Left Ventricular Assist Device-Associated Gastrointestinal Bleeding: Recognition of an Iatrogenic Etiology on 99mTc-Tagged Red Blood Cell Scintigraphy.

99mTc-tagged red blood cell scintigraphy is the imaging modality of choice in the diagnosis of active gastrointestinal bleeding. Continuous-flow left ventricular assist devices are the state-of-the-art treatment for advanced heart failure, with gastrointestinal bleeding as the most common complication. Recognition of the distinctive imaging feature of continuous-flow left ventricular assist devices on scintigraphic images can aid diagnosis of gastrointestinal bleeding.

An endovascular simulation exercise among radiology residents: comparison of simulation performance with and without practice.

PURPOSE: The purpose was to compare resident endovascular simulator performance with and without prior simulation. METHODS: Radiology residents were guided through a practice simulation and lectured on endovascular therapy, then randomized to simulate femoral arterial intervention with or without prior iliac simulation. Simulator measurements, performance grading and resident surveys were recorded. RESULTS: Prior simulation of iliac intervention significantly improved resident performance. In particular, it resulted in less catheter placement without a wire (P=.01), shorter time to proper catheter positioning (P=.045) and use of oblique digital subtraction angiography (P=.035). Survey respondents valued the experience. CONCLUSION: Endovascular simulator training improves simulation skills. Improvement of real-world performance and generalizability remain to be shown.

Construction and analyses of elastically coupled multiple-motor systems.

Precision analyses of the collective motor behaviors have become important to dissecting mechanisms underlying the trafficking of subcellular commodities in eukaryotic cells. Here, we describe a synthetic approach to create structurally defined multiple protein complexes containing two elastically coupled motor molecules. Motors are connected using a simple DNA-scaffolding molecule and DNA-conjugated, artificial protein polymers that function as tunable elastic linkers. The procedure to self-assemble these components produces complexes in high synthetic yield and allows individual multiple-motor systems to be interrogated at the single-complex level. Methods to evaluate cooperative motor responses in a static optical trap are also discussed. While enabling the average transport properties of single/noninteracting and coupled motors to be compared, these procedures can provide insight into the extent to which motors cooperate productively via load sharing as well as the roles loading-rate-dependent phenomena play in collective motor functions.

Analysis of Cooperative Behavior in Multiple Kinesins Motor Protein Transport by Varying Structural and Chemical Properties.

Intracellular transport is a fundamental biological process during which cellular materials are driven by enzymatic molecules called motor proteins. Recent optical trapping experiments and theoretical analysis have uncovered many features of cargo transport by multiple kinesin motor protein molecules under applied loads. These studies suggest that kinesins cooperate negatively under typical transport conditions, although some productive cooperation could be achieved under higher applied loads. However, the microscopic origins of this complex behavior are still not well understood. Using a discrete-state stochastic approach we analyze factors that affect the cooperativity among kinesin motors during cargo transport. Kinesin cooperation is shown to be largely unaffected by the structural and mechanical parameters of a multiple motor complex connected to a cargo, but much more sensitive to biochemical parameters affecting motor-filament affinities. While such behavior suggests the net negative cooperative responses of kinesins will persist across a relatively wide range of cargo types, it is also shown that the rates with which cargo velocities relax in time upon force perturbations are influenced by structural factors that affect the free energies of and load distributions within a multiple kinesin complex. The implications of these later results on transport phenomena where loads change temporally, as in the case of bidirectional transport, are discussed.

How the interplay between mechanical and nonmechanical interactions affects multiple kinesin dynamics.

Intracellular transport is supported by enzymes called motor proteins that are often coupled to the same cargo and function collectively. Recent experiments and theoretical advances have been able to explain certain behaviors of multiple motor systems by elucidating how unequal load sharing between coupled motors changes how they bind, step, and detach. However, nonmechanical interactions are typically overlooked despite several studies suggesting that microtubule-bound kinesins interact locally via short-range nonmechanical potentials. This work develops a new stochastic model to explore how these types of interactions influence multiple kinesin functions in addition to mechanical coupling. Nonmechanical interactions are assumed to affect kinesin mechanochemistry only when the motors are separated by less than three microtubule lattice sites, and it is shown that relatively weak interaction energies (~2 k(B)T) can have an appreciable influence over collective motor velocities and detachment rates. In agreement with optical trapping experiments on structurally defined kinesin complexes, the model predicts that these effects primarily occur when cargos are transported against loads exceeding single-kinesin stalling forces. Overall, these results highlight the interdependent nature of factors influencing collective motor functions, namely, that the way the bound configuration of a multiple motor system evolves under load determines how local nonmechanical interactions influence motor cooperation.

Cooperative responses of multiple kinesins to variable and constant loads.

Microtubule-dependent transport is most often driven by collections of kinesins and dyneins that function in either a concerted fashion or antagonistically. Several lines of evidence suggest that cargo transport may not be influenced appreciably by the combined action of multiple kinesins. Yet, as in previous optical trapping experiments, the forces imposed on cargos will vary spatially and temporally in cells depending on a number of local environmental factors, and the influence of these conditions has been largely overlooked. Here, we characterize the dynamics of structurally defined complexes containing multiple kinesins under the controlled loads of an optical force clamp. While demonstrating that there are generic kinetic barriers that restrict the ability of multiple kinesins to cooperate productively, the spatial and temporal properties of applied loads is found to play an important role in the collective dynamics of multiple motor systems. We propose this dependence has implications for intracellular transport processes, especially for bidirectional transport.

Productive cooperation among processive motors depends inversely on their mechanochemical efficiency.

Subcellular cargos are often transported by teams of processive molecular motors, which raises questions regarding the role of motor cooperation in intracellular transport. Although our ability to characterize the transport behaviors of multiple-motor systems has improved substantially, many aspects of multiple-motor dynamics are poorly understood. This work describes a transition rate model that predicts the load-dependent transport behaviors of multiple-motor complexes from detailed measurements of a single motor's elastic and mechanochemical properties. Transition rates are parameterized via analyses of single-motor stepping behaviors, load-rate-dependent motor-filament detachment kinetics, and strain-induced stiffening of motor-cargo linkages. The model reproduces key signatures found in optical trapping studies of structurally defined complexes composed of two kinesin motors, and predicts that multiple kinesins generally have difficulties in cooperating together. Although such behavior is influenced by the spatiotemporal dependence of the applied load, it appears to be directly linked to the efficiency of kinesin's stepping mechanism, and other types of less efficient and weaker processive motors are predicted to cooperate more productively. Thus, the mechanochemical efficiencies of different motor types may determine how effectively they cooperate together, and hence how motor copy number contributes to the regulation of cargo motion.

Two kinesins transport cargo primarily via the action of one motor: implications for intracellular transport.

The number of microtubule motors attached to vesicles, organelles, and other subcellular commodities is widely believed to influence their motile properties. There is also evidence that cells regulate intracellular transport by tuning the number and/or ratio of motor types on cargos. Yet, the number of motors responsible for cargo motion is not easily characterized, and the extent to which motor copy number affects intracellular transport remains controversial. Here, we examined the load-dependent properties of structurally defined motor assemblies composed of two kinesin-1 molecules. We found that a group of kinesins can produce forces and move with velocities beyond the abilities of single kinesin molecules. However, such capabilities are not typically harnessed by the system. Instead, two-kinesin assemblies adopt a range of microtubule-bound configurations while transporting cargos against an applied load. The binding arrangement of motors on their filament dictates how loads are distributed within the two-motor system, which in turn influences motor-microtubule affinities. Most configurations promote microtubule detachment and prevent both kinesins from contributing to force production. These results imply that cargos will tend to be carried by only a fraction of the total number of kinesins that are available for transport at any given time, and provide an alternative explanation for observations that intracellular transport depends weakly on kinesin number in vivo.

Coupling between motor proteins determines dynamic behaviors of motor protein assemblies.

Transport of intracellular cargos by multiple microtubule motor proteins is believed to be a common and significant phenomenon in vivo, yet signatures of the microscopic dynamics of multiple motor systems are only now beginning to be resolved. Understanding these mechanisms largely depends on determining how grouping motors affect their association with microtubules and stepping rates, and hence, cargo run lengths and velocities. We examined this problem using a discrete state transition rate model of collective transport. This model accounts for the structural and mechanical properties in binding/unbinding and stepping transitions between distinct microtubule-bound configurations of a multiple motor system. In agreement with previous experiments that examine the dynamics of two coupled kinesin-1 motors, the energetic costs associated with deformations of mechanical linkages within a multiple motor assembly are found to reduce the system's overall microtubule affinity, producing attenuated mean cargo run lengths compared to cases where motors are assumed to function independently. With our present treatment, this attenuation largely stems from reductions in the microtubule binding rate and occurs even when mechanical coupling between motors is weak. Thus, our model suggests that, at least for a variety of kinesin-dependent transport processes, the net 'gains' obtained by grouping motors together may be smaller than previously expected.