A comparative study of the membrane permeability properties of amphibian and cephalopod mollusc lenses

Lens crystallins of invertebrates--diversity and recruitment from detoxification enzymes and novel proteins

The major proteins (crystallins) of the transparent, refractive eye lens of vertebrates are a surprisingly diverse group of multifunctional proteins. A number of lens crystallins display taxon-specificity. In general, vertebrate crystallins have been recruited from stress-protective proteins (i.e. the small heat-shock proteins) and a number of metabolic enzymes by a gene-sharing mechanism. Despite the existence of refractive lenses in the complex and compound eyes of many invertebrates, relatively little is known about their crystallins. Here we review for the first time the state of knowledge of invertebrate crystallins. The major cephalopod (squid, octopus, and cuttlefish) crystallins (S-crystallins) have, like vertebrate crystallins, been recruited from a stress protective metabolic enzyme, glutathione S-transferase. The presence of overlapping AP-1 and antioxidant responsive-like sequences that appear functional in transfected vertebrate cells suggest that the recruitment of glutathione S-transferase to S-crystallins involved response to oxidative stress. Cephalopods also have at least two taxon-specific crystallins: omega-crystallin, related to aldehyde dehydrogenase, and omega-crystallin, related to a superfamily of lipid-binding proteins. L-crystallin (probably identical to O-crystallin) is the major protein of the lens of the squid photophore, a specialized structure for emitting light. The use of L/omega-crystallin in the ectodermal lens of the eye and the mesodermal lens of the photophore of the squid contrasts with the recruitment of different crystallins in the ectodermal lenses of the eye and photophore of fish. S-and omega-crystallins appear to be lens-specific (some S-crystallins are also expressed in cornea) and, except for one S-crystallin polypeptide (SL11/Lops4; possibly a molecular fossil), lack enzymatic activity. The S-crystallins (except SL11/Lops4) contain a variable peptide that has been inserted by exon shuffling. The only other invertebrate crystallins that have been examined are in one marine gastropod (Aplysia, a sea hare), in jellyfish and in the compound eyes of some arthropods; all are different and novel proteins. Drosocrystallin is one of three calcium binding taxon-specific crystallins found selectively in the acellular corneal lens of Drosophila, while antigen 3G6 is a highly conserved protein present in the ommatidial crystallin cone and central nervous system of numerous arthropods. Cubomedusan jellyfish have three novel crystallin families (the J-crystallins); the J1-crystallins are encoded in three very similar intronless genes with markedly different 5' flanking sequences despite their almost identical encoded proteins and high lens expression. The numerous refractive structures that have evolved in the eyes of invertebrates contrast markedly with the limited information on their protein composition, making this field as exciting as it is underdeveloped. The similar requirement of Pax-6 (and possibly other common transcription factors) for eye development as well as the diversity, taxon-specificity and recruitment of stress-protective enzymes as crystallins suggest that borrowing multifunctional proteins for refraction by a gene sharing strategy may have occurred in invertebrates as did in vertebrates.

Ion concentrations, fluxes and electrical properties of the embryonic chicken lens

The membrane properties of embryonic chicken lenses were characterized using isotopic and electrical techniques. The lenses had a relatively high water content (80%) and large extracellular space (12.5%). Isotopic uptake measurements indicated that the lens cytoplasm contained 118 mM K+ and 26 mM Cl-. A value for intracellular Na+ of 14 mM was obtained using Na(+)-sensitive microelectrodes. A double-exponential model was used to fit the efflux of 86Rb+, 22Na+, 36Cl- and [3H]mannitol (an extracellular space marker) from the lens. When perfused with artificial aqueous humor (AAH) solution, embryonic lenses exhibited membrane potentials of between -20 and -40 mV. The more negative values were generally observed in lenses from older embryos. A ouabain-sensitive component, contributing -7 mV to the membrane potential, was also identified. The relatively depolarized membrane potentials suggested that the lens membranes were only weakly selective for K+ over Na+. To test this further, lenses were perfused with AAH containing varying concentrations of K+. The resulting changes in potential were interpreted in terms of the Goldman model. The best fit of the Goldman potential equation indicated that, in the presence of ouabain, the chicken lens membranes had a relative permeability to K+, Na+ and Cl- of 1.0, 0.36, 0.51 respectively. Replacing most or all of the Na+ in the AAH caused only a small change in the membrane potential rather than the large hyperpolarization towards the K+ equilibrium potential predicted by the Goldman model. Including the K+ ionophore valinomycin in the low Na(+)-AAH solutions caused a large increase in 86Rb+ efflux but did not result in additional hyperpolarization. This suggested that the insensitivity of the membrane potential to reduced extracellular Na+ was not due to voltage or pH inactivation of lens K+ channels.

The ultrastructure of the lens of the cephalopod Sepiola: a scanning electron microscopic study

The ultrastructure of the eye lens of Sepiola atlantica was investigated using scanning electron microscopy. The main lens elements in both the anterior and posterior half of the Sepiola lens are plate-like configurations with fiber-like extensions at their margins. Anteriorly the plates are plano-convex, posteriorly subspherical. The central, primordial, posterior plates are spherical with no marginal extensions. The plates are mutually anchored by protrusions and invaginations and by push-button attachments. The posterior and anterior halves are separated by a septum which consists of concentric zones of radially orientated elongated cells. The marginal extensions of the plates and the septal elements are closely associated. The unique structure of the septum makes it a good candidate for the high resistance barrier between the posterior and anterior halves of the Sepiola lens (Jacob and Duncan, 1981).