Carnitine content and expression of mitochondrial β-oxidation enzymes in placentas of wild-type (OCTN2^+/+) and OCTN2 null (OCTN2 ^-/-) mice

Placenta requires energy to support its rapid growth, maturation, and transport function. Fatty acids are used as energy substrates in placenta, but little is known about the role played by carnitine in this process. We have investigated the role of carnitine in the expression of the enzymes involved in fatty acid β-oxidation in placenta of OCTN2^-/- mice with defective carnitine transporter (OCTN2). Heterozygous (OCTN2^+/-) female mice were mated with heterozygous (OCTN2^+/-) male mice. Pregnant mice were killed and fetuses and placentas were collected. Carnitine was measured using HPLC and tandem mass spectrometry. Immunohistochemistry was used to detect enzyme expression. Enzyme activities were measured spectrophotometrically. The fetal and placental weights were similar among the three genotypes (OCTN2^+/+, OCTN2^+/-, and OCTN2 ^-/-). The levels of carnitine were markedly reduced (<20%) in homozygous OCTN2^-/- null fetuses and placentas compared with wild-type OCTN2^+/+ controls. However, carnitine concentration in placenta was 2- to 7-fold higher than in the fetus in all three genotypes. Immunohistochemistry revealed that β-oxidation enzymes are expressed in trophoblast cells. Catalytic activities of these enzymes were present at comparable levels in wild-type (OCTN2^+/+) and homozygous (OCTN2 ^-/-) mouse placentas, with the exception of SCHAD, for which activity was significantly higher in OCTN2^-/- placentas than in OCTN2 ^+/+ placentas. These data show that placental OCTN2 is obligatory for accumulation of carnitine in placenta and fetus, that fatty acid β-oxidation enzymes are expressed in placenta, and that reduced carnitine levels up-regulate the expression of SCHAD in placenta.