Biomedical subjects

A Calado

Publications and source records attributed to A Calado.

10 recordsLinked to original sources

PubMed2004

Surface studies on acrylic bone cement.

Poly(methyl methacrylate) (PMMA) is used to fill the gap between the prosthesis and the surrounding bone in cemented arthroplasties. Biocompatibility problems related to bone cement application limit the clinical success of these cemented arthroplasties. Being the cement surface in close connection with the living bone, it is reasonable to assume that surface properties such as, surface composition and surface energy, will play a role in the biomaterial performance. X-ray photoelectron spectroscopy (XPS) analysis and surface energy studies were carried out during 4 months, in order to assess a possible correlation between aging time and surface changes. The aging of PMMA, in a biological model fluid, strongly influences the composition and wettability of the cement surface. These changes may be explained through the hydrolysis of PMMA ester groups and the subsequent hydrogen bonding. Although our study does not exactly reproduce the in vivo environment surrounding a prosthesis, it suggests that the changes in the composition and wettability of the surface may modulate the host response towards the implant, thus contributing to its loosening.

Bone Cements↗

PubMed2002

The effect of ethanol on acrylic bone cement.

Prosthesis loosening is a major problem associated with the use of poly(methyl methacrylate) (PMMA) bone cement that may be related to a peri-implant vacuolisation commonly observed at bone-cement interface. Methyl methacrylate (MMA) monomer may be one of the cement components partly responsible for the mentioned vacuolisation due to a cytotoxic effect associated to this compound. Alcoholism has been related to bone necrosis in predisposed individuals. Furthermore, ethanol has been shown to clean material with adherent cement debris during cleaning procedure in laboratory. Consequently, we have decided to study whether ethanol will also be related to an increased liberation of MMA from the polymer matrix. 'In vitro' release studies using PMMA plates were conducted to access the role of ethanol on the liberation of the monomer. Contact angle measurements and surface tension estimation were also carried out in order to find a possible effect of ethanol on surface cement properties. Results suggest that ethanol, even in small quantities, enhances the leaching of the monomer from the polymer matrix, but does not considerably change the wettability properties of the cement surface.

Acrylic Resins↗

PubMed2001

The influence of vacuum mixing on methylmethacrylate liberation from acrylic cement powder.

Polymethylmethacrylate (PMMA) bone cement is a biomaterial used to anchor prostheses during joint replacement surgery. Residual methylmethacrylate monomer (MMA) may be related with the cytotoxic effect of PMMA. The aim of the present paper was to investigate the effect of two different cement mixing methods: hand stirring at atmospheric pressure and under partial vacuum (0.330 and 0.154 bar) on residual monomer liberation in phosphate buffer saline solution from acrylic cement powder. Residual MMA content was determined by high-performance liquid chromatography. Mathematical models were applied to experimental dissolution data revealing that monomer release was significantly reduced in bone cement powder obtained at 0.154 bar vacuum pressure compared to the other mixing conditions. The kinetic models applied are consistent with a simple diffusion mechanism of the monomer from the polymer matrix.

Acrylates↗

PubMed2001

Transcription-dependent nucleocytoplasmic distribution of hnRNP A1 protein in early mouse embryos.

A unique feature of certain members of the heterogeneous nuclear ribonucleoprotein (hnRNP) family of proteins is that they shuttle continuously between nucleus and cytoplasm and their accumulation in the nucleus is transcription-dependent. An extensively characterised protein of this group is hnRNP A1. To date, most studies addressing the transcription-dependent transport of hnRNP A1 have been performed on cultured cell lines treated with transcription inhibitors. Here we have analysed the nucleocytoplasmic distribution of hnRNP A1 in early mouse embryos, where the haploid pronuclei remain transcriptionally inactive for a period of several hours. Consistent with its small molecular size (36 kDa), the hnRNP A1 protein diffuses passively through the nuclear pores and equilibrates between the nucleus and the cytoplasm of transcriptionally inactive embryos. In contrast, following transcriptional activation the A1 protein becomes accumulated in the nucleus. This accumulation of the A1 protein in the nucleus is blocked by the lectin wheat germ agglutinin (WGA), which binds to nuclear pore proteins and prevents translocation of receptor-cargo complexes through the pores. This indicates that a carrier-mediated transport pathway is required for the concentration of A1 in transcriptionally active nuclei. To further analyse how transcription is coupled to nucleocytoplasmic transport, we transplanted transcriptionally inactive pronuclei into the cytoplasm of transcriptionally active embryos. The results show that the presence of newly synthesised RNAs in the cytoplasm is not sufficient to induce the accumulation of hnRNP A1 in the nucleus. Rather, the appearance of nascent transcripts in the nucleus appears to be the crucial event. Since hnRNP A1 is a shuttling protein, an increase in its steady state nuclear concentration could be the result of either faster nuclear import or slower export to the cytoplasm. We propose that binding of A1 to nascent transcripts retards its export to the cytoplasm and therefore contributes to its concentration in the nucleus.

Animals↗

PubMed2000

Identification of two novel RanGTP-binding proteins belonging to the importin beta superfamily.

Nucleo-cytoplasmic transport comprises a large number of distinct pathways, many of which are defined by members of the importin beta superfamily of nuclear transport receptors. These transport receptors all directly interact with RanGTP to modulate the compartment-specific binding of their transport substrates. To identify new members of the importin beta family, we used affinity chromatography on immobilized RanGTP and isolated Ran-binding protein (RanBP) 16 from HeLa cell extracts. RanBP16 and its close human homologue, RanBP17, are distant members of the importin beta family. Like the other members of the transport receptor superfamily, RanBP16 interacts with the nuclear pore complex and is able to enter the nucleus independent of energy and additional nuclear transport receptors.

Amino Acid Sequence↗

PubMed2000

Nuclear inclusions in oculopharyngeal muscular dystrophy consist of poly(A) binding protein 2 aggregates which sequester poly(A) RNA.

Oculopharyngeal muscular dystrophy (OPMD) is an adult-onset disease characterized by progressive eyelid drooping, swallowing difficulties and proximal limb weakness. The autosomal dominant form of the disease is caused by short (GCG)(8-13) expansions in the PABP2 gene. This gene encodes the poly(A) binding protein 2 (PABP2), an abundant nuclear protein that binds with high affinity to nascent poly(A) tails, stimulating their extension and controlling their length. In this work we report that PABP2 is detected in filamentous nuclear inclusions, which are the pathological hallmark of OPMD. Using both immunoelectron microscopy and fluorescence confocal microscopy, the OPMD-specific nuclear inclusions appeared decorated by anti-PABP2 antibodies. In addition, the inclusions were labeled with antibodies directed against ubiquitin and the subunits of the proteasome and contained a form of PABP2 that was more resistant to salt extraction than the protein dispersed in the nucleoplasm. This suggests that the polyalanine expansions in PABP2 induce a misfolding and aggregation of the protein into insoluble inclusions, similarly to events in neurodegenerative diseases caused by CAG/polyglutamine expansions. No significant differences were observed in the steady-state poly(A) tail length in OPMD and normal myoblasts. However, the nuclear inclusions were shown to sequester poly(A) RNA. This raises the possibility that in OPMD the polyalanine expansions in the PABP2 protein may interfere with the cellular traffic of poly(A) RNA.

Cell Nucleus↗

PubMed2000

In vitro release studies of methylmethacrylate liberation from acrylic cement powder.

Bone cement or polymethylmethacrylate (PMMA) is commonly used for anchoring cemented prosthesis to the bone. Cytotoxic effect of culture media exposed to PMMA powder may be related with long term problems associated with acrylic cement application, being the monomer (methylmethacrylate) one of the cement's component partly responsible for the cytotoxic effect. The present work reports the studies of monomer release from acrylic bone cement powder under different experimental conditions: setting time of PMMA (in solution and air) and different culture media composition. High-performance liquid chromatography was used for the determination of residual monomer. Mathematical models were applied to experimental dissolution data revealing that monomer release is lightly affected by the studied variables. The monomer release seems to be a surface phenomena, suggesting that the possible actions of monomer will mainly be due to the initial loss of non polymerized monomer rather than to further depolymerization of the already polymerized cement.

Bone Cements↗

PubMed2000

Deciphering the cellular pathway for transport of poly(A)-binding protein II.

Poly(A)-binding protein II (PABP2) is an abundant nuclear protein that binds with high affinity to nascent poly(A) tails, stimulating their extension and controlling their length. In the cytoplasm, a distinct protein (PABP1) binds to poly(A) tails and participates in mRNA translation and stability. How cytoplasmic PABP1 substitutes for nuclear PABP2 is still unknown. Here we report that PABP2 shuttles back and forth between nucleus and cytoplasm by a carrier-mediated mechanism. A potential novel type of nuclear localization signal exists at the C-terminus of the protein, a domain that is highly enriched in methylated arginines. PABP2 binds directly to transportin in a RanGTP-sensitive manner, suggesting an involvement of this transport receptor in mediating import of the protein into the nucleus. Although PABP2 is small enough to diffuse passively through the nuclear pores, protein fusion experiments reveal the existence of a facilitated export pathway. Accordingly, no transport of PABP2 to the cytoplasm occurs at 4 degrees C. In contrast, export of PABP2 continues in the absence of transcription, indicating that transport to the cytoplasm is independent of mRNA traffic. Thus, rather than leaving the nucleus as a passive passenger of mRNAs, the data suggest that PABP2 interacts with the nuclear export machinery and may therefore contribute to mRNA transport.

3T3 Cells↗

PubMed2000

Localization of poly(A)-binding protein 2 (PABP2) in nuclear speckles is independent of import into the nucleus and requires binding to poly(A) RNA.

The nuclei of mammalian cells contain domains, termed nuclear speckles, which are enriched in splicing factors and poly(A) RNA. Although nuclear speckles are thought to represent reservoirs from which splicing factors are recruited to sites of transcription and splicing, the presence of poly(A) RNA in these structures remains enigmatic. An additional component of the speckles is poly(A) binding protein 2 (PABP2), a protein that binds with high affinity to nascent poly(A) tails, stimulating their extension and controlling their length. In this work we investigated whether PABP2 contributes to the targeting of poly(A) RNA to the speckles. The results show that localization of PABP2 in speckles is independent of import of the protein into the nucleus. Inhibition of transcription or poly(A) synthesis at the end of mitosis does not affect nuclear import of PABP2 but prevents its localization to speckles. Furthermore, PABP2 mutants with decreased ability to bind to poly(A) fail to localize to speckles. Taken together the results show that PABP2 localizes to the nuclear speckles as a consequence of its binding to poly(A) RNA, contrasting to splicing factors which assemble into speckles in the absence of newly synthesized transcripts.

Biological Transport↗

PubMed1999

Intranuclear trafficking of messenger RNA.

Within the nucleus, protein-encoding genes are transcribed into messenger RNA by RNA polymerase II. Messenger RNAs migrate to the cytoplasm, but before reaching their final destination the primary transcripts must undergo a series of modifications that include 5'-capping, splicing, and 3'-cleavage/polyadenylation. Errors in these processing events can originate aberrant products that, if translated, would produce abnormal proteins. Therefore, it is not surprising that eukaryotes have evolved a surveillance mechanism that recognizes and rapidly degrades aberrant mRNAs. Recent experiments provide exciting insights into how proper mRNAs are distinguished and selected for export. Transcription by RNA polymerase II is directly coupled to pre-mRNA processing, and the mechanism that targets the processing machinery to the polymerase complex suggests a model for co-transcriptional proofreading. Furthermore, there is evidence that at least some mRNAs move randomly throughout the nucleus, presumably by free diffusion. In this light, retention of aberrant mRNAs by the transcription/processing machinery is crucial to prevent their diffusion to the nuclear pores and eventual translocation to the cytoplasm.

Animals↗