Connexin Hemichannel Composition Determines the FGF-1–induced Membrane Permeability and Free [Ca²⁺]_iResponses

Abstract

Cell surface hemichannels (HCs) composed of different connexin (Cx) types are present in diverse cells and their possible role on FGF-1–induced cellular responses remains unknown. Here, we show that FGF-1 transiently (4–14 h, maximal at 7 h) increases the membrane permeability through HCs in HeLa cells expressing Cx43 or Cx45 under physiological extracellular Ca2+/Mg2+ concentrations. The effect does not occur in HeLa cells expressing HCs constituted of Cx26 or Cx43 with its C-terminus truncated at aa 257, or in parental nontransfected HeLa cells. The increase in membrane permeability is associated with a rise in HC levels at the cell surface and a proportional increase in HC unitary events. The response requires an early intracellular free Ca2+ concentration increase, activation of a p38 MAP kinase-dependent pathway, and a regulatory site of Cx subunit C-terminus. The FGF-1–induced rise in membrane permeability is also associated with a late increase in intracellular free Ca2+ concentration, suggesting that responsive HCs allow Ca2+ influx. The cell density of Cx26 and Cx43 HeLa transfectants cultured in serum-free medium was differentially affected by FGF-1. Thus, the FGF-1–induced cell permeabilization and derived consequences depend on the Cx composition of HCs.

Elevated Intracellular Ca2+ Signals by Oxidative Stress Activate Connexin 43 Hemichannels in Osteocytes

“…[Ca 2+ ]i rise induced by growth factors or 4-Br-A23187, a slow Ca 2+ ionophore, has been shown to induce the increase of Cx43 cell surface levels (21). This result is consistent with our observation that increased [Ca 2+ ]i promotes cell surface expression of Cx43 and the amount of Cx43 increased on the plasma membrane is well correlated with the increased availability of Cx43 in forming functional hemichannels.…”

FGF-1 induces ATP release from spinal astrocytes in culture and opens pannexin and connexin hemichannels

“…FGF-1 had much less effect on dye coupling in the presence of apyrase, implicating ATP as a major autocrine/ paracrine mediator of the effect. In HeLa cells, decrease in coupling occurs without significant change in total Cx43 levels (38), and changes in phosphorylation state of Cx43 could decrease open probability of GJ channels as previously demonstrated for other growth factors (41). Decrease in the amount of Cx43 in GJs would not increase availability of Cx43 HCs, because junctional degradation involves proteolysis of entire cell-cell channels and not separation of HCs making them available for reuse (42).…”

“…However, the transient increase in [Ca 2+ ] i due to influx through P2X 7 receptors or release from intracellular stores activated via FGF-1 receptors can activate intracellular signaling proteins, such as kinases (36). In HeLa Cx43 transfectants, this early [Ca 2+ ] i signaling appears to be essential for the increase in membrane permeability and surface expression of Cx43 HCs observed after 7 h of FGF-1 treatment, because increase in both is abolished by intracellular Ca 2+ chelation with BAPTA (38). However, in spinal astrocytes, increased permeability was accompanied by a minor reduction in surface Cx43 (Fig.…”

Role of Connexin Hemichannels in Neurodegeneration

“…In cultured cells under resting conditions hemichannels remain preferentially closed, but they can be activated by diverse physiological and pathological conditions, offering a diffusional transmembrane route between the intra and extracellular milieu. [Modified from Orellana et al 2011a] A main role in cellular proliferation and tissue remodelling has been attributed to hemichannels (Burra and Jiang 2009;Schalper et al 2008), while in the CNS they have been proposed to mediate ischemic tolerance Schock et al 2008) and establish adhesive interactions (Cotrina et al 2008). To date, most studies suggest that under normal brain conditions hemichannels release physiological molecules relevant for intercellular signalling, including propagation of intercellular Ca 2+ waves (Orellana et al 2011a).…”