Lag effect of microclimatic conditions on DNA integrity of frozen–thawed bovine sperm

Impact of bull age, sperm processing, and microclimatic conditions on the viability and DNA integrity of cryopreserved bovine sperm

Context Seasonal microclimatic fluctuations can cause changes in sperm quality even in dairy bulls bred under temperate climate. These changes can vary between sires of different age and affect sperm freezability. Aims We aimed to evaluate the modulating effect of bull age and equilibration time before freezing on the seasonal pattern of sperm viability and DNA integrity post-thaw. Methods In the frame of systematic sperm quality control, we assessed the integrity of sperm plasma membrane and acrosome (PMAI) in 15,496 cryopreserved bovine batches, and the percentage of sperm with high DNA fragmentation index (%DFI) after 0h and 3h incubation at 38°C post-thaw (3h) in 3422 batches. Semen was equilibrated for 24h before freezing if collected on Monday or Wednesday and 72h if produced on Friday. We investigated the effect of season, bull age, equilibration, and temperature-humidity index (THI) on the day of semen collection on sperm traits using mixed-effects linear models. Key results PMAI and %DFI (0h and 3h) deteriorated with increasing THI. The effect of THI on %DFI was detected with a 30-day time lag. Seasonal fluctuations of sperm quality were similar between young, mature, and older sires. Prolonged equilibration did not affect PMAI but was linked to elevated %DFI (3h) in summer. Conclusions Extending equilibration from 24 to 72h is compatible with commercial standards of bovine sperm quality post-thaw; however, it could interfere with the seasonal pattern of the latter. Implications Systematic monitoring of bovine sperm quality enables the prompt detection of stress factors related to microclimate and semen processing.

Heat stress and bull fertility

Since bull fertility may be adversely affected by hot humid conditions, the current increase in global temperature is of concern for future livestock production. Heat stress occurs when the body's normal physiological mechanisms to regulate body temperature cannot cope with external conditions. The testes and scrotum have their own complex regulatory mechanisms to protect developing sperm during their most vulnerable stages, but even these may be overwhelmed by unfavourable external conditions. The effects of mild, moderate and severe heat stress are somewhat different, with cattle exposed to mild and moderate heat stress apparently showing an adverse effect on fertility, whereas cattle in very hot, humid climates almost continuously may not exhibit any difference in sperm quality throughout the year. This apparent paradox may be due to differences in the cattle populations being studied, since they could differ in breed, age, purpose (beef versus dairy), or even in the methods used to assess sperm quality. The adverse effects on fertility may occur through the effects of reactive oxygen species on sperm DNA, or through perturbation of the production of antioxidants that usually protect sperm from oxidative attack. These effects can be mitigated to some extent by choosing breed and age of bulls with care, and adopting breeding strategies that avoid semen collection or ejaculation at the most adverse times of year. Husbandry measures such as controlled ventilation, misting, provision of shade or cool surfaces for lying down, could aid temperature regulation. Avoiding heat stress during late pregnancy aids calf growth in early life; careful feeding regimens for young bull calves create good conditions for sperm quality after puberty. Bull fertility is too important to be left to chance. Breeds should be chosen according to climate conditions and the purpose of livestock production.

Effect of season on bovine seminal plasma proteins in Thailand

Although season has been shown to affect bull sperm quality and fertility in some studies, the effect of season on seminal plasma proteins has not been examined. In the present study, seminal plasma proteins were analysed by Fast Protein Liquid Chromatography (FPLC), to separate the phosphorylcholine-binding proteins and heparin-binding proteins from the other proteins. Semen samples were collected from bulls in three seasons: winter, summer and the rainy season. Sperm quality was analysed by flow cytometry and computer assisted sperm analysis, and further aliquots of semen were used to prepare the seminal plasma for FPLC. Meteorological data were available from a location close to the bull station. There were slight differences in sperm kinematics between seasons, but other parameters of sperm quality were not different. Minor differences in the phosphorylcholine-binding proteins were detected according to season, being lower in summer than in winter or in the rainy season, although there were no changes in the heparin-binding proteins. Temperature, humidity and rainfall differed between winter and the rainy season, but no differences were observed between summer and the rainy season except in the temperature humidity index (THI). However, the THI was above the threshold indicative of heat stress in all seasons, which could explain why few seasonal differences in protein composition were detected in this study. Alternatively, the bulls could have been well-adapted to heat stress. In conclusion, there were only slight differences in bull sperm quality and seminal plasma proteins between seasons during this study.

Detection of protamine 2 in bovine spermatozoa and testicles

BACKGROUND

Sperm DNA integrity is crucial for transmission of genetic information to future generations and DNA damage can occur during chromatin packaging. Chromatin packaging involves the replacement of somatic nucleosomal histones by nuclear proteins called protamines. Protamine 1 (PRM1) is transcribed and translated in spermatids of all mammals; however, protamine 2 (PRM2) is transcribed in low levels in spermatids and it is not yet described in bull mature spermatozoa.

OBJECTIVES

The aim of this study was to assess gene and protein expression of PRM2 and corroborate gene and protein expression of PRM1 in bull spermatozoa and testis.

MATERIALS AND METHODS

For this purpose, absolute q-RT-PCR was performed to calculate the number of copies of PRM1 and PRM2 mRNAs in bovine epididymal spermatozoa and testicular tissue. Western blot and mass spectrometry were performed to identify PRM1 and PRM2 in samples of bovine epididymal spermatozoa. Samples of bovine testicular tissue were collected to identify PRM1 and PRM2 by immunohistochemistry.

RESULTS

We evaluated that the number of PRM1 mRNA copies was about hundred times higher than PRM2 mRNA copies in sperm and testicular samples (p < 0.0001). In addition, we estimated the PRM1: PRM2 ratio based on mRNA number of copies. In spermatozoa, the ratio was 1: 0.014, and in testicle, the ratio was 1: 0.009. We also evaluated the immunolocalization for PRM1 and PRM2 in bovine testis, and both proteins were detected in spermatids. Western blot and mass spectrometry in bovine epididymal spermatozoa confirmed these results.

CONCLUSION

Our work identifies, for the first time, PRM2 in bovine epididymal spermatozoa and in testis. Further studies are still needed to understand the role of PRM2 on the chromatin of the spermatozoa and to verify how possible changes in PRM2 levels may influence the bull fertility.

Effect of bovine sperm chromatin integrity evaluated using three different methods on in vitro fertility

In vitro fertility potential of individual bulls is still relatively uncharacterized. Classical sperm analysis does not include the evaluation of all sperm characteristics and thus, some cell compartments could be neglected. In humans, sperm DNA integrity has already proven to have major influence in embryo development and assisted reproduction techniques successfully. In bovine, some studies already correlated chromatin integrity with field fertility. However, none of those have attempted to relate DNA assessment approaches such as chromatin deficiency (CMA3), chromatin stability (SCSA; AO+) and DNA fragmentation (COMET assay) to predict in vitro bull fertility. To this purpose, we selected bulls with high and low in vitro fertility (n = 6/group), based on embryo development rate (blastocyst/cleavage rate). We then performed CMA3, SCSA test and COMET assay to verify if the difference of in vitro fertility may be related to DNA alterations evaluated by these assays. For the three tests performed, our results showed only differences in the percentage of cells with chromatin deficiency (CMA3+; high: 0.19 ± 0.03 vs low: 0.04 ± 0.04; p = 0.03). No difference for chromatin stability and any of COMET assay categories (grade I to grade IV) was observed between high and low in vitro fertility bulls. A positive correlation between AO + cells and grade IV cells was found. Despite the difference between groups in CMA3 analysis, our results suggest that protamine deficiency in bovine spermatozoa may not have a strong biological impact to explain the difference of in vitro fertility between the bulls used in this study.

Sperm cryodamage occurs after rapid freezing phase: flow cytometry approach and antioxidant enzymes activity at different stages of cryopreservation

Background

In order to improve the efficiency of bovine sperm cryopreservation process, it is important to understand how spermatozoa respond to differences in temperature as well as the ability to recover its own metabolism. The combination between flow cytometry approach and antioxidant enzymes activity allows a more sensible evaluation of sperm cell during cryopreservation. The aim of this study was to evaluate sperm attributes and antioxidant enzymes activity during different stages of cryopreservation process. Semen samples from Holstein bulls (n = 4) were separated in 3 treatments: fresh (37 °C); cooled (5 °C); and thawed. Evaluation occurred at 0 h and 2 h after incubation. Membrane integrity, mitochondrial membrane potential (MMP) and DNA damages were evaluated by flow cytometry; activities of antioxidant enzymes such as catalase, superoxide dismutase and gluthatione peroxidase were measured by spectrofotometry.

Results

There was an increase in the percentage of sperm with DNA damage in the thawed group, compared to fresh and cooled, and for 2 hs of incubation when compared to 0 h. Considering MMP, there was an increase in the percentage of cells with medium potential in thawed group when compared to fresh and cooled groups. Opposingly, a decrease was observed in the thawed group considering high mitochondrial potential. Also in the thawed group, there was an increase on cells with damaged acrosome and membrane when compared to fresh and cooled groups. Significant correlations were found between antioxidant enzymes activity and membrane or mitochondrial parameters.

Conclusion

Based on our results, we conclude that cryopreservation affects cellular and DNA integrity and that the critical moment is when sperm cells are exposed to freezing temperature. Also, our study indicates that intracellular antioxidant machinery (SOD and GPX enzymes) is not enough to control cryodamage.

Sperm Oxidative Stress Is Detrimental to Embryo Development: A Dose-Dependent Study Model and a New and More Sensitive Oxidative Status Evaluation

Our study aimed to assess the impact of sperm oxidative stress on embryo development by means of a dose-dependent model. In experiment 1, straws from five bulls were subjected to incubation with increasing H₂O₂ doses (0, 12.5, 25, and 50 μM). Motility parameters were evaluated by Computed Assisted System Analysis (CASA). Experiment 2 was designed to study a high (50 μM) and low dose (12.5 μM) of H₂O₂ compared to a control (0 μM). Samples were incubated and further used for in vitro fertilization. Analyses of motility (CASA), oxidative status (CellROX green and 2'-7' dichlorofluorescein diacetate), mitochondrial potential (JC-1), chromatin integrity (AO), and sperm capacitation status (chlortetracycline) were performed. Embryos were evaluated based on fast cleavage (30 h.p.i.), cleavage (D = 3), development (D = 5), and blastocyst rates (D = 8). We observed a dose-dependent deleterious effect of H₂O₂ on motility and increase on the percentages of positive cells for CellROX green, capacitated sperm, and AO. A decrease on cleavage and blastocyst rates was observed as H₂O₂ increased. Also, we detected a blockage on embryo development. We concluded that sperm when exposed to oxidative environment presents impaired motility traits, prooxidative status, and premature capacitation; such alterations resulting in embryo development fail.