Human multidrug resistance 3-P-glycoprotein expression in transgenic mice induces lens membrane alterations leading to cataract

Upregulations of Clcn3 and P-Gp Provoked by Lens Osmotic Expansion in Rat Galactosemic Cataract

OBJECTIVE

Lens osmotic expansion, provoked by overactivated aldose reductase (AR), is the most essential event of sugar cataract. Chloride channel 3 (Clcn3) is a volume-sensitive channel, mainly participating in the regulation of cell fundamental volume, and P-glycoprotein (P-gp) acts as its modulator. We aim to study whether P-gp and Clcn3 are involved in lens osmotic expansion of galactosemic cataract.

METHODS AND RESULTS

In vitro, lens epithelial cells (LECs) were primarily cultured in gradient galactose medium (10-60 mM), more and more vacuoles appeared in LEC cytoplasm, and mRNA and protein levels of AR, P-gp, and Clcn3 were synchronously upregulated along with the increase of galactose concentration. In vivo, we focused on the early stage of rat galactosemic cataract, amount of vacuoles arose from equatorial area and scattered to the whole anterior capsule of lenses from the 3rd day to the 9th day, and mRNA and protein levels of P-gp and Clcn3 reached the peak around the 9th or 12th day.

CONCLUSION

Galactosemia caused the osmotic stress in lenses; it also markedly leads to the upregulations of AR, P-gp, and Clcn3 in LECs, together resulting in obvious osmotic expansion in vitro and in vivo.

P-glycoprotein structure and evolutionary homologies

Analysis of multidrug resistant cell lines has led to the identification of the P-glycoprotein multigene family. Two of the three classes of mammalian P-glycoproteins have the ability to confer cellular resistance to a broad range of structurally and functionally diverse cytotoxic agents. P-glycoproteins are integral membrane glycoproteins comprised of two similar halves, each consisting of six membrane spanning domains followed by a cytoplasmic domain which includes a nucleotide binding fold. The P-glycoprotein is a member of a large superfamily of transport proteins which utilize ATP to translocate a wide range of substrates across biological membranes. This superfamily includes transport complexes comprised of multicomponent systems, half P-glycoproteins and P-glycoprotein-like homologs which appear to require approximately 12 alpha-helical transmembrane domains and two nucleotide binding folds for substrate transport. P-glycoprotein homologs have been isolated and characterized from a wide range of species. Amino acid sequences, the similarities between the halves and intron/exon boundaries have been compared to understand the evolutionary origins of the P-glycoprotein.

Up-regulation of P-glycoprotein expression by osmotic stress in rat sugar cataract

P-glycoprotein (P-gp), a plasma membrane protein, is thought to function in the export of cytotoxic drugs and to act as a modulator of chloride channels that regulate cell volume in many cell types. P-gp has been shown to play a role in lens volume regulation and initiation of osmotic cataract. We investigated the lenticular expression levels of P-gp in galactose-fed rats, an experimental model of sugar cataract. P-gp was overexpressed in lenses from galactose-fed rats with cortical sugar cataract, and in rat lens epithelial cells cultured in high-glucose medium. However, application of aldose reductase (AR) inhibitor was able to reverse the changes in P-gp levels in the lenses of galactose-fed rats, confirming the role of AR and involvement of the polyol pathway in cataract formation. Our findings suggest that P-gp may be induced by AR over-expression and/or osmotic stress, thus playing a regulatory role in maintaining lenticular osmotic balance in sugar cataract.

Structural and immunocytochemical alterations in eye lens fiber cells from Cx46 and Cx50 knockout mice

In the current study we describe the changes of overall organization of lens fiber cells in connexin 46 (Cx46) and connexin 50 (Cx50) knockout mice. Morphometric analyses and the application of immunocytochemical techniques revealed that in Cx46 knockout lens (Cx46 -/-), where Cx50 is expressed alone, the postnatal differentiation of secondary fiber cells proceeds faster and is characterized by an increased number of smaller fiber cells. Conversely, in Cx50 knockout mice (Cx50 -/-), the lenticular mass is considerably reduced and characterized by a small number of fiber cells added during the postnatal period. The process of terminal differentiation was impaired and generated larger fiber cells still possessing cytoplasmic organelles. Freeze-fracture and fracture labeling revealed that the junctional assembly, packing organization and topographic interactions between connexons and MP26 differed when Cx46 and Cx50 were co-assembled in the wild-type or expressed separately in the two distinct knockout phenotypes. Filipin cytochemistry provided indirect evidence that Cx46 and Cx50 expressed alone are recruited into different lipid environments. Our results represent the structural proof that interaction of connexins and MP26 contributes to the overall organization of the fiber cells.

Function and pathophysiological importance of ABCB4 (MDR3 P-glycoprotein)

Like several other ATP-binding cassette (ABC) transporters, ABCB4 is a lipid translocator. It translocates phosphatidylcholine (PC) from the inner to the outer leaflet of the canalicular membrane of the hepatocyte. Its function is quite crucial as evidenced by a severe liver disease, progressive familial intrahepatic cholestasis type 3, which develops in persons with ABCB4 deficiency. Translocation of PC makes the phospholipid available for extraction into the canalicular lumen by bile salts. The primary function of biliary phospholipid excretion is to protect the membranes of cells facing the biliary tree against these bile salts: the uptake of PC in bile salt micelles reduces the detergent activity of these micelles. In this review, we will discuss the functional aspects of ABCB4 and the regulation of its expression. Furthermore, we will describe the clinical and biochemical consequences of complete and partial deficiency of ABCB4 function.

Persistent, non‐cytolytic infection of neurons by Borna disease virus interferes with ERK 1/2 signaling and abrogates BDNF‐induced synaptogenesis

Infection of the central nervous system by Borna disease virus (BDV) provides a unique model to study the mechanisms whereby a persistent viral infection can impair neuronal function and cause behavioral diseases reminiscent of mood disorders, schizophrenia, or autism in humans. In the present work, we studied the effect of BDV infection on the response of hippocampal neurons, the main target for this virus, to the neurotrophin BDNF. We showed that persistent infection did not affect neuronal survival or morphology. However, it blocked BDNF-induced ERK 1/2 phosphorylation, despite normal expression of the TrkB BDNF receptor. In addition, BDNF-induced expression of synaptic vesicle proteins was abrogated, which resulted in severely impaired synaptogenesis and defects in synaptic organization. Thus, we provide the first evidence that a virus can interfere specifically with neurotrophin-regulated neuroplasticity, thereby hampering proper neuronal connectivity. These results may help to understand the behavioral disorders associated with BDV infection.

H-Ferritin Subunit Overexpression in Erythroid Cells Reduces the Oxidative Stress Response and Induces Multidrug Resistance Properties

The labile iron pool (LIP) of animal cells has been implicated in cell iron regulation and as a key component of the oxidative-stress response. A major mechanism commonly implied in the downregulation of LIP has been the induced expression of ferritin (FT), particularly the heavy subunits (H-FT) that display ferroxidase activity. The effects of H-FT on LIP and other physiological parameters were studied in murine erythroleukemia (MEL) cells stably transfected with H-FT subunits. Clones expressing different levels of H-FT displayed similar concentrations of total cell iron (0.3 ± 0.1 mmol/L) and of reduced/total glutathione. However, with increasing H-FT levels the cells expressed lower levels of LIP and reactive oxygen species (ROS) and ensuing cell death after iron loads and oxidative challenges. These results provide direct experimental support for the alleged roles of H-FT as a regulator of labile cell iron and as a possible attenuator of the oxidative cell response. H-FT overexpression was of no apparent consequence to the cellular proliferative capacity. However, concomitant with the acquisition of iron and redox regulatory capacities, the H-FT–transfectant cells commensurately acquired multidrug resistance (MDR) properties. These properties were identified as increased expression of MDR1 mRNA (by reverse transcription polymerase chain reaction [RT-PCR]), P-glycoprotein (Western immunoblotting), drug transport activity (verapamil-sensitive drug efflux), and drug cytotoxicity associated with increased MDR1 or PgP. Although enhanced MDR expression per se evoked no significant changes in either LIP levels or ROS production, it might be essential for the survival of H-FT transfectants, possibly by expediting the export of cell-generated metabolites.

Assembly of connexins and MP26 in lens fiber plasma membranes studied by SDS-fracture immunolabeling

The SDS-fracture immunolabeling technique, unlike conventional freeze-fracture, provides direct evidence for the biochemical nature of membrane constituents. SDS-fracture immunolabeling shows that during differentiation of lens fiber cells the onset of junctional assembly is characterized by the presence of small clusters and linear arrays comprising connexins alpha3 and alpha8. At this initial stage MP26, a major fiber membrane constituent, appears to be colocalized with these two connexins. The application of double-immunogold labeling reveals that when large junctional plaques are assembled MP26 becomes mainly associated with the periphery of the junctional domains. This type of distribution suggests that MP26 may play a role in the clustering and gathering of connexons. In aged nuclear fiber membranes connexins, MP26 and their proteolytic derivatives form an orthogonal lattice of repeating subunits.

Hepatocyte-specific expression of the human MDR3 P-glycoprotein gene restores the biliary phosphatidylcholine excretion absent in Mdr2 (-/-) mice

Mice homozygous for a disruption in the Mdr2 gene (Mdr2 (-/-) mice) lack the Mdr2 P-glycoprotein (P-gp) in the canalicular membrane of the hepatocyte and are unable to excrete phosphatidylcholine into the bile. These mice develop a nonsuppurative cholestatic liver disease, presumably caused by the high concentrations of free cytotoxic bile acids in bile. We generated transgenic mice that express the human homolog of Mdr2, MDR3, specifically in the liver by the use of an albumin promoter. In these mice the MDR3 P-gp is exclusively located in the canalicular membrane of hepatocytes and phospholipid excretion into bile is restored. Mice that contain the same amount of MDR3 P-gp as that of Mdr2 P-gp in wild-type mice, also excrete the same amount of phospholipids. No histopathological abnormalities were observed in the livers of these mice. In mice that express MDR3 at a higher or lower level, the phospholipid excretion correlated with the amount of MDR3 P-gp. We conclude that the human MDR3 P-gp is functionally homologous to the murine Mdr2 P-gp and that it can fully replace this P-gp in Mdr2 (-/-) mice, restoring the excretion of phospholipids into the bile. The phospholipid excretion is limited by the amount of MDR3 or Mdr2 P-gp. The excretion of cholesterol is not tightly coupled to the excretion of phospholipids in these mice, because a very low phospholipid excretion level is sufficient to give almost wild-type cholesterol excretion into the bile.

Disruption of alpha3 connexin gene leads to proteolysis and cataractogenesis in mice

Gap junction channels formed by alpha3 (Cx46) and alpha8 (Cx50) connexin provide pathways for communication between the fiber cells in the normal transparent lens. To determine the specific role of alpha3 connexin in vivo, the alpha3 connexin gene was disrupted in mice. Although the absence of alpha3 connexin had no obvious influence on the early stages of lens formation and the differentiation of lens fibers, mice homozygous for the disrupted alpha3 gene developed nuclear cataracts that were associated with the proteolysis of crystallins. This study establishes the importance of gap junctions in maintaining normal lens transparency by providing a cell-cell signaling pathway or structural component for the proper organization of lens membrane and cytoplasmic proteins.

Peripheral neuropathy in mice transgenic for a human MDR3 P-glycoprotein mini-gene

We have generated mice transgenic for a human MDR3 mini-gene, under control of a hamster vimentin promoter. Expression of the MDR3 transgene was found in mesenchymal tissues, peripheral nerves, and the eye lens. These MDR3 transgenic mice have a slowed motor nerve conduction and dysmyelination of their peripheral nerves. An extensive dysmyelination in some transgenic strains results in a severe peripheral neuropathy with paresis of the hind legs. How expression of the MDR3 transgene causes these abnormalities is unknown. The MDR3 gene encodes a large glycosylated plasma membrane protein with multiple transmembrane spanning domains, which are involved in the translocation of the phospholipid phosphatidylcholine through the hepatocyte canalicular membrane. The ability of the MDR3 P-glycoprotein to alter phsopholipid distribution in the plasma membrane of Schwann cells may cause the damage. It is also possible, however, that the presence of a large glycoprotein in the cell membrane may be sufficient to severely disturb myelination of peripheral nerves.

Peripheral neuropathy in mice transgenic for a human MDR3 P-glycoprotein mini-gene

We have generated mice transgenic for a human MDR3 mini-gene, under control of a hamster vimentin promoter. Expression of the MDR3 transgene was found in mesenchymal tissues, peripheral nerves, and the eye lens. These MDR3 transgenic mice have a slowed motor nerve conduction and dysmyelination of their peripheral nerves. An extensive dysmyelination in some transgenic strains results in a severe peripheral neuropathy with paresis of the hind legs. How expression of the MDR3 transgene causes these abnormalities is unknown. The MDR3 gene encodes a large glycosylated plasma membrane protein with multiple transmembrane spanning domains, which are involved in the translocation of the phospholipid phosphatidylcholine through the hepatocyte canalicular membrane. The ability of the MDR3 P-glycoprotein to alter phsopholipid distribution in the plasma membrane of Schwann cells may cause the damage. It is also possible, however, that the presence of a large glycoprotein in the cell membrane may be sufficient to severely disturb myelination of peripheral nerves.