Bistability in cell signaling: How to make continuous processes discontinuous, and reversible processes irreversible

Xenopus oocyte maturation is an example of an all-or-none, irreversible cel l fate induction process. In response to a submaximal concentration of the steroid hormone progesterone, a given oocyte may either mature or not matur e, but it can exist in intermediate states only transiently. Moreover, once an oocyte has matured, it will remain arrested in the mature state even af ter the progesterone is removed. It has been hypothesized that the all-or-n one character of oocyte maturation, and some aspects of the irreversibility of maturation, arise out of the bistability of the signal transduction sys tem that triggers maturation. The bistability, in turn, is hypothesized to arise from the way the signal transducers are organized into a signaling ci rcuit that includes positive feedback (which makes it so that the system ca nnot rest in intermediate states) and ultrasensitivity (which filters small stimuli out of the feedback loop, allowing the system to have a stable off -state). Here we review two simple graphical methods that are commonly used to analyze bistable systems, discuss the experimental evidence for bistabi lity in oocyte maturation, and suggest that bistability may be a common mea ns of producing all-or-none responses and a type of biochemical memory. (C) 2001 American Institute of Physics.