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Selective transfer of endogenous metabolites through gap junctions composed of different connexins

Nature Cell Biology volume 1, pages 457–459 (1999)Cite this article

Gap junctions, the only channels that allow direct exchange of small metabolites between cells, are composed of a family of integral membrane proteins, called connexins in vertebrates. Different connexins have been implicated in a diverse array of biological processes and diseases1, indicating that gap-junction properties may vary with connexin composition. For example, channels composed of different connexins have different conductances2 and permeabilities to ions3 and fluorescent dyes4,5. However, although the permeability of gap junctions to some nucleotides6, cyclic AMP7, calcium8 and possibly inositol-1,4,5-trisphosphate8 has been documented, the comparative selectivities of different connexins for biologically significant molecules remain an enigma. Here, using new techniques, we show that the rate of permeation of metabolites through gap junctions differs according to connexin composition.

One demonstration of the biological consequences of expression of different connexin isotypes is their variable effectiveness in suppressing growth of tumour cells9. Although transfection of C6 glioma cells with connexin-43 (Cx43) suppressed their in vitro growth potential10, Cx32 did not11. Paradoxically, however, Cx32 transfectants are far better coupled to each other, as assayed by calcein-dye transfer (Table 1; for methods see ref. 12). A diffusion constant for each cell interface was obtained from the time course of calcein transfer in two C6 cell transfectants12, using quantitative modelling of dye diffusion through cell monolayers5. Independently, we determined the ratio of calcein permeabilities of Cx43 and Cx32 channels to be 1.08 from quantitative measurements of calcein diffusion between Xenopus oocyte pairs expressing defined numbers of Cx43 or Cx32 channels, calculated from the junctional and single-channel conductance of each connexin (P.A. Weber, Y.I. Chen, J. Nitsche and B.J.N., unpublished observations). Applying the relative calcein permeability of the two connexins to the diffusion constants calculated above for the transfected C6 cell lines produced an estimate of the relative number of channels in the Cx32 versus the Cx43 transfectants of 16:1. Hence, the differential effects of these two connexins on cell growth were related not to their expression level but rather to some specific property of the channels, such as selective permeabilities for endogenous signals.

Table 1 Dye and metabolite coupling of C6 cell transfectants
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Figure 1: ADP, ATP, glutamate and glutathione constitute the majority of transjunctional material derived from glucose.
Figure 2: Cx43 mediates the transfer of ADP and ATP between C6 cells better than does Cx32, but neither shows selectivity for glutamate and glutathione.

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Acknowledgements

We thank P. Weber and J. Nitsche for development of the dye-diffusion model and permeability estimates that allowed quantification of channel number in the transfectants; D. Sheedy and C. Stewart for cell sorting; I. Silver and G. Koudelka for helpful discussions; Whatman and Molecular Dynamics for technical assistance with TLC; and C. Naus for continued help and interest. This work was supported by grants from PachTech and the Wendy Will Case Cancer Research Fund (to G.S.G.), the Whittaker Foundation (to B.J.N. and J. Nitsche), and the NIH (to B.J.N., grants CA480490 and GM48773, and P.D.L., grant GM55632).

Correspondence and requests for materials should be addressed to G.S.G.

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  1. Gary S. Goldberg

    Present address: Experimental Pathology and Chemotherapy, National Cancer Center Research Institute, 5-1-1 Tsukiji Chuo-ku, Tokyo, 104

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  1. Biological Sciences, Cooke Hall, State University of New York at Buffalo, Buffalo, 14260, New York, USA

    Gary S. Goldberg & Bruce J. Nicholson

  2. Fred Hutchinson Cancer Research Center, Seattle, 98109, Washington, USA

    Paul D. Lampe

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Goldberg, G., Lampe, P. & Nicholson, B. Selective transfer of endogenous metabolites through gap junctions composed of different connexins. Nat Cell Biol 1, 457–459 (1999). https://doi.org/10.1038/15693

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