GLYPICAN AND BIGLYCAN IN THE NUCLEI OF NEURONS AND GLIOMA-CELLS - PRESENCE OF FUNCTIONAL NUCLEAR-LOCALIZATION SIGNALS AND DYNAMIC CHANGES IN GLYPICAN DURING THE CELL-CYCLE

We have investigated the expression patterns and subcellular localizat ion in nervous tissue of glypican, a major glycosylphosphatidylinosito l-anchored heparan sulfate proteoglycan that is predominantly synthesi zed by neurons, and of biglycan, a small, leucine-rich chondroitin sul fate proteoglycan. By laser scanning confocal microscopy of rat centra l nervous tissue and C6 glioma cells, we found that a significant port ion of the glypican and biglycan immunoreactivity colocalized with nuc lear staining by propidium iodide and was also seen in isolated nuclei . In certain regions, staining was selective, insofar as glypican and biglycan immunoreactivity in the nucleus was seen predominantly in a s ubpopulation of large spinal cord neurons. The amino acid sequences of both proteoglycans contain potential nuclear localization signals, an d these were demonstrated to be functional based on their ability to t arget beta-galactosidase fusion proteins to the nuclei of transfected 293 cells. Nuclear localization of glypican beta-galactosidase or Fc f usion proteins in transfected 293 cells and C6 glioma cells was greatl y reduced or abolished after mutation of the basic amino acids or dele tion of the sequence containing the nuclear localization signal, and n o nuclear staining was seen in the case of heparan sulfate and chondro itin sulfate proteoglycans that do not possess a nuclear localization signal, such as syndecan-3 or decorin (which is closely related in str ucture to biglycan). Transfection of COS-1 cells with an epitope-tagge d glypican cDNA demonstrated transport of the full-length proteoglycan to the nucleus, and there are also dynamic changes in the pattern of glypican immunoreactivity in the nucleus of C6 cells both during cell division and correlated with different phases of the cell cycle. Our d ata therefore suggest that in certain cells and central nervous system regions, glypican and biglycan may be involved in the regulation of c ell division and survival by directly participating in nuclear process es.