An isoform-specific role of FynT tyrosine kinase in Alzheimer's disease.

Alzheimer's disease (AD) is the leading cause of dementia in old age and is characterized by the accumulation of ß-amyloid plaques and neurofibrillary tangles (NFT). Recent studies suggest that Fyn tyrosine kinase forms part of a toxic triad with ß-amyloid and tau in the disease process. However, it is not known whether Fyn is associated with the pathological features of AD in an isoform-specific manner. In this study, we identified selective up-regulation of the alternative-spliced FynT isoform with no change in FynB in the AD neocortex. Furthermore, gene ontology term enrichment analyses and cell type-specific localization of FynT immunoreactivity suggest that FynT up-regulation was associated with neurofibrillary degeneration and reactive astrogliosis. Interestingly, significantly increased FynT in NFT-bearing neurons was concomitant to decreased FynB immunoreactivity, suggesting an involvement of alternative splicing in NFT formation. Furthermore, cultured cells of astrocytic origin have higher FynT to FynB ratio compared to those of neuronal origin. Lastly, primary rat mixed neuron-astrocyte cultures treated with Aß25-35 showed selective up-regulation of FynT expression in activated astrocytes. Our findings point to an isoform-specific role of FynT in modulating neurofibrillary degeneration and reactive astrogliosis in AD. Fyn kinase is known to interact with ß-amyloid and tau, and contributes to Alzheimer's disease pathogenesis. In this study, it is shown that the alternatively spliced FynT isoform is specifically up-regulated in the AD neocortex, with no change in FynB isoform. The increased FynT correlated with markers of neurofibrillary degeneration and reactive astrogliosis. In primary mixed cultures, treatment with amyloid peptides specifically up-regulated FynT in activated astrocytes. This study points to altered alternative splicing as a potential pathogenic mechanism in AD.

Isoform-specific upregulation of FynT kinase expression is associated with tauopathy and glial activation in Alzheimer's disease and Lewy body dementias.

Cumulative data suggest the involvement of Fyn tyrosine kinase in Alzheimer's disease (AD). Previously, our group has shown increased immunoreactivities of the FynT isoform in AD neocortex (with no change in the alternatively spliced FynB isoform) which associated with neurofibrillary degeneration and reactive astrogliosis. Since both the aforementioned neuropathological features are also variably found in Lewy Body dementias (LBD), we investigated potential perturbations of Fyn expression in the post-mortem neocortex of patients with AD, as well as those diagnosed as having one of the two main subgroups of LBD: Parkinson's disease dementia (PDD) and dementia with Lewy bodies (DLB). We found selective upregulation of FynT expression in AD, PDD, and DLB which also correlated with cognitive impairment. Furthermore, increased FynT expression correlated with hallmark neuropathological lesions, soluble ß-amyloid, and phosphorylated tau, as well as markers of microglia and astrocyte activation. In line with the human post-mortem studies, cortical FynT expression in aged mice transgenic for human P301S tau was upregulated and further correlated with accumulation of aggregated phosphorylated tau as well as with microglial and astrocytic markers. Our findings provide further evidence for the involvement of FynT in neurodegenerative dementias, likely via effects on tauopathy and neuroinflammation.

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 PKCδ expression, PKCδ phosphorylation, as well as NFκB 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.

Decreased rabphilin 3A immunoreactivity in Alzheimer's disease is associated with Aß burden.

Synaptic dysfunction, together with neuritic plaques, neurofibrillary tangles and cholinergic neuron loss is an established finding in the Alzheimer's disease (AD) neocortex. The synaptopathology of AD is known to involve both pre- and postsynaptic components. However, the status of rabphilin 3A (RPH3A), which interacts with the SNARE complex and regulates synaptic vesicle exocytosis and Ca(2+)-triggered neurotransmitter release, is at present unclear. In this study, we measured RPH3A and its ligand Rab3A as well as several SNARE proteins in postmortem neocortex of patients with AD, and found specific reductions of RPH3A immunoreactivity compared with aged controls. RPH3A loss correlated with dementia severity, cholinergic deafferentation, and increased ß-amyloid (Aß) concentrations. Furthermore, RPH3A expression is selectively downregulated in cultured neurons treated with Aß25-35 peptides. Our data suggest that presynaptic SNARE dysfunction forms part of the synaptopathology of AD.