Midgestation metabolic constraint in purine metabolism drives distinct strategies for placenta and fetal growth

Background

Midgestation — the middle phase of pregnancy — is a critical stage of development, involving both embryonic organogenesis and the growth of extraembryonic tissues. Although the placenta is well established as a key regulator of normal fetal development (1), it remains unclear how, during midgestation, the placenta sustains its own rapid growth without compromising the nutrient supply to the embryo.

Previous work identified GD (gestation day)10.5–11.5 as a crucial metabolic transition in both the placenta and the embryo (2), marked by a sustained increase in purine synthesis in each compartment, especially in the embryo. Given that purine metabolites are essential for nucleotide synthesis, energy production and cell signalling, the key questions addressed by the preprint highlighted here are: how are purines synthesised within the placenta and the embryo during midgestation, and do these two compartments preferentially rely on different synthesis pathways?

Key findings

1. The placenta and embryo adopt different strategies for purine synthesis during midgestation.

In vivo infusion of GD10.5 mouse dams with [¹⁵N₄]-labelled salvage precursors showed that the placenta efficiently took up and interconverted the precursors via the salvage pathway, whereas embryos only catabolised them (preprint Fig. 1). To confirm this was intrinsic to the embryo rather than a consequence of placental transport, ex utero culture with labelled glutamine ± excess hypoxanthine (a metabolite at the intersection of purine salvage and catabolism) showed that embryos catabolised the hypoxanthine rather than salvaging it. Consistently, single-cell RNA-seq (GD8.5–12.5) revealed the embryo switching from salvage- to de novo purine synthesis enzyme dominance, while the placenta retained high salvage enzyme expression throughout (preprint Fig. 2).

This difference proved functionally important. When de novo synthesis was blocked, the placenta compensated via the salvage pathway, but the embryo could not. Yet this metabolic compensation in the placenta was incomplete: despite preserved GMP abundance, the syncytiotrophoblast layer II (SynII, which contacts fetal endothelium and mediates materno-fetal nutrient transfer) was significantly reduced, and this defect was not rescued by supplementing guanine or guanosine (GMP salvage precursors). Spongiotrophoblasts expanded reciprocally as SynII declined, a shift that guanine supplementation did rescue. Together, these findings show that purine metabolism governs placental cell-lineage determination in a cell-type-specific manner (preprint Fig. 4A–E).

2. In the human placenta, syncytiotrophoblast differentiation constrains purine synthesis and relies preferentially on the salvage pathway, and GMP synthesis is necessary for trophoblast syncytialisation.

As noted above, placental purine metabolism acts in a cell-type-specific manner. To dissect the underlying mechanism, the authors turned to the differentiation of human trophoblast stem cells (HTSCs). They first showed that, upon differentiation into syncytiotrophoblasts (STBs), cells became constrained to the salvage pathway and had limited capacity to re-induce de novo synthesis when salvage precursors were withdrawn (preprint Fig. 4A–F).

They next asked how GMP synthesis is linked to differentiation. GMP levels rose steadily during the HTSC-to-STB transition, and the authors found that GMP gated the initial activation of mTORC1. This GMP–mTORC1 checkpoint, validated in human placental organoids, ensures an adequate purine supply before cells commit to the STB lineage (preprint Fig. 5F–L).

3. Hypoxanthine declines during pregnancy and is further reduced in women with clinically small placentas.

Individuals with small-for-gestational-age (SGA) placentas had significantly lower circulating hypoxanthine at 14–20 weeks and modestly reduced placental GMP at delivery, suggesting that early hypoxanthine insufficiency may limit placental GMP synthesis and constrain placental growth (preprint Fig. 5M).

What I like about this preprint

What I really like about this preprint is how strategically it uses isotope-labelling methods to cleanly distinguish de novo and salvage purine synthesis, and how consistent the findings are across different models — mouse embryos both in vivo and in ex utero culture, human trophoblast stem cells and human placental organoids. I found the results clearly presented and carefully interpreted, and I was happy to see the study build on the same group’s earlier published finding of compartmentalised metabolism between embryo and placenta during midgestation, extending this concept into purine metabolism specifically. What I found most interesting, though, was the human data: the difference in circulating hypoxanthine levels between normal and small-for-gestational-age (SGA) pregnancies at 14–20 weeks, which to me really highlights the clinical relevance of purine metabolism for placental development.

Questions to the authors:

1. How would you explain the reduced circulating hypoxanthine in SGA compared with normal pregnancies, and what might cause this decrease? And is this reduced hypoxanthine level a consequence of SGA, or is SGA caused by the low hypoxanthine level?
2. It is interesting to note that when hypoxanthine is not available, STBs do not switch to the de novo synthesis pathway as HTSCs do, yet are still able to maintain their purine monophosphate pool. Besides the possibilities you mention (reduced GMP catabolism or conversion of GDP to GMP), could this pool also be sustained by other salvage precursors present in the medium?
3. In your whole-embryo scRNA-seq data (preprint Fig. 2D), apart from the decrease in salvage gene expression that you discussed in the preprint, de novo gene expression also decreases from GD8.5 to GD12.5. How would you interpret this change?

References

1. Perez-Garcia, V., Fineberg, E., Wilson, R. et al.Placentation defects are highly prevalent in embryonic lethal mouse mutants. Nature 555, 463–468 (2018). https://doi.org/10.1038/nature26002
2. Solmonson, A., Faubert, B., Gu, W. et al.Compartmentalized metabolism supports midgestation mammalian development. Nature 604, 349–353 (2022). https://doi.org/10.1038/s41586-022-04557-9