Brain transcriptome and connectome maps are being generated, but an equivalent effort on the proteome is currently lacking. We performed high-resolution mass spectrometry-based proteomics for in-depth analysis of the mouse brain and its major brain regions and cell types. Comparisons of the 12,934 identified proteins in oligodendrocytes, astrocytes, microglia and cortical neurons with deep sequencing data of the transcriptome indicated deep coverage of the proteome. Cell type-specific proteins defined as tenfold more abundant than average expression represented about a tenth of the proteome, with an overrepresentation of cell surface proteins. To demonstrate the utility of our resource, we focused on this class of proteins and identified Lsamp, an adhesion molecule of the IgLON family, as a negative regulator of myelination. Our findings provide a framework for a system-level understanding of cell-type diversity in the CNS and serves as a rich resource for analyses of brain development and function.

Regulatory protein phosphorylation controls normal and pathophysiological signaling in eukaryotic cells. Despite great advances in mass-spectrometry-based proteomics, the extent, localization, and site-specific stoichiometry of this posttranslational modification (PTM) are unknown. Here, we develop a stringent experimental and computational workflow, capable of mapping more than 50,000 distinct phosphorylated peptides in a single human cancer cell line. We detected more than three-quarters of cellular proteins as phosphoproteins and determined very high stoichiometries in mitosis or growth factor signaling by label-free quantitation. The proportion of phospho-Tyr drastically decreases as coverage of the phosphoproteome increases, whereas Ser/Thr sites saturate only for technical reasons. Tyrosine phosphorylation is maintained at especially low stoichiometric levels in the absence of specific signaling events. Unexpectedly, it is enriched on higher-abundance proteins, and this correlates with the substrate KM values of tyrosine kinases. Our data suggest that P-Tyr should be considered a functionally separate PTM of eukaryotic proteomes.

Granulomas are the pathological hallmark of tuberculosis (TB). However, their function and mechanisms of formation remain poorly understood. To understand the role of granulomas in TB, we analyzed the proteomes of granulomas from subjects with tuberculosis in an unbiased fashion. Using laser capture microdissection, mass spectrometry and confocal microscopy, we generated detailed molecular maps of human granulomas. We found that the centers of granulomas possess a pro-inflammatory environment characterized by anti-microbial peptides, ROS and pro-inflammatory eicosanoids. Conversely, the tissue surrounding the caseum possesses a comparatively anti-inflammatory signature. These findings are consistent across a set of six subjects and in rabbits. While the balance between systemic pro- and anti-inflammatory signals is crucial to TB disease outcome, here we find that these signals are physically segregated within each granuloma. The protein and lipid snapshots of human and rabbit lesions analysed here suggest that the pathologic response to TB is shaped by the precise anatomical localization of these inflammatory pathways during the development of the granuloma.

Protein kinases play important roles in tumor development and progression. A variety of members of this family of signal transduction enzymes serve as targets for therapeutic intervention in cancer. We have identified the receptor tyrosine kinase (RTK) AXL as a potential mediator of motility and invasivity of breast cancer cells. AXL is expressed in most highly invasive breast cancer cells, but not in breast cancer cells of low invasivity. Ectopic expression of AXL was sufficient to confer a highly invasive phenotype to weakly invasive MCF7 breast cancer cells. Experimental inhibition of AXL signaling by a dominant-negative AXL mutant, an antibody against the extracellular domain of AXL, or short hairpin RNA knockdown of AXL decreased motility and invasivity of highly invasive breast cancer cells. To selectively interfere with cancer cell properties defining the rate of disease progression, we identified 3-quinolinecarbonitrile compounds, which displayed potent inhibitory activity against AXL and showed strong interference with motility and invasivity of breast cancer cells. Our findings validated the RTK AXL as a critical element in the signaling network that governs motility and invasivity of breast cancer cells, and allowed the identification of experimental anti-AXL small molecular inhibitors that represent lead substances for the development of antimetastatic breast cancer therapy. [Cancer Res 2008;68(6):1905-15]

We report a proteomics strategy to both identify and quantify cellular target protein interactions with externally introduced ligands. We determined dissociation constants for target proteins interacting with the ligand of interest by combining quantitative mass spectrometry with a defined set of affinity purification experiments. We demonstrate the general utility of this methodology in interaction studies involving small-molecule kinase inhibitors, a tyrosine-phosphorylated peptide and an antibody as affinity ligands.