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Robert W Harrison

Publications and source records attributed to Robert W Harrison.

13 recordsLinked to original sources

Protein folding: search for basic physical models.

How a unique three-dimensional structure is rapidly formed from the linear sequence of a polypeptide is one of the important questions in contemporary science. Apart from biological context of in vivo protein folding (which has been studied only for a few proteins), the roles of the fundamental physical forces in the in vitro folding remain largely unstudied. Despite a degree of success in using descriptions based on statistical and/or thermodynamic approaches, few of the current models explicitly include more basic physical forces (such as electrostatics and Van Der Waals forces). Moreover, the present-day models rarely take into account that the protein folding is, essentially, a rapid process that produces a highly specific architecture. This review considers several physical models that may provide more direct links between sequence and tertiary structure in terms of the physical forces. In particular, elaboration of such simple models is likely to produce extremely effective computational techniques with value for modern genomics.

Models, Molecular↗

Disruption of leptin signaling contributes to cardiac hypertrophy independently of body weight in mice.

BACKGROUND: Whether left ventricular hypertrophy (LVH) in obesity results from increased hemodynamic load or altered neurohormonal signaling remains controversial. Dysregulation of leptin, a neurohormone essential to energy homeostasis, is implicated in the pathogenesis of obesity. Because leptin has cardiovascular bioactivity, we hypothesized that disruption of leptin signaling mediates the development of obesity-associated LVH. METHODS AND RESULTS: We measured left ventricular (LV) wall thickness and LV mass with echocardiography in mice lacking leptin (ob/ob, n=15) or functional receptor (db/db, n=10) and controls at 2, 4, and 6 months of age. None of the mice had LVH at 2 months. Progressive obesity developed in ob/ob and db/db mice. At 6 months, LVH occurred in ob/ob and db/db compared with controls. We observed corresponding myocyte hypertrophy by light microscopy. To separate the direct contribution of leptin deficiency from mechanical effects of obesity, we induced weight loss in 6- to 8-month-old ob/ob mice either by leptin infusion or caloric restriction. Mice in both groups lost similar weight compared with placebo-treated controls. Leptin infusion completely reversed the increase in wall thickness with partial resolution of myocyte hypertrophy, whereas calorie-restricted mice had no decrease in wall thickness and a lesser change in myocyte size. CONCLUSIONS: Together these data show that the effect of leptin on LV remodeling is not attributable to weight loss alone, indicating that leptin has antihypertrophic effects on the heart, either directly or through a leptin-regulated neurohumoral pathway. Disruption of leptin signaling may represent a novel mechanism in LVH and related cardiovascular disorders.

Animals↗

Nitric oxide regulation of myocardial contractility and calcium cycling: independent impact of neuronal and endothelial nitric oxide synthases.

The mechanisms by which nitric oxide (NO) influences myocardial Ca2+ cycling remain controversial. Because NO synthases (NOS) have specific spatial localization in cardiac myocytes, we hypothesized that neuronal NOS (NOS1) found in cardiac sarcoplasmic reticulum (SR) preferentially regulates SR Ca2+ release and reuptake resulting in potentiation of the cardiac force-frequency response (FFR). Transesophageal pacing (660 to 840 bpm) in intact C57Bl/6 mice (WT) stimulated both contractility (dP/dtmax normalized to end-diastolic volume; dP/dt-EDV) by 51+/-5% (P<0.001) and lusitropy (tau; tau) by 20.3+/-2.0% (P<0.05). These responses were markedly attenuated in mice lacking NOS1 (NOS1-/-) (15+/-2% increase in dP/dt-EDV; P<0.001 versus WT; and no change in tau; P<0.01 versus WT). Isolated myocytes from NOS1-/- (approximately 2 months of age) also exhibited suppressed frequency-dependent sarcomere shortening and Ca2+ transients ([Ca2+]i) compared with WT. SR Ca2+ stores, a primary determinant of the FFR, increased at higher frequencies in WT (caffeine-induced [Ca2+]i at 4 Hz increased 107+/-23% above 1 Hz response) but not in NOS1-/- (13+/-26%; P<0.01 versus WT). In contrast, mice lacking NOS3 (NOS3-/-) had preserved FFR in vivo, as well as in isolated myocytes with parallel increases in sarcomere shortening, [Ca2+]i, and SR Ca2+ stores. NOS1-/- had increased SR Ca2+ ATPase and decreased phospholamban protein abundance, suggesting compensatory increases in SR reuptake mechanisms. Together these data demonstrate that NOS1 selectively regulates the cardiac FFR via influences over SR Ca2+ cycling. Thus, there is NOS isoform-specific regulation of different facets of rate-dependent excitation-contraction coupling; inactivation of NOS1 has the potential to contribute to the pathophysiology of states characterized by diminished frequency-dependent inotropic responses.

Animals↗

Combined loss of neuronal and endothelial nitric oxide synthase causes premature mortality and age-related hypertrophic cardiac remodeling in mice.

Deficiency of either neuronal nitric oxide synthase (NOS1) or endothelial nitric oxide synthase (NOS3) leads to cardiac hypertrophy in mice. Loss of both produces concentric left ventricular (LV) remodeling, in which increased wall thickness is accompanied by reduced cavity size. In humans, this phenotype develops in elderly hypertensive patients and independently predicts mortality. Accordingly, we tested the hypothesis that NOS1/3(-/-) mice have reduced longevity compared to either NOS1(-/-) or NOS3(-/-). Survival data on colonies of NOS1(-/-) (n = 295), NOS3(-/-) (n = 525), and NOS1/3(-/-) (n = 331) mice were collected for 2 years. NOS1(-/-) mice had increased mortality compared to NOS3(-/-) (relative risk, RR 2.5, P < 0.001), whereas NOS1/3(-/-) fared significantly worse (RR 7.3, P < 0.001 vs. NOS3(-/-)). Importantly, gender did not affect survival in NOS1(-/-) or NOS3(-/-), but male NOS1/3(-/-) mice had 2-fold increased mortality compared to females. NOS1/3(-/-) mice developed progressive myocyte hypertrophy and interstitial fibrosis with age. NOS1/3(-/-) mice underwent in vivo hemodynamic analysis with a combined pressure-volume catheter to assess age-related cardiovascular changes. Compared with control, NOS1/3(-/-) demonstrated hypertension and hypercontractility at all ages, and developed passive diastolic dysfunction with increasing age. Thus, combined deficiency of NOS1 and NOS3 causes increased mortality, myocyte hypertrophy, and an age-associated increase in ventricular stiffness. These findings suggest that cardiac NO signals may play an essential role in successful cardiac aging.

Aging↗

Close pairs of carboxylates: a possibility of multicenter hydrogen bonds in proteins.

Covalent attachment of hydrogen to the donor atom may be not an essential characteristic of stable hydrogen bonds. A positively charged particle (such as a proton), located between the two negatively charged residues, may lead to a stable interaction of the two negative residues. This paper analyzes close Asp-Glu pairs of residues in a large set of protein chains; 840 such pairs of residues were identified, of which 28% were stabilized by a metal ion, 12% by a positive residue nearby and 60% are likely to be stabilized by a proton. The absence of apparent structural constraints, secondary structure preferences, somewhat lower B-factors and a distinct correlation between pH and the minimal O-O distance in carboxylate pairs suggest that most of the abnormally close pairs could indeed be stabilized by a shared proton. Implications for protein stability and modeling are discussed.

Amino Acids, Acidic↗

Novel bis-tetrahydrofuranylurethane-containing nonpeptidic protease inhibitor (PI) UIC-94017 (TMC114) with potent activity against multi-PI-resistant human immunodeficiency virus in vitro.

We designed, synthesized, and identified UIC-94017 (TMC114), a novel nonpeptidic human immunodeficiency virus type 1 (HIV-1) protease inhibitor (PI) containing a 3(R),3a(S),6a(R)-bis-tetrahydrofuranylurethane (bis-THF) and a sulfonamide isostere which is extremely potent against laboratory HIV-1 strains and primary clinical isolates (50% inhibitory concentration [IC(50)], approximately 0.003 micro M; IC(90), approximately 0.009 micro M) with minimal cytotoxicity (50% cytotoxic concentration for CD4(+) MT-2 cells, 74 micro M). UIC-94017 blocked the infectivity and replication of each of HIV-1(NL4-3) variants exposed to and selected for resistance to saquinavir, indinavir, nelfinavir, or ritonavir at concentrations up to 5 micro M (IC(50)s, 0.003 to 0.029 micro M), although it was less active against HIV-1(NL4-3) variants selected for resistance to amprenavir (IC(50), 0.22 micro M). UIC-94017 was also potent against multi-PI-resistant clinical HIV-1 variants isolated from patients who had no response to existing antiviral regimens after having received a variety of antiviral agents. Structural analyses revealed that the close contact of UIC-94017 with the main chains of the protease active-site amino acids (Asp-29 and Asp-30) is important for its potency and wide spectrum of activity against multi-PI-resistant HIV-1 variants. Considering the favorable pharmacokinetics of UIC-94017 when administered with ritonavir, the present data warrant that UIC-94017 be further developed as a potential therapeutic agent for the treatment of primary and multi-PI-resistant HIV-1 infections.

Amino Acid Substitution↗

Crystal structures of Tcl1 family oncoproteins and their conserved surface features.

Members of the TCL1 family of oncogenes are abnormally expressed in mature T-cell leukemias and B-cell lymphomas. The proteins are involved in the coactivation of protein kinase B (Akt/PKB), a key intracellular kinase. The sequences and crystal structures of three Tcl1 proteins were analyzed in order to understand their interactions with Akt/PKB and the implications for lymphocyte malignancies. Tcl1 proteins are approximately 15 kD and share 25-80% amino acid sequence identity. The tertiary structures of mouse Tcl1, human Tcl1, and Mtcp1 are very similar. Analysis of the structures revealed conserved semi-planar surfaces that have characteristics of surfaces involved in protein-protein interactions. The Tcl1 proteins show differences in surface charge distribution and oligomeric state suggesting that they do not interact in the same way with Akt/PKB and other cellular protein(s).

Amino Acid Sequence↗

Combining mutations in HIV-1 protease to understand mechanisms of resistance.

HIV-1 develops resistance to protease inhibitors predominantly by selecting mutations in the protease gene. Studies of resistant mutants of HIV-1 protease with single amino acid substitutions have shown a range of independent effects on specificity, inhibition, and stability. Four double mutants, K45I/L90M, K45I/V82S, D30N/V82S, and N88D/L90M were selected for analysis on the basis of observations of increased or decreased stability or enzymatic activity for the respective single mutants. The double mutants were assayed for catalysis, inhibition, and stability. Crystal structures were analyzed for the double mutants at resolutions of 2.2-1.2 A to determine the associated molecular changes. Sequence-dependent changes in protease-inhibitor interactions were observed in the crystal structures. Mutations D30N, K45I, and V82S showed altered interactions with inhibitor residues at P2/P2', P3/P3'/P4/P4', and P1/P1', respectively. One of the conformations of Met90 in K45I/L90M has an unfavorably close contact with the carbonyl oxygen of Asp25, as observed previously in the L90M single mutant. The observed catalytic efficiency and inhibition for the double mutants depended on the specific substrate or inhibitor. In particular, large variation in cleavage of p6(pol)-PR substrate was observed, which is likely to result in defects in the maturation of the protease from the Gag-Pol precursor and hence viral replication. Three of the double mutants showed values for stability that were intermediate between the values observed for the respective single mutants. D30N/V82S mutant showed lower stability than either of the two individual mutations, which is possibly due to concerted changes in the central P2-P2' and S2-S2' sites. The complex effects of combining mutations are discussed.

Amino Acid Substitution↗

Nitric oxide regulates the heart by spatial confinement of nitric oxide synthase isoforms.

Subcellular localization of nitric oxide (NO) synthases with effector molecules is an important regulatory mechanism for NO signalling. In the heart, NO inhibits L-type Ca2+ channels but stimulates sarcoplasmic reticulum (SR) Ca2+ release, leading to variable effects on myocardial contractility. Here we show that spatial confinement of specific NO synthase isoforms regulates this process. Endothelial NO synthase (NOS3) localizes to caveolae, where compartmentalization with beta-adrenergic receptors and L-type Ca2+ channels allows NO to inhibit beta-adrenergic-induced inotropy. Neuronal NO synthase (NOS1), however, is targeted to cardiac SR. NO stimulation of SR Ca2+ release via the ryanodine receptor (RyR) in vitro, suggests that NOS1 has an opposite, facilitative effect on contractility. We demonstrate that NOS1-deficient mice have suppressed inotropic response, whereas NOS3-deficient mice have enhanced contractility, owing to corresponding changes in SR Ca2+ release. Both NOS1-/- and NOS3-/- mice develop age-related hypertrophy, although only NOS3-/- mice are hypertensive. NOS1/3-/- double knockout mice have suppressed beta-adrenergic responses and an additive phenotype of marked ventricular remodelling. Thus, NOS1 and NOS3 mediate independent, and in some cases opposite, effects on cardiac structure and function.

Adrenergic beta-Agonists↗

Imbalance between xanthine oxidase and nitric oxide synthase signaling pathways underlies mechanoenergetic uncoupling in the failing heart.

Inhibition of xanthine oxidase (XO) in failing hearts improves cardiac efficiency by an unknown mechanism. We hypothesized that this energetic effect is due to reduced oxidative stress and critically depends on nitric oxide synthase (NOS) activity, reflecting a balance between generation of nitric oxide (NO) and reactive oxygen species. In dogs with pacing-induced heart failure (HF), ascorbate (1000 mg) mimicked the beneficial energetic effects of allopurinol, increasing both contractility and efficiency, suggesting an antioxidant mechanism. Allopurinol had no additive effect beyond that of ascorbate. Crosstalk between XO and NOS signaling was assessed. NOS inhibition with N(G)-monomethyl-L-arginine (L-NMMA; 20 mg/kg) had no effect on basal contractility or efficiency in HF, but prevented the +26.2+/-3.5% and +66.5+/-17% enhancements of contractility and efficiency, respectively, observed with allopurinol alone. Similarly, improvements in contractility and energetics due to ascorbate were also inhibited by L-NMMA. Because of the observed NOS-XO crosstalk, we predicted that in normal hearts NOS inhibition would uncover a depression of energetics caused by XO activity. In normal conscious dogs, L-NMMA increased myocardial oxygen consumption (MVO2) while lowering left ventricular external work, reducing efficiency by 31.1+/-3.8% (P<0.005). Lowered efficiency was reversed by XO inhibition (allopurinol, 200 mg) or by ascorbate without affecting cardiac load or systemic hemodynamics. Single-cell immunofluorescence detected XO protein in cardiac myocytes that was enhanced in HF, consistent with autocrine signaling. These data show that both NOS and XO signaling systems participate in the regulation of myocardial mechanical efficiency and that upregulation of XO relative to NOS contributes to mechanoenergetic uncoupling in heart failure.

Allopurinol↗

Effect of sequence polymorphism and drug resistance on two HIV-1 Gag processing sites.

The HIV-1 proteinase (PR) has proved to be a good target for antiretroviral therapy of AIDS, and various PR inhibitors are now in clinical use. However, there is a rapid selection of viral variants bearing mutations in the proteinase that are resistant to clinical inhibitors. Drug resistance also involves mutations of the nucleocapsid/p1 and p1/p6 cleavage sites of Gag, both in vitro and in vivo. Cleavages at these sites have been shown to be rate limiting steps for polyprotein processing and viral maturation. Furthermore, these sites show significant sequence polymorphism, which also may have an impact on virion infectivity. We have studied the hydrolysis of oligopeptides representing these cleavage sites with representative mutations found as natural variations or that arise as resistant mutations. Wild-type and five drug resistant PRs with mutations within or outside the substrate binding site were tested. While the natural variations showed either increased or decreased susceptibility of peptides toward the proteinases, the resistant mutations always had a beneficial effect on catalytic efficiency. Comparison of the specificity changes obtained for the various substrates suggested that the maximization of the van der Waals contacts between substrate and PR is the major determinant of specificity: the same effect is crucial for inhibitor potency. The natural nucleocapsid/p1 and p1/p6 sites do not appear to be optimized for rapid hydrolysis. Hence, mutation of these rate limiting cleavage sites can partly compensate for the reduced catalytic activity of drug resistant mutant HIV-1 proteinases.

Amino Acid Sequence↗

Geometric criteria of hydrogen bonds in proteins and identification of "bifurcated" hydrogen bonds.

Empirical criteria for identification of hydrogen bonds were analyzed to produce a set of geometrically consistent criteria. For a data set of 30 structures, application of a set of purely geometrical criteria, along with exclusion of abnormal backbone conformations, also excluded a common interaction of Ser/Thr side chains with Asp/Glu side chains ([ST]/[DE] pairs). These interactions were termed "bifurcated hydrogen bonds", which implies delocalization of a positively charged hydrogen of hydroxyl between the two acceptor atoms of the carboxylic group. These "bifurcated" interactions are among the most common packing patterns for [ST]/[DE] pairs of side chains. Therefore, the identification of hydrogen bonds cannot be based on geometrical criteria only and requires introduction of some physico-chemical criteria.

Amino Acids↗