Cellular promiscuity: explaining cellular fidelity in vivo against unrestrained pluripotency in vitro.

John Gurdon’s [1] and Shinya Yama­ naka’s [2] Nobel Prize involves discov­ eries that vex fundamental concepts about the stability of cellular identity [3,4], ageing as a rectified path and the differ­ ences between germ cells and somatic cells. The differentiation of pluripotent stem cells (PSCs) into progeny, including spermatids [5] and oocytes [6], is perplexing. In vivo, nature imposes strict fate constraints. Yet in vitro, reprogrammed PSCs liberated from the body government freely differentiate into any pheno type—except placenta—violat­ ing even somatic cell against germ cell seg­ regations. Albeit that it is anthropomorphic, might the concept of ‘cellular promiscuity’ explain these surprising behaviours? Fidelity to one’s differentiated state is nearly universal in vivo—even cancers retain some allegiance. Appreciating the mecha­ nisms in vitro that liberate reprogrammed cells from the numerous constraints govern­ ing development in vivo might provide new insights. Similarly to highway guiderails, a range of constraints preclude progeny cells within embryos and organisms from travelling too far away from the trajec­ tory set by their ancestors. Restrictions are imposed externally—basement membranes and intercellular adhesions; internally— chromatin, cytoskeleton, endomembranes and mitochondria ; and temporally by ageing. ‘Cellular promiscuity’ was glimpsed pre­ viously during cloning; it was seen when somatic cells successfully ‘fertilized’ enu­ cleated oocytes in amphibians [1] and later with ‘Dolly’ [7]. Embryonic stem cells (ESCs) corroborate this. The inner cell mass of the blasto cyst cells develops faithfully, but lib­ eration from the trophoectoderm generates pluripotent ESCs in vitro, which are freed from fate and polarity restrictions. These freedom­ seeking ESCs still abide by three­dimensional rules as they conform to chimaera body patterning when injected into blastocysts. Yet if transplanted elsewhere, this results in chaotic teratomas or helter­skelter in vitro differentiation—that is, pluripotency. August Weismann’s germ plasm theory, 130 years ago, recognized that gametes produce somatic cells, never the reverse. Primordial germ cell migrations into fetal gonads, and parent­of­origin imprints, explain how germ cells are sequestered, retaining genomic and epigenomic purity. Left uncontaminated, these future gametes are held in pristine form to parent the next generation. However, the cracks separat­ ing germ and somatic lineages in vitro are widening [5,6]. Perhaps, they are restrained within gonads not for their purity but to prevent wild, uncontrolled misbehaviours resulting in germ cell tumours. The ‘cellular promiscuity’ concept regard­ ing PSCs in vitro might explain why cells of nearly any desired lineage can be detected using monospecific markers. Are assays so sensitive that rare cells can be detected in heterogeneous cultures? Certainly popula­ tion heterogeneity is considered for trans­ plantable cells—dopaminergic neurons and islet cells—compared with applications needing few cells—sperm and oocytes. This dilemma of maintaining cellular identity in vitro after reprogramming is significant. If not addressed, the value of unrestrained induced PSCs (iPSCs) as reliable models for ‘diseases in a dish’, let alone for subsequent therapeutic transplantations, might be dimin­ ished. X­chromosome re­inactivation variants in differentiating human PSCs, epigenetic imprint errors and copy number variations are all indicators of in vitro infidelity. PSCs, which are held to be undifferentiated cells, are artefacts after all, as they undergo their programmed development in vivo. If correct, the hypothesis accounts for con­ cerns raised about the inherent genomic and epigenomic unreliability of iPSCs; they are likely to be unfaithful to their in vivo differ­ entiation trajectories due to both the freedom from in vivo developmental programmes, as well as poorly characterized modifications in culture conditions. ‘Memory’ of the PSC’s identity in vivo might need to be improved by using approaches that might not fully erase imprints. Regulatory authorities, including the Food & Drug Administration, require evidence that cultured PSCs do retain their original cellular identity. Notwithstanding fidelity lapses at the organismal level, the recognition that our cells have intrinsic freedom­loving tendencies in vitro might generate better approaches for only partly releasing somatic cells into probation, rather than full emancipation.