Differential distribution of alpha and beta isoforms of p21-activated kinase in the monkey cerebral neocortex and hippocampus.

The present study aimed to elucidate the subcellular distribution of the Cdc42 and Rac activated alpha and beta isoforms of p21-activated kinase (PAK) in the monkey cerebral neocortex and hippocampus. These proteins have been shown to play morphological roles through effects on the actin cytoskeleton. alphaPAK immunoreaction product was concentrated in regions of axon terminals or dendrites, some distance (0.2-1 microm) away from the synapse. The labeled portion of dendrite often appeared with "ruffled" cell membranes or resembling non-synapse forming "buds." betaPAK immunoreaction product was concentrated in cell bodies and larger diameter dendrites. Immunogold labeled sections showed that most of the label for both alphaPAK and betaPAK was present in a perisynaptic or extrasynaptic location, and relatively little staining was present on the postsynaptic density. Because alphaPAK has been shown to be associated with new membrane structures involving the Rho family GTPase Rac1, which controls dendritic morphology, these observations suggest alphaPAK positive regions of axons and dendrites may mark new areas of neurite extension.