Abstract In cells in general, regardless of their identity and functional status, the mediators of signal transduction (ST), the classic second messengers, are highly conserved: calcium, cAMP, nitric oxide, phosphorylation cascades, etc. At the same time, they are significantly less numerous than the extracellular signals (or first messengers) they represent, suggesting that this universal conversion of signals into second messengers follows the conventional rules of an organic code. Nevertheless, the way these second messengers are integrated and the consequences they trigger change dramatically according to cell organization - its structure and function. Here we examine ST beyond the generation of second messengers, and more as the ability of a cell in its different configurations to assign meaning to signals through discrimination of their context. In metabolism, cell cycle, differentiation, neuronal, and immune function the circuitry operating at cell level will proceed by the creation of conventional links between an increasing number of physiological activities, that is, changes in environment are progressively coupled to: transcription patterns; transcription and replication patterns; transcription, replication, and differentiation patterns; and transcription, replication, differentiation, and functional patterns.
The categorial framework  consisting of CELL/SELF/SENSE has been previously proposed  as an attempt to classify the levels of organization adopted by living systems. Our working hypothesis is that these categories reflect: (i) an improved comprehension of self-organization and the convergent gain of complexity that are crucial traits of biological systems, (ii) the possibility of a research agenda, which aims to identify organic codes at the transitions between levels. In the present work we shall use the CELL/SELF/SENSE categories to analyze the progressive complexity of cell fate control through evolution and through development, showing how it is related to switches in ST codes. The notion of a physical attractor  will be introduced to reframe the role of classical ST pathways in cell function. The notions of degeneracy and polisemy will also
Laboratory of History of Science, Instituto Butantan, Av Vital Brazil, 1500, Sao Paulo 05503-900, SP, Brazil e-mail: [email protected]
M. Barbieri (ed.), The Codes of Life: The Rules of Macroevolution. © Springer 2008
be examined as possible defining resources for the convergent gain of complexity taking place in biological systems.
Was this article helpful?