Expression of autofluorescent proteins reveals a novel protein permeable pathway between cells in the lens core

The lens of the eye is composed of concentric layers of tightly packed fibe r cells. The oldest fibers, those in the lens core, lose their nuclei and o ther organelles during terminal differentiation. This is thought to ensure the clarity of the lens. The anucleated core fibers are sustained by gap ju nction-mediated communication with metabolically active cells near the lens surface. In this study, we expressed autofluorescent proteins and microinj ected fluorescent markers to probe cell-to-cell communication in different regions of the developing lens. Our data indicate that a novel cell-cell di ffusion pathway becomes patent in the lens core during development. This pa thway is remarkable in that it is permeable to proteins and other large mol ecules and is thus distinct from gap junctions. Diffusion of large molecule s probably occurs through regions of membrane fusion observed between neigh boring cells in the lens core. Further direct evidence for a continuous pla sma membrane system was provided by the observation that exogenous membrane proteins expressed in one core fiber cell were able to diffuse laterally i nto the membranes of adjacent fibers. Thus, the lens core appears to repres ent a true syncytium within which both membrane proteins and cytoplasmic pr oteins freely diffuse. Significantly, the outermost edge of the core syncyt ium encompasses a shell of nucleated, transcriptionally-competent, fiber ce lls, This arrangement could facilitate the delivery of newly synthesized pr otein components to the aged and metabolically quiescent cells in the cente r of the lens.