Acrosomal integrity and capacitation are not influenced by sperm cryopreservation in the giant panda

Current knowledge in the biology of gametes and embryos from Carnivora

In addition to companion animals and laboratory species, about 270 carnivore species play fundamental ecological roles in different ecosystems. However, almost 40% of carnivore species are now threatened or endangered in the wild because of human activities. While protection of natural habitats is critical, it is equally important to better understand carnivore reproduction, including a solid knowledge in sperm, oocyte, and embryo biology, to maintain sustainable populations in the wild and in conservation breeding centers. Characterizing gamete and embryo biology is also needed to develop cryopreservation and assisted reproductive technologies to enhance conservation efforts. The objective of this review is to provide the most recent knowledge in the biology of sperm cells, oocytes, and early embryos across all carnivore families. Overall, most data originate from populations maintained in breeding centers or zoos. Characterizations of sperm biology and cryopreservation are far more advanced than for oocytes and embryos. Currently, sperm biology is mainly studied in Canids, Felids, Ursids, and Mustelids, with more emphasis on structural than functional properties. Importantly, fundamental studies of gamete and embryo biology in domestic dogs, cats, and ferrets have paved the way for more precise characterizations in wild counterparts as well as the development of cryopreservation and assisted reproductive technologies. A striking feature of spermatozoa across a wide range of Canids and Felids is the presence of teratospermia (>60% of abnormal sperm cells), which is related to the loss of genetic diversity in some populations. Although sperm structures differ across carnivore families, sperm biology remains difficult to compare because of the small amount of data in many species. Regarding oocyte biology and embryology, data are much scarcer than in sperm cells, with too few studies going beyond structural descriptions. More carnivore species and more individuals (especially from wild populations in addition to captive ones) must be studied to improve our understanding about comparative germplasm biology and develop adequate conservation breeding strategies including the use of cryobanking and assisted reproductive technologies.

Recent Progress in Spermatology Contributing to the Knowledge and Conservation of Rare and Endangered Species

There is a remarkable diversity in the animal kingdom regarding mechanisms underlying the production, maturation, structure, and function of sperm cells. Spermatology studies contribute to the knowledge of species diversity and also provide information about individual or population fitness. Furthermore, this fundamental research is required before collected spermatozoa can be used for conservation breeding, including assisted reproduction and cryobanking. This article aims to (a) review the most recent knowledge on sperm morphology and function in wild animal species, (b) analyze how this knowledge can be used to save species in their natural habitat or ex situ, and (c) propose future scientific directions in wildlife spermatology that could positively impact animal conservation. Variations in sperm structure and performance within and between species have multiple origins and significance. This collective body of knowledge enables the design and implementation of conservation strategies and action plans that integrate several disciplines. Expected final online publication date for the Annual Review of Animal Biosciences, Volume 10 is February 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Comparative Analysis of piRNA Profiles Helps to Elucidate Cryoinjury Between Giant Panda and Boar Sperm During Cryopreservation

Cryopreservation induces sperm cryoinjuries, including physiological and functional changes. However, the molecular mechanisms of sperm cryoinjury and cryoresistance are still unknown. Cryoresistance or the freeze tolerance of sperm varies across species, and boar sperm is more susceptible to cold stress. Contrary to boar sperm, giant panda sperm appears to be strongly freeze-tolerant and is capable of surviving repeated cycles of freeze-thawing. In this study, differentially expressed (DE) PIWI-interacting RNAs (piRNAs) of fresh and frozen-thawed sperm with different freeze tolerance capacity from giant panda and boar were evaluated. The results showed that 1,160 (22 downregulated and 1,138 upregulated) and 384 (110 upregulated and 274 downregulated) DE piRNAs were identified in giant panda and boar sperm, respectively. Gene ontology (GO) enrichment analysis revealed that the target DE messenger RNAs (mRNAs) of DE piRNAs were mainly enriched in biological regulation, cellular, and metabolic processes in giant panda and boar sperm. Moreover, Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis indicated that the target DE mRNAs of DE piRNAs were only distributed in DNA replication and the cyclic adenosine monophosphate (cAMP) signaling pathway in giant panda, but the cAMP, cyclic guanosine monophosphate (cGMP), and mitogen-activated protein kinase (MAPK) signaling pathways in boar sperm were considered as part of the olfactory transduction pathway. In conclusion, we speculated that the difference in the piRNA profiles and the DE piRNAs involved in the cAMP signaling pathway in boar and giant panda may have contributed to the different freeze tolerance capacities between giant panda and boar sperm, which helps to elucidate the molecular mechanism behind sperm cryoinjury and cryoresistance.

Effects of extender type on the quality of post-thaw giant panda (Ailuropoda melanoleuca) semen

Sperm cryopreservation is an essential approach for assisted reproduction and genetic resources conservation in captive giant pandas. Cryopreservation, however, leads to a significant decrease in sperm quality and, consequently, a low fertilization rate. Therefore, it is mandatory to disclose more suitable and efficient freezing strategies for sperm cryopreservation. In the present study, we compared for the first time the performance of two commercial freeze extender (INRA96 versus TEST) freezing methods on post-thawed semen quality. Semen cryopreserved with the INRA96 showed better total motility (73.00 ± 4.84% vs 57.56 ± 3.60%, P < 0.001), membrane integrity (60.92 ± 2.27% vs 40.53 ± 2.97%, P < 0.001) and acrosome integrity (90.39 ± 2.74% vs 84.26 ± 4.27%, P < 0.05) than stored with TEST. There was no significant difference in DNA integrity after thawing between the two extenders (95.69 ± 3.60% vs 94.26 ± 4.84%). In conclusion, the INRA96 method showed to be better for giant panda sperm cryopreservation and should therefore be recommended for use in order to increase success of artificial insemination.

Comparative Analysis of MicroRNA and mRNA Profiles of Sperm with Different Freeze Tolerance Capacities in Boar (Sus scrofa) and Giant Panda (Ailuropoda melanoleuca)

Post-thawed sperm quality parameters vary across different species after cryopreservation. To date, the molecular mechanism of sperm cryoinjury, freeze-tolerance and other influential factors are largely unknown. In this study, significantly dysregulated microRNAs (miRNAs) and mRNAs in boar and giant panda sperm with different cryo-resistance capacity were evaluated. From the result of miRNA profile of fresh and frozen-thawed giant panda sperm, a total of 899 mature, novel miRNAs were identified, and 284 miRNAs were found to be significantly dysregulated (195 up-regulated and 89 down-regulated). Combined analysis of miRNA profiling of giant panda sperm and our previously published data on boar sperm, 46, 21 and 4 differentially expressed (DE) mRNAs in boar sperm were believed to be related to apoptosis, glycolysis and oxidative phosphorylation, respectively. Meanwhile, 87, 17 and 7 DE mRNAs in giant panda were associated with apoptosis, glycolysis and oxidative phosphorylation, respectively. Gene ontology (GO) analysis of the targets of DE miRNAs showed that they were mainly distributed on membrane related pathway in giant panda sperm, while cell components and cell processes were tied to the targets of DE miRNAs in boar sperm. Finally, Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis of DE mRNAs indicated that most of these DE mRNAs were distributed in membrane signal transduction-related pathways in giant panda sperm, while those in boar sperm were mainly distributed in the cytokine-cytokine receptor interaction pathway and inflammatory related pathways. In conclusion, although the different freezing extenders and programs were used, the DE miRNAs and mRNAs involved in apoptosis, energy metabolism, olfactory transduction pathway, inflammatory response and cytokine-cytokine interactions, could be the possible molecular mechanism of sperm cryoinjury and freeze tolerance.

Transcriptome Sequencing Reveals the Differentially Expressed lncRNAs and mRNAs Involved in Cryoinjuries in Frozen-Thawed Giant Panda (Ailuropoda melanoleuca) Sperm

Sperm cryopreservation and artificial insemination are important methods for giant panda breeding and preservation of extant genetic diversity. Lower conception rates limit the use of artificial insemination with frozen-thawed giant panda sperm, due to the lack of understanding of the cryodamaging or cryoinjuring mechanisms in cryopreservation. Long non-coding RNAs (lncRNAs) are involved in regulating spermatogenesis. However, their roles during cryopreservation remain largely unexplored. Therefore, this study aimed to identify differentially expressed lncRNAs and mRNAs associated with cryodamage or freeze tolerance in frozen-thawed sperm through high throughput sequencing. A total of 61.05 Gb clean reads and 22,774 lncRNA transcripts were obtained. From the sequencing results, 1477 significantly up-regulated and 1,396 significantly down-regulated lncRNA transcripts from fresh and frozen-thawed sperm of giant panda were identified. GO and KEGG showed that the significantly dysregulated lncRNAs and mRNAs were mainly involved in regulating responses to cold stress and apoptosis, such as the integral component of membrane, calcium transport, and various signaling pathways including PI3K-Akt, p53 and cAMP. Our work is the first systematic profiling of lncRNA and mRNA in fresh and frozen-thawed giant panda sperm, and provides valuableinsights into the potential mechanism of cryodamage in sperm.

Mammalian fertility preservation through cryobiology: value of classical comparative studies and the need for new preservation options

Human-related fertility preservation strategies have enormous potential for helping sustain and protect other species, especially to assist managing or 'rescuing' the genomes of genetically valuable individuals, including endangered species. However, wider-scale applications are limited by significant physiological variations among species, as well as a lack of fundamental knowledge of basic reproductive traits and cryosensitivity. Systematic and comparative cryopreservation studies (e.g. on membrane biophysical properties and resilience to freezing temperatures) are required to successfully recover gametes and gonadal tissues after thawing and eventually produce healthy offspring. Such data are currently available for humans and a few laboratory and livestock animals, with virtually all other species, including wildlife, having gone unstudied. Interestingly, there also are commonalities among taxa that allow a protocol developed for one species to provide useful information or guidance for another. However, when a rare animal unexpectedly dies there is no time for a prospective understanding of that species' biophysical traits. Because the odds of success will be much lower in such instances, it is essential that more fundamental studies be directed at more species. But also worthwhile is thinking beyond these systematic characterisations to consider the potential of a 'universal preservation protocol' for animal biomaterials.

The antioxidant effects of soybean lecithin- or low-density lipoprotein-based extenders for the cryopreservation of brown-bear (Ursus arctos) spermatozoa

Egg yolk low-density lipoproteins (LDL) and soybean lecithin were evaluated as replacements for egg yolk in extenders used for the cryopreservation of brown-bear spermatozoa. The motility, viability and acrosomal status of post-thawed spermatozoa were analysed, and an egg-yolk extender was used as a control. The total antioxidant capacity of these extenders was tested. Soybean lecithin showed an effect that was dependent on the soybean concentration (2%, 3.5% and 5%) and source (Type A: 24% L-α-phosphatidylcholine, and Type B: 14-23% L-α-phosphatidylcholine). Only semen cryopreserved with 5% Type A soybean exhibited a sperm motility similar to that of semen cryopreserved in egg-yolk-based extender after thawing, although the sperm viability and acrosome status were not as high. Semen frozen in an extender containing LDL (10-15%) exhibited improved sperm viability in comparison with the control, but sperm motility was lower. The LDL-based extender exhibited a higher anti-oxidant activity than the egg-yolk extender and soy lecithin-based extenders. The extenders with higher anti-oxidant activity showed improvements in frozen sperm viability but lower semen motility. These results indicate that soybean lecithin did not have the same protective effect as egg yolk during the freezing of brown-bear spermatozoa but suggest that LDL (10-15%) could be a useful substitute for egg yolk in these extenders.

Comparative cryobiological traits and requirements for gametes and gonadal tissues collected from wildlife species

A major challenge to retaining viability of frozen gametes and reproductive tissues is to understand and overcome species-specificities, especially because there is substantial diversity in cryobiological properties and requirements among cell types and tissues. Systematic studies can lead to successful post-thaw recovery, especially after determining: 1) membrane permeability to water and cryoprotectant, 2) cryoprotectant toxicity, 3) tolerance to osmotic changes, and 4) resistance to cooling and freezing temperatures. Although species-dependency ultimately dictates the ability of specific cells and tissues to survive freeze-thawing, there are commonalities between taxa that allow a protocol developed for one species to be useful information for another. This is the reason for performing comparative cryopreservation studies among diverse species. Our laboratory has compared cellular cryotolerance, especially in spermatozoa, in a diverse group of animals-from corals to elephants-for more than 30 yrs. Characterizing the biophysical traits of gametes and tissues is the most efficient way to develop successful storage and recovery protocols, but, such data are only available for a few laboratory, livestock, and fish species, with virtually all others (wild mammals, birds, reptiles, and amphibians) having gone unstudied. Nonetheless, when a rare animal unexpectedly dies, there is no time to understand the fundamentals of biophysics. In these emergencies, it is necessary to rely on experience and the best data from taxonomically-related species. Fortunately, there are some general similarities among most species, which, for example, allow adequate post-thaw viability. Regardless, there is a priority for more information on biophysical traits and freezing tolerance of distinctive biomaterials, especially for oocytes and gonadal tissues, and even for common, domesticated animals. Our colleague, Dr John Critser was a pioneer in cryobiology, earning that moniker because of his advocacy and devotion to understanding the differences (and similarities) among species to better store living genetic material.

Giant panda (Ailuropoda melanoleuca) spermatozoon decondensation in vitro is not compromised by cryopreservation

Natural breeding of giant pandas in captivity is compromised, making artificial insemination and spermatozoa cryopreservation essential for genetic management. This study examined the influence of freeze-thawing on traditional parameters such as motility and spermatozoon functionality, specifically decondensation in vitro. Giant panda spermatozoa were assessed before and after rapid cryopreservation (4 degrees C to -130 degrees C over 2 min) in liquid nitrogen vapour. Spermatozoa pre-incubated in medium for 6 h were co-incubated with cat zonae (2 zonae microL(-1)) for 30 min to effect capacitation and an acrosome reaction. Spermatozoa were then mixed with mature cat oocyte cytoplasm (2 cytoplasm microL(-1)) for 4 h and evaluated for decondensation. Frozen spermatozoa were less motile (P < 0.05) than fresh counterparts immediately post-thawing, but not after 6 h incubation. There were more (P < 0.05) spermatozoa with completely diffused chromatin post-thaw (10.4 +/- 1.3%; mean +/- s.e.m.) compared to fresh counterparts (5.1 +/- 1.0%). However, there was no overall difference (P > 0.05) in the incidence of decondensation between fresh (4 h, 69.8 +/- 5.9%) and thawed (4 h, 71.5 +/- 4.9%) spermatozoa after exposure to cat oocyte cytoplasm. It is concluded that the 'rapid' method now used to cryopreserve giant panda spermatozoa has little impact on spermatozoon decondensation.

Challenges in cryopreservation of clouded leopard (Neofelis nebulosa) spermatozoa

The clouded leopard (Neofelis nebulosa) is an endangered species that is difficult to breed in captivity. Species management could benefit from the use of artificial insemination (AI) with frozen-thawed spermatozoa, but there have been no detailed studies of sperm cryosensitivity. The purposes of this study were to: (1) re-characterize seminal characteristics in the clouded leopard 20 years after the first descriptive studies Wildt et al., [Wildt DE, Howard JG, Hall LL, Bush M. Reproductive physiology of the clouded leopard. I. Electroejaculates contain high proportions of pleiomorphic spermatozoa throughout the year. Biol Reprod 1986; 34: 937-947]; and (2) conduct a comparative cryopreservation study on the feasibility of sperm from this species surviving a freeze-thawing stress. Ejaculates were collected from five adult males and subjected to standard analysis, followed by a two-step straw freezing protocol that evaluated the impact of thawing, dilution, centrifugation and in vitro culture (through 4 h) on sperm motility and acrosomal integrity. Additionally, we assessed the impact of both a traditional permeating cryoprotectant (glycerol at a final dilution of 4%) and an unconventional nonpermeating trisaccharide; raffinose (R) at a final dilution of 4% or 8%, with or without 4% glycerol on sperm cryosurvival. The clouded leopard produced an extremely poor quality ejaculate; although approximately 70% of fresh sperm were motile, >80% were malformed. Phase contrast microscopy revealed that 40% of all sperm had abnormal acrosomes, but Coomassie blue staining indicated that acrosomal abnormalities existed in almost 70% of spermatozoa. Upon freeze-thawing, sperm motility declined markedly (P < 0.05) by an average of 40%, regardless of diluent used. Interestingly, raffinose was as effective as glycerol in protecting both sperm motility and acrosomal integrity. Although no acrosomal damage was seen immediately after thawing, < 6% morphologically normal intact acrosomes were present by the last measured time point. In conclusion, the clouded leopard is a rare felid that (at least in North American zoos) is producing extraordinarily poor quality ejaculates. There are so many sperm with unexplained deranged acrosomes that it will be particularly challenging to use traditional AI with thawed sperm as an adjunct management tool.

The impact and potential etiology of teratospermia in the domestic cat and its wild relatives

Teratospermia (production of >60% morphologically abnormal sperm/ejaculate) is relatively common among various species in the family Felidae, which is comprised of 37 species. Over two decades of research in this area have produced a significant understanding of the phenotypic expression, its impacts on sperm function and etiology. There is good evidence suggesting that a reduction in genetic diversity contributes to this phenomenon. Results to date demonstrate that spermatozoa from teratospermic donors are compromised in the ability to undergo capacitation and the acrosome reaction, penetrate the zona-pellucida, fertilize conspecific oocytes and survive cryopreservation. Recent studies also reveal abnormalities in chromatin integrity in sperm from teratospermic donors, which, interestingly, fails to impact fertilization or embryo development after intracytoplasmic sperm injection. Through planned inbreeding studies, we now have established that teratospermic cats also produce more spermatozoa by virtue of more sperm producing tissue, more germ cells per Sertoli cell and reduced germ cell loss during spermatogenesis. Overall, it now is clear that gain in sperm quantity is achieved at the expense of sperm quality, suggesting an extensive disruption of normal testicular function in teratospermic donors. Preliminary studies on testicular gene expression in teratospermic cats have also revealed abnormal expression patterns. These findings have markedly increased our understanding of testis biology in the teratospermic donor and reaffirm the value of cats, including wild species, as models for studying novel regulatory mechanisms controlling spermatogenesis and spermiogenesis.

A Trial of Intrauterine Insemination Using a Fiberscope in the Giant Panda (Ailuropoda melanoleuca)

An intrauterine insemination technique using a fiberscope was investigated in the giant panda. A septal wall was present about 5 cm from the vulva, the external urethral orifice was present in the ventral region, and the entrance (pseudocervix) to the vagina was present in the dorsal region. The uterovaginal region protruded in the dorsal region about 15 cm from the pseudocervix. The external uterine orifice was present in the uterovaginal region, revealing that intrauterine insemination can be easily performed. This technique may greatly contribute to artificial reproduction of the giant panda.

Assessment of basic seminal characteristics, sperm cryopreservation and heterologous in vitro fertilisation in the fishing cat (Prionailurus viverrinus)

Conservation of the fishing cat, a threatened south-east Asian felid, could benefit from effective ex situ genetic management and breeding programmes, including the use of assisted reproduction. The aims of the present study were to: (1) characterise basal seminal traits of fishing cats in Thailand zoos; and (2) investigate the effect of cryopreservation on sperm motility, acrosomal integrity and in vitro function. Seminal traits were evaluated in electroejaculates collected from eight males. Spermatozoa were diluted in n-tris(hydroxymethyl)-methyl-2-aminoethanesulfonic acid Tris (TEST)-yolk buffer (TYB) without glycerol, then diluted further with TYB with glycerol (4% final concentration) at either 25°C or after slow cooling to 5°C and frozen in straws over liquid nitrogen vapour. After thawing, sperm function was assessed by insemination of viable domestic cat oocytes. Fishing cat ejaculates averaged (± s.e.m.) 43.6 ± 14.2 × 106 motile spermatozoa with 33.5 ± 6.8% normal sperm morphology. Semen processing had a negligible effect (P > 0.05) on sperm motility and acrosomal integrity, but values were reduced (P < 0.05) after thawing. All thawed samples fertilised domestic cat oocytes, with 62.1% (36/58) of mature oocytes cleaving. Glycerol addition at 5°C resulted in higher (P < 0.05) post-thaw motility and intact acrosomes than glycerol addition at 25°C. In conclusion, good-quality ejaculates can be obtained from Thai fishing cats and their spermatozoa exhibit adequate function after cryopreservation for in vitro fertilisation procedures.

Characteristics of Frozen-Thawed Spermatozoa Cryopreserved with Different Concentrations of Glycerol in Captive Japanese Black Bears (Ursus thibetanus japonicus)

Seven mature Japanese black bears were used as semen donors, and a total of 7 semen samples collected from the animals by the electroejaculation method were cryopreserved in liquid nitrogen. Egg yolk-TRIS-citrate-glucose extender was used, and the effects of different final concentrations of glycerol, at 4-12% (v/v), on frozen-thawed spermatozoa were examined. No significant difference was observed in percent motility or percent abnormal morphology of frozen-thawed spermatozoa among the different glycerol concentrations. Percent viability and percent intact acrosomes of spermatozoa cryopreserved with 4 and 6% glycerol were significantly higher than those with 10 and 12% glycerol. These results suggest that a suitable glycerol concentration for freezing Japanese black bear semen within the range tested would be 4-6%.

Atrial natriuretic peptide induces an acrosome reaction in giant panda spermatozoa and enhances their penetration of salt-stored porcine oocytes

Atrial natriuretic peptide (ANP) is a vasodilator peptide primarily produced in the heart. Locally synthesized ANP has been found in reproductive tissues of various mammals and humans, and plays an important role in rat oocyte maturation and human sperm function. The objective of the present study was to determine the effects of ANP on the function (acrosome reaction and zona penetration) of giant panda spermatozoa. In fresh and frozen-thawed spermatozoa that had been preincubated for 2.5h, treatment with ANP (for 60 min) significantly increased the proportion of acrosome-reacted spermatozoa; maximal response (an acrosome reaction in 18.3 and 21.8% of fresh and frozen-thawed spermatozoa, respectively) was detected at 1 nM ANP. Treatment with C-ANP-(4-23), an analogue of ANP and specific binder to natriuretic peptide receptors-C (NPRC), had no significant effect on the acrosome reaction. However, the cyclic guanosine 5'-monophosphate (cGMP)-dependent protein kinase (PKG) inhibitor KT5823 completely abolished the effect of ANP on acrosome reaction. The effects of ANP, caffeine and heparin on frozen-thawed sperm function were studied by insemination of porcine salt-stored oocytes in a modified Tris-buffered medium (mTBM). The presence of ANP, caffeine or heparin in the insemination medium resulted in a higher proportion (P < 0.05) of oocytes with spermatozoa in the zona and perivitelline space (PVS), and a higher average number of spermatozoa/oocyte (P < 0.05) in the zona and PVS. However, in the absence of ANP, caffeine and heparin, there were no oocytes with a spermatozoon in the PVS. There were no differences among ANP, caffeine or heparin treatments for the proportion of oocytes penetrated or average number of spermatozoa/oocyte in the zona and PVS. In conclusion, we inferred that ANP induced the acrosome reaction of preincubated giant panda spermatozoa by a PKG pathway. Furthermore, ANP enhanced the penetrability of porcine salt-stored oocytes by frozen-thawed giant panda spermatozoa.