Micro-domains of AQPO in lens equatorial fibers

To understand why the water channel aquaporin-0 (AQPO) replaces aquaporin-1(AQP1) during lens development, we studied its spatial arrangement and interactions with proteins in the plasma membrane of equatorial fibers. We used freeze-fracture-labelling; a method that can identify the individual intramembrane particle representing the AQPO channel. We found that AQPO was arranged in micro-domains that extended along the long axis of the equatorial fiber cell. One micro-domain consisted of AQPO channels intermingled with the normal complement of integral proteins of the fiber plasma membrane. We found that the density of AQPO channels varied along the long axis of the fiber. At the apical end of the fiber, the density was barely above background noise (approximately 50 mum(-2)). It increased first to 345 = 109 mum(-2) and then to 719 +/- 35 mum-2 in the region of the plasma membrane facing adjacent fibers (the lateral surface). Another micro-domain, located at the apical end of the fiber, was composed of AQPO channels within gap junctions ('mixed' junctions). This micro-domain contained approximately 1.5 x 10(5) cell-to-cell channels and approximately 3500 AQPO channels. A third micro-domain, located exclusively in the lateral surface of the fiber, was composed of clusters of channels abutted against an opposing, particle-free plasma membrane (AQPO junction). In equatorial fibers, the intramembrane particles in the AQPO junctions were densely packed (6747 +/- 1007 mum(-2)), but were not arranged In orthogonal arrays that are characteristic of equaporins. This micro-domain occupied 20-25% of the lateral surface of equatorial fibers and, more importantly, it was arranged in 'ribbons' that extended for long stretches (30-40 mum) along the apical-basal axis. We concluded that the ability of AQPO to arrange itself in micro-domains conferred functional properties that might contribute to the maintenance of lens transparency and homeostasis. (C) 2002 Elsevier Science Ltd.