The tyrosine kinase FER is responsible for the capacitation-associated increase in tyrosine phosphorylation in murine sperm

Only a subpopulation of mouse sperm displays a rapid increase in intracellular calcium during capacitation

Mammalian sperm must undergo a functionally defined process called capacitation to be able to fertilize oocytes. They become capacitated in vivo by interacting with the female reproductive tract or in vitro in a defined capacitation medium that contains bovine serum albumin, calcium (Ca²⁺ ), and bicarbonate (HCO₃ ^- ). In this work, sperm were double stained with propidium iodide and the Ca²⁺ dye Fluo-4 AM and analyzed by flow cytometry to determine changes in intracellular Ca²⁺ concentration ([Ca²⁺ ]_i ) in individual live sperm. An increase in [Ca²⁺ ]_i was observed in a subpopulation of capacitated live sperm when compared with noncapacitated ones. Sperm exposed to the capacitating medium displayed a rapid increase in [Ca²⁺ ]_i within 1 min of incubation, which remained sustained for 90 min. These rise in [Ca²⁺ ]_i after 90 min of incubation in the capacitating medium was evidenced by an increase in the normalized median fluorescence intensity. This increase was dependent on the presence of extracellular Ca²⁺ and, at least in part, reflected the contribution of a new subpopulation of sperm with higher [Ca²⁺ ]_i . In addition, it was determined that the capacitation-associated [Ca²⁺ ]_i increase was dependent of CatSper channels, as sperm derived from CatSper knockout (CatSper KO) or incubated in the presence of CatSper inhibitors failed to increase [Ca²⁺ ]_i . Surprisingly, a minimum increase in [Ca²⁺ ]_i was also observed in CatSper KO sperm suggesting the existence of other Ca²⁺ transport systems. Altogether, these results indicate that a subpopulation of sperm increases [Ca²⁺ ]_i very rapidly during capacitation mainly due to a CatSper-mediated influx of extracellular Ca²⁺ .

Shedding light on the role of cAMP in mammalian sperm physiology

Mammalian fertilization relies on sperm finding the egg and penetrating the egg vestments. All steps in a sperm's lifetime crucially rely on changes in the second messenger cAMP (cyclic adenosine monophosphate). In recent years, it has become clear that signal transduction in sperm is not a continuum, but rather organized in subcellular domains, e.g. the sperm head and the sperm flagellum, with the latter being further separated into the midpiece, principal piece, and endpiece. To understand the underlying signaling pathways controlling sperm function in more detail, experimental approaches are needed that allow to study sperm signaling with spatial and temporal precision. Here, we will give a comprehensive overview on cAMP signaling in mammalian sperm, describing the molecular players involved in these pathways and the sperm functions that are controlled by cAMP. Furthermore, we will highlight recent advances in analyzing and manipulating sperm signaling with spatio-temporal precision using light.

The actin cytoskeleton of the mouse sperm flagellum is organized in a helical structure

Conception in mammals is determined by the fusion of a sperm cell with an oocyte during fertilization. Motility is one of the features of sperm that allows them to succeed in fertilization, and their flagellum is essential for this function. Longitudinally, the flagellum can be divided into the midpiece, the principal piece and the end piece. A precise cytoskeletal architecture of the sperm tail is key for the acquisition of fertilization competence. It has been proposed that the actin cytoskeleton plays essential roles in the regulation of sperm motility; however, the actin organization in sperm remains elusive. In the present work, we show that there are different types of actin structures in the sperm tail by using three-dimensional stochastic optical reconstruction microscopy (STORM). In the principal piece, actin is radially distributed between the axoneme and the plasma membrane. The actin-associated proteins spectrin and adducin are also found in these structures. Strikingly, polymerized actin in the midpiece forms a double-helix that accompanies mitochondria. Our findings illustrate a novel specialized structure of actin filaments in a mammalian cell.

Signaling pathways involved in human sperm hyperactivated motility stimulated by Zn2

To fertilize the egg, sperm cells must reside in the female reproductive tract for several hours during which they undergo chemical and motility changes collectively called capacitation. During capacitation, the sperm develop a unique type of motility known as hyperactivated motility (HAM). The semen contains Zn²⁺ in millimolar concentrations, whereas in the female reproductive tract, the concentration is around 1 µM. In this study, we characterize the role of Zn²⁺ in human sperm capacitation focusing on its effect on HAM. Western blot analysis revealed the presence of GPR39-type Zn-receptor localized mainly in the sperm tail. Zn²⁺ at micromolar concentration stimulates HAM, which is mediated by a cascade involving GPR39-adenylyl cyclase (AC)-cyclic AMP (cAMP)-protein kinase A-tyrosine kinase Src (Src)-epidermal growth factor receptor and phospholipase C. Both the transmembrane AC and the soluble-AC are involved in the stimulation of HAM by Zn²⁺ . The development of HAM is precisely regulated by cAMP, in which relatively low concentration (5-10 µM) stimulated HAM, whereas at 30 µM no stimulation occurred. A similar response was seen when different concentrations of Zn²⁺ were added to the cells; low Zn²⁺ stimulated HAM, whereas at relatively high Zn²⁺ , no effect was seen. We further demonstrate that the Ca²⁺ -channel CatSper involved in Zn²⁺ -stimulated HAM. These data support a role for extracellular Zn²⁺ acting via GPR39 to regulate signaling pathways in sperm capacitation, leading to HAM induction.

Disruption of protein kinase A localization induces acrosomal exocytosis in capacitated mouse sperm

Protein kinase A (PKA) is a broad-spectrum Ser/Thr kinase involved in the regulation of several cellular activities. Thus, its precise activation relies on being localized at specific subcellular places known as discrete PKA signalosomes. A-Kinase anchoring proteins (AKAPs) form scaffolding assemblies that play a pivotal role in PKA regulation by restricting its activity to specific microdomains. Because one of the first signaling events observed during mammalian sperm capacitation is PKA activation, understanding how PKA activity is restricted in space and time is crucial to decipher the critical steps of sperm capacitation. Here, we demonstrate that the anchoring of PKA to AKAP is not only necessary but also actively regulated during sperm capacitation. However, we find that once capacitated, the release of PKA from AKAP promotes a sudden Ca²⁺ influx through the sperm-specific Ca²⁺ channel CatSper, starting a tail-to-head Ca²⁺ propagation that triggers the acrosome reaction. Three-dimensional super-resolution imaging confirmed a redistribution of PKA within the flagellar structure throughout the capacitation process, which depends on anchoring to AKAP. These results represent a new signaling event that involves CatSper Ca²⁺ channels in the acrosome reaction, sensitive to PKA stimulation upon release from AKAP.

Cryopreservation and egg yolk medium alter the proteome of ram spermatozoa

Cryopreservation causes significant lethal and sub-lethal damage to spermatozoa. In order to improve freezing outcomes, a comprehensive understanding of sub-lethal damage is required. Cryopreservation induced changes to sperm proteins have been investigated in several species, but few have employed currently available state of the art, data independent acquisition mass spectrometry (MS) methods. We used the SWATH LC-MS method to quantitatively profile proteomic changes to ram spermatozoa following exposure to egg yolk and cryopreservation. Egg yolk contributed 15 proteins to spermatozoa, including vitellogenins, apolipoproteins and complement component C3. Cryopreservation significantly altered the abundance of 51 proteins. Overall, 27 proteins increased (e.g. SERPINB1, FER) and 24 proteins decreased (e.g. CCT subunits, CSNK1G2, TOM1L1) in frozen thawed ram spermatozoa, compared to fresh spermatozoa. Chaperones constituted 20% of the proteins lost from spermatozoa following cryopreservation. These alterations may interfere with both normal cellular functioning and the ability of frozen thawed spermatozoa to appropriately respond to stress. This is the first study to apply SWATH mass spectrometry techniques to characterise proteins contributed by egg yolk based freezing media and to profile cryopreservation induced proteomic changes to ram spermatozoa.

SIGNIFICANCE

This study profiles changes to the sperm proteome induced by exposure to egg yolk based media and the process of cryopreservation, and the biological consequences are discussed.

TMEM16A inhibition impedes capacitation and acquisition of hyperactivated motility in guinea pig sperm

Ca²⁺ -activated Cl^- channels (CaCCs) are anionic channels that regulate many important physiological functions associated with chloride and calcium flux in some somatic cells. The molecular identity of CaCCs was revealed to be TMEM16A and TMEM16B (also known as Anoctamin or ANO1 and ANO2, respectively) in all eukaryotes. A recent study suggests the presence of TMEM16A in human sperm and a relationship with the rhZP-induced acrosome reaction. However, to the best of our knowledge, little is known about the role of TMEM16A in other spermatic processes such as capacitation or motility. In this study, we evaluated the effects of two TMEM16A antagonists on capacitation, acrosome reaction, and motility in guinea pig sperm; these antagonists were T16Ainh-A01, belonging to a second generation of potent antagonists of TMEM16A, and niflumic acid (NFA), a well-known antagonist of TMEM16A (CaCCs). First of all, we confirmed that the absence of Cl^- in the capacitation medium changes motility parameters, capacitation, and the progesterone-induced acrosome reaction. Using a specific antibody, TMEM16A was found as a protein band of ∼120 kDa, which localization was in the apical crest of the acrosome and the middle piece of the flagellum. Inhibition of TMEM16A by T16Ainh-A01 affected sperm physiology by reducing capacitation, blocking the progesterone-induced acrosome reaction under optimal capacitation conditions, inhibiting progressive motility, and the acquisition of hyperactivated motility, diminishing [Ca²⁺ ]i, and increasing [Cl^- ]i. These changes in sperm kinematic parameters provide new evidence of the important role played by TMEM16A in the production of sperm capable of fertilizing oocytes.

CatSper: A Unique Calcium Channel of the Sperm Flagellum

To overcome egg protective vestments and ensure successful fertilization, mammalian spermatozoa switch symmetrical progressive motility to a powerful, whip-like flagellar motion, known as hyperactivation. The latter is triggered by a calcium influx through the sperm-specific, voltage-dependent, and alkalization-activated calcium channel of sperm - CatSper. The channel comprises nine subunits which together form a heteromeric complex. CatSper-deficient male mice and men with mutations in CatSper genes are infertile. This calcium channel is regulated by various endogenous compounds, such as steroids, prostaglandins, endocannabinoids, and intracellular pH. Being a sperm-specific ion channel that is not expressed anywhere else in the body, CatSper represents an ideal target for the development of female and even male contraceptives. In this review, we discuss the recent advances in studying CatSper functional properties and discuss future steps that are required to take in order to achieve a deep understanding of the molecular basis of CatSper function.

SRC family kinases in hamster spermatozoa: evidence for the presence of LCK

Sperm capacitation is a prerequisite for successful fertilization. Increase in tyrosine phosphorylation is considered the hallmark of capacitation and attempts to understand its regulation are ongoing. In this regard, we attempted to study the role of SRC family kinases (SFKs) in the hamster sperm functions. Interestingly, we found the presence of the lymphocyte-specific protein tyrosine kinase, LCK, in mammalian spermatozoa and further characterized it in terms of its localization and function. LCK was found in spermatozoa of several species, and its transcript was identified in the hamster testis. Autophosphorylation of LCK at the Y394 residue increased as capacitation progressed, indicating an upregulation of LCK activity during capacitation. Inhibition of LCK (and perhaps the other SFKs) with the use of a specific inhibitor showed a significant decrease in protein tyrosine phosphorylation of several proteins, implying LCK/SFKs as key tyrosine kinase(s) regulating tyrosine phosphorylation during hamster sperm capacitation. Dihydrolipoamide dehydrogenase was identified as a substrate for LCK/SFK. LCK/SFKs inhibition significantly reduced the percentage fertilization (in vitro) but had no effect on sperm motility, hyperactivation and acrosome reaction. In summary, this is the first report on the presence of LCK, an SFK of hematopoietic lineage in spermatozoa besides being the first study on the role of SFKs in the spermatozoa of Syrian hamsters.

Computational flow cytometry reveals that cryopreservation induces spermptosis but subpopulations of spermatozoa may experience capacitation-like changes

The reduced lifespan of cryopreserved spermatozoa in the mare reproductive tract has been attributed to both capacitative and apoptotic changes. However, there is a lack of studies investigating both phenomena simultaneously. In order to improve our knowledge in this particular point, we studied in raw and frozen-thawed samples apoptotic and capacitative markers using a wide battery of test based in flow cytometry. Apoptotic markers evaluated were caspase 3 activity, externalization of phosphatidylserine (PS), and mitochondrial membrane potential. Markers of changes resembling capacitation were membrane fluidity, tyrosine phosphorylation, and intracellular sodium. Conventional and computational flow cytometry using nonlinear dimensionally reduction techniques (t-distributed stochastic neighbor embedding (t-SNE)) and automatic classification of cellular expression by nonlinear stochastic embedding (ACCENSE) were used. Most of the changes induced by cryopreservation were apoptotic, with increase in caspase 3 activation (P < 0.01), PS translocation to the outer membrane (P < 0.001), loss of mitochondrial membrane potential (P < 0.05), and increase in intracellular Na+ (P < 0.01). Average values of markers of capacitative changes were not affected by cryopreservation; however, the analysis of the phenotype of individual spermatozoa using computational flow cytometry revealed the presence of subpopulations of spermatozoa experiencing capacitative changes. For the first time advanced computational techniques were applied to the analysis of spermatozoa, and these techniques were able to disclose relevant information of the ejaculate that remained hidden using conventional flow cytometry.

Analysis of the effects of polyphenols on human spermatozoa reveals unexpected impacts on mitochondrial membrane potential, oxidative stress and DNA integrity; implications for assisted reproductive technology

The need to protect human spermatozoa from oxidative stress during assisted reproductive technology, has prompted a detailed analysis of the impacts of phenolic compounds on the functional integrity of these cells. Investigation of 16 individual compounds revealed a surprising variety of negative effects including: (i) a loss of mitochondrial membrane potential (Δψm) via mechanisms that were not related to opening of the permeability transition pore but associated with a reduction in thiol expression, (ii) a decline in intracellular reduced glutathione, (iii) the stimulation of pro-oxidant activity including the induction of ROS generation from mitochondrial and non-mitochondrial sources, (iv) stimulation of lipid peroxidation, (v) the generation of oxidative DNA damage, and (vi) impaired sperm motility. For most of the polyphenolic compounds examined, the loss of motility was gradual and highly correlated with the induction of lipid peroxidation (r=0.889). The exception was gossypol, which induced a rapid loss of motility due to its inherent alkylating activity; one consequence of which was a marked reduction in carboxymethyl lysine expression on the sperm tail; a post-translational modification that is known to play a key role in the regulation of sperm movement. The only polyphenols that did not appear to have adverse effects on spermatozoa were resveratrol, genistein and THP at doses below 100μM. These compounds could, therefore, have some therapeutic potential in a clinical setting.

Focal adhesion kinase is required for actin polymerization and remodeling of the cytoskeleton during sperm capacitation

Several focal adhesion proteins are known to cooperate with integrins to link the extracellular matrix to the actin cytoskeleton; as a result, many intracellular signaling pathways are activated and several focal adhesion complexes are formed. However, how these proteins function in mammalian spermatozoa remains unknown. We confirm the presence of focal adhesion proteins in guinea pig spermatozoa, and we explore their role during capacitation and the acrosome reaction, and their relationship with the actin cytoskeleton. Our results suggest the presence of a focal adhesion complex formed by β1-integrin, focal adhesion kinase (FAK), paxillin, vinculin, talin, and α-actinin in the acrosomal region. Inhibition of FAK during capacitation affected the protein tyrosine phosphorylation associated with capacitation that occurs within the first few minutes of capacitation, which caused the acrosome reaction to become increasingly Ca²⁺ dependent and inhibited the polymerization of actin. The integration of vinculin and talin into the complex, and the activation of FAK and paxillin during capacitation, suggests that the complex assembles at this time. We identify that vinculin and α-actinin increase their interaction with F-actin while it remodels during capacitation, and that during capacitation focal adhesion complexes are structured. FAK contributes to acrosome integrity, likely by regulating the polymerization and the remodeling of the actin cytoskeleton.

Summary: We describe the role of FAK and focal adhesion proteins in capacitation, acrosome reaction, polymerization and remodeling of actin cytoskeleton, and how inhibition of FAK affects sperm physiology.