Cytoskeleton involvement in the distribution of mRNP complexes and small cytoplasmic RNAs

Distinct RNP complexes of shuttling hnRNP proteins with pre-mRNA and mRNA: candidate intermediates in formation and export of mRNA

Nascent pre-mRNAs associate with hnRNP proteins in hnRNP complexes, the natural substrates for mRNA processing. Several lines of evidence indicate that hnRNP complexes undergo substantial remodeling during mRNA formation and export. Here we report the isolation of three distinct types of pre-mRNP and mRNP complexes from HeLa cells associated with hnRNP A1, a shuttling hnRNP protein. Based on their RNA and protein compositions, these complexes are likely to represent distinct stages in the nucleocytoplasmic shuttling pathway of hnRNP A1 with its bound RNAs. In the cytoplasm, A1 is associated with its nuclear import receptor (transportin), the cytoplasmic poly(A)-binding protein, and mRNA. In the nucleus, A1 is found in two distinct types of complexes that are differently associated with nuclear structures. One class contains pre-mRNA and mRNA and is identical to previously described hnRNP complexes. The other class behaves as freely diffusible nuclear mRNPs (nmRNPs) at late nuclear stages of maturation and possibly associated with nuclear mRNA export. These nmRNPs differ from hnRNPs in that while they contain shuttling hnRNP proteins, the mRNA export factor REF, and mRNA, they do not contain nonshuttling hnRNP proteins or pre-mRNA. Importantly, nmRNPs also contain proteins not found in hnRNP complexes. These include the alternatively spliced isoforms D01 and D02 of the hnRNP D proteins, the E0 isoform of the hnRNP E proteins, and LRP130, a previously reported protein with unknown function that appears to have a novel type of RNA-binding domain. The characteristics of these complexes indicate that they result from RNP remodeling associated with mRNA maturation and delineate specific changes in RNP protein composition during formation and transport of mRNA in vivo.

mRNA localization by a 145-nucleotide region of the c-fos 3'--untranslated region. Links to translation but not stability

The presence of a localization signal in the 3'-untranslated region of c-fos mRNA was investigated by in situ hybridization and cell fractionation techniques. Cells were transfected with chimeric gene constructs in which the beta-globin coding region was used as a reporter and linked to either its own 3'-untranslated region, the c-fos 3'-untranslated region, or the c-fos 3'-untranslated region containing different deletions. Replacement of the endogenous beta-globin 3'-untranslated region by that from c-fos caused a redistribution of the transcripts so that they were recovered in cytoskeletal-bound polysomes and seen localized in the perinuclear cytoplasm. Deletion of the AU-rich instability region did not affect transcript localization, but removal of a distinct 145-nucleotide region of the 3'-untranslated region abolished it. The prevention of transcript translation by desferrioxamine led to a marked loss of transcript localization, independent of mRNA instability. The data show that the 3'-untranslated region of c-fos mRNA, as c-myc, contains a localization signal, which targets the mRNA to the perinuclear cytoskeleton. We propose that this is important to ensure efficient nuclear import of these key regulatory proteins. mRNA localization by the fos 3'-untranslated region is independent of mRNA instability, and the two are determined by different regulatory elements.

Influence of metallothionein-1 localization on its function

Metallothioneins (MTs) have a major role to play in metal metabolism, and may also protect DNA against oxidative damage. MT protein has been found localized in the nucleus during S-phase. The mRNA encoding the MT-1 isoform has a perinuclear localization, and is associated with the cytoskeleton; this targeting, due to signals within the 3'-untranslated region (3'-UTR), facilitates nuclear localization of MT-1 during S-phase [Levadoux, Mahon, Beattie, Wallace and Hesketh (1999) J. Biol. Chem. 274, 34961-34966]. Using cells transfected with MT gene constructs differing in their 3'-UTRs, the role of MT protein in the nucleus has been studied. Chinese hamster ovary cells were transfected with either the full MT gene (MTMT cells) or with the MT 5'-UTR and coding region linked to the 3'-UTR of glutathione peroxidase (MTGSH cells). Cell survival following exposure to oxidative stress and chemical agents was higher in cells expressing the native MT gene than in cells where MT localization was disrupted, or in untransfected cells. Also, MTMT cells showed less DNA damage than MTGSH cells in response to either hydrogen peroxide or mutagen. After exposure to UV light or mutagen, MTMT cells showed less apoptosis than MTGSH cells, as assessed by DNA fragmentation and flow cytometry. The data indicate that the perinuclear localization of MT mRNA is important for the function of MT in a protective role against DNA damage and apoptosis induced by external stress.

A general RNA-binding protein complex that includes the cytoskeleton-associated protein MAP 1A

Association of mRNA with the cytoskeleton represents a fundamental aspect of RNA physiology likely involved in mRNA transport, anchoring, translation, and turnover. We report the initial characterization of a protein complex that binds RNA in a sequence-independent but size-dependent manner in vitro. The complex includes a approximately 160-kDa protein that is bound directly to mRNA and that appears to be either identical or highly related to a approximately 1600-kDa protein that binds directly to mRNA in vivo. In addition, the microtubule-associated protein, MAP 1A, a cytoskeletal associated protein is a component of this complex. We suggest that the general attachment of mRNA to the cytoskeleton may be mediated, in part, through the formation of this ribonucleoprotein complex.

The compartmentalization of protein synthesis: importance of cytoskeleton and role in mRNA targeting

Following the synthesis of mRNA molecules in eukaryotic cells, the transcripts are processed in the nucleus and subsequently transported through the nuclear membrane into the cytoplasm before being sequestered into polysomes where the information contained in the RNA molecule is translated into an amino acid sequence. Recent evidence suggests that an association of mRNAs with the cytoskeleton might be important in targeting mechanisms and, furthermore, in the transport of mRNA from the nucleus to its correct location in the cytoplasm. Until recently, polysomes have been considered to exist in two classes, namely free or membrane-bound. There is now compelling evidence, however, that ribosomes, in addition to being associated with endoplasmic reticulum membranes, also are associated with components of the cytoskeleton. Thus, a large number of morphological and biochemical studies have shown that mRNA, polysomes and translational factors are associated with cytoskeletal structures. Although the actual nature and significance of the interaction between components of the translational apparatus and the cytoskeleton is not yet understood in detail, it would seem evident that such interactions are important in both the spatial organization and control of protein synthesis. Recent work has shown that a subcellular fraction, enriched in cytoskeletal components, contains polysomes and these (cytoskeletal-bound) polysomes have been shown to contain specific mRNA species. Thus, a population of cytoskeletal-bound polysomes may provide a specialized mechanism for the sorting, targeting and topographical segregation of mRNAs. In this review, current knowledge of the subcellular compartmentalization of mRNAs is discussed.

A localisation signal in the 3′ untranslated region of c-myc mRNA targets c-myc mRNA and beta-globin reporter sequences to the perinuclear cytoplasm and cytoskeletal-bound polysomes

There is increasing evidence that in mammalian cells some mRNAs are localised to specific parts of the cytoplasm and a proportion of mRNAs and polyribosomes are associated with the cytoskeleton. It has been shown previously that c-myc mRNA is present in the perinuclear cytoplasm and associated with the cytoskeleton, and that this localisation is dependent upon the 3′ untranslated region of the mRNA. The present studies show that in transfected fibroblasts the c-myc 3′ untranslated region is able to localise beta-globin reporter sequences to the perinuclear cytoplasm. Studies with constructs containing deletions within the 3′ untranslated region identify the region between bases 194 and 280 as critical for localisation. Transfection of cells with constructs in which this region is linked to beta-globin sequences showed that it was sufficient to localise the chimaeric transcripts to the perinuclear cytoplasm and to cytoskeletal-bound polyribosomes. Transfection with constructs containing a mutated AUUUA sequence within the 194–280 base region showed that this conserved AUUUA is required for targeting of both c-myc mRNA and a chimaeric transcript of beta-globin transcripts linked to the c-myc 3′ untranslated region. The region between bases 194 and 280 did not induce instability of beta-globin transcripts and the AUUUA mutation had little effect upon mRNA stability. We propose that this 86 nt region of the 3′ untranslated region contains a localisation signal to target c-myc mRNA so that it is retained on cytoskeletal-bound polysomes in the perinuclear cytoplasm; a conserved AUUUA sequence appears to be a critical part of this signal.

The mRNAs for cyclin A, c-myc and ribosomal proteins L4 and S6 are associated with cytoskeletal-bound polysomes in HepG2 cells

Cytosolic, cytoskeleton and membrane fractions were extracted from HepG2 cells by a sequential detergent/salt extraction procedure. The cytosolic fraction contained 93% of the lactic dehydrogenase activity while the cytoskeleton fraction was enriched in actin and vimentin. The distribution of mRNAs for c-myc, glucose transporter 1, ribosomal proteins L4 and S6 and cyclin A were investigated by Northern hybridization of total RNA extracted from polysomes isolated from cytosolic, cytoskeleton and membrane fractions. The membrane-bound polysomes were enriched in the glucose transporter 1 mRNA and the cytoskeleton-bound polysomes were enriched in the mRNAs for the two ribosomal proteins, c-myc and cyclin A. The results suggest that the mRNAs for nuclear proteins are one class of mRNAs which are translated on polysomes associated with the cytoskeleton; this may be related to the requirement to transport the newly synthesized protein to the nucleus.

Selective induction of cell cycle regulatory genes cdk1 (p34cdc2), cyclins A/B, and the tumor suppressor gene Rb in transformed cells by okadaic acid

Genes encoding cdk1 (p34cdc2), cyclin A, cyclin B, and the tumor suppressor gene Rb are fundamental regulators of cell cycle progression which associate as a complex with the transcription factor E2F. Expression of many of these proteins has previously been shown to be repressed by okadaic acid, a specific inhibitor of protein phosphatases 1/2A (PP1/PP2A), resulting in growth arrest in nontransformed but immortalized cells. We have investigated levels of mRNA encoding cdk1 (p34cdc2), cyclin A, cyclin B, Rb, GAPDH, c-myc, and histone H4 genes for sensitivity to okadaic acid in HeLa cells to determine if transformation altered their regulation. Serum starvation slowed growth and diminished mRNA levels for all genes tested except c-myc and GAPDH. When starved cells were subsequently exposed to 19 nM okadaic acid or refed 10% serum, mRNA levels of cyclin A, cyclin B, cdk1, and Rb dramatically increased while mRNA levels for c-myc and GAPDH were largely unaffected. Histone H4 mRNA levels and the rate of DNA synthesis were greatly enhanced by serum addition but not affected appreciably by okadaic acid. Okadaic acid was also effective in blocking proliferation of exponentially growing HeLa cells at G2/M and S phase. Despite the cell cycle phase-specific block, elevated mRNA levels for cdk1, cyclin A, cyclin B, Rb, and suppression of H4 mRNA levels were detected and persisted for at least 12 hr following okadaic acid removal. The results demonstrate that cell cycle progression is blocked and several cell cycle regulatory genes, encoding transcription factor E2F-associated proteins, experience elevation of mRNA levels through mechanisms sensitive to okadaic acid likely through a PP1/PP2A-sensitive mechanism. Data from transformed cells contrast with data from immortalized but nontransformed cells in which okadaic acid also blocks cell cycle progression during G2/M phase but suppresses expression of these genes. Such contrasts may be correlated with reduced growth factor dependence and transformation.

Translation and the cytoskeleton: a mechanism for targeted protein synthesis

This review describes the critical evidence that in eukaryotic cells polyribosomes, mRNAs and components of the protein synthetic machinery are associated with the cytoskeleton. The role of microtubules, intermediate filaments and microfilaments are discussed; at present most evidence suggests that polyribosomes interact with the actin filaments. The use of non-ionic detergent/deoxycholate treatment in the isolation of cytoskeletal-bound polysomes is described and the conclusion reached that at low salt concentrations this leads to mixed preparations of polysomes derived from both the cytoskeleton and the endoplasmic reticulum. At present the best approach for isolation of cytoskeletal-bound polysomes appears to involve extraction with salt concentrations greater than 130 mM after an initial non-ionic detergent treatment. Such polysomes appear to be enriched in certain mRNAs and thus it is suggested that they are involved in translation of a unique set of proteins. The evidence for mRNA localisation is presented and the role of the cytoskeleton in transport and localisation of RNA discussed. Recent data on the role of the 3' untranslated region in the targeting of mRNAs both to particular regions of the cell and for translation on cytoskeletal-bound polysomes is described. The hypothesis is developed that the association of polysomes with the cytoskeleton is the basis of a mechanism for the targeting of mRNAs and the compartmentalization of protein synthesis.

Targeting of c-myc and beta-globin coding sequences to cytoskeletal-bound polysomes by c-myc 3' untranslated region

The influence of the 3' untranslated region on mRNA localization was investigated by measuring the distribution of myc, beta-globin and hybrid myc-globin mRNAs between free, cytoskeletal-bound and membrane-bound polysomes in cells transfected with either control or chimeric gene constructs. c-myc sequences and beta-globin-coding sequences linked to the myc 3' untranslated region were present at greatest enrichment in cytoskeletal-bound polysomes. beta-Globin mRNA and myc-coding sequences linked to the beta-globin 3' untranslated region were recovered largely in the free polysomes. In situ hybridization confirmed that replacement of the c-myc 3' untranslated region by that of globin caused a relocalization of the mRNA. The results suggest that mRNA localization in differentiated eukaryotic cells depends on a mechanism that involves directional information in the 3' untranslated region of mRNAs.

Differential detergent fractionation of isolated hepatocytes: biochemical, immunochemical and two-dimensional gel electrophoresis characterization of cytoskeletal and noncytoskeletal compartments

Two-dimensional (2-D) gel electrophoresis is often used in toxicologic and metabolic studies to assess treatment- or stage-specific changes in protein synthesis, degradation or posttranslational modification. When combined with cell fractionation studies the detectability of low abundance proteins is enhanced, and changes in subcellular distribution of proteins can also be monitored. Detergent fractionation is a simpler alternative to differential pelleting, which partitions cellular constituents into functionally distinct populations while preserving cytoskeletal integrity. We defined and characterized a differential detergent fractionation (DDF) protocol to enable protein dynamics in cytoskeletal and noncytoskeletal compartments of isolated hepatocytes to be monitored simultaneously. Rat hepatocytes were maintained in suspension culture and fractionated by sequential extraction with detergent-containing buffers (digitonin/EDTA, Triton/EDTA, Tween/deoxycholate). DDF reproducibly yielded four electrophoretically distinct fractions enriched in cytosolic, membrane-organelle, nuclear membrane and cytoskeletal-matrix markers, respectively. Immunoblotting with over 20 different antibodies corroborated the selectivity of fractionation and was used to characterize the distribution profiles of cytoskeletal (actin, tubulins, cytokeratins, vinculin, myosin, desmoplakins, fodrin, nuclear lamins) and noncytoskeletal proteins (heat-shock 70 proteins, glutathione-S-transferase, calpains, carbamoyl phosphate synthetase, etc.), as well as to identify spots in 2-D gels. Detergent buffers were compatible with equilibrium or nonequilibrium 2-D gel electrophoretic analysis. Extensive 2-D maps of acidic and basic proteins in each fraction were generated along with a tabular listing of M(r) and pI. Thus, DDF reproducibly partitions hepatocytic proteins into functionally distinct cytoskeletal and noncytoskeletal compartments that are readily analyzed by 2-D gel electrophoresis. DDF is simple, applicable to use with other cell types or culture systems and is especially useful when biomaterial is limited (i.e., clinical studies).

Induction of interleukin-2 receptor alpha-chain gene expression by cytochalasin B and 12-O-tetradecanoylphorbol-13-acetate in T lymphocytes

The high affinity form of interleukin-2 receptor (IL-2R) is composed of two subunits; the alpha (p55) and beta (p75). The alpha chain, unlike the beta, is expressed only on activated T lymphocytes. Therefore, high affinity binding of interleukin-2 (IL-2) is controlled by the expression of the IL-2R alpha-chain. In this study, we examined the effect of cytochalasin B (CB) plus 12-O-tetradecanoylphorbol-13-acetate (TPA) on expression of IL-2 and IL-2R. Northern blot and flow cytometric analysis showed that the IL-2R alpha-chain was expressed both at mRNA and protein levels. However, IL-2 gene expression was not induced by this treatment. Unlike the cells treated individually with CB or TPA, cells treated with CB plus TPA accumulated IL-2R mRNA at all the times examined. In order to determine the percentage of cells that incorporated tritiated thymidine ([3H]dT) in the presence of IL-2 after treatment with CB plus TPA, autoradiography was carried out. We found that about 11% of the cells were labelled. Because the percentage of labelled cells and cells expressing IL-2R alpha-chain was relatively low (11% and 9% respectively), perhaps CB plus TPA caused IL-2R expression in only a subset of T cells.

Beta-actin mRNA-binding proteins associated with the cytoskeletal framework

Association of mRNA with the cytoskeletal framework (CSK) is thought to play a strategic role in the placement of mRNA in the cytoplasm. However, the molecular determinants underlying mRNA/CSK association are completely unknown. To begin addressing this issue, we have employed a binding assay to identify proteins of the CSK compartment of NIH 3T3 cells that bind in-vitro-transcribed 32P-labelled beta-actin mRNA with high affinity. Three proteins, of approximate molecular masses 27, 50 and 97 kDa, were observed to exhibit strong binding. Binding to these proteins took place at physiological salt concentration and withstood washing in 0.5 M salt. Furthermore, binding was unaffected by heparin but was inhibited by unlabelled beta-actin mRNA. Treatment of isolated CSKs with the microfilament-severing agent DNase I abolished all beta-actin mRNA-binding activities, thus suggesting a possible association of beta-actin mRNA with the microfilament network in situ. Removal of the 3' untranslated region (UTR) significantly reduced beta-actin mRNA binding to all three CSK proteins but removal of the 5' UTR mainly affected binding to the 97-kDa species and that to a lesser extent. beta-Tubulin mRNA bound to the same three CSK proteins as did beta-actin mRNA, but with considerably less avidity. In contrast, vimentin mRNA strongly recognized these CSK proteins, and further bound to a group of smaller proteins (< 29 kDa). As beta-actin mRNA, beta-tubulin mRNA and vimentin mRNA have been observed to occupy separate cytoplasmic locales, the proteins detected here may be operative both in binding mRNAs to the CSK in situ, as well as in localizing mRNA in the cytoplasm.

Isolation and characterization of hamster brain polyribosome-cytomatrix complexes

We have developed a method for the isolation of a brain subcellular fraction enriched in both highly aggregated polyribosomes and cytoskeletal proteins. This method is based on gentle dispersion of brain tissue and low speed centrifugation. This fraction is enriched in typical cytoskeletal proteins as glial fibrillary protein, neurofilament proteins and actin. Messenger RNA did not seem to be involved in the polyribosome association to the cytomatrix as shown by the effect of exposure to micrococcal nuclease. On the other hand, in vivo disruption of protein synthesis by acute experimental phenylketonuria, hypothermia or heat-shock did not cause the release of ribosomes from the cytomatrix.

mRNA association with the cytoskeletal framework likely represents a physiological binding event

A multitude of studies has indicated that the vast majority of mRNA and polyribosomes is associated with the detergent-resistant cytoskeletal framework (CSK). However, the nature and purpose of this association remain unclear. To begin unraveling the factors which may mediate this phenomenon, we examined the extent of association of four mRNAs (tubulin, vimentin, actin, and histone mRNA) with the CSKs of NIH 3T3 cells over a wide range of salt concentrations. Results indicate that the vast majority (greater than 90%) of each of these mRNAs remains associated with the CSK after detergent extraction of cells in low ionic strength buffer (25 mM NaCl). This association is manifest under conditions that cause the complete depolymerization of microtubules but that leave microfilaments and intermediate filaments intact. Even after extensive washing in buffer of approximately physiological ionic strength (150 mM NaCl), 75-85% of these mRNAs still remain associated with the CSK. However, at least 50% of each of these mRNAs can be eluted from the CSK by washing with buffer containing 250 mM NaCl. Not all the mRNAs, though, display the same elution profile. This suggests that different binding sites and/or different binding affinities may exist for different mRNAs. Surprisingly, close to 50% of the polyribosome population remains bound to the CSK despite washing in as much as 1.0 M NaCl. These adherent polyribosomes appear to be of the same size as those that are eluted, allaying the possibility that they are retained by the CSK simply due to size exclusion. Collectively, these data strongly imply that mRNAs are neither weakly adsorbed to the CSK nor physically trapped within the meshwork of cytoskeletal filaments.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction between mRNA, ribosomes and the cytoskeleton

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c-myc mRNA in cytoskeletal-bound polysomes in fibroblasts

3T3 fibroblasts were treated sequentially with 25 mM-KCl/0.05% Nonidet P40, 130 mM-KCl/0.05% Nonidet P40 and finally with 1% Nonidet P40/1% deoxycholate in order to release free, cytoskeletal-bound and membrane-bound polysomes respectively. The membrane-bound fraction was enriched in the mRNA for the membrane protein beta 2-microglobulin, whereas the cytoskeletal-bound polysomes were enriched in c-myc mRNA. Actin mRNA was present in both free and cytoskeletal-bound polysomes. The results suggest that cytoskeletal-bound polysomes are involved in the translation of specific mRNA species.

The characterization of free, cytoskeletal and membrane-bound polysomes in Krebs II ascites and 3T3 cells

Polysomes from Krebs II ascites and 3T3 cells were separated into three populations by using a sequential extraction method. Free polysomes were released by using a combination of low salt (25 mM KCl) and NP-40 detergent in the lysis buffer. The cytoskeletal bound polysomes were subsequently released by raising the salt concentration to 130 mM and finally, polysomes bound to the membranes of the endoplasmic reticulum were extracted by the combined treatment with Triton X-100 and deoxycholate. The results presented here illustrate that the three polysome-containing fractions differ in many parameters such as polysome profiles, cytoskeletal components and phospholipid content. When polyA-containing mRNA was isolated from the three polysome fractions and translated in an in vitro system, some differences were observed in the patterns of proteins being synthesized.

"In situ" translation: use of the cytoskeletal framework to direct cell-free protein synthesis

We have developed a novel, "in situ" translation system derived from cultured cells that are subject to mild detergent extraction. By using a low concentration of nonionic detergent to gently permeabilize cells while they remain adherent to a substrate, cytoskeletal frameworks are obtained that are devoid of membraneous barriers yet retain much the same topological arrangement of mRNA, ribosomes and cytostructure that exists "in vivo". Data indicate that when these cytoskeletal frameworks are supported by a ribosome-depleted, nuclease-treated, reticulocyte lysate supernatant, they are capable of resuming translation of their attached polysomes for at least 40 minutes. Emulsion autoradiography of ongoing protein synthesis demonstrates that protein synthetic activity is ubiquitous throughout the population of extracted cells, and not confined to a less well-extracted subset. Computer-assisted, two-dimensional gel analysis reveals that the pattern of proteins produced by such extracted cells is approximately 70% coincident with that produced by unextracted cells, including proteins of molecular weight as great as 200 kilodaltons. Furthermore, a continued increase in intensity of almost all proteins during the first 40 minutes of translation suggests that translational re-initiation, in addition to polysome run-off, is also taking place. Collectively, these findings indicate that much of the translational machinery remains both intact and competent in this cytoskeletal-based translation system. As such, this system should prove extremely useful in identifying molecular factors operant during certain types of translation control and in further examining the role played by the cytoskeleton in regulating gene expression.

Immunological and chemical characterization of hamster brain polyribosomes-cytomatrix complexes

We have studied the cytoskeletal nature of a brain subcellular fraction previously shown to contain polyribosomes. We have identified the major proteins of this fraction by electrophoretic comparison to a standard cytoskeletal fraction and by immunodetection. These methods have shown the presence of actin, glial fibrillary acidic protein, and neurofilament triplet proteins. We have also studied the effect of various ions and nonionic detergents on the stability of this structure. It was stable in presence of Triton X-100 up to 2% but disrupted by 200 mM K+ acetate.

Ultrastructural visualization of cytoskeletal mRNAs and their associated proteins using double-label in situ hybridization

We have been able to visualize cytoskeletal messenger RNA molecules at high resolution using nonisotopic in situ hybridization followed by whole-mount electron microscopy. Biotinated cDNA probes for actin, tubulin, or vimentin mRNAs were hybridized to Triton-extracted chicken embryo fibroblasts and myoblasts. The cells were then exposed to antibodies against biotin followed by colloidal gold-conjugated antibodies and then critical-point dried. Identification of mRNA was possible using a probe fragmented to small sizes such that hybridization of several probe fragments along the mRNA was detected as a string of colloidal gold particles qualitatively and quantitatively distinguishable from nonspecific background. Extensive analysis showed that when eight gold particles were seen in this iterated array, the signal to noise ratio was greater than 30:1. Furthermore, these gold particles were colinear, often spiral, or circular suggesting detection of a single nucleic acid molecule. Antibodies against actin, vimentin, or tubulin proteins were used after in situ hybridization, allowing simultaneous detection of the protein and its cognate message on the same sample. This revealed that cytoskeletal mRNAs are likely to be extremely close to actin protein (5 nm or less) and unlikely to be within 20 nm of vimentin or tubulin filaments. Actin mRNA was found to be more predominant in lamellipodia of motile cells, confirming previous results. These results indicate that this high resolution in situ hybridization approach is a powerful tool by which to investigate the association of mRNA with the cytoskeleton.

Alkaline phosphatase: a marker of alveolar type II cell differentiation

In an effort to identify type II cells by a method independent of staining phospholipid inclusions, we evaluated a histochemical technique for alkaline phosphatase activity in normal rat lung, in freshly isolated type II cells, and in primary culture of type II cells. In the adult rat alveolus, alkaline phosphatase staining selectively identified type II cells, although nonciliated bronchiolar (Clara) cells and loose perivascular connective tissue also stained for alkaline phosphatase activity. In cell suspensions of type II cells and other dissociated lung cells, alkaline phosphatase staining correlated closely with the modified Papanicolaou technique and was particularly useful in distinguishing type II cells from alveolar macrophages. To determine if alkaline phosphatase was related to the differentiated phenotype of type II cells, we studied conditions known to affect other type II cell functions. When type II cells were cultured on plastic substrata, the intensity of alkaline phosphatase staining decreased with increasing time in culture. To quantitate the apparent decrease in alkaline phosphatase activity, we used a biochemical assay to study the expression of alkaline phosphatase by type II cells. The specific activity of alkaline phosphatase in type II cells declined with increasing time in tissue culture on plastic substrata. Alkaline phosphatase activity was maintained, however, by culturing cells on Englebreth-Holm-Swarm (EHS) tumor matrix. Cells that had reduced levels of alkaline phosphatase activity following 48 h of culture on plastic substrata could be "rescued" by removing them from the plastic substratum and reculturing them for 48 h on EHS matrix. Alkaline phosphatase activity was also increased by culturing type II cells in the presence of cAMP or sodium butyrate.(ABSTRACT TRUNCATED AT 250 WORDS)

Mitogenic activity in ovine uterine fluids: characterization of a growth factor activity which specifically stimulates myoblast proliferation

Fluids produced by the uterus of pregnant sheep (OUF-ovine uterine fluids) were assayed for mitogenic activity in a thymidine incorporation assay. A dose-dependent mitogenic activity was observed in OUF which exceeded that of adult ovine plasma or fetal bovine serum. Uterine fluids were capable of stimulating thymidine incorporation in mouse 3T3 fibroblasts, rat L6 myoblasts, ovine trophoblast-derived cells, HeLa S3 cells, and bovine aortic endothelial cells. The greatest stimulation was observed in L6 myoblasts. The name ovine uterine-derived growth factor (ovine UDGF) has been suggested for this activity.