Aspartate 171 is the major primate-specific determinant of human growth hormone. Engineering porcine growth hormone to activate the human receptor

Engineering a Supersecreting Strain of Escherichia coli by Directed Coevolution of the Multiprotein Tat Translocation Machinery

Escherichia coli remains one of the preferred hosts for biotechnological protein production due to its robust growth in culture and ease of genetic manipulation. It is often desirable to export recombinant proteins into the periplasmic space for reasons related to proper disulfide bond formation, prevention of aggregation and proteolytic degradation, and ease of purification. One such system for expressing heterologous secreted proteins is the twin-arginine translocation (Tat) pathway, which has the unique advantage of delivering correctly folded proteins into the periplasm. However, transit times for proteins through the Tat translocase, comprised of the TatABC proteins, are much longer than for passage through the SecYEG pore, the translocase associated with the more widely utilized Sec pathway. To date, a high protein flux through the Tat pathway has yet to be demonstrated. To address this shortcoming, we employed a directed coevolution strategy to isolate mutant Tat translocases for their ability to deliver higher quantities of heterologous proteins into the periplasm. Three supersecreting translocases were selected that each exported a panel of recombinant proteins at levels that were significantly greater than those observed for wild-type TatABC or SecYEG translocases. Interestingly, all three of the evolved Tat translocases exhibited quality control suppression, suggesting that increased translocation flux was gained by relaxation of substrate proofreading. Overall, our discovery of more efficient translocase variants paves the way for the use of the Tat system as a powerful complement to the Sec pathway for secreted production of both commodity and high value-added proteins.

Physiologic Aspects of Pig Kidney Transplantation in Nonhuman Primates

Xenotransplantation can provide a solution to the current shortage of human organs for patients with terminal renal failure. The increasing availability of genetically engineered pigs, effective immunosuppressive therapy, and antiinflammatory therapy help to protect pig tissues from the primate immune response and can correct molecular incompatibilities. Life-supporting pig kidney xenografts have survived in NHP for more than 6 mo in the absence of markers of consumptive coagulopathy. However, few reports have focused on the physiologic aspects of life-supporting pig kidney xenografts. We have reviewed the literature regarding pig kidney xenotransplantation in NHP. The available data indicate (1) normal serum creatinine, (2) normal serum electrolytes, except for a trend toward increased calcium levels and a transient rise in phosphate followed by a fall to slightly subnormal values, (3) minimal or modest proteinuria without hypoalbuminemia (suggesting that previous reports of proteinuria likely were due to a low-grade immune response rather than physiologic incompatibilities), (4) possible discrepancies between pig erythropoietin and the primate erythropoietin receptor, and (5) significant early increase in kidney graft size, which might result from persistent effects of pig growth hormone. Further study is required regarding identification and investigation of physiologic incompatibilities. However, current evidence suggests that, in the absence of an immune response, a transplanted pig kidney likely would satisfactorily support a human patient.

Role of Intrinsic Factors in the Growth of Transplanted Organs Following Transplantation

Shortages in the availability of transplantable organs have forced the transplant community to seek alternative methods to increase the supply of available organs. In our recent study following α-1,3-galactocyltransferase knockout (GalT-KO) pig-to-baboon kidney xenotransplantation, we found that certain recipients developed increased serum creatinine, possibly due to the rapid growth of orthotopic pig grafts in smaller baboon recipients. To test our hypothesis, we assessed whether the growth of outbred (Yorkshire) organ transplants (kidney and lung) in miniature swine was regulated by intrinsic (graft) factors. Yorkshire kidneys reached 3.7× their initial volume over 3 months vs. 1.2× for miniature swine kidneys over a similar time period. A similar pattern was seen in porcine lung allografts as well. Following xenotransplantation, a review of our results suggests that there is a threshold for kidney graft volume of 25 cm³/kg of recipient body weight at which cortical ischemia is induced in transplanted GalT-KO kidneys in baboons. These results suggest that intrinsic factors are in part responsible for the growth of donor organs and this should be taken into consideration for growth-curve-mismatched transplants.

Role of Intrinsic (Graft) Versus Extrinsic (Host) Factors in the Growth of Transplanted Organs Following Allogeneic and Xenogeneic Transplantation

In our studies of life-supporting α-1,3-galactocyltransferase knockout (GalT-KO) pig-to-baboon kidneys, we found that some recipients developed increased serum creatinine with growth of the grafts, without histological or immunological evidence of rejection. We hypothesized that the rapid growth of orthotopic pig grafts in smaller baboon recipients may have led to deterioration of organ function. To test this hypothesis for both kidneys and lungs, we assessed whether the growth of outbred (Yorkshire) organ transplants in miniature swine was regulated by intrinsic (graft) or extrinsic (host environment) factors. Yorkshire kidneys exhibited persistent growth in miniature swine, reaching 3.7 times their initial volume over 3 mo versus 1.2 times for miniature swine kidneys over the same time period. Similar rapid early growth of lung allografts was observed and, in this case, led to organ dysfunction. For xenograft kidneys, a review of our results suggests that there is a threshold for kidney graft volume of 25 cm³ /kg of recipient body weight at which cortical ischemia is induced in transplanted GalT-KO kidneys in baboons. These results suggest that intrinsic factors are responsible, at least in part, for growth of donor organs and that this property should be taken into consideration for growth-curve-mismatched transplants, especially for life-supporting organs transplanted into a limited recipient space.

The 20kDa and 22kDa forms of human growth hormone (hGH) exhibit different intracellular signalling profiles and properties

Human Growth Hormone (hGH) includes two main variants. The first is 22kDa GH (22K-GH), which is predominant in the blood circulation. The second most abundant variant is 20K-GH, which makes up 5-10% of the blood circulation. Both bind and activate the same receptor, called the human growth hormone receptor (GHR). However, the reason why 22K-GH and 20K-GH exhibit similar, but not identical physiological activities remains poorly understood. In this article, the intracellular signalling profiles between these two hormones were examined. Western blot analyses were performed in 3T3-F442A and CHO cells transfected with GHR (CHO-GHR). The results revealed that both 22K-GH and 20K-GH can activate Janus kinase 2 (JAK2) and signal transducers and activators of transcription 1, 3 and 5 (STATs 1/3/5). Both induced tyrosine phosphorylation of JAK2 and STAT/1/3/5 in a time-dependent and dose-dependent manner. However, there were significant differences in the intracellular signalling properties between 22K-GH and 20K-GH. In particular, the kinetics of signalling shown by 22K-GH and 20K-GH is different. In addition, we found that the 20K-GH-induced tyrosine phosphorylation of signalling proteins was weaker than that of 22K-GH. Together, these observations indicate that the levels and kinetics of phosphorylation mediated by the main signalling proteins triggered by 22K-GH or 20K-GH were not exactly the same. This may provide a possible explanation for the different biological activities exhibited by 22K-GH and 20K-GH.

Different intracellular signalling properties induced by human and porcine growth hormone

Growth hormone (GH) is reportedly species-specific. Primate growth hormone can trigger non-primate growth hormone receptor (GHR), but primates GHR cannot be activated by non-primate GH. However, it is also unclear that why primate GH and non-primate GH have different biological activities. Thus, we analysed primate growth hormone (human growth hormone (hGH)) or non-primate GH (porcine growth hormone (pGH))-induced intracellular signalling in 3T3-F442A cells and rat hepatocytes in a dose- and time-dependent manner to explore the different biological activities between them. The results revealed that both hGH and pGH can activate Janus kinase 2 (JAK2), Signal transducers and activators of transcription 1, 3 and 5 (STATs 1, 3 and 5) and extracellular signal-regulated kinase 1/2 (ERK1/2). There were no significant differences in JAK2 or ERK1/2 tyrosine phosphorylation after hGH and pGH treatment, but there were different between hGH and pGH in STAT/1/3/5 tyrosine phosphorylation, and JAK2, STAT/1/3/5 tyrosine phosphorylation was time-dependent and dose-dependent, whereas ERK1/2 was not. Both hGH and pGH demonstrated similar kinetics for STATs 1, 3 and 5 phosphorylation, but the pGH-mediated tyrosine phosphorylation was weaker than that mediated by hGH. Our observations indicated that the levels of main signalling proteins phosphorylation triggered by hGH or pGH were not exactly the same, which may explain the different biological activities showed by primate GH and non-primate GH.

Intracellular signaling transduction pathways triggered by a well-known anti-GHR monoclonal antibody, Mab263, in vitro and in vivo

A series of studies have reported that monoclonal antibody 263 (Mab263), a monoclonal antibody against the growth hormone receptor (GHR), acts as an agonist in vitro and in vivo. However, the intracellular signaling pathways triggered by Mab263 have not yet been delineated. Therefore, we examined the intracellular signaling pathways induced by Mab263 in vivo and in vitro in the present study. The results show that this antibody activated janus kinase 2 (JAK2), signal transducer and activator of transcription 3 (STAT3), STAT1 and extracellular signal-regulated kinase 1/2 (ERK1/2), but not STAT5. The phosphorylation kinetics of JAK2, STAT3/1 and ERK1/2 induced by Mab263 were subsequently analyzed in dose-response and time course experiments. Our observations indicate that Mab263 induced different intracellular signaling pathways than GH, which indicates that Mab263 is a signal-specific molecule and that Mab263 may be a valuable biological reagent to study the mechanism(s) of GHR-mediated intracellular signaling pathways.

Serum IGF-1 is insufficient to restore skeletal size in the total absence of the growth hormone receptor

States of growth hormone (GH) resistance, such those observed in Laron dwarf patients, are characterized by mutations in the GH receptor (GHR), decreased serum and tissue IGF-1 levels, impaired glucose tolerance, and impaired skeletal acquisition. IGF-1 replacement therapy in such patients increases growth velocity but does not normalize growth. Herein we combined the GH-resistant (GHR knockout [GHRKO]) mouse model with mice expressing the hepatic Igf-1 transgene (HIT) to generate the GHRKO-HIT mouse model. In GHRKO-HIT mice, serum IGF-1 levels were restored via transgenic expression of Igf-1, allowing us to study how endocrine IGF-1 affects growth, metabolic homeostasis, and skeletal integrity. We show that in a GH-resistant state, normalization of serum IGF-1 improved body adiposity and restored glucose tolerance but was insufficient to support normal skeletal growth, resulting in an osteopenic skeletal phenotype. The inability of serum IGF-1 to restore skeletal integrity in the total absence of GHR likely resulted from reduced skeletal Igf-1 gene expression, blunted GH-mediated effects on the skeleton that are independent of serum or tissue IGF-1, and poor delivery of IGF-1 to the tissues. These findings are consistent with clinical data showing that IGF-I replacement therapy in patients with Laron syndrome does not achieve full skeletal growth.

Genetic and structural characterization of the growth hormone gene and protein from tench, Tinca tinca

The analysis of the tench growth hormone gene structure revealed a comparable organization of coding and non-coding regions than other from cyprinid species. Based on the performed mRNA and amino acid sequence alignments, gh tench is related to Asian than to European representatives of Cyprinidae family. Second aim of the work was to characterize and predict protein structure of the tench growth hormone. Tinca tinca GH share many common features with human GH molecule. The Tench GH protein binds to the growth hormone receptor (GHR) using two regions I and II that are situated at opposite sites of molecule. Binding site I is placed in the central part of T. tinca GH and H 189 amino acid in the middle region of the IV helix is crucial for GH-GHR interactions.

Molecular mechanisms of prolactin and its receptor

Prolactin and the prolactin receptors are members of a family of hormone/receptor pairs which include GH, erythropoietin, and other ligand/receptor pairs. The mechanisms of these ligand/receptor pairs have broad similarities, including general structures, ligand/receptor stoichiometries, and activation of several common signaling pathways. But significant variations in the structural and mechanistic details are present among these hormones and their type 1 receptors. The prolactin receptor is particularly interesting because it can be activated by three sequence-diverse human hormones: prolactin, GH, and placental lactogen. This system offers a unique opportunity to compare the detailed molecular mechanisms of these related hormone/receptor pairs. This review critically evaluates selected literature that informs these mechanisms, compares the mechanisms of the three lactogenic hormones, compares the mechanism with those of other class 1 ligand/receptor pairs, and identifies information that will be required to resolve mechanistic ambiguities. The literature describes distinct mechanistic differences between the three lactogenic hormones and their interaction with the prolactin receptor and describes more significant differences between the mechanisms by which other related ligands interact with and activate their receptors.

Donor-graft compatibility tests in pig-to-primate xenotransplantation model: serum versus plasma in real-time cell analyzer trials

INTRODUCTION

Various strategies have been designed to assess in vitro donor-graft compatibility in pig-to-primate xenotransplantation models. Most of them are based on a cytolysis assessment by exposing donor tissue to host serum with investigations by flow cytometry, and photocolorimetric levels. The aim of this study was to analyze the difference in cytolysis produced by sera and plasma obtained using various anticoagulants, or containing high versus low levels of platelets.

METHODS

The cytolysis trials were performed using an xCELLigence real-time cell analyzer (RTCA) in a cell model involving transgenic pig fibroblasts exposed to sera (S) or plasma obtained using EDTA, Li-heparin, or Na-heparin in combination with plasma containing high versus low content of platelets. Samples were obtained from two baboons and five volunteer human donors. Evolution of fibroblast cell growth was assessed by RTCA as the cell index (CI). After 9 hours of growth, cells were exposed to 20 μL of each sample. The minimum CI (CImin), time to CImin (TCImin), and time to reach the CI observed before compound addition (Trec) were recorded for each microwell.

RESULTS

The lowest CImin, highest TCImin, and Trec observed for EDTA plasma showed significant differences from other samples (P < .001).

DISCUSSION

On the basis of this study, using the RTCA assay, heparinized plasma produced complement inhibition and with undervaluation of the cytolysis reaction. EDTA plasma produced total death of most of cultures. The most accurate sample matrix seems to be serum.

N-glycosylation increases the circulatory half-life of human growth hormone

Therapeutic use of recombinant GH typically involves daily sc injections. We examined the possibilities for prolonging the in vivo circulation of GH by introducing N-glycans. Human GH variants with a single potential N-glycosylation site (N-X-S/T) introduced by site-directed mutagenesis were expressed in HEK293 cells. In a scan of 15 different positions for N-glycosylation sites, four positions (amino acids 93, 98, 99, and 101) were efficiently utilized and did not influence GH in vitro activity. A GH variant (3N-GH) with all these sites was produced in CHOK1SV cells and contained up to three N-glycans. Two pools of 3N-GH were purified and separated according to their charge by anion-exchange chromatography. Anion-exchange HPLC revealed that the N-glycans in the two pools were very similar except for the extent of sialylation. Both 3N-GH pools circulated longer in rats than wild-type GH. The terminal half-life of 3N-GH after iv injection was 24-fold prolonged compared with wild-type GH for the pool with the most pronounced sialylation, 13-fold prolonged for the less sialylated pool, and similar to the wild-type for desialylated 3N-GH. The less sialylated 3N-GH pool exhibited a profound pharmacodynamic effect in GH-deficient rats. Over a 4-d period, a single injection of 3N-GH induced a stronger IGF-I response and a larger increase in body weight than daily injections with wild-type GH. Thus, N-glycans can prolong the in vivo circulation and enhance the pharmacodynamic effect of GH. Sialic acids seem to play a pivotal role for the properties of glycosylated GH.

Growth hormone: does it have a therapeutic role in fracture healing?

BACKGROUND

The role of growth hormone (GH) in augmenting fracture healing has been postulated for over half a century. GH has been shown to play a role in bone metabolism and this can be mediated directly or indirectly through IGF-I.

OBJECTIVES

The use of GH was evaluated as a possible therapeutic agent in augmenting fracture healing.

METHOD

A literature search was undertaken on GH and its effect on bone fracture healing primarily using MEDLINE/OVID (1950 to January 2009). Key words and phrases including 'growth hormone', 'insulin like growth factor', 'insulin like growth factor binding protein', 'insulin like growth factor receptor', 'fracture repair', 'bone healing', 'bone fracture', 'bone metabolism', 'osteoblast' and 'osteoclast' were used in different combinations. Manual searches of the bibliography of key papers were also undertaken.

RESULTS

Current evidence suggests a positive role of GH on fracture healing as demonstrated by in vitro studies on osteoblasts, osteoclasts and the crosstalk between the two. Animal studies have demonstrated a number of factors influencing the effect of GH in vivo such as dose, timing and method of administration. Application of this knowledge in humans is limited but clearly demonstrates a positive effect on fracture healing. Concern has been raised in the past regarding the safety profile of the pharmacological use of GH when used in critically ill patients.

CONCLUSION

The optimal dose and method of administration is still to be determined, and the safety profile of this novel use of GH needs to be investigated prior to establishing its widespread use as a fracture-healing agent.

The extracellular domain of the growth hormone receptor interacts with coactivator activator to promote cell proliferation

The presence of GH receptor (GHR) in the cell nucleus correlates with cell division, and targeting the GHR to the nucleus results in constitutive proliferation and transformation because of increased sensitivity to autocrine GH. Here we have sought additional mechanisms that might account for the enhanced proliferation seen with nuclear GHR, commencing with a yeast two-hybrid (Y2H) screen for interactors with the extracellular domain of the GHR [GH-binding protein (GHBP)]. We find that the GHBP is a transcriptional activator in yeast and mammalian cells, and this activity resides in the lower cytokine receptor module. Activity is dependent on S226, the conserved serine of the cytokine receptor consensus WSXWS box. By using parallel GHBP affinity columns and tandem mass spectrometry of tryptic digests of proteins bound to wild-type GHBP and S226A columns, we identified proteins that bind to the transcriptionally active GHBP. These include a nucleoporin and two transcriptional regulators, notably the coactivator activator (CoAA), which is also an RNA binding splicing protein. Binding of CoAA to the GHBP was confirmed by glutathione S-transferase pulldown and coimmunoprecipitation, and shown to be GH dependent in pro-B Ba/F3 cells. Importantly, stable expression of CoAA in Ba/F3 cells resulted in an increased maximum proliferation in response to GH, but not IL-3. Because CoAA overexpression has been identified in many cancers and its stable expression promotes cell proliferation and cell transformation in NIH-3T3 cells, we suggest CoAA contributes to the proliferative actions of nuclear GHR by the hormone-dependent recruitment of this powerful coactivator to the GHR.

Selected physiologic compatibilities and incompatibilities between human and porcine organ systems

The shortage of donor organs is a major barrier to clinical organ transplantation. Although xenotransplantation is considered one of the alternatives to human organ transplantation, there are immunologic and physiologic incompatibilities between humans and pigs. With the exception of coagulation, the major potential physiologic incompatibilities relating to function of the kidney, heart, liver, lungs, pancreatic islets, and hormones are reviewed. Some of these physiologic differences can be overcome by producing genetically altered pigs to improve compatibility with humans. The possibility of producing such pigs for organ transplantation is considered.

Reversal of diabetes in pancreatectomized pigs after transplantation of neonatal porcine islets

Neonatal porcine islets (NPIs) are able to grow and to reverse hyperglycemia after transplantation in immunoincompetent mice. The aim of this study was to demonstrate the feasibility of allogeneic NPI grafts to achieve normoglycemia in a pancreatectomized diabetic pig. NPIs were isolated from pancreases of 1- to 3-day-old pigs, cultured, and then transplanted via the portal vein into the liver of totally pancreatectomized pigs (mean body weight, 20.8 kg). Each pig received NPIs consisting of 3.1 +/- 0.3 x 10(6) beta-cells/kg (12,476 +/- 1,146 islet equivalent/kg). The six pigs that were given cyclosporine and sirolimus achieved normoglycemia by day 14 without insulin therapy. Three pigs died of surgical complications shortly after transplantation, whereas the other three remained insulin independent up to day 69. Of seven nonimmunosuppressed recipients, four pigs became normoglycemic by day 14 without insulin treatment, with two of the animals remaining normoglycemic long term. Well-preserved insulin-positive cells were found in the graft at the end of follow-up with a significant increase in insulin content in long-term survivors of both groups. This study demonstrates for the first time that allogeneic NPIs can reverse hyperglycemia in totally pancreatectomized diabetic pigs.

Expansion and divergence of the GH locus between spider monkey and chimpanzee

Growth hormone (GH) has been previously described as showing distinct evolutionary stories between primates and other mammals. A burst of changes and successive amplification events took place in the primate lineage giving rise to a multigene family in the three Anthropoidea lineages. Polymerase chain reaction (PCR) was used to obtain the genes and the intergenic regions comprising the GH loci of the spider monkey (Ateles geoffroyi), a New-World primate, and of the chimpanzee (Pan troglodytes), an ape. The intergenic sequences of both species were screened by hybridization to detect copies of the Alu family, which have been implicated in the formation of the human GH locus. The GH locus of the spider monkey contains at least six GH-related genes, four of them were cloned. Likewise, five short intergenic sequences of approximately 3 kb were amplified and cloned. On the other hand, in the chimpanzee four new placental lactogen (PL) genes as well as four intergenic regions were amplified. Consequently, in this ape, six genes (two GHs, previously obtained, and four PLs) are clustered, separated by intergenic sequences of different lengths (two short ones of about 5 kb, and at least two long ones between 9 and 13 kb). The presence of Alu sequences within the intergenic regions of both GH loci corroborates the current hypothesis that they acted as a driving force for the locus expansion. GH sequence comparisons reveal that several gene-conversion events might have occurred during the formation of this genome region, which has undergone independent evolution in the three Anthropoidea branches. To establish the GH's evolutionary history may prove to be a difficult task due to these gene-conversion events.

Increased Site 1 Affinity Improves Biopotency of Porcine Growth Hormone

Based on phage display optimization studies with human growth hormone (GH), it is thought that the biopotency of GH cannot be increased. This is proposed to be a result of the affinity of the first receptor for hormone far exceeding that which is required to trap the hormone long enough to allow diffusion of the second receptor to form the ternary complex, which initiates signaling. We report here that despite similar site 1 kinetics to the hGH/hGH receptor interaction, the potency of porcine GH for its receptor can be increased up to 5-fold by substituting hGH residues involved in site 1 binding into pGH. Based on extensive mutations and BIAcore studies, we show that the higher potency and site 1 affinity of hGH for the pGHR is primarily a result of a decreased off-rate associated with residues in the extended loop between helices 1 and 2 that interact with the two key tryptophans Trp104 and Trp169 in the receptor binding hot spot. Our mutagenic analysis has also identified a second determinant (Lys165), which in addition to His169, restricts the ability of non-primate hormones to activate hGH receptor. The increased biopotency of GH that we observe can be explained by a model for GH receptor activation where subunit alignment is critical for effective signaling.

Prolonged retention after aggregation into secretory granules of human R183H-growth hormone (GH), a mutant that causes autosomal dominant GH deficiency type II

Human R183H-GH causes autosomal dominant GH deficiency type II. Because we show here that the mutant hormone is fully bioactive, we have sought to locate an impairment in its progress through the secretory pathway as assessed by pulse chase experiments. Newly synthesized wild-type and R183H-GH were stable when expressed transiently in AtT20 cells, and both formed equivalent amounts of Lubrol-insoluble aggregates within 40 min after synthesis. There was no evidence for intermolecular disulfide bond formation in aggregates of wild-type hormone or the R183H mutant. Both wild-type and R183H-GH were packaged into secretory granules, assessed by the ability of 1 mM BaCl2 to stimulate release and by immunocytochemistry. The mutant differed from wild-type hormone in its retention in the cells after packaging into secretory granules; 50% more R183H-GH than wild-type aggregates were retained in AtT20 cells 120 min after synthesis, and stimulated release of R183H-GH or a mixture of R183H-GH and wild-type that had been retained in the cell was reduced. The longer retention of R183H-GH aggregates indicates that a single point mutation in a protein contained in secretory granules affects the rate of secretory granule release.

Functional promiscuity of squirrel monkey growth hormone receptor toward both primate and nonprimate growth hormones

Primate growth hormone (GH) has evolved rapidly, having undergone approximately 30% amino acid substitutions from the inferred ancestral eutherian sequence. Nevertheless, human growth hormone (hGH) is physiologically effective when administered to nonprimate mammals. In contrast, its functional counterpart, the human growth hormone receptor (hGHR), has evolved species specificity so that it responds only to Old World primate GHs. It has been proposed that this species specificity of the hGHR is largely caused by the Leu --> Arg change at position 43 after a prior His --> Asp change at position 171 of the GH. Sequence analyses supported this hypothesis and revealed that the transitional phase in the GH:GHR coevolution still persists in New World monkeys. For example, although the GH of the squirrel monkey has the His --> Asp substitution at position 171, residue 43 of its GHR is a Leu, the nonprimate residue. If the squirrel monkey truly represents an intermediate stage of GH:GHR coevolution, its GHR should respond to both hGH and nonprimate GH. Also, if the emergence of species specificity was a result of the selection for a more efficient GH:GHR interaction, then changing residue 43 of the squirrel monkey growth hormone receptor (smGHR) to Arg should increase its binding affinity toward higher primate GH. To test these hypotheses, we performed protein-binding assays between the smGHR and both human and rat GHs, using the surface plasmon resonance methodology. Furthermore, the effects of reciprocal mutations at position 43 of human and squirrel monkey GHRs are measured for their binding affinities toward human and squirrel monkey GHs. The results from the binding kinetic assays clearly demonstrate that the smGHR is in the intermediate state of the evolution of species specificity. Interestingly, the altered residue Arg at position 43 of the smGHR does not lead to an increased binding affinity. The implications of these results on the evolution of the GH:GHR interaction and on functional evolution are discussed.

Computational stabilization of human growth hormone

Recombinant human growth hormone (hGH) is used worldwide for the treatment of pediatric hypopituitary dwarfism and in children suffering from low levels of hGH. It has limited stability in solution, and because of poor oral absorption, is administered by injection, typically several times a week. Development has therefore focused on more stable or sustained-release formulations and alternatives to injectable delivery that would increase bioavailability and make it easier for patients to use. We redesigned hGH computationally to improve its thermostability. A more stable variant of hGH could have improved pharmacokinetics or enhanced shelf-life, or be more amenable to use in alternate delivery systems and formulations. The computational design was performed using a previously developed combinatorial optimization algorithm based on the dead-end elimination theorem. The algorithm uses an empirical free energy function for scoring designed sequences. This function was augmented with a term that accounts for the loss of backbone and side-chain conformational entropy. The weighting factors for this term, the electrostatic interaction term, and the polar hydrogen burial term were optimized by minimizing the number of mutations designed by the algorithm relative to wild-type. Forty-five residues in the core of the protein were selected for optimization with the modified potential function. The proteins designed using the developed scoring function contained six to 10 mutations, showed enhancement in the melting temperature of up to 16 degrees C, and were biologically active in cell proliferation studies. These results show the utility of our free energy function in automated protein design.

A novel bioassay for human somatogenic activity in serum samples supports the clinical reliability of immunoassays

OBJECTIVE

Because there is discordance between different immunoassay values for serum hGH, and because clinical state may not correlate with immunoreactive hGH, we have developed an assay to accurately measure serum hGH somatogenic bioactivity. The results of this assay were compared with the Elegance two-site ELISA assay across 135 patient samples in a variety of clinical states.

DESIGN

The somatogenic assay was based on stable expression of hGH receptor in the murine BaF line, allowing these cells to proliferate in response to hGH. To eliminate interference by other growth factors in serum, we created a specific antagonist of the hGH receptor (similar to Trovert or Pegvisomant) which allowed us to obtain a true measure of hGH somatogenic activity by subtraction of the activity in the presence of the antagonist. The assay was carried out in microtiter plates over 24 h, with oxidation of a chromogenic tetrazolium salt (MTT) as the endpoint.

PATIENTS

These encompassed a number of different clinical conditions related to short stature, including idiopathic short stature, neurosecretory dysfunction and renal failure, as well as obese patients on dietary restriction and normal volunteers.

MEASUREMENTS

In addition to the colourimetric (MTT) response to hGH, we measured free hGH by stripping out GHBP-bound hGH using beads coupled to a monoclonal antibody to the GHBP (GH binding protein). All samples were measured in both bioassay and ELISA assay.

RESULTS

This bioassay was sensitive (5 mU/l or 2 microg/l) and precise, and not subject to interference by the GHBP. There was a good correlation (r = 0.95) between bioactivity and immunoactivity across clinical states. There was, however, an increased bioactivity during secretory peaks (over 25 mU/l), which has been reported previously for the Nb2 bioassay. Free hGH did not correlate with clinical state.

CONCLUSIONS

Because the results of the Elegance ELISA and the bioassay correlate well, even though there is greater bioactivity at higher hormone concentrations, it is evident that an appropriate immunoassay is able to act as a reliable indicator for clinical assessment. In those rare cases where bio-inactive GH exists, our bioassay should provide an appropriate means to demonstrate this.

Physiological aspects of pig-to-primate renal xenotransplantation

BACKGROUND

Few data exist on the physiological aspects of pig-to-primate renal xenotransplantation.

METHODS

Use of organs transgenic for human decay accelerating factor has allowed assessment of the metabolic and hormonal functions of these xenografts.

RESULTS

Porcine renal xenografts largely maintain plasma electrolyte homeostasis. An increase in proteinuria was detected that may result from graft injury. In contrast to allotransplantation a severe anaemia developed requiring recipient treatment with exogenous human erythropoietin.

CONCLUSIONS

Our experience provides qualified encouragement for the likely physiological compatibility of pig and primate species, but identifies areas where a xenograft may not match the performance of an allograft.

Episodic Evolution of Growth Hormone in Primates and Emergence of the Species Specificity of Human Growth Hormone Receptor

Growth hormone (GH) evolution is very conservative among mammals, except for primates and ruminant artiodactyls. In fact, most known mammalian GH sequences differ from the inferred ancestral mammalian sequence by only a few amino acids. In contrast, the human GH sequence differs from the inferred ancestral sequence by 59 amino acids. However, it is not known when this rapid evolution of GH occurred during primate evolution or whether it was due to positive selection. Also, human growth hormone receptor (GHR) displays species specificity; i.e., it can interact only with human (or rhesus monkey) GH, not with nonprimate GHS: The species specificity of human GHR is largely due to the Leu-->Arg change at position 43, and it has been hypothesized that this change must have been preceded by the His-->Asp change at position 171 of GH. Is this hypothesis true? And when did these changes occur? To address the above issues, we sequenced GH and GHR genes in prosimians and simians. Our data supported the above hypothesis and revealed that the species specificity of human GHR actually emerged in the common ancestor of Old World primates, but the transitional phase still persists in New World monkeys. Our data showed that the rapid evolution of primate GH occurred during a relatively short period (in the common ancestor of higher primates) and that the rate of change was especially high at functionally important sites, suggesting positive selection. However, the nonsynonymous rate/synonymous rate ratio at these sites was <1, so relaxation of purifying selection might have played a role in the rapid evolution of the GH gene in simians, possibly as a result of multiple gene duplications. Similar to GH, GHR displayed an accelerated rate of evolution in primates. Our data revealed proportionally more amino acid replacements at the functionally important sites in both GH and GHR in simians but, surprisingly, showed few coincidental replacements of amino acids forming the same intermolecular contacts between the two proteins.

Bushbaby growth hormone is much more similar to nonprimate growth hormones than to rhesus monkey and human growth hormones

Unlike other mammals, Old World primates have five growth hormone-like genes that are highly divergent at the amino acid level from the single growth hormone genes found in nonprimates. Additionally, there is a change in the interaction of growth hormone with its receptor in humans such that human growth hormone functions in nonprimates, whereas nonprimate growth hormone is ineffective in humans. A Southern blotting analysis of the genome of a prosimian, Galago senegalensis, revealed a single growth hormone locus. This single gene was PCR-amplified from genomic DNA and sequenced. It has a rate of nonsynonymous nucleotide substitution less than one fourth that of the human growth hormone gene, while the rates of synonymous substitution in the two species are less different. Human and rhesus monkey growth hormones exhibit variation at a number of amino acid residues that can affect receptor binding. The galago growth hormone is conservative at each of these sites, indicating that this growth hormone is functionally like nonprimate growth hormones. These observations indicate that the amplification and rapid divergence of primate growth hormones occurred after the separation of the higher primate lineage from the galago lineage.

A novel bioassay based on human growth hormone (hGH) receptor mediated cell proliferation: measurement of 20K-hGH and its modified forms

Previously we introduced the full-length hGH receptor (hGHR) into the mouse pro-B cell line, Ba/F3, and obtained stable transfectant (Ba/F3-hGHR), which could grow in response to 20K- and 22K-hGH in a dose-dependent manner(1). In the present study, we established a new bioassay system based on the proliferation of the Ba/F3-hGHR in combination with the eluted stain assay (ESTA). The Ba/F3-hGHR assay is completed in 18 h and requires only 10(-6)-fold amount of GH sample (1.8 ng) as compared with the rat weight gain assay. The validation study shows that the Ba/F3-hGHR assay is specific for hGH, precise (RSD = 1.1-19.7%) and ultrasensitive (lower limit of working range = 18.7 pg/mL). Four modified forms of recombinant 20K-hGH (oxidized, deamidated, des-Phe(1)and cleaved form) all of which are newly identified were measured by the Ba/F3-hGHR assay and the rat weight gain assay with our in-house recombinant 20K-hGH as standard. The oxidized and deamidated 20K-hGH were fully active, however the des-Phe(1)and cleaved 20K-hGH had significantly reduced activities in both assays. These findings suggest that the Ba/F3-hGHR assay is useful as an alternative to the rat weight gain assay.

cDNA cloning and functional expression of growth hormone receptor from soft-shelled turtle (Pelodiscus sinensis japonicus)

The growth hormone receptor (GHR) cDNA was cloned from the liver of soft-shelled turtle (Pelodiscus sinensis japonicus) using the polymerase chain reaction (PCR). Although GHR has been cloned from several mammalian and avian species, this is the first description of the reptilian receptor. As deduced from the nucleotide sequence, the precursor GHR of soft-shelled turtle (tGHR) is a protein of 615 amino acids which presents 72% identity with the chicken receptor and 57-64% identity with GHRs of several mammals. The tGHR expressed in COS-7 cells specifically bound human growth hormone (hGH) and was able to transduce an activation of transcription in the transfected cells. Binding of (125)I-hGH to the expressed receptor was decreased by the addition of excess unlabeled hGH, pig GH, and bream GH but not by pig insulin. The open reading frame of tGHR cDNA was inserted into the pSINrep/gfp (green fluorescence protein) vector and the tGHR-gfp fusion protein was stably expressed in baby hamster kidney (BHK) cells. Confocal imaging showed that tGHR-gfp was largely concentrated on the plasma membrane. Western blot analysis and deglycosylation treatment with PNGase F demonstrated that tGHR was a glycoprotein in BHK cells.

Growth hormone (GH)-independent dimerization of GH receptor by a leucine zipper results in constitutive activation

Growth hormone initiates signaling by inducing homodimerization of two GH receptors. Here, we have sought to determine whether constitutively active receptor can be created in the absence of the extracellular domain by substituting it with high affinity leucine zippers to create dimers of the growth hormone receptor (GHR) signaling domain. The entire extracellular domain of the GHR was replaced by the hemagglutinin-tagged zipper sequence of either the c-Fos or c-Jun transcription factor (termed Fos-GHR and Jun-GHR, respectively). Transient transfection of Fos-GHR or Jun-GHR resulted in activation of the serine protease inhibitor 2.1 promoter in Chinese hamster ovary-K1 cells to a level equal to that achieved by fully activated wild type GHR. Furthermore, stable expression of Jun-GHR alone or Fos-GHR and Jun-GHR together in the interleukin 3-dependent BaF-B03 cell line resulted in cell proliferation after interleukin 3 withdrawal at a rate equal to maximally stimulated wild type GHR-expressing cells. Activation of STAT 5b was also observed in Fos-Jun-GHR-expressing cells at a level equal to that in chronically GH-treated GHR-expressing cells. Thus, forced dimerization of the transmembrane and cytoplasmic domains of the GHR in the absence of the extracellular domain can lead to the constitutive activation of known GH signaling end points, supporting the view that proximity of Janus kinase 2 (JAK2) kinases is the essential element in signaling. Such constitutively active GH receptors may have particular utility for transgenic livestock applications.

Growth of porcine kidneys in their native and xenograft environment

The increased survival of hDAF pig-to-primate renal xenografts for up to two months has afforded the opportunity to study physiological aspects such as organ growth. Experimental evidence exists of species restriction of the activity of growth hormone, although growth itself is also controlled by a number of other endocrine, paracrine and autocrine substances. This study consisted of four parts: (1) measurement of pig kidney size according to pig body weight; (2) measurement of pig kidney size according to pig age; (3) serial length measurement of pig-to-primate renal xenografts; (4) correlation of terminal weight of renal xenograft with age and histology. The xenografted pig kidneys in a primate recipient grow as they would in the pig for the first two weeks after transplantation. After this time there is a reduction in the rate of increase in the length of the xenograft. Over the same period, changes in weight are greatly increased by the presence of rejection. This observational study supports the notion that regulation of growth of a xenotransplanted porcine kidney occurs.

The species specificity of growth hormone requires the cooperative interaction of two motifs

Primate growth hormones (GH) activate both primate and non-primate somatotrophic receptors (GH receptors), but non-primate GHs do not activate primate GH receptors. Previous studies argued the interaction of Asp(171) of human GH and Arg(43) of the receptor produced an attractive ionic interaction. In non-primate GHs, His(170) replaces the homologous Asp(171), producing a repulsive interaction with Arg(43) of the primate receptor which was believed to reduce the attraction of non-primate GH for the human GH receptor, thus providing species specificity. In this report, H170D bovine GH had activity and affinity for human GH receptors approaching those of human GH. In contrast, replacing Asp(171) of human GH with His did not significantly reduce somatotrophic activity, indicating that species specificity is not wholly explained by this residue's interaction with Arg(43) of the receptor. Deletion of either Phe(44) (a residue present only in primate GHs) or residues 32-46 (20-kDa form of human GH) each only marginally reduced somatotrophic activities. But the combination of the D171H mutation with either DeltaPhe(44) or Delta32-46 in human GH reduced binding and activity in a greater than additive fashion, indicated a functional interaction between these distant structural features. In bovine GH addition of phenylalanine at position 44 increased the somatotrophic activity and receptor affinity in cells containing the human GH receptor. The combination of the H170D mutation and the addition of phenylalanine at position 44 created a bovine GH with activity indistinguishable from wild-type human GH. Based on evidence from both bovine and human GHs, the cooperative interaction of these two distant motifs determined the species specificity and indicated that structural plasticity was a critical feature necessary for the species specificity of somatotrophic activity.

Tyrosine 462 of the membrane-proximal F'-G' loop of murine Mpl is not essential for high-affinity binding of thrombopoietin

The ligand binding site of Mpl, the thrombopoietin (Tpo) receptor, has not been determined. Tyr(462)of murine Mpl corresponds to Tyr(421)of the common beta chain of the human IL-3, IL-5 and GM-CSF receptors. Tyr(421)has been identified as essential for high-affinity ligand binding. To determine whether Tyr(462)is similarly required for Tpo binding, wild-type murine Mpl (Mpl-WT) or mutant receptors containing an alanine (Y462A) or lysine (Y462K) in place of Tyr(462)were expressed in BaF3 cells. In proliferation studies, the Y462A mutation had no effect on Tpo-induced growth. In contrast, the Y462K mutation led to an attenuated proliferative response to Tpo. In single-point binding studies, both Mpl-WT and Y462A cells were able to bind [(125)I]Tpo in a specific manner. In contrast, there was a marked reduction in binding of [(125)I]Tpo by Y462K cells. Mpl-WT cells bound Tpo with a K(d)of approximately 330 pM, while Y462A cells bound Tpo with a K(d)of approximately 268 pM. The binding affinity of Y462K cells was below that quantifiable by Scatchard analysis. This study suggests that unlike the corresponding Tyr(421)of the common human beta chain, Tyr(462)of murine Mpl is not required for high-affinity ligand binding, although it may be located in proximity to the ligand binding site.

Molecular recognition events involved in the activation of the growth hormone receptor by growth hormone

Binding of growth hormone (GH) to its receptor (GHR) is a well-studied example of molecular recognition between a cytokine and its receptor. Extensive mutagenesis studies and several crystal structures have defined the key interactive amino acid residues that are involved in binding and subsequent receptor dimerization. This review encompasses each of the three molecular recognition events involved in GHR activation, namely binding of GH to its two receptors and the interactions that occur between these receptors. Particular attention is given to species and ligand specificity of hormone binding and to the molecular recognition events involved in receptor activation, including the possibility that a conformational change in the receptor is required.

Functional domains of human growth hormone necessary for the adipogenic activity of hGH/hPL chimeric molecules

Genetic analysis through construction of chimeric genes and their transfection in mammalian cells could provide a better understanding of biological functions of native or modified proteins, and would allow the design of new gene constructs encoding peptides that mimic or block ligand interaction with target tissues. To identify the hGH domains responsible for induction of adipose differentiation we constructed hGH/hPL chimeric molecules using homologous DNA mutagenesis, since hGH, but not human placental lactogen (hPL), promotes adipose differentiation in mouse 3T3-F442A cells. We assayed their adipogenic activity in an autocrine/paracrine biological model consisting of transiently transfected 3T3-F442A cells with the chimeric constructs. Plasmid DNAs carrying these constructs were transfected into growing 3T3-F442A cells, and cultures were further maintained for 7 days to differentiate into adipocytes. Secretion of transfected hGH/hPL chimeric proteins into the medium was in the range of 5–25 ng/ml. Adipogenic activity was a property only of those chimeric proteins that contained hGH exon III together with either hGH exon II or hGH IV. Our results also suggest that hGH binding site-2 is composed of two structural subdomains: subsite 2A encoded by exon II of hGH and subsite-2B encoded by exon IV. We also suggest that full adipogenic activity requires the presence of binding site-1 and any of the subsites of binding site-2. This simple autocrine/paracrine biological model of gene transfection allows the analysis of specific biological activity of products encoded by modified genes.

Identification of novel sites in the ovine growth hormone receptor involved in binding hormone and conferring species specificity

Using site-directed mutagenesis we mutated the extracellular domain of the ovine growth hormone receptor (oGHR) to the corresponding amino acids in the rat GHR at two different sites: site A is between Thr28 and Leu34 and represents a major immunogenic epitope, while site B is between Ser121 and Asp124 and is involved in the interaction of the human GHR with growth hormone (GH). Native and mutant receptors were bacterially expressed and refolded, and then RIA and GH-binding assays were carried out on the purified recombinant proteins. Mutations at the N-terminal site A of oGHR led to greatly reduced binding to bovine GH and, in addition, to significant loss of recognition by a polyclonal antiserum to bovine GHR which recognizes site A as a major epitope. The crystal structure of human GH bound to human GHR did not resolve this extreme N-terminal region of the receptor but our data indicate that the N-terminal loop undertakes a 180 degrees turn bringing it into close proximity to the hormone-binding domain in a fashion analogous to the prolactin receptor. A fourfold decrease in affinity for binding bovine GH was also observed after mutation of site B. However, this change from the ovine sequence to the equivalent sequence in the rat GHR at site B caused a 2.4-fold increase in the affinity of binding to rat GH. Taken together, the changes in binding affinity of the site-B mutant for rat and bovine GH demonstrate that this site is involved in conferring species specificity for binding GH.

Activation of chimeric and full-length growth hormone receptors by growth hormone receptor monoclonal antibodies. A specific conformational change may be required for full-length receptor signaling

Signal transduction by the growth hormone receptor (GHR) occurs through growth hormone (GH)-induced dimerization of two GHRs to form a trimeric complex. It is thought that dimerization alone is sufficient for signaling, since monoclonal antibodies (mAbs) against the extracellular domain of the GHR elicit proliferation of FDC-P1 cells transfected with a chimeric receptor comprising the extracellular domain of the GHR and the fibronectin and cytoplasmic domains of the murine granulocyte colony-stimulating factor receptor. We have screened 14 GHR mAbs for proliferative activity against characterized FDC-P1 and BaF-B03 cell lines stably expressing the full-length human, rabbit, or rat GHR, or the chimeric human GHR/granulocyte colony-stimulating factor receptor, and for transactivation of the c-fos promoter and STAT activation. With the chimeric receptor, eight mAbs were able to elicit proliferation, although there was no correlation between inhibition of hormone binding and agonist activity. In contrast, no mAbs were able to act as agonists with the full-length GHR FDC-P1 cell lines, although nine competed with GH for binding. A weak proliferative response was observed in the BaF-B03 cell lines with two of the mAbs (263 and 1C9), and the addition of anti-mouse F(ab)2 resulted in increased signaling in the hGHR BaF-B03 cell line to a plateau of 28 +/- 4% of the GH maximum for mAb 263. These data could indicate considerable stringency in the ability of mAbs to correctly dimerize the full-length GHR. However, the ability of mAb 263 to stimulate a mutant hGHR altered in the F'-G' loop of domain 2 was nearly abolished, concurrent with an increased affinity of this mAb for the receptor. Since the F'-G' loop undergoes a conformational change on GH binding and is necessary for full proliferative signaling, we propose that in addition to promoting receptor dimerization, mAb 263 may induce specific changes in receptor conformation similar to GH, which are required for the biological response.