Cytoskeletal structure and recovery in single human cardiac myocytes

Cardiac mechanics and reverse remodelling under mechanical support from left ventricular assist devices

In recent years, development of mechanical circulatory support devices has proved to be a new treatment modality, in addition to standard pharmacological therapy, for patients with heart failure or acutely depressed cardiac function. These include left ventricular assist devices, which mechanically unload the heart when implanted. As a result, they profoundly affect the acute cardiac mechanics, which in turn, carry long-term consequences on myocardial function and structural function. Multiple studies have shown that, when implanted, mechanical circulatory assist devices lead to reverse remodelling, a process whereby the diseased myocardium reverts to a healthier-like state. Here, we start by first providing the reader with an overview of cardiac mechanics and important hemodynamic parameters. We then introduce left ventricular assist devices and describe their mode of operation as well as their impact on the hemodynamics. Changes in cardiac mechanics caused by device implantation are then extrapolated in time, and the long-term consequences on myocardial phenotype, as well as the physiological basis for these, is investigated.

The physics of heart rhythm disorders

The global burden caused by cardiovascular disease is substantial, with heart disease representing the most common cause of death around the world. There remains a need to develop better mechanistic models of cardiac function in order to combat this health concern. Heart rhythm disorders, or arrhythmias, are one particular type of disease which has been amenable to quantitative investigation. Here we review the application of quantitative methodologies to explore dynamical questions pertaining to arrhythmias. We begin by describing single-cell models of cardiac myocytes, from which two and three dimensional models can be constructed. Special focus is placed on results relating to pattern formation across these spatially-distributed systems, especially the formation of spiral waves of activation. Next, we discuss mechanisms which can lead to the initiation of arrhythmias, focusing on the dynamical state of spatially discordant alternans, and outline proposed mechanisms perpetuating arrhythmias such as fibrillation. We then review experimental and clinical results related to the spatio-temporal mapping of heart rhythm disorders. Finally, we describe treatment options for heart rhythm disorders and demonstrate how statistical physics tools can provide insights into the dynamics of heart rhythm disorders.

Evidence for synergy between sarcomeres and fibroblasts in an in vitro model of myocardial reverse remodeling

We have created a novel in-vitro platform to study reverse remodeling of engineered heart tissue (EHT) after mechanical unloading. EHTs were created by seeding decellularized porcine myocardial sections with a mixture of primary neonatal rat ventricular myocytes and cardiac fibroblasts. Each end of the ribbon-like constructs was fixed to a plastic clip, allowing the tissues to be statically stretched or slackened. Inelastic deformation was introduced by stretching tissues by 20% of their original length. EHTs were subsequently unloaded by returning tissues to their original, shorter length. Mechanical characterization of EHTs immediately after unloading and at subsequent time points confirmed the presence of a reverse-remodeling process, through which stress-free tissue length was increased after chronic stretch but gradually decreased back to its original value within 9 days. When a cardiac myosin inhibitor was applied to tissues after unloading, EHTs failed to completely recover their passive and active mechanical properties, suggesting a role for actomyosin contraction in reverse remodeling. Selectively inhibiting cardiomyocyte contraction or fibroblast activity after mechanical unloading showed that contractile activity of both cell types was required to achieve full remodeling. Similar tests with EHTs formed from human induced pluripotent stem cell-derived cardiomyocytes also showed reverse remodeling that was enhanced when treated with omecamtiv mecarbil, a myosin activator. These experiments suggest essential roles for active sarcomeric contraction and fibroblast activity in reverse remodeling of myocardium after mechanical unloading. Our findings provide a mechanistic rationale for designing potential therapies to encourage reverse remodeling in patient hearts.

Building a bridge to recovery: the pathophysiology of LVAD-induced reverse modeling in heart failure

Heart failure mainly caused by ischemic or dilated cardiomyopathy is a life-threatening disorder worldwide. The previous work in cardiac surgery has led to many excellent surgical techniques for treating cardiac diseases, and these procedures are now able to prolong the human lifespan. However, surgical treatment for end-stage heart failure has been under-explored, although left ventricular assist device (LVAD) implantation and heart transplantation are options to treat the condition. LVAD can provide powerful circulatory support for end-stage heart failure patients and improve the survival and quality of life after implantation compared with the existing medical counterparts. Moreover, LVADs play a crucial role in the "bridge to transplantation", "bridge to recovery" and recently have served as "destination therapy". The structural and molecular changes that improve the cardiac function after LVAD implantation are called "reverse remodeling", which means that patients who have received a LVAD can be weaned from the LVAD with restoration of their cardiac function. This strategy is a desirable alternative to heart transplantation in terms of both the patient quality of life and due to the organ shortage. The mechanism of this bridge to recovery is interesting, and is different from other treatments for heart failure. Bridge to recovery therapy is one of the options in regenerative therapy which only a surgeon can provide. In this review, we pathophysiologically analyze the reverse remodeling phenomenon induced by LVAD and comment about the clinical evidence with regard to its impact on the bridge to recovery.

Hold or fold--proteins in advanced heart failure and myocardial recovery

Advanced heart failure (AHF) describes the subset of heart failure patients refractory to conventional medical therapy. For some AHF patients, the use of mechanical circulatory support (MCS) provides an intermediary "bridge" step for transplant-eligible patients or an alternative therapy for transplant-ineligible patients. Over the past 20 years, clinical observations have revealed that approximately 1% of patients with MCS undergo significant reverse remodeling to the point where the device can be explanted. Unfortunately, it is unclear why some patients experience durable, sustained myocardial remission, while others redevelop heart failure (i.e. which hearts "hold" and which hearts "fold"). In this review, we outline unmet clinical needs related to treating patients with MCS, provide an overview of protein dynamics in the reverse-remodeling process, and propose specific areas where we expect MS and proteomic analyses will have significant impact on our understanding of disease progression, molecular mechanisms of recovery, and provide new markers with prognostic value that can positively impact patient care. Complimentary perspectives are provided with the goal of making this important topic accessible and relevant to both a clinical and basic science audience, as the intersection of these disciplines is required to advance the field.

Psychophysiologic Remodeling of the Failing Human Heart

Autonomic imbalance is a therapeutic target in heart failure patients. Overactivation of the sympathetic branch of the autonomic nervous system has for years been treated with beta-adrenergic blocking drugs, although it cannot be long before invasive therapies such as renal sympathetic denervation, currently being tested for resistant hypertension, make their way to the heart failure arena. Insufficient parasympathetic input to the heart is already being augmented with implanted vagal nerve stimulators. It is our contention that biofeedback training could provide much of the same benefit as these more invasive therapies in improving autonomic imbalance, with the added advantage of increasing patient self-efficacy. This article describes a pilot study of biofeedback training in patients with end-stage heart failure awaiting heart transplantation. It was our goal to show not only that patients would appreciate this training and benefit from it but also that patient-controlled autonomic modulation could actually reverse the cellular and molecular markers of heart failure, or remodel the failing heart, similar to what we had previously shown for other types of heart failure therapy such as the left ventricular assist device. To our knowledge, this is the first study to demonstrate changes in the biology of the failing heart in response to a psychophysiologic intervention.

Type of desmin expression in cardiomyocytes - a good marker of heart failure development in idiopathic dilated cardiomyopathy

AIM

The aim of this study was to determine whether remodelling of the desmin (DES) cytoskeleton affects myocardial function and whether it could be a useful marker of disease progression in patients with idiopathic dilated cardiomyopathy (IDCM).

MATERIAL AND METHODS

Endomyocardial biopsy was performed in 195 IDCM patients, and five to six specimens were collected from the left ventricle. DES expression was evaluated using tissue immunostaining and Western blotting. The study population was assigned to four groups according to DES expression type: I, normal DES staining at Z-lines giving a regular pattern of cross-striation (n = 57); IIA, increased DES staining with a regular pattern of cross-striation (n = 40); IIB, increased DES staining with an irregular pattern of cross-striation and/or the presence of aggregates (n = 56); and III, decreased/lack of DES staining (n = 42). Fibrosis, cardiomyocyte hypertrophy and ultrastructure were assessed for the four types of DES expression.

RESULTS

The pathological types of DES expression (IIB or III) were associated with pathological changes in mitochondria and the contractile apparatus. Cardiomyocyte diameter and level of fibrosis were both significantly affected. DES expression type correlated with NYHA class, left ventricular end-diastolic diameter, left ventricular ejection fraction and the level of N-terminal pro-brain natriuretic protein.

CONCLUSION

The type of immunohistochemical DES expression correlated with the level of myocardial injury at the cellular and organ levels. This correlation was similar to that observed between DES expression and the well-established biochemical, echocardiographic and clinical parameters of heart failure (HF). DES expression type could be used as an important diagnostic feature of HF development.

Changes in the organization of excitation-contraction coupling structures in failing human heart

BACKGROUND

The cardiac myocyte t-tubular system ensures rapid, uniform cell activation and several experimental lines of evidence suggest changes in the t-tubular system and associated excitation-contraction coupling proteins may occur in heart failure.

METHODS AND RESULTS

The organization of t-tubules, L-type calcium channels (DHPRs), ryanodine receptors (RyRs) and contractile machinery were examined in fixed ventricular tissue samples from both normal and failing hearts (idiopathic (non-ischemic) dilated cardiomyopathy) using high resolution fluorescent imaging. Wheat germ agglutinin (WGA), Na-Ca exchanger, DHPR and caveolin-3 labels revealed a shift from a predominantly transverse orientation to oblique and axial directions in failing myocytes. In failure, dilation of peripheral t-tubules occurred and a change in the extent of protein glycosylation was evident. There was no change in the fractional area occupied by myofilaments (labeled with phalloidin) but there was a small reduction in the number of RyR clusters per unit area. The general relationship between DHPRs and RyR was not changed and RyR labeling overlapped with 51±3% of DHPR labeling in normal hearts. In longitudinal (but not transverse) sections there was an ∼30% reduction in the degree of colocalization between DHPRs and RyRs as measured by Pearson's correlation coefficient in failing hearts.

CONCLUSIONS

The results show that extensive remodelling of the t-tubular network and associated excitation-contraction coupling proteins occurs in failing human heart. These changes may contribute to abnormal calcium handling in heart failure. The general organization of the t-system and changes observed in failure samples have subtle differences to some animal models although the general direction of changes are generally similar.

Emerging concepts in the pharmacogenomics of arrhythmias: ion channel trafficking

Continuous, rhythmic beating of the heart requires exquisite control of expression, localization and function of cardiac ion channels - the foundations of the cardiac myocyte action potential. Disruption of any of these processes can alter the shape of the action potential, predisposing to cardiac arrhythmias. These arrhythmias can manifest in a variety of ways depending on both the channels involved and the type of disruption (i.e., gain or loss of function). As much as 1% of the population of developed countries is affected by cardiac arrhythmia each year, and a detailed understanding of the mechanism of each arrhythmia is crucial to developing and prescribing the proper therapies. Many of the antiarrhythmic drugs currently on the market were developed before the underlying cause of the arrhythmia was known, and as a result lack specificity, causing side effects. The majority of the available drugs target the conductance of cardiac ion channels, either by blocking or enhancing current through the channel. In recent years, however, it has become apparent that specific targeting of ion channel conductance may not be the most effective means for treatment. Here we review increasing evidence that suggests defects in ion channel trafficking play an important role in the etiology of arrhythmias, and small molecule approaches to correct trafficking defects will likely play an important role in the future of arrhythmia treatment.

Kif5b is an essential forward trafficking motor for the Kv1.5 cardiac potassium channel

We have investigated the role of the kinesin I isoform Kif5b in the trafficking of a cardiac voltage-gated potassium channel, Kv1.5. In Kv1.5-expressing HEK293 cells and H9c2 cardiomyoblasts, current densities were increased from control levels of 389 +/- 50.0 and 317 +/- 50.3 pA pF(1), respectively, to 614 +/- 74.3 and 580 +/- 90.9 pA pF(1) in cells overexpressing the Kif5b motor. Overexpression of the Kif5b motor increased Kv1.5 expression additively with several manipulations that reduce channel internalization, suggesting that it is involved in the delivery of the channel to the cell surface. In contrast, expression of a Kif5b dominant negative (Kif5bDN) construct increased Kv1.5 expression non-additively with these manipulations. Thus, the dominant negative acts by indirectly inhibiting endocytosis. The increase in Kv1.5 currents induced by wild-type Kif5b was dependent on Golgi function; a 6 h treatment with Brefeldin A reduced Kv1.5 currents to control levels in Kif5b-overexpressing cells but had little effect on the increase associated with Kif5bDN expression. Finally, expression of the Kif5bDN prior to induction of Kv1.5 in a tetracycline inducible system blocked surface expression of the channel in both HEK293 cells and H9c2 cardiomyoblasts. Thus, Kif5b is essential to anterograde trafficking of a cardiac voltage-gated potassium channel.

Reverse cardiac remodeling enabled by mechanical unloading of the left ventricle

Cardiac remodeling is a characteristic and basic component of heart failure progression and is associated with a poor prognosis. Attenuating or reversing remodeling is an accepted goal of heart failure therapy. Cardiac mechanical support with left ventricular assist devices, in addition to its established role as "bridge to transplantation" or "destination therapy" in patients not eligible for cardiac transplantation, offers the potential for significant and sustained myocardial recovery through reverse remodeling. This review discusses the emerging role of left ventricular assist devices as a "bridge to recovery". Clinical and basic aspects of cardiac remodeling and cardiac reverse remodeling enabled by mechanical unloading, potential candidates for this modality of treatment as well as unresolved issues regarding the use of mechanical circulatory support as a bridge to recovery are discussed.

Impact of left ventricular assist device (LVAD) support on the cardiac reverse remodeling process

With improved technology and expanding indications for use, left ventricular assist devices (LVADs) are assuming a greater role in the care of patients with end-stage heart failure. Following LVAD implantation with the intention of bridge to transplant, it became evident that some patients exhibit substantial recovery of ventricular function. This prompted explantation of some devices in lieu of transplantation, the so-called bridge-to-recovery (BTR) therapy. However, clinical outcomes following these experiences are not always successful. Patients treated in this fashion have often progressed rapidly back to heart failure. Special knowledge has emerged from studies of hearts supported by LVADs that provides insights into the basic mechanisms of ventricular remodeling and possible limits of ventricular recovery. In general, it was these studies that spawned the concept of reverse remodeling now recognized as an important goal of many heart failure treatments. Important examples of myocardial and/or ventricular properties that do not regress towards normal during LVAD support include abnormal extracellular matrix metabolism, increased tissue angiotensin levels, myocardial stiffening and partial recovery of gene expression involved with metabolism. Nevertheless, studies of LVAD-heart interactions have led to the understanding that although we once considered the end-stage failing heart of patients near death to be irreversibly diseased, an unprecedented degree of myocardial recovery is possible, when given sufficient mechanical unloading and restoration of more normal neurohormonal milieu. Evidence supporting and unsupporting the notion of reverse remodeling and clinical implications of this process will be reviewed.

Reverse remodeling during long-term mechanical unloading of the left ventricle

A significant proportion of patients placed on long-term mechanical circulatory support for end-stage heart failure can be weaned from mechanical assistance after functional recovery of their native heart ("bridge to recovery"). The pathophysiological mechanisms implicated in reverse remodeling that cause a sustained functional myocardial recovery have recently become the subject of intensive research, expected to provide information with a view to accurately identify reliable prognostic indicators of recovery. In addition, this kind of information will enable changes in the strategy of myocardial recovery by modifying the duration and scale of the unloading regimen or by combining it with other treatments that promote reverse remodeling.

Changes in sarcomeric and non-sarcomeric cytoskeletal proteins and focal adhesion molecules during clinical myocardial recovery after left ventricular assist device support

BACKGROUND

Reverse remodeling can occur after left ventricular assist device (LVAD) support, which is sufficient in some cases to allow explantation of the device without cardiac transplantation. The molecular mechanisms involved remain unknown. A specific pattern of expression of sarcomeric and non-sarcomeric proteins in the myocardium is thought to be essential for normal myocardial function. However, a detailed protein analysis of their role in recovery has not been performed previously.

METHODS

Myocardial samples were collected at implantation and explantation in 7 patients with dilated cardiomyopathy who had sufficient recovery for device explantation. Western blotting and immunoprobing were used to quantitate changes in the expression of sarcomeric and cytoskeletal proteins.

RESULTS

At implantation, all patients (6 men and 1 woman, age [mean +/- SD] 36.1 +/- 10.4 years) were inotrope-dependent; ejection fraction (EF) was 12.6 +/- 4.6%, cardiac index (CI) was 1.66 +/- 0.5 liters/min/m2 and pulmonary capillary wedge pressure (PCWP) was 26 +/- 6 mm Hg. Mean duration of LVAD support was 333 +/- 235 days. Prior to explantation, EF (pump off for 15 minutes) was 62.7 +/- 11.4%, CI was 2.7 +/- 0.7 liters/min/m2 and PCWP was 10.9 +/- 3.5 mm Hg. At explantation, the following statistically significant increases were noted: myosin heavy chain, 1.90-fold (p < 0.05); sarcomeric actin, 1.80-fold (p < 0.05); alphaII spectrin, 1.40-fold (p = 0.05); troponin C, 1.34-fold (p < 0.05); troponin T, 2.10-fold (p < 0.05); cytoskeletal actinin, 5.16-fold (p < 0.05); and smooth muscle alpha-actin, 4.10-fold (p = 0.05). Although not significant (NS), increases were also seen for: troponin I, 1.27-fold; myosin light chain 1, 1.28-fold; tropomyosin, 1.28-fold; and sarcomeric actinin at 3.24-fold. There was a decrease in talin of 2.01-fold (p = NS) between implant and explant. Vimentin was unchanged.

CONCLUSIONS

Our data suggest that reverse remodeling of the myocardium parallels improvements in hemodynamic function in LVAD patients showing clinical myocardial recovery.

Paxillin and ponsin interact in nascent costameres of muscle cells

Muscle differentiation requires the transition from motile myoblasts to sessile myotubes and the assembly of a highly regular contractile apparatus. This striking cytoskeletal remodelling is coordinated with a transformation of focal adhesion-like cell-matrix contacts into costameres. To assess mechanisms underlying this differentiation process, we searched for muscle specific-binding partners of paxillin. We identified an interaction of paxillin with the vinexin adaptor protein family member ponsin in nascent costameres during muscle differentiation, which is mediated by an interaction of the second src homology domain 3 (SH3) domain of ponsin with the proline-rich region of paxillin. To understand the molecular basis of this interaction, we determined the structure of this SH3 domain at 0.83 A resolution, as well as its complex with the paxillin binding peptide at 1.63 A resolution. Upon binding, the paxillin peptide adopts a polyproline-II helix conformation in the complex. Contrary to the charged SH3 binding interface, the peptide contains only non-polar residues and for the first time such an interaction was observed structurally in SH3 domains. Fluorescence titration confirmed the ponsin/paxillin interaction, characterising it further by a weak binding affinity. Transfection experiments revealed further characteristics of ponsin functions in muscle cells: All three SH3 domains in the C terminus of ponsin appeared to synergise in targeting the protein to force-transducing structures. The overexpression of ponsin resulted in altered muscle cell-matrix contact morphology, suggesting its involvement in the establishment of mature costameres. Further evidence for the role of ponsin in the maintenance of mature mechanotransduction sites in cardiomyocytes comes from the observation that ponsin expression was down-regulated in end-stage failing hearts, and that this effect was reverted upon mechanical unloading. These results provide new insights in how low affinity protein-protein interactions may contribute to a fine tuning of cytoskeletal remodelling processes during muscle differentiation and in adult cardiomyocytes.

LVAD-Induced Reverse Remodeling: Basic and Clinical Implications for Myocardial Recovery

BACKGROUND

With improved technology, increasing clinical experience, and expanding indications for use, left ventricular assist devices (LVADs) are assuming a greater role in the care of patients with end-stage heart failure. Early in the course of LVAD use as a bridge to transplant, it became evident that some patients exhibit substantial recovery of ventricular function, which led to the concept of reverse remodeling.

METHODS AND RESULTS

Herein we summarize and integrate insights derived from a multitude of studies that have investigated how LVAD support influences ventricular structural, cellular, extracellular matrix, molecular, biochemical, and metabolic characteristics of the end-stage failing heart. The focus includes a review of the extent and sustainability of reverse remodeling, the important advances in understanding of the pathophysiology of heart failure derived from these studies and the implications of these findings for development of new therapeutic strategies.

CONCLUSION

In brief, studies of LVAD-heart interactions have led to the understanding that although we once considered the end-stage failing heart of patients near death to be irreversibly diseased, when given sufficient mechanical unloading and restoration of more normal neurohormonal milieu, a relatively large degree of myocardial recovery is possible. Comparison of effects on right and left ventricles have provided mechanistic insights by implicating hemodynamic unloading as primarily regulating certain aspects of reverse remodeling, neurohormonal factors as regulating other aspects, and joint regulation of still other aspects. As such these observations have driven a shift of thinking of chronic heart failure as a progressive irreversible disease process to a potentially treatable entity.

Contractile responses to selective phosphodiesterase inhibitors following chronic beta-adrenoreceptor activation

Contractile responses to phosphodiesterase (PDE) inhibitors are attenuated in heart failure, an effect limiting the clinical value of these agents. In this study, we sought to determine whether abnormalities in the beta-adrenoreceptor (beta-AR)-cyclic adenosine monophosphate (cAMP) signal transduction are sufficient to account for downregulation of PDE inhibitor-induced inotropic responses following chronic sympathetic activation. Sustained beta-AR activation produced by administration of isoproterenol (ISO) (50 microg kg(-1) day(-1) i.p. for 1 month) to rats resulted in cardiac hypertrophy, but did not affect baseline cardiac systolic function, as assessed in vivo by echocardiography and ex vivo under controlled loading conditions and heart rate (left ventricular systolic pressure-volume and stress-strain relations). Moreover, chronic ISO administration did not alter the baseline myocardial norepinephrine release or inotropic responses to incremental concentrations of Ca(2+) in isolated, perfused heart preparations. However, left ventricular contractile responses to ISO, the PDE III inhibitor amrinone, and the PDE IV inhibitor rolipram were attenuated following chronic beta-AR activation. Myocardial cAMP concentrations after stimulation with amrinone and rolipram were similar in ISO-treated and control rats. However, in ISO-treated rats, a marked decrease in contractile responsiveness to the cell-permeable, PDE-resistant cAMP analogue, 8-bromoadenosine cAMP, was noted. In conclusion, these data suggest that in cardiac disease, sustained beta-AR activation, without producing ventricular systolic dysfunction or enhanced myocardial norepinephrine release, is sufficient to account for the downregulation of contractile responses to PDE inhibitors. This effect appears to be largely mediated through abnormalities in signal transduction between cAMP and Ca(2+)-induced Ca(2+) release.

Gene Profiling Changes in Cytoskeletal Proteins During Clinical Recovery After Left Ventricular–Assist Device Support

Background— After left ventricular–assist device (LVAD) support, a proportion of patients recover sufficient ventricular function to enable explantation of the device. The exact molecular mechanisms involved in myocardial recovery remain unknown. Cytoskeletal proteins are essential for the structure and function of the cardiac myocyte and might play a major role.

Methods and Results— A total of 15 patients with nonischemic cardiomyopathy who required LVAD implantation were studied; 6 recovered sufficiently to allow explantation of the device compared with 9 who did not recover and required transplantation. LV myocardial samples were collected at implantation and explantation/transplantation. Affymetrix microarray analysis was performed on the paired samples and analyzed with reference to sarcomeric and nonsarcomeric cytoskeletal proteins. In the recovery group, of the nonsarcomeric proteins, lamin A/C increased 1.5-fold ( P <0.05) and spectrin 1.6-fold ( P <0.05) between the times of implantation and explantation. Integrins β1, β6, and α7 decreased 1.7-fold (P<0.05), 2.4-fold (P<0.05), and 1.5-fold (P<0.05), respectively, but integrins α5 and β5 increased 2.3-fold (P<0.01) and 1.2-fold ( P <0.01) at explantation. The following sarcomeric proteins changed in the recovered group only: β-actin increased 1.4-fold ( P <0.05); α-tropomyosin, 1.3-fold ( P <0.05); α1-actinin, 1.8-fold ( P <0.01); and α-filamin A, 1.6-fold ( P <0.05). Both troponin T3 and α 2-actinin decreased by 1.6-fold at the time of explantation ( P <0.05). Vinculin decreased 1.7-fold ( P =0.001) in the recovered group but increased by 1.7-fold ( P <0.05) in the nonrecovered group. Vinculin protein levels decreased 4.1-fold in the recovered group.

Conclusions— Myocardial recovery was associated with a specific pattern of changes in sarcomeric, nonsarcomeric, and membrane-associated proteins, which could have important implications in understanding the mechanisms involved.

Kv1.5 surface expression is modulated by retrograde trafficking of newly endocytosed channels by the dynein motor

In this article we have investigated the mechanisms by which retrograde trafficking regulates the surface expression of the voltage-gated potassium channel, Kv1.5. Overexpression of p50/dynamitin, known to disrupt the dynein-dynactin complex responsible for carrying vesicle cargo, substantially increased outward K+ currents in HEK293 cells stably expressing Kv1.5 (0.57+/-0.07 nA/pF, n=12; to 1.18+/-0.2 nA/pF, n=12, P<0.01), as did treatment of the cells with a dynamin inhibitory peptide, which blocks endocytosis. Nocodazole pretreatment, which depolymerizes the microtubule cytoskeleton along which dynein tracks, also doubled Kv1.5 currents in HEK cells and sustained K+ currents in isolated rat atrial myocytes. These increased currents were blocked by 1 mmol/L 4-aminopyridine, and the specific Kv1.5 antagonist, DMM (100 nM). Confocal imaging of both HEK cells and myocytes, as well as experiments testing the sensitivity of the channel in living cells to external Proteinase K, showed that this increase of K+ current density was caused by a redistribution of channels toward the plasma membrane. Coimmunoprecipitation experiments demonstrated a direct interaction between Kv1.5 and the dynein motor complex in both heterologous cells and rat cardiac myocytes, supporting the role of this complex in Kv1.5 trafficking, which required an intact SH3-binding domain in the Kv1.5 N terminus to occur. These experiments highlight a pathway for Kv1.5 internalization from the cell surface involving early endosomes, followed by later trafficking by the dynein motor along microtubules. This work has significant implications for understanding the way Kv channel surface expression is regulated.