Multinucleated astrocytes in old demyelinated plaques in a patient with multiple sclerosis

The matrine derivate MASM inhibits astrocyte reactivity and alleviates experimental autoimmune encephalomyelitis in mice

Astrocytes (AST) play an important role in the pathogenesis of neurological disorders, and their activation is involved in the progression of multiple sclerosis (MS). (6aS, 10S, 11aR, 11bR, 11cS)-10-methylaminododecahydro-3a, 7a-diaza-benzo (de) anthracene-8-thione (MASM), a novel derivative of matrine, exhibits vast pharmacological activities, such as anti-tumor, anti-fibrosis and immune regulation. In this study, we demonstrate that MASM is a promising agent for the treatment of experimental autoimmune encephalomyelitis (EAE). MASM not only inhibited inflammatory responses in LPS-stimulated astrocytes, but also suppressed the formation of reactive A1 astrocyte and maintained astrocytic functions, including the ability to promote synapse formation and phagocytose synapses and myelin debris. Importantly, MASM could significantly alleviate the development of EAE, with significant inhibition of inflammation, demyelination, axon loss and the body weight loss. Meanwhile, MASM also inhibited the activation of astrocytes and improved the function of BBB in vivo. These findings provide novel insights into the protective effect of MASM on EAE, which may be a promising drug candidate for treatment of EAE.

Spectrum of sublytic astrocytopathy in neuromyelitis optica

Neuromyelitis optica is an autoimmune inflammatory disorder targeting aquaporin-4 water channels in CNS astrocytes. Histopathological descriptions of astrocytic lesions reported in neuromyelitis optica so far have emphasized a characteristic loss of aquaporin-4, with deposition of IgG and complement and lysis of astrocytes, but sublytic reactions have been underappreciated.

We performed a multi-modality study of 23 neuromyelitis optica autopsy cases (clinically and/or pathologically confirmed; 337 tissue blocks). By evaluating astrocytic morphology, immunohistochemistry and AQP4 RNA transcripts, and their associations with demyelinating activity, we documented a spectrum of astrocytopathy in addition to complement deposition, microglial reaction, granulocyte infiltration and regenerating activity.

Within advanced demyelinating lesions, and in periplaque areas, there was remarkable hypertrophic astrogliosis, more subtle than astrocytic lysis. A degenerative component was suggested by ‘dystrophic’ morphology, cytoplasmic vacuolation, Rosenthal fibres and associated stress protein markers. The abundance of AQP4 mRNA transcripts in sublytic reactive astrocytes devoid of aquaporin-4 protein supported in vivo restoration following IgG-induced aquaporin-4 endocytosis/degradation. Astrocytic alterations extending beyond demyelinating lesions speak to astrocytopathy being an early and primary event in the evolving neuromyelitis optica lesion. Focal astrocytopathy observed without aquaporin-4 loss or lytic complement component deposition verifies that astrocytic reactions in neuromyelitis optica are not solely dependent on IgG-mediated aquaporin-4 loss or lysis by complement or by IgG-dependent leucocyte mediators.

We conclude that neuromyelitis optica reflects a global astrocytopathy, initiated by binding of IgG to aquaporin-4 and not simply definable by demyelination and astrocytic lysis. The spectrum of astrocytic morphological changes in neuromyelitis optica attests to the complexity of factors influencing the range of astrocytic physiological responses to a targeted attack by aquaporin-4-specific IgG. Sublytic astrocytic reactions are no doubt an important determinant of the lesion’s evolution and potential for repair. Pharmacological manipulation of the astrocytic stress response may offer new avenues for therapeutic intervention.

Comparing astrocytic gap junction of genetic absence epileptic rats with control rats: an experimental study

The synchronization of astrocytes via gap junctions (GJ) is a crucial mechanism in epileptic conditions, contributing to the synchronization of the neuronal networks. Little is known about the endogenous response of GJ in genetic absence epileptic animal models. We evaluated and quantified astrocyte GJ protein connexin (Cx) 30 and 43 in the somatosensory cortex (SSCx), ventrobasal (VB), centromedian (CM), lateral geniculate (LGN) and thalamic reticular (TRN) nuclei of thalamus of genetic absence epilepsy rats from Strasbourg (GAERS), Wistar albino glaxo rats from Rijswijk (WAG/Rij) and control Wistar animals using immunohistochemistry and Western Blot. The Cx30 and Cx43 immunopositive astrocytes per unit area were quantified for each region of the three animal strains. Furthermore, Cx30 and Cx43 Western Blot was applied to the tissue samples from the same regions of the three strain. The number of Cx30 immunopositive astrocytes showed significant increase in both GAERS and WAG/Rij compared to control Wistar in all brain regions studied except LGN of WAG/Rij animals. Furthermore, Cx43 in both GAERS and WAG/Rij showed significant increase in SSCx, VB and TRN. The protein expression was increased in both Cx30 and Cx43 in the two epileptic strains compared to control Wistar animals. The significant increase in the astrocytic GJ proteins Cx30 and Cx43 and the differences in the co-expression of Cx30 and Cx43 in the genetically absence epileptic strains compared to control Wistar animals may suggest that astrocytic Cx's may be involved in the mechanism of absence epilepsy. Increased number of astrocytic Cx's in GAERS and WAG/Rij may represent a compensatory response of the thalamocortical circuitry to the absence seizures or may be related to the production and/or development of absence seizures.

Disorders of Astrocytes: Alexander Disease as a Model

Astrocytes undergo important phenotypic changes in many neurological disorders, including strokes, trauma, inflammatory diseases, infectious diseases, and neurodegenerative diseases. We have been studying the astrocytes of Alexander disease (AxD), which is caused by heterozygous mutations in the GFAP gene, which is the gene that encodes the major astrocyte intermediate filament protein. AxD is a primary astrocyte disease because GFAP expression is specific to astrocytes in the central nervous system (CNS). The accumulation of extremely large amounts of GFAP causes many molecular changes in astrocytes, including proteasome inhibition, stress kinase activation, mechanistic target of rapamycin (mTOR) activation, loss of glutamate and potassium buffering capacity, loss of astrocyte coupling, and changes in cell morphology. Many of these changes appear to be common to astrocyte reactions in other neurological disorders. Using AxD to illuminate common mechanisms, we discuss the molecular pathology of AxD astrocytes and compare that to astrocyte pathology in other disorders.

Human endogenous retroviruses and multiple sclerosis: innocent bystanders or disease determinants?

Human endogenous retroviruses (HERVs) constitute 5–8% of human genomic DNA and are replication incompetent despite expression of individual HERV genes from different chromosomal loci depending on the specific tissue. Several HERV genes have been detected as transcripts and proteins in the central nervous system, frequently in the context of neuroinflammation. The HERV-W family has received substantial attention in large part because of associations with diverse syndromes including multiple sclerosis (MS) and several psychiatric disorders. A HERV-W-related retroelement, multiple sclerosis retrovirus (MSRV), has been reported in MS patients to be both a biomarker as well as an effector of aberrant immune responses. HERV-H and HERV-K have also been implicated in MS and other neurological diseases but await delineation of their contributions to disease. The HERV-W envelope-encoded glycosylated protein, syncytin-1, is encoded by chromosome 7q21 and exhibits increased glial expression within MS lesions. Overexpression of syncytin-1 in glia induces endoplasmic reticulum stress leading to neuroinflammation and the induction of free radicals, which damage proximate cells. Syncytin-1's receptor, ASCT1 is a neutral amino acid transporter expressed on glia and is suppressed in white matter of MS patients. Of interest, antioxidants ameliorate syncytin-1's neuropathogenic effects raising the possibility of using these agents as therapeutics for neuroinflammatory diseases. Given the multiple insertion sites of HERV genes as complete and incomplete open reading frames, together with their differing capacity to be expressed and the complexities of individual HERVs as both disease markers and bioactive effectors, HERV biology is a compelling area for understanding neuropathogenic mechanisms and developing new therapeutic strategies.

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.

Diagnosis of inflammatory demyelination in biopsy specimens: a practical approach

Multiple sclerosis is the most frequent demyelinating disease in adults. It is characterized by demyelination, inflammation, gliosis and a variable loss of axons. Clinically and histologically, it shares features with other demyelinating and/or inflammatory CNS diseases. Diagnosis of an inflammatory demyelinating disease can be challenging, especially in small biopsy specimens. Here, we summarize the histological hallmarks and most important neuropathological differential diagnoses of early MS, and provide practical guidelines for the diagnosis of inflammatory demyelinating diseases.

TNF-alpha as an autocrine mediator and its role in the activation of Schwann cells

In the peripheral nervous system (PNS), tumor necrosis factor-alpha (TNF-alpha) derived from activated Schwann cells (SCs) play a critical role as a pleiotropic mediator. In this study, we examined the function of TNF-alpha as an inflammatory mediator in SCs activation. TNF-alpha exhibits its biological effect through two distinct surface receptors, TNF receptor 1 (TNFR1) and TNFR2. We show here that cultured SCs express both TNFR1 and TNFR2, and that activation of these receptors by TNF-alpha promotes expression of TNF-alpha. Meanwhile, TNF-alpha also increased the production of other inflammatory mediators. Furthermore, TNF-alpha is involved in the induction of apoptosis through binding to TNFR in SCs. The activation of SCs by lipopolysaccharide (LPS) is partially mediated by SCs-derived TNF-alpha. These findings suggest the existence of a positive feedback loop in the activation of SC via TNF-alpha. This loop may be involved in the prolonged activation of SCs. Acute or chronic stimulation of TNF-alpha by SC at sites of PNS inflammation may be critical in determining whether TNF-alpha has activational, inflammatory, or cytotoxic effects on these cells.

Tumor necrosis factor and its p55 and p75 receptors are not required for axonal lesion-induced microgliosis in mouse fascia dentata

Tumor necrosis factor (TNF) is a potent pro-inflammatory and neuromodulatory cytokine. In the CNS it is produced primarily by microglia and considered to regulate microglial activation. On the basis of previous observations of increased microglial TNF mRNA synthesis in areas of anterograde axonal and terminal degeneration in mice, we studied the effect of TNF and its p55 and p75 receptors on axonal lesion-induced microglial activation in fascia dentata following transection of the perforant path (PP) projection. Unexpectedly, cell counting showed that the axonal lesion-induced microglial response in TNF and TNF-p55p75 receptor knock out mice and C57BL/6 mice was similar 5 days after the lesion. In addition, the microglial expression of the lysosomal-associated antigen CD68, and the clearance of MBP(+) myelin debris appeared similar in TNF and TNF-p55p75 receptor knock out mice compared to C57BL/6 mice. Quantitative PCR and in situ hybridization showed the expression of TNF mRNA to be maximally upregulated 6 h after the lesion, and confirmed that TNF mRNA was still upregulated 5 days after lesion when microglial numbers, CD11b mRNA level, and cellular TNF-p55 and -p75 receptor mRNA level reached maximum. However, in spite of the induction of TNF mRNA, TNF protein level remained at base-line in fascia dentata using immunohistochemistry and ELISA. In conclusion, the results showed a lower than expected lesion-induced increase in TNF protein, and that neither TNF nor its receptors were required for the axonal lesion-induced microglial morphological transformation and proliferation or for the initial clearance of degenerated myelin in the PP-deafferented fascia dentata.

Human astrocytes express 14-3-3 sigma in response to oxidative and DNA-damaging stresses

The 14-3-3 protein family consists of seven isoforms, most of which are expressed abundantly in neurons and glial cells, although the sigma isoform, a p53 target gene originally identified as an epithelium-specific marker, has not been identified in the human central nervous system. Here, we show that human astrocytes in culture expressed 14-3-3sigma under stress conditions. By Western blot, the expression of 14-3-3sigma, p53 and p21 was coordinately upregulated in astrocytes following exposure to hydrogen peroxide, 4-hydroxy-2-nonenal (4-HNE) or etoposide, a topoisomerase II inhibitor. 14-3-3sigma was induced by treatment with 5-aza-2'-deoxycytidine, suggesting a hypermethylated status of the gene promoter in astrocytes. In vivo, a small subset of hypertrophic reactive astrocytes, often showing a multinucleated morphology, expressed 14-3-3sigma in active demyelinating lesions of multiple sclerosis (MS) and ischemic lesions of cerebral infarction, where the expression of 4-HNE and 8-hydroxy-2'-deoxyguanosine was enhanced in reactive astrocytes. Microarray analysis of etoposide-treated astrocytes verified upregulation of p53-responsive genes and concurrent downregulation of mitotic checkpoint-regulatory genes. These observations suggest that 14-3-3sigma might serve as a marker of oxidative and DNA-damaging stresses inducing the mitotic checkpoint dysfunction in reactive astrocytes under pathological conditions.

Autocrine activation of microglia by tumor necrosis factor-alpha

In the central nervous system (CNS), tumor necrosis factor-alpha (TNF-alpha) derived from activated microglia plays a critical role as an inflammatory mediator. In this study, we examined the function of TNF-alpha as an autocrine mediator in microglial activation. TNF-alpha induced TNF-alpha production by microglia through ligation of TNF receptor 1 (TNFR1). TNF-alpha also increased the production of other inflammatory mediators. The activation of microglia by lipopolysaccharide is partially mediated by microglia-derived TNF-alpha. These findings suggest the existence of a positive feedback loop in the activation of microglia via TNF-alpha. This autocrine loop may be involved in the prolonged activation of microglia.