LC–MS/MS identification of doublecortin as abundant beta cell-selective protein discharged by damaged beta cells in vitro

There is a clinical need for plasma tests that can directly detect injury to pancreatic beta cells in type 1 diabetes. Such tests require biomarkers that are abundantly and selectively released into plasma by damaged beta cells. We combined LC–MS/MS proteomics and tissue-comparative transcriptomics of FACS-purified beta cells for bottom-up identification of candidate markers. Less than 10% of 467 proteins detected in beta cells showed endocrine-enriched expression. One surprising candidate was the neuronal migration marker doublecortin: in situ analysis revealed uniform doublecortin expression in the cytoplasm of all beta cells. Western blotting and real-time PCR confirmed its strong beta cell-selectivity outside the brain and its high molar abundance, indicating promising biomarker properties in comparison to GAD65, a more established marker of beta cell injury. DCX potential was validated in vitro: chemically-induced necrosis of rat and human beta cells led to a discharge of intracellular doublecortin into the extracellular space, proportionate to the amount of injured cells, and similar to GAD65. In vivo, recombinant DCX showed favorable pharmacokinetic properties, with a half-life in plasma of around 3 h. Combined, our findings provide first proof-of-principle for doublecortin as biomarker for beta cell injury in vitro, advocating its further validation as biomarker in vivo.

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► There is a need for in vitro or in vivo markers of pancreatic beta cell injury.
► Proteomics and transcriptomics indicated doublecortin (DCX) as promising candidate.
► In situ analysis confirmed DCX as an abundant highly beta cell-selective protein.
► Human and rat beta cells in culture discharge DCX proportionate to degree of injury.