The neurogenic potential of astrocytes is regulated by inflammatory signals

Although the adult brain contains neural stem cells (NSCs) that generate new neurons throughout life, these astrocyte-like populations are restricted to two discrete niches. Despite their terminally differentiated phenotype, adult parenchymal astrocytes can re-acquire NSC-like characteristics following injury, and as such, these 'reactive' astrocytes offer an alternative source of cells for central nervous system (CNS) repair following injury or disease. At present, the mechanisms that regulate the potential of different types of astrocytes are poorly understood. We used in vitro and ex vivo astrocytes to identify candidate pathways important for regulation of astrocyte potential. Using in vitro neural progenitor cell (NPC)-derived astrocytes, we found that exposure of more lineage-restricted astrocytes to either tumor necrosis factor alpha (TNF-α) (via nuclear factor-κB (NFκB)) or the bone morphogenetic protein (BMP) inhibitor, noggin, led to re-acquisition of NPC properties accompanied by transcriptomic and epigenetic changes consistent with a more neurogenic, NPC-like state. Comparative analyses of microarray data from in vitro-derived and ex vivo postnatal parenchymal astrocytes identified several common pathways and upstream regulators associated with inflammation (including transforming growth factor (TGF)-β1 and peroxisome proliferator-activated receptor gamma (PPARγ)) and cell cycle control (including TP53) as candidate regulators of astrocyte phenotype and potential. We propose that inflammatory signalling may control the normal, progressive restriction in potential of differentiating astrocytes as well as under reactive conditions and represent future targets for therapies to harness the latent neurogenic capacity of parenchymal astrocytes

Astrocytes: biology and pathology

Astrocytes are specialized glial cells that outnumber neurons by over fivefold. They contiguously tile the entire central nervous system (CNS) and exert many essential complex functions in the healthy CNS. Astrocytes respond to all forms of CNS insults through a process referred to as reactive astrogliosis, which has become a pathological hallmark of CNS structural lesions. Substantial progress has been made recently in determining functions and mechanisms of reactive astrogliosis and in identifying roles of astrocytes in CNS disorders and pathologies. A vast molecular arsenal at the disposal of reactive astrocytes is being defined. Transgenic mouse models are dissecting specific aspects of reactive astrocytosis and glial scar formation in vivo. Astrocyte involvement in specific clinicopathological entities is being defined. It is now clear that reactive astrogliosis is not a simple all-or-none phenomenon but is a finely gradated continuum of changes that occur in context-dependent manners regulated by specific signaling events. These changes range from reversible alterations in gene expression and cell hypertrophy with preservation of cellular domains and tissue structure, to long-lasting scar formation with rearrangement of tissue structure. Increasing evidence points towards the potential of reactive astrogliosis to play either primary or contributing roles in CNS disorders via loss of normal astrocyte functions or gain of abnormal effects. This article reviews (1) astrocyte functions in healthy CNS, (2) mechanisms and functions of reactive astrogliosis and glial scar formation, and (3) ways in which reactive astrocytes may cause or contribute to specific CNS disorders and lesions

Photoluminescence of He-implanted ZnO

A study of the effects of ion-implanted He+ on the 4.2 K photoluminescence (PL) of ZnO is presented. This investigation is motivated by the need to further understand the effects of damage resulting from the implantation process on the PL of ZnO. For this study, 10 keV He+ ions were implanted with a fluence of 2.5 × 1013/cm2 in the (0 0 0 1) Zn-terminated surface. The implantation process is seen to reduce the overall luminescence efficiency, although the number and relative intensities of the bound-exciton peaks are observed to be similar to that of unimplanted ZnO. The 4.2 K PL of the implanted surface exhibits a broad orange/red peak near 1.86 eV nm and is attributed to damage introduced during the implantation process. This peak is identified as donor-acceptor pair (DAP) luminescence with a thermal activation energy of 11 meV. The 1.86 eV peak is not observed for H-implanted ZnO suggesting that H passivates the implantation-induced defects responsible for this luminescence. © 2005 Elsevier B.V. All rights reserved

Effects of hydrogen implantation on the photoluminescence and carrier mobility of ZnO films

A study of the effects of H+ implantation on the photoluminescence (PL) and carrier mobility of ZnO thin films is presented. The 4.2 K PL of the as-grown films exhibits free-exciton luminescence at 3.3755 eV and strong bound-exciton luminescence between 3.33 and 3.37 eV including a peak observed at 3.3313 eV which is identified as the collapse of excitons bound to structural defects. While the implantation process results in a slight reduction of the overall PL intensity due to the introduction of nonradiative centers, the intensity of the 3.3313 eV bound-exciton peak is reduced by more than two orders of magnitude. We attribute this reduction to preferential interaction of hydrogen with structural defects, such as passivation or the formation of complex defects. Room temperature Hall measurements show that changes in the optical properties of H-implanted ZnO are also accompanied by an increased carrier concentration and decreased carrier mobility. © 2006 Elsevier B.V. All rights reserved

Room temperature ferromagnetism of Co doped TiO<inf>2</inf> using ion implantation and defect engineering

Ferromagnetic (FM) semiconductors obtained by doping ferromagnetic elements into a nonmagnetic semiconductor matrix are essential for the second generation of spintronics devices. In this study, we investigate Co doping behavior and subsequent magnetic properties in Co implanted and thermally annealed TiO2. In TiO2 single crystals, a decrease in the oxygen partial pressure during thermal annealing is found to enhance the Co substitutional fraction by increasing the concentration of oxygen vacancies. Magnetic properties determined from superconducting quantum interference device magnetometer (SQUID) measurements show that TiO2 crystals with a large fraction of substitutional Co are ferromagnetic at room temperature. In addition to single crystals, the feasibility of Co doping via ion implantation is studied in sol-gel synthesized TiO2 thin films. Results from grazing incidence X-ray diffraction (GIXRD) show that the implantation can produce Co doped TiO2 thin films and that the Co incorporation into Ti lattice site accompanies the transition from rutile to anatase phase. These results show that ion beam synthesis is a useful tool for producing ferromagnetic TiO2 with a high Curie temperature (TC). © 2006 Elsevier B.V. All rights reserved

Photoluminescence and Raman Spectroscopy of Polycrystalline ZnO Nanofibers Deposited by Electrospinning

The technique of electrospinning offers the advantage of growing nanowires in bulk quantities in comparison with traditional methods. We report optical studies of polycrystalline zinc oxide (ZnO) nanofibers (∼100 nm thick and 5 μm long) deposited by electrospinning. Photoluminescence from the nanofibers shows a near-ultraviolet (near-UV) peak corresponding to near-band-edge emission and a strong broad peak in the visible region from oxygen antisite and interstitial defects. Temperature-dependent photoluminescence spectroscopy reveals that different carrier recombination mechanisms are dominant at low temperature. Our Raman spectroscopy results demonstrate that characterization of the quasimodes of longitudinal optical (LO) and transverse optical (TO) phonons present in an ensemble of polycrystalline nanofibers tilted at various angles in addition to the dominant E 2(high) mode provides a promising technique for assessing the quality of such randomly oriented nanowires