Extracellular S100β Disrupts Bergman Glia Morphology and Synaptic Transmission in Cerebellar Purkinje Cells

Astrogliosis is a pathological process that affects the density, morphology, and function of astrocytes. It is a common feature of brain trauma, autoimmune diseases, and neurodegeneration including spinocerebellar ataxia type 1 (SCA1), a poorly understood neurodegenerative disease. S100β is a Ca2+ binding protein. In SCA1, excessive excretion of S100β by reactive astrocytes and its uptake by Purkinje cells has been demonstrated previously. Under pathological conditions, excessive extracellular concentration of S100β stimulates the production of proinflammatory cytokines and induces apoptosis. We modeled astrogliosis by S100β injections into cerebellar cortex in mice. Injections of S100β led to significant changes in Bergmann glia (BG) cortical organization and affected their processes. S100β also changed morphology of the Purkinje cells (PCs), causing a significant reduction in the dendritic length. Moreover, the short-term synaptic plasticity and depolarization-induced suppression of synaptic transmission were disrupted after S100β injections. We speculate that these effects are the result of Ca2+-chelating properties of S100β protein. In summary, exogenous S100β induced astrogliosis in cerebellum could lead to neuronal dysfunction, which resembles a natural neurodegenerative process. We suggest that astrocytes play an essential role in SCA1 pathology, and that astrocytic S100β is an important contributor to this process.

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Progress in pathogenesis studies of spinocerebellar ataxia type 1

Philosophical Transactions of the Royal Society B Biological Sciences ◽

10.1098/rstb.1999.0462 ◽

1999 ◽

Vol 354 (1386) ◽

pp. 1079-1081 ◽

Cited By ~ 34

Author(s):

Christopher J. Cummings ◽

Harry T. Orr ◽

Huda Y. Zoghbi

Keyword(s):

Transgenic Mice ◽

Purkinje Cell ◽

Purkinje Cells ◽

Nuclear Localization ◽

Hela Cells ◽

Protein Misfolding ◽

Spinocerebellar Ataxia Type ◽

Spinocerebellar Ataxia Type 1 ◽

Cerebellar Purkinje Cells

Spinocerebellar ataxia type 1 (SCA1) is a dominantly inherited disorder characterized by progressive loss of coordination, motor impairment and the degeneration of cerebellar Purkinje cells, spinocerebellar tracts and brainstem nuclei. Many dominantly inherited neurodegenerative diseases share the mutational basis of SCA1: the expansion of a translated CAG repeat coding for glutamine. Mice lacking ataxin-1 display learning deficits and altered hippocampal synaptic plasticity but none of the abnormalities seen in human SCA1; mice expressing ataxin-1 with an expanded CAG tract (82 glutamine residues), however, develop Purkinje cell pathology and ataxia. These results suggest that mutant ataxin-1 gains a novel function that leads to neuronal degeneration. This novel function might involve aberrant interaction(s) with cell-specific protein(s), which in turn might explain the selective neuronal pathology. Mutant ataxin-1 interacts preferentially with a leucine-rich acidic nuclear protein that is abundantly expressed in cerebellar Purkinje cells and other brain regions affected in SCA1. Immunolocalization studies in affected neurons of patients and SCA1 transgenic mice showed that mutant ataxin-1 localizes to a single, ubiquitin-positive nuclear inclusion (NI) that alters the distribution of the proteasome and certain chaperones. Further analysis of NIs in transfected HeLa cells established that the proteasome and chaperone proteins co-localize with ataxin-1 aggregates. Moreover, overexpression of the chaperone HDJ-2/HSDJ in HeLa cells decreased ataxin-1 aggregation, suggesting that protein misfolding might underlie NI formation. To assess the importance of the nuclear localization of ataxin-1 and its role in SCA1 pathogenesis, two lines of transgenic mice were generated. In the first line, the nuclear localization signal was mutated so that full-length mutant ataxin-1 would remain in the cytoplasm; mice from this line did not develop any ataxia or pathology. This suggests that mutant ataxin-1 is pathogenic only in the nucleus. To assess the role of the aggregates, transgenic mice were generated with mutant ataxin-1 without the self-association domain (SAD) essential for aggregate formation. These mice developed ataxia and Purkinje cell abnormalities similar to those seen in SCA1 transgenic mice carrying full-length mutant ataxin-1, but lacked NIs. The nuclear milieu is thus a critical factor in SCA1 pathogenesis, but large NIs are not needed to initiate pathogenesis. They might instead be downstream of the primary pathogenic steps. Given the accumulated evidence, we propose the following model for SCA1 pathogenesis: expansion of the polyglutamine tract alters the conformation of ataxin-1, causing it to misfold. This in turn leads to aberrant protein interactions. Cell specificity is determined by the cell-specific proteins interacting with ataxin-1. Submicroscopic protein aggregation might occur because of protein misfolding, and those aggregates become detectable as NIs as the disease advances. Proteasome redistribution to the NI might contribute to disease progression by disturbing proteolysis and subsequent vital cellular functions.

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Calcium Dynamics and Electrophysiological Properties of Cerebellar Purkinje Cells in SCA1 Transgenic Mice

Journal of Neurophysiology ◽

10.1152/jn.2001.85.4.1750 ◽

2001 ◽

Vol 85 (4) ◽

pp. 1750-1760 ◽

Cited By ~ 36

Author(s):

Takafumi Inoue ◽

Xi Lin ◽

Kristi A. Kohlmeier ◽

Harry T. Orr ◽

Huda Y. Zoghbi ◽

...

Keyword(s):

Transgenic Mice ◽

Purkinje Cells ◽

Metabotropic Glutamate Receptor ◽

Membrane Resistance ◽

Spinocerebellar Ataxia Type ◽

Patch Clamp Recording ◽

Experimental Conditions ◽

Cerebellar Purkinje Cells ◽

Electrical Responses

Cerebellar Purkinje cells (PCs) from spinocerebellar ataxia type 1 (SCA1) transgenic mice develop dendritic and somatic atrophy with age. Inositol 1,4,5-trisphosphate receptor type 1 and the sarco/endoplasmic reticulum Ca2+ ATPase pump, which regulate [Ca2+]i, are expressed at lower levels in these cells compared with the levels in cells from wild-type (WT) mice. To examine PCs in SCA1 mice, we used whole-cell patch clamp recording combined with fluorometric [Ca2+]i and [Na+]i measurements in cerebellar slices. PCs in SCA1 mice had Na+ spikes, Ca2+ spikes, climbing fiber (CF) electrical responses, parallel fiber (PF) electrical responses, and metabotropic glutamate receptor (mGluR)-mediated, PF-evoked Ca2+ release from intracellular stores that were qualitatively similar to those recorded from WT mice. Under our experimental conditions, it was easier to evoke the mGluR-mediated secondary [Ca2+]i increase in SCA1 PCs. The membrane resistance of SCA1 PCs was 3.3 times higher than that of WT cells, which correlated with the 1.7 times smaller cell body size. Most SCA1 PCs (but not WT) had a delayed onset (about 50–200 ms) to Na+ spike firing induced by current injection. This delay was increased by hyperpolarizing prepulses and was eliminated by 4-aminopyridine, which suggests that this delay was due to enhancement of the A-like K+ conductance in the SCA1 PCs. In response to CF stimulation, most PCs in mutant and WT mice had rapid, widespread [Ca2+]i changes that recovered in <200 ms. Some SCA1 PCs showed a slow, localized, secondary Ca2+ transient following the initial CF Ca2+ transient, which may reflect release of Ca2+ from intracellular stores. Thus, with these exceptions, the basic physiological properties of mutant PCs are similar to those of WT neurons, even with dramatic alteration of their morphology and downregulation of Ca2+ handling molecules.

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Reactive astrocytosis and glial glutamate transporter clustering are early changes in a spinocerebellar ataxia type 1 transgenic mouse model

Neuron Glia Biology ◽

10.1017/s1740925x08000185 ◽

2007 ◽

Vol 3 (4) ◽

pp. 335-351 ◽

Cited By ~ 22

Author(s):

Roberto Giovannoni ◽

Nicola Maggio ◽

Maria Rosaria Bianco ◽

Carlo Cavaliere ◽

Giovanni Cirillo ◽

...

Keyword(s):

Mouse Model ◽

Synaptic Transmission ◽

Transgenic Mouse ◽

Glutamate Transporter ◽

Transgenic Mouse Model ◽

Spinocerebellar Ataxia Type ◽

Spinocerebellar Ataxia Type 1 ◽

Glial Glutamate Transporter ◽

Reactive Astrocytosis

AbstractSpinocerebellar ataxia type 1 (SCA1) is a neurodegenerative disorder caused by an expanded CAG trinucleotide repeats within the coding sequence of the ataxin-1 protein. In the present study, we used a conditional transgenic mouse model of SCA1 to investigate very early molecular and morphological changes related to the behavioral phenotype. In mice with neural deficits detected by rotarod performance, and simultaneous spatial impairments in exploratory activity and uncoordinated gait, we observed both significant altered expression and patchy distribution of excitatory amino acids transporter 1. The molecular changes observed in astroglial compartments correlate with changes in synapse morphology; synapses have a dramatic reduction of the synaptic area external to the postsynaptic density. By contrast, Purkinje cells demonstrate preserved structure. In addition, severe reactive astrocytosis matches changes in the glial glutamate transporter and synapse morphology. We propose these morpho-molecular changes are the cause of altered synaptic transmission, which, in turn, determines the onset of the neurological symptoms by altering the synaptic transmission in the cerebellar cortex of transgenic animals. This model might be suitable for testing drugs that target activated glial cells in order to reduce CNS inflammation.

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