The Ups and Downs of Thyrotropin-Releasing Hormone

The synthesis and secretion of thyrotropin-releasing hormone (TRH) is absolutely required for intact thyroid function in humans and rodents. Furthermore, feedback regulation of TRH production by thyroid hormone allows for tight maintenance of circulating thyroid hormone levels and thus establishes the hypothalamic-pituitary-thyroid (H-P-T) axis. Although the TRH gene is expressed in many regions of the brain, its regulation by thyroid hormone is restricted to neurons in the paraventricular nucleus of the hypothalamus (PVH) (1). This discrete set of neurons in the PVH, termed hypophysiotropic, project to the median eminence where mature TRH peptide is released into the portal capillary system bound for TRH-receptors present in the anterior pituitary (Fig. 1). Whereas it is clear that thyroid hormone regulates TRH production in the PVH both at the level of gene expression and posttranslational processing, in this issue of Endocrinology, Sanchez et al. (2) provide evidence that thyroid hormone also controls TRH peptide degradation by regulating the enzyme pyroglutamyl peptidase II (PPII). This enzyme is expressed in tanycytes; glial cells that line the third ventricle in the hypothalamus whose cytoplasmic processes extend into the median eminence. Thus, PPII appears to regulate TRH peptide bioavailability upstream of the pituitary.

Figure 1.

Thyroid hormone regulates TRH transcription, posttranslational modification, and degradation through the assistance of tanycytes. 1, Tanycytes take up T₄, which is converted to T₃ by dio2. 2, Hypophysiotropic TRH neurons in the PVH receive T₃ from tanycytes or from circulating thyroid hormone through the MCT8. 3, TRH, PC1/3, and PC2 are inversely regulated by T₃. 4, Posttranslational modifications to proTRH by PC1/3 and PC2, carboxypeptidases E and D (CPE and CPD), and peptidyl α-amidating monooxygenase (PAM) occur as it travels down the axon (dashed line). 5, TRH is released in the median eminence (ME) where it can be degraded by tanycyte-bound PPII, which is positively regulated by thyroid hormone.

The importance of this discovery is best highlighted by an understanding of the unique anatomic and biologic mechanisms by which TRH is regulated. As outlined, TRH is widely expressed in the hypothalamus but its production is only regulated by thyroid hormone in hypophysiotropic neurons present in the PVH. This observation suggests that these neurons possess either a singular molecular regulatory mechanism or, more likely, are uniquely positioned to sense thyroid hormone levels. Indeed, recent evidence suggests that local hypothalamic T₃ can be produced from circulating T₄ by the type 2 deiodinase (dio2) that is also expressed in tanycytes (3,4). Locally produced T₃ is then available for uptake by hypophysiotropic neurons to regulate TRH both transcriptionally and posttranslationally via specific transporters such as the monocarboxylate transporter 8 (MCT8). The physiological importance of MCT8 has been confirmed by mutations found in humans and in mouse knockout studies in which the regulation of TRH mRNA expression by thyroid hormone is impaired in the absence of MCT8 (5,6,7).

In addition to regulating TRH mRNA expression, thyroid hormone also regulates the production of the mature TRH tripeptide that is delivered to the pituitary. The TRH gene in humans and rodents encodes for multiple copies of TRH (pGlu-His-Pro-NH2). After its transcription and translation, proTRH, a 26-kDa protein, is sequentially modified by prohormone convertases 1/3 (PC1/3) and 2 (PC2) to a variety of cleavage products including TRH precursors (8,9,10). Importantly, these two prohormone convertases are negatively regulated by thyroid hormone such that their levels are high when thyroid hormone levels are low, which increases TRH production (11). After the actions of PC1/3 and PC2, TRH precursors are further modified by carboxypeptidases-E and -D (12). Finally, the immediate precursor to TRH undergoes cyclization at its N terminus and is amidated at its carboxyl terminus (13,14,15). Although all of these steps ensure the appropriate production of TRH, Sanchez et al. (2) demonstrate another mechanism by which thyroid hormone regulates TRH, through degradation by PPII.

PPII, a membrane-bound metallopepitdase with an extracellular active site, is widely distributed in the brain and in some peripheral tissues including the pituitary (16,17,18,19,20,21). PPII has high specificity for TRH (unlike the related PPI) hydrolyzing its pyroglutamyl-histidyl peptide bond. The findings of Sanchez et al. (2) update previous findings on the localization of PPII expression. Previously, PPII inactivation of TRH was thought to occur in the anterior pituitary; however, expression of PPII is limited to lactotrophs (22,23). Although prior work suggested that PPII was expressed in neurons (21,22,24), elegant use of in situ hybridization by Sanchez et al. (2) reveals a PPII mRNA expression pattern indicative of tanycyte localization. Earlier studies have shown that thyroid hormone could regulate PPII expression in the pituitary and frontal cortex (25,26,27). Now, Sanchez et al. (2) demonstrate that both PPII expression and enzymatic activity are strongly up-regulated in tanycytes in the presence of increased T₄ levels. Furthermore, confocal microscopy shows that axon terminals containing TRH in the median eminence are closely positioned to tanycytes, allowing for a model in which thyroid hormone levels dictate the amount of TRH peptide that reaches the portal circulation through the PPII-mediated degradation of TRH. The importance of these findings is accentuated by the fact that tanycytes also express dio2, suggesting that these cells function as key sensors of circulating thyroid hormone levels and are able to influence TRH production, either in the PVH by regulating T₃ levels via dio2 or by degradation in the median eminence by the actions of PPII.

Collectively, Sanchez et al. (2) demonstrate a novel and potentially critical mechanism by which thyroid hormone feeds back to regulate TRH production. This exciting observation now raises a number of questions including one posed by the authors; is the regulation of PPII expression and activity the first step in decreasing TRH action on the H-P-T axis after an increase in thyroid hormone? In addition, as TRH mRNA expression and peptide production play a key role in the response of the H-P-T axis to critical illness and nutritional deprivation, is PPII expression regulated in these states? Indeed, dio2 is strongly up-regulated by bacterial lipopolysaccharide in tanycytes, and a recent study demonstrated that hypothalamic dio2 responds to leptin in food-restricted rats (28,29). Thus, PPII may be poised to respond to these pathways and allow for rapid down regulation of thyroid hormone levels in these states. Lastly, do PPII mRNA expression and activity decrease in response to low thyroid hormone levels, and what is the molecular mechanism by which tanycytes sense thyroid hormone levels? The answers to these questions will allow for a better understanding of the complete role that PPII expression plays in regulating TRH action and will give new insight into the role of tanycytes in the regulation of the H-P-T axis.