Design Principles of Life, Disease, and Aging

# Systems biology: finding simplicity inside complexity *Evidence guide: Established biology and design-principle interpretation. Robustness refers to a specified function and perturbation.* A single bacterium makes thousands of different proteins. A human body has tens of trillions of cells, of hundreds of types, talking to each other through hormones and other signals. How can anyone hope to understand a system like that? Molecular biology has been spectacularly successful at one question: what molecules are present, and what does each one do? Systems biology asks a different question: how are the parts connected, and what does the circuit as a whole do? A parts list alone does not reveal the behavior. Knowing every resistor and capacitor in a radio does not tell you that the radio tunes to a station. The behavior lives in the wiring. Take the simplest example. A activates B, and B inhibits A. Is this a thermostat that settles, or something that overshoots, or oscillates? The diagram alone cannot say. It depends on the network structure, on how strong and how fast each interaction is, and on whether responses are gradual or switch-like. With quick, gentle inhibition, the pair settles to a steady level. Add a delay and a steep, switch-like response, and the same two arrows can oscillate. Same parts, same wiring diagram, different dynamics. The research program of Uri Alon looks for design principles: recurring ways that biological circuits are organized to carry out a function reliably. If many unrelated systems solve the same problem in the same way, that shared solution may be telling us something general. A central example is robustness: a function stays approximately the same even when protein levels, environmental conditions or physiological parameters vary. In 1999, Alon and colleagues showed that in bacterial chemotaxis, the precision of adaptation stayed robust when the levels of signaling proteins were changed, while other features, like adaptation time, did vary. Physiology has its own version, called homeostasis. Blood glucose, blood calcium and body temperature are held within narrow ranges, despite meals, exercise and weather. Later in the series we will meet circuits that achieve this, and see how the same circuits can fail. Across the series we follow one path: from many molecules, to an interaction network, to a recurring circuit, to its dynamical function, and finally to a design principle. And a second path, up in scale: from genes and proteins, to cells, tissues, organs, physiology, disease and aging. One caution. A design principle is an interpretation supported by models and data, not a law that biology must obey. Every scene in this series is labeled: established biology, experimental result, model result, interpretation, or hypothesis. Sources: [isb-book](https://www.routledge.com/An-Introduction-to-Systems-Biology-Design-Principles-of-Biological-Circuits/Alon/p/book/9781439837177), [sysmed-notes](https://www.weizmann.ac.il/mcb/alon/courses/system-medicine-2022-2023), [lab-site](https://www.weizmann.ac.il/mcb/alon/research), [alon1999](https://doi.org/10.1038/16483).