Unequal distribution of 16S mtrRNA at the 2-cell stage regulates cell lineage allocations in mouse embryos

Abstract

Cell lineage determination during early embryogenesis has profound effects on adult animal development. Pre-patterning of embryos, such as that of Drosophila and Caenorhabditis elegans, is driven by asymmetrically localized maternal or zygotic factors, including mRNA species and RNA binding proteins. However, it is not clear how mammalian early embryogenesis is regulated and what the early cell fate determinants are. Here we show that, in mouse, mitochondrial ribosomal RNAs (mtrRNAs) are differentially distributed between 2-cell sister blastomeres. This distribution pattern is not related to the overall quantity or activity of mitochondria which appears equal between 2-cell sister blastomeres. Like in lower species, 16S mtrRNA is found to localize in the cytoplasm outside of mitochondria in mouse 2-cell embryos. Alterations of 16S mtrRNA levels in one of the 2-cell sister blastomere via microinjection of either sense or antisense RNAs drive its progeny into different cell lineages in blastocyst. These results indicate that mtrRNAs are differentially distributed among embryonic cells at the beginning of embryogenesis in mouse and they are functionally involved in the regulation of cell lineage allocations in blastocyst, suggesting an underlying molecular mechanism that regulates pre-implantation embryogenesis in mouse.

Totipotency continuity from zygote to early blastomeres: a model under revision

“…Independently of the genome-wide studies reanalyzed above, Zheng and collaborators found that mtrRNAs located outside of mitochondria were differently abundant in the cytoplasm of sister mouse blastomeres at the end of the two-cell stage (Zheng et al 2016). This interblastomere imbalance was observed in 10-20% of the two-cell embryos examined.…”

“…(A) Asynchrony of developmental processes, such as mRNA degradation/deadenylation or EGA (embryonic genome activation) due to lack of coupling between sister blastomeres (Boni et al 1999, Brison et al 2014). (B) Unequal partition at 1st mitosis: (i) incomplete diffusion of the sperm content that is inherited preferentially by one blastomere (Piotrowska & Zernicka-Goetz 2002), (ii) unequal distribution of molecules such as the less abundant mRNAs (Shi et al 2015), (iii) unequal distribution of cytoplasmic organelles (Zheng et al 2016, Gao et al 2017), (iv) unequal segregation of chromosomes. rat (Zernicka-Goetz 1994), four-cell stage in pigs (Hyttel et al 2000), four-to eight-cell stage in human (Braude et al 1988), six-to eight-cell stage in rhesus monkeys (Schramm & Bavister 1999) and 8-to-16 cell stage in cow, sheep and rabbit (Crosby et al 1988, Plante et al 1994, Brunet-Simon et al 2001.…”

“…This interblastomere imbalance was observed in 10-20% of the two-cell embryos examined. When Zheng and colleagues tested the functional importance of 16S mtrRNA by gain-and loss-of-function, they found that those mtrRNAs seemed to drive the blastomere fate toward the ICM (Zheng et al 2016). Mechanistic understanding of the role of 16S mtrRNA warrants further investigation even if it seems unlikely that a single RNA could be in charge of regulating totipotency versus lineage commitment.…”

Instructions for Assembling the Early Mammalian Embryo

“…In the absence of transcription, partitioning errors likely account for the initial introduction of variability between cells. One such variably distributed factor may relate to mitochondrial rRNAs, which have recently been shown to become progressively heterogeneous by the end of the two-cell stage (Zheng et al, 2016). Whether and how this might influence or translate into asymmetries arising at the four-cell stage remains to be determined.…”

Single-embryo and single-blastomere immunoblotting reports protein expression heterogeneity in early-stage preimplantation embryos

“…The initiating events and specific proteins involved in the first cell fate commitment within preimplantation blastomeres still remain open questions in developmental biology. While functional studies and embryonic plasticity suggest that blastomeres remain equivalent until the compacted morula [1][2][3] , growing evidence of inter-blastomeric differences in early-stage embryos point to heterogeneous configurations at even the earliest multicellular stages [4][5][6][7][8][9][10][11][12][13][14] . Although measurement tools with single-embryo and single-blastomere resolution, including RNA-Seq, have greatly advanced our knowledge, companion protein expression and state measurements within single embryos are required to test and validate these transcript-based predictions.…”