Abstract
Purpose: :
To analyze the properties of the large molecule diffusion pathway (LMDP) in the mouse lens and test the hypothesis that the LMDP facilitates the movement of proteins between the metabolically active surface layers of the lens and the metabolically quiescent core.
Methods: :
GFP expression was induced in Cre-ERTM;Z/EG mice by tamoxifen injection. Fluorescent dextran (10 kDa) was microinjected into organ cultured lenses. The diffusion of fluorescent macromolecules within the lens was visualized by confocal microscopy. Membrane fusions, putative conduits for intercellular diffusion, were visualized in serial ultrathin sections of the lens. The role of Lim2, a claudin-like membrane protein, was evaluated by crossing the Cre-ERTM and Z/EG transgenes onto a Lim2-null background.
Results: :
Intercellular diffusion of GFP and dextran commenced early in fiber cell differentiation, in cells that had not yet reached the lens sutures. The initial intercellular spread of macromolecules was predominantly in the lateral direction (i.e. between cells in the same stratum of the lens). Long-term (>6 months) GFP labeling revealed that GFP diffused throughout a stratum of the lens, with relatively little centripetal spread. No significant intercellular diffusion of GFP was observed in Lim2-null lenses, indicating a requirement for Lim2 in the formation of the LMDP. The conduits for intercellular diffusion may be regions of limited cell fusion which were commonly observed in wildtype lenses but were absent in Lim2 deficient lenses.
Conclusions: :
Lens fiber cells are coupled by a Lim2-dependent pathway that permits the intercellular diffusion of macromolecules. The LMDP appears to couple cells of the same age rather than mediate the centripetal flow of macromolecules. The observation that Lim2-deficient lenses have disturbed optical properties suggests that the LMDP may sub-serve a refractive role in the lens.
Keywords: cell-cell communication • cell adhesions/cell junctions • cell membrane/membrane specializations