The role of the cysteine-rich region of the beta2 integrin subunit in the leukocyte function-associated antigen-1 (LFA-1, alphaLbeta2, CD11a/CD18) heterodimer formation and ligand binding

The cysteine-rich region (CRR) of the beta2 integrin subunit was replaced by that of beta1 to give the chimera beta2NV1. Beta2NV1 can combine with alphaL to form a variant leukocyte-function-associated antigen (LFA)-1 on COS cell surface, suggesting that the specificity of the beta2 interaction with alphaL does not lie in the CRR. Unlike those expressing wild-type LFA-1, COS cells expressing alphaL beta2NV1 are constitutively active in intercellular adhesion molecule (ICAM)-1 adhesion. These results suggest that activation of LFA-1 involves the release of an intramolecular constraint, which is maintained, in part, by the authentic beta2 CRR.

The phylogeny and evolution of the thioester bond-containing proteins C3, C4 and alpha 2-macroglobulin

The complement system is an effector of both the acquired and innate immune systems of the higher vertebrates. It has been traced back at least as far as the echinoderms and so predates the appearance of the antibodies, T-cell receptors and MHC molecules of adaptive immunity. Central to the function of complement is the reaction of the thioester bond located within the structure of complement components C3 and C4. The structural thioester first appeared in a protease inhibitor, alpha 2-macroglobulin, in which it is involved in the immobilisation and entrapment of proteases. An important development in the C3 molecule has been the acquisition of a catalytic His residue which greatly increases the rate of reaction of the thioester with hydroxyl groups and with water.

Expression of the H52 epitope on the beta2 subunit is dependent on its interaction with the alpha subunits of the leukocyte integrins LFA-1, Mac-1 and p150,95 and the presence of Ca2+

Integrin-mediated adhesion is a divalent cation-dependent process. Whether divalent cations directly participate in ligand binding or exert their effects indirectly by affecting the overall structure of the integrin heterodimers is not known. In this study we describe the epitope of the mAb H52 which has been mapped to a predicted disulfide-bonded loop (C386 and C400) in the beta2 integrin subunit. In the presence of Ca2+ and Mg2+, the H52 epitope is expressed on the monomeric beta2 subunit, the LFA-1 and Mac-1 heterodimers but not on p150,95, thus implying that this epitope is masked in p150,95. However, expression of the H52 epitope on Mac-1, but not on LFA-1, or the monomeric beta2 subunit, is dependent on the presence of Ca2+, thus suggesting that the chelation of Ca2+ causes a conformational change in Mac-1 which results in the loss of the epitope. These results suggest that expression of the H52 epitope on the beta2 subunit is dependent on its interaction with the different alpha subunits. Since the epitope itself is not required for heterodimer formation nor for ligand binding, occupancy of a Ca2+ binding site(s) must therefore affect the alphabeta subunit interactions, and thus the overall conformation of Mac-1.

The covalent binding reaction of complement component C3

The covalent binding of C3 to target molecules on the surfaces of pathogens is crucial in most complement-mediated activities. When C3 is activated, the acyl group is transferred from the sulfhydryl of the internal thioester to the hydroxyl group of the acceptor molecule; consequently, C3 is bound to the acceptor surface by an ester bond. It has been determined that the binding reaction of the B isotype of human C4 uses a two-step mechanism. Upon activation, a His residue first attacks the internal thioester to form an acyl-imidazole bond. The freed thiolate anion of the Cys residue of the thioester then acts as a base to catalyze the transfer of the acyl group from the imidazole to the hydroxyl group of the acceptor molecule. In this article, we present results which indicate that this two-step reaction mechanism also occurs in C3.

The internal thioester and the covalent binding properties of the complement proteins C3 and C4

The covalent binding of complement components C3 and C4 is critical for their activities. This reaction is made possible by the presence of an internal thioester in the native protein. Upon activation, which involves a conformational change initiated by the cleavage of a single peptide bond, the thioester becomes available to react with molecules with nucleophilic groups. This description is probably sufficient to account for the binding of the C4A isotype of human C4 to amino nucleophiles. The binding of the C4B isotype, and most likely C3, to hydroxyl nucleophiles, however, involves a histidine residue, which attacks the thioester to form an intramolecular acyl-imidazole bond. The released thiolate anion then acts as a base to catalyze the binding of hydroxyl nucleophiles, including water, to the acyl function. This mechanism allows the complement proteins to bind to the hydroxyl groups of carbohydrates found on all biological surfaces, including the components of bacterial cell walls. In addition, the fast hydrolysis of the thioester provides a means to contain this very damaging reaction to the immediate proximity of the site of activation.

Adhesion molecule CD11a/CD18-deficient Burkitt's lymphoma cells lack the transcript for the beta, but not the alpha, integrin subunit

Adhesion to cells and matrices participates in the regulation of lymphocyte proliferation, maturation and tissue localization. Consequently, abnormal patterns of adhesion molecule expression may contribute to the pathophysiology of lymphoproliferative disorders. Integrins are major cell-surface adhesive proteins composed by alpha and beta subunits. In contrast to normal lymphocytes, Burkitt's lymphoma (BL) cells lack the beta2 integrin CD11a/CD18. To study the molecular mechanism underlying this deficiency, presence of the transcript for each subunit was analysed by Northern blotting in group I BL lines (BL biopsy-like) and, for comparison, Epstein-Barr virus (EBV)-transformed lymphoblastoid cell lines (LCLs). While transcripts for both CD11a (alpha subunit) and CD18 (beta subunit) were readily detected in LCLs, BL lines contained the transcript for the alpha subunit only. Treatment of BL cells with phorbol ester for 72 h induced expression of the beta subunit mRNA and the CD11a and CD18 antigens on the cell surface. The results indicate that the CD11a/CD18 deficiency of BL is due to absence of the beta subunit transcript and that this defect is restored by stimulation of the cells.

Retinal complications after aqueous shunt surgical procedures for glaucoma

OBJECTIVES

To assess retinal complications and to identify risk factors for retinal complications following aqueous shunt procedures.

MATERIALS AND METHODS

Records of 38 consecutive aqueous shunt procedures that were performed on 36 patients at the Eye Institute of the Medical College of Wisconsin, Milwaukee, from June 1993 to March 1995 (minimum follow-up, 6 months) were reviewed. The mean +/- SD follow-up was 11.4 +/- 5.2 months (median, 10.5 months).

RESULTS

Twelve patients (32%) had the following retinal complications: 4 serous choroidal effusions (10%) that required drainage, 3 suprachoroidal hemorrhages (8%), 2 vitreous hemorrhages (5%), 1 rhegmatogenous retinal detachment (3%), 1 endophthalmitis (3%), and 1 scleral buckling extrusion (3%). Surgical procedures for retinal complications were required in 8 (67%) of these 12 patients. Visual acuity decreased 2 lines or more in 9 (75%) of these 12 patients. The median onset of a postoperative retinal complication was 12.5 days, with 10 patients (83%) experiencing complications within 35 days. Serous choroidal effusions developed in 10 other patients (26%), and these effusions resolved spontaneously. Visual acuity decreased 2 lines or more in 2 (20%) of these additional 10 patients. Patients who experienced serious retinal complications were significantly older, had a higher rate of hypertension, and postoperative ocular hypotony. Serious retinal complications were distributed evenly among patients with Krupin valves with discs and Molteno and Baerveldt devices. Experience with the Ahmed glaucoma valve implant was limited.

CONCLUSION

Aqueous shunt procedures may be associated with significant retinal complications and subsequent visual loss.

The reaction mechanism of the internal thioester in the human complement component C4

A key step in the elimination of pathogens from the body is the covalent binding of complement proteins C3 and C4 to their surfaces. Proteolytic activation of these proteins results in a conformational change, and an internal thioester is exposed which reacts with amino or hydroxyl groups on the target surface to form amide or ester bonds, or is hydrolysed. We report here that the binding of the human C4A isotype involves a direct reaction between amino-nucleophiles and the thioester. A two-step mechanism is used by the C4B isotype. The histidine at position 1,106(aspartic acid in C4A) first attacks the thioester to form an acyl-imidazole intermediate. The released thiol then acts as a base to catalyse the transfer of the acyl group to amino- and hydroxyl-nucleophiles, including water.

Reversible inactivation of purified leukocyte integrin CR3 (CD11b/CD18, alpha m beta 2) by removal of divalent cations from a cryptic site

Integrins exhibit reversible changes in their ability to bind ligands and these changes enable transient cell adhesion. We recently showed that leukocyte integrin CR3 (complement receptor type three, CD11b/CD18, alpha m beta 2) may be purified in a form that is either capable or incapable of binding soluble, monomeric ligand and that "inactive" CR3 may be rendered capable of binding ligand by addition of an anti-CR3 mAb known as KIM-127 (Cai and Wright, JBC. 270: 14358, 1995). Here, we demonstrate that active CR3 may be rendered inactive by treatment of immobilized receptor with EDTA. EDTA-treated CR3 failed to bind ligand even in the presence of mM Ca2+ and Mg2+, suggesting that EDTA-treatment caused a change in the receptor that is not readily reversed. EDTA-treated receptor did, however, bind ligand upon addition of KIM-127 plus Mg2+ with an affinity (17.8 +/- 4.5 nM) similar to untreated, active receptor (12.5 +/- 4.7 nM). EDTA-treated CR3 thus exhibits the properties of inactive CR3, in which the ligand binding site is cryptic but subject to exposure by KIM-127. A candidate for the cryptic ligand binding site is the I-domain, a Mg2+-binding region in the alpha chain of CR3. We found that monomeric C3bi binds directly to recombinant I-domain in a Mg(2+)-dependent fashion with an affinity of 300 +/- 113 nM. These results thus suggest that CR3 may be inactivated by removing tightly bound divalent cation from a cryptic site in CR3.

The effect of residue 1106 on the thioester-mediated covalent binding reaction of human complement protein C4 and the monomeric rat alpha-macroglobulin alpha 1 I3

The histidine at position 1106 of the C4B isotype of human complement is involved in catalyzing the covalent binding of the thioester to glycerol and water. By replacing the histidine with other residues, it was found that tyrosine is also capable of mediating the reaction. We propose that they act as nucleophiles by first attacking the thioester, upon activation, to form acyl intermediates, which subsequently react with the hydroxyl groups of glycerol or water. The monomeric alpha-macroglobulin, alpha 1I3 of the rat, was also studied. Unlike alpha 2-macroglobulin, which is a tetramer, alpha 1I3 has binding properties similar to those of C4A.

Molecular characterization of leukocyte adhesion deficiency in six patients

Leukocyte adhesion deficiency (LAD) is caused by defects in the CD18 gene, which codes for the common beta 2 subunit of the leukocyte integrins LFA-1, Mac-1 and p150,95. Failure to produce a functional beta 2 subunit results in the defective expression of all three leukocyte integrins, and the leukocytes of LAD patients have subnormal adhesion properties. Six patients with LAD were studied. Patient B was homozygous and carried a G284S mutation. A two-bp (GA) deletion at position 1256 (1256 delta GA) was found in the cDNA of patient C, who also had an abnormally large mRNA of 4.3 kb. Patients E and K were siblings and were heterozygous at the genomic level. One defective allele contained a mutation in intron 6/7 which created a preemptive 3' splice site. The resulting mRNA has 12 extra bases at the junction of exons 6 and 7, coding for four extra residues PSSQ in the protein. The same allele also carried a R586W mutation. The other allele was transcribed at a low level and was not characterized. Patient G carried a L149P mutation in one allele; again, the other allele was not characterized due to low transcription levels. Patient R carried two mutant alleles with G284S and R593C mutations respectively. The G284S mutation and the 1256 delta GA deletion have not been reported previously. CD18 cDNA carrying the abnormalities were cotransfected with normal CD11a or CD11b cDNA into COS cells. Expression of the LFA-1 (CD11a/CD18) and Mac-1 (CD11b/CD18) antigens on COS cells was not detected, suggesting that these two mutations are sufficient to account for LAD.

Thickness and resistivity variations over the upper surface of the human skull

A study of skull thickness and resistivity variations over the upper surface was made for an adult human skull. Physical measurements of thickness and qualitative analysis of photographs and CT scans of the skull were performed to determine internal and external features of the skull. Resistivity measurements were made using the four-electrode method and ranged from 1360 to 21400 Ohm-cm with an overall mean of 7560 +/- 4130 Ohm-cm. The presence of sutures was found to decrease resistivity substantially. The absence of cancellous bone was found to increase resistivity, particularly for samples from the temporal bone. An inverse relationship between skull thickness and resistivity was determined for trilayer bone (n = 12, p < 0.001). The results suggest that the skull cannot be considered a uniform layer and that local resistivity variations should be incorporated into realistic geometric and resistive head models to improve resolution in EEG. Influences of these variations on head models, methods for determining these variations, and incorporation into realistic head models, are discussed.

A new macrophage differentiation antigen which is a member of the scavenger receptor superfamily

The antibodies Ki-M8, Ber-Mac3, GHI/61 and SM4 define a human macrophage-associated antigen with a relative molecular mass of 130,000 which we designate M130. The protein was purified by immunoaffinity chromatography and an N-terminal and three internal amino acid sequences were obtained. A cDNA fragment was initially obtained by polymerase chain reaction (PCR) using reverse-translated primers. Several variant cDNA clones, derived from alternative spliced messages, were obtained from a lipopolysaccharide-stimulated human monocyte library and were sequenced. The relative abundance of these variants was evaluated by a series of overlapping PCR reactions. The size of the most representative cDNA is 3.7 kb and closely agrees with the mRNA size of 3.8 kb determined by Northern blot analysis. The membrane protein encoded contains a leader peptide of 40 residues, a putative extracellular domain of 1003 residues, followed by a hydrophobic segment of 24 residues and a cytoplasmic domain of 49 residues. The extracellular domain was found to contain nine repeating elements, of about 110 residues, which are similar to those of the scavenger receptor superfamily.

Improving spatial and temporal resolution in evoked EEG responses using surface Laplacians

Spline generated surface Laplacian temporal wave forms are presented as a method to improve both spatial and temporal resolution of evoked EEG responses. Middle latency and the N1 components of the auditory evoked response were used to compare potential-based methods with surface Laplacian methods in the time domain. Results indicate that surface Laplacians provide better estimates of underlying cortical activity than do potential wave forms. Spatial discrimination among electrode sites was markedly better with surface Laplacian than with potential wave forms. Differences in the number and latencies of peaks, and their topographic distributions, were observed for surface Laplacian, particularly during the time period encompassing the middle latency responses. Focal activities were observed in surface Laplacian wave forms and topographic maps which were in agreement with previous findings from auditory evoked response studies. Methodological issues surrounding the application of spline methods to the time domain are also discussed. Surface Laplacian methods in the time domain appear to provide an improved way for studying evoked EEG responses by increasing temporal and spatial resolution of component characteristics.

Covalent binding properties of the human complement protein C4 and hydrolysis rate of the internal thioester upon activation

The complement proteins C3 and C4 have an internal thioester. Upon activation on the surface of a target cell, the thioester becomes exposed and reactive to surface-bound amino and hydroxyl groups, thus allowing covalent deposition of C3 and C4 on these targets. The two human C4 isotypes, C4A and C4B, which differ by only four amino acids, have different binding specificities. C4A binds more efficiently than C4B to amino groups, and C4B is more effective than C4A in binding to hydroxyl groups. By site-directed mutagenesis, the four residues in a cDNA clone of C4B were modified. The variants were expressed and their binding properties studied. Variants with a histidine residue at position 1106 showed C4B-like binding properties, and those with aspartic acid, alanine, or asparagine at the same position were C4A-like. These results suggest that the histidine is important in catalyzing the reaction of the thioester with water and other hydroxyl group-containing compounds. When substituted with other amino acids, this reaction is not catalyzed and the thioester becomes apparently more reactive with amino groups. This interpretation also predicts that the stability of the thioester in C4A and C4B, upon activation, will be different. We measured the time course of activation and binding of glycine to C4A and C4B. The lag in the binding curve behind the activation curve for C4A is significantly greater than that for C4B. The hydrolysis rates (k0) of the thioester in the activated proteins were estimated to be 0.068 s-1 (t1/2 of 10.3 s) for C4A and 1.08 s-1 (t1/2 of 0.64 s) for C4B. These results indicate that the difference in hydrolysis rate of the thioester accounts, at least in part, for the difference in the binding properties of C4A and C4B.

High-level cryIVD and cytA gene expression in Bacillus thuringiensis does not require the 20-kilodalton protein, and the coexpressed gene products are synergistic in their toxicity to mosquitoes

Interactions among the 20-kDa protein gene and the cytA and cryIVD genes located in a 9.4-kb HindIII fragment were studied. A series of plasmids containing a combination of these different genes was constructed by using the Escherichia coli/Bacillus thuringiensis shuttle vector pHT3101. The plasmids were then used to transform an acrystalliferous strain, cryB, derived from B. thuringiensis subsp. kurstaki. The results from sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblot analyses suggest that although the 20-kDa protein is required for the efficient CytA protein production in E. coli, it is not required in B. thuringiensis. With or without the truncated 20-kDa protein gene, the CtyA and/or CryIVD proteins are produced and form parasporal inclusions in B. thuringiensis cells. However, more-efficient expression is obtained when a second protein, probably acting as a chaperonin, is present. In addition, the time course studies show that the CytA and CryIVD proteins are coordinately produced. Both the crude B. thuringiensis culture and purified inclusions from each recombinant B. thuringiensis strain are toxic to Culex quinquefasciatus larvae. The parasporal inclusions formed in B. thuringiensis cells are mosquitocidal, with CytA synergizing CryIVD toxicity.

High-resolution EEG using spline generated surface Laplacians on spherical and ellipsoidal surfaces

Spline generated surface Laplacians are introduced as an effective method for estimating neocortical source activity at moderate scales. The method appears to be robust to the unavoidable perturbations of measured potentials and errors of head geometry and resistivity that are certain to occur in clinical or research settings. In particular, we have derived the surface Laplacian for general ellipsoidal surfaces in terms of the spline function. The spline-Laplacian accurately estimates isolated dipoles or distributed sources, is insensitive to subcortical sources and to sources which originate outside the boundaries of the electrode array, and acts as a bandpass spatial filter whose characteristics appear to provide a good match to the volume conduction of intracranial sources through human heads. As a result, spatial resolution is improved over that obtained with conventional EEG by at least a factor of three. This improvement, whether obtained with spline-Laplacian or model-dependent methods, is likely to have a significant impact on both medical and cognitive studies involving EEG.

The thioester and isotypic sites of complement component C4 in sheep and cattle

The region inclusive of the thioester and the isotype-determining sites of the sheep C4 genes from a single animal was amplified by polymerase chain reaction (PCR). Two bands, at 880 base pairs (bp) and 1000 bp, were resolved by agarose gel electrophoresis. Four different clones were obtained for the 880 bp (type 1) product and two from the 1000 bp (type 2) product. Two of the type 1 clones (type 1H) and both type 2 clones (type 2H) code for the PCPVIH sequence at the isotypic site whereas the other two type 1 clones (type 1D) code for the PFPVMD sequence. By restriction mapping and Southern blot analysis, there appears to be four C4 gene loci for the sheep: two type 1H, one type 1D, and one type 2H. The type 1H and type 2H genes are likely to code for proteins with C4B-like properties whereas the type 1D genes for proteins with C4A-like properties. The same region of the sheep C4 genes of nine other breeds of sheep are also amplified by PCR and analyzed by restriction mapping and Southern hybridization. Each of the sheep has type 1H, type 2H, and type 1D genes and appears to have four C4 gene loci except for the Orkney, which may have five. A single band of 880 bp was obtained from the PCR product from the genomic DNA of a single cow. Five different clones were identified, two of which code for the PFPVMD sequence and three for the PCPVIH sequence at the isotypic site, which is consistent with previous finding that C4 proteins with A- and B-like activities could be purified from the plasma of the same animal. Comparison of the nucleotide sequences of the isotype-determining region of the sheep and cattle C4 genes with those of the primates and mouse suggests that the C4A-like genes evolved independently in the primates and the ungulates.

The gene organisation of the human beta 2 integrin subunit (CD18)

We have studied the gene of the human beta 2 integrin subunit (CD18) and found it to be organised into 16 exons spanning a region of about 40 kb. All exon/intron boundaries conform to the GT/AG splicing consensus. The exons coding for the cysteine-rich region, which has been postulated to consist of 3 or 4 repeating elements, are not organised correspondingly. Transcription of the gene initiates from multiple sites which may be due to the absence of an upstream TATA box. The polyadenylation site is also heterogeneous. Five different sites were identified over a stretch of 10 bases.

A 19-year-old man with leucocyte adhesion deficiency. In vitro and in vivo studies of leucocyte function

We describe a male patient with leucocyte adhesion molecule deficiency (LAD) of moderate phenotype. Although diagnosis was made only 2 years before his death, the patient survived until 19 years of age. This enabled us to perform a number of novel investigation, both in vivo and in vitro, relating to his leucocyte biology. Monocytes cultured in vitro matured into morphologically normal, phagocytically capable macrophages, which were able to recognize aged 'apoptotic' neutrophils. By injection of radiolabelled autologous neutrophils we demonstrated a prolonged neutrophil half-life, but normal margination, de-margination on exercise, and splenic pooling. Neutrophil adherence in vitro to vascular endothelium was normal. Histological examination of the patient's lungs at post-mortem showed intravascular aggregation of polymorphonuclear leucocytes but a paucity of cells in the interstitium and alveolar spaces. These findings indicate that the peripheral blood leucocytosis commonly observed in these patients may be due to prolonged intravascular neutrophil survival, and suggest that CD11/18 molecules have an important role in facilitating neutrophil emigration from blood vessels at sites of inflammation.

Quantitative representation of the upper surface of the human head

A three dimensional digitizer was used to locate 62 positions on an electrode cap placed on the heads of nine subjects. Nonlinear regression methods were used to obtain the best fit sphere, ellipsoid, and perturbed ellipsoid. RMS errors varied from approximately 1 cm to 2 mm, depending on subject and surface chosen. The spherical coordinates of the standard 10/20 electrodes on best fit spheres were obtained. A method to estimate ellipsoid axes using only three standard EEG head dimensions with an ordinary tape measure is derived from the data. The results have at least two potential applications: the forward/inverse problem in EEG and MEG, and the estimation of surface Laplacians from EEG data.

A visual study of surface potentials and Laplacians due to distributed neocortical sources: computer simulations and evoked potentials

A "picture book" of surface potentials, Laplacians, and magnetic fields due to distributed, neocortical sources is presented. The mathematically simulated data is based on 4200 current sources at the macrocolumn scale. Estimated scalp surface maps are based on the three-concentic spheres model of the head. Emphasis is placed on the effects of sampling with a limited number of electrodes, the choice of reference electrode, and the use of the spline Laplacian to improve spatial resolution. The spline Laplacian is applied to median and ulnar nerve somatosensory evoked potentials and to auditory evoked potentials including P300. Substantial improvement in spatial resolution over conventional methods is obtained. The implementation of practical high resolution EEG systems based on the spline Laplacian is considered.