Protein heterogeneity in the coiled body compartment

Coiled bodies are ubiquitous nuclear inclusions of unknown function. Although a considerable list of coiled body components has been assembled in recent years leading to several functional hypotheses, none have yet been borne out by experimentation. Pinpointing coiled body function is difficult in part because each known component molecule has been shown to be present at other sites in the nucleus. Using probes to individual coiled body molecules is therefore likely to yield ambiguous results. From direct observation of coiled body behavior we know that they are dynamic structures, changing in content, size, and number under different physiological conditions. In our studies, we have found that the number of coiled bodies in mammalian endothelial cells is relatively high. Depending on phenotype, quiescent or angiogenic, endothelial cells can average as few as 4 or as many as 15 coiled bodies per nucleus (as opposed to 2 or 3 for most cell types). This can provide certain advantages in the analysis of their dynamics and composition. Moreover, expression of the coiled body protein, pigpen, is sharply regulated as endothelial cells toggle back and forth between the two phenotypes. Using the endothelial cell system, we present several new observations in this report on the dynamics of coiled bodies and their constituent proteins and reinforce prior observations that we consider important but understated in the literature. With antibodies to p80-coilin, pigpen, and fibrillarin, we show that there may be heterogeneity in the coiled body population of individual cells. We demonstrate that the coiled body marker protein p80-coilin can also be found distributed in the nucleoplasm and in apparent association with the nuclear envelope. This suggests that coilin could play a role in some aspect of nucleocytoplasmic exchange. Finally, we correlate the presence of pigpen in a diffuse nucleoplasmic pool with the expression of a phosphatase-sensitive epitope, indicating that subnuclear localization may depend upon the phosphorylation state of the protein. Our results suggest to us that a viewpoint of coiled bodies as part of a fluid trafficking network may be helpful in discerning their cellular functions.