A unique central tryptophan hydroxylase isoform

Abstract

Serotonin (5-hydroxytryptophan, 5-HT) is a neurotransmitter synthesized in the raphe nuclei of the brain stem and involved in the central control of food intake, sleep, and mood. Accordingly, dysfunction of the serotonin system has been implicated in the pathogenesis of psychiatric diseases. At the same time, serotonin is a peripheral hormone produced mainly by enterochromaffin cells in the intestine and stored in platelets, where it is involved in vasoconstriction, haemostasis, and the control of immune responses. Moreover, serotonin is a precursor for melatonin and is therefore synthesized in high amounts in the pineal gland. Tryptophan hydroxylase (TPH) catalyzes the rate limiting step in 5-HT synthesis. Until recently, only one gene encoding TPH was described for vertebrates. By gene targeting, we functionally ablated this gene in mice. To our surprise, the resulting animals, although being deficient for serotonin in the periphery and in the pineal gland, exhibited close to normal levels of 5-HT in the brain stem. This led us to the detection of a second TPH gene in the genome of humans, mice, and rats, called TPH2. This gene is predominantly expressed in the brain stem, while the classical TPH gene, now called TPH1, is expressed in the gut, pineal gland, spleen, and thymus. These findings clarify puzzling data, which have been collected over the last decades about partially purified TPH proteins with different characteristics and justify a new concept of the serotonin system. In fact, there are two serotonin systems in vertebrates, independently regulated and with distinct functions.

Introduction

Serotonin is a monoaminergic neurotransmitter involved in a wide variety of brain functions such as mood control, the regulation of sleep and body temperature, anxiety, drug abuse, food intake, and sexual behavior [1], [2], [3], [4], [5], [6] with tryptophan hydroxylase (TPH; EC 1.14.16.4) as the first-step and rate-limiting enzyme in its biosynthesis [7], [8], [9]. TPH uses Fe²⁺ as cofactor and O₂ and tetrahydrobiopterin (BH4) as co-substrates to hydroxylate tryptophan generating 5-hydroxytryptophan (5-HT) (Fig. 1). This metabolite is decarboxylated by aromatic amino acid decarboxylase (AADC) to 5-HT or serotonin.

The serotonergic projection system is the most extensive monoaminergic system in the brain of vertebrates, but it is also the most difficult to study. The roots of this system are confined to a handful of selectively 5-HT-synthesizing neurons within the midbrain, pons, and medulla oblongata, which altogether constitute the several groups of raphe nuclei B1–B9 [10]. Furthermore, serotonin represents an intermediate product in melatonin synthesis and, therefore, the pineal gland expresses highest amounts of TPH under circadian control with maximal activity in the dark period [11]. Projections from the pineal gland to several brain regions have been described [12], [13], in which the rate-limiting enzyme in the biosynthesis of melatonin, serotonin N-acetyltransferase (AANAT) is present [14], suggesting local conversion of 5-HT to melatonin in the projection areas.

Besides the brain and the pineal gland, TPH has been found in enteric neurons [15], in preimplantation embryos [16], mast cells [17], and most prominently in enterochromaffin cells of the gastrointestinal tract [18]. These cells are supposed to be the source of 5-HT in the blood where it is almost exclusively located in the dense core storage vesicles of thrombocytes [19]. 5-HT has been implicated in different processes in peripheral tissues, such as regulation of vascular tone (“serotonin” [20], [21]) and intestinal motility [22], primary haemostasis [23], and T cell-mediated immune responses [24].

The enzyme TPH belongs to a superfamily of aromatic amino acid hydroxylases (AAAH), together with phenylalanine (PAH) and tyrosine hydroxylase (TH) [25], [26]. While the other family members have been studied in great detail concerning structure, characteristics, and regulation [27], TPH has been left behind, due to the extremely low abundance of TPH mRNA in the CNS and the difficulties to purify the protein [28], as well as to the lack of serotonin-producing neuronal cell lines suitable for in vitro studies [29].

TPH-cDNAs have been cloned from different mammalian species (rabbit: [30]; mouse: [31]; rat: [32]; human: [33]). The first TPH gene to be characterized contained 11 exons and was located on human chromosome 11 and mouse chromosome 7 [31], [34], [35], [36], [37]. For more than a decade this gene has been thought to be the only TPH gene in the vertebrate genome [38]. By targeted ablation of this TPH gene (now called Tph1) in mice, we recently discovered the existence of a second TPH isoform, TPH2, encoded by an additional gene on human chromosome 12 and mouse chromosome 10 [39]. TPH1 is mainly present in pineal gland, thymus, spleen, and gut while TPH2 predominates in brain stem. This commentary will summarize the history and the relevance of this discovery and the characteristics of TPH2.