Revisiting Pluripotency Acquisition: A Unified Framework for Reprogramming Strategies

The traditional hierarchical view of totipotent cells generating diverse lineages of terminally differentiated cells was challenged by the discovery of induced pluripotent stem cells (iPSCs). This breakthrough demonstrated that the ectopic expression of four transcription factors, Oct4, Sox2, Klf4, and c-Myc (OSKM), can reprogram somatic cells to a pluripotent state. Since then, studies have expanded into diverse reprogramming strategies employing different cell types, factor combinations, delivery methods, and microenvironmental cues, resulting in the generation of diverse pluripotent and multipotent states. iPSCs modeled as canonical pluripotent reprogramming are defined by a core transcriptional network, the capacity for tri-lineage differentiation, and a permissive epigenetic landscape. However, growing evidence has revealed other reprogramming trajectories, including direct lineage reprogramming (or transdifferentiation), atypical pluripotent reprogramming, and noncanonical pluripotent reprogramming-exemplified as bacterial protein-mediated multipotency. Here, we summarize emerging insights of canonical and alternative pluripotent reprogramming strategies to propose a more nuanced framework that views pluripotency as a spectrum of molecular and functional states. These unconventional states often exhibit incomplete erasure of somatic identity, altered regulatory networks, and restricted lineage potential, underscoring the plastic and context-dependent nature of pluripotency. This review aims to reconceptualize fundamental perspectives on the acquisition of pluripotency in mammalian fibroblasts based on recent advances. We delineate the criteria for direct lineage reprogramming, atypical pluripotent reprogramming and noncanonical pluripotent reprogramming, explore novel approaches like bacterial ribosome-mediated cell fate conversion. This study integrates these strategies into a unified resource to enhance conceptual clarity and establishes a platform for advancing regenerative medicine.