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.

Gap-junction-mediated propagation and amplification of cell injury.

Gap junctions are conductive channels that connect the interiors of coupled cells. We determined whether gap junctions propagate transcellular signals during metabolic stress and whether such signaling exacerbates cell injury. Although overexpression of the human proto-oncogene bcl2 in C6 glioma cells normally increased their resistance to injury, the relative resistance of bcl2+ cells to calcium overload, oxidative stress and metabolic inhibition was compromised when they formed gap junctions with more vulnerable cells. The likelihood of death was in direct proportion to the number and density of gap junctions with their less resistant neighbors. Thus, dying glia killed neighboring cells that would otherwise have escaped injury. This process of glial 'fratricide' may provide a basis for the secondary propagation of brain injury in cerebral ischemia.

ATP-mediated glia signaling.

Glia calcium signaling has recently been identified as a potent modulator of synaptic transmission. We show here that the spatial expansion of calcium waves is mediated by ATP and subsequent activation of purinergic receptors. Ectopic expression of gap junction proteins, connexins (Cxs), leads to an increase in both ATP release and the radius of calcium wave propagation. Cx expression was also associated with a phenotypic transformation, and cortical neurons extended longer neurites when co-cultured with Cx-expressing than with Cx-deficient cells. Purinergic receptor activation mediated both these effects, because treatment with receptor antagonists restored the glia phenotype and slowed neurite outgrowth. These results identify a key role of ATP in both short-term calcium signaling events and in long-term differentiation regulated by glia.

Expression and function of astrocytic gap junctions in aging.

Astrocytic gap junctions have been implicated in a variety of signaling pathways essential to normal brain function. However, no information exists on the prevalence of gap junction channels and their function in the aging brain. Here we have compared the expression of the two most abundant astrocytic gap junction proteins in young and senescent brains and quantified the extent of functional gap junction coupling. The expression level of Cx43 peaked in 7-month-old mice. The relative numbers of Cx43 immunoreactive plaques were 596+/-61, 734+/-62, and 755+/-114 in 3-, 7-, and 21-month-old mice, whereas plaques size averaged 0.9+/-0.1 microm(2) (3 months), 1.3+/-0.1 microm(2) (7 months), and 0.7+/-0.1 microm(2) (21 months). The expression level of Cx30 was also highest in 7-month-old animals (315+/-49 plaques, size 0.8+/-0.07 microm(2) vs. 585+/-51 plaques, size 0.9+/-0.1 microm(2) in 3- and 7-month-old mice, respectively), but only 262+/-63 plaques (size 0.4+/-0.04 microm(2)) in 21-month-old mice. Western blot analysis revealed that the content of both Cx43 and Cx30 remained relatively constant at 3, 7, and 21 months. The fluorescence recovery of photobleach technique (FRAP) was used to evaluate coupling in freshly prepared hippocampal slices. Gap junction coupling did not change significantly as a function of aging, but a tendency towards reduced coupling was observed as the animals aged. Average fluorescence recovery after 2 min was 63+/-6% in younger animals, 59+/-5% in adult animals, and 54+/-4% in old brain. These observations indicate that although astrocytic gap junction proteins are maintained at high levels through the entire lifespan of mice, aging is associated with changes in the number and size of both Cx30 and Cx43 gap junction plaques.

Cytoskeletal assembly and ATP release regulate astrocytic calcium signaling.

We have studied the role of actin fiber assembly on calcium signaling in astrocytes. We found that (1) after astrocytes have been placed in culture, it takes several hours for organization of the definitive actin cytoskeleton. Actin organization and the number of cells engaged in calcium signaling increased in parallel. (2) Disruption of the actin cytoskeleton attenuated the calcium wave propagation; cytochalasin D treatment reduced the number of astrocytes engaged in calcium signaling. (3) Propagation of calcium waves depends on cytoskeletal function; inhibition of myosin light chain kinase suppressed wave activity. (4) Astrocytic calcium signaling is mediated by release of ATP and purinergic receptor stimulation, because agents that interfere with this cascade attenuated or reduced calcium signaling. Because purinergic receptors are fully functional shortly after plating and not affected by cytochalasin D, these observations indicate that cytoskeleton organization is a prerequisite for interastrocytic calcium signaling mediated by release of ATP.

Meningeal cells can communicate with astrocytes by calcium signaling.

Mechanical stimulation of adult human and rat pia-arachnoid cell cultures (loaded with calcium indicator dye) produced an increase in calcium in the stimulated cell. This change then propagated rapidly among neighboring cells, producing a calcium wave with a maximum distance of propagation and velocity resembling calcium waves in astrocytes. The pia-arachnoid waves were blocked by either octanol or apyrase, suggesting that propagation might occur either by gap junction communication or extracellular movement of ATP. Calcium waves in pia-arachnoid cells could invade contiguous astrocytes, and vice versa. Gap junction coupling between pia-arachnoid cells and astrocytes was shown by dye transfer experiments, in conjunction with immunostaining for connexin43. We infer that calcium signals from cells in the cortical parenchyma may be transmitted to the pia-arachnoid and might then serve in the induction of neurovascular changes, including those postulated to be responsible for the pain of migraine headache.

Programmed cell death in the developing somites is promoted by nerve growth factor via its p75(NTR) receptor.

Neurotrophins control neuron number during development by promoting the generation and survival of neurons and by regulating programmed neuronal death. In the latter case, the cell death induced by nerve growth factor (NGF) in the developing chick retina is mediated by p75(NTR), the common neurotrophin receptor (J. M. Frade, A. Rodriguez-Tebar, and Y.-A. Barde, 1996, Nature 383, 166-168). Here we show that NGF also induces the programmed death of paraxial mesoderm cells in the developing somites. Both NGF and p75(NTR) are expressed in the somites of chick embryos at the time and the place of programmed cell death. Moreover, neutralizing the activity of endogenous NGF with a specific blocking antibody, or antagonizing NGF binding to p75(NTR) by the application of human NT-4/5, reduces the levels of apoptotic cell death in both the sclerotome and the dermamyotome by about 50 and 70%, respectively. Previous data have shown that Sonic hedgehog is necessary for the survival of differentiated somite cells. Consistent with this, Sonic hedgehog induces a decrease of NGF mRNA in somite explant cultures, thus showing the antagonistic effect of NGF and Sonic hedgehog with respect to somite cell survival. The regulation of programmed cell death by NGF/p75(NTR) in a mesoderm-derived tissue demonstrates the capacity of neurotrophins and their receptors to influence critical developmental processes both within and outside of the nervous system.

Glucocorticoids-potent modulators of astrocytic calcium signaling.

Glucocorticoids are the first line of choice in the treatment of cerebral edema associated with brain tumors. High-dose glucocorticoids reduce the extent of edema within hours, often relieving critical increases in intracranial pressure, but the mechanisms by which glucocorticoids modulate brain water content are not well-understood. A possible target of action may be glucocorticoid receptor-expressing astrocytes, which are the primary regulators of interstitial ion homeostasis in brain. In this study, we demonstrate that two glucocorticoids, methylprednisolone and dexamethasone, potentiate astrocytic signaling, via long-range calcium waves. Glucocorticoid treatment increased both resting cytosolic calcium (Ca2+i) level and the extent and amplitude of Ca2+ wave propagation two-fold, compared to matched controls. RU-486, a potent steroid receptor antagonist, inhibited the effects of methylprednisolone. The glucocorticoid-associated potentiation of Ca2+ signaling may result from upregulation of the cellular ability to mobilize Ca2+ and release ATP, because both agonist-induced Ca2+i increments (via ATP and bradykinin) and ATP release were proportionally enhanced by glucocorticoids. In contrast, neither gap junction expression (as manifested connexin 43 immunoreactivity) nor functional coupling was significantly affected by methylprednisolone. Confocal microscopy revealed both the expression of glucocorticoid receptors and nuclear translocation of these receptors when exposed to methylprednisolone. We postulate that the edemolytic effects of glucocorticoids may result from enhanced astrocytic calcium signaling.

Connexins regulate calcium signaling by controlling ATP release.

Forced expression of gap junction proteins, connexins, enables gap junction-deficient cell lines to propagate intercellular calcium waves. Here, we show that ATP secretion from the poorly coupled cell lines, C6 glioma, HeLa, and U373 glioblastoma, is potentiated 5- to 15-fold by connexin expression. ATP release required purinergic receptor-activated intracellular Ca2+ mobilization and was inhibited by Cl- channel blockers. Calcium wave propagation also was reduced by purinergic receptor antagonists and by Cl- channel blockers but insensitive to gap junction inhibitors. These observations suggest that cell-to-cell signaling associated with connexin expression results from enhanced ATP release and not, as previously believed, from an increase in intercellular coupling.

Astrocytic gap junctions remain open during ischemic conditions.

Gap junctions are highly conductive channels that allow the direct transfer of intracellular messengers such as Ca2+ and inositol triphosphate (IP3) between interconnected cells. In brain, astrocytes are coupled extensively by gap junctions. We found here that gap junctions among astrocytes in acutely prepared brain slices as well as in culture remained open during ischemic conditions. Uncoupling first occurred after the terminal loss of plasma membrane integrity. Gap junctions therefore may link ischemic astrocytes in an evolving infarct with the surroundings. The free exchange of intracellular messengers between dying and potentially viable astrocytes might contribute to secondary expansion of ischemic lesions.