- Open Access
The BSA-induced Ca(2+) influx during sperm capacitation is CATSPER channel-dependent
© Xia and Ren; licensee BioMed Central Ltd. 2009
- Received: 30 June 2009
- Accepted: 27 October 2009
- Published: 27 October 2009
Serum albumin is a key component in mammalian sperm capacitation, a functional maturation process by which sperm become competent to fertilize oocytes. Capacitation is accompanied by several cellular and molecular changes including an increased tyrosine phosphorylation of sperm proteins and a development of hyperactivated sperm motility. Both of these processes require extracellular calcium, but how calcium enters sperm during capacitation is not well understood.
BSA-induced changes in intracellular calcium concentration were studied using Fluo-4 and Fura-2 calcium imaging with wild-type and Catsper1 knockout mouse sperm.
We found that the fast phase of the BSA-induced rises in intracellular calcium concentration was absent in the Catsper1 knockout sperm and could be restored by an EGFP-CATSPER1 fusion protein. The calcium concentration increases were independent of G-proteins and phospholipase C but could be partially inhibited when intracellular pH was clamped. The changes started in the principal piece and propagated toward the sperm head.
We conclude that the initial phase of the increases in intracellular calcium concentration induced by BSA requires the CATSPER channel, but not the voltage-gated calcium channel. Our findings identify the molecular conduit responsible for the calcium entry required for the sperm motility changes that occur during capacitation.
- Bovine Serum Albumin
- Tyrosine Phosphorylation
- Zona Pellucida
- Acrosome Reaction
- Sperm Head
During mammalian fertilization, freshly ejaculated sperm do not have the ability to fertilize oocytes until after they undergo capacitation, a functionally defined, but poorly understood maturation process by which sperm become capable of fertilizing eggs [1–3]. Sperm become capacitated in vivo, by interacting with environmental stimuli in the female reproductive tract before encountering eggs. This process can also be mimicked in vitro by incubating sperm in defined capacitation media. Several commonly used components are essential for successful in vitro capacitation in sperm from many mammalian species. Among them are bovine serum albumin (BSA), Ca2+ and bicarbonate (HCO3-) . Capacitation leads to several cellular and behavioral changes, including an increase in tyrosine phosphorylation of sperm proteins, rises in intracellular pH (pHi) and Ca2+ concentration ([Ca2+]i), membrane hyperpolarization, and hyperactivated motility [4–6].
Increases in [Ca2+]i and intracellular [pH]i are believed to play central roles in both sperm capacitation and the acrosome reaction (AR) [3, 7, 8]. The capacitating agent BSA induces Ca2+ influx in sperm, but the molecular mechanisms underlying such an influx are not well understood. Multiple Ca2+-permeable ion channels have been detected in mammalian sperm, including voltage-gated Ca2+ channels (CaVs), transient receptor potential (TRP) channels, cyclic nucleic gated (CNG) channels and CATSPER channels [9–12]. Among these ion channel proteins, only the four mammalian CATSPER members (CATSPER 1-4) are specifically found in sperm and spermatogenic cells [13–17]. All four Catsper genes are required for male fertility as mice with any of these genes disrupted are infertile [15, 18–20]. Disruptions in Catsper1 and Catsper2 are also associated with male infertility in humans [21–23].
Using Ca2+-sensitive fluorescent probes, we and others have shown that CATSPERs are required for the Ca2+ entry induced by stimuli such as cyclic nucleotides, alkaline depolarizing medium and egg coat proteins [24–26]. Ca2+ entering the channel in sperm tail can trigger Ca2+ propagation toward the head [25, 26]. CATSPER's roles in the migration of sperm toward the oocyte and in penetrating the egg coat have been clearly established by studies showing that Catsper mutant sperm cannot migrate to the egg in vivo  and that, in in vitro fertilization (IVF), they cannot penetrate coat-intact eggs but can fertilize those without the zona pellucida . In contrast, CATSPER's function in sperm capacitation is less clear. Catsper mutant sperm do not develop hyperactive motility after incubation in capacitation medium, as do normal sperm. The mutant sperm also have a progressive decrease of motility under certain incubation conditions [14, 15, 19, 24, 28, 29]. This finding suggests that CATSPER has a role in the motility aspect of sperm capacitation. On the other hand, wild-type and Catsper mutant sperm do not differ in their patterns of protein tyrosine phosphorylation after sperm capacitation [19, 24] or in their capacitation and AR efficiency, as examined with the chlortetracycline (CTC) assay [25, 26]. In this study, we investigated CATSPER's potential role in sperm capacitation by studying the Ca2+ influx induced by BSA.
Fluo-4 AM, Fura-2 AM and pluronic F-127 were purchased from Molecular Probes (Invitrogen, Eugene, OR). Pertussis toxin (PTX) and ionomycin were from CalBiochem (Gibbstown, NJ) and Cell-Tak was from BD Biosciences (Bedford, MA). BSA (fraction V, fatty acid-depleted, Sigma #A3059), disodium salt ATP, and other reagents were purchased from Sigma. Similar Ca2+ responses in sperm were also observed with fatty acid-free BSA (Sigma #A8806; not shown).
Animals were treated according to institutional regulations. This study used Catsper1 knockout mice that were backcrossed to C57BL/6J for more than 10 generations . Sperm of the Catsper1 knockout mice lack not only CATSPER1, but also CATSPER2  and the associated auxiliary proteins CATSPERβ  and CATSPERγ . To reflect this fact, we do not distinguish CATSPER1 from the other CATSPERs throughout the paper. The EGFP-Catsper1 transgenic mice have a Catsper1 null background but carry an EGFP-CATSPER1 fusion protein gene that rescues the male sterile phenotype of the Catsper1 null mutant .
Non-capacitated caudal sperm were used for Ca2+ imaging, as previously described [25, 26]. Briefly, sperm were released into HS medium containing (mM): 135 NaCl, 5 KCl, 2 CaCl2, 1 MgCl2, 30 HEPES, 10 glucose, 10 lactic acid, and 1 pyruvic acid (pH adjusted to 7.4 with NaOH), and concentrated to 5 × 106 -1 × 107/ml. Cells were loaded with 10 μM Fluo-4 AM and 0.05% pluronic F-127 for 30 min at room temperature in the dark, followed by two washes in imaging medium (HS supplemented with 15 mM NaHCO3), each with a 4 min spin at 300 × g. Washed sperm were resuspended in imaging medium and loaded into a small-volume imaging chamber (~1 cm diameter, ~90 μl) formed with Sylgard on a Cell-Tak coated coverslip, and allowed to attach for ~10 min.
For imaging sperm from EGFP- Catsper1 transgenic mice, a ratiometric measurement with Fura-2 (5 μM for loading) was used because of the EGFP fluorescence. A monochromator (DeltaRAM V, PTI) with a 75-W Xenon lamp was used to generate the excitation at 488 nm for Fluo-4 (or 340 nm and 380 nm for Fura-2). A 60× objective and a 1.6× adaptor on an inverted microscope (IX-71, Olympus) were used for imaging. Emissions (515-565 nm) were bandpass filtered (HQ540/50, Chroma) and collected with a cooled CCD camera (CoolSNAP HQ, Roper Scientific) for 25 ms in every 0.5 s for fast recording, or 100 ms in every 6 s for slow recording. Online control, data collection, and image processing were conducted using commercial software (ImageMaster 3, PTI).
For imaging using Fluo-4, [Ca2+]i changes are presented as ΔF/F0 ratios after background subtraction, where ΔF was the change in fluorescence signal intensity and F0 was the baseline as calculated by averaging the 10 frames before stimulus application. In sperm loaded with Fura-2, [Ca2+]i changes are presented as the ratio of fluorescence from excitation at 340 nm to that at 380 nm (F340/F380) after background subtraction. All the imaging experiments were done at room temperature. Cells with uneven dye loading were excluded from the analysis. Motile sperm (~60% of the population) that had at least two points attached to the coverslip were used for analysis. Cells with peak changes of >50% in ΔF/F0 (for Fluo-4) or >0.1 in F340/380 (for Fura-2) after application of stimuli were counted as responsive. To detect the Ca2+ responses at "clamped" membrane potential (Vm), K+ ionophore valinomycin (1 μM) was added to the imaging buffer, with additional K+ as indicated to replace an equimolar amount of Na+. EK was calculated with the assumption of a 120 mM intracellular K+ concentration [30, 31]. To "clamp" intracellular pH, sperm were preincubated for 5 min with 3 μM carbonylcyanide-p-trifluoromethoxyphenyl hydrazone (FCCP) and 1 μM valinomycin .
Data analyses were performed with ImageMaster3, Excel and Origin. Student's t-tests and ANOVA were used for statistical comparisons between different treatment groups. P < 0.05 was considered statistically significant.
CATSPER channels are required for the BSA-induced [Ca2+]irise in mouse sperm
How BSA induces [Ca2+]i changes in sperm is not well understood, but one possibility is through CaVs [34–36]. Our recent studies, however, suggest that mature sperm do not have detectable functional CaV channels . To determine whether the BSA-induced [Ca2+]i rises are dependent on CATSPERs, we compared the [Ca2+]i changes in wild-type mouse sperm with those in the Catsper1 null mutants. Upon bath application of BSA (5 mg/ml), 98% of wild-type sperm (107/109) showed initial responses in the head within 20 s and 17% (10 of 58) had a 2nd response more than 2 min later (Figure 2A, E). In contrast, the initial [Ca2+]i changes were absent in CATSPER1-deficient sperm within 2 min of BSA stimulation (Figure 2B, E), although the delayed responses were present in some sperm (23%, Figure 2C). The BSA-induced [Ca2+]i rises were restored by a transgene encoding an EGFP-CATSPER1 fusion protein in the Catsper1 null background (Figure 2D, E). These results indicate that CATSPER1 is required for the initial [Ca2+]i responses.
Elevations in BSA -induced [Ca2+]istart in the sperm tail and propagate to the head
The BSA-induced increases in [Ca2+]i partially depend on pHichanges
Another effect of BSA in particular, and capacitation in general, is an intracellular alkalization by 0.4 pHi units [33, 43]. When pHi was clamped with a H+ ionophore FCCP and K+ ionophore valinomycin , the amplitude of the BSA-induced [Ca2+]i rise was decreased and the percentage of responsive cells was reduced (Figure 6E, F), suggesting that BSA induces [Ca2+]i increases, at least partially, through pHi changes.
The ATP-induced [Ca2+]irises in mouse sperm are independent of CATSPER channel
The major finding of our study is that CATSPER channels are required for the BSA induced [Ca2+]i increases in mouse sperm. Stimuli known to induce a CATSPER-dependent Ca2+ entry now include some of the most important mediators in sperm physiology: cyclic nucleotides, zona pellucida and serum albumin. There are clearly also CATSPER-independent Ca2+ entry paths that are responsible for the [Ca2+]i increases induced by stimuli such as extracellular ATP, as we showed in this study, and mechanical force (Xia and Ren, unpublished). Differences in signal transduction pathways that link various stimuli to CATSPER channels also exist. For example, the Ca2+ entry induced by the zona pellucid is dependent on G-proteins and phospholipase C [10, 25, 38] whereas the BSA-induced one is not (Figure 4).
Like several other stimuli such as 8-Br-cGMP  and the zona pellucida , BSA does not directly activate CATSPER channel currents in corpus sperm under whole cell voltage clamp patch clamp recording (with intracellular and extracellular pH buffered at 7.2 and 7.4, respectively [25, 37]; Xia and Ren, unpublished observation). We cannot exclude the possibility that BSA can directly activate CATSPER under more physiological conditions. During sperm capacitation with BSA-containing medium, there is an intracellular alkalization . Consistent with a role of the alkalization, the BSA-induced [Ca2+]i rises were reduced when pHi was "clamped" (Figure 6). The [Ca2+]i response, however, was not completely inhibited. The residual BSA-induced [Ca2+]i changes under "pHi clamp" condition are likely through pathways other than intracellular alkalization.
Although weakly voltage-dependent, whole cell CATSPER conductance can be increased by cell membrane depolarization . In addition, depolarization by increased extracellular K+ concentrations, especially coupled with alkalization medium (K8.6), can lead to a CATSPER-dependent Ca2+ influx . These results point to a possible role of membrane depolarization in the pathway leading to the BSA-induced Ca2+ entry. However, we consider this possibility unlikely for several reasons. First, we show here that the [Ca2+]i rises are not dependent on a membrane voltage change since they persist when the voltage is "clamped" with K+ ionophore. Second, inactivating the T-type CaV channel, that is a proposed candidate for the Ca2+ entry [34–36], by clamping the membrane at -20 mV does not decrease the BSA-induced [Ca2+]i rises. This result is consistent with our recent finding that mature sperm do not possess detectable functional CaV channels . Finally, BSA does not lead to depolarization during sperm capacitation, instead, a hyperpolarization of ~15 mV has been observed using voltage-sensitive fluorescence dyes . Along these lines, we used whole cell patch clamp under current clamp mode to directly measure acute membrane potential changes in corpus sperm upon BSA application and found that BSA (4 mg/ml) produced a fast and profound membrane hyperpolarization (11 ± 2 mV, n = 6). While the exact mechanism of such a membrane potential hyperpolization induced by BSA remains undetermined, we found no evidence for a role of membrane depolarization in the BSA-induced Ca2+ entry through CATSPER.
In summary, we determined the molecular identity of an ion channel responsible for the fast Ca2+ influx induced by BSA used in in vitro sperm capacitation. Three lines of evidence support that CATSPER is the aforementioned channel. First, the BSA-induced [Ca2+]i increases are absent in Catsper1 knockout sperm; second, such responses can be restored by an EGFP-CATSPER1 fusion protein; and finally, these [Ca2+]i rises start in the principal piece of sperm where CATSPER proteins and current through CATSPER channels are localized. The mechanisms by which BSA is coupled to the CATSPER channel are largely unknown. They are unlikely through a voltage change or a cholesterol efflux. An intracellular alkalization appears to be involved (Figure 6) but there is clearly a [pH]i change-independent component that remains to be uncovered.
Together with previous findings that the capacitation status and changes in the pattern of tyrosine phosphorylation during capacitation do not require CATSPER while the hyperactivated motility does [15, 20, 24, 28], our data genetically separate the Ca2+ requirements for the two aspects of sperm capacitation: one mediating the change of motility via the CATSPER channel and another mediating the increase of tyrosine phosphorylation via a source yet to be determined. One possible source for the latter is the second phase of [Ca2+]i rises that are also present in the Catsper1 mutant sperm.
We thank members of the Ren lab for discussion. This research was supported by NIH grant 1R01HD047578.
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