Regulation of chitosan-mediated differentiation of human olfactory receptor neurons by insulin-like growth factor binding protein-2

Olfaction is normally taken for granted in our lives, not only assisting us to escape from dangers, but also increasing our quality of life. Although olfactory neuroepithelium (ON) can reconstitute its olfactory receptor neurons (ORNs) after injury, no adequate treatment for olfactory loss has yet emerged. The present study investigates the role of glycosaminoglycans (GAGs) in modulating olfactory neuronal homeostasis and elucidates the regulatory mechanism. This work isolates and cultures human olfactory neuroepithelial cells (HONCs) with various GAGs for 7 days, and find that chitosan promotes ORN maturation, expressing olfactory marker protein (OMP) and its functional components. Growth factor protein array, ELISA and western blot analysis reveal that insulin-like growth factor binding protein 2 (IGFBP2) shows a higher level in chitosan-treated HONCs than in controls. Biological activity of insulin-like growth factor-1 (IGF-1), IGF-2 and IGF-1 receptor (IGF1R) is further investigated. Experimental results indicate that IGF-1 and IGF-2 enhance the growth of immature ORNs, expressing βIII tubulin, but decrease mature ORNs. Instead, down-regulation of phosphorylated IGF1R lifts the OMP expression, and lowers the βIII tubulin expression, by incubation with the phosphorylated inhibitor of IGF1R, OSI-906. Finally, the effect of chitosan on ORN maturity is antagonized by concurrently adding IGFBP2 protease, matrix metallopeptidase-1. Overall, our data demonstrate that chitosan promotes ORN differentiation by raising the level of IGFBP2 to sequestrate the IGFs-IGF1R signaling. STATEMENT OF SIGNIFICANCE: Olfactory dysfunction serves as a crucial alarm in neurodegenerative diseases, and one of its causes is lacking of sufficient mature olfactory receptor neurons to detect odorants in the air. However, the clinical treatment for olfactory dysfunction is still controversial. Chitosan is the natural linear polysaccharide and exists in rat olfactory neuroepithelium. Previously, chitosan has been demonstrated to mediate the differentiation of olfactory receptor neurons in an in vitro rat model, but the mechanism is unknown. The study aims to evaluate the role and mechanism of chitosan in an in vitro human olfactory neurons model. Overall, these results reveal that chitosan is a potential agent for treating olfactory disorder by the maintenance of olfactory neural homeostasis. This is the first report to demonstrate that chitosan promotes differentiation of olfactory receptor neurons through increasing IGFBP2 to sequestrate the IGFs-IGF1R.