Biologically Produced Optical Materials

The Voigt lab has been studying the S-Crystallin proteins responsible for GRadient INdex (GRIN) lens behavior in cephalopods, with the aim of adapting these proteins for use in optical materials or coatings. Transcriptome mining was leveraged against a Doryteuthis pealeii squid lens tissue RNA-seq dataset; the resulting S-Crystallin transcripts were truncated and showed evidence of RNA base-editing scrambling, frustrating further attempts to assemble viable coding sequences. Previously published S-Crystallin sequences from the closely related species Doryteuthis opalescenswere instead selected for further study, cloned and expressed in Escherichia coli, and purified for further materials characterization. We have also purchased and installed a 4 L benchtop bioreactor, optimized its operation via initial runs, and achieved 10x higher optical culture density than traditional shake flask culture. Preliminary computational studies have resolved predicted structures for several squid S-Crystallins, which align closely to a previously published Octopus vulgaris S-Crystallin.

GRadient INdex (GRIN) lenses are a class of optical lenses which focus light on a curved path through a medium with increasing density. These lenses have attractive optical properties including a robustness to spherical aberrations which mar image quality in standard single refractive index lenses. Traditional abiotic manufacturing techniques for GRIN lenses are expensive, rely on toxic production methods including vapor deposition and ion exchange, and can take up to months to produce. GRIN behavior is found in many animal lenses, but the strongest GRIN activity—denoted by the highest change in refractive index from the periphery to the core of the lens—is found in cephalopods. Previously the S-Crystallins which comprise these cephalopod lenses have been widely studied for their implication in cataracts and other developmental biology disorders. A recent study has confirmed the molecular and physical basis of GRIN behavior in squid lenses and suggests these lenses could be refactored in vitro and the GRIN behavior thereof exploited. S-Crystallins evolved from a highly conserved Glutathione S-Transferase (GST) housekeeping protein; the cephalopod transcriptome contains dozens of S-Crystallin sequences which differ by the length of a ‘spacer loop’ sequence, which determines hydrodynamic radius of the protein and the density of its packing in a gel. As gel packing density of protein affects refractive index by as much as 0.3, the length of the insert sequence is thought to be at least loosely proportional to the refractive index of a protein solution or material. To this end, GRIN protein sequences of various sizes will need to be mined or found, cloned, and purified in expression hosts. Protein expression should be characterized. Protein production should be scaled up to reach high production yields, as large amounts (grams) of protein will be needed to assess physical characteristics. Finally, protein samples should be characterized at MIT and with collaborators.