Neural stem/progenitor cells (NSPCs) exhibit therapeutic potential in neuronal diseases. Previously, we reported that a sulfated polysaccharide (HS) from the sea cucumber Stichopus japonicas increased the proliferation of NSPCs. Since the formation of neurospheres is related with NSPCs proliferation, we investigated the mechanism leading to neurosphere formation with and without HS. The results showed that HS significantly promoted neurosphere formation in a dose-dependent manner at concentrations between 2 and 8 mu g/ml. Cell cycle analysis showed that HS increased the percentage of cells in S phase by 2.8-fold, as compared with the control. On the other hand, we observed a significantly rapid aggregation of NSPCs, resulting in formation of neurospheres as early as 2 h after HS treatment. However, the aggregation was not caused by chemotactic migration of NSPCs, as evidenced by the transwell chamber assay. Furthermore, the effect of HS on NSPCs was similar to the tumor necrosis factor-alpha (TNF-alpha) that activated nuclear factor NF-kappa B. Thus, we demonstrated that HS was able to promote cell proliferation and aggregation of NSPCs which could lead to the formation of neurospheres, and suggested that HS can serve as an adjuvant for promoting proliferation of NSPCs and formation of neurospheres. (C) 2010, The Society for Biotechnology, Japan. All rights reserved.