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Oncoprotein18 (Op18) is a 19 kDa cytosolic phosphoprotein critical for cell growth and differentiation. Unphosphorylated Op18 associates with £\£]-tubulin heterodimer to form 2 tubulin-1 stathmin T2S complex and promotes microtubule catastrophe in interphase. Numerous cellular signals activate downstream protein kinases result in the phosphorylation of Ser16, Ser25, Ser38 and Ser 63 residues in Op18 that lowers its affinity for tubulin thereby increases the stability of microtubule and facilitates the formation of spindle during mitosis. Here, we found in addition to phosphorylation, Op18 could also be ubiquitin modified in vivo. An expression plasmid encodes for mutant EGFP-Op18-M5K protein whose potential lysine residues K42, K53, K75, K104, and K119 were mutated to arginines was generated to investigate the effect of ubiquitin modification of Op18 on the tubulin polymerization. Our results revealed a decrease of ubiquitin modification of mutant EGFP-Op18-M5K in comparison with that of wildtype EGFP-Op18. The expression of mutant but not the wildtype Op18 resulted in a significant increase of polymerized tubulin in mitotic cell implying that they might exhibit differential tubulin binding affinity. Moreover, the result of western blotting showed that the mutant Op18 detected in the mitotic cell corresponds to the phosphorylated version of Op18. In summary, these results imply the ubiquitination of Op18 might interfere with its phosphorylation and decrease its tubulin binding potential, thereby facilitates the polymerization of tubulin in mitotic cells. The in vitro tubulin polymerization assay will be performed to further confirm the above finding.

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The biological complexity of NGF action is achieved by binding two distinct Neurotrophin receptors, TrkA and p75NTR. While several reports have provided lines of evidence on the interaction between TrkA and p75NTR at the plasma membrane, much fewer data are available on the consequence of such an interaction in terms of intracellular signaling. In this study, we have focused on how p75NTR may affect TrkA downstream signaling with respect to neuronal differentiation. Here, we have shown that cooperation between p75NTR and TrkA results in an increased NGF-mediated TrkA autophosphorylation, leads to a sustained activation of ERK1/2 and accelerates neurite outgrowth. Interestingly, neurite outgrowth is concomitant with a selective enhancement of the AP-1 activity and the transcriptional activation of genes such as GAP-43 and p21(CIP/WAF), known to be involved in the differentiation process. Collectively, our results unveil a functional link between the specific expression profile of neurotrophin receptors in neuronal cells and the NGF-mediated regulation of the differentiation process possibly through a persistent ERKs activation and the selective control of the AP-1 activity. In our studies we discuss the functional role of the neurotrophin receptor p75NTR and TrkA in a ligand-dependent signal transduction. It is known that p75NTR is also involved in the mediation of cell death ligand dependent. Here we show for the first time that the membrane receptor p75NTR, upon binding to b- Amyloid (Ab) peptide, is able to transduce a cytotoxic signal through a mechanism very similar to the one adopted by Tumor Necrosis Factor Receptor 1 (TNFR1), when activated by TNFa. We define that in neuroblastoma cell line Ab cytotoxicity signals through a pathway depending on p75NTR death domain (DD), mostly through some specific conserved residues. We identified that TRADD is the first interactor recruiting to the membrane and activates JNK and NF-kB transcription factors. Since Ab is defined as the most important aetiologic element associated with the Alzheimer’s Disease (AD), characterization of the mechanism involved in the mediation of the neurodegeneration can suggest also new therapeutic approaches.