Expression of the barrier-associated proteins EAP-300 and claustrin in the developing central nervous system

Immunohistochemistry of embryonic chick central nervous system (CNS) and immunocytochemistry of retinal cells were performed to compare and map the expression of two barrier-associated molecules. EAP-300 (embryonic avian polypeptide of 300 kDa) and claustrin (a 320 kDa extracellular matrix keratan sulfate proteoglycan) were both transiently expressed in CNS regions that are considered non-permissive to either neuron migration or axon growth. In the developing spinal cord, EAP-300 and claustrin were both expressed by the marginal zone early in development and by the roof plate later in embryogenesis. In the developing rhombencephalon, immunoreactivity for both molecules was also observed first in the marginal zone, and later expression was restricted mostly to the midline. In the mesencephalon, EAP-300 and claustrin were also localized to the midline, and this expression represented a continuation of the expression observed in the spinal cord roof plate and hindbrain ventral midline. In the developing retina and cerebellum, EAP-300 and claustrin were differentially expressed. In retina, EAP-300 and claustrin were expressed by Müller cells and the optic fiber layer, respectively. In cerebellum at embryonic day 12 (E12), EAP-300 was expressed by Bergman glia, but claustrin was not expressed until E15. Immunocytochemical staining of retinal and cerebellar cultures indicated that EAP-300 was expressed by a subset of radial astrocytes, as confirmed by double labeling experiments with a specific marker for radial astrocytes. These data indicate that in the absence of claustrin expression, EAP-300 was expressed specifically by radial astrocytes during developmental periods of neuron migration. Also, the coexpression of EAP-300 and claustrin in CNS regions considered to be non-permissive to neurite extension suggests that these two developmentally regulated proteins may be associated with barrier function in the developing CNS.