A new gain-of-function allele in chimpanzee tryptophan hydroxylase 2 and the comparison of its enzyme activity with that in humans and rats

Species Differences in Tryptophan Metabolism and Disposition

Major species differences in tryptophan (Trp) metabolism and disposition exist with important physiological, functional and toxicity implications. Unlike mammalian and other species in which plasma Trp exists largely bound to albumin, teleosts and other aquatic species possess little or no albumin, such that Trp entry into their tissues is not hampered, neither is that of environmental chemicals and toxins, hence the need for strict measures to safeguard their aquatic environments. In species sensitive to toxicity of excess Trp, hepatic Trp 2,3-dioxygenase (TDO) lacks the free apoenzyme and its glucocorticoid induction mechanism. These species, which are largely herbivorous, however, dispose of Trp more rapidly and their TDO is activated by smaller doses of Trp than Trp-tolerant species. In general, sensitive species may possess a higher indoleamine 2,3-dioxygenase (IDO) activity which equips them to resist immune insults up to a point. Of the enzymes of the kynurenine pathway beyond TDO and IDO, 2-amino-3-carboxymuconic acid-6-semialdehyde decarboxylase (ACMSD) determines the extent of progress of the pathway towards NAD⁺ synthesis and its activity varies across species, with the domestic cat (Felis catus) being the leading species possessing the highest activity, hence its inability to utilise Trp for NAD⁺ synthesis. The paucity of current knowledge of Trp metabolism and disposition in wild carnivores, invertebrates and many other animal species described here underscores the need for further studies of the physiology of these species and its interaction with Trp metabolism.

Association between temperament related traits and single nucleotide polymorphisms in the serotonin and oxytocin systems in Merino sheep

Animal temperament is defined as the consistent behavioral and physiological differences that are seen between individuals in response to the same stressor. Neurotransmitter systems, like serotonin and oxytocin in the central nervous system, underlie variation in behavioral traits in humans and other animals. Variations like single nucleotide polymorphisms (SNPs) in the genes for tryptophan 5-hydroxylase (TPH2), the serotonin transporter (SLC6A4), the serotonin receptor (HTR2A), and the oxytocin receptor (OXTR) are associated with behavioral phenotype in humans. Thus, the objective of this study was to identify SNPs in those genes and to test if those variations are associated with the temperament in Merino sheep. Using ewes from the University of Western Australia temperament flock, which has been selected on emotional reactivity for more than 20 generations, eight SNPs (rs107856757, rs107856818, rs107856856 and rs107857156 in TPH2, rs20917091 in SLC6A4, rs17196799 and rs17193181 in HTR2A, and rs17664565 in OXTR) were found to be distributed differently between calm and nervous sheep. These eight SNPs were then genotyped in 260 sheep from a flock that has never been selected on emotional reactivity, followed by the estimation of the behavioral traits of those 260 sheep using an arena test and an isolation box test. We found that several SNPs in TPH2 (rs107856757, rs107856818, rs107856856 and rs107857156) were in strong linkage disequilibrium, and all were associated with behavioral phenotype in the nonselected sheep. Similarly, rs17196799 in HTR2A was also associated with the behavioral phenotype.

Polymorphism of the tryptophan hydroxylase 2 (TPH2) gene is associated with chimpanzee neuroticism

In the brain, serotonin production is controlled by tryptophan hydroxylase 2 (TPH2), a genotype. Previous studies found that mutations on the TPH2 locus in humans were associated with depression and studies of mice and studies of rhesus macaques have shown that the TPH2 locus was involved with aggressive behavior. We previously reported a functional single nucleotide polymorphism (SNP) in the form of an amino acid substitution, Q468R, in the chimpanzee TPH2 gene coding region. In the present study we tested whether this SNP was associated with neuroticism in captive and wild-born chimpanzees living in Japan and Guinea, respectively. Even after correcting for multiple tests (Bonferroni p = 0.05/6 = 0.008), Q468R was significantly related to higher neuroticism (β = 0.372, p = 0.005). This study is the first to identify a genotype linked to a personality trait in chimpanzees. In light of the prior studies on humans, mice, and rhesus macaques, these findings suggest that the relationship between neuroticism and TPH2 has deep phylogenetic roots.

Rhesus monkey tryptophan hydroxylase-2 coding region haplotypes affect mRNA stability

Tryptophan hydroxylase-2 (TPH2) synthesizes neuronal 5-HT and its genetic variance is associated with numerous behavioral traits and psychiatric disorders. This study characterized the functional significance of two nonsynonymous single nucleotide polymorphisms (SNPs) (C74A and G223A) in rhesus monkey TPH2 (mTPH2). Four haplotypes of mTPH2 were cloned into pcDNA3.1 and stably transfected into PC12 cells. The levels of mTPH2 mRNA and protein were assessed by quantitative real-time PCR and Western blot, respectively, while the intracellular 5-HT was measured by enzyme-linked immunosorbent assay (ELISA). The variant A-A haplotype showed significantly higher levels of mTPH2 mRNA and protein, as well as significantly higher 5-HT production than the wild-type C-G haplotype, while the other two variant haplotypes (C-A and A-G) also tended to produce more 5-HT than C-G haplotype when stably expressed in PC12 cells. Both C74A and G223A were predicted to change mRNA secondary structure, and analysis of the mRNA stability showed that the wild-type C-G haplotype mRNA degrades more quickly than mRNAs of the mutant mTPH2 haplotypes in both stable PC12 and transient HEK-293 cells. This study demonstrates that nonsynonymous SNPs in mTPH2 can affect mRNA stability. Our findings provide an additional mechanism by which nonsynonymous SNPs affect TPH2 function, and further our understanding of TPH2 gene expression regulation.

Role of GSK3 beta in behavioral abnormalities induced by serotonin deficiency

Dysregulation of brain serotonin (5-HT) neurotransmission is thought to underlie mental conditions as diverse as depression, anxiety disorders, bipolar disorder, autism, and schizophrenia. Despite treatment of these conditions with serotonergic drugs, the molecular mechanisms by which 5-HT is involved in the regulation of aberrant emotional behaviors are poorly understood. Here, we generated knockin mice expressing a mutant form of the brain 5-HT synthesis enzyme, tryptophan hydroxylase 2 (Tph2). This mutant is equivalent to a rare human variant (R441H) identified in few individuals with unipolar major depression. Expression of mutant Tph2 in mice results in markedly reduced ( approximately 80%) brain 5-HT production and leads to behavioral abnormalities in tests assessing 5-HT-mediated emotional states. This reduction in brain 5-HT levels is accompanied by activation of glycogen synthase kinase 3beta (GSK3beta), a signaling molecule modulated by many psychiatric therapeutic agents. Importantly, inactivation of GSK3beta in Tph2 knockin mice, using pharmacological or genetic approaches, alleviates the aberrant behaviors produced by 5-HT deficiency. These findings establish a critical role of Tph2 in the maintenance of brain serotonin homeostasis and identify GSK3beta signaling as an important pathway through which brain 5-HT deficiency induces abnormal behaviors. Targeting GSK3beta and related signaling events may afford therapeutic advantages for the management of certain 5-HT-related psychiatric conditions.

Phosphorylation and activation of tryptophan hydroxylase 2: identification of serine-19 as the substrate site for calcium, calmodulin-dependent protein kinase II

Tryptophan hydroxylase (TPH) is the initial and rate-limiting enzyme in the biosynthesis of serotonin. TPH was once thought to be a single-gene product but it is now known to exist in two isoforms. TPH1 is found in the periphery and pineal gland whereas TPH2 is expressed specifically in the CNS. Both TPH isoforms are known to be regulated by protein kinase-dependent phosphorylation and the sites of modification of TPH1 by protein kinase A have been identified. While TPH2 is activated by calcium, calmodulin-dependent protein kinase II (CaMKII), the sites at which this isoform is modified are not known. Treatment of wild-type TPH2 with CaMKII followed by mass spectrometry analysis revealed that the enzyme was activated and phosphorylated at a single site, serine-19. Mutagenesis of serine-19 to alanine did not alter the catalytic function of TPH2 but this mutant enzyme was neither activated nor phosphorylated by CaMKII. A phosphopeptide bracketing phosphoserine-19 in TPH2 was used as an antigen to generate polyclonal antibodies against phosphoserine-19. The antibodies are highly specific for phosphoserine-19 in TPH2. The antibodies do not react with wild-type TPH2 or TPH1 and they do not recognize phophoserine-58 or phosphoserine-260 in TPH1. These results establish that activation of TPH2 by CaMKII is mediated by phosphorylation of serine-19 within the regulatory domain of the enzyme. Production of a specific antibody against the CaMKII phosphorylation site in TPH2 represents a valuable tool to advance the study of the mechanisms regulating the function of this important enzyme.