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The biochemical basis of an all-or-none cell fate switch in Xenopus oocytes
1Department of Molecular Pharmacology, Stanford University School of Medicine, Stanford, CA 94305-5332, USA. ferrell@cmgm.stanford.edu
Abstract:
Xenopus oocytes convert a continuously variable stimulus, the concentration of the maturation-inducing hormone progesterone, into an all-or-none biological response-oocyte maturation. Here evidence is presented that the all-or-none character of the response is generated by the mitogen-activated protein kinase (MAPK) cascade. Analysis of individual oocytes showed that the response of MAPK to progesterone or Mos was equivalent to that of a cooperative enzyme with a Hill coefficient of at least 35, more than 10 times the Hill coefficient for the binding of oxygen to hemoglobin. The response can be accounted for by the intrinsic ultrasensitivity of the oocyte's MAPK cascade and a positive feedback loop in which the cascade is embedded. These findings provide a biochemical rationale for the all-or-none character of this cell fate switch.
Insights
Xenopus oocytes use a ultrasensitive mitogen-activated protein kinase (MAPK) cascade to achieve an all-or-none response to progesterone. This biochemical switch explains how a continuous hormone signal triggers a discrete cell fate decision.
Area of Science:
- Cell Biology
- Biochemistry
- Developmental Biology
Background:
- Xenopus oocytes exhibit an all-or-none maturation response to varying progesterone concentrations.
- Understanding the molecular mechanisms behind this switch is crucial for cell fate determination.
Purpose of the Study:
- To investigate the role of the mitogen-activated protein kinase (MAPK) cascade in generating the all-or-none oocyte maturation response.
- To elucidate the biochemical basis for the ultrasensitive switch in Xenopus oocytes.
Main Methods:
- Analysis of individual Xenopus oocytes.
- Quantification of MAPK cascade response to progesterone and Mos.
- Biochemical modeling of enzyme kinetics and feedback loops.
Main Results:
- The MAPK cascade exhibits extreme ultrasensitivity, with a Hill coefficient of at least 35.
- This ultrasensitivity, coupled with a positive feedback loop, accounts for the all-or-none response.
- The MAPK cascade acts as a biochemical switch for oocyte maturation.
Conclusions:
- The ultrasensitive MAPK cascade is the key driver of the all-or-none oocyte maturation.
- This mechanism provides a biochemical explanation for a critical cell fate decision in Xenopus.
- Findings offer insights into biological switches and signal amplification.