Lack of Effect of Sodium Benzoate at Reported Clinical Therapeutic Concentration on d-Alanine Metabolism in Dogs

Determination of D-serine and D-alanine Tissue Levels in the Prefrontal Cortex and Hippocampus of Rats After a Single Dose of Sodium Benzoate, a D-Amino Acid Oxidase Inhibitor, with Potential Antipsychotic and Antidepressant Properties

The effects of the N-methyl-D-aspartate receptor activators D-serine, D-alanine, and sarcosine against schizophrenia and depression are promising. Nevertheless, high doses of D-serine and sarcosine are associated with undesirable nephrotoxicity or worsened prostatic cancer. Thus, alternatives are needed. DAAO inhibition can increase D-serine as well as D-alanine and protect against D-serine-induced nephrotoxicity. Although several DAAO inhibitors improve the symptoms of schizophrenia and depression, they can increase the plasma levels but not brain levels of D-serine. The mechanism of action of DAAO inhibitors remains unclear. We investigated the effects of the DAAO inhibitor sodium benzoate on the prefrontal cortex and hippocampal level of D-alanine as known another substrate with antipsychotic and antidepressant properties and other NMDAR-related amino acids, such as, L-alanine, D-serine, L-serine, D-glutamate, L-glutamate, and glycine levels. Our results indicate that sodium benzoate exerts antipsychotic and antidepressant-like effects without changing the D-serine levels in the brain prefrontal cortex (PFC) and hippocampus. Moreover, D-alanine levels in the PFC and hippocampus did not change. Despite these negative findings regarding the effects of D-amino acids in the PFC and hippocampus, sodium benzoate exhibited antipsychotic and antidepressant-like effects. Thus, the therapeutic effects of sodium benzoate are independent of D-serine or D-alanine levels. In conclusion, sodium benzoate may be effective among patients with schizophrenia or depression; however, the mechanisms of actions remain to be elucidated.

Rational and Translational Implications of D-Amino Acids for Treatment-Resistant Schizophrenia: From Neurobiology to the Clinics

Schizophrenia has been conceptualized as a neurodevelopmental disorder with synaptic alterations and aberrant cortical–subcortical connections. Antipsychotics are the mainstay of schizophrenia treatment and nearly all share the common feature of dopamine D2 receptor occupancy, whereas glutamatergic abnormalities are not targeted by the presently available therapies. D-amino acids, acting as N-methyl-D-aspartate receptor (NMDAR) modulators, have emerged in the last few years as a potential augmentation strategy in those cases of schizophrenia that do not respond well to antipsychotics, a condition defined as treatment-resistant schizophrenia (TRS), affecting almost 30–40% of patients, and characterized by serious cognitive deficits and functional impairment. In the present systematic review, we address with a direct and reverse translational perspective the efficacy of D-amino acids, including D-serine, D-aspartate, and D-alanine, in poor responders. The impact of these molecules on the synaptic architecture is also considered in the light of dendritic spine changes reported in schizophrenia and antipsychotics’ effect on postsynaptic density proteins. Moreover, we describe compounds targeting D-amino acid oxidase and D-aspartate oxidase enzymes. Finally, other drugs acting at NMDAR and proxy of D-amino acids function, such as D-cycloserine, sarcosine, and glycine, are considered in the light of the clinical burden of TRS, together with other emerging molecules.

A Rationally and Computationally Designed Fluorescent Biosensor for d-Serine

Solute-binding proteins (SBPs) have evolved to balance the demands of ligand affinity, thermostability, and conformational change to accomplish diverse functions in small molecule transport, sensing, and chemotaxis. Although the ligand-induced conformational changes that occur in SBPs make them useful components in biosensors, they are challenging targets for protein engineering and design. Here, we have engineered a d-alanine-specific SBP into a fluorescence biosensor with specificity for the signaling molecule d-serine (D-serFS). This was achieved through binding site and remote mutations that improved affinity (K_D = 6.7 ± 0.5 μM), specificity (40-fold increase vs glycine), thermostability (T_m = 79 °C), and dynamic range (∼14%). This sensor allowed measurement of physiologically relevant changes in d-serine concentration using two-photon excitation fluorescence microscopy in rat brain hippocampal slices. This work illustrates the functional trade-offs between protein dynamics, ligand affinity, and thermostability and how these must be balanced to achieve desirable activities in the engineering of complex, dynamic proteins.

Metabolic and Neural Mechanisms Underlying the Associations Between Gut Bacteroides and Cognition: A Large-Scale Functional Network Connectivity Study

There is a proof-of-concept that microbial metabolites provide a molecular connection between the gut and the brain. Extensive research has established a link between gut Bacteroides and human cognition, yet the metabolic and neural mechanisms underlying this association remain largely unknown. Here, we collected fecal samples, resting-state functional MRI, and cognitive data from a large and homogeneous sample of 157 healthy young adults. 16S rRNA gene sequencing was conducted with abundances of Bacteroides and metabolic pathways quantified by species annotation and functional prediction analyses, respectively. Large-scale intra- and internetwork functional connectivity was measured using independent component analysis. Results showed that gut Bacteroides were related to multiple metabolic pathways, which in turn were associated with widespread functional network connectivity. Furthermore, functional network connectivity mediated the associations between some Bacteroides-related metabolic pathways and cognition. Remarkably, arginine and proline metabolism, phenylalanine metabolism, and biosynthesis of unsaturated fatty acids act as the key metabolic pathways that are most contributive, and the executive control and sensorimotor systems contribute most strongly at the neural level. Our findings suggest complex poly-pathway and poly-network processes linking Bacteroides to cognition, more generally yielding a novel conceptualization of targeting gut Bacteroides as an intervention strategy for individuals with cognitive impairment.

Advances in D-Amino Acids in Neurological Research

D-amino acids have been known to exist in the human brain for nearly 40 years, and they continue to be a field of active study to today. This review article aims to give a concise overview of the recent advances in D-amino acid research as they relate to the brain and neurological disorders. This work has largely been focused on modulation of the N-methyl-D-aspartate (NMDA) receptor and its relationship to Alzheimer’s disease and Schizophrenia, but there has been a wealth of novel research which has elucidated a novel role for several D-amino acids in altering brain chemistry in a neuroprotective manner. D-amino acids which have no currently known activity in the brain but which have active derivatives will also be reviewed.

Precision Medicine of Sodium Benzoate for the Treatment of Behavioral and Psychological Symptoms of Dementia (BPSD)

OBJECTIVE

Behavioral and psychological symptoms of dementia (BPSD) are associated with poorer prognosis of dementia. A 24-week study demonstrated that sodium benzoate, a D-amino acid oxidase (DAAO) inhibitor, surpassed placebo in improving cognitive function in early-phase Alzheimer's disease; however, benzoate did not excel placebo in another 6-week study on BPSD. The current study examined whether the precision medicine approach was able to identify specific individuals with BPSD who could benefit from benzoate treatment.

METHODS

In the randomized, double-blind, placebo-controlled, 6-week trial, 97 patients with BPSD were allocated to receive 250-1500 mg/day of sodium benzoate or placebo. Cognitive function was measured by the Alzheimer's disease assessment scale-cognitive subscale (ADAS-cog) and behavioral and psychological symptoms were mainly measured by Behavioral Pathology in Alzheimer's Disease Rating Scale (BEHAVE-AD). DAAO level, amino acids (L-serine, D-serine, L-alanine, and D-alanine, glycine), and two antioxidants (catalase, superoxide dismutase) were assayed in peripheral blood.

RESULTS

After benzoate treatment, DAAO inhibition was correlated with ADAS-cog decrease (p = 0.034), while baseline DAAO level was correlated with baseline BEHAVE-AD score. Multiple linear regression analyses showed that cognitive improvement after benzoate treatment was correlated with DAAO decrease, female gender, younger age, BMI, baseline BPSD severity, and antipsychotic use.

CONCLUSION

The finding suggests that sodium benzoate may have potential to benefit cognitive function in a fraction of BPSD patients after 6 weeks of treatment. Of note, the precision medicine approach may be helpful for identifying individuals who could respond to benzoate. More studies are warranted to confirm the preliminary findings.

TRIAL REGISTRATION

The trial was registered online (https://clinicaltrials.gov/ct2/show/NCT02103673).

Sodium benzoate for the treatment of behavioral and psychological symptoms of dementia (BPSD): A randomized, double-blind, placebo-controlled, 6-week trial

OBJECTIVE

Sodium benzoate, a D-amino acid oxidase (DAAO) inhibitor, improved cognitive function of early-phase Alzheimer's disease (AD) after 24-week treatment. This study examined benzoate treatment for behavioral and psychological symptoms of dementia (BPSD).

METHODS

In a double-blind, 6-week trial, 97 patients with BPSD were randomized to receive placebo or benzoate (mean dose: 622.0 mg/day). The primary outcomes were ADAS-cog and BEHAVE-AD.

RESULTS

Two treatments showed similar safety and primary and secondary outcomes.

CONCLUSIONS

Compared to antecedent 24-week, higher-dose treatment for early-phase AD, benzoate appeared ineffective in this 6-week trial. Longer-duration, higher-dose trials are warranted to clarify its efficacy for BPSD.

Recent Advances in the Early Intervention in Schizophrenia: Future Direction from Preclinical Findings

PURPOSE OF REVIEW

In the past decade, there has been increasing interest in the potential benefit of early intervention in schizophrenia. Patients with schizophrenia show cognitive impairment for several years preceding the onset of psychosis. The author discusses the recent topics on prevention of schizophrenia.

RECENT FINDINGS

Preclinical findings suggest that maternal immune activation (MIA) produces cognitive deficits as a prodromal symptom in juvenile offspring in rodents. Treatment with anti-inflammatory compounds, such as D-serine, 7,8-dihydroxyflavone (a TrkB agonist), sulforaphane (or its precursor glucoraphanin), and TPPU (1-trifluoromethoxyphenyl-3-(1-propionylpiperidin-4-yl) urea: a soluble epoxide hydrolase inhibitor), during adolescence might prevent the onset of behavioral abnormalities and parvalbumin immunoreactivity in the medial prefrontal cortex of adult offspring after MIA. Based on the role of inflammation and cognitive impairment in the prodromal state, early intervention using anti-inflammatory compounds (i.e., D-serine, sodium benzoate, TrkB agonist, Nrf2 agonist, soluble epoxide hydrolase inhibitor) may reduce the risk of subsequent transition to schizophrenia.

Striatal, Hippocampal, and Cortical Networks Are Differentially Responsive to the M4- and M1-Muscarinic Acetylcholine Receptor Mediated Effects of Xanomeline

Preclinical and clinical data suggest that muscarinic acetylcholine receptor activation may be therapeutically beneficial for the treatment of schizophrenia and Alzheimer's diseases. This is best exemplified by clinical observations with xanomeline, the efficacy of which is thought to be mediated through co-activation of the M1 and M4 muscarinic acetylcholine receptors (mAChRs). Here we examined the impact of treatment with xanomeline and compared it to the actions of selective M1 and M4 mAChR activators on in vivo intracellular signaling cascades in mice, including 3'-5'-cyclic adenosine monophosphate response element binding protein (CREB) phosphorylation and inositol phosphate-1 (IP1) accumulation in the striatum, hippocampus, and prefrontal cortex. We additionally assessed the effects of xanomeline on hippocampal electrophysiological signatures in rats using ex vivo recordings from CA1 (Cornu Ammonis 1) as well as in vivo hippocampal theta. As expected, xanomeline's effects across these readouts were consistent with activation of both M1 and M4 mAChRs; however, differences were observed across different brain regions, suggesting non-uniform activation of these receptor subtypes in the central nervous system. Interestingly, despite having nearly equal in vitro potency at the M1 and the M4 mAChRs, during in vivo assays xanomeline produced M4-like effects at significantly lower brain exposures than those at which M1-like effects were observed. Our results raise the possibility that clinical efficacy observed with xanomeline was driven, in part, through its non-uniform activation of mAChR subtypes in the central nervous system and, at lower doses, through preferential agonism of the M4 mAChR.