A role for the tyrosine kinase ACK1 in neurotrophin signaling and neuronal extension and branching.

Neurotrophins are involved in many crucial cellular functions, including neurite outgrowth, synapse formation, and plasticity. Although these events have long been known, the molecular determinants underlying neuritogenesis have not been fully characterized. Ack1 (activated Cdc42-associated tyrosine kinase) is a non-receptor tyrosine kinase that is highly expressed in the brain. Here, we demonstrate that Ack1 is a molecular constituent of neurotrophin signaling cascades in neurons and PC12 cells. We report that Ack1 interacts with Trk receptors and becomes tyrosine phosphorylated and its kinase activity is increased in response to neurotrophins. Moreover, our data indicate that Ack1 acts upstream of the Akt and MAPK pathways. We show that Ack1 overexpression induces neuritic outgrowth and promotes branching in neurotrophin-treated neuronal cells, whereas the expression of Ack1 dominant negatives or short-hairpin RNAs counteract neurotrophin-stimulated differentiation. Our results identify Ack1 as a novel regulator of neurotrophin-mediated events in primary neurons and in PC12 cells.

Ribose 1-phosphate and inosine activate uracil salvage in rat brain.

The purpose of this study was to determine the mechanism by which inosine activates pyrimidine salvage in CNS. The levels of cerebral inosine, hypoxanthine, uridine, uracil, ribose 1-phosphate and inorganic phosphate were determined, to evaluate the Gibbs free energy changes (deltaG) of the reactions catalyzed by purine nucleoside phosphorylase and uridine phosphorylase, respectively. A deltaG value of 0.59 kcal/mol for the combined reaction inosine+uracil <==> uridine+hypoxanthine was obtained, suggesting that at least in anoxic brain the system may readily respond to metabolite fluctuations. If purine nucleoside phosphorolysis and uridine phosphorolysis are coupled to uridine phosphorylation, catalyzed by uridine kinase, whose activity is relatively high in brain, the three enzyme activities will constitute a pyrimidine salvage pathway in which ribose 1-phosphate plays a pivotal role. CTP, presumably the last product of the pathway, and, to a lesser extent, UTP, exert inhibition on rat brain uridine nucleotides salvage synthesis, most likely at the level of the kinase reaction. On the contrary ATP and GTP are specific phosphate donors.