Astrocytes are the most abundant glia cell type in the central nervous system (CNS), and are known to constitute heterogeneous populations that differ in their morphology, gene expression and function. Although glial fibrillary acidic protein (GFAP) is the cardinal cytological marker of CNS astrocytes, GFAP-negative astrocytes can easily be found in the adult CNS. Astrocytes are also allocated to spatially distinct regional domains during development. This regional heterogeneity suggests that they help to coordinate post-natal neural circuit formation and thereby to regulate eventual neuronal activity. Here, during lineage-tracing studies of cells expressing Olig2 using Olig2CreER; Rosa-CAG-LSL-eNpHR3.0-EYFP transgenic mice, we found Olig2-lineage mature astrocytes in the adult forebrain. Long-term administration of tamoxifen resulted in sufficient recombinant induction, and Olig2-lineage cells were found to be preferentially clustered in some adult brain nuclei. We then made distribution map of Olig2-lineage astrocytes in the adult mouse brain, and further compared the map with the distribution of GFAP-positive astrocytes visualized in GFAPCre; Rosa-CAG-LSL-eNpHR3.0-EYFP mice. Brain regions rich in Olig2-lineage astrocytes (e.g., basal forebrain, thalamic nuclei, and deep cerebellar nuclei) tended to lack GFAP-positive astrocytes, and vice versa. Even within a single brain nucleus, Olig2-lineage astrocytes and GFAP astrocytes frequently occupied mutually exclusive territories. These findings strongly suggest that there is a subpopulation of astrocytes (Olig2-lineage astrocytes) in the adult brain, and that it differs from GFAP-positive astrocytes in its distribution pattern and perhaps also in its function. Interestingly, the brain nuclei rich in Olig2-lineage astrocytes strongly expressed GABA-transporter 3 in astrocytes and vesicular GABA transporter in neurons, suggesting that Olig2-lineage astrocytes are involved in inhibitory neuronal transmission.

Oxidative stress contributes to myocardial ischemia-reperfusion injury, which causes cardiomyocyte death and precipitate life-threatening heart failure. Propofol has been proposed to protect cells or tissues against oxidative stress. However, the mechanisms underlying its beneficial effects are not fully elucidated. In the present study, we employed an in vitro oxidative injury model, in which rat cardiac H9c2 cells were treated with H2O2, and investigated roles of propofol against oxidative stress. Propofol treatment reduced H2O2-induced apoptotic cell death. While H2O2 induced expression of the antioxidant enzyme HO-1, propofol further increased HO-1 mRNA and protein levels. Propofol also promoted nuclear localization of Nrf2 in the presence of H2O2. Knockdown of Nrf2 using siRNA suppressed propofol-inducible Nrf2 and expression of Nrf2-downstream antioxidant enzyme. Knockdown of Nrf2 suppressed the propofol-induced cytoprotection. In addition, Nrf2 overexpression induced nuclear localization of Nrf2 and HO-1 expression. These results suggest that propofol exerts antioxidative effects by inducing nuclear localization of Nrf2 and expression of its downstream enzyme in cardiac cells. Finally, we examined the effect of propofol on cardiomyocytes using myocardial ischemia-reperfusion injury models. The expression level of Nrf2 protein was increased at 15 min after reperfusion in the ischemia-reperfusion and propofol group compared with ischemia-reperfusion group in penumbra region. These results suggest that propofol protects cells or tissues from oxidative stress via Nrf2/HO-1 cascade.

Changes in astrocyte morphology are primarily attributed to the fine processes where intimate connections with neurons form the tripartite synapse and participate in neurotransmission. Recent evidence has shown that neurotransmission induces dynamic synaptic remodeling, suggesting that astrocytic fine processes may adapt their morphologies to the activity in their environment. To illustrate such a neuron-glia relationship in morphological detail, we employed a double transgenic Olig2CreER/WT; ROSA26-GAP43-EGFP mice, in which Olig2-lineage cells can be visualized and traced with membrane-targeted GFP. Although Olig2-lineage cells in the adult brain usually become mature oligodendrocytes or oligodendrocyte precursor cells with NG2-proteoglycan expression, we found a population of Olig2-lineage astrocytes with bushy morphology in several brain regions. The globus pallidus (GP) preferentially contains Olig2-lineage astrocytes. Since the GP exerts pivotal motor functions in the indirect pathway of the basal ganglionic circuit, we subjected the double transgenic mice to voluntary wheel running to activate the GP and examined morphological changes of Olig2-lineage astrocytes at both the light and electron microscopic levels. The double transgenic mice were divided into three groups: control group mice were kept in a cage with a locked running wheel for 3 weeks, Runner group were allowed free access to a running wheel for 3 weeks, and the Runner-Rest group took a sedentary 3-week rest after a 3-week running period. GFP immunofluorescence analysis and immunoelectron microscopy revealed that astrocytic fine processes elaborated complex arborization in the Runner mice, and reverted to simple morphology comparable to that of the Control group in the Runner-Rest group. Our results indicated that the fine processes of the Olig2-lineage astrocytes underwent plastic changes that correlated with overall running activities, suggesting that they actively participate in motor functions.

Sonic hedgehog (Shh), a member of the Hedgehog (Hh) family, plays essential roles in the development of the central nervous system. Recent studies suggest that the Hh signaling pathway also functions in mature astrocytes under physiological conditions. We first examined the expression of genes encoding Hh signaling molecules in the adult mouse cerebellum by in situ hybridization histochemistry. mRNA for Patched homolog 1 (Ptch1), a receptor for Hh family members, was expressed in S100β-positive astrocytes and Shh mRNA was expressed in HuC/D-positive neurons, implying that the Hh signaling pathway contributes to neuro-glial interactions. To test this hypothesis, we next examined the effects of recombinant SHH N-terminal protein (rSHH-N) on the functions of cultured cerebellar astrocytes. rSHH-N up-regulated Hh signal target genes such as Ptch1 and Gli-1, a key transcription factor of the Hh signaling pathway. Although activation of Hh signaling by rSHH-N or purmorphamine influenced neither glutamate uptake nor gliotransmitters release, inhibition of the Hh signaling pathway by cyclopamine, neutralizing antibody against SHH or intracellular Ca(2+) chelation decreased glutamate and ATP release from cultured cerebellar astrocytes. On the other hand, cyclopamine, neutralizing antibody against SHH or Ca(2+) chelator hardly affected D-serine secretion. Various kinase inhibitors attenuated glutamate and ATP release, while only U0126 reduced D-serine secretion from the astrocytes. These results suggested that the Hh signaling pathway sustains the release of glutamate and ATP and participates in neuro-glial interactions in the adult mouse brain. We also propose that signaling pathways distinct from the Hh pathway govern D-serine secretion from adult cerebellar astrocytes.

Single oligodendrocytes produce myelin sheaths around multiple axons in the central nervous system. Interfascicular oligodendrocytes (IOs) facilitate nerve conduction, but their detailed morphologies remain largely unknown. In the present study, we three‐dimensionally reconstructed IOs in the corpus callosum of adult mouse using serial block face scanning electron microscopy. The cell bodies of IOs were morphologically polarized and extended thick processes from the cytoplasm‐rich part of the cell. Processes originating from the cell body of each IO can be classified into two types: one myelinates an axon without branching, while the other type branches and each branch myelinates a distinct axon. Myelin sheaths originating from a particular IO have biased thicknesses, wrapping axons of a limited range of diameters. Consistent with this finding, IOs transduced and visualized with a rabies viral vector expressing GFP showed statistically significant variation in their myelination patterns. We further reconstructed the sheath immediately adjacent to that derived from each of the analyzed IOs; the thicknesses of the pair of sheaths were significantly correlated despite emanating from different IOs. These results suggest that a single axon could regulate myelin sheath thicknesses, even if the sheaths are derived from distinct IOs. Collectively, our results indicate that the IOs have their own myelin profiles defined by myelin thickness and axonal diameter although axons may regulate thickness of myelin sheath.