Functional analysis of uPAR alternatively spliced isoforms

The urokinase-type plasminogen activator receptor (uPAR) plays an essential role in cellular adhesion, migration, and differentiation. Alternative splicing of the PLAUR gene generates several isoforms, including ΔE5 and ΔE6, whose biological functions remain unclear. This study examined the localization and functional properties of these variants in comparison with membrane-bound uPAR (muPAR). Although ΔE5 and ΔE6 retain an intact GPI-anchor sequence, both isoforms were intracellularly retained and failed to localize to the cell membrane, unlike muPAR. In PMA-induced differentiation assays, only muPAR-expressing cells maintained adhesion after stimulation, whereas ΔE5, ΔE6, and PLAUR^-/- cells did not. All cell lines expressed the macrophage differentiation marker SR-A I, indicating preserved differentiation capacity. Functional analyses showed that only muPAR-expressing cells adhered to vitronectin and interacted with α5β1 integrin. Neither ΔE5 nor ΔE6 bound vitronectin, even after PMA or uPA stimulation, nor did they interact with α5β1 integrin. Functionally, both variants closely resembled uPAR-deficient cells. These results demonstrate that exons 5 and 6 are critical for proper uPAR localization and function. Dysregulated PLAUR splicing may therefore contribute to pathological processes, and modulation of uPAR splicing could represent a potential therapeutic approach.