Hippocampal microglia modifications in C57Bl/6 Pahenu2 and BTBR Pahenu2 phenylketonuria (PKU) mice depend on the genetic background, irrespective of disturbed sleep patterns

Clinical, genetic, and experimental research of hyperphenylalaninemia

Hyperphenylalaninemia (HPA) is the most common amino acid metabolism defect in humans. It is an autosomal-recessive disorder of the phenylalanine (Phe) metabolism, in which high Phe concentrations and low tyrosine (Tyr) concentrations in the blood cause phenylketonuria (PKU), brain dysfunction, light pigmentation and musty odor. Newborn screening data of HPA have revealed that the prevalence varies worldwide, with an average of 1:10,000. Most cases of HPA result from phenylalanine hydroxylase (PAH) deficiency, while a small number of HPA are caused by defects in the tetrahydrobiopterin (BH4) metabolism and DnaJ heat shock protein family (Hsp40) member C12 (DNAJC12) deficiency. Currently, the molecular pathophysiology of the neuropathology associated with HPA remains incompletely understood. Dietary restriction of Phe has been highly successful, although outcomes are still suboptimal and patients find it difficult to adhere to the treatment. Pharmacological treatments, such as BH4 and phenylalanine ammonia lyase, are available. Gene therapy for HPA is still in development.

High phenylalanine concentrations induce demyelination and microglial activation in mouse cerebellar organotypic slices

Phenylketonuria (PKU) is an inborn error of metabolism. Mutations in the enzyme phenylalanine hydroxylase (PAH)-encoding gene lead to a decreased metabolism of the amino acid phenylalanine (Phe). The deficiency in PAH increases Phe levels in blood and brain. Accumulation of Phe can lead to delayed development, psychiatric problems and cognitive impairment. White matter (WM) damage is a neuropathological hallmark of PKU and can be seen even in early detected and treated PKU patients. The mechanisms linking high Phe concentrations to WM abnormalities remain unclear. We tested the effects of high Phe concentrations on myelin in three in vitro models of increasing complexity: two simple cell culture models and one model that preserves local brain tissue architecture, a cerebellar organotypic slice culture prepared from postnatal day (P) 8 CD-1 mice. Various Phe concentrations (0.1–10 mM) and durations of exposure were tested. We found no toxic effect of high Phe in the cell culture models. On the contrary, the treatment promoted the maturation of oligodendrocytes, particularly at the highest, non-physiological Phe concentrations. Exposure of cerebellar organotypic slices to 2.4 mM Phe for 21 days in vitro (DIV), but not 7 or 10 DIV, resulted in a significant decrease in myelin basic protein (MBP), calbindin-stained neurites, and neurites co-stained with MBP. Following exposure to a toxic concentration of Phe, a switch to the control medium for 7 days did not lead to remyelination, while very active remyelination was seen in slices following demyelination with lysolecithin. An enhanced number of microglia, displaying an activated type morphology, was seen after exposure of the slices to 2.4 mM Phe for 10 or 21 DIV. The results suggest that prolonged exposure to high Phe concentrations can induce microglial activation preceding significant disruption of myelin.

Oral administration of phenylalanine molecularly imprinted polymer (MIP) benefits PKU mouse model

Phenylketonuria (PKU) is a rare genetic disorder caused by a defect in the metabolism of phenylalanine (Phe). Currently, the most commonly used treatment for PKU is dietary Phe restriction. Problems associated with Phe restricted diets include lack of universal availability, high treatment costs, and reduced adherence to continued treatment with age and finally the development of psychological and neurological problems in a significant proportion of patients despite early start of treatment. One possible approach to decreasing blood Phe level, is inhibition of GI tract absorption of this amino acid. We had previously shown that a Phe selective molecularly imprinted polymer was able to bind Phe in the GI tract and attenuate its plasma concentration. In this work, we used different orally administered Phe selective molecularly imprinted polymer doses in a PKU mouse model to further study the effects of this treatment on biochemical profile and cognitive function in test animals. Treatments started 21 days postnatally. After 3 weeks, brain and plasma amino acid profiles and brain monoaminergic neurotransmitter concentrations were measured. Behavioral profile was also evaluated. Treatment with 2% and 5% Phe selective molecularly imprinted polymer significantly reduced levels of blood Phe in PKU model animals (46% and 48% respectively) meanwhile levels of other amino acids remained unchanged. Brain dopamine concentrations in hippocampus was effectively restored by supplementation of Phe selective molecularly imprinted polymer. Finally, polymer treatment improved locomotor dysfunction in PKU model animals. Our data suggest that the Phe selective molecularly imprinted polymer can be a new candidate for treatment of PKU patients. Take home message: Orally administered Phenylalanine Selective Molecularly Imprinted Polymer is able to inhibit absorption of phenylalanine from the GI tract and may offer a new treatment, in conjunction with dietary restriction, for PKU patients.

Phenylketonuria

Phenylketonuria (PKU; also known as phenylalanine hydroxylase (PAH) deficiency) is an autosomal recessive disorder of phenylalanine metabolism, in which especially high phenylalanine concentrations cause brain dysfunction. If untreated, this brain dysfunction results in severe intellectual disability, epilepsy and behavioural problems. The prevalence varies worldwide, with an average of about 1:10,000 newborns. Early diagnosis is based on newborn screening, and if treatment is started early and continued, intelligence is within normal limits with, on average, some suboptimal neurocognitive function. Dietary restriction of phenylalanine has been the mainstay of treatment for over 60 years and has been highly successful, although outcomes are still suboptimal and patients can find the treatment difficult to adhere to. Pharmacological treatments are available, such as tetrahydrobiopterin, which is effective in only a minority of patients (usually those with milder PKU), and pegylated phenylalanine ammonia lyase, which requires daily subcutaneous injections and causes adverse immune responses. Given the drawbacks of these approaches, other treatments are in development, such as mRNA and gene therapy. Even though PAH deficiency is the most common defect of amino acid metabolism in humans, brain dysfunction in individuals with PKU is still not well understood and further research is needed to facilitate development of pathophysiology-driven treatments.

Insights from Animal Models on the Pathophysiology of Hyperphenylalaninemia: Role of Mitochondrial Dysfunction, Oxidative Stress and Inflammation

Phenylketonuria (PKU) is an inborn error of metabolism caused by phenylalanine hydroxylase (PAH) deficiency and characterized by elevated plasma levels of phenylalanine (hyperphenylalaninemia-HPA). In severe cases, PKU patients present with neurological dysfunction and hepatic damage, but the underlying mechanisms are not fully elucidated. Other forms of HPA also characterized by neurological symptoms occur in rare instances due to defects in the metabolism of the PAH cofactor tetrahydrobiopterin. This review aims to gather the knowledge acquired on the phenylalanine-induced toxicity focusing on findings obtained from pre-clinical studies. Mounting evidence obtained from PKU genetic mice, rats submitted to different HPA models, and cell cultures exposed to phenylalanine has shown that high levels of this amino acid impair mitochondrial bioenergetics, provoke changes in oxidative and inflammatory status, and induce apoptosis. Noteworthy, some data demonstrated that phenylalanine-induced oxidative stress occurs specifically in mitochondria. Further studies have shown that the metabolites derived from phenylalanine, namely phenylpyruvate, phenyllactate, and phenylacetate, also disturb oxidative status. Therefore, it may be presumed that mitochondrial damage is one of the most important mechanisms responsible for phenylalanine toxicity. It is expected that the findings reviewed here may contribute to the understanding of PKU and HPA pathophysiology and to the development of novel therapeutic strategies for these disorders.

Long-term dietary intervention with low Phe and/or a specific nutrient combination improve certain aspects of brain functioning in phenylketonuria (PKU)

INTRODUCTION

In phenylketonuria (PKU), a gene mutation in the phenylalanine metabolic pathway causes accumulation of phenylalanine (Phe) in blood and brain. Although early introduction of a Phe-restricted diet can prevent severe symptoms from developing, patients who are diagnosed and treated early still experience deficits in cognitive functioning indicating shortcomings of current treatment. In the search for new and/or additional treatment strategies, a specific nutrient combination (SNC) was postulated to improve brain function in PKU. In this study, a long-term dietary intervention with a low-Phe diet, a specific combination of nutrients designed to improve brain function, or both concepts together was investigated in male and female BTBR PKU and WT mice.

MATERIAL & METHODS

48 homozygous wild-types (WT, +/+) and 96 PKU BTBRPah2 (-/-) male and female mice received dietary interventions from postnatal day 31 till 10 months of age and were distributed in the following six groups: high Phe diet (WT C-HP, PKU C-HP), high Phe plus specific nutrient combination (WT SNC-HP, PKU SNC-HP), PKU low-Phe diet (PKU C-LP), and PKU low-Phe diet plus specific nutrient combination (PKU SNC- LP). Memory and motor function were tested at time points 3, 6, and 9 months after treatment initiation in the open field (OF), novel object recognition test (NOR), spatial object recognition test (SOR), and the balance beam (BB). At the end of the experiments, brain neurotransmitter concentrations were determined.

RESULTS

In the NOR, we found that PKU mice, despite being subjected to high Phe conditions, could master the task on all three time points when supplemented with SNC. Under low Phe conditions, PKU mice on control diet could master the NOR at all three time points, while PKU mice on the SNC supplemented diet could master the task at time points 6 and 9 months. SNC supplementation did not consistently influence the performance in the OF, SOR or BB in PKU mice. The low Phe diet was able to normalize concentrations of norepinephrine and serotonin; however, these neurotransmitters were not influenced by SNC supplementation.

CONCLUSION

This study demonstrates that both a long-lasting low Phe diet, the diet enriched with SNC, as well as the combined diet was able to ameliorate some, but not all of these PKU-induced abnormalities. Specifically, this study is the first long-term intervention study in BTBR PKU mice that shows that SNC supplementation can specifically improve novel object recognition.