[Chitinase-like protein Ym2 regulates olfactory epithelium homeostasis and olfactory behavior in mice].

<b>Objective:</b> To investigate the role of Ym2 in olfactory epithelium homeostasis and olfactory behavior. <b>Methods:</b> Differential expression of Ym2 in the olfactory epithelium of young and aged mice was analyzed using single-cell RNA sequencing. Ym2 and related genes in the aged olfactory epithelium were identified, and their biological function was analyzed by GO enrichment analysis. The expression of Ym2 and Ym2-related genes in the young and aged olfactory epithelium was detected by immunostaining and RNAscope in situ hybridization. Buried food pullet test was used to assess the impact of Ym2 knockout on olfactory function, while immunostaining was used to evalute the effect of Ym2 knockout of on the homeostasis of olfactory epithelium. Statistical analysis was performed using GraphPad Prism 8.0.1 software. <b>Results:</b> Single-cell RNA sequencing revealed that Ym2 was highly expressed in multiple cell types, including horizontal basal cells and respiratory ciliated cells of the olfactory epithelium in aged mice compared to young animals. In situ hybridization and immunostaining data showed that Ym2 mRNA level were higher in the aged olfactory epithelium than that in the young tissue (<i>t</i>=4.50, <i>P</i><0.001). At the protein level, Ym2 expression was higher in horizontal basal cells (<i>t</i>=3.03, <i>P</i><0.05) and supporting cells (<i>t</i>=7.76, <i>P</i><0.001) of the aged epithelium compared to the young tissue. Additionally, the mRNA levels of the two Ym2-related genes, Alox15 and Cxcl5, were also higher in the aged olfactory epithelium (<i>t</i>=2.72 and 2.68, respectively, both <i>P</i><0.05). Behavioral testing showed that Ym2-/- mice took significantly longer to find buried food pellets compared to wild-type (WT) mice (<i>t</i>=2.35, <i>P</i><0.05). Histological analysis revealed a significant reduction in the number of olfactory sensory neurons and supporting cells in the olfactory epithelium of Ym2-/- mice compared to WT mice (<i>t</i>=5.86 and 3.69, respectively, both <i>P</i><0.01). <b>Conclusions:</b> Ym2 plays a critical role in smell perception. Ym2 knockout leads to reduction in the number of olfactory epithelial cell and impairs olfactory behavior of food-searching in young mice.