D-serine reduces memory impairment and neuronal damage induced by chronic lead exposure

Although exogenous D-serine has been applied as a neural regulatory intervention in many studies,the role played by D-serine in hippocampal injuries caused by lead exposure remains poorly understood.Rat models of chronic lead exposure were established through the administration of 0.05%lead acetate for 8 weeks.Simultaneously,rats were administered 30 or 60 mg/kg D-serine,intraperitoneally,twice a day.Our results showed that D-serine treatment shortened the escape latency from the Morris water maze,increased the number of times that mice crossed the original platform location,and alleviated the pathological damage experienced by hippocampal neurons in response to lead exposure.Although D-serine administration did not increase the expression levels of the N-methyl-D-aspartate receptor subtype 2B(NR2B)in the hippocampi of lead-exposed rats,60 mg/kg D-serine treatment restored the expression levels of NR2A,which are reduced by lead exposure.These findings suggested that D-serine can alleviate learning and memory impairments induced by lead exposure and that the underlying mechanism is associated with the increased expression of NR2A in the hippocampus.This study was approved by the Animal Ethics Committee of North China University of Science and Technology,China(approval No.LX2018155)on December 21,2018.

D-serine对阿尔茨海默病模型小鼠海马区tau蛋白过度磷酸化的影响

目的:探讨D-丝氨酸(D-serine)对阿尔茨海默病(AD)模型小鼠海马区tau蛋白过度磷酸化的影响。方法:42只SPF级昆明小鼠随机分为6组:Control组、Saline组、Aβ组、Aβ+D-serine组、Aβ+DAAO组、Aβ+BE组;每组7只动物。AD模型组小鼠双侧海马CA1区微量注射Aβ1-42造模,之后连续10 d分别腹腔注射生理盐水,D-serine(30 mg/kg),D-氨基酸氧化酶(DAAO)(30 mg/kg),苯甲酸钠(BE)(30 mg/kg)。采用免疫组织化学以及Western Blot方法检测各组小鼠海马区IL-1β、p-MAPT(S404)、p-GSK3β以及DCX表达的具体变化。结果:与Control组相比,Aβ组小鼠海马区IL-1β以及p-MAPT(S404)蛋白表达明显增多(P<0.01),而p-GSK3β以及DCX蛋白表达明显减少(P<0.01);而与Aβ组相比,Aβ+D-serine组IL-1β以及p-MAPT(S404)蛋白表达明显减少(P<0.01);p-GSK3β以及DCX蛋白表达明显增多(P<0.01)。结论:D-serine可以改善Aβ1-42诱导的AD模型小鼠海马区tau蛋白过度磷酸化以及神经损伤。

Activity-dependent sulfhydration of serine racemase controls synaptic plasticity in the hippocampus via increasing D-serine availability

Aims More and more advances show that hydrogen sulfide （H2S） sulfhydrates functional proteins and regulates their actions. However, much less is known about the endogenous sulfhydration of key proteins under physiological conditions. Methods Biotin-switch assay, electrophysiological recording, RNA interference, full ESI-MS scan et al. Results Herein, we found that the protein sulfhydration was dynamically regulated by neuro- nal activity. H2S sulfhydrated and activated serine racemase （SR） in the hippocampus, resuhantly increased D- serine level. Notably, sulfhydration of SR was markedly promoted by neuronal stimulation and underlay activity-de- pendent changes of D-serine availability. Both genetic knockdown and pharmacological inhibition of cystathionine β-synthase （CBS）, the key H2S-producing enzyme in brain, attenuated hippoeampal long-term potentiation （LTP） , which can be reversed by exogenous supplement of H2S or D-serine. We also observed that the increase in LTP induced by H2S is dependent on D-serine and polysulfides. In aged rats, sulfhydration of SR was significantly decreased. Furthermore, exogenous supplement of H2S restored the level of sulfhydration and reversed the age-re- lated deficits in hippocampal LTP. Conclusion Our data provide direct evidence for the biological significance of endogenous sulfhydration in physiological conditions and demonstrate a pivotal role of activity-dependent sulfhydra- tion in synaptic plasticity.

Corticosterone induced D-serine release deficit play an important role in long-term potentiation impairment by corticosterone in perforant path-dentate gyrus pathway

OBJECTIVE Previous studies showed that over activation of NMDA receptors may be a crucial cause of long-term potentiation(LTP)and cognitive impairment induced by stress or corticosterone.However,other studies showed that the function of NMDA receptors is insufficient since the NMDA receptors co-agonist D-serine could improve stress-induced cognitive impairment.The purpose of this study is to clarify whether over activation of NMDA receptors or hypofunction of NMDA receptors is involved in hippocampal impairment of LTP by corticosterone and the underlying mechanisms.METHODS Cort was injected subcutaneously 1 h before the high-frequency stimulation(HFS)to induce LTP impairment.NMDA receptor antagonists and agonists were administrated by icv.RESULTS Hippocampal LTP and object location recognition memory were impaired in corticosterone-treated mice.Corticosterone increased the glutamate level in hippocampal tissues,neither NMDA receptors antagonist nor its subtype antagonists alleviated impairment of LTP,while enhancing the function of NMDA receptors by D-serine did alleviate impairment of LTP by corticosterone,suggesting that hypofunction of NMDA receptors might be one of the main reasons for impairment of LTP by corticosterone.Further results showed that the level of D-serine and its precursor L-serine did not change.D-serine release-related protein Na+-independent alanine-serine-cysteine transporter-1(ASC-1)in the cell membrane was decreased and increasing D-serine release by the selective activator of ASC-1 antiporter activity alleviated impairment of LTP by corticosterone.CONCLUSION Taken together,this study demonstrates that hypofunction of NMDA receptors may be involved in impairment of LTP by corticosterone and reduced D-serine release may be an important reason for its hypofunction,which is an important complement to existing mechanisms of corticosterone-induced LTP and cognitive impairment.

In vitro and in vivo activity of D-serine in combination with β-lactam antibiotics against methicillin-resistant Staphylococcus aureus

As D-amino acids play important roles in the physiological metabolism of bacteria,combination of D-amino acids with antibiotics may provide synergistic antibacterial activity. The aim of the study was to evaluate in vitro and in vivo activity of D-serine alone and in combination withβ-lactams against methicillin-resistant Staphylococcus aureus(MRSA) strains, and to explore the possible sensitization mechanisms. The activity of D-serine, β-lactams alone and in combinations was evaluated both in vitro by standard MICs, time–kill curves and checkerboard assays, and in vivo by murine systemic infection model as well as neutropenic thigh infection model. An in vitro synergistic effect was demonstrated with the combination of D-serine and β-lactams against MRSA standard and clinical strains.Importantly, the combinations enhanced the therapeutic efficacy in the animal models as compared toβ-lactam alone groups. Initial mechanism study suggested possible revision of D-alanine-D-alanine residue to D-alanine-D-serine in peptidoglycan by adding of D-alanine in the medium, which may cause decreased affinity to PBPs during transpeptidation. In conclusion, D-serine had synergistic activity in combination with β-lactams against MRSA strains both in vitro and in vivo. Considering the relatively good safety of D-serine alone or in combination with β-lactams, D-serine is worth following up as new anti-MRSA infection strategies.

Parvalbumin interneurons in anterior cingu⁃late cortex regulate cognitive behaviors in methylazoxymethanol acetate model of schizophrenia

OBJECTIVE Cognitive dysfunc⁃tion is a core disturbance of schizophrenia,appear to emerge from impaired neural activity.The anterior cingulate cortex(ACC)is an integra⁃tion hub for higher-order thalamic inputs impor⁃tant for complex cognitive tasks such as learning and memory processes,attention and social interaction.Parvalbumin(PV)interneurons could filter information at pyramidal neurons of ACC,and the abnormal PV interneurons have been observed in both humans and animal models of schizophrenia.However,the mechanisms of PV interneurons in ACC regulating cognition in schizophrenia is poorly understood.METHODS The pregnant mice were injected with methyl⁃azoxymethanol acetate(MAM)on gestational day(GD)16 for the neurodevelopmental MAM model of schizophrenia in our study.We investi⁃gated the cognitive behaviors by a serious of tests such as pre-pulse inhibition,Y maze,novel object and novel location recognition and the intrinsic excitability of PV interneurons and inhibi⁃tory synaptic transmission onto pyramidal cells localized in layer 5 of ACC by whole-cell record⁃ings.Further,the PV interneurons were regulat⁃ed by designer receptor exclusively activated by a designer drug(DREADD)system and the D-serine,a co-agonist of N-methyl-D-aspartate(NMDA)receptors.RESULTS①MAM mice showed the cognitive deficits and hypo-excitability of PV interneurons in ACC.②Restoration of PV interneuron activity in ACC improved cognitive function in MAM mice.③Inhibition of PV interneu⁃ron activity in ACC was sufficient to cause cogni⁃tive dysfunction in control mice.④NMDA recep⁃tors of PV interneurons in ACC were impaired in MAM mice.⑤Deficits of NMDA receptor sig⁃naling specifically in PV interneurons and of cog⁃nitive behaviors in MAM mice were rescued by D-serine.CONCLUSION PV interneurons in ACC are closely related to cognitive function in the MAM model of schizophrenia and D-serine maybe a potential therapy for schizophrenia.

S1A-5 Modulating the Balance of Synaptic and Extrasynaptic NMDA Receptors Shows Positive Effects Against Amyloid-Beta-Induced Neurotoxicity

The unbalance between synaptic(GluN2A,mediating the protective pathway)and extrasynaptic NMDA receptors(NMDARs)(GluN2B,mediating the excitotoxic pathway)has been found in Alzheimer’s disease(AD),indicating restoring the balance of GluN2A and GluN2B should be beneficial for AD.In this study,the GluN2B-selective antagonist,ifenprodil,and the non-selective NMDAR agonist,NMDA,had little effects on amyloid-beta(Abeta)-induced longterm potentiation(LTP)deficits.Enhancing the activity of GluN2A had a protective effect against Abeta,and specific activation of GluN2A and inhibition of GluN2B showed a better protective effect.The combination of ifenprodil and D-cycloserine(a co-activator of NMDRs similar to D-serine)led to greater improvement in behavior tests than ifenprodil or D-cycloserine alone,meanwhile,the combination of ifenprodil and D-cycloserine reversed the signal pathway more significantly than ifenprodil or D-cycloserine alone.These results indicate that enhancing synaptic NMDARs and inhibiting extrasynaptic NMDARs concurrently showed protective effects against Abeta-induced neurotoxicity,suggesting that modulation of the balance between GluN2A and GluN2B might be a good strategy for AD therapy.

Activation of glycine site and GluN2B subunit of NMDA receptors is necessary for ERK/CREB signaling cascade in rostral anterior cingulate cortex in rats:Implications for affective pain

Objective The rostral anterior cingulate cortex （rACC） is implicated in processing the emotional component of pain. N-methyl-D-aspartate receptors （NMDARs） are highly expressed in the rACC and mediate painrelated affect by activating a signaling pathway that involves cyclic adenosine monophosphate （cAMP）/protein ki- nase A （PKA） and/or extracellular regulated kinase （ERK）/cAMP-response element-binding protein （CREB）. The present study investigated the contributions of the NMDAR glycine site and GluN2B subunit to the activation of ERK and CREB both in vitro and in vivo in rat rACC. Methods Immunohistochemistry and Western blot analy- sis were used to separately assess the expression of phospho-ERK （pERK） and phospho-CREB （pCREB） in vitro and in vivo. Double immunostaining was also used to determine the colocalization of pERK and pCREB. Results Both bath application of NMDA in brain slices in vitro and intraplantar injection of formalin into the rat hindpaw in vivo induced significant up-regulation of pERK and pCREB in the rACC, which was inhibited by the NMDAR antago- nist DL-2-amino-5-phospho-novaleric acid. Selective blockade of the NMDAR GluN2B subunit and the glycine- binding site, or degradation of endogenous D-serine, a co-agonist for the glycine site, significantly decreased the up- regulation of pERK and pCREB expression in the rACC. Further, the activated ERK predominantly colocalized with CREB. Conclusion Either the glycine site or the GluN2B subunit of NMDARs participates in the phosphorylation of ERK and CREB induced by bath application of NMDA in brain slices or hindpaw injection of 5% formalin in rats, and these might be fundamental molecular mechanisms underlying pain affect.