Calmodulin antagonists inhibit T-type Ca(2+) currents in mouse spermatogenic cells and the zona pellucida-induced sperm acrosome reaction

The pair ceramide 1-phosphate/ceramide kinase regulates intracellular calcium and progesterone-induced human sperm acrosomal exocytosis

Before fertilization, spermatozoa must undergo calcium-regulated acrosome exocytosis in response to physiological stimuli such as progesterone and zona pellucida. Our laboratory has elucidated the signaling cascades accomplished by different sphingolipids during human sperm acrosomal exocytosis. Recently, we established that ceramide increases intracellular calcium by activating various channels and stimulating the acrosome reaction. However, whether ceramide induces exocytosis on its own, activation of the ceramide kinase/ceramide 1-phosphate (CERK/C1P) pathway or both is still an unsolved issue. Here, we demonstrate that C1P addition induces exocytosis in intact, capacitated human sperm. Real-time imaging in single-cell and calcium measurements in sperm population showed that C1P needs extracellular calcium to induce [Ca2+]i increase. The sphingolipid triggered the cation influx through voltage-operated calcium (VOC) and store-operated calcium (SOC) channels. However, it requires calcium efflux from internal stores through inositol 3-phosphate receptors (IP3R) and ryanodine receptors (RyR) to achieve calcium rise and the acrosome reaction. We report the presence of the CERK in human spermatozoa, the enzyme that catalyzes C1P synthesis. Furthermore, CERK exhibited calcium-stimulated enzymatic activity during the acrosome reaction. Exocytosis assays using a CERK inhibitor demonstrated that ceramide induces acrosomal exocytosis, mainly due to C1P synthesis. Strikingly, progesterone required CERK activity to induce intracellular calcium increase and acrosome exocytosis. This is the first report, implicating the bioactive sphingolipid C1P in the physiological progesterone pathway leading to the sperm acrosome reaction.

Mouse sperm membrane potential hyperpolarization is necessary and sufficient to prepare sperm for the acrosome reaction

Mammalian sperm are unable to fertilize the egg immediately after ejaculation; they acquire this capacity during migration in the female reproductive tract. This maturational process is called capacitation and in mouse sperm it involves a plasma membrane reorganization, extensive changes in the state of protein phosphorylation, increases in intracellular pH (pH(i)) and Ca(2+) ([Ca(2+)](i)), and the appearance of hyperactivated motility. In addition, mouse sperm capacitation is associated with the hyperpolarization of the cell membrane potential. However, the functional role of this process is not known. In this work, to dissect the role of this membrane potential change, hyperpolarization was induced in noncapacitated sperm using either the ENaC inhibitor amiloride, the CFTR agonist genistein or the K(+) ionophore valinomycin. In this experimental setting, other capacitation-associated processes such as activation of a cAMP-dependent pathway and the consequent increase in protein tyrosine phosphorylation were not observed. However, hyperpolarization was sufficient to prepare sperm for the acrosome reaction induced either by depolarization with high K(+) or by addition of solubilized zona pellucida (sZP). Moreover, K(+) and sZP were also able to increase [Ca(2+)](i) in non-capacitated sperm treated with these hyperpolarizing agents but not in untreated cells. On the other hand, in conditions that support capacitation-associated processes blocking hyperpolarization by adding valinomycin and increasing K(+) concentrations inhibited the agonist-induced acrosome reaction as well as the increase in [Ca(2+)](i). Altogether, these results suggest that sperm hyperpolarization by itself is key to enabling mice sperm to undergo the acrosome reaction.

Modulation of low-voltage-activated T-type Ca²⁺ channels

Low-voltage-activated T-type Ca²⁺ channels contribute to a wide variety of physiological functions, most predominantly in the nervous, cardiovascular and endocrine systems. Studies have documented the roles of T-type channels in sleep, neuropathic pain, absence epilepsy, cell proliferation and cardiovascular function. Importantly, novel aspects of the modulation of T-type channels have been identified over the last few years, providing new insights into their physiological and pathophysiological roles. Although there is substantial literature regarding modulation of native T-type channels, the underlying molecular mechanisms have only recently begun to be addressed. This review focuses on recent evidence that the Ca(v)3 subunits of T-type channels, Ca(v)3.1, Ca(v)3.2 and Ca(v)3.3, are differentially modulated by a multitude of endogenous ligands including anandamide, monocyte chemoattractant protein-1, endostatin, and redox and oxidizing agents. The review also provides an overview of recent knowledge gained concerning downstream pathways involving G-protein-coupled receptors. This article is part of a Special Issue entitled: Calcium channels.

Calcium Channels in the Development, Maturation, and Function of Spermatozoa

A proper dialogue between spermatozoa and the egg is essential for conception of a new individual in sexually reproducing animals. Ca2+ is crucial in orchestrating this unique event leading to a new life. No wonder that nature has devised different Ca2+-permeable channels and located them at distinct sites in spermatozoa so that they can help fertilize the egg. New tools to study sperm ionic currents, and image intracellular Ca2+ with better spatial and temporal resolution even in swimming spermatozoa, are revealing how sperm ion channels participate in fertilization. This review critically examines the involvement of Ca2+ channels in multiple signaling processes needed for spermatozoa to mature, travel towards the egg, and fertilize it. Remarkably, these tiny specialized cells can express exclusive channels like CatSper for Ca2+ and SLO3 for K+, which are attractive targets for contraception and for the discovery of novel signaling complexes. Learning more about fertilization is a matter of capital importance; societies face growing pressure to counteract rising male infertility rates, provide safe male gamete-based contraceptives, and preserve biodiversity through improved captive breeding and assisted conception initiatives.

Surfing the wave, cycle, life history, and genes/proteins expressed by testicular germ cells. Part 4: intercellular bridges, mitochondria, nuclear envelope, apoptosis, ubiquitination, membrane/voltage-gated channels, methylation/acetylation, and transcription factors

As germ cells divide and differentiate from spermatogonia to spermatozoa, they share a number of structural and functional features that are common to all generations of germ cells and these features are discussed herein. Germ cells are linked to one another by large intercellular bridges which serve to move molecules and even large organelles from the cytoplasm of one cell to another. Mitochondria take on different shapes and features and topographical arrangements to accommodate their specific needs during spermatogenesis. The nuclear envelope and pore complex also undergo extensive modifications concomitant with the development of germ cell generations. Apoptosis is an event that is normally triggered by germ cells and involves many proteins. It occurs to limit the germ cell pool and acts as a quality control mechanism. The ubiquitin pathway comprises enzymes that ubiquitinate as well as deubiquitinate target proteins and this pathway is present and functional in germ cells. Germ cells express many proteins involved in water balance and pH control as well as voltage-gated ion channel movement. In the nucleus, proteins undergo epigenetic modifications which include methylation, acetylation, and phosphorylation, with each of these modifications signaling changes in chromatin structure. Germ cells contain specialized transcription complexes that coordinate the differentiation program of spermatogenesis, and there are many male germ cell-specific differences in the components of this machinery. All of the above features of germ cells will be discussed along with the specific proteins/genes and abnormalities to fertility related to each topic.

CaMKIIalpha interacts with multi-PDZ domain protein MUPP1 in spermatozoa and prevents spontaneous acrosomal exocytosis

The success of acrosomal exocytosis, a complex process with a variety of inter-related steps, relies on the coordinated interaction of participating signaling molecules. Since the acrosome reaction resembles Ca(2+)-regulated exocytosis in neurons, we investigated whether cognate neuronal binding partners of the multi-PDZ domain protein MUPP1, which recruits molecules that control the initial tethering and/or docking between the acrosomal vesicle and the plasma membrane, are also expressed in spermatozoa, and whether they contribute to the regulation of acrosomal secretion. We observed that CaMKIIalpha colocalizes with MUPP1 in the acrosomal region of epididymal spermatozoa where the kinase selectively binds to a region encompassing PDZ domains 10-11 of MUPP1. Furthermore, we found that pre-treating mouse spermatozoa with a CaMKII inhibitor that directly blocks the catalytic region of the kinase, as well as a competitive displacement of CaMKIIalpha from PDZ domains 10-11, led to a significant increase in spontaneous acrosomal exocytosis. Since Ca(2+)-calmodulin releases CaMKIIalpha from the PDZ scaffolding protein, MUPP1 represents a central signaling platform to dynamically regulate the assembly and disassembly of binding partners pertinent to acrosomal secretion, thereby precisely adjusting an increase in Ca(2+) to synchronized fusion pore formation.

Chloride Is essential for capacitation and for the capacitation-associated increase in tyrosine phosphorylation

After epididymal maturation, sperm capacitation, which encompasses a complex series of molecular events, endows the sperm with the ability to fertilize an egg. This process can be mimicked in vitro in defined media, the composition of which is based on the electrolyte concentration of the oviductal fluid. It is well established that capacitation requires Na(+), HCO(3)(-), Ca(2+), and a cholesterol acceptor; however, little is known about the function of Cl(-) during this important process. To determine whether Cl(-), in addition to maintaining osmolarity, actively participates in signaling pathways that regulate capacitation, Cl(-) was replaced by either methanesulfonate or gluconate two nonpermeable anions. The absence of Cl(-) did not affect sperm viability, but capacitation-associated processes such as the increase in tyrosine phosphorylation, the increase in cAMP levels, hyperactivation, the zona pellucidae-induced acrosome reaction, and most importantly, fertilization were abolished or significantly reduced. Interestingly, the addition of cyclic AMP agonists to sperm incubated in Cl(-)-free medium rescued the increase in tyrosine phosphorylation and hyperactivation suggesting that Cl(-) acts upstream of the cAMP/protein kinase A signaling pathway. To investigate Cl(-) transport, sperm incubated in complete capacitation medium were exposed to a battery of anion transport inhibitors. Among them, bumetanide and furosemide, two blockers of Na(+)/K(+)/Cl(-) cotransporters (NKCC), inhibited all capacitation-associated events, suggesting that these transporters may mediate Cl(-) movements in sperm. Consistent with these results, Western blots using anti-NKCC1 antibodies showed the presence of this cotransporter in mature sperm.

Changes in calmodulin immunocytochemical localization associated with capacitation and acrosomal exocytosis of ram spermatozoa

The aim of this study was to determine the localization of calmodulin (CaM) in ram sperm and the possible changes during in vitro capacitation (CA) and the ionophore-induced acrosome reaction (AR). Likewise, changes in intracellular calcium levels ([Ca(2+)](i)) were also analysed by using flow cytometry. CA was induced in vitro in a medium containing BSA, CaCl(2), NaHCO(3), and AR by the addition of the calcium ionophore A23187. The acrosomal status was assessed by the chlortetracycline-fluorescence (CTC) assay. Flow cytometry (FC) analyses were performed by loading samples with Fluo-3 AM, that emits fluorescence at a high [Ca(2+)](i), combined with propidium iodide (PI) that allowed us to discriminate sperm with/without an integral plasma membrane both with high/low [Ca(2+)](i). Immunocytochemistry localized CaM to the flagellum, and some sperm also contained CaM in the head (equatorial and post-acrosomal regions). CA and AR resulted in a slight increase in the post-acrosomal labelling. The treatment of sperm with increasing concentrations of two CaM antagonists, W7 and calmidazolium (CZ), accounted for an increase in capacitated and acrosome-reacted CTC-sperm patterns. CZ induced a significant reduction in the content of three protein tyrosine-phosphorylated bands of approximately of 30, 40 and 45kDa. However, W7 showed no significant effect at any of the studied concentrations. Neither of them significantly influenced protein serine and threonine phosphorylation. FC analysis revealed that the main subpopulation in the control samples contained 70% of the total sperm with integral plasma membrane and a medium [Ca(2+)](i). After CA, 67.1% of the sperm preserved an integral membrane with a higher [Ca(2+)](i). After AR, only 7.2% of the total sperm preserved intact membranes with a very high [Ca(2+)](i). These results imply that CaM appears to be involved in ram sperm capacitation, and both treatments increased its localization in the post-acrosomal region.

When a sperm meets an egg: block to polyspermy

Embryonic development is initiated after the fertilizing spermatozoon enters the egg and triggers a series of events known as egg activation. Activation results in an increase in intracellular calcium concentration, cortical granule exocytosis (CGE), cell cycle resumption and recruitment of maternal mRNA. CGE is an evolutionary developed mechanism that causes modification of the zona pellucida to prevent penetration of additional spermatozoa, ensuring successful egg activation and embryo development. The egg CGE is a unique and convenient mammalian model for studying the different proteins participating at the membrane fusion cascade, which, unlike other secretory cells, occurs only once in the egg's lifespan. This article highlights a number of proteins, ascribed to participate in CGE and thus the block to polyspermy. CGE can be triggered either by a calcium dependent pathway, or via protein kinase C (PKC) activation that requires a very low calcium concentration. In a recent study, we suggested that the filamentous actin (F-actin) at the egg's cortex is a dynamic network. It can be maneuvered towards allowing CGE by activated actin associated proteins and/or by activated PKC and its down stream proteins, such as myristoylated alanine-rich C kinase substrate (MARCKS). MARCKS, a protein known to cross-link F-actin in other cell types, was found to be expressed and colocalized with actin in non-activated MII eggs. We further demonstrated MARCKS dissociation from actin after activation by ionomycin, a process that can lead to the breakdown of the actin network, thus allowing CGE. The more we know of the intricate process of CGE and of the proteins participating in it, the more the assisted reproductive procedures might benefit from that knowledge.

Fenvalerate modifies T-type Ca2+ channels in mouse spermatogenic cells

Prior to fertilization sperm must undergo the acrosome reaction that is initiated by opening of T-type Ca(2+) channel. Hence, T-type Ca(2+) channels could be a potential target for agents affecting the acrosome reaction. Our previous data have suggested that fenvalerate, a type II pyrethroid, inhibited the acrosome reaction in mice. To elucidate its potential mechanism we investigated fenvalerate's effect on T-type Ca(2+) channels in mouse pachytene spermatocytes using a whole-cell patch clamp technique. Fenvalerate significantly inhibited T-type Ca(2+) currents in a concentration-dependent manner. The maximal inhibitory concentration was 1 microM. The inhibitory effect of fenvalerate was slow and irreversible after washout of the drug. The curves of activation and inactivation simultaneously shifted to hyperpolarization, about 14 mV, suggesting the open time of Ca(2+) channel was unchanged. Voltage-dependent gating of Ca(2+) channel indicated a change in permeability to ions that contributed to fenvalerate's inhibition on Ca(2+) current. Taken together with our previous findings, these data suggest that the changes of T-type Ca(2+) currents contribute to the suppression of acrosome reaction and fertilizing ability caused by fenvalerate.

Moving to the beat: a review of mammalian sperm motility regulation

Because it is generally accepted that a high percentage of poorly motile or immotile sperm will adversely affect male fertility, analysis of sperm motility is a central part of the evaluation of male fertility. In spite of its importance to fertility, poor sperm motility remains only a description of a pathology whose underlying cause is typically poorly understood. The present review is designed to bring the clinician up to date with the most current understanding of the mechanisms that regulate sperm motility and to raise questions about how aberrations in these mechanisms could be the underlying causes of this pathology.

Sodium and epithelial sodium channels participate in the regulation of the capacitation-associated hyperpolarization in mouse sperm

In a process called capacitation, mammalian sperm gain the ability to fertilize after residing in the female tract. During capacitation the mouse sperm plasma membrane potential (E(m)) hyperpolarizes. However, the mechanisms that regulate sperm E(m) are not well understood. Here we show that sperm hyperpolarize when external Na(+) is replaced by N-methyl-glucamine. Readdition of external Na(+) restores a more depolarized E(m) by a process that is inhibited by amiloride or by its more potent derivative 5-(N-ethyl-N-isopropyl)-amiloride hydrochloride. These findings indicate that under resting conditions an electrogenic Na(+) transporter, possibly involving an amiloride sensitive Na(+) channel, may contribute to the sperm resting E(m). Consistent with this proposal, patch clamp recordings from spermatogenic cells reveal an amiloride-sensitive inward Na(+) current whose characteristics match those of the epithelial Na(+) channel (ENaC) family of epithelial Na(+) channels. Indeed, ENaC-alpha and -delta mRNAs were detected by reverse transcription-PCR in extracts of isolated elongated spermatids, and ENaC-alpha and -delta proteins were found on immunoblots of sperm membrane preparations. Immunostaining indicated localization of ENaC-alpha to the flagellar midpiece and of ENaC-delta to the acrosome. Incubations known to produce capacitation in vitro or induction of capacitation by cell-permeant cAMP analogs decreased the depolarizing response to the addition of external Na(+). These results suggest that increases in cAMP content occurring during capacitation may inhibit ENaCs to produce a required hyperpolarization of the sperm membrane.

Molecular regulation of voltage-gated Ca2+ channels

Voltage-gated Ca2+ (Ca(v)) channels are found in all excitable cells and many nonexcitable cells, in which they govern Ca2+ influx, thereby contributing to determine a host of important physiological processes including gene transcription, muscle contraction, hormone secretion, and neurotransmitter release. The past years have seen some significant advances in our understanding of the functional, pharmacological, and molecular properties of Ca(v) channels. Molecular studies have revealed that several of these channels are oligomeric complexes consisting of an ion-conducting alpha1 subunit and auxiliary alpha2delta, beta, and gamma subunits. In addition, cloning of multiple Ca(v) channel alpha1 subunits has offered the opportunity to investigate the regulation of these proteins at the molecular level. The regulation of Ca(v) channels by intracellular second messengers constitutes a key mechanism for controlling Ca2+ influx. This review summarizes recent advances that have provided important clues to the underlying molecular mechanisms involved in the regulation of Ca(v) channels by protein phosphorylation, G-protein activation, and interactions with Ca(2+)-binding and SNARE proteins.

Evidence of the presence of calcium/calmodulin-dependent protein kinase IV in human sperm and its involvement in motility regulation

The mechanisms involved in the regulation of mammalian sperm motility are not well understood. Calcium ions (Ca(2+)) have been suggested to play a key role in the maintenance of motility; nevertheless, how Ca(2+) modulates this process has not yet been completely characterized. Ca(2+) can bind to calmodulin and this complex regulates the activity of multiple enzymes, including Ca(2+)/calmodulin-dependent protein kinases (CaM kinases). Results from this study confirmed that the presence of Ca(2+) in the incubation medium is essential for maintaining human sperm motility. The involvement of CaM kinases in Ca(2+) regulation of human sperm motility was evaluated using specific inhibitors (KN62 and KN93) or their inactive analogues (KN04 and KN92 respectively). Sperm incubation in the presence of KN62 or KN93 led to a progressive decrease in the percentage of motile cells; in particular, incubation with KN62 also reduced sperm motility parameters. These inhibitors did not alter sperm viability, protein tyrosine phosphorylation or the follicular fluid-induced acrosome reaction; however, KN62 decreased the total amount of ATP in human sperm. Immunological studies showed that Ca(2+)/calmodulin-dependent protein kinase IV (CaMKIV) is present and localizes to the human sperm flagellum. Moreover, CaMKIV activity increases during capacitation and is inhibited in the presence of KN62. This report is the first to demonstrate the presence of CaMKIV in mammalian sperm and suggests the involvement of this kinase in the regulation of human sperm motility.

Calcium Channels and Ca2+ Fluctuations in Sperm Physiology

Generating new life in animals by sexual reproduction depends on adequate communication between mature and competent male and female gametes. Ion channels are instrumental in the dialogue between sperm, its environment, and the egg. The ability of sperm to swim to the egg and fertilize it is modulated by ion permeability changes induced by environmental cues and components of the egg outer layer. Ca(2+) is probably the key messenger in this information exchange. It is therefore not surprising that different Ca(2+)-permeable channels are distinctly localized in these tiny specialized cells. New approaches to measure sperm currents, intracellular Ca(2+), membrane potential, and intracellular pH with fluorescent probes, patch-clamp recordings, sequence information, and heterologous expression are revealing how sperm channels participate in fertilization. Certain sperm ion channels are turning out to be unique, making them attractive targets for contraception and for the discovery of novel signaling complexes.

Biophysical and pharmacological characterization of spermatogenic T‐type calcium current in mice lacking the Ca V 3.1 (α 1G ) calcium channel: Ca V 3.2 (α 1H ) is the main functional calcium channel in wild‐type spermatogenic cells

Mammalian acrosome reaction (AR) requires successive activation of three different types of calcium channels (T-type channels, Inositol-3-phosphate (InsP3) receptors, and TRPC2 channels). All the calcium signaling is under the control of the activation of the first-one, a T-type calcium channel. The molecular characterization of the T-type calcium channel is still a matter of debate, previous reports showing the presence of transcripts for Ca(V)3.1 and Ca(V)3.2 subunits. Using mice deficient for Ca(V)3.1 subunit, we show that the T-type current density in spermatogenic cells is not reduced in deficient mice versus control mice. We characterized the biophysical and pharmacological properties of T-type current in spermatogenic cells from Ca(V)3.1 deficient mice. Biophysical and pharmacological properties of spermatogenic T-type current from wild-type and Ca(V)3.1 deficient mice demonstrate that Ca(V)3.3 does not contribute to T-type current. Moreover, nickel and amiloride inhibit T-type currents in deficient and wild-type mice with similar potencies. These results demonstrate that T-type currents in spermatogenic cells is due to Ca(V)3.2 subunit and that Ca(V)3.1 contributes to a very negligible extent to the T-type currents. Thus, the deficient Ca(V)3.1 mouse model allows the characterization of native Ca(V)3.2 currents in spermatogenic cells. Spermatogenic Ca(V)3.2 currents present specific feature in comparison to the cloned Ca(V)3.2 current so far. More particularly, the time-dependence of recovery from short-term inactivation of native spermatogenic Ca(V)3.2 is close to 100 millisecond, a value expected for Ca(V)3.1 current.

Calcium regulation of the soluble adenylyl cyclase expressed in mammalian spermatozoa

In mammals, Ca2+ and HCO3- ions play a critical role in the regulation of sperm function, most likely by regulation of cAMP levels. Mammalian germ cells contain a soluble adenylyl cyclase (sAC) with properties distinct from the well characterized membrane-bound enzymes Here we investigated whether the cyclase expressed in mature spermatozoa has the properties of sAC and whether it is regulated by Ca2+. In addition to an HCO3--dependent activation, the cyclase endogenous to human spermatozoa is stimulated 2- to 3-fold by Ca2+ in a concentration-dependent manner (EC50 approximately 400 nM). In a similar fashion, Ca2+ activates the recombinant rat and human full-length sAC with similar EC50 values. The Ca2+ stimulation was also observed when sAC was activated with HCO3-, was independent of calmodulin, and was associated with an increase in Vmax without changes in Km for ATP-Mg2+. An increase in intracellular Ca2+ by ionophore or by a muscarinic cholinergic receptor agonist increases cAMP in cells transfected with FL-hsAC, but not in mock-transfected cells. Similarly, both Ca2+ and HCO3- stimulate cAMP accumulation in human spermatozoa. These findings provide evidence that human spermatozoa express a cyclase with the properties of sAC and that Ca2+ can substitute for HCO3- in the stimulation of this enzyme, underscoring an important role for sAC in the control of sperm functions.

Molecular physiology of low-voltage-activated t-type calcium channels

T-type Ca2+ channels were originally called low-voltage-activated (LVA) channels because they can be activated by small depolarizations of the plasma membrane. In many neurons Ca2+ influx through LVA channels triggers low-threshold spikes, which in turn triggers a burst of action potentials mediated by Na+ channels. Burst firing is thought to play an important role in the synchronized activity of the thalamus observed in absence epilepsy, but may also underlie a wider range of thalamocortical dysrhythmias. In addition to a pacemaker role, Ca2+ entry via T-type channels can directly regulate intracellular Ca2+ concentrations, which is an important second messenger for a variety of cellular processes. Molecular cloning revealed the existence of three T-type channel genes. The deduced amino acid sequence shows a similar four-repeat structure to that found in high-voltage-activated (HVA) Ca2+ channels, and Na+ channels, indicating that they are evolutionarily related. Hence, the alpha1-subunits of T-type channels are now designated Cav3. Although mRNAs for all three Cav3 subtypes are expressed in brain, they vary in terms of their peripheral expression, with Cav3.2 showing the widest expression. The electrophysiological activities of recombinant Cav3 channels are very similar to native T-type currents and can be differentiated from HVA channels by their activation at lower voltages, faster inactivation, slower deactivation, and smaller conductance of Ba2+. The Cav3 subtypes can be differentiated by their kinetics and sensitivity to block by Ni2+. The goal of this review is to provide a comprehensive description of T-type currents, their distribution, regulation, pharmacology, and cloning.

Stimulation of voltage-dependent calcium channels during capacitation and by progesterone in human sperm

To fertilize, mammalian sperm must undergo two sequential steps that require activation of calcium entry mechanisms, capacitation and acrosomal exocytosis, induced in the latter case by the egg zona pellucida glycoprotein ZP3 or by progesterone. Voltage-dependent calcium channels (VDCC) could participate in these processes. Since patch clamp recordings are extremely difficult in mature sperm, the activity of VDCC has been alternatively analyzed with optical detectors of membrane potential and intracellular calcium in sperm populations. Using this approach, we previously reported that in human sperm there is a voltage-dependent calcium influx system that strongly indicates that human sperm are endowed with functional VDCC. In this study we developed evidence indicating that calcium influx through VDCC is significantly stimulated during sperm in vitro capacitation and by progesterone action, which is present in the follicular fluid that surrounds the egg. The observed effects of capacitation and progesterone on VDCC may be physiologically significant for sperm-egg interaction.

Functional interaction between mouse spermatogenic LVA and thapsigargin-modulated calcium channels

The acrosome reaction in mouse is triggered by a long-lasting calcium signaling produced by a chain of openings of several calcium channels, a low-voltage-activated (LVA) calcium channel, an inositol trisphosphate receptor (IP(3)R), and the store-operated calcium channel TRP2. Since mature sperm cells are refractory to patch clamp experiments, we study the functional interactions among those sperm calcium channels in spermatogenic cells. We have studied the role of cytosolic calcium in voltage-dependent facilitation of low voltage-activated calcium channels. Calcium concentration was modified through the inclusion of the calcium buffers, EGTA and BAPTA, in the recording pipette solution, and by addition of calcium modulators like thapsigargin and the calcium ionophore A23187. We demonstrate that lowering calcium concentration below resting level allows to evidence a voltage-dependent facilitation. We also show that LVA calcium channels present strong voltage-dependent inhibition by thapsigargin. This effect is independent of cytosolic calcium elevation secondary to calcium store depletion and to the activation of TRP channels. Our data evidence an interesting functional relationship, in this cell type, between LVA channels and proteins whose activity is related to calcium filling state of the endoplasmic reticulum (presumably TRP channels and inositol triphosphate receptor). These relationships may contribute to the regulation of calcium signaling during acrosome reaction of mature sperm cell.

Rab3A and calmodulin regulate acrosomal exocytosis by mechanisms that do not require a direct interaction

The interaction between Rab3A and calmodulin is necessary for the inhibitory effect of Rab3A in neuroendocrine cells. Contrastingly, Rab3A triggers the exocytosis known as acrosome reaction in permeabilized spermatozoa. Here we show that a Rab3A mutant that cannot bind calmodulin was fully capable of triggering acrosomal exocytosis. Additionally, calmodulin by itself abrogated the exocytosis triggered by Rab3A. The effect was observed with both the wild type protein and the calmodulin binding deficient mutant. Our results indicate that the inhibitory and stimulatory effects of Rab3A in different exocytic processes are mediated by different effectors.

High molecular mass egg fucose sulfate polymer is required for opening both Ca2+ channels involved in triggering the sea urchin sperm acrosome reaction

A linear fucose sulfate polymer (FSP), >10(6) daltons, is a major component of sea urchin egg jelly. FSP induces the sperm acrosome reaction (AR), an exocytotic process required for animal fertilization. Two Ca(2+) channels activate during AR induction, the first opens 1 s after FSP addition, and the second opens 5 s after the first. Mild acid hydrolysis of FSP results in a linear decrease in polymer size. The ability of FSP to induce the AR and activate sperm Ca(2+) channels decreases with increasing time of hydrolysis. Hydrolyzed FSP of approximately 60 kDa blocks intact FSP from inducing the AR. At 44 microg/ml hydrolyzed FSP, Ca(2+) entry into sperm is almost equal to that occurring in 3.8 microg/ml intact FSP; however the AR is not induced. The shape of the [Ca(2+)](i) increase curve and use of the Ca(2+) channel blockers nifidipine and Ni(2+) indicate that hydrolyzed FSP opens the second Ca(2+) channel, but not the first, and thus does not induce the AR. The giant size of intact FSP is required to open both Ca(2+) channels involved in triggering the AR.