Chicken folliculo-stellate cells express thyrotropin receptor mRNA

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Abstract

We investigated the presence of thyrotropin receptor (TSHR) mRNA in chicken pituitary and brain, and quantified the changes in its expression during the last week of embryonic development. We found that in the pituitary gland, TSHR mRNA co-localizes with folliculo-stellate cells but not with thyrotropic cells, suggesting the existence of a paracrine ultra-short thyrotropin feedback loop. TSHR mRNA was also present throughout the diencephalon and various other brain regions, which implies a more general function for thyrotropin in the avian brain. During late embryogenesis, when the activity of the hypothalamo-pituitary-thyroidal axis increases markedly, a significant rise in TSHR mRNA expression was observed in pituitary, which may signify an important change in pituitary ultra-short thyrotropin feedback regulation around the period of hatching.

Introduction

The hypothalamo-pituitary-thyroidal (HPT) axis comprises several levels of interaction that regulate circulating thyroid hormone (TH) concentrations. Thyrotropin (TSH), being the direct stimulator of TH secretion, plays a critical role in adjusting plasma TH levels to the varying needs of the body. Several studies indicate that THs regulate TSH synthesis and release in the chicken: incubating pituitaries of 19-day-old chicken embryos (E19) with T3 reduces TSHβ mRNA levels significantly [1], and in embryonic (E19) as well as 8-day-old posthatch chicks (C8), THs suppress corticotropin-releasing hormone (CRH)-stimulated TSH release [2]. Recently, the chicken TSH receptor (TSHR) was cloned [3]. TSHR mRNA was found to be present in several extra-thyroidal sites including pituitary and diencephalon, two major control levels of the HPT axis. It is therefore possible that besides THs, also TSH itself is able to modulate its own production and secretion as well as that of the TSH-regulating hypothalamic hormones. In this study, we first localized the TSH mRNA expression sites in the chicken brain and pituitary. In addition, we investigated whether TSHR mRNA expression in pituitary changes during the last week of chicken embryonic development, a period associated with a strong activation of the HPT axis.

Section snippets

Materials and methods

All experiments were approved by the ethical committee for animal experiments of the Catholic University of Leuven.

Results

Using ISH combined with IHC, predominant TSHR mRNA expression was seen in between the pituitary thyrotropes, in embryos (Fig. 1A) as well as in post-hatch chicks (not shown). Hybridization with the sense probe did not produce any signal (not shown). Since no immunoreactivity to the S-100 protein is present in the embryonic [5] or C4 pituitary (own preliminary experiments, not shown), C8 pituitaries were used for the IHC detection of FS cells. Combining ISH and IHC with anti-S-100 antibody on C8

Discussion

In this study, we have shown that hypophyseal TSHR mRNA is expressed by FS cells but not by thyrotropes in posthatch chicks. FS cells are pituitary cells with cytoplasmic processes extending between the endocrine cells. They are connected by gap junctions and form a dense network throughout the pituitary, producing various paracrine factors that modulate the activity of the hormone-secreting cells [7]. Although FS cells cannot be visualized using anti-S-100 in embryonic pituitaries [5], it is

Disclosure statement

The authors have nothing to disclose. The authors declare that all authors made substantial contributions to the article; SVG, SG and BDG participated in the research, and SVG, VMD and BDG participated in the article preparation. All authors have approved of the final article.

Acknowledgements

The authors wish to thank L. Noterdaeme, W. Van Ham and F. Voets for their technical assistance and Dr. A. Iwasawa (Gifu University, Gifu, Japan) for kindly donating the anti-TSHβ antibody. Bert De Groef is supported by the Research Foundation – Flanders (FWO-Vlaanderen). This work was partially funded by FWO project G.0582.06.

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