Modulation of voltage-gated ion channels by sialylation

Control and modulation of electrical signaling is vital to normal physiology, particularly in neurons, cardiac myocytes, and skeletal muscle. The orchestrated activities of variable sets of ion channels and transporters, including voltage-gated ion channels (VGICs), are responsible for initiation, conduction, and termination of the action potential (AP) in excitable cells. Slight changes in VGIC activity can lead to severe pathologies including arrhythmias, epilepsies, and paralyses, while normal excitability depends on the precise tuning of the AP waveform. VGICs are heavily posttranslationally modified, with upward of 30% of the mature channel mass consisting of N- and O-glycans. These glycans are terminated typically by negatively charged sialic acid residues that modulate voltage-dependent channel gating directly. The data indicate that sialic acids alter VGIC activity in isoform-specific manners, dependent in part, on the number/location of channel sialic acids attached to the pore-forming alpha and/or auxiliary subunits that often act through saturating electrostatic mechanisms. Additionally, cell-specific regulation of sialylation can affect VGIC gating distinctly. Thus, channel sialylation is likely regulated through two mechanisms that together contribute to a dynamic spectrum of possible gating motifs: a subunit-specific mechanism and regulated (aberrant) changes in the ability of the cell to glycosylate. Recent studies showed that neuronal and cardiac excitability is modulated through regulated changes in voltage-gated Na(+) channel sialylation, suggesting that both mechanisms of differential VGIC sialylation contribute to electrical signaling in the brain and heart. Together, the data provide insight into an important and novel paradigm involved in the control and modulation of electrical signaling.

Sialic acids attached to O-glycans modulate voltage-gated potassium channel gating

Neuronal, cardiac, and skeletal muscle action potentials are produced and conducted through the highly regulated activity of several types of voltage-gated ion channels. Voltage-gated potassium (K(v)) channels are responsible for action potential repolarization. Glycans can be attached to glycoproteins through N- and O-linkages. Previous reports described the impact of N-glycans on voltage-gated ion channel function. Here, we show that sialic acids attached through O-linkages modulate gating of K(v)2.1, K(v)4.2, and K(v)4.3. The conductance-voltage (G-V) relationships for each isoform were shifted uniquely by a depolarizing 8-16 mV under conditions of reduced sialylation. The data indicate that sialic acids modulate K(v) channel activation through apparent electrostatic mechanisms that promote channel activity. Voltage-dependent steady-state inactivation was unaffected by changes in sialylation. N-Linked sialic acids cannot be responsible for the G-V shifts because K(v)4.2 and K(v)4.3 cannot be N-glycosylated, and immunoblot analysis confirmed K(v)2.1 is not N-glycosylated. Glycosidase gel shift analysis suggested that K(v)2.1, K(v)4.2, and K(v)4.3 were O-glycosylated and sialylated. To confirm this, azide-modified sugar residues involved specifically in O-glycan and sialic acid biosynthesis were shown to incorporate into all three K(v) channel isoforms using Cu(I)-catalyzed cycloaddition chemistry. Together, the data indicate that sialic acids attached to O-glycans uniquely modulate gating of three K(v) channel isoforms that are not N-glycosylated. These data provide the first evidence that external O-glycans, with core structures distinct from N-glycans in type and number of sugar residues, can modulate K(v) channel function and thereby contribute to changes in electrical signaling that result from regulated ion channel expression and/or O-glycosylation.

Expression of the sialyltransferase, ST3Gal4, impacts cardiac voltage-gated sodium channel activity, refractory period and ventricular conduction

The sequential glycosylation process typically ends with sialic acid residues added through trans-Golgi sialyltransferase activity. Individuals afflicted with congenital disorders of glycosylation often have reduced glycoprotein sialylation and present with multi-system symptoms including hypotonia, seizures, arrhythmia and cardiomyopathy. Cardiac voltage-gated Na(+) channel (Nav) activity can be influenced by sialic acids likely contributing to an external surface potential causing channels to gate at less depolarized voltages. Here, a possible pathophysiological role for reduced sialylation is investigated by questioning the impact of gene deletion of the uniformly expressed beta-galactoside alpha-2,3-sialyltransferase 4 (ST3Gal4) on cardiac Nav activity, cellular refractory period and ventricular conduction. Whole-cell patch-clamp experiments showed that ventricular Nav from ST3Gal4 deficient mice (ST3Gal4(-/-)) gated at more depolarized potentials, inactivated more slowly and recovered from fast inactivation more rapidly than WT controls. Current-clamp recordings indicated a 20% increase in time to action potential peak and a 30ms decrease in ST3Gal4(-/-) myocyte refractory period, concurrent with increased Nav recovery rate. Nav expression, distribution and maximal Na(+) current levels were unaffected by ST3Gal4 expression, indicating that reduced sialylation does not impact Nav surface expression and distribution. However, enzymatic desialylation suggested that ST3Gal4(-/-) ventricular Nav are less sialylated. Consistent with the shortened myocyte refractory period, epicardial conduction experiments using optical mapping techniques demonstrated a 27% reduction in minimum ventricular refractory period and increased susceptibility to arrhythmias in ST3Gal4(-/-) ventricles. Thus, deletion of a single sialyltransferase significantly impacts ventricular myocyte electrical signaling. These studies offer insight into diseases of glycosylation that are often associated with pathological changes in excitability and highlight the importance of glycosylation in cardiac physiology.

Intracellular O-linked glycosylation directly regulates cardiomyocyte L-type Ca2+ channel activity and excitation-contraction coupling

Cardiomyocyte L-type Ca²⁺ channels (Ca_vs) are targets of signaling pathways that modulate channel activity in response to physiologic stimuli. Ca_v regulation is typically transient and beneficial but chronic stimulation can become pathologic; therefore, gaining a more complete understanding of Ca_v regulation is of critical importance. Intracellular O-linked glycosylation (O-GlcNAcylation), which is the result of two enzymes that dynamically add and remove single N-acetylglucosamines to and from intracellular serine/threonine residues (OGT and OGA respectively), has proven to be an increasingly important post-translational modification that contributes to the regulation of many physiologic processes. However, there is currently no known role for O-GlcNAcylation in the direct regulation of Ca_v activity nor is its contribution to cardiac electrical signaling and EC coupling well understood. Here we aimed to delineate the role of O-GlcNAcylation in regulating cardiomyocyte L-type Ca_v activity and its subsequent effect on EC coupling by utilizing a mouse strain possessing an inducible cardiomyocyte-specific OGT-null-transgene. Ablation of the OGT-gene in adult cardiomyocytes (OGTKO) reduced OGT expression and O-GlcNAcylation by > 90%. Voltage clamp recordings indicated an ~ 40% reduction in OGTKO Ca_v current (I_Ca), but with increased efficacy of adrenergic stimulation, and Ca_v steady-state gating and window current were significantly depolarized. Consistently, OGTKO cardiomyocyte intracellular Ca²⁺ release and contractility were diminished and demonstrated greater beat-to-beat variability. Additionally, we show that the Ca_v α1 and β2 subunits are O-GlcNAcylated while α2δ1 is not. Echocardiographic analyses indicated that the reductions in OGTKO cardiomyocyte Ca²⁺ handling and contractility were conserved at the whole-heart level as evidenced by significantly reduced left-ventricular contractility in the absence of hypertrophy. The data indicate, for the first time, that O-GlcNAc signaling is a critical and direct regulator of cardiomyocyte I_Ca achieved through altered Ca_v expression, gating, and response to adrenergic stimulation; these mechanisms have significant implications for understanding how EC coupling is regulated in health and disease.

Aberrant sialylation causes dilated cardiomyopathy and stress-induced heart failure

Dilated cardiomyopathy (DCM), the third most common cause of heart failure, is often associated with arrhythmias and sudden cardiac death if not controlled. The majority of DCM is of unknown etiology. Protein sialylation is altered in human DCM, with responsible mechanisms not yet described. Here we sought to investigate the impact of clinically relevant changes in sialylation on cardiac function using a novel model for altered glycoprotein sialylation that leads to DCM and to chronic stress-induced heart failure (HF), deletion of the sialyltransferase, ST3Gal4. We previously reported that 12- to 20-week-old ST3Gal4 (-/-) mice showed aberrant cardiac voltage-gated ion channel sialylation and gating that contribute to a pro-arrhythmogenic phenotype. Here, echocardiography supported by histology revealed modest dilated and thinner-walled left ventricles without increased fibrosis in ST3Gal4 (-/-) mice starting at 1 year of age. Cardiac calcineurin expression in younger (16-20 weeks old) ST3Gal4 (-/-) hearts was significantly reduced compared to WT. Transverse aortic constriction (TAC) was used as a chronic stressor on the younger mice to determine whether the ability to compensate against a pathologic insult is compromised in the ST3Gal4 (-/-) heart, as suggested by previous reports describing the functional implications of reduced cardiac calcineurin levels. TAC'd ST3Gal4 (-/-) mice presented with significantly reduced systolic function and ventricular dilation that deteriorated into congestive HF within 6 weeks post-surgery, while constricted WT hearts remained well-adapted throughout (ejection fraction, ST3Gal4 (-/-) = 34 ± 5.2 %; WT = 53.8 ± 7.4 %; p < 0.05). Thus, a novel, sialo-dependent model for DCM/HF is described in which clinically relevant reduced sialylation results in increased arrhythmogenicity and reduced cardiac calcineurin levels that precede cardiomyopathy and TAC-induced HF, suggesting a causal link among aberrant sialylation, chronic arrhythmia, reduced calcineurin levels, DCM in the absence of a pathologic stimulus, and stress-induced HF.

Reduced sialylation impacts ventricular repolarization by modulating specific K+ channel isoforms distinctly

Voltage-gated K(+) channels (Kv) are responsible for repolarizing excitable cells and can be heavily glycosylated. Cardiac Kv activity is indispensable where even minimal reductions in function can extend action potential duration, prolong QT intervals, and ultimately contribute to life-threatening arrhythmias. Diseases such as congenital disorders of glycosylation often cause significant cardiac phenotypes that can include arrhythmias. Here we investigated the impact of reduced sialylation on ventricular repolarization through gene deletion of the sialyltransferase ST3Gal4. ST3Gal4-deficient mice (ST3Gal4(-/-)) had prolonged QT intervals with a concomitant increase in ventricular action potential duration. Ventricular apex myocytes isolated from ST3Gal4(-/-) mice demonstrated depolarizing shifts in activation gating of the transient outward (Ito) and delayed rectifier (IKslow) components of K(+) current with no change in maximum current densities. Consistently, similar protein expression levels of the three Kv isoforms responsible for Ito and IKslow were measured for ST3Gal4(-/-) versus controls. However, novel non-enzymatic sialic acid labeling indicated a reduction in sialylation of ST3Gal4(-/-) ventricular Kv4.2 and Kv1.5, which contribute to Ito and IKslow, respectively. Thus, we describe here a novel form of regulating cardiac function through the activities of a specific glycogene product. Namely, reduced ST3Gal4 activity leads to a loss of isoform-specific Kv sialylation and function, thereby limiting Kv activity during the action potential and decreasing repolarization rate, which likely contributes to prolonged ventricular repolarization. These studies elucidate a novel role for individual glycogene products in contributing to a complex network of cardiac regulation under normal and pathologic conditions.

Statistical Metamodeling and Sequential Design of Computer Experiments to Model Glyco-Altered Gating of Sodium Channels in Cardiac Myocytes

Glycan structures account for up to 35% of the mass of cardiac sodium ( Nav ) channels. To question whether and how reduced sialylation affects Nav activity and cardiac electrical signaling, we conducted a series of in vitro experiments on ventricular apex myocytes under two different glycosylation conditions, reduced protein sialylation (ST3Gal4(-/-)) and full glycosylation (control). Although aberrant electrical signaling is observed in reduced sialylation, realizing a better understanding of mechanistic details of pathological variations in INa and AP is difficult without performing in silico studies. However, computer model of Nav channels and cardiac myocytes involves greater levels of complexity, e.g., high-dimensional parameter space, nonlinear and nonconvex equations. Traditional linear and nonlinear optimization methods have encountered many difficulties for model calibration. This paper presents a new statistical metamodeling approach for efficient computer experiments and optimization of Nav models. First, we utilize a fractional factorial design to identify control variables from the large set of model parameters, thereby reducing the dimensionality of parametric space. Further, we develop the Gaussian process model as a surrogate of expensive and time-consuming computer models and then identify the next best design point that yields the maximal probability of improvement. This process iterates until convergence, and the performance is evaluated and validated with real-world experimental data. Experimental results show the proposed algorithm achieves superior performance in modeling the kinetics of Nav channels under a variety of glycosylation conditions. As a result, in silico models provide a better understanding of glyco-altered mechanistic details in state transitions and distributions of Nav channels. Notably, ST3Gal4(-/-) myocytes are shown to have higher probabilities accumulated in intermediate inactivation during the repolarization and yield a shorter refractory period than WTs. The proposed statistical design of computer experiments is generally extensible to many other disciplines that involve large-scale and computationally expensive models.

Reduced myocyte complex N-glycosylation causes dilated cardiomyopathy

Protein glycosylation is an essential posttranslational modification that affects a myriad of physiologic processes. Humans with genetic defects in glycosylation, which result in truncated glycans, often present with significant cardiac deficits. Acquired heart diseases and their associated risk factors were also linked to aberrant glycosylation, highlighting its importance in human cardiac disease. In both cases, the link between causation and corollary remains enigmatic. The glycosyltransferase gene, mannosyl (α-1,3-)-glycoprotein β-1,2- N-acetylglucosaminyltransferase (Mgat1), whose product, N-acetylglucosaminyltransferase 1 (GlcNAcT1) is necessary for the formation of hybrid and complex N-glycan structures in the medial Golgi, was shown to be at reduced levels in human end-stage cardiomyopathy, thus making Mgat1 an attractive target for investigating the role of hybrid/complex N-glycosylation in cardiac pathogenesis. Here, we created a cardiomyocyte-specific Mgat1 knockout (KO) mouse to establish a model useful in exploring the relationship between hybrid/complex N-glycosylation and cardiac function and disease. Biochemical and glycomic analyses showed that Mgat1KO cardiomyocytes produce predominately truncated N-glycan structures. All Mgat1KO mice died significantly younger than control mice and demonstrated chamber dilation and systolic dysfunction resembling human dilated cardiomyopathy (DCM). Data also indicate that a cardiomyocyte L-type voltage-gated Ca²⁺ channel (Ca_v) subunit (α2δ1) is a GlcNAcT1 target, and Mgat1KO Ca_v activity is shifted to more-depolarized membrane potentials. Consistently, Mgat1KO cardiomyocyte Ca²⁺ handling is altered and contraction is dyssynchronous compared with controls. The data demonstrate that reduced hybrid/complex N-glycosylation contributes to aberrant cardiac function at whole-heart and myocyte levels drawing a direct link between altered glycosylation and heart disease. Thus, the Mgat1KO provides a model for investigating the relationship between systemic reductions in glycosylation and cardiac disease, showing that clinically relevant changes in cardiomyocyte hybrid/complex N-glycosylation are sufficient to cause DCM and early death.-Ednie, A. R., Deng, W., Yip, K.-P., Bennett, E. S. Reduced myocyte complex N-glycosylation causes dilated cardiomyopathy.

In-Silico Modeling of the Functional Role of Reduced Sialylation in Sodium and Potassium Channel Gating of Mouse Ventricular Myocytes

Cardiac ion channels are highly glycosylated membrane proteins with up to 30% of the protein's mass containing glycans. Heart diseases often accompany individuals with congenital disorders of glycosylation (CDG). However, cardiac dysfunction among CDG patients is not yet fully understood. There is an urgent need to study how aberrant glycosylation impacts cardiac electrical signaling. Our previous works reported that congenitally reduced sialylation achieved through deletion of the sialyltransferase gene, ST3Gal4, leads to altered gating of voltage-gated Na⁺ and K⁺ channels ( and , respectively). However, linking the impact of reduced sialylation on ion channel gating to the action potential (AP) is difficult without performing computer experiments. Also, decomposing the sum of K⁺ currents is difficult because of complex structures and components of channels (e.g., , and ). In this study, we developed in-silico models to describe the functional role of reduced sialylation in both and gating and the AP using in vitro experimental data. Modeling results showed that reduced sialylation changes gating as follows: 1) The steady-state activation voltages of isoforms are shifted to a more depolarized potential. 2) Aberrant K⁺ currents ( and ) contribute to a prolonged AP duration, and altered Na⁺ current ( ) contributes to a shortened AP refractory period. This study contributes to a better understanding of the functional role of reduced sialylation in cardiac dysfunction that shows strong potential to provide new pharmaceutical targets for the treatment of CDG-related heart diseases.

Reduced O-GlcNAcylation diminishes cardiomyocyte Ca2+ dependent facilitation and frequency dependent acceleration of relaxation

Ca2+ dependent facilitation (CDF) and frequency dependent acceleration of relaxation (FDAR) are regulatory mechanisms that potentiate cardiomyocyte Ca2+ channel function and increase the rate of Ca2+ sequestration following a Ca2+-release event, respectively, when depolarization frequency increases. CDF and FDAR likely evolved to maintain EC coupling at increased heart rates. Ca2+/calmodulin-dependent kinase II (CaMKII) was shown to be indispensable to both; however, the mechanisms remain to be completely elucidated. CaMKII activity can be modulated by post-translational modifications but if and how these modifications impact CDF and FDAR is unknown. Intracellular O-linked glycosylation (O-GlcNAcylation) is a post-translational modification that acts as a signaling molecule and metabolic sensor. In hyperglycemic conditions, CaMKII was shown to be O-GlcNAcylated resulting in pathologic activity. Here we sought to investigate whether O-GlcNAcylation impacts CDF and FDAR through modulation of CaMKII activity in a pseudo-physiologic setting. Using voltage-clamp and Ca2+ photometry we show that cardiomyocyte CDF and FDAR are significantly diminished in conditions of reduced O-GlcNAcylation. Immunoblot showed that CaMKIIδ and calmodulin expression are increased but the autophosphorylation of CaMKIIδ and the muscle cell-specific CaMKIIβ isoform are reduced by 75% or more when O-GlcNAcylation is inhibited. We also show that the enzyme responsible for O-GlcNAcylation (OGT) can likely be localized in the dyad space and/or at the cardiac sarcoplasmic reticulum and is precipitated by calmodulin in a Ca2+ dependent manner. These findings will have important implications for our understanding of how CaMKII and OGT interact to impact cardiomyocyte EC coupling in normal physiologic settings as well as in disease states where CaMKII and OGT may be aberrantly regulated.

Simulation Modeling of Reduced Glycosylation Effects on Potassium Channels of Mouse Cardiomyocytes

Dilated cardiomyopathy (DCM) is the third most common cause of heart failure and the primary reason for heart transplantation; upward of 70% of DCM cases are considered idiopathic. Our in-vitro experiments showed that reduced hybrid/complex N-glycosylation in mouse cardiomyocytes is linked with DCM. Further, we observed direct effects of reduced N-glycosylation on Kv gating. However, it is difficult to rigorously determine the effects of glycosylation on Kv activity, because there are multiple Kv isoforms in cardiomyocytes contributing to the cardiac excitation. Due to complex functions of Kv isoforms, only the sum of K+ currents (IKsum) can be recorded experimentally and decomposed later using exponential fitting to estimate component currents, such as IKto, IKslow, and IKss. However, such estimation cannot adequately describe glycosylation effects and Kv mechanisms. Here, we propose a framework of simulation modeling of Kv kinetics in mouse ventricular myocytes and model calibration using the in-vitro data under normal and reduced glycosylation conditions through ablation of the Mgat1 gene (i.e., Mgat1KO). Calibrated models facilitate the prediction of Kv characteristics at different voltages that are not directly observed in the in-vitro experiments. A model calibration procedure is developed based on the genetic algorithm. Experimental results show that, in the Mgat1KO group, both IKto and IKslow densities are shown to be significantly reduced and the rate of IKslow inactivation is much slower. The proposed approach has strong potential to couple simulation models with experimental data for gaining a better understanding of glycosylation effects on Kv kinetics.

Cardiomyocyte Reduction of Hybrid/Complex N-Glycosylation in the Adult Causes Heart Failure With Reduced Ejection Fraction in the Absence of Cellular Remodeling

BACKGROUND

Heart failure (HF) presents a massive burden to health care with a complex pathophysiology that results in HF with reduced left ventricle ejection fraction (EF) or HF with preserved EF. It has been shown that relatively modest changes in protein glycosylation, an essential posttranslational modification, are associated with clinical presentations of HF. We and others previously showed that such aberrant protein glycosylation in animal models can lead to HF.

METHODS AND RESULTS

We develop and characterize a novel, tamoxifen-inducible, cardiomyocyte Mgat1 knockout mouse strain, achieved through deletion of Mgat1, alpha-1,3-mannosyl-glycoproten 2-beta-N-acetlyglucosaminyltransferase, which encodes N-acetylglucosaminyltransferase I. We investigate the role of hybrid/complex N-glycosylation in adult HFrEF pathogenesis at the ion channel, cardiomyocyte, tissue, and gross cardiac level. The data demonstrate successful reduction of N-acetylglucosaminyltransferase I activity and confirm that hybrid/complex N-glycans modulate gating of cardiomyocyte voltage-gated calcium channels. A longitudinal study shows that the tamoxifen-inducible, cardiomyocyte Mgat1 knockout mice present with significantly reduced systolic function by 28 days post induction that progresses into HFrEF by 8 weeks post induction, without significant ventricular dilation or hypertrophy. Further, there was minimal, if any, physiologic or pathophysiologic cardiomyocyte electromechanical remodeling or fibrosis observed before (10-21 days post induction) or after (90-130 days post induction) HFrEF development.

CONCLUSIONS

The tamoxifen-inducible, cardiomyocyte Mgat1 knockout mouse strain created and characterized here provides a model to describe novel mechanisms and causes responsible for HFrEF onset in the adult, likely occurring primarily through tissue-level reductions in electromechanical activity in the absence of (or at least before) cardiomyocyte remodeling and fibrosis.