Mitochondrial formate production and its impact on embryonic cranial tissue development in the mouse

Mammalian folate-dependent one-carbon (1C) metabolism provides the building blocks essential during development via amino acid interconversion, universal methyldonor production, regeneration of redox factors, and de novo purine and thymidylate synthesis. Folate supplementation prevents most neural tube defects (NTDs) that occur during the embryonic process of neurulation. The mechanism of how folate functions during neurulation is not well understood, and not all NTDs are preventable by folate supplementation. Mthfd1l is a mitochondrial 1C metabolism enzyme that produces formate, a 1C donor that fuels biosynthesis and the methyl cycle in the cytoplasm. Mthfdll-null (Mthfd1l [superscript lowercase z/z] ) mice are embryonic lethal and develop folate-resistant NTDs. These mice also have defects in cranial mesenchyme formation. In this work, the nature of how their mesenchyme is defective is explored. The extracellular matrix (ECM) of Mthfd1l [superscript lowercase z/z] embryos was found to be depleted in glycosaminoglycan (GAG) composition, as well as the basement membrane protein Collagen IV. Imaging mass spectrometry (IMS) was used to construct ion maps of the cranial mesenchyme that identified the spatial distribution and abundance of metabolites in Mthfd1l [superscript lowercase z/z] embryos compared to wild-type (WT). Purine and thymidylate derivatives, as well as amino acids, were diminished in the cranial mesenchyme of Mthfd1l [superscript lowercase z/z] embryos. Loss of Mthfd1l activity in this region also led to abnormal levels of methionine and dysregulated energy metabolism. These alterations in metabolism suggest possible approaches to preventing NTDs in humans. Finally, we created a mouse lacking both Mthfd1l and Aldh1l2, which is another mitochondrial enzyme associated with formate production. These embryos exhibit a more dramatic birth defect phenotype than Mthfd1l [superscript lowercase z/z] embryos. By associating metabolite abundance, gene expression, and tissue development, this study is focused on enhancing our current understanding of how folate-dependent 1C metabolism functions during neurulation.