Tyrosine hydroxylase is expressed during early heart development and is required for cardiac chamber formation.

AIMS Tyrosine hydroxylase (TH) is the first and rate-limiting enzyme in catecholamine biosynthesis. Whereas the neuroendocrine roles of cathecolamines postnatally are well known, the presence and function of TH in organogenesis is unclear. The aim of this study was to define the expression of TH during cardiac development and to unravel the role it may play in heart formation. METHODS AND RESULTS We studied TH expression in chick embryos by whole mount in situ hybridization and by quantitative reverse transcription-polymerase chain reaction and analysed TH activity by high-performance liquid chromatography. We used gain- and loss-of-function models to characterize the role of TH in early cardiogenesis. We found that TH expression was enriched in the cardiac field of gastrulating chick embryos. By stage 8, TH mRNA was restricted to the splanchnic mesoderm of both endocardial tubes and was subsequently expressed predominantly in the myocardial layer of the atrial segment. Overexpression of TH led to increased atrial myosin heavy chain (AMHC1) and T-box 5 gene (Tbx5) expression in the ventricular region and induced bradyarrhythmia. Similarly, addition of l-3,4-dihydroxyphenylalanine (l-DOPA) or dopamine induced ectopic expression of cardiac transcription factors (cNkx2.5, Tbx5) and AMHC1 as well as sarcomere formation. Conversely, blockage of dopamine biosynthesis and loss of TH activity decreased AMHC1 and Tbx5 expression, whereas exposure to retinoic acid (RA) induced TH expression in parallel to that of AMHC1 and Tbx5. Concordantly, inhibition of endogenous RA synthesis decreased TH expression as well as that of AMHC1 and Tbx5. CONCLUSION TH is expressed in a dynamic pattern during the primitive heart tube formation. TH induces cardiac differentiation in vivo and it is a key regulator of the heart patterning, conferring atriogenic identity.