Expression and distribution of two isoforms of tyrosine hydroxylase in macaque monkey brain

Abstract

In humans, the RNA for tyrosine hydroxylase (TH), the rate-limiting enzyme in the synthesis of catecholamines, can undergo alternative splicing to produce four different types of mRNA. Each of the predicted TH protein forms has been shown to be expressed in human brain, but constraints inherent in human postmortem studies require the use of an animal model to further explore the functional significance of multiple TH isoforms. Since the anatomical organization of catecholaminergic systems in Old World macaque monkeys appears to accurately predict their organization in humans, we used antibodies that selectively recognize each of the human TH isoforms to determine the expression and distribution of TH isoforms in monkey brain. Blot immunolabeling and immunocytochemical techniques demonstrated that catecholaminergic neurons of monkey brain express both type 1 and type 2 TH, but not type 3 or type 4. Thus, monkeys differ in the number of TH isoforms expressed in brain both from rodents (type 1 TH only) and humans (types 1–4 TH). In some catecholaminergic neurons and axon terminals of human brain, only type 1 TH immunoreactivity was detectable, suggesting that the selective expression of a single isoform could occur or that TH isoforms might differ in their accessibility to or engagement with cell trafficking mechanisms. However, in monkeys there was no evidence for the selective expression or distribution of either type 1 or type 2 TH in any of the catecholaminergic cell bodies or terminal fields examined. In addition, quantitative blot immunolabeling studies demonstrated that type 1 and type 2 TH were present in the same relative abundances in monkey substantia nigra and caudate nucleus. Furthermore, studies in monkeys designed to model the human postmortem state revealed that the apparently selective distribution of TH isoforms seen in the human study could be created by postmortem effects. In summary, in contrast to nonprimate species, multiple protein isoforms of TH are expressed in monkey brain as well as in human brain. Although the precise functional role of each isoform has not been determined, their presence suggests that the regulation of catecholamine biosynthesis may be more complex and subject to alternative modes of regulation in primates than in nonprimate species. In addition, the difference between monkeys and humans in the number of TH isoforms may provide insight into a molecular basis for species differences in cognitive and other brain functions. However, both type 1 and type 2 TH, the two predominant isoforms of human TH, are expressed in monkey brain. Thus, the macaque monkey is a useful model for clarifying questions that arise in human postmortem studies regarding TH isoforms and may prove to be an effective system for exploring the functional significance of multiple TH isoforms.