Subacute hepatic failure due to hepatitis E

BACKGROUND AND AIM

The data available on subacute hepatic failure due to hepatitis E virus is scarce. The aim of this study is to analyze the clinical spectrum and outcome of this condition.

METHODS

This is a retrospective hospital-based study of patients with acute hepatitis E and subacute hepatic failure from January 2001 to June 2006.

RESULTS

We encountered 12 patients with this condition during the study period. There were four females and eight males (age 39 +/- 16). Jaundice and ascites were present in all. The model for end stage liver disease (MELD) score was 25 +/- 8. All of them had normal-sized liver on ultrasonogram. Transjugular liver biopsies were done in nine patients and revealed extensive bridging, submassive necrosis and cholestasis. Complications included spontaneous bacterial peritonitis (four) and urinary tract infections (two), renal failure (three) and encephalopathy (three). The in-hospital mortality was 25% (3/12). The remaining nine patients left the hospital alive with normalization of liver functions in eight of them over the next few months.

CONCLUSION

Subacute hepatic failure caused by hepatitis E is a distinct entity with a better prognosis compared with the previously published series of subacute hepatic failure. Liver biopsy is useful to differentiate from hepatitis E virus superinfection on underlying chronic disease. Poor prognostic factors were female sex, younger age, encephalopathy and persistent renal failure. These patients should be considered for liver transplantation.

Serum biotinidase is a sensitive and specific biochemical marker of hepatic dysfunction: A preliminary report

Biotinidase is an enzyme synthesized predominantly by the liver. Serum activity of this enzyme has been shown to be low in chronic liver disease. In this study, we endeavored to assess the diagnostic value of serum biotinidase as a marker of hepatic biosynthetic function in acute and chronic liver dysfunction. Twenty-three patients with acute liver disease and 46 with chronic liver disease, as diagnosed by clinical examination, laboratory tests, histopathology and tests for viral markers, were inducted into the study. Forty-six healthy volunteers were selected as controls. Serum biotinidase activity was estimated in all the subjects. Biotinidase activity was found to be significantly lower in the serum of patients with acute (4.59 +/- 1.26 IU/L vs 7.56 +/- 0.82 IU/L in controls; P</= 0.001) and chronic (2.98 +/- 1.18 IU/L vs 7.56 +/- 0.82 IU/L in controls; P</= 0.001) liver disease. Using receiver-operator characteristic curves, serum biotinidase was found to have high values of sensitivity and specificity when applied as a diagnostic test in both acute and chronic liver disease. These results suggest that serum biotinidase may be a sensitive and specific diagnostic marker of hepatic biosynthetic function in both acute and chronic liver disease.

Autoimmune pancreatitis--an uncommon type of chronic pancreatitis

Autoimmune pancreatitis is a recently recognized clinical entity characterized by narrow strictured main pancreatic duct on ERCP, diffusely enlarged sausage-shaped pancreas on CT scan and MRI, seropositivity for antinuclear antibodies, hypergammaglobulinemia, and excellent response to steroids. We report a 25-year-old man and a 53-year-old man with this condition.

Hepatitis E superinfection produces severe decompensation in patients with chronic liver disease

BACKGROUND AND AIMS

The adverse effect of acute hepatitis A in chronic liver disease is well known. The outcome of acute hepatitis E in chronic liver disease has not been extensively studied. The present study aimed to examine the clinical profile and outcome of patients with chronic liver disease and hepatitis E virus (HEV) superinfection, and the seroprevalence of hepatitis A and E infections in patients with chronic liver disease and controls in India.

METHODS

A retrospective study of patients with chronic liver disease and acute icteric hepatitis E was performed. Acute hepatitis E was diagnosed by immunoglobulin (Ig)M ELISA. Seroprevalence studies were carried out using IgG ELISA in 100 patients with chronic liver disease and 79 age- and sex-matched controls.

RESULTS

From June 2001 to December 2002, nine patients with chronic liver disease were found to have superinfection with HEV. Out of these, six patients died of advanced liver failure. The etiology of liver disease was Wilson's disease in six, hepatitis B virus in one, autoimmune in one and cryptogenic in one case. The seroprevalence of hepatitis A was 99 and 100% and 56 and 21% for HEV in cases and controls, respectively.

CONCLUSIONS

Acute HEV in patients with chronic liver disease has a grave prognosis. Wilson's disease was the most common cause of chronic liver disease complicated by acute HEV. Seroprevalence studies showed that 44% of patients with chronic liver disease were at risk of developing hepatitis E. Hepatitis E vaccine, when available, is indicated for use in this group.

Role of Q-type Ca2+ channels in vasopressin secretion from neurohypophysial terminals of the rat

1. The nerve endings of rat neurohypophyses were acutely dissociated and a combination of pharmacological, biophysical and biochemical techniques was used to determine which classes of Ca2+ channels on these central nervous system (CNS) terminals contribute functionally to arginine vasopressin (AVP) and oxytocin (OT) secretion. 2. Purified neurohypophysial plasma membranes not only had a single high-affinity binding site for the N-channel-specific omega-conopeptide MVIIA, but also a distinct high-affinity site for another omega-conopeptide (MVIIC), which affects both N- and P/Q-channels. 3. Neurohypophysial terminals exhibited, besides L- and N-type currents, another component of the Ca2+ current that was only blocked by low concentrations of MVIIC or by high concentrations of omega-AgaIVA, a P/Q-channel-selective spider toxin. 4. This Ca2+ current component had pharmacological and biophysical properties similar to those described for the fast-inactivating form of the P/Q-channel class, suggesting that in the neurohypophysial terminals this current is mediated by a 'Q'-type channel. 5. Pharmacological additivity studies showed that this Q-component contributed to rises in intraterminal Ca2+ concentration ([Ca2+]i) in only half of the terminals tested. 6. Furthermore, the non-L- and non-N-component of Ca(2+)-dependent AVP release, but not OT release, was effectively abolished by the same blockers of Q-type current. 7. Thus Q-channels are present on a subset of the neurohypophysial terminals where, in combination with N- and L-channels, they control AVP but not OT peptide neurosecretion.

Determination of disulfide bridge pattern in omega-conopeptides

Synthetic versions of seven naturally occurring omega-conopeptides were subjected to structural analyses in order to determine their disulfide bridge pattern. The method applied in this study uses a combination of amino-acid composition and peptide sequence analysis of various peptide fragments generated by different enzymatic digestions. A temperature modification in the Edman degradation cycles of a protein sequencer allowed the unambiguous detection of the cleavage of cystine residues. The appearance of the cystine residues in particular cycles of the sequence analysis was characteristic of one or several of the theoretically possible 15 isomers. In the case of multiple choices, possible isomers were further eliminated by the amino-acid and sequence analysis of peptide fragments generated by the enzymatic digestion. All synthetic peptides, SNX-111, -157, -159, -183, -185, -230 and -231, were found to have the same disulfide bridge pattern as determined for the naturally occurring omega-conopeptide G-VI-A, i.e. disulfide bridges between the half-cystines 1-16, 8-20 and 15-25 (using the amino-acid numbering of SNX-111).

Solution structure of omega-conotoxin MVIIA using 2D NMR spectroscopy

The solution structure of omega-conotoxin MVIIA (SNX-111), a peptide toxin from the fish hunting cone snail Conus magus and a high-affinity blocker of N-type calcium channels, was determined by 2D NMR spectroscopy. The backbones of the best 44 structures match with an average pairwise RMSD of 0.59 angstroms. The structures contain a short segment of triple-stranded beta-sheet involving residues 6-8, 20-21, and 24-25. The structure of this toxin is very similar to that of omega-conotoxin GVIA with which is has only 40% sequence homology, but very similar calcium channel binding affinity and selectivity.

Structure-activity analysis of a Conus peptide blocker of N-type neuronal calcium channels

The synthetic peptide SNX-111 corresponding to the sequence of the omega-conopeptide MVIIA from the venom of the marine snail Conus magus is a highly potent and selective antagonist of N-type calcium channels. We have synthesized and characterized a large number of analogs of SNX-111 in order to elucidate the structural features of the peptide involved in blocking N-type calcium channels. Comparison of the binding of SNX-111 and its analogs to rat brain synaptosomal membranes rich in N-type channels revealed that, among the four lysines and two arginines in the molecule, lysine in position 2 and arginines at position 10 and 21 are important for the interaction of SNX-111 with N-type channels. The importance of the middle segment from residues 9 through 14 for this binding interaction was revealed by substitution of the individual residues as well as by the construction of hybrid peptides in which the residues 9-12 in SNX-111 and another conopeptide, SNX-183, corresponding to a peptide SVIB from Conus striatus, were interchanged. Introduction of the sequence SRLM from SNX-111 in place of RKTS in position 9-12 in SNX-183 resulted in a 38-fold increase in affinity.

Solution structure of omega-conotoxin MVIIC, a high affinity ligand of P-type calcium channels, using 1H NMR spectroscopy and complete relaxation matrix analysis

We have determined the solution structure of the omega-conotoxin MVIIC from Conus magus by 1H NMR. This conopeptide preferentially blocks P and Q type Ca2+ currents by binding with high affinity to voltage-sensitive Ca2+ channels in neurons. This 26 residue peptide with three disulfide bonds was chemically synthesized and refolded for NMR structural studies. The 1H NMR NOESY spectrum of this peptide was completely assigned, with stereospecific assignments made for 12 of the beta prochiral centers. Complete relaxation matrix analysis using MARDIGRAS was used to obtain initial interproton distances from peak intensities. The correlation time necessary for these calculations was determined by measuring 13C relaxation times using inversely detected natural abundance spectra. Distances were input to DG, which provided 15 starting structures which were then subjected to restrained molecular dynamics calculations using SANDER with the AMBER 91 force field in vacuo. 1H-1H vicinal coupling constants were obtained using a combination of line fitting of both E. COSY and NOESY spectra and used to generate angle restraints that were included explicitly in the restrained molecular dynamics calculations. The final set of the 15 best structures had a backbone rmsd of 0.84 A. The ensemble R1/6 factor calculated by CORMA for the final 15 structures was 11%. The final structure consists of an anti-parallel, triple-stranded beta-sheet, with four turns. In spite of significant differences in amino acid sequence and affinities for calcium channel subtypes, the backbone structure of omega-conotoxin MVIIC is very similar to the previously reported structure of omega-conotoxin GVIA.

Antagonists of neuronal calcium channels: structure, function, and therapeutic implications

This article reviews the structural and functional diversity of neuronal calcium channels and the therapeutic potential of antagonizing such channels. Through spatial and temporal control of intracellular calcium concentration, voltage-sensitive calcium channels regulate a host of neuronal processes, including neurotransmitter secretion, electrical activity, cytoskeletal function, cell metabolism and proliferation, and gene expression. Several genes elaborate a number of calcium channel isoforms or subtypes--each tailored to specific roles in neuronal function and possessing distinct biophysical properties, distribution, modulation, and pharmacological sensitivity. This diversity has raised the possibility that subtype-specific antagonists could provide novel treatments for some neuropathologies. In fact, neuroprotective and analgesic actions of N-type channel blockers in animals appear to confirm this supposition. These properties prompted human clinical studies evaluating these agents for prevention of neuronal degeneration following ischemic brain trauma and for relief of pain. Future medical applications for these blockers and antagonists of other channels subtypes are discussed.

Differential blockade of voltage-sensitive calcium channels at the mouse neuromuscular junction by novel omega-conopeptides and omega-agatoxin-IVA

This investigation assessed the ability of a variety of calcium channel blocking peptides to block synaptic transmission in the isolated mouse phrenic nerve-hemidiaphragm. The synthetic version of the naturally occurring N-type voltage-sensitive calcium channel (VSCC) blocker omega-conopeptide MVIIA (SNX-111) had no effect on nerve-evoked muscle contractions. The non-N-, non-L-type VSCC blocker, omega-conopeptide MVIIC (SNX-230), blocked neuromuscular transmission completely, as did the selective P-type VSCC blocker, omega-Aga-IVA. Subsequent evaluation of other synthetic omega-conopeptides and analogs disclosed a significant positive correlation between the test compounds' affinities for high-affinity SNX-230 brain binding sites and their neuromuscular blocking potencies. Quantal analysis of transmitter release showed that SNX-230 abolished evoked endplate potentials completely, but had little effect on the amplitude and frequency of spontaneous miniature endplate potentials. Perineural focal recordings of presynaptic currents showed that SNX-230 did not block the neuronal action potential. These and other findings indicated that SNX-230 prevents transmitter release at the mouse neuromuscular junction by blocking calcium channels at presynaptic nerve endings. These calcium channels correspond pharmacologically to VSCCs associated with high-affinity binding sites in rat brain and are most probably either of the P- or Q-type.

Calcium channel antagonist peptides define several components of transmitter release in the hippocampus

The use of subtype-selective voltage-sensitive calcium channel (VSCC) antagonists has established that neurotransmitter release in mammalian brain is mediated by N-like and P-like VSCCs, and that other subtypes also contribute significantly. To determine the roles presynaptic VSCCs play in nervous system function and to evaluate the therapeutic potential of their selective inhibition, it is necessary to define further the contributions of VSCC subtypes to neurotransmitter release. The novel conopeptide, SNX-230 (omega-conopeptide MVIIC), has revealed a new VSCC subtype, the Q-type, in cerebellar granule cells. We have compared the effects of SNX-230 on release of tritiated D-aspartate ([3H]D-Asp; a non-metabolizable analog of glutamate), gamma-aminobutyric acid ([3H]GABA), and norepinephrine ([3H]NE) from rat hippocampal slices to those of the N-type VSCC blocker, SNX-111 (omega-conopeptide MVIIA), and the P-type blocker, omega-agatoxin-IVA (AgaIVA). SNX-230 blocks both [3H]D-Asp and [3H]GABA release completely, whereas AgaIVA blocks them potently but partially and SNX-111 has no effect. These results suggest that glutamate and GABA release are mediated by two VSCC subtypes, a P-type and another, perhaps Q-like. SNX-111 blocks [3H]NE release potently but partially, while SNX-230 blockade is complete, consisting of one very potent phase and one less potent phase. AgaIVA also blocks [3H]NE release potently but partially. These results suggest that at least two VSCC subtypes, an N-type and a novel non-N-type, mediate NE release. Pair-wise combinations of the three ligands indicate that at least three pharmacologically distinct components comprise [3H]NE release in the hippocampus.

Neuroanatomical distribution of receptors for a novel voltage-sensitive calcium-channel antagonist, SNX-230 (omega-conopeptide MVIIC)

Neuronal voltage-sensitive calcium channels (VSCCs) are a diverse family of proteins that regulate entry of Ca2+ into neurons. Selective antagonists of VSCCs have proven to be powerful pharmacological tools for identifying and characterizing these channels. A new VSCC antagonist, SNX-230 (also known as omega-conopeptide MVIIC), binds with high affinity to receptors in rat brain and blocks one or more high-threshold VSCCs that are neither L- nor N-type. We have defined the neuroanatomical distribution of the high-affinity non-L, non-N VSCC receptors for SNX-230 using [125I]SNX-230 bound to rat brain sections and compared it with that of [125I]SNX-111, a reversible blocker of N-type VSCCs. Highest densities of binding for both ligands were seen in areas rich in synaptic connections, such as the oriens, radiatum and molecular layers of the hippocampus. In general, the density of [125I]SNX-230-binding was higher in cerebellum compared with that in forebrain. In contrast, this general distribution of density was reversed for [125I]SNX-111. In the glomeruli of the olfactory bulb, binding of [125I]SNX-230 was undetectable compared with the high density of [125I]SNX-111-binding. Differential localization of the two ligands was also seen in cervical spinal cord. The clearly different localization of [125I]SNX-230 compared with that of [125I]SNX-111 in the olfactory bulb and spinal cord suggested that the binding sites for [125I]SNX-230 in other brain regions, while co-localized macroscopically, are also distinct from those for [125I]SNX-111. This was confirmed when addition of saturating concentrations of SNX-111 did not affect the distribution pattern of [125I]SNX-230-binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium channel subtypes in rat brain: biochemical characterization of the high-affinity receptors for omega-conopeptides SNX-230 (synthetic MVIIC), SNX-183 (SVIB), and SNX-111 (MVIIA)

High-threshold voltage-sensitive calcium channels of the N-type, L-type, and P-type have been distinguished in the mammalian CNS predominantly on the basis of their sensitivity to selective antagonists. Matching them with genes identified by molecular cloning is an ongoing undertaking. Whereas L-type channels are characterized by their sensitivity to dihydropyridines and P-type channels by sensitivity to the funnel-web spider toxin AgaIVA, the N-type channel has been shown to be recognized by the omega-conopeptides GVIA and MVIIA. Recently, two new members of the family of omega-conopeptides--MVIIC from the marine snail Conus magus and SVIB from Conus striatus--have been described. Binding and electrophysiological data suggest that these two peptides, in addition to interacting with N-type calcium channels, interact with a widely distributed receptor in neuronal membranes that is distinct from N-type channels. In this report we demonstrate through biochemical and pharmacological differentiation at individual receptor polypeptide resolution, by affinity cross-linking, SDS-PAGE, and autoradiography, that SNX-230 (synthetic MVIIC) binds with high affinity to a calcium channel alpha 1 subunit distinct from the high-affinity alpha 1 target of SNX-111 (synthetic MVIIA). SNX-183 (synthetic SVIB) interacts with both alpha 1 subunits with lower affinity. Whereas the alpha 1 subunit recognized with high affinity by MVIIA corresponds to the N-type channel, the other represents a novel calcium channel distinct from N-, L-, and perhaps P-type channels.

Characterization of the binding of omega-conopeptides to different classes of non-L-type neuronal calcium channels

The interaction of two synthetic omega-conopeptides SNX-111 (MVIIA) and SNX-230 (MVIIC) both derived from the marine snail Conus magus, with non-L-type neuronal voltage-sensitive calcium channels (VSCC) in rat brain synaptosomal preparations has been investigated with the aid of well-characterized 125I derivatives of the two peptides. To assess the effects of iodination on the binding characteristics of SNX-111 and SNX-230, the corresponding peptides containing monoiodotyrosine in place of tyrosine, namely, SNX-259 ([127I]SNX-111) and SNX-260 ([127I]SNX-230), respectively, were prepared by solid-phase synthesis. Saturation analysis showed that [125I]SNX-111 and [125I]SNX-230 bound to two distinct classes of high-affinity sites with apparent Kd's of 9 and 11 pM and Bmax's of 0.54 and 2.2 pmol/mg protein, respectively. Kinetic analysis revealed that both peptides exhibited high association rates as well as rapid dissociation rates in contrast to the 125I derivative of the synthetic omega-conopeptide from Conus geographus, GVIA (SNX-124), which binds irreversibly to N-type channels in rat brain synaptosomes. Competition binding experiments with [125I]SNX-111 and [125I]SNX-124 established that both of them bind to the same site, namely, N-type VSCC. The site detected by the binding of [125I]SNX-230 is distinct from N-type VSCC since SNX-111 has very low affinity (K(i) = 135 nM) in competition studies. Recent findings that a novel high-voltage-activated calcium channel in rat cerebellar granule neurons is resistant to blockers of L-, N-, and P-type VSCC but is highly sensitive to SNX-230 suggest that the [125I]SNX-230 binding site may represent this novel type of calcium channel or another, as yet undescribed, VSCC.

Characterization and partial purification of the human receptor for the heat-stable enterotoxin

The receptor for the Escherichia coli heat-stable enterotoxin has been characterized and partially purified from the T84 human colonic cell line. Using a novel mutant heat-stable enterotoxin peptide as a radioligand (the C-terminal tyrosine residue is replaced by phenylalanine in the mutant), a single class of high-affinity receptor sites was detected in T84 cells, with a Kd of 0.1 nM, similar in affinity to the receptor described in human intestinal tissue. The receptor was solubilised from T84 cell membranes and affinity cross-linking of the solubilised preparation indicated that a single species of M(r) 160,000 served as the receptor. Freshly solubilised preparations of the receptor retained heat-stable enterotoxin-activable guanylyl cyclase activity. Purification of the receptor was achieved through sequential affinity chromatography on GTP--epoxy-Sepharose and wheat-germ-agglutinin columns resulting in purification of the receptor by 3000 fold. The heat-stable enterotoxin-binding characteristics of the receptor were unchanged during the purification and silver staining of the purified receptor preparation indicated a band of M(r) 160,000, which was specifically cross-linked to the 125I-labeled mutant peptide. The purified receptor retained guanylyl cyclase activity, but the activity was not stimulated on addition of human heat-stable enterotoxin, suggesting that accessory structural factors may be involved in the activation of the guanylyl cyclase/receptor.

A new Conus peptide ligand for Ca channel subtypes

A cDNA clone encoding a new omega-conotoxin was identified from Conus magus. The predicted peptide was chemically synthesized using a novel strategy that efficiently yielded the biologically active disulfide-bonded isomer. This peptide, omega-conotoxin MVIID, targets other voltage-gated calcium channels besides the N-subtype and exhibits greater discrimination against the N-channel subtype than any other omega-conotoxin variant to date. Consequently, omega-conotoxin MVIID may be a particularly useful ligand for calcium channel subtypes that are not of the L- or N-subclasses. Of the eight major sequence variants of omega-conotoxins that have been elucidated, four come from Conus magus venom. We suggest that sequence variants from the same venom may be designed to optimally interact with different molecular variants of calcium channels; such omega-conotoxin sets from a single venom may therefore be useful for helping to identify novel calcium channel subtypes.

Solution structure of omega-conotoxin GVIA using 2-D NMR spectroscopy and relaxation matrix analysis

We report here the solution structure of omega-conotoxin GVIA, a peptide antagonist of the N-type neuronal voltage-sensitive calcium channel. The structure was determined using two-dimensional NMR in combination with distance geometry and restrained molecular dynamics. The full relaxation matrix analysis program MARDIGRAS was used to generate maximum and minimum distance restraints from the crosspeak intensities in NOESY spectra. The 187 restraints obtained were used in conjunction with 23 angle restraints from vicinal coupling constants as input for the structure calculations. The backbones of the best 21 structures match with an average pairwise RMSD of 0.58 A. The structures contain a short segment of triple-stranded beta-sheet involving residues 6-8, 18-21, and 24-27, making this the smallest published peptide structure to contain a triple-stranded beta-sheet. Conotoxins have been shown to be effective neuroprotective agents in animal models of brain ischemia. Our results should aid in the design of novel nonpeptide compounds with potential therapeutic utility.

Novel alpha- and omega-conotoxins from Conus striatus venom

Three neurotoxic peptides from the venom of Conus striatus have been purified, biochemically characterized, and chemically synthesized. One of these, an acetylcholine receptor blocker designated alpha-conotoxin SII, has the sequence GCCCNPACGPNYGCGTSCS. In contrast to all other alpha-conotoxins, SII has three disulfide bonds (instead of two), has no net positive charge, and has a free C-terminus. The other two paralytic peptides are Ca channel-targeted omega-conotoxins, SVIA and SVIB. omega-SVIA is the smallest natural omega-conotoxin so far characterized and has the sequence CRSSGSPCGVTSICCGRCYRGKCT-NH2. Although omega-conotoxin SVIA is a potent paralytic toxic in lower vertebrate species, it was much less effective in mammals. The third toxin, omega-conotoxin SVIB, has the sequence CKLKGQSCRKTSYDCCSGSCGRSGKC-NH2. This peptide has a different pharmacological specificity from other omega-conotoxins previously purified from Conus venoms; only omega-conotoxin SVIB has proven to be lethal to mice upon ic injection. Binding competition experiments with rat brain synaptosomal membranes indicate that the high-affinity binding site for omega-conotoxin SVIB is distinct from the high-affinity omega-conotoxin GVIA or MVIIA site.

A new Conus peptide ligand for mammalian presynaptic Ca2+ channels

Voltage-sensitive Ca2+ channels that control neurotransmitter release are blocked by omega-conotoxin (omega-CgTx) GVIA from the marine snail Conus geographus, the most widely used inhibitor of neurotransmitter release. However, many mammalian synapses are omega-CgTx-GVIA insensitive. We describe a new Conus peptide, omega-CgTx-MVIIC, that is an effective inhibitor of omega-CgTx-GVIA-resistant synaptic transmission. Ca2+ channel targets that are inhibited by omega-CgTx-MVIIC but not by omega-CgTx-GVIA include those mediating depolarization-induced 45Ca2+ uptake in rat synaptosome preparations, "P" currents in cerebellar Purkinje cells, and a subset of omega-CgTx-GVIA-resistant currents in CA1 hippocampal pyramidal cells. The characterization of omega-CgTx-MVIIC by a combination of molecular genetics and chemical synthesis defines a general approach for obtaining ligands with novel receptor subtype specificity from Conus.

Hm1 muscarinic cholinergic receptor internalization requires a domain in the third cytoplasmic loop

Selected regions of the Hm1 muscarinic cholinergic receptor were mutated to analyze the molecular mechanisms of agonist-induced receptor internalization (or sequestration). The wild-type and mutant Hm1 genes were expressed, using pSG5, in U293 human kidney cells. Whereas surface receptor density measured with the polar tracer N-[3H]methylscopolamine was rapidly reduced by carbachol exposure, total receptor content measured with [3H]quinuclidinyl benzilate did not decline for at least 24 h, indicating the absence of extensive receptor down-regulation in U293 cells. Carbachol stimulation of phosphatidylinositol turnover paralleled receptor internalization, both with EC50 values of 10-20 microM. Furthermore, a D71N point mutation that prevented receptor activation also abolished carbachol-induced receptor internalization, indicating that receptor activation (but not necessarily second messenger stimulation) was required for internalization. Truncation of the COOH-terminal tail (K447 trunc) and point mutations of several potential Ser and Thr phosphorylation sites to Ala failed to affect receptor activation and internalization. In contrast, partial deletions of the third intracellular loop (i3) (Tyr208-Thr366) resulted in receptor mutants deficient in agonist-induced receptor internalization/sequestration. Various deletions caused either complete loss of internalization (d 232-358) or impaired internalization, ranging from 10 to 30% over 2 h, whereas wild-type Hm1 internalized to approximately 50%. Whereas the reason for the observed differences among the deficient deletion mutants remains unclear, the initial rate of N-[3H]methylscopolamine binding loss from the cell surface was much slower than that of wild-type Hm1 in each case. The deletion of only one single domain, 284-292 (SMESLTSSE), in the middle of i3 was consistently associated with impaired internalization. Domain 284-292 is partially conserved among closely related muscarinic receptors, whereas most of the remainder of i3 is not (except for the i3 membrane junctions), and similar Ser- and Thr-rich regions are present in many other G protein-coupled receptors. We propose that a small receptor domain in the middle of the i3 loop of Hm1 is involved in agonist-induced receptor internalization.

Structure and function of G protein coupled receptors

The G protein coupled receptors (GPC-Rs) comprise a large superfamily of genes encoding numerous receptors which all show common structural features, e.g., seven putative membrane spanning domains. Their biological functions are extremely diverse, ranging from vision and olfaction to neuronal and endocrine signaling. The GPC-Rs couple via multiple G proteins to a growing number of recognized second messenger pathway, e.g., cAMP and phosphatidyl inositol turnover. This review summarizes our current knowledge of the molecular mechanisms by which the GPC-Rs activate second messenger systems, and it addresses their regulation and structure.

Internalization of the Hm1 muscarinic cholinergic receptor involves the third cytoplasmic loop

The m1 muscarinic receptor was previously shown to stimulate phosphatidyl inositol (PI) turnover and to internalize rapidly upon agonist activation. Three receptor mutants with large deletions of the third cytoplasmic loop (i3) of human Hm1, leaving only 11 and 8 amino acids at the amino and carboxy terminal junctions of i3, respectively, retained full ability to stimulate PI turnover, when expressed in U293 cells, but receptor internalization was greatly reduced in two mutants with deletions reaching close to the NH2 terminal of i3. We propose that a receptor domain located toward the amino terminal junction of i3 plays a role in Hm1 internalization.

Functional diversity of muscarinic receptor subtypes in cellular signal transduction and growth

The regulation of cellular signal transduction and growth by four human muscarinic acetylcholine receptor (mAChR) subtypes has been studied comparatively. The four mAChRs fall into two functional sub-groups, based on their primary effects on second messenger formation; two of the receptors strongly inhibit adenylyl cyclase activity, whereas the other two strongly stimulate PI hydrolysis. Studies on mAChR regulation of two cellular events involved in cellular growth regulation, the transcription of proto-oncogene c-fos and DNA synthesis, indicate that these events are efficiently activated by those mAChRs which couple primarily to phospholipase C.

Peptide-based radioimmunoassay for insulin receptor. Detection of insulin-stimulated downregulation in IM-9 lymphocytes

To overcome the difficulties encountered in quantifying the insulin receptor number by Scatchard analysis, a radioimmunoassay (RIA) for the human insulin receptor (hIR) has been developed that uses an antibody raised against a synthetic peptide (Gly-Lys-Lys-Asn-Gly-Arg-Ile-Leu-Thr-Leu-Pro-Arg-Ser-Asn-Pro-Ser) corresponding to the carboxyl terminal of the hIR. A second peptide (Tyr-Gly-Arg-Ile-Leu-Thr-Leu-Pro-Arg-Ser-Asn-Pro-Ser) was used as a standard and allowed preparation of monoiodinated derivative of theoretical specific activity for use as the radioactive ligand. The assay is specific, highly reproducible, and sensitive, with a detection limit of 10 fmol of receptor. One mole of purified receptor, measured by Scatchard analysis or amino acid analysis, is read as one mole of receptor in the RIA with peptide being the standard. The assay is effective with receptor from multiple sources and could determine the decrease in number of insulin receptors seen in IM-9 lymphocytes after treatment with insulin (downregulation).

Acetylcholine analogue stimulates DNA synthesis in brain-derived cells via specific muscarinic receptor subtypes

Little is known about the factors which regulate the growth and development of the mammalian brain. Although proliferation of neuronal cells ceases relatively early in development, certain types of glial cells proliferate and differentiate mainly perinatally. In the perinatal period, the ability of acetylcholine to stimulate phosphoinositide (PI) hydrolysis in brain reaches peak levels, and indeed the stable acetylcholine analogue carbachol can stimulate PI hydrolysis of primary neonatal astroglial cells. As PI hydrolysis is thought to be important in the regulation of cell proliferation, we investigated whether cellular DNA synthesis can be induced by carbachol. Our results show that carbachol stimulates DNA synthesis via muscarinic acetylcholine receptors (mAChRs), in primary astrocytes derived from perinatal rat brain, in an age-dependent fashion. Carbachol is also mitogenic in certain brain-derived astrocytoma and neuroblastoma cell lines, as well as in chinese hamster ovary (CHO) cells expressing recombinant muscarinic receptors. DNA synthesis is strongly activated by carbachol in those brain-derived cell lines and transfected CHO cells that express mAChR subtypes which activate PI hydrolysis efficiently, and poorly activated in cells expressing mAChR subtypes which only weakly activate PI hydrolysis. These results strongly support a role for acetylcholine in regulating astroglial cell growth in the developing brain, and indicate that the specificity of acetylcholine-induced cell proliferation may be determined by the expression of those mAChR subtypes which activate PI hydrolysis.

Functionally distinct G proteins selectively couple different receptors to PI hydrolysis in the same cell

The number of G proteins identified by molecular cloning exceeds the number of known G protein functions. Here we show that a cell can possess multiple G proteins that carry out a similar function, the activation of phospholipase C, but couple selectively to different receptors, which are endogenous to the cell or introduced by DNA transfection. These G proteins (termed Gp) can be distinguished by their sensitivity to pertussis toxin. The assignment of a given Gp pathway to specific receptors is confirmed by the additivity relationships of the PI hydrolysis response mediated by the different receptors. Significantly different amounts of PI hydrolysis are activated through each Gp pathway, suggesting that Gp proteins also differ in their coupling to phospholipase C. These results indicate that distinct Gp pathways in a given cell exist to couple different receptors to PI hydrolysis selectively, and may specify the nature of the cellular response to different receptors by determining the magnitude of PI hydrolysis.

The structural and functional interrelationships of muscarinic acetylcholine receptor subtypes

Molecular cloning of the genes encoding the muscarinic acetylcholine receptors has shown that receptor subtypes classified on the basis of pharmacological properties are related polypeptides encoded by distinct genes. These studies have also revealed the existence of novel muscarinic receptor subtypes. Functional analysis of each of the subtypes expressed in mammalian cells indicates that the different subtypes activate distinct biochemical pathways, a finding that explains the tissue-specific physiological response elicited by the neurotransmitter, acetylcholine.

Structure and function changes in rat adrenal glands during aging

The current study examines corticosterone production in young and old rats with a view to understanding how hormone production is related to aging changes in the adrenal gland. For this purpose, we compared total (plasma volume-corrected) values for adrenocorticotropic hormone (ACTH)-induced maximal corticosterone production in young, mature (5-mo-old), and aging (18-mo-old) rats. These values were subsequently corrected for measured age-related differences in adrenal weight, adrenal cortex volume, specific adrenal zone volumes, and total number of corticosterone-producing cells in the adrenals of the differently aged rats. In addition, corticosterone disposal rates were measured in the two groups of rats, and adrenal cortical ACTH binding sites were identified by autoradiography after perfusion with 125I-labeled ACTH analogue. The results show that maximal serum ACTH-induced corticosterone concentrations (per ml serum) in the 18-mo-old rats are somewhat less than those seen in the younger animals. However, because the plasma volume is greater in the older animals (and corticosterone disposal rates do not vary), total circulating corticosterone production is, in fact, equivalent in the two groups of animals. When these total values for corticosterone are corrected for various structural changes in the adrenal gland with age, corticosterone production was found to be lower in the older group of rats: i.e., one sees an approximately 50% decline when corticosterone is expressed per adrenal weight or zonal volume and an approximately 20% decline when expressed per number of endocrine cells per adrenal.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential regulation of PI hydrolysis and adenylyl cyclase by muscarinic receptor subtypes

Muscarinic acetylcholine receptors (mAChRs), like many other neurotransmitter and hormone receptors, transduce agonist signals by activating G proteins to regulate ion channel activity and the generation of second messengers via the phosphoinositide (PI) and adenylyl cyclase systems. Human mAChRs are a family of at least four gene products which have distinct primary structures, ligand-binding properties and patterns of tissue-specific expression. To examine the question of whether functional differences exist between multiple receptor subtypes, we have investigated the ability of each subtype to regulate PI hydrolysis and adenylyl cyclase when expressed individually in a cell lacking endogenous mAChRs. We show that the HM2 and HM3 mAChRs efficiently inhibit adenylyl cyclase activity but poorly activate PI hydrolysis. In contrast, the HM1 and HM4 mAChRs strongly activate PI hydrolysis, but do not inhibit adenylyl cyclase, and in fact can substantially elevate cAMP levels. Interestingly, the subtypes that we find to be functionally similar are also more similar in sequence. Our results indicate that the different receptor subtypes are functionally specialized.

Carboxyl terminal domain of Gs alpha specifies coupling of receptors to stimulation of adenylyl cyclase

The alpha subunits of Gs and Gi link different sets of hormone receptors to stimulation and inhibition, respectively, of adenylyl cyclase. A chimeric alpha i/alpha s cDNA was constructed that encodes a polypeptide composed of the amino terminal 60% of an alpha i chain and the carboxyl terminal 40% of alpha s. The cDNA was introduced via a retroviral vector into S49 cyc- cells, which lack endogenous alpha s. Although less than half of the hybrid alpha chain is derived from alpha s, its ability to mediate beta-adrenoceptor stimulation of adenylyl cyclase matched that of the normal alpha s polypeptide expressed from the same retroviral vector in cyc- cells. This result indicates that carboxyl terminal amino acid sequences of alpha s contain the structural features that are required for specificity of interactions with the effector enzyme, adenylyl cyclase, as well as with the hormone receptor.

Photoaffinity labeling of pituitary GnRH receptors: significance of the position of photolabel on the ligand

Photoreactive derivatives of GnRH and its analogues were prepared by incorporation of the 2-nitro-4(5)-azidophenylsulfenyl [2,4(5)-NAPS] group into amino acid residues at positions 1, 3, 6, or 8 of the decapeptide sequence. The modification of Trp3 by the 2,4-NAPS group led to a complete loss of the luteinizing hormone (LH) releasing as well as LH-release-inhibiting activity of the peptide. The [D-Lys(2,4-NAPS)]6 analogue was a very potent agonist that, after covalent attachment by photoaffinity labeling, caused prolonged LH secretion at a submaximal rate. [Orn(2,4-NAPS)]8-GnRH, a full agonist with a relative potency of 7% of GnRH, after photoaffinity labeling caused prolonged maximal LH release from cultured pituitary cells. In contrast, [Orn(2,5-NAPS)]8-GnRH, although being equipotent with the 2,4-NAPS isomer in terms of LH releasing ability, was unable to cause prolonged LH release after photoaffinity labeling. Thus, [Orn(2,4-NAPS)]8-GnRH is a very effective photolabeling ligand of the functionally significant pituitary GnRH receptor. Based on this compound, a pituitary peptidase resistant derivative, D-Phe6,[Orn(2,4-NAPS)]8-GnRH-(1-9)-ethylamide, was synthesized. This derivative showed high-affinity binding to pituitary membranes with a Kd comparable to those of other GnRH analogues. A radioiodinated form of this peptide was used for pituitary GnRH-receptor labeling. This derivative labeled 59- and 57-kDa proteins in rat and 58- and 56-kDa proteins in bovine pituitary membrane preparations, respectively. This peptide also labeled pituitary GnRH receptors in the solubilized state and therefore appears to be a suitable ligand for the isolation and further characterization of the receptor.

Specificity of tyrosine protein kinases of the structurally related receptors for insulin and insulin-like growth factor I: Tyr-containing synthetic polymers as specific inhibitors or substrates

The receptors for insulin and insulin-like growth factor (IGF) I are structurally similar transmembrane proteins. Ligand binding to the extracellular domain of the receptor stimulates its cytoplasmic tyrosine protein kinase which phosphorylates its own beta subunit as well as exogenous substrates. It is believed, from several lines of evidence, that tyrosine-specific protein kinases are mediating some or all of the actions of insulin (or IGF-I). In order to gain insights into the substrate specificity of the structurally related insulin and IGF-I receptor kinases, we have studied the action of highly purified receptors isolated from human placental membranes. Present studies using selected tyrosine-containing polymers have revealed: (i) Polymers such as (Y,A,E)n and (Y-A-E)n inhibit beta subunit autophosphorylation and exogenous substrate phosphorylation by autophosphorylated receptors. (ii) Insulin receptor kinase is at least 10 times more sensitive to these inhibitors than IGF-I receptor kinase. (iii) (Y-A-E)n is approximately 8 times more potent an inhibitor than (Y,A,E)n toward both receptors. (iv) While (E4,Y1)n and (E6,A3,Y1)n are good substrates for both receptor kinases, the ratio of phosphate incorporation into the former to the latter is characteristically high (approximately 4) for the IGF-I receptor and low (approximately 1) for the insulin receptor. These results imply that the substrate specificity and enzymatic action of these two receptor kinases are distinct.

Distinct primary structures, ligand-binding properties and tissue-specific expression of four human muscarinic acetylcholine receptors

To investigate the molecular basis for the diversity in muscarinic cholinergic function, we have isolated the genes encoding the human M1 and M2 muscarinic receptors (mAChR) as well as two previously undiscovered mAChR subtypes, designated HM3 and HM4. The amino acid sequence of each subtype reflects a structure consisting of seven, highly conserved transmembrane segments and a large intracellular region unique to each subtype, which may constitute the ligand-binding and effector-coupling domains respectively. Significant differences in affinity for muscarinic ligands were detected in individual mAChR subtypes produced by transfection of mammalian cells. Each subtype exhibited multiple affinity states for agonists; differences among subtypes in the affinities and proportions of such sites suggest the capacity of mAChR subtypes to interact differentially with the cellular effector-coupling apparatus. Subtype-specific mRNA expression was observed in the heart, pancreas and a neuronal cell line, indicating that the regulation of mAChR gene expression contributes to the differentiation of cholinergic activity.

Dog mastocytoma tryptase: affinity purification, characterization, and amino-terminal sequence

A tryptic protease with the characteristics of a mast cell tryptase was purified from dog mastocytoma cells propagated in nude mice. Partial amino acid sequence of the mastocytoma tryptase revealed unexpected differences in comparison with other mast cell and leukocyte granule protease sequences. Extraction from mastocytoma homogenates at high ionic strength, followed by gel filtration and benzamidine affinity chromatography yielded a product with several closely spaced bands (Mr 30,000-32,000) on gel electrophoresis and a single N-terminal sequence. Nondenaturing analytical gel filtration revealed an apparent Mr of 132,000, suggesting noncovalent association as a tetramer. Studies with peptide p-nitroanilides indicated pronounced substrate preferences, with P1 arginine preferred to lysine. Benzoyl-L-Lys-Gly-Arg-p-nitroanilide was the best of the substrates screened. Inhibition by diisopropyl fluorophosphate and tosyllysine chloromethyl ketone indicated that the enzyme is a serine protease. Like the tryptases of human mast cells, mastocytoma tryptic protease was inhibited by NaCl, resistant to inactivation by alpha 1-proteinase inhibitor and plasma, and stabilized by heparin. Comparison of the N-terminal 24 residues of mastocytoma tryptase revealed 80% identity with the more limited sequence reported for human lung tryptase, and surprisingly, closer homology to serine proteases of digestion and clotting than to other leukocyte granule proteases sequenced to date, including mast cell chymase. The N-terminal isoleucine is the homolog of trypsinogen Ile-16 which becomes the new N-terminus upon cleavage of the activation peptide. Thus, the tryptase N-terminus is related to the catalytic domain of activated serine proteases, and lacks the N-terminal regulatory domains found in most clotting and complement serine proteases. These findings provide further evidence that tryptases are unique serine proteases and that they may be less closely related in evolution and function than are other leukocyte granule proteases described to date.

An M2 muscarinic receptor subtype coupled to both adenylyl cyclase and phosphoinositide turnover

To investigate whether a particular receptor subtype can be coupled to multiple effector systems, recombinant M2 muscarinic receptors were expressed in cells lacking endogenous receptor. The muscarinic agonist carbachol both inhibited adenylyl cyclase and stimulated phosphoinositide hydrolysis. The stimulation of phosphoinositide hydrolysis was significantly less efficient and more dependent on receptor levels than the inhibition of adenylyl cyclase. Both responses were mediated by guanine nucleotide binding proteins, as evidenced by their inhibition by pertussis toxin; the more efficiently coupled adenylyl cyclase response was significantly more sensitive. Thus, individual subtypes of a given receptor are capable of regulating multiple effector pathways.

Identification of the intact insulin receptor using a sequence-specific antibody directed against the C-terminus of the beta-subunit

An antibody was raised against a synthetic peptide corresponding to the carboxyl-terminal amino acids of the human insulin receptor (Anti-R beta C). Immunoprecipitation of the human insulin receptor and immunoblotting to the beta-subunit by Anti-R beta C could be inhibited by competition with the corresponding peptide. However, even at saturating concentrations, anti-R beta C could not completely immunoprecipitate or immunodeplete insulin receptors compared to a human autoantibody (anti-R B2). Using receptor labeled directly by 125I, evidence of multiple forms of the beta-subunit was found. When the receptor could be immunoprecipitated by anti-R beta C, the beta-subunit migrated with an apparent mol wt (MW) of 96,000 (at or above the phosphorylase b MW marker). However, in preparations where anti-R beta C was not able to immunoprecipitate the insulin receptor, the beta-subunit migrated at a significantly lower MW of 91,000 (below phosphorylase b), as detected by immunoprecipitation with Anti-R B2. Intermediate forms could also be detected. Phosphorylation of partially purified insulin receptor did not affect is ability to be immunoprecipitated by anti-R beta C, although insulin-stimulated phosphorylation increased the apparent MW of the beta-subunit. However, insulin receptor that was phosphorylated in solubilized extracts of whole cells had a beta-subunit that migrated at lower MW and was not immunoprecipitated by anti-R beta C. One possible explanation for this is that the beta-subunit may be degraded during preparation. When the MW of insulin receptor that has been purified to homogeneity from human placenta is compared to our data, it is clear that many of these insulin receptor preparations contain lower MW beta-subunits. These results must be taken into account when the sites of phosphorylation and kinase activity of purified insulin receptor preparations are studied.

Sequence and functional expression of the GABA A receptor shows a ligand-gated receptor super-family

Amino-acid sequences derived from complementary DNAs encoding the alpha- and beta-subunits of the GABA/benzodiazepine receptor from bovine brain show homology with other ligand-gated receptor subunits, suggesting that there is a super-family of ion-channel-containing receptors. Co-expression of the in vitro-generated alpha-subunit and beta-subunit RNAs in Xenopus oocytes produces a functional receptor and ion channel with the pharmacological properties characteristic of the GABAA receptor.

Primary structure and biochemical properties of an M2 muscarinic receptor

A partial amino acid sequence obtained for porcine atrial muscarinic acetylcholine receptor was used to isolate complementary DNA clones containing the complete receptor coding region. The deduced 466-amino acid polypeptide exhibits extensive structural and sequence homology with other receptors coupled to guanine nucleotide binding (G) proteins (for example, the beta-adrenergic receptor and rhodopsins); this similarity predicts a structure of seven membrane-spanning regions distinguished by the disposition of a large cytoplasmic domain. Stable transfection of the Chinese hamster ovary cell line with the atrial receptor complementary DNA leads to the binding of muscarinic antagonists in these cells with affinities characteristic of the M2 receptor subtype. The atrial muscarinic receptor is encoded by a unique gene consisting of a single coding exon and multiple, alternatively spliced 5' noncoding regions. The atrial receptor is distinct from the cerebral muscarinic receptor gene product, sharing only 38% overall amino acid homology and possessing a completely nonhomologous large cytoplasmic domain, suggesting a role for the latter region in differential effector coupling.