No abstract

Mitochondrial Ca2+ uptake is crucial for coupling receptor stimulation to cellular bioenergetics. Further, Ca2+ uptake by respiring mitochondria prevents Ca2+-dependent inactivation (CDI) of store-operated Ca2+ release-activated Ca2+ (CRAC) channels and inhibits Ca2+ extrusion to sustain cytosolic Ca2+ signaling. However, how Ca2+ uptake by the mitochondrial Ca2+ uniporter (MCU) shapes receptor-evoked interorganellar Ca2+ signaling is unknown. Here, we generated several cell lines with MCU-knockout (MCU-KO) as well as tissue-specific MCU-knockdown mice. We show that mitochondrial depolarization, but not MCU-KO, inhibits store-operated Ca2+ entry (SOCE). Paradoxically, despite enhancing Ca2+ extrusion and promoting CRAC channel CDI, MCU-KO increased cytosolic Ca2+ in response to store depletion. Further, physiological agonist stimulation in MCU-KO cells led to enhanced frequency of cytosolic Ca2+ oscillations, endoplasmic reticulum Ca2+ refilling, NFAT nuclear translocation and proliferation. However, MCU-KO did not affect inositol-1,4,5-trisphosphate receptor activity. Mathematical modeling supports that MCU-KO enhances cytosolic Ca2+, despite limiting CRAC channel activity.

Heterostructured nanowires of IV-IV materials have attracted attention for their potential as important building blocks to construct nanoscale electronics circuits and light-emitting devices. Recently, one-dimensional SiGe/Si heterostructures have been successfully generated by the metalcatalyst VLS mechanism, self-catalyst VLS mechanism and oxide-assisted growth (OAG) mechanism [1][2][3]. The elevated temperatures used for the processing of SiGe/Si devices may lead to interdiffusion between these miscible components, which would degrade the functionality of the device. Diffusion mechanisms in nanostructured materials are not well understood, and fundamental studies on diffusion mechanisms and kinetics in nanowires are necessary.We successfully grew metal-free SiGe tip / Si nanowire heterostructures by pulsed laser vaporization (PLV). This heterostructure provides a model structure for diffusions studies in the nanowire geometry. We focused on [111]-growth SiGe tip/Si nanowire heterostructures and studied Ge diffusion behavior at different annealing temperatures from 200 o C to 800 o C in N 2 /H 2 atmosphere. Structural and chemical characterizations of nanowire heterostructures were carried out using transmission electron microscopy (TEM) / scanning TEM (STEM) and complementary energydispersive x-ray spectroscopy (EDS). A dark-field STEM image of an as-grown sample is shown in Fig. 1A. High-resolution transmission electron microscopy (HRTEM) images of as-grown sample and sample annealed at 400 o C are shown in Fig. 1B and 1C, respectively. Corresponding to the area enclosed by the box shown in Figure 1B, Fig.1D shows a coherent twin boundary in the Si nanowire. Fig.2A and Fig. 2B show Ge concentration profiles along nanowire growth direction [111] determined by quantitative analysis of energy-dispersive x-ray spectroscopy for as-grown and 400 o C annealed samples. Ge compositions in Si nanowires were quantified by the ζ factor approach [4]. The ζ factor and absorption correction considered the Si and Ge Kα lines from at standard SiGe alloy thin-film sample.A limited-source diffusion mode was applied to determine the diffusion coefficient at different annealing temperatures from 200 o C to 800 o C. We evaluated the activation energy barrier for Ge diffusion in [111] growth Si nanowire to be 0.62eV by fitting an Arrhenius plot of the Ge diffusion coefficient. We found this activation barrier to be much lower than the value for Ge diffusion in Si bulk. In fact, the 0.62eV activation energy barrier is similar to the previous reported aviation energy barrier of 0.67eV for Ge diffusion on Si surface [5]. Proposed mechanisms for diffusion in the nanowire geometry will be discussed.

Si/Si 1-x Ge x axial heterostructure nanowires (hNWs) have potential as components in high performance electronic, photonic and thermoelectric devices[1, 2, 3]. Successful implementation in such devices, however, requires precise control of the Ge incorporation in the Si 1-x Ge x segments. Nanowires are often fabricated via the vapor-liquid-solid (VLS) process, whereby crystallization occurs from the liquid phase. For axial hNWs the axial composition is modulated and also requires the ability to form segments with abrupt interfaces. Such interfaces require the ability to rapidly vary the liquid-phase composition.