Gene X environment interaction of vigorous exercise and body mass index among male Vietnam-era twins

BACKGROUND

Secular trends over the past several decades suggest an environmental influence on body mass index (BMI). However, twin models that incorporate a gene-environment correlation and gene x environment interaction have not been applied to elucidate specific environmental factors that affect the heritability of BMI.

OBJECTIVE

Our aim was to determine whether one putative environmental predictor of obesity, vigorous exercise, shows evidence of a gene-environment correlation or gene x environment interaction with BMI among twins.

DESIGN

Twin structural equation modeling was used to examine a gene-environment correlation and a gene x environment interaction of vigorous exercise with BMI among 2710 monozygotic and 2327 dizygotic male-male twin pairs from the Vietnam Era Twin Registry -- a national registry of twin pairs who served in the military during the Vietnam War era.

RESULTS

Vigorous exercise significantly modified the additive genetic component of BMI, which indicated a gene x environment interaction (P < 0.001). BMI showed the greatest genetic influence among those who did not report vigorous exercise, with diminished genetic influence among those who did. Furthermore, vigorous exercise had a small but significant environmental effect on BMI (P = 0.006) -- a finding confirmed among monozygotic co-twins discordant for vigorous exercise.

CONCLUSIONS

Genetic influences on BMI are lower among those who report vigorous exercise. Consistent with an emerging literature, this suggests that vigorous exercise may mitigate some of the genetic influence on obesity. Molecular genetic studies of obesity should consider incorporating measures of behavioral and demographic factors to maximize the identification of novel obesity genes.

Educational attainment, smoking initiation and lifetime nicotine dependence among male Vietnam-era twins

BACKGROUND

Smoking initiation and persistence are clearly associated with factors commonly thought to be environmental in origin, including socio-economic status. However, twin models that incorporate gene-environment correlation and gene x environment interaction have not been applied to elucidate the genetic or environmental role that socio-economic status plays in smoking initiation and nicotine dependence.

METHOD

Twin structural equation modelling was used to examine gene-environment correlation and gene x environment interaction of one index of socio-economic status, educational attainment, with smoking initiation and nicotine dependence among 5119 monozygotic and 4295 dizygotic male-male Vietnam-era twins from the Vietnam Era Twin Registry, a national registry of twin pairs who served in the military during the Vietnam era.

RESULTS

Educational attainment correlated significantly with smoking initiation (r=-0.27, p<0.001). Additive genetic (p=0.011), shared environment (p=0.002) and unique environment (p=0.027) components contributed to the correlation between educational attainment and smoking initiation. Educational attainment also significantly moderated the variance in smoking initiation (p<0.001), suggestive of gene x environment interaction, although the interaction with the additive genetic, shared environmental and unique environmental components could not be resolved due to multi-collinearity. In contrast, educational attainment neither correlated with nor moderated nicotine dependence, once smokers had initiated.

CONCLUSIONS

Our study suggests that educational attainment is associated with smoking initiation, in part due to gene-environment correlation and gene x environment interaction. However, once smoking initiation is taken into account, there are no effects--be they gene-environment correlation or gene x environmental interaction--of educational attainment on nicotine dependence.

Genetics in psychosomatic medicine: research designs and statistical approaches

It has become increasingly clear that genetic factors influence many of the behaviors and disease endpoints of interest to psychosomatic medicine researchers. There has been increasing interest in incorporating genetic variation markers into psychosomatic research. In this Statistical Corner article, we build on the valuable experiences gained during two workshops for "starters in the field" at the American Psychosomatic Society and the Society for Psychophysiological Research to review two common genetically informative research designs for human studies: twin and genetic association studies. We outline statistical techniques for each and, for genetic association studies, address special topics, including the treatment of race and ethnicity, gene x gene and gene x environment interaction, haplotype analysis, and power and sample size. Finally, we discuss the issue of nonreplication and interpretation of results derived from genetic association studies. We hope this overview of twin and genetic association designs will support and stimulate thoughtful applications of genetic approaches within psychosomatic medicine.

Hostility and urine norepinephrine interact to predict insulin resistance: the VA Normative Aging Study

OBJECTIVE

Previous research has produced mixed results pertaining to the association between hostility and insulin resistance. These inconsistent findings may be the result of a lack of studies examining potential moderators of this relationship and inconsistent measures of insulin resistance and/or hostility. We hypothesized that hostility may interact with circulating norepinephrine (NEPI) levels, indexed by 24-hour urine concentrations, to affect insulin resistance.

METHODS

Six hundred forty-three men (mean age = 63.1 years) free of diabetic medications completed the Minnesota Multiphasic Personality Inventory and participated in a laboratory assessment. The Cook-Medley Hostility (Ho) and 24-hour urine NEPI were used to predict insulin resistance defined by the homeostatic model assessment (HOMA) index, 2-hour postchallenge glucose (PCGL), and insulin levels (PCIL) after controlling for nine common covariates.

RESULTS

Multiple regression showed that the two-way interaction between Ho and NEPI significantly predicted HOMA and PCIL, but not PCGL, after controlling for covariates. Simple regression slopes of Ho on HOMA and PCIL were explored and indicated that, at higher levels of NEPI, higher Ho was associated with higher HOMA (beta = 0.14, p < .05). Ho was not a significant predictor of HOMA at mean and lower levels of NEPI. Similar results were obtained for PCIL, but not PCGL. Cynicism, but not other subscales of Ho, was similarly related to insulin resistance and NEPI.

CONCLUSION

Individuals with high stress and high hostility were more likely to have insulin resistance. It is important to study moderators in the relationship between hostility and insulin resistance.

Common genetic vulnerability to depressive symptoms and coronary artery disease: a review and development of candidate genes related to inflammation and serotonin

OBJECTIVE

Although it is well established that depressive symptoms are associated with recurrent cardiac events among cardiac patients and novel cardiac events among participants with no known coronary artery disease (CAD), the nature of this association remains unclear. In this regard, little attention has been paid to the possibility that common genetic vulnerability contributes to both depressive symptoms and CAD. In this paper, we review the existing evidence for common genetic contributions to depression and CAD, primarily using evidence from twin and family studies, followed by a review of two major pathophysiological mechanisms thought to underlie covariation between depressive symptoms and CAD: inflammation and serotonin. We conclude with an overview of select candidate genes within these pathways.

METHODS

Literature review.

RESULTS

In twin studies, both depression and CAD appear heritable. In the only twin study to consider depression and CAD jointly, the correlation across heritabilities was 0.42, suggesting that nearly 20% of variability in depressive symptoms and CAD was attributable to common genetic factors. In addition, although it is plausible that genetic variation related to inflammation and serotonin may be associated with both depression and CAD, genetic variation related to inflammation has been primary examined in relation to CAD, whereas genetic variation in the serotonin system has been primarily examined in relation to depression.

CONCLUSIONS

It appears that the covariation of depressive symptoms and CAD may be attributable, in part, to a common genetic vulnerability. Although several pathways may be involved, genes within the inflammation and serotonin pathways may serve as good candidates for the first steps in identifying genetic variation important for depression, CAD or both.

Suppressed hostility predicted hypertension incidence among middle-aged men: the normative aging study

This study tested whether suppressed hostility predicted incident hypertension (HTN) in initially nonhypertensive men, using prospective data from the Normative Aging Study. Six hundred twenty-seven men who completed the MMPI and participated in a subsequent laboratory assessment were included in the study. The Cook-Medley Hostility scale (Ho), a suppression factor, and other risk factors were used to predict incident HTN over a 3-year period. Logistic regression showed a significant three-way interaction among Ho, suppression, and age in predicting incident HTN. Among middle-aged men (<or=60 years) with higher suppression, a 1-point decrease in the Ho score was associated with an 18% increase in HTN risk. This relationship remained significant after controlling for relevant risk factors. Among older participants, the HoxSuppression interaction was not predictive of HTN incidence. These results provide support for the Suppressed Hostility hypothesis in middle-aged men.

Dependence of Giardia lamblia encystation on novel transglutaminase activity

Earlier, we found that three protein disulfide isomerases (PDI) from Giardia lamblia (gPDI) also have transglutaminase (TGase) activity in vitro. We now show that differentiating Giardia cells contain isopeptide bonds (epsilon(gamma-glutamyl)lysine), the biological product of TGase activity that results in irreversible crosslinking of proteins in vivo. HPLC analyses showed the highest isopeptide bond content in cells encysting for 21 h, indicating an important role for TGase early in encystation. We were not able to detect isopeptide bonds in water-resistant cysts, possibly because they could not be extracted. One of the hallmarks of early encystation is the formation of encystation secretory vesicles (ESV) that transport nascent cyst wall proteins (CWPs) to the outer cell surface. ImmunoEM and live-cell immunofluorescence assays of encysting parasites revealed that gPDIs 1-3 are located in ESV and that gPDI-2 is also novel in that it is localized on the cell surface. Cystamine, a widely used TGase inhibitor, caused a dose-dependent inhibition of ESV formation by 21 h, thereby preventing development of trophozoites into cysts. Since cystamine (0.5-1 mM) inhibited the TGase activity of recombinant gPDIs 1-3 in vitro, PDIs appear to be the physiologic targets of cystamine. We found that when parasites were treated with cystamine, CWPs were not processed normally. These data suggest that TGase-catalyzed reactions may be needed for either the machinery that processes CWP precursors or their recruitment to ESV.

Anger-related personality traits and carotid artery atherosclerosis in untreated hypertensive men

OBJECTIVE

To determine whether anger-related personality traits are associated with carotid artery atherosclerosis in untreated hypertensive patients.

METHODS

Study participants were 237 men with elevated blood pressure (systolic 140-180 mm Hg and/or diastolic 90-110 mm Hg) but untreated for hypertension. Average age was 56 years; 80% of subjects were white. Eighty-six percent had no history of antihypertensive treatment, and the remainder reported median lifetime treatment exposure of 4 months. Subjects were administered the Spielberger State-Trait Anger Expression Inventory, which measures tendencies to experience anger (Trait Anger) and modes of anger expression (Anger-In, Anger-Out, Anger-Control). Mean and maximum intima-medial thickness (IMT) and plaque occurrence in the extracranial carotid arteries were measured by B-mode ultrasonography.

RESULTS

Trait Anger was marginally (p =.065) related to mean and significantly (p <.05) related to maximum IMT, independent of standard risk factors (age, race, body mass index, education, smoking, fasting glucose, total:high-density lipoprotein cholesterol ratio). A component of Trait Anger, Angry Temperament, similarly predicted mean (p =.062) and maximum IMT (p <.05) and plaque occurrence (p <.05). Anger-Out predicted both mean and maximum IMT (p values <.01).

CONCLUSIONS

An antagonistic disposition (Trait Anger), particularly a tendency to experience anger on minimal provocation (Angry Temperament) and a propensity to express anger outwardly (Anger-Out), are associated with heightened carotid atherosclerosis. These findings suggest that recently reported prospective associations between these anger dimensions and incident cerebrovascular disease may be mediated, in part, by increased atherosclerotic disease.

Intergenerational transmission of tobacco use and dependence: A transdisciplinary perspective

Numerous questions remain regarding the intergenerational transmission of tobacco use and dependence, and some of these questions are best approached from a transdisciplinary perspective. For example, considering both genetic and environmental influences on cigarette smoking promises to be a fruitful venue for future investigations. In this paper, we consider the evidence regarding intergenerational influences on the transmission of tobacco use and nicotine dependence in both humans and animal models; our focus will be on genetic influences, in utero exposure to nicotine, and some postnatal influences. Research gaps that exist between scientific disciplines are highlighted, and some directions for future research are suggested.

Allelic variation in the serotonin transporter gene-linked polymorphic region (5-HTTLPR) and cardiovascular reactivity in young adult male and female twins of European-American descent

OBJECTIVE

To examine the effect of length variation in the serotonin transporter gene-linked polymorphic region (5-HTTLPR) on individual differences in cardiovascular response to psychological challenge.

METHODS

Heart rate (HR) and systolic and diastolic blood pressure (SBP, DBP) responses to computerized versions of two psychological challenges, the Stroop Color-Word Interference Test and mental arithmetic, were measured among 131 monozygotic (MZ) and 60 dizygotic (DZ) male or female (same-sex) European-American twin pairs. Among the 382 participants, 140 were homozygous for the "long" allele (l/l) at 5-HTTLPR, 61 were homozygous for the "short" allele (s/s), and 181 participants had one long and one short allele (l/s). Association and sib-pair analyses were performed to characterize genetic associations.

RESULTS

In the full sample, 5-HTTLPR was associated with HR reactivity to psychological challenge, albeit in interaction with sex. Task-elicited HR responses of women homozygous for the short allele were significantly greater than among: a) men of the same genotype; and b) women having either one (l/s) or two (l/l) long alleles at 5-HTTLPR. SBP and DBP responsivity was unrelated to genotype. These results were corroborated on reanalysis in two genetically independent subsamples. Variability at 5-HTTLPR also predicted HR reactivity in sib-pair analyses among DZ twins.

CONCLUSIONS

These results suggest that the commonly observed sex difference in HR reactivity may be, in part, genetically mediated and perhaps occur only among individuals homozygous for the short allele at 5-HTTLPR.

Depressive symptoms and metabolic risk in adult male twins enrolled in the National Heart, Lung, and Blood Institute twin study

OBJECTIVE

To determine the extent to which depressive symptoms are associated with metabolic risk factors and whether genetic or environmental factors account for this association.

METHOD

Twin structural equation modeling was employed to estimate genetic and environmental contributions to the covariation of depressive symptoms, as indexed by the Centers for Epidemiological Studies-Depression Scale, and common variance among blood pressure, body mass index, waist-to-hip ratio, and serum triglycerides and glucose among 87 monozygotic and 86 dizygotic male twin pairs who participated in the NHLBI twin study.

RESULTS

Depressive symptoms were associated with individual components of the metabolic syndrome and common variance among the risk factors. Twin structural equation modeling indicated that the associations were attributable to environmental (nongenetic) factors.

CONCLUSIONS

These results support the hypothesis that depressive symptoms may increase risk for a pattern of physiological risk consistent with the metabolic syndrome.

Are metabolic risk factors one unified syndrome? Modeling the structure of the metabolic syndrome X

The metabolic syndrome, manifested by insulin resistance, obesity, dyslipidemia, and hypertension, is conceived to increase the risk for coronary heart disease and type II diabetes. Several studies have used factor analysis to explore its underlying structure among related risk variables but reported different results. Taking a hypothesis-testing approach, this study used confirmatory factor analysis to specify and test the factor structure of the metabolic syndrome. A hierarchical four-factor model, with an overarching metabolic syndrome factor uniting the insulin resistance, obesity, lipid, and blood pressure factors, was proposed and tested with 847 men who participated in the Normative Aging Study between 1987 and 1991. Simultaneous multi-group analyses were also conducted to test the stability of the proposed model across younger and older participants and across individuals with and without cardiovascular disease. The findings demonstrated that the proposed structure was well supported (comparative fit index = 0.97, root mean square error approximation = 0.06) and stable across subgroups. The metabolic syndrome was represented primarily by the insulin resistance and obesity factors, followed by the lipid factor, and, to a lesser extent, the blood pressure factor. This study provides an empirical foundation for conceptualizing and measuring the metabolic syndrome that unites four related components (insulin resistance, obesity, lipids, and blood pressure).

A study of depressive symptoms and smoking behavior in adult male twins from the NHLBI twin study

Self-report measures of depressive symptoms, such as the Center for Epidemiological Studies-Depression Scale (CES-D), correlate with current and lifetime smoking status. In one previous study of adult female twins, genetic factors accounted for the covariation of liability to a diagnosis of major depressive disorder and liability to lifetime smoking (Kendler, Neale, MacLean, Heath, Eaves, & Kessler, 1993b, Archives of General Psychiatry, 50, 36-43); however, it remained unclear whether genetic effects also account for the covariation between subclinical depressive symptomology and smoking behavior. In this study, we use twin structural equation modeling to explore whether genetic and/or environmental influences contribute to the covariation between depressive symptoms, as measured by the CES-D, and current and lifetime smoking status among 120 monozygotic and 114 dizygotic Caucasian male twin pairs (aged 59-69). In this sample, depressive symptoms showed small but significant correlations with current and lifetime smoking status. Univariate twin analyses indicated that additive genetic and non-shared environmental factors contributed significantly to liability to current and lifetime smoking. However, the majority of variance in CES-D scores was attributable to non-shared (individual) environment. In bivariate analyses, non-shared environmental factors accounted for the majority of covariation between liability to depressive symptoms (CES-D scores > or = 8; above the 75th percentile) and liability to current and lifetime smoking status. Taken together with the previous literature, these results suggest that the etiology of covariation among depressive symptoms and smoking behavior may vary by measurement and severity of depressive symptomology.

Is cardiovascular reactivity associated with atherosclerosis among hypertensives?

Exaggerated cardiovascular reactivity to behavioral challenges among otherwise healthy individuals has been associated with carotid atherosclerosis. We evaluated whether a similar relationship exists among hypertensives, who are at a heightened atherosclerotic risk. Untreated, hypertensive men (n=251; age range, 40 to 70 years; 197 white, 54 black) completed a standardized battery of behavioral challenges while their blood pressure responses to the battery were measured. Mean and maximum carotid intima-media thickness and the occurrence of carotid plaques were subsequently determined using B-mode ultrasonography. Although greater systolic and diastolic responses to the battery were associated with greater mean and maximum intima-media thickness in univariate analyses (P<0.01), only diastolic reactivity showed a unique association with mean and maximum carotid intima-media thickness after multivariate adjustment for age, race, socioeconomic status, smoking and alcohol use, body mass index, lipid profile, glucose and insulin concentrations, and resting blood pressure (P<0.05). Carotid plaque occurrence was associated with greater systolic reactivity (P=0.05) and was marginally associated with greater diastolic reactivity (P=0.07) in univariate analyses, but neither systolic nor diastolic reactivity was uniquely associated with the presence of carotid plaques after multivariate risk-factor adjustment. Among hypertensives, exaggerated behaviorally evoked cardiovascular reactivity appears to be uniquely associated with greater carotid intima-media thickness but not with carotid plaque occurrence.

Variability within alpha- and beta-adrenoreceptor genes as a predictor of cardiovascular function at rest and in response to mental challenge

OBJECTIVES

To investigate the association between polymorphic variation in alpha- and beta-adrenoreceptor genes and cardiovascular activity at rest and in response to psychological challenge in a sample in which the heritability of these cardiovascular phenotypes may be established.

METHODS

Several common polymorphisms were characterized within ADRA1B (alpha1B), ADRA2A (alpha2A), ADRB1 (beta1) and ADRB2 (beta2) and examined in relation to heart rate (HR) and systolic (SBP) and diastolic (DBP) blood pressure, both at rest and in response to stress. Participants were 309 European-American, young adult men and women (including 101 monozygotic and 44 dizygotic twin pairs).

RESULTS

In the full sample, participants carrying any G allele at base pair (bp) 1165 in ADRB1 exhibited elevated resting SBP and DBP and a larger DBP response to mental challenge compared to homozygotes for the C allele (P < 0.04). An AA genotype at bp 145 in ADRB1 was also associated with higher resting SBP and DBP than AG or GG genotypes (P < 0.03). At bp 46 in ADRB2, GG homozygotes had higher resting DBP than subjects possessing any A allele (P < 0.05). For the same polymorphism, however, AG heterozygotes showed lower SBP than both AA and GG homozygotes (P < 0.05). In a subsample of genetically unrelated individuals, ADRB1 (1165) continued to predict resting SBP, DBP and DBP response to stress (P < 0.03), while ADRB2 (46) was associated with resting SBP (P < 0.04) but not DBP. Finally, the degree of allele sharing at ADRB1 (1165) also predicted variability in SBP and DBP at rest among dizygotic twin pairs (P < 0.04).

CONCLUSIONS

These results indicate that some polymorphic variation within adrenoreceptor genes contributes to interindividual variability in resting SBP and DBP and in DBP response to mental challenge.

Covariation of psychosocial characteristics associated with cardiovascular disease: genetic and environmental influences

OBJECTIVE

Three psychosocial characteristics associated with cardiovascular disease (CVD)-depression, hostility, and social support-tend to correlate with one another. However, the causes of each characteristic and why they tend to co-occur are not completely understood. Therefore, the current study used a twin design to examine the relative contributions of genetic and environmental influences to the variation and covariation of these three psychosocial characteristics.

METHODS

The sources of variation and covariation among the Beck Depression Inventory, the Cook-Medley Hostility Scale, and the Interpersonal Support Evaluation List were examined in a young adult community sample of 157 monozygotic and 75 dizygotic twin pairs.

RESULTS

Phenotypic confirmatory factor analysis indicated that a single latent factor could account for their moderate intercorrelations. Twin analyses indicated that the Beck Depression Inventory and Interpersonal Support Evaluation List were each influenced by genetic and nonshared environmental factors, whereas the Cook-Medley Hostility Scale was influenced by familial (genetic and/or shared environmental) and nonshared environmental factors. Bivariate associations between these scales were largely determined by common genetic effects and, to a lesser degree, common nonshared environmental effects. Covariation among the three scales could be explained by a single common genetic factor and a common nonshared environmental factor. Environmental factors shared within families did not contribute to covariation among the psychosocial characteristics.

CONCLUSIONS

The results challenge the conventional approach of examining these psychosocial variables as independent risk factors for cardiovascular disease and argue for the importance of investigating specific causes for their covariation.

The nature of the association between diet and serum lipids in the community: a twin study

Diet is commonly thought to be an environmental influence on serum lipid concentrations. This study evaluated whether total caloric and fat intake predict total cholesterol (TC), low-density lipoprotein cholesterol (LDL), high-density lipoprotein cholesterol (HDL), and triglyceride (TRIG) concentrations for environmental, as compared with genetic, reasons among 137 monozygotic and 67 dizygotic young adult twins. When genetic influences were controlled by correlating differences between monozygotic co-twins, a significant association remained between diet and TC, LDL, and HDL, suggesting that these dietary and serum lipid measures correlate for environmental reasons. Twin structural equation modeling confirmed these results. Overall, these results provide additional support for the hypothesis that diet is an environmental influence on TC, LDL, and HDL.

A null mutation in the gene encoding the herpes simplex virus type 1 UL37 polypeptide abrogates virus maturation

The tegument is an integral and essential structural component of the herpes simplex virus type 1 (HSV-1) virion. The UL37 open reading frame of HSV-1 encodes a 120-kDa virion polypeptide which is a resident of the tegument. To analyze the function of the UL37-encoded polypeptide a null mutation was generated in the gene encoding this protein. In order to propagate this mutant virus, transformed cell lines that express the UL37 gene product in trans were produced. The null mutation was transferred into the virus genome using these complementing cell lines. A mutant virus designated KDeltaUL37 was isolated based on its ability to form plaques on the complementing cell line but not on nonpermissive (noncomplementing) Vero cells. This virus was unable to grow in Vero cells; therefore, UL37 encodes an essential function of the virus. The mutant virus KDeltaUL37 produced capsids containing DNA as judged by sedimentation analysis of extracts derived from infected Vero cells. Therefore, the UL37 gene product is not required for DNA cleavage or packaging. The UL37 mutant capsids were tagged with the smallest capsid protein, VP26, fused to green fluorescent protein. This fusion protein decorates the capsid shell and consequently the location of the capsid and the virus particle can be visualized in living cells. Late in infection, KDeltaUL37 capsids were observed to accumulate at the periphery of the nucleus as judged by the concentration of fluorescence around this organelle. Fluorescence was also observed in the cytoplasm in large puncta. Fluorescence at the plasma membrane, which indicated maturation and egress of virions, was observed in wild-type-infected cells but was absent in KDeltaUL37-infected cells. Ultrastructural analysis of thin sections of infected cells revealed clusters of DNA-containing capsids in the proximity of the inner nuclear membrane. Occasionally enveloped capsids were observed between the inner and outer nuclear membranes. Clusters of unenveloped capsids were also observed in the cytoplasm of KDeltaUL37-infected cells. Enveloped virions, which were observed in the cytoplasm of wild-type-infected cells, were never detected in the cytoplasm of KDeltaUL37-infected cells. Crude cell fractionation of infected cells using detergent lysis demonstrated that two-thirds of the UL37 mutant particles were associated with the nuclear fraction, unlike wild-type particles, which were predominantly in the cytoplasmic fraction. These data suggest that in the absence of UL37, the exit of capsids from the nucleus is slowed. UL37 mutant particles can participate in the initial envelopment at the nuclear membrane, although this process may be impaired in the absence of UL37. Furthermore, the naked capsids deposited in the cytoplasm are unable to progress further in the morphogenesis pathway, which suggests that UL37 is also required for egress and reenvelopment. Therefore, the UL37 gene product plays a key role in the early stages of the maturation pathway that give rise to an infectious virion.

The nature of the association between diet and serum lipids in the community: a twin study

Diet is commonly thought to be an environmental influence on serum lipid concentrations. This study evaluated whether total caloric and fat intake predict total cholesterol (TC), low-density lipoprotein cholesterol (LDL), high-density lipoprotein cholesterol (HDL), and triglyceride (TRIG) concentrations for environmental, as compared with genetic, reasons among 137 monozygotic and 67 dizygotic young adult twins. When genetic influences were controlled by correlating differences between monozygotic co-twins, a significant association remained between diet and TC, LDL, and HDL, suggesting that these dietary and serum lipid measures correlate for environmental reasons. Twin structural equation modeling confirmed these results. Overall, these results provide additional support for the hypothesis that diet is an environmental influence on TC, LDL, and HDL.

Micromechanics and ultrastructure of actin filament networks crosslinked by human fascin: a comparison with alpha-actinin

Fascin is an actin crosslinking protein that organizes actin filaments into tightly packed bundles believed to mediate the formation of cellular protrusions and to provide mechanical support to stress fibers. Using quantitative rheological methods, we studied the evolution of the mechanical behavior of filamentous actin (F-actin) networks assembled in the presence of human fascin. The mechanical properties of F-actin/fascin networks were directly compared with those formed by alpha-actinin, a prototypical actin filament crosslinking/bundling protein. Gelation of F-actin networks in the presence of fascin (fascin to actin molar ratio >1:50) exhibits a non-monotonic behavior characterized by a burst of elasticity followed by a slow decline over time. Moreover, the rate of gelation shows a non-monotonic dependence on fascin concentration. In contrast, alpha-actinin increased the F-actin network elasticity and the rate of gelation monotonically. Time-resolved multiple-angle light scattering and confocal and electron microscopies suggest that this unique behavior is due to competition between fascin-mediated crosslinking and side-branching of actin filaments and bundles, on the one hand, and delayed actin assembly and enhanced network micro-heterogeneity, on the other hand. The behavior of F-actin/fascin solutions under oscillatory shear of different frequencies, which mimics the cell's response to forces applied at different rates, supports a key role for fascin-mediated F-actin side-branching. F-actin side-branching promotes the formation of interconnected networks, which completely inhibits the motion of actin filaments and bundles. Our results therefore show that despite sharing seemingly similar F-actin crosslinking/bundling activity, alpha-actinin and fascin display completely different mechanical behavior. When viewed in the context of recent microrheological measurements in living cells, these results provide the basis for understanding the synergy between multiple crosslinking proteins, and in particular the complementary mechanical roles of fascin and alpha-actinin in vivo.

Reversible interruption of Giardia lamblia cyst wall protein transport in a novel regulated secretory pathway

To survive in the environment and infect a new host, Giardia lamblia secretes an extracellular cyst wall using a poorly understood pathway. The two cyst wall proteins (CWPs) form disulphide-bonded heterodimers and are exported via novel encystation-specific secretory vesicles (ESVs). Exposure of eukaryotic cells to dithiothreitol (DTT) blocks the formation of disulphide bonds in nascent proteins that accumulate in the endoplasmic reticulum (ER) and induces an unfolded protein response (UPR). Proteins that have exited the ER are not susceptible. Exposure to DTT inhibits ESV formation by > 85%. Addition of DTT to encysting cells causes rapid (t1/2 < 10 min), reversible disappearance of ESVs, correlated with reduction of CWPs to monomers and reformation of CWP oligomers upon removal of DTT. Neither CWPs nor ESVs are affected by mercaptoethanesulphonic acid, a strong reducing agent that does not penetrate cells. DTT does not inhibit the overall protein secretory pathway, and recovery does not require new protein synthesis. We found evidence of protein disulphide isomerases in the ESV and the surface of encysting cells, in which they may catalyse initial CWP folding and recovery from DTT. This is the first suggestion of non-CWP proteins in ESVs and of enzymes on the giardial surface. DTT treatment did not stimulate a UPR, suggesting that Giardia may have diverged before the advent of this conserved form of ER quality control.

Localization of the Rsp5p ubiquitin-protein ligase at multiple sites within the endocytic pathway

The Saccharomyces cerevisiae RSP5 gene encodes an essential HECT E3 ubiquitin-protein ligase. Rsp5p contains an N-terminal C2 domain, three WW domains in the central portion of the molecule, and a C-terminal catalytic HECT domain. A diverse group of substrates of Rsp5p and vertebrate C2 WW-domain-containing HECT E3s have been identified, including both nuclear and membrane-associated proteins. We determined the intracellular localization of Rsp5p and the determinants necessary for localization, in order to better understand how Rsp5p activities are coordinated. Using both green fluorescent protein fusions to Rsp5p and immunogold electron microscopy, we found that Rsp5p was distributed in a punctate pattern at the plasma membrane, corresponding to membrane invaginations that are likely sites of endosome formation, as well as at perivacuolar sites. The latter appeared to correspond to endocytic intermediates, as these structures were not seen in a sla2/end4-1 mutant, and double-immunogold labeling demonstrated colocalization of Rsp5p with the endosomal markers Pep12p and Vps32p. The C2 domain was an important determinant of localization; however, mutations that disrupted HECT domain function also caused mislocalization of Rsp5p, indicating that enzymatic activity is linked to localization. Deletion of the C2 domain partially stabilized Fur4p, a protein previously shown to undergo Rsp5p- and ubiquitin-mediated endocytosis; however, Fur4p was still ubiquitinated at the plasma membrane when the C2 domain was deleted from the protein. Together, these results indicate that Rsp5p is located at multiple sites within the endocytic pathway and suggest that Rsp5p may function at multiple steps in the ubiquitin-mediated endocytosis pathway.

Division of mitochondria requires a novel DNM1-interacting protein, Net2p

Mitochondria are dynamic organelles that undergo frequent division and fusion, but the molecular mechanisms of these two events are not well understood. Dnm1p, a mitochondria-associated, dynamin-related GTPase was previously shown to mediate mitochondrial fission. Recently, a genome-wide yeast two-hybrid screen identified an uncharacterized protein that interacts with Dnm1p. Cells disrupted in this new gene, which we call NET2, contain a single mitochondrion that consists of a network formed by interconnected tubules, similar to the phenotype of dnm1 Delta cells. NET2 encodes a mitochondria-associated protein with a predicted coiled-coil region and six WD-40 repeats. Immunofluorescence microscopy indicates that Net2p is located in distinct, dot-like structures along the mitochondrial surface, many of which colocalize with the Dnm1 protein. Fluorescence and immunoelectron microscopy shows that Dnm1p and Net2p preferentially colocalize at constriction sites along mitochondrial tubules. Our results suggest that Net2p is a new component of the mitochondrial division machinery.

A yeast DNA J protein required for uncoating of clathrin-coated vesicles in vivo

Clathrin-coated vesicles mediate diverse processes such as nutrient uptake, downregulation of hormone receptors, formation of synaptic vesicles, virus entry, and transport of biosynthetic proteins to lysosomes. Cycles of coat assembly and disassembly are integral features of clathrin-mediated vesicular transport (Fig. 1a). Coat assembly involves recruitment of clathrin triskelia, adaptor complexes and other factors that influence coat assembly, cargo sequestration, membrane invagination and scission (Fig. 1a). Coat disassembly is thought to be essential for fusion of vesicles with target membranes and for recycling components of clathrin coats to the cytoplasm for further rounds of vesicle formation. In vitro, cytosolic heat-shock protein 70 (Hsp70) and the J-domain co-chaperone auxilin catalyse coat disassembly. However, a specific function of these factors in uncoating in vivo has not been demonstrated, leaving the physiological mechanism and significance of uncoating unclear. Here we report the identification and characterization of a Saccharomyces cerevisiae J-domain protein, Aux1. Inactivation of Aux1 results in accumulation of clathrin-coated vesicles, impaired cargo delivery, and an increased ratio of vesicle-associated to cytoplasmic clathrin. Our results demonstrate an in vivo uncoating function of a J domain co-chaperone and establish the physiological significance of uncoating in transport mediated by clathrin-coated vesicles.

The dynamin-related GTPase, Mgm1p, is an intermembrane space protein required for maintenance of fusion competent mitochondria

Mutations in the dynamin-related GTPase, Mgm1p, have been shown to cause mitochondrial aggregation and mitochondrial DNA loss in Saccharomyces cerevisiae cells, but Mgm1p's exact role in mitochondrial maintenance is unclear. To study the primary function of MGM1, we characterized new temperature sensitive MGM1 alleles. Examination of mitochondrial morphology in mgm1 cells indicates that fragmentation of mitochondrial reticuli is the primary phenotype associated with loss of MGM1 function, with secondary aggregation of mitochondrial fragments. This mgm1 phenotype is identical to that observed in cells with a conditional mutation in FZO1, which encodes a transmembrane GTPase required for mitochondrial fusion, raising the possibility that Mgm1p is also required for fusion. Consistent with this idea, mitochondrial fusion is blocked in mgm1 cells during mating, and deletion of DNM1, which encodes a dynamin-related GTPase required for mitochondrial fission, blocks mitochondrial fragmentation in mgm1 cells. However, in contrast to fzo1 cells, deletion of DNM1 in mgm1 cells restores mitochondrial fusion during mating. This last observation indicates that despite the phenotypic similarities observed between mgm1 and fzo1 cells, MGM1 does not play a direct role in mitochondrial fusion. Although Mgm1p was recently reported to localize to the mitochondrial outer membrane, our studies indicate that Mgm1p is localized to the mitochondrial intermembrane space. Based on our localization data and Mgm1p's structural homology to dynamin, we postulate that it functions in inner membrane remodeling events. In this context, the observed mgm1 phenotypes suggest that inner and outer membrane fission is coupled and that loss of MGM1 function may stimulate Dnm1p-dependent outer membrane fission, resulting in the formation of mitochondrial fragments that are structurally incompetent for fusion.

Dnm1p GTPase-mediated mitochondrial fission is a multi-step process requiring the novel integral membrane component Fis1p

Yeast Dnm1p is a soluble, dynamin-related GTPase that assembles on the outer mitochondrial membrane at sites where organelle division occurs. Although these Dnm1p-containing complexes are thought to trigger constriction and fission, little is known about their composition and assembly, and molecules required for their membrane recruitment have not been isolated. Using a genetic approach, we identified two new genes in the fission pathway, FIS1 and FIS2. FIS1 encodes a novel, outer mitochondrial membrane protein with its amino terminus exposed to the cytoplasm. Fis1p is the first integral membrane protein shown to participate in a eukaryotic membrane fission event. In a related study (Tieu, Q., and J. Nunnari. 2000. J. Cell Biol. 151:353-365), it was shown that the FIS2 gene product (called Mdv1p) colocalizes with Dnm1p on mitochondria. Genetic and morphological evidence indicate that Fis1p, but not Mdv1p, function is required for the proper assembly and distribution of Dnm1p-containing fission complexes on mitochondrial tubules. We propose that mitochondrial fission in yeast is a multi-step process, and that membrane-bound Fis1p is required for the proper assembly, membrane distribution, and function of Dnm1p-containing complexes during fission.

E3-13.7 integral membrane proteins encoded by human adenoviruses alter epidermal growth factor receptor trafficking by interacting directly with receptors in early endosomes

Animal cell viruses provide valuable model systems for studying many normal cellular processes, including membrane protein sorting. The focus of this study is an integral membrane protein encoded by the E3 transcription region of human adenoviruses called E3-13.7, which diverts recycling EGF receptors to lysosomes without increasing the rate of receptor internalization or intrinsic receptor tyrosine kinase activity. Although E3-13.7 can be found on the plasma membrane when it is overexpressed, its effect on EGF receptor trafficking suggests that the plasma membrane is not its primary site of action. Using cell fractionation and immunocytochemical experimental approaches, we now report that the viral protein is located predominantly in early endosomes and limiting membranes of endosome-to-lysosome transport intermediates called multivesicular endosomes. We also demonstrate that E3-13.7 physically associates with EGF receptors undergoing E3-13.7-mediated down-regulation in early endosomes. Receptor-viral protein complexes then dissociate, and EGF receptors proceed to lysosomes, where they are degraded, while E3-13.7 is retained in endosomes. We conclude that E3-13.7 is a resident early endocytic protein independent of EGF receptor expression, because it has identical intracellular localization in mouse cells lacking endogenous receptors and cells expressing a human cytomegalovirus-driven receptor cDNA. Finally, we demonstrate that EGF receptor residues 675-697 are required for E3-13.7-mediated down-regulation. Interestingly, this sequence includes a known EGF receptor leucine-based lysosomal sorting signal used during ligand-induced trafficking, which is also conserved in the viral protein. E3-13.7, therefore, provides a novel model system for determining the molecular basis of selective membrane protein transport in the endocytic pathway. Our studies also suggest new paradigms for understanding EGF receptor sorting in endosomes and adenovirus pathogenesis.

The peroxisome biogenesis factors pex4p, pex22p, pex1p, and pex6p act in the terminal steps of peroxisomal matrix protein import

Peroxisomes are independent organelles found in virtually all eukaryotic cells. Genetic studies have identified more than 20 PEX genes that are required for peroxisome biogenesis. The role of most PEX gene products, peroxins, remains to be determined, but a variety of studies have established that Pex5p binds the type 1 peroxisomal targeting signal and is the import receptor for most newly synthesized peroxisomal matrix proteins. The steady-state abundance of Pex5p is unaffected in most pex mutants of the yeast Pichia pastoris but is severely reduced in pex4 and pex22 mutants and moderately reduced in pex1 and pex6 mutants. We used these subphenotypes to determine the epistatic relationships among several groups of pex mutants. Our results demonstrate that Pex4p acts after the peroxisome membrane synthesis factor Pex3p, the Pex5p docking factors Pex13p and Pex14p, the matrix protein import factors Pex8p, Pex10p, and Pex12p, and two other peroxins, Pex2p and Pex17p. Pex22p and the interacting AAA ATPases Pex1p and Pex6p were also found to act after Pex10p. Furthermore, Pex1p and Pex6p were found to act upstream of Pex4p and Pex22p. These results suggest that Pex1p, Pex4p, Pex6p, and Pex22p act late in peroxisomal matrix protein import, after matrix protein translocation. This hypothesis is supported by the phenotypes of the corresponding mutant strains. As has been shown previously for P. pastoris pex1, pex6, and pex22 mutant cells, we show here that pex4Delta mutant cells contain peroxisomal membrane protein-containing peroxisomes that import residual amounts of peroxisomal matrix proteins.

Organelle pH studies using targeted avidin and fluorescein-biotin

BACKGROUND

Mammalian organelles of the secretory pathway are of differing pH. The pH values form a decreasing gradient: the endoplasmic reticulum (ER) is nearly neutral, the Golgi is mildly acidic and the secretory granules are more acidic still ( approximately pH 5). The mechanisms that regulate pH in these organelles are still unknown.

RESULTS

Using a novel method, we tested whether differences in H(+) 'leak' and/or counterion conductances contributed to the pH difference between two secretory pathway organelles. A pH-sensitive, membrane-permeable fluorescein-biotin was targeted to endoplasmic-reticulum- and Golgi-localized avidin-chimera proteins in HeLa cells. In live, intact cells, ER pH (pH(ER)) was 7.2 +/- 0.2 and Golgi pH (pH(G)) was 6.4 +/- 0.3 and was dissipated by bafilomycin. Buffer capacities of the cytosol, ER and Golgi were all similar (6-10 mM/pH). ER membranes had an apparent H(+) permeability three times greater than that of Golgi membranes. Removal of either K(+) or Cl(-) did not affect ER and Golgi H(+) leak rates, or steady-state pH(G) and pH(ER).

CONCLUSIONS

The Golgi is more acidic than the ER because it has an active H(+) pump and fewer or smaller H(+) leaks. Neither buffer capacity nor counterion permeabilities were key determinants of pH(G), pH(ER) or ER/Golgi H(+) leak rates.

Behaviorally-evoked plasma catecholamine response and 24-hour excretion of urinary catecholamines among cardiac and vascular reactors

Individuals differ in the cardiac and vascular processes that underlie blood pressure elevations evoked by environmental stimuli; such differences may reflect variability in sympathoadrenal response. We separated 108 healthy, young-adult males into those with predominant elevations in either cardiac output or peripheral resistance when exposed to psychological challenges. We then asked if they differed on other measures of cardiovascular response, concomitant plasma catecholamine reactions or 24-h urinary excretion of catecholamines. Cardiac reactors, relative to vascular reactors, showed reduced cardiac pre-ejection period, a smaller reduction in stroke volume, and elevated plasma epinephrine response and 24-h urinary epinephrine excretion. Vascular reactors, relative to cardiac reactors, responded to mental stress with more elevated diastolic blood pressure, a rise in peripheral resistance and pulse wave velocity, and a greater reduction in stroke volume. Vascular reactors, however, did not show plasma norepinephrine response or 24-h urinary norepinephrine excretion that was greater than cardiac reactors. The results provide partial support for the hypothesis that variability in sympathoadrenal activity contributes to individual differences in cardiac and vascular reactivity, and extend prior observations by demonstrating covariation of behaviorally-elicited cardiac reactivity with the 24-h excretion of epinephrine.

Genetic and environmental causes of covariation among blood pressure, body mass and serum lipids during young adulthood: a twin study

OBJECTIVE

To determine the extent to which the correlation of systolic blood pressure, diastolic blood pressure, body mass, fasting total cholesterol and fasting triglycerides in young adulthood reflects common genetic or environmental influences.

DESIGN

Cardiovascular risk factors were measured in a community sample of 129 monozygotic and 67 dizygotic twin pairs, ages 18-30 years.

METHODS

Multivariate twin structural equation modelling allows estimation of the extent to which the covariation of two or more variables is attributable to common genetic and environmental factors and was used to analyse the correlation among systolic blood pressure, diastolic blood pressure, body mass index, fasting total cholesterol and triglycerides.

RESULTS

The covariation of risk factors was partially attributable to a single common genetic factor, while the covariation of systolic blood pressure, body mass index and triglycerides was also, in part, attributable to a common non-shared environmental factor.

CONCLUSIONS

Genetic and, to a lesser extent, non-shared environmental factors contribute to the covariation of cardiovascular risk factors in young adult twins. Nonetheless, it should be noted that these common influences account for a relatively small percentage of the variance in each risk factor compared to genetic and environmental factors that are risk factor-specific.

Novel protein-disulfide isomerases from the early-diverging protist Giardia lamblia

Protein-disulfide isomerase is essential for formation and reshuffling of disulfide bonds during nascent protein folding in the endoplasmic reticulum. The two thioredoxin-like active sites catalyze a variety of thiol-disulfide exchange reactions. We have characterized three novel protein-disulfide isomerases from the primitive eukaryote Giardia lamblia. Unlike other protein-disulfide isomerases, the giardial enzymes have only one active site. The active-site sequence motif in the giardial proteins (CGHC) is characteristic of eukaryotic protein-disulfide isomerases, and not other members of the thioredoxin superfamily that have one active site, such as thioredoxin and Dsb proteins from Gram-negative bacteria. The three giardial proteins have very different amino acid sequences and molecular masses (26, 50, and 13 kDa). All three enzymes were capable of rearranging disulfide bonds, and giardial protein-disulfide isomerase-2 also displayed oxidant and reductant activities. Surprisingly, the three giardial proteins also had Ca(2+)-dependent transglutaminase activity. This is the first report of protein-disulfide isomerases with a single active site that have diverse roles in protein cross-linking. This study may provide clues to the evolution of key functions of the endoplasmic reticulum in eukaryotic cells, protein disulfide formation, and isomerization.

Clathrin-coated vesicles bearing GAIP possess GTPase-activating protein activity in vitro

Galpha-interacting protein (GAIP) is a member of the RGS (regulators of G protein signaling) family, which serve as GAPs (GTPase-activating proteins) for Galpha subunits. Previously, we demonstrated that GAIP is localized on clathrin-coated vesicles (CCVs). Here, we tested whether GAIP-enriched vesicles could accelerate the GTPase activity of Galphai proteins. A rat liver fraction containing vesicular carriers (CV2) was enriched (4.5x) for GAIP by quantitative immunoblotting, and GAIP was detected on some of the vesicles in the CV2 fraction by immunoelectron microscopy. When liver fractions were added to recombinant Galphai3 and tested for GAP activity, only the CV2 fraction contained GAP activity. Increasing amounts of CV2 increased the activity, whereas immunodepletion of the CV2 fraction with an antibody against the C terminus of GAIP decreased GAP activity. CCV fractions were prepared from rat liver by using a protocol that maintains the clathrin coats. GAIP was enriched in these fractions and was detected on CCVs by immunogold labeling. Addition of increasing amounts of CCV to recombinant Galphai3 protein increased the GTPase activity. We conclude that CCVs possess GAP activity for Galphai3 and that membrane-associated GAIP is capable of interacting with Galphai3. The reconstitution of the interaction between a heterotrimeric G protein and GAIP on CCVs provides biochemical evidence for a model whereby the G protein and its GAP are compartmentalized on different membranes and come into contact at the time of vesicle fusion. Alternatively, they may be located on the same membrane and segregate at the time of vesicle budding.

Intracellular localization and membrane topology of 11-cis retinol dehydrogenase in the retinal pigment epithelium suggest a compartmentalized synthesis of 11-cis retinaldehyde

11-cis retinol dehydrogenase (EC 1.1.1.105) catalyses the last step in the biosynthetic pathway generating 11-cis retinaldehyde, the common chromophore of all visual pigments in higher animals. The enzyme is abundantly expressed in retinal pigment epithelium of the eye and is a member of the short chain dehydrogenase/reductase superfamily. In this work we demonstrate that a majority of 11-cis retinol dehydrogenase is associated with the smooth ER in retinal pigment epithelial cells and that the enzyme is an integral membrane protein, anchored to membranes by two hydrophobic peptide segments. The catalytic domain of the enzyme is confined to a lumenal compartment and is not present on the cytosolic aspect of membranes. Thus, the subcellular localization and the membrane topology of 11-cis retinol dehydrogenase suggest that generation of 11-cis retinaldehyde is a compartmentalized process.

A signal recognition particle receptor gene from the early-diverging eukaryote, Giardia lamblia

The molecular mechanisms for targeting and translocation of secreted proteins are highly conserved from bacteria to mammalian cells, although the machinery is more complex in higher eukaryotes. To investigate protein transport in the early-diverging eukaryote, Giardia lamblia, we cloned the gene encoding the alpha subunit (SRalpha) of the signal recognition particle (SRP) receptor. SRalpha is a small GTPase that functions in SRP-ribosome targeting to the ER. Sequence and phylogenetic analyses showed that SRalpha from G. lamblia is most homologous to SRalpha proteins from higher eukaryotes, although it lacks some conserved motifs. Specifically, giardial SRalpha has an N-terminal extension that enables SRalpha of higher eukaryotes to interact with a beta subunit that anchors it in the ER membrane. While the C-terminal regions are similar, giardial SRalpha lacks a prominent 13 amino acid regulatory loop that is characteristic of higher eukaryotic versions. Thus, giardial SRalpha resembles that of higher eukaryotes, but likely diverged before the advent of the regulatory loop. The 1.8 kb SRalpha transcript has extremely short untranslated regions (UTRs): a 1-2 nt 5'- and a 9 nt 3' UTR with the polyadenylation signal overlapping with the stop codon. RT-PCR, Northern and Western analyses showed that SRalpha is present at relatively constant levels during vegetative growth and encystation, even though there are extensive changes in endomembrane structures and secretory activity during encystation. Imnuno-EM showed that SRalpha localizes to ER-like structures, strengthening the observation of a typical ER in G. lamlia. Unexpectedly, SRalpha was also found in the lysosome-like peripheral vacuoles, suggesting unusual protein traffic in this early eukaryote. Our results indicate that the eukaryotic type of cotranslational transport appeared early in the evolution of the eukaryotic cell.

Differentiation-associated surface antigen variation in the ancient eukaryote Giardia lamblia

Encystation of Giardia lamblia is required for survival outside the host, whereas excystation initiates infection. The dormant cyst was considered an adaptation to external survival and passage through the stomach. However, we found previously that trophozoites which had recovered after completion of the life cycle had switched their major variant surface protein (VSP), called TSA 417, but neither the timing nor the molecular mechanism of switching had been elucidated. Here we demonstrate that TSA 417 predominates in cysts, but is downregulated during the stage of excystation that models cyst arrival in the small intestine. Transcripts of new VSPs appear late in encystation, and during and after excystation. Trophozoites appear to prepare for switching during encystation, when the major VSP on the cell surface diminishes and is internalized in lysosome-like vacuoles. As short-range DNA rearrangements were not detected, giardial VSP switching during differentiation appears to resemble the in situ switching of surface glycoproteins in African trypanosomes. We also report a unique extended 15 nucleotide polyadenylation signal in all VSP transcripts, but not in other known giardial genes. Antigenic variation during encystation-excystation may be a novel form of immune evasion that could help explain the common occurrence of reinfection by Giardia and other parasites with similar life cycles.

Aggression, impulsivity, and central nervous system serotonergic responsivity in a nonpatient sample

To test the hypothesis that traits of aggression and impulsivity correlate negatively with central serotonergic system function in a nonpatient population, a standard fenfluramine challenge (for assessment of serotonergic responsivity) and behavioral measurements germane to aggression/impulsivity were administered to a community-derived sample of 119 men and women. In men, peak prolactin responses to fenfluramine correlated significantly with an interview-assessed life history of aggression (r = -.40, p < .002), the Barratt Impulsiveness Scale (r = -.30, p < .03), and traits of Conscientiousness (r = +.30, p < .03), Neuroticism (r = -.31, p < .02) and Angry Hostility (r = -.35, p < .01) on the NEO-Personality Inventory. No significant relationships were observed across all women, although subanalyses restricted to postmenopausal subjects (in whom ovarian influences on prolactin secretion may be mitigated because of diminished estrogen) showed a pattern of behavioral associations somewhat similar to that seen in men. By extending documented relationships between an index of central serotonergic system function and traits of aggression and impulsivity to a more normative range of population variability than is represented in prior literature, this study supports speculation that these associations reflect a basic neurobehavioral dimension of individual differences.

A membrane coat complex essential for endosome-to-Golgi retrograde transport in yeast

We have recently characterized three yeast gene products (Vps35p, Vps29p, and Vps30p) as candidate components of the sorting machinery required for the endosome-to-Golgi retrieval of the vacuolar protein sorting receptor Vps10p (Seaman, M.N.J., E.G. Marcusson, J.-L. Cereghino, and S.D. Emr. 1997. J. Cell Biol. 137:79-92). By genetic and biochemical means we now show that Vps35p and Vps29p interact and form part of a multimeric membrane-associated complex that also contains Vps26p, Vps17p, and Vps5p. This complex, designated here as the retromer complex, assembles from two distinct subcomplexes comprising (a) Vps35p, Vps29p, and Vps26p; and (b) Vps5p and Vps17p. Density gradient fractionation of Golgi/endosomal/vesicular membranes reveals that Vps35p cofractionates with Vps5p/Vps17p in a vesicle-enriched dense membrane fraction. Furthermore, gel filtration analysis indicates that Vps35p and Vps5p are present on a population of vesicles and tubules slightly larger than COPI/coatomer-coated vesicles. We also show by immunogold EM that Vps5p is localized to discrete regions at the rims of the prevacuolar endosome where vesicles appear to be budding. Size fractionation of cytosolic and recombinant Vps5p reveals that Vps5p can self-assemble in vitro, suggesting that Vps5p may provide the mechanical impetus to drive vesicle formation. Based on these findings we propose a model in which Vps35p/Vps29p/Vps26p function to select cargo for retrieval, and Vps5p/Vps17p assemble onto the membrane to promote vesicle formation. Conservation of the yeast retromer complex components in higher eukaryotes suggests an important general role for this complex in endosome-to-Golgi retrieval.

The mammalian calcium-binding protein, nucleobindin (CALNUC), is a Golgi resident protein

We have identified CALNUC, an EF-hand, Ca2+-binding protein, as a Golgi resident protein. CALNUC corresponds to a previously identified EF-hand/calcium-binding protein known as nucleobindin. CALNUC interacts with Galphai3 subunits in the yeast two-hybrid system and in GST-CALNUC pull-down assays. Analysis of deletion mutants demonstrated that the EF-hand and intervening acidic regions are the site of CALNUC's interaction with Galphai3. CALNUC is found in both cytosolic and membrane fractions. The membrane pool is tightly associated with the luminal surface of Golgi membranes. CALNUC is widely expressed, as it is detected by immunofluorescence in the Golgi region of all tissues and cell lines examined. By immunoelectron microscopy, CALNUC is localized to cis-Golgi cisternae and the cis-Golgi network (CGN). CALNUC is the major Ca2+-binding protein detected by 45Ca2+-binding assay on Golgi fractions. The properties of CALNUC and its high homology to calreticulin suggest that it may play a key role in calcium homeostasis in the CGN and cis-Golgi cisternae.

Measurement of cytosolic, mitochondrial, and Golgi pH in single living cells with green fluorescent proteins

Many cellular events depend on a tightly compartmentalized distribution of H+ ions across membrane-bound organelles. However, measurements of organelle pH in living cells have been scarce. Several mutants of the Aequorea victoria green fluorescent protein (GFP) displayed a pH-dependent absorbance and fluorescent emission, with apparent pKa values ranging from 6.15 (mutations F64L/S65T/H231L) and 6.4 (K26R/F64L/S65T/Y66W/N146I/M153T/ V163A/N164H/H231L) to a remarkable 7.1 (S65G/S72A/T203Y/H231L). We have targeted these GFPs to the cytosol plus nucleus, the medial/trans-Golgi by fusion with galactosyltransferase, and the mitochondrial matrix by using the targeting signal from subunit IV of cytochrome c oxidase. Cells in culture transfected with these cDNAs displayed the expected subcellular localization by light and electron microscopy and reported local pH that was calibrated in situ with ionophores. We monitored cytosolic and nuclear pH of HeLa cells, and mitochondrial matrix pH in HeLa cells and in rat neonatal cardiomyocytes. The pH of the medial/trans-Golgi was measured at steady-state (calibrated to be 6.58 in HeLa cells) and after various manipulations. These demonstrated that the Golgi membrane in intact cells is relatively permeable to H+, and that Cl- serves as a counter-ion for H+ transport and likely helps to maintain electroneutrality. The amenability to engineer GFPs to specific subcellular locations or tissue targets using gene fusion and transfer techniques should allow us to examine pH at sites previously inaccessible.

RGS-GAIP, a GTPase-activating protein for Galphai heterotrimeric G proteins, is located on clathrin-coated vesicles

RGS-GAIP (Galpha-interacting protein) is a member of the RGS (regulator of G protein signaling) family of proteins that functions to down-regulate Galphai/Galphaq-linked signaling. GAIP is a GAP or guanosine triphosphatase-activating protein that was initially discovered by virtue of its ability to bind to the heterotrimeric G protein Galphai3, which is found on both the plasma membrane (PM) and Golgi membranes. Previously, we demonstrated that, in contrast to most other GAPs, GAIP is membrane anchored and palmitoylated. In this work we used cell fractionation and immunocytochemistry to determine with what particular membranes GAIP is associated. In pituitary cells we found that GAIP fractionated with intracellular membranes, not the PM; by immunogold labeling GAIP was found on clathrin-coated buds or vesicles (CCVs) in the Golgi region. In rat liver GAIP was concentrated in vesicular carrier fractions; it was not found in either Golgi- or PM-enriched fractions. By immunogold labeling it was detected on clathrin-coated pits or CCVs located near the sinusoidal PM. These results suggest that GAIP may be associated with both TGN-derived and PM-derived CCVs. GAIP represents the first GAP found on CCVs or any other intracellular membranes. The presence of GAIP on CCVs suggests a model whereby a GAP is separated in space from its target G protein with the two coming into contact at the time of vesicle fusion.

Cellular and transcriptional changes during excystation of Giardia lamblia in vitro

Excystation of Giardia lamblia entails differentiation of dormant cysts into parasitic trophozoites. Despite its importance for infection, this transformation is not understood at the cellular or molecular levels. In these studies, we report that excystation entails detection of environmental stimuli across the tough extracellular cyst wall leading to highly coordinated physiological, structural, and molecular responses. We found that novel cytoplasmic rearrangements and changes in individual species of mRNA and in cytoplasmic pH occur within the cyst wall in the earliest stage of excystation, in response to conditions modeling cyst ingestion and passage into the human stomach. This suggests that cysts do not contain all the mRNA needed for excystation and emergence and supports our hypothesis that external stimuli, including hydrogen ions, may penetrate or be perceived across the cyst wall. In contrast, changes in cyst wall structure or proteins were detected only later in excystation, in the stage that models passage into the human small intestine, where trophozoites can emerge and survive. These findings show that excystation of G. lamblia is a highly complex and active process and provide important insights into its cellular and molecular components.

A mobile PTS2 receptor for peroxisomal protein import in Pichia pastoris

Using a new screening procedure for the isolation of peroxisomal import mutants in Pichia pastoris, we have isolated a mutant (pex7) that is specifically disturbed in the peroxisomal import of proteins containing a peroxisomal targeting signal type II (PTS2). Like its Saccharomyces cerevisiae homologue, PpPex7p interacted with the PTS2 in the two-hybrid system, suggesting that Pex7p functions as a receptor. The pex7Delta mutant was not impaired for growth on methanol, indicating that there are no PTS2-containing enzymes involved in peroxisomal methanol metabolism. In contrast, pex7Delta cells failed to grow on oleate, but growth on oleate could be partially restored by expressing thiolase (a PTS2-containing enzyme) fused to the PTS1. Because the subcellular location and mechanism of action of this protein are controversial, we used various methods to demonstrate that Pex7p is both cytosolic and intraperoxisomal. This suggests that Pex7p functions as a mobile receptor, shuttling PTS2-containing proteins from the cytosol to the peroxisomes. In addition, we used PpPex7p as a model protein to understand the effect of the Pex7p mutations found in human patients with rhizomelic chondrodysplasia punctata. The corresponding PpPex7p mutant proteins were stably expressed in P. pastoris, but they failed to complement the pex7Delta mutant and were impaired in binding to the PTS2 sequence.

Behaviorally elicited heart rate reactivity and atherosclerosis in ovariectomized cynomolgus monkeys (Macaca fascicularis)

It has been hypothesized that atherogenesis is accelerated among individuals who exhibit heightened cardiovascular reactions to psychologic stress. We have reported previously that the coronary atherosclerosis of cholesterol-fed, male and reproductively intact (premenopausal) female cynomolgus monkeys was exacerbated in animals that experienced the largest heart rate (HR) reactions to a fear-eliciting laboratory stressor. In this article, we report a similar relationship among 20 female monkeys that were rendered estrogen-deficient (by ovariectomy) and subsequently treated with replacement of both estrogen and progesterone. At the beginning of a 30-month study period, animals were fitted with ECG telemetry devices, and their HRs were recorded under baseline and stressed conditions. Stress HR measurements were obtained during a standard challenge involving threatened capture and physical handling of the animals. As part of a related experiment, monkeys were then ovariectomized and, for the remainder of the study, administered 17 beta-estradiol (continuously) and progesterone (cyclically) by subcutaneous Silastic implant (Dow Corning). Animals consumed a cholesterol-containing diet throughout, and HR measurements were repeated in the 24th month. At necropsy, the magnitude of animals' HR responses to stress correlated significantly with intimal area measurements in the left anterior descending and circumflex coronary arteries (r = .59 and r = .57, respectively; P < .009). This association was due to a marked exacerbation of coronary atherosclerosis in animals comprising the upper third of the reactivity distribution. Although total and HDL cholesterol concentrations also covaried with HR reactivity, the greater atherosclerosis of "high" HR reactors persisted after statistical adjustment for concomitant variability in plasma lipids. HR reactivity was unrelated to blood pressure, body weight, or social behavior.

Fluorescent indicators for Ca2+ based on green fluorescent proteins and calmodulin

Important Ca2+ signals in the cytosol and organelles are often extremely localized and hard to measure. To overcome this problem we have constructed new fluorescent indicators for Ca2+ that are genetically encoded without cofactors and are targetable to specific intracellular locations. We have dubbed these fluorescent indicators 'cameleons'. They consist of tandem fusions of a blue- or cyan-emitting mutant of the green fluorescent protein (GFP), calmodulin, the calmodulin-binding peptide M13, and an enhanced green- or yellow-emitting GFP. Binding of Ca2+ makes calmodulin wrap around the M13 domain, increasing the fluorescence resonance energy transfer (FRET) between the flanking GFPs. Calmodulin mutations can tune the Ca2+ affinities to measure free Ca2+ concentrations in the range 10(-8) to 10(-2) M. We have visualized free Ca2+ dynamics in the cytosol, nucleus and endoplasmic reticulum of single HeLa cells transfected with complementary DNAs encoding chimaeras bearing appropriate localization signals. Ca2+ concentration in the endoplasmic reticulum of individual cells ranged from 60 to 400 microM at rest, and 1 to 50 microM after Ca2+ mobilization. FRET is also an indicator of the reversible intermolecular association of cyan-GFP-labelled calmodulin with yellow-GFP-labelled M13. Thus FRET between GFP mutants can monitor localized Ca2+ signals and protein heterodimerization in individual live cells.