Fyn Kinase Regulates Microglial Neuroinflammatory Responses in Cell Culture and Animal Models of Parkinson's Disease.

Sustained neuroinflammation mediated by resident microglia is recognized as a key pathophysiological contributor to many neurodegenerative diseases, including Parkinson's disease (PD), but the key molecular signaling events regulating persistent microglial activation have yet to be clearly defined. In the present study, we examined the role of Fyn, a non-receptor tyrosine kinase, in microglial activation and neuroinflammatory mechanisms in cell culture and animal models of PD. The well-characterized inflammogens LPS and TNFα rapidly activated Fyn kinase in microglia. Immunocytochemical studies revealed that activated Fyn preferentially localized to the microglial plasma membrane periphery and the nucleus. Furthermore, activated Fyn phosphorylated PKCδ at tyrosine residue 311, contributing to an inflammogen-induced increase in its kinase activity. Notably, the Fyn-PKCδ signaling axis further activated the LPS- and TNFα-induced MAP kinase phosphorylation and activation of the NFκB pathway, implying that Fyn is a major upstream regulator of proinflammatory signaling. Functional studies in microglia isolated from wild-type (Fyn(+/+)) and Fyn knock-out (Fyn(-/-)) mice revealed that Fyn is required for proinflammatory responses, including cytokine release as well as iNOS activation. Interestingly, a prolonged inflammatory insult induced Fyn transcript and protein expression, indicating that Fyn is upregulated during chronic inflammatory conditions. Importantly, in vivo studies using MPTP, LPS, or 6-OHDA models revealed a greater attenuation of neuroinflammatory responses in Fyn(-/-) and PKCδ (-/-) mice compared with wild-type mice. Collectively, our data demonstrate that Fyn is a major upstream signaling mediator of microglial neuroinflammatory processes in PD. SIGNIFICANCE STATEMENT: Parkinson's disease (PD) is a complex multifactorial disease characterized by the progressive loss of midbrain dopamine neurons. Sustained microglia-mediated neuroinflammation has been recognized as a major pathophysiological contributor to chronic degenerative processes in PD; however, the key molecular signaling mechanisms underlying microglial activation are not entirely clear. Herein, we identified a novel role for the non-receptor tyrosine kinase Fyn in regulating neuroinflammatory responses in microglia. Our data clearly suggest that the Fyn-PKCδ signaling axis acts as a major upstream signaling mediator of the sustained neuroinflammatory processes in cell culture and animal models of PD. Our finding has important clinical significance to PD because it identifies Fyn as a potential translational target for intervention of progressive neurodegenerative processes in PD.

The peptidyl-prolyl isomerase Pin1 up-regulation and proapoptotic function in dopaminergic neurons: relevance to the pathogenesis of Parkinson disease.

Parkinson disease (PD) is a chronic neurodegenerative disease characterized by a slow and progressive degeneration of dopaminergic neurons in substantia nigra. The pathophysiological mechanisms underlying PD remain unclear. Pin1, a major peptidyl-prolyl isomerase, has recently been associated with certain diseases. Notably, Ryo et al. (Ryo, A., Togo, T., Nakai, T., Hirai, A., Nishi, M., Yamaguchi, A., Suzuki, K., Hirayasu, Y., Kobayashi, H., Perrem, K., Liou, Y. C., and Aoki, I. (2006) J. Biol. Chem. 281, 4117-4125) implicated Pin1 in PD pathology. Therefore, we sought to systematically characterize the role of Pin1 in PD using cell culture and animal models. To our surprise we observed a dramatic up-regulation of Pin1 mRNA and protein levels in dopaminergic MN9D neuronal cells treated with the parkinsonian toxicant 1-methyl-4-phenylpyridinium (MPP(+)) as well as in the substantia nigra of the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD mouse model. Notably, a marked expression of Pin1 was also observed in the substantia nigra of human PD brains along with a high co-localization of Pin1 within dopaminergic neurons. In functional studies, siRNA-mediated knockdown of Pin1 almost completely prevented MPP(+)-induced caspase-3 activation and DNA fragmentation, indicating that Pin1 plays a proapoptotic role. Interestingly, multiple pharmacological Pin1 inhibitors, including juglone, attenuated MPP(+)-induced Pin1 up-regulation, α-synuclein aggregation, caspase-3 activation, and cell death. Furthermore, juglone treatment in the MPTP mouse model of PD suppressed Pin1 levels and improved locomotor deficits, dopamine depletion, and nigral dopaminergic neuronal loss. Collectively, our findings demonstrate for the first time that Pin1 is up-regulated in PD and has a pathophysiological role in the nigrostriatal dopaminergic system and suggest that modulation of Pin1 levels may be a useful translational therapeutic strategy in PD.

Pharmacological Inhibition of PTEN Rescues Dopaminergic Neurons by Attenuating Apoptotic and Neuroinflammatory Signaling Events.

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by the selective degeneration of dopaminergic neurons in the substantia nigra pars compacta resulting in an irreversible and a debilitating motor dysfunction. Though both genetic and idiopathic factors are implicated in the disease etiology, idiopathic PD comprise the majority of clinical cases and is caused by exposure to environmental toxicants and oxidative stress. Fyn kinase activation has been identified as an early molecular signaling event that primes neuroinflammatory and neurodegenerative events associated with dopaminergic cell death. However, the upstream regulator of Fyn activation remains unidentified. We investigated whether the lipid and tyrosine phosphatase PTEN (Phosphatase and Tensin homolog deleted on chromosome 10) could be the upstream regulator of Fyn activation in PD models as PTEN has been previously reported to contribute to Parkinsonian pathology. Our findings, using bioluminescence resonance energy transfer (BRET) and immunoblotting, indicate for the first time that PTEN is a critical early stress sensor in response to oxidative stress and neurotoxicants in in vitro models of PD. Pharmacological attenuation of PTEN activity rescues dopaminergic neurons from neurotoxicant-induced cytotoxicity by modulating Fyn kinase activation. Our findings also identify PTEN's novel roles in contributing to mitochondrial dysfunction which contribute to neurodegenerative processes. Interestingly, we found that PTEN positively regulates interleukin-1ß (IL-1ß) and the transcription of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB). Taken together, we have identified PTEN as a disease course altering pharmacological target that may be further validated for the development of novel therapeutic strategies targeting PD.

Environmental neurotoxicants and inflammasome activation in Parkinson's disease - A focus on the gut-brain axis.

Inflammasomes are multi-protein complexes expressed in immune cells that function as intracellular sensors of environmental, metabolic and cellular stress. Inflammasome activation in the brain, has been shown to drive neuropathology and disease progression by multiple mechanisms, making it one of the most attractive therapeutic targets for disease modification in Parkinson's Disease (PD). Extensive inflammasome activation is evident in the brains of people with PD at the sites of dopaminergic degeneration and synuclein aggregation. While substantial progress has been made on validating inflammasome activation as a therapeutic target for PD, the mechanisms by which inflammasome activation is triggered and sustained over the disease course remain poorly understood. A growing body of evidence point to environmental and occupational chemical exposures as possible triggers of inflammasome activation in PD. The involvement of the gastrointestinal system and gut microbiota in PD pathophysiology is beginning to be elucidated, especially the profound link between gut dysbiosis and immune activation. While large cohort studies confirmed specific changes in the gut microbiota in PD patients compared to age-matched healthy controls, recent research suggest that synuclein pathology could be initiated in the gastrointestinal tract. In this review, we present a summarized perspective on current understanding on inflammasome activation and the gut-brain-axis link during PD pathophysiology. We discuss multiple environmental toxicants that are implicated as the etiological agents in causing idiopathic PD and their mechanistic underpinnings during neuroinflammatory events. We additionally present future directions that needs to address the research questions related to the gut-microbiome-brain mechanisms in PD.

Discovery and Optimization of Biaryl Alkyl Ethers as a Novel Class of Highly Selective, CNS-Penetrable, and Orally Active Adaptor Protein-2-Associated Kinase 1 (AAK1) Inhibitors for the Potential Treatment of Neuropathic Pain.

Recent mouse knockout studies identified adapter protein-2-associated kinase 1 (AAK1) as a viable target for treating neuropathic pain. BMS-986176/LX-9211 (4), as a highly selective, CNS-penetrable, and potent AAK1 inhibitor, has advanced into phase II human trials. On exploring the structure-activity relationship (SAR) around this biaryl alkyl ether chemotype, several additional compounds were found to be highly selective and potent AAK1 inhibitors with good druglike properties. Among these, compounds 43 and 58 showed very good efficacy in two neuropathic pain rat models and had excellent CNS penetration and spinal cord target engagement. Both compounds also exhibited favorable physicochemical and oral pharmacokinetic (PK) properties. Compound 58, a central pyridine isomer of BMS-986176/LX-9211 (4), was 4-fold more potent than 4 in vitro and showed lower plasma exposure needed to achieve similar efficacy compared to 4 in the CCI rat model. However, both 43 and 58 showed an inferior preclinical toxicity profile compared to 4.

Discovery of (S)-1-((2',6-Bis(difluoromethyl)-[2,4'-bipyridin]-5-yl)oxy)-2,4-dimethylpentan-2-amine (BMS-986176/LX-9211): A Highly Selective, CNS Penetrable, and Orally Active Adaptor Protein-2 Associated Kinase 1 Inhibitor in Clinical Trials for the Treatment of Neuropathic Pain.

Recent mouse knockout studies identified adapter protein-2 associated kinase 1 (AAK1) as a viable target for treating neuropathic pain. Potent small-molecule inhibitors of AAK1 have been identified and show efficacy in various rodent pain models. (S)-1-((2',6-Bis(difluoromethyl)-[2,4'-bipyridin]-5-yl)oxy)-2,4-dimethylpentan-2-amine (BMS-986176/LX-9211) (34) was identified as a highly selective, CNS penetrant, potent AAK1 inhibitor from a novel class of bi(hetero)aryl ethers. BMS-986176/LX9211 (34) showed excellent efficacy in two rodent neuropathic pain models and excellent central nervous system (CNS) penetration and target engagement at the spinal cord with an average brain to plasma ratio of 20 in rat. The compound exhibited favorable physicochemical and pharmacokinetic properties, had an acceptable preclinical toxicity profile, and was chosen for clinical trials. BMS-986176/LX9211 (34) completed phase I trials with good human pharmacokinetics and minimum adverse events and is currently in phase II clinical trials for diabetic peripheral neuropathic pain (ClinicalTrials.gov identifier: NCT04455633) and postherpetic neuralgia (ClinicalTrials.gov identifier: NCT04662281).

Perspectives on the use and risk of adverse events associated with cytokine-storm targeting antibodies and challenges associated with development of novel monoclonal antibodies for the treatment of COVID-19 clinical cases.

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of the novel coronavirus disease 2019 (COVID-19) pandemic that lacks globally accessible effective antivirals or extensively available vaccines. Numerous clinical trials are exploring the applicability of repurposed monoclonal antibodies (mAbs) targeting cytokines that cause adverse COVID-19-related pathologies, and novel mAbs directly targeting SARS-CoV-2. However, comorbidities and the incidence of cytokine storm (CS)-associated pathological complexities in some COVID-19 patients may limit the clinical use of these drugs. Additionally, CS-targeting mAbs have the potential to cause adverse events that restrict their applicability in patients with comorbidities. Novel mAbs targeting SARS-CoV-2 require pharmacological and toxicological characterization before a marketable product becomes available. The affordability of novel mAbs across the global economic spectrum may seriously limit their accessibility. This review presents a perspective on antibody-based research efforts and their limitations for COVID-19.

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.

Fyn kinase mediates pro-inflammatory response in a mouse model of endotoxemia: Relevance to translational research.

Systemic inflammation resulting from the release of pro-inflammatory cytokines and the chronic activation of the innate immune system remains a major cause of morbidity and mortality in the United States. After having demonstrated that Fyn, a Src family kinase, regulates microglial neuroinflammatory responses in cell culture and animal models of Parkinson's disease, we investigate here its role in modulating systemic inflammation using an endotoxic mouse model. Fyn knockout (KO) and their wild-type (WT) littermate mice were injected once intraperitoneally with either saline or 5 mg/kg lipopolysaccharide (LPS) and were killed 48 h later. LPS-induced mortality, endotoxic symptoms and hypothermia were significantly attenuated in Fyn KO, but not WT, mice. LPS reduced survival in Fyn WT mice to 49% compared to 84% in Fyn KO mice. Fyn KO mice were also protected from LPS-induced deficits in horizontal and vertical locomotor activities, total distance traveled and stereotypic movements. Surface body temperatures recorded at 24 h and 48 h post-LPS dropped significantly in Fyn WT, but not in KO, mice. Importantly, endotoxemia-associated changes to levels of the serum pro-inflammatory cytokines tumor necrosis factor alpha (TNF-α) and interleukin-6 (IL-6), splenocyte apoptosis and inducible nitric oxide synthase (iNOS) production in hepatocytes were also significantly attenuated in Fyn KO mice. Likewise, pharmacologically inhibiting Fyn with 10 mg/kg dasatinib (oral) significantly attenuated LPS-induced increases in plasma TNF-α and IL-6 protein levels and hepatic pro-IL-1ß messenger ribonucleic acids (mRNAs). Collectively, these results indicate that genetic knockdown or pharmacological inhibition of Fyn dampens systemic inflammation, demonstrating for the first time that Fyn kinase plays a critical role in mediating the endotoxic inflammatory response.

Fyn kinase regulates misfolded α-synuclein uptake and NLRP3 inflammasome activation in microglia.

Persistent microglia-mediated neuroinflammation is a major pathophysiological contributor to the progression of Parkinson's disease (PD), but the cell-signaling mechanisms governing chronic neuroinflammation are not well understood. Here, we show that Fyn kinase, in conjunction with the class B scavenger receptor CD36, regulates the microglial uptake of aggregated human α-synuclein (αSyn), which is the major component of PD-associated Lewy bodies. αSyn can effectively mediate LPS-independent priming and activation of the microglial NLRP3 inflammasome. Fyn kinase regulates both of these processes; it mediates PKCδ-dependent NF-κB-p65 nuclear translocation, leading to inflammasome priming, and facilitates αSyn import into microglia, contributing to the generation of mitochondrial reactive oxygen species and consequently to inflammasome activation. In vivo experiments using A53T and viral-αSyn overexpression mouse models as well as human PD neuropathological results further confirm the role of Fyn in NLRP3 inflammasome activation. Collectively, our study identifies a novel Fyn-mediated signaling mechanism that amplifies neuroinflammation in PD.

Protein kinase D1 (PKD1) activation mediates a compensatory protective response during early stages of oxidative stress-induced neuronal degeneration.

BACKGROUND: Oxidative stress is a key pathophysiological mechanism contributing to degenerative processes in many neurodegenerative diseases and therefore, unraveling molecular mechanisms underlying various stages of oxidative neuronal damage is critical to better understanding the diseases and developing new treatment modalities. We previously showed that protein kinase C delta (PKCδ) proteolytic activation during the late stages of oxidative stress is a key proapoptotic signaling mechanism that contributes to oxidative damage in Parkinson's disease (PD) models. The time course studies revealed that PKCδ activation precedes apoptotic cell death and that cells resisted early insults of oxidative damage, suggesting that some intrinsic compensatory response protects neurons from early oxidative insult. Therefore, the purpose of the present study was to characterize protective signaling pathways in dopaminergic neurons during early stages of oxidative stress. RESULTS: Herein, we identify that protein kinase D1 (PKD1) functions as a key anti-apoptotic kinase to protect neuronal cells against early stages of oxidative stress. Exposure of dopaminergic neuronal cells to H2O2 or 6-OHDA induced PKD1 activation loop (PKD1S744/748) phosphorylation long before induction of neuronal cell death. Blockade of PKCδ cleavage, PKCδ knockdown or overexpression of a cleavage-resistant PKCδ mutant effectively attenuated PKD1 activation, indicating that PKCδ proteolytic activation regulates PKD1 phosphorylation. Furthermore, the PKCδ catalytic fragment, but not the regulatory fragment, increased PKD1 activation, confirming PKCδ activity modulates PKD1 activation. We also identified that phosphorylation of S916 at the C-terminal is a preceding event required for PKD1 activation loop phosphorylation. Importantly, negative modulation of PKD1 by the RNAi knockdown or overexpression of PKD1S916A phospho-defective mutants augmented oxidative stress-induced apoptosis, while positive modulation of PKD1 by the overexpression of full length PKD1 or constitutively active PKD1 plasmids attenuated oxidative stress-induced apoptosis, suggesting an anti-apoptotic role for PKD1 during oxidative neuronal injury. CONCLUSION: Collectively, our results demonstrate that PKCδ-dependent activation of PKD1 represents a novel intrinsic protective response in counteracting early stage oxidative damage in neuronal cells. Our results suggest that positive modulation of the PKD1-mediated compensatory protective mechanism against oxidative damage in dopaminergic neurons may provide novel neuroprotective strategies for treatment of PD.

Environmental neurotoxic pesticide dieldrin activates a non receptor tyrosine kinase to promote PKCδ-mediated dopaminergic apoptosis in a dopaminergic neuronal cell model.

Oxidative stress and apoptosis are two key pathophysiological mechanisms underlying dopaminergic degeneration in Parkinson's disease (PD). Recently, we identified that proteolytic activation of protein kinase C-delta (PKCδ), a member of the novel PKC family, contributes to oxidative stress-induced dopaminergic degeneration and that phosphorylation of tyrosine residue 311 (tyr311) on PKCδ is a key event preceding the PKCδ proteolytic activation during oxidative damage. Herein, we report that a non-receptor tyrosine kinase Fyn is significantly expressed in a dopaminergic neuronal N27 cell model. Exposure of N27 cells to the dopaminergic toxicant dieldrin (60 µM) rapidly activated Fyn kinase, PKCδ-tyr311 phosphorylation and proteolytic cleavage. Fyn kinase activation precedes the caspase-3-mediated proteolytic activation of PKCδ. Pre-treatment with p60-tyrosine-specific kinase inhibitor (TSKI) almost completely attenuated dieldrin-induced phosphorylation of PKCδ-tyr311 and its proteolytic activation. Additionally, TSKI almost completely blocked dieldrin-induced apoptotic cell death. To further confirm Fyn's role in the pro-apoptotic function of PKCδ, we adopted the RNAi approach. siRNA-mediated knockdown of Fyn kinase also effectively attenuated dieldrin-induced phosphorylation of PKCδ-tyr311, caspase-3-mediated PKCδ proteolytic cleavage, and DNA fragmentation, suggesting that Fyn kinase regulates the pro-apoptotic function of PKCδ. Collectively, these results demonstrate for the first time that Fyn kinase is a pro-apoptotic kinase that regulates upstream signaling of the PKCδ-mediated apoptotic cell death pathway in neurotoxicity models of pesticide exposure.