Beyond the Channels: Adhesion Functions of Aquaporin 0 and Connexin 50 in Lens Development

Abstract

Lens, an avascular tissue involved in light transmission, generates an internal microcirculatory system to promote ion and fluid circulation, thus providing nutrients to internal lens cells and excreting the waste. This unique system makes up for the lack of vasculature and distinctively maintains lens homeostasis and lens fiber cell survival through channels of connexins and other transporters. Aquaporins (AQP) and connexins (Cx) comprise the majority of channels in the lens microcirculation system and are, thus, essential for lens development and transparency. Mutations of AQPs and Cxs result in abnormal channel function and cataract formation. Interestingly, in the last decade or so, increasing evidence has emerged suggesting that in addition to their well-established channel functions, AQP0 and Cx50 play pivotal roles through channel-independent actions in lens development and transparency. Specifically, AQP0 and Cx50 have been shown to have a unique cell adhesion function that mediates lens development and transparency. Precise regulation of cell-matrix and cell-cell adhesion is necessary for cell migration, a critical process during lens development. This review will provide recent advances in basic research of cell adhesion mediated by AQP0 and Cx50.

Charged multivesicular body protein 4b forms complexes with gap junction proteins during lens fiber cell differentiation

“…At least a sub-population of CHMP4B on LFC membranes was found to lie in close proximity to Cx46 and/or Cx50 (Figure 5), suggesting possible proteinprotein interactions. Although Cx50 has been reported to interact with MIP/AQP0, 20 we did not find CHMP4B-MIP/ AQP0 complexes in wild-type lenses (Figure S4). Whether or not CHMP4B interacts with Cx46 and Cx50 either directly and/or indirectly via an intermediate protein(s) remains to be determined.…”

“…In addition to gap junction plaque formation, Cx46 and Cx50 have been associated with B&S doublemembrane, junctional domains that interlock adjacent LFCs and are believed to provide a rapid cytoplasmic exchange of ions, small solutes, and fluids coupled with cell-cell adhesion and mechanical integrity during rapid LFC elongation. 15,17,18,20 B&S junctions are characterized by neck-and-head-like regions (0.5-2 μm diameter) of plasma membrane curvature that protrudes from one LFC into neck-and-head-like invaginations in opposing LFC membranes-in a manner similar to jig-saw pieces. In addition to Cx46 and Cx50, several other proteins have been associated with B&S structures including zonula occludens-1 (ZO-1), beta-dystroglycan (β-DG), and unidentified postsynaptic density protein, Drosophila disc large tumor suppressor, and zonula occludens-1 protein (PDZ)-domain-containing proteins.…”

“…In addition to gap junction plaque formation, Cx46 and Cx50 have been associated with B&S double‐membrane, junctional domains that interlock adjacent LFCs and are believed to provide a rapid cytoplasmic exchange of ions, small solutes, and fluids coupled with cell–cell adhesion and mechanical integrity during rapid LFC elongation 15,17,18,20 . B&S junctions are characterized by neck‐and‐head‐like regions (0.5–2 μm diameter) of plasma membrane curvature that protrudes from one LFC into neck‐and‐head‐like invaginations in opposing LFC membranes—in a manner similar to jig‐saw pieces.…”

“…LFC gap junctions are arranged in large (micron‐sized) two‐dimensional arrays or plaques particularly on the broad cross‐sectional membrane faces and are often associated with so‐called “ball‐and‐socket” (B&S) double‐membrane junctions that interlock cortical LFCs 3,15–18 . Loss‐of‐function studies in mice have shown that Cx46 and/or Cx50 are critical for lens growth, differentiation, and homeostasis, in part, by facilitating cell–cell adhesion and by providing an outflow pathway for the lens ion/fluid microcirculation system that is essential for maintaining the transparency and high refractive index of the avascular lens 4,10,19,20 . Sequence variations in the genes coding for Cx46 and Cx50 have been associated with both rare forms of inherited cataracts and frequently acquired forms of age‐related cataracts in humans 12 (https://cat-map.wustl.edu), and gene‐targeted mice that are homozygous for human Cx46 or Cx50 mutations develop cataracts associated with calcium bio‐mineralization of the lens 21–23 …”

Dynamic changes in whole genome DNA methylation, chromatin and gene expression during mouse lens differentiation

“…Gap junction proteins Gja1 (connexin43), Gja3 (connexin46) and Gja8 (connexin50) and Mip (major intrinsic protein/aquaporin 0) are major proteins expressed in the lens [ 82 , 83 ]. Gja1 is already expressed in the lens placode and is enriched in lens epithelium.…”