Are gap junction membrane plaques implicated in intercellular vesicle transfer?

Abstract

Gap junction channels are concentrated in specialised plaques of plasma membrane where cells are in close apposition. In this communication evidence is provided showing that these specialised regions of membrane also provide a site for vesicular transfer between cells. Vesicle distribution in eye lenses was found to generally reflect the reported distribution of gap junction membrane plaques. In certain areas of the lens gap junction membrane plaques and vesicles could be seen to form combined, complex structures. Ultrastructure of the vesicle and gap junction membrane plaque complexes was consistent with the vesicles moving through membrane plaques from one lens fibre cell to the next. To investigate whether transport of substances was consistent with intercellular vesicle transfer, transport of various markers was investigated. Time course experiments showing the rate of uptake of various markers into the lens did not show dramatic differences for molecules smaller or larger then gap junction pores formed by connexons. While considered as a primary intercellular transport mechanism in the lens, connexon pores were not the sole agent mediating the observed transport. Other reported mechanisms of intercellular transport in the lens can only account for the movement of relatively small molecules. Vesicular transport may therefore be a major form of transport into the outer lens layers for larger molecules. Implicit in these observations is a new hypothesis for intercellular vesicle movement via gap junction membrane plaques. Intercellular vesicle movement could possibly provide a path for large molecules associated with intact vesicles to be transported into the eye lens tissue.