6R-Tetrahydrobiopterin induces dopamine synthesis in a human neuroblastoma cell line, LA-N-1

Abstract

Dopa-responsive dystonia (DRD) is an extrapyramidal disorder caused by deficit of 5,6,7,8-tetrahydrobiopterin (BH4), cofactor for tyrosine hydroxylase (TH). In these patients the nigrostriatal dopaminergic neurons normally express TH and the cellular machinery for the dopamine uptake. LA-N-1 is a human neuroblastoma cell line expressing tyrosine hydroxylase. Here we show that LA-N-1 cells are able to take up exogenous dopamine (DA) by an high-affinity mechanism; significant amounts of serotonin and its metabolite 5HIAA, but neither DA nor its metabolites, DOPAC and HVA, could be measured in the cell culture homogenate. 5,6,7,8-Tetrahydrobiopterin, cofactor for both tyrosine and tryptophan hydroxylases, is able to activate dopamine synthesis and also decreases the content of 5HIAA by 50%, indicating that LA-N-1 might be a useful model for studying dopamine–serotonin interaction in cultured cells and the neuronal mechanism of DRD.

Introduction

6R-Tetrahydrobiopterin (BH4) is the naturally occurring, essential cofactor for aromatic amino acid hydroxylases such as phenylalanine hydroxylase (PAH) tyrosine hydroxylase (TH) and tryptophan hydroxylase (TPH) [13], the rate-limiting enzymes for tyrosine, catecholamines and serotonin synthesis, respectively. Further to its key role in the synthesis of these biogenic amines, it is now known that BH4 is also a cofactor for all three forms of nitric oxide synthase (NOS) in the production of nitric oxide. In addition, BH4 is required by the enzyme glyceryl-ether monooxygenase to form α-hydroxyalkyl glycerol. BH4 is synthesized from guanosine triphosphate (GTP) by GTP cyclohydrolase I (GTPCH), 6-pyruvoyl-tetrahydropterin synthase (PTS), and sepiapterin reductase (SPD). GTPCH is the rate-limiting enzyme. The intracellular concentrations of BH4, which are mainly determined by GTPCH activity, may regulate the activity of amino acid hydroxylases as well the activity of NOS. More recently, other roles for BH4, independent of its cofactor function, have been defined and reviewed [1], [14].

The deficiency of BH4 has been amply demonstrated in patients affected by dominantly inherited progressive dystonia with marked diurnal fluctuation [20] also termed DOPA-responsive dystonia (DRD). The hallmark of the diseases is that patients benefit from low doses of l-dopa (l-dihydroxypheylalanine) [17]. Reduced levels of the dopamine metabolite homovanillic acid (HVA), tetrahydrobiopterin (BH4), and the BH4 precursor, neopterin, have been reported in the cerebrospinal fluid of DRD patients consistently with a deficiency in cofactor availability [9].

The illness is associated with mutation on a single allele of the gene for GTPCH [12]. Mutation in the GTPCH gene and in turn a reduced rate of synthesis of BH4 can explain many of the findings of the disease. However, no mutation has been identified in approximately 30% of families despite evidence of defective GTPCH enzyme function. Studies with 6-[¹⁸F]fluoro-l-dopa positron emission tomography (PET) suggest a normal number of presynaptic nigrostriatal dopaminergic neurons and adequate activity of dopa decarboxylase [19], [30]. Only one neuropathological and biochemical study has confirmed this result [18]. The mechanism for the reduction in TH protein concentration remains obscure but may involve a role for BH4 in the regulation of TH gene expression or TH protein stability [27].

The above pathophysiological explanation for DRD, still leaves several cues unexplained. It has been reported that BH4 (and not phenylalanine hydroxylase) deficiency is associated with a rare variant of hyperphenylalaninemia termed ‘atypical’ or ‘malignant’ phenylketonuria that is unresponsive to a low-phenylalanine diet. Phenotypically it is characterized by a deficit of biopterin, dopamine and serotonin and progressive neurological symptoms including mental retardation, muscle hypotonia, epilepsy, and severe episodical hyperthermia [16] and is caused by mutations in GTPCH gene [29]. DRD patients have normal levels of phenylalanine in the blood and urine and there are no evident clinical sign of dysfunctions in noradrenergic or serotoninergic neurons. These findings suggest a susceptibility of dopaminergic neurons to the decreased levels of BH4. However, decreased levels of BH4 in the cerebrospinal fluid have also been documented in other neurological diseases such as Parkinson’s disease [5], autism [28], depression [3], and Alzheimer’s disease [2], which present a different phenotypical picture respect to both ‘atypical’ phenylketonuria and DRD.

We have previously studied LA-N-1 cells, a cell line derived from a human bone marrow metastasis of neuroblastoma [20]. These cells express tyrosine hydroxylase, veratridine-sensitive sodium channels and M1 and M3 muscarinic receptors [22], [25]. In the present study we show that LA-N-1 cells are able to spontaneously synthesize serotonin but not dopamine even though they express the cellular machinery for the high affinity dopamine uptake. We also demonstrated that the supplementation of tetrahydrobiopterin to the culture medium is able to induce dopamine synthesis and to decrease levels of 5-hydroxyindolacetic acid (5HIAA), the major metabolite of serotonin. These results suggest that LA-N-1 cell line might be a suitable model for the study of the mechanisms of gene expression and protein function of biopterin synthesizing enzymes and may contribute to a better understanding of the pathophysiologic mechanisms in several diseases such as DRD.