15N-labeled brain enables quantification of proteome and phosphoproteome in cultured primary neurons.

Terminally differentiated primary cells represent a valuable in vitro model to study signaling events associated within a specific tissue. Quantitative proteomic methods using metabolic labeling in primary cells encounter labeling efficiency issues hindering the use of these cells. Here we developed a method to quantify the proteome and phosphoproteome of cultured neurons using (15)N-labeled brain tissue as an internal standard and applied this method to determine how an inhibitor of an excitatory neural transmitter receptor, phencyclidine (PCP), affects the global phosphoproteome of cortical neurons. We identified over 10,000 phosphopeptides and made accurate quantitative measurements of the neuronal phosphoproteome after neuronal inhibition. We show that short PCP treatments lead to changes in phosphorylation for 7% of neuronal phosphopeptides and that prolonged PCP treatment alters the total levels of several proteins essential for synaptic transmission and plasticity and leads to a massive reduction in the synaptic strength of inhibitory synapses. The results provide valuable insights into the dynamics of molecular networks implicated in PCP-mediated NMDA receptor inhibition and sensorimotor deficits.

A heterogeneous tRNA granule structure exhibiting rapid, bi-directional neuritic transport.

mRNA translation is regulated by diverse mechanisms that converge at the initiation and elongation steps to determine the rate, profile, and localization of proteins synthesized. A consistently relevant feature of these mechanisms is the spatial re-distribution of translation machinery, a process of particular importance in neural cells. This process has, however, been largely overlooked with respect to its potential role in regulating the local concentration of cytoplasmic tRNAs, even as a multitude of data suggest that spatial regulation of the tRNA pool may help explain the remarkably high rates of peptide elongation. Here, we report that Cy3/Cy5-labeled bulk tRNAs transfected into neural cells distribute into granule-like structures - "tRNA granules" - that exhibit dynamic mixing of tRNAs between granules and rapid, bi-directional vectorial movement within neurites. Imaging of endogenous tRNAgly and tRNAlys by fluorescent in situ hybridization revealed a similar granular distribution of tRNAs in somata and neurites; this distribution was highly overlapping with granules imaged by introduction of exogenous Cy5-tRNAthr and Cy3-tRNAval. A subset of tRNA granules located in the cell body, neurite branch points and growth cones displayed fluorescence resonance energy transfer (FRET) between Cy3 and Cy5-labeled tRNAs indicative of translation, and co-localization with elongation machinery. A population of smaller, rapidly trafficked granules in neurites lacked FRET and showed poor colocalization with translation initiation and elongation factors, suggesting that they are a translationally inactive tRNA transport particle. Our data suggest that tRNAs are packaged into granules that are rapidly transported to loci where translation is needed, where they may greatly increase the local concentration of tRNAs in support of efficient elongation. The potential implications of this newly described structure for channeling of elongation, local translation, and diseases associated with altered tRNA levels or function are discussed.

A cell-free expression and purification process for rapid production of protein biologics.

Cell-free protein synthesis has emerged as a powerful technology for rapid and efficient protein production. Cell-free methods are also amenable to automation and such systems have been extensively used for high-throughput protein production and screening; however, current fluidic systems are not adequate for manufacturing protein biopharmaceuticals. In this work, we report on the initial development of a fluidic process for rapid end-to-end production of recombinant protein biologics. This process incorporates a bioreactor module that can be used with eukaryotic or prokaryotic lysates that are programmed for combined transcription/translation of an engineered DNA template encoding for specific protein targets. Purification of the cell-free expressed product occurs through a series of protein separation modules that are configurable for process-specific isolation of different proteins. Using this approach, we demonstrate production of two bioactive human protein therapeutics, erythropoietin and granulocyte-macrophage colony-stimulating factor, in yeast and bacterial extracts, respectively, each within 24 hours. This process is flexible, scalable and amenable to automation for rapid production at the point-of-need of proteins with significant pharmaceutical, medical, or biotechnological value.