Neurons develop a highly polarized morphology consisting of dendrites and a long axon. Both axons and dendrites contain microtubules and microtubule-associated proteins (MAPs) with characteristic structures. Among MAPs, MAP2 is specifically expressed in dendrites whereas MAP2C and tau are abundant in the axon. But the influence of MAP2, MAP2C and tau on the organization of microtubule domains in dendrites versus axons is unknown. Both MAP2 and tau induce microtubule bundle formation in fibroblasts after transfection of complementary DNAs, and a long process resembling an axon is extended in Sf9 cells infected with recombinant baculovirus expressing tau. We have now expressed MAP2 and MAP2C in Sf9 cells in order to compare their morphology and the arrangement of their microtubules to that found in Sf9 cells expressing tau. We report here that the spacing between microtubules depends on the MAP expressed: in cells expressing MAP2, the distance is similar to that found in dendrites, whereas the spacing between microtubules in cells expressing MAP2C or tau is similar to that found in axons.

Impurity doping often alters or improves the properties of materials. In alumina, grain boundaries play a key role in deformation mechanisms, particularly in the phenomenon of grain boundary sliding during creep at high temperatures. We elucidated the atomic-scale structure in alumina grain boundaries and its relationship to the suppression of creep upon doping with yttrium by using atomic resolution microscopy and high-precision calculations. We find that the yttrium segregates to very localized regions along the grain boundary and alters the local bonding environment, thereby strengthening the boundary against mechanical creep.

The electronic structure and bonding of a realistic model of an intergranular glassy film ͑IGF͒ was studied with multiple computational methods. The model has a Si-O-N glassy region sandwiched between crystalline basal planes of ␤-Si 3 N 4 and contains a total of 798 atoms. It was constructed with periodic boundary conditions via classical molecular dynamics ͑MD͒ techniques using an accurate multibody atomic potential. The model was then further relaxed by the VASP ͑Vienna ab initio simulation package͒ program. It is shown that the VASP-relaxed structure reduces the total energy from the MD-relaxed structure by only 47.38 eV, validating the accuracy of the multiatom potential used. The calculated electronic structure shows the IGF model to be an insulator with a sizable gap of almost 3 eV. Quasidefectlike states can be identified near the band edges arising from the more strained Si-N and Si-O bonds at the interface. Calculation of the Mulliken effective charge and bond order values indicates that the bonds in the glassy region and at the interface can be enhanced and weakened by distortions in the bond length and bond angle. The states at the top of the valence band are derived mostly from the crystalline part of the Si-N bonding while the states at the bottom of the conduction band are dominated by the Si-O bonding in the glassy region. Calculation of the electrostatic potential across the interface shows an average band offset of about 1.5 eV between the crystalline ␤-Si 3 N 4 and the glassy Si-O-N region which could be related to the space charge model for IGF.