Amyloid-like protein structure in mammalian ocular lenses

Abstract

Over the past 30 years, extensive Raman and infrared spectroscopy studies determined that the major proteins in normal mammalian lenses exist predominantly as beta-pleated sheets in vivo. The present study examines the presence of amyloid protein supramolecular order of lens protein beta-pleated sheet arrays. The classic amyloidophilic stains Congo red and thioflavine were used in situ to identify lens regions with amyloid protein structure using brightfield microscopy. Birefringence in Congo red stained lenses was determined using polarizing light microscopy. Thioflavine stained lenses were also examined by fluorescence microscopy to detect a diagnostic fluorescence red-shift indicative of the intercalation of thioflavine into beta-sheet amyloid protein structures. The major findings are 1) Congo red staining begins in the lens interior at an abrupt boundary coinciding with the extensive reorganization of cell biology and physical environment that occurs during normal lens fiber cell differentiation. 2) Apple-green birefringence in Congo red stained lenses co-localizes with amyloidophilic dye affinity. 3) Thioflavine stains the lens interior beginning at the same boundary. 4) A red shift in thioflavine fluorescence was detected in the lens interior that co-localizes with amyloidophilic dye affinities and birefringence in organelle-free lens fibers. The present data demonstrating amyloidophilic dye binding, in situ red shift in thioflavine fluorescence, and apple-green birefringence provide evidence that lens protein beta-sheet arrays are organized in an amyloid-like supramolecular order in interior fiber cells of mammalian ocular lenses. The inherent stability of amyloid-like protein structure may contribute to the long-term structural integrity and transparency of the lens.