Decreased serum levels of D-serine in patients with schizophrenia: evidence in support of the N-methyl-D-aspartate receptor hypofunction hypothesis of schizophrenia

BACKGROUND: The hypofunction of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors has been implicated in the pathophysiology of schizophrenia. Several lines of evidence suggest that D-serine may function as an endogenous agonist of the glycine site of the NMDA receptor. The aim of this study was to examine whether serum levels of D- and L-serine in patients with schizophrenia are different from those of healthy controls. METHODS: Forty-two patients with schizophrenia and 42 age- and sex-matched healthy controls were enrolled in this study. Symptoms were assessed using the Brief Psychiatric Rating Scale. Serum levels of total serine and D- and L-serine were measured by high-performance liquid chromatography. RESULTS: Serum levels of D-serine in the patients with schizophrenia were significantly (z = -3.30, P =.001) lower than those of healthy controls. In contrast, serum levels of total (D and L) serine (z = -2.40, P =.02) and L-serine (z = -2.49, P =.01) in the schizophrenic patients were significantly higher than those of controls. In addition, the percentage of D-serine in the total serine in the schizophrenic patients was significantly (z = -4.78, P<.001) lower than that of controls, suggesting that the activity of serine racemase, an enzyme catalyzing the formation of D-serine from L-serine, may have been reduced in the schizophrenic patients. CONCLUSIONS: Reduced levels of D-serine may play a role in the pathophysiology of schizophrenia, and serum D- and L-serine levels might provide a measurable biological marker for schizophrenia.     

Reduced D-serine to total serine ratio in the cerebrospinal fluid of drug naive schizophrenic patients

Several lines of evidence suggest that D-serine, an endogenous agonist of the glycine site on the NMDA receptors, might play a role in the pathophysiology of schizophrenia. The purpose of this study was to determine whether levels of D- and L-serine or D-serine ratio (D-serine/total serine) in cerebrospinal fluid (CSF) were altered in first episode and drug-naive schizophrenic patients. The CSF levels of D- and L-serine in 25 male first episode and drug-naive schizophrenic patients and 17 age-matched male healthy subjects were measured using a column-switching high performance liquid chromatography system. The percentage of D-serine in the total serine of patients was significantly (z = -2.01, p = 0.044) lower than that of controls. This study suggests that synthetic or metabolic pathways of D-serine may be abnormal in the brain of drug-naive schizophrenic patients, supporting the NMDA receptor dysfunction hypothesis of schizophrenia.     

Increased serum levels of glutamate in adult patients with autism

BACKGROUND: Precise mechanisms underlying the pathophysiology of autism are currently unknown. Given the major role of glutamate in brain development, we have hypothesized that glutamatergic neurotransmission plays a role in the pathophysiology of autism. In this study, we studied whether amino acids (glutamate, glutamine, glycine, D-serine, and L-serine) related to glutamatergic neurotransmission are altered in serum of adult patients with autism. METHODS: We measured serum levels of amino acids in 18 male adult patients with autism and age-matched 19 male healthy subjects using high-performance liquid chromatography. RESULTS: Serum levels (mean = 89.2 microM, S.D. = 21.5) of glutamate in the patients with autism were significantly (t = -4.48, df = 35, p < 0.001) higher than those (mean = 61.1 microM, S.D. = 16.5) of normal controls. In contrast, serum levels of other amino acids (glutamine, glycine, d-serine, l-serine) in the patients with autism did not differ from those of normal controls. There was a positive correlation (r = 0.523, p = 0.026) between serum glutamate levels and Autism Diagnostic Interview-Revised (ADI-R) social scores in patients. CONCLUSIONS: The present study suggests that an abnormality in glutamatergic neurotransmission may play a role in the pathophysiology of autism.     

A CSF and postmortem brain study of D-serine metabolic parameters in schizophrenia

Clinical trials demonstrated that D-serine administration improves schizophrenia symptoms, raising the possibility that altered levels of endogenous D-serine may contribute to the N-methyl D-aspartate receptor hypofunction thought to play a role in the disease. We hypothesized that cerebro-spinal fluid (CSF) D-serine levels are decreased in the patients due to reduced synthesis and/or increased degradation in brain. We now monitored amino acid levels in CSF from 12 schizophrenia patients vs. 12 controls and in postmortem parietal-cortex from 15 control subjects and 15 each of schizophrenia, major-depression and bipolar patients. In addition, we monitored postmortem brain serine racemase and D-amino acid oxidase protein levels by Western-blot analysis. We found a 25% decrease in D-serine levels and D/L-serine ratio in CSF of schizophrenia patients, while parietal-cortex D-serine was unaltered. Levels of L-serine, L-glutamine and L-glutamate were unaffected. Frontal-cortex (39%) and hippocampal (21%) serine racemase protein levels and hippocampal serine racemase/D-amino acid oxidase ratio (34%) were reduced. Hippocampal D-amino-acid-oxidase protein levels significantly correlated with duration of illness (r=0.6, p=0.019) but not age. D-amino acid oxidase levels in patients with DOI>20 years were 77% significantly higher than in the other patients and controls. Our results suggest that reduced brain serine racemase and elevated D-amino acid oxidase protein levels may contribute to the lower CSF D-serine levels in schizophrenia.     

Effect of the intracerebroventricular and systemic administration of L-serine on the concentrations of D- and L-serine in several brain areas and periphery of rat

To gain further insight into the metabolic mechanism of endogenous D-serine, the effect of the intracerebroventricular and intraperitoneal administration of L-serine on the concentrations of D- and L-serine in several brain areas and periphery was investigated. The intracerebroventricular injection of L-serine caused a rapid and marked increase in the L-serine levels in almost all brain regions of adult rats. This administration also produced a gradual increase in the D-serine levels in the forebrain, whereas a slight but significant elevation of D-serine was found in the cerebellum and pons-medulla. The intraperitoneal administration of L-serine caused a marked increase in the L-serine levels in all brain regions of both infant and adult rats. The treatment induced a significant augmentation of the D-serine levels in all brain regions of infant rats with higher concentrations in the cerebellum and cortex, whereas no significant change was observed in the cerebellum and pons-medulla of adult rats. These in vivo observation, together with the fact that immunohistochemical studies have indicated that both D-serine and serine racemase are highly concentrated in Bergmann glia of developing cerebellum, suggest that D-serine can be synthesized not only in the forebrain but also in the hindbrain by serine racemase. Furthermore, because the drastic decline in the cerebellar D-serine level coincides well with a dramatic increase in the cerebellar D-amino acid oxidase during early postnatal development, synthesized D-serine may be metabolized by D-amino acid oxidase in the hindbrain of adult rats.     

Association of serum AACT levels and AACT signal polymorphism with late-onset Alzheimer's disease in Northern Ireland

alpha1-antichymotrypsin (AACT) is a serine protease inhibitor that has been associated with amyloid plaques in the brains of patients with Alzheimer's disease (AD). It has been reported that AACT serum levels are higher in AD patients than in age and sex matched controls. In addition, polymorphisms in the signal peptide and 5' of the AACT gene have been reported to increase the risk of developing AD. Serum AACT has also been suggested to be associated with cognitive decline in elderly subjects. Our objective was to investigate whether a relationship existed between serum AACT levels, AACT genotypes and risk for AD in a case control association study using 108 clinically well defined late onset AD cases and 108 age and sex matched controls from Northern Ireland. We also wished to determine whether higher serum AACT affected levels of cognition as had been previously reported. Serum AACT levels were found to be significantly raised in cases compared to controls (t=3.8, df=209, p<0.001). However, we detected no relationship between serum AACT levels and cognitive decline. We report allelic association of the AACT signal polymorphism with AD (chi(2)=3.70, df=1, p=0.04) but we failed to show any correlation between AACT serum levels and genotype.     

Increased midkine levels in sera from patients with Alzheimer's disease

Midkine (MK) is a heparin binding growth factor and promotes growth, survival and migration of various cells including neurons. It is also known to accumulate in senile plaques of patients with Alzheimer's disease (AD). To investigate the involvement of serum MK in the pathophysiology of AD, serum MK levels were determined in patients with AD (n=36) and age- and sex-matched healthy controls (n=32), using an enzyme-linked immunosorbent assay (ELISA). The serum MK values of the patients with AD (median 560 and interquartile range (500-755) pg/ml) were significantly (U=278.5, P=0.0003, Mann-Whitney U-test) higher than those of the controls (median 500 and interquartile range (385-520) pg/ml). Moreover, 17 patients (47.2%) had abnormally high values of more than 600 pg/ml, but no controls (0%) did. There was no correlation between serum MK level and the mini mental state examination (MMSE) score in the patients. The demonstration of elevated MK levels in sera of patients with AD may contribute toward an understanding the pathophysiology of this disease, and provide a novel potential therapeutic strategy for decreasing neuronal damages in patients with AD. We found that serum MK levels in patients with AD were increased in comparison with those of normal controls.     

Reduced serum levels of brain-derived neurotrophic factor in adult male patients with autism

BACKGROUND: The precise mechanisms underlying the pathophysiology of autism are currently unknown. Given the key role of brain-derived neurotrophic factor (BDNF) in brain development, we hypothesized that BDNF may play a role in the pathophysiology of autism. In this study, we studied whether serum levels of BDNF are altered in patients with autism. METHODS: We measured serum levels of BDNF in 18 adult male patients with autism and 18 age-matched healthy male control subjects. RESULTS: The serum levels of BDNF in patients with autism (25.6+/-2.15 ng/ml (mean+/-S.D.)) were significantly (z = -4.42, p < 0.001) lower than those of normal controls (61.6+/-10.9 ng/ml (mean+/-S.D.)). Nevertheless, we found no correlations between BDNF levels and clinical variables in autistic patients. CONCLUSIONS: This study suggests that reduced BDNF levels may play a role in the pathophysiology of autism.     

Alterations of serum levels of brain-derived neurotrophic factor (BDNF) in depressed patients with or without antidepressants.

BACKGROUND: Because researchers have reported that antidepressants increase the expression of brain-derived neurotrophic factor (BDNF) in the rat hippocampus, we investigated whether serum BDNF levels may be used as a putative biological marker for major depressive disorders (MDD). METHODS: We measured serum BDNF in the following three groups: antidepressant-naive patients with MDD (n = 16), antidepressant-treated patients with MDD (n = 17), and normal control subjects (n = 50). Patients were evaluated using the Hamilton Rating Scale for Depression (HAM-D). Serum BDNF was assayed with the sandwich ELISA method. RESULTS: We found that serum BDNF was significantly lower in the antidepressant-naive group (mean, 17.6 ng/mL; SD, 9.6) than in the treated (mean, 30.6 ng/mL; SD, 12.3; p =.001) or in the control group (mean, 27.7 ng/mL; SD, 11.4; p =.002). There was a significant negative correlation (r = -.350, z = -2.003, p =.045) between serum BDNF and HAM-D scores in all patients. In a preliminary examination, reduced BDNF values of three drug-naive patients recovered to basal levels after antidepressant treatment. CONCLUSIONS: Our study suggests that low BDNF levels may play a pivotal role in the pathophysiology of MDD and that antidepressants may increase BDNF in depressed patients.     

Meta-analysis of plasma amyloid-β levels in Alzheimer's disease

Plasma amyloid-β (Aβ) levels have been proposed as biomarkers of Alzheimer's disease (AD), but studies have produced inconsistent results. We present a meta-analytic review of cross-sectional studies that examined plasma Aβ levels in AD and cognitively normal subjects, and longitudinal studies that used baseline plasma Aβ levels to predict conversion from normal cognition to AD. Medline and EMBASE databases were searched to generate an initial list of relevant studies, and selected authors approached for additional data. Twelve cross- sectional studies (n = 1483) and seven longitudinal (n = 3920) met the inclusion criteria for meta-analysis. Random effects model was used to calculate the weighted mean difference (WMD) by Review Manager Version 4.2. In longitudinal studies, cognitively normal individuals who converted to AD had higher baseline Aβ1-40 and Aβ1-42 levels (WMD: 10.29, z = 3.80, p = 0.0001 and WMD: 8.01, z = 2.76, p = 0.006, respectively), and non-significantly increased Aβ1-42/Aβ1-40 ratio (WMD: 0.03, z = 1.65, p = 0.10). In cross sectional studies, compared to cognitively normal individuals, AD patients had marginally but non-significantly lower Aβ1-42 levels (WMD:-2.84, z = 1.73, p = 0.08), but Aβ1-40 levels were not significantly different (WMD: 3.43, z = 0.40, p = 0.69). Our systematic review suggests a model of differential longitudinal changes in plasma Aβ levels in cognitively stable individuals versus those who go on to develop AD dementia. Baseline Aβ1-40 and Aβ1-42 levels in cognitively normal elderly individuals might be predictors of higher rates of progression to AD, and should be further explored as potential biomarkers.     

Serum Progranulin Levels in Patients with Frontotemporal Lobar Degeneration and Alzheimer's Disease: Detection of GRN Mutations in a Spanish Cohort

Progranulin gene (GRN) mutations cause frontotemporal lobar degeneration (FTLD) with TDP43-positive inclusions, although its clinical phenotype is heterogeneous and includes patients classified as behavioral variant-FTLD (bvFTLD), progressive non-fluent aphasia (PNFA), corticobasal syndrome, Alzheimer's disease (AD), or Parkinson's disease (PD). Our main objective was to study if low serum progranulin protein (PGRN) levels may detect GRN mutations in a Spanish cohort of patients with FTLD or AD. Serum PGRN levels were measured in 112 subjects: 17 bvFTLD, 20 PNFA, 4 semantic dementia, 34 sporadic AD, 9 AD-PSEN1 mutation carriers, 10 presymptomatic-PSEN1 mutation carriers, and 18 control individuals. We detected 5 patients with PGRN levels below 94 ng/mL: two of them had a clinical diagnosis of bvFTLD, two of PNFA, and one of AD. The screening for GRN mutations detected two probable pathogenic mutations (p.C366fsX1 and a new mutation: p.V279GfsX5) in three patients and one mutation of unclear pathogenic nature (p.C139R) in one patient. The other patient showed a normal GRN sequence but carried a PRNP gene mutation. We observed no differences in serum PGRN levels between controls (mean = 145.5 ng/mL, SD = 28.5) and the other neurodegenerative diseases, except for the carriers of pathological GRN gene mutations (mean = 50.5 ng/mL, SD = 21.2). Null GRN mutation carriers also showed lower serum PGRN levels than the patient who was a carrier of p.C139R (92.3 ng/mL) and the one who was a carrier of the PRNP mutation (76.9 ng/mL). In conclusion, we detected GRN null mutations in patients with severely reduced serum PGRN levels, but not in patients with slightly reduced PGRN levels.     

Increased plasma levels of interleukin-1, interleukin-6 and alpha-1-antichymotrypsin in patients with Alzheimer's disease: peripheral inflammation or signals from the brain?

Plasma concentrations of interleukin-1beta (IL-1beta), interleukin-6 (IL-6), C reactive protein (CRP) and alpha-1-antichymotrypsin (ACT) in 145 patients with probable Alzheimer's disease (AD) and 51 non-demented controls were measured. To investigate the cellular activation of peripheral immune system, plasma levels of neopterin were also investigated. Plasma levels of IL-1 were detectable in 17 patients with AD (13%) and only in one control (2%) and average levels of IL-1 were higher in AD patients than in controls (p < 0.001). IL-6 plasma levels were detectable in a higher proportion of AD and controls (53% and 27%, respectively), and were increased in patients with AD (p < 0.001). Plasma levels of ACT were increased in patients with AD (p < 0.001) and CRP levels were in the normal range. Plasma levels of neopterin were slightly lower in AD patients than in controls, but differences were not statistically significant. No significant correlation was observed between IL-1 and IL-6 levels or neopterin and cytokine levels in plasma from AD patients. Plasma levels of ACT negatively correlated with cognitive performances, as assessed by the mini mental state examination (MMSE; R = -0.26, p < 0.02) and positively correlated with the global deterioration state (GDS) of AD patients (R = 0.30, p < 0.007). Present findings suggested that detectable levels of circulating cytokines and increased ACT might not be derived by activation of peripheral immune system of AD patients. Detection of these molecules might be used for monitoring the progression of brain inflammation associated with AD.     

Reduced serum BDNF levels in schizophrenic patients on clozapine or typical antipsychotics

Neurotrophic factors regulate neuronal development and synaptic plasticity, possibly playing a role in the pathophysiology of schizophrenia and other psychiatric disorders. Decreased brain-derived neurotrophic factor (BDNF) levels have been found in brains and in the serum of schizophrenic patients, but results are inconsistent. Also, clozapine may upregulate brain BDNF expression. In the present study, we assessed serum BDNF immunoreactivity in 44 schizophrenic patients (20 on clozapine and 24 on typical antipsychotics) and in 25 healthy volunteers. Serum BDNF levels were measured using an enzyme immunoassay. Healthy controls showed significantly higher levels of BDNF compared to the whole group of schizophrenic patients (p<0.001) as well as to the subgroups on typical antipsychotics and clozapine (p<0.001). Serum BDNF values for controls were 168.8+/-26.3pg/ml, for the clozapine group were 125.4+/-44.5pg/ml and for the group on typicals were 101.3+/-51.6pg/ml. BDNF values from patients on clozapine were non-significantly higher than values from patients on typical antipsychotics (p=0.09). Serum BDNF was strongly and positively correlated with clozapine dose (r=0.643; p=0.002) but not with other demographic characteristics. These results reinforce previous findings of reduced serum BDNF levels in schizophrenic patients and suggest a differential effect of clozapine compared to typical antipsychotics on such levels.     

Glial transport of the neuromodulator D-serine

D-Serine is an endogenous agonist of NMDA receptors that occurs in astrocytes in gray matter areas of the brain. D-Serine is synthesized from L-serine by the activity of a glial enriched serine racemase, but little is known on the properties of D-serine transport and factors regulating its synaptic concentration. In the present report we characterize the transport of D-serine in astrocytes. In primary astrocyte cultures, D-serine uptake is dependent on sodium ions and exhibits both low affinity and low specificity for D-serine. The kinetics of D-serine transport resembles that of ASCT type transporters as several small neutral amino acids strongly inhibit the uptake of D-serine. D-Serine fluxes are coupled to counter-movement of L-serine and to a less extent to other small neutral amino acids. Thus, addition of D-serine to cell cultures elicits robust efflux of intracellular L-serine. Conversely, physiological concentrations of L-serine induce efflux of preloaded D-serine from astrocytes. L-Serine was more effective than kainate, which have been previously shown to induce D-serine release from astrocytes upon stimulation of non-NMDA type of glutamate receptors. The features of D-serine transport we describe reveal possible new mechanisms controlling the synaptic concentration of D-serine.     

Decreased serum levels of transforming growth factor-beta1 in patients with autism

BACKGROUND: The neurobiological basis for autism remains poorly understood. Given the key role of transforming growth factor-beta1 (TGF-beta1) in brain development, we hypothesized that TGF-beta1 plays a role in the pathophysiology of autism. In this study, we studied whether serum levels of TGF-beta1 are altered in patients with autism. METHODS: We measured serum levels of TGF-beta1 in 19 male adult patients with autism and 21 age-matched male healthy subjects using enzyme-linked immunosorbent assay (ELISA). RESULTS: The serum levels (7.34+/-5.21 ng/mL (mean+/-S.D.)) of TGF-beta1 in the patients with autism were significantly (z=-5.106, p<0.001) lower than those (14.48+/-1.64 ng/mL (mean+/-S.D.)) of normal controls. However, there were no marked or significant correlations between serum TGF-beta1 levels and other clinical variables, including Autism Diagnostic Interview-Revised (ADI-R) scores, Yale-Brown Obsessive-Compulsive Scale (Y-BOCS), aggression, Theory of Mind, and Intellectual Quotient (IQ) in patients. CONCLUSIONS: These findings suggest that decreased levels of TGF-beta1 may be implicated in the pathophysiology of autism.     

Cellular origin and regulation of D- and L-serine in in vitro and in vivo models of cerebral ischemia

D-Serine is known to be essential for the activation of the N-methyl-D-aspartate (NMDA) receptor in the excitation of glutamatergic neurons, which have critical roles in long-term potentiation and memory formation. D-Serine is also thought to be involved in NMDA receptor-mediated neurotoxicity. The deletion of serine racemase (SRR), which synthesizes D-serine from L-serine, was recently reported to improve ischemic damage in mouse middle cerebral artery occlusion model. However, the cell type in which this phenomenon originates and the regulatory mechanism for D-/L-serine remain elusive. The D-/L-serine content in ischemic brain increased until 20 hours after recanalization and then leveled off gradually. The results of in vitro experiments using cultured cells suggested that D-serine is derived from neurons, while L-serine seems to be released from astroglia. Immunohistochemistry studies of brain tissue after cerebral ischemia showed that SRR is expressed in neurons, and 3-phosphoglycerate dehydrogenase (3-PGDH), which synthesizes L-serine from 3-phosphoglycerate, is located in astrocytes, supporting the results of the in vitro experiments. A western blot analysis showed that neither SRR nor 3-PGDH was upregulated after cerebral ischemia. Therefore, the increase in D-/L-serine was not related to an increase in SRR or 3-PGDH, but to an increase in the substrates of SRR and 3-PGDH.     

Serum elevated gamma glutamyltransferase levels may be a marker for oxidative stress in Alzheimer's disease

BACKGROUND: Gamma glutamyltransferase (GGT) plays a role in cellular glutathione uptake, which is an important element of antioxidant mechanisms. An increase in serum GGT is thought to be an early and sensitive marker of oxidative stress. Oxidative stress has a role in the pathogenesis of Alzheimer's disease (AD). The aim of this study was to investigate the GGT levels in AD. METHOD: In this cross-sectional study, 132 patients with AD (mean age: 74.1 +/- 7.4, female 62.9%) and 158 age- and gender-matched normal controls (mean age: 74.5 +/- 6.3, female 67.1%) were evaluated. For cognitive assessment, MMSE and clock drawing tests were performed; DSM-IV and NINCDS-ADRDA criteria were used. Serum GGT, aspartate aminotransferase, alanine aminotransferase and alkaline phosphatase concentrations were determined. RESULTS: Median (min-max) GGT levels were 18 (9-70) in AD group and 17 (5-32) in normal controls. Mann-Whitney U test showed that GGT levels were significantly higher in AD patients (p = 0.012). Linear regression analysis revealed AD was an independent correlate of elevated GGT levels. Hypertension, diabetes mellitus, total cholesterol, and low density lipoprotein cholesterol were not associated with GGT levels. CONCLUSION: GGT levels were increased significantly in AD patients. To evaluate the role of GGT as a marker of oxidative stress in AD, further studies are needed.     

Elevated serum superoxide dismutase and thiobarbituric acid reactive substances in schizophrenia: a study of patients treated with haloperidol or clozapine

There is strong evidence that oxygen free radicals may play an important role in the pathophysiology of schizophrenia. Impaired antioxidant defense and increased lipid peroxidation have been previously reported in drug-na?ve, first episode and chronically medicated schizophrenic patients using typical neuroleptics. We measured serum SOD and TBARS in two groups of chronic medicated DSM-IV schizophrenic patients, under haloperidol (n = 10) or clozapine (n = 7) and a group of healthy controls (n = 15). Serum SOD and TBARS were significantly higher (p = 0.001) in schizophrenic patients (7.1 +/- 3.0 and 3.8 +/- 0.8) than in controls (4.0 +/- 1.6 and 2.5 +/- 0.7). Among patients, serum TBARS was significantly higher (p = 0.008) in those under clozapine (4.4 +/- 0.7) than in those under haloperidol treatment (3.4 +/- 0.7). For SOD levels the difference between groups was not found. It is reasonable to argue that the difference found in TBARS levels might be due to the course of the disease, instead of medication. Further investigation on the role of oxidative tonus and lipid peroxidation in different schizophrenia subtypes and different outcome patterns in this disorder is warranted. Additionally it could also address questions concerning a possible oxidant/antioxidant imbalance in schizophrenia.