Glycosylation in the Thyroid Gland: Vital Aspects of Glycoprotein Function in Thyrocyte Physiology and Thyroid Disorders

Abstract

The key proteins responsible for hormone synthesis in the thyroid are glycosylated. Oligosaccharides strongly affect the function of glycosylated proteins. Both thyroid-stimulating hormone (TSH) secreted by the pituitary gland and TSH receptors on the surface of thyrocytes contain N-glycans, which are crucial to their proper activity. Thyroglobulin (Tg), the protein backbone for synthesis of thyroid hormones, is a heavily N-glycosylated protein, containing 20 putative N-glycosylated sites. N-oligosaccharides play a role in Tg transport into the follicular lumen, where thyroid hormones are produced, and into thyrocytes, where hyposialylated Tg is degraded. N-glycans of the cell membrane transporters sodium/iodide symporter and pendrin are necessary for iodide transport. Some changes in glycosylation result in abnormal activity of the thyroid and alteration of the metabolic clearance rate of hormones. Alteration of glycan structures is a pathological process related to the progression of chronic diseases such as thyroid cancers and autoimmunity. Thyroid carcinogenesis is accompanied by changes in sialylation and fucosylation, β1,6-branching of glycans, the content and structure of poly-LacNAc chains, as well as O-GlcNAcylation, while in thyroid autoimmunity the main processes affected are sialylation and fucosylation. The glycobiology of the thyroid gland is an intensively studied field of research, providing new data helpful in understanding the role of the sugar component in thyroid protein biology and disorders.

Keywords: NIS; TSHR; glycosylation; pendrin; thyroglobulin; thyroid; thyroid autoimmunity; thyroid cancers; thyroid-stimulating hormone.

Figure 1

Three post-translational modifications of proteins: N-glycosylation, O-glycosylation and O-GlcNAcylation. (A) N-oligosaccharides are attached via N-glycosidic bonds to asparagine (Asn) in the consensus sequence Asn-Xaa-Ser/Thr (Ser, serine; Thr, threonine; Xaa, any amino acid except proline). In the N-glycosylation process, three types of N-glycans are created: high-mannose (or oligomannose), hybrid-type, and complex-type bi-, tri- or tetraantennary, which share the same core structure (GlcNAc2Man3, dashed line) and differ in the external part, built of N-acetylglucosamine (GlcNAc), galactose (Gal), sialic acid (SA) and fucose (Fuc). Complex-type antennas can be extended with poly-N-acetyllactosamine (poly-LacNAc) chains. (B) O-glycan structures with mainly cores 1, 2, 3 and 4 are formed in the O-glycosylation pathway. O-glycans also contain poly-LacNAc chains or are terminated with SA. O-oligosaccharides are linked via N-acetylgalactosamine (GalNAc) to Ser or Thr in the protein sequence. (C) In the O-GlcNAcylation process, a single GlcNac is attached to Ser or Thr. Glycosylation processes are catalyzed by different glycosyltransferases, including fucosyltransferase 8 (Fut8), N-acetylglucosaminyltransferase V (GnTV), N-acetylgalactosamine-specific α2,6-sialyltransferase 2 (ST6GalNAc2) and O-GlcNAc transferase (OGT) [13,14,15,16,17,18,19].

Figure 2

Glycosylation of the key thyroid proteins. Synthesis of thyroid hormones is regulated by the hypothalamus-pituitary-thyroid axis. Thyrotropin-releasing hormone (TRH), produced by the hypothalamus, stimulates the pituitary gland to release thyroid-stimulating hormone (TSH). PD-TSH is secreted by the pars distalis (PD) and PT-TSH by the pars tuberalis of the pituitary gland. PT-TSH binds to the TSH receptor (TSHR) in the hypothalamus and regulates seasonality. PD-TSH binds to TSHR in the cell membrane of thyrocytes and induces signal transduction, resulting in thyroglobulin (Tg) synthesis [31]. Thyroperoxidase (TPO) catalyzes iodine oxidation, iodination of Tg, and production of monoiodotyrosine (MIT) and diiodotyrosine (DIT). The combination of MIT and DIT gives triiodothyronine (T3), while tetraiodothyronine, also called thyroxine (T4), consists of two coupled DITs [3,4,5]. Sodium/iodide symporter (NIS) is responsible for active transport of iodide ions through the thyroid follicular cell membrane into thyrocytes. Pendrin, an anion transporter located in the apical membrane of thyrocytes, is involved in iodide transport from follicular cells into the lumen of follicles [32]. All the above-mentioned human thyroid proteins are N-glycosylated and contain different numbers of N-glycosylation sites (red dots): TSH–3 (Asn23, Asn52, Asn78) [6], TSHR–6 (Asn77, Asn99, Asn113, Asn177, Asn198, Asn302) [33], Tg–16 (Asn57, Asn179, Asn465, Asn510, Asn729, Asn797, Asn928, Asn1200, Asn1329, Asn1345, Asn1696, Asn1754, Asn1993, Asn2230, Asn2275, Asn2562) [34], NIS–3 (Asn485, Asn497, Asn225) [32,35], pendrin–3 [36]. TSH is abundant in sulfated biantennary N-glycans [6]. TSHR contains high-mannose and complex-type structures [33]. High-mannose structures as well as galactosylated, fucosylated, and sialylated hybrid-type and complex-type N-glycans have been identified on Tg [34,37,38].