Thyrotropin-ganglioside interactions and their relationship to the structure and function of thyrotropin receptors.

Abstract

Gangliosides inhibit 1251-labeled thyrotropin binding to the thyrotropin receptors on bovine thyroid plasma membranes, on guinea pig retro-orbital tissue plasma membranes, and on human adipocyte membranes. This inhibition by gangliosides is critically altered by the number and location of the sialic acid residues within the ganglioside structure, the efficacy of inhibition having the following order: GD1b > GTI > GM1 > GM2 = GM3 > GD1a. The inhibition results from the interaction of thyrotropin and gangliosides, rather than the interaction of membrane and gangliosides. Fluorescence studies show that the inhibition is associated with a distinct conformational change of the thyrotropin molecule and that the progression from a "noninhibitory conformation" to an "inhibitory conformation" parallels exactly the order of effectiveness in inhibiting 125I-Yabeled thyrotropin binding. The ganglioside inhibition of 1251-labeled thyrotropin binding appears to be hormonally specific in that it is not affected by a umin, glucagon, insulin, prolactin, follicle-stimulating hormone, growth hormone, or corticotropin. The possibility that a ganglioside or ganglioside-like structure is a component of the thyrotropin receptor is suggested by the finding that gangliosides more complex than N-acetylneuraminylgalactosylglucosylceramide are present in bovine thyroid membranes in much higher quantities than have been previously found in extraneural tissue. The finding that the B component of cholera toxin, which also interacts with gangliosides, has a peptide sequence in common with the f, subunit of thyrotropin, suggests that thyrotropin and cholera toxin may be analogous in their mode of action on the membrane.In previous studies that detailed the solubilization of the thyrotropin (TSH) receptor from bovine thyroid plasma membranes, tryptic digestion was shown to yield a receptor fragment that exhibited specific TSH binding and had properties similar to those exhibited by the TSH receptor prior to solubilization (1, 2). This receptor fragment was purified by chromatography over TSH-Sepharose preparations (1, 2); it was shown to have a molecular weight of 25,000-30,000 by gel electrophoresis in the presence of sodium dodecyl sulfate (2), and to contain 30% carbohydrate and 10% sialic acid by weight (1). The sialic acid was vital to receptor function since neuraminidase digestion eliminated the ability of both the purified receptor fragment and the crude solubilized receptor preparation to specifically bind TSH (1).Recent studies have indicated that gangliosides, glycosphingolipids that contain sialic acid, specifically interact with cholera toxin and that variations in the oligosaccharide structure of the gangliosides influence this interaction (3-5). The present report demonstrates that TSH also interacts with gangliosides and that the location and number of the sialic acid residues on the ganglioside molecule is critical to this interaction. The report further shows that there is a sequence homology in the B component of chole...

Molecular cloning of the human thyrotropin‐β subunit gene

“…The nucleotide sequence, on the other hand, is poorly conserved with homologous regions only in exon 2 of the hLw gene ( fig.4b). This narrow but highly conserved region, both in terms of amino acid sequence and nucleotide sequence, was identical to the corresponding region in ,8 subunit of other glycoproteins [30] that is known as the CAGY region. From the standpoint of molecular evolution, it is of interest that the region of complete identity in nucleotide sequence is just limited to the CAGY region among other observed regions of homology in amino acid sequence.…”

Reevaluation of the role of gangliosides in the binding and action of thyrotropin.

“…In contrast to the complex array of gangliosides found in bovine thyroid (3,4), FRTL cells have a relatively simple ganglioside composition. The major ganglioside is GM3 with only trace amounts of GM, and GDLa.…”

“…Pretreatment of FRTL cells with mixed brain gangliosides resulted in a 10-fold increase in cholera toxin binding. (3,6) and that TSH also alters the membrane conductance oflipid bilayers containing gangliosides (9), analogous to the effects of cholera toxin on model membranes containing galactosyl-N-acetylgalactosaminyl-[N-acetylneuraminyl]-galactosylglucosylceramide (GM,) (10-12). The ability ofcholera toxin to inhibit TSH binding to thyroid membranes (13) and the existence of homologous amino acid sequences in the a and A subunits and the 3 and B subunits of TSH and cholera toxin, respectively (3, 14), raised the possibility that both the hormone and the toxin may utilize gangliosides as receptors and have similar mechanisms of action (3, 13).…”

“…Gangliosides have been shown to inhibit TSH binding to thyroid plasma membranes (3)(4)(5)(6), and TSH has been shown to bind to liposomes containing gangliosides (7,8). Further, it has been demonstrated that the interaction of TSH with gangliosides leads to a perturbation in the TSH molecule (3,6) and that TSH also alters the membrane conductance oflipid bilayers containing gangliosides (9), analogous to the effects of cholera toxin on model membranes containing galactosyl-N-acetylgalactosaminyl-[N-acetylneuraminyl]-galactosylglucosylceramide (GM,) (10)(11)(12).…”

GD1b-Derived Gangliosides Modulate FcεRI Endocytosis in Mast Cells

“…Phosphorylation of the insulin receptor is also inhibited by the exogenous administration of GM3 (Tagami et al 2002). It is now well established that selected glycosphingolipids bind to various receptors such as thyrotropin (Mullin et al 1976), vitronectin (Cheresh et al 1987), insulin (Kabayama et al 2007), epidermal growth factor (EGF; Miljan et al 2002), and platelet-derived growth factor (PDGF; Bremer et al 1984). Other studies have shown that the ganglioside GM1 also plays a crucial role in modulating excitatory opioid receptor functions Shen 1998, 2004).…”