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Biomedical subjects

A A Davis

Publications and source records attributed to A A Davis.

32 records · Page 2Linked to original sources
PubMed1992

Release of iron by human retinal pigment epithelial cells.

Retinal pigment epithelial cells, which form one aspect of the blood-retinal barrier, take up iron in association with transferrin by a typical receptor-mediated mechanism (Hunt et al., 1989. J. Cell Sci. 92:655-666). This iron is dissociated from transferrin in a low pH environment and uptake is sensitive to agents that inhibit endosomal acidification. The dissociated iron enters the cytoplasm as a low molecular weight (less than 10 kD) component and subsequently binds to ferritin. No evidence for recycling of iron in association with transferrin was found. Nevertheless, much of the iron that is taken up is recycled to the extracellular medium, primarily from the low molecular weight pool. This release of iron is not sensitive to inhibitors of energy production or of vesicular acidification but is increased up to a maximum of about 40% of the total 55Fe incorporated when cells are incubated with serum or the medium is changed. When a short loading time for 55Fe from 55Fe-transferrin is used (i.e., when the low molecular weight pool is proportionately larger), a much larger fraction of the cell-associated radiolabel is released than when longer loading times are used. The data suggest that a releasable intracellular iron pool is in equilibrium with the externalized material. The released iron may be separated into a high and a low molecular weight component. The former is similar on polyacrylamide gel electrophoresis to ferritin although it cannot be immune precipitated by anti-ferritin antibodies. The low molecular weight 55Fe which is heterogeneous in nature can be bound by external apo-transferrin and may represent a form that can be taken up by cells beyond the blood-retinal barrier.

Biological Transport↗
PubMed1990

Altered expression of keratin and vimentin in human retinal pigment epithelial cells in vivo and in vitro.

Actively proliferating human retinal pigment epithelial (RPE) cells grown in tissue culture possess keratin-containing intermediate filaments that react with a combination of AE1 and AE3 anti-keratin monoclonal antibodies. Antibody reactivity is lost, however, from RPE cells as the cell population ceases to proliferate when it approaches confluence and attains morphological characteristics more similar to those in vivo. In contrast, clone 8.13 anti-keratin antibody stains all cells in the culture at all stages of the growth cycle and cell densities. These findings were reflected in vivo using retinal pigment epithelium taken directly from the eye. Normal non-proliferating RPE cells bound 8.13 antibody to cytoskeletal structures, as judged by indirect immunofluorescence, but did not bind AE1/AE3 antibodies. However, proliferating dedifferentiated RPE cells from the vitreous humor of patients with proliferative vitreoretinopathy possess filaments that bind both AE1/AE3 and 8.13 antibodies. Thus it appears that structures detected by AE1/AE3 antibodies only occur in actively growing RPE cells in vitro and in vivo. Keratins produced by RPE cells were identified using Western blotting. Species with molecular masses of 54 (keratin 7), 52 (keratin 8), 42 (keratin 18), and 40 (keratin 19) kiloDaltons were the most abundant in proliferating cultured cells, but cells isolated directly from the eye were found to lack keratin 7 and 19. Keratin 19 was, however, observed in proliferating RPE cells from some patients with proliferative vitreoretinopathy. The latter findings explain the differential staining observed with AE1/AE3 antibodies in cells in culture and isolated directly from the eye since these antibodies interact primarily with keratin 19 which is absent from non-proliferating RPE cells. In contrast to the presence of keratin-containing intermediate filaments in human RPE cells in vivo, there are apparently no detectable vimentin-containing cytoskeletal structures. However, all RPE cells cultured in vitro develop filaments composed of vimentin which persist in cells that have reached confluence.

Antibodies, Monoclonal↗
PubMed1990

Selective inhibition of growing pigment epithelial cells by a receptor-directed immunotoxin.

An immunotoxin conjugate of a murine monoclonal antibody against human transferrin receptors and the A chain of ricin was examined for its potential to inhibit selectively the growth of retinal pigment epithelial (RPE) cells which grow in an uncontrolled manner in proliferative vitreoretinopathy. The probable efficacy of such an agent in vivo stems from the observation that actively proliferating cells possess many more transferrin receptors than normal quiescent cells. The authors showed in vitro that the immunoconjugate (454A12 MAB-rRA) inhibits protein synthesis in actively dividing RPE cells but has a smaller or no effect on protein synthesis by confluent, nondividing RPE cells. The effect was specific in that neither the free ricin A chain (rRA) nor the monoclonal antibody (454A12 MAB) alone has any inhibitory effect. Furthermore, the antibody competes with the immunotoxin and suppresses the latter's toxicity. This immunotoxin has applications for therapy in conditions in which the pathologic proliferation of RPE cells occurs.

Antibodies, Monoclonal↗
PubMed1989

Changes in glycosylation alter the affinity of the human transferrin receptor for its ligand.

When transferrin receptors of human erythroleukemic cells were pulse-labeled with [35S]methionine and then chased in the absence of radioactive precursor, the first detectable immunoprecipitable form of the receptor had a molecular mass of 85 kDa. This form of the receptor was converted to the mature form of 93 kDa with a half-time of about 40-60 min. Both the immature (85 kDa) and mature (93 kDa) receptors associated as dimers, the native form of the receptor. The 85-kDa, as well as the 93-kDa, receptors bound to a monoclonal antibody raised against the transferrin receptor or to transferrin-Sepharose. In order to determine whether glycosylation was necessary for ligand binding, purified receptors were isolated from cells grown in the presence of tunicamycin. When K562 cells were grown in the presence of tunicamycin, an 80-kDa nonglycosylated form of the receptor was synthesized. This nonglycosylated receptor was also capable of dimer formation; however, much less of it reached the cell surface than the fully glycosylated form, although both untreated and tunicamycin-grown cells appeared to synthesize transferrin receptors at similar rates. Although the number of receptor molecules/cell was similar in control and tunicamycin-treated cells, the nonglycosylated receptors exhibited a much lower affinity for transferrin than those of untreated cells; in contrast, when receptors were purified by immunoprecipitation and digested with bacterial alkaline phosphatase, no difference was observed between the affinity of these receptors and undigested immunoprecipitated receptors. These results suggest that glycosylation is not necessary for specific binding of transferrin to its receptor, but the affinity of this binding can be influenced greatly by the presence or absence of carbohydrate residues.

Alkaline Phosphatase↗
PubMed1989

Transferrin receptors on the surfaces of retinal pigment epithelial cells are associated with the cytoskeleton.

Retinal pigment epithelial cells, derived from human donor eyes, have been grown in culture as monolayers on membrane filters or plastic surfaces and shown to possess transferrin receptors with a monomeric molecular mass of 93,000. These receptors internalize 125I-labelled transferrin and recycle it to the surrounding medium in a similar manner to other cell types. Scatchard analyses show that there are about 100,000 high-affinity receptors on the surface of each cell and most of these receptors are associated with the cytoskeleton. In total cell extracts, there are additional low-affinity binding sites that do not appear to be strongly associated with the cytoskeleton. The apparent interaction of transferrin receptors with the cytoskeleton was confirmed in two ways: first, using 200 kV electron microscopy for stereo analyses, skeleton-associated transferrin receptors were detected by a monoclonal anti-receptor antibody and a colloidal gold-conjugated second antibody after Triton X-100 extraction of pigment epithelial cells grown directly on laminin-coated gold grids; and, second, when cell surface receptors were labelled with radioiodinated transferrin and then incubated for various periods of time, the labelled transferrin was observed to move from a Triton X-100-insoluble fraction (a putative cytoskeletal compartment) to a Triton-soluble compartment that was not associated with the cytoskeleton. Using either horseradish peroxidase or colloidal gold-labelled transferrin, it has been shown that basolateral and apical surface-located receptors participate in receptor-mediated endocytosis via clathrin-coated pits, endosomes and tubular structures. Initially, transferrin internalized from the apical surface is observed in small endosomes that often appear to be embedded in an apical layer of microfilaments. From these peripheral regions of the cells, the labelled receptors move to larger endosomes and multivesicular bodies deeper in the cytoplasm. These structures have no apparent association with cytoskeletal elements.

Cell Division↗
PubMed1986

Twelve years of clinical experience with lithium pulse generators.

The clinical performance of 9,651 lithium pulse generators, which were implanted at our registry hospitals since 1973, was assessed by analysis of actuarial survival data and information obtained at the time of pacemaker failure. Cumulative survival estimates for all units were: 5 years, 94 percent, 7 years, 81 percent, 10 years, 62 percent. Discriminant actuarial analysis revealed no differences between lithium iodine and lithium cupric sulfide longevity. Battery depletion was found in 49.1 percent of 277 units subjected to detailed explant analysis, while 31 percent and 8.7 percent had component and hermetic seal defects, respectively. Of the pulse generators with battery depletion, all but two units exhibited normal end-of-battery life behavior. We conclude that the lithium iodine and lithium cupric sulfide batteries are safe, long-term energy sources; however, the impact of reduced battery size and dual chamber pacing on these performance parameters is unknown.

Electronics, Medical↗
PubMed1982

"Montrose" Child Life Protection Unit: a treatment and assessment model in child abuse intervention.

In New South Wales, Australia, the Department of Youth and Community Services is empowered by statute to deal with cases of child abuse and neglect. Its child protection services include the Montrose Child Life Protection Unit which is designed to support and complement the work of both the Intake Service, Field Service and most importantly the work of child protective facilities in the community. The service has intake crisis counselling and call out resources; maintains a central register; provides a facility for crisis residential care for children and families, diagnostic assessment, and a day programme. The emphasis in service delivery in New South Wales in the area of child protection is on multidisciplinary intervention. The Department of Youth and Community Services is involved in promoting interagency cooperation and co-ordination. Over and above a philosophy that underlies service delivery is the Montrose Unit's involvement in sensitising the community to the difficult and often demanding role of parenting. Montrose represents an attempt to create an environment where parenting myths can be examined from a reality base and strategies implemented for reeducation.

Australia↗
PubMed1977

One hundred and eighty-seven cases of child abuse and neglect.

The case histories of 187 abused and neglected children, admitted to the Royal Alexandra Hospital for Children over a three-year-period, were reviewed. Multiproblem families from the lower socioeconomic groups predominated, reflecting the hospital's geographic catchment area. Eight children died and 30 were made state wards. A high proportion of mothers described problems during the pregnancy, a difficult delivery and neonatal problems. The majority of parents in this study were married and had small families. First-born male infants of young isolated mothers were found to be at greatest risk of child abuse and neglect.

Adolescent↗