Deepening of lipidome annotation by associating cross-metathesis reaction with mass spectrometry: application to an in vitro model of corneal toxicity

The in-depth knowledge of lipid biological functions needs a comprehensive structural annotation including a method to locate fatty acid unsaturations, which remains a thorny problem. For this purpose, we have associated Grubbs' cross-metathesis reaction and liquid chromatography hyphenated to tandem mass spectrometry to locate double bond positions in lipid species. The pretreatment of lipid-containing samples by Grubbs' catalyst and an appropriate alkene generates substituted lipids through cross-metathesis reaction under mild, chemoselective, and reproducible conditions. A systematic LC-MS/MS analysis of the reaction mixture allows locating unambiguously the double bonds in fatty acid side chains of phospholipids, glycerolipids, and sphingolipids. This method has been successfully applied at a nanomole scale to commercial standard mixtures consisting of 10 lipid subclasses as well as in lipid extracts of human corneal epithelial (HCE) cell line allowing to pinpoint double bond of more than 90 species. This method has also been useful to investigate the lipid homeostasis alteration in an in vitro model of corneal toxicity, i.e., HCE cells incubated with benzalkonium chloride. The association of cross-metathesis and tandem mass spectrometry appears suitable to locate double bond positions in lipids involved in relevant biological processes.

Mechanistic insights into the role of the chemokine CCL2/CCR2 axis in dorsal root ganglia to peripheral inflammation and pain hypersensitivity

BACKGROUND

Pain is reported as the leading cause of disability in the common forms of inflammatory arthritis conditions. Acting as a key player in nociceptive processing, neuroinflammation, and neuron-glia communication, the chemokine CCL2/CCR2 axis holds great promise for controlling chronic painful arthritis. Here, we investigated how the CCL2/CCR2 system in the dorsal root ganglion (DRG) contributes to the peripheral inflammatory pain sensitization.

METHODS

Repeated intrathecal (i.t.) administration of the CCR2 antagonist, INCB3344 was tested for its ability to reverse the nociceptive-related behaviors in the tonic formalin and complete Freund's adjuvant (CFA) inflammatory models. We further determined by qPCR the expression of CCL2/CCR2, SP and CGRP in DRG neurons from CFA-treated rats. Using DRG explants, acutely dissociated primary sensory neurons and calcium mobilization assay, we also assessed the release of CCL2 and sensitization of nociceptors. Finally, we examined by immunohistochemistry following nerve ligation the axonal transport of CCL2, SP, and CGRP from the sciatic nerve of CFA-treated rats.

RESULTS

We first found that CFA-induced paw edema provoked an increase in CCL2/CCR2 and SP expression in ipsilateral DRGs, which was decreased after INCB3344 treatment. This upregulation in pronociceptive neuromodulators was accompanied by an enhanced nociceptive neuron excitability on days 3 and 10 post-CFA, as revealed by the CCR2-dependent increase in intracellular calcium mobilization following CCL2 stimulation. In DRG explants, we further demonstrated that the release of CCL2 was increased following peripheral inflammation. Finally, the excitation of nociceptors following peripheral inflammation stimulated the anterograde transport of SP at their peripheral nerve terminals. Importantly, blockade of CCR2 reduced sensory neuron excitability by limiting the calcium mobilization and subsequently decreased peripheral transport of SP towards the periphery. Finally, pharmacological inhibition of CCR2 reversed the pronociceptive action of CCL2 in rats receiving formalin injection and significantly reduced the neurogenic inflammation as well as the stimuli-evoked and movement-evoked nociceptive behaviors in CFA-treated rats.

CONCLUSIONS

Our results provide significant mechanistic insights into the role of CCL2/CCR2 within the DRG in the development of peripheral inflammation, nociceptor sensitization, and pain hypersensitivity. We further unveil the therapeutic potential of targeting CCR2 for the treatment of painful inflammatory disorders.

Topical treatment with a mu opioid receptor agonist alleviates corneal allodynia and corneal nerve sensitization in mice

Corneal pain is considered to be a core symptom of ocular surface disruption and inflammation. The management of this debilitating condition is still a therapeutic challenge. Recent evidence supports a role of the opioid system in the management of corneal nociception. However, the functional involvement of the mu opioid receptor (MOR) underlying this analgesic effect is not known. We first investigated the expression of the MOR in corneal nerve fibers and trigeminal ganglion (TG) neurons in control mice and a mouse model of corneal inflammatory pain. We then evaluated the anti-nociceptive and electrophysiological effects of DAMGO ([D-Ala²,N-Me-Phe⁴,Gly⁵-ol] enkephalin), a MOR-selective ligand. MOR immunoreactivity was detected in corneal nerve fibers and primary afferent neurons of the ophthalmic branch of the TG of naive mice. MOR expression was significantly higher in both structures under conditions of inflammatory corneal pain. Topical ocular administration of DAMGO strongly reduced both the mechanical (von Frey) and chemical (capsaicin) corneal hypersensitivity associated with inflammatory ocular pain. Repeated instillations of DAMGO also markedly reversed the elevated spontaneous activity of the ciliary nerve and responsiveness of corneal polymodal nociceptors that were observed in mice with corneal pain. Finally, these DAMGO-induced behavioral and electrophysiological responses were totally blunted by the topical application of naloxone methiodide, an opioid receptor antagonist. Overall, these results provide evidence that topical pharmacological MOR activation may constitute a therapeutic target for the treatment of corneal pain and improve corneal nerve function to alleviate chronic pain.

Lipidomic analysis of human corneal epithelial cells exposed to ocular irritants highlights the role of phospholipid and sphingolipid metabolisms in detergent toxicity mechanisms

Detergent chemicals, widely used in household products, in pharmaceutical, medical, cosmetic and industrial fields, have been linked to side effects and involved in several eye diseases. On the ocular surface, detergents can interfere with the corneal epithelium, the most superficial layer of the cornea, representing a line of defence against external aggression. Despite its major role in numerous biological functions, there is still little data regarding disruption of lipid homeostasis induced by ocular irritants. To this purpose, a lipidomic analysis using UPLC-HRMS/MS-ESI ± was performed on human corneal epithelial (HCE) cells incubated with three widely known ocular irritants: benzalkonium chloride (BAK), sodium lauryl sulfate (SLS) and Triton X-100 (TXT). We found that these ocular irritants lead to a profound modification of the HCE cell lipidome. Indeed, the cell content of ceramide species increased widely while plasmalogens containing polyunsaturated fatty acid species, especially docosahexaenoic acids, decreased. Furthermore, these irritants upregulated the activity of phospholipase A₂. The present study demonstrates that BAK, SLS and TXT induced disruption of the cell lipid homeostasis, highlighting that lipids mediate inflammatory and cell death processes induced by detergents in the cornea. Lipidomics may thus be regarded as a valuable tool to investigate new markers of corneal damage.

Lipid Annotation by Combination of UHPLC-HRMS (MS), Molecular Networking, and Retention Time Prediction: Application to a Lipidomic Study of In Vitro Models of Dry Eye Disease

Annotation of lipids in untargeted lipidomic analysis remains challenging and a systematic approach needs to be developed to organize important datasets with the help of bioinformatic tools. For this purpose, we combined tandem mass spectrometry-based molecular networking with retention time (t_R) prediction to annotate phospholipid and sphingolipid species. Sixty-five standard compounds were used to establish the fragmentation rules of each lipid class studied and to define the parameters governing their chromatographic behavior. Molecular networks (MNs) were generated through the GNPS platform using a lipid standards mixture and applied to lipidomic study of an in vitro model of dry eye disease, i.e., human corneal epithelial (HCE) cells exposed to hyperosmolarity (HO). These MNs led to the annotation of more than 150 unique phospholipid and sphingolipid species in the HCE cells. This annotation was reinforced by comparing theoretical to experimental t_R values. This lipidomic study highlighted changes in 54 lipids following HO exposure of corneal cells, some of them being involved in inflammatory responses. The MN approach coupled to t_R prediction thus appears as a suitable and robust tool for the discovery of lipids involved in relevant biological processes.

Blue light phototoxicity toward human corneal and conjunctival epithelial cells in basal and hyperosmolar conditions

AIMS

The ocular surface is the very first barrier between the visual system and external environment. It protects the eye from the exposure to various light sources that significantly emit in blue spectrum. However, the impact of blue light on the ocular surface has been poorly explored so far. In this study, we investigated in vitro the phototoxicity of blue light illumination in human epithelial cells of the ocular surface. We worked either in basal conditions or under hyperosmolar stress, in order to mimic dry eye disease (DED) that is the most common disease involving the ocular surface.

RESULTS

Corneal and conjunctival epithelial cells suffered the most from violet-blue light but also from longer-wave blue light. Exposure to blue wavebands significantly decreased cellular viability, impacted on cellular morphology and provoked reactive oxygen species (ROS) over-production. Conjunctival epithelial cell line had a greater photosensitivity than the corneal epithelial one. Hyperosmolar stress potentiated the blue light phototoxicity, increasing inflammation, altering mitochondrial membrane potential, and triggering the glutathione-based antioxidant system.

INNOVATION

In human epithelial corneal and conjunctival cells of the ocular surface, we demonstrated the harmful impact of blue light on viability, redox state and inflammation processes, which was modified by hyperosmolarity.

CONCLUSION

Blue light induced cell death and significant ROS production, and altered the expression of inflammatory genes and operation of the cellular defensive system. We established for the first time that hyperosmolar stress impacted phototoxicity, further suggesting that DED patients might be more sensitive to blue light ocular toxicity.

[Retinal and trabecular degeneration in glaucoma: new insights into pathogenesis and treatment]

Academic and industrial research has brought new insights into the pathogenesis of glaucoma, aiming at identifying and targeting specific mechanisms to improve our current therapeutic strategy. Retinal neurodegeneration is still the main focus, whether in terms of extrinsic factors such as neurotrophin deprivation, glutamate toxicity, vascular deficiency and neuro-inflammation from glial cells, or in terms of retinal ganglion cell intrinsic sensibility to proapoptotic signals. However, glaucoma is not solely a retinal disease but also involves retinal and trabecular meshwork degeneration, extending into and/or even originating from the brain. The present review summarizes our current knowledge of key mechanisms involved in glaucoma degeneration, focusing on the direction of current research towards the future of glaucoma therapy.

[Chemokines and attraction of myeloid cells in peripheral neuropathic pains]

Chronic neuropathic pain has become a real social issue, due to the difficulty of its treatment and by the major impairment to quality of life that it causes in every day behavior. Understanding neurobiological basis and pathophysiological causes of diverse painful syndromes constantly evolves and reports the complexity of its mechanisms. Unfortunately this complexity makes it difficult to discover effective treatments against chronic pain syndromes, in particular as regards peripheral neuropathic pains. Recent studies reveal that, during chronic peripheral neuropathy, inflammatory mediators (in particular chemokines), besides their implications in the modulation of nociceptive messages and central neuroinflammatory mechanisms, play a critical role in the orchestration of the immune response induced by a peripheral nerve lesion. In this review, after a brief introduction about chemokines and their role in neuromodulation of the nociceptive message, we will attempt to define their functions and implications in the immune response associated to peripheral neuropathies. Thus, perfectly understanding the molecular and cellular communications between the nervous system and the immune system will be useful for the future development of novel and innovative therapeutic strategies against these highly disabling pathologies.

Functional up-regulation of Nav1.8 sodium channel in Aβ afferent fibers subjected to chronic peripheral inflammation

Background

Functional alterations in the properties of Aβ afferent fibers may account for the increased pain sensitivity observed under peripheral chronic inflammation. Among the voltage-gated sodium channels involved in the pathophysiology of pain, Na_v1.8 has been shown to participate in the peripheral sensitization of nociceptors. However, to date, there is no evidence for a role of Na_v1.8 in controlling Aβ-fiber excitability following persistent inflammation.

Methods

Distribution and expression of Na_v1.8 in dorsal root ganglia and sciatic nerves were qualitatively or quantitatively assessed by immunohistochemical staining and by real time-polymerase chain reaction at different time points following complete Freund’s adjuvant (CFA) administration. Using a whole-cell patch-clamp configuration, we further determined both total I_Na and TTX-R Na_v1.8 currents in large-soma dorsal root ganglia (DRG) neurons isolated from sham or CFA-treated rats. Finally, we analyzed the effects of ambroxol, a Na_v1.8-preferring blocker on the electrophysiological properties of Na_v1.8 currents and on the mechanical sensitivity and inflammation of the hind paw in CFA-treated rats.

Results

Our findings revealed that Na_v1.8 is up-regulated in NF200-positive large sensory neurons and is subsequently anterogradely transported from the DRG cell bodies along the axons toward the periphery after CFA-induced inflammation. We also demonstrated that both total I_Na and Na_v1.8 peak current densities are enhanced in inflamed large myelinated Aβ-fiber neurons. Persistent inflammation leading to nociception also induced time-dependent changes in Aβ-fiber neuron excitability by shifting the voltage-dependent activation of Na_v1.8 in the hyperpolarizing direction, thus decreasing the current threshold for triggering action potentials. Finally, we found that ambroxol significantly reduces the potentiation of Na_v1.8 currents in Aβ-fiber neurons observed following intraplantar CFA injection and concomitantly blocks CFA-induced mechanical allodynia, suggesting that Na_v1.8 regulation in Aβ-fibers contributes to inflammatory pain.

Conclusions

Collectively, these findings support a key role for Na_v1.8 in controlling the excitability of Aβ-fibers and its potential contribution to the development of mechanical allodynia under persistent inflammation.

Implication of the chemokine CCL2 in trigeminal nociception and traumatic neuropathic orofacial pain

BACKGROUND

Chemokine (C-C motif) ligand 2 (CCL2) participates in different mechanisms contributing to the spinal cord inflammation and pain development after sciatic nerve injury. Recent data also support its role in orofacial thermal hypersensitivity, although its implication in different phases of trigeminal pain emergence is unclear. We assessed the importance of CCL2 signalling in biochemical and behavioural alterations during the early and late stages following chronic constriction injury of infraorbital nerve (ION-CCI), a model of peripheral traumatic trigeminal pain.

METHODS

After evaluating the consequences of CCL2 intracisternal injection in naïve rats, we determined the expression changes for CCL2, inflammatory and glia activation markers in the somatosensory trigeminal complex (STC) and trigeminal ganglia (TG) after ION-CCI. The role of CCL2 signalling was assessed using pre-emptive or 'curative' intracisternal treatment with specific CCL2 receptor antagonist - INCB3344.

RESULTS

Exogenous CCL2 evoked spontaneous behaviour reminiscent of orofacial pain and marked mechanical hypersensitivity, associated with increased expression of proinflammatory cytokines and glial markers in STC and TG. CCL2-evoked changes were prevented by the co-administration of INCB3344. Two weeks after ION-CCI, mRNA for CCL2, glial and inflammatory markers were up-regulated, and CCL2-immunoreactivity accumulated in central and ganglionic tissues. At this time, repeated intracisternal administration of INCB3344 did not attenuate the ION-CCI-associated behavioural nor biochemical changes. By contrast, pre-emptive INCB3344 treatment delayed the emergence of trigeminal mechanical allodynia and associated biochemical alterations.

CONCLUSIONS

Our data suggest that CCL2 is involved principally in the early events accompanying the ION lesion rather than in long-term alterations and the maintenance of trigeminal mechanical hypersensitivity.

[Chemokines as new actors in the dopaminergic system]

Previous neuroanatomical studies realized in our team allowed us to demonstrate the neuronal and glial expression of various chemokines and their receptors in central dopaminergic (DA) pathways. In the light of these original observations, we questioned the role of chemokines on the physiology of DA neuron and on the neurodegenerative process in the DA nigro-striatal pathway, which characterizes Parkinson's disease. We focused our attention on two particular chemokines, the Stromal cell-Derived Factor-1 (SDF-1/CXCL12) and the Monocyte Chemoattractant Protein-1 (MCP-1/CCL2) and their cognate receptors CXCR4 and CCR2, as they are expressed constitutively in nearly all DA mesencephalic neurons. We demonstrated, by using in vivo and in vitro approaches, that SDF-1 and MCP-1 can modulate DA neurotransmission in the nigro-striatal pathway, modifying the electrophysiological state of the neuron and DA release, through their cognate receptors. These effects are produced through N-type high voltage-activated calcium currents for SDF-1 and potassium channels for MCP-1. We then discuss the possible implication of SDF-1 and its derivative SDF-1(5-67) in DA neurodegeneration.

[CCL2 chemokine and transmission of nociceptive information]

Chemokines are small proteins belonging to the family of inflammatory cytokines whose main property is to attract and activate immune cells. Chemokines produce their biological activity by interacting with receptors belonging to the family of G protein-coupled receptors (GPCR). It is now clearly demonstrated that, under non-pathological conditions, chemokines and their receptors are constitutively expressed in the nervous system. Exploration of the implication of chemokines and their receptors in pain is a rapidly expanding domain of research. Recent results demonstrate that chemokines are active as modulators of nociceptive information both in the peripheral and central nervous system. After a brief recall of the chemokinergic system, we will discuss recent work on CCL2, a chemo-attractive cytokine which binds to the CCR2 receptor. There is a large body of data supporting the hypothesis that the CCL2 chemokine, released in the dorsal horn of the spinal cord, could have a direct effect on the transmission of nociceptive information and thus participate in neuronal adaptation and probably in the plasticity of glial cells and attractivity of immune cells observed during the installation and development of chronic pain arising from an inflammatory and/or neuropathic mechanism.

Chemokines and chemokine receptors: new actors in neuroendocrine regulations

Chemokines are small secreted proteins that chemoattract and activate immune and non-immune cells. Their role in the immune system is well-known, and it has recently been suggested that they may also play a role in the central nervous system (CNS). Indeed, they do not only act as immunoinflammatory mediators in the brain but they also act as potential modulators in neurotransmission. Although we are only beginning to be aware of the implication of chemokines in neuroendocrine functions, this review aims at summarizing what is known in that booming field of research. First we describe the expression of chemokines and their receptors in the CNS with a focus on the hypothalamo-pituitary system. Secondly, we present what is known on some chemokines in the regulation of neuroendocrine functions such as cell migration, stress, thermoregulation, drinking and feeding as well as anterior pituitary functions. We suggest that chemokines provide a fine modulatory tuning system of neuroendocrine regulations.

Chemokines and neuromodulation

Chemokines are not only mediators of the immune system and expressed in inflammatory situations. They are also constitutively expressed in the brain in both glial cells and neurons. Several recent evidence suggest that they can have a neurotransmitter/neuromodulatory role on brain functions similar to several neuropeptides reported so far. The aim of this short review is to illustrate that point using two chemokine systems, SDF-1/CXCL12 and its receptor CXCR4 and MCP-1/CCL2 and its receptor CCR2.

Ontogenic and adult whole body distribution of aminopeptidase N in rat investigated by in vitro autoradiography

Aminopeptidase N (APN), which is widely distributed in mammalian tissues, is able to cleave numerous regulatory peptides. The selective inhibitor of APN, [(125)I] RB129, has been used to study the distribution of this exopeptidase during rat prenatal development and adult life by in vitro whole-body autoradiography. In the central nervous system, APN shows a weak labeling compared to the major part of the non-nervous tissues in the embryo and in the adult. APN is progressively expressed in kidney, intestine, heart, lung, sensory organs, eye, and thymus. In organs such as the liver, the cartilages and the bones, altered levels of APN expression are observed during the development, or in the embryo compared to the adult, suggesting a role of APN during the liver haematopoiesis and bone growth. At this time, all the physiological functions of APN are still incompletely known, however its developmental pattern of expression strongly suggests a function of modulation of this enzyme during the development, next in physiological and/or pathological situations in adult. In this way, APN could represent a new therapeutic target in pathological processes, such as tumoral proliferation and/or angiogenesis associated with cancer development, where an increase in the level of this enzyme has been observed.