Simple and efficient in vitro fertilization with cryopreserved C57BL/6J mouse sperm

Effects of CO₂ Euthanasia of C57BL/6 Mice on Sperm Motility, In Vitro Fertilization, and Embryonic Developmental Competence

Cryopreservation of epididymal sperm collected after euthanasia is a common method to preserve and distribute valuable mouse models worldwide. However, the euthanasia method used prior to sperm collection must not adversely affect sperm quality. The most common method of euthanasia in mice is CO₂ asphyxiation, but its effect on the quality of sperm collected postmortem is largely unknown. The objective of this study was to determine the effects of CO₂ euthanasia of C57BL/6 mice on both freshly recovered sperm and sperm subjected to freezing and thawing. First, sperm concentration, progressive motility, curvilineal velocity (VCL), average path velocity (VAP), and progressive velocity (VSL) were analyzed for mice euthanized by cervical dislocation (CD), high flow CO₂ (100%), or low flow CO₂ (30%) displacement/minute, respectively. Then, in-vitro fertilization and embryonic development rates were determined using frozen-thawed sperm from each euthanasia method. Neither fresh nor frozen-thawed sperm showed significant differences in sperm concentration, progressive motility, VAP, or VCL when compared to CD and CO₂ groups. However, frozen-thawed sperm collected from CD mice had higher VCL values than did those collected from the low flow mice (P = 0.039). VCL was not different in fresh or frozen-thawed sperm collected after mouse euthanasia by CD as compared with high flow CO₂ or by high flow as compared with low flow CO₂. Frozen-thawed sperm showed no differences among the 3 euthanasia groups for fertilization (P = 0.452) or blastocyst development rates (P = 0.298). The results indicate that CO₂ euthanasia can be used as an alternative to CD to obtain optimal quality mouse sperm for cryopreservation while remaining compliant with welfare requirements.

An optimized method for cryogenic storage of Xenopus sperm to maximise the effectiveness of research using genetically altered frogs

Cryogenic storage of sperm from genetically altered Xenopus improves cost effectiveness and animal welfare associated with their use in research; currently it is routine for X. tropicalis but not reliable for X. laevis. Here we compare directly the three published protocols for Xenopus sperm freeze-thaw and determine whether sperm storage temperature, method of testes maceration and delays in the freezing protocols affect successful fertilisation and embryo development in X. laevis. We conclude that the protocol is robust and that the variability observed in fertilisation rates is due to differences between individuals. We show that the embryos made from the frozen-thawed sperm are normal and that the adults they develop into are reproductively indistinguishable from others in the colony. This opens the way for using cryopreserved sperm to distribute dominant genetically altered (GA) lines, potentially saving travel-induced stress to the male frogs, reducing their numbers used and making Xenopus experiments more cost effective.

•

Xenopus cryopreservation is robust using an optimized method.

•

Success is dependent on the quality of animals from which the sperm are taken.

•

Frozen sperm may now be used to distribute lines and wild-type male gametes around the world.

Cryorecovery of Mouse Sperm by Different IVF Methods Using MBCD and GSH

Different protocols incorporating methyl-β-cyclodextrin (MBCD) and reduced glutathione (GSH) have been reported to improve IVF recovery of cryopreserved mouse sperm on a C57BL/6 (J and N) genetic background. However, it is not clear which IVF protocol is most appropriate when using the various methods to cryorecover sperm with different sperm quality and sample volumes. Therefore, in the present study we correlated sperm motility with fertilization rate and compared the efficiency of different IVF methods using various sperm samples so as to establish general guidelines for mouse sperm cryorecovery by IVF. High linear correlation between sperm fertilization rate and progressive motility was found, R² was 0.9623 and 0.9993 for pre-freezing and post-thaw progressive motility, respectively. High amounts of cryoprotective agent (CPA) were observed to impair both sperm capacitation and fertilization. Moreover, the presence of a large number of immotile sperm in the sperm-oocyte co-incubation drop was found to reduce IVF success which could be partially reversed by supplementation using monothioglycerol (MTG) during centrifugation. It was concluded that the efficiency of IVF using cryorecovered mouse sperm in media containing MBCD and GSH can be predicted from sperm progressive motility. High concentrations of CPA and immotile sperm should be mitigated prior to IVF. The optimum IVF method should be selected based on sperm sample volume and sperm parameters.

Mitochondrial hydrogen peroxide and defective cholesterol efflux prevent in vitro fertilization by cryopreserved inbred mouse sperm

Recent advances in the cryopreservation of mouse sperm have resulted in dramatically improved in vitro fertilization (IVF) rates, but the biological mechanisms underlying the techniques remain unclear. Two different classes of compounds have been widely utilized to improve the IVF rates of cryopreserved mouse sperm: antioxidants and cyclodextrins. To determine how cryopreservation reduces mouse sperm IVF and how antioxidants and cyclodextrins mitigate this effect, we examined sperm function and oxidative damage after cryopreservation, with and without treatments, in mouse strains important for biomedical research. Our investigation revealed mouse strain-specific effects on IVF by modulation of oxidative stress and cholesterol efflux of cryopreserved sperm. Antioxidants improved IVF rates of C57Bl6/J cryopreserved mouse sperm by reducing hydrogen peroxide produced by sperm mitochondria and ameliorating peroxidative damage to the sperm acrosome. Enhancing cholesterol efflux with cyclodextrin restored capacitation-dependent sperm function and IVF after cryopreservation of C57Bl/6J, C57Bl/6N, and 129X1 mouse sperm. Our results highlight two accessible pathways for continued development of IVF techniques for mouse sperm and provide novel endpoints prognostic of IVF success. These insights may improve sperm cryopreservation methods of other mouse strains and species.

Overview of new developments in and the future of cryopreservation in the laboratory mouse

The large-scale mutagenesis programmes underway around the world are generating thousands of novel GA mouse strains that need to be securely archived. In parallel with advances in mutagenesis, the procedures used to cryopreserve mouse stocks are being continually refined in order to keep pace with demand. Moreover, the construction of extensive research infrastructures for systematic phenotyping is fuelling demand for these novel strains of mice and new approaches to the distribution of frozen and unfrozen embryos and gametes are being developed in order to reduce the dependency on the transportation of live mice. This article highlights some contemporary techniques used to archive, rederive, and transport mouse strains around the world.

Large-scale mouse knockouts and phenotypes

Standardized phenotypic analysis of mutant forms of every gene in the mouse genome will provide fundamental insights into mammalian gene function and advance human and animal health. The availability of the human and mouse genome sequences, the development of embryonic stem cell mutagenesis technology, the standardization of phenotypic analysis pipelines, and the paradigm-shifting industrialization of these processes have made this a realistic and achievable goal. The size of this enterprise will require global coordination to ensure economies of scale in both the generation and primary phenotypic analysis of the mutant strains, and to minimize unnecessary duplication of effort. To provide more depth to the functional annotation of the genome, effective mechanisms will also need to be developed to disseminate the information and resources produced to the wider community. Better models of disease, potential new drug targets with novel mechanisms of action, and completely unsuspected genotype-phenotype relationships covering broad aspects of biology will become apparent. To reach these goals, solutions to challenges in mouse production and distribution, as well as development of novel, ever more powerful phenotypic analysis modalities will be necessary. It is a challenging and exciting time to work in mouse genetics.

Optimization of a protocol for cryopreservation of mouse spermatozoa using cryotubes

The rapid increase in the number of genetically modified mouse strains has produced a high demand for their frozen spermatozoa from laboratories and mouse banking facilities. Historically, plastic straws have been used preferentially as containers for frozen mammalian spermatozoa because spermatozoa frozen in plastic straws have a high survival rate after thawing. However, plastic straws are more fragile and are used less often than the cryotubes used for conventional cell freezing. In this study, we sought to develop a new protocol for sperm freezing using cryotubes as the container to increase the accessibility of mouse sperm cryopreservation. Epididymal spermatozoa were collected from mature ICR or C57BL/6J (B6) males and were suspended in 18% raffinose and 3% skim milk solution. We then optimized the following conditions using the sperm survival rate as an index: 1) distance of cryotubes from the surface of the liquid nitrogen at freezing, 2) volume of the sperm suspension in the cryotube and 3) temperature of warming sperm during thawing. The best result was obtained when cryotubes containing 10 µl of sperm suspension were immersed 1 cm below the surface of the liquid nitrogen and then thawed at 50 C. The fertilization rates using spermatozoa frozen and thawed using this method were 63.1% in ICR mice and 28.2% in B6 mice. The latter rate was increased to 62.3% by adding reduced glutathione to the fertilization medium. After embryo transfer, 68% and 62% of the fertilized oocytes developed into normal offspring in the ICR and B6 strains, respectively. These results show that cryotubes can be used for cryopreservation of mouse spermatozoa under optimized conditions. This protocol is easy and reproducible, and it may be used in laboratories that do not specialize in sperm cryopreservation.

Dissecting the molecular damage to stallion spermatozoa: the way to improve current cryopreservation protocols?

We review recent developments in the technology of freezing stallion sperm, paying special attention to the molecular lesions that spermatozoa suffer during freezing and thawing, such as osmotic stress, oxidative damage, and apoptotic changes. We also discuss the applicability of colloidal centrifugation in stallion sperm cryobiology. Increased knowledge about the molecular injuries that occur during cryopreservation may lead to improved protective techniques and thus to further improvements in fertility in the current decade.

Inhibition of in vitro fertilizing capacity of cryopreserved mouse sperm by factors released by damaged sperm, and stimulation by glutathione

Background

In vitro fertilization (IVF) of eggs by frozen and thawed C57BL/6J mouse sperm is inhibited by dead sperm and enhanced by preincubation of the sperm in calcium-free medium. In other species, the presence of sperm killed by freezing and thawing has been associated with the generation of hydrogen peroxide.

Methodology/Principal Findings

The proportion of eggs fertilized by cryopreserved C57BL/6J mouse sperm was increased significantly by increasing the volume of fertilization medium in which sperm and eggs were coincubated. Enhanced fertilization occurred even though the concentration of potentially fertile sperm was decreased fivefold. This suggested that if a putative soluble factor was inhibiting fertilization, dilution of that factor, but not the sperm, should increase the fertilization rate. This was achieved by coincubation of the gametes in cell culture inserts (Transwells^®) that during incubation were transferred progressively to wells containing fresh fertilization medium. Fertilization rates using inserts were high (66.6±2.4% versus 27.3%±2.8% in wells alone). On the assumption that the soluble factor could be H₂O₂, reduced glutathione was added to the fertilization medium. This enhanced fertilization rate significantly (76.6%±2.0% versus 21.2%±1.9%), while addition of oxidized glutathione did not (82.7%±6.5% with reduced glutathione; 44.5±8.8% with oxidized glutathione; 47.8%±12.1% with no glutathione). Positive effects of reduced glutathione on IVF were also seen with frozen 129S1, FVB, and C3H sperm, and sperm from two lines of genetically modified C57BL/6J mice.

Conclusions/Significance

IVF in cell culture inserts and addition of glutathione to fertilization medium significantly increased the proportion of eggs fertilized by cryopreserved mouse sperm from four inbred strains, suggesting that reactive oxygen species generated during fertilization inhibit fertilization. The modified IVF techniques developed here enhance the feasibility and efficiency of using cryopreserved sperm from genetically modified lines of inbred mice.

Archiving and distributing mouse lines by sperm cryopreservation, IVF, and embryo transfer

The number of genetically modified mouse lines has been increasing exponentially in the past few decades. In order to safeguard them from accidental loss and genetic drifting, to reduce animal housing cost, and to efficiently distribute them around the world, it is important to cryopreserve these valuable genetic resources. Preimplantation-stage embryos from thousands of mouse lines have been cryopreserved during the past two to three decades. Although reliable, this method requires several hundreds of embryos, which demands a sizable breeding colony, to safely preserve each line. This requirement imposes significant delay and financial burden for the archiving effort. Sperm cryopreservation is now emerging as the leading method for storing and distributing mouse lines, largely due to the recent finding that addition of a reducing agent, monothioglycerol, into the cryoprotectant can significantly increase the in vitro fertilization (IVF) rate in many mouse strains, including the most widely used C57BL/6 strain. This method is quick, inexpensive, and requires only two breeding age male mice, but it still remains tricky and strain-dependent. A small change in experimental conditions can lead to significant variations in the outcome. In this chapter, we describe in detail our sperm cryopreservation, IVF, and oviduct transfer procedures for storing and reviving genetically modified mouse lines.

A practical novel method for ensuring stable capacitation of spermatozoa from cryopreserved C57BL/6J sperm suspension

A large number of genetically modified mouse strains have been produced in recent years. Sperm cryopreservation is the most effective means of preserving these valuable strains, most of which have a C57BL/6 genetic background. However, the fertilization efficiency of sperm from several cryopreserved strains, including C57BL/6, is quite low. While new and improved methods of cryopreservation have been developed, the majority of sperm stocks have already been cryopreserved using traditional methods, such as storage in 18% raffinose and 3% skim milk (R18S3). Therefore, new thawing methods for these frozen stocks are needed. We have developed a new thawing method that involves selective collection of motile sperm and a preincubation medium that enhances capacitation. Motile sperm are selected simply by collecting a sample from the center of a dish, and capacitation is induced by the addition of methyl-beta-cyclodextrin, D-penicillamine, sodium citrate, and hypotaurine to modified Tyrode's solution. The fertilization rate of sperm prepared using this method was increased significantly compared to that of sperm thawed using the traditional method (63.9 vs 16.5%, P<0.01). These results demonstrate that this new in vitro fertilization method is an effective means of reviving C57BL/6 sperm cryopreserved in R18S3.

Biological methods for archiving and maintaining mutant laboratory mice. Part I: conserving mutant strains

The mouse is now firmly established as the model organism of choice for scientists studying mammalian biology and human disease. Consequently, a plethora of novel, genetically altered (GA) mouse lines have been created. In addition, the output from the large scale mutagenesis programmes currently under way around the world will increase the collection of GA mouse strains still further. Because of the implications for animal welfare and the constraints on resources, it would be unreasonable to expect anything other than those strains essential for ongoing research programmes to be maintained as breeding colonies. Unfortunately, unless the redundant strains are preserved using robust procedures, which guarantee their recovery, they will be lost to future generations of researchers.This chapter describes some of the preservation methods currently used in laboratories around the world to archive novel mouse strains.

Osmotic characteristics and fertility of murine spermatozoa collected in different solutions

Osmotic stress is an important factor that can result in cell damage during cryopreservation. Before ejaculation or collection for cryopreservation, murine spermatozoa are stored in epididymal fluid, a physiologically hyperosmotic environment (∼415 mmol/kg). The objectives of this study were to determine the osmotic tolerance limits of sperm motion parameters of ICR and C57BL/6 mouse spermatozoa collected in isosmotic (290 mmol/kg) and hyperosmotic (415 mmol/kg) media, and the effect of the osmolality of sperm collection media on sperm fertility after cryopreservation. Our results indicate that murine spermatozoa collected in media with different osmolalities (290 and 415 mmol/kg Dulbecco's phosphate buffered saline (DPBS)) appeared to have different osmotic tolerances for the maintenance of sperm motility and other motion parameters in both mouse strains. The hypo- and hyperosmotic treatments decreased motility and affected other motion parameters of spermatozoa collected in 290 mmol/kg DPBS. The extent of the change of motion parameters after treatments corresponded with the levels of osmotic stress. However, for spermatozoa collected in 415 mmol/kg DPBS, exposure to 290 mmol/kg DPBS tended to increase sperm motility and the quality of their motion parameters. The osmolality of sperm collection medium can affect murine sperm fertility. Spermatozoa collected in 415 mmol/kg medium showed higher fertility compared with spermatozoa collected in 290 mmol/kg as assessed by IVF. Results characterizing murine sperm osmotic tolerance collected in media with different osmolalities from different strains and the effect of collection media osmolality on sperm fertility after cryopreservation will be useful in designing cryopreservation protocols.

Conserving, distributing and managing genetically modified mouse lines by sperm cryopreservation

BACKGROUND

Sperm from C57BL/6 mice are difficult to cryopreserve and recover. Yet, the majority of genetically modified (GM) lines are maintained on this genetic background.

METHODOLOGY/PRINCIPAL FINDINGS

Reported here is the development of an easily implemented method that consistently yields fertilization rates of 70+/-5% with this strain. This six-fold increase is achieved by collecting sperm from the vas deferens and epididymis into a cryoprotective medium of 18% raffinose (w/v), 3% skim milk (w/v) and 477 microM monothioglycerol. The sperm suspension is loaded into 0.25 mL French straws and cooled at 37+/-1 degrees C/min before being plunged and then stored in LN(2). Subsequent to storage, the sperm are warmed at 2,232+/-162 degrees C/min and incubated in in vitro fertilization media for an hour prior to the addition of oocyte cumulus masses from superovulated females. Sperm from 735 GM mouse lines on 12 common genetic backgrounds including C57BL/6J, BALB/cJ, 129S1/SvImJ, FVB/NJ and NOD/ShiLtJ were cryopreserved and recovered. C57BL/6J and BALB/cByJ fertilization rates, using frozen sperm, were slightly reduced compared to rates involving fresh sperm; fertilization rates using fresh or frozen sperm were equivalent in all other lines. Developmental capacity of embryos produced using cryopreserved sperm was equivalent, or superior to, cryopreserved IVF-derived embryos.

CONCLUSIONS/SIGNIFICANCE

Combined, these results demonstrate the broad applicability of our approach as an economical and efficient option for archiving and distributing mice.

Generation of progeny from embryonic stem cells by microinsemination of male germ cells from chimeric mice

Mice chimeric for embryonic stem (ES) cells have not always successfully produced ES-derived offspring. Here we show that the male gametes from ES cells could be selected in male chimeric mice testes by labeling donor ES cells or host blastocytes with GFP. Male GFP-expressing ES-derived germ cells occurred as colonies in the chimeric testes, where the seminiferous tubules were separated into green and non-green regions. When mature spermatozoa from green tubules were used for microinsemination, GFP-expressing offspring were efficiently obtained. Using a reverse study, we also obtained ES-derived progeny from GFP-negative ES cells in GFP-labeled host chimeras. Furthermore, we showed this approach could be accelerated by using round spermatids from the testes of 20-day-old chimeric mice. Thus, this technique allowed us to generate the ES cell-derived progeny even from the low contributed chimeric mice, which cannot produce ES-origin offspring by natural mating.

Transgenic mouse colony management

This unit describes strategies and methods used to establish a colony of transgenic mice. Basic mating and genotyping strategies are introduced.

The improvement in fertilizing ability of cryopreserved mouse spermatozoa using laser-microdissected oocytes

Aim:  The C57BL/6 mouse strain is now commonly used for producing transgenic/knockout strains. However, the fertilizing ability of these spermatozoa decreases as a result of cryopreservaion. Although the micromanipulation technique has been established to increase their fertilizing ability, it requires a considerable degree of technical skill. In the present report, we investigate the simple microdissection of zona pellucida by laser to increase the fertilizing ability of cryopreserved spermatozoa. Methods:  C57BL/6J spermatozoa were cryopreserved using a solution consisting of 18% raffinose/3% skim milk. Oocytes of the same strain were placed in PB1 medium containing 0, 0.25, 0.50 or 0.75 mol sucrose. The zona pellucida of oocytes was microdissected by laser with different pulse lengths selected from 0.45 to 0.65 ms. Microdissected oocytes were then fertilized with cryopreserved spermatozoa, and the subsequent development of embryos was assessed. Results:  When oocytes were microdissected in PB1 medium without sucrose, 81.5% of the oocytes were fertilized. The fertilization rates increased significantly as the pulse length was lengthened when compared with oocytes with intact zona pellucida. Furthermore, normal offspring were obtained in all experiments. Conclusion:  The fertilizing ability of cryopreserved spermatozoa is improved when oocytes with their zona pellucida microdissected by laser were used. (Reprod Med Biol 2006; 5: 249-253).

MYC levels govern hematopoietic tumor type and latency in transgenic mice

Deregulated MYC expression has been implicated in the etiology of many human cancers, including hematopoietic malignancies. To explore the impact of widespread constitutive MYC expression in the hematopoietic compartment, we have used a vector containing regulatory elements of the Vav gene to generate transgenic mice. VavP-MYC mice are highly tumor-prone and the level of MYC was found to influence both the kinetics and nature of the malignancies that developed. Whereas aggressive T-cell lymphomas rapidly overwhelmed the highest-expressing line, late-onset monocytic tumors greatly predominated in 2 low-expressing lines. These monocytic tumors most likely arise from abnormal macrophage colony-stimulating factor (M-CSF)-dependent progenitor cells having enhanced self-generative capacity. There appears to be a sharp threshold for MYC-induced T-cell lymphomagenesis because merely doubling the MYC level in a low-expressing line by breeding homozygous transgenic animals switched the phenotype from primarily monocytic tumors to exclusively T-cell tumors. Even the low level of MYC, however, clearly affected T-cell cycling, size, and sensitivity to apoptosis, and coexpression of a BCL2 transgene promoted efficient T-cell lymphomagenesis. The implication is that MYC level affects the spontaneous acquisition of synergistic oncogenic mutations.

Osmotic tolerance of mouse spermatozoa from various genetic backgrounds: acrosome integrity, membrane integrity, and maintenance of motility

All cells have an intrinsic biophysical property related to their ability to undergo osmotically driven volume changes. This project is of fundamental importance to our understanding of the basic cryobiology of mouse spermatozoa. The objectives of this study were to determine the osmotic tolerance limits for (1) motility, (2) acrosome integrity, and (3) membrane integrity of mouse spermatozoa from multiple genetic backgrounds including: C57BL/6, BALB/c, FVB, C3H, 129/SVS2 hsd B6C3F1, CB6F1, and ICR. The maintenance of acrosomal and plasma membrane integrity was not affected by genetic background (p=0.13), however, there was an interaction between genetic background and osmolality. In addition, acrosome and plasma membrane integrity was highly correlated within each strain (p<0.01). In contrast to acrosome and plasma membrane integrity, the motility of spermatozoa from different genetic backgrounds fell sharply on both sides of isosmolality, both with and without return to isosmotic conditions. Exposure to hyposmotic conditions caused morphological changes in the spermatozoa, which inhibited motility. However, this morphological change was not reversible in all cases when returned to isosmotic conditions. The ability to maintain motility in an anisosmotic media was affected by genetic background, osmolality as well as the interaction between genetic background and osmolality (p<0.05). In conclusion, mice with different genetic backgrounds appear to have similar tolerance to osmotic changes in terms of sperm acrosome and plasma membrane integrity; however, the ability to maintain motility differs between genetic backgrounds.

T-cell lymphomas mask slower developing B-lymphoid and myeloid tumours in transgenic mice with broad haemopoietic expression of MYC

Deregulation of MYC expression occurs in many haematological malignancies. Previous studies modelling MYC-induced lymphomagenesis in the mouse used transgenic vectors that directed MYC overexpression in a lineage-specific manner. Here, we describe a transgenic mouse strain in which constitutive MYC expression is driven broadly in haemopoiesis by a vector containing regulatory elements of the Vav gene. Healthy young VavP-MYC17 mice had multiple haemopoietic abnormalities, most notably increased size and numbers of B-lymphoid cells, monocytes and megakaryocytes. The mice rapidly developed tumours and, surprisingly, these were exclusively T-cell lymphomas, mostly of mature CD4(+) CD8(-) T cells, a tumour type that is seldom seen in mouse models. To examine tumour development in the absence of the susceptible T cells, we bred VavP-MYC17 mice lacking the Rag1 recombinase. They survived longer and succumbed to tumours of several different haemopoietic cell types: pre-T cells, pro-B cells, macrophages and unusual progenitor cells. Thus, although T-lineage cells have the shortest latent period to transformation, the VavP-MYC17 transgene drives malignant transformation of multiple cell types and VavP-MYC17 mice provide a new model for tumours of multiple haemopoietic lineages.

Archiving mouse strains by cryopreservation

A great deal of time and energy goes into the creation of each new line of transgenic mice; established lines are expensive and labor-intensive to maintain. Archiving of mice by cryopreservation of germ cells or embryos represents a means to free up facility space, while protecting the line from loss due to environmental disasters, genetic drift, or infectious disease. The author reviews the available cryopreservation techniques and presents considerations for setting up a cryopreservation facility.

Long-term preservation of mouse spermatozoa after freeze-drying and freezing without cryoprotection

The widespread production of mice with transgenes, disrupted genes and mutant genes, has strained the resources available for maintaining these mouse lines as live populations, and dependable methods for gamete and embryo preservation in these lines are needed. Here we report the results of intracytoplasmic sperm injection (ICSI) with spermatozoa freeze-dried or frozen without a cryoprotectant after storage for periods up to 1.5 years. Freeze-dried samples were stored at 4 degrees C. Samples frozen without cryoprotection were maintained at -196 degrees C. After storage, spermatozoa were injected into the oocytes by ICSI. Zygotic chromosomes and fetal development at Day 15 of gestation were examined after 0, 1, 3, 6, 9, and 12 mo of sperm storage. When fresh spermatozoa were used for ICSI, 96% of resultant zygotes contained normal chromosomes, and 58% of two-cell embryos transferred developed to normal viable fetuses. Similar results were obtained when spermatozoa were frozen without cryoprotection and then used for ICSI (87% and 45%, respectively; P > 0.05) and after 12 mo of sperm storage (mean of six endpoints examined: 87% and 52%, respectively; P > 0.05). Freeze-drying decreased the proportion of zygotes with normal karyoplates (75% vs. 96%; P < 0.001) and the proportion of embryos that developed into fetuses (35% vs. 58%; P < 0.001), but similar to freezing, there was no further deterioration during 12 mo of storage (mean of six endpoints examined: 68% and 34%, respectively; P > 0.05). Live offspring were obtained from both freeze-dried and frozen spermatozoa after storage for 1.5 yr. The results indicate that 1) the freeze-drying procedure itself causes some abnormalities in spermatozoa but freezing without cryoprotection does not and 2) long-term storage of both frozen and freeze-dried spermatozoa is not deleterious to their genetic integrity. Freezing without cryoprotection is highly successful, simple, and efficient but, like all routine sperm storage methods, requires liquid nitrogen. Liquid nitrogen is also required for freeze-drying, but sperm can then be stored at 4 degrees C and shipped at ambient temperatures. Both preservation methods are successful, but rapid freezing without cryoprotection is the preferred method for preservation of spermatozoa from mouse strains carrying unique genes and mutations.

Oocyte activation after intracytoplasmic injection with sperm frozen without cryoprotectants results in live offspring from inbred and hybrid mouse strains

Re-establishment of mouse strains used for mutagenesis and transgenesis has been hindered by difficulties in freezing sperm. The use of intracytoplasmic sperm injection (ICSI) enables the production of embryos for the restoration of mouse lines using sperm with reduced quality. By using ICSI, simplified sperm-freezing methods such as snap freezing can be explored. We examined the capacity of embryos from the inbred C57Bl/6J and 129Sv/ImJ mouse strains, commonly used for transgenic and N-ethyl-N-nitrosourea mutagenesis purposes to develop to blastocysts in vitro and to term following ICSI with sperm frozen without cryoprotectant. The results were compared to F1 (C57BlxCBA) hybrid embryos. Following freezing, sperm were immotile but could fertilize oocytes at similar rates to fresh sperm. However, embryo development in vitro to the blastocyst stage was reduced in all three strains. No pups were born from C57Bl/6J or 129Sv/ImJ embryos obtained from frozen sperm following transfer to foster females, and only a limited number of F1 embryos developed to term. Activation of oocytes injected with frozen sperm with 1.7 mM Sr2+ (SrCl2) did result in the birth of pups in all three strains. We conclude that the inability of sperm frozen without cryoprotectants to effectively activate oocytes may affect embryo development to term and can be overcome by strontium activation. This may become an effective strategy for sperm preservation and the restoration of most popular strains used for genetic modifications.

Cryopreservation ofXenopus transgenic lines

Xenopus laevis has been widely used for molecular, cellular, and developmental studies. With the development of the sperm-mediated transgenic method, it is now possible to study gene function during vertebrate development by using this popular model. On the other hand, like other animal species, it is labor intensive, and the maintenance of transgenic lines is expensive. In this article, we investigated the possibility of using sperm-cryopreservation as a means to preserve transgenic frog lines. We demonstrated that cryopreserved sperms are viable but not fertile under our in vitro fertilization (IVF) conditions. However, by microinjecting cryopreserved sperm nuclei, we successfully regenerated a transgenic line carrying a double promoter transgene construct, where the marker gene encoding the green fluorescent protein (GFP) is driven by the gamma-crystallin gene promoter and a gene of interest, encoding a fusion protein of GFP with the matrix metalloproteinase stromelysin-3 (ST3-GFP), is driven by a heat shock-inducible promoter. We demonstrated the functional transmission of the ST3-GFP transgene by analyzing the phenotype of the F1 animals after heat-shock to induce its expression. Our method thus provides an inexpensive means to preserve transgenic frog lines and a convenient way for distribution of transgenic lines. Furthermore, the ease with which to microinject nuclei compared to the technically demanding transgenesis procedure with variable outcome should facilitate more laboratories to use transgenic Xenopus laevis for functional studies in vivo. Mol. Reprod. Dev. 67: 65-69, 2004.