Structural and functional compartmentalization of the cell nucleus in supraoptic neurons

It is well-established that the neuronal cell nucleus is organized in discrete compartments involved in transcription and RNA processing. The main nuclear compartments in neurons include the chromosome territories, the nucleolus, nuclear speckles of splicing factors, Cajal bodies, and nuclear rodlets. The supraoptic nucleus (SON) neurons provide a powerful model in vivo to study the organization of these nuclear compartments in response to variations of cellular activity. The upregulation of transcription in SON neurons under chronic hyperosmolar conditions is associated with 1) nuclear and nucleolar enlargement, 2) dispersion of chromatin, 3) reduction in the size of nuclear speckles, 4) increase in the number of Cajal bodies implicated in the maturation of splicing small nuclear ribonucleoproteins, and 5) proliferation of the fibrillar centers of the nucleolus, the sites of nucleolar transcription of ribosomal genes. These changes revert after the cessation of the activation by rehydration of animals. Under conditions of neuronal stress induced by hypertonic saline injection, SON neurons exhibit an early response of downregulation of transcription. This is accompanied by chromatin condensation, redistribution of splicing factors, reduction in the number of Cajal bodies, and microsegregation of the fibrillar and granular components of the nucleolus and disruption of its fibrillar centers, all of which are associated with a transitory expression of c-Fos. These changes progressively revert and at 24 hours after the stress induction a rebound upregulation of transcription is observed. These findings illustrate the transcription-dependent organization and behavior of nuclear compartments in the neuronal model of magnocellular neurosecretory cells of the hypothalamus.