This chapter will describe basic structural and functional features of the contractile apparatus of muscle cells of the heart, namely, cardiomyocytes and smooth muscle cells. Cardiomyocytes form the contractile myocardium of the heart, while smooth muscle cells form the contractile coronary vessels. Both muscle types have distinct properties and will be considered with respect to their cellular appearance (brick-like cross-striated versus spindle-like smooth), arrangement of contractile proteins (sarcomeric versus non-sarcomeric organization), calcium activation mechanisms (thin-filament versus thick-filament regulation), contractile features (fast and phasic versus slow and tonic), energy metabolism (high oxygen versus low oxygen demand), molecular motors (type II myosin isoenzymes with high adenosine diphosphate [ADP]-release rate versus myosin isoenzymes with low ADP-release rates), chemomechanical energy conversion (high adenosine triphosphate [ATP] consumption and short duty ratio versus low ATP consumption and high duty ratio of myosin II cross-bridges [XBs]), and excitation-contraction coupling (calcium-induced calcium release versus pharmacomechanical coupling). Part of the work has been published (Neuroscience - From Molecules to Behavior", Chap. 22, Galizia and Lledo eds 2013, Springer-Verlag; with kind permission from Springer Science + Business Media).

AbstractBackgroundAging is associated with a progressive reduction in cellular function leading to poor health and loss of physical performance. Mitochondrial dysfunction is one of the hallmarks of aging; hence, interventions targeting mitochondrial dysfunction have the potential to provide preventive and therapeutic benefits to elderly individuals. Meta‐analyses of age‐related gene expression profiles showed that the expression of Ahnak1, a protein regulating several signal‐transduction pathways including metabolic homeostasis, is increased with age, which is associated with low VO2MAX and poor muscle fitness. However, the role of Ahnak1 in the aging process remained unknown. Here, we investigated the age‐related role of Ahnak1 in murine exercise capacity, mitochondrial function, and contractile function of cardiac and skeletal muscles.MethodsWe employed 15‐ to 16‐month‐old female and male Ahnak1‐knockout (Ahnak1‐KO) and wild‐type (WT) mice and performed morphometric, biochemical, and bioenergetics assays to evaluate the effects of Ahnak1 on exercise capacity and mitochondrial morphology and function in cardiomyocytes and tibialis anterior (TA) muscle. A human left ventricular (LV) cardiomyocyte cell line (AC16) was used to investigate the direct role of Ahnak1 in cardiomyocytes.ResultsWe found that the level of Ahnak1 protein is significantly up‐regulated with age in the murine LV (1.9‐fold) and TA (1.8‐fold) tissues. The suppression of Ahnak1 was associated with improved exercise tolerance, as all aged adult Ahnak1‐KO mice (100%) successfully completed the running programme, whereas approximately 31% male and 8% female WT mice could maintain the required running speed and distance. Transmission electron microscopic studies showed that LV and TA tissue specimens of aged adult Ahnak1‐KO of both sexes have significantly more enlarged/elongated mitochondria and less small mitochondria compared with WT littermates (P < 0.01 and P < 0.001, respectively) at basal level. Further, we observed a shift in mitochondrial fission/fusion balance towards fusion in cardiomyocytes and TA muscle from aged adult Ahnak1‐KO mice. The maximal and reserve respiratory capacities were significantly higher in cardiomyocytes from aged adult Ahnak1‐KO mice compared with the WT counterparts (P < 0.05 and P < 0.01, respectively). Cardiomyocyte contractility and fatigue resistance of TA muscles were significantly increased in Ahnak1‐KO mice of both sexes, compared with the WT groups. In vitro studies using AC16 cells have confirmed that the alteration of mitochondrial function is indeed a direct effect of Ahnak1. Finally, we presented Ahnak1 as a novel cardiac mitochondrial membrane‐associated protein.ConclusionsOur data suggest that Ahnak1 is involved in age‐related cardiac and skeletal muscle dysfunction and could therefore serve as a promising therapeutical target.

Aim: Vascular remodeling following injury substantially accounts for restenosis and adverse clinical outcomes. In this study, we investigated the role of the giant scaffold protein Ahnak1 in vascular healing after endothelial denudation of the murine femoral artery. Methods: The spatiotemporal expression pattern of Ahnak1 and Ahnak2 was examined using specific antibodies and real-time quantitative PCR. Following wire-mediated endothelial injury of Ahnak1-deficient mice and wild-type (WT) littermates, the processes of vascular healing were analyzed. Results: Ahnak1 and Ahnak2 showed a mutually exclusive vascular expression pattern, with Ahnak1 being expressed in the endothelium and Ahnak2 in the medial cells in naïve WT arteries. After injury, a marked increase of Ahnak1- and Ahnak2-positive cells at the lesion site became evident. Both proteins showed a strong upregulation in neointimal cells 14 days after injury. Ahnak1-deficient mice showed delayed vascular healing and dramatically impaired re-endothelialization that resulted in prolonged adverse vascular remodeling, when compared to the WT littermates. Conclusion: The large scaffold and adaptor proteins Ahnak1 and Ahnak2 exhibit differential expression patterns and functions in naïve and injured arteries. Ahnak1 plays a nonredundant protective role in vascular healing.

The cardiac L-type Ca 2+ channel (Cav1.2) constitutes the main entrance gate for Ca 2+ that triggers cardiac contraction. It has been shown that the distal C-terminus fragment of Cav1.2 α1C subunit (α1C-dCT) is proteolytically cleaved and shuttles between the plasma membrane and the nucleus, which is regulated both developmentally and by Ca 2+ . However, the effects of sex and sex hormone 17β-Estradiol (E2) on α1C-dCT nuclear translocation are still unexplored. To analyze the sexual disparity in the α1C-dCT nuclear translocation, we first generated an antibody directed against a synthetic peptide (GRRASFHLE) located in α1C-dCT, and used it to probe ventricular myocytes from adult female and male mice. Immunocytochemistry of isolated mouse primary adult ventricular myocytes revealed both nuclear staining and cytosolic punctuate staining around the T-tubules. The ratio of nuclear to cytosolic intensity (I nuc /I cyt ) was significantly higher in isolated female cardiomyocytes (1.42±0.05) compared to male cardiomyocytes (1.05±0.02). Western blot analysis of nuclear fraction confirmed these data. Furthermore, we found a significant decrease in nuclear staining intensity of α1C-dCT in both female and male cardiomyocytes upon serum withdrawal for 18h (I nuc /I cyt 1.05±0.02 and 0.89± 0.02, respectively). Interestingly, subsequent E2 treatment (10 -8 M) for 8h normalized the intracellular distribution of α1C-dCT in male cardiomyocytes (I nuc /I cyt 1.04±0.02), but not in female cardiomyocytes. Acute treatment of male cardiomyocytes with E2 for 45min revealed a similar effect. This effect of E2 was revised by ICI 182780, an estrogen receptor (ER) inhibitor, (10 -5 M), indicating the involvement of ER in this signaling pathway. Taken together, we showed for the first time that the shuttling of α1C-CT in cardiomyocytes is regulated in a sex-dependent manner, and E2-activated ER may play a role in the nuclear shuttling of α1C-dCT in male cardiomyocytes. We suggest that this identified novel signaling mechanism may explain, at least partly, the observed sex differences in the regulation of cardiac Cav1.2 channel activity.

The cardiac voltage gated l-type Ca(2+) channel (Cav1.2) constitutes the main entrance gate for Ca(2+) that triggers cardiac contraction. Several studies showed that the distal C-terminus fragment of Cav1.2 α1C subunit (α1C-dCT) is proteolytically cleaved and shuttles between the plasma membrane and the nucleus, which is regulated both developmentally and by Ca(2+). However, the effects of sex and sex hormone 17β-estradiol (E2, estrogen) on α1C-dCT nuclear translocation are still unexplored. To investigate the sexual disparity in the α1C-dCT nuclear translocation, we first generated an antibody directed against a synthetic peptide (GRRASFHLE) located in α1C-dCT, and used it to probe ventricular myocytes from adult female and male mice. Immunocytochemistry of isolated mouse primary adult ventricular myocytes revealed both nuclear staining and cytosolic punctuate staining around the T-tubules. The ratio of nuclear to cytosolic intensity (Inuc/Icyt) was significantly higher in isolated female cardiomyocytes (1.42±0.05) compared to male cardiomyocytes (1.05±0.02). Western blot analysis of nuclear fraction confirmed these data. Furthermore, we found a significant decrease in nuclear staining intensity of α1C-dCT in both female and male cardiomyocytes upon serum withdrawal for 18h (Inuc/Icyt 1.05±0.02 and 0.89±0.02, respectively). Interestingly, subsequent E2 treatment (10(-8)M) for 8h normalized the intracellular distribution of α1C-dCT in male cardiomyocytes (Inuc/Icyt 1.04±0.02), but not in female cardiomyocytes. Acute treatment of male cardiomyocytes with E2 for 45min revealed a similar effect. This effect of E2 was revised by ICI indicating the involvement of ER in this signaling pathway. Taken together, our results showed that the shuttling of α1C-CT in cardiomyocytes is regulated in a sex-dependent manner, and E2-activated ER may play a role in the nuclear shuttling of α1C-dCT in male cardiomyocytes. This may explain, at least partly, the observed sex differences in the regulation of cardiac Cav1.2 channel activity.

Chronic increased workload of the human heart causes ventricular hypertrophy, re-expression of the atrial essential myosin light chain (hALC-1), and improved contractile function. Although hALC-1 is an important positive inotropic regulator of the human heart, little is known about its regulation. Therefore, we investigated the role of the sex hormone 17β-estradiol (E2) on hALC-1 gene expression, the underlying molecular mechanisms, and the impact of this regulatory process on cardiac contractile function. We showed that E2 attenuated hALC-1 expression in human atrial tissues of both sexes and in human ventricular AC16 cells. E2 induced the nuclear translocation of estrogen receptor alpha (ERα) and hALC-1 in AC16 cells, where they cooperatively regulate the transcriptional activity of hALC-1 gene promoter. E2-activated ERα required the estrogen response element (ERE) motif within the hALC-1 gene promoter to reduce its transcriptional activity (vehicle: 15.55 ± 4.80 vs. E2: 6.51 ± 3.69; ~2 fold). This inhibitory effect was potentiated in the presence of hALC-1 (vehicle: 11.13 ± 3.66 vs. E2: 2.18 ± 1.10; ~5 fold), and thus, hALC-1 acts as a co-repressor of ERα-mediated transcription. Yeast two-hybrid screening of a human heart cDNA library revealed that ERα interacts physically with hALC-1 in the presence of E2. This interaction was confirmed by Co-Immunoprecipitation and immunofluorescence in human atrium. As a further novel effect, we showed that chronic E2-treatment of adult mouse cardiomyocytes overexpressing hALC-1 resulted in reduced cell-shortening amplitude and twitching kinetics of these cells independent of Ca2+ activation levels. Together, our data showed that the expression of hALC-1 gene is, at least partly, regulated by E2/ERα, while hALC-1 acts as a co-repressor. The inotropic effect of hALC-1 overexpression in cardiomyocytes can be significantly repressed by E2.

ERβ (oestrogen receptor β) activation has been shown to be cardioprotective, but the cell types and mechanisms involved are not understood. To investigate whether ERβ restricted to cardiomyocytes contributes to the observed cardioprotection, we tested the effects of cardiomyocyte-specific ERβ-OE (ERβ overexpression) on survival, cardiac remodelling and function after MI (myocardial infarction) and studied the molecular pathways potentially involved. Female and male mice with cardiomyocyte-specific ERβ-OE and WT (wild-type) littermates were subjected to chronic anterior coronary artery ligation or sham surgery. Two weeks after MI, ERβ-OE mice showed improved survival (100% and 83% compared with 76% and 58% in WT females and males respectively). ERβ-OE was associated with attenuated LV (left ventricular) dilatation, smaller increase in heart weight, less lung congestion at similar MI size, and improved systolic and diastolic function in both sexes. We identified two potential pathways for ERβ-mediated myocardial protection. First, male and female ERβ-OE mice had a lower reduction of SERCA2a (sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase 2a) expression after MI, suggesting less reduction in diastolic Ca(2+)-reuptake into the sarcoplasmic reticulum post-MI. Secondly, male ERβ-OE revealed attenuated cardiac fibrosis in the remote LV tissue and expression of fibrosis markers collagen I and III, periostin and miR-21. Cardiomyocyte-specific ERβ-OE improved survival associated with reduced maladaptive remodelling, improved cardiac function and less heart failure development after MI in both sexes. These effects seem to be related, at least in part, to a better maintenance of Ca(2+) cycling in both sexes and a lower induction of cardiac fibrosis in males after MI.

BACKGROUND:The Muscle-specific RING-finger (MuRF) protein family of E3 ubiquitin ligases is important for maintenance of muscular structure and function. MuRF proteins mediate adaptation of striated muscles to stress. MuRF2 and MuRF3 bind to microtubules and are implicated in sarcomere formation with noticeable functional redundancy. However, if this redundancy is important for muscle function in vivo is unknown. Our objective was to investigate cooperative function of MuRF2 and MuRF3 in the skeletal muscle and the heart in vivo.METHODS:MuRF2 and MuRF3 double knockout mice (DKO) were generated and phenotypically characterized. Skeletal muscle and the heart were investigated by morphological measurements, histological analyses, electron microscopy, immunoblotting, and real-time PCR. Isolated muscles were subjected to in vitro force measurements. Cardiac function was determined by echocardiography and working heart preparations. Function of cardiomyocytes was measured in vitro. Cell culture experiments and mass-spectrometry were used for mechanistic analyses.RESULTS:DKO mice showed a protein aggregate myopathy in skeletal muscle. Maximal force development was reduced in DKO soleus and extensor digitorum longus. Additionally, a fibre type shift towards slow/type I fibres occurred in DKO soleus and extensor digitorum longus. MuRF2 and MuRF3-deficient hearts showed decreased systolic and diastolic function. Further analyses revealed an increased expression of the myosin heavy chain isoform beta/slow and disturbed calcium handling as potential causes for the phenotype in DKO hearts.CONCLUSIONS:The redundant function of MuRF2 and MuRF3 is important for maintenance of skeletal muscle and cardiac structure and function in vivo.

Induced pluripotent stem cell-derived cardiomyocytes (iPS-CMs) may be suitable for myocardial repair. While their functional and structural properties have been extensively investigated, their response to ischemia-like conditions has not yet been clearly defined.

BACKGROUND:Adipocyte fatty acid-binding protein (FABP4) is a member of a highly conserved family of cytosolic proteins that bind with high affinity to hydrophobic ligands, such as saturated and unsaturated long-chain fatty acids and eicosanoids. Recent evidence has supported a novel role for FABP4 in linking obesity with metabolic and cardiovascular disorders. In this context, we identified FABP4 as a main bioactive factor released from human adipose tissue that directly suppresses heart contraction in vitro. As FABP4 is known to be a transport protein, it cannot be excluded that lipid ligands are involved in the cardiodepressant effect as well, acting in an additional and/or synergistic way.OBJECTIVE:We investigated a possible involvement of lipid ligands in the negative inotropic effect of adipocyte factors in vitro.RESULTS:We verified that blocking the CYP epoxygenase pathway in adipocytes attenuates the inhibitory effect of adipocyte-conditioned medium (AM) on isolated adult rat cardiomyocytes, thus suggesting the participation of epoxyeicosatrienoic acids (EETs) in the cardiodepressant activity. Analysis of AM for EETs revealed the presence of 5,6-, 8,9-, 11,12- and 14,15-EET, whereas 5,6-EET represented about 45% of the total EET concentration in AM. Incubation of isolated cardiomyocytes with EETs in similar concentrations as found in AM showed that 5,6-EET directly suppresses cardiomyocyte contractility. Furthermore, after addition of 5,6-EET to FABP4, the negative inotropic effect of FABP4 was strongly potentiated in a concentration-dependent manner.CONCLUSIONS:These data suggest that adipocytes release 5,6-EET and FABP4 into the extracellular medium and that the interaction of these factors modulates cardiac function. Therefore elevated levels of FABP4 and 5,6-EET in obese patients may contribute to the development of heart dysfunction in these subjects.

The essential myosin light chain (ELC) is involved in modulation of force generation of myosin motors and cardiac contraction, while its mechanism of action remains elusive. We hypothesized that ELC could modulate myosin stiffness which subsequently determines its force production and cardiac contraction. Therefore, we generated heterologous transgenic mouse (TgM) strains with cardiomyocyte-specific expression of ELC with human ventricular ELC (hVLC-1; TgM(hVLC-1)) or E56G-mutated hVLC-1 (hVLC-1(E56G); TgM(E56G)). hVLC-1 or hVLC-1(E56G) expression in TgM was around 39% and 41%, respectively of total VLC-1. Laser trap and in vitro motility assays showed that stiffness and actin sliding velocity of myosin with hVLC-1 prepared from TgM(hVLC-1) (1.67 pN/nm and 2.3 mu m/s, respectively) were significantly higher than myosin with hVLC-1(E56G) prepared from TgM(E56G) (1.25 pN/nm and 1.7 mu m/s, respectively) or myosin with mouse VLC-1 (mVLC-1) prepared from C57/BL6 (1.41 pN/nm and 1.5 mu m/s, respectively). Maximal left ventricular pressure development of isolated perfused hearts in vitro prepared from TgM(hVLC-1) (80.0 mmHg) were significantly higher than hearts from TgM(E56G) (66.2 mmHg) or C57/BL6 (59.3 +/- 3.9 mmHg). These findings show that ELCs decreased myosin stiffness, in vitro motility, and thereby cardiac functions in the order hVLC-1 > hVLC-1(E56G) approximate to mVLC-1. They also suggest a molecular pathomechanism of hypertrophic cardiomyopathy caused by hVLC-1 mutations. (C) 2014 Elsevier Inc. All rights reserved.

Experimental studies showed that 17β-estradiol (E2) and activated Estrogen Receptors (ER) protect the heart from ischemic injury. However, the underlying molecular mechanisms are not well understood. To investigate the role of ER-alpha (ERα) in cardiomyocytes in the setting of myocardial ischemia, we generated transgenic mice with cardiomyocyte-specific overexpression of ERα (ERα-OE) and subjected them to Myocardial Infarction (MI). At the basal level, female and male ERα-OE mice showed increased Left Ventricular (LV) mass, LV volume and cardiomyocyte length. Two weeks after MI, LV volume was significantly increased and LV wall thickness decreased in female and male WT-mice and male ERα-OE, but not in female ERα-OE mice. ERα-OE enhanced expression of angiogenesis and lymphangiogenesis markers (Vegf, Lyve-1), and neovascularization in the peri-infarct area in both sexes. However, attenuated level of fibrosis and higher phosphorylation of JNK signaling pathway could be detected only in female ERα-OE after MI. In conclusion, our study indicates that ERα protects female mouse cardiomyocytes from the sequelae of ischemia through induction of neovascularization in a paracrine fashion and impaired fibrosis, which together may contribute to the attenuation of cardiac remodelling.

Binding of the utmost N-terminus of essential myosin light chains (ELC) to actin slows down myosin motor function. In this study, we investigated the binding constants of two different human cardiac ELC isoforms with actin. We employed circular dichroism (CD) and surface plasmon resonance (SPR) spectroscopy to determine structural properties and protein-protein interaction of recombinant human atrial and ventricular ELC (hALC-1 and hVLC-1, respectively) with alpha-actin as well as alpha-actin with alanin-mutated ELC binding site (alpha-actin(ala3)) as control. CD spectroscopy showed similar secondary structure of both hALC-1 and hVLC-1 with high degree of alpha-helicity. SPR spectroscopy revealed that the affinity of hALC-1 to alpha-actin (K-D = 575 nM) was significantly (p < 0.01) lower compared with the affinity of hVLC-1 to alpha-actin (K-D = 186 nM). The reduced affinity of hALC-1 to alpha-actin was mainly due to a significantly (p < 0.01) lower association rate (k(on): 1018 M-1 s(-1)) compared with kon of the hVLC-1/alpha-actin complex interaction (2908 M-1 s(-1)). Hence, differential expression of ELC isoforms could modulate muscle contractile activity via distinct alpha-actin interactions. (C) 2014 Elsevier Inc. All rights reserved.

The C-terminus of myosin-S1 forms the alpha-helical lever arm which is associated with an essential and a regulatory light chain. The lever arm is believed to work as an elastic ratchet which moves during force-generation from an up into a down position. The ELC in the heart tethers the tick and thin filaments by binding with its four EF-hand domains to the myosin lever arm, while the antenna-like N-terminal domain binds to a distinct actin binding site. Using cell-penetrating peptides which compete for actin interaction with the N-terminal ELC domain we showed that this interaction regulates contraction velocity of the myosin motor.The interaction of the ELC with the lever arm seems to be more complex. Albeit it is known that the ELC are not essential for myosin ATPase activity, ELC are in particular critical for force generation of a single myosin molecule. Therefore, an increased affinity of ELC should raise the fraction of highforce-generating myosin and force of muscle contraction – without the need of additional rise in activating calcium levels. In addition, increasing the ELC-affinity to myosin could modulate the high-force myosin state and, therefore, force generation. We tested this prediction by investigating the myosin affinity of different cardiac ELC isoforms. We observed that the human ALC-1 has an almost three-fold higher myosin affinity than the human or rat VLC-1 isoform. In a recent study we produced adenoviral vectors with human ALC1 as expression cassette and infected adult rat cardiomyocytes. After two days in culture, we investigated shortening amplitude and intracellular Ca2+ during electrical stimulation of the cardiomyocytes. As predicted, shortening amplitude of ALC-1 producing cells was significantly higher than those cells infected with a β-gal-adenovirus. Please note, that this positive inotropic effect occurred without a secondary increase in activating systolic free calcium levels, as monitored by investigating the Fura-fluorescence. We observed consistent inotropic effects of hALC-1 in cardiac preparations with increasing amounts of expressed ALC-1, namely i) in isolated perfused hearts from transgenic rats overexpressing human ALC-1, ii) skinned human ventricular fibers, and iii) in vivo in patients with heart diseases, who showed an improved ejection fraction with increasing amounts of ALC-1 expressed in the left ventricle. If strong ELC binding to the lever arm → (increased stiffness) → increased myosin force generation, than weak ELC binding to the lever arm → (decreased stiffness) → decreased myosin force generation. We tested this prediction with the help of disease-causing ELC mutations. At the moment, there are seven mutations of the human MYL3 gene coding the VLC-1. They are clustered in exon Session 5. Molecular Mechanisms of Motility

Adipocyte fatty acid-binding protein (FABP4) is a member of a highly conserved family of cytosolic proteins that bind with high affinity to hydrophobic ligands. Recent evidence has supported a novel role for FABP4 in linking obesity with metabolic and cardiovascular disorders. In this context, we verified in previous studies a direct bioactive impact of FABP4 on heart function. We demonstrated that human adipocytes release factors that directly suppress heart contraction in vitro. Subsequently, we identified FABP4 as the main cardiodepressant factor. Since FABP4 is known to be a transport protein, it cannot be excluded that lipid ligands are involved in the cardiodepressant effect as well, acting in an additional and/or synergetic way. Therefore, we investigated in the present study a possible involvement of lipid ligands on the negative inotropic effect of adipocyte-factors in interaction with FABP4 in vitro. First, we verified that blocking the CYP epoxygenase pathway in adipocytes attenuates the inhibitory effect of adipocyte-conditioned medium (AM) on isolated adult rat cardiomyocytes, thus suggesting the participation of epoxyeicosatrienoic acids (EETs) in the cardiodepressant activity. Analysis of AM for EETs revealed the presence of 5,6-, 8,9-, 11,12- and 14,15-EET, whereas 5,6-EET represented about 45% of total EET-concentration in AM. Incubation of isolated cardiomyocytes with EETs in similar concentrations as found in AM showed that 5,6-EET directly suppresses cardiomyocytes contractility. Furthermore, after addition of 5,6-EET to FABP4, the negative inotropic effect of FABP4 was strongly potentiated in a concentration-dependent manner. These data suggest that adipocytes release 5,6-EET and FABP4 into the extracellular medium and the interaction of these factors modulates the cardiac function. Therefore, elevated levels of FABP4 and 5,6-EET in obese subjects may partially explain the development of heart dysfunction in those subjects.

Estrogen (17β-estradiol, E2) is one of the key regulators of growth, differentiation and physiological function in different tissues, including the heart. The effects of E2 are mainly mediated by estrogen receptor (ER) alpha and beta, which act in concert with many cofactors to mediate estrogenic effects. So far, only a few cofactors of ER have been described in the human heart. To gain a better understanding of E2-mediated ERα action in the human heart, we identified and characterized the novel interaction partners of ERα Yeast two hybrid screening of a human heart cDNA library revealed that ERα interacts with the cardiac myosin essential light chain isoform 4 (ALC1) in presence of E2. ALC1, as a member of contractile proteins, is expressed in the fetal heart and becomes restricted to the atria of the adult heart under physiological conditions, and is reexpressed in ventricle of adult hypertrophic hearts. This switch is accompanied by alteration of contractile performance, thus improving the heart function. Retransformation experiments showed that ALC1 interacts with full-length ERα and ERα-EF domain in presence of E2. The interaction of ERα with ALC1 was also confirmed by Co-IP in human atrium. Double irnmunofluorescence (IF) analysis of paraffin-embedded sections from human atrium tissues showed co-localization of ERα and ALC1 proteins in a striated sarcomeric pattern. Co-localization corresponds most likely to the H-zone of sarcomere. IF analysis of AC16 cells (a human cardiomyocytes cell line) showed that ERα and ALC1 were mainly localized in the cytoplasm in the absence of E2. However, E2 treatment of AC16 cells led to a translocation of ALC1 and ERα into the nuclei of AC16 cells, where they also co-localized. Expression analysis in AC16 cells showed that E2 increases the expression of ALC1 gene (MYL4). Using ERE-based luciferase reporter assays we showed that E2-induced interaction of ERα with the ALC1 represses the transcriptional activity of ERα. We characterize for the first time an E2-regulated interaction of ALC1 with ERα in cardiomyocytes that may be crucial in physiological and/or pathological processes by regulating of transcriptional activity of ERa in the heart and/or by modulating contractile properties of cardiomyocytes.