beta-Actin is confined to structures having high capacity of remodelling in developing and adult rat cerebellum

Neurons undergo complex morphological changes during differentiation and in cases of plasticity. A major determinant of cell morphology is the actin c ytoskeleton, which in neurons is comprised of two actin isoforms, non-muscl e gamma- and beta-actin. To better understand their respective roles during differentiation and plasticity, their cellular and subcellular localizatio n was examined in developing and adult cerebellar cortex. It was observed t hat gamma-actin is expressed at a constant level throughout development, wh ile the level of beta-actin expression rapidly decreases with age. At the l ight microscopic level, gamma-actin staining is ubiquitous and the only dev elopmental change observed is a relative reduction of its concentration in cell bodies and white matter. In contrast, beta-actin staining almost compl etely disappears from the cytoplasm of cell bodies, primary dendrites and a xons. In young cerebellar cultures, gamma-actin is found in the cell body, neurites and growth cones, while beta-actin is mainly found in growth cones , as previously reported in other primary neuronal culture systems [Kaech e t al. (1997), J. Neuroscience, 17, 9565-9572; Bassell et al., (1998), J. Ne uroscience, 18, 251-265]. Electron microscopy of post-embedding immunogold- labelled tissue confirms the widespread distribution of gamma-actin, and al so reveals an increased concentration of gamma-actin in dendritic spines in the adult, During development, beta-actin accumulation is observed in acti vely growing structures, e.g. growth cones, filopodia, cell bodies and axon al tracts. In the adult cerebellar cortex, beta-actin is preferentially fou nd in dendritic spines, structures which are known to retain their capacity for morphological modifications in the adult brain. This differential subc ellular localization and developmental regulation of the two actin isoforms point to their different roles in neurons.