Maturation of Xenopus oocytes, the irreversible, progesterone-induced resumption of meiosis I (1, 2), provides a classic example of a cell-fate decision with a well-defined threshold: Oocytes treated with a sufficient concentration of progesterone undergo maturation, whereas those treated with a lower concentration do not. Suprathreshold progesterone stimulus up-regulates the translation and gradual accumulation of the protein kinase Mos through a largely unidentified pathway. Mos is the essential initiator of meiosis (3); it signals through the mitogen-activated protein kinase (MAPK) cascade (composed of Mos, the MAPK kinase MEK1, and p42 MAPK) and promotes the activation of the cyclin-dependent kinase 1 (Cdk1)•cyclin B complex (Fig. 1A). Nonlinearity in the relationship between Mos concentration and MAPK activation conveys switchlike character to the decision to mature (4). In mature, M-phase cells, high Mos concentrations are maintained by continual Mos translation. Mos translation, in turn, is critically dependent on the activities of Mos, MEK, MAPK, and Cdk1, forming a positive feedback loop (5). The net effect of this nonlinearity and positive feedback is to enforce a stable separation of interphase and M phase at steady state. Thus, Xenopus oocyte maturation is bistable (4, 5). An oocyte crosses its progesterone threshold when Mos concentrations are sufficient to initiate M-phase positive feedback, hours after the signal is received at the cell surface. We explain how oocytes specify the progesterone concentration that initiates M-phase entry.

Depending on environmental factors and a frog's physiology, the dose of hormone required to induce ovulation can vary. We reasoned that, although receptor number and ligand affinity establish a basal median effective concentration (EC₅₀) for the system, specific intracellular kinases might tune the oocytes’ progesterone thresholds. Progesterone-dependent inactivation of glycogen synthase kinase–3β (GSK-3β) (by phosphorylation on Ser⁹) is an obligatory step in determining the cell-fate of the oocyte. Over-expression of GSK-3β can prevent progesterone-induced translation of Mos (6), and it uncoupled MAPK and Cdk1 activation, rendering M-phase entry incomplete (fig. S1). Because it regulates the transition between interphase and M phase, we tested whether GSK-3β also regulates the progesterone threshold of Xenopus oocytes. We measured progesterone dose responses in the presence and absence of 7-azaindolyl-pyrazinyl-maleimide (7AIPM), a specific GSK-3β inhibitor (7) (Fig. 1B), and observed that constant inhibition of GSK-3β doubled the system's responsiveness to progesterone (Fig. 1C and fig. S2) (8). Single-cell experiments indicated that maturation retained its all-or-none, irreversible character in 7AIPM-treated oocytes (Fig. 1D and fig. S3), suggesting that GSK-3β dampens overall progesterone responsiveness by raising the thresholds of individual oocytes. The 7AIPM treatment brought the oocytes’ in vitro EC₅₀ closer to the low nM progesterone concentrations measured in intact Xenopus ovaries (9, 10).

We envisioned two possible threshold regulation mechanisms that center on two negative regulatory connections placed in series. First, GSK-3β could dampen interphase signaling until GSK-3β is inactivated by progesterone-dependent signaling in a feed-forward manner, before meiotic entry (Fig. 2A, top). Alternatively, M-phase feedback could inactivate GSK-3β and relieve its repression of Mos translation as the meiotic switch activates (Fig. 2A, bottom). To verify that these mechanisms can raise a bistable system's threshold, we modeled them mathematically and mimicked 7AIPM treatment by setting GSK-3β activity to zero (11). This analysis revealed that both mechanisms provide a general solution for dampening a system's responsiveness (Fig. 2A, far right). In an analytical treatment, we proved mathematically that a raised threshold is a structural feature of bistable systems containing two negative regulatory connections placed in series; this is not dependent on parameter choice (fig. S8) (8).

To distinguish between these threshold regulation mechanisms, we experimentally defined GSK-3β's position in the progesterone-responsive signaling network relative to M-phase entry. Unlike most kinases, phosphorylation of GSK-3β on Ser⁹ inhibits its activity; this occurs through pseudo-substrate inhibition (12). We conducted a time course of phospho-GSK-3β accumulation at high progesterone concentration and found that maximum GSK-3β phosphorylation occurred during M phase (Fig.2B) (13). Accordingly, phosphorylation of GSK-3β correlated precisely with the decision to enter M phase in individual progesterone-stimulated oocytes (Fig. 2C). To determine whether M-phase signaling is necessary for phosphorylation of GSK-3β, we initiated maturation in the presence of PD98059, a specific MEK inhibitor. PD98059 treatment significantly reduced the population's average amount of phospho-GSK-3β (Fig. 2D, left); single-oocyte analysis revealed a precise correlation between MEK inhibition, very low amounts of phospho-GSK-3β, and failure of most cells to enter M phase (Fig. 2D, right). To verify that GSK-3β is not inactivated by progesterone-dependent interphase signals, we initiated meiosis in the absence of progesterone by microinjecting oocytes with Mos or nondegradable cyclin B1 (Δ90). In the microinjected cells, phospho-MAPK confirmed meiotic entry, and phosphorylation of GSK-3β was indistinguishable from progesterone-treated controls (Fig. 2E), suggesting that signals originating within the M-phase feedback loop are sufficient to inactivate GSK-3β. Given these data, we hypothesized that GSK-3β regulates oocytes’ progesterone responsiveness by participating in M-phase network itself.

We tested whether phosphorylation of GSK-3β mirrors the hallmark characteristics of MAPK activation during Xenopus oocyte maturation: nonlinear progesterone dependence and hysteresis (4, 5). In equivalently stimulated cells, phosphorylation of GSK-3β on Ser⁹ was either nearly undetectable or maximal at steady state (Fig. 3, A and B top). No intermediate responses were observed in single cells, demonstrating that GSK-3β phosphorylation is ultrasensitive with respect to progesterone concentration. In mature oocytes, positive, Mos-dependent feedback is sufficiently strong to maintain meiotic commitment after progesterone is removed (5). If GSK-3β inactivation relies on the same M-phase feedback, GSK-3β's phospho-regulation should display the same extreme hysteresis. We observed that phosphorylation of GSK-3β was sustained at its maximum 18 hours after removal of progesterone (Fig. 3C). Moreover, this hysteresis depended on the activity of MEK: When PD98059 was added as progesterone was removed, phosphorylation of GSK-3β was significantly reduced (Fig. 3D). In total, our data show that induction and maintenance of GSK-3β inactivation requires feedback through the meiotic kinase network. GSK-3β also displays the same bistable behavior observed in MAPK and Cdk1. Taken together, these results define GSK-3β as a node in a MEK-dependent, double-negative feedback loop that is likely driven by the bistable, positive feedback within M-phase oocytes. These results provide experimental verification of our feedback regulation mechanism (Fig. 2A, bottom images).

GSK-3β–dependent desensitization might allow a cell to integrate several signals, an impossibility if it were maximally responsive to progesterone alone. The branched-chain amino acid l-leucine (Leu), a nutritional marker, regulates translation through kinases that require inhibition of GSK-3β for activity (14, 15). To test whether Leu-dependent signals can be integrated into oocytes’ decision to mature, we conducted progesterone dose-responses in the presence and absence of extracellular Leu. Although Leu does not alter oocytes’ baseline response at low progesterone concentrations (Fig. 4A), Leu lowered the oocytes’ progesterone threshold, demonstrating cooperation between nutritional signals and cell-cycle entry (Fig. 4, A and B). If Leu signaling and GSK-3β form a common pathway during maturation, simultaneous treatment with Leu and 7AIPM would have no greater effect on progesterone threshold than either 7AIPM or Leu alone. However, if nutrient signaling was mediated through a GSK-3β–independent pathway, the two stimuli would have an additive or synergistic effect. We observed that the effect of Leu and 7AIPM together on progesterone responsiveness was statistically indistinguishable from that of 7AIPM alone (Fig. 4C), suggesting that GSK-3β regulation is a point of signal integration during Xenopus oocyte maturation.

Much attention has focused on the ubiquity of simple dynamical motifs in biological systems (16–23). Here, we demonstrate that layering apparently redundant motifs (positive feedback and double-negative feedback) tunes a bistable system's response threshold. This illustrates a remarkable separation of functions in bistable systems: Input EC₅₀ is regulated by relationships that are not strictly input-dependent. A striking number of biological networks contain nested feedback loops (17–19, 21), suggesting that this may be a general mechanism for modulating biological responsiveness. If multiple stimuli act on components of linked feedback loops, cells may tune their responsiveness to respond appropriately to their environment.

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