Lawrence Brako

Aquaporin-0 targets interlocking domains to control the integrity and transparency of the eye lens.

PURPOSE Lens fiber cell membranes contain aquaporin-0 (AQP0), which constitutes approximately 50% of the total fiber cell membrane proteins and has a dual function as a water channel protein and an adhesion molecule. Fiber cell membranes also develop an elaborate interlocking system that is required for maintaining structural order, stability, and lens transparency. Herein, we used an AQP0-deficient mouse model to investigate an unconventional adhesion role of AQP0 in maintaining a normal structure of lens interlocking protrusions. METHODS The loss of AQP0 in AQP0(-/-) lens fibers was verified by Western blot and immunofluorescence analyses. Changes in membrane surface structures of wild-type and AQP0(-/-) lenses at age 3 to 12 weeks were examined with scanning electron microscopy. Preferential distribution of AQP0 in wild-type fiber cell membranes was analyzed with immunofluorescence and immunogold labeling using freeze-fracturing transmission electron microscopy. RESULTS Interlocking protrusions in young differentiating fiber cells developed normally but showed minor abnormalities at approximately 50 μm deep in the absence of AQP0 in all ages studied. Strikingly, protrusions in maturing fiber cells specifically underwent uncontrolled elongation, deformation, and fragmentation, while cells still retained their overall shape. Later in the process, these changes eventually resulted in fiber cell separation, breakdown, and cataract formation in the lens core. Immunolabeling at the light microscopy and transmission electron microscopy levels demonstrated that AQP0 was particularly enriched in interlocking protrusions in wild-type lenses. CONCLUSIONS This study suggests that AQP0 exerts its primary adhesion or suppression role specifically to maintain the normal structure of interlocking protrusions that is critical to the integrity and transparency of the lens.

A developmental defect in astrocytes inhibits programmed regression of the hyaloid vasculature in the mammalian eye

Previously we reported the novel observation that astrocytes ensheath the persistent hyaloid artery, both in the Nuc1 spontaneous mutant rat, and in human PFV (persistent fetal vasculature) disease (Developmental Dynamics 234:36-47, 2005). We now show that astrocytes isolated from both the optic nerve and retina of Nuc1 rats migrate faster than wild type astrocytes. Aquaporin 4 (AQP4), the major water channel in astrocytes, has been shown to be important in astrocyte migration. We demonstrate that AQP4 expression is elevated in the astrocytes in PFV conditions, and we hypothesize that this causes the cells to migrate abnormally into the vitreous where they ensheath the hyaloid artery. This abnormal association of astrocytes with the hyaloid artery may impede the normal macrophage-mediated remodeling and regression of the hyaloid system.

A Correlative Immunoconfocal and Electron Microscopic Study of Gap Junctions in Interlocking Domains of the Lens

Ball-and-sockets and protrusions are specialized interlocking membrane domains between fiber cells of the avascular lenses in all species studied [1, 2]. Although both domains are similar in their shape, surface morphology and are highly enriched with aquaporin-0 water channel protein, our previous studies reveal that ball-and-sockets and protrusions possess important structural and functional differences during fiber cell differentiation and maturation [2, 3]. Specifically, gap junctions are regularly associated with all ball-and-sockets examined in metabolically active young cortical fibers, but not with protrusions. Also, while many ball-and-sockets are distributed primarily on the broad and narrow surfaces of young hexagonal fiber cells, numerous protrusions are located along the angles of the cells throughout the entire lens. It was proposed that the unique ball-and-socket-associated gap junctions may significantly facilitate cell-to-cell communication between young cortical fiber cells since they often protrude deeply into neighboring cells to increase membrane surface areas [2]. In contrast, protrusions are shown to play important interlocking role in maintaining fiber-to-fiber stability during visual accommodation [3].