Estudio comparativo de la estructura de orden superior al interior del núcleo celular (NHOS) en dos especies cercanas de mamífero (rata y ratón)

Abstract

ABSTRACT In metazoans, DNA is organized in supercoiled loops that are stabilized by their association with a nuclear compartment or substructure known as the nuclear matrix (NM). The interactions between DNA and the NM are more stable than those between DNA and chromatin proteins and define a nuclear higher order structure (NHOS). The establishment of anchorages between DNA and the NM is a necessary and fundamental condition in the organization of the genome, which preserves the integrity and coherence of such a massive genome within the nucleus. There is evidence that the NHOS is necessary for cell viability and that it has relevant implications for the appropriate nuclear physiology. Current evidence suggests that thermodynamic and structural constraints drive the actualization of the interactions between DNA and the 3 NM. To investigate whether functional factors may also determine the NHOS, we decided to study the NHOS from an evolutionary perspective, since in case that the NHOS is conserved during evolution, this would suggest that biological factors are involved in its establishment, since any functional factor that is determinant for the survival and/or adaptation of the species is a target of natural selection. We carried out a coarse-grained, comparative evaluation of the DNA-NM topological relationships in primary hepatocytes from two closely related mammals: rat and mouse, by determining the relative position to the NM of eight target sequences corresponding to highlyconserved genomic regions that also represent a sample of distinct chromosome territories within the interphase nucleus. Our results indicate that the pattern of topological relationships between DNA and NM is not conserved in two closely related species, suggesting that the NHOS is species-specific. This suggests that biological factors are not direct determinants of the interactions between DNA and the NM. In order to further investigate whether functional factors may participate in determining the NHOS, we compared the NHOS of neurons with that from hepatocytes and naïve B-lymphocytes in the rat. Neurons retain the capacity to replicate the genome under certain circumstances despite being postmitotic, and remain transcriptionally very active. Therefore, if the NHOS of neurons is similar to that of hepatocytes or naïve B-lymphocytes, this would suggest that functional factors (such as replication and/or transcription) have a role in determining this structure. Our results indicate that the NHOS of neurons is completely different from that one observed in hepatocytes or in naïve B-lymphocytes, suggesting that the NHOS is independent of functional factors. Therefore, now we know that NHOS is tissue-specific and species-specific, and that there is no obvious causal relationship between the functional processes of the nucleus (replication and transcription) and the NHOS; and this suggests that physical and thermodynamic factors are more important for establishing the NHOS. This structure represents a baseline level of organization on which the functional processes adapted to it unfold.