Betaine is accumulated via transient choline dehydrogenase activation during Mouse oocyte meiotic maturation [Cell Biology]

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, but reached high levels in the mature, ovulated egg. However, no betaine transport into oocytes was detected during meiotic maturation. Since betaine can be synthesized in mammalian cells via choline dehydrogenase (CHDH; EC1.1.99.1), 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 if CHDH was required. Isolated maturing oocytes autonomously synthesized betaine in vitro in the presence of choline, while 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.

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