Glial influence on neuronal signaling

Brain Structure and Function: Insights from Chemical Neuroanatomy

We present a brief historical and epistemological outline of investigations on the brain’s structure and functions. These investigations have mainly been based on the intermingling of chemical anatomy, new techniques in the field of microscopy and computer-assisted morphometric methods. This intermingling has enabled extraordinary investigations to be carried out on brain circuits, leading to the development of a new discipline: “brain connectomics”. This new approach has led to the characterization of the brain’s structure and function in physiological and pathological conditions, and to the development of new therapeutic strategies. In this context, the conceptual model of the brain as a hyper-network with a hierarchical, nested architecture, arranged in a “Russian doll” pattern, has been proposed. Our investigations focused on the main characteristics of the modes of communication between nodes at the various miniaturization levels, in order to describe the brain’s integrative actions. Special attention was paid to the nano-level, i.e., to the allosteric interactions among G protein-coupled receptors organized in receptor mosaics, as a promising field in which to obtain a new view of synaptic plasticity and to develop new, more selective drugs. The brain’s multi-level organization and the multi-faceted aspects of communication modes point to an emerging picture of the brain as a very peculiar system, in which continuous self-organization and remodeling take place under the action of external stimuli from the environment, from peripheral organs and from ongoing integrative actions.

Glial overexpression of Dube3a causes seizures and synaptic impairments in Drosophila concomitant with down regulation of the Na+/K+ pump ATPα

Duplication 15q syndrome (Dup15q) is an autism-associated disorder co-incident with high rates of pediatric epilepsy. Additional copies of the E3 ubiquitin ligase UBE3A are thought to cause Dup15q phenotypes, yet models overexpressing UBE3A in neurons have not recapitulated the epilepsy phenotype. We show that Drosophila endogenously expresses Dube3a (fly UBE3A homolog) in glial cells and neurons, prompting an investigation into the consequences of glial Dube3a overexpression. Here we expand on previous work showing that the Na+/K+ pump ATPα is a direct ubiquitin ligase substrate of Dube3a. A robust seizure-like phenotype was observed in flies overexpressing Dube3a in glial cells, but not neurons. Glial-specific knockdown of ATPα also produced seizure-like behavior, and this phenotype was rescued by simultaneously overexpressing ATPα and Dube3a in glia. Our data provides the basis of a paradigm shift in Dup15q research given that clinical phenotypes have long been assumed to be due to neuronal UBE3A overexpression.

Modeling glial contributions to seizures and epileptogenesis: cation-chloride cotransporters in Drosophila melanogaster

Flies carrying a kcc loss-of-function mutation are more seizure-susceptible than wild-type flies. The kcc gene is the highly conserved Drosophila melanogaster ortholog of K+/Cl- cotransporter genes thought to be expressed in all animal cell types. Here, we examined the spatial and temporal requirements for kcc loss-of-function to modify seizure-susceptibility in flies. Targeted RNA interference (RNAi) of kcc in various sets of neurons was sufficient to induce severe seizure-sensitivity. Interestingly, kcc RNAi in glia was particularly effective in causing seizure-sensitivity. Knockdown of kcc in glia or neurons during development caused a reduction in seizure induction threshold, cell swelling, and brain volume increase in 24-48 hour old adult flies. Third instar larval peripheral nerves were enlarged when kcc RNAi was expressed in neurons or glia. Results suggest that a threshold of K+/Cl- cotransport dysfunction in the nervous system during development is an important determinant of seizure-susceptibility in Drosophila. The findings presented are the first attributing a causative role for glial cation-chloride cotransporters in seizures and epileptogenesis. The importance of elucidating glial cell contributions to seizure disorders and the utility of Drosophila models is discussed.

CORTICAL PLASTICITY AND ITS IMPLICATIONS FOR FOCAL HAND DYSTONIA

BACKGROUND

The exact origin of focal dystonias has not been elucidated so far. Aberrant plasticity of the brain cortex is suspected to be a crucial factor in the development of this group of movement disorders. The aim of this article is to summarize recent findings on the etiopathogenesis of focal hand dystonias with a focus on the role of abnormal cortical plasticity.

METHODS AND RESULTS

A search of the literature mainly from 1995 to 2005 was done using the PubMed and Ovid search engines. English-language articles were identified using the following keywords: focal hand dystonia or writer's cramp and cortical plasticity, sensorimotor, imaging. Additional references were found through bibliography reviews of relevant articles. The data from neurophysiological and imaging studies, as well as clinical observation, in focal hand dystonia suggest multiple failures at different levels of the somatosensory and motor systems, particularly in the brain cortex. This disorders lead to attenuation of inhibitory and fortification of excitatory processes.

CONCLUSIONS

The emerging theory presumes that a maladaptive plasticity of brain cortex with abnormal sensorimotor intergration can evolve in predisposed individuals. Consequent methods of management of focal hand dystonias are outlined.

Estrogen, cognition and female ageing

Starting from fetal life, estrogens are crucial in determining central gender dimorphism, and an estrogen-induced synaptic plasticity is well evident during puberty and seasonal changes as well as during the ovarian cycle. Estrogens act on the central nervous system (CNS) both through genomic mechanisms, modulating synthesis, release and metabolism of neurotransmitters, neuropeptides and neurosteroids, and through non-genomic mechanisms, influencing electrical excitability, synaptic function and morphological features. Therefore, estrogen's neuroactive effects are multifaceted and encompass a system that ranges from the chemical to the biochemical to the genomic mechanisms, protecting against a wide range of neurotoxic insults. Clinical evidences show that, during the climacteric period, estrogen withdrawal in the limbic system gives rise to modifications in mood, behaviour and cognition and that estrogen administration is able to improve mood and cognitive efficiency in post-menopause. Many biological mechanisms support the hypothesis that estrogens might protect against Alzheimer's disease (AD) by influencing neurotransmission, increasing cerebral blood flow, modulating growth proteins associated with axonal elongation and blunting the neurotoxic effects of beta-amyloid. On the contrary, clinical studies of estrogen replacement therapy (ERT) and cognitive function have reported controversial results, indicating a lack of efficacy of estrogens on cognition in post-menopausal women aged >or=65 years. These findings suggest the presence of a critical period for HRT-related neuroprotection and underlie the potential importance of early initiation of therapy for cognitive benefit. In this review, we shall first describe the multiple effects of steroids in the nervous system, which may be significant in the ageing process. A critical update of HRT use in women and a discussion of possible prospectives for steroid use are subsequently proposed.

Analysis of K+ accumulation reveals privileged extracellular region in the vicinity of glial cells in situ

Astrocytes and oligodendrocytes in rat and mouse spinal cord slices, characterized by passive membrane currents during de- and hyperpolarizing stimulation pulses, express a high resting K+ conductance. In contrast to the case for astrocytes, a depolarizing prepulse in oligodendrocytes produces a significant shift of reversal potential (Vrev) to positive values, arising from the larger accumulation of K+ in the vicinity of the oligodendrocyte membrane. As a result, oligodendrocytes express large tail currents (Itail) after a depolarizing prepulse due to the shift of K+ into the cell. In the present study, we used a mathematical model to calculate the volume of the extracellular space (ECS) in the vicinity of astrocytes and oligodendrocytes (ESVv), defined as the volume available for K+ accumulation during membrane depolarization. A mathematical analysis of membrane currents revealed no differences between glial cells from mouse (n = 59) or rat (n = 60) spinal cord slices. We found that the Vrev of a cell after a depolarizing pulse increases with increasing Itail, expressed as the ratio of the integral inward current (Qin) after the depolarizing pulse to the total integral outward current (Qout) during the pulse. In astrocytes with small Itail and Vrev ranging from -50 to -70 mV, the Qin was only 3-19% of Qout, whereas, in oligodendrocytes with large Itail and Vrev between -20 and 0 mV, Qin/Qout was 30-75%. On the other hand, ESVv decreased with increasing values of Vrev. In astrocytes, ESVv ranged from 2 to 50 microm3, and, in oligodendrocytes, it ranged from 0.1 to 2.0 microm3. Cell swelling evoked by the application of hypotonic solution shifted Vrev to more positive values by 17.2 +/- 1.8 mV and was accompanied by a decrease in ESVv of 3.6 +/- 1.3 microm3. Our mathematical analysis reveals a 10-100 times smaller region of the extracellular space available for K+ accumulation during cell depolarization in the vicinity of oligodendrocytes than in the vicinity of astrocytes. The presence of such privileged regions around cells in the CNS may affect the accumulation and diffusion of other neuroactive substances and alter communication between cells in the CNS.

Confocal imaging of changes in glial calcium dynamics and homeostasis after mechanical injury in rat spinal cord white matter

Periaxonal glia play an important role in maintaining axonal function in white matter. However, little is known about the changes that occur in glial cells in situ immediately after traumatic injury. We used fluo-3 and confocal microscopy to examine the effects of localized (<0.5 mm) mechanical trauma on intracellular calcium (Ca(i)(2+)) levels in glial cells in a mature rat spinal cord white matter preparation in vitro. At the injury site, the glial Ca(i)(2+) signal increased by 300-400% within 5 min and then irreversibly declined indicating cell lysis and death. In glial cells at sites adjacent to the injury (1.5-2 mm from epicenter), Ca(i)(2+) levels peaked at 10-15 min, and thereafter declined but remained significantly above rest levels. At distal sites (6-9 mm), Ca(i)(2+) levels rose and declined even slower, peaking at 80-90 min. Injury in zero calcium dampened Ca(i)(2+) responses, indicating a role for calcium influx in the generation and propagation of the injury-induced Ca(i)(2+) signal. By 50-80 min post-injury, surviving glial cells demonstrated an enhanced ability to withstand supraphysiological Ca(i)(2+) loads induced by the calcium ionophore A-23187. Glial fibrillary acidic protein (GFAP) and CNPase immunolabeling determined that the glial cells imaged with fluo-3 included both astrocytes and oligodendrocytes. These data provide the first direct evidence that the effects of localized mechanical trauma include a glial calcium signal that can spread along white matter tracts for up to 9 mm within less than 3 h. The results further show that trauma can enhance calcium regulation in surviving glial cells in the acute post-injury period.

Brain plasticity in health and disease

Research during the last decades has greatly increased our understanding of brain plasticity, i.e. how neuronal circuits can be modified by experience, learning and in response to brain lesions. Currently available neuroimaging techniques that make it possible to study the function of the human brain in vivo have had an important impact. Cross-modal plasticity during development is demonstrated by cortical reorganization in blind or deaf children. Early musical training has lasting effects in shaping the brain. Albeit the plasticity is largest during childhood, the adult brain retains a capacity for functional and structural reorganization that earlier has been underestimated. Recent research on Huntington's disease has revealed the possibility of environmental interaction even with dominant genes. Scientifically based training methods are now being applied in rehabilitation of patients after stroke and trauma, and in the sensory retraining techniques currently applied in the treatment of focal hand dystonia as well as in sensory re-education after nerve repair in hand surgery. There is evidence that frequent participation in challenging and stimulating activities is associated with reduced cognitive decline during aging. The current concept of brain plasticity has wide implication for areas outside neuroscience and for all human life.

Water ADC, extracellular space volume, and tortuosity in the rat cortex after traumatic injury

The diffusion parameters in rat cortex were studied 3-35 days following a cortical stab wound, using diffusion-weighted MR to determine the apparent diffusion coefficient of water (ADC(W)) in the tissue, and the real-time iontophoretic tetramethylammonium (TMA) method to measure the extracellular space (ECS) diffusion parameters: ECS volume fraction alpha and the ADC of TMA(+) (ADC(TMA)). Severe astrogliosis was found close to the wound, and mild astrogliosis was found in the ipsilateral but not the contralateral cortex. Chondroitin sulfate proteoglycan (CSPG) expression was increased throughout the ipsilateral cortex. In the hemisphere contralateral to the wound, alpha, ADC(TMA), and ADC(W) were not significantly different from control values. ECS volume fraction was increased only in the vicinity of the wound, in the region of cell death and severe astrogliosis, at 3 and 7 days after injury. However, both ADC(TMA) and ADC(W) were significantly decreased after lesion in the vicinity of the wound as well as in the rest of the ipsilateral hemisphere distant from the wound. Thus, both ADC(W) and ADC(TMA) decreased in regions wherein alpha did not change but CSPG increased. An increase in extracellular matrix expression may therefore impose diffusion barriers for water as well as for TMA molecules.

Effects of cortical ischemia and postischemic environmental enrichment on hippocampal cell genesis and differentiation in the adult rat

The study aimed to elucidate the effects of cortical ischemia and postischemic environmental enrichment on hippocampal cell genesis. A cortical infarct was induced by a permanent ligation of the middle cerebral artery distal to the striatal branches in 6-month-old spontaneously hypertensive rats. Bromodeoxyuridine (BrdU) was administered as 7 consecutive daily injections starting 24 hours after surgery and animals were housed in standard or enriched environment. Four weeks after completed BrdU administration, BrdU incorporation and its co-localization with the neuronal markers NeuN and calbindin D28k, and the astrocytic marker glial fibrillary acidic protein in the granular cell layer and subgranular zone of the hippocampal dentate gyrus were determined with immunohistochemistry and were quantified stereologically. Compared with sham-operated rats, rats with cortical infarcts had a five-to sixfold ipsilateral increase in BrdU-labeled cells. About 80% of the new cells were neurons. Differential postischemic housing did not influence significantly the total number of surviving BrdU-labeled cells or newborn neurons. However, postischemic environmental enrichment increased the ipsilateral generation of astrocytes normalizing the astrocyte-to-neuron ratio, which was significantly reduced in rats housed in standard environment postischemically.

Patch-clamp recordings from white matter glia in thin longitudinal slices of adult rat spinal cord

We developed a technique of whole cell patch-clamp recordings from white matter oligodendrocytes and astrocytes in 200-250 microm-thick horizontal slices of adult (>2 months, 240-260 g) rat thoracic spinal cord. The viability of the white matter, sectioned in Na(+)-free, low Ca(2+) media, and the function of axons were preserved for >8 h, as demonstrated by the propagation of TTX-sensitive compound action potentials (CAPs) and the sensitivity of their refractory period to K(+) channel blocker 4-aminopyridine (1 microM). Glial cells were visually identified within the slices with a 40 x water immersion objective using infra-red differential interference contrast (IR-DIC) video microscopy, and the details of their morphology were further elucidated after filling the cells with Lucifer Yellow or Alexa 350 fluorescent dyes during whole-cell recording. Using voltage steps and ramps, we revealed pronounced non-linearity of I-V relationships in both oligodendrocytes and astrocytes. Both types of cells expressed TEA-sensitive outward delayed rectifier-type currents activated at positive voltages but showed little, if any, signs of inward rectification at voltages up to -140 mV. At -70 mV holding voltage, bath-applied kainic acid (100 microM) activated inward currents in both types of cells. This novel horizontal slice preparation of adult rat thoracic cord will facilitate the examination of mature glial cell physiology, glial-axonal signaling and the pathophysiology of spinal cord trauma and ischemia.

Peripheral astrocyte processes: monitoring by selective immunostaining for the actin-binding ERM proteins

Astrocytes extend thin lamellate processes in the neuropil, in particular around synapses, where they can modulate synaptic function or mediate glial-neuronal communication. Previous studies have shown that these lamellate perisynaptic processes change their shape in response to neuronal activity, but the underlying mechanisms have remained unclear. Similarly, the molecular composition of these fine, sheet-like astrocytic processes (often 50-100 nm wide) is not understood but has to be related to their dynamic properties. To this end, we have studied the presence of ezrin, radixin, and moesin (ERM proteins) in the rat hippocampus and in primary cultured astrocytes, applying immunoperoxidase, immunofluorescence, and immunogold techniques. These three ERM proteins are known as actin-binding proteins that link the cell membrane to the actin cytoskeleton, particularly in microvillus-bearing epithelial cells. In cell culture, anti-ezrin and antiradixin, but not antimoesin, antibodies were specific for astrocytes, which often displayed selective staining of filopodia and microvilli. Nonoverlapping visualization of astrocytic peripheral and stem processes was obtained by immunocytochemical double labeling for ezrin and GFAP, respectively. In sections of rat hippocampus, homogeneous labeling of the neuropil, but not of cell layers, resulted from immunostaining of fine, peripheral astrocyte processes, as confirmed ultrastructurally. Our data show that the fine peripheral processes of astrocytes, which also constitute the perisynaptic glial sheath, are specialized in that they contain characteristic actin-associated molecules, likely to contribute to their dynamic properties. Applying anti-ezrin and anti-radixin as selective markers, plasticity of these perisynaptic glial processes can be analyzed.

Effect of elevated K(+), hypotonic stress, and cortical spreading depression on astrocyte swelling in GFAP-deficient mice

Glial fibrillary acidic protein (GFAP) is the main component of intermediate filaments in astrocytes. To assess its function in astrocyte swelling, we compared astrocyte membrane properties and swelling in spinal cord slices of 8- to 10-day-old wild-type control (GFAP(+/+)) and GFAP-knockout (GFAP(-/-)) mice. Membrane currents and K(+) accumulation around astrocytes after a depolarizing pulse were studied using the whole-cell patch-clamp technique. In vivo cell swelling was studied in the cortex during spreading depression (SD) in 3 to 6-month-old animals. Swelling-induced changes of the extracellular space (ECS) diffusion parameters, i.e., volume fraction alpha and tortuosity lambda, were studied by the real-time iontophoretic tetramethylammonium (TMA(+)) method using TMA(+)-selective microelectrodes. Morphological analysis using confocal microscopy and quantification of xy intensity profiles in a confocal plane revealed a lower density of processes in GFAP(-/-) astrocytes than in GFAP(+/+) astrocytes. K(+) accumulation evoked by membrane depolarization was lower in the vicinity of GFAP(-/-) astrocytes than GFAP(+/+) astrocytes, suggesting the presence of a larger ECS around GFAP(-/-) astrocytes. Astrocyte swelling evoked by application of 50 mM K(+) or by hypotonic solution (HS) produced a larger increase in [K(+)](e) around GFAP(+/+) astrocytes than around GFAP(-/-) astrocytes. No differences in alpha and lambda in the spinal cord or cortex of GFAP(+/+) and GFAP(-/-) mice were found; however, the application of either 50 mM K(+) or HS in spinal cord, or SD in cortex, evoked a large decrease in alpha and an increase in lambda in GFAP(+/+) mice only. Slower swelling in GFAP(-/-) astrocytes indicates that GFAP and intermediate filaments play an important role in cell swelling during pathological states.

Glutamate, NMDA, and AMPA induced changes in extracellular space volume and tortuosity in the rat spinal cord

Glutamate release, particularly in pathologic conditions, may result in cellular swelling. The authors studied the effects of glutamate, N-methyl-D-aspartate (NMDA), and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) on extracellular pH (pH(e)), extracellular potassium concentration ([K(+)](e)), and changes in extracellular space (ECS) diffusion parameters (volume fraction alpha, tortuosity lambda) resulting from cellular swelling. In the isolated spinal cord of 4-to 12-day-old rats, the application of glutamate receptor agonists induced an increase in [K(+)](e), alkaline-acid shifts, a substantial decrease in alpha, and an increase in lambda. After washout of the glutamate receptor agonists, alpha either returned to or overshot normal values, whereas lambda remained elevated. Pretreatment with 20 mmol/L Mg(++), MK801, or CNQX blocked the changes in diffusion parameters, [K(+)](e) and pH(e) evoked by NMDA or AMPA. However, the changes in diffusion parameters also were blocked in Ca(2+)-free solution, which had no effect on the [K(+)](e) increase or acid shift. The authors conclude that increased glutamate release may produce a large, sustained and [Ca(2+)](e)-dependent decrease in alpha and increase in lambda. Repetitive stimulation and pathologic states resulting in glutamate release therefore may lead to changes in ECS volume and tortuosity, affecting volume transmission and enhancing glutamate neurotoxicity and neuronal damage.

Effect of osmotic stress on potassium accumulation around glial cells and extracellular space volume in rat spinal cord slices

In rat brain and spinal cord slices, the local extracellular accumulation of K(+), as indicated by K(+) tail currents (I(tail)) after a depolarization step, is greater in the vicinity of oligodendrocytes than that of astrocytes. It has been suggested that this may reflect a smaller extracellular space (ECS) around oligodendrocytes compared to astrocytes [Chvátal et al. [1997] J. Neurosci. Res. 49:98-106; [1999] J. Neurosci. Res. 56:493-505). We therefore compared the effect of osmotic stress in spinal cord slices from 5-11-day-old rats on the changes in reversal potentials (V(rev)) of I(tail) measured by the whole-cell patch-clamp technique and the changes in ECS volume measured by the real-time iontophoretic method. Cell swelling induced by a 20 min perfusion of hypoosmotic solution (200 mmol/kg) decreased the ECS volume fraction from 0.21 +/- 0.01 to 0.15 +/- 0.02, i.e., by 29%. As calculated from V(rev) of I(tail) using the Nernst equation, a depolarizing prepulse increased [K(+)](e) around astrocytes from 11.0 to 44.7 mM, i.e., by 306%, and around oligodendrocytes from 26.1 to 54.9 mM, i.e., by 110%. The ECS volume fraction decrease had the same time course as the changes in V(rev) of I(tail). Cell shrinkage in hyperosmotic solution (400 mmol/kg) increased ECS volume fraction from 0.24 +/- 0.02 to 0.32 +/- 0.02, i.e., by 33%. It had no effect on [K(+)](e) evoked by a depolarizing prepulse in astrocytes, whereas in oligodendrocytes [K(+)](e) rapidly decreased from 52 to 26 mM, i.e., by 50%. The increase in ECS volume was slower than the changes in [K(+)](e). These data demonstrate that hypoosmotic solution has a larger effect on the ECS volume around astrocytes than around oligodendrocytes and that hyperosmotic solution affects the ECS volume around oligodendrocytes only. This indicates that increased K(+) accumulation in the vicinity of oligodendrocytes could be due to a restricted ECS. Oligodendrocytes in the CNS are therefore most likely surrounded by clusters of "compacted" ECS, which may selectively affect the diffusion of neuroactive substances in specific areas and directions and facilitate spatial K(+) buffering.

Membrane currents and morphological properties of neurons and glial cells in the spinal cord and filum terminale of the frog

Using the patch-clamp technique in the whole-cell configuration combined with intracellular dialysis of the fluorescent dye Lucifer yellow (LY), the membrane properties of cells in slices of the lumbar portion of the frog spinal cord (n=64) and the filum terminale (FT, n=48) have been characterized and correlated with their morphology. Four types of cells were found in lumbar spinal cord and FT with membrane and morphological properties similar to those of cells that were previously identified in the rat spinal cord (Chvátal, A., Pastor, A., Mauch, M., Syková, E., Kettenmann, H., 1995. Distinct populations of identified glial cells in the developing rat spinal cord: Ion channel properties and cell morphology. Eur. J. Neurosci. 7, 129-142). Neurons, in response to a series of symmetrical voltage steps, displayed large repetitive voltage-dependent Na(+) inward currents and K(+) delayed rectifying outward currents. Three distinct types of non-neuronal cells were found. First, cells that exhibited passive symmetrical non-decaying currents were identified as astrocytes. These cells immunostained for GFAP and typically had at least one thick process and a number of fine processes. Second, cells with the characteristic properties of rat spinal cord oligodendrocytes, with passive symmetrical decaying currents and large tail currents after the end of the voltage step. These cells exhibited either long parallel or short hairy processes. Third, cells that expressed small brief inward currents in response to depolarizing steps, delayed rectifier outward currents and small sustained inward currents identical to rat glial precursor cells. Morphologically, they were characterized by round cell bodies with a number of finely branched processes. LY dye-coupling in the frog spinal cord gray matter and FT was observed in neurons and in all glial populations. All four cell types were found in both the spinal cord gray matter and FT. The glia/neuron ratio in the spinal cord was 0.78, while in FT it was 2.0. Moreover, the overall cell density was less in the FT than in the spinal cord. The present study shows that the membrane and morphological properties of glial cells in the frog and rat spinal cords are similar. Such striking phylogenetic similarity suggests a significant contribution from distinct glial cell populations to various spinal cord functions, particularly ionic and volume homeostasis in both mammals and amphibians.