[초청강연] Molecular mechanisms for human embryonic germ layer formation

Molecular mechanisms for human embryonic germ layer formation
Jiwon Jang
Department of Life Sciences, Pohang University of Science and Technology (POSTECH)
Abstract
The developmental hourglass model suggests that embyos from different species are more divergent
at the earliest and latest stages of development, suggesting the importance of species-specific
models for developmental studies. To understand molecular mechanisms for early human
development, we utilized human embryonic stem cells (hESCs) as an experimental model for pre-
gastrulation epiblasts and discovered new mechanisms and regulators underlying human embryonic
germ layer formation. First, we identified an evolutionarily conserved BTB domain-containing zinc
finger protein, ZBTB12, as a molecular barrier for dedifferentiation of hESCs. Single cell RNA
sequencing revealed that ZBTB12 was essential for three germ layer differentiation by blocking
dedifferentiation of hESCs toward a more primitive state. Mechanistically, ZBTB12 fine-tunes the
expression of transcriptionally active human endogenous retrovirus H (HERVH), a primate-specific
retrotransposon. Although ZBTB12 depletion induced global upregulation of HERVH, ZBTB12
targeted specific transcripts that utilize HERVH as a regulatory element. In particular, downregulation
of HERVH-overlapping long non-coding RNAs (lncRNAs) by ZBTB12 is necessary for successful exit
from a pluripotent state and lineage derivation. Collectively, we have identified ZBTB12 as a molecular
barrier that safeguards the unidirectional transition of metastable stem cell fates toward the three
embryonic germ layers. In the second part, we investigated how the three different germ layers are
emerged within a seemingly homogenous hESCs. In a hESC epithelium, we found that epithelial
expansion-driven local crowding initiates symmetry breaking that ultimately drives multi-lineage
derivation. Mechanistically, ETV4, a well-known oncogene, is identified as a new mechanotransducer
linking cell density to lineage fate. The mechanosensitive ETV4 expression is regulated by dynamic
receptor endocytosis and most importantly, ETV4 showed higher sensitivity to mechanical stimuli than
the well-established mechanotransducers, YAP/TAZ. Mathematical modeling demonstrated that the
dynamics of epithelial expansion precisely determine the spatiotemporal ETV4 expression pattern and
consequently the timing and geometry of lineage development. This mechanism underlies multi-
lineage derivation from a hESC epithelium with ETV4 as a highly sensitive mechanotransducer.
Overall, our discovery of a molecular barrier and a mechanotransducer as critical regulators for
embryonic germ layers will provide new insights to understanding early human development.