Mitochondria-Associated Transcription Precedes Oxidative Phosphorylation Activation During Human Pre-Implantation Embryogenesis

Abstract

Mitochondria undergo significant structural and functional changes during human pre-implantation embryogenesis, yet the transcriptional activity of both nuclear-encoded mitochondria-associated genes and mitochondrially transcribed genes across this developmental window remains poorly characterized. While mitochondria are established as the primary energy source for the early embryo, emerging evidence suggests they may also influence lineage specification through epigenetic regulation and metabolite availability. To investigate this, we reanalyzed two publicly available human single-cell RNA sequencing datasets filtered for mitochondria-associated genes using the MitoCarta 3.0 reference database, with separate analyses conducted on the nuclear-encoded and mitochondrially transcribed subsets. The first dataset spanned individual blastomeres from the oocyte through blastocyst stage, and the second compared trophectoderm and inner cell mass cells isolated from blastocysts. Mitochondria-associated gene expression was sufficient to cluster human blastomeres by developmental stage, with morula and blastocyst stage cells forming well-defined clusters. Mitochondrially transcribed genes were found to be the primary drivers of clustering in earlier developmental stages, while nuclear-encoded mitochondria-associated genes drove clustering at the blastocyst stage. A pronounced shift in the expression of both gene sets was identified at the transition from the 4-cell to the 8-cell stage, with 115 unique differentially expressed genes identified across the two stages immediately following this transition, compared to only 5 across the two prior stages. The timing of this transcriptional upregulation, preceding the known onset of oxidative phosphorylation at approximately the 32-cell stage, suggests a mitochondrial role in early embryogenesis beyond energy production. Analysis of trophectoderm and inner cell mass cells showed that mitochondrial gene expression profiles partially distinguished these two lineages, consistent with known differences in mitochondrial activity between them. These findings suggest that both nuclear-encoded and mitochondrially transcribed gene expression is upregulated prior to the first lineage specification event in the human embryo, potentially contributing to epigenetic regulation and cell fate determination through altered metabolite availability. A limitation of this study is its reliance on transcriptomic data alone; future work incorporating functional metabolite measurements will be needed to establish causality. Nonetheless, these data reframe mitochondria as active participants in early human developmental programming rather than passive energy suppliers.