RRDCD is a clinically intractable ophthalmic disease. The patients with RRDCD have high surgical difficulty and poor prognosis. However, there are few studies on this disease. In our study, the IOP of RRDCD group was significantly lower than that of IEMM group, and RRDCD patients had different degrees of PVR. Investigating potential sensitive molecular events in vitreous fluid of RRDCD patients can provide clues for exploring the pathogenesis of RRDCD.
Metabolomics has been widely used in screening clinical biomarkers. Compared with the IEMM group, we found that the levels of 125 metabolites in vitreous fluid of RRDCD patients were up-regulated, and the levels of 10 metabolites were down-regulated. L-Palmitoylcarnitine, N-Docosanoyl-4-sphingenyl-1-O-phosphorylcholine, adenosine, sphingomyelin and 4,7,10,13,16,19-docosahexaenoic acid may be potential biomarkers. Protein digestion and absorption, ABC transporters, aminoacyl-tRNA biosynthesis, sphingolipids metabolism and other pathways were significantly interfered in RRDCD group. Revealing the changes of metabolites in vitreous fluid of RRDCD patients can help us discover the pathogenesis of RRDCD, find specific biological targets, and formulate treatment strategies.
Sphingomyelin (SM) is the main component of cell membrane. It has been proved that RPE cells, glial cells and inflammatory cells migrate to vitreous cavity after retinal detachment 13. SM in vitreous fluid of RRDCD patients may come from migrating retinal cells. Several studies have shown that the SM-rich microdomains on the cell membrane act as receptors to regulate a variety of cellular processes such as cell death, proliferation 14, 15 and migration 16. The up-regulated SM level will induce more cell migration and cell death. Broken cells release intracellular components to induce the increase of inflammatory mediators 17, such as IL-6, CCL4, TGF-β3 and VEGF 10.
In addition, sphingolipid metabolism participates in inflammatory reaction through various mechanisms. Sphingolipid metabolism can affect the activation of pro-inflammatory gene-NFB. Ceramide can activate TLR4 signal 18. In sphingomyelin synthase 2 knockout mice, toll like receptor 4-MD2 complex levels on the surface of macrophages were also significantly reduced 19. Exogenous addition of SM can directly trigger the inflammatory response of mouse macrophages in an NFB independent manner 18. Abnormal activity of sphingolipid metabolism can lead to the intensification of intraocular inflammation and eye tissue damage.
L-Palmitoylcarnitine is a long-chain acyl carnitine that accumulates in the muscle membrane during ischemia and disturbs the membrane lipid environment. It has been proved that acylcarnitine is a metabolite with biological activity and inflammatory properties. Palmitoylcarnitine increases the stress indicators of muscle cells, such as cell permeability, caspase-3 cleavage, IL-6 production, and MAP kinase pathway 20. When the concentration of L-palmitoylcarnitine increases, it can inhibit carnitine palmitoyltransferase activity 21, 22 and limit the transport of long-chain fatty acids in the mitochondria. More than 60% of retinal mitochondria are in photoreceptor cells 23. When mitochondrial dysfunction occurs, it may lead to the death of photoreceptor cells 24, and denaturation of neurons 25. After retinal detachment, the up-regulation of L-palmitoylcarnitine level caused by hypoxia in the inner retina may aggravate the damage of visual function. We also detected that many fatty acid metabolites were up-regulated in the eyes of RRDCD patients, such as 1-stearoyl-2-oleoyl-sn-glycerol 3-phosphocholine, 1-stearoyl-2-arachidonoyl-sn-glycerol, linoleic acid and 4,7,10,13,1 6,19-docosahexaenoic acid. The up-regulation of L-palmitoylcarnitine and various lipid metabolites may indicate that the retinal or choroidal lipid oxidation process in RRDCD patients was damaged 26. The retina is rich in lipids and requires high metabolism 27. lipid β-oxidation is an important pathway of retina obtaining energy, especially in RPE cells, muller cells and photoreceptor cells 23. Damaged energy pathway and the accumulation of long-chain fatty acids in cells can induce apoptosis 28. Previous studies have shown that when the oxidative utilization and storage of long-chain fatty acids are unbalanced, it can cause cellular stress and inflammation 29. Accumulation of long-chain fatty acids leads to the accumulation of reactive oxygen species (ROS) 30. ROS can induce inflammation as a metabolic stressor. It has been shown that the overproduction of ROS after retinal detachment causes oxidative stress, activates inflammasomes, and interacts with the inflammatory mediators, including IL-1β, TNF-α, and CCL2 31. Previous studies have shown that PVR is related to the increase of free radical formation and the decrease of antioxidant activity in human vitreous 32.
The limitation of this study is that non-target metabolites cannot be quantitatively to analyze metabolomics. Our results need to be verified by more experiments. However, our results revealed the possible pathways involved in RRDCD. The study helps us to explore the pathogenesis of RRDCD and find biomarkers.