Accelerated hippocampal spreading depression and enhanced locomotory activity in mice with astrocyte-directed inactivation of connexin43

Impaired astrocytic gap junction coupling and potassium buffering in a mouse model of tuberous sclerosis complex

Abnormalities in astrocytes occur in the brains of patients with Tuberous Sclerosis Complex (TSC) and may contribute to the pathogenesis of neurological dysfunction in this disease. Here, we report that knock-out mice with Tsc1 gene inactivation in glia (Tsc1(GFAP)CKO mice) exhibit decreased expression of the astrocytic connexin protein, Cx43, and an associated impairment in gap junction coupling between astrocytes. Correspondingly, hippocampal slices from Tsc1(GFAP)CKO mice have increased extracellular potassium concentration in response to stimulation. This impaired potassium buffering can be attributed to abnormal gap junction coupling, as a gap junction inhibitor elicits an additional increase in potassium concentration in control, but not Tsc1(GFAP)CKO slices. Furthermore, treatment with a mammalian target of rapamycin inhibitor reverses the deficient Cx43 expression and impaired potassium buffering. These findings suggest that Tsc1 inactivation in astrocytes causes defects in astrocytic gap junction coupling and potassium clearance, which may contribute to epilepsy in Tsc1(GFAP)CKO mice.

Determinants of regional and local diversity within the astroglial lineage of the normal central nervous system

Astrocytes are a major component of the resident non-neuronal glial cell population of the CNS. They are ubiquitously distributed throughout the brain and spinal cord, where they were initially thought to function in both structural and homeostatic capacities, providing the framework and environment in which neurons performed their parenchymal duties. However, this stroma-like view of astrocytes is no longer satisfactory. Mounting evidence particularly within the last decade indicates that astrocytes do not simply support neuronal activity but directly contribute to it. Congruent with this evolving view of astrocyte function in information processing is the emergent notion that these glial cells are not a homogeneous population of cells. Thus, astrocytes in various anatomically distinct regions of the normal CNS possess unique phenotypic characteristics that may directly influence the particular neuronal activities that define these regions. Remarkably, regional populations of astrocytes appear to exhibit local heterogeneity as well. Many phenotypic traits of the astrocyte lineage are responsive to local environmental cues (i.e., are adaptable), suggesting that plasticity contributes to this diversity. However, compelling evidence suggests that astrocytes arise from multiple distinct progenitor pools in the developing CNS, raising the intriguing possibility that some astrocyte heterogeneity may result from intrinsic differences between these progenitors. The purpose of this review is to explore the evidence for and mechanistic determinants of regional and local astrocyte diversity.

An Update on Connexin Genes and their Nomenclature in Mouse and Man

Gap junctions, composed of connexin protein subunits, allow direct communication through conduits between neighboring cells. Twenty and twenty-one members of the connexin gene family are likely to be expressed in the mouse and human genome, respectively, 19 of which can be grouped into sequence-orthologous pairs. Their gene structure appears to be relatively simple. In most cases, an untranslated exon1 is separated by an intron of different lengh from exon2 that includes the uninterrupted coding region and the 3'-untranslated region. However, there are several exceptions to this scheme, since some mouse connexin genes contain different 5'-untranslated regions spliced either in an alternative and/or consecutive manner. Additionally, in at least 3 mouse and human connexin genes (mCx36, mCx39, mCx57 and hCx31.3, hCx36, as well as hCx40.1) the reading frame is spliced together from 2 different exons. So far, there are two nomenclatures to classify the known connexin genes: The "Gja/Gjb" nomenclature, as it is currently adopted by the NCBI data base, contains some inconsistencies compared to the "Cx" nomenclature. Here we suggest some minor corrections to co-ordinate the "Gja/Gjb" nomenclature with the "Cx" nomenclature. Furthermore, this short review contains an update on phenotypic correlations between connexin deficient mice and patients bearing mutations in their orthologous connexin genes.

Connexin expression by radial glia-like cells is required for neurogenesis in the adult dentate gyrus

In the adult dentate gyrus, radial glia-like cells represent putative stem cells generating neurons and glial cells. Here, we combined patch-clamp recordings, biocytin filling, immunohistochemistry, single-cell transcript analysis, and mouse transgenics to test for connexin expression and gap junctional coupling of radial glia-like cells and its impact on neurogenesis. Radial glia-like cells were identified in mice expressing EGFP under control of the nestin and gfap promoters. We show that a majority of Radial glia-like cells are coupled and express Cx43. Neuronal precursors were not coupled. Mice lacking Cx30 and Cx43 in GFAP-positive cells displayed almost complete inhibition of proliferation and a significant decline in numbers of radial glia-like cells and granule neurons. Inducible virus-mediated ablation of connexins in the adult hippocampus also reduced neurogenesis. These findings strongly suggest the requirement of connexin expression by radial glia-like cells for intact neurogenesis in the adult brain and point to possible communication pathways of these cells.

A quantitative model of cortical spreading depression due to purinergic and gap-junction transmission in astrocyte networks

Spreading depression (SD), a propagating wave of electrical silence in the cortex and archicortex, involves depolarization of neurons and astrocytes for approximately 1 min, due principally to a large increase in extracellular K+. SD is accompanied by large increases in extracellular ATP and is blocked by glutamate N-methyl-D-aspartate receptor antagonists. As a principal means of transmission between astrocytes is through their release of ATP, we have investigated if a model in which SD is driven by the effects of astrocyte waves of ATP interacting with waves of glutamate release from neurons and astrocytes can give a quantitative account of experimental observations on SD. We show that the characteristics of SD and the accompanying extracellular ionic changes can be accommodated by such a model-whether astrocyte transmission is principally through the release of ATP, as in archicortex (hippocampus) and spinal cord, or via gap junctions, as in the neocortex. Furthermore, these models give quantitative accounts of the effects on the characteristics of SD of agents toxic for astrocytes and of gap-junction blockers. Finally, an additional series of critical tests of the model is suggested.

Adhesive properties of connexin hemichannels

Gap junctions are intercellular channels formed by hemichannels (or connexons) from two neighboring cells. Hemichannels, which are composed of proteins called connexins, can function as conduits of ATP and glutamate, and interact with adhesion molecules and other signaling elements. As a result, their functional repertoire is expanding into other roles, such as control of cell growth or cell migration. Here we further elucidate the involvement of hemichannels in cell-cell adhesion by analyzing how connexins regulate cell adhesion without the need of gap junction formation. Using a short-term aggregation assay with C6-glioma and HeLa cells stably transfected with connexin (Cx) 43 or Cx32, we found that the connexin type dictates the ability of these cells to aggregate, even though these two cell types do not usually adhere to each other. We have also found that high expression of Cx43, but not Cx32 hemichannels, can drive adhesion of cells expressing low levels of Cx43. Aggregation was not dependent on high levels of extracellular Ca(2+), as Ca(2+) removal did not change the aggregation of Cx43-expressing cells. Our data confirm that connexin hemichannels can establish adhesive interactions without the need for functional gap junctions, and support the concept that connexins act as adhesion molecules independently of channel formation.

The hyperactive syndrome: metanalysis of genetic alterations, pharmacological treatments and brain lesions which increase locomotor activity

The large number of transgenic mice realized thus far with different purposes allows addressing new questions, such as which animals, over the entire set of transgenic animals, show a specific behavioural abnormality. In the present study, we have used a metanalytical approach to organize a database of genetic modifications, brain lesions and pharmacological interventions that increase locomotor activity in animal models. To further understand the resulting data set, we have organized a second database of the alterations (genetic, pharmacological or brain lesions) that reduce locomotor activity. Using this approach, we estimated that 1.56% of the genes in the genome yield to hyperactivity and 0.75% of genes produce hypoactivity when altered. These genes have been classified into genes for neurotransmitter systems, hormonal, metabolic systems, ion channels, structural proteins, transcription factors, second messengers and growth factors. Finally, two additional classes included animals with neurodegeneration and inner ear abnormalities. The analysis of the database revealed several unexpected findings. First, the genes that, when mutated, induce hyperactive behaviour do not pertain to a single neurotransmitter system. In fact, alterations in most neurotransmitter systems can give rise to a hyperactive phenotype. In contrast, fewer changes can decrease locomotor activity. Specifically, genetic and pharmacological alterations that enhance the dopamine, orexin, histamine, cannabinoids systems or that antagonize the cholinergic system induce an increase in locomotor activity. Similarly, imbalances in the two main neurotransmitters of the nervous system, GABA and glutamate usually result in hyperactive behaviour. It is remarkable that no genetic alterations pertaining to the GABA system have been reported to reduce locomotor behaviour. Other neurotransmitters, such as norepinephrine and serotonin, have a more complex influence. For instance, a decrease in norepinephrine synthesis usually results in hypoactive behaviour. However, a chronic increase in norepinephrine may result in hypoactivity too. Similarly, changes in both directions of serotonin levels may reduce locomotor activity, whereas alterations in specific serotonin receptors can induce hyperactivity. The lesion of at least 12 different brain regions can increase locomotor activity too. Comparatively, few focal lesions decrease locomotor activity. Finally, a large number of toxic events can increase locomotor activity, particularly if delivered during the prepuberal time window. These data show that there is a net imbalance in the number of altered genes/brain lesions/toxics that induce hyperactivity versus hypoactive behaviour. Although some of these data may be explained in terms of the activating role of subcortical systems (such as catecholamines), the larger number of alterations that induce hyperactivity suggests a different scenario. Specifically, we hypothesize (i) the existence of a control system that continuously inhibit a basally hyperactive locomotor tone and (ii) that this control system is highly vulnerable (intrinsic fragility) to any change in the genetic asset or to any toxic/drug delivered during prepuberal stages. Brain lesion studies suggest that the putative control system is located along an axis that connects the olfactory bulb and the enthorhinal cortex (enthorhinal-hippocampal-septal-prefrontal cortex-olfactory bulb axis). We suggest that the increased locomotor activity in many psychiatric diseases may derive from the interference with the development of this brain axis during a specific postnatal time window.

Layer specific changes of astroglial gap junctions in the rat cerebellar cortex by persistent Borna Disease Virus infection

Neonatal Borna Disease Virus (BDV) infection of the Lewis rat brain, leads to Purkinje cell degeneration, in association with astroglial activation. Since astroglial gap junctions (GJ) are known to influence neuronal degeneration, we investigated BDV dependent changes in astroglial GJ connexins (Cx) Cx43, and Cx30 in the Lewis rat cerebellum, 4, and 8 weeks after neonatal infection. On the mRNA level, RT-PCR demonstrated a BDV dependent increase in cerebellar Cx43, and a decrease in Cx30, 8, but not 4 weeks p.i. On the protein level, Western blot analysis revealed no overall upregulation of Cx43, but an increase of its phosphorylated forms, 8 weeks p.i. Cx30 protein was downregulated. Immunohistochemistry revealed a BDV dependent reduction of Cx43 in the granular layer (GL), 4 weeks p.i. 8 weeks p.i., Cx43 immunoreactivity recovered in the GL, and was induced in the molecular layer (ML). Cx30 revealed a BDV dependent decrease in the GL, both 4, and 8 weeks p.i. Changes in astroglial Cxs correlated not with expression of the astrogliotic marker GFAP, which was upregulated in radial glia. With regard to functional coupling, primary cerebellar astroglial cultures, revealed a BDV dependent increase of Cx43, and Cx30 immunoreactivity and in spreading of the GJ permeant dye Lucifer Yellow. These results demonstrate a massive, BDV dependent reorganization of astroglial Cx expression, and of functional GJ coupling in the cerebellar cortex, which might be of importance for the BDV dependent neurodegeneration in this brain region.

Glial connexins and gap junctions in CNS inflammation and disease

Gap junctions facilitate direct cytoplasmic communication between neighboring cells, facilitating the transfer of small molecular weight molecules involved in cell signaling and metabolism. Gap junction channels are formed by the joining of two hemichannels from adjacent cells, each composed of six oligomeric protein subunits called connexins. Of paramount importance to CNS homeostasis are astrocyte networks formed by gap junctions, which play a critical role in maintaining the homeostatic regulation of extracellular pH, K+, and glutamate levels. Inflammation is a hallmark of several diseases afflicting the CNS. Within the past several years, the number of publications reporting effects of cytokines and pathogenic stimuli on glial gap junction communication has increased dramatically. The purpose of this review is to discuss recent observations characterizing the consequences of inflammatory stimuli on homocellular gap junction coupling in astrocytes and microglia as well as changes in connexin expression during various CNS inflammatory conditions.

Astrocytes discriminate and selectively respond to the activity of a subpopulation of neurons within the barrel cortex

Sensory information from single whiskers in rodents projects to defined morphological units in the cortex, the barrels. We found that astrocytes selectively respond with an increase in the cytosolic Ca(2+) concentration to activation of layer 4 neurons, the input cells of the barrel columns. The neuronal Ca(2+) signal also spread across barrel column borders mainly in layer 2/3, but the glutamate-mediated astrocyte response stayed restricted to the barrel column. In contrast, when interfering with inhibitory pathways by blocking either purinergic, adenosine or gamma-aminobutyric acid(A) receptors, the stimulation activated a Ca(2+) response in a much larger astrocyte population no longer restricted to the borders of the barrel column. We also observed spontaneous and evoked Ca(2+) activity in the synaptic target cells of layer 4 neurons, the layer 2/3 pyramidal cells, but again, we never recorded Ca(2+) responses in astrocytes following activity in this neuronal population. Our data show that astrocytes can discriminate and selectively respond to the activity of a subpopulation of excitatory neurons within a given brain region. This selectivity in the astrocyte response describes a new level of complexity and integration in the reaction of astrocytes to neuronal activity.

Acute astrocyte activation in brain detected by MRI: new insights into T(1) hypointensity

Increases in the T(1) of brain tissue, which give rise to dark or hypointense areas on T(1)-weighted images using magnetic resonance imaging (MRI), are common to a number of neuropathologies including multiple sclerosis (MS) and ischaemia. However, the biologic significance of T(1) increases remains unclear. Using a multiparametric MRI approach and well-defined experimental models, we have experimentally induced increases in tissue T(1) to determine the underlying cellular basis of such changes. We have shown that a rapid acute increase in T(1) relaxation in the brain occurs in experimental models of both low-flow ischaemia induced by intrastriatal injection of endothelin-1 (ET-1), and excitotoxicity induced by intrastriatal injection of N-methyl-D-aspartate (NMDA). However, there appears to be no consistent correlation between increases in T(1) relaxation and changes in other MRI parameters (apparent diffusion coefficient, T(2) relaxation, or magnetisation transfer ratio of tissue water). Immunohistochemically, one common morphologic feature shared by the ET-1 and NMDA models is acute astrocyte activation, which was detectable within 2 h of intracerebral ET-1 injection. Pretreatment with an inhibitor of astrocyte activation, arundic acid, significantly reduced the spatial extent of the T(1) signal change induced by intrastriatal ET-1 injection. These findings suggest that an increase in T(1) relaxation may identify the acute development of reactive astrocytes within a central nervous system lesion. Early changes in T(1) may, therefore, provide insight into acute and reversible injury processes in neurologic patients, such as those observed before contrast enhancement in MS.

Astrocytic connexin distributions and rapid, extensive dye transfer via gap junctions in the inferior colliculus: implications for [(14)C]glucose metabolite trafficking

The inferior colliculus has the highest rates of blood flow and metabolism in brain, and functional metabolic activity increases markedly in response to acoustic stimulation. However, brain imaging with [1- and 6-(14)C]glucose greatly underestimates focal metabolic activation that is readily detected with [(14)C]deoxyglucose, suggesting that labeled glucose metabolites are quickly dispersed and released from highly activated zones of the inferior colliculus. To evaluate the role of coupling of astrocytes via gap junctions in dispersal of molecules within the inferior colliculus, the present study assessed the distribution of connexin (Cx) proteins in the inferior colliculus and spreading of Lucifer yellow from single microinjected astrocytes in slices of adult rat brain. Immunoreactive Cx43, Cx30, and Cx26 were heterogeneously distributed; the patterns for Cx43 and Cx 30 differed and were similar to those of immunoreactive GFAP and S100beta, respectively. Most Cx43 was phosphorylated in resting and acoustically stimulated rats. Dye spreading revealed an extensive syncytial network that included thousands of cells and perivasculature endfeet; with 8% Lucifer yellow VS and a 5-min diffusion duration, about 6,100 astrocytes (range 2,068-11,939) were labeled as far as 1-1.5 mm from the injected cell. The relative concentration of Lucifer yellow fell by 50% within 0.3-0.8 mm from the injected cell with a 5-min diffusion interval. Perivascular dye labeling was readily detectable and often exceeded dye levels in nearby neuropil. Thus, astrocytes have the capability to distribute intracellular molecules quickly from activated regions throughout the large, heterogeneous syncytial volume of the inferior colliculus, and rapid trafficking of labeled metabolites would degrade resolution of focal metabolic activation.

Persistent Borna Disease Virus infection changes expression and function of astroglial gap junctions in vivo and in vitro

Neonatal Borna Disease Virus (BDV) infection of the Lewis rat brain leads to dentate gyrus (DG) degeneration, underlying mechanisms are not fully understood. Since astroglial gap junction (GJ) coupling is known to influence neurodegenerative processes, the question arose whether persistent BDV infection influences astroglial connexins (Cx) Cx43 and Cx30 in the hippocampal formation (HiF) of Lewis rats. RT-PCR and Western blot analysis of forebrain (FB) samples revealed a virus dependent reduction of both Cx types 8 but not 4 weeks post infection (p.i.). Immunohistochemistry revealed an increase of Cx43 in the DG and a decrease in the CA3 region 4 and 8 weeks p.i. Cx30, which was detectable only 8 weeks p.i., revealed a BDV dependent increase in DG and CA3 regions. BDV dependent astrogliosis as revealed by immunodetection of glial fibrillary acidic protein (GFAP) correlated not with astroglial connexin expression. With regard to functional coupling as revealed by scrape loading, BDV infection resulted in increased spreading of the GJ permeant dye Lucifer yellow in primary hippocampal astroglial cultures, and in increased expression of Cx43 and Cx30 as revealed by immunocytochemistry. In conclusion, persistent BDV infection of the Lewis rat brain leads to changes in astroglial Cx expression both in vivo and in vitro and of functional coupling in vitro. Distribution and time course of these changes suggest them to be a direct result of neurodegeneration in the DG and an indirect effect of neuronal deafferentiation in the CA3 region.

A role for Connexin43 during neurodevelopment

Connexin43 (Cx43) is the predominant gap junction protein expressed in premitotic radial glial cells and mature astrocytes. It is thought to play a role in many aspects of brain development and physiology, including intercellular communication, the release of neuroactive substances, and neural and glial proliferation and migration. To investigate the role of Cx43 in brain physiology, we generated a conditional knockout (cKO) mouse expressing Cre recombinase driven by the human GFAP promoter and a floxed Cx43 gene. The removal of Cx43 from GFAP-expressing cells affects the behavior of the mice and the development of several brain structures; however, the severity of the phenotype varies depending on the mouse background. One mouse subline, hereafter termed Shuffler, exhibits cellular disorganization of the cortex, hippocampus, and cerebellum, accompanied by ataxia and motor deficits. The Shuffler cerebellum is most affected and displays altered distribution and lamination of glia and neurons suggestive of cell migration defects. In all Shuffler mice by postnatal day two (P2), the hippocampus, cortex, and cerebellum are smaller. Disorganization of the ventricular and subventricular zone of the cortex is also evident. Given that these are sites of early progenitor cell proliferation, we suspect production and migration of neural progenitors may be altered. In conclusion, neurodevelopment of Shuffler/Cx43 cKO mice is abnormal, and the observed cellular phenotype may explain behavioral disturbances seen in these animals as well as in humans carrying Cx43 mutations.

Gap junctional control of glial glutamate transporter expression

The uptake of glutamate into astroglia is the predominant mechanism to terminate glutamatergic neurotransmission and to prevent neurotoxic extracellular glutamate concentrations. Here, we show that uncoupling cultured cortical astrocytes with the gap junction blocker, propofol, or the Cx43 mimetic peptide, Gap27, inhibits the expression of GLT-1, the major glutamate transporter subtype in the cortex. The dependence of GLT-1 expression on gap junctions was further confirmed by the use of astrocytes in which either the expression of Cx43, the major astrocytic gap junction protein, was inhibited by RNA interference or which were derived from animals carrying an astrocyte-specific deletion of the Cx43 gene. In both cases, reduced astrocytic coupling was associated with a pronounced decline in GLT-1 expression. Finally, a luciferase reporter gene assay demonstrated that blockade of gap junctions/connexins suppressed transcriptional activity of GLT-1 promoter. These observations unravel a previously unrecognized role of gap junctions in the control of glial glutamate transport.

Conditional gene targeting in the mouse nervous system: Insights into brain function and diseases

Conditional gene knockout represents an extremely powerful approach to study the function of single genes in the nervous system. The Cre-LoxP system is the most advanced technology for spatial and temporal control of genetic inactivation, and there is rapid progress using this methodology in neuroscience research. In this approach, mice with LoxP sites flanking the gene of interest (floxed mice) are bred with transgenic mice expressing Cre recombinase under the control of a selected promoter (Cre mice). This promoter is critical in that it determines the time and site of Cre expression. Cre enzyme, in turn, recombines the floxed gene and produces gene knockout. Here we review Cre mouse lines that have been developed to target either the entire brain, selected brain areas, or specific neuronal populations. We then summarize phenotypic consequences of conditional gene targeting in the brain for more than 40 genes, as reported to date. For many broadly expressed genes, brain-restricted knockout has overcome lethality of conventional knockout (KO) and has highlighted a specific role of the encoded protein in some aspect of brain function. In the case of neural genes, data from null mutants in specific brain sites or neurons has refined our understanding of the role of individual molecules that regulate complex behaviors or synaptic plasticity within neural circuits. Among the many developing functional genomic approaches, conditional gene targeting in the mouse has become an excellent tool to elucidate the function of the approximately 5000 known or unknown genes that operate in the nervous system.

The TLR3 ligand polyI: C downregulates connexin 43 expression and function in astrocytes by a mechanism involving the NF-kappaB and PI3 kinase pathways

Toll-like receptor 3 (TLR3) is a component of the innate immune response that responds to dsRNA viruses and virus replication intermediates. In this study we show that activation of astrocytes with the dsRNA mimetic polyinosinic-cytidylic acid (pI:C) results in loss of expression of connexin43 (Cx43) mRNA and protein while upregulating the expression of the ionotropic P2 receptor P2X(4)R. Analysis of the signaling pathways involved failed to demonstrate a role for the p38 MAP kinase, ERK, or JNK signaling pathways whereas an inhibitor of the PI3 kinase/Akt pathway effectively blocked the action of pI:C. Using adenoviral vectors containing a super-repressor of NF-kappaB (NF-kappaB SR) construct or a dominant negative interferon regulatory factor 3 (dnIRF3) construct showed that inhibition of both transcription factors also blocked the effects of pI:C. To explore the functional consequences of pI:C activation we used a pore-forming assay for P2X(4)R activity and a scrape loading assay for gap junction intercellular communication (GJIC). No pore-forming activity consistent with functional P2X(4)R expression was detected in either control or activated astrocytes. In contrast, robust Lucifer yellow transfer indicative of GJIC was detected in resting cells that was lost following pI:C activation. The dnIRF3 construct failed to restore GJIC whereas the NF-kappaB SR or the NF-kappaB inhibitor BAY11-7082 and the PI3K inhibitor LY294002 all significantly reversed the effect of pI:C on GJ connectivity. We conclude that activation of the innate immune response in astrocytes is associated with functional loss of GJIC through a pathway involving NF-kappaB and PI3 kinase.

Low peak bone mass and attenuated anabolic response to parathyroid hormone in mice with an osteoblast-specific deletion of connexin43

Connexin43 (Cx43) is involved in bone development, but its role in adult bone homeostasis remains unknown. To overcome the postnatal lethality of Cx43 null mutation, we generated mice with selective osteoblast ablation of Cx43, obtained using a Cx43fl allele and a 2.3-kb fragment of the alpha1(I) collagen promoter to drive Cre in osteoblasts (ColCre). Conditionally osteoblast-deleted ColCre;Cx43-/fl mice show no malformations at birth, but develop low peak bone mass and remain osteopenic with age, exhibiting reduced bone formation and defective osteoblast function. By both radiodensitometry and histology, bone mineral content increased rapidly and progressively in adult Cx43+/fl mice after subcutaneous injection of parathyroid hormone (PTH), an effect significantly attenuated in ColCre;Cx43-/fl mice, with Cx43-/fl exhibiting an intermediate response. Attenuation of PTH anabolic action was associated with failure to increase mineral apposition rate in response to PTH in ColCre;Cx43-/fl, despite an increased osteoblast number, suggesting a functional defect in Cx43-deficient bone-forming cells. In conclusion, lack of Cx43 in osteoblasts leads to suboptimal acquisition of peak bone mass, and hinders the bone anabolic effect of PTH. Cx43 represents a potential target for modulation of bone anabolism.

Inducible gene deletion in astroglia and radial glia--a valuable tool for functional and lineage analysis

Astrocytes are thought to play a variety of key roles in the adult brain, such as their participation in synaptic transmission, in wound healing upon brain injury, and adult neurogenesis. However, to elucidate these functions in vivo has been difficult because of the lack of astrocyte-specific gene targeting. Here we show that the inducible form of Cre (CreERT2) expressed in the locus of the astrocyte-specific glutamate transporter (GLAST) allows precisely timed gene deletion in adult astrocytes as well as radial glial cells at earlier developmental stages. Moreover, postnatal and adult neurogenesis can be targeted at different stages with high efficiency as it originates from astroglial cells. Taken together, this mouse line will allow dissecting the molecular pathways regulating the diverse functions of astrocytes as precursors, support cells, repair cells, and cells involved in neuronal information processing.

Normal embryonic development and cardiac morphogenesis in mice with Wnt1-Cre-mediated deletion of connexin43

Mice harboring a null mutation in the gap junction protein connexin43 (Cx43) die shortly after birth due to an obstruction of the right ventricular outflow tract of the heart. These hearts exhibit prominent pouches at the base of the pulmonary outlet, i.e., morphological abnormalities that were ascribed to Cx43-deficiency in neural crest cells. In order to examine the Cx43 expression pattern in neural crest cells and derived tissues and to test whether neural crest-specific deletion of Cx43 leads to the conotruncal defects seen in Cx43null mice, we ablated Cx43 using a Wnt1-Cre transgene. Deletion of Cx43 was complete and occurred in neural crest cells as well as in neural crest-derived tissues. Nevertheless, hearts of mice lacking Cx43 specifically in neural crest cells were indistinguishable from controls. Thus, the morphological heart abnormalities of Cx43 null mice are most likely not caused by lack of Cx43 in neural crest cells.

A role for glutamate and glia in the fast network oscillations preceding spreading depression

The mechanism of the propagation of spreading depression is unclear. Classical theories proposed a self-maintained cycle fed by elevated potassium and/or glutamate in the extracellular space. Earlier we found in vivo a characteristic oscillatory field activity that is synchronous in a strip of tissue ahead of the oncoming wave of neuron depolarization and that occurs before the extracellular potassium level begins to rise [Herreras O, Largo C, Ibarz JM, Somjen GG, Marrín del Río R (1994) Role of neuronal synchronizing mechanisms in the propagation of spreading depression in the in vivo hippocampus. J Neurosci 14:7087-7098]. We investigated here the possible participation of glutamate and the role of glia in the prodromal field oscillations using extra and intracellular recordings and pharmacological manipulations in rat hippocampal slices. As earlier shown in vivo, field oscillations propagated ahead of the negative potential shift covering distances of up to 1 mm. The oscillatory prodromals were initially subthreshold but then each wave became crowned by a population spike. The frequency of the oscillatory prodromals was variable among slices (80-115 Hz), but constant in individual slices. The blockade of ionotropic glutamate receptors decreased the frequency of prodromal oscillations, retarded spreading depression propagation, and shortened the duration of depolarization. Blocking the glutamate membrane transport increased the oscillatory frequency. The selective metabolic poisoning of astrocytes led to gradual disorganization of prodromal oscillations whose frequency first increased and then decreased. Also, the amplitude of the population spikes within the burst diminished as individual cells fired fewer action potentials, although still phase-locked with population spikes. The effects of glial metabolic impairment were observed within the period when neuron electrical properties were still normal, and were blocked by glutamate receptor antagonists. These data suggest that glutamate released from glial cells and possibly also from neurons has a role in the generation of oscillations and neuron firing synchronization that precede the spreading depression-related depolarization, but additional mechanisms are required to fully explain the onset and propagation of spreading depression.

Connexin-47 and connexin-32 in gap junctions of oligodendrocyte somata, myelin sheaths, paranodal loops and Schmidt-Lanterman incisures: implications for ionic homeostasis and potassium siphoning

The subcellular distributions and co-associations of the gap junction-forming proteins connexin 47 and connexin 32 were investigated in oligodendrocytes of adult mouse and rat CNS. By confocal immunofluorescence light microscopy, abundant connexin 47 was co-localized with astrocytic connexin 43 on oligodendrocyte somata, and along myelinated fibers, whereas connexin 32 without connexin 47 was co-localized with contactin-associated protein (caspr) in paranodes. By thin-section transmission electron microscopy, connexin 47 immunolabeling was on the oligodendrocyte side of gap junctions between oligodendrocyte somata and astrocytes. By freeze-fracture replica immunogold labeling, large gap junctions between oligodendrocyte somata and astrocyte processes contained much more connexin 47 than connexin 32. Along surfaces of internodal myelin, connexin 47 was several times as abundant as connexin 32, and in the smallest gap junctions, often occurred without connexin 32. In contrast, connexin 32 was localized without connexin 47 in newly-described autologous gap junctions in Schmidt-Lanterman incisures and between paranodal loops bordering nodes of Ranvier. Thus, connexin 47 in adult rodent CNS is the most abundant connexin in most heterologous oligodendrocyte-to-astrocyte gap junctions, whereas connexin 32 is the predominant if not sole connexin in autologous ("reflexive") oligodendrocyte gap junctions. These results clarify the locations and connexin compositions of heterologous and autologous oligodendrocyte gap junctions, identify autologous gap junctions at paranodes as potential sites for modulating paranodal electrical properties, and reveal connexin 47-containing and connexin 32-containing gap junctions as conduits for long-distance intracellular and intercellular movement of ions and associated osmotic water. The autologous gap junctions may regulate paranodal electrical properties during saltatory conduction. Acting in series and in parallel, autologous and heterologous oligodendrocyte gap junctions provide essential pathways for intra- and intercellular ionic homeostasis.

Gap junctions and the propagation of cell survival and cell death signals

Gap junctions are a unique type of intercellular channels that connect the cytoplasm of adjoining cells. Each gap junction channel is comprised of two hemichannels or connexons and each connexon is formed by the aggregation of six protein subunits known as connexins. Gap junction channels allow the intercellular passage of small (< 1.5 kDa) molecules and regulate essential processes during development and differentiation. However, their role in cell survival and cell death is poorly understood. We review experimental data that support the hypothesis that gap junction channels may propagate cell death and survival modulating signals. In addition, we explore the hypothesis that hemichannels (or unapposed connexons) might be used as a paracrine conduit to spread factors that modulate the fate of the surrounding cells. Finally, direct signal transduction activity of connexins in cell death and survival pathways is addressed.

Ultrastructural localization of connexins (Cx36, Cx43, Cx45), glutamate receptors and aquaporin-4 in rodent olfactory mucosa, olfactory nerve and olfactory bulb

Odorant/receptor binding and initial olfactory information processing occurs in olfactory receptor neurons (ORNs) within the olfactory epithelium. Subsequent information coding involves high-frequency spike synchronization of paired mitral/tufted cell dendrites within olfactory bulb (OB) glomeruli via positive feedback between glutamate receptors and closely-associated gap junctions. With mRNA for connexins Cx36, Cx43 and Cx45 detected within ORN somata and Cx36 and Cx43 proteins reported in ORN somata and axons, abundant gap junctions were proposed to couple ORNs. We used freeze-fracture replica immunogold labeling (FRIL) and confocal immunofluorescence microscopy to examine Cx36, Cx43 and Cx45 protein in gap junctions in olfactory mucosa, olfactory nerve and OB in adult rats and mice and early postnatal rats. In olfactory mucosa, Cx43 was detected in gap junctions between virtually all intrinsic cell types except ORNs and basal cells; whereas Cx45 was restricted to gap junctions in sustentacular cells. ORN axons contained neither gap junctions nor any of the three connexins. In OB, Cx43 was detected in homologous gap junctions between almost all cell types except neurons and oligodendrocytes. Cx36 and, less abundantly, Cx45 were present in neuronal gap junctions, primarily at "mixed" glutamatergic/electrical synapses between presumptive mitral/tufted cell dendrites. Genomic analysis revealed multiple miRNA (micro interfering RNA) binding sequences in 3'-untranslated regions of Cx36, Cx43 and Cx45 genes, consistent with cell-type-specific post-transcriptional regulation of connexin synthesis. Our data confirm absence of gap junctions between ORNs, and support Cx36- and Cx45-containing gap junctions at glutamatergic mixed synapses between mitral/tufted cells as contributing to higher-order information coding within OB glomeruli.

Glial and neuronal connexin expression patterns in the rat spinal cord during development and following injury

Spinal cord injury induces a complex cascade of degenerative and remodeling events evolving over time. The possible roles of changed intercellular communication via gap junctions after spinal cord injury (SCI) have remained relatively unexplored. We investigated the temporospatial expression patterns of gap junctional genes and proteins, connexin 43 (Cx43), Cx36, and Cx32, by in situ hybridization and immunohistochemistry in the rat neonatal, adult normal, and adult injured spinal cord. Cx36 was strongly expressed in immature neurons, and levels declined markedly during development, whereas Cx43 and Cx32 persisted throughout adulthood. After a complete transection of the adult spinal cord, the levels of Cx43 mRNA and protein were up-regulated within hours, especially in gray matter rostral to the lesion, reaching over three times normal levels at 4 weeks postinjury. Cx43 immunoreactivity was seen primarily in astrocytes and rarely in microglia. In contrast, Cx36 and Cx32 mRNA and proteins were relatively sparse and unchanged after spinal cord injury along the entire axis of the spinal cord. Cx43 is the most abundant gap junctional protein in the adult CNS and has been shown to form channels between astrocytes as well as between astrocytes and oligodendrocytes. Long-term up-regulation of Cx43 in reactive astrocytes may be one critical component in the rearrangement of the local astroglial network following SCI.

Genetic alteration of anxiety and stress-like behavior in mice lacking CaMKIV

Calcium-calmodulin-dependent protein kinase IV (CaMKIV) phosphorylates the major transcription factor cyclic AMP-response element binding protein (CREB), which plays a role in emotional behavior. Here, CaMKIV knockout mice (CaMKIV^-/-) were tested in a battery of stress and anxiety-related behavioral tests, to determine if CaMKIV plays a role in emotional behavior. CaMKIV^-/-exhibited a decrease in anxiety-like behavior in both the elevated plus maze and dark-light emergence tests when compared to wild-type mice. Both the acoustic startle response and prepulse inhibition of startle were decreased with the deletion of CaMKIV. In addition, CaMKIV^-/- mice displayed a lack of stress-induced analgesia following restraint or cold swim stress. Our results demonstrate a key role for CaMKIV in anxiety and stress-related behavior.

Sensitivity of the brain transcriptome to connexin ablation

Extensive studies on mice with total or partial disruption of either connexin43 (Cx43) or connexin32 (Cx32) have detected only subtle changes in central nervous system structure, growth, development, or function. We have used high density cDNA arrays to analyze the regulation, control, and coordination of the abundances of 7446 distinct transcripts in four brains, each of Cx43 null (K43), Cx43 heterozygous (H43), and Cx32 null (K32) mice as compared to the brains of wildtype (W) mice. The use of multiple samples allowed the determination of the statistical significance of gene regulation. Significantly regulated genes encoded proteins of all functional categories, extending beyond those that might be expected to depend on junctional communication. Moreover, we found a high degree of similarity between genes regulated in the K43 and H43 brains and a remarkable overlap between gene regulation in brains of K43 and K32. The regulated genes in both K43 and H43 brains showed an outstanding inverse coordination with the levels of expression of Cx43 in W brain, indicating that the regulated genes are largely predictable from their co-variance with Cx43 in the wildtype samples. These findings lead to the hypothesis that connexin expression may represent a central node in the regulation of gene expression patterns in brain.

Activity-dependent ATP-waves in the mouse neocortex are independent from astrocytic calcium waves

In the corpus callosum, astrocytic calcium waves propagate via a mechanism involving ATP-release but not gap junctional coupling. In the present study, we report for the neocortex that calcium wave propagation depends on functional astrocytic gap junctions but is still accompanied by ATP-release. In acute slices obtained from the neocortex of mice deficient for astrocytic expression of connexin43, the calcium wave did not propagate. In contrast, in the corpus callosum and hippocampus of these mice, the wave propagated as in control animals. In addition to calcium wave propagation in astrocytes, ATP-release was recorded as a calcium signal from 'sniffer cells', a cell line expressing high-affinity purinergic receptors placed on the surface of the slice. The astrocyte calcium wave in the neocortex was accompanied by calcium signals in the 'sniffer cell' population. In the connexin43-deficient mice we recorded calcium signals from sniffer cells also in the absence of an astrocytic calcium wave. Our findings indicate that astrocytes propagate calcium signals by two separate mechanisms depending on the brain region and that ATP release can propagate within the neocortex independent from calcium waves.

Emerging complexities in identity and function of glial connexins

Recent research results indicate that glial gap-junction communication is much more complex and widespread than originally thought, and has diverse roles in brain homeostasis and the response of the brain to injury. The situation is far from clear, however. Pharmacological agents that block gap junctions can abolish neuron-glia long-range signaling and can alleviate neuronal damage whereas, intriguingly, opposite effects are observed in mice lacking connexin43, a major gap-junction subunit protein in astrocytes. How can the apparently contradictory results be explained, and how is specificity achieved within the glial gap-junction system? Another key issue in understanding glial connexin function is that oligodendrocytes and astrocytes, each of which express distinct connexin isotypes, are thought to participate in brain homeostasis by forming a panglial syncytium. Molecular analysis has revealed a surprising diversity of connexin expression and function, and this has led to new hypotheses regarding their roles in the brain, which could be tested using new approaches.

New insights into the expression and function of neural connexins with transgenic mouse mutants

Gap junctions represent direct intercellular conduits between contacting cells. The subunit proteins of these conduits are called connexins. To date, 20 and 21 connexin genes have been described in the mouse and human genome, respectively, many of them represent sequence-orthologous pairs. Targeted deletion of connexin genes in the mouse genome opened new insights into the biological function of these channel forming proteins, which, in some cases, could be correlated to phenotypic abnormalities in humans, suffering from inherited diseases caused by mutations in the corresponding orthologous connexin gene. Replacing the connexin coding DNA by an appropriate reporter gene has clarified in several cases its cell type specific expression in mouse brain. Various studies demonstrated that connexin36 is mainly expressed in interneurons of retina and brain. Targeted deletion of connexin36 evoked a loss of electrical signal transduction and interferes with synchrony which probably leads to defects in visual transmission and memory. Deletion of connexin43 in astrocytes of mouse brain resulted in increased spreading depression consistent with the notion of altered "spatial buffering" of K(+) ions and glutamate secreted by active neurons. General connexin30-deficiency led to hearing impairment and apoptosis of hair cells, similar to that observed in mice with cochlea specific deletion of connexin26. Reporter gene expression in connexin30-deficient mice indicated that astrocytes in certain brain regions and leptomeningeal as well as ependymal cells are labelled. Reporter gene expression in connexin45- and connexin47-deficient mice was used to reassign connexin45 expression to certain CNS neurons and connexin47 expression to oligodendrocytes.

Spontaneous ectopic recombination in cell-type-specific Cre mice removes loxP-flanked marker cassettes in vivo

Conditional gene targeting using the Cre/loxP technology generally includes integration of a selection marker cassette flanked by loxP recognition sites (floxed) in the target gene locus. Subsequent marker removal avoids possible impairment of gene expression or mosaicism due to partial and total deletions after Cre-mediated recombination in vivo. The use of deleter Cre mice for in vivo marker removal in floxed connexin43 mice revealed considerable mosaicism, but no selective marker removal. In addition, we noted that several Cre transgenic lines displayed spontaneous ectopic activity, reminiscent of deleter Cre mice, and required the confirmation of cell type-specific deletion in every individual mouse. When we used myosin heavy chain promoter Cre (alphaMyHC-Cre) mice for cardiomyocyte specific deletion, we observed, in addition to cardiomyocyte-restricted or complete excision, selective marker removal in a subgroup of mice as well. Thus, selective marker removal can be achieved as a byproduct of cell-type restricted deletion.

Stimulus complexity dependent memory impairment and changes in motor performance after deletion of the neuronal gap junction protein connexin36 in mice

Gap junction channels, composed of connexin (Cx) proteins, are conduits for intercellular communication and metabolic exchange in the central nervous system. Connexin36 (Cx36) is expressed in distinct subpopulations of neurons throughout the mammalian brain. Deletion of the Cx36 gene in the mouse affected power and frequency of gamma and sharp wave-ripple oscillations, putative correlates of memory engram inscription. Here, we present a behavioral analysis of Cx36-deficient mice. Activity patterns, exploratory- and anxiety-related responses were largely unaffected by elimination of Cx36, while sensorimotor capacities and learning and memory processes were impaired. Repeated testing on the rotarod suggested that the Cx36-deficient mice showed slower motor-coordination learning. After a retention interval of 24 h the Cx36-deficient mice showed habituation to an open-field, but failed to habituate to a more complex spatial environment (Y-maze). A more pronounced memory impairment was found when Cx36 knockout mice had to remember recently explored objects. Cx36-deficient mice were unable to recognize objects after short delays of 15 and 45 min. These data suggest that lack of Cx36 induces memory impairments that vary in dependence of the complexity of the stimuli presented. Our results suggest that neuronal gap junctions incorporating Cx36 play a role in learning and memory.

Genes controlling multiple functional pathways are transcriptionally regulated in connexin43 null mouse heart

We have used mouse 27k cDNA arrays to compare gene expression patterns in four sets of three hearts each of neonatal wild types and four sets of three hearts each of littermates lacking the major cardiac gap junction protein, connexin43 (Cx43). Each individual set of hearts was hybridized against aliquots of an RNA standard prepared from selected mouse tissues, allowing calculation of variability and coordination of gene expression among the samples from both genotypes. Overall variance of gene expression was found to be markedly higher in wild-type hearts than in those from Cx43 null littermates. Expression levels of 586 of 5,613 adequately quantifiable distinct genes with known protein products were statistically altered in the Cx43 null hearts, 38 upregulated and 548 downregulated compared with wild types. Downregulation was confirmed for seven tested genes by quantitative RT-PCR. Functions of proteins encoded by the altered genes encompassed all functional categories, with largest percent changes in genes involved in intracellular transport and transcription factors. Among the downregulated genes in the Cx43 null hearts were those related to neuronal and glial function, suggesting that cardiac innervation might be compromised as a consequence of Cx43 deletion. This was supported by immunodetection of sympathetic innervation, using antibodies to the synaptic vesicle protein synaptophysin and to the adrenergic nerve terminal marker tyrosine hydroxylase. These findings reinforce the proposal that the cardiac abnormality in Cx43 null animals may be contributed by altered innervation and indicate that Cx43 deletion has consequences in addition to reduced intercellular communication.

Development of mice with osteoblast-specific connexin43 gene deletion

Genetic deficiency of Cx43 in vivo causes skeletal developmental defects, osteoblast dysfunction and perinatal lethality. To determine the role of Cx43 in the adult skeleton, we developed two models of osteoblast-specific Cx43 gene deletion using Cre mediated replacement of a "floxed" Cx43 allele with a LacZ reporter gene. Cre recombinase expression in osteoblasts was driven by either the osteocalcin OG2 promoter or the 2.3 kb fragment of the Colalpha1(I) promoter. Homozygous Cx43(fl/fl) mice, in which the Cx43 coding region is flanked by two loxP sites, were crossed with Cre expressing mice in a heterozygous Cx43-null background [Cx43(+/-); Colalpha1(I)-Cre or Cx43(+/-); OG2-Cre]. Cx43 gene ablation was demonstrated in tissues by selective X-gal staining of cells lining the endosteal surface, and in cultured osteoblastic cells from calvaria using different approaches. Although no LacZ expression was observed in proliferating calvaria cells, before osteoblast differentiation begins, post-proliferative cells isolated from conditional knockout mice [Cx43(fl/-); Colalpha1(I)-Cre or Cx43(fl/-); OG2-Cre] developed strong LacZ expression as they differentiated, in parallel to a progressive disappearance of Cx43 mRNA and protein abundance relative to controls. Selective Cre mediated Cx43 gene inactivation in bone forming cells will be useful to determine the role of Cx43 in adult skeletal homeostasis and overcome the perinatal lethality of the conventional null model.

Neuroprotective role of astrocytic gap junctions in ischemic stroke

The role of astrocytic gap junctions in ischemia remains controversial. Several studies support that astrocytic gap junctions play a role in the spread of hypoxic injury, while other reports have demonstrated that blocking astrocytic gap junctions increases neuronal death. Using a stroke model on animals in which the astrocytic gap junction protein connexin43 (Cx43) was compromised, we explored the neuroprotective role of astrocytic gap junctions. A focal brain stroke was performed on heterozygous Cx43 null [Cx43(+/-)] mice, wild type [Cx43(+/+)] mice, astrocyte-directed Cx43 deficient [Cx43(fl/ fl)/hGFAP-cre] mice (here designated as Cre(+) mice), and their corresponding controls [Cx43(fl/fl)] (here designated as Cre(-) mice). Four days following stroke, ischemic lesions were measured for size and analyzed immunohistochemically. Stroke volume was significantly larger in Cx43(+/-) and Cre(+) mice compared to Cx43(+/+) and Cre(-) mice, respectively. Apoptosis as detected by TUNEL labeling and caspase-3 immunostaining was amplified in Cx43(+/-) and Cre(+) mice compared to their control groups. Furthermore, increased inflammation as characterized by the immunohistochemical staining of the microglial marker CD11b was observed in the Cre(+) mice penumbra. Astrocytic gap junctions may reduce apoptosis and inflammation in the penumbra following ischemic insult, suggesting that coupled astrocytes fulfill a neuroprotective role under ischemic stroke conditions.

Inhomogeneous propagation of cortical spreading depression—detection by electro- and magnetoencephalography in rats

Spreading depression (SD) propagates in cortical regions that are different in their morphological and functional characteristics. We tested whether the propagation pattern of spreading depression was different between parts of the cortex. In six adult rats, we recorded the ECoG by a 4 x 4 electrode array that covered parts of the frontal, parietal cortex and the cingulate cortex. Simultaneously a 16-channel magnetoencephalogram was recorded to characterize the development and direction of intracortical ion movements accompanying this phenomenon. Spreading depression was initiated by occipital application of 0.3 molar KCl solution. Depolarization was observed, at first, at lateral cortical regions and then at medial cortical regions. Thereafter, the propagation velocity increased in medial cortical regions and was faster than in lateral regions. Negative potential shifts were detected by all electrodes, but the depolarization reached a maximum over lateral and caudal cortical regions. The recorded magnetic fields indicated the same orientation of currents underlying these fields, which was perpendicular to the wave front and points away from the depolarization region. Overall, the data indicated that propagation patterns of spreading depression differed between parts of the cortex and, thus, propagation was inhomogeneous. This propagation was accompanied by strong currents parallel to the cortical surface.

The complex of ciliary neurotrophic factor-ciliary neurotrophic factor receptor alpha up-regulates connexin43 and intercellular coupling in astrocytes via the Janus tyrosine kinase/signal transducer and activator of transcription pathway

Cytokines regulate numerous cell processes, including connexin expression and gap junctional coupling. In this study, we examined the effect of ciliary neurotrophic factor (CNTF) on connexin43 (Cx43) expression and intercellular coupling in astrocytes. Murine cortical astrocytes matured in vitro were treated with CNTF (20 ng/ml), soluble ciliary neurotrophic factor receptor alpha (CNTFRalpha) (200 ng/ml), or CNTF-CNTFRalpha. Although CNTF and CNTFRalpha alone had no effect on Cx43 expression, the heterodimer CNTF-CNTFRalpha significantly increased both Cx43 mRNA and protein levels. Cx43 immunostaining correlated with increased intercellular coupling as determined by dye transfer analysis. By using the pharmacological inhibitor alpha-cyano-(3,4-dihydroxy)-N-benzylcinnamide (AG490), the increase in Cx43 was found to be dependent on the Janus tyrosine kinase/signal transducer and activator of transcription (JAK/STAT) pathway. Immunocytochemical analysis revealed that CNTF-CNTFRalpha treatment produced nuclear localization of phosphorylated STAT3, whereas CNTF treatment alone did not. Transient transfection of constructs containing various sequences of the Cx43 promoter tagged to a LacZ reporter into ROS 17/2.8 cells confirmed that the promoter region between -838 to -1693 was deemed necessary for CNTF-CNTFRalpha to induce heightened expression. CNTF-CNTFRalpha did not alter Cx30 mRNA levels, suggesting selectivity of CNTF-CNTFRalpha for connexin signaling. Together in the presence of soluble receptor, CNTF activates the JAK/STAT pathway leading to enhanced Cx43 expression and intercellular coupling.

Neurons set the tone of gap junctional communication in astrocytic networks

A number of studies have contributed to demonstrate that neurons and astrocytes tightly and actively interact. Indeed, the presence of astrocytes in neuronal cultures increases the number of synapses and their efficiency, and thanks to enzymatic and uptake processes, astrocytes play a role in neuroprotection. A typical feature of astrocytes is that they establish cell-cell communication in vitro, as well as in situ, through intercellular channels forming specialized membrane areas defined as gap junctions. These channels are composed of junctional proteins termed connexins (Cxs): in astrocytes connexin 43 (Cx43) and 30 (Cx30) have been shown to prevail. Several recent works indicate that gap junctional communication (GJC) and/or connexin expression in astrocytes are controlled by neurons. Altogether, these observations lead to the concept that neuronal and astrocytic networks interact through mutual setting of their respective mode of communication and that astrocyte gap junctions represent a target in neuroglial interaction.

Increased apoptosis and inflammation after focal brain ischemia in mice lacking connexin43 in astrocytes

Astrocytes secrete cytokines and neurotrophic factors to neurons, consistent with a neurosupportive role for astrocytes. However, in ischemic or metabolic insults, the function of astrocytic gap junctions composed mainly from connexin43 (Cx43) remains controversial. We have previously shown that heterozygous Cx43 null mice subjected to middle cerebral artery occlusion exhibited significantly enhanced stroke volume and apoptosis compared to wild-type mice. In this study, we used mice in which the human GFAP promoter-driven cre transgene deletes the floxed Cx43 gene in astrocytes, excluding the effects from reduced Cx43 expression in many other cell types as well as astrocytes. We induced focal brain ischemia in mice lacking Cx43 in astrocytes [Cre(+)] and control littermates [Cre(-)]. Cre(+) mice showed a significantly increased stroke volume and enhanced apoptosis, detected by terminal dUTP nick-end labeling and caspase-3 immunostaining, compared to Cre(-) mice. Inflammatory response assessed by the microglial marker CD11b was amplified in the penumbra of Cre(+) mice compared to that of Cre(-) mice. Our results suggest that astrocytic gap junctions could be important for the regulation of neuronal apoptosis and the inflammatory response after stroke. These findings support the view that astrocytes play a critical role in neuroprotection during ischemic insults.

Distinct types of astroglial cells in the hippocampus differ in gap junction coupling

Previous studies have shown that two subpopulations of cells with astrocytic properties coexist in the mouse hippocampus, which display distinct morphological and functional characteristics, specifically a nonoverlapping expression of either AMPA-type glutamate receptors (GluR cells) or glutamate transporters (GluT cells). Use of transgenic mice with hGFAP promoter-controlled EGFP expression and patch-clamp recordings allow reliable identification of the two cell types in hippocampal slices. Extending functional characterization, we report here the complete lack of gap junctional tracer coupling in GluR cells, while GluT cells are shown to be extensively coupled. This distinction is valid in immature as well as adult animals. Analysis of transgenic mice expressing beta-Gal under regulatory elements of the Cx43 promoter revealed the absence of Cx43 in GluR cells. Experiments using gap junction blockers demonstrated that passive currents, displayed primarily by GluT cells, do not reflect intercellular coupling but are attributable to intrinsic membrane properties of individual cells. This study supports the notion that the two subpopulations of hGFAP-EGFP-positive cells represent distinct cell types with contrasting physiological properties. Since GluR cells do not participate in the astrocytic gap junctional network, their functional role must be different from spatial buffering of ions or signaling molecules, i.e., properties generally assigned to astrocytes.

Gap junctions and neurological disorders of the central nervous system

Gap junctions are intercellular channels which directly connect the cytoplasm between neighboring cells. In the central nervous system (CNS) various kinds of cells are coupled by gap junctions, which play an important role in maintaining normal function. Neuronal gap junctions are involved in electrical coupling and may also contribute to the recovery of function after cell injury. Astrocytes are involved in the pathology of most neuronal disorders, including brain ischemia, Alzheimer's disease and epilepsy. In the pathology of brain tumors, gap junctions may be related to the degree of malignancy and metastasis. However, the role of connexins, gap junctions and hemichannels in the pathology of the diseases in the CNS is still ambiguous. Of increasing importance is the unraveling of the function of gap junctions in the neural cell network, involving neurons, astrocytes, microglia and oligodendrocytes. A better understanding of the role of gap junctions may contribute to the development of new therapeutic approaches to treating diseases of the CNS.

Comprehensive strategies to study neuronal function in transgenic animal models

In the last 20 years of transgenic research, transgenic technology has been developed and refined to provide a seemingly comprehensive range of experimental approaches to questions of neuronal gene function; however, parallel developments in other technologies, such as whole genome sequencing and microarray analysis of gene expression, has meant that the questions have gotten harder. The complexity of neuronal systems and the difficulty of modeling neurologic diseases in rodents also present considerable challenges to molecular neuroscientists. Future functional genomic studies of brain and behavior will involve the full range of available transgenic methods and the incorporation of new technologies, including RNA interference.

Endothelins regulate astrocyte gap junctions in rat hippocampal slices

Gap junctional communication (GJC) is a typical feature of astrocytes proposed to contribute to the role played by these glial cells in brain physiology and pathology. In acutely isolated hippocampal slices from rat (P11-P19), intercellular diffusion of biocytin through gap junction channels was shown to occur between hundreds of cells immuno-positive for astrocytic markers studied in the CA1/CA2 region. Single-cell RT-PCR demonstrated astrocytic mRNA expression of several connexin (Cx) subtypes, the molecular constituent of gap junction channels, whereas immunoblotting confirmed that Cx43 and Cx30 are the main gap junction proteins in hippocampal astrocytes. In the brain, astrocytes represent a major target for endothelins (Ets), a vasoactive family of peptides. Our results demonstrate that Ets decrease the expression of phosphorylated Cx43 forms and are potent inhibitors of GJC. The Et-induced effects were investigated using specific Et receptor agonists and antagonists, including Bosentan (Tracleer trade mark ), an EtA/B receptor antagonist, and using hippocampal slices and cultures from EtB-receptor-deficient rats. Interestingly, the pharmacological profile of Ets effects did not follow the classical profile established in cardiovascular systems. The present study therefore identifies Ets as potent endogenous inhibitory regulators of astrocyte networks. As such, the action of these peptides on astrocyte GJC might be involved in the contribution of astrocytes to neuroprotective processes and have a therapeutic potential in neuropathological situations.

Replacement by a lacZ reporter gene assigns mouse connexin36, 45 and 43 to distinct cell types in pancreatic islets

Transcripts of three connexin isoforms (Cx36, Cx43 and Cx45) have been reported in rodent pancreatic islets, but the precise distribution of the cognate proteins is still unknown. We determined expression of Cx36 in a cell-autonomous manner using mice with a targeted replacement of the Cx36 coding region by a lacZ reporter gene. For cell-autonomous monitoring of Cx43 expression, we used the Cre/loxP system: Mice carrying the Cx43 coding region flanked by loxP sites (floxed) also carried an embedded lacZ gene that is activated after Cre-mediated recombination in cells with transcriptional activity of the Cx43 gene. Deletion of the Cx43 coding region in beta-cells did not result in the activation of the embedded lacZ reporter gene. Instead, Cx43 expression was found in endothelial cells of the islets of Langerhans in mice with endothelium-specific deletion. Ubiquitous deletion of Cx43 led to a similar endothelial lacZ expression, but again, activity of the reporter gene was not detected in beta-cells. Mice with targeted replacement of the Cx45 coding region by lacZ showed a vascular expression similar to Cx43. The data show that native insulin-producing cells express a connexin isoform (Cx36) which differs from those (Cx43 and Cx45) expressed by vascular islet cells.

IL-1-regulated responses in astrocytes: Relevance to injury and recovery

In the central nervous system (CNS), the cellular processes of astrocytes make intimate contact with essentially all areas of the brain. They have also been shown to be functionally coupled to neurons, oligodendrocytes, and other astrocytes via both contact-dependent and non-contact-dependent pathways. These observations have led to the suggestion that a major function of astrocytes in the CNS is to maintain the homeostatic environment, thus promoting the proper functioning of the neuronal network. Inflammation in the CNS disrupts this process either transiently or permanently and, as such, is thought to be tightly regulated by both astrocytes and microglia. The remarkable role that single cytokines, such as TNF and IL-1, may play in this process has now been well accepted, but the extent of the reprogramming of the transcriptional machinery initiated by these factors remains to be fully appreciated. With the advent of microarray technology, a more comprehensive analysis of this process is now available. In this report we review data obtained with this technology to provide an overview of the extent of changes induced in astrocytes by the cytokine IL-1.

Astrocyte cultures from conditional connexin43-deficient mice

Connexin43 (Cx43) mainly provides the molecular basis for astrocytic gap junctions. Interastrocytic coupling is thought to mediate extracellular ion homeostasis, long-range signaling, and neuroprotection in the brain. Cx43 has been implicated in astrocytic growth control and is also expressed in other cell types in the brain, such as leptomeningeal and vascular cells. Cx43 function has been studied in astrocyte cultures of Cx43-deficient mice, which lack Cx43 in all cell types. We have generated conditionally deficient mice with an astrocyte-directed inactivation of Cx43, which leaves expression in other cell types unaffected. Other connexins have been detected in astrocytes. For the study of astrocytes lacking Cx45 and Cx26 in vitro, which deficiencies are embryonic lethal, conditionally deficient astrocyte cultures are essential. In the present study, we describe the developmental kinetics of Cx43 inactivation and loss of intercellular communication in astrocyte cultures derived from conditional Cx43-deficient mice. Conditional ablation of Cx43 is efficient, reaches a plateau at 4 weeks in culture, but retains Cx43 expression in contaminating nonastrocytic cells. Our findings indicate that conditional knockout astrocytes are a promising tool for the study of embryonic lethal genes in astrocyte cultures.