The embryonic lethal abnormal vision (ELAV)-like proteins are mRNA-binding proteins that regulate mRNA stability. The neuronal members of this family are required for neuronal differentiation. We identified the binding region of purified HuD protein to a target neuronal mRNA encoding for the tau microtubule-associated protein and demonstrated an in vivo interaction between the ELAV-like protein and its target tau mRNA. We show that treatment of neuronal cells with antisense oligodeoxynucleotides directed against HuD blocks the induction of neurite outgrowth and decreases the levels of tau mRNAs, indicating that the ELAV-like proteins are required for neuronal differentiation.

Tau mRNA is axonally localized mRNA that is found in developing neurons and targeted by an axonal localization signal (ALS) that is located in the 3¢UTR of the message. The tau mRNA is trafficked in an RNA-protein complex (RNP) from the neuronal cell body to the distal parts of the axon, reaching as far as the growth cone. This movement is microtubule-dependent and is observed as granules that contain tau mRNA and additional proteins. A major protein contained in the granule is HuD, an Elav protein family member, which has an identified mRNA binding site on the tau 3¢UTR and stabilizes the tau message and several axonally targeted mRNAs. Using GST-HuD fusion protein as bait, we have identified four proteins contained within the tau RNP, in differentiated P19 neuronal cells. In this work, we studied two of the identified proteins, i.e. IGF-II mRNA binding protein 1 (IMP-1), the orthologue of chick b-actin binding protein-ZBP1, and RAS-GAP SH3 domain binding protein (G3BP). We show that IMP-1 associates with HuD and G3BP-1 proteins in an RNA-dependent manner and binds directly to tau mRNA. We also show an RNA-dependent association between G3BP-1 and HuD proteins. These associations are investigated in relation to the neuronal differentiation of P19 cells.

Subcellular mRNA localization, a fundamental mechanism for regulating gene expression, leads to local protein translation that results in the generation of neuronal cell polarity. In this study, we have used P19 embryonic carcinoma cells, which are amenable to transfection, and selection of clonal stable cell lines that are not overexpressing the constructs. We identified the 3Ј untranslated region (3ЈUTR) tau axonal localization signal and examined its effect on tau protein localization in nondifferentiated and neuronally differentiated P19 cells. Using GFPtagged tau constructs combined with in situ hybridization analysis, we demonstrated colocalization of the targeted tau mRNA and its translated protein in the axon and growth cone. Absence of or mutation in the 3ЈUTR axonal targeting region of tau mRNA resulted in suppression of tau mRNA localization, and both tau mRNA and tau protein remained in the cell body. Swapping between the 3ЈUTR tau mRNA axonal localization signal and the 3ЈUTR MAP2 mRNA dendritic targeting signal proved that the localization of the proteins into the axon or dendrites depends on the specific 3ЈUTR targeting signals. Moreover, the identification of ribosomal proteins in the axon lends further support to the presence of protein synthetic machinery in the axons, a prerequisite for local translation. It is suggested therefore that the P19 cell system can be used to analyze mutations that affect mRNA transport and local translation and that it has the potential of being used to examine the onset of the neuronal differentiation process. Key words: tau protein; tau mRNA; axonal targeting signal; ribosomes; P19 EC cells; neuronal differentiationNeuronal polarity results from the segregated distribution of molecules and organelles and depends on cytoskeletal organization (Bassell and Singer, 1997;Kiebler et al., 1999). It has been established that MAP2, the high molecular-weight microtubuleassociated protein (MAP), is localized in the cell body and dendrites, whereas tau MAP is found mainly in the cell body and axons (Matus et al., 1981;Binder et al., 1985).The molecular mechanisms responsible for the segregation of MAP proteins into the axons and dendrites are not yet fully understood. Subcellular mRNA localization and local translation within dendrites and axons are posttranscriptional control mechanisms that can explain this segregation and may play a key role in generation and maintenance of neuronal polarity. Recent data, which were obtained using molecular approaches and visualization, have demonstrated the presence of unique mRNA species and local protein synthesis in the dendrites, axons, and their growth cones

Localization of tau mRNA to the axon requires the axonal localization cis signal (ALS), which is located within the 3′ untranslated region, and trans-acting binding proteins, which are part of the observed granular structures in neuronal cells. In this study, using both biochemical and morphological methods, we show that the granules contain tau mRNA, HuD RNA-binding protein, which stabilizes mRNA, and KIF3A, a member of the kinesin microtubuleassociated motor protein family involved in anterograde transport. The granules are detected along the axon and accumulate in the growth cone. Inhibition of KIF3A expression caused neurite retraction and inhibited tau mRNA axonal targeting. Taken together, these results suggest that HuD and KIF3A proteins are present in the tau mRNA axonal granules and suggest an additional function for the kinesin motor family in the microtubuledependent translocation of RNA granules. Localized tau-GFP expression was blocked by a protein synthesis inhibitor, and upon release from inhibition, nascent tau-GFP 'hot spots' were directly observed in the axon and growth cones. These observations are consistent with local protein synthesis in the axon resulting from the transported tau mRNA.

Tau microtubule-associated protein is a neuron specific protein found primarily in axons and is developmentally regulated. The function of tau is in stabilization of microtubules, which is important in establishing and maintaining neuronal morphology. Axonal localization of tau involves a multistep process which is studied in differentiating primary neuronal culture. The initial step involves sorting and subcellular localization of its encoding mRNA into the proximal portion of the axon. Using the transfection assay into neuronal cells, we have demonstrated that sequences located in the 3'-untranslated region include a cis-acting signal which is involved in tau mRNA targeting. In addition, using ultraviolet cross-linking assay, two RNA-binding proteins of 43 and 38 kDa were identified, that exhibit specific binding to a minimal sequence of 91 nucleotides located within the same functional region, which is involved in targeting. The 43 and 38-kDa RNA-binding proteins are present in cytoplasmic extracts, prepared from neuronal cells, and in isolated microtubule preparations. Our results support a novel model in which cis-acting signals, together with RNA-binding proteins are involved in the targeting of tau mRNA, that may ultimately lead to its axonal localization.