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Biomedical subjects

R Maitra

Publications and source records attributed to R Maitra.

At least 19 recordsLinked to original sources
PubMed2002

Intramedullary ancient schwannoma of the cervical spinal cord: case report and review of literature.

We report a rare case of intramedullary ancient schwannoma of cervical spinal cord in a 68 year old patient. About 49 cases of intramedullary schwannomas and neurofibromas have been reported in the literature but to our knowledge there is no report of the 'ancient' variety of intramedullary schwannoma. The cell of origin of these tumours is the schwann cell, which normally does not exist in the parenchyma of the central nervous system. Many theories have been advanced to explain this paradox. According to one theory, these tumours arise from the perivascular nerve plexus of the pial vessels. This plexus was found mostly to exist along the branches of the anterior spinal artery. In our case, the tumour was supplied by two branches of the anterior spinal artery, which may add further support to the above theory.

Aged↗
PubMed2001

The initial steps of biogenesis of cyanobacterial photosystems occur in plasma membranes.

During oxygenic photosynthesis in cyanobacteria and chloroplasts of plants and eukaryotic algae, conversion of light energy to biologically useful chemical energy occurs in the specialized thylakoid membranes. Light-induced charge separation at the reaction centers of photosystems I and II, two multisubunit pigment-protein complexes in the thylakoid membranes, energetically drive sequential photosynthetic electron transfer reactions in this membrane system. In general, in the prokaryotic cyanobacterial cells, the thylakoid membrane is distinctly different from the plasma membrane. We have recently developed a two-dimensional separation procedure to purify thylakoid and plasma membranes from the genetically widely studied cyanobacterium Synechocystis sp. PCC 6803. Immunoblotting analysis demonstrated that the purified plasma membrane contained a number of protein components closely associated with the reaction centers of both photosystems. Moreover, these proteins were assembled in the plasma membrane as chlorophyll-containing multiprotein complexes, as evidenced from nondenaturing green gel and low-temperature fluorescence spectroscopy data. Furthermore, electron paramagnetic resonance spectroscopic analysis showed that in the partially assembled photosystem I core complex in the plasma membrane, the P700 reaction center was capable of undergoing light-induced charge separation. Based on these data, we propose that the plasma membrane, and not the thylakoid membrane, is the site for a number of the early steps of biogenesis of the photosynthetic reaction center complexes in these cyanobacterial cells.

Cell Membrane↗
PubMed2001

Amino acid residues that are critical for in vivo catalytic activity of CtpA, the carboxyl-terminal processing protease for the D1 protein of photosystem II.

CtpA, a carboxyl-terminal processing protease, is a member of a novel family of endoproteases that includes a tail-specific protease from Escherichia coli. In oxygenic photosynthetic organisms, CtpA catalyzes C-terminal processing of the D1 protein of photosystem II, an essential event for the assembly of a manganese cluster and consequent light-mediated water oxidation. We introduced site-specific mutations at 14 conserved residues of CtpA in the cyanobacterium Synechocystis sp. PCC 6803 to examine their functional roles. Analysis of the photoautotrophic growth capabilities of these mutants, their ability to process precursor D1 protein and hence evolve oxygen, along with an estimation of the protease content in the mutants revealed that five of these residues are critical for in vivo activity of CtpA. Recent x-ray crystal structure analysis of CtpA from the eukaryotic alga Scenedesmus obliquus (Liao, D.-I., Qian, J., Chisholm, D. A., Jordan, D. B. and Diner, B. A. (2000) Nat. Struct. Biol. 7, 749-753) has shown that the residues equivalent to Ser-313 and Lys-338, two of the five residues mentioned above, form the catalytic center of this enzyme. Our in vivo analysis demonstrates that the three other residues, Asp-253, Arg-255, and Glu-316, are also important determinants of the catalytic activity of CtpA.

Algal Proteins↗
PubMed2001

Differential effects of mitomycin C and doxorubicin on P-glycoprotein expression.

Previous studies have demonstrated that mitomycin C (MMC) and other DNA cross-linking agents can suppress MDR1 (multidrug resistance 1) gene expression and subsequent functional P-glycoprotein (Pgp) expression, whereas doxorubicin and other anthracyclines increase MDR1 gene expression. In the present study, with stably transfected Madin-Darby canine kidney C7 epithelial cells expressing a human Pgp tagged with green fluorescent protein under the proximal human MDR1 gene promoter, we demonstrated that MMC and doxorubicin have differential effects on Pgp expression and function. Doxorubicin caused a progressive increase in the cell-surface expression of Pgp and function. In contrast, MMC initially increased plasma membrane expression and function at a time when total cellular Pgp was constant and Pgp mRNA expression had been shown to be suppressed. This was followed by a rapid and sustained decrease in cell-surface expression at later times, presumably as a consequence of the initial decrease in mRNA expression. These studies imply that there are at least two independent chemosensitive steps that can alter Pgp biogenesis: one at the level of mRNA transcription and the other at the level of Pgp trafficking. Understanding the combined consequences of these two mechanisms might lead to novel chemotherapeutic approaches to overcoming drug resistance in human cancers by altering either Pgp mRNA expression or trafficking to the membrane.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed2001

Increased functional cell surface expression of CFTR and DeltaF508-CFTR by the anthracycline doxorubicin.

Cystic fibrosis (CF) is a disease that is caused by mutations within the cystic fibrosis transmembrane conductance regulator (CFTR) gene. The most common mutation, DeltaF508, accounts for 70% of all CF alleles and results in a protein that is defective in folding and trafficking to the cell surface. However, DeltaF508-CFTR is functional when properly localized. We report that a single, noncytotoxic dose of the anthracycline doxorubicin (Dox, 0.25 microM) significantly increased total cellular CFTR protein expression, cell surface CFTR protein expression, and CFTR-associated chloride secretion in cultured T84 epithelial cells. Dox treatment also increased DeltaF508-CFTR cell surface expression and DeltaF508-CFTR-associated chloride secretion in stably transfected Madin-Darby canine kidney cells. These results suggest that anthracycline analogs may be useful for the clinical treatment of CF.

Adenocarcinoma↗
PubMed2001

Functional enhancement of CFTR expression by mitomycin C.

Cystic fibrosis is caused by mutations in the CFTR gene. The most common of these mutations, DeltaF508, results in a protein that is not trafficked to the apical plasma membrane but instead is retained and degraded in the endoplasmic reticulum (ER) by the 26S proteosome. However, this protein is functional upon plasma membrane expression. It has been theoretically estimated that even a modest ( approximately 10%) increase in CFTR-associated chloride conductance can be beneficial in a clinical setting. Thus, understanding basic CFTR biogenesis is important, and identification of prototypical compounds that can increase CFTR expression and trafficking is potentially useful in the development of novel therapeutic strategies to treat cystic fibrosis. We report that mitomycin C (MMC) elicits such a response by increasing CFTR mRNA and protein expression in T-84 and HT-29 cells at very low, non-cytotoxic, pharmacologically relevant concentrations (0.1 microM) leading to enhanced chloride secretion. Thus, MMC may be a useful compound for understanding CFTR regulation and biogenesis.

ATP-Binding Cassette Transporters↗
PubMed2000

Maf transcriptionally activates the mouse p53 promoter and causes a p53-dependent cell death.

An increase in the level of the tumor suppressor protein p53 can induce cell cycle arrest or cell death. Although mechanisms for regulating the life span of p53 have been described, there is growing evidence that transcriptional regulation of the p53 gene contributes significantly to controlling p53 protein levels and therefore the fate of a cell. However, the signal transduction pathways that lead to transcriptional activation of the p53 gene are poorly understood. The oncoprotein v-Maf and its cellular counterparts belong to the large combinatorially complex basic leucine zipper family of transcription factors, which include the AP1 family. To date few cellular targets of c-Maf have been identified. It is demonstrated here that v-Maf can bind as a homodimer to a variant Maf recognition element located between -66 and -54 upstream in the mouse p53 promoter. V-Maf and its cellular counterparts are shown to activate p53 expression through this site. The ability of v-Maf to activate p53 expression is modulated by AP1 family members. In addition, overexpression of v-Maf in primary cells leads to a p53-dependent cell death. Thus, Maf and members of the AP1 family are able to regulate p53 expression through this site in the p53 promoter.

Animals↗
PubMed1999

Subunit dependent modulation of GABAA receptor function by neuroactive steroids.

Neurosteroids are potent, endogenous modulators of GABAA receptor function in the central nervous system. The endogenous progesterone metabolite allopregnanolone (ALP) and the synthetic steroid compound alphaxalone (AFX) have been shown to both directly activate and potentiate GABAA receptor-activated membrane current (IGABA). The role of different alpha and gamma subunit subtypes in modulation of IGABA by ALP and AFX was investigated using recombinant GABAA receptor isoforms expressed in Xenopus oocytes. Changing or removal of the alpha subunit subtype altered the efficacy of both ALP and AFX (alpha2beta1gamma2L>alpha1beta1gamma2L>>beta1gamma2L) to potentiate IGABA, but did not alter the potency of the neuroactive steroids at these receptor isoforms. The efficacy of ALP to enhance IGABA was also dependent on the gamma subunit subtype (alpha1beta1gamma3>alpha1beta1gamma2L = alpha1beta1gamma1). AFX also had higher efficacy in the alpha1beta1gamma3 receptor isoform compared to alpha1beta1gamma1. In contrast to ALP, the potency of AFX was greater in the alpha1beta1gamma3 and alpha1beta1gamma1 receptor isoforms compared to alpha1beta1gamma2L. This study provides evidence that the alpha subunit subtype determines the efficacy, but not the potency, of these neuroactive steroids to potentiate IGABA. The gamma3 subunit subtype increases the maximal efficacy of neuroactive steroids compared to other gamma subunit subtypes. These results suggest that the heteromeric assembly of different GABAA receptor isoforms containing different subunit subtypes results in multiple steroid recognition sites on GABAA receptors that in turn produce distinctly different modulatory interactions between neuroactive steroids acting at the GABAA receptor.

Animals↗
PubMed1999

Selective modulation of collagenase 1 gene expression by the chemotherapeutic agent doxorubicin.

Matrix metalloproteinases (MMPs) play a crucial role in tumor cell invasion and metastasis due to their ability to digest basement membrane and extracellular matrix components, thereby facilitating cell movement through connective tissues. At noncytotoxic concentrations, i.e., concentrations lower than those normally used in cancer chemotherapy, the anthracycline doxorubicin specifically inhibited collagenase 1 (MMP-1) gene expression in the highly invasive and metastatic human melanoma cell line A2058. This inhibition was specific for collagenase 1 because it did not affect the expression of two other MMPs, gelatinase A (MMP-2) and gelatinase B (MMP-9). The reduction in collagenase 1 expression correlated with a decrease in the invasive ability of tumor cells through a collagen type I matrix and was independent of the cytotoxic and antiproliferative effects usually associated with this anticancer drug. The selective modulation of collagenase 1 expression by nontoxic doses of doxorubicin suggests a novel application for this chemotherapeutic agent, perhaps in combination therapy, because it decreases the invasive/metastatic potential of melanoma cells that are otherwise unaffected by this drug.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1998

[Heparin-induced thrombocytopenia. Diagnosis, clinical course and therapeutic alternatives].

Heparins are common and have been widely used in prophylaxis and therapy of thromboembolic disorders for many years. Nevertheless, the serious side effect of heparin-induced thrombocytopenia type II (HIT II) has attracted attention only recently. First evidence of HIT II is a drop in platelet count below 100,000/mm3. Subsequently, thromboembolic complications occur 6-20 days after beginning heparin therapy. Overall mortality is 20-30% of patients with HIT II. Therefore, frequent platelet counts are required for early diagnosis. However, in cases of moderate or absent thrombocytopenia the diagnosis of HIT II can be difficult. Laboratory tests such as heparin-induced platelet activation (HIPA) test or heparin/PF4-antibody-ELISA have limited sensitivity and specificity. Therefore, with typical clinical findings, divergent laboratory results should be interpreted with caution. If HIT II is suspected, all heparins should be discontinued immediately. Due to a high cross reactivity rate with the heparin-dependent antibody, subsequent therapy with low molecular weight heparins (LMWH) is contraindicated. As treatment of choice danaparoid or recombinant hirudin may be administered. Oral anticoagulation should be started cautiously with an alternative immediate-acting antithrombotic agent. More widespread use of LMWH may reduce the incidence of HIT II in the future. Nevertheless, the main and most important factor in the prevention of this life-threatening complication is the awareness and caution of the clinician.

Anticoagulants↗
PubMed1998

An approximate bootstrap technique for variance estimation in parametric images.

Parametric imaging procedures offer the possibility of comprehensive assessment of tissue metabolic activity. Estimating variances of these images is important for the development of inference tools in a diagnostic setting. However, these are not readily obtained because the complexity of the radio-tracer models used in the generation of a parametric image makes analytic variance expressions intractable. On the other hand, a natural extension of the usual bootstrap resampling approach is infeasible because of the expanded computational effort. This paper suggests a computationally practical, approximate simulation strategy to variance estimation. Results of experiments done to evaluate the approach in a simplified model one-dimensional problem are very encouraging. Diagnostic checks performed on a single real-life positron emission tomography (PET) image to test for the feasibility of applying the procedure in a real-world PET setting also show some promise. The suggested methodology is evaluated here in the context of parametric images extracted by mixture analysis; however, the approach is general enough to extend to other parametric imaging methods.

Algorithms↗
PubMed1998

Modulation of GABA(A) receptor function by neuroactive steroids: evidence for heterogeneity of steroid sensitivity of recombinant GABA(A) receptor isoforms.

Neuroactive steroids are potent, selective allosteric modulators of gamma-aminobutyric acid type A (GABA(A)) receptor function in the central nervous system, and may serve as endogenous anxiolytic and analgesic agents. In order to study the influence of subunit subtypes of the GABA(A) receptor on modulation of receptor function by neuroactive steroids, we expressed human recombinant GABA(A) receptors in Xenopus oocytes. GABA-activated membrane current, and the modulatory effects of the endogenous neurosteroid 5alpha-pregnan-3alpha-ol-20-one (allopregnanolone) and the synthetic steroid anesthetic 5alpha-pregnan-3alpha-ol-11,20-dione (alphaxalone) were measured using two-electrode voltage-clamp recording techniques. Allopregnanolone had similar effects to potentiate GABA-activated membrane current in the alpha1beta1gamma2L and alpha1beta2gamma2L receptor isoforms. In contrast, alphaxalone was much more effective as a positive allosteric modulator on the alpha1beta1gamma2L receptor isoform. In the absence of the gamma2L subunit subtype, allopregnanolone had much greater efficacy, but its potency was decreased. Allopregnanolone was much more effective on the alpha1beta1 receptor isoform compared with the alpha1beta2 receptor isoform. The potency for alphaxalone to potentiate the GABA response was not altered in the absence of the gamma2L subunit subtype, although its efficacy was greatly enhanced. Both allopregnanolone and alphaxalone produced nonparallel leftward shifts in the GABA concentration-response relationship in the absence of the gamma2L subunit, decreasing the EC50 concentration of GABA and increasing the maximal response. Only alphaxalone increased the maximal GABA response when the gamma2L subunit subtype was present. The 3beta-pregnane isomers epipregnanolone and isopregnanolone both inhibited the ability of allopregnanolone and alphaxalone to potentiate GABA(A) receptor function. However, the degree of block produced by the 3beta-pregnane steroid isomers was dependent on the type of receptor isoform studied and the neuroactive steroid tested. Isopregnanolone, the 3beta-isomer of allopregnanolone, was significantly more effective as a blocker of potentiation caused by allopregnanolone compared with alphaxalone in all receptor isoforms tested. Epipregnanolone had a greater efficacy as a blocker at the alpha1beta2gamma2L receptor isoform compared with the alpha1beta1gamma2L receptor isoform, and also produced a greater degree of block of potentiation caused by allopregnanolone compared with alphaxalone. Our results support the hypothesis that the heteromeric assembly of different GABA(A) receptor isoforms containing different subunit subtypes results in multiple steroid recognition sites on GABA(A) receptors, which in turn produces distinctly different modulatory interactions between neuroactive steroids acting at the GABA(A) receptor. The alpha and gamma subunit subtypes may have the greatest influence on allopregnanolone modulation of GABA(A) receptor function, whereas the beta and gamma subunit subtypes appear to be most important for the modulatory effects of alphaxalone.

Animals↗
PubMed1997

Isolated dislocation of the middle cuneiform in a farmer: a case report and review of the literature.

Isolated dislocations of the middle cuneiform are uncommon. There have been four reported previously. Significant force is required to produce these injuries, and they can have serious neurovascular consequences. In this case report and review of the literature, we present an isolated middle cuneiform dislocation in a 69-year-old farmer with impending skin and soft tissue loss over the dislocated bone.

Accidental Falls↗
PubMed1996

Propofol and flurazepam act synergistically to potentiate GABAA receptor activation in human recombinant receptors.

The intravenous general anaesthetic propofol (2,6-di-isopropylphenol) is frequently combined with a benzodiazepine. There are clinical reports of synergism between these two agents for induction of general anaesthesia. To investigate a possible mechanism of this synergistic interaction between propofol and benzodiazepines, the effect of propofol and flurazepam on GABAA receptor function was examined in Xenopus oocytes expressing recombinant alpha 1 beta 2 gamma 2L and alpha 2 beta 2 gamma 2L receptor constructs. Potentiation of GABA receptor-activated current by low (1-10 microM) concentrations of propofol together with flurazepam (0.25-0.5 microM) was significantly greater than predicted by an additive response. Isobolographic analysis indicated a strong synergistic interaction between propofol and flurazepam at either of the receptor constructs examined. In contrast, the cyclopyrrolone derivative zopiclone, which produced a similar facilitation of GABA receptor-activated current compared to flurazepam, produced a less than additive potentiation when combined with propofol. Flurazepam significantly decreased the EC50 concentration of propofol for potentiation of GABA responses. Thus, flurazepam, in addition to facilitating GABA receptor activity on its own, also increases on its own, also increases the apparent affinity of the GABAA receptor complex to propofol, resulting in a greater than expected potentiation by the combination of propofol plus flurazepam.

Anesthetics, Intravenous↗
PubMed1996

Modulation of GABAA receptor function by alcohols: effects of subunit composition and differential effects of ethanol.

We sought to test the hypotheses that closely related alcohols would have effects on GABAA receptor function that were not predicted by differences in lipid solubility, and that the subunit structure of the GABAA receptor would significantly affect the actions of different alcohols. Cloned subunits of human GABAA receptors were expressed in Xenopus oocytes, and two-electrode voltage-clamp recording was used to quantify the membrane current response to GABA in the presence and absence of different alcohols. 1-Butanol and 2-butanol differentially potentiated the response to 20 microM GABA in oocytes expressing the alpha 1 beta 2 gamma 2L and alpha 2 beta 2 gamma 2L receptor isoforms. In the alpha 1 beta 2 gamma 2L receptor construct, 1-butanol was more potent than 2-butanol to potentiate GABAA receptor function, but 2-butanol had a greater efficacy. In the alpha 2 beta 2 gamma 2L receptor construct, 1-butanol and 2-butanol were equipotent, but 2-butanol again had a greater efficacy. In the alpha 2 beta 2 receptor construct, both 1-butanol and 2-butanol produced large potentiations of the current response to 3 microM GABA. The efficacy for butanol potentiation of GABA responses in the absence of a gamma 2L subunit was greater, but the potency was greatly reduced. Low concentrations (20 mM) of ethanol potentiated GABA responses in the alpha 1 beta 2 gamma 2L receptor construct. Ethanol potentiation of GABAA receptor function was completely blocked by the benzodiazepine receptor partial inverse agonist RO15-4513 at a concentration (0.5 microM) that did not alter the control GABA response. In contrast, RO15-4513 did not block potentiation of GABAA receptor activity induced by n-propanol, 1-butanol, 2-butanol, 1-heptanol, or propofol (2,6-diisopropylphenol). These results suggest that alcohols have specific interactions with GABAA receptors, and that ethanol may have unique effects not shared by other longer chain alcohols.

1-Butanol↗
PubMed1994

Multiple fragment ligation on glass surface: a novel approach.

Binding of DNA to pure silicon dioxide involving formation of hydrogen bonds by disruption of the hydration shell of DNA using chaotropic agents can be easily reversed with water or buffers of low ionic strength. When powdered glass was used instead of pure silicon dioxide the binding of DNA to glass was less easily reversed. DNA bound to glass permits digestion by restriction enzyme, ligation and transformation. The method opens up the possibility of enhancing transformation efficiency especially for DNA digests containing more than two pieces.

DNA, Bacterial↗