Selective induction of alternatively spliced FynT isoform by TNF facilitates persistent inflammatory responses in astrocytes

Fyn tyrosine kinase has been implicated in the pathogenesis of Alzheimer's disease (AD). We have previously reported that upregulation of the FynT isoform in AD brains was partly associated with astrocyte activation. In this study, we demonstrated selective FynT induction in murine cortex and primary astrocyte culture after prolonged exposure to inflammatory stimulants, suggesting that FynT may mediate persistent neuroinflammation. To delineate the functional role of astrocytic FynT in association with TNF-mediated inflammatory responses, immortalized normal human astrocytes (iNHA) stably expressing FynT kinase constitutively active (FynT-CA) or kinase dead (FynT-KD) mutants were treated with TNF and compared for inflammatory responses using high-throughput real-time RT-PCR and Luminex multi-analyte immunoassays. FynT-CA but not FynT-KD mutant exhibited drastic induction of proinflammatory cytokines and chemokines after prolonged exposure to TNF, which could be attenuated by treating with Fyn kinase inhibitor PP2 or silencing via FynT-specific DsiRNA. FynT kinase activity-dependent induction of PKC delta expression, PKC delta phosphorylation, as well as NF kappa B activation was detected at the late phase but not the early phase of TNF signaling. In conclusion, selective FynT induction by TNF may facilitate persistent inflammatory responses in astrocytes, which is highly relevant to chronic neuroinflammation in neurodegenerative diseases including but not limited to AD.