Transcriptomic integrity of human oocytes used in ARTs: technical and intrinsic factor effects

BACKGROUND

Millions of children have been born throughout the world thanks to ARTs, the harmlessness of which has not yet been fully demonstrated. For years, efforts to evaluate the specific effects of ART have focused on the embryo; however, it is the oocyte quality that mainly dictates first and foremost the developmental potential of the future embryo. Ovarian stimulation, cryopreservation, and IVM are sometimes necessary steps to obtain a mature oocyte, but they could alter the appropriate expression of the oocyte genome. Additionally, it is likely that female infertility, environmental factors, and lifestyle have a significant influence on oocyte transcriptomic quality, which may interfere with the outcome of an ART attempt.

OBJECTIVE AND RATIONALE

The objective of this review is to identify transcriptomic changes in the human oocyte caused by interventions specific to ART but also intrinsic factors such as age, reproductive health issues, and lifestyle. We also provide recommendations for future good practices to be conducted when attempting ART.

SEARCH METHODS

An in-depth literature search was performed on PubMed to identify studies assessing the human oocyte transcriptome following ART interventions, or in the context of maternal aging, suboptimal lifestyle, or reproductive health issues.

OUTCOMES

ART success is susceptible to external factors, maternal aging, lifestyle factors (smoking, BMI), and infertility due to endometriosis or polycystic ovary syndrome. Indeed, all of these are likely to increase oxidative stress and alter mitochondrial processes in the foreground. Concerning ART techniques themselves, there is evidence that different ovarian stimulation regimens shape the oocyte transcriptome. The perturbation of processes related to the mitochondrion, oxidative phosphorylation, and metabolism is observed with IVM. Cryopreservation might dysregulate genes belonging to transcriptional regulation, ubiquitination, cell cycle, and oocyte growth pathways. For other ART laboratory factors such as temperature, oxygen tension, air pollution, and light, the evidence remains scarce. Focusing on genes involved in chromatin-based processes such as DNA methylation, heterochromatin modulation, histone modification, and chromatin remodeling complexes, but also genomic imprinting, we observed systematic dysregulation of such genes either after ART intervention or lifestyle exposure, as well as due to internal factors such as maternal aging and reproductive diseases. Alteration in the expression of such epigenetic regulators may be a common mechanism linked to adverse oocyte environments, explaining global transcriptomic modifications.

WIDER IMPLICATIONS

Many IVF factors and additional external factors have the potential to impair oocyte transcriptomic integrity, which might not be innocuous for the developing embryo. Fortunately, it is likely that such dysregulations can be minimized by adapting ART protocols or reducing adverse exposure.

Cubilin- and megalin-mediated uptake of albumin in cultured proximal tubule cells of opossum kidney

BACKGROUND

Reabsorption of albumin from the glomerular filtrate occurs via receptor-mediated endocytosis in the proximal tubule. This process is initiated by binding of albumin in apical clathrin-coated pits, followed by endocytosis and degradation in lysosomes. Although binding sites have been characterized by kinetic studies, the receptors responsible for the binding of albumin have not been fully identified. Two giant glycoproteins, cubilin and megalin, constitute important endocytic receptors localized to the kidney proximal tubule.

METHODS

In the present study, we examined the colocalization of cubilin and megalin in the endocytic pathway and the relationship between the uptake of albumin and the expression of cubilin and megalin in opossum kidney (OK) proximal tubule cells by immunocytochemistry and immunoblotting.

RESULTS

OK cells expressed both cubilin and megalin. The light microscope labeling patterns for cubilin and megalin were almost identical and were mainly located at the surface area of the cells. Cubilin and megalin were also shown to colocalize on cell surface microvilli, in coated pits, and in endocytic compartments at the electron microscope level. Endocytosed bovine serum albumin (BSA) was identified exclusively in cells expressing megalin and cubilin. Uptake of BSA-FITC was saturable and inhibited by receptor-associated protein (RAP) and by intrinsic factor-vitamin B12 complex (IF-B12) at high concentrations. Significant inhibition was also observed by specific antibodies to cubilin, and megalin and cubilin antisense oligonucleotides likewise significantly reduced albumin uptake. Egg albumin did not affect the uptake of BSA.

CONCLUSION

The present observations suggest that the two receptors cubilin and megalin are both involved in the endocytic uptake of albumin in renal proximal tubule cells.

Mechanism of uptake of cobalamin into enterocyte mitochondria

To study the mechanism which cobalamin is taken up by the mitochondria, 57Co-cyanocobalamin-binder complex and freshly prepared mitochondria were prepared from the enterocytes of rats. Subsequently, the binder complex was incubated together with mitochondria in a calcium-containing medium in vitro. Uptake of cobalamin was determined by measuring the radioactivities bound to the mitochondria. Consequently, lysosomal and microsomal binders enhanced cobalamin uptake into the mitochondria, but intrinsic factor did not. It was found that the uptake into the mitochondria was inhibited by previous treatment with calcium-chelating agents. The uptake was completely restored after addition of calcium ions to the mitochondria.

Intrinsic factor-mediated intestinal absorption of cobalamin in the dog

The purpose of these studies was to determine whether gastric intrinsic factor and the ileal intrinsic factor receptor participate in the process of cobalamin absorption in the dog. Physicochemical analysis of gastrointestinal fluids and mucosal extracts obtained 3-5 h after cyano[57Co]-cobalamin was fed to dogs demonstrated that 1) all cyano-[57Co]cobalamin became bound to proteins during intraluminal transport; and 2) mucosal cyano[57Co]cobalamin in the extract of the ileal mucosa was bound to intrinsic factor, to intrinsic factor coupled to receptor protein, and to proteins with properties similar to R protein and transcobalamin II. A significant fraction of the cyano[57Co]cobalamin in the mucosal extract was membrane bound and, upon solubilization with Triton X-100, was found to contain immunoreactive intrinsic factor that, however, could no longer couple to the isolated receptor. The formation of the complex of cobalamin with intrinsic factor and the receptor protein and the selective accumulation of cobalamin in the ileum indicate that the intrinsic factor-mediated mechanism for absorption of this vitamin is active in the dog.

Clinical significance of serum vitamin B12 measured by radioassay using pure intrinsic factor

Serum vitamin B12 (B12) levels of 53 patients (15 with pernicious anemia) and 42 healthy volunteers were determined using crude intrinsic factor (IF), pure IF, and a mixture of crude IF + R-protein blocking agent (block IF). The radioassay using pure IF showed less sample-to-sample variation in nonspecific binding than the radioassay using block IF. The mean B12 levels in 42 healthy subjects were significantly higher with crude IF (499 +/- 23 pg/ml, 1 s.e.m.) than with pure IF (408 +/- 29 pg/ml) or with block IF (407 +/- 22 pg/ml). B12 levels were abnormally low in all 15 patients with pernicious anemia by pure IF (less than 100 pg/ml), in 14 patients by block IF (less than 150 pg/ml), and in only seven patients by crude IF (less than 200 pg/ml). Our data confirm previous reports that B12 deficiency can be diagnosed more reliably by measuring serum B12 levels with etiher pure IF or block IF.

Clinically significant vitamin B12 deficiency secondary to malabsorption of protein-bound vitamin B12

Protein- (chicken serum) bound [57Co]cyanocobalamin absorption was evaluated in five hypochlorhydric patients who had developed B12 deficiency despite having normal absorption of unbound crystalline vitamin B12. All five patients had decreased urinary excretion of protein-bound B12 (0.06--0.34%) as compared to twelve normal controls (0.61--5.6%), P less than .001. Improvement in protein-bound B12 absorption in four of the five patients occurred with the exogenous administration of hydrochloric acid, pepsin, gastric intrinsic factor, or a combination thereof. Vitamin B12 deficiency developing in the setting of hypochlorhydria may result from deficiency of acid-peptic digestion of B12 bound to protein and/or a relative deficiency of intrinsic factor. This digestive defect is not detected with tests which measure the absorption of unbound crystalline B12 but is detected by a simple test which employs B12 bound to chicken serum as the form of protein-bound B12.

[Blocking and binding antibodies to intrinsic factor and their behaviour during oral therapy with intrinsic factor-vitamin B 12 complex (author's transl)]

13 patients with pernicious anaemia and 2 patients who had been subjected to total gastrectomy were treated orally with a hog instrinsic factor preparation (IF) for 1 to 4.5 years. During this therapy 11 of the patients with pernicious anaemia and both gastrectomized patients developed blocking and binding antibodies to IF. Antibodies already present before the commencement of therapy showed an increase in titre. All patients remained in complete haematological remission. The conclusion is drawn that circulating antibodies to IF do not play any significant role in the absorption of the hog IF-B 12 complex.

Factors affecting the absorption of vitamin B 12

1. The absorption of radioactive cobalamin was measured by a whole-body counting technique in control subjects and in patients with pernicious anaemia.

2. The absorption of cyanocobalamin by patients with pernicious anaemia was decreased by charcoal but not by bile or saliva.

3. The absorption of cyanocobalamin by control subjects was not affected by food but was significantly increased by pentagastrin. With pentagastrin the absorption of cyanocobalamin was significantly greater than that of hydroxocobalamin. The hog intrinsic factor-mediated absorption of cyanocobalamin by patients with pernicious anaemia was significantly depressed by pentagastrin.

4. The effect of increasing the mass of hog intrinsic factor concentrate on the absorption of cyanocobalamin by patients with pernicious anaemia could be described by a function relating the amount absorbed, the mass of intrinsic factor and two constants. The relationship implies that when the mass of intrinsic factor is small the amount of cyanocobalamin absorbed is directly proportional to the mass of intrinsic factor but that absorption approaches a saturation value with increasing mass of intrinsic factor. In physiological terms the function implies that absorption is proportional to the amount of cyanocobalamin attached to receptor sites but that cyanocobalamin attached to receptor sites may become detached and either reattached or lost to absorption.

5. With oral doses of 25 μg and 50 μg, control subjects absorbed more cyanocobalamin and hydroxocobalamin than patients with pernicious anaemia. At both dose levels control subjects absorbed more cyanocobalamin than hydroxocobalamin but no difference was observed in patients with pernicious anaemia. The intrinsic factor mechanism therefore influences amounts absorbed at such dose levels and appears to be a factor in the differences in absorption of cyanocobalamin and hydroxocobalamin.

6. The use of double-tracer techniques makes it possible for each subject to act as his own control in studies of vitamin B12 absorption. The value of this technique is stressed.

2,4-diaminovaleric acid: an intermediate in the anaerobic oxidation of ornithine by Clostridium sticklandii

The oxidation of ornithine in the presence of proline by crude extracts of Clostridium sticklandii cells was stimulated by nicotinamide adenine dinucleotide, coenzyme A, alpha-ketoglutarate, dimethylbenzimidazolyl cobamide (DBC) coenzyme, MgCl(2), and adenosine diphosphate. Deletion of various cofactors resulted in the accumulation of a new dibasic amino acid which was identified as 2,4-diaminovaleric acid. Both the oxidation of ornithine to alanine and acetate and the conversion of ornithine to 2,4-diaminovaleric acid were stimulated by addition of DBC coenzyme, and both were inhibited by intrinsic factor, an inhibitor of cobamide coenzyme-dependent reactions. This inhibition was reversed by addition of DBC coenzyme. However, the oxidation of 2,4-diaminovaleric acid was insensitive to added intrinsic factor. The data indicate that 2,4-diaminovaleric acid represents the first intermediate in the oxidation of ornithine by C. sticklandii.

Vitamin B12 absorption in dogs with chronic isolated intestinal loops

The absorption of physiological amounts of vitamin B12 is thought to occur only from the ileum, but the nature of the mechanism involved has not been established. A method utilizing the detection of radioactive vitamin B12 in plasma was used to investigate the mechanism of vitamin B12 absorption in dogs with isolated Thiry–Vella loops of small intestine, either ileum or jejunum. An animal with a Thiry–Vella loop of the entire ileum failed to absorb orally administered vitamin B12, but did absorb the vitamin from the isolated ileal loop when it was given with intrinsic factor, but not when given without. Attempts to restore absorption of orally administered vitamin B12 by the administration of ileal juice (either resting or stimulated), by the administration of an extract of ileal mucosa, or by the simultaneous absorption of non-radioactive vitamin B12 from the ioslated loop, all failed. In contrast, the animal with a jejunal Thiry–Vella loop absorbed vitamin B12 given orally but not when administered into the isolated jejunum alone, in combination with intrinsic factor, or with intrinsic factor and ileal juice. Under the conditions of these experiments the results confirm that vitamin B12 absorption is limited to the ileum, but fail to provide support for the hypothesis that the ileum liberates, either locally or systemically, a vitamin B12 absorption factor. Thus the hypothesis that ileal mucosa contains a special cellular mechanism for vitamin B12 absorption gains indirect support from these results.

Malabsorption of hemoglobin iron in pernicious anemia: correction with intrinsic factor--containing substances

Hemoglobin iron absorption in patients with treated prenicious anemia (PA) and concomitant iron deficiency was low compared to absorption in patients with iron deficiency alone. Crude and purified hog intrinsic factor (IF) concentrates doubled the absorption of hemoglobin iron in these patients as did normal (neutralized depepsinized) human gastric juice. Hemoglobin iron absorption was not significantly enhanced by PA gastric juice. Absorption of heme iron, like that of hemoglobin iron, was enhanced by normal neutralized depepsinized gastric juice. No enhancement of hemoglobin iron absorption by these substances was obtained in the normal or iron-deficient non-PA control subjects. Preincubation of the hog IF concentrate with antisera to IF significantly reduced the enhancement of hemoglobin iron absorption due to the concentrate. In vitro studies suggest that heme complexes with a substance present in IF-containing materials. Whether a gastric glycoprotein similar to IF serves as an intestinal transport factor for heme, similar to transport of vitamin B(12), or whether normal gastric juice acts by another mechanism cannot be determined at this time.

Intrinsic factor-mediated attachment of vitamin B12 to brush borders and microvillous membranes of hamster intestine

Hamster intrinsic factor (IF) preparations markedly enhanced the uptake of (57)cobalt-labeled cyanocobalamin (B(12)-(57)Co) by brush borders and microvillous membranes isolated from villous absorptive cells obtained from the distal but not the proximal half of hamster intestine. A similar effect was observed with rat and rabbit IF preparations, but IF preparations obtained from man, dog, and hog were ineffective. After fractionation of hamster IF preparations by gel filtration or ion exchange chromatography, the extent to which each fraction enhanced B(12)-(57)Co uptake by brush borders correlated closely with the vitamin B(12) binding capacity of the fraction. IF-mediated attachment of B(12)-(57)Co to brush borders occurred rapidly, was not diminished by removal of glucose or oxygen from the incubation medium, and was not significantly altered when incubation temperatures were reduced from 37 degrees C to 7 degrees C. Marked reduction in uptake occurred, however, in the absence of divalent cations. IF enhanced B(12)-(57)Co uptake by brush borders isolated from the proximal half of the intestine when these proximal brush borders were preincubated with supernatant fluid obtained after centrifugation of homogenates of distal intestinal mucosa at 28,500 g. The factor in this supernate responsible for the effect on proximal brush borders was shown to be particulate in nature upon centrifugation at speeds of 54,500 g or greater. The resultant pellet contained ribosomes and membranous fragments. Prolonged incubation of brush borders with crude saline extracts of hamster gastric mucosa resulted in decreased uptake of B(12)-(57)Co and marked lysis of brush borders with concomitant release of tissue nitrogen. Neither lysis of brush borders nor decreased uptake of B(12)-(57)Co with prolonged incubation was observed when hamster IF was partially purified. Furthermore, uptake of B(12)-(57)Co by brush borders increased with increasing purity of the IF preparation used. These results demonstrate IF-mediated attachment of B(12)-(57)Co to brush borders and microvillous membranes of hamster intestinal cells and provide further support for the presence of a specific receptor for IF-bound vitamin B(12) at the microvillous surface of the intestinal cell. IF-mediated attachment to the intestinal cell surface appears to be facilitated by divalent cations and to result from adsorption rather than an energy-requiring enzymatic reaction. Crude sources of hamster IF contain a factor which causes lysis of brush borders in vitro and which may explain in part the inhibitory effects of IF excess previously observed in vitro.

A student laboratory exercise illustrating active transport across the small intestine

A 7-year experience with student laboratory exercises dealing with intestinal transport in vitro is described. A single experiment which has evolved from trial and error in the student laboratory is given in detail. Everted sacs of small intestine from the golden hamster are incubated in flasks containing bicarbonate-saline with 10 mm glucose and 2 mm d- or l-tyrosine. Glucose and l-tyrosine are transported across the intestinal wall against concentration gradients; d-tyrosine is not, illustrating the stereospecificity of the amino acid transport system. A second experiment which illustrates the stimulation of vitamin B12 absorption by gastric intrinsic factor is briefly recorded. The student interest aroused and the high degree of satisfactory results make such experiments useful additions to the repertoire of student laboratory exercises in physiology.

intestinal transport; amino acid transport; B12 transport

Submitted on November 12, 1964