The co-receptor Tspan12 directly captures Norrin to promote ligand-specific β-catenin signaling

Wnt/β-catenin signaling directs animal development and tissue renewal in a tightly controlled, cell- and tissue-specific manner. In the central nervous system, the atypical ligand Norrin controls angiogenesis and maintenance of the blood-brain barrier and blood-retina barrier through the Wnt/β-catenin pathway. Like Wnt, Norrin activates signaling by binding and heterodimerizing the receptors Frizzled (Fzd) and Low-density lipoprotein receptor-related protein 5 or 6 (LRP5/6), leading to membrane recruitment of the intracellular transducer Dishevelled (Dvl); this ultimately results in the stabilization of the transcriptional coactivator β-catenin. Unlike Wnt, the cysteine-knot ligand Norrin only signals through Fzd4 and additionally requires the co-receptor Tspan12; however, the mechanism underlying Tspan12-mediated signal enhancement is unclear. It has been proposed that Tspan12 integrates into the Norrin-Fzd4 complex to enhance Norrin-Fzd4 affinity or otherwise allosterically modulate Fzd4 signaling. Here, we measure direct, high-affinity binding between purified Norrin and Tspan12 in a lipid environment and use AlphaFold models to interrogate this interaction interface. We find that Tspan12 and Fzd4 can simultaneously bind Norrin and that a pre-formed Tspan12/Fzd4 heterodimer, as well as cells co-expressing Tspan12 and Fzd4, more efficiently capture low concentrations of Norrin than Fzd4 alone. We also show that Tspan12 competes with both heparan sulfate proteoglycans and LRP6 for Norrin binding and that Tspan12 does not impact Fzd4-Dvl affinity in the presence or absence of Norrin. Our findings suggest that Tspan12 does not allosterically enhance Fzd4 binding to Norrin or Dvl, but instead functions to directly capture Norrin upstream of signaling.

PI(4,5)P2-stimulated positive feedback drives the recruitment of Dishevelled to Frizzled in Wnt-β-catenin signaling

In the Wnt-β-catenin pathway, Wnt binding to Frizzled (Fzd) and LRP5 or LRP6 (LRP5/6) co-receptors inhibits the degradation of the transcriptional coactivator β-catenin by recruiting the cytosolic effector Dishevelled (Dvl). Polymerization of Dvl at the plasma membrane recruits the β-catenin destruction complex, enabling the phosphorylation of LRP5/6, a key step in inhibiting β-catenin degradation. Using purified Fzd proteins reconstituted in lipid nanodiscs, we investigated the factors that promote the recruitment of Dvl to the plasma membrane. We found that the affinity of Fzd for Dvl was not affected by Wnt ligands, in contrast to other members of the GPCR superfamily for which the binding of extracellular ligands affects the affinity for downstream transducers. Instead, Fzd-Dvl binding was enhanced by increased concentration of the lipid PI(4,5)P₂, which is generated by Dvl-associated lipid kinases in response to Wnt and which is required for LRP5/6 phosphorylation. Moreover, binding to Fzd did not promote Dvl DEP domain dimerization, which has been proposed to be required for signaling downstream of Fzd. Our findings suggest a positive feedback loop in which Wnt-stimulated local PI(4,5)P₂ production enhances Dvl recruitment and further PI(4,5)P₂ production to support Dvl polymerization, LRP5/6 phosphorylation, and β-catenin stabilization.

Reconstitution of purified membrane protein dimers in lipid nanodiscs with defined stoichiometry and orientation using a split GFP tether

Many membrane proteins function as dimers or larger oligomers, including transporters, channels, certain signaling receptors, and adhesion molecules. In some cases, the interactions between individual proteins may be weak and/or dependent on specific lipids, such that detergent solubilization used for biochemical and structural studies disrupts functional oligomerization. Solubilized membrane protein oligomers can be captured in lipid nanodiscs, but this is an inefficient process that can produce stoichiometrically and topologically heterogeneous preparations. Here, we describe a technique to obtain purified homogeneous membrane protein dimers in nanodiscs using a split GFP (sGFP) tether. Complementary sGFP tags associate to tether the coexpressed dimers and control both stoichiometry and orientation within the nanodiscs, as assessed by quantitative Western blotting and negative-stain EM. The sGFP tether confers several advantages over other methods: it is highly stable in solution and in SDS-PAGE, which facilitates screening of dimer expression and purification by fluorescence, and also provides a dimer-specific purification handle for use with GFP nanobody–conjugated resin. We used this method to purify a Frizzled-4 homodimer and a Frizzled-4/low-density lipoprotein receptor–related protein 6 heterodimer in nanodiscs. These examples demonstrate the utility and flexibility of this method, which enables subsequent mechanistic molecular and structural studies of membrane protein pairs.

Structure of an endosomal signaling GPCR-G protein-β-arrestin megacomplex

Classically, G-protein-coupled receptors (GPCRs) are thought to activate G protein from the plasma membrane and are subsequently desensitized by β-arrestin (β-arr). However, some GPCRs continue to signal through G protein from internalized compartments, mediated by a GPCR-G protein-β-arr 'megaplex'. Nevertheless, the molecular architecture of the megaplex remains unknown. Here, we present its cryo-electron microscopy structure, which shows simultaneous engagement of human G protein and bovine β-arr to the core and phosphorylated tail, respectively, of a single active human chimeric β₂-adrenergic receptor with the C-terminal tail of the arginine vasopressin type 2 receptor (β₂V₂R). All three components adopt their canonical active conformations, suggesting that a single megaplex GPCR is capable of simultaneously activating G protein and β-arr. Our findings provide a structural basis for GPCR-mediated sustained internalized G protein signaling.

Measuring ligand efficacy at the mu-opioid receptor using a conformational biosensor

The intrinsic efficacy of orthosteric ligands acting at G-protein-coupled receptors (GPCRs) reflects their ability to stabilize active receptor states (R*) and is a major determinant of their physiological effects. Here, we present a direct way to quantify the efficacy of ligands by measuring the binding of a R*-specific biosensor to purified receptor employing interferometry. As an example, we use the mu-opioid receptor (µ-OR), a prototypic class A GPCR, and its active state sensor, nanobody-39 (Nb39). We demonstrate that ligands vary in their ability to recruit Nb39 to µ-OR and describe methadone, loperamide, and PZM21 as ligands that support unique R* conformation(s) of µ-OR. We further show that positive allosteric modulators of µ-OR promote formation of R* in addition to enhancing promotion by orthosteric agonists. Finally, we demonstrate that the technique can be utilized with heterotrimeric G protein. The method is cell-free, signal transduction-independent and is generally applicable to GPCRs.

Mechanistic insights into GPCR-G protein interactions

G protein-coupled receptors (GPCRs) respond to extracellular stimuli and interact with several intracellular binding partners to elicit cellular responses, including heterotrimeric G proteins. Recent structural and biophysical studies have highlighted the dynamic nature of GPCRs and G proteins and have identified specific conformational changes important for receptor-mediated nucleotide exchange on Gα. While domain separation within Gα is necessary for GDP release, opening the inter-domain interface is insufficient to stimulate nucleotide exchange. Rather, an activated receptor promotes GDP release by allosterically disrupting the nucleotide-binding site via interactions with the Gα N-termini and C-termini. Highlighting the allosteric nature of GPCRs, recent studies suggest that agonist binding alone poorly stabilizes an active conformation of several receptors. Rather, full stabilization of the receptor in an active state requires formation of the agonist-receptor-G protein ternary complex. In turn, nucleotide-free Gα is able to stabilize conformational changes around the receptor's agonist-binding site to enhance agonist affinity.

GPCR-G Protein-β-Arrestin Super-Complex Mediates Sustained G Protein Signaling

Classically, G protein-coupled receptor (GPCR) stimulation promotes G protein signaling at the plasma membrane, followed by rapid β-arrestin-mediated desensitization and receptor internalization into endosomes. However, it has been demonstrated that some GPCRs activate G proteins from within internalized cellular compartments, resulting in sustained signaling. We have used a variety of biochemical, biophysical, and cell-based methods to demonstrate the existence, functionality, and architecture of internalized receptor complexes composed of a single GPCR, β-arrestin, and G protein. These super-complexes or "megaplexes" more readily form at receptors that interact strongly with β-arrestins via a C-terminal tail containing clusters of serine/threonine phosphorylation sites. Single-particle electron microscopy analysis of negative-stained purified megaplexes reveals that a single receptor simultaneously binds through its core region with G protein and through its phosphorylated C-terminal tail with β-arrestin. The formation of such megaplexes provides a potential physical basis for the newly appreciated sustained G protein signaling from internalized GPCRs.

Fetal rhabdomyomatous nephroblastoma with a renal pelvic mass simulating sarcoma botryoides

Fetal rhabdomyomatous nephroblastomas are very rare histologic variants of Wilms' tumor. These neoplasms are composed predominantly of fetal striated muscle and isolated regions of typical trimorphic nephroblastoma. This case is presented because in addition to its rarity, part of the tumor formed a renal pelvic mass which simulated sarcoma botryoides. However, ultrastructural analysis of this area revealed nephronogenic epithelial differentiation rather than embryonal rhabdomyoblasts, a distinction of important clinical significance. The literature is reviewed, and the clinicopathologic importance of distinguishing this variant from classic Wilms' tumor, the rhabdomyosarcomatous type of Wilms' tumor and congenital mesoblastic nephroma is stressed.

Does aspirin prolong bleeding from gastric biopsies in man?

Aspirin prolongs skin bleeding time in man by inducing abnormal platelet function. Prolongation of gastric bleeding time has been postulated as a mechanism for gastric hemorrhage after aspirin in man. To determine whether endoscopic gastric biopsy is safe in patients taking aspirin, we studied the effects of acute and chronic aspirin use on gastric bleeding time in four groups of subjects. Gastric bleeding time was assessed directly following endoscopic biopsy. Skin bleeding time was done by the Mielke method. Control subjects (group I) were studied twice at one-week intervals to determine reproducibility of the gastric bleeding time technique. The effect of aspirin on gastric and skin bleeding time when given to normal volunteers for 24 hours (group II) and for two weeks (group III) and to rheumatic disease patients on a chronic basis (group IV) was also studied. In normal volunteers given aspirin for 24 hours or two weeks, gastric bleeding time was not affected in spite of skin bleeding time being significantly prolonged over baseline. Gastric bleeding time was less then skin bleeding time in all groups including patients with rheumatoid arthritis (p less than 0.05). We conclude that aspirin ingestion does not prolong gastric bleeding time in man and that gastric biopsy is not contraindicated on th basis of recent aspirin ingestion.

Follicular adenoacanthoma of the thyroid gland

A 67-year-old euthyroid black woman presented with a painful palpable thyroid nodule. Microscopic examination revealed an extensively infarcted follicular carcinoma with minimal capsular and vascular invasion and advanced squamous metaplasia. From the regions of squamous metaplasia, ultrastructural analysis demonstrated both squamous and adenomatous features within the same individual cells. We stress the prognostic importance of histologic distinction between adenoacanthomas and adenosquamous or squamous cell carcinomas. The literature is reviewed and the aetiology, pathology, classification and prognosis of primary thyroid cancers with squamous epithelium are discussed.

Midgut carcinoid tumor with carcinoid heart disease. Its presence in the absence of hepatic metastases--a case report

A 56-year-old man with a midgut carcinoid tumor had extensive metastatic involvement of lymphatic spaces and carcinoid heart disease. Unlike similar previously reported cases with carcinoid heart disease, hepatic metastases and history of the carcinoid syndrome were absent in this patient. Tumor involvement of cardiac lymphatic spaces may have been responsible for the development of carcinoid heart disease due to a local fibrogenic effect similar to the extensive fibrosis associated with tumor seen within the abdominal cavity at autopsy.

Primary histiocytic lymphoma of bone: a light and ultrastructural study of four cases

The light and ultrastructural features of primary histiocytic lymphoma of bone are indistinguishable from similar studies of respective cell types of nodular and diffuse large cell nodular and extranodal lymphomas. These ultrastructural features add further support to their lymphocytic histogenesis. There was no relationship between increasing nuclear clefts and convolutions in individual cases and the ultrastructural presence of increasing "histiocytic" features. Primary histiocytic lymphoma of bone is not a completely uniform monomorphic tumor cell population, but may vary in percentage of large noncleaved, large cleaved, large convoluted lymphocytic, histiocytic, and small atypical lymohocytic cells. Detailed study of four cases enabled division into two primary subgroups: Type I, consisting of a predominance of large noncleaved cells; and Type II, consisting of a mixture of cleaved, noncleaved, and convoluted cells. Further cases are being studied to determine whether these two groups are distinct or merely represent part of a continuous spectrum of cellular pleomorphism. In contrast to nodal lymphomas, there is no evidence at this time that proportional variations in the tumor cell population bear any relationship to biologic behavior or survival. Glycogen may be present in histiocytic lymphomas but was very rare (less than 1% total cells). Therefore, this feature as an isolated observation will not differentiate histiocytic lymphoma from Ewing's sarcoma.

Multifocal osteosarcoma: a case report with review of the literature

Multifocal osteosarcoma with unique patho-biological features in a 23-year old white male is reported. The natural history and classification of multiple osteosarcomas are reviewed. A proposal is presented for evaluation of the multicentric origin in future cases.

So-called extraskeletal Ewing's sarcoma. Report of a case with ultrastructural analysis

The light and the electron microscopic features of an extraskeletal round-cell tumor resembling Ewing's sarcoma are described. Ultrastructural observation revealed features similar to Ewing's sarcoma of bone. Variable agrees of nuclear complexity are described. These soft tissue tumors are probably composed of undifferentiated mesenchymal cells; there is no ultrastructural evidence to indicate the cell of origin.

Ewing's sarcoma. A light- and electron-microscopic study of 21 cases

Twenty-one cases of Ewing's sarcoma were analyzed by light- and electron-microscopy and the fine structure compared to that described in previous publications. In the predominant "primary" tumor cells, glycogen was abundant in 53% of cases, infrequent in 33%, and rare in 14%. In three cases, microtubules, in association with glycogen, were demonstrated. The so-called differentiated "secondary" reticular tumor cells were sparsely populated in eight cases. Evidence is presented to suggest that these so-called "secondary" reticular cells are merely "primary" tumor cells in a state of regression. Secondary cells and cells with nuclear identations or convolutions were of no discernible prognostic significance. The histogenesis of Ewing's sarcoma remains an enigma but present findings support a primitive mesenchymal origin.

Admission screening for hepatitis B surface antigen in a university hospital

Upon admission to the hospital, 7,984 patients were tested for hepatitis B surface antigen (HBsAg) by radioimmunoassay. Seventy-one patients had sera positive for HBsAg. Twenty-four (34%) were possible asymptomatic carriers in whom liver function was not further evaluated and occult liver disease could not be excluded: 16 (23%) had either a previous history or admitting diagnosis of hepatic dysfunction; eight(11%) had occult liver disease, revealed after HBsAg antigenemia was discovered; and 25 (34%) were unsuspected asymptomatic carriers whose liver function was normal. We concluded that screening for HBsAg was an effective preventive tool in identifying HBsAg-positive patients. Screening solely for the detection of occult liver disease is not an effective method because of the high cost. Perhaps because of the unsolicited nature of this data collection, screening for HBsAg was not clinically effective for the majority of patients, as evidenced by the high incidence of inadequate clinical evaluations and lack of serologic follow-up. Proposals to alleviate ineffectiveness are discussed.

Studies on manganese. 3. The biological half-life of radiomanganese in man and factors which affect this half-life

The biological half-life of manganese and some factors influencing it have been studied in man. The disappearance of manganese from the body in normal subjects is described by a curve having two exponential components. An average of 70% of the injected material was eliminated by the "slow" pathway. The half-time characterizing this component showed a small variation in normal subjects and had an average value of 39 days. The half-time for the "fast" component also showed a small variation and had an average value of 4 days. In a normal subject presumed to have a low manganese intake due to a voluntary low caloric intake, the percentage eliminated by the slow pathway increased to 84% and the half-time characterizing the pathway increased to 90 days. The half-time of the "fast" component was the same as for the normal group. 2 months after initiation of the study in this subject, a large "flushing" dose of manganese markedly increased the elimination rate which was described by a single exponential curve.A mildly iron-deficient subject showed a marked decrease in the percentage of manganese eliminated by the "slow" pathway accompanied by a less dramatic decrease in the half-time characterizing this pathway. Oral iron therapy, which corrected the mild anemia, caused a decrease in the elimination rate and the altered curve was described by a single exponential component. Preloading two subjects with manganese resulted in a great decrease in the fraction eliminated by the "slow" pathway with less effect on the half-time. The subject with the largest preloading dose showed no "slow" component at all. Observations on the red cells of some of these subjects showed that a small but definite fraction was incorporated into the erythrocytes. In the mildly iron-deficient subject, our observations suggest an interrelationship between manganese and iron metabolism.