Rna as code makers
a biosemiotic view of rnai and cell immunity
pp. 347-364
in: Marcello Barbieri (ed), Introduction to biosemiotics, Berlin, Springer, 2007Abstract
The development of the adaptive immune system as it is known in vertebrates relies on the highly coordinated program of cell differentiation achieved by such multicell organisms during their embryonic development, as well as during their functional physiology. This paper discusses the acquisition of an immune response by means of cell function specialization (recognizers, presenters, killers) in the light of biosemiosis. In particular, it will be argued that self/nonself differentiation rises in multicell organisms by a switch of organic codes and operating logic. In fact, double-stranded RNA molecules that induce a highly specific and selective mRNA degradation in non-vertebrates bring about an ubiquitary silencing of transcription and translation in differentiated vertebrate cells. This last response requires elements which are common to cell immunity, the so called interferon response machinery which is responsible by preserving cell genomes from mobile DNA fragments often generated during viral infection. This particular phenomenon will be extensively discussed to show the general point of how a major evolutionary change - invertebrates to vertebrates, in this particular case – requires the development and fixation of new organic codes. The pattern of embryonic and functional cell differentiation achieved by vertebrates' immune system will only be possible whenever, in evolution, cells are able to discriminate, recognize and integrate signs. We propose that the performance of these increasingly complex skills by cells is the hallmark of different levels of stabilization for living systems, the levels of CELL/SELF/SENSE. The way double-stranded RNA is dealt with by each of the levels proposed will be analyzed as a case study of a broader phenomenon: the contextual meaning of molecular signs as fixed by the combination of natural convention and natural selection as component mechanisms of the evolutionary process