Differential effects of low-phenylalanine protein sources on brain neurotransmitters and behavior in C57Bl/6-Pahenu2 mice

Abstract

Phenylketonuria (PKU) is an inborn error of metabolism caused by a deficiency of the enzyme phenylalanine hydroxylase, which metabolizes phenylalanine (phe) to tyrosine. A low-phe diet plus amino acid (AA) formula is necessary to prevent cognitive impairment; glycomacropeptide (GMP) contains minimal phe and provides a palatable alternative to the AA formula. Our objective was to assess neurotransmitter concentrations in brain and the behavioral phenotype of PKU mice (Pahenu2 on the C57Bl/6 background) and how this is affected by low-phe protein sources. Wild type (WT) and PKU mice, both male and female, were fed high-phe casein, low-phe AA, or low-phe GMP diets between 3–18 weeks of age. Behavioral phenotype was assessed using the open field and marble burying tests, and brain neurotransmitter concentration measured using HPLC with electrochemical detection system. Data were analyzed by 3-way ANOVA with genotype, sex, and diet as the main treatment effects. Brain mass and the concentrations of catecholamines and serotonin were reduced in PKU mice compared to WT mice; the low-phe AA and GMP diets improved these parameters in PKU mice. Relative brain mass was increased in female PKU mice fed the GMP diet compared to the AA diet. PKU mice exhibited hyperactivity and impaired vertical exploration compared to their WT littermates during the open field test. Regardless of genotype or diet, female mice demonstrated increased vertical activity time and increased total ambulatory and horizontal activity counts compared with male mice. PKU mice fed the high-phe casein diet buried significantly fewer marbles than WT control mice fed casein; this was normalized in PKU mice fed the low-phe AA and GMP diets. In summary, C57Bl/6-Pahenu2 mice showed an impaired behavioral phenotype and reduced brain neurotransmitter concentrations that were improved by the low-phe AA or GMP diets. These data support lifelong adherence to a low-phe diet for PKU.

The Behavioral Consequence of Phenylketonuria in Mice Depends on the Genetic Background

“…Both strains showed a vast increase in Phe levels and disrupted serotonin and norepinephrine levels in the brain (for norepinephrine a trend was observed for BTBR statistically) which is in accordance with previous studies (Andolina et al, 2011; Pascucci et al, 2013; Sawin et al, 2014; van Vliet et al, 2015). How these PKU-related changes can result in a different functional outcome is not clear.…”

Voluntary Exercise Prevents Oxidative Stress in the Brain of Phenylketonuria Mice

“…As shown before (Ney et al 2008;Solverson et al 2012;Sawin et al 2014), Phe levels in plasma and brain of SedPKU mice were higher in comparison to respective levels in SedWT group (p < 0.001). Exercise did not modify Phe levels when comparing each Exe group to its Sed control (Fig.…”

Phenylketonuria Pathophysiology: on the Role of Metabolic Alterations

“…Recently, it has also been demonstrated that these patients are more susceptible to neurological symptoms caused by cerebral dopamine deficiency, such as Parkinsonism [2]. Indeed, several studies have shown that high Phe concentrations is associated with decreased serotonin, dopamine, and norepinephrine levels in human and murine PKU [48][49][50][51][52]. The decrease of these neurotransmitters levels could be related to the effect of high Phe concentration on amino acids transport through the blood-brain barrier (BBB) (such as Tyr and tryptophan -Trp) or on enzymes involved in neurotransmitters synthesis [48].…”

“…The decrease of these neurotransmitters levels could be related to the effect of high Phe concentration on amino acids transport through the blood-brain barrier (BBB) (such as Tyr and tryptophan -Trp) or on enzymes involved in neurotransmitters synthesis [48]. It is important to stress out that large neutral amino acid (LNNA) transporter has high affinity for Phe, which competes with other amino acids to cross BBB [53], consequently reducing the amount of Trp and Tyr available for neurotransmitter synthesis [52,54,55]. Phe is also a competitive inhibitor of Tyr hydroxylase (EC # 1.14.16.2) and Trp hydroxylase (EC # 1.14.16.4), important enzymes for the brain synthesis of the neurotransmitters dopamine and serotonin, respectively [56][57][58][59][60].…”