Flow cytometric evaluation of mitochondrial function and membrane integrity of marine invertebrate sperm

Quality assessment of cryopreserved Portuguese oyster (Crassostrea angulata) sperm through ultrastructural and flow cytometry analysis

We investigated the effects of cryopreservation on the quality of Portuguese oyster (Crassostrea angulata) sperm, which were examined before and after freezing; sperm motility, fertilizing capacity, and ultrastructural morphology were analyzed. The motility percentage and fertilizing capacity of the cryopreserved sperm (mean ± standard error) were 16% ± 1% and 17% ± 8%, respectively. In the pre-freezing sperm, these were 58% ± 2% and 76% ± 4%, respectively. The sperm sustained substantial morphological and ultrastructural damage during cryopreservation. The morphological changes varied considerably in nature and extent, ranging from no apparent damage to virtual disintegration. Sperm were stained with fluorescent dyes to assess viability, plasma membrane integrity, mitochondrial activity, acrosomal membrane integrity, oxidation level, and DNA compaction and examined through flow cytometry. The methods used for the flow cytometry assays were slightly modified from those used for evaluating the semen quality of livestock. Relative to the pre-freezing sperm, the frozen-thawed sperm exhibited lower acrosomal membrane integrity (acrosomal damage, 59.86 ± 5.29; P < 0.05) and substantially higher oxidation levels (free radicals, 60.06 ± 0.82; P < 0.003). Oxidation level was found to be the most sensitive indicator of cryodamage. Along with ultrastructural analysis, we used flow cytometry to measure the qualitative and quantitative characteristics of Portuguese oyster sperm before and after cryopreservation rapidly, objectively, and accurately. This is the first study to assess the quality of Portuguese oyster sperm through these methods.

Sperm viability assessment in marine invertebrates by fluorescent staining and spectrofluorimetry: A promising tool for assessing marine pollution impact

The viability of spermatozoa is a crucial parameter to evaluate their quality that is an important issue in ecotoxicological studies. Here, a new method has been developed to rapidly determine the viability of spermatozoa in three marine invertebrates: the ascidian Ciona intestinalis, the sea urchin Paracentrotus lividus and the mollusc Mytilus galloprovincialis. This method employed the dual DNA fluorescent staining coupled with spectrofluorimetric analysis. The dual fluorescent staining used the SYBR-14 stained live spermatozoa and propidium iodide stained degenerated cells that had lost membrane integrity. Stain uptake was assessed by confocal microscopy and then the percentage of live and dead spermatozoa was quantified by spectrofluorimetric analysis. The microscopic examination revealed three populations of spermatozoa: living-SYBR-14 stained, dead-PI stained, and dying-doubly stained spermatozoa. The fluorescence emission peak values recorded in a spectrofluorimeter provide the portion of live and dead spermatozoa showing a significant negative correlation. The stain combination was further validated using known ratios of live and dead spermatozoa. The present study demonstrated that the dual DNA staining with SYBR-14 and propidium iodide was effective in assessing viability of spermatozoa in marine invertebrates and that spectrofluorimetric analysis can be successfully employed to evaluate the percentage of live and dead spermatozoa. The method develop herein is simple, accurate, rapid, sensitive, and cost-effective, so it could be a useful tool by which marine pollutants may be screened for spermiotoxicity.

Involvement of Mitochondrial Activity and OXPHOS in ATP Synthesis During the Motility Phase of Spermatozoa in the Pacific Oyster, Crassostrea gigas

In the Pacific oyster, spermatozoa are characterized by a remarkably long movement phase (i.e., over 24 h) sustained by a capacity to maintain intracellular ATP level. To gain information on oxidative phosphorylation (OXPHOS) functionality during the motility phase of Pacific oyster spermatozoa, we studied 1) changes in spermatozoal mitochondrial activity, that is, mitochondrial membrane potential (MMP), and intracellular ATP content in relation to motion parameters and 2) the involvement of OXPHOS for spermatozoal movement using carbonyl cyanide m-chlorophenyl hydrazone (CCCP). The percentage of motile spermatozoa decreased over a 24 h movement period. MMP increased steadily during the first 9 h of the movement phase and was subsequently maintained at a constant level. Conversely, spermatozoal ATP content decreased steadily during the first 9 h postactivation and was maintained at this level during the following hours of the movement phase. When OXPHOS was decoupled by CCCP, the movement of spermatozoa was maintained 2 h and totally stopped after 4 h of incubation, whereas spermatozoa were still motile in the control after 4 h. Our results suggest that the ATP sustaining flagellar movement of spermatozoa may partially originate from glycolysis or from mobilization of stored ATP or from potential phosphagens during the first 2 h of movement as deduced by the decoupling by CCCP of OXPHOS. However, OXPHOS is required to sustain the long motility phase of Pacific oyster spermatozoa. In addition, spermatozoa may hydrolyze intracellular ATP content during the early part of the movement phase, stimulating mitochondrial activity. This stimulation seems to be involved in sustaining a high ATP level until the end of the motility phase.

Flow cytometric assessment of morphology, viability, and production of reactive oxygen species of Crassostrea gigas oocytes. Application to toxic dinoflagellate (Alexandrium minutum) exposure

The Pacific oyster Crassostrea gigas accounts for a large part of shellfish aquaculture production worldwide. Aspects of morphological and functional characteristics of oyster oocytes remain poorly documented, and traditional techniques, such as microscopic observations of shape or fertilization rate, are time and space consuming. The purpose of this study was to assess for the first time viability and reactive oxygen species (ROS) production of Pacific oyster oocytes using flow cytometry (FCM) and to apply this method to determine oocyte responses to in vitro exposure to the toxic dinoflagellate Alexandrium minutum. A culture of A. minutum caused a significant increase in oocyte ROS production, which gradually increased with the age of the culture, but viability was not affected. Effect of the supernatant of the same A. minutum culture did not cause any significant modifications of oocyte morphology, viability, or ROS level. This study confirmed that some oocyte cellular characteristics can be assessed using FCM techniques.

Cryopreservation of sperm in farmed Australian greenlip abalone Haliotis laevigata

This study investigated factors important to the development of the liquid nitrogen (LN) vapor sperm cryopreservation technique in farmed greenlip abalone Haliotis laevigata, including (1) cryoprotectant agent (CPA) toxicity; (2) cooling temperature (height above LN surface); (3) thawing temperature; (4) sperm to egg ratio; and (5) sugar supplementation, using sperm motility, fertilization rate or integrity/potential of sperm components and organelles as quality assessment indicators. Results suggested that among the single CPAs evaluated 6% dimethyl sulfoxide (Me2SO) would be the most suitable for sperm cryopreservation in this species. The highest post-thaw sperm motility was achieved with the sperm that had been exposed to LN vapor for 10min at 5.2cm above the LN surface, thawed and recovered in 60 and 18°C seawater bathes, respectively after at least 2h storage in LN. The highest fertilization rates were achieved at a sperm to egg ratio of 10,000:1 or 15,000:1. Addition of 1% glucose or 2% sucrose produced significantly higher post-thaw sperm motility than 6% Me2SO alone. Among the three cryoprotectant solutions further trialled, 6% Me2SO+1% glucose produced the highest fertilization rate of 83.6±3.7%. Evaluation of sperm has shown that the addition of glucose could significantly improve the sperm plasma membrane integrity and mitochondrial membrane potential. These results demonstrated a positive role of glucose in the improvement of sperm cryopreservation in farmed greenlip abalone.

Evaluation of sperm mitochondrial function using rh123/PI dual fluorescent staining in asthenospermia and oligoasthenozoospermia

Objective

The recent advent of flow cytometry (FCM), coupled with fluorescent dyes, has been successfully applied to assess mitochondrial function. The aim of this study was to investigate the feasibility and clinical significance of detecting sperm mitochondrial function and to evaluate sperm mitochondrial function by using Rhodamine 123/propidium (Rh123/PI) dual fluorescent staining and FCM in asthenospermia and oligoasthenozoospermia.

Methods

Twenty-five fertile men (with normal sperm parameters) and 230 infertile patients were examined. Fifty-five patients of the above 230 patients were selected for idiopathic infertility samples and were divided into two groups: asthenospermia (n = 30) and oligoasthenozoospermia (n = 25). Rh123/PI dual fluorescent staining and FCM were carried out to examine sperm mitochondrial function.

Results

Significant differences were found between the normal and abnormal semen samples (P < 0.05) when Rh123⁺/PI⁻, Rh123⁻/PI⁺ and Rh123⁻/PI⁻ sperm were examined by FCM, but there was no significant difference between the asthenospermia (P = 0.469) and oligoasthenozoospermia group (P = 0.950) when Rh123⁺/PI⁻ and Rh123⁻/PI⁺ sperm were then examined; however, a significant difference was found between the 2 groups (P = 0.003) when Rh123⁻/PI⁻ sperm were examined. There was no correlation between Rh123⁻/PI⁻ sperm and semen parameters in the normal group, but there was a significant negative correlation between the sperm concentration and Rh123⁻/PI⁻ sperm in asthenospermia and oligoasthenozoospermia patients (r = -0.509, -0.660; P = 0.018, 0.038).

Conclusion

Rh123/PI dual fluorescent staining and FCM can provide reliable information to assess the quality of sperm and reveal differences in mitochondrial membrane potential in asthenospermia and oligoasthenozoospermia.

Flow-cytometric analyses of viability biomarkers in pesticide-exposed sperm of three aquatic invertebrates

Toxicity studies on sperm often use fertilization success as the end point. This type of assay can be affected by sperm density, egg quality, and sperm-egg compatibility. Testing sperm viability biomarkers with flow cytometry is a fast, high-throughput technique for seminal analysis. In this study, we detected sperm viability biomarkers with several fluorescent reporter dyes using flow cytometry in three aquatic invertebrates (Crassostrea virginica, Dreissena polymorpha, and Lytechinus variegatus) after exposure to a pesticide and herbicide. The pesticide, Bayluscide, appeared to affect mitochondrial membrane potential in the sperm of all three species, as measured with MitoTracker Red CMXRos. A decrease in the percentage of sperm stained with SYBR-14 (indicating uncompromised plasma membrane) was observed in C. virginica and D. polymorpha sperm exposed to Bayluscide, but propidium iodide staining (indicating compromised plasma membranes) appeared to be inhibited by Bayluscide. Acrosome-reacted sperm, as measured by FITC-PNA, decreased after Bayluscide exposure in C. virginica and D. polymorpha sperm. The herbicide, Roundup Ready To-Use-Plus, did not affect the overall percentages of sperm stained with MitoTracker but did cause an increase in MitoTracker fluorescence intensity at 16 mg/L in D. polymorpha. Roundup also caused significant decreases in SYBR-14 staining, significant increases in propidium iodide staining, and significant increases in FITC-PNA staining in D. polymorpha sperm. By not having to rely on egg availability and optimal sperm density, sperm toxicity can be more accurately assessed with flow cytometry as being directly correlated to sperm viability rather than the possibility of altered toxicity results due to sperm-to-egg compatibility.

Assessment of gamete quality for the eastern oyster (Crassostrea virginica) by use of fluorescent dyes

Evaluation of sperm motility is the single most widely used parameter to determine semen quality in mammals and aquatic species. While a good indicator for fresh sperm viability, post-thaw motility is not always effective at predicting fertilizing ability. Techniques using fluorescent dyes can assess functionality of mammalian sperm, but have not been widely applied in aquatic organisms. The eastern oyster Crassostrea virginica is an important mollusk in the United States, and cryopreservation protocols have been developed to preserve sperm and larvae to assist research and hatchery production. In this study, protocols were developed to assess sperm cell membrane integrity and mitochondrial function by flow cytometry and to assess viability of eggs by fluorescence microscopy. The fluorescent dyes SYBR 14 and propidium iodide (PI) (to assess membrane integrity) and rhodamine 123 (R123) (to assess mitochondrial membrane potential) were used to evaluate the quality of thawed oyster sperm previously cryopreserved with different cryoprotectant and thawing treatments. Membrane integrity results were correlated with motility of thawed sperm and mitochondrial membrane potential with fertilizing ability. Fluorescein diacetate (FDA) was used to assess cytotoxicity of cryoprotectant solutions and post-thaw damage to oyster eggs. The results indicated that membrane integrity (P=0.004) and thawing treatments (P=0.04), and mitochondrial membrane potential (P=0.0015) were correlated with motility. Fertilizing ability was correlated with cryoprotectant treatments (P=0.0258) and with mitochondrial membrane potential (P=0.001). The dye FDA was useful in indicating structural integrity of fresh and thawed eggs. Exposure of eggs, without freezing, to dimethyl sulfoxide yielded higher percentages of stained eggs and fertilization rate than did exposure to propylene glycol (P=0.002). Thawed eggs were not stained with FDA (<1%) and larvae were not produced.

Ultraviolet damages sperm mitochondrial function and membrane integrity in the sea urchin Anthocidaris crassispina

Effects of ultraviolet A (UVA) and ultraviolet B (UVB) on mitochondrial function and membrane integrity of sea urchin sperm were investigated using flow cytometry and fluorescent probes. Both UVA and UVB impaired sperm mitochondrial function in a dose-dependent manner. Covariance analysis further showed that the slopes of change in mitochondrial function in relation to UVA and UVB were significantly different, suggesting that the modes of action were different. UVA did not affect membrane integrity, while membrane integrity showed a linear reduction with increasing UVB doses. Sperm mitochondria function showed significant positive correlations with sperm motility and subsequent fertilization success. Overall, our results showed that both UVA and UVB could decrease sperm motility and fertilization success through impairment of mitochondrial function, whereas UVB alone could cause additional damage through impairing the functional integrity of sperm membrane. Mitochondrial function of sperm may also offer a reliable ecotoxicological biomarker for predicting fertilization success in urchins.