A role for γS‐crystallin in the organization of actin and fiber cell maturation in the mouse lens

Abstract

γS-crystallin (γS) is a highly conserved component of the eye lens. To gain insights into the functional role(s) of this protein, the mouse gene (Crygs) was deleted. Although mutations in γS can cause severe cataracts, loss of function of γS in knockout (KO) mice produced no obvious lens opacity, but was associated with focusing defects. Electron microscopy showed no major differences in lens cell organization, suggesting that the optical defects are primarily cytoplasmic in origin. KO lenses were also grossly normal by light microscopy but showed evidence of incomplete clearance of cellular organelles in maturing fiber cells. Phalloidin labeling showed an unusual distribution of F-actin in a band of mature fiber cells in KO lenses, suggesting a defect in the organization or processing of the actin cytoskeleton. Indeed, in wild-type lenses, γS and F-actin colocalize along the fiber cell plasma membrane. Relative levels of F-actin and G-actin in wild-type and KO lenses were estimated from fluorescent staining profiles and from isolation of actin fractions from whole lenses. Both methods showed a twofold reduction in the F-actin/G-actin ratio in KO lenses, whereas no difference in tubulin organization was detected. In vitro experiments showed that recombinant mouse γS can directly stabilize F-actin. This suggests that γS may have a functional role related to actin, perhaps in ‘shepherding’ filaments to maintain the optical properties of the lens cytoplasm and normal fiber cell maturation.

Functions of crystallins in and out of lens: Roles in elongated and post-mitotic cells

“…Trefoil domains are widespread and may have many different functions, but some are known to be important in organization of actin filaments (Jansen et al, 2011). Recently, examination of a knockout mouse model for γS-crystallin has revealed a role for this highly conserved protein in the organization of actin filaments (Fan et al, 2012) (Figure 1), suggesting that the protein and its relatives can act to stabilize and prevent aggregation of large macromolecular complexes such as cytoskeletal filaments. In the lens this would help prevent the formation of light scattering centers.…”

“…It could also help maintain normal trafficking networks that rely on such structures. Indeed, clearance of degraded organelles in the maturing lens fiber cells is also defective in the γS KO mouse (Fan et al, 2012). Such a role could be useful in retinal neurons, particularly in axons.…”

“…Extensive cell-cell contacts are formed through intercalation of complex junctions. (For methods, see (Fan et al, 2012))…”

“…D: Co-localization of γS-crystallin and F-actin along the plasma membranes of mature mouse fiber cells (adapted from (Fan et al, 2012)).…”

“…This might indicate another chaperone-like role, distinct from that of the sHSPs, in preventing formation of aggregates through “shepherding” other protein complexes (Fan et al, 2012). …”