DISTRIBUTION OF SNRNPS, SPLICING FACTOR-SC-35 AND ACTIN IN INTERPHASENUCLEI - IMMUNOCYTOCHEMICAL EVIDENCE FOR DIFFERENTIAL DISTRIBUTION DURING CHANGES IN FUNCTIONAL-STATES

Small nuclear ribonucleoproteins (snRNPs) play an integral role in the processing of pre-mRNA in eukaryotic nuclei. snRNPs often occur in a speckled intranuclear distribution, together with the non-snRNP splici ng factor SC-35. snRNPs have also been shown to be associated with act in in the nuclear matrix, suggesting that both actin and snRNPs may be involved in the processing and transport of transcripts. The work rep orted here was undertaken to compare the spatial relationship of snRNP s, SC-35, and intranuclear actin in neuronal and non-neuronal cell typ es. In undifferentiated PC12 cells and in non-neuronal cells growing i n association with dorsal root ganglion neurons, confocal immunocytoch emistry revealed a typical, speckled distribution of snRNP aggregates, which colocalized with the SC-35 splicing factor. In contrast, a uniq ue snRNP distribution was observed in dorsal root ganglion neurons in vitro and in PC12 cells differentiated by nerve growth factor. In nucl ei of these cells, snRNPs were predominantly located at the periphery where they formed a spherical shell apposed to the nuclear envelope. U ltrastructural immunogold labelling of snRNPs in dorsal root ganglion neurons in vitro confirmed this distribution. In contrast, SC-35 remai ned distributed in a speckled pattern throughout nuclei of dorsal root ganglion neurons and PC12 cells, even in cases where snRNPs were almo st exclusively positioned at the nuclear periphery. In non-neuronal ce lls in dorsal root ganglion cultures and in undifferentiated PC12 cell s, snRNP aggregates were frequently associated with actin aggregates, as determined by Nearest Neighbor Analyses. In PC12 cells, this spatia l relationship was altered during nerve growth factor-induced differen tiation, prior to the time at which these cells showed morphological e vidence of differentiation. Specifically, Nearest Neighbor Analyses be tween snRNP and actin aggregates in PC12 cells exposed to nerve growth factor for 4 hours revealed that snRNP and actin aggregates exhibited a closer association than in undifferentiated cells. These results su ggest that sites of pre-mRNA processing and transcription may differ b etween cell types, and that the functions of snRNPs and actin within i nterphase nuclei may be related. The results also indicate that the di stribution of snRNPs is dynamic and that it may depend upon the functi onal state of the cell as well as upon its state of differentiation.