The spatial distribution of polyribosomes in 3T3 cells and the associated assembly of proteins into the skeletal framework

De-centralizing the Central Dogma: mRNA translation in space and time

As our understanding of the cell interior has grown, we have come to appreciate that most cellular operations are localized, that is, they occur at discrete and identifiable locations or domains. These cellular domains contain enzymes, machines, and other components necessary to carry out and regulate these localized operations. Here, we review these features of one such operation: the localization and translation of mRNAs within subcellular compartments observed across cell types and organisms. We describe the conceptual advantages and the "ingredients" and mechanisms of local translation. We focus on the nature and features of localized mRNAs, how they travel and get localized, and how this process is regulated. We also evaluate our current understanding of protein synthesis machines (ribosomes) and their cadre of regulatory elements, that is, the translation factors.

Cotranslational Mechanisms of Protein Biogenesis and Complex Assembly in Eukaryotes

The formation of protein complexes is crucial to most biological functions. The cellular mechanisms governing protein complex biogenesis are not yet well understood, but some principles of cotranslational and posttranslational assembly are beginning to emerge. In bacteria, this process is favored by operons encoding subunits of protein complexes. Eukaryotic cells do not have polycistronic mRNAs, raising the question of how they orchestrate the encounter of unassembled subunits. Here we review the constraints and mechanisms governing eukaryotic co- and posttranslational protein folding and assembly, including the influence of elongation rate on nascent chain targeting, folding, and chaperone interactions. Recent evidence shows that mRNAs encoding subunits of oligomeric assemblies can undergo localized translation and form cytoplasmic condensates that might facilitate the assembly of protein complexes. Understanding the interplay between localized mRNA translation and cotranslational proteostasis will be critical to defining protein complex assembly in vivo.

Microtubules orchestrate local translation to enable cardiac growth

Hypertension, exercise, and pregnancy are common triggers of cardiac remodeling, which occurs primarily through the hypertrophy of individual cardiomyocytes. During hypertrophy, stress-induced signal transduction increases cardiomyocyte transcription and translation, which promotes the addition of new contractile units through poorly understood mechanisms. The cardiomyocyte microtubule network is also implicated in hypertrophy, but via an unknown role. Here, we show that microtubules are indispensable for cardiac growth via spatiotemporal control of the translational machinery. We find that the microtubule motor Kinesin-1 distributes mRNAs and ribosomes along microtubule tracks to discrete domains within the cardiomyocyte. Upon hypertrophic stimulation, microtubules redistribute mRNAs and new protein synthesis to sites of growth at the cell periphery. If the microtubule network is disrupted, mRNAs and ribosomes collapse around the nucleus, which results in mislocalized protein synthesis, the rapid degradation of new proteins, and a failure of growth, despite normally increased translation rates. Together, these data indicate that mRNAs and ribosomes are actively transported to specific sites to facilitate local translation and assembly of contractile units, and suggest that properly localized translation – and not simply translation rate – is a critical determinant of cardiac hypertrophy. In this work, we find that microtubule based-transport is essential to couple augmented transcription and translation to productive cardiomyocyte growth during cardiac stress.

New contractile units are required during cardiac hypertrophy, though it remains unclear precisely where and how these new sarcomeres are added. Here the authors reveal that in the heart, microtubules spatiotemporally regulate mRNAs and ribosomes to build new sarcomeres, a role which is essential for growth.

Localized translation on the mitotic apparatus: A history and perspective

Localized translation is a proposed biological event that allows mRNA to be translated on site, providing an additional level of protein regulation within a cell. Examples of localized translation have been found or proposed in a variety of cellular contexts from neurons to cancer cells and implicated in both normal development and disease for over a half century. For example, mRNA translation on the mitotic apparatus (MA) was initially hypothesized in the 1950-60s. However, its proof of existence, biological significance and mechanistic details have remained sparse and it is still unclear how well conserved this mechanism may be among different cell types or organisms. In this review, we provide a brief historic summary of translation on the MA and discuss how current and future work may help us understand this biological process that provides a subcellular level of regulation in protein synthesis within a cell.

Mechanical Forces and Their Effect on the Ribosome and Protein Translation Machinery

Mechanical forces acting on biological systems, at both the macroscopic and microscopic levels, play an important part in shaping cellular phenotypes. There is a growing realization that biomolecules that respond to force directly applied to them, or via mechano-sensitive signalling pathways, can produce profound changes to not only transcriptional pathways, but also in protein translation. Forces naturally occurring at the molecular level can impact the rate at which the bacterial ribosome translates messenger RNA (mRNA) transcripts and influence processes such as co-translational folding of a nascent protein as it exits the ribosome. In eukaryotes, force can also be transduced at the cellular level by the cytoskeleton, the cell’s internal filamentous network. The cytoskeleton closely associates with components of the translational machinery such as ribosomes and elongation factors and, as such, is a crucial determinant of localized protein translation. In this review we will give (1) a brief overview of protein translation in bacteria and eukaryotes and then discuss (2) how mechanical forces are directly involved with ribosomes during active protein synthesis and (3) how eukaryotic ribosomes and other protein translation machinery intimately associates with the mechanosensitive cytoskeleton network.

Differential requirement of F-actin and microtubule cytoskeleton in cue-induced local protein synthesis in axonal growth cones

BACKGROUND

Local protein synthesis (LPS) via receptor-mediated signaling plays a role in the directional responses of axons to extrinsic cues. An intact cytoskeleton is critical to enact these responses, but it is not known whether the two major cytoskeletal elements, F-actin and microtubules, have any roles in regulating axonal protein synthesis.

RESULTS

Here, we show that pharmacological disruption of either microtubules or actin filaments in growth cones blocks netrin-1-induced de novo synthesis of proteins, as measured by metabolic incorporation of labeled amino acids, implicating both elements in axonal synthesis. However, comparative analysis of the activated translation initiation regulator, eIF4E-BP1, revealed a striking difference in the point of action of the two elements: actin disruption completely inhibited netrin-1-induced eIF4E-BP1 phosphorylation while microtubule disruption had no effect. An intact F-actin, but not microtubule, cytoskeleton was also required for netrin-1-induced activation of the PI3K/Akt/mTOR pathway, upstream of translation initiation. Downstream of translation initiation, microtubules were required for netrin-1-induced activation of eukaryotic elongation factor 2 kinase (eEF2K) and eEF2.

CONCLUSIONS

Taken together, our results show that while actin and microtubules are both crucial for cue-induced axonal protein synthesis, they serve distinct roles with F-actin being required for the initiation of translation and microtubules acting later at the elongation step.

Kinesin molecular motor Eg5 functions during polypeptide synthesis

The microtubule motor Eg5 is well known for its functions during mitosis. It is shown that during interphase, Eg5 associates with ribosomes and is required for efficient protein synthesis.

The kinesin-related molecular motor Eg5 plays roles in cell division, promoting spindle assembly. We show that during interphase Eg5 is associated with ribosomes and is required for optimal nascent polypeptide synthesis. When Eg5 was inhibited, ribosomes no longer bound to microtubules in vitro, ribosome transit rates slowed, and polysomes accumulated in intact cells, suggesting defects in elongation or termination during polypeptide synthesis. These results demonstrate that the molecular motor Eg5 associates with ribosomes and enhances the efficiency of translation.

Emerging role for the cytoskeleton as an organizer and regulator of translation

The cytoskeleton is an intricate and dynamic fibrous network that has an essential role in the generation and regulation of cell architecture and cellular mechanical properties. The cytoskeleton also evolved as a scaffold that supports diverse biochemical pathways. Recent evidence favours the hypothesis that the cytoskeleton participates in the spatial organization and regulation of translation, at both the global and local level, in a manner that is crucial for cellular growth, proliferation and function.

What determines the degradation rate of an injected protein?

The fusion of cultured mammalian cells to red blood cells loaded with specific proteins provides a powerful system for the study of intracellular proteolysis. During the past four years the degradation rates of more than 30 proteins have been examined after their injection into HeLa cells. Results from these studies support the legitimacy of the microinjection approach. They also provide insight into the mechanism of intracellular proteolysis.

Physiological factors influencing the growth of skeletal muscle

The growth of muscle can be regulated by developmental changes or by alterations in hormone levels or in the rate or amount of work demanded. The mechanisms and structures involved in growth processes can be studied by controlling these factors. The models used are chicken anterior latissimus dorsi (ALD) muscle under the influence of overloading and rabbit tibialis anterior (TA) muscle under the influence of chronic nerve stimulation. Both models involve changes in the isoform of myosin that is expressed. Methods of study include quantitative ultrastructural analysis, immunofluorescence and in situ mRNA hybridization. In overloaded chick ALD fibres polysomes are nonuniformly distributed between the myofibrils and in a peripheral annulus even though subcellular concentrations of the new isoform are not found. In normal rabbit muscle the highest concentration of myosin mRNA detected by in situ hybridization is found in the subsarcolemmal zone. In stimulated TA polysomes are found between myofibrils. It appears that the myosin mRNA accumulates at specific cell locations before translation; then diffusion of isomyosin and rapid exchange into myofibrils follows. Therefore, regulation of growth may be possible at the transcriptional, translational and assembly stages.

Kinectin-dependent assembly of translation elongation factor-1 complex on endoplasmic reticulum regulates protein synthesis

Kinectin is an integral membrane protein with many isoforms primarily found on the endoplasmic reticulum. It has been found to bind kinesin, Rho GTPase, and translation elongation factor-1delta. None of the existing models for the quaternary organization of the elongation factor-1 complex in higher eukaryotes involves kinectin. We have investigated here the assembly of the elongation factor-1 complex onto endoplasmic reticulum via kinectin using in vitro and in vivo assays. We established that the entire elongation factor-1 complex can be anchored to endoplasmic reticulum via kinectin, and the interacting partners are as follows. Kinectin binds EF-1delta, which in turn binds EF-1gamma but not EF-1beta; EF-1gamma binds EF-1delta and EF-1beta but not kinectin. In vivo splice blocking of the kinectin exons 36 and 37 produced kinectin lacking the EF-1delta binding domain, which disrupted the membrane localization of EF-1delta, EF-1gamma, and EF-1beta on endoplasmic reticulum, similar to the disruptions seen with the overexpression of kinectin fragments containing the EF-1delta binding domain. The disruptions of the EF-1delta/kinectin interaction inhibited expression of membrane proteins but enhanced synthesis of cytosolic proteins in vivo. These findings suggest that anchoring the elongation factor-1 complex onto endoplasmic reticulum via EF-1delta/kinectin interaction is important for regulating protein synthesis in eukaryotic cells.

The identification of a new actin-binding region in p57

The actin-binding protein p57 is a member of mammalian coronin-like proteins. The roles of this protein in phagocytic processes conceivably depend on its interactions with F-actin. Two regions, p57(1-34) and p57(111-204), were previously reported to be actin-binding sites. In this study, we found that the C-terminal region of p57, p57(297-461), also possessed F-actin binding activity. Furthermore, the leucine zipper domain at the C-terminus of p57(297-461) was essential for this actin-binding activity. The F-actin cross-linking assay revealed that the region contained in p57(297-461) was sufficient to cross-link actin filaments. Our results strongly suggested that there was a new actin-binding region at the C-terminus of p57.

The role of the cytoskeleton in cell body enlargement, increased nuclear eccentricity and chromatolysis in axotomized spinal motor neurons

BACKGROUND

When spinal motor axons are injured, the nucleolus, nucleus and cell body of the injured cell transiently increase in size, the nucleus becomes more eccentrically placed, and the organization of polyribosomes into Nissl bodies is temporarily disrupted. The mechanisms for these classical morphological responses to axotomy have not been satisfactorily explained.

RESULTS

In this study we address the role of the cell body cytoskeleton in these structural changes. We show that the cytoskeleton of uninjured lumbar motor neuron cell bodies maintains nucleolar, nuclear and cell body size and nuclear position. When isolated, the relatively insoluble cell body cytoskeleton contains Nissl bodies and lipofuscin granules. After axotomy, protein labeling increases markedly and the cytoskeleton enlarges, increasing nucleolar, nuclear and cell body size, as well as nuclear eccentricity. Nearly all of the protein mass that accumulates in the cell body after axotomy appears to be added to the cytoskeleton.

CONCLUSION

We conclude that axotomy causes the conjugate enlargement of the nucleolus, nucleus and cell body and increases nuclear eccentricity in spinal motor neurons by adding protein to the cytoskeleton. The change in nuclear position, we propose, occurs when cytoskeletal elements of the axon cannot enter the shortened axon and "dam up" between the nucleus and axon hillock. As a consequence, we suggest that Nissl body-free axonal cytoskeleton accumulates between the nucleus and axon, displaces Nissl body-containing cytoskeleton, and produces central chromatolysis in that region of the cell.

The case for extending storage and secretion functions of human mast cell granules to include synthesis

Ultrastructural studies using standard procedures have for years indicated close associations of ribosomes and secretory granules in human mast cells. These descriptive studies have informed new studies, using established and new ultrastructural methods based on different principles, designed to investigate the possible role of RNA metabolism in secretory granules of human mast cells. In aggregate, these studies indicate human mast cell secretory granule associations with ribosomes, the protein synthetic machine of cells, with ribosomal proteins, with RNA, with poly(A)-positive mRNA and with various long-lived, or short-lived, uridine-rich, and poly(A)-poor RNA species with key roles in RNA processing and splicing. These studies indicate that secretory-storage granules in human mast cells may also be synthetic granules.

Cytoskeleton-dependent transport and localization of mRNA

Messenger RNAs are localized in both somatic and germ cells as a means of focusing the translation of proteins at specific cellular sites. The signals for this lie within the mRNA, and these are recognized by proteins in the cell. The latter appear to be attached via linker proteins to the transport machinery for localization. In some instances it is a myosin motor which translocates along actin microfilaments, and in others kinesin or dynein motors appear to be responsible for driving the movement of mRNA along microtubule substrates. The way that cytoskeleton-based mRNA translocation is regulated is speculated upon.

A nonerythroid isoform of protein 4.1R interacts with components of the contractile apparatus in skeletal myofibers

The approximately 80-kDa erythroid 4.1R protein is a major component of the erythrocyte cytoskeleton, where it links transmembrane proteins to the underlying spectrin/actin complexes. A diverse collection of 4.1R isoforms has been described in nonerythroid cells, ranging from approximately 30 to approximately 210 kDa. In the current study, we identified the number and primary structure of 4.1R isoforms expressed in adult skeletal muscle and characterized the localization patterns of 4.1R message and protein. Skeletal muscle 4.1R appears to originate solely from the upstream translation initiation codon (AUG-1) residing in exon 2'. Combinations of alternatively spliced downstream exons generate an array of distinct 4.1R spliceoforms. Two major isoform classes of approximately 105/110 and approximately 135 kDa are present in muscle homogenates. 4.1R transcripts are distributed in highly ordered signal stripes, whereas 4.1R protein(s) decorate the sarcoplasm in transverse striations that are in register with A-bands. An approximately 105/110-kDa 4.1R isoform appears to occur in vivo in a supramolecular complex with major sarcomeric proteins, including myosin, alpha-actin, and alpha-tropomyosin. In vitro binding assays showed that 4.1R may interact directly with the aforementioned contractile proteins through its 10-kDa domain. All of these observations suggest a topological model whereby 4.1R may play a scaffolding role by anchoring the actomyosin myofilaments and possibly modulating their displacements during contraction/relaxation.

Ultrastructural cytochemical, immunocytochemical and in situ hybridization methods with polyuridine probes detect mRNA in human mast cell granules

Mature human mast cells are classical secretory cells that are filled with secretory-storage granules but are poorly endowed with visible free or membrane-bound cytoplasmic ribosomes. We recently reported close associations of ribosomes and various components essential to RNA metabolism in and close to human mast cell granules using multiple ultrastructural imaging methods. In view of these findings and an increased awareness of RNA sorting and localization to specific subcellular sites and organelles, we used human mast cells purified from non-tumour portions of lung samples resected at surgery for carcinoma and ultrastructural methods to investigate this further. Poly(U) probes were used to detect direct en grid binding, and radiolabelled as well as non-radiolabelled poly(U) probes were used in in situ hybridization protocols to detect poly(A)-positive pre-mRNA and mRNA in nuclear, cytoplasmic and granular compartments of mature human mast cells. Negative controls verified specificity of label; expected nuclear and cytoplasmic locations of poly(A)-positive RNA served as positive controls for each sample. These findings lend support to the hypothesis that site-specific synthesis in secretory-storage granules may occur in secretory cells.

RNA is closely associated with human mast cell secretory granules, suggesting a role(s) for granules in synthetic processes

The distribution of ribosomes in mature human mast cells, a major granulated secretory cell, does not resemble that in other secretory cells, such as pancreatic acinar cells and plasma cells. By routine ultrastructural analysis, ribosomes in human mast cells are often close to, attached to, or even appear to be within secretory granules. To document better these relationships, we used multiple electron microscopic imaging methods, based on different principles, to define RNA, ribosome, and granule relationships in mature human mast cells. These methods included EDTA regressive staining, RNase digestion, immunogold labeling of ribonucleoproteins or uridine, direct binding or binding after ultrastructural in situ hybridization of various polyuridine probes to polyadenine in mRNA, and ultrastructural autoradiographic localization of [3H]-uridine incorporated into cultured human mast cells. These different labeling methods demonstrated ribosomes, RNA, U1SnRNP (a small nuclear RNP specific for alternative splicing of mRNA), mRNA, and uridine to be associated with secretory granules in human mast cells, implicating granules in a larger synthetic role in mast cell biology.

Control of translation initiation in animals

Regulation of translation initiation is a central control point in animal cells. We review our current understanding of the mechanisms of regulation, drawing particularly on examples in which the biological consequences of the regulation are clear. Specific mRNAs can be controlled via sequences in their 5' and 3' untranslated regions (UTRs) and by alterations in the translation machinery. The 5'UTR sequence can determine which initiation pathway is used to bring the ribosome to the initiation codon, how efficiently initiation occurs, and which initiation site is selected. 5'UTR-mediated control can also be accomplished via sequence-specific mRNA-binding proteins. Sequences in the 3' untranslated region and the poly(A) tail can have dramatic effects on initiation frequency, with particularly profound effects in oogenesis and early development. The mechanism by which 3'UTRs and poly(A) regulate initiation may involve contacts between proteins bound to these regions and the basal translation apparatus. mRNA localization signals in the 3'UTR can also dramatically influence translational activation and repression. Modulations of the initiation machinery, including phosphorylation of initiation factors and their regulated association with other proteins, can regulate both specific mRNAs and overall translation rates and thereby affect cell growth and phenotype.

Assembly of tropomyosin isoforms into the cytoskeleton of avian muscle cells

Tropomyosin (TM) is a component of microfilaments of most eukaryotic cells. In striated muscle, TM helps confer calcium sensitivity to the actin-myosin interaction. TM is a fibrillar, self-associating protein that binds to the extended actin filament system. We hypothesized that these structural features would permit TM to undergo assembly into the cytoskeleton during translation, or cotranslational assembly. Pulse-chase experiments with [35S]methionine and pulse experiments with [3H]puromycin followed by extraction and immunoprecipitation of TM were performed to examine the mechanism of assembly of TM into the cytoskeleton in cultured avian muscle cells. Pulse-chase experiments provide kinetic evidence for cotranslational assembly of TM in skeletal and cardiac muscle. Demonstration of a large majority of completed TM on purified skeletal muscle microfilaments after a short labeling period confirms that these kinetic data are not related to trapping of TM within the actin network of the cytoskeleton. Nascent TM peptides are demonstrated on the cytoskeleton of muscle cells after a short metabolic pulse followed by puromycin treatment to release nascent peptides from ribosomes or after labeling with [3H]puromycin. Nascent chain localization to the cytoskeleton independent of ribosomal attachment further confirms the high degree of cotranslational assembly of this protein. The extent of cotranslational assembly is similar before and after the formation of significant myofibril in myotubes, suggesting that cotranslational assembly of TM is active during contractile apparatus assembly in muscle differentiation. This is the first report where assembly mechanism has been predicted to be cotranslational based upon structural features of a cytoskeletal protein.

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 role for Rev in the association of HIV-1 gag mRNA with cytoskeletal beta-actin and viral protein expression

Human immunodeficiency virus type-1 (HIV-1) Rev acts by inducing the specific nucleocytoplasmic transport of a class of incompletely spliced RNAs that encodes the viral structural proteins. The transfection of HeLa cells with a rev-defective HIV-1 expression plasmid, however, resulted in the export of overexpressed, intron-containing species of viral RNAs, possibly through a default process of nuclear retention. Thus, this system enabled us to directly compare Rev+ and Rev+ cells as to the usage of RRE-containing mRNAs by the cellular translational machinery. Biochemical examination of the transfected cells revealed that although significant levels of gag and env mRNAs were detected in both the presence and absence of Rev, efficient production of viral proteins was strictly dependent on the presence of Rev. A fluorescence in situ hybridisation assay confirmed these findings and provided further evidence that even in the presence of Rev, not all of the viral mRNA was equally translated. At the early phase of RNA export in Rev+ cells, gag mRNA was observed throughout both the cytoplasm and nucleoplasm as uniform fine stippling. In addition, the mRNA formed clusters mainly in the perinuclear region, which were not observed in Rev+ cells. In the presence of Rev, expression of the gag protein was limited to these perinuclear sites where the mRNA accumulated. Subsequent staining of the cytoskeletal proteins demonstrated that in Rev+ cells gag mRNA is colocalized with beta-actin in the sites where the RNA formed clusters. In the absence of Rev, in contrast, the gag mRNA failed to associate with the cytoskeletal proteins. These results suggest that in addition to promoting the emergence of intron-containing RNA from the nucleus, Rev plays an important role in the compartmentation of translation by directing RRE-containing mRNAs to the beta-actin to form the perinuclear clusters at which the synthesis of viral structural proteins begins.

Testis/brain RNA-binding protein attaches translationally repressed and transported mRNAs to microtubules

We have previously identified a testicular phosphoprotein that binds to highly conserved sequences (Y and H elements) in the 3' untranslated regions (UTRs) of testicular mRNAs and suppresses in vitro translation of mRNA constructs that contain these sequences. This protein, testis/brain RNA-binding protein (TB-RBP) also is abundant in brain and binds to brain mRNAs whose 3' UTRs contain similar sequences. Here we show that TB-RBP binds specific mRNAs to microtubules (MTs) in vitro. When TB-RBP is added to MTs reassembled from either crude brain extracts or from purified tubulin, most of the TB-RBP binds to MTs. The association of TB-RBP with MTs requires the assembly of MTs and is diminished by colcemid, cytochalasin D, and high levels of salt. Transcripts from the 3' UTRs of three mRNAs that contain the conserved sequence elements (transcripts for protamine 2, tau protein, and myelin basic protein) are linked by TB-RBP to MTs, whereas transcripts that lack the conserved sequences do not bind TB-RBP. We conclude that TB-RBP serves as an attachment protein for the MT association of specific mRNAs. Considering its ability to arrest translation in vitro, we propose that TB-RBP functions in the storage and transportation of mRNAs to specific intracellular sites where they are translated.

Spnr, a murine RNA-binding protein that is localized to cytoplasmic microtubules

Previous studies in transgenic mice have established the importance of the 3' untranslated region (UTR) of the spermatid-specific protamine-1 (Prm-1) mRNA in its translational control during male germ cell development. To clone genes that mediate the translational repression or activation of the Prm-1 mRNA, we screened cDNA expression libraries made with RNA from pachytene spermatocytes and round spermatids, with an RNA probe corresponding to the 3' UTR of Prm-1. We obtained six independent clones that encode Spnr, a spermatid perinuclear RNA-binding protein. Spnr is a 71-kD protein that contains two previously described RNA binding domains. The Spnr mRNA is expressed at high levels in the testis, ovary, and brain, and is present in multiple forms in those tissues. Immunolocalization of the Spnr protein within the testis shows that it is expressed exclusively in postmeiotic germ cells and that it is localized to the manchette, a spermatid-specific microtubular array. Although the Spnr protein is expressed too late to be directly involved in the translational repression of Prm-1 specifically, we suggest that the Spnr protein may be involved in other aspects of spermatid RNA metabolism, such as RNA transport or translational activation.

Substratum surface topography alters cell shape and regulates fibronectin mRNA level, mRNA stability, secretion and assembly in human fibroblasts

The regulation of cell shape, fibronectin mRNA level, secretion and assembly by substratum surface topography was investigated in early passage human gingival fibroblasts cultured on titanium-coated smooth or V-shaped grooved substrata produced by micromachining. Cells on grooved surfaces were significantly elongated and orientated along the grooves of the substratum, while cell height, measured using confocal scanning laser microscopy, was approximately 1.5-fold greater than that of cells on smooth surfaces. Northern hybridization analysis revealed that on a per cell basis the grooved surface increased the amounts of fibronectin mRNA/cell approximately 3.5-fold at 16 hours, approximately 1.9-fold at 40 hours and approximately 2.2-fold at 90 hours, while the mRNA levels of the house-keeping gene glyceraldehyde-3-phosphate dehydrogenase (GAPD) were constant. The amounts of secreted fibronectin on the grooved surface were increased approximately 2-fold for all time points. The stability of fibronectin mRNA was also altered by substratum surface topography. The half-life of fibronectin mRNA on smooth surfaces was estimated to be approximately 5 hours, but on the grooved surfaces the half-life of fibronectin mRNA showed a two-phase response: a rapid 60% reduction in the first half-life (t1/2 approximately 2 hours) and a 2.4-fold increase in the second half-life (t1/2 approximately 12 hours) relative to that observed on the smooth surface. The GAPD mRNA half-lives were essentially unaffected by the surface topography of the substrata. The grooved surface was also found to alter the amount of fibronectin assembled into the extracellular matrix, producing a approximately 2-fold increase in the cultures at all time points. It thus appears that substratum surface topography alters cell shape and modulates fibronectin at the transcriptional and post-transcriptional levels, as well as the amount of fibronectin assembled into extracellular matrix. Micromachining, which has the ability to precisely control surface topography over a wide range of dimensions and shapes, appears to be a useful technique in investigating the relationship between cell shape and function.

Single mRNAs visualized by ultrastructural in situ hybridization are principally localized at actin filament intersections in fibroblasts

Considerable evidence indicates that mRNA associates with structural filaments in the cell (cytoskeleton). This relationship would be an important mechanism to effect mRNA sorting since specific mRNAs could be sequestered at sites within the cell. In addition, it can provide a mechanism for spatial regulation of mRNA expression. However, the precise structural interactions between mRNA and the cytoskeleton have yet to be defined. An objective of this work was to visualize "individual" poly(A) mRNA molecules in situ by electron microscopy to identify their relationship to individual filaments. Poly(A) RNA and filaments were identified simultaneously using antibodies to detect hybridized probe and filaments or actin-binding proteins. In human fibroblasts, most of the poly(A) mRNA (72%) was localized within 5 nm of orthogonal networks of F-actin filaments. Poly(A) mRNA also colocalized with vimentin filaments (29%) and microtubules (< 10%). The sites of mRNA localization were predominantly at filament intersections. The majority of poly(A) mRNA and polysomes colocalized with the actin crosslinking proteins, filamin, and alpha-actinin, and the elongation factor, EF-1 alpha (actin-binding protein; ABP-50). Evidence that intersections contained single mRNA molecules was provided by using a labeled oligo dT probe to prime the synthesis of cDNA in situ using reverse transcriptase. Both the poly(A) and cis sequences of the same mRNA molecule could then be visualized independently. We propose that the cytoskeletal intersection is a mRNA receptor and serves as a "microdomain" where mRNA is attached and functionally expressed.

Physical properties of cytoplasm

The physical properties of cytoplasm differ considerably from dilute aqueous solutions. Recent research has improved our understanding of the properties of the fluid phase and provided a more detailed picture of cytoarchitecture and its relation to cytomechanics. Several recent holistic models indicate novel directions for future research.

Effect of cytoarchitecture on the transport and localization of protein synthetic machinery

The emerging picture of cytoarchitecture imposes constraints on the transport and localization of several components of the protein synthetic machinery. The range in which "free" polysomes can diffuse through the cytoplasm may be restricted to about 50 nm due to obstruction by cytoskeletal barriers. Individual ribosomes and large transcripts will diffuse at least 4-10 times slower in cytoplasm than in dilute aqueous solution and may be sterically excluded from some cytoplasmic domains. The transport of these components from the nucleus to the cell periphery may be restricted to microtubule-containing channels that traverse the excluding domains. In the peripheral cytoplasm, mitochondria, endoplasmic reticulum, and other membrane-bound organelles are found only in nonexcluding channels, while actin, nonmuscle filamin (ABP280), and fodrin are concentrated in excluding domains. This suggests that the cytoplasmic volume may be functionally compartmentalized by local differentiations of cytoarchitecture. Excluding compartments may play a structural role, while nonexcluding compartments are the site of vesicle traffic and protein synthesis.

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.

Spatial and temporal dissection of immediate and early events following cadherin-mediated epithelial cell adhesion

Cell-cell adhesion is at the top of a molecular cascade of protein interactions that leads to the remodeling of epithelial cell structure and function. The earliest events that initiate this cascade are poorly understood. Using high resolution differential interference contrast microscopy and retrospective immunohistochemistry, we observed that cell-cell contact in MDCK epithelial cells consists of distinct stages that correlate with specific changes in the interaction of E-cadherin with the cytoskeleton. We show that formation of a stable contact is preceded by numerous, transient contacts. During this time and immediately following formation of a stable contact, there are no detectable changes in the distribution, relative amount, or Triton X-100 insolubility of E-cadherin at the contact. After a lag period of approximately 10 min, there is a rapid acquisition of Triton X-100 insolubility of E-cadherin localized to the stable contact. Significantly, the total amount of E-cadherin at the contact remains unchanged during this time. The increase in the Triton X-100 insoluble pool of E-cadherin does not correlate with changes in the distribution of actin or fodrin, suggesting that the acquisition of the Triton X-100 insolubility is due to changes in E-cadherin itself, or closely associated proteins such as the catenins. The 10 minute lag period, and subsequent prompt and localized nature of E-cadherin reorganization indicate a form of signaling is occurring.

The cytoskeleton and the cellular traffic of the progesterone receptor

Previous studies on glucocorticoid receptors have suggested the existence of interactions between the receptor and microtubule or actin networks. It was hypothesized that such interactions may contribute to the guidance of steroid hormone receptors towards the nucleus. We used a permanent L cell line expressing the delta 638-642 progesterone receptor. This mutant has all the characteristics of the wild type receptor except that the deletion of five amino acids inactivates the constitutive karyophilic signal. Consequently, the receptor is cytoplasmic in the absence of hormone but is shifted into the nucleus when administration of hormone activates the second karyophilic signal. Optical microscopy and confocal laser microscopy were used in intact cells or in cells depleted of soluble elements by permeabilization with detergents. By immunofluorescence, the receptor was found to be mainly concentrated in the perinuclear area. A small fraction of progesterone receptor (PR) persisted in this region after Triton X100 treatment. These observations suggested that the receptor could interact with some insoluble constituent(s) of the cytoplasm. However, careful colocalization studies showed that this heterogenous distribution was not due to interactions with microtubule, microfilament, or intermediate filament networks. Functional involvement of these networks in the translocation of the receptor into the nucleus was studied after cell treatment with cytoskeletal drugs such as nocodazole, demecolcine and cytochalasin. None of these compounds prevented or even delayed the hormone-dependent transfer of delta 638-642 PR into the nucleus. Similar conclusions were reached with the wild type receptor expressed by transfection in Cos-7 cells. PR was shifted from the nucleus into the cytoplasm by administration of energy-depleting drugs. After disruption of the various cytoskeletal networks normal nuclear reaccumulation of the receptor was observed when these drugs were removed. The results thus suggest that the progesterone receptor is not colocalized with the main cytoskeletal components. Disruption of the cytoskeletal networks does not prevent its nuclear translocation. Thus, karyophilic signals and interactions with the nuclear pore seem to be the primary determinants of the cellular traffic of the progesterone receptor.

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.

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.

Association of glycolytic enzymes with the cytoskeleton

The diverse physical associations of the glycolytic enzymes with structural components of the cell suggest that the glycolytic enzymes are not entirely soluble in the cell. The relatively low affinities of the associations are likely responsible for the apparently transient interactions. The binding phenomenon is suggested to regulate metabolism through changes in enzymatic activity and facilitates localized enrichment of the enzymes.

Regulation of expression and intracellular distribution of calreticulin, a major calcium binding protein of nonmuscle cells

In the present study we have demonstrated the presence of calreticulin, a major Ca(2+)-sequestering protein of nonmuscle cells, in a variety of cell types in tissue culture. The protein localizes to the endoplasmic reticulum in most cell types and also to the nuclear envelope or nucleoli-like structures in some cell types. Calreticulin is enriched in the rough endoplasmic reticulum, suggesting a possible involvement in protein synthesis. Calreticulin terminates with the KDEL-COOH sequence, which is likely responsible for its endoplasmic reticulum localization. Unlike some other KDEL proteins, calreticulin expression is neither heat-shock nor Ca(2+)-shock dependent. Using a variety of metabolic inhibitors, we have shown that the pool of calreticulin in L6 cells has a relatively slow turnover and a stable intracellular distribution. In proliferating muscle cells in culture (both L6 and human skeletal muscle) calreticulin is present in the endoplasmic reticulum, and additional intranuclear staining is observed. When fusion of the L6 cells is inhibited with either a high serum concentration or TGF-beta or TPA, the nucleolar staining by anticalreticulin antibodies is diminished, although the presence of calreticulin in the endoplasmic reticulum remains unchanged. In contrast, in differentiated (i.e., fused) muscle cells neither intranuclear nor intracellular staining for calreticulin is present. We conclude, therefore, that calreticulin is abundant in the endoplasmic reticulum in proliferating myoblasts, while it is present in only small amounts in sarcoplasmic reticulum membranes in terminally differentiated myotubes. We propose a model for the domain structure of calreticulin that may explain the differential subcellular distribution of this protein. Because of its widespread distribution in nonmuscle tissues, we postulate that calreticulin is a multifunctional protein that plays an important role in Ca(2+) sequestering and thus that it is the nonmuscle analog of calsequestrin.

Interaction between mRNA, ribosomes and the cytoskeleton

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Post-translational incorporation of actin into myofibrils in vitro: evidence for isoform specificity

The incorporation of actin into myofibrils has been examined in a cell-free system [Bouché et al.: Journal of Cell Biology 107:587-596, 1988; Goldfine et al.: Cellular and Molecular Biology of Muscle Development, 1989]. Actin was translated in a reticulocyte lysate in the presence of 35S-methionine (35S-actin) or purified from muscle and labeled with fluorescein-5-isothiocyanate (FITC-actin). Myofibrils were incubated with either 35S-actin or FITC-actin and then analyzed by gel electrophoresis or fluorescence microscopy. When myofibrils were incubated with FITC-actin monomer in the reticulocyte lysate buffer, strong fluorescent labeling was observed in Z-band regions and less so in I-bands. No fluorescence was detected in non-overlap regions of A-bands. Confocal microscopic analysis of these myofibrils indicated that FITC-actin was distributed evenly across the diameter of the myofibrils. These observations suggest that actin incorporation in the reticulocyte lysate buffer occurred at sites in the sarcomere which contain actin. In contrast, FITC-actin showed a variety of non-physiological incorporation patterns when incubated with myofibrils in the presence of an isotonic buffer (I-buffer). However, when ATP was added to I-buffer, FITC-actin showed a pattern of incorporation into myofibrils similar to that seen in the reticulocyte lysate buffer. Immunoblots indicated that actin of native size was released from myofibrils during incubation in the reticulocyte lysate buffer. No actin release was detected when the myofibrils were incubated in I-buffer lacking ATP. We used this system to compare the incorporation of actin isoforms into myofibrils. Both alpha- and beta-actins exhibited incorporation into the myofibrils but there was a three-fold greater incorporation of the alpha isoform. We propose that the differential affinities of actin isoforms for myofibrils and other cytoskeletal structures could provide a mechanism for actin isoform targeting within the cytoplasm.

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.

Differential association of membrane-bound and non-membrane-bound polysomes with the cytoskeleton

We report here a differential release of specific mRNAs from the cytoskeleton by cytochalasin D treatment. Non-membrane-bound polysomal mRNAs, such as histone mRNA and c-fos mRNA, are readily released from the cytoskeleton of HeLa cells during cytochalasin D treatment. Over 90% of H3 and H4 histone mRNA is associated with the cytoskeleton in control cells and only 25% in cells treated with cytochalasin D (40 micrograms/ml). In contrast, the membrane-bound polysomal mRNAs for HLA-B7 and chorionic gonadotropin-alpha are inefficiently released from the cytoskeletal framework by cytochalasin D alone; approximately 98% of the HLA-B7 mRNA in control cells is associated with the cytoskeleton, whereas approximately 65% of the HLA-B7 mRNA is retained on the cytoskeleton in cells treated with cytochalasin D (40 micrograms/ml). Disruption of polysome structure with puromycin during cytochalasin D treatment results in the efficient release of HLA-B7 mRNA from the cytoskeleton. Under these conditions, only 25% of the HLA-B7 mRNA remains associated with the cytoskeletal framework. Thus, membrane-bound polysomes appear to be attached to the cytoskeleton through a cytochalasin D-sensitive site as well as through association with the nascent polypeptide and/or ribosome. These results demonstrate a complex association of polysomes with the cytoskeleton and elements of the endoplasmic reticulum.

Cytomatrix synthesis in MDCK epithelial cells

Detailed information regarding the synthesis rates of individual protein components is important in understanding the assembly and dynamics of the cytoskeletal matrix of eukaryotic cells. As an approach to this topic, the dual isotope technique of Clark and Zak (J. Biol. Chem., 256:4863-4870, 1981), was employed to measure fractional synthesis rates (FSRs) in growing and quiescent cultures of MDCK epithelial cells. Cell protein was labeled to equilibrium with [14C]leucine over several days and then pulse-labeled for 4 hours with [3H]leucine. FSRs (as percent per hour) were calculated from the 3H/14C ratio of cell extracts or individual proteins separated by two-dimensional polyacrylamide gel electrophoresis and the 3H/14C ratio of free leucine in the medium. Synthesis of total cell protein rose from approximately 1.4%/hour in quiescent cells to 3.5%/hour in the growing cultures. The latter rate was sufficient to account for the rate of protein accumulation and a low level of turnover in the growing cultures. The FSR of the buffered-Triton soluble extract was higher and the cytoskeletal FSR significantly lower than that for total protein in quiescent monolayers. This difference, however, was not observed in growing cultures. A distinct pattern of differences was seen in the FSRs of individual cytoskeletal proteins in the quiescent cultures. Vimentin synthesis was significantly lower than that of the keratins and the keratin FSRs were not obviously matched in pairwise fashion. Unexpectedly, the FSRs of alpha- and beta-tubulin diverged in quiescent cells with alpha-tubulin turnover exceeding beta-tubulin. Likewise, components of the microfilament lattice showed unequal fractional synthesis rates, myosin and alpha-actinin being faster than actin. In addition, the FSR for globular actin exceeded that of the cytoskeletal associated form. The results suggest that metabolic coupling between individual cellular filament systems is not strict. The data are, however, consistent with models that predict that assembly of a subcellular structure influences the turnover of its component proteins.

Subcellular localization of transcripts in Drosophila photoreceptor neurons: chaoptic mutants have an aberrant distribution

Photoreceptor neurons in the Drosophila retina are long (100 mu) and narrow, providing a system for the study of the intracellular distribution of transcripts and proteins. The chaoptic gene is expressed exclusively in photoreceptor neurons, and mutations of the gene result in reduced developmental competence of cells to generate normal rhabdomeric membranes. The mutant protein exhibited altered distribution both in developing and adult photoreceptor neurons. Furthermore, the transcript distribution in mutants was altered, decreasing with distance from the nucleus, instead of the normal uniform distribution throughout the cell soma. The deficit of transcript concentration correlated with the severity of developmental defect in rhabdomere formation along the cell. In contrast, the distribution of the opsin transcript was not affected by the chaoptic mutation. To observe RNA localization at the ultrastructural level, a high-resolution, electron microscopic in situ hybridization protocol was developed. The results indicate that the normal chaoptic transcript is present on the rough endoplasmic reticulum, which may be a vehicle for specific transcript distribution.

Distribution of myelin basic protein and P2 mRNAs in rabbit spinal cord oligodendrocytes

Myelin basic protein (MBP) and P2 protein are small positively charged proteins found in oligodendrocytes of rabbit spinal cord. Both proteins become incorporated into compact myelin. We have begun investigations into the mechanisms by which MBP and P2 become incorporated into the myelin membrane. We find that P2, like the MBPs, is synthesized on free polysomes in rabbit spinal cord. Cell fractionation experiments reveal that rabbit MBP mRNAs are preferentially segregated to the peripheral myelinating regions whereas P2 mRNAs are predominantly localized within the perikaryon of the cell. In vitro synthesized rabbit MBP readily associates with membranes added to translation mixtures, whereas P2 protein does not. It is possible that P2 requires a "receptor" molecule, perhaps a membrane-anchored protein, for association with the cytoplasmic face of the myelin membrane.

Intracellular assembly of newly synthesized canine cardiac myosins

To investigate how newly synthesized cardiac myosins are assembled into myofilaments, we analysed the distribution of newly produced alpha-myosin heavy chain isozyme in sarcomeres by immunoelectron microscopy using a monoclonal antibody (CMA19), which is specific for alpha-myosin heavy chain. Isozymic changes in myosin heavy chains from beta to alpha type were induced in canine ventricular muscles and cultured ventricular myocytes by administration of 1-thyroxine. We incubated the glycerinated ventricular muscles or cultured ventricular myocytes with the enzyme (horseradish peroxidase) labelled Fab fragment of CMA19. After the reaction with 3, 3'-diaminobenzidine and osmification, we prepared ultrathin sections of the ventricular muscles or cultured ventricular myocytes and analysed their staining patterns by electron microscopy. There was apparent heterogeneity in the staining intensity of the myofilaments among different cells, among different myofibrils and even intramyofibrillarly. Higher magnification revealed that there were scattered foci of strong reaction which appeared to be foci of assembly of the newly synthesized alpha-myosin heavy chain. Immunocytochemical study also showed heterogeneous reactions within myofilaments and that there were scattered foci of myofilament assembly, which were closely associated with polyribosomes producing newly induced alpha-myosin heavy chain. These data suggest that newly synthesized cardiac myosins are assembled into myofilaments from the sites of synthesis, that is polyribosomes. This may explain the heterogeneity of the assembly pattern of newly synthesized cardiac myosins at the subcellular level.

Organization of intermediate filaments and their association with collagen-containing phagosomes in mouse decidual cells

We have analyzed the distribution of intermediate filaments (IF) in the cytoplasm of mature decidual cells of mice. IF were scattered throughout the cytoplasm of these cells although there was a preferential accumulation around the nuclei. In many cells a large area of the cytoplasm was occupied by a rich network of IF that extended from the perinuclear region toward the cell surface. Thin bundles of IF crossed the cytoplasm without a preferential orientation. IF were also seen in close association with nuclear pore complexes, gap junctions, mitochondria, and lysosomes. A very developed network of IF surrounded phagosomes that contained collagen fibrils. Longitudinal and cross sections of these phagosomes showed a very close association of IF with the phagosome membrane.

Cytoskeleton-bound mRNA for a 40-kDa polypeptide in rat L6 cells is not always translated

The relationship between attachment of mRNA to the cytoskeletal framework and its translation was examined using the mRNA for a polypeptide of 40 kDa (P-40) which is translated in rat L6 myoblasts but not in the myotubes. In both myoblasts and myotubes this mRNA was found to be associated with the cytoskeletal framework. Furthermore, the stability of the association between P-40 mRNA and the cytoskeletal framework in absence of RNA and protein synthesis was examined by using actinomycin D and NaF to block RNA and protein synthesis, respectively. In absence of RNA synthesis portions of both nontranslated P-40 mRNA and translated actin mRNA of myotubes were released into the soluble fraction. In myoblasts, however, both mRNAs remained associated with the cytoskeletal framework following inhibition of RNA synthesis. Inhibition of protein synthesis, on the other hand, had a more dramatic effect on the association between the cytoskeletal framework and P-40 mRNA in myoblasts but not in myotubes. In contrast, the association between actin mRNA and cytoskeletal framework was unaffected by inhibition of protein synthesis in both myoblasts and myotubes. The results of these studies show that the molecular nature of association between cytoskeletal framework and mRNA may differ among mRNAs and may also depend on whether the cells are dividing or are terminally differentiated. Furthermore, no direct relationship between the translation of mRNA and its attachment to the cytoskeletal framework was observed.