AXONAL MEMBRANE-SKELETAL PROTEIN A60 - ASSOCIATION WITH A BRAIN SPECTRIN-BINDING ACTIVITY AND ENTRY INTO CEREBELLAR AXONS AT A STAGE AFTER THE INITIATION OF AXONAL GROWTH

A60 is a 60-kDa component of the axonal cortical cytoskeleton in CNS n eurones. It appears to be neurone specific and is tightly bound to bra in membranes. In this study the cytoskeletal activities and developmen tal expression of A60 in rat cerebellum have been examined using the m onoclonal antibody DR1. A60 in a partially purified soluble extract of brain membranes interacts selectively with brain but not erythrocyte spectrin. Because erythrocyte spectrin is more closely related to the dendritic form of spectrin than the axonal form, this raises the possi bility that A60 localises in axons by interaction with the axonal form of spectrin only. A60 is not found in rat cerebellum before the day o f birth. However, during postnatal development of the cerebellum (days 1-13) DR1 reactivity appears progressively. On postnatal day 1, a sma ll population of cells in the mantle layer (presumptive Purkinje cells ) is DR1 positive. There is no DR1 reactivity found in Purkinje cell a xons during their initial phase of growth. By postnatal day 7, Purkinj e cell bodies, initial dendritic segments, and the cerebellar white ma tter are all positive. This pattern of labelling is strengthened up un til postnatal day 13. By contrast, in adult rat cerebellum, the locati on of A60 has changed so that it is most concentrated in axons, and de ndritic staining is lost. These data indicate that A60 is a spectrin-b inding component of the adult axonal membrane skeleton, the presence o f which is only required in axons after the initial phase of growth.