Membrane-associated sodium channels and cytoplasmic precursors in glial cells. Immunocytochemical, electrophysiological, and pharmacological studies

Potentiometric study of resting potential, contributing K+ channels and the onset of Na+ channel excitability in embryonic rat cortical cells

Resting membrane potential (RMP), K+ channel contribution to RMP and the development of excitability were investigated in the entire population of acutely dissociated embryonic (E) rat cortical cells over E11-22 using a voltage-sensitive fluorescent indicator dye and flow cytometry. During the period of intense proliferation (E11-13), two cell subpopulations with distinct estimated RMPs were recorded: one polarized at approximately -70 mV and the other relatively less-polarized at approximately -40 mV. Ca2+o was critical in sustaining the RMP of the majority of less-polarized cells, while the well-polarized cells were characterized by membrane potentials exhibiting a approximately Nernstian relationship between RMP and [K+]o. Analysis of these two subpopulations revealed that > 80% of less-polarized cells were proliferative, while > 90% of well-polarized cells were postmitotic. Throughout embryonic development, the disappearance of Ca2+o-sensitive, less-polarized cells correlated with the disappearance of the proliferating population, while the appearance of the K+o-sensitive, well-polarized population correlated with the appearance of terminally postmitotic neurons, immuno-identified as BrdU-, tetanus toxin+ cells. Differentiating neurons were estimated to contain increased K+i relative to less-polarized cells, coinciding with the developmental expression of Cs+/Ba2+-sensitive and Ca2+-dependent K+ channels. Both K+ channels contributed to the RMP of well-polarized cells, which became more negative toward the end of neurogenesis. Depolarizing effects of veratridine, first observed at E11, progressively changed from Ca2+o-dependent and tetrodotoxin-insensitive to Na+o-dependent and tetrodotoxin-sensitive response by E18. The results reveal a dynamic development of RMP, contributing K+ channels and voltage-dependent Na+ channels in the developing cortex as it transforms from proliferative to primarily differentiating tissue.

Glial ontogeny and glial neoplasia: the search for closure

The last ten years have seen rapid progress in both our understanding of the normal progression and control of gliogenesis and in the laboratory techniques necessary to sustain and study most glial cell types, including progenitor cells of both type-1 astrocyte (T1A) and oligodendrocyte-type-2 astrocyte (T2A) lineage. These studies have direct relevance for the lineage analysis of human gliomas, optimizing in vitro glioma models, and suggesting potentially fertile new grounds for glioma biology research. We do not yet known whether malignant transformation occurs only in mature glia that then 'de-differentiate' into cells with glial precursor phenotypes and behavior characteristics, whether neoplastic transformation occurs in O-2A progenitor cells, or whether both mechanisms may occur in different patients. However, preliminary results suggest that astrocytomas can arise from two different glial lineages, that oligodendrogliomas and mixed oligo-astrocytomas arise exclusively from the O-2A lineage, and that medulloblastomas may also have a connection with the O-2A lineage. An ontogeny-based glioma classification system may lead to better prognostic patient data and better predict patient response to treatment than currently available classification systems. Available data from the study of developmental glial biology raises serious doubts about the fidelity and relevance of in vitro glioma models that rely on culture media supplemented with animal serum and suggest that relying on chemically-defined media conditioned by astrocytes may be the better research strategy. Findings from the study of normal gliogenesis also suggest that growth factors are likely to act as much more than simple mitogens in glioma biology. Potentially fertile areas of research for glioma biology include studying the cooperative effect of multiple growth factors, potential growth factor effects as survival factors, inhibitors of differentiation, and differentiation inducers, and studying potential positive humoral feedback loops between glioma cells and normal glial cells, as well as normal non-glial cells, within and surrounding each glioma.

Tissue extract from dopamine-depleted striatum enhances differentiation of cultured striatal type-1 astrocytes

The effects of tissue extract from dopamine (DA)-depleted striatum (lesion extract, L-ext) on morphological and electrophysiological natures of cultured striatal astrocytes were investigated. L-ext treatment suppressed the proliferation of type-1 astrocytes. They became fibrous in a concentration-dependent manner. These changes were not observed in type-2 astrocytes. By whole cell patch-clamp recording, two kinetically and pharmacologically distinct voltage-activated potassium currents, A current and delayed rectifier, were identified. L-ext treatment enhanced both currents in type-1 astrocytes, but only A current in type-2. Data suggest that in tissue extract from DA-depleted striatum, there are increased trophic activities that promote the differentiation of type-1 astrocytes.

Cortical type 2 astrocytes are not dye coupled nor do they express the major gap junction genes found in the central nervous system

The O-2A progenitor cell first described from the rat optic nerve is a bipotential precursor of oligodendrocytes and type 2 astrocytes. Each cell expresses specific markers that distinguish them as unique cell types. O-2A progenitors cultured in high serum preferentially differentiate into type 2 astrocytes and when exposed to defined medium or low serum develop along the oligodendrocyte lineage. We analyzed the gap junction gene expression of type 2 astrocytes to determine if they are coupled to form a syncytium, like their type 1 astrocyte counterparts. Dye coupling experiments demonstrated that cortical type 2 astrocytes are not coupled, while type 1 astrocytes in the same culture dish are highly coupled. Immunocytochemistry revealed the presence of Cx43 in type 1 astrocytes but we could not detect Cx26, 32, or 43 protein in type 2 astrocytes. In situ hybridization did not detect mRNA for any of the three connexin genes in type 2 astrocytes. These data demonstrate that type 2 astrocytes do not express the major gap junction genes found in the central nervous system. The precise function of type 2 astrocytes is not known but the lack of gap junction genes expression suggests that their functions are different from the spatial buffering capacity of type 1 astrocytes.

Voltage-gated ion channels in Schwann cells and glia

In the past decade, particularly its last half, it has become clear that the satellite cells of the nervous system (oligodendrocytes and astrocytes in the CNS, Schwann cells in the PNS) are liberally endowed with a vast array of voltage-gated ion channels. These original observations of the various channel types have been followed by the elucidation in considerable detail of the electrophysiological characteristics of the channels. Virtually all of the types of voltage-gated channel found in neurons have now also been found in the various populations of satellite cells. In spite of this, the intriguing questions concerning the physiological function of these channels remain to be fully elucidated.