Neuronal body size correlates with the number of nucleoli and cajal bodies, and with the organization of the splicing machinery in rat trigeminal ganglion neurons

Trigeminal ganglion neurons comprise three main cell body-size types. This cell size heterogeneity provides an excellent neuronal model to study the c ell size-dependent organization and dynamics of the nucleoli, Cajal (coiled ) bodies (CBs), and nuclear speckles of pre-mRNA splicing factors, nuclear structures that play a key role in the normal neuronal physiology. We have analyzed the number of nucleoli and CBs and the structural and molecular or ganization of CBs and nuclear speckles in the three neuronal types by using immunofluorescence with antibodies that recognize nucleoli (fibrillarin), CBs (coilin), and nuclear speckles (snRNPs), confocal microscopy, and elect ron microscopy. Whereas the mean number of nucleoli per neuron decreases as a function of cell size, the number of CBs per cell significantly increase s in large neurons in comparison with the small ones. In addition, large ne urons have a higher proportion of CBs associated with the nucleolus. In all neuronal types, CBs concentrate coilin, fibrillarin, snRNPs, and the survi val motor neuron protein (SMN). Immunostaining for snRNPs shows small speck le domains and extensive areas of diffuse nucleoplasmic signal in large neu rons, in contrast with the large nuclear speckles found in small neurons. F urthermore, flow cytometric analysis shows that all neurons are in the rang e of diploid cells. These findings indicate that the fusion behavior of nuc leoli, the formation of CBs and their relationships with the nucleolus, as well as the compartmentalization of the pre-mRNA splicing machinery, is rel ated to cell body size in the trigeminal ganglion neurons. Because transcri ptional activity is a basic determinant mechanism of cell size in diploid c ells, we suggest that our findings reflect a distinct transcription-depende nt organization of the nucleolus and splicing machinery in the three cell t ypes of trigeminal ganglion neurons. J. Comp. Neurol. 430:250-263, 2001. (C ) 2001 Wiley-Liss, Inc.