Siyi Fu

Cardiac Telocytes and Fibroblasts in Primary Culture: Different Morphologies and Immunophenotypes

Telocytes (TCs) are a peculiar type of interstitial cells with very long prolongations termed telopodes. TCs have previously been identified in different anatomic structures of the heart, and have also been isolated and cultured from heart tissues in vitro. TCs and fibroblasts, both located in the interstitial spaces of the heart, have different morphologies and functionality. However, other than microscopic observation, a reliable means to make differential diagnosis of cardiac TCs from fibroblasts remains unclear. In the present study, we isolated and cultured cardiac TCs and fibroblasts from heart tissues, and observed their different morphological features and immunophenotypes in primary culture. Morphologically, TCs had extremely long and thin telopodes with moniliform aspect, stretched away from cell bodies, while cell processes of fibroblasts were short, thick and cone shaped. Furthermore, cardiac TCs were positive for CD34/c-kit, CD34/vimentin, and CD34/PDGFR-β, while fibroblasts were only vimentin and PDGFR-β positive. In addition, TCs were also different from pericytes as TCs were CD34 positive and α-SMA weak positive while pericytes were CD34 negative but α-SMA positive. Besides that, we also showed cardiac TCs were homogenously positive for mesenchymal marker CD29 but negative for hematopoietic marker CD45, indicating that TCs could be a source of cardiac mesenchymal cells. The differences in morphological features and immunophenotypes between TCs and fibroblasts will provide more compelling evidence to differentiate cardiac TCs from fibroblasts.

Telocytes in human liver fibrosis.

Liver fibrosis is a wound‐healing response which engages a variety of cell types to encapsulate injury. Telocyte (TC), a novel type of interstitial cell, has been identified in a variety of tissues and organs including liver. TCs have been reported to be reduced in fibrotic areas after myocardial infarction, human interstitial walls fibrotic remodelling caused either by ulcerative colitis or Crohns disease, and skin of systemic sclerosis. However, the role of TCs in human liver fibrosis remains unclear. Liver samples from human liver biopsy were collected. All samples were stained with Massons trichrome to determine fibrosis. TCs were identified by several immunofluorescence stainings including double labelling for CD34 and c‐kit/CD117, or vimentin, or PDGF Receptor‐α, or β. We found that hepatic TCs were significantly decreased by 27%–60% in human liver fibrosis, suggesting that loss of TCs might lead to the altered organization of extracellular matrix and loss the control of fibroblast/myofibroblast activity and favour the genesis of fibrosis. Adding TCs might help to develop effective and targeted antifibrotic therapies for human liver fibrosis.

Cardiac telocytes are double positive for CD34/PDGFR-α

Telocytes (TCs) are a distinct type of interstitial cells, which are featured with a small cellular body and long and thin elongations called telopodes (Tps). TCs have been widely identified in lots of tissues and organs including heart. Double staining for CD34/PDGFR‐β (Platelet‐derived growth factor receptor β) or CD34/Vimentin is considered to be critical for TC phenotyping. It has recently been proposed that CD34/PDGFR‐α (Platelet‐derived growth factor receptor α) is actually a specific marker for TCs including cardiac TCs although the direct evidence is still lacking. Here, we showed that cardiac TCs were double positive for CD34/PDGFR‐α in primary culture. CD34/PDGFR‐α positive cells (putative cardiac TCs) also existed in mice ventricle and human cardiac valves including mitral valve, tricuspid valve and aortic valve. Over 87% of cells in a TC‐enriched culture of rat cardiac interstitial cells were positive for PDGFR‐α, while CD34/PDGFR‐α double positive cells accounted for 30.25% of the whole cell population. We show that cardiac TCs are double positive for CD34/PDGFR‐α. Better understanding of the immunocytochemical phenotypes of cardiac TCs might help using cardiac TCs as a novel source in cardiac repair.

MicroRNAs in diabetic cardiomyopathy and clinical perspectives

Diabetes is a progressive metabolic disorder that can ultimately lead to serious chronic vascular complications including renal failure, vision loss, and cardiac dysfunction (Ruiz and Chakrabarti, 2013). Diabetic cardiomyopathy is responsible for higher incidence of sudden cardiac death and represents the leading cause of morbidity and mortality among the diabetic patients (Aksnes et al., 2007; Chavali et al., 2013). Previous studies have indicated that oxidative stress and mitochondrial dysfunction were critically involved in the etiology of diabetes-induced cardiac dysfunction (Sugamura and Keaney, 2011; Styskal et al., 2012), that could subsequently induce a cascade of complex pathophysiological events characterized by early impairments of diastolic function, development of cardiomyocyte hypertrophy, myocardial fibrosis and cardiomyocyte apoptosis, eventually leading to heart failure (Huynh et al., 2014). However, the underlying mechanisms of diabetic cardiomyopathy are far from understood and current therapeutic strategies do not specifically aim at diabetic cardiomyopathy and diabetes-induced heart failure. MicroRNAs (miRNAs, miRs), a novel class of non-coding RNAs of 22~24 nucleotides in length, act as post-transcriptional regulators of gene expression by binding to the 3′-untranslated region (3′-UTR) of target mRNA that induces mRNA degradation and/or translational repression (Lim et al., 2005; Van Rooij, 2011). Given that miRNAs are crucially involved in many critical biological processes including cell proliferation, apoptosis, necrosis, migration and differentiation (Bartel, 2004), desregulated miRNAs contribute to many human diseases including diabetes (Tyagi et al., 2011; Shantikumar et al., 2012; McClelland and Kantharidis, 2014) and cardiovascular diseases (Xiao et al., 2012; Fu et al., 2013; Vickers et al., 2014). Recent studies demonstrate that aberrant expression of miRNAs also participates in the pathogenetic processes mediating diabetic cardiomyopathy, where miR-1, -133, -141, -206, -223 have been reported upregulated, whereas miR-133a, -373, and -499 downregulated (Shen et al., 2011; Shantikumar et al., 2012; Asrih and Steffens, 2013). Thus, it is of crucial importance to gain insight into the role of miRNAs in the development of diabetic cardiomyopathy which will help clarify the molecular mechanisms as well as identify novel therapeutic strategies for diabetic cardiomyopathy. Cardiomyocyte hypertrophy, myocardial fibrosis, and cardiomyocyte apoptosis are important features of diabetic cardiomyopathy (Ruiz and Chakrabarti, 2013). Downregulation of miR-133a induces cardiomyocyte hypertrophy via upregulating the expression of MEF2A and MEF2C, two transcription factors involved in myocardial hypertrophy (Feng et al., 2010). While upregulation of miR-1 and -206 contributes to increased cardiomyocyte apoptosis, via repressing the expression of heat shock protein (Hsp) 60, PIM 1, and IGF-1 receptor (Yu et al., 2008; Shan et al., 2010; Katare et al., 2011). In addition, miR-373 is downregulated in diabetic heart, which is supposed to induce cardiac fibrosis via regulating the expression of p300 (Feng et al., 2008; Chen et al., 2010; Shen et al., 2011; Chavali et al., 2014). Thus, these reports indicate the critical contribution of miRNAs in cardiomyocyte hypertrophy, myocardial fibrosis, and cardiomyocyte apoptosis during the development of diabetic cardiomyopathy. Hyperglycemia, oxidative stress and mitochondrial damage are involved in the etiology of diabetes-induced cardiac dysfunction and diabetic cardiomyopathy (Shantikumar et al., 2012; Asrih and Steffens, 2013; McClelland and Kantharidis, 2014). Previous study has shown that miR-499 and -133a were markedly downregulated in the diabetic cardiomyocytes, while normalization of oxidant/antioxidant level by the treatment of N-acetylcysteine (NAC) restored the impaired expression of these miRNAs, indicating that hyperglycemia-induced downregulation of miR-499 and -133a was oxidative stress dependent (Yildirim et al., 2013). Similarly, miR-373 was downregulated by hyperglycemia-induced oxidative stress in diabetic cardiomyopathy via p38 MAPK pathway (Shen et al., 2011). MiR-141, a critical regulator of the inner mitochondrial phosphate carrier (Slc25a3), has been shown upregulated in diabetic heart, thus leading to the impaired mitochondrial ATP production in the pathogenesis of diabetic cardiomyopathy (Baseler et al., 2012). In terms of cardiomyocyte glucose metabolism, miR-223 was shown to be upregulated in the left ventricular biopsies of diabetic patients, which induced Glut4 protein level in cardiomyocytes and contributed to cardiomyocyte glucose uptake in vitro, indicating that overexpression of miR-223 might be a compensatory response to restore glucose metabolism in diabetic heart (Lu et al., 2010). Accumulating evidence has indicated that circulating miRNAs can be used as sensitive biomarkers for certain diseases such as cardiovascular diseases and cancers (Fabbri, 2010; Tijsen et al., 2012; Xu et al., 2012). Despite that diabetes is among the major risk factors for cardiovascular complications, researches investigating circulating miRNAs in diabetic patients are quite limited. Zampetaki et al. reported deregulation of 12 plasma miRNAs (miR-24, -21, -20b, -15a, -126, -191, -197, -223, -320, -486, -150, and -28-3p) in diabetic subjects, among which miR-126 emerged as a predictor of diabetes mellitus (Zampetaki et al., 2010). A separate study identified 7 upregulated serum miRNAs (miR-9, -29a, -30d, -34, -124, -146a, and -375) in newly diagnosed type 2 diabetic patients as compared to susceptible controls (Kong et al., 2011). Another study identified elevation of miR-144, 192, and 29a in the whole blood of diabetic patients (Karolina et al., 2011), whereas no change was found in miR-126 level which was inconsistent with the report published by Zampetaki et al. (2010). This may be explained by different biosamples detected (plasma vs. whole blood) in these two studies (Zampetaki et al., 2010; Karolina et al., 2011). In addition, miR-503 was found to be enriched in the plasma of diabetic patients with critical limb ischemia (Caporali et al., 2011). However, to date, no specific circulating miRNA has been identified in diabetic cardiomyopathy. The diagnostic and predicted value of circulating miRNAs as biomarkers for diabetic cardiomyopathy remains to be further explored. Taken together, desregulated miRNAs are potentially involved in the etiology and pathogenetic processes of diabetic cardiomyopathy. An in-depth understanding of their functional roles and molecular mechanisms in the development of diabetic cardiomyopathy will provide better prospects to identify sensitive clinical biomarkers and novel therapeutic targets for diabetic cardiomyopathy.

Telocytes in exercise‐induced cardiac growth

Exercise can induce physiological cardiac growth, which is featured by enlarged cardiomyocyte cell size and formation of new cardiomyocytes. Telocytes (TCs) are a recently identified distinct interstitial cell type, existing in many tissues and organs including heart. TCs have been shown to form a tandem with cardiac stem/progenitor cells in cardiac stem cell niches, participating in cardiac regeneration and repair. Although exercise‐induced cardiac growth has been confirmed as an important way to promote cardiac regeneration and repair, the response of cardiac TCs to exercise is still unclear. In this study, 4 weeks of swimming training was used to induce robust healthy cardiac growth. Exercise can induce an increase in cardiomyocyte cell size and formation of new cardiomyocytes as determined by Wheat Germ Lectin and EdU staining respectively. TCs were identified by three immunofluorescence stainings including double labelling for CD34/vimentin, CD34/platelet‐derived growth factor (PDGF) receptor‐α and CD34/PDGF receptor‐β. We found that cardiac TCs were significantly increased in exercised heart, suggesting that TCs might help control the activity of cardiac stem/progenitor cells, cardiomyocytes or endothelial cells. Adding cardiac TCs might help promote cardiac regeneration and renewal.

Desregulated microRNAs in aging-related heart failure

Heart failure is the major cause of death in the western world. Despite the development and use of standard evidence-based therapeutic strategies for heart failure like inhibition of the activity of the β-adrenergic signaling and renin-angiotensin-aldosterone system, the prevalence of heart failure is still increasing, while morbidity and mortality have not been satisfactorily improved (Hofmann and Frantz, 2013). Growing evidence has indicated that the rising incidence of heart failure is substantially associated with age. In the United States, a high proportion of the estimated 5 million heart failure patients are older people, and a vast majority of heart failure-related hospitalization and death occurred in patients over 65 years old (Go et al., 2014). With the tendency of global aging, it is necessary to go deeper in exploring the aging-related heart failure. Cardiac aging is characterized by a series of complex events of ventricle and valvular changes involving left ventricular hypertrophy, diastolic dysfunction, increased risk of atrial fibrillation, valvular degeneration and fibrosis, and decreased maximal exercise capacity. These changes make the aged heart more susceptible to stress, leading to a high prevalence of cardiovascular diseases and heart failure (Correia et al., 2002; Dai et al., 2012a). The mechanisms of progression to heart failure in the aged heart have been previously described. The oxidative stress and mitochondrial damage are responsible for triggering the increased cardiomyocyte death including necrosis, apoptosis and autophagy, accompanied by hypertrophy of remaining cells and impaired structure of extracellular matrix (ECM), thus leading to ventricular remodeling and reduced cardiac contractility (Nadal-Ginard et al., 2003; Sarkar et al., 2004; Lindsey et al., 2006; Dai et al., 2012b; Venkataraman et al., 2013). Meanwhile, cardiac hypertrophy leads to a mismatch in oxygen supply and demand, which contributes to endothelial dysfunction and angiogenesis (Shiojima et al., 2005; Izumiya et al., 2006; Heineke et al., 2007). In response to these chronic stress, the aged heart undergoes a complex pathophysiological changes and finally progresses to symptomatic heart failure (Foo et al., 2005; Dai et al., 2012a). MicroRNAs (miRNAs, miRs) are a novel class of small non-coding RNAs with approximately 20-24 length of base, which function as endogenous suppressors of gene expression through mRNA degradation and/or translational inhibition mainly by binding to 3′-untranslated region (3′-UTR) of target mRNAs (Lim et al., 2005; Van Rooij, 2011). Nowadays over 2000 miRNAs have been identified in human genome and each miRNA can modulate numerous target genes and build complex signaling networks (Kim and Nam, 2006; Liang, 2009). As a center player of gene regulation, many essential biological processes are regulated by miRNAs, including proliferation, apoptosis, necrosis, autophagy, differentiation, and stress responses (Bartel, 2004). Due to these multiple roles, miRNAs are critically involved in the development of multifarious heart diseases, such as heart hypertrophy, arrhythmia, acute myocardial infarction, and heart failure (Latronico and Condorelli, 2009; Xiao et al., 2012, 2014; Fu et al., 2013; Vickers et al., 2014). Several microarray studies have revealed expression profiles of specific miRNAs that are aberrantly expressed in heart failure. MiR-1, -29, -30, -133, and -150 were found to be downregulated in heart failure, whereas miR-21, -23, -27, -125, -132, -146, -195, -199, -214, -223, and 342 were upregulated (Van Rooij et al., 2006; Cheng et al., 2007; Ikeda et al., 2007; Sayed et al., 2007; Tatsuguchi et al., 2007; Thum et al., 2007; Sucharov et al., 2008; Matkovich et al., 2009; Naga Prasad et al., 2009). In addition, several circulating miRNAs including miR-423-5P have been considered as putative biomarkers for heart failure (Tijsen et al., 2010). Some distinguished reviews have summarized it in detail (Elzenaar et al., 2013; Kumarswamy and Thum, 2013; De Rosa et al., 2014; Harada et al., 2014). As we know, cardiac aging is among the predominant risk factors for the development of heart failure (Correia et al., 2002; Dai et al., 2012a). Recent advances suggest that miRNAs may also play a role in the regulation of gene expression in cardiovascular aging processes (Zhang et al., 2012; Olivieri et al., 2013; Menghini et al., 2014). It has been previously demonstrated that 65 miRNAs were differentially expressed in the old versus young mouse adult hearts, approximately half of which belong to 11 miRNA clusters, indicating that these clusters contribute to the complex regulation of gene expression during heart aging (Zhang et al., 2012). In addition, miR-22 was shown to be involved in aging-related cardiac fibrosis, whose overexpression contributed to cellular senescence and migration of cardiac fibroblasts (Jazbutyte et al., 2013). More recently, it was demonstrated that aging-induced expression of miR-34a and inhibition of its target PNUTs lead to increased cardiomyocyte death and reduced cardiac contractility function, by inducing telomere shortening and DNA damage responses (Boon et al., 2013). However, the role of miRNAs in aging-related heart failure is far from elucidated. A previous study showed that the members of miR-17-92 cluster, including miR-18a, -19a, and -19b, were all downregulated in failure-prone heart of aged mice as well as in cardiac biopsies of idiopathic cardiomyopathy patients at old age with severely impaired cardiac function (ejection fraction, EF<30%), accompanied by increased expression of the ECM proteins connective tissue growth factor (CTGF) and thrombospondin-1 (TSP-1). Furthermore, the in vitro studies showed that these expression changes were specific in aged cardiomyocytes but not in cardiac fibroblasts, and the inhibition of miR-18/19 in cardiomyocytes contributed to collagen synthesis (Collagen 1A1 and 1A3) via the regulation of pro-fibrotic CTGF and TSP-1. Although the mechanisms underlying these regulations are still unknown, it provides a close relationship between miR-18/19 and aging-induced cardiac remodeling and heart failure (Van Almen et al., 2011). With the development of the research for roles of miRNAs in aging-related heart failure, its cellular and molecular mechanisms as well as pathophysiological changes will be further clarified, which will help develop novel miRNA-targeted therapeutic strategies for heart failure in aged people.

Cardiac cell proliferation is not necessary for exercise-induced cardiac growth but required for its protection against ischaemia/reperfusion injury

The adult heart retains a limited ability to regenerate in response to injury. Although exercise can reduce cardiac ischaemia/reperfusion (I/R) injury, the relative contribution of cardiac cell proliferation including newly formed cardiomyocytes remains unclear. A 4‐week swimming murine model was utilized to induce cardiac physiological growth. Simultaneously, the antineoplastic agent 5‐fluorouracil (5‐FU), which acts during the S phase of the cell cycle, was given to mice via intraperitoneal injections. Using EdU and Ki‐67 immunolabelling, we showed that exercise‐induced cardiac cell proliferation was blunted by 5‐FU. In addition, the growth of heart in size and weight upon exercise was unaltered, probably due to the fact that exercise‐induced cardiomyocyte hypertrophy was not influenced by 5‐FU as demonstrated by wheat germ agglutinin staining. Meanwhile, the markers for pathological hypertrophy, including ANP and BNP, were not changed by either exercise or 5‐FU, indicating that physiological growth still developed in the presence of 5‐FU. Furthermore, we showed that CITED4, a key regulator for cardiomyocyte proliferation, was blocked by 5‐FU. Meanwhile, C/EBPβ, a transcription factor responsible for both cellular proliferation and hypertrophy, was not altered by treatment with 5‐FU. Importantly, the effects of exercise in reducing cardiac I/R injury could be abolished when cardiac cell proliferation was attenuated in mice treated with 5‐FU. In conclusion, cardiac cell proliferation is not necessary for exercise‐induced cardiac physiological growth, but it is required for exercise‐associated protection against I/R injury.

MicroRNA basis of physiological hypertrophy.

Cardiac hypertrophy is a distinguished feature of several physiological and pathological remodeling (Frey et al., 2004). Pathological hypertrophy is commonly seen in patients with heart injury or stress, such as myocardial infarction, hypertension, and valve disease (Frey et al., 2004). Physiological hypertrophy is typically induced by exercise or pregnancy. Interestingly, physiological hypertrophy is generally accepted as an adaptive beneficial response while pathological hypertrophy can ultimately decompensate to heart failure, a common, costly, disabling, and deadly disease. Thus, dissecting the mechanisms for physiological hypertrophy will help identify novel effective therapies for a large spectrum of cardiovascular diseases (Da Costa Martins and De Windt, 2012). Distinct underlying signaling pathways for physiological and pathological hypertrophy have been identified (Maillet et al., 2013). The classic pathway for physiological hypertrophy is IGF-1/PI3K (p110a)/Akt1 while the key one for pathological hypertrophy is AngII (ET-1)/Gaq/Calcineurin/NFAT (Maillet et al., 2013). MicroRNAs (miRNAs, miRs) are a novel class of non-coding RNAs with 20–24 length of base, which posttranscriptional regulates gene expression via base-pairing with complementary sequences within mRNA (Xiao et al., 2012). It is estimated that over 1000 miRNAs were encoded by the human genome. Individual miRNAs can regulate several target genes while one gene can also be regulated by several miRNAs. Being a center player of gene regulation, miRNAs participate in many essential biological processes, including proliferation, differentiation, apoptosis, necrosis, autophagy, and stress responses (Xiao et al., 2012; Kumarswamy and Thum, 2013). Due to these multiple roles, it is naturally that miRNAs are critical in the development of various heart diseases, such as hypertrophy, heart failure, acute myocardial infarction, and arrhythmia (Xiao et al., 2011; Kumarswamy and Thum, 2013). In addition, circulating miRNAs have also been indicated to be promising biomarkers for cardiovascular diseases (Xu et al., 2012). Among them, pathological hypertrophy is the most widely studied one. Accumulating evidence has indicated that a lot of miRNAs such as miR-1, miR-133, miR-26, miR-9, miR-98, miR-29, miR-199a, miR-199b, miR-208, miR-23a, miR-499, miR-21, and mir-19b contribute to pathological hypertrophy (Da Costa Martins and De Windt, 2012). Some distinguished reviews have summarized it in detail (Da Costa Martins and De Windt, 2012; Ellison et al., 2012). Unlike pathological hypertrophy, only a little studies described how miRNAs response to physiological hypertrophy (Soci et al., 2011; Diniz et al., 2013). It has been reported that miR-1, miR-133, mir-29c, miR-27a, mir-27b, and miR-143 response to physiological hypertrophy (Soci et al., 2011; Da Costa Martins and De Windt, 2012; Ellison et al., 2012). However, these miRNAs are either compensatory or lack of direct evidence for regulating cell size. Thus, these results might only set the beginning of filling the gap in miRNAs and physiological hypertrophy. Although these studies have suggested some potential roles of miRNAs in physiological hypertrophy, more functional studies are highly needed to establish miRNAs as contributors for physiological hypertrophy. Traditionally, the adult mammalian heart is recognized as a post-mitotic organ with no regenerative capacity for cardiomyogenesis (Rosenzweig, 2012). Recently, resident endogenous cardiac stem-progenitor cells (eCSCs) in the adult heart challenged this dogma (Rosenzweig, 2012). Moreover, adult cardiomyocytes have also been reported to proliferate in response to specific stimuli (Rosenzweig, 2012). Two studies regarding physiological hypertrophy are of great importance (Bostrom et al., 2010; Waring et al., 2012). A transcriptional factor named CEBPB has been found to be down-regulated with exercise and reduction of CEBPB induces cardiomyocyte hypertrophy and proliferation (Bostrom et al., 2010). Further studies show that CEBPB promotes cardiomyocyte proliferation via increasing CITED4 (Bostrom et al., 2010). This study indicates that besides the generally accepted idea that physiological hypertrophy is solely due to the hypertrophy of existing cardiomyocytes, physiological hypertrophy also has the phenotype of new cardiomyocytes formation (Bostrom et al., 2010). A more recent study shows that c-Kit positive eCSCs increases their number and activated state in exercise-induced physiological hypertrophy, indicating that c-Kit positive eCSCs might be a source of new cardiomyocyte formation (Waring et al., 2012). Therefore, myocyte-restricted lineage tracing studies are highly needed to definitively unravel this question. Anyway, both studies indicate that it is necessary to check miRNAs roles in promoting new cardiomyocyte formation either in cardiomyocytes or in eCSCs in physiological hypertrophy. With the strategies for checking myocyte hypertrophy and new cardiomyocyte formation, the miRNA basis of physiological hypertrophy will be revealed, which will help develop a miRNA-based therapy for heart failure.

Telocytes in Cardiac Protection

Telocytes (TCs) are a distinct type of stromal cells with extremely thin and long prolongations called telopodes (Tps). TCs have been ubiquitously reported in almost all tissues and organs across species including heart. TCs are distinct from fibroblasts as evidenced by ultrastructural characteristics, immunohistochemistry features, gene profiles, proteome features, and miRNA signatures. By means of heterocellular junctions and extracellular vesicles, TCs may be able to regulate cardiac stem cells, angiogenesis, and anti-fibrosis. Therapeutic effects of cardiac TCs in myocardium infarction have been demonstrated. Cardiac TCs could be a source of cardiac repair and protection.

MicroRNA as a novel player in atrial fibrillation

Atrial fibrillation (AF), with an extremely highly age-dependent prevalence, is a most frequent type of sustained arrhythmia in the clinic both in developing and developed countries (Dobrev and Nattel, 2011). AF patients have a lot of symptoms including palpitations, dizziness, breathlessness, and chest pain. AF will lead to an increased risk of stroke and aggravate congestive heart failure (Shi et al., 2013). Compared with familial AF, the non-familial AF occupies the majority of AF. In China, at least 10 million people are affected by it. Therefore, AF is associated with a remarkable morbidity and mortality, leading to a large socio-economic burden. Unfortunately, the underling mechanisms of AF are still exclusive (Wang et al., 2011; Liu et al., 2012a,b). Several hypotheses have been proposed for AF. Focal activity, single-circuit reentry, and multiple-circuit reentry have been generally considered as three classic models for AF. In addition, different substrates which facilitate the formation of AF have also been revealed including electrical remodeling, structure remodeling, and intracellular Ca2+ handling remodeling. Moreover, various genetic mutations such as chromosomal loci (10q22-q24 and 6q14-16) mutations, ion channel (Ka+ and Na+) mutations, connexin (GJA5 and GJA1) mutations in the familial AF and more recently several common variants on 4q25, 16q22, and 1q21 in the non-familial AF have been identified (Xiao et al., 2011a,b; Liu et al., 2012a,b). Despite this, the definitive elucidation of AF still remains relatively limited and unclear (Zhang et al., 2011). MicroRNAs (miRNAs, miRs) are a novel class of endogenous non-coding RNAs of around 22 nucleotides in length, which are widely accepted to possess a key role in the gene expression regulatory network at the post-transcriptional level (Wang et al., 2011; Fu et al., 2013). So far, at least one thousand miRNAs have been identified and they have been reported to participate in many fundamental biological processes including cell proliferation, growth, differentiation, apoptosis, and tissue remodeling. Dysregulated miRNAs have been shown to be related to the genesis of many cardiovascular diseases, including arrhythmia, hypertrophy, and heart failure. Moreover, miRNAs have also been identified to be necessary for the differentiation of human-derived cardiomyocyte progenitor cells (Xiao et al., 2012). Furthermore, circulating miRNAs can also be served as promising biomarkers for acute myocardial infarction and heart failure etc. (Dimmeler and Zeiher, 2010; Li et al., 2013). Thus, it is not surprising that miRNAs gain a critical position in the physiological and pathological processes of the cardiovascular system (Wang et al., 2011; Xiao et al., 2011a,b; Liu et al., 2012a,b). Several studies have presented interesting connections between miRNAs and AF. Of note, a group of miRNAs has been identified to regulate target genes encoding cardiac ion channels/transporters/Ca2+-handling proteins, which may participate in the genesis of AF (Wang et al., 2011; Dobrev, 2012). Interestingly, many available data have demonstratedthatmiR-1, miR-21, miR-26, miR-29, miR-30, miR-133, miR-208, miR-328, miR-499, and miR-590 might take part in the genesis of AF (Dawson et al., 2013; Shi et al., 2013). Most of these miRNAs promote the electrical or structural remodeling in the atrium. MiR-26 family contributes to AF via repressing the expression of KCNJ2/Kir2.1/IK1 while miRNA-1 via regulating IK1 expression and Ca2+ handling proteins. In addition, miR-499 regulates KCNN3/SK3 while miR-328 targets CACNA1C and CACNB, contributing to adverse electrical remodeling (Lu et al., 2010). Moreover, miR-133 and miR-590 have been found to target transforming growth factor-β 1 (TGF-β 1) and TGF-β receptor type II (TGF-β RII), leading to structure remodeling in AF (Shan et al., 2009). Interestingly, circulating levels of miR-328 and miR-150 have been reported to be down-regulated in AF patients though the functional role is still unclear (Liu et al., 2012a,b; McManus et al., 2014). With more systematic and insightful studies exploring the miRNAs basis for AF, novel therapeutics will be developed and ultimately lead to advanced treatment (Zhao et al., 2013).