**1. Introduction**

Pre-eclampsia (PE) is a pregnancy-specific hypertensive disorder that a ffects about 5% of all pregnancies and is responsible for more than 50,000 maternal deaths each year worldwide [1–4]. It is hypothesized that the pathophysiology of early-onset pre-eclampsia (<34 weeks' gestation) has its origins in the placental and uterine vasculature, where inadequate vascular remodeling leads to repeated ischemic-reperfusion (I/R) events within placental tissue [5]. This chronic impaired placental perfusion is thought to promote the production and release of inflammatory and anti-angiogenic factors that are known to be elevated in the maternal circulation of PE cases [6]. These factors damage the maternal vasculature inducing wide-spread endothelial dysfunction, hypertension and, in severe cases, organ injury and maternal death [7].

Impaired oxygen delivery to the placenta also destabilizes placental metabolism and bioenergetics, which in turn limits ATP-dependent processes and promotes the production of reactive oxygen species, often leading to functional insu fficiencies in the placenta that can negatively impact the developing fetus. Indeed, the risk of fetal growth restriction and preterm birth, either spontaneous or through iatrogenic delivery, is increased with PE [2,8].

A recent wide-spectrum metabolomic analysis identified increased creatine and creatinine in cord plasma in PE pregnancies [9] indicating that, among other metabolic changes, fetal and potentially placental creatine homeostasis is altered in PE. Creatine— *N*-(aminoiminomethyl) *N*-methyl glycine—is a nitrogenous amino acid derivative obtained from our diet and endogenously synthesized by some tissues, including the placenta [10]. Through a reversible phosphorylation/dephosphorylation reaction catalyzed by mitochondrial (CKMT1A) and cytosolic (BBCK) creatine kinases, creatine stabilizes cellular bioenergetics through both spatial and temporal maintenance of adenosine diphosphate and triphosphate (ADP/ATP) ratios [11].

Our understanding of the importance of creatine provision to the developing placenta and fetus in healthy pregnancies has expanded in recent years [12–16]. We have previously demonstrated that the term human placenta expresses both enzymes (arginine:glycine aminotransferase [AGAT] and guanidinoacetate methyltransferase [GAMT]) required to synthesize creatine *de novo*, and can produce both creatine and the creatine precursor guanidinoacetate (GAA) ex vivo, thus bypassing a complete reliance on dietary creatine or endogenous synthesis by the maternal kidney and liver for its creatine supply [10]. Our further investigations into creatine metabolism in cases of third trimester placental insu fficiency and fetal growth restriction in the absence of PE sugges<sup>t</sup> that the compromised placenta adapts by increasing tissue creatine concentrations, which may reflect a heightened reliance on the creatine kinase circuit to stabilize placental bioenergetics under chronic hypoxic conditions [17]. Whether placental metabolic changes associated with PE induce changes in placental creatine metabolism, and potentially creatine transfer to the fetus, is unknown.

This study examined whether early-onset PE altered placental creatine metabolism. Specifically, we assessed placental creatine and GAA content, the expression of the enzymes required for creatine synthesis, creatine intracellular uptake and utilization via the creatine kinase circuit. We hypothesized that PE would have e ffects on placental creatine homeostasis that would include pathways involved in its synthesis and transport. Our findings are discussed in relation to a possible placental response to minimize PE-induced injury, which could have the dual benefit of reducing the maternal syndrome of PE and improving fetal wellbeing.
