Haptoglobin Phenotype and Intensive Glycemic Control for Coronary Artery Disease Risk Reduction in People With Type 2 Diabetes: The ADVANCE Study

OBJECTIVE

Intensive glycemic control reduced coronary artery disease (CAD) events among the Action to Control Cardiovascular Risk in Diabetes (ACCORD) study participants with the haptoglobin (Hp)2-2 phenotype but not in participants without the Hp2-2 phenotype. It is unknown whether and how these results translate across different demographic/clinical characteristics and treatment strategies.

RESEARCH DESIGN AND METHODS

Haptoglobin phenotype was measured in available samples from the Action in Diabetes and Vascular Disease: Preterax and Diamicron MR Controlled Evaluation (ADVANCE) biomarker case-cohort study. Weighted multivariable-adjusted Cox regression models were used to evaluate the association between intensive glycemic control (HbA1c target of ≤6.5%) versus standard therapy (based on local guidelines) and major CAD events among participants with (n = 1,327) and without (n = 2,077) the Hp2-2 phenotype separately and within prespecified stratifications by sex, race, previous cardiovascular disease (CVD), diabetes duration, and HDL-cholesterol.

RESULTS

While the hazard ratios (HRs) were in the hypothesized differing directions, compared with standard therapy, intensive glycemic control was not significantly associated with risk of CAD events among participants without (1.04, 95% CI 0.82-1.32) or with (0.84, 0.63-1.14, Pinteraction = 0.27) the Hp2-2 phenotype overall. Intensive therapy was associated with lower CAD risk among participants with the Hp2-2 phenotype who had no previous CVD (0.47, 0.29-0.76, Pinteraction = 0.01).

CONCLUSIONS

Our findings suggest that intensive glycemic control contributes to the prevention of major CAD events among ADVANCE participants with the Hp2-2 phenotype and no previous CVD and are in alignment with our hypothesis that intensive glycemic control may be beneficial in a subset of people with the Hp2-2 phenotype.

Intensive lifestyle intervention in type 2 diabetes and risk of incident coronary artery disease for the common haptoglobin phenotypes: the Look AHEAD study

BACKGROUND

Intensive glycemic control reduced coronary artery disease (CAD) events among the Action to Control Cardiovascular Disease Risk in Diabetes (ACCORD) participants with the haptoglobin (Hp) 2-2 phenotype only. It remains unknown whether Hp phenotype modifies the effect of an intensive lifestyle intervention (ILI) on CAD in type 2 diabetes.

METHODS

Haptoglobin phenotype was measured in 4542 samples from the Action for Health in Diabetes (Look AHEAD) study. Cox regression models assessed the effect of ILI (focused on weight loss from caloric restriction and physical activity) versus diabetes support and education (DSE) on CAD events in each phenotype group, and within pre-specified subgroups including race/ethnicity, sex, history of cardiovascular disease, diabetes medication use, and diabetes duration.

RESULTS

1590 (35%) participants had the Hp2-2 phenotype. The ILI did not lower glycated hemoglobin (%HbA1c) to < 6.5% in either phenotype, with a peak significant difference between treatment arms of 0.5% [non-Hp2-2] and 0.6% [Hp2-2]. The cumulative CAD incidence was 13.4% and 13.8% in the DSE arm and 12.2% and 13.6% in the ILI arm for non-Hp2-2 and Hp2-2 groups, respectively. Compared to DSE, the ILI was not associated with CAD among participants without (HR = 0.95, 95% CI 0.78-1.17) or with (0.89, 0.68-1.19) the Hp2-2 phenotype (p-interaction between Hp phenotype and ILI = 0.58). After Bonferroni correction, there were no significant results among any subgroups.

CONCLUSIONS

Hp phenotype did not modify the effect of the weight loss ILI on risk of CAD in Look AHEAD, potentially because it did not substantially impact glycemic control among participants with or without the Hp2-2 phenotype. Further research is needed to determine if these results are conclusive.

The Relationship Between Time-Varying Achieved HbA1c and Risk of Coronary Events Depends on Haptoglobin Phenotype Among White and Black ACCORD Participants

OBJECTIVE

Intensive glycemic therapy reduced coronary artery disease (CAD) events among White participants in the Action to Control Cardiovascular Risk in Diabetes (ACCORD) study with the haptoglobin (Hp)2-2 phenotype, while participants without the Hp2-2 phenotype had no CAD benefit. The association between achieved glycated hemoglobin (HbA1c) and CAD for each Hp phenotype remains unknown.

RESEARCH DESIGN AND METHODS

Achieved HbA1c was similar in each phenotype throughout the study. Prospectively collected HbA1c data (categorized as <6.0%, 6.0-6.5%, 6.6-6.9%, or ≥8.0% compared with 7.0-7.9%) from the ACCORD study, updated every 4 months over a median of 4.7 years, were analyzed in relation to CAD in the Hp2-2 (n = 3,322) and non-Hp2-2 (n = 5,949) phenotypes separately overall, and within White (63%, 37% Hp2-2) and Black (19%, 26% Hp2-2) participants using Cox proportional hazards regression with time-varying covariables.

RESULTS

Compared with HbA1c of 7.0-7.9%, having HbA1c ≥8.0% was associated with CAD risk among White (adjusted HR [aHR] 1.43, 95% CI 1.03-1.98) and Black (2.86, 1.09-7.51) participants with the Hp2-2 phenotype, but not when all Hp2-2 participants were combined overall (1.30, 0.99-1.70), and not among participants without the Hp2-2 phenotype. HbA1c <7.0% was not associated with a lower risk of CAD for any Hp phenotype.

CONCLUSIONS

Achieving HbA1c >8.0% compared with 7.0-7.9% was consistently associated with incident CAD risk among White and Black ACCORD participants with the Hp2-2 phenotype, while no association was observed among participants without the Hp2-2 phenotype. We found no evidence that HbA1c concentration <7.0% prevents CAD in either Hp phenotype group.

Relationship Between Time-Varying Achieved High-Density Lipoprotein Cholesterol and Risk of Coronary Events Depends on Haptoglobin Phenotype Within the ACCORD Lipid Study

Background The Hp (haptoglobin)2-2 phenotype (~40% of people) is associated with dysfunctional high-density lipoprotein (HDL) that is heavily oxidized in hyperglycemia, which may explain why raising HDL-cholesterol (HDL-C) does not reliably prevent coronary artery disease (CAD) in diabetes. Methods and Results In this observational study using longitudinal data from the ACCORD (Action to Control Cardiovascular Risk in Diabetes) lipid trial, time-varying (achieved) HDL-C updated at 4, 8, and 12 months, and annually thereafter over a mean of 4.7 years, was analyzed in relation to risk of CAD and secondary outcomes using Cox proportional hazards regression with time-varying covariables among participants with (n=1781) and without (n=3191) the Hp2-2 phenotype. HDL-C did not differ between the phenotypes throughout the study. Having low HDL-C (<40 mg/dL for male participants and <50 mg/dL for female participants) was associated with a greater risk of CAD compared with non-low HDL-C among participants with the non-Hp2-2 phenotype (hazard ratio [HR], 1.48 [95% CI, 1.18-1.87]) but not among the Hp2-2 phenotype (HR, 0.97 [95% CI, 0.70-1.35]; P interaction=0.03). Similarly, an inverse relationship was observed between HDL-C quintiles and CAD risk among participants without the Hp2-2 phenotype, whereas no significant inverse relationship was observed among participants with the Hp2-2 phenotype (P interaction=0.38). Among the Hp2-2 phenotype group, having low HDL-C was associated with higher risk of CVD mortality (HR, 2.09 [95% CI, 1.05-4.13]), and compared with the lowest HDL-C quintile, higher quintiles were associated with lower risk of CVD mortality and congestive heart failure. Conclusions Hp phenotype modified the association between HDL-C and risk of CAD in the ACCORD lipid study, suggesting that HDL dysfunction in the Hp2-2 phenotype may hinder CAD-protective properties of HDL-C.

Incidence and Progression of Fibrotic Lung Disease in an At-Risk Cohort

Rationale: Relatives of patients with familial interstitial pneumonia (FIP) are at increased risk for pulmonary fibrosis and develop preclinical pulmonary fibrosis (PrePF). Objectives: We defined the incidence and progression of new-onset PrePF and its relationship to survival among first-degree relatives of families with FIP. Methods: This is a cohort study of family members with FIP who were initially screened with a health questionnaire and chest high-resolution computed tomography (HRCT) scan, and approximately 4 years later, the evaluation was repeated. A total of 493 asymptomatic first-degree relatives of patients with FIP were evaluated at baseline, and 296 (60%) of the original subjects participated in the subsequent evaluation. Measurements and Main Results: The median interval between HRCTs was 3.9 years (interquartile range, 3.5-4.4 yr). A total of 252 subjects who agreed to repeat evaluation were originally determined not to have PrePF at baseline; 16 developed PrePF. A conservative estimate of the annual incidence of PrePF is 1,023 per 100,000 person-years (95% confidence interval, 511-1,831 per 100,000 person-years). Of 44 subjects with PrePF at baseline, 38.4% subjects had worsening dyspnea compared with 15.4% of those without PrePF (P = 0.002). Usual interstitial pneumonia by HRCT (P < 0.0002) and baseline quantitative fibrosis score (P < 0.001) are also associated with worsening dyspnea. PrePF at the initial screen is associated with decreased survival (P < 0.001). Conclusions: The incidence of PrePF in this at-risk population is at least 100-fold higher than that reported for sporadic idiopathic pulmonary fibrosis (IPF). Although PrePF and IPF represent distinct entities, our study demonstrates that PrePF, like IPF, is progressive and associated with decreased survival.

Haptoglobin Phenotype Modifies the Effect of Fenofibrate on Risk of Coronary Event: ACCORD Lipid Trial

OBJECTIVE

The haptoglobin (Hp)2-2 phenotype (∼35-40% of people) is associated with increased oxidation and dysfunctional HDL in hyperglycemia and may explain why drugs designed to pharmacologically raise HDL cholesterol and lower triglycerides have not reliably prevented cardiovascular disease in diabetes. We aimed to determine whether the effect of adding fenofibrate versus placebo to simvastatin on the risk of coronary artery disease (CAD) events depends on Hp phenotype in the Action to Control Cardiovascular Risk in Diabetes (ACCORD) lipid trial.

RESEARCH DESIGN AND METHODS

Cox proportional hazards regression models quantified the relationship between fenofibrate therapy and CAD events in the ACCORD lipid trial in participants with the Hp2-2 phenotype (n = 1,795) separately from those without (n = 3,201).

RESULTS

Fenofibrate therapy successfully lowered the risk of CAD events in participants without the Hp2-2 phenotype (multivariable adjusted hazard ratio 0.74 [95% CI 0.60-0.90] compared with no fenofibrate therapy) but not in participants with the Hp2-2 phenotype (1.16 [0.87-1.56]; P interaction = 0.009). Subgroup analyses revealed that this protective effect of fenofibrate against CAD events among the non-Hp2-2 phenotype group was pronounced in participants with severe dyslipidemia (P interaction = 0.01) and in males (P interaction = 0.02) with an increased CAD risk from fenofibrate treatment observed in females with the Hp2-2 phenotype (P interaction = 0.002).

CONCLUSIONS

The effect of fenofibrate added to simvastatin on risk of CAD events depends on Hp phenotype in the ACCORD lipid trial.

Quantification of Trichoderma afroharzianum, Trichoderma harzianum and Trichoderma gamsii inoculants in soil, the wheat rhizosphere and in planta suppression of the crown rot pathogen Fusarium pseudograminearum

AIMS

Develop quantitative assays (qPCR) to determine the detection threshold limits, colonization and persistence of Trichoderma gamsii, Trichoderma afroharzianum and T. harzianum inoculants in cropping soils, the wheat rhizosphere and their in planta suppressive efficacy against the crown rot pathogen Fusarium pseudograminearum.

METHODS AND RESULTS

Trichoderma qPCR primers were designed from the internal transcribed spacer region of 5.8S rDNA and from sequences of DNA fragments diagnostic for each inoculant genotype. The minimum detection thresholds of qPCR assays varied between 1 × 10³ (log 3) and 8 × 10⁴ (log 4·9) conidia (genome) equivalents per gram of soil for multi- and single-copy target sequences, respectively and were independent of soil type. There was a strong correlation (r > 0·974) between culture-dependent and culture-independent (qPCR) quantification methods. In wheat bioassays, Trichoderma inoculants colonized rhizosphere soils and wheat roots at 56-112 days postemergence to a depth of 20 cm but were more abundant (P < 0·001) at 0-10 cm root depth, means ranging from 2 × 10² (log 2·3) to 4 × 10⁵ (log 5·6) conidia equivalents per gram of rhizosphere soil or root tissue. Inoculants reduced (P < 0·001) F. pseudograminearum biomass in wheat crown and root tissue by up to 5754-fold and increased (P = 0·008) plant biomass, relative to the pathogen control.

CONCLUSIONS

The qPCR assays provided sensitive and accurate assessment of wheat root and rhizosphere soil colonization of Trichoderma inoculants. Strains persisted through to grain maturity at levels shown to significantly suppress F. pseudograminearum in planta.

SIGNIFICANCE AND IMPACT OF THE STUDY

The qPCR assays developed here were used to determine the wheat rhizosphere dynamics of T. harzianum, T. afroharzianum and T. gamsii inoculants and their suppressive efficacy against F. pseudograminearum in planta. These assays can be applied to monitor inoculant dynamics in suppressing crown rot and other wheat root diseases in the field.

Mallet Finger: A Trial of Two Splints

Over a period of twelve months, 116 cases of mallet finger were allocated randomly to treatment with either a Stack or Abouna splint. The two splints were equally effective, producing a cure or a significant improvement in approximately 50% of cases. However, the Stack splint was much preferred by the patients, who found it more comfortable, more robust and easier to keep clean.

Expression in Escherichia coli of the Cellulomonas fimi Structural Gene for Endoglucanase B

Endoglucanase B (EB) of Cellulomonas fimi has an M(r) of 110,000 when it is produced in Escherichia coli. The level of expression of the cenB gene (encoding EB) was significantly increased by replacing its normal transcriptional and translational regulatory signals with those of the E. coli lac operon. EB was purified to homogeneity from the periplasmic fraction of E. coli in one step by affinity chromatography on microcrystalline cellulose (Avicel). Alignment of the NH(2)-terminal amino acid sequence with the partial nucleotide sequence of a fragment of C. fimi DNA showed that EB is preceded by a putative signal polypeptide of 33 amino acids. The signal peptide functions and is processed correctly in E. coli, even when its first 15 amino acids are replaced by the first 7 amino acids of beta-galactosidase. The intact EB polypeptide is not required for enzymatic activity. Active polypeptides with M(r)s of 95,000 and 82,000 also appear in E. coli, and a deletion mutant of cenB encodes an active polypeptide with an M(r) of 72,000.

The cellulose-binding domain (CBD(Cex)) of an exoglucanase from Cellulomonas fimi: production in Escherichia coli and characterization of the polypeptide

The gene fragment encoding the cellulose-binding domain (CBD) of an exoglucanase (Cex) from Cellulomonas fimi was subcloned and expressed in Escherichia coli. Transcription from the lac promoter coupled with translation from a consensus prokaryotic ribosome binding site led to the production of large quantities of CBD(Cex) (up to 25% total soluble cell protein). The polypeptide leaked into the culture supernatant (up to 50 mg . L(-1)), facilitating one-step purification by affinity chromatography on cellulose. The 11-kDa polypeptide reacted with Cex antiserum. Absence of free thiols indicated that the two Cys residues of CBD(Cex) form a disulfide bridge. It had the same N-terminal amino acid sequence as CBD(Cex) prepared from Cex by proteolysis, plus two additional N-terminal amino acid residues (Ala and Ser) encoded by the Nhel site introduced during plasmid construction. CBD(Cex) bound to a variety of beta-1, 4-glycans with different affinities and saturation levels. Adsorption to bacterial microcrystalline cellulose was dependent on the temperature, but not on the pH.

Population Analysis of Single Neurons in Cat Somatosensory Cortex

Single neurons in the somatosensory cortex are divisible into a population with receptive fields and a population without receptive fields. These two populations display different laminar distributions, and their respective functions are unknown. We compared other physiological characteristics of these two neuronal populations in an attempt to understand why some neurons lack a receptive field. Only 23% of 465 neurons isolated in the somatosensory cortex of halothane-anesthetized cats displayed a cutaneous receptive field. The iontophoretic administration of glutamate uncovered input from the periphery in another 34% of the sample, leaving 43% of the neurons without evidence of peripheral input under these experimental conditions. Neurons with a receptive field were spontaneously active much more often than neurons lacking peripheral inputs, and their rates of discharge were higher. No differences were found between neurons having a receptive field uncovered with glutamate and those unaffected by glutamate. In all classes of neurons, those cells with spontaneous activity were excited by smaller amounts of glutamate than were silent neurons, but sensitivity to glutamate was not correlated with the presence or absence of a receptive field. We infer that some classes of somatosensory cortical neurons receive strong thalamocortical inputs, whereas others have only relatively weak or no thalamocortical connections. In other experiments we have shown also that those neurons lacking a receptive field and/or spontaneous activity were more likely to be plastic than those with stronger inputs (see Warren and Dykes, 1993a,b), suggesting that neurons having weaker afferent inputs can be more readily modified under certain circumstances.

Postnatal development of excitatory postsynaptic currents in nucleus accumbens medium spiny neurons

We have recorded excitatory postsynaptic currents (EPSCs) evoked by local electrical stimulation in 243 nucleus accumbens (nAcb) neurons in vitro during postnatal development from the day of birth (postnatal day 0; P0) to P27 and in young adults rats (P59-P71). An EPSC sensitive to glutamatergic antagonists was found in all neurons. In the majority of cases (189/243), the EPSC had two distinct components: an early one sensitive to 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and a late one that was sensitive to D-2-amino-5-phosphonovaleric acid (APV) showing that early and late components of the EPSC were mediated by AMPA/kainate (KA) and N-methyl-D-aspartate (NMDA) receptors respectively. During the first four postnatal days, the amplitudes of both the AMPA/KA and NMDA components of the EPSC were relatively small and then began to increase until the end of the second postnatal week. Whereas the amplitude of the early component appeared to stabilize from that point on, the late component began to decrease and became virtually undetectable in preparations from animals older than 3 weeks unless the AMPA/KA response was blocked with CNQX. In addition, the ratio between the amplitude of the NMDA and AMPA/KA receptor-mediated components of the EPSC followed a developmental pattern parallel to that of the NMDA receptor component showing an increase during the first two postnatal weeks followed by a decrease. Together, these results show that, during postnatal development, there is a period when NMDA receptor-mediated EPSC are preeminent and that time frame might represent a period during which the development of the nAcb might be sensitive to environmental manipulation.

Do cellulose binding domains increase substrate accessibility?

This article provides an overview of various theories proposed during the past five decades to describe the enzymatic hydrolysis of cellulose highlighting the major shifts that these theories have undergone. It also describes the effect of the cellulose-binding domain (CBD) of an exoglucanase/xylanase from bacterium Cellulomonas fimi on the enzymatic hydrolysis of Avicel. Pretreatment of Avicel with CBDCex at 4 and 37 degrees C as well as simultaneous addition of CBDCex to the hydrolytic enzyme (Celluclast, Novo, Nordisk) reduced the initial rate of hydrolysis owing to irreversible binding of CBD proteins to the substrate's binding sites. Nonetheless, near complete hydrolysis was achieved even in the presence of CBDCex. Protease treatment of both pure and CBDCex-treated Avicel reduced the substrates' hydrolyzability, perhaps owing to proteolysis of the hydrolyzing enzyme (Celluclast) by the residual Proteinase K remaining in the substrate. Better protocols for complete removal of CBD proteins from the substrate need to be developed to investigate the effect of CBD adsorption on cellulose digestibility.

Mechanistic consequences of mutation of the active site nucleophile Glu 358 in Agrobacterium .beta.-glucosidase

The replacement of the active site nucleophile Glu 358 in Agrobacterium beta-glucosidase by Asn and Gln by site-directed mutagenesis results in essentially complete inactivation of the enzyme, while replacement by Asp generates a mutant with a rate constant for the first step, formation of the glycosylenzyme, some 2500 times lower than that of the native enzyme. This low activity is shown to be a true property of the mutant and not due to contaminating wild-type enzyme by active site titration studies and also through studies of its thermal denaturation and of the pH dependence of the reaction catalyzed. Binding of ground-state inhibitors is affected relatively little by the mutation, while binding of transition-state analogues is greatly impaired, consistent with a principal role for Glu 358 being in transition-state stabilization, not substrate binding. Determination of kinetic parameters for a series of aryl glucosides revealed that the glycosylation step is rate determining for all these substrates in contrast to the native enzyme, where a switch from rate-limiting glycosylation to rate-limiting deglycosylation was observed as substrate reactivity was increased. These results coupled with secondary deuterium kinetic isotope effects of kH/kD = 1.17 and 1.12 measured for the 2,4-dinitrophenyl and p-nitrophenyl glucosides point to a principal role of the nucleophile in stabilizing the cationic transition states and in formation of the covalent intermediate.(ABSTRACT TRUNCATED AT 250 WORDS)

beta-1,3-Glucan binding by a thermostable carbohydrate-binding module from Thermotoga maritima

The C-terminal 155 amino acids of the putative laminarinase, Lam16A, from T. maritima comprise a highly thermostable family 4 CBM that binds beta-1,3- and beta-(1,3)(1,4)-glucans. Laminarin, a beta-1,3-glucan, presented two classes of binding sites for TmCBM4-2, one with a very high affinity (3.5 x 10(7) M(-1)) and one with a 100-fold lower affinity (2.4 x 10(5) M(-1)). The affinities for laminarioligosaccharides and beta-(1,3)(1,4)-glucans ranged from approximately 2 x 10(5) to approximately 2.5 x 10(6) M(-1). Cellooligosaccharides and laminariobiose were bound only very weakly (K(a)s approximately 5 x 10(3) M(-1)). Spectroscopic and mutagenic studies implicated the involvement of three tryptophan residues (W28, W58, and W99) and one tyrosine residue (Y23) in ligand binding. Binding was enthalpically driven and associated with large negative changes in heat capacity. Temperature and osmotic conditions profoundly influenced binding. For the first time in solution, the direct uptake and release of water in CBM binding are demonstrated.

Experimental gonococcal urethritis and reinfection with homologous gonococci in male volunteers

BACKGROUND

Reinfection, a common occurrence with gonorrhea, may result from a lack of protective immune response, or from the tremendous gonococcal strain variation.

GOAL

A two-phase study in human volunteers tested whether experimental infection with Neisseria gonorrhoeae MS11mkC would protect against reinfection with the same organisms.

STUDY DESIGN

In phase 1, an intraurethral inoculum of 57,000 piliated, transparent (opacity protein-negative [Opa-]) MS11mkC N gonorrhoeae infected 14 of 15 (93%) volunteers. The volunteers were encouraged to delay treatment for at least 5 days. In phase 2, which began 2 weeks after treatment for the initial infection, volunteers were inoculated with 7,100 piliated, Opa- MS11mkC.

RESULTS

The phase 2 challenge infected 6 of 14 (43%) previously infected volunteers and 5 of 10 (50%) naïve control subjects. Phase 1 volunteers who resisted reinfection were significantly more likely to have had a fourfold or greater increase in lipooligosaccharide immunoglobulin G during phase 1 than those who did not resist reinfection (P = 0.026).

CONCLUSIONS

Although infection did not provide protection from reinfection under the conditions used, the results suggest that immunity to reinfection is more complex than anticipated by the experimental design.

The actin-binding protein Hip1R associates with clathrin during early stages of endocytosis and promotes clathrin assembly in vitro

Huntingtin-interacting protein 1 related (Hip1R) is a novel component of clathrin-coated pits and vesicles and is a mammalian homologue of Sla2p, an actin-binding protein important for both actin organization and endocytosis in yeast. Here, we demonstrate that Hip1R binds via its putative central coiled-coil domain to clathrin, and provide evidence that Hip1R and clathrin are associated in vivo at sites of endocytosis. First, real-time analysis of Hip1R-YFP and DsRed-clathrin light chain (LC) in live cells revealed that these proteins show almost identical temporal and spatial regulation at the cell cortex. Second, at the ultrastructure level, immunogold labeling of 'unroofed' cells showed that Hip1R localizes to clathrin-coated pits. Third, overexpression of Hip1R affected the subcellular distribution of clathrin LC. Consistent with a functional role for Hip1R in endocytosis, we also demonstrated that it promotes clathrin cage assembly in vitro. Finally, we showed that Hip1R is a rod-shaped apparent dimer with globular heads at either end, and that it can assemble clathrin-coated vesicles and F-actin into higher order structures. In total, Hip1R's properties suggest an early endocytic function at the interface between clathrin, F-actin, and lipids.

Glycosylation by Pichia pastoris decreases the affinity of a family 2a carbohydrate-binding module from Cellulomonas fimi: a functional and mutational analysis

When produced by Pichia pastoris, three of the five Asn-Xaa-Ser/Thr sequences (corresponding to Asn-24, Asn-73 and Asn-87) in the carbohydrate-binding module CBM2a of xylanase 10A from Cellulomonas fimi are glycosylated. The glycans are of the high-mannose type, ranging in size from GlcNAc(2)Man(8) to GlcNAc(2)Man(14). The N-linked glycans block the binding of CBM2a to cellulose. Analysis of mutants of CBM2a shows that glycans on Asn-24 decrease the association constant (K(a)) for the binding of CBM2a to bacterial microcrystalline cellulose approx. 10-fold, whereas glycans on Asn-87 destroy binding. The K(a) of a mutant of CBM2a lacking all three N-linked glycosylation sites is the same when the polypeptide is produced by either Escherichia coli or P. pastoris and is approx. half that of wild-type CBM2a produced by E. coli.

Binding specificity and thermodynamics of a family 9 carbohydrate-binding module from Thermotoga maritima xylanase 10A

The C-terminal family 9 carbohydrate-binding module of xylanase 10A from Thermotoga maritima (CBM9-2) binds to amorphous cellulose, crystalline cellulose, and the insoluble fraction of oat spelt xylan. The association constants (K(a)) for adsorption to insoluble polysaccharides are 1 x 10(5) to 3 x 10(5) M(-1). Of the soluble polysaccharides tested, CBM9-2 binds to barley beta-glucan, xyloglucan, and xylan. CBM9-2 binds specifically to the reducing ends of cellulose and soluble polysaccharides, a property that is currently unique to this CBM. CBM9-2 also binds glucose, xylose, galactose, arabinose, cellooligosaccharides, xylooligosaccharides, maltose, and lactose, with affinities ranging from 10(3) M(-1) for monosaccharides to 10(6) M(-1) for disaccharides and oligosaccharides. Cellooligosaccharides longer than two glucose units do not bind with improved affinity, indicating that cellobiose is sufficient to occupy the entire binding site. In general, the binding reaction is dominated by favorable changes in enthalpy, which are partially compensated by unfavorable entropy changes.

The identification of the catalytic nucleophiles of two beta-galactosidases from glycoside hydrolase family 35

The beta-galactosidases from Xanthomonas manihotis (beta-Gal Xmn) and Bacillus circulans (beta-Gal-3 Bcir) are retaining glycosidases that hydrolyze glycosidic bonds through a double displacement mechanism involving a covalent glycosyl-enzyme intermediate. The mechanism-based inactivator 2,4-dinitrophenyl 2-deoxy-2-fluoro-beta-D-galactopyranoside was shown to inactivate beta-Gal Xmn and beta-Gal-3 Bcir through the accumulation of 2-deoxy-2-fluorogalactosyl enzyme intermediates with half lives of 40 and 625 h, respectively. Peptic digestion of these labeled enzymes and analysis by LC-MS identified Glu(260) and Glu(233) as the catalytic nucleophiles involved in the formation of the glycosyl-enzyme intermediate during catalysis by beta-Gal Xmn and beta-Gal-3 Bcir, respectively. These findings confirm the previous prediction of the position of these residues based on primary sequence similarities to other members of the glycoside hydrolase family 35.

Directed evolution of new glycosynthases from Agrobacterium β-glucosidase: a general screen to detect enzymes for oligosaccharide synthesis

BACKGROUND

Oligosaccharide synthesis is becoming increasingly important to industry as diverse therapeutic roles for these molecules are discovered. The chemical synthesis of oligosaccharides on an industrial scale is often prohibitively complex and costly. An alternative, that of enzymatic synthesis, is limited by the difficulty of obtaining an appropriate enzyme. A general screen for enzymes that catalyze the synthesis of the glycosidic bond would enable the identification and engineering of new or improved enzymes.

RESULTS

Glycosynthases are nucleophile mutants of retaining glycosidases that efficiently catalyze the synthesis of the glycosidic linkage by condensing an activated glycosyl fluoride donor with a suitable acceptor sugar. A novel agar plate-based coupled-enzyme screen was developed (using a two-plasmid system) and used to select an improved glycosynthase from a library of mutants.

CONCLUSIONS

Plate-based coupled-enzyme screens of this type are extremely valuable for identification of functional synthetic enzymes and can be applied to the evolution of a range of glycosyl transferases.

Identification of Glu-519 as the catalytic nucleophile in beta-mannosidase 2A from Cellulomonas fimi

Incubation of the beta-mannosidase Man2A from Cellulomonas fimi with 2-deoxy-2-fluoro-beta-D-mannosyl fluoride (2FMan beta F) resulted in time-dependent inactivation of the enzyme (inactivation rate constant k(i)=0.57 min(-1), dissociation constant for the inactivator K(i)=0.41 mM) through the accumulation of a covalent 2-deoxy-2-fluoro-alpha-D-mannosyl-beta-mannosidase 2A (2FMan-Man2A) enzyme intermediate, as observed by electrospray ionization mass spectrometry. The stoichiometry of inactivation was 1:1. Removal of excess inactivator and regeneration of active enzyme by transglycosylation of the covalently attached inhibitor to gentiobiose [Glc beta(1-6)Glc] demonstrated that the covalent intermediate was catalytically competent. Comparison by MS of the peptic digests of 2FMan-Man2A with peptic digests of native Man2A revealed a peptide of m/z 1520 that was unique to 2FMan-Man2A, and one of m/z 1036.5 that was unique to a Man2A peptide. Their sequences, determined by collision-induced fragmentation, were CSEFGFQGPPTW and FGFQGPPTW, corresponding to residues 517-528 and 520-528 of Man2A respectively. The difference in mass of 483.5 between the two peptides equals the sum of the masses of the tripeptide CSE plus that of 2-fluoromannose. It was concluded that in 2FMan-Man2A, the 2-fluoromannose esterified to Glu-519 blocks hydrolysis of the Glu-519-Phe-520 peptide bond, and that Glu-519 is the catalytic nucleophile in this enzyme. This residue is conserved in all members of family 2 of the glycosyl hydrolases. This represents the first ever labelling and identification of an active-site nucleophile in a beta-mannosidase.

Factor X fusion proteins: improved production and use in the release in vitro of biologically active hirudin from an inactive alpha-factor-hirudin fusion protein

Many recombinant proteins are synthesized as fusion proteins containing affinity tags to aid in the downstream processing. After purification, the affinity tag is often removed by using a site-specific protease such as factor Xa (FXa). However, the use of FXa is limited by its expense and availability from plasma. To develop a recombinant source of FXa, we have expressed two novel forms of FXa using baby hamster kidney (BHK) cells as host and the expression vector pNUT. The chimeric protein FIIFX consisted of the prepropeptide and the Gla domain of prothrombin linked to the activation peptide and protease region of FXa, together with a cellulose-binding domain (CBD(Cex)) as an affinity tag. A second variant consisted of the transferrin signal peptide linked to the second epidermal growth factor-like domain and the catalytic domain of FX and a polyhistidine tag. Both FX variants were secreted into the medium, their affinity tags were functional, and following activation, both retained FXa-specific proteolytic activity. However, the yield of the FIIFX-CBD(Cex) fusion protein was 10-fold higher than that of FX-CBD(Cex) and other forms of recombinant FX reported to date. The FXa derivatives were used to cleave two different fusion proteins, including a biologically inactive alpha-factor-hirudin fusion protein secreted by Saccharomyces cerevisiae. After cleavage, the released hirudin demonstrated biological activity in a thrombin inhibition assay, suggesting that this method may be applicable to the production of toxic or unstable proteins. The availability of novel FX derivatives linked to different affinity tags allows the development of a versatile system for processing fusion proteins in vitro.

Postnatal development of electrophysiological properties of nucleus accumbens neurons

We have studied the postnatal development of the physiological characteristics of nucleus accumbens (nAcb) neurons in slices from postnatal day 1 (P1) to P49 rats using the whole cell patch-clamp technique. The majority of neurons (102/108) were physiologically identified as medium spiny (MS) projection neurons, and only these were subjected to detailed analysis. The remaining neurons displayed characteristics suggesting that they were not MS neurons. Around the time of birth and during the first postnatal weeks, the membrane and firing characteristics of MS neurons were quite different from those observed later. These characteristics changed rapidly during the first 3 postnatal weeks, at which point they began to resemble those found in adults. Both whole cell membrane resistance and membrane time constant decreased more than fourfold during the period studied. The resting membrane potential (RMP) also changed significantly from an average of -50 mV around birth to less than -80 mV by the end of the third postnatal week. During the first postnatal week, the current-voltage relationship of all encountered MS neurons was linear over a wide range of membrane potentials above and below RMP. Through the second postnatal week, the proportion of neurons displaying inward rectification in the hyperpolarized range increased steadily and after P15, all recorded MS neurons displayed significant inward rectification. At all ages, inward rectification was blocked by extracellular cesium and tetra-ethyl ammonium and was not changed by 4-aminopyridine; this shows that inward rectification was mediated by the same currents in young and mature MS neurons. MS neurons fired single and repetitive Na(+)/K(+) action potentials as early as P1. Spike threshold and amplitude remained constant throughout development in contrast to spike duration, which decreased significantly over the same period. Depolarizing current pulses from rest showed that immature MS neurons fired action potentials more easily than their older counterparts. Taken together, the results from the present study suggest that young and adult nAcb MS neurons integrate excitatory synaptic inputs differently because of differences in their membrane and firing properties. These findings provide important insights into signal processing within nAcb during this critical period of development.

Specificity and affinity of substrate binding by a family 17 carbohydrate-binding module from Clostridium cellulovorans cellulase 5A

The C-terminal carbohydrate-binding module (CBM17) from Clostridium cellulovorans cellulase 5A is a beta-1,4-glucan binding module with a preference for soluble chains. CBM17 binds to phosphoric acid swollen Avicel (PASA) and Avicel with association constants of 2.9 (+/-0.2) x 10(5) and 1.6 (+/-0.2) x 10(5) M(-1), respectively. The capacity values for PASA and Avicel were 11.9 and 0.4 micromol/g of cellulose, respectively. CBM17 did not bind to crystalline cellulose. CBM17 bound tightly to soluble barley beta-glucan and the derivatized celluloses HEC, EHEC, and CMC. The association constants for binding to barley beta-glucan, HEC, and EHEC were approximately 2.0 x 10(5) M(-1). Significant binding affinities were found for cello-oligosaccharides greater than three glucose units in length. The affinities for cellotriose, cellotetraose, cellopentaose, and cellohexaose were 1.2 (+/-0.3) x 10(3), 4.3 (+/-0.4) x 10(3), 3.8 (+/-0.1) x 10(4), and 1.5 (+/-0.0) x 10(5) M(-1), respectively. Fluorescence quenching studies and N-bromosuccinimide modification indicate the participation of tryptophan residues in ligand binding. The possible architecture of the ligand-binding site is discussed in terms of its binding specificity, affinity, and the participation of tryptophan residues.

Substrate specificity in glycoside hydrolase family 10. Tyrosine 87 and leucine 314 play a pivotal role in discriminating between glucose and xylose binding in the proximal active site of Pseudomonas cellulosa xylanase 10A

The Pseudomonas family 10 xylanase, Xyl10A, hydrolyzes beta1, 4-linked xylans but exhibits very low activity against aryl-beta-cellobiosides. The family 10 enzyme, Cex, from Cellulomonas fimi, hydrolyzes aryl-beta-cellobiosides more efficiently than does Xyl10A, and the movements of two residues in the -1 and -2 subsites are implicated in this relaxed substrate specificity (Notenboom, V., Birsan, C., Warren, R. A. J., Withers, S. G., and Rose, D. R. (1998) Biochemistry 37, 4751-4758). The three-dimensional structure of Xyl10A suggests that Tyr-87 reduces the affinity of the enzyme for glucose-derived substrates by steric hindrance with the C6-OH in the -2 subsite of the enzyme. Furthermore, Leu-314 impedes the movement of Trp-313 that is necessary to accommodate glucose-derived substrates in the -1 subsite. We have evaluated the catalytic activities of the mutants Y87A, Y87F, L314A, L314A/Y87F, and W313A of Xyl10A. Mutations to Tyr-87 increased and decreased the catalytic efficiency against 4-nitrophenyl-beta-cellobioside and 4-nitrophenyl-beta-xylobioside, respectively. The L314A mutation caused a 200-fold decrease in 4-nitrophenyl-beta-xylobioside activity but did not significantly reduce 4-nitrophenyl-beta-cellobioside hydrolysis. The mutation L314A/Y87A gave a 6500-fold improvement in the hydrolysis of glucose-derived substrates compared with xylose-derived equivalents. These data show that substantial improvements in the ability of Xyl10A to accommodate the C6-OH of glucose-derived substrates are achieved when steric hindrance is removed.

Mannanase Man26A from Cellulomonas fimi has a mannan-binding module

A modular mannanase (Man26A) from the bacterium Cellulomonas fimi contains a mannan-binding module (Man26Abm) that binds to soluble but not to insoluble mannans. Man26Abm does not bind to cellulose, chitin or xylan. The K(d) for binding of Man26Abm to locust bean gum (LBG) is approximately 0.2 microM. Man26A is the first mannanase reported to contain a mannan-binding module.

Mannanase Man26A from Cellulomonas fimi has a mannan-binding module

A modular mannanase (Man26A) from the bacterium Cellulomonas fimi contains a mannan-binding module (Man26Abm) that binds to soluble but not to insoluble mannans. Man26Abm does not bind to cellulose, chitin or xylan. The K(d) for binding of Man26Abm to locust bean gum (LBG) is approximately 0.2 microM. Man26A is the first mannanase reported to contain a mannan-binding module.

The E358S mutant of Agrobacterium sp. beta-glucosidase is a greatly improved glycosynthase

Glycosynthases are nucleophile mutants of retaining glycosidases that catalyze the glycosylation of sugar acceptors using glycosyl fluoride donors, thereby synthesizing oligosaccharides. The 'original' glycosynthase, derived from Agrobacterium sp. beta-glucosidase (Abg) by mutating the nucleophile glutamate to alanine (E358A), synthesizes oligosaccharides in yields exceeding 90% [Mackenzie, L.F., Wang, Q., Warren, R.A.J. and Withers, S.G. (1998) J. Am. Chem. Soc. 120, 5583-5584]. This mutant has now been re-cloned with a His(6)-tag into a pET-29b(+) vector, allowing gram scale production and single step chromatographic purification. A dramatic, 24-fold, improvement in synthetic rates has also been achieved by substituting the nucleophile with serine, resulting in improved product yields, reduced reaction times and an enhanced synthetic repertoire. Thus poor acceptors for Abg E358A, such as PNP-GlcNAc, are successfully glycosylated by E358S, allowing the synthesis of PNP-beta-LacNAc. The increased glycosylation activity of Abg E358S likely originates from a stabilizing interaction between the Ser hydroxyl group and the departing anomeric fluorine of the alpha-glycosyl fluoride.

Genetic characterization of a Tn5-disrupted glycosyltransferase gene homolog in Brucella abortus and its effect on lipopolysaccharide composition and virulence

We constructed a rough mutant of Brucella abortus 2308 by transposon (Tn5) mutagenesis. Neither whole cells nor extracted lipopolysaccharide (LPS) from this mutant, designated RA1, reacted with a Brucella O-side-chain-specific monoclonal antibody (MAb), Bru-38, indicating the absence of O-side-chain synthesis. Compositional analyses of LPS from strain RA1 showed reduced levels of quinovosamine and mannose relative to the levels in the parental, wild-type strain, 2308. We isolated DNA flanking the Tn5 insertion in strain RA1 by cloning a 25-kb XbaI genomic fragment into pGEM-3Z to create plasmid pJM6. Allelic exchange of genomic DNA in B. abortus 2308 mediated by electroporation of pJM6 produced kanamycin-resistant clones that were not reactive with MAb Bru-38. Southern blot analysis of genomic DNA from these rough clones revealed Tn5 in a 25-kb XbaI genomic fragment. A homology search with the deduced amino acid sequence of the open reading frame disrupted by Tn5 revealed limited homology with various glycosyltransferases. This B. abortus gene has been named wboA. Transformation of strain RA1 with a broad-host-range plasmid bearing the wild-type B. abortus wboA gene resulted in the restoration of O-side-chain synthesis and the smooth phenotype. B. abortus RA1 was attenuated for survival in mice. However, strain RA1 persisted in mice spleens for a longer time than the B. abortus vaccine strain RB51, but as expected, neither strain induced antibodies specific for the O side chain.

Mannan-degrading enzymes from Cellulomonas fimi

The genes man26a and man2A from Cellulomonas fimi encode mannanase 26A (Man26A) and beta-mannosidase 2A (Man2A), respectively. Mature Man26A is a secreted, modular protein of 951 amino acids, comprising a catalytic module in family 26 of glycosyl hydrolases, an S-layer homology module, and two modules of unknown function. Exposure of Man26A produced by Escherichia coli to C. fimi protease generates active fragments of the enzyme that correspond to polypeptides with mannanase activity produced by C. fimi during growth on mannans, indicating that it may be the only mannanase produced by the organism. A significant fraction of the Man26A produced by C. fimi remains cell associated. Man2A is an intracellular enzyme comprising a catalytic module in a subfamily of family 2 of the glycosyl hydrolases that at present contains only mammalian beta-mannosidases.

Spatial control of coated-pit dynamics in living cells

Here we visualize new aspects of the dynamics of endocytotic clathrin-coated pits and vesicles in mammalian cells by using a fusion protein consisting of green fluorescent protein and clathrin light chain a. Clathrin-coated pits invaginating from the plasma membrane show definite, but highly limited, mobility within the membrane that is relaxed upon treatment with latrunculin B, an inhibitor of actin assembly, indicating that an actin-based framework may be involved in the mobility of these pits. Transient, motile coated vesicles that originate from coated pits can be detected, with multiple vesicles occasionally appearing to emanate from a single pit. Despite their seemingly random distribution, coated pits tend to form repeatedly at defined sites while excluding other regions. This spatial regulation of coated-pit assembly and function is attributable to the attachment of the coated pits to the membrane skeleton.

Cellulose as an inert matrix for presenting cytokines to target cells: production and properties of a stem cell factor-cellulose-binding domain fusion protein

A chimaera of stem cell factor (SCF) and a cellulose-binding domain from the xylanase Cex (CBDCex) effectively immobilizes SCF on a cellulose surface. The fusion protein retains both the cytokine properties of SCF and the cellulose-binding characteristics of CBDCex. When adsorbed on cellulose, SCF-CBDCex is up to 7-fold more potent than soluble SCF-CBDCex and than native SCF at stimulating the proliferation of factor-dependent cell lines. When cells are incubated with cellulose-bound SCF-CBDCex, activated receptors and SCF-CBDCex co-localize on the cellulose matrix. The strong binding of SCF-CBDCex to the cellulose surface permits the effective and localized stimulation of target cells; this is potentially significant for long-term perfusion culturing of factor-dependent cells. It also permits the direct analysis of the effects of surface-bound cytokines on target cells.

The topology of the substrate binding clefts of glycosyl hydrolase family 10 xylanases are not conserved

The crystal structures of family 10 xylanases indicate that the distal regions of their active sites are quite different, suggesting that the topology of the substrate binding clefts of these enzymes may vary. To test this hypothesis, we have investigated the rate and pattern of xylooligosaccharide cleavage by the family 10 enzymes, Pseudomonas fluorescens subsp. cellulosa xylanase A (XYLA) and Cellulomonas fimi exoglucanase, Cex. The data showed that Cex contained three glycone and two aglycone binding sites, while XYLA had three glycone and four aglycone binding sites, supporting the view that the topologies of substrate binding clefts in family 10 glycanases are not highly conserved. The importance of residues in the substrate binding cleft of XYLA in catalysis and ligand binding were evaluated using site-directed mutagenesis. In addition to providing insight into the function of residues in the glycone region of the active site, the data showed that the aromatic residues Phe-181, Tyr-255, and Tyr-220 play important roles in binding xylose moieties, via hydrophobic interactions, at subsites +1, +3, and +4, respectively. Interestingly, the F181A mutation caused a much larger reduction in the activity of the enzyme against xylooligosaccharides compared with xylan. These data, in conjunction with a previous study (Charnock, S. J., Lakey, J. H., Virden, R., Hughes, N., Sinnott, M. L., Hazlewood, G. P., Pickersgill, R., and Gilbert, H. J. (1997) J. Biol. Chem. 272, 2942-2951), suggest that the binding of xylooligosaccharides at the -2 and +1 subsites ensures that the substrates occupy the -1 and +1 subsites and thus preferentially form productive complexes with the enzyme. Loss of ligand binding at either subsite results in small substrates forming nonproductive complexes with XYLA by binding to distal regions of the substrate binding cleft.

Characterization and affinity applications of cellulose-binding domains

Cellulose-binding domains (CBDs) are discrete protein modules found in a large number of carbohydrolases and a few nonhydrolytic proteins. To date, almost 200 sequences can be classified in 13 different families with distinctly different properties. CBDs vary in size from 4 to 20 kDa and occur at different positions within the polypeptides; N-terminal, C-terminal and internal. They have a moderately high and specific affinity for insoluble or soluble cellulosics with dissociation constants in the low micromolar range. Some CBDs bind irreversibly to cellulose and can be used for applications involving immobilization, others bind reversibly and are more useful for separations and purifications. Dependent on the CBD used, desorption from the matrix can be promoted under various different conditions including denaturants (urea, high pH), water, or specific competitive ligands (e.g. cellobiose). Family I and IV CBDs bind reversibly to cellulose in contrast to family II and III CBDs which are in general, irreversibly bound. The binding of family II CBDs (CBD(Cex)) to crystalline cellulose is characterized by a large favourable increase in entropy indicating that dehydration of the sorbent and the protein are the major driving forces for binding. In contrast, binding of family IV CBDs (CBD(N1)) to amorphous or soluble cellulosics is driven by a favourable change in enthalpy which is partially offset by an unfavourable entropy change. Hydrogen bond formation and van der Waals interactions are the main driving forces for binding. CBDs with affinity for crystalline cellulose are useful tags for classical column affinity chromatography. The affinity of CBD(N1) for soluble cellulosics makes it suitable for use in large-scale aqueous two-phase affinity partitioning systems.

Insights into transition state stabilization of the beta-1,4-glycosidase Cex by covalent intermediate accumulation in active site mutants

The catalytic mechanism of 'retaining' beta-glycosidases has been the subject of considerable interest and debate for many years. The visualization of a covalent glycosyl enzyme intermediate by X-ray crystallography was first accomplished with a saccharide substrate substituted with fluorine at its 2-position. The structure implicated major roles for residue His 205 and for the 2-hydroxyl position of the proximal saccharide in binding and catalysis. Here we have studied the kinetic behavior of various His 205 mutants. One of these mutants, a double mutant H205N/E127A, has been used to stabilize a covalent glycosyl-enzyme intermediate involving an unsubstituted sugar, permitting crystallographic analysis of the interactions between its 2-hydroxyl group and the enzyme.

Distinct saturable pathways for the endocytosis of different tyrosine motifs

Endocytosis of surface proteins through clathrin-coated pits requires an internalization signal in the cytoplasmic domain. Two types of internalization signal have been described: one requiring a tyrosine as the critical residue (tyrosine-based motif), and the other consisting of either two consecutive leucines or an isoleucine and leucine (dileucine motif). Although it seems that these signals are necessary and sufficient for endocytic targeting, the mechanism of recognition is not well understood. To examine this question, tetracycline-repressible cell lines were used to overexpress one of several receptors bearing a tyrosine-based internalization signal. By measuring the rates of endocytosis for either the overexpressed receptor, or that of other endogenous receptors, we were able to show that the endocytosis of identical receptors could be saturated, but a complete lack of competition exists between the transferrin receptor (TfR), the low-density lipoprotein receptor, and the epidermal growth factor receptor. Overexpression of any one of these receptors resulted in its redistribution toward the cell surface, implying that entry into coated pits is limited. During high levels of TfR expression, however, a significant increase in the amount of surface Lamp1, but not low-density lipoprotein receptor, epidermal growth factor receptor, or Lamp2, is detected. This suggests that Lamp1 and TfR compete for the same endocytic sites. Together, these results support the idea that there are at least three distinct saturable components involved in clathrin-mediated endocytosis.

Analysis of molecular size distributions of cellulose molecules during hydrolysis of cellulose by recombinant Cellulomonas fimi beta-1,4-glucanases

Four beta-1,4-glucanases (cellulases) of the cellulolytic bacterium Cellulomonas fimi were purified from Escherichia coli cells transformed with recombinant plasmids. Previous analyses using soluble substrates had suggested that CenA and CenC were endoglucanases while CbhA and CbhB resembled the exo-acting cellobiohydrolases produced by cellulolytic fungi. Analysis of molecular size distributions during cellulose hydrolysis by the individual enzymes confirmed these preliminary findings and provided further evidence that endoglucanase CenC has a more processive hydrolytic activity than CenA. The significant differences between the size distributions obtained during hydrolysis of bacterial microcrystalline cellulose and acid-swollen cellulose can be explained in terms of the accessibility of beta-1,4-glucan chains to enzyme attack. Endoglucanases and cellobiohydrolases were much more easily distinguished when the acid-swollen substrate was used.

Progression of change in NMDA, non-NMDA, and metabotropic glutamate receptor function at the developing corticothalamic synapse

The development of receptor function at corticothalamic synapses during the first 20 days of postnatal development is described. Whole cell excitatory postsynaptic currents (EPSCs) were evoked in relay neurons of the ventral posterior nucleus (VP) by stimulation of corticothalamic fibers in in vitro slices of mouse brain from postnatal day 1 (P1). During P1-P12, excitatory postsynaptic conductances showed strong voltage dependence at peak current and at 100 ms after the stimulus and were almost completely antagonized by -2-amino-5-phosphonopentoic acid (APV), indicating that N-methyl--aspartate (NMDA) receptor-mediated currents dominate corticothalamic EPSCs at this time. After P12, in 42% of cells, excitatory postsynaptic conductances showed no voltage-dependence at peak current but still showed voltage-dependence 100-ms poststimulus. This voltage-dependent conductance was antagonized by APV. The nonvoltage-dependent component was APV resistant, showed fast decay, and was antagonized by the nonNMDA antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). In the remaining 58% of cells after P12, excitatory postsynaptic conductances showed moderate voltage dependence at peak conductance and strong voltage dependence 100 ms after the stimulus. Analysis of EPSCs before and after APV showed a significant increase in the relative contribution of the non-NMDA conductance after the second postnatal week. From P1 to P16, there was a significant decrease in the time constant of decay of the NMDA EPSC but no change in the voltage dependence of the NMDA response. After P8, slow EPSPs, 1.5-30 s in duration and mediated by metabotropic glutamate receptors (mGluRs), could be evoked by high-frequency stimulation of corticothalamic fibers in the presence of APV and CNQX. Similar slow depolarizations could be evoked by local application of the mGluR agonist (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (t-ACPD) but from P0. Both conductances were blocked by the mGluR antagonist, (RS)-alpha-methyl-4-carboxyphenylglycine. Hence functional mGluR receptors are present on VP cells from birth, but their synaptic activation at corticothalamic synapses can only be detected after P8. In voltage clamp, the extrapolated reversal potential of the t-ACPD current, with potassium gluconate-based internal solution, was +12 +/- 10 (SE) mV, and the measured reversal potential with cesium gluconate-based internal solution was 1.5 +/- 9.9 mV, suggesting that the mGluR-mediated depolarization was mediated by a nonselective cation current. Replacement of NaCl in the external solution caused the reversal potential of the current to shift to -18 +/- 2 mV, indicating that Na+ is a charge carrier in the current. The current amplitude was not reduced by application of Cs+, Ba2+, and Cd2+, indicating that the t-ACPD current was distinct from the hyperpolarization-activated cation current (IH) and distinct from certain other previously characterized mGluR-activated, nonselective cation conductances.

Exploring the cellulose/xylan specificity of the beta-1,4-glycanase cex from Cellulomonas fimi through crystallography and mutation

The retaining beta-1,4-glycanase Cex from Cellulomonas fimi, a family 10 glycosyl hydrolase, hydrolyzes xylan 40-fold more efficiently than cellulose. To gain insight into the nature of its preference for xylan, we determined the crystal structure of the Cex catalytic domain (Cex-cd) trapped as its covalent 2-deoxy-2-fluoroxylobiosyl-enzyme intermediate to 1.9 A resolution. Together with the crystal structure of unliganded Cex-cd [White, A., et al. (1994) Biochemistry 33, 12546-12552] and the previously determined crystal structure of the covalent 2-deoxy-2-fluorocellobiosyl-Cex-cd intermediate [White, A., et al. (1996) Nat. Struct. Biol. 3, 149-154], this structure provides a convincing rationale for the observed substrate specificity in Cex. Two active site residues, Gln87 and Trp281, are found to sterically hinder the binding of glucosides and must rearrange to accommodate these substrates. Such rearrangements are not necessary for the binding of xylobiosides. The importance of this observation was tested by examining the catalytic behavior of the enzyme with Gln87 mutated to Met. This mutation had no measurable effect on substrate affinity or turnover number relative to the wild type enzyme, indicating that the Met side chain could accommodate the glucoside moiety as effectively as the wild type Gln residue. Subsequent mutagenesis studies will address the role of entropic versus enthalpic contributions to binding by introducing side chains that might be more rigid in the unliganded enzyme.

A scheme for designating enzymes that hydrolyse the polysaccharides in the cell walls of plants

A scheme is proposed for designating enzymes that hydrolyse the polysaccharides in the cell walls of plants. These enzymes are predominantly beta-1,4-glycanases. The scheme is based on the classification of the catalytic domains of glycoside hydrolases into families of related amino acid sequences. The new designation for an enzyme indicates its family and, because all members of a family have these characteristics in common, its three-dimensional fold and stereospecificity of hydrolysis. The scheme is intended to simplify comparison of the systems of enzymes produced by different microorganisms for the hydrolysis of plant cell walls.

Mechanistic consequences of replacing the active-site nucleophile Glu-358 in Agrobacterium sp. beta-glucosidase with a cysteine residue

Retaining glycosidases achieve the hydrolysis of glycosidic bonds through the assistance of two key active-site carboxyls. One carboxyl functions as a nucleophile/leaving group, and the other acts as the acid-base catalyst. It has been suggested that a cysteine residue could fulfil the role of the active site nucleophile [Hardy and Poteete (1991) Biochemistry 30, 9457-9463]. To test the validity of this proposal, a kinetic evaluation was conducted on the active-site nucleophile cysteine mutant (Glu-358-->Cys) of the retaining beta-glucosidase from Agrobacterium sp. The Glu-358-->Cys mutant was able to complete the first step (glycosylation) of the enzymic mechanism, forming a covalent glycosyl-enzyme intermediate, but the rate constant for this step was decreased to 1/10(6) of that of the native enzyme. The subsequent hydrolysis (deglycosylation) step was also severely affected by the replacement of Glu-358 with a cysteine residue, with the rate constant being depressed to 1/10(7) or less. Thus Cys-358 functions inefficiently in both the capacity of catalytic nucleophile and leaving group. On the basis of these results it seems unlikely that the role of the active-site nucleophile in retaining glycosidases could successfully be filled by a cysteine residue.

Cytoskeletal protein ABP-280 directs the intracellular trafficking of furin and modulates proprotein processing in the endocytic pathway

Furin catalyzes the proteolytic maturation of many proproteins within the trans-Golgi network (TGN)/endosomal system. Furin's cytosolic domain (cd) directs both the compartmentalization to and transit between its manifold processing compartments (i.e., TGN/biosynthetic pathway, cell surface, and endosomes). Here we report the identification of the first furin cd sorting protein, ABP-280 (nonmuscle filamin), an actin gelation protein. The furin cd was used as bait in a yeast two-hybrid screen to identify ABP-280 as a furin-binding protein. Binding analyses in vitro and coimmunoprecipitation studies in vivo showed that furin and ABP-280 interact directly and that ABP-280 tethers furin molecules to the cell surface. Quantitative analysis of both ABP-280-deficient and genetically replete cells showed that ABP-280 modulates the rate of internalization of furin but not of the transferrin receptor, a cycling receptor. However, although ABP-280 directs the rate of furin internalization, the efficiency of sorting of the endoprotease from the cell surface to early endosomes is independent of expression of ABP-280. By contrast, efficient sorting of furin from early endosomes to the TGN requires expression of ABP-280. In addition, ABP-280 is also required for the correct localization of late endosomes (dextran bead uptake) and lysosomes (LAMP-1 staining), demonstrating a pleiotropic role for this actin binding protein in the organization of cellular compartments and directing protein traffic. Finally, and consistent with the trafficking studies on furin, we showed that ABP-280 modulates the processing of furin substrates in the endocytic but not the biosynthetic pathways. The novel roles of ABP-280 and the cytoskeleton in the sorting of furin in the TGN/ endosomal system and the formation of proprotein processing compartments are discussed.

Excitotoxic lesions of the prefrontal cortex reduce dopamine D1-like receptors in the ventral tegmental area

Ventral mesencephalic dopamine D1-like receptors were quantified in brains of male rats ten days after unilateral microinjections of ibotenic acid (2 or 10 microg/microl) or its vehicle into the medial prefrontal cortex. The density of dopamine D1-like receptors was reduced by more than 40% in the ipsilateral ventral tegmental area (both doses) and by 15% (low dose) and 44% (high dose) in the contralateral side; no significant reduction was observed in the substantia nigra. These results suggest that a significant number of ventral tegmental D1-like receptors are localized on afferent terminals from the medial prefrontal cortex.

GABA(B)-receptor-mediated inhibition in developing mouse ventral posterior thalamic nucleus

Inhibitory postsynaptic potentials (IPSPs) generated by activation of the thalamic reticular nucleus (RTN) were recorded in neurons of the ventral posterior nucleus (VP) in vitro in slices from mice aged postnatal day (P)1-P17. An early IPSP peaking 41 +/- 2.5 (SE) ms after electrical stimulation of the internal capsule or RTN was found in 96% of VP neurons. This early IPSP was blocked by bicuculline, showing its dependence on gamma-aminobutyric acid-A (GABA(A)) receptors. A late IPSP peaking 357 +/- 27 ms after the stimulus was observed in 22% of VP neurons in control medium but was uncovered in 38% of neurons when bicuculline was added. The late IPSP was blocked by addition of a GABA(B) antagonist, 2-hydroxysaclofen, to the medium (n = 7); it had a reversal potential of -98 +/- 1.3 mV, 14 mV negative to the early component. In contrast to the early IPSP, whose reversal potential became more negative during postnatal development, the reversal potential of the late IPSP remained constant throughout the postnatal period studied. The most significant change in the late IPSP was shortening in duration, with reduction in latency-to-peak by >400 ms, between P1 and P10. No changes of comparable magnitude were observed in the duration of the earlier GABA(A) response. These results show that both GABA(A) and GABA(B) IPSPs are present very early in the postnatal thalamus and that their characteristics evolve along independent paths during postnatal development.

Saturation of the endocytic pathway for the transferrin receptor does not affect the endocytosis of the epidermal growth factor receptor

Cell-surface receptors that undergo clathrin-mediated endocytosis contain short amino acid sequences in their cytoplasmic domain that serve as internalization signals. Interactions between these sequences and components of the endocytic machinery should become limiting upon overexpression of the constitutively recycling transferrin receptor (TfR). A tetracycline-responsive system was used to induce overexpression of the TfR up to 20-fold in HeLa cells. Internalization assays indicate the rate of 125I-transferrin uptake per surface TfR is reduced by a factor of 4 in induced cells. Consistent with endocytosis being the rate-limiting step, TfRs shift from an endosomal to more of a plasma membrane distribution with TfR overexpression. The clathrin-associated protein AP-2 has been proposed to interact directly with the cytoplasmic domain of many receptors, yet no changes in the amount or distribution of AP-2 were detected in induced cells. The internalization rate for the epidermal growth factor receptor was also measured, with or without induction of TfR expression. Even though endocytosis of the TfR is saturated in induced cells, 125I-labeled epidermal growth factor continues to be internalized at a rate identical to that seen in uninduced cells. We propose that there are different limiting steps for the endocytosis of these two receptors.

Maturation of neuronal form and function in a mouse thalamo-cortical circuit

Postnatal development of physiological properties underlying slow intrathalamic oscillations was studied by whole-cell recording from synaptically coupled neurons of the reticular nucleus (RTN) and ventral posterior nucleus (VPN) of mouse brain slices in vitro and compared with the morphological development of dye-injected cells. Between postnatal days 3 and 11 (P3-P11), progressive changes in RTN and VPN neurons included shortening of the membrane time constant, decreasing input resistance, and lowering of the resting membrane potential (RMP). Low-threshold Ca2+ spikes (LTS) were present from P3, but their capacity to sustain multispike bursts was limited before P11. Synaptic responses were evoked in RTN and VPN neurons by electrical stimulation of the internal capsule from P3. Younger RTN neurons responded with a single spike, but their capacity to fire bursts gradually improved as the RMP reached levels below the LTS activation potential. Concomitantly, as the reversal potential of the inhibitory postsynaptic potential in VPN neurons became more negative, its capacity to deinactivate the LTS increased, and rebound bursts that could maintain oscillations were produced; sustained oscillations became the typical response to internal capsule stimulation at P12. The functional maturation of the intrathalamic circuitry, particularly between P10 and P14, occurs in parallel with the morphological maturation (size, dendritic growth, and dendritic field structure) of individual RTN and VPN neurons, as studied by confocal microscopy. Maturation of RTN cells led that of VPN cells by 2-3 d. The appearance of intrathalamic oscillations is probably correlated with the appearance of slow-wave sleep in postnatal animals.

Probing the role of tryptophan residues in a cellulose-binding domain by chemical modification

The cellulose-binding domain (CBDCex) of the mixed function glucanase-xylanase Cex from Cellulomonas fimi contains five tryptophans, two of which are located within the beta-barrel structure and three exposed on the surface (Xu GY et al., 1995, Biochemistry 34:6993-7009). Although all five tryptophans can be oxidized by N-bromosuccinimide (NBS), stopped-flow measurements show that three tryptophans react faster than the other two. NMR analysis during the titration of CBDCex with NBS shows that the tryptophans on the surface of the protein are fully oxidized before there is significant reaction with the two buried tryptophans. Additionally, modification of the exposed tryptophans does not affect the conformation of the backbone of CBDCex, whereas complete oxidation of all five tryptophans denatures the polypeptide. The modification of the equivalent of one and two tryptophans by NBS reduces binding of CBDCex to cellulose by 70% and 90%, respectively. This confirms the direct role of the exposed aromatic residues in the binding of CBDCex to cellulose. Although adsorption to cellulose does afford some protection against NBS, as evidenced by the increased quantity of NBS required to oxidize all of the tryptophan residues, the polypeptide can still be oxidized completely when adsorbed. This suggests that, whereas the binding appears to be irreversible overall [Ong E et al., 1989, Bio/Technology 7:604-607], each of the exposed tryptophans interacts reversibly with cellulose.