Cells Find Signal in the Noise
Parents at an amusement park know the challenge of picking out their child’s voice, or even hearing their own hollering, in the noise of the crowd. Yelling won’t help much if the rest of the crowd is yelling also. Acoustic engineers know that raising the volume while playing back a noisy tape amplifies the noise as well as the signal. Cells have a novel way of meeting this challenge, as two Japanese mathematical biologists discuss in PNAS.1 Cells are continuously sending and receiving chemical messages, a process called signal transduction. Treating the cell signal transduction network like a physical system of receivers and amplifiers, the researchers noted that a cell, like an amusement park, is an intrinsically noisy place, yet some of the reactions are very sensitive. “How cells respond properly to noisy signals by using noisy molecular networks is an important problem in elucidating the underlying ‘design principle’ of cellular systems,” they say in the introduction. How do the sensitive reactions get their messages through all that noise?
Because intracellular processes are inherently noisy, stochastic reactions process noisy signals in cellular signal transduction. One essential feature of biological signal transduction systems is the amplification of small changes in input signals. However, small random changes in the input signals could also be amplified, and the transduction reaction can also generate noise. Here, we show theoretically how the abrupt response of ultrasensitive signal-transduction reactions results in the generation of large inherent noise and the high amplification of input noise. The inherently generated noise propagates with amplification through intracellular molecular network. We discuss how the contribution of such transmitted noise can be shown experimentally. Our results imply that the switch-like behavior of signal transduction could be limited by noise; however, high amplification reaction could be advantageous to generate large noise, which would be essential to maintain behavioral variability.
They categorized the noise as intrinsic, coming from the reaction itself, to extrinsic, coming from other reactions. This is somewhat like hearing your own voice vs. the yelling of those around you. The intrinsic noise has higher frequency than the extrinsic noise. As one source of noise becomes dominant, it reaches a crossover point where the other source is less dominant. This provides a kind of signal, or switch, which the cell can use to advantage:
From our result, it can be further suggested that if the extrinsic noise dominates, the upstream reactions affect the fluctuation of the most downstream reaction, which determines the cellular behavior. As a result, the behavioral fluctuations are made up of the contributions of the fluctuations of several upstream reactions. On the other hand, if the intrinsic noise dominates, only the intrinsic noise of the most downstream reaction determines the behavioral fluctuations. As a result, the behavior could be simpler than the case in which extrinsic noise is dominant….
….Consequently, the low-frequency modulations in the downstream reactions can be affected by the behaviors of upstream reactions, whereas the high-frequency modulations are expected to be independent of upstream reactions.
As a result, a bacterium can respond to chemicals in the environment, the hemoglobin in your blood can respond to changing conditions in the capillaries, genes can respond correctly to requests for expression, and complex cascades of cellular reactions can respond to the signal from any reaction in the series, in the midst of all the noise. “Therefore,” they conclude, “the result implies that the extrinsic noise is essential to maintain the behavioral variability in wild-type bacteria.” Their experiments related to three relatively simple reactions, and their analysis considered primarily linear response. Many cellular reactions involve nonlinear behavior. “In these cases,” they admit, “the relation between the response and the fluctuations can be more complicated than the relations we studied.” The authors made no attempt to explain how these capabilities evolved.
1Tatsuo Shibata and Koichi Fujimoto, “Noisy signal amplification in ultrasensitive signal transduction,” Proceedings of the National Academy of Sciences USA, 10.1073/pnas.0403350102, published online before print December 29, 2004.
Evolutionists accuse the intelligent design movement of never publishing anything, and then cry foul when they do (see 09/08/2004 and 12/28/2004 headlines). Actually, there are thousands of ID papers, and they are published regularly, not in obscure outlets, but in the major, high-impact journals. They may not mention the buzzword “intelligent design” explicitly, but they do everything the ID movement advocates: explore the design of a phenomenon as if it has a purpose, follow the evidence where it leads, and leave the philosophical or religious implications to the reader. We regularly highlight such articles right here (see 11/10/2004, 10/27/2004, 10/27/2004, and 09/22/2004 headlines for a few recent examples). Notice how these authors used the phrase “design principle” but had no use for the evolutionary hypothesis. Very few papers try to explain in any detail how a complex feature evolved. Most, if they mention evolution at all, merely assume it in passing, as if fulfilling the obligatory pinch of incense to Father Charlie (see 11/18/2004 and 11/04/2004 and 10/01/2004 recent examples). If the criteria were rearranged with these considerations in mind, the ID movement could claim the vast majority of scientific papers as their own, and the Darwin Party would be left with a handful of just-so stories. Demand a recount.