Optineurin suppression causes neuronal cell death via NF‐κB pathway

Mutations in more than 10 genes are reported to cause familial amyotrophic lateral sclerosis (ALS). Among these genes, optineurin (OPTN) is virtually the only gene that is considered to cause classical ALS by a loss‐of‐function mutation. Wild‐type optineurin (OPTN^WT) suppresses nuclear factor‐kappa B (NF‐κB) activity, but the ALS‐causing mutant OPTN is unable to suppress NF‐κB activity. Therefore, we knocked down OPTN in neuronal cells and examined the resulting NF‐κB activity and phenotype. First, we confirmed the loss of the endogenous OPTN expression after siRNA treatment and found that NF‐κB activity was increased in OPTN‐knockdown cells. Next, we found that OPTN knockdown caused neuronal cell death. Then, overexpression of OPTN^WT or OPTN^E50K with intact NF‐κB‐suppressive activity, but not overexpression of ALS‐related OPTN mutants, suppressed the neuronal death induced by OPTN knockdown. This neuronal cell death was inhibited by withaferin A, which selectively inhibits NF‐κB activation. Lastly, involvement of the mitochondrial proapoptotic pathway was suggested for neuronal death induced by OPTN knockdown. Taken together, these results indicate that inappropriate NF‐κB activation is the pathogenic mechanism underlying OPTN mutation‐related ALS.

     

Identification of P‐glycoprotein co‐fractionating proteins and specific binding partners in rat brain microvessels

Drug delivery to the brain for the treatment of pathologies with a CNS component is a significant clinical challenge. P‐glycoprotein (PgP), a drug efflux pump in the endothelial cell membrane, is a major factor in preventing therapeutics from crossing the blood‐brain barrier (BBB). Identifying PgP regulatory mechanisms is key to developing agents to modulate PgP activity. Previously, we found that PgP trafficking was altered concomitant with increased PgP activity and disassembly of high molecular weight PgP‐containing complexes during acute peripheral inflammatory pain. These data suggest that PgP activity is post‐translationally regulated at the BBB. The goal of the current study was to identify proteins that co‐localize with PgP in rat brain microvessel endothelial cell membrane microdomains and use the data to suggest potential regulatory mechanisms. Using new density gradients of microvessel homogenates, we identified two unique pools (1,2) of PgP in membrane fractions. Caveolar constituents, caveolin1, cavin1, and cavin2, co‐localized with PgP in these fractions indicating the two pools contained caveolae. A chaperone (Hsc71), protein disulfide isomerase and endosomal/lysosomal sorting proteins (Rab5, Rab11a) also co‐fractionated with PgP in the gradients. These data suggest signaling pathways with a potential role in post‐translational regulation of PgP activity at the BBB.

     

Zinc transporter‐1: a novel NMDA receptor‐binding protein at the postsynaptic density

Zinc (Zn²⁺) is believed to play a relevant role in the physiology and pathophysiology of the brain. Hence, Zn²⁺ homeostasis is critical and involves different classes of molecules, including Zn²⁺ transporters. The ubiquitous Zn²⁺ transporter‐1 (ZNT‐1) is a transmembrane protein that pumps cytosolic Zn²⁺ to the extracellular space, but its function in the central nervous system is not fully understood. Here, we show that ZNT‐1 interacts with GluN2A‐containing NMDA receptors, suggesting a role for this transporter at the excitatory glutamatergic synapse. First, we found that ZNT‐1 is highly expressed at the hippocampal postsynaptic density (PSD) where NMDA receptors are enriched. Two‐hybrid screening, coimmunoprecipitation experiments and clustering assay in COS‐7 cells demonstrated that ZNT‐1 specifically binds the GluN2A subunit of the NMDA receptor. GluN2A deletion mutants and pull‐down assays indicated GluN2A(1390–1464) domain as necessary for the binding to ZNT‐1. Most importantly, ZNT‐1/GluN2A complex was proved to be dynamic, since it was regulated by induction of synaptic plasticity. Finally, modulation of ZNT‐1 expression in hippocampal neurons determined a significant change in dendritic spine morphology, PSD‐95 clusters and GluN2A surface levels, supporting the involvement of ZNT‐1 in the dynamics of excitatory PSD.

     

Hippocampal nuclear factor kappa B accounts for stress‐induced anxiety behaviors via enhancing neuronal nitric oxide synthase (nNOS)‐carboxy‐terminal PDZ ligand of nNOS‐Dexras1 coupling

Anxiety disorders are associated with a high social burden worldwide. Recently, increasing evidence suggests that nuclear factor kappa B (NF‐κB) has significant implications for psychiatric diseases, including anxiety and depressive disorders. However, the molecular mechanisms underlying the role of NF‐κB in stress‐induced anxiety behaviors are poorly understood. In this study, we show that chronic mild stress (CMS) and glucocorticoids dramatically increased the expression of NF‐κB subunits p50 and p65, phosphorylation and acetylation of p65, and the level of nuclear p65 in vivo and in vitro , implicating activation of NF‐κB signaling in chronic stress‐induced pathological processes. Using the novelty‐suppressed feeding (NSF) and elevated‐plus maze (EPM) tests, we found that treatment with pyrrolidine dithiocarbamate (PDTC; intra‐hippocampal infusion), an inhibitor of NF‐κB, rescued the CMS‐ or glucocorticoid‐induced anxiogenic behaviors in mice. Microinjection of PDTC into the hippocampus reversed CMS‐induced up‐regulation of neuronal nitric oxide synthase (nNOS), carboxy‐terminal PDZ ligand of nNOS (CAPON), and dexamethasone‐induced ras protein 1 (Dexras1) and dendritic spine loss of dentate gyrus (DG) granule cells. Moreover, over‐expression of CAPON by infusing LV‐CAPON‐L‐GFP into the hippocampus induced nNOS‐Dexras1 interaction and anxiety‐like behaviors, and inhibition of NF‐κB by PDTC reduced the LV‐CAPON‐L‐GFP‐induced increases in nNOS‐Dexras1 complex and anxiogenic‐like effects in mice. These findings indicate that hippocampal NF‐κB mediates anxiogenic behaviors, probably via regulating the association of nNOS‐CAPON‐Dexras1, and uncover a novel approach to the treatment of anxiety disorders.

     

Arrestin‐dependent but G‐protein coupled receptor kinase‐independent uncoupling of D2‐dopamine receptors

We reconstituted D2 like dopamine receptor (D2R) and the delta opioid receptor (DOR) coupling to G‐protein gated inwardly rectifying potassium channels (K_ir3) and directly compared the effects of co‐expression of G‐protein coupled receptor kinase (GRK) and arrestin on agonist‐dependent desensitization of the receptor response. We found, as described previously, that co‐expression of a GRK and an arrestin synergistically increased the rate of agonist‐dependent desensitization of DOR. In contrast, only arrestin expression was required to produce desensitization of D2R responses. Furthermore, arrestin‐dependent GRK‐independent desensitization of D2R‐K_ir3 coupling could be transferred to DOR by substituting the third cytoplasmic loop of DOR with that of D2R. The arrestin‐dependent GRK‐independent desensitization of D2R desensitization was inhibited by staurosporine treatment, and blocked by alanine substitution of putative protein kinase C phosphorylation sites in the third cytoplasmic loop of D2R. Finally, the D2R construct in which putative protein kinase C phosphorylation sites were mutated did not undergo significant agonist‐dependent desensitization even after GRK co‐expression, suggesting that GRK phosphorylation of D2R does not play an important role in uncoupling of the receptor.

     

In vivo induction of P‐glycoprotein expression at the mouse blood–brain barrier: an intracerebral microdialysis study

Intracerebral microdialysis was utilized to investigate the effect of P‐glycoprotein (a drug efflux transporter) induction at the mouse blood–brain barrier (BBB) on brain extracellular fluid concentrations of quinidine, an established substrate of P‐glycoprotein. Induction was achieved by treating male CD‐1 mice for 3 days with 5 mg/kg/day dexamethasone (DEX), a ligand of the nuclear receptor, pregnane X receptor, and a P‐glycoprotein inducer. Tandem liquid chromatography mass spectrometric method was used to quantify analytes in dialysate, blood and plasma. P‐glycoprotein, pregnane X receptor and Cyp3a11 (metabolizing enzyme for quinidine) protein expression in capillaries and brain homogenates was measured by immunoblot analysis. Following quinidine i.v. administration, the average ratio of unbound quinidine concentrations in brain extracellular fluid (determined from dialysate samples) to plasma at steady state (375–495 min) or K_{p, uu, ECF/Plasma} in the DEX‐treated animals was 2.5‐fold lower compared with vehicle‐treated animals. In DEX‐treated animals, P‐glycoprotein expression in brain capillaries was 1.5‐fold higher compared with vehicle‐treated animals while Cyp3a11 expression in brain capillaries was not significantly different between the two groups. These data demonstrate that P‐gp induction mediated by DEX at the BBB can significantly reduce quinidine brain extracellular fluid concentrations by decreasing its brain permeability and further suggest that drug–drug interactions as a result of P‐gp induction at the BBB are possible.

     

Tuftsin signals through its receptor neuropilin‐1 via the transforming growth factor beta pathway

Tuftsin (Thr‐Lys‐Pro‐Arg) is a natural immunomodulating peptide found to stimulate phagocytosis in macrophages/microglia. Tuftsin binds to the receptor neuropilin‐1 (Nrp1) on the surface of cells. Nrp1 is a single‐pass transmembrane protein, but its intracellular C‐terminal domain is too small to signal independently. Instead, it associates with a variety of coreceptors. Despite its long history, the pathway through which tuftsin signals has not been described. To investigate this question, we employed various inhibitors to Nrp1's coreceptors to determine which route is responsible for tuftsin signaling. We use the inhibitor EG00229, which prevents tuftsin binding to Nrp1 on the surface of microglia and reverses the anti‐inflammatory M2 shift induced by tuftsin. Furthermore, we demonstrate that blockade of transforming growth factor beta (TGFβ) signaling via TβR1 disrupts the M2 shift similar to EG00229. We report that tuftsin promotes Smad3 phosphorylation and reduces Akt phosphorylation. Taken together, our data show that tuftsin signals through Nrp1 and the canonical TGFβ signaling pathway.

     

Platelet activating factor induces transient blood‐brain barrier opening to facilitate edaravone penetration into the brain

The blood–brain barrier (BBB) greatly limits the efficacy of many neuroprotective drugs' delivery to the brain, so improving drug penetration through the BBB has been an important focus of research. Here we report that platelet activating factor (PAF) transiently opened BBB and facilitated neuroprotectant edaravone penetration into the brain. Intravenous infusion with PAF induced a transient BBB opening in rats, reflected by increased Evans blue leakage and mild edema formation, which ceased within 6 h. Furthermore, rat regional cerebral blood flow (rCBF) declined acutely during PAF infusion, but recovered slowly. More importantly, this transient BBB opening significantly increased the penetration of edaravone into the brain, evidenced by increased edaravone concentrations in tissue interstitial fluid collected by microdialysis and analyzed by Ultra‐performance liquid chromatograph combined with a hybrid quadrupole time‐of‐flight mass spectrometer (UPLC‐MS/MS). Similarly, incubation of rat brain microvessel endothelial cells monolayer with 1 μM PAF for 1 h significantly increased monolayer permeability to ¹²⁵I‐albumin, which recovered 1 h after PAF elimination. However, PAF incubation with rat brain microvessel endothelial cells for 1 h did not cause detectable cytotoxicity, and did not regulate intercellular adhesion molecule‐1, matrix‐metalloproteinase‐9 and P‐glycoprotein expression. In conclusion, PAF could induce transient and reversible BBB opening through abrupt rCBF decline, which significantly improved edaravone penetration into the brain.

     

Diet‐induced obesity accelerates the onset of terminal phenotypes in α‐synuclein transgenic mice

Parkinson's disease (PD) and diabetes belong to the most common neurodegenerative and metabolic syndromes, respectively. Epidemiological links between these two frequent disorders are controversial. The neuropathological hallmarks of PD are protein aggregates composed of amyloid‐like fibrillar and serine‐129 phosphorylated (pS129) α‐synuclein (AS). To study if diet‐induced obesity could be an environmental risk factor for PD‐related α‐synucleinopathy, transgenic (TG) mice, expressing the human mutant A30P AS in brain neurons, were subjected after weaning to a lifelong high fat diet (HFD). The TG mice became obese and glucose‐intolerant, as did the wild‐type controls. Upon aging, HFD significantly accelerated the onset of the lethal locomotor phenotype. Coinciding with the premature movement phenotype and death, HFD accelerated the age of onset of brainstem α‐synucleinopathy as detected by immunostaining with antibodies against pathology‐associated pS129. Amyloid‐like neuropathology was confirmed by thioflavin S staining. Accelerated onset of neurodegeneration was indicated by Gallyas silver‐positive neuronal dystrophy as well as astrogliosis. Phosphorylation of the activation sites of the pro‐survival signaling intermediate Akt was reduced in younger TG mice after HFD. Thus, diet‐induced obesity may be an environmental risk factor for the development of α‐synucleinopathies. The molecular and cellular mechanisms remain to be further elucidated.

     

Divergent effects of the H50Q and G51D SNCA mutations on the aggregation of α‐synuclein

The discoveries of mutations in SNCA were seminal findings that resulted in the knowledge that α‐synuclein (αS) is the major component of Parkinson's disease‐associated Lewy bodies. Since the pathologic roles of these protein inclusions and SNCA mutations are not completely established, we characterized the aggregation properties of the recently identified SNCA mutations, H50Q and G51D, to provide novel insights. The properties of recombinant H50Q, G51D, and wild‐type αS to polymerize and aggregate into amyloid were studied using (trans,trans)‐1‐bromo‐2,5‐bis‐(4‐hydroxy)styrylbenzene fluorometry, sedimentation analyses, electron microscopy, and atomic force microscopy. These studies showed that the H50Q mutation increases the rate of αS aggregation, whereas the G51D mutation has the opposite effect. However, H50Q and G51D αS could still be similarly induced to form intracellular aggregates from the exposure to exogenous amyloidogenic seeds under conditions that promote their cellular entry. Both mutant αS proteins, but especially G51D, promoted cellular toxicity under cellular stress conditions. These findings reveal that the novel pathogenic SNCA mutations, H50Q and G51D, have divergent effects on aggregation properties relative to the wild‐type protein, with G51D αS demonstrating reduced aggregation despite presenting with earlier disease onset, suggesting that these mutants promote different mechanisms of αS pathogenesis.

     

Sulforaphane enhances temozolomide‐induced apoptosis because of down‐regulation of miR‐21 via Wnt/β‐catenin signaling in glioblastoma

Temozolomide (TMZ) has been widely used in the treatment of glioblastoma (GBM), although inherent or acquired resistance restricts the application. This study was aimed to evaluate the efficacy of sulforaphane (SFN) to TMZ‐induced apoptosis in GBM cells and the potential mechanism. Biochemical assays and subcutaneous tumor establishment were used to characterize the function of SFN in TMZ‐induced apoptosis. Our results revealed that β‐catenin and miR‐21 were concordantly expressed in GBM cell lines, and SFN significantly reduced miR‐21 expression through inhibiting the Wnt/β‐catenin/TCF4 pathway. Furthermore, down‐regulation of miR‐21 enhanced the pro‐apoptotic efficacy of TMZ in GBM cells. Finally, we observed that SFN strengthened TMZ‐mediated apoptosis in a miR‐21‐dependent manner. In conclusion, SFN effectively enhances TMZ‐induced apoptosis by inhibiting miR‐21 via Wnt/β‐catenin signaling in GBM cells. These findings support the use of SFN for potential therapeutic approach to overcome TMZ resistance in GBM treatment.

     

Fastigial nucleus stimulation regulates neuroprotection via induction of a novel microRNA,rno‐miR‐676‐1, in middle cerebral artery occlusion rats

Previous studies have shown that fastigial nucleus stimulation (FNS) reduces tissue damage resulting from focal cerebral ischemia. Although the mechanisms of neuroprotection induced by FNS are not entirely understood, important data have been presented in the past two decades. MicroRNAs (miRNAs) are a newly discovered group of non‐coding small RNA molecules that negatively regulate target gene expression and are involved in the regulation of cell proliferation and cell apoptosis. To date, no studies have demonstrated whether miRNAs can serve as mediators of the brain's response to FNS, which leads to endogenous neuroprotection. Therefore, this study investigated the profiles of FNS‐mediated miRNAs. Using a combination of deep sequencing and microarray with computational analysis, we identified a novel miRNA in the rat ischemic cortex after 1 h of FNS. This novel miRNA (PC‐3p‐3469_406), herein referred to as rno‐miR‐676‐1, was upregulated in rats with cerebral ischemia after FNS. In vivo observations indicate that this novel miRNA may have antiapoptotic effects and contribute to neuroprotection induced by FNS. Our study provides a better understanding of neuroprotection induced by FNS.

     

Phosphoproteomic evaluation of pharmacological inhibition of leucine‐rich repeat kinase 2 reveals significant off‐target effects of LRRK‐2‐IN‐1

Genetic mutations in leucine‐rich repeat kinase 2 (LRRK2) have been linked to autosomal dominant Parkinson's disease. The most prevalent mutation, G2019S, results in enhanced LRRK2 kinase activity that potentially contributes to the etiology of Parkinson's disease. Consequently, disease progression is potentially mediated by poorly characterized phosphorylation‐dependent LRRK2 substrate pathways. To address this gap in knowledge, we transduced SH‐SY5Y neuroblastoma cells with LRRK2 G2019S via adenovirus, then determined quantitative changes in the phosphoproteome upon LRRK2 kinase inhibition (LRRK2‐IN‐1 treatment) using stable isotope labeling of amino acids in c ulture combined with phosphopeptide enrichment and LC‐MS/MS analysis. We identified 776 phosphorylation sites that were increased or decreased at least 50% in response to LRRK2‐IN‐1 treatment, including sites on proteins previously known to associate with LRRK2. Bioinformatic analysis of those phosphoproteins suggested a potential role for LRRK2 kinase activity in regulating pro‐inflammatory responses and neurite morphology, among other pathways. In follow‐up experiments, LRRK2‐IN‐1 inhibited lipopolysaccharide‐induced tumor necrosis factor alpha (TNFα) and C‐X‐C motif chemokine 10 (CXCL10) levels in astrocytes and also enhanced multiple neurite characteristics in primary neuronal cultures. However, LRRK2‐IN‐1 had almost identical effects in primary glial and neuronal cultures from LRRK2 knockout mice. These data suggest LRRK2‐IN‐1 may inhibit pathways of perceived LRRK2 pathophysiological function independently of LRRK2 highlighting the need to use multiple pharmacological tools and genetic approaches in studies determining LRRK2 function.

     

Neuroprotective role of Nrf2 for retinal ganglion cells in ischemia‐reperfusion

Retinal ischemia plays a critical role in multiple vision‐threatening diseases and leads to death of retinal neurons, particularly ganglion cells. Oxidative stress plays an important role in this ganglion cell loss. Nrf2 (NF‐E2‐related factor 2) is a major regulator of the antioxidant response, and its role in the retina is increasingly appreciated. We investigated the potential retinal neuroprotective function of Nrf2 after ischemia‐reperfusion (I/R) injury. In an experimental model of retinal I/R, Nrf2 knockout mice exhibited much greater loss of neuronal cells in the ganglion cell layer than wild‐type mice. Primary retinal ganglion cells isolated from Nrf2 knockout mice exhibited decreased cell viability compared to wild‐type retinal ganglion cells, demonstrating the cell‐intrinsic protective role of Nrf2. The retinal neuronal cell line 661W exhibited reduced cell viability following siRNA‐mediated knockdown of Nrf2 under conditions of oxidative stress, and this was associated with exacerbation of increase in reactive oxygen species. The synthetic triterpenoid CDDO‐Im (2‐Cyano‐3,12‐dioxooleana‐1,9‐dien‐28‐imidazolide), a potent Nrf2 activator, inhibited reactive oxygen species increase in cultured 661W under oxidative stress conditions and increased neuronal cell survival after I/R injury in wild‐type, but not Nrf2 knockout mice. Our findings indicate that Nrf2 exhibits a retinal neuroprotective function in I/R and suggest that pharmacologic activation of Nrf2 could be a therapeutic strategy.