Histochemically demonstrable phosphotyrosine protein phosphatase in the rat hippocampal formation

Laminin stimulates protein tyrosine dephosphorylation in PC12 cells

Laminin stimulates neurite outgrowth in rat pheochromocytoma cells (PC12 cells). Here, we investigated laminin signal transduction mechanisms by adding the tyrosine kinase/phosphatase modulators, genistein, quercetin, aurin tricarboxylic acid (ATA), and vanadate to PC12 cells. At 10 microM both genistein and quercetin enhanced laminin-mediated neurite outgrowth by 1.7- and 2.3-fold, respectively, while at 10 microM, ATA inhibited laminin-mediated neurite outgrowth by 92%. Vanadate inhibited neurite outgrowth by 63% at 10 microM. Immunoblot analysis revealed four proteins of approximately 240, 22, 110, and 35 kDa, which were dephosphorylated on tyrosine residues in laminin-treated PC12 cells, but not in NIH 3T3 cells. These results demonstrate that laminin-mediated neurite outgrowth involves protein tyrosine dephosphorylation and suggests that this mechanism may have specificity to neuronal cells.

Subcellular localization of the F5 protein to the neuronal membrane-associated cytoskeleton

F5 was identified originally as an interleukin-2-regulated gene in the murine helper T-lymphocyte clone L2. Subsequent studies demonstrated high levels of F5 mRNA and protein in mature neurons in adult mouse central and peripheral nervous systems. The F5 protein was present in dendrites and perikarya but not in axons. In the present studies, the intracellular localization of the F5 protein in adult mouse brain was determined by subcellular fractionation and Western blotting. Although the deduced F5 sequence predicts a soluble protein, virtually no F5 immunoreactivity was found in the cytosol. The F5 protein was restricted to the P2 crude mitochondrial and P3 crude microsomal particulate fractions. Within the P2 fraction, F5 protein was enriched in the P2B synaptosomal subfraction. The results of temperature-dependent phase separation with Triton X-114 and alkaline extraction with sodium carbonate of the P2 and P3 fractions were consistent with the F5 protein being an extrinsic membrane-associated protein. Although essentially all of the F5 protein in the P3 fraction was membrane-associated, a substantial proportion of P2-associated F5 protein and nearly all of the synaptosomal F5 protein was detergent-insoluble. Direct isolation and subfractionation of brain cytoskeleton confirmed colocalization of F5 immunoreactivity with the membrane-associated cytoskeleton and postsynaptic densities. These studies suggest that the F5 protein, which has a large number of potential phosphorylation sites, plays a role in membrane-cytoskeletal interactions and in dynamic aspects of synaptic structure or function.

Characterization of the neuroimmune protein F5: localization to the dendrites and perikarya of mature neurons and the basal aspect of choroid plexus epithelial cells

F5 was identified originally as an interleukin-2-regulated gene in L2 cells, a murine helper T-lymphocyte clone. In adult mouse, F5 mRNA was expressed at a modest level in lymphoid tissues, at a high level in mature neurons in the nervous system, but not in other tissues. Although the F5 sequence is highly conserved over evolution, the function of the F5 protein is unknown. In the present studies, the putative F5 protein-coding region was translated in vitro using a reticulocyte lysate system and in Escherichia coli, yielding a protein with the predicted molecular weight of 42 kDa. Polyclonal rabbit anti-F5 antibody was generated against a synthetic peptide corresponding to the C-terminus of the F5 protein. This antibody specifically recognized recombinant F5 protein. Western blot studies demonstrated a strongly-reactive 42-kDa band and a faint 39-kDa band in extracts of adult mouse brain regions, the levels of which paralleled F5 mRNA expression. Immunoperoxidase studies of adult mouse brain demonstrated F5 immunoreactivity in neuronal perikarya and dendrites but not axons. Neurons expressing the highest levels of F5 protein corresponded to those with the highest levels of F5 mRNA. Choroid plexus epithelial cells also exhibited strong reactivity localized to their basal aspect. These observations suggest that the F5 protein, expression of which appears to be regulated predominantly at the RNA level, may be involved in the maintenance of the functional or anatomic polarity of neurons and choroid plexus epithelial cells.

The role of protein phosphatases in synaptic transmission, plasticity and neuronal development

In the past year significant advances have been made in our understanding of the role of protein dephosphorylation in the control of neuronal function. Molecular cloning has identified a large number of serine/threonine and tyrosine protein phosphatases in the nervous system. Many of these enzymes are selectively enriched in the nervous system, some are localized to specific neurons, and yet others are expressed only during specific periods of neuronal development. The availability of purified protein phosphatases and selective inhibitors has facilitated the analysis of these enzymes and their role in the regulation of neurotransmitter receptors and ion channels.