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MicroRNAs (miRNAs) are small, endogenous, non-coding RNA molecules involved in the post-transcriptional regulation of many genes1 (Friedman et al. 2009). Initially, genes encoding miRNAs are transcribed in the nucleus via the activation of RNA polymeraseII/III and managed through a series of biogenesis steps2 (Shomron & Levy 2009) which has been extensively reviewed3, 4 (Bartel 2004; Rutnam et al. 2013). In the initial step, the primary transcript (pri-miRNA) with characteristic 5′ m7G cap structure and 3′ poly(A) tail is created5 (Wahid et al. 2010).The pri-miRNA is then cleaved to a pre-miRNA in two stages by the action of two RNase III-type proteins: Drosha in the nucleus and Dicer in the cytoplasm, where pre-miRNA is transported to by Exportin-5 protein. In the cytosol, miRNA duplexes are created. The less stable of the two strands in the duplex is incorporated into a multiple-protein nuclease complex, the RNA-induced silencing complex (RISC) and Argonaute (Ago) subfamily proteins, which regulate protein expression5, 6 (Wahid et al. 2010; Gregory et al. 2006). The mechanism of action of the miRNA is associated with the post-transcriptional regulation of gene expression, depending on the degree of complementarity with the target site, often, but not exclusively, situated in the 3′- untranslated region (UTR) of the mRNA, at least 12 nucleotides from the stop codon7 (Matoulkova et al. 2012). If the target region is fully complementary, protein Ago2 (RISC complex component) can cleave the target mRNA molecule leading to its degradation. In the case of partial target mRNA complementarity, the interaction is performed on the basis of inhibition of translation8 (Nakanishi 2016). Thus, the interaction between the miRNA and its target mRNA may occur in two ways: target mRNA degradation or translational repression, or sometimes via combination of both processes. If the target region is fully complementary, protein Ago2 (RISC complex component) can cleave the target mRNA molecule leading to its degradation. Furthermore, miRNAs are able to either be excreted out into the circulation via microparticles or be membrane-free, bound to either protein complexes or high-density lipoproteins9, 10 (Creemers et al. 2012; Bayraktar et al. 2017). Although precursors of mature or immature miRNAs have been observed outside of the cell and in the circulation, little is known about the mechanism of miRNA release outside of the cell, into the bloodstream as well as its stability during these processes. The biogenesis and mechanisms of miRNA action are summarised in Figure 1. Figure 1. MicroRNA biogenesis and mechanisms of action (Guo et al. 201011, modified). MicroRNAs (miRNAs) are initially transcribed by polymerase II (Pol II) as primary-miRNA (pri-miRNA) transcripts which are processed by Drosha to generate pre-miRNAs. DGCR8 binds to Drosha, an RNase III enzyme, to form the Microprocessor complex that cleaves a primary transcript known as pri-miRNA to a characteristic stem-loop structure known as a pre-miRNA, which is then further processed to miRNA fragments by the enzyme Dicer. TRBP is a double strand RNA binding protein (dsRBP) that is required for the recruitment of Ago2 to the small interfering RNA (siRNA) bound by Dicer.  Pre-miRNAs are exported from nucleus to cytoplasm by exportin 5 (EXPO5). The Dicer complex is recruited to pre-miRNAs to remove the stem loop from pre-miRNAs, and then mature miRNAs which is one strand of the miRNA duplex are incorporated into RNA-induced silencing complex (RISC). Within the RISC, miRNAs bind to complementary sequences of target mRNAs to repress their translation or induce their degradation.Currently, in the human genome, over 700 miRNAs are described as important regulators of various cellular processes, via many signaling pathways, – including development, proliferation and apoptosis12 (Bueno & Malumbres 2011). Studies have indicated that miRNAs may be expressed as tissue-specific, and alteration of their expression may be reflective of many pathological processes13 (Ardekani et al. 2010). Additionally, some miRNAs are released into the systemic circulation resulting from the normal physiological responses to activities such as physical effort. This collective response is further influenced by the prevailing immune response together with the skeletal and heart muscle fitness modulation under activity14, 15 (Bi et al. 2009; Eisenberg et al. 2007). Recently, it was suggested that changes in the miRNA expression profile within in the systemic circulation can serve as molecular markers for the physiological adaptive response of the body to physical activity16 (Polakovi?ová 2016).The aim of this review is to underline the importance of miRNAs in the process of adaptation to physical effort since a relationship is emerging between miRNA expression profiles and particular signaling pathways activated during exercise.2. MicroRNAs and physiologically important pathways in physical activity  MicroRNAs play an imperative role in the maintenance of healthy cellular physiology, mainly via inhibition of the expression of target genes or their protein translation9, 17, 18 (Creemers et al. 2012; Humphreys et al. 2005; Pillai et al. 2005). Recently, miRNAs have been shown to play an important regulatory role in several signaling pathways, specifically pathways implicated in the response and adaptation to physical effort during training19 (Silva et al 2017).Among them, the most important pathway, the IGFI/PI3K/AKT/mTOR signaling pathway, is involved in the regulation of cell muscle growth, proliferation, differentiation, survival, and skeletal protein synthesis20 (Schiaffino & Mammucari 2011). Insulin-like growth factor 1 (IGF1) is  a hormone with autocrine and paracrine functions, acting as both a mitogen and a differentiation factor which has been implicated in the mitogenic and myogenic processes during muscle development, regeneration and hypertrophy21 (Philippou et al. 2007). Numerous studies have identified the members of the various signaling pathway as key factors involved in the regulation of muscle protein metabolism, in particular Protein Kinase B (AKT), its phosphorylation (pAKT) and the Forkhead box (FOXO) family of transcription factors22 (Hannenhalli & Kaestner 2009). AKT plays a key role in multiple cellular processes such as glucose metabolism, apoptosis, cell proliferation, transcription and cell migration23 (Altomare et al. 2012) whereas the  FOXO family of transcription factors  play important roles in regulating the expression of genes involved in cell growth, proliferation, differentiation, and longevity24 (Martins et al. 2016). In muscles, FOXO1 and FOXO3 regulate the expression of E3 ubiquitin-protein ligase TRIM63 and muscle atrophy F-Box protein (MAFbx), which are critical for skeletal muscle atrophy (metabolism)25 (Datta et al. 1999). The IGFI/PI3K/AKT/mTOR signaling pathway is activated by many factors under various conditions such as growth, glucose homeostasis regulators and muscle atrophy/hypertrophy factors secreted especially during resistance exercise training20, 26 (Schiaffino & Mammucari 2011; Velloso 2008). Generally, the binding of IGF1 to IGF1R/insulin receptors (IRS) results in muscle protein synthesis and thereby regulates muscle tissue mass20 (Schiaffino & Mammucari 2011). Current evidence suggests that initiation of IGF-I/PI3K/AKT/mTOR signaling can work under activation of Ser-Thr phosphatidylinositol-regulated kinase 3 (PI3K), which produces phosphatidylinositol-3,4,5-triphosphate (PIP3). This signaling pathway induces activation of AKT and subsequently glycogen synthase kinase 3 (GSK3) phosphorylation as downstream targets20 (Schiaffino & Mammucari 2011). Using animal models, AKT expression was shown to be necessary for increasing the size of myofibrils27, 28, 29, 30 (Lai et al. 2004; Izumiya et al. 2008; Mammucari et al. 2007; Blaauw et al. 2009). The activity of IGFI/PI3K/AKT/mTOR signaling is however controlled by several feedback loops including the FOXO transcription factor family or the influence of kinase mTOR2, mechanistic-target-of-rapamycin complex 2, on protein degradation or synthesis20, 31 (Schiaffino & Mammucari 2011; Greer et al. 2007). Furthermore, the activity of the IGFI/PI3K/AKT/mTOR pathway can be controlled by a diversity of factors, including integrin-linked kinase (ILK) 32 (Wang et al. 2008) or myostatin, also called growth and differentiation factor 8 (GDF8), which acts as negative regulator of muscle growth33 (McPherron et al. 1997).Of note, 5’AMP-activated protein kinase (AMPK) is an important element participating in the regulation of the IGFI/PI3K/AKT/mTOR signaling34, 35 (O’Neill & Hardie 2013; O’Neill 2013).  Studies have demonstrated that AMPK is a key regulator of carbohydrate (glucose uptake and glycogen synthesis) and lipid (fatty acid uptake and oxidation) metabolism34, 36 (O’Neill & Hardie 2013; Watt et al. 2006). In addition, it influences mitochondrial biogenesis37 (Fernandes et al. 2006) and insulin sensitivity during or following exercise34, 35 (O’Neill & Hardie 2013; O’Neill 2013).  The role of AMPK in the control of muscle fibre size through mTOR inhibition, via the switching off of mTOR activity, has also been recognized38, 39 (Gwinn et al. 2008; Vissing et al. 2013). Furthermore, as a “metabolic sensor”, AMPK directly regulates peroxisome proliferator-activated receptor-gamma co-activator 1 alpha (PGC-1?) activity through phosphorylation and deacetylation40 (Cantó & Auwerx 2009). In this way, AMPK may influence mitochondrial biogenesis important in aerobic exercise and energy utilization. PGC-1? is a transcriptional co-activator that controls energy homeostasis and mitochondrial biogenesis through interaction and activation of NRF-1, NRF-2, PPAR? and ERR?: these are regulators of mitochondrial DNA expression, fatty acid ?-oxidation, and tricarboxylic acid cycle41 (Baar et al. 2002). Moreover, PGC-1? controls the mitochondrial electron transport chain, thereby influencing cellular AMP:ATP and NAD+:NADH homeostasis41, 42, 43 (Baar et al. 2002; Pilegaard et al. 2003; Perry et al. 2010).  Interestingly, recent studies have shown that IGF1/PI3K/AKT/mTOR signaling pathway is directly and/or indirectly regulated by many families of miRNAs, namely miR-1, miR-21, miR-23a, miR-124, miR-125b, miR-133a/b, miR-144, miR-145, miR-206, miR-486 and miR-69644 (Wang et al. 2016) (see Figure 2).  Expression levels of all mentioned miRNAs may fluctuate, increase or decrease, depending on the regulation of a particular biological process involved in physical training. Figure 2. Schematic representation of miRNAs involved in the regulation of theIGF1/PI3K/AKT and AMPK/PGC-1?/mTOR signaling pathways. IGF -1 functions as a ligand to interact with IGF -1 receptor (IGF -1R) in the cellular membrane, which leads to autophosphorylation and recruitment of the adaptor proteins IRS-1, IRS-2, and Shc. The interaction of IRS-1 and IRS-2 with IGF -1R induces the activation of the class I phosphatidyl inositol 3′ kinase (PI3K). PI3K converts PIP2 to the lipid second messenger PIP3. AKT family of kinases is activated by PDK1 and by mTOR-containing complex mTORC2 resulting in the phosphorylation at Threonine 308 (Thr308) and Serine 473 (Ser473), respectively. Activated AKT then regulates downstream signaling molecules including Tuberous sclerosis protein 1/2 (TSC1/2) which inhibit mTORC1 complex and regulate S6K1/2 and 4EB-P1 phosphorylation, FOXO transcription factors, GSK-3?. MiR-1, miR-21, miR-23a, miR-124, miR-125b, miR-133a/b, miR-144, miR-145, miR-206, miR-486 and miR-696 regulate IGF1/PI3K/AKT/mTOR signaling pathway. Modified from 45, 46, 47 (Nielsen  et al. 2010; Russell et al. 2013; Wang et al. 2014;modified).MicroRNA-1 and-133a/bMicroRNA-1 and -133a belong to the family of muscle-specific miRNAs (myomiRs)48, 49 (McCarthy & Esser 2007; Callis et al. 2008), which play a central role in the regulation of myogenesis50 (Ge et al. 2011). This type of miRNAs is able to modulate fiber type I/II synthesis and muscle mass regulation in response to activity and is thus important for skeletal muscle plasticity  Predominantly, miR-133a is a key factor for proliferation and differentiation of cultured myoblasts in vitro51 (Chen et al. 2006). A study, conducted in a mouse model, has confirmed that miR-1 and miR-133a/b expression is strongly modified during multiple biological processes specific to skeletal muscle, including growth, development, maintenance, atrophy and hypertrophy48 (McCarthy & Esser 2007). The expression levels of miR-1 and miR-133a/b are significantly increased during myogenesis50 (Ge et al. 2011), while reduced in mice and human muscle tissue during the growth of muscle mass (skeletal muscle hypertrophy) in response to resistance exercise training48, 52 (McCarthy & Esser 2007; Drummond et al. 2008). Moreover, cardiac hypertrophy has been inversely correlated with the expression of miR-153 (Care et al. 2007). Studies focused on the role of miR-1 in cardiac and skeletal muscle have shown that miR-1 expression level is decreased in mouse with cardiac hypertrophy, while IGF-1 protein level is significantly increased53, 54 (Care et al. 2007; Elia et al. 2009). MicroRNA-1 and -133a are proposed to contribute to muscle hypertrophy by the removal of their transcriptional inhibitory effect on growth factors and their receptors, such as IGF1/IGFR55, 56 (Adams et al. 1999; Huang et al. 2011) or inhibition of HSP70 thereby influencing AKT. Therefore, it has been proposed that miR-1 and -133a target IGF1/IGFR in the IGF1/PI3K/AKT pathway where their reduced expression correlates with signaling pathway activation. In addition, there is a functional feedback loop between IGF-1and AKT/FOXO354 (Elia et al. 2009). Reduced IGF1 protein level increases the level of miR-1 via FOXO3a57 (Rao et al. 2006). It has also been documented that miR-1 and miR-133a/b may control skeletal muscle myogenesis and regeneration, mainly via influence on genes encoding myoblast precursors: proteins belonging to the myogenic regulatory factors (MRFs), such as MyoD, a protein which plays a major role in regulating muscle differentiation, myogenin, a transcription factor involved in the coordination of skeletal muscle development or myogenesis and repair, Myf5, a protein which regulates muscle differentiation or myogenesis – specifically the development of skeletal muscle, and MRF4, a myogenic regulatory factor in the process myogenesis56, 58 (Huang et al. 2011; Sabourin & Rudnicki 2000).MicroRNA-206Like miR-1 and -133, miR-206 belongs to the family of muscle-specific miRNAs which play a central role in myogenesis. The expression of miR-206 is restricted to skeletal myoblasts and cardiac tissue during embryonic development and muscle cell differentiation, which suggests a regulation by MRFs59 (Sweetman et. al 2008). MicroRNA-206 promotes differentiation of primary line of murine myoblasts C2C12 cells60 (Kim et al. 2006) and participates in skeletal muscle regeneration following injury in mice61 (Liu et al. 2012). It is also described as a regulator of myotube width62 (Johnson et al. 2013), but gain or loss of miR-206 function does not disturb skeletal muscle size in vivo63 (Winbanks et al. 2013). On the other hand, in tilapia, inhibition of miR-206 in skeletal muscle promotes body growth with an increase in IGF1 expression64 (Yan et al. 2013). It is possible that direct or indirect regulatory effect of miR-206 expression may contribute to the above, for example myostatin, FOXO1 and atrogin-1 are muscle atrophy regulators65, 66 (Bodine et al. 2001; Kim & Kim 2012). In particular, decreased miR-206 expression was linked to upregulation of myostatin mRNA67 (Allen et al. 2009).MicroRNA-124 MicroRNA-124 may regulate cell cycle protein levels via the TSC2/PI3K/AKT/mTOR pathway in many types of tissues68 (Moss et al. 2015). This in vitro study performed in mice suggests that miR-124 may be a negative regulator of myogenic differentiation of mesenchymal stem cells (MSCs). Downregulation of miR-124 expression by myogenic stimuli may be essential for the progression of myogenic differentiation69 (Qadir et al. 2014). This study performed on animal model, demonstrates that myogenic stimuli significantly decrease the expression level of miR-124 in bone marrow-derived MSCs and C2C12 cells. Moreover, expression of miR-124 represses the expression of certain myogenic marker genes such as: Myf5, Myod1, myogenin and myosin heavy chain and multinucleated myotube formation69 (Qadir et al. 2014).MicroRNA-125bMicroRNA-125b and its paralogue miR-125a, have been described as a homologue of Caenorhabditis elegans lin-4, the first miRNA to be discovered70 (Lee et al. 1993). It has also been documented as a negative regulator of myoblast differentiation and muscle regeneration on the kinase-independent mTOR signaling level50 (Ge et al. 2011). This regulation takes place on the transcriptional as well as the post-transcriptional level71 (Erbay et al. 2003). The study performed by Ge et al. (2011)50 show that miR-125b biogenesis is negatively controlled by kinase mTOR. Moreover, decreased miR-125b expression was shown to regulate IGF-2 mRNA level through targeting lin-28 and/or regulating distinct muscle-enriched transcription factors50 (Ge et al. 2011). Using a cardiomyocyte model, Lozano-Velasco et al. (2015)72 confirmed that miR-125 can selectively upregulate and downregulate various target mRNAs in a cell-type specific. Overexpression of miR-125 results in a selective upregulation of Mef2d in HL1 atrial cardiomyocytes. During cardiac muscle development there is an increased expression of miR-125 which is postulated to play an essential role in stem cell differentiation72 (Lozano-Velasco et al. 2015). Interestingly, it has been shown that miR-125 protects the myocardium from myocardial ischaemia/reperfusion (I/R) injury47 (Wang et al. 2014). Furthermore, miR-125b targets p53 in stress-induced apoptosis in different cell types73 (Le et al. 2009). In mice, miR-125 inhibits p53-mediated apoptotic signaling and suppresses TRAF6-mediated nuclear factor kappa-light-chain-enhancer of activated B cells (NF-?B) activation47 (Wang et al. 2014). Moreover, miR-125b may activate neutrophils and macrophages after physical stimulation; therefore the removal of damaged muscle tissue during exercise is possible. Simultaneously, growth factors involved in repair processes are secreted74 (Chaudhuri et al. 2011). In a study focused on miR-125b expression in Ewing’s sarcoma, a rare cancer, Cui et al. (2012)75 observed that its overexpression may inhibit expression level of PIK3CD, by targeting its 3′-UTR directly which in turn suppresses phosphorylation of downstream genes, AKT and mTOR. These results suggest that miR-125b suppresses tumor growth activity by targeting the PI3K/ Akt/mTOR signaling pathway, and may therefore provide a target for effective therapies.MicroRNA- 21MicroRNA-21 plays a protective role in myocardial ischemia-reperfusion (I/R) injury through the “phosphatase and tensin homolog deleted on chromosome ten” (PTEN)/AKT signaling pathway in animal model. MicroRNA-21 is abnormally expressed (upregulated) in ischemic mouse hearts in response to I/R injury as a protective effect of miR-21 on myocardial apoptosis76 (Tu et al. 2013). MiRNAs are expressed at a constant level under physiological conditions; however, the endogenous expression levels of miRNAs are often altered in response to the physiological and pathological stimuli, tissue injury or milieu intérieur disorders. More recently, studies show that miR-21 has a protective role in hearts in response to I/R injury77, 78 (Cheng et al. 2010; Yin et al. 2009). Moreover, miR-21 is involved in the vascular remodeling that affects both vascular-smooth-muscle-cell (VSMC) and vascular endothelial cells (VEC). Abnormal VSMC and VEC proliferation is thought to play an important role in the pathogenesis of both atherosclerosis and restenosis. These pathological vascular smooth muscle cell conditions are related to the production of ROS, which lead to VSMC apoptosis and death. A recent study identified, that the treatment of VSMCs with hydrogen peroxide increases miR-21 expression in a dose-dependent manner and causes an anti-apoptotic effect on the cells79 (Lin et al. 2009). The overexpression of miR-21 induces an artificial VSMC phenotype as observed after vascular injury, besides being a critical miRNA for angiogenesis80 (Hartmann et al. 2011). These studies suggest that miR-21 modulation could be a therapeutic option to improve the function of diverse vascular cells.MicroRNA-23aMicroRNA-23a can protect muscles from glucocorticoid-induced skeletal muscle atrophy81 (Wada et al. 2011). It suppresses translation of MAFbx and TRIM63, which are critical for muscle-protein breakdown, depending on AKT phosphorylation and downregulation of FOXO factors82 (Wang et al. 2014). The high expression of TRIM63 and MAFbx simulates the degradation of MyoD. Consequently, muscle regeneration is impaired by this mechanism. However, miR-23a upregulation may inhibit muscle-specific protein degradation via downregulation of TRIM63 and MAFbx82 (Wang et al. 2014). In muscle these are regulated via the interaction of miR-23a with FOXO at the 3?-UTRs. MicroRNA-144MicroRNA-144 has been reported as a regulator of cell proliferation, survival and cell migration, senescence, and aging via suppression of the natural inhibitor of the PI3K/AKT pathway, PTEN, which main function is to limit cell proliferation83 (Dong et al. 2014). It has been reported that miR-144 is involved in regulation of insulin signaling pathway, as a key modulator of IRS184 (Karolina et al. 2011). Its expression correlates with down-regulation of its target, IRS1, at both mRNA and protein levels. As a result, miR-144 is postulated as an indicator of insulin resistance, which shows an approximately linear relationship with increasing glycemic state84 (Karolina et al. 2011). Additionally, an increased expression of miR-144 was observed in muscle cells of older rhesus monkeys as an age-associated biological modification and its levels were dampened after caloric restriction85 (Mercken et al. 2013). Muscle degeneration involves different biological processes such as mass and strength reduction, inflammation and reduced ability to cell regeneration86 (Frontera et al. 2012). The upregulation of miR-144 in aging muscles should be further investigated as a potential marker of muscle degeneration.MicroRNA-145It is predicted that miR-145 targets the tuberous sclerosis 2 gene (TSC2)87 (Sachdeva et al 2012), encoding a protein also known as tuberin but whose function has not been fully characterised. Tuberin may interact with a hamartin protein (TSC1) which is involved in cell growth and size control88, 89 (Huang et al. 2008; Gan et al. 2005). Interestingly in muscle tissue, upregulation of miR-145 is accompanied by decreased TSC2 gene expression after exercise90 (Ma et al. 2013).  Moreover, miR-145 and miR-143 can negatively regulate the insulin signaling pathway91 (Nielsen et al. 2014).MicroRNA-486A decreased activity of the IGF1/PI3K/AKT signaling pathway by stimulation of the ubiquitin proteasome system and concurrent activation of the FOXO transcription factor family may accelerate muscle protein degradation92, 93 (Sandri et al. 2004; Small et al. 2010). It has been observed that increased expression of miR-486 enhances the activity of the abovementioned pathway through a direct impact on the inhibition of negative pathway regulators such as PTEN and FOXOs 56, 93 (Small et al. 2010; Huang et al. 2011).MicroRNA-696MicroRNA-696 may play an important role in regulation of peroxisome proliferator-activated receptor – ? coactivator -1 ? (PGC-1 ?; PPAR- ?), which is a key regulator of mitochondrial biogenesis and respiratory cell function94, 95 (Oishi et al. 2008; Russell et al. 2003). According to a study by Aoi et al. (2010) in mice96, miR-696 is involved in transcriptional regulation of PGC-1? in skeletal muscle in response to physical activity. Thus, MiR-696 is postulated to be a physical activity-dependent regulator of metabolic adaptation to exercise.

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