Potassium currents in primary cultured astrocytes from the rat corpus callosum

Intrinsic organization of the corpus callosum

The corpus callosum-the largest commissural fiber system connecting the two cerebral hemispheres-is considered essential for bilateral sensory integration and higher cognitive functions. Most studies exploring the corpus callosum have examined either the anatomical, physiological, and neurochemical organization of callosal projections or the functional and/or behavioral aspects of the callosal connections after complete/partial callosotomy or callosal lesion. There are no works that address the intrinsic organization of the corpus callosum. We review the existing information on the activities that take place in the commissure in three sections: I) the topographical and neurochemical organization of the intracallosal fibers, II) the role of glia in the corpus callosum, and III) the role of the intracallosal neurons.

Dynamic expression of homeostatic ion channels in differentiated cortical astrocytes in vitro

The capacity of astrocytes to adapt their biochemical and functional features upon physiological and pathological stimuli is a fundamental property at the basis of their ability to regulate the homeostasis of the central nervous system (CNS). It is well known that in primary cultured astrocytes, the expression of plasma membrane ion channels and transporters involved in homeostatic tasks does not closely reflect the pattern observed in vivo. The individuation of culture conditions that promote the expression of the ion channel array found in vivo is crucial when aiming at investigating the mechanisms underlying their dynamics upon various physiological and pathological stimuli. A chemically defined medium containing growth factors and hormones (G5) was previously shown to induce the growth, differentiation, and maturation of primary cultured astrocytes. Here we report that under these culture conditions, rat cortical astrocytes undergo robust morphological changes acquiring a multi-branched phenotype, which develops gradually during the 2-week period of culturing. The shape changes were paralleled by variations in passive membrane properties and background conductance owing to the differential temporal development of inwardly rectifying chloride (Cl⁻) and potassium (K⁺) currents. Confocal and immunoblot analyses showed that morphologically differentiated astrocytes displayed a large increase in the expression of the inward rectifier Cl⁻ and K⁺ channels ClC-2 and Kir4.1, respectively, which are relevant ion channels in vivo. Finally, they exhibited a large diminution of the intermediate filaments glial fibrillary acidic protein (GFAP) and vimentin which are upregulated in reactive astrocytes in vivo. Taken together the data indicate that long-term culturing of cortical astrocytes in this chemical-defined medium promotes a quiescent functional phenotype. This culture model could aid to address the regulation of ion channel expression involved in CNS homeostasis in response to physiological and pathological challenges.

Morphological and Calcium Signaling Alterations of Neuroglial Cells in Cerebellar Cortical Dysplasia Induced by Carmustine

Cortical dysplasias are alterations in the organization of the layers of the brain cortex due to problems in neuronal migration during development. The neuronal component has been widely studied in experimental models of cortical dysplasias. In contrast, little is known about how glia are affected. In the cerebellum, Bergmann glia (BG) are essential for neuronal migration during development, and in adult they mediate the control of fine movements through glutamatergic transmission. The aim of this study was to characterize the morphology and intracellular calcium dynamics of BG and astrocytes from mouse cerebellum and their modifications in a model of cortical dysplasia induced by carmustine (BCNU). Carmustine-treated mice were affected in their motor coordination and balance. Cerebellar dysplasias and heterotopias were more frequently found in lobule X. Morphology of BG cells and astrocytes was affected, as were their spontaneous [Ca²⁺]ⁱ transients in slice preparation and in vitro.

Kir 4.1 inward rectifier potassium channel is upregulated in astrocytes in a murine multiple sclerosis model

Multiple sclerosis (MS) is a high prevalence degenerative disease characterized at the cellular level by glial and neuronal cell death. The causes of cell death during the disease course are not fully understood. In this work we demonstrate that in a MS model induced by Theiler's murine encephalomyelitis virus (TMEV) infection, the inward rectifier (Kir) 4.1 potassium channel subunit is overexpressed in astrocytes. In voltage clamp experiments the inward current density from TMEV-infected astrocytes was significantly larger than in mock-infected ones. The cRNA hybridization analysis from mock- and TMEV-infected cells showed an upregulation of a potassium transport channel coding sequence. We validated this mRNA increase by RT-PCR and quantitative PCR using Kir 4.1 specific primers. Western blotting experiments confirmed the upregulation of Kir 4.1, and alignment between sequences provided the demonstration that the over-expressed gene encodes for a Kir family member. Flow cytometry showed that the Kir 4.1 protein is located mainly in the cell membrane in mock and TMEV-infected astrocytes. Our results demonstrate an increase in K⁺ inward current in TMEV-infected glial cells, this increment may reduce the neuronal depolarization, contributing to cell resilience mechanisms.

GABAρ selective antagonist TPMPA partially inhibits GABA-mediated currents recorded from neurones and astrocytes in mouse striatum

The neostriatum plays a central role in motor coordination where nerve cells operate neuronal inhibition through GABAergic transmission. The neostriatum expresses a wide range of GABA-A subunits, including GABAρ1 and ρ2 which are restricted to a fraction of GABAergic interneurons and astrocytes. Spontaneous postsynaptic currents (sPSCs) evoked by 4-aminopyridine (4-AP) were recorded from neurones of the dorsal neostriatum, and their frequency was reduced > 50% by the selective GABAρ antagonist (1,2,5,6-Tetrahydropyridine-4-yl) methylphosphinic acid (TPMPA). Additionally, we recorded GABA evoked currents from astrocytes in vitro and in situ. Astrocytes in vitro showed modulation by pentobarbital and desensitization upon consecutive applications of GABA. However, modulation by pentobarbital was absent and no significant desensitization was detected from astrocytes in situ. Moreover, TPMPA-sensitive GABA-currents that were insensitive to bicuculline were also recorded from astrocytes in situ, consistent with our previous study where GABAρ expression was demonstrated. Finally, we assessed the mRNA expression of GABAρ3, through different stages of postnatal development; double immunofluorescence disclosed GABAρ3 expression in calretinin-positive interneurons as well as in astrocytes (>70%). These results add new information about the participation of GABAρ subunits in neostriatal interneurons and astrocytes.

Novel Poly(3-hydroxybutyrate-g-vinyl alcohol) Polyurethane Scaffold for Tissue Engineering

The design of new synthetic grafted poly(3-hydroxybutyrate) as composite 3D-scaffolds is a convenient alternative for tissue engineering applications. The chemically modified poly(3-hydroxybutyrate) is receiving increasing attention for use as biomimetic copolymers for cell growth. As of yet, these copolymers cannot be used efficiently because of the lack of good mechanical properties. Here, we address this challenge, preparing a composite-scaffold of grafted poly(3-hydroxybutyrate) polyurethane for the first time. However, it is unclear if the composite structure and morphology can also offer a biological application. We obtained the polyurethane by mixing a polyester hydroxylated resin with polyisocyanate and the modified polyhydroxyalkanoates. The results show that the poly(3-hydroxybutyrate) grafted with poly(vinyl alcohol) can be successfully used as a chain extender to form a chemically-crosslinked thermosetting polymer. Furthermore, we show a proposal for the mechanism of the polyurethane synthesis, the analysis of its morphology and the ability of the scaffolds for growing mammalian cells. We demonstrated that astrocytes isolated from mouse cerebellum, and HEK293 can be cultured in the prepared material, and express efficiently fluorescent proteins by adenoviral transduction. We also tested the metabolism of Ca(2+) to obtain evidence of the biological activity.

GABAρ subunits confer a bicuculline-insensitive component to GFAP+ cells of cerebellum

GABA-A receptors mediating synaptic or extrasynaptic transmission are molecularly and functionally distinct, and glial cells are known to express a plethora of GABA-A subunits. Here we demonstrate that GFAP(+) cells of the granular layer of cerebellum express GABAρ subunits during early postnatal development, thereby conferring peculiar pharmacologic characteristics to GABA responses. Electron microscopy revealed the presence of GABAρ in the plasma membrane of GFAP(+) cells. In contrast, expression in the adult was restricted to Purkinje neurons and a subset of ependymal cells. Electrophysiological studies in vitro revealed that astrocytes express functional receptors with an EC50 of 52.2 ± 11.8 μM for GABA. The evoked currents were inhibited by bicuculline (100 μM) and TPMPA (IC50, 5.9 ± 0.6 μM), indicating the presence of a GABAρ component. Coimmunoprecipitation demonstrated protein-protein interactions between GABAρ1 and GABAα1, and double immunofluorescence showed that these subunits colocalize in the plasma membrane. Three populations of GABA-A receptors in astrocytes were identified: classic GABA-A, bicuculline-insensitive GABAρ, and GABA-A-GABAρ hybrids. Clusters of GABA-A receptors were distributed in the perinuclear space and along the processes of GFAP(+) cells. Time-lapse microscopy showed GABAρ2-GFP accumulation in clusters located in the soma and along the processes. The clusters were relatively immobile, with mean displacement of 9.4 ± 0.9 μm and a net distance traveled of 1-2 μm, owing mainly to directional movement or simple diffusion. Modulation of GABAρ dynamics may be a novel mechanism of extrasynaptic transmission regulating GABAergic control of GFAP(+) cells during early postnatal development.

Alteration in rectification of potassium channels in perinatal hypoxia ischemia brain damage

Oligodendrocyte progenitor cells (OPCs) are susceptible to perinatal hypoxia ischemia brain damage (HIBD), which results in infant cerebral palsy due to the effects on myelination. The origin of OPC vulnerability in HIBD, however, remains controversial. In this study, we defined the HIBD punctate lesions by MRI diffuse excessive high signal intensity (DEHSI) in postnatal 7-day-old rats. The electrophysiological functional properties of OPCs in HIBD were recorded by patch-clamp in acute cerebral cortex slices. The slices were intracellularly injected with Lucifer yellow and immunohistochemically labeled with NG2 antibody to identify local OPCs. Passive membrane properties and K(+) channel functions in OPCs were analyzed to estimate the onset of vulnerability in HIBD. The resting membrane potential, membrane resistance, and membrane capacitance of OPCs were increased in both the gray and white matter of the cerebral cortex. OPCs in both the gray and white matter exhibited voltage-dependent K(+) currents, which consisted of the initiated rectified potassium currents (IA) and the sustained rectified currents (IK). The significant alternation in membrane resistance was influenced by the diversity of potassium channel kinetics. These findings suggest that the rectification of IA and IK channels may play a significant role in OPC vulnerability in HIBD.

Modulation of GABA-A receptors of astrocytes and STC-1 cells by taurine structural analogs

Taurine activates and modulates GABA receptors in vivo as well as those expressed in heterologous systems. This study aimed to determine whether the structural analogs of taurine: homotaurine and hypotaurine, have the ability to activate GABA-A receptors that include GABAρ subunits. The expression of GABA-A receptors containing GABAρ has been reported in the STC-1 cells and astrocytes. In both cell types, taurine, homo-, and hypotaurine gated with low efficiency a picrotoxin-sensitive GABA-A receptor. The known bimodal modulatory effect of taurine on GABAρ receptors was not observed; however, differences between the activation and deactivation rates were detected when they were perfused together with GABA. In silico docking simulations suggested that taurine, hypo-, and homotaurine do not form a cation-π interaction such as that generated by GABA in the agonist-binding site of GABAρ. This observation complements the electrophysiological data suggesting that taurine and its analogs act as partial agonists of GABA-A receptors. All the observations above suggest that the structural analogs of taurine are partial agonists of GABA-A receptors that occupy the agonist-binding site, but their structures do not allow the proper interaction with the receptor to fully gate its Cl(-) channel.