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Major regulatory mechanisms involved in sperm motility

The genetic bases and molecular mechanisms involved in the assembly and function of the flagellum components as well as in the regulation of the flagellar movement are not fully understood, especially in humans. There are several causes for sperm immotility, of which some can be avoided and corrected, whereas other are related to genetic defects and deserve full investigation to give a diagnosis to patients. This review was performed after an extensive literature search on the online databases PubMed, ScienceDirect, and Web of Science. Here, we review the involvement of regulatory pathways responsible for sperm motility, indicating possible causes for sperm immotility. These included the calcium pathway, the cAMP-dependent protein kinase pathway, the importance of kinases and phosphatases, the function of reactive oxygen species, and how the regulation of cell volume and osmolarity are also fundamental components. We then discuss main gene defects associated with specific morphological abnormalities. Finally, we slightly discuss some preventive and treatments approaches to avoid development of conditions that are associated with unspecified sperm immotility. We believe that in the near future, with the development of more powerful techniques, the genetic causes of sperm immotility and the regulatory mechanisms of sperm motility will be better understand, thus enabling to perform a full diagnosis and uncover new therapies.

Ca 2+ sensor, and the calmodulin-dependent kinase may mediate this Ca 2+ signal. These complexes, localized at the sperm axoneme, are regulated by the central pair complex and radial spokes. Calmodulin regulates motility through direct interaction with protein kinases, 17 phosphatases, 18 and sAC (Figure 1). 7,8 Bicarbonate (HCO 3 − ) ions also play a critical role in the regulation of sperm function (Figure 1). It is an anion of the female reproductive tract transported into sperm during capacitation. An increase in Ca 2+ stimulates sAC; it converts ATP to cAMP and increases cAMP levels. 19,20 As HCO 3 − is required for Ca 2+ uptake, it causes the same effects. The treatment in vitro with HCO 3 − evokes Ca 2+ entry, which rapidly increases flagellar beat frequency but decreases flagellar beat asymmetry. 21 As a result, serine/threonine PKA is activated, which then phosphorylates serine and threonine residues on neighboring proteins to trigger a cascade of protein phosphorylation events. 22 The presence of proteins in the fibrous sheath (FS) with PKA anchoring sites strongly suggests that one of the major roles of this structure is to anchor PKA in the principal piece of the flagellum. Cyclic AMP promotes both capacitation and the acrosome reaction and activates PKA (Figure 1). [22][23][24] PKA subunits are expressed differentially. The regulatory subunit RIα is expressed throughout male germ cell development, RIIα only appears at the late stages in spermatogenesis, and the catalytic subunit Cα2 is only expressed in sperm. It is believed that the activation of PKA increases flagellar beat frequency and tyrosine  3 − levels. 167 The activation of the sperm Na + /H + exchanger (sNHE) aligned with the activation of the proton channel (Hv1) leads to a raise of the pH i , which activates CatSper, a cation channel of sperm that enables the entry of Ca 2+ and thus increases the internal Ca 2+ concentration ([Ca 2+ ]). 168 Progesterone, a steroid hormone synthesized by the cumulus/granulosa cells, activates CatSper either by binding to the channel itself or to an associated protein. 27 Further, albumin, the main protein of human blood plasma and oocyte glycoproteins, together with alkalinization of the sperm cytoplasm also, elevates the internal [Ca 2+ ]. 169,170 The overall Ca 2+ increase may influence glycolysis and the axoneme activity promoting hyperactivation of motility. 171 Further, HCO 3 − and Ca 2+ regulate the atypical soluble adenylyl cyclase (sAC), which generates cAMP and that by its turn activates protein kinase A (PKA). PKA induces phosphorylation of axonemal dynein, leading to consumption of ATP and thus increases the pH i . PKA activates sperm tyrosine kinases (with serine and threonine residues) to trigger a cascade of protein phosphorylation involved in sperm motility. 24,35,47 Increased cAMP may activate PKA that in turn activates tyrosine kinase and seems to inhibit tyrosine phosphatase. 172 The Ca 2+ levels are regulated by a plasma membrane Ca 2+ -ATPase pump (PMCA4), expressed in the principal piece of the axoneme, which extrudes Ca 2+ and is essential for hyperactivated motility and male fertility. 173

Figure 2:
The increase in protein tyrosine phosphorylation during capacitation has been shown to be regulated by a cAMP-dependent pathway involving protein kinase A (PKA), receptor tyrosine kinase pathway, and by the nonreceptor protein tyrosine kinase pathway. 35 cAMP has been shown to activate PKA, which in turn regulates protein tyrosine phosphorylation. 48 The binding of PKA regulatory subunit to the AKAP family of proteins promotes an increase in tyrosine phosphorylation of sperm proteins by indirect activation of Tyrosine kinases (TKs). 174 In human sperm, AKAPs proteins, namely AKAP3 and FSP95, are the most prominent tyrosine phosphorylated proteins during capacitation. Receptors TK is transmembrane proteins having an extracellular ligand binding domain and an intracellular tyrosine kinase domain. Upon extracellular ligand binding, a receptor TK are activated and then phosphorylates it (autophosphorylation) or other proteins. By contrast, nonreceptor protein TK lacks a transmembrane domain, most are soluble intracellular proteins located in the cytoplasm, nucleus, or anchored to the inner leaflet of the plasma membrane. Tyrosine and protein phosphorylation of the sperm flagellar proteins leads to capacitation of human sperm. 47,175,176 phosphorylation to prepare the capacitated sperm for fertilization. 22 PKA localizes at the principal piece of the flagellum, and Cα2 null males are completely infertile. 25,26 Several Ca 2+ -permeable-specific channels have been found in sperm based on immunostaining or on the presence of transcripts in spermatogenic cells, such as high voltage-gated Ca 2+ channels, cyclic nucleotide-gated channels, cation channels of sperm (CatSper), and transient receptor potential channels (Figure 1). These are a family of alkalinization-activated cation channels (CATSPER-1-4) that are highly conserved in humans. They are the principal Ca 2+ channels activated by progesterone in human sperm. 27 Mutations in these channels were associated with human infertility and also suggested as a target for development of a male contraceptive. [28][29][30][31] Thus, it is likely that Ca 2+ plays different roles in distinct stages of the sperm journey.
Phosphorylation is essential in almost every aspect of the cell life, and protein kinases are known to regulate important signaling pathways and cellular processes such as transcription, cell-cycle progression, cell movement, apoptosis, and immunological functions. Protein kinases share a conserved catalytic domain that transfers a phosphate group from ATP and covalently attaches it to specific amino acids with a free hydroxyl group, frequently on both serine and threonine amino acids (serine/threonine kinases). Calcium is important to activate the kinase through limited proteolysis by Ca 2+ -dependent protease. 32 Phosphorylation (Figures 1 and 2) substantially contributes to proper functioning of sperm proteins, 33,34 and it seems to be a necessary prerequisite for a sperm to fertilize an oocyte. [35][36][37][38] During capacitation, it was detected an increase in the phosphotyrosine content of human FS proteins, 39 which makes evident involvement of protein tyrosine phosphorylation in the control of sperm motility.
In human sperm, the A-kinase anchoring proteins (AKAPs) (AKAP3 was formerly called FSP95), Ca 2+ -binding and tyrosine phosphorylation-regulated protein (CABYR), which is localized in the FS, are the most prominent tyrosine phosphorylated proteins during capacitation. [40][41][42][43] Immotile sperm with deficiency in tyrosine phosphorylation do not capacitate properly, and this has been related to altered sperm membrane lipid composition, particularly due to high cholesterol content, which would impair the ability of this sperm to respond to capacitation-inducing stimuli. [44][45][46] Sperm protein phosphorylation is highly regulated, and there are several pathways involved. 34,35,47 The cAMP-dependent pathway (Figures 1 and 2) can also regulate protein tyrosine phosphorylation by stimulation of PKA activity, because PKA activates some intermediate tyrosine kinases involved in sperm motility. 20,48 It was demonstrated that the presence and activity of a kinase (PI 3-kinase) in human sperm and its inhibition results in an increase in intracellular cAMP levels and in tyrosine phosphorylation of the protein AKAP3. This results in the binding of PKA to AKAP3, which is important for motility. These results provide a confirmation that PKA can be targeted to sperm tails by interaction with tyrosine phosphorylated form of AKAP3. 49 AKAP scaffolding proteins are thus very important in regulating sperm motility as they sequester enzymes, such as protein kinases and phosphatases with the appropriate substrates to coordinate phosphorylation and dephosphorylation events. 50 In humans, AKAP3 and AKAP4 are the most abundant structural FS proteins that anchor cAMP-dependent PKA. 43

The role of reactive oxygen species in the acquisition and control of sperm motility
Reactive oxygen species (ROS), such as the superoxide anion (O 2 − ), hydrogen peroxide (H 2 O 2 ), and nitric oxide (NO − ), are chemically reactive molecules resulting from oxygen consumption. At certain concentrations, ROS are of extreme importance to sperm function. 51,52 It was shown that O 2 − triggers hyperactivation and capacitation, since the presence of superoxide dismutase (enzyme that catalyzes the dismutation of O 2 − into oxygen and H 2 O 2 ) blocks both events. 53,54 Other studies also gave evidence of the involvement of ROS on sperm function by demonstrating that low levels of NO − induce capacitation and that at higher levels, it blocks sperm motility. 55,56 Thus, low levels of O 2 − are required to the capacitation process with H 2 O 2 acting as an inductor of the acrosome reaction with high O 2 − levels being deleterious for sperm function 57 and higher H 2 O 2 levels adversely affect sperm motility parameters. 58 The cAMP/PKA pathway is also dependent of ROS. 52,59 ROS are naturally originated in the human ejaculate: (a) sperm by themselves from spontaneous production through the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase system at the level of the sperm plasma membrane, [60][61][62][63] or by the natural production of ROS by mitochondria, which is considered the main source of ROS in spermatozoa; 64,65 (b) from leukocytes infiltrated into semen. 66 When ROS production is heightened or a compelling reduction in the effectiveness of antioxidant defenses arises, an imbalance between ROS production and the biological system's aptitude to withdraw or repair the ROS damage occurs, which is known as oxidative stress (Figure 3). This can be caused by several factors. 67 Cytoplasmic droplets (excess of residual cytoplasm) are a result of defective spermiogenesis and a considerable source of ROS. 68 Leukocytospermia is also positively correlated with an increase in ROS production. 69,70 Genitourinary tract infections, 71 chronic inflammation, 72 and pathologies such as varicocele 73 induce ROS production and contribute to sperm immotility. The lifestyle, including smoking, 74 dietary deficiencies, 75 excessive alcohol consumption, 76 psychological stress, 77 the contact with environmental pollutants, 78 and age, 79 are all positively correlated with oxidative stress and linked with sperm immotility. Thus, increased ROS levels have been correlated with decreased sperm motility and with the proportion of various sperm head and tail anomalies, 80,81 and some hypotheses have been proposed for this correlation (Figure 3).
One hypothesis is that ROS inhibit the activity of some enzymes such as glucose-6-phosphate dehydrogenase (G6PD), which through the hexose monophosphate pathway controls the intracellular availability of NADPH. Inhibition of G6PD leads to a decrease in the availability of NADPH and a parallel accumulation of oxidized glutathione and reduced glutathione. This leads to a reduction in the antioxidant defenses of the sperm and peroxidation of membrane phospholipids. 62,82 Another way of injury may be due to the fact that high ROS levels induce a cascade of events that result in a decrease in axoneme protein phosphorylation and sperm immobilization. 83 Advantages of this effect of ROS in sperm function are being used to develop contraceptives. Besides the direct influence on sperm motility, the increase of ROS is also related with an increase of DNA damage. 84,85 The control of cell volume and osmolarity is fundamental for sperm motility The maintenance of a correct cell volume and osmolarity is vital. During maturation and at ejaculation, sperm experiences great changes in its environment, namely rapid changes in the osmotic environment, once the osmolarity of cauda epididymidal fluid (osmol: 342 mmol kg −1 ) is higher than the contents of uterus (osmol: 284 mmol kg −1 ). 86,87 When sperm encounters hypo or hypertonic environments, they tend to swell or shrink owing to the influx or efflux of water during reestablishment of the osmotic equilibrium. To maintain cell functionality in face of these osmotic changes, sperm possesses volume regulatory abilities, particularly regulatory volume decrease in response to hypotonic challenge (Figure 4). 88 Defects in the mechanisms of volume regulation and in the epididymal osmolyte uptake cause an abnormal increase in sperm head volume and angulation of sperm tail that leads to an alteration of movement patterns, compromising forward progression resulting in defects in sperm motility and fertility. 86 The cytoplasmic droplet found at the midpiece of some sperm is a portion of excessive residual cytoplasm that is normally lost during the final maturation phase of spermiogenesis. The normal connecting piece and upper midpiece contain a small portion of cytoplasm with a few endoplasmic reticulum vesicles. The midpiece is the major site of water influx and cell volume regulation, and these vesicles are important when sperm face hypo-osmotic challenges. 89 The cytoplasmic droplet is indeed, really important for sperm function given that spermatozoa without it were immotile due to a defective spermatogenesis. 90 In addition to cytoplasmic droplet, it has been reported that sperm osmolytes, namely glutamate and K +91 and K + and Cl − channels are involved in mechanisms of sperm regulatory volume decrease. 92,93 Calcium is also known to be involved in the regulation of cell volume, namely by the activation of Ca 2+ -dependent K + channels (Figure 4). 94,95 Ejaculated sperm is immersed in the seminal plasma, a medium composed of aliquots of the fluid of the testis, epididymal tail, and the secretions of the accessory sexual glands. It also contains a wide variety of factors that influence the functionality of sperm. Sperm motility is also negatively influenced by seminal osmolarity, as patients with normal motility exhibit a mean value of semen osmolarity significantly lower (Ca 2+ : 3.36 mmol l −1 ; osmol: 318 mmol kg −1 ) than that of patients with low sperm motility (Ca 2+ : 3.10 mmol l −1 ; osmol: 345 mmol kg −1 ). 96 Seminal plasma proteins are also considered modulators of sperm function and several important biological roles have been attributed to them. 97 The striking reduction of cell volume is one of the most distinct morphological changes during the differentiation of spermatids into sperm and is largely due to osmotically driven fluid efflux. Aquaporins (AQP) may be involved in the rapid reduction of spermatid volume during spermiogenesis, the final step of spermatogenesis. 98 In humans, it was detected the presence of the water transport protein AQP3 in the principal piece of ejaculated sperm, 99 and of AQP7 in the tail of spermatids and testicular spermatozoa, as well as at the midpiece and the anterior flagellum portion of ejaculated sperm. 100 The AQP3 was shown to be essential for sperm volume regulation, which is important for the balance between sperm motility and swelling in response to physiological hypotonicity, since AQP3-deficient sperm exhibited hampered migration in the oviduct, resulting in reduced male fertility. 99 Regarding the AQP7 expression, it was observed that its absence in ejaculated sperm of some infertile patients was directly correlated with motility rate. 100

Molecular abnormalities and associated flagellar sperm structure
Normal sperm morphology is one of the most informative semen parameters used for infertility diagnosis, 101 It is correlated with poor sperm motility, 102 as an isolated event was not associated with a decreased probability of pregnancy. 103 Abnormalities in the sperm structure can occur as a single defect or attain different sperm components. 104 In the large majority of the cases, the ultrastructural analysis of immotile sperm reveals nonspecific flagellar anomalies that include disruption of the normal axoneme pattern in association with other components of the sperm tail. 105 Specific defects are however found in dysplasia of FS (DFS) and primary ciliary dyskinesia (PCD).
DFS is characterized by a marked hypertrophy and hyperplasia of the FS. Typically, the annulus is not formed, and the abnormal FS invades the midpiece. It is also frequent to observe absence of the central pair complex and dynein arms (DA) (Figure 5). 106,107 It has been estimated that about 20% of DFS cases have a familiar incidence and family tree analysis seems to indicate an autosomic recessive inheritance. 108,109 However, there are no consensus if DFS is a genetic disorder. Indeed, no association between DFS and defects in genes that code for AKAP3 and AKAP4 proteins were found. [110][111][112] Regarding PCD, it is a genetic, heterogeneous, and autosomal recessive disease that is characterized by cilia immotility due to absence of DA (Figure 5), resulting in recurrent infections of the upper respiratory tract. Absence or dislocation of the central pair complex, defects of radial spokes, and doublet abnormalities are also common. In about 50% of PCD cases, patients present Kartagener syndrome, which is characterized by the combination of situs inversus. 106,107 Therefore, investigations into the genetic basis of PCD have started by analysis of DA proteins. 113 However, nowadays, several genes associated with flagellar structures are known to be associated with PCD. 114 The sperm flagellum is a highly complex structure with several molecular components that are responsible for its assembly, composition, and function (Figure 6). The molecular composition of the human flagellum is still not completely understood and what is known came mainly from the study of model species such as mouse, 115 marine invertebrates, and protists. 116 As motor of sperm, the axoneme is one of the most studied structures. Due to the high complexity of sperm, any alteration in external and/or internal factors regulating sperm motion as well as in the cellular structure and metabolism involved in generating flagellar beat may result in defects in sperm motility, which consequently results in male infertility. In humans, a strict association between mutations in some genes and alteration in sperm motility is not simple to Figure 4: Transmembrane ion transport proteins and mechanisms thought to be involved in the regulatory volume decrease (RVD) of sperm. Reduction of cell volume is characterized by H 2 O removal via aquaporins (blue tubules) due to K + and Cl − efflux via K + and Cl − channels and co-transporters (red tubules) and coupled Cl − /HCO 3 − exchangers (brown tubules). 86,92,94,177 Protein kinase-C (PKC) and tyrosine kinase (TK) are believed to be involved in the signaling sequence that leads to deactivation of the regulatory mechanism by closing and keeping closed the anion channel (inactive state). Inhibition of PKC, probably by dephosphorylation of the residues phosphorylated by PKC, which results in activation of the channel, increases the isotonic cell volume. Protein phosphatase 1 (PP1) may be involved in the signaling sequence that leads to the activation of the regulatory mechanism, possibly by the opening the ion channels (active state) and thus reducing the cell volume. The inhibition of PP1 results in blocked RVD. The TK and tyrosine phosphatase (TP) are believed to be involved in the activation and regulation of PP1 activity. Besides PP1, PKC, TK, and TP regulation is also assumed that cAMP-dependent pathway and Ca 2+ channels (not represented) are also involved in regulation of sperm volume. Under hypotonic conditions, forskolin, a potent stimulator of soluble adenylyl cyclase (sAC), activates sAC that in turn leads to an increase in cAMP levels. This increase of cAMP levels may cause an activation of the mechanism of RVD (opening the ion channels). By contrast, activation of phosphodiesterase, an enzyme belonging to a group of enzymes that degrades the phosphodiester bond in the second messenger molecules cAMP, results in decrease of cAMP levels with opposite effects. 178  make due to the high number of variables. Consequently, a list of genes and its relation to sperm motility are still scarce. Nevertheless, due to the degree of conservation of many genes, some, isolated or associated with syndromes, have already been proved to be responsible for some cases of human infertility associated with poor sperm motility. 114,117 Dyneins are motor proteins that convert the chemical energy contained in ATP into the mechanical energy of movement. The outer and inner DA are composed of heavy chains (HCs), intermediate chains (ICs), and light chains (LCs). 116,118 The HC contains the motor machinery that is responsible for transducing chemical energy into directed mechanical force applied to the microtubule surface as it possesses the sites of both ATP hydrolysis and ATP-sensitive microtubule binding; the IC participates in the structural attachment of the DA to flagellar microtubules; and the LC participates in several functions, such as redox-sensitive vicinal dithiols, Ca 2+ -binding, and intraflagellar transport. The variety of structure and function of these chains indicates that many regulatory mechanisms are present and needed for the proper sperm motility. 118,119 Multiple dynein genes are found in the genomes of organisms with motile cilia and flagella. 120 At least, five human genes are known to encode for outer DA HC genes such as DNAH5 (dynein, axonemal, and heavy chain 5), DNAH8, DNAH9, DNAH11, and DNAH17. Relative to the inner DA, there are eight human genes such as DNAH1, DNAH2, DNAH3, DNAH6, DNAH7, DNAH10, DNAH12, and DNAH14. The intermediate and light chains are thought to contain at least five genes, including DNAI1 (dynein, axonemal, and intermediate chain 1), DNAI2, DNAL1 (dynein, axonemal, and light chain 1), DNAL4, DNALI1, and NME8 (NME/NM23 family member 8) (National Center for Biotechnology Information-NCBI-database-accessed in October 2014). As these chains regulate DA activity, mutations may result in abnormal ciliary ultrastructure and function and were already associated with syndromes such as PCD. 114,117 Another essential structure for sperm function is the dynein regulatory complex (DRC), which functions for dynein regulation and limitation of doublet sliding. 121 Some components of the DRC serve primarily to regulate DA activity while others play a role in mediating structural interactions between the DA and the radial spokes. 122,123 Four genes were identified as components of the DRC in humans such as DRC1 (dynein regulatory complex subunit 1), DRC7, CCDC39 (coiled-coil domain containing 39), and CCDC40.
The radial spokes and central pair complex are also essential for sperm function since they are important regulators of DA. Mutations that disrupt assembly of the central pair complex generally result in abnormal motility. 124,125 Radial spokes and central pair complex are involved in converting simple symmetric bends into the asymmetric waveforms required for forward swimming and in the release of ATP inhibition in a controlled manner. 126,127 In addition, the central pair may function as a distributor to provide a local signal to the radial spokes that selectively activates subsets of DA. 128 In humans, it has been already described at least seven radial spokes proteins and its encoding genes are RSPH1 (radial spoke-head 1 homolog), RSPH3, RSPH4A, RSPH6A, RSPH9, RSPH10B, and RSPH10B2 (NCBI and UniProt databases, accessed July 2014). One of the most known radial spokes genes is the RSPH1 gene that encodes a radial spoke-head protein that is mainly expressed in respiratory and testis cells. It is important for the proper building of the central pair complex and radial spokes, since mutations in RSPH1 lead to an abnormal axoneme configuration with central pair complex and radial spokes defects, 129,130 whereas mutations in RSPH4A and RSPH9 were associated with anomalies in central pair complex. 125 Using next generation sequencing, mutations in CCDC39 and CCDC40 genes were also found among individuals with PCD with IDA and central pair complex defects. 131 As mitochondria provide part of the energy for motility, dysfunctions of the human mitochondrial sheath as well as of mitochondrial membrane integrity represent the main feature of sperm immotility. 65,132 Mitochondrial DNA (mtDNA) mutations/deletions might have several implications to male fertility. [133][134][135][136] The integrity and copy number of mtDNA were significantly correlated with sperm count and motility, as they were related to an increase of excessive ROS formation through increased lipid peroxidation in men presenting large-scale mtDNA deletions. [135][136][137][138][139] In humans, the lack of the annulus causes a disorganization of the midpiece-principal piece junction with associated sperm immotility, altogether with mitochondrial structural and functional disability. 140 Septins (SEPT) are essential structural components of the human annulus. 141 It was shown that SEPT4 an SEPT12 are essential for the structural and mechanical integrity of sperm, including proper mitochondrial architecture and establishment of the annulus, 141,142 and SEPT7 was shown to be involved in the regulation of sperm morphology and maturation. 143 In patients with sperm immotility and annulus defects, a defective labeling for SEPT4 and/or SEPT7 was observed, and these proteins were suggested as biomarkers for monitoring the status of spermiogenesis and sperm quality. 144

PREVENTIVE AND THERAPEUTIC APPROACHES TO IMPROVE SPERM MOTILITY
There is no present treatment to sperm immotility due to genetic causes. However, the quality of sperm, which includes sperm motility, can be protected. As discussed above, unhealthy lifestyle habits (recreational toxics, physical inactivity, and excessive use of personal technologies), specific toxic environmental exposures, and several pathologies related to endocrine and cardiovascular diseases are correlated with sperm oxidative stress and can be totally avoided. [145][146][147][148][149][150] Other conditions that also increase sperm oxidative stress can be treated by surgery (varicocele) 73 by the use of corticosteroids (presence of anti-sperm antibodies following chronic genital tract inflammations) [151][152][153] and the correct and timely use of antibiotics for genital tract infections. [154][155][156] Based on the knowledge that the human body developed an antioxidant system to keep ROS at an optimum level, several antioxidants have been used to improve sperm motility both in healthy 75 and infertile men. [157][158] For instance, Vitamin E is a potent peroxyl radical scavenger that functions as a chain-breaking antioxidant. This prevents the propagation of free radicals in membranes and plasma lipoproteins (prevents lipid peroxidation), and decreases the levels of malondialdehyde (an organic compound that is used as a marker for oxidative stress), 159 thus improving sperm motility. 158,160 Vitamin C (L-ascorbic acid) acts as a reducing agent by donating electrons to various enzymatic reactions. It protects against oxidative stress behaving as a scavenger of ROS (prevents lipid pexoxidation). In addition, by recycling Vitamin E, it protects against DNA damage induced by the H 2 O 2 radical. This molecule is also widely used in preventive treatments. 159 Another example is Coenzyme Q 10 (ubiquinone), which is a component of the electron transport chain in the mitochondrial respiratory chain. The energy generated is dependent on its availability in the human body. It is also an antioxidant that acts by stabilizing membranes and recycling Vitamin E. It is currently used for treatment of sperm immotility, especially in idiopathic asthenozoospermia. 161 Hormonal agents and sperm vitalizers can also be used to improve sperm motility. 162 For instance, pentoxifylline, a phosphodiesterase inhibitor, was shown to increase sperm motility 163,164 by interfering with the metabolism of cAMP. 165 However, although the use of these agents has been positively correlated with sperm motility, 166 they can cause adverse effects. 155 Consequently, more studies are needed to determine the optimal doses for each compound and establish a solid link with the desired effects.

CONCLUSIONS
This review has explored a little of the complex process underlying sperm motility, which has several pathways and genes involved. In a well-designed process, a minimal alteration may lead to male infertility. Besides preventive measures and some empirical therapies, there is the urgent need for developing a safe and directed therapy based on the genetic causes of sperm immotility and on the pathways that govern sperm motility.

AUTHOR CONTRIBUTIONS
RP Literature search; data analysis and interpretation; text writing. RS Critical discussion; manuscript critical review. AB Patient samples; critical review of the manuscript. MS Study conception and design; data analysis and interpretation; final text writing. All authors read and approved the final manuscript.

COMPETING INTERESTS
All authors declared no competing financial interests.