Structure, function, and localization of aquaporins: their possible implications on gamete cryopreservation. Sales AD 2014 et al.
The discovery of water channels (aquaporins, AQPs) was a landmark event for the clarification of water transport through the plasma membrane. AQPs belong to a family of intrinsic membrane proteins that act as selective channels for water and for solutes such as glycerol and urea. AQPs were found in different tissues and organs, including male and female reproductive systems. In the swine female reproductive system, the AQPs were localized in the uterus, oviduct, and ovary, as well as in the granulosa cells from primordial follicles. Knowing the involvement of AQPs with the male and female germ cells, as well as their acknowledged role in transporting water through the plasma membrane, the research of these proteins in cryopreservation processes becomes essential. Thus, this review aims to describe the structure and function of AQPs in membranes, highlighting their role in the reproductive system (male and female). We also discuss the involvement of AQPs in cryopreservation, focusing on the effect and importance of these proteins on the rates of vitrification protocols for preantral follicles present in the ovarian tissue of domestic mammals.
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-CHIP is a 28-kD integral protein purified from the plasma membranes of red cells and renal tubules by Denker et
al. (1988) . The
protein exists as a homotetramer which physically resembles channel proteins and was the first molecular water channel
identified. The AQP-CHIP cDNA isolated from a human bone marrow cDNA library was found to be related to the major
intrinsic protein of lens . Two other related proteins were found to be water transporters (Fushimi et al., 1993 and these 3 proteins were referred to as the aquaporins.
NCBI Summary:
Aquaporins are a family of small integral membrane proteins related to the major intrinsic protein (MIP or AQP0). This gene encodes an aquaporin which functions as a molecular water channel protein. It is a homotetramer with 6 bilayer spanning domains and N-glycosylation sites. The protein physically resembles channel proteins and is abundant in erythrocytes and renal tubes. The gene encoding this aquaporin is a possible candidate for disorders involving imbalance in ocular fluid movement. Alternative splicing results in two transcript variants which encode the same protein.
General function
Channel/transport protein
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Cellular localization
Plasma membrane
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Ovarian function
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Effect of the expression of aquaporins 1 and 3 in mouse oocytes and compacted eight-cell embryos on the nucleation temperature for intracellular ice formation. Seki S et al. The occurrence of intracellular ice formation (IIF) is the most important factor determining whether cells survive a cryopreservation procedure. What is not clear is the mechanism or route by which an external ice crystal can traverse the plasma membrane and cause the heterogeneous nucleation of the supercooled solution within the cell. We have hypothesized that one route is through preexisting pores in aquaporin (AQP) proteins that span the plasma membranes of many cell types. Since the plasma membrane of mature mouse oocytes expresses little AQP, we compared the ice nucleation temperature of native oocytes with that of oocytes induced to express AQP1 and AQP3. The oocytes were suspended in 1.0?M ethylene glycol in PBS for 15?min, cooled in a Linkam cryostage to -7.0??C, induced to freeze externally, and finally cooled at 20??C/min to -70??C. IIF that occurred during the 20??C/min cooling is manifested by abrupt black flashing. The mean IIF temperatures for native oocytes, for oocytes sham injected with water, for oocytes expressing AQP1, and for those expressing AQP3 were -34, -40, -35, and -25??C respectively. The fact that the ice nucleation temperature of oocytes expressing AQP3 was 10-15??C higher than the others is consistent with our hypothesis. AQP3 pores can supposedly be closed by low pH or by treatment with double-stranded Aqp3 RNA. However, when morulae were subjected to such treatments, the IIF temperature still remained high. A possible explanation is suggested.
Expression regulated by
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Ovarian localization
Oocyte, Granulosa, Theca
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The Water Permeability Channels Aquaporins 1-4 Are Differentially Expressed in Granulosa and Theca Cells of the Preovulatory Follicle during Precise Stages of Human Ovulation. Thoroddsen A et al. Context: Changes in vascular permeability and expansion of the fluid-filled antrum are major events in the LH-induced ovulatory process. Objectives: Our objective was to investigate the presence and expression levels of aquaporins (AQPs) in the granulosa and theca cell compartments of the follicle during defined phases of human ovulation. Design and Setting: We conducted a prospective experimental study at the Department of Obstetrics and Gynaecology at a university hospital. Participants: Twenty-eight women underwent laparoscopic sterilization and at the same time follicle retrieval at four periovulatory phases. Main Outcome Measures: mRNA levels of AQP1-4 were measured in separated granulosa and theca cells from preovulatory phase, early ovulatory (EO) phase, late ovulatory phase, and postovulatory phase. Immunohistochemistry was done for AQP1-4 in intact human follicles. Results: All four AQPs were expressed in both the theca and granulosa cells during ovulation. In granulosa cells, AQP1 levels increased in the late ovulatory and postovulatory phases. Expression of AQP2-3 followed a similar pattern with a marked increase in the EO phase, whereas AQP4 levels decreased from preovulatory to the EO phase. The presence of AQP1-4 in the human follicle was verified by immunohistochemistry. Conclusions: The results show for the first time the presence of AQP1-4 in human follicles during ovulation. The marked early rise in expression of AQP2 and AQP3 suggests a role during the process leading to follicular rupture, and the late rise of AQP1 suggests a role in corpus luteum formation.
Expression and localization of aquaporin 1b during oocyte development in the Japanese eel (Anguilla japonica). Kagawa H et al. ABSTRACT: To elucidate the molecular mechanisms underling hydration during oocyte maturation, we characterized the structure of Japanese eel (Anguilla japonica) novel-water selective aquaporin 1 (AQP1b) that thought to be involved in oocyte hydration. The aqp1b cDNA encodes a 263 amino acid protein that includes the six potential transmembrane domains and two Asn-Pro-Ala motifs. Reverse transcription-polymerase chain reaction showed transcription of Japanese eel aqp1b in ovary and testis but not in the other tissues. In situ hybridization studies with the eel aqp1b cRNA probe revealed intense eel aqp1b signal in the oocytes at the perinucleolus stage and the signals became faint during the process of oocyte development. Light microscopic immunocytochemical analysis of ovary revealed that the Japanese eel AQP1b was expressed in the cytoplasm around the yolk globules which were located in the peripheral region of oocytes during the primary yolk globule stage; thereafter, the immunoreactivity was observed throughout the cytoplasm of oocyte as vitellogenesis progressed. The immunoreactivity became localized around the large membrane-limited yolk masses which were formed by the fusion of yolk globules during the oocyte maturation phase. These results together indicate that AQP1b, which is synthesized in the oocyte during the process of oocyte growth, is essential for mediating water uptake into eel oocytes.