Cleavage of the thyrotropin receptor

Cleavage of the human thyrotropin receptor by ADAM10 is regulated by thyrotropin

The thyrotropin receptor (TSHR) has a unique 50 residue (317-366) ectodomain insertion that sets it apart from other glycoprotein hormone receptors (GPHRs). Other ancient members of the leucine-rich repeat G protein-coupled receptor (GPCR) (LGR) family do exhibit ectodomain insertions of variable lengths and sequences. The TSHR-specific insert is digested, apparently spontaneously, to release the ectodomain (A-subunit) leaving the balance of the ectodomain attached to the serpentine (B-subunit). Despite concerted efforts for the last 12 years by many laboratories, the enzyme involved in TSHR cleavage has not been identified and a physiologic role for this process remains unclear. Several lines of evidence had suggested that the TSHR protease is likely a member of the a disintegrin and metalloprotease (ADAM) family of metalloproteases. We show here that the expression of ADAM10 was specific to the thyroid by specially designed DNA microarrays. We also show that TSH increases TSHR cleavage in a dose-dependent manner. To prove that ADAM10 is indeed the TSHR cleavage enzyme, we investigated the effect of TSH-induced cleavage by a peptide based on a motif (TSHR residues 334-349), shared with known ADAM10 substrates. TSH increased dose dependently TSHR ectodomain cleavage in the presence of wild-type peptide but not a scrambled control peptide. Interestingly, TSH increased the abundance of non-cleaved single chain receptor, as well higher molecular forms of the A-subunit, despite their enhancement of the appearance of the fully digested A-subunit. This TSH-related increase in TSHR digested forms was further increased by wild-type peptide. We have identified for the first time ADAM10 as the TSHR cleavage enzyme and shown that TSH regulates its activation.

Evolutionary divergence of thyrotropin receptor structure

The availability of 18 thyrotropin receptor (TSHR) sequences, including two recent entries for primates and seven from fish, have allowed us to investigate diversification of residues or domains during evolution. We used a likelihood ratio test for evolutionary rate shifts [Proc. Natl. Acad. Sci. 98 (2001) 14512] using LH/CGR sequences as an out-group. At each residue in the alignment, a statistical test was performed for a rate shift at the divergence between mammals and fish. Eighty-two rate shift sites were found, significantly more than was expected (p < 0.0001). The occurrence of rate shifts was highest in the intracellular tail, lowest in the transmembrane serpentine and intermediate in the ectodomain. In 52 mammalian sites, the rates were significantly faster than for the corresponding sites in fish. We have identified rate shift in sites important to TSHR function or in intimate proximity to such regions. The former category includes residues 53 and 55 (of LLR1 beta strand) and 253 and 255 (of LLR9 beta strand), crucial to TSH thyrotropic activity, residue 113, the site of N-linked glycosylation limited to humans, residue 310, an important switch in the hinge region for receptor binding and constitutive activity and residue 382 which centres a motif important for TSH-mediated receptor activation. The rate shifts positions close to functional region include a site proximal to a TSHR-specific motif on LLR3 beta strand, sites important in TM helix structure and homodimerization as well as, in the case of the third intracellular loop, to TSHR/G protein coupling. Rate shift analyses have identified residues whose manipulation in the human TSHR may lead to better understanding of receptor functions and help in the creation of designer analogues.

Evolution of the thyrotropin receptor: a G protein coupled receptor with an intrinsic capacity to dimerize

The rapidly escalating number of genome sequences has emphasized the basic tenants of the schema of life. By the same token comparisons according to specialized function or niche within nature expose genomic strategies to optimize the use of resources and ensure biological success. Increasing complexity may result from diversification, shuffling, and re-arrangement of an otherwise limited functional genomic complement. To further test the concept of relative structural plasticity of the TSH receptor we sequenced the TSHR gene of two Old World monkey species Macaca mulatta and Cercopithecus aethiops, evolutionary removed from Homo sapiens by >20Myr. Both genes encoded a protein of 764 residues. This structure was 99% homologous between the two species of Old World monkeys while C. aethiops was 97% and M. mulatta was 96% homologous to H. sapiens. TSHR sequence comparisons were sought for an additional eight mammals as well as four (two Salmon, Tilapia, and Sea Bass) from teleosts. The amino-acid sequences of the 14 TSH receptors were similar. The most variable sequences were those of the intracellular tail and the distal cysteine-rich C-terminus flanking region of the ectodomain, whereas the trans-membrane domain was most preserved. Some sequences were decidedly H. sapiens specific, while others were primate specific or showed the changes expected of evolutionary descent. Others, however, exhibited "cross-species polymorphism," sometimes at quite remarkable evolutionary distances. As opposed to H. sapiens the sequence differences may have subtle influences on TSHR function or may affect long-range compensation for radical changes in adducts. The two Old World monkeys share with other lower mammals the absence of a glycosylation site at 113-115. Sea Bass and Tilapia have four glycosylation sites, whereas the two salmon receptors have only three. Changes in some critical residues raise questions about variation in function: thus S281 is conserved in all mammals and an important determinant of negative agonist function of TSHR is replaced by R in Sea Bass. Likewise the K183, found at an important transitional region at LRR 6 conserved in all mammals, is represented by M in fish and may contribute to TSHR lutenization in fish. There is no evidence that evolutionary changes in primate receptors are more rapid than that in other mammals and the separation times of different mammals based on silent nucleotide changes of TSHR are closely parallel to archaeological estimates. Results of correlated mutation analysis, referenced to the rhodopsin crystal structure, affirms dimerization of TSHR transmembrane helices. In addition, it suggests the involvement of critical lipid-facing residues in the helices in receptor dimerization and oligomerization. We highlight the value of evolutionary informatics and set the stage for dissecting out potential subtle differences in TSHR function associated with structural variations.