Lipid droplet analysis using in vitro bovine oocytes and embryos

Zooming into lipid droplet biology through the lens of electron microscopy

Electron microscopy (EM), in its various flavors, has significantly contributed to our understanding of lipid droplets (LD) as central organelles in cellular metabolism. For example, EM has illuminated that LDs, in contrast to all other cellular organelles, are uniquely enclosed by a single phospholipid monolayer, revealed the architecture of LD contact sites with different organelles, and provided near-atomic resolution maps of key enzymes that regulate neutral lipid biosynthesis and LD biogenesis. In this review, we first provide a brief history of pivotal findings in LD biology unveiled through the lens of an electron microscope. We describe the main EM techniques used in the context of LD research and discuss their current capabilities and limitations, thereby providing a foundation for utilizing suitable EM methodology to address LD-related questions with sufficient level of structural preservation, detail, and resolution. Finally, we highlight examples where EM has recently been and is expected to be instrumental in expanding the frontiers of LD biology.

Beyond energy provider: multifunction of lipid droplets in embryonic development

Since the discovery, lipid droplets (LDs) have been recognized to be sites of cellular energy reserves, providing energy when necessary to sustain cellular life activities. Many studies have reported large numbers of LDs in eggs and early embryos from insects to mammals. The questions of how LDs are formed, what role they play, and what their significance is for embryonic development have been attracting the attention of researchers. Studies in recent years have revealed that in addition to providing energy for embryonic development, LDs in eggs and embryos also function to resist lipotoxicity, resist oxidative stress, inhibit bacterial infection, and provide lipid and membrane components for embryonic development. Removal of LDs from fertilized eggs or early embryos artificially leads to embryonic developmental arrest and defects. This paper reviews recent studies to explain the role and effect mechanisms of LDs in the embryonic development of several species and the genes involved in the regulation. The review contributes to understanding the embryonic development mechanism and provides new insight for the diagnosis and treatment of diseases related to embryonic developmental abnormalities.

Free fatty acid treatment of mouse preimplantation embryos demonstrates contrasting effects of palmitic acid and oleic acid on autophagy

Treatment of mouse preimplantation embryos with elevated palmitic acid (PA) reduces blastocyst development, while co-treatment with PA and oleic acid (OA) together rescues blastocyst development to control frequencies. To understand the mechanistic effects of PA and OA treatment on early mouse embryos, we investigated the effects of PA and OA, alone and in combination, on autophagy during preimplantation development in vitro. We hypothesized that PA would alter autophagic processes and that OA co-treatment would restore control levels of autophagy. Two-cell stage mouse embryos were placed into culture medium supplemented with 100 μM PA, 250 μM OA, 100 μM PA and 250 μM OA, or KSOMaa medium alone (control) for 18 - 48 h. The results demonstrated that OA co-treatment slowed developmental progression after 30 h of co-treatment but restored control blastocyst frequencies by 48 h. PA treatment elevated LC3-II puncta and p62 levels per cell while OA co-treatment returned to control levels of autophagy by 48 h. Autophagic mechanisms are altered by non-esterified fatty acid (NEFA) treatments during mouse preimplantation development in vitro, where PA elevates autophagosome formation and reduces autophagosome degradation levels, while co-treatment with OA reversed these PA-effects. Autophagosome-lysosome co-localization only differed between PA and OA alone treatment groups. These findings advance our understanding of the effects of free fatty acid exposure on preimplantation development, and they uncover principles that may underlie the associations between elevated fatty acid levels and overall declines in reproductive fertility.

Ovarian cholesterol efflux: ATP-binding cassette transporters and follicular fluid HDL regulate cholesterol content in mouse oocytes†

High density lipoproteins (HDL) take up cholesterol from peripheral tissues via ABC transporters and deliver it to the liver via scavenger receptor class B type I (SR-B1). HDL are the main lipoproteins present in follicular fluid (FF). They are thought to derive from plasma, but their origin is still controversial. SR-B1 knock-out (KO) mice have provided important evidence linking HDL metabolism and female fertility. These mice have cholesterol-rich circulating HDL and female infertility that can be restored by treating mice with the cholesterol-lowering drug probucol. Ovulated oocytes from SR-B1 KO females are dysfunctional and show excess cholesterol. The mechanisms explaining the contribution of FF HDL to oocyte cholesterol homeostasis are unknown. Here, using quantitation of filipin fluorescence we show that in SR-B1 KO ovaries, cholesterol excess is first observed in immature oocytes in antral follicles. By performing cross-transplant experiments between WT and apolipoprotein A-I deficient (ApoA-I KO) mice, which lack the main protein component of HDL, we provide evidence supporting the plasmatic origin of FF HDL. Also, we demonstrate that probucol treatment in SR-B1 KO females results in lowering of cholesterol content in their oocytes. Incubation of oocytes from SR-B1 KO mice with purified WT HDL reduces their cholesterol content, suggesting that HDL promote efflux of excess cholesterol from oocytes. In agreement with this hypothesis, we identified ABC transporters in oocytes and observed that ABCA1 KO oocytes have excess cholesterol and lower viability than WT oocytes.

Distribution and size of lipid droplets in oocytes recovered from young lamb and adult ovine ovaries

This study evaluated the distribution and size of lipid droplets (LDs) in oocytes recovered from young and adult ovine ovaries. Collected oocytes were categorised on the basis of their major diameter (small (SO), 70-90 µm; medium (MO), >90-110 µm; large (LO), >110-130µm) and were stained with Nile red to detect LDs. In adult and young oocytes, a diffuse pattern distribution of LDs was dominant in all classes except adult LO and young SO and LO. Larger LDs (i.e. >3µm) were mostly present in young SO and LO, whereas smaller LDs (1-3µm) were detected in the other adult and young oocyte categories.

Lipid profile of bovine grade-1 blastocysts produced either in vivo or in vitro before and after slow freezing process

Currently, in vitro embryo production (IVP) is successfully commercially applied in cattle. However, the high sensitivity of embryos to cryopreservation in comparison to in vivo (IVD) embryos slows the dissemination of this biotechnology. Reduced cryotolerance is frequently associated with lipid accumulation in the cytoplasm mainly due to in vitro culture conditions. The objective of this study was to evaluate the lipid composition of biopsied and sexed embryos, produced either in vivo or in vitro from the same Holstein heifers before and after a slow freezing protocol. Lipid extracts were analysed by liquid chromatography-high resolution mass spectrometry, which enabled the detection of 496 features. Our results highlighted a lipid enrichment of IVP embryos in triglycerides and oxidised glycerophospholipids and a reduced abundance in glycerophospholipids. The slow freezing process affected the lipid profiles of IVP and IVD embryos similarly. Lysophosphatidylcholine content was reduced when embryos were frozen/thawed. In conclusion, the embryonic lipid profile is impacted by IVP and slow freezing protocols but not by sex. Lysophosphatidylcholine seemed highly sensitive to cryopreservation and might contribute to explain the lower quality of frozen embryos. Further studies are required to improve embryo freezability by modulating the lipidome.

Lipid Metabolism in Bovine Oocytes and Early Embryos under In Vivo, In Vitro, and Stress Conditions

Lipids are a potential reservoir of energy for initial embryonic development before activation of the embryonic genome and are involved in plasma membrane biosynthesis. Excessive lipid droplet formation is detrimental to cryotolerance and is related to alterations in mitochondrial function, which likely affects lipid metabolism. Increased lipid accumulation in in vitro produced embryos is a consequence of the stress during in vitro embryonic development process. There are several open questions concerning embryo lipid metabolism and developmental potential. Oocyte maturation and embryo development in vivo and in vitro may vary if the donors are subjected to any type of stress before follicle puncture because crucial changes in oocyte/embryonic metabolism occur in response to stress. However, little is known about lipid metabolism under additional stress (such as heat stress). Therefore, in this review, we aimed to update the information regarding the energy metabolism of oocytes and early bovine embryos exhibiting developmental competence, focusing on lipid metabolic pathways observed under in vivo, in vitro, and stress conditions.

Effect of pterostilbene on development, equatorial lipid accumulation and reactive oxygen species production of in vitro-produced bovine embryos

The pterostilbene (PT) molecule is a phytoalexin with a reducing effect on reactive oxygen species (ROS) and with a capacity to block lipogenesis. However, the potential reducing effects of PT on equatorial lipid accumulation and ROS have not yet been elucidated for in vitro-derived bovine embryos. The present study evaluated the effects of concentrations of 3, 1, 0.33, 0.11 μM PT, and a vehicle group on the percentage of cleaved embryos, embryos with more than 6 cells, percentage of blastocyst on Day 7 and 8, percentage of transferable embryos on Day 7, the cell count and relative concentration of lipids. In the second experiment, the effects of 0.33 μM PT and a vehicle group within two different O₂ environments (5% and 20%) were evaluated for ROS generation and the percentage of Day 8 blastocysts. In the first experiment, no significant differences were found between the treatments with PT and the vehicle group (p > .05) concerning the percentage of cleaved embryos and embryos with more than 6 cells. Lipid reduction was observed in the groups treated with PT versus the vehicle group (p < .05). The vehicle group showed a higher rate of blastocyst production on Days 7 and 8 (p < .05) and an increase in the percentage of transferable embryos on Day 7 compared to the PT treatment groups (p < .05). Cell counts were not significantly different between treatments with PT and the vehicle group (p > .05). In the second experiment, the O₂ concentration did not significantly affect ROS generation (p > .05); however, the groups treated with PT (0.33 μM) had a reduction in ROS (p < .05). The O₂ concentration also did not significantly affect the rate of blastocyst production on Day 8 (p = .7696). Future research should be conducted to ascertain whether the reduction of lipids could enhance the cryopreservation and post-thaw viability of PT-treated embryos.

Recent progress in bovine in vitro-derived embryo cryotolerance: Impact of in vitro culture systems, advances in cryopreservation and future considerations

Cryopreservation of in vitro-derived bovine embryos is a crucial step for the widespread reproduction and conservation of valuable high-merit animals. Given the current popularity of bovine in vitro embryo production (IVP), there is a demand for a highly efficient ultra-low temperature storage method in order to maximize donor ovum pickup (OPU) turn-over, recipient availability/utilization and domestic/overseas commercial trading opportunities. However, IVP bovine embryos are still very sensitive to chilling and cryopreservation, and despite recent progress, a convenient (simple and robust) protocol has not yet been developed. At the moment, there are two methods for bovine IVP embryo cryopreservation: slow programmable freezing and vitrification. Both of the aforementioned techniques have pros and cons. While controlled-rate slow cooling can easily be adapted for direct transfer (DT), ice crystal formation remains an issue. On the other hand, vitrification solved this problem but the possibility of successful DT commercial incorporation remains to be determined. Moreover, simplification of the vitrification protocol (including warming) through the use of an in-straw dilution without the use of a microscope is a prerequisite for its use under farm conditions. This review summarizes the bovine IVP embryo cryopreservation achievements, strengths and limitations of both freezing systems and prospective improvements to enhance cryosurvival, as well as perspectives on future directions of this assisted reproductive technology.

Breed composition does not influence the performance of Holstein-Gyr crossbred as oocyte donors for OPU/IVP

Holstein-Gyr crossbred cattle are strategic for dairy systems in tropical countries, since they combine milk yield genetics with adaptability to tropical climate. However, Holstein (Bos taurus) and Gyr (Bos indicus) breeds present remarkable differences regarding reproductive physiology. Brazil stands out as the world’s largest user of embryo in vitro production (IVP) in bovine, and the use of this technique is increasing in dairy systems. As Holstein-Gyr crossbreds are important oocyte donors for IVP, the present work aimed at investigating whether increased Gyr or Holstein breed composition influences donor’s performance. Sixteen Holstein-Gyr crossbred females presenting increased (HG, 71.4 to 87.5% Holstein; n = 9) or decreased (GH, 40.2 to 46.6% Holstein; n = 7) Holstein composition were submitted to three ovum pick up (OPU) sessions. We observed similar (P = 0.2946) antral follicle count between HG and GH donors (24.8 ± 3.2 vs 29.4 ± 2.8 respectively; mean ± SEM). Groups also display similar morphological oocyte grading (Grade I: 0.1 ± 0.1 vs 0.1 ± 0.1 – P = 0.9680; Grade II: 0.9 ± 0.5 vs 1.9 ± 0.5 – P = 0.1942; Grade III, 4.0 ± 1.2 vs 7.2 ± 1.4 – P = 0.1047, HG vs GH respectively; mean ± SEM). Additionally, the proportion of viable oocyte was similar between HG and GH groups (27.8% vs 31.9%, respectively, P = 0.3500) and oocyte lipid area fraction (6.8% vs 9.5%, respectively; P = 0.1539). Our results indicate that the individual variation has more influence than breed composition of crossbred oocyte donors.

Effects of fetal calf serum on cGMP pathway and oocyte lipid metabolism in vitro

Intracellular levels of cyclic nucleotides, such as cGMP, are involved in the regulation of adipocyte lipolysis. Cumulus-oocyte complexes (COCs) express enzymes that both synthesise (guanylate cyclase) and degrade (phosphodiesterase (PDE) 5A) cGMP. Because serum interferes with lipid metabolism, its effects on the cGMP pathway and lipid content in bovine COCs were examined. COCs were matured in medium containing fetal calf serum (FCS; 2% or 10%) or 0.4% bovine serum albumin (BSA; control). At both 2% and 10%, FCS decreased cGMP levels in COCs compared with BSA (0.64 and 1.04 vs 9.46 fmol per COC respectively; P<0.05) and decreased transcript levels of guanylate cyclase 1, soluble, beta 3 (GUCY1B3), whereas PDE5A levels were increased. FCS also affected the expression of genes related to lipolysis, increasing relative expression of perilipin 2 (PLIN2) and carnitine palmitoyltransferase 1B (CPT1B) in cumulus cells. Effects of FCS and cGMP on the lipid content of oocytes and embryos were evaluated by Nile red staining. COCs were matured with 10% FCS, FCS+10^-5 M sildenafil (SDF), a PDE5 inhibitor, or 0.4% BSA. The lipid content was increased in oocytes matured in FCS compared with BSA (fluorescence intensity 20.1 vs 17.61 respectively; P<0.05), whereas the lipid content in oocytes matured in FCS+SDF (fluorescence intensity 16.33) was similar to that in the BSA-treated group (P>0.05). In addition, lipid content was higher in embryos from oocytes matured with FCS than BSA (fluorescence intensity 31.12 vs 22.31 respectively; P<0.05), but was increased even further in the FCS+SDF-treated group (fluorescence intensity 40.35; P<0.05), possibly due to a compensatory mechanism during embryo culture without SDF for the reduction in lipid content during IVM. The present study provides, for the first time, evidence that the cGMP pathway may be involved in lipid metabolism in bovine COCs and that this pathway is affected by FCS.

Interactions of bovine oocytes with follicular elements with respect to lipid metabolism

Among many factors, lipid metabolism within the follicular environment emerges as an important indicator of oocyte quality. In the literature a crucial significance is described concerning follicular fluid (FF) composition as well as messenger RNA (mRNA) expression in follicular cells. The aim of this study was to describe the relationship between oocyte, FF and follicular cells with regard to lipid metabolism. The set of data originating from individual follicles comprised: lipid droplets (LD) number in oocytes (BODIPY staining), mRNA expression of seven genes in cumulus and granulosa cells (SCD, FADS2, ELOVL2, ELOVL5, GLUT1, GLUT3, GLUT8; real time polymerase chain reaction) and fatty acid (FA) composition in FF (gas chromatography). Obtained results demonstrate significant correlation between oocyte lipid droplets number and FA composition in FF. However, gene expression studies show significant correlation between LD number and GLUT1 gene only. Moreover, the present experiment revealed correlations between FA content in FF and expression of several genes (SCD, FADS2, ELOVL5, GLUT8) in granulosa cells, whereas only the SCD gene in cumulus cells. We suggest that the results of our experiment indicate the importance of glucose : lipid metabolism balance, which contributes to better understanding of energy metabolism conversion between oocytes and the maternal environment.