Leukotrienes and the brain

A novel therapeutic potential of cysteinyl leukotrienes and their receptors modulation in the neurological complications associated with Alzheimer's disease

Cysteinyl leukotrienes (cysLTs) are member of eicosanoid inflammatory lipid mediators family produced by oxidation of arachidonic acid by action of the enzyme 5-lipoxygenase (5-LOX). 5-LOX is activated by enzyme 5-Lipoxygenase-activating protein (FLAP), which further lead to production of cysLTs i.e. leukotriene C4 (LTC4), leukotriene D4 (LTD4) and leukotriene E4 (LTE4). CysLTs then produce their potent inflammatory actions by activating CysLT1 and CysLT2 receptors. Inhibitors of cysLTs are indicated in asthma, allergic rhinitis and other inflammatory disorders. Earlier studies have associated cysLTs and their receptors in several neurodegenerative disorders diseases like, multiple sclerosis, Parkinson's disease, Huntington's disease, epilepsy and Alzheimer's disease (AD). These inflammatory lipid mediators have previously shown effects on various aggravating factors of AD. However, not much data has been elucidated to test their role against AD clinically. Herein, through this review, we have provided the current and emerging information on the role of cysLTs and their receptors in various neurological complications responsible for the development of AD. In addition, literature evidences for the effect of cysLT inhibitors on distinct aspects of abnormalities in AD has also been reviewed. Promising advancement in understanding on the role of cysLTs on the various neuromodulatory processes and mechanisms may contribute to the development of newer and safer therapy for the treatment of AD in future.

Leukotriene D4 induces amyloid-β generation via CysLT(1)R-mediated NF-κB pathways in primary neurons

Although the pathogenesis of sporadic Alzheimer's disease (AD) is not clearly understood, neuroinflammation has been known to play a role in the pathogenesis of AD. To investigate a functional link between the neuroinflammation and AD, the effect of leukotriene D4 (LTD4), an inflammatory lipid mediator, was studied on amyloid-β generation in vitro. Application of LTD4 to cell monolayers at concentrations up to 40 nM LTD4 caused increases in the Aβ releases. Concentrations ≥ 40 nM LTD4 decreased neuronal viability. Application of 20 nM LTD4 caused a significant increase in Aβ generation, as assessed by ELISA or Western blotting, without significant cytotoxicity. At this concentration, exposure of neurons to LTD4 for 24h produced maximal effect in the Aβ generation, and significant increases in the expressions of cysteinyl leukotriene 1 receptor (CysLT(1)R) and activity of β- or γ-secretase with complete abrogation by the selective CysLT(1)R antagonist pranlukast. Exposure of neurons to LTD4 for 1h showed activation of NF-κB pathway, by assessing the levels of p65 or phospho-p65 in the nucleus, and either CysLT(1)R antagonist pranlukast or NF-κB inhibitor PDTC prevented the nuclear translocation of p65 and the consequent phosphorylation. PDTC also inhibited LTD4-induced elevations of β- or γ-secretase activity and Aβ generation in vitro. Overall, our data show for the first time that LTD4 causes Aβ production by enhancement of β- or γ-secretase resulting from activation of CysLT(1)R-mediated NF-κB signaling pathway. These findings provide a novel pathologic link between neuroinflammation and AD.

5-Lipoxygenase gene disruption reduces amyloid-beta pathology in a mouse model of Alzheimer's disease

5-Lipoxygenase (5LO), by producing leukotrienes, is a proinflammatory enzyme, and there is evidence suggesting that it is up-regulated with aging and may be involved in Alzheimer's disease (AD). In this paper, we studied the effect of 5LO-targeted gene disruption on the amyloid phenotype of a transgenic mouse model of AD, the Tg2576. Amyloid-beta (Abeta) deposition in the brains of Tg2576 mice lacking 5LO was reduced by 64-80% compared with Tg2576 controls. This reduction was associated with a similar significant decrease in Abeta levels measured by sandwich ELISA. Absence of 5LO did not induce any significant change in amyloid-beta precursor protein (APP) levels and processing, or Abeta catabolic pathways. Furthermore, in vitro studies showed that 5LO activation or 5LO metabolites increase, whereas 5LO inhibition decreases, Abeta formation, secondary to correspondent changes in gamma-secretase activity. These data establish for the first time a novel functional role for 5LO in the pathogenesis of AD-like amyloidosis, thereby modulating gamma-secretase activity. Our work suggests that pharmacological inhibition of 5LO could provide a novel therapeutic tool for AD.

Highly selective localization of leukotriene C4 synthase in hypothalamic and extrahypothalamic vasopressin systems of mouse brain

While leukotriene C4 (LTC4) was originally identified as a potent bronchoconstrictor, the compound has versatile biological activities besides inflammatory reactions. Although the high content of LTC4 has been reported in the hypothalamus and median eminence, the precise localization of the compound remained obscure. To elucidate its possible functions in the neuroendocrine systems, we determined immunohistochemical localization of LTC4 synthase, a key enzyme to produce LTC4 using mouse brains. Light microscopy and confocal laser scanning microscopy showed that LTC4 synthase was selectively localized in the vasopressinergic magnocellular neurons of the hypothalamic paraventricular, supraoptic and suprachiasmatic nuclei and in the retrochiasmatic area, as well as in axons that emanated from these neurons to the pars nervosa of the pituitary gland. At subcellular level, however, LTC4 synthase and arginine vasopressin appeared to localize differently within individual neurons. LTC4 synthase immunoreactivity was also observed in the axons of the extrahypothalamic system including the bed nucleus of the stria terminalis, lateral habenular nucleus, midbrain central gray, medial amygdaloid nucleus and ventral septal area, although this immunoreactivity was relatively minor. The other brain regions did not contain LTC4 synthase immunoreactivity. The distribution of LTC4 synthase did not overlap with that of either oxytocin or luteinizing hormone releasing hormone. Therefore, LTC4 is considered to be involved in neural functions of the brain magnocellular vasopressinergic system such as water retention. LTC4 may also be involved in extrahypothalamic vasopressinergic neural functions including the regulation of learning and memory, social recognition memory, sexual and aggressive behavior, etc.

Enhanced activity of Ca2+-activated K+ channels by 1-[2-hydroxy-3-propyl-4-[(1H-tetrazol-5-yl)butoxyl]phenyl] ethanone (LY-171883) in neuroendocrine and neuroblastoma cell lines

The effects of LY-171883, an orally active leukotriene antagonist, on membrane currents were examined in pituitary GH(3) and in neuroblastoma IMR-32 cells. In GH(3) cells, LY-171883 (1-300 microM) reversibly increased the amplitude of Ca(2+)-activated K(+) current in a concentration-dependent manner with an EC(50) value of 15 microM. In excised inside-out patches recorded from GH(3) cells, the application of LY-171883 into cytosolic face did not modify single channel conductance of large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels; however, it did increase the channel activity. The LY-171883-stimulated activity of BK(Ca) channels is dependent on membrane potential, and results mainly from an increase in mean open time and a decrease in mean closed time. However, REV-5901 (30 microM) suppressed the activity of BK(Ca) channels and MK-571 (30 microM) did not have any effect on it. Under the current-clamp condition, LY-171883 (30 microM) caused membrane hyperpolarization as well as decreased the firing rate of action potentials in GH(3) cells. In neuroblastoma IMR-32 cells, the application of LY-171883 (30 microM) also stimulated BK(Ca) channel activity in a voltage-dependent manner. However, neither clofibrate (30 microM) nor leukotriene D(4) (10 microM) affected the channel activity in IMR-32 cells. Troglitazone (30 microM) decreased the channel activity, but ciglitazone (30 microM) enhanced it. This study clearly demonstrates that LY-171883 stimulates the activity of BK(Ca) channels in a manner unlikely to be linked to its blockade of leukotriene receptors or stimulation of peroxisome proliferator-activated receptors. The stimulatory effects on these channels may, at least in part, contribute to the underlying cellular mechanisms by which LY-171883 affects neuronal or neuroendocrine function.

Involvement of the 5-lipoxygenase pathway in the neurotoxicity of the prion peptide PrP106-126

Transmissible spongiform encephalopathies are characterised by the transformation of the normal cellular prion protein (PrP(C)) into an abnormal isoform (PrP(TSE)). Previous studies have shown that N-methyl-D-aspartate (NMDA) receptor antagonists can inhibit glutathione depletion and neurotoxicity induced by PrP(TSE) and a toxic prion protein peptide, PrP106-126, in vitro. NMDA receptor activation is known to increase intracellular accumulation of Ca(2+), resulting in up-regulation of arachidonic acid (AA) metabolism. This can stimulate the lipoxygenase pathways that may generate a number of potentially neurotoxic metabolites. Because of the putative relationship between AA breakdown and PrP106-126 neurotoxicity, we investigated AA metabolism in primary cerebellar granule neuron cultures treated with PrP106-126. Our studies revealed that PrP106-126 exposure for 30 min significantly up-regulated AA release from cerebellar granule neurons. PrP106-126 neurotoxicity was mediated through the 5-lipoxygenase (5-LOX) pathway, as shown by abrogation of neuronal death with the 5-LOX inhibitors quinacrine, nordihydroguaiaretic acid, and caffeic acid. These inhibitors also prevented PrP106-126-induced caspase 3 activation and annexin V binding, indicating a central role for the 5-LOX pathway in PrP106-126-mediated proapoptosis. Interestingly, inhibitors of the 12-lipoxygenase pathway had no effect on PrP106-126 neurotoxicity or proapoptosis. These studies clearly demonstrate that AA metabolism through the 5-LOX pathway is an important early event in PrP106-126 neurotoxicity and consequently may have a critical role in PrP(TSE)-mediated cell loss in vivo. If this is so, therapeutic intervention with 5-LOX inhibitors may prove beneficial in the treatment of prion disorders.

Characterization of ionotropic glutamate receptors in human lymphocytes

The effect of L-glutamate (Glu) on human lymphocyte function was studied by measuring anti-CD(3) monoclonal antibody (mAb) or phytohaemagglutinin (PHA)-induced intracellular Ca(2+) ([Ca(2+)](i)) rise (Fura-2 method), and cell proliferation (MTT assay). Glu (0.001 - 100 microM) did not modify basal lymphocyte [Ca(2+)](i), but significantly potentiated the effects of anti-CD(3) mAb or PHA. Maximal [Ca(2+)](i) rises over resting cells were: 165+/-8 and 247+/-10 nM at 3.0x10(-2) mg ml(-1) anti-CD(3) mAb; 201+/-4 and 266+/-9 nM at 5.0x10(-2) mg ml(-1) PHA, in the absence or presence of 1 microM Glu, respectively. The Glu effect showed a bell-shape concentration-dependent relationship, with a maximum (+90+/-3% for anti-CD(3) mAb and +57+/-2% for PHA over Glu-untreated cells) at 1 microM. Non-NMDA receptor agonists (1 microM) showed a greater efficacy (+76+/-2% for (S)-AMPA; +78+/-4% for KA), if compared to NMDA (+46+/-2%), or Glu itself. Ionotropic Glu receptor antagonists completely inhibited the effects of the corresponding specific receptor agonists (1 microM). The IC(50) values calculated were: 0.9 microM for D-AP5; 0.6 microM for (+)-MK801; 0.3 microM for NBQX. Both NBQX and KYNA were able to abolish Glu effect. The IC(50s) calculated were: 3.4 microM for NBQX; 0.4 microM for KYNA. Glu (0.1 - 1 mM) did not change the resting cell proliferation, whereas Glu (1 mM) significant inhibited (-27+/-4%) PHA (1.0x10(-2) mg ml(-1))-induced lymphocyte proliferation at 72 h. In conclusion, human lymphocytes express ionotropic Glu receptors functionally operating as modulators of cell activation.

Defective metabolism of leukotriene B4 in the Sjögren-Larsson syndrome

The Sjögren-Larsson Syndrome (SLS) is a neurocutaneous disorder, caused by deficient activity of the microsomal enzyme fatty aldehyde dehydrogenase (FALDH). FALDH catalyzes the oxidation of medium- and long-chain fatty aldehydes to their corresponding carboxylic acids. SLS is diagnosed by demonstrating the enzyme deficiency or by mutation analysis of the FALDH gene, while laboratory investigations of plasma, urine, and cerebrospinal fluid do not reveal any diagnostic abnormality. Leukotriene (LT) B4 is a pro-inflammatory mediator synthesized from arachidonic acid. LTB4 is inactivated by microsomal omega-oxidation, successively yielding 20-OH-LTB4, 20-CHO-LTB4 and 20-COOH-LTB4. Since FALDH is involved in LTB4 degradation, we have analyzed LTB4 and its metabolites in urine and cerebrospinal fluid as well as the degradation capacity for LTB4 in fresh polymorphonuclear leukocytes (PMN) of SLS patients. The urinary concentrations of LTB4, 20-OH-LTB4 and 20-COOH-LTB4 are below the detection limit in healthy controls. The urine of all SLS patients (n=13) exhibited highly elevated concentrations of LTB4 and 20-OH-LTB4, while 20-COOH-LTB4 was absent. Cerebrospinal fluid levels of LTB4, 20-OH-LTB4 and 20-COOH-LTB4 were found to be normal (n=7). PMN isolated from four patients were shown to be unable to convert 20-OH-LTB4 to 20-COOH-LTB4. Our findings provide unambiguous evidence for defective LTB4 degradation in SLS patients, and offer new and non-invasive diagnostic tools. Moreover, they open new pathophysiological considerations, with the prospect of rational treatment strategies.