BBB seminar: Jan Haavik

Jan Haavik
Department of Biomedicine, University of Bergen

Serotonin is an important neurotransmitter that is involved in a range of physiological functions, including regulation of sleep, gastrointestinal functions, hemostasis, food intake and mood. In humans, it also appears to be associated with impulsivity, suicidality, depression and anxiety disorders . Thus, there is an intense interest in the serotonergic system as a drug target for these and several other disease states. As tryptophan hydroxylase (TPH, EC 1.14.16.4 ) is the rate-limiting enzyme in serotonin biosynthesis, a role for TPH in the pathophysiology of mental disorders has long been suspected. In all vertebrates examined, two different TPH enzymes, TPH1 and TPH2, appear to be responsible for the serotonin production . Whereas both enzymes are expressed in the brain, their exact expression patterns appear to be tissue specific. TPH1 is mainly expressed in non-neuronal serotonergic tissues, e.g. enterochromaffin cells of the gut and the pineal body, whereas TPH2 expression is highest in neuronal serotonergic tissues, e.g. the raphe nuclei of the central nervous system. We have recently shown that TPH1 and TPH2 have different molecular properties, including different substrate specificities, phosphorylation sites and kinetic properties.

Several rare variants of TPH2 have recently been reported to be associated with reduced serotonin production and behavioural alterations in man and mice. We have expressed the human forms of these enzymes in eukaryotic cells and in E. coli and examined the effects of mutations on their expression levels, thermal stability, secondary structure, and catalytic properties. Distinct differences between the wild-type and mutant enzymes were found in their thermal stability and cellular turnover. In addition, some human TPH2 variants displayed significant changes in K m and K si -values for tryptophan and tetrahydrobiopterin compared to the wild type enzyme. These findings provide potential molecular explanations for disorders related to the central serotonergic system, such as depression or suicidality. Based on literature data and our own experience, we have proposed a standardized battery of biochemical tests that can be used to identify the functional consequences of mutations affecting aromatic amino acid hydroxylases, including TPH.