title: Identification and Functional Assessment of Age-Dependent Truncations to Cx46 and Cx50 in the Human Lens.
abstract: OBJECTIVE
  Many proteins in the lens undergo extensive posttranslational modifications (PTMs) with age, leading to alterations in their function. The extent to which lens gap junction proteins, Cx46 and Cx50, accumulate PTMs with aging is not known. In this study, we identified truncations in Cx46 and Cx50 in the human lens using mass spectrometry. We also examined the effect of truncations on channel function using electrophysiological measurements.
  
  
  METHODS
  Human lenses were dissected into cortex, outer nucleus, and nucleus regions, and fiber cell membranes were subjected to trypsin digestion. Tryptic peptides were analyzed by liquid chromatography (LC)-electrospray tandem mass spectrometry (ESI/MS/MS). Effects of truncations on channel conductance, permeability, and gating were assessed in transfected cells.
  
  
  RESULTS
  Cleavage sites were identified in the C-terminus, the cytoplasmic loop, and the N-terminus of Cx46 and Cx50. Levels of C-terminal truncations, which were found at residues 238 to 251 in Cx46 and at residues 238 to 253 and 274 to 284 in Cx50, were similar in different lens regions. In contrast, levels of truncations in cytoplasmic loop and N-terminal domains of Cx46 and Cx50 increased dramatically from outer cortex to nucleus. Most of the C-terminally truncated proteins were functional, whereas truncations in the cytoplasmic loop did not result in the formation of functional channels.
  
  
  CONCLUSIONS
  Accumulation of cytoplasmic loop and N-terminal truncations in the core might lead to decreases in coupling with age. This reduction is expected to lead to an increase in intracellular calcium and a decrease in levels of glutathione in the nucleus. These changes may ultimately lead to age-related nuclear cataracts.
authors: Department of Biological and Vision Sciences and the Graduate Center for Vision Research, SUNY College of Optometry, New York, New York, United States.
authors: Nefeli
authors: N
authors: Slavi
authors: Department of Biochemistry and Mass Spectrometry Research Center Vanderbilt University School of Medicine, Nashville, Tennessee, United States.
authors: Zhen
authors: Z
authors: Wang
authors: Department of Biological and Vision Sciences and the Graduate Center for Vision Research, SUNY College of Optometry, New York, New York, United States.
authors: Lucas
authors: L
authors: Harvey
authors: Department of Biochemistry and Mass Spectrometry Research Center Vanderbilt University School of Medicine, Nashville, Tennessee, United States.
authors: Kevin L
authors: KL
authors: Schey
authors: Department of Biological and Vision Sciences and the Graduate Center for Vision Research, SUNY College of Optometry, New York, New York, United States.
authors: Miduturu
authors: M
authors: Srinivas
mesh_terms: D000375:Aging / Q000378:metabolism*; D002386:Cataract / Q000175:diagnosis / Q000378:metabolism*; D002463:Cell Membrane Permeability; D002478:Cells, Cultured; D017630:Connexins / Q000378:metabolism*; D006801:Humans; D007908:Lens, Crystalline / Q000378:metabolism* / Q000473:pathology; D008562:Membrane Glycoproteins; D008875:Middle Aged; D018408:Patch-Clamp Techniques; D053719:Tandem Mass Spectrometry; D000099264:Connexin 50
chemical_list: D017630:Connexins; D008562:Membrane Glycoproteins; D000099264:Connexin 50; C082917:GJA3 protein, human