Date Published: April 17, 2019
Publisher: Public Library of Science
Author(s): Maximilian Breuer, Luca Guglielmi, Matthias Zielonka, Verena Hemberger, Stefan Kölker, Jürgen G. Okun, Georg F. Hoffmann, Matthias Carl, Sven W. Sauer, Thomas Opladen, Ryan Thummel.
Dihydropteridine reductase (QDPR) catalyzes the recycling of tetrahydrobiopterin (BH4), a cofactor in dopamine, serotonin, and phenylalanine metabolism. QDPR-deficient patients develop neurological symptoms including hypokinesia, truncal hypotonia, intellectual disability and seizures. The underlying pathomechanisms are poorly understood. We established a zebrafish model for QDPR deficiency and analyzed the expression as well as function of all zebrafish QDPR homologues during embryonic development. The homologues qdpra is essential for pigmentation and phenylalanine metabolism. Qdprb1 is expressed in the proliferative zones of the optic tectum and eye. Knockdown of qdprb1 leads to up-regulation of pro-proliferative genes and increased number of phospho-histone3 positive mitotic cells. Expression of neuronal and astroglial marker genes is concomitantly decreased. Qdprb1 hypomorphic embryos develop microcephaly and reduced eye size indicating a role for qdprb1 in the transition from cell proliferation to differentiation. Glutamine accumulation biochemically accompanies the developmental changes. Our findings provide novel insights into the neuropathogenesis of QDPR deficiency.
Dihydropteridine reductase (human: DHPR; mouse / zebrafish: Qdpr) is the key recycling enzyme of the cofactor tetrahydrobiopterin (BH4). The homodimer uses NADH to supply two hydrogen atoms to BH2 to recover BH4 . BH4 is initially formed in a three-step pathway de-novo from GTP and is then salvaged in a recycling pathway via pterin-4a-carbinolamine dehydratase (PCBD) and DHPR [2, 3]. This pathway is highly conserved among species [4–6]. The zebrafish genome contains three DHPR homologs, Qdpra, Qdprb1 and Qdprb2, the function of which has remained largely unknown.
The zebrafish genome contains three annotated QDPR homologues, Qdpra, Qdprb1 and Qdprb2. Therefore, we tested their function individually.
QDPR is required for the regeneration of BH4 that is a critical co-factor for the generation of dopamine and serotonin. Defects in the synthesis and recycling of BH4 lead to severe infantile Parkinsonism. In comparison to defects in BH4 biosynthesis, patients with inborn variants of QDPR present a higher frequency of severe neurological symptoms, including muscular hypotonia, dystonia, microcephaly, epilepsy and brain atrophy [11, 12]. The aim of our study was to establish a suitable model to investigate the pathophysiology of QDPR deficiency. To this end, we characterized expression and function of the three zebrafish QDPR homologues Qdpra, Qdprb1, and Qdprb2.
This study is the first to characterize the expression and function of the three zebrafish QDPR homologues qdpra, qdprb1, and qdprb2. While we could not identify a zygotic function for qdprb2 in embryonic development, qdpra executes the expected BH4 recycling function in melanin producing cells and in the liver. Our results further suggest that qdprb1 is required for neuronal and glial differentiation of neural progenitor cells emerging from the CMZ and OT.