Decreased tyrosine transport in fibroblasts from schizophrenics: implications for membrane pathology

Clozapine but not lithium reverses aberrant tyrosine uptake in patients with bipolar disorder

RATIONALE

Availability of the dopamine and noradrenaline precursor tyrosine is critical for normal functioning, and deficit in tyrosine transport across cell membrane and the blood-brain barrier has been reported in bipolar disorder and schizophrenia. Clozapine and lithium are two psychoactive agents used to treat psychosis, mood disorders and suicidal behavior, but their mechanism of action remains largely unknown.

OBJECTIVE

To characterize immediate and delayed differences in tyrosine uptake between healthy controls (HC) and bipolar patients (BP) and see if these differences could be normalized by either clozapine, lithium or both. A second objective was to see if clozapine and lithium have additive, antagonistic or synergistic effects in this.

METHOD

Fibroblasts from five HC and five BP were incubated for 5 min or 6 h with clozapine, lithium, or combination of both. Radioactive labelled tyrosine was used to quantify tyrosine membrane transport.

RESULTS

There was significantly reduced tyrosine uptake at baseline in BP compared to HC, a deficit that grew with increasing incubation time. Clozapine selectively increased tyrosine uptake in BP and abolished the deficit seen under baseline conditions, while lithium had no such effect. Combination treatment with clozapine and lithium was less effective than when clozapine was used alone.

CONCLUSIONS

There was significant deficit in tyrosine transport in BP compared to HC that was reversed by clozapine but not lithium. Clozapine was more effective when used alone than when added together with lithium. Potential clinical implications of this will be discussed.

Genetic and Functional Study of L-Type Amino Acid Transporter 1 in Schizophrenia

Schizophrenia involves neural catecholaminergic dysregulation. Tyrosine is the precursor of catecholamines, and its major transporter, according to studies on fibroblasts, in the brain is the L-type amino acid transporter 1 (LAT1). The present study assessed haplotype tag single-nucleotide polymorphisms (SNPs) of the SLC7A5/LAT1 gene in 315 patients with psychosis within the schizophrenia spectrum and 233 healthy controls to investigate genetic vulnerability to the disorder as well as genetic relationships to homovanillic acid (HVA) and 3-methoxy-4-hydroxyphenylglycol (MHPG), the major catecholamine metabolites in the cerebrospinal fluid (CSF). Moreover, the involvement of the different isoforms of the system L in tyrosine uptake and LAT1 tyrosine kinetics were studied in fibroblast cell lines of 10 patients with schizophrenia and 10 healthy controls. The results provide suggestive evidence of individual vulnerability to schizophrenia related to the LAT1 SNP rs9936204 genotype. A number of SNPs were nominally associated with CSF HVA and MHPG concentrations but did not survive correction for multiple testing. The LAT1 isoform was confirmed as the major tyrosine transporter in patients with schizophrenia. However, the kinetic parameters (maximal transport capacity, affinity of the binding sites, and diffusion constant of tyrosine transport through the LAT1 isoform) did not differ between patients with schizophrenia and controls. The present genetic findings call for independent replication in larger samples, while the functional study seems to exclude a role of LAT1 in the aberrant transport of tyrosine in fibroblasts of patients with schizophrenia.

Human dermal fibroblasts in psychiatry research

In order to decipher the disease etiology, progression and treatment of multifactorial human brain diseases we utilize a host of different experimental models. Recently, patient-derived human dermal fibroblast (HDF) cultures have re-emerged as promising in vitro functional system for examining various cellular, molecular, metabolic and (patho)physiological states and traits of psychiatric disorders. HDF studies serve as a powerful complement to postmortem and animal studies, and often appear to be informative about the altered homeostasis in neural tissue. Studies of HDFs from patients with schizophrenia (SZ), depression, bipolar disorder (BD), autism, attention deficit and hyperactivity disorder and other psychiatric disorders have significantly advanced our understanding of these devastating diseases. These reports unequivocally prove that signal transduction, redox homeostasis, circadian rhythms and gene*environment (G*E) interactions are all amenable for assessment by the HDF model. Furthermore, the reported findings suggest that this underutilized patient biomaterial, combined with modern molecular biology techniques, may have both diagnostic and prognostic value, including prediction of response to therapeutic agents.

A simplified method to quantify dysregulated tyrosine transport in schizophrenia

BACKGROUND

Schizophrenia is associated with altered tyrosine transport across plasma membranes. This is typically demonstrated by measuring the uptake of radiolabeled tyrosine in cultured human fibroblasts. Our primary goal was to determine whether tyrosine uptake could be characterized using unlabeled tyrosine. A secondary goal was to assess the effect of antipsychotic drugs added during the incubation.

METHOD

Epithelium-derived fibroblast cultures were generated from patients with schizophrenia (n=6) and age-matched controls (n=6). Cells between cycles 8-12 were exposed to an amino acid free medium for 60min and then for 1min to media containing tyrosine (0.008-1.0mM). Amino acid levels were measured and Michaelis-Menten parameters determined. Uptake of tyrosine (0.5mM) was also measured in control cells after antipsychotic drugs were introduced during the depletion or uptake phases.

RESULTS

Tyrosine uptake was sodium-independent. The maximal transport velocity (Vmax) was significantly lower in patients with schizophrenia than in controls (p<0.01). The transporter affinity (Km) did not differ between the groups. Tyrosine uptake was differentially affected (p<0.001) by inclusion of 10(-4)M haloperidol, chlorpromazine or clozapine during different periods of incubation.

CONCLUSION

Dysregulated tyrosine kinetics in schizophrenia can be readily studied without the use of radiolabeled tracers. The data also indicate that tyrosine uptake may be subject to complex pharmacological effects.

A comparative genomic study in schizophrenic and in bipolar disorder patients, based on microarray expression profiling meta-analysis

Schizophrenia affecting almost 1% and bipolar disorder affecting almost 3%–5% of the global population constitute two severe mental disorders. The catecholaminergic and the serotonergic pathways have been proved to play an important role in the development of schizophrenia, bipolar disorder, and other related psychiatric disorders. The aim of the study was to perform and interpret the results of a comparative genomic profiling study in schizophrenic patients as well as in healthy controls and in patients with bipolar disorder and try to relate and integrate our results with an aberrant amino acid transport through cell membranes. In particular we have focused on genes and mechanisms involved in amino acid transport through cell membranes from whole genome expression profiling data. We performed bioinformatic analysis on raw data derived from four different published studies. In two studies postmortem samples from prefrontal cortices, derived from patients with bipolar disorder, schizophrenia, and control subjects, have been used. In another study we used samples from postmortem orbitofrontal cortex of bipolar subjects while the final study was performed based on raw data from a gene expression profiling dataset in the postmortem superior temporal cortex of schizophrenics. The data were downloaded from NCBI's GEO datasets.

Altered tryptophan and alanine transport in fibroblasts from boys with attention-deficit/hyperactivity disorder (ADHD): an in vitro study

Background

The catecholaminergic and serotonergic neurotransmitter systems are implicated in the pathophysiology of attention-deficit/hyperactivity disorder (ADHD). The amino acid tyrosine is the precursor for synthesis of the catecholamines dopamine and norepinephrine, while tryptophan is the precursor of serotonin. A disturbed transport of tyrosine, as well as other amino acids, has been found in a number of other psychiatric disorders, such as schizophrenia, bipolar disorder and autism, when using the fibroblast cell model. Hence, the aim of this study was to explore whether children with ADHD may have disturbed amino acid transport.

Methods

Fibroblast cells were cultured from skin biopsies obtained from 14 boys diagnosed with ADHD and from 13 matching boys without a diagnosis of a developmental disorder. Transport of the amino acids tyrosine, tryptophan and alanine across the cell membrane was measured by the cluster tray method. The kinetic parameters, maximal transport capacity (V_max) and affinity constant (K_m) were determined. Any difference between the two groups was analyzed by Student's unpaired t-test or the Mann Whitney U test.

Results

The ADHD group had significantly decreased V_max (p = 0.039) and K_m (increased affinity) (p = 0.010) of tryptophan transport in comparison to controls. They also had a significantly higher V_maxof alanine transport (p = 0.031), but the Km of alanine transport did not differ significantly. There were no significant differences in any of the kinetic parameters regarding tyrosine transport in fibroblasts for the ADHD group.

Conclusions

Tryptophan uses the same transport systems in both fibroblasts and at the blood brain barrier (BBB). Hence, a decreased transport capacity of tryptophan implies that less tryptophan is being transported across the BBB in the ADHD group. This could lead to deficient serotonin access in the brain that might cause disturbances in both the serotonergic and the catecholaminergic neurotransmitter systems, since these systems are highly interconnected. The physiological importance of an elevated transport capacity of alanine to the brain is not known to date.

Tryptophan transport in human fibroblast cells-a functional characterization

There are indications that serotonergic neurotransmission is disturbed in several psychiatric disorders. One explanation may be disturbed transport of tryptophan (precursor for serotonin synthesis) across cell membranes. Human fibroblast cells offer an advantageous model to study the transport of amino acids across cell membranes, since they are easy to propagate and the environmental factors can be controlled. The aim of this study was to functionally characterize tryptophan transport and to identify the main transporters of tryptophan in fibroblast cell lines from healthy controls.

Tryptophan kinetic parameters (V_max and K_m) at low and high concentrations were measured in fibroblasts using the cluster tray method. Uptake of ³H (5)-L-tryptophan at different concentrations in the presence and absence of excess concentrations of inhibitors or combinations of inhibitors of amino acid transporters were also measured. Tryptophan transport at high concentration (0.5 mM) had low affinity and high V_max and the LAT1 isoform of system-L was responsible for approximately 40% of the total uptake of tryptophan. In comparison, tryptophan transport at low concentration (50 nM) had higher affinity, lower V_max and approximately 80% of tryptophan uptake was transported by system-L with LAT1 as the major isoform. The uptake of tryptophan at the low concentration was mainly sodium (Na⁺) dependent, while uptake at high substrate concentration was mainly Na⁺ independent. A series of different transporter inhibitors had varying inhibitory effects on tryptophan uptake.

This study indicates that tryptophan is transported by multiple transporters that are active at different substrate concentrations in human fibroblast cells. The tryptophan transport trough system-L was mainly facilitated by the LAT1 isoform, at both low and high substrate concentrations of tryptophan.

Aberrant tyrosine transport across the fibroblast membrane in patients with schizophrenia--indications of maternal inheritance

BACKGROUND

In previous studies of the present patients with schizophrenia, aberrant tyrosine transport across the fibroblast membrane was found. A low K(m), a kinetic factor indicating high affinity between tyrosine and the binding site at the cell membrane, was found to be associated with poor cognitive functions in patients. The present study aimed at investigating possible relationships between patients with schizophrenia and their first-degree relatives in aberrant tyrosine transport indicating that it may be a biological marker for the genetic susceptibility.

METHODS

Thirty-three parents, 13 fathers and 20 mothers, from 23 families with a schizophrenic patient agreed to enter the study. They underwent skin biopsies for fibroblast cultivation, neuropsychological and psychiatric investigations and were classified as family history positive or negative. Tyrosine transport kinetics (K(m) and V(max)) were calculated from in vitro trials of gradients of extracellular tyrosine concentrations in fibroblast cultures.

RESULTS

An association between patients with schizophrenia and their mothers were found for a low K(m) indicating maternal inheritance. Mothers displaying a low K(m) performed worse on the neuropsychological tests compared to mothers with normal K(m). Corresponding relationships between a low K(m) and neurocognitive dysfunction had previously been found for the patients.

CONCLUSIONS

An aberrant tyrosine transport across plasma membrane may constitute a biological marker for an endophenotype within the schizophrenia spectrum with low cognitive functioning. A plausible mode for genetic transmission is maternal inheritance.

Aberrant amino acid transport in fibroblasts from patients with bipolar disorder

Aberrant tyrosine transport is a repeated finding in fibroblasts from schizophrenic patients. The transport aberration could lead to disturbances in the dopaminergic and noradrenergic neurotransmitter systems. Tyrosine and tryptophan are the precursors of the neurotransmitters dopamine and serotonin. Disturbed dopaminergic, noradrenergic and serotoninergic systems are implicated as causes of bipolar disorder. Hence, the aim of this study was to explore whether patients with bipolar disorder have an aberrant transport of tyrosine and/or tryptophan. Fibroblast cell lines from patients with bipolar type-1 disorder (n=10) and healthy controls (n=10) were included in this study. All patients fulfilled the DSM-IV diagnostic criteria. The transport of amino acids across the cell membranes was measured by the cluster tray method. The kinetic parameters, maximal transport velocity (V(max)) and affinity constant (K(m)) were determined. A significantly lower V(max) for tyrosine (p=0.027) was found in patients with bipolar type-1 disorder in comparison to healthy controls. No significant differences in K(m) for tyrosine and in the kinetic parameters of tryptophan between patients with bipolar type-1 disorder and healthy controls were observed. The decreased tyrosine transport (low V(max)) found in this study may indicate less access of dopamine in the brain, resulting in disturbed dopaminergic and/or noradrenergic neurotransmission, that secondarily could lead to disturbances in other central neurotransmitter systems, such as the serotoninergic system. However, as sample size was small in this study and an age difference between patients and controls existed, the present findings should be considered as pilot data. Further studies with larger sample number are needed to elucidate the transport aberration and the significance of these findings.

Functional characterization of tyrosine transport in fibroblast cells from healthy controls

Human fibroblast cells are an advantageous model to study the transport of amino acids across cell membranes, since one can control the environmental factors. A major problem in all earlier studies is the lack of precise and detailed knowledge regarding the expression and functionality of tyrosine transporters in human fibroblasts. This motivated us to perform a systematic functional characterization of the tyrosine transport in fibroblast cells with respect to the isoforms of system-L (LAT1, LAT2, LAT3, LAT4), which is the major transporter of tyrosine. Ten (n=10) fibroblast cell lines from healthy volunteers were included in the study. Uptake of L-[U-14C] tyrosine in fibroblasts was measured using the cluster tray method in the presence and absence of excess concentrations of various combinations of inhibitors. This study demonstrated that LAT1 is involved in 90% of total uptake of tyrosine and also around 51% of alanine. Not more than 10% can be accounted for by LAT2, LAT3 and LAT4 isoforms. LAT2 seems to be functionally weak in uptake of tyrosine while LAT3 and LAT4 contributed around 7%. 10% could be contributed by system-A (ATA2 isoform). Alanine consequently inhibited the tyrosine transport by up to 60%. Tyrosine transport through the LAT1 isoform has a higher affinity compared to system-L. In conclusion, the LAT1 isoform is the major transporter of tyrosine in human fibroblast cells. Competition between tyrosine and alanine for transport is shown to exist, probably between LAT1 and LAT2 isoforms. This study established fibroblast cells as a suitable experimental model for studying amino acid transport defects in humans.

Tyrosine depletion lowers dopamine synthesis and desipramine-induced prefrontal cortex catecholamine levels

The relationship between limited tyrosine availability, DA (dopamine) synthesis and DA levels in the medial prefrontal cortex (MPFC) of the rat was examined by in vivo microdialysis. We administered a tyrosine- and phenylalanine-free mixture of large neutral amino acids (LNAA-) IP to lower brain tyrosine, and the norepinephrine transporter inhibitor desipramine (DMI) 10 mg/kg IP to raise MPFC DA levels without affecting DA synthesis. For examination of DOPA levels, NSD-1015 20 microM was included in perfusate. Neither NSD-1015 nor DMI affected tyrosine levels. LNAA- lowered tyrosine levels by 45%, and lowered DOPA levels as well; this was not additionally affected by concurrent DMI 10 mg/kg IP. In parallel studies DMI markedly increased extracellular levels of DA (420% baseline) and norepinephrine (NE) (864% baseline). LNAA- had no effect on baseline levels of DA or NE but robustly lowered DMI-induced DA (176% baseline) as well as NE (237% baseline) levels. Even when DMI (20 microM) was administered in perfusate, LNAA- still lowered DMI-induced DA and NE levels. We conclude that while baseline mesocortical DA synthesis is indeed dependent on tyrosine availability, the MPFC maintains normal extracellular DA and NA levels in the face of moderately lower DA synthesis. During other than baseline conditions, however, tyrosine depletion can lower ECF DA and NE levels in MPFC. These data offer a potential mechanism linking dysregulation of tyrosine transport and cognitive deficits in schizophrenia.

Tyrosine transport in fibroblasts from healthy volunteers and patients with schizophrenia

Aberrant tyrosine transport across the fibroblast membrane, as measured by lower Vmax and/or lower Km is a repeated finding in patients with schizophrenia. The aim of this study was to investigate the importance of two major transporters, the L- and A-systems and tyrosine transport in fibroblast cell lines from patients with schizophrenia and healthy volunteers. Fibroblast cell lines, n=6 from healthy volunteers and n=6 from patients with schizophrenia, were included in the study. Uptake of [14-C] L-tyrosine in fibroblasts was measured using the cluster tray method in absence and presence of inhibitors. The uptake of tyrosine by the L-system was evaluated with the inhibitor 2-aminobicyclo heptane-2-carboxylic acid (BCH) and the A-system with the inhibitor nonmetabolized methyl-aminoisobutyric acid (MeAIB). Using [14-C] MeAIB the functionality of system A isoform 2, ATA2, was tested. BCH inhibited the uptake of tyrosine with 90%, showing that tyrosine transport in fibroblasts is mainly transported by the L-system. Not more than 10% could be contributed by the A-system. Excess of MeAIB did not influence tyrosine kinetics. Moreover, MeAIB kinetics did not differ between the patients and the controls. In conclusion, aberrant tyrosine transport observed in patients with schizophrenia is probably linked to the one of the L-systems and does not seem to involve the ATA2 transporter.

Phenylalanine hydroxylase gene in psychiatric patients: screening and functional assay of mutations

BACKGROUND

Reports relating phenylalanine kinetics and metabolism to psychiatric disorders led us to undertake the comprehensive screening of the phenylalanine hydroxylase (PAH) coding region and functional testing of discovered mutations in a sample of psychiatric patients and healthy control subjects.

METHODS

Genomic DNA from psychiatric patients and control subjects was assayed for sequence variants in all PAH coding regions and splice junctions. In vivo functional analysis of mutations was conducted by assessing the kinetics and conversion to tyrosine of a standardized phenylalanine dose and by measuring fasting pterin levels.

RESULTS

A known missense mutation was observed in a schizoaffective subject, and a novel missense mutation was discovered in four subjects with schizophrenia and one normal subject. The schizoaffective patient heterozygous for the known A403V mutation showed the lowest rate of phenylalanine kinetics and lowest conversion to tyrosine in the patient sample. The four schizophrenic patients heterozygous for the novel K274E mutation showed significantly decreased phenylalanine kinetics, reduced conversion to tyrosine, and increased synthesis of the PAH cofactor tetrahydrobiopterin compared with schizophrenic subjects without the mutation.

CONCLUSIONS

The study findings suggest that larger scale studies are warranted to test the relationship of the PAH genotype with a psychiatric phenotype.

Superoxide anion production by neutrophils derived from peripheral blood of schizophrenic patients

Evidence indicates that excess free radicals formation may occur in patients with schizophrenia. A study comparing the production of superoxide anion by peripheral blood neutrophils of 29 schizophrenic patients with that of 17 healthy volunteers detected a significant statistical increase in superoxide anion production in schizophrenic patients compared to the healthy control group. Despite the fact that oxidative mechanisms may play a role in schizophrenia, further studies are needed to define their involvement. Such studies would shed light on the etiology and pathogenesis of schizophrenia and may lead to new therapeutic approaches using antioxidants, which might partially alleviate or prevent the symptoms of schizophrenia.