Reactive astroglia-neuron relationships in the human cerebellar cortex: a quantitative, morphological and immunocytochemical study in Creutzfeldt-Jakob disease

Blunted mGluR Activation Disinhibits Striatopallidal Transmission in Parkinsonian Mice

The prevailing circuit model predicts that hyperactivity of the striatopallidal pathway and subsequently increased inhibition of external globus pallidus (GPe) neurons lead to the hypokinetic symptoms of Parkinson's disease (PD). It is believed that hyperactivity of the striatopallidal pathway is due to inactivity of dopamine receptors on the somatodendritic membrane of striatopallidal neurons, but the exact cellular underpinnings remain unclear. In this study, we show that mouse GPe astrocytes critically control ambient glutamate level, which in turn gates striatopallidal transmission via the activation of presynaptic metabotropic glutamate receptors. This presynaptic inhibition of striatopallidal transmission is diminished after the chronic loss of dopamine. Elevation of intracellular glutamate content in astrocytes restores the proper regulation of the striatopallidal input in PD models. These findings argue that astrocytes are key regulators of the striatopallidal synapse. Targeting this cell class may serve as an alternative therapeutic strategy for PD.

Bergmann glia translocation: a new disease marker for vanishing white matter identifies therapeutic effects of Guanabenz treatment

AIM

Vanishing White Matter (VWM) is a devastating leucoencephalopathy without effective treatment options. Patients have mutations in the EIF2B1-5 genes, encoding the five subunits of eIF2B, a guanine exchange factor that is an important regulator of protein translation. We recently developed mouse models for VWM that replicate the human disease. To study disease improvement after treatment in these mice, it is essential to have sensitive biomarkers related to disease stage. The Bergmann glia of the cerebellum, an astrocytic subpopulation, translocate into the molecular layer in symptomatic VWM mice and patients. This study looked at the prospects of using Bergmann glia pathology as an objective disease marker for VWM.

METHODS

We defined a new quantitative measurement of Bergmann glia pathology in the cerebellum of VWM mice and patients. To test the sensitivity of this new marker for improvement, VWM mutant mice received long-term treatment with Guanabenz, an FDA-approved anti-hypertensive agent affecting eIF2B activity.

RESULTS

Bergmann glia translocation was significantly higher in symptomatic VWM mice and VWM patients than in controls and worsened over the disease course. Both Bergmann glia pathology and cerebellar myelin pathology improved with Guanabenz treatment in mice, showing that Bergmann glia translocation is a sensitive measurement for improvement.

CONCLUSIONS

Bergmann glia translocation can be used to objectively assess effects of treatment in VWM mice. Future treatment strategies involving compounds regulating eIF2 phosphorylation might benefit VWM patients.

Dynamic distribution and stem cell characteristics of Sox1-expressing cells in the cerebellar cortex

Bergmann glia cells are a discrete radial glia population surrounding Purkinje cells in the cerebellar cortex. Although Bergmann glia are essential for the development and correct arborization of Purkinje cells, little is known about the regulation of this cell population after the developmental phase. In an effort to characterize this population at the molecular level, we have analyzed marker expression and established that adult Bergmann glia express Sox1, Sox2 and Sox9, a feature otherwise associated with neural stem cells (NSCs). In the present study, we have further analyzed the developmental pattern of Sox1-expressing cells in the developing cerebellum. We report that before becoming restricted to the Purkinje cell layer, Sox1-positive cells are present throughout the immature tissue, and that these cells show characteristics of Bergmann glia progenitors. Our study shows that these progenitors express Sox1, Sox2 and Sox9, a signature maintained throughout cerebellar maturation into adulthood. When isolated in culture, the Sox1-expressing cerebellar population exhibited neurosphere-forming ability, NSC-marker characteristics, and demonstrated multipotency at the clonal level. Our results show that the Bergmann glia population expresses Sox1 during cerebellar development, and that these cells can be isolated and show stem cell characteristics in vitro, suggesting that they could hold a broader potential than previously thought.

Loss of cerebellar granule neurons is associated with punctate but not with large focal deposits of prion protein in Creutzfeldt-Jakob disease

Whether aggregates of prion protein (PrP) reflect neurotoxicity or are neuroprotective in prion diseases is unclear. To address this question, we performed a clinicopathologic study of cerebellar granular neurons in 100 patients affected with sporadic Creutzfeldt-Jakob disease (CJD). There was significant loss of these neurons in the subset of cases with Val/Val genotype at PRNP Codon 129 and Molecular Isotype 2 of abnormal PrP (sporadic CJD-VV2) (n=32) compared with both the other CJD subtypes and to controls. Pathological PrP deposits of the punctate-type (synaptic-type) in this subgroup correlated with neuronal loss and proliferation of astrocytes and microglia. By contrast, the numbers of large deposits (5- to 50-microm-diameter) and numbers of amyloid plaques did not correlate with neuronal loss. These findings are consistent with the view that large aggregates may protect neurons by sequestering neurotoxic PrP oligomers, whereas punctate deposits may indicate the location of neuronal death processes in CJD.

Canine cognitive dysfunction and the cerebellum: acetylcholinesterase reduction, neuronal and glial changes

The specific functional and pathological alterations observed in Alzheimer's disease are less severe in the cerebellum than in other brain areas, particularly the entorhinal cortex and hippocampus. Since dense core amyloid-beta plaque formation has been associated with an acetylcholinesterase heterogeneous nucleator action, we examined if an acetylcholinesterase imbalance was involved in cerebellum plaque deposition. By using the canine counterpart of senile dementia of the Alzheimer's type, a promising model of human brain aging and early phases of Alzheimer's disease, we investigated how cerebellar pathology and acetylcholinesterase density could be related with cognitive dysfunction. As in Alzheimer's disease, the late affectation of the cerebellum was evidenced by its lack of amyloid-beta plaque and the presence of diffuse deposition throughout all cortical grey matter layers. The highest acetylcholinesterase optic density corresponded to cerebellar islands of the granular layer and was predominantly associated with synaptic glomeruli and the somata of Golgi cells. Its reduction correlated with aging and loss of granule cells, whereas cognitive deficit only correlated with loss of Purkinje cells. The observed Bergmann glia alterations may correspond to a reactive response to the loss and damage of the Purkinje cells, their specific neuronal partner. Regarding the role of acetylcholinesterase mediation in amyloid-beta deposition, our data argue against an interaction between these two proteins because acetylcholinesterase reduction correlates with aging but not with cognitive deficit. Finally, our data support the use of companion dogs of all breeds to study aging and early phases of Alzheimer's disease.

Meningothelial hyperplasia: a detailed clinicopathologic, immunohistochemical and genetic study of 11 cases

Meningothelial hyperplasia is a poorly characterized entity, often associated with advanced age, chronic renal failure, trauma, hemorrhage, and neoplasia. In order to elucidate the nature of this lesion, 11 cases defined by the presence of nests of 10 or more cell layers thick, were compared with normal arachnoidal cap cells and meningiomas. Immunohistochemistry and FISH were performed to determine NF2 (merlin), protein 4.1B, EMA, progesterone receptor (PR), EGFR, survivin, VEGF, PDGF-BB, PDGFR-beta, E-cadherin, and cathepsin D status. All cases had at least one putative predisposing factor, including hemorrhage (7), chronic renal disease (5), old age (5), trauma (1), and an adjacent optic nerve pilocytic astrocytoma (1). There was typically a discontinuous growth pattern, with no invasion of surrounding normal tissue. No gene deletions were found, though scattered polyploid cells were seen in 2 cases. The immunoprofile was similar to normal cap cells with one exception; whereas normal cells were uniformly negative for PR, nuclear positivity was seen in 64% of hyperplasias, a frequency similar to that of benign meningiomas. Our data suggest that meningothelial hyperplasia is a reactive process that is usually distinguishable from meningioma based on clinicopathologic and genetic features. It may be preneoplastic in some, though further studies are needed to test this hypothesis.

The brain-specific protein MLC1 implicated in megalencephalic leukoencephalopathy with subcortical cysts is expressed in glial cells in the murine brain

The human MLC1 gene (also known as KIAA0027 and WKL1) and its murine orthologue (Mlc1) encode a putative transmembrane protein expressed primarily in brain. Recessive mutations within human MLC1 cause megalencephalic leukoencephalopathy with subcortical cysts (MLC), whereas a missense mutation resulting in a methionine substitution within a transmembrane leucine string of MLC has been implicated in catatonic schizophrenia in a large pedigree. To gain insight into the function of the MLC protein and to elucidate the pathophysiology of these severe neurodegenerative disorders, information on the cellular and regional distribution of the murine Mlc1, as well as the developmental pattern of Mlc1 expression in brain, is required. Using in situ hybridization (ISH), Mlc1 mRNA was exclusively detected in glial cells of the adult murine brain, such as astrocytes, Bergmann glia, and ependymal cells. ISH, Northern blot analysis, and quantitative real-time polymerase chain reaction (PCR) demonstrated that Mlc1 mRNA is broadly distributed in the adult mouse brain, with highest concentrations of expression in the cerebellum and olfactory bulb. Furthermore, differential expression patterns during brain development were revealed. Overall brain Mlc1 mRNA concentrations exhibited a substantial increase in the perinatal period reaching adult concentrations at postnatal day 5. At the cellular level, highest Mlc1 expression was found during the pre- and perinatal period in multipotential neural precursor cells, especially in the subventricular zone of the lateral ventricle, whereas in adulthood highest Mlc1 mRNA concentrations were revealed in Bergmann glia cells. Because the temporal expression profile of Mlc1 indicates that, in contrast to developing and mature astrocytes, oligodendrocytes are devoid of Mlc1 expression, white matter tract abnormalities observed in these disorders may result from a primary astrocytic defect. Detailed information on Mlc1 expression in brain is likely to lead to a better understanding of Mlc1 involvement in the pathogenesis of both MLC and catatonic schizophrenia.

Dendritic spine pathology: cause or consequence of neurological disorders?

Altered dendritic spines are characteristic of traumatized or diseased brain. Two general categories of spine pathology can be distinguished: pathologies of distribution and pathologies of ultrastructure. Pathologies of spine distribution affect many spines along the dendrites of a neuron and include altered spine numbers, distorted spine shapes, and abnormal loci of spine origin on the neuron. Pathologies of spine ultrastructure involve distortion of subcellular organelles within dendritic spines. Spine distributions are altered on mature neurons following traumatic lesions, and in progressive neurodegeneration involving substantial neuronal loss such as in Alzheimer's disease and in Creutzfeldt-Jakob disease. Similarly, spine distributions are altered in the developing brain following malnutrition, alcohol or toxin exposure, infection, and in a large number of genetic disorders that result in mental retardation, such as Down's and fragile-X syndromes. An important question is whether altered dendritic spines are the intrinsic cause of the accompanying neurological disturbances. The data suggest that many categories of spine pathology may result not from intrinsic pathologies of the spiny neurons, but from a compensatory response of these neurons to the loss of excitatory input to dendritic spines. More detailed studies are needed to determine the cause of spine pathology in most disorders and relationship between spine pathology and cognitive deficits.

Astrocytic and microglia cells reactivity induced by neonatal administration of glutamate in cerebral cortex of the adult rats

Recent studies confirm that astrocytes and neurons are associated with the synaptic transmission, particularly with the regulation of glutamate (Glu) levels. Therefore, they have the capacity to modulate the Glu released from neurons into the extracellular space. It has also been demonstrated an intense astrocytic and microglia response to physical or chemical lesions of the central nervous system. However, the persistence of the response of the glial cells in adult brain had not been previously reported, after the excitotoxic damage caused by neonatal dosage of monosodium glutamate (MSG) to newborn rats. In this study, 4 mg/g body weight of MSG were administered to newborn rats at 1, 3, 5, and 7 days after birth, at the age of 60 days the astrocytes and the microglia cells were analyzed with immunohistochemical methods in the fronto-parietal cortex. Double labeling to glial fibrillary acidic protein (GFAP) and BrdU, or isolectin-B(4) and BrdU identified astrocytes or microglia cells that proliferated; immunoblotting and immunoreactivity to vimentin served for assess immaturity of astrocytic intermediate filaments. The results show that the neonatal administration of MSG-induced reactivity of astrocytes and microglia cells in the fronto-parietal cortex, which was characterized by hyperplasia; an increased number of astrocytes and microglia cells that proliferated, hypertrophy; increased complexity of the cytoplasm extension of both glial cells and expression of RNAm to vimentin, with the presence of vimentin-positive astrocytes. This glial response to neuroexcitotoxic stimulus of Glu on the immature brain, which persisted to adulthood, suggests that the neurotransmitter Glu could trigger neuro-degenerative illnesses.

Identification of upregulated genes in scrapie-infected brain tissue

The pathogenesis of scrapie, and of neurodegenerative diseases in general, is still insufficiently understood and is therefore being intensely researched. There is abundant evidence that the activation of glial cells precedes neurodegeneration and may thus play an important role in disease development and progression. The identification of genes with altered expression patterns in the diseased brain may provide insight on the molecular level into the process which ultimately leads to neuronal loss. Differentially expressed genes in scrapie-infected brain tissue were enriched by the suppression subtractive hybridization technique, molecularly cloned, and further characterized. Northern blotting and nucleotide sequencing confirmed the identities of 19 upregulated genes, 11 of which were unknown to be affected by scrapie. A considerable number of these 19 genes, namely those encoding interferon-inducible protein 10 (IP-10), 2',5'-oligo(A) synthetase, Mx protein, IIGP protein, major histocompatibility complex classes I and II, complement, and beta(2)-microglobulin, were inducible by interferons (IFNs), suggesting that an IFN response is a possible mechanism of gene activation in scrapie. Among the newly found genes, that coding for 2',5'-oligo(A) synthetase is of special interest because it could contribute to the apoptotic loss of neuronal cells via RNase L activation. In addition, upregulation of the chemokine IP-10 and B-lymphocyte chemoattractant mRNAs was seen at relatively early stages of the disease and was sustained throughout disease development.

Long-term evolution of local, proximal and remote astrocyte responses after diverse nucleus basalis lesioning (an experimental Alzheimer model): GFAP immunocytochemical study

A study on long-term astrocytic responses (from 1 day to 20 months after lesioning in 4-month-old rats, and from 1 day to 6 months in 20-month-old rats) to diverse unilateral damage of the nucleus basalis (nbM) by injection of 40 nmol of ibotenic acid, or 50 or 100 nmols of quisqualic acid was performed using a histochemical method (immunoreactivity against the glial fibrillary acidic protein GFAP). Glial reactivity (i.e., isolated or clustered hypertrophic and/or hyper-reactive astrocytes) was evaluated in several ipsilateral and contralateral brain regions: the 'local response' within the damaged nbM region; the 'proximal response' (a new concept proposed by us) in the non-damaged structures neighbouring the nbM; and the 'remote response' in the ipsilateral brain cortex and in the contralateral cortex and nbM. In 4-month-old animals, the remote cortical glial responses, independent of the involution of cortical cholinergic activity and randomly located in layers I-V of motor and somatosensory cortical regions, were similar in appearance over a long period (13-20 months), with the highest reactivity 45 days after lesioning. The proximal response lasted from 1 day to 13 months and afterwards tended to disappear. Contralateral reactivity and ipsilateral cortical scars were observed. The local (nbM) glial response was maintained throughout the period studied. Subsets of astrocytes of different reactivities were observed, most of their elements being highly intermeshed. In 20-month-old animals, nbM lesions produced less positive, but similar, glial reactive patterns. This glial reactivity was superposed onto the glial reactivity of old age. All these results are discussed. The maintenance of reactive astrocytes many months after lesioning suggests the existence of cellular factors other than those produced by damaged nbM neurons. Taking into account the role of glial cells under pathological conditions, it is possible that these reactive astrocytes in humans could promote neurodegenerative processes, such as amyloid plaque formation and neurodegeneration (Alzheimer's disease). Along this line, nbM cholinergic involution could then originate cortical involution through induced reactive astrocytosis.

Reactive microglia in Creutzfeldt-Jakob disease

Creutzfeldt-Jakob disease (CJD) is characterized by a loss of neurons accompanied by astrogliosis and spongiform changes in the neuropil. It has been recognized that reactive microglia occur in CJD but little is known about the regional distribution and extent of the microglial activation. We have, therefore, examined six brains from cases of sporadic CJD by immunohistochemical labelling of grey and white matter microglia from frontal, parietal, temporal, and occipital lobes, striatum, thalamus, cerebellum and brain stem with RCA-1, LCA, CD68, HLA-DR, and HAM56. Microglial activation occurred in the grey matter where astrogliosis and prion protein (PrP) deposits were prominent. Processes of activated microglia surrounded the outer rim of spongy vacuoles. A diffuse microglial response was seen in the white matter that was immunophenotypically different from grey matter. Double-labelling with microglial markers and anti-PrP showed that activated microglia did not contain PrP-immunoreactivity. Therefore a primary role of microglia in PrP processing seems unlikely. Activated microglia may contribute to neuronal damage in CJD due to their cytotoxic potential.

Astrocytic gap junction removal, connexin43 redistribution, and epitope masking at excitatory amino acid lesion sites in rat brain

We previously reported that kainic acid (KA) lesion sites in rat brain exhibit an absence of astrocytic gap junctions at 1 week post-lesion. Loss of immunocytochemical reactivity with a sequence-specific antibody against the astrocytic gap junctional protein connexin43 (Cx43) suggested epitope masking since persistence of Cx43 was observed on Western blots. Here, we determined the fate of Cx43 at various times after thalamic KA and striatal NMDA lesions. In normal tissue and at 6 hr post-KA lesion, Cx43 immunoreactivity predominated at typical astrocytic gap junctions. Immunolabelled junctions were still seen at 3 days, with epitope masking already present, and were virtually absent by 6 days post-lesion. Gap junction remodeling was indicated by the appearance of intracellular immunostained annular profiles and uncharacteristically extensive gap junctions between symmetrically immunolabelled membranes and between labelled astrocytic and unlabelled oligodendrocytic membranes. Labelled multivesicular clusters emerged at 2 days, were numerous at 3 days and constituted the sole Cx43 sequestration site by post-lesion day 6. Ultrastructural disruption and gap junction disassembly progressed more slowly in NMDA-injected tissue where immunoreactivity persisted, albeit at markedly decreasing levels until the final survival time examined (16 days). Intense Cx43 immunolabelling was seen in filopodia of putative reactive astrocytes at the lesion periphery at 6-8 days and was associated at 16 days with an increased number of gap junctions primarily between fine astrocytic processes. These results demonstrate that massive neuronal loss alone or in conjunction with direct actions of excitotoxins on astrocytes precipitates an astrocytic reaction accompanied initially by removal of their gap junctions followed by redistribution of Cx43, and suggest that the astrocytic syncytium may undergo reorganization in a manner leading to isolation of the lesion site.

Food restriction delays the age-related increase in GFAP mRNA in rat hypothalamus

Astrogliosis with advancing age is correlated with increased expression of glial fibrillary acidic protein (GFAP). Hypothalamic GFAP mRNA prevalence was determined in male F344 rats of different ages that were fed ad lib (AL) and compared with that of rats that were food-restricted (FR) to 60% of AL levels. Hypothalamic GFAP mRNA increased 3-fold at 24 to 25 months in AL rats compared with 3 and 6 month groups. There were no differences in GFAP mRNA levels between AL and FR rats from 3 to 18 months. However, GFAP mRNA was significantly lower in FR than in AL rats at 24 to 25 months; FR rats reached the level of GFAP mRNA in 24 to 25 months AL rats by 33 months. Hypothalamic glutamine synthetase mRNA also increased with age in both dietary groups but did not differ between dietary groups at any age. The observation that FR delays the increased expression of GFAP in the hypothalamus during aging lends support to the hypothesis that upregulation of GFAP mRNA is a biomarker of brain aging.

GFAP and astrogliosis

One of the most remarkable characteristics of astrocytes is their vigorous response to diverse neurologic insults, a feature that is well conserved across a variety of different species. The astroglial response occurs rapidly and can be detected within one hour of a focal mechanical trauma (Mucke et al., 1991). Prominent reactive astrogliosis is seen; in AIDS dementia; a variety of other viral infections; prion associated spongiform encephalopathies; inflammatory demyelinating diseases; acute traumatic brain injury; neurodegenerative diseases such as Alzheimer's disease. The prominence of astroglial reactions in various diseases, the rapidity of the astroglial response and the evolutionary conservation of reactive astrogliosis indicate that reactive astrocytes fulfill important functions of the central nervous system (CNS). Yet, the exact role reactive astrocytes play in the injured CNS has so far remained elusive. This chapter summaries the various experimental models and diseases that exhibit astrogliosis and increase in glial fibrillary acidic protein (GFAP). Recent in vitro studies to inhibit GFAP synthesis are also presented.