Differential proteomics and early neuronal differentiation of MEHMO patient-derived iPSCs

Abstract

The eukaryotic initiation factor 2 (eIF2) is a protein complex which is part of the cellular translation machinery. eIF2 comprises three subunits (α, β, γ), the γ subunit being the one which contains a GTP-binding domain and binding sites for the other two subunits α and β to interact. eIF2 acts in the initial part of the protein synthesis, forming a complex with GTP and Met-tRNAMeti. This ternary complex associates itself with the 40S subunit of the ribosome and, in a cap-dependent manner, scans the mRNA with the help of other factors. The recognition of the first AUG codon leads to the final assembly of the ribosome and the translation of the protein. Due to eIF2 importance, mutations in highly conserved regions of their genes might be lethal; however, a EIF2S3 mutation was recently found to be responsible for intellectual disability in male patients. In those patients, signs of problems in nervous system development are accompanied by a broad spectrum of other symptoms such as obesity, microgenitalism and ataxia gait, a syndrome known as MEHMO. This finding has motivated the present study in investigating why function disruption in a central player of protein translation mainly impacts on nervous system development. Patient-derived iPS cells were generated, and served as the model to learn more about eIF2, its impact on the cell proteome and its function in early neuronal development. The global translation profile from the iPSCs was obtained by SILAC-based LC-MSMS analysis, and the expression of candidate genes was assessed in a high-throughput manner at different time points of neuronal differentiation. A specific increase in APOE and CRABP1 (retinoic acid related proteins) translation and transcription was observed in patient cells, whereas CBS and TKT protein levels were decreased. Interestingly, other genes correlated to translation regulatory mechanisms were also differentially expressed. All this data indicates an imbalance of survival/apoptotic pathways activation due to translational impairment. Although the present results need further confirmation, this alteration on translation rates (iPSCs) suggests a very early embryonic development effect of the mutation. The findings on altered candidate proteins/mRNAs might open new avenues for future research. This hopefully will contribute to the understanding of both, the eIF2S3 roles so far not described and the exact functional consequences of the mutation.