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. 2003 Jan 21;13(2):161-167.
doi: 10.1016/s0960-9822(02)01436-7.

Single mRNA molecules demonstrate probabilistic movement in living mammalian cells

Dahlene Fusco #  1 , Nathalie Accornero #  2 , Brigitte Lavoie  3 , Shailesh M Shenoy  1 , Jean-Marie Blanchard  2 , Robert H Singer  1 , Edouard Bertrand  2
Affiliations

Single mRNA molecules demonstrate probabilistic movement in living mammalian cells

Dahlene Fusco et al. Curr Biol. .

Abstract

Cytoplasmic mRNA movements ultimately determine the spatial distribution of protein synthesis. Although some mRNAs are compartmentalized in cytoplasmic regions, most mRNAs, such as housekeeping mRNAs or the poly-adenylated mRNA population, are believed to be distributed throughout the cytoplasm. The general mechanism by which all mRNAs may move, and how this may be related to localization, is unknown. Here, we report a method to visualize single mRNA molecules in living mammalian cells, and we report that, regardless of any specific cytoplasmic distribution, individual mRNA molecules exhibit rapid and directional movements on microtubules. Importantly, the beta-actin mRNA zipcode increased both the frequency and length of these movements, providing a common mechanistic basis for both localized and nonlocalized mRNAs. Disruption of the cytoskeleton with drugs showed that microtubules and microfilaments are involved in the types of mRNA movements we have observed, which included complete immobility and corralled and nonrestricted diffusion. Individual mRNA molecules switched frequently among these movements, suggesting that mRNAs undergo continuous cycles of anchoring, diffusion, and active transport.

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Figures

Figure1
Figure1. Visualization of Single mRNA Molecules in Living Mammalian Cells
(A) A schematic of the constructs used in this study. The cassettes expressing the MS2-GFP fusion protein and the reporter mRNAs are shown. (B and C) Visualization of the reporter mRNA with the MS2-GFP fusion protein. Cos cells transiently cotransfected with the pMS2GFP and pRSV-Z-6-SV plasmids and hybridized in situ with a probe against the MS2 binding sites are shown. GFP, green; in situ, red; nuclei (DAPI), blue. (B) A cell expressing both the MS2-GFP fusion and the reporter showing a GFP signal in the cytoplasm colocalizing with the probe (yellow). The scale bar represents 20 μm. (C) A cell expressing the MS2-GFP fusion alone, showing only a nuclear signal. The scale bar represents 20 μm. (D) Improvement of the sensitivity of RNA detection. Cos cells transiently cotransfected with pMS2-GFP and pRSV-Z-24-SV and treated as in (B), except that images were deconvolved. GFP (green) and the in situ hybridization signal using probes to MS2 (red) colocalize in particles (yellow). The scale bar represents 2 μm. (E–G) Quantification of the number of RNA molecules per particle. Cos cells transiently cotransfected with pMS2-GFP and pGRE-Z-24-hGH. (E) Automated selection and analysis of the cytoplasmic GFP particles. The scale bar represents 10 μm. Left: acquired image. Right: deconvolved image with automatically selected objects that correspond to the GFP particles. (F) The histogram depicts the number of GFP molecules per particle. The results are from 8 cells ( >600 particles). (G) The histogram depicts the number of mRNA molecules per GFP particle detected by in situ hybridization, using a single probe to LacZ. The results are from 5 cells (125 particles).
Figure 2
Figure 2. Dynamics of mRNA Molecules in the Cytoplasm of Mammalian Cells
All images were obtained at a rate of nine images per second, for periods of 22 s, and were deconvolved. (A) Movements of LacZ-24-hGH mRNAs. Cos cells transiently expressing LacZ-24-hGH mRNAs and the MS2-GFP protein were imaged live. Left: a maximum intensity image projection of 200 time frames on 1 image (“maximal projection”). The scale bar represents 10 μm. Right: panel magnifications: the scale bar represents 2 μm. mRNA track superimposed (green) from each of the indicated boxed regions. See Movies 1 and 2 in the Supplementary Material. The blue arrow points to a “static” particle in the vicinity of a “corralled” mRNA. (B) Movements of LacZ-24-SV mRNAs. Cos cells were transiently cotransfected with pRSV-Z-24-SV and pMS2-GFP and were imaged as in (A). The scale bar represents 10 μm. Right: panel magnifications: track of mRNA movement superimposed (green) on an enlargement from each of the indicated boxed regions. The scale bar represents 2 μm. (See Movies 1 and 2 in the Supplementary Material). (C) Velocities of directed motion. For each directed movement of either the LacZ-24-hGH or the LacZ-24-SV mRNA particles, the mean velocity was calculated. The corresponding histograms show a peak at 1.0–1.5 μm/s.
Figure 3
Figure 3. Role of the Cytoskeleton in Movement of Nonlocalized mRNA
(A) Retention of the LacZ-hGH mRNA after triton extraction. Cos cells transfected with pGRE-Z-24-GH were extracted with 0.1% triton for 1 min at room temperature (right) or were fixed directly (left), and they were hybridized in situ with an MS2 probe. Unextracted cells show a stronger cytoplasmic staining (6.5x ). (68 68 μm). (B–D) Simultaneous localization of microtubules and mRNA movements in live cells. Cos cells were transiently cotransfected with pMS2-YFP, pCFP-tubulin, and pGRE-Z-24-hGH and were imaged live in the CFP and YFP wavelengths. Images were captured in the CFP channel, immediately followed by a movie in the YFP channel (3 images per second for 15 s) and, again, a CFP image. This reduced the artifacts due to cytoskeletal remodeling during recording of mRNA movements. (B) Left: CFP image; right: maximum intensity image projection of YFP movie. The scale bar represents 10 μm. (C) Magnification of merged images of microtubules (red) with YFP (green). The scale bar represents 2 μm. Arrows point to the center of mass of a “directed” particle at t 0 and t 1.67 s. The distance between the center of mass at these time points is 1.3 μm (particle speed 780 nm/s). (D) Magnification of merged images of microtubules (red) with YFP (green). The scale bar represents 2 μm. Arrows point to the center of mass of two “static” particles that colocalize with microtubules.
Figure 4
Figure 4. Cytoplasmic Movements of ß-actin mRNA Reporter
(A) Movements of LacZ-24-ßact mRNAs. Cos cell transiently cotransfected with pMS2-GFP and pRSV-Z-24-ßact, imaged live. Left: a maximum intensity image projection of 200 frames. The scale bar represents 10 μm. Right: panel magnifications: track of mRNA movement superimposed (green) on an enlargement from each of the indicated boxed regions. The scale bar represents 2 μm. (See Movies 3 and 4 in the Supplementary Material). (B) Influence of a zipcode sequence on moving particles. Moving particles of GFP-labeled RNA, observed in the image sequences acquired, were classified as either directed, diffusional, corralled, or static. The average distribution from cells transfected with an mRNA reporter either containing (LacZ-24-ßact) or not containing (LacZ-24-hGH, LacZ-24-SV) the ß-actin zipcode sequence are shown. (n 9 image sequences each, 162 particles. The bars report the standard errors). (C and D) Colocalization of directed motion with microtubules. (C) Cos cells transiently cotransfected with pMS2-YFP, pCFP-tubulin, and pRSV-Z-24act, imaged live. Left: maximum intensity image projection of YFP; right, CFP. The scale bar represents 10 μm. (D) Merged images of CFP microtubules (red) with sequences of movement of the RNA labeled with MS2-YFP (green). The scale bar represents 2 μm. The arrowheads point to the particle center of mass at t 0 and t 2.67 s. The distance between the arrowheads is 2.7 μm (particle speed 990 nm/s).
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