4 ISOFORMS OF TYROSINE-HYDROXYLASE ARE EXPRESSED IN HUMAN BRAIN

In contrast to nonprimate species, the RNA for human tyrosine hydroxyl ase, the rate-limiting enzyme in catecholamine biosynthesis, can under go alternative splicing to produce four different types of mRNA. Altho ugh types 1 and 2 of these human tyrosine hydroxylase mRNAs have been identified in human brain, whether types 3 and 4 human tyrosine hydrox ylase mRNAs are present in the central nervous system remains controve rsial. Furthermore, little is known about the expression of the protei n products of these mRNAs in human brain. In this study we used antibo dies raised against different octapeptide sequences from each of the p redicted human tyrosine hydroxylase protein forms to determine the pre sence and distribution of each human tyrosine hydroxylase isoforms in several regions of human brain. Control immunocytochemical and blot im munolabeling experiments demonstrated that each antibody selectively r ecognized the human tyrosine hydroxylase isoform against which it was directed. In immunocytochemical studies, all four human tyrosine hydro xylase isoforms were clearly detectable in neurons of both the substan tia nigra and locus coeruleus. The presence of all four isoforms in th ese nuclei was confirmed with blot immunolabeling studies. Single-labe l immunocytochemical studies of adjacent sections as well as dual-labe l comparisons of immunoreactivity for human tyrosine hydroxylase type 1 with type 2, type 3, or type 4 suggested that at least some neurons in these brain regions contain all four human tyrosine hydroxylase iso forms. In contrast, some neurons of the mesencephalon appeared to be s electively immunoreactive with the antibodies against type 1. In the c audate nucleus and putamen, the terminal zones of the dopaminergic pro jection from the substantia nigra, all four isoforms were detected, al though in immunocytochemical studies type 1 appeared to be the predomi nant isoform present in axons and terminals. These findings demonstrat e that human brain contains four distinct isoforms of human tyrosine h ydroxylase and that the presence or relative amount of each isoform ma y differ among catecholaminergic cell populations and between catechol aminergic neurons and terminal fields. These patterns of expression ma y have important implications for understanding the regulation of cate cholamine biosynthesis in human brain both in normal and pathological states.