Filament-directed intercellular contacts during differentiation of cultured chick myoblasts

RNA transfer through tunneling nanotubes

It was already suggested in the early '70's that RNA molecules might transfer between mammalian cells in culture. Yet, more direct evidence for RNA transfer in animal and plant cells was only provided decades later, as this field became established. In this mini-review, we will describe evidence for the transfer of different types of RNA between cells through tunneling nanotubes (TNTs). TNTs are long, yet thin, open-ended cellular protrusions that are structurally distinct from filopodia. TNTs connect cells and can transfer many types of cargo, including small molecules, proteins, vesicles, pathogens, and organelles. Recent work has shown that TNTs can also transfer mRNAs, viral RNAs and non-coding RNAs. Here, we will review the evidence for TNT-mediated RNA transfer, discuss the technical challenges in this field, and conjecture about the possible significance of this pathway in health and disease.

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

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

Strategies for ultrastructural visualization of biotinated probes hybridized to messenger RNA in situ

A progressive development of the application of in situ methodology to ultrastructural procedures has resulted in the ability to detect individual molecules of mRNA with high probability. Beginning with whole-mount cells and then developing myotubes, both in culture and detergent extracted before fixation, we were able to progress to methods which allow detection of mRNA in tissue sections. Initial results confirm that the detection of mRNA in thin-sectioned tissue is very similar to observations on the extracted, cultured cells, and that the same methods of data analysis apply. Current work is devoted to the application of the methodology to other cellular structures, such as the nucleus, and to other tissue-probe systems, such as brain. Acknowledgements. The authors appreciate the skilled help from John McNeil and Shirwin Pockwinse in the laborious and time-consuming preparations of material and photography. FS was on sabbatical leave from the Department of Pathology at Southwestern Medical Center.

Microheterogeneity of actin gels formed under controlled linear shear

The diffusion coefficients and fluorescence polarization properties of actin subjected to a known shear have been determined both during and after polymerization, using a modification of a cone-plate Wells-Brookfield rheometer that allows monitoring of samples with an epifluorescence microscope. Fluorescence polarization and fluorescence photobleaching recovery experiments using rhodamine-labeled actin as a tracer showed that under conditions of low shear (shear rates of 0.05 s-1), a spatial heterogeneity of polymerized actin was observed with respect to fluorescence intensity and the diffusion coefficients with actin mobility becoming quite variable in different regions of the sample. In addition, complex changes in fluorescence polarization were noted after stopping the shear. Actin filaments of controlled length were obtained using plasma gelsolin (gelsolin/actin molar ratios of 1:50 to 1:300). At ratios of 1:50, neither spatial heterogeneity nor changes in polarization were observed on subjecting the polymerized actin to shear. At ratios of approximately 1:100, a decrease on the intensity of fluorescence polarization occurs on stopping the shear. Longer filaments exhibit spatial micro-heterogeneity and complex changes in fluorescence polarization. In addition, at ratios of 1:100 or 1:300, the diffusion coefficient decreases as the total applied shear increased. This behavior is interpreted as bundling of filaments aligned under shear. We also find that the F-actin translational diffusion coefficients decrease as the total applied shear increases (shear rates between 0.05 and 12.66 s-1), as expected for a cumulative process. When chicken gizzard filamin was added to gelsolin-actin filaments (at filamin/actin molar ratios of 1:300 to 1:10), a similar decrease in the diffusion coefficients was observed for unsheared samples. Spatial microheterogeneity might be related to the effects of the shear field in the alignment of filaments, and the balance between a three-dimensional network and a microheterogeneous system (containing bundles or anisotropic phases) appears related to both shear and the presence of actin-binding proteins.

Cytoskeletal changes induced in HEp-2 cells by the cytotoxic necrotizing factor of Escherichia coli

The effect of the cytotoxic necrotizing factor of Escherichia coli on HEp-2 cells was studied by fluorescence and scanning electron microscopy. This cytotoxin, known for inducing the formation of giant multinucleated cells in several cell lines, caused changes in actin and tubulin organization. The presence of membrane ruffles at the cell border and of numerous thick bundles of actin crossing the cell body, suggests that the factor promotes cell spreading; this probably interferes with cytokinesis, ultimately leading to the formation of very large flattened multinucleated cells. Moreover, the nuclear segmentation observed in treated cells seems to be associated with a rearrangement of actin in the perinuclear region and with the presence of tubulin bundles in proximity to nuclear clefts. Although the primary target is still unknown, these findings suggest that the cytoskeleton is affected accounting for the multinucleation process induced by the factor.

Formation and alignment of Z lines in living chick myotubes microinjected with rhodamine-labeled alpha-actinin

We have used fluorescence analogue cytochemistry in conjunction with time lapse recording to study the dynamics of alpha-actinin, a major component of the Z line, during myofibrillogenesis. Rhodamine-labeled alpha-actinin microinjected into living cultured chick skeletal myotubes became localized in discrete cellular structures within 1 h and remained specifically associated with structures for up to 4 d, allowing individual identified structures to be followed during development. In the most immature cells used, alpha-actinin was found in diffuse aggregates, some of which displayed sarcomeric periodicity. Aggregates were observed to coalesce into better defined structures (Z bands) that were approximately 1.0-micron wide. Z bands condensed into narrow, more intensely fluorescent Z lines in 4-48 h. During this period, Z lines grew laterally, primarily by the addition of small beads of alpha-actinin to existing Z lines or by the merging of small Z lines. In more mature cells, alpha-actinin added to Z lines without going through a visible intermediary structure. Mean sarcomere length did not change significantly during the stages examined, although the variability of sarcomere length did decrease markedly over time for identified sets of sarcomeres. At early stages, myofibrils frequently shifted position in both the longitudinal and lateral directions. Neighboring myofibrils were frequently associated for one or more sarcomeres sporadically along their length, such that the intervening sarcomeres were often misaligned. Associations between myofibrils were often transitory. Shifts in myofibril location in conjunction with the formation, breaking, and reformation of lateral associations between myofibrils facilitated the alignment of Z lines through a trial and error process.

Possible translocation of actin and alpha-actinin along stress fibers

We have employed fluorescent analogue cytochemistry and fluorescence photobleaching to study the mobility of actin and alpha-actin along stress fibers. Rhodamine-labeled actin or alpha-actinin microinjected into embryonic chick cardiac fibroblasts soon became incorporated into stress fibers. A pulse of a laser microbeam was used to photobleach small spots on the fluorescent stress fibers. Images of the bleached fiber were recorded with an intensified image processing system at 2-3 min intervals. The distance between the bleached spot and the terminus of the stress fiber, which remained stationary throughout the experiment, was then measured in the successive images. Movement of bleached spots was detected along stress fibers located in the apparently trailing processes of polygonal fibroblasts, and only occurred in one direction: away from the distal tip of the stress fiber. The rate of movement calculated for alpha-actinin-injected cells was 0.24 +/- 0.12 micron/min, for actin-injected cells, 0.29 +/- 0.11 micron/min. The rate did not seem to be affected by the location of the spot relative to the distal end of the stress fiber unless the spot was located within the most distal 5 microns of the stress fiber. Anti-myosin antibody staining indicated that stress fibers which demonstrated translocation were relatively depleted of myosin. The apparent translocation of proteins along stress fibers, possibly generated by stretching, may be related to the retraction of cell processes during locomotion.