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Tetrahydrobiopterin treatment reduces brain L-Phe but only partially improves serotonin in hyperphenylalaninemic ENU1/2 mice

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Journal of Inherited Metabolic Disease

Abstract

Hyperphenylalaninemia (HPA) caused by hepatic phenylalanine hydroxylase (PAH) deficiency has severe consequences on brain monoamine neurotransmitter metabolism. We have studied monoamine neurotransmitter status and the effect of tetrahydrobiopterin (BH4) treatment in Pahenu1/enu2 (ENU1/2) mice, a model of partial PAH deficiency. These mice exhibit elevated blood L-phenylalanine (L-Phe) concentrations similar to that of mild hyperphenylalaninemia (HPA), but brain levels of L-Phe are still ~5-fold elevated compared to wild-type. We found that brain L-tyrosine, L-tryptophan, BH4 cofactor and catecholamine concentrations, and brain tyrosine hydroxylase (TH) activity were normal in these mice but that brain serotonin, 5-hydroxyindolacetic acid (5HIAA) and 3-methoxy-4-hydroxyphenylglycol (MHPG) content, and brain TH protein, as well as tryptophan hydroxylase type 2 (TPH2) protein levels and activity were reduced in comparison to wild-type mice. Parenteral L-Phe loading conditions did not lead to significant changes in brain neurometabolite concentrations. Remarkably, enteral BH4 treatment, which normalized brain L-Phe levels in ENU1/2 mice, lead to only partial recovery of brain serotonin and 5HIAA concentrations. Furthermore, indirect evidence indicated that the GTP cyclohydrolase I (GTPCH) feedback regulatory protein (GFRP) complex may be a sensor for brain L-Phe elevation to ameliorate the toxic effects of HPA. We conclude that BH4 treatment of HPA toward systemic L-Phe lowering reverses elevated brain L-Phe content but the recovery of TPH2 protein and activity as well as serotonin levels is suboptimal, indicating that patients with mild HPA and mood problems (depression or anxiety) treated with the current diet may benefit from supplementation with BH4 and 5-OH-tryptophan.

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Abbreviations

PAH:

phenylalanine hydroxylase

LNAA:

large neutral amino acid

BH4 :

6R–L-erythro-5,6,7,8-tetrahydrobiopterin

GTPCH:

GTP cyclohydrolase I

GFRP:

GTPCH feedback regulatory protein (gene Gchfr)

HPA:

hyperphenylalaninemia

PKU:

phenylketonuria

L-Phe:

L-phenylalanine

TH:

tyrosine hydroxylase

TPH:

tryptophan hydroxylase

L-Dopa:

L-3,4-dihydroxyphenylalanine

3OMD:

3-o-methyldopa

HVA:

homovanillic acid

5HTP:

5-hydroxytryptophan

5HIAA:

5-hydroxyindolacetic acid

MHPG:

3-methoxy-4-hydroxyphenylglycol

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Acknowledgements

We thank Dr. Ernst Werner for helpful discussions and the Institute for Animal Science at the University of Zürich for cooperation. Financial support was given by the Stiftung für wissenschaftliche Forschung University of Zürich (Baumgarten Stiftung to B.T.), the Swiss National Science Foundation (to B.T.) SNF 310030-162547, the NIH R01NS080866 to C.O.H., the Novartis “Stiftung für medizinisch-biologische Forschung” (to B.T.), The Research Council of Norway (Programmes FORNY and FRIMEDBIO to A.M.), and the K. G. Jebsen Centre for Neuropsychiatric Disorders (to A.M.).

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Author notes

  1. Tanja Scherer, Gabriella Allegri, Christineh N. Sarkissian and Ming Ying contributed equally to this work.

Authors and Affiliations

  1. Department of Pediatrics, Divisions of Metabolism and of Clinical Chemistry and Biochemistry, University of Zürich, Zürich, Switzerland

    Tanja Scherer, Gabriella Allegri, Hiu Man Grisch-Chan, Anahita Rassi & Beat Thöny

  2. Department of Biology, McGill University, Montreal, QC, H3A 1B1, Canada

    Christineh N. Sarkissian

  3. Department of Biomedicine and K.G. Jebsen Centre for Neuropsychiatric Disorders, University of Bergen, Bergen, Norway

    Ming Ying & Aurora Martinez

  4. Department of Molecular and Medical Genetics, School of Medicine, Oregon Science & Health University, Portland, OR, USA

    Shelley R. Winn & Cary O. Harding

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  1. Tanja Scherer
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Conflict of interest

T.Scherer, G.Allegri, C. Sarkissian, M. Ying, H.M. Grisch-Chan, A. Rassi, S. Winn, C. Harding, A. Martinez and B. Thöny declare that they have no conflict of interest.

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All institutional and national guidelines for the care and use of laboratory animals were followed (see also chapter “Materials, methods, and animal husbandry”).

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Communicated by: Viktor Kožich

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Suppl. Fig. S1

Inhibition of GPTCH activity by 2,4,-diamino-6-hydroxypyrimidine (DAHP) in crude brain extracts from either wild-type or ENU2/2 (PKU) mice. Activity was assayed as the quantity of 7,8,-dihydroneopterin triphosphate produced by 80 mg of brain extract from either wild-type or ENU1/2 mice in the presence of increasing amounts of added DAHP. Assays were performed in triplicates. Two tailed Student‘s t-test, ***p < 0.005, **p < 0.05, *p < 0.1; n = 3 (wild-type and PKU) (GIF 10 kb)

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Scherer, T., Allegri, G., Sarkissian, C.N. et al. Tetrahydrobiopterin treatment reduces brain L-Phe but only partially improves serotonin in hyperphenylalaninemic ENU1/2 mice. J Inherit Metab Dis 41, 709–718 (2018). https://doi.org/10.1007/s10545-018-0150-y

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