MALDI tissue profiling of integral membrane proteins from ocular tissues

MALDI tissue profiling and imaging have become valuable tools for rapid, direct analysis of tissues to investigate spatial distributions of proteins, potentially leading to an enhanced understanding of the molecular basis of disease. Sample preparation methods developed to date for these techniques produce protein expression profiles from predominantly hydrophilic, soluble proteins. The ability to obtain information about the spatial distribution of integral membrane proteins is critical to more fully understand their role in physiological processes, including transport, adhesion, and signaling. In this article, a sample preparation method for direct tissue profiling of integral membrane proteins is presented. Spatially resolved profiles for the abundant lens membrane proteins aquaporin 0 (AQP0) and MP20, and the retinal membrane protein opsin, were obtained using this method. MALDI tissue profiling results were validated by analysis of dissected tissue prepared by traditional membrane protein processing methods. Furthermore, direct tissue profiling of lens membrane proteins revealed age related post-translational modifications, as well as a novel modification that had not been detected using conventional tissue homogenization methods.