) and late, end-stage HF (28 wk post-Mi; L-pMI) were studied. HF was associated with decreased sarcoplasmic reticulum Ca2+ ATPase protein levels (28% E-pMI; 52% L-pMI). HF affected neither sodium/calcium exchange, ryanodine receptor, nor phospholamban protein levels. Twitch force at saturating extracellular [Ca2+] was depressed in HF (30% E-pMI; 38% L-pMI), concomitant with a marked increase in sensitivity of twitch force toward extracellular [Ca2+] (26% E-pMI; 68% L-pMI). Ca2+-saturated myofilament force development in skinned trabeculae was unchanged in E-pMI but significantly depressed in L-pMI (45%). Tension-dependent ATP hydrolysis rate was depressed in L-pMI (49%), but not in E-pMI. Our results suggest a hierarchy of cellular events during the development of HF, starting with altered calcium homeostasis during the early phase followed by myofilament dysfunction at end-stage HF.

Modulation of Bone Morphogenetic Protein (BMP) 2 gene expression by Sp1 transcription factors

Changes in Bone Morphogenetic Protein (BMP) 2 gene expression and activity have been linked to many pathological conditions including cancer, osteoarthritis, and birth defects. BMP2 gene polymorphisms have been linked to osteoporosis and osteoarthritis. Sp1 and related proteins are widely expressed regulators of gene expression whose transcription activating abilities vary in different cells and on different genes. We present data indicating that the ratio of Sp1 and Sp3 isoforms varies in cells that express or do not express BMP2. Furthermore, the orientation of Sp1 sites conserved between four orders of mammals influences BMP2 expression. Together our data indicate that the stoichiometry and orientation of Sp1 and Sp3 complexes on the BMP2 promoter influence BMP2 expression.