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Signaling Pathways

One clear advantage to a complex multi-component signaling pathway is the fact that a complex pathway has the potential to better amplify a small, transient extracellular sample in order to produce a robust cellular response. If each level of a theoretical hierarchical signaling cascade were to activate twice as many signaling molecules than the previous level, then a single receptor in a three component pathway would ultimately activate only 8 terminal signals. In a ten component cascade, however, a single receptor would activate 1024. While these numbers are not realistic and true signaling pathways are more complex, it remains true that a more robust response can potentially be generated by complex signaling cascades than by simple ones in response to comparable amounts of signal.

Another advantage of a complex system is that an increase in the number of components offers several means by which the response can be regulated. If a signal transduction pathway were composed of three components acting in a stepwise manner then only these three components could be targeted by regulatory mechanisms, likely assuring that the response only be “on” or “off”. In contrast, a complex system with multiple parallel mechanisms and myriad components could be regulated at many points, fine tuning signaling events so as to better direct the intended response based on specific intracellular and extracellular stimuli.

While an abundance of signaling components may allow for fine regulation of the signaling cascade, one disadvantage to a complex pathway is the fact that the larger number of activated signal molecules offer more instances for misregulation and disruption of aspects of this pathway. For example, if one pathway were to use four components and another were to use twenty then in cells exposed to mutagenic stress the complex pathway would be five times more likely to be disrupted by mutagenesis than the simple pathway (assuming an equal chance of disruptive mutations for all genes).

A final significant disadvantage of a complex signaling pathway is that signal termination has the potential to be both more time consuming and less energy efficient than for a simple pathway. If signal termination were to require a blockade of a protein which is activated early in the pathway then it would take time for this blockade to disseminate through the signaling cascade, terminating the signal. In contrast, because a simple pathway has few signaling intermediates this signal termination can occur more rapidly. Likewise, if a pathway terminates signal by deactivating all signaling intermediates then this process will be far more energetically costly in a complex pathway (as will the costs of activating these signals initially).

Signaling and DNA Damage

The DNA damage checkpoint prior to G1-S phase transition uses reversible mechanisms to limit damaged cell growth. DNA damage activates the ATM/ATR kinases, which activate the Chk1/Chk2 kinases, leading to p53 stabilization and production of p21 which inhibits the G1/S-Cdk and S-Cdk complexes thus preventing cell cycle progression. If DNA damage is repaired then this arrest is revered and the cell cycle can proceed. This mechanism presumably evolved as a means of limiting growth of mutated cells. If DNA damage triggered an irreversible block in the cell cycle then a majority of cells would die under unfavorable conditions. By instead giving the cell a chance to repair DNA damage, this reversible checkpoint enables a larger proportion of cells to survive in an evolutionarily favorable manner.

The G1-S transition is an irreversible step after which cells are committed to cell cycle progression. If conditions are favorable, the ubiquitin ligase SCFSkp2 targets cyclin inhibitors for irreversible proteasomal degradation leading to Cdk4/cyclin D accumulation. This drives the activation of E2F, which the promotes its own transcription creating a positive feedback loop which further commits the cell to G1-S transition and leads to Cdk2/cyclin E accumulation. From an evolutionary perspective the irreversible nature of this checkpoint is essential; if cells were to replicate their chromosomes in S phase and then regress to the G1 phase then these cells would become polyploid. Daughter cells would then receive an altered number of chromosomes, leading to misregulation of various signaling processes and possibly resulting in cell death.

To assess whether DNA damage mechanisms play a role in cell cycle arrest in E. histolytica, one should first establish whether DNA damage can trigger a state of arrest in these cells. To determine this, cells should be exposed to a range of doses of X-ray radiation, pulsed with BrdU and propidium iodide and then fixed after a range of times, treated with anti-BrdU antibody, and assessed by flow cytometry to determine whether cells have entered S phase (as evidenced by BrdU incorporation [1]. To identify genes altered during this arrested growth phase, DNA microarrays should next be utilized to identify differentially transcribed genes in growth-arrested cells relative to normally growing cells. As a final experiment, bacteria should be transfected with defective forms of the proteins identified in this screen, X-ray irradiated, and assessed for DNA synthesis/cell cycle progression as above. If knockdown of specific proteins can allow cells to progress through the cell cycle despite accumulated DNA damage then this would suggest that these cells utilize a DNA damage checkpoint analogous to the p53 checkpoint in eukaryotic cells.

References

  • Kuerbitz SJ, et al. Wild-type p53 is a cell cycle checkpoint determinant following irradiation. Proc Nat Acad Sci. 1992; 89: 7491-7495.


Science | Biology


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