Structural analysis of conserved oligomeric Golgi complex subunit 2

The conserved oligomeric Golgi (COG) complex is strongly implicated in retrograde vesicular trafficking within the Golgi apparatus. Although its mechanism of action is poorly understood, it has been proposed to function by mediating the initial physical contact between transport vesicles and their membrane targets. An analogous role in tethering vesicles has been suggested for at least six additional large multisubunit complexes, including the exocyst, a complex essential for trafficking to the plasma membrane. Here we report the solution structure of a large portion of yeast Cog2p, one of eight subunits composing the COG complex. The structure reveals a six-helix bundle with few conserved surface features but a general resemblance to recently determined crystal structures of four different exocyst subunits. This finding provides the first structural evidence that COG, like the exocyst and potentially other tethering complexes, is constructed from helical bundles. These structures may represent platforms for interaction with other trafficking proteins including SNAREs (soluble N-ethylmaleimide factor attachment protein receptors) and Rabs.     

WISE-2005: developing a non-invasive method to monitor cardiovascular deconditioning

We have determined the gain of the cardiopulmonary baroreflex (CPBR) by measuring the central venous pressure (CVP) by venous catheter and calculating the total peripheral resistance (TPR) from mean arterial pressure (MAP) by Finometer and cardiac output (QD) by Doppler ultrasound. We tested the hypothesis that the BeatScope software of the Finometer, which calculates cardiac stroke volume (SVF) and cardiac output (QF) from the arterial pulse wave, could provide data with which to estimate CVP and TPR. The estimate of QF was linearly related to QD with a correction factor. Further, we found linear relationships between CVP and SVF that allowed us to establish a prediction of CVP from the SVF as the new input to the CPBR. To test the ability of this method to monitor changes in CPBR we are testing the subjects of the WISE-2005 study before and after 50-days of bed rest. We conclude that the TPR can be assessed with the Finometer and without any invasive method to record the CVP.     

Short-term variability of blood pressure: effects of lower-body negative pressure and long-duration bed rest

Mild lower-body negative pressure (LBNP) has been utilized to selectively unload cardiopulmonary baroreceptors, but there is evidence that arterial baroreceptors can be transiently unloaded after the onset of mild LBNP. In this paper, a black box mathematical model for the prediction of diastolic blood pressure (DBP) variability from multiple inputs (systolic blood pressure, R-R interval duration, and central venous pressure) was applied to interpret the dynamics of blood pressure maintenance under the challenge of LBNP and in long-duration, head-down bed rest (HDBR). Hemodynamic recordings from seven participants in the WISE (Women's International Space Simulation for Exploration) Study collected during an experiment of incremental LBNP (-10 mmHg, -20 mmHg, -30 mmHg) were analyzed before and on day 50 of a 60-day-long HDBR campaign. Autoregressive spectral analysis focused on low-frequency (LF, ~0.1 Hz) oscillations of DBP, which are related to fluctuations in vascular resistance due to sympathetic and baroreflex regulation of vasomotor tone. The arterial baroreflex-related component explained 49 ± 13% of LF variability of DBP in spontaneous conditions, and 89 ± 9% (P < 0.05) on day 50 of HDBR, while the cardiopulmonary baroreflex component explained 17 ± 9% and 12 ± 4%, respectively. The arterial baroreflex-related variability was significantly increased in bed rest also for LBNP equal to -20 and -30 mmHg. The proposed technique provided a model interpretation of the proportional effect of arterial baroreflex vs. cardiopulmonary baroreflex-mediated components of blood pressure control and showed that arterial baroreflex was the main player in the mediation of DBP variability. Data during bed rest suggested that cardiopulmonary baroreflex-related effects are blunted and that blood pressure maintenance in the presence of an orthostatic stimulus relies mostly on arterial control.     

Intermittent claudication: factors determining outcome.

Two groups of patients were followed up for four to eight years after first referral or admission to hospital for intermittent claudication (IC) in a study of the natural history of the disease and of factors determining its outcome. In one series of 60 patients, those who stopped or reduced smoking after referral had a much improved prognosis. Thus even after the diagnosis of IC it is extremely important that patients should be encouraged to stop smoking, since this correctable factor appears to be of greater importance in determining outcome than other medical risk factors for the disease that are less amenable to treatment. In the second study, 160 patients were followed up for eight years after first hospital admission. They had a total of 480 hospital admissions and had spent 11 190 days in hospital; their life expectancy after the age of 60 was about half that of the general population. Age, coronary artery disease, cerebrovascular disease, and diabetes were associated with an adverse outcome.     

Dependence of muscle VO2 on blood flow dynamics at onset of forearm exercise

Hughson, R. L., J. K. Shoemaker, M. E. Tschakovsky, and J. M. Kowalchuk. Dependence muscle ofO₂on blood flow dynamics at the onset of forearm exercise.J. Appl. Physiol. 81(4):1619-1626, 1996.The hypothesis that the rate of increase inmuscle O₂ uptake (O_{2 mus})at the onset of exercise is influenced by muscle blood flow was testedduring forearm exercise with the arm either above or below heart levelto modify perfusion pressure. Ten young men exercised at a power of~2.2 W, and five of these subjects also worked at 1.4 W. Blood flowto the forearm was calculated from the product of blood velocity andcross-sectional area obtained with Doppler techniques. Venous blood wassampled from a deep forearm vein to determineO₂ extraction. The rate ofincrease inO_{2 mus}and blood flow was assessed from the mean response time (MRT), which isthe time to achieve ~63% increase from baseline to steady state. Inthe arm below heart position during the 2.2-W exercise, blood flow andO_{2 mus}both increased, with a MRT of ~30 s. With the arm above the heart atthis power, the MRTs for blood flow [79.8 ± 15.7 (SE)s] and O_{2 mus}(50.2 ± 4.0 s) were both significantly slower. Consistent withthese findings were the greater increases in venous plasma lactateconcentration over resting values in the above heart position (2.8 ± 0.4 mmol/l) than in the below heart position (0.9 ± 0.2 mmol/l). At the lower power, both blood flow andO_{2 mus}also increased more rapidly with the arm below compared with above theheart. These data support the hypothesis that changes in blood flow atthe onset of exercise have a direct effect on oxidative metabolismthrough alterations in O₂transport.

     

Interactions within the yeast t-SNARE Sso1p that control SNARE complex assembly

In the eukaryotic secretory and endocytic pathways, transport vesicles shuttle cargo among intracellular organelles and to and from the plasma membrane. Cargo delivery entails fusion of the transport vesicle with its target, a process thought to be mediated by membrane bridging SNARE protein complexes. Temporal and spatial control of intracellular trafficking depends in part on regulating the assembly of these complexes. In vitro, SNARE assembly is inhibited by the closed conformation adopted by the syntaxin family of SNAREs. To visualize this closed conformation directly, the X-ray crystal structure of a yeast syntaxin, Sso1p, has been determined and refined to 2.1 A resolution. Mutants designed to destabilize the closed conformation exhibit accelerated rates of SNARE assembly. Our results provide insight into the mechanism of SNARE assembly and its intramolecular and intermolecular regulation.     

Operation Everest II: An indication of deterministic chaos in human heart rate variability at simulated extreme altitude

It has been shown that fluctuation of human heartbeat intervals (heart rate variability, HRV) reflects variations in autonomic nervous system activity. We studied HRV at simulated altitudes of over 6000 m from Holter electrocardiograms recorded during the Operation Everest II study (Houston et al. 1987). Stationary, 30-min segments of HRV data from six subjects at sea level and over 6000m were supplied to (1) spectral analysis to evaluate sympathetic and parasympathetic nervous system (SNS, PNS) activity, (2) the analysis of Poincaré section of the phase space trajectory reconstructed on a delayed coordinate system to evaluate whether there was fluctuation with deterministic dynamics, (3) the estimation of the correlation dimension to evaluate a static property of putative attractors, and (4) the analysis of nonlinear predictability of HRV time series which could reflect a dynamic property of the attractor. Unlike HRV at sea level, the recordings at over 6000 m showed a strong periodicity (period of about 20 s) with small cycle-to-cycle perturbation. When this perturbation was expressed on a Poincaré section, it seemed to be likely that the perturbation itself obeyed a deterministic law. The correlation dimensions of these recordings showed low dimensional values (3.5 ± 0.4, mean±SD), whereas those of the isospectral surrogates showed significantly (P < 0.05) higher values (5.3 ±0.5) with embedding dimensions of 5.6 ± 0.9. At over 6000 m, the correlation coefficients between the observed and the predicted time series with the prediction time of < 4 beats were significantly (P < 0.01) higher than those for the surrogate data, whereas there was no significant difference in the nonlinear predictability between the observed and the surrogate data at sea level. The results of the spectral analyses showed that, at over 6000 m, there was hardly any power > 0.15 Hz in the HRV spectra possibly due to PNS withdrawal. Hence, these deterministic and/or chaotic dynamics might be mediated by variations in SNS activity at over 6000 m. Present address and address for correspondence: Laboratory for Exercise Physiology and Biomechanics, Faculty of Education, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan     

Retrograde transport on the COG railway

The conserved oligomeric Golgi (COG) complex is essential for establishing and/or maintaining the structure and function of the Golgi apparatus. The Golgi apparatus, in turn, has a central role in protein sorting and glycosylation within the eukaryotic secretory pathway. As a consequence, COG mutations can give rise to human genetic diseases known as congenital disorders of glycosylation. We review recent results from studies of yeast, worm, fly and mammalian COG that provide evidence that COG might function in retrograde vesicular trafficking within the Golgi apparatus. This hypothesis explains the impact of COG mutations by postulating that they impair the retrograde flow of resident Golgi proteins needed to maintain normal Golgi structure and function.