Transient activation of the UPRER is an essential step in the acquisition of pluripotency during reprogramming

Somatic cells can be reprogrammed into pluripotent stem cells using the Yamanaka transcription factors. Reprogramming requires both epigenetic landscape reshaping and global remodeling of cell identity, structure, basic metabolic processes, and organelle form and function. We hypothesize that variable regulation of the proteostasis network and its influence upon the protein-folding environment within cells and their organelles is responsible for the low efficiency and stochasticity of reprogramming. We find that the unfolded protein response of the endoplasmic reticulum (UPR ER ), the mitochondrial UPR, and the heat shock response, which ensure proteome quality during stress, are activated during reprogramming. The UPR ER is particularly crucial, and its ectopic, transient activation, genetically or pharmacologically, enhances reprogramming. Last, stochastic activation of the UPR ER predicts reprogramming efficiency in naive cells. Thus, the low efficiency and stochasticity of cellular reprogramming are due partly to the inability to properly initiate the UPR ER to remodel the ER and its proteome.