Biomedical subjects

B Rachelle Merriman-Smith

Publications and source records attributed to B Rachelle Merriman-Smith.

4 recordsLinked to original sources

PubMed2004

Functional imaging: new views on lens structure and function.

1. We have developed an experimental imaging approach that allows the distribution of lens membrane proteins to be mapped with subcellular resolution over large distances as a function of fibre cell differentiation. 2. Using this approach in the rat lens, we have localized precisely histological sites of connexin 46 cleavage, quantitatively mapped changes in gap junction distribution and fibre cell morphology and correlated these changes to differences in intercellular dye transfer. 3. Profiling of glucose transporter isoform expression showed that lens epithelial cells express GLUT1, whereas deeper cortical fibre cells express the higher-affinity GLUT3 isoform. Near the lens periphery, GLUT3 was located in the cytoplasm of fibre cells, but it underwent a differentiation-dependent membrane insertion. 4. Similarly, the putative adhesion protein membrane protein 20 is inserted into fibre cell membranes at the stage when the cells lose their nuclei. This redistribution is strikingly rapid in terms of fibre cell differentiation and correlates with a barrier to extracellular diffusion. 5. Our imaging-orientated approach has facilitated new insights into the relationships between fibre cell differentiation and lens function. Taken together, our results indicate that a number of strategies are used by the lens during the course of normal differentiation to change the subcellular distribution, gross spatial location and functional properties of key membrane transport proteins.

Animals↗

PubMed2004

Cl- influx into rat cortical lens fiber cells is mediated by a Cl- conductance that is not ClC-2 or -3.

PURPOSE: Exposure of organ-cultured lenses to Cl(-) channel blockers under isotonic conditions induces a localized cortical zone of extracellular space dilations. The purpose of this study was to investigate whether elongated lens fiber cells from this zone contain an anion conductance that mediates Cl(-) influx and whether two chloride channel isoforms known to be expressed in the lens (ClC-2 and -3) are responsible. METHODS: Fiber cells were isolated by enzymatic dissociation in the presence of Gd(3+) and Co(2+) and their electrical properties analyzed by whole-cell patch clamping. Cells from the zone of extracellular space dilations were selected for analysis on the basis of cell length. RT-PCR and immunocytochemistry were used to determine whether ClC-2 or -3 channel isoforms are expressed in fiber cells located in the zone of extracellular space dilations. RESULTS: Cells from the zone of extracellular space dilations were typically >120 microm in length and exhibited an outwardly rectifying Cl(-) conductance that was blocked by DIDS (4,4'-diisothiocyanostilbene-2,2'-disulfonic acid) and displayed an anion selectivity sequence of I(-) > Cl(-) >> gluconate. ClC-2 and -3 were found to be expressed at the transcript and protein level in lens fiber cells, but subsequent immunocytochemical studies indicated that expressed proteins did not colocalize with cell membranes in the zone of extracellular space dilations, being predominately cytoplasmic in nature. CONCLUSIONS: Taken together, the data indicate that extracellular space dilations are due to the inhibition of a Cl(-) channel(s) that normally mediates Cl(-) influx into cortical lens fiber cells under isotonic conditions. The molecular identity of this channel remains to be determined.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗

PubMed2003

Expression patterns for glucose transporters GLUT1 and GLUT3 in the normal rat lens and in models of diabetic cataract.

PURPOSE: To determine whether the expression levels and cellular distribution of the facilitative glucose transporters GLUT1 and -3 undergo changes in the hyperglycemic lens. METHODS: Hyperglycemia was induced in vivo by injecting rats with streptozotocin or in vitro by culturing lenses in the presence of 50 mM glucose. Northern blot analysis and quantitative RT-PCR were used to detect changes in GLUT1 and -3 transcript levels, and Western blot analysis was used to monitor changes in GLUT3 protein expression levels in diabetic rats. Immunocytochemistry was used to map the cellular distribution of GLUT3 in normal and hyperglycemic lenses. RESULTS: GLUT1 and -3 were found to be differentially expressed in the epithelial and fiber cells, respectively. In the fiber cells, the distribution of GLUT3 protein changed as a function of fiber cell differentiation. In young differentiating fiber cells, GLUT3 was mainly found in the cytoplasm, but with increasing depth into the lens became inserted into the narrow sides of older fiber cells, before becoming completely dispersed around the entire membrane of the oldest fiber cells. Hyperglycemia had similar effects on tissue damage and transporter expression in both the in vitro and in vivo models. Tissue damage was characterized by an initial local cell swelling that with prolonged insult gradually spread and resulted in the creation of large areas of tissue liquefaction. Northern blot analysis and quantitative RT-PCR showed that transcript for GLUT3 but not GLUT1 was upregulated under hyperglycemic conditions. This increase in GLUT3 expression was confirmed at the protein level by both Western blot analysis and immunocytochemistry. In hyperglycemic lenses, GLUT3 antibody labeling was localized to the region of tissue liquefaction. CONCLUSIONS: GLUT3 in the lens exhibits dynamic changes in expression levels and cellular localization as a function of fiber cell differentiation and hyperglycemia. In the lens cortex, regions of GLUT3 overexpression and hyperglycemic tissue damage overlap, suggesting a functional relationship.

Animals↗

PubMed2002

Molecular identification of P-glycoprotein: a role in lens circulation?

PURPOSE: To determine whether P-glycoprotein is expressed in the rat lens and to assess what type of damage occurs when P-glycoprotein inhibitors are applied to organ-cultured lenses. METHODS: An initial screening for the P-glycoprotein isoforms multidrug resistance (mdr)1a, mdr1b, and mdr2 was performed by RT-PCR on RNA extracted from rat lens fiber cells. Northern blot analysis was used to determine whether transcript levels detected by RT-PCR were significant. The presence of P-glycoprotein in the lens was confirmed by Western blot analysis and immunocytochemistry. Organ-cultured lenses, maintained in isotonic artificial aqueous humor, were exposed to various concentrations of the P-glycoprotein inhibitor tamoxifen. Lens opacification was assessed by dark-field microscopy, and the underlying cellular changes were visualized by confocal microscopy of lens sections, using a fluorescent membrane marker. Initial cellular damage was assessed after a 6-hour exposure to 100 micro M tamoxifen. Other P-glycoprotein inhibitors, verapamil, and 1,9-dideoxyforskolin (DDFK) were assessed, and the damage phenotypes were compared with those seen for tamoxifen. RESULTS: Transcript for all three P-glycoprotein isoforms was detected with RT-PCR, but only mdr1a and mdr2 could be detected by Northern blot analysis. P-glycoprotein was localized in the plasma membrane of lens epithelial and fiber cells. Treatment of organ-cultured lenses with increasing doses of the P-glycoprotein inhibitor tamoxifen for 18 hours showed that two distinct damage phenotypes were evident. At a dose of 20 micro M tamoxifen, tissue damage was found in a discrete zone that initially started approximately 100 micro m from the capsule, whereas at higher doses (60-100 micro M tamoxifen), extensive vesiculation of fiber cell membranes occurred throughout the entire lens cortex. Decreasing tamoxifen (100 micro M) exposure to 6 hours showed that the inner zone of damage was caused by the dilation of extracellular space between fiber cells. The extracellular space dilution and fiber cell vesiculation could be reproduced by varying the concentrations of other P-glycoprotein inhibitors, verapamil and DDKF. CONCLUSIONS: The P-glycoproteins mdr1a and mdr2 are expressed in the lens and appear to be functional. The initial cellular damage phenotype of extracellular space dilations caused by the P-glycoprotein inhibitors was identical with that caused by chloride channel inhibitors, indicating that P-glycoprotein may play a role in regulating cell volume in the lens. Whether the secondary damage phenotype of fiber cell vesiculation, induced by high doses of P-glycoprotein inhibitors, was due to the inhibition of additional regulatory activities of P-glycoprotein or to nonspecific effects of the drugs remains to be determined. However, regardless of the precise mode of action, these results indicate that P-glycoprotein should be considered in the regulatory mechanisms associated with the control of lens volume and in the initiation of osmotic cataract.

ATP Binding Cassette Transporter, Subfamily B↗