The role of astrocytes in oxidative stress of central nervous system: A mixed blessing

Central nervous system (CNS) maintains a high level of metabolism, which leads to the generation of large amounts of free radicals, and it is also one of the most vulnerable organs to oxidative stress. Emerging evidences have shown that, as the key homeostatic cells in CNS, astrocytes are deeply involved in multiple aspects of CNS function including oxidative stress regulation. Besides, the redox level in CNS can in turn affect astrocytes in morphology and function. The complex and multiple roles of astrocytes indicate that their correct performance is crucial for the normal functioning of the CNS, and its dysfunction may result in the occurrence and progression of various neurological disorders. To date, the influence of astrocytes in CNS oxidative stress is rarely reviewed. Therefore, in this review we sum up the roles of astrocytes in redox regulation and the corresponding mechanisms under both normal and different pathological conditions.     

Spontaneous reactive astrogliosis in the dentate gyrus of Bax-deficient mice

Astrocytes play critical roles in many aspects of brain functions via modulation of neurotransmission, metabolism, and structural remodeling in response to physiological or pathological stimuli. Activation of astrocytes is a common phenomenon in many brain pathologies such as stroke, trauma, and neurodegenerative diseases. In this study, we found that gene deletion of the pro-apoptotic gene Bax (Bax-knockout) resulted in a spontaneous reactive astrogliosis in the dentate gyrus, as evidenced by the increased number/volume of astrocytes and cytoplasmic localization of the Olig2 protein. On the other hand, there was no evidence for microglial activation in the dentate gyrus of Bax-knockout mice. Previously, we reported that Bax-knockout mice failed to execute programmed cell death of adult-produced neurons, but the surplus neurons eventually impaired normal synaptic connections and dendritic arborization of dentate gyrus neurons. Therefore, we propose that the reactive astrocytes in the Baxknockout mice may play a role in tissue remodeling of the dentate gyrus following a failure in the programmed cell death of adult-produced neurons.     

P2Y nucleotide receptor interaction with alpha integrin mediates astrocyte migration

Astrocytes become activated in response to brain injury, as characterized by increased expression of glial fibrillary acidic protein (GFAP) and increased rates of cell migration and proliferation. Damage to brain cells causes the release of cytoplasmic nucleotides, such as ATP and uridine 5'-triphosphate (UTP), ligands for P2 nucleotide receptors. Results in this study with primary rat astrocytes indicate that activation of a G protein-coupled P2Y(2) receptor for ATP and UTP increases GFAP expression and both chemotactic and chemokinetic cell migration. UTP-induced astrocyte migration was inhibited by silencing of P2Y(2) nucleotide receptor (P2Y(2)R) expression with siRNA of P2Y(2)R (P2Y(2)R siRNA). UTP also increased the expression in astrocytes of alpha(V)beta(3/5) integrins that are known to interact directly with the P2Y(2)R to modulate its function. Anti-alpha(V) integrin antibodies prevented UTP-stimulated astrocyte migration, suggesting that P2Y(2)R/alpha(V) interactions mediate the activation of astrocytes by UTP. P2Y(2)R-mediated astrocyte migration required the activation of the phosphatidylinositol-3-kinase (PI3-K)/protein kinase B (Akt) and the mitogen-activated protein kinase/extracellular signal-regulated kinase (MEK/ERK) signaling pathways, responses that also were inhibited by anti-alpha(V) integrin antibody. These results suggest that P2Y(2)Rs and their associated signaling pathways may be important factors regulating astrogliosis in brain disorders.     

Regulation of A2B adenosine receptor functioning by tumour necrosis factor a in human astroglial cells

Low-affinity A2B adenosine receptors (A2B ARs), which are expressed in astrocytes, are mainly activated during brain hypoxia and ischaemia, when large amounts of adenosine are released. Cytokines, which are also produced at high levels under these conditions, may regulate receptor responsiveness. In the present study, we detected A2B AR in human astrocytoma cells (ADF) by both immunoblotting and real-time PCR. Functional studies showed that the receptor stimulated adenylyl cyclase through Gs proteins. Moreover, A2B ARs were phosphorylated and desensitized following stimulation of the receptors with high agonist concentration. Tumour necrosis factor alpha (TNF-alpha) treatment (24- h) increased A2B AR functional response and receptor G protein coupling, without any changes in receptor protein and mRNA levels. TNF-alpha markedly reduced agonist-dependent receptor phosphorylation on threonine residues and attenuated agonist-mediated A2B ARs desensitization. In the presence of TNF-alpha, A2B AR stimulation in vitro induced the elongation of astrocytic processes, a typical morphological hallmark of in vivo reactive astrogliosis. This event was completely prevented by the selective A2B AR antagonist MRS 1706 and required the presence of TNF-alpha. These results suggest that, in ADF cells, TNF-alpha selectively modulates A2B AR coupling to G proteins and receptor functional response, providing new insights to clarify the pathophysiological role of A2B AR in response to brain damage.     

The inhibition of mammalian target of rapamycin (mTOR) in improving inflammatory response after traumatic brain injury

Traumatic brain injury (TBI) provokes primary and secondary damage on endothelium and brain parenchyma, leading neurons die rapidly by necrosis. The mammalian target of rapamycin signalling pathway (mTOR) manages numerous aspects of cellular growth, and it is up-regulated after moderate to severe traumatic brain injury (TBI). Currently, the significance of this increased signalling event for the recovery of brain function is unclear; therefore, we used two different selective inhibitors of mTOR activity to discover the functional role of mTOR inhibition in a mouse model of TBI performed by a controlled cortical impact injury (CCI). Treatment with KU0063794, a dual mTORC1 and mTORC2 inhibitor, and with rapamycin as well-known inhibitor of mTOR, was performed 1 and 4 hours subsequent to TBI. Results proved that mTOR inhibitors, especially KU0063794, significantly improved cognitive and motor recovery after TBI, reducing lesion volumes. Also, treatment with mTOR inhibitors ameliorated the neuroinflammation associated with TBI, showing a diminished neuronal death and astrogliosis after trauma. Our findings propose that the involvement of selective mTORC1/2 inhibitor may represent a therapeutic strategy to improve recovery after brain trauma.     

Astrogliosis in the hippocampus and cortex and cognitive deficits in rats with streptozotocin-induced diabetes: Effects of melatonin

We examined, using a Western blot technique, the contents and compositions of a specific neuronal protein, NCAM, and of an astrocyte marker, GFAP, in the hippocampus and cortex of rats with streptozotocin (STZ)-induced diabetes and compared these indices with those in control (intact) animals and STZ-diabetic rats treated with melatonin. Behavioral cognitive indices manifested in the passive avoidance test (PAT) and Morris water maze (MWM) learning performance were also estimated in the above groups of animals. As was found, STZ-diabetic rats demonstrated clear cognitive deficits according to the values of the retention latency in the PAT and time of reaching the escape platform in the MWM performance. In these animals, the GFAP content was elevated, and the amount of degraded products of this protein increased, as compared with the control. Simultaneously, considerable down-regulation of the NCAM expression and modifications of NCAM isoform composition were found in diabetic animals. In addition, significantly increased levels of lipid peroxidation (according to the amounts of malondialdehyde + 4-hydroxyalkenals) were measured in the cortex and hippocampus of rats with stable diabetic hyperglycemia. All the above-mentioned shifts were significantly smoothed or even nearly completely compensated in the case of treatment of STZ-diabetic rats with melatonin (10 mg/kg per day). The role of diabetes-related changes in the amount and composition of specific neural and glial proteins in the development of cognitive deficits, the involvement of oxidative stress in the mechanisms of the respective shifts, and possible mechanisms of the neuroprotective effect of melatonin with respect to diabetes-related pathological biochemical and behavioral shifts are discussed. Neirofiziologiya/Neurophysiology, Vol. 40, No. 2, pp. 105–111, March–April, 2008.     

分子疫苗防治老年性痴呆小鼠模型的研究进展

老年斑是老年性痴呆(Alzheimer's disease, AD)的特征性病理改变,β-淀粉样(β-amyloid,Aβ)是其主要成分.将Aβ作为疫苗对动物进行免疫,有明显抑制转基因小鼠脑内Aβ的沉积,清除原有斑块的作用,从而减少异常Aβ沉积对认知功能的损害.疫苗没有引起针对动物自身抗原的免疫反应,对中枢神经系统的正常功能也没有明显损害.     

Cross‐talk between monocyte invasion and astrocyte proliferation regulates scarring in brain injury

Scar formation after brain injury is still poorly understood. To further elucidate such processes, here, we examine the interplay between astrocyte proliferation taking place predominantly at the vascular interface and monocyte invasion. Using genetic mouse models that decrease or increase reactive astrocyte proliferation, we demonstrate inverse effects on monocyte numbers in the injury site. Conversely, reducing monocyte invasion using CCR2^?/? mice causes a strong increase in astrocyte proliferation, demonstrating an intriguing negative cross‐regulation between these cell types at the vascular interface. CCR2^?/? mice show reduced scar formation with less extracellular matrix deposition, smaller lesion site and increased neuronal coverage. Surprisingly, the GFAP⁺ scar area in these mice is also significantly decreased despite increased astrocyte proliferation. Proteomic analysis at the peak of increased astrocyte proliferation reveals a decrease in extracellular matrix synthesizing enzymes in the injury sites of CCR2^?/? mice, highlighting how early key aspects of scar formation are initiated. Taken together, we provide novel insights into the cross‐regulation of juxtavascular proliferating astrocytes and invading monocytes as a crucial mechanism of scar formation upon brain injury.     

Prolactin induced expression of interleukin-1 alpha,tumor necrosis factor-alpha,and transforming growth factor-alpha in cultured astrocytes

Prolactin (PRL) is a potent mitogen in cultured astrocytes. Because one of the major effects of astrocyte proliferation is the expression of inflammatory cytokines, we examined the effect of PRL-induced mitogenesis on the expression of interleukin-1 (IL-1α), tumor necrosis factor-α (TNF-α), and transforming growth factor-α (TGF-α) in cultured astrocytes. Astrocytes were stimulated with PRL or growth hormone (GH), and the expression of cytokines was determined by immunohistochemistry and Western blot analysis. Following incubation of astrocytes with 1 nM PRL for 6 h, strong positive staining of IL-1α and TNF-α, but not TGF-α, was found. No detectable staining for the above cytokines was found in vehicle, or GH treated astrocytes. When astrocytes were incubated in the presence of 1 nM PRL for 18 h, strong positive staining for IL-1α and TGF-α was found. Immunocytochemical analysis of the expression of TNF-α and IL-1α in PRL stimulated astrocytes suggested that the expression of IL-1α preceded the expression of TNF-α. To confirm this observation, Western blot analyses were performed on extracts from astrocytes incubated with 1 nM PRL. In unstimulated astrocytes, IL-1α levels were not detectable. In astrocytes stimulated with 1 nM PRL, expression of IL-1α was clearly detected after 1 h of incubation, and IL-1α levels continued to increase during the course of the experiment (6 h). In contrast, in astrocytes stimulated with 1 nM PRL, an increase in the expression of TNF-α was first apparent after 2 h of incubation. TNF-α levels peaked 3 to 4 h after the addition of PRL, and returned to near control levels after 6 h. Finally, injection of PRL into a wound site in female rats increased the expression of glial fibrillary acid protein (GFAP), an astrocyte specific protein. These data suggest that PRL can stimulate astrogliosis at the wound site in vivo. These data clearly indicate that PRL can stimulate the expression of TNF-α and IL-1α in cultured astrocytes and suggest that PRL may play a role in the regulation of the neuroimmune response in vivo.