The Development-associated Cleavage of Lens Connexin 45.6 by Caspase-3-like Protease Is Regulated by Casein Kinase II-mediated Phosphorylation

Abstract

Gap junctions are important in maintaining lens transparency and metabolic homeostasis. In this paper, we report that the gap junction-forming protein, connexin (Cx) 45.6, was specifically truncated during lens development and that the majority of the truncated fragments were located in the differentiated lens fibers. When isolated lens membranes were treated by caspase-3, the truncated fragments of Cx45.6 were reproduced, and this truncation occurred at the COOH terminus of Cx45.6. Moreover, when primary lens cells were treated with apoptosis-inducing reagents, Cx45.6 was cleaved similarly as the in vitro treatment by caspase-3, and this cleavage was blocked by a caspase-3 inhibitor. These results suggest that caspase-3 is responsible for the development-associated cleavage of Cx45.6. The cleavage site of Cx45.6 was identified between amino acid residues Glu367 and Gly368. We have shown previously that Ser363 is an in vivophosphorylated site by casein kinase II, and this specific phosphorylation leads to a rapid turnover of Cx45.6. Interestingly, we found here that when Ser363 was phosphorylated by casein kinase II, the cleavage of Cx45.6 catalyzed by caspase-3 was inhibited. This study, for the first time, demonstrates that a connexin can be a direct target of an apoptotic protease and that cleavage by caspase-3-like protease leads to the development-associated truncation of a lens connexin. Finally, caspase-3-mediated cleavage can be regulated by casein kinase II-mediated phosphorylation, suggesting that Cx45.6 turnover and specific cleavage by caspase-3-like protease is alternatively modulated. Gap junctions are important in maintaining lens transparency and metabolic homeostasis. In this paper, we report that the gap junction-forming protein, connexin (Cx) 45.6, was specifically truncated during lens development and that the majority of the truncated fragments were located in the differentiated lens fibers. When isolated lens membranes were treated by caspase-3, the truncated fragments of Cx45.6 were reproduced, and this truncation occurred at the COOH terminus of Cx45.6. Moreover, when primary lens cells were treated with apoptosis-inducing reagents, Cx45.6 was cleaved similarly as the in vitro treatment by caspase-3, and this cleavage was blocked by a caspase-3 inhibitor. These results suggest that caspase-3 is responsible for the development-associated cleavage of Cx45.6. The cleavage site of Cx45.6 was identified between amino acid residues Glu367 and Gly368. We have shown previously that Ser363 is an in vivophosphorylated site by casein kinase II, and this specific phosphorylation leads to a rapid turnover of Cx45.6. Interestingly, we found here that when Ser363 was phosphorylated by casein kinase II, the cleavage of Cx45.6 catalyzed by caspase-3 was inhibited. This study, for the first time, demonstrates that a connexin can be a direct target of an apoptotic protease and that cleavage by caspase-3-like protease leads to the development-associated truncation of a lens connexin. Finally, caspase-3-mediated cleavage can be regulated by casein kinase II-mediated phosphorylation, suggesting that Cx45.6 turnover and specific cleavage by caspase-3-like protease is alternatively modulated. connexin casein kinase II chicken embryonic fibroblast Dulbecco's modified Eagle's medium glutathione S-transferase polyacrylamide gel electrophoresis polymerase chain reaction benzyloxycarbonyl fluoromethyl ketone 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid embryonic day postnatal day The vertebrate lens is one of the most important model systems used in the study of the function and regulation of gap junctions. The lens is an avascular organ composed of an anterior epithelial cell layer and highly differentiated fibers ranging from the outer cortex toward the central core region. The terminal differentiation and aging of lens fibers are marked by dramatic morphological changes. As new cells arise on the outside of the lens, older cells are pushed inward where nuclei and organelles are lost (1Rafferty N. Esson E. J. Ultrastruct. Res. 1974; 46: 239-253Crossref PubMed Scopus (39) Google Scholar). Because these cells are never lost from the lens, the most central cells are as old as the organism itself. The survival of lens cells relies on the intercellular communications between these cells and the cells at the lens surface through a large network of gap junctions that facilitate the exchange of ions and metabolites throughout the organ (2Mathias R.T. Rae J.L. Sperelakis N. Cole W.C. Cell Interactions and Gap Junctions. CRC Press, Inc., Boca Raton, FL1989: 29-50Google Scholar, 3Cooper K. Mathias R.T. Rae J.L. Peracchia C. Biophysics of Gap Junction Channels. CRC Press, Inc., Boca Raton, FL1991: 57-74Google Scholar). Gap junctions are intercellular channels between two adjacent cells, which allow passage of small molecules (≤1000 Da) such as small metabolites, ions, and second messengers. The structural components of gap junctions are the members of a protein family called connexins, which consist of four conserved transmembrane domains and two extracellular loops, whereas their cytoplasmic regions are unique. The COOH terminus, the most variable region among connexins, contains several kinase and protease consensus sequences. Connexins are highly dynamic proteins that undergo rapid turnover both in cell lines (4Musil L.S. Cunningham B.A. Edelman G.M. Goodenough D.A. J. Cell Biol. 1990; 111: 2077-2088Crossref PubMed Scopus (594) Google Scholar) and in animal organs (5Dermietzel R. Traub O. Hwang T.K. Beyer E. Bennett M.V.L. Spray D.C. Willecke K. Proc. Natl. Acad. Sci. U. S. A. 1989; 86: 10148-10152Crossref PubMed Scopus (457) Google Scholar, 6Beardslee M.A. Laing J.G. Beyer E.C. Saffitz J.E. 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Phosphorylation has been demonstrated to play an important role in regulation of gap junction stability and turnover (13Hertlein B. Butterweck A. Haubrich S. Willecke K. Traub O. J. Membr. Biol. 1998; 162: 247-257Crossref PubMed Scopus (57) Google Scholar, 14Berthoud V.M. Bassnett S. Beyer E.C. Exp. Eye Res. 1999; 68: 475-484Crossref PubMed Scopus (34) Google Scholar, 15Hossain M.Z. Ao P. Boynton A.L. J. Cell. Physiol. 1998; 174: 66-77Crossref PubMed Scopus (73) Google Scholar, 16Elvira M. Diez J.A. Wang K.K.W. Villalobo A. J. Biol. Chem. 1993; 268: 14294-14300Abstract Full Text PDF PubMed Google Scholar). We have recently shown that an in vivo phosphorylated lens connexin undergoes a faster turnover than its unphosphorylated counterpart (17Yin X. Jedrzejewski P.T. Jiang J.X. J. Biol. Chem. 2000; 275: 6850-6856Abstract Full Text Full Text PDF PubMed Scopus (48) Google Scholar). Differentiating lens fibers share a number of morphological and biochemical characteristics with cells undergoing apoptosis such as nucleus degeneration, loss of organelles, and activation of members of a cysteine protease family named caspase (18Wride M.A. Parker E. Sanders E.J. Dev. Biol. 1999; 213: 142-156Crossref PubMed Scopus (76) Google Scholar, 19Wride M.A. Apoptosis. 2000; 5: 203-209Crossref PubMed Scopus (79) Google Scholar, 20Dahm R. Ophthalmic Res. 1999; 31: 163-183Crossref PubMed Scopus (95) Google Scholar, 21Lee A. Morrow J.S. Fowler V.M. J. Biol. Chem. 2001; 276: 20735-20742Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar). However, unlike apoptotic cells, which are rapidly digested, the organelle-free lens fibers retain their basic cell integrity and metabolism throughout the lifetime of the organ. Poly(ADP-ribose) polymerase, an enzyme involved in DNA repair and maintenance of genomic integrity, is cleaved by caspases in the developing lens (18Wride M.A. Parker E. Sanders E.J. Dev. Biol. 1999; 213: 142-156Crossref PubMed Scopus (76) Google Scholar). Caspase-3 is predominantly activated during staurosporine-induced apoptosis in lens cells (22Andersson M. Sjöstrand J. Petersen A. Honarvar A.K.S. Karlsson J. Invest. Ophthalmol. Vis. Sci. 2000; 41: 2623-2632PubMed Google Scholar). The function of caspases has been reported to be responsible for nucleus degradation during the later stage of lens fiber differentiation (18Wride M.A. Parker E. Sanders E.J. Dev. Biol. 1999; 213: 142-156Crossref PubMed Scopus (76) Google Scholar). Although activation of caspases has been demonstrated during lens fiber differentiation, only a couple of caspase substrates have been identified, such as cytoskeletal proteins named α- and β-spectrins (21Lee A. Morrow J.S. Fowler V.M. J. Biol. Chem. 2001; 276: 20735-20742Abstract Full Text Full Text PDF PubMed Scopus (59) Google Scholar). Connexin (Cx)1 45.6 is one of the connexins expressed in chick lens fibers (23Jiang J.X. White T.W. Goodenough D.A. Paul D.L. Mol. Biol. Cell. 1994; 5: 363-373Crossref PubMed Scopus (81) Google Scholar, 24Jiang J.X. White T.W. Goodenough D.A. Dev. Biol. 1995; 168: 649-661Crossref PubMed Scopus (55) Google Scholar). Cx45.6, which is different from the other two chick lens connexins, Cx43 and Cx56, seems to be involved in lens development and differentiation. Mutations in Cx50, the mammalian ortholog of Cx45.6, have been identified in human cataract families (25Shiels A. Mackay D. Ionides A. Berry V. Moore A. Bhattacharya S. Am. J. Hum. Genet. 1998; 62: 526-532Abstract Full Text Full Text PDF PubMed Scopus (317) Google Scholar, 26Berry V. Mackay D. Khaliq S. Francis P.J. Hameed A. Anwar K. Mehdi S.Q. Newbold R.J. Ionides A. Shiels A. Moore T. Bhattacharya S.S. Hum. Genet. 1999; 105: 168-170Crossref PubMed Scopus (114) Google Scholar). Mice deficient in Cx50 not only develop cataracts, but with a of lens T.W. Goodenough D.A. Paul D.L. J. Cell Biol. 1998; PubMed Scopus Google Scholar). of chick lens in cell differentiation in which the of the lens epithelial cells called of differentiated lens fibers J. S.S. L.M. Dev. Biol. PubMed Scopus Google Scholar, Dev. Biol. PubMed Scopus (95) Google Scholar). We have recently shown that of Cx45.6 in lens primary lens cell differentiation and of fibers S. X. Jiang J.X. Mol. Biol. Cell. 1999; Scholar). Cx45.6 is to Cx45.6 is a (23Jiang J.X. White T.W. Goodenough D.A. Paul D.L. Mol. Biol. Cell. 1994; 5: 363-373Crossref PubMed Scopus (81) Google Scholar, J.X. Goodenough D.A. J. 1998; PubMed Scopus Google Scholar). We have recently shown that Cx45.6 is phosphorylated in vivo by at Ser363 located at the and this phosphorylation the turnover of Cx45.6 (17Yin X. Jedrzejewski P.T. Jiang J.X. J. Biol. Chem. 2000; 275: 6850-6856Abstract Full Text Full Text PDF PubMed Scopus (48) Google Scholar). report that Cx50, the ortholog of Cx45.6, is truncated at the COOH terminus in the of the lens by a protease called J.S. S. C. P. J. J. Cell Biol. 1997; Google Scholar). Although such as phosphorylation and specific truncation of lens connexins, have been the and of these in lens differentiation and development are In this we identified a specific cleavage of Cx45.6 with lens that the protease involved in this cleavage is caspase-3, an apoptotic Moreover, the cleavage site by caspase-3 in Cx45.6 was identified to be to the phosphorylated site by This specific phosphorylation Cx45.6 from cleavage by This is the first report that the apoptotic caspase-3, is responsible for cleavage of a and this specific cleavage is regulated by a specific chicken were from and and caspase-3 were from Caspase-3 was from and were from were from were from was from was from was from proteins with of Cx45.6 were as (23Jiang J.X. White T.W. Goodenough D.A. Paul D.L. Mol. Biol. Cell. 1994; 5: 363-373Crossref PubMed Scopus (81) Google Scholar, Jiang J.X. Proc. Natl. Acad. Sci. U. S. A. 1999; PubMed Scopus Google Scholar). The proteins the with COOH terminus of Cx45.6, and its site and with COOH terminus of Cx45.6, and with region of Cx45.6, DNA fragments of Cx45.6 or its were by with specific The was to and Glu367 was to in one of the two for DNA was the The proteins were expressed by and isolated with the were at the DNA protein of Cx45.6 was used to in and The were by passage through two and as (23Jiang J.X. White T.W. Goodenough D.A. Paul D.L. Mol. Biol. Cell. 1994; 5: 363-373Crossref PubMed Scopus (81) Google Scholar, Jiang J.X. Proc. Natl. Acad. Sci. U. S. A. 1999; PubMed Scopus Google Scholar). The proteins and were in of a on were to membranes to the of White T.W. R. Goodenough D.A. Paul D.L. Mol. Biol. Cell. PubMed Scopus Google Scholar). were with the or Cx45.6 The primary were either with or and by to the The membranes were to and by and were on previously J.X. R. C. Connexin and Inc., Scholar). the Cx45.6 was by and was the as (17Yin X. Jedrzejewski P.T. Jiang J.X. J. Biol. Chem. 2000; 275: 6850-6856Abstract Full Text Full Text PDF PubMed Scopus (48) Google Scholar, J.X. Goodenough D.A. Invest. Ophthalmol. Vis. Sci. 1998; Google Scholar). the DNA as a of Cx45.6 were with the to the The for site and were was by Glu367 to were at the DNA The the and the Cx45.6 were as we have previously J.X. R. C. Connexin and Inc., Scholar, J.X. Goodenough D.A. Invest. Ophthalmol. Vis. Sci. 1998; Google Scholar). The cell membranes were isolated and as J.X. Goodenough D.A. Proc. Natl. Acad. Sci. U. S. A. 1996; PubMed Scopus Google Scholar). from embryonic day or cells with were in and and membranes were at for The membranes from cells and or the proteins were to in vitro cleavage by membranes or of proteins were at in of caspase-3 reaction and with and of caspase-3 at were by the of electrophoresis and and for lens were to the previously (17Yin X. Jedrzejewski P.T. Jiang J.X. J. Biol. Chem. 2000; 275: 6850-6856Abstract Full Text Full Text PDF PubMed Scopus (48) Google Scholar, L.S. Beyer E.C. Goodenough D.A. J. Membr. Biol. 1990; PubMed Scopus Google Scholar). chick from embryonic day were and the cells that from the were at cells in medium cell primary were treated either with in medium or with for at When caspase-3 was were with this for to or cells were at cells in with and The day cell cells were with either the or the Cx45.6 When cells were either for or for treatment with apoptosis-inducing cells were treated in the of for at The isolated embryonic lens membranes were in of reaction and of of at for The reaction was by the of by at for of caspase-3 reaction were to the in vitro phosphorylated lens and the reaction was two In of caspase-3 or caspase-3 and was and at for The reaction was by the for in the of to the with Cx45.6 as previously J.X. Goodenough D.A. J. 1998; PubMed Scopus Google Scholar, J.X. Goodenough D.A. Proc. Natl. Acad. Sci. U. S. A. 1996; PubMed Scopus Google Scholar). for the embryonic day were with medium deficient in to the with medium with and of for The medium was and were in medium with and for lens membranes were isolated and treated at the as the the of in reaction by the treatment by caspase-3, and as at embryonic and and postnatal and were with Cx45.6 that the region of Cx45.6. The Cx45.6 was phosphorylated in vivo and as at the of and as demonstrated previously (23Jiang J.X. White T.W. Goodenough D.A. Paul D.L. Mol. Biol. Cell. 1994; 5: 363-373Crossref PubMed Scopus (81) Google Scholar). a Cx45.6 was in the lens at an at and its during the development of the lens as shown at and postnatal In a with the at which is a than at embryonic The of this at the later and a as the protein be not the that these have occurred during we from embryonic and postnatal at for and not of these fragments not one of the fragments was the phosphorylated of membranes from lens at were treated with The of these two fragments were not not suggesting that of the fragments is the phosphorylated of the The of the postnatal chick lens epithelial cells and the fibers of the outer cortex were from the central core of the lens composed of lens fibers. The lens both the Cx45.6 and the cleaved fragments However, the cleaved fragments predominantly in the fibers of the central core and to a in the outer cortex in was composed of Cx45.6 Because cells are in the lens and new cells arise on the outside of the lens, most cells in the central core regions are differentiated lens fibers (1Rafferty N. Esson E. J. Ultrastruct. Res. 1974; 46: 239-253Crossref PubMed Scopus (39) Google Scholar). with the in that truncated Cx45.6 fragments were with lens the results that the specific cleavage of Cx45.6 in differentiated lens fibers be to the of lens cell differentiation and of fibers. the involved in the cleavage of Cx45.6 during lens we first with cysteine protease This protease at the COOH terminus of Cx50, the counterpart of Cx45.6 in the central region of the lens J.S. S. C. P. J. J. Cell Biol. 1997; Google Scholar). of the membranes from embryonic day chick lens a Cx45.6 by to a cleavage from a Cx45.6 a different from the in the chick lens This that unlike Cx50 is involved in the in vivo truncation of Cx45.6 in chick protease involved in apoptosis has been reported to be activated when lens epithelial cells fibers in vivo J. Cell Biol. 1998; PubMed Scopus Google and caspase-3 is in the outer fibers (18Wride M.A. Parker E. Sanders E.J. Dev. Biol. 1999; 213: 142-156Crossref PubMed Scopus (76) Google Scholar). Caspase-3 is activated between embryonic and in chick lens (18Wride M.A. Parker E. Sanders E.J. Dev. Biol. 1999; 213: 142-156Crossref PubMed Scopus (76) Google which is at the as when the cleavage of Cx45.6 was caspase-3 is the protease responsible for the embryonic lens membranes were with As shown in the Cx45.6 was cleaved fragments which to the as the truncated Cx45.6 of the lens from the chick with caspase-3, the majority of the Cx45.6 proteins was cleaved These results that Cx45.6 be a direct for Cx45.6 can be cleaved by other in the lens that be activated by these proteins with of Cx45.6 were used for in vitro caspase-3 The of Cx45.6 the the of Cx45.6 and COOH terminus and and COOH terminus These with the protein and were treated with and were cleaved by caspase-3 whereas protein and were not These results suggest that Cx45.6 is a of caspase-3 and that the cleavage site is located the COOH terminus between amino and Cx45.6 can be cleaved by caspase-3 in lens cells, primary lens were with apoptosis-inducing (22Andersson M. Sjöstrand J. Petersen A. Honarvar A.K.S. Karlsson J. Invest. Ophthalmol. Vis. Sci. 2000; 41: 2623-2632PubMed Google Scholar) or Wang K.K.W. J. 1999; PubMed Scopus Google Scholar). These two are for apoptosis (22Andersson M. Sjöstrand J. Petersen A. Honarvar A.K.S. Karlsson J. Invest. Ophthalmol. Vis. Sci. 2000; 41: 2623-2632PubMed Google Scholar, Wang K.K.W. J. 1999; PubMed Scopus Google Scholar). by both fragments and The cleavage was by a caspase-3 The results that caspase-3 is to be involved in the cleavage of Cx45.6 in The amino acid was identified in the region of Cx45.6 This is to a caspase-3 consensus C. S. J.A. D. T. J. Biol. Chem. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar) with the of with at cleavage by caspase-3 between Glu367 and the of one of the cleaved fragments that of the The two residues in the consensus of caspase-3 are and cleavage either is to M. D. J. Biol. Chem. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar). is an this of the in Cx45.6 is a target for As in were with in the conserved amino acid residues to the consensus of proteins and its and were to caspase-3 cleavage in vitro was cleaved by caspase-3 two and to cleavage by These suggest that either or Glu367 seems to be in the cleavage by The COOH terminus of Cx45.6 in protein not have as with the one in the Cx45.6 and its of conserved residues to caspase-3 were expressed in cells through and the isolated cell membranes were to caspase-3 treatment to the results shown for in and cleavage of Cx45.6 by caspase-3 and the of a cleavage in Cx45.6 to the consensus for caspase-3, cleavage The of the fragments to the of is than that to the Cx45.6 results were when cells and Cx45.6 were treated by the apoptosis-inducing was cleaved whereas the Cx45.6 was In to the Cx45.6, the cleavage of was the results from proteins and Cx45.6 expressed in cells suggest that the cleavage site of Cx45.6 by caspase-3 is between Glu367 and Gly368. We have reported that an amino acid to the caspase-3 cleavage is phosphorylated in vivo by (17Yin X. Jedrzejewski P.T. Jiang J.X. J. Biol. Chem. 2000; 275: 6850-6856Abstract Full Text Full Text PDF PubMed Scopus (48) Google Scholar). Phosphorylation of substrates has been shown to be important in their cleavage by caspase-3 M. D. J. Biol. Chem. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar, J. A. J. C. Proc. Natl. Acad. Sci. U. S. A. 1999; PubMed Scopus Google Scholar). phosphorylation at Ser363 the of Cx45.6 to as a for caspase-3, Cx45.6 in the membranes of embryonic was treated in the of and by by caspase-3 The results that Cx45.6 phosphorylated by was from caspase-3 that Cx45.6 in both and truncated be embryonic were with and in medium and of and The isolated membranes with were treated in the and and and of caspase-3 and by The Cx45.6 we used here was of the as as the truncated Cx45.6 cleaved by the suggest that the specific cleavage of Cx45.6 by caspase-3 is to be regulated by In this we identified caspase-3 as the protease responsible for the cleavage of Cx45.6 during chick lens the cleavage on Cx45.6 was to the site to the amino acid phosphorylated by (17Yin X. Jedrzejewski P.T. Jiang J.X. J. Biol. Chem. 2000; 275: 6850-6856Abstract Full Text Full Text PDF PubMed Scopus (48) Google Scholar) have shown that Cx45.6 is phosphorylated in vivo by at and this phosphorylation be a for Cx45.6 degradation by we found that phosphorylation of Ser363 by Cx45.6 from cleavage by caspase-3, suggesting that the specific cleavage of Cx45.6 is regulated by on a model is for the of phosphorylation in the regulation of Cx45.6 turnover and specific cleavage by caspase-3-like In the outer cortex of the lens, when a Cx45.6 is phosphorylated by at and undergoes degradation through the proteasome pathway (17Yin X. Jedrzejewski P.T. Jiang J.X. J. Biol. Chem. 2000; 275: 6850-6856Abstract Full Text Full Text PDF PubMed Scopus (48) Google Scholar). The phosphorylation site Ser363 is located a (17Yin X. Jedrzejewski P.T. Jiang J.X. J. Biol. Chem. 2000; 275: 6850-6856Abstract Full Text Full Text PDF PubMed Scopus (48) Google Scholar, Dev. 1997; Scopus Google and this has been reported to be with a rapid protein turnover degradation M. Sci. 1996; Full Text PDF PubMed Scopus Google Scholar). phosphorylation is to protein degradation by proteasome T. Proc. Natl. Acad. Sci. U. S. A. 2000; PubMed Scopus Google Scholar). When Ser363 is not phosphorylated by Cx45.6 a for caspase-3, which at Glu367 at the COOH terminus of Cx45.6 and truncated fragments of Cx45.6. These fragments were to the central core region of the We two fragments from Cx45.6 in the of and These fragments were during lens The was during the embryonic whereas the of the and a as with the at the postnatal of two fragments was when was treated with However, of a cleavage site in blocked the of either suggesting that this site of these two Because of the of their these two fragments were on the We a than at and this be from cleavage or of suggest that caspase-3-like protease is responsible for vivo cleavage of Cx45.6. In to in a study has shown that a large of the COOH terminus of Cx50, the ortholog of Cx45.6, is by J.S. S. C. P. J. J. Cell Biol. 1997; Google Scholar). This for the in or of these two expressed in these two the of one protease be by the other during is that the cleaved of Cx50 in is by in We found of lens primary cells were Cx45.6 be cleaved a an as the cleaved by not We have shown that the truncated Cx45.6 was during lens development and was to the differentiated lens fibers. The activation of in the differentiated lens fibers and is involved in later stage cell differentiation, such as nucleus (18Wride M.A. Parker E. Sanders E.J. Dev. Biol. 1999; 213: 142-156Crossref PubMed Scopus (76) Google Scholar). the Cx45.6 is to a of gap different from the by Cx45.6. These intercellular channels facilitate the passage of important for fiber differentiation the of fibers. gap junction channels by truncated Cx50 of lens J.S. S. C. P. J. J. Cell Biol. 1997; Google Scholar). These channels are to be to with by by Le and L.S. Dev. Biol. 1998; PubMed Scopus Google Scholar) have shown that lens primary the channels are blocked with the gap junction acid has been for gap junctions by connexins X.J. S. Ruch R.J. Mol. 1996; PubMed Scopus Google the channels of truncated connexins not be similarly by this Cx45.6 is the first connexin identified as a for In to lens fibers other cell such as undergo of nuclei and organelles during cell differentiation. activation has been identified in the differentiation of and is for the loss of the nucleus M. Biol. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar). is of to connexins in cells are to regulation by The of the regulation of connexins by caspases We D. and Wang for and for We members of the Jiang for of the