Deficient chloride transport through cystic fibrosis (CF) transmembrane conductance regulator (CFTR) causes lethal complications in CF patients. CF is the most common autosomal recessive genetic disease, which is caused by mutations in the CFTR gene; thus, CFTR mutants can serve as primary targets for drugs to modulate and rescue the ion channel’s function. The first step of drug modulation is to increase the expression of CFTR in the apical plasma membrane (PM); thus, accurate measurement of CFTR in the PM is desired. This work reports a tandem enrichment strategy to prepare PM CFTR and uses a stable isotope labeled CFTR sample as the quantitation reference to measure the absolute amount of apical PM expression of CFTR in CFBE 41o- cells. It was found that CFBE 41o- cells expressing wild-type CFTR (wtCFTR), when cultured on plates, had 2.9 ng of the protein in the apical PM per million cells; this represented 10% of the total CFTR found in the cells. When these cells were polarized on filters, the apical PM expression of CFTR increased to 14%. Turnover of CFTR in the apical PM of baby hamster kidney cells overexpressing wtCFTR (BHK-wtCFTR) was also quantified by targeted proteomics based on multiple reaction monitoring mass spectrometry; wtCFTR had a half-life of 29.0 ± 2.5 h in the apical PM. This represents the first direct measurement of CFTR turnover using stable isotopes. The absolute quantitation and turnover measurements of CFTR in the apical PM can significantly facilitate understanding the disease mechanism of CF and thus the development of new disease-modifying drugs. Absolute CFTR quantitation allows for direct result comparisons among analyses, analysts, and laboratories and will greatly amplify the overall outcome of CF research and therapy.

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A targeted mass spectrometry-based method is presented that adopts the fast photochemical oxidation of proteins (FPOP) for footprinting of cystic fibrosis transmembrane conductance regulator (CFTR) membrane transporter at its original plasma membrane location. Two analytical imperatives were sought: (1) overall simplification in data acquisition and analysis and (2) lower quantitation limits, which enabled direct analysis of intrinsically low-abundance transmembrane proteins. These goals were achieved by using a reversed-footprinting technique that monitored the unoxidized peptides remaining after the FPOP treatment. In searching for structurally informative peptides, a workflow was designed for accurate and precise quantitation of CFTR peptides produced from proteolytically digesting the plasma membrane subproteome of cells. This sample preparation strategy mitigated the need for challenging purification of large quantities of structurally intact CFTR. On the basis of the interrogated peptides, it was proposed a concept of the structural marker peptide that could report CFTR structure and function in cells. The reversed-footprinting mass spectrometry extends the FPOP technology to study conformation and interaction changes of low-abundance proteins directly in their endogenous cellular locations.

Proteomics, the large-scale study of protein expression in cells and tissues, is a powerful tool to study the biology of clinical conditions and has provided significant insights in many experimental systems. Herein, we review the basics of proteomic methodology and discuss challenges in using proteomic approaches to study autism. Unlike other experimental approaches, such as genomic approaches, there have been few large-scale studies of proteins in tissues from persons with autism. Most of the proteomic studies on autism used blood or other peripheral tissues; few studies used brain tissue. Some studies found dysregulation of aspects of the immune system or of aspects of lipid metabolism, but no consistent findings were noted. Based on the challenges in using proteomics to study autism, we discuss considerations for future studies. Apart from the complex technical considerations implicit in any proteomic analysis, key nontechnical matters include attention to subject and specimen inclusion/exclusion criteria, having adequate sample size to ensure appropriate powering of the study, attention to the state of specimens prior to proteomic analysis, and the use of a replicate set of specimens, when possible. We conclude by discussing some potentially productive uses of proteomics, potentially coupled with other approaches, for future autism research including: (1) proteomic analysis of banked human brain specimens; (2) proteomic analysis of tissues from animal models of autism; and (3) proteomic analysis of induced pluripotent stem cells that are differentiated into various types of brain cells and neural organoids. Autism Res 2017, 10: 1460-1469. © 2017 International Society for Autism Research, Wiley Periodicals, Inc.