The Role of Thin Filament Cooperativity in Cardiac Length-Dependent Calcium Activation

Length-dependent activation (LDA) is a prominent feature of cardiac muscle characterized by decreases in the Ca2+ levels required to generate force (i.e., increases in Ca2+ sensitivity) when muscle is stretched. Previous studies have concluded that LDA originates from the increased ability of (strong) cross-bridges to attach when muscle is lengthened, which in turn enhances Ca2+ binding to the troponin C (TnC) subunit of the troponin complex. However, our results demonstrate that inhibition of strong cross-bridge attachment with blebbistatin had no effect on the length-dependent modulation of Ca2+ sensitivity (i.e., EC50) or Ca2+ cooperativity, suggesting that LDA originates upstream of cross-bridge attachment. To test whether LDA arises from length dependence of thin-filament activation, we replaced native cTnC with a mutant cTnC (DM-TnC) that is incapable of binding Ca2+. Although progressive replacement of native cTnC with DM-TnC caused an expected monotonic decrease in the maximal force (Fmax), DM-TnC incorporation induced much larger increases in EC50 and decreases in Ca2+ cooperativity at short lengths than at long lengths. These findings support the conclusion that LDA arises primarily from the influence of length on the modulation of the Ca2+ cooperativity arising from interaction between adjacent troponin-tropomyosin complexes on the thin filament

Bringing Space Capabilities to the Warfighter: Virtual Mission Operations Center (VMOC)

The Virtual Mission Operations Center (VMOC) is a joint DoD and U.S. intergovernmental initiative to exploit Internet Protocol (IP) based systems in space or nearspace, allowing any computer linked to the Internet to conduct dynamic tasking of payloads, interact with databases, and Tracking, Telemetry, and Control (TT&C) operations. The use of IP systems enables disadvantaged field users to access and task space capabilities. At the same time, it encourages common interfaces that lead to reconfigurable and standardized vehicle design for operationally responsive space missions. As such, IP can be an enabler for constellations of tactical small satellites. In order to utilize IP and distributed tasking, one must consider issues such as security, prioritization, and contention control. This is the role of the VMOC. Each user must log on through a VMOC server that authenticates the user, validates the operations that user is authorized to perform, and verifies the prioritization level that user holds. Once these parameters are established the user makes a request. VMOC first searches its database for the requested information to prevent tasking an on-orbit asset. If the data is unavailable, the user’s request is prioritized and executed according to the prioritization. The Air Force Space Battlelab is the project manager of the VMOC demonstration team that includes Army Space and Missile Defense Command Battle Lab and NASA Glenn Research Center. The team will demonstrate the capability of VMOC via a Surrey Satellite Technologies Conner 2 18th Annual AIAA/USU Conference on Small Satellites Limited (SSTL) micro-satellite. Using the Army Space Support Element Toolset, we will demonstrate that a field user can log into the Internet and perform various dynamic tasking operations on the satellite. The field portion of the demonstration is scheduled to begin in May 2004 and will be completed in June 2004

Metabolically Activated Adipose Tissue Macrophages Perform Detrimental and Beneficial Functions during Diet-Induced Obesity

During obesity, adipose tissue macrophages (ATMs) adopt a metabolically activated (MMe) phenotype. However, the functions of MMe macrophages are poorly understood. Here, we combine proteomic and functional methods to demonstrate that, in addition to potentiating inflammation, MMe macrophages promote dead adipocyte clearance through lysosomal exocytosis. We identify NADPH oxidase 2 (NOX2) as a driver of the inflammatory and adipocyte-clearing properties of MMe macrophages and show that, compared to wild-type, Nox2−/− mice exhibit a time-dependent metabolic phenotype during diet-induced obesity. After 8 weeks of high-fat feeding, Nox2−/− mice exhibit attenuated ATM inflammation and mildly improved glucose tolerance. After 16 weeks of high-fat feeding, Nox2−/− mice develop severe insulin resistance, hepatosteatosis, and visceral lipoatrophy characterized by dead adipocyte accumulation and defective ATM lysosomal exocytosis, a phenotype reproduced in myeloid cell-specific Nox2−/− mice. Collectively, our findings suggest that MMe macrophages perform detrimental and beneficial functions whose contribution to metabolic phenotypes during obesity is determined by disease progression

Myosin head orientation: a structural determinant for the Frank-Starling relationship

The cellular mechanism underlying the Frank-Starling law of the heart is myofilament length-dependent activation. The mechanism(s) whereby sarcomeres detect changes in length and translate this into increased sensitivity to activating calcium has been elusive. Small-angle X-ray diffraction studies have revealed that the intact myofilament lattice undergoes numerous structural changes upon an increase in sarcomere length (SL): lattice spacing and the I1,1/I1,0 intensity ratio decreases, whereas the M3 meridional reflection intensity (IM3) increases, concomitant with increases in diastolic and systolic force. Using a short (∼10 ms) X-ray exposure just before electrical stimulation, we were able to obtain detailed structural information regarding the effects of external osmotic compression (with mannitol) and obtain SL on thin intact electrically stimulated isolated rat right ventricular trabeculae. We show that over the same incremental increases in SL, the relative changes in systolic force track more closely to the relative changes in myosin head orientation (as reported by IM3) than to the relative changes in lattice spacing. We conclude that myosin head orientation before activation determines myocardial sarcomere activation levels and that this may be the dominant mechanism for length-dependent activation