L-Serine Decreases Taurine Concentration in the Extracellular Fluid of Brain Slices

L-Serine is considered a functional amino acid in the central nervous system, and induces sedation and hypnotic effects in some animal models of acute and chronic stress. Accordingly, while L-serine is a candidate anti-stress factor, the central mechanism of L-serine is not clear. The present study clarifies the action of L-serine using acute chick brain slices. We investigated the changes in some extracellular fluid amino acid concentrations in response to L-serine perfusion. Taurine concentration decreased while L-alanine concentration increased following L-serine perfusion. To examine the involvement of the taurine transporter, the effect of L-serine on the taurine concentration in the presence and absence of Na+ was also investigated. Na+ had no effect on taurine concentration induced by L-serine perfusion. These results suggest that L-serine has an ability to promote L-alanine synthesis facilitating the catabolism of taurine. In conclusion, L-serine modifies the metabolism of taurine and L-alanine in the extracellular space in chick brain.

Metabolic engineering and flux analysis of Corynebacterium glutamicum for L-serine production

L-Serine plays a critical role as a building block for cell growth, and thus it is difficult to achieve the direct fermentation of L-serine from glucose. In this study, Corynebacterium glutamicum ATCC 13032 was engineered de novo by blocking and at- tenuating the conversion of L-serine to pyruvate and glycine, releasing the feedback inhibition by L-serine to 3-phosphoglycerate dehydrogenase （PGDH）, in combination with the co-expression of 3-phosphoglycerate kinase （PGK） and feedback-resistant PGDH （PGDHr）. The resulting strain, SER-8, exhibited a lower specific growth rate and significant differ- ences in L-serine levels from Phase I to Phase V as determined for fed-batch fermentation. The intracellular L-serine pool reached （14.22_＋1.41） ~trnol gcoM-1, which was higher than glycine pool, contrary to fermentation with the wild-type strain. Furthermore, metabolic flux analysis demonstrated that the over-expression of PGK directed the flux of the pentose phosphate pathway （PPP） towards the glycolysis pathway （EMP）, and the expression of PGDHr improved the L-serine biosynthesis pathway. In addition, the flux from L-serine to glycine dropped by 24%, indicating that the deletion of the activator GlyR re- sulted in down-regulation of serine hydroxymethyltransferase （SHMT） expression. Taken together, our findings imply that L-serine pool management is fundamental for sustaining the viability of C. glutamicum, and improvement of C1 units genera- tion by introducing the glycine cleavage system （GCV） to degrade the excessive glycine is a promising target for L-serine pro- duction in C. glutamicum.

Efficacy of a novel endotoxin adsorber polyvinylidene fluoride fiber immobilized with L-serine ligand on septic pigs

A novel adsorber,polyvinylidene fluoride matrix immobilized with L-serine ligand (PVDF-Ser),was developed in the present study to evaluate its safety and therapeutic efficacy in septic pigs by extracorporeal hemoperfusion.Endotoxin adsorption efficiency (EAE) of the adsorber was firstly measured in vitro.The biocompatibility and hemodynamic changes during extracorporeal circulation were then evaluated.One half of 16 pigs receiving lipopolysaccharide (Escherichia coli O111:B4,5 μg/kg) intravenously in 1 h were consecutively treated by hemoperfusion with the new adsorber for 2 h.The changes of circulating endotoxin and certain cytokines and respiratory function were analyzed.The 72 h-survival rate was assessed eventually.EAE reached 46.3% (100 EU/ml in 80 ml calf serum) after 2 hcirculation.No deleterious effect was observed within the process.The plasma endotoxin,interleukin-6 (IL-6),and tumor necrosis factor-α (TNF-α) levels were decreased during the hemoperfusion.Arterial oxygenation was also improved during and after the process.Furthermore,the survival time was significantly extended (>72 h vs.47.5 h for median survival time).The novel product PVDF-Ser could adsorb endotoxin with high safety and efficacy.Early use of extracorporeal hemoperfusion with the new adsorber could reduce the levels of circulating endotoxin,IL-6,and TNF-α,besides improve respiratory function and consequent 72 h-survival rate of the septic pigs.Endotoxin removal strategy with blood purification using the new adsorber renders a potential promising future in sepsis therapy.

Metabolic engineering of an industrial bacterium Zymomonas mobilis for anaerobic l-serine production

Due to the complicated metabolic and regulatory networks of l-serine biosynthesis and degradation,microbial cell factories for l-serine production using non-model microorganisms have not been reported.In this study,a combination of synthetic biology and process optimization were applied in an ethanologenic bacterium Zymomonas mobilis for l-serine production.By blocking the degradation pathway while introducing an exporter EceamA from E.coli,l-serine titer in recombinant Z.mobilis was increased from 15.30 mg/L to 62.67 mg/L.It was further increased to 260.33 mg/L after enhancing the l-serine biosynthesis pathway.Then,536.70 mg/L l-serine was achieved by removing feedback inhibition with a SerA mutant,and an elevated titer of 687.67 mg/L was further obtained through increasing serB copies while enhancing the precursors.Finally,855.66 mg/L l-serine can be accumulated with the supplementation of the glutamate precursor.This work thus not only constructed an l-serine producer to help understand the bottlenecks limiting l-serine production in Z.mobilis for further improvement,but also provides guidance on engineering non-model microbes to produce biochemicals with complicated pathways such as amino acids or terpenoids.