MP26 in the bovine lens: a post‐embedding immunocytochemical study

Abstract

Gold immunolabeling of bovine lens tissue embedded in Lowicryl K4M, using a polyclonal antibody specific for a major component of lens fiber plasma membrane of 26 K molecular weight, shows that this constituent is absent from the epithelial cell plasma membrane and associated only with the junctional and non-junctional domains of the lens fiber plasma membrane.

Expression of the gene for main intrinsic polypeptide (MIP): separate spatial distributions of MIP and beta-crystallin gene transcripts in rat lens development.

“…Alternately, this punctate staining could reflect regions of higher protein concentration. The distribution of antibody binding sites is thus most compatible with the view that MIP is present in both the nonjunctional and the junctional membrane, as proposed from immunocytochemical studies by electron microscopy of intact lenses (Fitzgerald et al, 1983;Vallon et al, 1985) and of isolated junctions (Bok et al, I982; Sas et al., 1985). Interestingly, Paul and Goodenough (1983a) also found that by immunofluorescence MIP appeared to be present throughout the fiber cell membrane, junctional and nonjunctional, but failed to detect it in junctions in the isolated membranes.…”

“…The cells of the anterior epithelium appear to be devoid of either MIP or 13-A1/A3 transcripts at all time points studied. We, as others (Waggoner and Maisel, 1978;Vermorken et al, 1977, Broekhuyse et al, 1979Paul and Goodenough, 1983a;Vallon et al, 1985), also found no suggestion that MIP is present in the membranes of the epithelial cells. This lack of immunostaining of epithelial cell membranes is particularly striking at the early stages of lens morphogenesis before the cells of the posterior wall fill the lumen of the lens vesicle and make solid contact with the anterior epithelial cells.…”

“…The cells of the lens epithelium follow a single pathway of differentiation, from the earliest stages of lens development and throughout life, maturing to become fiber cells which form the mass of the lens. This differentiation of epithelium to fiber, both during embryonic development and in the mature lens, is characterized by the appearance and subsequent massive accumulation of the cytoplasmic crystallins (tz, 13, and ~, in mammalians [Harding and Crabbe, 1984]) and of an integral membrane protein known as the main intrinsic polypeptide (MIP) ~ (Alcala et al, 1975, Broekhuyse et al, 1976Waggoner and Maisel, 1978;Bloemendal, 1979;Vallon et al, 1985). The specific function of MIP in the lens fiber cell membrane is still an unsolved problem.…”

Channel reconstitution in liposomes and planar bilayers with HPLC-purified MIP26 of bovine lens

“…Furthermore, only thin (13-nm)junctions showed antibody binding, while junctions of normal thickness (17 nm) did not. In contrast, with monoclonal antibodies Fitzgerald, Bok and Horwitz (1982), Sas et al (1985) and Vallon et al (1985) localized MIP26 at both types of junctions.…”

Cellular distribution of the aquaporins: A family of water channel proteins

“…It has also been located in plasma membranes of virtually all nonfenestrated capillary endothelial cells, but is not found in fenestrated capillaries (Nielsen et al 1993b;Sabolic et al 1992). Lens and corneal epithelium, but not lens fibers, which contain MIP26 (Dunia et al 1987;Vallon et al 1985), are also rich in AQP1 (Nielsen et al 1993b;Stamer et al 1994;Verbavatz et al 1994). Recently, several cell types in the inner ear have also been shown to express high levels of AQP1 (Stankovic et al 1995).…”