Brevican in the developing hippocampal fimbria: Differential expression inmyelinating oligodendrocytes and adult astrocytes suggests a dual role forbrevican in central nervous system fiber tract development

Brevican is one of the most abundant extracellular matrix proteoglycans in the mammalian brain. We have previously shown that brevican produced by gra y matter astrocytes constitutes a major component of perineuronal extracell ular matrix in the adult brain. In this paper, we investigate the expressio n of brevican in the postnatal hippocampal fimbria to explore the role of t he proteoglycan in central nervous system fiber tract development. We demon strate that brevican is expressed by both oligodendrocytes and white matter astrocytes in the fimbria, but the expression of brevican in these two gli al cell types is differently regulated during development. At P14, brevican immunoreactivity was observed throughout the fimbria, with particularly st rong immunoreactivity in the developing interfascicular glial rows. In situ hybridization showed that oligodendrocytes in the glial rows strongly expr ess brevican during the second and third postnatal weeks. Expression in oli godendrocytes was then down-regulated after P21. In the adult fimbria, no b revican expression was observed in oligodendrocytes. The time window of bre vican expression coincides with the phase in which immature oligodendrocyte s actively extend membrane processes and enwrap axon fibers. In contrast, t he expression in astrocytes started around P21 as oligodendrocytes began to down-regulate the expression. In the adult fimbria, brevican expression wa s restricted to astrocytes. In situ hybridization with isoform-specific pro bes and RNase protection assays showed that the authentic, secreted form of brevican, not the glycosylphosphatidylinositol-anchored variant, is the pr edominant species expressed in the developing fimbria. Our results suggest that brevican plays a dual role in developing and adult fiber tracts. (C) 2 001 Wiley-Liss, Inc.