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Neuropathy target esterase-related esterase and its potential role in maintaining the membrane homeostasis of the endoplasmic reticulum in human skeletal muscle cells

The neuropathy target esterase-related enzyme (NRE, PNPLA7) is a member of the PNPLA (patatin- like phospholipase domain containing proteins) family of nine enzymes sharing a patatin-like domain. NRE has been identified as a lysophospholipase that hydrolyses sn-1 esters in lysophosphatidylcholine and lysophosphatidic acid, but its physiological roles are not fully understood. The crystal structure of NRE has not been resolved yet, but according to the sequence analysis and homology modelling, it consists of an N-terminal domain with a transmembrane segment and three cyclic nucleotide binding sites, and a C- terminal domain where the active site with catalytic dyad Ser-Asp is located. It is a transmembrane protein associated with the endoplasmic reticulum (ER) through the transmembrane segment and non-enzymatically after cyclic nucleotides stimuli with lipid droplets. In our research, we used human skeletal muscle cells as a model to investigate the physiological role of NRE. We conducted an experiment in which we silenced gene encoding NRE in human myoblasts, and transfected it with “small interfering” RNA for mRNA NRE by lipofection. As a positive control, we used “non-targeting scrambled” RNA. Afterwards, we examined how silencing NRE affects the expression of the ionic transporter Na+/K+ ATP-ase (NKAα1) and phosphorylation of S6RP, p70 and 4E-BP1, proteins involved in the mTOR signalling pathway and protein translation. Firstly, our results showed that the introduction of scrambled RNA upregulates NRE expression. We believe that lipofection activates the defence mechanisms of the cell against excess lipids and that NRE may have a function of protecting cells from excessive phospholipids. Also, our results showed that silencing of NRE downregulates the expression of NKAα1, an important membrane protein, and downregulates the phosphorylation of pS6RP and p70, activators of protein translation. NRE silencing did not have any effect on the phosphorylation of the protein translation inhibitor, p4E-BP1, which implies that downregulation of NRE inhibits protein translation occurs at the ER. In summary, our results imply that NRE plays an important role in maintaining ER membrane homeostasis.

NRE, myoblasts, inhibition, signaling

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