Role of Fyn Kinase Inhibitors in Switching Neuroinflammatory Pathways

Fyn kinase is a member of the Src non-receptor tyrosine kinase family. Fyn is involved in multiple signaling pathways extending from cell proliferation and differentiation to cell adhesion and cell motility, and it has been found to be overexpressed in various types of cancers. In the central nervous system, Fyn exerts several different functions such as axon–glial signal transduction, oligodendrocyte maturation and myelination, and it is implicated in neuroinflammatory processes. Based on these premises, Fyn emerges as an attractive target in cancer and neurodegenerative disease therapy, particularly Alzheimer disease (AD), based on its activation by Aβ via cellular prion protein and its interaction with tau protein. However, Fyn is also a challenging target since the Fyn inhibitors discovered so far, due to the relevant homology of Fyn with other kinases, suffer from off-target effects. This review covers the efforts performed in the last decade to identify and optimize small molecules that effectively inhibit Fyn, both in enzymatic and in cell assays, including drug repositioning practices, as an opportunity of therapeutic intervention in neurodegeneration.

Fyn kinase regulates dopaminergic neuronal apoptosis in animal and cell models of high glucose (HG) treatment

Background High glucose (HG) is linked to dopaminergic neuron loss and related Parkinson’s disease (PD), but the mechanism is unclear. Results Rats and differentiated SH-SY5Y cells were used to investigate the effect of HG on dopaminergic neuronal apoptotic death. We found that a 40-day HG diet elevated cleaved caspase 3 levels and activated Fyn and mTOR/S6K signaling in the substantia nigra of rats. In vitro, 6 days of HG treatment activated Fyn, enhanced binding between Fyn and mTOR, activated mTOR/S6K signaling, and induced neuronal apoptotic death. The proapoptotic effect of HG was rescued by either the Fyn inhibitor PP1 or the mTOR inhibitor rapamycin. PP1 inhibited mTOR/S6K signaling, but rapamycin was unable to modulate Fyn activation. Conclusions HG induces dopaminergic neuronal apoptotic death via the Fyn/mTOR/S6K pathway.

PI3K/AKT Activation Mediates B-Cell Transformation By the "T" Splice-Variant of Fyn Kinase

The SRC-family kinase FYN has key roles downstream of growth-factor receptor activation and is implicated in various cancers. Activating FYNutations were reported in angioimmunoblastic T-cell lymphoma (AITL), but its role in transforming lymphocytes is poorly characterized. FYN also may play a role in signaling downstream of the B cell Receptor (BCR) receptor in Diffuse Large B-Cell Lymphoma (DLBCL), and laboratory studies show inhibition of FYN and other SRC family kinases (SFKs) can block BCR-mediated lymphomagenesis. To investigate FYN's role in oncogenic singling, we introduced FYN alleles to IL3-dependent FL5.12 murine pro-B cells and tested for selective advantage upon cytokine withdrawal and ability to transform the cells to cytokine independence. FYN B and T variants result from alternate splicing of exon 7, with B originally detected in brain and T in hematopoietic cells, but both are expressed and may be deregulated associated with disease states in diffuse tissues. We employed wildtype (WT) alleles for both spliceoforms along with mutations detected recurrently in AITL (L174R and R176C), kinase active (Y528H), and kinase dead (K299A). No FYN B alleles showed any selective advantage in the prolymphocyte cells subjected to cytokine withdrawal. In contrast, all FYN T alleles other than kinase dead were strongly enriched through multiple rounds of IL3 withdrawal and rescue. Indeed, complete withdrawal of cytokine resulted in transformation to IL3-independent growth, generating stable cell lines addicted to FYN kinase activity for survival. The SFK inhibitor PP2 was toxic to all FYN-T transformed cells but not to baseline FL5.12 cells growing in cytokine. WT- and L147R-transformed cells were similarly sensitive to PP2, while the other AITL-derived mutant R176C demonstrated reduced sensitivity. Immunoblotting demonstrated PP2-sensitive phosphorylation of AKT and the MTORC1 target 4EBP1, suggesting PI3K/AKT activation as a key survival pathway downstream from transformation by FYN T alleles. Consistently, the AKT inhibitor ipatasertib and PDK1 inhibitor GSK2334470 were highly active against all FYN-transformed lines, and AKT phosphorylation was sensitive to PDK1 inhibition, showing convergence on AKT activation in all transformants. Pan-PI3K inhibition was active against FYN transformants, while the delta isoform-specific inhibitor idelalisib showed only partial activity, suggesting activation of multiple PI3K isoforms downstream of FYN activity. These results suggest that AKT, PI3K, and PDK1 are important targets downstream of FYN, and that the PI3K/AKT pathway is the key survival mechanism in lymphomagenesis mediated by FYN. We show that WT and activating mutants of the FYN T spliceoform specifically, but not FYN B, induce drug-sensitive B lymphocyte malignant transformation by activating PI3K/AKT/MTORC1 signaling. Disclosures No relevant conflicts of interest to declare.

Regulation of Phosphorylated State of NMDA Receptor by STEP61 Phosphatase after Mild-Traumatic Brain Injury: Role of Oxidative Stress

Traumatic Brain Injury (TBI) mediates neuronal death through several events involving many molecular pathways, including the glutamate-mediated excitotoxicity for excessive stimulation of N-methyl-D-aspartate receptors (NMDARs), producing activation of death signaling pathways. However, the contribution of NMDARs (distribution and signaling-associated to the distribution) remains incompletely understood. We propose a critical role of STEP61 (Striatal-Enriched protein tyrosine phosphatase) in TBI; this phosphatase regulates the dephosphorylated state of the GluN2B subunit through two pathways: by direct dephosphorylation of tyrosine-1472 and indirectly via dephosphorylation and inactivation of Fyn kinase. We previously demonstrated oxidative stress’s contribution to NMDAR signaling and distribution using SOD2+/− mice such a model. We performed TBI protocol using a controlled frontal impact device using C57BL/6 mice and SOD2+/− animals. After TBI, we found alterations in cognitive performance, NMDAR-dependent synaptic function (decreased synaptic form of NMDARs and decreased synaptic current NMDAR-dependent), and increased STEP61 activity. These changes are reduced partially with the STEP61-inhibitor TC-2153 treatment in mice subjected to TBI protocol. This study contributes with evidence about the role of STEP61 in the neuropathological progression after TBI and also the alteration in their activity, such as an early biomarker of synaptic damage in traumatic lesions.

Saracatinib Fails to Reduce Alcohol-Seeking and Consumption in Mice and Human Participants

More effective treatments to reduce pathological alcohol drinking are needed. The glutamatergic system and the NMDA receptor (NMDAR), in particular, are implicated in behavioral and molecular consequences of chronic alcohol use, making the NMDAR a promising target for novel pharmacotherapeutics. Ethanol exposure upregulates Fyn, a protein tyrosine kinase that indirectly modulates NMDAR signaling by phosphorylating the NR2B subunit. The Src/Fyn kinase inhibitor saracatinib (AZD0530) reduces ethanol self-administration and enhances extinction of goal-directed ethanol-seeking in mice. However, less is known regarding how saracatinib affects habitual ethanol-seeking. Moreover, no prior studies have assessed the effects of Src/Fyn kinase inhibitors on alcohol-seeking or consumption in human participants. Here, we tested the effects of saracatinib on alcohol consumption and craving/seeking in two species, including the first trial of an Src/Fyn kinase inhibitor to reduce drinking in humans. Eighteen male C57BL/6NCrl mice underwent operant conditioning on a variable interval schedule to induce habitual responding for 10% ethanol/0.1% saccharin. Next, mice received 5 mg/kg saracatinib or vehicle 2 h or 30 min prior to contingency degradation to measure habitual responding. In the human study, 50 non-treatment seeking human participants who drank heavily and met DSM-IV criteria for alcohol abuse or dependence were randomized to receive 125 mg/day saracatinib (n = 33) or placebo (n = 17). Alcohol Drinking Paradigms (ADP) were completed in a controlled research setting: before and after 7–8 days of treatment. Each ADP involved consumption of a priming drink of alcohol (0.03 mg%) followed by ad libitum access (3 h) to 12 additional drinks (0.015 g%); the number of drinks consumed and craving (Alcohol Urge Questionnaire) were recorded. In mice, saracatinib did not affect habitual ethanol seeking or consumption at either time point. In human participants, no significant effects of saracatinib on alcohol craving or consumption were identified. These results in mice and humans suggest that Fyn kinase inhibition using saracatinib, at the doses tested here, may not reduce alcohol consumption or craving/seeking among those habitually consuming alcohol, in contrast to reports of positive effects of saracatinib in individuals that seek ethanol in a goal-directed manner. Nevertheless, future studies should confirm these negative findings using additional doses and schedules of saracatinib administration.

Fyn kinase inhibition using AZD0530 improves recognition memory and reduces depressive-like behaviour in an experimental model of Parkinson's disease

Fyn kinase has recently been established as a major upstream regulator of neuroinflammation in PD. This study aimed to determine if inhibition of Fyn kinase could lead to reduced neuroinflammation and improvements in motor and non-motor impairments in an early-stage model of PD. An experimental model of PD was produced using intra-striatal injection (4ul) of the neurotoxin 6-OHDA (5ug/ul). Sprague Dawley rats (n=42) were given either vehicle, 6mg/kg or 12mg/kg of Fyn kinase inhibitor (AZD0530) daily for 32 days via oral gavage and tested on a battery of tasks assessing motor, cognitive and neuropsychiatric outcomes. AZD 0530 administration led to improvement in volitional locomotion and recognition memory, as well as a reduction in depressive-like behaviour. Pathologically, an inflammatory response was observed; however, there were no significant differences in markers of neuroinflammation between treatment groups. Taken together, results indicate a potential therapeutic benefit for use of Fyn kinase inhibition to treat non-motor symptoms of PD, although mechanisms remain to be elucidated.

Fyn Kinase-Mediated PKCδ Y311 Phosphorylation Induces Dopaminergic Degeneration in Cell Culture and Animal Models: Implications for the Identification of a New Pharmacological Target for Parkinson’s Disease

Oxidative stress, neuroinflammation and apoptosis are some of the key etiological factors responsible for dopamin(DA)ergic degeneration during Parkinson’s disease (PD), yet the downstream molecular mechanisms underlying neurodegeneration are largely unknown. Recently, a genome-wide association study revealed the FYN gene to be associated with PD, suggesting that Fyn kinase could be a pharmacological target for PD. In this study, we report that Fyn-mediated PKCδ tyrosine (Y311) phosphorylation is a key event preceding its proteolytic activation in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of Parkinsonism. MPP+/MPTP induced Fyn kinase activation in N27 DAergic neuronal cells and the mouse substantia nigra. PKCδ-Y311 phosphorylation by activated Fyn initiates the apoptotic caspase-signaling cascade during DAergic degeneration. Pharmacological attenuation of Fyn activity protected DAergic neurons from MPP+-induced degeneration in primary mesencephalic neuronal cultures. We further employed Fyn wild-type and Fyn knockout (KO) mice to confirm whether Fyn is a valid pharmacological target of DAergic neurodegeneration. Primary mesencephalic neurons from Fyn KO mice were greatly protected from MPP+-induced DAergic cell death, neurite loss and DA reuptake loss. Furthermore, Fyn KO mice were significantly protected from MPTP-induced PKCδ-Y311 phosphorylation, behavioral deficits and nigral DAergic degeneration. This study thus unveils a mechanism by which Fyn regulates PKCδ′s pro-apoptotic function and DAergic degeneration. Pharmacological inhibitors directed at Fyn activation could prove to be a novel therapeutic target in the delay or halting of selective DAergic degeneration during PD.