A cytoskeletal structure with associated polyribosomes obtained from HeLa cells

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.

Deadenylation of the unstable mRNA encoded by the yeast MFA2 gene leads to decapping followed by 5'-->3' digestion of the transcript

The first step in the decay of some eukaryotic mRNAs is the shortening of the poly(A) tail. To examine how the transcript body was degraded after deadenylation, we followed the decay of a pulse of newly synthesized MFA2 transcripts while utilizing two strategies to trap intermediates in the degradation pathway. First, we inserted strong RNA secondary structures, which can slow exonucleolytic digestion and thereby trap decay intermediates, into the MFA2 5' UTR. Following deadenylation, fragments of the MFA2 mRNA trimmed from the 5' end to the site of secondary structure accumulated as full-length mRNA levels decreased. In addition, in cells deleted for the XRN1 gene, which encodes a major 5' to 3' exonuclease in yeast, the MFA2 transcript is deadenylated normally but persists as a full-length mRNA lacking the 5' cap structure. These results define a mRNA decay pathway in which deadenylation leads to decapping of the mRNA followed by 5'-->3' exonucleolytic degradation of the transcript body. Because the poly(A) tail and the cap structure are found on essentially all mRNAs, this pathway could be a general mechanism for the decay of many eukaryotic transcripts.

Association of poly(A) mRNA with microtubules in cultured neurons

The structural basis for the synthesis of specific proteins within distinct intraneuronal compartments is unknown. We studied the distribution of poly(A) mRNA within cultured cerebrocortical neurons using high resolution in situ hybridization to identify cytoskeletal components that may anchor mRNA. After 1 day in culture, poly(A) mRNA was distributed throughout all of the initial neurites, including the axon-like process. At 4 days in culture, poly(A) mRNA was distributed throughout the cell body and dendritic processes, but confined to the proximal segment of the axon. Poly(A) mRNA was bound to the cytoskeleton as demonstrated by resistance to detergent extraction. Perturbation of microtubules with colchicine resulted in a major reduction of dendritic poly(A) mRNA; however, this distribution was unaffected by cytochalasin. Ultrastructural in situ hybridization revealed that poly(A) mRNA and associated ribosomes were excluded from tightly bundled microtubules.

Supramolecular organization of the mammalian translation system

Although evidence suggests that the protein synthetic machinery is organized within cells, this point has been difficult to prove because any organization that might exist is lost upon preparation of the cell-free systems usually used to study translation in vitro. To examine this process under conditions more representative of the intact cell, we have developed an active protein-synthesizing system using Chinese hamster ovary (CHO) cells permeabilized with the plant glycoside saponin. This procedure renders cells permeable to trypan blue and exogenous tRNA, but there is little release of endogenous macromolecules. Protein synthesis in this system proceeds at the same rate as that in intact cells and is about 40-fold faster than that in a cell-free system prepared from the same cells. Active protein synthesis in this system requires the addition of only Mg2+, K+, and creatine phosphate, with a small further stimulation by ATP and an amino acid mixture; no exogenous macromolecules are necessary. The proteins synthesized in this system are indistinguishable from those made by the intact cell, and the channeling of aminoacyl-tRNA observed in vivo is maintained. Our data suggest that the permeabilized cell system retains the protein-synthesizing capabilities of the intact cell and presumably its internal structure as well. Studies with this system demonstrate that the protein-synthesizing apparatus is highly organized and that its macromolecular components are not freely diffusible in mammalian cells.

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).

Ontogeny of the cytoskeleton during mammalian oogenesis

Mammalian oogenesis is a process which requires a variety of changes in the structure and function of the specialized female germ cell. Evidence suggests that the cytoskeleton may mediate several of these structural and functional changes. In this review we evaluate what is known of cytoskeletal function during oogenesis, with emphasis on specialized cytoskeletal features in mammals. Existing investigations suggest that the oocyte, as a highly specialized cell, contains unique cytoskeletal elements which exhibit functions restricted to the process of early development.

Intracellular distribution of beta-actin mRNA is polarized in embryonic corneal epithelia

The intracellular distribution of filamentous actin (F-actin), all actin isoforms and beta-actin mRNA were analyzed in whole-mount preparations of freshly isolated corneal epithelia. Filamentous actin distribution was analyzed with fluorescently tagged phalloidin. An antibody that recognizes an epitope on both globular (G-actin) and F-actin was used in an immunohistochemical analysis of actin protein distribution. Whole-mount epithelial tissues were examined with a confocal laser scanning microscope (CLSM). Biotinylated oligonucleotide probes specific for the beta-actin mRNA were used, and visualized with avidin-FITC. The intracellular localization of the beta-actin mRNA was similar to the F-actin protein distribution. In the most apical optical sections of embryonic cornea, actin staining delineated the cell borders and microvilli of the periderm cells. The actin is also detected as an organized network at the interface between the basal and periderm cells. At the level of the basal cell nucleus, F-actin is sparse, associating only with the lateral cell membranes. However, at the optical plane below the nuclei, the actin forms an elaborate actin cortical mat. Actin mRNA staining was visualized as discrete punctate areas. The beta-actin mRNA was positive at the optical plane just below the periderm cell apical membrane surface, similar to actin in microvilli. These cells also contained punctate staining near the cell membranes and in the periderm-basal cell junction area. At the level of the basal cell nucleus the actin mRNA was present in a punctate pattern along the cell membranes. Below the basal cell nuclei the actin mRNA staining increased at the level of the actin cortical mat. These experiments are the first demonstration that actin mRNA is polarized in embryonic corneal epithelia and co-localized with actin protein in an intact tissue.

Inhibition of protein synthesis alters the subcellular distribution of mRNA in neurons but does not prevent dendritic transport of RNA

This study evaluates whether protein synthesis plays a role in targeting RNA molecules to different subcellular domains within neurons. Transport of newly synthesized RNA (labeled with [3H]uridine) was examined in the presence of the protein synthesis inhibitors puromycin and cycloheximide. In situ hybridization was used to determine whether inhibition of protein synthesis altered the subcellular distribution of mRNAs. Transport of recently synthesized RNA was not disrupted after prolonged exposure to either inhibitor. However, inhibition of protein synthesis caused several mRNAs that are normally confined to the cell body to appear in dendrites. The distribution of mRNAs that are normally present in dendrites was unaffected. These findings suggest that protein synthesis is not required to translocate RNA into the dendrites but may play a role in restricting particular mRNAs to the neuronal cell body.

The distribution of myelin basic protein mRNAs within myelinating oligodendrocytes

The nervous system contains oligodendrocytes with processes that are greatly extended in space. It is now clear that there are numerous complex, poorly understood mechanisms by which polypeptides are synthesized and delivered to their sites of function in these cells. One mechanism is by the active positioning of mRNAs encoding certain proteins to restricted intracellular subdomains. Perhaps the best studied example of this in the vertebrate CNS is the translocation of myelin basic protein mRNAs to the forming myelin sheath, where the newly synthesized polypeptides, which avidly associate with membranes, can be directly incorporated into the myelin membrane. Evidence for this conclusion is presented here in the context of related work on the general phenomenon of mRNA translocation that is under analysis in other systems.

Rice prolamine protein body biogenesis: a BiP-mediated process

Rice prolamines are sequestered within the endoplasmic reticulum (ER) lumen even though they lack a lumenal retention signal. Immunochemical and biochemical data show that BiP, a protein that binds lumenal polypeptides, is localized on the surface of the aggregated prolamine protein bodies (PBs). BiP also forms complexes with nascent chains of prolamines in polyribosomes and with free prolamines with distinct adenosine triphosphate sensitivities. Thus, BiP retains prolamines in the lumen by facilitating their folding and assembly into PBs.

Organization of the double-stranded RNA-activated protein kinase DAI and virus-associated VA RNAI in adenovirus-2-infected HeLa cells

We have examined the cellular distribution of the double-stranded RNA-activated protein kinase DAI in adenovirus 2 (Ad2)-infected and uninfected HeLa cells. In uninfected cells DAI was found to be concentrated in the cytoplasm. In addition, DAI was localized in the nucleoli and diffusely distributed throughout the nucleoplasm. Cells treated with alpha-interferon displayed a similar pattern of distribution for DAI. When RNA polymerase I activity was inhibited by the drug actinomycin D, nucleoli segregated and DAI was found to colocalize with the dense fibrillar region of the nucleoli. During mitosis, the distribution of DAI paralleled that of rRNA. In adenovirus-infected cells the localization of DAI was similar to that in uninfected interphase cells. VA RNAI was detected in Ad2-infected cells by 10–14 hours post-infection as fine dots in the nucleoplasm. By 18–24 hours post-infection, VA RNAI appeared in bigger and more abundant dots in the nucleoplasm and the cytoplasm was intensively labeled. Transient expression of the VA RNAI gene in uninfected cells resulted in a similar localization of the RNA. Our results are consistent with a role for DAI and VA RNAI in protein synthesis and suggest that DAI may play an early role in ribosome biogenesis in the nucleolus in addition to its cytoplasmic role in translation.

A turnover pathway for both stable and unstable mRNAs in yeast: evidence for a requirement for deadenylation

To determine pathways of mRNA turnover in yeast, we have followed the poly(A) tail removal and degradation of a pulse of newly synthesized transcripts from four different genes. Before decay of both stable and unstable mRNAs initiated, there was a temporal lag during which the poly(A) tail was deadenylated to an oligo(A) length. Altering the deadenylation rate of an mRNA led to a corresponding change in the length of this lag. The rate of deadenylation and the stability of the oligo(A) species varied between mRNAs, explaining the differences in mRNA half-lives. To examine how the transcript body was degraded following deadenylation, we used the strategy of inserting strong RNA secondary structures, which can slow exonucleolytic digestion and thereby trap decay intermediates, into the 3' UTR of mRNAs. Fragments lacking the 5' portion of two different mRNAs accumulated after deadenylation as full-length mRNA levels decreased. Therefore, these results define an mRNA decay pathway in which deadenylation leads to either internal cleavage or decapping followed by 5'-->3' exonucleolytic degradation of the mRNA.

High resolution distribution of mRNA within the cytoskeleton

It has been well documented that mRNA is associated with the cytoskeleton, and that this relationship is involved in translation and mRNA sorting. The molecular components involved in the attachment of mRNA to the cytoskeleton are only poorly understood. The objective of this research was to directly visualize the interaction of mRNA with the cytoskeleton, with sufficient resolution to identify the filament systems involved. This work required the development of novel in situ hybridization methods for use with electron microscopy.

Suppression of protein synthesis in the reperfused brain

BACKGROUND

Brain ischemia and reperfusion produce profound protein synthesis alterations, the extent and persistence of which are dependent on the nature of the ischemia, the brain region, the cell layer within a region, and the particular proteins studied. After transient ischemia, most brain regions recover their protein synthesis capability; however, recovery in the selectively vulnerable areas is poor. It is unknown whether this phenomenon itself provokes or is a consequence of the process of neuronal death.

SUMMARY OF REVIEW

Protein synthesis suppression during ischemia is due to energy depletion, but this is quickly reversed upon recirculation. Reperfusion does not appear to damage DNA or transcription mechanisms, although there are changes in the profile of transcripts being made. Similarly, purified ribosomes isolated from reperfused brains can make the normal repertoire of proteins and heat-shock proteins. However, during early reperfusion, newly synthesized messenger RNAs appear to accumulate in the nucleus; this alteration in RNA handling could reflect disruption at any of several steps, including posttranscriptional processing, nuclear pore transport, cytoskeletal binding, or formation of the translation initiation complex. Another mechanism that may be responsible for protein synthesis suppression during late reperfusion is progressive membrane destruction, with consequent shifts in the concentration of ions crucial for ribosomal function.

CONCLUSIONS

Protein synthesis suppression after ischemia likely involves a progression of multiple mechanisms during reperfusion. Although the recent work reviewed here offers new insight into the potential mechanisms disrupting protein synthesis, detailed understanding will require further investigation.

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.

Actin isoforms

The actin supergene family encodes a number of structurally related, but perhaps functionally distinct, protein isoforms that regulate contractile potential in muscle tissues and help to control the shape as well as the motility of non-muscle cells. In spite of the documented conservation amongst isoactin genes and their encoded proteins, recent results of biochemical, antibody localization, molecular mutagenesis and isoactin gene replacement studies lend credence to the notion that functional differences amongst muscle and non-muscle actin isoforms exist. Furthermore, the discovery of a new class of actin isoforms, the actin-related proteins, reveals that the actin gene and protein isoform family is more complex than was previously believed.

Poly(A) RNA codistribution with microfilaments: evaluation by in situ hybridization and quantitative digital imaging microscopy

The distribution of poly(A) RNA has been visualized in single cells using high-resolution fluorescent in situ hybridization. Digital imaging microscopy was used to quantitate the signal in various cellular compartments. Most of the poly(A) signal remained associated with the cellular filament systems after solubilization of membranes with Triton, dissociation of ribosomes with puromycin, and digestion of non-poly(A) RNA with ribonuclease A and T1. The actin filaments were shown to be the predominant cellular structural elements associating with the poly(A) because low doses of cytochalasin released about two- thirds of the poly(A). An approach to assess the extent of colocalization of two images was devised using in situ hybridization to poly(A) in combination with probes for ribosomes, membranes, or F- actin. Digital imaging microscopy showed that most poly(A) spatially distributes most significantly with ribosomes, slightly less with F- actin, and least of all with membranes. The results suggest a mechanism for anchoring (and perhaps moving) much of the cellular mRNA utilizing the interaction between actin filaments and poly(A).

Restricted distribution of mRNA produced from a single nucleus in hybrid myotubes

Although the proteins encoded by a single nucleus in multinucleated myotubes have a wide range of distributions within the myofiber, little is known about the distributions of their mRNAs. We have used hybrid myotubes in which one or a few nuclei are derived from myoblasts that express nonmuscle proteins to investigate this question. We find that three different mRNAs, encoding proteins that are, respectively, nuclear, cytoplasmic, and targeted to the ER, have similar distributions within myotubes. Each is confined to an area within approximately 100 microns of the nucleus that expresses it.

Poliovirus infection results in structural alteration of a microtubule-associated protein

Poliovirus infection results in profound changes in cellular metabolism and architecture. To identify alterations in cellular proteins following poliovirus infection which might account for these changes, monoclonal antibodies were prepared by screening for differences in antigen pattern in infected and uninfected cell lysates. Further characterization of the antigen of one such antibody (25 C C1) is described in this report. The 25 C C1 antigen is a cytoskeleton-associated protein which decreases in size 4 to 5 h postinfection. It copurifies with some of the protein synthesis initiation factors but not with eucaryotic initiation factor (eIF)-4F, the p220 subunit of which is cleaved following infection (D. Etchison, S. C. Milburn, I. Edery, N. Sonenberg, and J. W. B. Hershey, J. Biol. Chem. 257:14806-14810, 1982). Unlike alteration of p220, alteration of the 25 C C1 antigen is not due to a protease which can be detected by cell lysate mixing experiments. Alteration of the antigen occurs during purification, suggesting progressive proteolysis, but the alteration is more extensive in preparations from infected cells than in those from uninfected cells. A recombinant phage expressing the antigenic determinant was isolated from a human fibroblast cDNA library, and the sequence of the cDNA insert was found to be entirely contained within the established sequence of microtubule-associated protein (MAP) 4 (R. R. West, K. M. Tenbarge, and J. B. Olmsted, J. Biol. Chem. 266:21886-21896, 1991). The antigen distribution, as detected by indirect immunofluorescence, was similar to, but more diffuse than, the distribution of tubulin. The antibody recognized the largest abundant HeLa cell MAP, which copurified with tubulin after three cycles of polymerization-depolymerization, thus confirming the identity of the antigen as MAP 4. These results indicate that poliovirus infection of HeLa cells affects the structural integrity of a cytoskeletal protein, MAP 4.

Preservation and visualization of molecular structure in detergent-extracted whole mounts of cultured cells

Today's electron microscopes have a resolution sufficient to resolve supramolecular structures. However, the methods used to prepare biological samples for electron microscopy often limit our ability to achieve the resolution that is theoretically possible. We use whole mounts of detergent-extracted cells grown on Formvar-coated gold grids as a model system to evaluate various steps in the preparation of biological samples for high resolution scanning electron microscopy (SEM). Factors that are important in determining the structure and composition of detergent-extracted cells include the nature of the detergent and the composition of the extraction vehicle. Chelation of calcium is extremely important to stabilize and preserve the cytoskeletal filaments. We have also demonstrated both morphologically and by gel electrophoresis that treatment of cells with bifunctional protein crosslinkers before or during extraction with detergent can significantly enhance the preservation of both proteins and supramolecular structures. The methods used to dry samples are a major determinant of the quality of structural preservation. For cytoskeletons freeze-drying (FD) is superior to critical point-drying (CPD), one reason being that CPD samples have to be dehydrated, thereby causing more shrinkage as compared to FD samples. The high pressures to which samples are exposed during CPD may also cause increased shrinkage, and water contamination during CPD causes severe structural damage. We have obtained the best structural preservation of detergent-extracted and fixed cells by manually plunging them into liquid propane and drying over night in a freeze-dryer. The factor that most limits achievement of high resolution in SEM is the metal coat, which has to be very thin, uniform, and free of grain in order not to hide structures or to create artifactual ones. We have found that sputter-coating with 1-3 nm of tungsten (W) or niobium (Nb) gives extremely fine-grained films as well as satisfactory emission of secondary electrons. These samples can also be examined at high resolution by transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The best preservation and visualization of supramolecular structures have been obtained using cryosputtering, in which the samples are freeze-dried and then sputter-coated within the freeze-dryer while still frozen.

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.

Role of the cytoskeleton during early development

Oocytes, eggs, and embryos from a diverse array of species have evolved cytoskeletal specializations which allow them to meet the needs of early embryogenesis. While each species studied possesses one or more specializations which are unique, several cytoskeletal features are widely conserved across different animal phyla. These features include highly-developed cortical cytoskeletal domains associated with developmental information, microtubule-mediated pronuclear transport, and rapid intracellular signal-regulated control of cytoskeletal organization.

A 33-kDa polypeptide with homology to the laminin receptor: component of translation machinery

A 33-kDa polypeptide (termed p40), which shares an antigenic determinant with a laminin receptor and is under translational control, is believed to serve as a precursor to the receptor and to be related to the neoplastic state. The present study of subcellular localization of this protein shows it to be a cytoplasmic component not associated with the plasma membrane. Most of the cellular p40 was found to be associated with polyribosomes as well as with 40S to 60S cytoplasmic particles. Conditions that lead to polysome disruption also caused release of the polysomal form of p40 as smaller particles, and polysome reconstitution was accompanied by uptake of p40 into these structures. Because of the large abundance of this protein in the cells (six to eight copies per ribosome), it is unlikely that it represents a factor that associates with the 40S preinitiation complex. The p40-containing particles appear to represent a newly discovered structure involved in the process of polysome formation.

Role of cytoplasmic mRNP proteins in translation

Polyribosomal and free mRNPs from rabbit reticulocytes were isolated and characterized. Translation of mRNPs was studied in the rabbit reticulocyte and wheat germ cell-free systems. Both classes of mRNPs were active in rabbit reticulocyte lysates. However, considerable differences between mRNPs and mRNA have been revealed. High concentrations of mRNA in the form of mRNP did not inhibit protein biosynthesis, whereas the same amounts of deproteinized mRNA caused inhibition of this process. Polyribosomal mRNPs and deproteinized mRNA, but not free mRNPs, are active in the wheat germ cell-free translation system. Translation of free mRNPs in this system can be restored by addition of 0.5 M KCl-wash of rabbit reticulocyte ribosomes. These results suggest the existence of a special repressor/activator regulatory system which controls mRNA distribution between free mRNPs and polyribosomes in rabbit reticulocytes. This regulatory system should include: i) a translation repressor associated with mRNA within free mRNPs, preventing its translation; and ii) a translation activator associated with ribosomes, overcoming the effect of the repressor. Both classes of cytoplasmic mRNPs contain a major 50 kDa protein (p50). The content of this protein per mol of mRNA in free mRNPs is twice as much as in polyribosomal ones. The method of p50 isolation has been developed and some properties of this protein were investigated. It has been shown that small amounts of p50 stimulate, whereas high amounts inhibit mRNA translation. We suggest that p50 has a dual role in protein biosynthesis. In polyribosomal mRNPs (p50:mRNA approximately 2:1, mol/mol), this protein promotes the translation process.(ABSTRACT TRUNCATED AT 250 WORDS)

Glial fibrillary acidic protein and its mRNA: ultrastructural detection and determination of changes after CNS injury

We have previously demonstrated that glial fibrillary acidic protein (GFAP) containing intermediate filaments in retinal Müller cells undergo both quantitative induction and subcellular reorganization as a response to long-term retinal detachment (an induced CNS degeneration wherein the Müller cells form a multicellular scar). This study demonstrates by RNA blotting analysis that normal retina expresses a low basal level of GFAP mRNA, which is induced approximately 500% within 3 days of retinal detachment. At the cellular level, electron microscopic in situ hybridization analysis readily detects GFAP mRNA in Müller cells of detached retinas, but not in normal retinas. On the other hand, GFAP mRNA was readily detected in retinal astrocytes (which appear to express GFAP mRNA at high, constitutive levels). In both cell types, the ultrastructural localization of GFAP mRNA was the same. In the nuclei, the GFAP mRNA was associated with amorphous, electron-dense regions within the euchromatin. In the cytoplasm, the GFAP mRNA was associated with intermediate filaments near the nuclear pores, along the filaments when no other structures were apparent, and when the filaments appeared to be associated with ribosomes and polysomes. The ultrastructural location of the GFAP mRNA (especially along the intermediate filaments) may be unique to this mRNA or may represent a more generalized mRNA phenomenon.

The cytoskeleton and mRNA localization

The localization of mRNA appears to facilitate protein sorting, so that proteins are synthesized in specific cellular regions. The spatial information on the mRNA may be transduced by proteins that recognize specific localizing sequences on the 3' end and then chaperone the mRNA, presumably along filaments, to its destination. Additional sequences such as poly(A), or the nascent chains of cytoskeleton-associated proteins, may then anchor mRNAs on the cytoskeleton.

Mechanisms for intracellular distribution of mRNA: in situ hybridization studies in muscle

The intracellular distribution of mRNA in striated muscle fibers is highly ordered, as is the structural organization of the fibers' contractile apparatus. Results from in situ hybridization of muscle mRNA are reviewed in an attempt to discern the mechanisms involved in mRNA distribution and to determine its relationship to developmental, growth, and repair processes in muscle. Nonradioactively labeled complementary RNA probes allow anatomic localization of mRNA at the light and electron microscopic level. Myosin mRNA in striated muscle is concentrated around transcriptionally active nuclei, myosin mRNA is excluded by the myofibrillar mass, myosin mRNA distribution correlates with that of cytoskeletal elements, and myosin mRNA is concentrated in regions of rapid growth and repair. The even distribution of myosin mRNA along the length of myofibrils gives no indication of specific association with either the thick or thin filaments. Of the possible mechanisms directing mRNA distribution, results from in situ hybridization and other analyses support a restricted diffusion model. Diffusion of mRNA (and polysomes) is severely limited by the myofibrillar lattice. It is possible that myosin mRNA is also associated with a cytoskeletal element, which may direct the mRNA to specific intracellular locations and affect translational activity.

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)

Posttranscriptional regulation of myelin protein gene expression

Regulation of myelin protein gene expression occurs at many different levels including transcription, mRNA translocation, translation, and posttranslational modification of myelin proteins prior to their assembly into the membrane. Translocation of myelin basic protein (MBP) mRNAs into oligodendrocyte processes was observed in vivo and in primary cultures, but no such translocation was observed for the mRNAs encoding the proteolipid protein (PLP) or myelin-associated glycoprotein. More than 99% of the mRNAs encoding 2'3'-cyclic nucleotide phosphodiesterase (CNP) remained associated with cell bodies. In the jimpy mutant, MBP mRNA translocation appeared to be impaired, but translocation occurred normally in quaking brains in vivo. We have found that steroids, such as glucocorticoids, stimulate the translation of MBP and PLP mRNAs in cell-free systems and inhibit the translation of CNP mRNA. This pattern of regulation is consistent with compositional changes noted in myelin during development. We have localized a nine nucleotide segment within the 5'-untranslated region of the MBP mRNA that is involved in the action of steroids on translation of this mRNA. We have also determined that the protein synthetic step modulated by the steroids is chain initiation, enhancing the rate at which new ribosomal subunits bind to the MBP mRNAs.

The cap and poly(A) tail function synergistically to regulate mRNA translational efficiency

The cap structure and the poly(A) tail are important regulatory determinants in establishing the translational efficiency of a messenger RNA. Although the mechanism by which either determinant functions remains poorly characterized, the interaction between the poly(A) tail-poly(A)-binding protein complex and events occurring at the 5' terminus during translation initiation has been an intriguing possibility. In this report, the mutual dependence of the cap and the poly(A) tail was studied. Poly(A)+ and poly(A)- luciferase (Luc) mRNAs generated in vitro containing or lacking a cap were translated in vivo in tobacco protoplasts, Chinese hamster ovary cells, and yeast following delivery by electroporation. The poly(A) tail-mediated regulation of translational efficiency was wholly dependent on the cap for function. Moreover, cap function was enhanced over an order of magnitude by the presence of a poly(A) tail. The relative differences in stability between the mRNAs could not account for the synergism. The synergism between the cap and poly(A) tail was not observed in yeast cells in which active translation had been disrupted. In addition, the synergism was not observed in in vitro translation lysates. These data demonstrate that the cap and the poly(A) tail are interdependent for optimal function in vivo and suggest that communication between the two regulatory determinants may be important in establishing efficient translation.

Requirement of microfilaments in sorting of actin messenger RNA

Specific messenger RNAs (mRNAs) can be sequestered within distinct cellular locations, but little is known about how this is accomplished. The participation of the three major cellular filaments in the localization of actin mRNA was studied in chicken embryo fibroblasts. Movement of actin mRNA to the cell periphery and maintenance of that regionalization required intact microfilaments (composed of actin) but not microtubules or intermediate filaments. The results presented here suggest that actin-binding proteins may participate in mRNA sorting.

Interaction between mRNA, ribosomes and the cytoskeleton

Images

Animal cell shape changes and gene expression

Cell shape and cell contacts are determined by transmembrane receptor-mediated associations of the cytoskeleton with specific extracellular matrix proteins and with ligands on the surface of adjacent cells. The cytoplasmic domains of these microfilament-membrane associations at the adherens junction sites, also localize a variety of regulatory molecules involved in signal transduction and gene regulation. The stimulation of cells with soluble polypeptide factors leads to rapid changes in cell shape and microfilament component organization. In addition, this stimulation also activates the phosphoinositide signaling pathway. Recently, a linkage between actin-binding proteins and the phosphoinositide signaling pathway, was discovered. It is suggested that by the association with the second messenger system, and/or by controlling the localization of regulatory molecules, the cytoskeleton may regulate gene expression.

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.

Beta actin and its mRNA are localized at the plasma membrane and the regions of moving cytoplasm during the cellular response to injury

Previous work in our laboratory has shown that microvascular pericytes sort muscle and nonmuscle actin isoforms into discrete cytoplasmic domains (Herman, I. M., and P. A. D'Amore. 1985. J. Cell Biol. 101:43-52; DeNofrio, D.T.C. Hoock, and I. M. Herman. J. Cell. Biol. 109:191-202). Specifically, muscle (alpha-smooth) actin is present on the stress fibers while nonmuscle actins (beta and gamma) are located on stress fibers and in regions of moving cytoplasm (e.g., ruffles, lamellae). To determine the form and function of beta actin in microvascular pericytes and endothelial cells recovering from injury, we prepared isoform-specific antibodies and cDNA probes for immunolocalization, Western and Northern blotting, as well as in situ hybridization. Anti-beta actin IgG was prepared by adsorption and release of beta actin-specific IgG from electrophoretically purified pericyte beta actin bound to nitrocellulose paper. Anti-beta actin IgGs prepared by this affinity selection procedure showed exclusive binding to beta actin present in crude cell lysates containing all three actin isoforms. For controls, we localized beta actin as a bright rim of staining beneath the erythrocyte plasma membrane. Anti-beta actin IgG, absorbed with beta actin bound to nitrocellulose, failed to stain erythrocytes. Simultaneous localization of beta actin with the entire F-actin pool was performed on microvascular pericytes or endothelial cells and 3T3 fibroblasts recovering from injury using anti-beta actin IgG in combination with fluorescent phalloidin. Results of these experiments revealed that pericyte beta actin is localized beneath the plasma membrane in association with filopods, pseudopods, and fan lamellae. Additionally, we observed bright focal fluorescence within fan lamellae and in association with the ends of stress fibers that are preferentially associated with the ventral plasmalemma. Whereas fluorescent phalloidin staining along the stress fibers is continuous, anti-beta actin IgG localization is discontinuous. When injured endothelial and 3T3 cells were stained through wound closure, we localized beta actin only in motile cytoplasm at the wound edge. Staining disappeared as cells became quiescent upon monolayer restoration. Appearance of beta actin at the wound edge correlated with a two- to threefold increase in steady-state levels of beta actin mRNA, which rose within 15-60 min after injury and returned to noninjury levels during monolayer restoration. In situ hybridization revealed that transcripts encoding beta actin were localized at the wound edge in association with the repositioned protein. Results of these experiments indicate that beta actin and its encoded mRNA are polarized at the membrane-cytoskeletal interface within regions of moving cytoplasm.

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.

Distribution of myosin heavy chain mRNA in embryonic muscle tissue visualized by ultrastructural in situ hybridization

We have localized myosin heavy chain (MHC) mRNAs in cells of intact embryonic chick muscle using high resolution in situ hybridization. Blocks of muscle were aldehyde-fixed prior to detergent treatment and hybridized with a biotinated cDNA probe, followed by colloidal gold-labeled antibodies, before embedment. Labeling was determined to represent MHC mRNA by extensive quantitative comparisons of electron micrographs from experimental and four different types of control samples. MHC mRNA was localized primarily to peripheral regions of 14-day chick pectoral muscle cells, where the majority of developing myofibrils were found. MHC mRNAs were consistently associated with the nonmyofibrillar cytoskeletal filaments which had diameters ranging from 4 to 10 nm. They were often oriented parallel to the longitudinal axis of the cell. The resolution of the ultrastructural approach allowed us to demonstrate that the mRNA molecules visualized were not directly associated with myofilaments, suggesting that nascent chains read from those messages do not assemble directly into myofilaments simultaneous with translation.

Actin mRNA localizes in the absence of protein synthesis

Actin mRNA is localized in chicken embryo fibroblasts to the distal regions of leading lamellae, but not within the ruffling edges. In this investigation we have addressed the role of actin translation in this process. The translocation of actin mRNA to the cell periphery was studied by monitoring the distribution of actin mRNA in cells during spreading. Within 90 min, actin mRNA moved from a perinuclear to a peripheral distribution. Formation of lamellipodia preceded actin mRNA localization, indicating that localization is not a prerequisite for this event. Neither puromycin (which dissociates ribosomes from mRNA) nor cycloheximide (which stabilizes ribosomes on mRNA) had any effect on this movement of actin mRNA. Anchoring of actin mRNA was studied using cells with peripherally localized actin mRNA. No change in actin mRNA localization was observed for 30 min in the same inhibitors. These data indicate that the presence of the nascent polypeptide is not necessary for translocation of actin mRNA to the cell periphery, or anchoring at that site. This suggests that the mRNA contains information concerning its spatial distribution within the cytoplasm.

Distinct spatial localization of specific mRNAs in cultured sympathetic neurons

We examined the subcellular distribution of specific mRNAs in cultured sympathetic neurons. Under appropriate conditions, sympathetic neurons extend both axons and dendrites that are distinguishable by light microscopic and immunocytochemical criteria. In situ hybridization revealed a differential localization of mRNA within dendrites. mRNA encoding MAP2 was abundant in cell bodies and distributed nonhomogeneously throughout the dendritic compartment, but was not detected in axons. In contrast, mRNAs encoding GAP-43 and alpha-tubulin were restricted to the cell body and largely excluded from dendrites as well as axons. Detergent extraction revealed that most dendrite-associated mRNA encoding MAP2 was associated with the Triton X-100 insoluble fraction of the cell. The subset of mRNAs present in the dendritic compartment may encode proteins involved in the morphogenesis and remodeling of dendrites.