In neurons, the distinct molecular composition of axons and dendrites is established through polarized targeting mechanisms, but it is currently unclear how nonpolarized cargoes, such as mitochondria, become uniformly distributed over these specialized neuronal compartments. Here, we show that TRAK family adaptor proteins, TRAK1 and TRAK2, which link mitochondria to microtubule-based motors, are required for axonal and dendritic mitochondrial motility and utilize different transport machineries to steer mitochondria into axons and dendrites. TRAK1 binds to both kinesin-1 and dynein/dynactin, is prominently localized in axons, and is needed for normal axon outgrowth, whereas TRAK2 predominantly interacts with dynein/dynactin, is more abundantly present in dendrites, and is required for dendritic development. These functional differences follow from their distinct conformations: TRAK2 preferentially adopts a head-to-tail interaction, which interferes with kinesin-1 binding and axonal transport. Our study demonstrates how the molecular interplay between bidirectional adaptor proteins and distinct microtubule-based motors drives polarized mitochondrial transport.

The room-temperature luminescence of single CdSe/ZnS core-shell quantum dots is investigated by spectrally and temporally resolved confocal microscopy. A large (30 nm) blue shift is observed in the emission wavelength during illumination in air. In nitrogen, no blue shift is observed. The blue shift in air is ascribed to a 1 nm shrinkage of the CdSe core by photooxidation. Photobleaching occurs about 4 times faster in air than in nitrogen, indicating the formation of nonradiative recombination centers during photooxidation. The initial light output is higher in air than in nitrogen, which may be due to a reduction of the defect state lifetime by oxygen.

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The fundamental and applied physics of noble-metal nanoparticles is currently attracting much attention. To a great extent this is due to promising new applications of noble-metal colloidal nanoparticles in fields such as materials science, [1] biophysics, [2] molecular electronics, and fluorescence-spectral engineering based on surface-enhancement effects. [3] In particular the nanoparticles have promising applications as bright fluorescent markers with enhanced photostability in fluorescence microscopy, sensor technology, and microarrays. The enhancement of the fluorescence emission of molecules near a metal surface arises from interactions with surface plasmon (SP) resonances in the metal particles. [4][5][6] These interactions may also result in shortening of the excited-state lifetime thus improving the photostability of the dye.[7]The optical properties of a fluorescent molecule located near a metal nanoparticle are affected by the near-field electrodynamical environment. [4][5][6] This can cause an enhancement or quenching of the fluorescence depending on the distance between the molecule and the metal surface. In the case of fluorescent molecules located at very short distances from a metal surface, non-radiative energy transfer to SPs in the metal takes place. [8,9] Electromagnetic-field enhancement due to SPs, however, still occurs at longer distances from the metal core. As a result, there is an optimal fluorescent molecule to metal-core distance for fluorescence enhancement. Important factors affecting the strength of the fluorescence enhancement are the size and shape of the nanoparticle, the orientation of the dye dipole moments relative to the nanoparticle surface normal, the overlap of the absorption and emission bands of the dye with the plasmon band of the metal, and the radiative decay rate and quantum yield (Q) of the fluorescent molecules.

A new global analysis algorithm to analyse (hyper-) spectral images is presented. It is based on the phasor representation that has been demonstrated to be very powerful for the analysis of lifetime imaging data. In spectral phasor analysis the fluorescence spectrum of each pixel in the image is Fourier transformed. Next, the real and imaginary components of the first harmonic of the transform are employed as X and Y coordinates in a scatter (spectral phasor) plot. Importantly, the spectral phasor representation allows for rapid (real time) semi-blind spectral unmixing of up to three components in the image. This is demonstrated on slides with fixed cells containing three fluorescent labels. In addition the method is used to analyse autofluorescence of cells in a fresh grass blade. It is shown that the spectral phasor approach is compatible with spectral imaging data recorded with a low number of spectral channels.