The effects of chronic hyperphenylalaninaemia on mouse brain protein synthesis can be prevented by other amino acids

Post-Translational Incorporation of L-Phenylalanine into the C-Terminus of α-Tubulin as a Possible Cause of Neuronal Dysfunction

α-Tubulin C-terminus undergoes post-translational, cyclic tyrosination/detyrosination, and L-Phenylalanine (Phe) can be incorporated in place of tyrosine. Using cultured mouse brain-derived cells and an antibody specific to Phe-tubulin, we showed that: (i) Phe incorporation into tubulin is reversible; (ii) such incorporation is not due to de novo synthesis; (iii) the proportion of modified tubulin is significant; (iv) Phe incorporation reduces cell proliferation without affecting cell viability; (v) the rate of neurite retraction declines as level of C-terminal Phe incorporation increases; (vi) this inhibitory effect of Phe on neurite retraction is blocked by the co-presence of tyrosine; (vii) microtubule dynamics is reduced when Phe-tubulin level in cells is high as a result of exogenous Phe addition and returns to normal values when Phe is removed; moreover, microtubule dynamics is also reduced when Phe-tubulin is expressed (plasmid transfection). It is known that Phe levels are greatly elevated in blood of phenylketonuria (PKU) patients. The molecular mechanism underlying the brain dysfunction characteristic of PKU is unknown. Beyond the differences between human and mouse cells, it is conceivable the possibility that Phe incorporation into tubulin is the first event (or among the initial events) in the molecular pathways leading to brain dysfunctions that characterize PKU.

Gene Therapy for the Treatment of Neurological Disorders: Metabolic Disorders

Metabolic disorders comprise a large group of heterogeneous diseases ranging from very prevalent diseases such as diabetes mellitus to rare genetic disorders like Canavan Disease. Whether either of these diseases is amendable by gene therapy depends to a large degree on the knowledge of their pathomechanism, availability of the therapeutic gene, vector selection, and availability of suitable animal models. In this book chapter, we review three metabolic disorders of the central nervous system (CNS; Canavan Disease, Niemann-Pick disease and Phenylketonuria) to give examples for primary and secondary metabolic disorders of the brain and the attempts that have been made to use adeno-associated virus (AAV) based gene therapy for treatment. Finally, we highlight commonalities and obstacles in the development of gene therapy for metabolic disorders of the CNS exemplified by those three diseases.

High dose sapropterin dihydrochloride therapy improves monoamine neurotransmitter turnover in murine phenylketonuria (PKU)

Central nervous system (CNS) deficiencies of the monoamine neurotransmitters, dopamine and serotonin, have been implicated in the pathophysiology of neuropsychiatric dysfunction in phenylketonuria (PKU). Increased brain phenylalanine concentration likely competitively inhibits the activities of tyrosine hydroxylase (TH) and tryptophan hydroxylase (TPH), the rate limiting steps in dopamine and serotonin synthesis respectively. Tetrahydrobiopterin (BH4) is a required cofactor for TH and TPH activity. Our hypothesis was that treatment of hyperphenylalaninemic Pah(enu2/enu2) mice, a model of human PKU, with sapropterin dihydrochloride, a synthetic form of BH4, would stimulate TH and TPH activities leading to improved dopamine and serotonin synthesis despite persistently elevated brain phenylalanine. Sapropterin (20, 40, or 100mg/kg body weight in 1% ascorbic acid) was administered daily for 4 days by oral gavage to Pah(enu2/enu2) mice followed by measurement of brain biopterin, phenylalanine, tyrosine, tryptophan and monoamine neurotransmitter content. A significant increase in brain biopterin content was detected only in mice that had received the highest sapropterin dose, 100mg/kg. Blood and brain phenylalanine concentrations were unchanged by sapropterin therapy. Sapropterin therapy also did not alter the absolute amounts of dopamine and serotonin in brain but was associated with increased homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA), dopamine and serotonin metabolites respectively, in both wild type and Pah(enu2/enu2) mice. Oral sapropterin therapy likely does not directly affect central nervous system monoamine synthesis in either wild type or hyperphenylalaninemic mice but may stimulate synaptic neurotransmitter release and subsequent metabolism.

Large neutral amino acids supplementation in phenylketonuric patients

Phenylketonuria is an inborn error of amino acid metabolism that results in severe mental retardation if not treated early and appropriately. The traditional treatment, consisting of a low-phenylalanine diet, is usually difficult to maintain throughout adolescence and adulthood, resulting in undesirable levels of blood phenylalanine and consequent neurotoxicity. The neurotoxicity of phenylalanine is enhanced by its transport mechanism across the blood-brain barrier, which has the highest affinity for phenylalanine compared with the other large neutral amino acids that share the same carrier. The supplementation of large neutral amino acids in phenylketonuric patients has been showing interesting results. Plasma phenylalanine levels can be reduced, which may guarantee important metabolic and clinical benefits to these patients. Although long-term studies are needed to determine the efficacy and safety of large neutral amino acids supplements, the present state of knowledge seems to recommend their prescription to all phenylketonuric adult patients who are non-compliant with the low-phenylalanine diet.

Phenylketonuria: High plasma phenylalanine decreases cerebral protein synthesis

Left untreated, phenylketonuria biochemically results in high phenylalanine concentrations in blood and tissues, and clinically especially in severe mental retardation. Treatment consists of severe dietary restriction of phenylalanine with more or less normal intellectual outcome as result when started early enough. It is unclear whether treatment for life is necessary. A clear relationship between plasma phenylalanine concentrations and cerebral outcome exists, but the precise pathophysiological mechanism is not understood. In studies in mice with phenylketonuria, the cerebral protein synthesis rate is decreased when compared to controls. The aim of the present study was to determine the protein synthesis rate in relation to the plasma phenylalanine concentrations in-vivo in patients with phenylketonuria by positron emission tomography brain studies after an intravenous l-[1-(11)C]-tyrosine bolus. Results showed a significant negative relationship (R(2)=0.40, p<0.01) between plasma phenylalanine concentration and the cerebral protein synthesis rate in 19 patients with phenylketonuria. At increased plasma phenylalanine concentrations, i.e. above 600-800micromol/l, the cerebral protein synthesis rate is clearly decreased compared to lower phenylalanine concentrations. These data suggest that cerebral protein metabolism in untreated adults with phenylketonuria can be abnormal due to high plasma phenylalanine concentrations. Hence, we speculate that it is important to continue dietary treatment into adulthood, aiming at plasma phenylalanine concentrations <600-800micromol/l.

Effect of acute tyrosine depletion in using a branched chain amino-acid mixture on dopamine neurotransmission in the rat brain

Central dopamine function is reduced by decreasing the availability of the catecholamine precursor, tyrosine, using a tyrosine-free amino acid mixture containing multiple large neutral as well as branched chain amino-acids, which compete with tyrosine for uptake into the brain. Current mixtures are cumbersome to make and administer, and unpalatable to patients and volunteers. Here, we investigate whether individual or limited amino-acid combinations could reduce brain tyrosine levels and hence dopamine function. Measurements of regional brain tyrosine levels, catecholamine and indoleamine synthesis (L-DOPA and 5-HTP accumulation, respectively) were used to identify an effective paradigm to test in neurochemical, behavioral and fos immunocytochemical models. Administration of leucine or isoleucine, or a mixture of leucine, isoleucine, and valine reduced tyrosine and 5-HTP, but not L-DOPA accumulation. A mixture of leucine, valine, and isoleucine supplemented with tryptophan reduced brain tyrosine and L-DOPA, but not 5-HTP. In microdialysis experiments this amino-acid mixture reduced basal and amphetamine-evoked striatal dopamine release, as well as amphetamine-induced hyperactivity. This mixture also reduced amphetamine-induced fos expression in striatal areas. In conclusion, the present study identified a small combination of amino acids that reduces brain tyrosine and dopamine function in a manner similar to mixtures of multiple amino acids. This minimal mixture may have use as a dopamine reducing paradigm in patient and volunteer studies.

Human LAT1 single nucleotide polymorphism N230K does not alter phenylalanine transport

The deleterious effects on the brain of phenylketonuria are caused by the saturation of the blood-brain barrier large neutral amino acid transporter type 1 (LAT1) by high plasma phenylalanine concentrations. There is only one known single nucleotide polymorphism (SNP) of the open reading frame of human LAT1, N230K. Site-directed mutagenesis of the wild type human LAT1 cDNA replicated the N230K SNP, and the corresponding cloned RNA encoding either the wild type or N230K human LAT1 were injected into frog oocytes. The kinetics of phenylalanine transport via either form of the human LAT1 was not significantly different. Similarly, there was no difference in the kinetics of phenylalanine transport via the wild type rabbit LAT1 or the corresponding K226N mutant of rabbit LAT1. These studies demonstrate that the only known SNP in the open reading frame of human LAT1 has no effect on the kinetics of large neutral amino acid transport via this carrier.

Site-directed mutagenesis of rabbit LAT1 at amino acids 219 and 234

The availability of amino acids in the brain is regulated by the blood-brain barrier (BBB) large neutral amino acid transporter type 1 (LAT1) isoform, which is characterized by a high affinity (low Km) for substrate large neutral amino acids. The hypothesis that brain amino acid transport activity can be altered with single nucleotide polymorphisms was tested in the present studies with site-directed mutagenesis of the BBB LAT1. The rabbit has a high Km LAT1 large neutral amino acid transporter, as compared to the low Km neutral amino acid transporter at the human or rat BBB. The rabbit LAT1 was cloned from a rabbit brain capillary cDNA library. Alignment of the amino acid sequences of rabbit, human, and rat LAT1 revealed two radical amino acid residues that differ in the rabbit relative to the rat or human LAT1. The G219D mutation had a modest effect on the Km and Vmax of tryptophan transport via cloned rabbit LAT1 in frog oocytes, but the W234L variant reduced the Km by 64% and the Vmax by 96%. Conversely, LAT1 transport of either tryptophan or phenylalanine was nearly normalized when the double mutation W234L/G219D variant was produced. These studies show that marked changes in the affinity and capacity of the LAT1 are caused by single nucleotide polymorphisms and that phenotype can be restored with a double mutation.

Cerebral protein synthesis in a genetic mouse model of phenylketonuria

Local rates of cerebral protein synthesis (lCPS(leu)) were measured with the quantitative autoradiographic [1-(14)C]leucine method in a genetic mouse model (Pah(enu2)) of phenylketonuria. As in the human disease, Pah(enu2) mice have a mutation in the gene for phenylalanine hydroxylase. We compared adult homozygous (HMZ) and heterozygous (HTZ) Pah(enu2) mice with the background strain (BTBR). Arterial plasma concentrations of phenylalanine (Phe) were elevated in both HMZ and HTZ mutants by 21 times and 38%, respectively. In the total acid-soluble pool in brain concentrations of Phe were higher and other neutral amino acids lower in HMZ mice compared with either HTZ or BTBR mice indicating a partial saturation of the l-amino acid carrier at the blood brain barrier by the elevated plasma Phe concentrations. In a series of steady-state experiments, the contribution of leucine from the arterial plasma to the tRNA-bound pool in brain was found to be statistically significantly reduced in HMZ mice compared with the other groups, indicating that a greater fraction of leucine in the precursor pool for protein synthesis is derived from protein degradation. We found reductions in lCPS(leu) of about 20% throughout the brain in the HMZ mice compared with the other two groups, but no reductions in brain concentrations of tRNA-bound neutral amino acids. Our results in the mouse model suggest that in untreated phenylketonuria in adults, the partial saturation of the l-amino acid transporter at the blood-brain barrier may not underlie a reduction in cerebral protein synthesis.

Large neutral amino acids block phenylalanine transport into brain tissue in patients with phenylketonuria

Large neutral amino acids (LNAAs), including phenylalanine (Phe), compete for transport across the blood-brain barrier (BBB) via the L-type amino acid carrier. Accordingly, elevated plasma Phe impairs brain uptake of other LNAAs in patients with phenylketonuria (PKU). Direct effects of elevated brain Phe and depleted LNAAs are probably major causes for disturbed brain development and function in PKU. Competition for the carrier might conversely be put to use to lower Phe influx when the plasma concentrations of all other LNAAs are increased. This hypothesis was tested by measuring brain Phe in patients with PKU by quantitative 1H magnetic resonance spectroscopy during an oral Phe challenge with and without additional supplementation with all other LNAAs. Baseline plasma Phe was approximately 1,000 micromol/l and brain Phe was approximately 250 micromol/l in both series. Without LNAA supplementation, brain Phe increased to approximately 400 micromol/l after the oral Phe load. Electroencephalogram (EEG) spectral analysis revealed acutely disturbed brain activity. With concurrent LNAA supplementation, Phe influx was completely blocked and there was no slowing of EEG activity. These results are relevant for further characterization of the LNAA carrier and of the pathophysiology underlying brain dysfunction in PKU and for treatment of patients with PKU, as brain function might be improved by continued LNAA supplementation.

Effects of dietary mixtures of amino acids on fetal growth and maternal and fetal amino acid pools in experimental maternal phenylketonuria

BACKGROUND

Branched-chain amino acids have been reported to improve fetal brain development in a rat model in which maternal phenylketonuria (PKU) is induced by the inclusion of an inhibitor of phenylalanine hydroxylase, DL-p-chlorophenylalanine, and L-phenylalanine in the diet.

OBJECTIVE

We studied whether a dietary mixture of several large neutral amino acids (LNAAs) would improve fetal brain growth and normalize the fetal brain amino acid profile in a rat model of maternal PKU induced by DL-alpha-methylphenylalanine (AMPhe).

DESIGN

Long-Evans rats were fed a basal diet or a similar diet containing 0.5% AMPhe + 3.0% L-phenylalanine (AMPhe + Phe diet) from day 11 until day 20 of gestation in experiments to test various mixtures of LNAAs. Maternal weight gains and food intakes to day 20, fetal body and brain weights at day 20, and fetal brain and fetal and maternal plasma amino acid concentrations at day 20 were measured.

RESULTS

Concentrations of phenylalanine and tyrosine in fetal brain and in maternal and fetal plasma were higher and fetal brain weights were lower in rats fed the AMPhe + Phe diet than in rats fed the basal diet. However, fetal brain growth was higher and concentrations of phenylalanine and tyrosine in fetal brain and in maternal and fetal plasma were lower in rats fed the AMPhe + Phe diet plus LNAAs than in rats fed the diet containing AMPhe + Phe alone.

CONCLUSION

LNAA supplementation of the diet improved fetal amino acid profiles and alleviated most, but not all, of the depression in fetal brain growth observed in this model of maternal PKU.

Blood-brain barrier carrier-mediated transport and brain metabolism of amino acids

The transport of neutral amino acids through the brain capillary endothelial wall, which makes up the blood-brain barrier (BBB) in vivo, is an important control point for the overall regulation of cerebral metabolism, including protein synthesis and neurotransmitter production. The Michaelis-Menten kinetics of BBB amino acid transport have been investigated in vivo with the brain uptake index (BUI) technique, and in vitro with the isolated human brain capillary preparation. The only amino acid that is albumin-bound is tryptophan, and the majority of albumin-bound tryptophan in the plasma is available for transport through the BBB via an enhanced dissociation mechanism that operates at the surface of the brain capillary endothelium. The availability in brain of amino acids is predicted from the BBB Km values to be sharply influenced by supra-physiological concentrations of phenyalanine in the 200-500 microM range. Moreover, the measurement of cerebral protein synthesis with an internal carotid artery perfusion technique and HPLC-based measurements of aminoacyl-transfer RNA specific activities shows an inverse relationship between cerebral protein synthesis and plasma phenyalanine concentrations in the 200-500 microM range. These findings indicate the neurotoxicity of hyperphenylalninemia is not restricted to the phenylketonuria range of approximately 2000 microM, but is exerted in the supra-physiological range of 200-500 microM.

The effect of elevated plasma phenylalanine levels on protein synthesis rates in adult rat brain

Increasing the plasma phenylalanine concentration to levels as high as 0.560-0.870 mM (over ten times normal levels) had no detectable effect on the rate of brain protein synthesis in adult rats. The average rates for 7-week-old rats were: valine, 0.58 +/- 0.05%/h, phenylalanine, 0.59 +/- 0.06%/h, and tyrosine, 0.60 +/- 0.09%/h, or 0.59 +/- 0.06%/h overall. Synthesis rates calculated on the basis of the specific activity of the tRNA-bound amino acid were slightly lower (4% lower for phenylalanine) than those based on the brain free amino acid pool. Similarly, the specific activities of valine and phenylalanine in microdialysis fluid from striatum were practically the same as those in the brain free amino acid pool. Thus the specific activities of the valine and phenylalanine brain free pools are good measures of the precursor specific activity for protein synthesis. In any event, synthesis rates, whether based on the specific activities of the amino acids in the brain free pool or those bound to tRNA, were unaffected by elevated levels of plasma phenylalanine. Brain protein synthesis rates measured after the administration of quite large doses of phenylalanine (> 1.5 mumol/g) or valine (15 mumol/g) were in agreement (0.62 +/- 0.01 and 0.65 +/- 0.01%/h respectively) with the rates determined with infusions of trace amounts of amino acids. Thus the technique of stabilizing precursor-specific activity, and pushing values in the brain close to those of the plasma, by the administration of large quantities of precursor, appears to be valid.

Phenylketonuria due to phenylalanine hydroxylase deficiency: an unfolding story. Medical Research Council Working Party on Phenylketonuria

Efficient neonatal screening for phenylketonuria and the availability of complex diets for lifelong use have virtually eliminated severe mental handicap from the disease. Nevertheless, there remains a high risk of fetal damage in offspring of women with the disease, and the possibility that the diets themselves may be harmful cannot be excluded. Search for a preventive treatment for the disease has been greatly aided by advances in molecular genetics. For example, in mice modified liver cells have been implanted, which have not only corrected the phenylalanine defect but have remained healthy for the normal life span of the animal. Overall, however, prevention and treatment have not progressed as quickly as was hoped, and research and development must be pursued vigorously to take account of contemporary perceptions of the disorder.

Golgi-Kopsch silver study of the brain of a patient with untreated phenylketonuria, seizures, and cortical blindness

This report describes the morphological changes observed in the brain of an untreated 27-year-old man with phenylketonuria, cortical blindness, and seizures. Golgi-Kopsch silver, cresyl violet, and hematoxylin and eosin stains were used to study cell structure and organization of the cerebellum, the lateral geniculate nuclei, the visual cortex, frontal cortex, and hippocampus. Extensive neuronal losses occurred in the right lateral geniculate nucleus (LGN), the visual cortex, and hippocampus. The left LGN, cerebellum, and frontal cortex retained neuronal components; there was a reduction in the number of dendritic processes on the Purkinje cells of the PKU subject. The loss of neurons in the LGN and occipital cortex is related to the blindness and the neuronal loss in the hippocampus is related to seizure activity.

Decreases in brain protein synthesis elicited by moderate increases in plasma phenylalanine

Large increases in plasma phenylalanine concentration (greater than 1 mM) adversely affect brain function and inhibit cerebral protein synthesis. The threshold hypothesis predicts that moderate increases in plasma phenylalanine concentrations below a 1 mM threshold have no adverse effects on brain function or protein synthesis. Using a new in situ internal carotid artery perfusion technique and measurement of the amino acyl-tRNA specific activities by high performance liquid chromatography (HPLC) separation of phenylisothiocyanate (PITC) derivatized amino acids, the present studies demonstrate a linear decrease in cerebral protein synthesis in vivo in proportion to moderate increases in plasma phenylalanine. A 50% inhibition of brain protein synthesis in vivo is observed at a plasma phenylalanine concentration of approximately 0.40 mM. Since moderate increases in plasma phenylalanine concentrations may be achieved in humans with ingestion of phenylalanine, the present findings should be considered in evaluating the safety of liberal and selective increases in dietary phenylalanine.

Tryptophan distribution and metabolism in experimental chronic renal insufficiency

Several aspects of tryptophan distribution and metabolism in chronic renal insufficiency (CRI) were investigated in a rat model prepared by partial nephrectomy. Partially nephrectomized (pNx) rats with a moderate degree of CRI demonstrated a 40% decrease in plasma total tryptophan concentration between 5 and 11 weeks after the acute reduction of renal functional mass. This decrease was accompanied by hypoalbuminemia, polyuria, albuminuria, and tryptophanuria. After 5 weeks of sustained plasma total tryptophan deficiency (from Week 6 to Week 11), the plasma free tryptophan concentration, the plasma concentrations of large neutral amino acids, and the tryptophan levels in red cells, liver, and kidney of the pNx rats were similar to those of the controls. However, evidence for abnormal brain tryptophan metabolism in pNx rats after 11 weeks of CRI included 16% reductions of tryptophan levels in the midbrain and pons and 65% increases in the serotonin contents of the hypothalamus and medulla. Monoamine oxidase activities in hypothalamus and cerebellum of pNx rats were the same as those of the controls. These studies indicate that tryptophanuria is an important factor in the development of the plasma tryptophan deficiency in the pNx model. In addition, the results support the hypothesis that regional abnormalities in tryptophan metabolism contribute to the neurological and neuroendocrine dysfunction of CRI.

Clofibric acid, phenylpyruvate, and dichloroacetate inhibition of branched-chain alpha-ketoacid dehydrogenase kinase in vitro and in perfused rat heart

Branched-chain alpha-ketoacid dehydrogenase kinase, purified from rabbit liver, was inhibited by clofibric acid, phenylpyruvate, and dichloroacetate in a mixed manner relative to ATP. I40 values relative to 75 microM ATP were 0.33, 1.7, and 3.0 mM, respectively. Inhibition of the kinase by acetate, pyruvate, and lactate was minimal; whereas a p-hydroxyphenyl substitution of these compounds increased their potency as kinase inhibitors, a phenyl substitution gave the most potent inhibitors. Clofibric acid, phenylpyruvate, and dichloroacetate activated branched-chain alpha-ketoacid dehydrogenase in perfused rat hearts. Perfusate concentrations that gave 50% activation (A50) were 0.1, 0.32, and 0.63 mM, respectively. A50 concentrations of clofibric acid and phenylpyruvate also increased flux (decarboxylation of alpha-keto[1-14C]isovalerate) through branched-chain alpha-ketoacid dehydrogenase in perfused rat heart. These findings suggest that, although clofibric acid and phenylpyruvate can inhibit substrate utilization by the branched-chain alpha-ketoacid dehydrogenase complex, the major effect of these compounds on branched-chain amino acid metabolism is due to inhibition of branched-chain alpha-ketoacid dehydrogenase kinase with subsequent activation of and increased flux through the complex.

Inhibition of protein and aminoacyl-tRNA synthesis, and binding and transport sites for aromatic amino acids in the brain in vitro with aromatic acids

The influx of [3H]phenylalanine, [3H]tyrosine and [3H]tryptophan into brain slices and synaptosomes, their binding to synaptic membranes and their incorporation into protein and aminoacyl-tRNA were studied in the presence of an excess of a second aromatic amino acid or some other aromatic acid, viz., phenylpyruvate, phenyllactate, phenylacetate, homogentisate, salicylate or benzoate. The influx into brain slices was strongly inhibited by a second aromatic amino acid and in general also by phenylpyruvate and homogentisate, but the effects of these substances upon the influx into synaptosomes were slight. The binding of phenylalanine and tyrosine to the synaptic membranes was affected mainly by phenylpyruvate and homogentisate, and these were also effective in preventing the formation of aminoacyl-tRNA, and thus apparently inhibited the biosynthesis of proteins and polyphenylalanine. In all cases phenyllactate, phenylacetate salicylate and benzoate had virtually no effect. Phenylalanine seemed to be a noncompetitive, and tyrosine a competitive inhibitor, while tryptophan had both properties, as was also the case with phenylpyruvate and homogentisate. Under phenylketonuric conditions high excesses of phenylalanine and phenylpyruvate, and also certain other aromatic compounds, seemed to occupy the cellular transport sites for amino acids on the cellular membranes and prevent the formation of aminoacyl-tRNAs, thus inhibiting brain protein synthesis. The reduced supply of intracellular amino acids and the inhibition of protein synthesis may constitute one reason for the development of biochemical phenylketonuric abnormalities.