Human antigen R transfers miRNA to Syntaxin 5 to synergize miRNA export from activated macrophages

Intercellular miRNA exchange acts as a key mechanism to control gene expression post-transcriptionally in mammalian cells. Regulated export of repressive miRNAs allows the expression of inflammatory cytokines in activated macrophages. Intracellular trafficking of miRNAs from the endoplasmic reticulum to endosomes is a rate-determining step in the miRNA export process and plays an important role in controlling cellular miRNA levels and inflammatory processes in macrophages. We have identified the SNARE protein Syntaxin 5 (STX5) to show a synchronized expression pattern with miRNA activity loss in activated mammalian macrophage cells. STX5 is both necessary and sufficient for macrophage activation and clearance of the intracellular pathogen Leishmania donovani from infected macrophages. Exploring the mechanism of how STX5 acts as an immunostimulant, we have identified the de novo RNA-binding property of this SNARE protein that binds specific miRNAs and facilitates their accumulation in endosomes in a cooperative manner with human ELAVL1 protein, Human antigen R. This activity ensures the export of miRNAs and allows the expression of miRNA-repressed cytokines. Conversely, in its dual role in miRNA export, this SNARE protein prevents lysosomal targeting of endosomes by enhancing the fusion of miRNA-loaded endosomes with the plasma membrane to ensure accelerated release of extracellular vesicles and associated miRNAs.

HuR-miRNA complex activates RAS GTPase RalA to facilitate endosome targeting and extracellular export of miRNAs

Extracellular vesicles-mediated exchange of miRNA cargos between diverse types of mammalian cells is a major mechanism of controlling cellular miRNA levels and activity, thus regulating the expression of miRNA-target genes in both donor and recipient cells. Despite tremendous excitement related to extracellular vesicles-associated miRNAs as biomarkers or having therapeutic potential, the mechanism of selective packaging of miRNAs into endosomes and multivesicular bodies for subsequent extracellular export is poorly studied due to the lack of an in vitro assay system. Here, we have developed an in vitro assay with endosomes isolated from mammalian macrophage cells to follow miRNA packaging into endocytic organelles. The synthetic miRNAs, used in the assay, get imported inside the isolated endosomes during the in vitro reaction and become protected from RNase in a time- and concentration-dependent manner. The selective miRNA accumulation inside endosomes requires both ATP and GTP hydrolysis and the miRNA-binding protein HuR. The HuR-miRNA complex binds and stimulates the endosomal RalA GTPase to facilitate the import of miRNAs into endosomes and their subsequent export as part of the extracellular vesicles. The endosomal targeting of miRNAs is also very much dependent on the endosome maturation process that is controlled by Rab5 protein and ATP. In summary, we provide an in vitro method to aid in the investigation of the mechanism of miRNA packaging process for its export from mammalian macrophage cells.

Accelerated export of Dicer1 from lipid-challenged hepatocytes buffers cellular miRNA-122 levels and prevents cell death

Hepatocytes on exposure to high levels of lipids reorganize the metabolic program while fighting against the toxicity associated with elevated cellular lipids. The mechanism of this metabolic reorientation and stress management in lipid-challenged hepatocytes has not been well explored. We have noted the lowering of miR-122, a liver-specific miRNA, in the liver of mice fed with either a high-fat diet or a methionine-choline-deficient diet that is associated with increased fat accumulation in mice liver. Interestingly, low miR-122 levels are attributed to the enhanced extracellular export of miRNA processor enzyme Dicer1 from hepatocytes in the presence of high lipids. Export of Dicer1 can also account for the increased cellular levels of pre-miR-122-the substrate of Dicer1. Interestingly, restoration of Dicer1 levels in the mouse liver resulted in a strong inflammatory response and cell death in the presence of high lipids. Increasing death of hepatocytes was found to be caused by increased miR-122 levels in hepatocytes restored for Dicer1. Thus, the Dicer1 export by hepatocytes seems to be a key mechanism to combat lipotoxic stress by shunting out miR-122 from stressed hepatocytes. Finally, as part of this stress management, we determined that the Ago2-interacting pool of Dicer1, responsible for mature microribonucleoprotein formation in mammalian cells, gets depleted. miRNA-binder and exporter protein HuR is found to accelerate Ago2-Dicer1 uncoupling to ensure export of Dicer1 via extracellular vesicles in lipid-loaded hepatocytes.

Leishmania survives by exporting miR-146a from infected to resident cells to subjugate inflammation

Leishmania donovani, the causative agent of visceral leishmaniasis, infects and resides within tissue macrophage cells. It is not clear how the parasite infected cells crosstalk with the noninfected cells to regulate the infection process. During infection, Leishmania adopts a dual strategy for its survival by regulating the intercellular transport of host miRNAs to restrict inflammation. The parasite, by preventing mitochondrial function of host cells, restricts the entry of liver cell derived miR-122-containing extracellular vesicles in infected macrophages to curtail the inflammatory response associated with miR-122 entry. On contrary, the parasite up-regulates the export of miR-146a from the infected macrophages. The miR-146a, associated with the extracellular vesicles released by infected cells, restricts miR-122 production in hepatocytes while polarizing neighbouring naïve macrophages to the M2 state by affecting the cytokine expression. On entering the recipient macrophages, miR-146a dominates the miRNA antagonist RNA-binding protein HuR to inhibit the expression of proinflammatory cytokine mRNAs having HuR-interacting AU-rich elements whereas up-regulates anti-inflammatory IL-10 by exporting the miR-21 to polarize the recipient cells to M2 stage.

Inhibition of extracellular vesicle-associated MMP2 abrogates intercellular hepatic miR-122 transfer to liver macrophages and curtails inflammation

Hepatic miRNA, miR-122, plays an important role in controlling metabolic homeostasis in mammalian liver. Intercellular transfer of miR-122 was found to play a role in controlling tissue inflammation. miR-122, as part of extracellular vesicles released by lipid-exposed hepatic cells, are taken up by tissue macrophages to activate them and produce inflammatory cytokines. Matrix metalloprotease 2 or MMP2 was found to be essential for transfer of extracellular vesicles and their miRNA content from hepatic to non-hepatic cells. MMP2 was found to increase the movement of the extracellular vesicles along the extracellular matrix to enhance their uptake in recipient cells. Inhibition of MMP2 restricts functional transfer of hepatic miRNAs across the hepatic and non-hepatic cell boundaries, and by targeting MMP2, we could reduce the innate immune response in mammalian liver by preventing intra-tissue miR-122 transfer. MMP2 thus could be a useful target to restrict high-fat-diet-induced obesity-related metaflammation.

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Hepatocytes on exposure to high lipid export proinflammatory miR-122 in mouse liver

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Uptake of extracellular miR-122 induces inflammatory signals in liver macrophages

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MMP2 on extracellular vesicles is essential for intercellular transfer of miRNA

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Inhibition of MMP2 prevents miR-122 transfer and stops activation of macrophages

Amyloid-β oligomers block lysosomal targeting of miRNPs to prevent miRNP recycling and target repression in glial cells

Upon exposure to amyloid-β oligomers (Aβ1-42), glial cells start expressing proinflammatory cytokines, despite an increase in levels of repressive microRNAs (miRNAs). Exploring the mechanism of this potential immunity of target cytokine mRNAs against repressive miRNAs in amyloid-β-exposed glial cells, we have identified differential compartmentalization of repressive miRNAs in glial cells that explains this aberrant miRNA function. In Aβ1-42-treated cells, whereas target mRNAs were found to be associated with polysomes attached to endoplasmic reticulum (ER), the miRNA ribonucleoprotein complexes (miRNPs) were found to be present predominantly with endosomes that failed to recycle to ER-attached polysomes, preventing repression of mRNA targets. Aβ1-42 oligomers, by masking Rab7a proteins on endosomal surfaces, affected Rab7a interaction with Rab-interacting lysosomal protein (RILP), restricting the lysosomal targeting and recycling of miRNPs. RNA-processing body (P-body) localization of the miRNPs was found to be enhanced in amyloid-β-treated cells as a consequence of enhanced endosomal retention of miRNPs. Interestingly, depletion of P-body components partly rescued the miRNA function in glial cells exposed to amyloid-β and restricted the excess cytokine expression. This article has an associated First Person interview with the first author of the paper.

Rheb-mTOR activation rescues Aβ-induced cognitive impairment and memory function by restoring miR-146 activity in glial cells

Deposition of amyloid beta plaques in adult rat or human brain is associated with increased production of proinflammatory cytokines by associated glial cells that are responsible for degeneration of the diseased tissue. The expression of these cytokines is usually under check and is controlled at the post-transcriptional level via several microRNAs. Computational analysis of gene expression profiles of cortical regions of Alzheimer’s disease patients’ brain suggests ineffective target cytokine mRNA suppression by existing micro-ribonucleoproteins (miRNPs) in diseased brain. Exploring the mechanism of amyloid beta-induced cytokine expression, we have identified how the inactivation of the repressive miR-146 miRNPs causes increased production of cytokines in amyloid beta-exposed glial cells. In exploration of the cause of miRNP inactivation, we have noted amyloid beta oligomer-induced sequestration of the mTORC1 complex to early endosomes that results in decreased Ago2 phosphorylation, limited Ago2-miRNA uncoupling, and retarded Ago2-cytokine mRNA interaction in rat astrocytes. Interestingly, constitutive activation of mTORC1 by Rheb activator restricts proinflammatory cytokine production by reactivating miR-146 miRNPs in amyloid beta-exposed glial cells to rescue the disease phenotype in the in vivo rat model of Alzheimer’s disease.

Non-canonical ago loading of EV-derived exogenous single stranded miRNA in recipient cells

MicroRNAs, the tiny regulators of gene expression, can be transferred between neighbouring cells via Extracellular Vesicles (EV) to control the expression of genes in both donor and recipient cells. How the EV-derived miRNAs get internalized and become functional in target cells is an unresolved question. We have expressed liver specific microRNA, miR-122, in non-hepatic cells for packaging in the released EVs. With these EVs, we have followed the trafficking of miR-122 to recipient HeLa cells that otherwise don't express this miRNA. We found that EV-associated miR-122 are primarily single stranded and, to become functional, get loaded onto the recipient cell Ago proteins without requiring host Dicer1. Following endocytosis, EV-associated miR-122 get loaded onto the host cell Ago on the endosomal membrane where the release of internalized miRNAs occurs in a pH-dependent manner facilitating the formation of the exogenous miRNP pool in the recipient cells. Endosome maturation defect affects EV-mediated entry of exogeneous miRNAs in mammalian cells.

Mitochondria control mTORC1 activity-linked compartmentalization of eIF4E to regulate extracellular export of microRNAs

Defective intracellular trafficking and export of microRNAs (miRNAs) have been observed in growth-retarded mammalian cells having impaired mitochondrial potential and dynamics. Here, we found that uncoupling protein 2 (Ucp2)-mediated depolarization of mitochondrial membrane also results in progressive sequestration of miRNAs within polysomes and lowers their release via extracellular vesicles. Interestingly, the impaired miRNA-trafficking process in growth-retarded human cells could be reversed in the presence of Genipin, an inhibitor of Ucp2. Mitochondrial detethering of endoplasmic reticulum (ER), observed in cells with depolarized mitochondria, was found to be responsible for defective compartmentalization of translation initiation factor eIF4E to polysomes attached to ER. This caused a retarded translation process accompanied by enhanced retention of miRNAs and target mRNAs within ER-attached polysomes to restrict extracellular export of miRNAs. Reduced compartment-specific activity of the mammalian target of rapamycin complex 1 (mTORC1), the master regulator of protein synthesis, in cells with defective mitochondria or detethered ER, caused reduced phosphorylation of eIF4E-BP1 and prevented eIF4E targeting to ER-attached polysomes and miRNA export. These data suggest how mitochondrial membrane potential and dynamics, by affecting mTORC1 activity and compartmentalization, determine the subcellular localization and export of miRNAs.

Probing the molecular mechanism of aggressive infection by antimony resistant Leishmania donovani

The disease visceral leishmaniasis (VL) or kala azar is caused by the protozoan parasite, Leishmania donovani (LD). For many decades the pentavalent antimonial drugs countered the successive epidemics of the disease in the Indian sub-continent and elsewhere. With time, antimony resistant LD (LD^R) developed and the drug in turn lost its efficacy. Infection of mammals with LD^R gives rise to aggressive infection as compared to its sensitive counterpart (LD^S) coupled with higher surge of IL-10 and TGF-β. The IL-10 causes upregulation of multidrug resistant protein-1 which causes efflux of antimonials from LD^R infected cells. This is believed to be a key mechanism of antimony resistance. MicroRNAs (miRNAs) are tiny post-transcriptional regulators of gene expression in mammalian cells and in macrophage play a pivotal role in controlling the expression of cytokines involved in infection process. Therefore, a change in miRNA profiles of macrophages infected with LD^S or LD^R could explain the differential cytokine response observed. Interestingly, the outcome of LD infection is also governed by the critical balance of pro- and anti-inflammatory cytokines which is inturn regulated by miRNA-Ago2 or miRNP complex and its antagonist RNA binding protein HuR. Here Ago2 plays the fulcrum whose phosphorylation and de-phosphorylation dictates the process; which in turn is controlled by PP2A and HuR. LD^S and LD^R upregulate PP2A and downregulate HuR at different magnitude leading to various levels of anti-inflammatory to proinflammatory cytokine production and resulting pathology in the host. While ectopic HuR expression alone is sufficient to clear LD^S infection, simultaneous upregulation of HuR and inhibition of PP2A is required to inhibit LD^R mediated infection. Therefore, tampering with miRNA pathway could be a new strategy to control infection caused by LD^R parasite.

MicroRNA exporter HuR clears the internalized pathogens by promoting pro-inflammatory response in infected macrophages

HuR is a miRNA derepressor protein that can act as miRNA sponge for specific miRNAs to negate their action on target mRNAs. Here we have identified how HuR, by inducing extracellular vesicles‐mediated export of miRNAs, ensures robust derepression of miRNA‐repressed cytokines essential for strong pro‐inflammatory response in activated mammalian macrophages. Leishmania donovani, the causative agent of visceral leishmaniasis, on the contrary alters immune response of the host macrophage by a variety of complex mechanisms to promote anti‐inflammatory response essential for the survival of the parasite. We have found that during Leishmania infection, the pathogen targets HuR to promote onset of anti‐inflammatory response in mammalian macrophages. In infected macrophages, Leishmania also upregulate protein phosphatase 2A that acts on Ago2 protein to keep it in dephosphorylated and miRNA‐associated form. This causes robust repression of the miRNA‐targeted pro‐inflammatory cytokines to establish an anti‐inflammatory response in infected macrophages. HuR has an inhibitory effect on protein phosphatase 2A expression, and mathematical modelling of macrophage activation process supports antagonistic miRNA‐modulatory roles of HuR and protein phosphatase 2A which mutually balances immune response in macrophage by targeting miRNA function. Supporting this model, ectopic expression of the protein HuR and simultaneous inhibition of protein phosphatase 2A induce strong pro‐inflammatory response in the host macrophage to prevent the virulent antimonial drug‐sensitive or drug‐resistant form of L. donovani infection. Thus, HuR can act as a balancing factor of immune responses to curtail the macrophage infection process by the protozoan parasite.

Retrograde trafficking of Argonaute 2 acts as a rate-limiting step for de novo miRNP formation on endoplasmic reticulum-attached polysomes in mammalian cells

Intracellular trafficking of Argonaute 2 controls de novo miRNP formation on endoplasmic reticulum–attached polysomes in mammalian cells.

microRNAs are short regulatory RNAs in metazoan cells. Regulation of miRNA activity and abundance is evident in human cells where availability of target messages can influence miRNA biogenesis by augmenting the Dicer1-dependent processing of precursors to mature microRNAs. Requirement of subcellular compartmentalization of Ago2, the key component of miRNA repression machineries, for the controlled biogenesis of miRNPs is reported here. The process predominantly happens on the polysomes attached with the endoplasmic reticulum for which the subcellular Ago2 trafficking is found to be essential. Mitochondrial tethering of endoplasmic reticulum and its interaction with endosomes controls Ago2 availability. In cells with depolarized mitochondria, miRNA biogenesis gets impaired, which results in lowering of de novo–formed mature miRNA levels and accumulation of miRNA-free Ago2 on endosomes that fails to interact with Dicer1 and to traffic back to endoplasmic reticulum for de novo miRNA loading. Thus, mitochondria by sensing the cellular context regulates Ago2 trafficking at the subcellular level, which acts as a rate-limiting step in miRNA biogenesis process in mammalian cells.

Biological membranes in EV biogenesis, stability, uptake, and cargo transfer: an ISEV position paper arising from the ISEV membranes and EVs workshop

Paracrine and endocrine roles have increasingly been ascribed to extracellular vesicles (EVs) generated by multicellular organisms. Central to the biogenesis, content, and function of EVs are their delimiting lipid bilayer membranes. To evaluate research progress on membranes and EVs, the International Society for Extracellular Vesicles (ISEV) conducted a workshop in March 2018 in Baltimore, Maryland, USA, bringing together key opinion leaders and hands-on researchers who were selected on the basis of submitted applications. The workshop was accompanied by two scientific surveys and covered four broad topics: EV biogenesis and release; EV uptake and fusion; technologies and strategies used to study EV membranes; and EV transfer and functional assays. In this ISEV position paper, we synthesize the results of the workshop and the related surveys to outline important outstanding questions about EV membranes and describe areas of consensus. The workshop discussions and survey responses reveal that while much progress has been made in the field, there are still several concepts that divide opinion. Good consensus exists in some areas, including particular aspects of EV biogenesis, uptake and downstream signalling. Areas with little to no consensus include EV storage and stability, as well as whether and how EVs fuse with target cells. Further research is needed in these key areas, as a better understanding of membrane biology will contribute substantially towards advancing the field of extracellular vesicles.

Leishmania donovani restricts mitochondrial dynamics to enhance miRNP stability and target RNA repression in host macrophages

The miRNA complex with Argonaute protein recognizes and down-regulates target mRNA. The role of interorganellar interactions in controlling miRNA activity in mammalian macrophages infected with Leishmania is explored.

MicroRNAs (miRNAs), the tiny regulatory RNAs, form complexes with Argonaute (Ago) proteins and inhibit gene expression in metazoan cells. While studying parasite-invaded macrophages, we identify a unique mode of gene regulation in which the parasite Leishmania donovani (Ld) causes mitochondrial depolarization, reduces mitochondrial dynamics, and restricts turnover of cellular microRNA ribonucleoprotein (miRNP) complexes in infected host cells. This leads to increased stability of miRNPs along with elevated levels of Ago2-bound cytokine mRNA in Ld-infected macrophages. Thus the increase of miRNP stability in Ld-infected cells curtails production of proinflammatory cytokines, which are otherwise detrimental for survival of the parasite within the infected macrophages. Loss of mitochondrial membrane potential is accompanied by reduced juxtaposition of endoplasmic reticulum (ER) and mitochondria as well as endosomes. This is likely coupled with enhanced sequestration and stabilization of ER- associated miRNPs observed in infected macrophage cells. Mitofusin 2 (Mfn2), a membrane protein implicated in ER–mitochondria tethering, also shows reduced expression in Ld-infected cells. A mitochondrial role in Ld-induced alteration of miRNA activity and stability is further corroborated by impaired compartmentalization and stabilization of miRNP components in Mfn2-depleted mammalian cells.

Obstacles and opportunities in the functional analysis of extracellular vesicle RNA - an ISEV position paper

The release of RNA-containing extracellular vesicles (EV) into the extracellular milieu has been demonstrated in a multitude of different in vitro cell systems and in a variety of body fluids. RNA-containing EV are in the limelight for their capacity to communicate genetically encoded messages to other cells, their suitability as candidate biomarkers for diseases, and their use as therapeutic agents. Although EV-RNA has attracted enormous interest from basic researchers, clinicians, and industry, we currently have limited knowledge on which mechanisms drive and regulate RNA incorporation into EV and on how RNA-encoded messages affect signalling processes in EV-targeted cells. Moreover, EV-RNA research faces various technical challenges, such as standardisation of EV isolation methods, optimisation of methodologies to isolate and characterise minute quantities of RNA found in EV, and development of approaches to demonstrate functional transfer of EV-RNA in vivo. These topics were discussed at the 2015 EV-RNA workshop of the International Society for Extracellular Vesicles. This position paper was written by the participants of the workshop not only to give an overview of the current state of knowledge in the field, but also to clarify that our incomplete knowledge – of the nature of EV(-RNA)s and of how to effectively and reliably study them – currently prohibits the implementation of gold standards in EV-RNA research. In addition, this paper creates awareness of possibilities and limitations of currently used strategies to investigate EV-RNA and calls for caution in interpretation of the obtained data.

Target-dependent biogenesis of cognate microRNAs in human cells

Extensive research has established how miRNAs regulate target mRNAs by translation repression and/or endonucleolytic degradation in metazoans. However, information related to the effect of target mRNA on biogenesis and stability of corresponding miRNAs in animals is limited. Here we report regulated biogenesis of cognate miRNAs by their target mRNAs. Enhanced pre-miRNA processing by AGO-associated DICER1 contributes to this increased miRNP formation. The processed miRNAs are loaded onto AGO2 to form functionally competent miRISCs both in vivo and also in a cell-free in vitro system. Thus, we identify an additional layer of posttranscriptional regulation that helps the cell to maintain requisite levels of mature forms of respective miRNAs by modulating their processing in a target-dependent manner, a process happening for miR-122 during stress reversal in human hepatic cells.

MicroRNAs are a widespread regulatory mechanism and are themselves extensively regulated. Here the authors show regulated miRNA biogenesis by the target mRNA, a layer of regulation that modulates miRNA levels dependent on target availability.

Reversible HuR-microRNA binding controls extracellular export of miR-122 and augments stress response

microRNAs (miRNAs), the tiny but stable regulatory RNAs in metazoan cells, can undergo selective turnover in presence of specific internal and external cues to control cellular response against the changing environment. We have observed reduction in cellular miR-122 content, due to their accelerated extracellular export in human hepatic cells starved for small metabolites including amino acids. In this context, a new role of human ELAV protein HuR has been identified. HuR, a negative regulator of miRNA function, accelerates extracellular vesicle (EV)-mediated export of miRNAs in human cells. In stressed cells, HuR replaces miRNPs from target messages and is both necessary and sufficient for the extracellular export of corresponding miRNAs. HuR could reversibly bind miRNAs to replace them from Ago2 and subsequently itself gets freed from bound miRNAs upon ubiquitination. The ubiquitinated form of HuR is predominantly associated with multivesicular bodies (MVB) where HuR-unbound miRNAs also reside. These MVB-associated pool of miRNAs get exported out via EVs thereby delimiting cellular miR-122 level during starvation. Therefore, by modulating extracellular export of miR-122, HuR could control stress response in starved human hepatic cells.

High Rayleigh number convection in a one-dimensional model

A model for one-dimensional convection is proposed by adding a buoyancy term to the Burgers' equation and including an equation for the temperature perturbation. A linear stability analysis shows onset of instability at a critical Rayleigh number. Computation in the unstable region shows steady convection with only one convection cell. Computations up to 10^{5} times the critical Rayleigh number do not show transition to an oscillatory state or to turbulence. Using a large Rayleigh number approximation, closed form solutions for the spectrum and the scaling for the heat transport due to nonlinear convection are obtained up to two orders. These are shown to be in good agreement with numerical results at high Rayleigh number.

mRNA Targeting to Endoplasmic Reticulum Precedes Ago Protein Interaction and MicroRNA (miRNA)-mediated Translation Repression in Mammalian Cells

MicroRNA (miRNA) binds to the 3′-UTR of its target mRNAs to repress protein synthesis. Extensive research was done to understand the mechanism of miRNA-mediated repression in animal cells. Considering the progress in understanding the mechanism, information about the subcellular sites of miRNA-mediated repression is surprisingly limited. In this study, using an inducible expression system for an miRNA target message, we have delineated how a target mRNA passes through polysome association and Ago2 interaction steps on rough endoplasmic reticulum (ER) before the miRNA-mediated repression sets in. From this study, de novo formed target mRNA localization to the ER-bound polysomes manifested as the earliest event, which is followed by Ago2 micro-ribonucleoprotein binding, and translation repression of target message. Compartmentalization of this process to rough ER membrane ensures enrichment of miRNA-targeted messages and micro-ribonucleoprotein components on ER upon reaching a steady state.

Polysome arrest restricts miRNA turnover by preventing exosomal export of miRNA in growth-retarded mammalian cells

MicroRNAs (miRNAs) are tiny posttranscriptional regulators of gene expression in metazoan cells, where activity and abundance of miRNAs are tightly controlled. Regulated turnover of these regulatory RNAs is important to optimize cellular response to external stimuli. We report that the stability of mature miRNAs increases inversely with cell proliferation, and the increased number of microribonucleoproteins (miRNPs) in growth-restricted mammalian cells are in turn associated with polysomes. This heightened association of miRNA with polysomes also elicits reduced degradation of target mRNAs and impaired extracellular export of miRNA via exosomes. Overall polysome sequestration contributes to an increase of cellular miRNA levels but without an increase in miRNA activity. Therefore miRNA activity and turnover can be controlled by subcellular distribution of miRNPs that may get differentially regulated as a function of cell growth in mammalian cells.

Insulin-like growth factor-1 prevents miR-122 production in neighbouring cells to curtail its intercellular transfer to ensure proliferation of human hepatoma cells

miRNAs are 20-22 nt long post-transcriptional regulators in metazoan cells that repress protein expression from their target mRNAs. These tiny regulatory RNAs follow tissue and cell-type specific expression pattern, aberrations of which are associated with various diseases. miR-122 is a liver-specific anti-proliferative miRNA that, we found, can be transferred via exosomes between human hepatoma cells, Huh7 and HepG2, grown in co-culture. Exosomal miR-122, expressed and released by Huh7 cells and taken by miR-122 deficient HepG2 cells, was found to be effective in repression of target mRNAs and to reduce growth and proliferation of recipient HepG2 cells. Interestingly, in a reciprocal process, HepG2 secretes Insulin-like Growth Factor 1 (IGF1) that decreases miR-122 expression in Huh7 cells. Our observations suggest existence of a reciprocal interaction between two different hepatic cells with distinct miR-122 expression profiles. This interaction is mediated via intercellular exosome-mediated miR-122 transfer and countered by a reciprocal IGF1-dependent anti-miR-122 signal. According to our data, human hepatoma cells use IGF1 to prevent intercellular exosomal transfer of miR-122 to ensure its own proliferation by preventing expression of growth retarding miR-122 in neighbouring cells.

Leishmania donovani targets Dicer1 to downregulate miR-122, lower serum cholesterol, and facilitate murine liver infection

Leishmania donovani causes visceral leishmaniasis (VL) where the parasite infects and resides inside liver and spleen tissue macrophages. Given the abnormal lipid profile observed in VL patients, we examined the status of serum lipids in an experimental murine model of VL. The murine VL liver displayed altered expression of lipid metabolic genes, many of which are direct or indirect targets of the liver-specific microRNA-122. Concomitant reduction of miR-122 expression was observed in VL liver. High serum cholesterol caused resistance to L. donovani infection, while downregulation of miR-122 is coupled with low serum cholesterol in VL mice. Exosomes secreted by the infective parasites caused reduction in miR-122 activity in hepatic cells. Leishmania surface glycoprotein gp63, a Zn-metalloprotease, targets pre-miRNA processor Dicer1 to prevent miRNP formation in L. donovani-interacting hepatic cells. Conversely, restoration of miR-122 or Dicer1 levels in VL mouse liver increased serum cholesterol and reduced liver parasite burden.

A transient reversal of miRNA-mediated repression controls macrophage activation

During macrophage activation, cytokine mRNAs are translated despite high levels of counteracting miRNAs. Here, phosphorylation of Ago2 is shown to impair its binding to miRNAs and cognate mRNAs, enabling macrophage activation and prevention of pathogen invasion.

In mammalian macrophages, the expression of a number of cytokines is regulated by miRNAs. Upon macrophage activation, proinflammatory cytokine mRNAs are translated, although the expression of miRNAs targeting these mRNAs remains largely unaltered. We show that there is a transient reversal of miRNA-mediated repression during the early phase of the inflammatory response in macrophages, which leads to the protection of cytokine mRNAs from miRNA-mediated repression. This derepression occurs through Ago2 phosphorylation, which results in its impaired binding to miRNAs and to the corresponding target mRNAs. Macrophages expressing a mutant, non-phosphorylatable AGO2—which remains bound to miRNAs during macrophage activation—have a weakened inflammatory response and fail to prevent parasite invasion. These findings highlight the relevance of the transient relief of miRNA repression for macrophage function.

HuR protein attenuates miRNA-mediated repression by promoting miRISC dissociation from the target RNA

The microRNA (miRNA)-mediated repression of protein synthesis in mammalian cells is a reversible process. Target mRNAs with regulatory AU-rich elements (AREs) in their 3′-untranslated regions (3′-UTR) can be relieved of miRNA repression under cellular stress in a process involving the embryonic lethal and altered vision family ARE-binding protein HuR. The HuR-mediated derepression occurred even when AREs were positioned at a considerable distance from the miRNA sites raising questions about the mechanism of HuR action. Here, we show that the relief of miRNA-mediated repression involving HuR can be recapitulated in different in vitro systems in the absence of stress, indicating that HuR alone is sufficient to relieve the miRNA repression upon binding to RNA ARE. Using in vitro assays with purified miRISC and recombinant HuR and its mutants, we show that HuR, likely by its property to oligomerize along RNA, leads to the dissociation of miRISC from target RNA even when miRISC and HuR binding sites are positioned at a distance. Further, we demonstrate that HuR association with AREs can also inhibit miRNA-mediated deadenylation of mRNA in the Krebs-2 ascites extract, in a manner likewise depending on the potential of HuR to oligomerize.

Stability of cylindrical Couette flow in the presence of an oscillating axial magnetic field

The linear stability of cylindrical Couette flow of an electrically conducting fluid in the presence of an axial magnetic field is examined, where the magnetic field has a small oscillatory component imposed on a steady value. The effect of the field modulation on the threshold of instability is studied for different values of gap width, Chandrasekhar number, magnetic Prandtl number, and oscillation frequency. Modulation is found to have a stabilizing effect for low values of the Chandrasekhar number, a destabilizing effect for intermediate values, and again a stabilizing effect for still higher values of the Chandrasekhar number. The effect of modulation is found to be almost independent of the magnetic Prandtl number and the modulation frequency.

Stress-induced reversal of microRNA repression and mRNA P-body localization in human cells

In metazoa, microRNAs (miRNAs) imperfectly base-pair with the 3'-untranslated region (3'UTR) of mRNAs and prevent protein accumulation by either repressing translation or inducing mRNA degradation. Examples of specific mRNAs undergoing miRNA-mediated repression are numerous, but whether the repression is a reversible process remains largely unknown. Here, we show that cationic amino acid transporter 1 (CAT-1) mRNA and reporters bearing the CAT-1 3'UTR or its fragments can be relieved from the miRNA miR-122-induced inhibition in human hepatoma cells in response to different stress conditions. The derepression of CAT-1 mRNA is accompanied by its release from cytoplasmic processing bodies (P bodies) and its recruitment to polysomes, indicating that P bodies act as storage sites for mRNAs inhibited by miRNAs. The derepression requires binding of HuR, an AU-rich-element-binding ELAV family protein, to the 3'UTR of CAT-1 mRNA. We propose that proteins interacting with the 3'UTR will generally act as modifiers altering the potential of miRNAs to repress gene expression.

Argonautes and company: sailing against the wind

The 3'-untranslated region (UTR) of mRNA is crucial for posttranscriptional regulation. In this issue, Vasudevan and Steitz (2007) report a new function for AGO2 and FXR1--two proteins that have been linked to the regulation of microRNAs and translation repression. AGO2 and FXR1 bind to the AU-rich element in the 3'-UTR of TNFalpha mRNA, unexpectedly activating its translation in a cell growth-dependent manner.

Scaling in magnetohydrodynamic convection at high Rayleigh number

The theory of Grossmann and Lohse [J. Fluid Mech. 407, 27 (2000)] is extended to include the effect of a magnetic field on convection of an electrically conducting fluid. Different scaling laws are obtained depending on whether the bulk or the boundary layers make the major contribution to the dissipation. Scalings are obtained for both weak and strong magnetic fields. The predictions are shown to be in better agreement with experimental data than earlier theoretical models.

Relief of microRNA-mediated translational repression in human cells subjected to stress

In metazoans, most microRNAs imperfectly base-pair with the 3' untranslated region (3'UTR) of target mRNAs and prevent protein accumulation by either repressing translation or inducing mRNA degradation. Examples of specific mRNAs undergoing microRNA-mediated repression are numerous, but whether the repression is a reversible process remains largely unknown. Here we show that cationic amino acid transporter 1 (CAT-1) mRNA and reporters bearing its 3'UTR can be relieved from the microRNA miR-122-induced inhibition in human hepatocarcinoma cells subjected to different stress conditions. The derepression of CAT-1 mRNA is accompanied by its release from cytoplasmic processing bodies and its recruitment to polysomes. The derepression requires binding of HuR, an AU-rich-element binding protein, to the 3'UTR of CAT-1 mRNA. We propose that proteins interacting with the 3'UTR will generally act as modifiers altering the potential of miRNAs to repress gene expression.

Formation of density waves in traffic flow through intersecting roads

The formation of density waves in two intersecting roads, with a traffic circle at the intersection, is studied. It is found that, depending on the traffic densities in the two roads, density waves can form in the traffic circle and in one or both of the roads. Depending on the expression chosen for the optimal velocity, either the congestion moves entirely to the traffic circle or the congestion becomes confined to the traffic circle and a part of the road approaching the traffic circle.

The chromatoid body of male germ cells: similarity with processing bodies and presence of Dicer and microRNA pathway components

The chromatoid body is a perinuclear, cytoplasmic cloud-like structure in male germ cells whose function has remained elusive. Here we show that the chromatoid body is related to the RNA-processing body of somatic cells. Dicer and components of microRNP complexes (including Ago proteins and microRNAs) are highly concentrated in chromatoid bodies. Furthermore, we show that Dicer interacts with a germ cell-specific chromatoid body component, the RNA helicase MVH (mouse VASA homolog). Thus, chromatoid bodies seem to operate as intracellular nerve centers of the microRNA pathway. Our findings underscore the importance of posttranscriptional gene regulation and of the microRNA pathway in the control of postmeiotic male germ cell differentiation.

Inhibition of Translational Initiation by Let-7 MicroRNA in Human Cells

MicroRNAs (miRNAs) are approximately 21-nucleotide-long RNA molecules regulating gene expression in multicellular eukaryotes. In metazoa, miRNAs act by imperfectly base-pairing with the 3' untranslated region of target messenger RNAs (mRNAs) and repressing protein accumulation by an unknown mechanism. We demonstrate that endogenous let-7 microribonucleoproteins (miRNPs) or the tethering of Argonaute (Ago) proteins to reporter mRNAs in human cells inhibit translation initiation. M(7)G-cap-independent translation is not subject to repression, suggesting that miRNPs interfere with recognition of the cap. Repressed mRNAs, Ago proteins, and miRNAs were all found to accumulate in processing bodies. We propose that localization of mRNAs to these structures is a consequence of translational repression.

Synthesis of CdS nanoparticles in colloidal state and its possible interaction with tyrosine

The basic objective is to develop a simpler method of preparation of the colloidal CdS nanoparticles with greater stability and to study interaction with tyrosine molecules. Average size of the particles in the colloid is found to be about 3 nm as probed by transmission electron microscopy (TEM) and dynamic light scattering (DLS) measurements. Effect of both sulfide enriched CdS as well as Cd(2+) enriched CdS on tyrosine is investigated both through absorbance and emission spectroscopy. Quenching of tyrosine emission followed Stern-Volmer relation and was found to be independent of temperature, indicating possible static quenching. However, Forster transfer between tyrosine and CdS can be suspected to mimic static quenching in addition to the charge transfer complexes. In the presence of Cd(2+) enriched CdS nanoparticles, the emission of tyrosine in phosphate buffer shows typical spectral broadening and a long wavelength increase in fluorescence emission. This may be attributed to the sensitized emission of CdS itself.

Different hypotensive responses to intravenous bovine and human thrombin preparations in swine

BACKGROUND

Accidental intravenous introduction of commercial bovine thrombin (BT) during use of fibrin glue may result in profound hypotension. Commercial human thrombin (HT) is now available. This study compared the effects of intravenous BT versus HT in swine.

METHODS

Swine received 30 U/kg BT, 60 U/kg BT, 30 U/kg HT, or 60 U/kg HT intravenously. Mean arterial pressure (MAP) and survival were monitored for 30 minutes. Thrombin purities and in vitro activities were examined.

RESULTS

MAP nadir was lower (p < 0.05) after BT, 27.7 +/- 3.3% (mean +/- SEM) of pretreatment MAP, compared with 41.1 +/- 3.7% after HT. Five of six animals died after 60 U/kg BT, whereas all others survived (p < 0.05). Histology suggested more severe disseminated intravascular coagulation after BT. HT was purer than BT. In vitro activities were similar.

CONCLUSION

Both BT and HT produced hypotension. HT appeared safer, because of higher purity. Regardless of source and purity, thrombin must be used with caution.

Mutations in a tRNA import signal define distinct receptors at the two membranes of Leishmania mitochondria

Nucleus-encoded tRNAs are selectively imported into the mitochondrion of Leishmania, a kinetoplastid protozoan. An oligoribonucleotide constituting the D stem-loop import signal of tRNA(Tyr)(GUA) was efficiently transported into the mitochondrial matrix in organello as well as in vivo. Transfer through the inner membrane could be uncoupled from that through the outer membrane and was resistant to antibody against the outer membrane receptor TAB. A number of mutations in the import signal had differential effects on outer and inner membrane transfer. Some mutants which efficiently traversed the outer membrane were unable to enter the matrix. Conversely, restoration of the loop-closing GC pair in reverse resulted in reversion of transfer through the inner, but not the outer, membrane, and binding of the RNA to the inner membrane was restored. These experiments indicate the presence at the two membranes of receptors with distinct specificities which mediate stepwise transfer into the mitochondrial matrix. The combination of oligonucleotide mutagenesis and biochemical fractionation may provide a general tool for the identification of tRNA transport factors.

Stepwise transfer of tRNA through the double membrane of Leishmania mitochondria

Import of tRNA into Leishmania mitochondria involves transfer through a double membrane barrier. To examine whether specific sorting mechanisms for individual tRNAs direct them to different mitochondrial compartments, the distribution of tRNA transcripts, internalized in vitro, was examined by suborganellar fractionation. Significant amounts of tRNA(Tyr) were localized in the matrix and on the outer face of the inner mitochondrial membrane. With time, the matrix:membrane ratio increased. Translocation through the inner membrane apparently required the presence of a specific signal in the D arm of tRNA(Tyr), and tRNA(Gln)(CUG), lacking this sequence, was excluded. Hydrolysis of ATP was necessary at both the outer and inner membranes. However, the protonophores carbonylcyanide m-chlorophenylhydrazone and nigericin, the K(+) ionophore valinomycin, and the F(1)F(0) ATPase inhibitor oligomycin had only marginal effects on uptake through the outer membrane but severely inhibited inner membrane translocation, indicating the unusual requirement of both the electrical and chemical components of the electromotive force generated across the inner membrane. The results are consistent with a mechanism involving stepwise transfer of tRNA through distinct outer and inner membrane channels.

Smoke-induced inhalation injury: effects of retinoic acid and antisense oligodeoxynucleotide on stability and differentiated state of the mucociliary epithelium

Rabbit tracheal explants, exposed to burning pine wood smoke, were cultured in a chemically defined medium with and without retinoic acid (+/- RA). Exposures of 15-20 minute led to RA-independent degeneration of the mucociliary epithelial sheath. In 10 minute exposures tissue integrity was retained, but epithelial morphology changed from normal pseudostratified columnar to the flattened appearance typical of the squamous phenotype. Despite the dramatic shift in morphology, explants exhibited normal RA-dependent mucin gene expression characteristic of the mucociliary phenotype. Furthermore, electron micrographs showed continued presence of both secretory granules and cilia. RA(+) cultures also showed a normal pattern of adherent epithelial cells. In RA(-) cultures, however, there were prominent intercellular spaces indicating an RA dependence for maintaining adhesive contacts following smoke exposure. An 18-mer mucin antisense oligomer that suppressed mucin gene expression also unexpectedly blocked the smoke induced metaplasia in RA(+) cultures, but the sense oligomer had no effect.

Differentiation of respiratory epithelium: the effects of retinoic acid and carcinogens on the expression of mucociliary vs. squamous phenotype

Changes in ultrastructural characteristics and mucin gene expression were examined in rat tracheal explants cultured in a synthetic medium +/- retinoic acid (RA), benzo[a]pyrene (B[a]P) and N-methyl-N-nitrosourea (NMNU). In the RA(+) cultures, no changes in either ultrastructural features or mucin gene expression were detected after 48 h incubation. After 96 h incubation, however, the ultrastructural features associated with the squamous phenotype were characteristics of cultures containing the two carcinogens and the mucin gene expression was slightly reduced. Thus, in the presence of retinoic acid, the carcinogen induced changes in cytology to the squamous phenotypes were not matched by a marked loss of mucin gene expression. Explants cultured for 48 h without RA and +/- carcinogens showed none of the cytological changes associated with onset of the squamous phenotype. While mucin mRNA was still detected, it was clearly reduced compared to 48 h cultures in RA(+) medium. However, 48 h later, all explants exhibited pronounced squamous metaplasia and the mucin message decreased to trace levels. Thus, the results of these experiments with B[a]P and NMNU in RA(+) and RA(-) media indicates that at least the early carcinogen induced changes may be distinct from those associated with the retinoid pathway controlling expression of the mucin component of the mucociliary epithelium.

Reactions of OH and eaq- adducts of cytosine and its nucleosides or nucleotides with Cu(II) ions in dilute aqueous solutions: a steady-state and pulse radiolysis study

The reactions of OH and eaq- adducts of cytosine, cytidine and deoxycytidine in the presence of Cu(II) ions have been studied by product analysis and pulse radiolysis. The product analysis studies show that the degradation of the base is enhanced in N2O-saturated conditions in the presence of Cu(II) ions and the major radiolytic products are Cu(I), cytosine glycols and 5(6)-hydroxycytosine. It is also interesting to note that the yields of Cu(I) are equivalent to cytosine degradation yields, which suggests that the interaction of the OH adducts with Cu(II) ions restricts the radical recombination reactions (known to be the major physicochemical repair process) which partly regenerate the parent cytosine. The rate constants of the reactions of cytosine OH adducts with Cu(II) ions determined by pulse radiolysis lie between 10(7) and 10(8) dm3 mol-1 s-1. The growth in the transient absorption spectra of cytosine OH adducts in the range 330-400 nm, observed in the presence of copper(II) ions in free and complexed state, suggests formation of copper radical adduct which decays by water insertion at the copper-carbon bond to give glycol as the major product. Such copper radical adduct formation was also observed in the case of cytidine and deoxycytidine. The protonated electron adducts (at the hetero atoms) of cytosine, cytidine and deoxycytidine transfer electrons to the Cu(II) ions with rate constants of 10(8) and 10(9) dm3 mol-1 s-1. Here no adduct formation is observed. The steady-state results show that such electron transfer reactions regenerate the parent molecules themselves. Hence such electron transfer reactions do not contribute to enhanced base degradation in the presence of copper ions.

Radiosensitization of thymine by copper(II) and nickel(II) complexes of metronidazole

Aqueous solutions of thymine (10(-3) mol dm(-3)) were irradiated with Co 60 gamma-rays in the absence and presence of metronidazole as well as its Cu(II) and Ni(II) complexes (10(-4) mol dm(-3)) under different conditions. The yields for the loss of thymine and those for the formation of the products of radiolysis were determined by hplc. The degradation of thymine when compared with that in the absence of metronidazole was not significantly altered. However, it increased significantly when the radiolysis of thymine was carried out in the presence of the Cu(II) and Ni(II) complexes of metronidazole. All three compounds were found to promote the formation of thymine glycol although the increase was more significant for the metal complexes than for free metronidazole. The higher radiosensitizing efficiency observed with the metal complexes as compared with that for free metronidazole is due to their higher rate of oxidation of the transient thymine-OH radical adduct. The reduction of the sensitizers results in the formation of the nitro-anion radical which in the case of the Cu(II) complex undergoes intramolecular electron transfer to the metal centre leading to the formation of Cu(I).

Retinoic acid-regulated cellular differentiation and mucin gene expression in isolated rabbit tracheal-epithelial cells in culture

Rabbit tracheal epithelial cells were cultured in a serum-free and hormone-supplemented medium with and without retinoic acid. The cells showed time-dependent mucin gene expression when cultured in the medium with retinoic acid. In the absence of retinoic acid, however, mucin mRNA was barely detectable in the cells. When retinoic acid was added back to the medium, the mucin message was prominent again. Actinomycin D and cycloheximide did not inhibit mucin gene expression. The mucin message was slightly elevated by cAMP agonists. A mucin antisense oligomer inhibited the retinoic acid-induced mucin mRNA expression and secretion, thus offering an alternate approach in the management of mucus hypersecretion in upper airway respiratory diseases such as chronic bronchitis, asthma, and cystic fibrosis.

Retinoic acid modulation of mucin mRNA in rat tracheal explants: response to actinomycin D, cycloheximide, signal transduction effectors and antisense oligodeoxynucleotide

Factors affecting the retinoic acid modulated expression of mucin mRNA in rat tracheal cultures were studied. Actinomycin D had no effect on mucin mRNA in cultures grown with retinoic acid (RA+). The usual precipitous drop in mucin mRNA in cultures lacking retinoic acid (RA-) was prevented by actinomycin D. Cycloheximide also had no effect on mucin mRNA in RA+ cultures, but, like actinomycin D, it prevented the precipitous drop in mucin mRNA in RA- cultures. cAMP agonists had some marginal effects on the mucin mRNA, but none as dramatic as those noted by actinomycin D and cycloheximide in the RA- cultures. An antisense oligomer (18 bases) to rat mucin cDNA inhibited the mucin mRNA expression in RA+ cultures.

Effect of retinoic acid on mucin gene expression in rat airways in vitro

Ultrastructural examination of rat tracheal explants at various times of culture in a serum-free and hormone-supplemented medium containing retinoic acid showed that the cytological characteristics of the epithelium were well preserved for at least 192 h. Hybridization analyses for mucin core protein mRNA in the explants were performed with a 30-base oligonucleotide probe, the design of which was based on the tandem repeat sequence of the rat intestine mucin core protein. The probe reacted with total RNA prepared from trachea, intestine and colon, but not with total RNA obtained from liver or alveolar region of the lung. Type-I keratin expression was observed in the explant grown at different periods of time in a medium with and without retinoic acid. The hybridization probe gave a prominent reaction with RNA preparations obtained from tracheal explants incubated for as long as 192 h in a medium containing retinoic acid. In the absence of retinoic acid, however, the mucin message was evident at the 24 h time point but thereafter decreased to barely detectable levels. When retinoic acid was added at 96 h to the latter cultures, the mucin mRNA was prominent again after additional incubation for 24 and 48 h. Northern-blot analyses of tracheal RNA showed a diffuse band at approx. 7.5 kb. Addition of a variety of chemical and pharmacological agents to explants cultured in the presence of retinoic acid had no dramatic induction or inhibitory effects on the mucin mRNA. Only the steroid prednisolone had a reproducible inhibitory effect.

Tyrosine residues in unirradiated and gamma-irradiated dihydroorotate dehydrogenase: fluorimetric and second-derivative absorption spectrophotometric studies

The effect of gamma-radiation on tyrosine residues of dihydroorotate dehydrogenase under different conditions was investigated by means of fluorescence spectroscopy and second-derivative spectrophotometry. No change in the fluorescence spectral distribution was observed when unirradiated and irradiated enzyme were denatured in guanidine hydrochloride. However, decreases in fluorescence intensity in irradiated samples indicates a radiation-induced modification of tyrosine residues. The fluorescence intensity at 340 nm decreased exponentially with radiation dose in aerated medium but non-exponentially under argon and nitrous oxide-saturated conditions. The percentage loss of tyrosine fluorescence under different conditions was determined. The number of tyrosine residues left intact following irradiation at a dose for 50% inactivation under different conditions was measured by second-derivative absorption spectrophotometry. The results obtained from both these methods show that the hydroxyl radical is less efficient in inducing radiation damage of tyrosine in aerated conditions compared with that under deoxygenated conditions. This lower efficiency of the hydroxyl radical in aerated medium has been attributed to the protective effect of oxygen and/or the superoxide radical anion.

Radiation-induced inactivation of dihydroorotate dehydrogenase in dilute aqueous solution

The inactivation of dihydroorotate dehydrogenase by gamma irradiation in dilute aqueous solution has been investigated. The activity of the enzyme decreased exponentially as a function of the absorbed dose under aerated and nitrous oxide-saturated conditions. The contributions of the individual radical species derived from water radiolysis were estimated from the inactivation results observed under aerated, argon-saturated, and nitrous oxide-saturated conditions. The hydrogen atom and hydroxyl radical were found to be important in enzyme inactivation. The effect of selected inorganic radical anions such as Br.2-, I.2-, and (SCN).2- on the enzyme activity was also studied, and the results implicate the possible involvement of cysteine and tyrosine residues in the catalytic activity of dihydroorotate dehydrogenase. Changes in the kinetic parameters (Michaelis-Menten constant, Km, and maximal velocity, Vmax) due to irradiation under the conditions investigated suggest that radiation-induced inactivation is due to modification of the substrate binding sites and that of the active site residues in the enzyme. Evidence for the reduction of iron-sulfur centers in the enzyme during the inactivation process has been put forward from the difference spectrum of the irradiated dihydroorotate dehydrogenase. It has also been shown by electrophoretic studies that radiation-induced inactivation was not due to any fragmentation of the protein structure or the formation of any intermolecular crosslinking.

In vitro effects of drugs on production of mucins in rabbit tracheal epithelial cells expressing mucin gene: a model system for studying upper airway respiratory diseases

Rabbit tracheal epithelial cells, cultured on collagen-coated dishes in serum-free and hormone-supplemented medium, were found to incorporate [3H]glucosamine into high-molecular-weight components that were secreted in the medium. The chemical analysis of the secreted products resulted in a profile that resembled that of mucous glycoproteins (mucins). When examined by dot blot analysis, the total RNA isolated from these cells hybridized to an antisense 30-mer oligonucleotide corresponding to a rat intestine mucin peptide sequence, indicating that mucin gene was expressed in these cell lines. Lung and liver tissues of rabbit did not express this gene. Transmission electron microscopy exhibited secretory granules in these cells. The incorporation of [3H]glucosamine into mucins was inhibited by three aryl-N-acetyl-galactosaminides and a chemical carcinogen, N-nitroso-N-ethyl urea, whereas 5-azacytidine enhanced the proliferation of cells as well as the radiolabeling of mucins. Parasympathetic agent (pilocarpine), cholinergic antagonist (atropine), and beta-adrenergic agonist (isoproterenol) alone have little effect on the secretion of mucins. The cholinergic agonist, methacholine, was found to increase the production of mucins and addition of atropine to the medium before methacholine blocked this stimulation. Histamine was found to stimulate mucin production in these cells.