HnRNP L inhibits CD44 V₁₀ exon splicing through interacting with its upstream intron

CD44 is a complex cell adhesion molecule that mediates communication and adhesion between adjacent cells as well as between cells and the extracellular matrix. CD44 pre-mRNA produces various mRNA isoforms through alternative splicing of 20 exons, among which exons 1-5 (C₁-C₅) and 16-20 (C₆-C₁₀) are constant exons, whereas exons 6-15 (V₁-V₁₀) are variant exons. CD44 V₁₀ exon has important roles in breast tumor progression and Hodgkin lymphoma. Here we show that increased expression of hnRNP L inhibits V₁₀ exon splicing of CD44 pre-mRNA, whereas reduced expression of hnRNP L promotes V₁₀ exon splicing. In addition, hnRNP L also promotes V₁₀ splicing of endogenous CD44 pre-mRNA. Through mutation analysis, we demonstrate that the effects of hnRNP L on V₁₀ splicing are abolished when the CA-rich sequence on the upstream intron of V₁₀ exon is disrupted. However, hnRNP L effects are stronger if more CA-repeats are provided. Furthermore, we show that hnRNP L directly contacts the CA-rich sequence. Importantly, we provide evidences that hnRNP L inhibits U2AF⁶⁵ binding on the upstream Py tract of V₁₀ exon. Our results reveal that hnRNP L is a new regulator for CD44 V₁₀ exon splicing.