Project description:The subcellular localization of specific mRNAs is an evolutionary conserved mechanism that underlies the establishment of cellular polarity and specialized cell functions. In neurons, mRNA trafficking and local protein translation in dendrites provides an important mechanism that mediates synaptic development and plasticity. The significance of mRNA targeting and protein synthesis in axons, however, is still unclear. Only a small number of transcripts have been identified in axons to date, and their contribution to axon growth and neuronal survival remains largely unknown. Here, we report the results of a novel screen that allowed the separate identification of mRNAs localized in cell bodies and in axons of developing neurons. Using compartmentalized cultures of sympathetic neurons and Sequential Analysis of Gene Expression (SAGE), the screen identified more than 200 axonal mRNAs, including ones that encode cytoskeletal proteins and proteins that function in neural development and signal transduction. Importantly, several classes of transcripts were selectively enriched in axons, indicating that an active process drives the targeting of specific mRNAs from the cell bodies to the axons. This study is the first comprehensive and unbiased analysis of mRNA localization in subcellular domains of any neuronal cell type. We used compartmentalized chambers to culture neonatal rat sympathetic neurons (Campenot, 1977). In these cultures, the cell bodies are separated from the distal axons by a 1 mm wide Teflon divider, which maintains the cell bodies and axon terminals in separate fluid compartments. Primary rat sympathetic neurons are especially suitable for compartmentalized culture because they can be grown as a highly homogeneous population without glial cells. Neurons were seeded in the central compartment with nerve growth factor (NGF), and after a few days, the NGF was lowered in this compartment and supplied only to the peripheral compartment to stimulate axon growth. The anti-mitotic agent cytosine arabinoside C (Ara-C) was added to both compartments to remove non-neuronal cells. mRNA was then isolated after 12 days in culture (DIV) from cell body or axon compartments. As the initial mRNA content in axons was not sufficient to perform the SAGE analysis, both axon and cell body mRNAs were subjected to two rounds of linear amplification to obtain antisense RNA (aRNA). The amplified aRNA was then reverse transcribed and second strand synthesis was performed to proceed with the SAGE assay using the LongSAGE kit (Invitrogen) according to the manufacturer’s protocol.

Project description:The nerve growth factor NGF has been shown to cause cell fate decisions towards either differentiation or proliferation depending on the relative activity of downstream pERK, pAKT or pJNK signaling. However, how these protein signals are translated into and fed back from transcriptional activity to complete cellular differentiation over a time span of hours to days is still an open question. Using dynamic proteome and transcriptome data from NGF-stimulated PC12 cells over a time span of 24 hours we inferred a dynamic Boolean model capturing the temporal sequence of protein signaling, transcriptional response and subsequent autocrine feedback. Optimal model topology was achieved by introducing multiple time scales for fast cellular signaling and slower gene response and subsequent fitting to the experimental data. The integrated model confirmed the parallel use of MEK/ERK, AKT/PI3K and JNK/JUN for PC12 cell differentiation. Redundancy of cell signaling is demonstrated from the inhibition of the different MAPK pathways. As suggested in silico and confirmed in vitro, differentiation was substantially suppressed under JNK/JUN inhibition, yet delayed only under MEK/ERK inhibition. Most importantly, we found that sustained transcriptional feedback is necessary to induce bistability in the cell fate switch. De novo Gene expression was necessary to activate autocrine feedback that caused integrin signaling to perpetuate the MAPK activity, finally resulting in the expression of late, differentiation related genes. Thus, the cellular decision towards differentiation depends on the establishment of a transcriptome-induced positive feedback between protein signaling and gene expression thereby constituting a robust control between proliferation and differentiation. To eludicate the transcriptome response of PC12 cells time-resolved gene expression data of NGF or EGF simulated PC12 were recorded at t = [0.5h 1h, 2h, 3h, 4h, 5h, 6h, 8h, 12h, 24h, 48h] for NGF and at t=[1h, 2h, 3h, 4h, 6h, 8h, 12h, 24h]. Control time points of unstimulated PC12 cells were taken at t=[0h, 2h, 4h, 6h, 8h, 24h, 48h]. Additionally, the MEK-inhibitor UO126 was added together with NGF and the gene response was measured at t=[1h, 2h, 4h, 6h, 8h, 12h, 24h, 48h] and for UO126 alone at t=[1h, 2h, 4h, 8h, 12h, 48h]. Total RNA was isolated, labeled and hybridized to an Illumina ratRef-12 bead array (Illumina, San Diego, CA, USA) according to the manufacturerâ??s protocol.

Project description:We have undertaken a genome-wide study of transcriptional activity in embryonic superior cervical ganglia (SCG) and dorsal root ganglia (DRG) during a time course of neurite outgrowth in vitro. Gene expression observed in these models likely includes both developmental gene expression patterns and regenerative responses to axotomy, which occurs as the result of tissue dissection. Comparison across both models revealed many genes with similar gene expression patterns during neurite outgrowth. These patterns were minimally affected by exposure to the potent inhibitory cue Semaphorin3A, indicating that this extrinsic cue does not exert major effects at the level of nuclear transcription. We also compared our data to several published studies of DRG and SCG gene expression in animal models of regeneration, and found the expression of a large number of genes in common between neurite outgrowth in vitro and regeneration in vivo. Experiment Overall Design: We wished to determine the transcriptional profiles of neurons undergoing neurite outgrowth in vitro. We were particularly interested in finding genes whose expression is generally associated with the process of neurite outgrowth, rather than with cell type-specific effects. Thus, in order to avoid focusing on transcripts unique to one tissue type versus another, we used a comparative strategy to look for effects that were common to two tissue types and therefore more likely to be involved in the general process of neurite outgrowth. While these explants contain multiple cell types, we felt this was preferable to the more disruptive conditions required to dissociate neurons or obtain a pure neuron population. To this end, we monitored gene expression in cultured explants from SCG and DRG using DNA microarrays. We initiated our studies by culturing embryonic day 13 (E13) mouse SCG in vitro and harvesting tissue for RNA isolation at time points from 2 to 65 hours. Time points were selected to detect both fast, short-term responses (2, 5 and 12 hours), as well as sustained, long-term changes (24, 40, and 65 hours). Samples were hybridized to Affymetrix MG-U74v2 A and B microarrays, with RNA from acutely dissected explants serving as a baseline reference. We followed these experiments with a parallel analysis of a more heterogeneous tissue type, the DRG, which is more frequently used than SCG for in vivo studies of neurite regeneration. Cervical and upper thoracic DRG from E12 embryos were cultured with NGF (the same trophic support as in SCG cultures), harvested at time points from 2 to 40 hours, and hybridized to Affymetrix MOE 430A microarrays.

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Global analysis of gene expression in NGF-deprived sympathetic neurons identifies molecular pathways associated with cell death

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