Age-associated alteration of oocyte-specific gene expression in polar bodies: potential markers of oocyte competence

Abstract

Objective: To confirm that oocyte-specific messenger RNAs are detectable in the polar body (PB) of metaphase II (MII) oocytes and determine the effect of age on oocyte-specific transcript levels.

Design: Prospective study.

Setting: Hospital-based academic research laboratory.

Animal(s): CD1 female mice.

Intervention(s): Aged (40-50 weeks) and young (7-9 weeks) mice were administered pregnant mare serum gonadotropin (PMSG) and hCG. Oocytes were fertilized in vitro to assess fertilization and developmental competence. The MII oocytes were obtained and first PBs were removed. Messenger RNAs from each PB and its sibling oocyte were reverse transcribed and analyzed by real-time quantitative polymerase chain reaction (PCR).

Main outcome measure(s): Fertilization and developmental rates and expression of six oocyte-specific genes (Bmp15, Gdf9, H1foo, Nlrp5, Tcl1, and Zp3) in PBs and sibling oocytes from young versus aged mice.

Result(s): Oocytes from aged mice had lower developmental competence. Four genes (H1foo, Nlrp5, Tcl1, and Zp3) were differentially expressed in aged versus young oocytes. All six transcripts were present in PBs from aged and young mice at lower levels than in the sibling oocytes; transcript levels were lower in aged PBs compared with young PBs.

Conclusion(s): There is a significant difference in the transcript levels of oocyte-specific genes in aged versus young PB that correlates with age-related decreases in oocyte competence. Differences in gene expression in PB may be potential biomarkers of MII oocyte competence.

FIGURE1

Morphology of the first PB (A-C) and the separation of the first PB from its associated MII oocyte (D, E). A: intact PB of normal size; B: fragmented PB; C: enlarged PB; D: Remove ZP; E: Separation of first PB from its sibling oocyte. Bar=50μm.

FIGURE2

Morphology of MII and fertilzed oocytes from young (A–C) and aged (D–E) mice. (A,D) MII oocyte; (B,E) 2-cell embryo; (C,F) blastocyst. Bar=50μm

FIGURE 3

A: Oocyte-specific genes expression in MII oocytes from young and aged mice. Results are normalized to each transcript in MII oocyte from young mice as 1 (the calibrator). Compared with young oocytes, there was a significantly lower level of H1foo, Nlrp5, Tcl1, and Zp3 transcripts in aged oocytes. Error bars represent SD. * significance relative to young mice (P < 0.05) B: Oocyte-specific genes expression in PB from young and aged mice. Results are normalized to each transcript in PB from young mice as 1 (the calibrator). PBs from aged mice had significantly lower levels of all 6 transcripts compared with PBs from young mice. Error bars represent SD. * significance relative to young mice (P < 0.05) C: Relative abundance of each single gene in PBs compared with its sibling oocyte. If the normalized mRNA level of each transcript in sibling oocytes is set at 1, the abundance of the transcript in PBs was consistently lower than that in the sibling oocytes. In addition, transcript levels were significantly lower in aged animals compared with young animals Error bars represent SD. Horizontal lineindicate the normalized level in sibling oocyte. * significance relative to its sibling oocyte (P < 0.05)