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A novel role for FGF and extracellular signal-regulated kinase in gap junction-mediated intercellular communication in the lens.

Le AC, Musil LS - J. Cell Biol. (2001)

Bottom Line: Insulin and insulin-like growth factor 1, as potent as FGF in inducing lens cell differentiation, had no effect on gap junctions.These findings support a model in which regional differences in FGF signaling through the ERK pathway lead to the asymmetry in gap junctional coupling required for proper lens function.Our results also identify upregulation of intercellular communication as a new function for sustained ERK activation and change the current paradigm that ERKs only negatively regulate gap junction channel activity.

View Article: PubMed Central - PubMed

Affiliation: Molecular Medicine Division, Oregon Health Sciences University, 3181 SW Sam Jackson Park Road, Portland, OR 97201, USA.

ABSTRACT
Gap junction-mediated intercellular coupling is higher in the equatorial region of the lens than at either pole, a property believed to be essential for lens transparency. We show that fibroblast growth factor (FGF) upregulates gap junctional intercellular dye transfer in primary cultures of embryonic chick lens cells without detectably increasing either gap junction protein (connexin) synthesis or assembly. Insulin and insulin-like growth factor 1, as potent as FGF in inducing lens cell differentiation, had no effect on gap junctions. FGF induced sustained activation of extracellular signal-regulated kinase (ERK) in lens cells, an event necessary and sufficient to increase gap junctional coupling. We also identify vitreous humor as an in vivo source of an FGF-like intercellular communication-promoting activity and show that FGF-induced ERK activation in the intact lens is higher in the equatorial region than in polar and core fibers. These findings support a model in which regional differences in FGF signaling through the ERK pathway lead to the asymmetry in gap junctional coupling required for proper lens function. Our results also identify upregulation of intercellular communication as a new function for sustained ERK activation and change the current paradigm that ERKs only negatively regulate gap junction channel activity.

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Model of the role of FGF in establishing regional differences in gap junction–mediated intercellular communication in the lens. (A) The concentration of FGF is known to be higher in the vitreous than in the aqueous humor. The level of FGF in the aqueous humor is too low to stimulate either gap junctional communication or fiber differentiation in the central epithelium. (B) Both the equatorial region and polar/core fiber cells have access to the high levels of FGF that diffuse out of the vitreous body, but only the former cell population can efficiently respond to this FGF by sustained activation of ERKs. (C) Sustained ERK activation leads to an increase in gap junctional intercellular communication (GJIC) in equatorial region cells. The decreased FGF signaling in polar and core fiber cells results in a reduction in intercellular coupling, thereby producing the observed equator-to-pole gradient of lenticular communication. See Discussion for details.
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fig11: Model of the role of FGF in establishing regional differences in gap junction–mediated intercellular communication in the lens. (A) The concentration of FGF is known to be higher in the vitreous than in the aqueous humor. The level of FGF in the aqueous humor is too low to stimulate either gap junctional communication or fiber differentiation in the central epithelium. (B) Both the equatorial region and polar/core fiber cells have access to the high levels of FGF that diffuse out of the vitreous body, but only the former cell population can efficiently respond to this FGF by sustained activation of ERKs. (C) Sustained ERK activation leads to an increase in gap junctional intercellular communication (GJIC) in equatorial region cells. The decreased FGF signaling in polar and core fiber cells results in a reduction in intercellular coupling, thereby producing the observed equator-to-pole gradient of lenticular communication. See Discussion for details.

Mentions: In a previous study, we reported that purified recombinant FGF (either FGF-2 or FGF-1 plus heparin) at ≥10 ng/ml stimulates the expression of fiber differentiation markers in primary cultures of E10 chick lens epithelial cells (Le and Musil, 2001). In the current investigation, we found that these same concentrations of FGF also increased gap junction–mediated intercellular dye transfer in the same system. Upregulation of intercellular coupling required sustained activation of ERK, was not accompanied by a detectable increase in either connexin synthesis or gap junction assembly, and was reversed upon removal of FGF. We also found that vitreous humor contains a factor with heparin-binding properties, ERK activation kinetics, and communication- and differentiation-promoting activity indistinguishable from purified recombinant FGF. Lastly, we have demonstrated that FGF-induced ERK activation in the intact lens is higher in the equatorial region than in polar and core fibers (Fig. 10). Taken together, these results support (although cannot prove) a model in which regional differences in FGF signaling through the ERK pathway lead to the observed asymmetry of gap junctional intercellular coupling in the lens (Fig. 11) . We propose that the low level of FGF in the aqueous humor (Tripathi et al., 1992; Schulz et al., 1993) is inadequate to support either epithelial-to-fiber differentiation or enhanced intercellular coupling in the central epithelium. In contrast, cells in the equatorial region are in close physical proximity to the vitreous humor and respond to the high levels of FGF that diffuse out of the vitreous body by sustained ERK activation that upregulates gap junctional coupling. The posterior aspect of polar and core fiber cells also faces the vitreous body. Unlike equatorial region cells, these fibers do not, however, efficiently activate ERKs in response to FGF (Fig. 10). During the ongoing process of fiber formation, cells once in the equatorial region gradually become displaced towards the lens core as they are buried by newer generations of fiber cells. If exposure of cells to FGF in the equatorial region permanently changed their gap junctional phenotype, then intercellular coupling would be expected to remain high throughout the lifetime of the fiber regardless of its location in the lens. However, our results in cultured lens cells demonstrate that FGF-induced upregulation of gap junction function is reversed within 2 d of FGF withdrawal. We propose that the reduction in FGF signaling in polar fiber cells in vivo leads to a similar drop in intercellular coupling, thereby producing the observed equator-to-pole gradient of lenticular communication.


A novel role for FGF and extracellular signal-regulated kinase in gap junction-mediated intercellular communication in the lens.

Le AC, Musil LS - J. Cell Biol. (2001)

Model of the role of FGF in establishing regional differences in gap junction–mediated intercellular communication in the lens. (A) The concentration of FGF is known to be higher in the vitreous than in the aqueous humor. The level of FGF in the aqueous humor is too low to stimulate either gap junctional communication or fiber differentiation in the central epithelium. (B) Both the equatorial region and polar/core fiber cells have access to the high levels of FGF that diffuse out of the vitreous body, but only the former cell population can efficiently respond to this FGF by sustained activation of ERKs. (C) Sustained ERK activation leads to an increase in gap junctional intercellular communication (GJIC) in equatorial region cells. The decreased FGF signaling in polar and core fiber cells results in a reduction in intercellular coupling, thereby producing the observed equator-to-pole gradient of lenticular communication. See Discussion for details.
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Related In: Results  -  Collection

Show All Figures
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fig11: Model of the role of FGF in establishing regional differences in gap junction–mediated intercellular communication in the lens. (A) The concentration of FGF is known to be higher in the vitreous than in the aqueous humor. The level of FGF in the aqueous humor is too low to stimulate either gap junctional communication or fiber differentiation in the central epithelium. (B) Both the equatorial region and polar/core fiber cells have access to the high levels of FGF that diffuse out of the vitreous body, but only the former cell population can efficiently respond to this FGF by sustained activation of ERKs. (C) Sustained ERK activation leads to an increase in gap junctional intercellular communication (GJIC) in equatorial region cells. The decreased FGF signaling in polar and core fiber cells results in a reduction in intercellular coupling, thereby producing the observed equator-to-pole gradient of lenticular communication. See Discussion for details.
Mentions: In a previous study, we reported that purified recombinant FGF (either FGF-2 or FGF-1 plus heparin) at ≥10 ng/ml stimulates the expression of fiber differentiation markers in primary cultures of E10 chick lens epithelial cells (Le and Musil, 2001). In the current investigation, we found that these same concentrations of FGF also increased gap junction–mediated intercellular dye transfer in the same system. Upregulation of intercellular coupling required sustained activation of ERK, was not accompanied by a detectable increase in either connexin synthesis or gap junction assembly, and was reversed upon removal of FGF. We also found that vitreous humor contains a factor with heparin-binding properties, ERK activation kinetics, and communication- and differentiation-promoting activity indistinguishable from purified recombinant FGF. Lastly, we have demonstrated that FGF-induced ERK activation in the intact lens is higher in the equatorial region than in polar and core fibers (Fig. 10). Taken together, these results support (although cannot prove) a model in which regional differences in FGF signaling through the ERK pathway lead to the observed asymmetry of gap junctional intercellular coupling in the lens (Fig. 11) . We propose that the low level of FGF in the aqueous humor (Tripathi et al., 1992; Schulz et al., 1993) is inadequate to support either epithelial-to-fiber differentiation or enhanced intercellular coupling in the central epithelium. In contrast, cells in the equatorial region are in close physical proximity to the vitreous humor and respond to the high levels of FGF that diffuse out of the vitreous body by sustained ERK activation that upregulates gap junctional coupling. The posterior aspect of polar and core fiber cells also faces the vitreous body. Unlike equatorial region cells, these fibers do not, however, efficiently activate ERKs in response to FGF (Fig. 10). During the ongoing process of fiber formation, cells once in the equatorial region gradually become displaced towards the lens core as they are buried by newer generations of fiber cells. If exposure of cells to FGF in the equatorial region permanently changed their gap junctional phenotype, then intercellular coupling would be expected to remain high throughout the lifetime of the fiber regardless of its location in the lens. However, our results in cultured lens cells demonstrate that FGF-induced upregulation of gap junction function is reversed within 2 d of FGF withdrawal. We propose that the reduction in FGF signaling in polar fiber cells in vivo leads to a similar drop in intercellular coupling, thereby producing the observed equator-to-pole gradient of lenticular communication.

Bottom Line: Insulin and insulin-like growth factor 1, as potent as FGF in inducing lens cell differentiation, had no effect on gap junctions.These findings support a model in which regional differences in FGF signaling through the ERK pathway lead to the asymmetry in gap junctional coupling required for proper lens function.Our results also identify upregulation of intercellular communication as a new function for sustained ERK activation and change the current paradigm that ERKs only negatively regulate gap junction channel activity.

View Article: PubMed Central - PubMed

Affiliation: Molecular Medicine Division, Oregon Health Sciences University, 3181 SW Sam Jackson Park Road, Portland, OR 97201, USA.

ABSTRACT
Gap junction-mediated intercellular coupling is higher in the equatorial region of the lens than at either pole, a property believed to be essential for lens transparency. We show that fibroblast growth factor (FGF) upregulates gap junctional intercellular dye transfer in primary cultures of embryonic chick lens cells without detectably increasing either gap junction protein (connexin) synthesis or assembly. Insulin and insulin-like growth factor 1, as potent as FGF in inducing lens cell differentiation, had no effect on gap junctions. FGF induced sustained activation of extracellular signal-regulated kinase (ERK) in lens cells, an event necessary and sufficient to increase gap junctional coupling. We also identify vitreous humor as an in vivo source of an FGF-like intercellular communication-promoting activity and show that FGF-induced ERK activation in the intact lens is higher in the equatorial region than in polar and core fibers. These findings support a model in which regional differences in FGF signaling through the ERK pathway lead to the asymmetry in gap junctional coupling required for proper lens function. Our results also identify upregulation of intercellular communication as a new function for sustained ERK activation and change the current paradigm that ERKs only negatively regulate gap junction channel activity.

Show MeSH