Differentiation requires continuous regulation

IFFERENTIATION has long been conceived of as a series of binary decisions. Wsddington's epigenetic landscape (1940) provides a particularly vivid visual image of this concept: differentiation is likened to the path taken by a ball as it rolls down a sloped surface grooved by valleys. Similarly, according to Stuart Kauffman's binary model (1973), early choices limit later probabilities for changes in cell fate that accompany determination and transdetermina-tion. These theories have led to the still widely held view that progression along a developmental pathway entails changes that exclude future possibilities. This view is also embodied in commonly used terms such as "committed stem cell" and "ter-minally differentiated cell7 It suggests that cell states are locked in place by mechanisms that are not easily disrupted. Two molecular mechanisms that could silence genes that are inappropriate to a given differentiated cell type are "passive" and ~activeY It seems simplest to silence genes by a "passive control" mechanism: closing down unneeded genes so that they do not require active consideration in a given cell lineage for the life of the organism. Thus, commitment, like Lyoniza-tion of the X chromosome, would result in the permanent inac-tivation of many unnecessary genes. Indeed, what would be the advantage of keeping muscle genes accessible in the liver? Alternatively, differentiation could be governed by an ~active control" mechanism: the expression state of each gene being determined by the dynamic interaction of regulatory proteins present in the cell at any given time. This second possibility is often dismissed for the following reasons: (a) differentiation appears to be stable, (b) plasticity seems unnecessary, and (c) the number of regulators required appears cumbersome. In particular, the investment in negative regulators necessary to maintain the majority of genes in a silent state seems disproportionately large. In spite of the appeal of passive control, accumulating evidence suggests that differentiation is stably maintained by continuous regulation, both by positive (Britten and Davidson, 1969), and by negative regulators. We first present the evidence and then the implications of the continuous mode of regulation. Gurdon's (1962) nuclear transplantation experiments showed that genes were neither lost nor permanently inactivated during development. Upon transfer of an intestinal cell nucleus into an enucleated egg, entire swimming tadpoles developed. However, the frequency of this event was low, unless nuclei were first injected into oocytes (DiBerardino et al., 1986), a step that might allow reprogramming by stripping the DNA of mitotically …