Ternary complex between placental lactogen and the extracellular domain of the prolactin receptor

Natriuretic peptide receptor A activation stabilizes a membrane-distal dimer interface

We have shown previously (Rondeau, J.-J., McNicoll, N., Gagnon, J., Bouchard, N., Ong, H., and De Léan, A. (1995) Biochemistry 34, 2130-2136) that atrial natriuretic peptide (ANP) stabilizes a dimeric form of the natriuretic peptide receptor A (NPRA) by simultaneously interacting with both receptor subunits. However, the first crystallographic study of unliganded NPRA extracellular domain documented a V-shaped dimer involving a membrane-proximal dimer interface and separate binding sites for ANP on each monomer. We explored the possibility of an alternative A-shaped dimer involving a membrane-distal dimer interface by substituting an unpaired solvent-exposed cysteine for Trp(74) in the amino-terminal lobe of full-length NPRA. The predicted spacing between Trp(74) from both subunits drastically differs, depending on whether the V-shaped (84 A) or the A-shaped (8 A) dimer model is considered. In contrast with the expected results for the reported V-shaped dimer, the NPRA(W74C) mutant was constitutively covalently dimeric. Also, the subunits spontaneously reassociated following transient disulfide reduction by dithiothreitol and reoxidation. However, ANP could neither bind to nor activate NPRA(W74C). Permanent disulfide opening by reduction with dithiothreitol and alkylation with N-ethylmaleimide rescued ANP binding to NPRA(W74C). The NPRA mutant could be maintained as a covalent dimer while preserving its function by crosslinking with the bifunctional alkylating agent phenylenedimaleimides (PDM), the ortho-substituted oPDM being more efficient than mPDM or pPDM. These results indicate that the membrane-distal lobe of the NPRAM extracellular domains are dynamically interfacing in the unliganded state and that ANP binding stabilizes the receptor dimer with more stringent spacing at the dimer interface.

Determination of the energetics governing the regulatory step in growth hormone-induced receptor homodimerization

Signaling in the human growth hormone (hGH)-human GH receptor system is initiated by a controlled sequential two-step hormone-induced dimerization of two hGH receptors via their extracellular domains (ECDs). Little is currently known about the energetics governing the important regulatory step in receptor signaling (step 2) because of previously existing experimental barriers in characterizing the binding of the second receptor (ECD2). A further complication is that ECD2 binds through contacts from two spatially distinct sites: through its N-terminal domain to hGH, and to ECD1 through its C-terminal domain, which forms a pseudo-2-fold symmetrical interaction between the stems of the two receptors. We report here a detailed evaluation of the energetics of step 2 binding using a modified surface plasmon resonance method that is able to measure accurately the kinetics of the trimolecular binding process and separate the effects of the two binding sites. The binding kinetics of 23 single and 126 ECD1-ECD2 pair-wise alanine mutations was measured. Although both of the ECD2 binding interfaces were found to be important, the ECD1-ECD2 stem-stem contact is the stronger of the two. It was determined that most residues in the binding interfaces act in additive fashion, and that the six residues common in both ECDs contribute very differently to homodimerization depending on which ECD they reside in. This interface is characterized by a binding "hot-spot" consisting of a core of three residues in ECD1 and two in ECD2. There is no similar hot-spot in the N-terminal domain of ECD2 binding to Site2 of hGH. This study suggests ways to engineer ECD molecules that will bind specifically to either Site1 or Site2 of hGH, providing novel reagents for biophysical and biological studies.

Structure of the extracellular domains of the human interleukin-6 receptor alpha -chain

Dysregulated production of IL-6 and its receptor (IL-6R) are implicated in the pathogenesis of multiple myeloma, autoimmune diseases and prostate cancer. The IL-6R complex comprises two molecules each of IL-6, IL-6R, and the signaling molecule, gp130. Here, we report the x-ray structure (2.4 A) of the IL-6R ectodomains. The N-terminal strand of the Ig-like domain (D(1)) is disulfide-bonded to domain D(2), and domains D(2) and D(3), the cytokine-binding domain, are structurally similar to known cytokine-binding domains. The head-to-tail packing of two closely associated IL-6R molecules observed in the crystal may be representative of the configuration of the physiological dimer of IL-6R and provides new insight into the architecture of the IL-6R complex.

Subcloning, expression, purification, and characterization of recombinant human leptin-binding domain

A subdomain of the human leptin receptor encoding part of the extracellular domain (amino acids 428 to 635) was subcloned, expressed in a prokaryotic host, and purified to homogeneity, as evidenced by SDS-PAGE, with over 95% monomeric protein. The purified leptin-binding domain (LBD) exhibited the predicted beta structure, was capable of binding human, ovine, and chicken leptins, and formed a stable 1:1 complex with all mammalian leptins. The binding kinetics, assayed by surface plasmon resonance methodology, showed respective k(on) and k(off) values (mean +/- S.E.) of 1.20 +/- 0.23 x 10(-5) mol(-1) s(-1) and 1.85 +/- 0.30 x 10(-3) s(-1) and a K(d) value of 1.54 x 10(-8) m. Similar results were achieved with conventional binding experiments. LBD blocked leptin-induced, but not interleukin-3-induced, proliferation of BAF/3 cells stably transfected with the long form of human leptin receptor. The modeled LBD structure and the known three-dimensional structure of human leptin were used to construct a model of 1:1 LBD.human leptin complex. Two main residues, Phe-500, located in loop L3, and Tyr-441, located in L1, are suggested to contribute to leptin binding.

Dimerization, ubiquitylation and endocytosis go together in growth hormone receptor function

Internalization of membrane proteins has been studied for more than three decades without solving all the underlying mechanisms. Our knowledge of the clathrin-coated endocytosis is sufficient to understand the basic principles. However, more detailed insight is required to recognize why different proteins enter clathrin-coated pits with different rates and affinities. In addition to clathrin coat components, several adapter systems and even more accessory proteins have been described to preselect membrane proteins before they can enter cells. Recent experimental data have identified the ubiquitin-proteasome system as a regulatory system both in endocytic and lysosomal membrane traffic. This system is well-known for its basic regulatory function in protein degradation, and controls a magnitude of key events. In this review, we will discuss the complexity and implications of this mechanism for membrane trafficking with emphasis on the growth hormone receptor.

Treatment of immune neuropathies

PURPOSE OF THE REVIEW

Guillain-Barré syndrome (GBS), chronic inflammatory demyelinating poly(radiculo)neuropathy (CIDP) and multifocal motor neuropathy (MMN) are potentially treatable disorders. The use of appropriate assessment scales to evaluate the effects of treatment is essential. Recent therapeutic trials and the question of whether patients with mild disease or other variants of these disorders need to be treated are discussed.

RECENT FINDINGS

Recent clinical trials and Cochrane reviews give new information on the effect of various treatments in patients with GBS, CIDP and MMN. Intravenous immunoglobulin remains the only treatment proven to be effective in MMN. Combinations of treatment may be even more effective in GBS. Studies on prognostic factors related to improvement have been reported. Whether patients with Miller-Fisher syndrome or those with mild GBS should also be treated is still debated. New assessment scales at the disability and handicap level have now been evaluated for GBS and CIDP, and are ready for use. Results of studies in experimental models contribute to our understanding of the mechanism of action of intravenous immunoglobulin.

SUMMARY

Recent new information on the use of intravenous immunoglobulin and steroids indicates that the former should remain the cornerstone of treatment for GBS and MMN, and probably also for CIDP. Whether steroids not only suppress disease activity in CIDP but also eradicate the disease remains to be established. Some GBS patients have secondary deterioration or finally turn out to have CIDP; additional information in this group of patients may lead to more appropriate disease management. Most patients with CIDP and those with MMN need long-term treatment. New treatment strategies should now focus also on the effect and the costs of treatment over long-term follow up.

Characterization of a novel and functional human prolactin receptor isoform (deltaS1PRLr) containing only one extracellular fibronectin-like domain

Prolactin (PRL)-dependent signaling occurs as the result of ligand-induced homodimerization of the PRL receptor (PRLr). To date, short, intermediate, and long human PRLr isoforms have been characterized. To investigate the expression of other possible human PRLr isoforms, RT-PCR was performed on mRNA isolated from the breast carcinoma cell line T47D. A 1.5-kb PCR fragment was isolated, subcloned, and sequenced. The PCR product exhibited a nucleotide sequence 100% homologous to the human long isoform except bp 71-373 were deleted, which code for the S1 motif of the extracellular domain. Therefore, this isoform was designated the deltaS1 PRLr. Northern analysis revealed variable deltaS1 PRLr mRNA expression in a variety of tissues. Transfection of Chinese hamster ovary cells with deltaS1 cDNA showed the isoform is expressed at the protein level on the cell surface with a molecular mass of approximately 70 kDa. Kinetic studies indicated the deltaS1 isoform bound ligand at a lower affinity than wild-type receptor. The deltaS1 PRLr was also shown to activate the proximal signaling molecule Jak2 upon addition of ligand to transfected cells, and, unlike the long PRLr, high concentrations of ligand did not function as a self-antagonist to signaling during intervals of PRL serum elevation, i.e. stress and pregnancy. Given its apparent widespread expression, this PRLr isoform may contribute to PRL action. Furthermore, the functionality of this receptor raises interesting questions regarding the minimal extracellular domain necessary for ligand-induced receptor signaling.

Sequence conservation in families whose members have little or no sequence similarity: the four-helical cytokines and cytochromes

Proteins for which there are good structural, functional and genetic similarities that imply a common evolutionary origin, can have sequences whose similarities are low or undetectable by conventional sequence comparison procedures. Do these proteins have sequence conservation beyond the simple conservation of hydrophobic and hydrophilic character at specific sites and if they do what is its nature? To answer these questions we have analysed the structures and sequences of two superfamilies: the four-helical cytokines and cytochromes c'-b(562). Members of these superfamilies have sequence similarities that are either very low or not detectable. The cytokine superfamily has within it a long chain family and a short chain family. The sequences of known representative structures of the two families were aligned using structural information. From these alignments we identified the regions that conserve the same main-chain conformation: the common core (CC). For members of the same family, the CC comprises some 50% of the individual structures; for the combination of both families it is 30%. We added homologous sequences to the structural alignment. Analysis of the residues occurring at sites within the CCs showed that 30% have little or no conservation, whereas about 40% conserve the polar/neutral or hydrophobic/neutral character of their residues. The remaining 30% conserve hydrophobic residues with strong or medium limitations on their volume variations. Almost all of these residues are found at sites that form the "buried spine" of each helix (at sites i, i+3, i+7, i+10, etc., or i, i+4, i+7, i+11, etc.) and they pack together at the centre of each structure to give a pattern of residue-residue contacts that is almost absolutely conserved. These CC conserved hydrophobic residues form only 10-15% of all the residues in the individual structures.A similar analysis of the cytochromes c'-b(562), which bind haem and have a very different function to that of the cytokines, gave very similar results. Again some 30% of the CC residues have hydrophobic residues with strong or medium conservation. Most of these form the buried spine of each helix and play the same role as those in the cytokines. The others, and some spine residues bind the haem co-factor.

The relationship of Campylobacter jejuni infection and the development of Guillain-Barré syndrome

Campylobacter jejuni is recognized as the most common infectious agent associated with the development of Guillain-Barré syndrome. Available information on the complete genome sequence of C. jejuni NCTC 11168 has helped researchers to identify polysaccharide capsules as well as genetic mechanisms in the synthesis of ganglioside-like cell surface molecules in this bacteria. Toxins may contribute to the host's inflammatory response seen in Guillain-Barré syndrome.

Prolactin: the new biology of an old hormone

Prolactin (PRL) is a paradoxical hormone. Historically known as the pituitary hormone of lactation, it has had attributed to it more than 300 separate actions, which can be correlated to the quasi-ubiquitous distribution of its receptor. Meanwhile, PRL-related knockout models have mainly highlighted its irreplaceable role in functions of lactation and reproduction, which suggests that most of its other reported target tissues are presumably modulated by, rather than strictly dependent on, PRL. The multiplicity of PRL actions in animals is in direct opposition to the paucity of arguments that suggest its involvement in human pathophysiology other than effects on reproduction. Although many experimental data argue for a role of PRL in the progression of some tumors, such as breast and prostate cancers, drugs lowering circulating PRL levels are ineffective. This observation opens new avenues for research into the understanding of whether local production of PRL is involved in tumor growth and, if so, how extrapituitary PRL synthesis is regulated. Finally, the physiological relevance of PRL variants, such as the antiangiogenic 16K-like PRL fragments, needs to be elucidated. This review is aimed at critically discussing how these recent findings have renewed the manner in which PRL should be considered as a multifunctional hormone.

Mechanism of ruminant placental lactogen action: molecular and in vivo studies

Ruminant placental lactogens (PLs) are structurally related to prolactins (PRLs) and growth hormones (GHs) and are secreted by placentae. Ruminant PLs are unusual in their capacity to bind and activate PRL and GH receptors (Rs) from other species. The present minireview summarizes several works showing that unlike in heterologous species (rat, rabbit, human), in homologous (ruminant) species, PLs act by activating PRLRs or by heterodimerizing GHRs and PRLRs, and suggests that this may be the main mechanism of PL action in vivo. Mutations impairing the ability of ovine (o)PL or bovine (b)PL to form complexes with PRLRs (but not with GHRs) do not cause loss of biological activity, because the transient existence of the homodimeric complex is still sufficient to initiate the signal transduction; however, mutants do lose their ability to activate homologous PRLRs. To explain this difference, we proposed a novel term-minimal time of homodimer persistence-which assumes that to initiate the signal transduction, a "minimal time" of homodimer existence is required for transphosphorylation of associated JAK2s. In interactions between ruminant PLs and homologous PRLRs, this minimal time is met through the interaction with homologous PRLRs, which has a shorter half-life than with heterologous PRLRs. Thus oPL or bPL are active in cells possessing both homologous and heterologous PRLRs. Mutations of PLs decrease the affinity, shortening the "time of homodimer persistence." In heterologous interactions, the minimal time is still sufficient to initiate the biological activity, whereas in homologous interactions, which in any case are weaker, further destabilization of the complex shortens its persistence below the minimal time, causing loss of biological activity.

Structure of a phage display-derived variant of human growth hormone complexed to two copies of the extracellular domain of its receptor: evidence for strong structural coupling between receptor binding sites

The structure of the ternary complex between the phage display- optimized, high-affinity Site 1 variant of human growth hormone (hGH) and two copies of the extracellular domain (ECD) of the hGH receptor (hGHR) has been determined at 2.6 A resolution. There are widespread and significant structural differences compared to the wild-type ternary hGH hGHR complex. The hGH variant (hGH(v)) contains 15 Site 1 mutations and binds>10(2) tighter to the hGHR ECD (hGH(R1)) at Site 1. It is biologically active and specific to hGHR. The hGH(v) Site 1 interface is somewhat smaller and 20% more hydrophobic compared to the wild-type (wt) counterpart. Of the ten hormone-receptor H-bonds in the site, only one is the same as in the wt complex. Additionally, several regions of hGH(v) structure move up to 9A in forming the interface. The contacts between the C-terminal domains of two receptor ECDs (hGH(R1)- hGH(R2)) are conserved; however, the large changes in Site 1 appear to cause global changes in the domains of hGH(R1) that affect the hGH(v)-hGH(R2) interface indirectly. This coupling is manifested by large changes in the conformation of groups participating in the Site 2 interaction and results in a structure for the site that is reorganized extensively. The hGH(v)- hGH(R2) interface contains seven H-bonds, only one of which is found in the wt complex. Several groups on hGH(v) and hGH(R2) undergo conformational changes of up to 8 A. Asp116 of hGH(v) plays a central role in the reorganization of Site 2 by forming two new H-bonds to the side-chains of Trp104(R2) and Trp169(R2), which are the key binding determinants of the receptor. The fact that a different binding solution is possible for Site 2, where there were no mutations or binding selection pressures, indicates that the structural elements found in these molecules possess an inherent functional plasticity that enables them to bind to a wide variety of binding surfaces.

Progress in Guillain-Barré syndrome

This editorial review summarizes and critically analyses reports published in the preceding 18 months on the pathogenesis of Guillain-Barré syndrome, with particular emphasis on the role of ganglioside antibodies, antecedent infections, and the concept of molecular mimicry. It concludes with an appraisal of currently available and proposed therapies.

Relative localization of the prolactin receptor binding sites for lactogenic hormones

A monoclonal antibody termed MAb R7B4, directed to an epitope present in prolactin receptors (PRLRs), was used as a tool to map the receptor binding sites for human growth hormone (hGH), ovine prolactin (oPRL) and human placental lactogen (hPL). Although the three hormones completely inhibited the binding of each other to Nb2 cells or rat liver receptors, MAb R7B4 behaviour was different depending on the hormone tested and the receptor source. According to the MAb effects, PRLR from Nb2 cells would locate both hGH and oPRL close to R7B4 epitope, whereas hPL would bind far from the MAb binding site. On the other hand, PRLR from rat liver should bind hGH close to the R7B4 epitope but oPRL and hPL would be recognized by a separate region of the same receptor. Thus, results presented in this paper suggest that PRLR binding sites for hGH, oPRL and hPL do not exactly overlap in spite of full competition between ligands.

The structure and activity of a monomeric interferon-gamma:alpha-chain receptor signaling complex

BACKGROUND

Interferon-gamma (IFN-gamma) is a homodimeric cytokine that exerts its various activities by inducing the aggregation of two different receptors. The alpha chain receptor (IFN-gammaRalpha) is a high affinity receptor that binds to IFN-gamma in a symmetric bivalent manner to form a stable, intermediate 1:2 complex. This intermediate forms a binding template for the subsequent binding of two copies of the second receptor, beta chain receptor (IFN-gammaRbeta), producing the active 1:2:2 signaling complex.

RESULTS

A single chain monovalent variant of IFN-gamma (scIFN-gamma) was constructed and complexed to one copy of the extracellular domain (ECD) of IFN-gammaRalpha. The structure of this 1:1 complex was determined and the hormone-receptor interface shown to be characterized by a number of hydrophilic interactions mediated by several highly ordered water networks. The scIFN-gamma interface consists of segments from each of the monomer chains of the homodimer. The principal hydrophobic contact of the receptor involves a tripeptide segment of the receptor having an unusual and high energy conformation. Despite containing only one binding site for IFN-gammaRalpha, the monovalent scIFN-gamma molecule has significant activity in antiviral biological assays.

CONCLUSIONS

ScIFN-gamma binds the ECD of IFN-gammaRalpha through a highly hydrated interface with an important set of hormone-receptor contacts mediated through structured waters. Although the interface is highly hydrated, it supports tight binding and has a considerable degree of specificity. The biological activity of scIFN-gamma confirms that the scIFN-gamma:IFN-gammaRalpha complex represents a productive intermediate and that it can effectively recruit the other required component, IFN-gammaRbeta, to signal based on the 1:1:1 complex.

Analysis of biological effects and signaling properties of Flt-1 (VEGFR-1) and KDR (VEGFR-2). A reassessment using novel receptor-specific vascular endothelial growth factor mutants

Endothelial cells express two related vascular endothelial growth factor (VEGF) receptor tyrosine kinases, KDR (kinase-insert domain containing receptor, or VEGFR-2) and Flt-1 (fms-like tyrosine kinase, or VEGFR-1). Although considerable experimental evidence links KDR activation to endothelial cell mitogenesis, there is still significant uncertainty concerning the role of individual VEGF receptors for other biological effects such as vascular permeability. VEGF mutants that bind to either KDR or Flt-1 with high selectivity were used to determine which of the two receptors serves to mediate different VEGF functions. In addition to mediating mitogenic signaling, selective KDR activation was sufficient for the activation of intracellular signaling pathways implicated in cell migration. KDR stimulation caused tyrosine phosphorylation of both phosphatidylinositol 3-kinase and phospholipase Cgamma in primary endothelial cells and stimulated cell migration. KDR-selective VEGF was also able to induce angiogenesis in the rat cornea to an extent indistinguishable from wild type VEGF. We also demonstrate that KDR, but not Flt-1, stimulation is responsible for the induction of vascular permeability by VEGF.