Intercellular Communication in Spinal Cord Astrocytes: Fine Tuning between Gap Junctions and P2 Nucleotide Receptors in Calcium Wave Propagation

Abstract

Electrophysiological properties of gap junction channels and mechanisms involved in the propagation of intercellular calcium waves were studied in cultured spinal cord astrocytes from sibling wild-type (WT) and connexin43 (Cx43) knock-out (KO) mice. Comparison of the strength of coupling between pairs of WT and Cx43 KO spinal cord astrocytes indicates that two-thirds of total coupling is attributable to channels formed by Cx43, with other connexins contributing the remaining one-third of junctional conductance. Although such a difference in junctional conductance was expected to result in the reduced diffusion of signaling molecules through the Cx43 KO spinal cord syncytium, intercellular calcium waves were found to propagate with the same velocity and amplitude and to the same number of cells as between WT astrocytes. Measurements of calcium wave propagation in the presence of purinoceptor blockers indicate that calcium waves in Cx43 KO spinal cord astrocytes are mediated primarily by extracellular diffusion of ATP; measurements of responses to purinoceptor agonists revealed that the functional P2Y receptor subtype is shifted in the Cx43 KO astrocytes, with a markedly potentiated response to ATP and UTP. Thus, the reduction in gap junctional communication in Cx43 KO astrocytes leads to an increase in autocrine communication, which is a consequence of a functional switch in the P2Y nucleotide receptor subtype. Intercellular communication via calcium waves therefore is sustained in Cx43 null mice by a finely tuned interaction between gap junction-dependent and independent mechanisms.

Pannexin 1 in erythrocytes: Function without a gap

“…Hemichannel function has been described also for some connexins under certain experimental conditions (18)(19)(20)(21)(22). In particular, Cx43 has been implicated as an ATP release channel (9,23,24), although calcium wave propagation was found to proceed in Cx43-deficient cells at normal rates (25,26). Similarly, erythrocytes release ATP in the absence of detectable Cx43 expression (Fig.…”

Folliculostellate cell network: A route for long-distance communication in the anterior pituitary

“…Purine nucleotides have been identified as an extracellular messenger in various cells including FS cells (31), and there is evidence that they may act in concert with gap junction signaling for wave propagation in cell preparations (17)(18)(19)(20)22). Bath application of both pyridoxal phosphate-6-azophenyl-2Ј,4Ј-disulfonic acid (PPADS) and suramin (100 M each), antagonists known to depress purinergic receptor transmission in glial cell networks (18), failed to suppress propagated [Ca 2ϩ ] i waves (Fig. 4c) (n ϭ 9 fields), whereas they abolished [Ca 2ϩ ] i responses to ATP (10 M, 10-s application, n ϭ 23 cells, data not shown).…”

“…Very little is known about the functioning of this FS cell network, in particular with regard to the dynamics of cellular͞intercellular messages. To study the behavior of this network, we measured multicellular changes in [Ca 2ϩ ] i , a messenger involved in a wide range of cell-to-cell communication mechanisms (17)(18)(19)(20)(21)(22)(23), electrophysiological properties of FS cells, and intercellular diffusion of dyes in acute pituitary slices. We show here that the FS cell network forms a functional intrapituitary circuitry in which information-Ca 2ϩ signals and small diffusible molecules-can be transferred over long distances (millimeter range) within the intact pituitary tissue.…”

Extracellular Nucleotides Differentially Regulate Interleukin-1β Signaling in Primary Human Astrocytes: Implications for Inflammatory Gene Expression

“…Astrocytes function to maintain the homeostatic environment of the CNS and also play an important role in immune regulation, acting as a source of chemokines, cytokines, and effector molecules (Ransom and Sontheimer, 1992;Norenberg, 1997). Coordination of function in astrocyte populations is believed to occur via at least two different mechanisms: an intercellular pathway mediated by gap junctions composed of connexin43 subunits and an extracellular pathway mediated by receptors of the P2 family that respond to nucleotides such as ATP, ADP, UTP, and UDP (Guthrie et al, 1999;John et al, 1999;Scemes et al, 2000). These pathways are thought to provide a mechanism whereby astrocytes are able to sense and respond to changes in the state and activity of neighboring cells and the surrounding CNS microenvironment.…”