Alternative splicing of CHI3L1 regulates protein secretion through conformational changes.

Background: Alternative splicing (AS) plays a crucial role in regulating protein function through the generation of structurally distinct isoforms.

Objective: We identify a novel splicing event in Chitinase 3-like 1 (CHI3L1) that modulates its secretion through conformational changes.

Methods: CHI3L1 alternative splicing was analyzed using the GTEx dataset. The regulation of CHI3L1 splicing was examined in response to THP-1 and BEAS-2B cells using RT-PCR. Structural modeling of CHI3L1 isoforms was conducted with AlphaFold to predict conformational changes caused by exon 8 exclusion. Protein expression and secretion levels of CHI3L1 isoforms were analyzed by Western blotting.

Results: Analysis of the GTEx dataset revealed tissue-specific regulation of CHI3L1 exon 8, with pronounced exclusion in lung tissue. The splicing pattern of CHI3L1 was dynamically regulated during THP-1 macrophage differentiation and by cell density in lung-derived epithelial BEAS-2B cells, suggesting its responsiveness to cellular context. While both full-length and exon 8-excluded CHI3L1 proteins showed cytoplasmic localization, structural analysis using AlphaFold revealed that exon 8 exclusion significantly altered the orientation of the signal peptide. Consequently, exon 8-excluded CHI3L1 exhibited minimal secretion into the culture medium compared to the full-length protein.

Conclusion: These findings demonstrate that alternative splicing-mediated exclusion of exon 8 serves as a novel regulatory mechanism controlling CHI3L1 secretion through conformational changes, providing new insights into the post-transcriptional regulation of secreted proteins.