Lipids are key molecules in various biological processes, thus their quantification is a crucial point in a lot of studies and should be taken into account in lipidomics development. This family is complex and presents a very large diversity of structures, so analyzing and quantifying all this diversity is a real challenge. In this review, the different techniques to analyze lipids will be presented: from nuclear magnetic resonance (NMR) to mass spectrometry (with and without chromatography) including universal detectors. First of all, the state of the art of quantification, with the definitions of terms and protocol standardization, will be presented with quantitative lipidomics in mind, and then technical considerations and limitations of analytical chemistry’s tools, such as NMR, mass spectrometry and universal detectors, will be discussed, particularly in terms of absolute quantification.

The energy cost of neuronal activity is mainly sustained by glucose^1,2. However, in an apparent paradox, neurons modestly metabolize glucose through glycolysis^3–6, a circumstance that can be accounted for by the constant degradation of 6-phosphofructo-2-kinase–fructose-2,6-bisphosphatase-3 (PFKFB3)^3,7,8, a key glycolysis-promoting enzyme. To evaluate the in vivo physiological importance of this hypoglycolytic metabolism, here we genetically engineered mice with their neurons transformed into active glycolytic cells through Pfkfb3 expression. In vivo molecular, biochemical and metabolic flux analyses of these neurons revealed an accumulation of anomalous mitochondria, complex I disassembly, bioenergetic deficiency and mitochondrial redox stress. Notably, glycolysis-mediated nicotinamide adenine dinucleotide (NAD⁺) reduction impaired sirtuin-dependent autophagy. Furthermore, these mice displayed cognitive decline and a metabolic syndrome that was mimicked by confining Pfkfb3 expression to hypothalamic neurons. Neuron-specific genetic ablation of mitochondrial redox stress or brain NAD⁺ restoration corrected these behavioural alterations. Thus, the weak glycolytic nature of neurons is required to sustain higher-order organismal functions.

Rheumatic diseases are often associated to debilitating chronic pain, which remains difficult to treat and requires new therapeutic strategies. We had previously identified lysophosphatidyl-choline (LPC) in the synovial fluids from few patients, and shown its effect as a positive modulator of Acid-Sensing Ion Channel 3 (ASIC3) able to induce acute cutaneous pain in rodents. However, the possible involvement of LPC in chronic joint pain remained completely unknown. Here we show, from two independent cohorts of patients with painful rheumatic diseases, that the synovial fluid levels of LPC are significantly elevated, especially the LPC16:0 species, compared to post mortem controls. Moreover, LPC16:0 levels are correlated with pain outcomes in a cohort of osteoarthritis (OA) patients. However, LPC16:0 but do not appear to be the hallmark of a particular joint disease, since similar levels are found in the synovial fluids of a second cohort of patients with various rheumatic diseases. The mechanism of action was next explored by developing a pathology-derived rodent model. Intra-articular injections of LPC16:0 is a triggering factor of chronic joint pain in both male and female mice, ultimately leading to persistent pain and anxiety-like behaviors. All these effects are dependent on ASIC3 channels, which drive sufficient peripheral inputs to generate spinal sensitization processes. This study brings evidences from mouse and human supporting a role for LPC16:0 via ASIC3 channels in chronic pain arising from joints, with potential implications for pain management in OA and possibly across other rheumatic diseases.

Several bone conditions e.g., bone cancer, osteoporosis and rheumatoid arthritis (RA) are associated with a risk of developing persistent pain. Increased osteoclast activity is often the hallmark of these bony pathologies and leads not only to bone remodeling but is also a source of pronociceptive factors that sensitize the bone-innervating nociceptors. Although historically bone loss in RA has been thought to be a consequence of inflammation, both bone erosion and pain can occur years before the symptom onset. Here we have addressed the disconnection between inflammation, pain and bone erosion by using a combination of two monoclonal antibodies isolated from B-cells of RA patients. We have found that mice injected with B02/B09 mAbs developed a long-lasting mechanical hypersensitivity which was accompanied by bone erosion in the absence of joint edema or synovitis. Intriguingly, we have noted a lack of analgesic effect of naproxen and a moderate elevation of few inflammatory factors in the ankle joints suggesting that B02/B09-induced pain-like behavior does not depend on inflammatory processes. In contrast, we found that inhibiting osteoclast activity and ASIC3 signaling prevented the development of B02/B09-mediated mechanical hypersensitivity. Moreover, we have identified phospholipase A₂ (sPLA₂) and lysophospatidylcholine 16:0 as critical components of B02/B09-induced pain-like behavior and shown that treatment with sPLA₂ inhibitor reversed B02/B09-induced mechanical hypersensitivity and bone erosion. Taken together, our study suggests a potential link between bone erosion and pain in a state of subclinical inflammation and offers a step forward in understanding the mechanisms of bone pain in diseases like RA.

Elisa Vidal, Bokkyoo Jun, William C. Gordon, Marie-Annick Maire, Lucy Martine, Stéphane Grégoire, Spiro Khoury, Stephanie Cabaret, Olivier Berdeaux, Niyazi Acar, Lionel Bretillon, Nicolas G. Bazan