Osmotic Regulation of Rab-Mediated Organelle Docking

SummaryOsmotic gradients across organelle and plasma membranes modulate the rates of membrane fission and fusion; sufficiently large gradients can cause membrane rupture [1–6]. Hypotonic gradients applied to living yeast cells trigger prompt (within seconds) swelling and fusion of Saccharomyces cerevisiae vacuoles, whereas hypertonic gradients cause vacuoles to fragment on a slower time scale [7–11]. Here, we analyze the influence of osmotic strength on homotypic fusion of isolated yeast vacuoles. Consistent with previously reported in vivo results, we find that decreases in osmolyte concentration increase the rate and extent of vacuole fusion in vitro, whereas increases in osmolyte concentration prevent fusion. Unexpectedly, our results reveal that osmolytes regulate fusion by inhibiting early Rab-dependent docking or predocking events, not late events. Our experiments reveal an organelle-autonomous pathway that may control organelle surface-to-volume ratio, size, and copy number: Decreasing the osmolyte concentration in the cytoplasmic compartment accelerates Rab-mediated docking and fusion. By altering the relationship between the organelle surface and its enclosed volume, fusion in turn reduces the risk of membrane rupture

Intrinsic tethering activity of endosomal Rab proteins.

Rab small G proteins control membrane trafficking events required for many processes including secretion, lipid metabolism, antigen presentation and growth factor signaling. Rabs recruit effectors that mediate diverse functions including vesicle tethering and fusion. However, many mechanistic questions about Rab-regulated vesicle tethering are unresolved. Using chemically defined reaction systems, we discovered that Vps21, a Saccharomyces cerevisiae ortholog of mammalian endosomal Rab5, functions in trans with itself and with at least two other endosomal Rabs to directly mediate GTP-dependent tethering. Vps21-mediated tethering was stringently and reversibly regulated by an upstream activator, Vps9, and an inhibitor, Gyp1, which were sufficient to drive dynamic cycles of tethering and detethering. These experiments reveal a previously undescribed mode of tethering by endocytic Rabs. In our working model, the intrinsic tethering capacity Vps21 operates in concert with conventional effectors and SNAREs to drive efficient docking and fusion

Microwave Lens for Polar Molecules

We here report on the implementation of a microwave lens for neutral polar molecules suitable to focus molecules both in low-field-seeking and in high-field-seeking states. By using the TE_11m modes of a 12 cm long cylindrically symmetric microwave resonator, Stark-decelerated ammonia molecules are transversally confined. We investigate the focusing properties of this microwave lens as a function of the molecules' velocity, the detuning of the microwave frequency from the molecular resonance frequency, and the microwave power. Such a microwave lens can be seen as a first important step towards further microwave devices, such as decelerators and traps.Comment: 4 pages, 3 figure

Brachial Artery Constriction during Brachial Artery Reactivity Testing Predicts Major Adverse Clinical Outcomes in Women with Suspected Myocardial Ischemia: Results from the NHLBI-Sponsored Women's Ischemia Syndrome Evaluation (WISE) Study

Background:Limited brachial artery (BA) flow-mediated dilation during brachial artery reactivity testing (BART) has been linked to increased cardiovascular risk. We report on the phenomenon of BA constriction (BAC) following hyperemia.Objectives:To determine whether BAC predicts adverse CV outcomes and/or mortality in the women's ischemic Syndrome Evaluation Study (WISE). Further, as a secondary objective we sought to determine the risk factors associated with BAC.Methods:We performed BART on 377 women with chest pain referred for coronary angiography and followed for a median of 9.5 years. Forearm ischemia was induced with 4 minutes occlusion by a cuff placed distal to the BA and inflated to 40mm Hg > systolic pressure. BAC was defined as >4.8% artery constriction following release of the cuff. The main outcome was major adverse events (MACE) including all-cause mortality, non-fatal MI, non-fatal stroke, or hospitalization for heart failure.Results:BA diameter change ranged from -20.6% to +44.9%, and 41 (11%) women experienced BAC. Obstructive CAD and traditional CAD risk factors were not predictive of BAC. Overall, 39% of women with BAC experienced MACE vs. 22% without BAC (p=0.004). In multivariate Cox proportional hazards regression, BAC was a significant independent predictor of MACE (p=0.018) when adjusting for obstructive CAD and traditional risk factors.Conclusions:BAC predicts almost double the risk for major adverse events compared to patients without BAC. This risk was not accounted for by CAD or traditional risk factors. The novel risk marker of BAC requires further investigation in women. © 2013 Sedlak et al

Experimental Studies Towards a DC-DC Conversion Powering Scheme for the CMS Silicon Strip Tracker at SLHC

The upgrade of the CMS silicon tracker for the Super-LHC presents many challenges. The distribution of power to the tracker is considered particularly difficult, as the tracker power consumption is expected to be similar to or higher than today, while the operating voltage will decrease and power cables cannot be exchanged or added. The CMS tracker has adopted parallel powering with DC-DC conversion as the baseline solution to the powering problem. In this paper, experimental studies of such a DC-DC conversion powering scheme are presented, including system test measurements with custom DC-DC converters and current strip tracker structures, studies of the detector susceptibility to conductive noise, and simulations of the effect of novel powering schemes on the strip tracker material budget