The transmembrane adapter LAT plays a central role in immune receptor signalling.

The transmembrane adapter LAT (linker for activation of T cells) plays a central role in signalling by ITAM bearing receptors expressed on T cells, natural killer cells, mast cells and platelets. Receptor engagement leads to the phosphorylation of tyrosine residues present in the intracellular domain of LAT and formation of a multiprotein complex with other adapter molecules and enzymes including Grb2, Gads/SLP-76 and PLCgamma isoforms. These signalling events predominantly take place in glycolipid-enriched membrane domains. The constitutive presence of LAT in GEMs enables its function as the main scaffolding protein for the organization of GEM-localized signalling. The study of LAT-deficient mice and LAT-deficient cell lines further emphasizes the importance of LAT for these signalling cascades but also defines the existence of LAT-independent events downstream of the Syk-family kinase-ITAM complex.

The role of ITAM- and ITIM-coupled receptors in platelet activation by collagen.

The major activation-inducing collagen receptor glycoprotein VI (GPVI) has been cloned within the last two years. It is a member of the Ig superfamily of proteins and is constitutively associated with the ITAM-bearing Fc receptor gamma-chain (FcR gamma-chain). GPVI signals through a pathway that involves several of the proteins used by Fc, B- and T-lymphocyte receptors and which takes place in glycolipid-enriched membrane domains in the plasma membrane known as GEMs. Responses to GPVI are regulated by PECAM-1 (CD31) and possibly other ITIM-bearing receptors. Despite a pivotal role for GPVI, there are important differences between signalling events to collagen and GPVI-specific ligands. This may reflect a role for co-receptors in the response to collagen.

Detection of thyroid-stimulating hormone receptor and Gsalpha mutations: in 75 toxic thyroid nodules by denaturing gradient gel electrophoresis.

The actual frequency of constitutively activating thyrotropin receptor or Gsalpha mutations in toxic thyroid nodules (TTNs) remains controversial as considerable variation in the prevalence of these mutations has been reported. We studied a series of 75 consecutive TTNs and performed mutation screening by the more sensitive method of denaturing gradient gel electrophoresis (DGGE) in addition to direct sequencing. Furthermore, the likelihood of somatic mutations occurring in genes other than that for the thyroid-stimulating hormone receptor (TSHR) and exons 7-9 of the Gsalpha protein gene was determined by clonality analysis of TTNs, which did not harbor mutations in the investigated genes. In 43 of 75 TTNs (57%) constitutively active TSHR mutations were identified. Six TSHR mutations were detected only by DGGE, underlining the importance of a sensitive screening method. Novel, constitutively activating mutations were identified at positions 425 (Ser-->Leu) and 512 (Leu-->Glu/Arg). Furthermore, a new base substitution was detected at position Pro639Ala (CCA-->GCA). Ten of 20 TSHR or Gsalpha mutation negative cases (50%) showed nonrandom X-chromosome inactivation, indicating clonal origin. In conclusion, somatic, constitutively activating TSHR mutations appear to be a major cause of TTNs (57%), while mutations in Gsalpha play a minor role (3%). The mutation negative but clonal cases indicate a probable involvement of somatic mutations other than in the TSH receptor or Gsalpha genes as the molecular cause of these hot nodules.

Thyrotropin receptor mutations as a tool to understand thyrotropin receptor action.

A large number of mutations have been identified in the thyrotropin (TSH) receptor (TSHR) gene causing human diseases. Toxic thyroid nodules are frequently associated with somatic constitutively activating TSHR mutations. Autosomal dominant non-autoimmune hyperthyroidism is caused by activating TSHR germline mutations. Inactivating germline mutations cause TSH unresponsiveness. Discovery of the different TSHR mutations in various regions of the receptor molecule has led to the identification of important domains for intramolecular TSHR signal transduction. However, despite the functional characterization of the naturally occurring mutations the precise molecular mechanisms of receptor activation including the processes of hormone binding, intramolecular signaling between the different TSHR domains and of G protein coupling are not completely understood. This review discusses the importance of the various receptor domains for TSHR activation identified on the basis of the naturally occurring gain or loss of function mutations and in vitro investigations performed with site-directed mutagenesis, synthetic peptides, or antibodies. Several in vitro studies have provided new insights into structure-function relationships by site-directed mutagenesis in combination with molecular modeling. These in vitro investigations have often been guided by naturally occurring mutations and have provided new insights into intramolecular changes during receptor activation. This has led to progress in understanding the mechanism of TSHR activation.

Functional characterization of five constitutively activating thyrotrophin receptor mutations.

OBJECTIVE: Gain of function mutations of the thyrotrophin receptor (TSHR) affect several functional characteristics, such as cAMP and inositol phosphate (IP) accumulation, cell surface expression and TSH affinity. In this study we compared five constitutively activating TSHR mutations, four receptors with a point mutation (S505N, L629F, I630L, V656F) and a nine amino acid (aa) deletion mutant (aa positions 613-621) for these functional parameters in parallel transfection experiments. METHODS: The wild-type TSHR (wt) and TSHRs containing the mutations S505N, L629F, I630L, V656F and the deletion 613-621 (all cloned in the expression vector pSVL) were transiently expressed in COS-7 cells in parallel experiments. Forty-eight hours after transfection the basal and stimulated cAMP and inositol phosphate accumulation as well as the cell surface expression (by FACS and ELISA), KD-values and TSHR down regulation by different stimuli were determined. RESULTS: In contrast to the very different values for specific constitutive activity (sca) (ranging from 7.5 to 100.3-fold wt) and very different levels of receptor cell surface expression (11-94% wt level) the basal cAMP accumulation determined in transfected COS-7 cells was surprisingly uniform (6.5-8.0 over wt basal). None of the point mutated receptors constitutively activates the phospholipase C cascade. In contrast the deletion 613-621 mutant showed constitutive activity for the IP pathway with a twofold increase in basal IP accumulation compared to the wild type TSHR. All investigated TSHR-mutants showed a TSH-stimulated receptor down-regulation, which seems to be independent of the phospholipase C pathway. CONCLUSIONS: The uniform basal cAMP values in spite of the large variation in specific constitutive activity values suggest that the COS-7 cell overexpression system used for the in vitro characterization is partly regulated. This regulation is most likely due to receptor down regulation. The TSHR deletion mutant (613-621) showed a constitutive activity for both the Galphas and the Galphaq/11 pathways. The TSH-mediated IP-stimulation by this mutant contrasts with its unresponsiveness to TSH for cAMP accumulation and therefore supports the model of different active conformations of the TSHR.

Variable phenotype associated with Ser505Asn-activating thyrotropin-receptor germline mutation.

Constitutively activating thyrotropin-receptor (TSHR) germline mutations have been identified as a molecular cause of hereditary nonautoimmune hyperthyroidism. To date, seven cases of familial and six cases of sporadic nonautoimmune hyperthyroidism have been described associated with 13 different TSHR germline mutations, with a variable clinical course. We report the case of a 12.3-year-old girl with a history of thyrotoxicosis since the age of 11 months who developed diffuse thyroid hyperplasia at the age of 4.5 years. The patient has required continuous moderate-dose antithyroid medication, to maintain euthyroidism. There were no clinical signs of autoimmune thyroid disease and autoantibodies were negative. An activating germline mutation in the TSHR gene was suspected and was found in TSHR exon 10 (Ser505Asn) but was absent in the girl's mother. This same mutation, was first reported in a patient with severe intrauterine hyperthyroidism with early and progressive goiter development. Our patient had a significantly less severe clinical course with later onset compared to the original patient with the same TSHR germline mutation.

Autosomal dominant nonautoimmune hyperthyroidism. Clinical features-diagnosis-therapy.

Autosomal dominant nonautoimmune hyperthyroidism is a hereditary form of hyperthyroidism caused by constitutively activating germline mutations in the TSH-receptor gene. Clinical features comprise familial prevalence of thyroid autonomy in more than 2 generations and conditions of persisting neonatal hyperthyroidism or nonautoimmune hyperthyroidism of childhood onset with frequent relapses of hyperthyroidism under thyrostatic therapy and after thyroid surgery. Once clinically suspected the diagnosis can be confirmed by mutation analysis of genomic DNA extracted from a routinely obtainable EDTA blood sample. In patients with hereditary nonautoimmune hyperthyroidism a near total thyroidectomy is recommended as the first line treatment to avoid relapses from residual thyroid tissue with the activating TSHR mutation. Furthermore, genetic counselling of the affected patients is advised.

Deletions in the third intracellular loop of the thyrotropin receptor. A new mechanism for constitutive activation.

Gain-of-function mutations of the thyrotropin receptor (TSHR) gene have been invoked as one of the major causes of toxic thyroid adenomas. In a toxic thyroid nodule, we recently identified a 9-amino acid deletion (amino acid positions 613-621) within the third intracellular (i3) loop of the TSHR resulting in constitutive receptor activity. This finding exemplifies a new mechanism of TSHR activation and raises new questions concerning the function of the i3 loop. Because the i3 loop is thought to be critical for receptor/G protein interaction in many receptors, we systematically reexamined the role of the TSHR's i3 loop for G protein coupling. Thus, various deletion mutants were generated and functionally characterized. We identified an optimal deletion length responsible for constitutive activity. If the number of deleted amino acids was reduced, elevated basal cAMP accumulation was found to be concomitantly diminished. Expansion of the deletion dramatically impaired cell surface expression of the receptor. Shifting the deletion toward the N terminus of the i3 loop resulted in unaltered strong constitutive receptor activity. In contrast, translocation of the deletion toward the C terminus led to significantly reduced basal cAMP formation, most probably due to destruction of a conserved cluster of amino acids. In this study, we show for the first time that amino acid deletions within the i3 loop of a G protein-coupled receptor result in constitutive receptor activity. In the TSHR, 75% of the i3 loop generally assumed to play an essential role in G protein coupling can be deleted without rendering the mutant receptor unresponsive to thyrotropin. These findings support a novel model explaining the molecular events accompanying receptor activation by agonist.

Identification of constitutively activating somatic thyrotropin receptor mutations in a subset of toxic multinodular goiters.

Constitutively activating mutations in the TSH receptor (TSHR) gene and in the Gs alpha gene are frequent molecular causes for solitary toxic nodules of the thyroid. However, the etiology of toxic multinodular goiter is still largely unknown. Therefore, DNA from nodular and quiescent surrounding tissue of six patients with toxic multinodular goiters was screened for mutations in exons 9 and 10 of the TSHR gene and exons 7-10 of the Gs alpha gene by direct automated sequencing. In one patient, two different somatic TSHR mutations were identified in two different toxic nodules (L632I and F631L). In another patient, two different toxic nodules harbored the same TSHR mutation (I630L), whereas only one TSHR mutation (F631L) was identified in one of the two toxic nodules of an additional patient. In the other three patients, no mutations could be found in exons 9 and 10 of the TSHR gene or in exons 7-10 of the Gs alpha gene. Our results demonstrate that not only solitary toxic adenomas but also toxic multinodular goiters can be caused by constitutively activating mutations of the TSHR. In addition to mutations in the TSHR and possibly in Gs alpha, there are probably other still unknown mechanisms that cause hot nodules in toxic multinodular goiters.

Identification of a new thyrotropin receptor germline mutation (Leu629Phe) in a family with neonatal onset of autosomal dominant nonautoimmune hyperthyroidism.

Constitutively activating germline mutations in the TSH receptor (TSHR) gene have been identified as a cause of autosomal dominant nonautoimmune hyperthyroidism and sporadic congenital hyperthyroidism. We report a 10-yr-old boy and his 31-yr-old mother, both presenting with a history of recurring toxic thyroid hyperplasia and no evidence for autoimmune thyroid disease. In the boy, onset of hyperthyroidism and goiter was neonatal. In the mother, onset of thyroid disease dates back to early childhood. There was no history of thyroid disease in the rest of the family. Screening for germline mutations in exon 10 of the TSHR was performed by direct sequencing of genomic DNA extracted from peripheral blood leukocytes of both patients. In the boy and his mother, an identical heterozygous TSHR mutation was identified, exchanging leucine for phenylalanine at residue 629 of the TSHR (TTG-->TTT). Transient expression of the mutated TSHR construct in COS-7 cells confirmed the constitutive activity of the new TSHR germline mutation. This is the second family displaying congenital manifestation of hyperthyroidism in familial nonautoimmune hyperthyroidism.

Sporadic congenital hyperthyroidism due to a spontaneous germline mutation in the thyrotropin receptor gene.

Neonatal hyperthyroidism in the absence of maternal autoimmune thyroid disease and without thyroid-stimulating antibodies in the child is rare. We here describe a boy with severe intrauterine hyperthyroidism and advanced bone age in the absence of thyroid-stimulating autoantibodies. After long term antithyroid treatment and relapse of hyperthyroidism, a near-total thyroid resection was performed. The necessity to progressively decrease postoperative thyroid hormone replacement indicates thyroid tissue regrowth in the small thyroid remnant. Analysis of the genomic DNA of the child's peripheral leukocytes showed a G to A base exchange that led to a heterozygous Ser to Asn conversion at position 505 in the third transmembrane region of the TSH receptor (TSHR). The absence of the Ser505 Asn mutation in all other family members identifies the child's TSHR mutation as a sporadic germline mutation. Transient expression of the mutated TSH receptor in COS-7 cells showed a constitutively activated cAMP cascade. We thus identified a new constitutively activating germline mutation. Neonates with persistent nonautoimmune hyperthyroidism should be investigated for TSHR germline mutations. Because of frequent relapses, patients with sporadic congenital nonautoimmune hyperthyroidism should be treated with early subtotal to near-total thyroid resection. Moreover, post-operative radioiodine treatment should be considered.

Somatic mutations in the thyrotropin receptor gene and not in the Gs alpha protein gene in 31 toxic thyroid nodules.

Studies on frequency and distribution pattern of TSH receptor (TSHR) and Gs alpha protein (gsp) mutations in toxic thyroid nodules (TTNs) reported conflicting results, most likely also related to the different screening methods applied and the investigation of only part of exon 10 of the TSHR. Therefore, we screened a consecutive series of 31 TTNs for both TSHR and gsp mutations by direct sequencing of exon 9 and the entire exon 10 of the TSHR gene and exons 7-10 of the gsp gene. Somatic TSHR mutations were identified in 15 of 31 TTNs. TSHR mutations were localized in the third intracellular loop (Asp619Gly and Ala623Val), the sixth transmembrane segment (Phe631Leu and Thr632Ile, Asp633Glu) and the second extracellular loop (Ile568Thr). One mutation was found in the extracellular TSHR domain (Ser281Asn). Two new TSHR mutations were identified. One involves codon 656 in the third extracellular loop (Val656Phe). The other new mutation is a 27-bp deletion in the third intracellular loop resulting in deletion of 9 amino acids at codons 613-621. Transient expression of the new TSHR mutations in COS-7 cells demonstrated their constitutive activity. No mutation was found in exons 7-10 of the gsp gene. This finding was confirmed by an allele-specific PCR for mutations in gsp codons 201 (Arg-->His, Cys) and 227 (Gln-->His, Arg). Our data indicate that constitutively activating TSHR mutations can be found in 48% of TTNs and thus currently represent the most frequent molecular mechanism known in the etiopathogenesis of TTNs. Moreover, the absence of gsp mutations in our series argues for an only minor role of these mutations in TTNs. Constitutive activation of the TSHR by a deletion in a region that might be involved in G protein coupling of the TSHR offers new insights into TSHR activation.

Constitutively activating mutations of the thyrotropin receptor and thyroid disease.

Recent advances in the understanding of the molecular biology of the TSH receptor had a considerable impact on several aspects of thyroidology. Identification and functional characterization of somatic mutations in the TSH receptor gene that activate the receptor ligand-independently. Provide an explanation for the molecular pathomechanisms of solitary toxic thyroid adenomas. Moreover, we have recently demonstrated, that multiple hyperfunctioning adenomas in the same thyroid gland can be caused by identical or different somatic Thyrotropin receptor mutations. Furthermore, germline mutations with similar characteristics have been reported in the TSH receptor gene of families with autosomal dominant forms of non-autoimmune hyperthyroidism and in two cases of sporadic congenital non-autoimmune hyperthyroidism. Interestingly, constitutively activating Thyrotropin receptor alterations have also been identified in differentiated thyroid carcinoma, thus implying a possible role of the constitutively active TSH receptor in the aetiopathology of both benign and malignant thyroid neoplasia.