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George Cooper

Publications and source records attributed to George Cooper.

13 recordsLinked to original sources
PubMed2006

Comet 81P/Wild 2 under a microscope.

The Stardust spacecraft collected thousands of particles from comet 81P/Wild 2 and returned them to Earth for laboratory study. The preliminary examination of these samples shows that the nonvolatile portion of the comet is an unequilibrated assortment of materials that have both presolar and solar system origin. The comet contains an abundance of silicate grains that are much larger than predictions of interstellar grain models, and many of these are high-temperature minerals that appear to have formed in the inner regions of the solar nebula. Their presence in a comet proves that the formation of the solar system included mixing on the grandest scales.

Journal Article↗
PubMed2006

Organics captured from comet 81P/Wild 2 by the Stardust spacecraft.

Organics found in comet 81P/Wild 2 samples show a heterogeneous and unequilibrated distribution in abundance and composition. Some organics are similar, but not identical, to those in interplanetary dust particles and carbonaceous meteorites. A class of aromatic-poor organic material is also present. The organics are rich in oxygen and nitrogen compared with meteoritic organics. Aromatic compounds are present, but the samples tend to be relatively poorer in aromatics than are meteorites and interplanetary dust particles. The presence of deuterium and nitrogen-15 excesses suggest that some organics have an interstellar/protostellar heritage. Although the variable extent of modification of these materials by impact capture is not yet fully constrained, a diverse suite of organic compounds is present and identifiable within the returned samples.

Carbon↗
PubMed2006

Enhanced ubiquitination of cytoskeletal proteins in pressure overloaded myocardium is accompanied by changes in specific E3 ligases.

Ubiquitin conjugation of proteins is critical for cell homeostasis and contributes to both cell survival and death. Here we studied ubiquitination of proteins in pressure overloaded (PO) myocardium in the context of cardiomyocyte survival. Analysis using a feline right ventricular pressure overload (RVPO) model revealed a robust and transient increase in ubiquitination of proteins present in the Triton X-100-insoluble fraction in 24 to 48 h PO myocardium, and confocal micrographs indicate this increase in ubiquitination occurs subsarcolemmaly near the intercalated disc area of cardiomyocytes. The ubiquitination was accompanied by changes in E3 ligases including Cbl, E6AP, Mdm2 and cIAP in the same period of PO, although atrophy-related E3 ligases, MuRF1 and MuRF3 were unaltered. Furthermore, Cbl displayed a substantial increase in both levels of expression and tyrosine phosphorylation in 48 h PO myocardium. Confocal studies revealed enrichment of Cbl at the intercalated discs of 48 h PO cardiomyocytes, as evidenced by its colocalization with N-cadherin. Although apoptosis was observed in 48 h PO myocardium by TUNEL staining, cardiomyocytes showing ubiquitin staining were not positive for TUNEL staining. Furthermore, 48 h PO resulted in the phosphorylation of inhibitor of nuclear factor kappa B (IkappaB), suggesting its ubiquitin-mediated degradation and the nuclear localization of NFkappaB for the expression of specific cell survival factors such as cIAPs. Together these data indicate that increased levels of E3 ligases that regulate cell homeostasis and promote cell survival could ubiquitinate multiple cytoskeletal protein targets and that these events that occur during the early phase of PO may contribute to both cardiomyocyte survival and hypertrophy.

Animals↗
PubMed2006

Microtubule-associated protein-4 (MAP-4) inhibits microtubule-dependent distribution of mRNA in isolated neonatal cardiocytes.

OBJECTIVES: Active mRNA distribution in the form of ribonucleoprotein particles moving along microtubules has been shown in several cell types, but not yet in cardiocytes. This study addresses two hypotheses: 1) a similar mRNA distribution mechanism operates in cardiocytes; 2) decoration of microtubules with microtubule-associated proteins compromises this distribution. METHODS: To visualize ribonucleoproteins in cultured neonatal rat cardiocytes, they were transfected with vectors encoding zipcode binding protein-1 and Staufen fused with GFP. The velocity of microtubular transport and elongation were calculated on time-lapse confocal pictures. RESULTS: ZBP-1 and Staufen labeled particles co-localized with each other and with microtubules and moved along microtubules over a distance of 1-20 microm with a mean speed of 80 nm/s. The average speed decreased about 50% after decoration of microtubules by adenoviral microtubule-associated protein-4 (MAP-4). The elongation speed measured using the GFP-tagged end-binding protein-1 exceeded 200 nm/s and was not influenced by MAP-4. CONCLUSIONS: We demonstrate for the first time ribonucleoprotein particles in cardiocytes, their microtubular-related movement, and its inhibition (but not of the microtubular elongation), by the MAP-4 decoration of microtubules.

Animals↗
PubMed2006

Cytoskeletal networks and the regulation of cardiac contractility: microtubules, hypertrophy, and cardiac dysfunction.

The cytoskeleton as classically defined for eukaryotic cells consists of three systems of protein filaments: the microtubules, the intermediate filaments, and the microfilaments. In mature striated muscle such as the heart of the adult mammal, these three types of cytoskeletal filaments are superimposed spatially on the myofilaments, a specialized system of contractile protein filaments. Each of these systems of protein filaments has the potential to respond in an adaptive or maladaptive manner during load-induced hypertrophic cardiac growth. However, the extent to which such hypertrophy is compensatory is also critically dependent on the type of hemodynamic overload that serves as the hypertrophic stimulus. Thus cardiac hypertrophy is not intrinsically maladaptive; rather, it is the nature of the inducing load rather than hypertrophy itself that is responsible, through effects on structural and/or regulatory proteins, for the frequent deterioration of initially compensatory hypertrophy into the congestive heart failure state. As one example reviewed here of this load specificity of maladaptation, increased microtubule network density is a persistent feature of severely pressure-overloaded, hypertrophied, and failing myocardium that imposes a primarily viscous load on active myofilaments during contraction.

Actin Cytoskeleton↗
PubMed2006

Mutations at leucine 215 of beta-tubulin affect paclitaxel sensitivity by two distinct mechanisms.

Paclitaxel resistance mutations in Chinese hamster ovary cells frequently alter a cluster of leucine residues in the H6-H7 loop region of beta-tubulin. To gain further insight into the role of this region in microtubule assembly and drug resistance, site-directed mutagenesis was used to systematically change amino acid L215. The mutated genes were cloned into a tetracycline-regulated expression vector and transfected into wild-type cells. Most of the mutations destabilized microtubule assembly, causing a decreased fraction of tubulin to appear in the microtubule cytoskeleton. In each case, the decreased level of assembly was associated with paclitaxel resistance and increased colcemid sensitivity. In two cases, however, the alteration did not significantly perturb the level of assembled tubulin or confer resistance to paclitaxel. One of these, L215V, produced little or no detectable phenotype, while the other, L215I, conferred increased sensitivity to paclitaxel. The increased drug sensitivity did not extend to epothilone A, a drug that binds to the same site and has a mechanism of action similar to that of paclitaxel, or colcemid, a drug with an opposing mechanism of action and a distinct binding site. Moreover, L215I conferred enhanced paclitaxel sensitivity at very low levels of expression, and sensitivity was not further enhanced in cells with higher levels of expression, implying that paclitaxel acts substoichiometrically. These properties, along with the proximity of L215 to the drug binding site, suggests that the L215I substitution may enhance the binding or effectiveness of paclitaxel. Our studies confirm the importance of the H6-H7 loop of beta-tubulin in microtubule assembly and resistance to antimitotic drugs. They also identify the first mammalian mutation shown to specifically increase sensitivity to paclitaxel.

Amino Acid Substitution↗
PubMed2004

Inhibition of beta-adrenergic receptor trafficking in adult cardiocytes by MAP4 decoration of microtubules.

Decreased beta-adrenergic receptor (beta-AR) number occurs both in animal models of cardiac hypertrophy and failure and in patients. beta-AR recycling is an important mechanism for the beta-AR resensitization that maintains a normal complement of cell surface beta-ARs. We have shown that 1) in severe pressure overload cardiac hypertrophy, there is extensive microtubule-associated protein 4 (MAP4) decoration of a dense microtubule network; and 2) MAP4 microtubule decoration inhibits muscarinic acetylcholine receptor recycling in neuroblastoma cells. We asked here whether MAP4 microtubule decoration inhibits beta-AR recycling in adult cardiocytes. [(3)H]CGP-12177 was used as a beta-AR ligand, and feline cardiocytes were isolated and infected with adenovirus containing MAP4 (AdMAP4) or beta-galactosidase (Adbeta-gal) cDNA. MAP4 decorated the microtubules extensively only in AdMAP4 cardiocytes. beta-AR agonist exposure reduced cell surface beta-AR number comparably in AdMAP4 and Adbeta-gal cardiocytes; however, after agonist withdrawal, the cell surface beta-AR number recovered to 78.4 +/- 2.9% of the pretreatment value in Adbeta-gal cardiocytes but only to 56.8 +/- 1.4% in AdMAP4 cardiocytes (P < 0.01). This result was confirmed in cardiocytes isolated from transgenic mice having cardiac-restricted MAP4 overexpression. In functional terms of cAMP generation, beta-AR agonist responsiveness of AdMAP4 cells was 47% less than that of Adbeta-gal cells. We conclude that MAP4 microtubule decoration interferes with beta-AR recycling and that this may be one mechanism for beta-AR downregulation in heart failure.

Adenoviridae↗
PubMed2004

Veterans Administration support for medical research: opinions of the endangered species of physician-scientists.

Over the past three decades the Veterans Affairs (VA) Research program has evolved into a powerful, peer-reviewed funding mechanism for basic and translational research that has resulted in numerous important contributions to medical science and improvements in patient care. Continuity in VA Merit Review funding has fostered and nurtured the scientific careers of a large number of physician-scientists who have remained devoted to the mission of performing creative and innovative research that affects the patient care mission of the VA. VA medical research policies have undergone a major overhaul in the past year. Although many of these changes (de-emphasizing bench research and revamping the peer review process) have recently been reversed, the future direction of VA research remains in flux. The goal of this manuscript is to demonstrate the importance of the Merit Review medical research funding mechanism not just to the VA, but to the entire nation's health care system. To achieve this goal, the opinions of 65 established VA medical investigators were obtained regarding the past success and future direction of VA research. The conclusions reached include the following. 1) Merit Review research funding has been essential to the training, recruitment, and retention of productive VA physician-scientists. 2) The VA research program has contributed both basic and clinical innovations that have led to improvements in medical care. Contributions of VA researchers to excellence in many aspects of patient care at VA hospitals have been extraordinary. 3) Development of initiatives that entice outstanding Ph.D.'s to develop their careers in the VA has been crucial to the success of the program. 4) The VA research program has fostered a mutually beneficial relationship with affiliated medical schools. 5) Better methods to quantify VA research contributions and outcomes are essential for future program development.

Biomedical Research↗
PubMed2003

Phenotypic consequences of beta1-tubulin expression and MAP4 decoration of microtubules in adult cardiocytes.

In pressure-overload cardiac hypertrophy, microtubule network densification is one cause of contractile dysfunction. Cardiac transcriptional upregulation of beta1-tubulin rather than the constitutive beta4-tubulin and of microtubule-associated protein (MAP)4 accompanies hypertrophy, with extensive microtubule decoration by MAP4. Because MAP4 stabilizes microtubules, and because the isoform-variable carboxy terminus of beta-tubulin binds to MAP4, we wished to determine whether one or both of these proteins has etiologic significance for cardiac microtubule network densification. Recombinant adenoviruses encoding beta1-tubulin, beta4-tubulin, and MAP4 were used to infect isolated cardiocytes. Overexpressed MAP4 caused a shift of tubulin dimers to the polymerized fraction and formation of a dense, stable microtubule network. Overexpressed beta1- or beta4-tubulin had neither any independent effect on these variables nor any effect additive to that of simultaneously overexpressed MAP4. Results from transgenic mice with cardiac overexpression of beta1-tubulin or MAP4 were confirmatory, but unlike the effects of brief adenovirus-mediated MAP4 overexpression in isolated cardiocytes, MAP4 transgenic hearts showed a marked increase in total alpha- and beta-tubulin. Thus MAP4 overexpression caused increased tubulin expression, formation of stable microtubules, and altered microtubule network properties, such that MAP4 upregulation may be one cause for the dense, stable microtubule network characteristic of pressure-overloaded, hypertrophied cardiocytes.

Adenoviridae↗
PubMed2003

Role of microtubules versus myosin heavy chain isoforms in contractile dysfunction of hypertrophied murine cardiocytes.

In large mammals there is a correlation between microtubule network densification and contractile dysfunction in severe pressure-overload hypertrophy. In small mammals there is a similar correlation for the shift to beta-myosin heavy chain (MHC), a MHC isoform having a slower ATPase Vmax. In this study, murine left ventricular (LV) pressure overload invoked both mechanisms: microtubule network densification and beta-MHC expression. Cardiac beta-MHC was also augmented without altering tubulin levels by two load-independent means, chemical thyroidectomy and transgenesis. In hypertrophy, contractile function of the LV and its cardiocytes decreased proportionally; microtubule depolymerization restored normal cellular contraction. In hypothyroid mice having a complete shift from alpha-MHC to beta-MHC, contractile function of the LV and its cardiocytes also decreased, but microtubule depolymerization had no effect on cellular contraction. In transgenic mice having a cardiac beta-MHC increase similar to that in hypertrophy, contractile function of the LV and its cardiocytes was normal, and microtubule depolymerization had no effect. Thus, although both mechanisms may cause contractile dysfunction, for the extent of MHC isoform switching seen even in severe murine LV pressure-overload hypertrophy, microtubule network densification appears to have the more important role.

Animals↗
PubMed2002

Inhibition of G protein-coupled receptor trafficking in neuroblastoma cells by MAP 4 decoration of microtubules.

One mechanism for the reappearance of G protein-coupled receptors after agonist activation is microtubule-based transport. In pressure-overload cardiac hypertrophy, there is downregulation of G protein-coupled receptors and the appearance of a densified microtubule network extensively decorated by a microtubule-associated protein, MAP 4. Our hypothesis is that overdecoration of a dense microtubule network with this structural protein, as in hypertrophied myocardium, would impede receptor recovery. We tested this hypothesis by studying muscarinic acetylcholine receptor (mAChR) internalization and recovery after agonist stimulation in neuroblastoma cells. Exposure of cells to carbachol, a muscarinic receptor agonist, decreased membrane receptor binding activity. After carbachol withdrawal, receptor binding recovered toward the initial value. When microtubules were depolymerized before carbachol withdrawal, mAChR recovery was only 44% of that in intact cells. Cells were then infected with an adenovirus containing MAP 4 cDNA. MAP 4 protein decorated the microtubules extensively, and receptor recovery upon carbachol withdrawal was reduced to 54% of control. Thus muscarinic receptor recovery after agonist exposure is microtubule dependent, and MAP 4 decoration of microtubules inhibits receptor recovery.

Actin Cytoskeleton↗
PubMed2002

Constitutive properties of hypertrophied myocardium: cellular contribution to changes in myocardial stiffness.

Recent studies have suggested that pressure overload hypertrophy (POH) alters the viscoelastic properties of individual cardiocytes when studied in isolation. However, whether these changes in cardiocyte properties contribute causally to changes in the material properties of the cardiac muscle as a whole is unknown. Accordingly, a selective, isolated, acute change in cardiocyte constitutive properties was imposed in an in vitro system capable of measuring the resultant effect on the material properties of the composite cardiac muscle. POH caused an increase in both myocardial elastic stiffness, from 20.5 +/- 1.3 to 28.4 +/- 1.8, and viscous damping, from 15.2 +/- 1.1 to 19.8 +/- 1.5 s (normal vs. POH, P < 0.05), respectively. Recent studies have shown that cardiocyte constitutive properties could be acutely altered by depolymerizing the microtubules with colchicine. Colchicine caused a significant decrease in the viscous damping in POH muscles (19.8 +/- 1.5 s at baseline vs. 14.7 +/- 1.3 s after colchicine, P < 0.05). Therefore, myocardial material properties can be altered by selectively changing the constitutive properties of one element within this muscle tissue, the cardiocyte. Changes in the constitutive properties of the cardiocytes themselves contribute to the abnormalities in myocardial stiffness and viscosity that develop during POH.

Animals↗