# tRNA and tsRNA: From Heterogeneity to Multifaceted Regulators
## 5 Regulators in Cell Fate Determination and Embryonic Development  
The differentiation of stem cells into various cell types is modulated by gene temporal and spatial transcription and translation and determined by cell-specific proteomics. During this process, the composition and abundance of the tRNA pools are adjusted to meet the translational demand of cells and are thereby involved in cell fate determination. Evidence has shown that genes associated with proliferation and differentiation exhibit distinct codon usage patterns. Correspondingly, the tRNA pools in cells show different profiles during proliferation and differentiation, reinforcing either state [160]. For instance, during the differentiation of human induced pluripotent stem cells (hiPSCs), the tRNA transcript pool undergoes extensive remodeling [34]. Moreover, distinct subsets of the tRNA pools were observed under different cell states to respond to biological activities in human cells [32], among which, some tRNAs were defined as \"proliferation-tRNAs\", while some tRNA populations were defined as \"differentiation-tRNAs\", according to their expression signatures in distinct cell types. [PERSON] et al. found that the \"proliferation-tRNAs\" were upregulated during normal and cancerous cell division, whereas \"differentiation-tRNAs\" were more active in non-dividing but differentiated cells, suggesting that tRNA plays a crucial role in regulating cell fate determination. Importantly, the heterogeneity of tRNA in cells is precisely regulated to maintain cell-specific tRNA pools and fulfill the translation demand during cell fate determination [161]. During hiPSC differentiation, the chromatin state of tRNA genes modulated by histone modifications, such as H3K4 me3 and H3K27 me3, determines the recruiting of RNA pol III on specific tRNA genes, thus controlling tRNA isodecoder transcripts in cellular tRNA pools. However, the RNA pol III repressor MAF1 could stabilize cellular tRNA isotype pools by regulating the activation of RNA pol III through the mechanistic target of the rapamycin kinase (mTOR) signal pathway. This stabilization minimizes the risk of ribosomal errors and protein misfolding due to fluctuations in decoding rates [34]. During embryonic development, the expression profiles of tRNAs and tsRNAs show dynamic regulation. After fertilization and zygotic genome activation (ZGA), the transcription activity is largely enhanced to support the developmental process, triggering extensive reprogramming and a global increase in translation activity. At the same time, the repertoire of tRNA undergoes necessary alterations to adapt to the translation process during embryonic development. For example, during the embryonic development of zebrafish, the repertoire of tRNA isodecoder and tRNA modification signatures undergoes substantial alteration around the onset of gastrulation [162]. Repression of the translation of tRNA\({}^{\text{Gly(GCC)}}\) and tRNA\({}^{\text{Glu(CTC)}}\) in zebrafish embryos leads to early embryonic lethality [23]. Similarly, the deletion of tRNA\({}^{\text{Phe}}\) isodecoder genes impairs the embryonic development of mice, in which the isodecoder tRNA\({}^{\text{Phe}}\)1-1, required for neuronal function,shows the most serious lethal effect [17]. This evidence suggests a dispensable role of proper tRNA pools in maintaining early embryonic development. In addition to the modulation of tRNA abundance, the regulation of tRNA modification levels also plays pivotal roles in controlling cellular translation efficiency in stem cells. For instance, the depletion of the \(\Psi\) \"writer\" PUS7 significantly impairs mesoderm specification in hESCs [113]. Similarly, the knockout of the m\({}^{7}\)G methyltransferase _Mettl1_ promotes differentiation towards the mesoderm and endoderm while impairing differentiation towards the neural lineage in mESCs [16]. Moreover, a study showed that pluripotent cells exhibit distinct codon usage signatures, with a preference for translation by inosin-modified tRNAs in self-renewing hESCs.