Roles and regulation of lens epithelial cell connexins

Abstract

The avascular lens of the eye is covered anteriorly by an epithelium containing nucleated, metabolically active cells. This epithelium contains the first lens cells to encounter noxious external stimuli and cells that can develop compensatory or protective responses. Lens epithelial cells express the gap junction proteins, connexin43 (Cx43) and connexin50 (Cx50). Cx43 and Cx50 form gap junction channels and hemichannels with different properties. Although they may form heteromeric hemichannels, Cx43 and Cx50 probably do not form heterotypic channels in the lens. Cx50 channels make their greatest contribution to intercellular communication during the early postnatal period; subsequently, Cx43 becomes the predominant connexin supporting intercellular communication. Although epithelial Cx43 appears dispensable for lens development, Cx50 is critical for epithelial cell proliferation and differentiation. Cx43 and Cx50 hemichannels and gap junction channels are regulated by multiple different agents. Lens epithelial cell connexins contribute to both normal lens physiology and pathology.

Keywords: Cataract; Connexin43; Connexin50; Lens.

Figure 1

Immunofluorescence of Cx43 and Cx50 in the epithelium. Confocal images showing the distribution of Cx50 (green) and Cx43 (red) in a flat mount of the epithelium removed from the lens of a 1.9 month old C3H mouse. These images illustrate the variations in relative proportions of Cx43 and Cx50 and their co-localization. In some areas, cells have an increased proportion of Cx43 whereas in other areas, cells show an increased proportion of Cx50 punctate staining. While some of the cells show a high degree of co-localization between the two connexins, others show a more uniform punctate staining with some co-localization between Cx43 and Cx50. Bar, xx μm

Figure 2

Photomicrographs show the distribution of immunoreactive Cx50 in HLE-Cx50 (A) and HLE-Cx50P88S (B). Arrows indicate staining at appositional membranes consistent with the expected distribution of gap junctions; this staining is prominent in cells expressing Cx50, but not in those expressing Cx50P88S. Arrowheads indicate cytoplasmic accumulations seen only in cells expressing Cx50P88S. C-E. Immunoblots show levels of unmodified and glycosylated CADM1 (C), NQO1 (D) and MURF2 (E) in total homogenates of untransfected HLE cells or cells transfected with Cx50 or Cx50P88S. Levels of CADM1 were decreased in HLE-Cx50P88S compared to untransfected and HLE-Cx50 cells and levels of NQO1 were increased in HLE-Cx50 compared to untransfected and HLE-Cx50P88S cells. Levels of MURF2 were similar between HLE-Cx50 and HLE-Cx50P88S.