Betaine is accumulated via transient choline dehydrogenase activation during mouse oocyte meiotic maturation

Taylor Mcclatchie,Megan Meredith,Mariame O Ouédraogo,Sandy Slow,Michael Lever,Mellissa R.W Mann,Steven H Zeisel,Jacquetta M Trasler,Jay M Baltz

doi:10.1074/jbc.m117.803080

Abstract

Betaine (N,N,N-trimethylglycine) plays key roles in mouse eggs and preimplantation embryos first in a novel mechanism of cell volume regulation and second as a major methyl donor in blastocysts, but its origin is unknown. Here, we determined that endogenous betaine was present at low levels in germinal vesicle (GV) stage mouse oocytes before ovulation and reached high levels in the mature, ovulated egg. However, no betaine transport into oocytes was detected during meiotic maturation. Because betaine can be synthesized in mammalian cells via choline dehydrogenase (CHDH; EC 1.1.99.1), we assessed whether this enzyme was expressed and active. Chdh transcripts and CHDH protein were expressed in oocytes. No CHDH enzyme activity was detected in GV oocyte lysate, but CHDH became highly active during oocyte meiotic maturation. It was again inactive after fertilization. We then determined whether oocytes synthesized betaine and whether CHDH was required. Isolated maturing oocytes autonomously synthesized betaine in vitro in the presence of choline, whereas this failed to occur in Chdh-/- oocytes, directly demonstrating a requirement for CHDH for betaine accumulation in oocytes. Overall, betaine accumulation is a previously unsuspected physiological process during mouse oocyte meiotic maturation whose underlying mechanism is the transient activation of CHDH.

Highlights

Betaine (N,N,N-trimethylglycine) plays key roles in mouse eggs and preimplantation embryos first in a novel mechanism of cell volume regulation and second as a major methyl donor in blastocysts, but its origin is unknown
Higher levels of betaine are present in mouse eggs than in earlier stage oocytes Substantial betaine is present in preimplantation embryos and ovulated MII eggs of mice [1, 20], but whether betaine is present in oocytes during meiotic maturation was unknown
Betaine levels had not increased in MI oocytes at 3 h after ovulation was induced with human chorionic gonadotropin, and only a small increase was seen in MI oocytes at 6 h

Summary

To whom correspondence should be addressed

Lular concentrations (5–10 mM) that are much higher than in most other cells [1]. It serves at least two established roles in embryos, first as a key contributor to cell volume regulation and second as a major source of methyl groups in the blastocyst [1,2,3,4,5,6]. GV oocytes are released from arrest and progress through first meiotic metaphase (MI), become uncoupled from granulosa cells, and are re-arrested in second meiotic metaphase (MII) as mature eggs awaiting fertilization [21,22,23], a process that is collectively termed meiotic maturation These nuclear changes have been extensively studied, much less is known about other physiological processes during oocyte meiotic maturation that are important for producing a mature MII egg. As we show in the present study, GV oocytes and early to mid-MI oocytes contain lower endogenous levels of betaine in contrast to the high levels in MII eggs, demonstrating that betaine accumulation must occur during meiotic maturation. To investigate the mechanism underlying the production of the high levels of betaine that are present in MII mouse eggs, which are important during early embryonic development, we have tested the hypotheses that CHDH is present and active in oocytes during meiotic maturation, that betaine is synthesized autonomously by oocytes, and that CHDH is required for betaine accumulation

Results

Discussion

Experimental procedures