Studies on membrane permeability of zebrafish (Danio rerio) oocytes in the presence of different cryoprotectants

Development of sperm cryopreservation protocol for patin buah, Pangasius nasutus

The development of sperm cryopreservation for Pangasius nasutus is necessary in order to serve the growing demand of this species through artificial fertilization and the preservation of valuable strains of male broodstocks. In the present study, the basic protocol of sperm cryopreservation for P. nasutus was established by identifying the optimal conditions for optimum cryoprotectant, toxicity of cryoprotectants, extenders, freezing condition and dilution ratio. Methanol (MeOH) at 10% concentration had the best post-thaw motility (26.3 ± 0.9%) and curvilinear velocity (VCL) compared to dimethyl acetamide and dimethyl sulfoxide. MeOH was the least toxic cryoprotectant; sperm suspended in 5 and 10% MeOH maintained motility up to 50 min. No significant differences were detected between the three types of extenders tested (0.9% sodium chloride, Calcium-free Hanks' Balance salt solution and ringer solution). P. nasutus sperm had a narrow range of optimal cooling rate. Significantly higher post-thaw motility was identified when cooling at 9.23 °C min-1, obtained by freezing at height of 14 cm above liquid nitrogen vapor for 7 min, showing lower cooling rate is suitable for this species. However, when cooling below and above the optimal cooling rate, post-thaw motility dropped drastically. There were no significant differences among the dilution ratios investigated, indicating the volume of cryodiluent at all tested ratios (1:9, 1:19 and 1:49) was sufficient for the protection of cells during the cryopreservation process. The development of the protocol for cryopreserved P. nasutus sperm will assist artificial seed production and provide an important tool for genetic and breeding research.

Determination of cell membrane permeability coefficients: Comparison of models in the case of oocytes

Values of cell membranes permeability coefficients for water and molecules of cryoprotective agents (CPAs) are the necessary characteristics for developing physical-mathematical models describing mass transfer processes through cell membranes in order to predict optimal cell cooling rates. We carried out a comparative analysis of the permeability coefficients of mouse oocyte membranes for molecules of water, ethylene glycol (EG), propane-1,2-diol (1,2-PD) and dimethyl sulfoxide (Me2SO), determined by applying the classical Kedem-Katchalsky model, which considers only the penetration of non-electrolyte molecules (water and CPA) through the membrane, and the model developed by us, which takes into account the transmembrane transfer of ions and the associated changes in the transmembrane electric potential. We shown that calculations based on the developed modified model provide lower values of the permeability coefficients of the oocyte membrane for water and CPA molecules. What is important that the obtained by our modified model permeability coefficients for water molecules do not depend on the type of cryoprotectant, while the application of the classical model both in our studies and works of other authors always gave different values of these coefficients in solutions with different cryoprotectants. Our modified model also makes it possible to determine the dynamics of the transmembrane electric potential of the cell under the conditions of transmembrane mass transfer and the duration of the membrane being influenced by the changes in electric potential, that is a parameter that can directly affect the viability of cells.

An Effective Chemical Permeabilization of Silkworm Embryos

The lipid layer surrounding the vitelline membrane of insect eggs has a critical role in the waterproofing and desiccation resistance of embryos. However, this lipid layer also prevents the flux of chemicals into the embryos, such as cryoprotectants, which are required for successful cryopreservation. The permeabilization studies of silkworm embryos remain insufficient. Therefore, in this study, we developed a permeabilization method to remove the lipid layer in the silkworm, Bombyx mori, and examined factors affecting the viability of dechorionated embryos, including the types and exposure times of chemicals and embryonic stages. Among the chemicals used, hexane and heptane were effective for permeabilization, whereas Triton X-100 and Tween-80 were less effective. Regarding the embryonic stages, there were significant differences between 160 and 166 h after egg laying (AEL) at 25 °C. Consequently, we found that the treatment of 160 AEL embryos with hexane for 30 s was the best condition for the permeability and viability of embryos, in which over 62% of the permeabilized embryos grew up to the second larval instar and their moths could lay fertilized eggs. Our method can be used for various purposes, including permeability investigations using other chemicals and embryonic cryopreservation.

Hydrogel encapsulation as a handling and vitrification tool for zebrafish ovarian tissue

Zebrafish is an important animal model, thousands lines have been developed, thus having a great need for their preservation. However, the cryopreservation of fish oocytes is still limited and needs improvement. The sodium alginate hydrogel, in addition to providing support for the cells, has been shown to be a potential cryoprotectant. Therefore, the aim of this study was to evaluate the sodium alginate hydrogel encapsulation technique efficiency during zebrafish ovarian tissue vitrification. The encapsulation methodology was standardized in the first experiment. In Experiment 2, we evaluated four vitrified groups: standard protocol without encapsulation (VS); encapsulated with cryoprotectants (VS1-A); encapsulated with half the cryoprotectants concentration (VS2-A); encapsulated without cryoprotectants (VA). VS treatment (54.6 ± 12.3%; 23.7 ± 9.9%; 12.6 ± 5.0%) did not differ from the VS1-A and VA showed a lower membrane integrity percentage (1.2 ± 1.4%; 0.3 ± 0.6%; 0.5 ± 1.5%). Mitochondrial activity was significantly greater in non-encapsulated treatment (VS) when compared to the encapsulated treatments. VS1-A and VS obtained the lowest lipid peroxidation (39.4 ± 4.4 and 40.5 ± 3.3 nmol MDA/mg respectively) in which VS was not significantly different from the VS2-A treatment (63.6 ± 3.1 nmol MDA/mg), unlike, VA obtained the highest lipid peroxidation level (124.7 ± 7.9 nmol MDA/mg). The results obtained in this study demonstrate that the sodium alginate hydrogel encapsulation technique did not have a cryoprotective action, but maintained the membrane integrity when used the standard concentration of cryoprotectants. However, halving the cryoprotectant concentration of fragments encapsulated in alginate hydrogel did not cause an increase in lipid peroxidation. In addition, it provided support and prevented the oocytes from loosening from the tissue during the vitrification process, being an interesting alternative for later in vitro maturation.

Wild coral oocytes are more amenable to low temperature preservation than cultured counterparts

Few studies have examined the biochemical differences between cultured and wild coral after undergoing low-temperature preservation. The present study aimed to explore the differences in the biochemical characteristics of cultured and wild coral cells and oocytes (Echinopora gemmacea and Oxypora lacera) in cryopreservation conditions. Wild and cultured coral cells were extracted and subjected to freezing experiments involving multiple types and concentrations of cryoprotectant, and the oocytes from the cultured and wild corals were subjected to chilling experiments. Cultured and wild coral cells exhibited no significant differences in viability or cell density after cryopreservation, whereas the oocytes from the cultured corals E. gemmacea and O. lacera exhibited lower chilling tolerance compared with their wild counterparts. Significant differences were observed between the oocytes from the cultured and wild corals after low-temperature preservation, particularly in their metabolic activity and vital status, which could be possibly attributed to food consumption and environmental factors. The study provides a foundation for research promoting the technological development of artificial coral propagation and cryopreservation.

Chilling sensitivity of Steindachneridion parahybae (Siluriformes: Pimelodidae) oocytes in different cryoprotectants

The viability of post-thaw fish oocytes can be affected by different stages of the freezing process, such as cryoprotectant toxicity, cold sensitivity, freezing curves and thawing. Therefore, these steps need to be investigated for the development of a protocol. In the present study, the aim was to investigate chilling sensitivity at different oocyte stages of Steindachneridion parahybae. Immature and mature oocytes were incubated in Hanks' or 90% L15 solutions containing different CPAs (cryoprotectant solutions) per experiment: (1) 0.1-0.4 M sucrose + 1-2 M methanol and (2) 1-4 M methanol X 1-4 M propylene glycol X 1-4 M DMSO for mature oocytes; (3) 0.5 M sucrose or fructose + 2 M methanol or PG or DMSO and (4) 0.25-1 M fructose + 1-4 M DMSO for immature oocytes. All treatments were kept for 120 min at -5.9 ± 2.8°C. For the control treatment, only Hanks' or 90% L15 solutions were carried out. Evaluations were made by viability tests: membrane integrity staining in 0.4% Trypan blue (TB) and fertilization rate (%F) sole for mature oocytes. Results presented that mature oocytes were the most sensitive to lower temperatures, because there was no %F. All cryoprotectants tested in the different concentrations can be used for immature oocytes, however the statistically superior cryoprotectant was CPA with fructose and DMSO, with the low concentration of this CPA being was the best statistically. This may indicate that for this species the immature stages have presented a lower chilling sensitivity than the mature stages.

Amphibian Assisted Reproductive Technologies: Moving from Technology to Application

Amphibians have experienced a catastrophic decline since the 1980s driven by disease, habitat loss, and impacts of invasive species and face ongoing threats from climate change. About 40% of extant amphibians are under threat of extinction and about 200 species have disappeared completely. Reproductive technologies and biobanking of cryopreserved materials offer technologies that could increase the efficiency and effectiveness of conservation programs involving management of captive breeding and wild populations through reduced costs, better genetic management and reduced risk of species extinctions. However, there are relatively few examples of applications of these technologies in practice in on-the-ground conservation programs, and no example that we know of where genetic diversity has been restored to a threatened amphibian species in captive breeding or in wild populations using cryopreserved genetic material. This gap in the application of technology to conservation programs needs to be addressed if assisted reproductive technologies (ARTs) and biobanking are to realise their potential in amphibian conservation. We review successful technologies including non-invasive gamete collection, IVF and sperm cryopreservation that work well enough to be applied to many current conservation programs. We consider new advances in technology (vitrification and laser warming) of cryopreservation of aquatic embryos of fish and some marine invertebrates that may help us to overcome factors limiting amphibian oocyte and embryo cryopreservation. Finally, we address two case studies that illustrate the urgent need and the opportunity to implement immediately ARTs, cryopreservation and biobanking to amphibian conservation. These are (1) managing the biosecurity (disease risk) of the frogs of New Guinea which are currently free of chytridiomycosis, but are at high risk (2) the Sehuencas water frog of Bolivia, which until recently had only one known surviving male.

Development of molecular markers for zebrafish (Danio rerio) ovarian follicle growth assessment following in-vitro culture in cryopreservation studies

Development of in vitro culture protocol for early stage ovarian follicles of zebrafish is important since cryopreserved early stage ovarian follicles would need to be matured in vitro following cryopreservation before they can be fertilised. Development of molecular markers for zebrafish (Danio rerio) ovarian follicle growth assessment following in vitro culture of early stage zebrafish ovarian follicles in ovarian tissue fragments is reported here for the first time although some work has been reported for in vitro culture of isolated early stage zebrafish ovarian follicles. The main aim of the present study was to develop molecular markers in an optimised in vitro culture protocol for stage I and stage II zebrafish ovarian follicles in ovarian tissue fragments. The effect of concentration of the hormones human chorionic gonadotropin and follicle stimulating hormones, and additives such as Foetal Bovine Serum and Bovine Serum Albumin were studied. The results showed that early stage zebrafish ovarian fragments containing stage I and stage II follicles which are cultured in vitro for 24 h in 20% FBS and 100mIU/ml FSH in 90% L-15 medium at 28 °C can grow to the size of stage II and stage III ovarian follicles respectively. More importantly the follicle growth from stage I to stage II and from stage II to stage III were confirmed using molecular markers such as cyp19a1a (also known as P450aromA) and vtg1 genes respectively. However, no follicle growth was observed following cryopreservation and in vitro culture.

Viability assessment of primary growth oocytes following ovarian tissue vitrification of neotropical teleost pacu (Piaractus mesopotamicus)

Vitrification of ovarian tissue containing immature oocytes provides an important tool for protecting the endangered species and genetic diversity in aquatic species. Therefore, the main objective was to assess primary growth (PG) oocytes viability following ovarian tissue vitrification using histological analysis, two staining protocols (trypan blue or fluorescein diacetate combined with propidium iodide) and mitochondrial activity assay (MTT assay). In addition, oocyte histomorphometry was performed to evaluate the morphometric parameters after vitrification and the relationship with the occurrence of damage (nucleus and/or membrane) in PG oocytes. There was no significant difference among the vitrified oocytes using trypan blue dye or FDA + IP staining. Oocyte viability assessed using histological analysis showed that vitrification solution 2.0 M Me₂SO + 2.5 M etilenoglycol +0.5 M sucrose (VS3; 66.43 ± 4.68%) and 1.5 M methanol + 5.5 M Me₂SO + 0.5 M sucrose (VS5; 74.14 ± 3.71%) had the lowest viability rate. Similar results were observed in MTT assay where VS3 (1.63 ± 0.12) and VS5 (1.58 ± 0.09) had the lowest averages when compare with VS1 (2.39 ± 0.14), VS2 (1.78 ± 0.06) and VS4 (2.34 ± 0.19) (P = 0.0002). In membrane damage evaluation by histology, there was no difference among vitrified oocytes and control. However, the highest percentages of nucleus damage were observed in treatments VS3 (26.00 ± 5.55) and VS5 (26.00 ± 5.55). Oocyte diameter did not change after vitrification; however, nucleus diameter was significantly higher in control group (49.03 ± 1.07). Oocyte viability by histological analysis was positive-correlated to the occurrence of nucleus (r² = 0.78) and membrane (r² = 0.45) damage after vitrification/warming. The high viability of PG oocytes obtained after ovarian tissue vitrification of Piaractus mesopotamicus suggests that the protocol applied here might be used successfully in other teleost species for food production.

Oocyte viability and cortical activation under different salt solutions in Prochilodus lineatus (Teleostei: Prochilodontidae)

This study aimed to evaluate the effect of five salt solutions in the maintenance of morphological features of cortical alveolus, hydration and fertilization capacity of Prochilodus lineatus oocytes. For this purpose, five saline solutions were tested: Ringer's solution, Ringer's lactate solution, Hank's balanced salt solution (HBSS), Hank's balanced salt solution without calcium (HBSS without calcium) and solution for salmonid eggs. Oocytes were maintained for 2 hr in saline solution with controlled temperature subsequently evaluated for hydration, cortical activation and fertilization ability. In the evaluation of the fertilization ability, two controls were used: C1-fertilized oocytes after extrusion-and C2-oocytes kept in ovarian fluid and fertilized after 2 hr. There was a significant reduction in the viability of oocytes C2 (28.8% ± 12.9%) compared to C1 (65.3% ± 26.7%), and no significant differences were found between treatments HBSS and HBSS without calcium and C2. Only HBSS and HBSS without calcium maintained the non-activated state of the gametes, with a fertilization rate of 16.4% ± 6.7% and 5.6% ± 2.3%, respectively; however, they did not extend the viability of oocytes, such that they continued to undergo degradation during the storage period, similar to oocytes retained only in ovarian fluid.

Cryobanking of aquatic species

This review is focused on the applications of genome cryobanking of aquatic species including freshwater and marine fish, as well as invertebrates. It also reviews the latest advances in cryobanking of model species, widely used by the scientific community worldwide, because of their applications in several fields. The state of the art of cryopreservation of different cellular types (sperm, oocytes, embryos, somatic cells and primordial germ cells or early spermatogonia) is discussed focusing on the advantages and disadvantages of each procedure according to different applications. A special review on the need of standardization of protocols has also been carried out. In summary, this comprehensive review provides information on the practical details of applications of genome cryobanking in a range of aquatic species worldwide, including the cryobanks established in Europe, USA, Brazil, Australia and New Zealand, the species and type of cells that constitute these banks and the utilization of the samples preserved.

STATEMENT OF RELEVANCE

This review compiles the last advances on germplasm cryobanking of freshwater and marine fish species and invertebrates, with high value for commercial aquaculture or conservation. It is reviewed the most promising cryopreservation protocols for different cell types, embryos and larvae that could be applied in programs for genetic improvement, broodstock management or conservation of stocks to guarantee culture production.

A trial to cryopreserve immature medaka (Oryzias latipes) oocytes after enhancing their permeability by exogenous expression of aquaporin 3

Fish oocytes have not been cryopreserved successfully, probably because it is difficult to prevent intracellular ice from forming. Previously, we have shown in medaka that immature oocytes are more suitable for cryopreservation than mature oocytes or embryos, in terms of permeability. We have also shown in immature medaka oocytes that the exogenous expression of aquaporin 3 (AQP3), a water/cryoprotectant channel, promotes the movement of water and cryoprotectants through the plasma membrane. In the present study, we attempted to cryopreserve immature medaka oocytes expressing AQP3. We first examined effects of hypertonic stress and the chemical toxicity of cryoprotectants on the survival of the AQP3-expressing oocytes. Exposure to hypertonic solutions containing sucrose decreased the survival of oocytes, but the expression of AQP3 did not affect sensitivity to hypertonic stress. Also, AQP3 expression did not markedly increase sensitivity to the toxicity of cryoprotectants. Of the four cryoprotectants tested, propylene glycol was the least toxic. Using a propylene glycol-based solution, therefore, we tried to cryopreserve immature oocytes by vitrification. During cooling with liquid nitrogen, all intact oocytes became opaque, but many AQP3-expressing oocytes remained transparent. This indicates that the expression of AQP3 is effective in preventing intracellular ice from forming during cooling. During warming, however, all the AQP3-expressing oocytes became opaque, indicating that intracellular ice formed. Therefore, the dehydration and permeation by propylene glycol were still insufficient. Further studies are necessary to realize the cryopreservation of fish oocytes.

The effect of chilling and cryoprotectants on hard coral (Echinopora spp.) oocytes during short-term low temperature preservation

Understanding chilling sensitivity and chilling injury of coral oocytes, in the presence and absence of a cryoprotectant, is important in developing cryopreservation protocols, as well as for short-term storage and transport (e.g., for species conservation). The objective of this study was to investigate the chilling sensitivity of hard coral (Echinopora spp.) oocytes and the effectiveness of methanol (as a cryoprotectant) in protecting these oocytes during short-term, low temperature preservation. Oocytes were exposed to 0.5, 1, or 2 m methanol at 5, 0, or -5 °C for 1, 2, 4, 6, 8, 16, or 32 h, and their quality determined based on adenosine triphosphate (ATP) content. Methanol at 0.5 m was the most effective means to reduce chilling-induced reduction in ATP concentrations. Coral oocytes can be stored at room temperature for 4 h in filtered nature seawater with no detrimental effect on oocyte quality; however, in the present study, oocyte survival was extended for 8 h by addition of methanol in low concentrations (0.5 or 1 m) at low temperatures (5 and 0 °C). These findings should enhance conservation efforts and facilitate low-temperature transport of endangered and threatened coral species.

Design and characterization of genetically engineered zebrafish aquaporin-3 mutants highly permeable to the cryoprotectant ethylene glycol

Background

Increasing cell membrane permeability to water and cryoprotectants is critical for the successful cryopreservation of cells with large volumes. Artificial expression of water-selective aquaporins or aquaglyceroporins (GLPs), such as mammalian aquaporin-3 (AQP3), enhances cell permeability to water and cryoprotectants, but it is known that AQP3-mediated water and solute permeation is limited and pH dependent. To exploit further the possibilities of using aquaporins in cryobiology, we investigated the functional properties of zebrafish (Danio rerio) GLPs.

Results

Water, glycerol, propylene glycol and ethylene glycol permeability of zebrafish Aqp3a, -3b, -7, -9a, -9b, -10a and -10b, and human AQP3, was examined. Expression in Xenopus laevis oocytes indicated that the permeability of DrAqp3a and -3b to ethylene glycol was higher than for glycerol or propylene glycol under isotonic conditions, unlike other zebrafish GLPs and human AQP3, which were more permeable to glycerol. In addition, dose-response experiments and radiolabeled ethylene glycol uptake assays suggested that oocytes expressing DrAqp3b were permeated by this cryoprotectant more efficiently than those expressing AQP3. Water and ethylene glycol transport through DrAqp3a and -3b were, however, highest at pH 8.5 and completely abolished at pH 6.0. Point mutations in the DrAqp3b amino acid sequence rendered two constructs, DrAqp3b-T85A showing higher water and ethylene glycol permeability at neutral and alkaline pH, and DrAqp3b-H53A/G54H/T85A, no longer inhibited at acidic pH but less permeable than the wild type. Finally, calculation of permeability coefficients for ethylene glycol under concentration gradients confirmed that the two DrAqp3b mutants were more permeable than wild-type DrAqp3b and/or AQP3 at neutral pH, resulting in a 2.6- to 4-fold increase in the oocyte intracellular concentration of ethylene glycol.

Conclusion

By single or triple point mutations in the DrAqp3b amino acid sequence, we constructed one mutant with enhanced ethylene glycol permeability and another with reduced pH sensitivity. The DrAqp3b and the two mutant constructs may be useful for application in cryobiology.

On proper linearization, construction and analysis of the Boyle-van't Hoff plots and correct calculation of the osmotically inactive volume

The Boyle-van't Hoff (BVH) law of physics has been widely used in cryobiology for calculation of the key osmotic parameters of cells and optimization of cryo-protocols. The proper use of linearization of the Boyle-vant'Hoff relationship for the osmotically inactive volume (v(b)) has been discussed in a rigorous way in (Katkov, Cryobiology, 2008, 57:142-149). Nevertheless, scientists in the field have been continuing to use inappropriate methods of linearization (and curve fitting) of the BVH data, plotting the BVH line and calculation of v(b). Here, we discuss the sources of incorrect linearization of the BVH relationship using concrete examples of recent publications, analyze the properties of the correct BVH line (which is unique for a given v(b)), provide appropriate statistical formulas for calculation of v(b) from the experimental data, and propose simplistic instructions (standard operation procedure, SOP) for proper normalization of the data, appropriate linearization and construction of the BVH plots, and correct calculation of v(b). The possible sources of non-linear behavior or poor fit of the data to the proper BVH line such as active water and/or solute transports, which can result in large discrepancy between the hyperosmotic and hypoosmotic parts of the BVH plot, are also discussed.

Kinetics of osmotic water flow across cell membranes in non-ideal solutions during freezing and thawing

Cryopreservation requires quantitatively analytical models to simulate the biophysical responses of biomaterials during cryopreservation. The Mazur model and other improved ones, such as Karlsson model concerning solutions containing cryoprotectants (CPA), are somehow precluded by some minor points, particularly, the assumption of ideal solutions. To avoid the ideal solution assumption, in this study a new method is developed to simulate water transport across cell membranes in non-ideal solutions during cooling and thawing. The comparison between osmolalities calculated by the linear freezing-point depression used in this new method and other non-ideal ones is conducted and a good agreement is achieved. In addition, in an ideal case, besides a theoretical agreement, this new approach has been validated by its numerical simulation results. Comparisons between this new approach and the traditional ones with an ideal solution assumption have been conducted based on a spherical hypothetical cell. The main results are (1) the predicted non-ideal intracellular water content is larger than the ideal results; (2) the concentration of CPA solutions is directly proportional to the deviation between the non-ideal and ideal curves. In the end, this study presents a direct description of the degree of subcooling of the protoplasm during dynamic cooling. This study demonstrates that our experimental data-based method is a valid one with clear physical interpretations, convenient expressions and a more extensive application room than traditional ones.

Membrane permeability characteristics of bovine oocytes and development of a step-wise cryoprotectant adding and diluting protocol

Membrane permeability is very helpful for the optimization of effective cryopreservation protocols. In this study, experiments were performed to determine these characteristics for immature (germinal vesicle (GV)) and in vitro matured (metaphase II (MII)) bovine oocytes within 4-37 degrees C, and a new step-wise adding and diluting protocol for ethylene glycol (EG) was developed and verified. Osmotically inactive volumes (V(b)) of GV and MII oocytes were calculated to be 16.1% and 26.1%. The membrane permeability of the oocytes to water (L(p)) in the presence of EG were between 0.08-0.18 and 0.14-0.28 microm/min/atm, and the membrane permeability of the oocytes to solutes (Ps) were between 0.0011-0.0038 and 0.0029-0.0061 cm/min for GV and MII oocytes, respectively. The activation energies (E(a)) for L(p) and P(s) in the presence of EG were 3.68 and 6.84 kcal/mol for GV oocyte, while 3.62 and 0.83-9.08 kcal/mol for MII oocyte. The data indicated that L(p) and P(s) varied significantly between developmental stages and among temperatures evaluated. Based on these results, different protocols for EG adding and diluting from oocytes were developed and tested. The assessment of cleavage rate and embryonic development in vitro confirmed that the designed 4-step adding 2-step diluting protocol indicated a better outcome. The present study is helpful for better understanding of cryobiological properties and the design of cryopreservation protocols for bovine oocytes.

Effect of methanol and Me2SO exposure on mitochondrial activity and distribution in stage III ovarian follicles of zebrafish (Danio rerio)

In this study the effect of cryoprotectants that have been shown to be the least toxic to zebrafish ovarian follicles (methanol and Me(2)SO), on mitochondria of stage III ovarian follicles was evaluated. The mitochondrial distributional arrangement, mitochondrial membrane potential, mtDNA copy number, ATP levels and ADP/ATP ratios were assessed following exposure to cryoprotectants for 30 min at room temperature. Results obtained by confocal microscopy showed that 30 min exposure to 2M methanol induced a loss of membrane potential, although viability tests showed no decrease in survival even after 5h post-exposure incubation. Higher concentrations of methanol (3 and 4M) induced not only a decrease in mitochondrial membrane potential but also the loss of mitochondrial distributional arrangement, which suggested a compromised mitochondrial function. Furthermore 3 and 4M treatments resulted in a decrease in viability assessed by Fluorescein diacetate-Propidium iodide (FDA-PI) and in a decrease in mtDNA copy number and ADP/ATP ratio after 5h incubation following methanol exposure, indicating a delayed effect. The use of Me(2)SO, which is considered to be a more toxic CPA to zebrafish ovarian follicles than methanol, caused a decrease in viability and a sustained decrease in ATP levels accompanied by failure to maintain mtDNA copy number within 1h post-exposure incubation. These results indicated that even CPAs that are considered to have no toxicity as determined by Trypan blue (TB) and FDA-PI tests can have a deleterious effect on mitochondrial activity, potentially compromising oocyte growth and embryo development.

Development of cryopreservation protocols for early stage zebrafish (Danio rerio) ovarian follicles using controlled slow cooling

Cryopreservation of germplasm of aquatic species offers many benefits to the fields of aquaculture, conservation and biomedicine. Although successful fish sperm cryopreservation has been achieved with many species, there has been no report of successful cryopreservation of fish embryos and late stage oocytes which are large, chilling sensitive and have low membrane permeability. In the present study, cryopreservation of early stage zebrafish ovarian follicles was studied for the first time using controlled slow freezing. The effect of cryoprotectant, freezing medium, cooling rate, method for cryoprotectant removal, post-thaw incubation time and ovarian follicle developmental stage were investigated. Stages I and II ovarian follicles were frozen in 4M methanol and 3M DMSO in either L-15 medium or KCl buffer. Ovarian follicle viability was assessed using trypan blue, FDA+PI staining and ADP/ATP assay. The results showed that KCl buffer was more beneficial than L-15 medium, methanol was more effective than DMSO, optimum cooling rates were 2-4 degrees C/min, stepwise removal of cryoprotectant improved ovarian follicle viability significantly and stage I ovarian follicles were more sensitive to freezing. The results also showed that FDA+PI staining and ADP/ATP assay were more sensitive than TB staining. The highest follicle viabilities after post-thaw incubation for 2h obtained with FDA+PI staining were 50.7+/-4.0% although ADP/ATP ratios of the cryopreserved follicles were significantly increased indicating increased cell damage. Studies are currently being carried out on in vitro maturation of these cryopreserved ovarian follicles.

Studies on chilling sensitivity of early stage zebrafish (Danio rerio) ovarian follicles

Cryopreservation of fish gametes is of great importance in aquaculture, conservation and human genomic research. The creation of gamete cryobanks allows the storage of genetic material of targeted species for almost unlimited time periods. Cryopreservation has been successfully applied to fish sperm of many species, but there has been no success with fish embryos and oocytes. One of the obstacles to fish oocyte cryopreservation is their high chilling sensitivity and especially at subzero temperatures. Although studies on late stage oocyte cryopreservation has been carried out, there have been no reported studies on cryopreservation of early stage ovarian follicles. The aim of this study is to investigate the chilling sensitivity of early stage zebrafish ovarian follicles before developing protocols for their cryopreservation. Experiments were conducted with stage I (primary growth), stage II (cortical alveolus) and stage III (vetillogenesis) ovarian follicles, which were chilled in KCl buffer and L-15 medium for up to 144h at -1 degrees C in a low temperature bath. Ovarian follicles were also exposed to 2M methanol or 2M DMSO in L-15 medium for up to 168h at -1 and -5 degrees C, respectively. Control follicles were kept at 28 degrees C. Ovarian follicle viability was assessed using trypan blue staining. The results showed that stage I and II ovarian follicles are less sensitive to chilling than stage III follicles. These results were also confirmed following in vitro maturation of the chilled ovarian follicles. The results also showed that L-15 medium is more beneficial than KCl buffer for ovarian follicles at all stages. The presence of both methanol and DMSO reduced chilling sensitivity of ovarian follicles at all stages with methanol being the most effective. The study indicated that stage I and II follicles are less sensitive to chilling than stage III follicles, and that early stage zebrafish ovarian follicles may be better candidates for cryopreservation.

Cryopreservation of zebrafish (Danio rerio) oocytes using improved controlled slow cooling protocols

Cryopreservation of gametes provides a promising method to preserve fish genetic material. Previously we reported some preliminary results on cryopreservation of zebrafish (Danio rerio) oocytes using controlled slow cooling and determined the optimum cryoprotective medium and cooling rate for stage III zebrafish oocytes. In the present study, the effects of two different cryopreservation media, cryoprotectant removal method, final sample freezing temperature before LN(2) plunge, warming rate, and the post-thaw incubation time on oocyte viability were investigated. Commonly used cryoprotectant methanol and glucose were used in this study. Stage III zebrafish oocytes were frozen in standard culture medium 50% L-15 or in a sodium-free KCl buffer medium. Oocyte viability was assessed using trypan blue staining and ATP assay. The viability of oocytes frozen in KCl buffer was significantly higher than oocytes frozen in L-15 medium. The results also showed that fast thawing and stepwise removal of cryoprotectant improved oocyte survival significantly, with highest viability of 88.0+/-1.7% being obtained immediately after rapid thawing when assessed by trypan blue staining. However, after 2h incubation at 22 degrees C the viability of freeze-thawed oocytes decreased to 29.5+/-5.1%. Results also showed that the ATP level in oocytes decreased significantly immediately after thawing. All oocytes became translucent after freezing which complicated the use of GVBD test (in vitro maturation of oocytes followed by observation of germinal vesicle breakdown which results in oocytes becoming translucent). New oocyte viability assessment methods are urgently needed.

Preliminary studies on the vitrification of red sea bream (Pagrus major) embryos

The objective was to identify an appropriate cryoprotectant and protocol for vitrification of red sea bream (Pagrus major) embryos. The toxicity of five single-agent cryoprotectants, dimethyl sulfoxide (DMSO), propylene glycol (PG), ethylene glycol (EG), glycerol (GLY), and methyl alcohol (MeOH), as well as nine cryoprotectant mixtures, were investigated by comparing post-thaw hatching rates. Two vitrifying protocols, a straw method and a solid surface vitrification method (copper floating over liquid nitrogen), were evaluated on the basis of post-thaw embryo morphology. Exposure to single-agent cryoprotectants (10% concentration for 15 min) was not toxic to embryos, whereas for higher concentrations (20 and 30%) and a longer duration of exposure (30 min), DMSO and PG were better tolerated than the other cryoprotectants. Among nine cryoprotectant mixtures, the combination of 20% DMSO+10% PG+10% MeOH had the lowest toxicity after exposure for 10 min or 15 min. High percentages of morphologically intact embryos, 50.6+/-16.7% (mean+/-S.D.) and 77.8+/-15.5%, were achieved by the straw vitrifying method (20.5% DMSO+15.5% acetamide+10% PG, thawing at 43 degrees C and washing in 0.5M sucrose solution for 5 min) and by the solid surface vitrification method (40% GLY, thawing at 22 degrees C and washing in 0.5M sucrose solution for 5 min). After thawing, morphological changes in the degenerated embryos included shrunken yolks and ruptured chorions. Furthermore, thawed embryos that were morphologically intact did not consistently survive incubation.

The permeability to water and cryoprotectants of immature and mature oocytes in the zebrafish (Danio rerio)

To identify a stage feasible for the cryopreservation of zebrafish oocytes, we investigated the permeability to water and cryoprotectants of immature (stage III) and mature (stage V) oocytes. The permeability to water (microm/min/atm) of immature oocytes at 25 degrees C (0.37) was significantly higher than that of mature oocytes (0.10). The permeability (x10(-3)cm/min) of immature oocytes to ethylene glycol, propylene glycol, and Me(2)SO (1.49-3.03) at 25 degrees C was substantially higher than that of mature oocytes approximately 0. The permeability of immature oocytes to glycerol was also high (1.75), although the permeability could not be measured in mature oocytes. Immature oocytes would be more suitable than mature oocytes for conservation of the zebrafish.

Expression of aquaporin-3 improves the permeability to water and cryoprotectants of immature oocytes in the medaka (Oryzias latipes)

The permeability of the plasma membrane plays a crucial role in the successful cryopreservation of oocytes and embryos. Several efforts have been made to facilitate the movement of water and cryoprotectants across the plasma membrane of fish oocytes/embryos because of their large size. Aquaporin-3 is a water/solute channel that can also transport various cryoprotectants. In this study, we tried to improve the permeability of immature medaka (Oryzias latipes) oocytes to water and cryoprotectants by artificially expressing aquaporin-3. The oocytes were injected with aquaporin-3 cRNA and cultured for 6-7 h. Then, hydraulic conductivity (L(P)) and cryoprotectant permeability (P(S)) were determined from volume changes in a hypertonic sucrose solution and various cryoprotectant solutions, respectively, at 25 degrees C. The L(P) value of the cRNA-injected oocytes was 0.22+/-0.04 microm/min/atm, nearly twice larger than that of intact or water-injected oocytes (0.14+/-0.02 and 0.14+/-0.03 microm/min/atm, respectively). P(S) values of intact oocytes for ethylene glycol, propylene glycol, and DMSO were 1.36+/-0.34, 1.97+/-0.20, and 1.17+/-0.52 x 10(-3) cm/min, respectively. The permeability to glycerol could not be calculated because oocytes remained shrunken in the glycerol solution. On the other hand, cRNA-injected oocytes had significantly higher P(S) values (glycerol, 2.20+/-1.29; ethylene glycol, 2.98+/-0.36; propylene glycol, 3.93+/-1.70; DMSO, 3.11+/-0.74 x 10(-3) cm/min) than intact oocytes. When cRNA-injected oocytes were cultured for 12-14 h, 51% matured to the metaphase II stage, and 43% of the matured oocytes were fertilized and hatched following in vitro fertilization and 14 days of culture. Thus, the permeability of medaka oocytes to water and cryoprotectants was improved by the artificial expression of aquaporin-3, and the oocytes retained the ability to develop to term.

Methods for reverse genetic screening in zebrafish by resequencing and TILLING

Animal models provide an in vivo system to study gene function by transgenic and knockout approaches. Targeted knockout approaches have been very successful in mice, but are currently not feasible in zebrafish due to the inability to grow embryonic stem cells. As an alternative, a reverse genetic approach that utilizes screening by resequencing and/or TILLING (Targeting Induced Local Lesions INGenomes) of mutagenized genomes has recently gained popularity in the zebrafish field. Spermatogonia of healthy males are mutagenized using ENU (N-ethyl-N-nitrosourea) and F1 progeny is collected by breeding treated males with healthy wild type females. Sperm and DNA banks are generated from F1 males. DNA is screened for ENU-induced mutations by sequencing or TILLING. These mutations can then be studied by in vitro fertilization (IVF) from the cryopreserved sperm of the corresponding F1 male followed by breeding to homozygosity. A high-throughput method of screening for rare heterozygotes and efficient recovery of mutant lines are important in identification of a large number of mutations using this approach. This article provides optimized protocols for resequencing and TILLING based on our experiences. We performed a pilot screen on 1235 F1 males by resequencing 54 exons from 17 genes and analyzed the sequencing data using multiple programs to maximize the mutation detection with minimal false positive detection. As an alternative to sequencing, we developed the protocols for TILLING by capillary electrophoresis using an ABI Genetic analyzer 3100 platform followed by fragment analysis using GeneScan and Genotyper softwares. PCR products generated by fluorescently labeled universal primers and tailed exon-specific primers were pooled 4-fold prior to heteroduplex formation. Overall, our pilot screen shows that a combination of TILLING and sequencing is optimal for achieving cost-effective, high-throughput screening of a large number of samples. Amplicons with fewer common SNPs are ideal for TILLING whereas amplicons with multiple SNPs and in/del polymorphisms are best suited for sequencing followed by analysis with SNPdetector.

Methanol alters ecto-nucleotidases and acetylcholinesterase in zebrafish brain

Methanol is a neurotoxic compound that is responsible for serious damage on CNS. Besides being found as an environmental contaminant, this alcohol is also employed as a component of cryoprotector solutions for zebrafish embryos. Here we tested the acute effect of methanol on ecto-nucleotidase (NTPDase, ecto-5'-nucleotidase) and acetylcholinesterase (AChE) activities in zebrafish brain. After acute treatment, there were significant decreases on ATP (26% and 45%) and ADP hydrolysis (26% and 30%) at 0.5% and 1.0%, respectively. However, no significant alteration on ecto-5'-nucleotidase activity was verified in zebrafish brain. A significant inhibition on AChE activity (39%, 33% and 30%) was observed at the range of 0.25% to 1.0% methanol exposure. Four NTPDase sequences were identified from phylogenetic analyses, which one is similar to NTPDase1 and the others to NTPDase2. Methanol was able to inhibit NTPDase1, two isoforms of NTPDase2 and AChE transcripts. To evaluate if methanol affects directly these enzymes activities, we have performed in vitro assays. ATP hydrolysis presented a significant inhibition (19% and 34%) at 1.5% and 3.0%, respectively, and ADP hydrolysis decreased only at 3.0% (29.2%). Nevertheless, AMP hydrolysis and AChE were not altered after in vitro exposure. The inhibitory effect observed on these enzymes could contribute to the neurodegenerative events promoted by methanol in zebrafish brain.