Agrin accumulates in the brain microvascular basal lamina during development of the blood-brain barrier

The blood-brain barrier (BBB) is an essential feature of the microvasculature in neural tissues. Agrin, a synapse organizing molecule at the neuromuscular junction, also accumulates on brain microvasculature and may be involved in BBB formation and function. We investigated the developmental expression of agrin at rat and chick brain capillaries by immunohistochemistry and immunoblotting. Anti-agrin immunoreactivity (IR) completely ensheathes all observed microvessels labeled with anti-von Willebrand factor in adult brains of both species. Brain microvascular agrin IR codistributes with anti-laminin IR, consistent with agrin localization in the microvessel basal lamina. On microvessels in testis and thymus, tissues which also contain blood-tissue barriers, the pattern of IR is indistinguishable from brain microvessels. In contrast, little or no agrin IR is observed on capillaries in muscle and other tissues. During chick and rat development, agrin accumulates on brain microvessels around the time the vasculature becomes impermeable. Differential staining and electroblotting suggest that the agrin isoforms expressed on brain microvessels lack the 8- and 11-amino acid sequences that confer on agrin high potency in acetylcholine receptor clustering. Taken in context with the organizing role of agrin in synaptogenesis, these results indicate that agrins may function as important players in the formation and maintenance of cerebral microvascular impermeability.