Use of therapeutic anticoagulation in splanchnic vein thrombosis associated with acute pancreatitis: a systematic review and meta-analysis

Background

Splanchnic vein thrombosis is a well-recognized local vascular complication of acute pancreatitis (AP), estimated to occur in approximately 15% of patients. While splanchnic vein recanalization occurs spontaneously in approximately one third of patients, severe complications such as bowel ischemia and liver failure have also been reported. At present, there is no consensus on whether patients presenting with AP-associated splanchnic vein thrombosis should receive therapeutic anticoagulation.

Methods

We searched multiple databases from inception through December 2020 to collect studies that compared the clinical outcomes of patients with AP and splanchnic vein thrombosis who received therapeutic anticoagulation (AC group) with those who did not (N-AC group). A meta-analysis was performed to calculate the relative risk (RR) of vessel recanalization, bleeding complications, collateral formation and death in the 2 groups.

Results

Seven studies with 8353 patients, 339 of whom had splanchnic vein thrombosis, were included in the final analysis. A total of 154 patients (45.4%) had acute severe pancreatitis. A significantly higher proportion of patients had vessel recanalization in the AC group: RR 1.6, 95% confidence interval 1.17-2.27; I²=0%; P=0.004. There was no difference between the 2 groups in the RR of bleeding complications, collateral formation and death.

Conclusions

Our analysis demonstrated that, among patients with AP-associated splanchnic vein thrombosis, therapeutic anticoagulation resulted in recanalization of the involved vessels without significantly increasing the risk of bleeding complications. There was no difference in the RR of death or the rates of collateral vessel formation during the follow up.

Choice of sedation in endoscopic retrograde cholangiopancreatography: is monitored anesthesia care as safe as general anesthesia? A systematic review and meta-analysis

Background

Monitored anesthesia care (MAC) and general anesthesia (GA) are the 2 most common methods of sedation used for endoscopic retrograde cholangiopancreatography (ERCP). We performed a systematic review and meta-analysis to compare the overall safety between MAC vs. GA in ERCP.

Methods

We conducted a comprehensive search of electronic databases to identify studies reporting the use of MAC or GA as a choice of sedation for ERCP. The primary outcome was to compare the overall rate of sedation-related adverse events in MAC vs. GA groups. The secondary endpoint was to investigate the total duration of the procedure, recovery time, ERCP cannulation rates, and conversion rate of MAC to GA. The meta-analysis was performed using a Der Simonian and Laird random-effects model.

Results

A total of 21 studies reporting on 11,592 patients were included. The overall sedation-related side-effects were similar in the GA (12.76%, 95% confidence interval [CI] 5.80-21.73; I²=95%) and MAC (12.08%, 95%CI 5.38-20.89; I²=99%) groups (P=0.956). Hypoxia, arrhythmias, hypotension, aspiration and other sedation-related side-effects were similar between the 2 groups. The mean duration of the procedure was longer in the MAC group, but the mean recovery time was shorter. Significant heterogeneity was noted in our meta-analysis.

Conclusions

In our meta-analysis, the overall sedation-related side-effects were similar between the MAC and GA groups. MAC could be used as a safer alternative to GA when performing ERCP. However, large multicenter randomized control trials are needed to further validate our findings.

EUS-guided gastroenterostomy versus enteral stenting for gastric outlet obstruction: Systematic review and meta-analysis

Background and study aims  Endoscopic and surgical techniques have been utilized for palliation of gastric outlet obstruction (GOO). Enteral stenting (ES) is an established technique with high clinical success and low morbidity rate. Endoscopic ultrasound-guided gastroenterostomy (EUS-GE) is a novel approach that aims to provide sustained palliation of GOO. We conducted a comprehensive review and meta-analysis to evaluate the effectiveness in terms of clinical and technical success, as well as the safety profile of EUS-GE and ES. Methods  We searched multiple databases from inception through July 2020 to identify studies that reported on safety and effectiveness of EUS-GE in comparison to ES. Pooled rates of technical success, clinical success, and adverse events (AEs) were calculated. Study heterogeneity was assessed using I 2 % and 95 % confidence interval. Results  Five studies including 659 patients were included in our final analysis. Pooled rate of technical and clinical success for EUS-GE was 95.2 % (CI 87.2-.98.3, I 2  = 42) and 93.3 % (CI 84.4-97.3, I 2  = 59) while for ES it was 96.9 % (CI 90.9-99, I 2  = 64) and 85.6 % (CI 73-92.9, I 2  = 85), respectively. Pooled rate of re-intervention was significantly lower with EUS-GE i. e. 4 % (CI 1.8-8.7, I 2  = 35) compared to ES, where it was 23.6 % (CI 17.5-31, I 2  = 35), p = 0.001 . Pooled rates of overall and major AEs were comparable between the two techniques. Conclusion  EUS-GE is comparable in terms of technical and clinical effectiveness and has a similar safety profile when compared to ES for palliation of GOO.

Efficacy and Safety of Intragastric Balloon (IGB) in Non-alcoholic Fatty Liver Disease (NAFLD): a Comprehensive Review and Meta-analysis

Intragastric balloon (IGB) therapy has shown efficacy in weight loss but its role in NAFLD remains unknown. We conducted a systematic review and meta-analysis to evaluate the efficacy of IGB in NAFLD. Meta-analysis was performed to estimate the pooled proportion of patients with improvement in steatosis as determined by imaging and histology following IGB placement. Nine studies were included in our analysis. Four hundred forty-two IGBs were placed. Improvement in steatosis was seen in 79.2% of patients and NAS in 83.5% of patients, and HOMA-IR score improved in 64.5% of patients. A reduction in liver volume by CT scan was noticed in 93.9% of patients undergoing IGB placement. IGB is an effective and safe short-term therapeutic modality for patients with NAFLD.

Adenoma and polyp detection rates during insertion versus withdrawal phase of colonoscopy: a systematic review and meta-analysis of randomized controlled trials

BACKGROUND AND AIMS

Colonoscopy is the preferred modality for colorectal cancer screening because it has both diagnostic and therapeutic capabilities. Current consensus states that colonoscopy should be performed with initial rapid passage of the instrument to the cecum, followed by thorough evaluation for and removal of all polyps during a deliberate slow withdrawal. Reports have suggested that polyps that are seen but not removed during insertion are sometimes quite difficult to find during withdrawal.

METHODS

We performed a comprehensive literature search of several major databases (from inception to March 2020) to identify randomized controlled trials comparing inspection and polypectomy during the insertion phase as opposed to the traditional practice of inspection and polypectomy performed entirely during the withdrawal phase. We examined differences in terms of adenoma detection rate (ADR), polyps detected per patient (PDPP), cecal intubation time (CIT), withdrawal time, and total procedure time.

RESULTS

Seven randomized controlled trials, including 3834 patients, were included in our final analysis. The insertion/withdrawal cohort had 1951 patients and the withdrawal-only cohort 1883 patients. Pooled odds of adenoma detection in the insertion/withdrawal cohort was .99 (P = .8). ADR was 47.2% in the insertion/withdrawal cohort and 48.6% in the withdrawal-only cohort. Although total procedure and withdrawal times were shorter in the insertion/withdrawal cohort, PDPP in both cohorts were not statistically significant (1.4 vs 1.5, P = .7).

CONCLUSIONS

Additional inspection and polypectomy during the insertion and withdrawal phases of colonoscopy offer no additional benefit in terms of ADR or PDPP.

Safety and adequacy of endoscopic ultrasound-guided random liver biopsy in comparison with transjugular and percutaneous approaches

Background and study aims  Endoscopic ultrasound-guided liver biopsy (EUS-LB) is an accepted technique for tissue acquisition. Traditionally, random LB has been performed with percutaneous (PC-LB) and transjugular (TJ-LB) approaches. The purpose of this study was to compare the safety profile and efficacy of EUS-LB, PC-LB, and TJ-LB. Patients and methods  A retrospective analysis was performed at a tertiary academic medical center. Inclusion criteria for analysis were all adult patients who underwent EUS-LB since inception and TJ-LB/PC-LB over a 3-year span (June 2016 to June 2019). The primary outcome assessed was any adverse events. Secondary outcomes included technical success resulting in tissue acquisition and diagnostic adequacy of the sample for histologic analysis. Results  A total of 513 patients were included for analysis. There were 135 EUS-LB, 287 PC-LB, and 91 TJ-LB. The most common indication for LB was abnormal liver function tests. For the primary outcome, the rate of adverse events was low with five reported (< 1 %). There were two in the EUS-LB group, two in the PC-LB group, and one in TJ-LB group, and this difference was not statistically significant ( P  = 0.585). The technical success rate was 100 % in each group. The rate of diagnostic adequacy was 100 % in TJ-LB group and 99 % in both EUS-LB and PC-LB groups. This difference was not statistically significant ( P  = 1.000). The most common histologic finding was non-specific changes (33.7 %) followed by non-alcoholic steatohepatitis (15.60 %). Conclusion  In comparison with PC-LB and TJ-LB, EUS-LB has comparable safety profile, technical success rate, and diagnostic adequacy. EUS-LB should be considered as an option for random liver biopsy.

Comparison of EUS-guided conventional smear and liquid-based cytology in pancreatic lesions: A systematic review and meta-analysis

Background and study aims  Endoscopic ultrasound (EUS)-guided fine-needle aspiration (EUS-FNA) has limitations of inadequate sampling and false-negative results for malignancy. It has been performed using conventional smear (CS) cytology with rapid on-site evaluation (ROSE) with reasonable diagnostic accuracy. An alternative to ROSE is liquid-based cytology (LBC). Commonly used LBC techniques include precipitation-based (SurePath™) and filtration-based (ThinPrep ^® , CellPrep ^® ). Data regarding the diagnostic efficacy of LBC compared with CS are limited. Methods  Multiple databases were searched through March 2020 to identify studies reporting diagnostic yield of EUS-guided CS and LBC in pancreatic lesions. Pooled diagnostic odds and rates of performance for the cytologic diagnoses of benign, suspicious, and malignant lesions were calculated. Diagnostic efficacy was evaluated by pooled rates of accuracy, sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV). Results  Nine studies with a total of 1308 patients were included in our final analysis. Pooled diagnostic odds of CS cytology were 1.69 (CI 1.02-2.79) and 0.39 (CI 0.19-0.8) for malignant lesions when compared to filtration-based and precipitation-based LBC techniques, respectively. For CS, precipitation-based and filtration-based LBC, pooled diagnostic accuracy was 79.7 %, 85.2 %, 77.3 %, sensitivity was 79.2 %, 83.6 %, 68.3 %, and specificity was 99.4 %, 99.5 %, 99.5 %, respectively. Conclusions  The precipitation-based LBC technique (SurePath™) had superior diagnostic odds for malignant pancreatic lesions compared with CS cytology in the absence of ROSE. It showed superior accuracy and sensitivity, but comparable specificity and PPV. Diagnostic odds of CS cytology in the absence of ROSE were superior to the filtration-based LBC technique (ThinPrep ^® , Cellprep ^® ) for diagnosing malignant pancreatic lesions.

Dry Beriberi Manifesting as Acute Inflammatory Demyelinating Polyneuropathy in a Patient With Decompensated Alcohol-Induced Cirrhosis

Thiamine (vitamin B1) deficiency is uncommon in developed countries and is most commonly seen in patients with poor dietary intake, malabsorption syndromes, and alcoholism. With the increasing rates of alcohol use, thiamine deficiency is likely an under-recognized and potentially reversible cause of sensorimotor dysfunction called dry beriberi. We present a case of profound lower extremity weakness in a 28-year-old female from Nepal with decompensated alcohol-induced cirrhosis. Based on laboratory testing, it was determined that the cause of her neuropathy was dry beriberi. She was subsequently started on thiamine replacement therapy with slow improvement over the next six months. 

Efficacy of EUS-RFA in pancreatic tumors: Is it ready for prime time? A systematic review and meta-analysis

Background and study aims  Endoscopic ultrasound-guided radiofrequency ablation (EUS-RFA) can be used in patients with unresectable pancreatic ductal adenocarcinoma (PDAC). We performed a systematic review and meta-analysis to evaluate the efficacy of EUS-RFA in treatment of locally advanced unresectable PDAC and other pancreatic tumors. Patients and methods  A comprehensive search was done of multiple electronic databases and conference proceedings including PubMed, EMBASE, Web of Science databases, Google Scholar and manual search of references (from inception through May 2019) to identify the studies reporting use of EUS-RFA for pancreatic lesions. The primary outcome was to evaluate technical and clinical success of the procedure. The secondary outcome was to study overall adverse events (AEs). Results  Thirteen studies reporting 165 EUS-RFA procedures on 134 patients were included. Of 134 patients, 27.94 % (38) had unresectable locally advanced PDAC, 40 % (53) had PNETs, 3 % (4) had metastasis to the pancreas and 30 % (41) had other lesions. The pooled technical success rate calculated out of the total number of procedures was 100 % (95 % CI [99.18 - 100], I2 = 0 %). The pooled clinical success rate calculated out of the total number of patients was 91.58 % (95 % CI [82.5 - 98.08], I2 = 21.5 %). The pooled overall AE rates were 14.67 % (95 % CI [4.77 - 27.46], I2 = 56.19 %) out of which abdominal pain was the most common with 9.82 % (95 % CI [3.34 - 18.24], I2 = 23.76 %). Low to moderate heterogeneity was noted. Conclusion  EUS-RFA has high technical (100 %) and clinical success (91.5 %) rates. Further multicenter trials are needed to further validate our findings.

Efficacy of pancreatoscopy for pancreatic duct stones: A systematic review and meta-analysis

BACKGROUND

Pancreatic duct stones can lead to significant abdominal pain for patients. Per oral pancreatoscopy (POP)-guided intracorporal lithotripsy is being increasingly used for the management of main pancreatic duct calculi (PDC) in chronic pancreatitis. POP uses two techniques: Electrohydraulic lithotripsy (EHL) and laser lithotripsy (LL). Data on the safety and efficacy are limited for this procedure. We performed a systematic review and meta-analysis with a primary aim to calculate the pooled technical and clinical success rates of POP. The secondary aim was to assess pooled rates of technical success, clinical success for the two individual techniques, and adverse event rates.

AIM

To perform a systematic review and meta-analysis of POP, EHL and LL for management of PDC in chronic pancreatitis.

METHODS

We conducted a comprehensive search of multiple electronic databases and conference proceedings including PubMed, EMBASE, Cochrane, Google Scholar and Web of Science databases (from 1999 to October 2019) to identify studies with patient age greater than 17 and any gender that reported on outcomes of POP, EHL and LL. The primary outcome assessed involved the pooled technical success and clinical success rate of POP. The secondary outcome included the pooled technical success and clinical success rate for EHL and LL. We also assessed the pooled rate of adverse events for POP, EHL and LL including a subgroup analysis for the rate of adverse event subtypes for POP: Hemorrhage, post-endoscopic retrograde cholangiopancreatography pancreatitis (PEP), perforation, abdominal pain, fever and infections. Technical success was defined as the rate of clearing pancreatic duct stones and clinical success as the improvement in pain. Random-effects model was used for analysis. Heterogeneity between study-specific estimates was calculated using the Cochran Q statistical test and I² statistics. Publication bias was ascertained, qualitatively by visual inspection of funnel plot and quantitatively by the Egger test.

RESULTS

A total of 16 studies including 383 patients met the inclusion criteria. The technical success rate of POP was 76.4% (95%CI: 65.9-84.5; I² = 64%) and clinical success rate was 76.8% (95%CI: 65.2-85.4; I² = 66%). The technical success rate of EHL was 70.3% (95%CI: 57.8-80.3; I² = 36%) and clinical success rate of EHL was 66.5% (95%CI: 55.2-76.2; I² = 19%). The technical success rate of LL was 89.3% (95%CI: 70.5-96.7; I² = 70%) and clinical success rate of LL was 88.2% (95%CI: 66.4-96.6; I² = 77%). The incidence of pooled adverse events for POP was 14.9% (95%CI: 9.2-23.2; I² = 49%), for EHL was 11.2% (95%CI: 5.9-20.3; I² = 15%) and for LL was 13.1% (95%CI: 6.3-25.4; I² = 31%). Subgroup analysis of adverse events showed rates of PEP at 7% (95%CI: 3.5-13.6; I² = 38%), fever at 3.7% (95%CI: 2-6.9; I² = 0), abdominal pain at 4.7% (95%CI: 2.7-7.8; I² = 0), perforation at 4.3% (95%CI: 2.1-8.4; I² = 0), hemorrhage at 3.4% (95%CI: 1.7-6.6; I² = 0) and no mortality. There was evidence of publication bias based on funnel plot analysis and Egger’s test.

CONCLUSION

Our study highlights the high technical and clinical success rates for POP, EHL and LL. POP-guided lithotripsy could be a viable option for management of chronic pancreatitis with PDC.

Efficacy and safety of underwater endoscopic mucosal resection for superficial non-ampullary duodenal epithelial tumors: a systematic review and meta-analysis

Background:

Superficial non-ampullary duodenal epithelial tumors (SNADET) are increasingly found during upper endoscopy. Underwater endoscopic mucosal resection (UEMR) is an emerging technique for the endoscopic resection of SNADET. We performed a systematic review and meta-analysis to evaluate the efficacy and safety of this technique.

Methods:

We conducted a comprehensive search of several databases from inception to August 2019, which included Ovid Cochrane Database of Systematic Reviews, Ovid Embase, Scopus, Ovid Cochrane Central Register of Controlled trials, Ovid MEDLINE^®, and In-Process and other non-indexed citations. The primary outcome assessed was the pooled clinical success rate of UEMR. Secondary outcomes included rate of en bloc resection, pooled rate of high-grade dysplasia or intramucosal carcinoma (HGIC), and pooled rate of adverse events. Meta-regression analysis was performed based on tumor size.

Results:

A total of 8 study arms were included for analysis with UEMR performed in a total of 258 lesions. The pooled clinical success rate was 89.9% (95% confidence interval [CI] 83.4-94.1). En-bloc removal was achieved in 84.6% of treated lesions (95%CI 75.5-90.7). The pooled rate of HGIC was 24.7% (95%CI 10.3-48.3). The pooled rate of adverse events was 6.9% (95%CI 2.5-17.9). This included 10 total adverse events, with the majority being self-limited delayed bleeding. There were no duodenal perforations.

Conclusions:

UEMR for endoscopic resection of SNADET has a high efficacy. In addition, this technique has a high rate of en bloc resection and an acceptable adverse event profile. Given these data, UEMR should be considered as a method for endoscopic resection of SNADET.

Clinical efficacy and safety of palliative esophageal stenting without fluoroscopy: a systematic review and meta-analysis

Background and study aims  Despite advances in curative treatments for esophageal cancer, many patients often present with advanced disease. Dysphagia resulting in significant weight loss and malnutrition leads to poor quality of life. Palliative esophageal stenting with self-expanding metal stents (SEMS) helps alleviate symptoms and prolongs survival. However, access to fluoroscopy may be limited at certain centers causing delay in patient care. Methods  We searched multiple databases from inception to November 2019 to identify studies evaluating the efficacy and safety of endoscopic palliative esophageal stenting and selected only those studies where fluoroscopic guidance was not used. Our primary aim was to calculate the overall technical as well as clinical success. Using meta-regression analysis, we also evaluated the effect of tumor location and obstruction length on overall technical and clinical success. Results  A total of 1778 patients from 17 studies were analyzed. A total of 2036 stents were placed without the aid of fluoroscopy. The pooled rate of technical success was 94.7 % (CI 89.9-97.3, PI 55-99; I ²  = 85) and clinical success was 82.1 % (CI 67.1-91.2, PI 24-99; I ²  = 87). Based on meta-regression analysis both the length of obstruction and tumor location did not have any statistically significant effect on technical and clinical success. The pooled rate of adverse events was 4.1 % (CI 2.4-7.2; I ²  = 72) for stent migration, 8.1 % (CI 4.1-15.4; I ²  = 89) for tumor overgrowth and 1.2 % (CI 0.7-2; I ²  = 0) for perforation. The most frequent clinical adverse event was retro-sternal chest pain. Conclusion  Palliative esophageal stenting without fluoroscopy using SEMS is both safe and effective in patients with advanced esophageal cancer.

Diagnostic yield of deep biopsy via endoscopic submucosal dissection for the diagnosis of upper gastrointestinal subepithelial tumors: a systematic review and meta-analysis

Background:

Conventionally, endoscopic ultrasound-guided fine-needle aspiration and biopsy (EUS-FNA)/EUS-FNB) has been used for tissue diagnosis of upper gastrointestinal (GI) subepithelial tumors (SETs). However, deep biopsy (DB) via endoscopic submucosal dissection (ESD) is emerging as an alternative technique, given the inadequate tissue sampling with EUS-FNA/EUS-FNB. Our aim was to conduct a systematic review and meta-analysis to report the overall diagnostic yield of DB via ESD for upper GI SETs.

Methods:

PubMed, Cochrane Library and Web of Science databases were searched to identify studies (from commencement to Oct 2017) that reported the DB via ESD technique for diagnosis of upper GI SETs. The primary outcome of interest was the method’s overall diagnostic yield and the secondary outcome was to the occurrence of complications. The meta-analysis was performed using the DerSimonian and Laird random-effects model.

Results:

A total of 7 studies, comprising 209 patients with a mean age of 57.3 years, were included in the final meta-analysis. The overall pooled diagnostic yield of DB via ESD for upper GI SETs was 95% (95% confidence interval [CI] 84.91-99.98, I²=78.2%). Overall, pooled outcomes of major bleeding and perforation in our meta-analysis was noted in 0.07% (95%CI 0.00-2.32, I²=0%) and 0% (95%CI 0.00-1.70, I²=0%) respectively. Data regarding major bleeding and perforation rates were not reported in 2 studies. Substantial heterogeneity was observed in our meta-analysis.

Conclusion:

DB via ESD is an effective and safe procedure for diagnosing upper GI SETs. Further multicenter randomized controlled trials are needed to validate these findings.

Impact of psychosocial comorbidities on clinical outcomes after liver transplantation: Stratification of a high-risk population

BACKGROUND

Liver transplantation is the accepted standard of care for end-stage liver disease due to a variety of etiologies including decompensated cirrhosis, fulminant hepatic failure, and primary hepatic malignancy. There are currently over 13000 candidates on the liver transplant waiting list emphasizing the importance of rigorous patient selection. There are few studies regarding the impact of additional psychosocial barriers to liver transplant including financial hardship, lack of caregiver support, polysubstance abuse, and issues with medical non-compliance. We hypothesized that patients with certain psychosocial comorbidities experienced worse outcomes after liver transplantation.

AIM

To assess the impact of certain pre-transplant psychosocial comorbidities on outcomes after liver transplantation.

METHODS

A retrospective analysis was performed on all adult patients from 2012-2016. Psychosocial comorbidities including documented medical non-compliance, polysubstance abuse, financial issues, and lack of caregiver support were collected. The primary outcome assessed post-transplantation was survival. Secondary outcomes measured included graft failure, episodes of acute rejection, psychiatric decompensation, number of readmissions, presence of infection, recidivism for alcohol and other substances, and documented caregiver support failure.

RESULTS

For the primary outcome, there were no differences in survival. Patients with a history of psychiatric disease had a higher incidence of psychiatric decompensation after liver transplantation (19% vs 10%, P = 0.013). Treatment of psychiatric disorders resulted in a reduction of the incidence of psychiatric decompensation (21% vs 11%, P = 0.022). Patients with a history of polysubstance abuse in the transplant evaluation had a higher incidence of substance abuse after transplantation (5.8% vs 1.2%, P = 0.05). In this cohort, 15 patients (3.8%) were found to have medical compliance issues in the transplant evaluation. Of these specific patients, 13.3% were found to have substance abuse after transplantation as opposed to 1.3% in patients without documented compliance issues (P = 0.03).

CONCLUSION

Patients with certain psychosocial comorbidities had worse outcomes following liver transplantation. Further prospective and multi-center studies are warranted to properly determine guidelines for liver transplantation regarding this high-risk population.

Immunostimulatory antibodies: Challenging the drug testing paradigm

The recently failed first-in-man clinical trial of TGN1412 raises concerns about whether the existing drug testing paradigm is suited to the safety assessment of drugs based on immunostimulatory antibodies that have complex and novel mechanisms of action. In particular, there is a need to consider whether animal studies are relevant and, if so, how the resulting information can be used to best inform clinical studies. The preclinical testing of TGN1412 is considered in relation to the selection of a suitable test species, deficiencies in an understanding of the similarities and differences between human and other primate immune functioning and species extrapolation. It is concluded that more emphasis should be placed on the development and use of in vitro and computational methods to identify potentially important species differences in the activity of immunostimulatory antibodies. Such approaches are useful with regards to species extrapolation, mechanistic studies and the design of both preclinical tests in animals and clinical studies in humans.

Evidence to support a spectrum of active states for the glucagon receptor

The ternary complex model suggests that G-protein-coupled receptors resonate between inactive (R) and active (R*) forms. Physiologically, R sites ordinarily predominate with a few R* sites giving rise to basal activity. Agonists recognize, stabilize and increase the R* population, thus altering intracellular activity. There is evidence to suggest the possibility of a spectrum of conformations between R and R*. Our aim is to study the consequences of putative GR (glucagon receptor)-activating mutations using glucagon and partial agonist des-His(1)-[Glu(9)]glucagon amide (glucagon-NH(2)). Alanine substitution in TM (transmembrane) helix 2 of Arg(173) or of His(177) detrimentally affected glucagon and glucagon-NH(2) response maxima. TM2 receptor mutant, Phe(181)-Ala, displayed reduced maximum cAMP accumulation in response to glucagon-NH(2). Thr(353)-Cys (TM6) and Glu(406)-Ala (TM7) receptors demonstrated constitutive activity and enhanced EC(50) values for glucagon-NH(2); Arg(346)-Ala (TM6) and Asn(404)-Ala (TM7) receptors were activated by sub-fmol glucagon concentrations, yet were not constitutively active and demonstrated wild-type receptor-like EC(50) values for glucagon-NH(2). Unlike Arg(346)-Ala receptors, Thr(353)-Cys, Asn(404)-Ala and Glu(406)-Ala receptors demonstrated improved EC(50) values for glucagon, whereas their maximal responses to and their affinity for glucagon were comparable with the wild-type receptor. In contrast, despite slightly reduced glucagon-NH(2) affinity, Arg(346)-Ala, Thr(353)-Cys, Asn(404)-Ala and Glu(406)-Ala receptors displayed glucagon-NH(2) response maxima that exceeded those seen for wild-type receptors. Interestingly, we observed biphasic glucagon-mediated signalling responses. Our results are consistent with the concept of different agonists promoting the formation of distinct active states from partially active R*(low) to fully active R*(high) forms.

Interaction of Met297 in the seventh transmembrane segment of the tachykinin NK2 receptor with neurokinin A

We report the use of thiol chemistry to define specific and reversible disulfide interactions of Cys-substituted NK2 receptor mutants with analogues of neurokinin A (NKA) containing single cysteine substitutions. The NKA analogues were N-biotinylated to facilitate the rapid detection of covalent analogue-receptor interactions utilizing streptavidin reactivity. N-biotinyl-[Tyr1,Cys9]NKA, N-biotinyl-[Tyr1,Cys10]NKA were both found to reversibly disulfide bond to the NK2 receptor mutant Met297 --> Cys. This is consistent with the improved affinities of these particular analogues for the Met297 --> Cys receptor as compared with those for the wild-type and Met297 --> Leu receptors. In our three-dimensional model, Met297 occupies the equivalent position in helix 7 to the retinal binding Lys296 in rhodopsin. Binding of the NK2 receptor antagonist [3H]SR 48968 and of 125I-NKA was used to characterize additional receptor mutants. It seems that the aromatic residues Trp99 (helix 3), His198 (helix 5), Tyr266, His267, and Phe270 play an important role in NKA binding as structural determinants. The existence of overlapping SR 48968 and NKA binding sites is also evident. These data suggest that the peptide binding site of the NK2R is at least in part formed by residues buried deep within the transmembrane bundle and that this intramembranous binding domain may correspond to the binding sites for substantially smaller endogenous GPCR ligands.

Spatial requirements of the antagonist binding site of the NK2 receptor

Computer-aided modelling has been used to identify a putative antagonist binding site in the tachykinin NK2 receptor. In order to validate the implied spatial requirements for this region, a series of compounds, based on the potent antagonist GR 149861 have been synthesised and their binding affinities established. Our findings suggest the presence of a large hydrophobic cavity in the putative binding crevice of GR 149861.

The ligand binding site of the neurokinin 2 receptor. Site-directed mutagenesis and identification of neurokinin A binding residues in the human neurokinin 2 receptor

Thirteen residues in the human neurokinin 2 (NK2) receptor were identified as potential ligand-binding residues by molecular modeling and amino acid sequence analysis. Site-directed mutagenesis was used to alter these residues in order to ascertain their importance in binding neurokinin A (NKA), the physiological peptide ligand for the NK2 receptor, and the non-peptide NK2 receptor selective antagonist SR48968. Four sites appear to be critical for NKA binding (Gln109, His198, Ile202, and Gly273). The mutant receptors Gln109-->His, Ile202-->Val, Gly273-->Pro, and Gly273-->Thr maintain their affinity for SR48968, despite being unable to bind the peptide ligand. His198-->Ala and His198-->Leu no longer bind NKA or SR48968. We have also identified a residue (Leu292) which appears to play a minor role in the binding of substance P (SP) and neurokinin B (NKB) to the NK2 receptor. The mutant receptor Leu292-->Ser binds NKB and SP with approximately a 5-fold greater affinity in comparison with the wild type receptor while the affinity of NKA remains unaffected. The results suggest that intramembranous residues, as well as residues which lie close to the extracellular side of transmembrane helices 3, 5, and 6, form part of the NK2 receptor binding site. Binding of SP and NKB to the NK2 receptor may also be influenced by residues near the extracellular side of helix 7. These results suggest that some regions of the binding site for NKA in the NK2 receptor are not used for binding SP in the NK1 receptor. However, it also seems that the NKA binding site includes regions that are also used by other G-protein-coupled receptors such as rhodopsin and the beta 2-adrenergic receptors.