Expression of the tight junction protein ZO-1 in the olfactory system: presence of ZO-1 on olfactory sensory neurons and glial cells.

The olfactory system is a unique part of the central nervous system since it retains neuronal turnover and regenerative capacities in adulthood. Thus it provides an ideal model to study plasticity of membrane moities involved in cell-cell interactions. One structure particularly involved in cell-cell interaction is the tight junction, which establishes polarization of epithelial cells and creates diffusion barriers to paracellular passages. ZO-1 is a phosphoprotein peripherally associated with tight junctions. We have studied expression of ZO-1 protein in the developing and adult olfactory system of the mouse in order to get information about the localization and developmental expression of this tight junction component. ZO-1 expression has also been determined in cell cultures of olfactory bulbs. ZO-1 was present in the olfactory placode prior to formation of tight junctions. ZO-1 was localized in the developing and mature olfactory epithelium at heterotypic contacts between supporting cells and olfactory neurons as well as at homotypic contacts between both these cell types. Confocal microscopy showed quantitative differences in the ZO-1 expression among different olfactory dendrites. In the olfactory nerves ZO-1 immunolabeling was detectable between olfactory ensheathing cells. From the seventh postnatal day ZO-1 immunolabeling was detected at the mitral cell layer of the bulb on cells tentatively identified as oligodendrocytes. Myelinated tracts of the bulb were ZO-1 negative. Cell cultures of olfactory bulbs showed ZO-1 immunoreaction, mostly localized on glial fibrillary acidic protein (GFAP)-positive cells. Our results provide further evidence that ZO-1 serves functions unrelated to the tight junction complex and indicate molecular heterogeneity of these cell-cell contacts.