Sperm cells are equipped with a limited repertoire of behaviors that exclusively subserve their purpose to fertilize eggs (mature ova). When produced in the testis, sperm are immotile; they acquire the ability to swim as they transit through the epididymal tract of mammals or after ejaculation from invertebrates. But motility alone is not sufficient to direct a sperm to an egg. The egg itself (or associated structures) must “lure” the sperm by releasing diffusible chemotactic factors and/or through ligand/receptor interactions. After mammalian sperm become motile, they mature (a process called Capacitation) and are able to fertilize eggs within the female reproductive tract. Sperm and egg initially interact through surface receptors, and then the proteolytic contents of the sperm’s acrosomal vesicle are released. This proteolytic cocktail helps the sperm to penetrate the outer coat of the egg and to reach the egg’s plasma membrane (fertilization) (Ref.1 & 2). Human spermatozoa are broadly divided into head and tail (flagellum) segments. Tail segment is further sub-divided into middle piece, principal piece and end piece. The basic parts of human spermatozoa include cell membrane, acrosome, nucleus, centriole, redundant nuclear envelope, mitochondria and axoneme (Ref.3).
In many animals and lower plant groups, sperm are guided by chemoattractants released from eggs. Chemoattractant signals that initiate sperm chemotaxis are synthesized and secreted by the egg or, in some cases, by somatic cells associated with the egg, and are believed to exert influence over short distances to enhance the efficiency of sperm-egg contact. In species with internal fertilization, the need for such precise chemotactic mechanisms seems to be less vital, as gamete encounter is apparently easier in a “closed” system such as the female internal genital apparatus (Ref.2). However, sperm chemotaxis evidently exists in mammals, and the acquisition of chemotactic responsiveness is likely to represent a part of the activation process that the spermatozoa of mammals undergo in their trip through the female genital tract. The phenomenon seems to have been conserved through to mammals, where sperm attractants may occur in human ovarian follicular fluids. The three principal events in sperm mobilization are motility (and chemotaxis), capacitation, and the acrosome reaction-each depend on the intracellular cyclic nucleotides; cAMP (Cyclic Adenosine 3,5-Monophosphate) and cGMP (Cyclic Guanosine 3,5-Monophosphate) (Ref.4 & 5).
In this regard, intracellular Ca2+ (Calcium), K+ (Potassium), Na+ (Sodium) and Cl- (Chloride) levels also control the waveform asymmetry of mammalian sperm flagellar beating and acrosomal reaction. It is an established fact that only about 1 in 25,000 spermatozoa seminated into the vagina reaches the fallopian tubes and, the existence of some communication at distance between the gametes positively affects their chance of meeting and, finally, the chance of fertilization. The main physiological inducer of the mammalian sperm acrosomal reaction is the zona pellucida (Ref.1). Three sulfated glycoproteins (ZP1 (Zona Pellucida Glycoprotein-1 (Sperm Receptor), ZP2 and ZP3) principally constitute zona pellucida. ZP3 exhibits most of the sperm binding and acrosomal reaction-inducing activity. Both protein and carbohydrate regions of ZP3 appear to be involved in its acrosomal reaction-inducing activity. The main candidates on sperm membrane that act as primary receptors for ZP3 includes Zan (Zonadhesin)/Spermadhesins and B4GALT (UDP-GAL:Beta-GlcNAc Beta-1,4-Galactosyltransferase Polypeptide). Upon binding these activate several G-proteins, such as GN-AlphaI (Guanine Nucleotide Binding Protein-Alpha Inhibiting Activity Polypeptide) and GN-AlphaZ (Guanine Nucleotide Binding Protein (G-Protein)-Alpha-Z Polypeptide) present in mammalian sperm. This in turn induces acrosomal reaction and release of H+ (Hydrogen) ion fluxes (H+ Transporter) associated with it, thereby increasing intracellular pH (pHi). Increase in pHi generates T-type Ca2+ currents (through activation of T-Type Calcium channels) that trigger acrosomal reaction and hyperactivation by catalysis of acrosomal enzymes (Ref.6).
The concept of chemotaxis as part of the Capacitation process in mammals is well understood, and although the substance(s) (Female/Egg factors) responsible for sperm chemotaxis in mammals is yet to be confirmed, it seems likely that AC(Adenylate Cyclase) and GC (Guanylate Cyclase) activation is an essential part of the process. AC and GC are the enzymes that synthesize cAMP and cGMP from ATP (Adenosine Triphosphate) and GTP (Guanosine Triphosphate), respectively. AC and GC occur in both soluble and membrane bound forms, however the soluble forms predominantly occur in mature spermatozoa. Bicarbonate ions (that pass through Anion Transporters) directly activate the soluble form of Adenylyl Cyclase (sAC), whereas, Female/Egg factors stimulate activation of sAC by release of Calcium ions through CatSper (Sperm Associated Cation Channels), localized in sperm flagellum, the principal piece of the sperm tail. Membrane bound ACs (mACs) generate cAMP following interaction of the agonist, ZRK (Zona Receptor Tyrosine Kinase)/SPRMTK (Sperm Protein Zona Receptor Tyrosine Kinase) with ZP3. This increases protein tyrosine phosphorylation and autophosphorylation of the receptor that stimulates G-Protein mediated signal transduction to activate AC and PLC (Phospholipase-C) (Ref.7 & 8). Similarly, extracellular cues like ANP (Atrial Natriuretic Peptides) and Nitric Oxide (NO)/Carbon Monoxide (CO) activate mGCs (Membrane bound GCs) and sGCs (Soluble GCs), respectively to synthesize cGMP. The concomitant increase of both cyclic nucleotides (cAMP and cGMP) leads to a cross-talk between the cAMP and cGMP signaling pathways, a phenomenon facilitated in many tissues by the presence of cGMP-regulated PDE (Phosphodiesterase) isoforms. Of the known PDE isoforms, PDE3 is inhibited by the binding of cGMP to its active site. Increases in cGMP level evoke a concomitant increase in cAMP by inhibiting its PDE3-catalyzed hydrolysis of cAMP to 5’AMP. An increase in Ca2+ level also catalyzes cAMP hydrolysis by activating PDE1, PDE2 and PDE4 (Ref.9 & 10).
Activation of these enzymes (AC, GC and PLC) leads to increased generation of the second messengers like cAMP, cGMP, IP3 (Inositol Trisphosphate) and DAG (Diacylglycerol). PLC generates IP3 and DAG by cleaving PIP2 (Phosphatidylinositol 4,5-bisphosphate). IP3 binding to IP3R(IP3 Receptor) increase intracellular Ca2+ by liberation of the ion from intracellular Ca2+ stores. A consequence of the increase of second messengers is the activation of protein kinases such as PKA (cAMP-dependent Protein Kinase-A), PKC (Ca2+ and Phospholipid-dependent Protein Kinase-C) and PKG (cGMP-Dependent Protein Kinase). These kinases in turn facilitate gating of ion channels which includes T-Type Calcium channels, HCn (Hyperpolarization-Activated Cyclic Nucleotide-Gated Potassium Channel), KCn (Potassium Channel), and CNG (Cyclic-Nucleotide Gated Ion Channel) along with the activation of tyrosine kinases like the STKs (Serine/Threonine Protein Kinase) and PTKs (Protein-Tyrosine Kinases) with increased protein phosphorylation, that has an important concomitant role in human ZP-induced acrosome reaction (Ref.9 & 10). Acrosome reaction is also primed by Progesterone and GABA (Gamma-Aminobutyric Acid). Progesterone trigger human sperm responses by interacting with GABA(A) Receptor and along with its metabolites enhance the interaction of GABA with the GABA(A) Receptor, regulating Cl- fluxes. Chloride ions increase membrane potential to enhance influx of Na+, K+ and Ca2+. Ca2+-dependent activation of Calm (Calmodulin), PLA2 (Phospholipase-A2) and PLD (Phospholipase-D) (with increased generation of other second messengers as Arachidonic Acid, Lysophosphatidylcholine and Phosphatidic Acid from membrane phospholipids) occur during acrosome reaction. Elevations of sperm Na+, K+, Ca2+, Cl- and H+ together with increased protein phosohorylation induces signaling pathways that brings about the capacitation response and is responsible for an wave form asymmetry of motility called hyperactivation, which first occurs during the time of fertilization. Hyperactivation and acrosomal reaction enhances flagellar beating, ultimately resulting in the penetration of outer egg coat and subsequent fertilization of mature ovum (Ref.6).
Hyperactivated motility is critical to the process of fertilization, allowing spermatozoa to reach the oocyte through the narrow, mucus-filled, labyrinthine lumen of the oviduct and assisting spermatozoa in penetrating the outer coat of oocytes (Ref.6). The signal initiating hyperactivation in the oviduct has not been identified; however, ion channels and second messengers play a cardinal role in the dialogue between gametes and in the generation of a new individual in many species. They are critical elements in the signaling pathways to convert symmetrical bending to the asymmetrical bending that is characteristic of hyperactivation. Determining how hyperactivation is initiated in the oviduct can provide valuable information in the diagnosis and treatment of infertility or in the development of new methods of contraception that prevent spermatozoa from reaching the oocyte/ovum (Ref.1 & 11).