Project Grant R21EY036127

QUANTITATIVE PROTEOME OF THE PHOTORECEPTOR OUTER-INNER SEGMENT JUNCTION - RETINAL PHOTORECEPTORS ARE SPECIALIZED NEURONS RESPONSIBLE FOR DETECTION AND PRIMARY PROCESSING OF THE INFORMATION ENTERING THE EYE IN THE FORM OF LIGHT. THESE FIRST STEPS OF VISION TAKE PLACE IN THE PHOTORECEPTOR OUTER SEGMENT, WHICH CONTAINS PROTEINS PERFORMING VISUAL SIGNAL TRANSDUCTION. IT IS CONNECTED TO THE CELL SOMA (OR THE INNER SEGMENT) THROUGH THE CONNECTING CILIUM, WHICH SERVES AS A TRAFFICKING ROUTE FOR THE OUTER SEGMENT PROTEINS SYNTHESIZED IN THE INNER SEGMENT. THE CELLULAR STRUCTURES IMMEDIATELY ADJACENT TO THE CONNECTING CILIUM ARE ALSO ENGAGED IN PERFORMING CRITICAL PHOTORECEPTOR FUNCTIONS. AT THE DISTAL END OF THE CONNECTING CILIUM IS THE SITE OF PHOTORECEPTOR DISC MORPHOGENESIS WHERE SEVERAL DOZEN NEW DISCS ARE FORMED ON A DAILY BASIS TO SUSTAIN THE EVER-GOING PROCESS OF OUTER SEGMENT RENEWAL. AT THE PROXIMAL END OF THE CONNECTING CILIUM, THERE ARE STRUCTURES FORMING A SORTING GATE THAT REGULATES THE OUTER SEGMENT ENTRY AND EXCLUSION OF MEMBRANE PROTEINS. DEFECTS IN ANY OF THESE PROCESSES HAVE BEEN IMPLICATED IN A WIDE RANGE OF PHOTORECEPTOR DEGENERATIVE DISEASES. DESPITE MANY PROTEINS ALREADY BEING MAPPED TO THE CONNECTING CILIUM AND OTHER PARTS OF THE OUTER-INNER SEGMENT JUNCTION USING CONVENTIONAL BIOCHEMICAL AND GENETIC APPROACHES, OUR KNOWLEDGE OF THE PROTEIN COMPOSITION OF THIS CELLULAR REGION REMAINS FAR FROM COMPLETE, WHICH HINDERS THE PROGRESS IN ELUCIDATING THE MOLECULAR MECHANISMS OF PROTEIN TRAFFICKING, SORTING, OUTER SEGMENT MAINTENANCE AND DISC MORPHOGENESIS. THE GOAL OF THIS PROPOSAL IS TO REDUCE THIS KNOWLEDGE GAP BY CHARACTERIZING THE UNIQUE PROTEOME OF THE PHOTORECEPTOR OUTER-INNER SEGMENT JUNCTION. IN AIM 1, WE WILL EMPLOY A CUTTING-EDGE APPLICATION OF QUANTITATIVE PROTEOMICS, CALLED PROTEIN CORRELATION PROFILING. UNLIKE TRADITIONAL BIOCHEMICAL TECHNIQUES, THIS METHODOLOGY ALLOWS ANALYZING THE UNIQUE PROTEIN COMPOSITION OF SUBCELLULAR STRUCTURES THAT CAN BE ENRICHED BUT NOT PURIFIED, AS IN THE CASE OF THE CONNECTING CILIUM AND ITS ADJACENT REGIONS. WE WILL OBTAIN THIN SERIAL TANGENTIAL SECTIONS THROUGH THE OUTER-INNER SEGMENT REGION OF A FROZEN FLAT-MOUNTED RETINA AND CHARACTERIZE THE PROTEIN COMPOSITION OF THESE SECTIONS USING LABEL-FREE QUANTITATIVE MASS SPECTROMETRY. THE RESULTING PROTEIN DISTRIBUTION PROFILES WILL BE COMPARED TO THOSE OF WELL-CHARACTERIZED CONNECTING CILIUM MARKERS. PROTEINS DISTRIBUTED SIMILARLY TO THESE MARKERS WILL BE CONSIDERED AS CANDIDATE UNIQUE COMPONENTS OF THE OUTER- INNER SEGMENT JUNCTION, AND THEIR LOCALIZATION WILL BE VERIFIED USING IMMUNOLOCALIZATION TECHNIQUES. IN AIM 2, WE WILL EMPLOY A NOVEL, HIGHLY ACCURATE APPROACH OF MULTIPLEX PROTEIN QUANTIFICATION TO SIMULTANEOUSLY DETERMINE ABSOLUTE AMOUNTS OF LARGE GROUPS OF PREVIOUSLY KNOWN AND NEWLY IDENTIFIED PROTEINS RESIDING AT THIS CELLULAR LOCATION. THIS ANALYSIS WILL SUGGEST WHICH PROTEINS MAY EXIST WITHIN MULTI-SUBUNIT STOICHIOMETRIC COMPLEXES AND/OR SPECIFIC SUBCELLULAR STRUCTURES, THEREBY FACILITATING UNCOVERING THEIR FUNCTIONAL ROLES. THE INFORMATION GENERATED IN THIS RESOURCE- AND HYPOTHESIS-GENERATING STUDY WILL UNDOUBTEDLY FACILITATE THE EFFORTS OF MANY LABORATORIES ADDRESSING A BROAD ARRAY QUESTIONS RELATED TO PHOTORECEPTOR BIOLOGY AND DISEASE.